Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROI~ND OF THE INVENTION
Field of the Invention
The present invention relates to a galvannealed steel
l sheet, suitable for producing body parts of automobiles.
05 The invention is also concerned with a method of producing
-1 it.
Description of the Related Art
Galvannealed Steel sheets exhibit superior corrosion
; resistance and, hence, are broadly used as the material of
automobile body parts. Materials of automobile body parts
are required to have corrosion resistance property as well
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Il as other characteristics such as press-workability,
resistance to peeling of plating layer during press work,
and spot-weldability~
In general, a continuous hot dip galvannealing
process does not allowed a lengthy time period for heating
and soaking. Therefore, in the production of plates steel
sheets for automobile body parts which are required to
have high press-workability, steel sheets having very low
carbon content, which generally exhibit excellent press-
workability with short heating and annealing,are used as
~ the base materials, as d/isclosed, for example, in Japanese
't~ Patent Publicat.ion No. 60-48571.
;~ However, galvannealed steel sheets having very low
carbon content, exhibit inferior spot-weldability
characteristics for reasons which will be explained later.
The result is that the efficiency of the automobile body
assembly process is seriously impaired.
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In order to obviate these problems, it has been
proposed to increase the Fe content in the plating layer
or to coat the surface of the plating layer with a ferrous
alloy. The first-mentioned method, however, is
05 disadvantageous in that exfoliation or peeling of the
plating layer tends to occur when the Fe content is
increased to a level which provides the desired level of
spot-weldability. On the other hand, the second-mentioned
method causes the production cost to be raised seriously
and reduces corrosion resistance after painting.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to
provide a galvannealed steel sheet which employs, as the
base sheet material, a steel sheet having very low carbon
content which exhibits superior press-workability and
which exhibits improved resistance to exfoliation or
peeling of the plating layer during press-work, as well as
superior spot-weldability in terms of spot welding at
successive spots, thereby overcoming the above-described
problems of the prior art.
To this end, according to one aspect of the present
invention, there is provided a galvannealed steel sheet
having superior spot-weldability, comprising a cold rolled
base steel sheet having a composition consisting
essentially of 0.005 wt% or less of~ C, 0.005 to 0.05 wt%
of Ti, 0.01 to 0.1 wt% of Ae~ 0.005 to 0.015 wt% of Nb and
0.0002 to 0.002 wt~ of B, and a hot-dip galvannealed layer
containing 9 to 12 wt% of Fe.
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According to another aspect of the present invention,
there is provided a method of producing a galval-nealed
steel sheet having superior spot-weldability, comprising
the steps of: producing a cold rolled steel sheet
05 containing 0.005 wt% or less of C, 0.005 to 0.05 wt~ of
Ti, 0.01 to 0.1 wt% of Ae~ 0.005 to 0.015 wt% of Nb and
0.0002 to 0.002 wt% of B, annealing said base steel sheet
at a temperature ranging between 770 and gOOoc; rapidly
cooling the annealed sheet to a temperature ranging
between 380OC and 530OC at a cooling rate of lOoc/se~ or ` - ;
greater; dipping said base steel sheet in a hot melt of
plating zinc having an Ae content of 0.13 wt% or greater
so as to form a plating layer; and effecting an alloying
heat-treatment on said plating layer to obtain an Fe
content ranging between 9 and 12 wt% in said plating
layer.
The present inventors have found that the inferior
spot-weldability in terms of welding at successive welding
spots exhibited by galvannealed steel sheet is
attributable to the following facts. ~teel having very
small carbon content is drastically softened by heating as
compared with ordinary low-carbon steels. Therefore, the
area of contact between the electrode and the plate
surface i5 increased when spot welding is conducted and,
in addition,the reaction between the electrode and zinc is
promoted to deteriorate the state of end of the electrode.
Therefore, in order to produce a galvannealed steel
sheet having a good press-workability and spot-weldability
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characteristic, it is advantageous to use a steel sheet
which is soft enough at normal temperature to exhibit
excellent press-workability and which is less liable to be
softened when heated.
