U.S. patent application number 15/226287 was filed with the patent office on 2016-11-24 for duplex stainless steel, duplex stainless steel slab, and duplex stainless steel material.
The applicant listed for this patent is Nippon Steel & Sumikin Stainless Steel Corporation. Invention is credited to Haruhiko Kajimura, Yuusuke OIKAWA, Shinji Tsuge, Hiroshi Urashima.
Application Number | 20160340764 15/226287 |
Document ID | / |
Family ID | 48140919 |
Filed Date | 2016-11-24 |
United States Patent
Application |
20160340764 |
Kind Code |
A1 |
Tsuge; Shinji ; et
al. |
November 24, 2016 |
DUPLEX STAINLESS STEEL, DUPLEX STAINLESS STEEL SLAB, AND DUPLEX
STAINLESS STEEL MATERIAL
Abstract
One aspect of this duplex stainless steel contains, in mass %,
C: 0.03% or less, Si: 0.05% to 1.0%, Mn: 0.1% to 7.0%, P: 0.05% or
less, S: 0.0001% to 0.0010%, Ni: 0.5% to 5.0%, Cr: 18.0% to 25.0%,
N: 0.10% to 0.30%, Al: 0.05% or less, Ca: 0.0010% to 0.0040%, and
Sn: 0.01% to 0.2%, with the remainder being Fe and inevitable
impurities, wherein a ratio Ca/O of the amounts of Ca and O is in a
range of 0.3 to 1.0, and a pitting index PI shown by formula (1) is
in a range of less than 30, PI=Cr+3.3Mo+16N (1).
Inventors: |
Tsuge; Shinji; (Hikari-shi,
JP) ; OIKAWA; Yuusuke; (Hikari-shi, JP) ;
Urashima; Hiroshi; (Kitakyushu-shi, JP) ; Kajimura;
Haruhiko; (Hikari-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Nippon Steel & Sumikin Stainless Steel Corporation |
Tokyo |
|
JP |
|
|
Family ID: |
48140919 |
Appl. No.: |
15/226287 |
Filed: |
August 2, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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14347437 |
Mar 26, 2014 |
|
|
|
PCT/JP2012/076821 |
Oct 17, 2012 |
|
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15226287 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/008 20130101;
C22C 38/48 20130101; C22C 38/06 20130101; C22C 38/005 20130101;
C22C 38/004 20130101; C22C 38/46 20130101; C21D 8/0263 20130101;
C22C 38/001 20130101; C21D 8/0226 20130101; C22C 38/50 20130101;
C22C 38/42 20130101; C22C 38/002 20130101; C22C 38/04 20130101;
C22C 38/54 20130101; C21D 6/004 20130101; C22C 38/44 20130101; C22C
38/58 20130101; C22C 38/00 20130101; C22C 38/02 20130101; C22C
38/52 20130101 |
International
Class: |
C22C 38/58 20060101
C22C038/58; C22C 38/52 20060101 C22C038/52; C22C 38/50 20060101
C22C038/50; C22C 38/48 20060101 C22C038/48; C22C 38/00 20060101
C22C038/00; C22C 38/44 20060101 C22C038/44; C22C 38/42 20060101
C22C038/42; C22C 38/06 20060101 C22C038/06; C22C 38/02 20060101
C22C038/02; C22C 38/54 20060101 C22C038/54; C22C 38/46 20060101
C22C038/46 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 21, 2011 |
JP |
2011-231352 |
Dec 6, 2011 |
JP |
2011-266351 |
Claims
1-4. (canceled)
5. A duplex stainless steel comprising, in mass %: C: 0.03% or
less; Si: 0.05% to 1.0%; Mn: 0.1% to 4.0%; P: 0.05% or less; S:
0.0001% to 0.0010%; Cr: 23.0% to 28.0%; Ni: 2.0% to 6.0%; Co: 0% to
1.0%; Cu: 0.2% to 3.0%; Sn: 0.01% to 0.2%; N: 0.20% to 0.30%; Al:
0.05% or less; and Ca: 0.0010% to 0.0040%, with the remainder being
Fe and inevitable impurities, wherein Ni+Co is in a range of 2.5%
or more and a ratio Ca/O of the amounts of Ca and O is in a range
of 0.3 to 1.0, and PI shown by formula (1) is in a range of 30 or
more to less than 40, PI=Cr+3.3Mo+16N (1), (the chemical symbols in
the formula (1) indicate the amounts of the elements).
6. The duplex stainless steel according to claim 5, further
comprising: either one or both of Mo: 2.0% or less, and W: 1.0% or
less.
7. The duplex stainless steel according to claim 5 or 6, further
comprising: one or more selected from V: 0.05% to 0.5%, Nb: 0.01%
to 0.15%, and Ti: 0.003% to 0.05%.
8. The duplex stainless steel according to any one of claims 5 to
7, further comprising: one or more selected from B: 0.0050% or
less, Mg: 0.0030% or less, and REM: 0.10% or less.
9-10. (canceled)
Description
TECHNICAL FIELD
[0001] The present invention relates to an inexpensive
Sn-containing duplex stainless steel. In addition, the present
invention relates to an inexpensive duplex stainless steel which
contains a combination of Cu and Sn and which is excellent in
corrosion resistance. In detail, the present invention relates to a
duplex stainless steel, a duplex stainless steel slab (a cast steel
of a duplex stainless steel), and a duplex stainless steel material
which are able to be used in a seawater desalination unit, tanks
for a transport ship, various types of containers, or the like.
[0002] This application is a national stage application of
International Application No. PCT/JP2012/076821, filed on Oct. 17,
2012, which claims priority to Japanese Patent Application No.
2011-231352 filed on Oct. 21, 2011 in Japan, and Japanese Patent
Application No. 2011-266351 filed on Dec. 6, 2011 in Japan, the
contents of which are incorporated herein by reference.
BACKGROUND ART
[0003] A general-purpose duplex stainless steel contains a large
amount of Cr, Mo, Ni, and N and has favorable corrosion resistance.
However, as a result of containing Mo and Ni, which are expensive,
the alloy cost is high and the manufacturability is not favorable.
As a result, the price of steel material is not cheap and the
duplex stainless steel is not widely used in place of 316 grade
stainless steel or 317 grade stainless steel. Here, the
general-purpose duplex stainless steel referred to in the present
invention indicates duplex stainless steel having the pitting index
PI (represented by the following formula which is the sum of the
amounts of the alloy elements: PI=Cr+3.3Mo+16N) of approximately 30
or more to less than 40 (mass %). From the circumstances described
above, in such steels, it is considered that there is a need for
steels where the alloy cost is lower than that in the related art
and the manufacturing costs are inexpensive and which have
favorable hot manufacturability while exhibiting the same level of
corrosion resistance as the general-purpose duplex stainless steel
of the related art.
[0004] On the other hand, recently, an alloy-saving type duplex
stainless steel in which amounts of Cr, Ni, Mo, and the like are
reduced has been developed. Here, the alloy-saving type duplex
stainless steel indicates a stainless steel which exhibits a
pitting resistance equivalent to those of SUS 304 and 316L and
where the pitting resistance index PI (=Cr+3.3Mo+16N), which is
indexed by the amounts of the alloy elements, is approximately in a
range of less than 30. In these steels where the amounts of alloy
elements which are effective for pitting resistance and acid
resistance are reduced, it is difficult to obtain the same level of
corrosion resistance as that of the general-purpose duplex
stainless steel. However, it is considered that it is possible to
develop improved steels by using inexpensive alternative
elements.
[0005] Various types of duplex stainless steels which contain Sn
have been proposed in the related art. For example, duplex
stainless steels are disclosed which contain 25% or more of Cr and
contain 0.01% to 0.1% of Sn as a selected element (refer to Patent
Documents 1 and 2 described below). In addition, alloy-saving type
duplex stainless steels are disclosed which contain 1% or less or
0.1% of Sn (refer to Patent Documents 3 and 4 described below). In
the Patent Documents, an object is to improve the corrosion
resistance by means of the amount of Sn; however, the relationship
between the hot manufacturability of the steel material and the
amount of Sn was not investigated.
[0006] In addition, in the Patent Documents described above, the
subject is a steel where the amount of N is in a range of 0.2% or
less. N is an element which decreases the hot workability of the
stainless steel. Ensuring a desired level of hot workability of a
duplex stainless steel which contains 0.2% or more of N is more
difficult than ensuring a desired level of hot workability of a
duplex stainless steel which contains less than 0.2% of N.
Technical literature which makes a disclosure regarding the hot
workability of a duplex stainless steel which contains 0.20% or
more of N and further contains a combination of Sn and Cu is not to
be found.
[0007] The present inventors focused on the possibility of
improving the acid resistance and the pitting resistance using Sn
in an alloy-saving type duplex stainless steel. Then, the present
inventors investigated the relationship between the amount of Sn
and the corrosion resistance and the hot manufacturability. As a
result, it was found that it was possible to improve the corrosion
resistance by 0.01% to 0.2% of Sn being contained. However, it was
learned that the hot manufacturability decreased in duplex
stainless steels which contained a large amount of Sn. For this
reason, the frequency of decreases in the yield of the steel
material will increase and a significant cost increase is
predicted.
[0008] In addition, the present inventors focused on the
possibility of improving the acid resistance and the pitting
resistance using Sn and Cu in the general-purpose duplex stainless
steel. Then, with regard to the duplex stainless steel where the
amounts of Mo and Ni are reduced and which contains 0.20% or more
of N, the present inventors investigated the relationship between
the amounts of Sn and Cu, the corrosion resistance, and the hot
manufacturability. As a result, it was found that it was possible
to improve the corrosion resistance by 0.01% to 0.2% of Sn and 0.2%
to 3.0% of Cu being contained. However, it was learned that the hot
manufacturability decreased in duplex stainless steels which
contained a large amount of Sn and Cu. For this reason, the
frequency of decreases in the yield of the steel material will
increase and a significant cost increase is predicted.
[0009] The present inventors investigated the knowledge of the
related art relating to the manufacturing techniques for
Sn-containing duplex stainless hot-rolled steel material of the
related art starting with Patent Documents 1 to 4. As a result, it
was found that there was little knowledge with regard to the
relationship between the temperature range where hot embrittlement
occurs due to Sn which is included in the duplex stainless steel
and the amount of Sn and the relationship with the amounts of other
elements.
PRIOR ART DOCUMENT
Patent Documents
[0010] Patent Document 1: Japanese Unexamined Patent Application,
First Publication No. H3-158437 [0011] Patent Document 2: Japanese
Unexamined Patent Application, First Publication No. H4-072013
[0012] Patent Document 3: Japanese Unexamined Patent Application,
First Publication No. 2010-222593 [0013] Patent Document 4: PCT
International Publication No. W02009-119895 [0014] Patent Document
5: Japanese Unexamined Patent Application, First Publication No.
