DESCRIPTION
HOT DIP GALVANNEALED STEEL SHEET AND METHOD OF
PRODUCTION OF THE SAME
TECHNICAL FIELD
The present invention relates to hot dip
galvannealed steel sheet made using an ultra-low carbon
steel sheet excellent in corrosion resistance,
workability, and coatability as a sheet material and a
method of production of the same. Further, the present
invention relates to a method of production of hot dip
galvannealed steel sheet extremely excellent in
appearance.
BACKGROUND ART
Conventional hot dip galvannealed steel sheet is
known as steel sheet for automobiles or buildings
excellent in coating adhesion and corrosion resistance
after coating. In recent years, in particular for
automobile applications, deep drawability has been
required, so large amounts of hot dip galvannealed steel
sheet made using ultra-low carbon steel sheet as the
sheet material are being used. In this case, the naked,
corrosion resis.tance and corrosion resistance of
2 5 scratched parts of coatings cannot necessarily be said to
be sufficient. Further, there have been the problem of
the difficulty in achieving both suppression of powdering
and suppression of flaking at the time of working and the
problem of the ease of occurrence of flaws in appearance
3 0 at the time of electrodeposition coating.
Japanese Patent Publication (A) No. 9-3417 discloses
hot dip galvannealed steel sheet excellent in corrosion
resistance comprised of steel sheet having a first layer
made of a Zn-Fe alloy layer and a second layer made of
3 5 Fe: 8 to 15%, Ni: 0.1 to 2%, and Al: 1% or less. Further,
Japanese Patent No. 2783452 discloses a method of
production of hot dip galvannealed steel sheet excellent
in corrosion resistance characterized by preplating the
surface of the steel sheet with 0.2 to 2 g/m2 of Ni, then
rapidly heating the sheet to 430 to 500°C, hot dip coating
the sheet in a galvanization bath containing A1 in an
amount of 0.05 to 0.25%, wiping the sheet, then heat
treating the sheet right over it at 470 to 550°C for 10 to
40 seconds for alloying. The above Japanese Patent
Publication (A) No. 9-3417 and Japanese Patent No.
2783452 disclose hot rolled low carbon A1 killed steel
10 sheet and are not discoveries regarding the ultra-low
carbon steel sheet aimed at by the present invention.
\i Ultra-low carbon steel sheet, compared with low
carbon steel sheet, has a higher degree of cleanliness of
the ferrite grain boundaries, uneven progress of
15 alloying, and easy growth of the r layer, so discoveries
relating to low carbon steel sheet cannot be applied as
they are. Further, said Japanese Patent Publication (A)
No. 9-3417 and Japanese Patent No. 2783452 have no
discoveries relating to workability and coating.
2 0 Japanese Patent No. 2804167 discloses a hot dip
galvannealed steel sheet plating bath obtained by hot dip
coating and alloying a sheet in a bath containing less
than 0.2% of A1 and 0.01 to 0.5% of Ni to give a coating
containing Fe: 8 to 13%, Al: less than 0.5%, Ni: 0.02 to
j 2 5 I%, and the balance of Zn and having a r layer thickness
of the base iron boundary of 0.5 p or less. Japanese
Patent No. 2804167 discloses low carbon steel sheet. It
has no discovery regarding the ultra-low carbon steel
sheet aimed at by the present invention. Even if applying
3 0 the method of production disclosed here to ultra-low
carbon steel sheet, the I' layer thickness substantially
cannot be made 0.5 p or less and the corrosion
resistance, workability, coatability are also all
completely insufficient.
35 Japanese Patent No. 2800285 discloses a method of
production of hot dip galvannealed steel sheet comprising
plating ultra-low carbon steel sheet with 20 to 70 mg/m2
of Ni, then annealing, hot dip galvanizing, and
galvannealing it. However, with this method, there is no
effect of improvement of the corrosion resistance and,
further, the workability is also not sufficient.
Japanese Patent No. 3557810 discloses a hot dip
galvannealed steel sheet excellent in slidability and
coatability obtained by plating in a hot dip
galvanization bath containing Al: 0.1 to 0.2% and Ni:
0.04 to 0.2%, alloying it by a 10 to 20°C/s rate of
temperature rise, and covering 1 to 40% of the surface
with a 1 to 10 pn 6 layer. However, with this technology,
the workability, in particular the anti-powdering
property and corrosion resistance, are not sufficient.
Japanese Patent No. 3498466 discloses plating in a
hot dip galvanization bath containing A1 to which Ni and
further at least one type of Pb, Sb, Bi, and Sn is added
and alloying under predetermined conditions to obtain hot
dip galvannealed steel sheet containing Al: 0.1 to 0.25%,
Fe: 6 to 18%, Ni: 0.05 to 0.3%, and 0.001 to 0.01% of at
least one type of Pb, Sb, Bi, and Sn. However, with this
technology, not only does the bath contain four elements
and control become troublesome, but also dross comprised
of Ni and A1 easily is formed in the bath. When this is
caught up in the plating layer, it becomes a factor
behind deterioration of the corrosion resistance, so this
is not preferred.
Further, ultra-low carbon steel sheet containing Ti
features extremely excellent deep drawability and
ductility obtained stably over a wide range of
ingredients. However, when hot dip galvanizing and
further alloying this steel sheet, the Ti in the steel
causes the crystal grain boundaries to be cleaned, so the
alloying reaction is promoted at the crystal grain
boundaries. As a result, an outburst reaction occurs
easily, overalloying proceeds easily, and the antipowdering
property deteriorates.
