Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02799696 2012-11-16
Description
[Title of the Invention]
STRUCTURAL STAINLESS STEEL SHEET HAVING EXCELLENT
CORROSION RESISTANCE AT WELD AND METHOD FOR MANUFACTURING SAME
[Technical Field]
[0001]
The present invention relates to a structural stainless
steel sheet having excellent welded part corrosion resistance
which is suitably used as a material for a body of a railway
wagon which carries coal or iron ore, for example, and a method
of manufacturing the structural stainless steel sheet.
[Background Art]
[0002]
As a material for a body of a railway wagon which carries
coal or iron ore, stainless steel has been popularly used.
Since mined coal contains large sulfur content, the material
for the body of the railway wagon is required to possess
sulfuric acid corrosion resistance, and particularly
intergranular corrosion resistance of the welded part.
[0003]
As the stainless steel which possesses both corrosion
resistance and weldability, for example, patent document 1
discloses a Ti-containing ferritic stainless steel which
exhibits excellent weld toughness thereof. However, in the
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technique disclosed in patent document 1, components are
designed such that the structure of the welded part has a
ferrite phase and hence, there exists a drawback that weld
toughness and corrosion resistance of the welded part are not
sufficient.
[0004]
On the other hand, patent document 2 and patent document
3 disclose a technique where a proper quantity of martensitic
phase is formed in a welded part by controlling a phase fraction
at a high temperature thus improving workability and corrosion
resistance of the welded part. Further, patent document 4
discloses stainless steel which is suitable for a welding
method using a carbon dioxide gas. Further, one of inventors
of the present invention has proposed previously a structural
stainless steel sheet which improves corrosion resistance of
a welded part by properly regulating the composition using
parameters which can accurately predict the structure of the
welded part (patent document 5).
[Prior art literature]
[Patent document]
[0005]
[Patent document 1] JP-A-3-249150
[Patent document 2] JP-A-2002-167653
[Patent document 3] JP-A-2009-13431
[Patent document 4] JP-A-2002-30391
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[Patent document 5] JP-A72009-280850
[Summary of the Invention]
[Task to be solved by the Invention]
[0006]
However, in the techniques disclosed in these patent
documents 2 to 5, studies on an optimum component range have
not been entirely sufficient. Particularly,
manufacturability has been hardly taken into consideration in
these techniques. Accordingly, the occurrence of cracks in
a slab stage and the occurrence of a surface defect called as
scabs are conspicuous and hence, it is difficult to obviate
a cost rise caused by lowering of a yield ratio.
[0007]
The present invention has been made under such
circumstances, and it is an object of the present invention
to provide a structural stainless steel sheet which can be
manufactured at a low cost with high efficiency, and possesses
excellent welded-part corrosion resistance.
[Means for solving the Task]
[0008]
One of inventors of the present invention has made
extensive studies to overcome the above-mentioned drawback,
and has found that intergranular corrosion caused by depletion
of Cr in the vicinity of a grain boundary can be suppressed
and a welded heat affected zone can be formed into the structure
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which is mainly formed of martensite by adjusting chemical
components, particularly, contents of Mn and Ti, and a balance
between the respective components within proper ranges, and
has proposed a parameter (F value) shown in patent document
5. Then, the inventors of the present invention have continued
detailed studies particularly on the manufacturability based
on the finding and, as a result of the studies, have found that
slab cracks and scabs (surface defects) caused by inclusions
can be remarkably reduced when a proper quantity of Al is added
to the composition, contents of V, Ca, 0 are reduced to
predetermined ranges or less, and an FFV value is set within
a proper range as a new parameter indicative of whether or not
manufacturability is favorable, and have completed the present
invention.
[0009]
That is, the present invention provides the structural
stainless steel sheet having excellent welded part corrosion
resistance, the structural stainless steel sheet having a
composition which contains by mass% 0.01 to 0.03% C, 0.01 to
0.03% N, 0.10 to 0.40% Si, 1.5 to 2.5% Mn, 0.04% or less P,
0.02% or less S, 0.05 to 0.15% Al, 10 to 13% Cr, 0.5 to 1.0%
Ni, from 4x(C+N) to 0.3% Ti (C, N indicating contents (mass%) of C and N), and
Fe and
unavoidable impurities as a balance, V, Ca and 0 in the unavoidable impurities
being
regulated to 0.05% or less V, 0.0030% or less Ca and 0.0080%
_____________________
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or less 0, wherein an F value and an FFV value expressed by
following formulae satisfy a condition that F value<11 and FFV
value9Ø
F value = Cr+2xSi+4xTi-2xNi-Mn-30x(C+N)
FFV value =
Cr+3xSi+16xTi+Mo+2xA1-2xMn-4x(Ni+Cu)-40x(C+N)+20xV
In the formulae, the respective element symbols are
contents of the elements (mass%).
