U.S. patent application number 09/310831 was filed with the patent office on 2001-11-01 for method and apparatu for carrying out the annealing step of a galvannealing process.
Invention is credited to BRISBERGER, ROLF.
Application Number | 20010035240 09/310831 |
Document ID | / |
Family ID | 7868091 |
Filed Date | 2001-11-01 |
United States Patent
Application |
20010035240 |
Kind Code |
A1 |
BRISBERGER, ROLF |
November 1, 2001 |
METHOD AND APPARATU FOR CARRYING OUT THE ANNEALING STEP OF A
GALVANNEALING PROCESS
Abstract
A method and an apparatus for carrying out the annealing step of
a galvannealing process in which strips and sheets, particularly of
steel, are subjected after galvanizing to an annealing step by
heating the coated material and subsequently holding at final
annealing temperature, wherein, during annealing, the heating step
is interrupted by at least one additional holding step and, thus, a
stepwise increase of the temperature over time is adjusted.
Inventors: |
BRISBERGER, ROLF; (ISSUM,
DE) |
Correspondence
Address: |
FRIEDRICH KUEFFNER
342 MADISON AVENUE
SUITE 1921
NEW YORK
NY
10173
|
Family ID: |
7868091 |
Appl. No.: |
09/310831 |
Filed: |
May 12, 1999 |
Current U.S.
Class: |
148/527 ;
148/533 |
Current CPC
Class: |
C23C 2/28 20130101 |
Class at
Publication: |
148/527 ;
148/533 |
International
Class: |
C21D 001/00; C22F
001/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 16, 1998 |
DE |
198 22 156.8 |
Claims
I claim:
1. In a method of carrying out an annealing stage of a
galvannealing process in which, after being coated by a hot
galvanizing process, strip or sheet material, particularly of
steel, is annealed by carrying out a heating step of the coated
material, wherein the coated material is subsequently held at final
end temperature, wherein the improvement comprises interrupting
during annealing the heating step with at least one additional
holding step, so that a stepwise increase of the temperature is
adjusted over time.
2. The method according to claim 1, wherein the at least one
additional holding step is carried out at a temperature below the
final annealing temperature.
3. An apparatus for carrying out an annealing stage of a
galvannealing process in which, after being coated by a hot
galvanizing process, strip or sheet material, particularly of
steel, is annealed by carrying out a heating step of the coated
material, wherein the coated material is subsequently held at final
end temperature, the apparatus comprising a zone for inductively
heating the coated material and a second zone for holding the strip
at final temperature, further comprising at least one additional
holding zone in the zone for inductively heating the material.
4. The apparatus according to claim 3, comprising a first induction
zone, a subsequent first holding zone, a subsequent second
induction zone, and a subsequent second holding zone.
5. The apparatus according to claim 4, wherein the induction zones
are each comprised of a plurality of induction coils.
6. The apparatus according to claim 4, wherein the induction zones
are each comprised of one induction coil.
7. The apparatus according to claim 3, wherein the holding zones
are resistor-heated.
8. The apparatus according to claim 3, wherein the holding zones
are gas-heated.
9. A coated fine sheet of higher-strength steel produced by
carrying out a method of carrying out an annealing stage of a
galvannealing process in which strip or sheet material,
particularly of steel, after being coated by a hot galvanizing
process, is annealed by carrying out a heating step of the coated
material, wherein the coated material is subsequently held at final
end temperature, and wherein during annealing the heating step is
interrupted with at least one additional holding step, so that a
stepwise increase of the temperature is adjusted over time.
10. A coated fine sheet of IF-steel produced by carrying out a
method of carrying out an annealing stage of a galvannealing
process in which strip or sheet material, particularly of steel,
after being coated by a hot galvanizing process, is annealed by
carrying out a heating step of the coated material, wherein the
coated material is subsequently held at final end temperature, and
wherein during annealing the heating step is interrupted with at
least one additional holding step, so that a stepwise increase of
the temperature is adjusted over time.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method and an apparatus
for carrying out the annealing step of a galvannealing process in
which strips and sheets, particularly of steel, are subjected after
galvanizing to an annealing step by heating the coated material and
subsequently holding at final annealing temperature.
[0003] 2. Description of the Related Art
[0004] When hot-galvanized sheet or strip steel is annealed after
dipping at temperatures above the melting point of zinc, the
resulting product is called galvannealed sheet or strip and the
process is called galvannealing, i.e., by combining the expressions
"galvanizing" and "annealing".
[0005] The coating of the coated strip treated in this manner is
composed only of iron/zinc compounds with about 10-12% Fe.
