TECHNICAL FIELD
The present invention relates to a process of
production and production system of a high strength
galvannealed steel sheet, more particularly relates to a
plated steel sheet able to be utilized for various
applications, for example, a steel sheet for a building
material or an automobile.
BACKGROUND ART
As a plated steel sheet with a good corrosion
resistance, there is a galvannealed steel sheet. This
galvannealed steel sheet usually is produced by
degreasing the steel sheet, then preheating it in a non-
oxidizing furnace, reduction annealing it in a reducing
furnace to clean the surface and secure the quality,
dipping it in a hot-dip galvanizing bath, controlling the
cunount of deposition, then alloying. Due to its
characteristics of superior corrosion resistance, plating
adhesion, etc., it is widely used for automobile,
building material, and other applications.
In particular, in recent years, in the automobile
sector, galvanized steel sheets have to be made higher in
strength to achieve both the function of protecting
passengers at the time of collision and reducing weight
for the purpose of improving fuel efficiency.
Further, recently, to make the reaction at the
surface of the steel sheet at the time of annealing more
uniform and improve the plating appearance, production
systems for galvanized steel sheet using all radiant tube
type annealing furnaces have spread in use.
To make the steel sheet higher in strength without
reducing the workability, addition of elements like Si,
Mn, and P is effective. These elements are selectively
oxidized in the reduction annealing step and become
concentrated at the surface of the steel sheet. In
particular, oxides of Si concentrated at the surface of
the steel sheet cause the wettability of the steel sheet
and molten zinc to drop. In extreme cases, the molten
zinc will not adhere to the steel sheet.
Therefore, to plate with molten zinc a steel sheet
to which an element like P has been added, use has been
made of the method of making the thickness of the oxide
film of the iron a suitable range to keep down the
production of oxide layers of elements such as Si, Mn,
and P and improve the wettability (for example, see
Japanese Patent No. 2513532) or the method of pre-plating
to improve the plating wettability (for example, see
Japanese Unexamined Patent Publication (Kokai) No. 2-
38549).
Further, the inventors proposed the method of
production comprising suitably controlling the reducing
atmosphere to cause internal oxidation of SiO so as to
improve the plating wettability (for exeunple, see
Japanese Unexamined Patent Publication (Kokai) No. 2001-
323355).
However, the technology disclosed in Japanese Patent
No. 2513532 and Japanese Unexamined Patent Publication
(Kokai) No. 2001-323355 is technology using a Sendzimir
type hot-dip galvanizing steel sheet production system
for heating in a non-oxidizing atmosphere and annealing
in a reducing atmosphere and cannot be used in a
manufacturing equipment of hot-dip galvanized steel sheet
using an all radiant tube type annealing furnace.
Further, in the technology disclosed in Japanese
Unexamined Patent Publication (Kokai) No. 2-38549, a pre-
plating system is necessary. When there is no
installation space, it cannot be used. Further, a rise in
cost due to the installation of the pre-plating system is
unavoidable.
DISCLOSURE OF INVENTION
Therefore, the present invention solves the above
problem and proposes a process of production of a high
strength galvannealed steel sheet by a manufacturing
equipment of hot-dip galvanized steel sheet using an all
radiant tube type annealing furnace and a production
system for the same.
The inventors engaged in intensive research on a
process of production for producing a high strength
galvannealed steel sheet by a manufacturing equipment of
hot-dip galvanized steel sheet using an all radiant tube
type annealing furnace and as a result discovered that by
making the atmosphere in the reducing zone an atmosphere
containing H2 in an amount of 1 to 60 wt% and comprising
the balance of N2, H2O, O2, CO2, CO, and unavoidable
impurities, controlling the log(PCO2/PH2) of the carbon
dioxide partial pressure and hydrogen partial pressure in
the atmosphere to log(PCO2/PH2)=-0.5 and the log(PH2O/PH2)
of the water partial pressure and hydrogen partial
pressure to log(PH2O/PH2)=-0.5, and controlling the
log(PT/PH2) of the total partial pressure PT of the carbon
dioxide partial pressure PCO2 and water partial pressure
PH2O and the hydrogen partial pressure to
-3=1og(PT/PH2)=-0.5, it is possible to produce a high
strength galvannealed steel sheet. Further, they
discovered that by filling the all radiant tube type
annealing furnace with a gas comprised 1 to 100 wt% of
CO2 and the balance of N2, H2O, O2, CO, and unavoidable
impurities, it is possible to produce a high strength
galvannealed steel sheet.
