U.S. patent application number 11/816015 was filed with the patent office on 2009-01-29 for high tensile strength, refractory steel having excellent weldability and gas cuttability and method for producing same.
This patent application is currently assigned to Nippon Steel Corporation. Invention is credited to Kiyoshi Ishibashi, Ryuuji Uemori, Yoshiyuki Watanabe, Kenichi Yoshii.
Application Number | 20090025839 11/816015 |
Document ID | / |
Family ID | 36941312 |
Filed Date | 2009-01-29 |
United States Patent
Application |
20090025839 |
Kind Code |
A1 |
Watanabe; Yoshiyuki ; et
al. |
January 29, 2009 |
HIGH TENSILE STRENGTH, REFRACTORY STEEL HAVING EXCELLENT
WELDABILITY AND GAS CUTTABILITY AND METHOD FOR PRODUCING SAME
Abstract
Accordingly to an exemplary embodiment of the present invention,
a high tensile strength, refractory steel can be provided which
comprises, in mass %, approximately C: 0.04 to 0.14%, Si: 0.50% or
less, Mn: 0.50 to 2.00%, P: 0.020% or less, S: 0.010% or less, Nb:
0.01 to 0.05%, Mo: 0.30% or more and less than 0.70%, Al: 0.060% or
less, N: 0.0010 to 0.0060%, and the balance consisting of iron and
unavoidable impurities. For example, a weld crack sensitive
composition P.sub.CM can be defined by the following equation may
be about 0.25% or less:
P.sub.CM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B. An area
fraction of polygonal ferrite or pseudo polygonal ferrite in a 1/4
thick position in the plate thickness direction of the steel plate
of the final rolling product is about 10% or less.
Inventors: |
Watanabe; Yoshiyuki;
(Kimitsu-shi, JP) ; Uemori; Ryuuji; (Kisarazu-shi,
JP) ; Ishibashi; Kiyoshi; (Kimitsu-shi, JP) ;
Yoshii; Kenichi; (Kimitsu-shi, JP) |
Correspondence
Address: |
DORSEY & WHITNEY LLP;INTELLECTUAL PROPERTY DEPARTMENT
250 PARK AVENUE
NEW YORK
NY
10177
US
|
Assignee: |
Nippon Steel Corporation
Tokyo
JP
|
Family ID: |
36941312 |
Appl. No.: |
11/816015 |
Filed: |
March 3, 2006 |
PCT Filed: |
March 3, 2006 |
PCT NO: |
PCT/JP2006/304127 |
371 Date: |
August 10, 2007 |
Current U.S.
Class: |
148/653 ;
148/330; 148/331; 148/332; 148/337; 148/654 |
Current CPC
Class: |
C22C 38/04 20130101;
C22C 38/001 20130101; C21D 8/02 20130101; C21D 2211/005 20130101;
C22C 38/12 20130101 |
Class at
Publication: |
148/653 ;
148/337; 148/332; 148/330; 148/331; 148/654 |
International
Class: |
C21D 8/00 20060101
C21D008/00; C22C 38/12 20060101 C22C038/12; C22C 38/20 20060101
C22C038/20; C22C 38/32 20060101 C22C038/32; C22C 38/08 20060101
C22C038/08; C22C 38/16 20060101 C22C038/16 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 4, 2005 |
JP |
2005-060601 |
Claims
1-16. (canceled)
7. A high tensile strength, refractory steel having a particular
weldability and a particular gas cuttability, comprising: at least
one portion of a steel plate of a rolling product comprising
approximately, in mass %, C: 0.04 to 0.14%, Si: 0.50% or less, Mn:
0.50 to 2.00%, P: 0.020% or less, S: 0.010% or less, Nb: 0.01 to
0.05%, Mo: 0.30% or more and less than 0.70%, Al: 0.060% or less,
N: 0.0010 to 0.0060%, and a balance consisting of iron and
unavoidable impurities, wherein a weld crack sensitive composition
P.sub.CM is about 0.25% or less, P.sub.CM being defined as:
P.sub.CM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B, and wherein
an area fraction of polygonal ferrite or pseudo polygonal ferrite
in a 1/4 thick position in a plate thickness direction of the steel
plate is about 10% or less.
8. The steel according to claim 7, wherein the at least one portion
further comprises approximately, in mass %, Ni: 0.05 to 1.0%, Cu:
0.05 to 1.0%, and one or two or more selected from Cr: 0.05 to
1.0%, V: 0.01 to 0.06%, B: 0.0002 to 0.0030%, Ti: 0.005 to 0.025%,
and Mg: 0.0002 to 0.0050%, and wherein a content of Ni is equal to
or greater than half of a content of Cu.
9. The steel according to claim 7, wherein the at least one portion
further comprises approximately, in mass %, at least one of Ca:
0.0005 to 0.0040% or REM: 0.0005 to 0.0100%.
10. The steel according to claim 7, wherein a yield strength of the
steel is about 440 MPa or more.
11. A method for manufacturing a high tensile strength, refractory
steel having a particular weldability and a particular gas
cuttability, comprising: heating a steel member at a temperature of
about 1100 to 1300.degree. C. in a form of at least one of a billet
or a slab comprising approximately, in mass %, C: 0.04 to 0.14%,
Si: 0.50% or less, Mn: 0.50 to 2.00%, P: 0.020% or less, S: 0.010%
or less, Nb: 0.01 to 0.05%, Mo: 0.30% or more and less than 0.70%,
Al: 0.060% or less, N: 0.0010 to 0.0060%, and a balance consisting
of iron and unavoidable impurities; rolling the steel member at a
temperature of about 800 to 950.degree. C.; directly quenching the
steel member from a temperature greater or equal to a greater of
about 750.degree. C. or a temperature of about 150.degree. C. lower
than a temperature at a time of completing the rolling; and
tempering the steel member at a temperature lower than or equal to
Ac.sub.1.
12. A method for manufacturing a high tensile strength steel having
a particular weldability and a particular gas cuttability,
comprising: hot rolling a steel member in a form of at least one of
a billet or a slab, the steel member comprising approximately, in
mass %, C: 0.04 to 0.14%, Si: 0.50% or less, Mn: 0.50 to 2.00%, P:
0.020% or less, S: 0.010% or less, Nb: 0.01 to 0.05%, Mo: 0.30% or
more and less than 0.70%, Al: 0.060% or less, N: 0.0010 to 0.0060%,
and a balance consisting of iron and unavoidable impurities;
self-cooling the steel member: quenching the steel member after
reheating the steel member at a temperature of about 900 to
950.degree. C.; and tempering the steel member at a temperature not
higher than Ac.sub.1.
