DESCRIPTION
TITLE OF INVENTION
AUSTENITIC STAINLESS STEEL AND METHOD FOR PRODUCING THE
SAME
TECHNICAL FIELD
[0001]
The present invention relates to a stainless steel and a method for producing
the same, more specifically to an austenitic stainless steel and a method for
producing the same.
BACKGROUND ART
[0002]
In recent years, fuel cell powered vehicles that run on hydrogen as fuel, and
hydrogen stations where fuel cell powered vehicles are supplied with hydrogen, have
been under development. A stainless steel is one of the candidates for a material
used for fuel cells.
[0003]
When a stainless steel is used for fuel cells, the stainless steel is used in a
high-pressure hydrogen gas environment. For this reason, a stainless steel used for
·fuel cells needs to have an excellent hydrogen brittleness resistance. At present,
according to the standards of compressed hydrogen containers for automobiles
provided by the High Pressure Gas Safety Act, SUS316Lis accredited as a stainless
steel having an excellent hydrogen brittleness resistance.
[0004]
However, in consideration oflowering weight of fuel cell powered vehicles,
downsizing hydrogen stations, and operations under high pressure in a hydrogen
station, it is preferable that a stainless steel used for the above applications has a high
strength.
[0005]
2
As previously described, a stainless steel used for fuel cells needs to have an
excellent hydrogen brittleness resistance and a high strength. Meanwhile, when a
stainless steel is to be used for fuel cells, the stainless steel is processed into a desired
shape. For example, machining such as cutting may be performed on stainless steel
products of high strength. In this case, it is preferable that the stainless steel further
has an excellent machinability.
[0006]
International Application Publication No. W02004/083476 (Patent Literature
1), International Application Publication No. W02004/083477 (Patent Literature 2),
International Application Publication No. W02004/111285 (Patent Literature 3), and
International Application Publication No. W02012/132992 (Patent Literature 4)
propose stainless steels that are used in high-pressure hydrogen environments and
have high strengths.
[0007]
The stainless steel for hydrogen gas disclosed in Patent Literature I contains:
in mass%, C: 0.02% or less; Si: 1.0% or less; Mn: 3 to 30%; Cr: more than 22% to
30%; Ni: 17 to 30%; V: 0.001 to 1.0%; N: 0.10 to 0.50%; and AI: 0.10% or less, with
the balance being Fe and impurities. Of the impurities, P is 0.030% or less, S is
0.005% or less, and Ti, Zr, and Hf are each 0.01% or less. The contents of Cr, Mn,
and N satisfY the following formula.
5Cr + 3.4Mn:.:; SOON
- [0008]
The stainless steel for high-pressure hydrogen gas disclosed in Patent
Literature 2 contains: in mass%, C: 0.04% or less; Si: 1.0% or less; Mn: 7 to 30%;
Cr: 15 to 22%; Ni: 5 to 20%; V: 0.001 to 1.0%; N: 0.20 to 0.50%; and AI: 0.10% or
less, with the balance being Fe and impurities. Of the impurities, Pis 0.030% or
less, Sis 0.005% or less, and Ti, Zr, and Hf are each 0.01% or less, which satisfY the
following formula.
2.5Cr + 3.4Mn :<; 300N
[0009]
The austenitic stainless steel for hydrogen gas disclosed in Patent Literature 3
has a chemical composition including: in mass%, C: 0.10% or less; Si: 1.0% or less;
3
Mn: 0.01 to 30%; P: 0.040% or less; S: 0.01% or less; Cr: 15 to 30%; Ni: 5.0 to 30%;
soLA!: 0.10% or less; and N: 0.001 to 0.30%, with the balance being Fe and
impurities. The austenitic stainless steel includes a micro-structure in which an Xray
integrated intensity I (Ill) on a cross section along a direction perpendicular to a
processing direction is five times or less as much as that in a random orientation, and
an X-ray integrated intensity I (220) on a cross section along the processing direction
satisfies I (220)/I (Ill) :S 10.
[0010)
The austenitic stainless steel for high-pressure hydrogen gas disclosed in
Patent Literature 4 contains: in mass%, C: 0.10% or less; Si: 1.0% or less; Mn: 3% or
more and less than 7%; Cr: 15 to 30%; Ni: 10% or more and less than 17%; AI:
0.10% or less; N: 0.10 to 0.50%; and at least one ofV: 0.01 to 1.0% and Nb: 0.01 to
0.50%, with the balance being Fe and impurities. Of the impurities, P is 0.0050%
or less, and S is 0.050% or less. The austenitic stainless steel contains an alloy
carbo-nitride at 0.4/J.Lm2 or more in cross section observation, the alloy carbo-nitride
having a tensile strength of 800 MPa or more, a grain size number (ASTM Ell2) of
8 or more, and a maximum diameter of 50 to 1000 nm.
CITATION LIST
PATENT LITERATURE
[0011)
·Patent Literature 1: International Application Publication No. W02004/083476
Patent Literature 2: International Application Publication No. W02004/083477
Patent Literature 3: International Application Publication No. W020041111285
Patent Literature 4: International Application Publication No. W02012/132992
[0012)
The stainless steels disclosed in Patent Literatures 1 and 2 have strengths of
700 MPa or more even after solution treatment is performed thereon. However, the
stainless steel of Patent Literature I is produced at a high cost because it has a high
content ofNi. The stainless steel of Patent Literature 2 may fail to provide an
excellent workability because it has a high content of Mn. As to the stainless steels
of Patent Literature 3 and Patent Literature 4, solution treatment and cold working
4
--
are performed to achieve high strengths. However, the cold working may lead to a
decrease in hydrogen brittleness resistance. Furthermore, as to the stainless steel
disclosed in Patent Literatures I to 4 described above, there are no studies conducted
on machinability. Therefore, even with the stainless steels described in Patent
Literature 1 to 4 mentioned above, an excellent hydrogen brittleness resistance, a
high strength, and an excellent machinability are not provided in some cases.
