Note: Descriptions are shown in the official language in which they were submitted.
CA 02791878 2014-07-16
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DESCRIPTION
MARTENSITIC STAINLESS STEEL WITH EXCELLENT WELD
CHARACTERISTICS, AND MARTENSITIC STAINLESS STEEL MATERIAL
TECHNICAL FIELD
[0001]
The present invention relates to a martensitic stainless steel suitably used
for
portions that are needed to be welded in welded structures such as
construction structures,
ship structures, and the like, and the present invention also relates to a low-
cost
martensitic stainless steel material which is manufactured using the
martensitic stainless
steel, and which is excellent in impact characteristics and corrosion
resistance of a base
material and welded portions, and the cost is low since the Ni content is
redcued in the
martensitic stainless steel material.
BACKGROUND ART
[0002]
The martensitic stainless steel is widely used for things such as blades,
springs,
brake discs, and the like since the strength can be easily enhanced through a
quenching
thermal treatment. However, the martensitic stainless steel has a low
toughness and poor
weldability; and therefore, the martensitic stainless steel is not used for
welded structures.
Meanwhile, a steel material having improved toughness, weldability, and
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corrosion resistance is developed by reducing the content of C and adding
approximately
3% or more of Ni to a steel containing 13% to 17% of Cr, and the steel
material is used for
water wheel runners for hydroelectric power generation or steel pipes for oil
well (for
example, Patent Documents 1 to 4).
[0003]
However, even in the improved martensitic stainless steel as described above,
tempering resistance is extremely large. Therefore, a problem still remains in
that
thermal treatment facility capacities are impaired since a long-term treatment
or the like is
required in a tempering thermal treatment for tempering the characteristics of
a final
product, and the manufacturing costs are high.
[0004]
Therefore, a martensitic stainless steel for which a thermal treatment for
tempering is not required or manufacturing conditions for which a
dehydrogenation
treatment is not required are studied, and Patent Document 4 is disclosed
which aims to
obtain a martensitic single-phase structure, and Patent Document 5 is
disclosed which has
a multiphase structure mainly including a martensite phase and including a
ferrite phase or
a residual austenite phase.
[0005]
As disclosed in Patent Document 4, in the majority of martensitic stainless
steels,
the amount of Cr is in a range of 11% to 15%, the corrosion resistance is poor
compared to
a ferrite stainless steel such as SUS430, and there are cases in which rusting
(occurrence
of rusts) is caused even in an indoor environment. Therefore, in order to
obtain excellent
corrosion resistance, it is necessary to add Mo or increase the amount of Cr.
[0006]
In addition, Patent Document 5 discloses that 15% or more of Cr or 1% or more
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of Mo is preferably included in order to enhance corrosion resistance.
However, the
martensitic stainless steel of Patent Document 5 has a metallic structure
mainly including
a martensite phase that includes a ferrite phase, the hot workability is not
favorable, and
there was a problem in that the manufacturing yield of the steel material
frequently
degraded. In addition, in order to secure mechanical characteristics, it is
necessary to add
austenite-forming elements at amounts that commensurate with the increment in
amounts
of Cr and Mo; and thereby, an increase in the alloy costs is caused.
That is, as the steel that can favorably maintain the characteristics of a
base
material and welded portions, a steel containing a large amount of Ni is in
practical use.
However, there was no practical steel which had favorable hot workability,
corrosion
resistance equivalent to that of SUS430 in a base material and welded
portions, and
excellent mechanical characteristics, and in which the amount of Ni was
reduced so as to
be inexpensive.
PRIOR ART DOCUMENT
Patent Document
[0007]
Patent Document 1: Japanese Unexamined Patent Application, First Publication
No. H06-306549
Patent Document 2: Japanese Unexamined Patent Application, First Publication
No. H06-306551
Patent Document 3: Japanese Unexamined Patent Application, First Publication
No. H02-243739
Patent Document 4: Japanese Unexamined Patent Application, First Publication
No. H02-243740
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Patent Document 5: Japanese Unexamined Patent Application, First Publication
No. 2001-279392
Non Patent Document
[0008]
Non-Patent Document 1: Current Advances in Materials and Processes, Vol. 3
(1990), 1840
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0009]
In consideration of the above-described problems, the present inventors
address
an object of the invention of clarifying the component system and metallic
structure of
inexpensive martensitic stainless steel having favorable hot workability,
mechanical
characteristics, and corrosion resistance equivalent to that of SUS430, and
developing a
practical steel material.
Means for Solving the Problems
[0010]
C, N, Mn, Cu, Co, or the like is considered as an element that can replace Ni;
however, with regard to the above-described martensitic stainless steel, there
is a few
publications in relation to a steel containing large amounts of Mn, Cu, and
Co. As an
example, Non-Patent Document 1 shows an example in which a high-purity
stainless steel
containing 17% of Cr is used as the base, and Ni or Mn is added. However, Non-
Patent
Document 1 does not disclose an example in which both of Ni and Mn are added,
and
does not consider corrosion resistance either.
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[0011]
Meanwhile, generally, Mn is an element that degrades corrosion resistance.
Therefore, there are only a few examples in which active addition of Mn is
attempted to a
martensitic stainless steel having poor corrosion resistance compared to an
ordinary
5 stainless steel. In the case where the amount of C is increased and, at
the same time, the
amount of Mn is increased, whether or not a desired corrosion resistance can
be obtained
is still under question. Therefore, based on technical aspects or experience
thus far
obtained, it was impossible to employ a method of adjusting the above-
described alloy
elements in order to develop a practical steel material that can secure
excellent hot
workability and mechanical characteristics as well as corrosion resistance.