Description
CROSS REFERENCE TO RELATED APPLICATION(S)
[0001] The present application is a national phase application of
International Application PCT/JP2006/304127 filed on Mar. 3, 2006
and published as International Publication WO 2006/093282 on Sep.
8, 2006. This application claims priority from the International
Application pursuant to 35 U.S.C. .sctn. 365. .The present
application also claims priority from Japanese Patent Application
No. 2005-060601 filed on Mar. 4, 2005 under 35 U.S.C. .sctn. 119.
The disclosures of these applications are incorporated herein in
their entireties.
FIELD OF THE INVENTION
[0002] The present invention relates to a high tensile strength,
refractory steel having excellent weldability and gas cuttability,
and a method of producing the same.
BACKGROUND INFORMATION
[0003] As a refractory steel for architectural construction,
intended to provide a high temperature strength at a time of fire
or the like, a refractory steel obtained by hot rolling a billet or
slab has been described (see, for example, Japanese Unexamined
Patent Application , First Publication No. H2-77523).
[0004] This refractory steel generally belongs to so called 400 MPa
class steel or 490 MPa class steel, and can include several
examples of so called 590 MPa class steel having an yield strength
of 440 MPa (45 kgf/mm.sup.2) or more.
[0005] On the other hand, as a refractory steel corresponding to
the 590 MPa class steel, steel containing Mo of 0.7% or more has
been described (see, e.g., Japanese Unexamined Patent Application,
First Publication No.2002-12939).
SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION
[0006] In "rolled steel for architectural construction" JIS G 3136
according to Japanese Industrial Standard and "high performance 590
N/mm.sup.2 steel for architectural construction (SA440B,C)"
qualified by the Minister of Land, Infrastructure and Transport of
Japan as examples of construction steel, the plate thickness can be
regulated up to 100 mm. However, in the conventional types of
refractory steel mainly constituting 400 MPa class steel and 490
MPa class steel, the plate thickness of the 590 MPa class steel can
be at most, e.g., 40 mm, and thicker steel may not be supplied.
[0007] Recently, there has been an increasing demand for steel
having a yield strength of 440 MPa or more and belonging to a class
not lower than so-called 590 MPa class. Such a steel can be
subjected to thermal refining. After the thermal refining, a hot
rolled steel can have a metallic texture mainly composed of, e.g.,
polygonal ferrite or pseudo polygonal ferrite having low strength.
Therefore, even when a thick steel plate having a thickness of
about 100 mm is produced by hot rolling, it may be difficult to
ensure the strength of the steel stably by a technical control.
[0008] On the other hand, a refractory steel corresponding to 590
MPa class steel may have a steel composition containing Mo of 0.7%
or more, likely resulting in inferior cuttability by gas-cutting
and high production cost. In addition, although weld crack
sensitive composition (P.sub.CM) can be controlled in this
refractory steel, Mo generally enhances the hardenability of the
steel. From the view point of weldability, it may be preferable
that the Mo content can be controlled to a low level.
[0009] Exemplary embodiments of the present invention may take the
above issues into consideration. Thus, one of the objects of the
present invention is to provide a high tensile strength, refractory
steel and method of producing the same which has an excellent
weldability and cuttability by gas cutting so as to allow mass
production at low cost of a high tensile strength steel having a
yield strength of 440 MPa or more and possibly having sufficient
high temperature strength under a high temperature environment,
such as, e.g., a fire.
[0010] Research for the above-described exemplary issue has been
performed. Based on such research, it has been determined that by
compound addition of Nb while suppressing the content of Mo, it is
possible to stably provide and/or ensure the high temperature
strength of a high tensile strength steel having a yield strength
of 440 MPa or more. By suppressing the Mo content in the steel, it
is possible to suppress the deterioration of weldability and gas
cuttability of the steel to a minimum level. At the same time, by
controlling PCM and the contents of respective alloy elements such
as C, Si, Mn, and by further limiting the microstructure of the
steel and manufacturing conditions for the microstructure, it is
possible to obtain consistently complex properties including
excellent high temperature strength, weldability, and gas
cuttability.
[0011] According to one exemplary embodiment of the present
invention, a high tensile strength, refractory steel having
excellent weldability and gas cuttability can be provided. For
example, this steel can include, in mass %, approximately, : 0.04
to 0.14%, Si: 0.50% or less, Mn: 0.50 to 2.00%, P: 0.020% or less,
S: 0.010% or less, Nb: 0.01 to 0.05%, Mo: 0.30% or more and less
than 0.70%, Al: 0.060% or less, N: 0.0010 to 0.0060%, and the
balance consisting of iron and unavoidable impurities. A weld crack
sensitive composition PCM of the steel is 0.25% or less, and can be
defined by the following equation:
P.sub.CM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B,
and the area fraction of polygonal ferrite or pseudo polygonal
ferrite in a 1/4 thick position in the plate thickness direction of
a steel plate of the final rolling product can be 10% or less.
[0012] This exemplary embodiment of the steel can further include,
in mass %, approximately, Ni: 0.05 to 1.0%, Cu: 0.05 to 1.0%, and
one or two or more selected from Cr: 0.05 to 1.0%, V: 0.01 to
0.06%, B: 0.0002 to 0.0030%, Ti: 0.005to 0.025%, Mg: 0.0002 to
0.0050%, and the Ni content may be at least half of the Cu content.
Such exemplary steel can further comprise, in mass %,
approximately, one or two selected from Ca: 0.0005 to 0.0040% and
REM: 0.0005 to 0.0100%. The yield strength of the steel can be 440
MPa or more.