[0006] A conventional hot galvanizing process is carried out before
the annealing step of a galvannealing process. For this purpose,
the steel surface is cleaned first. Subsequently, a recrystallizing
annealing of the initial material which is hot from rolling is
carried out in a furnace in a protective gas atmosphere. The strip
is then cooled to galvanizing temperature and is hot galvanized by
means of an aluminum-containing zinc melt. Finally, the excessive
zinc melt is stripped off by means of air or nitrogen.
[0007] In a steel strip whose surface has been coated in this
manner the galvannealing process is completed by a subsequent
annealing step in an additional furnace.
[0008] During annealing, a diffusion-controlled process takes place
between the steel matrix and the zinc coating. In dependence on the
temperature adjusted during annealing and the annealing time,
different FeZn phases in accordance with the zinc/iron phase
diagram are formed. The respective phase components determine the
total iron content of the coating.
[0009] The phase composition adjusted during this annealing step is
a decisive factor for the quality of the coating and the usefulness
of the basic material treated by galvannealing, for example, for
the later deep-drawing process in the forging press.
[0010] In conventional plants this galvannealing furnace is
composed of two zones: first, the zone for inductively heating the
strip and, second, the subsequent zone for holding at the desired
final temperature. The holding zone is usually heated by
resistance-heated or gas-fired furnace portions.
[0011] The annealing step of the galvannealing process and, thus,
obtaining a defined base composition of the coating material are
dependent in particular on the parameters temperature and time.
These important parameters can be influenced by the plant
parameters, the strip entry temperature into the zinc melt, the
temperature of the zinc melt, the aluminum concentration in the
zinc melt as well as the thickness of the coating. The most
important variable is the basic material, i.e. the alloy
composition of the steel and its condition.
[0012] Fine sheet metal treated by galvannealing is used
predominantly in the automobile industry and is distinguished by a
good weldability and by the fact that it can be easily
varnished.
[0013] In the past, IF-steels (interstitial-free steels) have been
used almost exclusively in this field as a base material for
deep-drawn products, special-deep drawn products and extra-deep
drawn products for a galvannealing treatment.
[0014] IF-steels are those steels which in the iron lattice do not
have any interstitially dissolved atoms. The C-atoms and the
N-atoms are bound by specifically adding carbonitride formers (Ti,
Nb, V) to the alloy. IF-steels have no significant contents of
strength-increasing elements such as P, Mn or B. On the other hand,
the element Si can be added to the alloy (up to about 0.10%) for
improving the adherence of the galvannealed coating.
[0015] In order to meet the requirement for a weight reduction of
automobiles, increasingly thinner sheet steels are used which,
however, must have the same strength properties as conventional
sheet steels. This requirement can only be met by using
higher-strength steels and also higher-strength IF-steels.
Higher-strength IF-steels have significant portions of the
above-mentioned elements. When higher-strength steels are mentioned
in the following, they are intended to also include higher-strength
IF-steels, BH-steels and TRIP-steels.
[0016] However, the two steel groups IF-steels and higher-strength
steels have in relation to the applied zinc coating a significantly
different alloying behavior, particularly with respect to their
speed. The alloy formation takes place in the higher-strength
steels substantially slower than in the IF-steels.
SUMMARY OF THE INVENTION
[0017] Therefore, it is the primary object of the present invention
to propose a method and an apparatus for the annealing step of a
galvannealing process to which sheets and steels of different base
materials, particularly of higher-strength steels can be subjected
without negatively affecting the output.
[0018] In accordance with the present invention, during annealing,
the heating step is interrupted by at least one additional holding
step and, thus, a stepwise increase of the temperature over time is
adjusted.
[0019] The apparatus according to the present invention for
carrying out the annealing step of a galvannealing process includes
a zone for inductively heating the strip and an additional zone for
holding the strip at final temperature, wherein the zone for
inductively heating the strip is interrupted by at least one
additional holding zone.
[0020] The basic concept of the present invention is the adjustment
of the annealing cycle with respect to the parameters temperature
and time to the base materials, particularly higher-strength
steels, for taking into consideration the material-specific
alloying progress. The proposed process technology including the
stepwise annealing treatment provides the possibility of a
controlled adjustment of the properties between the base material
and the coating material and the coating material itself.
[0021] This annealing treatment is advantageously carried out in
such a way that the heating process with subsequent holding at
final temperature is interrupted by a second holding process at a
temperature below the final temperature.
[0022] A furnace suitable for the apparatus according to the
present invention includes a zone for inductively heating the strip
and an additional zone for holding the strip at the heating
temperature, wherein at least one additional holding zone is
provided between the zone for inductively heating the strip and the
final holding zone.
[0023] In accordance with an advantageous embodiment which is
particularly suitable for higher-strength steel strips, this
furnace is composed of four zones, namely, a first induction zone
with a subsequent first holding zone, and a second subsequent
induction zone with a subsequent second holding zone.
