U.S. patent application number 12/671300 was filed with the patent office on 2010-08-12 for al-plated steel sheet for exhaust gas passageway members of motorcycles excellent in high-temperature strength and the members.
Invention is credited to Yasunori Hattori, Yoshiaki Hori, Sadayuki Nakamura, Manabu Oku, Shin Ueno.
Application Number | 20100203357 12/671300 |
Document ID | / |
Family ID | 40304469 |
Filed Date | 2010-08-12 |
United States Patent
Application |
20100203357 |
Kind Code |
A1 |
Hori; Yoshiaki ; et
al. |
August 12, 2010 |
AL-PLATED STEEL SHEET FOR EXHAUST GAS PASSAGEWAY MEMBERS OF
MOTORCYCLES EXCELLENT IN HIGH-TEMPERATURE STRENGTH AND THE
MEMBERS
Abstract
Provided is an Al-plated steel sheet for motorcycle exhaust gas
passageway members excellent in high-temperature strength and red
scale resistance, which is produced by dipping a substrate steel
sheet comprising, in terms of % by mass, at most 0.02% of C, at
most 2% of Si, at most 2% of Mn, from 5 to 25% of Cr, from more
than 0.1 to 1% of Nb, at most 0.3% of Ti, at most 0.02% of N, and
optionally at least one of at most 0.6% of Ni, at most 0.2% of Al,
at most 3% of Mo, at most 3% of Cu, at most 3% of W, at most 0.5%
of V, at most 0.5% of Co and at most 0.01% of B, with a balance of
Fe and inevitable impurities, in a hot-dip Al-base plating bath to
thereby form a hot-dip plating layer having a mean thickness of
from 3 to 20 .mu.m on the surface thereof.
Inventors: |
Hori; Yoshiaki; (Shunan-shi,
JP) ; Nakamura; Sadayuki; (Shunan-shi, JP) ;
Oku; Manabu; (Shunan-shi, JP) ; Ueno; Shin;
(Sakai-shi, JP) ; Hattori; Yasunori; (Sakai-shi,
JP) |
Correspondence
Address: |
CLARK & BRODY
1700 Diagonal Road, Suite 510
Alexandria
VA
22314
US
|
Family ID: |
40304469 |
Appl. No.: |
12/671300 |
Filed: |
July 29, 2008 |
PCT Filed: |
July 29, 2008 |
PCT NO: |
PCT/JP2008/063939 |
371 Date: |
January 29, 2010 |
Current U.S.
Class: |
428/653 |
Current CPC
Class: |
C23C 2/12 20130101; C22C
21/02 20130101; Y10T 428/12757 20150115; C23C 2/20 20130101; F01N
2590/04 20130101; C22C 38/26 20130101; F01N 13/16 20130101 |
Class at
Publication: |
428/653 |
International
Class: |
C23C 2/12 20060101
C23C002/12 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 31, 2007 |
JP |
2007-198903 |
Claims
1. An Al-plated steel sheet for motorcycle exhaust gas passageway
members excellent in red scale resistance, which is produced by
dipping a substrate steel sheet comprising, in terms of % by mass,
at most 0.02% of C, at most 2% of Si, at most 2% of Mn, from 5 to
25% of Cr, from more than 0.1 to 1% of Nb, at most 0.3% of Ti and
at most 0.02% of N with a balance of Fe and inevitable impurities,
in a hot-dip plating bath containing, in terms of % by mass, from 3
to 12% of Si with a balance of Al and inevitable impurities, then
pulling it up, and controlling the plating amount to thereby form a
plating layer having a mean thickness of from 3 to 20 .mu.m on the
surface thereof.
2. The Al-plated steel sheet for motorcycle exhaust gas passageway
members excellent in red scale resistance as claimed in claim 1,
wherein the steel sheet of the substrate further contains at least
one of at most 0.6% of Ni, at most 0.2% of Al, at most 3% of Mo, at
most 3% of Cu, at most 3% of W, at most 0.5% of V, at most 0.5% of
Co and at most 0.01% of B.
3. The Al-plated steel sheet for motorcycle exhaust gas passageway
members excellent in red scale resistance as claimed in claim 1,
wherein the hot-dip plating bath further contains at least one of
Ti, B, Sr, Cr, Mn, Mg and Zr in a total amount of at most 1%.
4. A motorcycle exhaust gas passageway member which is formed of
the plated steel sheet of claim 1 as the constitutive material
thereof and which is so designed that the plating layer thereof is
kept in contact with exhaust gas and that the maximum service
temperature is 400.degree. C. or higher.
5. The Al-plated steel sheet for motorcycle exhaust gas passageway
members excellent in red scale resistance as claimed in claim 2,
wherein the hot-dip plating bath further contains at least one of
Ti, B, Sr, Cr, Mn, Mg and Zr in a total amount of at most 1%.
6. A motorcycle exhaust gas passageway member which is formed of
the plated steel sheet of claim 2 as the constitutive material
thereof and which is so designed that the plating layer thereof is
kept in contact with exhaust gas and that the maximum service
temperature is 400.degree. C. or higher.
7. A motorcycle exhaust gas passageway member which is formed of
the plated steel sheet of claim 3 as the constitutive material
thereof and which is so designed that the plating layer thereof is
kept in contact with exhaust gas and that the maximum service
temperature is 400.degree. C. or higher.
