U.S. patent application number 10/790712 was filed with the patent office on 2004-09-16 for process for the controlled oxidation of a strip before continuous galvanizing, and galvanizing line.
This patent application is currently assigned to STEIN HEURTEY. Invention is credited to Francois, Mignard.
Application Number | 20040177903 10/790712 |
Document ID | / |
Family ID | 32749781 |
Filed Date | 2004-09-16 |
United States Patent
Application |
20040177903 |
Kind Code |
A1 |
Francois, Mignard |
September 16, 2004 |
Process for the controlled oxidation of a strip before continuous
galvanizing, and galvanizing line
Abstract
Process for the continuous hot-dip galvanizing of a steel strip
(1) containing oxidizable addition elements in a proportion
allowing the mechanical properties of the steel to be improved, in
which process the strip passes through a galvanizing furnace (3) in
a reducing atmosphere, this furnace consisting of heat treatment
sections, for heating, soaking and cooling, and is then dipped into
a galvanizing bath (2). The strip is subjected, upstream of the
inlet section of the furnace, to an oxidation treatment under
conditions as regards temperature, duration and oxygen content of a
gas in which the strip is immersed, such that the oxidizable
addition elements are essentially oxidized within the strip, before
they can migrate to the surface in order to form thereat an oxide
layer.
Inventors: |
Francois, Mignard; (Mennecy,
FR) |
Correspondence
Address: |
CONNOLLY BOVE LODGE & HUTZ LLP
SUITE 800
1990 M STREET NW
WASHINGTON
DC
20036-3425
US
|
Assignee: |
STEIN HEURTEY
Ris Orangis
FR
|
Family ID: |
32749781 |
Appl. No.: |
10/790712 |
Filed: |
March 3, 2004 |
Current U.S.
Class: |
148/533 |
Current CPC
Class: |
C23C 2/003 20130101;
C23C 2/02 20130101; C21D 9/561 20130101; C21D 9/52 20130101 |
Class at
Publication: |
148/533 |
International
Class: |
C21D 001/68 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 12, 2003 |
FR |
03-03058 |
Claims
1. Process for the continuous hot-dip galvanizing of a steel strip
(1) containing oxidizable addition elements in a proportion
allowing the mechanical properties of the steel to be improved, in
which process the strip passes through a galvanizing furnace (3) in
a reducing atmosphere, this furnace consisting of heat treatment
sections, for heating, soaking and cooling, and is then dipped into
a galvanizing bath (2), the strip having been subjected to an
oxidation treatment under conditions as regards temperature,
duration and oxygen content of a gas in which the strip is
immersed, such that the oxidizable addition elements are
essentially oxidized within the strip, before they can migrate to
the surface in order to form thereat a layer of oxides of a kind
liable to create galvanizing defects, characterized in that the
strip is subjected to the oxidation treatment upstream of the inlet
section of the furnace, in that the gas in which the strip is
immersed for the oxidation treatment is air, in that this strip is
heated to a temperature between approximately 150.degree. C. and
400.degree. C. for the oxidation treatment and in that the
oxidation at the surface and immediately beneath the surface of the
strip is controlled by controlling the temperature/time pair in
such a way that the temperature of the steel strip (1) is increased
when the line speed increases and the treatment time decreases, and
vice versa.
2. Process according to claim 1, characterized in that the steel
strip is heated to a temperature between approximately 150.degree.
C. and 300.degree. C. for the oxidation treatment.
3. Process according to claim 1 or 2, characterized in that the
temperature is controlled by varying the power of a means (8) for
heating the strip upstream of the galvanizing furnace.
4. Process according to one of claims 1 to 3, characterized in that
the oxidation treatment time is controlled by modifying the length
of strip (1) between the outlet of a heating means (8) located
upstream of the furnace and the inlet of the galvanizing furnace
(3).
5. Process according to claim 4, characterized in that the length
of strip between the outlet of the heating means (8) and the inlet
of the galvanizing furnace (3) is modified by moving the heating
means (8) along the direction of the strip.
