JP4367259B2 - Seamless steel pipe for oil wells with excellent pipe expandability - Google Patents

Description

  The present invention relates to a seamless steel pipe for oil wells used in oil wells or gas wells (hereinafter simply referred to as “oil wells”). More specifically, the pipes are expanded in wells and used as casings or tubing as they are. It is related with the seamless steel pipe for oil wells which is excellent in pipe expandability with a tensile strength of 600 MPa or more and a yield ratio of 85% or less.

  In recent years, due to the demand for lowering the cost of oil well drilling, a construction method using pipe expansion by expansion processing in a well has been developed (see, for example, Patent Documents 1 and 2). According to this construction method so-called expanded pipe burying method, by expanding the casing in the radial direction in the well, it is possible to suppress the diameter of each casing having a multi-stage structure, and as a result, the casing size of the upper part of the well can be suppressed small, Costs for drilling wells can be reduced.

  In such pipe burying method, the steel pipe is exposed to the environment of oil and gas while being processed by pipe expansion, so heat treatment cannot be applied after processing, and the pipe has been subjected to cold pipe expansion processing. Corrosion resistance up to is required. In order to meet this requirement, in mass%, C: 0.10 to 0.45%, Si: 0.1 to 0.5%, Mn: 0.10 to 3.0%, P: 0.03% Hereinafter, S: 0.01% or less, sol. Al: 0.05% or less and N: 0.010% or less, with the balance being Fe and impurities, and the strength (yield strength YS (MPa)) and crystal grain size (d (Μm)) satisfying the relationship of formula (1): ln (d) ≦ −0.0067YS + 8.09, and an oil well steel pipe for pipe expansion excellent in corrosion resistance after pipe expansion processing, and part of Fe in the steel pipe Instead of (A) in mass%, Cr: 0.2-1.5%, Mo: 0.1-0.8%, V: 0.005-0.2%, one or more (B) by mass%, Ti: 0.005 to 0.05%, Nb: 0.005 to 0.03%, or (C) Ca: 0.001 to 0.005%, Patent Document 3 discloses one or more of the above.

  Further, in Patent Document 4, in order to prevent the uneven thickness ratio from expanding and the crushing strength from being reduced by expanding the tube, the uneven thickness ratio E0 (%) before expanding the tube is 30 / (1 + 0.018α) or less ( However, it is limited to α (: tube expansion ratio) = (inner diameter after tube expansion / inner diameter before tube expansion−1) × 100), and the difference in the expansion amount in the circumferential direction is converted into the difference in contraction amount in the length direction. In order to suppress the bending of the steel pipe, the eccentric thickness deviation (primary thickness deviation) rate (%) (= {(maximum thickness of the eccentric thickness deviation component−the same minimum thickness) / average thickness} × 100 ) Is limited to 10% or less.

In Patent Documents 3 and 4 mentioned above, a manufacturing method in which the ERW steel pipe and the seamless steel pipe after pipe making are subjected to a treatment such as quenching-tempering or quenching (repeating twice or more) -tempering is preferable, and the expansion ratio is 30% or less. Examples in the range of are disclosed.
JP 7-567610 A International Publication No. WO98 / 00626 JP 2002-266055 A JP 2002-349177 A

  However, due to further cost reduction requirements, there is a demand for inexpensive steel pipes that can withstand the expansion process with a tube expansion rate exceeding 30%. This is because the casing size can be further reduced and the excavation cost can be reduced if the expansion ratio of the steel pipe can be further increased from the conventional 30% in the well.

  In order to meet this requirement, the present invention does not rely on the quenching-tempering (Q / T) process as disclosed in Patent Documents 3 and 4, but is controlled as-rolled or at the time of cooling after rolling, or To provide a seamless steel pipe for oil wells with excellent pipe expandability, which exhibits excellent pipe expandability with respect to pipe expansion processing with a pipe expansion ratio exceeding 30%, while having high strength of tensile strength (TS) of 600 MPa or more by cheaper heat treatment. With the goal.

