JP2020059892A - Electroseamed steel pipe for oil wells and method for producing the same - Google Patents

Abstract

To provide an electroseamed steel pipe for oil wells and a method for producing the same.SOLUTION: An electroseamed steel pipe for oil wells contains specific amounts of C, Mn, Ti, Nb, N, Si, Al, B, P, S, with the balance being Fe and inevitable impurities; the Mn%/Si% ratio being 2.0 or more; SK value of formula (1) being 0.10 or more and BH value of formula (2) being 1.3-2.7; a base metallographic structure containing bainite or martensite with an area ratio of 80% or more and ferrite with an area ratio of 5% or more; an electroseamed welded part being a mixed structure of bainite and martensite; a tensile yield strength of 655 MPa or more and 758 MPa or less; a tensile strength of 724 MPa or more; a Charpy absorption energy at -20°C being 100 J or more. SK=Nb%+Ti%+(V%+Mo%)/5 formula (1), BH=2.7C%+0.4Si%+Mn%+0.45(Ni%+Cu%) +0.8Cr%+2Mo% formula (2).SELECTED DRAWING: None

Description

  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 electric resistance welded steel pipe for oil wells having 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 a manufacturing method thereof.

Oil well pipes are steel pipes used for extracting gas and oil from the ground. In recent years, the characteristics required for oil well pipes have been changing with the harshness of natural resource excavation areas.
As one example of this, deep wells are being advanced, and improvement in crushing characteristics (characteristics that do not buckle against external pressure) and higher toughness have begun to be demanded.

The crushing property is improved by the high yield strength in the circumferential direction of the steel pipe and the high shape accuracy (especially uneven thickness / roundness) of the steel pipe. ERW steel pipe is known to have high crushing characteristics compared to other pipe types of the same size (outer diameter and wall thickness) because of its high shape accuracy.
Another measure for improving the crush strength is to increase the strength, but strength and toughness are generally reciprocal properties, and it is difficult to achieve both at the same time.
Further, in the electric resistance welded steel pipe, the increase in toughness of the electric resistance welded portion, which makes it difficult to refine the crystal grains, is a factor that hinders the increase in the toughness of the steel pipe.

Patent Documents 1 and 2 disclose a method for manufacturing an electric resistance welded well tube having a yield strength of 655 MPa, which utilizes Mo.
Patent Document 1 discloses a hot-rolled steel sheet containing specified amounts of C, Si, Mn, Ti, B, Mo, V, and Nb, and suppressing P, S, and O to a low level, and having a tempered upper bainite as the metal structure. Disclosed is a hot-rolled steel sheet for electric resistance welded pipe in which the short diameter of the old γ-shaped particles is 25 μm or less.
In Patent Document 2, in an electric resistance welded steel pipe containing C, Si, Mn, Nb, V, Ti, Mo, Ni, and Al in a specified amount and setting the total value of the Mo amount and the Ni amount in a specified range, 10 area% The following discloses a high-strength electric resistance welded steel pipe comprising polygonal ferrite and the balance bainitic ferrite and having tensile strength, yield strength, and Charpy absorbed energy in specific ranges.

JP, 2005-168864, A Japanese Patent No. 6048621

  Both of the electric resistance welded steel pipes described in Patent Documents 1 and 2 are characterized in that the Charpy absorbed energy at 0 ° C. is 22 J or more, and in Patent Document 1, the Charpy absorbed energy at 0 ° C. is 46 to 76 J. Examples are disclosed, and Patent Document 2 discloses an example of Charpy absorbed energy of 75 to 170 J at 0 ° C. However, as described above, in recent years, further toughness has been required, specifically, 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 requirement. I can't.

  The present invention has been made to solve the above-mentioned problems, and has an yield strength of 655 MPa or more in both the base metal portion and the electric resistance welded portion, and an oil well having a Charpy toughness value of 100 J or more at -20 ° C. An object of the present invention is to provide an electric resistance welded steel pipe and a manufacturing method thereof.

The inventors of the present invention have made extensive studies in order to solve the above-mentioned problems, and as a result, have obtained the following findings.
The strength and toughness characteristics of steel materials are closely related to their metallographic structure, and refining the crystal grain size is one of the few ways to improve both strength and toughness. On the other hand, in an electric resistance welded steel pipe produced from a hot-rolled steel sheet, precipitation strengthening is generally the main strengthening mechanism, but it is known that precipitation strengthening deteriorates toughness. In the base metal part of ERW steel pipe, crystal grains can be refined by performing low temperature rolling etc. in the production of hot rolled steel plate, and even with precipitation strengthened steel, steel pipe with relatively high toughness can already be produced. Is.

  On the other hand, with respect to the electric resistance welded portion, a metal microstructure is formed by the heat treatment after the electric resistance welding, and therefore a microstructure refinement technique such as low temperature rolling cannot be used. In the heat treatment after electric resistance welding, the metal structure is controlled by simple water cooling after reheating (austenizing), and the control range of the metal structure is smaller than that in the hot rolling process. In view of such process restrictions, the present inventors diligently studied the relationship between the structure factor of the base metal / electric resistance welded portion and the strength / toughness characteristics.

  As a result, we have clarified the feasible and optimal metallographic structure for both strength and toughness of both the base metal part and the electric resistance welded part. Specifically, in the base metal part, it is important to optimize precipitation strengthening and control of hot rolling conditions and cooling conditions after hot rolling to optimize the microstructure fraction. It was found that it is important to suppress the precipitation of precipitates and form a metal structure mainly composed of bainite-martensite structure by containing and then transforming at low temperature. This is because cracks are less likely to occur in the electric resistance welded portion by reducing the precipitates that contribute to strengthening as much as possible.