05 The base steel sheet used in the present invention
has been developed from the above-described point of view.
A description will be given of the reasons of limitation
of the contents of the respective components of the steel. ~;
C is an element which adversely affects press-
workability. The C content,therefore, should be not
greater than 0.005 wt%, in order to obtain a steel sheet
having excellent press-workability under a condition where
heating and soaking have to be done in short time as in
the case of annealing in a continuous hot-dip galvanizing
process.
Ti is an element which reacts with inevitably
existing elements such as N and C so as to form TiN and
TiC, thereby fixing such elements, thus eliminating any
undesirable effect of such elements on press-workability,
and thus enhancing the effect of B which will be mentioned
later. In order to attain an appreciable effect of
addition of Ti, the Ti content should be at least 0.005 ~ -
wt~. On the other hand, however, addition of Ti in excess
of 0.05 wt~ causes burning defects in galvannealing
process. The Ti content therefore should not exceed 0.05
wt%.
A~ is an element which is added to prevent oxidation -
of elements such as Ti, Nb and B which are added to the
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molten steel. In order to sufficiently deoxidize the
molten steel, it is necessary that the A~ is added in an
amount which is not smaller than 0.01 wt%. On the other
hand, addition of A~ in excess of 0.1 wt~ causes a rise in
05 the cost The A~ content, therefore, should be not
smaller than 0.01 wt% and not greater than 0.1 wt%.
Nb and B are elements which are effective in
preventing softening of steel sheet at high temperature.
This advantageous effect is obtained only when both Nb and
B coexist. In order to attain appreciable effect in
preventing softening at high temperature, the Nb content
should not be smaller than 0.005 wt% and the B content
should not be smaller than 0.0002 wt%. However, addition
of Nb in excess of 0.015 wt% undesirably reduces the
ductility of the steel sheet at normal temperature, thus
impairing press-workability. On the other hand, any B
content exceeding 0.002 wt% causes a reduction in the
Lankford value r which is an index of deep-drawability in
press work, thus impairing press-workability. The Nb and
B contents, therefore, are limited to be from 0.005 to
0.015 wt~ and from 0.0002 to 0.002 wt%, respectively.
Si is an element which is effective in strengtheniing
the steel and is added in accordance with the demand for
strengthening. Addition of Si 1n excess of 0.1 wt%, ~ `-
however7 adversely affects the deep-drawability and
elongation so that Si content is determined to be not ;~
greater than 0.1 wt%.
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Mn also is an element which strengthen the steel.
The Mn content, however, is limited to be not greater than
1.0 wt%, because Mn content exceeding 1.0 wt% undesirably
reduces deep-drawability.
05 A cold-rolled steel with the contents of components
controlled as described above exhibits superior press-
workability when annealed by being reheated to a
temperature ranging between 770 and 900oC. When the
annealing temperature is below 7700C, it is impossible to
obtain sufficient recrystallization effect. On the other
hand, when the annealing temperature exceeds 900oC, a
transformation takes place to reduce the Lankford value r,
thus causing reduction in ductility. The annealing
temperature, therefore, should be determined to be from
lS 7700C to 900oC.
The rate of cooling of the annealed cold-rolled steel
sheet before entering a molten zinc bath should be
lOoC/sec. This cooling rate causes a moderate level of
internal stress to be generated in the steel sheet, thus
imparting greater resistance to softening of the portions
of the steel sheet thermally affected during spot welding.
In order to énhance!this advantageous effect, it is
preferred that the cooling be conducted at a rate which is
200C/sec or greater.
The cooling at such a fast rate, i.e.,quenching, is
ceased when the steel sheet is dipped in the molten zinc ~
bath. It is necessary that the steel sheet is cooled to -
5300C at the highest before entering the molten zinc bath. ~
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On the other hand, cooling down below a lower li~it
temperature of 380OC causes plating failure.
