Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2116984
ALLOYED HOT DIP GALVANIZED STEEL SHEET
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to an alloyed hot dip
galvanized steel sheet which is used for external vehicle
body plates having excellent pressing workability and
plating separation resistance.
Descri~tion of Related Art
Recently, increased anticorrosive properties for
vehicle bodies have been demanded. Industry has
therefore been trying to develop adequate galvanized
surface treatment steel sheets to meet these demands.
One development has been hot dip galvanized steel sheets,
which are economically excellent. Further, it has become
possible to improve weldability and corrosion resistance
after coating steel sheets by composing a plating layer
of Fe-Zn (alloyed hot dip galvanized steel plate) and
sub~ecting it to heat treatment.
Higher ductility (elongation El) and higher
drawabil~ty (Lankford value, r value) have also been
demanded because alloyed hot dip galvanized steel sheets
used for external vehicle body sheets (hereinafter
sometimes referred to as "GA") are subjected to high-
level press processing to improve their design
characteristics... To solve problems associated with
achieving high ductility and high drawability, a variety
of methods for reducing the content of C, N, P, S and the
like contained in steels from the viewpoint of the
property of the steel sheet as a material have been
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developed. Optimum hot rolling and cold rolling
processes have also been developed.
In conjunction with the demands for the steel sheet,
the characteristics demanded for the plating layer of GA
steel sheets are that the plating layer must not exhibit
~ powdering~ in which the structure thereof is powdered
and iseparated because it cannot follow the deformation of
the steel sheet during processing, and 2) "flaking" in
which the structure is separated in the form of scales
when it is ironed by means of pressing dies. If such
phenomena occur, separated plated particles are collected
in the pressing dies, thereby causing highly undesirable
indentations on the surface of steel sheets. Also, the
corrosion resistance of the plating itself may be lost.
Generally, the plating layer of GA steel sheet is
said to be composed of three Zn-Fe alloy phases ~, ~I r r
from the viewpoint of low percentage content of Fe. The
reason for powdering is believed to originate from the r
phase and the reason for flaking is believed to originate
from the ~ phase. If GA steel sheet is produced by hot-
dip galvanizing the C, N, P, S reduced material, the
ductility and r value are satisfied. However, if such a
treatment is performed, alloying in the grain boundary of
the s~eel sheet is excessively accelerated and increases
the amount of r phase formation, thereby reducing
powdering resistance. To secure a satisfactory powderlng
resistance, it is necessary to restrict the degree of
alloying to the level (concentration of Fe) in which
2116984
substantially no r phase is generated. However, in this
case, as disclosed in Japanese Patent Laid-Open No.2-
11745, if only the percentage content of Fe is restricted
to cause no r phase, the ~ phase may form in a thick
layer on the surface of the plating layer depending on
production conditions, so that flaking becomes likely to
occur when the layer is strongly flattened during
pressing.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present
invention to obtain an alloyed hot dip galvanized steel
sheet in which the powdering resistance and flaking
resistance can be satisfied at the same time by using a :
steel sheet obtained by reducing the content of C, N, P,
S contained in the steel plate.
According to the present invention, there is
provided an alloyed hot dip galvanized steel sheet having
excellent press workability and plating separation
resistance, wherein an alloyed hot dip galvanized layer
which contains about 9 weight % or more and about 12
weight % or less of Fe, about 0.3 weight % or more and : ::
about 1.5 weight % or less of Al, about 0.1 weight % or
less of Pb is formed on the surface of a steel sheet :
which contains about 0.0015 weight % or less of C, about
0.1 weight % or less of Si, about 0.03 weight % or more
and about 0.3 weight % or less of Mn, about 0.01 weight % : ;
or more and about 0.1 weight % or less of Al, about 0.01
weight % or less of P, about 0.005 weight % or less of S,
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about 0.005 weight % or less of O, about 0.005 weight %
or less of N and further contains at least one of about
0.03 weight 'i~ or less of Ti or about 0.03 weight % or
less of Nb in the range in which C/12<Ti*/48 + Nb/93~C/2,
the layer being in the thickness of about 25g/m2or more
and about 70g/m2or less.