Based on the technical idea based on these findings, the present inventors have a yield strength of 655 MPa class for both the base metal part and the electric resistance welded part, and satisfy the toughness in which the absorbed energy in the Charpy test at −20 ° C. is 100 J or more. The present invention has been accomplished by developing a technology that enables manufacturing of an electric resistance welded steel pipe.
The gist of the present invention aiming to solve the above problems is as follows.
[1] The electric resistance welded steel pipe for oil wells of the present embodiment is, in mass%, C: 0.020 to 0.100%, Mn: 0.60 to 2.00%, Ti: 0.015 to 0.150%, Nb: 0.015 to 0.100%, N: 0.0010 to 0.0200%, Si: 0.010 to 0.500%, Al: 0.001 to 0.100%, B: 0.0010. It contains 0.0025% and is limited to P: 0.030% or less and S: 0.010% or less, the balance being Fe and unavoidable impurities, and the Mn% / Si% ratio is 2. 0 or more, the SK value represented by the formula (1) is 0.10 or more, and the BH value represented by the formula (2) is in the range of 1.3 to 2.7, and the metal structure of the base metal has an area. Structure composed of bainite and / or martensite of 80% or more in area ratio, and ferrai of 5% or more in area ratio Including the structure, the metal structure of the electric resistance welded part is mainly a mixed structure of bainite and martensite, 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 base material and the electric resistance welding The Charpy absorbed energy at -20 ° C of the welded portion is 100 J or more, and the hardness of the electric resistance welded portion is 240 Hv or more.
SK = Nb% + Ti% + (V% + Mo%) / 5 (Equation 1)
BH = 2.7C% + 0.4Si% + Mn% + 0.45 (Ni% + Cu%) + 0.8Cr% + 2Mo% ... Formula (2)

[2] In the oil-well electric resistance welded steel pipe of the present embodiment, the plate thickness is preferably 10 mm or more and 25 mm or less.

"3" In the electric resistance welded steel pipe for oil wells of the present embodiment, in mass%, Mo: 0.01 to 0.50%, Cu: 0.05 to 0.50%, Ni: 0.05 to 0.50%, One or more of Cr: 0.05 to 0.50%, V: 0.01 to 0.10%, Ca: 0.0001 to 0.0100%, REM: 0.0001 to 0.0100%. May be included.

"4" The manufacturing method of the electric resistance welded steel pipe for oil wells according to the present embodiment is, in mass%, C: 0.020 to 0.100%, Mn: 0.60 to 2.00%, Ti: 0.015 to 0. 150%, Nb: 0.015 to 0.100%, N: 0.0010 to 0.0200%, Si: 0.010 to 0.500%, Al: 0.001 to 0.100%, B: Including 0.0010 to 0.0025%, P: 0.030% or less, S: 0.010% or less, with the balance being Fe and unavoidable impurities, the composition is Mn% / Si%. A slab having a ratio of 2.0 or more, an SK value represented by the formula (1) of 0.10 or more, and a BH value represented by the formula (2) of 1.3 to 2.7 is 950 ° C. After finishing rolling under the conditions that the cumulative rolling ratio below is 2.0 or more and the rolling end temperature is 850 ° C. or less, a flatness of 35 ° C./s or more is obtained. After hot-rolling a hot-rolled steel sheet cooled to 450 to 650 ° C. at a uniform cooling rate and rolled up, pipe-welded and electric-welded, the electric-welded welded portion is heated between 900 ° C. and 1050 ° C., and then the average cooling rate is 15 It is characterized in that the cooling is carried out at a temperature of 500 ° C./s or more and the cooling is stopped in the range of 500 ° C. to room temperature.
SK = Nb% + Ti% + (V% + Mo%) / 5 ... Formula (1)
BH = 2.7C% + 0.4Si% + Mn% + 0.45 (Ni% + Cu%) + 0.8Cr% + 2Mo% ... Formula (2)

[5] In the method for manufacturing an electric resistance welded steel pipe for oil wells according to this embodiment, Mo: 0.01 to 0.50%, Cu: 0.05 to 1.00%, Ni: 0.05 to 1 in mass%. 0.000%, Cr: 0.05 to 1.00%, V: 0.01 to 0.10%, Ca: 0.0001 to 0.0100%, REM: 0.0001 to 0.0100%, one kind Alternatively, two or more kinds can be contained.

  According to the present invention, it is possible to improve the toughness and crushing property of the base material portion and the electric resistance welded portion, and it is possible to provide the electric resistance welded steel pipe for oil wells and the manufacturing method thereof, which greatly contributes to industry. Has a remarkable effect.

The graph which shows the relationship between SK value and yield strength. The graph which shows the relationship between BH value and Vickers hardness.

Hereinafter, an embodiment of an electric resistance welded steel pipe for oil wells according to the present invention will be described.
First, the component composition of steel suitable for the electric resistance welded steel pipe for oil wells 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. In addition, in the numerical range in the component composition, the numerical range represented by "to" means a range including the numerical values described before and after "to" as the lower limit value and the upper limit value, unless otherwise specified. Therefore, for example, 0.020 to 0.10% means a range of 0.020% or more and 0.10% or less.

  The electric resistance welded steel pipe for oil wells according to the present embodiment is, as described below, in mass%, C: 0.020 to 0.100%, Mn: 0.60 to 2.00%, Ti: 0.015. .About.0.150%, Nb: 0.015 to 0.100%, N: 0.0010 to 0.0200%, Si: 0.010 to 0.500%, Al: 0.001 to 0.100%, B: 0.0010 to 0.0025% is included, P: 0.030% or less and S: 0.010% or less, with the balance being Fe and inevitable impurities.