6. Process according to claim 5, characterized in that the length
of strip between the outlet of the heating means (8) and the inlet
of the galvanizing furnace (3) is modified by adjusting the length
of at least one vertical or horizontal strand of the strip, or a
combination of the two.
7. Line for the continuous hot-dip galvanizing of a steel strip (1)
containing oxidizable addition elements in a proportion allowing
the mechanical properties of the steel to be improved, in which
line the strip passes through a galvanizing furnace (3) in a
reducing atmosphere before being dipped into a galvanizing bath
(2), this line being characterized in that it comprises, upstream
of the galvanizing furnace, a means (8) for heating the strip to a
temperature of between approximately 150.degree. C. and 400.degree.
C., and a zone for exposing the strip to an oxidizing gas, the
oxygen content of which is such that, owing to the temperature and
the duration of treatment, the oxidizable addition elements in the
steel strip are oxidized within this strip before they can migrate
to the surface in order to form thereat an oxide layer.
8. Galvanizing line according to claim 7, characterized in that the
heating means (8) consists of an induction furnace which also
constitutes the zone for exposing the strip to an oxidation
gas.
9. Galvanizing line according to claim 7 or 8, characterized in
that the heating zone (8) is sealably connected to the inlet of the
furnace (3) by a chamber (13) in which the oxygen concentration may
be monitored and adjusted to the treatment to be obtained.
10. Galvanizing line according to claim 8, characterized in that
the induction furnace includes at least one induction coil that can
be moved closer to or further away from the galvanizing furnace in
order to vary the heating rate produced.
11. Galvanizing line according to claim 7, characterized in that
the heating means consists of a gas furnace.
12. Galvanizing line according to one of claims 7 to 11,
characterized in that it includes a control means (7) suitable for
acting on the heating means (8) in order to maintain the strip at a
defined temperature at the outlet of the heating means, in response
to information provided by sensors (4, 5, 6).
Description
[0001] The invention relates to a process for the continuous
hot-dip galvanizing of a steel strip, the steel containing
oxidizable addition elements in a proportion allowing the
mechanical properties of the steel to be improved.
[0002] The improvement in mechanical properties of the steel goes
either towards increasing the mechanical strength, for example for
the purpose of reducing the thickness and therefore the weight of
steel, or towards increasing the drawability, or else towards both
these criteria. This has resulted in the development of multiphase
grades of steel, for example of the DP (dual phase) and TRIP
(transformation-induced plasticity) type.
[0003] These very high-strength multiphase grades are generally
obtained by the addition of hardening elements, such as Si, Mn, Cr,
Mo, etc.
[0004] Hot-dip galvanizing furnaces according to the prior art
usually comprise several sections equipped to carry out various
steps of the heat treatment, these being, in general: heating,
soaking and cooling. The heat treatment furnace is conditioned
using an inert or reducing atmosphere, generally consisting of a
nitrogen/hydrogen mixture intended to reduce the iron oxides
present on the surface of steel sheets before they are
galvanized.
[0005] It has been observed that, in the case of multiphase steels,
elements such as Si, Mn, Cr, Mo, etc. present in the steel, which
are more oxidizable than iron, preferentially combine with the
oxygen atoms present in the furnace to form oxides on the surface
of the strip.
[0006] The very high oxidation potential of these components even
results in migration of their atoms towards the surface of the
strip so that they can be oxidized by the oxygen present in the
furnace.
[0007] The result is the formation of a thin oxide layer on the
surface of the strip. These oxides are stable and are not reduced
during its passage through the various zones of the furnace--they
are therefore still on the surface of the strip when it is dipped
into the zinc bath and obstruct the adhesion of the zinc during the
galvanizing operation. Reducing the dew point of the atmosphere
inside the furnace to limits compatible with the current prior art
has not eliminated this phenomenon, and the presence on the surface
of a galvanized strip of defects caused by the local presence of
these oxides is still observed.
[0008] It follows that, at the present time, the steel strip
hot-dip galvanizing process does not allow correct galvanizing of
multiphase steel grades having a content of oxidizable elements,
such as Si, Cr, Mn, Mo, etc., that is sufficient to improve the
mechanical properties of the steel.
