Note: Descriptions are shown in the official language in which they were submitted.
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Process for manufacturing a gaivanneated steel sheet by DFF
regulation
The present invention relates to a process for manufacturing a hot-dip
galvannealed steel sheet having a TRIP microstructure.
To meet the requirement of lightening power-driven ground vehicle
structures, it is known to use TRIP steels (the term TRIP standing for
io transformation-induced plasticity), which combine very high mechanical
strength
with the possibility of very high levels of deformation. TRIP steels have a
microstructure comprising ferrite, residual austenite and optionally
martensite
and/or bainite, which allows them to achieve tensile strength from 600 to 1000
MPa. This type of steel is widely used for production of energy-absorbing
parts,
ls such as for example structural and safety parts such as longitudinal
members
and reinforcements.
Before the delivery to car-makers, steel sheets are coated with a zinc-
based coating generally performed by hot-dip galvanizing, in order to increase
the resistance to corrosion. After leaving the zinc bath, galvanized steel
sheets
2o are often submitted to an annealing which promotes the alloying of the zinc
coating with the iron of the steel (so-called galvannealing). This kind of
coating
made of a zinc-iron alloy offers a better weldability than a zinc coating.
Most of TRIP steel sheets are obtained by adding a large amount of
silicon to steel. Silicon stabilizes the ferrite and the austenite at room
25 temperature, and prevents residual austenite from decomposing to form
carbide. However, TRIP steel sheets containing more than 0.2% by weight of
silicon, are galvanized with difficulty, because silicon oxides are formed on
the
surface of the steel sheet during the annealing taking place just before the
coating. These silicon oxides show a poor wettability toward the molten zinc,
so and deteriorate the plating performance of the steel sheet.
To solve this problem, it is known to use TRIP steel having low silicon
content (less than 0.2% by weight). However, this has a major drawback: a high
level of tensile strength, that is to say about 800 MPa, can be achieved only
if
C NFIRMATI N COPY
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the content of carbon is increased. But, this has the effect to lower the
mechanical resistance of the welded points.
On the other hand, alloying speed during the gaivannealing process is
strongly slowed down whatever the TRIP steel composition because of
external selective oxidation acting as a diffusion barrier to iron, and the
temperature of the gaivannealing has to be increased. The increase of the
temperature of the galvannealing is detrimental to the preservation of the
TRIP
effect because of the decomposition of the residual austenite at high
temperature. In order to preserve the TRIP effect, a large quantity of
molybdenum (more than 0.15 % by weight) has to be added to the steel, so
that the precipitation of carbide can be delayed. However, this has an effect
on
the cost of the steel sheet.
Indeed, the TRIP effect is observed when the TRIP steel sheet is being
deformed, as the residual austenite is transformed into martensite under the
effect of the deformation, and the strength of the TRIP steel sheet increases.
The purpose of the present invention is therefore to remedy the
aforementioned drawbacks and to propose a process for hot-dip galvannealing
a steel sheet having a high silicon content (more than 0.5% by weight) and a
TRIP microstructure showing high mechanical characteristics, that guarantees a
good wettability of the surface steel sheet and no non-coated portions, and
thus
guarantees a good adhesion and a nice surface appearance of the zinc alloy
coating on the steel sheet, and that preserves the TRIP effect.
The first subject of the invention is a process for manufacturing a hot-dip
galvannealed steel sheet having a TRIP microstructure comprising ferrite,
residual austenite and optionally martensite and/or bainite, said process
comprising the steps consisting in:
- providing a steel sheet whose composition comprises, by weight:
0.01 :5 C5 0.22%
0.50:5 Mn<2.0%
0.5<Si<2.0%
0.005 < AI <_ 2.0%
Mo<0.01%
Cr _< 1.0%
P<0.02%
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Ti<_0.20%
V <_ 0.40%
Ni_1.0%
Nb<_0.20%,
the balance of the composition being iron and unavoidable impurities
resulting from the smelting,
- oxidizing said steel sheet in order to form a layer of iron oxide on the
surface of the steel sheet, and to form an internal oxide of at least
one type of oxide selected from the group consisting of Si oxide, Mn
oxide, Al oxide, complex oxide comprising Si and Mn, complex oxide
comprising Si and Al, complex oxide comprising Al and Mn, and
complex oxide comprising Si, Mn and Al is formed,
- reducing said oxidized steel sheet in order to reduce the layer of iron
oxide,
- hot-dip galvanising said reduced steel sheet to form a zinc-based
coated steel sheet, and
- subjecting said zinc-based coated steel sheet to an alloying treatment
to form a galvannealed steel sheet.
