Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
MARTENSITIC STAINLESS STEEL SEAMLESS PIPE
AND A MANUFACTURING METHOD THEREOF
TECHNICAL FIELD
The present invention relates to a martensitic stainless steel seamless
pipe, such as a pipe for an oil well, which ensures no generation of cracks
resulting from a delayed fracture. The present invention also relates to a
method
for manufacturing such a martensitic stainless steel pipe without any
generation
of inner surface defects such as internal scabs.
BACKGROUND ART
A martensitic stainless steel such as API-13% Cr, which is used as a pipe
for an oil well, normally includes a carbon content of about 0.2%, which needs
a
high yield strength of 80 ksi (552 MPa) or more and a hot workability. Due to
a
high C and Cr content, an as-rolled stainless steel pipe has an extreme
hardness,
therefore has a reduced toughness. Consequently, an as-rolled conventional
martensitic stainless steel pipe might have a crack resulting from a delayed
fracture in "the impact-worked portion", where an impact load or static load
was
worked before a heat treatment. Accordingly, it is necessary to restrict the
piling
height in "a rack" and/or the dropping height into a rack of the steel pipes
during
transportation or storage. Moreover, the stand-by time before a heat treatment
after hot-rolling must be shortened.
The above-mentioned restrictions during transportation or storage could
result in various disadvantages such as a large stockyard because of the
restriction of the piling height and/or the dropping height of pipes, a
reduction in
working efficiency resulting from the careful handling of the steel pipes
without
excessive loading impact and a restricted time schedule from hot rolling to a
heat
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treatment in order to finish the heat treatment within a restricted working
period.
Japanese Patent Unexamined Publication No. H8-120415 discloses a
martensitic stainless steel having a restricted N content. In this patent
specification, only the improvement of toughness after a heat treatment is
described. However, neither the relationship between the N content and a
delayed fracture in the impact-worked portions of an as-rolled steel pipe nor
the
measures for suppressing such inner surface defects as internal scabs due to
poor
hot workability resulting from the decreased N content is described. It is not
practical to manufacture a seamless steel pipe without any measures to
suppress
internal scabs.
Japanese Patent Unexamined Publication No. H6-306551 discloses an
invention, in which the hydrogen content is restricted to improve the
toughness in
the heat affected zone by welding of a martensitic stainless steel pipe having
low
carbon content. Furthermore, Japanese Patent Unexamined Publication No.
H5-255734 describes an invention of dehydrogenating a martensitic stainless
steel having low carbon content in order to prevent a delayed fracture. These
inventions deal with a martensitic stainless steel having low carbon content.
However, no description is given regarding the relationship between the
hydrogen
content and a delayed fracture in the impact-worked portions of an as-rolled
martensitic stainless steel pipe containing such high C of about 0.2%. cn
DISCLOSURE OF INVENTION
The present invention provides a manufacturing stainless steel ,
seamless pipe, containing C of about 0.2 %, which suppresses a delayed
fracture
in the impact-worked portions after rolling as well as after a heat treatment,
and
also generates no internal scab.
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It is a second objective of the present invention to provide a method for
manufacturing a martensitic stainless steel pipe, without an internal scab
generation, which suppresses a delayed fracture in the impact-worked portions
before a heat treatment after rolling.
The present inventors have attained the first objective by restricting the
correlation of the contents of C (carbon), H (hydrogen), N (nitrogen) and S
(sulfur)
in addition to specifying the contents of various elements in steel properly.
Moreover, the present inventors have attained the second object by
specifying the condition to roll a steel pipe.
The present invention is characterized by the following martensitic
stainless steel (A) and the following method (B) for manufacturing martensitic
stainless steel. In this specification, "%" implies "mass %" regarding a
content of
each element. Furthermore, "as-rolled pipe" means a pipe which is formed by a
hot rolling and to which a heat treatment has not been applied yet.
