U.S. patent application number 12/557892 was filed with the patent office on 2010-09-09 for high tensile strength steel and marine structure having excellent weld toughness.
Invention is credited to Masahiko Hamada, Takahiro Kamo, Hirofumi Nakamura, Kazushi Ohnishi, Takeshi Urabe.
Application Number | 20100226813 12/557892 |
Document ID | / |
Family ID | 34631540 |
Filed Date | 2010-09-09 |
United States Patent
Application |
20100226813 |
Kind Code |
A1 |
Kamo; Takahiro ; et
al. |
September 9, 2010 |
HIGH TENSILE STRENGTH STEEL AND MARINE STRUCTURE HAVING EXCELLENT
WELD TOUGHNESS
Abstract
In order to provide a high tensile strength steel having
excellent low temperature toughness and which can withstand large
heat input welding, a steel comprises, in mass percent, C:
0.01-0.10%, Si: at most 0.5%, Mn: 0.8-1.8%, P: at most 0.020%, S:
at most 0.01%, Cu: 0.8-1.5%, Ni: 0.2-1.5%, Al: 0.001-0.05%, N:
0.0030-0.0080%, O: 0.0005-0.0035%, if necessary at least one of Ti:
0.005-0.03%, Nb: 0.003-0.03%, and Mo: 0.1-0.8%, and a remainder of
Fe and impurities, and the N/Al ratio is 0.3-3.0.
Inventors: |
Kamo; Takahiro;
(Kashima-shi, JP) ; Urabe; Takeshi; (Kashima-shi,
JP) ; Nakamura; Hirofumi; (Amagasaki-shi, JP)
; Ohnishi; Kazushi; (Kobe-shi, JP) ; Hamada;
Masahiko; (Kobe-shi, JP) |
Correspondence
Address: |
CLARK & BRODY
1700 Diagonal Road, Suite 510
Alexandria
VA
22314
US
|
Family ID: |
34631540 |
Appl. No.: |
12/557892 |
Filed: |
September 11, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11443849 |
May 26, 2006 |
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12557892 |
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PCT/JP2004/017575 |
Nov 26, 2004 |
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11443849 |
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Current U.S.
Class: |
420/83 ; 148/330;
148/331; 148/332; 420/84; 420/91; 420/92 |
Current CPC
Class: |
C22C 38/02 20130101;
C22C 38/44 20130101; C22C 38/48 20130101; C22C 38/42 20130101 |
Class at
Publication: |
420/83 ; 420/84;
420/91; 420/92; 148/331; 148/330; 148/332 |
International
Class: |
C22C 38/42 20060101
C22C038/42; C22C 38/16 20060101 C22C038/16; C22C 38/08 20060101
C22C038/08; C22C 38/02 20060101 C22C038/02; C22C 38/04 20060101
C22C038/04; C22C 38/06 20060101 C22C038/06; C22C 38/14 20060101
C22C038/14; C22C 38/20 20060101 C22C038/20 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 27, 2003 |
JP |
2003-397531 |
Claims
1. A high tensile strength steel consisting essentially, in mass
percent, of C: 0.01-0.10%, Si: at most 0.5%, Mn: 0.8-1.8%, P: at
most 0.020%, S: at most 0.01%, Cu: 0.8-1.5%, Ni: 0.2-1.5%, Al:
0.001-0.05%, N: 0.003-0.008%, O: 0.0005-0.0035%, Ti: 0-0.03%, Nb:
0-0.03%, Mo: 0-0.8%, Cr: 0-0.80%, B: 0-0.002%, V: 0-0.05%, Ca:
0-0.005%, Mg: 0-0.005%, REM: 0-0.01%, and a reminder of Fe and
impurities, wherein the N/Al ratio is 0.3-3.0, wherein the value of
Pcm represented by the following equation (I) is at most 0.25, and
for Cu particles having a major axis with a length of at least 1 nm
dispersed in the steel, the average equivalent circle diameter of
the Cu particles is in the range of 4-25 nm and the plane-converted
area fraction thereof is in the range of 3-20%.
Pcm=C+(Si/30)+(Mn/20)+(Cu/20)+(Ni/60)+(Cr/20)+(Mo/15)+(V/10)+5B
(I)
2. A high tensile strength steel as set forth in claim 1 which
contains, in mass percent, Ti: 0.005-0.03%.
3. A high tensile strength steel as set forth in claim 1 which
contains, in mass percent, Nb: 0.003-0.03%.
4. A high tensile strength steel as set forth in claim 1 which
contains, in mass percent, Mo: 0.1-0.8%.
5. A high tensile strength steel as set forth in claim 1 which
contains, in mass percent, at least one of Cr: 0.03-0.80% and B:
0.0002-0.002%.
6. A high tensile strength steel as set forth in claim 1 which
contains, in mass percent, V: 0.001-0.05%.
7. A high tensile strength steel as set forth in claim 1 which
contains, in mass percent, at least one of Ca: 0.0005-0.005%, Mg:
0.0001-0.01%, and REM: 0.0001-0.01%.
8. A high tensile strength steel as set forth in claim 1 wherein
the steel is manufactured by heating a billet at a temperature of
950.degree. C. or less, hot rolling the heated billet, cooling the
resulting hot rolled steel, and tempering the cooled steel.
9. A steel plate comprising a high tensile strength steel as set
forth in claim 1, wherein the ferrite fraction of the plate is at
least 70%, the average diameter is 30 micro-meters or less, and the
flatness is 1.5 or less at a depth of 1/2t in the thickness
direction of the plate.
10. A marine structure using a high tensile strength steel as set
forth in claim 1.
Description
[0001] This is a continuation in part of application Ser. No.
11/443,849 filed on May 26, 2006, which is a continuation of
International Patent Application No. PCT/W2004/017575, filed Nov.
26, 2004. The PCT application was not in English as published under
PCT Article 21(2).
TECHNICAL FIELD
[0002] The present invention relates to a high tensile strength
steel and a marine structure, and particularly to a high tensile
strength steel for welding and a marine structure having excellent
weld toughness.
[0003] More specifically, the present invention relates to a high
tensile strength steel for welding for use in welded structures
such as buildings, civil engineering projects, construction
equipment, ships, pipes, tanks, and marine structures, and
particularly to a high tensile strength steel for welding to
construct marine structures as well as to a marine structure thus
constructed. For example, it relates to a thick, high strength
steel plate with a thickness of at least 50 mm and a yield strength
of at least 420 N/mm.sup.2 and to a marine structure using such a
plate.
BACKGROUND ART
[0004] In recent years, as energy demands show a tendency to
increase more and more, the search for undersea oil resources is
being actively carried out. Marine structures used for this purpose
such as platforms and jackup rigs are becoming large in size, and
at the same time the thickness of steel materials such as steel
plates used in such structures is increasing, so attaining higher
safety has become an important issue.
[0005] In usual marine structures, medium strength steel having a
yield stress on the order of 300-360 MPa has been used, but in
large structures like those described above, extremely thick high
tensile strength steel having a high strength on the order of
460-700 MPa and a plate thickness exceeding 100 mm is sometimes
used.
[0006] The regions where searches for undersea oil resources are
conducted have in recent years shifted towards cold regions and
deep water regions. Marine structures which operate in such land
regions or sea regions are exposed to extremely severe weather and
ocean conditions.
[0007] Therefore, steel used in such marine structures is required
to have toughness in extremely severe low temperature ranges such
as at -40.degree. C. or below. Weldability is of course also
required.