In order to obtain the hot-dip gaivannealed steel sheet having a TRIP
microstructure according to the invention, a steel sheet comprising the
following
elements is provided:
- Carbon with a content between 0.01 and 0.22% by weight. This
element is essential for obtaining good mechanical properties, but it
must not be present in too large amount in order not to tear the
weldability. To encourage hardenability and to obtain a sufficient yield
strength Re, and also to form stabilized residual austenite the carbon
content must not be less than 0.01% by weight. A bainitic
transformation takes place from an austenitic microstructure formed
at high temperature, and ferrite/bainite lamellae are formed. Owing to
the very low solubility of carbon in ferrite compared with austenite,
the carbon of the austenite is rejected between the Iamellae. Owing
to silicon and manganese, there is very little precipitation of carbide.
Thus, the interlamellar austenite is progressively enriched with
carbon without any carbides being precipitated. This enrichment is
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such that the austenite is stabilized, that is to say the martensitic
transformation of this austenite does not take place upon cooling
down to room temperature.
- Manganese with a content between 0.50 and 2.0% by weight.
Manganese promotes hardenability, making it possible to achieve a
high yield strength Re. Manganese promotes the formation of
austenite, contributes to reducing the martensitic transformation start
temperature Ms and to stabilizing the austenite. However, it is
necessary to avoid the steel having too high a manganese content in
order to prevent segregation, which may be demonstrated during
heat treatment of the steel sheet. Furthermore, an excessive addition
of manganese causes the formation of a thick internal manganese
oxide layer which causes brittleness, and the adhesion of the zinc
based coating will not be sufficient.
- Silicon with a content of more than 0.5% by weight, preferably more
than 0.6% by weight, and less or equal to 2.0% by weight. Silicon
improves the yield strength Re of the steel. This element stabilizes
the ferrite and the residual austenite at room temperature. Silicon
inhibits the precipitation of cementite upon cooling from austenite,
considerably retarding the growth of carbides. This stems from the
fact that the solubility of silicon in cementite is very low and the fact
that silicon increases the activity of the carbon in austenite. Thus, any
cementite nucleus that forms will be surrounded by a silicon-rich
austenitic region, and rejected to the precipitate-matrix interface. This
silicon-enriched austenite is also richer in carbon, and the growth of
the cementite is slowed down because of the reduced diffusion
resulting from the reduced carbon activity gradient between the
cementite and the neighbouring austenitic region. This addition of
silicon therefore contributes to stabilizing an amount of residual
austenite sufficient to obtain a TRIP effect. During the annealing step
to improve the wettability of the steel sheet, internal silicon oxides
and complex oxide comprising silicon and/or manganese and/or
aluminium are formed and dispersed under the surface of the sheet.
However, an excessive addition of silicon causes the formation of a
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thick internal silicon oxide layer and possibly complex oxide
comprising silicon and/or manganese and/or aluminium which causes
brittleness and the adhesion of the zinc based coating will not be
sufficient.
5 - Aluminium with a content between 0.005 and 2.0% by weight. Like
the silicon, aluminium stabilizes ferrite and increases the formation of
ferrite as the steel sheet cools down. It is not very soluble in
cementite and can be used in this regard to avoid the precipitation of
cementite when holding the steel at a bainitic transformation
temperature and to stabilize the residual austenite. A minimum
amount of aluminium is required in order to deoxidize the steel.
- Molybdenum with a content less than 0.01% by weight, and
preferably not exceeding 0.006% by weight. Conventional process
requires the addition of Mo to prevent carbide precipitation during re-
heating after galvanizing. Here, thanks to the internal oxidation of
silicon, manganese and aluminium, the alloying treatment of the
galvanized steel sheet can be performed at a lower temperature than
that of conventional galvanized steel sheet comprising no internal
oxide. Consequently, the content of molybdenum can be reduced and
be less than 0.01% by weight, because it is not necessary to delay
the bainitic transformation as it is the case during the alloying
treatment of conventional galvanized steel sheet.
- Chromium with a content not exceeding 1.0% by weight. The
chromium content must be limited in order to avoid surface
appearance problems when galvanizing the steel
- Phosphorus with a content not exceeding 0.02% by weight, and
preferably less than 0.010% by weight. Phosphorus in combination
with silicon increases the stability of the residual austenite by
suppressing the precipitation of carbides.
- Titanium with a content not exceeding 0.20% by weight. Titanium
improves the yield strength of Re, however its content must be limited
to 0.20% by weight in order to avoid degrading the toughness.
- Vanadium with a content not exceeding 0.40% by weight. Vanadium
improves the yield strength of R. by grain refinement, and improves
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the weldability of the steel. However, above 0.40% by weight, the
toughness of the steel is degraded and there is a risk of cracks
appearing in the weld zones.
- Nickel with a content not exceeding 1.0% by weight. Nickel increases
the yield strength of Re. Its content is generally limited to 1.0% by
weight because of its high cost.