Description
TECHNICAL FIELD
[0001] The present invention relates to an aluminium-plated steel
sheet excellent in high-temperature strength and red scale
resistance for use in motorcycle engine exhaust gas passageway
members (e.g., mufflers, catalyst carriers, exhaust pipes, etc.),
and to a motorcycle exhaust gas passageway member comprising
it.
PRIOR ART
[0002] The engine exhaust gas passageway in motorcycles is shorter
than that in four-wheel vehicles, and not unusually, even the
downstream members such as mufflers and others may be often exposed
to high temperatures not lower than 400.degree. C. With recent
reinforcement for exhaust gas regulation, an exhaust gas
purification device comprising a ternary catalyst is being built
also in motorcycles; and in motorcycles, the purification device is
often incorporated in the muffler therein. Heat-resistant ferritic
stainless steel such as SUH409L, SUS436 or the like is used for
motorcycle exhaust gas passageway members from the viewpoint of
heat resistance.
[0003] Exhaust gas passageway members comprising stainless steel
may often have red-brown scale (hereinafter referred to as "red
scale"). This is a phenomenon often occurring in a low-oxygen
high-moisture atmosphere at an exhaust gas temperature of from 400
to 700.degree. C. When red scale has formed and when it is mixed
with dew condensation water, then it gives a red-brown liquid. The
dew condensation water in an exhaust gas passageway may be
discharged out through an exhaust port along with exhaust gas
therethrough, but it is extremely unfavorable to discharge the
above-mentioned red-brown liquid in the appearance.
[0004] Of exhaust gas passageway members, a muffler is a site where
dew condensation water may readily remain. In ordinary four-wheel
vehicles, the muffler is rarely exposed to exhaust gas at
400.degree. C. or higher, and the corrosion caused by internal dew
condensation water is often problematic rather than the red scale
to form inside the muffler. However, in motorcycles, even the
muffler may be frequently exposed to 400.degree. C. or higher as so
mentioned in the above, and therefore red scale forms inside the
muffler and the release of a red-brown liquid caused by it is often
problematic.
[0005] Patent Reference 1 discloses a technique of inhibiting red
scale formation in stove combustion cylinders and others by
previously forming a Cr oxide-base oxide film on the surface of the
structure. However, the oxide film is poor in corrosion resistance
and therefore requires a countermeasure to enhance the corrosion
resistance of the steel base, which brings about the increase in
the material cost. Accordingly, the technique is difficult to apply
to exhaust gas passageway members. Patent References 2 and 3
disclose a technique of inhibiting red scale formation by the use
of steel with much Al and Si added thereto. The shapability of such
a high-Al, Si steel may be good as compared with that of SUH21
(18Cr-3Al steel of good scale resistance), but is much inferior to
that of ferritic stainless steels such as SUH409L, SUS410L,
SUS430LX and the like. Therefore, the technique is unsuitable to
motorcycle exhaust gas passageway members.
[0006] On the other hand, Al-plated steel sheets of Cr-containing
steel such as stainless steel or the like exhibit good oxidation
resistance, and various types of such steel sheets have been
developed for automobile exhaust system members (Patent References
4 to 13).
[0007] Patent Reference 1: JP-A 2001-240911
[0008] Patent Reference 2: JP-A 2001-316773
[0009] Patent Reference 3: JP-A 2003-160844
[0010] Patent Reference 4: JP-A 8-319543
[0011] Patent Reference 5: JP-A 5-112859
[0012] Patent Reference 6: JP-A 5-295513
[0013] Patent Reference 7: JP-A 61-147866
[0014] Patent Reference 8: JP-A 61-147865
[0015] Patent Reference 9: JP-A 7-233451
[0016] Patent Reference 10: JP-A 63-47356
[0017] Patent Reference 11: JP-A 3-277761
[0018] Patent Reference 12: JP-A 7-188887
[0019] Patent Reference 13: JP-A 8-325691
PROBLEMS THAT THE INVENTION IS TO SOLVE
[0020] According to the present inventors' investigations,
Al-plating is effective for enhancing the resistance to red scale
formation (hereinafter referred to as "red scale resistance") of a
Cr-containing steel sheet such as stainless steel or the like.
However, the present inventors' further detailed investigations on
the above-mentioned known Al-plated steel sheets have revealed that
these have some problems to be solved in realizing their
broad-range practicable application to motorcycle exhaust gas
passageway members.