[0010]
Further, the present invention provides the structural
stainless steel sheet having excellent welded part corrosion
resistance which is characterized by further containing 1.0%
or less Cu by mass% in addition to the above-mentioned
components.
[0011]
Further, the present invention provides the structural
stainless steel sheet having excellent welded part corrosion
resistance which is characterized by further containing 1.0%
or less Mo by mass% in addition to the above-mentioned
components.
[0012]
Further, the present invention provides a method of
manufacturing a structural stainless steel sheet, wherein a
steel slab having a composition which contains by mass% 0.01
to 0.03% C, 0.01 to 0.03% N, 0.10 to 0.40% Si, 1.5 to 2.5% Mn,
CA 02799696 2014-11-10
0.04% or less P, 0.02% or less S, 0.05 to 0.15% Al, 10 to 13% Cr, 0.5 to 1.0%
Ni, from
4 x (C + N) to 0.3% Ti (C, N indicating contents (mass%) of C and N), and Fe
and
N indicating contents (mass%) of C and N), and Fe and
unavoidable impurities as a balance, V, Ca and 0 in the
unavoidable impurities being regulated to 0.05% or less V,
0.0030% or less Ca and 0.0080% or less 0, wherein an F value
and an FFV value expressed by following formulae satisfy a
condition that F valuell and FFV value9.0 is heated at a
temperature of 1100 to 1300 C and, thereafter, hot rolling which
includes a rough hot rolling where rolling is performed for
at least 1 pass or more at a reduction rate of 30% or more in
a temperature range exceeding 1000 C, or the hot rolling is
performed without annealing the hot-rolled sheet or after
annealing the hot-rolled sheet at a temperature of 600 to 1000 C.
And, thereafter, pickling is applied to a hot-rolled sheet or
an annealed hot-rolled sheet.
F value = Cr+2xSi+4xTi-2xNi-Mn-30x(C+N)
FFV value =
Cr+3xSi+16xTi+Mo+2xA1-2xMn-4x(Ni+Cu)-40x(C+N)+20xV
In the formulae, the respective element symbols are
contents (mass%) of the elements.
[0013]
Further, the present invention provides the method of
manufacturing a structural stainless steel sheet having
excellent welded part corrosion resistance which is
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characterized by further containing 1.0% or less Cu by mass%
in addition to the above-mentioned components.
[0014]
Further, the present invention provides the method of
manufacturing a structural stainless steel sheet having
excellent welded part corrosion resistance which is
characterized by further containing 1.0% or less Mo by mass%
in addition to the above-mentioned components.
[Advantage of the Invention]
[0015]
According to the present invention, it is possible to
provide the structural stainless steel sheet having excellent
welded part corrosion resistance which is manufactured at a
low cost and with high efficiency, and is suitably used as a
material for a body of a railway wagon which carries coal or
iron ore, for example.
[Brief description of the drawings]
[0016]
Fig. 1 is a graph showing the relationship between an
FFV value and a surface defect occurrence rate.
Fig. 2 is an optical micrograph showing an observation
example when deep pit-shaped corrosion is recognized in a
welded heat affected zone in cross section of a specimen after
a sulfuric acid-copper sulfate corrosion test.
[Best Mode for carrying out the Invention]
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[0017]
The present invention is explained in detail
hereinafter.
Firstly, the composition of the present invention is
explained. In the
explanation made hereinafter, the %
indication is mass%.
[0018]
C: 0.01 to 0.03%
N: 0.01 to 0.03%
It is necessary for a structural stainless steel sheet
to contain both at least 0.01 or more C and 0.01 or more N for
acquiring strength necessary for the structural stainless
steel sheet. On the other hand, when the contents of C, N exceed
0.03%, Cr carbide or Cr carbonitride tends to precipitate so
that corrosion resistance, and particularly, corrosion
resistance of a welded heat affected zone is deteriorated.
Further, the welded heat affected zone is hardened thus also
deteriorating toughness. Accordingly, both contents of C and
N are limited to values which fall within a range from 0.01
to 0.03%. The content of C is preferably limited to a value
which falls within a range from 0.015 to 0.025%, and the content
of N is preferably limited to a value which falls within a range
from 0.012 to 0.02%.
[0019]
Si: 0.10 to 0.40%
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Si is an element which is used as a deoxidizer, and it
is necessary to contain 0.10% or more Si to acquire such an
advantage brought about by Si. On the other hand, when the
content of Si exceeds 0.40%, toughness of a hot-rolled steel
sheet is deteriorated. Accordingly, the content of Si is
limited to a value which falls within a range from 0.10 to 0 . 40%.
A lower limit of the Si content is preferably set to 0.20%,
and an upper limit of the Si content is preferably set to 0.30%.
