U.S. patent application number 12/219390 was filed with the patent office on 2008-11-20 for high strength seamless steel pipe excellent in hydrogen-induced cracking resistance and its production method.
Invention is credited to Nobuyuki Hisamune, Kunio Kondo, Nobutoshi Murao, Hajime Osako.
Application Number | 20080283161 12/219390 |
Document ID | / |
Family ID | 34811763 |
Filed Date | 2008-11-20 |
United States Patent
Application |
20080283161 |
Kind Code |
A1 |
Murao; Nobutoshi ; et
al. |
November 20, 2008 |
High strength seamless steel pipe excellent in hydrogen-induced
cracking resistance and its production method
Abstract
The present invention relates to a high strength seamless steel
pipe excellent in hydrogen-induced cracking resistance,
characterized by consisting of, by mass %, C: 0.03-0.11%, Si:
0.05-0.5%, Mn: 0.8-1.6%, P: 0.025% or less, S: 0.003% or less, Ti:
0.002-0.017%, Al: 0.001-0.10%, Cr: 0.05-0.5%, Mo: 0.02-0.3%, V:
0.02-0.20%, Ca: 0.0005-0.005%, N: 0.008% or less and O (Oxygen):
0.004% or less, and the balance Fe and impurities, and also
characterized in that the microstructure of the steel is bainite
and/or martensite, ferrite is precipitated at grain boundaries and
yield stress is 483 MPa or more. Further, to ensure high strength
of the steel, the seamless steel pipe preferably contains, by mass
%, at least one of Cu: 0.05-0.5% and Ni: 0.05-0.5%. To produce the
above-mentioned steel pipe, it is desirable to limit a starting
temperature of quenching after rolling, a cooling rate and a
tempering temperature. By this configuration a seamless steel pipe
having an yield stress of 483 MPa or more and excellent HIC
resistance, which is suitable for a pipeline, can be provided.
Inventors: |
Murao; Nobutoshi;
(Wakayama-shi, JP) ; Hisamune; Nobuyuki;
(Naga-gun, JP) ; Osako; Hajime; (Wakayama-shi,
JP) ; Kondo; Kunio; (Sanda-shi, JP) |
Correspondence
Address: |
CLARK & BRODY
1090 VERMONT AVENUE, NW, SUITE 250
WASHINGTON
DC
20005
US
|
Family ID: |
34811763 |
Appl. No.: |
12/219390 |
Filed: |
July 22, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11088740 |
Mar 25, 2005 |
7416617 |
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12219390 |
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PCT/JP2003/012373 |
Sep 26, 2003 |
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11088740 |
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Current U.S.
Class: |
148/593 |
Current CPC
Class: |
C22C 38/02 20130101;
C22C 38/24 20130101; C21D 2211/002 20130101; C22C 38/002 20130101;
C21D 8/10 20130101; C22C 38/04 20130101; C21D 2211/005 20130101;
C21D 2211/008 20130101; C22C 38/22 20130101; C21D 9/08
20130101 |
Class at
Publication: |
148/593 |
International
Class: |
C21D 9/08 20060101
C21D009/08 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 1, 2002 |
JP |
2002-288661 |
Feb 27, 2003 |
JP |
2003-051427 |
Claims
1-2. (canceled)
3. A production method of a high strength seamless steel pipe
excellent in hydrogen-induced cracking resistance, characterized in
that after rolling a billet having a composition according to claim
1 to a seamless steel pipe by hot rolling, said seamless steel pipe
is immediately soaked and then cooled at a starting temperature of
quenching of (Ar.sub.3 point+50.degree. C.) to 1100.degree. C. and
at a cooling rate of 5.degree. C./sec or more, and then said
seamless steel pipe is tempered at 550.degree. C. to Ac.sub.1
points, whereby a seamless steel pipe in which the microstructure
of steel is bainite and/or martensite, ferrite is precipitated at
grain boundaries and yield stress is 483 MPa or more is
produced.
4. A production method of a high strength seamless steel pipe
excellent in hydrogen-induced cracking resistance, characterized in
that after rolling a billet having a composition according to claim
2 to a seamless steel pipe by hot rolling, said seamless steel pipe
is immediately soaked and then cooled at a starting temperature of
quenching of (Ar.sub.3 point+50.degree. C.) to 1100.degree. C. and
at a cooling rate of 5 .quadrature./sec or more, and then said
seamless steel pipe is tempered at 550.degree. C. to Ac.sub.1
points, whereby a seamless steel pipe in which the microstructure
of steel is Bainite and/or martensite, ferrite is precipitated at
grain boundaries and yield stress is 483 MPa or more is produced.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a seamless steel pipe
excellent in hydrogen-induced cracking resistance (hereinafter
referred to as "HIC resistance"), which is used as a line pipe
having 5L-X70 grade or higher of American Petroleum Institute (API)
Standard in strength level.