[0013] In accordance with another exemplary embodiment of the
present invention, a method for manufacturing a high tensile
strength, refractory steel having excellent weldability and gas
cuttability, can be provided. For example, according to this
exemplary method, a steel member in a form of billet or slab can be
heated at a temperature of about 1100 to 1300.degree. C., with the
billet and/or slab having the steel composition that includes, in
mass %, approximately, : 0.04 to 0.14%, Si: 0.50% or less, Mn: 0.50
to 2.00%, P: 0.020% or less, S: 0.010% or less, Nb: 0.01 to 0.05%,
Mo: 0.30% or more and less than 0.70%, Al: 0.060% or less, N:
0.0010 to 0.0060%, and the balance consisting of iron and
unavoidable impurities. The steel member is rolled at a temperature
of 800 to 950.degree. C. The steel member is directly quenched from
a temperature not lower than a higher one selected from about
750.degree. C. or a temperature about 150.degree. C. lower than a
temperature at a time of completing the rolling. The steel member
is tempered at a temperature not higher than Ac.sub.1(e.g., a
temperature at which generation of austenite starts at a time of
heating).
[0014] According to still another exemplary embodiment of the
present invention, a method for manufacturing high tensile
strength, refractory steel having excellent weldability and gas
cuttability can be provided. For example, a steel member can be
hot-rolled in a form of billet or slab, which has the steel
composition that includes, in mass %, approximately, : 0.04 to
0.14%, Si: 0.50% or less, Mn: 0.50 to 2.00%, P: 0.020% or less, S:
0.010% or less, Nb: 0.01 to 0.05%, Mo: 0.30% or more and less than
0.70%, Al: 0.060% or less, N: 0.0010 to 0.0060%, and the balance
consisting of iron and unavoidable impurities. The steel member is
self-cooled, and quenched after reheating the steel member at a
temperature of about 900 to 950.degree. C. The steel member is
tempered at a temperature not higher than Ac.sub.1.
[0015] According to a further exemplary embodiment of the present
invention, a high tensile strength, refractory steel having
excellent weldability and gas cuttability can be provided which may
have a weld crack sensitive composition PCM of about 0.25% or less
and the balance consisting of iron and unavoidable impurities. For
example, PCM can be defined by the following equation:
P.sub.CM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B. Further, an
area fraction of polygonal ferrite or pseudo polygonal ferrite in a
1/4 thick position in the plate thickness direction of a steel
plate of the final rolling product can be about 10% or less.
According to such a high tensile strength, refractory steel, it may
be possible to perform mass production at low cost of high tensile
strength steel having a yield strength of about 440 MPa or more,
having excellent weldability and gas cuttability, and having
sufficient high temperature strength under a high temperature
environment such as a fire.
[0016] A construction steel for architectural construction
according to yet another exemplary embodiment of the present
invention can be provided, composed of, e.g., the high tensile
strength, refractory steel which can be applied as general
construction steel for various applications including civil
engineering, marine structures, ships and vessels, various storage
tanks, industrial facilities such as plate-mills or the like. Since
the exemplary embodiment of the high tensile strength, refractory
steel according to the present invention has sufficient high
temperature strength even under a severe environment, for example,
at a time of fire, in which the steel is exposed to high
temperature conditions, it may be possible to further enhance the
safety of weld constructions.
[0017] In accordance with yet a further exemplary embodiment of a
method for manufacturing high tensile strength, refractory steel
having excellent weldability and gas cuttability according to the
present invention, a steel member in a form of billet or slab
having the steel composition can be heated at a temperature of
about 1100 to 1300.degree. C., and rolled at a temperature of about
800 to 950.degree. C. After that, the steel member may be directly
quenched from a temperature of not lower than a higher one selected
from about 750.degree. C. or a temperature about 150.degree. C.
lower than a temperature at a time of completing the rolling, and
tempered at a temperature not higher than Ac.sub.1. Therefore, it
is possible to perform mass production at low cost of a high
tensile strength steel having a yield strength of 440 MPa or more
and having excellent weldability and gas cuttability and sufficient
high temperature strength under a high temperature environment such
as a fire, where the steel is exposed to high temperature
conditions.
[0018] In accordance with still another aspect of a method for
manufacturing high tensile strength, refractory steel having
excellent weldability and gas cuttability according to the present
invention, after hot-rolling a steel member in a form of billet or
slab having the steel composition as described herein above, the
exemplary steel member can be self-cooled, and quenched after being
reheated at a temperature of about 900 to 950.degree. C., and
tempered at a temperature of not higher than Ac.sub.1. Therefore,
it is possible to perform mass production at low cost of a high
tensile strength steel having a yield strength of about 440 MPa or
more and having excellent weldability and gas cuttability and
sufficient high temperature strength under a high temperature
environment such as a fire, where the steel is exposed to high
temperature conditions.
[0019] These and other objects, features and advantages of the
present invention will become apparent upon reading the following
detailed description of embodiments of the invention, when taken in
conjunction with the appended claims.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF INVENTION
[0020] An exemplary embodiment of high tensile strength, refractory
steel having excellent weldability and gas cuttability according to
the present invention and the method for manufacturing the same are
described herein.
[0021] It should be understood that the exemplary embodiment of the
present invention described herein are not limited to the
description, and can be used and/or applicable in various other
ways and for various applications.
[0022] For example, an exemplary embodiment of a high tensile
strength steel according to the present invention can includes, in
mass %, approximately, C: 0.04 to 0.14%, Si: 0.50% or less, Mn:
0.50 to 2.00%, P: 0.020% or less, S: 0.010% or less, Nb: 0.01 to
0.05%, Mo: 0.30% or more and less than 0.70%, Al: 0.060% or less,
N: 0.0010 to 0.0060%, and the balance consisting of iron and
unavoidable impurities. A weld crack sensitive composition PCM can
be defined by the following exemplary equation can be 0.25% or
less:
P.sub.CM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B,
[0023] and an area fraction of polygonal ferrite or pseudo
polygonal ferrite in a 1/4 thick position in the plate thickness
direction of a steel plate of the final rolling product can be
about 10% or less.
[0024] Further, it is possible to limit the composition of the high
tensile strength, refractory steel. For example, C likely has an
influence on the property of the steel. The preferably minimum
value of approximately 0.04% can be a minimum content so as to
ensure the strength, and may be used to reduce or inhibit the over
softening than necessary of a heat-affected portion such as a weld
portion. On the other hand, too high a C content may enhance the
hardenability of the steel to an unnecessary level, and can have a
negative influence on the strength, toughness balance, and
weldability as intrinsic properties of the steel. Therefore, the
upper limit of the C content may be set to be about 0.14%.
[0025] Si can have an influence on cleanability, weldability, and
weld-portion toughness of the steel. Therefore, it may be used to
control its upper limit value. Therefore, the Si content can be set
to be about 0.50% or less. Si may also have an effect of
deoxidizing of the steel. However, deoxidization of the steel can
be performed by Ti or Al. Therefore, where weldability and
weld-portion toughness are preferably used, it may not be necessary
to add Si to the steel.
