U.S. patent application number 15/765031 was filed with the patent office on 2018-10-25 for stainless steel pipe with excellent corrosion resistance and manufacturing method thereof.
The applicant listed for this patent is NISSHIN STEEL CO., LTD.. Invention is credited to Kazunari IMAKAWA, Tomoaki SAIDA, Yoshikazu TAI.
Application Number | 20180304433 15/765031 |
Document ID | / |
Family ID | 58487533 |
Filed Date | 2018-10-25 |
United States Patent
Application |
20180304433 |
Kind Code |
A1 |
SAIDA; Tomoaki ; et
al. |
October 25, 2018 |
STAINLESS STEEL PIPE WITH EXCELLENT CORROSION RESISTANCE AND
MANUFACTURING METHOD THEREOF
Abstract
A stainless steel pipe may be provided which exhibits excellent
corrosion resistance so as not to rust at an early stage even in a
waterfront environment affected by sea salt particles. The
stainless steel pipe may have a polishing mark on a surface, an
oxide film exhibiting color is not present on the surface, and an
average number of surface defects including covering by a metal
base of 5 .mu.m or more on the surface is suppressed to 5 or fewer
per 0.01 mm.sup.2.
Inventors: |
SAIDA; Tomoaki; (Chiyoda-ku
Tokyo, JP) ; TAI; Yoshikazu; (Chiyoda-ku Tokyo,
JP) ; IMAKAWA; Kazunari; (Chiyoda-ku Tokyo,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NISSHIN STEEL CO., LTD. |
Chiyoda-ku Tokyo |
|
JP |
|
|
Family ID: |
58487533 |
Appl. No.: |
15/765031 |
Filed: |
September 6, 2016 |
PCT Filed: |
September 6, 2016 |
PCT NO: |
PCT/JP2016/076142 |
371 Date: |
March 30, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/58 20130101;
C22C 38/00 20130101; B24B 29/08 20130101 |
International
Class: |
B24B 29/08 20060101
B24B029/08 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 5, 2015 |
JP |
2015-197977 |
Claims
1. A stainless steel pipe exhibiting excellent corrosion
resistance, wherein the stainless steel pipe has a polishing mark
on a surface, an oxide film exhibiting color is not present on the
surface, and an average number of surface defects including
covering by a metal base of 5 .mu.m or more on the surface is
suppressed to 5 or fewer per 0.01 mm.sup.2.
2. A method of manufacturing the stainless steel pipe according to
claim 1, the method comprising a polishing step of polishing a
surface of a stainless steel pipe with a solid polishing agent.
3. The manufacturing method according to claim 2, wherein the
surface of the stainless steel pipe is polished by attaching the
solid polishing agent to a polishing flap wheel in the polishing
step.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is the U.S. national stage of application No.
PCT/JP2016/076142, filed on Sep. 6, 2016. Priority under 35 U.S.C.
.sctn. 119(a) and 35 U.S.C. .sctn. 365(b) is claimed from Japanese
Application No. 2015-197977, filed on Oct. 5, 2015, the disclosure
of which is also incorporated herein by reference.
TECHNICAL FIELD
[0002] The present invention relates to a stainless steel pipe
exhibiting excellent corrosion resistance and a method of
manufacturing the same.
BACKGROUND ART
[0003] Stainless steel is widely used in building material
applications such as roofing materials, wall materials, and
building components since it exhibits excellent weather resistance,
processability, weldability, and the like. In addition, a stainless
steel pipe is used in applications such as handrails, fences, and
pipe shutters after being subjected to surface polishing since it
exhibits excellent designing property as well.
[0004] In the general industrial polishing of this stainless steel
pipe, scratch-removing polishing is first conducted in order to
remove scratches and the like on the original pipe before being
polished and then finish polishing, glossy polishing, and the like
are conducted. Dry polishing using a flap wheel, a polishing belt,
or the like is conducted in the rough polishing and finish
polishing in this polishing operation. Furthermore, there is a case
in which wet polishing by buffing is conducted after the above
process in order to obtain a desired surface.
