Note: Descriptions are shown in the official language in which they were submitted.
~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'
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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
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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
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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
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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.
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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
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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.
1 2 1 2
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.
- 10 - ~23956~ ''
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Example 2 (95 ski ERR oil-well pipe)
ERR oil-well pipes 5-l/2" outer diameter and 0.361"
in wall thickness were produced under the conditions
given in Table 2. The properties are all also shown in
Table 2. As apparent from Table 2, 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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