U.S. patent application number 14/971248 was filed with the patent office on 2016-06-23 for twin-roll strip caster, method for manufacturing thin duplex stainless steel sheet using the same and thin duplex stainless steel sheet.
The applicant listed for this patent is POSCO. Invention is credited to Suk Kyun HWANG, Seong In JEONG, Cheol Min PARK.
Application Number | 20160177415 14/971248 |
Document ID | / |
Family ID | 56128748 |
Filed Date | 2016-06-23 |
United States Patent
Application |
20160177415 |
Kind Code |
A1 |
JEONG; Seong In ; et
al. |
June 23, 2016 |
TWIN-ROLL STRIP CASTER, METHOD FOR MANUFACTURING THIN DUPLEX
STAINLESS STEEL SHEET USING THE SAME AND THIN DUPLEX STAINLESS
STEEL SHEET
Abstract
Provided are a twin-roll strip caster, a method for
manufacturing a duplex stainless steel sheet using the same, and a
thin duplex stainless steel sheet. The twin-roll strip caster
includes a pair of casting rolls rotating in opposite directions,
an edge dam installed such that a molten steel pool is formed on
respective sides of the casting rolls, and a meniscus shield
provided to cover the molten steel pool such that a contact between
the molten steel pool and air is blocked, wherein hills and valleys
are alternately arranged on surfaces of the cast rolls in
circumferential directions thereof and a hill area in edge sections
thereof is higher than that in centers sections thereof. Since cast
rolls in which a hill area ratio of edge sections is higher than
that of center sections are used, a thin stainless steel sheet with
improved edge quality may be provided.
Inventors: |
JEONG; Seong In; (Pohang-si,
KR) ; HWANG; Suk Kyun; (Pohang-si, KR) ; PARK;
Cheol Min; (Pohang-si, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
POSCO |
Pohang-si |
|
KR |
|
|
Family ID: |
56128748 |
Appl. No.: |
14/971248 |
Filed: |
December 16, 2015 |
Current U.S.
Class: |
148/541 ;
148/327; 164/428 |
Current CPC
Class: |
C22C 38/001 20130101;
C21D 9/46 20130101; C21D 6/008 20130101; B22D 11/0651 20130101;
C21D 6/004 20130101; C21D 8/0226 20130101; C21D 6/005 20130101;
C21D 8/0263 20130101; C22C 38/42 20130101; C22C 38/58 20130101;
C22C 38/02 20130101; B22D 11/0622 20130101 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C21D 6/00 20060101 C21D006/00; C22C 38/00 20060101
C22C038/00; C22C 38/58 20060101 C22C038/58; C22C 38/42 20060101
C22C038/42; C22C 38/02 20060101 C22C038/02; B22D 11/06 20060101
B22D011/06; C21D 8/02 20060101 C21D008/02 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 22, 2014 |
KR |
10-2014-0186477 |
Claims
1. A twin-roll strip caster comprising: a pair of casting rolls
rotating in opposite directions; an edge dam installed such that a
molten steel pool is formed on respective sides of the pair of
casting rolls; and a meniscus shield provided to cover the molten
steel pool such that a contact between the molten steel pool and
air is blocked, wherein hills and valleys are alternately arranged
on surfaces of the pair of cast rolls in circumferential directions
thereof and a hill area ratio (an area ratio of hills) in edge
sections thereof is higher than that in centers sections
thereof.
2. The twin-roll strip caster of claim 1, wherein the hill area
ratio is constant in the center section and is continuously
increased in the edge section in a direction away from a boundary
between the edge section and the center section.
3. The twin-roll strip caster of claim 1, wherein the hill area
ratio in the center section is in a range of about 10-40% and the
hill area ratio in the edge section increases up to 70%.
4. The twin-roll strip caster of claim 1, wherein the edge sections
have a width of 50-200 mm from one ends of the pair of casting
rolls.
5. The twin-roll strip caster of claim 1, wherein a gas discharge
index G of the center section has a value of 80-130, the gas
discharge index G of the edge section continuously decreases to a
range of at least 50-70 from a boundary of the center section, and
G=width (W) of valley depth (d)/pitch (p).
