Note: Descriptions are shown in the official language in which they were submitted.
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CONTINUOUS PROCESS OF ROLL-FORMING S'1'AMPE'1) SHE'L+ 1'
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is a continuation-in-part application of co-assigned, co-pending
utility
application serial no. 11/330,301, filed February 11, 2006, entitled
CONTINUOUS
PROCESS OF ROLL-FORMING PRE-STAMPED VARYING SHAPES, which is turn
claims benefit of provisional application Serial No. 60/723,393, filed October
4, 2005,
entitled CONTINUOUS PROCESS OF ROLL-FORMING PRE-STAMPED VARYING
SHAPES
BACKGROUND
[0001] The present invention relates to a continuous process of roll-forming
pre-stamped
varying shapes, and more particularly relates to a process combining a
stamping/deforming process with a roll-forming apparatus to obtain advantages
of both
in a continuous process.
[0002] The advantages associated with roll-forming have made roll-forming a
preferred
manufacturing method for many open structural sections and tubular structural
sections.
Automotive bumpers beams are one example of a product that has capitalized on
the
benefits of roll-forming to produce light weight, low cost, and performance
based
designs that are widely accepted and used commercially. Roll-formed tubular
bumper
beams represent a majority of the bumper beam market and continue to gain in
popularity as bumper beams move from Class A surface designs such as stamped
and
chrome plated beams to structural beams that are positioned behind plastic
fascias.
Another style of bumpers are open sections (sometimes called "C" section
bumpers) are
also commonly used for bumper beam designs. These open sections typically
require
additional secondary processing to insert and weld strategically placed
internal bulkheads
and strapping. The use of bulkheads and strapping is to improve structural
integrity of
the section with the addition of a minimal amount of weight. These bulkheads
and straps
improve bending stiffness of the open section but achieve this increased
performance at
the cost of secondary processing.
[0003] The roll-forming process has many benefits that make it a cost-
effective method
of manufacturing tubular beams. Some advantages of roll-forming include the
abilities
to forni high strength steels and Ultra High Strength Steels (UHSS). Pre-
pierce, mid-
pierce, and/or post-pierce operations can be used on flat sections of the
sheet material
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nerore ron-rormmg. Also, operations can oe uune in-ime wir.n me roii-rorming
process,
such as in-line welding and cutoff. However, roll-forming has limitations,
such as the
inability to change material properties within the part and the inability to
change the
cross section along the length of the part. The inability to change the cross
section along
the length of the part typically results in excessive material being used in
areas where it
is not needed to meet performance requirements in other areas where it is
needed. The
excessive material also adds to the weight of the part and to material cost.
Specifically,
testing and computer analysis shows that a tubular beam that is constrained at
its ends
and loaded in the center will exhibit a free body diagram where greater
section depth,
(i.e., moment of inertia) at the beam's center produces a stiffer beam. The
inability to
change the cross section of a roll-formed tubular beam along its length
results in a
tubular beam with a constant moment of inertia. The constant moment inertia
results in
excess material and geometry in areas along the length of the tubular beam
that do not
contribute to the overall bending stiffness of the beam.
[0004] Stamping processes have an advantage over roll-forming in that beams
with non-
uniform cross sections can be made. However, stamping operations are limited
and
generally less efficient than roll-forming since individual sheet blanks must
be laterally
moved and accurately positioned after each die stroke. Also, dies cannot make
tubular
beams without significant difficulty or complexity, or with secondary
operations.
Further, high strength and ultra high strength sheet material is very wearing
on dies
depending on the amount of shape deformation being imparted onto the sheet
material
and depending on a strength of the material.
[0005] It is noted that current roll-forming processes sometimes utilize
relatively-lower-
force, fast-acting stamping presses before the roll-forming process, but these
presses are
used to piece and cut features such as holes, slots, etc. into the material
while it is flat
and before roll-forming is used to shape the part.