[0005] Conventionally, stainless steel exhibits excellent weather
resistance as a material, but there is a case in which the inherent
weather resistance of the material is not exerted and remarkable
rusting occurs depending on the state of polishing finish, and this
is one of the factors to eliminate the stability (reliability) of
the weather resistance of stainless steel. For example, there is a
case in which rusting occurs in a short period of about one month
after stainless steel is constructed into outdoor handrails and the
like.
[0006] It is considered that the oxide film and polishing marks
remaining on the surface of the stainless steel pipe after being
polished are the starting points of rusting. The remaining oxide
film is a film formed due to heat generation at the time of
polishing and a Cr-depleted layer is formed right under the oxide
film. Hence, rusting proceeds from the oxide film and the
Cr-depleted layer right under the oxide film and corrosion
resistance is likely to deteriorate when the oxide film remains. In
addition, with regard to the polishing marks which are scratches
engraved on the surface of the stainless steel pipe by polishing as
well, the possibility that it is difficult to remove the oxide film
formed by polishing using a flap wheel or the like by buffing and
the oxide film remains is higher as the concave portion of the
polishing marks is deeper, and rusting proceeds and corrosion
resistance is likely to deteriorate since the concave portion of
the polishing marks becomes a starting point of rusting.
[0007] Patent Document 1 proposes a stainless steel pipe capable of
maintaining glossiness and weather resistance for a long period by
polishing the surface to be in a state in which rusting does not
occur in a short period even in an outdoor environment.
[0008] Patent Document 1: Japanese Unexamined Patent Application,
Publication No. 2003-56755
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0009] The invention described in Patent Document 1 is a stainless
steel pipe having a surface roughness after the final polishing of
Ry 0.6 .mu.m or less and an area ratio of the remaining oxide film
of 7.0% or less. In other words, it is intended to decrease the
oxide films remaining in the concave portions of the polishing
marks by setting the surface roughness after final polishing to Ry
0.6 .mu.m or less. In addition, it is intended to suppress progress
of rusting starting from the oxide film and the Cr-depleted layer
right under the oxide film and deterioration in corrosion
resistance by setting the area ratio of the remaining oxide film to
7.0% or less.
[0010] Here, referring to Examples of Patent Document 1, the area
ratio of the remaining oxide film is from 3.1 to 6.8% and the oxide
film thus remains in a weather resistance-passed product. Hence,
there is still a problem that rusting may proceed from the
remaining oxide film and the Cr-depleted layer right under the
oxide film and the corrosion resistance may deteriorate.
[0011] Furthermore, the demand for construction has increased in
association with the redevelopment of urban areas and the demand
for construction in the waterfront environment has increased in
recent years. In the waterfront environment, there is a problem
that building components are likely to be affected by sea salt
particles which are a kind of aerosol particles contained in the
atmosphere and are fine particles composed of salts derived from
seawater. Hence, needs for highly corrosion resistant building
components have increased.
[0012] In Patent Document 1, SUS304 is mentioned as one steel type
of stainless steel pipes exhibiting excellent weather resistance.
However, SUS304 has a problem that it rusts at an early stage and
requires maintenance in a waterfront environment affected by sea
salt particles.
[0013] The present invention has been made to solve the problem
described above and an object thereof is to provide a stainless
steel pipe which exhibits excellent corrosion resistance so as not
to rust at an early stage even in a waterfront environment affected
by sea salt particles and a method of manufacturing the same.
Means for Solving the Problems
[0014] The present inventors have carried out investigations on the
stainless steel pipe described in Patent Document 1. In Examples of
Patent Document 1, dry polishing using a flap wheel is conducted.
The oxide film on the surface of the stainless steel pipe of
Examples of Patent Document 1 using this polishing method remains
at an area ratio of 3.1% or more. As a result of investigations on
the factors of this, it has been found out that the temperature of
the surface of the stainless steel pipe increases high and an oxide
film is formed at the time of polishing using a flap wheel, which
is a dry polishing and surface defects are caused together with
polishing marks which are scratches engraved by high polishing
resistance by dry polishing. The term "surface defect" as referred
to herein is a defect having a form in which the metal on the
surface partly peels off and covers the base portion as the
polishing material and polishing paper are continuously brought
into contact with the surface of the steel pipe and polished when
polishing the surface of the steel pipe, and it is referred to as
"burr" or "covering". A surface defect includes portions at which
the metal is turned up as a strip shape or a bamboo leaf shape, and
it is a defect having a maximum length from one end portion of the
portion bonded to the base to the other end portion at the peeling
tip of 5 .mu.m or more. The surface defect forms a microgap with
the surface base portion of the stainless steel pipe, and thus
crevice corrosion is likely to occur and a decrease in corrosion
resistance of the steel pipe is caused.