[0009] The object of the proposed invention is to provide a
continuous hot-dip galvanizing device and process that allow
correct treatment of a strip containing oxidizable addition
elements whose content is sufficient to improve the mechanical
properties of the steel.
[0010] The invention relates to a line for the continuous hot-dip
galvanizing of a steel strip containing oxidizable addition
elements in a proportion allowing the mechanical properties of the
steel to be improved, in which line the strip passes through a
galvanizing furnace in a reducing atmosphere before being dipped
into a galvanizing bath, this line being characterized in that it
comprises, upstream of the galvanizing furnace, a means for heating
the strip to a suitable temperature followed by a zone for exposing
the strip to an oxidizing atmosphere, the oxygen content of which
is such that, owing to the temperature of the strip and the
duration of the treatment, the oxidizable addition elements in the
steel strip are oxidized at the surface and immediately beneath the
surface of the strip before they can migrate to the said surface,
in order to form thereat a layer of oxides capable of causing
galvanizing defects. The iron oxides produced during this operation
will be reduced while the strip is passing through the furnace.
[0011] Advantageously, the strip is heated to a temperature of
between 150.degree. C. and 400.degree. C., preferably between
approximately 150.degree. C. and 300.degree. C., for the oxidation
treatment. For a given grade of steel, the oxidation of its surface
will be controlled, for a given oxidizing atmosphere, by the choice
of a pair of parameters, namely the temperature and the residence
time of the strip in the oxidizing atmosphere.
[0012] This temperature/residence time pair will be continuously
monitored and will take the operating speed of the line into
account, in particular the instantaneous run speed of the strip.
The strip oxidation treatment may be controlled by regulating the
heating power upstream of the furnace (thus varying the temperature
of the strip) or by varying the distance between the heating
element located upstream of the furnace and the inlet of the
furnace (which varies the oxidation time).
[0013] The oxidizing atmosphere in which the controlled oxidation
operation is carried out on the surface of the strip may be the
ambient air or any other confined atmosphere in a chamber which is
installed upstream of the furnace and the oxygen content of which
will be controlled.
[0014] The invention consists, apart from the arrangements
mentioned above, of a certain number of other arrangements which
will be more explicitly mentioned below with regard to illustrative
examples described in detail with reference to the appended
drawings, but which are in no way limiting:
[0015] FIG. 1 is a diagram of a continuous hot-dip galvanizing line
for implementing the process of the invention;
[0016] FIG. 2 is a graph showing the variation in temperature of a
point on the strip, plotted on the y-axis as a function of the
position of the point on the line plotted on the x-axis;
[0017] FIG. 3 is a diagram of an alternative embodiment of the
galvanizing line; and
[0018] FIGS. 4 to 6 are other alternative embodiments.
[0019] In the case of FIGS. 1 to 4, the strip is moving from the
left to the right.
[0020] Shown schematically in FIG. 1 of the drawings is a line for
the continuous hot-dip galvanizing of a steel strip 1 in a
molten-zinc galvanizing bath 2.
[0021] The line includes a galvanizing furnace 3 according to the
prior art, for treating the strip 1 before it is dipped into the
bath 2. The furnace comprises several sections equipped for
carrying out in succession the various steps of the heat treatment,
which are in general heating, soaking and then cooling down to a
temperature suitable for depositing the zinc on the surface of the
strip. The atmosphere in the furnace 3 is reducing, produced by a
conventional nitrogen/hydrogen gas mixture with a dew point
maintained as low as possible.
[0022] The steel strip 1 contains oxidizable addition elements,
such as Si, Cr, Mn and Mo, in proportions sufficient to improve its
mechanical properties. Hitherto, this type of galvanizing line has
not allowed a steel containing such oxidizable elements in such
proportions to be correctly galvanized in a continuous hot-dip
operation since, as explained above, during the high-temperature
heating and soaking treatment, a very thin layer of oxides of these
addition elements forms on the surface and remains, right in the
molten zinc, thereby causing defects in the coating.
