Note: Descriptions are shown in the official language in which they were submitted.
1
METHOD FOR PRODUCING A ZINC-COATED STEEL COMPONENT FOR A VEHICLE
The invention relates to a method of producing a component for a vehicle,
comprising the
following steps:
providing a workpiece composed of a heat-treatable steel material provided
with a zinc-
containing coating on both sides,
at least partly heating the workpiece to a temperature above Ad.,
inserting the at least partly heated workpiece into a hot-forming and/or press-
hardening
mold comprising at least one punch and at least one die,
closing the mold by relative movement of the punch and/or the die toward one
another and
hot-forming and/or press-hardening the workpiece, with cooling of at least a
region of the
heated workpiece in the closed mold in such a way that there is at least
partial formation of a
hardened microstructure.
The invention further relates to a component, especially produced by the
method of the
invention, and to a corresponding use of the component.
Modern automobile construction without press-hardened components is hard to
imagine.
Conventional steel components have been replaced by new high- and higher-
strength steel
materials, and the increase in strength or the high strength of the material
has enabled a
reduction in the material thickness with the same mechanical properties, such
that a positive
effect on a reduction in the total weight of the vehicle is possible and hence
an associated
reduction in the CO2 output is also possible. The steel material used is a
heat-treatable steel,
for example a manganese-boron steel, the most common representative at present
being
22 MnB5. Press-hardened components are generally produced from coated formed
blanks or
from coated blanks which are first cold-formed to give preformed semifinished
products. The
coating may be organic in nature, but inorganic coatings based on aluminum or
aluminum/silicon and based on zinc have become established in practice. In the
case of
coatings based on aluminum or aluminum/silicon, during the heating process
(austenitization), diffusion of iron out of the base material (22MnB5) into
the coating takes
place and an AlSi-Fe layer is formed, which has what is called a barrier
effect with regard to
the anticorrosion properties. There is no active cathodic corrosion protection
in the case of
an aluminum-based
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coating. However, active cathodic corrosion protection is provided by zinc-
based
coatings. Diffusion of iron out of the base material (22MnB5) into the coating
does again
take place during the heating process (austenitization). The diffusion does
raise the
melting point of the iron-enriched zinc coating, but formation of liquid zinc
phases (also
known as "liquid embrittlement") cannot be completely suppressed. Liquid
embrittlement, in the course of subsequent hot forming and/or press hardening,
according to the degree of forming that results from the forming at the
surface of the
coating, results in formation of cracks which can propagate further in the
direction of
the base material owing to the material stress (compression-tension) and
extend into
the base material. The propagation of the cracks is due to the presence of
liquid zinc
phases at the particle boundaries of the material, which weaken the material
and
promote propagation of cracks in the coating by virtue of the
compressive/tensile stress
down to the base material. Press-hardened components of this kind that are
afflicted by
cracking can cause a strength reduction in the component in the event of a
crash and
shortening of the expected lifetime, especially under cyclical stress.
The phenomenon of crack formation in the hot forming of zinc-coated blanks is
disclosed, for example, in the following publication: Drillet et al. "Study of
cracks
propagation inside the steel on press hardened steel zinc based coatings", La
Metallurgia
Italiana - n. 1/2012, pages 3-8). Studies were conducted on an omega-shaped
profile
cross section which was to be hot-formed and press-hardened. It was found that
cracks
promoted by pockets of liquid zinc present in the material as a result of
heating,
particularly in the region of the frame to be produced (critical region), can
arise at first
owing to compressive stress and/or subsequent tensile stress on the side
facing the die
and can extend into the base material. The more complex the degree of forming,
especially in the frame region, the higher the propensity to stress cracking
and,
associated with this, the deeper the crack that can be formed in the base
material.
It is an object of the present invention, proceeding from the prior art, to
specify a
method of producing a component for a vehicle, a component for a vehicle and a
use of
the component, wherein not only active corrosion protection but also
sufficient
component strength in the event of a crash and expected lifetime, especially
under
cyclical stress, can be assured.
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The object is achieved in a method of the invention in that the workpiece has
a first
surface having a smaller coating thickness of the zinc-containing coating
compared to
the second surface of the workpiece, wherein the workpiece is inserted into
the hot-
forming and/or press-hardening mold in such a way that the first surface of
the
workpiece is positioned on the side predominantly subjected to compression
and/or
tension in the component manufacture, especially the predominantly concave
mold side.