[0024] The proposed method for the annealing treatment carried out
step-by-step or in stages in a galvannealing process and the
proposed configuration of the annealing furnace have the following
advantages.
[0025] By carrying out the increase of the temperature in a
stepwise manner, an adjustment of the annealing treatment to the
slower diffusion processes and, thus, alloying speeds in
higher-strength steels is achieved. The alloying process can be
controlled and regulated. This makes possible a uniform product
quality under controllable production conditions. This stepwise
heating process does not exhibit any disadvantages in
IF-steels.
[0026] The annealing parameters, particularly the heating
temperature and heating speed, are adapted to the alloying sequence
of the combination steel/coating material. Consequently,
overheating in the coating material does not take place without an
alloy formation. Moreover, the possibility of an increased
evaporation of zinc is reduced. This constitutes a significant
advantage for the operation of the galvannealing furnace as well as
for the morphology of the galvannealed coating.
[0027] For avoiding an overheating of the zinc coating in
conventional galvannealing furnaces, which have only one single
heating zone composed of several or only one single induction coil,
and for adjusting a controlled alloying process, the capacity of
the induction zone would have to be lowered. In order to still be
able to reach the desired galvannealing temperature, this would
make it necessary to reduce the speed of the plant. However, this
would result in a reduction of the output of the hot galvanizing
plant.
[0028] In contrast, the method and apparatus proposed in accordance
with the present invention do not result in a decrease of the
output of the hot galvanizing plant.
[0029] The various features of novelty which characterize the
invention are pointed out with particularity in the claims annexed
to and forming a part of the disclosure. For a better understanding
of the invention, its operating advantages, specific objects
attained by its use, reference should be had to the drawing and
descriptive matter in which there are illustrated and described
preferred embodiments of the invention.
BRIEF DESCRIPTION OF THE DRAWING
[0030] In the drawing:
[0031] FIG. 1 is a schematic illustration of an embodiment of the
furnace configuration according to the present invention for
carrying out an annealing step during a galvannealing process;
[0032] FIGS. 2a and 2b are schematic illustrations of conventional
furnaces for carrying out an annealing step during a galvannealing
process; and
[0033] FIG. 3 is a diagram showing the strip temperature pattern
over time in the different embodiments of galvannealing furnaces
shown in FIG. 1 and FIGS. 2a and 2b.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0034] The configuration of the galvannealing furnace according to
the present invention with an interrupted heating zone is
schematically illustrated in FIG. 1. The galvannealing furnace 1
includes a first zone 2a for carrying out an inductive heating
step. This first zone is followed by a holding zone 3a. Following
this holding zone 3a, the strip is once again conducted through a
heating zone 2b. Subsequently, the coated strip is held at final
temperature in a second holding zone 3b.
[0035] The diagram of FIG. 3 shows in a broken line (curve c) the
stepwise heating curve resulting from the furnace configuration
according to the present invention. The speed of the plant is 90
m/min. The strip enters the furnace with an initial temperature of
420 .degree.C. and is quickly heated in a first stage to 470
.degree.C. The strip then enters the first holding zone 3a and is
held for 7 s at the intermediate temperature. Subsequently, a
second heating process to the final annealing temperature of
520.degree. C. takes place.
[0036] FIGS. 2a and 2b schematically illustrate the configurations
of conventional galvannealing furnaces. Both embodiments are
composed of a first zone 2 for inductively heating and a second
subsequent zone 3 for holding the strip at final temperature.
Conventional plants with inductive strip heating are provided in
the inductive section 2 either with several induction coils 2a, 2b,
2c, 2d, as a rule four to seven coils, as shown in FIG. 2a, or they
have only a single induction coil 2, as shown in FIG. 2b. This
single coil 2 has the same installed output as the several coils
previously used together. The difference is the substantially
smaller strip surface area in the inductor, so that the specific
output or output density is significantly increased which, in turn,
results in a higher heating rate.
[0037] The temperature/time curves of the annealing treatments
carried out in the furnace embodiments of FIGS. 2a and 2b are also
illustrated in FIG. 3. In contrast to the furnace according to the
present invention, the final annealing temperature is reached
quickly. This is advantageous for IF-steels whose full alloying
point is reached already after a short time.
[0038] In higher-strength steels the alloying point is reached only
after a longer annealing which is also due to the higher alloying
contents in the steel. In order to prevent an overheating of the
coating and to adjust a controlled alloying sequence between the
base material and the coating material and in the coating material
itself, an intermediate annealing step is carried out and heating
to final annealing temperature is carried out subsequently.
Moreover, the possibility of evaporation of zinc in the
galvannealing furnace is reduced.
[0039] While specific embodiments of the invention have been shown
and described in detail to illustrate the inventive principles, it
will be understood that the invention may be embodied otherwise
without departing from such principles.
* * * * *