U.S. patent application number 10/594660 was filed with the patent office on 2007-08-09 for high-strength thick steel plate excellent in low temperature toughness at heat affected zone resulting from large heat input welding.
Invention is credited to Toshiei Hasegawa, Minoru Ito, Akihiko Kojima, Masanori Minagawa, Jun Otani, Yoichi Tanaka.
Application Number | 20070181223 10/594660 |
Document ID | / |
Family ID | 35125101 |
Filed Date | 2007-08-09 |
United States Patent
Application |
20070181223 |
Kind Code |
A1 |
Ito; Minoru ; et
al. |
August 9, 2007 |
High-strength thick steel plate excellent in low temperature
toughness at heat affected zone resulting from large heat input
welding
Abstract
The present invention provides a high-strength thick steel plate
having a plate thickness of 50 to 80 mm and a tensile strength of
490 to 570 MPa which is able to realize an excellent HAZ toughness
even when welding with a heat input of 20 to 100 kJ/mm is conducted
and is characterized by containing, by wt %, 0.03-0.14% of C, 0.30%
or less of Si, 0.8-2.0% of Mn, 0.02% or less of P, 0.005% or less
of S, 0.8-4.0% of Ni, 0.003-0.040% of Nb, 0.001-0.040% of Al,
0.0010-0.0100% of N, and 0.005-0.030% of Ti, where Ni and Mn
satisfy equation [1], and the balance of iron and unavoidable
impurities: Ni/Mn.gtoreq.10.times.Ceq-3 (0.36<Ceq<0.42)
[1]where, Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15
Inventors: |
Ito; Minoru; (Chiba, JP)
; Kojima; Akihiko; (Chiba, JP) ; Minagawa;
Masanori; (Oita, JP) ; Tanaka; Yoichi; (Chiba,
JP) ; Hasegawa; Toshiei; (Chiba, JP) ; Otani;
Jun; (Oita, JP) |
Correspondence
Address: |
KENYON & KENYON LLP
ONE BROADWAY
NEW YORK
NY
10004
US
|
Family ID: |
35125101 |
Appl. No.: |
10/594660 |
Filed: |
April 6, 2005 |
PCT Filed: |
April 6, 2005 |
PCT NO: |
PCT/JP05/07109 |
371 Date: |
September 27, 2006 |
Current U.S.
Class: |
148/336 ;
420/119 |
Current CPC
Class: |
C22C 38/08 20130101 |
Class at
Publication: |
148/336 ;
420/119 |
International
Class: |
C22C 38/08 20060101
C22C038/08; C22C 38/14 20060101 C22C038/14 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 7, 2004 |
JP |
2004-113278 |
Mar 31, 2005 |
JP |
2005-10241 |
Claims
1. A high-strength thick steel plate excellent in low temperature
toughness at heat affected zone resulting from large heat input
welding, characterized by containing, by wt %, C: 0.03-0.14%, Si:
0.30% or less, Mn: 0.8-2.0%, P: 0.02% or less, S: 0.005% or less,
Al: 0.001-0.040%, N: 0.0010-0.0100%, Ni: 0.8-4.0%, Ti:
0.005-0.030%, and Nb: 0.003-0.040%, where Ni and Mn satisfy
equation [1], and the balance of iron and unavoidable impurities:
Ni/Mn.gtoreq.10.times.Ceq-3 (0.36<Ceq<0.42) [1] where,
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15
2. A high-strength thick steel plate excellent in low temperature
toughness at heat affected zone resulting from large heat input
welding according to claim 1, characterized by fturther containing,
by wt %, one or more of: Ca: 0.0003-0.0050%, Mg: 0.0003-0.0050%,
and REM: 0.001-0.030% and contains at least 1 00/mm.sup.2 of oxide
particles containing O: 0.0010-0.0050% and having a equivalent
circle diameter of 0.005 to 0.5 .mu.m.
3. A high-strength thick steel plate excellent in low temperature
toughness at heat affected zone resulting from large heat input
welding according to claim 1, characterized by further containing,
by wt %, B: 0.0005-0.0050%.
4. A high-strength thick steel plate excellent in low temperature
toughness at heat affected zone resulting from large heat input
welding according to claim 1, characterized by further containing,
by wt %, one or more of: Cr: 0.1-0.5%, Mo: 0.01-0.5%, V:
0.005-0.10%, and Cu: 0.1-1.0%.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a high strength thick steel
plate excellent in low temperature toughness at heat affected zone
(hereinafter referred to as an "HAZ") used for ships, offshore
structures, medium/high rise buildings, bridges, and so forth, more
particularly relates to a steel plate having a thickness of 50 mm
or more and a tensile strength of 490 to 570 MPa and having an
excellent welded joint even in a case where welding with a welding
heat input of 20 to 100 kJ/mm is conducted.
[0003] 2. Description of the Related
[0004] In recent years, the demands on the material properties of
steel materials for welding used in large sized structures such as
ships, offshore structures, medium/high rise buildings, and bridges
have become increasingly tough. Especially, among these structures,
the use of steel plates having large thicknesses exceeding 50 mm
and having tensile strength of around 570 MPa has been increasing.
Further, in order to promote higher efficiency of the welding, for
the welding of such high-strength thick steel plates, one-pass
welding by a large heat input welding process such as electro-gas
welding, electo-slag welding, etc. has been investigated. The
demands on the HAZ toughness have become increasingly tough in the
same way as on the toughness of the base material per se.
[0005] Many proposals have been made hitherto paying attention to
the HAZ toughness of steel plate to which a large heat input
welding process is applied. For example, Japanese Unexamined Patent
Publication (Kokai) No. 55-026164 discloses an invention of
securing fine Ti nitrides in the steel so as to reduce the
austenite grain size in the HAZ and thereby improve the toughness.
Further, Japanese Unexamined Patent Publication (Kokai) No.
03-264614 proposes an invention making use of complex precipitates
of Ti nitrides and MnS as transformation nuclei of ferrite so as to
improve the HAZ toughness. Further, Japanese Unexamined Patent
Publication (Kokai) No. 04-143246 proposes an invention making use
of complex precipitates of Ti nitrides and BN as precipitation
nuclei of grain boundary ferrite so as to improve the HAZ
toughness.
[0006] However, Ti nitrides end up becoming almost completely
dissolved in the vicinity of the border with a welded metal in HAZ
where the highest temperature reached exceeds 1400.degree. C.
