U.S. patent application number 13/698483 was filed with the patent office on 2013-05-23 for structural stainless steel sheet having excellent corrosion resistance at weld and method for manufacturing same.
This patent application is currently assigned to JFE STEEL CORPORATION. The applicant listed for this patent is Kenichi Fujita, Yasushi Kato, Hiroki Ota. Invention is credited to Kenichi Fujita, Yasushi Kato, Hiroki Ota.
Application Number | 20130126052 13/698483 |
Document ID | / |
Family ID | 45066832 |
Filed Date | 2013-05-23 |
United States Patent
Application |
20130126052 |
Kind Code |
A1 |
Ota; Hiroki ; et
al. |
May 23, 2013 |
STRUCTURAL STAINLESS STEEL SHEET HAVING EXCELLENT CORROSION
RESISTANCE AT WELD AND METHOD FOR MANUFACTURING SAME
Abstract
A structural stainless steel sheet which can be manufactured at
a low cost and with high efficiency, and possesses excellent
welded-part corrosion resistance and a manufacturing method thereof
are provided. The structural stainless steel sheet has 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, 4.times.(C+N) or
more and 0.3% or less Ti, and Fe and unavoidable impurities as a
balance, V, Ca and O in the unavoidable impurities being regulated
to 0.05% or less V, 0.0030% or less Ca and 0.0080% or less O,
wherein an F value expressed by
Cr+2.times.Si+4.times.Ti-2.times.Ni-Mn-30.times.(C+N) satisfies a
condition that F value.ltoreq.11 and an FFV value expressed by
Cr+3.times.Si+16.times.Ti+Mo+2.times.Al-2.times.Mn-4.times.(Ni+Cu)-40.tim-
es.(C+N)+20.times.V satisfies a condition that FFV
value.ltoreq.9.0.
Inventors: |
Ota; Hiroki; (Chiba, JP)
; Fujita; Kenichi; (Hyogo, JP) ; Kato;
Yasushi; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ota; Hiroki
Fujita; Kenichi
Kato; Yasushi |
Chiba
Hyogo
Tokyo |
|
JP
JP
JP |
|
|
Assignee: |
JFE STEEL CORPORATION
Tokyo
JP
|
Family ID: |
45066832 |
Appl. No.: |
13/698483 |
Filed: |
May 26, 2011 |
PCT Filed: |
May 26, 2011 |
PCT NO: |
PCT/JP2011/062640 |
371 Date: |
January 16, 2013 |
Current U.S.
Class: |
148/506 ;
148/325; 420/41; 72/200 |
Current CPC
Class: |
C21D 6/005 20130101;
C21D 9/0068 20130101; C22C 38/50 20130101; C22C 38/58 20130101;
C22C 38/02 20130101; C22C 38/42 20130101; C22C 38/06 20130101; C22C
38/001 20130101; B21B 9/00 20130101; C21D 9/46 20130101; C22C
38/002 20130101; C22C 38/46 20130101 |
Class at
Publication: |
148/506 ;
148/325; 420/41; 72/200 |
International
Class: |
C21D 9/00 20060101
C21D009/00; C22C 38/50 20060101 C22C038/50; B21B 9/00 20060101
B21B009/00; C22C 38/06 20060101 C22C038/06; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00; C22C 38/46 20060101
C22C038/46; C22C 38/58 20060101 C22C038/58; C22C 38/42 20060101
C22C038/42 |
Foreign Application Data
Date |
Code |
Application Number |
May 31, 2010 |
JP |
2010-124059 |
Claims
1. A 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 5, 0.05 to 0.15%
Al, 10 to 13% Cr, 0.5 to 1.0% Ni, 4.times.(C+N) or more and 0.3% or
less Ti (C, N indicating contents (mass %) of C and N), and Fe and
unavoidable impurities as a balance, V, Ca and O in the unavoidable
impurities being regulated to 0.05% or less V, 0.0030% or less Ca
and 0.0080% or less O, wherein an F value and an FFV value
expressed by following formulae satisfy a condition that F
value.ltoreq.11 and FFV value.ltoreq.9.0. F
value=Cr+2.times.Si+4.times.Ti-2.times.Ni-Mn-30.times.(C+N) FFV
value=Cr+3.times.Si+16.times.Ti+Mo+2.times.Al-2.times.Mn-4.times.(Ni+Cu)--
40.times.(C+N)+20.times.V In the formulae, the respective element
symbols are contents of the elements (mass %).
2. The structural stainless steel sheet further containing 1.0% or
less Cu by mass % in addition to the components of claim 1.
3. The structural stainless steel sheet further containing 1.0% or
less Mo by mass % in addition to the components of claim 1.