SUMMARY OF INVENTION
[0013]
An objective of the present invention is to provide an austenitic stainless steel
having an excellent hydrogen brittleness resistance and a high strength, and further
having an excellent machinability.
[0014]
The austenitic stainless steel of the present embodiment has a chemical
composition including: in mass%, C: 0.10% or less; Si: 1.0% or less; Mn: 2.1 to
6.0%; P: 0.045% or less; S: 0.1% or less; Ni: 8.0 to 16.0%; Cr: 15.0 to 30.0%; Mo:
1.0 to 5.0%; N: 0.05 to 0.45%; Nb: 0 to 0.50%; and V: 0 to 0.50%, with the balance
being Fe and impurities, and satisfying Formula (1 ). The austenitic stainless steel
of the present embodiment has a grain size number ofless than 8.0 and a tensile
strength of 690 MPa or more.
15 ~ 12.6C + 1.05Mn + Ni + 15N (1)
The symbols of elements in Formula (1) are to be substituted by the contents
of corresponding elements (in mass%).
[0015]
A method for producing the previously-mentioned austenitic stainless steel
includes a step of preparing a starting material having the previously-mentioned
chemical composition and a step of performing hot working on the starting material
one or a plurality of times. In the step of performing the hot working, a reduction of
area in the hot working after last heating is 70% or less.
[0016)
5
--
The austenitic stainless steel according to the present embodiment has an
excellent hydrogenbrittleness resistance and a high strength and further has an
excellent machinability.
BRIEF DESCRIPTION OF DRAWINGS
[0017]
[FIG. 1] FIG. 1 is a diagram illustrating the relation between the grain size number of
a steel and a relative amount ratio of wear which is an index of machinability.
· [FIG. 2] FIG. 2 is a diagram illustrating the relation between the formula defined
with F1 = 12.6C + 1.05Mn + Ni + 15N, and a relative rupture elongation, which is an
index of hydrogen brittleness resistance.
[FIG. 3] FIG. 3 is a diagram illustrating the relation between a reduction of area and
a tensile strength in the austenitic stainless steel of the present embodiment.
DESCRIPTION OF EMBODIMENTS
[0018]
The present inventors conducted investigations and studies on the
machinability, hydrogen brittleness resistance, and strength of an austenitic stainless
steel, and obtained the following findings.
[0019]
(A) A machinability is linked to cutting resistance and chip disposability (indicating
• how easy chips can be detached from a cutting tool) in cutting. If a cutting
resistance is low, and a chip disposability is high, wear in a cutting tool is suppressed.
That is, the machinability of a steel is increased.
[0020]
A cutting resistance depends on the tensile strength of a work material under
specified cutting conditions. A chip disposability can be determined based on the
length of a chip at the time when the chip snaps in cutting. The brittler a chip is, the
higher the chip disposability is. The brittleness of a chip depends on the elongation
and tensile strength of a work material. Therefore, suppressing the tensile strength
and elongation of an austenitic stainless steel, which is a work material, decreases the
6
cutting resistance and increases the chip disposability. As a result, the
machinability thereof is increased.
[0021)
If the diameters of crystal grains in a steel are large, it is possible to suppress
the tensile strength and the elongation of the steel. FIG. I is a diagram illustrating
the relation between the grain size number of a steel and a relative amount ratio of
wear which is an index of machinability. FIG. I is a plot of the results of the
example, which will be described later. A relative amount ratio of wear is a ratio of
the amount of wear of a cutting tool in the case where a steel having a chemical
composition equivalent to that of SUS316 in JIS Standard is subjected to peeling
treatment (a reference amount of wear) with respect to the amount of wear of the
cutting tool in the case where an austenitic stainless steel is subjected to peeling
treatment, under the same conditions. A higher relative amount ratio of wear means
a less wear in a cutting tool as compared with the reference amount of wear, namely,
a higher machinability of a steel.
[0022]
Referring to FIG. 1, if a grain size number is more than 8.0, a machinability
does not vary significantly. On the other hand, if a grain size number is 8.0 or less,
the relative amount ratio of wear is increased significantly as the grain size number
becomes small. Therefore, in the case of an austenitic stainless steel having the
chemical composition accordingto the present embodiment, an excellent
·machinability can be obtained by making the grain size number 8.0 or less.
[0023)
(B) Meanwhile, coarsening crystal grains may incur the risk of decreasing a
hydrogen brittleness resistance. Thus, the present embodiment is intended to
increase the stabilization of an austenite so as to increase the hydrogen brittleness
resistance even if the crystal grains are coarse. The diffusion coefficient of
hydrogen in an austenite is low. Stabilizing an austenite makes hydrogen difficult
to diffuse in steel. Furthermore, the generation of a martensite having a high
susceptibility to hydrogen brittleness is suppressed. As a result, the hydrogen
brittleness resistance is increased.
[0024]
7
Carbon (C), nitrogen (N), manganese (Mn), and nickel (Ni) are elements that
stabilize an austenite. Thus, by making these elements contained in appropriate
amounts, the hydrogen brittleness resistance is increased.