(A) A martensitic stainless steel seamless pipe, characterized by consisting
of,
by mass %, C: 0.15 to 0.22%, Si: 0.1 to 1.0%, Mn: 0.10 to 1.00%, Cr: 12.00 to
14.00%, P: 0.020% or less, S: 0.010% or less, N: 0.05% or less, 0 (Oxygen):
0.0060% or less, Al: 0 to 0.1%, Ni: 0 to 0.5%, Cu: 0 to 0.25%, Ca: 0 to
0.0050% and
at least one alloying element selected from at least one group of those
mentioned
below (totally 0.005 to 0.200 mass % in case of including two or more kinds of
these alloying elements), and the balance Fe and impurities:
The first group: V, Nb and Ti of 0.005 to 0.200 mass %, respectively,
The second group: B of 0.0005 to 0.0100 mass %,
and is also characterized by satisfying either of the following inequalities
(1), (2),
(4) and (5) or the following inequalities (1), (3), (4) and (5):
C* + 10N* 5 0.45, (1)
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Hl < -0.003(C* + 10N*) + 0.0016, (2)
H2< -0.0018(C* + ION*) + 0.00096, (3)
Cr* < 9.0, (4)
S< 0.088N* + 0.00056, (5)
where C* is an effective solute carbon content (mass %) defined by the
following
equation (6), N* is an effective solute nitrogen content (mass %) defined by
equation (7), and Cr* is a Cr equivalent defined by equation(8), H1 of
inequality(2) is the amount (mass %) of residual hydrogen in an as-rolled
steel
pipe, and H2 of inequality (3) is the amount (mass %) of residual hydrogen in
the
steel pipe after a heat treatment, and a symbol of an element in each equation
or
inequality is a content(mass %) of the respective element:
C* = C - [12{(Cr/52) x (6/23)}/10], (6)
N* = N - [14{(V/51) + (Nb/93)}/10]
- [14{(Ti/48) + (B/11) + (Al/27)}/2], (7)
Cr* = Cr + 4Si - (22C + 0.5Mn + 1.5Ni + 30N) (8)
Furthermore, it is preferable that the steel pipe wherein has a C content of
0.18 to 0.21%, a Si content of 0.20 to 0.35%, a Cr content of 12.40 to 13.10%,
a S
content of 0.003% or less, and a N content of 0.035% or less.
(B) A method for manufacturing a martensitic stainless steel seamless pipe,
characterized by pierce-rolling a stainless steel with an inclined roller type
piercing mill under conditions of satisfying the inequality(9) below,
which consists of, by mass %, C: 0.15 to 0.22%, Si: 0.1 to 1.0%, Mn: 0.10 to
1.00%,
Cr: 12.00 to 14.00%, P: 0.020% or less, S: 0.010% or less, N: 0.05% or less,
O(Oxygen): 0.0060% or less, Al: 0 to 0.1%, Ni: 0 to 0.5%, Cu: 0 to 0.25%, Ca:
0 to
0.0050% and at least one alloying element selected from at least one group of
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those mentioned below (totally 0.005 to 0.200 mass % in case of including two
or
more kinds of these alloying elements), and the balance Fe and impurities:
The first group: V, Nb and Ti of 0.005 to 0.200 mass %, respectively.
The second group: B of 0.0005 to 0.0100 mass %.
and which also satisfies all the following inequalities (1), (4) and (5):
C* + 10N* 5 0.45, (1)
Cr* <- 9.0, (4)
S<- 0.088N* + 0.00056, (5)
Cr* < 0.00009(C.A. + F.A.) 3 - 0.0035(C.A. + F.A.) 2
+ 0.0567(C.A. + F.A.) + 8.0024 (9)
wherein C* is an effective solute carbon content (mass %) defined by the
following
equation (6), N* is an effective solute nitrogen content (mass %) defined by
equation (7), and Cr* is a Cr equivalent defined by equation (8), C.A. and
F.A. in
inequality (9) express a toe angle and a feed angle, respectively, a symbol of
an
element in each equation or inequality represents a content (mass %) of the
respective element:
C* = C - [12{(Cr/52) x (6/23)}/10], (6)
N* = N - [14{(V/51) + (Nb/93)}/10]
- [14{(Ti/48) + (B/11) + (Al/27)}/2], (7)
Cr* = Cr + 4Si - (22C + 0.5Mn + 1.5Ni + 30N) (8)
Furthermore, it is preferable that the steel pipe wherein has a C content of
0.18 to 0.21%, a Si content of 0.20 to 0.35%, a Cr content of 12.40 to 13.10%,
a S
content of 0.003% or less, and a N content of 0.035% or less, and also that
method
for manufacturing the martensitic stainless steel seamless pipe comprises the
following steps (10) and (11) after pierce-rolling:
(10) soaking a pipe at a temperature 920 C or more,
(11) carrying out the hot rolling.