  In addition, the electric resistance welded steel pipe for oil wells having the above-described composition has a Mn% / Si% ratio of 2.0 or more, an SK value of 0.10 or more represented by a formula (1) described later, and a formula (2) described below. The BH value indicated by) is in the range of 1.3 to 2.7.

SK = Nb% + Ti% + (V% + Mo%) / 5 (Equation 1)
BH = 2.7C% + 0.4Si% + Mn% + 0.45 (Ni% + Cu%) + 0.8Cr% + 2Mo% ... Formula (2)
In the formulas (1) and (2), C%, Si%, Mn%, Ni%, Cu%, Cr%, Mo%, Nb%, Ti%, and V% are C, Si, Mn, and It is the content (mass%) of Ni, Cu, Cr, Mo, Nb, Ti, and V. Mo, Cu, Ni, Cr, and V are arbitrary contained elements, and when they are not intentionally contained, they are calculated as 0 in the formula (1). In addition, if the compositional composition does not have a lower limit, it indicates that the content is up to the impurity level.
Hereinafter, the reasons for limiting the component composition of the steel material of the present invention will be described.

"C: carbon (0.020 to 0.100%)"
C is an important element that contributes to the improvement of the hardenability of steel and the development of strength, and the C content is 0.020% or more. If the carbon content is lower than this range, the strength of the base material decreases. On the other hand, if the C content exceeds 0.100%, the strength of the steel exceeds, so the upper limit of the C content is made 0.100%.
"Mn: manganese (0.60 to 2.00%)"
Mn is an element that enhances the hardenability of steel and is essential for making S harmless, and the Mn content is set to 0.60% or more. When Mn is excessively contained, coarse MnS is generated in the central portion of the plate thickness, which may impair the toughness of the base steel plate and the electric resistance welded portion. Therefore, the upper limit of the Mn content is 2.00%.

"Ti: Titanium (0.015 to 0.150%)"
Ti is an element that forms carbonitrides and improves the strength of the base steel sheet, and also contributes to the refinement of crystal grains. The Ti content is 0.015% or more. However, if the Ti content exceeds 0.150%, coarse carbonitrides are generated, and the toughness of the base steel sheet and the electric resistance welded portion is deteriorated. Therefore, the upper limit of the Ti content is 0.150%. .

"Nb: niobium (0.015 to 0.100%)"
Nb is contained in order to enhance the toughness of the base steel sheet and to contribute to the improvement of the strength of the base steel sheet. The Nb content is 0.015% or more in order to improve the toughness by the non-recrystallization rolling. If the Nb content exceeds 0.100%, the coarse carbide deteriorates the toughness, so the upper limit of the Nb content is set to 0.100%.
"N: Nitrogen (0.0010 to 0.0200%)"
N suppresses the coarsening of crystal grains by forming an alloy nitride in the steel and improves the toughness of the base steel sheet. In order to obtain the effect, the N content is set to 0.0010% or more. On the other hand, if the N content exceeds 0.0200%, the amount of alloy nitride produced increases, and the toughness of the base steel sheet and the electric resistance welded portion deteriorates. Therefore, the upper limit of the N content is 0.0200%. To do.

"Si: Silicon (0.010 to 0.500%)"
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 steel sheet and the electric resistance welded portion and improving the toughness. In order to obtain the effect, the Si content is set to 0.010% or more. On the other hand, if the Si content exceeds 0.500%, inclusions may be generated in the electric resistance welded portion, the Charpy absorbed energy may decrease, and the toughness may decrease, so the upper limit of the Si content is set to 0. 500%.
"Al: Aluminum (0.001 to 0.100%)"
Al, like Si, is contained as a deoxidizing material for steel. To prevent cracking due to free oxygen, the Al content is set to 0.001% or more. On the other hand, if the Al content exceeds 0.100%, the toughness of the electric resistance welded portion is reduced due to the formation of Al-based oxide during electric resistance welding, so the upper limit of the Al content is set to 0.100%. .

"B: Boron (0.0010 to 0.0025%)"
B is an element that enhances the hardenability of steel in a small amount. In order to obtain the effect, the B content is set to 0.0010% or more. On the other hand, if the B content exceeds 0.0025%, the effect of improving the hardenability is reduced due to the formation of B precipitates, so the upper limit of the B content is made 0.0025%.

"P: Phosphorus (0.030% or less)"
P is an element existing as a target impurity in steel, and if the P content exceeds 0.030%, segregation at grain boundaries impairs toughness, so the upper limit of P content is made 0.030%. .
"S: Sulfur (0.010% or less)"
S is an element existing as an impurity in the steel, and if contained in excess, it deteriorates the toughness of the steel, so the upper limit of the S content is made 0.010%.

  In the present embodiment, in addition to the above elements, the base steel sheet is further provided with Mo: 0.01 to 0.50%, Cu: 0.05 to 0.50%, Ni: 0. 05-0.50%, Cr: 0.05-0.50%, V: 0.01-0.10%, Ca: 0.0001-0.0100%, REM: 0.0001-0.0100% One or more elements selected from the above may be contained.

"Mo: molybdenum (0.01 to 0.50%)"
The reason for containing Mo is to improve the hardenability of the steel material and to obtain high strength by precipitation strengthening. In order to obtain the effect, the Mo content is 0.01% or more. If a large amount of Mo is contained, the toughness may be reduced due to the formation of Mo carbonitrides, so the upper limit of the Mo content was made 0.50%.
"Cu: Copper (0.05 to 0.50%)"
Cu is an element effective in improving the strength of the base material, and in order to obtain the effect, the Cu content is set to 0.05% or more. However, if too much Cu is contained, fine Cu particles may be generated and the toughness may be significantly deteriorated. Therefore, the upper limit of the Cu content is 0.50%.