  Here, pipe expandability is to be evaluated with a limit pipe expansion rate that allows pipe expansion without causing uneven deformation during pipe expansion. Specifically, in the present invention, the wall thickness ratio after pipe expansion is the wall thickness before pipe expansion. The tube expansion rate did not exceed the rate + 5%.

Expansion rate (%) = [(inner diameter of the tube after expansion−inner diameter of the tube before expansion) / inner diameter of the tube before expansion] × 100
Unevenness ratio (%) = [(maximum wall thickness−minimum wall thickness) / average wall thickness] × 100

  As a result of intensive studies to achieve the above object, the present inventors have determined that the steel composition of the material is a low C-high Si-high Mn system and the residual austenite (residual γ) phase fraction is 5% by volume or more. As a result, the steel pipe has a low YR and excellent uniform elongation, and particularly the non-uniform deformation is suppressed. In particular, it was also found that if the residual γ phase fraction was 8% by volume or more, superior tube expandability was exhibited.

  The details of these reasons are not clear, but by setting the residual γ phase fraction to 5% by volume or more, the work hardening rate increases due to the processing strain-induced transformation of the residual γ. It is presumed that the deformation has a deformation strength equal to or greater than that of the thick part due to curing, and subsequently promotes the deformation of the thick part, thereby making the processing rate uniform. On the other hand, in the single phase steel with high YR and low work hardening rate such as Q / T material, it is presumed that the deformation of the thin-walled portion preferentially progresses along with the expansion processing and reaches the limit tube expansion rate at an early stage.

The present invention has been made based on these findings, and the gist thereof is as follows.
(Invention Item 1) The composition is mass%,
C: 0.05 to 0.30%
Si: 0.2-2%
Mn: 0.7 to 4.0%,
P: 0.03% or less,
S: 0.015% or less,
N: 0.007% or less,
O: 0.005% or less, and a balance of Fe and unavoidable impurities, have a phase fraction 5 vol% or more of residual γ phase in the structure, the balance of Rukoto such a low-temperature transformation phase and the ferrite A seamless steel pipe for oil wells with excellent pipe expandability.
(Invention 2) In addition to the above composition,
Al: 0.06% or less,
Cr: 0.05 to 1%,
Ni: 0.05-2%,
Cu: 0.05 to 1%,
Nb: 0.005 to 0.2%,
V: 0.005 to 0.2%,
Ti: 0.005 to 0.2%,
Mo: 0.05-0.5%
B: 0.0005 to 0.0035%,
Ca: 0.001 to 0.005%
The seamless steel pipe for oil wells having excellent pipe expandability according to claim 1, characterized by containing one or more of them.

  According to the present invention, even if the pipe expansion rate exceeds 30%, a steel pipe of TS600 MPa or more that is excellent in pipe expandability can be supplied at low cost.

  First, the reason why the composition of the steel pipe is limited as described above will be described. The component ratio (component content) of the composition is expressed in mass% and is abbreviated as%. Further, (% X) represents a mass% value of component X.

C: 0.05-0.30%
In order to form the residual γ phase, higher C is more advantageous, but when C exceeds 0.30%, the strength-toughness balance deteriorates. On the other hand, if C is less than 0.05%, it is difficult to form a residual γ phase. Therefore, C is set to 0.05 to 0.30%. Preferably it is 0.08 to 0.20%.

Si: 0.2-2%
In general, Si is added as a deoxidizer and can contribute to an increase in strength. However, in the present invention, 0.2% or more is added to form a residual γ phase advantageously by controlled cooling or two-phase heat treatment. is required. In particular, during controlled cooling, the concentration of C into the γ phase is promoted, which is advantageous for the formation of a residual γ phase. On the other hand, even if added in excess of 2%, the effect is not only saturated, but the hot workability is significantly deteriorated. Preferably it is 0.5 to 1.5%.

Mn: 0.7-4.0%
Mn is important for the formation of the residual γ phase. By containing 0.7% or more under the combination with low C and high Si, it is concentrated to the γ phase during controlled cooling or heat treatment in the two-phase region, and it is 5% by volume or more. The residual γ phase fraction is efficiently achieved. However, if added over 4.0%, segregation increases and the toughness and tube expandability are reduced. Therefore, Mn is set to 0.7 to 4.0%. Preferably it is 1.0 to 3.5%.