[0012]
Regarding a steel in which a steel containing 16% of Cr and 2% of Ni is used
as
the base and 2% or more of Mn is added, the inventors studied in detail the
influence of
the component elements and the metallic structure of a steel material on a
variety of the
above-described characteristics. As a result, it was found that the amount of
Ni, whose
price widely varies, can be suppressed, and both of toughness and corrosion
resistance of
welded portions can be satisfied by setting the amounts of Cr, Ni, Mn, and
other elements
described below in predetermined ranges. Furthermore, it was found that the
mechanical
characteristics of a base material can be secured by setting the phase
fractions in a steel
material even when thermal treatments of quenching and tempering which were
required
in the related art are not carried out. Based on the above findings, the
invention has been
completed.
[0013]
The features of the invention are as follows.
(1) Martensitic stainless steel with excellent weld characteristics according
to an
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aspect of the invention contains, in terms of percent by mass, C: 0.003% to
0.03%, Si:
0.01% to 1.0%, Mn: 3.0% to 6.0%, P: 0.05% or less, S: 0.003% or less, Ni: 1.0%
to 3.0%,
Cr: 15.0% to 18.0%, Mo: 0% to 1.0%, Cu: 0% to 2.0%, Ti: 0% to 0.05%, N: 0.05%
or less,
Al: 0.001% to 0.1%, and 0: 0.005% or less with a remainder being Fe and
inevitable
impurities. A total amount of C and N is in a range of 0.060% or less, yp,a,
represented
by the formula 1 is in a range of 80 or more, and '(pot represented by the
formula 2 is in a
range of 60 to 90.
'(max = 420 x C% + 470 x N% + 23 x Ni% + 9 x Cu% + 7 x Mn% ¨11.5 x Cr% ¨
11.5 x Si% ¨ 52 x Al% + 189 ¨ Formula 1
Ypot = 700 x C% + 800 x N% + 10 x (Mn% + Cu%) + 20 x Ni% ¨9.3 x Si% ¨6.2
x Cr% ¨ 9.3 x Mo% ¨ 74.4 x Ti% ¨ 37.2 x Al% + 63.2 ¨ Formula 2
Here, C%, N%, Ni%, Cu%, Mn%, Cr%, Si%, A1%, Mo%, and Ti% represent the
contents (mass%) of the respective elements.
[0014]
(2) The martensitic stainless steel with excellent weld characteristics
according to
the aspect of the invention described in the above (1) may further contain Nb,
and '(pot that
is calculated by the formula 3 instead of the formula 2 may be in a range of
60 to 90.
7p0t = 700 x C% + 800 x N% + 10 x (Mn% + Cu%) + 20 x Ni% ¨ 9.3 x Si% ¨ 6.2
x Cr% ¨ 9.3 x Mo% ¨ 3.1 x Nb% ¨ 74.4 x Ti% ¨ 37.2 x Al% + 63.2 ¨ Formula 3
Here, C%, N%, Mn%, Cu%, Ni%, Si%, Cr%, Mo%, Nb%, Ti%, and Al%
represent the content (mass%) of the respective elements.
[0015]
(3) The martensitic stainless steel with excellent weld characteristics
according to
the aspect of the invention described in the above (1) or (2) may further
contain either one
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or both of V: 0.5% or less and W: 1.0% or less.
(4) The martensitic stainless steel with excellent weld characteristics
according to
the aspect of the invention described in any one of the above (1) to (3) may
further contain
Co: 1.0% or less.
(5) The martensitic stainless steel with excellent weld characteristics
according to
the aspect of the invention described in any one of the above (1) to (4) may
further contain
one or more selected from B: 0.0050% or less, Ca: 0.0050% or less, Mg: 0.0030%
or less,
and REM: 0.10% or less.
[0016]
(6) A martensitic stainless steel material according to an aspect of the
invention
has a composition described in any one of the above (1) to (5), and has a
structure
including 5% to 30% of a ferrite phase and 0% to 20% of a residual austenite
phase with a
remainder being a martensite phase.
(7) The martensitic stainless steel material according to the aspect of the
invention described in the above (6) may have a yield strength in a range of
400 MPa to
800 MPa.
Effects of the Invention
[0017]
The martensite steel having the composition of the aspect of the invention
exhibits an effect of excellent toughness and excellent corrosion resistance
of welded
portions. In addition, according to the aspect of the invention, it is
possible to provide a
inexpensive martensitic stainless steel material that can be used for large-
scale welded
structures such as construction structures, ship structures, and the like. In
addition, since
desired characteristics can be obtained even when long-term thermal treatments
of
Amended Sheet (Article 34 of Patent Cooperation Treaty)
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quenching and termpering are not carried out, mass productivity can be
improved.
Therefore, the aspect of the invention can significantly contribute to the
industry.
BEST MODE FOR CARRYING OUT THE INVENTION
[0018]
Hereinafter, firstly, the reasons why the chemical composition of the
martensitic
stainless steel of the present embodiment is limited will be described.
Meanwhile, the
unit of the contents of the respective components in the following is mass%.
C is included at a content of 0.003% or more in order to secure the strength
of a
steel. However, in the case where more than 0.03% of C is included, the
strength
increases more than necessary, and corrosion resistance and toughness
deteriorate in
welded portions. Therefore, the content of C is limited to 0.003% to 0.03%.
The
content of C is preferably in a range of 0.005% to 0.025%.
[0019]
Si is added at a content of 0.01% or more for deoxidization. However, in the
case where more than 1.0% of Si is added, toughness deteriorates. Therefore,
the upper
limit of the content of Si is limited to 1.0%. The content of Si is preferably
in a range of
0.2% to 0.5%.
[0020]
Mn is added at a content of 3.0% or more in order to improve the toughness of
welded portions. However, an increase in the content of Mn deteriorates
corrosion
resistance. In the steel of the embodiment, the content of Mn, imm, Ypot, and
the fraction
of a ferrite phase in a steel material of the embodiment which will be
described below
have a close relationship, and deterioration of the corrosion resistance due
to an increase
in the content of Mn is suppressed by controlling a metallic structure.
However, in the
Amended Sheet (Article 34 of Patent Cooperation Treaty)
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'
case where more than 6.0% of Mn is included, desired corrosion resistance
cannot be
secured. Therefore, the upper limit of the content of Mn is limited to 6.0%.
