Note: Descriptions are shown in the official language in which they were submitted.
~101772
SPECIFICATION
MANUFACTURING METHOD FOR I~MERSION MEMBERS
FOR USE IN MOLTEN ~ETAL BATHS
Technical Field
The present invention relates to a manufacturing method
for immersion members for use in immersion over long periods in
a high temperature molten metal bath ~uch as one of mol~en zinc,
molten aluminum, molten tin, and the like. In particular, the
present invention relates to a manufacturing method fo~
immersion members for use in molten metal baths in molten zinc
plating production lines, molten aluminum plating production
lines, molten tin plating production lines, or the like; for
example, sink rolls and support rolls which are used in an
immersed ~tate in a molten zinc plating bath or a molten
aluminum plating bath.
Background Art
It is apparent that a resistance to corrosion resulting
from molten metals is in great demand with respect to immersion
members which can be used over a long period of time in an
immersed state in high temperature molten metal baths such as
one of molten zinc, molten aluminum, or molten tin, or the like.
In particular, in sink rolls and 3upport rolls, it has been
desirable not merely that resi~tance to corro3ion resulting from
molten metals be present, but also that abrasion resulting from
the contact between the roll and the substrate to be plated,
such as a steel plate or the like, which is immersed in the
bath, be unlikely to occur, and that adhesion of metals also be
21~1772
unlikely to occur.
When metal adhesion occurs on immersion rolls such as
sink rolls, 3upport rolls or the like, damage is caused to the
substrate to be plated, or to the plating ~urface of the steel
plate or the like, which is guided by these rolls and immersed
in the bath. Furthermore, for this reason, immersion rolls such
as sink rolls and support rolls have become unfit for use.
Conventionally, in response to these varying demands,
immersion members having various cermet materials thermal
sprayed thereon have been developed and used; however, such
members are as yet insuffici2nt. ~or example, a WC-Co cermet
thermal sprayed coating is used as an immersion member for use
in molten metal baths; however, such a member is insufficient
from the point of view of molten metal corrosion resistance.
Furthermore, the above-described demands have become more
and more pressing in concert with demands for increasing quality
of plated products, demands for a reduction in manufacturing
costs, and demands for extended service life of immersion rolls.
In response to these demands, the present inventors have
previously invented an immersion member for use in molten zinc
baths and the like, in which the surface coating of the
immersion member itself comprises one or more o~ tungsten
carbides, tungsten borides, and molybdenum borides, in addition
to Co, and this was disclosed in Japanese Patent Application Hei
1-231293 (Japane~e Patent Application, Laid-Open No. ~ei
3-94048, laid open date: April 18, 1991), Corrosion resistance
of the immersion member with respect to molten metal baths was
achieved by means of this invention; however, there was a
problem in that corrosive peeling occurred during use over a
2 ~ 2
long period of time.
In general, cracks and micropores are present in a
thermal sprayed coating. During use of an immersion member in a
molten metal bath over a long period of time, the molten metal
penetrates to the interior of the thermal sprayed layer through
these crack~ and micropores and breaks down the thermal sprayed
coating, corroding this thermal sprayed coating from below the
surface, so that a phenomenon is noted in which the thermal
sprayed coating peels away. This is termed corrosive peeling.
In order to solve this problem, the present inventors
have tested immersion members in which the cracks and micropores
present in the thermal sprayed coating are filled~with coal tar
however, under the conditions of high temperature present in the
molten metal baths, the organic substances present in the coal
tar broke down and became gassified, and for this reason, the
quality of the thermal sprayed coating was deteriorated, so that
an immersion member having a long service life could not be
obtained. Furthermore, the gas produced by the breakdown of the
organic substances in the molten metal bath produced undesirable
effects.
Furthermore, in order to avoid this phenomenon, an
attempt was made to subject the immersion member to heat
processing immediately prior to use in the molten metal bath
after filling the cracks and micropores of the thermal sprayed
coating of the i~mer~ion member for use in molten metal baths
with coal tar; however, gas was produced by the breakdown of the
organic substances contained in the coal tar during heat
proceRsing, and for this reason~ micropitting was produced, and
the coal tar filling material itself was lost, so that the
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de3irable properties could not be obtained.