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BRIEF DESCRIPTION OF DRAWINGS
Fig. 1 is a diagram showing the relationship between a crack resulting
from a delayed fracture and two parameters: the effective solute carbon
content
(C*) and the effective solute nitrogen content (N*).
Fig. 2 is a diagram showing the relationship between the amount of
residual hydrogen in an as-rolled steel pipe (H1) and that in a heat-treated
(H2).
Fig. 3 is a diagram showing the relationship between a crack resulting
from a delayed fracture and two parameters: "C* + 10N*" and the amount of
residual hydrogen in an as-rolled steel pipe (H1).
Fig. 4 is a diagram showing the relationship between a crack resulting
from a delayed fracture and two parameters, "C* + 10N*" and the amount of
residual hydrogen in a heat-treated steel pipe (H2).
Fig. 5 is a diagram of occurrence of internal scabs in a correlation of
effective solute nitrogen content (N*) and sulfur content.
Fig. 6 is a diagram of occurrence of both internal scabs and external
defects in correlation of "toe angle (C.A.) + feed angle (F.A.)" and Cr
equivalent
(Cr*).
BEST MODE FOR CARRYING OUT THE INVENTION
The present inventors assumed that a delayed fracture of the
impact-worked portions in a martensitic stainless steel depends on the amounts
of
solute C (carbon), solute N (nitrogen) and solute H (hydrogen), which are
interstitial elements. Following many experiments and the following facts (a)
to (d) were confirmed:
(a) Sensitivity of a delayed fracture in the impact-worked portions of an as-
rolled
steel pipe depends upon the amount of both solute C and solute N, and
especially
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upon that of solute N.
(b) The amount of solute C strongly influences the mechanical strength after a
heat treatment, whereas the amount of solute N has less influence on it.
However, N provides a remarkable reduction in the delayed fracture resistance
for
the impact-worked portions of an as-rolled steel pipe.
(c) When the N content is decreased in order to enhance the delayed fracture
resistance for the impact-worked portions of an as-rolled steel pipe, the
austenite
structure becomes unstable at a high temperature, which causes numerous
internal scabs during the manufacturing of the pipe because of poor hot
workability. Therefore, it is necessary to suppress scabs.
(d) In order to solve this problem, a piercing angle (toe angle) and a feed
angle for
the piercing mill is specified, according to the contents of the austenite
generating
elements and the ferrite generating elements in order to minimize the amount
of
work strain in the material. Thus, this procedure makes it possible to prevent
an
internal scab.
Various conditions, such as the chemical composition of the steel pipe and
the manufacturing method, according to the present invention, will be
explained
in detail below.
1. Chemical composition of steel pipe
The chemical composition of the martensitic stainless steel pipe according
to the invention is determined as follows.
C:
C provides a solid-solution hardening of an as-rolled steel pipe together
with N. The content of C should be 0.22% or less, and is preferably 0.21% or
less,
in order to suppress the delayed fracture of the impact-worked portions by the
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solid-solution hardening. However, such a reduced C content makes it difficult
to
attain the aimed mechanical strength after a heat treatment. Moreover, an
excessive reduction in the C content causes internal scabs generated after
making
a steel pipe due to S -ferrite since C is an austenite-generating element.
Accordingly, the content of C should be 0.15% or more, and the content of
effective
solute C should satisfy the inequality (1) above. The reason for this will be
explained later. It is preferable that the C content is 0.18% or more.
Si:
Si is added as deoxidant during steel making. A content of less than 0.1%
provides no effect on deoxygenating whereas more than 1.0% causes a low
toughness. Accordingly, the content should be 0.1 tol.0%. A preferable content
is
0.75% or less in order to obtain a high toughness. A more preferable content
is
0.20 to 0.35%.
Mn:
Mn is an element effective for enhancing the mechanical strength of steel,
and is added as a deoxidant during steel making. In addition, it fixes the S
in steel
by forming MnS, and causes a good hot workability. A content of less than
0.10%
provides no effect on a hot workability, whereas more than 1.00% causes a low
toughness. Accordingly, the content should be 0.1 to 1.0%. It is preferable
that
the Mn content is 0.7% or less.
Cr:
Cr is a basic element for enhancing a corrosion resistance of steel. In
particular, a content of more than 12.00% improves a corrosion resistance for
a
pitting, and further greatly enhances a corrosion resistance under a C02
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environment. On the other hand, because Cr is a ferrite- generating element,
the
Cr content of more than 14.00% is apt to generate 6 ferrite in the process at
a
high temperature, causing a reduced hot workability . Moreover, an excessive
Cr content results in high cost of production. Accordingly, the content should
be
12.00% to 14.00%, and is more preferably 12.40% to 13.10%.
