U.S. patent application number 15/775335 was filed with the patent office on 2018-12-27 for austenitic stainless steel having excellent orange peel resistance and manufacturing method therefor.
The applicant listed for this patent is POSCO. Invention is credited to Gyu Jin JO, Hyung Gu KANG, Sang Hoon KIM, Byoung Jun SONG.
Application Number | 20180371575 15/775335 |
Document ID | / |
Family ID | 58695848 |
Filed Date | 2018-12-27 |
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United States Patent
Application |
20180371575 |
Kind Code |
A1 |
KANG; Hyung Gu ; et
al. |
December 27, 2018 |
AUSTENITIC STAINLESS STEEL HAVING EXCELLENT ORANGE PEEL RESISTANCE
AND MANUFACTURING METHOD THEREFOR
Abstract
An austenitic stainless steel having excellent orange peel
resistance and a method for producing the same are disclosed. In
the austenitic stainless steel having excellent orange peel
resistance, according to an embodiment of the present disclosure, a
ratio Gs/Gi of an average crystal grain size Gs of surface crystal
grains included in a first area corresponding to a depth of 10% or
less of a total thickness of the austenitic stainless steel from
the surface of the austenitic stainless steel with respect to an
average crystal grain size Gi of internal crystal grains included
in a second area corresponding to a depth that is deeper than 10%
of the total thickness of the austenitic stainless steel from the
surface of the austenitic stainless steel may be 0.5 or smaller.
Therefore, it is possible to prevent deterioration of surface
roughness due to orange peel of the steel surface even after
post-processing of the austenitic stainless steel while increasing
the sizes of crystal grains in order to reduce strength of the
austenitic stainless steel, and also to reduce cost by replacing
copper pipes
Inventors: |
KANG; Hyung Gu; (Pohang-si,
Gyeonsangbuk-do, KR) ; KIM; Sang Hoon; (Pohang-si,
Gyeonsangbuk-do, KR) ; JO; Gyu Jin; (Pohang-si,
Gyeongsangbuk-do, KR) ; SONG; Byoung Jun; (Pohang-si,
Gyeongsangbuk-do, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
POSCO |
Pohang-si, Gyeongsangbuk-do |
|
KR |
|
|
Family ID: |
58695848 |
Appl. No.: |
15/775335 |
Filed: |
November 9, 2016 |
PCT Filed: |
November 9, 2016 |
PCT NO: |
PCT/KR2016/012874 |
371 Date: |
May 10, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 8/0226 20130101;
C21D 9/46 20130101; C22C 38/58 20130101; B22D 11/0682 20130101;
C22C 38/02 20130101; C21D 9/08 20130101; C22C 38/04 20130101; C22C
38/42 20130101; C22C 38/001 20130101; C21D 6/004 20130101; B22D
11/00 20130101; C21D 8/0236 20130101; C21D 6/008 20130101; B22D
11/0622 20130101; C21D 6/005 20130101; C21D 8/0205 20130101; B22D
11/002 20130101; C21D 2211/001 20130101; C21D 8/105 20130101; C21D
2211/005 20130101; B22D 11/06 20130101 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C21D 8/02 20060101 C21D008/02; C21D 6/00 20060101
C21D006/00; B22D 11/06 20060101 B22D011/06; C22C 38/42 20060101
C22C038/42; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 12, 2015 |
KR |
10-2015-0158665 |
Claims
1. Austenitic stainless steel having excellent orange peel
resistance, wherein a ratio Gs/Gi of an average crystal grain size
Gs of surface crystal grains included in a first area corresponding
to a depth of 10% or less of a total thickness of the austenitic
stainless steel from the surface of the austenitic stainless steel
with respect to an average crystal grain size Gi of internal
crystal grains included in a second area corresponding to a depth
that is deeper than 10% of the total thickness of the austenitic
stainless steel from the surface of the austenitic stainless steel
is 0.5 or smaller.
2. The austenitic stainless steel of claim 1, comprising silicon
(Si) of 0.1 to 0.65 weight %, manganese (Mn) of 1.0 to 3.0 weight
%, nickel (Ni) of 6.5 to 10.0 weight %, chrome (Cr) of 16.5 to 18.5
weight %, copper (Cu) of 6.0 weight % or less (except for zero),
carbon (C) and nitrogen (N) of 0.13 weight % or less (except for
zero), iron (Fe), and other inevitable impurities.
3. The austenitic stainless steel of claim 1, wherein the average
crystal grain size Gs of the surface crystal grains is 100 .mu.m or
less.
4. The austenitic stainless steel of claim 1, manufactured by strip
casting.
5. The austenitic stainless steel of claim 4, wherein when the
austenitic stainless steel is casted by strip casting, a delta
ferritic phase content remaining upon solidification is 5 weight %
or more
6. The austenitic stainless steel of claim 5, wherein a delta
ferrite phase content of a cold-rolled structure of the austenitic
stainless steel is 0.5 weight % or more.