2002-69592 [0015] Patent Document 6: Japanese Unexamined Patent
Application, First Publication No. H7-118805
Non-Patent Document
[0015] [0016] Non-Patent Document 1: "Effect of Cu and Ni on Hot
Workability of Hot-rolled Mild Steel" ISIJ, Vol. 37, p. 217 to 223
(1997)
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0017] The present invention finds a measure for solving the
problems described above by clarifying the relationship between the
amount of Sn and hot manufacturability in an alloy-saving type
duplex stainless steel. In addition, the present invention finds a
measure for solving the problems described above by clarifying the
relationship between the amounts of Sn and Cu and hot
manufacturability in a general-purpose duplex stainless steel. Due
to this, the object of the present invention is to provide an
Sn-containing duplex stainless steel, a cast steel of a duplex
stainless steel, and a duplex stainless steel material which are
inexpensive and have favorable hot manufacturability. Such a duplex
stainless steel is expected to have an excellent balance between
corrosion resistance and cost. For this reason, it is considered
that the possibility that the duplex stainless steel will be widely
used in various fields is high.
[0018] In particular, an object of a second aspect (a second
embodiment) of the invention is to develop an inexpensive
general-purpose duplex stainless steel where the amounts of Ni and
Mo, which are expensive elements, are reduced by increasing the
amounts of N and Mn and adding a combination of Cu and Sn.
Means for Solving the Problems
[0019] In order to solve the problems described above, for the
alloy-saving type duplex stainless steel which is the subject of
the present invention, the present inventors prepared melted
materials where the amount of Sn and the amounts of Ca, B, rare
earth elements (REM), or the like were changed and performed the
following experiments. Here, the amounts of Ca, B, rare earth
elements (REM), or the like are said to improve the hot
manufacturability.
[0020] Tensile test pieces were collected from cast steels which
were cast from the melted materials. High temperature tensile test
was performed at a temperature of 1200 to 700.degree. C. with
respect to the tensile test pieces, and the high temperature
ductility was evaluated by measuring the reduction of area
(cross-sectional reduction ratio of the fracture surface). In
addition, a hot-rolled steel plate with a plate thickness of 12 mm
was obtained by hot forging and hot rolling and the edge cracking
resistance was evaluated. The edge cracking resistance was
evaluated by changing the heating temperature and the rolling
temperature of the hot rolling with respect to a part of the steel,
and a correlation of the heating temperature and the rolling
temperature of the hot rolling with the high temperature ductility
was determined.
[0021] As disclosed in Patent Documents 5 and 6 described above,
generally, in duplex stainless steels, it is known that significant
edge cracking is generated in the hot rolling of the cast steel in
most cases where the reduction of area of the cast steel, which is
evaluated by high temperature tensile test, falls below 60%. For
this reason, engineers in this field often subject steels to
refining, casting, and hot working for the purpose of setting the
reduction of area of the cast steel at high temperatures to be in a
range of 60% or more. Here, when the present inventors evaluated
the high temperature ductility of the alloy-saving type duplex
stainless steel (base composition: 21% Cr--2% Ni--3% Mn --0.18% N)
cast steel which contains around 0.1% of Sn, it was clear that all
the reductions of area fell below 60% in several melting
experiments. The evaluation of high temperature ductility was
performed as follows. First, a parallel section of a round bar of 8
mm.phi. was heated to 1200.degree. C. using a high frequency. Next,
the temperature was lowered to a temperature for performing a break
test, and tensile rupture was performed at a rate of 20 mm/second
at this temperature. Then, the shrinkage ratio of the cross section
was determined. An example of the data is shown in FIG. 1. From
these results, it was considered that there was almost no hope of
obtaining an inexpensive alloy-saving type duplex stainless steel
with added Sn in practice.
[0022] The present inventors observed an edge cracking length which
was generated when a cast steel of an alloy-saving type
Sn-containing duplex stainless steel, which was obtained by vacuum
melting and casting, was subjected to hot rolling. As a result, it
was found that there rarely exists a cast steel of an Sn-containing
duplex stainless steel in which a number of edge cracks is small.
Hot rolling experiments were performed as follows. First, a cast
steel with a thickness of 90 to 44 mm was heated to 1200.degree. C.
Next, the thickness of the cast steel was reduced to a thickness of
12 to 6 mm by a plurality of rolling passes. The finishing rolling
temperature was controlled to be approximately 900.degree. C. Edge
cracking was generated on the left and right sides and the maximum
lengths on both sides were totaled to obtain the edge cracking
length. Even when the edge cracking length of the steel material
was looked upon as being related to the minimum value (the minimum
value is obtained at approximately 900.degree. C. in FIG. 1) of the
reduction of area of the high temperature ductility of the cast
steel, it was not possible to obtain a clear correlation. However,
when the edge cracking length was looked upon as being related to
the reduction of area at 1000.degree. C. as shown in FIG. 2, it was
clear that a good correlation is exhibited regardless of whether or
not Sn is contained. Here, in FIG. 2, the points which are plotted
by .smallcircle. (open circles) correspond to the results of Sn-A
and Sn-B of FIG. 1, and the points which are plotted by
.diamond-solid. (black diamonds) are the other experiment results
(the experiment results examined regardless of whether or not Sn is
contained).
[0023] The present inventors performed melting, casting, and
rolling experiments while further changing the amounts of various
elements in order to find the conditions for reliably obtaining a
cast steel with little edge cracking as described above. Then, the
evaluation of the high temperature ductility of the cast steel and
the evaluation of edge cracking of the steel material after hot
rolling were actively performed. The first aspect of the present
invention where the inexpensive Sn-containing alloy-saving type
duplex stainless steel is specified was completed on the basis of
the findings which were obtained through the above experiments.
[0024] The requirements of the first aspect of the duplex stainless
steel of the present invention are shown below.
(1) A duplex stainless steel which includes, in mass %: C: 0.03% or
less; Si: 0.05% to 1.0%; Mn: 0.1% to 7.0%; P: 0.05% or less; S:
0.0001% to 0.0010%; Ni: 0.5% to 5.0%; Cr: 18.0% to 25.0%; N: 0.10%
to 0.30%; Al: 0.05% or less; Ca: 0.0010% to 0.0040%; and Sn: 0.01%
to 0.2%, with the remainder being Fe and inevitable impurities,
wherein a ratio Ca/O of the amounts of Ca and O is in a range of
0.3 to 1.0, and a pitting index PI shown by formula (1) is in a
range of less than 30.
PI=Cr+3.3Mo+16N (1)
[0025] (The chemical symbols in the formula (1) indicate the
amounts of the elements).
(2) The duplex stainless steel according to (1), which further
includes one or more selected from Mo: 1.5% or less, Cu: 2.0% or
less, W: 1.0% or less, and Co: 2.0% or less. (3) The duplex
stainless steel according to (1) or (2), which further includes one
or more selected from V: 0.05% to 0.5%, Nb: 0.01% to 0.20%, and Ti:
0.003% to 0.05%. (4) The duplex stainless steel according to any
one of (1) to (3), which further includes one or more selected from
B: 0.0050% or less, Mg: 0.0030% or less, and REM: 0.10% or
less.
[0026] In addition, in order to solve the problems described above,
with regard to the general-purpose duplex stainless steel which is
the subject of the present invention, the present inventors
prepared melted materials where the amount of Sn, the amounts of
Ca, B, rare earth elements (REM), and the like and the amount of Ni
were changed and where Co was further added, and they performed the
following experiments. Here, it is said that the hot
manufacturability is improved by containing Ca, B, rare earth
elements (REM), and the like.
[0027] Tensile test pieces were collected from a cast steel which
was cast from the melted materials. The tensile test pieces were
subjected to high temperature tensile test at a temperature of 1200
to 700.degree. C., and the high temperature ductility was evaluated
by measuring the reduction of area (cross-sectional reduction ratio
of the fracture surface). In addition, a hot-rolled steel plate
with a plate thickness of 12 mm was obtained by hot forging and hot
rolling, and the edge cracking resistance was evaluated. The edge
cracking resistance was evaluated by changing the heating
temperature and the rolling temperature of the hot rolling with
respect to a part of the steel, and a correlation of the heating
temperature and the rolling temperature of the hot rolling with the
high temperature ductility was determined.
[0028] As disclosed in Patent Documents 5 and 6 described above,
generally, in duplex stainless steels, it is known that significant
edge cracking is generated in the hot rolling of the cast steel in
most cases where the reduction of area of the cast steel, which is
evaluated by high temperature tensile test, falls below 60%. For
this reason, engineers in this field often subject steels to
refining, casting, and hot working for the purpose of setting the
reduction of area of the cast steel at high temperatures to be in a
range of 60% or more. Here, when the present inventors evaluated
the high temperature ductility of the general-purpose cast steel of
a duplex stainless steel (base composition: 25% Cr--4% Ni --1.2%
Mo--1.5% Cu--0.25% N) which contains around 0.1% of Sn, it was
clear that the minimum values of all the reductions of area fell
below 60% in several melting experiments. The evaluation of high
temperature ductility was performed as follows. First, a parallel
section of a round bar of 8 mm.phi. was heated to 1200.degree. C.
using a high frequency. Next, the temperature was lowered to a
temperature for performing a break test, and tensile rupture was
performed at a rate of 20 mm/second at this temperature. Then, the
shrinkage ratio of the cross section was determined. An example of
the data is shown in FIG. 3. From these results, it was considered
that there was almost no hope of obtaining an inexpensive
general-purpose duplex stainless steel with added Sn in
practice.
[0029] The present inventors observed an edge cracking length which
was generated when a cast steel of a general-purpose duplex
stainless steel, which was obtained by vacuum melting and casting,
was subjected to hot rolling. As a result, it was discovered that
there rarely exists an Sn-containing duplex stainless steel
material in which a number of edge cracks is small. Hot rolling
experiments were performed as follows. First, a cast steel with a
thickness of 90 to 44 mm was heated to 1200.degree. C. Next, the
thickness of the cast steel was reduced to a thickness of 12 to 6
mm by a plurality of rolling passes. The finishing rolling
temperature was controlled to be approximately 900.degree. C. Edge
cracking was generated on the left and right sides and the maximum
lengths on both sides were totaled to obtain the edge cracking
length. Even when the edge cracking length of the steel material
was looked upon as being related to the minimum value (the minimum
value is obtained at approximately 900.degree. C. in FIG. 3) of the
reduction of area of the high temperature ductility of the cast
steel, it was not possible to obtain a clear correlation. However,
when the edge cracking length was looked upon as being related to
the reduction of area at 1000.degree. C. as shown in FIG. 4, it was
clear that a good correlation is exhibited regardless of whether or
not Sn is contained. Here, in FIG. 4, the points which are plotted
by .smallcircle. (open circles) correspond to the results of Sn-A
and Sn-B of FIG. 3, and the points which are plotted by
.diamond-solid. (black diamonds) are the other experiment results
(the experiment results examined regardless of whether or not Sn is
contained).