[0003] 2. Related Art
[0004] In recent years, well conditions of an oil well for crude
oil and a gas well for natural gas (hereinafter referred to as only
"oil well and the like" generally) become severe and the
transportation of the crude oil and natural gas has been performed
under a severe environment. As the depth of water is increased, the
well condition of the oil well and the like tends to contain
CO.sub.2, H.sub.2S, Cl.sup.-, and the like in the ambient, and
H.sub.2S is often contained in the crude oil and natural gas.
[0005] When the oil well and the like are in the seabed, as the
depth of water is increased, an off shore pipe line is demanded for
high strength and thick wall thickness to stand the water pressure
on the seabed. As the off shore pipeline in such deep sea seamless
steel pipes are usually used.
[0006] In a pipeline used for the transportation of crude oil or
natural gas containing much H.sub.2S, not only corrosion of a
surface of a steel material due to H.sub.2S, but also a fracture
phenomenon of the steel material such as hydrogen-induced cracking
or hydrogen-induced blistering or the like (hereinafter referred to
as "HIC" generally) due to absorption of hydrogen generated by the
corrosion into steel, are generated. This HIC is different from the
sulfide stress corrosion cracking, which is conventionally
recognized in a high strength steel, and does not depend on
external stress so that the occurrence of HIC is recognized without
external stress.
[0007] When such an HIC is occurred in a transporting pipeline, it
may lead to a breakage accident of the pipeline. As a result a
large scale environmental breakage due to leakage of crude oil or
natural gas tends to occur. Accordingly, in the transporting
pipelines for crude oil and natural gas, it is an important matter
to prevent the occurrence of HIC.
[0008] The above-mentioned HIC is a steel material fracture
phenomenon that inclusions such as MnS, Al.sub.2O.sub.3, CaO, CaS
and the like existing in steel are changed, during the rolling of a
steel material, to elongated ones in the rolling direction or
crushed cluster-like ones, hydrogen absorbed into the interfaces
between these inclusions and matrix steel is accumulated and
gasified, cracks are generated by the gas pressure of the
accumulated hydrogen, and these cracks propagate in steel.
[0009] To prevent the HIC, which exhibits such behaviors in steel,
various steel materials for a line pipe has been proposed. For
example, Japanese Patent Application Laid-open No. S50-97515
proposes steel for a line pipe in which Cu: 0.2-0.8% is added to
steel having strength of X42-X80 grade in the API standard to form
an anticorrosive film thereby preventing hydrogen from absorbing
into the matrix steel.
[0010] Further, Japanese Patent Application Laid-open No.
S53-106318 proposes a steel material for a line pipe in which Ca:
excess 0.005%-0.020 or less %, which is comparatively a large
amount, is added to steel and inclusion (MnS) in steel is
spheroidized by a shape control by Ca treatment thereby reducing
cracking sensitivity. Even at present HIC resistant steel has been
produced based on these proposed technologies.
[0011] Further, since the principal use of the HIC resistant steel
is a transporting pipeline for crude oil and natural gas,
weldability is important. Thus a low-carbon steel is applied to the
HIC resistant steel, but high strength steel is difficult to obtain
due to the low C content of the steel. On the other hand, as
mentioned above, consumers require for high strength materials.
Thus, to satisfy the requirement, the following steps are often
performed: after finish rolling a steel pipe by hot rolling, the
steel pipe is heated and quenched, and subsequently tempered.
[0012] Such quenching and tempering treatment of a rolled steel
pipe is effective for avoiding a ferrite and pearlite band-shaped
microstructure in which HIC is liable to occur.
[0013] As mentioned above, in the steel material for a line pipe
the weldability is important and high strength is required. Thus,
after hot rolling, a rolled steel pipe is often subjected to be
quenched and tempered. Further, in producing a seamless steel pipe,
from the view points of a suppression of an increase in equipment
costs and the production efficiency, it has been considered to
adopt a treatment applying quenching and tempering after soaking,
without cooling a finish-rolled steel pipe to Ar.sub.3 point, by
directly connecting a pipe rolling line to a heat treatment line
(hereinafter sometimes referred to as only "inline
quenching/tempering (QT)").
[0014] Accordingly, to improve the HIC resistance of a high
strength steel material for a line pipe, a seamless steel pipe of a
high strength material was produced by quenching and tempering
after soaking without cooling the rolled steel pipe to Ar.sub.3
point after hot rolling by the use of a previously proposed steel
in which inclusions (MnS) are shape-controlled by Ca treatment.
