U.S. patent application number 15/560677 was filed with the patent office on 2018-11-08 for thick steel plate for structural pipes or tubes, method of producing thick steel plate for structural pipes or tubes, and structural pipes and tubes.
This patent application is currently assigned to JFE STEEL CORPORATION. The applicant listed for this patent is JFE STEEL CORPORATION. Invention is credited to Shigeru ENDO, Nobuyuki ISHIKAWA, Shusaku OTA, Junji SHIMAMURA.
Application Number | 20180320257 15/560677 |
Document ID | / |
Family ID | 56978030 |
Filed Date | 2018-11-08 |
United States Patent
Application |
20180320257 |
Kind Code |
A9 |
OTA; Shusaku ; et
al. |
November 8, 2018 |
THICK STEEL PLATE FOR STRUCTURAL PIPES OR TUBES, METHOD OF
PRODUCING THICK STEEL PLATE FOR STRUCTURAL PIPES OR TUBES, AND
STRUCTURAL PIPES AND TUBES
Abstract
Disclosed is, as a high-strength steel plate of API X80 grade or
higher with a thickness of 38 mm or more, a thick steel plate for
structural pipes or tubes that exhibits high strength in the
rolling direction and excellent Charpy properties at its
mid-thickness part without addition of large amounts of alloying
elements. The thick steel plate for structural pipes or tubes
disclosed herein has: a specific chemical composition; a
microstructure at its mid-thickness part that is a dual-phase
microstructure of ferrite and bainite with an area fraction of the
ferrite being less than 50%, and that contains ferrite grains with
a grain size of 15 .mu.m or less in an area fraction of 80% or more
with respect to the whole area of the ferrite; a tensile strength
of 620 MPa or more; and a Charpy absorption energy vE.sub.-20+ C.
at -20.degree. C. at the mid-thickness part of 100 J or more.
Inventors: |
OTA; Shusaku; (Chiyoda-ku,
Tokyo, JP) ; SHIMAMURA; Junji; (Chiyoda-ku, Tokyo,
JP) ; ISHIKAWA; Nobuyuki; (Chiyoda-ku, Tokyo, JP)
; ENDO; Shigeru; (Chiyoda-ku, Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE STEEL CORPORATION |
Chiyoda-ku, Tokyo |
|
JP |
|
|
Assignee: |
JFE STEEL CORPORATION
Chiyoda-ku, Tokyo
JP
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20180105907 A1 |
April 19, 2018 |
|
|
Family ID: |
56978030 |
Appl. No.: |
15/560677 |
Filed: |
March 25, 2016 |
PCT Filed: |
March 25, 2016 |
PCT NO: |
PCT/JP2016/001763 PCKC 00 |
371 Date: |
September 22, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 8/0263 20130101;
C21D 9/46 20130101; C22C 38/44 20130101; C21D 1/19 20130101; C22C
38/00 20130101; C22C 38/50 20130101; C22C 38/28 20130101; C22C
38/48 20130101; C21D 2211/002 20130101; C22C 38/14 20130101; C22C
38/06 20130101; C22C 38/001 20130101; C22C 38/02 20130101; C21D
8/0205 20130101; C22C 38/04 20130101; C22C 38/58 20130101; C21D
8/02 20130101; C22C 38/42 20130101; C21D 2211/005 20130101; C22C
38/24 20130101; C22C 38/26 20130101; C21D 1/20 20130101; C21D
8/0226 20130101; C22C 38/002 20130101; C22C 38/12 20130101; C22C
38/46 20130101; C22C 38/22 20130101; C22C 38/38 20130101 |
International
Class: |
C22C 38/58 20060101
C22C038/58; C21D 8/02 20060101 C21D008/02; C22C 38/50 20060101
C22C038/50; C22C 38/48 20060101 C22C038/48; C22C 38/46 20060101
C22C038/46; C22C 38/44 20060101 C22C038/44; C22C 38/42 20060101
C22C038/42; C22C 38/06 20060101 C22C038/06; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00; C21D 9/46 20060101
C21D009/46 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 26, 2015 |
JP |
PCT/JP2015/001750 |
Claims
1-7. (canceled)
8. A thick steel plate for structural pipes or tubes, comprising: a
chemical composition that contains, in mass%, C: 0.030% to 0.100%,
Si: 0.01% to 0.50%, Mn: 1.50% to 2.50%, Al: 0.080% or less, Mo:
0.05% to 0.50%, Ti: 0.005% to 0.025%, Nb: 0.005% to 0.080%, N:
0.001% to 0.010%, O: 0.0050% or less, P: 0.010% or less, S: 0.0010%
or less, and the balance consisting of Fe and incidental
impurities, with the chemical composition having a carbon
equivalent C.sub.eq as defined by the following Expression (1) of
0.42 or more: C.sub.eq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 (1), where
each element symbol indicates content in mass % of the element in
the steel plate and has a value of 0 if the element is not
contained in the steel plate; and a microstructure at a
mid-thickness part of the thick steel plate that is a dual-phase
microstructure of ferrite and bainite with an area fraction of the
ferrite being less than 50%, and that contains ferrite grains with
a grain size of 15 .mu.m or less in an area fraction of 80% or more
with respect to the whole area of the ferrite, wherein the steel
plate satisfies a set of conditions including: a tensile strength
being 620 MPa or more; and a Charpy absorption energy
vE.sub.-20.degree. C. at -20.degree. C. at the mid-thickness part
of 100 J or more.
9. The thick steel plate for structural pipes or tubes according to
claim 8, wherein the chemical composition further contains, in mass
%, V: 0.005% to 0.100%.
10. The thick steel plate for structural pipes or tubes according
to claim 8, wherein the chemical composition further contains, in
mass %, one or more selected from the group consisting of Cu: 0.50%
or less, Ni: 0.50% or less, Cr: 0.50% or less, Ca: 0.0005% to
0.0035%, REM: 0.0005% to 0.0100%, and B: 0.0020% or less.
11. The thick steel plate for structural pipes or tubes according
to claim 9, wherein the chemical composition further contains, in
mass %, one or more selected from the group consisting of Cu: 0.50%
or less, Ni: 0.50% or less, Cr: 0.50% or less, Ca: 0.0005% to
0.0035%, REM: 0.0005% to 0.0100%, and B: 0.0020% or less.