7. A method of manufacturing austenitic stainless steel having
excellent orange peel resistance by strip casting of passing
stainless steel between a pair of rotating rolls to freeze the
stain steel into a solid, the method comprising, casting austenitic
stainless steel, and controlling elements of the austenitic
stainless steel according to Equation (1) such that a content Delta
of delta ferritic phase remaining upon solidification is 5 wt % or
more to manufacture a hot-rolled steel sheet:
Delta=((Cr+Mo+1.5Mn+0.5Nb+2Ti+18)/(Ni+0.3Cu+30*(C+N)+0.5Mn+36)+0.-
262).times.161-161, Equation (1) wherein element symbols of
Equation (1) represent weight percentages (weight %) of the
corresponding elements
8. The method of claim 7, wherein the austenitic stainless steel
comprises silicon (Si) of 0.1 to 0.65 weight %, manganese (Mn) of
1.0 to 3.0 weight %, nickel (Ni) of 6.5 to 10.0 weight %, chrome
(Cr) of 16.5 to 18.5 weight %, copper (Cu) of 6.0 weight % or less
(except for zero), carbon (C) and nitrogen (N) of 0.13 weight % or
less (except for zero), iron (Fe), and other inevitable
impurities.
9. The method of claim 7, further comprising performing
heat-treatment on the hot-rolled steel sheet, and then performing
cold rolling on the hot-rolled steel sheet at a total reduction
ratio of 50% or higher, thereby manufacturing a cold-rolled steel
sheet, wherein a delta ferritic phase content of a cold-rolled
structure of the cold-rolled steel sheet is 0.5 weight % or more.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to austenitic stainless steel
having excellent orange peel resistance and a manufacturing method
thereof, and more particularly, to austenitic stainless steel
having excellent orange peel resistance, which is capable of
preventing deterioration of surface roughness due to orange peel of
the steel surface even after post-processing of the stainless steel
by controlling average crystal grain sizes at different depths of
the stainless steel while increasing the sizes of crystal grains in
order to reduce strength, and a method of manufacturing the
austenitic stainless steel.
BACKGROUND ART
[0002] There was a trial for applying stainless steel to
refrigerant pipes for household and car air conditioners, since
stainless steel has excellent corrosion resistance and a low
price.
[0003] However, since refrigerant pipes for air conditioner are
limited in installation space, in many cases, constructing pipes
essentially accompanies the work of bending the pipes with human
power. Copper pipes or aluminum pipes used typically do not make
any problem since they are sufficiently soft, but general stainless
steel has a problem that it does not have flexibility essentially
required for construction of pipes.
[0004] Metal materials are work hardened when they are subject to
deformation, such as extension or compression, and as they are
subject to higher deformation, they become stronger. Bending pipes
is a combined action of extension and compression, and as the
degree of bending is greater, the material will be more hardened.
Particularly, when 304 steel which is most widely used as
austenitic stainless steel is used in pipes for air conditioner, it
is very difficult to bend the pipes with human power in space for
installing the pipes since the degree of work hardening is
high.
[0005] In order to overcome the problem, a method of reducing the
strength of stainless steel by increasing the size (that is, grain
size) of crystal grains is used. The strength of stainless steel
decreases as the grain size increases. However, as the size of
crystal grains increases, orange peel being uneven defects appears
on the surface after processing. Therefore, it is difficult to
reduce the strength of stainless steel only by increasing the size
of crystal grains. The orange peel should be removed since it
impairs the beauty, and removing the orange peel through polishing
requires additional cost and time.
(Patent Document 0001) Korean Laid-open Patent Application No.
10-2010-0020446
DISCLOSURE
Technical Problem
[0006] The embodiments of the present disclosure are directed to
providing austenitic stainless steel having excellent orange peel
resistance, which is capable of preventing deterioration of surface
roughness due to orange peel of the steel surface even after
post-processing of the stainless steel by controlling average
crystal grain sizes at different depths of the stainless steel
while increasing the sizes of crystal grains in order to reduce
strength.
[0007] Further, the embodiments of the present disclosure is
directed to providing a method of manufacturing austenitic
stainless steel having excellent orange peel resistance, which is
capable of preventing deterioration of surface roughness due to
orange peel, wherein the austenitic stainless steel is casted
through strip casting to control a delta ferritic phase content and
thereby suppress the growth of surface crystal grains.
Technical Solution
[0008] In austenitic stainless steel having excellent orange peel
resistance, according to an embodiment of the present disclosure, a
ratio Gs/Gi of an average crystal grain size Gs of surface crystal
grains included in a first area corresponding to a depth of 10% or
less of a total thickness of the austenitic stainless steel from
the surface of the austenitic stainless steel with respect to an
average crystal grain size Gi of internal crystal grains included
in a second area corresponding to a depth that is deeper than 10%
of the total thickness of the austenitic stainless steel from the
surface of the austenitic stainless steel may be 0.5 or
smaller.