[0006] Typical (most) stamping press methods for forming metals can be
referred to as
cold stamping. In cold stamping, the material is formed between dies, with the
material
and dies being kept generally at room temperature. Another type of forming is
called hot
stamping. In hot stamping processes, the metal is heated to its austenite
temperature
(Ac3) and then cooled rapidly after forming is completed in the die. The
required
austenite temperature and time required to quench the material are dependent
on the
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enemisiry or ine rnaRriai, aiiu ine nnai properties aesireu. ror exampie, it
is xnown to
heat steels containing 0.2% mass percent of carbon to 850 C (or higher such as
880 C to
950 C) to reach their austenitic region, and then to pass them into cooled
dies in a
stamping press for forming while the steel is still sufficiently hot. The hot
steel is formed
using the stamping dies and then quickly cooled by the temperature of the dies
and/or by
quenching with water. For example, the material can remain in the die with the
press in
its full stroked position until the steel has reached a low enough temperature
to maintain
the desired material strength. However, the above-described hot stamping
technique
tends to be an intermittent "stop-and-go" process which historically is not
considered to
be well-suited for use in a continuous processes operating at relatively high
line speeds
(i.e. not well-suited for being used in-line with rollforming equipment).
Further, I, the
present inventor, am not aware of anyone combining stamping with roliforming,
for the
reasons discussed above.
[0007] Thus, a system having the aforementioned advantages and solving the
aforementioned problems is desired. '
SUMMARY OF THE PRESENT INVENTION
[0008] In one aspect of the present invention, an apparatus adapted to form a
reinforcement beam includes a press having dies configured to deform a flat
strip of
heated metal material into a deformed strip with a three-dimensional shape by
drawing
and moving material, and a cooling system associated with the press and
configured to
cool the heated metal material of the deformed strip. A roll-former is located
downstream of the press and cooling system and has rolls configured to shape
edge
portions of the deformed strip into a continuous beam.
[0009] In another aspect of the present invention, an apparatus adapted to
form a
reinforcement beam includes a heater configured to heat steel material to a
temperature
of at least about 850 C. A press includes dies configured to deform a
continuous strip of
hot steel material. A cooling system is associated with the press for
quenching and
hardening the hot steel material. A roll-former is positioned in-line with the
press and
downstream of the cooling system and has rolls configured and arranged to
receive the
strip in the feed direction and shape edge portions of the deformed strip into
a
reinforcement beam.
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Lvvi.vj Ali a11VLlll.1 aON%Ai, vl L11G 1J1GOG11L 111VG11L1U11, A. 111GL11U11
1:V111.Pr15es steps VI IIUt-
stamping a sheet of heated steel material having a first tensile strength,
including
drawing and stretching the material. The method further includes quenching the
stamped
sheet to cause the material to have a higher tensile strength. The method
still further
includes roll-forming portions of the stamped sheet by use of a roll-forming
mill into a
beam.
[0011] In still another aspect of the present invention, a method of forming a
reinforcement beam comprises steps of providing a heater, providing a press
including
forming dies at a location downstream of the heater and also a cooling system
associated
with the press, and providing a roll-former having rolls in-line and
downstream of the
press. The method includes heating a strip of material, operating the dies to
deform the
strip of material by drawing the hot material and thereafter quickly cooling
the material,
and operating rolls of a roll-forming mill to receive the strip in a feed
direction and
shape edge portions of the deformed strip into a reinforcement beam.
[0012] The present inventive concepts focus on tubular sections, but it should
be noted
that the concepts and their uniqueness can also be applied to open sections.
[0013] These and other aspects, objects, and features of the present invention
will be
understood and appreciated by those skilled in the art upon studying the
following
specification, claims, and appended drawings.
BRIEF DESCRIPTION OF DRAWINGS
[0014] Fig. 1 is a flow chart showing a process embodying the present
invention.
[0015] Fig. 1A is a side view of an apparatus incorporating a roll-forming
mill, a
stamping station, a welding station, and a sweeping station for the process of
Fig. 1.
[0016] Fig. 2 is a side view of a vehicle bumper beam embodying the present
invention.
[0017] Figs. 3-6 are cross sections taken through Fig. 2.
[0018] Fig. 7 is a side view of the vehicle bumper beam after being roll-
formed but
prior to being longitudinally swept.