[0026] Mn can be an important element for ensuring strength and
toughness of the steel, and its minimum content may be, e.g., about
0.50%. On the other hand, too high a Mn content can enhance the
hardenability of the steel, deteriorate weldability and toughness
of weld-heat affected portion of the steel, as well as enhance the
segregation of the central portion of the slab during continuous
casting process. Therefore, the upper limit of the Mn can be set to
be about 2.00%.
[0027] P can constitute an impurity in the steel of an exemplary
embodiment of the present invention. By reducing the P content,
grain boundary deformation in the weld heat-affected portion may be
reduced. Therefore, it may be preferable to control the P content
to as low as possible. Therefore, so as not to deteriorate low
temperature toughness of the base metal and weld heat-affected
portion, the upper limit of the P content can be set to be about
0.020%.
[0028] As with P, S may constitute an impurity in an exemplary
embodiment of the steel of the present invention. In order to
ensure low temperature toughness of the steel, it may be preferable
to control the S content to as low as possible. Therefore, so as
not to deteriorate low temperature toughness of the base metal and
weld heat-affected portion, the upper limit of the S content can be
set to be about 0.010%.
[0029] Nb is an element which can play a role in an exemplary
embodiment of the present invention where Mo content can be
depressed as far as possible. Firstly, as a general effect, Nb can
be an important element for elevating the recrystallization
temperature of austenite, and exerting the effect of controlled
rolling at a time of hot-rolling. In order to realize such effects,
it may be preferable for the steel to contain Nb of at least about
0.01%.
[0030] Nb may also contribute to grain refining of heated austenite
at a time of reheating preceding the rolling and has an effect of
enhancing strength of the steel by precipitation hardening. In
addition, by composite addition with Mo, Nb can contribute to the
high temperature strength of the steel. However, too high a Nb
contents may result in a deterioration of toughness of the weld
portion. Therefore, so as not to generate the deterioration of
toughness of the weld portion, the upper limit of the Nb content
can be set to be about 0.005%.
[0031] Mo can be an important element for ensuring high temperature
toughness of the steel.
[0032] In order for the exemplary embodiment of the steel to have
sufficient high temperature strength under an environment, for
example, at a time of fire, where the steel is exposed to high
temperature conditions, it may be preferable for the steel to
contain Mo of about 0.30% or more. On the other hand, too high of a
Mo content can deteriorate weldability and gas cuttability of the
steel. Therefore, the upper limit of the steel can be set to be
less than 0.70%.
[0033] Al is a deoxidizing element. However, deoxidization of the
steel can be sufficiently performed, e.g., solely by Si or by Ti.
Therefore, the lower limit of Al content may not be set according
to an exemplary embodiment of the present invention. On the other
hand, too high an Al content may impair cleanability of the steel,
deteriorate toughness of the base metal, and deteriorate toughness
of the weld-heat affected portion. Therefore, the upper limit of Al
content may be set to be about 0.060%.
[0034] N can be contained as an unavoidable impurity in the steel.
By bonding with the above-described Nb, N can form carbonitride and
enhance the strength of the steel. In addition, where the
below-mentioned Ti is added, N can enhance strength of the steel by
forming TiN. In order to obtain such effects, it may be preferable
for the steel to contain N of at least about 0.0010%.
[0035] On the other hand, an increased N content has an adverse
effect on toughness of the weld heat-affected portion and
weldability. Therefore, the upper limit of N content can be set to
be about 0.0060%.
[0036] In addition to the above-described exemplary composition,
the high tensile strength steel according to the exemplary
embodiment of the present invention can further include, in mass %,
about Ni: 0.05 to 1.0%, Cu: 0.05 to 1.0%, and one or two or more
selected from Cr: 0.05 to 1.0%, V: 0.01 to 0.06%, B: 0.0002 to
0.0030%, Ti: 0.005to 0.025%, Mg: 0.0002 to 0.0050%, wherein the Ni
content may be at least half of the Cu content.
[0037] One of the reason for adding these elements to the
above-described exemplary basal composition can be to improve the
properties such as strength and toughness of the steel without
impairing excellent characteristics of the steel according to the
exemplary embodiment of the present invention. Therefore, loadings
of the exemplary elements may be restricted.
[0038] Where too much loading is avoided, Ni can improve strength
and toughness of the steel without having a negative influence on
weldability and toughness of the weld heat-affected portion of the
steel. To realize such exemplary effects, it may be preferable for
the steel to contain Ni of at least about 0.05% or more. On the
other hand, too high a loading of Ni may elevate the price of the
steel and also has an undesirable effect on weldability. Therefore,
the upper limit of Ni content can be set to be about 1.0%.
[0039] Where Cu is added, in order to prevent the occurrence of
Cu-cracks during the hot-rolling, it is necessary to control the Ni
content to be not lower than 1/2 of the Cu content while
controlling the Ni content in the above-described range.
[0040] Cu can show nearly similar functions and effects as those of
Ni. However, in addition to deteriorating the weldability, too high
a loading of Cu may cause Cu-cracks to occur at the time of
hot-rolling and makes it difficult to produce the exemplary steel.
Therefore, the upper limit of the Cu content may be set to be about
1.0%. On the other hand, in order to obtain a substantial effect,
it may be preferable for the steel to contain a minimum amount of
Cu. Therefore, the lower limit of the Cu content can be set to be
about 0.05%.
[0041] Cr may improve strength and toughness of the base metal.
However, too high a Cr content can deteriorate the toughness of the
base metal and weld portion and weldability. Therefore, the upper
limit of the Cr content may be set to be about 1.0%. On the other
hand, in order to obtain a substantial effect, it may be preferable
for the steel to contain a minimum amount of Cr. Therefore, the
lower limit of the Cr content can be set to be about 0.05%.
[0042] The above-described Ni, Cu and Cr can be effective for
improving weather resistance as well as for improving strength and
toughness of the base metal. For this purpose, it can be preferable
for the steel to contain these elements while controlling their
amounts in a range not impairing weldability.
[0043] V has similar effects as Nb. However, its effect may be less
than that of Nb. V may have an influence on hardenability and
contributes to improvement of high temperature strength.