"Ni: nickel (0.05 to 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 in order to obtain its effect, the Cr content is set to 0.05% or more. On the other hand, Cr is also an element that deteriorates the weldability, and when it is contained in a large amount, the toughness of the electric resistance welded part is reduced by the Cr-based inclusions generated in the electric resistance welded part. Therefore, the upper limit of the Cr content is 0.50%.

"V: Vanadium (0.01-0.10%)"
V has almost the same effect as Nb, and in order to obtain the effect, the V content is set to 0.01% or more. V also has an effect of suppressing softening of the electric resistance welded portion. However, if a large amount of V is contained, the toughness decreases due to the precipitation of V carbonitride. Therefore, the upper limit of the V content is 0.10%.
"Ca: Calcium (0.0001-0.0100%)"
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 set to 0.0001% or more. If the Ca content exceeds 0.0100%, coarse Ca-based inclusions or clusters are generated, which may adversely affect the toughness. Therefore, the upper limit of the Ca content is 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 0.0001% or more. On the other hand, if REM is contained in excess of 0.0100%, coarse oxides may be generated and the toughness of the base steel sheet may be reduced. Therefore, the upper limit of the REM content is 0.0100%.
Here, REM refers to a total of 17 elements of Sc, Y and lanthanoid, and the content of the REM means the total content of these elements. For the REM, for example, a misch metal in which a plurality of these elements are mixed can be used.

The balance other than the above elements is Fe and inevitable impurities. In addition to the above elements, a trace amount of an element that does not impair the action and effect of this embodiment may be contained.
Further, in the present embodiment, the SK value represented by the above-described formula (1) is set to 0.10. The SK value is a parameter related to the precipitation strengthening amount, and the higher the SK value, the larger the precipitation strengthening amount.
In the present embodiment, the cooling stop temperature of the steel sheet after hot rolling is 450 to 650 ° C, and precipitation strengthening is the main strengthening mechanism in the steel material in which cooling is stopped in this temperature range and rolled up. From this, this parameter (SK value) is related to the yield strength of the base material as shown in FIG.
FIG. 1 is a graph showing the relationship between the SK value and the base material yield strength selected from a plurality of samples from the examples and comparative examples described later.
Taking the yield strength as an example, the yield strength falls below 655 MPa when the SK value falls below 0.10. Therefore, the SK value is set to 0.10. The SK value affects the tensile strength as well as the yield strength.

In this embodiment, the BH value represented by the equation (2) is set to 1.3 or more. The BH value is related to the hardenability of steel, and the higher the BH value, the higher the hardenability.
In the present embodiment, the strength of the electric resistance welded portion forms a metal structure by rapid cooling to 500 ° C. or less, so that precipitates do not substantially precipitate and the strength is almost determined by the hardenability. The BH value) is related to the strength of the electric resistance welded portion as shown in FIG.
FIG. 2 is a graph showing the relationship between the BH value and the Vickers hardness of the electric resistance welded portion selected from a plurality of samples and displayed from the examples and comparative examples described later.
By setting the BH value to 1.3 or more, the hardness of the electric resistance welded portion becomes 240 Hv or more. Therefore, the BH value is set to 1.3 or more. If the BH value is too high, the hardenability is too high, so that ferrite is not generated in the base material and the toughness deteriorates. Therefore, the upper limit was set to 2.7.
In the steel sheet having the above composition, the hardness of 240 Hv means that the hardness is equivalent to 655 MPa in terms of yield strength in the tensile test. Although B is not included in the expression (2) of this parameter (BH value), this parameter is premised on the inclusion of B (boron), and is not applicable to steel materials not containing B.

  In the present embodiment, when the parameter of the Mn% / Si% ratio becomes smaller than 2.0, the toughness of the electric resistance welded portion deteriorates. It is considered that this is because the MnSi-based inclusions generated in the electric resistance welded portion serve as the starting point of fracture. When the Mn% / Si% ratio is 2.0 or more, the Charpy absorbed energy becomes stable at a high level, so the Mn% / Si% ratio is 2.0 or more in the electric resistance welded steel pipe of the present embodiment.

The required metallographic structure and the ratio thereof of the electric resistance welded steel pipe for oil wells of the present embodiment are as follows.
In the component range of the present embodiment, the metal structure of the base material includes a structure composed of bainite and / or martensite having an area ratio of 80% or more and ferrite having an area ratio of 5% or more. As a result, toughness is improved. When the area ratio of the structure composed of bainite and / or martensite is less than 80%, carbon concentration in bainite and martensite becomes remarkable, and the addition of B increases hardness. As a result, the toughness of the base material deteriorates. Further, generating a predetermined amount of ferrite has an effect of improving toughness through grain refinement, and in order to obtain the effect, the area ratio of ferrite is 5% or more. If the ferrite content is less than 5%, the toughness of the base material deteriorates.
The metallographic structure of the electric resistance welded portion is mainly a mixed structure of bainite and martensite. It should be noted that both the base material portion and the electric resistance welded portion may inevitably contain residual austenite in an area ratio of 3% or less, but this does not affect the characteristics of the steel material.

  These metal structures can be produced by highly controlling the hot rolling process and the heat treatment of the electric resistance welded portion described later. In addition, the area ratio of the metal structure is measured in the electric resistance welded steel pipe by a cross section at a position deviated from the electric resistance welded portion by 90 ° in the pipe circumferential direction (specifically, a cross section perpendicular to the pipe axis direction). ), The weld line on the surface consisting of the plate thickness direction and the pipe circumferential direction is exposed by nital etching for the electric resistance welded portion, and then EBSD (500 μm left and right from the weld line). It can be determined by grain average misorientation analysis (hereinafter referred to as GAM analysis) based on the crystal orientation information obtained by the Electron Back Scatter Diffraction patterns) method. Specifically, after the surface is mirror-polished, finish polishing with colloidal silica is performed, and then a Field-Emission Scanning Electron Microscope (7001F manufactured by JEOL) is used in a step of 0.3 μm for a region of 200 μm × 300 μm. Crystal orientation analysis is performed by the EBSD method.