12. A method of producing a thick steel plate for structural pipes
or tubes, comprising at least: heating a steel raw material having
the chemical composition as recited in claim 8 to a heating
temperature of 1100.degree. C. to 1300.degree. C.; hot-rolling the
heated steel raw material, with a cumulative rolling reduction
ratio at 800.degree. C. or lower being set to 70% or more, to
obtain a hot-rolled steel plate; accelerated-cooling the hot-rolled
steel plate under a set of conditions including a cooling start
temperature being no lower than 650.degree. C., a cooling end
temperature being lower than 400.degree. C., and an average cooling
rate being 5.degree. C./s or higher.
13. A method of producing a thick steel plate for structural pipes
or tubes, comprising at least: heating a steel raw material having
the chemical composition as recited in claim 9 to a heating
temperature of 1100.degree. C. to 1300.degree. C.; hot-rolling the
heated steel raw material, with a cumulative rolling reduction
ratio at 800.degree. C. or lower being set to 70% or more, to
obtain a hot-rolled steel plate; accelerated-cooling the hot-rolled
steel plate under a set of conditions including a cooling start
temperature being no lower than 650.degree. C., a cooling end
temperature being lower than 400.degree. C., and an average cooling
rate being 5.degree. C./s or higher.
14. A method of producing a thick steel plate for structural pipes
or tubes, comprising at least: heating a steel raw material having
the chemical composition as recited in claim 10 to a heating
temperature of 1100.degree. C. to 1300.degree. C.; hot-rolling the
heated steel raw material, with a cumulative rolling reduction
ratio at 800.degree. C. or lower being set to 70% or more, to
obtain a hot-rolled steel plate; accelerated-cooling the hot-rolled
steel plate under a set of conditions including a cooling start
temperature being no lower than 650.degree. C., a cooling end
temperature being lower than 400.degree. C., and an average cooling
rate being 5.degree. C./s or higher.
15. A method of producing a thick steel plate for structural pipes
or tubes, comprising at least: heating a steel raw material having
the chemical composition as recited in claim 11 to a heating
temperature of 1100.degree. C. to 1300.degree. C.; hot-rolling the
heated steel raw material, with a cumulative rolling reduction
ratio at 800.degree. C. or lower being set to 70% or more, to
obtain a hot-rolled steel plate; accelerated-cooling the hot-rolled
steel plate under a set of conditions including a cooling start
temperature being no lower than 650.degree. C., a cooling end
temperature being lower than 400.degree. C., and an average cooling
rate being 5.degree. C./s or higher.
16. The method producing a thick steel plate for structural pipes
or tubes according to claim 12, further comprising, immediately
after the accelerated cooling, reheating the steel plate to a
temperature range of 400.degree. C. to 550.degree. C. at a heating
rate from 0.5.degree. C./s to 10.degree. C./s.
17. The method producing a thick steel plate for structural pipes
or tubes according to claim 13, further comprising, immediately
after the accelerated cooling, reheating the steel plate to a
temperature range of 400.degree. C. to 550.degree. C. at a heating
rate from 0.5.degree. C./s to 10.degree. C./s.
18. The method producing a thick steel plate for structural pipes
or tubes according to claim 14, further comprising, immediately
after the accelerated cooling, reheating the steel plate to a
temperature range of 400.degree. C. to 550.degree. C. at a heating
rate from 0.5.degree. C./s to 10.degree. C./s.
19. The method producing a thick steel plate for structural pipes
or tubes according to claim 15, further comprising, immediately
after the accelerated cooling, reheating the steel plate to a
temperature range of 400.degree. C. to 550.degree. C. at a heating
rate from 0.5.degree. C./s to 10.degree. C./s.
20. A structural pipe or tube formed from the thick steel plate for
structural pipes or tubes as recited in claim 8.
21. A structural pipe or tube formed from the thick steel plate for
structural pipes or tubes as recited in claim 9.
22. A structural pipe or tube formed from the thick steel plate for
structural pipes or tubes as recited in claim 10.
23. A structural pipe or tube formed from the thick steel plate for
structural pipes or tubes as recited in claim 11.
24. A structural pipe or tube obtainable by forming the steel plate
for structural pipes or tubes as recited in claim 8 into a tubular
shape in its longitudinal direction, and then joining butting faces
by welding from inside and outside to form at least one layer on
each side along the longitudinal direction.
25. A structural pipe or tube obtainable by forming the steel plate
for structural pipes or tubes as recited in claim 9 into a tubular
shape in its longitudinal direction, and then joining butting faces
by welding from inside and outside to form at least one layer on
each side along the longitudinal direction.
26. A structural pipe or tube obtainable by forming the steel plate
for structural pipes or tubes as recited in claim 10 into a tubular
shape in its longitudinal direction, and then joining butting faces
by welding from inside and outside to form at least one layer on
each side along the longitudinal direction.
27. A structural pipe or tube obtainable by forming the steel plate
for structural pipes or tubes as recited in claim 11 into a tubular
shape in its longitudinal direction, and then joining butting faces
by welding from inside and outside to form at least one layer on
each side along the longitudinal direction.
Description
TECHNICAL FIELD
[0001] This disclosure relates to a thick steel plate for
structural pipes or tubes, and in particular, to a thick steel
plate for structural pipes or tubes that has strength of API X80
grade or higher and that exhibits excellent Charpy properties at
its mid-thickness part even with a plate thickness of 38 mm or
more.
[0002] This disclosure also relates to a method of producing a
thick steel plate for structural pipes or tubes, and to a
structural pipe or tube produced from the thick steel plate for
structural pipes or tubes.
BACKGROUND
[0003] For excavation of oil and gas by seabed resource drilling
ships and the like, structural pipes or tubes such as conductor
casing steel pipes or tubes, riser steel pipes or tubes, and the
like are used. In these applications, there has been an increasing
demand for high-strength thick steel pipes or tubes of no lower
than American Petroleum Institute (API) X80 grade from the
perspectives of improving operation efficiency with increased
pressure and reducing material costs.
[0004] Such structural pipes or tubes are often used with forged
products containing alloying elements in very large amounts (such
as connectors) subjected to girth welding. For a forged product
subjected to welding, post weld heat treatment (PWHT) is performed
to remove the residual stress caused by the welding from the forged
product. In this case, there may be a concern about deterioration
of mechanical properties such as strength after heat treatment.
Accordingly, structural pipes or tubes are required to retain
excellent mechanical properties, in particular high strength, in
their longitudinal direction, that is, rolling direction, even
after subjection to PWHT in order to prevent fractures during
excavation by external pressure on the seabed.