That is, the gist of the present invention is as
follows:
(1) A process of production of a high strength
galvannealed steel sheet comprising continuously plating
by molten zinc a high strength steel sheet having a
content of Si of 0.4 to 2.0 wt% during which making the
atmosphere of the reducing zone an atmosphere containing

H2 to 1 to 6 0 wt% and comprised of the balance of N2, H2O,
O2, CO2, CO, and unavoidable impurities, controlling, in
the atmosphere, the log(PCO2/PH2) of the carbon dioxide
partial pressure and hydrogen partial pressure to
log(PCO2/PH2)=-0.5, the log(PH2O/PH2) of the water partial,
pressure and hydrogen partial pressure to
log(PH2O/PH2)=-0.5, and the log(PT/PH2) of the total
partial pressure PT of the carbon dioxide partial
pressure PCO2 and water partial pressure PH2O and the
hydrogen partial pressure to -3=log(PT/PH2)=-0.5,
performing the annealing in the reducing zone in a
ferrite-austenite two-phase temperature region at 720°C
to 880°C, then cooling by a plating bath and performing
the galvanizing so as to form a hot-dip galvanizing layer
on the surface of the cold rolled steel sheet, then
heating for alloying the steel sheet on which the hot-dip
galvanizing layer is formed at 460 to 550°C, it is
possible to produce a high strength galvannealed steel
sheet.
(2) A process of production of a high strength
galvannealed steel sheet as set forth in (1),
characterized by performing the galvannealed in a hot-dip
galvanizing bath of a composition comprised of an
effective Al concentration in the bath of at least 0.0 7
wt% and the balance of Zn and unavoidable impurities and
performing the alloying at a temperature (°C) satisfying
450:=T=410xexp(2x[Al%] )
where, [Al%]: effective Al concentration {wt%)
in the hot-dip galvanizing bath
(3) A process of production of a high strength
galvannealed steel sheet as set forth in (1) or (2)
superior in bondability, characterized by being performed
at an effective Al concentration (wt%) in the bath
satisfying the effective Al concentration in the bath of [A1% ]=0 . 092-0 . 001x( Si% ]'

where, [Si%]: Si content in steel sheet (wt%)
(4) A manufacturing equipment of hot-dip galvanized
steel sheet comprising providing a hot-dip galvanizing
bath and continuously plating a steel sheet by molten
zinc, said equipment for production of a hot-dip
galvanizing steel sheet for working the process of
production of a high strength galvannealed steel sheet
described in (1) characterized by making the annealing
furnace an all radiant tube type annealing furnace and
providing an apparatus for introducing into the annealing
furnace a gas containing CO2 in an amount of 1 to 10 0 wt%
and comprised of the balance of N2, H2O, Oj, CO, and
unavoidable impurities.
(5) A manufacturing equipment of hot-dip galvanized
steel sheet comprising providing a hot-dip galvanizing
bath and continuously plating a steel sheet by molten
zinc, said equipment for production of a hot-dip
galvanizing steel sheet for working the process of
production of a high strength galvannealed steel sheet
described in (1) characterized by making the annealing
furnace an all radiant tube type annealing furnace and
providing an apparatus for burning CO or a hydrocarbon in
the annealing furnace and producing a gas containing CO2
in an amount of 1 to 100 wt% and comprised of the balance
of N2, H2O, O2, CO, and unavoidable impurities.
Further, in the present invention, it is possible to
produce a high strength galvannealed steel sheet aimed at
by the present invention under the conditions defined
below:
1) In the process of production of a high strength
galvannealed steel sheet as set forth in any of the above
(1) to (5), the sheet is cooled from the maximum reached
temperature to 650°C by an average cooling rate of 0.5 to
10°C/sec and then from 650°C to the plating bath by an
average cooling rate of at least 3°C/sec.