[0025]
Let F1 = 12.6C + 1.05Mn + Ni + 15N. FIG. 2 is a diagram illustrating the
relation between F1 and the hydrogen brittleness resistance. FIG. 2 is a plot of the
results of the example, which will be described later. The term "relative rupture
elongation"(%) in FIG. 2 is a ratio of a rupture elongation in a high-pressure
hydrogen environment with respect to a rupture elongation in the atmosphere. A
higher relative rupture elongation means a more excellent hydrogen brittleness
resistance.
[0026]
Referring to FIG. 2, if Fl is less than 15, the relative rupture elongation
rapidly increases with an increase in Fl. Then, ifF1 becomes 15 or more, the
relative ruptUre elongation does not increase significantly even with an increase in F1
but is substantially constant. That is, in the graph of FIG. 2, there is an inflection
point at about F1 = 15. Therefore, when Fl is 15 or more, an excellent hydrogen
brittleness resistance can be obtained.
[0027]
(C) Coarsening crystal grains increases a machinability but incurs the risk of
decreasing a strength. Thus, in the present embodiment, 1.0% or more ofMo is
• contained. This provides a high tensile strength even if the grain size number is less
than 8.0.
[0028]
The austenitic stainless steel of the present embodiment completed based on
the findings described above has a chemical composition that contains, in mass%, C:
0.10% or less, Si: 1.0% or less, Mn: 2.1 to 6.0%, P: 0.045% or less, S: 0.1% or less,
Ni: 8.0 to 16.0%, Cr: 15.0 to 30.0%, Mo: 1.0 to 5.0%, N: 0.05 to 0.45%, Nb: 0 to
0.50%, and V: 0 to 0.50%, with the balance being Fe and impurities, and satisfies
Formula (I). The austenitic stainless steel of the present embodiment has a grain
size nnmber ofless than 8.0 and a tensile strength of 690 MPa or more.
15 ~ 12.6C + 1.05Mn + Ni + 15N (1)
8
The symbols of elements in Formula (1) are to be substituted by the contents
of corresponding elements (in mass%).
[0029]
The austenitic stainless steel described above may contain one or more
elements selected from the group consisting ofNb: 0.01 to 0.50% and V: 0.01 to
0.50%.
[0030]
The grain size number of the austenitic stainless steel described above is
preferably 3.0 or more. In this case, the austenitic stainless steel described above
has a still more excellent tensile strength.
[0031]
The mixed grain ratio of the crystal grain micro-structure of the austenitic
stainless steel described above is preferably 20% or less. In this case, it is possible
to suppress the variations in strength of the austenitic stainless steel described above.
[0032]
A starting material having the chemical composition described above is
subjected to hot working one or more times, and after the last heating, subjected to
hot working at a reduction of area of 70%, whereby the austenitic stainless steel
described above is produced.
[0033]
In this case, an austenitic stainless steel having the chemical composition
- described above can be made to have a grain size number ofless than 8.0.
[0034]
Hereinafter, the austenitic stainless steel of the present embodiment will be
described in detail.
[0035]
[Chemical Composition]
The austenitic stainless steel of the present embodiment has a chemical
composition including the following elements.
[0036]
C: 0.10% or less
9
--
Carbon (C) stabilizes an austenite having an fcc structure, where a hydrogen
brittleness hardly occurs. However, an excessively high content of C results in the
precipitation of carbide in grain boundaries, decreasing the toughness of a steel.
Therefore, the content of C is made to be 0.10% or less. The upper limit of the
content of Cis preferably less than 0.1 0%, more preferably 0.08%, still more
preferably 0.06%.
[0037]
Si: 1.0% or less
Silicon (Si) is an impurity. Si is bonded toNi and Cr to form intermetallic
compounds. Furthermore, Si facilitates the growth of intermetallic compounds such
as a sigma phase. These intermetallic compounds decrease the hot workability of
steel. Therefore, the content of Si is made to be 1.0% or less. The upper limit of
the content of Si is preferably 0.8%. The content of Si is preferably as low as
possible.
[0038]
Mn: 2.1 to 6.0%
Manganese (Mn) stabilizes an austenite and suppresses the generation of a
martensite, which has a high susceptibility to hydrogen brittleness. Furthermore,
Mn is bonded to S to form MnS, increasing the machinability of a steel. An
excessively low content ofMn results in failure to provide the effects described
above. On the other hand, an excessively high content of Mn results in a decrease
·in the ductility and hot workability of a steel. Therefore, the content of Mn is made
to be 2.1 to 6.0%. The lower limit of the content ofMn is preferably more than
2.1 %, more preferably 2.5%, still more preferably 3.0%. The upper limit of the
content ofMn is preferably less than 6.0%.
[0039]
P: 0.045% or less
Phosphorus (P) is an impurity. P decreases the hot workability and
toughness of a steel. Therefore, the content of P is made to be 0.045% or less.
The upper limit of the content ofP is preferably less than 0.045%, more preferably
0.035%, still more preferably 0.020%. The content ofP is preferably as low as
possible.
10
[0040]
S: 0.1% or less
Sulfur (S) is bonded to Mn to form MnS, increasing the machinability of a
steel. However, an excessively high content of S results in a decrease in toughness
of a steel. Therefore, the content of S is made to be 0.1% or less. The upper limit
of the content of S is preferably less than 0.1 %, more preferably 0.09%, still more
preferably 0.07%. The content of S is preferably as low as possible.
[0041]
Ni: 8.0 to 16.0%
Nickel (Ni) stabilizes an austenite. Furthermore, Ni increases the ductility
and toughness of a steel. An excessively low content ofNi results in failure of
providing the effects described above. On the other hand, an excessively high
content of Ni results in saturation of the effects described above, increasing
manufacturing costs. Therefore, the content ofNi is made to be 8.0 to 16.0%. The
lower limit of the content ofNi is preferably more than 8.0%, more preferably 9.0%,
still more preferably 10.5%. The upper limit of the content ofNi is preferably less
than 16.0%, more preferably 15.0%.