[0021] Specifically, in the Al-plated steel sheet in Patent
Reference 4, Mn and the like is thickened in the alloy layer, and
the plating layer thickness could not be said to be sufficiently
thin, and therefore, it is recognized that the peeling resistance
of the plating layer (in this description, a hot-dip plating layer
including an alloy layer is referred to as "plating layer") is
insufficient. In the plated steel sheets in Patent References 5 and
6, the plating layer is thick, and therefore also in these, the
peeling resistance of the plating layer is insufficient. The plated
steel sheets in Patent References 7, 8 and 10 are produced by Al
plating after Ni pre-plating; however, the Ni pre-plating employed
therein much increases the production costs and therefore could not
be directly applied to exhaust gas passageway members such as
mufflers, catalyst carriers and the like for which cost reduction
is much desired. In these, in addition, the peeling resistance of
the plating layer when heated up to a range of from 400 to
700.degree. C. is not always on a satisfactory level. In the plated
steel sheet in Patent Reference 9, the substrate steel
indispensably contains a rare earth element or Y added thereto, and
is therefore protected from abnormal oxidation at a high
temperature of from 1150 to 1250.degree. C.; however, since the
adhesiveness between the plating layer and the substrates is
insufficient, the plating layer tends to readily peel away when the
plated steel sheet is exposed to cycles of heating to a range of
from 400 to 700.degree. C. followed by cooling. In the plated steel
sheets in Patent References 11 and 12, the plating layer is not
sufficiently thin, and therefore the peeling resistance of the
plating layer to cycles of heating to a range of from 400 to
700.degree. C. followed by cooling is insufficient. "Red rust"
described in Patent Reference 11 is typical red rust generally seen
in ordinary steel that has been much corroded at room temperature,
and this differs from "red scale" as referred to herein.
[0022] As in the above, basically in known Al-plated stainless
steel sheets, the peeling resistance of the plating layer could not
be said to be satisfactory when heated in a temperature range of
from 400 to 700.degree. C. Accordingly, when conventional Al-plated
stainless steel sheets are applied to mufflers, catalyst carriers
and the like in motorcycles that are used in a temperature range of
from 400 to 700.degree. C., they could exhibit good corrosion
resistance and red scale resistance in the early days; however,
while used for a long period of time, the plating layer may peel
away, therefore causing reduction in the corrosion resistance and
reduction in the red scale resistance of the steel sheets. In other
words, they involve some risk factors in point of the durability
thereof. On the other hand, various types of motorcycles with an
exhaust gas purification catalyst built therein are increasing
owing to the recent tendency toward reinforcement for exhaust gas
regulation. When such a catalyst is built in motorcycles, the
temperature of the exhaust gas from them becomes higher due to the
reaction and the exhaust gas passageway members such as mufflers
and others therein shall be exposed to further high temperatures.
Accordingly, it has become desired to apply materials having much
better high-temperature strength than before to exhaust gas
passageway members of motorcycles. In addition, the steel sheets
for exhaust gas members are required to have good shapability and
low-temperature toughness. Further, low-cost production is an
important factor for industrial applicability.
[0023] An object of the present invention is to provide an
Al-plated steel sheet for motorcycle exhaust gas passageway
members, which is inexpensive and has good high-temperature
strength and is excellent in red scale resistance, shapability and
low-temperature toughness and in which the peeling resistance of
the plating layer in repeated heating in a temperature range of
from 400 to 700.degree. C. has been significantly enhanced.
MEANS FOR SOLVING THE PROBLEMS
[0024] As a result of detailed investigations, the present
inventors have found that, when the thickness of the Al-base
plating layer is controlled to be at most 20 .mu.m, then the
peeling resistance of the Al-base plating layer can be
significantly enhanced, and have completed the present
invention.
[0025] Specifically, the invention provides an Al-plated steel
sheet for motorcycle exhaust gas passageway members excellent in
red scale resistance, which is produced by dipping a substrate
steel sheet having a base steel composition comprising, in terms of
% by mass, at most 0.02% of C, at most 2% of Si, at most 2% of Mn,
from 5 to 25% of Cr, from more than 0.1 to 1% of Nb, at most 0.3%
of Ti, at most 0.02% of N, and optionally at least one of at most
0.6% of Ni, at most 0.2% of Al, at most 3% of Mo, at most 3% of Cu,
at most 3% of W, at most 0.5% of V, at most 0.5% of Co, at most
0.01% of B, with a balance of Fe and inevitable impurities, in a
hot-dip plating bath containing, in terms of % by mass, from 3 to
12% of Si and optionally at least one of Ti, B, Sr, Cr, Mn, Mg and
Zr in a total amount of at most 1%, with a balance of Al and
inevitable impurities, then pulling it up, and controlling the
plating amount to thereby form a plating layer having a mean
thickness of from 3 to 20 .mu.m on the surface thereof.
[0026] The invention also provides a motorcycle exhaust gas
passageway member which is formed of the above-mentioned plated
steel sheet and which is so designed that the above-mentioned
plating layer thereof is kept in contact with exhaust gas and that
the maximum service temperature is 400.degree. C. or higher.
[0027] The Al-plated steel sheet of the invention is excellent in
red scale resistance and is excellent in peeling resistance of the
plating layer in repeated heating in a temperature range of from
400 to 700.degree. C., and therefore, it is favorable for
motorcycle exhaust gas passageway members (e.g., muffler members)
that are used in an environment where red scale readily forms in
heating in that temperature range. Since a substrate steel sheet
having good high-temperature strength is used, the latitude in
planning the exhaust gas passageway member is broadened, and this
is especially advantageous in some types of motorcycles which are
equipped with an exhaust gas purification catalyst built therein
and in which the temperature of the exhaust gas may be therefore
high. In addition, the production costs may be reduced to at most
the same level as that for ordinary Al-plated stainless steel
sheets. Accordingly, the invention may contribute toward improving
and enhancing the quality and the durability of motorcycle exhaust
gas passageway members.
