BACKGROUND OF THE INVENTION
Field of the Invention
[0001] The present invention concerns a structural material for cryogenic temperature use
and, more in particular, it relates to a non-magnetic structural material for cryogenic
temperature use, required for constituting superconductive magnets. The steel sheet
referred to this invention includes steel sheets and steel strips.
Description of the Related Art
[0002] In various techniques utilizing super conductivity such as nuclear fusion power generation,
particle accelerators, and superconductive power storage, superconductive magnets
are used in view of the requirement of supplying a large amount of current for generating
strong magnetic fields. Since large electromagnetic forces are induced in the superconductive
magnet and it is usually cooled to a cryogenic temperature at 2 - 4 K by liquid helium,
structural materials supporting the superconductive magnet require high strength capable
of withstanding the large electromagnetic forces under the cryogenic temperature.
In addition, since it is a basic object to generate a strong magnetic field at a uniform
and stable distribution and in a range as wide as possible, it is essential to minimize
the effects of the structural materials on the magnetic fields. Accordingly, it is
an essential condition for the materials that they are non-magnetic materials not
causing interaction with the magnetic fields.
[0003] In view of the above, the structural materials used at the inside or the periphery
of the superconductive magnet are required to have high mechanical characteristics
and extremely low magnetic permeability at a cryogenic temperature and, further, it
is also necessary to take a consideration on thermal deformation in order to firmly
hold the superconductive magnet in a composite structure. Further, in the manufacture
of the superconductive magnet, it is required for the structural materials that they
are excellent in machinability such as punching or boring property or weldability
and, further, also excellent in surface flatness or fitness required for laminating
a plurality of sheets.
[0004] Existent materials considered as structural materials for supporting the superconductive
magnet can include austenitic stainless steels, high Mn steels, aluminum alloys, titanium
alloys and fiber-reinforced plastics. The mechanical strength, the magnetic permeability
and the thermal expansion coefficient required for the structural materials for supporting
the superconductive magnet vary depending on the designed intensity of the magnetic
fields in the superconductive magnet to be manufactured or the aimed uniformity for
the distribution of the magnetic fields and it is important for the selection of the
materials that the strength is high, and the permeability and the thermal expansion
coefficient are low at a cryogenic temperature.
[0005] The fiber-reinforced plastics are non-magnetic and easy to handle with being of low
specific gravity and have lower thermal expansion coefficient compared with austenitic
stainless steels but the strength per unit cross sectional area is lower. Further,
while titanium alloys are low in the specific gravity and high in the strength and
have high specific strength, they involve a problem that the toughness is low at a
low temperature and is expensive.
[0006] Aluminum alloys are used in various applications at cryogenic temperatures since
they are light in weight, and have high specific strength and extremely low permeability
but they lack in the strength when the designed magnetic fields are applied as in
large scale particle accelerators and also involve a problem in the weldability.
[0007] Since usual austenitic stainless steels are insufficient in the strength and the
toughness at low temperatures, stainless steels of low carbon content with addition
of nitrogen have been developed. However, since the stability in the austenitic phase
is insufficient in such stainless steels, a portion of the austenitic phase is transformed
into a ferromagnetic martensitic phase by deformation at a low temperature. Accordingly,
this results in lowering of the toughness and involves a problem that the permeability
can not be lowered sufficiently at a cryogenic temperature.
[0008] Subsequently, austenitic stainless steels with further increased Ni content have
been developed but they involve a problem of increased cost and high thermal expansion
coefficient as the structural material for cryogenic temperature use.
[0009] In view of the problems described above, Japanese Patent Publications No. 11661/1984
and No. 18887/1993 propose relatively inexpensive high Mn non-magnetic steels and
manufacturing methods thereof. However, the high Mn non-magnetic steels described
in Japanese Patent Publication No. 11661/1984 have high permeability at a cryogenic
temperature and involve problems as a large scale particle accelerator use. The technique
disclosed in Japanese Patent Publication No. 18887/1993 involves problems of requiring
long time aging treatment and lowering the productivity.