[0008] From the standpoint of safety, the inspection standards of
users have become more severe, and not only is the conventional
Charpy impact value prescribed for base metals and welds, but the
CTOD (crack tip opening displacement) value at the lowest
temperature of use has also come to be prescribed to evaluate
toughness. Namely, even when stable properties are obtained in a
Charpy test which is an evaluation test performed on a minute test
piece cut into a size of 10 mm.times.10 mm, there are many cases in
which the CTOD properties, which are evaluated on a test piece
having the actual thickness of a structure, cannot satisfy required
properties. Today, even stricter CTOD properties are being
demanded.
[0009] Thus, there is a strong demand for an increase in low
temperature toughness of weld heat affected zones (referred to
below as HAZ) not only for steels used in marine structures
installed in icy waters but also for steels used in line pipe for
use in cold regions having a milder environment as well as for
large welded structures such as ships and LNG tanks.
[0010] In order to obtain a high toughness in a low temperature
range of -40.degree. C. or below, it is necessary to perform
welding under low heat input welding conditions, which have poor
welding efficiency. Welding costs represent a large portion of the
construction cost of marine structures. The most direct method for
decreasing welding costs is to employ a high performance welding
method which can perform welding with a high heat input so as to
decrease the number of welding passes.
[0011] Accordingly, today, in structures intended for cold regions
where there are severe demands for low temperature toughness, it is
important to carry out welding so that welding costs are as low as
possible while taking into consideration the toughness of HAZ.
[0012] From in the past, it has been known that decreasing the
level of C is effective in order to dramatically improve the
toughness of a HAZ of a steel. In order to compensate for a
decrease in strength due to the decrease in C, it has been
attempted to increase the strength by adding various alloying
elements or by utilizing the effect of age precipitation hardening.
For example, ASTM A710 discloses a steel which utilizes the age
precipitation hardening effect of Cu, and there have been a number
of reports based on this concept.
[0013] For example, JP H07-81164 B, JP H05-186820 A, and JP
H05-179344 A propose a Cu-precipitated steel having improved weld
toughness.
[0014] However, JP H07-81164 B merely evaluates the Charpy
properties of a welded joint obtained from a plate with a thickness
of 30 mm using a welding heat input of 40 kJ/cm, and such steel
cannot be truly considered one which can cope with high heat input
welding.
[0015] JP H05-186820 A proposes a high tensile strength steel
having a tensile strength of at least 686 MPa to which 0.5%-4.0% Cu
is added. Regarding the low temperature toughness of this steel,
however, in view of its transition temperature in a Charpy test
which is only 30.degree. C., the steel cannot really be considered
to have low temperature CTOD properties in extremely thick steel
plates.
[0016] JP H05-179344 A proposes a Cu-precipitated steel having
improved Charpy toughness in welds. However, it merely evaluates
Charpy properties of a welded joint obtained with a weld heat input
of 5 kJ/mm, and it cannot be truly viewed as a technique for fully
satisfying the safety of a structure at the time of high heat input
welding.
DISCLOSURE OF THE INVENTION
[0017] Accordingly, the object of the present invention is to
provide a high tensile strength steel for welding which is improved
generally in low temperature toughness of welds and particularly in
low temperature toughness in HAZ.
[0018] As a result of various experiments concerning a steel
composition and a method for its manufacture with the object of
developing a thick, high strength steel plate having excellent weld
toughness, the present inventors made the following findings.
[0019] (i) Using Cu-containing steel as a base, the contents of N
and Al are adjusted with controlling the N/Al ratio.
[0020] In a steel having a high Cu content, in order to improve the
toughness of a HAZ with high heat input, it is effective to finely
disperse carbides/nitrides such as TiN, Ti(C,N), and MN. As a
result of studying steels with a high content of Cu and Ti, it was
found that it is effective to adjust the N and Al content with
controlling the N/Al ratio. This is thought to be because when the
N/Al ratio is too small, coarse AlN precipitates, and not only does
this itself have an adverse effect on toughness, but also it
impedes fine dispersion of TiN in large amounts. On the other hand,
if the N/Al ratio is too large, the amount of solid solution N
increases, and the density of AlN and TiN dispersed in the steel
becomes very small.
[0021] (ii) In order to increase yield strength, it is necessary to
disperse finely precipitated Cu particles as uniformly as
possible.
[0022] (iii) In order to improved toughness and particularly low
temperature CTOD properties, it is necessary to coarsen the Cu
particles to a certain extent and to suppress the amount of the
dispersed Cu particles.
[0023] (iv) In order to uniformly disperse the Cu particles, the
formation of Cu particles in any stage before aging is suppressed
as much as possible, and the state of dispersion of Cu particles is
controlled by controlling the conditions of aging.
[0024] (v) Concerning the distribution of Cu particles, by taking
as factors the average value of the equivalent circle diameter of
the Cu particles and the plane-converted area fraction occupied by
the Cu particles which are both determined on a TEM photograph, it
is possible to control the balance of strength and toughness.
[0025] (vi) Cu particles readily form on crystal defects (primarily
on dislocations) in steel, and if the density of dislocations is
high, precipitation of Cu particles is promoted. Cu particles
precipitated on dislocations impede the movement of dislocations
and increase yield strength.
[0026] (vii) The density of dislocations in steel can be controlled
by the rolling and water cooling conditions. A decrease in the
rolling temperature, an increase in the overall rolling reduction,
an increase in the temperature at the start of water cooling, an
increase in the cooling rate, and a decrease in the temperature at
the completion of water cooling each increase the density of
dislocations.
[0027] (viii) Using a high-Cu steel composition as a base, it is
possible to stabilize the toughness of HAZ of welds formed with
high heat input by controlling the hardenability by adjusting the
contents of C, Mn, and Mo in the steel.
[0028] Namely, in a high-Cu steel, the more the weld cracking
parameter Pcm decreases, the more HAZ toughness can be improved. It
was found that a decrease in C and Mn is effective for this
purpose. However, in order to achieve high strength, addition of
other element is necessary. It was also found that addition of Mo
in a controlled amount makes it possible to stabilize the balance
between strength and toughness.
[0029] The present invention is based on such findings, and in its
essence it is as follows.
[0030] (1) A high tensile strength steel comprising, in mass
percent, C: 0.01-0.10%, Si: at most 0.5%, Mn: 0.8-1.8%, P: at most
0.020%, S: at most 0.01%, Cu: 0.8-1.5%, Ni: 0.2-1.5%, Al:
0.001-0.05%, N: 0.0030-0.0080%, O: 0.0005-0.0035%, and a reminder
of Fe and impurities, wherein the N/Al ratio is 0.3-3.0.
[0031] (2) A high tensile strength steel as described above in (1)
which further contains, in mass percent, Ti: 0.005-0.03%.
[0032] (3) A high tensile strength steel as described above in (1)
or (2) further containing, in mass percent, Nb: 0.003-0.03%.
[0033] (4) A high tensile strength steel as described above in any
of (1)-(3) further containing, in mass percent, Mo: 0.1-0.8%.
[0034] (5) A high tensile strength steel as described above in any
of (1)-(4) further containing, in mass percent, at least one of Cr:
0.03-0.80% and B: 0.0002-0.0020%.
[0035] (6) A high tensile strength steel as described above in any
of (1)-(5) further containing, in mass percent, V: 0.001-0.05%.
[0036] (7) A high tensile strength steel as described above in any
of (1)-(6) further containing, in mass percent, at least one of Ca:
0.0005-0.005%, Mg: 0.0001-0.005%, and REM: 0.0001-0.01%.
