JP2018168425A - Seamless steel pipe for low alloy oil well - Google Patents

Abstract

To provide a seamless steel pipe for low alloy oil well having necessary strength and toughness even when a regulation of S content is alleviated.SOLUTION: A seamless steel pipe for low alloy oil well has a chemical composition with, by mass%, C:0.20 to 0.50%, Si:0.05 to 0.50%, Mn:0.05 to 1.0%, P:0.030% or less, S:0.0025 to 0.0060%, Al:0.005 to 0.10%, Cr:0.1 to 1.2%, Mo:0.25 to 1.0%, Ti:0.002 to 0.05%, N:0.01% or less, O:0.0030% or less, Ca:0.0010 to 0.0030%, REM:0.0010 to 0.0040%, V:0 to 0.30%, Nb:0 to 0.10%, Cu:0 to 1.0%, Ni:0 to 1.0%, B:0 to 0.0040% and the balance:Fe with impurities, average particle diameter of sulfide-based inclusion and oxysulfide-based inclusion of 2.0 μm or less and aspect ratio of 2.0 or less.SELECTED DRAWING: Figure 4

Description

  The present invention relates to a seamless steel pipe for a low alloy oil well.

  The oil well steel pipe is used as a casing or tubing of an oil well or a gas well (hereinafter, the oil well and the gas well are collectively referred to as an oil well). In recent years, the development environment for oil wells has become harsh due to concerns over the depletion of petroleum resources and energy. Yield strength of 758 MPa (110 ksi) class or higher is required for deepening the oil well to be mined, and the demand for low-temperature toughness is increasing in consideration of use in cold regions.

The oil well steel pipe is manufactured by performing heat treatment by quenching and tempering after hot pipe making. There are two types of heat treatment patterns: offline heat treatment and in-line heat treatment. In the offline heat treatment, the steel pipe once cooled to near room temperature after hot pipe making is reheated and quenched in an off-line heat treatment furnace to the austenite temperature range above the Ac ₃ transformation point and rapidly cooled, and below the Ac ₁ transformation point. Including tempering. In-line heat treatment does not cool to room temperature after hot pipe forming, but directly quenches from the temperature above the Ar ₃ transformation point using the temperature of the steel pipe after hot rolling, and temper below the Ac ₁ transformation point. Including.

  Oil-well steel pipes that have been off-line heat treated have excellent strength and toughness stability and corrosion resistance compared to oil-well steel pipes that have been in-line heat-treated because the austenite grains are refined by reverse transformation during reheating. . However, in-line heat treatment is often adopted because off-line heat treatment has a long lead time and has a problem with productivity. In particular, oil well steel pipes used in an environment not containing hydrogen sulfide are generally manufactured by in-line heat treatment.

  Since in-line heat treatment makes it difficult to refine crystal grains, it is effective to reduce inclusions in order to achieve both high strength and high toughness. Specifically, (1) low S by desulfurization treatment, and (2) suppression of formation of alumina clusters by addition of CaSi are performed.

  As a technique related to (1), Japanese Patent No. 3385966 describes a method of manufacturing a steel material that can obtain strength and excellent toughness without refining the structure. Specifically, this document describes reducing inclusions such as MnS and TiN.

  As a technique related to (2), Japanese Patent Application Laid-Open No. 2004-52076 discloses a form of oxide inclusions by adding rare earth elements (REM) to Al deoxidized or Al-Si deoxidized molten steel. It is described that the production of alumina clusters can be suppressed by controlling the above.

  Japanese Patent No. 5765497 discloses an electric resistance welded steel pipe in which the form of inclusions is controlled to improve the quality of the welded portion. Specifically, it is described that at least one of Ce and La is added before adding Ca in the steel sheet smelting step. It is described that, by this, inclusions can be made hard and fine, and a decrease in toughness due to the extension of the inclusions during upsetting can be suppressed.