[0019] Fig. 8 is an end view of the beam of Fig. 7 moving through an exemplary
roll-
forming station, the view showing the beam, the rolls, and a weld joint.
[0020] Figs. 9-9A are a flow chart and side view of a corresponding machine
similar to
Figs. 1 and 1A, but adapted for hot stamping steel sheet material prior to the
roll
forming portion of the machine.
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1JL'.11-11Lr-1J Lzal_iCli" 11V1V Vr rl[nrP~iCLCP~L n1V1DVL11V1L"1V 13
[0021] The current invention defines a way to produce roll-formed tubular
beams with
varying cross sections from high-strength sheet material, such as materials of
over 80 ksi
tensile strength and even ultra-high-strength steel of over 140 ksi tensile
strength
(sometimes called "UHSS" or "AUHSS" material). For example, a DP980 (DF140)
material has successfully been used. The ability to change cross sections
along the
length of the beam is achieved by combining a pre-forming process via stamping
with
the roll-forming. The stamping process and roll-forming operations are done in-
line and
sequentially. Sequential processes in-line without the need for secondary
handling
results in a very cost efficient manufacturing process. The stamping press is
used to pre-
form material as well as pre-pierce the sheet material and add features before
it travels
through the roll-forming tooling. The stamping of the material while it is in
the flat
produces a shape with varying depth and geometry across the length of the
part. For
example, the formed shape could represent the rearward section of a tubular
bumper
section. The stamping press that combines the stamping of features and forming
of
varying shapes would have a greater tonnage than a typical pre-pierce press
that stamps
features into the material. The present stamping press would also have to
include
stamping dies beyond the required punches and buttons necessary for the
stamping of
features. Since it is envisioned that material grades above mild steel such as
UHSS
material would be used to fabricate tubular bumper systems, it is assumed that
a one-hit
stamping operation must be sufficient to form the shape. Higher grade
materials such as
UHSS undergo a sizable amount of work hardening which makes the material
susceptible to cracking if multiple hits are required to produce the desired
final shape. If
the stamping process requires a large amount of forming and in turn causes the
materials
to significantly work harden, it is likely that the stamped shape would be
formed in a
one-hit operation to avoid any material cracking that might result from
multiple stamping
of the work hardened material. Aggressive forming via stamping can be done in
multiple hits if caution is exercised in speed of forming to keep work
hardening at a
minimum. A good understanding of the application of design guidelines with
respect to
bend radii should be exercised when stamping UHSS. It should also be noted
that the
shape when formed by UHSS material is formed primarily by moving material and
not
stretching or extruding the material. Materials such as UHSS have high yield
and tensile
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bI.IC11gu1J allu 111 6ur11 d lUw C1Ui1gdL1U11. 1110 lUw G1U11gd61U11
LranslaLes alrecLly LU iow
ductility which prohibits the forming through material thinning, i.e.,
extrusion/drawing.
[0022] The material that is used to form the shape during the stamping process
draws
from the width of the coil and not down the length of the coil. This will
require that the
width of the coil that feeds the stamping press to be oversized to allow for
shape forming
via stamping and still provide sufficient blank width for the roll-forming
operation which
is used to finish the shape into a tubular geometry. It is also envisioned
that relief slots
will need to be introduced at the ends of the formed shape. These slots will
assist in
forcing material from the edges of the blank width and not down the length of
the
formed shape. The relief slots also will maintain blank flatness of the
unstamped non-
deformed areas of the blank before and after the shape is stamped into each
part. The
forming operation in the press will produce an irregular width blank since
material
moves across the coil width and will need to be resized before the material
can pass
through the roll-former. Roll-forming uses the edges of the material to guide
the
material from pass to pass and to trap the material while bending is being
performed at
each of the passes. The resizing of the blank width can be done at one or more
locations
along the manufacturing process. It is contemplated that the irregular blank
will be slit
immediately after the material leaves the stamping press. The slitting
operation may
have to be offset where one edge is slit and then used as a reference edge to
slit the other
edge. The material can also be slit after one or more of the early passes of
the roll-
former operation. It is contemplated that there may be some advantages to
begin some
initial forming in the roll-form mill before the irregular blank is resized
with a slitting
operation and before the roll-forming process is finalized. An additional
option is to
resize the irregular blank in a station of a multiple station stamping
operation. One
option would be to have the bed of the stamping press be at least twice the
length of a
typical part, or the other option would be to use two presses in-line. It is
envisioned that
the first station of the stamping press or the first press would stamp the
relief slots
needed at the ends of the part length and form the complete shape. The second
station of
the press or the second press would stamp features (holes, slots, etc...) into
the formed
shape and trim the irregular blank to a size appropriate for the roll-forming
process
which will finish the part into a tubular beam section with varying cross-
sectional
geometries along the length of the part.