[0042]
Cr: 15.0 to 30.0%
Chromium (Cr) increases the corrosion resistance of a steel. An excessively
low content of Cr results in failure to provide this effect. On the other hand, an
·excessively high content of Cr results in the generation of M23C6 carbide, decreasing
in ductility and toughness of a steel. Therefore, the content of Cr is made to be 15.0
to 30.0%. The lower limit of the content ofCr is preferably more than 15.0%, more
preferably 16.0%, still more preferably 17.0%, even still more preferably 18.0%.
The upper limit of the content of Cr is preferably less than 30.0%, more preferably
25.0%.
[0043]
Mo: 1.0 to 5.0%
Molybdenum (Mo) subjects an austenite to solid-solution strengthening.
Furthermore, Mo increases the corrosion resistance of a steel. An excessively low
content of Mo results in failure to provide the effect described above. On the other
II
hand, an excessively high content ofMo is liable to result in the precipitation of
intermetallic compounds, decreasing in ductility and toughness of a steel.
Therefore, the content ofMo is made to be 1.0 to 5.0%. The lower limit of the
content ofMo is preferably more than 1.0%, more preferably 1.2%. The upper
limit of the content ofMo is preferably less than 5.0%, more preferably 4.0%, still
more preferably 3.5%.
[0044)
N: 0.05 to 0.45%
Nitrogen (N) stabilizes an austenite. Furthermore, N increases the strength
of a steel through solid-solution strengthening. An excessively low content ofN
results in failure to provide the effects described above. On the other hand, an
excessively high content ofN causes the generation of coarse nitrides, decreasing the
mechanical properties of a steel such as toughness. Therefore, the content ofN is
made to be 0.05 to 0.45%. The lower limit of the content ofN is preferably more
than 0.05%, more preferably 0.1 0%, still more preferably 0.15%, even still more
preferably 0.21 %. The upper limit of the content ofN is preferably less than 0.45%,
more preferably 0.40%.
[0045)
The balance ofthe chemical composition of the austenitic stainless steel
according to the present embodiment is Fe and impurities. The term "impurities"
herein means elements that are mixed from ores and scraps used as raw material of a
·steel, or from the environment of a producing step.
[0046)
The austenitic stainless steel of the present embodiment may further contain,
in place of a part of Fe, one or more elements selected from the group consisting of
Nband V.
[0047]
Nb: 0 to 0.50%
Nb is an optional element and may not be contained.. If being contained, Nb
causes the generation of alloy carbides, increasing the strength of a steel. However,
an excessively high content ofNb results in saturation of the effect, increasing
manufacturing costs. Therefore, the content ofNb is made to be 0 to 0.50%. The
12
lower limit of the content ofNb is preferably 0.01 %, more preferably 0.05%. The
upper limit of the content ofNb is preferably less than 0.50%, more preferably
0.40%, still more preferably 0.30%.
[0048]
V: 0 to 0.50%
V is an optional element and may not be contained. If being contained, V
causes the generation of alloy carbides, increasing the strength of a steel. However,
an excessively high content ofV results in saturation of the effect, increasing
manufacturing costs. Therefore, the content ofV is made to be 0 to 0.50%. The
lower limit of the content ofV is preferably 0.01 %, more preferably 0.05%. The
upper limit of the content ofV is preferably less than 0.50%, more preferably 0.35%,
still more preferably 0.30%.
[0049]
[Formula (1)]
The chemical composition described above further satisfies Formula (1 ).
15 s 12.6C + 1.05Mn + Ni + 15N (1)
The symbols of elements in Formula (l) are to be substituted by the contents
of corresponding elements (in mass%).
[0050]
As mentioned previously, C, Mn, Ni, and N stabilize an austenite. The
diffusion coefficient of hydrogen in an austenite is low. For this reason, hydrogens
·are difficult to diffuse in an austenite.
[0051]
Let Fl = 12.6C + 1.05Mn + Ni + 15N. As illustrated in FIG. 2, when F1 is
less than 15, an austenite difficult to stabilize, and thus the hydrogen brittleness
resistance is low. On the other hand, when Fl is 15 or more, the hydrogen
brittleness resistance becomes significantly high. Therefore, Fl is 15 or more. Fl
is preferably 16 or more, more preferably 17 or more.
[0052]
[Grain Size]
Furthermore, the grain size number specified in JIS G055l (2005) ofthe
austenitic stainless steel of the present embodiment is less than 8.0. For this reason,
13
the austenitic stainless steel of the present embodiment has a low cutting resistance.
When the cutting resistance is low, it is possible to suppress wear of a cutting tool,
increasing the productivity. Furthermore, when the grain size number is less than 8,
it is easy for chips to be detached from a work material and a cutting tool in cutting,
which increases the chip disposability. As seen from the above, when the grain size
number is less than 8.0, the machinability of the steel is increased. On the other
hand, when the grain size number is excessively low, the tensile strength of a steel
may be decreased. For this reason, the grain size number is preferably 3.0 or more,
more preferably 4.0 or more.