[0010] Further, in the superconductive magnet, a non-magnetic member referred to as a collar
is required as fixing members for superconducting wires as conductor coils and the
collar is formed by laminating a plurality of non-magnetic steel sheets. Then, the
collar also requires an appropriate mechanical strength in order to withstand strong
electromagnetic forces caused when it is cooled to a cryogenic temperature and a large
amount of current is supplied as the superconductive magnet. However, when the mechanical
strength of the non-magnetic steel sheet is excessively high or the residual stress
therein is excessive, the working life of a punching die is shortened or warps are
caused after punching the non-magnetic steel sheet into a predetermined shape of the
collar.
[0011] In the superconductive magnet, the collar is often manufactured by precision punching
as fine blanking. With the view point as described above, the mechanical strength
of the material used for the collar is determined while taking the strength and the
distribution of the designed magnetic field into a consideration. Accordingly, it
has been demanded for a method of manufacturing a non-magnetic steel sheet that can
easily control the strength of the non-magnetic steel sheet as the material to a desired
strength demanded in the design.
OBJECT OF THE INVENTION
[0012] An object of this invention is to effectively overcome the foregoing problems in
the prior art and provide a method of manufacturing a high Mn non-magnetic steel sheet
for cryogenic temperature use, capable of manufacturing, with industrial stability
and high productivity, and a high Mn non-magnetic steel sheet which is suitable for
use in large scale particle accelerators, and has a high yield point at a cryogenic
temperature and low permeability at the cryogenic temperature.
SUMMARY OF THE INVENTION
[0013] In order to attain the foregoing subject, the present inventors have investigated
characteristics required for supporting structural members used in superconductive
magnets for use in large scale particle accelerators and have made an earnest study
for the factors giving effects on the permeability and the yield stress at a cryogenic
temperature of high Mn non-magnetic steel sheets. As a result, it has been found that
the permeability of the high Mn non-magnetic steel at the cryogenic temperature can
be lowered by further stabilizing the austenitic phase by increasing the content of
Mn. Further, it has been found that the yield stress of the high Mn non-magnetic steel
at the cryogenic temperature can be controlled easily to 900 MPa or more by applying
temper rolling to a steel sheet after intermediate annealing.
[0014] This invention has been constituted based on the findings described above. That is,
this invention provides a method of manufacturing a hot rolled high Mn non-magnetic
steel sheet for cryogenic temperature use, which comprises:
heating a steel material containing, on the weight percent basis:
from 0.05 to 0.18% of C,
from 26.0 to 30.0% of Mn,
from 5.0 to 10.0% of Cr,
from 0.05 to 0.15% of N and, optionally, 0.02% or less of Ca
and hot rolling the material into a hot rolled steel sheet, in which a hot rolling
start temperature is from 1050 to 1200°C and the rolling end temperature is 700 to
1000°C for the hot rolling. Further, in a preferred embodiment of this invention,
the steel material preferably contains, on the weight percent basis: from 0.05 to
0.18% of C, from 26.0 to 30.0% of Mn, from 5.0 to 10.0% of Cr, from 0.50 to 5.0 of
Ni and from 0.05 to 0.15% of N and, optionally, 0.02% or less of Ca.
[0015] Further, this invention provides a method of manufacturing a cold rolled high Mn
non-magnetic steel sheet for cryogenic temperature use, which comprises:
heating a steel material containing, on the weight percent basis:
from 0.05 to 0.18% of C,
from 26.0 to 30.0% of Mn,
from 5.0 to 10.0% of Cr,
from 0.05 to 0.15% of N and, optionally, 0.02% or less of Ca
and hot rolling the material into a hot rolled steel sheet, applying hot rolled plate
annealing for the hot rolled sheet then applying cold rolling to form a cold rolled
sheet and then applying annealing to the cold rolled sheet, in which a hot rolling
start temperature is from 1050 to 1200°C and a rolling end temperature is 700 to 1000°C
for the hot rolling and, further, the annealing temperature for the cold rolled sheet
annealing is from 1050 to 1200°C.
[0016] Further, in this invention, the steel material is, preferably, a steel material containing,
on the weight percent basis, from 0.05 to 0.18% of C, from 26.0 to 30.0% of Mn, from
5.0 to 10.0% of Cr, from 0.50 to 5.0 of Ni and from 0.05 to 0.15% of N and, optionally,
0.02% or less of Ca. Further, in this invention, temper rolling at a draft ratio of
30% or lower is preferably applied after the cold rolled sheet annealing.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0017] At first, the reason for defining the chemical compositions of the steel material
is to be explained.