6. A method for manufacturing a thin duplex stainless steel sheet,
the method comprising: forming a cast strip by pouring molten steel
between a pair of cast rolls rotating in opposite directions; and
manufacturing a hot rolled strip by rolling the cast strip in a
rolling mill, wherein hills and valleys are alternately arranged on
surfaces of the pair of cast rolls in circumferential directions
thereof and a hill area ratio (an area ratio of hills) in edge
sections thereof is higher than that in centers sections
thereof.
7. The method of claim 6, wherein the reduction ratio is in a range
of 15-60%.
8. The method of claim 6, further comprising annealing the hot
rolled strip, wherein the annealing temperature is in a range of
1,000-1,250.degree. C.
9. A thin duplex stainless steel sheet manufactured by the method
as set forth in claim 6.
10. The duplex stainless steel sheet of claim 9, wherein the thin
duplex stainless steel sheet comprises, by weight: 0.1% or less
carbon (C) (exclusive of 0%), 0.2-3.0% silicon (Si), 1.0-4.0%
manganese (Mn), 19.0-23.0% chromium (Cr), 0.3-2.5% nickel (Ni),
0.15-0.3% nitrogen (N), 0.3-2.5% copper (Cu), a balance of iron
(Fe), and inevitable impurities.
11. The duplex stainless steel sheet of claim 9, wherein the thin
duplex stainless steel sheet may has an elongation of 25-55% in a
direction perpendicular to the rolling direction and a yield
strength of 350-700 MPa.
12. The duplex stainless steel sheet of claim 9, wherein the thin
duplex stainless steel sheet has a recrystallized grain size of
about 4-9 .mu.m and a necking-down width of 10 mm or less.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the priority of Korean Patent
Application No. 10-2014-0186477 filed on Dec. 22, 2014, in the
Korean Intellectual Property Office, the disclosure of which is
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a twin-roll strip caster, a
method for manufacturing a duplex stainless steel sheet using the
same, and a thin duplex stainless steel sheet.
[0004] 2. Description of the Related Art
[0005] In general, austenitic stainless steels having good
workability and corrosion resistance contain iron (Fe) as a matrix
metal, and chromium (Cr) and nickel (Ni) as major ingredients, and
in this regard, various types of steel have been developed from
such austenitic stainless steels by adding other elements such as
molybdenum (Mo), copper (Cu) or the like, and since 304 and 316
series stainless steels with good corrosion resistance and
workability contain relatively expensive ingredients such as Ni, Mo
or the like, 200 series and 400 series stainless steels have
increased in popularity as alternatives. However, 200 series and
400 series stainless steels do not have superior characteristics to
300 series stainless steels in terms of formability and corrosion
resistance.
[0006] Meanwhile, duplex stainless steels in which an austenite
phase and a ferrite phase are mixed have both the advantages of
austenitic stainless steels and the advantages of the ferritic
stainless steels, and thus, various kinds of duplex stainless
steels have been developed. Since duplex stainless steels commonly
contain a large amount of nitrogen to increase corrosion
resistance, duplex stainless steels exhibit superior corrosion
resistance in various corrosive environments, as compared with
austenitic stainless steels such as 304 series and 316 series
stainless steels. However, such duplex stainless steels commonly
contain relatively expensive elements such as nickel (Ni),
molybdenum (Mo), or the like, and thus, the manufacturing costs
thereof may be increased.
[0007] To increase the price competitiveness of such duplex
stainless steels, interest in lean duplex stainless steels in which
relatively expensive alloy elements such as Ni, Mo or the like
contained in the duplex stainless steels are excluded and
relatively inexpensive alloying elements are added has increased.
However, such lean duplex stainless steels have limitations in
terms of surface cracks and edge cracks, due to poor hot
workability caused by a difference in strength between a ferrite
phase and an austenite phase.
PATENT DOCUMENTS
[0008] Patent 1: U.S. Pat. No. 5,624,504 entitled `Duplex structure
stainless steel having high strength and elongation and a process
for producing the steel` published on Apr. 29, 1997
[0009] Patent 2: Korean Patent Publication No. 2013-0135575
entitled `Method for producing high nitrogen thin duplex stainless
steel sheet` published on Dec. 11, 2013
SUMMARY OF THE INVENTION
[0010] An aspect of the disclosure may provide a twin-roll strip
caster capable of manufacturing a thin stainless steel sheet having
improved edge quality, a method for manufacturing a thin duplex
stainless steel sheet using the same, and a thin duplex stainless
steel sheet.