To solve this problem, a method of production of hot
dip galvannealed steel sheet comprising complexly adding
Nb together with Ti so as to control the alloying
reaction occurring at the crystal grain boundaries and
5 thereby improving the anti-powdering property has been
disclosed (Japanese Patent Publication (B2) No. 61-32375,
Japanese Patent Publication (A) No. 59-67319, Japanese
Patent Publication (A) No. 59-74231, and Japanese Patent
Publication (A) No. 5-106003). This further complexly
adds Nb to Ti, but the addition of Nb is costly, so this
has the defect of not being economical.
As technology for improving the anti-powdering
property of Ti-containing ultra-low carbon steel sheet
without complexly adding Nb, Japanese Patent Publication
(A) No. 10-287964 discloses controlling the steam
atmosphere in the cooling process after the
recrystallization annealing so as to cause the crystal
grain boundaries to oxidize and suppressing outburst at
the time of the alloying reaction. With this method, not
only is it difficult to control the oxidation, but also
the plating appearance is liable to be detrimentally
affected.
Japanese Patent Publication (A) No. 8-269665
discloses the method of raising the concentration of A1
in the hot dip plating bath to 0.12 to 0.2% or higher
than usual and creating locally high A1 concentration
phases at the base iron-plating boundary, but in this
case the plating layer easily becomes uneven and the
appearance easily deteriorates.
Further, when hot dip galvannealed steel sheet is
used for automobile body panel applications, the uneven
appearance of galvannealing often remains even after
painting the automobile, so an extremely high quality of
appearance is required. Most of this unevenness is
unevenness of the oxide film of the plated sheet
material, unevenness of the fine ingredients, and other
unevenness arising due to the previous processes, but the
causes are almost always difficult to identify. Basic
solutions were therefore difficult. The documents
mentioned above do not disclose any guidelines for
obtaining an extremely excellent appearance able to
5 withstand use for automobile body panels aimed at by the
present invention.
DISCLOSURE OF THE INVENTION
As explained above, the object is to provide hot dip
galvannealed steel sheet using an ultra-low carbon steel
sheet excellent in corrosion resistance, workability, and
coatability as a sheet material and a method of
production of the same. Further, in general, in
production of hot dip galvannealed steel sheet, an Fe-A1-
Zn alloy layer (so-called barrier layer) is formed in a
hot dip galvanization bath at the base iron-plating
boundary, the alloy layer is removed by later heat
treatment, and an Zn-Fe alloy layer in which A1 is
diffused is formed. The Fe-Al-Zn alloy layer plays an
extremely important role from the viewpoint of control of
the subsequent Zn-Fe alloying reaction and securing the
plating adhesion. However, the speed of formation of the
Fe-Al-Zn alloy layer is finely affected by the surface
conditions of the plated sheet material, the flow of
solution in the plating bath, etc., the fine differences
in thickness of the Fe-Al-Zn alloy layer directly have an
effect on the alloying reaction behavior, and fine
unevenness in plating appearance is induced, so it has
not been easy to produce hot dip galvannealed steel sheet
extremely excellent in appearance. Therefore, the present
invention has as its object the provision of a method of
production of hot dip galvannealed steel sheet extremely
excellent in appearance.
The inventors studied hot dip galvannealed steel
sheet excellent in corrosion resistance, workability, and
coatability using ultra-low carbon steel sheet as a sheet
material based on the discoveries of the technologies
disclosed in the above-mentioned Japanese Patent
Publication (A) No. 9-3417 and Japanese Patent No.
2783452 and thereby completed the present invention. That
is, the present invention provides hot dip galvannealed
steel sheet excellent in corrosion resistance,
workability, and coatability comprised of an ultra-low
carbon steel sheet having on at least one surface a
plating layer comprised of, by mass%, Fe: 8 to 13%, Ni:
0.05 to 1.0%, Al: 0.15 to 1.5%, and a balance of Zn and
unavoidable impurities, having a ratio of Al/Ni of 0.5 to
5.0, having an average thickness of a layer of the base
iron boundary of 1 p or less, and having a variation of
the same of f0.3 p or less.
Further, the present invention provides a method of
production of hot dip galvannealed steel sheet comprising
cleaning a surface of an annealed ultra-low carbon steel
sheet, preplating it by 0.1 to 1.0 g/m2 of Ni, rapidly
heating the sheet in a nonoxidizing or reducing
atmosphere to a sheet temperature of 430 to 500°C by a
30°C/sec or more rate of temperature rise, then plating it
in a hot dip galvanization bath containing Al: 0.1 to 0.2
mass%, wiping it, then rapidly heating it to 470 to 600°C
by a 30°C/sec or more rate of temperature rise, cooling it
without any soaking time or soaking and holding it for
less than 15 seconds, then cooling it.
