Note: Descriptions are shown in the official language in which they were submitted.
'I`l.tlc o[ ~ he .Lnventic)n
E'rocess to~- prepdrln(3 hot-clip zinc-plated steel stlelLs
I`ecl1nicc~ cld of the Invention
I`hls invention relates to a process for preparirlc3
hot-dip zinc-plated steel sheets which have excellent
corrosion resistance and excellent coatability.
Background of the Invention
Ln recent years, use of hot-dip zinc-plated steeL
sheets has become rnore diversified and sophis-ticated.
Whereas these steel sheets have conventionally been used
for roo~s and walls, they are now used as materials for
autornobiles, as precoated materials for colored sheets,
e-tc. That is, in these uses the steel sheets are subjected
to severe plastic worlcing such as bending, deep drawing,
etc. and the finished produc-ts must have excellent surface
properties and appearance. So, there is increasing demand
for products which are far better than the conventional
ones not only in the formabilitv of the substrate sheet
but also in the formability of -the alloyed layer, as well
as in the corrosion resistance of the alloyed layer of the
formed parts.
In order to improve the corrosion resistance of
hot-dip zinc-plated steel sheets, two me-thods have hitherto
been resorted to. One is to increase the thickness of
plated zinc layer, and the other is to improve the quality
of the plated zinc layer i-tself by incorporating o-ther
elements in the plating bath
The former will enhance the protec-tive effect
of the zinc layer for the iron subs-trate, but a thick layer
is susceptible to cracking when the plated sheet is worked,
and flaking o~ the plated layer and deteriora-tion in the
surface appearance rnay result Therefore, this method is
prac-tically unacceptable.
~ q Regarding the latter rnethod, ~. S Patent No.
3'5 4,02~,~78 proposes use of a zinc bath containing 0.2 - 17
3~
-- 2 -
Al, 0.003 - ().15 % M~ and 0.02 - 0.15 ~ Pb. Use of ti1iS
zinc bath ~ives considerdhly improved corrosion resistance.
~lowever, a close study revealed thcl~ the resultir1cJ corrosio
resistance is not entirely sufficient and -that this method
does not yive sheets with satisfactory coatability. The
reason is surmised to be that the balance among the added
elements such as Al, Mg and Pb is not completely
satisfactory. More specifically -the Pb content is rather
too high and the Mg content is insufficient rela-tive to
the Pb content.
In order -to irnprove the surface appearance and
mechanical properties of zinc plated steel sheets, it is
also generally practiced to cool the plated sheet by
spraying water mist immediately before the plated zinc
solidifies after plated so as to minimize the size o~ the
zinc crystal grains, or to smooth the surface of the sheet
by stretcher leveling and temper rolling in combination.
However, rapid cooling of the plated layer or mechanical
smoothing increases the number of ac-tivated points at which
corrosion may be initiated. Therefore, these me-thods are
not desirable from the viewpoint of corrosion resistance,
and especially excessive temper rolling should be avoided.
Further, when use of zinc-plated steel sheets
for precoated sheets or automobile materials is considered,
a thinner plated layer is preferred because these materials
have to be subjected to shaping and welding and thinly
plated sheets are advantageous in this respect. Therefore,
a steel sheet with a thinner plated layer and high corrosion
resistance is desired. Modernly, the thickness of plated
layers is controlled by gas-wiping under high speed
operation (160 - 200 m/min). Under such high speed
operation, the amoun-t of plated zinc is in the range of
~5 ~ 60 g/m~ per side (the amount of plated zinc will be
indicated per surface hereinafter) with the conventional
plating bath (0.15 - 0.18 ~ ~l*). This is far from the
* All composition percentages yiven by weigh-t unless otherwise
speci~ied.
"j
~i'l, ~,;,~
--3--
targe-t oE 30 g/m or less, which is desirable from the
standpoin-t of working of the plated sheets. The measure
generally employed in order to reduce the amount of plated
~:inc is to slow the plating speed by 20 - 30 % below the
standard speed to increase gas wiping effect. However,
this lowers the productivity and therefore is no-t desir-
able in an industrial process.
We have studied -the above problem and ~ound that
the key to the solution fundamen-tally resides in the
property of the plating bath. Thus we have completed
this invention.
Summary of the Invention
According to this invention, there is provided
a novel zinc plating bath essentially consisting oE 0.35 -
3.0 % Al, 0.15 - 1.0 % Mg, and the balance of Zn and
inevitable impurities, whereby the Pb content is not
more than 0.015 %.
Further, a process for preparing hot-dip
zinc-plated sheets characterized in that a plating bath
essentially consisting of 0.35 - 3.0 % Al, 0.15 - 1.0 %
My, balance Zn and inevitable impurities, whereby the
Pb content is no-t more than 0.015 %.
In the preferred embodiment, the amount of the
plated zinc is not more than 30 g/m per side. Also the
plated shee-ts are heated so as to form alloyed layers.
A preferred bath composition is: 0.35 - 2.0 %
Al, 0.15 - 0.8 % Mg, balance Zn and inevitahle impurities,
whereby the Pb content is not more than 0.01 %
A more preferred bath composition is: 0.35 -
1.0 % Al, greater than 0.20 - 0.5 % Mg, balance Zn and
inevitable impurities, whereby the Pb content is not
more than 0.007 %.
By inevitable impurities is meant impurities
contained in th~ commercially available ~inc, that is,
not more than 0.01 % Cd, not more than 0.01 % Sn and not
more than 1.0 % Fe.
Lower limits for the amount of Mg in the bath
composltion include greater than 0.20 %.
t~
~rllis inv~u~tion has the ~oLlowing advantages.
1. The~ corrosion resistance oE -the products is more than
three times t~lat of the products produced with the~
conventional plating ba-tn containing 0.15 - 0.18
Al.
2. Smooth plated sheets with mlnimized spangles are
obtained wi-thout particularly rapid cooling aEter the
sheets have passed through the plating bath.
3. Produc-ts having excellent surface appearance and
mechan:ical properties are obtained by sub~ecting the
sheets to a light temper rolling (aroun~ 1 % reduction)
aEter plating.
4. As a result of increasing the Al content in -the ba-th
2.5 - 20 times, -the Eluidity of the bath is increased
and the wlping effect in gas wiping is enhanced under
the same gas wiping condition at the same temperature
and thus products with a thinner plated layer are
obtained.
~ow each component of the bath composition and
the concentration thereof are explained.
1. Aluminum
As the Al content increases in the ho-t-dip
zinc-plating bath, the fluidity of the bath at the same
-temperature increases remarkably over that of -the
conventional plating bath. When the Al content is increased
from 0.15 % of the conventional bath to 0.3 % or more, the
fluidity of the plating bath is increased by 1.5 - 2.0 %
or so. So -the lower limit of the Al content of the bath
in this invention is defined as 0.35 %. The upper limit
is defined as 3.0 %, since if the Al content exceeds this
value, -the Zn-Al eutectic structure becomes remarkable and
local cells are formed between the eutectic phase and the
zinc phase and this induces deterioration in corrosion
resistance.
2. Magnesium
- 4
.~¢3~ 3
-- 5 ~
~ g is one of the elements added for the purpose
of increasing corrosion resistance. The corrosion
resistance of the ~ormed alloyed layer marked:Ly improved
when about 0.15 ~ or more Mg is contained. As the Mg
content increases, the corrosion resistance increases
accordingly, but if it exceeds 1.0 %, deterioration in the
surface appearance of the alloyed layer by ormation of
wrinkles, and oxidation of the surface gradually becomes
pronounced, and at the same time oxida-tion (formation of
dross) on the surface of the pLa-ting bath is promoted, which
results in wasting of -the pla-ting bath. The allowable upper
limit of the Mg content is 1.0 %.
3. Lead
Pb has almost no solid-solubility in Zn at
ordinary temperature and therefore, it precipitates usually
within the crystal grains or at the grain boundaries, as
minu-te par-ticles which form local cells. This is one of
the causes of deterioration in corrosion resistance. In
this sense, it is desirable that the Pb content be as low
as possible. It has been confirmed, as the result of close
investigation, -that when the Pb content is not more than
0.015 %, microscopically almost no precipitation of Pb in
grains and at grain boundaries of the alloyed layer is
found, prac-tically no intergranular corrosion is found;
and almost no crystal spangles of Zn can be observed by
the naked eye. Thus the Pb content is defined as not more
than 0.015 ~.
By using the Zn plating ba-th of the above-
mentioned composition, Zn-plated steel plates with a very
-thin plated layer (not more than 30 g/m2) and having
excellent appearance and corrosion resistance can be
obtained under the conven-tional operation conditions.
It should also be noted that although the thinness
of the plated layer is one of the characteristics of this
inven-tion, the amount of the plated Zn can be varied by
.33~3
6 ~
changing the conditions o~ gas wiping. Therefore, thicker
plated layers can also he obt~ined.
Detailed Description of the Embodiments
Now the invention is explained in detal:L b~ way
of working exampl.es.
Example 1
Using a Zn plating bath, the composition oE which
is shown in Table 1, non-annealed rimmed steel sheets 0.4 mm
in thickness and 300 mm in wldth were plated with Zn with
a conventional gas reduction plating apparatus under -the
following conditions.
Pre-treatment:
Temperature of sheets at -the outle-t
of the non-oxidizing furnace: 590 - 600C
Reduction furnace gas: H2 75 %I N2 25 %
Temperature of the gas at the outlet oE
the reduction furnace: 700 - 720C
Bath temperature: 460C + 5 C
Thickness of pla-ted layer: 120 g/m per side
After-treatmen-t:
I'emper rolling: not employed
After-treatment with chromic acid- not employed
The methods for testing of charac-teristics of
the obtained products and for evaluation of the test results
were as follows:
Size of spangles: Observation by naked eye
Adherence of plated layer: Plated sheets were
lock-formed and a cellophane adhesive -tape was
applied on the worked portion, whereafter
flaking of the p.Lated layer was observed when
the cellophane -tape was peeled off. No flaking
is rated as 'good'.
Existence of Pb in grains and at grain boundaries:
Determined by means of a scanning electron
microscope and rated as 'large', 'medium',
'small' and 'minute'.
Corrosion resistance:
(1) Time (hour) until generat.ion of red rust
was observed in the salt water spray test
(ASTM B117-73) was measured.
(2) Corros:ion weight loss (g/m2) was determined
200 hours after the salt water
(ASTM B117-73).
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lrhe test results are sumrnarized in Iable 1
together with the bath composition. The characteris-tics
of the products of the process oE this invention and ~hose
of the conventional process (comparative samples) are
explained i.n comparison.
1. Spangle size:
The products of the process of t`r,is invention
exhibited very minute spangles despite the Eact that the
pla-ted sheets were allowed to spontaneously cool by
themselves. The ra-ting 'minute' in the -tabl.e means that
spangles were almost unrecognizable by naked eye and the
surface appearance was very smoo-th. In contrast, appa:ren-t
'medium' and 'large' spangles could be observed by naked
eye in all the comparative samples in wh.i.ch the plated
sheets were cooled spontaneously, The spangles were 'small'
only in sample No. 10 in which the sheets were forcibly
cooled (water cooling). It is no-ted -that water cooling
makes the spangles minute in all the comparative samples,
although inferior in homogenuity. When water cooling was
not employed, apparent spangles were observed and the
surfaces were considerably rough. Therefore rather high
level temper rolling was necessary ln order to smooth the
surfaces.
2. Adherence of plated layer
Although there was no problem in either the
products of the comparative samples or those of -this
invention in the adherence of the plated layer per se,
occurrence of cracking in the plated layer at -the bent
portion was very rare in the products of the process of
this invention, while in the comparative samples
considerably remarkable cracking in the plated layer within
the crystal grains and at the grain boundaries were
detected. The products of the process of this invention
were much superior to the compara-ti,ve samples.
3. Existence of Pb in grains and at grain boundaries
-- 1 0 --
As the resuLt of restric-ting the Pb content to
not more than 0.015 ~, existence of Pb in grair1s and at
grain boundaries was very slight and could not be clearly
discerned in the scal1ning electron microscopic photographs.
5 In contrast, in -the comparative samples in which the Pb
content was high, exlstence oE Pb in grains and at grain
boundaries was clearly observed, in all the samples except
No. 4.
4. Corrosion resistance
In dll the produc-ts of the process of -this
invention, more than 1000 hours passed before red rust was
generated, and the corrosion weight loss was less than 30
g/m2. On the other hand, red rust was generated within
800 hours in the rather better sample No. 11 and wi-thin
200 hours in the inferior samples No. 10 and 12. The
corrosion weight loss of the comparative samples were 2 -
6 times as great as that of the products of the process
of this invention.
Example 2
Using a hot-dip zlnc plating bath of the
composition shown in Table 2, very thinly plated zinc-plated
steel sheets were prepared with the same apparatus as used
in Example 1 under the following conditions.
Substrate sheets: 0.4 mm thick rimmed steel sheets
Pre-treatment: the same as in Example 1
Thickness of plated layer (condition of gas wiping)
Gas pressure: 0.35 kg/cm
Position of nozzle: 150 mm from the bath surface
Distance between strip and nozzle tip. 6 mm
After-treatment:
Temper rolling: not employed
After treatment with chromic acid: not employed
The test results are summarized in Table 2
together with the bath composition. The charac-teristics
oE the products of the process of this invention and those
- 10 -
o~ the convent.ional process (comparative samples) are
explained in comparison.
- 11 -
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- 13 -
1. Amount oE pla-ted zinc
It is apparent from the table that the dmount
of the pla-ted zinc is around 10 g/m2 per side in the
products of this invention, meaning that it is very thin
plating, while the amount of the plated zinc is about twice
that, in -the comparative samples. This means that the
process of this inven-tion is very effective in controlling
the thickness of the plated layer by gas wiping.
2. Corrosion resistance
As the plated layer is very thin in the process
of this invention, the corrosion resistance oE the products
was a matter of concern. ~lowever, i-t was conEirmed tha-t
considerably high level corrosion resistance was acquired
as seen in the table. That is, the -time required for
generation of red rus-t was 270 hours in Samples No. 17,
140 hours and 180 hours respective in Nos. 16 and 18. The
time before the generated red rust spread to about 30 %
of the tested area was 840 hours, 520 hours and 580 hours
respectively in Nos. 17, 16 and 18. That is, development
of corrosion was considerably slow and good corrosion
resistance was recognized. On -the other hand, the amount
of the plated zinc in the comparative samples was about
twice tha-t of -the samples of the process of -this invention,
and yet the area covered by red rust exceeded 30 % in 100
hours in No. 19 and exceeded 30 % in 450 hours even in No.
20, which had a thick pla-ted layer. This fact means that
the process of this invention is superior to the
conventional process even in the corrosion resistance of
the products.
It has been also confirmed that alloyed zinc
plated steel sheets which are obtained by heat--treating
the plated sheets made in accordance with the process of
this invention are provided with far better corrosion
resistance than the products produced by using the
conventional plating bath.
3~3
Also i.t has been confirmed that single side
zinc-plated steel shee-ts Eor automobiles produced on the
trial base by using a plating inhibitor are e~cellent in
bo-th corrosion resistance and workability.
~
The zinc-plated steel shee-ts produced by the
process of -this invention are expec-ted to find use as
pre-coated sheets for colored sheets, automobi.le materials,
materials for householcl electric appliances as well as in
industrial fields which will be developed from now on, in
addition to the conventional use as materials for roofs
and walls. The invention of this application will be highly
evaluated in its industrial application.
