Precoated steel sheet having improved corrosion resistance and formability

A precoated steel sheet having improved corrosion resistance and formability is disclosed, which comprises a Zn- or Zn alloy-plated steel sheet having on the plated surface either a colloidal silica-containing chromate undercoat layer and a polyhydroxypolyether resin-based topcoat of 0.3 to 10 .mu.m in thickness in which resin is derived by polycondensation of a dihydric phenol component selected from a mononuclear dihydric phenol, dinuclear dihydric phenol, and a mixture of both, with an epihalohydrin or a non-colloidal material-containing chromate underlayer and an epoxy resin based topcoat containing colloidal silica of a thickness of from 0.3-1.6 .mu.m in thickness. In spite of the absence of zinc powder, the precoated steel sheet can be satisfactorily welded by resistance welding when the thickness of the topcoat layer is not greater than 2.5 .mu.m, and even with such a thin topcoat, the precoated steel sheet retains its improved corrosion resistance and formability. The precoated steel sheet can be satisfactorily finish-coated by electrodeposition. The undercoat layer is produced by a two stage reduction of Cr.sup.+6 to Cr.sup.+3 in an aqueous suspension containing chromic acid.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a precoated corrosion-resistant steel sheet 
having a chromate undercoat and an organic topcoat. More particularly, it 
relates to such a duplex coated steel sheet which possesses good corrosion 
resistance and formability, can be finish coated by electrodeposition, and 
is preferably weldable by means of resistance welding so that it is highly 
suitable for use in automobile bodies. 
2. Description of the Prior Art 
Weldable precoated steel sheets which can be welded by electrical 
resistance welding have been increasingly used in automobile bodies in 
order to prevent them from rusting due to salt which is spread on roads 
for melting snow in snowy areas. 
Typical weldable precoated steel sheets are Zincrometal (a registered 
trademark of Diamond Shamrock) and similar precoated steel sheets having a 
coating of a zinc-rich primer. Zincrometal comprises a steel sheet having 
an undercoat of a zinc-chromate solution (Dacromet, a registered trademark 
of Diamond Shamrock), and a topcoat of a zinc-rich epoxy resin-based 
primer (Zincromet, a registered trademark of Diamond Shamrock) and 
exhibits a significantly higher corrosion resistance than cold rolled 
steel sheets. Similar weldable precoated steel sheets called "Z-coat steel 
sheets" have an undercoat made by phosphate treatment and a topcoat of a 
zinc-rich primer such as Zincromet. 
It is known that various additives may be incorporated in the zinc-chromate 
undercoat of Zincrometal. Such additives include reducing agents, metal 
chromates, oxides and hydroxides of an amphoteric metal, and hydrophilic 
colloids. See Japanese Patent Publications Nos. 47-6882(1972), 
52-904(1977), and 52-4286(1977), and Japanese Patent Laid-Open 
Applications Nos. 49-74137(1974), 49-74138(1974), and 49-74139(1974). 
In general, precoated steel sheets for use in automobile bodies or the like 
are required to have good formability, weldability, and corrosion 
resistance. In this connection, however, the properties, particularly the 
formability and corrosion resistance of the above-mentioned Zincrometal 
and Z-coat steel sheets are not satisfactory. This is because the 
zinc-rich primer used to form the topcoat of these precoated steel sheets 
contains a large amount of zinc powder or dust (hereinafter referred to as 
zinc powder) equal to around 50% on a volume basis or approximately 85% to 
90% on a weight basis so that the topcoat films are brittle and tend to be 
readily peeled off during working or forming such as press forming. Such 
peeling or removal of the topcoat results in a significant loss of 
corrosion resistance of the precoated steel sheet. In addition, the 
removed pieces of the topcoat readily adhere to the die of the press 
machine, which may cause formation of flaws or scratches on the coated 
surfaces of precoated steel sheets being formed on the machine thereafter. 
Therefore, the die must be cleaned more frequently and the working 
efficiency is significantly decreased. 
Another disadvantage of a zinc-rich primer is that the dry film thereof has 
a relatively large water permeability, which is also responsible for the 
propensity of its corrosion resistance to decrease. These problems, i.e., 
peeling of the coated film deterioration in corrosion resistance can be 
effectively alleviated by decreasing the content of zinc powder in the 
epoxy resin-based primer. However, this results in an increase in 
electrical resistance of the film, which makes it difficult or impossible 
to apply resistance welding to the precoated steel sheet. 
In the above-mentioned precoated steel sheets, it is necessary to cure the 
topcoat of a zinc-rich primer by baking at a high temperature in the range 
of from 250.degree. to 280.degree. C., resulting in a loss of 
bake-hardenability of the base steel sheet if the base steel is of the 
bake-hardening type. The term "bake-hardening" used herein indicates that 
the yield stress of the steel is increased during baking of a finish 
coated applied, for example, by electrodeposition after press forming. 
As another type of corrosion-resistant steel sheet, Japanese Patent 
Laid-Open Application No. 57-108292(1982) discloses a precoated steel 
sheet comprising a plated steel sheet with a Zn- or Al-based plating, the 
steel sheet having a chromate film formed on the plated surface and an 
organic composite coating formed on the chromate film. The organic 
composite coating comprises an organic water-soluble or water-dispersible 
resin such as an acrylic copolymer, epoxy resin, polyvinyl alcohol or 
starch and a silica sol (hydrophilic colloidal silica). The precoated 
steel sheet has improved corrosion resistance before and after finish 
coating and provides the finish coating with good adhesion. 
It is also known that silica sol or colloidal silica may be incorporated 
into a chromate solution in order to improve the corrosion resistance of 
the chromated steel sheet and to increase the adhesion to a finish coating 
formed thereon. See, for example, Japanese Patent Publication No. 
42-14050(1967). 
It has been proposed to use a chromate solution in which a part of the 
hexavalent chromic acid has been reduced to trivalent chromium in order to 
decrease the solubility of the resulting chromate film, thereby improving 
the corrosion resistance of the steel sheet (Japanese Patent Publication 
No. 52-2851(1977)). 
Japanese Patent Laid-Open Application No. 54-161549(1979) discloses a 
chromate solution which comprises partially reduced chromic acid and 
silica sol. A galvanized steel sheet treated with this solution has 
improved corrosion resistance due to the presence of Cr.sup.3+ and silica 
sol in the chromate film. 
Japanese Patent Laid-Open Application No. 60-86281(1985) discloses a highly 
corrosion-resistant precoated steel sheet comprising a plated steel sheet 
having thereon a chromate undercoat layer and a topcoat layer of, e.g., a 
zinc-rich primer in which the chromate undercoat is formed from an aqueous 
suspension containing chromic acid, an iron phosphide powder, and 
optionally one or more substances selected from a dicarboxylic acid or a 
diol, zinc chromate or strontium chromate, oxides or hydroxides of zinc or 
strontium, and phosphoric acid. 
Japanese Patent Laid-Open Application No. 61-239941(1986) discloses a 
weldable precoated steel sheet comprising a steel sheet plated with zinc 
or zinc base alloy, the steel sheet having a chromate film on the plated 
surface which is formed from an aqueous suspension containing chromic 
acid, an iron phosphide powder, and optionally a metal chromate, and a 
topcoat layer on the chromate film which is based on a 
polyhydroxypolyether resin formed by polycondensation of a mononuclear 
dihydric phenol or a mixture of a mononuclear dihydric phenol and a 
dinuclear dihydric phenol with an epihalohydrin. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a precoated steel sheet 
having an organic topcoat layer which is substantially free from zinc 
powder and which exhibits improved corrosion resistance and formability 
and good adhesion to a finish coating formed on the topcoat, for example, 
by electrodeposition coating. 
Another object of the invention is to provide a precoated steel sheet which 
is weldable by electrical resistance welding and which is free from the 
above-mentioned disadvantages of the prior-art weldable precoated steel 
such as Zincrometal and Z-coat steel sheets having a coating of a 
zinc-rich primer. 
A further object of the invention is to provide a precoated steel sheet 
having a chromate undercoat layer and an organic topcoat layer in which 
the topcoat can be baked at a relatively low temperature so as not to 
interfere with the bake-hardenability of the base steel. 
According to one aspect of the present invention, there is provided a 
precoated steel sheet having improved corrosion resistance and 
formability, which comprises a Zn- or Zn alloy-plated steel sheet having 
on the plated surface an undercoat of a chromate film with a weight of 
10-600 mg/m.sup.2 as Cr and a topcoat of 0.3-10 .mu.m in thickness, 
wherein the undercoat is formed from an aqueous suspension containing 
partially-reduced chromic acid, colloidal silica and at least one reducing 
agent selected from the group consisting of a polyhydric alcohol, a 
polycarboxylic acid, and a hydroxycarboxylic acid, in amounts such that 
the weight ratio of silica to total chromic acid is in the range of from 
0.1:1 to 5:1, said partially-reduced chromic acid has a ratio of Cr.sup.3+ 
/(Cr.sup.3+ +Cr.sup.6+) in the range of from 0.1 to 0.6, and the molar 
ratio of reducing agent to unreduced chromic acid is in the range of from 
0.01:1 to 2.0:1 and the topcoat is formed from a coating composition 
containing as a base resin a polyhydroxypolyether resin prepared by 
polycondensation of a dihydric phenol component selected from a 
mononuclear dihydric phenol, dinuclear dihydric phenol, and a mixture of 
both with an epihalohydrin, said topcoat being baked at a temperature of 
from 80.degree. to 200.degree. C., and both of said undercoat and topcoat 
layers being free of substantial amount of zinc powder. 
In a preferred embodiment of this aspect of the invention, the aqueous 
suspension used to form the undercoat layer may contain, in addition to 
partially-reduced chromic acid, colloidal silica and one or more reducing 
agents selected from the group consisting of a polyhydric alcohol, a 
polycarboxylic acid and a hydroxycarboxylic acid, an iron phosphide 
powder, and a metal chromate or its precursor, and the coating composition 
used to form the topcoat layer may further contain at least one additive 
selected from an inorganic filler and a cross-linking agent. Also a 
plasticizer such as an acrylate or methacrylate ester or a flexible resin 
such as butyral resin, or a mixture of these may be incorporated in the 
topcoating composition. 
According to another aspect of the present invention, there is provided a 
precoated steel sheet having improved corrosion resistance and 
weldability, which comprises a Zn or Zn alloy-plated steel sheet having on 
the plated surface an undercoat of a chromate film with a weight of 20-100 
mg/m.sup.2 as Cr and a topcoat of 0.3-1.6 .mu.m in thickness, wherein said 
undercoat is formed by applying or firing after application a chromate 
solution, the chromate solution is an aqueous suspension which contains 
chromic acid partially reduced to give a ratio of Cr.sup.3+ /(Cr.sup.3+ 
+Cr.sup.+6) of 0.4-0.6 and a reducing agent in an amount of 1-4 times 
larger than that required to reduce the remaining Cr.sup.+6 to Cr.sup.3+, 
the aqueous solution is substantially free from colloidal materials, and 
said topcoat is formed by applying or firing after application a 
resin-containing solution which contains as a base resin an epoxy resin 
together with colloidal silica in amounts of 10-25% by weight based on the 
total amount of resin solids and colloidal silica in the resin-containing 
solution.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Base Material 
The base material of the precoated steel sheet of the present invention is 
a steel sheet plated with zinc or a zinc-based alloy. The zinc or zinc 
alloy plating may be carried out by hot dipping, electroplating, or 
electroless plating. The plating weight is preferably in the range of 
5-100 g/m.sup.2, and more preferably in the range of 10-60 g/m.sup.2. 
Examples of a zinc alloy useful for plating of the steel sheet include 
Zn-Ni, Zn-Fe, and Zn-Al. Alloyed galvanized steel sheet which is prepared 
by heating a galvanized steel sheet sufficiently to form an Ni-Fe alloy in 
the plating layer is also included in the zinc alloy-plated steel sheet. 