Further, the inventors engaged in studies and as a
result discovered that if using an Fe-Ni-Al-Zn alloy
layer instead of an Fe-Al-Zn alloy layer as the alloy
layer formed at the base iron-plating in the hot dip
galvanization bath, the variation in behavior for
formation of the alloy layer due to the surface
conditions of the plated sheet material, flow of the
solution in the plating bath, etc. becomes smaller and
that, further, even if the alloy layer varies in
thickness, this does not have that much of an effect on
the subsequent Zn-Fe alloying reaction behavior and as a
result an extremely good appearance is obtained and
thereby reached the present invention. That is, the
present invention provides a method of production of a
hot dip galvannealed steel sheet comprising forming an
Fe-Ni-Al-Zn alloy layer on a base iron boundary in a hot
5 dip galvanization bath, then heat treating this to
eliminate the Fe-Ni-Al-Zn alloy layer and form a Zn-Fe
alloy layer in which Ni and A1 are diffused.
According to the present invention, it is possible
to provide hot dip galvannealed steel sheet using an
10 ultra-low carbon steel sheet excellent in corrosion
resistance, workability, and coatability as a sheet
material and a method of production of the same. Further,
according to the present invention, a method of
production of hot dip galvannealed steel sheet with an
15 extremely excellent appearance able to be used for
automobile body panels is obtained.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the results of analysis of the platingbase
iron boundary alloy layer formed in a hot dip
2 0 galvanization bath according to the present invention.
FIG. 2 shows the results of analysis of the platingbase
iron boundary alloy layer formed in a hot dip
galvanization bath in the prior art.
FIG. 3 shows the results of analysis of a hot dip
galvannealed layer structure according to the present
invention.
FIG. 4 shows the results of analysis of a hot dip
galvannealed layer structure according to the prior art.
FIG. 5 is a view of the preferable ranges of the A1
concentration in the bath and the amount of deposition of
Ni preplating in the present invention.
BEST MODE FOR WORKING THE INVENTION
Below, the present invention will be explained in
detail.
First, the ultra-low carbon steel sheet covered by
the present invention is one to which Ti, Nb, etc. are
added alone or complexly to eliminate the solute carbon,
one to which P, Mn, Si, are further added to improve the
strength, etc. Further, one containing an extremely small
amount of Ni, Cu, Sn, Cr, or other so-called trump
element may be used.
5 As the ultra-low carbon steel sheet to which Ti, Nb,
etc. are added alone or complexly to eliminate the solute
carbon, specifically one containing, by mass%, C: 0.005%
or less, Si: 0.03% or less, Mn: 0.05 to 0.5%, P: 0.02% or
less, S: 0.02% or less, and Ti (and/or Nb): 0.001 to 0.2%
10 may be used. Even with Ti (or Nb) added alone, inclusion
of Nb (or Ti) in an extent of 0.001% or less entering as
unavoidable impurities is deemed included.
-)
Further, as the ultra-low carbon steel sheet
improved in strength through the addition of P,
15 specifically one containing C: 0.005% or less, Si: 0.03%
or less, Mn: 0.05 to 0.58, P: 0.02 to 0.1%, and S: 0.02%
may be used. This may be used as the sheet material for
high strength hot dip galvannealed steel sheet good in
drawability able to be applied even for 340 MPa to 390
2 0 MPa class automobile body panel applications. Further, a
sheet of the above composition further containing Mn to
0.5 to 2.5% and further containing Si to 0.5% or less may
be used. This may be used as tbe sheet material for high
strength hot dip galvannealed steel sheet good in
J 2 5 drawability able to be applied even for 390 MPa to 440
MPa class automobile body panel applications.
Next, the reasons for limitation of the composition
and structure of the plating layer will be explained. Fe
was made 8 to 13% because if less than the lower limit,
3 0 the corrosion resistance is liable to deteriorate, while
if over the upper limit, the anti-powdering property is
liable to deteriorate.
Ni was made 0.05 to 1.0% because if less than the
lower limit, the corrosion resistance is liable to
3 5 deteriorate, while if over the upper limit, the antipowdering
property is liable to deteriorate. Note that
when seeking a better anti-powdering property, Ni is
preferably made 0 . 1 t o 0.5%.
A1 was made 0.15 t o 1.5% because i f l e s s than the
lower l i m i t , the anti-powdering property and corrosion
resistance are l i a b l e t o d e t e r i o r a t e , while i f over the
5 upper l i m i t , the c o a t a b i l i t y and f u r t h e r the corrosion
resistance are l i a b l e t o d e t e r i o r a t e . Note t h a t when
seeking a b e t t e r anti-powdering property, the A1 lower
l i m i t is preferably made 0.3%, while when seeking a s t i l l
b e t t e r c o a t a b i l i t y , the A1 upper l i m i t is preferably made
10 0.8%.
Further, the A l / N i r a t i o was defined as 0.5 t o 5.0
because i f l e s s than the lower l i m i t , the anti-powdering
property is l i a b l e t o d e t e r i o r a t e , while i f over the
upper l i m i t , the c o a t a b i l i t y and f u r t h e r t h e corrosion
15 r e s i s t a n c e are l i a b l e t o d e t e r i o r a t e . When seeking a
b e t t e r anti-powdering property, the lower l i m i t of the
A l / N i r a t i o is preferably made 1 . 0 .
The present invention features an r layer of the
base iron boundary having an average thickness of 1 p or
l e s s and having a v a r i a t i o n of the same of f0.3 p or
l e s s . Here, as the means for measuring the r layer
thickness, for example, the e l e c t r o l y t i c peeling method
of dissolving everything except the r layer in an
ammonium chloride aqueous solution by constant p o t e n t i a l
e l e c t r o l y s i s , then quantifying the r layer by constant
current e l e c t r o l y s i s , the method of etching a crosssection
of the p l a t i n g by a Nital (alcohol + n i t r i c acid)
or other known etching solution and d i r e c t l y observing it
by an o p t i c a l microscope e t c . , the method of finding it
from the X-ray d i f f r a c t i o n strength, e t c . may be used.