Japanese Patent No. 3385966 JP 2004-52076 A Japanese Patent No. 5765497

  In desulfurization treatment for low sulfidation (for example, desulfurization treatment for reducing S content to 20 ppm or less), CaO is introduced into molten steel in a smelting process. However, CaO is expensive and causes an increase in cost and a decrease in productivity. Therefore, it is preferable that necessary strength and toughness can be ensured even if the regulation of the S content is relaxed and the desulfurization process is reduced.

  An object of the present invention is to provide a seamless steel pipe for a low alloy oil well that has the required strength and toughness even if the regulation of the S content is relaxed.

  The seamless steel pipe for a low alloy oil well according to an embodiment of the present invention has a chemical composition of mass%, C: 0.20 to 0.50%, Si: 0.05 to 0.50%, Mn: 0.00. 05-1.0%, P: 0.030% or less, S: 0.0025-0.0060%, Al: 0.005-0.10%, Cr: 0.1-1.2%, Mo: 0.25 to 1.0%, Ti: 0.002 to 0.05%, N: 0.01% or less, O: 0.0030% or less, Ca: 0.0010 to 0.0030%, REM: 0 0010 to 0.0040%, V: 0 to 0.30%, Nb: 0 to 0.10%, Cu: 0 to 1.0%, Ni: 0 to 1.0%, B: 0 to 0. 0040%, balance: Fe and impurities. The average particle diameter of sulfide inclusions and oxysulfide inclusions is 2.0 μm or less, and the aspect ratio is 2.0 or less.

  According to the present invention, a seamless steel pipe for low alloy oil wells having the required strength and toughness can be obtained even if the restriction on the S content is relaxed.

FIG. 1 is a diagram for explaining the detoxification mechanism of sulfide inclusions. FIG. 2 is a diagram for explaining the detoxification mechanism of sulfide inclusions. FIG. 3 is a diagram for explaining the detoxification mechanism of sulfide inclusions. FIG. 4 is a photomicrograph of the composite inclusion. FIG. 5 is a photomicrograph of sulfide inclusions.

  The present inventors have studied means for solving the above-mentioned problems. As a result, it has been found that the following effects (A) to (C) can be obtained by adding REM to molten steel and further adding Ca in the refining process.

(A) Al ₂ O ₃ is bonded by low melting point inclusions such as FeO and FeO · Al ₂ O ₃ acting as a binder to form an alumina cluster (see FIG. 1). Coarse alumina clusters reduce the toughness of the steel. By adding REM, FeO is reduced, and the alumina cluster becomes Al ₂ O ₃ and X ₂ O ₃ and / or X ₂ OS (X is REM) (see FIG. 2). Thereby, the production | generation of a coarse alumina cluster can be suppressed.

(B) At this time, sulfides such as MnS crystallize using the produced non-stretchable X ₂ O ₃ and / or X ₂ OS as nuclei and become composite inclusions (see FIG. 3). X ₂ O ₃ and / or X ₂ OS is a hard and fine oxide, and suppresses stretching of soft sulfide inclusions such as MnS to be combined. As a result, the sulfide inclusions can be rendered harmless.

(C) When REM is added alone, the nozzle is likely to close during casting. By adding Ca after REM addition, clogging of the nozzle can be suppressed. Ca also has the effect of spheroidizing inclusions. Therefore, it contributes to further modification of MnS.

  By properly controlling the amount of REM and Ca added, most of the S in the steel is produced as a composite inclusion even if the desulfurization process is reduced by relaxing the S content regulation, especially by the effect of (B) above. Is rendered harmless. Therefore, the amount of harmful S that affects toughness can be made equal to that of low-sulfur steel. This can reduce costs and improve productivity.

  Based on the above findings, the present invention has been completed. Hereinafter, a seamless steel pipe for a low alloy oil well according to an embodiment of the present invention will be described in detail.

[Chemical composition]
The seamless steel pipe for a low alloy oil well according to the present embodiment has a chemical composition described below. In the following description, “%” of the element content means mass%.