In this case, it is assumed that T* is Ti - (48N/14
+ 48S/32) when Ti - (48N/14 + 48S/32)20 and Ti* is 0 when
Ti - (48N/14 + 48S/32)~0. Additionally, the steel sheet
having the aforementioned composition may contain about
0.001 weight % or less of B.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic, explanatory drawing of a bead
type pulling testing apparatus.
pETAILED DESCRIPTION OF THE INVENTION
The alloyed hot dip galvanized steel sheet which has
excellent press workability and plating separation
resistance which is an object of the present invention
will be described below. However, it will be appreciated
that the following description is intended to refer to
specific embodiments of the invention selected for
illustration in the drawing and is not intended to define
or limit the invention, other than in the appended
claims.
First, the components contained in steel sheet which
is to be used as the material for plating are determined ~
as follows in order to satisfy necessary properties and
achieve efficient economic production.
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C: C is an element which directly determines the
strength of steel. To obtain an extremely high
workability (high El, r value) which is an object of the
present invention, a smaller content thereof is better.
The content thereof should be about 0.0015 weight % or
less.
N, P and S: N, P and S are in the structure of
steel to reduce the El and r values. As in the case of
C, a smaller content of these elements is better. It is
considered that N, P and S must be about 0.005 weight %
or less, about 0.01 weight ~ or less, and about 0.005
weight % or less, respectively.
O: O is precipitated as an oxide if it is
excessively contained in steel, thereby reducing the El
and r values. Therefore, the content thereof must be
about 0.005 weight % maximum.
Mn: If Mn is added into steel, it is combined with
S and then precipitated to become non-effective. Thus,
when a small amount of Mn is added, there is no
remarkable effect from the viewpoint of the material.
However, if the content thereof exceeds about 0.3 weight
%, the El and r values decrease gradually. Thus, the
content of Mn must be about 0.03 weight ~ or more and
about 0.3 weight % or less.
Si: As in the case of Mn, a large amount of Si
contain0d in steel reduces the El and r values and blocks
the wettability of plating. Thus, the amount of Si is
about 0.1 weight % maximum.
r~ :. ::: t.,. - : . '' ' .' ' . " '' '
2~1 ~98~
Ti and Nb: Ti and Nb are combined with C and
precipitated in the form of TiC and NbC, thereby
improving workability. Thus, the atomic ratio of Ti and
Nb with respect to C needs to be more than about 1.
However, because excessive addition of the elements
increases cost, the maximum atomic ratio is about 6.
Additionally, it is desirable that the maximum amount of
each component is about 0.03 weight ~. However, because
Ti is more likely to be combined with N or S than C, it
is necessary to determine the amount of Ti so that it is
minus N and S equivalents. Specifically, it is assumed
that the amounts of Ti and Nb satisfy the expression
described below:
C/12~Ti*/48 + Nb/93<C/2
(where Ti* is Ti - (48N/14 + 48S/32) when Ti -
(48N/14 + 48S/32)~0 and Ti* is 0 when (Ti - 48N/14 +
48S/32)<0)
Al: The amount of Al needs to be about 0.01 weight
% or more to prevent Ti and Nb from being oxidized and
lost when Ti and Nb are added. Al is combined with N and
S contained in steel, thereby eliminating the effects
thereof. However, if the amount of added Al exceeds
about 0.1 weight %, the effect is saturated, so that
excessive addition of Al is economically meaningless.
B: In the steel sheet according to the present
invention, besides the basic composition described above,
it is preferable that the amount of B is less than about
0.001 weight %. The reason is that B is effective in
2116984
strengthening the grain boundary and improving spot
weldability and cold working embrittlement. However, if
the amount of added B exceeds 0.001 weight %, the
drawability is lost. Thus, the maximum amount is about
0.001 weight %.
Next, the reasons for the components of plating
layer will be described below.
The alloyed hot dip galvanized steel sheet is
produced by immersing a steel sheet in a molten zinc bath
and then heating the steel sheet to diffuse Fe contained
in the steel sheet into plated layers, thereby forming a
Zn-Fe alloy layer. Consequently, the corrosion
resistance, phosphatability and spot weldability thereof
are markedly better than ordinary galvanized steel
plates. These functions are preferably achieved by
adjusting the content of Fe to about 9 weight % or more.