The base material may be of the duplex plating type having two or more 
plating layers on the substrate steel sheet as long as the uppermost layer 
is a Zn or Zn alloy plating. In such cases, the underlying plating layers 
may be comprised of other metals or alloys. 
The zinc- and zinc alloy-plated steel sheets as the base material may be 
hereinafter collectively referred to as galvanized steel sheets. 
Undercoat Chromate Layer 
The following description is of the undercoat chromate layer for the 
embodiment of the first aspect of the present invention which contains 
colloidal silica. 
In general, a chromate film is formed from an aqueous chromic acid solution 
by reduction of chromic acid and evaporation of water during baking of the 
applied wet coating. 
According to the present invention, an aqueous suspension which contains 
partially-reduced chromic acid and colloidal silica is used to form the 
undercoat chromate layer in order to promote reduction of chromic acid and 
film formation so as to enable a chromate film to be efficiently formed at 
a lower temperature. 
The use of partially-reduced chromic acid decreases the amount of chromic 
acid which has to be reduced during baking of the applied wet coating, and 
accelerates film formation. The ratio of partial reduction of chromic acid 
as defined by Cr.sup.3+ /(Cr.sup.3+ +Cr.sup.6+) in the chromating solution 
is preferably in the range of 0.1-0.6 and more preferably in the range of 
0.3-0.6. If this ratio is less than 0.1, it is difficult to efficiently 
carry out the reduction of chromic acid in the wet chromate coating during 
baking. On the other hand, if the ratio is greater than 0.6, it is 
difficult to maintain the chromium ions as a stable solution due to the 
instability of Cr.sup.3+ in solution. 
Partial reduction of chromic acid may be carried out by reacting an aqueous 
chromic acid solution with a suitable reducing agent such as those 
described below at an elevated temperature prior to addition of colloidal 
silica and other optional additives. 
Colloidal silica serves to increase the wetting power of the chromic acid 
solution, thereby accelerating the film formation of the chromate wet 
coating, and for this purpose it is added to the partially-reduced chromic 
acid solution in an amount such that the weight ratio of silica to total 
chromic acid is in the range of from 0.1:1 to 5:1. The term "total chromic 
acid" means the total weight as CrO.sub.3 of Cr.sup.3+ and Cr.sup.6+ ions 
present in the aqueous medium. If the above weight ratio is less than 
0.1:1, the effect of colloidal silica on acceleration of film formation is 
inadequate. If the ratio is greater than 5:1, the resulting chromate film 
becomes brittle due to the presence of too much silica. 
The colloidal silica which is present in the undercoat chromate layer may 
be either of the dry type or wet type. Typical colloidal silica of the dry 
type is commercially available under the registered trademark "Aerosil". 
Wet-type colloidal silica is commercially available in the form of a 
stable aqueous suspension, for example, sold under the trade names Ludox 
(du Pont), Nalcoag (Nalco Chemical), Syton (Monsanto), Snowtex (Nissan 
Kagaku), and Cataloid (Shokubai Kasei). 
The average particle diameter of the colloidal silica is not critical, and 
it is preferably within the range of 1-100 nm. 
The following additives (a)-(e) may be optionally added to the aqueous 
suspension used in the present invention to form the undercoat chromate 
film. 
(a) Silane coupling agent: 
A silane coupling agent serves to strengthen the colloidal 
silica-containing chromate film by hydrolysis to form a polysiloxane, 
thereby improving the adhesion between silica particles and the chromate 
film matrix and between the topcoat and the undercoat layers. It is also 
advantageous in that hydrolysis of the silane coupling agent results in 
the formation of an alcohol, which acts as a reducing agent for chromic 
acid. 
Examples of useful silane coupling agents include vinyltriethoxysilane, 
vinyl-tris(beta-methoxyethoxy)silane, 
gamma-methacryloxypropyltrimethoxysilane, 
gamma-glycidoxypropyltrimethoxysilane, gamma-aminopropyltriethoxysilane, 
N-beta-(aminoethyl)-gamma-aminopropyltrimethoxysilane, 
beta-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, and the like. 
When a silane coupling agent is added to the aqueous suspension, it is 
preferably used in an amount such that the molar ratio of silane to 
unreduced chromic acid is at least 0.01, i.e., in an amount of at least 1 
mole % based on the unreduced chromic acid remaining in the suspension. If 
the amount of a silane coupling agent is less than 1 mole % of the 
unreduced chromic acid, the above-mentioned advantages of the silane 
coupling agent will not be attained sufficiently. Addition of a silane 
coupling agent in an excessively large amount will be disadvantageous from 
an economical viewpoint. 
(b) Polyhydric alcohol, polycarboxylic acid, hydroxycarboxylic acid 
(reducing agent): 
One or more compounds selected from polyhydric alcohols, polycarboxylic 
acids, and hydroxycarboxylic acids may be added in the aqueous suspension 
as an reducing agent in order to enhance the efficiency of reduction of 
chromic acid at a relatively low baking temperature. 
Examples of polyhydric alcohols useful in the present invention include 
ethylene glycol, propylene glycol, and glycerol. Examples of useful 
polycarboxylic acids include succinic acid, glutaric acid, and adipic 
acid. Examples of useful hydroxycarboxylic acids are citric acid and 
lactic acid. 
Part of the above reducing agents may be replaced by a sugar. 
These reducing agents are preferably added in an amount such that the molar 
ratio of total reducing agents to unreduced chromic acid is in the range 
of from 0.01:1 to 2.0:1. If the molar ratio is less than 0.01:1, the 
efficiency of reduction of chromic acid will not be enhanced adequately. 
If the reducing agent is added in a molar amount greater than twice the 
molar amount of unreduced chromic acid, further enhancement of reduction 
efficiency will not be obtained and moreover the reducing agent will be 
retained in the chromate film after baking, thereby deteriorating the 
water resistance of the film. 
(c) Iron phosphide powder: 
When an iron phosphide in the form of fine powder is present in an aqueous 
chromate solution, it reacts with free hexavalent chromium ions in the wet 
chromate coating during baking, thereby decreasing the amount of these 
ions in the chromate film. The hexavalent chromium ions are soluble in 
water which penetrates into the chromate film through the finish coating 
and topcoat layers formed thereon. A decrease in the amount of these ions 
in the chromate film is therefore effective in maintaining the corrosion 
resistance and adhesion of the chromate film in a corrosive environment. 
Since an iron phosphide is electrically conductive, the incorporation of an 
iron phosphide powder facilitates electrodeposition performed on the 
precoated steel sheet to form a finish coating, and resistance welding of 
the precoated steel sheet is also facilitated in spite of the absence of 
zinc powder, making the welding operation more efficiently. Therefore, it 
is desirable to add an iron phosphide powder to the aqueous chromate 
suspension, particularly in the case of a precoated steel sheet having a 
relatively thick topcoat organic layer on which electrodeposition coating 
and/or resistance welding is to be performed. 
An iron phosphide powder is water-insoluble and when it is present in an 
aqueous chromic acid solution it forms a suspension. Therefore, in order 
to allow it to efficiently react with free hexavalent chromium ions, it is 
preferable to add it in an amount of at least 10% by weight based on the 
total chromic acid. On the other hand, addition of an excessively large 
amount of an iron phosphide powder results in loss of adhesion of the iron 
phosphide particles to the chromate film, which may readily cause peeling 
of the coating during working or forming, thereby deteriorating 
formability and corrosion resistance. Due to the above-mentioned 
conductive nature, addition of an excessively large amount of an iron 
phosphide powder is also disadvantageous in that an electric current can 
readily pass between the base steel sheet and the surface of the coating, 
resulting in a significant reduction of the ability of the coating to 
function as a corrosion barrier. These phenomena are prominent when the 
weight ratio of iron phosphide to total chromic acid exceeds 20:1. 
Accordingly, when an iron phosphide powder is added, it is preferably used 
in an amount such that the weight ratio of iron phosphide to total chromic 
acid is in the range of from 0.1:1 to 20:1, and more preferably is in the 
range of from 1:1 to 10:1. 
In addition to the most common iron phosphide in the form of Fe.sub.2 P 
[ferrous (II) phosphide], several other compositions of iron phosphide are 
known, such as FeP, Fe.sub.3 P, and FeP.sub.2. All of these iron 
phosphides may be used in the present invention singly or in combination. 
It is preferable to use an iron phosphide in the form of a fine powder 
having an average particle diameter of not greater than 5 .mu.m. 
(d) Metal chromate: 
The aqueous suspension which contains partially reduced chromic acid and 
colloidal silica may further contain a metal chromate. A metal chromate, 
when incorporated in the chromate film, serves as a rust-preventive 
pigment, increasing the rust-preventing properties of the film. More 
specifically, a metal chromate can passivate iron and zinc metals in the 
base galvanized steel sheet and suppress dissolution of these metals in a 
corrosive environment, thereby contributing to further improvement in 
corrosion resistance of the precoated steel sheet. Therefore, it is 
preferred to incorporate a metal chromate in the undercoat chromate film. 
Examples of a metal chromate useful for this purpose are zinc chromate and 
strontium chromate. A precursor of a metal chromate can also be used. Such 
a precursor includes metal oxides and hydroxides such as zinc oxide and 
hydroxide and strontium oxide and hydroxide. In an aqueous medium 
containing chromic acid, these metal oxides or hydroxides react with 
chromate ions to form a metal chromate. 
Addition of an excessive amount of a metal chromate inhibits film formation 
of a chromating solution and decreases the adhesion of the resulting 
chromate film to the base steel sheet. Accordingly, when a metal chromate 
is added, it is preferably used in a molar amount less than or equal to 
the amount of residual unreduced chromic acid present in the aqueous 
suspension. When a precursor of a metal chromate in the form of an oxide 
or hydroxide is used, it is preferable to add the precursor in an amount 
of at most 50 mole % based on the unreduced chromic acid, since the 
precursor reacts with chromic acid and consumes it as described above. 
When a metal chromate is added to the aqueous suspension, the Cr values 
originating from such chromate are excluded from the total chromic acid 
referred to in the above. 
(e) Other optional additive: 
In order to further improve the adhesion between the chromate film and the 
galvanized base steel sheet, the aqueous suspension comprising 
partially-reduced chromic acid and colloidal silica may further contain 
phosphoric acid in a molar amount less than or equal to the molar amount 
of the unreduced chromic acid present in the aqueous suspension. 
The above-mentioned aqueous suspension is applied to a galvanized steel 
sheet so as to give a chromate film having a weight of at least 10 
mg/m.sup.2 as Cr on the plated surface. Preferably, the weight of the 
chromate film is in the range of 10-600 mg/m.sup.2 as Cr, more preferably 
30-300 mg/m.sup.2 as Cr, and most preferably 30-100 mg/m.sup.2 as Cr. 
The Cr weight referred to herein means the weight of Cr coming from the 
partially reduced chromic acid component in the suspension, and it does 
not take account of the Cr values coming from the metal chromate component 
(d) when it is added. 
If the chromate film has a weight of less than 10 mg/m.sup.2 as Cr, the 
precoated steel sheet will not have satisfactory corrosion resistance. A 
chromate film having a weight far beyond 100 mg/m.sup.2 as Cr may 
sometimes cause increased damage to tip electrodes during spot welding of 
the precoated steel sheet. In a precoated steel sheet having a thick 
chromate film with a weight exceeding 600 mg/m.sup.2 as Cr, peeling of the 
coating may readily occur during severe working such as press forming or 
deep drawing. However, when severe working or forming is not applied to 
the precoated steel sheet, as in the case of precoated steel sheets for 
use as building materials, such a thick chromate film with a weight 
exceeding 600 mg/m.sup.2 as Cr may be applied as the undercoat layer. 