In the subsequent GAM analysis, a region surrounded by a crystal orientation difference of 15 ° is defined as one crystal grain, and one having an average crystal orientation difference of 1 ° or less is determined as ferrite. The value obtained by arithmetically averaging the area ratio of ferrite in each field obtained by measuring 5 or more fields of view in the above-mentioned measurement is taken as the ferrite area ratio of the electric resistance welded steel pipe.
The EBSD method can also obtain crystal structure information together with crystal orientation information, and can classify ferrite / bainite / martensite and retained austenite from the crystal structure information. Therefore, it is possible to measure the area ratio of retained austenite by performing EBSD measurement.
The area ratio of the structure composed of one or both of bainite and martensite is the balance of the area ratio of ferrite and retained austenite.

Next, a method for manufacturing an electric resistance welded steel pipe for an oil well according to the present invention will be described.
First, a slab obtained by continuous casting from molten steel adjusted to the above composition is charged into a heating furnace and heated. Since the steel of the present embodiment has a large content of Ti and Nb, if the heating temperature of the slab is low, undissolved Nb carbide is generated and the toughness deteriorates. Therefore, the heating temperature is 1100 ° C or higher. Preferably. 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 roughly rolling the heated slab, finish rolling is performed under the conditions that the cumulative reduction ratio at 950 ° C or lower is 2.0 or more and the finish rolling end temperature is 850 ° C or lower.
These conditions are necessary for refining the metal structure of the base material and improving both strength and toughness, and particularly important when utilizing a structure mainly composed of bainite / martensite as in the present embodiment. Process.

After finish rolling, it is preferable to start accelerated cooling at a temperature of 3 points or higher of Ar. This is because after finish rolling, if air-cooled to less than the Ar3 point at which ferrite transformation starts, coarse polygonal ferrite is generated and strength decreases, which may deteriorate toughness.
Accelerated cooling is started at a temperature of the Ar 3 point or higher to suppress the formation of ferrite, whereby a structure containing a predetermined amount of bainite and martensite is generated.
The Ar3 point can be calculated by the following formula (3) from the components of the base steel sheet.
Ar3 (° C) = 910-310C% -80Mn% -55Ni% -20Cu% -15Cr% -80Mo% Formula (3)
In the formula (3), C%, Mn%, Ni%, Cu%, Cr%, and Mo% are contents (mass%) of C, Mn, Ni, Cu, Cr, and Mo, respectively. Ni, Cu, Cr, and Mo are arbitrary contained elements, and when they are not intentionally contained, they are calculated as 0 in the above formula (3).

In the present embodiment, controlling the area ratio of bainite martensite which is the main phase and ferrite which is the subphase is indispensable for balancing strength and toughness.
Steel material undergoes ferrite transformation from hot-worked austenite, and then bainite transformation or martensite transformation completes the transformation. The transformation behavior of steel is determined by the composition and hot rolling conditions / cooling pattern, and the effect of the composition / cooling pattern is particularly large.
The range of the components and the cooling rate of the electric resistance welded steel pipe for oil wells in the present embodiment is a technique for controlling the area ratio of the metal structure. Cooling is performed at an average cooling rate of 35 ° C./s or more to a cooling stop temperature of 450 to 650 ° C. When the cooling stop temperature exceeds 650 ° C., the area ratio of ferrite becomes high, and the carbon concentration in the balance becomes remarkable, so that hard martensite and bainite are generated and the toughness deteriorates. If the cooling stop temperature is lower than 450 ° C., no precipitate is formed in the subsequent winding step, and the strength cannot be satisfied. It is preferable to wind up within 10 seconds after stopping cooling.
The steel sheet of the embodiment is particularly effective for a steel pipe having a plate thickness of 10 to 25 mm.

The hot-rolled steel sheet obtained as described above is continuously roll-formed into an open pipe, and then the vicinity of the butted portion is heated to a melting point or higher, and electric resistance welding is performed by pressure welding with a squeeze roll. After electric resistance welding, heat the outer surface of the electric resistance welded portion so as to be in the range of 900 ° C to 1050 ° C, then cool it so that the average cooling rate is 15 ° C / s or more, and from 500 ° C to room temperature. By stopping the cooling within the range, the target ERW steel pipe can be obtained.
The temperature during heating is measured from the outer surface of the steel pipe with a radiation thermometer. These conditions are necessary processes to secure the strength of the electric resistance welded portion and to prevent the toughness from deteriorating by allowing the precipitated element to remain in a solid state as much as possible.
If the heating temperature of the electric resistance welded portion is lower than 900 ° C., the coarse structure generated during welding remains, and the toughness deteriorates. Further, if the heating temperature of the electric resistance welded portion exceeds 1050 ° C., the crystal grain size becomes coarse and the toughness deteriorates. The cooling rate needs to be 15 ° C./s or more in order to generate martensite bainite and obtain a desired hardness of the electric resistance welded portion without depositing precipitates. If the cooling rate is less than 15 ° C / s, it becomes difficult to secure the toughness of the electric resistance welded portion. When the cooling stop temperature exceeds 500 ° C., precipitates are deposited during subsequent cooling, making it difficult to secure the toughness of the electric resistance welded portion. In the present embodiment, the lower limit of the cooling stop temperature does not particularly affect the characteristics.
After the heat treatment and cooling is completed, it is cooled to room temperature and reduced-sized by a sizer roll. The diameter reduction ratio of the diameter reduction rolling is preferably in the range of 0.3% to 5.0%.