[0005] Thus, for example, JPH1150188A (PTL 1) proposes a process
for producing a high-strength steel plate for riser steel pipes or
tubes that can exhibit excellent strength even after subjection to
stress relief (SR) annealing, which is one type of PWHT, at a high
temperature of 600.degree. C. or higher, by hot rolling a steel to
which 0.30% to 1.00% of Cr, 0.005% to 0.0030% of Ti, and 0.060% or
less of Nb are added, and then subjecting it to accelerated
cooling.
[0006] In addition, JP2001158939A (PTL 2) proposes a welded steel
pipe or tube that has a base steel portion and weld metal with
chemical compositions in specific ranges and both having a yield
strength of 551 MPa or more. PTL 2 describes that the welded steel
pipe or tube has excellent toughness before and after SR in the
weld zone.
CITATION LIST
Patent Literature
[0007] PTL 1:JPH1150188A
[0008] PTL 2: JP2001158939A
SUMMARY
Technical Problem
[0009] In the steel plate described in PTL 1, however, Cr carbide
is caused to precipitate during PWHT in order to compensate for the
decrease in strength due to PWHT, which requires adding a large
amount of Cr. Accordingly, in addition to high material cost,
weldability and toughness may deteriorate.
[0010] In addition, the steel pipes or tubes described in PTL 2
focus on improving the characteristics of seam weld metal, without
giving consideration to the base steel, and inevitably involve
decrease in the strength of the base steel by PWHT. To secure the
strength of the base steel, it is necessary to increase the
strength before performing PWHT by controlled rolling or
accelerated cooling.
[0011] It could thus be helpful to provide, as a high-strength
steel plate of API X80 grade or higher with a thickness of 38 mm or
more, a thick steel plate for structural pipes or tubes that
exhibits high strength in a direction perpendicular to the rolling
direction and excellent Charpy properties at its mid-thickness part
without addition of large amounts of alloying elements. It could
also be helpful to provide a method of producing the
above-described thick steel plate for structural pipes or tubes,
and a structural pipe or tube produced from the thick steel plate
for structural pipes or tubes.
Solution to Problem
[0012] For thick steel plates having a thickness of 38 mm or more,
we conducted detailed studies on the influence of rolling
conditions on their microstructures in order to determine how to
balance Charpy properties at the mid-thickness part and strength.
In general, the steel components for welded steel pipes or tubes
and steel plates for welded structures are strictly limited from
the viewpoint of weldability. Thus, high-strength steel plates of
X65 grade or higher are manufactured by being subjected to hot
rolling and subsequent accelerated cooling. Thus, the steel plate
has a microstructure that is mainly composed of bainite or a
microstructure in which martensite austenite constituent
(abbreviated MA) is formed in bainite, yet, as the plate thickness
increases, deterioration of Charpy properties at the mid-thickness
part would be inevitable. In view of the above, we conducted
intensive studies on a microstructure capable of exhibiting
excellent Charpy properties at the mid-thickness part, and as a
result, arrived at the following findings: [0013] (a) Refinement of
the steel microstructure is effective for improving the Charpy
properties at the mid-thickness part. It is thus necessary to
increase the cumulative rolling reduction ratio in the
non-recrystallization region. [0014] (b) On the other hand, if the
cooling start temperature is excessively low, the ferrite area
fraction increases to 50% or more and the strength decreases. It is
thus necessary to set a high cooling start temperature.
[0015] Based on the above findings, we made intensive studies on
the chemical compositions and microstructures of steel as well as
on the production conditions, and completed the present
disclosure.
[0016] Specifically, the primary features of the present disclosure
are as described below.
[0017] 1. A thick steel plate for structural pipes or tubes,
comprising: a chemical composition that contains (consists of), in
mass%, C: 0.030% to 0.100%, Si: 0.01% to 0.50%, Mn: 1.50% to 2.50%,
Al: 0.080% or less, Mo: 0.05% to 0.50%, Ti: 0.005% to 0.025%, Nb:
0.005% to 0.080%, N: 0.001% to 0.010%, 0: 0.0050% or less, P:
0.010% or less, S: 0.0010% or less, and the balance consisting of
Fe and inevitable impurities, with the chemical composition having
a carbon equivalent C.sub.eq as defined by the following Expression
(1) of 0.42 or more:
C.sub.eq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 (1),
where each element symbol indicates content in mass % of the
element in the steel plate and has a value of 0 if the element is
not contained in the steel plate; and a microstructure at a
mid-thickness part of the thick steel plate that is mainly a
dual-phase microstructure of ferrite and bainite composed of
bainite with an area fraction of the ferrite being less than 50%,
and that contains ferrite grains with a grain size of 15 .mu.m or
less in an area fraction of 80% or more with respect to the whole
area of the ferrite, with the steel plate satisfying a set of
conditions including: a tensile strength being 620 MPa or more; and
a Charpy absorption energy vE.sub.-20.degree. C. at -20.degree. C.
at the mid-thickness part being 100 J or more.
[0018] 2. The thick steel plate for structural pipes or tubes
according to 1., wherein the chemical composition further contains,
in mass %,
[0019] V: 0.005% to 0.100%.
[0020] 3. The thick steel plate for structural pipes or tubes
according to 1. or 2., wherein the chemical composition further
contains, in mass %, one or more selected from the group consisting
of Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Ca:
0.0005% to 0.0035%, REM: 0.0005% to 0.0100%, and B: 0.0020% or
less.
[0021] 4. A method of producing a thick steel plate for structural
pipes or tubes, comprising at least: heating a steel raw material
having the chemical composition as recited in any one of 1. to 3.
to a heating temperature of 1100.degree. C. to 1300.degree. C.;
hot-rolling the heated steel raw material, with a cumulative
rolling reduction ratio at 800.degree. C. or lower being set to 70%
or more, to obtain a hot-rolled steel plate; accelerated-cooling
the hot-rolled steel plate under a set of conditions including a
cooling start temperature being no lower than 650.degree. C., a
cooling end temperature being lower than 400.degree. C., and an
average cooling rate being 5.degree. C./s or higher.
[0022] 5. The method producing a thick steel plate for structural
pipes or tubes according to 4., further comprising, immediately
after the accelerated cooling, reheating the steel plate to a
temperature range of 400.degree. C. to 550.degree. C. at a heating
rate from 0.5.degree. C./s to 10.degree. C./s.