Further, t h e v a r i a t i o n of the r layer means a maximum
value and minimum value of within f 0 . 3 p with respect t o
the average value of the r layer when measuring several
points t o tends of points of the s t e e l sheet in the width
d i r e c t i o n . The upper l i m i t of the average thickness of
the r layer of the present invention of 1 p is a
relatively large value, but for the anti-powdering
property and workability, control of the above-mentioned
variation is important. Further, together with the above-
5 mentioned suitable plating composition, a good
performance can be obtained.
Next, a method of production of the hot dip
galvannealed steel sheet of the present invention will be
explained.
10 In the present invention, an annealed ultra-low
carbon steel sheet is used as the sheet material. First,
the surface has to be cleaned. The method is not
particularly limited. Alkali degreasing, brushing, acid
treatment, or another known method may be performed alone
or in combination in accordance with the state of dirt or
oxide film on the sheet material. From the viewpoint of
the uniformity of the Ni plating explained later, alkali
degreasing (for example, NaOH aqueous solution treatment)
and acid treatment (for example, sulfuric acid aqueous
solution treatment) are preferably used in combination in
that order.
In the present invention, the sheet is preplated by
0.1 to 1.0 g/m2 of Ni. While also depending on the
previously explained cleaning pretreatment, if less than
the lower limit, the wettability of this later hot dip
coating is insufficient and, further, the corrosion
resistance is also sufficient, while if over the upper
limit, the anti-powdering property is liable to
deteriorate. When seeking a better anti-powdering
property, the upper limit of the Ni preplating is
preferably made 0.8 g/m2.
After the Ni preplating, the sheet is rapidly heated
in a nonoxidizing or reducing atmosphere to a sheet
temperature of 430 to 500°C by a 30°C/sec or higher rate
of temperature rise. This treatment is required for
securing the wettability of the hot dip coating and
further the plating adhesion. When seeking a better antipowdering
property, the upper l i m i t of the sheet
temperature during heating is p r e f e r a b l y made 480°C.
The hot dip galvanization b a t h used is a b a t h
comprised of A l : 0.1 t o 0.2%, unavoidable impurities, and
5 the balance of Zn. This is because i f the A1 is l e s s than
the lower l i m i t , the anti-powdering property and
corrosion r e s i s t a n c e e a s i l y d e t e r i o r a t e , while i f over
the upper l i m i t , the c o a t a b i l i t y and the corrosion
r e s i s t a n c e a l s o e a s i l y d e t e r i o r a t e . I n t h e present
10 invention, N i is not d e l i b e r a t e l y added t o t h e p l a t i n g
bath. This point d i f f e r s from Patent Documents 5 and 6.
As the source of N i f o r t h e p l a t i n g l a y e r , N i p r e p l a t i n g
is used, so the problem of the Ni-A1 dross formed i n the
p l a t i n g bath being c a r r i e d t o t h e p l a t i n g layer causing
15 the p l a t i n g l a y e r t o become uneven and r e s u l t i n g i n
d e t e r i o r a t i o n of performance and other problems does not
occur. When seeking a b e t t e r anti-powdering property, the
lower l i m i t of the bath A1 concentration is p r e f e r a b l y
made 0.12%.
After p l a t i n g , the sheet is wiped, then rapidly
heated t o 470 t o 600°C by a 30°C/sec or more r a t e of
temperature r i s e , then cooled without any soaking time o r
soaked and held f o r l e s s t h a n 15 seconds, then cooled f o r
a l l o y i n g . This is extremely important f o r suppressing the
I- l a y e r , i n p a r t i c u l a r f o r suppressing v a r i a t i o n . In
p a r t i c u l a r , i f the r a t e of temperature rise is less than
30°C/seconds, both the r l a y e r and its v a r i a t i o n increase.
After t h e rapid h e a t i n g , cooling without any soaking time
o r soaking and holding f o r a short time ( l e s s t h a n 15
s e c o n d s ) , , t h e n cooling is important. In t h i s case, i f
t h i s condition is deviated from, both the r layer and its
v a r i a t i o n i n c r e a s e . Note t h a t an ordinary ultra-low
carbon s t e e l sheet is p r e f e r a b l y cooled without any
soaking time. Since no soaking time is necessary, the
furnace f a c i l i t y can be shortened and the speed does not
have t o be reduced f o r soaking. These are advantageous
from the viewpoint of the productivity. Further, an
ultra-low carbon steel sheet improved in strength by the
addition of P etc. tends to be slower to alloy, so it
should be soaked and held for a short time in accordance
5 with need. When seeking a better anti-powdering property,
the sheet is preferably rapidly heated to 470 to 550°C by
a 30°C/sec or more rate of temperature rise, cooled
without any soaking time, or soaked and held for less
than 10 seconds, then cooled for alloying.
10 Next, the method for obtaining an extremely good
appearance of a hot dip galvanized steel sheet will be
explained.
The plated sheet material used in the present
invention may be any sheet material, but the present
invention has as its object to obtain an extremely good
appearance such as required for automobile body panel
applications, so use of the ultra-low carbon steel sheet
often used for automobile body panel applications is
effective.