C: 0.20 to 0.50%
Carbon (C) increases the hardenability of the steel and increases the strength of the steel. If the C content is less than 0.20%, this effect cannot be sufficiently obtained. On the other hand, if the C content exceeds 0.50%, the steel is more susceptible to fire cracking. Therefore, the C content is 0.20 to 0.50%. The lower limit of the C content is preferably 0.22%. The upper limit of the C content is preferably 0.40%, more preferably 0.30%.

Si: 0.05 to 0.50%
Silicon (Si) deoxidizes steel. If the Si content is less than 0.05%, this effect cannot be obtained sufficiently. On the other hand, if the Si content exceeds 0.50%, the toughness of the steel decreases. Therefore, the Si content is 0.05 to 0.50%. The lower limit of the Si content is preferably 0.10%, more preferably 0.20%. The upper limit of the Si content is preferably 0.45%, more preferably 0.40%.

Mn: 0.05 to 1.0%
Manganese (Mn) increases the hardenability of steel and contributes to the improvement of strength. If the Mn content is less than 0.05%, this effect cannot be sufficiently obtained. On the other hand, when the Mn content exceeds 1.0%, the hot workability of the steel decreases. Therefore, the Mn content is 0.05 to 1.0%. The lower limit of the Mn content is preferably 0.1%, more preferably 0.2%. The upper limit of the Mn content is preferably 0.8%, more preferably 0.6%.

P: 0.030% or less Phosphorus (P) is an impurity. P segregates at the grain boundaries and lowers the toughness of the steel. Therefore, the P content is 0.030% or less. The P content is preferably as low as possible. The P content is preferably 0.020% or less.

S: 0.0025 to 0.0060%
Sulfur (S) is an impurity. S combines with Mn and the like to form soft sulfide inclusions and oxysulfide inclusions, thereby reducing the toughness of the steel. On the other hand, from the viewpoint of productivity and cost, it is preferable to reduce the desulfurization process. In the present embodiment, the S content is set to 0.0025 to 0.0060%. The lower limit of the S content is preferably 0.0030%, more preferably 0.035%. The upper limit of the S content is preferably 0.0055%, more preferably 0.0050%.

Al: 0.005-0.10%
Aluminum (Al) deoxidizes steel. If the Al content is less than 0.005%, this effect cannot be sufficiently obtained. On the other hand, when the Al content exceeds 0.10%, inclusions become coarse and the toughness of the steel decreases. Therefore, the Al content is 0.005 to 0.10%. The lower limit of the Al content is preferably 0.01%. The upper limit of the Al content is preferably 0.08%, more preferably 0.06%. The Al content in the present specification means the content of acid-soluble Al (so-called Sol. Al).

Cr: 0.1-1.2%
Chromium (Cr) increases the hardenability of steel and contributes to the improvement of strength. If the Cr content is less than 0.1%, this effect cannot be obtained sufficiently. On the other hand, if the Cr content exceeds 1.2%, the toughness of the steel decreases. Therefore, the Cr content is 0.1 to 1.2%. The lower limit of the Cr content is preferably 0.2%, and more preferably 0.4%. The upper limit of the Cr content is preferably 1.0%, more preferably 0.8%.

Mo: 0.25 to 1.0%
Molybdenum (Mo) improves the strength of steel by transformation strengthening and solid solution strengthening. If the Mo content is less than 0.25%, this effect cannot be obtained sufficiently. On the other hand, if the Mo content exceeds 1.0%, the toughness of the steel decreases. Therefore, the Mo content is 0.25 to 1.0%. The lower limit of the Mo content is preferably 0.27%, and more preferably 0.3%. The upper limit of the Mo content is preferably 0.8%, more preferably 0.6%.

Ti: 0.002 to 0.05%
Titanium (Ti) suppresses billet cracking. If the Ti content is less than 0.002%, this effect cannot be sufficiently obtained. On the other hand, if the Ti content exceeds 0.05%, carbide (TiC) is generated and the toughness of the steel is lowered. Therefore, the Ti content is 0.002 to 0.05%. The lower limit of the Ti content is preferably 0.003%, more preferably 0.005%. The upper limit of the Ti content is preferably 0.03%, more preferably 0.02%.