However, the occurrence of HIC exhibiting a form of an
intergranular fracture was observed. Thus, even if the HIC
resistant steel proposed in the above-described Japanese Patent
Application Laid-open No. S53-106318 and the like was applied to a
high strength steel, the HIC resistance is not necessarily
improved.
SUMMARY OF THE INVENTION
[0015] The present invention was made in consideration to the
production of a seamless steel pipe having high strength and HIC
resistance, and an object of the present invention is to provide a
high strength seamless steel pipe, which can exhibit excellent HIC
resistance and its production method.
[0016] The present inventors have collated the knowledge about
behaviors of HIC, which occurs in a line pipe, to solve the
above-mentioned problem.
[0017] As explained above, HIC is a breakage of steel by
hydrogen-induced cracking or hydrogen-induced blistering, which is
generated by the facts that hydrogen generated by corrosion absorbs
into the steel and accumulates at the interface between the
inclusions in the steel and the matrix steel and gasifies, and that
the gas pressure is increased more than the yield strength of the
steel to generate cracks, which propagate in the steel.
[0018] Therefore, in a conventional technology, an inclusion shape
control and the like, for example, were performed so that the
absorbed hydrogen hardly gasificates. However, for the high
strength steel having 5L-X70 grade or higher of API, all of
starting point of HIC is not at inclusions, and an HIC fracture
exhibits a fracture like sulfide stress-corrosion cracking and can
exhibit a form of intergranular fracture.
[0019] Hence, the relationships between HIC resistance of steel and
a quenched microstructure thereof were further reviewed. As a
result it has been newly found that even in a bainite and/or
martensite quenched microstructure, the brittleness of a grain
boundary is prevented by precipitating ferrite on the grain
boundaries and even if a minute crack is occurred in steel, the
propagation of the crack can be suppressed whereby a seamless steel
pipe having excellent HIC resistance can be obtained.
[0020] The present invention has been completed based on the
above-mentioned knowledge and the gist of the present invention is
the following high strength seamless steel pipes (1) and (2) and
the following production method of the high strength seamless steel
pipe (3).
[0021] (1) A high strength seamless steel pipe excellent in HIC
resistance, characterized by consisting of, by mass %, C,
0.03-0.11%, Si: 0.05-0.5%, Mn: 0.8-1.6%, P: 0.025% or less, S:
0.003% or less, Ti: 0.002-0.017%, Al: 0.001-0.1%, Cr: 0.05-0.5%,
Mo: 0.02-0.3%, V: 0.02-0.20%, Ca: 0.0005-0.005%, N: 0.008% or less
and O (Oxygen): 0.004% or less, and the balance Fe and impurities,
and also characterized in that the microstructure of steel is
bainite and/or martensite, ferrite is precipitated on grain
boundaries and yield stress is 483 MPa or more.
[0022] (2) A high strength seamless steel pipe, in addition to the
above-mentioned seamless steel pipe (1), further preferably
containing, by mass %, at least one of Cu: 0.05-0.5% and Ni:
0.05-0.5%.
[0023] (3) A production method of a high strength seamless steel
pipe excellent in HIC resistance, characterized in that after
rolling a billet having a composition described in the
above-mentioned (1) or (2) to a seamless steel pipe by hot rolling,
said seamless steel pipe is immediately soaked and then cooled at a
starting temperature of quenching of (Ar.sub.3 point+50.degree. C.)
to 1100.degree. C. and at a cooling rate of 5.degree. C./sec or
more, and then said seamless steel pipe is tempered at 550.degree.
C. to Ac.sub.1 points, whereby a seamless steel pipe in which the
microstructure of steel is bainite and/or martensite, ferrite is
precipitated at grain boundaries and yield stress is 483 MPa or
more is produced.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a view showing a microstructure photograph of a
seamless steel pipe inferior in HIC resistance; and
[0025] FIG. 2 is a view showing a microstructure photograph of a
seamless steel pipe excellent in HIC resistance.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0026] Reasons for defining a chemical composition, a steel pipe
microstructure and a production method as those described above in
the present invention, will be explained. First, the reason for
defining chemical composition of a seamless steel pipe according to
the present invention will be described. In the following
descriptions, the chemical composition is shown by mass %.