[0009] According to an embodiment of the present invention, the
austenitic stainless steel may include silicon (Si) of 0.1 to 0.65
wt % (weight percentage), manganese (Mn) of 1.0 to 3.0 weight %,
nickel (Ni) of 6.5 to 10.0 wt %, chrome (Cr) of 16.5 to 18.5 wt %,
copper (Cu) of 6.0 wt % or less (except for zero), carbon (C) and
nitrogen (N) of 0.13 wt % or less (except for zero), iron (Fe), and
other inevitable impurities.
[0010] According to an embodiment of the present invention, the
average crystal grain size Gs of the surface crystal grains may be
100 .mu.m or less.
[0011] According to an embodiment of the present invention, the
austenitic stainless steel may be manufactured by strip
casting.
[0012] According to an embodiment of the present invention, when
the austenitic stainless steel is casted by strip casting, a delta
ferritic phase content remaining upon solidification may be 5
weight % or more.
[0013] According to an embodiment of the present invention, a delta
ferrite phase content of a cold-rolled structure of the austenitic
stainless steel may be 0.5 weight % or more.
[0014] In a method of manufacturing austenitic stainless steel
having excellent orange peel resistance by strip casting of passing
stainless steel between a pair of rotating rolls to freeze the
stain steel into a solid, according to an embodiment of the present
disclosure, the method comprises; casting austenitic stainless
steel, and controlling elements of the austenitic stainless steel
according to Equation (1) such that a content Delta of delta
ferritic phase remaining upon solidification is 5 wt % or more to
manufacture a hot-rolled steel sheet:
Delta=((Cr+Mo+1.5Mn+0.5Nb+2Ti+18)/(Ni+0.3Cu+30*(C+N)+0.5Mn+36)+0.262).ti-
mes.161-161, Equation (1) [0015] Herein, element symbols of
Equation (1) represent weight percentages (weight %) of the
corresponding elements.
[0016] According to an embodiment of the present invention, the
austenitic stainless steel may include silicon (Si) of 0.1 to 0.65
wt % (weight percentage), manganese (Mn) of 1.0 to 3.0 weight %,
nickel (Ni) of 6.5 to 10.0 wt %, chrome (Cr) of 16.5 to 18.5 wt %,
copper (Cu) of 6.0 wt % or less (except for zero), carbon (C) and
nitrogen (N) of 0.13 wt % or less (except for zero), iron (Fe), and
other inevitable impurities.
[0017] According to an embodiment of the present invention, the
method further comprises; performing heat-treatment on the
hot-rolled steel sheet, and then performing cold rolling on the
hot-rolled steel sheet at a total reduction ratio of 50% or higher,
thereby manufacturing a cold-rolled steel sheet, wherein a delta
ferritic phase content of a cold-rolled structure of the
cold-rolled steel sheet is 0.5 weight % or more.
Advantageous Effects
[0018] The embodiments of the present disclosure may prevent
deterioration of surface roughness due to orange peel of the
surface of austenitic stainless steel even after post-processing of
the austenitic stainless steel, while increasing the sizes of
crystal grains in order to reduce the strength of the austenitic
stainless steel, and may reduce cost by replacing copper pipes or
aluminum pipes used as air conditioner refrigerant pipes with
stainless pipes.
[0019] Also, the embodiments of the present disclosure can prevent
deterioration of surface roughness due to orange peel by casting
austenitic stainless steel through strip casting to control a delta
ferritic phase content and thereby suppress the growth of surface
crystal grains.
DESCRIPTION OF DRAWINGS
[0020] FIG. 1 is a schematic diagram of an apparatus provided to
describe strip casting for manufacturing austenitic stainless steel
according to an embodiment of the present disclosure.
[0021] FIG. 2 is a cross-sectional view for describing an
austenitic stainless cold-rolled sheet according to an embodiment
of the present disclosure.
[0022] FIG. 3 is a picture showing an austenitic stainless steel
pipe according to an embodiment of the present disclosure, bent 90
degrees.
[0023] FIG. 4 is a picture showing the surface of an austenitic
stainless steel pipe of FIG. 1.
[0024] FIG. 5 is a picture showing the surface of a typical
austenitic stainless steel pipe.
[0025] FIG. 6 is a picture showing a section microstructure of the
austenitic stainless steel pipe of FIG. 1.
[0026] FIG. 7 is a picture showing a section microstructure of the
typical austenitic stainless steel pipe.
[0027] FIG. 8 is a graph for describing the size of a crystal grain
of austenitic stainless steel according to an embodiment of the
present disclosure and surface roughness when an austenitic
stainless steel pipe is bent 90 degrees.