The method of measuring the characteristics of the electric resistance welded steel pipe for oil well manufactured as described above is as follows.
A full-thickness test piece in the axial direction (rolling direction) of the steel pipe is taken as a tensile test piece from the electric resistance welded steel pipe, a tensile test is performed, and the yield strength (YS: 0.2% offset) and the tensile strength (TS) are measured. . Here, the full-thickness test piece and the tensile test piece are sampled from a portion corresponding to a position of 90 ° in the circumferential direction from the seam portion of the electric resistance welded steel pipe.
The value of the compressive yield strength calculated by the compression test in the circumferential direction is important for the crushing strength, but in the electric resistance welded steel pipe after pipe making, the compressive yield strength in the circumferential direction and the tensile yield strength in the axial direction have a correlation. Since it has, it is possible to evaluate the crush strength by a tensile test in the axial direction.

Furthermore, the method of measuring the toughness of the electric resistance welded steel pipe is as follows.
Regarding toughness, a full-size V-notch Charpy test piece in the circumferential direction (vertical rolling direction) was applied to the ERW welded portion of the electric resistance welded steel pipe and the base material (a portion corresponding to a position 90 ° in the circumferential direction from the seam portion of the electric resistance welded steel pipe). ), A V-notch Charpy test was performed, and the absorbed energy (CVN value) at −20 ° C. was measured. In the Charpy test of the electric resistance welded portion, a test piece provided with a V notch at the electric resistance comparison portion in the plane formed by the plate thickness direction and the circumferential direction is used.

  The hardness of the electric resistance welded portion is measured using a hardness tester (FV-300) manufactured by Future Tech Co., Ltd. After exposing the weld line of the surface consisting of the plate thickness direction and the pipe circumferential direction by nital etching, 6 points of the outer surface 1 / 4t, 1 / 2t and the inner surface 3 / 4t position 500 μm away from the weld line on the left and right sides. Is measured with a load of 1 kg, and the arithmetic mean value of all values is taken as the hardness of the electric resistance welded portion.

The electric resistance welded steel pipe manufactured as described above has a structure in which the metal structure of the base material is composed of one or both of bainite and martensite with an area ratio of 80% or more, and a ferrite structure with an area ratio of 5% or more. And the metal structure of the electric resistance welded part is mainly a mixed structure of bainite and martensite.
This ERW 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 absorbed energy at -20 ° C of the base material and the electric resistance welded portion of 100 J or more. It is an excellent electric resistance welded steel pipe having a hardness of 240 Hv or more at the sewn welded portion.

Examples will be described below. However, the examples described below are described along with specific examples, and the present invention is not limited to the conditions used in the following examples.
The slabs of the invention examples of No. 1 to No. 16 having the composition shown in Table 1 and the slabs of the comparative examples of No. 17 to No. 48 shown in Table 1 and Table 2 were manufactured by continuous casting, and 1200 to 1250 ° C. After heating to rough rolling, the finish rolling was performed under the conditions of a cumulative reduction ratio of 950 ° C. or less and a finish rolling finish temperature shown in Tables 3 and 4 to obtain a steel sheet having a thickness of 17.5 mm.
In the ROT (runout table) after finish rolling, the steel sheet after hot rolling was subjected to accelerated cooling to the cooling stop temperature shown in Tables 3 and 4 at the average cooling rate shown in Tables 3 and 4 and winding. , Hot rolled steel sheet was obtained.

The hot-rolled steel sheet thus obtained was continuously roll-formed into an open pipe, and then the vicinity of the butted portion was heated to a temperature equal to or higher than the melting point, and electric resistance welding was performed by pressure welding with a squeeze roll to obtain an electric resistance welded steel pipe.
The electric resistance welded portion of the electric resistance welded steel tube is 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 After cooling to the stop temperature, the cooling was stopped and the mixture was allowed to cool. After cooling to room temperature, diameter reduction rolling was performed using a sizer roll to obtain an electric resistance welded steel pipe having an outer diameter of 406 mm and a wall thickness of 17.5 mm.

  The base metal of the obtained electric resistance welded steel pipe and the metal structure of the electric resistance welded portion were investigated by the method described above. Further, the tensile test of the base material, the Charpy test of the base material and the electric resistance welded portion, and the hardness measurement of the electric resistance welded portion were carried out by the methods described above.

Tables 3 and 4 show the bainite and martensite area ratio of the base material, the ferrite area ratio of the base material, the base material yield strength of the electric resistance welded steel pipe, the base material tensile strength, the base material Charpy absorbed energy (J), and the electric resistance welded portion. The hardness (Hv) and the Charpy absorbed energy (J) of the electric resistance welded portion are collectively shown.
The bainite and martensite area ratio of the base material indicates the area ratio (%) occupied by the structure composed of one or both of bainite and martensite with respect to the entire structure of the base material.

  As shown in Table 1 and Table 3, No. 1 of the present invention example. 1 to No. Sample No. 16 has a tensile yield strength of 655 MPa or more and 758 MPa or less of a base material suitable for oil wells, a tensile strength of 724 MPa or more of the base material, and a Charpy absorbed energy of −20 ° C. of the base material and the electric resistance welded portion of 100 J. The above was an excellent electric resistance welded steel tube having a hardness of the electric resistance welded portion of 240 Hv or more.