(hereinafter also referred to as a "weld bond portion"). As a
result, there is a problem that the effect of improvement of the
toughness is lowered. For this reason, in steel utilizing the Ti
nitrides as described above, it is difficult to meet the recent
tough demands for the HAZ toughness or the required characteristics
of the HAZ toughness in ultra-large heat input welding.
[0007] Steels containing Ti oxides as a method of improving the
toughness in the vicinity of this weld bond portion are being used
in various fields such as thick plates and steel shapes. For
example, in the field of thick steel plates, as described in the
inventions disclosed in Japanese Unexamined Patent Publication
(Kokai) No. 61-079745 and Japanese Unexamined Patent Publication
(Kokai) No. 61-117245, steel containing Ti oxides is very effective
for improving toughness at the large heat input weld portion, and
is promising in application to high tensile steels. This principle
is that the Ti nitrides, MnS, etc. precipitate in the middle of
temperature drop after welding using the Ti oxides stable even at
the melting point of the steel as precipitation sites, fine ferrite
is generated using these as sites, and as a result the production
of coarse ferrite harmful to the toughness is suppressed and
deterioration of the toughness can be prevented.
[0008] However, such Ti oxides involve a problem that the number of
particles dispersed into the steel cannot be increased that much.
The reason is the coarsening or aggregation of the Ti oxides. It is
believed that if the Ti oxides particles are increased, coarse Ti
oxide particles of 5 .mu.m or more, i.e., so-called inclusions, end
up increasing. These inclusions of 5 .mu.m or more size become
initiation sites for fracture of a structure or cause a drop of the
toughness and therefore are harmful. Therefore, this should be
avoided. For this reason, in order to achieve a further improvement
of the HAZ toughness, it was necessary to make use of oxides which
are hard to coarsen and agglomerate and are more finely dispersed
than Ti oxides.
[0009] Further, many such methods of dispersion of such Ti oxides
into steel are based on adding Ti into molten steel substantially
not containing any Al or other strong deoxidizing elements.
However, it is difficult to control the number and degree of
dispersion of Ti oxide particles in steel by just adding Ti into
the melt. Further, it is also difficult to control the number and
degree of dispersion of precipitates such as TiN and MnS. For this
reason, in steel in which Ti oxide particles are dispersed by only
Ti deoxidation, there was the problem for example that neither a
sufficient number of Ti oxide particles nor a stable toughness of
thick plate in the thickness direction could be obtained.
[0010] With respect to such problems, Japanese Unexamined Patent
Publication (Kokai) No. 06-293937 and Japanese Unexamined Patent
Publication (Kokai) No. 10-183295 disclose inventions making use of
Ti--Al complex oxides and Ti, Al, and Ca complex oxides produced by
the addition of Al immediately after the addition of Ti or the
complex addition of Al and Ca. By such inventions, it became
possible to greatly improve HAZ toughness in the large heat input
welding.
SUMMARY OF THE INVENTION
[0011] However, with the conventional means of reducing the
austenite grain size of the HAZ or generating ferrite by using
precipitates as transformation nuclei of the ferrite, it is
necessary to increase the alloy elements in order to secure a
tensile strength of 490 MPa or more when the plate thickness is 50
mm or more. In this case, the hardness of the HAZ rises and, at the
same time, the production of MA (martensite-austenite constituent)
degrading the toughness becomes remarkable. Therefore, a sufficient
HAZ toughness such as the E grade (-20.degree. C. guarantee) in for
example the shipbuilding field cannot be stably secured. In
addition, when the tensile strength becomes more than 570 MPa, the
required HAZ toughness cannot be obtained.
[0012] Therefore, an object of the present invention is to provide
a high-strength thick steel plate excellent in the low temperature
toughness of the heat affected zone resulting from large heat input
welding, which can realize excellent HAZ toughness even in a case
of welding with a heat input of 20 to 100 kJ/mm for steel plate
having a thickness of 50 to 80 mm, and a tensile strength of 490 to
570 MPa.
[0013] The inventors discovered that by defining the amount of
addition of Ni and Ni/Mn ratio, the above problems could be
advantageously solved. They engaged in extensive study and thereby
completed the present invention for the first time. The gist
thereof is as follows:
[0014] (1) A high-strength thick steel plate excellent in low
temperature toughness at a heat affected zone resulting from large
heat input welding characterized by containing, by wt %, 0.03-0.14%
of C, 0.30% or less of Si, 0.8-2.0% of Mn, 0.02% or less of P,
0.005% or less of S, 0.001-0.040% of Al, 0.0010-0.0100% of N,
0.8-4.0% of Ni, 0.005-0.030% of Ti, and 0.003-0.040% of Nb, where
Ni and Mn satisfy Equation [1], and a balance of iron and
unavoidable impurities: Ni/Mn.gtoreq.10.times.Ceq-3
(0.36<Ceq<0.42) [1]
[0015] where, Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15
[0016] (2) A high-strength thick steel plate excellent in low
temperature toughness at a heat affected zone resulting from large
heat input welding as set forth in (1), characterized by further
containing, by wt %, one or more of 0.0003-0.0050% of Ca,
0.0003-0.0050% of Mg, 0.001-0.030% of an REM and containing at
least 100/mm.sup.2 of grains of an oxide containing 0.0010-0.0050%
of O and having an equivalent circle diameter of 0.005 to 0.5
.mu.m.
[0017] (3) A high-strength thick steel plate excellent in low
temperature toughness at a heat affected zone resulting from large
heat input welding as set forth in (1) or (2), characterized by
further containing, by wt %, 0.0005-0.0050% of B.
[0018] (4) A high-strength thick steel plate excellent in low
temperature toughness at a heat affected zone resulting from large
heat input welding as set forth in any one of (1) to (3),
characterized by further containing, by wt %, one or more of
0.1-0.5% of Cr, 0.01-0.5% of Mo, 0.005-0.10% of V, and 0.1-1.0% of
Cu.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a graph of a welding heat cycle corresponding to
45 kJ/mm.
[0020] FIG. 2 is a graph of the relationships among Ni/Mn, Ceq, and
the simulated HAZ toughness.
[0021] FIG. 3 is a graph of an effect of improvement of the
simulated HAZ toughness due to dispersion of fine oxides or B
addition.
BEST MODE FOR CARRYING OUT THE INVENTION
[0022] A detailed explanation of the present invention will be
given below.