Additionally, the amou~t of Fe needs to be about 9 weight
% or more to prevent a ~ phase layer from growing. On
the other hand, if the content of Fe exceeds about 12
weight %, a hard, brittle r phase layer is developed even
if the content of Al in the plating layer is controlled
in a range described later, thereby blocking press
workability. Thus, the content of Fe contained in the
plating layer must be about 9-12 weight %.
~he content of Al contained in the plating layer
affects the phase composition of the Zn-Fe alloy which is
formed at the time of alloying. If the amount of Al is
less than about 0.3 weight %, the r phase layer is
2116984
developed so that undesirable powdering becomes likely.
If the amount of Al exceeds about 1.5 weight %,
insufficient alloying is achieved. ~hus, the amount of
Al contained in the plating layer is about 0.3 weight % -
about 1.5 weight ~.
Pb contained in the plating layer is restricted to
about 0.2 weight % or less because it badly affects the
corrosion resistance of the plating layer.
The amount of the plating layer applied needs to be
about 25g/m2 from the viewpoint of corrosion resistance.
However, if the plating layer is too thick, the layer
cannot follow the deformation of the steel sheet when
pressing is performed, thereby resulting in powdering.
Thus, the maximum amount of plating applied to the steel
plate i8 determined to be about 70g/cm2.
Although the steel sheet production method according
to the present invention is not particularly restricted
to a specific method, a preferred production example will
be described below.
Molten steel which is adjusted to the aforementioned
composition i8 processed into a slab by means of a
continuous casting method. The slab is then processed
into cold finished steel plates through hot rolling and
cold rolling. In hot rolling, it is desirable that the
finishing temperature is about 850C - 920C which is
near the Ar3 transformation point to obtain high
processing properties. It is desirable that the winding
temperature is about 600C or more. Further, in the cold
2116984
rolling step, it is desirable that the rolling reduction
is about 50% or more.
In hot dip galvanizing, the surface of a steel sheet
is purified before annealing reduction is performed.
Degreasing, pickling or burning are permissible methods.
The steel sheet is then subjected to annealing reduction.
It is appropriate to use an H2 atmosphere containing
between several % and several tens ~ of N2. It is also
desirable that the dew point be 0C or less. Although
the annealing reduction temperature needs to be higher
than the recrystallization temperature to secure a
preferred material, it is desirable that the annealing ~,
reduction temperature is about 780C or more.
After annealing reduction is performed, the steel
sheet is cooled in reducing gas and introduced to a hot
dip galvanizing bath. The components and the temperature
of the bath are determined as follows.
Concentration of Al in the bath: One purpose of the
present invention is to secure powdering resistance and
flaking resistance by controlling the amount of the Al-Fe
alloy layer generated in the galvanizing bath to achieve
alloying of mainly ~1 phase. The amount of the Al-Fe
alloy layer should be ad~usted so that the amount of Al
contained in the alloy is 0.15g/m2 or more for this
purpose. Thus, the amount of Al in the bath needs to be
about 0.13 weight % or more. To form the Al-Fe alloy
layer effectively, it is desirable that the amount of Al
is about 0.145 weight % or more. On the other hand, if
---, .. ' "' :' ', ' ~ ' ."' '' '' ' .' . `'` :
211698~
the amount of the Al-Fe layer is increased so that the
amount of Al exceeds about O.Sg/m2, alloying is
excessively restricted, so that productivity may be
blocked. Namely, in the plating layer after alloying is
S performed, it is desirable that the amount of Al
including Al contained in layers other than the Al-Fe
layer is about 1.5 weight % at most. Thus, the maximum
concentration of Al contained in the bath is about 0.2
weight %.
Concentration of Pb in the bath: Unlike Al, Pb in
the bath is not concentrated on the plating during hot
dipping. However, if the concentration of Pb in the ~ ~
plating layer exceeds about 0.1 weight %, corrosion ~ r '
resistance may drop. Thus, the upper limit of the
lS concentration of Pb in the bath is about 0.1 weight %.