N: 0.01% or less Nitrogen (N) is an impurity. N forms nitride inclusions and lowers the toughness of the steel. Therefore, the N content is 0.01% or less. The N content is preferably as low as possible. The upper limit of the N content is preferably 0.008%, more preferably 0.006%. From the viewpoint of cost, the lower limit of the N content is preferably 0.001%.

O: 0.0030% or less Oxygen (O) is an impurity. O forms oxides and reduces the toughness of the steel. Therefore, the O content is 0.0030% or less. The O content is preferably as low as possible. The O content is preferably 0.0025% or less, more preferably 0.0020% or less.

Ca: 0.0010 to 0.0030%
Calcium (Ca) suppresses nozzle clogging during casting. Ca also contributes to the improvement of toughness by spheroidizing inclusions. If the Ca content is less than 0.0010%, these effects cannot be obtained sufficiently. On the other hand, if the Ca content exceeds 0.0030%, coarse oxide inclusions are generated, and the toughness of the steel decreases. Therefore, the Ca content is 0.0010 to 0.0030%. The lower limit of the Ca content is preferably 0.0015%, more preferably 0.0020%. The upper limit of the Ca content is preferably 0.0028%.

REM: 0.0010 to 0.0040%
Rare earth elements (REM) reduce FeO and suppress the formation of alumina clusters. REM also forms non-stretchable oxides and / or oxysulfides, traps harmful S and renders it harmless. If the REM content is less than 0.0010%, these effects cannot be obtained sufficiently. On the other hand, when the REM content exceeds 0.0040%, coarse oxide inclusions are generated, and the toughness of the steel decreases. Moreover, the fluidity of the molten steel is lowered, and the nozzle is likely to be blocked during casting. Therefore, the REM content is 0.0010 to 0.0040%. The lower limit of the REM content is preferably 0.0012%, more preferably 0.0015%. The upper limit of the REM content is preferably 0.0035%, more preferably 0.0030%.

  Note that REM is a generic name for 17 elements in which Y and Sc are combined with 15 elements of lanthanoid, and one or more of these elements can be contained. The REM content means the total content of these elements. Among REM, La and Ce are preferable.

  The balance of the chemical composition of the low alloy oil well seamless steel pipe according to the present embodiment is Fe and impurities. The impurity here refers to an element mixed from ore and scrap used as a raw material of steel, or an element mixed from the environment of the manufacturing process.

  The chemical composition of the seamless steel pipe for a low alloy oil well according to the present embodiment may contain the elements described below instead of a part of Fe. All elements described below are selective elements. That is, the chemical composition of the low alloy oil well seamless steel pipe according to the present embodiment may not include some or all of the following elements.

V: 0 to 0.30%
Nb: 0 to 0.10%
Vanadium (V) and niobium (Nb) form carbides and increase the strength of the steel. This effect can be obtained if any of these elements is contained. On the other hand, when the content of these elements becomes excessive, the toughness decreases. Therefore, the V content is 0 to 0.30%, and the Nb content is 0 to 0.10%. The lower limit of the V content is preferably 0.01%. The upper limit of V content is preferably 0.20%, more preferably 0.15%. The lower limit of the Nb content is preferably 0.002%. The upper limit of the Nb content is preferably 0.08%, more preferably 0.05%.

Cu: 0 to 1.0%
Ni: 0 to 1.0%
B: 0 to 0.0040% or less Copper (Cu), nickel (Ni), and boron (B) increase the hardenability of steel and contribute to the improvement of strength. This effect can be obtained if any of these elements is contained. On the other hand, when the content of these elements becomes excessive, the toughness decreases. Therefore, each content of Cu and Ni is 0 to 1.0%, and the B content is 0 to 0.0040%. The lower limit of each content of Cu and Ni is preferably 0.01%. The upper limit of each content of Cu and Ni is preferably 0.5%, more preferably 0.2%. The lower limit of the B content is preferably 0.0001%. The upper limit of the B content is preferably 0.0020%, more preferably 0.0010%.

