CA1239568A

CA1239568A - Erw oil-well pipe and process for producing same

Info

Publication number: CA1239568A
Application number: CA000475496A
Authority: CA
Inventors: Zensaku Chano; Hiroshi Murayama; Yasushi Yamamoto
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1984-03-01
Filing date: 1985-02-28
Publication date: 1988-07-26
Also published as: GB2155950B; GB8504858D0; DE3507124A1; GB2155950A; DE3507124C2

Abstract

ERW OIL-WELL PIPE AND PROCESS FOR PRODUCING SAME

ABSTRACT OF THE DISCLOSURE

An ERW oil-well pipe is strain-aged to exhibit a low hardness and a high yield strength, so that both sour-environment resistance and collapse resistance are obtained at better levels, as compared with quench and tempered ERW oil-well pipes.

Description

~L23~56~

PEW OIL-WELL PIPE AND PROCESS FOR PRODUCING SAVE

BACKGROUND OF THE INVENTION
1. Field of the Invention The present invention relates to an electric resistance welded (ERR) oil-well pipe having a low hardness and a high yield strength and to a process for producing the same.

2. Description of the Related Art The demand for high sour-environment resistance and high collapse-strength type oil pipes has been increasing year by year along with the greater depths to which gas or oil wells have been drilled in recent years. All such deep oil wells are repeatedly subjected to sour gas environments. For example the hydrostatic pressure at an underground depth of 9000 m is approxi-mutely 900 atmospheres. According to the definition of the National Association of Chemical Engineers (NICE), an environment in which the hydrogen sulfide partial pressure is 0.05 psi or more is "sour". Thus, a hydrogen sulfide content of 5 ppm or more renders an environment sour, since the hydrogen sulfide partial pressure becomes 0.064 psi at a pressure of 900 atmospheres.
It is therefore indispensable for oil well pipes used in a deep well to have both excellent sour-environment resistance and collapse resistance.
The sour-environment resistance is enhanced by lessening the hardness and strength, while the collapse resistance is enhanced by enhancing the strength, particularly yield strength. Japanese Unexamined Patent Publication (Cook) No. 53-138,916 discloses a method for producing ERR pipe utilizing quenching and tempering.
In this method, an ERR pipe having welds is quenched from a temperature of from 800C to 1000C and tempered at a temperature of from 550C to the A 1 point. It is, however, very difficult to obtain compatibly excel-lent sour-environment and crushing resistances by I' - 2 - i239568 quenching and tempering. Also, deformation of pipes due to quenching and tempering must be rectified by straightening to improve the straightness and roundness.
Deformation of pipes by quenching and tempering renders the production yield of pipes low.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an ERR oil-well pipe having both improved sour-environ-mint resistance and collapse resistance, that is, a low hardness and high yield strength.
It is another object of the present invention to provide an ERR oil-well pipe having a high yield ratio and a satisfactorily high strength.
It is a further object of the present invention to provide a method for producing an ERR oil-well pipe by other than quenching and tempering.
An ERR oil-well pipe according to the present invention consists of 0.22% or less of C, 0.50~ or less of Six from 1.0 to 2.0% of My, 0.0S~ or less of Nub, and a balance of iron and unavoidable incidental elements including N. It is characterized by being placed in a - strain aged state to have the required hardness and yield strength.
The process for producing an ERR oil-well pipe according to the present invention includes the features of: carrying out hot-rolling at a low temperature to refine the crystal grains; after hot-rolling, rapidly cooling and coiling at a low temperature so as to retain stably the solute carbon and nitrogen in the matrix of steel; introducing, during the formation of a pipe from a sheet, plastic strain into the pipe material in an amount greater than the prior art, thereby increasing the number of dislocations; and fixing the solute carbon and nitrogen to the dislocations by heat treatment for a short period of time at low temperature.
The method for producing an ERR oil-well pipe according to the present invention is characterized by