[0037] (8) A high tensile strength steel as described above in any
of (1)-(7) wherein the value of Pcm given by the following equation
(I) is at most 0.25, and for Cu particles dispersed in the steel
having a major axis measuring at least 1 nm, they have an average
equivalent circle diameter in the range of 4-25 nm and a
plane-converted area fraction in the range of 3-20%.
Pcm=C+(Si/30)+(Mn/20)+(Cu/20)+(Ni/60)+(Cr/20)+(Mo/15)+(V/10)+5B
(I)
[0038] (9) A marine structure using a high tensile strength steel
as described above in any of (1)-(8).
[0039] According to the present invention, it is possible to
manufacture a high tensile strength steel having excellent
weldability and a yield stress of at least 420 N/mm.sup.2 which can
be welded with a welding heat input of at least 300 kJ/cm by a
welding method such as electrogas arc welding, although it is not
particularly limited to this application. As a result, the
efficiency and safety of on-site welding are enormously increased.
In addition, it is possible to provide a high tensile strength
steel which can be used in extremely severe environments such as in
marine structures.
BEST MODE FOR CARRYING OUT THE INVENTION
[0040] The present invention will next be explained in detail.
First, the reasons why the present invention limits a steel
composition as described above will be explained. In this
description, percent with respect to a steel composition refers to
mass percent.
[0041] C is added in order to attain the strength of steel as well
as to produce the effect of refining the structure when Nb, V, and
the like are added. If it is less than 0.01%, these effects are not
adequate. However, if the amount of C is too large, a hardened
structure referred to as island martensite (abbreviated as
M-A=martensite-austenite constituent) forms in welds and worsens
the toughness of HAZ and has a harmful effect on the toughness and
weldability of the base metal. Accordingly, C is at most 0.10%.
Preferably it is 0.02-0.08% and more preferably it is
0.02-0.05%.
[0042] Si is an element which is effective at preliminary
deoxidation of molten steel, but since it does not dissolve in
cementite, if a large amount of Si is added, untransformed
austenite grains are prevented from breaking down into ferrite
grains and cementite, and the formation of island martensite is
promoted. For these reasons, the content of Si in the steel is at
most 0.5%. Preferably it is at most 0.2% and more preferably it is
at most 0.15%.
[0043] Mn is an element which is necessary for attaining strength,
and it is also effective as a deoxidizing agent. However, addition
of too much Mn excessively increases hardenability and causes
weldability and HAZ toughness to deteriorate. Mn is also an element
which is known to promote center segregation, and from the
standpoint of suppressing center segregation, its content should
not exceed 1.8%. Accordingly, the content of Mn is 0.8-1.8%.
Preferably it is 0.9-1.5%.
[0044] P is an impurity element which is unavoidably contained in
steel. It is an element which segregates at grain boundaries, and
hence it causes grain boundary cracks in HAZ. In order to increase
the toughness of weld metals and HAZ and to reduce segregation at
the center of a slab, the content of P is at most 0.020%.
Preferably it is at most 0.015% and more preferably it is at most
0.01%.
[0045] When a large amount of S is present, precipitates in the
form of MnS which act as starting points for weld cracking are
formed. Therefore, the content of S is at most 0.01%. Preferably it
is at most 0.008% and more preferably it is at most 0.005%.
[0046] Cu has the effect of increasing the strength and toughness
of the steel, and its adverse effect on HAZ toughness is small. In
particular, it is necessary to add at least 0.8% Cu in order to
expect an effect of increasing strength by epsilon (.epsilon.)-Cu
precipitation at the time of aging. However, as the Cu content
increases, susceptibility to weld cracking at high temperatures
increases, and welding procedures such as preheating become
complicated. Therefore, the Cu content is made at most 1.5%.
Preferably it is 0.9-1.1%.
[0047] Ni is an element which effectively increases the strength
and toughness of steel and which is also effective at increasing
HAZ toughness. However, if it is 0.2% or less, these effects are
not obtained, while if it exceeds 1.5%, an effect commensurate with
the cost increase is not obtained. Therefore, the content of Ni is
0.2-1.5%. Preferably it is 0.4-1.2%.
[0048] Al is an element which is necessary for deoxidation.
However, as its content increases, it becomes easy for toughness to
deteriorate, particularly in HAZ. This is thought to be because
coarse cluster-shaped alumina-based inclusion particles are easily
formed. Therefore, the content of Al is 0.001-0.05%. Preferably it
is 0.001-0.03%. More preferably it is 0.001-0.015%.
[0049] N contributes to refining structure by forming nitrides, but
when too much N is added, it causes toughness to deteriorate due to
aggregation of nitrides. Accordingly, the content of N is
0.003-0.008%. Preferably it is 0.0035-0.0065%.
[0050] By controlling the N/Al ratio to 0.3-3.0, it is possible to
increase the toughness of high heat input HAZ and particularly the
CTOD properties of weld joints.
[0051] This is thought to be because if the N/Al ratio is smaller
than 0.3, coarse AlN precipitates. As a result, not only does this
have an adverse effect on toughness, but also it impedes dispersion
of TiN in fine forms and a large amount. On the other hand, when
the N/Al ratio exceeds 3.0, the amount of solid solution N
increases, HAZ toughness deteriorates, and the density of dispersed
AlN and TiN becomes low. In order to attain better results, a
preferred range of the N/AI ratio is 0.4-2.5.
[0052] O (oxygen) is effective at forming oxides which become
nuclei for the formation of ferrite. If it is present in a large
amount, steel cleanliness markedly deteriorates, and it becomes
difficult to attain a practical level of toughness in the base
metal, the weld metal, and HAZ. Accordingly, the content of 0 is
0.0005-0.0035%. Preferably it is 0.0008-0.0018%.
[0053] The following elements are optional elements which can be
added to the steel if desired.
[0054] Ti forms nitrides and has the effect of suppressing
coarsening of crystal grains and refining the structure formed by
transformation. However, these effects are not exhibited with a
small amount of Ti, and addition of a large amount of Ti has an
adverse effect on the toughness of the base metal and of welds.
Accordingly, when Ti is added, the content of Ti is 0.005-0.03%.
Preferably it is 0.007-0.015%.
[0055] Nb increases the strength and toughness of a base metal by
grain refinement and precipitation of carbides. However, addition
of too much Nb causes the effect of improving the properties of the
base metal saturate and the toughness of HAZ's to markedly reduce.
Accordingly, when Nb is added, the content of Nb is 0.003-0.03%.
Preferably it is 0.003-0.015%.
[0056] Mo is effective at attaining hardenability and at increasing
HAZ toughness, but if too much is added, it leads to marked
hardening of HAZ and deteriorates toughness. Accordingly, when Mo
is added, the content of Mo is 0.1-0.8%. Preferably it is
0.1-0.5%.
[0057] Cr is effective at increasing the hardenability of steel and
at attaining strength. The effect of improving these properties is
not exhibited with addition of Cr in a minute amount, while if too
much is added, there is a tendency for it to prevent hardening of
weld metals and HAZ and increase susceptibility to weld cracking at
low temperatures. Accordingly, when Cr is added, the content of Cr
is 0.03-0.80%. Preferably it is 0.05-0.60%.
[0058] B has the effect of improving hardenability and increasing
strength. If too much thereof is added, the effect of increasing
strength saturates, and there is a marked tendency for the
toughness of the base metal and HAZ to deteriorate. Accordingly,
when B is added, the content of B is 0.0002-0.002%. Preferably it
is 0.003-0.0015%.
[0059] V forms carbonitrides and has the effect of suppressing
coarsening of crystal grains and refining the structure formed by
transformation. However, these effects are not exhibited with a
small amount of V, and addition of a large amount of V has an
adverse effect on the toughness of the base metal and of welds.