BEST MODE
[0028] In austenitic stainless steel having excellent orange peel
resistance, according to an embodiment of the present disclosure, a
ratio Gs/Gi of an average crystal grain size Gs of surface crystal
grains included in a first area corresponding to a depth of 10% or
less of a total thickness of the austenitic stainless steel from
the surface of the austenitic stainless steel with respect to an
average crystal grain size Gi of internal crystal grains included
in a second area corresponding to a depth that is deeper than 10%
of the total thickness of the austenitic stainless steel from the
surface of the austenitic stainless steel may be 0.5 or
smaller.
MODES OF THE INVENTION
[0029] Hereinafter, the embodiments of the present disclosure will
be described in detail with reference to the accompanying drawings.
The following embodiments are provided to transfer the technical
concepts of the present disclosure to one of ordinary skill in the
art. However, the present disclosure is not limited to these
embodiments, and may be embodied in another form. In the drawings,
parts that are irrelevant to the descriptions may be not shown in
order to clarify the present disclosure, and also, for easy
understanding, the sizes of components are more or less
exaggeratedly shown.
[0030] Austenitic stainless steel having excellent orange peel
resistance, according to an embodiment of the present disclosure,
may include silicon (Si) of 0.1 to 0.65 wt % (weight percentage),
manganese (Mn) of 1.0 to 3.0 weight %, nickel (Ni) of 6.5 to 10.0
wt %, chrome (Cr) of 16.5 to 18.5 wt %, copper (Cu) of 6.0 wt % or
less (except for zero), carbon (C) and nitrogen (N) of 0.13 wt % or
less (except for zero), iron (Fe), and other inevitable
impurities.
[0031] Hereinafter, a reason for numerical limitation of elements
constituting austenitic stainless steel having excellent
flexibility according to the present disclosure will be
described.
[0032] Silicon (Si) may be added within the range of 0.1 to 0.65 wt
%.
[0033] Since silicon (Si) is an element essentially added for
deoxidation, silicon (Si) of 0.1 wt % or more may be added.
[0034] However, if the silicon (Si) content is excessively high,
the material may be hardened, and combined with oxygen to form
inclusions, resulting in a deterioration of corrosion resistance.
Therefore, the maximum silicon (Si) content may be limited to 0.65
wt %.
[0035] Manganese (Mn) may be added within the range of 1.0 to 3.0
wt %.
[0036] Since manganese (Mn) is an element essentially added for
deoxidation and increasing the stabilization of the austenite
phase, manganese (Mn) of 1.0 wt % or more may be added to maintain
the austenite balance. However, if the manganese (Mn) content is
excessively high, the corrosion resistance of the material may
deteriorate, and accordingly, the maximum manganese (Mn) content
may be limited to 3.0 wt %.
[0037] Nickel (Ni) may be added within the range of 6.5 to 10.0 wt
%.
[0038] If nickel (Ni) and chrome (Cr) are added together, corrosion
resistance such as pitting corrosion resistance may be improved,
and also if the nickel (Ni) content increases, the austenitic steel
may be softened.
[0039] Also, since nickel (Ni) is an element contributing to an
improvement of phase-stabilization of the austenitic stainless
steel, nickel (Ni) of 6.5 wt % or more may be added to maintain the
austenite balance. However, if the nickel (Ni) content is
excessively high, manufacturing cost of the austenitic stainless
steel may increase, and accordingly, the maximum nickel (Ni)
content may be limited to 10.0 wt %.
[0040] Chrome (Cr) may be added within the range of 16.5 to 18.5 wt
%.
[0041] Since chrome (Cr) is an element essentially added for
improving corrosion resistance, chrome (Cr) of 16.5 wt % or more
may need to be added for general use. However, if the chrome (Cr)
content is excessively high, the austenite phase may be hardened,
and manufacturing cost may increase. Accordingly, the maximum
chrome (Cr) content may be limited to 18.5 wt %.
[0042] Copper (Cu) may be added within the range of 6.0 wt %.
[0043] Copper (Cu) may soften the austenitic stainless steel.
However, if the copper (Cu) content is excessively high, hot
workability may deteriorate, and the austenitic phase may be
hardened. Accordingly, the maximum copper (Cu) content may be
limited to 6.0 wt %.
[0044] Carbon (C) and nitrogen (N) may be added by 0.13 wt % or
less.
[0045] Carbon (C) and nitrogen (N), which are interstitial solid
solution hardening elements, may harden the austenitic stainless
steel, and if the Carbon (C) and nitrogen (N) content is high,
strain induced martensite generated during processing may be
hardened to increase work hardness of the material. Accordingly,
the Carbon (C) and nitrogen (N) content may need to be limited, and
in the present disclosure, the Carbon (C) and nitrogen (N) content
may be limited to 0.13 w % or less.