2) In the process of production of a high strength
galvannealed steel sheet as set forth in any of the above
(1) to (5), the sheet is cooled from the maximum reached
temperature to 650°C by an average cooling rate of 0.5 to
10°C/sec and then from 650°C to 500°C by an average
cooling rate of at least 3°C/sec and further from 500°C
by an average cooling rate of at 0.5°C/sec from 420°C to
460°C and held from 500°C to the plating bath for 25 sec
to 240 sec, then hot-dip galvanizing is carried out.
3)In the process of production of a high strength
galvannealed steel sheet as set forth in any of the above
(1) to (5), the time until cooling to a temperature of
not more than 400°C after the hot-dip galvanizing is made
30 sec to 120 sec.
4)In the process of production of a high strength
galvannealed steel sheet as set forth in any of the above
(1) to (5), the sheet is cooled to 400°C to 450°C after
annealing, then reheated from 430°C to 470°C and hot-dip
galvanizing is carried out.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side view of an exaunple of a production
system for hot-dip galvanized steel sheet according to
the present invention.
FIG. 2 is a side view of an example of a production
system for hot-dip galvanized steel sheet according to
the present invention.
BEST MODE FOR WORKING THE INVENTION
Below, the present invention will be explained in
further detail.
The present invention comprises continuously hot-dip
galvanized high strength steel sheet having a content of
Si of 0.4 to 2.0 wt% by a hot-dip galvanized steel sheet
production system using an all radiant tube type
annealing furnace during which making the atmosphere of
the reducing zone is made one which does not cause iron
to oxidize and causes internal oxidation of SiO2. Here,
"internal oxidation of Si" is a phenomenon where the
oxygen diffused in the steel sheet reacts with Si near
the surface layer of the alloy and precipitates as an

oxide. The phenomenon of internal oxidation occurs when
the rate of diffusion of the oxygen inward is far faster
than the rate of diffusion of the Si outward, that is,
when the oxygen potential in the atmosphere is relatively
high. At this time, the Si does not move much at all and
is oxidized in place, so the cause of the drop in plating
adhesion, that is, the concentration of Si at the surface
of the steel sheet, can be prevented.
Specifically, the invention comprises making the
atmosphere of the reducing zone an atmosphere containing
H2 to 1 to 60 wt% and comprised of the balance of N2, H2O,
Oj, CO2, CO, and unavoidable impurities, controlling the
log(PCO2/PH2) of the carbon dioxide partial pressure and
hydrogen partial pressure in the atmosphere to
log(PCO2/PH2)=-0.5 and the log(PH2O/PH2) of the water
partial pressure and hydrogen partial pressure to
log(PH2O/PH2)=-0.5, controlling the log(PT/PH2) of the
total partial pressure PT of the carbon dioxide partial
pressure PCO2 and water partial pressure PH2O and the
hydrogen partial pressure to -3=log(PT/PH2)=-0.5, and
performing the annealing in the reducing zone in a
ferrite-austenite two-phase temperature region at 720°C
to 880°C.
In the reducing zone, a gas including H2 in the
range of 1 to 60 wt% is used. The reason for limiting the
Hj to 1% to 60% is that if less than 1%, the oxide film
produced at the surface of the steel sheet before
annealing cannot be sufficiently reduced and the plating
wettability cannot be secured, while if over 60%, no
improvement in the reducing action can be seen and the
cost is increased.
Further, in the reducing zone, for the purpose of
causing internal oxidation of SiO2, one or two or more of
H2O, O2, CO2, and CO are introduced into the reducing
atmosphere, the log(PCO2/PH2) of the carbon dioxide
partial pressure and hydrogen partial pressure in the

atmosphere is controlled to log(PCO2/PH2)=-0.5 and the
log(PH2O/PH2) of the water partial pressure and hydrogen
partial pressure to log(PH2O/PH2)=-0.5, and the
log(PT/PH2) of the total partial pressure P= of the carbon
dioxide partial pressure PCO2 and water partial pressure
PH2O and the hydrogen partial pressure is controlled to
-3=log(pT/PH2)=-0.5.
The log(PCO2/PH2) of the carbon dioxide partial
pressure and hydrogen partial pressure and the
log(PHaO/PH2) of the water partial pressure and hydrogen
partial pressure are controlled by introducing CO2 and
water vapor into the furnace.