[0023] Hitherto, as the means for improving the HAZ toughness, as
previously mentioned, it has been considered to suppress an
austenite grain growth at a high temperature. The most effective
method as that means is pinning an austenite grain boundaries by
dispersed particles so as to stop the movement of the grain
boundaries. This pinning is extremely effective for reduction of
the reheated austenite grain size at the HAZ even in the case where
a large heat input such as 20 to 100 kJ/mm is applied. However, in
steel material wherein the amount of alloy elements added is
increased in order to raise the strength and a carbon equivalent
(Ceq) indicating both the weldability of the steel and a quench
bardenability in terms of chemical composition becomes 0.36 or
more, the hardness of the HAZ becomes higher. Therefore, there
arises the new problem that a sufficient HAZ toughness cannot be
obtained even when the re-heated austenite grains become finer due
to the pinning. In the case where the hardness of the HAZ becomes
high, it is necessary to improve the toughness of the base material
per se.
[0024] Therefore, the inventors intensively studied the optimal
chemical composition for improving the toughness of the base
material itself for improvement of the HAZ toughness in the case
where the Ceq was as high as 0.36 to 0.42, which becomes necessary
for the high-strength thick steel plate. It has been conventionally
known that Ni is an effective element improving the toughness of
the matrix. However, it has not been known whether it is effective
for improving the HAZ toughness in case of a high Ceq of 0.36 to
0.42. Therefore, the inventors first studied the influence of the
amount of the Ni addition. For the study, they predetermined that
an 0.003% or more of Nb addition is effective for securing the base
material strength. For evaluation of the HAZ toughness, they
employed the ductility/brittleness transition temperature in a
Charpy impact test (vTrs) when imparting a heat cycle corresponding
to electro-gas welding (heat input of 45 kJ/mm) shown in FIG.
1.
[0025] As a result of their studying the influence of the amount of
addition of Ni, first they learned that the required toughness
could not be obtained when the Ni was less than 0.8%. Further, even
when the Ni was 0.8% or more, they observed cases where the HAZ
toughness was not improved and cases where conversely the HAZ
toughness was lowered. Therefore, they engaged in further intensive
studies including the relationships with the other added elements
and Ceq. As a result, they discovered that the HAZ toughness was
related to the Ceq and Ni/Mn ratio as shown in FIG. 2 when the Ceq
is 0.36 to 0.42 in this way. FIG. 2 plots the synthetic HAZ
toughness (vTrs) of the steel material used for the study
classified for each Ceq with the Ni/Mn ratio plotted on the
abscissa. From FIG. 2, in steel wherein the following relationship
stands: Ni/Mn.gtoreq.10.times.Ceq-3 [1] a good toughness in terms
of vTrs of -15.degree. C. or less was obtained. As the reason why
steel not satisfying equation [1] cannot obtain a sufficient HAZ
toughness, it can be considered that the amount of addition of Ni
is not sufficient and the effect of increasing the toughness of the
matrix is small, or that even if a large amount of Ni is contained,
MA is produced in the HAZ due to an excessive addition of Mn and
the effect of Ni of increasing the toughness disappears. Note that
the inventors engaged in a similar study on the heat cycle of the
heat input corresponding to a heat input of 100 kJ/mm for the steel
used in the above described study, and as a result confirmed that a
good synthetic HAZ toughness is obtained in steel satisfying the
relationship of equation [1] even in the case of a heat input of
100 kJ/mm.
[0026] By the above mentioned studies, the inventors found that the
HAZ toughness was improved by the addition of Ni in an amount of
0.8% or more satisfying equation [1]. The inventors further studied
regarding the improvement of the HAZ toughness. They studied the
following three methods as methods of improving the HAZ toughness.
First is the method of suppressing the coarsening of the austenite
grains at a high temperature, since in large heat input welding,
the holding time at a high temperature becomes a long period.
Therefore, the austenite grains coarsen, which lowers the HAZ
toughness. Second is the method of suppressing the coarsening of
the grain boundary ferrite, since the cooling time after the
welding is long in large heat input welding, so the ferrite
generated from the austenite grain boundaries coarsen. The coarse
grain boundary ferrite becomes the cause of a drop in the HAZ
toughness. Third is the method of refining the HAZ microstructure
itself.
[0027] Concerning the first method of suppressing the coarsening of
the austenite grains, for example, as described in Japanese
Unexamined Patent Publication (Kokai) No. 10-183295, the method of
dispersing fine oxides is effective. In this publication, in the
dispersion of the fine oxides, the amount of dissolved oxygen in
the steel melt is adjusted by an equilibrium reaction with Si in
the deoxidation process. Further, the following deoxidation is
conducted in a sequence of Ti, Al, and Ca. Then, by this method,
oxides having a particle size of 0.01-1.0 .mu.m are dispersed to
5.times.10.sup.3 to 1.times.10.sup.5/mm.sup.2.
[0028] Therefore, the inventors intensively studied a method of
further improving the HAZ toughness by dispersing fine oxides in a
process containing 0.03% of Nb and adding 0.8% or more of Ni in the
case of a Ceq as high as 0.36 to 0.42. First, for the method of
dispersing fine oxides, they found the fact that, in such a
process, by adjusting the amount of dissolved oxygen of the steel
melt to 0.0010-0.0050% in the deoxidation process, then first
deoxidizing the steel melt with Ti, and then deoxidizing the steel
melt with Al, and further adding one or more of Ca, Mg, and REM, it
is possible to disperse fine oxide particles having an equivalent
circle diameter of 0.005 to 0.5 .mu.m to 100/mm.sup.2 or more.
Further, by this dispersion of the fine oxides, the coarsening of
the austenite grains at the holding time at a high temperature in
the welding was suppressed, and the HAZ toughness could be further
improved. As an example, the result of comparison with the HAZ
toughness obtained by only adding a proper amount of Ni is shown in
FIG. 3. Note that, the larger the amount of Ni, the finer the
produced oxides, and the larger the number of particles. When the
amount of Ni is 1.5% or more, it even becomes 1000/mm.sup.2 or
more. This is discovered this time. Further, for the amount of Si
in the steel melt, the larger the amount of Si, the harder the
oxide to form. Therefore, it was clarified from this study that the
amount of Si was preferably 0.30% or less and further preferably
0.20% or less. On the other hand, in the case where the amount of
dissolved oxygen before the Ti deoxidation exceeds 0.050% and the
case where the sequence of the deoxidation elements is different,
the oxide coarsens and the sufficient amount of fine oxide cannot
be obtained. Therefore, almost no effect of suppression of
coarsening of the austenite grains can be obtained. Note that the
number of grains of the oxides having the equivalent circle
diameter of 0.005 to 0.5 .mu.m was obtained by preparing an
extraction replica from the steel plate as the base material.