[0015] The present inventors have found out a stainless steel pipe
exhibiting excellent corrosion resistance and a method of
manufacturing the same based on the analysis results.
[0016] In other words, the present invention provides a stainless
steel pipe exhibiting excellent corrosion resistance and a method
of manufacturing the same of the following (1) to (3).
(1) A stainless steel pipe exhibiting excellent corrosion
resistance, in which the stainless steel pipe has a polishing mark
on a surface, an oxide film exhibiting color is not present on the
surface, and an average number of surface defects including
covering by a metal base of 5 .mu.m or more on the surface is
suppressed to 5 or fewer per 0.01 mm.sup.2.
[0017] The stainless steel pipe of the present invention exhibits
excellent design property and antiglare property since it has
polishing marks on the surface thereof. In addition, rusting
starting from the oxide film and the Cr-depleted layer right under
the oxide film hardly proceeds and the corrosion resistance hardly
deteriorates since the oxide film exhibiting color is not present
on the surface of the stainless steel pipe. Furthermore, crevice
corrosion is suppressed and a stainless steel pipe exhibiting
excellent corrosion resistance is obtained since the average number
of surface defects including covering by the metal base of 5 .mu.m
or more on the surface of the stainless steel pipe is suppressed to
5 or fewer per 0.01 mm.sup.2.
(2) A method of manufacturing the stainless steel pipe of (1), the
method including a polishing step of polishing a surface of a
stainless steel pipe with a solid polishing agent. (3) The
manufacturing method of (2), in which the surface of the stainless
steel pipe is polished by attaching the solid polishing agent to a
polishing flap wheel in the polishing step.
Effects of the Invention
[0018] According to the present invention, it is possible to
provide a stainless steel pipe which exhibits excellent corrosion
resistance so as not to rust at an early stage even in a waterfront
environment affected by sea salt particles and a method of
manufacturing the same.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a photograph of the surface enlarged using an
optical microscope of a stainless steel pipe, FIG. 1A is a surface
on which the generation of surface defects is suppressed, and FIG.
1B is a surface having surface defects.
[0020] FIG. 2 is a view illustrating the relationship between
surface defects and a change in current density, FIG. 2A is an
enlarged photograph illustrating surface defects of a stainless
steel pipe, and FIG. 2B is a graph illustrating a change in current
density in pitting potential measurement.
[0021] FIG. 3 is a view illustrating the relationship between
surface defects and a change in current density, FIG. 3A is an
enlarged photograph illustrating the surface of a stainless steel
pipe on which the generation of surface defects is suppressed, and
FIG. 3B is a graph illustrating a change in current density in
pitting potential measurement.
[0022] FIG. 4 is a photograph of the surface enlarged using an
optical microscope of the stainless steel pipe of Comparative
Example 2.
[0023] FIG. 5 is a photograph of the surface enlarged using an
optical microscope of the stainless steel pipe of Reference Example
1.
[0024] FIG. 6 is a photograph of the surface of a stainless steel
pipe after a CCT test, FIG. 6A illustrates the surface of Example
1, and FIG. 6B illustrates the surface of Comparative Example
1.
PREFERRED MODE FOR CARRYING OUT THE INVENTION
[0025] Hereinafter, embodiments for carrying out the present
invention will be described. It should be noted that the present
invention is not to be interpreted restrictively by the
embodiments.
(Stainless Steel Pipe)
[0026] The stainless steel pipe of the present invention is a
stainless steel pipe exhibiting excellent corrosion resistance
since it has a polishing mark on the surface, an oxide film
exhibiting color is not present on the surface, and an average
number of surface defects including covering by the metal base of 5
.mu.m or more on the surface is suppressed to 5 or fewer per 0.01
mm.sup.2.
[0027] In the present invention, the surface of the stainless steel
pipe is subjected to polishing finish so as to impart unevenness
and gloss to the surface. By this, the stainless steel pipe becomes
a stainless steel pipe which have polishing marks and exhibits
excellent design property and antiglare property. The polishing
marks are scratches engraved on the surface of the stainless steel
pipe by polishing.