The reason for making the log(PCO2/PH2) not more
than -0.5 is that if the log(PCO2/PH2) is over -0.5, the
oxide film which had been produced on the surface of the
steel sheet before annealing cannot be sufficiently
reduced and the plating wettability cannot be secured.
Further, the reason for making the log(PH2O/PH2) not more
than -0.5 is that if the log(PH2O/PH2) is over -0.5, the
oxide film which had been produced on the surface of the
steel sheet before annealing cannot be sufficiently
reduced and the plating wettability cannot be secured.
The reason for making the log(PT/PH2) of the carbon
dioxide partial pressure PCO2 and the water partial
pressure PH2O and the hydrogen partial pressure not more
than -0.5 is that if the log(PT/PH2) is over -0.5, the
oxide film which had been produced on the surface of the
steel sheet before annealing cannot be sufficiently
reduced and the plating wettability cannot be secured.
Further, the reason for making the log(PT/PH2) not less
than -3 is that if the log(PT/PH2) is less than -3,
external oxidation of the Si occurs, SiO2 is produced on
the surface of the steel sheet, and the plating
wettability is caused to fall.
O2 and CO do not have to be deliberately introduced,
but when introducing H2O and CO, into the furnace of the
main annealing temperature and atmosphere, parts are
reduced by H2 and O2 and CO are produced.
H2O and CO2 need only be introduced in the required
amounts. The method of introduction is not particularly
limited, but the method of burning a gas comprised of a
mixture of for example CO and H2 and introducing the
produced H2O and CO2, the method of burning a gas of CH4,
CsHg, CgHe, or another hydrocarbon or a mixture of LNG or
another hydrocarbon and introducing the produced H2O and
CO2, the method of burning a mixture of gasoline, light
oil, heavy oil, or another liquid hydrocarbon and
introducing the produced H2O and CO2, a method of burning
CH2OH, C2H5OH, or another alcohol or its mixture or
various types of organic solvents and introducing the
produced H2O and CO2, etc. may be mentioned.
The method of burning only CO and introducing the
produced CO2 may also be considered, but when introducing
CO2 into the furnace of the main annealing temperature
and atmosphere, part is reduced by the Hg. There is no
inherent difference from the case of introducing H2O and
CO2 to produce CO and H2O.
Further, in addition to the method of introducing
the H2O and CO2 produced by burning, the method may also
be used of introducing a gas of a mixture of CO and H2, a
gas of CH4, C2H6, C8H8, or another hydrocarbon, a mixture
of LNG or another hydrocarbon, a mixture of gasoline,
light oil, heavy oil, or another liquid hydrocarbon,
CH3OH, C2H5OH, or another alcohol or their mixtures, and
various types of organic solvents etc. simultaneously
with oxygen into the annealing furnace and burning them
in the furnace to produce H2O and CO2.
When annealing by an in-line annealing type
continuous hot-dip galvanizing system, the annealing
temperature is made a ferrite-austenite two-phase region
of 720°C to 880°C. If the annealing temperature is less
than 720°C, the recrystallization is insufficient. The
press workability required for steel sheet cannot be

provided. By annealing by a temperature over 880°C, a
rise in cost is invited, so this is not preferable.
Next, the steel strip is cooled by a process of
dipping in a plating bath, but when not aiming at use of
a member with particularly strict processing
requirements, no special cooling process not be gone
through. Hot-dip galvanizing is performed so as to form a
hot-dip galvanizing layer on the surface of the steel
sheet, then the steel sheet on which said hot-dip
galvanizing layer is formed is heat treated for alloying
at 460 to 550°C so as to fabricate a high strength
galvannealed steel sheet.
In particular, to achieve both a high strength and
good press workability, the steel sheet to which Si or Mn
has been added in a large amount is annealed, then cooled
in the process of dipping into the plating bath from the
maximum reached temperature to 650°C by an average of 0.5
to 10°C/sec then cooled from 650°C to the plating bath by
an average of at least 3°C/sec. The cooling rate down to
650°C is made an average 0.5 to 10°C/sec to increase the
percent volume of the ferrite for improving the
workability and simultaneously increase the C
concentration of the austenite to lower the free energy
produced and make the temperature of start of the
martensite transformation not more than the plating bath
temperature. To make the average cooling rate down to
650°C less than 0.5°C/sec, it is necessary to make the
line length of the continuous hot-dip galvanizing
manufacturing equipment longer and the cost becomes high,
so the average cooling rate down to 650°C is made at
least o-5'fe/sec.