The aqueous suspension which contains partially reduced chromic acid, 
colloidal silica, and optionally other additives may be applied by any 
conventional coating means, for example, by use of a wire-wound rod 
coater, roll coater, or spray coater, or by dipping. 
As is apparent to those skilled in the art, the galvanized steel sheet 
having a wet chromate coating applied on the plated surface as above is 
then baked to form an insoluble chromate film in the conventional manner. 
The baking is preferably carried out at a temperature of 
60.degree.-200.degree. C., and more preferably 100.degree.-150.degree. C. 
for a time sufficient to obtain a dry film. 
In regard to the second embodiment of the present invention (in which there 
is no colloidal silica or other colloids in the undercoat) the basic 
principles in regard to the formation of the undercoat chromate layer as 
set forth above are also applicable with the exceptions noted hereafter. 
First, the aqueous solution is substantially free from any collodial 
material including colloidal silica. In addition, the undercoat is applied 
to a film weight of 20-100 mg/m.sup.2 as Cr by applying or firing after 
application a chromate solution which contains chromic acid partially 
reduced to give a ratio of Cr.sup.3+ /(Cr.sup.3 +Cr.sup.+6) of 0.4-0.6 and 
a reducing agent in the amount of 1 to 4 times larger than that required 
to reduce the remaining Cr.sup.+6 to Cr.sup.3+. Preferably, the chromate 
solution further includes a silane coupling agent of the same type as 
discussed above in regard to the first aspect of the first aspect of the 
present invention in an amount such that the molar ratio of silane 
coupling agent to unreduced chromic acid (Cr.sup.+6) is at least 0.01:1. 
Organic Topcoat Layer 
The following description is of the organic topcoat layer for the 
embodiment of the first aspect of the present invention. This organic 
topcoat layer is based on a polyhydroxypolyether resin applied on the 
undercoat colloidal silica-containing chromate film. The topcoating 
composition may contain, in addition to the above base resin, an inorganic 
filler, a cross-linking agent, and/or a monomeric or polymeric 
plasticizer. Additional resins other than the polyhydroxypolyether resin 
may be added in a total amount of less than 50% by weight of the resin 
solids in the topcoating composition. 
The polyhydroxypolyether resin which is used as a base resin of the topcoat 
in accordance with the invention is prepared by polycondensation of a 
dihydric phenol and an epihalohydrin in the presence of an alkaline 
catalyst. The dihydric phenol may be either a mononuclear one having one 
benzene nucleus, e.g., resorcinol, hydroquinone, or catechol, or a 
dinuclear one having two benzene nuclei, e.g., bisphenol A 
(2,2-bis(4'-hydroxyphenyl)propane), bisphenol F 
(bis(4'-dydroxyphenyl)methanel), or a mixture of a mononuclear and a 
dinuclear phenols. The epihaloydrin includes epichlorohydrin, 
epibromohydin, and epibromohydrin, and epiiodohydrin. 
Epichlorohydrin is preferred. A diepoxide compound may be used in place of 
an epihalohydrin. 
A polyhydroxypolyether resin in which the dihydric phenol component is 
comprised of an equimolar mixture of resorcinol (mononucelar) and 
bisphenol A (dinuclear) is characterized by recurring units of the 
following formula: 
##STR1## 
A polyhydroxypolyether resin in which the dihydric phenol component is 
comprised solely of resorcinol is characterized by recurring units of the 
following formula: 
##STR2## 
A high molecular-weight polyhydroxypolyether resin in which the dihydric 
phenol component is comprised solely of bisphenol A is also known as a 
phenoxy resin and sold by Union Carbide Corp. under the trade name "PKHH". 
PKHH is characterized by recurring units of the following formula: 
##STR3## 
The polyhydroxypolyether resins, particularly high molecular-weight 
polyhydroxypolyether resin, and their preparation are described in 
Japanese Patent Laid-Open Application No. 57-102925 (1982). 
Also included in the polyhydroxypolyether resin useful as the base resin of 
the topcoat layer are epoxy resins of the glycidyl ether type which are 
prepared by polycondensation of a mononuclear or dinuclear dihydric phenol 
or a mixture of both and an epihalohydrin. The epoxy resins of this type 
have the same recurring units as illustrated above although they have 
terminal epoxy groups at the ends of the polymer chain. Epoxy resins 
useful in the present invention include common epoxy resins derived from 
bisphenol A, bisphenol F, or a dinuclear brominated epoxide and an 
epihalohydrin. Modified epoxy resins such as epoxy esters, epoxy 
urethanes, and epoxy acrylates are also included in the epoxy resins. 
Epoxy esters are prepared by using a fatty acid derived from a drying oil 
and reacting epoxy and hyrdroxyl groups in an epoxy resin with carboxyl 
groups in the fatty acid. Epoxy urethanes can be prepared by reacting an 
epoxy resin with an isocyanate compound. Epoxy acrylates can be prepared 
by modifying an epoxy resin with acrylic acid, methacrylic acid, or a 
similar unsaturated carboxylic acid. 
Particularly suitable for use as the base resin of the topcoat layer is a 
high molecular-weight polyhydroxypolyether resin having a number-average 
molecular weight of at least 5,000, and preferably in the range of 
8,000-50,000. Such a high molecular-weight polyhydroxypolyether resin may 
be prepared by reacting a lower molecular-weight epoxy resin derived from 
a dihydric phenol component and an epihalohydrin, e.g., bisphenol A di- or 
poly-glycidyl ether, with an additional amount of a dihydric phenol. 
In the case of using a common epoxy resin as a polyhydroxypolyether resin, 
the molecular weight of the base resin may be much lower. However, the 
molecular weight of the epoxy resin should preferably be at least 1000 so 
that a tack-free film can be readily obtained by baking at a relatively 
low temperature which is not sufficient to completely cure the epoxy 
resin. Of course, an epoxy resin having a higher molucular weight, for 
example, on the order of 5,000 or higher may be used. 
As shown in the above structural formulas of recurring units, 
polyhydroxypolyether resins including epoxy resins have many --OH groups 
and --O-- groups in the polymer chain. Hydroxyl groups (--OH) can form 
hydrogen bonding with the underlying chromate film and assure that the 
topcoat layer has improved adhesion to the chromate film, while oxy groups 
(--O--) allow easy rotation of the polymer chain and assure that the 
topcoat layer has enhanced flexibility. 
Regarding the number of these functional groups in a given weight of a 
polymer, a polyhydroxypolyether resin derived from a mononuclear dihydric 
phenol such as resorcinol has a number greater than that derived from a 
dinuclear dihydric phenol such as bisphenol A, because the molecular 
weight of resorcinol is lower than that of bisphenol A. For example, when 
resorcinol and bisphenol A are used in molar ratios of 0/1, 1/1, and 1/0 
in polycondensation with an equimolar amount of an epihalohydrin, the 
numbers of --OH and --O-- functional groups present in each 100 molecular 
weight of the resulting polyhydroxypolyether resin are as follows: 
______________________________________ 
Molar ratio 
of resorcinol/ 
Weight % Number of Number of 
bisphenol A 
resorcinol --OH groups 
--O-- groups 
______________________________________ 
0/1 0 0.35 0.70 
1/1 23 0.44 0.89 
1/0 66 0.60 1.20 
______________________________________ 
Thus, as the content of a mononulear phenol in the dihydric phenol 
component is increased, the resulting resin contains --OH and --O-- 
functional groups at an increased concentration, and, as a general trend, 
a coating formed therefrom has an increased adhesion and flexibility. 
Therefore, in order to enhance the corrosion resistance and formability of 
the precoated steel sheet, it is generally advantageous to use a 
polyhydroxypolyether resin in which at least part of the dihydric phenol 
component is comprised of a mononuclear phenol such as resorcinol. 
However, even in the cases where the base resin is a polyhydroxypolyether 
resin in which a dinuclear phenol such as bisphenol A comprises 100% of 
the dihydric phenol component, the resin has many --OH and --O-- groups as 
shown in the above Formula (III), and a precoated steel sheet having a 
topcoat of such a base resin still possesses satisfactory corrosion 
resistance and adhesion. 
The topcoating composition may be prepared by dissolving one or more 
polyhydroxypolyether resins (including an epoxy resins and modified epoxy 
resins) in an organic solvent. The organic solvent may be selected 
depending on the properties required for the topcoat layer such as drying 
rate and film smoothness as well as the type and molecular weight of the 
polyhydroxypolyether resin. For dissolution of a high molecular-weight 
polyhydroxypolyether resin, solvents such as cellosolves, ketones, 
glycol-ethers, and mixtures of these can be used. When the base resin is a 
polyhydroxypolyether resin of lower molecular weight, for example, not 
greater than 10,000, any solvent commonly used in epoxy coating 
compositions, for example, cellosolves, ketones, esters, alcohols, 
hydrocarbons, halogenated hydrocarbons, and mixtures of these may be used. 
The topcoating composition may further contain at least one additive 
selected from the following groups (A) to (C). 
(A) Inorganic filler: 
One or more inorganic fillers may be added to the topcoating composition in 
order to further improve the corrosion resistance of the precoated steel 
sheet. 
Examples of inorganic fillers useful in the present invention include the 
above-mentioned metal chromates such as zinc chromate and strontium 
chromate, as well as other inorganic fillers such as calcium carbonate, 
alumina, various silicates, zinc phosphate, calcium phosphate, zinc 
phosphomolybdate, aluminum phosphomolybdate, silica powder, colloidal 
silica, and the like. 
Any type of the colloidal silica described previously as an additive to the 
chromate undercoat layer may be used as an inorganic filler to be added to 
the organic topcoat layer. When colloidal silica is present as an 
inorganic filler in the organic topcoat layer, a silane coupling agent as 
mentioned previously may be added in a small amount to the topcoating 
composition in order to increase the adhesion between the silica particles 
and the resin matrix, thereby further improving corrosion resistance of 
the organic coating. 
Metal chromates such as zinc chromate and strontium chromate serve as 
rust-preventive pigments as described above and are highly effective for 
improving the corrosion resistance of the coating when it is present in 
the organic topcoat layer. However, when the resulting precoated steel 
sheet is pretreated by degreasing or chemical conversion treatment prior 
to finish coating, some of the chromate ions present in the topcoat layer 
tend to dissolve in the aqueous solution used in the pretreatment, causing 
rapid contamination of the solution. Therefore, if the precoated steel 
sheet is subsequently treated with a degreasing solution or a chemical 
conversion solution, it is preferred that the amount of a metal chromate 
added to the topcoat layer be minimized. 
The amount of inorganic filler added to the topcoating composition is at 
most 40% by volume, and preferably in the range of 1-20% by volume, based 
on the total resin solids in the coating composition. If it is less than 
1% by volume, the improvement in corrosion resistance will not be 
significant. Addition of an inorganic filler in excess of 40% by volume 
may cause deterioration in the adhesion or corrosion resistance of the 
organic coating, and may increase the electrical resistance of the coating 
to such a degree that electrodeposition or resistance welding such as spot 
welding becomes difficult. 
(B) Cross-linking agent: 
One or more cross-linking agents may be added in order to further improve 
corrosion resistance of the precoated steel sheet. It is believed that 
cross-linking of the base resin can strengthen the coating, thereby 
improving the corrosion resistance thereof. 
For this purpose, any cross-linking agent or curing agent which is known as 
effective in curing epoxy resins may be used. Examples of such 
cross-linking agents include a phenolic resin, an amino resin, a 
polyamide, an amine, an isocyanate including a blocked isocyanate, and an 
acid anhydride. Preferred cross-linking agents are blocked isocyanates. 