PREFERRED EMBODIMENTS OF THE INVENTION
[0028] When a high-Cr steel such as ferritic stainless steel or the
like is heated at a high temperature, in general, an oxide having a
high Cr concentration (Cr-base oxide) is formed in the surface of
the steel base. Since the Cr-base oxide has a high protecting
capability, the high-Cr steel may generally have good
high-temperature oxidation resistance as compared with low-Cr
steel. However, it is known that, when heated in a low-oxygen
high-moisture atmosphere in a temperature range of from 400 to
700.degree. C., even such a high-Cr steel readily forms an Fe-base
oxide rather than a Cr-base oxide in the initial stage of the
oxidation process thereof. This is considered because Cr could not
rapidly diffuse in the surface of the steel base in an amount
enough to cover the surface in the form of the Cr-base oxide
thereof but rather Fe existing in a large amount in the surface
would be predominantly oxidized in the low-oxygen high-moisture
atmosphere. Red scale is composed of the oxide having a high Fe
concentration formed in the surface in the manner as above.
[0029] For preventing the red scale formation, the Fe-base oxide
may be prevented from forming in the surface of the steel base in
the atmosphere and the temperature range mentioned above. As one
means for it, Al-base plating may be effective. In this case, Al in
the surface of the plating layer may be rapidly oxidized, and the
surface of the steel sheet may be covered with the Al-base oxide
film.
[0030] However, the present inventors' investigations have revealed
that the Al-base plating layer may readily peel off from the
surface of the steel base in repeated cycles of heating up to a
temperature of from 400 to 700.degree. C. followed by cooling. This
is a significant risk factor of not always exhibiting good
durability in application of conventional Al-plated steel sheets to
motorcycle muffler members or the like. When a hot-dip Al-plated
steel sheet is heated at 400 to 700.degree. C., Al in the plating
layer and Fe in the steel base interdiffuse to give an Fe--Al
intermetallic compound layer, and the intermetallic compound layer
peels off from the steel base. In that manner, the plating layer
peels off from the steel base, from which red scale begins to newly
form.
[0031] The present inventors have made detailed studies about the
method of preventing the plating layer from peeling off from the
Al-plated steel sheet. As a result, the inventors have found that
reducing the thickness of the Al-base plating layer is extremely
effective for preventing the Al-base plating layer from peeling off
from the steel base in heating in a low-oxygen high-moisture
atmosphere in a temperature range of from 400 to 700.degree. C. In
this case, there is no necessity of providing any specific
limitation on the chemical composition of the base steel sheet, on
the composition and the texture condition of the Al-base plating
layer and on the composition of the alloy layer.
[Thickness of Al-Base Plating Layer]
[0032] Concretely, in the hot-dip Al-plated steel sheet, the mean
thickness of the Al-base plating layer is defined to be at most 20
.mu.m per one surface, whereby the plating layer can exhibit
excellent peeling resistance. The mean thickness of the Al-base
plating layer is the thickness including no alloy layer. As well
known, the hot-dip plating amount can be controlled according to a
gas wiping method or the like, and therefore, in case where the
plating amount itself per one surface is controlled to be at most
20 .mu.m, then a part thereof could react with the steel base to
form an alloy layer, and the mean thickness of the formed Al-base
plating layer is not more than 20 .mu.m. Under the operation
condition under which a relatively thick alloy layer could be
formed, the mean thickness of the plating layer could be at most 20
.mu.m as the case may be, even when the plating amount is set to be
somewhat larger than 20 .mu.m. When the mean thickness of the
Al-base plating layer is less than 15 .mu.m, then the peeling
resistance may better further more. Accordingly, a case where the
mean thickness of the Al-base plating layer per one surface is less
than 15 .mu.m is an especially preferred embodiment of the
invention.
[0033] On the other hand, from the viewpoint of preventing red
scale, the thickness of the Al-base plating layer is preferably
larger. As a result of various investigations, the mean thickness
of the Al-base plating layer per one surface must beat least 3
.mu.m on the precondition of using a base steel sheet having a
controlled composition as described below. When the plating layer
is thinner than the range, it may be difficult to stably prevent
the red scale formation. More preferably, the mean thickness is at
least 4 .mu.m, even more preferably at least 5 .mu.m.
[Thickness of Alloy Layer]
[0034] The alloy layer is relatively more brittle as compared with
the steel base and the plating layer. Accordingly, the thickness of
the alloy layer is preferably thinner in consideration of the
workability of the plated steel sheet. Any specific consideration
may be unnecessary for use of the plated steel for low working
ratio members; however, when the plated steel sheet is severely
deformed, then the mean thickness of the alloy layer per one
surface is preferably at most 5 .mu.m, more preferably at most 4
.mu.m. In a hot-dip Al-base plating line using a gas-wiping method,
it is well possible to produce plated steel sheets having such a
thin alloy layer. Regarding the total thickness of the plating
layer and the alloy layer, the thickness of the alloy layer is
preferably so controlled that the "mean thickness of the Al-base
plating layer+mean thickness of the alloy layer" per one surface
could be at most 24 .mu.m.