4. The 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,
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, 4.times.(C+N) or more and 0.3% or less Ti (C, N
indicating contents (mass %) of C and N), and Fe and unavoidable
impurities as a balance, V, Ca and O in the unavoidable impurities
being regulated to 0.05% or less V, 0.0030% or less Ca and 0.0080%
or less O, wherein an F value and an FFV value expressed by
following formulae satisfy a condition that F value.ltoreq.11 and
FFV value.ltoreq.9.0 is heated at a temperature of 1100.degree. C.
to 1300.degree. 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.degree. 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.degree. C. and, thereafter,
pickling is applied to the hot-rolled sheet or the annealed
hot-rolled sheet. F
value=Cr+2.times.Si+4.times.Ti-2.times.Ni-Mn-30.times.(C+N) FFV
value=Cr+3.times.Si+16.times.Ti+Mo+2.times.Al-2.times.Mn-4.times.(Ni+Cu)--
40.times.(C+N)+20.times.V In the formulae, the respective element
symbols are contents of the elements (mass %).
5. The method of manufacturing a structural stainless steel sheet
further containing 1.0% or less Cu by mass % in addition to the
components of the steel slab according to claim 4.
6. The method of manufacturing a structural stainless steel sheet
further containing 1.0% or less Mo by mass % in addition to the
components of the steel slab according to claim 4.
7. The structural stainless steel sheet further containing 1.0% or
less Mo by mass % in addition to the components of claim 2.
8. The method of manufacturing a structural stainless steel sheet
further containing 1.0% or less Mo by mass % in addition to the
components of the steel slab according to claim 5.
Description
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 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
[0006] [Patent document 2] JP-A-2002-167653
[0007] [Patent document 3] JP-A-2009-13431
[0008] [Patent document 4] JP-A-2002-30391
[0009] [Patent document 5] JP-A-2009-280850
SUMMARY OF THE INVENTION
Task to be Solved by the Invention
[0010] 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.
[0011] 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
[0012] 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 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, O 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.
[0013] 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, 4.times.(C+N) or
more and 0.3% or less Ti (C, N indicating contents (mass %) of C
and N), and Fe and unavoidable impurities as a balance, V, Ca and O
in the unavoidable impurities being regulated to 0.05% or less V,
0.0030% or less Ca and 0.0080% or less O, wherein an F value and an
FFV value expressed by following formulae satisfy a condition that
Fvaluell and FFV value.ltoreq.9.0.
F value=Cr+2.times.Si+4.times.Ti-2.times.Ni-Mn-30.times.(C+N)
FFV
value=Cr+3.times.Si+16.times.Ti+Mo+2.times.Al-2.times.Mn-4.times.(Ni-
+Cu)-40.times.(C+N)+20.times.V
[0014] In the formulae, the respective element symbols are contents
of the elements (massa).
[0015] 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.
[0016] 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.
[0017] 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, 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,
4.times.(C+N) or more and 0.3% or less Ti (C, N indicating contents
(mass %) of C and N), and Fe and unavoidable impurities as a
balance, V, Ca and O in the unavoidable impurities being regulated
to 0.05% or less V, 0.0030% or less Ca and 0.0080% or less O,
wherein an F value and an FFV value expressed by following formulae
satisfy a condition that F valuell and FFV value.ltoreq.9.0 is
heated at a temperature of 1100 to 1300.degree. 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.degree. 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.degree. C. And, thereafter, pickling is applied to a
hot-rolled sheet or an annealed hot-rolled sheet.
F value=Cr+2.times.Si+4.times.Ti-2.times.Ni-Mn-30.times.(C+N)
FFV
value=Cr+3.times.Si+16.times.Ti+Mo+2.times.Al-2.times.Mn-4.times.(Ni-
+Cu)-40.times.(C+N)+20.times.V
[0018] In the formulae, the respective element symbols are contents
(mass %) of the elements.
[0019] 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 Cu by mass % in addition to the
above-mentioned components.
[0020] 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
[0021] 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
[0022] FIG. 1 is a graph showing the relationship between an FFV
value and a surface defect occurrence rate.
[0023] 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
[0024] The present invention is explained in detail
hereinafter.
[0025] Firstly, the composition of the present invention is
explained. In the explanation made hereinafter, the % indication is
mass %.
C: 0.01 to 0.03%
N: 0.01 to 0.03%
[0026] 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%.
Si: 0.10 to 0.40%
[0027] 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%.
Mn: 1.5 to 2.5%
[0028] 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 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%.
P: 0.04% or Less
[0029] 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.
[0030] S: 0.02% or Less
[0031] 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.