1. Chemical Composition of Steel
C: 0.03-0.11%
[0027] C (Carbon) is an element necessary to enhance hardenability
and to increase the strength of the steel. When the content of C is
less than 0.03%, the hardenability are lowered, and high strength
is difficult to ensure. On the other hand, when the content of C
exceeds 0.11%, in a case where QT is applied, the steel tends to
have a fully quenched microstructure such as bainite and/or
martensite or the like, whereby the HIC resistance of the steel is
not only lowered but also weldability is lowered.
Si: 0.05-0.5%
[0028] Si (Silicon) is added to steel for the purpose of
deoxidation of steel, and contributes to an increase in strength
and enhancing a softening resistance during tempering the steel. To
obtain these effects the addition of 0.05% or more Si is needed.
However, since the excess addition of Si decreases toughness of the
steel, the Si content was set to 0.5% or less.
Mn: 0.8-1.6%
[0029] Mn (Manganese) is an effective element for increasing
hardenability of the steel to increase strength thereof and for
enhancing hot workability of the steel. Particularly, to enhance
the hot workability of steel 0.8% or more Mn is needed. However,
since the excess addition of Mn decreases toughness and weldability
of steel, the Mn content was set to 1.6% or less.
P: 0.025% or Less
[0030] P (Phosphorus) exists in the steel as impurities. Since the
segregation of P in grain boundaries deteriorates toughness of
steel, the P content was set to 0.025% or less. The P content is
preferably 0.015% or less, and more preferably 0.009% or less.
S: 0.003% or Less
[0031] S (Sulfur) exists in the steel as impurities. Since S
generates sulfides such as MnS and the like and deteriorates HIC
resistance, the S content was set to 0.003% or less. The S content
is preferably 0.002% or less, and more preferably 0.001% or
less.
Al: 0.002-0.017%
[0032] Al (Aluminium) is an element effective to prevent cracking
of the billet. To exhibit the effect the Al content of 0.002% or
more is needed. On the other hand, since excessive addition of Ti
deteriorates toughness of the steel, the Ti content was set to
0.017% or less, and preferably 0.010% or less.
Al: 0.001-0.10%
[0033] Al (Aluminum) is an indispensable element for deoxidation of
the steel. When the Al content is too small, deoxidation becomes
insufficient and surface defects are generated on the billet to
deteriorate the property of steel. Thus, the Al content was set to
0.001% or more. On the other hand, since excessive addition of Al
generates cracks in the billet, which leads to deterioration of the
steel property. Thus the Al content was set to 0.10% or less, and
preferably 0.040% or less.
Cr: 0.05-0.5%
[0034] Cr (Chromium) is an element for enhancing the strength of
the steel. The significant effect can be obtained by addition of
0.05% or more Cr. However, since even excessive addition of Cr
saturates the effect, the Cr content was set to 0.5% or less.
Mo: 0.02-0.3%
[0035] Mo (Molybdenum) is an element for enhancing the strength of
the steel. The significant effect can be obtained by addition of
0.02% or more Mo. However, since even excessive addition of Mo
saturates the effect, the Mo content was set to 0.3% or less.
V: 0.02-0.20%
[0036] V (Vanadium) is an element for enhancing the strength of the
steel. The significant effect can be obtained by addition of 0.02%
or more V. However, since even excessive addition of V saturates
the effect, the V content was set to 0.20% or less and preferably
0.09% or less.
Ca: 0.0005-0.005%
[0037] Ca (Calcium) is used for the shape controlling of inclusion.
To enhance the HIC resistance by sphering the MnS inclusions, the
Ca content of 0.0005% or more is needed. On the other hand, when
the Ca content exceeds 0.005%, the effect is saturated and further
effects cannot be exhibited. Additionally, Ca inclusions tend to be
clusters so that the HIC resistance is lowered. Accordingly, the
upper limit of Ca content was set to 0.005%.
N: 0.008% or Less
[0038] N (Nitrogen) exists in the steel as impurities. When the N
content is increased, cracks are generated in the billet so that
the steel property deteriorates. Thus the N content was set to
0.008% or less. Preferably, the N content is 0.006% or less.
O (Oxygen): 0.004% or Less
[0039] The O content means a total content of soluble oxygen in the
steel and oxygen in oxide inclusions. This O content is
substantially the same as the O content in oxide inclusions in the
sufficiently deoxidized steel. Therefore, as the O content is
increased, there exist increased oxide inclusions in the steel
thereby decreasing HIC resistance. Accordingly, smaller O content
is better and the O content was set to 0.004% or less.
Cu (Copper): 0.05-0.5%, Ni (Nickel): 0.05-0.5%
[0040] These elements are for enhancing the strength of the steel.
Thus, when the strength of the steel should be ensured one of the
elements or both of the elements can be contained. The effect
becomes significant in each Cu, Ni content of 0.05% or more.
