[0001] The invention relates to the continuous galvanizing of steel strips especially high
strength steels with high content of elements like silicon, manganese, aluminium and
chromium and, in particular, to the facilities comprising a direct fire or non oxidising
furnace and radiant tubes furnace.
[0002] New steel grades with a very high yield point having a high elongation capacity have
been developed over the years to meet the demands of higher safety and lower weight
in the car industry. Such steels, sometimes referred to as Advanced High Strength
Steels (AHSS) comprise families of steel such as "DP" or Dual Phase steels (DP) and
or TRansformation Induced Plasticity steels (TRIP). Unfortunately these steels raise
some problems for steel manufacturers because some of their alloying elements such
as manganese, silicon, aluminium, chromium may result in a thin layer of oxides on
the steel surface during the annealing operation preceding the dipping in the galvanizing
bath. This (selective) oxidation harms the zinc "wettability" and thus the quality
of the coating. These phenomena are due to diffusion processes of the highly oxidisable
alloy components towards the strip surface where they can oxidize even in the furnaces
radiant tubes zones wherein the atmosphere is yet reducing for the iron oxides.
[0003] A solution which has been proposed is to subject the strips surface to temperatures
and atmosphere conditions fit for quickly and deeply oxidizing the alloy components
in the direct fired (DFF) part of the annealing furnace, thereby avoiding later migration
of the oxidisable elements towards the surface followed by reducing the iron oxide
back to iron in the radiant tube section (RTF). For this oxidation to take place it
is necessary that direct fired furnace zones are used.
[0004] However, the galvanizing furnaces do not comprise all the required DFF zones to easily
perform the oxidizing and many are only using radiant tubes. Now these furnaces, despite
their controlled atmosphere, do not prevent the selective oxidizing of the alloy components.
Patent
WO 2005/017214 recommends two possibilities to solve the problem. The first one consists in using
a direct flame combustion chamber separated from the RTF annealing furnace and from
which the burnt gasses are collected in order to inject them in the furnace. This
method requires adjusting the air to combustible gas ratio to provide excess oxygen
after combustion which is then subsequently used for oxidation of the steel strip
surface. The second one consists in setting up a direct flame burner in a section
of the furnace enclosure. In both cases, the burnt gasses supply the necessary oxidizing
atmosphere. The subsequent reduction of the oxides is then commonly obtained by going
passing the strip through a nitrogen and hydrogen mixture. These two possibilities
require a modification of the existing facilities.
[0005] The object of this invention is to provide an improved method of avoiding selective
oxidation of alloying elements in AHSS in a direct fire or non oxidising furnace and
radiant tube fired continuous hot dip galvanizing line.
[0006] One or more of these objects are reached by a method of galvanizing a steel strip
in a continuous hot dip galvanizing line comprising a direct fired furnace or non
oxidising furnace section and a subsequent radiant tube furnace section, the method
comprising injecting a gas mixture of nitrogen and air or a gas mixture of nitrogen
and oxygen into the galvanizing furnace by a nozzle system to cause one or both of
the steel strip surfaces to oxidise in a controlled manner 1. in the direct fired
furnace section or non oxidising furnace, wherein the direct fired furnace section
is operated in a substantially non-oxidising manner or in the connection chamber between
the direct fired furnace section or non-oxidising section and radiant tube section
and 2. before the radiant tube section, the method further comprising at least partly
reducing the oxide back to iron in the radiant tube furnace section and the method
further comprising hot dip galvanizing the steel strip in the hot dip galvanizing
line, wherein the gas mixture comprises an oxygen content of 0.5 to 10%.
[0007] The invention consists in projecting an oxidizing medium consisting of a gas mixture
of nitrogen and oxygen or a gas mixture of nitrogen and air onto one or both of the
surfaces of the uncoated strip exiting the non-oxidising or direct fired furnace section.
In the prior art the air to combustible gas ratio had to be adjusted in the DFF-section
which leads to a compromise between the conditions in the DFF section and the subsequent
oxidation of the strip. This leads to control problems and stability problems, leading
in turn to bad oxide homogeneity across the strip width caused by the burner pattern.
By using a separate nozzle system dedicated to injecting either 1). a gas mixture
of nitrogen and air or 2). a gas mixture of nitrogen and oxygen so that a controlled
oxygen content is injected into the furnace to cause the oxidation of the hot steel
strip to take place in a controlled manner and to prevent selective oxidizing of the
steel alloy components. To that end the nozzles are designed such as to distribute
the gas mixture evenly thereby causing the hot steel strip surface to oxidise evenly
and reproducibly.