When a cross-linking agent of the blocked type such as a blocked isocyanate 
is used, it is advantageous that the cross-linking agent does not release 
the functional groups, e.g., isocyanate groups in a blocked isocyanate, at 
the baking tempearture of the topcoat layer. In other words, it is 
preferred that the releasing temperature of the blocked-type cross-linking 
agent be higher than the baking temperature of the topcoat. In such a 
case, cross-linking of the base resin does not occur during baking of the 
topcoat layer, resulting in the formation of a topcoat layer which still 
fully retains the flexible nature of the base resin, and the formability 
of the precoated steel sheet obtained after baking is not deteriorated in 
spite of the presence of the cross-linking agent. After the precoated 
steel sheet is formed into a desired shape and then finish-coated, for 
example, by electrodeposition, the finish coating is baked. By selecting a 
baking temperature of the finish coating which is sufficiently high to 
activate the blocked-type cross-linking agent in the topcoat layer of the 
precoated steel sheet and which is higher than the baking temperature of 
the topcoat layer, the functional groups in the cross-linking agent are 
released and cross-linking of the topcoat layer proceeds as the finish 
coating is baked, thereby strengthening the topcoat layer. In this manner, 
corrosion resistance of the precoated steel sheet can be highly improved 
without a sacrifice of formability. 
When a cross-linking agent is added, it is used in an amount such that the 
ratio of the total number of functional groups in the cross-linking agent 
to the total number of epoxy and hydroxyl groups in the 
polyhydroxypolyether base resin is at most 2.0:1, preferably in the range 
of from 0.1:1 to 2.0:1. If this ratio is less than 0.1, the effect of the 
cross-linking agent will not be significant. On the other hand, if the 
ratio exceeds 2.0:1, the flexibility of the resulting organic coating will 
be significantly lost and the coating will tend to readily crack during 
forming of the precoated sheet, resulting in a substantial decrease in 
corrosion resistance. 
(C) Others: 
In addition to the above-described inorganic filler and cross-linking 
agent, various other additives such as additional resins other than epoxy 
resins, conductive pigments, plasticizers, and the like may be added to 
the topcoating composition in order to further improve various properties 
of the coating, e.g., formability, plasticity or flexibility, 
electrodeposition coating properties, and weldability. 
One such useful additive is a plasticizer which is added to improve the 
flexibility of the topcoat layer. For this purpose, flexible resins such 
as a butyral resin can be used. When a butyral resin or other non-reactive 
plasticizer is added in a large amount, it tends to bleed out of the resin 
matrix while the precoated steel sheet is exposed to a relatively high 
temperature for a prolonged period. 
Such bleeding of a plasticizer can be effectively prevented by addition of 
an acrylate or methacrylate ester, preferably a di- or higher functional 
acrylates or methacrylates, as a reactive plasticizer. Of course, an 
acrylate or methacrylate may be added by itself as a plasticizer. An 
acrylate or methacrylate ester plasticizer is finally fixed in the resin 
matrix through cross-linking caused by cleavage of the double bond in the 
ester which occurs with the elapse of time. The fixation of the acrylate 
or methacrylate plasticizer is accelerated when heat is applied to the 
precoated steel sheet after forming, such as during baking of a finish 
coating. Acrylate or methacrylate esters which are useful as a reactive 
plasticizer include pentaerythritol triacrylate or methacrylate, and 
trimethylolpropane triacrylate or methacrylate. 
In order to facilitate electrodeposition applied to the precoated steel 
sheet for finish coating, a water-soluble resin such as polyvinyl alcohol, 
polyacrylic or polymethacrylic acid, or acrylamide or methacrylamide may 
be added. 
When one or more additional resins are added as a plasticizer or other 
additive to the polyhydroxypolyether resin-based topcoating composition, 
the total amount of additional resins other than polyhydroxypolyether 
resins should be at most 50% by weight based on the total resin solids in 
the coating composition in order to avoid a substantial decrease in 
corrosion resistance of the resulting coating. 
The topcoating compositin may also be applied by a conventional method, for 
example, by use of a wire-wound rod coater or roll coater. The thickness 
of the organic topcoat layer is in the range of 0.3-10 .mu.m, and 
preferably 0.3-2.5 .mu.m as a dry film thickness. If the dry film 
thickness of the topcoat layer is less than 0.3 .mu.m, satisfactory 
improvement in corrosion resistance and adhesion cannot be achieved and 
the coating tends to be peeled off during forming. When the precoated 
steel sheet is to be welded by resistance welding, the thickness of the 
topcoat layer is preferably at most 2.5 .mu.m, since with a topcoat 
thickness greater than 2.5 .mu.m it is difficult or even impossible to 
perform resistance welding on the precoated steel sheet. A precoated steel 
sheet having an organic topcoat layer with a thickness greater than 10 
.mu.m is disadvantageous from an economical viewpoint. 
In regard to the organic topcoat layer of the second aspect of the present 
invention (in which the topcoat layer contains colloidal silica), the 
topcoat is formed by applying or firing after application a 
resin-containing solution which contains as a base resin an epoxy resin 
such as conventionally used and as described above together with colloidal 
silica in amounts of 10-25% by weight based on the total amount of resin 
solids and colloidal silica in the resin-containing solution. A solution 
is applied so as to result in a topcoat of 0.3-1.6 .mu.m in thickness. 
The resin-containing solution in this aspect of the present invention 
further can include a cross-linking agent in an amount such that the molar 
ratio of the total number of functional groups in the cross-linking agent 
to the total number of epoxy and hydroxyl groups in the epoxy resin is 0.1 
to 2.0:1. In addition, an additional resin other than epoxy resin (such as 
conventional resins as disclosed above) can be added in an amount of 50% 
by weight or less based on the total amount of resin solids in the 
resin-containing solution and in such a fashion that it does not 
deleteriously effect the properties obtained from the compositions of the 
present invention. 
Regardless of which organic topcoating is used, the wet organic topcoating 
is formed on the chromate undercoat film is baked at a temperature of from 
80.degree. to 300.degree. C. By employing such a baking temperature, it is 
possible not only to dry the topcoat layer but to accelerate reduction of 
the chromate ions remaining in the underlying chromate film so as to make 
the chromate film insoluble and tough. 
The baking temperature of the organic topcoat layer is preferably above the 
boiling temperature of the solvent used in the topcoating composition in 
order to prevent blocking of the precoated steel sheet product. However, 
when the dry film thickness of the organic layer is not greater than 5 
.mu.m, substantially no blocking will occur even if the baking temperature 
is below the boiling temperature of the solvent. Therefore, more 
specifically, the baking temperature is preferably between the boiling 
temperature of the solvent and 300.degree. C. for a topcoat layer having a 
dry film thickness of 5-10 .mu.m, and between 80.degree. and 300.degree. 
C. for a topcoat layer having a dry film thickness of less than 5 .mu.m. 
As the baking temperature is elevated, of course, a more uniform coating 
which exhibits better corrosion resistance and formability is readily 
obtained. When the steel substrate is of the bake-hardening type, however, 
the maximum baking temperature is preferably 200.degree. C., since such a 
steel sheet will lose the desirable bake-hardenability after being heated 
at a temperature above 200.degree. C. as described above. According to the 
present invention, since the undercoat chromate film is formed with 
partially-reduced chromic acid in order to accelerate formation of an 
insoluble chromate film, it is possible to bake the organic topcoat layer 
in a relatively low temperature below 200.degree. C. 
The thus-prepared precoated steel sheet of the present invention has the 
following multilayers on the substrate steel sheet: (1) first embodiment: 
a first or undermost layer of Zn or Zn alloy plating, a second or 
intermediate layer of a colloidal silica-containing chromate film, and a 
third or uppermost layer of an organic polyhydroxypolyether resin-based 
coating; and (b) the second embodiment: a first or an undermost layer of 
Zn or Zn alloy plating, a second or intermediate layer of a non-colloidal 
material-containing chromate film, and a third or uppermost layer of an 
organic epoxy resin-colloidal silica based coating. In the case of a 
precoated steel sheet for use in automobile bodies, such multilayer 
coating is typically applied to one surface of the substrate steel sheet. 
Depending on the end use, of course, it may be applied to both surfaces of 
the substrate steel sheet. 
The following examples illustrate the superior performance of the precoated 
steel sheet of the present invention. It should be understood, however, 
that the invention is not limited to the specific details set forth in the 
examples. In the examples, all the percents are by weight unless otherwise 
indicted. 
EXAMPLE 1 
This example illustrates the preparation of precoated steel sheets of the 
present invention in which the organic topcoat layer contains no inorganic 
filler or cross-linking agent. 
(a) Base steel sheet: 
The base steel sheet used in this example was a Zn alloy-electroplated 
steel sheet comprising a 0.8 mm-thick cold-rolled steel sheet having an 
electroplated coating of 12% Ni-Zn alloy with a weight of 20 g/m.sup.2 on 
one surface thereof. Prior to use, the base steel sheet was degreased with 
Fine Cleaner 4336 (manufactured by Nihon Parkerizing) to clean the plated 
surface. 
In some runs, a cold-rolled steel sheet of the bake-hardening type having 
the same Zn-Ni alloy plating as above on one surface was used as the base 
steel sheet. 
(b) Aqueous suspension for chromating: 
To an aqueous chromic acid solution containing 120 g/l of CrO.sub.3, 
ethylene glycol in an aqueous solution was added as a reducing agent and 
the mixture was heated at 80.degree. C. for 6 hours to partially reduce 
the chromic acid. After cooling, the reaction mixture was diluted with an 
aqueous chromic acid solution containing 40 g/l of CrO.sub.3 in an amount 
sufficient to adjust the Cr.sup.3+ /(Cr.sup.3+ +Cr.sup.6+) ratio to a 
predetermined value. The aqueous solution of partially-reduced chromic 
acid was further diluted with water sufficient to adjust the concentration 
of total chromic acid (total Cr concentration as CrO.sub.3) to 40 g/l 
(0.4M as CrO.sub.3). 
To the resulting aqueous solution of partially reduced chromic acid, a 
predetermined amount of colloidal silica having an average particle 
diameter of 12 nm (Aerosil 200 manufactured by Nippon Aerosil) was added. 
In some runs, one or more of the following optional additives were added in 
predetermined amounts: 
Silane coupling agent: 
Vinyltriethoxysilane (A-151 manufactured by Nippon Unicar); 
Gamma-glycidoxypropyltrimethoxysilane (A-187 manufactured by Nippon 
Unicar); 
Gamma-aminopropyltriethoxysilane (A-1101 manufactured by Nippon Unicar); 
Polyhydric alcohol: 
Glycerol (reagent grade); 
Oxycarboxylic acid: 
Citric acid (reagent grade); 
Iron phosphide powder: 
Ferrous (II) phosphide ((Fe.sub.2)P) powder having an average particle 
diameter of 3 .mu.m (HRS-2132 manufactured by Occidental Chemical); and 
Metal chromate: 
Strontium chromate (reagent grade). 
The resulting mixture was thoroughly agitated by a High-Speed Disper to 
form an aqueous suspension prior to use. 
(c) Polyhydroxypolyether resin-based coating composition: 
A flask fitted with a condenser was charged with 230 parts by weight of 
bisphenol A diglycidyl ether (Epikote 828 manufactured by Yuka Shell 
Epoxy), 55 parts by weight of resorcinol, 200 parts by weight of methyl 
ethyl ketone, and 4 parts by volume of an aqueous 5N NaOH solution. The 
mixture was heated to reflux and allowed to react at that temperature for 
18 hours. The resulting resinous mass was poured into water and stirred to 
precipitate a water-insoluble resin. The precipitates were collected by 
filtration and dried in vacuo to yield a high molecular-weight 
polyhydroxypolyether resin having a number-average molecular weight of 
approximately 35,000 as powder in which the dihydric phenol component was 
comprised of an equimolar mixture of resorcinol (mononuclear) and 
bisphenol A (dinuclear). 