[0023] 6. A structural pipe or tube formed from the thick steel
plate for structural pipes or tubes as recited in any one of 1. to
3.
[0024] 7. A structural pipe or tube obtainable by forming the steel
plate for structural pipes or tubes as recited in any one of 1. to
3. into a tubular shape in its longitudinal direction, and then
joining butting faces by welding from inside and outside to form at
least one layer on each side along the longitudinal direction.
Advantageous Effect
[0025] According to the present disclosure, it is possible to
provide, as a high-strength steel plate of API X80 grade or higher,
a thick steel plate for structural pipes or tubes that exhibits
high strength in the rolling direction and excellent Charpy
properties at its mid-thickness part without addition of large
amounts of alloying elements, and a structural pipe or tube formed
from the steel plate for structural pipes or tubes. As used herein,
the term "thick" means that the plate thickness is 38 mm or
more.
DETAILED DESCRIPTION
Chemical Composition
[0026] Reasons for limitations on the features of the disclosure
will be explained below.
[0027] In the present disclosure, it is important that a thick
steel plate for structural pipes or tubes has a specific chemical
composition. The reasons for limiting the chemical composition of
the steel as stated above are explained first. The %
representations below indicating the chemical composition are in
mass % unless otherwise noted.
[0028] C: 0.030% to 0.100%
[0029] C is an element for increasing the strength of steel. To
obtain a desired microstructure for desired strength and toughness,
the C content needs to be 0.030% or more. However, if the C content
exceeds 0.100%, weldability deteriorates, weld cracking tends to
occur, and the toughness of base steel and HAZ toughness are
lowered. Therefore, the C content is set to 0.100% or less. The C
content is preferably 0.050% to 0.080%.
[0030] Si: 0.01% to 0.50%
[0031] Si is an element that acts as a deoxidizing agent and
increases the strength of the steel material by solid solution
strengthening. To obtain this effect, the
[0032] Si content is set to 0.01% or more. However, Si content of
greater than 0.50% causes noticeable deterioration in HAZ
toughness. Therefore, the Si content is set to 0.50% or less. The
Si content is preferably 0.05% to 0.20%.
[0033] Mn: 1.50% to 2.50%
[0034] Mn is an effective element for increasing the hardenability
of steel and improving strength and toughness. To obtain this
effect, the Mn content is set to 1.50% or more. However, Mn content
of greater than 2.50% causes deterioration of weldability.
Therefore, the Mn content is set to 2.50% or less. The Mn content
is preferably from 1.80% to 2.00%.
[0035] Al: 0.080% or less
[0036] Al is an element that is added as a deoxidizer for
steelmaking. However, Al content of greater than 0.080% leads to
reduced toughness. Therefore, the Al content is set to 0.080% or
less. The Al content is preferably from 0.010% to 0.050%.
[0037] Mo: 0.05% to 0.50%
[0038] Mo is a particularly important element for the present
disclosure that functions to greatly increase the strength of the
steel plate by forming fine complex carbides with Ti, Nb, and V,
while suppressing pearlite transformation during cooling after hot
rolling. To obtain this effect, the Mo content is set to 0.05% or
more. However, Mo content of greater than 0.50% leads to reduced
toughness at the heat-affected zone (HAZ). Therefore, the Mo
content is set to 0.50% or less.
[0039] Ti: 0.005% to 0.025%
[0040] In the same way as Mo, Ti is a particularly important
element for the present disclosure that forms complex precipitates
with Mo and greatly contributes to improvement in the strength of
steel. To obtain this effect, the Ti content is set to 0.005% or
more. However, adding Ti beyond 0.025% leads to deterioration in
HAZ toughness and toughness of base steel. Therefore, the Ti
content is set to 0.025% or less.
[0041] Nb: 0.005% to 0.080%
[0042] Nb is an effective element for improving toughness by
refining microstructural grains. In addition, Nb forms composite
precipitates with Mo and contributes to improvement in strength. To
obtain this effect, the Nb content is set to 0.005% or more.
However, Nb content of greater than 0.080% causes deterioration of
HAZ toughness. Therefore, the Nb content is set to 0.080% or
less.
[0043] N: 0.001% to 0.010%
[0044] N is normally present in the steel as an inevitable impurity
and, in the presence of Ti, forms TiN. To suppress coarsening of
austenite grains caused by the pinning effect of TiN, the N content
is set to 0.001% or more. However, TiN decomposes in the weld zone,
particularly in the region heated to 1450.degree. C. or higher near
the weld bond, and produces solute N. Accordingly, if the N content
is excessively increased, a decrease in toughness due to the
formation of the solute N becomes noticeable. Therefore, the N
content is set to 0.010% or less. The N content is more preferably
0.002% to 0.005%.
[0045] O: 0.0050% or less, P: 0.010% or less, S: 0.0010% or
less
[0046] In the present disclosure, O, P, and S are inevitable
impurities, and the upper limit for the contents of these elements
is defined as follows. O forms coarse oxygen inclusions that
adversely affect toughness. To suppress the influence of the
inclusions, the O content is set to 0.0050% or less. In addition, P
lowers the toughness of the base metal upon central segregation,
and a high P content causes the problem of reduced toughness of
base metal. Therefore, the P content is set to 0.010% or less. In
addition, S forms MnS inclusions and lowers the toughness of base
metal, and a high S content causes the problem of reduced toughness
of the base material. Therefore, the S content is set to 0.0010% or
less. It is noted here that the O content is preferably 0.0030% or
less, the P content is preferably 0.008% or less, and the S content
is preferably 0.0008% or less. No lower limit is placed on the
contents of O, P, and S, yet in industrial terms the lower limit is
more than 0%. On the other hand, excessively reducing the contents
of these elements leads to longer refining time and increased cost.
Therefore, the O content is 0.0005% or more, the P content is
0.001% or more, and the S content is 0.0001% or more.
[0047] In addition to the above elements, the thick steel plate for
structural pipes or tubes disclosed herein may further contain V:
0.005% to 0.100%.
[0048] V: 0.005% to 0.100%
[0049] In the same way as Nb, V forms composite precipitates with
Mo and contributes to improvement in strength. When V is added, the
V content is set to 0.005% or more to obtain this effect. However,
V content of greater than 0.100% causes deterioration of HAZ
toughness. Therefore, when V is added, the V content is set to
0.100% or less.