FIG. 1 shows the state of the alloy layer formed in
the hot dip galvanization bath in the present invention.
FIG. 1 shows the distribution of elements (Ni, Al, Zn,
and Fe). in the plating depth direction measured by EPMA
analysis of a cross-section of a sample rapidly cooled
right after being lifted out from the hot dip
galvanization bath and polished embedded. It is learned
that an alloy layer comprised of Fe-Ni-Al-Zn is formed at
the base iron-plating layer. Note that FIG. 2 shows the
case, for comparison, of an ordinary Fe-Al-Zn alloy layer
formed at the base iron-plating boundary as observed by a
similar method.
Next, FIG. 3 shows the distribution of elements (Ni,
Al, Zn, and Fe) in the plating depth direction after
heating and alloying in the present invention. The Fe-Ni-
Al-Zn alloy layer of the base iron-plating boundary as
seen in FIG. 1 disappears and a Zn-Fe alloy layer in
which Ni and A1 are diffused is formed. Further, FIG. 4
shows, by comparison, the distribution of elements (Ni,
Al, Zn, and Fe) in the plating depth direction of a sheet
having an alloy layer in the ordinary state of FIG. 2
after heating and alloying.
5 In the present invention, the state of FIG. 1 is
formed in a hot dip galvanization bath, then the state of
FIG. 3 is changed by heating and alloying. The reason why
going through these steps gives a better appearance
compared with the usual steps (that is, the steps from
10 FIG. 2 to FIG. 4) is not necessarily clear, but is
believed to be because of the following reason. That is,
the step of forming the boundary alloy layer of FIG. 1 is
believed to involve a precipitation reaction of Ni, Al,
Zn, and Fe in the bath, but since Ni is included, the Ni
15 acts as the nucleus for crystallization. Even if there is
some unevenness in the base sheet material, it is
believed that this is concealed as an effect. Further,
with the Fe-Ni-Al-Zn alloy layer, it is believed the
barrier action on the Zn-Fe alloying reaction is less
2 0 dependent on the thickness of the alloy layer compared
with a Fe-Al-Zn alloy layer and therefore unevenness of
the thickness of the alloy layer does not easily become
unevenness after alloying.
Next, a method of production of hot dip galvannealed
steel sheet from the state of FIG. 1 to FIG. 3 of the
present invention explained above will be explained in
greater detail. The A1 of the base iron-plating boundary
alloy layer of the present invention is supplied from the
hot dip galvanization bath. Further, Ni can also be
supplied from the hot dip galvanization bath, but in this
case, a large amount of Ni has to be included in the bath
and a large amount of Ni-A1 dross is formed, so this is
not preferred. To avoid this problem, the Ni is
preferably supplied by preplating the steel sheet.
Below, a specific method in the case of applying Ni
preplating will be explained.
In the present invention, first, the surface has to
be cleaned, but the method is not particularly limited.
Alkali degreasing, brushing, acid treatment, or another
known method may be performed alone or in combination in
accordance with the state of dirt or oxide film on the
5 sheet material. From the viewpoint of the uniformity of
the Ni plating explained later, alkali degreasing (for
example, NaOH aqueous solution treatment) and acid
treatment (for example, sulfuric acid aqueous solution
treatment) are preferably used in combination in that
10 order.
In the present invention, the sheet is preplated by
0.05 to 1.0 g/m2 of Ni. If less than the lower limit, the
wettability of this later hot dip coating is
insufficient, while if over the upper limit, a boundary
alloy layer as shown in FIG. 1 becomes difficult to form
in the Zn bath and as a result a good appearance is hard
to obtain.
After the Ni preplating, the sheet is rapidly heated
in a nonoxidizing or reducing atmosphere to a sheet
temperature of 430 to 500°C by a 30°C/sec or higher rate
of temperature rise. This treatment is required for
securing the wettability of the hot dip coating and
further the plating adhesion.
The hot dip galvanization bath used is a bath
comprised of Al: 0.07 to 0.2%, unavoidable impurities,
and the balance of Zn. This is because if the A1 is less
than the lower limit, a boundary alloy layer as shown in
FIG. 1 becomes difficult to form and as a result a good
appearance is hard to obtain.
Note that formation of the boundary alloy layer as
shown in FIG. 1 depends on the amount of preplating of Ni
and the concentration of A1 in the bath. The inventors
used ultra-low carbon steel sheets, changed the amounts
of Ni preplating in various ways, rapidly heated the
sheets to 460°C by a 50°C/sec rate of temperature rise,
dipped the sheets in 455OC hot dip galvanization baths
containing various concentrations of Al, took them out
after 3 seconds, and rapidly cooled them so as to
investigate if there were Fe-Ni-Al-Zn alloy layers at the
base iron-plating boundaries. The results are shown in
FIG. 5. In the figure, the "0" marks show samples where
the Fe-Ni-Al-Zn alloy layer was confirmed. The trend was
observed of the upper limit of the amount of Ni
preplating dropping as the bath A1 dropped. The region
below the broken line in the figure (when assuming the
amount of Ni preplating to be yg/m2 and the A1
concentration in the galvanization bath to be [XI%, the
relationship Y=15x[X]-1 stands) is the suitable region in
the present invention.