[0030] The present inventors performed melting, casting, and
rolling experiments while further changing the amounts of various
elements in order to find the conditions for reliably obtaining a
steel material with little edge cracking as described above. Then,
the evaluation of the high temperature ductility of the cast steel
and the evaluation of the edge cracking of the steel material after
hot rolling were actively performed. The second aspect of the
present invention where the inexpensive Sn-containing duplex
stainless steel is specified was completed on the basis of the
findings which were obtained through the above experiments.
[0031] The requirements of the second aspect of the duplex
stainless steel of the present invention are shown below.
(5) A duplex stainless steel which includes, in mass %: C: 0.03% or
less; Si: 0.05% to 1.0%; Mn: 0.1% to 4.0%; P: 0.05% or less; S:
0.0001% to 0.0010%; Cr: 23.0% to 28.0%; Ni: 2.0% to 6.0%; Co: 0% to
1.0%; Cu: 0.2% to 3.0%; Sn: 0.01% to 0.2%; N: 0.20% to 0.30%; Al:
0.05% or less; and Ca: 0.0010% to 0.0040%, with the remainder being
Fe and inevitable impurities, wherein Ni+Co is in a range of 2.5%
or more and a ratio Ca/O of the amounts of Ca and O is in a range
of 0.3 to 1.0, and PI shown by formula (1) is in a range of 30 or
more and less than 40.
PI=Cr+3.3Mo+16N (1)
[0032] (The chemical symbols in the formula (1) indicate the
amounts of the elements).
(6) The duplex stainless steel according to (5), which further
includes either one or both of Mo: 2.0% or less, and W: 1.0% or
less. (7) The duplex stainless steel according to (5) or (6), which
further includes one or more selected from V: 0.05% to 0.5%, Nb:
0.01% to 0.15%, and Ti: 0.003% to 0.05%. (8) The duplex stainless
steel according to any one of (5) to (7), which further includes
one or more selected from B: 0.0050% or less, Mg: 0.0030% or less,
and REM: 0.10% or less.
[0033] The requirements of one aspect of the cast steel of the
duplex stainless steel and the duplex stainless steel material of
the present invention are shown below.
(9) A cast steel of a duplex stainless steel which has a
composition according to any one of (1) to (8), wherein a fracture
reduction of area at 1000.degree. C. is in a range of 70% or more.
(10) A duplex stainless steel material which is manufactured by hot
working the cast steel of the duplex stainless steel according to
(9).
Effects of the Invention
[0034] According to an aspect of the present invention, it is
possible to provide a duplex stainless steel, a cast steel of a
duplex stainless steel, and a duplex stainless steel material which
have improved corrosion resistance compared to a steel used in the
related art as the material for seawater desalination unit, tanks
for a transport ship, various types of containers, or the like in
addition to an excellent balance with cost. For this reason, the
aspects of the present invention make a significant contribution to
industrial development.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIG. 1 is a diagram which illustrates the high temperature
ductility of Sn-containing and Sn-free duplex stainless steels
associated with the first aspect of the duplex stainless steel (an
alloy-saving type duplex stainless steel).
[0036] FIG. 2 is a diagram which shows the relationship between the
edge cracking length after hot rolling and the reduction of area at
1000.degree. C. associated with the first aspect of the duplex
stainless steel (the alloy-saving type duplex stainless steel).
[0037] FIG. 3 is a diagram which illustrates the high temperature
ductility of Sn-containing and cast steels of Sn-free duplex
stainless steels associated with the second aspect of the duplex
stainless steel (a general-purpose duplex stainless steel).
[0038] FIG. 4 is a diagram which shows the relationship between the
edge cracking length after hot rolling and the reduction of area at
1000.degree. C. associated with the second aspect of the duplex
stainless steel (the general-purpose duplex stainless steel).
EMBODIMENTS OF THE INVENTION
First Embodiment
[0039] Below, description will be given of the reasons for limiting
the first aspect (the alloy-saving type duplex stainless steel) of
the duplex stainless steel of the present invention. Here, the
amounts of the respective components are shown in terms of mass
%.
[0040] Here, in the present embodiment, the cast steel of the
stainless steel indicates a steel in a state after casting and
before processing such as hot working, forging, or the like is
performed, and the stainless steel material indicates a
semi-finished product, a hot-rolled steel plate, a cold-rolled
steel plate, a steel wire, a steel pipe, or the like after
processing the cast steel by various methods. In addition, the
stainless steel indicates general forms for a steel such as a cast
steel, a steel material, and the like. The processing described
above includes hot and cold processings.
[0041] In order to ensure the corrosion resistance of the stainless
steel, the amount of C is limited to be in a range of 0.03% or
less. When more than 0.03% of C is contained, the corrosion
resistance and toughness are degraded due to the generation of Cr
carbides during hot rolling.
[0042] 0.05% or more of Si is added for deoxidation. However, when
more than 1.0% of Si is added, the toughness is degraded.
Therefore, the upper limit for the amount of Si is limited to 1.0%.
The preferable range for the amount of Si is in a range of 0.2% to
0.7%.
[0043] Mn has the effect of improving the toughness by increasing
the austenite phase. In addition, since Mn has the effect of
decreasing the nitride precipitation temperature TN, it is
preferable to actively add Mn to the steel material of the present
embodiment. For the toughness of the base material and the welding
sections, 0.1% or more of M is added. However, when more than 7.0%
of Mn is added, the corrosion resistance and the toughness are
degraded. Therefore, the upper limit for the amount of Mn is
limited to 7.0%. The amount of Mn is preferably in a range of 1.0%
to 6.0%, and more preferably in a range of 2.0% to 5.0%.
[0044] P is an element which is inevitably mixed in from raw
materials and the amount of P is limited to be in a range of 0.05%
or less since P degrades the hot workability and the toughness. The
amount of P is preferably in a range of 0.03% or less.
[0045] S is an element which is inevitably mixed in from the raw
materials and the amount of S is limited to be in a range of
0.0010% or less since S degrades the hot workability, the
toughness, and the corrosion resistance. In addition, reducing the
amount of S to less than 0.0001% increases the costs due to
desulfurization refining. For this reason, the amount of S is set
to be in a range of 0.0001% to 0.0010%. The amount of S is
preferably in a range of 0.0002% to 0.0006%.
[0046] Since Ni stabilizes the austenitic structure and improves
the toughness and the corrosion resistance with respect to various
types of acid, 0.5% or more of Ni is contained. By increasing the
amount of Ni, it is possible to decrease the precipitation
temperature of nitrides. On the other hand, Ni is an expensive
alloy, and from the point of view of costs, the amount of Ni is
limited to be in a range of 5.0% or less in the steel of the
present embodiment where the subject is an alloy-saving type duplex
stainless steel. The amount of Ni is preferably in a range of 1.0%
to 4.0%, and more preferably in a range of 1.5% to 3%.
[0047] In order to ensure the basic corrosion resistance, 18.0% or
more of Cr is contained. On the other hand, when more than 25.0% of
Cr is contained, the ferrite phase fraction increases and the
toughness and the corrosion resistance of the welding sections are
inhibited. For this reason, the amount of Cr is set to be in a
range of 18.0% or more and 25.0% or less. The amount of Cr is
preferably in a range of 19.0% to 23.0%.
[0048] N is an element which is effective for increasing the
strength and the corrosion resistance by being solid-solubilized in
the austenite phase. For this reason, 0.10% or more of N is
contained. On the other hand, the solid solubility limit is
increased according to the amounts of Cr and Mn; however, when more
than 0.30% of N is contained in the steel of the present
embodiment, Cr nitrides are precipitated such that the toughness
and the corrosion resistance are inhibited and the hot
manufacturability is inhibited. For this reason, the upper limit of
the amount of N is set to 0.30%. The amount of N is preferably in a
range of 0.10% to 0.25%.
[0049] Al is an element which deoxidizes a steel and reduces the
oxygen in the steel according to necessity. For this reason, Al is
contained together with 0.05% or more of Si. In an Sn-containing
steel, the reduction of the oxygen amount is essential in order to
ensure the hot manufacturability, and for this reason, it is
necessary that 0.003% or more of Al be contained according to
necessity. On the other hand, Al is an element having comparatively
large affinity with N, and when an excessive amount of Al is added,
the toughness of the stainless steel is inhibited due to the
generation of AlN. The degree also depends on the amount of N;
however, when the amount of Al exceeds 0.05%, the toughness is
greatly decreased. For this reason, the upper limit of the amount
of Al is set to 0.05%. The amount of Al is preferably in a range of
0.04% or less.
[0050] Ca is an important element for the hot manufacturability of
the steel, and it is necessary that Ca be contained in order to fix
S and O in the steel as inclusions and to improve the hot
manufacturability. In the steel of the present embodiment, 0.0010%
or more of Ca is contained for this purpose. In addition, addition
of an excessive amount thereof decreases the pitting resistance.
For this reason, the upper limit of the amount of Ca is set to
0.0040%.
[0051] Sn is contained in order to improve the corrosion resistance
of the steel of the present embodiment. For this reason, it is
necessary that at least 0.01% of Sn be contained. Furthermore, it
is preferable that 0.02% or more of Sn be contained. On the other
hand, Sn is an element which inhibits the hot manufacturability of
the steel, and decreases the hot strength of the interface between
the ferrite phase and the austenite phase, particularly at a
temperature of 900.degree. C. or less in the alloy element saving
type duplex stainless steel which is the subject of the present
embodiment. The degree of the decrease depends on the amounts of S,
Ca, and O; however, when more than 0.2% of Sn is contained, it is
not possible to prevent the decrease in the hot manufacturability
even by restricting other limits in the present embodiment.
Therefore, the upper limit of the amount of Sn is set to 0.2%.
[0052] The ratio Ca/O of the amounts of O and Ca is an important
component index in order to improve the hot manufacturability and
the corrosion resistance of the steel of the present embodiment.
The lower limit of Ca/O is limited in order to improve the hot
manufacturability of the Sn-containing steel. The high temperature
ductility of the Sn-containing steel is decreased, particularly at
a temperature of 900.degree. C. or less. When the value of Ca/O is
in a range of less than 0.3, the high temperature ductility at
1000.degree. C. is also decreased and the hot manufacturability is
greatly impaired. For this reason, Ca/O is limited to be in a range
of 0.3 or more in the steel of the present embodiment. On the other
hand, when an excessive amount of Ca is added and Ca/O exceeds 1.0,
the pitting resistance is impaired. In addition, when the amount of
Ca is excessive, the high temperature ductility at a temperature of
1000 to 1100.degree. C. is also impaired. For this reason, the
upper limit of Ca/O is set to be in a range of 1.0. Ca/O is
preferably in a range of 0.4 to 0.8.