C: 0.05 - 0.18%, N: 0.05 - 0.15%
[0018] Both of C and N are interstitial solute elements, which are effective for increasing
the strength of steels by solid-solution hardening. For obtaining a desired yield
stress at a cryogenic temperature, it is necessary to contain C and N by 0.05% or
more. On the other hand, when C exceeds 0.18%, the austenitic phase becomes instable
to precipitate carbides, and the permeability can no more be kept lower at a cryogenic
temperature, and the weldability and the workability are deteriorated. Accordingly,
C is defined within a range from 0.05 to 0.18%. A preferred range for C is from 0.07
to 0.15%.
[0019] Further, N is an addition element useful for stabilizing the austenitic phase and
increasing of the strength at a cryogenic temperature but, if the content exceeds
0.15%, the weldability is deteriorated and abrasion of a tool upon punching fabrication
is accelerated, as well as the permeability is increased by precipitation of nitrides
or carbonitrides. Accordingly, N is defined within a range from 0.05 to 0.15%. A preferred
range for N is from 0.07 to 0.13%.
Mn: 26.0 - 30.0%
[0020] Mn is an important element in this invention, which is useful for stabilizing the
austenitic phase and attaining an extremely low permeability even at a cryogenic temperature.
In order to obtain such an effect, it is necessary to contain Mn by 26.0% or more.
On the other hand, if it exceeds 30.0%, the toughness and the weldability, as well
as the productivity are deteriorated, so that Mn is defined within a range from 26.0
to 30.0%.
Cr: 5.0 - 10.0%
[0021] Cr contributes to the increase of the strength by solid-solution hardening and also
functions effectively to the improvement of corrosion resistance. Such an effect is
recognized at the content of 5.0% or more but the content in excess of 10.0% hinders
stabilization of the austenitic phase and results in increase of the permeability
at a low temperature. Therefore, Cr is defined within a range from 5.0 to 10.0%. The
circumstance in which the material as a target of this invention is used is basically
at cryogenic temperature and in high vacuum where chemical reactions proceed extremely
slowly, which is not so severe in view of corrosion and a sufficient corrosion resistance
can be ensured by the Cr content at such a level. A preferred range for Cr is from
6 to 8%.
Ni: 0.50 - 5.0%
[0022] Ni contributes to the stabilization of the austenitic phase and improvement of the
toughness at a cryogenic temperature, as well as improves the corrosion resistance.
It can be contained optionally in this invention. Such effect is recognizable at the
content of at least 0.50% or more, a great amount of content is not industrially desirable
since Ni is expensive. Therefore, Ni is preferably within a range from 0.50 to 5.0%.
According to this, the steel material of this invention have remarkable advantages
not only in the thermal expansion coefficient but also in view of the cost, as compared
with high Ni austenite stainless steels such as SUS 316LN.
Ca: 0.02% or less
[0023] Ca can be added optionally with a purpose of suppressing damages caused by S mingled
as an inevitable impurity, to thereby improve hot workability. A preferred addition
amount for S is within a range from 0.004 to 0.01%, and it is effective for ensuring
the hot workability to satisfy the following equation (1):

in which the content for each of elements Ca, S and O is indicated on the weight
ppm basis. As a more simple criterion for judgement, Ca/S ≥ 2, preferably, Ca/S ≥
3 may also be used.
[0024] The balance other than the chemical compositions described above substantially comprises
Fe and inevitable impurities. As the inevitable impurities, S: 0.005% or less, P:
0.05% or less and O: 0.005% or less are permissible with a view point of the industrial
economy. Further, it is desirable that the contents for precipitates such as carbides,
nitrides and carbonitrides, particularly, Fe
3C and Fe
4N that form ferromagnetic precipitates or deteriorate the stability of the austenitic
phase are as low as possible.
[0025] In the method of manufacturing the high Mn non-magnetic steel sheet of this invention;
the steel material of the chemical composition as described above is at first heated
and hot rolled into a hot rolled sheet.