To make the average cooling rate down to 650°C less
than 0.5°C/sec, it may be considered to lower the maximum
reached temperature and anneal at a temperature with a
small percent volume of austenite, but in this case the
suitable temperature range is narrower than the
temperature range allowed in actual operation and if the
annealing temperature is even slightly low, austensite
will not be formed and the object will not be achieved.
On the other hand, if the average cooling rate up to
650°C is made to exceed 10°C/sec, not only will the
increase in the percent volume of the ferrite be
insufficient, but also the increase in the C
concentration in the austenite will be small, so before
the steel strip is dipped in the plating bath, part of it
will transform to martensite and that martensite will be
tempered and precipitate as cementite by the subsequent
heating for alloying, so achievement of both high
strength and good workability will become difficult.
The average cooling rate from 650°C to the plating
bath is made at least 3°C/sec to avoid the austenite
being transformed to pearlite in the middle of the
cooling. With a cooling rate of less than 3°C/sec, the
sheet is annealed at a temperature defined in the present
invention. Further, even if cooling down to 650°C,
formation of pearlite is unavoidable. The upper limit of
the average cooling rate is not particularly limited, but
cooling the steel strip so that the average cooling rate
does not exceed 20°C/sec is difficult in a dry
atmosphere.
Further, to produce a high strength galvannealed
steel sheet with good workability, the sheet is cooled by
an average cooling rate from 650°C to 500°C of at least
3°C/sec, further cooled by an average cooling rate from
500°C of at least 0.5°C/sec down to 420°C to 460°C, held
from 500°C to the plating bath for 25 sec to 240 sec,
then hot-dip galvanizing is carried out.
The average cooling rate from 650°C to 500°C was
made at least 3°C/sec to avoid the austenite being
transformed to pearlite in the middle of the cooling.
With a cooling rate of less than 3°C/sec, even if
annealing at the temperature defined in the present
invention or cooling down to 650°C, formation of pearlite
is unavoidable. The upper limit of the average cooling
rate is not particularly limited, but cooling the steel
strip so as not to exceed an average cooling rate of
20°C/sec is difficult in a dry atmosphere.
The average cooling rate from 500°C is made at least
0.5°C/sec so as to avoid the austenite transforming to
pearlite in the middle of the cooling. With a cooling
rate of less than 0.5°C/sec, even if annealing at the
temperature defined in the present invention or cooling
down to 500°C, formation of pearlite is unavoidable. The
upper limit of the average cooling rate is not
particularly limited, but cooling the steel strip so as
not to exceed an average cooling rate of 20°C/sec is
difficult in a dry atmosphere. Further, the cooling end
temperature was made 420 to 460°C so as to promote
concentration of C in the austenite and obtain a high
strength alloyed molten zinc plating superior in
workability.
The reason for limiting the maintaining time of
below 25 seconds and less than 240 seconds between 500°C
and a temperature of the plating bath is that when the
maintaining tine is below 25 seconds, the concentration
of C in the austenite is insufficient and the
concentration of C in the austenite does not reach the
level enabling residual presence of austenite at room
temperature. If over 240 sec, the bainite transformation
does not proceed too much, the amount of austenite
becomes smaller, and a sufficient amount of residual
austenite cannot be produced.
Further, the sheet is cooled all at once to a
temperature of 400 to 450°C while being held from 500°C
to the plating bath. When held, the concentration of C in
the austenite is promoted and a high strength alloyed
molten zinc plating superior in workability is obtained.
However, if continuing to immerse the sheet in the
plating bath at under 430°C, the plating bath is cooled
and solidifies, so it is necessary to reheat it to a
temperature of 430 to 470°C, then perform the hot-dip
galvanizing.
