Note: Descriptions are shown in the official language in which they were submitted.
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A METHOD OF FORMING PARTS FROM SHEET STEEL
FIELD
The present invention relates to the forming of parts from metal. In
embodiments, it relates to the forming of parts from metal sheet, such as
steel and steel
alloys.
BACKGROUND
Processes using "hot stamping" are emerging as preferred solutions for forming
high-strength parts from steel sheet for applications in, for example,
automotive "body
in white" (BiW), and chassis and suspension (C&S) parts. The development of
Boron
steel makes such process feasible for the production of automotive safety
critical panel
-- parts, such as A-pillars, B-pillars, bumpers, roof rails, rocker rails and
floor tunnels for
Body-in-White and tubular parts and twist beams for C&S. The global demand for
such
ultra-high-strength steel parts has been growing sharply in recent years.
A typical Boron steel hot stamping process is shown in Figure 1. Essentially
it
comprises the steps of:
(1) Heating the steel blank to above its austenitisation temperature, say
925 C, and soaking at that temperature to enable all the metal to be
transformed into
austenite. In this state the metal is soft and has high ductility (easy to
form);
(2) Quickly transferring the austenitised material blank to the press;
(3) Forming the blank into the shape of the component using a cold die set,
-- which is normally water cooled;
(4) Holding the formed part within the cold die set for a certain period
(e.g.
6-10 seconds depending on geometry, sheet thickness, pressure, etc.) for
quenching,
enabling the hard phase of the material, e.g. martensite, (for a high strength
component)
to be formed; and
(5) Releasing the die when the part temperature has dropped to a
sufficiently
low level, say 250 C, and taking the component out.
Such a process is sometimes referred to as a "hot stamping, cold die forming
and quenching" process.
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Most of the heat in the work-piece goes to the die in the hot stamping
process.
The cooling rate is largely related to the tool surface temperature. Even if
the die set is
water cooled, under mass production conditions, it is difficult to keep the
tool surface
temperature sufficiently low. A high tool surface temperature causes the
following
problems:
In this conventional hot stamping process for forming complex parts from sheet
steel, a sheet work-piece is transferred, as quickly as possible, from a
furnace to tools at
room temperature in which it is deformed and quenched simultaneously. The
quench
rate is sufficiently rapid to produce a martensitic microstructure in the
steel, which form
the basis for high strength products.
(i) The cooling rate in die quenching might become too low, which would
cause undesirable soft phases to be formed in the case of steel (a low
strength part
produced); in the case of a light alloy, e.g. aluminium, a die quenching rate
that is too
low could cause undesirable grain boundary precipitation which can lead to
stress
corrosion cracking and a low strength part;
(ii) The cold die holding period required may be too long (because the heat
transfer from the sheet is slower as a result of a warmer die, hence a greater
time is
required to achieve the final temperature), which reduces the productivity
(increased
forming cycle time);
(iii) The requirement for adequate die cooling is important, but providing it
artificially (by ad hoc methods, i.e. cooling ducts with forced cooling fluid,
etc.)
increases tooling costs making an efficient method difficult to design and
install, and
can raise the tooling and maintenance costs significantly.
(iv) Tool wear and or die surface distortion are accelerated when the tool
surface temperature is high, reducing tool life, the costs of which are
exacerbated by ad
hoc cooling systems described in (iii).
Thus, in summary, when parts are produced using this process in rapid
succession, the continual contact of work-pieces from the furnace causes the
temperature of the tools to increase. As a result, the quenching rate reduces,
which can
lead to finished products with a sub-standard microstructure. To avoid this,
tool
temperature can be kept low either by reducing production rate, or by using
cooling
systems, such as internal coolant-carrying conduits or sprays of coolant onto
the tools.
Often, a combination of these two methods is used to achieve a desired
microstructure
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at the highest production rate possible for the given cooling strategy. A
drawback is that
all of these measures increase cost.