P: 0.03% or less P is an element contained in the steel as an impurity and easily segregates at the grain boundary. If it exceeds 0.03%, the grain boundary strength is remarkably lowered and the toughness is lowered. Therefore, P is restricted to 0.03% or less. Preferably it is 0.015% or less.

S: 0.015% or less S is an element contained as an impurity in steel and exists mainly as an inclusion of Mn-based sulfide. When the content exceeds 0.015%, the inclusions are present as coarse and extended inclusions, and the toughness and tube expandability are significantly reduced. Therefore, S is restricted to 0.015% or less. Preferably it is 0.006% or less. Moreover, the form control of the inclusion by Ca is also effective.

N: 0.007% or less N is an element contained as an impurity in the steel, but if contained in excess of 0.007%, coarse nitrides are formed and the toughness and tube expandability are reduced. Therefore, N is restricted to 0.007% or less. Preferably it is 0.005% or less.

O: 0.005% or less O is present as an inclusion in the steel. If the content exceeds 0.005%, inclusions tend to aggregate and exist, and the toughness and tube expandability deteriorate. Therefore, O is regulated to 0.005% or less. Preferably it is 0.003% or less.

  In addition to the above elements, the following elements may be added as necessary.

Al: 0.06% or less Al is used as a deoxidizer as needed, but if added over 0.06%, the effect is saturated, and alumina inclusions increase toughness and tube expansion. Therefore, when added, the content is preferably 0.06% or less.

Cr: 0.05 to 1%
Cr suppresses the formation of pearlite, contributes to the formation of a low temperature transformation phase, and also contributes to an increase in strength due to hardening due to a decrease in the transformation point of the low temperature transformation phase. However, if it is less than 0.05%, the effect cannot be obtained. On the other hand, even if added over 1%, the effect is saturated, so 0.05 to 1% is preferable.

Ni: 0.05-2%
Ni is an element effective for improving strength, toughness, and corrosion resistance. Further, in the present invention, like Mn, during the controlled cooling or the two-phase heat treatment, the γ phase is concentrated to greatly contribute to the formation of the residual γ phase. When Cu is added, it is effective to prevent Cu cracking during hot rolling. These effects are exhibited by addition of 0.05% or more, but on the other hand, it is expensive, and even if added excessively, the effects are saturated, so 0.05 to 2% is preferable. From the viewpoint of Cu cracking, it is preferable to add ((% Cu) × 0.3)% or more.

Cu: 0.05 to 1%
Cu is added to improve strength and corrosion resistance. The effect is exhibited by addition of 0.05% or more. On the other hand, if it exceeds 1%, hot embrittlement tends to occur and toughness also decreases, so 0.05 to 1% is preferable.

Nb: 0.005 to 0.2%
Nb suppresses the formation of pearlite, contributes to the formation of a low-temperature transformation phase, and contributes to an increase in strength by forming a carbonitride. However, if it is less than 0.005%, the effect cannot be obtained. On the other hand, even if added over 0.2%, the effect is saturated, so 0.005 to 0.2% is preferable.

V: 0.005-0.2%
V has an effect of increasing strength by forming a carbonitride to refine the structure and strengthening precipitation, but if less than 0.005%, the effect is unclear, while exceeding 0.2% When added, the effect is saturated, and problems such as continuous casting cracks are caused, so 0.005 to 0.2% is preferable.

Ti: 0.005 to 0.2%
Ti is a strong carbonitride-forming element, and N aging is suppressed by the addition of about N ((% N) × 48/14)%, and when B is added, B depends on N in the steel. BN may be precipitated and fixed so that the effect is not suppressed. Furthermore, the addition increases the strength by forming fine carbides. If it is less than 0.005%, there is no effect, and it is particularly preferable to add ((% N) × 48/14)% or more. However, if it exceeds 0.2%, coarse nitrides are likely to be formed, and the toughness and tube expandability are deteriorated.