[0011] According to an aspect of the present invention, there is
provided a twin-roll strip caster including: a pair of casting
rolls rotating in opposite directions; an edge dam installed such
that a molten steel pool is formed on respective sides of the pair
of casting rolls; and a meniscus shield provided to cover the
molten steel pool such that a contact between the molten steel pool
and air is blocked, wherein hills and valleys are alternately
arranged in circumferential directions on the pair of casting rolls
and a hill area ratio (an area ratio of hills) in edge sections
thereof is higher than that in center sections thereof.
[0012] The hill area ratio may be constant in the center section
and may continuously increase in the edge section in a direction
away from a boundary between the edge section and the center
section.
[0013] The hill area ratio in the center section may be in a range
of about 10-40% and the hill area ratio in the edge section may
increase up to 70%.
[0014] The edge section may have a width of 50-200 mm from one ends
of the pair of casting rolls.
[0015] A gas discharge index G of the center section may have a
value of 80-130 and the gas discharge index G of the edge section
may continuously decrease to a range of at least 50-70 from a
boundary of the center section, in which G=width (W) of valley
depth (d)/pitch (p).
[0016] According to another aspect of the preset disclosure, a
method for manufacturing a thin duplex stainless steel sheet, the
method includes: forming a cast strip by pouring molten steel
between a pair of cast rolls rotating in opposite directions; and
manufacturing a hot rolled strip by rolling the cast strip in a
rolling mill, in which hills and valleys are alternately arranged
on surfaces of the pair of cast rolls in circumferential directions
thereof and a hill area ratio (an area ratio of hills) in edge
sections thereof is higher than that in centers sections
thereof.
[0017] The reduction ratio may be in a range of 15-60%.
[0018] The above method may further include annealing the hot
rolled strip in which the annealing temperature maybe in a range of
1,000-1,250.degree. C.
[0019] According to another aspect of the present invention, a
duplex stainless steel sheet manufactured by the above-described
method is provided.
[0020] The thin duplex stainless steel sheet may include, by
weight: 0.1% or less carbon (C) (exclusive of 0%); 0.2-3.0% silicon
(Si); 1.0-4.0% manganese (Mn); 19.0-23.0% chromium (Cr); 0.3-2.5%
nickel (Ni); 0.15-0.3% nitrogen (N); 0.3-2.5% copper (Cu); a
balance of iron (Fe); and inevitable impurities.
[0021] The thin duplex stainless steel sheet may have an elongation
of 25-55% in a direction perpendicular to the rolling direction,
and a yield strength of 350-700 MPa.
[0022] The thin duplex stainless steel sheet may have a
recrystallized grain size of about 4-9 .mu.m and a necking-down
width of 10 mm or less.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] The above and other aspects, features and other advantages
of the present invention will be more clearly understood from the
following detailed description taken in conjunction with the
accompanying drawings, in which:
[0024] FIG. 1 is a schematic view of a twin-roll strip caster;
[0025] FIG. 2 is a schematic view illustrating a surface of a cast
roll in a twin-roll strip caster according to an exemplary
embodiment of the present invention;
[0026] FIG. 3 is a three dimensional image showing a surface of a
cast roll in a twin-roll strip caster according to an exemplary
embodiment of the present invention;
[0027] FIG. 4 is a graph showing a hill area ratio and a gas
discharge index in a width direction of a cast roll according to an
exemplary embodiment of the present invention;
[0028] FIGS. 5A and 5B are high temperature photographs of slabs
according to a comparative example and an example of the present
invention;
[0029] FIGS. 6A and 6B are surface photographs of casting materials
according to a comparative example and an example of the present
invention; and
[0030] FIGS. 7A and 7B are microstructure photographs after cold
rolled annealing of casting materials according to a comparative
example and an example of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0031] Exemplary embodiments of the present invention will now be
described in detail with reference to the accompanying
drawings.
[0032] Hereinafter, exemplary embodiments will be described in
detail with reference to the accompanying drawings.
[0033] Embodiments of the disclosure may, however, be embodied in
many different forms or combined and the scope of the present
invention should be construed as being limited to the embodiments
set forth herein. These embodiments are provided so that this
disclosure will more fully convey the concept of the invention to
those skilled in the art. In the drawings, the shapes and sizes of
elements may be exaggerated for clarity. Like reference numerals in
the drawings denote like elements.
