Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF ~HE INVEN~ION
FI~LD OF ~HE INVENTION
.. .. ..
The present invention relates to steels which are
useful in fabricating pressure vessels, and more particularly
it relates to improved chromium-molybdenum type steels which
can be formed into thick plates that are weldable together to
make chemical pressure vessels.
~HE PRIOR AR~
Chemical pressure vessels, such as pressure vessels
used as oil-refining reactors, must be fabricated from materials
which excel in high temperature strength, creep strength and
hydrogen-attach resistivity.~ The most widely used materials
; have been 14Cr-0.5Mo type steels, whlch are standarized steels
as discussed, for example, ln AS~M A387 G11.
15In recent years, however, pressure vessels have been
growing larger and larger in size, thus requiring the use of
steel plates having increasing thlcknesses (e.g. of 100 mm or
more). Conventional 14Cr-0.5Mo type steels, which are generally
produced using normalizing and tempering treatments, have not
produced completely satisfactory steel plates having such great
thicknesses. ~his lS due to the fact that~as the plate thick-
nesses increase, the steel plate cooling speed is reduced and
the ferrlte content will lncrease during the normalizlng
treatment. ~he temperature of the stress relief annealing after
welding or post weld heat treatment (hereinafter abbreviated as
PWH~)`thus must be elevated and the treatment duration lengthened~
As a result, the strength of the~produced steel will be drastical-
ly lowered~ Consequently, the produced steel plates will often
times not have the required strengths for use in making pressure
3o Ivessel . ~t the s me time, howev~ , increasing the content of
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alloying elements such as chromium and molybdenum will not
solve the problem because the weldability of the steel
plates will be reduced.
In addition, it has recently been discovered that the
occurrence of creep damage in pressure vessels in welded
heat affected ZOlle (hereinafter abbreviated as HAZ),
accompanied by stress concentration, has become a serious
problem.
Investigations on the creep damage have revealed that
at normal operation temperatures of pressure vessels
(400 - 550C), the hardened zones suffer from much
degradation of grain-boundary strength, and that owing to
the concentration of strain in the grain boundaries, cracks
develop at the boundaries which grow and mature into rup-
tures. The most effective way to prevent creep ruptur~s
is to enhance the strength of the base metal and to soften
the HAZ by applying a high-temperature PWHT (6S0 - 720C).
With respect to enhancing the strength of the base metal,
this can be achieved by lowering the phosphorus content.
However, drastically decreasing the phosphorus content
(e.g., to less than 0.010%) entails huge cost increases
because a special treatment step in the steelmaking
operation becomes necessary.
One proposal for solving the noted problems is
disclosed in JA-OS 41962/1980 (laid open for public
inspection on March 25, lg80). In this proposal, the
- employed Cr-Mo steels have aluminum and boron or boron
and titanium added thereto. However, this proposal is
basically directed to two ~Cr-lMo type steels which
suffer from severe embrittlement from tempering treatment,
and it is also directed to preventing ~his embrittlement
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~7~1 ~Y17
by lowering the silicon and manganese contents. When
this prior art proposal for improving steel proper~ies is
applied to ~Cr-0.5Mo steel plates which have ~hicknesses
exceeding, for example, 50 mm, the steel plates will have
insufficient strengths and toughnesses because of the low
manganese contents. Indeed, because the silicon contents
are kept low, the chromium and molybdenum contents must
be increased to achieve the necessary s~rengths. This
detrimentally increases the cost of production, and the
produced steels still lack the extra strength needed for
withstanding the high temperature PWHT and do not show
improved creep damage resistivity.
It is thus an object of the present invention to
provide improved Cr-Mo type steels which can be formed
into extremely thick plates for use in fabricating
pressure vessels, which plates will have increased
strengths (even after high temperature PWHT) and
enhanced creep damage resistivity, yet will not have
reduced weldability or increased cost.
SUMMARY OF THE INVENTION
We, the present inventors, have found that Cr-Mo type
steels, and especially 1 ~r~0.5Mo steels, will display
all of the required properties when 0.00015 to 0.0015% of
boron and 0.01 to 0.100% of Sol. Al have been incorporated
therein after addition of at least 0.46~ of Mn. We have
found that the addition of this specific amount o Sol. A1
and the noted minute amount of boron to the Cr-Mo steel
curbs the ferrite transformation during the normalizing
treatment, gives rise to a bainite structure and enhances
the steells strength. We have found that when the cooling
speed is lowered, the excess amount of boron is precipi-
tated in the austenite grain boundary which promotes the
ferrite transformation and lowers the strength. Therefore,
the very minute boron addition proves advantageous in
enhancing hardenability during normal.izing.