However, since excessive addition of any element saturates the
effect, the content of each element was set to 0.5% or less.
[0041] Nb: The Nb (Niobium) content does not influence on the HIC
resistance and strength of the steel. Thus the Nb element can be
cared as an impurity element and its content is not be defined in
the present invention. However, when the Nb content exceeds 0.1%,
undesirable effects such as deterioration of the toughness of the
steel become significant. Thus the Nb content range is preferably
0.1% or less.
2. Steel Pipe Microstructure and its Production Method
[0042] In the seamless steel pipe of the present invention, a steel
pipe microstructure must be a quenched microstructure such as
bainite and/or martensite to ensure the strength of 5L-X70 grade or
more by use of a comparatively low C steel as shown by the
above-mentioned chemical compositions. To obtain the microstructure
the inline QT is preferably applied.
[0043] However, since only a bainite and/or martensite fully
quenched microstructure tends to generate HIC, which exhibits a
form of an intergranular fracture such as sulfide stress-corrosion
cracking, it is important to precipitate ferrite on the grain
boundary.
[0044] In the present invention the precipitation of ferrite on the
bainite and/or martensite grain boundary has an effect to prevent
the generation of HIC, which exhibits a form of a intergranular
fracture such as sulfide stress-corrosion cracking, while ensuring
the strength of 5L-X70 grade or more.
[0045] FIG. 1 is a view showing a microstructure photograph of a
seamless steel pipe inferior in HIC resistance. The microstructure
in FIG. 1 is a structure etched by a nital and exhibits a bainite
and/or martensite fully quenched microstructure in which prior
austenite grain boundaries can be clearly recognized. In a case of
such a microstructure an HIC, which exhibits a form of
intergranular fracture such as sulfide stress-corrosion cracking,
tends to generate.
[0046] On the contrary, FIG. 2 is a view showing a microstructure
photograph of a seamless steel pipe excellent in HIC resistance
relating to the present invention. FIG. 2 shows a microstructure
etched by a nital as in FIG. 1. Because a ferrite phase is
generated in the grain boundary, the prior austenitic grain
boundaries are not clear in the microstructure. In a case of such a
microstructure, the HIC, which shows a form of intergranular
fracture, is not occurred.
[0047] In the present invention, by defining the above-described
microstructure while using a billet containing the chemical
composition defined by the present invention as a material, a
seamless steel pipe excellent in an aimed performance i.e. HIC
resistance can be obtained. A preferable production method for
obtaining a seamless steel pipe, which satisfies the microstructure
and the high strength simultaneously, is shown as follows.
[0048] That is, after heating a billet and finish rolling it to a
shape of a steel pipe by hot working, the obtained steel pipe is
soaked-immediately to a temperature of (Ar.sub.3 point+50.degree.
C.) or more by use of soaking furnace without cooling it to the
Ar.sub.3 point and is quenched.
[0049] When the starting temperature of quenching is less than
(Ar.sub.3 point+50.degree. C.), variation is generated in strength.
On the other hand, when the starting temperature of quenching is
increased, toughness of the steel pipe is significantly lowered.
Thus, the starting temperature of quenching must be 1100.degree. C.
or less. Therefore, the starting temperature of quenching is set to
(Ar.sub.3 point+50.degree. C.) to 1100.degree. C.
[0050] The quenching of the finish rolled steel pipe is performed
by cooling it to room temperature, for example, while keeping the
cooling rate of 5.degree. C./sec. When the cooling rate during this
quenching is less than 5.degree. C./sec, a microstructure including
martensite and bainite required for obtaining necessary strength
cannot be ensured. Thus, the cooling rate of 5.degree. C./sec or
more should be kept.
[0051] To prevent the reduction of strength in a heat affected zone
of welding a tempering temperature of 550.degree. C. or more is
needed. However, when the tempering temperature exceeds Ac.sub.1
point, the strength of the steel pipe is decreased. Accordingly,
the tempering must be performed under a temperature condition of
550.degree. C. to Ac.sub.1 point.
[0052] The present invention does not limit production steps until
finish rolling a steel pipe from a billet, which is a starting
material. Alternatively, by adopting, for example, a
Mannesmann-mandrel mill process a billet cast by a continuous
casting machine or a billet obtained by rolling in a blooming mill
after casting is heated and a hollow shell is obtained by a piercer
such as a inclined rolling mill. After that a mandrel bar is
inserted into the pipe to roll it, a finish rolling is performed by
use of a sizer or reducer.