Comparative example No. 1 shown in Table 1. In the sample of No. 1, since the C content was below the lower limit of the desirable range, as shown in Table 3, the base material yield strength was below the lower limit of the desirable range.
Comparative example No. 1 shown in Table 1. In the sample of No. 2, since the C content exceeded the upper limit of the desirable range, the base material yield strength exceeded the desirable range as shown in Table 3.
Comparative example No. 1 shown in Table 1. In the sample of No. 3, since the Mn content was below the lower limit of the desirable range, embrittlement due to S occurred, and as shown in Table 3, the toughness of the base material and the toughness of the electric resistance welded portion deteriorated.
Comparative example No. 1 shown in Table 1. In the sample of No. 4, since the Mn content exceeded the upper limit of the desirable range, embrittlement due to MnS occurred, and as shown in Table 3, the toughness of the base metal and the toughness of the electric resistance welded portion deteriorated.

Comparative example No. 1 shown in Table 1. In the sample of No. 5, the Ti content was below the lower limit of the desirable range, so the crystal grain size became large and the toughness of the base material deteriorated as shown in Table 3.
Comparative example No. 1 shown in Table 1. In the sample of No. 6, since the Ti content exceeded the upper limit of the desirable range, a large amount of Ti-based carbonitride was produced, and as shown in Table 3, the toughness of the base metal and the toughness of the electric resistance welded portion were deteriorated.
Comparative example No. 1 shown in Table 1. In the sample of No. 7, the Nb content was below the lower limit of the desirable range, so that the crystal grain size became large and the toughness of the base material deteriorated as shown in Table 3.
Comparative example No. 1 shown in Table 1. In the sample of No. 8, the Nb content exceeded the upper limit of the desirable range, so that a large amount of Nb-based carbonitride was produced, and as shown in Table 3, the toughness of the base metal and the toughness of the electric resistance welded portion were deteriorated.

Comparative example No. 1 shown in Table 1. In the sample of No. 9, the N content was below the lower limit of the desirable range, so carbonitride was not formed, the crystal grain size became coarse, and the base material toughness deteriorated as shown in Table 3.
Comparative example No. 1 shown in Table 1. In the sample of No. 10, the N content exceeded the upper limit of the desirable range, so that the generation of alloy carbide increased, and as shown in Table 3, the toughness of the base metal and the toughness of the electric resistance welded portion deteriorated.

Comparative example No. 3 shown in Table 2. In the sample of No. 11, the Si content was below the lower limit of the desirable range, so deoxidation was insufficient, and as shown in Table 4, the toughness of the base metal and the toughness of the electric resistance welded portion deteriorated.
Comparative example No. 3 shown in Table 2. In sample No. 12, the Si content exceeded the upper limit of the desirable range, so a large amount of Si oxide was generated in the electric resistance welded portion, and as shown in Table 4, the toughness of the electric resistance welded portion was deteriorated.
Comparative example No. 3 shown in Table 2. In the sample of No. 13, the Al content was below the lower limit of the desirable range, so deoxidation became insufficient, and as shown in Table 4, the toughness of the base metal and the toughness of the electric resistance welded portion deteriorated.
Comparative example No. 3 shown in Table 2. In the sample of No. 14, since the Al content exceeded the upper limit of the desirable range, a large amount of Si oxide was generated in the electric resistance welded portion, and as shown in Table 4, the toughness of the electric resistance welded portion was deteriorated.

Comparative example No. 3 shown in Table 2. In the sample of No. 15, the B content was below the lower limit of the desirable range, so the hardenability was insufficient, and the hardness of the electric resistance welded portion decreased as shown in Table 4.
Comparative example No. 3 shown in Table 2. In the samples of No. 16, since the B content exceeded the upper limit of the desirable range, the formation of B precipitates deteriorated the hardenability, and as shown in Table 4, the hardness of the electric resistance welded part decreased.
Comparative example No. 3 shown in Table 2. In the sample of No. 17, since the P content exceeded the upper limit of the desirable range, grain boundary embrittlement occurred, and as shown in Table 4, both the base metal toughness and the toughness of the electric resistance welded portion deteriorated.
Comparative example No. 3 shown in Table 2. In the sample of No. 18, the S content exceeded the upper limit of the desirable range, so that coarse inclusions were generated, and as shown in Table 4, the toughness of the base material and the toughness of the electric resistance welded portion were deteriorated.

Comparative example No. 3 shown in Table 2. In the sample of No. 19, Mn / Si was below the lower limit of the desirable range, so that MnSi-based oxide was generated in the electric resistance welded portion, and as shown in Table 4, the toughness of the electric resistance welded portion was deteriorated.
Comparative example No. 3 shown in Table 2. In the sample of No. 20, the SK value was below the lower limit of the desirable range, so that the base material tensile yield strength decreased as shown in Table 4.
Comparative example No. 3 shown in Table 2. In the sample of No. 21, the BH value was below the lower limit of the desirable range, so that the hardness of the electric resistance welded portion was decreased as shown in Table 4.

Comparative example Nos. Shown in Tables 2 and 4. In the sample of No. 22, the finish rolling end temperature exceeded the upper limit of the desirable range, so that the base material toughness deteriorated as shown in Table 4.
Comparative example Nos. Shown in Tables 2 and 4. In the sample of No. 23, the cumulative reduction ratio was below the lower limit of the desirable range, so that the base material toughness deteriorated as shown in Table 4.

Comparative example Nos. Shown in Tables 2 and 4. In the sample No. 24, the first stage cooling rate after hot rolling fell below the lower limit of the desirable range, so that the base material toughness deteriorated as shown in Table 4.
Comparative example Nos. Shown in Tables 2 and 4. In the sample of No. 25, the cooling stop temperature was below the lower limit of the desirable range, so that precipitation did not sufficiently occur and the strength did not reach the specified value as shown in Table 4.
Comparative example Nos. Shown in Tables 2 and 4. In the sample of No. 26, the cooling stop temperature exceeded the upper limit of the desirable range, so the structural fraction did not satisfy the regulation, and the base material toughness deteriorated as shown in Table 4.