- 3 - 1~3~5~

hot-rolling steel at a finishing temperature of from 740C to 830C, cooling the steel at an average cooling temperature of 15C/sec or more down to the coiling temperature, coiling the steel at a temperature of 500C
or less, and; during a subsequent ERR pipe-forming process, applying a high reduction to cause 3% or more strain in a longitudinal direction. The pipe is subset quaintly heated to a temperature of from 100C to 550C
for a period of from 30 seconds to 30 minutes.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure l is a graph showing the yield strength at the abscissa and the hardness (Arc) at the ordinate.
Figure 2 is a graph showing the relationship between the longitudinal strain of a pipe at the abscissa and the So value and collapse pressure at the ordinate.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The gist of the present invention lies on strain-aging of the ERR pipe material, in which the solute carbon and solute nitrogen retained in the ERR pipe material stick to and pin the dislocations which are introduced to the pipe material curing the formation of the pipe.
The strain-aging provides compatibly excellent sour-environment resistance and collapse-resistance.
Strain-aging refers to changes in the mechanical properties of metals as a result of room or moderately elevated temperatures after plastic deformation.
Strain-aging is usually avoided for steel, since it drastically deteriorates its mechanical properties, especially impact property. Strain-aging herein means that the solute carbon and solute nitrogen slick to and pin the dislocations induced in the steel due to plastic working, particularly cold plastic working. A strain-aged state herein means the state of steel of an ERR
pipe in which the solute carbon and nitrogen stick to and pin the dislocations induced by plastic working.
- The s-train-aging and strain-aged state bring about ~;~395~

outstanding changes in the relationship between the hardness and yield strength and in the relationship between the tensile and yield strength. The relation-ship between the tensile strength and yield strength is frequently expressed by the yield ratio, i.e., (the yield strength/tensile strength) x 100 (%). When the yield ratio is high, the sour-environment resistance and collapse resistance are compatible, since the collapse pressure is increased with an increase in the yield strength, and, further the sour-environment resistance is increased with a decrease in the tensile strength.
Generally, the hardness and tensile strength vary in direct proportion to one another. Low hardness and thus low tensile strength provide a high sour resistance.
The collapse pressure is not dependent on the tensile strength or hardness but depends greatly on the yield strength. Accordingly, a high yield ratio is India-pen sable for compatible sour resistance and collapse resistance. Desirably, the variation in the yield ratio of ERR oil-well pipes should be as small as possible.
Steel materials having a high yield ratio tend to feature lower ductility and toughness. Strain-aging, which is associated with the impairment of ductility, particularly impact strength, is usually not employed for improvement of steel properties.
The composition of an ERR oil-weld pipe according to the present invention will now be described.
Carbon dissolved in the matrix of steel and slicked to the dislocations is used to provide both excellent sour-environment and crush resistances. An increase in the carbon content tends to reduce the yield ratio.
Therefore, the highest carbon content is limited to 0.22%. Carbon effectively strengthens the steel when the carbon content is at least 0.0~.
Silicon also strengthens steel in a minor but effective content. When the silicon content exceeds 9.5%, however, yield ratio is lessened.

~X39S6~3 Manganese also strengthens the steel and enhances the yield ratio due to refinement of ferrite grains, at a content of at least 1.0~. The highest content of manganese should be 2.0% and is set so as not to impair the ductility and toughness.
Niobium refines the ferrite grains and enhances the yield ratio at a content of 0.05% at the highest. When the niobium content exceeds 0.05%, the dissolution of niobium in the matrix becomes difficult, and, thus, the ferrite grains cannot be refined by the precipitation of niobium.
Aluminum, vanadium, and titanium are optional alloying elements and strengthen the steel due to precipitation within the ferrite grains and/or refinement of the ferrite grains. These elements enhance the yield strength by precipitation hardening and/or refinement of the ferrite grains. The highest contents are 0.050% for aluminum, 0.050% for vanadium, and 0.040% for titanium.
If these elements exceed the highest contents, they exceed the volubility limit.
The mechanical properties of the ERR oil-well pipe according to the present invention are shown in Fig. 1.
Referring to Fig. 1, line A refers to y = 2x + 33, line AD y = 55, line AND' y = 70, line BY y = 80, line 25 DUD' y = 2.14x + 40, and line DO y = 2x + 41. The ERR
oil-well pipe according to the present invention, i.e., the stringed pipe, has a hardness and yield strength falling within the range defined by the points A, A', B, C, D", D', and D. The black dots above this range show the hardness and yield strength of conventional quenched and tempered ERR oil-well pipes. From the comparison of the mechanical properties of the quenched and tempered ERR oil-well pipes with the strain-aged pipe, it is apparent that a low hardness and a high yield strength, us as well as a high yield ratio are provided by the present invention.
- The ERR oil-well pipe having the hardness and the - 6 _ ~239~