SUMMARY
In general terms, a two-stage cooling method is proposed to improve the
productivity of high-strength sheet parts. In the proposed two-stage cooling
method, the
heated sheet is rapidly cooled between heating and forming. It is envisaged
that this
rapid cooling is by some artificial means, rather than just by ambient, still,
air. For
example, a high heat conductivity transfer device, an air jet or air/liquid
mist spray may
be used. In this way, the temperature of the blank can be reduced by the time
it starts to
be fonned in the die. Therefore, in the forming process (in which further
quenching
ensues) less heat is absorbed by the tools and the rise in their temperature
is reduced.
Thus, maintaining a low base-line temperature is made easier, costs are
reduced and
productivity is increased. Other beneficial effects result from optional
features.
According to a first aspect of this invention, there is provided a method of
founing a part from sheet steel, the method comprising the steps of:
(a) heating the sheet to a temperature at which austenitisation occurs; and
(b) forming the sheet between dies into the part;
wherein there is an additional step between (a) and (b) of applying cooling
means to the sheet to extract heat therefrom.
The additional step may include applying the cooling means to rapidly cool the
sheet.
By rapidly cooling the heated sheet before forming the sheet between the dies,
the sheet can be Ruined in the cold dies at a lower starting temperature than
is
conventional. This has the following effects: the sheet can cool sufficiently
quickly in
the dies that the hardest phase, martensite, is founed; the sheet can reach
the
temperature at which it is suitable for release from the dies more quickly
than in the
conventional process, speeding up production; the damage to tools from
elevated
surface temperature is reduced, increasing tool life; and reducing the need
for tool
cooling structures such as cooling ducts and thereby reducing the cost of the
dies.
The additional step may comprise extracting heat using cooling means such as
high conductivity transfer devices or by impinging cooling means such as
cooling
medium on the heated sheet
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The cooling medium may be a fluid. It may be a gas, for example air. The
cooling fluid may be a liquid, for example water. The cooling fluid of may
comprise
gas and liquid, for example air and water. The cooling fluid may be directed
as a
pressurised flow of the fluid. The cooling fluid may be directed as a jet. The
cooling
fluid may be directed as a mist spray. The cooling fluid may be used to cool
the dies. It
may be used to clean the dies. It may be used to both cool and clean the dies.
The
cooling fluid may be directed at the dies. It may be directed at the dies
subsequently to
being directed at the heated sheet and/or it may be directed simultaneously at
the dies
and at the heated sheet.
The cooling means may be a high heat conductivity solid, such as a copper
transfer grip or plate.
The cooling means may be applied when the blank is between the dies.
The cooling between (a) and (b) may also be achieved by increasing the
transfer
time between the two steps, for example from the furnace to the dies.
The additional step may comprise directing the cooling fluid at the heated
sheet
such that the sheet is cooled sufficiently rapidly to avoid the steel entering
the bainite
phase. The additional step may comprise directing the cooling fluid at the
heated sheet
such that the sheet is cooled at more than 25 C/second on average. The
additional step
may comprise directing the cooling fluid at the heated sheet. The cooling
fluid may be
directed with duration, temperature and/or mass flow such that the sheet is
cooled
sufficiently rapidly to avoid the steel entering the bainite phase. The
cooling fluid may
be directed with duration, temperature and/or mass flow such that the sheet is
cooled at
more than 25 C/second on average.
The additional step may comprise directing the cooling fluid at the heated
sheet
such that the temperature of the sheet remains above the austenitisation
temperature for
the steel while being cooled in this way. The additional step may comprise
directing the
cooling fluid at the heated sheet such that the sheet is cooled to between 500
C and
600 C. The cooling fluid may be directed with duration, temperature and/or
mass flow
such that temperature of the sheet maintains the austenitisation state for the
steel while
being cooled in this way. The cooling fluid may be directed with duration,
temperature
and/or mass flow such that that the sheet is cooled to between 500 C and 600
C.