[0046] In the austenitic stainless steel having excellent orange
peel resistance, according to an embodiment of the present
disclosure, a ratio Gs/Gi of an average crystal grain size Gs of
surface crystal grains included in a first area corresponding to a
depth of 10% or less of a total thickness of the austenitic
stainless steel from the surface of the austenitic stainless steel
with respect to an average crystal grain size Gi of internal
crystal grains included in a second area corresponding to a depth
that is deeper than 10% of the total thickness of the austenitic
stainless steel from the surface of the austenitic stainless steel
may be 0.5 or smaller.
[0047] That is, by maintaining the size of the surface crystal
grains distributed in the first area adjacent to the surface of the
austenitic stainless steel smaller than that of the internal
crystal grains of the austenitic stainless steel, a pipe made of
the austenitic stainless steel may prevent the generation of orange
peel on the surface when it is bent.
[0048] For example, the average crystal grain size Gs of the
surface crystal grains may be 100 .mu.m or smaller. Meanwhile, the
average crystal grain size Gi of the internal crystal grains may be
larger than 100 .mu.m.
[0049] The austenitic stainless steel having excellent orange peel
resistance, according to an embodiment of the present disclosure,
may be manufactured by strip casting of passing stainless steel
between a pair of rotating rolls to freeze it into a solid.
[0050] FIG. 1 is a schematic diagram of an apparatus provided to
describe strip casting for manufacturing austenitic stainless steel
according to an embodiment of the present disclosure.
[0051] The strip casting is a steel casting process for producing
hot-rolled thin strips directly from molten steel. The strip
casting can innovatively reduce manufacturing cost, equipment
investment cost, energy consumption, and pollutant gas emissions,
etc. by omitting a hot-rolling process.
[0052] In FIG. 1, a twin roll strip caster used for the strip
casting is shown. Referring to FIG. 1, molten steel may be put in a
ladle 2, and the molten steel may enter a turn dish 3 along a
nozzle. The molten steel entered the turn dish 3 may be supplied
between edge dams 6 installed at both ends of casting rolls 1 and
between the casting rolls 1 through a molten injecting nozzle 4,
and then solidified. In a molten metal area between the casting
rolls 1, the surfaces of the molten metal may be protected by a
meniscus shield 5 for preventing oxidation, and proper gas may be
injected to adjust an atmosphere appropriately. When the molten
steel escapes from a roll nip at which the casting rolls 1 meet, a
thin sheet 7 may be produced and drawn. Then, the thin sheet 7 may
pass through a rolling mill 8 to be rolled, and then cooled and
wound in winding equipment 9.
[0053] The strip casting may be performed to cast liquid molten
steel directly into a sheet having a thickness of 1 to 5 mm, and
then to apply high cooling speed to the sheet. That is, the strip
casting may be used to manufacture a hot coil using the twin roll
strip caser. The twin roll strip caster may supply molten steel
between the casting rolls 1 rotating in opposite directions and
between the edge dams 6 installed at the sides of the casting rolls
1, and emit a large amount of heat through the surfaces of the
casting rolls cooled with water, thereby casting steel. At this
time, solidified shells may be formed on the surfaces of the
casting rolls due to high cooling speed, and after casting, in-line
rolling may be sequentially performed to manufacture a thin
hot-rolled steel sheet having a thickness of 1 to 5 mm. Since the
strip casting casts a thin sheet directly, the strip casting may
omit a process of manufacturing a slab through continuous casting
and a hot-rolling process.
[0054] Particularly, when austenitic stainless steel is
manufactured through typical continuous casting, delta ferritic
phase may be generated in an initial time of solidification in
order to secure stability of solidification, and then
solidification to austenitic phase may be performed. A content of
delta ferritic phase remaining during casting may be within the
range of about 1 wt % to about 10 wt % according to the kind of
steel. That is, when a typical slab is casted, delta ferritic phase
may remain in the slab.
[0055] However, the slab may be heated in a reheating furnace for
two hours or more for hot rolling. At this time, most of the delta
ferritic phase may be resolved to austenitic phase by solid state
transformation, and then hot rolling may be performed at high
temperature so that most of the delta ferritic phase existing in
the slab casting structure may be resolved. Accordingly, a delta
ferrite content of a typical hot coil made of austenitic stainless
steel may be 0.5 wt % or less.
[0056] Since the strip casting casts a thin sheet directly from
molten steel using water-cooling rolls, a process of reheating
slabs and a process of hot rolling may be omitted, and accordingly,
a high delta ferritic phase content of 5 wt % or more may be
obtained. Also, most of the delta ferritic phase may remain on the
surface of the stainless steel sheet, so that the delta ferritic
phase remaining on the surface of the stainless steel sheet may
suppress the growth of crystal grains.
[0057] Accordingly, by controlling the content of the delta
ferritic phase remaining in the structure of the austenitic
stainless hot-rolled steel manufactured according to strip casting
to 5 wt % or more, it is possible to increase the content of
in-structure delta ferritic phase remaining in a final product.