The steel sheet of the present invention can be used
for various applications including automobiles, household
electric appliances, construction materials, and the like
in bare condition and/or in a condition which undergoes
pre-coating, po~t-coating, laminating, chromate ~;
treatment, phosphate treatment or the like. Moreover, if
the top layer of the alloyed hot dip galvanized plating
layer is further coated with a plating layer containing
at least one of Fe, Zn and Ni, the corrosion resistance
is further improved.
After the steel sheet is immersed in the plating
bath, it is subjected to alloying processing to obtain a
GA steel sheet in which the degree of alloying (Fe) is
2~ 16984
9 - 12~.
In the process described above, alloyed hot dip
galvanized steel sheet which has excellent press
processing and plating separation resistance can be
obtained.
EXAMPLES:
Advantages of the present invention will be
described with reference to examples thereof. Using a
vertical type hot dip galvanizing experimental apparatus
as a plating apparatus, a steel of 70mm x 200 mm was ;
plated in an atmosphere containing an-annealing reduction
gas of 5%-hydrogen containing nitrogen. A heating oven
which controlled the amount of heat generated by
resistance by directly feeding power to a plated steel
sheet was used for the alloying treatment of the plating.
A specimen of steel sheet wa~ softened by means of a
vacuum melting furnace, and hot rolled and cold rolled to
adjust the thickness of the sheet to 0.7 mm. The sheet
wa~ sub~ected to electrolytic degreasing and pickling
with hydrochloric acid before it was inserted into a
plating apparatus. The hot rolling finish temperature
was 900C. After temporary cooling, the sheet was
equally heated at 700C for an hour according to the heat
history obtained after it was coiled. The sheet was then
cold rolled under a rolling reduction of 75~ after it was
cooled and pickled with acid.
Table 1 shows the components of the specimen steel
sheet, the condition for plating and the composition of a
..... . ... . . . . . .... . . . . ...
21~698~
plated layer provided before alloying treatment. Table 2
shows the characteristics of the plated steel sheet after
the alloying treatment was performed. The material of
the steel sheet was obtained by heat treatment according
to CGL(Continuous Galvanizing Line) in an alloyed hot dip
galvanizing cycle after which cold rolling was performed.
The steel sheet was then annealed at 850C for 20 seconds
and cooled at 500C for 30 seconds. Table 1 shows the
components of the steel sheet as well.
The measurement of the amount of Al-Fe shown in
Table 2 was performed by immersing a plated steel sheet
before it was subjected to alloying processing in fuming
nitric acid to remove zinc (~) phase, solving the Al-Fe
alloy layer which was left unsolved in the passive state
in hydrochloric acid and then measuring the amount of Al
according to the atomic absorption method.
The elongation percentage tEl) and r values of the
steel sheet were obtained by tensile testing to evaluate
the characteristics of the plated steel sheet. Powdering
resistance and flaking resistance were obtained to
investigate the characteristics of the plated layer.
..
Powdering resistance was evaluated according to a five-
step evaluation system by bending a plated steel sheet
which had been sub~ected to alloying processing at 90
degrees, restoring it, and collecting separated plating
particles using a preliminarily attached cellophane tape
to measure the amount thereof. "1" indicated acceptable
and "5" indicated unacceptable in the test.
2116984
Flaking resistance was measured by means of the bead
type drawing test apparatus shown in Fig. 1 by using 10
mm wide cut pieces of the steel sheet which had undergone
alloying processing. In the bead type drawing test
apparatus, a test piece 2 was drawn through a bent path
between an indented member 1 and a protruded member 3.
The test piece coated with no oil was drawn under the
condition in which the pressing load was lOOkgf and that
the drawing velocity was 500mm/min. Separated plated
particles were collected using a cellophane tape to
visually recognize whether flaking had occurred according
to a two-level (yes/no) evaluation system.
As is evident from Tables 1 and 2, the present
invention has succeeded in realizing production of
alloyed hot dip galvanized steel sheet which has high
workability and excellent platiny separation resistance.
Consequently, according to the present invention, it is
possible to produce an alloyed hot dip galvanized steel
8heet which has high workabLlity and excellent plating
separation resistance.
13
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211698~
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