Note: Descriptions are shown in the official language in which they were submitted.
3~;6
The present invention relates to cast steel and to
a method of producing the same. More particularly, the
invention relates to medium chromium (Cr) low nickel (Ni)
stainless cast steel having good corrosion resistance
against strong acids, and to a method of producing the
cast steel.
This application is a division of co-pending patent
application Serial No. 294,192 filed on December 30, 1977.
With the recent remarkable developments in the chemi-
cal industry and paper-manufacturing industry, etc., the
requirement for materials having good corrosion resistance
has been rapidly increasing. Although bronze has conven~
tionally been employed extensively as a reliable material
having good corrosion resistance against strong acids, it
has problems when required for large size facilities, due
to its low allowable stress, elastic modulus and yield
strength. Accordingly, martensite stainless steel of the
13 Cr group is generally employed for such purposes, while
stainless steels of the 18-8 and 18-8-Mo groups are gen-
erally used when subjected to strong acids. Meanwhile,in the field of stainless steel, high Cr low Ni two phase
stainless steels having higher strength and corrosion re-
sistance than the conventional stainless steels have been
developed, and recently have been used for tubes for sea
water heat exchangers, rolls for paper manufacturing, etc.
The two phase stainless steel as described above,
however, has not yet been put into wide actual use, with
various characteristics thereof still being left to be
found. Accordingly, at the present stage, stainless steel
of the 18-8 group or the 18-8-Mo group mentioned earlier
is mainly used, but since stainless steels of the above
~ 3~6i~i;
kinds have an allowable stress lower than 13 Cr steel,
they have not yet been brought into actual use with full
confidence.
Accordingly, an essential object of the present
invention is to provide a stainless cast steel of high
corrosion resistance and high strength of the medium
Cr low Ni group which is superior in yield strength to
conventional stainless steels of the 18-8 group or lB-8-Mo
group and which can be used in environments requiring
contact with strong acids.
According to one aspect of the invention there is
provided a high corrosion resistant and high strength
medium Cr and low Ni stainless cast steel which consists
! essentially of, in weight percentage, C: 0.1% and below,
Si: 1.5% and below, Mn: 2.0~ and below, P: 0.04% and
below, S: 0.04% and below, Cr: 17.0% to 20.0%, Ni: 3.0% to
7.0%, Mo: 1.5% to 2.5%, Cu: 2.5% to 5.0%, W: 0.2% to 2.0%,
N: 0.1% and below and the remaining portion substantially
of Fe to form the material of said stainless cast steel,
with said Mo and Cu contents in the weight percentage
being set to be in range of Mo~Cu: 5.0% to 7.0%.
According to another aspect of the invention there is
provided a method of producing a high corrosion resistant
and high strength medium Cr and low Ni stainless cast
steel which consists of the steps of preparing a molten
material substantially of Fe, adding to said Fe material
the following components: C: 0.1~ and below, Si: 1.5% and
below, Mn: 2.0% and below, P: 0.04% and below, S: 0.04%
and below, Cr: 17.0% to 20.0%, Ni: 3.0% to 7.0%, Mo: 1.5%
to 2.5%, Cu: 2.5% to 5.0%, W: 0.2% to 2.0%, N: 0.1% and
below in weight percentage to form the base material,
setting the said Mo and Cu contents in the weight
3~
percentage to be in the range Mo + Cu: 5.0~ to 7.0% and
casting the base material to form a stainless cast steel.
An advantage of the present invention, at least in
preferred forms, is that it can provide a stainless cast
steel of the above described type which is stable and
reliable in structure and performance, and can be readily
manufactured through simple processings at low cost.
These and other objects and features of the present
invention will become apparent from the following descrip-
tion of preferred embodiments thereof with reference tothe accompanying drawings, in which;
Fig. 1 is a graph showing the results of comparative
tests between conventional steels and steels according to
embodiments of the present invention in which the weight
reduction due to the corrosion of the sample stainless
steels maintained for six hours in boiling 5% sulfuric
acid are shown; and
Fig. 2 is a graph also showing the results of com-
parative tests between conventional steels and steels
according to the present invention in which the hydro-
chloric acid density and speed of corrosion ~g/cm2/24 ~;
hours) of the stainless steel samples maintained for
twenty-four hours in 3% NaCQ + MoQ HC~ solution are shown.