The
content of Mn is preferably in a range of 3.5% to 5.5%.
[0021]
Since P deteriorates hot workability and toughness, the content of P is
limited to
0.05% or less. The content of P is preferably in a range of 0.03% or less. P
is an
element that is inevitably included in a steel, and the smaller the content
thereof, the more
preferable. However, extreme reduction causes an increase in the costs; and
therefore,
generally, P is inevitably included at a content of approximately 0.005% or
more.
[0022]
Since S deteriorates hot workability, toughness, and corrosion resistance, the
content of S is limited to 0.003% or less. The content of S is preferably in a
range of
0.001% or less. In addition, S is also an element that is inevitably included
in a steel, and
the smaller the content thereof, the more preferable. However, extreme
reduction causes
an increase in the costs; and therefore, generally, S is inevitably included
at a content of
approximately 0.0001% or more.
[0023]
Ni stabilizes an austenite structure, and Ni improves corrosion resistance
with
respect to a variety of acids and, furthermore, Ni improves toughness.
Therefore, Ni is
included at a content of 1.0% or more. On the other hand, Ni is an expensive
alloy, and
the content of Ni is limited to 3.0% or less from the viewpoint of costs. The
content of
Ni is preferably in a range of 1.5% to 2.5%.
[0024]
Cr is included at a content of 15.0% or more in order to secure basic
corrosion
resistance. On the other hand, in the case where more than 18.0% of Cr is
included,
Amended Sheet (Article 34 of Patent Cooperation Treaty)
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toughness and corrosion resistance in welded portions are impaired. Therefore,
the
content of Cr is set to be in a range of 15.0% to 18.0%. The content of Cr is
preferably
in a range of 16% to 17%.
[0025]
5 Mo is an element extremely effective for incrementally enhancing the
corrosion
resistance of a stainless steel, and Mo is an arbitrary component (selective
component)
that is included as necessary. Since Mo is an extremely expensive element, in
the case
where Mo is added to enhance corrosion resistance, the upper limit of the
content of Mo is
set to 1.0% or less from the viewpoint of costs. In the case where Mo is
added, the
10 content of Mo is preferably in a range of 0.1% to 0.5%.
[0026]
Cu is an element having actions of incrementally enhancing corrosion
resistance
of a stainless steel with respect to acids and improving toughness, and Cu is
an arbitrary
component (selective component) that is included as necessary. In the case
where more
than 2.0% of Cu is included, the content of Cu exceeds the solid solubility
such that 6Cu
precipitates and embrittlement occurs. Therefore, in the case where Cu is
included, the
upper limit of the content of Cu is set to 2.0%. Cu has effects of stabilizing
an austenite
phase and improving toughness. In the case where Cu is included, the content
of Cu is
preferably in a range of 0.2% to 1.5%.
[0027]
An extremely small amount of Ti forms oxides, nitrides, and sulfides,
solidifies
steel, and Ti is a grain refining element in a solidified and high-temperature-
heated
structure, and Ti is an arbitrary component (selective component) that is
included as
necessary. In the case where more than 0.05% of Ti is included, a ferrite
phase is
generated, and TiN is generated; and thereby, the toughness of a steel is
impaired.
Amended Sheet (Article 34 of Patent Cooperation Treaty)
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Therefore, in the case where Ti is included, the upper limit of the content of
Ti is set to be
0.05%. In the case where Ti is included, the content of Ti is preferably in a
range of
0.003% to 0.020%.
[0028]
N is included at a content of 0.01% or more as necessary in order to enhance
the
strength of a martensite phase. However, in the case where more than 0.05% of
N is
included, the strength is excessively increased; and thereby, toughness is
deteriorated.
Therefore, the content of N is limited to 0.05% or less. The content of N is
preferably in
a range of 0.01% to 0.04%.
[0029]
Al is an element important for deoxidization of a steel, and Al is included
with Si
in order to reduce oxygen in a steel. It is essential to reduce the content of
oxygen so as
to secure toughness; and therefore, it is necessary to include 0.001% or more
of Al. On
the other hand, Al is an element that increases an amount of a ferrite phase,
and, in the
case where Al is added excessively, toughness is impaired. In the case where
the content
of Al exceeds 0.1%, toughness becomes greatly degraded. Therefore, the upper
limit of
the content of Al is set to be 0.1%. The content of Al is preferably in a
range of 0.01% to
0.05%.
[0030]
0 is an element that composes oxides which are representative non-metallic
inclusions, and 0 is inevitably included in a steel. Therefore, the smaller
the content of
0, the more preferable. However, extreme reduction causes an increase in the
costs.
Therefore, generally, 0 is inevitably included at a content of approximately
0.001% or
more. On the other hand, in the case where an excessive amount of 0 is
included,
toughness is impaired. In addition, when coarse cluster-shaped oxides are
generated, the
Amended Sheet (Article 34 of Patent Cooperation Treaty)
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oxides cause surface cracking. Therefore, the upper limit of the content of 0
is set to
0.005%.
[0031]
The sum (C + N) of the contents of C and N has a relationship with the
strength
of a steel. In the case where the sum (C + N) of the contents of C and N
exceeds 0.060%,
the strength enhances excessively, and toughness is impaired. Therefore, the
upper limit
of the sum (C + N) of the contents of C and N is set to 0.060%. The sum (C +
N) of the
contents of C and N is preferably in a range of 0.015% to 0.050%.
[0032]
yomx represented by the following formula 1 is a computation formula that
estimates the maximum value of the fraction of an austenite phase generated in
a
temperature range of 900 C to 1000 C. The toughness of a steel can be enhanced
by
increasing the value of the ymax. In the embodiment, in the case where the
value of ym. is
less than 80%, an amount of a ferrite phase becomes excessive large, and a
ferrite band
structure remains; and thereby, desired toughness cannot be secured.
Therefore, ymax is
set to be in a range of 80% or more. ymax is preferably in a range of 85% or
more.