The method of the invention for production of a component for a vehicle
firstly
comprises the step of providing a workpiece composed of a heat-treatable steel
material
provided with a zinc-containing coating on both sides. Heat-treatable steels
used are
essentially manganese-boron steels. It is also conceivable to use other steel
qualities in
which a higher strength can be generated as a result of a heat treatment and
by
comparison with the condition as supplied. A further step comprises at least
partial
heating of the workpiece to a temperature above An, especially above Ac3. The
workpiece is first heated partially or completely to austenitization
temperature, and
(partially) different microstructures or a homogeneous microstructure
throughout can
be established in the workpiece according to the requirement on the component
to be
produced and the end use thereof in the motor vehicle. This can be effected by
means of
appropriate furnaces and/or by means of appropriate hot-forming and/or press-
hardening tools. If different microstructures in the workpiece have to be
taken into
account, this is referred to as "tailored tempering", meaning that at least
one region
having a hard microstructure and at least one region which has a soft
microstructure
and is more ductile compared to the hard microstructure are established. A
further step
comprises the insertion of the at least partly heated workpiece into a hot-
forming
and/or press-hardening mold comprising at least one punch and at least one
die. At least
one side of the mold is predominantly concave, preferably the die, and at
least one side
of the mold is predominantly convex, preferably the punch. The closure of the
mold by
relative movement of the punch and/or the die toward one another and hot-
forming
and/or press-hardening of the workpiece comprises a further step, with cooling
of at
least one region of the heated workpiece in the closed mold in such a way that
there is at
least partial formation of a hardened microstructure. The workpiece is
completely or at
least partially hardened, which is effected by rapid cooling in a mold,
especially an
actively cooled mold, in the course of hot forming and press-hardening (direct
hot
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forming) or in the course of press-hardening (indirect hot forming], with
conversion of
the microstructure of the at least partly austenitic region of the workpiece
by abrupt
cooling to a martensitic and/or bainitic microstructure, particular preference
being
given to the achievement of a martensitic microstructure.
The inventors have found that, surprisingly, the reduction in the coating
thickness of the
zinc-containing coating on the first surface of the workpiece, which is
positioned on the
side predominantly subjected to compression and/or tension in the component
manufacture, especially the predominantly concave mold side, preferably
contacted with
the die of the mold, can reduce the cracks or crack depth in the critical
region compared
to coating thicknesses that are customary in practice to a degree which
satisfies the
demands on component strength in the event of a crash and the expected
lifetime,
especially under cyclical stresses. Crack formation cannot be entirely avoided
owing to
the aforementioned phenomenon. The lowering of the coating thickness also
reduces the
zinc supply, and hence a lower level of liquid zinc phases, which weaken the
material,
arises in the material during the heating process.
In a first configuration of the method of the invention, the workpiece, prior
to the
provision thereof, is separated from a steel material in strip form which has
been
provided with a zinc-containing coating applied electrolytically or by hot dip
coating,
which is especially applied in a continuous coating process. Continuous
coating
processes are firstly economically viable and, secondly, the required coating
thicknesses
can be established in a controlled manner. The way in which the application of
the
different coating thicknesses (differential coating) on the surfaces of the
steel material in
strip form is conducted is known in the art.
Preferably, the steel material in strip form, after the application of the
zinc-containing
coating, is subjected to a heat treatment at a temperature especially between
200 C and
Act preferably between 350 C and An, for a period of time between 5 and 300 s,
preferably between 20 and 240 s. The heat treatment (galvannealing) conducted
additionally prior to the hot forming and/or press hardening enriches the
coating with
iron in a controlled manner, which raises the melting point of the zinc-
containing
coating and can reduce the formation of liquid zinc phases during the
austenitization in
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the material. The heat treatment is preferably effected continuously,
preferably inline
after the coating process. The heat treatment can also alternatively be
conducted on a
steel material in strip form that has been wound up to form a coil, which, for
example, is
batch-annealed, in which case the heat treatment time may be several minutes
to several
hours and the temperature range is within the aforementioned order of
magnitude.
In a preferred configuration of the process of the invention, a workpiece
having a first
surface having a coating thickness of < 4 gm, especially < 3.5 gm, more
preferably
<3 gm, and a second surface having a coating thickness > 4 gm, especially >
4.5 gm,
more preferably? 5 gm, of the zinc-containing coating is used, in each case in
the as yet
non-press-hardened condition (condition as supplied). A reduction in the
cracks or the
crack depth, especially in the critical region, is perceptible essentially
when the coating
thickness is <4 gm on the first surface of the workpiece. In order to assure
sufficient
cathodic corrosion protection, the coating thickness on the first surface
should be
1 gm, especially 1.5 gm, more preferably 2 gm. The coating thickness on the
second
surface is 5 25 gm, especially 5. 20 gm, more preferably 15 gm, in order to
keep the
diffusion pathway for iron enrichment in the applied layer short.