[0053] O is an inevitable impurity and an upper limit thereof is
not particularly set; however, O is an important element which
configures oxides which are the representative of non-metallic
inclusions. Composition control of the oxides is extremely
important for the improvement of the hot manufacturability. In
addition, surface defects are caused when coarse cluster-shaped
oxides are generated. For this reason, it is necessary to limit the
amount of O so as to be low. In the present embodiment, as
described above, by setting the ratio of the amount of Ca and the
amount of O to be in a range of 0.3 or more, the amount of O is
limited. The upper limit of the amount of O is preferably in a
range of 0.005% or less.
[0054] In order to incrementally increase the corrosion resistance,
one or more selected from Mo: 1.5% or less, Cu: 2.0% or less, W:
1.0% or less, and Co: 2.0% or less may be contained according to
necessity. Description will be given of the reasons for these
limits.
[0055] Mo is an element which is extremely effective at
incrementally increasing the corrosion resistance of the stainless
steel, and Mo can be contained according to necessity. In order to
improve the corrosion resistance, it is preferable that 0.2% or
more of Mo be contained. On the other hand, Mo is an element which
promotes precipitation of intermetallic compounds, and the upper
limit of the amount of Mo is set to 1.5% from the point of view of
suppressing precipitation in the steel of the present embodiment
during hot rolling.
[0056] Cu is an element which incrementally increase the corrosion
resistance of the stainless steel with respect to acid, and Cu has
an effect of improving the toughness; and therefore, it is
recommended that 0.3% or more be contained according to necessity.
When more than 2.0% of Cu is contained, the amount of Cu exceeds
the solid solubility; and thereby, .epsilon.-Cu is precipitated
during hot rolling to cause embrittlement. For this reason, the
upper limit of the amount of Cu is set to 2.0%. In a case where Cu
is contained, the amount is preferably in a range of 0.3% to
1.5%.
[0057] W is an element which incrementally increases the corrosion
resistance of the stainless steel in the same manner as Mo, and W
can be added according to necessity. For the purpose of increasing
the corrosion resistance in the steel of the present embodiment,
the upper limit of the amount of W is set to 1.0%. The amount of W
is preferably in a range of 0.05% to 0.5%.
[0058] Co is an element which is effective for increasing the
toughness and the corrosion resistance of the steel and which is
selectively added. The amount of Co is preferably in a range of
0.03% or more. When more than 2.0% of Co is contained, an effect
which is commensurate with the cost is not exhibited as Co is an
expensive element. For this reason, the upper limit of the amount
of Co is set to 2.0%. In a case where Co is added, the amount is
preferably in a range of 0.03% to 1.0%.
[0059] Furthermore, one or more selected from V: 0.05% to 0.5%, Nb:
0.01% to 0.20%, and Ti: 0.003% to 0.05% may be contained. These are
elements which are more likely to generate nitrides rather than Cr.
V, Nb, and Ti can be added according to necessity, and there is a
tendency for the corrosion resistance to be improved in cases where
these are contained in trace amounts.
[0060] Nitrides and carbides which are formed by V are generated in
the hot working and the cooling process of the steel material, and
these have the effect of increasing the corrosion resistance. The
reasons therefor are not sufficiently confirmed; however, it is
considered that there is a probability of suppressing the
generation speed of the chromium nitrides at a temperature of
700.degree. C. or less. 0.05% or more of V is contained in order to
improve the corrosion resistance. When more than 0.5% of V is
contained, coarse V carbonitrides are generated, and toughness is
degraded. Therefore, the upper limit of the amount of V is limited
to 0.5%. In a case where V is added, the amount is preferably in a
range of 0.1% to 0.3%.
[0061] Nitrides and carbides which are formed by Nb are generated
in the hot working and the cooling process of the steel material,
and these have the effect of increasing the corrosion resistance.
The reasons therefor are not sufficiently confirmed; however, it is
considered that there is a probability of suppressing the
generation speed of the chromium nitrides at a temperature of
700.degree. C. or less. 0.01% or more of Nb is contained in order
to improve the corrosion resistance. On the other hand, in the case
where an excessive amount of Nb is added, Nb is precipitated as
non-solid-solubilized precipitates during heating before the hot
rolling; and thereby, the toughness is inhibited. For this reason,
the upper limit of the amount of Nb is set to 0.20%. In a case
where Nb is added, the range of the amount is preferably in a range
of 0.03% to 0.10%.
[0062] Ti is an element which forms oxides, nitrides, and sulfides
in very small amounts and Ti refines crystal grains in the
solidified structure and the structure heated at a high temperature
of the steel. In addition, in the same manner as V and Nb, Ti also
has the property of replacing a part of the chromium in the
chromium nitrides. With an amount of Ti of 0.003% or more, Ti
precipitates are formed. On the other hand, when more than 0.05% of
Ti is contained in the duplex stainless steel, the toughness of the
steel is impaired due to the generation of coarse TiN. For this
reason, the upper limit of the amount of Ti is set to 0.05%. A
suitable amount of Ti is in a range of 0.005% to 0.020%.
[0063] Furthermore, one or more selected from B: 0.0050% or less,
Mg: 0.0030% or less, and REM: 0.10% or less may be contained. In
order to achieve further improvement of the hot workability, the B,
Mg, and REM to be contained according to necessity are limited as
follows.
[0064] B, Mg, and REM are all elements which improve the hot
workability of the steel, and one or more thereof is added for this
purpose. The addition of an excessive amount of any one of B, Mg,
and REM has the opposite effect of decreasing the hot workability
and the toughness. For this reason, the upper limits of the above
amounts are set as follows. The upper limit of the amount of B is
0.0050%. The upper limit of the amount of Mg is 0.0030%. The upper
limit of the amount of REM is 0.10%. Preferable amounts of
respective elements are B: 0.0005% to 0.0030%, Mg: 0.0001% to
0.0015%, and REM 0.005% to 0.05%. Here, REM is the sum of the
amounts of lanthanoid rare earth elements such as Ce, La, and the
like.
[0065] By having the characteristics of the duplex stainless steel
of the present embodiment described above, it is possible to
greatly improve the hot manufacturability of the alloy-saving
duplex stainless steel which contains Sn.
[0066] In the cast steel stage, a fracture reduction of area at
1000.degree. C. is in a range of 70% or more. In addition, by
subjecting the cast steel to the processes which include the hot
working, it is possible to obtain a duplex stainless steel material
with a high yield and few surface defects.
Second Embodiment
[0067] Below, description will be given of the reasons for the
limits of the second aspect (a general-purpose duplex stainless
steel) of the duplex stainless steel of the present invention.
Here, the amounts of the respective components are shown in terms
of mass %.
[0068] Here, in the present embodiment, the cast steel of the
stainless steel indicates a steel in a state after casting and
before processing such as hot working, forging, or the like is
performed, and the stainless steel material indicates a
semi-finished product, a hot-rolled steel plate, a cold-rolled
steel plate, a steel wire, a steel pipe, or the like after
processing the cast steel by various methods. In addition, the
stainless steel indicates the general forms for a steel such as a
cast steel, a steel material, and the like. The processing
described above includes hot and cold processings.
[0069] In order to ensure the corrosion resistance of the stainless
steel, the amount of C is limited to be in a range of 0.03% or
less. When more than 0.03% of C is contained, the corrosion
resistance and toughness are degraded due to the generation of Cr
carbides during hot rolling.
[0070] 0.05% or more of Si is added for deoxidation. However, when
more than 1.0% of Si is added, the toughness is degraded.
Therefore, the upper limit for the amount of Si is limited to 1.0%.
The preferable range for the amount of Si is in a range of 0.2% to
0.7%.
[0071] Mn has the effect of improving the toughness by increasing
the austenite phase. In addition, since Mn has the effect of
suppressing the precipitation of nitrides, it is preferable to
actively add Mn to the steel material of the present embodiment.
For the toughness of the base material and the welding sections,
0.1% or more of Mn is added. However, when more than 4.0% of Mn is
added, the corrosion resistance and the toughness are degraded.
Therefore, the upper limit for the amount of Mn is limited to 4.0%.
The amount of Mn is preferably in a range of 1.0% to 3.5%, and more
preferably in a range of 2.0% to 3.0%.
[0072] P is an element which is inevitably mixed in from raw
materials and the amount of P is limited to be in a range of 0.05%
or less since P degrades the hot workability and the toughness. The
amount of P is preferably in a range of 0.03% or less.
[0073] S is an element which is inevitably mixed in from the raw
materials and the amount of S is limited to in a range of 0.0010%
or less since S degrades the hot workability, the toughness, and
the corrosion resistance. In addition, reducing the amount of S to
less than 0.0001% increases the costs due to desulfurization
refining. For this reason, the amount of S is set to be in a range
of 0.0001% to 0.0010%. The amount of S is preferably in a range of
0.0002% to 0.0006%.
[0074] 23.0% or more of Cr is contained in order to ensure basic
corrosion resistance. On the other hand, when more than 28.0% of Cr
is contained, the ferrite phase fraction increases and the
toughness and the corrosion resistance of the welding sections are
inhibited. For this reason, the amount of Cr is set to be in a
range of 23.0% or more to 28.0% or less. The amount of Cr is
preferably in a range of 24.0% to 27.5%.
[0075] Ni stabilizes the austenitic structure and improves the
toughness and the corrosion resistance with respect to various
types of acid. Furthermore, Ni suppresses a decrease in hot
workability due to the addition of Sn and Cu. For this reason, 2.0%
or more of Ni is contained. By increasing the amount of Ni, it is
possible to decrease the nitride precipitation temperature. On the
other hand, since Ni is an expensive alloy, the amount of Ni is
limited to be in a range of 6.0% or less. The amount of Ni is
preferably in a range of 2.5% to 5.5%, and more preferably in a
range of 3.0% to 5.0%.
[0076] Co is an element which is effective for increasing the
toughness and the corrosion resistance of the steel and which
suppresses a decrease in the hot workability due to the addition of
Sn and Cu, and it is desirable that Co be contained together with
Ni. In addition, in a case where Co is added, it is preferable that
0.1% or more of Co be contained. When more than 1.0% of Co is
contained, an effect which is commensurate with the cost is not
exhibited as Co is an expensive element. For this reason, the upper
limit of the amount of Co is set to 1.0%. In a case where Co is
added, the amount is preferably in a range of 0.1% to 0.5%.