[0053] It is noted that even in a production method other than the
production methods described in said (3) of the present invention,
a seamless steel pipe having the chemical compositions and
microstructure defined in said (1) or (2) of the present invention
can obtain the HIC resistance of the present invention.
Example 1
[0054] Some kinds of steels, having chemical compositions shown in
Table 1, were melted by a converter. Billets produced by continuous
casting were heated to 1100.degree. C. or more and hollow shells
were obtained by use of a tilting roller piercer. These hollow
shells were finish rolled to steel pipes by a mandrel mill and a
sizer. After that without cooling the steel pipes to Ar.sub.3 point
or less, they were soaked at 950.degree. C. and subjected to
quenching and tempering treatment to produce seamless steel pipes.
The steel pipe sizes and heat treatment conditions are shown in
Table 2. In this case the cooling rate was set to 30.degree.
C./sec.
[0055] Tensile test specimens of JIS 12 were taken from the
obtained steel pipes as tensile tests and tensile strength (TS) and
yield strength (YS) were measured. It is noted that the tensile
tests were performed in accordance with JIS Z 2241.
[0056] Further, specimens having thickness of 12 to 20 mm, width of
20 mm and length of 100 mm were taken for HIC resistance tests. The
specimens were immersed into a H.sub.2S-saturated 0.5%
CH.sub.3COOH-5% NaCl water solution (temperature of 25.degree. C.,
pH=2.7-4.0, so called NACE environment) for 96 hours, and crack
area ratios (CAR (%)) were measured. These results are shown in
Table 2.
[0057] Further, after the HIC resistance tests, cross-sections of
the HIC test specimens were cut off and their microstructure were
observed by an optical microscope. The obtained observation results
are shown in Table 2.
[0058] As can be seen from Table 2 the all steels of Nos. 1 to 14
according to example of the present invention satisfy strength of
5L-X70 grade, and have an excellent condition of CAR=0%.
[0059] On the other hand, the steel of No. 15 in comparative
examples has C and O content, which are outside their definition of
the present invention, and ferrite is not precipitated at the
interface whereby a deteriorated result of CAR=12.6% was obtained.
Also the C content of the steel of No. 16 is outside the specified
values of the present invention, and ferrite does not exist at the
grain boundary whereby a deteriorated result of CAR=7.9% was
obtained.
[0060] Further, the steel of No. 17 in comparative examples has 0
content outside the specified values of the present invention and a
deteriorated result of CAR=6.2% was obtained by inclusion. The
steel of No. 18 has Ca content outside the definition of the
present invention and a deteriorated result of CAR=3.6% was
obtained due to inclusion.
TABLE-US-00001 TABLE 1 Steel Chemical composition (mass %) Balance:
Fe and impurities No. C Si Mn P S Ti Al Ca N O Cu Cr Ni Mo V 1 0.06
0.09 1.29 0.007 0.002 0.008 0.033 0.0020 0.0055 0.0017 -- 0.28 --
0.21 0.05 2 0.06 0.33 1.43 0.011 0.002 0.008 0.029 0.0036 0.0047
0.0025 -- 0.27 0.19 0.22 0.06 3 0.06 0.29 1.36 0.008 0.001 0.007
0.034 0.0032 0.0045 0.0020 -- 0.26 0.07 0.21 0.05 4 0.08 0.27 1.29
0.021 0.002 0.003 0.022 0.0025 0.0048 0.0019 -- 0.49 -- 0.02 0.02 5
0.08 0.28 1.02 0.024 0.001 0.008 0.027 0.0010 0.0039 0.0016 0.28
0.26 0.26 0.16 0.05 6 0.11 0.22 1.24 0.014 0.002 0.009 0.039 0.0018
0.0052 0.0017 -- 0.21 -- 0.12 0.04 7 0.07 0.32 1.41 0.008 0.002
0.008 0.037 0.0005 0.0043 0.0012 -- 0.05 -- 0.02 0.10 8 0.09 0.29
1.36 0.019 0.001 0.017 0.032 0.0023 0.0077 0.0020 -- 0.26 0.07 0.21
0.03 9 0.04 0.41 0.82 0.006 0.003 0.012 0.026 0.0027 0.0064 0.0029
0.44 0.37 0.48 0.24 0.03 10 0.04 0.34 1.16 0.016 0.002 0.008 0.044
0.0048 0.0056 0.0033 -- 0.12 -- 0.04 0.08 11 0.03 0.24 1.48 0.011