Observing this under an electron microscope with .times.10000
magnification in at least 100 fields (10000 .mu.m.sup.2 or larger
observation area), and observing particles less than 0.1 .mu.m by
properly raising the magnification. The inventors conducted
elemental analysis at each particle having a diameter of 0.005 to
0.5 .mu.m observed and counted the oxide particles.
[0029] Next, the inventors intensively studied the suppression of
the coarsening of the grain boundary ferrite and the refining of
the HAZ microstructure as above described second and third methods
of improvement of the HAZ toughness. As a result, the inventors
clarified that the addition of B was effective particularly in the
case where large heat input welding corresponding to 20 to 100
kJ/mm was conducted in a process where the Ceq was as high as 0.36
to 0.42 and Ni was added in an amount of 0.8% or more. The reason
for that, in terms of the suppression of the coarsening of the
grain boundary ferrite, is the suppression of the production of
grain boundary ferrite by a segregation of the solute B at the
re-heated austenite grain boundaries. Further, in terms of the
increased refined HAZ microstructure, in the case where the cooling
rate is slow in the large heat input welding, the B nitrides
precipitated at the austenite grain boundaries and in the
inclusions in the austenite grains due to the addition of B, and a
large number of fine ferrite grains of several micrometers using
those as nuclei are present at the austenite grain boundaries and
in the grains, whereby the HAZ structure is made finer. The
inventors compared the improvement of the HAZ toughness by the
addition of B with the HAZ toughness obtained by only properly
adding Ni. The results are shown in FIG. 3. It is seen that the HAZ
toughness is further improved by the addition of B. Further, FIG. 3
shows the HAZ toughness in the case where the B is added in
addition to the method of dispersing the above mentioned fine
oxides. The HAZ toughness is further improved by the dispersion of
the fine oxides and the addition of B. It is considered that the
improvement is due to the increase of the oxides acting as
precipitation sites of the BN and thereby the greater refined HAZ
microstructure due to the increase of the ferrite using the BN as
the nuclei.
[0030] Further, from the viewpoints of securing the strength and
improvement of the corrosion resistance, in addition to the above
described conditions, the inventors also studied the HAZ toughness
when Cu, Cr, Mo, and V were added. As a result, they found that the
HAZ toughness was not greatly lowered when they were added within
ranges of 0.1-0.4%, 0.1-0.5%, 0.03-0.2%, and 0.005-0.050%.
[0031] Note that, the method of production of the steel plate
according to the present invention is not particularly limited. The
steel plate may be produced by any known method. For example, a
slab is formed from steel melt adjusted to the preferred
composition described above by a continuous casting method, then is
heated to 1000 to 1250.degree. C., then is hot rolled.
[0032] Next, an explanation will be given on the reasons for
limitation of the ingredients of the steel materials used in the
present invention. Below, the wt % in the compositions will be
simply described as %.
[0033] C is an ingredient effective for improving the strength of
the steel, so the lower limit is made 0.03%. An excess addition
produces large amounts of carbide and MA and remarkably lowers the
HAZ toughness, therefore the upper limit was made 0.14%.
[0034] Si is an ingredient necessary for securing the strength of
the base material and deoxidation, but in order to prevent the drop
in the toughness due to the hardening of the HAZ, the upper limit
was made 0.30%. When utilizing an oxide, the upper limit of the
content is made 0.20% or less in order to prevent the reduction of
the oxygen concentration in the molten steel.
[0035] Mn is an ingredient effective for securing the strength and
toughness of the base material and must be added in an amount of
0.8% or more, but the upper limit was made 2.0% within the range
where the toughness, cracking property etc. of the welding zone
were permissible. Further, concerning the upper limit of Mn, it is
necessary to satisfy equation [1] indicating the relationship among
the Ceq, Mn amount, and the Ni amount. This is based on the newly
found fact by this study that the increase of Mn becomes the cause
of production of a large amount of MA in the HAZ microstructure in
the case where the Ceq is high and the effect of improvement of the
HAZ toughness by Ni disappears. Ni/Mn.gtoreq.10.times.Ceq-3 [1]
[0036] P is desirably as little as possible, but in order to reduce
this industrially, enormous costs are entailed, so the range of
content was made 0.02 or less.
[0037] S is desirably as little as possible, but in order to reduce
this industrially, enormous costs are entailed, so the range of
content was made 0.005 or less.
[0038] Ni is an important element in the present invention and must
be added in an amount of at least 0.8%. Further, concerning the
lower limit of Ni, it is necessary to satisfy equation [1] showing
the relationship of Ceq, the amount of Mn, and the amount of Ni.
The upper limit was made 4.0% from the viewpoint of the production
cost. Ni/Mn.gtoreq.10.times.Ceq-3 [1]
[0039] Nb is an element effective for improving the strength of the
base material by improving the quench bardenability, so is added in
an amount of 0.003% or more. However, if a lot of Nb is added, the
MA becomes easy to be produced in the HAZ regardless of the Ni/Mn
ratio, while when it is added in an amount larger than 0.040%, a
large amount of coarse MA having a long axis of 5 .mu.m is produced
in the HAZ and greatly reduces the HAZ toughness. Therefore, the
upper limit of Nb is made 0.040%. Note that, in order to obtain a
higher toughness, preferably the amount of Nb is suppressed to
0.020% or less where almost no coarse MA having a long axis of 5
.mu.m is produced in the case of the Ni/Mn ratio satisfying the
above mentioned equation [1]. In order to stably obtain further
higher toughness, it is preferred to suppress the amount of Nb to
0.010% or less where almost no MA having a long axis of 3 .mu.m or
more is generated in the case of an Ni/Mn ratio satisfying the
above mentioned equation [1].
[0040] Al is an important deoxidation element, so the lower limit
was made 0.001%. Further, when a large amount of Al is present, the
surface quality of the slab is deteriorated, so the upper limit was
made 0.040%.
[0041] Ti is added in an amount of 0.005% or more according to need
in order to produce the Ti nitride and the Ti-containing oxide
particles which become pinning sites necessary for suppressing the
coarsening of the re-heated austenite grains. However, its excess
addition increases the amount of dissolved Ti and induces a drop in
the HAZ toughness, therefore 0.030% was made the upper limit.