Accordingly, the surface crystal grains distributed in the area
adjacent to the surface of the austenitic stainless steel may be
formed with a size that is smaller than that of the internal
crystal grains of the austenitic stainless steel.
[0058] That is, the content of delta ferritic phase remaining in
the austenitic stainless hot-rolled steel according to an
embodiment of the present disclosure may be 5 wt % or more.
[0059] In order to control the content Delta of delta ferritic
phase remaining in the casted austenitic stainless steel upon
solidification to 5 wt % or more, elements may be controlled
according to Equation (1) below:
Delta=((Cr+Mo+1.5Mn+0.5Nb+2Ti+18)/(Ni+0.3Cu+30*(C+N)+0.5Mn+36)+0.262).ti-
mes.161-161, Equation (1)
[0060] Herein, element symbols of Equation (1) represent weight
percentages (wt %) of the corresponding elements.
[0061] Thereafter, the hot-rolled steel sheet may be subject to
heat treatment, and then to cold rolling at a total reduction ratio
of 50% or higher, thereby manufacturing an austenitic stainless
cold-rolled steel sheet.
[0062] A cold-rolled structure of the austenitic stainless
cold-rolled sheet, according to an embodiment of the present
disclosure, cold-rolled by the cold-rolling process described above
may have a delta ferritic phase content of 0.5 wt % or smaller.
[0063] Hereinafter, the present disclosure will be described in
more detail through embodiments.
[0064] As shown in Table 1, by controlling a content Delta of delta
ferritic phase of a hot-rolled steel sheet casted by strip casting
to 5 wt % or more, austenitic stainless steel was manufactured
according to the present disclosure. Then, cold-rolling was
performed at a total reduction ratio of 50% or higher to
manufacture a cold-rolled steel sheet, and the cold-rolled steel
sheet extended by 15%. Thereafter, surface roughness (orange peel)
of the resultant steel sheet was measured. For tension, standard
tension pieces such as JIS13B are manufactured, or the sheet
material may extend as it is. 15% was used for comparison under the
same condition, and the characteristics of the present disclosure
are not necessarily obtained only upon extension of 15%.
[0065] An average crystal grain size Gs of surface crystal grains
included in an area corresponding to a depth of 10% or less of a
total thickness of the austenitic stainless steel from the surface
of the austenitic stainless steel, and an average crystal grain
size Gi of internal crystal grains included in an area
corresponding to a depth that is deeper than 10% of the total
thickness of the austenitic stainless steel from the surface of the
austenitic stainless steel were measured, and a ratio Gs/Gi of the
average crystal grain size Gs with respect to the average crystal
grain size Gi was represented.
[0066] The average crystal grain size Gs of the surface crystal
grains means an average crystal grain size of crystal grains
included in an area corresponding to a depth of 10% or less of the
total thickness of the austenitic stainless steel from the surface
of the austenitic stainless steel. Also, the average crystal grain
size Gi of the internal crystal grains means an average crystal
grain size of crystal grains included in an area corresponding to a
depth that is deeper than 10% of the total thickness of the
austenitic stainless steel from the surface of the austenitic
stainless steel.
TABLE-US-00001 TABLE 1 Existence or Absence of Delta Process Gs/Gi
Orange Peel Embodiment 1 5.1 Strip Casting 0.47 .largecircle.
Embodiment 2 6.2 Strip Casting 0.42 .largecircle. Embodiment 3 7.2
Strip Casting 0.37 .largecircle. Embodiment 4 8.5 Strip Casting
0.42 .largecircle. Comparative 5.5 Continuous 1.03 X Example 1
Casting Comparative 6.4 Continuous 0.95 X Example 2 Casting
Comparative 4.1 Strip Casting 0.74 X Example 3 Comparative 6.2
Continuous 0.97 X Example 4 Casting
[0067] Referring to Table 1, when austenitic stainless steel was
manufactured by strip casting, and the content of delta ferritic
phase was controlled to 5 wt % or more, no orange peel was
generated in the final product. Also, when the content of the delta
ferritic phase was controlled to 5 wt %, the ratio Gs/Gi of the
average crystal grain size Gs of the surface crystal grains with
respect to the average crystal grain size Gi of the internal
crystal grains was 0.5 or smaller.
[0068] Table 3 shows measured surface grain sizes of Embodiments 1
to 4 and Comparative Examples 1 to 4.
TABLE-US-00002 TABLE 2 Surface Grain size (Gs, .mu.m) Orange Peel
Embodiment 1 14 .largecircle. Embodiment 2 40 .largecircle.
Embodiment 3 58 .largecircle. Embodiment 4 85 .largecircle.