[0026] Since the steel material suitable to this invention contains a great amount of Mn
and Mn is easily oxidized at a high temperature, it is not desired to excessively
increase the temperature for heating slabs since this not only increases scale losses
but also results in excessive formation of Mn fumes. Further, the hot workability
of the steel material of the chemical composition described above is not always excellent.
DESCRIPTION OF THE ACCOMPANYING DRAWINGS
[0027]
Fig. 1 is a graph showing a relation between a cross sectional shrinkage at rupture
and a heating temperature in tensile test; and
Fig. 2 is a graph showing a relation between a hardness (Hv) and a draft ratio in
temper rolling.
[0028] Then, the hot workability of the steel material suitable to this invention (C: 0.12%,
Si: 0.05%, Mn: 27.9%, P: 0.029%, S: 0.002%, Cr: 7.0%, N: 0.10%, Ni: 0.15% and Ca:
0.006%) was evaluated at first by a high temperature tensile test. The results are
shown in Fig. 1. It can be seen from Fig. 1, that the cross sectional shrinkage decreases
as the temperature exceeds 1200°C and the trend of the hot shortness develops.
[0029] Then, for suppressing occurrence of edge cracks, the upper limit for the rolling
start temperature in hot rolling is preferably 1200°C. On the contrary, when the rolling
start temperature for hot rolling is lower than 1050°C, melting of carbides is insufficient
and it results in disadvantages of increasing the deformation resistance. Therefore,
the rolling start temperature for the hot rolling is within a range from 1050 to 1200°C.
It is preferably from 1100 to 1180°C.
[0030] Further, it can be seen from Fig. 1 that the cross sectional shrinkage decreases
to 60% or less when the tensile (heating) temperature is 700°C or lower to deteriorate
the hot workability.
[0031] Therefore, the rolling end temperature for the hot rolling is defined to 700°C or
higher in this invention. Further, when the rolling end temperature for the hot rolling
exceeds 1000°C, crystal grains undesirably grow coarser by recrystallization. Therefore,
the rolling end temperature for the hot rolling is defined within a range from 700
to 1000°C. It is preferably from 800 to 950°C in view of prevention for edge cracks.
[0032] As is apparent, the hot rolled sheet can be used as a product sheet as it is or after
application of hot rolled sheet annealing.
[0033] The hot rolled sheet is then applied with hot rolled sheet annealing. The hot rolled
sheet annealing is conducted for homogenizing the structure. The hot rolled sheet
annealing is desirably conducted within a temperature range from 950 to 1200°C. When
the annealing temperature is lower than 950°C, the cross sectional shrinkage is decreased,
whereas scales are formed excessively together with embrittlement if the temperature
exceeds 1200°C.
[0034] Then, the hot rolled sheet is applied with cold rolling into a cold rolled sheet.
In this invention, the rolling condition is not necessarily restricted so long as
a predetermined sheet thickness can be obtained by the cold rolling.
[0035] The cold rolled sheet fabricated to a predetermined sheet thickness is then applied
with cold rolled sheet annealing.
[0036] The cold rolled sheet annealing is conducted mainly for the purpose of relieving
internal strains caused by cold rolling, recrystallization and solid solution of precipitates.
Particularly, this is an indispensable process for complete solid solution of carbides,
nitrides and carbonitrides into an austenitic matrix phase to eliminate a precipitation
phase disadvantageous to the keeping of the low permeability. The annealing temperature
is from 1050 to 1200°C. When the annealing temperature is lower than 1050°C, solid
solution of precipitates is insufficient. On the other hand, if it exceeds 1200°C,
continuous annealing can not be conducted industrially stably. A preferred annealing
temperature is from 1050 to 1180°C. Further, the holding time for annealing is desirably
such that the temperature of the sheet is kept at the temperature described above
for 10 to 120 sec.
[0037] Further, in this invention, the cold rolled sheet is cooled after it has been maintained
at the annealing temperature within the range described above. Cooling is conducted
with an aim of preventing precipitation of carbides or carbonitrides and there is
no particular restriction for the cooling manner so long as the cooling is conducted
at a cooling rate of 5 to 30°C/s.
[0038] In this invention, temper rolling may further be applied after the cold rolled sheet
annealing. Combination of cold rolled sheet annealing and subsequent temper rolling
can easily control the collar as the fixing members for superconductive magnet conductor
wires to a required mechanical strength. The temper rolling is conducted in a cold
state, preferably, at a room temperature to 150°C, and the draft ratio is controlled
in accordance with the desired strength. The draft ratio is desirably 30% or lower.