Disclo~ure of the Invention
In order to solve the problems described above, the
present inventors have conducted extensive research as described
above, and as a result of this research, have arrived at the
present invention.
First, an important feature of the present invention is
the addition, in the thermal sprayed coating composition, of
tungsten borides (WB and the like), the production of a Cr203-B203
system glass in at least the cracks and micropores, by means of
an oxidation reaction with H2CrO4, or the like, and the formation
of a fine and strong thermal sprayed pore-sealing layer using
this effect. In accordance with the present invention, it is
possible to obtain a superior immersion member for use in molten
metals which is provided with a fine and strong surface film
layer not found in the conventional art.
Hereinbelow, the present invention will be explained in
detail.
Conventionally, a WC-Co cermet was employed in immersion
members for use in molten metal baths; however, a~ a result of
the research of the present inventors, it was determined that,
in addition to WC, WB is superior from the point of view of
corroYion reYi~tance in molten metal. Next, it wa~ determined
that WB has a higher coefficient of thermal expan~ion and that
the re~lting thermal sprayed coating has a stronger thermal
shock resistance than that of WC. Furthermore, it was
determined that in an oxidizing atmosphere, borides form B203 on
the surface thereof, and that at high temperatures, a portion of
7 7 2
this Bz03 is volatilized; however, a certain amount remains on
the surface.
Furthermore, the present inventors have determined that
it is possible to obtain a superior coating when a thermal
spraying material consisting of a cermet in which WC and WB are
combined with at least one of Ni, Co, Cr, and Mo, or a thermal
spraying material consisting of WC and WB which are coated with
Ni, Co, or the like, or a thermal qpraying material consisting
of WC and WB which are agglomerated with at least one of Ni, Co,
Cr, and Mo, and are subjected to granulation, and sint~ring in
a neutral atmosphere, is subjected to thermal spray by a
high-velocity oxygen fuel gun method or a plasma spraying
method.
Furthermore, WB-WC is superior to WC in molten metal
wettability, so that adhesion is unlikely to occur with respect
to, for example, molten zinc. However, it was discovered that
when the amount of WB added becomes large, satisfactory thermal
spraying becomes difficult in a standard atmosphere.
Accordingly, it is preferable that the limitation on the
amount of WB contained in the thermal sprayed coating be set to
less than 50 weight %. Furthermore, when the amount thereof is
too small, the desired effects cannot be realized. Accordingly,
the amount of WB contained should be within a range of 1-50
weight %. It is more preferable that the amount contained be
within a range of 10-40 wt %.
The reason for the addition of at least one of Ni, Co,
Cr, and Mo as a metal phase is to increase resistance to
peeling, and to increase hardness, so that superior layer may be
obtained. The amount contained of at least one of Ni, Co, Cr,
2~1772
and ~o should preferably be within a range of 3-25 wt %. At
amounts of less than 3 wt %, no cermet effects can be obtained.
Furthe~more, when the metal phase exceeds 25 wt %, the effect of
adding ceramics which are WC, WB or the like is lost. If at
least one of Cr and Mo is added in an amount of less than 15 wt
~, it is possible to improve the molten metal corrosion
resistance of the metal phase. It is therefore necessary to
limit the total amount of Ni, Co, Cr, and Mo to less than 25 wt
%.
The immersion member for use in molten metal baths i5
subjected to surface polishing after thermal spraying; in the
manufacturing method of the present invention, it is possible to
conduct final polishing after thermal spray coating, prior to
processing fluid impregnation processing, or after baking
processing. A strong acid solution in which chromic acid
comprises the main component is used as the processing fluid.