[0033]
ymax = 420 x C% + 470 x N% + 23 x Ni% + 9 x Cu% + 7 x Mn% ¨11.5 x Cr% ¨
11.5 x Si% ¨ 52 x Al% + 189 ... Formula 1
Here, C%, N%, Ni%, Cu%, Mn%, Cr%, Si%, and Al% represent the contents
(mass%) of the respective elements.
[0034]
Ypot represented by the following formula 2 is a computation formula that
shows
the fraction of a martensite phase in a cast state, and ypot also corresponds
to the fraction of
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an austenite phase during hot working. In the embodiment, a range ofypot is
determined
in order to secure hot workability. When ypot increases, a fraction of a soft
ferrite phase
decreases excessively; and thereby, strains concentrate in the ferrite phase
during hot
working, and cracking is promoted. The upper limit of ypot is dependent on the
content of
Mn or the content of Si that influences hot workability. In the embodiment, in
the case
where ypot exceeds 90%, there is a problem in that the manufacturing yield of
the steel
material decreases greatly. Therefore, the upper limit of ypot is set to be
90%. On the
other hand, in the case where ypot is less than 60%, C and N concentrate in a
martensite
phase which is generated in welded portions; and thereby, the martensite phase
becomes
hard and turns into a heterogeneous structure. In addition, since the
corrosion resistance
of the martensite phase in which alloying elements of C, N, Mn, and the like
concentrate
degrades, the lower limit of ypot is set to be 60%. v
pot is preferably in a range of 65% to
85%.
[0035]
7pot = 700 x C% + 800 x N% + 10 x (Mn% + Cu%) + 20 x Ni% ¨9.3 x Si% ¨6.2
x Cr% ¨ 9.3 x Mo% ¨ 74.4 x Ti% ¨ 37.2 x Al% + 63.2 Formula 2
Here, C%, N%, Ni%, Cu%, Mn%, Cr%, Si%, A1%, Mo%, and Ti% represent the
contents (mass%) of the respective elements.
[0036]
Next, the reasons why the arbitrary components (selective components) of the
embodiment are limited will be described. The elements that will be described
below are
arbitrary components (selective components) that are added as necessary.
Nb is an element effective for miniaturizing crystal grains in a hot-rolled
structure.
Furthermore, Nb also has an action of enhancing corrosion resistance. Nitrides
and
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=
carbides that Nb generates are generated during processes of hot working and a
thermal
treatment, and have an action of suppressing growth of crystal grains and
strengthening a
steel and a steel material. Therefore, Nb may be included at a content of
0.01% or more.
On the other hand, when an excessive amount of Nb is added, Nb precipitates in
the form
of a non-dissolved precipitate during heating before hot rolling, and Nb
impairs toughness.
Therefore, the upper limit of the content of Nb is set to be 0.2%. In the case
where Nb is
included, the content of Nb is preferably in a range of 0.03% to 0.10%.
[0037]
Ypot represented by the following formula 3 is a computation formula that
shows
the fraction of a martensite phase in a cast state in the case where Nb is
included, and Ypot
also corresponds to the fraction of an austenite phase during hot working. In
the case
where Nb is included, ypot calculated using the formula 3 instead of the above
formula 2 is
set to be in a range of 60% to 90%, and the formula 3 includes the Nb element.
Even in
the case where Nb is included, Ypot is preferably in a range of 65% to 85%.
[0038]
Ypot = 700 x C% + 800 x N% + 10 x (Mn% + Cu%) + 20 x Ni% ¨ 9.3 x Si% ¨ 6.2
x Cr% ¨ 9.3 x Mo% ¨ 3.1 x Nb% ¨ 74.4 x Ti% ¨ 37.2 x Al% + 63.2 ¨ Formula 3
Here, C%, N%, Mn%, Cu%, Ni%, Si%, Cr%, Mo%, Nb%, Ti%, and Al%
represent the contents (mass%) of the respective elements.
[0039]
V and W are elements that are added in order to incrementally enhance the
corrosion resistance of a duplex stainless steel.
V may be included at a content of 0.05% or more for the purpose of enhancing
corrosion resistance. However, in the case where more than 0.5% of V is
included,
CA 02791878 2012-08-31
'
coarse V-based carbonitrides are generated, and toughness deteriorates.
Therefore, the
upper limit of the content of V is limited to 0.5%. In the case where V is
included, the
content of V is preferably in a range of 0.1% to 0.3%.
Similarly to Mo, W is an element that incrementally enhances the corrosion
5 resistance of a stainless steel, and W has a large solid solubility
compared to V. In the
embodiment, W may be included at a content of 1.0% or less for the purpose of
enhancing
corrosion resistance. In the case where W is included, the content of W is
preferably in a
range of 0.05% to 0.5%.
That is, either one or both of the above-specified amounts of V and W may be
10 included.
[0040]
Co is an element effective for enhancing toughness and corrosion resistance of
a
steel, and Co may be selectively added. The content of Co is preferably in a
range of
0.03% or more. In the case where more than 1.0% of Co is included, effects
that are
15 worth the costs are not exhibited since Co is an expensive element.
Therefore, the upper
limit of the content of Co is set to be 1.0%. In the case where Co is
included, the content
of Co is preferably in a range of 0.03% to 0.5%.
[0041]
Furthermore, in order to improve hot workability, B, Ca, Mg, and REM may be
included as necessary.
Here, REM refers to rare earth metals, and is one or more selected from Sc, Y,
La,
Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.
B, Ca, Mg, and REM are all elements that improve the hot workability of a
steel,
and one or more may be added for that purpose. In the case where an excessive
amount
of B, Ca, Mg, or REM is added, hot workability and toughness degrade; and
therefore, the
CA 02791878 2012-08-31
16
=
upper limit thereof is set as follows. The upper limit of the content of each
of B and Ca
is 0.0050%. The upper limit of the content of Mg is 0.0030%. The upper limit
of the
content of REM is 0.10%. The content of each of B and C is preferably in a
range of
0.0005% to 0.0030%. The content of Mg is preferably in a range of 0.0001% to
0.0015%. The content of REM is preferably in a range of 0.005% to 0.05%.