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LuuLjI liue to tne amount ot rorming tnat is requirea ror tne part, it may be
necessary to
accumulate material after the press and before the roll-forming process. In a
typical pre-
pierce operation where features are added to the material while it is flat,
the die is
configured to travel with the material, (i.e., flying pre-pierce die).
Preferably, the
forming die would be configured to stamp while moving longitudinally along
with the
moving material, i.e., flying die configuration. Alternatively, if the shape-
forming die
is too heavy to move with the material, it may become necessary to accumulate
the
material after the press and before the roll-forming process. Another option
for
excessively heavy dies would be to forgo the accumulation of material after
the press and
use a stop-start process with the roll-forming operation. Roll-form mills can
be run
successfully and efficiently using a stop-start repetitive motion, but stop-
start motion is
not a very complimentary process if in-line high frequency induction welding
is used. A
stop-start process may be more suited for either laser welding or contact
welding.
[0024] The formed shape and either the untrimmed blank or the trimmed blank
can now
be fed into the roll-forming process. The roll-form tooling would be designed
to provide
clearance for the formed shape. It is envisioned that only upper roll tooling
be used to
complete constant part geometry via roll forming, with the stamp formed
perimeter of
the part would be supported while the rest of the perimeter is roll formed. It
is
envisioned that the depth of the formed shape and the shape itself would
change along
the length of the part. The width at some given location and most preferably a
location
near the transition between the stamped formed section and the roll-formed
section
would remain constant along the length of the part. This common width is
important
because it provides a location where the material can be constrained as the
remaining
material is formed in the roll-forming process. The ability to constrain the
formed part
as the roll-forming process continues will cease because at some point along
the roll-
forming process, the individual legs of the part will converge to form a
tubular shape.
At this point, internal mandrels may be necessary to constrain the shape
formed from
stamping while the roll-forming process completes the tubular geometry.
[0025] The individual legs of the part are brought to a point of contact via
the roll-
forming process and at this point, the two individual legs are welded to form
a one-piece
tubular part with varying cross sections along the length of the part. Welding
can be
done in various ways where the type of the welding process used is based on
numerous
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1[lGLU1J, 1.G., gGV111CLIy, LLC511CU Gyl:1C Lll11C, CLC. . . . vpuons ior
weiaing may inciuae
high frequency induction welding, contact welding, or laser for tubular
section. Other
welding options such as impulse seam welding (rotary spot), plasma arc
welding, or
laser may be more suitable for irregular shaped geometries. All of these
welding
methods are widely used today to produce commercially available tubular and
irregular
shapes of constant cross section.
[0026] An automotive buinper beam typically requires some degree of curvature
which
is complimentary to vehicle styling. As styling changes and becomes more
aggressive, a
bumper beam with multiple radii becomes easier to package in the available
envelope.
Imparting of multiple radii in a bumper beam is referred to as a compound
swept
bumper. This ability to produce compound swept bumper beams in-line and during
the
manufacturing process can be accomplished with an apparatus that uses servos,
is
driven, and is computer controlled. This type of apparatus allows for real
time
adjustment of a bending roller which is used to impart curvature into the
beam. Because
of the real time adjustment of the bending roller, multiple radii can be
imparted at given
locations along the length of the beam. The last in-line operation is to cut
the beam
section to length. The cutoff operation can be done in various ways. The most
common
method of cutting bumper beams to length is to use a flying cutoff apparatus.