In order to conduct the impregnation of the processing fluid
into the thermal sprayed coating, it is possible to immerse the
member for use in molten metal baths, having formed thereon the
thermal sprayed coating, into the processing fluid, or to brush
the processing fluid onto the thermal sprayed coating formed on
the surface of the member for use in molten metal baths. By
means of the impregnation processing, the processing fluid
penetrates the cracks and micropores, and it is thus possible to
fill these cracks and micropores. Next, by means of the initial
heating during baking, the chromic acid ~H2CrO4 and ~2Cr207)
present in the processing fluid within the cracks and micropores
is converted to CrO3, and a filling of these ~racks and
micropores results. The chromic acid solution is desicc~ted by
2101772
means of the heating, and the moi~ture component thereof is
removed; however, if heating is continued, in the vicinity of
200 C , molten CrO3 (chromic acid anhydride) results, and it is
possible to conduct CrO3 molten salt processing in the thermal
~prayed coating. The thermal ~prayed coating in contact with
this is oxidized, and the CrO3 i9 finely bonded with the thermal
sprayed coating. That is to say, by means of the reaction using
CrO3, the CrzO3 which is formed and the inner surfaces of the
cracks and micropores are chemically bonded, and a fine
ceramic-filled thermal sprayed coating is formed. The baking
temperature should preferably be greater than 400 C , at which
temperature Cr203 conversion can be sufficiently conducted, and
less than 500 C ; at these temperatures, almost all CrO3 is
converted to CrzO3.
Furthermore, it has been determined that the reason that
the immersion member produced in accordance with the present
invention exhibits superior corrosion resistance with respect to
molten metals is that, after the impregnation processing with
processing fluid and baking processing, the borides, such as WB
and the like, which are present in the thermal coating sprayed
coating are finely and strongly bound with CrzO3.
In particular, in the present invention, the
vitrification reaction of the B203 produced by the oxidation of
the borides present in the thermal sprayed coating and the CrO3
i~ important. That is to say, thé vitrification of B203 beginQ
at a temperature of approximately 300 C during heating; however,
at this temperature, CrO3 becomes a molten oxide, and the
vitrified Bz03 and the CrO3, which has become a molten oxide,
oxidize the surface of the thermal sprayed coating and the layer
2 ~ 7 2
within the cracks and micropores, Yo that fine fusion occurs so
as to produce a CrO3-Cr203-B203 glass substance. Furthermore, when
heating is conti~ued and the temperature reaches a level above
400 C , the CrO3 is converted to Cr203 and Rolidifies completely
however, the B203 component becomes softer, a portion thereof
reacts with the Cr203, becomes more finely bound thereto, and the
cracks and micropores are filled. The melting point of B203 iS
approximately 450C .
Accordingly, the combination of the thermal sprayed
coating and the processing of the present invention should be
termed "glass sealing", and the oxide bonds between the thermal
sprayed coating and CrO3, and the bond resulting from
vitrification of CrO3 and B203 produce combined function to
provide a strong and complete crack-and-micropore-filling
effect, as well as an effect of an increase in layer bonding,
are exhibited. Furthexmore, no volatilization or combustion of
the moisture component or alcohol component occurs during the
thermal reaction (in the present invention, a dehydration
reaction occurs; however, the moisture component is removed
prior to the formation of molten CrO3), and there is no formation
of micropitting during heating. For this reason, it is thought
that a fine and strong surface layer can be formed.
Furthermore, heating to a temperature in excess of 500 C
produces strain or residual stress in immersion members for use
in molten metal baths, so that such heating is not preferable.
AS a result of the above, it iR recommended that the
heating temperature during baking processing be within a range
of 400C to 500C .
Furthermore, a strongly acidic fluid comprising primarily
2101772
chromic acid is used as the impregnation processing fluid of the
present invention; and the addition of Na+ and K~ ions may
improve the permeability of this fluid and apply the solubility
of the metallic oxides on the surface of the layer to B203, a
small amount of the salts thereof may be added. For example, a
small amount of sodium hydroxide (NaOH) or potassium hydroxide
(KOH) may be added.
Furthermore, it is possible to add sodium molybdate or
ammonium molybdate, or both sodium molybdate and ammonium
molybdate, to the processing fluid 3. By means of this, the
vitrification described above is improved, and furthermore, as a
result of the presence of Mo~3, it is possible to obtain a finer
and stronger bonding and diminution effect of micropores and
increa~ing fineness of layer microstructures. Thi~ is thought
to occur because the components filling the cracks or micropores
form a Cr203-B203-MoO3-borate system compound (for example, Na2B407
)-
Furthermore, it lS also possible to blend a water-soluble
coating agent; however, in this case, an oxidation reaction is
carried out by mean~ of chromic acid, so that such an agent
should be blended immediately prior to the use thereof in the
impregnation processing.