[0053]
The grain size number is determined by the following method. A test
specimen for microscopy is taken from an austenitic stainless steel. On the taken
test specimen, the microscopic test method on grain size specified in JIS 00551
(2005) is performed to evaluate the grain size number. Specifically, etching is
performed on a surface of the test specimen using a well-known etching reagent (e.g.,
Olyceregia, Kalling's reagent, or Marble's reagent) so as to cause a crystal grain
boundary on the surface to appear. In ten visual fields on the etched surface, a grain
size number is determined for each visual field. The area of each visual field is
about.40 mm2. By performing a comparison with .the grain size number standard
chart specified in the section 7.1.2 ofJIS 00551 (2005), the grain size number in
each visual field is evaluated. The average of the grain size numbers of the
·respective visual fields is defined as a grain size number of the austenitic stainless
steel of the present embodiment.
[0054]
[Tensile Strength]
The tensile strength of the austenitic stainless steel of the present embodiment
is 690 MPa or more. It is possible to make the tensile strength 690 MPa or more by
making the austenitic stainless steel contain Mo at the content previously mentioned
and further adjusting working conditions in the final operation of hot working, which
will be described later. The tensile strength is preferably made to be 720 MPa or
more. In order to increase the machinability of a steel, the tensile strength is
preferably made to be 880 MPa or less.
14
[0055]
[Mixed Grain Ratio]
If the crystal grain micro-structure is of mixed grain size, there is the risk of
causing variations in strength or machinability. Therefore, the upper limit of a
mixed grain ratio is preferably 25%, more preferably 20%. The lower the mixed
grain ratio is, the more preferable it is. The mixed grain size refers to a state in
which, in the microscopy mentioned previously, there are unevenly distributed grains
having a grain size number that is higher or lower, by three or more, than the grain
size number of grains with a maximum frequency in one visual field and the
unevenly distributed grains occupy 20% or more of the area of the visual field, or a
state in which, among the visual fields, there is a visual field having a grain size
number higher or lower, by three or more, than that of the another visual field.
[0056]
The mixed grain ratio can be measured by, for example, the following method.
A test specimen for microscopy is taken from an austenitic stainless steel, and the
previously-mentioned microscopic test method is performed. The mixed grain ratio
can be determined by substituting, into Formula (2), the number of all visual fields
observed in the microscopic test method, which is denoted by N, and the number of
visual fields determined to be mixed grain size, which is denoted by n.
Mixed grain ratio(%)= (n!N) x 100 (2)
[0057]
By performing the producing step to be described later, it is possible to make
the grain size number less than 8.0 and to make the tensile strength 690 MPa or more.
[0058]
[Producing Method]
The method for producing the austenitic stainless steel of the present
embodiment includes a step of preparing starting material and a step of subjecting the
starting material to hot working. The producing method will be described below.
[0059]
[Step of Preparing Starting Material]
Molten steel having the previously-mentioned chemical composition is
produced. As necessary, well-known degassing is performed on the produced
15
molten steel. From the degassed molten steel, starting material is produced. A
method for producing the starting material is, for example, a continuous casting
process. By the continuous casting process, continuous casting material (starting
material) is produced. The continuous casting material is, for example, slab, bloom,
billet, and the like. The molten steel may be subjected to an ingot-making process
to be made into an ingot.
[0060]
[Step of Hot Working]
The starting material (continuous casting material or an ingot) is subjected to
hot working by a method well-known to be made into an austenitic stainless steel
product. The austenitic stainless steel product is, for example, a steel pipe, a steel
plate, a steel bar, a wire rod, a forged steel, or the like. The austenitic stainless steel
product may be made by, for example, hot extrusion working according to the U gineSejoumet
process.
[0061]
The austenitic stainless steel product may be produced by a single operation
of hot working, or by a plurality of operations of hot working. When the hot
working is performed by the plurality of operations, reheating is performed before
every operation of hot working after the second operation so as to perform the
working on the entire steel uniformly. This makes the mixed grain ratio of the
crystal grain micro-structure of the steel low.
- [0062]
In the final operation of hot working (the h6t working if the hot working is
performed only once, otherwise the final operation of the hot working), heating
conditions and the reduction of area by the hot working are as follows.
[0063]
Heating temperature: I 000 to 1250°C
An excessively low heating temperature is liable to result in a crack
attributable to impurity elements such asP. On the other hand, an excessively high
heating temperature is liable to result in a crack inside a steel product due to the
occurrence of grain boundary melting. Therefore, a preferable heating temperature
ranges from 1000 to 1250°C.
16
--
[0064]
Reduction of area: 70% or less
When the cross-sectional area of the starting material before the final
operation of the hot working is denoted by AO (mm2
), and the cross-sectional area of
the starting material after the final operation of the hot working is denoted by Al
(mm2), a reduction of area RA (%)is defined with Formula (3).
RA=(AO-Al)/AOxlOO (3)
[0065]
When the reduction of area previously described is excessively high, crystal
grains are made to be fine by the hot working, and the grain size number becomes
8.0 or more. Therefore, the reduction of area is made to be 70% or less.
[0066]
Meanwhile, as illustrated in FIG. 3, in a steel product satisfying the chemical
composition described above and Formula (1), the reduction of area RAin the final
operation of the hot working has a proportional relation with a tensile strength TS.
For this reason, an excessively low reduction of area RA may lead to a low tensile
strength although the steel product is an austenitic stainless steel product satisfying
the chemical composition described above and Formula (1). Therefore, in order to
increase the tensile strength, the reduction of area is set as appropriate.
[0067]
Preferably, the reduction of area RA is made to be 20% or more for an
·austenitic stainless steel product satisfying the chemical composition described above
and Formula (1). In this case, the tensile strength TS of the austenitic stainless steel
product after the final operation of the hot working is 690 MPa or more. More
preferably, the reduction of area RA is made to be 30% or more. In this case, the
mixed grain ratio of the austenitic stainless steel can be made to be further low.