- Niobium with a content not exceeding 0.20% by weight. Niobium
promotes the precipitation of carbonitrides, thereby increasing the
yield strength of Re. However, above 0.20% by weight, the weldability
and the hot formability are degraded.
The balance of the composition consists of iron and other elements that
are usually expected to be found and impurities resulting from the smelting of
the steel, in proportions that have no influence on the desired properties.
The steel sheet having the above composition is first subjected to an
oxidation followed by a reduction, before being hot-dip galvanized in a bath
of
molten zinc and heat-treated to form said gaivannealed steel sheet.
The aim is to form an oxidized steel sheet having an outer layer of iron
oxide with a controlled thickness which will protect the steel from the
selective
outer oxidation of silicon, manganese and aluminium, while the steel sheet is
annealed before the hot-dip galvanization.
Said oxidation of the steel sheet is performed under conditions that allow
the formation, on the surface of the steel sheet, of a layer of iron oxide
containing no superficial oxides selected from the group consisting of silicon
oxide, manganese oxide, aluminium oxide, complex oxide comprising silicon
and/or manganese and/or aluminium. During this step, internal selective
oxidation of silicon, manganese and aluminium will develop under the iron
oxide
layer, and leads to a deep depletion zone in metallic silicon, manganese and
aluminium which will minimize the risk of superficial selective oxidation when
the further reduction will be achieved. A layer of an internal oxide of at
least one
type of oxide selected from the group consisting of silicon oxide, manganese
oxide, aluminium oxide, complex oxide comprising Si and Mn, complex oxide
comprising Si and Al, complex oxide comprising Mn and Al and complex oxide
comprising Si, Mn and Al is thus formed.
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The oxidation is preferably performed by heating said steel sheet from
ambient temperature to a heating temperature T1 which is between 680 and
800 C, in a direct flame furnace where the atmosphere comprises air and fuel,
with a ratio air-to-fuel preferably between 1 and 1.2.
When the,temperature T1 is above 800 C, the iron oxide layer formed on
the surface of the steel sheet will contain manganese coming from the steel,
and the wettability will be impaired. If the temperature T1 is below 680 C,
the
internal oxidation of silicon, manganese and aluminium will not be favoured,
and
the galvanizability of the steel sheet will be insufficient.
An atmosphere having a ratio air-to-fuel less than 1 leads to the
formation of superficial oxidation of silicon, manganese and aluminium, and
thus a superficial layer of oxides selected from the group consisting of
silicon
oxide, manganese oxide, aluminium oxide, and complex oxide comprising
silicon and/or manganese and/or aluminium, possibly in combination with iron
oxide is formed, and the wettability is impaired. However, with a ratio air-to-
fuel
above 1.2, the layer of iron oxide is too thick, and will not be completely
reduced. Thus, the wettability will also be impaired.
When leaving the direct flame furnace, the oxidized steel sheet is
reduced in conditions permitting the achievement of the complete reduction of
the iron oxide into iron. This reduction step can be performed in a radiant
tube
furnace or in a resistance furnace. Said oxidized steel sheet is thus heat
treated
in an atmosphere comprising preferably more than 15% by volume of hydrogen,
the balance being nitrogen and unavoidable impurities. Indeed, if the content
of
hydrogen in the atmosphere is less than 15% by volume, the layer of iron oxide
can be insufficiently reduced and the wettability is impaired.
Said oxidized steel sheet is heated from the heating temperature TI to a
soaking temperature T2, then it is soaked at said soaking temperature T2 for a
soaking time t2, and is finally cooled from said soaking temperature T2 to a
cooling temperature T3.
Said soaking temperature T2 is preferably between 770 and 850 C.
When the steel sheet is at the temperature T2, a dual phase microstructure
composed of ferrite and austenite is formed. When T2 is above 850 C, the
volume ratio of austenite grows too much, and external selective oxidation
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occurs on the steel surface. But when T2 is below 770 C, the time required to
form a sufficient volume ratio of austenite is too high.
In order to obtain the desired TRIP effect, sufficient austenite must be
formed during the soaking step, so that sufficient residual austenite is
s maintained during the cooling step. The soaking is performed for a time t2,
which is preferably between 20 and 180s. If the time t2 is longer than 180s,
the
austenite grains coarsen and the yield strength Re of the steel after forming
will
be limited. Furthermore, the hardenability of the steel is low. However, if
the
steel sheet is soaked for a time t2 less than 20s, the proportion of austenite
io formed will be insufficient and sufficient residual austenite and bainite
will not
form when cooling.
The reduced steel sheet is finally cooled at a cooling temperature T3
near the temperature of the bath of molten zinc, in order to avoid the cooling
or
15 the re-heating of said bath. T3 is thus preferably between 460 and 510 C.