When the draft ratio in the temper rolling exceeds 30%, the internal strains increase
excessively to deteriorate the flatness after slitting/punching.
[0039] Fig. 2 shows a relation between the draft ratio in the temper rolling and the hardness
after temper rolling. As can be seen from Fig. 2, the hardness Hv increases from 170
to 270 and 0.2% yield point increases from about 300 MPa to about 700 MPa by changing
the draft ratio from 0.5 to 15%. Even if the temper rolling at such a draft ratio
is applied, since the austenitic phase is extremely stable in the high Mn non-magnetic
steel sheet according to this invention, the permeability is kept at as low as about
1.001 and this low permeability scarcely changes even at a cryogenic temperature such
as 4 K.
EXAMPLE
[0040] Steel materials of chemical compositions shown in Table 1 were melted in a converter
and formed into slabs by a continuous casting process. The slabs were applied with
hot rolling under the conditions shown in Table 2 to form hot rolled sheets of 5.0
mm thickness. Then, the hot rolled sheets were applied with hot rolled sheet annealing
under the conditions shown in Table 2, applied with pickling treatment and then to
cold rolling into cold rolled sheets of 1 to 3 mm thickness. The cold rolled sheets
were applied with cold rolled sheet annealing under the conditions as shown in Table
2 and rapid cooling was conducted after the annealing. A dry AX gas was used as the
annealing atmosphere for the cold rolled sheet annealing. The cooling rate after the
cold rolled sheet annealing was about 15°C/s.
[0041] Then, after applying pickling to the cold rolled sheets after annealing, temper rolling
was further applied under the conditions shown in Table 2.
[0042] The thus obtained steel sheets were subjected to: (1) observation for the appearance
of hot rolled sheet with naked eyes, (2) tensile test at room temperature and at 4
K, (3) measuring test for permeability at room temperature and 4 K using a vibrating
sample magnetometer, (4) measuring test for average thermal expansion coefficient
at a temperature range from room temperature to liquid nitrogen temperature and (5)
precision punching test by fine blanking. The flatness was evaluated for the entire
portion of 200 × 200 mm steel sheets as: ○ where warp was 0.2mm or less, as △ where
it was from 0.2 mm to 0.5 mm and as X where it was more than 0.5 mm. In the precision
punching test, circular 50 mm⌀ test pieces were punched and the punching accuracy
for the punched test specimens was measured. The punching accuracy was measured by
the height of burrs and evaluated as ○ in a case of 20 µm or lower, as △ in a case
of from 20 µm to 50 µm and as X in a case of higher than 50 µm.
[0043] As existent examples, tests (2) - (5) were conducted for thin Ti alloy (5% Al - 2.5%
Sn-Ti) sheets of 2.5 mm thickness, thin Al alloy (5% Mg - 0.6% Mn-Al) sheets and thin
SUS 304 cold rolled sheets.
[0044] The result of the test are shown in Table 3.