yield strength falling within the range defined by the points A, A', D', and D (hereinafter referred to as 80 ski ERR oil-well pipe) and the ERR oil-well pipe having the hardness and the yield strength falling within the range defined by points A', B, C and D
(hereinafter referred to as 95 ski ERR oil-well pipe) are produced by adjusting the chemical composition and production conditions as follows.
80 ski ERR oil-well pipe lo The carbon content is from 0.08~ to 0.19~ and the average cooling rate at hot rolling is from 15 to 35C/sec.
95 ski ERR oil-well pipe The carbon content is from 0.12~ to 0.22~, and the average cooling rate at hot rolling is from 25 to 45C/sec.
The above described carbon content and the average cooling rate are adjusted depending upon the thickness - and outer diameter of the ERR oil-well pipe. For producing the 80 ski ERR oil-well pipe, the carbon content should be as low as possible in the range of from 0.08~ to 0.12~. At least 0.12~ of carbon is necessary for producing the 95 ski ERR oil-well pipe.
When the carbon content is determined, the average cooling rate at hot-rolling is then determined.
As is described above, strain-aging results in a high yield ratio, that is, a small difference between the tensile strength and yield strength. In other words, the tensile strength becomes relatively low.
This is not advantageous from the viewpoint of strengthening steel. In the present invention, however, the carbon, silicon, and manganese in the content set as described above can satisfactorily strengthen the steel.
In addition, the steel is also strengthened by the ferrite refinement. The ferrite grain size of the ERR
oil-well pipe according to the present invention usually ranges from ASTM No. 13 to 14.

- 7 _ 1%~56~

The production of an ERR oil-well pipe according to the present invention will now be described.
Slabs are produced by either the ingot-making and stabbing method or the continuous casting method. The continuous casting method is preferred from the viewpoint of fine-graining.
In hot-rolling of the slabs, the finishing temper-azure should be as low as possible, 830C at the maximum, since the austenite grains are refined by low-temperature rolling, resulting in less probability an intermediate structure which reduces the yield ratio.
In addition, the low-temperature annealing allows generation of fine ferrite grains and rolled products with a high yield ratio. However, when the finishing temperature of hot-rolling is less than 740C, the ferrite grains coarsen and thus the yield ratio is enhanced.
Cooling conditions after hot-rolling are important for minimizing the scatter of the strength and for retaining the solute carbon and solute nitrogen in the matrix of steel. The average cooling rate in a period between the finishing-rolling and coiling should be 15C/sec or higher. Such an average cooling rate causes the puerility transformation to complete at a given high rate while the steel strip travels on the run-out table.
The completion of ferrite transformation at a given high rate on the run-out table results in minimum scatter of the strength. In addition, the cooling rate mentioned above results in a rapid austenite-ferrite transform motion, so that the solute carbon and solute nitrogen of the austenite phase are retained in the ferrite. The average cooling rate should be generally high (low) for producing the 95 ski (80 ski) ERR oil-well pipe.
The coiling temperature should be 500C or less to ensure stable retainment of the solute carbon and solute nitrogen in the ferrite phase. When the coiling temper-azure exceeds 500C, carbon and nitrogen precipitate due ~23~56~

to aging during the coiling and become inactive as to the strain-aging.
Now, the forming process is described. "Forming"
process herein refers not only to forming or shaping the rolled product, i.e., a strip into a tubular form, but also to inducing strain in an amount appropriate for the strain-aging, which is carried out later than the forming process. The strain herein is the one in the longitudinal direction of an ERR oil-well pipe. Refer-ring to Fig. 2, the So value and the collapse pressure are enhanced by the longitudinal strain of the pipe.
The So value expresses the durability-evaluatlon value in a "Shell Bent Beam Test". A longitudinal strain of a pipe of at least 3% is effective for inducing a number of dislocations to which the solute carbon and nitrogen stick, thereby improving the sour-environment resistance and collapse resistance.
The longitudinal strain is determined by the elongation percentage of an ERR oil-well pipe in the longitudinal direction, hereinafter referred to as the longitudinal elongation I The longitudinal eon-gallon E 3 is determined by the strip width We. The strip width We for providing 3% or more of the longitudinal elongation En is calculated using the following formulas.