Accordingly, when V is added, the content of V is 0.001-0.05%.
Preferably it is 0.005-0.04%.
[0060] Ca, Mg, and REM are elements which form oxides and sulfides
which become nuclei for precipitation of intergranular ferrite. In
addition, they control the form of sulfides and increase low
temperature toughness. In order to obtain these effects of Ca, Mg,
and REM, it is necessary to contain at least 0.0005% of Ca or at
least 0.0001% of Mg or REM. If the content of Ca exceeds 0.005% or
the content of Mg or REM exceed 0.01%, large inclusions or clusters
composed mainly of Ca or Mg are formed, thereby deteriorating the
cleanliness of steel. Accordingly, when Ca is added, the content of
Ca is 0.0005-0.005%, and when Mg or REM is added, the content of Mg
or REM is 0.0001-0.01%.
[0061] In a preferable steel according to the present invention,
the value of Pcm represented by the following equation (I) is at
most 0.25, and for Cu particles having a major axis measuring at
least 1 nm which are dispersed in the steel, the average equivalent
circle diameter of the particles is 4-25 nm and the plane-converted
area fraction thereof is 3-20%.
Pcm=C+(Si/30)+(Mn/20)+(Cu/20)+(Ni/60)+(Cr/20)+(Mo/15)+(V/10)+5B
(I)
[0062] Pcm (weld cracking parameter) is indicative of the
susceptibility to weld cracking. If its value is 0.25 or less, weld
cracking does not occur under usual welding conditions.
Accordingly, Porn is at most 0.25. If Pcm is decreased, it is
possible to omit preheating at the time of welding. Preferably it
is at most 0.22 and more preferably it is at most 0.20.
[0063] The average equivalent circle diameter and the
plane-converted area fraction of precipitated Cu particles are now
described. The reason why Cu particles having a major axis
measuring at least 1 nm are of interest is because Cu particles of
smaller than 1 nm contribute little to increasing strength. There
are no particular upper limits on the length of the major axis of
Cu particles, but when the average value thereof is in the range of
4-25 nm, no particles have a major axis exceeding 100 nm. Cu
particles precipitate generally in the form of spheres, but
measuring a solid shape is not easy, so projected shapes of Cu
particles are measured.
[0064] The term "equivalent circle diameter" used herein is the
diameter of a circle having an area which is the same as the
projected area of a particle. Specifically, it is found from the
equation
d=- {square root over ( )}(4a/.pi.)
wherein a is the projected area (nm.sup.2), d is the equivalent
circle diameter (nm), and .pi. is 3.14.
[0065] For the plane-converted area fraction of Cu particles, a
steel material is worked to form a membrane, a portion of the
membrane having a thickness of approximately 0.2 micrometers is
observed with a TEM, and it is calculated by measuring the percent
area occupied by Cu particles when Cu particles which are
three-dimensionally distributed in the membrane test piece are
projected onto a plane using TEM photograph at a magnification of
100,000.
[0066] The reasons why the equivalent circle diameter and the
plane-converted area fraction of Cu particles are prescribed in the
above manner will be further described.
[0067] A characteristic of steel used in marine structures is that
in many cases it is an extremely thick high tensile strength steel
with a maximum thickness close to 100 mm such that it can withstand
external forces applied by waves in a storm. In the future, it will
be used for severe conditions, so it will need to satisfy an even
more severe CTOD value.
[0068] If the strength becomes too high due to Cu precipitation,
the CTOD value decreases, and if Cu precipitation becomes
inadequate, the strength becomes inadequate even if the CTOD value
is high.
[0069] There were almost no attempts of a conventional
Cu-containing steel being applied to marine structures. Therefore,
such steel was not required to satisfy a severe CTOD value, and
there was no need to strictly control the average diameter or area
fraction of precipitated Cu particles.
[0070] In a preferred embodiment of the present invention, in order
to achieve a balance between an increase in strength due to Cu
precipitation and a decrease in CTOD value, the average diameter
and the area fraction of precipitated Cu particles is prescribed in
the above manner.
[0071] The average equivalent circle diameter of Cu particles is
4-25 nm in order to achieve a balance between strength and
toughness, and the plane-converted area fraction thereof is 3-20%
also in order to achieve a balance between strength and
toughness.
[0072] The following are conceivable as factors which can be used
for controlling the average diameter and area fraction of Cu
particles.
[0073] (1) The larger the amount of added Cu, the higher is the
area fraction of Cu particles. As for its effect on particle
diameter, if the amount of Cu which is added is in a suitable
range, the average particle diameter is determined primarily by the
structure prior to aging and the temperature and duration of aging.
If the added amount is smaller than a suitable amount, the growth
of precipitated Cu particles is inadequate and the particle
diameter becomes small, while if it is large, there is a tendency
for the particle diameter to become large.
[0074] (2) The structure before aging has a large influence. The
structure before aging is preferably a fine structure comprised
primarily of ferrite and bainite.
[0075] Since dislocations and particle boundaries become sites of
precipitation of Cu particles, a structure including many such
precipitation sites facilitates refinement of the Cu particle
diameter and increases the area fraction. For this purpose, it is
necessary to suitably control the steel composition and make the
rolling conditions suitable, and then to select the cooling
conditions so as to obtain a fine structure which is comprised
primarily of ferrite and bainite.
[0076] (3) The temperature and duration of aging are important
factors. A desired distribution of Cu particles can be achieved by
carefully controlling the diffusion speed of Cu and the growth rate
of Cu particles by the aging conditions.
[0077] A structure after aging is basically derived from a
structure before aging. According to the present invention, a
structure after aging comprises at least 70% of a ferrite fraction
with the ferritic grain size being 30 micro-meters or less and the
flatness of ferrite particles being 1/3 or less.
[0078] When the ferrite fraction is at least 70% and the average
grain diameter is 30 micro-meters or smaller, stable CTOD
characteristics can be obtained at low temperatures.
[0079] The flatness of the particles is restricted to 1.5 or less
in order to ensure improved resistance to ductile fracture at low
temperatures.
[0080] The metallurgical structure of a steel plate of the present
invention is defined by that in the middle portion of the plate in
the thickness direction, i.e., 1/2t (t: thickness of the plate).
The reason why the portion at a depth of 1/2 of the thickness of
the plate is selected is that a metallurgical structure at the
middle portion is less sensitive to effects of rolling and cooling
after rolling, i.e., the portion can exhibit the worst properties.
Especially, when the thickness is as large as 50-100 mm, the mid
portion of a plate in the thickness direction is not affected
substantially by reduction during rolling and cooling after
rolling.
[0081] The average diameter of ferrite grains is defined by a
diameter of a circle having the same area as that of the grain,
i.e., a diameter of a corresponding equivalent circle.
[0082] The flatness of the particles is defined by the formula:
(grain diameter in the thickness direction of a plate)/(grain
diameter in the direction of rolling).
[0083] By suitably controlling the above-described three factors
while manufacturing the steel according to the present invention,
from the above disclosure, a person skilled in the art can carry
out the present invention without difficulty.
[0084] Next, a method of manufacturing a high tensile strength
steel according to the present invention will be explained.
[0085] Even with a steel composition as described above, in order
to adequately exhibit precipitation hardening by Cu and in order to
uniformly provide a high strength and toughness with an increased
yield strength at each position in the thickness direction of a
thick steel plate with a thickness of at least 50 mm, the
manufacturing method must be appropriate.
[0086] Steelmaking itself may be carried out by a conventional
method and there are no particular restrictions thereon in the
present invention. After steelmaking, a steel slab is obtained.