In order to increase the reliability of the coating and
strengthening effects of the thermal sprayed coating resulting
from the manufacturing method of the present in~ention, it is
also possible to repeat the cycle of the processing fluid
impregnation processing and baking processing two or more times.
2 i 0 1 7 7 2
Best Mode for Carrying Out the Invention
~ ereinbelow, an embodiment of the present invention will
be explained.
Embodiment I
A plurality of metal plates conforming to American Iron
and Steel Institute standard AISI 316 (corresponding to the JIS
standard SUS 316~ having a thickness of 5 mm, a width of 30 mm
and a length of 100 mm were prepared, and on one side of each
metal plate, a thermal sprayed coating was formed by means of a
high velocity oxygen fuel gun method, and as shown in Table 1,
metal plates having formed thereon thermal sprayed coating
having the compositions a-k, o, p, q, and r were produced. The
compositions of the thermal sprayed coating formed on the sample
metal plate surfaces are shown in Table 1. The compositions
having the reference letters a-k fulfill the conditions of the
present invention. The compositions referenced o and p do not
fulfill the conditions of the present invention and are
presented as Comparative Examples. The sample metal plates
referenced q and r are Conventional Examples corresponding to
~tandard conventional products; they employ WC~Co system cermet
thermal sprayed coating.
Next, a3 shown in Table 2, impregnation processing in
proces3ing fluid and baking processing were conducted on the
sample metal plate~ prepared a~ described above, and a molten
zinc bath immersion test was conducted. In concert with this, a
molten zinc immersion test was conducted with respect to the
sample metal plates which had not been subjected to impregnation
processing in processing fluid or baking processing, and
7 2
11 `
comparison was made with the examples of the present invention.
The plating bath employed in the test was a zinc aluminum
(Zn-Al) plating bath containing 3% aluminum. In this test, each
sample metal plate was continuously immer~ed in this plating
bath, and the bath temperature was maintained at 500 C ; the
state of the thermal sprayed coating of each sample metal plate
was then visually evaluated. As a result of this evaluation,
those plates which exhibited no corrosive peeling even after a
period of 30 days of continuous immersion are indicated by the
designation ~, plates which exhibited no corrosive peeling after
10 days of continuous immersion but which exhibited corrosive
peeling after 15 days of continuous immersion are indicated by
the designation O , while plates which exhibited corrosive
peeling after a period of 10 days of continuous immersion are
indicated by the designation ~ .
In Table 2, Examples 1-28 correspond to examples of the
present invention, while Comparative Examples 31-42 are examples
having thermal sprayed coating, identical to those of 1-28,
which were not subjected to impregnation processing in the
processing fluid or to baking procPssing. As is clear from the
results shown in the Table, even immersion members possessing
thermal sprayed coating having identical compositions did not
have long service lives if not subjected to impregnation
processing in the processing fluid and baking processing.
Furthermore, even if impregnation proces~ing in the processing
fluid and baking processing were conducted with respect to
immersion members having a conventional WC-Co cerme~ thermal
sprayed coating formed thereon, satisfactory effects could not
be obtained, as shown by Comparative Examples 45 and 46.
~0~772
12
Furthermore, as is clear from Comparative Examples 43 and 44, in
cases in which the metal phase of the thermal sprayed coating
was 2 wt % and 3~ wt ~, these examples were unacceptable in
spite of the fact that WB was contained in an amount of lO wt %.
This was found to be so because, in the case in which the metal
phase is too small, the caramic material peels easily away from
the thermal sprayed coating, while when the metal phase is too
large, the metal phase is corroded by the molten metal.
From the above Examples, Comparative Examples, and
Conventional Examples, it was found that the effects of the
present invention are great.