[0020]
Mn: 1.5 to 2.5%
Mn is a useful element as a deoxidizer and also as a
reinforcing element for securing strength necessary for a
structural stainless steel sheet, and Mn is also an austenite
stabilizing element at a high temperature. Further, in the
present invention, Mn is an important element for controlling
the microstructure of the welded heat affected zone to the
martensitic structure having desired volume fraction. To
allow Mn to exhibit such function, it is necessary to set the
content of Mn to 1.5% or more. On the other hand, even when
the content of Mn exceeds 2.5%, not only the advantage of Mn
is saturated but also the excessive content of Mn deteriorats
toughness of the steel sheet, adversely influences a surface
property by deterioration descaling property during a
manufacturing step, and pushes up an alloy cost. Accordingly,
the content of Mn is limited to a value which falls within a
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range from 1.5 to 2.5%. The content of Mn is preferably limited
to a value which falls within a range from 1.8 to 2.5%. The
content of Mn is more preferably limited to a value which falls
within a range from 1.85 to 2.0%.
[0021]
P: 0.04% or less
The content of P is preferably set small from a viewpoint
of hot workability, and an allowable upper limit of the content
of P is set to 0.04%. The upper limit of the content of P is
more preferably set to 0.035% or less.
[0022]
S: 0.02% or less
The content of S is preferably set small from a viewpoint
of hot workability and corrosion resistance, and an allowable
upper limit of the content of S is set to 0.02%. The upper
limit of the content of S is more preferably set to 0.005% or
less.
[0023]
Al: 0. 05 to 0.15%
Although Al is an element which is added to the
composition for deoxidization in general, according to the
present invention, the inventors of the present invention have
found that Al enhances manufacturability, and effectively
functions to suppress the occurrence of cracks in a slab stage
particularly, and a proper quantity of Al is added for allowing
CA 02799696 2012-11-16
Al to exhibit such a function. To suppress the occurrence of
cracks in a slab, in addition to the containing of Al, the
reduction of V, Ca and 0, and the optimization of an FFV value
are necessary as described later. Although the mechanism
where the occurrence of cracks in a slab is suppressed due to
the containing of Al is not entirely clarified, it is estimated
that such improvement is brought about by properly regulating
a phase fraction and by controlling a morphology of inclusion.
To acquire such an advantage, it is necessary to set the content
of Al to 0.05% or more. On the other hand, when the content
of Al exceeds 0.15%, large-sized Al-based inclusion is
generated thus causing a surface defect. Accordingly, the
content of Al is limited to a value which falls within a range
from 0. 05 to 0.15%. The content of Al is preferably limited
to a value which falls within a range from 0.080 to 0.150%.
The content of Al is more preferably limited to a value which
falls within a range from 0.085 to 0.120%.
[0024]
Cr: 10 to 13%
Cr is an element which forms a passive film, and is
inevitable for securing corrosion resistance, particularly,
corrosion resistance of a welded heat affected zone. It is
necessary to set the content of Cr to 10% or more to acquire
such an advantage. On the other hand, when the content of Cr
exceeds 13%, not only a cost is pushed up but also it is difficult
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to secure a sufficient austenite phase at a high temperature
in a welded part and hence, it is difficult to acquire the
martensitic structure of a fraction necessary for a welded heat
affected zone after welding. As a result, deterioration of
intergranular corrosion resistance at the welded heat affected
zone is brought about. Accordingly, the content of Cr is
limited to a value which falls within a range from 10 to 13%.
The content of Cr is preferably limited to a value which falls
within a range from 10.5 to 12.5%.
[0025]
Ni: 0.5 to 1.0%
The content of Ni is set to 0.5% or more to secure strength
and toughness. On the other hand, Ni is an expensive element
and hence, an upper limit of the content of Ni is set to 1.0%
from an economical point of view. Ni is, in the same manner
as Mn, an austenite stabilizing element at a high temperature
and hence, Ni is useful in controlling the microstructure of
a welded heat affected zone to the martensitic structure having
desired volume fraction. However, this advantage can be
sufficiently acquired due to the addition of Mn and hence, it
is reasonable to limit the content of Ni to a value which falls
within a range from 0.5 to 1.0%. The content of Ni is preferably
limited to a value which falls within a range from 0.60 to 1.0%.
The content of Ni is more preferably limited to a value which
falls within a range from 0.60 to 0.90%.