Referring now to the drawings, preferred embodiments of
the present invention are described in detail hereinbelow.
In order to overcome the strength-wise disadvan-
tages inherent in the 18-8 and 18-8-Mo group stainless
steels mentioned earlier, the present inventors have
carried out various studies on the characteristics
of the stainless steels in question, and as a result,
have developed novel stainless cast steels which
are superior in yield strength to conventional
~3~ :
18-8 and 18-8-Mo group stainless steels and which are resistant
to corrosion by strong acids in actual use.
Before the detailed description, it is to be noted
that the invention is particularly characterized by the
following points.
- In a first embodiment, the stainless steel is
composed in weight % of C: 0.1% or below, Si: 1.5% or below,
Mn: 2.0% or below, P: 0.04% or below, S: 0.04% or below,
Cr: 17.0% to 20.0%, Ni: 3.0% to 7.0%, Mo: 1.5% to 2.5%,
Cu: 5.0% to 7.0%, N: 0.1% or below and the remainder
substantially Fe.
In a second embodiment, the material of the first
embodlment i6 subjected to a solution heat treatment at
temperatures at least in the region of from 900 to 1,150C.
In a third embodiment, the resultant material of
the second embodiment thus subjected to the solution heat
treatment is further heated to 600 to 700C with subsequent
cooling.
In a fourth embodiment, the resultant material of
the third embodiment is further sub;ected to a precipitation
hardening treatment at temperatures of 450 to 600C.
In a fifth embodiment, the stainless steel is
composed in weight % of C: 0.1% or below, Si: 1.5% or below,
Mn: 2.0% or below, P: 0.04% or below, S: 0.04% or below, Cr:
17.0% to 20.0%, Ni: 3.0% to 7.0%, Mo: 1.5% to 2.5%, Cu: 2.5%
to 5.0%, W: 0.2% to 2.0%, N: 0.1% or below and the remainder
substantially Fe, while said Mo and Cu are present in a range
of Mo+Cu: 5.0 to 7.0 weight %.
In a sixth embodiment, the material of the fifth
embodiment is subjected to a solution heat treatment at
temperatures at least in the region from 900 to 1,150C.
-- 5 --
~3~36~ :
In a seventh embodiment, the resultant material of the
sixth embodiment is further heated to temperatures of 600 to
700c with subsequent cooling,
In an eighth embodiment, the resultant material of
the seventh embodiment is further subjected to a precipitation
- hardening treatment at temperatures of 450 to 600C.
The reasons for limiting the range of the elements
as described above will be described in detail hereinbelow.
It is preferable that the amount of the element C
is as small as possible, and more than 0.1% reduces the
corrosion resistance. Although the element Si improves
resistance against oxidation, more than 1.5% tends to reduce
the tenacity. ~n is necessary for desulfurization, but
inclusion of more than 2.0% reduces the corrosion resistance.
Inclusion of more than 0.04% of the element P obstructs the
welding performance, while the amount of S should preferably
be as small as possible from the viewpoint of resistance
a~ainst pitting and is not more than 0.04%.
While Cr, which is the important element for forming
stainless steel, remarkably improves the corrosion resistance,
inclusion thereof up to 17.0% is not very effective, and if !~,~
there is more than 20.0%, the tenacity is reduced.
For improving the mechanical properties and general
corrosion resistance of the steel to form martensite and
ferrite structures, inclusion of Ni should preferably be in
the region of 3.0 to 7.0%.