Meanwhile, the content of REM refers to the sum of the contents of lanthanoid-
based rare
earth elements such as La, Ce, and the like.
[0042]
Next, the reasons why the martensitic stainless steel material of the
embodiment
is limited will be described.
The martensitic stainless steel material of the embodiment has a composition
of
the above-described martensitic stainless steel of the embodiment, and has a
metallic
structure that meets the following requirements. Mechanical characteristics
and strength
of a base material can be secured by adjusting the phase fraction of the steel
material.
A ferrite phase is soft, and, in the case where a certain amount of a ferrite
phase is
included, an excessive increase in strength is suppressed, and crystal grains
are finely
controlled through a dual-phase microstructure. Thereby, the toughness of the
martensitic stainless steel material of the embodiment can be improved. A
least 5% of a
ferrite phase is required to improve the toughness. On the other hand, a
ferrite phase is
poor in toughness; and therefore, in the case where an excessive amount of a
ferrite phase
is included, the toughness of the martensitic stainless steel material of the
embodiment
degrades. In order to prevent the degradation of toughness, the fraction of a
ferrite phase
is set to be in a range of 30% or less. The fraction of a ferrite phase is
preferably in a
range of 5% to 20%.
This fraction of a ferrite phase is realized through the manufacturing
conditions
CA 02791878 2012-08-31
17
of the steel material in addition to the chemical composition, ymax and ypot.
The
above-described fraction range of a ferrite phase can be realized by selecting
the
manufacturing conditions from the manufacturing conditions of an ordinary
stainless steel
material depending on the chemical composition. For example, for rolling
conditions,
the heating temperature of hot rolling may be selected from 1150 C to 1250 C.
The
finishing temperature of hot rolling may be selected from 950 C to 700 C. In
addition,
in the case where a thermal treatment is carried out as necessary, the
temperature of a
quenching thermal treatment may be selected from 850 C to 950 C. The
temperature of
a tempering thermal treatment may be selected from 550 C to 750 C. In
addition, the
holding time of the temperature of the quenching thermal treatment is
preferably in a
range of approximately 5 minutes to 30 minutes. In addition, the holding time
of the
temperature of the tempering thermal treatment is preferably in a range of
approximately
10 minutes to 1 hour.
[0043]
In addition, a residual austenite phase is generated by remaining an austenite
phase that is present at high temperatures in a non-transformation state. The
residual
austenite phase is soft; and therefore, the residual austenite phase enhances
the toughness
of the steel material. On the other hand, in the case where a residual
austenite phase
remains in excess, yield strength of the steel material decreases; and
thereby, strength
characteristics of the martensitic stainless steel material of the embodiment
are impaired.
Therefore, the upper limit of the fraction of a residual austenite phase is
set to be 20%.
In order to control the amount of a residual austenite phase, it is necessary
to
control an Ms value ( C) represented by the following formula 4. The chemical
components are set so that the formula 4 becomes in a range of 200 C or
higher. In the
CA 02791878 2012-08-31
18
case where the value of the formula 4 is lower than 200, the fraction of a
residual austenite
phase exceeds 20% which is the upper limit value specified in the embodiment.
In
addition, since the fraction of a residual austenite phase may be 0%, it is
not necessary to
set the upper limit of the Ms value ( C) represented by the formula 4. In the
composition
range of the present embodiment, the Ms value can be set high in the permitted
range.
Meanwhile, the fraction of a residual austenite phase can be obtained through
X-ray
measurement. The amount of the residual austenite phase is preferably in a
range of 3%
to 15%.
[0044]
Ms = 1305¨ 41.7% x (Cr% + Mo% + Cu%) ¨61 x N% ¨33 x Mn% ¨27.8 x
Si% ¨ 1667 x (C% + N%) ¨ Formula 4
Here, Cr%, Mo%, Cu%, Ni%, Mn%, Si%, C%, and N% represent the contents
(mass%) of the respective elements.
[0045]
In addition, the remainder other than the ferrite phase and the residual
austenite
phase is a martensite phase, and the sum of the fractions of three phases
becomes 100%.
[0046]
The yield strength of the martensitic stainless steel material of the
embodiment is
preferably in a range of 400 MPa to 800 MPa.
The embodiment relates to a martensitic stainless steel and a steel material
which
mainly include a martensite phase structure, and the embodiment has a high
strength and
excellent toughness. Therefore, in the case where the yield strength is less
than 400 MPa,
the value of the embodiment for applying to high-strength structure members,
which is the
object of the embodiment, is insufficient. On the other hand, in the case
where the yield
strength exceeds 800 MPa, even when a metallic structure is appropriately
controlled,
CA 02791878 2012-08-31
19
=
desired weld toughness cannot be secured. Therefore, the yield strength of the
martensitic stainless steel material of the embodiment is preferably in a
range of 400 MPa
to 800 MPa.
EXAMPLES
[0047]
Hereinafter, examples will be described.
Tables 1 to 4 show the chemical compositions of test specimen steels and the
evaluation results of joint characteristics. These steels were manufactured by
the
following method. Steel ingots (50 kg) were manufactured through vacuum
melting in a
laboratory, and each of the steel ingots was cast so as to obtain rolling test
specimens
having dimensions of thickness 60 mm x width 110 mm x length 150 mm. After
that,
the rolling test specimens were hot-rolled such that the thickness became 12
mm.
The chemical compositions in Tables 1 to 3 are the analysis results of test
specimens taken from the hot-rolled steel sheets.
Meanwhile, components other than the components described in Tables 1 to 3
(remainder) are Fe and inevitable impurity elements. In addition, in the
components
shown in Tables 1 to 3, the content of the component which is not described is
an impurity
level. In addition, REM in the tables refers to lanthanoid-based rare earth
elements, and
the content of REM shows the total content of the elements. In addition, Steel
Nos. A to
U are examples, and Steel Nos. V to AG are comparative examples.