The chemical composition of the seamless steel pipe for a low alloy oil well according to the present embodiment preferably satisfies the following formula (1).
(Ca / O + Ca / S + 0.285 × REM / O + 0.285 × REM / S) × (Al / Ca)> 20 Formula (1)
In Ca, O, S, REM, and Al of the formula (1), the content of each element is substituted by mass%.

Of the formula (1), (Ca / O + Ca / S + 0.285 × REM / O + 0.285 × REM / S) is an index of the ability of Ca and REM to trap O and S. The coefficient 0.285 is a numerical value close to the atomic weight ratio 0.289 of Ca and La and the atomic weight ratio 0.286 of Ca and Ce, and rounded for convenience of calculation. Moreover, (Al / Ca) is a composition ratio of CaO and Al ₂ O ₃ and is an index of the melting point of inclusions. If the left side of formula (1) (Ca / O + Ca / S + 0.285 × REM / O + 0.285 × REM / S) × (Al / Ca) is small, the inclusions are easily stretched.

  If the left side of Formula (1) is made larger than 20, the extension of inclusions can be easily suppressed, and the aspect ratio of inclusions can be reduced more stably. The left side of formula (1) is more preferably greater than 25, and even more preferably greater than 30.

[Inclusion]
The seamless steel pipe for a low alloy oil well according to this embodiment has an average particle diameter of 2.0 μm or less and an aspect ratio of 2.0 or less of sulfide inclusions and oxysulfide inclusions.

Specifically, the average particle diameter and aspect ratio of the sulfide inclusions and oxysulfide inclusions are measured as follows. An observation specimen is collected from a seamless steel pipe for low alloy oil wells so that the observation surface is parallel to the rolling direction. The observation surface is polished, and a scanning electron microscope (SEM) having a particle analysis function observes 100 visual fields (total 4.35 mm ² ) with a magnification of 500 times and an area of one visual field of 0.236 mm × 0.184 mm. The acceleration voltage during measurement is 15 kV. For the observation of inclusions, a Schottky field emission scanning electron microscope (FE-SEM, for example, “JSM-7800F” manufactured by JEOL Ltd.) can be used.

  Only sulfide inclusions and oxysulfide inclusions are extracted from data observed in 100 fields of view. Specifically, composition analysis is performed by an energy dispersive X-ray analyzer (EDS), and an S content of 10% by mass or more is extracted. The size of the extracted inclusion is calculated by image processing. Specifically, the major axis and minor axis of the inclusion are measured by elliptic approximation, and the equivalent circle diameter is obtained based on the area calculated from the major axis and minor axis. The major axis / minor axis is the aspect ratio of the inclusion. At this time, a circle equivalent diameter of 0.5 to 100 μm is used in the calculation, and a circle outside this range is ignored. For example, “GENESIS Particle Analysis, Version 5.10” manufactured by AMETEK can be used for data collection and analysis.

  The average value of the equivalent circle diameters of these inclusions is defined as the average particle diameter of the sulfide inclusions and oxysulfide inclusions in the seamless steel pipe for low alloy wells. Similarly, the average value of the aspect ratios of these inclusions is defined as the aspect ratio of the sulfide-type inclusions and oxysulfide-type inclusions of the low alloy oil well seamless steel pipe.

  When the average particle size of sulfide inclusions and oxysulfide inclusions is larger than 2.0 μm, or when the aspect ratio of sulfide inclusions and oxysulfide inclusions is larger than 2.0, Excellent toughness cannot be obtained. The average particle size of the sulfide inclusions and oxysulfide inclusions is preferably 1.8 μm or less, and more preferably 1.7 μm or less. The aspect ratio of the sulfide inclusions and oxysulfide inclusions is preferably 1.95 or less, and more preferably 1.9 or less.

  The seamless steel pipe for a low alloy oil well according to this embodiment preferably has a structure whose main phase is tempered martensite. More preferably, the seamless steel pipe for a low alloy oil well according to the present embodiment has a volume fraction of tempered martensite of 95% or more.