The amount of Cu, known as the element for improving
the corrosion resistance of stainless steel against non- -
oxidizing acids, is conventionally from 0.2 to 1.3% (the
solid solubility phase in the ferrite phase i5 1.25% at 840C),
and if the amount exceeds the above level, the Curich phase
3~6~
(~ phase) is precipitated for precipitation hardening, thus
the strength of the material is remarkably improved, although
excessive precipitation expedites the development of local
corrosion and is not desirable from the viewpoint of tenacity.
Accordingly, the proper amount of Cu is between 2.5 and 7.0%
and is set to be in the region from 5.0 to 7.0% in the first to
fourth embodiments, taking into account the composite addition
effect with respect to Mo mentioned later, and in the region
from 2.5 to 5.0% in the fifth to eighth embodiments from the
viewpoint of the composite addition effect with respect to Mo
and the addition of W referred to later.
The element Mo, which remarkably imp~roves the
resistance against local corrosion, is required to be included
in at least 1.5 to 2.5%, but more than 2.5% is not preferable
from the viewpoint of strength, since martensite transformation
is then started at normal temperatures or at temperatures less
than the normal temperature, and thus the improvement of the
corrosion resistance by composite addition together with Cu
becomes important, with the proper amount of Cu for optimum
result being in the region of 5.0 to 7.0% as described earlier.
The element W (tungsten), which is important in
the fifth to eighth embodiments, has a particular effect for
improvement of the corrosion resistance against strong acids
when present together with Cu, Mo, etc. When 2.5 to 5% of Cu
is present, the above effect is particularly conspicuous at
the weight percentages of Mo+Cu from 5.0 to 7.0% and W of
0.2 to 2.0% as is clear from Example mentioned later.
Although the element N is important for improving
the resistance against pitting, the tenacity tends to be
reduced if N is contained in more than 0.1%, due to the
precipitation of nitrides, and therefore, the amount of N is
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set at no more than 0.1%.
It should be noted here that in the foregoing
description, although the reasons for limiting the range of
composition are described with reference to the effects of
individual elements, the present invention is not based on
the mere addition of these elements, but is characterized
by exhibiting effécts greater than the sum of the effects of
the individual elements in corrosion resistance and yield
strength through interaction of the various elemente, as is
clear from the Example explained in detail hereinbelow.
Example
Table 1 below shows the chemical compositions and
conditions of heat treatment for samples of comparative steels
and steels according to the present invention.
- 8 -
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__ _ _ _.
~1~3~6
In order to assess the corrosion resistance of each
of the steels in Table 1 against boiling 5% sulfuric acid,
corrosion tests were carried out on a laboratory scale, in which
test pieces each having the dimensions of lO~ x 30 mmQ were
immersed for 6 hours in the boiling acid with subsequent
- measurement of the weight reduction.
In Fig. 1, showing the amount of weight reduction
in %, the corrosion resistance against acids may be said to
be extremely superior at weight reductions of less than 0.06~.
From the test results for the steels AISI 304, 321 and 316
given for comparison in Fig. 1, which are generally thought
to be superior in corrosion resistance, it is noticed that
the samples for 304 and 321 have quite large amounts of
corrosion, while the sample 316 is still subjected to an
appreciable amount of corrosion, although the corrosion
resistance thereof is improved to a considerable extent by
the addition of Mo. On the other hand, each of the steels
according to the present invention is found to be superior
in corrosion resistance.
Referring also to Fig. 2, showing the speed o~
corrosion of each of the steels of Table 1 in a three %
solution of NacQ + HcQ, the results of which were obtained by
short-time accelerated evaluation of the resistance against
pitting by CQ concentration, the comparative steels AISI
304 and 321 had such large corrosion speeds at 0.02HcQ to
0.1 NHCQ, as the passive state thereof was difficult to
maintain, i.e. they were subjected to active dissolution,
while the AISI 316 steel and steels according to the present
invention were in the passive state up to 0.06 NHCQ, with a
consequent very slow corrosion speed. It is particularly to
be noticed that the steels according to the present invention
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have a passive state which is even nore stable than the AISI
316 steel.
Table 2 below shows the effect of the precipitation
hardening of Cu by the heat treatment of each of the steels
in Table 1.