This allows the suppression of variations in strength and machinability. Still more
preferably, the reduction of area RA is made to be more than 35%. In this case, the
tensile strength of the austenitic stainless steel can be further increased.
[0068}
17
In the producing step of the present embodiment, solution treatment and cold
working after the hot working are omitted. That is, the austenitic stainless steel of
the present embodiment is a material as subjected to the hot working.
[0069]
The austenitic stainless steel produced by the above producing steps is
excellent in hydrogen brittleness resistance and machinability and has a high strength.
EXAMPLE
[0070]
[Test Method]
Molten steels having chemical compositions of test numbers AI to A20 and
Bl to B9 shown in Table 1 were produced with a vacuum furnace.
[0071]
18
~
[Table 1]
Test Chemical composition (mass%, the balance being Fe and impurities)
Fl
number c Si Mn p s Ni Cr Mo N Nb v
AI 0.030 0.70 4.59 0.013 0.0002 11.70 22.05 2.00 0.32 0.18 0.22 21.7
A2 0.030 0.36 4.44 0.015 0.0002 11.80 21.70 1.20 0.31 0.21 0.21 21.5
A3 0.033 0.39 4.51 0.016 0.0003 12.21 22.07 2.22 0.33 0.20 0.21 22.3
A4 0,031 0.44 4.00 0,017 0.0005 10.17 21.57 1.98 0.21 0.18 0.23 17.9
AS 0.030 0.43 4.49 0.014 0.0003 12.04 21.80 1.99 0.32 0.20 0.22 21.9
A6 0.010 0.34 5.70 0.016 0.0002 12.35 24.34 2.23 0.32 0.21 0.20 23.3
A7 0.027 0.33 4.23 0.012 0.0006 11.98 22.06 2.12 0.31 0.20 0.20 21.4
A8 0.055 0.36 3.10 0.015 0.0002 14.50 . 23.40 1.75 0.18 - 0.30 21.1
A9 0.029 0.38 4.46 0.016 0.0003 12.11 22.41 1.98 0.32 0.20 0.19 22.0
A10 0.035 0.39 4.44 0,017 0.0650 11.98 22.16 2.07 0.31 0.20 0.20 21.7
All 0.055 0.41 4.55 0.016 0.0050 12.13 20.11 2.10 0.33 0.19 0.20 22.6
A12 0.032 0.41 4.50 0,017 0.0030 12.24 22.08 2.07 0.32 0.32 - 22.2
A13 0.029 0.39 4.68 0,015 0.0002 12.10 22.04 2.07 0.32 0.21 0.24 22.2
A14 0.034 0.42 4.40 0.004 0.0025 12.08 18.10 3.40 0.31 0.09 0.23 21.8
A15 0.042 0.39 4.35 0.014 0.0008 13.00 21.91 2.55 0.32 0.19 0.18 22.9
A16 0.060 0.40 5.91 0.014 0.0004 10.55 21.83 2.10 0.38 0.45 0.07 23.2
A17 0.028 0.35 4.11 0.016 0.0006 10.30 21.34 2.04 0.26 - - 18.9
A18 0.040 0.30 2.10 0.028 0.0008 13.50 21.40 1.30 0.10 - - 17.7
A19 0.035 0.35 4.69 0,011 0.0002 13.52 21.99 2.23 0.31 0.18 0.19 23.5
A20 0.031 0.48 4.33 0.013 0.0005 12.79 20.28 2.21 0.38 0.41 0.32 23.4
B1 0.033 0.35 5.50 0.016 0.0011 12.36 22.06 2.05 0.38 0.30 0.22 24.3
B2 0.031 0.36 5.65 0,015 0.0009 12.45 21.80 2.49 0.32 0.28 0.21 23.6
B3 0.034 0.35 5.70 0,015 0.0010 11.75 21.90 2.04 0.31 0.28 0.03 22.8
19
~
B4 0.050 0.48 0.86 0.020 0.0008 10.23 16.07
B5 0.041 0.31 3.15 O.Ql5 0.0002 8.15 17.50
B6 0.030 0.30 1.95 0.028 0.0008 9.50 18.63
B7 0.023 0.42 3.32 0.017 0.0006 5.02 17.61
B8 0.025 0.41 3.35 0.017 0.0004 5.12 17.80
B9 0.027 0.33 4.23 0.012 0.0006 11.98 22.06
20
2.07 0.04 - -
1.15 0.12 0.11 -
0.61 0.09 - -
1.81 0.25 0.19 0.21
2.05 0.29 - -
2.12 0.31 0.20 0.20
•'
12.4
13.8
13.3
12.5
13.3
21.4
''C•~<=';'•"-~'.v'CW';=,<"~'-~"~'-'"
t
~
r I i
[0072]
Fl in Table 1 is the value ofFl by the definition previously mentioned.
From the molten steel of each test number, a 50-kg ingot was produced. The ingot
was subjected to the hot forging to produce into a block having a thickness of 70 mm.
[0073]
The produced block was subjected to the final operation of hot working (hot
rolling) to produce an austenitic stainless steel plate. A heating temperature (0 C)
and a reduction of area RA (%)in the final operation of hot working were set as
shown in Table 2. Only on the test number B9, solution heat treatment was
performed. In the solution heat treatment, the temperature was 1 060°C, and the
heating time period was 30 minutes.