[0077] It is known from Non-Patent Document 1 that Ni increases the
solid solubility of Cu and has an effect of suppressing the
generation of a liquid phase having a low melting point due to the
addition of Cu and Sn. In addition, Co is an element which belongs
to the same group as Ni. For this reason, it is considered that the
decrease in the hot workability due to Cu and Sn is suppressed by
increasing the sum of the amounts of Ni and Co. The present
inventors learned that the edge cracking of the steel material
increases in the case where the total amount of Ni and Co is in a
range of less than 2.5% when the hot workability of the steel which
is the subject of the present embodiment is arranged on the sum of
the amounts of Ni and Co. For this reason, the range of Ni+Co is
set to be in a range of 2.5% or more.
[0078] Cu is an element which increases the corrosion resistance of
the stainless steel with respect to acid and has an effect of
improving the toughness. In the present embodiment, in order to
increase the corrosion resistance, 0.2% or more of Cu is contained
together with 0.01% or more of Sn. When more than 3.0% of Cu is
contained, the amount of Cu exceeds the solid solubility; and
thereby, .epsilon.-Cu is precipitated during hot rolling to cause
embrittlement. For this reason, the upper limit of the amount of Cu
is set to 3.0%. In the case where Cu is contained, the amount is
preferably in a range of 0.5% to 2.0%.
[0079] Sn is contained in order to improve the corrosion resistance
of the steel of the present embodiment. For this reason, it is
necessary that at least 0.01% of Sn be contained. Furthermore, it
is preferable that 0.02% or more of Sn be contained. On the other
hand, Sn is an element which inhibits the hot manufacturability of
the steel, and decreases the hot strength of the interface between
the ferrite phase and the austenite phase, particularly at a
temperature of 900.degree. C. or less in the alloy element saving
type duplex stainless steel which is the subject of the present
embodiment. The degree of the decrease depends on the amounts of S,
Ca, and O; however, when more than 0.2% of Sn is contained, it is
not possible to prevent the decrease in the hot manufacturability
even by restricting other limits in the present embodiment.
Therefore, the upper limit of the amount of Sn is set to 0.2%.
[0080] N is an element which is effective for increasing the
strength and the corrosion resistance by being solid-solubilized in
the austenite phase. For this reason, 0.20% or more of N is
contained. Since it is possible to decrease the amount of Ni by
increasing the amount of N, N is an element which it is desirable
to actively add. On the other hand, it is necessary to limit the
upper limit of the amount of N to be within the solubility limit of
N. The solubility limit of N is increased according to the amounts
of Cr and Mn. When more than 0.30% of N is contained in the steel
of the present embodiment, Cr nitrides are precipitated such that
the toughness and the corrosion resistance are inhibited and the
hot manufacturability is inhibited. For this reason, the upper
limit of the amount of N is set to 0.30%. The amount of N is
preferably in a range of 0.20% to 0.28%.
[0081] Al is an element which deoxidizes a steel and Al is
contained together with 0.05% or more of Si in order to reduce the
oxygen in the steel according to necessity. In an Sn-containing
steel, the reduction of the oxygen amount is essential in order to
ensure the hot manufacturability, and for this reason, it is
necessary that 0.003% or more of Al be contained according to
necessity. On the other hand, Al is an element having comparatively
large affinity with N, and when an excessive amount of Al is added,
the toughness of the stainless steel is inhibited due to the
generation of AlN. The degree also depends on the amount of N;
however, when the amount of Al exceeds 0.05%, the toughness is
greatly decreased. For this reason, the upper limit of the amount
of Al is set to 0.05%. The amount of Al is preferably in a range of
0.04% or less.
[0082] Ca is an important element for the hot manufacturability of
the steel, and it is necessary that Ca be contained in order to fix
the S and O in the steel as inclusions and to improve the hot
manufacturability. In the steel of the present embodiment, 0.0010%
or more of Ca is contained for this purpose. In addition, addition
of an excessive amount thereof decreases the pitting resistance.
For this reason, the upper limit of the amount of Ca was set to
0.0040%.
[0083] The ratio Ca/O of the amounts of O and Ca is an important
component index in order to improve the hot manufacturability and
the corrosion resistance of the steel of the present embodiment.
The lower limit of Ca/O is limited in order to improve the hot
manufacturability of the Sn-containing steel. The high temperature
ductility of the Sn-containing steel is decreased, particularly at
a temperature of 900.degree. C. or less. When the value of Ca/O is
in a range of less than 0.3, the high temperature ductility at
1000.degree. C. is also decreased and the hot manufacturability is
greatly impaired. For this reason, in the steel of the present
embodiment, Ca/O is limited to be in a range of 0.3 or more. On the
other hand, when an excessive amount of Ca is added and Ca/O
exceeds 1.0, the pitting resistance is impaired. In addition, when
the amount of Ca is excessive, the high temperature ductility at a
temperature of 1000 to 1100.degree. C. is also impaired. For this
reason, the upper limit of Ca/O is set to be in a range of 1.0.
Ca/O is preferably in a range of 0.4 to 0.8.
[0084] O is an inevitable impurity and an upper limit thereof is
not particularly set; however, O is an important element which
configures oxides which are representatives of non-metallic
inclusions. Composition control of the oxides is extremely
important for the improvement of the hot manufacturability. In
addition, surface defects are caused when coarse cluster-shaped
oxides are generated. For this reason, it is necessary to limit the
amount of O so as to be low. In the present embodiment, as
described above, by setting the ratio of the amount of Ca and the
amount of O to be in a range of 0.3 or more, the amount of O is
limited. The upper limit of the amount of O is preferably in a
range of 0.005% or less.
[0085] Furthermore, either one or both of Mo: 2.0% or less, and W:
1.0% or less may be contained. These are elements which
incrementally increase the corrosion resistance. Description will
be given of the reasons for these limits.
[0086] Mo is an element which is extremely effective at
incrementally increasing the corrosion resistance of the stainless
steel, and Mo can be contained according to necessity. In order to
improve the corrosion resistance, it is preferable that 0.2% or
more of Mo be contained. On the other hand, Mo is an expensive
element, and from the point of view of suppressing the cost of the
alloy in the steel of the present embodiment, the upper limit of
the amount of Mo is set to 2.0%.
[0087] W is an element which incrementally increases the corrosion
resistance of the stainless steel in the same manner as Mo, and it
is possible to add W according to necessity. For the purpose of
increasing the corrosion resistance in the steel of the present
embodiment, the upper limit of the amount of W is set to 1.0%. The
amount of W is preferably in a range of 0.1% to 0.8%.
[0088] Furthermore, one or more selected from V: 0.05% to 0.5%, Nb:
0.01% to 0.15%, and Ti: 0.003% to 0.05% may be contained. These are
elements which are more likely to generate nitrides rather than Cr.
It is possible to add any of V, Nb, and Ti according to necessity,
and there is a tendency for the corrosion resistance to be improved
in cases where these are contained in trace amounts.
[0089] Nitrides and carbides which are formed by V are generated in
the hot working and the cooling process of the steel material, and
these have the effect of increasing the corrosion resistance. The
reasons therefor are not sufficiently confirmed; however, it is
considered that there is a probability of suppressing the
generation speed of the chromium nitrides at a temperature of
700.degree. C. or less. It is desirable that 0.05% or more of V be
contained in order to improve the corrosion resistance. When more
than 0.5% of V is contained, coarse V carbonitrides are generated
and the toughness is degraded. Therefore, the upper limit of the
amount of V is limited to 0.5%. In a case where V is added, the
amount is preferably in a range of 0.1% to 0.3%.
[0090] Nitrides and carbides which are formed of Nb are generated
in the hot working and the cooling process of the steel material,
and these have the effect of increasing the corrosion resistance.
The reasons therefor are not sufficiently confirmed; however, it is
considered that there is a probability of suppressing the
generation speed of the chromium nitrides at a temperature of
700.degree. C. or less. It is desirable that 0.01% or more of Nb be
contained in order to improve the corrosion resistance. On the
other hand, in the case where an excessive amount of Nb is added,
Nb is precipitated as non-solid-solubilized precipitates during
heating before the hot rolling; and thereby, the toughness is
inhibited. For this reason, the upper limit of the amount of Nb is
set to 0.15%. In a case where Nb is added, the range of the amount
is preferably in a range of 0.03% to 0.10%.
[0091] Ti is an element which forms oxides, nitrides, and sulfides
in very small amounts and Ti refines crystal grains in the
solidified structure and the structure heated at a high temperature
of the steel. In addition, in the same manner as V and Nb, Ti also
has the property of replacing a part of the chromium in the
chromium nitrides. With an amount of Ti of 0.003% or more, Ti
precipitates are formed. On the other hand, when more than 0.05% of
Ti is contained in the duplex stainless steel, the toughness of the
steel is impaired due to the generation of coarse TiN. For this
reason, the upper limit of the amount of Ti is set to 0.05%. A
suitable amount of Ti is in a range of 0.005% to 0.020%.
[0092] Furthermore, one or more selected from B: 0.0050% or less,
Mg: 0.0030% or less, and REM: 0.10% or less may be contained. In
order to achieve further improvement of the hot workability, the B,
Mg, and REM to be contained according to necessity are limited as
follows.
[0093] B, Mg, and REM are all elements which improve the hot
workability of the steel, and it is desirable that one or more be
added for this purpose. The addition of an excessive amount of any
of B, Mg, and REM has the opposite effect of decreasing the hot
workability and the toughness. For this reason, the upper limits of
the above amounts are set as follows. The upper limit of the amount
of B is 0.0050%. The upper limit of the amount of Mg is 0.0030%.
The upper limit of the amount of REM is 0.10%. Preferable amounts
of respective elements are B: 0.0005% to 0.0030%, Mg: 0.0001% to
0.0015%, and REM 0.005% to 0.05%. Here, REM is the sum of the
amounts of lanthanoid rare earth elements such as Ce, La, and the
like.
[0094] Above, by having the characteristics of the duplex stainless
steel of the present embodiment described above, it is possible to
greatly improve the hot manufacturability of the general-purpose
duplex stainless steel which contains Sn.
[0095] In the cast steel stage, a fracture reduction of area at
1000.degree. C. is in a range of 70% or more. In addition, by
subjecting the cast steel to the processes which include the hot
working, it is possible to obtain a duplex stainless steel material
with a high yield and few surface defects.
EXAMPLES
Example 1
[0096] Description will be given of examples of the alloy-saving
type duplex stainless steel below. The chemical compositions of
test steels are shown in Tables 1 to 4. Here, the remainder other
than the components which are described in Table 1 is Fe and
inevitable impurity elements. In addition, for the components which
are shown in Tables 1 to 4, portions where the amounts are not
described show the impurity levels. REM indicates lanthanoid rare
earth elements, and the amount of REM shows the total of these
elements. The numbers which are underlined in the tables indicate
values outside of the ranges which are defined in the first
embodiment.