[0028] With regard to the polishing marks on the surface after
polishing, the possibility that the oxide film formed by polishing
using a flap wheel or the like remains is higher as the concave
portion of the polishing marks is deeper, and rusting proceeds and
corrosion resistance is likely to deteriorate since the concave
portion of the polishing marks becomes a starting point of rusting.
Accordingly, the surface roughness Ra of the surface of the
stainless steel pipe in the present invention after polishing is
preferably from 0.1 to 1.0 .mu.m and more preferably from 0.2 to
0.5 .mu.m. The surface roughness after polishing is measured in
conformity to JIS B0601, and it can be measured using a contact
type surface roughness tester, for example.
[0029] As polishing finish, dry polishing using a flap wheel or the
like has been conventionally conducted, but the temperature of the
surface of the stainless steel pipe increases high and an oxide
film is formed when dry polishing is conducted. Meanwhile, the
stainless steel pipe of the present invention is characterized in
that an oxide film exhibiting color is not present on the surface.
The present inventors believe that the reason for this is because
the oxide film on the surface is removed as the stainless steel
pipe of the present invention is polished with a solid polishing
agent. In addition, the formation of oxide film is further
suppressed as the solid polishing agent is attached to the
polishing flap wheel.
[0030] In the present invention, the fact that an oxide film
exhibiting color is present refers to a case in which an oxide film
which is a spot-like substance exhibiting color is present at an
area ratio of 5% or more in 50 .mu.m square when arbitrary 10
points on the surface of the stainless steel pipe are observed at a
magnification of 400-fold using an optical microscope. Here, the
color is not limited to a specific one, and it may be a color which
can be visually distinguished from the metal base or metallic
luster of the stainless steel pipe. A typical color as the color is
dark reddish-brown.
[0031] In addition, when dry polishing using a flap wheel or the
like is conducted as polishing finish, the polishing material and
the polishing paper are continuously brought into contact with the
surface of the stainless steel pipe and a surface defect which is
burr or covering that the metal constituting the surface partly
peels off and covers the base portion is caused. The surface defect
is a factor of crevice corrosion since it forms a microgap with the
surface base portion of the stainless steel pipe.
[0032] FIG. 1 is a photograph of the surface enlarged using an
optical microscope of the stainless steel pipe, FIG. 1A is a
surface on which the generation of surface defects is suppressed,
and FIG. 1B is a surface having surface defects. FIG. 1A is the
surface of the stainless steel pipe of the present invention, which
has polishing marks but on which the generation of surface defects
is suppressed. Meanwhile, FIG. 1B is a dry polished surface of a
stainless steel pipe, and surrounded portions 1 to 9 represent
surface defects that the metal constituting the surface partly
peels off and covers the base portion. The present inventors have
analyzed the reason why the generation of surface defects is
suppressed on the surface of the stainless steel pipe of the
present invention after polishing as illustrated in FIG. 1A is
because a solid polishing agent is used at the time of polishing.
In addition, the generation of surface defects is further
suppressed by attaching the solid polishing agent to the polishing
flap wheel. Incidentally, the white horizontal line in FIG. 1
represents a convex portion formed during polishing, and the
concave portion between the white horizontal line which is a convex
portion and the white horizontal line adjacent thereto is a
polishing mark.
[0033] In the present invention, the surface defects refer to those
having covering by the metal base in which the maximum length
portion of the defect has a size of 5 .mu.m or more. In addition,
the surface is defined to be in a state in which the generation of
surface defects is suppressed in the present invention in a case in
which the average number of measured surface defects is 5 or fewer
when a range of 100 .mu.m.times.100 .mu.m (0.01 mm.sup.2) at
arbitrary 10 points on the polished surface of a stainless steel
pipe is enlarged by 200-fold and observed using an optical
microscope. The number of surface defects on the polished surface
of the stainless steel pipe is more preferably 3 or fewer and still
more preferably 2 or fewer per unit area of 100 .mu.m.times.100
.mu.m (0.01 mm.sup.2). Incidentally, there is no upper limit to the
maximum length portion of surface defects, but the upper limit may
be set to 50 .mu.m as a reference value in the measurement.