Surprisingly, this has the effect of increasing the formability of the alloy
since the strain
hardening of the steel increases while the ductility remains substantially the
same. The
method may comprise commencing step (b) while the sheet is at a temperature at
which
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it is in the austenite phase. The method may also comprise carrying out step
(b) until
the temperature of the sheet is such that it is in the martensite phase.
Step (a) may contain some or all of the features of that step of the
conventional
process described herein.
The method may be a method of forming parts for automotive applications. The
method may be a method of forming panel parts for automotive applications. The
method may be a method of forming load-bearing parts and parts adapted to
bearing
load in automotive applications; for example, the method may be a method of
forming
one or more of: pillars including A-pillars and B-pillars, bumpers, door
beams, roof
rails, rocker rails and floor tunnels. The method may be a method of forming
Chassis
and Suspension parts; for example tubular parts and twist beams.
The sheet steel may be of an alloy that contains boron.
In another aspect of the invention, a method of forming a part is provided in
which the part is formed from a material other than steel. For example, the
material
may be an aluminium alloy. It may be in sheet form. It is therefore envisaged
that the
method of the first aspect may be used with aluminium alloys, for example
those in
sheet form. In the method of this other aspect, step (a) may comprise heating
the sheet
to a temperature at which a change in crystal structure substantially
equivalent to
austenitisation occurs.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows in schematic form an existing hot-stamping process;
Figure 2 shows a CCT diagram for a typical Boron steel;
Figure 3 shows a temperature profile in cold die quenching; and
Figure 4 shows the stress-strain relationships for a Boron steel tested at
temperatures of 500, 600, 700 and 800 C at a strain rate of 1.0s-1.
SPECIFIC DESCRIPTION OF CERTAIN EXAMPLE EMBODIMENTS
As described above, an existing method is shown in Figure 1. A very important
aspect in this existing method is that, as the hot stamped part is held in
cold dies, the
cooling rate should be sufficiently high, e.g. more than 25 C/second on
average, as
shown in Figure 2, to enable the hardest phase of the material, martensite, to
be formed.
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In this way, high strength components can be made. The cooling rate is not
constant
during the cold die quenching, as shown in Figure 3. Initially, the
temperature
difference between the work-piece and the die is high and a high cooling rate
can be
achieved. As the work-piece temperature drops close to the tool surface
temperature
(which increases due to heat transfer), the cooling rate reduces
significantly. In a
continuous hot stamping process, the tool surface temperature can be as high
as 150 C.
This results in a low work-piece cooling rate in the temperature range 500 C
to 250 C.
This is the sensitive range for metallurgical transformation and a low
quenching rate
could result in the formation of lower strength bainite instead of martensite
(see Figure
2). Thus, a low strength part would be formed.
The present embodiment provides a method in which the amount of heat
transferred from the workpiece to the cold die is reduced when compared with
such an
existing method, thereby reducing the tool temperature in comparison with the
existing
method and addressing the problems of the existing method described above.
This
embodiment reduces the amount of heat absorbed by the die while maintaining
the
necessary rate of quenching, and of production.
In overview, in the present embodiment, the sheet of boron steel is rapidly
cooled as it is transported from furnace to die by a solid medium of high heat
conductivity, or by a fluid such as an air jet or air/liquid mist spray, and
thus its
temperature is reduced by the time it is placed on the die. Therefore, in the
forming
process (in which further quenching ensues) less heat is absorbed by the tools
and the
rise in their temperature is reduced. Thus, maintaining a low base-line
temperature of
the tools is made easier, costs are reduced and productivity is increased.
The new method involves the following steps.
First, a sheet metal blank of boron steel is heated in a furnace to above its
austenitisation temperature. In the present embodiment, the blank is heated to
925 C.
The blank is then soaked at this temperature to ensure the material is
transformed
entirely into the austenite phase. In this state the metal is soft and has
high ductility
(easy to form), as in the conventional process.