TABLE-US-00001 TABLE 1 Steel No. C Si Mn P S Ni Cr N Al Ca Sn Other
O PI Ca/O 1-1 Invention Examples 0.015 0.39 3.21 0.022 0.0005 2.15
20.9 0.173 0.015 0.0013 0.05 0.0038 23.7 0.34 1-2 0.020 0.34 3.01
0.024 0.0004 2.08 21.0 0.165 0.042 0.0022 0.09 0.0024 23.6 0.92 1-3
0.018 0.42 4.93 0.021 0.0006 2.13 20.9 0.186 0.025 0.0019 0.06
0.0032 23.9 0.59 1-4 0.018 0.35 3.02 0.023 0.0007 2.35 20.8 0.178
0.023 0.0022 0.13 Mo: 0.32 0.0030 24.7 0.73 1-5 0.018 0.35 3.05
0.023 0.0007 2.35 20.9 0.168 0.013 0.0028 0.02 Cu: 1.05 0.0048 23.6
0.58 1-6 0.018 0.35 3.02 0.024 0.0007 2.35 20.8 0.181 0.015 0.0018
0.07 W: 0.25 0.0039 23.7 0.46 1-7 0.018 0.35 3.05 0.023 0.0007 2.35
20.9 0.176 0.012 0.0019 0.03 Co: 0.23 0.0048 23.7 0.40 1-8 0.021
0.42 2.56 0.031 0.0005 1.53 18.5 0.125 0.043 0.0013 0.05 Mo: 1.22,
0.0026 24.5 0.50 Cu: 0.95 1-9 0.021 0.42 2.54 0.031 0.0005 1.42
18.5 0.132 0.047 0.0012 0.05 Mo: 1.38, 0.0024 25.2 0.50 Cu: 1.03,
Co: 0.02 1-10 0.021 0.42 2.53 0.031 0.0005 1.44 18.5 0.115 0.049
0.0015 0.05 Mo: 0.12, 0.0028 20.7 0.54 Cu: 1.23, W: 0.23 1-11 0.025
0.64 4.89 0.026 0.0006 1.52 21.3 0.215 0.023 0.0023 0.06 V: 0.12
0.0034 24.7 0.68 1-12 0.025 0.64 5.12 0.026 0.0006 1.52 21.5 0.205
0.021 0.0015 0.06 Nb: 0.052 0.0034 24.8 0.44 1-13 0.025 0.64 4.96
0.026 0.0006 1.51 21.7 0.218 0.015 0.0018 0.06 Ti: 0.012 0.0038
25.2 0.47 1-14 0.025 0.64 5.32 0.026 0.0006 1.53 21.5 0.232 0.019
0.0022 0.06 V: 0.11, 0.0036 25.2 0.61 Nb: 0.035 1-15 0.028 0.56
1.74 0.023 0.0006 4.53 23.4 0.106 0.035 0.0023 0.17 Mo: 0.34,
0.0027 26.2 0.85 V: 0.35, Ti: 0.032
TABLE-US-00002 TABLE 2 Steel No. C Si Mn P S Ni Cr N Al Ca Sn Other
O PI Ca/O 1-16 Invention Examples 0.014 0.45 2.95 0.015 0.0003 1.95
20.7 0.175 0.023 0.0013 0.06 Mo: 0.35, 0.0026 24.7 0.50 Cu: 1.04,
V: 0.12, Ti: 0.007 1-17 0.007 0.44 2.98 0.014 0.0003 1.97 20.7
0.175 0.012 0.0036 0.07 W: 0.35, 0.0046 23.5 0.78 Co: 0.03, V:
0.11, Ti: 0.006 1-18 0.005 0.46 2.96 0.013 0.0003 1.96 20.6 0.173
0.025 0.0021 0.08 Mo: 0.28, 0.0027 24.3 0.78 Cu: 1.05, V: 0.14, Nb:
0.048, Ti: 0.011 1-19 0.022 0.15 4.03 0.033 0.0002 2.03 21.3 0.155
0.023 0.0011 0.10 B: 0.0026 0.0033 23.8 0.33 1-20 0.023 0.14 3.26
0.036 0.0010 2.00 21.2 0.165 0.023 0.0031 0.10 Mg: 0.0012 0.0032
23.8 0.97 1-21 0.024 0.16 3.33 0.023 0.0009 2.04 20.9 0.166 0.022
0.0023 0.10 REM: 0.065 0.0034 23.6 0.68 1-22 0.023 0.13 3.12 0.021
0.0008 2.03 20.0 0.164 0.024 0.0022 0.10 B: 0.0032, 0.0035 22.6
0.63 Mg: 0.0006 1-23 0.021 0.07 2.86 0.019 0.0001 2.05 21.1 0.175
0.021 0.0016 0.10 B: 0.0023, 0.0023 23.9 0.70 REM: 0.032 1-24 0.022
0.12 2.75 0.016 0.0002 2.01 20.9 0.177 0.023 0.0024 0.05 Mo: 0.56,
0.0036 25.6 0.67 Cu: 1.45, B: 0.0028
TABLE-US-00003 TABLE 3 Steel No. C Si Mn P S Ni Cr N Al Ca Sn Other
O PI Ca/O 1-25 Invention Examples 0.026 0.76 2.89 0.018 0.0005 2.45
20.8 0.172 0.022 0.0028 0.05 Mo: 0.38, 0.0032 24.8 0.88 Cu: 1.06,
Co: 0.04, V: 0.13, Ti: 0.006, B: 0.0024 1-26 0.024 0.78 3.01 0.015
0.0003 2.56 21.9 0.179 0.021 0.0023 0.05 Mo: 0.35, 0.0033 25.9 0.70
Cu: 1.01, W: 0.12, Co: 0.03, V: 0.16, Nb: 0.015, Ti: 0.004, B:
0.0016, Mg: 0.0003 1-27 0.016 0.43 6.53 0.021 0.0004 0.75 18.3
0.182 0.016 0.0016 0.04 Mo: 1.35, 0.0034 25.7 0.47 Cu: 1.23 1-28
0.024 0.37 2.43 0.023 0.0006 4.58 24.4 0.245 0.023 0.0019 0.06 V:
0.13 0.0036 28.3 0.53 1-29 0.013 0.42 3.15 0.022 0.0004 4.13 24.5
0.235 0.016 0.0022 0.07 0.0046 28.3 0.48 1-30 0.025 0.36 0.23 0.012
0.0003 3.02 21.1 0.165 0.005 0.0023 0.04 Co: 1.52 0.0047 23.7 0.49
1-31 0.018 0.26 0.85 0.031 0.0002 4.23 21.3 0.201 0.0021 0.08 W:
0.75 0.0052 24.5 0.40 1-32 0.023 0.32 2.45 0.024 0.0005 3.24 18.2
0.112 0.003 0.0016 0.12 Mo: 1.43 0.0042 24.7 0.38 1-33 0.019 0.39
0.31 0.021 0.0006 1.68 21.3 0.164 0.013 0.0014 0.06 Cu: 1.83 0.0038
23.9 0.37
TABLE-US-00004 TABLE 4 Steel No. C Si Mn P S Ni Cr N Al Ca Sn Other
O PI Ca/O 1-A Comparative 0.016 0.38 2.96 0.022 0.0006 1.96 20.9
0.174 0.026 0.0006 0.08 0.0036 23.7 0.17 1-B Examples 0.016 0.38
2.98 0.022 0.0006 1.96 20.9 0.174 0.0012 0.08 0.0052 23.7 0.23 1-C
0.015 0.39 2.96 0.023 0.0006 1.98 21.0 0.172 0.023 0.0016 <0.01
0.0032 23.8 0.50 1-D 0.016 0.38 2.98 0.022 0.0006 1.97 21.0 0.172
0.023 0.0018 0.26 0.0032 23.8 0.56 1-E 0.021 0.42 3.12 0.023 0.0005
2.02 21.1 0.175 0.021 0.0045 0.08 0.0021 23.9 2.14 1-F 0.043 0.54
2.86 0.025 0.0006 2.01 21.0 0.182 0.017 0.0020 0.08 0.0036 23.9
0.56 1-G 0.025 1.54 3.13 0.029 0.0006 2.00 21.0 0.183 0.017 0.0020
0.07 0.0036 23.9 0.56 1-H 0.024 0.39 7.85 0.028 0.0006 2.03 21.0
0.175 0.017 0.0020 0.07 0.0032 23.8 0.63 1-I 0.023 0.46 3.24 0.065
0.0005 2.00 21.0 0.186 0.018 0.0020 0.06 0.0033 24.0 0.61 1-J 0.026
0.48 3.16 0.022 0.0012 1.99 21.0 0.165 0.019 0.0018 0.07 0.0041
23.6 0.44 1-K 0.025 0.42 3.08 0.031 0.0006 0.32 21.0 0.159 0.017
0.0007 0.07 0.0035 23.5 0.20 1-L 0.024 0.41 2.56 0.022 0.0007 2.23
17.2 0.184 0.016 0.0018 0.12 0.0038 20.1 0.47 1-M 0.025 0.43 2.66
0.023 0.0006 1.85 20.9 0.330 0.015 0.0018 0.08 0.0033 26.2 0.55 1-N
0.016 0.44 2.36 0.021 0.0007 1.96 21.0 0.174 0.021 0.0019 0.07 V:
0.63 0.0035 23.8 0.54 1-O 0.017 0.42 2.86 0.025 0.0007 1.94 21.0
0.175 0.019 0.0019 0.07 Nb: 0.24 0.0036 23.8 0.53 1-P 0.016 0.38
2.94 0.024 0.0007 1.95 21.0 0.173 0.023 0.0019 0.07 Ti: 0.062
0.0034 23.8 0.56 1-Q 0.018 0.39 3.11 0.024 0.0007 1.93 21.1 0.172
0.022 0.0019 0.07 B: 0.0076 0.0036 23.9 0.53 1-R 0.015 0.41 3.13
0.023 0.0007 1.92 20.9 0.170 0.021 0.0019 0.07 Mg: 0.0041 0.0037
23.6 0.51 1-S 0.015 0.42 3.06 0.022 0.0007 1.94 21.0 0.169 0.020
0.0019 0.30 REM: 0.150 0.0042 23.7 0.45 1-T 0.023 0.38 2.98 0.024
0.0006 2.15 21.3 0.165 0.072 0.0022 0.05 0.0018 23.9 1.22 1-U 0.023
0.39 2.99 0.023 0.0006 2.18 21.1 0.078 0.024 0.0021 0.06 0.0036
22.3 0.58
[0097] For all the steels, firstly, a cast steel with a thickness
of 100 mm was prepared, and the fracture reduction of area was
evaluated. The evaluation was performed as follows. First, a
parallel section of a round bar of 8 mm.phi. was heated to
1200.degree. C. using a high frequency. Next, the temperature was
lowered to a temperature (1000.degree. C.) at which a break test
was performed. Tensile rupture was performed at a speed of 20
mm/second at this temperature, and the shrinkage of the cross
section was measured. Steels where the fracture reduction of area
was in a range of 70% or more were evaluated as A (good), steels
where the reduction of area was in a range of 60% or more to less
than 70% were evaluated as B (fair), steels where the reduction of
area was in a range of less than 60% were evaluated as C (bad), and
the results are given in Tables 5 and 6.