The powdery high molecular-weight polyhydroxypolyether resin obtained above 
was dissolved in a mixed solvent of cellosolve acetate and cyclohexanone 
(1/1 by volume) to form a resin solution containing 20% resin solids. 
In the cases where the resin topcoat layer was baked at a low temperature 
below 100.degree. C., a resin solution having the same resin solids 
content as above was prepared by using methyl ethyl ketone as a solvent. 
A commercially-available high molecular-weight polyhydroxypolyether resin 
in which the dihydric phenol component was a dinuclear phenol (bisphenol 
A), i.e., Bakelite (registered trademark) phenoxy resin PKHH manufactured 
by Union Carbide (molecular weight about 30,000) was also used in some 
runs and it was dissolved in the same mixed solvent as above to form a 
resin solution having 20% resin solids content. 
As a reactive plasticizer, pentaerythritol triacrylate (Aronix M-305 
manufactured by Toa Gosei Chemical) was added to some resin solutions. 
(d) Preparation of precoated steel sheet: 
On a cleaned plated surface of the above-mentioned base steel sheet, the 
aqueous suspension prepared in (b) above which contained partially-reduced 
chromic acid, colloidal silica and optionally one or more other additives 
was applied by a wire-wound rod coater at varying coating weights, and the 
coated steel sheet was then baked for 30 seconds at a predetermined 
temperature of the steel sheet to form a colloidal silica-containing 
chromate film on the plated surface. After the steel sheet was allowed to 
cool to room temperature, the resin solution preared in (c) above was 
applied with varying thicknesses on the chromate film by a wire-wound rod 
coater and baked for 60 seconds at a predetermined temperature of the 
steel sheet to form an organic topcoat layer. 
The thus-prepared precoated steel sheet was evaluated with respect to 
corrosion resistance, formability, and weldability by the testing 
procedures described below. For the precoated steel sheets in which the 
substrate steel was of the bake-hardening type, the bake-hardenability of 
the precoated steel sheets was also evaluated. 
(e) Testing procedures: 
(i) Corrosion resistance: 
The corrosion resistance of the precoated steel sheet was evaluated by an 
altenate wet and dry test in which a test piece of the precoated steel 
sheet was subjected to repeated cycles consisting of dipping in 5% NaCl 
solution at 35.degree. C. for 1 hour and subsequent air drying at 
50.degree. C. for 1 hour. After exposure to 480 cycles (total exposure 
period: 960 hours), the percent area of blisters observed on the coating 
and the average diameter of the blisters were determined as measures of 
corrosion resistance. 
(ii) Formability (adhesion after press forming): 
In order to evaluate formability of the precoated steel sheet, a test piece 
was subjected to a beaded U-bend press forming test shown in FIG. 1. In 
FIG. 1 only the left half of the test piece is shown because the right 
half is the same. Referring to FIG. 1, on a die 1 a test piece 2 having a 
coating 3 on one surface was placed with the coating 3 facing the die 1 
and was supported with the aid of a spacer 4 by a blank holder 5. 
Thereafter a punch 6 was forced downward as indicated by the arrow to 
perform press forming on the test piece between the die and punch so as to 
make a U-bend. As shown in FIG. 2, the evaluation was made by determining 
the percent area of peeled-off portions 7 of the coating produced by the 
U-bend forming, which was calculated by the following equation: 
##EQU1## 
Although only a half of the test piece is shown in FIGS. 1 and 2, the 
percent area of peeled-off coating was calculated by the above equation 
based on the measurements of the entire test piece. The die shoulders were 
cleansed with trichloroethylene and polished with a #120 Emery paper prior 
to each press forming test in order to keep a constant surface roughness 
of the shoulder portions. 
(iii) Weldability: 
Two test pieces of each precoated steel sheet were placed one on the other 
with the coated surface of one test piece facing the uncoated surface of 
the other, and spot welding was performed thereon with an AC single spot 
welder with electrodes having a tip diameter of 5.0 mm by impressing a 
welding current of 8000A for 10 cycles under a load of 200 kg. The 
weldability was evaluated as follows: 
.largecircle.: Completely welded with no surface flashes; 
.DELTA.: Completely welded with surface flashes; 
X: Incompletely welded or unwelded. 
(iv) Bake-hardenability: 
A test piece of a precoated steel sheet was stretched with 2% elongation 
and then heated at 170.degree. C. for 30 minutes. The tensile properties 
of the heated test piece were determined and the bake-hardenability was 
evaluated in terms of the difference of the yield stress (yield point) 
before heating substracted from that after heating. 
The results are summarized in Tables 1-3 below, in which Table 1 shows the 
compositions, weight or thickness, and baking temperatures of the 
undercoat chromate layer and the organic topcoat layer employed in the 
preparation of each precoated steel sheet. The run numbers bearing an 
asterisk indicate comparative examples in which one or more parameters are 
outside the ranges defined herein. 
Table 2 shows the test results for corrosion resistance, press formability, 
and weldability of each precoated steel sheet. Table 3 shows the test 
results for bake-hardenability of a precoated steel sheet having a 
substrate steel of the bake-hardening type. The chemical composition of 
the bake-hardening-type steel used as a substrate is also shown in Table 
3. 
EXAMPLE 2 
This example illustrates the preparation of precoated steel sheets in which 
the organic topcoat layer contains an inorganic filler and/or a 
cross-linking agent. 
(a) Base steel sheet: 
The base steel sheet used in this example was the same as that used in 
Example 1. Namely, it was comprised of a 0.8 mm-thick cold-rolled steel 
sheet having an electroplated coating of 12% Ni-Zn alloy with a weight of 
20 g/m.sup.2 on one surface thereof. Prior to use, the base steel sheet 
was degreased with Fine Cleaner 4336 (manufactured by Nihon Parkerizing) 
to clean the plated surface. 
(b) Aqueous suspension for chromating: 
To an aqueous chromic acid solution containing 120 g/l of CrO.sub.3, an 
aqueous ethylene glycol solution was added as a reducing agent and the 
mixture was heated at 80.degree. C. for 6 hours to partially reduce the 
chromic acid. After cooling, the reaction mixture was diluted with an 
aqueous chromic acid solution containing 40 g/l of CrO.sub.3 in an amount 
sufficient to adjust the Cr.sup.3+ /Cr.sup.6+ ratio to 2/3 [Cr.sup.3+ 
/(Cr.sup.3+ +Cr.sup.6+)=0.4]. The aqueous solution of partially-reduced 
chromic acid was further diluted with water sufficient to adjust the 
concentration of total chromic acid to 40 g/l (0.4M as CrO.sub.3). 
To the resulting aqueous solution of partially reduced chromic acid, the 
following additives were added: 
(a) 40 g/l of colloidal silica having an average particle diameter of 12 nm 
(Aerosil 200 manufactured by Nippon Aerosil); 
(b) 11.5 g/l of glycerol as a polyhydric alcohol; 
(c) 6.5 g/l of citric acid as a hydroxycarboxylic acid; 
(d) 15 g/l of gamma-glycidoxypropyltrimethoxysilane as a silane coupling 
agent; and 
(e) a predetermined amount of iron phosphide (Fe.sub.2 P) powder having an 
average particle diameter of 3 .mu.m (HRS-2132 manufactured by Occidental 
Chemical). 
In some runs, (f) strontium chromate as a metal chromate was also added in 
a predetermined amount. 
The resulting mixture was thoroughly agitated by a High-Speed Disper to 
form an aqueous suspension prior to use. 
(c) Polyhydroxypolyether resin-based coating composition: 
The polyhydroxypolyether resins used in this example were the same as those 
employed in Example 1. Namely, one was a powdery high molecular-weight 
polyhydroxypolyether resin having a number-average molecular weight of 
approximately 35,000 prepared as described in Example 1 in which the 
dihydric phenol component was comprised of resorcinol (mononuclear) and 
bisphenol A (dinuclear) at a molar ratio of 1:1, and the other was the 
commercially-available Bakelite phenoxy resin PKHH described in Example 1 
(M.W.=about 30,000) in which the dihydric phenol component was comprised 
solely of dinuclear bisphenol A. These resins were dissolved in the same 
manner as described in Example 1 to form coating compositions. 
When a cross-linking agent (blocked isocyanate) and/or a plasticizer 
(butyral resin) was incorporated in the resin solution, it was added with 
stirring. When an inorganic filler was added to the resin solution, it was 
dispersed in the solution by using glass beads of 2 mm in diameter in a 
sand mill as follows: A predetermined amount of the inorganic filler was 
added to 80 g of the resin solution and the mixture was stirred with the 
glass beads for 10-30 minutes until there was no particle larger than 5 
.mu.m in diameter as measured by a grindometer. 
(d) Preparation of precoated steel sheet: 
On a clean plated surface of the above-mentioned base steel sheet, the 
aqueous suspension prepared in (b) above was applied by a wire-wound rod 
coater with varying coating weights, and the coated steel sheet was then 
baked for 30 seconds at a temperature of the steel sheet between 
120.degree.-140.degree. C. to form a colloidal silica-containing chromate 
film. After the steel sheet was allowed to cool to room temperature, the 
resin solution prepared in (c) above was applied with varying thicknesses 
on the chromate film by a wire wound rod coater and baked for 60 seconds 
at a predetermined temperature of the steel sheet to form an organic 
topcoat layer. 
The thus-prepared precoated steel sheet was evaluated with respect to 
corrosion resistance, formability, electrodeposition coating property, 
weldability, and chromium solve-out according to the testing procedures 
described below. 
(e) Testing procedures: 
(i) Corrosion resistance: 
The corrosion resistance of each precoated steel sheet was measured with a 
flat test piece with no working applied thereto and a test piece which had 
been subjected to cylindrical deep drawing with a diameter of 50 mm. The 
shoulder of the die used in the cylindrical drawing was washed with 
trichloroethylene and polished with a #120 Emery paper prior to each test 
so as to maintain a constant surface roughness of the shoulder portion. 
Both test pieces were immersed in a degreasing solution FC-4357 
(manufactured by Nihon Parkerizing) at 60.degree. C. for 2 minutes, then 
rinsed with water, and dried by heating at 165.degree. C. for 25 minutes. 
Thereafter, each test piece was subjected to an altenate wet and dry test 
in which the test piece was exposed to repeated cycles consisting of salt 
spraying with a 5% NaCl solution at 35.degree. C. for 4 hour, air drying 
at 60.degree. C. for 2 hour, and exposure to a wet atmosphere at 
50.degree. C. and 95 % relative humidity for 2 hours. After exposure to 
200 cycles (total exposure period: 1600 hours), the percent of the coating 
area covered by red rust was determined as a measure of corrosion 
resistance. 
(ii) Formability (adhesion after press forming): 
Formability was evaluated in the same manner as described in Example 
1-(ii). 
(iii) Electrodeposition coating property: 
A test piece was degreased in the same manner as described in the Corrosion 
Resistance Test (i) above. Subsequently, electrodeposition coating was 
applied to the coated surface of the test piece using a coating 
composition U-100 (manufactured by Nippon Paint) under such conditions 
that a 20 .mu.m-thick coating would be deposited on a cold-rolled steel 
sheet which had been treated by chemical conversion (usually for 3 minutes 
at 200 V), and the electrodeposited coating was baked at 165.degree. C. 
for 25 minutes. The appearance of the electrodeposited coating was 
visually evaluated and assigned the following ratings: 
.largecircle.: Good appearance 
.DELTA.: Significantly roughened surface; 
X: Formation of craters or incapable of electrodeposition. 
Secondary adhesion of the electrodeposited coating was also evaluated by 
the cross cut adhesion peeling test after the test piece was immersed in 
warm water at 40.degree. C. for 10 days. When all the cross-cut sections 
of the coating remained on the steel sheet after the peeling test, the 
rating ".largecircle." was assigned. 
