Note: Descriptions are shown in the official language in which they were submitted.
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NSC-S898
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DESCRIPTION
STEEL EXCELLENT IN TOUGHNESS OF WELD HEAT AFFECTED ZONE
TECHNICAL FIELD
The present invention relates to steel excellent in
toughness of the weld heat affected zone (HAZ) in small
heat input welding to medium heat input welding and a
method of production thereof.
BACKGROUND ART
The HAZ toughness of a low alloy steel is governed
by various factors such as (1) the size of the crystal
grains, (2) the state of dispersion of hard phases such
as high-carbon martensite (M*), upper bainite (Bu), and
ferrite sideplate (FSP), (3) the state of precipitation
hardening, (4) the presence of any intergranular
embrittlement, and (5) the microsegregation of the
elements. These factors are known to have a large effect
on the toughness. Many technologies are being
commercialized in order to improve the HAZ toughness.
It is safe to say that such toughness inhibiting
factors are caused by additive elements. Reduction of the
alloy element content increases the toughness. However,
higher strength is always being sought in structural
steel. Because of that, the addition of alloy elements is
necessary. That is, the demands of strength and toughness
are contradictory from the viewpoint of the alloy element
content. Toughness increasing technology which does not
depend on alloy elements has been sought.
As particularly excellent technology, it is known to
use steel which does not substantially include any Al so
as to make the microstructure finer and in addition
correctly balance the Ti, 0, and N to suppress the
precipitation of TiC and reduce precipitation hardening
and thereby improve the toughness (Japanese Patent
Publication (A) No. 5-247531). In this case, the
toughness of the weld heat affected zone is determined by
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the balance of the effects of the microstructure and the
effects of the hardened layer which includes M*. In the
prior art, this was solved by improving the toughness of
the base material matrix by Ni and the like. However, the
addition of large amounts of the Cu, Ni, and other
expensive alloy elements necessary for the realization of
this technology invited an increase in the production
costs. This became an obstacle in producing high strength
steel excellent in CTOD property.
The point of the steel according to this invention
not substantially including any Al and Nb is made use of
in the present invention as well. However, in this
invention, the C content is high, so the problem of the
drop in toughness when increasing the Mn content remains
unsolved. Further, there was a concern over the
impurities Nb and V having a detrimental effect on the
toughness.
Further, Japanese Patent Publication (A) No. 2003-
147484 follows the thinking of Japanese Patent
Publication (A) No. No. 5-247531 and, while making use of
Ti oxides, adds Nb and raises the Mn content. This causes
the austenite-ferrite transformation start temperature to
drop to thereby suppress the formation of the hard phases
and simultaneously to obtain a suitable microstructure to
thereby satisfy the -10 C CTOD property. However, the
invention of this Japanese Patent Publication (A) No.
2003-147484 did not sufficiently satisfy the required
CTOD property of weld joints at the much tougher level of
-40 C or less.
DISCLOSURE OF THE INVENTION
The present invention provides technology which
inexpensively produces high strength steel excellent in
toughness in multi-layer welding of small to medium heat
input. The steel produced by the present invention is
extremely good in the CTOD property of multi-layer weld
zones of small to medium heat input among the levels of
weld heat affected zone toughness. The gist of the
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present invention is as follows.
(1) A steel excellent in toughness of a weld heat
affected zone characterized by containing, by mass%, C:
0.02 to 0.06%, Si: 0.05 to 0.30%, Mn: 1.7 to 2.7%, P:
0.015% or less, S: 0.010% or less, Ti: 0.005 to 0.015%,
0: 0.0010 to 0.0045, and N: 0.0020 to 0.0060% and
comprising a balance of iron and unavoidable impurities,
having an amount of intermixture of impurities limited to
Al: 0.004% or less, Nb: 0.003% or less, and V: 0.030% or
less, and having a CeH represented by formula (A) in the
range of 0.04 or less:
CeH = C+1/4Si-
1/24Mn+1/48Cu+1/32Ni+1/0.4Nb+1/2V...(A)
where, C, Si, Mn, Cu, Ni, Nb, and V show steel
compositions (mass%).
(2) A steel excellent in toughness of a weld heat
affected zone as set forth in (1), characterized in that
the CeH is in the range of 0.01 or less.
(3) A steel excellent in toughness of a weld heat
affected zone as set forth in (1) or (2), characterized
by further containing, by mass%, one type or two types of
Cu: 0.25% or less and Ni: 0.50% or less.
(4) A method of production of steel excellent in
toughness of a weld heat affected zone characterized by
heating a slab satisfying the steel ingredients and CeH
of (1) or (g) to a temperature of 1100 C or less, then
treating it by thermo-mechanical control process.
(5) A method of production of steel excellent in
toughness of a weld heat affected zone characterized by
heating a slab satisfying the steel ingredients and CeH
of (3) to a temperature of 1100 C or less, then treating
it by thermo-mechanical control process.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view showing the relationship of a
cooling time of 800 to 500 C and an M* fraction.
FIG. 2 is a view showing the relationship of the CeH
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and CTOD properties.