[0034] FIG. 1 is a schematic view of a twin-roll strip caster
according to an exemplary embodiment of the present invention, and
the twin-roll strip caster includes a ladle 1 receiving molten
steel, a tundish 2 into which the received molten steel is
introduced, a pair of cast rolls 5 rotating in opposite directions,
an injection nozzle 3 supplying molten steel to a space between the
pair of cast rolls 5, an edge dam 6 forming a sump 4 such that a
molten steel pool is formed on respective sides of the pair of cast
rolls 5, and a meniscus shield 7 provided to cover an upper side of
the molten steel pool and to block contact between the molten steel
pool and air. Also, the twin-roll strip caster may further include
a rolling mill 8, a cooling machine 9, and a coiling machine
10.
[0035] In a twin-roll strip casting process according to an
exemplary embodiment of the present invention, molten steel is
received in the ladle 1 by using the twin-roll strip caster
illustrated in FIG. 1, and the received molten steel is introduced
into the tundish 2 through a nozzle. The molten steel introduced
into the tundish 2 is supplied to the edge dam 6, i.e., between the
cast rolls 5, through the molten steel injection nozzle 3 and
starts to be solidified. At this time, the meniscus shield 7 may
prevent oxidation of the molten steel in the molten steel pool
between the cast rolls 5 at an upper surface of the molten steel
pool, and the atmosphere surrounding the molten steel pool may be
adjusted by injecting a predetermined gas. The molten steel may be
extruded and manufactured into a strip while passing through a roll
nib at a point at which the cast rolls 5 meet. Then, the strip is
rolled into a thin steel sheet while passing through the rolling
mill 8, and the thin steel sheet is cooled while passing through
the cooling machine 9 and is wound by the coiling machine 10.
[0036] In the twin-roll strip casting process directly
manufacturing a strip having a thickness of 10 mm or less from
molten steel, it is important that molten steel is supplied through
the injection nozzle 3 between the internal cooling type cast rolls
5 rotating in opposite directions at a rapid rate to manufacture
the strip at a desired thickness without cracks with an improved
actual yield.
[0037] Also, in order to manufacture, by using a twin-roll strip
casting process, a thin steel sheet such as a high
nitrogen-containing duplex stainless steel sheet in which gas
exhaust occurs, a cast roll surface treatment technique that
enables gas to be exhausted is necessary and a control for uniform
cooling in the width direction is required.
[0038] Referring to FIG. 2, a portion of a surface 5S of the cast
roll 5 in a twin-roll strip caster according to an exemplary
embodiment of the present invention is illustrated. The surface 5S
of the cast roll 5 may include edge sections having a predetermined
width from one end, and center sections between the edges sections,
and FIG. 2 illustrates a region including a boundary between the
edge section and the center section.
[0039] Hills 110 and valleys 120 extending in a casting direction
or a rolling direction of the cast roll 5 may be alternately formed
on the surface 5S of the cast roll 5. A three dimensional image of
a portion of the surface of the cast roll 5 is shown in FIGS. 5A
and 5B. That is, the hills 110 and the valleys 120 may be arranged
in linear manner in a circumferential direction of the cast roll 5.
In the case of high nitrogen duplex stainless steel, since nitrogen
gas is exhausted due to a solubility difference during
solidification of molten steel, the surface 5S of the cast roll may
be processed to have the valleys 120 so that gas may be easily
exhausted.
[0040] Particularly, according to an exemplary embodiment of the
present invention, the area ratio of the hills 110 may decrease in
a direction from the edge section toward the center section. Thus,
a width (L1) of any one of the hills 110 of the edge section may be
wider than a width (L2) of the hill 110 adjacent to the center
section. Also, the widths L1 and L2 of the hills 110 of the edge
section may be wider than a width L3 of the hill 110 of the center
section.
[0041] Referring to FIG. 4, a hill area ratio and a gas discharge
index Gin a width direction of a cast roll according to an
exemplary embodiment of the present invention are illustrated. The
graph of FIG. 4 shows variations according to a distance from one
end of the edge section, i.e., one end of the cast roll from one
end of the edge section toward the center section.