[0044] In order to realize the same effect as Nb, it is necessary
for the steel to contain at least 0.01% V. On the other hand, where
the steel contains excessive V, toughness of the weld portion may
be deteriorated. Therefore, so as not to deteriorate the toughness
of the weld portion, the upper limit of the V content can be set to
be about 0.06%.
[0045] B can be segregated in the grain boundary of austenite, may
depress occurrence of ferrite and thereby improves hardenability
and strength of the steel. In order to realize such effects, it may
be preferable for the steel to contain at least about 0.0002% of B.
However, where too much B is contained, hardenability-improving
effect is saturated, and there is a possibility of occurrence of B
precipitates possibly having an adverse effect on the toughness of
the exemplary steel. Therefore, the upper limit of the B content
can be set to be about 0.003%.
[0046] When being used as steel for a tank or the like, there is a
possibility of stress corrosion cracking of the exemplary steel. In
such a case, reduction of hardness of the base metal and weld
heat-affected portion are often important. For example, in order to
prevent sulfide stress corrosion cracking (SSC), hardness of
HRC.ltoreq.22 (HV.ltoreq.248) may be important. In such a case, it
may be not as preferable to add B which enhances hardenability.
[0047] Where high toughness is preferred in the base metal and in
the weld portion, it is preferable to load Ti in the exemplary
steel. Where the Al content is low, for example, where the Al
content is 0.03% or less, Ti can be bonded with O and may form
precipitates mainly composed of Ti.sub.2O.sub.3 likely acting as
nuclei of generation of ferrite by transgranular transformation and
improving toughness of the weld portion. In addition, Ti can be
effective by being bonded with N and forms TiN constituting fine
precipitates in the slab, thereby depressing coarsening of
austenite grain at the time of heating and reducing grain size of
the rolled texture. In addition, fine TiN existing in a steel plate
may reduce the grain size of the weld heat-affected portion at the
time of welding.
[0048] In order to obtain such an effect, at least about 0.005% of
Ti can be used. However, excessive Ti can form TiC and may
deteriorate low-temperature toughness and weldability. Therefore,
the upper limit of the Ti content may be set to be about
0.025%.
[0049] Mg can depress the grain growth of austenite grains in the
weld heat-affected portion and reduce the grain size. As a result,
the weld portion may be given a high toughness. In order to realize
such an effect, it may be preferable for the exemplary steel to
contain Mg of about 0.0002% or more. On the other hand, where the
Mg content can be increased, it may not be cost effective, because
there may be less enhancement of the effect of Mg compared with the
increase of Mg content. Therefore, the upper limit of the Mg
content can be set to be about 0.0050%.
[0050] In addition to the above-described exemplary composition,
the high tensile strength, refractory steel of an exemplary
embodiment of the present invention can further include, in mass %,
one or two selected from Ca: about 0.0005 to 0.0040% and REM (Rare
Earth Metal): about 0.0005 to 0.0100%.
[0051] As the REM, one or more selected from rare earth metals such
as Ce, La, and Nd or the like may be used.
[0052] Ca and REM may be effective in controlling MnS morphologies
and improving low-temperature toughness of the base metal. In
addition, Ca and REM may be effective in reducing sensitivity for
hydrogen-induced cracking, for example, hydrogen-induced cracking
(HIC) under a humid hydrogen sulfide environment, SSC, and stress
oriented HIC (SOHIC). In order to express such effects, at least a
content of about 0.0005% can be used.
[0053] However, if too much Ca or REM is contained, cleanability of
steel may be deteriorated, and toughness of the base metal and
hydrogen induced cracking (HIC, SSC, SOHIC) sensitivity under a
humid hydrogen sulfide environment may be enhanced. Therefore, the
upper limit of the Ca content can be set to be about 0.0040% and
the upper limit of the REM content may be set to be about 0.0100%.
Since Ca and REM can express a nearly similar effect, it is
possible to load one of Ca and REM in the above-described range, or
it is possible to load a mixture of Ca and REM in the
above-described exemplary range.
[0054] In the high tensile strength, refractory steel according to
an exemplary embodiment of the present invention, in order to
ensure the yield strength of about 440 MPa or more, and yield
strength at about 600.degree. C. of not less than about 2/3 of the
yield strength at room temperature, that is, not less than about
294 MPa, while controlling Mo content to be lower than about 0.70%,
it can be preferable to inhibit or reduce a control of the
exemplary steel composition also to control its microstructure.
[0055] In the exemplary microstructure of the high tensile
strength, refractory steel according an exemplary embodiment of the
present invention, an area fraction of polygonal ferrite or pseudo
polygonal ferrite in a 1/4 thick position in the plate thickness
direction of a steel plate of the final rolling product may be
controlled to be about 10% or less.
[0056] In the steel having the steel composition according to the
exemplary embodiment of the present invention in which the Mo
content may be restricted to be lower than about 0.70%, especially
in a thick steel plate thicker than about 40 mm, it may be
difficult to ensure not only the room temperature strength, but
also the high temperature strength of the steel, when the area
fraction of polygonal ferrite or pseudo polygonal ferrite exceeds
about 10%.
[0057] In the present invention, the microstructure is represented
by the texture on the plane along the final rolling direction,
where the plane is in a 1/4 thick position with respect to the
section of the plate thickness.
[0058] Although each component of the exemplary steel can be
limited, it may be difficult to obtain preferable properties if the
component system as a whole is not appropriately controlled.
Therefore, a weld crack sensitive composition PCM can be limited to
be about 0.25% or less, where the PCM may be defined by the
following exemplary equation:
P.sub.CM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B.
[0059] The weld crack sensitive composition PCM can be a parameter
indicating weldability, and weldability may be satisfactory as PCM
shows a low value. In the steel according to the exemplary
embodiment of the present invention, where the weld crack sensitive
composition PCM may be 0.25% or less, it is possible to ensure
excellent weldability while ensuring excellent high temperature
strength.
[0060] Further, a method for manufacturing a high tensile strength,
refractory steel according to an exemplary embodiment of the
present invention is explained.
[0061] The high tensile strength, refractory steel according to the
present invention can be manufacture by the first or by the second
exemplary embodiment of a manufacturing method according to the
present invention.