From a standpoint of decreasing costs, it is preferable to prepare
a slab by continuous casting.
[0087] The conditions for heating the slab, hot rolling it,
cooling, and tempering will be explained. First, a steel slab
having the above-described composition is heated to 900-950.degree.
C., and hot rolling is then carried out. In the present invention,
in order to obtain a high toughness, it is necessary for austenite
grains to be refined sufficiently so as to allow for the formation
of the upper bainite structure at the center of the thickness of
the resulting thick steel plate. Therefore, in the heating stage,
it is important to refine austenite grains inside the thickness of
the steel slab. Heating at a temperature lower than 900.degree. C.,
the formation of solid solution is not adequate, and sufficient
precipitation hardening cannot be expected in the tempering step.
However, at a heating temperature exceeding 950.degree. C.,
austenite grains prior to rolling cannot be maintained in a fine
and uniform state, and in subsequent rolling, the austenite grains
are not uniformly refined. Accordingly, the heating temperature of
the steel slab is 900-950.degree. C.
[0088] Preferably rolling is performed such that the overall
rolling reduction at 900.degree. C. or below is at least 50%. After
hot rolling, quench hardening is conducted by water cooling which
is commenced at a temperature of at least the Ar.sub.1 point of the
steel and terminated at a temperature of 600.degree. C. or below.
This is in order to achieve refinement of the structure and to
suppress precipitation of Cu particles as much as possible in the
stages before aging. If water cooling is commenced at a temperature
lower than the Ar.sub.1 point or if cooling is carried out by air
cooling, work strains are eliminated, and this causes a decrease in
strength and toughness.
[0089] The finishing temperature of the hot rolling is preferably
at least 700.degree. C., the temperature at the start of cooling is
preferably 680-750.degree. C., and the cooling speed up to the
temperature at which water cooling is terminated is preferably
1-50.degree. C. per second. If the temperature at the end of water
cooling exceeds 600.degree. C., the precipitation strengthening
effect in the tempering stage becomes inadequate.
[0090] The Ar.sub.1 point is found by a method in which the change
in volume of a minute test piece is measured.
[0091] After hot rolling and subsequent water cooling, the
resulting steel plate is then subjected to aging at a temperature
of at least 540.degree. C. and at most the Ac.sub.1 point of the
steel, after heating, if necessary, followed by cooling.
[0092] The average heating speed up to the aging temperature minus
100.degree. C. in the case where heating is carried out to elevate
the temperature to the aging temperature, and the average cooling
speed up to 500.degree. C. are controlled. This aging is performed
in order to achieve adequate precipitation hardening by
precipitated Cu particles, and control of heating/cooling speeds is
carried out in order to obtain uniform dispersion of Cu particles.
In this respect, heating is preferably performed at an average
heating speed of 5-50.degree. C. per minute up to the target aging
temperature minus 100.degree. C. with a temperature holding time of
at least 1 hour, and cooling is preferably carried out at an
average cooling speed of at least 5-60.degree. C. per minute up to
500.degree. C.
[0093] In the present specification, the heating temperature is the
temperature of the atmosphere inside the furnace used for heating,
and the temperature holding time after heating is the length of
time for which the steel is kept in the atmosphere inside the
furnace. The finishing temperature of hot rolling and the
temperatures at the start and completion of water cooling are the
surface temperatures of the steel, and the average heating and
cooling speeds at the time of reheating are calculated from the
temperature calculation at a position at one-half the thickness t
of the steel.
[0094] In order to construct a large marine structure from a high
tensile strength steel according to the present invention, steel
materials such as plates, pipes, and shapes are assembled by
welding, but in general the steel is used in the form of steel
plates.
[0095] In the present specification, "excellent weldability"
normally means that arc welding with a weld heat input of at least
300 kJ/cm is possible, but other welding methods may also be used
such as submerged arc welding and shielded metal arc welding.
[0096] Marine structures include not only platforms and jackup rigs
which are installed on the sea floor but also semisubmersible rigs
(semisubmersible oil drilling rigs) and the like. As long as it is
a marine structure requiring weldability and low temperature
toughness, there are no particular restrictions. The term "large"
structure means that steel used therein has a thickness of at least
50 mm.
EXAMPLES
[0097] In this example, steel slabs having a thickness of 300 mm
and having the chemical compositions shown in Table 1 and Table 2
were prepared by continuous casting. In order to control inclusions
at the center of the thickness of a steel plate, during continuous
casting, the temperature of molten steel was not made too high, and
the difference thereof from the solidification temperature which
was determined by the molten steel composition was controlled so as
to be at most 50.degree. C., and electromagnetic stirring just
before solidification and reduction in thickness at the time of
solidification were carried out.
[0098] Table 3 and Table 4 show the working conditions of steel
slabs having the chemical compositions shown in Table 1 and Table
2. The working conditions shown in Table 3 and Table 4 are for the
steel slabs having the chemical compositions shown in Table 1 and
Table 2, respectively.
[0099] After a slab with a thickness of 300 mm was heated at the
indicated heating temperature for the indicated period, it was
subjected to hot rolling and then cooled at an average cooling
speed of 5.degree. C. per second by water cooling from the starting
temperature to the ending temperature of water cooling. The
resulting steel plate had a thickness of 77 mm. (These conditions
are shown as the initial heating and rolling conditions in Table 3
and Table 4.)
[0100] Reheating was then carried out to the indicated aging
temperature, and the temperature was held for the indicated
duration (holding time). The heating speed was controlled such that
the average heating speed up to the aging temperature minus
100.degree. C. was 10.degree. C. per minute, and the cooling speed
was controlled so as to attain an average cooling speed of
10.degree. C. per minute up to 500.degree. C. (These conditions are
shown as the aging treatment conditions in Table 3 and Table
4.)
[0101] For each of the resulting steel plate, a tensile test piece
was taken for a tensile test in accordance in ASTM standards from
the center of a plate thickness so that a tensile test piece having
a parallel portion with a diameter of 12.5 mm was perpendicular to
the rolling direction.
[0102] Similarly, a CTOD test of the resulting steel plate was
carried out at -40.degree. C. in accordance with BS7448 standards
using a 3-point bending test piece which had the full thickness of
the plate and which was taken in a direction perpendicular to the
rolling direction.
[0103] A welded joint was obtained by performing FCAW (flux cored
arc welding) at 10.0 kJ/cm on the butt portions of steel plates
prepared so as to form a K-shaped groove in accordance with BS7448
standards. From the joint obtained in this manner, a CTOD test
piece was obtained by working so as to form a fatigue notch of the
CTOD test piece in alignment with the weld line on the straight
side of the V-shaped edge portion of the joint, and it was
subjected to a CTOD test at -40.degree. C.
[0104] In order to ascertain the responsiveness to large heat input
welding, the same steel plates having a 20.degree.-shaped beveled
edge were abutted and a welded joint was prepared by electrogas arc
welding (EGW) with a heat input of 350 kJ/cm. A CTOD test was
carried out on the welded joint which was prepared in this manner
in accordance with ASTM E1290. The CTOD test piece was obtained by
working so as to form a fatigue notch in alignment with the weld
line of the joint, and the critical CTOD value was measured at a
test temperature of -10.degree. C.