[0035] [Composition of Al-Base Plating Bath]
[0036] In an Al--Si alloy system, the liquidus-line temperature
lowers with addition of Si to Al, and the system may have an
eutectic composition when the Si content thereof reaches about 12%
by mass. In the invention, a hot-dip Al-base plating bath
containing Si in an amount of at least 3% by mass is used. An
Al-richer composition than those of the bath will need a higher
bath temperature, and in such a plating bath, it may be difficult
to control the mean thickness of the alloy layer to be thin (for
example, at most 5 .mu.m). In addition, the increase in the bath
temperature may cause the increase in the production cost. However,
when the Si content is more than 12% by mass, then the workability
of the Al-base plating layer itself may worsen, therefore often
bringing about a problem. Accordingly, in the invention, a hot-dip
Al-base plating bath that contains from 3 to 12% by mass of Si is
used to produce the intended plated steel sheet.
[0037] The Al-base plating bath may contain at least one of Ti, B,
Sr, Cr, Mn, Mg and Zr in a total amount of at most 1%. In addition,
the bath may contain Fe as an inevitable impurity, in which Fe is
allowable within a range of at most 2.5% by mass.
[Substrate Steel Sheet]
[0038] The substrate steel sheet to be plated is a high-Cr steel
sheet containing from 5 to 25% by mass of Cr. Since the corrosion
resistance and the oxidation resistance of the steel sheet could be
enhanced by Al-plating, the substrate steel sheet is not always
required to have a Cr content on a level of stainless steel;
however, in order that the plated steel sheet could secure the
necessary corrosion resistance and red scale resistance for
motorcycle exhaust gas passageway members that are exposed to an
environment where they are kept in contact with dew condensation
water and water vapor therein, the substrate steel sheet must have
a Cr content of at least 5% by mass. More preferably, the Cr
content is at least 10% by mass. With the increase in the Cr
content, the corrosion resistance and the heat resistance of the
steel sheet may better; however, too much Cr existing in the steel
sheet is uneconomical and may cause a factor of interfering the
shapability and the low-temperature toughness of steel.
Accordingly, the Cr content of the substrate steel sheet is defined
to fall within a range of at most 25% by mass.
[0039] Nb has an effect of enhancing the high-temperature strength
of steel. In case where steel contains relatively much C and N,
then Nb therein may have an effect of enhancing the low-temperature
toughness of the steel. In order to fully attain these effects, in
the invention, a ferritic steel having an Nb content of more than
0.1% by mass is employed. However, the present inventors' studies
have revealed that Nb in a substrate steel sheet may detract from
the peeling resistance of the hot-dip Al-base plating layer. As a
result of detailed investigations, the inventors have found that
the Nb content of the substrate steel sheet is allowable to a level
of up to 1% on the precondition of controlling the mean thickness
of the Al-base plating layer to be at most 20 .mu.m as so mentioned
in the above. Accordingly, in the invention, the substrate steel
sheet to be used contains Nb in an amount falling within a range of
from more than 0.1 to 1%.
[0040] Ti is an element effective for fixing C and N in steel and
for stabilizing the ferrite phase and further for enhancing the
low-temperature toughness and the shapability of steel. In order to
make the element fully exhibit these effects thereof, preferably
the Ti content is at least 0.05% by mass, more preferably at least
0.1% by mass. However, too much Ti may harden steel and therefore
may rather worsen the workability and the low-temperature toughness
of steel. Accordingly, the Ti content is limited to fall within a
range of at most 0.3% by mass, and more preferably, it is within a
range of at most 0.2% by mass.
[0041] C, Si, Mn and N are basic elements in steel. C may be in an
amount falling within a range of at most 0.02% by mass, and N may
be in an amount of at most 0.02% by mass. When the content of Si
and Mn increases, then the low-temperature toughness of steel may
lower; but in the invention, both Si and Mn are allowable within a
range of up to 2% by mass each. As other elements, the substrate
steel may contain at least one of at most 0.6% of Ni, at most 0.2%
of Al, at most 3% of Mo, at most 3% of Cu, at most 3% of W, at most
0.5% of V, at most 0.5% of Co and at most 0.01% of B; however, when
the content of these elements is more than the above-mentioned
limitation, then they may have some negative influences on the
shapability and the low-temperature toughness of steel. As
inevitable impurities, P may be allowable in an amount of at most
0.10% by mass or so, and S may be in an amount of at most 0.03% by
mass or so.
[Production Method]
[0042] The substrate steel sheet may be produced according to an
ordinary steel sheet production process, and the production method
for it is not specifically defined. For example, pickling-finished
cold-rolled steel sheet may be used as the substrate; and while the
surface of the substrate steel sheet is kept activated, the sheet
is dipped in a hot-dip Al-base plating bath and then pulling it up,
and the plating amount is controlled to produce the hot-dip
Al-plated steel sheet of the invention. For enhancing the
platability thereof, an Fe pre-plated substrate steel sheet maybe
employed. A strip of the substrate steel sheet is introduced into a
continuous hot-dip plating line, and a hot-dip Al-plated steel
sheet of high quality can be thereby produced stably in a mode of
industrial-scale mass production. Regarding the plating condition,
it is important that the line speed and the wiping condition are so
controlled that the mean thickness of the plating layer could be at
most 20 .mu.m per one surface; but for the others, ordinary
conditions may be employed. Thus obtained, the plated steel sheet
may be processed in a predetermined shaping and deforming process
to give exhaust gas passageway members for motorcycles. For some
members, the sheet may be welded into pipes and then and deformed
shaped.