[0074]
21
i·'.·
' [Table 2]
Test Heating Reduction of Solution heat Grain size TS Relative rupture Relative amount Mixed grain
number temperature (0 C) area(%) treatment number (MPa) elongation (%) ratio of wear ratio(%)
AI 1200 66 No 7.5 825.3 90 0.44 0
A2 1250 63 No 7.3 787.9 89 0.48 5
A3 1250 58 No 6.8 809.3 94 0.47 0
A4 1200 51 No 6.4 779.3 91 0.48 5
AS 1200 61 No 6.8 774.3 93 0.49 5
A6 1200 45 No 5.4 764.2 97 0.49 5
A7 1200 45 No 5.7 761.7 92 0.50 5
A8 1200 38 No 5.1 730.1 86 0.52 10
A9 1200 56 No 6.8 761.3 90 0.50 5
AlO 1250 47 No 5.7 736.5 88 0.55 10
All 1250 40 No 5.2 772.4 86 0.51 5
A12 1250 42 No 5~0 756.1 87 0.52 5
Al3 1200 54 No 6.5 779.3 91 0.49 0
Al4 1250 55 No 6.1 815.6 92 0.52 0
Al5 1200 57 No 7.3 824.0 90 0.43 0
Al6 1250 68 No 7.8 875.4 91 0.42 0
A17 1200 44 No 5.0 751.1 86 0.49 10
A18 1250 30 No 5.0 706.0 85 0.53 10
Al9 1250 35 No 2.7 692.6 90 0.58 5
A20 1200 20 No 3.1 713.2 81 0.53 25
Bl 1200 88 No I 0.1 891.2 90 0.35 0
B2 1200 83 No 9.5 872.5 91 0.33 0
B3 1200 75 No 9.1 851.2 92 0.36 0
22
~
B4 1200 32 No 4.7
B5 1250 45 No 5.2
B6 1200 38 No 4.8
B7 1250 55 No 6.6
B8 1200 60 No 6.5
B9 1250 34 Yes 2.2
23
556.7 46 0.78
655.1 53 0.62
586.1 58 0.70
791.3 39 0.49
772.1 43 0.49
687.3 90 0.60
•'
15
15
15
10
10
0
i
1
I
I
I I
I
I
I
[0075]
[Measurement Test on Grain Size]
The steel plate of each test number was cut in a direction perpendicular to a
rolling direction. From the resultant section, a portion the surface of which is the
center of the section in a width and thickness directions (hereinafter, referred to as an
observed surface) was taken as a sample. The observed surface of each sample was
subjected to well-known electropolishing. For the observed surface after the
electropolishing, a grain size number was determined based on the previouslymentioned
method.
[0076]
[Measurement Test on Mixed Grain Ratio]
The steel plate of each test number was subjected to the microscopy
previously mentioned, and the mixed grain ratio thereof was determined by the
method previously mentioned. For each test number, the observation was
performed on ten visual fields.
[0077]
[Tensile Test]
For each test number, a round-bar tensile test specimen was taken from the
central portion of the steel plate. The round-bar tensile test specimen includes the
central axis of the steel plate, and the parallel portion of the round bar test specimen
was parallel to the rolling direction of the steel plate. The diameter of the parallel
• portion was 5 mm. On the round bar test specimen, a tensile test was performed at
a normal temperature (25°C) in the atmosphere, and a tensile strength TS (MPa) of
the steel plate was determined for each test number.
[0078]
[Hydrogen Brittleness Resistance Evaluation Test]
For each test number, two round-bar tensile test specimens (first and second
test specimens) were taken from the central portion of the steel plate. Each of the
round-bar tensile test specimens includes the central axis of the steel plate, and the
parallel portion of the round bar test specimen was parallel to the rolling direction of
the steel plate. The diameter of the parallel portion was 3 mm.
[0079]
24
On the first test specimen, a tensile test was performed at a normal
temperature (25°C) in the atmosphere (referred to as an atmospheric tensile test) to
measure a rupture elongation BEo. Furthermore, on the second test specimen, a
tensile test was performed at a normal temperature (25°C) in a high-pressure
hydrogen atmosphere at 45 MPa (referred to as a hydrogen tensile test) to measure a
rupture elongation BEH. In both of the atmospheric tensile test and the hydrogen
tensile test, a strain rate was set at 3x 1 o·6 IS. The effect of hydrogen brittleness
manifests in the form of rupture elongation. Thus, the relative rupture elongation
(%)was defined with Formula (4).
Relative rupture elongation= BEHI BEo x 100 (4)
[0080]
When a test specimen had a relative rupture elongation of 80% or more, the
test specimen was determined to be excellent in hydrogen brittleness resistance.
[0081]
[Machinability Evaluation Test]
For each test number, a bar test specimen was taken from the central portion
of the steel plate. Each of the bar test specimens includes the central axis of the
steel plate, and the parallel portion of the bar test specimen was parallel to the rolling
direction of the steel plate. The bar test specimen had a round cross section, and the
diameter thereof was 8 mm.
[0082]
On the bar test specimen, peeling treatment was performed. The bar test
specimen having the diameter of 8 mm was subjected to the peeling treatment for
five minutes. In the peeling treatment, a cemented carbide tool equivalent to P20
specified in JIS Standard was used, the cemented carbide tool not being subjected to
coating treatment. A cutting speed was 100m/min, and a feed was 0.2 mm/rev, and
a depth of cut was 1.0 mm. No lubricant was used in the peeling. The peeling
treatment was performed with the above conditions, and an amount of flank wear W1
(mm) of the cemented carbide tool after the test was measured.