0.002 0.015 0.041 0.0026 0.0074 0.0022 0.29 0.23 0.33 0.09 0.04 12
0.08 0.26 1.59 0.013 0.001 0.009 0.032 0.0023 0.0047 0.0016 -- 0.22
-- 0.06 0.03 13 0.04 0.29 1.51 0.007 0.001 0.006 0.026 0.0033
0.0042 0.0040 0.16 0.24 0.12 0.08 0.05 14 0.05 0.38 1.46 0.012
0.001 0.007 0.046 0.0029 0.0044 0.0018 -- 0.05 -- 0.30 0.06 15
*0.13 0.26 1.31 0.009 0.001 0.008 0.037 0.0030 0.0070 *0.0053 --
0.19 0.03 0.09 0.03 16 *0.12 0.22 1.36 0.012 0.003 0.007 0.028
0.0026 0.0038 0.0027 -- 0.17 0.03 0.04 0.04 17 0.07 0.30 1.25 0.008
0.001 0.007 0.035 0.0034 0.0036 *0.0049 0.04 0.17 0.03 0.13 0.04 18
0.06 0.29 1.36 0.008 0.001 0.012 0.042 *0.0003 0.0056 0.0026 --
0.14 -- 0.04 0.05 19 0.05 0.28 *1.78 0.011 0.002 0.008 0.034 0.0032
0.0046 0.0020 -- 0.11 -- 0.02 0.05 20 *0.02 0.34 1.33 0.016 0.002
0.008 0.046 0.0028 0.0054 0.0029 0.28 0.23 0.43 0.15 0.03 21 0.05
0.23 1.36 0.013 0.001 0.009 0.039 *0.0052 0.0063 0.0034 -- 0.16 --
0.05 0.06 Note: *in Table shows out of range specified in the
present invention.
TABLE-US-00002 TABLE 2 Heat treatment condition Test result Steel
pipe size Cooling Ferrite Outer Wall starting Tempering
precipitation Steel diameter thickness temperature temperature TS
YS at grain Micro- CAR No. (mm) (mm) (.degree. C.) (.degree. C.)
(MPa) (MPa) boundary structure (%) Present 1 323.9 40.0 950 650 583
509 Yes F + B + M 0 invention 2 219.1 29.2 950 650 641 569 Yes F +
B + M 0 example 3 219.1 37.8 950 650 602 526 Yes F + B + M 0 4
219.1 19.1 950 650 604 520 Yes F + B + M 0 5 323.9 34.1 950 650 605
522 Yes F + B + M 0 6 323.9 20.5 950 650 622 530 Yes F + B + M 0 7
323.9 35.2 950 650 618 545 Yes F + B + M 0 8 323.9 21.1 950 650 620
534 Yes F + B + M 0 9 219.1 16.7 950 650 584 525 Yes F + B + M 0 10
219.1 20.6 950 650 579 519 Yes F + B + M 0 11 219.1 19.1 950 650
588 537 Yes F + B + M 0 12 219.1 28.6 950 650 606 523 Yes F + B + M
0 13 219.1 39.7 950 650 584 525 Yes F + B + M 0 14 219.1 31.8 950
650 598 534 Yes F + B + M 0 Comparative 15 219.1 28.9 950 650 644
520 *No B + M 12.6 example 16 355.6 23.8 950 650 681 591 *No B + M
7.9 17 323.9 17.5 950 650 653 566 Yes F + B + M 6.2 18 219.1 16.7
950 650 600 530 Yes F + B + M 3.6 19 323.9 34.1 950 650 590 524 *No
B + M 10.8 20 219.1 37.8 950 650 523 *474 Yes F + B 0 21 323.9 20.5
950 650 585 519 Yes F + B + M 9.4 Note: Structure in Table shows B:
Bainite, M: Martensite, F: Ferrite. *in Table shows out of range
specified in the present invention.
[0061] In the comparative examples, the steel of No. 19 has Mn
content outside the specified values of the present invention and
ferrite does not exist at the interface whereby a deteriorated
result of CAR=10.8% was obtained. Further, the steel of No. 20 has
C content outside the specified values of the present invention, so
cannot satisfy strength of 5L-X70 grade, even if the result of
CAR=0% was excellent.
[0062] Further, in the comparative examples, the steel of No. 21
has Ca content outside the specified values of the present
invention and a deteriorated result of CAR=9.4% was obtained due to
inclusions.
Example 2
[0063] To confirm effects of heat treatment conditions, the steel
of No. 3 in Table 1 was melted by converter, and an billet produced
by continuous casting was heated to 1100.degree. C. or more and a
hollow shell was obtained by use of a inclined rolling mill. The
hollow shell was finish rolled to a steel pipe by a mandrel mill
and a sizer. After that the steel pipe was cooled in a range of
920.degree. C. to 20.degree. C., and seamless steel pipes were
produced by changing the cooling starting temperature, cooling rate
and tempering temperature. The sizes of the produced steel pipes
and heat treatment conditions are shown in Table 3. In this case,
the Ar.sub.3 point of the tested steel of No. 3 was 768.degree. C.,
and the Ac.sub.1 point thereof was 745.degree. C.