[0034] FIG. 2 and FIG. 3 are views illustrating the relationship
between surface defects and a change in current density, FIG. 2A is
an enlarged photograph illustrating surface defects of a stainless
steel pipe, FIG. 3A is an enlarged photograph illustrating the
surface of a stainless steel pipe on which the generation of
surface defects is suppressed, and FIG. 2B and FIG. 3B are graphs
illustrating a change in current density in pitting potential
measurement of the stainless steel pipes of FIG. 2A and FIG. 3A,
respectively.
[0035] The pitting potential measurement method of stainless steel
conforms to JIS G0577, and the method B is used. The method B is a
pitting potential measurement method by a potentiodynamic method in
a 3.5 mass % sodium chloride aqueous solution. The pH of the sodium
chloride aqueous solution is set to 7 and the temperature is set to
30.degree. C. In addition, the potential sweep rate is set to 20
mV/min.
[0036] As illustrated in FIG. 3A and FIG. 3B, in the case of a
stainless steel pipe having a surface on which the generation of
surface defects is suppressed, a change in the value of the current
density at a potential lower than the pitting potential is small
and the portion at which the rate of change (maximum current
density/minimum current density) in the current density in a range
of from the natural potential to the pitting potential, namely, a
potential range of from 0.1 to 0.5 (part B in FIG. 3B) is 10 or
more is not recognized in the change in current density in the
pitting potential measurement.
[0037] Meanwhile, as illustrated in FIG. 2A and FIG. 2B, in the
case of a stainless steel pipe having surface defects, the change
in the value of the current density at a potential lower than the
pitting potential is great and the portion at which the rate of
change in the current density in a range of from the natural
potential to the pitting potential, namely, a potential range of
from 0.1 to 0.3 V (part A in FIG. 2B) exceeds 10 is at ten or more
potential positions in the change in current density in the pitting
potential measurement. This great change in current density is
caused by occurrence of corrosion. Therefore, the present inventors
presume that a great change in current density indicates the
occurrence of crevice corrosion caused by the presence of surface
defects. Accordingly, in the present invention, the portion at
which the rate of change (maximum current density/minimum current
density) in the current density in a range of from the natural
potential to the pitting potential is 10 or more is preferably at
fewer than ten potential positions and more preferably at five or
fewer places in the change in current density in pitting potential
measurement.
[0038] As a composition in the case of using ferritic stainless
steel as the material for the stainless steel pipe of the present
invention, for example, C is contained preferably at 0.02% by mass
or less since it tends to decrease corrosion resistance when being
contained in a great amount although it is an element useful for
obtaining the strength of steel. Si is contained preferably at
1.00% by mass or less since it tends to harden the steel when being
contained in a great amount although it is an element useful as a
deoxidizer and a heat source in the steelmaking process. Mn is
contained preferably at 2.00% by mass or less, and more preferably
at 1.00% by mass or less since it tends to form an austenite phase
when being contained in a great amount although it is an element
useful as deoxidation in the steelmaking process. Cr is contained
preferably at from 17.00% to 30.00% by mass and more preferably at
from 20.00% to 24.00% by mass since it tends to not only increase
the cost but also decrease the processability when being contained
in a great amount although it is an element useful for securing
corrosion resistance. Mo is contained preferably at from 1.00% to
20.50% by mass and more preferably at from 1.00% to 1.50% by mass
since it tends to not only increase the cost but also decrease the
processability when being contained in a great amount although it
is an element useful for improving the corrosion resistance of
stainless steel in the presence of Cr. It is more preferable as P
is contained in a smaller amount since it decreases corrosion
resistance, and P is contained preferably at 0.040% by mass or
less. It is more preferable as S is contained in a smaller amount
since it decreases corrosion resistance, and S is contained
preferably at 0.030% by mass or less. Ni is contained preferably at
0.6% by mass or less since it causes the formation of an austenite
phase and high cost when being contained too much although it is