[0083]
Furthermore, a bar test specimen having a chemical composition equivalent to
SUS316 specified in JIS Standard (referred to as a reference test specimen) was
25
--
prepared. The reference test specimen had the same shape as that of the bar test
specimen of each test number. On the reference test specimen, the peeling
treatment was performed under the same conditions as those of the above, and an
amount of flank wear WO (mm) of the cemented carbide tool after the test was
measured. Based on the results of the measurement, a relative ·amount ratio of wear,
which is defined with the following Formula (5), was determined.
Relative amount ratio of wear= WO I WI (5)
[0084)
When a test specimen had a relative amount ratio of wear of0.40 or more, the
test specimen was determined to be excellent in machinability.
[0085)
[Test Result]
Referring to Table 2, the chemical compositions of the steels of the test
numbers AI to A20 were appropriate, satisfYing Formula (1). Furthermore, the
steels of the test numbers AI to A20 were produced under appropriate conditions and
had grain size numbers ofless than 8.0. For this reason, the relative rupture
elongations of the steels of these test numbers were 80% or more, exhibiting
excellent hydrogen brittleness resistances. Furthermore, the relative amount ratios
of wear of the steels of these test numbers were 0.4 or more, exhibiting excellent
machinabilities. Furthermore, the tensile strengths of the steels of these test
numbers were 690 MPa or more, exhibiting high strengths .
• [0086]
As to the test numbers A I to A 19, the reductions of area in the final operation
of hot working were 30% or more. For this reason, the test numbers AI to Al9
were low in mixed grain ratio of grain size as compared with the test number A20
having a reduction of area of 20%.
[0087)
As to the test numbers AI to Al7, the reductions of area in the final operation
of hot working were more than 35%. For this reason, the test numbers AI to Al7
were high in tensile strength, 720 MPa or more, as compared with A 18 to A20
having reductions of area of 3 5% or less.
[0088)
26
As to the test numbers A 1 to A 18 and the test number A20, the grain size
numbers were 3.0 or more. For this reason, the test numbers AI to A18 and the test
number A20 were high in tensile strength TS as compared with the test number A19
having a grain size number ofless than 3.0.
[0089]
In contrast, the test numbers B 1 to B3 had appropriate chemical compositions
but were too high in reduction of area in the final operation of hot working. As a
result, the grain size numbers thereof were more than 8.0. For this reason, the test
numbers B 1 to B3 had relative amount ratios of wear ofless than 0.40, which is low
in machinability.
[0090]
The chemical composition of the test number B4 included excessively low
contents ofMn and Nand did not satisfY Formula (1). For this reason, the test
number B4 had a relative rupture elongation ofless than 80%, which is low in
hydrogen brittleness resistance.
[0091]
The test number B5 had an appropriate content for each element but did not
satisfY Formula (1 ). For this reason, the test number B5 had a relative rupture
elongation ofless than 80%, which is low in hydrogen brittleness resistance.
[0092]
The chemical composition of the test number B6 included excessively low
contents ofMn and Mo and did not satisfY Formula (1). For this reason, the test
number B6 had a relative rupture elongation ofless than 80%, which is low in
hydrogen brittleness resistance.
[0093]
The chemical compositions of the test numbers B7 and B8 included an
excessively low content ofNi and did not satisfY Formula (1 ). For this reason, the
test numbers B7 and B8 had relative rupture elongations of less than 80%, which is
low in hydrogen brittleness resistance.
[0094]
27
The test number B9 had an appropriate content for each element, satisfYing
Formula (1), but was subjected to the solution heat treatment after the hot working.
For this reason, the tensile strength of the test number B9 became less than 690 MPa.
[0095)
As seen from the above, the embodiment according to the present invention
has been described. However, the embodiment previously mentioned is merely an
example for practicing the present invention. Therefore, the present invention is not
limited to the previously-mentioned embodiment, and the previously-mentioned
embodiment can be modified and practiced as appropriate without departing from the
scope of the present invention.

We claim:
I. An austenitic stainless steel comprising a chemical composition that includes,
in mass%:
C: 0.10% or less;
Si: 1.0% or less;
Mn: 2.1 to 6.0%;
P: 0.045% or less;
S: 0.1% or less;
Ni: 8.0 to 16.0%;
Cr: 15.0 to 30.0%;
Mo: 1.0 to 5.0%;
N: 0.05 to 0.45%;
Nb: 0 to 0.50%; and
V: 0 to 0.50%,
with the balance being Fe and impurities, and
satisfies Formula (I),
the austenitic stainless steel having a grain size number ofless than 8.0 and a
tensile strength of 690 MPa or more:
15,; 12.6C + 1.05 Mn + Ni + 15N (!)
where symbols of elements in Formula(!) are to be substituted by contents of
·the corresponding elements (in mass%).
2. The austenitic stainless steel according to claim I, further comprising one or
more elements selected from the group consisting of
Nb: 0.01 to 0.50% and
V: 0.01 to 0.50%.
3. The austenitic stainless steel according to claim I or claim 2, wherein the
grain size number is 3.0 or more.
29
--
4. The austenitic stainless steel according to any one of claim 1 to claim 3,
wherein a mixed grain ratio of a crystal grain micro-structure is 20% or less.
5. The austenitic stainless steel according to any one of claim 1 to claim 4,
wherein a starting material having the chemical composition is subjected to hot
working one or a plurality of times, and after last heating, subjected to hot working at
a reduction of area 70% or less.
6. A method for producing the austenitic stainless steel according to any one of
claim 1 to claim 4, comprising:
a step of preparing a starting material having the chemical composition; and
a step of performing hot working on the starting material one or a plurality of
times, wherein
in the step of performing the hot working, a reduction of area in the hot
working after last heating is 70% or less.