Note: Descriptions are shown in the official language in which they were submitted.
9647
The present invention relates to the use of fine
grain manganese structural steel which contains 0.04 to 0.09%
carbon 1.2 to 1.8% manganese; 0.1 to 0.4 silicon 0.03 to
0.08% niobium up to 0.025% aluminum: up to 0.015% sulfur
0.5 to 1.5% nickel and, optionally 0.2 to 0.4% copper the
residue being iron including the impurities resulting frorn
smelting.
An alloy steel of the above type has already been
known from German Patent no. 24 07 338. This steel contains
0.01 to 0.10% carbon, 0.5 to 2% manganese, 0.1 to 0.9% silicon,
0.001 to 0.10% niobium, 0.01 to 0.3% aluminum and 1.4 to 3.5%
nickel. If this steel undergoes a controlled hot rolling
which is dependent on its nickel content, it will acquire a
certain strength at low temperatures. However, in practice,
a hot rolling, which is dependent on the nickel-content of
a particular material, has been found to be difficult and
expensive. In addition, the ductility in the cold state of
such steel was found to be insufficient to enable it to come
into contact with liquid methane and, in particular, liquid
ethylene.
~ or transporting and storing liquid gases, there must
be used structural materials which possess sufficient strength
and ductility at temperatures of at least -196C. In addition,
these materials should be weldable in order to ensure economical
production of pipes and reservoirs.
It is known that stainless steel can be used at
working temperatures of at least -270C. In this case, it is
nickel that may be considered responsible of cold ductility.
The large amount of expensive components9 however, limits the
use of stainless steel and to solve the problem the use of less
expensive alloy steels should be considered. As a result, a
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series of steels has been developed, which contain about 9%
nickel 0.1% carbon, 0.80% manganese and 0.020% phosphorus,
and which as compared with stainless steels are characterized
by higher strength and cold ductility at temperatures of at
least -200C. The precondition for high cold ductility,
however, is a two-stage normalizing and air-cooling intended
to bring a suitable portion of austenite into the ferretic
matrix. This process is based on the recognition that
ductility increases with an increase in the portion of
austenite.
Tests have shown that cold ductility increases
with a decrease in the contents of carbon, phosphorus, and
manganese. Furthermore, it has also been established that
a gradual reduction of the nickel content to values as low
as 2.1% gradually increases cold ductility. For example,
the notch impact strength of normalized and air-cooled steel
containing 8.5% to 9.5% nickel is 34 J at -196C it decreases
to 20 J at -100C in the case of steels containing 3.25 to
3.75% nickel, ancl to 18 J at -68C in the case of steels
containing 2.1 to 2.5% nickel. Therefore, steels with a
nickel content less than 9% cannot be regarded as suitable
for use at low temperatures.
The aim of the present invention is to provide an
alloy steel which can be welded, has a high yield point at
room temperature, cold ductility, resistance to hydrogen-
cracks and which, as a result, is particularly suitable to
be used as a material for welded pieces, such as pipes and
reservoirs, useful for transporting and storing liquid yases
even in the presence of hydrogen sulfide or water. In
particular, this steel should be resistant to ethylene and
should be suitable to be used at temperatures of at least
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-120C
It is recommended that the solution to this problem
consists of providing a steel having the composition mentioned
in the introduction, thereby achieving the above aim.
Despite its very low nickel content, even in the
hard-rolled and air-cooled state, this steel has such a high
notch impact strength and transition t~mperature that it can
be used at temperatures of at least -70C. However, the proper-
ties of this material will be fully developed only after the
steel which is proposed has been normalized and, if necessary,
again tempered. With a heat-treatment of this kind, this steel
has a yield point of at least 420 N/mm at room temperature and
a transition temperature of at least -120C with a notch impact
strength of 51 J/cm2 across the rolling direction, and a notch
impact strength of at least 280 J/cm at room temperature.
If this steel contains 0.2 to 0.4% copper, its
resistan~e to crack-formation in the presence of traces of
hydrogen sulfide will be particularly high. This fact is of
considerable importance since liquid gases often contain
traces of hydrogen sulfide which, in the case of a simultaneous
presence of water, has a corroding effect and results in
cracks due to the presence of hydrogen.