In a further configuration of the method of the invention, the workpiece,
after being
heated, is inserted as an essentially flat workpiece into a hot-forming and
press-
hardening mold (direct hot forming), or as an already cold-formed workpiece of
near
finished geometry into a press-hardening mold (indirect hot forming). Indirect
hot
forming offers the advantage that there are no liquid zinc phases in the
material, and the
cold (pre)forming to give a workpiece of near finished geometry results in
barely any
cracks or any significant propagation of cracks into the base material through
the stress
on the material, especially in the critical region during the cold forming.
After the
austenitization, quenching and calibration are effected in the press-hardening
mold,
which may include a low degree of forming. The disadvantage is that an
additional
method step, namely the preforming of the workpiece, is required, and
additional
investment in plant is also associated therewith. Particular preference is
given to using
direct hot forming. A workpiece means either a flat steel sheet or a cold,
preformed steel
part that has yet to be hardened.
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In an alternative configuration of the method of the invention, the workpiece
is
subjected to hot forming in a first mold and at least to partial press
hardening in a
second mold. The division of the "hot forming" and "press hardening" processes
between two molds can advantageously increase the cycle time, with an
associated
enhancement in economic viability. However, the division of the process into
two means
that it is necessary to ensure that the temperature does not go below the Ms
temperature (martensite start temperature) on insertion of the already hot-
formed
workpiece into the press-hardening mold. Preferably, the temperature on
insertion is at
least Ms+20K, especially Ms+50K.
In a further configuration of the method of the invention, the workpiece is
trimmed in
the hot-forming and/or press-hardening mold. This has the advantage that the
workpiece, preferably in the still-hot state, can be trimmed relatively easily
when the
temperature has not yet gone below the Ms temperature. This makes it possible
to
dispense with additional mechanical cutting tools which, because of the high
hardness in
the finished workpiece (component), are prone to wear and have a short service
life, or
alternative separating apparatuses, for example costly hard trimming by laser.
In a further configuration of the method of the invention, a workpiece which
is a tailored
product is used. Tailored products are understood to mean tailored blanks or
tailored
welded blanks, tailored strips or tailored welded strips and tailored rolled
blanks or
tailored rolled strips, which are known in the art. Tailored blanks and
tailored strips
with different sheet thicknesses and tailored rolled blanks can additionally
be used to
save mass by comparison with workpieces having a uniform material thickness.
In the
tailored blanks and tailored strips, as well as different sheet thicknesses,
it is also
possible to use different steel materials in order to take account of
different
microstructures in the workpiece, which are not established by the "tailored
tempering"
already mentioned; in other words, a heat-treatable steel material which has a
hard
microstructure after the hardening is welded to at least one non-heat-
treatable, non-
hardenable steel material which, after hardening, essentially retains its soft
microstructure, along the joining edge of each, preferably by means of butt
laser
welding. Complex delayering of the joining zone, which is absolutely necessary
in the
case of AlSi coatings, can be dispensed with in the case of zinc-containing
coatings.
7
The heat-treatable steel is a magnesium-boron steel having a tensile strength
of at least
1500 MPa in the hardened state. Its alloy constituents in % by weight are
preferably
limited as follows:
s0.5
Si s 0.7
Mn s.2.5
0.01
Al 0.015
Ti 5 0.05
Cr+Mo 5 1.0
.s0.05
balance: iron and unavoidable impurities.
In a further configuration of the method of the invention, a component having
a profile
cross section which is hat-shaped or omega-shaped at least in some regions is
produced.
More particularly, the component produced has the form of a half shell. Half
shells are
components which, in the installed state, are preferably parts of an A, B, C,
D pillar, of a
door sill, of a longitudinal beam, of a transverse beam, of a crash box or of
a chassis
component.
In a second aspect of the invention, a component for a vehicle which has a
profile cross
section which is top hat-shaped or omega-shaped at least in some regions, and
which has
been at least partly press-hardened, especially produced by the process of the
invention
is specified, wherein the component has a first surface having a smaller
coating thickness
of a zinc-containing coating compared to the second surface of the component.
In order
to avoid repetition, reference is made to the above statements at this point.
In a first configuration of the component of the invention, the component has
been formed
from a tailored product, in order especially to be able to influence the
weight. If a
component is to have different microstructures, it may have been produced
alternatively
or cumulatively by a "tailored tempering" process.