Industrial ~pplicability
As stated above, the manufacturing mPthod for immersion
members for use in molten metal baths in accordance with the
present invention i5 capable of producing immersion members for
use in molten metal baths which possess corrosion resistance
with respect to molten metals, have superior resistance to
corrosive peeling, have superior resistance to abrasion, have a
long service life, have superior wettability with respect to
molten metals, and exhibit little metal adhesion, so that such
members are extremely useful in industry.
21 ~1772
13
Table l
R :
~ . Metal-Phase
e Ceramic Composition (wt %)
r Composition (wt %)
C
WB CrB2 Other WCCo Ni Mo Cr
. ~ (W2B5) Borides _
a 10 _ _ ~n- 10 _ _ _
Used b 10 MoB 3 der 10 _ 3
in c 20 _ _ ~nd~---_ 13 _ _
Composi- Embodi- d 20 5 ZrB2 5 der 12 _ _
tion mentse 20 = TiB2 10 = 11 _ _ 5
of of f 2 0 _ MoB 2 5 der 8 _ 5 _
Thermal Present g 20 _ _ trhdn- 5 7 _
Sprayed Inven- h 30 = = 10 3 5
Coating tion i 3 0 5 _ der _ 12 3 5
_ 30 5TiB2 5 tmdn- 12 5 3
k 40 _ . ._ _ ~3~in- 12 _ - _
C~pa- O 10 _ _ R~i3~- 2 _ _
rative ~ . der _ _
E~ple5 P 10 _ ....... __ ~_ 35 _ 3 _
Cc~n- ~ _ _ _ Ranain- 10 _ _
to~ r = = ~ d~ 12 _ _ 5
ote l: Thermal spraying on one surface of an AISI316 sample
having dimensions of 5mm X 30 mm X lOO mm
ote 2: In the Table, (W2Bs) indicates that a small amount of
W2Bs is contained in the WB.
2 ~ 7 7 2
14
Table 2
_ThermalImpregnation Baking Molten Zn
No.Sp~ed Processing Proces- Bath
CoatingCom~Fluid sing Immersion
sition(froc Test
1 a 30~ Chrrdc ACid 450C, Ba~ g (~
_ 30 mi~ tes _ _
2 a30~ Chmoie Aeid, 2i 450C, Baki~ (~
di~sn ~1~3ate MiXn~re 30 mirlutes _
3 b3096 C~mie Aeid450C, ~ g
4 30~ Chrcmic Aeid, 26 450C, ~id~
S~di~n ~l}~te Mixturo 30 m~r~tes
C30~ Cbmnie Acid450C, Bak~
_ _ _ .... _ _ 30 m~l~tes
Embodi- 6 c303 Chranie Acid, 296gsoC, Baki~g
_ Sodit~ date ~cture30 n~i~tes .
ments 7 c30~ mieAeid, 2~Am- -gSODC Ba~ g ~}1
m ni~n ~l~te ~ixture30 mi~tes
of 8 d30t Ch~alLie Acid950C, Ba.lc~
_ . 30 mi~utes
Present 9 d30~ Chrcmie Aeid, 2~450C, Bakir~ @~
_ = 30 m~nutes
Invention 10 e 3096 Chrcll~ic Aeid gSOC, Ba~cing
_ 30 m~tes . _
11 e3096 Chranic Aeid, 2~gSOC, Baki~ ~)
Sodium r~l~a3ate Mixture 30 mi~tes _ .
12 e30~ Chrcmie Aeid, 2i450C, Bakir~
Sodiun ~l~ate, 296 Am-30 min~ltes ~})
mlli~n l~ly}x~ato ~re .