(iv) Weldability: 
Two test pieces of each precoated steel sheet were placed one on the other 
with the coated surface of one test piece facing the uncoated surface of 
the other, and spot welding was performed thereon with an AC single spot 
welder with electrodes having a tip diameter of 5.0 mm by impressing a 
welding current of 8000A for 12 cycles under a load of 200 kg. The 
weldability was evaluated as follows: 
.largecircle.: Weldable with 5000 consecutive spots 
.DELTA.: Weldable with less than 5000 consecutive spots 
X: Non-weldable 
(v) Chromium solve-out: 
Two test pieces of each precoated steel sheet were immersed in a degreasing 
solution FC-L4410 (manufactured by Nihon Parkerizing) at 43.degree. C. for 
2 minutes and 30 seconds, and thereafter one of the test pieces was 
further immersed in a zinc phosphate-containing chemical conversion 
solution PB-L3020 (manufactured by Nihon Parkerizing) at 43.degree. C. for 
2 minutes. The weight of chromium dissolved out of the coating into each 
solution during immersion was determined based on the measurements of the 
Cr weight of the coating before and after the immersion which were carried 
out by fluorescent X-ray analysis. 
The compositions, weight or thickness, and baking temperatures of the 
undercoat chromate layer and the organic topcoat layer employed in the 
preparation of each precoated steel sheet are summarized in Table 4, while 
Table 5 shows the test results for each precoated steel sheet. The run 
numbers bearing an asterisk indicate comparative examples in which one or 
more parameters are outside the range defined herein. 
EXAMPLE 3 
This example illustrates the preparation of precoated steel sheets of the 
present invention in which the organic topcoat layer contains colloidal 
silica and the undercoat is free from colloidal silica. 
(a) Base steel sheet: 
The base steel sheet used in this example was a Zn alloy-electroplated 
steel sheet comprising a 0.8 mm-thick cold-rolled steel sheet having an 
electroplated coating of 12% Ni-Zn alloy with a weight of 20 g/m.sup.2 on 
one surface thereof. Prior to use, the base steel sheet was degreased with 
Fine Cleaner 4336 (manufactured by Nihon Parkerizing) to clean the plated 
surface. 
(b) Aqueous suspension for chromating: 
To an aqueous chromic acid solution containing 120 g/l of CrO.sub.3, 
ethylene glycol in an aqueous solution was added as a reducing agent and 
the mixture was heated at 80.degree. C. for 6 hours to partially reduce 
the chromic acid. After cooling, the reaction mixture was diluted with an 
aqueous chromic acid solution containing 40 g/l of CrO.sub.3 in an amount 
sufficient to adjust the Cr.sup.3+ /(Cr.sup.3+ +Cr.sup.6+) ratio to a 
predetermined value. The aqueous solution of partially-reduced chromic 
acid was further diluted with water sufficient to adjust the concentration 
of total chromic acid (total Cr concentration as CrO.sub.3) to 40 g/l 
(0.4M as CrO.sub.3). 
To the resulting aqueous solution of partially reduced chromic acid, a 
predetermined amount of glycerol (polyhydric alcohol) as a reducing agent. 
In some, runs as a silane coupling agent 
gamma-glycidoxypropyltrimethoxysilane was added. 
For comparison an aqueous solution for chromating which contains colloidal 
silica was prepared. 
(c) Polyhydroxypolyether resin-based coating composition: 
A flask fitted with a condenser was charged with 230 parts by weight of 
bisphenol A diglycidyl ether (Epikote 828 manufactured by Yuka Shell 
Epoxy), 55 parts by weight of resorcinol, 200 parts by weight of methyl 
ethyl ketone, and 4 parts by volume of an aqueous 5N NaOH solution. The 
mixture was heated to reflux and allowed to react at that temperature for 
18 hours. The resulting resinous mass was poured into water and stirred to 
precipitate a water-insoluble resin. The precipitates were collected by 
filtration and dried in vacuo to yield a high molecular-weight 
polyhydroxypolyether resin having a number-average molecular weight of 
approximately 35,000 as powder in which the dihydric phenol component was 
comprised of an equimolar mixture of resorcinol (mononuclear) and 
bisphenol A (dinuclear) in a molar ratio of 1/1 (hereunder referred as 
Resin-A). 
The powdery high molecular-weight polyhydroxypolyether resin obtained above 
was dissolved in a mixed solvent of cellosolve acetate and cyclohexanone 
(1/1 by volume) to form a resin solution containing 20% resin solids. 
A commercially-available high molecular-weight polyhydroxypolyether resin 
in which the dihydric phenol component was a dinuclear phenol (bisphenol 
A), i.e., Bakelite (registered trademark) phenoxy resin PKHH manufactured 
by Union Carbide (molecular weight about 30,000) was also used in some 
runs and it was dissolved in the same mixed solvent as above to form a 
resin solution having 20% resin solids content (hereunder referred to as 
Resin-B). 
As a general-purpose expoxy resin, Epikote 1009 (molecular weight of 3750) 
by Yuka Shell was dissolved in a xylene/methyl ethyl keton solvent (weight 
ratio 6/4) to form a resin-containing solution (hereunder referred to as 
Resin-C). 
Colloidal silica (average particle size 10-20 m.mu., "Oskal 1432", trade 
name of Shokubai Kasei Co. Ltd.), a cross-linking agent (blocked 
isocyanate having a dissociation temperature of 80.degree. C. for Resin-A 
and -B, and phenol resin for Resin-C), and a plasticizer (butyral resin) 
were added, mixed, and dispersed in the resin-containing solution. 
(d) Preparation of precoated steel sheet: 
On a cleaned plated surface of the above-mentioned base steel sheet, the 
aqueous suspension prepared in (b) above which contained partially-reduced 
chromic acid, 
and optionally one or more other additives was applied by a wire-wound rod 
coater at varying coating weights, and the coated steel sheet was then 
baked for 30 seconds at a temperature of the steel sheet of 140.degree. C. 
to form a chromate film on the plated surface. After the steel sheet was 
allowed to cool to room temperature, the resin solution prepared in (c) 
above was applied with varying thicknesses on the chromate film by a 
wire-wound rod coater and baked for 60 seconds at a temperature of the 
steel sheet of 140.degree. C. to form an organic topcoat layer. 
The thus-prepared precoated steel sheet was evaluated with respect to 
corrosion resistance, electrodeposition applicability, solving-out of 
chromium, and weldability by the testing procedures described below. 
(i)) Weldability: 
Two test pieces of each precoated steel sheet were placed one on the other 
with the coated surface of one test piece facing the uncoated surface of 
the other, and spot welding was performed thereon with an AC single spot 
welder with electrodes having a tip diameter of 6.0 mm by impressing a 
welding current of 10000 A for 12 cycles under a load of 200 kg. The 
weldability was evaluated as follows: 
(A) Uniformness of welding spots: 
After performing spot welding with consecutive 1,000 spots, 100 spot 
samples were taken at random out of 1,000 spots. The number of irregular 
spots which were caused by local concentration of current was determined. 
FIG. 3 schematically shows welding spots; one is good and the other one is 
bad. 
(B) Applicability of spot welding: 
After performing spot welding with 1,000 consecutive spots, the diameter of 
the electrode was measured: 
0: Diameter&lt;7.0 mm 
.DELTA.: Diameter=7.0-8.0 mm 
X: Diameter&gt;8.0 mm 
(ii) Corrosion resistance: 
The corrosion resistance of each precoated steel sheet was measured with a 
flat test piece with no working applied thereto and a test piece which had 
been subjected to cylindrical deep drawing with a diameter of 50 mm. The 
shoulder of the die used in the cylindrical drawing was washed with 
trichloroethylene and polished with a #120 Emery paper prior to each test 
so as to maintain a constant surface roughness of the shoulder portion. 
Both test pieces were immersed in a degreasing solution FC-L4410 
(manufactured by Nihon Parkerizing) at 43.degree. C. for 2.5 minutes, then 
rinsed with water, and dried by heating at 165.degree. C. for 25 minutes. 
Thereafter, each test piece was subjected to an altenate wet and dry test 
in which the test piece was exposed to repeated cycles consisting of salt 
spraying with a 5% NaCl solution at 35.degree. C. for 4 hour, air drying 
at 60.degree. C. for 2 hour, and exposure to a wet atmosphere at 
50.degree. C. and 95 % relative humidity for 2 hours. After exposure to 
200 cycles (total exposure period: 1600 hours), the percent of the coating 
area covered by red rust was determined as a measure of corrosion 
resistance. 
(iii) Electrodeposition coating property: 
A test piece was degreased in the same manner as described in the Corrosion 
Resistance Test (i) above. Subsequently, electrodeposition coating was 
applied to the coated surface of the test piece using a coating 
composition U-100 (manufactured by Nippon Paint) under such conditions 
that a 20 .mu.m-thick coating would be deposited on a cold-rolled steel 
sheet which had been treated by chemical conversion (usually for 3 minutes 
at 200 V), and the electrodeposited coating was baked at 165.degree. C. 
for 25 minutes. The appearance of the electrodeposited coating was 
visually evaluated and assigned the following ratings: 
.largecircle.: Good appearance 
.DELTA.: Significantly roughened surface; 
X: Formation of craters or incapable of electrodeposition. 
(iv) Chromium solve-out: 
Two test pieces of each precoated steel sheet were immersed in a degreasing 
solution FC-L4410 (manufactured by Nihon Parkerizing) at 43.degree. C. for 
2 minutes and 30 seconds, and thereafter one of the test pieces was 
further immersed in a zinc phosphate-conataining chemical conversion 
solution PB-L3020 (manufactured by Nihon Parkerizing) at 43.degree. C. for 
2 minutes. The weight of chromium dissolved out of the coating into each 
solution during immersion was determined based on the measurements of the 
Cr weight of the coating before and after the immersion which were carried 
out by fluorescent X-ray analysis. 
The test results are summarized in Table 6. 
3 TABLE 1 
Undercoat chromate layer (Cr weight, Cr reduction rate, amounts of 
additives, baking temp.) Organic topcoat layer Run No. 