A flying
cutoff will travel with the part and perform the cut-off operation as the part
is moving at
line speed. A typical flying cut-off apparatus would include part clamping
steel forms,
air or hydraulic cylinders, and shearing blade.
[0027] The uniqueness of the present invention is the ability to produce a one-
piece roll-
formed structural beam or an open section roll-formed structural beam that has
varying
cross section along the length of the part. The ability to change the cross
section along
the length of the beam is achieved by the use of a forming die and a stamping
press
positioned upstream and in-line with the roll-forming process. The innovative
design of
the roll tooling allows the stamped pre-formed shape to move its coplanar flat
edge
sections through the individual roll-forming passes freely without distortion
or shape
change of its now-stamp-formed center section. Each pass of the roll-forming
process
continues to form the material of the edge sections into a final shape that
incorporates
the stamped form. Current roll-forming processes utilize relatively-lower-
force, fast-
acting stamping presses before the roll-forming process, but these presses are
used to
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piece ana cut reaiures sucn as noles, s10Ls, etc. into r.ne maLerlal wniie 1L
is rlat ana
before roll-forming is used to shape the part. The present invention increases
the
tonnage of the upstream stamping press and incorporates forming dies into the
stamping
press. The stamping press or presses are used to form the steel with a shape
that when
closed in geometry with the use of roll-forming, produces a tubular bearn
section with
varying cross-sectionals along the length of the part. The same concept can be
applied
to open sections where the stamping operation imparts an irregular shape
across the
length of the part and the roll-forming operations process the rest of the
part shape. The
varying of cross sections along the length of the part provides the
opportunity to
efficiently use material and geometry to achieve performance requirements. The
result
is a weight and performance optimized structural member that is produced in a
continuous cost-effective manufacturing process.
[0028] Fig. 1 is a flow chart showing a process embodying the present
invention, and
Fig. 1A shows a corresponding apparatus. As noted, the process starts with a
step 30
(Fig. 1) where a coiled sheet 31 (Fig. 1A) is unrolled and the uncoiled
strip/sheet 32 is
fed forward. The process proceeds to a step 33 to pre-pierce and shape form
the sheet 32
through the use of a stamping press 34 that actuates dies 35 and 36 together
against the
sheet 32. The press 34 and/or dies 35 and 36 preferably are movable laterally
along
with the sheet 32 during the stamping process and also are relatively fast-
acting.
Alternatively, the roll-forming operation can be slowed or stopped during the
stamping
operation. The dies 35 and 36 are configured and designed to stamp the sheet
material,
primarily pulling and moving material from a width direction and not from a
longitudinal
direction of the sheet 32. It is noted that a preferred material is ultra high
strength steel
(UHSS), which can be bent but basically not drawn or stretched. Accordingly,
pulling
and moving material laterally (rather than longitudinally) is a significant
concept when
USHH material is used. Slits in the sheet at each end of a (future) bumper
section can
be used to reduce longitudinal movement of sheet material during the three-
dimensional
deformation process of step 33.
[0029] The process proceeds in a step 38 through slitter 39 that slits/cuts
edges of the
formed sheet 32 to a particular known width dimension, with edge portions of
the sheet
32 still in a coplanar flat condition.
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lnc PrVCCNS uicn proeecu5 ni a 5Lep 1+1 rnrougn a ron-rorinmg operation, as
illustrated by the roll mill 42 having roll-forming stations 43-48'. The
particular
illustrated roll mill 42 has rolls constructed to form the sheet into a
tubular shape (see
Fig. 7) which is welded in process step 50 at abutting edges 51, 52 (Fig. 8)
and welded
at location 82 in a welding station 53 (Fig. 1A). Where necessary or
desirable, an
internal mandrel can be positioned upstream or downstream to help maintain
cross
sectional shape or to provide support for bending or forming through the roll
forming
operation. For example, the internal mandrel can be anchored at location 54
and an
anchoring tie-bar or cord can extend downstream (or upstream) to provide
mandrel
support. Due to the stamping operation, the middle or lower portion of the
cross section
changes shape longitudinally and laterally, causing the final cross section of
the final
beam 72 to vary in a depth dimension and shape along its length (see Figs. 3-
6).