Mo: 0.05-0.5%
Mo has the effect of increasing the strength at normal temperature and high temperature by solid solution and carbide formation, but if it exceeds 0.5%, the effect is saturated and expensive, so 0.5% or less Is preferred. In order to exhibit the effect of increasing the strength, addition of 0.05% or more is preferable.

B: 0.0005 to 0.0035%
B contributes to toughness improvement by suppressing grain boundary cracking as a grain boundary strengthening element. In order to exhibit the effect, 0.0005% or more is preferable. On the other hand, even if added excessively, the effect is saturated, so 0.0035% or less is preferable.

Ca: 0.001 to 0.005%
Ca is added for the purpose of controlling the shape of inclusions in a spherical shape, and the effect is exhibited at 0.001% or more, but when it exceeds 0.005%, it is saturated, so 0.001 to 0.00%. 005% is preferable.

  Next, the reason for limiting the structure of the steel pipe will be described.

  In order to ensure low YR and uniform elongation effective for pipe expandability and to suppress non-uniform deformation, it is necessary that a residual γ phase be formed in the steel pipe structure at a phase fraction of 5% by volume or more. The other phases are not particularly limited, but in order to ensure TS600 MPa or more, the total sum of the phase fractions of the residual γ phase and the low temperature transformation phase is preferably 50% by volume or more. It is more preferable that the residual γ-phase fraction is 8 to 20% by volume, since particularly good tube expandability can be obtained.

Here, examples of the low temperature transformation phase include bainite, martensite, bainitic ferrite (synonymous with acicular ferrite), and the like. The remaining structure excluding the residual γ phase and the low temperature transformation phase is mainly ferrite . Pearlite and cementite are not preferable from the viewpoint of balance between strength and toughness.

  Next, in the present invention, as long as the composition and structure defined in the present invention are achieved, the manufacturing method is not particularly limited, but a manufacturing method that is favorable in terms of productivity and the like will be described.

  It is preferable that the molten steel having the above composition is melted by a known melting method such as a converter or an electric furnace to form a steel pipe material such as a billet by a known casting method such as a continuous casting method or an ingot forming method. Note that a slab may be formed by a continuous casting method or the like, and the slab may be formed into a billet by rolling.

  Further, from the viewpoint of reducing inclusions, it is preferable to take reduction measures such as floatation of inclusions and suppression of aggregation during steelmaking-casting. Further, the center segregation may be reduced by forging pressure during continuous casting or heat treatment in a soaking furnace.

  Next, the obtained steel pipe material is heated and hot-worked and formed using a normal Mannesmann-plug mill method or Mannesmann-Mandrel mill manufacturing process to obtain a seamless steel pipe having a desired size.

  In the low carbon low alloy steel having the composition according to the present invention, it is difficult to obtain a sufficient residual γ phase as it is rolled, and it cannot be obtained by Q / T treatment.

  As a method for obtaining the residual γ phase, at the time of cooling after rolling or normal heat treatment, (α / γ) rapid cooling to the two-phase region and slow cooling or holding in the two-phase region to C to the γ phase. There is a method of performing controlled cooling in which rapid cooling is performed after concentration.

  A simpler method of forming a residual γ phase is to use a heating temperature during normal heat treatment in an (α / γ) two-phase region, and a γ stabilizing component such as C, Mn, or Ni is sufficiently concentrated in the γ phase. And a method of rapidly cooling it. This method is preferable in terms of strength and property management because the phase fraction can be easily controlled.

  Of course, even if normal normal heat treatment is performed before these suitable treatments to make the material characteristics uniform and the structure refined, the residual γ phase formation effect of the preferred treatments does not change.

  It is also preferable in terms of characteristics to make the structure during two-phase region heating finer and homogenized by performing Q treatment as a pretreatment for two-phase region heating.

  Steel of the composition shown in Table 1 is cast into a 100 kg steel ingot by vacuum melting, made into a billet by hot rolling, hot-worked by a model seamless rolling mill, piped, outer diameter 4 in (101.6 mm), A seamless steel pipe having a wall thickness of 0.262 in (6.65 mm) was obtained.