Comparative example Nos. Shown in Tables 2 and 4. In the sample No. 27, the reheating temperature of the electric resistance welded portion was below the lower limit of the desirable range, so that the coarse structure remained as welded and the toughness of the electric resistance welded portion deteriorated as shown in Table 4.
Comparative example Nos. Shown in Tables 2 and 4. In the sample No. 28, the reheating temperature of the electric resistance welded portion exceeded the upper limit of the desired range, so that the structure was coarsened and the toughness of the electric resistance welded portion was deteriorated as shown in Table 4.

Comparative example Nos. Shown in Tables 2 and 4. In the sample No. 29, the cooling rate during heat treatment of the electric resistance welded portion was below the lower limit of the desirable range, so that the toughness of the electric resistance welded portion deteriorated as shown in Table 4.
Comparative example Nos. Shown in Tables 2 and 4. In sample No. 30, the cooling stop temperature during heat treatment of the electric resistance welded portion exceeded the upper limit of the desirable range, so that the toughness of the electric resistance welded portion was deteriorated as shown in Table 4.
Comparative example Nos. Shown in Tables 2 and 4. In the sample No. 31, the BH value exceeded the upper limit of the desired range, so that the ferrite area ratio fell below the standard as shown in Table 4 and the base material toughness deteriorated.
From the above description, it was found that manufacturing under the above-mentioned conditions is important when manufacturing the electric resistance welded steel pipe for oil wells having the above-mentioned excellent properties.

Claims (5)

In mass%,
C: 0.020 to 0.100%,
Mn: 0.60 to 2.00%,
Ti: 0.015 to 0.150%,
Nb: 0.015 to 0.100%,
N: 0.0010 to 0.0200%,
Si: 0.010 to 0.500%,
Al: 0.001 to 0.100%,
B: 0.0010 to 0.0025%
Including,
P: 0.030% or less,
S: limited to 0.010% or less, with the balance being a component composition consisting of Fe and impurities,
The Mn% / Si% ratio is 2.0 or more, the SK value represented by the formula (1) is 0.10 or more, and the BH value represented by the formula (2) is in the range of 1.3 to 2.7. The metal structure of the base metal includes a structure composed of one or both of bainite and martensite having an area ratio of 80% or more, and ferrite having an area ratio of 5% or more, and the metal structure of the electric resistance welded portion is Mainly a mixed structure of bainite and martensite, the tensile 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 absorbed energy at -20 ° C of the base material and the electric resistance welded portion is An electric resistance welded steel pipe for an oil well having a hardness of 100 J or more and a hardness of the electric resistance welded portion of 240 Hv or more.
SK = Nb% + Ti% + (V% + Mo%) / 5 (Equation 1)
BH = 2.7C% + 0.4Si% + Mn% + 0.45 (Ni% + Cu%) + 0.8Cr% + 2Mo% ... Formula (2)   The electric resistance welded steel pipe for oil wells according to claim 1, wherein the plate thickness is 10 mm or more and 25 mm or less. In mass%,
Mo: 0.01 to 0.50%,
Cu: 0.05 to 0.50%,
Ni: 0.05 to 0.50%,
Cr: 0.05 to 0.50%,
V: 0.01 to 0.10%,
Ca: 0.0001 to 0.0100%,
REM: 0.0001 to 0.0100%
The electric resistance welded steel pipe for oil wells according to claim 1 or 2, containing one or more of the above. In mass%,
C: 0.020 to 0.100%,
Mn: 0.60 to 2.00%,
Ti: 0.015 to 0.150%,
Nb: 0.015 to 0.100%,
N: 0.0010 to 0.0200%,
Si: 0.010 to 0.500%,
Al: 0.001 to 0.100%,
B: 0.0010 to 0.0025%
Including,
P: 0.030% or less,
S: limited to 0.010% or less, with the balance being a component composition consisting of Fe and impurities,
The Mn% / Si% ratio is 2.0 or more, the SK value represented by the formula (1) is 0.10 or more, and the BH value represented by the formula (2) is in the range of 1.3 to 2.7. The slab is finish-rolled under the conditions that the cumulative reduction ratio at 950 ° C or lower is 2.0 or higher and the rolling end temperature is 850 ° C or lower, and then cooled to 450 to 650 ° C at an average cooling rate of 35 ° C / sec or higher and wound. After pipe forming and electric resistance welding of the taken hot rolled steel sheet, the electric resistance welding portion is heated between 900 ° C. and 1050 ° C., then cooled at an average cooling rate of 15 ° C./s or more, and 500 ° C. or less. A method for producing an electric resistance welded steel pipe for oil wells, characterized in that cooling is stopped within a range.
SK = Nb% + Ti% + (V% + Mo%) / 5 ... Formula (1)
BH = 2.7C% + 0.4Si% + Mn% + 0.45 (Ni% + Cu%) + 0.8Cr% + 2Mo% ... Formula (2) In mass%,
Mo: 0.01 to 0.50% or less,
Cu: 0.05 to 1.00% or less,
Ni: 0.05 to 1.00% or less,
Cr: 0.05 to 1.00% or less,
V: 0.01 to 0.10% or less,
Ca: 0.0001 to 0.0100% or less,
REM: 0.0001-0.0100% or less 1 type or 2 types or more are contained, The manufacturing method of the electric resistance welded steel pipe for oil wells of Claim 4 characterized by the above-mentioned.

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