[0042] The hill area ratio in the center section may be constant in
a range of 10-40% but the embodiments of the present invention are
not limited thereto. If the hill area ratio in the center section
is less than 10%, the cast roll and a solidification shell maybe
adhered to each other to make it difficult to perform a casting
operation, and if the area ratio is more than 40%, a solidification
ability difference between the center section and the edge section
is not significantly high, so that it may be difficult to prevent
solidification delay of the edge section.
[0043] The hill area ratio in the edge section may be larger than
that in the center section. Also, the hill area ratio in the edge
section may increase in a direction away from the center section
but the embodiments of the present invention are not limited
thereto. For example, the hill area ratio in the edge section may
be in a range of 10-70% and may be continuously changed. The
maximum hill area ratio of 70% in the edge section is a value
designed in consideration of gas exhausting.
[0044] A transition boundary of the hill area ratio between the
edge section and the center section, i.e., a boundary at which the
hill area ratio is changed and is then made constant, may be in a
range of 50-200 mm from one end of the cast roll. That is, the
width of the edge section may be in a range of 50-200 mm from one
end of the cast roll. The transition boundary may correspond to a
position at which solidification delay occurs along the edge
section.
[0045] The gas discharge index G of the center section may be in a
range of 80-130 and the gas discharge index G of the edge section
may be continuously decreased to a minimum range of 50-70. Herein,
the gas discharge index G indicates the area of hill per unit pitch
and is expressed by the following equation: G=width (w) of valley x
depth (d)/pitch (p).
[0046] When the gas discharge index G is lower than 80,
micro-cracks or depressions may be generated on a surface of a
casting material, and when the gas discharge index G is 130 or
higher, the depth of valley is so deep that the cast roll and the
solidification shell may be adhered to each other to make it
difficult to perform the casting.
[0047] If a cast roll having a constant hill area ratio along the
width direction is used, solidification delays may be generated in
the edge section so that edge bulging or the leakage of molten
steel may be generated. However, according to an exemplary
embodiment of the present invention, the degree of solidification
may be controlled by manufacturing high nitrogen lean duplex
stainless steel with the cast rolls in which hills and valleys are
adjusted as above. It could be understood from experimental results
that the higher the hill area ratio, the more the solidification
ability is enhanced, and edge bulging was prevented by increasing
the hill area ratio of the edge section based on such fact. Also,
valleys shaped in fine grooves were formed such that gas discharge
index G had a predetermined value or higher, and the valleys were
differently applied to the edge section and the center section so
that casting materials with good surface and edge qualities may be
manufactured.
[0048] Hereinafter, a method for manufacturing a thin duplex
stainless steel sheet according to an exemplary embodiment of the
present invention will be described in more detail.
[0049] According to an exemplary embodiment of the preset
disclosure, a method for manufacturing a thin duplex stainless
steel sheet, the method includes: forming a cast strip by pouring
molten steel between a pair of cast rolls rotating in opposite
directions; and manufacturing a hot rolled strip by rolling the
cast strip in a rolling mill.
[0050] Hills and valleys are alternately arranged on surfaces of
the pair of cast rolls in circumferential directions thereof and a
hill area ratio (i.e., an hill area ratio) in edge sections thereof
is higher than that in centers sections thereof.
[0051] The cast strip may have a thickness of 1-6 mm and a width of
1,000-1,400 mm.
[0052] The reduction ratio in the rolling may be in a range of
15-60%.
[0053] If the reduction ratio is less than 15%, pores may be
generated in a central segregation section so that product quality
may be deteriorated, and if the reduction ratio is more than 60%,
rolling may be impossible due to the limitations of specifications
of rolling facilities.
[0054] The hot rolled strip may have a thickness of 0.7-4 mm and a
width of 1,000-1,400 mm.
[0055] The above method may further include annealing the hot
rolled strip in which the annealing temperature maybe in a range of
1,000-1,250.degree. C.
[0056] Hereinafter, a thin duplex stainless steel sheet
manufactured according to an exemplary embodiment of the present
invention will be described in more detail.
[0057] A thin duplex stainless steel sheet according to an
exemplary embodiment of the present invention may include, by
weight: 0.1% or less carbon (C) (exclusive of 0%), 0.2-3.0% silicon
(Si), 1.0-4.0% manganese (Mn), 19.0-23.0% chromium (Cr), 0.3-2.5%
nickel (Ni), 0.15-0.3% nitrogen (N), 0.3-2.5% copper (Cu), a
balance of iron (Fe), and inevitable impurities. However, minimum
amounts of phosphorous (P) and sulfur (S) may be included in order
to suppress segregation.