BEST MODE FOR CARRYING OUT THE INVENTION
According to the research of the present inventors,
the CTOD property of the HAZ at the time of small to
medium heating input (1.5 to 6.0 kJ/mm with a sheet
thickness of 50 mm) welding (CTOD property at temperature
of -40 C or less) is governed by the toughness of
extremely local regions. Control of the microstructure of
this portion and reduction of the embrittlement elements
are important. In other words, the CTOD property is not
the average property of the material, but is governed by
the local embrittlement zones. If there are regions which
cause embrittlement, even in just parts of the steel
material, the CTOD property of the steel sheet will be
remarkably impaired.
Specifically, the local regions which exert the
greatest effects on the CTOD property are the M*, ferrite
sideplate (FSP), and other hard phases. In order to
suppress the formation of this kind of hard phase, in the
past it had been necessary to keep the hardenability of
the steel low. This became a factor inhibiting higher
strength.
The present invention is characterized by the
following discoveries and their embodiment in a steel of
a high HAZ toughness. Specifically,
1) In a small to medium heat input welded HAZ,
generally the cooling time after welding is within 60
seconds. The inventors discovered that under such cooling
conditions, if the C content is sufficiently low, by
adequately controlling other embrittlement elements, even
if adding Mn to 27%, the M* which exerts a negative
effect on toughness is no longer formed. FIG. 1 shows the
M* fraction when changing the amount of.Mn from 1.7% to
2.7% with 0.05%C-0.15% Si. It is learned that even if the
Mn content changes, if the cooling time of 800 to 500 C is
within 60 seconds or so, the M* fraction becomes very
small. As a result, it becomes possible to raise the
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content of Mn for which addition in a large quantity had
been thought to be impossible in the past due to causing
deterioration of the toughness.
2) The inventors discovered that the steel
ingredients could be made suitable in an Al-less based
steel.
3) The inventors eliminated the unexpected factors
reducing toughness by limiting the Al, Nb, and V present
as impurities in the steel to certain limits or less.
That is, by employing Al-less based steel, it became
possible to reliably form TiO and effectively improve the
toughness.
By combining these three points, it became possible
to realize a good CTOD property under difficult
temperature conditions of -20 C or less in a small to
medium heat input welded HAZ which could not be achieved
until now.
Even when very little M* is formed, control of the
embrittlement elements C, Si, Cu, Ni, Nb, V, and the like
is essential. Specifically, it is essential to control
the value (CeH) of C+1/4Si-
1/24Mn+1/48Cu+1/32Ni+1/0.4Nb+1/2V to a predetermined
range.
FIG. 2 shows the results when producing 20 kg of
steel of the steel ingredients of 0.05% C-0.15aSi-1.7 to
2.7%Mn by vacuum melting, rolling it to steel sheet,
imparting a heat history of an actual welded joint three
times by a simulated thermal cycle device, then running a
CTOD test.
TSc 0.1 (670.9 CeH-67.6) is the temperature when the
lowest value of three CTOD test values at different test
temperatures is 0.1 mm. There is a clear trend for the TSc
0.1 (CTOD property) to excellent substantially linearly
as the CeH drops. If the CeH drops to around 0.01, it is
learned that the T8c 0.1 reaches -60 C.
That is, by satisfying the requirements of the
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present invention steel and controlling the CeH, the
intended CTOD property can be obtained. With the present
invention steel, control of the value of CeH according to
the required CTOD property is one of the characterizing
features of the invention. In addition to the control of
the value of CeH, rectifying the contents of the other
alloy elements is required for realizing steel provided
with both high strength and a superior CTOD property.
Below, the ranges of limitation and the reasons will be
explained.
C has to be 0.02% or more in order to obtain
strength, but if over 0.06%, it degrades the toughness of
the welding HAZ and does not allow satisfaction of a good
CTOD property, so 0.06% is made the upper limit.
Si inhibits the HAZ toughness, so a smaller amount
is preferable in order to obtain a good HAZ toughness.
However, with the invention steel, no Al is added, so
addition of 0.05% or more is necessary for deoxidation.
However, if the content is over 0.30%, the HAZ toughness
is harmed, so 0.30% is made the upper limit.
Mn is an inexpensive element with a large effect of
rectifying the microstructure and lowers the CeH, so
addition does not harm the HAZ toughness of small to
medium heat input, therefore it is desirable to make the
content large to and obtain a high strength. However, if
over 2.7%, it promotes the segregation of the slab and
facilitates formation of Bu harmful to toughness, so the
content was made to an upper limit of 2.7%. Further, if
less than 1.7%, the effect is small, so the lower limit
was made 1.7%. Note that from the viewpoint of toughness,
over 2.0% is more preferable.
P and S should both be small in amount from the
viewpoints of base material toughness and HAZ toughness,
but there are limits to their reduction in industrial
production. 0.015% and 0.010%, preferably 0.008% and
0.005%, were therefore made the upper limits.
Al is not deliberately added in the present
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invention, but inclusion as an impurity in the steel is
unavoidable. This forms Al oxides which inhibit the
formation of Ti oxides, so a smaller content is
desirable, but there are limits to its reduction in
industrial production. 0.004% is therefore the upper
limit.