[0098] The cast steel was subjected to hot forging to obtain a
semi-finished product with a thickness of 60 mm, and this
semi-finished product was used as a hot-rolled material. The
semi-finished product was heated to a predetermined temperature of
1150 to 1250.degree. C., and then the hot rolling was performed
using a two stage rolling machine in a laboratory under the
following conditions. First, reduction was repeatedly performed so
as to adjust the plate thickness to be 25 mm. Then, finishing
rolling was performed from 1000.degree. C., and the final finishing
rolling was carried out at 900.degree. C. This rolling was
performed such that the final plate thickness became 12 mm and the
plate width became 120 mm to obtain a hot-rolled steel plate. The
maximum lengths of the edge crackings which were generated in the
left and right edge sections of the obtained hot-rolled steel plate
were measured, and the sum of the maximum lengths of the edge
crackings in the left and right edge sections was determined.
Steels where the sum of the edge crackings was in a range of less
than 5 mm were evaluated as A (good), steels where the sum of the
edge crackings was in a range of 5 to 10 mm were evaluated as B
(fair), steels where the sum of the edge crackings exceeds 10 mm
were evaluated as C (bad), and the results are given in Tables 5
and 6.
[0099] Furthermore, the steel plates were subjected to a
solutionizing heat treatment in the following manner. The steel
plate was inserted into a heat treatment furnace at 1000.degree. C.
and heated for approximately 5 minutes. Next, the steel plate was
taken out, and then was subjected to water cooling to room
temperature.
[0100] The corrosion resistance of the steel plate was evaluated by
the corrosion rate in sulfuric acid.
[0101] The corrosion rate in the sulfuric acid was measured as
follows. Test pieces of 3 mm thick.times.25 mm wide.times.25 mm
long were subjected to an immersion test for 6 hours in boiling 5%
sulfuric acid. The weight before and after immersion was measured,
and the rate of decrease in weight was calculated. Steels where the
corrosion rate in the sulfuric acid was in a range of less than 0.3
g/m.sup.2 per hour were evaluated as A (good), steels where the
corrosion rate in the sulfuric acid was in a range of 0.3 to 1
g/m.sup.2 per hour were evaluated as B (fair), steels where the
corrosion rate in the sulfuric acid was in a range of 1 g/m.sup.2
per hour or more were evaluated as C (bad), and the evaluation
results are given in Tables 5 and 6.
[0102] The impact characteristics were measured using Charpy test
pieces which were taken a long in the width direction. The test
pieces were prepared by processing 2 mm V notches at full size in
the rolling direction. Testing was carried out at -20.degree. C.
using two test pieces for each of the steels, and the impact
characteristics were evaluated by the average values of the
obtained impact values. Steels where the impact value was in a
range of more than 100 J/cm.sup.2 were evaluated as A (good),
steels where the impact value was in a range of 50 to 100
J/cm.sup.2 were evaluated as B (fair), steels where the impact
value was less than 50 J/cm.sup.2 were evaluated as C (bad), and
the evaluation results are given in Tables 5 and 6.
TABLE-US-00005 TABLE 5 Edge Sulfuric Impact Cracking Acid
Character- Reduction Resistance Resistance istics Steel of Area of
of Steel of Steel of Steel No. Cast Steel Material Material
Material Invention 1-1 A A A A Examples 1-2 A A A A 1-3 A A A A 1-4
A A A A 1-5 A A A A 1-6 A A A A 1-7 A A A A 1-8 A A A A 1-9 A A A A
1-10 A A A A 1-11 A A A A 1-12 A A A A 1-13 A A A A 1-14 A A A A
1-15 A A A A 1-16 A A A A 1-17 A A A A 1-18 A A A A 1-19 A A A A
1-20 A A A A 1-21 A A A A 1-22 A A A A 1-23 A A A A 1-24 A A A A
1-25 A A A A 1-26 A A A A 1-27 A A A A 1-28 A A A A 1-29 A A A A
1-30 A A A A 1-31 A A A A 1-32 A A A A 1-33 A A A A
TABLE-US-00006 TABLE 6 Edge Sulfuric Impact Cracking Acid
Character- Reduction Resistance Resistance istics Steel of Area of
of Steel of Steel of Steel No. Cast Steel Material Material
Material Comparative 1-A C C A A Examples 1-B B C A A 1-C A A B A
1-D C C A B 1-E C C A A 1-F B B A B 1-G A A A B 1-H B C B A 1-I C C
A B 1-J C C A A 1-K C C C C 1-L C C C A 1-M C C A A 1-N C B A C 1-O
B B A C 1-P B B A C 1-Q B B A C 1-R B B A B 1-S C C A B 1-T B B A C
1-U A A A C
[0103] From the examples which are shown in Table 5 and 6, steels
No. 1-1 to 1-33 which satisfy the conditions of the first
embodiment have favorable hot manufacturability, corrosion
resistance, and impact characteristics. On the other hand, the
steels No. 1-A to 1-U which do not satisfy the conditions of the
first embodiment were inferior in all of hot manufacturability,
corrosion resistance, and impact characteristics.
[0104] As seen from the above examples, it is clear that it is
possible to obtain an inexpensive alloy-saving type duplex
stainless steel with favorable hot manufacturability where the
corrosion resistance is improved by the addition of Sn according to
the first embodiment.
Example 2
[0105] Description will be given of examples of the general-purpose
duplex stainless steel below. The chemical compositions of the test
steels are shown in Tables 7 to 10. Here, the remainder other than
the components which are described in Tables 7 to 10 is Fe and
inevitable impurity elements. In addition, for the components which
are shown in Tables 7 to 10, portions where the amounts are not
described show the impurity levels. REM indicates lanthanoid rare
earth elements and the amount of REM shows the total of these
elements. The numbers which are underlined in the table indicate
values outside of the ranges which are defined in the second
embodiment.
TABLE-US-00007 TABLE 7 Steel No. C Si Mn P S Cr Ni Co N Al Ca Cu Sn
Other O 2-1 Invention Examples 0.015 0.39 2.45 0.022 0.0005 26.5
4.48 0.254 0.015 0.0021 1.43 0.07 0.0038 2-2 0.012 0.35 3.25 0.021
0.0007 27.3 4.83 0.65 0.235 0.023 0.0016 1.52 0.08 0.0032 2-3 0.021
0.42 3.45 0.023 0.0004 25.3 4.05 0.12 0.253 0.018 0.0018 1.03 0.05
Mo: 1.23 0.0034 2-4 0.024 0.22 3.65 0.023 0.0005 23.5 2.35 0.32
0.245 0.025 0.0023 1.53 0.13 Mo: 1.75 0.0028 2-5 0.023 0.53 1.52
0.024 0.0002 26.4 4.52 0.01 0.265 0.003 0.0021 0.52 0.14 W: 0.35
0.0038 2-6 0.016 0.65 2.43 0.025 0.0006 25.1 3.85 0.23 0.245 0.016
0.0015 1.53 0.06 Mo: 1.25, 0.0042 W: 0.24 2-7 0.007 0.24 0.25 0.021
0.0005 26.5 4.03 0.53 0.246 0.012 0.0016 1.45 0.07 V: 0.12 0.0043
2-8 0.026 0.74 3.35 0.023 0.0006 26.5 4.53 0.24 0.224 0.017 0.0017
1.23 0.08 Nb: 0.034 0.0038 2-9 0.015 0.44 2.56 0.031 0.0005 26.4
4.52 0.21 0.236 0.021 0.0023 1.52 0.13 Ti: 0.007 0.0032 2-10 0.014
0.42 2.75 0.033 0.0005 26.6 4.51 0.23 0.245 0.022 0.0021 1.48 0.09
V: 0.07, 0.0037 Nb: 0.024 2-11 0.023 0.39 3.21 0.015 0.0004 26.4
4.36 0.85 0.234 0.028 0.0013 1.03 0.10 Nb: 0.047, 0.0028 Ti: 0.011
2-12 0.022 0.36 2.35 0.034 0.0006 26.3 4.42 0.03 0.253 0.013 0.0024
0.95 0.08 V: 0.13, 0.0034 Nb: 0.015, Ti: 0.005 2-13 0.025 0.35 2.64
0.026 0.0005 23.8 3.85 0.15 0.238 0.024 0.0023 1.05 0.12 Mo: 1.52,
0.0040 V: 0.12 2-14 0.018 0.31 2.48 0.024 0.0009 25.6 4.15 0.19
0.247 0.018 0.0019 1.12 0.08 Mo: 0.52, 0.0042 V: 0.07, Nb: 0.034
2-15 0.019 0.28 2.54 0.026 0.0007 26.4 4.62 0.06 0.265 0.023 0.0022
1.33 0.06 B: 0.0023 0.0044 2-16 0.013 0.33 2.53 0.024 0.0006 26.6
4.58 0.14 0.267 0.021 0.0024 1.22 0.07 Mg: 0.0012 0.0031
TABLE-US-00008 TABLE 8 Steel No. C Si Mn P S Cr Ni Co N Al Ca Cu Sn
Other O 2-17 Invention Examples 0.024 0.37 2.54 0.025 0.0005 26.4
4.05 0.51 0.258 0.028 0.0023 1.45 0.06 REM: 0.035 0.0034 2-18 0.025
0.45 2.56 0.023 0.0005 26.5 4.45 0.25 0.265 0.0021 1.03 0.07 B:
0.0026, 0.0048 Mg: 0.0007 2-19 0.027 0.51 2.51 0.025 0.0005 24.8
4.01 0.15 0.244 0.016 0.0025 1.49 0.05 Mo: 1.23, 0.0036 V: 0.12, B:
0.0031, Mg: 0.0005 2-20 0.022 0.23 2.58 0.024 0.0005 23.3 3.52 0.36