[0058] Carbon (C) is an element for forming an austenite phase and
is an effective element for increasing strength of a material by
solid-solution strengthening. When C is, however, added
excessively, C is easily bonded to a carbide-forming element such
as chromium (Cr) that is effective for corrosion resistance in a
boundary between a ferrite phase and an austenite phase to decrease
the content of Cr and the corrosion resistance. Therefore, C may be
added in an amount of 0.1% or less to maximize the corrosion
resistance.
[0059] Silicon (Si) is an element which is partially added to
achieve a deoxidizing effect, used to forma ferrite phase, and is
concentrated on ferrite during an annealing treatment. Therefore,
Si is added in an amount of 0.2% or more to secure an appropriate
ferrite phase fraction. However, if Si is added in an amount of
3.0% or more, Si sharply increases the hardness of the ferrite
phase and decreases the elongation, thus making it difficult to
secure the austenite phase affecting the securement of the
elongation. Also, an excessive amount of Si decreases the fluidity
of slag in a steel making process and may be bonded to oxygen to
form inclusions, thus decreasing corrosion resistance. Therefore,
the content of Si may be determined in a range of 0.2-3.0%.
[0060] Nitrogen (N) is an element which greatly attributes to
stabilization of the austenite phase and is one of elements which
are concentrated on the austenite phase together with nickel (Ni)
during an annealing treatment. Therefore, corrosion resistance and
strength may be incidentally improved by increasing the content of
nitrogen but solubility of nitrogen may be changed according to the
content of manganese (Mn) added. So, it is necessary to adjust the
content of nitrogen. In a range of Mn according to an exemplary
embodiment of the present invention, when the content of nitrogen
exceeds 0.3%, blow holes, pin holes or the like are generated
during casting due to an excessive amount of nitrogen exceeding the
solubility, so that surface defects may be caused in final
products. Also, 0.150 or more of nitrogen should be added in order
to secure corrosion resistance corresponding to that of 304 steel.
When the content of nitrogen is relatively low, it may be difficult
to secure a proper phase fraction. Therefore, the content of
nitrogen may be determined within a range of 0.15-0.30%.
[0061] Manganese (Mn) is an element which serves as a deoxidizing
agent, increases solubility of nitrogen, and forms austenite, and
is added in replacement of expensive nickel (Ni). When the content
of Mn exceeds 4%, it maybe difficult to secure corrosion resistance
corresponding to the level of 304 steel. Also, when Mn is added in
excess of 4%, solubility of nitrogen may be improved but corrosion
resistance may be decreased because Mn bonds to sulfur (S) in steel
to form MnS. When the content of Mn is less than 1%, it is
difficult to secure an appropriate austenite phase fraction even by
adjusting the content of an austenite-forming element such as Ni,
Cu, N or the like and it fails to obtain a sufficient solubility of
nitrogen at atmospheric pressure because the solubility of nitrogen
added is low. Therefore, the content of Mn may be made in a range
of 1-4%.
[0062] Chromium (Cr) is an element which stabilizes ferrite
together with silicon (Si), plays a main role of securing a ferrite
phase of two phase stainless steel, and is an essential element for
securing corrosion resistance. The increase of content of Cr
increases corrosion resistance but the content of relatively
expensive nickel or other austenite-forming elements should be
increased to maintain the phase fraction. Therefore, the content of
Cr may be in a range of 19-23% so as to secure corrosion resistance
as well as to maintain phase fraction.
[0063] Nickel (Ni) is an element which stabilizes austenite
together with Mn, Cu, and N and plays a main role of securing
austenite phase of duplex stainless steel. However, when nickel is
excessively added, the austenite phase fraction increases to make
it possible to secure an appropriate austenite fraction and
manufacturing costs of products may be increased due to the use of
relatively expensive nickel to make it difficult to secure
competitiveness against 304 steel. Therefore, the balance of phase
fraction may be sufficiently maintained by increasing other
austenite-forming elements, e.g., Mn and N instead of decreasing
the content of relatively expensive nickel as much as possible for
cost savings. However, since it is possible to secure sufficient
stabilization of austenite phase by suppressing the formation of
plastic induced martensite generated in cold working with nickel,
nickel may be added in an amount of 0.3% or more. Therefore, the
content of nickel (Ni) may be made within a range of 0.3-2.5%.