[0042] N is adjusted in the amount of addition, if necessary, in
order to produce the Ti nitride and the B nitride particles at the
austenite grain boundaries and in the grains during the cooling
after the welding. In order to form the B nitride by binding with
B, N must be added in an amount of 0.0010% or more, but its excess
addition increases the amount of dissolved N and induces a drop in
the HAZ toughness, so 0.0100% was made the upper limit.
[0043] Ca is added in an amount of 0.0003% or more, if necessary,
in order to produce the Ca-based oxide particles acting as pinning
grains necessary for suppressing the coarsening of the re-heated
austenite grains. However, excess addition produces coarse
inclusions, so 0.0050% was made the upper limit.
[0044] Mg is added in an amount of 0.0003% or more, if necessary,
in order to generate the Mg-based oxide particles acting as pinning
grains necessary for suppressing the coarsening of the re-heated
austenite grains. However, excess addition produces coarse
inclusions, so 0.0050% was made the upper limit.
[0045] A REM is added in the amount of 0.0001% or more, if
necessary, in order to produce the REM-based oxide particles acting
as pinning sites necessary for suppressing the coarsening of the
re-heated austenite grains. However, excess addition produces
coarse inclusions, so 0.030% was made the upper limit. Further, the
"REM" mentioned here represent Ce and La, and the amount of
addition is the total amount of the two.
[0046] B is added in an amount of 0.0005% or more, if necessary, in
order to cause the dissolved B to segregate at the austenite grain
boundaries during the cooling after the welding and suppress the
production of the grain boundary ferrite and further to produce BN
at the austenite grain boundaries and in the grains. However, its
excess addition increases the amount of dissolved B, greatly raises
the HAZ hardness, and induces a drop in the HAZ toughness, so
0.0050% was made the upper limit.
[0047] Cu is added in an amount of 0.1% or more, if necessary, in
order to improve the strength and corrosion resistance of the
steel. The effect thereof is saturated at 1.0%, so the upper limit
was made 1.0, but when it exceeds 0.4%, MA becomes easy to be
generated and the HAZ toughness is lowered, therefore 0.4% or less
is preferred.
[0048] Cr is added in an amount of 0.1% or more, if necessary, in
order to improve the corrosion resistance of the steel, but its
excess addition induces a drop in the HAZ toughness due to the
generation of MA, so 0.5% was made the upper limit.
[0049] Mo is an element effective for improving the strength and
the corrosion resistance of the base material and is added in an
amount of 0.01% or more, if necessary. The effect thereof is
saturated at 0.5%, so the upper limit was made 1.0, but its excess
addition induces a drop in the HAZ toughness due to the generation
of MA, so 0.2% or less is preferred.
[0050] V is an element effective for improving the strength of the
base material and is added in an amount of 0.005% or more, if
necessary. The effect thereof is saturated at 0.5%, so the upper
limit was made 0.10%, but its excess addition induces a drop in the
HAZ toughness due to the generation of MA, so 0.050% or less is
preferred.
EXAMPLE 1
[0051] Slabs were prepared by continuously casting the steel melt
having the chemical compositions shown in Table 1. For D23-D31 and
D46-D49, the amounts of dissolved oxygen of the steel melt were
adjusted to 0.0010%-0.0050% by Si before charging the Ti, then Ti
was used for deoxidization, then Al was used for deoxidation, then
any of Ca, Mg, or REM was added for deoxidation. The slabs were
re-heated at 1100 to 1250.degree. C., then were hot rolled by the
following two methods to produce steel plates having plate
thicknesses of 50 to 80 mm. One method was to roll the plate at a
surface temperature within a range of 750-900.degree. C., then cool
it by water at a plate surface temperature within the temperature
range of 200-400.degree. C. after recalescence (described as TMCP
in Table 2). The other method of production is cooling with water
down to room temperature after hot rolling, then tempering within a
range of 500-600.degree. C. (described as DQ-T in Table 2).
[0052] Table 2 shows the production conditions, plate thicknesses,
and mechanical properties of the steel plates. Further, for D23-D31
and D46-D49, the numbers of fine oxide particles having equivalent
circle diameters of 0.005 to 0.5 .mu.m measured at any location of
the steel plates were additionally described. The number of the
oxide particles are found by preparing an extraction replica from
any portion of the steel plate, observing this under an electron
microscope with X10000 magnification in 100 fields or more (10000
.mu.m.sup.2 or more in observation area), and observing particles
less than 0.1 .mu.m by properly raising the magnification.
Elemental analysis was conducted for each observed particle having
a diameter of 0.005 to 0.5 .mu.m and the oxide particles were
counted. All of the steel plates among D23-D31 and D46-D49 had fine
oxide particles having equivalent circle diameters of 0.01 to 0.5
.mu.m dispersed to 100/mm.sup.2 within the range of the present
invention. Note that it is seen from the comparison of D46 and D47
and D48 and D49 wherein elements other than Si are almost equal,
that the smaller the amount of Si, i.e. 0.20% or less, the larger
the amount of the oxides.
[0053] Each of these steel plates was made to abut against another
steel plate and subjected to vertical one-pass butt welding in
using electro-gas welding (EGW) or electro-slag welding (ESW)
having welding heat inputs of 20 to 100 kJ/mm. Then, in the HAZ
located at the center portion of the plate thickness (t/2), notches
were formed at two locations, that is, the HAZ separated from the
FL (Fusion Line) by 1 mm (HAZ 1 mm) and the FL. A Charpy impact
test was conducted at -40.degree. C. Table 2 shows the welding
conditions and HAZ toughnesses. In the Charpy impact test here, use
was made of JIS No. 4 2 mm V-notch full size test pieces. Further,
Table 2 also describes the former austenite grain size in FL-HAZ 1
mm. The "former austenite grain size in FL-HAZ 1 mm" described here
is the average grain size obtained by measuring the grain size of
the former austenite grains contained within a 2 mm range in the
thickness direction centered by t/2 and the FL-HAZ 1 mm range by
the cross-sectional method. Note that, the measurement was
conducted by using particulate ferrite connected in a form of a net
as the grain boundaries of the former austenite grains.
[0054] D1 to D49 are steels of the present invention. The chemical
compositions of the steels are properly controlled, therefore the
large heat input HAZ toughness at -40.degree. C. is good while
satisfying the predetermined base material performances. Further,
in D23-D31 and D46-D49 obtained by dispersing the fine oxide
particles, the former austenite grain size in FL-HAZ 1 mm becomes
finer than those of the others, i.e. 200 .mu.m or less, and the
large heat input HAZ toughness at -40.degree. C. becomes further
higher. Further, D20 aiming at increasing the refined HAZ structure
by adding B has a better HAZ toughness in comparison with D19
without addition of B and containing addition elements other than B
in the equal amounts and exhibits a higher performance for also the
large heat input HAZ toughness at -40.degree. C.