Al: O. 05 to 0.15%
[0032] 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 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 O, 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%.
Cr: 10 to 13%
[0033] 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 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%.
Ni: 0.5 to 1.0%
[0034] 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%.
Ti: 4.times.(C+N) or More and 0.3% or Less
[0035] 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 4.times.(C+N) or more (C, N indicating
contents (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 4.times.(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
4.times.(C+N) or more.
[0036] 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 O as
described hereinafter.
V: 0.05% or Less
[0037] 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 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.
Ca: 0.0030% or Less
[0038] 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.
O: 0.0080% or Less
[0039] It is necessary to suppress the content of O so as to
suppress the generation of an oxide-based inclusion thus securing
high productivity and hence, an upper limit of the content O is set
to 0.0080%. The upper limit of the content of O is more preferably
set to 0.060% or less.
[0040] 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.
F Value.ltoreq.11
[0041] The F value is expressed by
Cr+2.times.Si+4.times.Ti-2.times.Ni-Mn-30.times.(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 .delta. 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 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).
[0042] 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 F value is preferably set to 5.0 or more, and is more
preferably set to 6.0 or more.
FFV Value.ltoreq.9.0.
[0043] The FFV value is expressed by
Cr+3.times.Si+16.times.Ti+Mo+2.times.Al-2.times.Mn-4.times.(Ni+Cu)-40.tim-
es.(C+N)+20.times.V (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, O 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
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.
[0044] 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%.
[0045] FIG. 1 shows the relationship between the FFV value and a
surface defect occurrence rate. The surface defect 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.
[0046] In the present invention, the steel may contain Cu within a
following range when necessary in addition to the above-mentioned
components.
Cu: 1.0% or Less
[0047] 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%.
Mo: 1.0% or Less
[0048] 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%.
[0049] 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.
[0050] In the steel sheet of the present invention, by setting the
above-mentioned F value to11 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 .gamma. 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.
[0051] Next, a method of manufacturing a stainless steel sheet
according to the present invention is explained.
[0052] 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.
[0053] Hereinafter, the method of the present invention is
explained in detail.
[0054] 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 VOD (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 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 200 mm or more.
[0055] Next, the steel slab is heated up to a temperature of 1100
to 1300.degree. 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.degree. 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.degree. 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.
[0056] 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.degree. C. for at least 1 pass or more. Due to this
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).
[0057] A hot-rolled sheet having a sheet thickness of approximately
2.0 to 8.0 mm 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.degree. C. When an annealing temperature of the hot-rolled
sheet is below 600.degree. C., there may be a case where a
martensite phase or a residual .gamma. 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.degree. 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.degree. C. for 1 hour or more by
so-called box annealing. Further, when the annealing temperature
becomes excessively high, there is a case where the hot-rolled
sheet enters a temperature at which the .gamma. 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.degree. 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.
[0058] As welding a stainless steel sheet according to the present
invention, all usual welding methods including arc welding such as
TIG 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
[0059] Stainless steel having the composition shown in Table 1 is
formed into slabs having a thickness of 200 mm through a steel
converter, VOD and continuous casting. These slabs are heated at a
temperature of 1180.degree. C. and, thereafter, the slab is formed
into a coil-shaped hot-rolled sheet having a sheet thickness of 5.0
mm by hot rolling. A hot rolling finish (delivering) temperature is
set to 900.degree. C., and a coiling temperature after hot rolling
is set to 700.degree. C. The obtained hot-rolled steel sheet is
subjected to annealing at a temperature of 690.degree. C. for 10
hours and, thereafter, scales are removed from the hot-rolled steel
sheet by shot blasting and pickling.
[0060] 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 % O.sub.2, 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 0.8 kJ/mm.
[0061] 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% CuSO.sub.4/0.5%
H.sub.2SO.sub.4, 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 heat affected zone is
observed.
[0062] 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.
[0063] 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 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.
[0064] 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 hot-rolled annealed sheet
as a product.
TABLE-US-00001 TABLE 1 chemical composition (mass %) F FFV No. C Si
Mn P S Al Cu Ni Cr Ti V N O 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 5 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 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 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 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 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 11.2 12.3 comparison steel 10 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 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 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 0.80 12.1 0.205 0.01 0.0178
0.0049 0.0002 8.7 7.9 present invention steel 14 0.025 0.38 1.12
0.034 0.003 0.250 0.60 13.0 0.297 0.03 0.0193 0.0056 0.0005 11.3
13.6 comparison steel 15 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
TABLE-US-00002 TABLE 2 sulfuric acid-copper No. 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 5 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 10 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 15 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%
* * * * *