[0031] The process then proceeds in a step 60 where the continuous welded
tubular
beam 61 is longitudinally swept as it moves through stabilizing/motivating
rolls 49 and a
sweep station 62 with adjustable external mandrels 63 and with (if needed)
internal
mandrels 64. Thereafter, the continuous beam 61 is cut to length in step 70 by
a cut-off
apparatus 71 into individual beams 72 useful as impact reinforcement bumper
beams on
a vehicle. The entire process is controlled by a controller for optimal,
coordinated, and
simultaneous operation.
[0032] Notably, one overall apparatus and process of rolling, welding, and
sweeping a
tubular beam of a constant cross section is shown and described in Sturrus
5,092,512
and 5,454,504, and an exemplary cut-off device is shown in Heinz 5,305,625.
The
reader is referred to these disclosures if additional detail about those
processes are
desired. Also, their teachings are incorporated herein in their entirety for
the purpose of
fully disclosing and teaching the present invention. The present concepts can
be used
with a power adjusted variable sweep station, such as those known in the art.
[0033] Fig. 2 is a side view of a vehicle bumper beam embodying the present
invention
as it comes off the cut-off apparatus 71.
[0034] Figs. 3-6 are cross sections taken through Fig. 2. Notably, these cross
sections
are similar to cross sections taken through similar locations in the
continuous beam 61
(see Fig. 7) prior to the step of sweeping the beam 61.
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Luu35I Fig. 7 is a side view ot tne venicle bumper neam arEer neing roii-
rormea nut
prior to being longitudinally swept. Notably, the top "half" section of the
beam is linear
and constant in cross section (compare the upper portion of Figs. 3-6 and also
see Fig.
8), while the bottom "half" section of the beam is deformed into different
three-
dimensional changing longitudinal shapes (compare the Figs. 3-6). It is
contemplated
that various features can be incorporated into the beams 72. For example, the
angled
surfaces 73 (Fig. 2) become coplanar and aligned mounting surfaces 74 when the
beam
61 is swept to become individual beams 72. The coplanar mounting surfaces 74
are
adapted to be attached to a front of vehicle frame rails, such as by
attachment bolts or
fasteners, and can include pre-pierced holes for the attachment bolts and
fasteners.
[0036] Fig. 8 is an end view of the beam of Fig. 7 moving through an exemplary
roll-
forming station, the view showing the beam 61 engaged by bottom forming roll
80 and
side containment rolls 81. A weld joint 82 found at station 53 is shown at
abutting edges
51 and 52. Sequential formation of the top "wings" or side flanges 84 of the
sheet are
illustrated by the positions 85-88. It is contemplated that the bottom forming
roll 80
and/or side constraint roll 81 can also be fully or partially replaced with
stationary guide
blocks and/or roll(s). In particular, I envision a modification where only top
roll form
tooling is used, and the bottom is braced with either blocks, or (non-forming)
rolls, or a
combination of blocks and rolls.
[0037] It is contemplated that the present process can be used to manufacture
sophisticated beams with non-uniform cross section along their length,
allowing the
beams to be "customized" and optimized for various applications, such as for
interior
cross car structural beam, frame components, exterior cross car structural
beams, roof
bows, windshield header, rocker panels/sills, door beams, engine cradles, and
instrument panel supports. It is contemplated that the present concept would
be more
cost competitive and have a more efficient/higher through-put than hydro-
forming
processes and also provide design flexibility over stamping processes.