P:
P is an impurity contained in steel. An excessive P content causes a low
toughness of products after a heat treatment. An allowable upper limit of the
P
content should be 0.020%. It is preferable to minimize the P content as small
as
possible.
S:
Because S is an impurity that decreases a hot workability, the S content
should be minimized. An allowable upper limit of the S content is 0.010%. The
S content should satisfy the inequality (5) above. It is preferable that the S
content is 0.003% or less.
N:
N is an austenite-stabilizing element that improves the hot workability of
steel. However, N causes a delayed fracture in the impact-worked portions of
an
as-rolled steel pipe. Accordingly, the upper limit of the N content should be
0.05%. The reduction in a hot workability resulting from a decreased N content
is
compensated by other elements, so that the N content should be minimized. It
is
preferable that the N content is 0.035% or less.
O (oxygen):
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In case of an incomplete deoxygenating during the steel making process,
the number of cracks or streaks on the surface of a billet are increased and
an
external scabs are generated in a hot-rolled steel. Accordingly, the content
of 0
should be minimized to be 0.0060% or less.
V, Ti, Nb and B:
These elements combine with N to form nitrides. An inclusion of more
than one selected from these elements provides a reduced the number of solute
N
solubility as if N content is decreased. However, an excessive N content
causes
extremely high hardness by the nitrides formed after a heat treatment and
results
in a reduction of a corrosion resistance and toughness. Accordingly, the V, Ti
or
Nb content should be 0.005 to 0.200%, respectively, and the B content should
be
0.0005 to 0.0100%. The total content of these elements should be 0.005 to
0.200% in case of including two or more kinds of these alloying elements.
Al, Ni, Cu and Ca
These elements can be included if necessary. The numerical value "0" in
the content for one of the elements implies that the element is not
intentionally
added into the steel.
Al:
Al can be added when deoxygenating during the steel making process and
is effective for suppressing an external scab in a steel pipe. However, an
excessive Al content causes a reduced cleanness of steel and also causes
clogging
of an immersion nozzle in the process of a continuous casting. Accordingly, it
is
preferable that the Al content is 0 to 0.1%.
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Ni:
Ni is an austenite-stabilizing element and improves the hot workability
of steel. However, an excessive Ni content causes a reduced sulfide stress
corrosion cracking resistance. Accordingly, it is preferable that the Ni
content is
0to0.5%.
Cu:
Cu is effective for enhancing corrosion resistance and is an
austenite-stabilizing element to improve the hot workability of steel.
However,
Cu has a low melting point, and an excessive Cu content causes a reduced hot
workability. Accordingly, it is preferable that the Cu content is 0 to 0.25%.
Ca:
Ca combines with S in steel and prevents a sulfur segregation in grain
boundaries, which caused a reduced hot workability. However, an excessive Ca
content causes macro-streak-flaws. Accordingly, it is preferable that the Ca
content is 0 to 0.0050%.
2. As for inequalities (1) to (5)
First, the inequality (1) will be described. In order to suppress cracks in
the impact-worked portions, it is necessary to improve the delayed fracture
resistance. An interstitial element such as C and N enhances the mechanical
strength of steel, but it deteriorates the delayed fracture resistance in the
impact-worked portions. In an as-rolled steel pipe, there remains a residual
stress resulting from a hot rolling by a sizing mill or a stretch reducing
mill, which
reduces a delayed fracture resistance more.
The present inventors studied the effect of C and N on a delayed fracture
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in the impact-worked portions of an as-rolled API-13% Cr steel pipe. In a
delayed
fracture test, an impact load was applied to the steel pipes whose conditions
will
be described in "EXAMPLES". The results are shown in Fig. 1 and Tables 1 to 4,
in which an effective solute carbon content (C*) and an effective solute
nitrogen
content (N*) were used. The reason for using C* and N* is described below.
Some of C atoms combine with Cr atoms to form carbides. The content of
C, acting as an interstitial element, can be obtained by subtracting the
content of
C in the carbide from the total content of C. Accordingly, an effective solute
carbon content (C*) is defined by the equation (6).
Similarly, some of N atoms combine with V, Nb, Ti, B and Al atoms to form
nitrides. The content of N, acting as an interstitial element, can be obtained
by
subtracting the content of N in the nitride from the total content of N.