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In a third aspect, the invention relates to use of the component of the
invention as
bodywork component of a vehicle, especially as part of an A, B, C, D pillar,
door sill,
longitudinal beam, transverse beam, crash box, or in the form of a chassis
part of a
vehicle, especially as part of a chassis component, more preferably in
passenger vehicles,
utility vehicles, heavy goods vehicles, specialty vehicles, buses, omnibuses,
whether
driven by a combustion engine and/or electrically, but also in rail-bound
vehicles, for
example trams or passenger-carrying wagons.
The invention is elucidated hereinafter with reference to a diagram that shows
working
examples. Identical parts are given identical reference numerals. The figures
show:
Figure 1): a schematic sequence of steps for production of a component for
a vehicle
in a first configuration of a method of the invention,
Figure 2): a partial cross-sectional view of a first working example of a
component of
the invention,
Figure 3a, b): micrographs of the critical region, shown in fig. 2, of hot-
formed and press-
hardened components, wherein the coating thickness was 5 i.tm and 3 pm
in the non-press-hardened condition (condition as supplied).
Figure 1 shows, in schematic form, a sequence (E) of steps for production of a
component for a vehicle in a first configuration of a method of the invention.
The method
of the invention firstly comprises the step (A) of providing a workpiece
composed of a
heat-treatable steel material provided with a zinc-containing coating on both
sides. The
coating has a first surface 3 having a coating thickness < 4 p.m, especially <
3.5 pm, more
preferably < 3 pm, and a second surface 4 having a coating thickness of? 4 pm,
especially? 4.5 pm, more preferably? 5 pm, of the zinc-containing coating, in
each case
in the as yet non-press-hardened condition (condition as supplied).
What is not shown here is that the workpiece, prior to provision thereof, is
separated
from a steel material in strip form, which has been provided with a zinc-
containing
coating applied electrolytically or by hot dip coating, which has especially
been applied
in a continuous coating process. Heat-treatable steel materials are
essentially
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manganese-boron steels. A further step (B) comprises at least partial heating,
preferably
complete heating, of the workpiece to a temperature above Ad, especially above
Ac3. The
workpiece is first partially or completely heated to austenitization
temperature, and
different microstructures or a homogeneous microstructure throughout can be
established in the workpiece according to the requirement on the component to
be
produced and the end use thereof in the motor vehicle. This can be effected by
means of
appropriate furnaces. Workpieces used include monolithic steel materials
having a
homogeneous material thickness, for example having material thicknesses
between 0.5
and 6 mm, especially between 0.8 and 4 mm, or tailored products.
The temperature for heating (through-heating), preferably in a furnace
(continuous
furnace), is, for example, 850 to 930 C with a residence time, for example,
between 3
and 12 min. The heating is followed by the insertion of the at least partly,
preferably
completely, heated workpiece into a hot-forming and/or press-hardening mold
(step C)
comprising at least one punch and at least one die. It has to be ensured that
the
workpiece having a first surface having a lower coating thickness of the zinc-
containing
coating compared to the second surface of the workpiece is inserted into the
hot-
forming and/or press-hardening mold in such a way that the first surface of
the
workpiece is brought into contact with the predominantly concave side of the
mold,
preferably with the die of the mold, and the second surface of the workpiece
with the
predominantly convex side of the mold, preferably with the punch of the mold.
This can
be verified, for example, by suitable means that are not shown here, for
example
measurement systems, for example thermal imaging cameras etc., namely at an
early
stage in the region of charging of the furnace and/or at the outlet of the
furnace and/or
prior to the insertion into the mold, in order to avoid incorrect insertion.
The reduction
in the coating thickness of the zinc-containing coating on the first surface 3
of the
workpiece, which is preferably brought into contact with the die of the mold,
can reduce
the cracks or excessively high crack depths in the critical region 1 by
comparison with
coating thicknesses that are customary in practice to a degree which meets the
demands
on component strength in the event of a crash and the expected lifetime,
especially
under cyclical stresses. The lowering of the coating thickness also reduces
the zinc
supply and hence a lower level of liquid zinc phases, which weaken the
material, forms
during the heating process in the material.
10
The closure of the mold by relative movement of the punch and/or the die
relative to one another
and hot forming and/or press hardening of the workpiece are encompassed by a
further step
(D), wherein at least one region of the hot workpiece in the closed mold is
cooled in such a way
that there is at least partial formation of a hardened microstructure. The
workpiece is at least
completely or partially hardened, which is effected by rapid cooling in a
mold, especially an
actively cooled mold, in the course of hot forming and press-hardening (direct
hot forming) or
in the course of press-hardening (indirect hot forming), with conversion of
the microstructure
of the at least partly austenitic region of the workpiece by abrupt cooling to
a martensitic and/or
bainitic microstructure, particular preference being given to the achievement
of a martensitic
microstructure. If different microstructures in the workpiece are required, it
is possible by the
"tailored tempering" process or alternatively through the use of, for example,
a tailored blank
composed at least of one heat-treatable steel material and at least one non-
heat-treatable steel
material, to establish at least one region having a hard microstructure and at
least one region
which has a soft microstructure and is more ductile compared to the hard
microstructure.