Examples
[0043] Ferritic steels each having the composition shown in Table 1
were produced through melting, and then processed according to an
ordinary method to give cold-rolled annealed steel sheets
(pickling-finished steel sheets) having a thickness of 1.2 mm. As
the inevitable impurities in all these steels, P was at most 0.10%
by mass and S was at most 0.01% by mass. These steel sheets were
used as substrates, and variously plated in a mode of hot-dip
Al-base plating. In Nos. 11 and 27, the surface of the cold-rolled
annealed steel sheet was pre-plated with Fe (2 g/m.sup.2), and
these were used as substrates. In all Examples, the plating bath
contained an inevitable impurity Fe in an amount of 1.7% by mass or
so. The cross section of the obtained, hot-dip Al-plated steel
sheet (sample sheet) was observed with SEM (scanning electronic
microscope), and the mean thickness of the Al-base plating layer
was determined. On that occasion, the mean thickness of the alloy
layer was also determined, and as a result, it was at most 4 .mu.m
in all cases except some comparative examples.
TABLE-US-00001 TABLE 1 Steel Chemical Composition (% by mass)
Classification Code C Si Mn Ni Cr Nb Ti Al N Mo Cu W V Co B Steel
of the X1 0.009 0.46 0.80 0.08 12.05 0.51 0.07 0.03 0.012 -- -- --
-- -- Invention X2 0.006 0.93 0.25 0.10 10.94 0.25 0.15 0.02 0.008
-- -- -- 0.03 -- 0.003 X3 0.007 0.25 0.36 0.22 18.10 0.18 0.19 0.08
0.010 -- -- -- 0.04 -- -- X4 0.006 0.24 0.49 0.30 24.60 0.33 0.07
0.09 0.007 -- -- -- 0.03 -- -- X5 0.010 0.38 1.50 0.34 10.21 0.80
0.18 0.06 0.009 -- -- -- 0.04 -- -- X6 0.018 0.60 0.99 -- 17.50
0.45 0.20 0.05 0.011 -- -- -- -- -- -- X7 0.016 1.10 0.28 0.50
18.32 0.60 0.08 0.04 0.009 -- -- -- 0.07 -- -- X8 0.012 0.44 0.60
-- 13.80 0.77 0.15 0.10 0.015 -- -- -- 0.03 -- -- X9 0.009 0.39
0.55 0.26 12.91 0.39 0.18 -- 0.008 1.11 -- -- 0.04 -- -- X10 0.007
0.74 0.31 0.37 14.60 0.33 0.09 0.06 0.007 -- 2.10 -- 0.05 -- -- X11
0.005 0.25 1.15 0.44 19.62 0.47 0.17 0.07 0.009 -- -- 2.220 0.06 --
-- X12 0.011 1.00 0.33 0.21 16.29 0.69 0.06 0.08 0.010 -- -- --
0.31 -- -- X13 0.013 0.83 0.28 0.19 17.40 0.24 0.05 0.09 0.005 --
-- -- 0.07 0.490 -- X14 0.014 0.25 1.10 0.15 12.99 0.89 0.11 --
0.009 -- -- -- 0.04 -- 0.009 X15 0.010 0.72 0.65 -- 17.42 0.33 0.12
-- 0.010 -- -- -- -- -- -- Comparative Y1 0.009 0.60 0.50 0.26
13.55 1.04 0.29 0.04 0.009 -- 0.12 -- 0.04 -- -- Steel Y2 0.007
0.80 0.85 0.36 27.30 0.30 0.18 0.04 0.009 0.09 0.11 -- 0.05 -- --
Y3 0.011 0.33 0.69 0.19 19.26 0.69 0.38 0.03 0.010 0.07 0.09 --
0.03 -- -- Y4 0.009 0.56 1.25 0.11 18.63 0.05 0.16 0.03 0.012 3.10
0.18 -- -- -- -- Y5 0.008 0.90 1.09 0.09 4.88 0.59 0.13 0.02 0.008
0.10 -- -- 0.04 -- -- Y6 0.010 0.48 0.69 0.51 15.39 0.43 0.07 0.06
0.009 0.08 3.55 -- 0.05 -- -- Underlined: falling outside the scope
of the invention.
[0044] The sample sheets were evaluated for the red scale
resistance, the peeling resistance of the plating layer, the
shapability and the low-temperature toughness in the manner
mentioned below.
[Red Scale Resistance]
[0045] A test piece of 25 mm.times.35 mm was cut out of the sample
sheet; and this was tested in the following two heating modes of
oxidation test. In every test mode, the number of the test pieces
was n=3. [0046] Continuous heating: In a nitrogen atmosphere having
a dew point of 80.degree. C., the test piece is continuously heated
at 600.degree. C. for 100 hours. [0047] Cycle heating: "Heating in
a nitrogen atmosphere having a dew point of 80.degree. C., at
600.degree. C. for 30 minutes (in which the soaking time is 25
minutes).fwdarw.cooling in air for 5 minutes" is one cycle; and the
test piece is exposed to 500 cycles.