13 f30i Chmnie Aeid 450C, ~ g
. 30 minutes
14 f30~ Chrnie Aeid, 2i450C, Ba~ g ~3
_ S~di~n ~ly~date ~e30 mi~lutes
f303 Chr~mic Acid, 23 Am- 450C, Bakir~g ~a
m ni~n ~l~ate Mixture30 ~utes
16 303 C}~mic Acid 450C, ~aki~
_ 30 m~tes
17 g303 Ch~ ic Acid, 23450C, Bakir~g O
scdiun ~l~ate Mixture30 mir~tes
18 g303 Ch~cmic Acid, 23 Am- 450C, Bak~
~ni~ l~te Mi~cture 30 mi~tes _
19 g303 Chrcmic Acid, 23450C, E~
Scdiun Mcl~te, 296 Am-30 mir~tes ~3
nmi~ Dl~te ~ixture
h303 Chr~mic Acid450C, 8akir~g
. . . . 30 mirD~tes
21 h 303 Chrcmic ACid, 23 450C, Bak~
. S~ n l~l~te Mix~re 30 mir~tes .. .__. .
2 2 i303 Chrcmic Acid450C, Ba~cir~g (~
30 oiD~ _ __
23 i303 Chrcmic Acid, 23 450C, 8a~cing _
_ Scdi~m 1~1~3ate ~ctyre 30 mi~tes
(continued)
Note 1: The evaluation of the zinc bath immersion test (molten Zn bath
containing 3% Al, 500C, an AISI 316 sample tharmal sprayed on one surface
and having dimensions of 5 mm X 30 mm X 100 mm) was as follows:
~: No corrosive peeling after 30 daysl immersion
0: No peeling after 10 days, corrosive peeling after 15
days~ immersion
~: Corrosive peeling after 10 days' immersion
2101772
Table 2 (continued)
_ .
ThermalImpregnationBaking Molten Zn
No.Sp~edProcessing Proces- Bath
~ting ~ i~ Fluid s~ng Immerslon
Embodiments 24 i303 Chranic ACid 450C, ~a~cing
_ _ _ _ 30 minutes
of Present 25 i30~ChromicAcid, 2~ 450C, Bak~
dium ~l~te Mixture30 mir~tes
Invention26 k 30~Chrnic Acid4S0C, ~)cir~ 0
27 k30~ Chrcmic Acid, 2~ 450C, Baking
_Sodi~m~l~e ~ure30 minutes
28 k30~ Chmrdc Acid, 2~ Am-450C, 8aking ~)
. _ msniu~ l~te Mi~cture 30 minutes
Comparative 31 _Prccessina Fluid Prccessin~ O
Examples 32 b r~ ~pr~til inN~ lcirg O
Pr~xessir~r FluidProcessirg
33 c~ ~p~ati~ in ~ a~ O
_ Pr~essir~ Fluid ~
34 d~ ~p~ati~ in ~ ~ O
Prccessi~ FluidPrccessi~
e~ ~p~ation in ~ ~ O
_ Prccessirg FluidPr~cessing
3 6 f~ ~pr~nation in~ ~dng O
P~cessir~ Fluid _Prccess~ng
37 g~ ~p~ticn in ~ ~ O
.Pmcessir~LFluidPrccessinSL
38 h~ ~p~ati~ in ~ ~ O
E~rccessing FluidProcessir~
3 9 i1~ ~pr~pation in~ 33kina O
Prccessim FluidPrccessir~
41 . ~b ~p~tial in . _ _
Prccessing Fluid Prccessir~T
42 k~ ~p~ati~ in ~ ~ina O
Processina Fluid Pmcessir~
a~3 30~ Chr~nic Acid 450C, Ba)cin
. _ . 30 nunutes
44 P 30~Chmmic Acid 450C, ~ ~
e . _ 30 mi~es
~1 30~ mic Acid 450C, Bakina
. 30 Ir~utes
46 r 30~ Ci~cmic Acid 450C,
_ . .. 30 ~uCe~ .
Convention- S1 q ~ ~p~ati~ in ~ ~ ~
.~cessir a Fluid P~e5s~3 _
al Examples 52 r ~ ~a~ion in ~ a~ ~
ccessirg Fluid Prccessir~
Note 1: The evaluation of the zinc bath immersion test (molten Zn bathcontaining 3% A1, 500C, an AISI 316 sample thermal sprayed on one surface
and having dimensions of 5 mm X 30 mm X 10,0 mm~ was as follows:
~: No corrosive peeling after 30 days' immersion
o: No peeling after 10 days, corrosive peeling after 15
days' immersion
~: Corrosive peeling after 10 days~ immersion