Thus, according to one aspect of the invention there
is provided a high strength steel which also displays
high hydrogen attack resistivity, good high-temperature
ductility and high creep rupture strength, said steel
being useful in fabricating pressure vessels, said steel
consisting essentially of 0.11 to 0.17% of C, 0.50 to
1.20~ of Si, 0.46 to 1.00% of Mn, 1.00 to 1.50% of Cr,
0.45 to 0.65% of Mo, C.00015 to 0.0015% of B, 0.010 to
0.100~ of Sol, Al, 0 to 0.006% of N as an impurity and 0
to 0.010% of P as an impurity, said percentages being by
weight, and the balance of Fe.
According to another aspect of the invention there
is provided a steel plate being useful in fabricating
pressure vessels, said steel plate having essentially the
composition of 0.11 to 0.17% of C, 0.50 to 1.20% of Si,
0.46 to 1.00% of Mn, 1.00 to 1.50% of Cr, 0.45 to 0.65% of
Mo, 0.00015 to 0.0015~ of B, 0.010 to 0.100% of Sol. Al,
0 to 0.006% of N as an impurity and 0 to 0.010% of P as
an impurity, said percentages being by weight, and the
balance of Fe and being used after a post weld heat
treatment at a temperature within the range of 650 to
720C.
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97
BRIEF DESCRIPTION OF TH~ DRAWINGS
..
Figure 1 is a diagram showing a heat pattern of "step
cooling" that is an accelerating process to learn a temper
embrittlement for short time, and
Figure 2 is a diagram showing the relationship of
tempering parameter and mechanical property between the con~
ventional steel and the steel of the invention.
DETAILED DESCRIPTION OF THE INVENTION
The inventors have discovered that a steel composition
containing 0.11 to 0.17% of C~ 0.50 to 1.20% of Si9 0.46 to 1.00%
of Mn, 1.00 to 1.50% of Cr, 0.45 to 0.65% of Mo, 0.00015 to
0.0015% of B, 0.010 to 0.100% Of Sol. Al and the balance-(to make
up 100%) of Fe (together with impurities inevitably entrained by
the components of the compositlon) gives highly favorable results.
A further explanation of the reason~s behind the required ranges
noted for the carbon, silicon, manganese, chromium, polybdenum,
boron and Sol. aluminum are as follows.
Carbon is a primary element for achieving the necessary
product strength. To fulfil this purpose, carbon must be added
in an amount of not less than 0.11%. If the carbon content
exceeds 0.17%~ however, the excess carbon results ln reduced
toughness and weldability. Thus, the carbon content must be
from 0.11 to 0.17%.
Silicon is necessary, if not solely9 for deoxidation.
With the steel of the present invention, an important purpose
of the use of silicon is to help ~the base metal acquire strength
both at room temperature and at e evated temperatures. For this
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purpose, the silicon must be added in an amount of not less than
On50k~ If the silicon content exceeds 1.20%~ however, the excess
silicon causes serious degradation in product toughness. Thus,
the silicon content must be from 0.50 to 1.20%. ~urther, since
the toughness of the mother matexial will be reduced to a slight
extent when the silicon content exceeds 0.70/0, the preferred
silicon content is from 0.50 to 0.70%. ~his amount will be used
in steels in which the product toughness will b-e of greatest
importance.
In order to have &n enhanced hardenability during the
normalizing treatment &nd in order to help very thick steel plates
to acquire strength and toughness, m&nganese must be added in &n
amount of no-t less th&n 0.46%. If the mang&nese content exceeds
1.00%, however, the excess mang&nese degrades the product weld-
ability. ~hus, the m&ng&nese content must be from 0.46 to 1.00%.
Molybdenum is necessary for heightening the product
hardenability during the normalizing treatment &nd for enh&ncing
its strength both at room temperature &nd at elevated tempera-
tures. For this purpose, molybdenum must be added in &n amount
of not less th&n 0.45%. If the molybdenum conten-t exceeds 0.65%,
however, the excess molybdenum adds to the product's strength
but causes a loss in its toughness. ~hus, the molybdenum content
must be from 0.45 to 0.65%.