  The seamless steel pipe for low alloy oil wells according to this embodiment is not limited to this, but is particularly suitable when the size of the prior austenite grains is 5.5 or less in terms of the crystal grain size number in accordance with ASTM E112-13. This is because in the case of a coarse-grained structure having a grain size number of 5.5 or less, the effect of improving toughness due to refinement cannot be sufficiently obtained, and improvement of toughness by controlling inclusions becomes more important. It is more preferable when the crystal grain size number is 5.0 or less. The lower limit of the grain size number is preferably 4.0.

  The grain size number of the prior austenite grains is determined by the Bechet-Beaujard method in which a test piece is cut out from each steel tube, embedded in a resin, and corroded with a saturated aqueous solution of picric acid so that the cross section perpendicular to the rolling direction becomes the test surface. Grain boundaries are revealed and measured according to ASTM E112-13.

  The crystal grain size number of the prior austenite grains may be measured using a steel material before quenching and before tempering (so-called as-quenched material), or may be measured using a tempered steel material. Regardless of which steel material is used, the crystal grain size number of the prior austenite grains hardly changes.

For steel materials after tempering, the ASTM grain size number of the prior austenite crystal grains can be obtained from the crystal orientation relationship using a method such as electron beam backscatter diffraction (EBSD). In this case, the metal structure of the seamless steel pipe after tempering is measured by EBSD as follows. A sample is taken from the center of the thickness of the cross section (cross section perpendicular to the rolling direction) of the seamless steel pipe after tempering. Using the collected sample, crystal orientation analysis is performed by EBSD in the observation range of 500 × 500 μm ² , and the boundary between grains in which the misalignment angle is in the range of 15 to 51 ° is defined as the old austenite grain boundary, and line drawing is performed. Based on the drawing, the crystal grain size number is obtained in accordance with ASTM E112-13.

  The seamless steel pipe for a low alloy oil well according to the present embodiment preferably has a yield strength of 758 MPa (110 ksi) or more. The yield strength of the low alloy oil well seamless steel pipe is more preferably 827 MPa (120 ksi) or more, and still more preferably 896 MPa (130 ksi) or more.

  The seamless steel pipe for low alloy oil wells according to this embodiment has excellent low temperature toughness. The seamless steel pipe for a low alloy oil well according to this embodiment preferably has an absorbed energy obtained by a Charpy impact test at 0 ° C. in accordance with ASTM E23 of 55 J or more. The absorbed energy of the low alloy oil well seamless steel pipe is more preferably 65 J or more, and further preferably 75 J or more.

[Production method]
Hereinafter, an example of the manufacturing method of the seamless steel pipe for low alloy oil wells by this embodiment is demonstrated. This manufacturing method is merely an example, and the manufacturing method of the low alloy oil well seamless steel pipe according to the present embodiment is not limited thereto.

In the refining process, the chemical composition of the molten steel excluding REM and Ca is adjusted to the above-described range. Thereafter, at least one element of REM is added, and then Ca is added. When Ca is added first, calcium aluminate (mCaO.nAl ₂ O ₃ , m and n are natural numbers) is formed as inclusions, and dispersed as fine inclusions (XCaAlOS, X is REM) containing REM Can not be.

  Cast molten steel into billets. Or you may manufacture a slab, a bloom, an ingot, etc., and make these into billets by hot processing.

  The manufactured billet is heated to sufficiently dissolve the carbide, thereby preventing the coarsening of crystal grains. The heating temperature is, for example, 1000 to 1300 ° C.

  The heated billet is hot-worked into a raw tube of a predetermined shape and size. Hot working is, for example, a Mannesmann-mandrel mill system or a Mannesmann-plug mill system. The final finishing temperature for hot working is preferably set to 900 to 1100 ° C. in consideration of prevention of coarsening of crystal grains.

The manufactured tube is subjected to heat treatment by quenching and tempering. More specifically, quenching is performed to quench the base tube from the austenite temperature range, and tempering is performed by heating to a temperature equal to or lower than the Ac ₁ transformation point and holding for a predetermined time. The seamless steel pipe for low alloy oil wells is manufactured by the above process.