[0105] The average value of the equivalent circle diameter of Cu
particles was calculated by observing a transmission electron
microscope (TEM) photograph at a magnification of 100,000 so as to
measure the equivalent circle diameter for each precipitate having
a major axis of at least 1 nm and finding the average value of this
diameter in each field of view. In order to decrease the variation
of measurement, ten fields of view in a TEM photograph (each field
of view was a rectangle measured 900.times.700 nm) taken at a
position of one quarter of the initial thickness of the steel
material were observed, and the average value was used. In
addition, a microphotograph was taken of each of samples of No. 2,
No. 8, and No. 36 of Table 1 and No. 61 of Table 2. Namely, a
sample was cut from an area including the 1/2t portion. The sample
was embedded in a resin and was ground and buffed to give a minor
surface. After etching the surface, the surface was investigated
using a microscope at a magnification of 500 times. Image analysis
of each of 20 fields (900.times.700 nm) of microphotography was
carried out for each of these samples so as to determine the
ferrite fraction ratio and the like.
[0106] Table 1 shows test materials satisfying the chemical
composition in the present invention. As shown in Table 5, each of
these test steels which was manufactured under the working
conditions shown in Table 3 had a state of dispersion of Cu
particles satisfying a prescribed range.
[0107] Regarding microstructures, for No. 2 steel, the ferrite
fraction is 75%, the average grain diameter is 24 micro-meters, and
the particle flatness is 1.3.
[0108] For No. 8 steel, the ferrite fraction is 78%, the average
grain diameter is 26 micro-meters, and the particle flatness is
1.2.
[0109] For No. 36 steel, the ferrite fraction is 80%, the average
grain diameter is 23 micro-meters, and the particle flatness is
1.1.
[0110] It is noted from the above that since the billet was heated
to 950.degree. C. or less, growth of the crystal grains thereof did
not occur so much that the average diameter was kept under 30
micro-meters.
[0111] Therefore, the base metal strength, the base metal CTOD
properties, and the joint CTOD properties (-40.degree. C. and
-10.degree. C.) were high values for each test steel.
[0112] In Table 2, No. 40 is a test material which has satisfies
the chemical composition and Pcm in the present invention, while
Nos. 41-61 are test materials for which the range of chemical
composition is outside the range in the present invention. When
these test steels were manufactured under the working conditions
shown in Table 4, the state of dispersion of Cu particles shown in
Table 6 was obtained.
[0113] No. 40 satisfied the chemical composition prescribed in the
present invention, but the state of dispersion of Cu particles did
not satisfy the prescribed range, so the base metal strength was a
low value. Accordingly, in order to satisfy both high heat input
welding properties and base metal strength, it is desirable to
satisfy the condition for dispersion of Cu particles according to
the present invention.
[0114] Nos. 41-61 did not satisfy the chemical composition in the
present invention, and they could not simultaneously satisfy the
base metal strength, the base metal CTOD properties, and the joint
CTOD properties (-40.degree. C. and -10.degree. C.). According to
the present invention, all of these properties must be satisfied
simultaneously.
[0115] The microstructure of No. 61 test steel was observed. It was
noted that the ferrite fraction ratio thereof was 50%, the average
grain diameter was 54 micro-meters, and the flatness of particles
was 1.8. This microstructure was obtained when the heating
temperature of billets was increased to 1160.degree. C. Such a high
heating temperature promoted growth of crystal grains. Thus, the
ferrite fraction ratio and the flatness of particles of No. 61 test
steel are outside the range of the present invention.
[0116] Although the present invention has been described with
respect to preferred embodiments, they are mere illustrative and
not intended to limit the present invention. It should be
understood by those skilled in the art that various modifications
of the embodiments described above can be made without departing
from the scope of the present invention as set forth in the
claims.
TABLE-US-00001 TABLE 1 Steel C Si Mn P S Cu Ni Mb Mo Al N Ti 1
0.040 0.11 1.45 0.004 0.004 0.93 0.49 0.012 0.19 0.003 0.0041 0.012
2 0.039 0.07 0.87 0.008 0.003 1.01 1.46 0.005 0.37 0.010 0.0047
0.012 3 0.035 0.11 0.98 0.006 0.003 0.96 1.21 -- 0.45 0.013 0.0060
0.015 4 0.031 0.09 1.11 0.008 0.004 1.02 1.32 -- 0.35 0.005 0.0053
0.012 5 0.031 0.12 1.21 0.004 0.004 0.95 1.41 0.005 0.45 0.003
0.0052 0.015 6 0.030 0.12 1.09 0.004 0.004 0.95 0.95 -- 0.42 0.003
0.0052 -- 7 0.023 0.12 1.09 0.004 0.004 0.95 0.95 0.003 0.42 0.003
0.0052 0.015 8 0.023 0.12 1.09 0.004 0.004 0.95 0.95 0.003 0.42
0.003 0.0052 0.015 9 0.041 0.15 1.20 0.008 0.002 0.98 1.20 -- --
0.004 0.0060 -- 10 0.045 0.11 1.10 0.008 0.001 0.97 1.10 0.008 --
0.005 0.0055 -- 11 0.039 0.10 1.31 0.009 0.002 0.98 0.87 -- 0.32
0.007 0.0056 -- 12 0.045 0.09 1.12 0.008 0.001 0.95 1.09 -- --
0.008 0.0062 0.011 13 0.038 0.10 1.25 0.009 0.002 0.94 0.63 0.007
0.41 0.007 0.0057 0.007 14 0.037 0.06 1.05 0.009 0.003 0.95 0.52
0.005 0.45 0.003 0.0063 0.008 15 0.035 0.09 1.31 0.009 0.002 0.97
0.59 0.012 0.39 0.006 0.0051 0.007 16 0.041 0.10 1.21 0.009 0.002
0.95 0.56 0.010 0.42 0.005 0.0049 0.009 17 0.043 0.14 1.17 0.009
0.002 1.05 0.42 0.015 0.30 0.007 0.0052 0.008 18 0.042 0.11 1.25
0.008 0.002 0.98 0.48 0.008 0.26 0.006 0.0061 0.011 19 0.037 0.12
1.24 0.008 0.002 0.96 0.64 0.009 0.31 0.009 0.0061 0.010 20 0.041