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[0026]
Ti: 4x(C+N) or more and 0.3% or less
Ti is an important element for acquiring excellent welded
part corrosion resistance in the present invention, and is an
element particularly inevitable for enhancing intergranular
corrosion resistance of a welded heat affected zone. Ti has
an advantage that Ti precipitates and fixes C, N in steel as
carbide, nitride or carbonitride of Ti (hereinafter three kinds
of compositions consisting of carbide, nitride and
carbonitride being collectively referred to as carbonitride
or the like) thus suppressing the generation of carbonitride
or the like of Cr. In the present invention, in a welded heat
affected zone of a steel sheet which has the structure formed
of ferrite and martensite, from a viewpoint of corrosion
resistance, deterioration of corrosion resistance of a ferrite
phase part which causes the precipitation of carbonitride or
the like during cooling becomes a problem. In the steel sheet
according to the present invention, carbonitride or the like
of Cr precipitates in the welded heat affected zone at the time
of welding so that Cr depletion occurs in the vicinity of the
grain boundary whereby, particularly, a drawback that
intergranular corrosion resistance of the ferrite phase part
is deteriorated can be overcome due to the containing of Ti.
To allow Ti to exhibit such function, it is necessary to set
the content of Ti to 4x(C+N) or more (C, N indicating contents
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(mass%) of C and N). On the other hand, even when the content
of Ti exceeds 0.3%, not only the advantage of Ti is saturated
but also a large quantity of carbonitride or the like of Ti
precipitates in the steel thus bringing about the deterioration
of toughness of the steel sheet. Accordingly, the content of
Ti is limited to 4x(C+N) or more and 0.3% or less. The content
of Ti is more preferably limited to a value which falls within
a range from 0.180 to 0.230%. That is, it is effective for
the steel sheet to reduce C, N such that the content of Ti
simultaneously satisfies 4x(C+N) or more.
[0027]
In the present invention, to increase productivity
(yield rate) or manufacturability, and particularly to
suppress the occurrence of scabs (surface defects) which occur
due to cracks or inclusion in a slab stage, it is important
to reduce V, Ca and 0 as described hereinafter.
[0028]
V: 0.05% or less
It is often the case that V is added to a steel sheet
as an impurity in a Cr raw material or the like, and there may
be case where V is added to a steel sheet unintentionally.
However, to suppress the occurrence of cracks particularly in
a slab stage, it is necessary to strictly regulate the content
of V. From such a viewpoint, it is necessary to limit the
content of V to 0.05% or less. It is more preferable to limit
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to the content of V to 0.03% or less. It is still more
preferable to limit to the content of V to less than 0.03%.
Although a larger crack suppression effect can be obtained by
limiting the content of V to 0.01% or less, the selection of
a raw material or the like becomes necessary and hence, such
limitation of the content of V becomes economically
disadvantageous.
[0029]
Ca: 0.0030% or less
Calcium forms an inclusion of a low melting point and
hence, Ca becomes a cause of a surface defect particularly
attributed to the inclusion. Accordingly, in the present
invention, it is necessary to strictly restrict the content
of Ca, and an upper limit of the content of Ca is limited to
0.0030%. It is preferable that the content of Ca is as small
as possible, and the content of Ca may be preferably limited
to 0.0010% and may be more preferably limited to 0.0002% or
less. However, the selection of the raw material or the like
becomes necessary and hence, such limitation of the content
of Ca becomes economically disadvantageous.
[0030]
0: 0.0080% or less
It is necessary to suppress the content of 0 so as to
suppress the generation of an oxide-based inclusion thus
securing high productivity and hence, an upper limit of the
CA 02799696 2012-11-16
content 0 is set to 0.0080%. The upper limit of the content
of 0 is more preferably set to 0.060% or less.
[0031]
Further, in the present invention, corrosion resistance
and productivity can be largely improved by setting an F value
and an FFV value described hereinafter to within proper ranges.
[0032]
F value5_.11
The F value is expressed by
Cr+2xSi+4xTi-2xNi-Mn-30x(C+N) (respective element symbols
being contents of the elements (mass%)), and is a parameter
for estimating the microstructure of a welded heat affected
zone at the time of welding. To be more specific, the F value
is a parameter for estimating a volume fraction of the
martensitic structure (a residual rate of the ferrite
structure). In a part of a steel sheet such as a welded heat
affected zone which is exposed to a high temperature, a part
of the zone is transformed into austenite (or a portion of the
part is further transformed into 6 ferrite (delta ferrite)),
and these phase are transformed into martensite in a cooling
step. The rate is influenced by a quantitative balance between
ferrite stabilizing elements (ferrite formation elements) and
austenite stabilizing elements (austenite formation elements) .
In the above-mentioned formula expressing the F value , elements
having a positive coefficient (Cr, Si, Ti) are the ferrite
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stabilizing elements and elements having a negative
coefficient (Ni, Mn, C, N) are the austenite stabilizing
elements. That is, the larger the F value , the more the ferrite
structure is likely to remain (the larger a volume fraction
of the ferrite structure becomes, that is, the smaller a volume
fraction of the martensitic structure becomes), while the
smaller the F value, the more scarcely the ferrite structure
remains (the smaller a volume fraction of the ferrite becomes,
that is, the larger a volume fraction of the martensitic
structure becomes).