Table 1
Steel No. |
Chemical composition (wt%) |
|
C |
Mn |
Cr |
Ni |
N |
S |
Ca |
O |
A |
0.09 |
28.5 |
8.0 |
1.2 |
0.11 |
0.002 |
0.006 |
0.004 |
B |
0.09 |
29.0 |
7.5 |
- |
0.11 |
0.002 |
0.0065 |
0.004 |
C |
0.09 |
28.5 |
8.0 |
- |
0.11 |
0.003 |
0.007 |
0.005 |
D |
0.14 |
29.5 |
6.0 |
2.5 |
0.10 |
0.002 |
0.006 |
0.004 |
E |
0.09 |
29.0 |
7.5 |
0.8 |
0.11 |
0.003 |
0.0075 |
0.003 |
F |
0.10 |
27.5 |
7.0 |
2.5 |
0.10 |
0.003 |
0.007 |
0.005 |
G |
0.20 |
22.0 |
11.5 |
- |
0.03 |
0.003 |
0.008 |
0.005 |
H |
0.12 |
25.0 |
8.0 |
- |
0.10 |
0.002 |
0.006 |
0.004 |
I |
0.02 |
28.5 |
8.0 |
- |
0.10 |
0.003 |
0.008 |
0.005 |
J |
0.10 |
28.5 |
8.0 |
- |
0.20 |
0.002 |
0.007 |
0.004 |
Table 2
Steel sheet No. |
Steel No. |
Hot rolling condition |
Hot rolled sheet annealing |
Cold rolled sheet annealing |
Temper rolling |
|
|
Start temp(°C) |
End temp(°C) |
Annealing temp(°C) |
Annealing temp(°C) |
Draft % |
Sheet thickness mm |
1 |
A |
1100 |
900 |
1050 |
1100 |
5 |
2.5 |
2 |
B |
1080 |
850 |
1050 |
1050 |
3.5 |
1.5 |
3 |
A |
1080 |
800 |
1050 |
1050 |
10 |
2.5 |
4 |
C |
1030 |
750 |
1050 |
1100 |
5 |
2.5 |
5 |
D |
1100 |
900 |
1050 |
1100 |
10 |
2.5 |
6 |
D |
1080 |
790 |
1050 |
1050 |
3 |
2.5 |
7 |
B |
1120 |
900 |
1050 |
1050 |
5 |
2.5 |
8 |
E |
1050 |
850 |
1050 |
1100 |
10 |
2.5 |
9 |
F |
1080 |
850 |
1050 |
1050 |
3 |
2.5 |
10 |
F |
1120 |
900 |
1050 |
1050 |
5 |
2.5 |
11 |
E |
1050 |
850 |
1050 |
1100 |
15 |
2.5 |
12 |
F |
1120 |
900 |
1050 |
1050 |
1 |
2.5 |
13 |
G |
1100 |
900 |
1050 |
1050 |
5 |
2.5 |
14 |
H |
1100 |
920 |
1050 |
900 |
10 |
2.5 |
15 |
A |
1020 |
650 |
1050 |
1050 |
3 |
2.5 |
16 |
A |
1250 |
950 |
1050 |
1100 |
10 |
2.5 |
17 |
I |
1100 |
900 |
1050 |
1050 |
3 |
2.5 |
18 |
J |
1100 |
900 |
1050 |
1100 |
10 |
2.5 |
22 |
K |
Ti alloy (Al: 5%, Sn: 2.5%) cold rolled sheet |
2.5 |
23 |
L |
Al alloy (Mg: 5%, Mn 0.6%) cold rolled sheet |
2.5 |
24 |
M |
SUS 304 (Cr: 18%, Ni: 8%, C: 0.02%. Mn: 1.5%) cold rolled sheet |
2.5 |

[0045] In the examples of this invention, occurrence of edge cracks and fine cracks were
scarcely observed on the surface of hot rolled sheets and the appearance of the hot
rolled sheets was favorable. In the examples of this invention, tensile strength at
a cryogenic temperature (4 K) is high and it has a sufficient strength as structural
materials for large scale particle accelerators. Further, the average thermal expansion
coefficient in the examples of this invention is smaller compared with that of austenitic
stainless steels (about 11 × 10
-6), and it shows a value closely approximate to that of pure iron used generally as
yoke materials for superconductive magnets.
[0046] The permeability in the examples of this invention is low also at room temperature
and cryogenic temperature, and shows less temperature dependence. Further, in the
examples of this invention, defects such as warps and burrs did not occur even when
conducting precision punch and, further, the flatness and punching accuracy were also
satisfactory (○).
[0047] On the contrary, comparative examples out of this invention caused cracks on the
surface of steel sheets to show defective appearance, and had high permeability at
cryogenic temperature and poor flatness and poor punching accuracy in a precision
punching test.
[0048] Further, the examples of this invention show lower permeability and lower thermal
expansion coefficient compared with existent examples and have sufficient performance
for use at cryogenic temperature.
[0049] According to this invention, a high Mn non-magnetic steel sheet showing high yield
stress (yield point) at cryogenic temperature, low magnetic permeability at cryogenic
temperature and low average thermal expansion coefficient can be industrially manufactured
stably and at high productivity to provide outstanding industrial effects. Further,
the high Mn non-magnetic steel sheet according to this invention has sufficient characteristics
for large scale particle accelerators and it is industrially useful.