[0064] When the content of copper (Cu) is 2.5% or more, it is
difficult to process products due to hot shortness and thus the
content of Cu may be set to a minimal amount in consideration of
cost savings. However, copper may be added in an amount of 0.3% or
more so as to secure sufficient stabilization of the austenite
phase by suppressing the formation of the plastic induced
martensite generated in cold working. Therefore, the content of Cu
may be adjusted in a range of 0.3-2.5%.
MODE FOR CARRYING OUT THE PRESENT INVENTION
[0065] Hereinafter, the present invention will be described in more
detail with examples thereof.
[0066] To identify the influence of nitrogen contained above the
solubility limit in molten steel on strips, casting strips were
manufactured by a casting method of Table 1 using molten steel
having compositions listed in Table 1 and then were rolled to
manufacture hot rolled strips. The content of each composition in
Table 1 below indicates a value expressed in % by weight.
[0067] Examples (Comparative Example 2 and Examples 1-6)
corresponding to rapid casting of Table 1 were conducted with 90
tons of molten steel using a twin-roll strip casting (i.e., rapid
casting) method to manufacture casting strips having a width of
1,300 mm and a thickness of 4.0 mm, and directly after the casting,
the casting strips were hot rolled at a high temperature to
manufacture hot rolled strip coils having a thickness of 2.5
mm.
[0068] Meanwhile, an example (Comparative Example 1) indicated by
an existing continuous casting method in Table 1 was cast using a
continuous casting method.
[0069] Whether internal pores were generated or not was observed
with respect to the hot rolled strips manufactured as above, and
results are summarized in Table 1.
TABLE-US-00001 TABLE 1 Casting N Internal Item C Si Mn Cr: Ni Cu N
method exhaust pore Compar- 0.05 1.35 2.8 20.3 1.06 1.0 0.23
Existing X .largecircle. ative continuous Example casting 1 Compar-
0.05 1.35 2.8 20.3 1.06 1.0 0.33 Rapid X .largecircle. tive casting
Example 2 Example 0.045 1.08 3.02 19.63 0.98 0.98 0.272 Rapid
.largecircle. X 1 casting Example 0.071 1.3 3.81 19.69 1.14 0.5
0.24 Rapid .largecircle. X 2 casting Example 0.051 1.28 3.07 20.02
1.0 0.503 0.24 Rapid .largecircle. X 3 casting Example 0.051 1.27
3.09 20.41 1.03 0.5 0.25 Rapid .largecircle. X 4 casting Example
0.02 1.21 2.63 20.53 0.85 0.793 0.22 Rapid .largecircle. X 5
casting Example 0.05 0.7 2.73 20.5 0.95 0.7 0.15 Rapid
.largecircle. X 6 casting
[0070] As summarized in Table 1, the content of nitrogen (N) in
Comparative Example 1 manufactured by an existing continuous
casting method was 0.23%, but it could be understood that internal
pores were generated in the hot rolled strip because nitrogen was
not exhausted during the casting.
[0071] Since the nitrogen content of nitrogen (N) in Comparative
Example 2 was 0.33% and high, it could be also understood that
although the twin-roll strip casting method was applied, nitrogen
was not sufficiently exhausted so that internal pores were
generated in the hot rolled strip.
[0072] Meanwhile, the content of nitrogen (N) in Examples 1-6
corresponding to the present invention was 0.15-0.3%, and it could
be understood that hot rolled strips were able to be cast without
generation of internal pores by applying a twin-roll strip casting
method of the present invention.
[0073] Meanwhile, a high temperature photograph of a hot rolled
strip (Conventional Example) manufactured by an existing twin-roll
strip casting method and a high temperature photograph of the hot
rolled strip of Example 2 in Table 1 were observed and observation
results were shown in FIGS. 5A and 5B, respectively. That is, FIG.
5A indicates the hot rolled strip of the conventional example in
which solidification delay was generated in an edge section and
FIG. 5B indicates the hot rolled strip of Example 2.