[0033]
In patent document 5, the optimization of content is
attempted by investigating the relationship between the F value
and a volume fraction of the martensitic structure of the welded
heat affected zone and by evaluating corrosion resistance of
an area in the vicinity of the welded heat affected zone by
a sulfuric acid-copper sulfate corrosion test. Also in this
embodiment, in the same manner as the above-mentioned patent
document 5, to enhance the corrosion resistance of the welded
heat affected zone, the above-mentioned F value is limited to
11 or less (martensite volume fraction: 40% or more). The
above-mentioned F value is preferably limited to 10.5 or less
(martensite volume fraction: 60% or more), and is more
preferably limited to 10 or less. Here, from a viewpoint of
corrosion resistance at the welded part, a lower limit of the
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F value is preferably set to 5.0 or more, and is more preferably
set to 6.0 or more.
[0034]
FFV value5_9. 0 .
The FFV value is expressed by
Cr+3xSi+16xTi+Mo+2xA1-2xMn-4x (Ni+Cu) -40x (C+N) +20xV
(the respective element symbols being contents of the elements
(mass%) ) . The FFV is newly introduced in the present invention
as an index for indicating manufacturability. The FFV value
is set by taking a phase balance during hot rolling into
consideration. By adjusting the components as described above,
particularly by regulating the content of Al and upper limits
of V, Ca, 0 and, thereafter, by setting this FFV value smaller,
the occurrence of surface defects caused by cracks in a slab
stage or inclusions can be remarkably reduced. The
significant technical feature of present invention lies in
succeeding in largely suppressing the lowering of a yield rate
caused by the occurrence of a surface defect by optimizing a
new parameter which takes an Al quantity which was not taken
into consideration at the time of inventing the F value into
consideration. Although the mechanism of the improvement of
the manufacturability by optimization of the FFV value is not
entirely clarified, since the manufacturability is largely
improved by limiting the FFV value to 9.0 or less, the FFV value
is set to 9.0 or less. The FFV value is preferably set to 8.5
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CA 02799696 2012-11-16
or less. Although it is effective to decrease a Cr quantity
or to increase C, N quantities to make the FFV value small,
there is a possibility that the reduction of Cr quantity or
the increase of C, N quantities deteriorats corrosion
resistance. Accordingly, it is preferable to set the lower
limit of the FFV value to 5.0 or more, and it is more preferable
to set the lower limit of the FFV value to 6.0 or more.
[0035]
For the steel sheet of the present invention which is
used in a state of a hot-rolled sheet or a hot-rolled annealed
sheet, the control of cracks in a slab stage and inclusions
is important for reducing surface defects. It is because, with
respect to the occurrence of surface defects, portions such
as cracks or scabs which largely lower a yield rate not only
deteriorate the appearance but also become a starting point
of the occurrence of rust and hence, it is necessary to cut
off the portions where cracks or scab occur at the time of
shipping the steel sheet as a product. Although the
above-mentioned formula on the FFV value includes Mo, V, Cu,
there may be a case where these components are not added to
the steel. When these contents are not added to the steel,
the FFV value is calculated by setting the contents of the
components not contained in the steel to 0%.
Fig. 1 shows the relationship between the FFV value and
a surface defect occurrence rate. The surface defect
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occurrence rate was calculated based on a length of a portion
where defects occur with respect to a total length of a coil.
It is understood that by limiting the FFV value within a range
of 9.0 or below, the occurrence of surface defects can be
remarkably suppressed.
[0036]
In the present invention, the steel may contain Cu within
a following range when necessary in addition to the
above-mentioned components.
[0037]
Cu: 1.0% or less
Cu is an element which enhances corrosion resistance,
and is an element which particularly reduces crevice corrosion.
Accordingly, Cu can be added when the steel is requested to
possess high corrosion resistance. However, when the content
of Cu exceeds 1.0%, hot workability is deteriorated, and also
a phase balance at a high temperature collapses and hence, it
is difficult for a welded heat affected zone to acquire the
desired microstructure. Accordingly, when Cu is added to the
composition, an upper limit of the content of Cu is set to 1.0%.
To allow Cu to exhibit a sufficient corrosion resistance
enhancing effect, it is effective to set the content of Cu to
0.3% or more. The content of Cu is more preferably set to a
value which falls within a range from 0.3 to 0.5%.
[0038]
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Mo: 1.0% or less
Mo is an element which enhances corrosion resistance,
and can be added to the composition when a steel sheet is
requested to possess high corrosion resistance particularly.
However, when the content of Mo exceeds 1.0%, cold workability
is deteriorated, and also a rough surface occurs in hot rolling
so that surface quality is extremely deteriorated.
Accordingly, when Mo is added to the composition, an upper limit
of the content of Mo is set to 1.0%. To allow Mo to exhibit
sufficient corrosion resistance, it is effective to set the
content of Mo to 0.03% or more. The content of Mo is more
preferably set to a value which falls within a range from 0.1
to 1.0%.