The next step is to transfer the austenitised material blank to the press in
which
it is to be formed into the shape of the part. During the transfer or, in
other
embodiments, after the transfer to the die but before the hot metal blank
touches the die,
the blank is cooled quickly by contacting it with a substance with high heat
conductivity. This substance, that is this cooling means, may take the form of
one, more
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or all of: copper grips, blowing air, directing an air-water mist or other
fluid/liquid
cooling medium at the blank. In the present embodiment, an air-water mist is
applied to
the blank. This is done by directing a fine spray of pressurized water at the
blank
through a plurality of nozzles. In this way, the blank is cooled to a
temperature of about
600 C. The cooling rate is adjusted to be sufficiently rapid to maintain an
austenite
structure per the CCT diagram in Figure 2. During this stage of work-piece
cooling, it is
envisaged that the same cooling medium is also used to cool and clean the
tools.
The remainder of the method is the same as in the conventional method
described herein. Thus, the method may be illustrated as the conventional
process
shown in Figure 1, but with additional cooling during the transfer between the
furnace
and the die.
From the typical-stress strain curves for Boron steel shown in Figure 4, it
can be
observed that when temperature decreases from 800 C to about 600 C, the
ductility of
the alloy does not change very much. However, the strain hardening of the
alloy leads
to a near doubling of the strength. This strain hardening feature increases
the
formability of the alloy significantly, by causing the deformation to be more
uniform
(i.e. an area deformed more becomes stronger, causing deformation to occur in
other
areas, which then become stronger, etc.), thereby mitigating the tendency for
localised
necking. This is particularly important in hot stamping, since friction is
normally high
and the strain hardening feature could reduce the friction effects. Thus, if,
as is the case
in the present embodiment, a part can be formed at a temperature starting at
about
600 C in the dies, rather than 800 C as is done conventionally; more complex-
shaped
components can be formed. It should be emphasized that this effect cannot be
achieved
by simply heating the sheet to a lower initial temperature, as it must first
be fully
austenitised.
The CCT diagram for Boron steel in Figure 2 shows that the alloy is still in
the
austenite state if it is cooled quickly to about 500-600 C. If the cooling is
too slow, the
lower-strength bainite phase begins to form; the present method, however,
avoids this.
In the present method, as the blank is transferred to cold dies while in this
temperature
range and maintained at a temperature between 450-500 C during the entire
forming
process, all phase transformation takes place during the cold die holding
period and the
austenite is entirely converted to martensite to produce a high strength part.
In existing methods, the formed part is released from the die as soon as the
part
temperature drops to about 250 C. At this temperature, phase transformation
has been
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completed and no obvious thei _________________________________________ ual
distortion is observed by further cooling in the air
without the tool constraint. The cold die quenching period (i.e. the time for
which the
part is held in the die) required to cool a part from about 800 C to about 250
C (550 C
difference), is about 5 to 15 seconds in these existing methods, depending on
the
thickness and shape of the work-piece and part shape. Thus, a significant
amount of
heat has to be absorbed by the die directly, which makes cooling the die
difficult.
In the present embodiment, the part is formed at about 600-500 C. Thus, in the
cold die quenching period, the only need is to bring the part temperature down
from, at
the lower end of this range, 500 C to about 250 C (250 C difference). Only
about half
the amount of heat therefore needs to be extracted from the die, and so the
cooling
requirement for the tool is much lower. The tool design can therefore be
simpler and the
tool can be cheaper. The lower temperature of the tool surface reduces the
cold die
holding period significantly, and also increases the cooling rate
significantly during the
temperature range of 500 C to 250 C. The holding time can be reduced to about
2 to 8
seconds. Thus, productivity can be increased significantly. This is vital for,
for
example, a competitive automotive company. In addition, the lower tool surface
temperature reduces tool wear, thus increasing tool life significantly, which
is an
additional benefit for reducing production costs.
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