In the present invention, after plating and wiping,
the sheet is preferably rapidly heated to 470 to 600°C by
a 30°C/sec or more rate of temperature rise, then cooled
with any soaking time or is soaked and held for less than
15 seconds, then cooled for alloying. This provision is
important for obtaining a good appearance and securing a
suitable alloying degree and plating adhesion.
EXAMPLES
Below, examples will be used to explain the present
invention in detail.
(Examples 1 to 13 and Comparative Examples 1 to 11)
Table 1 shows the ingredients of annealed ultra-low
carbon steel sheets used for the tests. These were
pretreated by the conditions shown in Table 2, then
preplated by Ni in plating baths shown in Table 3 by
electroplating (bath temperature 60°C, current density
30~/dm') .
After this, the sheets were heated in a 3%H2+N2
atmosphere by a 50°C/sec rate of temperature rise to
450°Cl then immediately dipped in a hot dip galvanization
bath warmed to 450°C and held for 3 seconds, then wiped
and adjusted in basis weight, and alloyed right above the
wiping by a predetermined rate of temperature rise,
16
temperature, and soaking time. The sheets were cooled by
gradual cooling of Z°C/sec over 10 seconds, then were
rapidly cooled by 20°C/sec. After this, the sheets were
temper rolled by a reduction rate of 0.5%.
Samples were produced by the various types of
conditions (amount of preplating of Ni, A1 concentration
of plating bath, and alloying conditions). Note that the
basis weight was 50 g/m2 in each case.
The inventors measured the compositions and r layer
thicknesses of the plating layers of the samples of Table
4. The results are shown in Table 5. Each plating layer
was dissolved in hydrochloric acid to find the
concentrations of the different ingredients. Further, the
r layer was measured at 10 points by the electrolytic
peeling method to find the average value, maximum value,
and minimum value. Regarding the variation of the r
layer, samples with either the maximum value-average
value or average value-minimum value over 0.3 pm were
indicated as "Poor".
Table 6 shows the results of evaluation of the
performance. The performance was evaluated as follows:
(1) Plating appearance: Visual observation with
samples with no nonplating or other defects evaluated as
"Good", some as "Fair", and remarkable amounts as "Poor".
(2) Workability (anti-powdering property): A sample
coated with rustproofing oil was pressed (drawn) by a 40
mm+ cylinder press under conditions of a draw ratio of 2.2
and was evaluated for the degree of blackening by tape
peeling at its side surface. Samples with a degree of
blackening of 0 to less 20% were evaluated as "Good", 20
to less than 30% as "Fair", and 30% or more as "Poor".
(3) Workability (slidability): A sample coated with
rustproofing oil was subjected to a flat plate continuous
sliding test. It was slid by a compressive load of 500
kgf consecutively five times and the frictional
coefficient at the fifth time was evaluated. Samples with
a frictional coefficient of less than 0.15 were evaluated
as "Good1', 0.15 to less than 0.2 as "Fair", and 0.2 or .
more as "Poor".
(4) Corrosion resistance (rust resistance at
5 scratched parts of coating): A sample of a steel sheet
was chemically converted by the trication process for
automobiles*', cationically electrodeposition coatedt2 (20
p), then the coating was peeled off in a 5 mm x 50 mm
slit shape to expose the plating surface and a corrosion
10 cycle test*3 was conducted. The resistance was evaluated
by the appearance after 10 days. Samples with no rust or
only yellow rust were evaluated as "Good", with less than
20% of red rust as "Fair", and with 20% or more of red
rust as "Poor".
(5) Corrosion resistance (pitting resistance) : A
sample pressed to a U-shape with a bead was flattened,
then, while masking 40 mm x 40 mm, was chemically
converted by the trication process for aut~mobiles*~a,n d
was cationically electrodeposition ~oated*(~20 p ). A
bent plate and flat plate were joined by 0.5 mm spacers
so that the uncoated part from which the mask was removed
became the inside so as to create a chassis hem model.
This sample was then subjected to a corrosion cycle
testt3. The resistance was evaluated by the appearance
after 30 days. Samples with less than 20% red rust were
evaluated as "Good", with 20 to less than 50% red rust as
"Fair", and with 50% or more red rust as "Poor".
( 6 ) Coatability: A sample of a steel sheet was
chemically converted by the trication process for
automobilesa1 and was cationically electrodeposition
~oated*~T.he electrodeposition coating was performed
under conditions of a voltage of 220V, an upslope of 0.5
minutes, and a total conduction time of 3 minutes. The
number of craters and other abnormalities in the test
piece 70 x 150 mm) were counted. Samples with no
abnormalities were evaluated as "Good", one to less than
three as "Fair", and three or more as "Poor".
/S
*1: SD5000 made by Nippon P a i n t ,
*2: PN120M made by Nippon P a i n t ,
*3: SST (6 h) => dry 50°C 45% RH (3 h) => w e t 50°C
95% RH ( 1 4 h) => dry 50°C 45% RH (1 h)
Table 1. Types of T e s t S t e e l
Table 2. Pretreatment Conditions
Steel type 1
Steel type 2
Table 3. Pre-Ni P l a t i n g Solution
Ingredients (mass%)
Alkali degreasing
Pickling
Ingredients Concentration
iS04 - 6H20 300 g/l
40 g/l
100 g/l
NaOH 50 g/l
Solution temperature 6 S°C
Dipping 10 sec
100 g/l
Solution temperature 3 O°C
Dipping 5 sec
C
0.0016
0.0020
Ti
0.015 -
Mn
0.170
0.381
Nb
0.018
0.003
S i
0.011
0.003
P
0.012
0.059
S
0.006
0.006
- M -
Table 4. Sample Production Conditions
Table 5. Composition of Plating Layer and l- Layer
Thickness of Test Samples
-%-
Table 6. Results of Evaluation of Performance
In the above way, sheets in the scope of the present
invention exhibited superior properties.