[0048]
Welding for evaluating the joint characteristics was carried out as follows.
The width center portion of the steel sheet was cut in a rolling length
direction,
and the end surface was cut so as to form a V-shape groove. Next, a joint was
produced
CA 02791878 2012-08-31
through two passes of welding under a heat input condition of 3.5 kJ/mm using
welding
rods for submerged arc welding of SUS329J3L and flux. From the welded joint, a
Charpy test specimen was taken, and in the Charpy test specimen, a 2 mm V-
shape groove
was notched at a location 1 mm away from an interface between the weld metal
and a
5 heat-affected zone towards the heat-affected zone side. The test was
carried out on two
specimens at -20 C for each of the test specimen steels. The average value of
the
obtained impact values was shown as the impact value 1 in Table 4.
[0049]
Corrosion resistance was evaluated in the following manner.
10 A pitting potential measurement sample including the weld metal and the
heat-affected zone was produced. Next, a silver-silver chloride electrode
(SSE) was used
as a reference electrode, and a pitting potential Vc' 100 was measured in 3.5%
NaCl at
C according to JIS G0577. The results were shown in Table 4.
[0050]
15 In the case where the impact value 1 was 35 J/cm2 (= 27 J) or more, the
evaluation result was determined to be favorable. In addition, in the case
where the
pitting potential Vc' 100 was 0.10 V or more which is the average pitting
potential level of
the base material of SUS430 steel, the corrosion resistance was determined to
be
favorable.
20 As a result, it was found that steels having the composition of the
embodiment
were all excellent in impact value 1 and corrosion resistance. In contrast, in
Comparative
Examples having compositions outside the range of the embodiment, steels were
all poor
in impact value 1 and corrosion resistance, which indicates the superiority of
the steel of
the embodiment.
25 [0051]
CA 02791878 2012-08-31
21
In addition, Tables 5 to 8 show the manufacturing conditions, hot workability,
metallic structures, and base material characteristics of the steel materials
of the examples.
A rolling test specimen having dimensions of thickness 60 mm x width 110 mm x
length 150 mm was heated to a predetermined hot rolling heating temperature,
and then
the rolling test specimen was subjected to a plurality of rolling such that
the thickness
became 12 mm. The temperatures at the final rolling were described as the hot
rolling
finishing temperature in Tables 5 and 6. The size of a cracked edge caused at
the edge
portion of the steel sheet obtained after the hot rolling was measured, and
the hot
workability was evaluated to be good in the case where the maximum cracked
edge was 5
mm or less, the hot workability was evaluated to be bad in the case where the
maximum
cracked edge exceeded 5 mm, and evaluation results are shown in the "hot
workability"
column in Tables 5 and 6.
[0052]
With regard to the obtained steel sheets or the steel sheets obtained by
carrying
out either one or both of a quenching thermal treatment and a tempering
thermal treatment,
metallic structures were investigated by the following method. A sheet
thickness
cross-section was etched so as to develop the microstructure. The metallic
structure was
observed using an optical microscope, and the area fraction of a ferrite phase
was obtained
through an image analysis. In addition, a test specimen having a measurement
surface at
a portion of 1/4 of the sheet thickness and dimensions of 3 mm x 23 mm x 23 mm
was
produced, and the fraction of a residual austenite phase was quantified by the
X-ray
diffraction method. The results were shown in the "metallic structure" column
in Tables
7 and 8.
[0053]
Next, a tensile test and an impact test were carried out by the following
method.
CA 02791878 2012-08-31
22
A tensile test specimen having a circular parallel portion with a diameter of
10
mm and a length of 60 mm was taken perpendicularly to the rolling direction.
The test
specimen was subjected to a tensile test, and the 0.2% yield strength was
measured.
JIS No. 4 full-size Charpy test specimens having a 2 mm V groove were
produced.
The test was carried out on two specimens at -60 C for each of the test
specimen steels,
and average impact values were measured. The average value of the obtained
impact
values was shown as the impact value 2.
[0054]
In the case where the yield strength was 400 MPa or more, the yield strength
was
higher than that of an austenite stainless steel, and the yield strength was
determined to be
favorable. In the case where the impact value was 35 J/cm2 (= 27 J) or more,
the impact
value was determined to be favorable. As a result, it is found that Examples
that
correspond to the embodiment were all favorable in hot workability, base
material strength,
and toughness. In addition, from the results of Examples 34 to 37, it is found
that the
strength and the toughness of the base material can be secured without
carrying out a
quenching or a tempering thermal treatment. On the other hand, in Comparative
Examples, the hot workability was insufficient, or either one of the base
material yield
strength or the impact value 2 was out of the desired value. From the results
of
Comparative Examples 39 and 40, it is found that, even for steels that meet
the
requirements regarding the chemical composition of the embodiment, in the case
where
the manufacturing conditions were not appropriate, and the metallic structure
did not meet
the requirements of the embodiment, desired characteristics were not obtained.
[0055]
As is found from the above-described Examples and Comparative Examples, it
became clear that a martensitic stainless steel with excellent weld
characteristics can be
CA 02791878 2012-08-31
23
obtained from the embodiment, and a martensitic stainless steel material with
excellent
characteristics in a base material and welded portions can be obtained by
meeting the
requirements regarding the metallic structure as well.
...
[0056]
Table 1
Steel Chemical composition (mass%)
Type
No. C Si Mn P S Cu Ni Cr Mo Ti Al 0 N
Others
A 0.015 0.40 4.98 0.025 0.001 1.95 16.28
0.015 0.004 0.025
B 0.015 0.40 5.85 0.025 0.001
1.72 16.28 0.12 0.015 0.004 0.025
C 0.015 0.40 4.25 0.025 0.001 0.23 1.85 16.28
0.015 0.004 0.025
D 0.013 0.42 4.05 0.023 0.001 0.85 2.01 16.15 0.32 0.012 0.007 0.005 0.025
V:0.05
V:0.08,
E
0.012 0.40 4.23 0.024 0.002 0.45 2.12 16.62 0.005 0.012 0.003
0.028
B:0.0012
n
Nb:0.03,
0
F 0.013 0.35 3.12 0.019 0.001 2.83 16.35
0.035 0.002 0.022 I.)