In the production of the galvannealed steel sheet of
the present invention, to produce a high strength
galvannealed steel sheet with a good workability, the
hot-dip galvanizing bath used should be adjusted to an Al
concentration of an effective Al concentration in the
bath of 0.07 to 0.092 wt%. Here, the effective Al
concentration in the plating bath is the value of the Al
concentration in the bath minus the concentration of Fe
in the bath.
The reason for limiting the effective Al
concentration 0.07 to 0.092 wt% is that if the effective
Al concentration is less than 0.07%, the formation of the
Fe-Al-Zn phase serving as the alloying barrier at the
start of plating is insufficient and a brittle T phase
is formed thickly at the interface of the plated steel
sheet at the time of the plating, so only an galvannealed
steel sheet with an inferior plating coating bonding
force at the time of working can be obtained. On the
other hand, if the effective Al concentration is higher
than 0.092%, alloying at a high temperature for a long
time becomes necessary, the austenite remaining in the
steel transforms into pearlite, and therefore realization
of both high strength and good workability become
difficult. Further, making the alloying temperature at
the time of the alloying in the present invention a
temperature T (°C) satisfying
450=:T:£410xexp(2x[Al%] )
where, [Al%]: effective Al concentration (wt%) in
hot-dip galvanizing bath
is effective for the production of high strength
galvannealed steel sheet with a good workability.
The reason for making the alloying temperature at
least 450°C to not more than 410xexp(2x[Al%])°C is that
if the alloying temperature T is lower than 450°C, the
alloying will not proceed or the alloying will proceed
insufficiently, the alloying will be incomplete, and the
plating layer will be covered with an r\ phase inferior
in bondability. Further, if T is higher than
410xexp{2x[Al%])°C, the alloying will proceed too much
and a brittle phase is formed thickly at the interface
of the plated steel sheet, so the plating bonding
strength at the time of the working falls.
In the present invention, if the alloying
temperature is too high, the austenite remaining in the
steel transforms to pearlite and it is difficult to
obtain steel sheet achieving both high strength and good
workability. Therefore, the greater the amount of Si
added and the more difficult the alloying, the more
effective lowering the effective Al concentration in the
bath and lowering the alloying temperature are for
improving the workability.
Specifically, the plating is performed at an
effective Al concentration (wt%) in the bath satisfying
[Al%l=0.092-0.001x[Si%]2
where, [Si%]: Si content in steel sheet (wt%).
The reason for limiting the effective Al
concentration to not more than 0.092-0.001x[Si%]2% is
that if the effective Al concentration is higher than
0.092-0.001x[Si%]2%, alloying at a high temperature and a
long time becomes required, the austenite remaining in
the steel transforms to pearlite, and the workability
deteriorates.
The reason for making the time until cooling to a
temperature of not more than 400°C after hot-dip
galvanizing to 30 sec to 120 sec is that if less than 30
sec, the alloying is insufficient, the alloying becomes
incomplete, and the surface layer of the plating is
covered by an phase inferior in bondability, while if
over 12 0 sec, the pearlite transformation proceeds too
much, the amount of austenite becomes small, and a
sufficient amount of residual austenite cannot be
produced.
FIG. 1 and FIG. 2 show an example of a manufacturing
equipment of hot-dip galvanized steel sheet according to
the present invention by a side view. In the figures, 1
indicates a high strength steel sheet having a content of
Si of 0.4 to 2.0 wt%, 2 a heating zone of the annealing
furnace, 3 a soaking zone of the annealing furnace, 4 a
cooling zone of the annealing furnace, 5 an in-furnace
roll, 6 a steel sheet advance direction, 7 a hot-dip
galvanizing tank, 8 molten zinc, 9 a snout, 10 a sink
roll, 11 a gas wiping nozzle, 12 an alloying furnace, 13
a gas flow adjustment valve, 14 a reducing gas pipe, 15 a
reducing gas flow direction, 16 a burner, 17 a combustion
gas pipe, 18 a combustion gas flow direction, 19 a fuel
gas pipe, 20 a fuel gas flow direction, 21 an air pipe,
22 an air flow direction, and 23 a burner provided in the
furnace.