MODIFICATION
[0038] A flow chart (Fig. 9) and apparatus (Fig. 9A) are illustrated similar
to Figs. 1
and 1A respectively, but modified and adapted for hot stamping ahead of the
roll
forming portion of the machine. Specifically, uncoiled sheet material 32
capable of
being hot stamped and quenched for high material strength is heated to a
temperature of
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at least about 850 C (and preferably to about 880 C to 950 C, depending on
material
chemistry) in a step (100) by heaters 101. The duration of the heating process
can be
varied, depending on the BTU capability of the heaters, and depending on
whether the
process is a stop-start roll forming process or a continuous process. A length
of the
heaters is extended as necessary to match a speed of the machine/process. The
heaters
101 are positioned immediately prior to the step 33 of piercing and shaping of
the sheet
32, at a location adjacent and upstream of the stamping press 34. The dies
35/36 are
cooled by a coolant system 103 and/or water/coolant is applied by a quenching
system
103 to the formed sheet while the formed sheet is still between the dies 35/36
or as the
formed sheet exits the dies 35/36. The timing of cooling is critical. Cooling
must be
done before the material temperature drops unacceptably. Typically, the
quenching is
done during or within seconds of the forming operation, such as by cooled dies
and/or
coolant applied to the steel while compressed between the dies.
[0039] A typical steel suitable for the hot stamping process is Usinor pre-
coated
USIBOR 1500. The material as received and formed in the press 34 is preferably
at a
yield strength greater than 370 MPa, preferably at a tensile strength greater
than 550
MPA, and has an elongation of greater than 14%. The properties of the "as-
received"
USIBOR 1500 are very suitable for press forming, because the material is able
to be
drawn (i.e. "stretched" or extruded such that a thickness of the material is
reduced). At
the same time, the post quench properties of this material are significantly
higher; with a
yield in the range of 1000 MPa, a tensile strength in the range of 1500MPa,
and an
elongation in the range of 5%. These higher material properties are well-
suited for
energy absorption, improved part performance in crashworthiness, and provide
an
excellent strength to weight ratio. By using this material, it is possible to
make deeper
deformations in the sheet in the stamping process without losing as much width
in the
final sheet (i.e. prior to the step of slitting 38 in slitter 39) ... thus
reducing material
waste. Also, it is noted that ultra high strength steels work harden when
bent, thus
limiting the ability to bend them more than once. This problem is avoided by
the hot
stamping process, since the material is able to be drawn and does not have the
same
problems associated with rapid work hardening of the sheet material. By hot
stamping
mild steel material, there are more options in how to move material in the
stamping die,
thus facilitating the stamping process and allowing deeper sections to be
drawn/formed
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WO 2007/044327 PCT/US2006/038609
into the sheet material. For example, 30 % to 50 % deeper sections can be
drawn into the
steel sheet.
[0040] The uniqueness of the present concept includes the ability to combine
the benefits
of two very different manufacturing processes to produce a more competitive
part.
Benefits include the ability to change cross section along the length of a
part. Higher part
strength and reduced part weight represent benefits that are gained when
material
properties are increased and cross sections are tubular. The stamping process
either cold
stamping or hot stamping has the ability to vary shape across the part length.
The hot
stamped process has the added ability to work with as received mild steel
which is
suitable for forming and then transform the material during the processing
into a
material with extremely high properties. This transformation is achieved after
the
quenching of the heated and formed steel. This avoids the problems associated
with
work-hardening which can occur when high strength materials are stamped ...
since the
sheet material becomes high strength only when queiiched after the stamping
operation.
In the present process, roll forming is used to receive the stamped sheet and
finish the
forming of the part. The gradual forming stages associated with the roll
forming process
are capable of forming materials of very high strength level, including high
strength and
ultra high strength steels. This makes roll forming a desirable method for
final forming
of open or tubular pressed forms that are formed hot in a stamping press.
[0041] The hot stamped process provides greater capability over cold stamping
since
forming is done with one material grade/strength and the final part is at
another higher
grade/strength. Material as received for the hot stamping processes is
generally
classified as mild steel and possesses properties very suitable for stamp
forming. The hot
steel is formed and then quenched in the die (or immediately quickly
thereafter), which
produces a martensitic grain structure and material properties more common to
martensitic steels with tensile strengths greater than 1500 MPa.
[0042] It is to be understood that variations and modifications can be made on
the
aforementioned structure without departing from the concepts of the present
invention,
and further it is to be understood that such concepts are intended to be
covered by the
following claims unless these claims by their language expressly state
otherwise.
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