[0074] As shown in FIGS. 5A and 5B, in the case of the conventional
example, a phenomenon in which a solidification shell is lifted off
from the cast roll occurs, so that cooling ability of the cast roll
in the edge section is lowered and thus the temperature of the edge
section of the hot rolled strip (casting material) is elevated.
Also, when such a phenomenon is severe, molten steel may flow
without solidification. However, in the case of Example 1 of the
present invention, since the temperature is uniform in the width
direction, the hot rolled strip (casting material) may be
manufactured without the generation of depressions.
[0075] Also, a high temperature photograph of a hot rolled strip
(Conventional Example) manufactured by an existing twin-roll strip
casting method and a high temperature photograph of the hot rolled
strip of Example 2 in Table 1 were observed and observation results
were shown in FIGS. 6A and 6B, respectively. That is, FIG. 6A shows
a photograph of the conventional example in which an edge
depression defect was generated, and FIG. 6B shows a photograph of
the hot rolled strip (casting material) manufactured according to
Example 2 of the present invention.
[0076] As shown in FIGS. 6A and 6B, it can be understood that in
the case of the conventional example, a depression defect was
generated due lack of gas exhaust or abrupt gas exhaust by
deformation of the solidification shell. Such a depression defect
may be generated in a vertical form or a horizontal form in a
boundary between hill and valley. Also, since the depression may
include micro-cracks that may be a cause of strip breakage, when
such depression is generated in the edge section, cold rolling is
conducted after the edge section is removed. However, it can be
understood that Example 2 of the present invention has no
depression defect.
[0077] Meanwhile, in order to manufacture hot rolled strips of
Examples 2 and Comparative Example 1 in Table 1, hot annealing,
cold rolling and cold annealing were conducted at a hot annealing
temperature of 1,100.degree. C. and a cold annealing temperature of
1,150.degree. C.
[0078] After the cold annealing, microstructures of steel sheets
were investigated and investigation results are shown FIGS. 7A and
7B.
[0079] FIG. 7A shows a photograph of a recrystallized structure of
a cold annealed product manufactured by a continuous casting method
and corresponding to Comparative Example 1 and FIG. 7B shows a
microstructure of a cold annealed product manufactured by a
twin-roll strip casting method according to Example 2 of the
present invention.
[0080] In Comparative Example 1, grains elongated in the rolling
direction were observed and ferrite and austenite were stacked and
arranged. By virtue of such microstructure arrangement, the
elongation was high when a tensile test was performed in the
rolling direction but the elongation was low when the tensile test
was performed in a direction perpendicular to the rolling
direction. As an analysis result obtained by using an image
analysis tool, the grains elongated in the rolling direction had an
average length ranging from 9 to 10 .mu.m and an average diameter
of about 5 .mu.m.
[0081] In Example 2, it could be confirmed that microstructures
were randomly arranged without having a specific orientation and
the plastic anisotropy was minimized due to the microstructures.
Also, in the case of cold annealing, a necking-down width was shown
to be 10 mm or less, which was as good as general 304 series
stainless steel. As an analysis result obtained by using an image
analysis tool in order to check the distribution and size of
recrystallized grains, the grains elongated in the rolling
direction had an average length ranging from about 4 to 9 .mu.m and
an average diameter of about 4 .mu.m.
[0082] Furthermore, hot rolled strips of Examples 1 to 6 in Table 1
were subjected to hot annealing, cold rolling, and cold annealing,
and then the elongations and yield strengths of the hot annealed
product and the cold annealed product were respectively measured.
As a result, the elongation of the cold annealed product was
30-55%, which was higher than that of the hot rolled product by
about 5%. The yield strength of the cold annealed product was
320-680 MPa, which was slightly lower than that of the hot rolled
product.
[0083] Since cast rolls in which a hill area ratio of edge sections
is higher than that of center sections are used, a twin-roll strip
caster capable of a thin stainless steel sheet with improved
quality, a method for manufacturing a thin duplex stainless steel
sheet using the same, and a thin duplex stainless steel sheet may
be provided.
[0084] Various and advantageous advantages and effects of the
present invention are not limited to the above description and will
be more easily understood through description of exemplary
embodiments of the present invention.
[0085] While the present invention has been shown and described in
connection with the exemplary embodiments, it will be apparent to
those skilled in the art that modifications and variations can be
made without departing from the spirit of the present invention as
defined by the appended claims.
* * * * *