Chromium serves to heighten the product's hardenability
~25 during the normalizing treatment and to enhance hydrogen attack
resistivity. To achieve these results, the chromium must be
added in &n amount of not less th&n 1.00%. If the chromium
content exceeds 1.50%, however, the excess chromium acts to
degrade weldability (also, 1. 50% is the upper limit specified
by the AS~M standard). ~hus, the chromium content must be from
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117~ 697
.00 to ~.~0/o. ~urther, since hydrogen attach resistivity
increases in direct proportion to an increase in the chromium
content (possible dispersion of chromium content under normal
production conditions being also be taken into account), the
preferred chromium content is from 1.25 to 1. 50%.
Boron is capable of enhancing the product's hardenabili-
ty during the normalizing treatment as described above. To help
the steel of this invention acquire the micro-structure consisting
essentially of baitnite, boron must be added in an amount of not
less than 0.00015%. If the boron content exceeds 0.0015%~ however
the excess boron heightens susceptibility to PWH~ cracking,
induces precipitation of ferrite during the normalizing treatment
and degrades strength. ~hus, the boron content is from 0.00015
to 0.0015%. Further, to ensure the effect of boron upon the
hardenability and lessen possible dispersion of strength, boron
is preferably added in an amount of not less than 0~000~/0. When
the boron content exceeds 0O0009%~ the excess boron acts to
impair the toughness of the heat affected zone. ~herefore, boron
is preferably used within the range of from 0.0002 to 0.0009%.
~0 ~he Sol. Al is necessary for fixing nitrogen in the
form of AlN and for ensuring thorough deoxidation of the steel.
To fulfil this purpose, the Sol. Al must be added in an amount
of not less than 0.010%. If the So. Al content exceeds 0.100%,
however, the excess Sol. Al acts to impair hot workability and
causes sùrface cracks on the steel slabs. ~hus, the Sol. Al
content must be from 0.010 to 0.100%. ~0 ensure the possitive
effects of boron, the Sol. Al is preferably added in an amount
of not less than 0.035%. If the~Sol. Al content exceeds 0.065%,
however, the excess Sol. Al acts to degrade the toughness of HAZ.
3Q ~hus, the preferred Sol. Al content is from 0~035 to 0~065%.
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~` 1171697
~itrogen, being one of the inevi-tably entrained im-
purities, is generally contained in an amount of not more than
0.010~. If the nitrogen content exceeds 0~0060%, the eventual
AlN content increases sufficiently to degrade hot workability.
Thus, the nitrogen content is preferably kept below 000060%.
Phosphorus, another of the ine~ritably entrained
impurities, segregates itself in the austenite grain boundary
and impairs ductility at elevated temperatures. ~o preclude
the degradation of ductility, prevent creep damage and enhance
high-temperature ductility and creep duc-tility, the phosphorus
content is preferably not more than 0.010%~
~he heat treatment involved in the production of thick
plates of steel according to the present invention generally
comprises a normalizing step (for example, at 900 to 970 C)
and a tempering step (for example, at 650- to 750 C for 0.5 to
10-odd hours).
In situations where the nitrogen content is high (for
example, when it exceeds 0.0060%), it is desirable to take any
one of the following processes in the course of production for
the purpose of ensuring the desired effects of boron and in
securing the necessary strength. ~
(1) ~he heating temperature of slabs for the plate
rolling is below 1150 C; ~ ~
(2) After the rolllng step, the steel plate is
gradually cooled at a rate of no-t more than 30 C/hour;
- (3) ~he normalizlng step is performed twice, wlth
the first normalizing operation performed at a higher
temperature (950 to 1000 C) and the last one at a
regular temperature ~(900 to~970 C).
In order to enhance the hardening effect of boron,
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.,
it is necessary for the nitrogen to be fixed (or com-
pounded) with aluminum in the form of Al~ and ~or the
boron to be converted to free boron prior to normalizing.
Since both boron and nitrogen are readily segregated in
the austenite grain boundary and conversely aluminum is
segregated very little, boron and nitrogen which are in
a non-equilibrium state tend to be precipitated, partic-
ularly when the nitrogen content is high. Further, when
the slab heating temperature is above 1150C, Al, B and N
dissolve into the steel during the slab heating process
and BN is easy to be precipitated in proportion to fall of
the temperature during and after the plate rolling. The
reaction of BN + Al + B + AlN can be completed by proceed-
ing it at 1000C for 30 minutes or a gradual cooling
(lower than 30C/hr.). Therefore, when N content exceeds
0.0060%, by performing any one process of heating the slab
at below 1150C, gradual cooling after rolling and high
temperature normalizing (900 to 1000C) prior to the
regular temperature normali~ing (900 to 970C), nitrogen
is fixed with aluminum to be in the form of AlN and boron
is converted to free boron so that the desired hardening
effect of boron can be obtained.