[0048] After tested in each heating mode, the test piece was
visually checked for the presence or absence of red-brown
coloration therein. In addition, the oxidation weight increase and
the oxidation weight reduction (scale peel loss) were measured. All
the test pieces of n=3 were tested in the manner as above, and
those having cleared the three requirements of (i) presence of no
red-brown coloration, (ii) oxidation weight increase of less than
0.2 mg/cm.sup.2, and (iii) oxidation weight reduction of less than
0.2 mg/cm.sup.2 were evaluated as good (O) and the others were as
not good (x).
[Peeling Resistance of Plating Layer]
[0049] A test piece of 25 mm.times.35 mm was cut out of the sample
sheet; and this was tested in a mode of 600.degree. C. cycle
heating and 700.degree. C. cycle heating as mentioned below. In
every test mode, the number of the test pieces was n=5. [0050]
600.degree. C. cycle heating: "Heating in a nitrogen atmosphere
having a dew point of 80.degree. C., at 600.degree. C. for 30
minutes (in which the soaking time is 25 minutes) cooling in air
for 5 minutes" is one cycle; and the test piece is exposed to 1000
cycles. [0051] 700.degree. C. cycle heating: "Heating in a nitrogen
atmosphere having a dew point of 80.degree. C., at 700.degree. C.
for 30 minutes (in which the soaking time is 25
minutes).fwdarw.cooling in air for 5 minutes" is one cycle; and the
test piece is exposed to 1000 cycles.
[0052] After tested at the predetermined temperature, the test
piece was visually checked for the presence or absence of peeling
of the plating layer. Of all the test pieces of 5 at different
temperatures (n=5).times.two-level temperatures, totaling 10,
tested in the manner as above, those with no peeling of the plating
layer were evaluated as good (O) and the others were as not good
(x).
[0053] The test pieces evaluated as good (O) in the above-mentioned
1000-cycle tests were further tested up to 2000 cycles of
600.degree. C. cycle heating and 700.degree. C. cycle heating. Of
all the test pieces of 5 at different temperatures
(n=5).times.two-level temperatures, totaling 10, tested in the
manner as above, those with no peeling of the plating layer were
evaluated as very good (.THETA.).
[Shapability]
[0054] A tensile test piece (JIS 13B) was cut out of the sample
sheet (hot-dip Al-plated steel sheet having a thickness of 1.2 mm)
in such a manner that its lengthwise direction could be the rolling
direction of the sheet; and the test pieces of n 3 were tested for
elongation in a tensile test according to JIS 22241. The data of
the elongation of the test pieces of n=3 were averaged to give the
mean elongation of the sample sheet. From various experiments, it
is known that, when a steel sheet having a thickness of 1.2 mm
could have an elongation of at least 30%, then it may satisfy the
necessary shapability in forming it into motorcycle exhaust gas
passageway members. Accordingly, the sample sheets having an
elongation of at least 30% were evaluated as good (0) and the
others were as not good (x).
[Low-Temperature Toughness]
[0055] A test piece of 55 mm.times.10 mm was cut out of the sample
sheet (hot-dip Al-plated steel sheet having a thickness of 1.2 mm)
in such a manner that its lengthwise direction could be
perpendicular to the rolling direction of the sheet, and its center
was notched to have a 2-mm V-notch, thereby preparing a notched
impact test piece. According to the definition by JIS 22202, the
height is 10 mm, the width is 1.2 mm, the length is 55 mm and the
height below the notch is 8 mm. The test pieces were tested in a
Charpy impact test according to JIS 22242; and those determined to
have a nil ductility temperature (a brittle fracture occurrence
temperature) of not higher than -50.degree. C. were evaluated as
good (O), and the others were as not good (x)
[0056] These results are shown in Table 2 and Table 3.
TABLE-US-00002 TABLE 2 Mean Plating Bath Thickness Red Scale
Plating Composition of Plating Resistance Layer Low- Sample Steel
(% by mass) Layer continuous cycle Peeling Temperature
Classification No. Code Al Si Others (.mu.m) heating heating
Resistance Shapability Toughness Example of 1 X1 balance 9.0 -- 4
.largecircle. .largecircle. .THETA. .largecircle. .largecircle. the
Invention 2 X1 balance 4.5 -- 20 .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. 3 X1 balance 4.5 -- 14
.largecircle. .largecircle. .THETA. .largecircle. .largecircle. 4
X1 balance 11.5 -- 13 .largecircle. .largecircle. .THETA.
.largecircle. .largecircle. 5 X1 balance 8.8 Ti: 0.15 18
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. 6 X1 balance 8.8 Ti: 0.15 11 .largecircle.
.largecircle. .THETA. .largecircle. .largecircle. 7 X1 balance 9.0
Sr: 0.40 8 .largecircle. .largecircle. .THETA. .largecircle.
.largecircle. 8 X1 balance 8.7 B: 0.25 14 .largecircle.
.largecircle. .THETA. .largecircle. .largecircle. 9 X1 balance 8.5
10 .largecircle. .largecircle. .THETA. .largecircle. .largecircle.
10 X1 balance 8.2 B: 0.15 13 .largecircle. .largecircle. .THETA.
.largecircle. .largecircle. 11 X2 balance 9.0 -- 6 .largecircle.
.largecircle. .THETA. .largecircle. .largecircle. 12 X2 balance 3.9
-- 14 .largecircle. .largecircle. .THETA. .largecircle.