  Although the seamless steel pipe for low alloy oil wells according to the present embodiment is not limited to this, it is particularly suitable when manufactured by in-line heat treatment. In the in-line heat treatment, the crystal grains are less likely to be refined than the offline heat treatment, and the effect of improving toughness due to the refinement cannot be sufficiently obtained. Therefore, in the in-line heat treatment, it is more important to improve toughness by controlling inclusions.

  In the above, the seamless steel pipe for low alloy oil wells by this embodiment was demonstrated. According to this embodiment, even if the restriction on the S content is relaxed, a low alloy oil well seamless steel pipe having the necessary strength and toughness can be obtained.

  Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited to these examples.

  Steel having the chemical composition shown in Table 1 was melted in a vacuum melting furnace, further subjected to RH degassing treatment, and then a billet having a diameter of 360 mm was manufactured by a continuous casting method. REM and Ca were added in this order in the refining process, except for steel D1, steel D2, and steel D5, to which REM was not added. Steel D1 is a low-sulfur steel for comparison, in which CaO is introduced and the S content is reduced to less than 20 ppm.

  The manufactured billet was hot rolled into a raw tube having an outer diameter of 346.08 mm and a wall thickness of 15.88 mm. Then, the in-line heat processing which performs a quenching tempering process immediately after hot pipe making was performed at the temperature shown in Table 2, and the seamless steel pipe was manufactured. In Table 2, Q is the quenching temperature and T is the tempering temperature.

[Nozzle blockage]
During continuous casting, the opening area of the sliding nozzle was automatically controlled so that the difference between the amount flowing into the mold and the amount flowing out from the mold was within a predetermined range. The change with time of the opening area of the sliding nozzle was examined, and it was determined that the nozzle blockage occurred when the opening area became zero. The results are shown in the column of “No / no nozzle clogging” in Table 2. “◯” in the same column indicates that no nozzle clogging has occurred, and “x” indicates that nozzle clogging has occurred.

[Grain size number]
After quenching, each austenitic steel pipe before tempering was subjected to the Bechet-Beaujard method described in the embodiment to reveal old austenite grains, and the grain size number was measured according to ASTM E112-13. The measurement results are shown in the column of “old γ grain size” in Table 2.

[Inclusion measurement]
The seamless steel pipe after tempering was cut and the inclusions were observed. The average particle diameter and aspect ratio of sulfide inclusions and oxysulfide inclusions were measured by the method described in the examples. A Schottky field emission scanning electron microscope (FE-SEM, “JSM-7800F” manufactured by JEOL Ltd.) was used for the observation of inclusions, and “GENESIS Particle Analysis, Version 5” manufactured by AMETEK was used for data collection and analysis. 10 "was used. The types of typical inclusions, average particle diameters and aspect ratios of sulfide inclusions and oxysulfide inclusions are shown in the corresponding columns of Table 2, respectively.

[Tensile test]
An arc-shaped tensile test in accordance with ASTM E8 (parallel part width 38.1 mm, GL 50.8 mm) so that the longitudinal direction of the test piece is parallel to the rolling direction of the seamless steel pipe from the seamless steel pipe after tempering. ) Was collected. Using this test piece, a tensile test was performed at room temperature (25 ° C.) in the air. The stress at 0.6% elongation obtained in the tensile test was taken as the yield strength of the seamless steel pipe, and the maximum stress during uniform elongation was taken as the tensile strength of the seamless steel pipe. The measured yield strength and tensile strength are shown in the “YS” and “TS” columns of Table 2, respectively.

[Charpy impact test]
Full size test piece according to ASTM E23 (dimension: width 10 mm x height 10 mm x length 55 mm) so that the length direction of the test piece is perpendicular to the rolling direction of the seamless steel pipe from the seamless steel pipe after tempering. Were collected. Using this test piece, a Charpy impact test was performed at 0 ° C. with 3 pieces / set, and the absorbed energy was measured. The results are shown in Table 2. When the average absorbed energy (hereinafter simply referred to as “average absorbed energy”) of the three test pieces was 55 J or more, it was evaluated that the toughness was excellent.