(Examples 14 to 22 and Comparative Examples 12 and
13)
Table 7 shows the ingredients of annealed ultra-low
carbon steel sheets used for the tests. The sheets were
pretreated by the conditions shown in Table 2, then were
preplated by Ni in the plating bath shown in Table 3 by
electroplating (bath temperature 60°C, current density
30~/dm~. )
After this, the sheets were heated in a 4%H2+N2
atmosphere by a 50°C/sec rate of temperature rise to
455OC, then immediately dipped in a hot dip galvanization
bath warmed to 450°C, held there for 2.5 seconds, then
wiped to adjust the basis weight, raised in temperature
by 50°C/sec right above the wiping, held for 4 seconds,
and rapidly cooled by 50°C/sec. After this, the sheets
were temper rolled at a reduction rate of 0.5%.
(Comparative Example 14)
The ingredients of the annealed ultra-low carbon
steel sheets used for the test are shown in Table 7. The
sheets were pretreated by the conditions shown in Table
2, then were heated in a 4%H2+N2 atmosphere by a 20°C/sec
rate of temperature rise to 650°C, held at 60 seconds,
gradually cooled to 455OC, then dipped in a galvanization
bath warmed to 450°C, held there for 2.5 seconds, then
wiped to adjust the basis weight, raised in temperature
by 50°C/sec right above the wiping, held for 4 seconds,
and rapidly cooled by 50°C/sec. After this, the sheets
were temper rolled at a reduction rate of 0.5%.
Samples were produced by the various types of
conditions shown in Table 8 (amount of preplating of Ni,
A1 concentration of plating bath, alloying conditions).
Note that the basis weighf was 50 g/m2 in each case.
The compositions of the plating layers of the
samples of Table 8 and the results of measurement of the
r layer thickness are shown in Table 9. Each plating
layer was dissolved in hydrochloric acid to find the
concentrations of the different ingredients. Further, the
r layer was measured at 10 points by the electrolytic
peeling method to find the average value, maximum value,
and minimum value. Regarding the variation of the r
layer, samples with either the maximum value-average
value or average value-minimum value over 0.3 pm were
indicated as "Poor".
Table 10 shows the results of evaluation of the
performance. The performance was evaluated in the same
way as above. However, the workability (anti-powdering
property) was evaluated under tougher conditions (draw
ratio 2.3). The evaluation criteria etc. were the same as
above. Further, in addition to the evaluation of the
above examples, the low temperature chipping property was
added. The low temperature chipping property was
5 evaluated in the following way. The method of the abovementioned
evaluation item (6) was used up to the
electrodeposition coating, a polyester-based midcoat was
coated to 30 p and a topcoat was coated to 40 p, then
the sample was allowed to stand for one day (size 70 mm x
10 150 mm). The coated sample was cooled by dry ice to -20°C,
then approximately 0.4 g pebbles (10) were dropped on it
vertically by an air pressure of 2 kgf/cm2. The coating
raised up by the chipping was removed, then the maximum
value of the peeling diameters was measured. Samples with
15 a peeling diameter of less than 4 mm were evaluated as
"Good", 4 mm to less than 6 mm as "Fair", and 6 mm or
more as "Poor".
Table 7. Types of Test Steel
- w-
Table 8. Sample Production Conditions
Table 9. Composition of Plating Layer and r Layer
Thickness of Test Samples
2 1
22
12
13
14
7
7
3
4
4
0.3
0.6
1.1
1.1
0
0.16
0.18
0.1
0.11
0.18
50
50
50
50
50
550
570
480
470
580
4
4
4
4
4
Inv. ex.
Inv. ex.
Comp. Ex.
Comp. Ex.
Comp. Ex.
- FTable
10. Results of Evaluation of Performance
In the above way, sheets in the scope of the present
invention exhibited superior properties.
5 Next, examples for obtaining an extremely good GA
appearance will be explained.
(Examples 19 to 25 and Comparative Examples 15 to
17
The cold rolled annealed sheet materials shown in
10 Table 1 were pretreated as shown in Table 2, then
preplated by Ni in the plating bath shown in Table 3 by
electroplating (bath temperature 60°C, current density
30~/dm'). After this, the sheets were heated in a 3%H2+N2
atmosphere by a 50°C/sec rate of temperature rise to
15 460°C, then immediately dipped in a hot dip galvanization
bath warmed to 455OC and held for 3 seconds, then wiped
and adjusted in basis weight. The basis weight was 60
g/m2. After this, the sheets were heated and alloyed under
predetermined conditions. After the heating, the sheets
2 0 were cooled by gradual cooling of 2OC/sec over 10 seconds,
then were rapidly cooled by 20°C/sec. After this, the
sheets were temper rolled by a reduction rate of 0.5%.
Note that the samples for observation of the boundary
alloy layer were ones dipped in the hot dip galvanization
bath, held there for 3 seconds, then rapidly cooled.