In a pressure vessel made of the steels of this
invention, the hardened portion ln the HAZ can be soft-
ened and the creep damage resistivity can be improved by
performing a high-temperature PWHT. It is, therefore,
desirable to give a high-temperature PWHT (650~ - 720C)
to such pressure vessels.
The steels o~ this invention use chromium in an amount not
exceeding the specified upper limit to improve the hydrogen
attack reslstivity and permit a decrease in the silicon content
proportionately to the increase of strength obtained by the
chromium. They further use Mn aluminum and boron in the
_ g _
~ .
6~7
respectively specified optimu~ amounts. Consequently, there are
obtained very tough Cr-Mo steels which have a high hydrogen
attac~ resistivity.
~urther, by limiting the content of nitrogen and
phosphorus, among the other inevitably entrained impurities,
the AlN content can be decreased, the effect of the boron can
be ensured and the hot workability can be improved. ~y lowering
the phosphorus content ? the creep rupture resistivity can be
heightened. Consequently, there are provided steels which
excel in high-temperature ductility and creep ductility and
which will endure under harsh use conditions.
Example 1
Steels having the chemical composition as shown in
Table 1 were prepared by induction melting, '~hese steels were
formed into the plates under the manufacturing conditions shown
in ~able 2, i.e. by rolling and treatment after the rolling.
~he mechanical properties of thus formed plates are also shown
in Table 2. In ~ables 1 and 2, steel No.1 to No.4 denote
respectively the steel produced by the~invention and steel No.5
and No.6 denote respectively the steels which are given for
comparison.
As shown in '~able 2, the steels of the invention
displayed the enhanced strength even after the high temperature
PWH~ and, partlcularly, superior impact property to that of the
comparison steels after the step cooling No.1 and No.~ steels
containing higher N content were subJected respectively to the
different special treatments, as indicated by 3-1 and 3-2 in
'~able 2. '~able 2 can be featurlzed by that double normalizing,
heating at a lower temperature for rolling and slower cooling
3o after rolling secure the favourable results.
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Il. -"`
~ 7
~he steels with high strength and high ductility
, according to the invention can be annealed at a high temperature
after welding (PWHT) so that the cree~ damage was effectively
prevented.
~able 1. Chemical composition of steels
..____ . ........................... . . .
Stee: _ C Si Mn P Cr Mo Bol. N
steels 1 0~13 0~72 0~49 0~015 1 ~18 Or48 0~0012 0~072 0~0072
infVten
tion 20.15 o .57 0 ~ 58 0.012 1 ~ 24 - 59 0 ~ 0005 0 ~ 039 000058
_
30~12 0~65 0~61 0~013 1 ~45 0~62 0~0006 0~042 0~0062
_ ...
0.15 0 ~ 53 0~ 56 0 ~004 1.40 0~600~0007 0~043 0~0032
_ . _
fOrels 0.16 0.21 0.35 0.015 ~9 0.450. 0033 O ~ 038 0. 0073
com-
parison 0.13 0.63 0.560.008 1.22 0.52 _ 0.027 ~.0064
. ,
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11'716~7
~xample 2.
14Cr-0.5Mo steel according to the invention and the
conventional 14Cr-0.5Mo steel, both of which having respectively
the chemical compositions as shown in Ta~le 3, were prepared.
~hese steels were heated at 1230 C and ~alized at 930'C for
one hour to investigate the relationship of the tempering
parameter with the mechanical properties. ~he results of the
investigation are shown in Figure 2. As can be seen from Figure
2, as compared with the conventional steel, the steel of the
invention becomes less deteriorated in the strength and impact
value in proportion to the rise of the tempering temperature.
In other words~ the steel of the invention is provided with
sufficient strength and toughness even by high temperature
annealing after welding (PWH~) so as to prevent the creep damage
in HAZ.
~able 3. Chemical composition of 1-Cr-1,~Mo steels
__._ _ - . .... ...
Steel C Sl Mn Cr Mo ASlol B
.
steel 0.160.65 0.60 1.40 0.60 00025 _
........ _ ~ _
Steel of the 0.140.65 0.60 1.40 0.55 0.055 0.0006
invention
. . -- ~
Variations in the present invention will be apparent
to those of ordinary skill in this art and yet still fall within
the scope of the appended claims. ~
I ~ .
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