[0050] In addition to the above elements, the thick steel plate for
structural pipes or tubes may further contain Cu: 0.50% or less,
Ni: 0.50% or less, Cr: 0.50% or less, Ca: 0.0005% to 0.0035%, REM:
0.0005 to 0.0100%, and B: 0.0020% or less.
[0051] Cu: 0.50% or less
[0052] Cu is an effective element for improving toughness and
strength, yet excessively adding Cu causes deterioration of
weldability. Therefore, when Cu is added, the Cu content is set to
0.50% or less. No lower limit is placed on the Cu content, yet when
Cu is added, the Cu content is preferably 0.05% or more.
[0053] Ni: 0.50% or less
[0054] Ni is an effective element for improving toughness and
strength, yet excessively adding Ni causes deterioration of
resistance to PWHT. Therefore, when Ni is added, the Ni content is
set to 0.50% or less. No lower limit is placed on the Ni content,
yet when Ni is added, the Ni content is preferably to 0.05% or
more.
[0055] Cr: 0.50% or less
[0056] In the same way as Mn, Cr is an effective element for
obtaining sufficient strength even with a low C content, yet
excessive addition lowers weldability. Therefore, when Cr is added,
the Cr content is set to 0.50% or less. No lower limit is placed on
the Cr content, yet when Cr is added, the Cr content is preferably
set to 0.05% or more.
[0057] Ca: 0.0005% to 0.0035%
[0058] Ca is an effective element for improving toughness by
morphological control of sulfide inclusions. To obtain this effect,
when Ca is added, the Ca content is set to 0.0005% or more.
However, adding Ca beyond 0.0035% does not increase the effect, but
rather leads to a decrease in the cleanliness of the steel, causing
deterioration of toughness. Therefore, when Ca is added, the Ca
content is set to 0.0035% or less.
[0059] REM: 0.0005% to 0.0100%
[0060] In the same way as Ca, a REM (rare earth metal) is an
effective element for improving toughness by morphological control
of sulfide inclusions in the steel. To obtain this effect, when a
REM is added, the REM content is set to 0.0005% or more. However,
excessively adding a REM beyond 0.0100% does not increase the
effect, but rather leads to a decrease in the cleanliness of the
steel, causing deterioration of toughness. Therefore, the REM is
set to 0.0100% or less.
[0061] B: 0.0020% or less
[0062] B segregates at austenite grain boundaries and suppresses
ferrite transformation, thereby contributing particularly to
preventing reduction in HAZ strength. However, adding B beyond
0.0020% does not increase the effect. Therefore, when B is added,
the B content is set to 0.0020% or less. No lower limit is placed
on the B content, yet when B is added, the B content is preferably
0.0002% or more.
[0063] The thick steel plate for structural pipes or tubes
disclosed herein consists of the above-described components and the
balance of Fe and inevitable impurities. As used herein, the phrase
"consists of . . . the balance of Fe and inevitable impurities" is
intended to encompass a chemical composition that contains
inevitable impurities and other trace elements as long as the
action and effect of the present disclosure are not impaired.
[0064] In the present disclosure, it is important that all of the
elements contained in the steel satisfy the above-described
conditions and that the chemical composition has a carbon
equivalent C.sub.eq of 0.42 or more, where C.sub.eq is defined
by:
C.sub.eq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 (1),
where each element symbol indicates content in mass % of the
element in the steel plate and has a value of 0 if the element is
not contained in the steel plate.
[0065] C.sub.eq is expressed in terms of carbon content
representing the influence of the elements added to the steel,
which is commonly used as an index of strength as it correlates
with the strength of base metal. In the present disclosure, to
obtain a high strength of API X80 grade or higher, C.sub.eq is set
to 0.42 or more. C.sub.eq is preferably 0.43 or more. No upper
limit is placed on C.sub.eq, yet a preferred upper limit is
0.50.
Microstructure at Mid-Thickness Part
[0066] Next, the reasons for limitations on the steel
microstructure according to the disclosure are described.
[0067] In the present disclosure, it is important for the steel
plate to have a microstructure at its mid-thickness part that is a
dual-phase microstructure of ferrite and bainite with an area
fraction of the ferrite being less than 50%, and that contains
ferrite grains with a grain size of 15 .mu.m or less in an area
fraction of 80% or more with respect to the whole area of the
ferrite. Controlling the microstructure in this way makes it
possible to ensure Charpy properties at the mid-thickness part
while providing high strength of API X80 grade. In the case of a
thick steel plate with a plate thickness of 38 mm or more according
to the disclosure, if these microstructural conditions are
satisfied at the mid-thickness part, it is considered that the
resulting microstructure meets the microstructural conditions
substantially over the entire region in the plate thickness
direction, and the effects of the present disclosure may be
obtained
[0068] As used herein, the phrase "a dual-phase microstructure of
ferrite and bainite" refers to a microstructure that consists
essentially of only ferrite and bainite, yet as long as the action
and effect of the present disclosure are not impaired, those
containing other microstructural constituents are intended to be
encompassed within the scope of the disclosure. Specifically, the
total area fraction of ferrite and bainite in the microstructure of
steel is preferably 90% or more, and more preferably 95% or more.
Specifically, the total area fraction of ferrite and bainite in the
steel microstructure is preferably 90% or more, and more preferably
95% or more. On the other hand, the total area fraction of ferrite
and bainite is desirably as high as possible without any particular
upper limit. The area fraction of bainite may be 100%.
[0069] The amount of microstructural constituents other than
ferrite and bainite is preferably as small as possible. However,
when the area fraction of ferrite and bainite is sufficiently high,
the influence of the residual microstructural constituents is
almost negligible, and an acceptable total area fraction of one or
more of the microstructural constituents other than ferrite and
bainite in the microstructure is up to 10%. A preferred total area
fraction of these microstructural constituents other than ferrite
is up to 5%. Examples of the residual microstructural constituents
include pearlite, cementite, martensite, and martensite austenite
constituent.
[0070] In addition, the area fraction of ferrite in the
microstructure at the mid-thickness part needs to be less than 50%.
The area fraction of ferrite is preferably 40% or less. On the
other hand, no lower limit is placed on the area fraction of
ferrite, yet a preferred lower limit is 5%.
[0071] Furthermore, to secure Charpy properties at the
mid-thickness part of the steel plate, it is necessary for the
microstructure at the mid-thickness part to contain ferrite grains
with a grain size of 15 .mu.m or less in an area fraction of 80% or
more with respect to the whole area of the ferrite. The area
fraction of ferrite grains with a grain size of 15 .mu.m or less is
preferably as high as possible without any particular upper limit,
and may be 100%.