Comparative 105 X Example 1 Comparative 145 X Example 2 Comparative
192 X Example 3 Comparative 280 X Example 4
[0069] Referring to Table 2, excellent orange peel appeared in a
stainless steel sheet having a surface grain size Gs of 100 .mu.m
or smaller, whiled orange peel appeared significantly in a
stainless steel sheet having a surface grain size Gs that is larger
than 100 .mu.m, like the Comparative Examples.
[0070] The surface grain sizes and internal grain sizes of the
Embodiments 1 to 4 and Comparison Examples 1 to 4 were measured,
and ratios of the surface grain sizes with respect to the internal
grain sizes are shown in Table 3.
TABLE-US-00003 TABLE 3 Surface Internal Existence or Grain size
Grain size Absence of (Gs, .mu.m) (Gi, .mu.m) Gs/Gi Orange Peel
Embodiment 1 14 30 0.47 .largecircle. Embodiment 2 40 95 0.42
.largecircle. Embodiment 3 58 155 0.37 .largecircle. Embodiment 4
85 201 0.42 .largecircle. Comparative 105 102 1.03 X Example 1
Comparative 145 153 0.95 X Example 2 Comparative 192 260 0.74 X
Example 3 Comparative 280 290 0.97 X Example 4
[0071] As shown in Table 3, under the condition that the surface
grain size Gs is 100 .mu.m or smaller, and the ratio Gs/Gi of the
surface grain size Gs to the internal grain size Gi is 0.5 or
smaller, excellent orange peel resistance appeared.
[0072] In Table 4, by controlling the content Delta of delta
ferritic phase of a hot-rolled steel sheet casted by strip casting
to 5 wt % or more, austenitic stainless steel was manufactured.
[0073] Then, cold-rolling was performed at a total reduction ratio
of 50% to manufacture an austenitic stainless steel pipe. The
austenitic stainless steel pipe was bent 90 degrees using a hand
bender, and then surface roughness (orange peel) of the bent
portion was measured. The hand bender was used to apply deformation
with a uniform radius of curvature, and another apparatus may be
used instead of the hand blender. Also, the angle of 90 degrees was
used to apply uniform deformation, and the characteristics of the
present disclosure may be obtained although another angle is
used.
[0074] FIG. 2 is a cross-sectional view for describing an
austenitic stainless cold-rolled sheet according to an embodiment
of the present disclosure. FIG. 3 is a picture showing an
austenitic stainless steel pipe according to an embodiment of the
present disclosure, bent 90 degrees.
[0075] An average crystal grain size Gs of surface crystal grains
included in a first area A1 corresponding to a depth of 10% or less
of a total thickness of austenitic stainless steel 100 from the
surface of the austenitic stainless steel 100, and an average
crystal grain size Gi of internal crystal grains included in a
second area A2 corresponding to a depth that is deeper than 10% of
the total thickness of the austenitic stainless steel 100 from the
surface of the austenitic stainless steel 100 were measured, and a
ratio Gs/Gi of the average crystal grain size Gs with respect to
the average crystal grain size Gi was represented.
TABLE-US-00004 TABLE 4 Surface Grain size Existence or Absence of
(Gs, .mu.m) Orange Peel Embodiment 1 35 .largecircle. Embodiment 2
50 .largecircle. Embodiment 3 69 .largecircle. Embodiment 4 95
.largecircle. Comparative 110 X Example 1 Comparative 150 X Example
2 Comparative 170 X Example 3 Comparative 250 X Example 4
[0076] As shown in the Embodiments of Table 4, in a pipe having a
surface grain size of 100 .mu.m or smaller, excellent orange peel
appeared, and as shown in the Comparative Examples of Table 4, in a
pipe having a surface grain size that is larger than 100 .mu.m,
orange peel appeared significantly.
[0077] That is, orange peel appeared in the pipe having the surface
grain size that is larger than 100 .mu.m. Therefore, by maintaining
the surface grain size Gs at 100 .mu.m or smaller, and increasing
the size of the internal crystal grains Gi, the strength of the
stainless steel may be reduced through an increase in size of the
crystal grains of the stainless steel.
[0078] For example, the average crystal grain size Gi of the
internal crystal grains of the austenitic stainless steel may be
larger than 100 .mu.m.
TABLE-US-00005 TABLE 5 Surface Internal Existence or Grain size
Grain size Absence of (Gs, .mu.m) (Gi, .mu.m) Gs/Gi Orange Peel
Embodiment 1 35 101 0.35 .largecircle. Embodiment 2 50 152 0.33
.largecircle. Embodiment 3 69 169 0.41 .largecircle. Embodiment 4
95 210 0.45 .largecircle. Comparative 110 102 1.08 X Example 1
Comparative 150 153 0.98 X Example 2 Comparative 170 260 0.65 X
Example 3 Comparative 250 201 1.24 X Example 4
[0079] If the surface grain size Gs is 100 .mu.m or smaller as in
the Embodiments of Table 5, and the ratio Gs/Gi of the surface
grain size Gs to the internal grain size Gi is 0.5 or smaller,
excellent orange peel resistance appeared.