0.12 1.22 0.008 0.002 0.97 0.71 0.010 0.45 0.008 0.0059 0.012 21
0.033 0.06 1.12 0.006 0.004 0.93 0.81 0.010 0.20 0.009 0.0060 0.013
22 0.047 0.04 1.31 0.006 0.003 1.00 0.51 0.015 0.26 0.004 0.0052
0.009 23 0.049 0.08 1.23 0.009 0.002 0.96 0.62 0.012 0.41 0.009
0.0054 0.009 24 0.048 0.09 0.85 0.009 0.002 0.98 0.94 0.009 0.46
0.009 0.0061 0.010 25 0.041 0.11 1.17 0.009 0.004 1.00 0.88 0.005
0.26 0.009 0.0041 0.010 26 0.042 0.10 1.15 0.008 0.002 0.97 0.84
0.007 0.36 0.007 0.0048 0.011 27 0.041 0.10 1.20 0.008 0.002 0.97
0.67 0.009 0.36 0.008 0.0046 0.009 28 0.031 0.06 1.06 0.010 0.003
0.95 1.00 0.013 0.18 0.006 0.0063 0.012 29 0.044 0.04 1.17 0.005
0.002 0.90 0.46 0.005 0.26 0.003 0.0040 0.008 30 0.049 0.04 0.92
0.005 0.002 0.93 0.62 0.005 0.42 0.009 0.0037 0.010 31 0.036 0.11
1.23 0.009 0.003 1.00 0.64 0.008 0.26 0.007 0.0063 0.008 32 0.041
0.11 1.28 0.005 0.002 0.90 0.43 0.008 0.26 0.006 0.0057 0.009 33
0.041 0.08 0.91 0.008 0.002 1.01 1.18 0.005 0.37 0.010 0.0050 0.010
34 0.038 0.10 0.98 0.008 0.002 0.96 0.87 -- 0.45 0.009 0.0060 0.009
35 0.030 0.10 1.02 0.008 0.002 1.02 0.67 -- 0.35 0.006 0.0051 0.012
36 0.032 0.11 1.20 0.007 0.002 0.95 1.00 0.005 0.45 0.004 0.0052
0.011 Steel O N/Al Cr V B Ca Mg REM Pcm 1 0.0013 1.3667 0.09 -- --
-- -- -- 0.188 2 0.0015 0.4700 0.12 -- 0.0003 -- -- -- 0.192 3
0.0010 0.4615 0.10 0.005 -- 0.002 -- -- 0.191 4 0.0016 1.0600 0.21
0.005 -- -- 0.002 -- 0.197 5 0.0013 1.7333 -- -- -- -- -- 0.002
0.197 6 0.0013 1.7333 0.31 -- -- -- -- -- 0.195 7 0.0013 1.7333
0.31 -- -- -- -- -- 0.188 8 0.0013 1.7333 0.31 -- -- -- -- -- 0.188
9 0.0009 1.5000 -- -- -- -- -- -- 0.175 10 0.0008 1.1000 -- -- --
-- -- -- 0.171 11 0.0011 0.8000 -- -- -- -- -- -- 0.193 12 0.0010
0.7750 -- -- -- -- -- -- 0.170 13 0.0010 0.8143 -- 0.012 -- -- --
-- 0.190 14 0.0011 2.1000 -- -- 0.0005 -- -- -- 0.180 15 0.0009
0.8500 -- -- -- 0.002 -- -- 0.188 16 0.0009 0.9800 -- -- -- --
0.0025 -- 0.190 17 0.0013 0.7429 0.20 0.008 -- -- -- -- 0.196 18
0.0010 1.0167 -- 0.011 0.0005 -- -- -- 0.186 19 0.0011 0.6778 0.10
-- 0.0006 -- -- -- 0.190 20 0.0010 0.7375 0.15 0.016 0.0007 -- --
-- 0.209 21 0.0014 0.6667 0.31 -- -- 0.002 -- -- 0.180 22 0.0016
1.3000 -- 0.035 -- -- -- 0.0016 0.193 23 0.0012 0.6000 -- -- 0.0008
0.003 -- -- 0.203 24 0.0011 0.6778 0.20 0.022 -- 0.002 -- -- 0.201
25 0.0016 0.4556 -- 0.015 0.0012 0.001 -- -- 0.193 26 0.0013 0.6857
0.21 -- 0.0015 0.002 -- -- 0.207 27 0.0009 0.5750 0.19 0.010 0.0010
0.001 -- -- 0.204 28 0.0016 1.0500 0.10 -- -- -- -- -- 0.167 29
0.0014 1.3333 -- -- -- -- -- -- 0.174 30 0.0010 0.4111 0.10 -- --
-- -- -- 0.186 31 0.0013 0.9000 0.20 -- -- -- -- -- 0.189 32 0.0018
0.9500 -- -- -- -- 0.0016 0.0025 0.178 33 0.0010 0.5000 0.12 --
0.0003 -- -- 0 0.192 34 0.0012 0.6667 0.10 0.005 -- 0.002 -- --
0.188 35 0.0013 0.8500 0.21 0.005 -- -- 0.002 -- 0.181 36 0.0013
1.3000 -- -- -- -- -- 0.002 0.190
TABLE-US-00002 TABLE 2 Steel C Si Mn P S Cu Ni Nb Mo Al N Ti 40
0.023 0.12 1.09 0.004 0.004 0.95 0.95 0.003 0.42 0.003 0.0052 0.015
41 0.110 0.15 1.01 0.007 0.005 0.97 0.89 -- 0.30 0.012 0.0049 0.010
42 0.061 0.05 1.94 0.005 0.003 0.94 0.95 -- 0.20 0.003 0.0052 -- 43
0.052 0.08 1.21 0.008 0.002 0.98 1.21 -- -- 0.019 0.0006 -- 44
0.041 0.16 1.05 0.009 0.003 0.95 1.03 0.019 0.26 0.005 0.0055 0.017
45 0.029 0.21 1.24 0.021 0.006 0.47 0.52 0.012 -- 0.031 0.0071
0.015 46 0.042 0.35 1.02 0.009 0.006 0.87 0.86 -- 0.30 0.112 0.0065
-- 47 0.110 0.06 1.28 0.010 0.002 0.93 0.88 0.008 0.50 0.006 0.0041
0.013 48 0.033 0.04 1.39 0.008 0.002 1.94 0.64 0.013 0.42 0.009
0.0052 0.012 49 0.043 0.09 1.39 0.010 0.002 0.98 0.10 0.005 0.42
0.003 0.0063 0.008 50 0.030 0.11 1.39 0.009 0.003 0.90 1.00 0.005
0.34 0.003 0.0041 0.011 51 0.041 0.14 1.06 0.005 0.003 0.93 0.88
0.015 0.85 0.004 0.0046 0.008 52 0.030 0.06 1.39 0.010 0.002 0.90
0.88 0.013 0.18 0.009 0.0057 0.009 53 0.030 0.06 1.50 0.008 0.003
0.90 0.64 0.052 0.42 0.003 0.0063 0.009 54 0.040 0.14 1.06 0.005
0.003 1.00 0.76 0.005 0.42 0.007 0.0041 0.034 55 0.030 0.09 1.17
0.009 0.002 0.90 0.52 0.015 0.26 0.003 0.0052 0.008 56 0.047 0.04
1.17 0.008 0.002 0.90 0.52 0.013 0.18 0.062 0.0032 0.012 57 0.040
0.04 1.39 0.009 0.002 0.90 0.64 0.008 0.50 0.007 0.0110 0.013 58
0.035 0.11 1.06 0.008 0.002 0.90 0.52 0.013 0.26 0.006 0.0046 0.008
59 0.051 0.04 1.17 0.005 0.002 0.90 0.52 0.005 0.26 0.034 0.0031
0.012 60 0.033 0.06 1.39 0.009 0.003 0.95 0.52 0.005 0.42 0.002
0.0078 0.008 61 0.040 0.11 1.45 0.004 0.004 0.95 0.49 0.012 0.19
0.003 0.0041 0.012 Steel O N/Al Cr V B Ca Mg REM Pcm 40 0.0013
1.7333 0.31 -- -- -- -- -- 0.188 41 0.0015 0.4083 -- -- -- -- -- --
0.249 42 0.0013 1.7333 0.15 -- -- -- -- -- 0.243 43 0.0015 0.0316
0.2 0.01 -- -- -- -- 0.195 44 0.0051 1.1000 -- -- -- -- -- -- 0.181
45 0.0071 0.2290 -- -- -- -- -- -- 0.130 46 0.0065 0.0580 0.2 -- --
-- -- -- 0.193 47 0.0016 0.6833 -- 0.035 -- -- 0.0021 -- 0.274 48
0.0017 0.5778 0.22 -- 0.0009 -- -- -- 0.255 49 0.0017 2.1000 0.40
-- -- -- -- -- 0.214 50 0.0014 1.3667 0.85 0.008 0.0012 -- 0.0025
-- 0.237 51 0.0017 1.1500 -- 0.035 -- 0.0007 -- -- 0.220 52 0.0016
0.6333 -- 0.061 0.0010 -- -- -- 0.184 53 0.0017 2.1000 -- -- --
0.0012 -- -- 0.191 54 0.0016 0.5857 -- 0.035 0.0021 -- -- -- 0.202
55 0.0014 1.7333 0.16 -- 0.0045 -- -- 0.0021 0.193 56 0.0018 0.0516
-- -- 0.0026 -- -- -- 0.186 57 0.0017 1.5714 -- -- 0.0012 -- 0.0007
-- 0.206 58 0.0041 0.7667 -- 0.015 0.0023 -- -- -- 0.176 59 0.0014
0.0912 -- -- -- -- -- -- 0.182 60 0.0013 3.9000 -- 0.015 -- -- --
-- 0.190 61 0.0013 1.3667 0.09 -- -- -- -- -- 0.185
TABLE-US-00003 TABLE 3 Initial Heating and Rolling Conditions Water
cooling Aging Condition Heating Heating Finishing started ended
Aging Dura- Steel temp. period temp. at at temp. tion No. (.degree.