[0023] According to the invention, the strip 1 is subjected,
upstream of the furnace 3, in a zone 8 to an oxidation treatment
under atmosphere/temperature and residence time conditions such
that the oxidizable addition elements, especially Si, Cr, Mn or Mo,
are oxidized beneath the surface of the strip before they can
migrate to this surface in order to form an oxide layer capable of
causing galvanizing defects.
[0024] Under these conditions, during the treatment in the furnace
3, the oxides of the addition elements remain trapped within the
material and there is no longer any migration of addition elements
to the surface of the strip capable of enriching the oxide layer up
to the point of causing of galvanizing defects.
[0025] Iron oxides are formed on the surface of the strip during
treatment in the zone 8 and going from the zone 8 as far as the
inlet of the furnace. These iron oxides are reduced within the
chamber of the furnace 3 in such a way that the strip 1, when it
enters the molten zinc bath 2, has a surface with a layer of
reduced oxides of the addition elements, which allows correct
galvanizing to occur.
[0026] The zone 8 includes a heating means for raising the strip 1
to the desired temperature, typically between 150.degree. C. and
400.degree. C. A control means 7 consisting of a computer is
provided in order to adjust the heating of the strip on the basis
of sensors, such as a strip speed sensor 4, a strip surface
temperature sensor 5 and a strip surface emissivity sensor 6.
[0027] The oxidation rate is controlled, for a given oxidizing
atmosphere, as a result of controlling the final temperature of the
strip 1 as it leaves the heating means 8 and the residence time of
the strip 1 in the zone 8 and between the zone 8 and the inlet of
the furnace 3. The combination of these parameters is optimized
depending on the grade of steel to be treated, the speed of the
line and the thickness and width of the strip.
[0028] The heating means 8 is chosen to have a low thermal inertia
and a high reactivity so as to maintain control of the surface
oxidation of the strip during transient phases brought about by
changes in the speed of the line or changes in geometry of the
strip 1. This heating means 8 may consist of a gas furnace, of the
naked flame or indirect heating type, but preferably his heating
means will consist of an electromagnetic induction furnace. The
induction furnace has at least one induction coil that can be moved
up to or away from the galvanizing furnace in order to vary the
heating rate produced.
[0029] The oxidation treatment of the strip 1 in the zone 8 and
between the zone 8 and the inlet of the furnace 3 will preferably
be carried out in air. The oxidation of the strip will then be
controlled by controlling two parameters, namely the temperature of
the strip leaving the zone 8 and the residence time of the strip in
air between its entry into the zone 8 and its entry into the
furnace 3. The temperature will have to be increased when the speed
of the line increases, so as to compensate for the shorter
residence time of the strip at high temperature in the air.
[0030] FIG. 2 shows the temperature variation of a point on the
strip 1 plotted on the y-axis as a function of the position of this
point on the line plotted on the x-axis. Upstream of the heating
means 8, the temperature of the strip is low, for example below
100.degree. C., and corresponds to the segment 9. As the strip 1
passes through the heating means 8, its temperature increases, for
example as per the inclined segment 10. The temperature of the
strip 1, from the point where it leaves the heating means 8 up to
the point where it enters the furnace 3 remains approximately
constant, as shown schematically by the segment 11--the oxidation
treatment continues during this phase. Within the chamber of the
furnace 3, the strip 1 will continue to be heated in a cycle
tailored to its metallurgy and shown symbolically by 12.
[0031] The oxidation of the strip may be controlled by varying one
or more of the parameters presented in FIG. 2. It is possible to
vary the temperature of the strip by varying the mean slope of the
segment 10, in order to obtain a variable temperature hold level of
the segment 11. It is also possible to vary the duration of the
temperature hold 11 or to modify the effectiveness of the strip
oxidation during the temperature hold 11, for example by varying
the concentration of oxygen in the oxidizing atmosphere to which
the strip is exposed during this temperature hold.
[0032] FIG. 3 shows a variation of FIG. 1 in which the heating zone
8 is connected in a sealed manner to the inlet of the furnace 3 by
the chamber 13. It will be understood that, within the chamber 13,
it is possible to control the oxygen concentration so as to tailor
the oxidation of the strip to the specific type of steel, to the
speed of the strip and to any other parameter necessary for
controlling the oxidation rate of the strip. The oxygen content of
the chamber 13 and the means for sealing this chamber with respect
to the outside or with respect to the chamber of the furnace 3 will
be controlled using the means of the prior art.