[0072] The area fraction of ferrite and bainite and the grain size
of ferrite may be determined by mirror-polishing a test piece
sampled from the mid-thickness part (location of half the plate
thickness), etching its surface with nital, and observing five or
more fields randomly selected on the surface under a scanning
electron microscope (at 1000 times magnification), In this
disclosure, equivalent circle radius is used as the grain size.
Mechanical Properties
[0073] The thick steel plate for structural pipes or tubes
disclosed herein has mechanical properties including: a tensile
strength of 620 MPa or more; and a Charpy absorption energy
vE.sub.-20.degree. C. at -20.degree. C. at its mid-thickness part
of 100 J or more. In this respect, tensile strength and Charpy
absorption energy can be measured with the method described in
examples explained later. No upper limit is placed on tensile
strength, yet an exemplary upper limit is 825 MPa or less for X80
grade and 990 MPa or less for X100 grade. Similarly, the upper
limit for vE.sub.-20.degree. C. is also not particularly limited,
yet it is normally 500 J or less.
Steel Plate Production Method
[0074] Next, a method of producing a steel plate according to the
present disclosure is described. In the following explanation, it
is assumed that the temperature is the average temperature in the
thickness direction of the steel plate unless otherwise noted. The
average temperature in the plate thickness direction can be
determined by, for example, the plate thickness, surface
temperature, or cooling conditions through simulation calculation
or the like. For example, the average temperature in the plate
thickness direction of the steel plate can be determined by
calculating the temperature distribution in the plate thickness
direction using a finite difference method.
[0075] The thick steel plate for structural pipes or tubes
disclosed herein may be produced by sequentially performing
operations (1) to (3) below on the steel raw material having the
above chemical composition. Additionally, optional operation (4)
may be performed. [0076] (1) heating the steel raw material to a
heating temperature of 1100.degree. C. to 1300.degree. C.; [0077]
(2) hot-rolling the heated steel material, with a cumulative
rolling reduction ratio at 800.degree. C. or lower being set to 70%
or more, to obtain a hot-rolled steel plate; [0078] (3)
accelerated-cooling the hot-rolled steel plate under a set of
conditions including a cooling start temperature being no lower
than 650.degree. C., a cooling end temperature being lower than
400.degree. C., and an average cooling rate being 5.degree. C./s or
higher; [0079] (4) immediately after the accelerated cooling,
reheating the steel plate to a temperature range of 400.degree. C.
to 550.degree. C. at a heating rate from 0.5.degree. C./s to
10.degree. C./s.
[0080] Specifically, the above-described operations may be
performed as described below.
Steel Raw Material
[0081] The above-described steel raw material may be prepared with
a regular method. The method of producing the steel raw material is
not particularly limited, yet the steel raw material is preferably
prepared with continuous casting.
Heating
[0082] The steel raw material is heated prior to rolling. At this
time, the heating temperature is set from 1100.degree. C. to
1300.degree. C. Setting the heating temperature to 1100.degree. C.
or higher makes it possible to cause carbides in the steel raw
material to dissolve, and to obtain the target strength. The
heating temperature is preferably set to 1120.degree. C. or higher.
However, a heating temperature of higher than 1300.degree. C.
coarsens austenite grains and the final steel microstructure,
causing deterioration of toughness. Therefore, the heating
temperature is set to 1300.degree. C. or lower. The heating
temperature is preferably set to 1250.degree. C. or lower.
Hot Rolling
[0083] Then, the heated steel raw material is rolled to obtain a
hot-rolled steel plate. At this point, if the cumulative rolling
reduction ratio at 800.degree. C. or lower is below 70%, it is not
possible to optimize the microstructure at the mid-thickness part
of the steel plate after the rolling. Therefore, the cumulative
rolling reduction ratio at 800.degree. C. or lower is set to 70% or
more. No upper limit is placed on the cumulative rolling reduction
ratio at 800.degree. C. or lower, yet a normal upper limit is 90%.
The rolling finish temperature is not particularly limited, yet
from the perspective of ensuring a cumulative rolling reduction
ratio at 800.degree. C. or lower as described above, a preferred
rolling finish temperature is 780.degree. C. or lower, and more
preferably 760.degree. C. or lower. In addition, to ensure the
cooling start temperature as described above, the rolling finish
temperature is preferably set to 700.degree. C. or higher, and more
preferably to 720.degree. C. or higher.
Accelerated Cooling
[0084] After completion of the hot rolling, the hot-rolled steel
plate is subjected to accelerated cooling. At that time, if the
accelerated cooling start temperature is below 650.degree. C.,
ferrite increases to 50% or more, causing a large decrease in
strength. Therefore, the cooling start temperature is set to
650.degree. C. or higher. The cooling start temperature is
preferably 680.degree. C. or higher from the perspective of
ensuring a certain area fraction of ferrite. On the other hand, no
upper limit is placed on the cooling start temperature, yet a
preferred upper limit is 780.degree. C.
[0085] On the other hand, if the cooling finish temperature is
excessively high, transformation to bainite does not proceed
sufficiently and a large amount of pearlite or martensite austenite
constituent is generated, which may adversely affect the toughness.
Therefore, the cooling finish temperature is set to lower than
400.degree. C. No lower limit is placed on the cooling end
temperature, yet a preferred lower limit is 200.degree. C.
[0086] In addition, if the cooling rate is excessively low,
transformation to bainite does not proceed sufficiently and a large
amount of pearlite is generated, which may adversely affect the
toughness. Therefore, the average cooling rate is set to 5.degree.
C./s or higher. No upper limit is placed on the average cooling
rate, yet a preferred upper limit is 25.degree. C./s.
Reheating
[0087] After completion of the accelerated cooling, reheating may
be performed. Even if the accelerated cooling stop temperature is
low and a large amount of low-temperature transformed
microstructure other than bainite, such as martensite, is produced,
performing reheating and tempering makes it possible to ensure
specific toughness. In the case the reheating is performed, the
reheating is carried out, immediately after the accelerated
cooling, to a temperature range of 400.degree. C. to 550.degree. C.
at a heating rate from 0.5.degree. C./s to 10.degree. C./s. As used
herein, the phrase "immediately after the accelerated cooling"
refers to starting reheating at a heating rate from 0.5.degree.