[0008] The gas mixture must have an oxygen content such that the steel surface can be oxidised
in a controlled way and a controlled oxide thickness. This allows alloy components
like silicon, manganese, aluminium and chromium to be oxidized and to not have the
possibility to migrate towards the surface anymore during further annealing. To reach
this goal, the inventors found that the mixture had to be 1). a mixture of nitrogen
and air or 2). a mixture of nitrogen and oxygen. In the first case there is additional
nitrogen from the air as well as smaller amounts of other gases present in air. These
smaller amounts do not affect the oxidation process. In the second case residual amounts
of other gases may be present as a result of their presence in the oxygen and nitrogen
to be mixed. These residual amounts also do not affect the oxidation process. It was
found that the gas mixture resulting from the combustion of an overstoichiometric
air or of an oxygen enriched air or of an oxygen/fuel in a burner like those proposed
in the prior art was inadequate to provide a controlled oxidation.
[0009] The inventors found that the oxygen content of the gas mixture needed to be between
0.5 and 10% in volume. At oxygen values above 10% the oxide layer did not have the
desired composition and the growth rate of the oxide layer is too high, resulting
in thick oxide layers. At oxygen values below 0.5% the oxidation process was too slow
and the oxide layer remained too thin.
[0010] In an embodiment a spray bar with specifically designed nozzle is mounted, preferably
in the DFF or in the connection chamber between the DFF and RTF, on one or on both
sides of the strip which sprays a jet of oxidising medium onto the strip surface so
that the strip surface is evenly oxidised. An illustration of the system is provided
in a schematic drawing in figure 1.
[0011] In preferred embodiment the maximum oxygen content is 7%. This results in an oxide
layer of consistent composition, thickness and homogeneity to provide a good quality
galvanised coating. In a preferable embodiment the gas mixture comprises an oxygen
content of between 2 to 4.5%.
[0012] In an embodiment a method is provided wherein the control of the oxidation of the
steel strip surface or surfaces is based on the measurement of the oxygen content
in the mixture. This measurement can be performed by oxygen transducers set up a fixed
way and running in closed loop with the flow control valves regulating the flow rate
of the mixture injected by the nozzles. This results in an oxide layer of consistent
composition, thickness and homogeneity to provide a good quality galvanised coating.
[0013] In an embodiment the oxidation of the steel strip surface or surfaces takes place
between 650°C and 900°C. In a preferred embodiment the oxidation of the steel strip
surface or surfaces takes place at a temperature of at most 800°C and more preferably
of at most 750°C.
[0014] By means of non-limiting examples commercial trials were performed on 1 and 1.5 mm
thick and 1200 mm wide coiled strip material of the DP600 and DP800 type. The dew
point during the trials was between -30 and -24°C. The oxygen content of the gas mixture
was varied between 2.26 to 3.61%. The results in terms of strip wettability and coating
adhesion after the annealing and coating were excellent.
[0015] In figure 2 a schematic indication is given where the oxidation of the steel substrate,
for instance using the system in figure 1, is performed in a furnace comprising a
direct fired furnace and a radiant tube furnace.
1. Method of galvanizing a steel strip in a continuous hot dip galvanizing line comprising
a direct fired furnace section or non oxidising furnace and a subsequent radiant tube
furnace section, the method comprising injecting a gas mixture of nitrogen and air
or a gas mixture of nitrogen and oxygen into the galvanizing furnace by a nozzle system
to cause one or both of the steel strip surfaces to oxidise in a controlled manner
1. in the direct fired furnace section or non oxidising furnace, wherein the direct
fired furnace section is operated in a substantially non-oxidising manner or in the
connection chamber between the direct fired furnace section or non-oxidising section
and radiant tube section and 2. before the radiant tube section, the method further
comprising at least partly reducing the oxide back to iron in the radiant tube furnace
section and the method further comprising hot dip galvanizing the steel strip in the
hot dip galvanizing line, wherein the gas mixture comprises an oxygen content of 0.5
to 10%.
2. Method according to claim 1 wherein the gas mixture comprises an oxygen content of
at most 7%.
3. Method according to claim 1 wherein the gas mixture comprises an oxygen content of
between 2 to 4.5%.
4. Method according to any one of the preceding claims wherein the oxidation of the steel
strip surface or surfaces takes place between 650°C and 900°C.
5. Method according to any one of the preceding claims wherein the oxidation of the steel
strip surface or surfaces takes place at a temperature of at most 750°C.
6. Method according to any one of the preceding claims wherein the control of the oxidation
of the steel strip surface or surfaces is based on the measurement of the oxygen content
of the gas mixture.
REFERENCES CITED IN THE DESCRIPTION
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It does not form part of the European patent document. Even though great care has
been taken in compiling the references, errors or omissions cannot be excluded and
the EPO disclaims all liability in this regard.
Patent documents cited in the description