These steel pipes were subjected to heat treatment for forming a residual γ phase. The heat pattern of this heat treatment is (a) a control cooling pattern or (b) a two-phase region heat treatment pattern shown in FIG. 1 and was simulated by a model cooling device. Some samples were subjected to pretreatment before heating at 940 ° C. for 15 minutes and then cooling at a cooling rate of 5 ° C./s or higher. At this time, A ₁ and A ₃ were values obtained by the following simple empirical formula.

A ₃ (° C.) = 910−230 × √ (% C) + 44.7 × (% Si) −30 × (% Mn) −15.2 × (% Ni) −20 × (% Cu) −11 × ( % Cr) + 31.5 × (% Mo) + 104 × (% V) + 700 × (% P) + 400 × (% Al) + 400 × (% Ti)
A1 (° C.) = 723 + 29.1 × (% Si) −10.7 × (% Mn) −16.9 × (% Ni) + 16.9 × (% Cr)
Further, some of these steel pipes were subjected to the following normalization treatment or Q / T treatment.
Normalization: 890 ° C. × 20 min after heating and air cooling Q / T treatment: 920 ° C. × 20 min after heating, water cooling → 430-530 ° C. × 30 min tempering For each steel tube after each heat treatment, the residual γ phase fraction is determined by the X-ray diffraction intensity ratio. The structure was quantified, and the morphology of the structure was examined by observation with an optical microscope and SEM (scanning electron microscope), and further, the tensile properties and tube expandability were investigated.

  Here, the tensile test was performed according to the tensile test method specified in JIS Z 2241, and the test piece used was a JIS No. 12B test piece specified in JIS Z 2201.

  Expandability is evaluated by the tube expansion rate that can be expanded without causing uneven deformation during tube expansion (limit tube expansion rate). Specifically, the tube expansion rate after tube expansion does not exceed the wall thickness rate before tube expansion + 5%. Rate. The thickness deviation rate was determined by measuring the 16 cross-sections at 22.5 ° intervals with an ultrasonic wall thickness meter.

  In the pipe expansion test, as shown in FIG. 2, a plug 2 having various maximum outer diameters D1 larger than the inner diameter D0 before the pipe expansion of the steel pipe 1 is inserted into the steel pipe 1 and mechanically pulled out in the plug drawing direction 3. The expansion ratio was calculated from the average inner diameter before and after the expansion.

  Table 2 shows the results of these investigations. In the tensile properties shown in Table 2, YS is the yield strength, TS is the tensile strength, YR is the yield ratio (= YS / TS (× 100%)), u-El is the uniform elongation, and El is the total elongation. . From Table 2, it can be seen that according to the present invention, an excellent tube expandability with a limit tube expansion rate of 45% or more is obtained, and a steel tube with a small tube expansion load with a YR of 65% or less is obtained.

  The present invention can be used for excavation of oil wells (including gas wells).

It is a heat pattern figure which shows the example of the heat processing for forming a residual (gamma) phase. It is sectional drawing which shows the expansion processing method of a steel pipe.

Explanation of symbols

1 Steel pipe 2 Plug 3 Plug pulling direction

Claims (2)

The composition is mass%,
C: 0.05 to 0.30%
Si: 0.2-2%
Mn: 0.7 to 4.0%,
P: 0.03% or less,
S: 0.015% or less,
N: 0.007% or less,
O: 0.005% or less, and a balance of Fe and unavoidable impurities, have a phase fraction 5 vol% or more of residual γ phase in the structure, the balance of Rukoto such a low-temperature transformation phase and the ferrite A seamless steel pipe for oil wells with excellent pipe expandability. In addition to the above composition,
Al: 0.06% or less,
Cr: 0.05 to 1%,
Ni: 0.05-2%,
Cu: 0.05 to 1%,
Nb: 0.005 to 0.2%,
V: 0.005 to 0.2%,
Ti: 0.005 to 0.2%,
Mo: 0.05-0.5%
B: 0.0005 to 0.0035%,
Ca: 0.001 to 0.005%
1 or 2 types or more are contained, The seamless steel pipe for oil wells which is excellent in pipe expandability of Claim 1 characterized by the above-mentioned.

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