C./s to 10.degree. C./s within 120 seconds after the completion of
the accelerated cooling.
[0088] Through the above process, it is possible to produce a thick
steel plate for structural pipes or tubes that has strength of API
X80 grade or higher and that is excellent in Charpy properties at
its mid-thickness part. As described above, the thick steel plate
for structural pipes or tubes disclosed herein is intended to have
a plate thickness of 38 mm or more. Although no upper limit is
placed on the plate thickness, a preferred plate thickness is 60 mm
or less because it may be difficult to satisfy the production
conditions described herein if the plate thickness is greater than
60 mm.
Steel Pipe or Tube
[0089] A steel pipe or tube can be produced by using the steel
plate thus obtained as a material. The steel pipe or tube may be,
for example, a structural pipe or tube that is obtainable by
forming the thick steel plate for structural pipes or tubes into a
tubular shape in its longitudinal direction, and then joining
butting faces by welding. The method of producing a steel pipe or
tube is not limited to a particular method, and any method is
applicable. For example, a UOE steel pipe or tube may be obtained
by forming a steel plate into a tubular shape in its longitudinal
direction by U press and O press following a conventional method,
and then joining butting faces by seam welding. Preferably, the
seam welding is performed by performing tack welding and
subsequently submerged arc welding from inside and outside to form
one layer on each side. The flux used for submerged arc welding is
not limited to a particular type, and may be a fused flux or a
bonded flux. After the seam welding, expansion is carried out to
remove welding residual stress and to improve the roundness of the
steel pipe or tube. In the expansion, the expansion ratio (the
ratio of the amount of change in the outer diameter before and
after expansion of the pipe or tube to the outer diameter of the
pipe or tube before expansion) is normally set from 0.3% to 1.5%.
From the viewpoint of the balance between the roundness improving
effect and the capacity required for the expanding device, the
expansion rate is preferably from 0.5% to 1.2%. Instead of the
above-mentioned UOE process, a press bend method, which is a
sequential forming process to perform three-point bending
repeatedly on a steel plate, may be applied to form a steel pipe or
tube having a substantially circular cross-sectional shape before
performing seam welding in the same manner as in the
above-described UOE process. In the case of the press bend method,
as in the UOE process, expansion may be performed after seam
welding. In the expansion, the expansion ratio (the ratio of the
amount of change in the outer diameter before and after expansion
of the pipe or tube to the outer diameter of the pipe or tube
before expansion) is normally set from 0.3% to 1.5%. From the
viewpoint of the balance between the roundness increasing effect
and the capacity required for the expanding device, the expansion
rate is preferably from 0.5% to 1.2%. Optionally, preheating before
welding or heat treatment after welding may be performed.
EXAMPLES
[0090] Steels having the chemical compositions presented in Table 1
(each with the balance consisting of Fe and inevitable impurities)
were prepared by steelmaking and formed into slabs by continuous
casting. The obtained slabs were used as raw material to produce
steel plates with a thickness of 38 mm to 51 mm. For each obtained
steel plate, the area fraction of ferrite and bainite in the
microstructure and the mechanical properties were evaluated as
described below. The evaluation results are presented in Table
3.
[0091] The area fraction of ferrite and bainite was evaluated by
mirror-polishing a test piece sampled from the mid-thickness part,
etching its surface with nital, and observing five or more fields
randomly selected on the surface under a scanning electron
microscope (at 1000 times magnification).
[0092] Among the mechanical properties, 0.5% yield strength (YS)
and tensile strength (TS) were measured by preparing full-thickness
test pieces sampled from each obtained thick steel plate in a
direction perpendicular to the rolling direction, and then
conducting a tensile test on each test piece in accordance with JIS
Z 2241 (1998).
[0093] As for Charpy properties, among the mechanical properties,
three 2 mm V notch Charpy test pieces were sampled from the
mid-thickness part with their longitudinal direction parallel to
the rolling direction, and the test pieces were subjected to a
Charpy impact test at -20.degree. C. energy (vE.sub.-20.degree.
C.), to obtain absorption energy vE.sub.-20.degree. C., and the
average values were calculated.
[0094] For evaluation of heat affected zone (HAZ) toughness, a test
piece to which heat hysteresis corresponding to heat input of 40
kJ/cm to 100 kJ/cm was applied by a reproducing apparatus of weld
thermal cycles was prepared and subjected to a Charpy impact test.
Measurements were made in the same manner as in the evaluation of
Charpy absorption energy at -20.degree. C. described above, and the
case of Charpy absorption energy at -20.degree. C. being 100 J or
more was evaluated as "Good", and less than 100 J as "Poor".
[0095] Further, for evaluation of PWHT resistance, PWHT treatment
was performed on each steel plate using a gas atmosphere furnace.
At this time, heat treatment was performed on each steel plate at
600.degree. C. for 2 hours, after which the steel plate was removed
from the furnace and cooled to room temperature by air cooling.
Each steel plate subjected to PWHT treatment was measured for 0.5%
YS, TS, and vE.sub.-20.degree. C. in the same manner as in the
above-described measurements before PWHT.
[0096] As can be seen from Table 3, examples (Nos. 1 to 7) which
satisfy the conditions disclosed herein exhibited excellent
mechanical properties before and after subjection to PWHT. In
contrast, comparative examples (Nos. 8 to 18) which do not satisfy
the conditions disclosed herein were inferior in mechanical
properties before and/or after subjection to PWTH. For example,
Nos. 8 to 12 were inferior in strength of base metal, and Charpy
properties, although their steel compositional ranges met the
conditions of the present disclosure. Of these, for No. 9, Charpy
properties are considered to be deteriorated due to a low
cumulative rolling reduction ratio at 800.degree. C. or lower and
accordingly to a lower area fraction of ferrite grains with a grain
size of 15 .mu.m or less. For No. 10, the microstructure of the
steel plate contained ferrite in an area fraction of greater than
50%, which is considered as a cause of lower strength of base
metal. Nos. 13 to 18 were inferior in at least one of the strength
of base metal, Charpy properties, and HAZ toughness because their
steel compositional ranges were outside the range of the present
disclosure.