Ca:0.0025
-1
Example
,0
,
V:0.06, W:0.32,
0
G
0.019 0.38 4.53 0.013 0.001 0.36 1.52 15.26 0.23 0.022 0.003
0.021 -1
Mg:0.0018
1.,...) op
.p.
I.)
H 0.012 0.43
4.95 0.024 0.002 0.45 1.98 16.23 0.035 0.014 0.003 0.022 Co:0.12,
REM:0.040 0
H
IV
Nb:0.13, B:0.0009,
1
I 0.005 0.44 4.89 0.022 0.001 0.45 2.13 16.35
0.015 0.003 0.026 0
Ca:0.0021
0
i
us,
V:0.05, Co:0.05,
H
J 0.024 0.45 4.95 0.021 0.001 2.05 16.23
0.018 0.003 0.019
Ca:0.0015, Mg:0.0008
K
0.013 0.46 3.52 0.021 0.001 1.35 2.12 17.25 0.013 0.003 0.024
=
[0057]
Table 2
Steel Chemical composition (mass%)
Type
No. C Si Mn P S Cu Ni Cr Mo Ti Al 0 N
Others
0.013 0.40 4.07 0.015 0.001 0.82 1.99 16.20 0.31 0.005 0.008 0.005 0.025
W:0.30
0.014 0.41 4.03 0.020 0.001 0.79 2.03 16.13 0.29 0.003 0.007 0.003 0.025
Nb:0.03, W:0.31
0.013 0.43 4.05 0.025 0.001 0.87 2.01 16.19 0.33 0.011 0.010 0.003 0.022
V:0.12, Nb:0.02, W:0.29
0 0.012 0.42 4.04 0.019 0.001 0.85 2.02 16.15 0.35 , 0.004 0.009
0.004 0.025 V:0.11, Nb:0.03
0.013 0.39 4.02 0.023 0.002 0.83 1.95 16.22 0.32 0.013 0.007 0.004 0.021
Nb:0.02, Co:0.10
0.013 0.41 3.98 0.026 0.001 0.85 2.00 16.18 0.31 0.003 0.007 0.005 0.026
W:0.30, Co:0.11
R Example 0.015 0.42 4.10 0.024 0.001 0.86 2.03 16.20 0.28 0.005 0.012
0.005 0.025 V:0.12, W:0.31, Co:0.12 0
V:0.11, Nb:0.04, W:0.29,
0.014 0.38 4.05 0.022 0.001 0.88 2.05 16.16 0.30 0.004 0.015 0.003 0.028
CO:0.10
CO
0.013 0.40 4.06 0.021 0.002 0.82 1.98 16.06 0.34 0.008 0.006 0.004 0.020
B:0.0020, Mg:0.0011,
REM:0.035
0
B:0.0018, Ca:0.0017,
0.012 0.42 4.08 0.023 0.001 0.85 1.97 16.14 0.33 0.012 0.007 0.005 0.025
0
4g
40
UJ
.,.
[0058]
Table 3
Chemical composition (mass%)
Steel No. Type
C _ Si Mn P S Cu Ni Cr Mo Ti
Al 0 N
V _ 0.015 0.40 5.02 0.025
0.001 1.25 15.45 0.015 0.004 0.010
W 0.013 0.45 2.48 0.024
0.001 2.05 16.21 0.005 0.005 0.038
_
X 0.015 0.40 6.52 0.025
0.001 1.95 16.28 0.011 0.004 0.024
Y , 0.015 0.41 4.85 0.023
0.001 0.53 16.24 0.012 0.004 0.025
Z 0.016 0.35 4.53 0.021
0.001 2.01 14.52 0.012 0.004 0.025
AA Comparative _ 0.002 0.40
4.98 0.025 0.001 1.95 16.28 0.003 _ 0.007 0.015
AB Example 0.013 0.42 4.89 _ 0.021
0.001 1.98 16.25 0.09 0.015 0.003 0.032 n
AC 0.010 0.41 4.95 0.024
0.001 1.93 18.23 0.35 0.015 0.004 0.034 0
I.)
AD- 0.017 0.45 4.98 0.025 0.002 1.95 16.28 r- 0.015
0.004 0.045 -1
,0
H
AE 0.014 0.44 5.01 0.023
0.004 1.96 16.32 0.014 0.003 0.023 0
t=-)
-..1
AF 0.021 0.42 4.86 0.022
0.002 1.97 18.32 0.013 0.003 0.024 I.)
_
AG 0.005 0.35 3.25 0.023
0.001 1.96 16.21 0.012 , 0.005 0.065 0
H
"
1
0
CO
I
UJ
H
.,
[0059]
Table 4
Joint characteristics
Steel Ypot Ymax C+N
Type Impact value 1
Corrosion resistance
No. (mass%) (mass%) (mass%) (J/cm2)
(V vs. SSE)
A 77.3 94.2 0.040 55
0.15
B 80.3 93.5 0.040
70 0.13
C 70.3 88.8 0.040 35
0.14
D 73.3 93.7 0.038
45 0.19
E 75.6 93.3 0.040
55 0.13
F 71.7 97.9 0.035 35
0.14 0
G 71.5 93.0 0.040
35 0.15 0
I.)
-1
H 75.1 96.3 0.034
50 0.16 ,0
,
0
I 77.1 96.7 0.031 60
0.14 -1
J 80.2 97.1 0.043 55
0.14
0
K Example 70.9 86.9 0.037
75 0.19 H
"
I
L 73.3 92.8 0.038
40 0.20 0
0
1
M 74.7 94.6 0.039 40
0.20 UJ
H
N 70.6 91.6 0.035
45 0.20
O 72.9 93.0 0.037
40 0.19
P 68.1 89.6 0.034
45 0.19
Q 73.9 93.3 0.039 40
0.20
R 76.1 95.1 0.040 45
0.20
S 78.2 96.9 0.042 50
0.19
T 69.4 91.5 0.033 40
0.20
U 72.1 92.5 0.037
40 0.20
,
.,
,
Table 4 (Continued)
Joint characteristics
Steel 7pot lmax C+N
Type Impact value 1
Corrosion resistance
No. (mass%) (mass%) (mass%) (J/cm2) (V vs.