[0064] As in Example 1, tensile test specimens of JIS 12 were taken
and as tensile tests, tensile strength (TS) and yield strength (YS)
were measured. Further, HIC resistance tests were performed under
the same conditions as in Example 1, and crack area ratios (CAR
(%)) were measured. Further, after HIC resistance testing,
cross-sections of HIC test specimens were cut off and
microstructure observation was performed by an optical microscope.
These results are shown in Table 3.
TABLE-US-00003 TABLE 3 Heat treatment condition Test result Steel
pipe size Temper- Cooling Ferrite Outer Wall ature starting Cooling
Tempering precipi- diam- thick- before temper- rate temper- tation
Steel Test eter ness soaking ature (.degree. C./ ature TS YS at
grain Micro- CAR No. No. (mm) (mm) (.degree. C.) (.degree. C.) sec)
(.degree. C.) (MPa) (MPa) boundary structure (%) Present 3 22 219.1
37.8 920 1080 30 650 614 553 Yes F + B + M 0 invention 3 23 219.1
37.8 920 980 30 650 609 542 Yes F + B + M 0 example 3 24 219.1 37.8
920 900 30 650 590 513 Yes F + B + M 0 3 25 219.1 37.8 920 850 30
650 564 491 Yes F + B + M 0 3 26 219.1 37.8 920 950 30 650 668 494
Yes F + B + M 0 3 27 219.1 37.8 920 950 30 600 605 532 Yes F + B +
M 0 3 28 219.1 37.8 920 950 30 570 610 537 Yes F + B + M 0
Comparative 3 29 219.1 37.8 920 *1150 30 650 647 589 *No B + M *7.4
example 3 30 219.1 37.8 920 950 30 *750 521 *443 Yes F + B + M 0 3
31 219.1 37.8 920 950 *4 650 478 *287 Yes *F + P 0 3 32 219.1 37.8
920 *750 30 650 509 *438 Yes F + B + M 0 3 33 219.1 37.8 920 950 30
*500 642 578 Yes F + B + M 0 Note: Structure in Table shows B:
Bainite, M: Martensite, F: Ferrite, P: Pearlite. *in Table shows
out of range specified in the present invention.
[0065] As apparent from the results in Table 3, the steels of test
Nos. 22 to 28 according to examples of the present invention
satisfy heat treatment conditions specified in the present
invention, and the all steels thereof satisfy strength of 5L-X70
grade, and have an excellent condition of CAR=0%.
[0066] On the other hand, the steel of test No. 29 in comparative
examples adopts a quenching temperature, which is outside the
specified values of the present invention, and ferrite is not
precipitated at the grain boundaries whereby a deteriorated result
of CAR=7.4% was obtained. Also the steel of test No. 30 adopts a
tempering temperature, which is outside the specified values of the
present invention, and the strength could not satisfy 5L-X70
grade.
[0067] Further, in the comparative examples, the steel of test No.
31 adopts a cooling rate outside the specified values of the
present invention and the microstructure of the steel is a
ferrite-pearlite microstructure whereby the strength of the steel
could not satisfy 5L-X70 grade.
[0068] Further, since in the steel of test No. 32 the starting
temperature of quenching was less than (Ar.sub.3 point+50.degree.
C.), the strength of the steel could not satisfy 5L-X70 grade.
[0069] Furthermore, in the comparative examples the steel of test
No. 33 could not ensure a tempering temperature of 550.degree. C.
or more, an additional welding test was performed and it was found
that the strength was decreased in a welding heat affected
zone.
INDUSTRIAL APPLICABILITY
[0070] In the seamless steel pipe and its production method
according to the present invention, the chemical compositions of
the steels, the microstructure of the steel, and the precipitation
of ferrite at grain boundaries in the steels are specified.
Accordingly, the steel can obtain high strength and stable,
excellent HIC resistance. Further, by specifying the conditions in
a case where an inline QT is applied a pipeline having excellent
HIC resistance and high yield stress of 483 MPa or more can be
provided without inhibiting the cost down or cost saving of heat
treatment process and the improvement of productivity. Therefore,
the seamless steel pipe and its production method of the present
invention can be utilized widely in technical fields requiring for
a high strength seamless steel pipe excellent in HIC
resistance.
* * * * *