[0062] The first exemplary embodiment of the manufacturing method
can include: heating a steel member in a form of billet or slab
having the steel composition according to the exemplary embodiment
of the present invention at a temperature of about 1100 to
1300.degree. C.; rolling the steel member at a temperature of about
800 to 950.degree. C.; directly quenching the steel member from a
temperature of not lower than a higher one selected from about
750.degree. C. or a temperature about 150.degree. C. lower than a
temperature at a time of completing the rolling; and tempering the
steel member at a temperature not higher than Ac.sub.1.
[0063] The second embodiment of the manufacturing method can
include: hot rolling a steel member in a form of billet or slab
having the steel composition according to the exemplary embodiment
of the present invention; self-cooling the steel member: quenching
the steel member after reheating the steel member at a temperature
of about 900 to 950.degree. C.; and tempering the steel member at a
temperature not higher than Ac.sub.1.
[0064] Firstly, the first exemplary embodiment of the manufacturing
method as follows. A steel member in a form of billet or slab
having the steel composition according to the exemplary embodiment
of the present invention can be heated at a temperature of about
1100 to 1300.degree. C. The exemplary reason for controlling the
heating temperature preceding the rolling to be about 1100 to
1300.degree. may be to inhibit the coarsening of austenite grains
to an unnecessary size in the time of heating, and to refine the
rolled texture. The temperature of approximately 1300.degree. C.
can be an upper limit of the temperature at which extreme
coarsening of austenite is inhibited at the time of heating. Where
the heating temperature exceeds the exemplary upper limit
temperature, coarse grained austenite may be mixed in the texture,
and rolled austenite grains also have a relatively large size. As a
result, the metallographic structure after the phase transformation
can be relatively coarse grained. In addition, in the
transformation from the coarse austenite, the microstructure may
tend to become a bainitic structure, possibly resulting in
remarkable deterioration of toughness of the exemplary steel. On
the other hand, the lower limit of the heating temperature can be
set to be about 1100.degree. C. based on the consideration of
solution treatment of Nb so as to express the effect of controlled
rolling at the time of hot-rolling and precipitation hardening.
[0065] The exemplary steel member thus heated can be rolled at a
temperature of about 800 to 950.degree. C. The rolling temperature
may be limited to be in the range of about 800 to 950.degree. C.
for the following exemplary reason. Where the rolling is performed
at a temperature exceeding about 950.degree. C., even though Mo and
Nb are compositely loaded, grain size refining of the rolled
austenite may not be sufficient, and therefore low-temperature
toughness may not be ensured stably even by performing subsequent
direct quenching and tempering. On the other hand, at a temperature
lower than about 800.degree. C., depending on plate thickness,
precipitation of ferrite can occur before the direct quenching and
may cause difficulty in ensuring the microstructure, or Nb
precipitates during rolling and does not contribute to the high
temperature strength.
[0066] After completing the rolling, the steel member is directly
quenched from a temperature of not lower than a higher one selected
from 750.degree. C. or a temperature 150.degree. C. lower than a
temperature at a time of completing the rolling, that is a rolling
finish temperature minus 150.degree. C.
[0067] For example, the direct quenching temperature can be limited
in the above-described range. Firstly, in order to control the
microstructure with a purpose of ensuring the microstructure, the
temperature can be at least about 750.degree. C. or more. On the
other hand, even where the temperature is not lower than about
750.degree. C., when a temperature drop from the rolling finish
temperature exceeds about 150.degree. C., there may be a high
possibility of recovery and recrystallization after the rolling or
precipitation of Nb. In such a case, there is a possibility of
deterioration of toughness or reduction of strength including high
temperature strength.
[0068] Therefore, the starting temperature of the direct quenching
can be limited to be not lower than a higher one selected from
about 750.degree. C. or a temperature about 150.degree. C. lower
than a temperature at a time of completing the rolling.
[0069] After the direct quenching, a tempering treatment can be
performed at a temperature not higher than Ac.sub.1.
[0070] In the exemplary embodiment of a steel member having a steel
composition according to the present invention, in certain cases, a
temperature of about 700.degree. C. or less is not higher than
Ac.sub.1. The practical treatment temperature can be set in
accordance with target properties such as strength.
[0071] Considering productivity and controllability of the heat
treatment furnace in the industrial production, the preferable
temperature of the tempering treatment may be about 450 to
650.degree. C.
[0072] In the above-description, rolling temperature or the like
can denote a surface temperature of the steel plate which may be
monitored.
[0073] By the above-described exemplary method, high tensile
strength, refractory steel according to an exemplary embodiment of
the present invention can be manufactured.
[0074] According to the second exemplary embodiment of the
manufacturing method, after hot rolling a steel member in a form of
billet or slab having a steel composition according to the
exemplary embodiment of the present invention, the steel member may
be self-cooled. In this exemplary case, the conditions of
hot-rolling and self-cooling may not be limited because the
metallographic structure and material quality of the steel member
is determined depending on the subsequent treatments including
reheating, quenching and tempering.
[0075] Further, the hot-rolled and self-cooled steel member can be
reheated at a temperature of about 900 to 950.degree. C. and
subjected to quenching.
[0076] It may be preferable to control the reheating and quenching
temperature to be higher than Ac.sub.3 ( a temperature at which
transformation of ferrite to austenite is completed at the time of
heating) in terms of metallurgical definition.
[0077] In the steel member having a steel composition according to
an exemplary embodiment of the present invention, a temperature of
about 900.degree. C. or more may be sufficient as the temperature
not lower than Ac.sub.3.
[0078] On the other hand, where reheating and quenching
temperatures are too high, the metallic structure can be coarsened
and low temperature toughness may be deteriorated. Therefore, the
maximum temperature of the reheating and quenching can be set to be
about 950.degree. C.
[0079] In addition, the reheated and quenched steel member may be
subjected to tempering treatment at a temperature of not higher
than Ac.sub.1.
[0080] The conditions of the tempering treatment or the like may be
similar or exactly the same as the above-described first exemplary
embodiment of the manufacturing method.
[0081] By the above-described method, high tensile strength,
refractory steel according to the exemplary embodiment of the
present invention can be manufactured.
[0082] The high tensile strength, refractory steel according to the
exemplary embodiment of the present invention can be applied to
general weld construction steel not only for architectural
construction but also for various applications including civil
engineering, marine structures, ships and vessels, various storage
tanks, or the like.
EXAMPLE
[0083] Further, high tensile strength, refractory steel having
excellent weldability and gas cuttability according to an exemplary
embodiment of the present invention is explained with reference to
Examples 1 to 15 and Comparative Examples 16 to 22.