[0055] On the other hand, the Comparative Steels C1 to C17 do not
contain sufficient Ni for satisfying equation [1] or properly
control the chemical compositions of the steels, so the large heat
input HAZ toughness is insufficient. TABLE-US-00001 TABLE 1 Class
Sym. C Si Mn P S Ni Nb Al Ti N Ca Mg Inv. D1 0.04 0.13 1.31 0.008
0.002 1.6 0.006 0.015 0.008 0.0035 steel D2 0.04 0.17 0.81 0.008
0.002 2.8 0.006 0.015 0.008 0.0035 D3 0.07 0.17 1.40 0.007 0.002
0.9 0.006 0.015 0.007 0.0035 D4 0.07 0.05 0.81 0.006 0.002 2.4
0.006 0.014 0.008 0.0035 D5 0.10 0.19 1.11 0.007 0.002 1.1 0.004
0.015 0.007 0.0032 D6 0.13 0.05 0.91 0.007 0.003 1.2 0.005 0.014
0.006 0.0037 D7 0.06 0.19 1.14 0.007 0.002 1.3 0.005 0.012 0.006
0.0042 D8 0.06 0.22 1.11 0.006 0.003 2.1 0.005 0.014 0.006 0.0035
D9 0.09 0.15 1.21 0.007 0.003 1.4 0.006 0.015 0.006 0.0035 D10 0.12
0.11 1.01 0.007 0.002 1.4 0.005 0.014 0.009 0.0035 D11 0.13 0.14
0.91 0.006 0.002 1.5 0.006 0.015 0.008 0.0035 D12 0.06 0.14 1.41
0.007 0.002 1.6 0.005 0.014 0.008 0.0032 D13 0.06 0.22 1.11 0.007
0.002 2.4 0.005 0.014 0.007 0.0035 D14 0.09 0.21 1.31 0.008 0.002
1.4 0.005 0.014 0.008 0.0035 D15 0.13 0.19 1.01 0.007 0.002 1.6
0.005 0.014 0.007 0.0035 D16 0.12 0.22 0.91 0.007 0.003 2.1 0.006
0.015 0.006 0.0035 D17 0.13 0.17 1.01 0.006 0.002 1.7 0.005 0.014
0.006 0.0032 D18 0.12 0.15 1.11 0.008 0.002 1.5 0.005 0.014 0.006
0.0037 D19 0.06 0.14 1.41 0.007 0.002 1.6 0.005 0.014 0.009 0.0041
D20 0.06 0.14 1.40 0.007 0.002 1.6 0.005 0.014 0.009 0.0041 D21
0.06 0.10 1.21 0.007 0.002 1.7 0.006 0.015 0.010 0.0035 D22 0.06
0.22 1.11 0.007 0.002 1.8 0.005 0.014 0.008 0.0055 D23 0.06 0.16
1.31 0.008 0.004 1.8 0.005 0.014 0.008 0.0035 0.0018 D24 0.06 0.17
1.40 0.008 0.004 1.6 0.005 0.014 0.008 0.0037 0.0019 D25 0.06 0.12
1.20 0.007 0.002 2.1 0.006 0.015 0.007 0.0033 0.0016 D26 0.06 0.15
1.00 0.007 0.002 2.6 0.005 0.014 0.008 0.0036 D27 0.06 0.11 1.21
0.007 0.002 1.8 0.015 0.015 0.007 0.0035 0.0014 D28 0.06 0.23 1.00
0.007 0.002 1.9 0.005 0.014 0.006 0.0035 0.0018 D29 0.06 0.17 1.20
0.008 0.004 1.8 0.005 0.014 0.006 0.0035 0.0036 D30 0.06 0.12 1.20
0.007 0.002 1.7 0.006 0.015 0.006 0.0035 0.0009 D31 0.06 0.12 1.20
0.007 0.002 1.9 0.006 0.015 0.006 0.0035 0.0016 D32 0.07 0.15 1.21
0.007 0.003 1.4 0.028 0.014 0.009 0.0032 0.0015 D33 0.08 0.13 1.21
0.009 0.002 1.3 0.013 0.014 0.010 0.0052 0.0017 D34 0.08 0.19 1.21
0.008 0.003 1.5 0.013 0.025 0.008 0.0076 0.0012 D35 0.08 0.19 1.21
0.010 0.002 1.2 0.021 0.014 0.008 0.0035 Class REM O B Cu Cr Mo V
Ceq Nl/Mn 10 .times. Ceq-3 Judg.* Inv. 0.37 1.22 0.7 .largecircle.
steel 0.36 3.46 0.6 .largecircle. 0.36 0.64 0.6 .largecircle. 0.37
2.96 0.7 .largecircle. 0.36 0.99 0.6 .largecircle. 0.36 1.32 0.6
.largecircle. 0.38 0.92 0.8 .largecircle. 0.39 1.89 0.9
.largecircle. 0.39 1.16 0.9 .largecircle. 0.38 1.39 0.8
.largecircle. 0.38 1.65 0.8 .largecircle. 0.40 1.13 1.0
.largecircle. 0.41 2.16 1.1 .largecircle. 0.40 1.07 1.0
.largecircle. 0.41 1.58 1.1 .largecircle. 0.41 2.31 1.1
.largecircle. 0.41 1.68 1.1 .largecircle. 0.41 1.35 1.1
.largecircle. 0.40 1.13 1.0 .largecircle. 0.0012 0.40 1.14 1.0
.largecircle. 0.4 0.40 1.40 1.0 .largecircle. 0.0023 0.2 0.41 1.62
1.1 .largecircle. 0.0019 0.40 1.37 1.0 .largecircle. 0.0019 0.0008
0.40 1.14 1.0 .largecircle. 0.0017 0.0009 0.40 1.75 1.0
.largecircle. 0.0220 0.0020 0.0011 0.40 2.60 1.0 .largecircle.
0.0017 0.3 0.40 1.49 1.0 .largecircle. 0.0030 0.0009 0.2 0.017 0.40
1.90 1.0 .largecircle. 0.0029 0.0009 0.3 0.40 1.50 1.0
.largecircle. 0.0028 0.0009 0.2 0.05 0.40 1.42 1.0 .largecircle.