(mg/m.sup.3 as Cr)Cr weight 
##STR4## 
silica.sup.1)Colloidal agent.sup.2),5)couplingSilane Glycerol.sup.2) 
Citric acid.sup.2) Fe.sub.2 
P.sup.1) SrCrO.sub.4.sup.1) temp. (.degree.C.)Baking Base resin.sup.3) 
plasticizer.sup.4)Reactive (.mu.m)thicknessDry film temp. (.degree.C.)Bak 
ing 
1 50 0.4 1.5 0.1 G 100 A 1 120 2 100 0.4 1.5 0.1 G 100 A 
1 120 3 200 0.4 1.5 0.1 G 100 A 1 120 4 300 0.4 1.5 0.1 G 100 
A 1 120 5 100 0.4 1.5 0.1 100 A 1 120 6 100 0.4 1.5 0.1 
100 A 1 120 7 100 0.4 1.5 0.1 G 0.1 0.1 100 A 1 120 8 100 0.4 1.5 
0.1 V 60 A 1 120 9 100 0.4 1.5 0.1 60 A 1 120 10 100 0.4 
1.5 0.1 60 A 1 120 11 100 0.4 1.5 0.1 V 0.1 0.1 60 A 1 120 12 
100 0.4 1.5 0.01 A 100 A 1 120 13 100 0.4 1.5 0.01 100 A 1 
120 14 100 0.4 1.5 0.01 100 A 1 120 15 100 0.4 1.5 0.01 A 0.01 
0.01 100 A 1 120 16 100 0.1 1.5 100 A 1 120 17 100 0.1 1.5 0.1 
A 0.1 0.1 100 A 1 120 18 100 0.4 0.5 100 A 1 120 19 100 0.4 
1.5 100 A 1 120 20 100 0.4 5.0 100 A 1 120 21 100 0.4 0.5 
0.1 A 0.1 0.1 100 A 1 120 22 100 0.4 5.0 0.1 A 0.1 0.1 100 A 1 120 
23 100 0.4 1.5 0.1 A 0.3 0.3 100 A 1 120 24 300 0.4 1.5 5 100 A 
1 120 25 100 0.4 1.5 0.1 G 5 100 A 1 120 26 100 0.4 1.5 0.1 5 
100 A 1 120 27 100 0.4 1.5 0.1 5 100 A 1 120 28 100 0.4 1.5 0.1 G 
0.1 0.1 5 100 A 1 120 29 100 0.4 1.5 0.1 G 0.1 0.1 1 100 A 1 120 30 
100 0.4 1.5 0.1 G 0.1 0.1 10 100 A 1 120 31 100 0.4 1.5 0.1 G 0.1 0.1 
5 0.4 100 A 1 120 32 100 0.4 1.5 0.1 G 0.1 0.1 100 A 5 120 33 100 
0.4 1.5 0.1 G 0.1 0.1 5 100 A 5 120 34 100 0.4 1.5 0.1 G 0.1 0.1 5 
100 A 10 150 35 100 0.4 1.5 100 A 1 1 120 36 100 0.4 1.5 
100 A 5 1 120 37 100 0.4 1.5 100 A 10 1 120 38 100 0.4 1.5 
100 A 20 1 120 39 100 0.4 5.0 0.1 G 0.1 0.1 5 0.4 100 A 10 1 120 40 100 
0.4 1.5 0.1 G 0.1 0.1 100 A 10 1 120 41 100 0.4 1.5 100 B 1 1 
120 42 100 0.4 1.5 100 B 5 1 120 43 300 0.4 1.5 100 B 10 1 
120 44 100 0.4 1.5 100 A 20 1 120 45 100 0.4 1.5 0.1 G 0.1 0.1 5 
0.4 80 A 1 80 46 100 0.4 1.5 0.1 G 0.1 0.1 5 0.4 120 A 1 120 47 100 
0.4 1.5 0.1 G 0.1 0.1 5 0.4 150 A 1 150 48 100 0.4 1.5 0.1 G 0.1 0.1 5 
0.4 180 A 1 180 49* 100 0.05** 1.5 100 A 1 120 50* 100 0.4 
0.01** 100 A 1 120 51* 5** 0.4 1.5 0.1 G 0.1 0.1 100 A 1 
120 52* 630** 0.4 1.5 0.1 G 0.1 0.1 100 A 1 120 53* 100 0.4 1.5 
0.1 G 0.1 0.1 5 100 A 15** 150 54* 100 0.4 7.0** 100 A 1 120 
55* 100 0.4 1.5 3.0** 100 A 1 120 56* 100 0.4 1.5 3.0** 100 
A 1 120 57* 100 0.4 1.5 0.1 G 0.1 0.1 5 0.4 210** A 1 210** 58* 
100 0.4 
(Notes) 
.sup.1) Weight ratio relative to total chromic acid (excluding 
SrCrO.sub.4); Regarding the amount of SrCrO.sub.4, the weight ratio of 
SrCrO.sub.4 /total chromic acid of 0.4 indicated in the table corresponds 
to the molar ratio of SrCrO.sub.4 /unreduced chromic acid of 0.46. 
.sup.2) Molar ratio relative to residual unreduced chromic acid 
(Cr.sup.6+); 
.sup.3) Base resin; A = High molecularweight polyhydroxypolyether resin i 
which the dihydric phenol is comprised of resorcinol and bisphenol A in a 
molar ratio of 1:1; B = High molecularweight polyhydroxypolyether resin 
derived from bisphenol A as a dihydric phenol; 
.sup.4) Reactive plasticizer: pentaerythritol triacrylate; Weight % based 
on the total resin solids; 
.sup.5) Silane coupling agent: V = Vinyltriethoxysilane; G = 
.gamma.-Glycidoxypropyltrimethoxysilane; A = 
.gamma.-Aminopropyltriethoxysilane. 
**Outside the range defined herein. 
In the precoated steel sheets of Runs Nos. 45-48 and 57, the substrate 
steel sheet used was made from a steel of the bakehardening type. 
TABLE 2 
______________________________________ 
Formability 
Corrosion resistance % Area of 
Run % Area of Blister peeled-off 
Weld- 
No. blisters diameter coating ability 
______________________________________ 
1 5 0.5 0 .DELTA. 
2 0 -- 0 .DELTA. 
3 0 -- 0 .DELTA. 
4 0 -- 5 .DELTA. 
5 0 -- 0 .DELTA. 
6 0 -- 2 .DELTA. 
7 0 -- 0 .DELTA. 
8 5 0.5 2 .DELTA. 
9 5 0.5 2 .DELTA. 
10 5 0.5 2 .DELTA. 
11 2 0.5 0 .DELTA. 
12 5 0.5 5 .DELTA. 
13 5 0.5 5 .DELTA. 
14 5 0.5 5 .DELTA. 
15 5 0.5 2 .DELTA. 
16 10 0.5 5 .DELTA. 
17 5 0.5 2 .DELTA. 
18 10 0.5 10 .DELTA. 
19 10 0.5 5 .DELTA. 
20 5 0.5 10 .DELTA. 
21 2 0.5 5 .DELTA. 
22 0 -- 5 .DELTA. 
23 0 -- 0 .DELTA. 
24 2 0.5 5 .largecircle. 
25 2 0.5 0 .largecircle. 
26 2 0.5 0 .largecircle. 
27 2 0.5 0 .largecircle. 
28 0 -- 0 .largecircle. 
29 0 -- 0 .largecircle. 
30 0 -- 0 .largecircle. 
31 0 -- 0 .largecircle. 
32 0 -- 0 .DELTA. 
33 0 -- 0 .largecircle. 
34 0 -- 0 .DELTA. 
35 5 0.5 2 .DELTA. 
36 2 0.5 0 .DELTA. 
37 2 0.5 0 .DELTA. 
38 2 0.5 0 .DELTA. 
39 0 -- 0 .largecircle. 
40 0 -- 0 .DELTA. 
41 10 0.5 5 .DELTA. 
42 5 0.5 0 .DELTA. 
43 5 0.5 0 .DELTA. 
44 5 0.5 0 .DELTA. 
45 5 0.5 0 .largecircle. 
46 0 -- 0 .largecircle. 
47 0 -- 0 .largecircle. 
48 0 -- 0 .largecircle. 
49* 40 3 20 .DELTA. 
50* 10 0.5 30 .DELTA. 
51* 60 3 10 .DELTA. 
52* 0 -- 50 .DELTA. 
53* 0 -- 0 X 
54* 0 -- 60 .DELTA. 
55* 30 3 0 .DELTA. 
56* 30 3 0 .DELTA. 
57* -- -- -- -- 
58* 20 2 30 O 
______________________________________ 
TABLE 3 
______________________________________ 
Composition of bake hardening-type steel (weight %) 
C Si Mn P S sol.Al 
N 
______________________________________ 
0.01 0.02 0.12 0.075 
0.005 0.0049 
0.0069 
______________________________________ 
Tensile properties and bake-hardenability 
Yield Tensile Elon- Bake hard- 
point strength gation 
YPE.sup.(1) 
enability 
Run No. 
(kgf/mm.sup.2) 
(kgf/mm.sup.2) 
(%) (%) (kgf/mm.sup.2) 
______________________________________ 
45 20.5 35.6 39.2 0 4.3 
46 20.5 35.2 39.5 0 4.3 
47 20.5 35.2 39.0 0 4.5 
48 21.0 35.4 39.2 0.2 4.5 
57* 23.0 35.5 36.2 0.8 1.5 
Unbaked 
20.0 35.2 40.1 0 4.3 
stock 
______________________________________ 
(Note) 
.sup.(1) YPE: Yield point elongation 
TABLE 4 
__________________________________________________________________________ 
Undercoat chromate layer 
Organic topcoat layer 
Cr weight Polybutyral 
Dry 
Baking 
Run (mg/ni Base 
Inorganic filler 
Cross-link. agent 
plasticizer 
thickness 
temp. 
No. as Cr) 
Fe.sub.2 P.sup.(1) 
SrCrO.sub.4.sup.(1) 
Resin.sup.(2) 
Class.sup. (3) 
vol %.sup.(4) 
Class.sup. (4) 
Amount.sup.(5) 
(weight %).sup.(6) 
(.mu.m) 
(.degree.C.) 
__________________________________________________________________________ 
1 60 5 -- A Zn phosphate 
10 -- -- -- 1.2 130 
2 60 5 -- A Ca phosphate 
10 -- -- -- 1.2 130 
3 60 5 -- A Zn phospho- 
10 -- -- -- 1.2 130 
molybdate 
4 60 5 -- A Al phospho- 
10 -- -- -- 1.2 130 
molybdate 
5 60 5 -- A silica A 
5 -- -- -- 1.2 130 
6 60 5 -- A silica A 
10 -- -- -- 1.2 130 
7 60 5 -- A silica A 
15 -- -- -- 1.2 130 
8 60 5 -- A silica B 
5 -- -- -- 1.2 130 
9 60 5 -- A silica B 
10 -- -- -- 1.2 130 
10 60 5 -- A silica B 
15 -- -- -- 1.2 130 
11 60 5 0.4 A silica A 
10 -- -- -- 1.2 130 
12 60 5 -- A -- -- A 0.5 -- 1.2 130 
13 60 5 -- A silica B 
10 A 0.5 -- 1.2 130 
14 60 5 -- A silica B 
10 A 0.5 10 1.2 130 
15 60 5 -- A -- -- B 0.5 -- 1.2 130 
16 60 5 -- A silica B 
10 B 0.5 -- 1.2 130 
17 40 5 -- A silica B 
10 -- -- -- 1.2 130 
18 60 5 -- A silica B 
10 -- -- -- 0.7 130 
19 60 -- -- A silica B 
10 -- -- -- 0.7 130 
20 150 5 -- A silica B 
10 -- -- -- 1.2 130 
21 60 5 -- A silica B 
10 -- -- -- 3.0 130 
22 60 5 -- B silica B 
10 -- -- -- 1.2 130 
23 60 5 -- B silica B 
10 A 0.5 -- 1.2 130 
24 60 -- -- B silica B 
10 A 0.5 -- 0.7 130 
25 60 5 -- A SrCrO.sub.4 
10 -- -- -- 1.2 130 
26* 
60 5 -- A silica B 
45** 
-- -- -- 1.2 130 
27* 
5** 5 -- A silica B 
10 -- -- -- 1.2 130 
28* 
630** 
5 -- A silica B 
10 -- -- -- 1.2 130 
__________________________________________________________________________ 
(Notes) 
.sup.(1) Weight ratio relative to total chronic acid as CrO.sub.3 ; 
.sup.(2) Base Resin A: Polyhydroxypolyether resin in which the molar rati 
of mononuclear/dinuclear phenol is 1/1 (a 1/1 mixture of resorcinol and 
bisphenol A) Base Resin B: Polyhydroxypolyether resin in which the 
dihydric phenol is bisphenol A (PKHH phenoxy resin); 
.sup.(3) Silica A: colloidal silica with an average particle diameter of 
10-20 nm (OSCAL 1432, Shokubai Kasei): Silica B: colloidal silica with an 
average particle diameter of 10-20 nm (OSCAL 1622, Shokubai Kasei); 
.sup.(4) Cross-linking agent A: Blocked isocyanatetype epoxy curing agent 
(releasing temerature 80.degree. C.); Crosslinking agent B: Blocked 
isocyanatetype epoxy curing agent (releasing temerature 145.degree. C.); 
.sup.(5) Ratio of the total number of the functional groups in the 
crosslinking agent to the total number of hydroxyl and epoxy functional 
groups in the resin; 
.sup.(6) Percent based on the total resin solids in the coating 
composition. 