Therefore, a zinc-based coating having a homogenous microstructure can be
obtained.
When the steel sheet is cooled, it is hot dipped in the bath of molten zinc
whose temperature is preferably between 450 and 500 C. This bath can contain
20 0.08 to 0.135% by weight of dissolved aluminium, the balance being zinc and
unavoidable impurities. Aluminium is added in the bath in order to deoxidize
the
molten zinc, and to make it easier to control the thickness of the zinc-based
coating. In that condition, precipitation of delta phase (FeZn7) is induced at
the
interface of the steel and of the zinc-based coating.
25 When leaving the bath, the steel sheet is wiped by projection of a gas, in
order to adjust the thickness of the zinc-based coating. This thickness, which
is
generally between 3 and 10 pm, is determined according to the required
resistance to corrosion.
The hot-dip galvanized steel sheet is finally heat-treated so that a coating
30 made of a zinc-iron alloy is obtained, by diffusion of the iron from steel
to the
zinc of the coating. This alloying treatment can be performed by maintaining
said steel sheet at a temperature T4 between 460 and 510 C for a soaking time
t4 between 10 and 30s. Thanks to the absence of external selective oxidation
of
silicon, manganese and aluminium, this temperature T4 is lower than the
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conventional alloying temperatures. For that reason, large quantities of
molybdenum to the steel are not required, and the content of molybdenum in
the steel can be limited to less than 0.01% by weight. If the temperature T4
is
below 460 C, the alloying of iron and zinc is not possible. If the temperature
T4
is above 510 C, it becomes difficult to form stable austenite, because of the
unwished carbide precipitation, and the TRIP effect cannot be obtained. The
time t4 is adjusted so that the average iron content in the alloy is between 8
and
12% by weight, which is a good compromise for improving the weldability of the
coating and limiting the powdering while shaping.
The invention will now be illustrated by examples given by way of non-
limiting indication and with reference to figures 1 and 2.
Trial was carried out using samples A and B coming from 0.8 mm thick
sheet manufactured from a steel sheet whose composition is given in table I.
Samples A and B are pre-heated from ambient temperature (20 C) to
750 C, in a direct flame furnace. They are subsequently and continuously
annealed in a radiant tube furnace, where they are heated from 750 to 800 C,
then they are soaked at 800 C for 60 s, and finally they are cooled to 460 C.
2o The atmosphere in the radiant tube furnace comprises 30% by volume of
hydrogen, the balance being nitrogen and unavoidable impurities.
After cooling, samples A and B are hot dip galvanized in a molten zinc-
based bath comprising 0.12% by weight of aluminium, the balance being zinc
and unavoidable impurities. The temperature of said bath is 460 C. After
wiping
with nitrogen and cooling the zinc-based coating, the thickness of the zinc-
based coating is 7 pm.
First, the aim is to compare the wettablilty and the adherence of
these samples, when the air-to-fuel ratio in the direct flame furnace
fluctuates.
The air-to-fuel ratio is 0.90 for sample A, and 1.05 according to the
invention for
sample B. The results are shown in table II.
The wettability is visually controlled by an operator. The adherence of the
coating is also visually controlled after a 180 bending test of samples.
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Table I: chemical composition of the steel of samples A and B, in % by
weight, the balance of the composition being iron and unavoidable impurities
(sample A and B).
Table I
C Mn Si Al Mo Cr P Ti V Ni Nb
0.20 1.73 1.73 0.01 0.005 0.02 0.01 0.005 0.005 0.01 0.005
5
Table II
Weitabilty Adherence Aspect of the
surface
Sample A** Bad Bad Bad
Sample B* Good Good Good
* according to the invention
** according to the conventional process
10 Figure 1 is a photography of sample A after the pre-heating step and
before the annealing step, and figure 2 is a photography of sample B after the
pre-heating step and before the annealing step.
Then, the aim is to show the effect of the internal selective oxidation of
Is silicon and manganese on the temperature of alloying. Thus, the temperature
of
alloying treatment applied to sample B in order to obtain a gaivannealed steel
sheet according to the invention is compared with the temperature of alloying
of
sample A.
Sample B which has been hot dip galvanized is then subjected to an
2o alloying treatment by heating it to 480 C, and by maintaining it at this
temperature for 19 s . The inventors have checked that the TRIP microstructure
of the obtained hot dip galvannealed steel sheet according to the invention
was
not lost by this alloying treatment.
In order to obtain the alloying of the zinc-based coating of sample A, it is
25 necessary to heat it to 540 C, and to maintain it at this temperature for
20 s
With such a treatment, the inventors have checked that carbide precipitation
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occurs, residual austenite is no more kept during cooling down to room
temperature and that the TRIP effect has disappeared.