The A~ content in the bath is not a factor which
directly affects the spot-weldability, but produces an
05 effect to effectively suppress exfoliation or peeling of
the plating layer during the press work particularly when
the Fe content of the plating layer is comparatively
large. More specifically, it is possible to obtain a
resistance to exfoliation or peeling of the plating layer
during press work, high enough to enable the plated steel
sheet to be used as an automotive body part when the Fe
content of the plating layer ranges between 9 and 12 wt%,
provided that the A~ content in the plating bath is 0.13
wt% or more, and preferably is equal to or higher than
0.15 wt%.
It is a critical feature of the present invention
that the plating layer has an Fe content not smaller than
9 wt%. When the Fe content is below 9 wt%,it is
impossible to obtain the required spot-weldability even
when the contents of the components of the base steel
sheet are controlled ~s specified above. This is
attributed to the fact that Fe content below 9 wt%
undesirably allows presence of ~ phase of low melting
point in the plating layer so as to seriously promote the
consumption of the spot welding electrode. On the other
hand, any Fe content in the plating layer exceeding 12 wt~
reduces the resistance to exfoliational peeling of the
plating layer during press work, tending to cause a
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phenomenon known as "powdering". For these reasons, the
Fe content in the plating layer is limited to be from 9
wt% to 12 wt%.
~ EXAMPLES
05 Practical examples of the invention will be described
hereinunder. Hot dip galvanizing was conducted on each of
the steel sheets (0.7 mm thick) having compositions as
shown in Table 1, followed by galvannealing. Plating
characteristics (anti-powdering in relation to Fe content
( wt%) in plating layer), press-workability (mechanical
properties, in particular elongation E~ and Lankford value
r) and spot-weldability (number of spots welded
continuously) were examined and the results are shown in
Table 2 together with the annealing and plating
conditions.
From Table 2, it will be understood that the
galvannealed steel sheet prepared in accordance with the
present invention is excellent in all aspects of anti-
powdering, press-workability and spot-weldability
characteristics.
The Fe content in the plating layer was measured by
dissolving the plating layer in an acid and measuring the
Fe content by atomic spectral absorption.
The anti-powdering characteristic was measured by
bending the plated steel sheet at gOo~ straightening it
again, applying an adhesive tape to the plating layer
exfoliated, and subjecting the exfoliated plating layer on
the tape to a fluorescent X-ray analysis so as to measure
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the number of the X-rays peculiar to zinc per second (Zn
cps). The anti-powdering characteristic was then
evaluated in the following five ranks.
Evaluation ranks Zn cps
1 < 2000
2 2001 to 4000
3 4001 to 6000
4 6001 to 10000
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The spot-weldability was measured by counting the
number of spots welded continuously under the following
welding conditions.
Welding electrode :~:
Type: CF .
Top end diameter: 4.5 mm
Top end angle: 120
Outside diameter: 13 mm .
Material: Cu-Cr
Welding Conditions `
Welding current. 8.8 KA ~ .
Period of current supply: 0.2 second (at 50 Hz)
Pressing force: 170 kgf :~
Pressing conditions :~
Before supply of current: 0.6 second (at 50 Hz)
After supply of current: 0.14 second (at 50 Hz) .
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The evaluation of the spot-weldability was made in
the following four ranks a, b, c and d in terms of the
number of spots continuously welded to nugget diameters
not smaller than 4 ~/t, where t (mm) represents the sheet
thickness.
Evaluation Number of welding spots
a 3000 or more
b 2000 to 3000
c 1000 to 2000
d 1000 or less
As will be understood from the foregoing description,
according to the present invention, it is possible to
produce a galvannealed steel sheet which is superior in
press-workability, anti-powdering characteristic and spot-
weldability, thus offering an anti-rust steel sheet -
suitable for use as automotive body parts.
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Table 1
Type c Si Mn P S A e n Nb N B
A 0.001 0.031 0.06 0.009 0.005 0.060.03 0.010 0.0030.0004
__ . _ , _ _ _ _ .. ,
B 0.002 O 029 0.07 0.007 0.004 0.080.02 0.008 0.0020.0009
C 0.004 ~ 043 0.08 0.011 0.006 0.03 0.009 0.012 0.003 0.0006
D 0.0030.035~ 0.07 0.008 o.oos 0.060.02 o.oll 0.003
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