.largecircle. 13 X2 balance 10.8 -- 19 .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. 14 X2 balance 8.9 Ti:
0.13 9 .largecircle. .largecircle. .THETA. .largecircle.
.largecircle. 15 X2 balance 9.0 Sr: 0.50 5 .largecircle.
.largecircle. .THETA. .largecircle. .largecircle. 16 X2 balance 8.2
Cr: 0.13 10 .largecircle. .largecircle. .THETA. .largecircle.
.largecircle. 17 X2 balance 8.2 Mn: 0.20 13 .largecircle.
.largecircle. .THETA. .largecircle. .largecircle. 18 X3 balance 9.0
-- 18 .largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. 19 X3 balance 8.7 Mg: 0.15 9 .largecircle.
.largecircle. .THETA. .largecircle. .largecircle. 20 X4 balance 8.5
Zr: 0.14 14 .largecircle. .largecircle. .THETA. .largecircle.
.largecircle. 21 X4 balance 9.0 -- 18 .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. 22 X4 balance 9.0 -- 10
.largecircle. .largecircle. .THETA. .largecircle. .largecircle. 23
X5 balance 9.0 -- 11 .largecircle. .largecircle. .THETA.
.largecircle. .largecircle. 24 X6 balance 9.0 -- 8 .largecircle.
.largecircle. .THETA. .largecircle. .largecircle. 25 X7 balance 9.0
-- 19 .largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. 26 X7 balance 9.0 -- 9 .largecircle. .largecircle.
.THETA. .largecircle. .largecircle. 27 X8 balance 9.0 -- 14
.largecircle. .largecircle. .THETA. .largecircle. .largecircle. 28
X9 balance 9.0 -- 20 .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. 29 X9 balance 9.0 -- 10 .largecircle.
.largecircle. .THETA. .largecircle. .largecircle. 30 X10 balance
9.0 -- 8 .largecircle. .largecircle. .THETA. .largecircle.
.largecircle. 31 X11 balance 9.0 -- 13 .largecircle. .largecircle.
.THETA. .largecircle. .largecircle. 32 X12 balance 9.0 -- 10
.largecircle. .largecircle. .THETA. .largecircle. .largecircle. 33
X13 balance 9.0 -- 7 .largecircle. .largecircle. .THETA.
.largecircle. .largecircle. 34 X14 balance 9.0 -- 17 .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle. 35 X14
balance 9.0 -- 8 .largecircle. .largecircle. .THETA. .largecircle.
.largecircle. 36 X15 balance 9.0 -- 13 .largecircle. .largecircle.
.THETA. .largecircle. .largecircle.
TABLE-US-00003 TABLE 3 Plating Bath Mean Red Scale Plating
Composition Thickness Resistance Layer Low- Sample Steel (% by
mass) of Plating continuous cycle Peeling Temperature
Classification No. Code Al Si Others Layer (.mu.m) heating heating
Resistance Shapability Toughness Comparative 51 X1 balance 9.0 --
42 .largecircle. .largecircle. X .largecircle. .largecircle.
Example 52 X1 balance 9.0 -- 2 X X .THETA. .largecircle.
.largecircle. 53 X1 balance 9.0 -- 23 .largecircle. .largecircle. X
.largecircle. .largecircle. 54 X2 balance 9.0 -- 1 X X .THETA.
.largecircle. .largecircle. 55 X2 balance 9.0 -- 60 .largecircle.
.largecircle. X .largecircle. .largecircle. 56 X2 balance 9.0 -- 25
.largecircle. .largecircle. X .largecircle. .largecircle. 57 Y1
balance 9.0 -- 15 .largecircle. .largecircle. X .largecircle. X 58
Y2 balance 9.0 -- 18 .largecircle. .largecircle. .largecircle. X X
59 Y3 balance 9.0 -- 20 .largecircle. .largecircle. .largecircle. X
X 60 Y4 balance 9.0 -- 9 .largecircle. .largecircle. .THETA. X X 61
Y5 balance 9.0 -- 16 X X .largecircle. .largecircle. .largecircle.
62 Y6 balance 9.0 -- 19 .largecircle. .largecircle. .largecircle. X
X Underlined: falling outside the scope of the invention.
[0057] As known from Table 2, the examples of the invention in
which the mean thickness of the Al-base plating layer falls within
a range of from 3 to 20 .mu.m were all excellent in the red scale
resistance and the plating layer peeling resistance. In particular,
those in which the mean thickness of the plating layer was less
than 15 .mu.m exhibited more excellent peeling resistance.
[0058] As opposed to these, Nos. 52 and 54 of comparative examples
were poor in the red scale resistance since the mean thickness of
the Al-base plating layer therein was too small. Nos. 51, 53, 55
and 56 were poor in the plating layer peeling resistance since the
mean thickness of the Al-base plating layer was larger than 20
.mu.m. No. 57 was poor in the peeling resistance of the plating
layer since the Nb content of the substrate steel sheet was too
high. Nos. 58, 59, 60 and 62 were all poor in the shapability and
the low-temperature toughness since the content of Cr, Ti, Mo and
Cu in the substrate steel sheet was too high. No. 61 was poor in
the red scale resistance since the Cr content of the substrate
steel sheet was too low.
* * * * *