Accordingly, an effective solute nitrogen content (N*) is defined by the
equation (7).
In the equation (7), a coefficient of 1/10 is applied for Nb and V nitrides
because of
the lower precipitation temperature and a coefficient of 1/2 for Ti, B and Al
nitrides because of the higher precipitation temperature.
Both C and N are interstitial elements in steel. If they have the same
content, they provide approximately the same influence on the mechanical
strength and the hardness. However, the content of C is restricted within a
range of 0.18 to 0.21% in a 13% Cr martensitic stainless steel seamless pipe
specified in the API-L80 grade, which is used for oil well. On the contrary,
if the
content of N is restricted only by "0.1% or less", then the content of N is
widely
selective. Usually, the N content is 0.01 to 0.05%, which is one tenth smaller
than the C content. Therefore, the properties of steel were investigated on
the
relationship of the effective solute carbon content (C*) and ten times of the
effective solute nitrogen content (N*).
As can be seen in Fig. 1, a delayed fracture in the impact-worked portions
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(crack) decreases as the content of both of C* and N* decreases. The
inequality
(1) above is determined by applying a linear interpolation to the result.
An interstitial element such as C and N influences on the work hardening
due to a cold working when a steel pipe is subjected to the impact work. In
particular, N provides pining of dislocations in order to increase the work
hardening. From the experimental results, the inventors found that the work
hardening and the delayed fracture due to hydrogen were suppressed remarkably
when the amount of "C* + 10 N*" is restricted to 0.45 or less.
The delayed fracture of the impact-worked portions is influenced by the
hydrogen amount and the hardness of the portions. It is necessary to reduce
the
effective solute carbon content (C*) and the effective solute nitrogen content
(N*)
and thereby reduce hardness in order to suppress the generation of cracks.
When steel is work-hardened by cold working due to a handling impact, hydrogen
cracking is generated even if the initial hardness is low. Accordingly, the
amount
of residual hydrogen in a steel pipe should be decreased to prevent hydrogen
cracking.
The amount of residual hydrogen in an as-rolled steel pipe is different
from that in a heat-treated steel pipe. In a 13% Cr steel, there is a
correlation
between the amount of residual hydrogen in an as-rolled steel pipe and that in
a
heat-treated steel pipe because a heat treatment temperature is substantially
fixed. The quenching temperature is 920 to 980 C and the tempering
temperature is 650 to 750 C.
Fig. 2 is a diagram showing the relationship of the amount of residual
hydrogen between H1 (as-rolled) and H2 (after heat-treated) regarding the 13%
Cr steel pipe used in the EXAMPLES below. For instance, at a point of the sign
of 0 marked by "a", the amount of residual hydrogen (H1) in an as-rolled steel
pipe was approximately 3 ppm, and the amount of residual hydrogen (H2) after a
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heat treatment was approximately 2 ppm.
The inequality (2) above restricts the relationship between "C* + 1ON*"
and Hl, and the inequality (3) above restricts the relationship between "C* +
10N*" and H2. As described above, an increased amount of C* and N* causes an
increase in strength and a decrease in toughness, and then increases delayed
fracture sensitivity due to hydrogen in the impact-worked portions. As a
result,
it is necessary to take into account a total relationship of the contents of
C* and N*
and the amount of residual hydrogen, in order to suppress a delayed fracture.
Fig. 3 shows the result which is obtained by investigating a delayed
fracture sensitivity of the impact-worked portions for an as-rolled steel pipe
of a
13% Cr martensitic stainless steel having the C content of 0.19% and plotting
the
results on the correlation of "C* + 10N*" and H1. Fig. 4 shows a result of a
similar investigation and plots on the correlation of "C* + ION*" and H2 after
a
heat treatment. These results were obtained in EXAMPLES below.
From the diagrams of Figs. 3 and 4, it can be recognized that a delayed
fracture (crack) is no longer generated in the impact-worked portions if the
inequality (1) above and the following inequalities (2) or (3) are satisfied,
where
H1 is the amount of residual hydrogen in an as-rolled pipe and H2 is the
amount
of residual hydrogen after a heat treatment:
Hl < -0.003 (C* + ION*) + 0.0016 (2)
H2< -0.0018 (C* + ION*) + 0.00096 (3)
On the other hand, the inequalities (4) and (5) below represent the ranges
of the Cr and S contents effective for suppressing an inner surface defect,
which is
called an internal scab. The satisfaction of the inequalities (2) and (3)
above
makes it possible to suppress a delayed fracture in the impact-worked
proportions
for an as-rolled steel pipe and after a heat treatment. Nevertheless, there is
a
possibility that an internal scab could be generated in the process of
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manufacturing a steel pipe.