[0084] Firstly, using a steel converter, steel slabs having various
compositions shown in Table 1 were produced as ingots. Next, the
slabs were subjected to various steel manufacturing processes using
conditions shown in Table 2, and steel plates each having a
thickness (50 to 100 mm) shown in Table 2 were manufactured.
[0085] Next, as shown in Table 2, each steel plate of Examples 1 to
15 and Comparative Examples 16 to 22 was subjected to evaluation of
the base metal structure, mechanical properties, toughness of weld
heat-affected portion and roughness of gas-cut face.
[0086] In addition, as the mechanical properties, three parameters,
that is, yield strength, tensile strength, and yield strength at
600.degree. C. were measured, and yield ratio (yield
strength/tensile strength (%)) was determined from the yield
strength and tensile strength. Then the mechanical properties were
evaluated.
[0087] With respect to the structure of the base metal, on a plane
in a 1/4 thick position with respect to the section of the plate
thickness, 10 fields of view were observed using a microscope with
a magnification ratio of 500. Thus, the area fraction (%) of
polygonal ferrite (.alpha..sub.p) and area fraction of pseudo
polygonal ferrite (.alpha..sub.q) were calculated.
[0088] With respect to yield strength and tensile strength, a test
piece was sampled from a direction perpendicular to the rolling
direction in the central portion of the plate thickness.
Configuration of the test piece was in accordance with a No.4 round
bar for a test piece for testing tensile strength standardized by
Japanese Industrial Standard JIS Z 2201 "metallic material tensile
strength test piece". After that, yield strength and tensile
strength were evaluated based on measurements in accordance with
Japanese Industrial Standard JIS Z 2241 "Method for tensile test of
metallic material".
[0089] For the evaluation of toughness of the base metal, a test
piece was sampled from a direction perpendicular to the rolling
direction in the central portion of the plate thickness.
Configuration of the test piece was in accordance with a 2 mmV
notch impact test specimen standardized by Japanese Industrial
Standard JIS Z 2202 "impact test specimen of metallic material".
After that, toughness was evaluated based on the measurement of
fracture appearance transition temperature (vTrs (.degree. C.)) of
the impact test specimen in accordance with Japanese Industrial
Standard JIS Z 2242 "Method for impacting test of metallic
material".
[0090] For the evaluation of toughness of the weld heat-affected
portion, a test piece was sampled from a 1/4 thick position of the
plate thickness. Configuration of the test piece was in accordance
with an impact test specimen standardized by Japanese Industrial
Standard JIS Z 2202 "impact test specimen of metallic material".
Each test piece was subjected to a heat cycle corresponding to
submerged arc welding (plate thickness 50 mm) of energy input of 60
kJ/mm. The toughness was evaluated based on the measurement of
absorbed energy (vE.sub.0) of the test piece at 0.degree. C.
[0091] For the evaluation of roughness of gas-cut face, the highest
height (Ry) of the surface roughness of the surface of each steel
plate was measured, where the definition of Ry was in accordance
with Japanese Industrial Standard JIS B 0601 "Geometrical Property
Standard (GPS) of a product-Surface Property: profile curve
method-term, definition, and surface property parameter". Where the
maximum height (Ry) was 50 .mu.m or less, roughness was evaluated
as satisfactory (A), and where the maximum height (Ry) exceeded 50
.mu.m, roughness was evaluated as unsatisfactory (B).
[0092] Target values of respective properties were 440 MPa for
yield strength, -40.degree. C. or less for fracture appearance
transition temperature (vTrs), 294MPa or more for yield strength at
600.degree. C., and 100 J or more for absorbed energy (vE.sub.0) at
0.degree. C.
[0093] Compositions of steels are shown in Table 1 and
manufacturing processes of steel plates and various properties are
shown in Table 2.
TABLE-US-00001 TABLE 1 Table CLASS OF COMPOSITION (mass %) STEEL C
Si Mn P S Nb Mo Al N Ni EXAMPLE 1 0.04 0.36 1.98 0.009 0.005 0.04
0.68 0.031 0.0042 2 0.04 0.45 1.80 0.012 0.004 0.05 0.64 0.035
0.0035 3 0.05 0.32 1.62 0.010 0.003 0.04 0.60 0.003 0.0038 0.50 4
0.05 0.30 1.58 0.011 0.005 0.03 0.55 0.026 0.0032 5 0.06 0.24 1.55
0.008 0.006 0.01 0.58 0.042 0.0029 6 0.06 0.30 1.40 0.012 0.003
0.02 0.52 0.020 0.0034 0.14 7 0.07 0.35 1.46 0.012 0.004 0.02 0.48
0.002 0.0048 0.35 8 0.07 0.30 1.35 0.011 0.006 0.02 0.37 0.033
0.0036 9 0.08 0.30 1.50 0.012 0.005 0.03 0.45 0.040 0.0035 10 0.08
0.56 1.52 0.010 0.005 0.01 0.40 0.055 0.0055 0.25 11 0.09 0.41 1.24
0.010 0.003 0.02 0.31 0.025 0.0038 0.20 12 0.09 0.35 0.80 0.011
0.001 0.03 0.36 0.020 0.0064 13 0.10 0.25 0.85 0.012 0.004 0.02
0.45 0.029 0.0040 14 0.12 0.33 0.55 0.010 0.003 0.02 0.50 0.019
0.0048 15 0.14 0.31 0.71 0.009 0.005 0.03 0.49 0.020 0.0078 COMPAR-
16 0.03 0.35 1.48 0.012 0.005 0.04 0.50 0.025 0.0039 ATIVE 17 0.04
0.36 1.39 0.012 0.005 0.00 0.51 0.024 0.0041 18 0.04 0.08 0.45
0.014 0.004 0.03 0.60 0.026 0.0041 0.15 19 0.05 0.35 1.41 0.011
0.003 0.07 0.50 0.026 0.0038 20 0.15 0.43 1.18 0.010 0.006 0.02
0.49 0.030 0.0014 21 0.06 0.35 1.30 0.013 0.008 0.02 0.25 0.029
0.0028 22 0.06 0.35 1.31 0.013 0.007 0.02 0.75 0.028 0.0028 CLASS
OF COMPOSITION (mass %) STEEL Cu Cr V B Ti Mg Ca P.sub.CM * EXAMPLE
1 0.014 0.196 2 0.24 0.0016 0.200 3 0.028 0.012 0.193 4 0.176 5
0.009 0.184 6 0.14 0.38 0.203 7 0.34 0.010 0.0013 0.210 8 0.15
0.0008 0.184 9 0.195 10 0.19 0.015 0.215 11 0.25 0.010 0.202 12
0.12 0.039 0.018 0.0014 0.176 13 0.044 0.185 14 0.005 0.192 15
0.010 0.0018 0.219 COMPAR- 16 0.149 ATIVE 17 0.156 18 0.60 0.138 19
0.166 20 0.256 21 0.153 22 0.187 * P.sub.CM = C + Si/30 + Mn/20 +
Cu/20 + Ni/60 + Cr/20 + Mo/15 + V/10 + 5B ** Underline denotes that
the data is outside of the range of the present invention.