C.) (hour) (.degree. C.) (.degree. C.) (.degree. C.) (.degree. C.)
(hour) 1 950 10 730 710 350 650 5 2 900 10 735 705 350 580 5 3 950
10 750 720 350 575 5 4 900 10 730 720 350 590 5 5 900 10 720 710
350 590 5 6 900 10 730 710 350 650 5 7 900 10 720 710 350 590 5 8
900 10 730 710 350 650 5 9 950 5 730 710 250 600 5 10 950 5 730 710
250 600 5 11 950 5 730 710 250 600 5 12 900 5 730 710 250 600 5 13
900 5 730 710 250 600 5 14 950 5 720 700 250 600 5 15 950 5 730 710
250 600 5 16 950 5 730 710 250 600 5 17 950 5 730 710 250 590 5 18
900 5 710 690 250 600 5 19 950 5 740 720 250 600 5 20 950 5 730 710
250 590 5 21 950 5 730 710 250 630 5 22 950 5 720 700 250 650 5 23
950 5 730 710 250 600 5 24 950 5 740 720 250 600 5 25 900 5 730 710
250 600 5 26 900 5 730 710 250 600 5 27 900 5 720 700 250 600 5 28
900 5 730 710 250 600 5 29 950 5 730 710 250 600 5 30 950 5 730 710
250 600 5 31 950 5 730 710 250 600 5 32 950 5 730 710 250 600 5 33
900 10 735 705 250 580 5 34 950 10 750 720 250 575 5 35 900 10 730
720 250 590 5 36 900 10 720 710 250 590 5
TABLE-US-00004 TABLE 4 Initial Heating and Rolling Conditions Water
cooling Aging Condition Heating Heating Finishing started ended
Aging Dura- Steel temp. period temp. at at temp. tion No. (.degree.
C.) (hour) (.degree. C.) (.degree. C.) (.degree. C.) (.degree. C.)
(hour) 40 950 10 730 720 350 720 5 41 950 10 750 705 350 600 5 42
950 10 740 710 350 580 5 43 950 10 715 710 350 550 5 44 950 10 720
720 350 620 5 45 950 10 725 710 350 550 5 46 950 10 730 715 350 590
5 47 1000 5 730 720 250 600 5 48 950 5 730 710 250 580 5 49 1000 5
730 720 250 600 5 50 1000 5 720 700 250 600 5 51 950 5 730 710 250
600 5 52 950 5 730 720 250 590 5 53 950 5 730 700 250 590 5 54 1000
5 730 700 250 600 5 55 1000 5 720 700 250 600 5 56 1000 5 730 700
250 600 5 57 950 5 730 700 250 600 5 58 1000 5 730 710 250 600 5 59
950 5 730 700 250 600 5 60 950 5 730 700 250 600 5 61 1160 8 780
760 350 600 5
TABLE-US-00005 TABLE 5 Cu particles CTOD of joint Average
equivalent Plane-converted CTOD of base low heat high heat Steel
circle diameter area fraction Ys Ts metal -40.degree. C. input
-40.degree. C. input -10.degree. C. No. (nm) (%) (N/mm.sup.2)
(N/mm.sup.2) (mm) (mm) (mm) 1 16 5 510 571 >1.3 0.61 0.31 2 14
16 580 627 >1.3 0.98 0.41 3 15 17 589 640 1.10 0.79 0.32 4 15 14
563 637 1.09 0.61 0.40 5 15 15 571 641 >1.3 1.10 0.42 6 16 10
512 572 0.89 0.62 0.29 7 14 16 592 646 >1.3 0.83 0.31 8 17 17
509 578 >1.3 1.12 0.28 9 16 15 506 564 1.20 0.62 0.29 10 15 16
499 552 1.00 0.70 0.31 11 16 15 520 579 >1.3 0.56 0.30 12 14 14
497 552 >1.3 0.80 0.42 13 14 16 521 578 >1.3 0.72 0.42 14 15
16 493 572 1.10 0.37 0.41 15 13 14 523 589 >1.3 0.46 0.31 16 14
15 512 584 >1.3 0.53 0.37 17 12 13 530 600 >1.3 0.26 0.27 18
14 14 524 591 >1.3 0.53 0.32 19 15 17 519 587 1.20 0.41 0.33 20
13 16 541 612 1.30 0.63 0.41 21 17 9 497 558 1.10 0.35 0.29 22 18 8
489 553 >1.3 0.31 0.41 23 13 14 531 603 1.10 0.56 0.36 24 14 13
528 600 1.20 0.71 0.31 25 16 15 512 574 >1.3 0.38 0.29 26 14 16
510 581 >1.3 0.42 0.31 27 15 14 507 572 >1.3 0.61 0.36 28 13
13 482 551 >1.3 0.31 0.29 29 12 14 500 571 >1.3 0.42 0.31 30
13 16 510 581 >1.3 0.35 0.31 31 12 15 502 573 >1.3 0.32 0.36
32 14 14 501 569 >1.3 0.31 0.34 33 14 16 580 627 1.10 0.94 0.40
34 15 17 584 642 0.94 0.79 0.35 35 15 14 560 637 1.09 0.54 0.41 36
15 15 569 640 1.10 0.63 0.42
TABLE-US-00006 TABLE 6 Cu particles CTOD of joint Average
equivalent Plane-converted CTOD of base low heat high heat Steel
circle diameter area fraction Ys Ts metal -40.degree. C. input
-40.degree. C. input -10.degree. C. No. (nm) (%) (N/mm.sup.2)
(N/mm.sup.2) (mm) (mm) (mm) 40 28 2 417 472 >1.3 0.67 0.30 41 17
16 626 711 0.12 0.09 0.006 42 14 14 642 720 0.19 0.12 0.006 43 14
13 632 691 0.41 0.06 0.007 44 17 17 578 632 0.08 0.09 0.007 45 10
15 384 420 0.84 0.71 0.006 46 17 15 574 623 0.12 0.04 0.006 47 16
15 621 716 0.10 0.03 0.007 48 18 23 619 701 0.08 0.02 0.007 49 14
16 512 589 0.08 0.03 0.006 50 15 15 623 712 0.08 0.04 0.007 51 14
16 621 700 0.09 0.01 0.008 52 13 14 552 636 0.08 0.02 0.008 53 13
15 541 617 0.09 0.01 0.006 54 17 13 543 675 0.09 0.03 0.008 55 14
17 567 648 0.08 0.04 0.009 56 14 13 511 652 0.04 0.03 0.008 57 14
14 498 612 0.06 0.02 0.008 58 16 15 470 530 0.06 0.03 0.009 59 16
16 502 580 0.21 0.16 0.018 60 15 15 491 565 0.22 0.17 0.019 61 31
15 570 630 0.12 0.53 0.56
* * * * *