3 (1 - ~1)(1 + 2)} (x 100(%)) .. (1) (3 97 _ 0~0476) (x 100(%)) ... (2) We - ~(~ - t) 1 { Do - t) } (x 100(%)) ... (3) In the formulas, 1 is the size-reduction in the circular circumferential direction of the pipe, I is the thickness increase in the direction across the pipe wall, D is the diameter of pipe, t is the thickness of the pipe wall, and We is the strip width. Formulas 9 123~S6~

(1) and (3) are theoretical formulas, while formula (2) is an empirical formula including the inherent constants of an ERR mill.
The longitudinal strain is induced by working the strip by an ERR mill including breakdown rolls, sizing rolls, fin-pass rolls, and squeeze rolls.
As to the strain-aging process, the conditions of the strain-aging treatment vary depending upon the amount of solute carbon and nitrogen and the longitudinal elongation I A temperature of from 100C to 550C
and time of from 30 seconds to 30 minutes are preferred.
A low temperature and long time within the above ranges are preferred. Clearly, the conditions of the strain-aging treatment must be adjusted within the above temperature and time ranges, so that, depending upon the amount of solute carbon and nitrogen and the longitudinal elongation I , the stress correlated with and generated by the strain is not appreciably reduced by the thermal activation. In addition, the conditions for the strain-aging treatment should also be adjusted from an economic point of view and adjusted so as not to deteriorate the roundness and straightness of an ERR oil-well pipe.
The present invention is now explained by way of examples.
Example 1 (80 ski ERR oil-well pipe) ERR oil-well pipes 5 1/2" in outer diameter and 0.361" in wall thickness were produced under the con-dictions given in Table 1. The properties are also shown in Table 1. As apparent from Table 1, both the sour-environment resistance and collapse resistance of the pipes according to the present invention were excellent as compared with the comparative ERR oil-well pipes.

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Example 2 (95 ski ERR oil-well pipe) ERR oil-well pipes 5-l/2" outer diameter and 0.361"
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Claims

- 15 -

1. An ERW oil-well pipe consisting of 0.22% or less of C, 0.50% or less of Si, from 1.0 to 2.0% of Mn, 0.05% or less of Nb, and the balance of iron and unavoid-able incidental elements including N, characterized by being in a strain-aged state and having a hardness and yield strength falling within the range defined by points A, A', B, C, D", D', and D shown in the attached Fig. 1.

2. An ERW oil-well pipe according to claim 1, characterized by containing at least one member selected from the group consisting of aluminum in an amount of 0.050% or less, vanadium in an amount of 0.050% or less, and titanium in an amount of 0.040% or less.

3. An ERW oil-well pipe according to claim 2, characterized by having the hardness and yield strength falling within the range defined by the points A, A', D', D.

4. An ERW oil-well pipe according to claim 3, wherein the carbon content is 0.19% or less.

5. An ERW oil-well pipe according to claim 2, characterized by having the hardness and yield strength falling within the points A', B, C, and D".

6. An ERW oil-well pipe according to claim 5 wherein the carbon content is from 0.12% to 0.22%.

7. A process for producing an ERW oil-pipe according to claim 1, characterized by hot-rolling steel at a finishing temperature of from 740°C to 830°C, cooling the steel at an average cooling temperature of 15°C/sec or more down to the coiling temperature, coiling the steel at a temperature of 500°C or less, and, during a subsequent forming process of a pipe, inducing 3% or more strain in a longitudinal direction of the pipe during formation of the pipe and subsequently heating the pipe to a temperature of from 100°C to 550°C
for a period of from 30 seconds to 30 minutes.

8. A process according to claim 7, wherein the ERW oil-well pipe has hardness and yield strength falling within the range defined by the points A, A', D', D, and, a carbon content of from 0.08% to 0.19%, and the average cooling rate is from 15 to 35°C/sec.

9. A process according to claim 7, wherein the ERW oil-well pipe has a hardness and yield strength falling within the points A', B, C, and D", and a carbon content from 0.12% to 0.22%, and the average cooling rate is from 25 to 45°C/sec.