It is to be noted that in Tables 1 and 2, the symbols
(a), (b) and (c) represent the following conditions employed
in the heat treatment.
(a) Cooling by water after maintaining at a temperature
of 1,050 C for 4 hours.
(b) Cooling by water after maintaining at a temperature
of 1,050C for 4 hours, followed by reheating~ up to 680C with
subsequent cooling with air.
(c) Cooling by water after maintaining at a temperature
of 1,050 C for 4 hours, followed by reheating up to 680C with
subsequent cooling by air, and further followed by reheating
up to 550C with subsequent cooling in a furnace.
Table 2 Effect of the precipitation hardening of
Cu by the heat treatment
Classlficatlon 8eat0 2% yield strengeh kg/mm2
Comparativë AISI 304 (a)24.4
steels AISI 321 (a) 25~8
AISI316 (a)28.6
__ __ (a)38.5
B ~ (b)42.6
C (c)50.6
_ _ (a)37.4
E _ ( )40.8
F (c)49.1
. ._
Steels ofG _ (a) 35.6
H (b) 39.4
the present _
invention_ I _ (c)
K (b) 46.3
L (c) 52.6
M (b) 44.7
. .
, ; (C) .... _ _ :
From the above Table 2, it can be seen that in the
2% yield strength, although the comparative AISI 304, 321 and
316 steels have extremely low values in the region of 24 to
27 kg/mm J the steels of the present invention each have high
values. More particularly, the heat treatment conditions (b)
show higher effects than those of (a), while the heat treatment
conditions (c) show also higher effects than those of (b).
Table 3 below shows the 0.2% yield strength in
kg/mm2 of the stainless steels of the present invention without
any heat treatment (i.e., in the state as they are cast), and
it can be seen from Table 3 that the steels of the present
invention are superior to the comparative steels in this respect
also.
Table 3 Yield strengths of steels of the present
invention without heat treatment
.. .. .
Classification 0.2% yield strength Kg/mm
_ ..... ._ .. , .
D' 36.8
, . ... ___
Steels of the G' 35.0
present J' 38.7
invention _
M' 39.1 ~;
__ _ _ -
As ls clear from the foregolng description, although
the strength increase in the steels according to the present
invention is mainly attributable to the inclusion of Cu in
the amounts of the predetermined range and also to the specific
heat treatment, the effect is particularly conspicuous in the
steels subjected to the heat treatment under the conditions
(c) mentioned earlier, i.e., a solution heat treatment at a
temperature of 900 to 1,150C, heating to a temperature from
600 to 700C with subsequent cooling, and further a precipitation
hardening treatment at temperatures of 450 to 600C. In the
above case, the reasons for limiting the temperatures for the
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.
~1~3~6
' second treatment to 600 to 700C are that the martensite
transformation rate of the steels according to the present
invention (temperatures for starting counter-transformation
are in the region of 700 to 750C) in the first solid solution
heat treatment is 80 to 85~, and that the rate of martensite
formation is much improved by heating to the temperature
immediately below the above counter-transformation temperature
with subsequent cooling after the first heat treatment mentioned
above. The temperatures are specified to be from 600 to 700C,
because such temperature range is best suited for the purpose. ~'-
Steels including elements in the composition range
of the stainless cast steel according to the present invention
and those further sub~ected to the heat treatment are much
superior in yield strength than the conventional stainless
steels, and thus practical stainless steels having stable
corrosion resistance against strong acids, especially in the
chemical industry, paper manufacturing industry, etc., are
advantageously;produced. The steels of the present invention
which are particularly suitable for use in suction roll shells
for paper manufacturing can also be used for any industrlal
components and parts which require the varlous characteristics t'~j
as described in the foregoing.
Although the present invention has been fully described
by way of examples with reference to the accompanying drawings,
it is to be noted that various changes and modifications will be
apparent to those skilled in the ar~. Therefore, unless otherwise
such changes and modifications depart from the scope of the present
invention as defined by the appendant claims, they should be
construed as included therein.
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