preferred from the viewpoint of an effect of suppressing the
progress of corrosion and of being effective in improvement of
toughness of ferritic stainless steel pipes. With regard to Ti and
Nb, it is preferable to contain one kind or two kinds of these. Ti
is contained preferably at from 0.05% to 0.5% by mass since a large
amount of Ti content tends to decrease the surface quality of steel
although it is preferred from the viewpoint of having a strong
affinity for C and N and suppressing intergranular corrosion of
ferritic stainless steel pipes. Nb is contained preferably at from
0.1% to 0.6% by mass since a large amount of Nb content tends to
hinder toughness although it is preferred from the viewpoint of
having a strong affinity for C and N and suppressing intergranular
corrosion of ferritic stainless steel pipes. N is contained
preferably at 0.025% by mass or less since it, like C, tends to
decrease corrosion resistance when being contained in a great
amount. Al is contained preferably at from 0.01% to 0.50% by mass
since it decreases the weldability and low temperature toughness of
steel as well as deteriorates the surface quality when being
excessively added although it is an element effective in refining
and casting as a deoxidizer. The balance is preferably Fe and
inevitable impurities. In addition, for example, it is also
possible to use one containing C at 0.02% by mass or less, Si at
0.40% by mass or less, Mn at 0.40% by mass or less, Cr at from
21.00% to 23.00% by mass, Mo at from 1.00% to 1.50% by mass, P at
0.040% by mass or less, S at 0.030% by mass or less, Ni at 0.60% by
mass or less, Ti at from 0.05% to 0.5% by mass, Nb at from 0.10% to
0.6% by mass, N at 0.025% by mass or less, Al at 0.15% by mass or
less, and Fe as the balance as the stainless steel pipe of the
present invention.
[0039] As the material for the stainless steel pipe of the present
invention, a material having a pitting (corrosion) index (PI) of 20
or more is preferable. PI is determined by the following Equation
(1).
PI=Cr+3Mo Equation (1)
[0040] The stainless steel pipe of the present invention having a
pitting index (PI) of 20 or more exhibits excellent corrosion
resistance. Hence, rusting of the stainless steel pipe of the
present invention can be suppressed whereas SUS304 having a low
pitting index of 19 rusts at an early stage in a waterfront
environment affected by sea salt particles. The pitting index (PI)
is more preferably 24 or more and still more preferably 30 or more
from the viewpoint of corrosion resistance.
(Manufacturing Method)
[0041] The method of manufacturing a stainless steel pipe of the
present invention is a manufacturing method including a polishing
step of polishing the surface of a stainless steel pipe with a
solid polishing agent.
[0042] The solid polishing agent is not particularly limited and
can be used as long as it contains a fatty acid and mineral fat and
oil.
[0043] It is preferable that the solid polishing agent contains
oxides such as SiO.sub.2, Al.sub.2O.sub.3, and CrO.sub.2. The
content of oxides such as SiO.sub.2, Al.sub.2O.sub.3, and CrO.sub.2
is preferably from 50% to 80% by mass, more preferably from 55% to
75% by mass, and particularly preferably from 60% to 70% by
mass.
[0044] As the fatty acid, it is preferable to use stearic acid,
myristic acid, and the like. As the mineral fat and oil, it is
preferable to use palmitic acid and the like.
[0045] In the present method of manufacturing a stainless steel
pipe, it is preferable to polish the surface of the stainless steel
pipe using a polishing flap wheel to which the solid polishing
agent is attached in the polishing step.
[0046] As described above, when dry polishing using a flap wheel or
the like is conducted as polishing finish, the polishing material
and the polishing paper are continuously brought into contact with
the surface of the stainless steel pipe and a surface defect which
is burr or covering that the metal constituting the surface partly
peels off and covers the base portion is caused. Meanwhile, it is
preferable to conduct wet polishing by attaching the solid
polishing agent to the polishing flap wheel in the method of
manufacturing a stainless steel pipe of the present invention. This
makes it possible to decrease the polishing resistance even in a
case in which the polishing material and the polishing paper are
continuously brought into contact with the surface of the stainless
steel pipe, and the generation of a surface defect which is burr or
covering that the metal constituting the surface partly peels off
and covers the base portion is likely to be further suppressed.