TABLE 5 
__________________________________________________________________________ 
Formability 
Corrosion resistance 
(% peeled- 
Electrodeposition 
Dissolved Cr (mg/m.sup.2) 
Run 
(% red rusted area) 
off area 
Appear- 
Secondary 
Weld- 
Degreasing 
Zn phosphate 
No. 
Flat sheet 
After drawing 
of coating) 
ance adhesion 
ability 
solution 
solution 
__________________________________________________________________________ 
1 0 0 0 .largecircle. 
.largecircle. 
.largecircle. 
0 0.3 
2 0 0 0 .largecircle. 
.largecircle. 
.largecircle. 
0 0 
3 0 0 0 .largecircle. 
.largecircle. 
.largecircle. 
0.2 0.3 
4 0 0.about.2 
0 .largecircle. 
.largecircle. 
.largecircle. 
0.3 0.2 
5 0 0 0 .largecircle. 
.largecircle. 
.largecircle. 
0 0 
6 0 0 0 .largecircle. 
.largecircle. 
.largecircle. 
0.3 0.2 
7 0 0 2 .largecircle. 
.largecircle. 
.largecircle. 
0.2 0.3 
8 0 0 0 .largecircle. 
.largecircle. 
.largecircle. 
0.2 0 
9 0 0 0 .largecircle. 
.largecircle. 
.largecircle. 
0.3 0 
10 0 0 1 .largecircle. 
.largecircle. 
.largecircle. 
2.7 0 
11 0 0 0 .largecircle. 
.largecircle. 
.largecircle. 
0.8 0.6 
12 0 0 0 .largecircle. 
.largecircle. 
.largecircle. 
1.0 0.3 
13 0 2 2 .largecircle. 
.largecircle. 
.largecircle. 
1.2 0.3 
14 0 0 1 .largecircle. 
.largecircle. 
.largecircle. 
0.8 0.2 
15 0 0 3 .largecircle. 
.largecircle. 
.largecircle. 
0.7 0.4 
16 0 1 1 .largecircle. 
.largecircle. 
.largecircle. 
0.6 0.3 
17 0 5 0 .largecircle. 
.largecircle. 
.largecircle. 
0.2 0 
18 0 2 0 .largecircle. 
.largecircle. 
.largecircle. 
0.5 0.2 
19 0 0 0 .largecircle. 
.largecircle. 
.largecircle. 
0.8 0.2 
20 0 0 5 .largecircle. 
.largecircle. 
.DELTA. 
2.2 0.3 
21 0 0 3 X -- X 0 0 
22 0 1 2 .largecircle. 
.largecircle. 
.largecircle. 
1.0 0.2 
23 0 0 1 .largecircle. 
.largecircle. 
.largecircle. 
0.7 0.2 
24 0 2 1 .largecircle. 
.largecircle. 
.largecircle. 
1.2 0.3 
25 0 0 0 .largecircle. 
.largecircle. 
.largecircle. 
2.3 23 
26* 
0 20 10 .DELTA. 
.largecircle. 
X 0.8 0.2 
27* 
50 70 2 .largecircle. 
.largecircle. 
.largecircle. 
0 0 
28* 
0 0 50 .largecircle. 
.largecircle. 
X 20 3 
__________________________________________________________________________ 
TABLE 6 
__________________________________________________________________________ 
Chromate Solution, Undercoat Chromate Layer 
Resin Solution, Top coat 
Initial Coupling 
Cr Cross-linking 
Thick- 
SiO.sub.2 /CrO.sub.3 
C.sup.3+ / 
Glycerin 
OH group/Cr.sup.6+ 
Agent 
Deposition 
SiO.sub.2 
Agent Butyral 
ness 
No. 
in Solution 
Total Cr 
(g/l) 
in Glycerin 
(g/l) 
(mg/m.sup.2) 
Resin 
(%) (Molar Ratio) 
Resin 
(.mu.m) 
__________________________________________________________________________ 
1 0 0.4 15 2 0 60 A 15 0.5 -- 0.8 
2 0 0.5 6 1 0 60 A 15 0.5 -- 0.8 
3 0 0.5 12 2 0 30 A 15 0.5 -- 0.8 
4 0 0.5 12 2 0 60 A 15 0.5 -- 0.8 
5 0 0.5 12 2 0 90 A 15 0.5 -- 0.8 
6 0 0.5 12 2 10 60 A 15 0.5 -- 0.8 
7 0 0.5 12 2 0 60 A 15 0.5 -- 0.6 
8 0 0.5 12 2 0 60 A 15 0.5 -- 1.2 
9 0 0.5 12 2 0 60 A 15 0 -- 0.8 
10 0 0.5 12 2 0 60 A 15 0.5 10 0.8 
11 0 0.5 12 2 0 60 B 15 0.5 -- 0.8 
12 0 0.5 12 2 0 60 C 15 0.5 -- 0.8 
13 0 0.5 12 2 0 60 A 25 0.5 -- 0.8 
14 0 0.5 18 3 0 60 A 15 0.5 -- 0.8 
15 0 0.6 10 2 0 60 A 15 0.5 -- 0.8 
16 0.5* 0.5 12 2 0 60 A 15 0.5 -- 0.8 
17 1.0* 0.5 12 2 0 60 A 15 0.5 -- 0.8 
18 1.5* 0.5 12 2 0 60 A 15 0.5 -- 0.8 
19 0 0.5 0 0* 0 60 A 15 0.5 -- 0.8 
20 0 0.5 31 5* 0 60 A 15 0.5 -- 0.8 
21 0 0.5 12 2 0 10* A 15 0.5 -- 0.8 
22 0 0.5 12 2 0 150* A 15 0.5 -- 0.8 
23 0 0.5 12 2 0 60 A 0* 0.5 -- 0.8 
24 0 0.5 12 2 0 60 A 40* 
0.5 -- 0.8 
25 0 0.5 12 2 0 60 A 15 0.5 -- 0.2* 
26 0 0.5 12 2 0 60 A 15 0.5 -- 2.0* 
__________________________________________________________________________ 
Weldability Corrosion Solved Cr 
Electrode 
Overall 
Resistance % 
Electro- 
(mg/m.sup.2) 
Uniformness 
Diameter 
Evalua- 
Area of Blisters 
deposition 
During 
During Chemical 
No. 
of Welding 
After Welding 
tion Plate 
Cup Appearance 
Degreasing 
Treatment 
__________________________________________________________________________ 
1 0/100 .largecircle. 
.largecircle. 
0 0.about.1 
.largecircle. 
0.7 0.5 
2 0/100 .largecircle. 
.largecircle. 
0 0.about.1 
.largecircle. 
0.8 0.4 
3 0/100 .largecircle. 
.largecircle. 
0 3.about.5 
.largecircle. 
0.3 0.2 
4 0/100 .largecircle. 
.largecircle. 
0 0 .largecircle. 
0.4 0.4 
5 0/100 .largecircle. 
.largecircle. 
0 0 .largecircle. 
0.8 0.7 
6 0/100 .largecircle. 
.largecircle. 
0 0 .largecircle. 
0.2 0.1 
7 0/100 .largecircle. 
.largecircle. 
0.about.1 
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__________________________________________________________________________ 
COMATIVE EXAMPLE 
This example was carried out so as to prove effectiveness of two 
stage-reduction of chromium, i.e., the presence of partially-reduced 
chromic acid in the chromate solution with respect to improvement in 
corrosion resistance. 
As in the preceeding Examples 1 and 2, a steel sheet 0.8 mm thick was 
electroplated with a Zn-Ni alloy in an amount of 20 g/m.sup.2. Prior to 
the chromate formation treatment, the steel sheet was subjected to 
degreasing with an alkaline cleaner (FC-L 4480, trade name of Nihon 
Parkerizing). 
A chromate solution having the following basic composition was applied to 
the degreased one surface of the sheet. 
______________________________________ 
Basic Composition 
CrO.sub.3 
H.sub.3 PO.sub.4 
H.sub.2 SiF.sub.6 
SiO.sub.2 * 
______________________________________ 
g/l 50 12 2 50 
______________________________________ 
Note: *Snowtex of Nissan Kagaku 
After coating with the chromate solution, the steel sheet was dried at 
120.degree. C. A topcoating comprising urethane-modified high-molecular 
epoxy resin which contains phenol resin and colloidal silica in amounts of 
16% and 15% respectively was applied and then hardened at 140.degree. C. 
The resulting coated steel sheet was dipped in a degreasing solution (20 
g/l) at 60.degree. C. for 15 minutes so as to determine the solve-out of 
chromium from the coated layyer. The resistance to corrosion was also 
determined in the same manner as in the preceeding examples. 
The amount of Cr was determined using fluorescent X-ray analysis. The fixed 
Cr ratio was calculated by the following equation: 
EQU Fixed Cr Ratio=Cr content after dipping/Cr content before dipping 
The test results are summarized in the following Table. 
__________________________________________________________________________ 
Run 
Chromate Formation Treatment 
Topcoating 
Fixed Cr ratio 
Corrosion Resistance 
No. 
Additive *1 
Additive *2 
Cr content 
(Thickness) 
(%) Flat Sheet 
After Drawing 
Remarks 
__________________________________________________________________________ 
1 Ethylene 
None 70 mg/m.sup.2 
None 40 -- -- Pretest 
glycol 6 g/l 
2 Ethylene 
Glycerin 
" None 61 -- -- " 
glycol 6 g/l 
27 g/l 
3 None Glycerine 
" None 59 -- -- " 
43 g/l 
4 Ethylene 
None " 0.8 .mu.m 
70 30 100 Comparative 
glycol 6 g/l 
5 Ethylene 
Glycerin 
" " 95 0 0 Invention 
glycol 6 g/l 
27 g/l 
6 None Glycerin 
" " 90 5 20 Comparative 
43 g/l 
__________________________________________________________________________ 
Note:- 
*1 Added one day before the application. 
*2 Added one hour before the application. 
As is apparent from the results shown in the Table above, when the 
topcoating is not provided, the fixed Cr ratio after degreasing is 60% 
even if the two-stage reduction is applied. On the other hand, when the 
topcoating is applied, as shown in No. 5, the fixed Cr ratio is very high, 
i.e., 95%. 
In addition, Run Nos. 4 and 6 show the cases in which reduction was carried 
out at once, i.e., single-stage reduction was carried out, and Run No. 4 
allows the presence of partially-reduced chromic acid. In these cases, 
however, the fixed Cr ratio is remarkably small in comparison with that in 
Run No. 5 which falls within the range of the present invention. 
Thus, according to the present invention, the resistance to corrosion can 
be improved much more than the conventional chromate formation treatment. 
Compare Run Nos. 4 and 6 with Run No. 5. This is because two-stage 
reduction is caried out before application in the present invention. 
As described and demonstrated above, the precoated steel sheets of the 
present invention can be successfully welded by resistance welding when 
the organic topcoat has a thickness of about 22.5 .mu.m or less, and even 
with such a thin film thickness of the topcoat, they still maintain the 
properties of good corrosion resistance and formability. Therefore, they 
are particularly suitable for use in automobile bodies. The precoated 
steel sheets of the present invention are also useful in the manufacture 
of household appliances, business machines, and the like, and as building 
materials. 
Although the invention has been described with respect to preferred 
embodiments, it is to be understood that variations and modifications may 
be employed without departing from the concept of the invention as defined 
in the following claims. 
The principles, preferred embodiments and modes of operation of the present 
invention have been described in the foregoing specification. The 
invention which is intended to be protected herein, however, is not to be 
construed as limited to the particular forms disclosed, since these are to 
be regarded as illustrative rather than restrictive. Variations and 
changes may be made by those skilled in the art without departing from the 
spirit of the invention.