[0033] The duration of the oxidation treatment may be
advantageously controlled, according to the operating parameters of
the line, by modifying the length of strip 1 between the outlet of
the heating means 8 and the inlet of the furnace 3. This length
variation may be accomplished in various ways.
[0034] One possible way consists in moving the heating means 8
along the direction of the strip 1, as illustrated schematically in
FIG. 4 by the dashed arrow 14. For a given strip speed, when the
heating means 8 is brought closer to the furnace 3, the treatment
type decreases, whereas when the heating means 8 is moved further
away from the furnace the treatment time increases.
[0035] A second possible way is illustrated by FIG. 5. The heating
means 8 are stationary and, between the heating means 8 and the
furnace 3, the strip 1 passes over a fixed roll 15 and over a
moving roll 16, which can be moved parallel to the direction of the
strip as illustrated schematically by the arrow 17. When the moving
roll 16 is moved to the right, the length of strip between the
heating means 8 and the furnace 3 increases, thereby increasing the
duration of the oxidation treatment. Conversely, when the moving
roll 16 is moved to the left in FIG. 5, the length of strip
decreases, thereby reducing the treatment time. This arrangement
with a moving roll 16 and two horizontal strip strands may be
repeated several times with several rolls and several strands of
variable length, so as to increase the length of strip between 8
and 3 and to increase the possible variation in this length.
[0036] FIG. 6 shows an alternative embodiment of FIG. 5, in which
the heating means 8 are stationary and the strip 1 passes over two
fixed rolls 20 and 21 and over one moving roll 19, which can be
moved perpendicular to the main direction of the strip as
illustrated schematically by the arrow 18. When the moving roll 19
is moved upwards, the length of strip between the heating means 8
and the furnace 3 increases, thereby increasing the oxidation
treatment time. Conversely, when the moving roll 19 is moved
downwards in FIG. 6, the length of strip decreases, thereby
reducing the treatment time. This arrangement with a roll 19 and
two vertical strands may be repeated several times so as to
increase the length of strip between 8 and 3 and to increase the
possible variation in this length.
[0037] It will be understood that all the combinations of fixed
rolls and moving rolls allowing the length of strip between the
heating means 8 and the inlet of the furnace 3 to be varied make it
possible to vary the strip oxidation time and may be implemented
within the context of this invention.
[0038] It is also possible to place the rolls 15 and 17 of FIG. 5
or the rolls 19, 20 and 21 of FIG. 6 in a chamber such as 13, in
which the oxygen concentration may be controlled and adjusted to
the treatment to be obtained.
[0039] It will also be understood that it is possible to combine
controlling the temperature of the strip as it leaves the heating
means 8 with controlling the duration of oxidation according to the
characteristics of the material and to the intended objectives.
This control of the temperature and of the treatment time, and also
the operation of the corresponding actuators, is performed by the
computer 7 according to the product data input by the operator and
also the measurements carried out by the sensors, such as 4, 5, and
6 for example.
[0040] Thanks to the use of these devices, the strip 1 enters the
molten zinc bath 2 with a surface on which the formation of oxides
has been limited, including in the case of the oxides of the
addition elements, in such a way that the adhesion of the zinc to
this surface can be optimal.
[0041] The galvanizing line according to the invention constitutes
a flexible production tool allowing economic galvanizing of various
grades of steel, irrespective of the nature of their additives,
without any defect in the zinc coating on their surface. The
control means 7 and the heating means 8, owing to the speed with
which they can be adapted, allow the oxidation control process to
be adapted to products of any dimensions and to any variation in
the speed of the production line.
[0042] It may also be noted that the devices needed to implement
the method of controlling the oxidation of a strip containing
additives such as Si, Cr, Mn, Mo, etc. may be easily added to an
existing plant in order to extend its production range or, in a
plant in which they are installed, they can be readily neutralized
for the production of grades of steel not containing these
additives.
* * * * *