TABLE-US-00001 TABLE 1 Steel Chemical composition (mass %)* ID C Si
Mn P S Mo Ti Nb V Al Cu Ni A 0.072 0.24 1.78 0.008 0.0008 0.28
0.011 0.024 0.023 0.032 -- -- B 0.065 0.16 1.82 0.008 0.0008 0.14
0.018 0.044 0.066 0.035 0.10 0.20 C 0.060 0.20 1.79 0.008 0.0008
0.20 0.017 0.036 0.045 0.038 0.21 0.23 D 0.061 0.19 1.85 0.008
0.0008 0.19 0.008 0.043 0.036 0.034 -- -- E 0.062 0.10 1.78 0.008
0.0008 0.14 0.011 0.044 -- 0.035 0.31 0.14 F 0.065 0.10 1.87 0.008
0.0008 0.12 0.014 0.012 -- 0.037 0.20 0.09 G 0.068 0.22 1.67 0.008
0.0008 0.15 0.020 0.036 0.052 0.041 0.15 0.21 H 0.024 0.35 1.85
0.008 0.0008 0.26 0.012 0.042 0.038 0.030 0.40 0.40 I 0.065 0.32
2.22 0.008 0.0008 0.02 0.015 0.035 0.063 0.032 0.15 0.40 J 0.106
0.25 1.86 0.008 0.0008 0.11 0.012 0.031 -- 0.028 -- -- K 0.065 0.19
1.71 0.008 0.0008 0.19 0.043 0.038 0.047 0.041 0.30 0.22 L 0.058
0.14 1.84 0.008 0.0008 0.15 0.011 0.020 -- 0.033 0.10 0.15 Steel
Chemical composition (mass %)* Ceq ID Cr Ca REM B O N (mass %)
Remarks A -- -- -- -- 0.002 0.004 0.43 Conforming B 0.03 -- 0.0012
-- 0.002 0.005 0.44 steel C -- -- -- 0.0005 0.002 0.005 0.44 D 0.12
-- -- -- 0.002 0.004 0.44 E 0.0015 -- -- 0.002 0.004 0.42 F 0.02 --
-- -- 0.002 0.005 0.42 G 0.10 0.0023 -- -- 0.002 0.004 0.43 H -- --
-- -- 0.002 0.004 0.45 Comparative I -- -- -- -- 0.002 0.005 0.49
steel J -- -- -- -- 0.002 0.004 0.44 K -- -- -- -- 0.002 0.005 0.43
L -- -- -- -- 0.002 0.004 0.41 *The balance consists of Fe and
inevitable impurities.
TABLE-US-00002 TABLE 2 Hot rolling Cumulative rolling reduction
ratio Accelerated cooling Heating at or below Rolling Cooling start
Cooling end Steel temp. 800.degree. C. finish temp. temp. Cooling
rate temp. No. ID (.degree. C.) (%) (.degree. C.) (.degree. C.)
(.degree. C./s) (.degree. C.) 1 A 1250 75 760 720 20 290 2 B 1180
75 750 710 15 260 3 C 1180 70 770 710 14 280 4 D 1180 75 780 730 12
250 5 E 1150 80 760 740 15 230 6 F 1180 80 750 720 14 210 7 G 1190
75 770 750 15 270 8 C 1050 75 780 750 15 240 9 C 1150 65 770 720 16
280 10 C 1180 75 750 640 12 260 11 C 1180 75 780 760 4 280 12 C
1200 80 760 730 12 500 13 H 1150 75 760 710 15 210 14 I 1200 75 750
740 12 250 15 J 1180 75 760 730 14 280 16 K 1150 75 780 740 14 220
17 L 1150 75 760 720 15 250 Reheating Heating Reheating Plate rate
temp. thickness No. Reheating apparatus (.degree. C./s) (.degree.
C.) (mm) Remarks 1 -- 51 Example 2 -- 51 3 -- 38 4 -- 51 5
gas-fired furnace 1 480 51 6 induction heating furnace 3 420 51 7
-- 51 8 -- 51 Comparative 9 -- 51 Example 10 -- 51 11 -- 51 12 --
51 13 induction heating furnace 9 400 51 14 -- 51 15 -- 51 16 -- 51
17 -- 51
TABLE-US-00003 TABLE 3 Microstructure at mid-thickness part Area
fraction Area of F with Mechanical properties Mechanical properties
Area fraction grain size (before PWHT) (after PWHT) fraction of
Residual of 15 .mu.m 0.5% 0.5% Steel of F* F + B* microstructural
or less YS TS vE.sub.-20.degree. C. HAZ YS TS vE.sub.-20.degree. C.
No. ID (%) (%) constituents* (%) (MPa) (MPa) (J) toughness (MPa)
(MPa) (J) Remarks 1 A 18 100 -- 90 610 675 186 Good 604 671 174
Example 2 B 12 96 MA 85 627 705 157 Good 612 670 133 3 C 20 97 MA
90 643 725 195 Good 635 717 174 4 D 25 95 MA 95 696 765 184 Good
677 745 152 5 E 17 98 MA, C 100 665 750 178 Good 653 727 159 6 F 16
96 MA, C 95 630 711 163 Good 616 695 139 7 G 22 97 MA 95 657 741
165 Good 642 715 167 8 C 13 95 MA 100 544 615 155 Good 540 600 156
Comparative 9 C 10 100 -- 65 600 685 66 Good 610 694 155 Example 10
C 55 100 -- 80 470 611 166 Good 514 610 142 11 C 40 86 P 70 610 634
67 Good 630 682 140 12 C 30 88 MA, C 90 620 651 85 Good 622 678 135
13 H 15 97 MA, C 90 545 610 150 Good 540 605 132 14 I 15 96 MA 90
600 665 120 Good 544 640 115 15 J 20 98 MA 95 640 760 102 Good 635
710 66 16 K 22 97 MA 95 655 735 62 Poor 660 722 45 17 L 26 97 MA
100 651 712 121 Good 624 695 137 *F: ferrite, B: bainite, P:
pearlite, C: cementite, MA: martensite austenite constituent
INDUSTRIAL APPLICABILITY
[0097] According to the present disclosure, it is possible to
provide, as a high-strength steel plate of API X80 grade or higher
with a thickness of 38 mm or more, a thick steel plate for
structural pipes or tubes that exhibits high strength in the
rolling direction and excellent Charpy properties at its
mid-thickness part without addition of large amounts of alloying
elements, and a structural pipe or tube formed from the thick steel
plate for structural pipes or tubes. The structural pipe or tube
maintains excellent mechanical properties even after subjection to
PWHT, and thus is extremely useful as a structural pipe or tube for
a conductor casing steel pipe or tube, a riser steel pipe or tube,
and so on.
* * * * *