Example 1
A slab comprised of the composition shown by R in
Table 1 was heated to 1150oC to obtain a hot rolled steel
strip of 4.5 mm of a finishing temperature of 910 to
930°C. This was wound up at 580 to 680°C. The strip was
pickled, then cold rolled to obtain a cold rolled steel
strip of 1.6 mm, then a continuous hot-dip galvanizing
equipment using an all radiant tube type annealing
furnace was used for the heat treatment and plating under
the conditions such as shown in Table 2 to produce a
galvannealed steel sheet. The continuous hot-dip
galvanizing equipment was provided with an apparatus for
burning a gas comprised of a mixture of CO and H2 and
introducing the produced H2O and CO2 and adjusted the
log(PT/PH2) of the total partial pressure PT of the carbon
dioxide partial pressure PCO2 and water partial pressure
PH2O and the hydrogen partial pressure to become the
value shown in Table 2.
The tensile strength (TS) and elongation (El) were
found by cutting out JIS No. 5 test pieces from the steel
sheets and performing tensile tests at ordinary
temperature.
The amount of deposition of the plating was
determined by dissolving the coating film in hydrochloric
acid in an inhibitor and measuring it by the weight
method.
The wettability was judged by scoring the percent
area of plating gaps of the rolled coil as follows. A
score of 3 or more was judged as passing.
4: percent area of plating gaps of less than 1%
3: percent area of plating gaps of 1% to less than
5%
2: percent area of plating gaps of 5% to less than
10%
1: percent area of plating gaps of 10% or more
The results of evaluation are shown in Table 2. No.
1 had a log(PT/PH2) of the total partial pressure P,. of
the carbon dioxide partial pressure PCO2 and water
partial pressure PH2O and the hydrogen partial pressure
outside of the scope of the present invention, so the
oxide film produced on the surface of the steel sheet
before annealing could not be sufficiently reduced and
the plating wettability was judged as failing. No. 7 had
a log(PT/PH2) of the total partial pressure PT of the
carbon dioxide partial pressure PCO2 and water partial
pressure PH2O and the hydrogen partial pressure outside
of the scope of the present invention, so external
oxidation of Si occurred, SiO2 was produced on the
surface of the steel sheet, and the plating wettability
was judged as failing.
The rest of the steel sheets, those produced by the
process of the present invention, were high strength
galvannealed steel sheets superior in plating
wettability.
Example 2
A slab comprised of the composition shown in Table 1
was heated to 1150°C to obtain a hot rolled steel strip
of 4.5 mm of a finishing temperature of 910 to 930°C.
This was wound up at 580 to 680°C. The strip was pickled,
then cold rolled to obtain a cold rolled steel strip of
1.6 mm, then a continuous hot-dip galvanizing equipment
using an all radiant tube type annealing furnace was used
for the heat treatment and plating under the conditions
such as shown in Table 3 to produce a galvannealed steel
sheet. The continuous hot-dip galvanizing equipment was
provided with an apparatus for burning a gas comprised of
a mixture of CO and H2 and introducing the produced H2O
and CO2 and adjusted the log(PT/PH2) of the total partial
pressure Pj of the carbon dioxide partial pressure PCO2
and water partial pressure PH2O and the hydrogen partial
pressure to become -1 to -2.
The tensile strength (TS) and elongation (El) were
found by cutting out JIS No. 5 test pieces from the steel
sheel:s and performing tensile tests at ordinary
temperature.
The amount of deposition of the plating was
det=ermined by dissolving the coating film in hydrochloric
acid in an inhibitor and measuring it by the weight
method.
The wettability was judged by scoring the percent
area of plating gaps of the rolled coil as follows:
4: percent area of plating gaps of less than 1%
3: percent area of plating gaps of 1% to less than
5%
2: percent area of plating gaps of 5% to less than
10%
1: percent area of plating gaps of 10% or more
The results of evaluation are shown in Table 3.
Using the process of the present invention, it becomes
possible to produce a high strength galvannealed steel
sheet superior in plating wettability.
In particular, the processes of production shown in
Nos. 4, 5, 6, 10, 11, 13, 14, 16, 17, 20, 21, 22, 25, 31,
32, 34, 35, and 36 had suitable cooling rates in the
annealing furnace, effective Al concentrations in the
hot-dip galvanizing bath, and alloying temperatures, so
were able to produce high strength hot-dip galvanizing
steel sheet with good workabilities.