(Comparative Example 18)
A cold rolled unannealed sheet material of the same
ingredients and same sheet thickness as the sheet
material 1 of Table 1 was used as the sheet material, was
just alkali degreased among the pretreatments shown in
Table 2, then was annealed and reduced in a 10% hydrogen
atmosphere at 800°C for 30 seconds, then cooled to 460°C,
then dipped in a hot dip galvanization bath warmed to
455OC and held for 3 seconds, then wiped and adjusted in
basis weight. The basis weight was 60 g/m2. After this,
the sheet was heated and alloyed under predetermined
conditions. After the heating, the sheet was cooled by
gradual cooling of 2OC/sec over 10 seconds, then was
rapidly cooled by 20°C/sec. After this, the sheet was
temper rolled by a reduction rate of 0.5%. Note that the
sample for observation of the boundary alloy layer was
one dipped in the hot dip galvanization bath, held there
for 3 seconds, then rapidly cooled.
In each of Examples 19 to 25 and Comparative
Examples 15 to 18, as shown in Table 11, the hot dip
galvanization bath concentration and Ni preplating amount
were adjusted.
The performance was evaluated as follows:
1) Hot dip galvanized base iron-plating boundary
alloy layer: A cross-section of the sample was polished
embedded and analyzed by EPMA to investigate the state of
the alloy layer. Samples with an Fe-Ni-Al-Zn alloy layer
were evaluated as "Good" and others as "Poor".
2) Plating appearance (visual): The sample was
irradiated with fluorescent light at a slant and the
presence of any small plating unevenness was examined.
Samples with no unevenness were evaluated as "Good".
3) Plating appearance (SEM observation): Samples
were observed under 500X power for 20 fields and the
ratio of area of the parts crushed and smoothed by the
temper rolling was found. Samples with a larger of the
difference between the average value and maximum value of
the area ratios or the difference of the average value
5 and minimum value of less than 10% was evaluated as
"Good", 10% to less than 20% as "Fair", and over 20% as
"Poor".
4) Alloying degree: The plating layer was dissolved
in hydrochloric acid and chemically analyzed to find the
10 amounts of ingredients and calculate the Fe% in the
plating layer. Samples with an Fe of %9 to 12% were
> evaluated as "Good1' and others as "Poor".
5) Plating adhesion: A sample coated with
rustproofing oil was pressed (drawn) by a 40 cylinder
15 press under conditions of a draw ratio of 2.2 and was
evaluated for degree of blackening by tape peeling at its
side surface. Samples.with a degree of blackening of 0 to
less 20% were evaluated as "Good", 20 to less than 30% as
"Fair", and 30% or more as "Poor".
2 0 Table 11. Sample Production Conditions and Interface
Alloy Layer
Fe-Ni-A1 -Zn
2 5
15
16
17
18
* In Comparative Example 15, remarkable nonplating
occurred, so the boundary alloy layer was difficult to
identify. For this reason, the performance was not
evaluated after GA.
2
1
1
1
0.2
0.01,
1.5
0.5
1 (not yet
annealed)
0.15
0.15
0.15
0.05
0.11
Good
.h
Poor
Poor
Poor
530 -
500
500
500
5 -
0
0
Inv. ex.
Comp. Ex.
Comp. Ex.
Comp. Ex.
0 Comp. Ex.
- 2tf-
Table 12. Results of Evaluation of Performance
18 1 Poor Poor Good Good I Comp. Ex.
As shown in Table 12, samples in the range of the
present invention give superior characteristics.
INDUSTRIAL APPLICABILITY
According to the present invention, a hot dip
galvannealed steel sheet having excellent corrosion
resistance, workability, and coatability using an ultralow
carbon steel sheet mainly used for automobiles as a
sheet material is obtain. The value of utilization in
industry is tremendous. Further, according to the present
invention, a method of production of hot dip galvannealed
steel sheet extremely excellent in appearance able to be
used for automobile body panels is obtained.
We claim:
1. Hot dip galvannealed steel sheet excellent in corrosion resistance, workability, and
coatability cornpriscd of an ultra-low carbon steel sheet having on at lcast onc surfacc a
plating laycr cornpriscd of, by mas?, l:c: 8 to 13", Ni: 0.05 to 1.0"", A I: 0.15 to 1.5"". and a
balance of Zn and unavoidable impurities, having a ratio of AINi of 0.5 to 5.0, having an
average thickness of a 'I' layer of the base iron boundary of 1 pm of Icss, and a having a
variation of the same of t0.3 pm or less.
2. A method of production of a hot dip galvannealed steel sheet comprising clcaning of
surfacc of a stccl shcct. prcplating it by 0.05 to 1.0 g/m%f Ni. rapidly hcating thi5 in a
nonoxidizing or reducing atmosphere to a sheet temperature of 430 to 500°C by a 3O0C/scc or
more rate of temperature rise, hot dip coating the sheet in galvanization bath containing Al in
a concentration of 0.07 to 0.2", wiping it, then immediately rapid hcating to 470 to 600°C by
a 30°C/sec or more rate of temperature rise, cooling it without any soaking time or 5oak1ng
and holding it for less than 15 seconds, then cooling it, wherein the amount of Ni prcplating
(Y dm2) and the concentration of Al in the galvanization bath (1x1 mass") satisfies the
relationship of YI15x[X1-1.