0.228 0.026 0.0017 0.99 0.07 Mo: 1.36, 0.0035 W: 0.75, V: 0.06, Ti:
0.004, B: 0.0026 2-21 0.011 0.26 2.48 0.023 0.0004 25.0 4.49 0.13
0.240 0.018 0.0022 1.05 0.06 Mo: 1.22, 0.0034 V: 0.13, Nb: 0.045,
Ti: 0.004, B: 0.0024, Mg: 0.0001 2-22 0.016 0.12 2.36 0.025 0.0006
25.0 4.00 0.12 0.242 0.024 0.0023 1.48 0.02 Mo: 1.35, 0.0035 V:
0.12, Nb: 0.015, Ti: 0.006, B: 0.0023, Mg: 0.0003
TABLE-US-00009 TABLE 9 Steel No. C Si Mn P S Cr Ni Co N Al Ca Cu Sn
Other O 2-23 Invention Examples 0.024 0.46 2.44 0.026 0.0005 25.3
4.23 0.16 0.248 0.023 0.0025 1.46 0.05 Mo: 1.02, 0.0042 W: 0.32, V:
0.10, Nb: 0.021, Ti: 0.005, B: 0.0024, Mg: 0.002 2-A Comparative
Examples 0.016 0.37 2.13 0.022 0.0006 25.6 3.25 0.246 0.023 0.0008
2.85 0.12 0.0042 2-B 0.013 0.41 2.65 0.027 0.0008 25.4 3.24 0.05
0.273 0.021 0.0014 2.23 0.24 0.0035 2-C 0.015 0.40 3.01 0.025
0.0006 25.1 4.00 0.10 0.251 0.016 0.0021 0.05 0.10 0.0037 2-D 0.014
0.40 2.99 0.025 0.0006 25.0 4.02 0.10 0.249 0.017 0.0020 0.50
0.0036 2-E 0.036 0.39 2.98 0.024 0.0005 24.8 3.98 0.10 0.248 0.026
0.0021 0.47 0.04 0.0033 2-F 0.015 1.26 3.02 0.024 0.0007 26.8 3.88
0.10 0.233 0.026 0.0017 0.49 0.06 0.0028 2-G 0.014 0.42 5.12 0.025
0.0005 25.1 3.87 0.11 0.256 0.020 0.0020 0.48 0.05 0.0043 2-H 0.016
0.41 2.97 0.062 0.0005 26.5 3.76 0.09 0.232 0.014 0.0015 0.48 0.08
0.0040 2-I 0.016 0.43 2.98 0.024 0.0013 26.3 4.04 0.12 0.255 0.013
0.0015 0.52 0.07 0.0041 2-J 0.012 0.45 2.42 0.026 0.0006 29.1 4.53
0.08 0.262 0.018 0.0020 0.53 0.09 0.0051 2-K 0.013 0.39 2.89 0.024
0.0007 24.9 1.78 0.35 0.249 0.019 0.0018 0.55 0.06 0.0038 2-L 0.016
0.42 2.51 0.025 0.0008 24.9 3.98 1.35 0.244 0.018 0.0023 1.02 0.08
0.0045 2-M 0.014 0.38 2.47 0.025 0.0006 25.0 3.42 0.06 0.321 0.015
0.0016 0.49 0.05 0.0052 2-N 0.015 0.42 2.42 0.023 0.0007 24.8 4.02
0.07 0.253 0.062 0.0022 0.52 0.06 0.0042 2-O 0.015 0.39 2.52 0.024
0.0007 24.9 4.01 0.09 0.245 0.008 0.0012 0.50 0.06 0.0053 2-P 0.014
0.40 2.46 0.023 0.0006 25.0 3.99 0.10 0.246 0.007 0.0021 3.53 0.11
0.0044
TABLE-US-00010 TABLE 10 Steel No. C Si Mn P S Cr Ni Co N Al Ca Cu
Sn Other O 2-Q Comparative Examples 0.015 0.40 2.50 0.025 0.0007
25.0 4.00 0.09 0.251 0.021 0.0021 0.04 Mo: 1.02 0.0038 2-R 0.014
0.39 2.48 0.026 0.0006 24.8 4.02 0.12 0.246 0.019 0.0017 0.03 0.02
Mo: 0.52, 0.0043 V: 0.06, B: 0.0021 2-S 0.016 0.41 2.52 0.025
0.0005 25.1 2.35 0.01 0.265 0.018 0.0023 1.83 0.16 Mo: 0.48, 0.0033
W: 0.12, Nb: 0.012, Ti: 0.006, B: 0.0023 2-T 0.014 0.42 2.49 0.026
0.0006 25.1 4.03 0.03 0.262 0.023 0.0048 1.02 0.07 Mo: 0.32 0.0032
2-U 0.013 0.48 1.65 0.024 0.0006 22.5 5.83 0.178 0.013 0.0023 0.05
Mo: 3.03 0.0035
[0106] Under the same conditions as Example 1, the manufacturing of
the cast steel, the evaluation of the fracture reduction of area of
the cast steel, the manufacturing of the hot-rolled material, the
performing of the hot rolling with respect to the hot-rolled
material, and the evaluation of the edge cracking were performed.
The obtained evaluation results are given in Tables 11 and 12.
[0107] Furthermore, the steel plates were subjected to a
solutionizing heat treatment in the following manner. The steel
plate was inserted into a heat treatment furnace at 1050.degree. C.
and heated for approximately 5 minutes. Next, the steel plate was
taken out, and then was subjected to water cooling to room
temperature.
[0108] The corrosion resistance of the steel plate was evaluated by
the corrosion rate in the sulfuric acid.
[0109] The corrosion rate in the sulfuric acid was measured as
follows. Test pieces of 3 mm thick.times.25 mm wide.times.25 mm
long, were subjected to an immersion test for 6 hours in sulfuric
acid including 2000 ppm of Cl ions, where the concentration was 15%
and the temperature was 40%. The weight before and after immersion
was measured, and the rate of decrease in weight was calculated.
Steels where the corrosion rate in the sulfuric acid was in a range
of less than 0.1 g/m.sup.2 per hour were evaluated as A (good),
steels where the corrosion rate in the sulfuric acid was in a range
of 0.1 to 0.3 g/m.sup.2 per hour were evaluated as B (fair), steels
where the corrosion rate in the sulfuric acid was in a range of
more than 0.3 g/m.sup.2 per hour were evaluated as C (bad), and the
evaluation results are given in Tables 11 and 12.
[0110] Under the same conditions as Example 1, the impact
characteristics were measured. The obtained evaluation results are
given in Tables 11 and 12.
TABLE-US-00011 TABLE 11 Edge Sulfuric Cracking Acid Impact
Reduction Resistance Resistance Characteristics Steel Ni + of Area
of of Steel of Steel of Steel No. Co Ca/O PI Cast Steel Material
Material Material Invention Examples 2-1 4.48 0.55 30.6 A A A A 2-2
5.48 0.50 31.1 A A A A 2-3 4.17 0.53 33.4 A A A A 2-4 2.67 0.82
33.2 A A A A 2-5 4.53 0.55 30.6 A A A A 2-6 4.08 0.36 33.1 A A A A
2-7 4.56 0.37 30.4 A A A A 2-8 4.77 0.45 30.1 A A A A 2-9 4.73 0.72
30.2 A A A A 2-10 4.74 0.57 30.5 A A A A 2-11 5.21 0.46 30.1 A A A
A 2-12 4.45 0.71 30.3 A A A A 2-13 4.00 0.58 32.6 A A A A 2-14 4.34
0.45 31.3 A A A A 2-15 4.68 0.50 30.6 A A A A 2-16 4.72 0.77 30.9 A
A A A 2-17 4.56 0.68 30.5 A A A A 2-18 4.70 0.44 30.7 A A A A 2-19
4.16 0.69 32.8 A A A A 2-20 3.88 0.49 31.4 A A A A 2-21 4.62 0.65
32.9 A A A A 2-22 4.12 0.66 33.3 A A A A 2-23 4.39 0.60 32.6 A A A
A
TABLE-US-00012 TABLE 12 Edge Sulfuric Cracking Acid Impact
Reduction Resistance Resistance Characteristics Steel Ni + of Area
of of Steel of Steel of Steel No. Co Ca/O PI Cast Steel Material
Material Material Comparative Examples 2-A 3.25 0.19 29.5 C C A A
2-B 3.29 0.40 29.8 C C A A 2-C 4.10 0.57 29.1 A A C A 2-D 4.12 0.56
29.0 A A C A 2-E 4.08 0.64 28.8 A A B B 2-F 3.98 0.61 30.5 A A A C
2-G 3.98 0.47 29.2 A A C A 2-H 3.85 0.38 30.2 A A B B 2-I 4.16 0.37
30.4 C C B A 2-J 4.61 0.39 33.3 A A A C 2-K 2.13 0.47 28.9 C C B C
2-L 5.33 0.51 28.8 A A A A 2-M 3.48 0.31 30.1 C C A B 2-N 4.09 0.52
28.8 A A A C 2-O 4.10 0.23 28.8 C C A B 2-P 4.09 0.48 28.9 C C A B
2-Q 4.09 0.55 32.4 A A C A 2-R 4.14 0.40 30.5 A A C A 2-S 2.36 0.70
30.9 C C A A 2-T 4.06 1.50 30.3 B B B B 2-U 5.83 0.66 35.4 A A A
A
[0111] From the examples which are shown in Table 11 and 12, the
general-purpose duplex stainless steels No. 2-1 to 2-23 which
satisfy the conditions of the second embodiment have favorable hot
manufacturability, corrosion resistance, and impact
characteristics. On the other hand, steels No. 2-A to 2-K and 2-M
to 2-T which do not satisfy the conditions of the second embodiment
were inferior in hot manufacturability, corrosion resistance, and
impact characteristics. In addition, comparative example 2-L
satisfied the characteristics; however, since a large amount of Co
was contained, comparative example 2-L was inferior in terms of
cost. In addition, comparative example 2-U is S31803 steel and is
favorable in all of hot manufacturability, corrosion resistance,
and manufacturability. However, the amounts of Ni and Mo are high
and comparative example 2-U is inferior in terms of cost for the
purpose of the second embodiment.
[0112] As seen from the above examples, it is clear that it is
possible to obtain an inexpensive general-purpose duplex stainless
steel with favorable hot manufacturability where the corrosion
resistance is improved due to the addition of Sn and Cu according
to the second embodiment.
INDUSTRIAL APPLICABILITY
[0113] According to the first and second embodiments, it is
possible to provide an alloy-saving type duplex stainless steel and
a general-purpose duplex stainless steel which are inexpensive and
where the corrosion resistance is improved. These duplex stainless
steel materials make an extremely significant contribution to
industries because it is possible to use the duplex stainless steel
materials in seawater desalination unit, tanks for a transport
ship, various types of containers, or the like.
* * * * *