[0047] Incidentally, the present invention is not limited by the
above embodiment. For example, buffing using a solid polishing
agent may be conducted after wet polishing is conducted by
attaching a solid polishing agent to a polishing flap wheel. In
addition, it is possible to manufacture a stainless steel pipe
which has random polishing marks on the surface of the stainless
steel pipe and on which generation of an oxide film exhibiting
color and surface defects is suppressed by manually conducting
polishing by the movement formed by combining an eccentric motion
and a rotational motion using a polishing apparatus (air sander) to
which a nonwoven fabric is attached after a solid polishing agent
is applied and wet polishing is conducted as well.
EXAMPLES
[0048] Piping and shape modification of a stainless steel pipe were
conducted, and polishing finish for decoration was conducted. The
following two types of stainless steel pipes were used. The
composition (mass %) and dimensions are as follows.
[0049] Steel Type 1 (SUS445J1) Cr: 22%, Mo: 1.05%, Ti: 0.2%, Nb:
0.2%, Al: 0.09%, and Fe: balance
[0050] Steel Type 2 (SUS304) Cr: 18%, Ni: 8%, Si: 0.6%, Mn: 0.8%,
and Fe: balance
[0051] Dimensions: 34 mm in diameter.times.1.5 mm in
thickness.times.4000 mm in length.
[0052] Polishing was conducted in Lines 1 to 4 as follows. In
addition, the polishing conditions are as follows.
[0053] Line 1 is a line in which five flap wheels (#240, #240,
#240, #400, and #600) are arranged so as to polish the surface of
steel pipe in the circumferential direction (to impart polishing
marks in the circumferential direction). Line 2 is a line in which
four flap wheels (#240, #240, #240, and #400) are arranged so as to
polish the surface of steel pipe in the longitudinal direction (to
impart polishing marks in the longitudinal direction). Line 3 is a
line in which four flap wheels (#150, #150, #150, and #320) are
arranged so as to polish the surface of steel pipe in the
longitudinal direction (to impart polishing marks in the
longitudinal direction). Line 4 is a line composed of three flap
wheels (#320, #400, and #600) arranged so as to polish the surface
of steel pipe in the longitudinal direction (to impart polishing
marks in the longitudinal direction) and two cotton buffs (#400 and
#400) arranged so as to polish the surface of steel pipe in the
circumferential direction (to impart polishing marks in the
circumferential direction). Here, a solid polishing agent was
applied to the flap wheel in Line 1 and Line 4. Meanwhile, a solid
polishing agent was not applied in Line 2 and Line 3. Incidentally,
"#240" and the like represent the mesh grading.
(Polishing Conditions)
[0054] Line speed: 1.8 m/min
[0055] Number of rotations of pipe: 380 rpm
[0056] Number of rotations of wheel: 1500 rpm
[0057] Wheel diameter: 400 mm
(Solid Polishing Agent)
[0058] The solid polishing agent had a SiO.sub.2 content of 75% by
mass, a content of stearic acid which was a fatty acid of 16% by
mass, and a content of palmitic acid which was a mineral fat and
oil of 3.8% by mass.
Example 1
[0059] Polishing of Steel Type 1 was conducted in Line 1 (applied
with a solid polishing agent).
Example 2
[0060] Polishing of Steel Type 1 was conducted in Line 3 (not
applied with a solid polishing agent) and then in Line 4 (applied
with a solid polishing agent). Thereafter, polishing to uniformly
adjust the random polishing marks was manually conducted by the
movement formed by combining an eccentric motion and a rotational
motion using a polishing apparatus (air sander) to which a nonwoven
fabric (#80) was attached without applying a solid polishing
agent.
Comparative Example 1
[0061] Polishing of Steel Type 1 was conducted in Line 2 (not
applied with a solid polishing agent).
Comparative Example 2
[0062] Polishing of Steel Type 2 was conducted in Line 2 (not
applied with a solid polishing agent).
Reference Example 1
[0063] Polishing of Steel Type 2 was conducted in Line 1 (applied
with a solid polishing agent).
(Surface Defects)
[0064] The polished surface of a stainless steel pipe was enlarged
by 200-fold and observed in a range of 100 .mu.m.times.100 .mu.m
(0.01 mm.sup.2) using an optical microscope. The surface was
evaluated as ".largecircle." to be in a state in which the
generation of surface defects was suppressed in a case in which the
number of surface defects having covering by the metal base of 5
.mu.m or more was 5 or fewer, and the surface was evaluated as "x"
to be in a state in which the generation of surface defects was
suppressed in a case in which the number of surface defects was
more than 5 (see Table 1).