Particular preference is given to direct hot forming.
By differential coating, the workpiece is used to produce, by the indirect and
preferably direct
hot forming, components which have a top hat-shaped or omega-shaped profile
cross section 5
in some regions. More particularly, the component produced has the form of a
half shell. Half
shells are components which, in the installed state, are preferably parts of
an A, B, C, D pillar, of
a door sill, of a longitudinal beam, of a transverse beam, of a crash box or
of a chassis component.
What is not shown here is that hot-formed parts can also have other profile
cross sections which
are used, for example, as attachments, especially as part of a wheel rim,
preferably as the wheel
disk of a wheel rim. Figure 2 shows a partial cross-sectional view of a first
working example of a
component of the invention, for example in the form of a B pillar. What is
shown is a cross section
along an axis of the component F, where the component may be formed
symmetrically about the
axis F at least in this cross section. A B pillar has a cross section of
variable length along its
component axis. Especially in the case of direct hot forming, which is
preferred, the first surface
3 of the workpiece, which is in contact with the die during the hot forming
and press hardening,
experiences high compressive/tensile stress, as a result of which
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cracks with high crack depths which extend into the base material form in the
critical
region (1) by virtue of the liquid zinc phases, which weaken the material,
that result
from the heating. The second surface 4 of the workpiece, which is in contact
with the
punch during the hot forming and press hardening, experiences lower
compressive/tensile stress compared to the first surface, as a result of which
there is no
risk of excessively deep crack formation on the side facing the aunch.
Components or
half shells of this kind are preferably joined to further components or half
shells to form
a profile having a cavity. The first surface 3 of the component having the
reduced coating
thickness of the zinc-containing coating (in the condition as supplied)
accordingly has an
external side for component-related reasons. The second surface 4, having a
higher
coating thickness of the zinc-containing coating compared to the first surface
3,
accordingly has an internal side present within the cavity for component-
related
reasons. Especially in cavities, in the case of entry of a corrosive medium,
there can be
elevated risk of corrosion. In general, secondary measures, for example cavity
sealing by
means of wax, are undertaken in these regions. With a corresponding (higher)
coating
thickness of the zinc-containing coating on the second surface 4, it is
possible in
accordance with the invention to provide active long-term corrosion
protection.
For the purpose of studying the formation of cracks, two samples were taken
from the
critical region 1 of hot-formed and press-hardened components 5, the coating 7
having
been enriched with iron for temperature-related reasons after the annealing
treatment
and the subsequent hot forming and press hardening, in order to be able to
create
micrographs. The furnace temperature was 880 C with a residence time of 6 min.
The
press-hardened components were produced from a manganese-boron steel (22MnB5)
having a zinc-containing coating applied electrolytically at least on the
first surface 3
and having a coating thickness of 3 pm (figure 3a) and 5 pm (figure 3b) prior
to the
press hardening in the condition as supplied. In the iron-enriched coating, it
can be seen
in the micrographs that, with coating thicknesses of the zinc-containing
coating > 4 pm
(in the condition as supplied), there are cracks with high crack depths 6',
which extend
down to the base material 2. The crack depths 6' in the base material are > 10
jun (figure
3b), which means that it is no longer possible to assure sufficient component
strength in
the event of a crash and expected lifetime, especially under cyclical stress.
Crack
formation behavior is different with low coating thicknesses of the zinc-
containing
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coating < 4 pm (in the condition as supplied). The crack depth 6 in the base
material 2
can be reduced to a maximum of 10 pm, which means that it is possible to
assure
sufficient component strength in the event of a crash and expected lifetime,
especially
under cyclical stress. In order to ensure sufficient cathodic corrosion
protection, the
coating thickness on the first surface 3 is ?_ 1 p.m, especially 1.5 gm, more
preferably
2 m, in the as yet non-press-hardened condition (in the condition as
supplied). The
coating thickness on the second surface 4 is restricted to 5. 25 pm,
especially 5. 20 pm,
more preferably 15 m.
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List of reference signs
A, B, C, D step sequence, method steps
process direction
component axis
1 critical region
2 heat-treatable steel material, base material
3 first surface of the steel material
4 second surface of the steel material
press-hardened component
6, 6' crack depth
7 coating after the annealing treatment and the subsequent hot forming
and
press hardening