A generation of an internal scab results from a shear deformation in a
circumferential direction in the process of pierce rolling with a piercing
mill. The
shear strain causes cracks on such a portion that has a different deformation
resistance in a billet as ferrite/austenite grain boundaries, segregations of
sulfur
and inclusions. These cracks deform and cause internal scabs in the course of
rolling.
In order to suppress cracks in the ferrite/austenite grain boundaries,
the amount of 6 ferrite has to be minimized. The amount of S ferrite depends
on the Cr equivalent (Cr*), and in fact, an increase of Cr* causes an increase
of
ferrite. Cr* can be expressed by the following equation (8), which represents
a
linear correlation between ferrite-forming elements and austenite-forming
elements:
Cr* = Cr + 4Si - (22C + 0.5Mn + 1.5Ni + 30N). (8)
As can be seen in the equation (8), N provides a significant contribution to
Cr*. When the N content is decreased to enhance the toughness of an as-rolled
steel pipe, the Cr equivalent increases and the amount of ferrite increases,
which
causes an internal scab. In view of these facts, the satisfaction of the
following
inequality (4) suppresses a ferrite and an internal scab:
Cr* < 9Ø (4)
A sulfur- segregated portion also becomes an origin of generating a crack.
In order to suppress such segregation, it is desirable to minimize S content.
For
this purpose, S content should be 0.010% or less, and it is preferable that S
content is 0.003% or less. It is preferable that the content of oxygen (0) is
0.0060% or less in order to reduce inclusions in steel, macro-streak-flaw and
the S
content during steel making.
When N* is decreased in order to satisfy the inequality (1) to suppress
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cracks, Cr* expressed by equation (8) is increased. This causes an increase of
the
ferrite phase, which causes a reduced hot workability. In order to recover a
hot
workability, the S content has to be reduced.
Fig. 5 illustrates a diagram of occurrence of internal scabs less than 2%
(shown by a sign of 0) or not less than 2% (shown by a sign of X) in the
correlation of N* in abscissa and S content in ordinate. This diagram leads to
a
recognition that restricting S content by the following inequality (5)
suppresses an
internal scab. The criteria line is decided to be 2 % of an internal scab
generation
from the viewpoint of work efficiency without interrupting manufacturing.
S<_ 0.088 N* + 0.00056. (5)
3. As for the manufacturing method
In the method of manufacturing a seamless steel pipe according to the
invention, the steel having the above-mentioned chemical composition and
satisfying the inequalities (1), (4) and (5) is pierce-rolled under conditions
restricted by the inequality (9) with the aid of a cross roller type piercing
mill.
In order to suppress an internal scab during pierce rolling, it is important
to select the proper rolling conditions, taking into account the hot
workability of
the steel to be rolled.
Various factors influence generating internal scabs. Among these factors,
feed angle and toe angle of main rollers in a piercing mill play an essential
role.
Generally, an increase in both a feed angle and a toe angle reduces the
additional
shear deformation in the process of pierce rolling, and makes it possible to
roll the
steel without generating cracks even if it has a poor hot workability.
However, feed angle and toe angle cannot always be easily increased. In
order to attain an increase in these angles, the replace of a main motor is
required,
and even a replace of the mill may be required. If the steel has a proper hot
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workability during rolling, it would be possible to choose a relatively small
feed
and toe angles. The relationship between an index regarding a hot workability
during rolling and an index suppressing an internal scab i.e. an additional
shear
deformation, can lead to a possible optimal manufacturing conditions of design
of
material of steel and conditions for pierce rolling from the viewpoint of
economy in
the manufacturing.
The present inventors researched the past experimental data to
investigate the influence of feed and toe angles on the additional shear
deformation, and further studied the relationship between the Cr* and the sum
of
"C.A. (toe angle) + F.A. (feed angle)". As a result, an explicit correlation
between
Cr* and "C.A. + F.A." was found on the basis that both of feed and toe angles
contribute to the same extent to an additional shear stress
Fig. 6 illustrates a diagram of the occurrence of both an internal scab and
an external defect less than 2% (shown by 0) or not less than 2% (shown by a
sign of =) in a correlation of "C.A. + F.A." in abscissa and Cr* in ordinate.