SSE)
V 56.8 80.8 0.025 5
0.08
W 63.6 85.0 0.051 5
0.15
X 92.0 104.7 0.039 50
0.06
Y 47.9 61.1 0.040 5
0.08
Z 86.2 113.8 0.041 25
0.08
AA Comparative 60.6 84.6 0.017 5 0.12
AB Example 74.5 96.8 0.045 20
0.11 n
AC 64.8 68.9 0.044 25
0.24
0
AD 94.2 103.8 0.062 5
0.15 I.)
-1
l0
AE 74.9 92.4 0.037 25
0.07 H
CO
AF 67.1 72.2 0.045 45
0.22
00
I.)
AG 86.2 98.4 0.070 5
0.15 0
,
"
1
0
0
1
UJ
H
..
[0060]
Table 5
Steel Hot rolling Hot rolling H
Quenching Tempering
Sl
material Type
tee No. heating finishing
ot
workability
thermal treatment thermal treatment
No. temperature ( C) temperature ( C)
temperature ( C) temperature ( C)
1 A 1200 900 good
900 None
2 B 1200 900 good
880 None
3 C 1200 900 good
900 None
4 D 1200 900 good
900 None
E 1200 900 good 900
None
n
6 F 1200 900 good
900 None
0
7 G 1200 900 good
900 None I.)
-1
8 H 1180 900 good
900 None ,0
H
0
9 I 1180 900 good
900 None -1
iv op
J 1220 900 good 900
None I.)
0
H
11 Example K 1220 900 good
900 None "
1
12 L 1200 900 good
900 None 0
0
1
13 M 1200 900 good
900 None
H
14 N 1200 900 good
900 None
0 1200 900 good 900
None
16 P 1200 900 good
900 None
17 Q 1200 900 good
900 None
18 R 1200 900 good
900 None
19 S 1200 900 good
900 None
T 1200 900 good 900
None
21 U 1200 900 good
900 None
,
[0061]
Table 6
Steel St l Hot rolling Hot rolling
Quenching Tempering
ee
material Type No heating finishing Hot workability
thermal treatment thermal treatment
.
No. temperature ( C) temperature ( C)
temperature ( C) temperature ( C)
22 V 1200 900 good
900 None
23 W 1200 900 good
900 None
24 X 1200 900 bad
900 None
25 Y 1200 900 good
900 None
26 Z 1200 900 bad
900 None
n
27 Comparative AA 1200 900 good
900 None
0
28 Example AB 1200 900 good
900 None I.)
-1
29 AC 1200 900 good
900 None ,0
H
0
30 AD 1200 900 bad
900 None -1
(...)
co
31 AE 1200 900 bad
900 None
0
H
32 , AF 1200 900 good
900 None "
1
33 AG 1200 900 good
900 None 0
0
1
34 A 1200 900 good
None None UJ
H
35 A 1200 950 good
None None
36 Example A 1200 900 good
None 700
37 B 1200 900 good
None 700
38 B 1200 900 good
880 700
39 Comparative C 1270 950 good
None None
40 Example J 1100 800 bad
None None
.,
[0062]
Table 7
Steel Metallic structure Base
material characteristics
Steel
material Type Fraction of ferrite Fraction of residual
Yield strength Impact value 2
No.
No. phase (%) austenite phase (%) (MPa)
(J/cm2)
1 A 8 2 600
120
2 B 6 6 520
_ 160
3 C 15 0 680
85
_
4 D 10 5 640
135
E 8 7 460 145
n
6 F 8 0 720
75
0
7 G 10 0 710
70 "
-1
,0
8 H 10 4 630
135 H
0
-1
9 I 10 6 540
140
¨
J 10 5 520 130
I.)
0
H
11 Example K 10 15 420
185 "
,
0
12 L 10 5 640
130 0
,
L.,..
13 M 10 10 660
130 H
14 N 10 5 650
130
0 10 5 660 135
16 P 15 5 650
140
17 Q 10 5 640
140
18 R 10 10 640
140
19 S 8 10 680
135
T 15 5 640 120
,
21 U 10 5 640
125
.õ
[0063]
Table 8
Steel St Metallic structure Base
material characteristics
eel
material Type No Fraction of ferrite
Fraction of residual Yield strength Impact value 2
.
No. phase (%) austenite phase (%)
(MPa) (J/cm2)
22 V 25 0
530 15
23 W 35 0
660 5
24 X 3 15
420 145
25 Y 40 0
530 5
26 Z 3 0
780 30 n
27 Comparative AA 15 0
740 25 0
I.)
28 Example AB 15 0
600 30 -1
,0
H
29 AC 35 25
320 55 0
-1 _
30 AD 2 10
850 25 w 0
t,.)
N)
31 AE 10 1
710 65 0
H
"
I
32 AF 35 25
320 140 0
0
1
33 AG 5 0
900 5
-
34 A 10 2
620 110 H
35 A 15 3
580 130
36 Example A 12 0
720 120
_
37 B 10 0
670 150
38 B 10 0
660 140
39 Comparative C 35 0
580 _ 10
40 Example J 4 2
840 30
CA 02791878 2012-08-31
33
INDUSTRIAL APPLICABILITY
[0064]
In accordance with the embodiment of the invention, it is possible to provide
an
economic martensitic stainless steel material having favorable weld
characteristics and a
small content of Ni. Therefore, it is possible to provide a low-cost high-
strength steel
material that can be applied to large-scale structures. In addition, since
long-term
thermal treatments that were required in the related art can be skipped, the
embodiment of
the invention can improve mass productivity and greatly contribute to the
industry.