[0080] FIG. 4 is a picture showing the surface of an austenitic
stainless steel pipe of FIG. 1. FIG. 5 is a picture showing the
surface of a typical austenitic stainless steel pipe. FIG. 6 is a
picture showing a section microstructure of the austenitic
stainless steel pipe of FIG. 1. FIG. 7 is a picture showing a
section microstructure of the typical austenitic stainless steel
pipe.
[0081] More specifically, FIGS. 4 and 6 show the surface and
section microstructure of an austenitic stainless steel pipe
according to the Embodiment 3, and FIGS. 5 and 7 show the surface
and section microstructure of an austenitic stainless steel pipe
according to the Comparative Example 3.
[0082] Referring to Table 4, Table 5, and FIGS. 5 to 7, it can be
seen that no orange peel is generated on the surface of the
austenitic stainless steel pipe according to an embodiment of the
present disclosure, the size of crystal grains adjacent to the
surface of the stainless steel is smaller than that of the other
crystal grains since in the section microstructure, crystal grains
of an area adjacent to the surface of the stainless steel have an
average size of 100 .mu.m or smaller, and crystal grains of the
inside of the stainless steel have an average size that is larger
than 100 .mu.m. Accordingly, the size of the crystal grains of the
area adjacent to the surface of the stainless steel is smaller than
that of the crystal grains of the other area. Also, in the surface
of the austenitic stainless steel pipe according to the Comparison
Example, orange peel was generated so that the outer appearance
deteriorates, and in the section microstructure, the size of
crystal grains of an area adjacent to the surface of the stainless
steel was a little smaller than that of internal crystal grains of
the stainless steel, wherein the crystal grains of the area
adjacent to the surface of the stainless steel averagely have a
size that is larger than 100 .mu.m.
[0083] FIG. 8 is a graph for describing the size of a crystal grain
of austenitic stainless steel according to an embodiment of the
present disclosure and surface roughness when an austenitic
stainless steel pipe is bent 90 degrees.
[0084] Referring to FIG. 8, it can be seen that by limiting the
size Gs of surface crystal grains, a stainless steel pipe capable
of suppressing orange peel while increasing an average crystal
grain size of the stainless steel in order to reduce the strength
of the stainless steel is manufactured. That is, it can be seen
from FIG. 8 that in the Comparative Examples, the surface roughness
Rz exceeds 10 .mu.m so that orange peel is generated, while in the
Embodiments, the surface roughness Rz is 10 .mu.m or smaller so
that orange peel is suppressed although the size Gi of internal
crystal grains increases like the Comparative Examples.
[0085] More specifically, as shown in FIG. 8, Embodiment in which
the average crystal grain size Gi of internal crystal grains is 100
.mu.m corresponds to an austenitic stainless steel pipe
manufactured according to the Embodiment 1 in which the surface
roughness Rz is about 4.5 .mu.m so that no orange peel is
generated, Embodiment in which the average crystal grain size Gi of
internal crystal grains is 150 .mu.m corresponds to an austenitic
stainless steel pipe manufactured according to the Embodiment 2 in
which the surface roughness Rz is about 4.8 .mu.m so that no orange
peel is generated, and Embodiment in which the average crystal
grain size Gi of the internal crystal grains is 200 .mu.m
corresponds to an austenitic stainless steel pipe manufactured
according to the Embodiment 4 in which the surface roughness Rz is
about 5.2 .mu.m so that no orange peel is generated.
[0086] Unlike this, Comparative Example in which the average
crystal grain size Gi of internal crystal grains is 100 .mu.m
corresponds to an austenitic stainless steel pipe manufactured
according to the Comparative Example 1 in which the surface
roughness Rz is about 12 .mu.m so that orange peel is generated,
Comparative Example in which the average crystal grain size Gi of
internal crystal grains is 150 .mu.m corresponds to an austenitic
stainless steel pipe manufactured according to the Comparison
Example 2 in which the surface roughness Rz is about 19 .mu.m so
that orange peel is generated, and Embodiment in which the average
crystal grain size Gi of the internal crystal grains is 200 .mu.m
corresponds to an austenitic stainless steel pipe manufactured
according to the Embodiment 4 in which the surface roughness Rz is
about 22 .mu.m so that orange peel is generated.
[0087] While the present disclosure has been particularly described
with reference to exemplary embodiments, it should be understood by
those of skilled in the art that various changes in form and
details may be made without departing from the spirit and scope of
the present disclosure.
INDUSTRIAL APPLICABILITY
[0088] The austenitic stainless steel according to the embodiments
of the present disclosure may be applied to refrigerant pipes for
household or car air conditioners.
* * * * *