TABLE-US-00002 TABLE 2 TEMPER- TEMPER- ATURE ATURE QUENCH- HEATING
AT AT ING TEMPERING PLATE .alpha.p OR CLASS PRODUC- TEMPER-
FINISHING STARTING TEMPER- TEMPER- THICK- .alpha.q OF TION ATURE
ROLLING ROLLING ATURE ATURE NESS FRACTION STEEL PROCESS * (.degree.
C.) (.degree. C.) (.degree. C.) (.degree. C.) (.degree. C.) (mm)
(%) ** EXAMPLE 1 DQT 1150 900 850 -- 600 60 1 2 DQT 1100 810 760 --
580 50 2 3 QT 1200 -- -- 910 550 80 0 4 DQT 1150 940 870 -- 620 75
0 5 DQT 1150 870 800 -- 600 60 0 6 QT 1200 -- -- 930 580 50 0 7 QT
1250 -- -- 900 640 55 0 8 DQT 1200 920 880 -- 600 75 1 9 QT 1200 --
-- 920 500 100 0 10 DQT 1100 900 860 -- 580 100 0 11 DQT 1200 930
850 -- 470 85 0 12 QT 1250 -- -- 910 520 80 0 13 QT 1200 -- -- 940
600 75 0 14 QT 1250 -- -- 900 550 100 0 15 DQT 1250 900 860 -- 620
75 0 COMPAR- 16 DQT 1150 930 770 -- 600 50 12 ATIVE 17 DQT 1100 850
730 -- 600 50 16 18 QT 1100 -- -- 900 600 50 3 19 AR 1200 860 -- --
-- 75 79 20 QT 1250 -- -- 900 600 75 0 21 QT 1200 -- -- 900 600 75
1 22 AR 1250 900 -- -- -- 75 72 TOUGHNESS TOUGH- OF ROUGH- YIELD
NESS WELD-HEAT NESS CLASS YIELD TENSILE YIELD STRENGTH OF BASE
AFFECTED OF OF STRENGTH STRENGTH RATIO AT 600.degree. C. METAL
PORTION GAS CUT STEEL (MPa) (MPa) (%) (MPa) vTrs(.degree. C.)
vE0(J) SURFACE EXAMPLE 1 520 623 83 345 -68 165 A 2 515 610 84 365
-72 171 A 3 567 663 86 381 -64 154 A 4 482 581 83 344 -59 149 A 5
525 619 85 372 -62 151 A 6 562 654 86 386 -57 160 A 7 543 628 86
370 -61 204 A 8 538 613 88 368 -55 143 A 9 496 616 81 325 -51 153 A
10 509 602 85 336 -54 167 A 11 485 619 78 317 -59 132 A 12 525 622
84 322 -53 129 A 13 533 614 87 371 -50 152 A 14 468 593 79 316 -56
187 A 15 531 636 83 353 -47 137 A COMPAR- 16 433 552 78 261 -62 174
A ATIVE 17 404 535 76 303 -58 168 A 18 436 554 79 327 -51 155 A 19
313 468 67 235 -74 21 A 20 507 598 85 299 -12 17 A 21 472 572 83
274 -52 104 A 22 324 491 66 243 -32 24 B * DQT: DIRECT QUENCHING -
TEMPERING, QT: REHEATING QUENCHING-TEMPERING, AR: AS ROLLED (AIR
COOL) ** .alpha.p: polygonal ferrite, .alpha.q: pseudo polygonal
ferrite *** Underline denotes that the data is outside of the range
of the present invention.
In accordance with the results of evaluation, all of Examples 1 to
15 showed satisfactory properties.
[0094] On the other hand, compared with Examples 1 to 15,
Comparative Examples 16 to 22 having compositions out of the
composition range of the present invention showed inferior values
in fundamental properties such as strength and toughness, and in
high temperature strength, toughness of weld-heat affected portion,
gas cuttability or the like.
[0095] In Comparative Example 18, since the Ni content is lower
than the Cu content, cracks occurred during the hot-rolling, making
it difficult to produce the steel.
[0096] In Comparative Example 20, because of not only high C
content but also of high PCM, root cracks were generated by an
oblique y shape weld crack test at room temperature.
[0097] According to the exemplary embodiment of the present
invention, by composite loading of Nb while suppressing the content
of Mo, a high temperature strength of a high tensile strength steel
having a yield strength of 440 MPa or more can be stably ensured.
By suppressing the content of Mo, a deterioration of weldability
and gas cuttability can be limited to a minimum level. At the same
time, by limiting each of alloy elements such as C, Si, and Mn, as
well as by limiting P.sub.CM, and further by limiting the
microstructure of the steel and manufacturing conditions for same,
composite properties such as excellent high temperature strength,
weldability, gas cuttability are compatibly ensured. Such high
tensile strength, refractory steel having excellent weldability and
gas cuttability can be widely applied as general weld construction
steel for architectural constructions, civil engineering, marine
structures, ships and vessels, various storage tanks or the like,
and therefore has very large industrial applicability.
[0098] The foregoing merely illustrates the exemplary principles of
the present invention. Various modifications and alterations to the
described embodiments will be apparent to those skilled in the art
in view of the teachings herein. It will thus be appreciated that
those skilled in the art will be able to devise numerous
modification to the exemplary embodiments of the present invention
which, although not explicitly shown or described herein, embody
the principles of the invention and are thus within the spirit and
scope of the invention. All publications, applications and patents
cited above are incorporated herein by reference in their
entireties.
* * * * *