This
map leads to the recognition that a boundary line of whether both an internal
scab and an external defect are less than 2% (shown by 0) or not (shown by a
sign of =) can be expressed by the cubic curve. A condition satisfying the
following inequality (9) leads to a suppressed generation of internal scabs.
Cr* < 0.00009 (C.A. + F.A.) 3 - 0.0035 (C.A. + F.A.) 2
+ 0.0567 (C.A. + F.A.) + 8.0024, (9)
where the right side of the inequality (9) is determined by interpolating the
obtained data and represents the boundary above.
A manufacturing method according to the invention may include a process
of re-heating before finishing rolling wherein a stretch reducer is used. It
is
preferable, in this case, that soaking is held at a temperature of 920 C or
more
during re-heating. A decreased soaking temperature during re-heating causes a
17
CA 02491834 2005-01-06
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reduced toughness of an as-rolled steel in T direction, which is perpendicular
to a
rolling direction, because of the incomplete recrystallization of flat grains,
formed
during working. Furthermore, C and N enriched areas are generated around Nb
and/or V carbides/nitrides because of the incomplete solid solution or
diffusion of
the carbides and/or nitrides. Then, a hardening and a brittleness take place
in
the areas, which cause a delayed fracture. It is preferable that the lower
limit of
a soaking temperature during re-heating is 920 C, or more preferable 1000 C,
and it is preferable that the upper limit of a soaking temperature is 1100 C
or so.
EXAMPLES
Seamless pipes, having a 60.3 mm outer diameter and a 4.83 mm
thickness, were produced from 43 kinds of steel having the chemical
composition
shown in Tables 1 and 2. Then the following tests were carried out for these
steel
pipes.
(1) Delayed fracture test
Drop test pieces having a 250 mm length were prepared from as-rolled
steel pipes. A weight test element, having 150 kg weight and a 90 mm curvature
at its tip, was dropped from a 0.2 in height onto a test piece, which is
deformed
under an impact load (294J). After one week each piece was inspected as to
whether or not cracks were generated. An inspection of cracks was carried out
by a visual check and also by an ultrasonic test (UST). The results are listed
in
Tables 3 and 4.
Fig. 1 is a diagram showing the relationship between the generated cracks
and both effective solute carbon content (C*) and effective solute nitrogen
content
(N*). As shown in the diagram, a straight line "a" implies a boundary of
generating cracks. The straight line "a" can be expressed by "C* + ION* =
0.45".
18
CA 02491834 2005-01-06
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Accordingly, the condition of generating no delayed fracture can be expressed
by
C* + ION* < 0.45.
(2) Measurement the amount of residual hydrogen (H1 and H2)
The amount of residual hydrogen of an as-rolled steel pipe and the amount
of the same after a heat treatment were measured using an analyzing method
specified in JIS Z2614. In the heat treatment, a test piece was water-quenched
at the temperature of 950 C and then tempered at 7009C. The results of
measurement are listed in Tables 3 and 4.
Fig. 2 is a diagram showing the relationship between H1 and H2 of the
test piece. It can be ascertained that there is a linear relationship which
can be
expressed approximately by "H2 = 0.6H1".
(3) The relationship between a delayed fracture and three parameters, C*, N*
and
the amount of residual hydrogen.
The data listed in Tables 3 and 4 regarding whether a delayed fracture
was generated or not are represented in the diagram Fig. 3 for an as-rolled
steel
pipe and in the diagram Fig. 4 for a heat-treated steel pipe, respectively,
where
the abscissa means "C* + ION*" and the ordinate means the amount of residual
hydrogen. The straight lines for the boundaries of whether a crack is
generated
or not is expressed by the following equations (2)-1 and (3)-1 below,
respectively.
Accordingly, a condition of generating no delayed fracture is to satisfy the
inequalities (2) or (3) above. Moreover, even if the inequalities (2) and (3)
are
satisfied, there is a possibility that a delayed fracture could take place
when "C* +
10N*" is more than 0.45. Then, the inequality (1) above should be satisfied.