[Test results]
As shown in Table 2, all of the seamless steel pipes manufactured from steels A1 to A10, steels B1 to B10, and steels C1 to C4 have an average particle size of sulfide inclusions and oxysulfide inclusions. The aspect ratio was 2.0 or less. The average absorbed energy of these seamless steel pipes was 55 J or more, and a value similar to the average absorbed energy of the low-sulfur steel (steel D1) was obtained.

  The seamless steel pipes manufactured from the steels D2 to D5 had an average particle size of sulfide inclusions and oxysulfide inclusions larger than 2.0 μm and an aspect ratio larger than 2.0. Seamless steel pipes manufactured from these steels had an average absorbed energy of less than 55J.

  Steel D2 does not contain REM, and it is considered that sulfide inclusions and oxysulfide inclusions were not modified. Steel D3 has too little Ca content, and it is considered that the modification of sulfide inclusions and oxysulfide inclusions was insufficient. Steel D4 has too much REM content, and it is considered that sulfide inclusions and oxysulfide inclusions are coarsened. Steel D5 is considered to have decreased toughness due to the influence of stretched sulfide-based inclusions and oxysulfide-based inclusions, since S content was too high, and Ca content and REM content were too low.

  In addition, nozzle blockage was also observed in steel D4. This is presumably because the REM content containing REM coarsened and the fluidity in molten steel deteriorated because the REM content was too high.

  The seamless steel pipe produced from steel D6 had an aspect ratio of sulfide inclusions and oxysulfide inclusions greater than 2.0. The seamless steel pipe produced from steel D6 had an average absorbed energy of less than 55J. Further, in the steel D6, nozzle clogging was also observed. This is presumably because the steel D6 had too much O content.

FIG. 4 is an example of a micrograph of composite inclusions observed in steels A1 to A10, steels B1 to B10, and steels C1 to C4. As shown in FIG. 4, Mn (Ca, S) is attached to the composite inclusion of Ce ₂ OS and Al ₂ O ₃ .

  FIG. 5 is an example of a micrograph of sulfide inclusions (MnS) observed in steels D2 to D5. As shown in FIG. 5, this inclusion has a shape that is extended in the rolling direction (left-right direction in FIG. 5).

From the comparison between FIG. 4 and FIG. 5, it is understood that the extension of sulfide inclusions can be suppressed by attaching the sulfide inclusions to non-extensible X ₂ O ₃ and / or X ₂ OS. As a result, the sulfide inclusions can be rendered harmless.

  The embodiment of the present invention has been described above. The above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit thereof.

Claims (3)

Chemical composition is mass%,
C: 0.20 to 0.50%,
Si: 0.05 to 0.50%,
Mn: 0.05 to 1.0%
P: 0.030% or less,
S: 0.0025 to 0.0060%,
Al: 0.005 to 0.10%,
Cr: 0.1 to 1.2%,
Mo: 0.25 to 1.0%,
Ti: 0.002 to 0.05%,
N: 0.01% or less,
O: 0.0030% or less,
Ca: 0.0010 to 0.0030%,
REM: 0.0010 to 0.0040%,
V: 0 to 0.30%,
Nb: 0 to 0.10%,
Cu: 0 to 1.0%
Ni: 0 to 1.0%,
B: 0 to 0.0040%,
Balance: Fe and impurities,
A seamless steel pipe for low alloy wells in which the mean particle diameter of sulfide inclusions and oxysulfide inclusions is 2.0 μm or less and the aspect ratio is 2.0 or less. The low alloy oil well seamless steel pipe according to claim 1,
The chemical composition is mass%,
V: 0.01-0.30%, and Nb: 0.002-0.10%,
A seamless steel pipe for low alloy oil wells, comprising one or two selected from the group consisting of: A seamless steel pipe for a low alloy oil well according to claim 1 or 2,
The chemical composition is mass%,
Cu: 0.01 to 1.0%,
Ni: 0.01-1.0%, and B: 0.0001-0.0040%,
A seamless steel pipe for low alloy oil wells containing one or more selected from the group consisting of: