Hot dip coated steel sheet and process for producing the same

Disclosed is a hot dip coated steel sheet obtained by hot dipping a steel sheet in a hot-dip metal bath comprising zinc-based or zinc/aluminum based alloy, and a process for producing the same. The hot dip coated steel sheet has a film of oxides which are compounds of cobalt or iron, or both of these, with oxygen, and the film can prevent the hot dip coated steel sheet from undergoing color change into grayish black in the case where spangles are minimized, where a coating contains corrosion resistance-improving elements such as magnesium and aluminum, or where a chromate treatment is applied after the mechanical processings were carried out. Formation of oxide film on the surface of the hot dip coated steel sheet is carried out by a method in which an aqueous solution obtained by dissolving a salt of cobalt or a salt of iron or both of these is sprayed on the surface of a coating of the steel sheet when the temperature of the coating is 170.degree. C. or higher. The salt is preferably a nitrate or a chloride for both the salt of cobalt and the salt of iron.

TECHNICAL FIELD 
This invention relates to a hot dip coated steel sheet, coated with 
zinc-based or zinc/aluminum-based alloy, which is provided with a metal 
oxide film formed on the surface of a coating so that the surface of the 
coating may hardly undergo a color change into grayish black even when 
spangles are minimized or a chromate film is formed on the surface. 
BACKGROUND ART 
As the hot dip coated steel sheets comprising a zinc-based coating, there 
are known, in addition to the ordinary ones whose coating contains Al 
added in a very small amount for the purpose of inhibiting the growth of 
an alloy layer in the coating, those in which an element for improving 
corrosion resistance is added thereto, for example, those in which the 
coating comprises 4 to 53% of Al, 0 to 0.5% of Mg, 0.002 to 0.3% of Pb and 
the balance of Zn and inevitable impurities. As the hot dip coated steel 
sheets, coated with zinc/aluminum-based alloy, there are known those in 
which the coating comprises 0.1 to 0.3% of Al, 0 to 0.5% of Mg, 0 to 10% 
of Si, 0.002 to 0.3% of Pb and the balance of Zn and inevitable 
impurities. These hot dip coated steel sheets are usually continuously 
produced through the steps of heating reduction of a steel belt, dipping 
of the belt into a hot-dip metal bath and regulation of the coverage. 
However, in order to improve appearance, flatness, formability, etc., it 
is often carried out that spangles are minimized by spraying water on the 
coating while that formed is still in the unsolidified state immediately 
after the step of regulating the coverage, or that various mechanical 
treatment processings, for example, buffing, levelling and skin pass 
rolling treatments are applied after the coating has been solidified. 
However, of those in which the spangles are minimized, or, even in the case 
of regular spangles, those which are obtained by processing in a hot-dip 
metal bath of zinc-based alloy or a zinc/aluminum-based alloy with 
additional elements such as Mg, or those in which mechanical treatment 
processings are further applied thereto to make chromate treatment for the 
purpose of improving the corrosion resistance or the paint film adherence, 
the surfaces are subject to color change into grayish black (hereinafter 
referred to as "blackening") in a relatively short period of several 
months even stored under ordinary conditions, thereby losing the 
appearance of silver white. 
This blackening is presumed to be caused by changes in the state of 
elemental distribution or the crystal orientation on the surface of a 
coating, due to the treatment for minimizing spangles or the mechanical 
treatment processings, to form a film (an oxide layer mainly composed of 
Al, Cr and Zn) which is liable to absorb light, said film having a blacky 
appearance. This blackening becomes extreme particularly when a chromate 
treatment has been applied. Therefore, in order to lessen or make quiet 
the blackening, a post-treatment may be carried out in a non-chromate 
system. However, such post-treatment in the non-chromate system results in 
high cost for the treatment. 
On the other hand, as a process which can prevent the blackening even when 
the chromate treatment has been applied for the post-treatment, there is 
known a process in which a coating is treated, after the mechanical 
treatment processings and before the chromate treatment, with an acidic 
aqueous solution of pH 1 to 4 or an alkaline aqueous solution of pH 11 to 
13.5 containing Ni ions or Co ions or ions of both of these (as disclosed 
in Japanese unexamined patent publication No. 177381/1984). According to 
this process, although more inexpensive than the post-treatment of the 
non-chromate system, it is required to provide an additional treatment 
tank anterior to a chromate treatment tank, and also, since the treatment 
solution is strongly acidic or strongly alkaline, a washing tank is needed 
to wash out the solution. Thus, equipment cost becomes high and also the 
steps are made complicated. 
Also, this process can prevent the blackening by deposition of Ni or Co on 
the surface of an active coating in the form of metals or oxides. However, 
since the deposition taking place here is effected by the displacement 
plating, the adhesion of the deposited metals or oxides is weak. 
Accordingly, the deposits are readily removed to cause the blackening on 
the removed face in the shape of spots, streaks or stripes, if slippage 
occurs between sheets when the steel sheet is wound into a coil, or when 
cut sheets are overlapped, handled or transported, or if friction is 
applied by roll forming, pressing or bending processings. 
Further, if the aqueous solution, which is strongly acidic or strongly 
alkaline as mentioned above, is not sufficiently washed after the 
treatment, the remaining acid or alkali may be a factor for corrosion to 
cause the blackening or corrosion or the development of white rust on the 
contrary, or to cause the formation of local cells between Zn and the 
deposited Ni or Co, resulting in the microscopic corrosion. 
This invention provides a hot dip coated steel sheet comprising a stable, 
blackening-preventive film toughly formed on the surface of a coating, and 
a process for producing the hot dip coated steel sheet, which makes it 
possible to form inexpensively and toughly the film on the surface of a 
coating. 
DISCLOSURE OF THE INVENTION 
This invention prevents the blackening by forming a film of oxides of 
cobalt or iron, or both of these, on the surface of a hot dip coated steel 
sheet whose coating is comprised of zinc-based or zinc/aluminum-based 
alloy. The formation of the oxide film is carried out, after subjecting a 
steel sheet to hot dipping of zinc-based or zinc/aluminum-based alloy, by 
regulating the coverage and thereafter spraying an aqueous solution 
containing a salt of cobalt or a salt of iron, or both of these, on the 
surface of a coating while the coating is kept at a temperature of 
170.degree. C. or higher or after the coating is heated to 170.degree. C. 
or higher, to effect thermal decomposition of the salt. 
BEST MODE FOR WORKING THE INVENTION 
The Japanese unexamined patent publication mentioned before discloses a 
process in which a hot dip, zinc-based alloy coated steel sheet is treated 
with an aqueous solution containing Co ions, whereupon oxides of Co are 
deposited. However, since the oxides mentioned in this process are those 
having been formed by the displacement plating, the oxides are not the 
compounds reacted with oxygen, but the compounds in which the valence of a 
metal increased, and thus they are different from the oxide film according 
to this invention. The film of oxides according to the invention is 
comprised of a compound reacted with oxygen, such as, in the case of Co 
for example, Co.sub.3 O.sub.4, CoO or spinel type CoAl.sub.2 O.sub.4, 
which is chemically stable enough to act as a barrier to protect the 
coating for a long period of time, thereby inhibit the formation or growth 
of a blackened film composed mainly of ZnO.sub.2 and containing Al.sub.2 
O.sub.3, Cr.sub.2 O.sub.3, MgAl.sub.2 O.sub.4, etc. 
The coating quantity on the oxide film should be controlled to be 1 to 100 
mg/m.sup.2 calculated in terms of metal cobalt or iron. If it is less than 
1 mg/m.sup.2, the blackening preventive effect is insufficient. If it 
exceeds 100 mg/m.sup.2, the film will be colored. 
A chromate film may be formed on the oxide film in order to further surely 
prevent the blackening and impart the paint film adherence to the film. If 
the oxide film is present, the blackening never proceeds contrary to the 
conventional case, even if the chromate film is formed. 
The blackening is not effectively prevented or an adverse effect will be 
exhibited, if the oxides comprise other metals such as Ni, Zn, Ti, W and 
Nb. 
The formation of the oxide film on the surface of a coating is carried out 
by keeping or adjusting the temperature of a coating to a temperature 
higher than the temperature at which a salt of cobalt or a salt of iron 
can effectively undergo the thermal decomposition in a short time, i.e., 
170.degree. C. or higher, and then spraying on the coating an aqueous 
solution containing the salt to effect the decomposition of the salt. If 
the salt is a nitrate or the like, the thermal decomposition takes place 
even at about 20.degree. C. to form oxides, but such a low temperature may 
lower the efficiency of the thermal decomposition. 
The spraying of the aqueous solution may be carried out either while the 
coating is kept at 170.degree. C. or higher after the step of regulating 
the coverage in the production of a hot dip coated steel sheet, or by 
heating to 170.degree. C. or higher a hot dip coated steel sheet once 
having been cooled after its production. In particular, it is very 
convenient if the spraying is carried out at the stage where a coating is 
still in the unsolidified state, since oxides of cobalt or iron produced 
by the thermal decomposition are dispersed in the surface of a coating to 
be made integral with the coating, whereupon a tough oxide film is formed 
on the surface of the coating and also spangles are uniformly minimized 
because of salts acting as nuclei of them. When spangles are minimized, it 
has bee known even in conventional arts to add a minimized spangle-forming 
agent such as phosphates and sodium chloride to an aqueous solution. In 
this invention, however, the salt of cobalt or the salt of iron can also 
play a role as the minimized spangle-forming agent. It is also possible to 
mix the conventional minimized spangle-forming agent together with these 
salts. By spraying an aqueous solution of a salt in the manner mentioned 
above, the oxide film is melt adhered or firmly sticked to the surface of 
a coating, and may not be readily removed or melted even when the 
mechanical treatment processing such as leveling or skin passing rolling 
or the chromate treatment is applied on the resultant steel sheet. 
Further, the aqueous solution may be left as it was sprayed, and no washing 
out thereof is required. Accordingly, a continuous hot dipping equipment 
presently used may be additionally provided with only a spraying device. 
The salt preferably includes nitrates and chlorides. This is because other 
salts such as acetates, sulfates and fluorides, although they can form a 
metal oxide film, do not show a satisfactory blackening preventive effect. 
Moreover, they are chemically unstable and tend to be readily melted out, 
and also an unreacted residual product or decomposed product may act as a 
corrosion factor to inhibit the blackening resistance. 
There is no limitation in the concentration of the salt of cobalt or the 
salt of iron in the aqueous solution, the temperature of the aqueous 
solution, etc., but it is preferable to control the salt concentration to 
0.1 to 20 g/lit. in terms of ion concentration. The reason therefor is as 
follows: If the metal ion concentration is less than 0.1 g/lit., the 
amount of the metal oxide formed on the surface of a coating is so minute 
that the blackening preventive effect may become small and also that, when 
spangles are to be minimized by spraying the aqueous solution to the 
coating having not yet been solidified, it may become impossible to 
achieve uniform minimization, and, if it exceeds 20 g/lit., a color 
inherent to molten salts comes out to color the whole area of the surface, 
although the blackening preventive effect can be improved. 
The pH of the aqueous solution should be adjusted to 5 or 7, different from 
the pH in the process dsclosed in the above-mentioned Japanese unexamined 
patent publication. This is because, if the pH is too low or too high, the 
solution is instantaneously reacted with the coating because of the high 
temperature of the coating, thereby causing etching or microscopic 
corrosion on it, resulting in not only the change in color but also the 
corrosion. When a salt or chloride of cobalt is used as the salt, it may 
be dissolved in water so as to give 0.1 to 20 g/lit. of cobalt ion 
concentration, whereby the pH is turned to be of 5 to 7 which is of from 
weakly acidic side to neutral side and thus it is made possible to use the 
solution as it is without particularly adjusting the pH. 
The treatment solution used when a chromate film is formed on the oxide 
film includes most preferably a solution in which chromic anhydride is 
mainly dissolved to have such composition as being Cr.sup.3+ /(Cr.sup.3+ 
+Cr.sup.6+)=0 to 0.6. The chromate treatment solution in which Cr.sup.3+ 
and Cr.sup.6+ are composed like this is a known chromate treatment 
solution (such as a reaction type chromate treatment solution, a coating 
type chromate treatment solution and an electrolytic chromate treatment 
solution). However, in the case of this invention, there may be some 
problems if such treatment solution is used as it is. This is because a 
reaction accelerating agent or an etching component to appropriately etch 
the zinc surface are added to almost all of the known chromate treatment 
solution, whereby zinc is melted to make the metal oxide film liable to be 
removed, resulting in lowering the blackening preventive effect. For this 
reason, it is preferable to use, as the chromate treatment solution, those 
which do not contain any etching component constituted of anions such as 
F.sup.-, Cl.sup.-, SO.sub.4.sup.2-, NO.sub.3.sup.-, PO.sub.4.sup.3-, 
CH.sub.3 COO.sup.- and oxalate anions. Additionally speaking, it is 
possible to add to the aqueous solution such compounds that may not melt 
the zinc surface or the metal oxide film (for example, Al.sub.2 O.sub.3, 
SiO.sub.2, etc.) regardless of whether they are inorganic or organic.

EXAMPLE 1 
Steel sheets were subjected to hot dipping with use of a hot-dip metal bath 
comprising, in % by weight, 0.17% of Al, 0.30% of Pb, 0.03% of Fe and the 
balance being Zn and inevitable impurities, and thereafter the plating 
coverage was regulated to 200 to 250 g/m.sup.2 by a gas wiping method, and 
then an aqueous solution of a salt as shown in Table 1 was sprayed on each 
of the steel sheets with use of compressed air of 2 to 3 kg/cm.sup.2 to 
produce hot dip zinc coated steel sheets. Thereafter, part of these steel 
sheets was dipped in an aqueous solution of chromic acid anhydride, 
containing 20 g/lit. of CrO.sub.3 and kept at a temperature of 40.degree. 
C., for 5 seconds to effect a chromate treatment. 
Next, from these non-chromate treated hot dip zinc coated steel sheets and 
the chromate treated ones, test pieces were collected and were allowed to 
stand for 30 days in an atmosphere of a temperature of 50.+-.1.degree. C. 
and a relative humidity of 98% to carry out accelerated blackening tests. 
Results of the accelerated tests are shown in Table 2. 
Judgement on the blackening was in accordance with the following standard: 
A: No blackening occurred. 
B: Very faintly gray colored. 
C: Less than 30% of the total area was blackened. 
D: 30% or more of the total area was blackened. 
TABLE I 
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Temperature of 
Aqueous solution coating at the 
Content start of spraying 
No. Co or Fe salt (g/lit) pH (.degree.C.) 
______________________________________ 
Process of this invention: 
1 Co(NO.sub.3).sub.2.6H.sub.2 O 
5 6.0 470 
2 " 5 6.0 470 
3 " 5 6.0 170 
4 " 0.5 6.8 420 
5 Fe(NO.sub.3).sub.3.9H.sub.2 O 
10 5.0 420 
6 " 10 5.0 420 
7 " 10 5.0 170 
8 CoCl.sub.2 5 6.0 420 
9 FeCl.sub.3 20 6.3 470 
10 Co(NO.sub.3).sub.2.6H.sub.2 O 
5 5.7 470 
CoCl.sub.2 5 
11 Co(NO.sub.3).sub.2.6H.sub.2 O 
10 4.8 420 
Fe(NO.sub.3).sub.3.9H.sub.2 O 
10 
12 CoSO.sub.4 6H.sub.2 O 
10 6.0 420 
13 (CH.sub.3 COO).sub.2 Co.4H.sub.2 O 
10 6.2 420 
14 FeSO.sub.4 10 6.3 200 
Conventional process: 
15 Water only -- 7.0 470 
16 Water only -- 7.0 470 
17 Natural air cooling -- 
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TABLE 2 
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Adhesion Amount of 
amount of Cr in Accelerated 
Co, Fe in chromate blackening 
oxide film film test 
No. (mg/m.sup.2) 
(mg/m.sup.2) 
1 day 30 days 
Spangles 
______________________________________ 
Process of this invention: 
1 Co 30 -- C -- Minimized 
2 Co 30 20 -- A Minimized 
3 Co 26 19 -- A Regular 
4 Co 1.3 18 -- A Regular* 
5 Fe 50 -- C -- Regular* 
6 Fe 50 30 -- A Regular* 
7 Fe 52 15 -- A Regular 
8 Co 26 19 -- A Regular* 
9 Fe 95 21 -- A Minimized 
10 Co 52 20 -- A Minimized 
11 Co 30 20 -- A Regular* 
Fe 30 
12 Co 32 20 -- C Regular* 
13 Co 47 23 -- C Regular* 
14 Fe 33 19 -- C Regular 
Conventional process: 
15 0 -- D -- Minimized 
16 0 17 -- D Minimized 
17 0 21 -- D Regular 
______________________________________ 
*(small) 
EXAMPLE 2 
Steel sheets were subjected to hot dipping in plating baths having the 
composition as shown in Table 3, and, after regulating the coverages, 
aqueous solutions in which 5 g/lit. of Co(NO.sub.3).sub.2.6H.sub.2 O were 
dissolved or aqueous solutions in which 10 g/lit. of 
Fe(NO.sub.3).sub.3.9H.sub.2 O were dissolved were sprayed on the steel 
belts. Thereafter, chromate treatment was carried out in the same manner 
as in Example 1 to produce hot dip coated steel belts. 
Next, test pieces were collected from these hot dip coated steel sheet, and 
allowed to stand for 30 days in an atmosphere of a temperature of 
50.+-.1.degree. C. and a relative humidity of 98% or more to carry out 
accelerated blackening tests. Results of the accelerated tests are shown 
in Table 4. Judgement on blackening was made according to the same 
standard as in Example 1. 
TABLE 3 
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Composition of hot-dip metal bath 
Aqueous 
No. Al Mg Si Pb La Ce Zn solution 
______________________________________ 
Process of this invention: 
1 0.3 0.5 0.2 0.3 0 0 Bal. Co solution 
2 " " " " " " Bal. Fe solution 
3 4.1 0 0.2 0.002 0 0 Bal. Co solution 
4 " " " " " " Bal Fe solution 
5 5.0 0.1 0 0.005 0.005 
0.001 
Bal. Co solution 
6 5.0 0.3 1.0 0.2 0 0 Bal. Fe solution 
7 14 0.1 1.3 0.3 0 0 Bal. Co solution 
8 " " " " " " Bal. Fe solution 
9 53 0.1 0 0.003 0 0 Bal. Co solution 
10 56 0.3 0 0.15 0 0 Bal. Fe solution 
Conventional process: 
11 0.3 0.5 0.2 0.3 0 0 Bal. Natural air- 
cooling 
12 4.1 0 0.2 0.002 0 0 Bal. Water only 
13 5.0 0.3 1.0 0.2 0.005 
0.001 
Bal. Natural air- 
cooling 
14 14 0.1 1.3 0.3 0 0 Bal. Natural air- 
cooling 
15 53 0.1 0 0.003 0 0 Bal. Water only 
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TABLE 4 
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Temperature of 
Amount of 
coating at Co--, Fe-- 
Accelera- 
the start of adhesion in 
ted black- 
spraying oxide layer 
ening test 
No. (.degree.C.) (mg/m.sup.2) 
(30 days) 
Spangles 
______________________________________ 
Process of this invention: 
1 370 Co 29 A Regular* 
2 190 Fe 47 A Regular 
3 470 Co 31 A Minimized 
4 470 Fe 42 A Minimized 
5 240 Co 32 A Regular 
6 320 Fe 53 A Regular 
7 180 Co 34 A Regular 
8 410 Fe 45 A Regular* 
9 470 Co 28 A Minimized 
10 470 Fe 50 A Minimized 
Conventional process: 
11 -- 0 D Regular 
12 470 0 D Minimized 
13 -- 0 D Regular 
14 -- 0 D Regular 
15 470 0 D Minimized 
______________________________________ 
*(small) 
POSSIBILITY OF INDUSTRIAL UTILIZATION 
This invention can be utilized not only when a hot dip coated steel sheet, 
coated with zinc-based or zinc/aluminum-based alloy, is produced, but also 
when the hot dip coated steel sheet is blackened because of cutting or 
processing of a member after the production.