H1= -0.003 (C* + ION*) + 0.0016, (2)-1
H2 = -0.0018 (C* + ION*) + 0.00096. (3)-1
19
CA 02491834 2005-01-06
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"0 0 r 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Z O C C
'v=
O CD 0 CD 0 0 0 0 0 CD 0 CD 0 0 0 0 0 0 CD
CO ='C3 `-' 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ,D
D > Cq 0 0 0 0 0 0 0 0 0 C) 0 0 0 0 0 0 0 0 m 41
N N
CO N
41 41
w
O 0 0 0 x x x x x O O O x O O O O O O o LIO
4-4
+ M It M M N LC') M ~t CO M CO M d= M CO d= r O N T O 0
Z N d' d' CO 0) N- M " t' CO N C) 'd' CO 00 N- CO > 41 t
-a -a
* M M M G d: LC) d= CO N C') La) Cl) Cl) N N N N 41 -11
O 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3 0 0
0 0
r N C) O n 00 N LC) C) N - La) N O O N .~ 2
# O N M CO C) O La) = tt La) CO CO M CO 00 N- N co N
N N N M M M I~t m N r N M N N r r r N 0>>
Z O O O O O O O O O O O O O O O O O O d 4, a) CD 0 C) O CD 0 0 CD C O CD C7 O
CD O C 0 C~ aai m 41 m
N N d' M LL d' La) M N CO) 'It N N et N r t aL-+ 7 7
' N N N C') N O O C') M o r CM C) r 0 0 CD C) I 0 C) 0
T r r r r r r T r r r r r r r r O rn ...~
O O O O O O CD O O O O CD O O O O O O C 3 0 C0i
0 CO N- CO C) O N M d^ La) CO N- 00 Cn O r= N M 0
Z N C4 N CV CO M M M M M CM M M M Z Or OO
23
CA 02491834 2005-01-06
WO 2004/007780 PCT/JP2003/008625
(4) Inspection of internal scabs
By selecting several kinds of steel in Tables 1 and 2 which have various
contents of effective solute of N and sulfur, 500 steel pipes were produced
under
condition of "C.A. + F.A." = 9, and were inspected whether an internal scab
was
generated or not. The result is shown in Fig. 5. The inclined straight line
implies a boundary of whether an internal scab generation is less than 2% or
not.
It can be expressed by the following equation (5)-1. Therefore, an internal
scab
can be suppressed by satisfying the inequality (5) above.
S = 0.088 N* + 0.00056. (5)-1
By selecting several kinds of steel in Tables 1 and 2, 50 steel pipes which
have various Cr equivalent (Cr*) listed in Table 5, were produced from billets
under the following conditions, which were inspected to determine whether an
internal scab was generated or not:
(1) Heating temperature of billet: 1200 to 1250 C
(2) Reduction rate of billet diameter at the top of plug: 5.0 to 8.0%
(3) C.A. + F.A.: 10, 17, 21 and 30
Table 5 shows a relationship between an internal scab generation and two
parameters, Cr* and "C.A. + F.A.". In Table 5 and Fig. 6, a sign of 0
indicates
that both an internal scab and an external scab are less than 2%, and a sign
of =
indicates that either an internal scab or an external scab is not less than
2%.
Fig. 6 is a diagram of the results in Table 5 using the parameters, "C.A. +
F.A." and Cr*. A cubic line in the diagram is expressed by the following
equation
(9)-1. Accordingly, the condition of suppressing an internal scab generation
is to
satisfy the inequality (9) above.
Cr* = 0.00009 (C.A. + F.A.) 3 - 0.0035 (C.A. + F.A.) 2
+ 0.0567 (C.A. + F.A.) + 8.0024. (9)-1
24
CA 02491834 2005-01-06
WO 2004/007780 PCT/JP2003/008625
Table 5
C.A.+F.A.
No. Cr*
17 21 30
9 7.735 0 0 0 0
4 7.815 0 0 0 0
6 7.980 0 0 0
39 8.120 0 0 0 0
7 8.225 0
11 8.280 0 0 0
40 8.395 = 0
41 8.515 = = = 0
42 8.795 = = = 0
35 9.155 = = 0
34 9.230 = =
43 9.515 =
5 INDUSTRIAL APPLICABILITY
A 13% Cr martensitic steel seamless pipe according to the invention
prevents a delayed fracture generation when it is subjected to an impact cold
working during handling after manufacturing the pipe. This steel pipe has an
excellent corrosion resistance and is particularly available for oil well. A
13% Cr
10 martensitic seamless steel pipe can be produced without an internal scab
generation according to a manufacturing method of the invention.