[0039]
In the present invention, besides the improvement of
corrosion resistance acquired by adding 1.0% or less of Cu or
Mo described above, other elements may be added based on
conventional finding for improving ductility or the like due
to addition of 0.005% or less B. Also in this case, it is
important to take a phase balance at a high temperature into
consideration. Nb is a strong stabilizing element and largely
collapses a phase balance by combining with C or N and hence,
Nb is not added in the present invention. A balance other than
the above-prescribed elements is constituted of Fe and
unavoidable impurities.
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[0040]
In the steel sheet of the present invention, by setting
the above-mentioned F value to 11 or less to enhance corrosion
resistance of a welded heat affected zone, a martensite in
volume fraction of the welded heat affected zone becomes 40%
or more. By preferably setting the above-mentioned F value
to 10.5 or less, the martensite fraction of the welded heat
affected zone becomes 60% or more. By further preferably
setting the above-mentioned F value to 10 or less, the
martensite in volume fraction of the welded heat affected zone
becomes 80% or more in this case. Also in the steel sheet
according to the present invention, 50% or more of a matrix
steel (base material) portion in volume fraction is formed of
the ferrite structure. The remaining structure is formed of,
particularly in a hot-rolled state, the structure where a
martensite phase and a residual y phase are present and
partially contains carbonitride or the like. Particularly,
with respect to the structure of a hot-rolled annealed sheet
which is manufactured as described later such that contents
of components are set to fall within a proper composition range
and hot-rolled-sheet annealing is applied under a proper
annealing condition, almost 100% of the structure has the
ferrite-phase structure in volume fraction and hence, the
structure possesses the excellent workability.
[0041]
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Next, a method of manufacturing a stainless steel sheet
according to the present invention is explained.
The method of manufacturing a stainless steel sheet of
the present invention may be performed in accordance with a
given method and is not specifically limited. However, as a
method which can manufacture a stainless steel sheet with high
efficiency, a method where a molten steel having the
above-mentioned composition is formed into a slab by continuous
casting or the like, the slab is formed into a hot-rolled coil,
the hot-rolled coil is annealed when necessary and, thereafter,
descaling (shot blasting, pickling and the like) is performed
thus manufacturing a stainless steel sheet according to the
present invention is recommended.
[0042]
Hereinafter, the method of the present invention is
explained in detail.
Firstly, a molten steel adjusted to the composition of
the present invention is produced by a known commonly used
melting furnace such as a steel converter or an electric furnace
and, thereafter, the molten steel is refined by a known refining
method such as a vacuum degassing method (RH method) , a VOID
(Vacuum Oxygen Decarburization) method or an AOD (Argon Oxygen
Decarburization) method, and the molten steel is formed into
a steel slab (raw steel material) by a continuous casting or
an ingot-making/blooming method. It is preferable to adopt
23
CA 02799696 2012-11-16
continuous casting as a casting method from a viewpoint of
productivity and quality. Further, a thickness of a slab may
preferably be set to 100mm or more for securing a reduction
ratio in hot coarse rolling described later. It is more
preferable to set the thickness of the slab within a range of
200mm or more.
[0043]
Next, the steel slab is heated up to a temperature of
1100 to 1300 C and, thereafter, is subjected to hot rolling
whereby a hot-rolled steel sheet is formed. It is desirable
to set the slab heating temperature high for enhancing surface
roughness resistance of the hot-rolled sheet or anti-ridging
property or ridging property after annealing in cold rolling.
However, when the slab heating temperature exceeds 1300 C, slag
sag becomes conspicuous, and crystal grains become coarse thus
deteriorating toughness of the hot-rolled sheet. On the other
hand, when the slab heating temperature is below 1100 C, a load
in the hot rolling becomes high and hence, rough surface in
hot rolling becomes conspicuous, and also the
recrystallization during hot rolling becomes insufficient
thus also deteriorating toughness of the hot-rolled sheet.
[0044]
In a hot rough rolling step, it is preferable to perform
rolling at a reduction rate of 30% or more in a temperature
range exceeding 1000 C for at least 1 pass or more. Due to this
24
CA 02799696 2012-11-16
rolling with a high reduction rate, the grain (crystal)
structure of a steel sheet is made fine so that toughness of
the steel sheet is enhanced. After hot rough rolling, hot
finish rolling is performed in accordance with a given method
(under a condition of usual hot finish rolling).