INDUSTRIAL APPLICABILITY
According to the present invention, it becomes
possible to provide a process for production for plating
a high strength steel sheet having a content of Si of 0.4
to 2.0 wt% using a continuous hot-dip galvanizing
equipment using an all radiant tube type annealing
furnace and an apparatus for the same. The contribution
to development of the industry is extremely great.
WE CLAIM:
1. A process and a system for production of a high strength galvannealed steel
sheet using all radiant tube type armealing furnace comprising continuously
hot-dip galvanizing a high strength steel sheet having a content of Si of 0.4 to
2.0 wt% during which making the atmosphere of the reducing zone an
atmosphere containing H2 to 1 to 60 wt% and comprised of the balance of N2,
H2O, O2,CO2, CO, and unavoidable impurities, controlling, in the
atmosphere, the log(PCO2/PH2) of the carbon dioxide partial pressure and
hydrogen partial pressure to log(PCO2/PH2) = -0.5, the log(PH02/PH2) of the
water partial pressure and hydrogen partial pressure to log(PH2O/PH2) = -
0.5, and the log(PT/PH2) of the total partial pressure Px of the carbon dioxide
partial pressure PCO2 and water partial pressure PH2O and the hydrogen
partial pressure to -3 = log (Px/PHa) = -0.5, performing the annealing in the
reducing zone in a ferrite-austenite two-phase temperature region at 720°C to
880°C, then cooling by a plating bath and performing the molten zinc plating
so as to form a hot-dip galvanizing layer on the surface of the cold rolled steel
sheet, then heating for alloying the steel sheet on which the hot-dip
galvanizing layer is formed at 450 to 550°C, it is possible to produce a high
strength galvannealed steel sheet.
2. A process and a system as claimed in claim 1, wherein the hot-dip
galvanizing performed in a hot-dip galvanizing bath of a composition
comprised of an effective Al concentration in the bath of at least 0.07 wt% and
the balance of Zn and unavoidable impurities and performing the alloying at
a temperature (°C) satisfying
450 = T = 410 X exp (2x[Al%])
where, [Al%] : effective Al concentration (wt%) in the hot-dip
galvanizing bath.
A process and a system as claimed in claim 1 or 2, wherein being performed
at an effective Al concentration (wt%) in the bath satisfying the effective Al
concentration in the bath of:
[Al%] = 0.092 - 0.001 X [Si%]2
where, [Si%]: Si content in steel sheet (wt%).
4. A system for production of a hot-dip galvanized steel sheet, comprising
providing a hot-dip galvanizing bath and continuously plating a steel sheet
by molten zinc, said system for production of hot-dip galvanized steel sheet
for carrying out the process as claimed in claini 1, comprises an all radiant
tube type annealing furnace and a means for providing into the annealing
furnace a gas containing CO2, said means preferably being a means for
burning CO or hydrocarbon into the annealing furnace for producing the gas
containing CO2, in an amount of 1 to 100 wt% and comprised of the balance
of N2, H2O, O2, CO and unavoidable impurities.
5. A process for production of a high strength galvannealed steel sheet
substantially as herein described with reference to the accompanying
drawings.
6. A system for production of a high strength galvannealed steel sheet
substantially as herein described with reference to the accompanying
drawings.

A process of production for producing a high
strength galvannealed steel sheet by a hot-dip galvanized steel sheet production equipment using an all radiant tube type annealing furnace and a production equipment for the same are provided, comprising continuously hotdip
galvanizing a high strength steel sheet having a
content of Si of 0.4 to 2.0 wt% during which making the atmosphere of the reducing zone an atmosphere containing H2 to 1 to 60 wt% and comprised of the balance of N2, H2O,
O2, CO2, CO, and unavoidable impurities, controlling the log(PCO2/PH2) of the carbon dioxide partial pressure and hydrogen partial pressure in the atmosphere to log(PCO2/PH2)≤-0.5 and the log(PH2O/PH2) of the water partial pressure and hydrogen partial pressure to
log(PCO2/PH2)≤-0.5, and controlling the log(PT/PH2) of the total partial pressure PT of the carbon dioxide partial pressure PCO2 and water partial pressure PH2O and the hydrogen partial pressure to -3≤log(PT/PH2)≤-0.5.