[0065] As presented in Table 1, the surface of the stainless steel
pipe of Example 1 did not have surface defects as illustrated in
FIG. 1A. Meanwhile, the surface of the stainless steel pipe of
Comparative Example 1 had at least nine surface defects as
illustrated in FIG. 1B, and thus it was not in a state in which the
generation of surface defects was suppressed. In addition, the
surface of the stainless steel pipe of Comparative Example 2 had at
least 6 surface defects as illustrated in FIG. 4, and thus it was
not in a state in which the generation of surface defects was
suppressed. Incidentally, in Reference Example 1, there was no
surface defect as illustrated in FIG. 5.
(Oxide Film)
[0066] The surface of the stainless steel pipe was observed at a
magnification of 400-fold using an optical microscope, and to what
extent an oxide film which was a spot-like substance exhibiting
dark reddish-brown was present in 50 .mu.m square as an area ratio
was calculated. The surface was evaluated as ".largecircle. (Good)"
not to have an oxide film exhibiting color in a case in which the
area ratio of the remaining oxide film was 3% or more and less than
5%, the surface was evaluated as ".circle-w/dot. (Excellent)" in
the case of a more preferred state in which the area ratio of the
remaining oxide film was less than 3%, and the surface was
evaluated as "x (Failure)" to have an oxide film exhibiting color
in a case in which the area ratio was 5% or more (see Table 1).
[0067] As presented in Table 1, the area ratio of the oxide film
was 1% or less in Example 1, the area ratio of the oxide film was
3% in Example 2, and an oxide film exhibiting color was not thus
present on the surface of the stainless steel pipes. Meanwhile, the
area ratio of the oxide film was 15% and 20% in Comparative
Examples 1 and 2, respectively and an oxide film exhibiting color
was thus present on the surface of the stainless steel pipes.
Incidentally, the area ratio of the oxide film was 2% and an oxide
film exhibiting color was not thus present on the surface of the
stainless steel pipe in Reference Example 1.
(Corrosion Resistance Test)
[0068] The stainless steel pipes of Examples 1 and 2, Comparative
Examples 1 and 2, and Reference Example 1 were subjected to a
corrosion resistance test (salt-dry-wet combined cyclic corrosion
test (CCT test)) under the following conditions. Conditions: (1)
Salt water spray (35.degree. C., 5% NaCl, 15 minutes)
(2) Drying (60.degree. C., 30% RH, 60 minutes) (3) Wetting
(50.degree. C., 95% RH, 3 hours) The above conditions (1) to (3)
constituted one cycle, and the cycle was repeatedly conducted by 30
cycles. Evaluation: the stainless steel pipe was evaluated as
".largecircle. (Good)" to exhibit good corrosion resistance when
the rusting area after the test was within 5% of the entire surface
of the steel pipe, as ".DELTA. (Passing)" when the rusting area was
more than 5% and 15% or less, and as "x (Failure)" to exhibit poor
corrosion resistance when the rusting area was more than 15% (see
Table 1).
[0069] The surface photographs after the CCT test of Example 1 and
Comparative Example 1 are illustrated in FIG. 6. In Example 1, it
has been indicated that rusting has not occurred on the surface
even after the CCT test as illustrated in FIG. 6A and the corrosion
resistance is excellent. Meanwhile, in Comparative Example 1,
rusting has occurred on the surface after the CCT test as
illustrated in FIG. 6B and the corrosion resistance is inferior.
Incidentally, in Reference Example 1, the corrosion resistance is
.DELTA. since the corrosion resistant level of the base material
itself is low. The corrosion resistant level of the base material
in a waterfront environment affected by sea salt particles is
preferably 24 or more as a pitting index (PI).
TABLE-US-00001 TABLE 1 Corrosion Surface defects Oxide film
resistance Example1 .largecircle. .circle-w/dot. .largecircle.
Example2 .largecircle. .largecircle. .largecircle. Comparative
Example1 X X X Comparative Example2 X X X Reference Example1
.largecircle. .circle-w/dot. .DELTA.
EXPLANATION OF REFERENCE NUMERALS
[0070] 1 to 9 . . . Surface defect [0071] A and B . . . Region
having change in current density
* * * * *