The low carbon content ensures a good behavior in
welding on the one hand, and it promotes the notch impact
resistance, on the other. As a whole, the reason for the
excellent properties of the recommended steel is the synergistic
action of nickel, niobium, and manganese.
Advantageously, the steel is normalized until the core
temperature is about 30 to 50C higher than the AC3- point
after which it is tempered at 550-650C, preferably at 630C,
for two to four minutes per 2 mm thickness of the material, in
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order to adjus-t cold ductility.
The invention is better explained with reference to
the drawings which describe diagrams and explain preferred
embodiments. In the drawings:
Figure 1 illustrates the notch impact strength at
room temperature as a ~unction of the nickel content and the
type of heat-treatment.
Figure 2 illustxates the transition temperature as
a function of the nickel content and of the heat-treatment.
Figure 3 illustrates the notch impact strength and
the deformation at break of a steel in the invention as
compared with known steels at the testing temperature.
Figure 4 illustrates the amount of dissolved
hydrogen as a function of the copper content after a 96 hour
immersion into sea water saturated with hydrogen sulfide.
Figure 5 illustrates the length of hydrogen induced
cracks as a function of the hydrogen content.
The tests which served aQ a basis for the diagrams
of Figures 1 and 2 wer~ conducted on steels 1 to 5 having
20 composition given in the Table below. Out o~ these steels,
steels 2 and 3 are steels in the invention.
TABLE
C Si Mn P S N Al Nb Ni
Steel % (%) (%) (%) (%) (%) (%) (%) (%)
1 o.og 0.28 I.37 0.015 0.013 0.0061 0.050 0.07 0.05
2 o.og 0.31 1.52 0.014 0.013 0.0077 0.051 0.08 0.61
3 0.09 0.31 1.46 0.013 0.013 0.0077 0.030 0.0~ 1.40
4 0.09 0.33 1.46 0.015 0.013 0.0079 0.046 0.08 2.23
0.0~ 0,32 1.43 0.015 0.014 0.0078 0.038 0.08 3.10
Samples taken ~rom the experimental steels
underwent the heat-treatment given in the diagrams and tests
were made to determine the notch impact strength and the cold
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ductility. The results illustrated in the diagrams of Figures
1 and 2 show that, within the 0.5-1.5% nickel-content range,
both the notch impact strength at room temperature as well as
the transition temperature, without taking any special measure
and independently of the type of heat-treatment are
optimum. This is a surprising fact since, in accordance
with traditional considerations, a reduced nickel content goes
together with a reduction of cold ductility and notch irnpact
strength, unless of course special measures, such as controlled
hot rolling, are taken to adjust the cold ductility.
As compared with traditional normal steels, the
superiority of the steel of the invention is evidenced from
the diagrams of Figure 3, whereby it should be remembered that
in the case of the steel of the invention the samples are in
transverse direction while in the other samples (with one
exception), are in the longitudinal direction.
In all cases, the steels under study have a
yield point at room temperature of at least 420 n/mm and
a notch impact strength of at least 280 j/cm2.
Furthermore, the diagrams of Figures S and 6 show
that the sensitivity to crack formation in the presence of
hydrogen sulfide is particularly low when a copper content
is above about 0.02%. In this manner, the suggested steel is
also particularly suitable for transport and storage of
contaminated liquid gases. The high resistance to crack-
formation is explained by the fact that, under working
conditions, a weak acid is formed under the action of hydrogen
sulfide and water. The hydrogen ions generated in this process
travel along the material and a molecular precipitation takes
place on the grain boundaries. In the case of traditional
steel, it is a pressure that results from this phenomenon which
~1~9647
leads to the formation of cracks. On the other hand, in the
case of the steel of the invention, a portion of copper is
dissolved in the acid. The ions formed in this process
travel, by way of ion exchange, along the surface of the
material and form thereon a molecular protecting copper layer.
This copper layer acts as an inhibiting layer against further
intrusion of hydrogen and explains the high resistance against
hydrogen of the steel of the invention, as it may be seen in
Figure 4.