0.0023 0.0009 0.2 0.037 0.41 1.58 1.1 .largecircle. 0.0018 0.0012
0.4 0.39 1.16 0.9 .largecircle. 0.0020 0.0009 0.3 0.39 1.07 0.9
.largecircle. 0.0015 0.0035 0.4 0.41 1.24 1.1 .largecircle. 0.36
0.99 0.6 .largecircle. Class Sym. C Si Mn P S Ni Nb Al Ti N Ca Mg
Inv. D36 0.09 0.19 1.21 0.010 0.003 1.5 0.015 0.015 0.008 0.0035
steel D37 0.10 0.19 1.11 0.006 0.002 1.9 0.015 0.035 0.007 0.0035
D38 0.11 0.14 0.91 0.006 0.003 1.7 0.031 0.014 0.008 0.0035 D39
0.12 0.14 1.11 0.008 0.002 1.8 0.029 0.032 0.007 0.0032 D40 0.10
0.14 1.21 0.009 0.002 1.6 0.030 0.014 0.006 0.0037 D41 0.10 0.24
1.21 0.006 0.002 1.5 0.031 0.028 0.006 0.0042 D42 0.08 0.24 1.31
0.005 0.003 1.6 0.032 0.014 0.006 0.0035 D43 0.03 0.22 1.31 0.008
0.003 2.5 0.035 0.038 0.006 0.0035 D44 0.06 0.24 0.81 0.007 0.003
3.2 0.035 0.014 0.009 0.0035 D45 0.03 0.11 1.51 0.007 0.002 1.8
0.035 0.014 0.010 0.0035 D46 0.07 0.11 1.21 0.007 0.003 1.4 0.028
0.014 0.009 0.0032 0.0015 D47 0.07 0.28 1.21 0.007 0.003 1.4 0.028
0.014 0.009 0.0032 0.0015 D48 0.06 0.11 1.20 0.008 0.004 1.8 0.005
0.014 0.006 0.0035 0.0018 D49 0.06 0.28 1.20 0.008 0.004 1.8 0.005
0.014 0.006 0.0035 0.0017 Comp. C1 0.04 0.14 1.90 0.007 0.002 0.1
0.008 0.012 0.008 0.0032 Steel C2 0.04 0.09 1.60 0.006 0.003 0.8
0.009 0.019 0.008 0.0037 C3 0.06 0.09 1.70 0.007 0.003 0.2 0.005
0.012 0.008 0.0042 C4 0.09 0.11 1.60 0.008 0.003 0.0 0.006 0.015
0.007 0.0035 C5 0.10 0.14 1.30 0.008 0.002 0.7 0.005 0.014 0.008
0.0035 C6 0.13 0.22 1.20 0.007 0.002 0.5 0.003 0.012 0.008 0.0035
C7 0.06 0.11 1.90 0.007 0.002 0.1 0.006 0.015 0.008 0.0035 C8 0.06
0.14 1.60 0.007 0.004 0.8 0.005 0.014 0.007 0.0032 C9 0.09 0.17
1.40 0.008 0.002 0.9 0.005 0.014 0.008 0.0037 C10 0.11 0.23 1.30
0.007 0.002 0.8 0.006 0.015 0.007 0.0042 C11 0.06 0.16 2.00 0.008
0.004 0.1 0.005 0.014 0.006 0.0035 C12 0.06 0.11 1.60 0.007 0.002
1.1 0.006 0.015 0.006 0.0035 C13 0.10 0.25 1.70 0.006 0.003 0.2
0.006 0.015 0.006 0.0035 C14 0.12 0.14 1.40 0.007 0.002 0.7 0.005
0.014 0.006 0.0035 C15 0.09 0.12 1.60 0.008 0.003 0.8 0.015 0.013
0.009 0.0032 C16 0.08 0.24 1.50 0.008 0.003 0.6 0.035 0.013 0.010
0.0037 C17 0.09 0.14 1.20 0.007 0.002 1.4 0.045 0.014 0.008 0.0042
Class REM O B Cu Cr Mo V Ceq Ni/Mn 10 .times. Ceq-3 Judg.* Inv.
0.03 0.40 1.24 1.0 .largecircle. steel 0.41 1.71 1.1 .largecircle.
0.045 0.38 1.87 0.8 .largecircle. 0.43 1.62 1.3 .largecircle. 0.2
0.42 1.32 1.2 .largecircle. 0.40 1.24 1.0 .largecircle. 0.41 1.22
1.1 .largecircle. 0.42 1.91 1.2 .largecircle. 0.41 3.95 1.1
.largecircle. 0.40 1.19 1.0 .largecircle. 0.0018 0.4 0.39 1.16 0.9
.largecircle. 0.0018 0.4 0.39 1.16 0.9 .largecircle. 0.0026 0.0012
0.3 0.40 1.50 1.0 .largecircle. 0.0026 0.0012 0.3 0.40 1.50 1.0
.largecircle. Comp. 0.36 0.05 0.6 X steel 0.36 0.50 0.6 X 0.36 0.12
0.6 X 0.36 0.00 0.6 X 0.36 0.54 0.6 X 0.36 0.42 0.6 X 0.38 0.05 0.8
X 0.38 0.50 0.8 X 0.38 0.64 0.8 X 0.38 0.62 0.8 X 0.40 0.05 1.0 X
0.40 0.69 1.0 X 0.40 0.12 1.0 X 0.40 0.50 1.0 X 0.41 0.50 1.1 X
0.37 0.40 0.7 X 0.38 1.17 0.8 .largecircle. *.largecircle. is
described where Ni/Mn .gtoreq. 10 .times. Ceq-3 is satisfied, and X
is described where it is not satisfied.