Ti forms Ti oxides and makes the microstructure
finer, so greatly contributes to improvement of the
toughness, but if the content is too great, it forms TiC.
This degrades the HAZ toughness, so 0.005 to 0.015% is a
suitable range.
0 is necessary for the formation of a large amount
of oxides of Ti. If less than 0.0010%, the effect is
small, while if over 0.0045%, it forms coarse Ti oxides
and sharply degrades the toughness, so the range of
content was made 0.0010 to 0.0045%.
N is necessary to form fine Ti nitrides and improve
the base material toughness and HAZ toughness, but if
less than 0.002%, the effect is small, while if over
0.006%, surface defects are formed at the time of billet
production, so the upper limit was made 0.006%.
Further, Nb and V are inherently embrittlement
elements. As shown by the large coefficient in formula
(A), their presence causes the CeH to greatly rise and
made the HAZ toughness remarkably fall, so these are not
deliberately added in the present invention. Even when
included as impurities in the steel, to secure toughness,
Nb has to be limited to 0.003% or less. Further, V has to
be limited to 0.030% or less, preferably 0.020% or less.
Cu and Ni result in little deterioration of the HAZ
toughness due to their addition, have the effect of
increasing the strength of the base material, and are
effective for the further improvement of the properties,
but increase the production costs, so the upper limits of
the contents when added were made Cu: 0.25% and Ni:
0.500.
Even if limiting the ingredients of the steel in the
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above way, if not forming a suitable structure by a
suitable method of production, the desired effects cannot
be exhibited. Due to this, the production conditions
also have to be considered.
The present invention steel is preferably produced
industrially by continuous casting. The reasons are that
the solidification cooling rate of the molten steel is
fast and it is possible to form fine Ti oxides and Ti
nitrides in large amounts in the slab. When rolling the
slab, the reheating temperature has to be made 1100 C or
less. If the reheating temperature exceeds 1100 C, the Ti
nitrides becomes coarser, the toughness of the base
material decreases, and the effect of improvement of the
HAZ toughness cannot be expected.
Next, the method of production after reheating
requires treatment by thermo-mechanical control process.
The reason is that even if a superior HAZ toughness is
obtained, if the toughness of the base material is
inferior, the steel product is insufficient. As methods
of treatment by thermo-mechanical control process, 1)
controlled rolling, 2) control rolling-accelerated
cooling, 3) direct quenching-tempering after rolling,
etc. can be mentioned, but the preferred methods are
controlled rolling-accelerated cooling and the direct
quenching-tempering after rolling.
Note that after producing the steel, even if
reheating to a temperature of the Ar3 transformation
point or more for the purpose of dehydrogenation etc.,
the characterizing features of the present invention are
not impaired.
Further, the above method is one example of a method
of production of the present invention steel. The method
of production of the present invention steel is not
limited to the above method.
EXAMPLES
Thick-gauge steel sheet of various steel ingredients
were produced by the converter-continuous casting-thick-
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gauge sheet process. The base material strength was
determined and a CTOD test of the weld joints was run.
The welding was performed by the submerged arc welding
(SAW) method, generally used for test welding, with a
welding heat input of 4.5 to 5.0 kJ/mm at the K groove so
that the weld fusion line (FL) became perpendicular. The
CTOD test was run by a sheet of a size of t (sheet
thickness) x 2t notched by introducing a 50% fatigue
crack in the FL location. Table 1 shows examples of the
present invention and comparative examples.
The steel sheet produced by the present invention
(Invention Steels 1 to 20) had yield strengths (YS) of
430 N/mm2 or more and exhibited good breaking toughness of
CTOD values at -20 C, -40 C, and -60 C all of 0.27 mm or
more.
As opposed to this, Comparative Steels 21 to 26 had
strengths and CTOD values inferior to the invention
steels and did not possess the properties necessary as
steel sheet used under harsh environments. Comparative
Steel 21 had Nb added, therefore the Nb content of the
steel sheet became too great. The value of CeH also
became high, so the CTOD value was a low value.
Comparative Steel 22 had too great a C content and also
too great a value of CeH, so the CTOD value was a low
value. Comparative Steels 23 and 24 had low CeH's, but
the Al content was too high, Ti oxides were
insufficiently formed, and the microstructure was not
sufficiently made finer. Comparative Steel 25 had a CeH
of about the same extent as the invention steel, but the
C was too low and the 0 was too great, so the base
material strength was low and the CTOD value was a low
value. Comparative Steel 26 had an excessively large
amount of Nb mixed in as an impurity, so despite CeH
being low, the base material strength and CTOD value were
low values.
CA 02602076 2007-08-30
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CA 02602076 2007-08-30
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CA 02602076 2007-08-30
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INDUSTRIAL APPLICABILITY
The steel produced by the present invention is high
in strength, has an extremely good CTOD property of the
FL part where the toughness degrades the most at the time
of welding, and exhibits superior toughness. Due to this,
production of a high strength steel product that can be
used in offshore structures, earthquake resistant
buildings, and other harsh environments became possible.