[0045]
A hot-rolled sheet having a sheet thickness of
approximately 2. 0 to 8. Omm which is manufactured by hot rolling
is used as a structural material directly or through pickling
without annealing. Pickling may be applied to the hot-rolled
sheet after the hot-rolled sheet is annealed at a temperature
of 600 to 1000 C. When an annealing temperature of the
hot-rolled sheet is below 600 C, there may be a case where a
martensite phase or a residual y phase which has a possibility
of existing in a hot-rolled state remains and hence, the ferrite
structure becomes 50% or less in terms of a volume fraction
whereby the steel sheet cannot acquire the sufficient
workability. On the other hand, when the annealing
temperature exceeds 1000 C, the coarsening of grain size
becomes conspicuous and hence, toughness of the hot-rolled
sheet is deteriorated. Annealing of the hot-rolled sheet may
preferably be performed such that the hot-rolled sheet is held
at a predetermined temperature of 600 to 1000 C for 1 hour or
more by so-called box annealing. Further, when the annealing
temperature becomes excessively high, there is a case where
CA 02799696 2012-11-16
the hot-rolled sheet enters a temperature at which the y
transformation occurs and hence, the excessively high
temperature is not preferable. Accordingly, it is necessary
to adjust the composition within a proper range and to select
a proper temperature range corresponding to the composition.
In the composition range of the steel of the present invention,
when the annealing temperature is mainly set to a value which
falls within 600 to 900 C, almost 100% of the hot-rolled sheet
becomes a ferrite phase in terms of a volume fraction and hence,
it is preferable to set the annealing temperature within this
temperature range.
[0046]
As welding a stainless steel sheet according to the
present invention, all usual welding methods including arc
welding such as TIC welding or MIG welding, seam welding,
resistance welding such as spot welding, laser welding and the
like are applicable to the steel of the present invention.
[Embodiment]
[0047]
Stainless steel having the composition shown in Table
1 is formed into slabs having a thickness of 200mm through a
steel converter, VOD and continuous casting. These slabs are
heated at a temperature of 1180 C and, thereafter, the slab
is formed into a coil-shaped hot-rolled sheet having a sheet
thickness of 5.0mm by hot rolling. A hot rolling finish
26
CA 02799696 2012-11-16
(delivering) temperature is set to 900 C, and a coiling
temperature after hot rolling is set to 700 C. The obtained
hot-rolled steel sheet is subjected to annealing at a
temperature of 690 C for 10 hours and, thereafter, scales are
removed from the hot-rolled steel sheet by shot blasting and
pickling.
[0048]
Flat plate samples are cut out from the steel sheet after
removing scales, T-shaped specimens each of which is formed
of a lower plate and a vertical plate are assembled, and both
side one pass fillet welding (gas metal arc welding, shielding
gas: 98 volume% Ar-2 volume%02, flow rate: 20 litter/min) is
applied to the T-shaped specimens thus forming three fillet
welding specimens. MGS-309LS made by Kobe steel limited is
used as a welding rod, and a welding input heat is set to a
value which falls within a range from 0.4 to 0.8kJ/mm.
[0049]
Corrosion test specimens are sampled from these filled
welded parts of these fillet welding specimens, and the
corrosion specimens are subjected to a sulfuric acid-copper
sulfate corrosion test (Modified Strauss test in accordance
with ASTM A262 practice E and ASTM A763 practice Z, a test
liquid: Cu/6%CuSO4/0.5%H2S06, a specimen with polished end
surfaces being immersed in the boiling test liquid for 20 hours) ,
and a corrosion state of an area in the vicinity of a welded
27
CA 02799696 2012-11-16
heat affected zone is observed.
[0050]
Fig. 2 is an optical micrograph showing an observation
example of a cross section of the specimen after the sulfuric
acid-copper sulfate corrosion test. The evaluation "C" is
given to a case where intergranular corrosion is observed or
pit-shaped corrosion far deeper than intergranular corrosion
is observed in the welded heat affected zone as shown in the
photograph. The evaluation "B" is given to a case where slight
corrosion is observed in the welded heat affected zone. The
evaluation "A" is given to a case where corrosion is not
observed by the observation using an optical microscope.
Further, a surface state of the hot-rolled annealed sheet after
pickling is observed over the whole length of the sheet. Using
a rate of a length of the hot-rolled annealed sheet along which
a surface defect caused by cracks in a slab or inclusion is
observed with respect to the whole length of the hot-rolled
annealed sheet as an index, the evaluation is made by giving
"a" to a case where the defect occurrence rate is 3% or less,
"b" to a case where the defect occurrence rate is 3% or more
and 30% or less, and "c" to a case where the defect occurrence
rate is more than 30%. These results are shown in Table 2.
[0051]
As a result, with respect to present invention examples
No.1 to 5, 10 to 13 and 15 which fall within the scope of the
28
CA 02799696 2012-11-16
present invention, these examples exhibit favorable welded
part corrosion resistance and a surface state of the welded
part is also extremely favorable. To the contrary, with
respect to comparison examples No. 9 and 14 where the F value
falls outside the scope of the present invention, a martensite
generation quantity in the welded heat affected zone is small
and hence, these examples exhibit the intergranular corrosion
resistance clearly inferior to the intergranular corrosion
resistance of the present invention examples. Further, with
respect to a comparison example No. 6 where an Si content is
higher than a range of Si content of the present invention and
an Al content is lower than a range of Al content of the present
invention and comparison examples No. 7, 8, 9 and 14 where the
FFV value falls outside a range of the FFV value of the present
invention, in the surface observation carried out after hot
rolling and annealing, many cracks attributed to slab and many
scabs attributed to inclusions are observed.