[0056] TABLE-US-00002 TABLE 2 Plate Base material (t/2).sup.1)
Number.sup.2) Produc- thick- Tensile Yield of oxide tion ness
strength stress particles Class Symbol method (mm) (Mpa) (Mpa)
vE.sub.-40(J) (per mm.sup.2) Inv. D1 TMCP 60 576 476 231 steel D2
TMCP 65 565 465 229 D3 DQ-T 70 576 456 225 D4 TMCP 60 576 476 231
D5 DQ-T 55 605 485 238 D6 TMCP 65 565 465 229 D7 TMCP 70 560 460
219 D8 TMCP 80 541 441 213 D9 DQ-T 60 601 481 225 D10 TMCP 65 570
470 223 D11 TMCP 75 550 450 216 D12 TMCP 80 545 445 208 D13 TMCP 55
596 496 224 D14 DQ-T 65 595 475 217 D15 TMCP 70 566 466 213 D16
TMCP 65 578 478 214 D17 DQ-T 70 588 468 211 D18 TMCP 75 556 456 210
D19 TMCP 70 565 465 214 D20 TMCP 70 575 482 214 D21 DQ-T 70 585 465
214 D22 TMCP 70 566 466 213 D23 TMCP 65 575 475 218 900 D24 DQ-T 60
605 485 221 1200 D25 TMCP 70 565 465 214 1300 D26 TMCP 80 545 445
209 1100 D27 TMCP 70 565 465 214 900 D28 TMCP 65 574 474 218 1800
D29 TMCP 60 585 485 221 2100 D30 DQ-T 65 594 474 218 2400 D31 TMCP
60 587 487 219 1900 D32 TMCP 70 563 463 217 700 D33 TMCP 65 572 472
221 600 D34 DQ-T 80 567 447 207 1400 D35 TMCP 70 555 455 225
one-pass butt welding condition.sup.3) Average .gamma. grain HAZ
toughness.sup.5) Welding Heat input size in FL-HAZ 1 FL/vE.sub.-40
FL + 1 mm/ Class method (kJ/mm) mm (.mu.m).sup.4) (J) vE.sub.-40(J)
Inv. EGW 39 480 140 128 steel EGW 42 520 135 124 ESW 85 770 116 106
ESW 73 660 123 113 ESW 67 605 127 117 EGW 42 520 135 124 ESW 85 770
116 106 EGW 51 640 124 114 EGW 39 480 140 128 ESW 79 715 119 109
EGW 48 600 128 117 EGW 51 640 124 114 EGW 35 440 144 132 ESW 79 715
119 109 ESW 85 770 116 106 EGW 42 520 135 124 EGW 45 560 131 120
EGW 48 600 128 117 EGW 45 560 131 120 EGW 45 560 171 156 ESW 85 770
116 106 EGW 45 560 184 180 ESW 79 180 207 189 EGW 39 165 214 196
EGW 45 152 221 203 EGW 51 185 204 187 ESW 45 180 207 189 ESW 79 167
213 195 EGW 39 184 205 188 EGW 42 165 214 196 EGW 39 184 205 188
EGW 45 180 207 189 EGW 42 164 214 197 ESW 98 180 196 180 ESW 85 660
123 113 Plate Base material (t/2).sup.1) Number.sup.2) Produc-
thick- Tensile Yield of oxide tion ness strength stress particles
Class Symbol method (mm) (Mpa) (Mpa) vE.sub.-40(J) (per mm.sup.2)
Inv. D36 DQ-T 70 584 464 215 steel D37 TMCP 65 578 478 214 D38 TMCP
60 581 481 226 D39 TMCP 70 571 471 208 D40 DQ-T 80 570 450 203 D41
TMCP 70 565 465 214 D42 TMCP 65 576 476 216 D43 TMCP 60 589 489 217
D44 TMCP 65 577 477 215 D45 DQ-T 60 605 485 221 D46 TMCP 70 553 465
217 900 D47 TMCP 70 579 481 217 400 D48 TMCP 60 578 485 221 2300
D49 TMCP 60 592 485 221 1500 Comp. C1 TMCP 70 556 456 225 steel. C2
DQ-T 60 595 475 233 C3 TMCP 75 544 444 224 C4 TMCP 60 574 474 234
C5 TMCP 60 576 476 232 C6 TMCP 55 586 486 236 C7 DQ-T 60 601 481
226 C8 TMCP 60 580 480 227 C9 TMCP 60 581 481 226 C10 TMCP 60 580
480 227 C11 DQ-T 70 585 465 214 C12 TMCP 60 585 485 221 C13 TMCP 70
564 464 215 C14 TMCP 65 575 475 218 C15 TMCP 55 598 498 222 C16
DQ-T 65 588 468 226 C17 TMCP 60 581 481 226 one-pass butt welding
condition.sup.3) Average .gamma. grain HAZ toughness.sup.5) Welding
Heat input size in FL-HAZ 1 FL/vE.sub.-40 FL + 1 mm/ Class method
(kJ/mm) mm (.mu.m).sup.4) (J) vE.sub.-40(J) Inv. ESW 85 605 127 117
steel EGW 42 520 135 124 ESW 73 770 116 106 EGW 45 640 124 114 EGW
51 480 140 128 ESW 85 715 119 109 EGW 42 600 128 117 EGW 39 640 124
114 EGW 42 440 144 132 ESW 73 715 119 109 EGW 45 145 225 206 EGW 45
195 200 183 EGW 39 164 214 236 EGW 39 185 204 225 Comp. ESW 85 770
36 25 steel EGW 39 480 57 40 EGW 48 600 46 32 EGW 39 480 57 40 EGW
39 480 57 40 ESW 67 605 45 32 EGW 39 480 57 40 EGW 39 480 57 40 EGW
39 480 57 40 EGW 39 480 57 40 ESW 85 770 36 25 EGW 39 480 57 40 EGW
45 560 49 34 EGW 42 520 53 37 ESW 67 605 45 32 EGW 42 502 53 37 EGW
39 480 57 40 .sup.1)Plate thickness center position; YS and TS are
average values of two test pieces; Charpy absorption energy at
-40.degree. C. (vE-40) is an average value of three test pieces.
.sup.2)Extraction replica was prepared from any portion of steel
plate, and observed under electron microscope by .times.10000
magnification in 100 fields or more (10000 .mu.m.sup.2 or more in
observation area). However, particles of less than 0.1 .mu.m were
obtained at properly raising magnification. # Particles are counted
including oxide by elemental analysis among particles of equivalent
circle diameter of 0.005 to 0.5 .mu.m and convert to number per 1
mm.sup.2. .sup.3)EGW: Electro-gas welding; ESW: Electro-slag
welding; Welding heat input is an average value in total length of
welding; Common welding material is used in welding processes
.sup.4)Average grain size of former austenite grains contained
within a 2 mm range in the thickness direction centered by t/2 and
the FL-HAZ 1 mm range was measured by the cross-sectional method.
Measurement was conducted by using particulate ferrite connected in
a form of a net as the grain boundaries of the former austenite
grains. .sup.5)FL notches are layed down so as to equally divide WM
and HAZ. vE - 40- at each notch location is an average value of
three test pieces. As described above, the present invention
provides thick steel plate satisfying the excellent toughness
demands regarding destruction for ships, offshore structures,
medium/high rise buildings, etc.
* * * * *