[0052]
Since the present invention steel is used in a state of
a hot-rolled sheet or a hot-rolled annealed sheet, the
occurrence of scabs largely lowers a yield rate. This is
because the scab portions not only exhibit poor appearance but
also become a starting point of the occurrence of rust and hence,
it is necessary to cut off portions corresponding to the scab
portions at the time of shipping the hot-rolled sheet or the
29
CA 02799696 2012-11-16
hot-rolled annealed sheet as a product.
[0053]
Table 1
N chemical composition (mass%)
F FFV
o.
C Si Mn P S Al Cu Ni Cr , Ti _ V _ N
0 Ca value value
1 0.022 0.24 1.87 0.034 0.005 0.105 0.65 11.2
0.194 0.01 0.0150 0.0052 0.0010 8.2 7.6 present invention
steel
2 0.025 0.30 _ 1.53 0.029 , 0.001 0.120 0.85 12.6
0.180 0.01 0.0242 0.0050 0.0005 9.2 8.4 present invention
steel
-
3 0.015 0.28 1.90 0.031 0.004 0.119 0.70
11.4 0.210 0.01- 0.0195 0.0065 0.0001 8.5 8.1 present
invention steel
-
-
4 0.020 0.21 1.64 0.034 0.003 0.082 0.40 0.80
12.0 0.192 0.01 0.0165 0.0055- 0.0024 8.9 6.5 present
invention steel
- -
0.018 0.24 1.95 0.030 0.003 0.103 , 0.60 11.0
0.185 0.03 0.0145 0.0050 0.0001 8.1 7.9
present invention steel n
_ _
_
6 0.018 0.45 1.70 0.030 0.010 0.013 0.91 11.2-
0.240 0.01 0.0130 0.0062 0.0001 8.6 8.3
comparison steel 0
I.)
7 0.022 0.40 1.70 0.025 0.002 0.005 0.40 11.1
0.200 0.01 0.0140 0.0054 0.0010 9.1 9.3
comparison steel ko
ko
uu
_______________________________________________________________________________
________________________________________ (5)
1-- 8 0.020 0.40 1.9 0.030 0.006 0.014 0.91-
11.2 0.251 0.10 0.0100 0.0055 0.0001 8.4
9.9 comparison steel ko
(5)
-
9 0.01 0.50 1.20 0.029 0.002 0.004 0.30 11.9
0.200 0.01 0.0120 0.0057 0.0002 1t2 12.3
comparison steel I.)
0
- -H
0.020 0.29 1.91 0.026 0.002 0.113 0.86 11.5
0.221 0.008 0.0171 0.0054 0.0004 8.2 7.5
present invention steel I.)
1
- - -
- H
H
11 0.019 0.40 1.81 0.030 0.001 0.107 0.45 0.95
13.0 0.298 0.02 0.0171 0.0054 0.0004 10.2 8.9
present invention steel 1
- .
- H
(5)
12 0.025 0.19 1.95 0.031 0.002 0.150 0.95 10.1
0.194 0.04 0.0198 0.0054 0.0004 6.1 5.4 present
invention steel
- - -
13 0.022 0.22 1.89 0.031 0.002 0.122 0.8012.1
0.205 0.01 0.0178 0.0049 0.0002 8.7 7.9 present
invention steel
- -
- -
14 0.025 0.38 1.12
0.60 13.0 0.297 0.03
0.0193 0.0056 0.0005 11.3 13.6 comparison steel
- 0.034 0.003 0.250 - - - -
0.023 0.25 1.85 0.030 0.002 0.110 0.71 11.5
0.216 0.02 0.0154 0.0051 0.0005 8.4 8.4 present invention
steel
-
CA 02799696 2012-11-16
[0054]
Table 2
No. sulfuric acid-copper sulfate test result surface quality
1 A a present invention steel
2 A a present invention steel
3 A a present invention steel
4 A a present invention steel
A a present invention steel
6 A b comparison steel
7 A b comparison steel
8 B b comparison steel
9 C c comparison steel
A a present invention steel
11 A a present invention steel
12 A a present invention steel
13 A a present invention steel
14 C b comparison steel
A a present invention steel
A: no corrosion
B: slight corrosion
C: intergranular corrosion or deep pit-shaped corrosion
a: defect occurrence rate of 3% or less
b: defect occurrence rate of exceeding 3% and 30% or less
c: defect occurrence rate exceeding 30%
32
