U.S. patent application number 13/046251 was filed with the patent office on 2012-03-15 for method of producing press-hardened structural parts.
This patent application is currently assigned to Benteler Automobiltechnik GmbH. Invention is credited to Friedrich Bohner, Otto Buschsieweke, Ulrich Huschen, Paul Kaupmann.
Application Number | 20120060982 13/046251 |
Document ID | / |
Family ID | 44276033 |
Filed Date | 2012-03-15 |
United States Patent
Application |
20120060982 |
Kind Code |
A1 |
Bohner; Friedrich ; et
al. |
March 15, 2012 |
METHOD OF PRODUCING PRESS-HARDENED STRUCTURAL PARTS
Abstract
In a method, a blank of unhardened steel sheet is subjected in a
press tool to a hot forming and press-hardening process to produce
a structural part. After press-hardening, at least one linear zone
of the structural part is heat treated in order to increase
ductility and reduce strength in the linear zone in relation to
adjacent regions of the structural part. After heat treatment, a
bending or cutting operation is carried out on the structural part
along the linear zone.
Inventors: |
Bohner; Friedrich;
(Oerlinghausen, DE) ; Buschsieweke; Otto;
(Paderborn, DE) ; Huschen; Ulrich; (Lichtenau,
DE) ; Kaupmann; Paul; (Anroechte, DE) |
Assignee: |
Benteler Automobiltechnik
GmbH
Paderborn
DE
|
Family ID: |
44276033 |
Appl. No.: |
13/046251 |
Filed: |
March 11, 2011 |
Current U.S.
Class: |
148/653 ;
72/364 |
Current CPC
Class: |
C21D 2221/00 20130101;
C21D 1/673 20130101; B32B 15/01 20130101; B21D 22/022 20130101;
C21D 2261/00 20130101; B32B 15/018 20130101; B21D 5/00
20130101 |
Class at
Publication: |
148/653 ;
72/364 |
International
Class: |
C21D 8/02 20060101
C21D008/02; B21D 53/00 20060101 B21D053/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 12, 2010 |
DE |
102010011368.9-14 |
Aug 2, 2010 |
EP |
10 008 035.7 |
Claims
1. A method, comprising the steps of: subjecting a blank of
unhardened steel sheet in a press tool to a hot forming and
press-hardening process to produce a structural part; heat treating
at least one linear zone of the structural part; and subjecting the
structural part after the heat treating step to a bending or
cutting operation along the linear zone.
2. The method of claim 1, wherein the structural part has a tensile
strength of at least 700 MPa after undergoing press-hardening.
3. The method of claim 1, wherein the structural part has a
ductility which is increased in the linear zone in relation to a
region adjacent to the linear zone.
4. The method of claim 3, wherein the ductility in the linear zone
is increased by at least 5% in relation to the adjacent region of
the structural part.
5. The method of claim 1, wherein the heat treating step is
executed at a temperature between 100.degree. C. and 700.degree. C.
over a time period of less than or equal to 30 seconds.
6. The method of claim 1, wherein the heat treating step is
executed inductively.
7. The method of claim 1, wherein the heat treating step is
executed through infrared radiation.
8. The method of claim 1, wherein the heat treating step is
executed while the structural part is clamped in a holding
device.
9. The method of claim 8, wherein the structural part is clamped in
the holding device until the structural part has cooled down to a
temperature level that does not cause distortion.
10. The method of claim 8, wherein at least one area of the
structural part is maintained in the holding device under elastic
tension.
11. The method of claim 8, wherein the structural part is secured
in the holding device in such a way as geometrical changes of the
structural part are compensated during the heat treating step so
that the structural part has a desired geometry after undergoing
heat treatment and/or removal from the holding device.
12. The method of claim 1, further comprising the step of coating a
surface of the structural part.
13. The method of claim 1, further comprising the step of coating
the blank before undergoing heat treatment and press-hardening.
14. The method of claim 1, wherein the structural part is used for
a body or component of a motor vehicle.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application claims the priorities of German Patent
Application, Serial No. 10 2010 011 368.9-14, filed Mar. 12, 2010,
and European Patent Application, Serial No. 10 008 035.7, filed
Aug. 2, 2010, pursuant to 35 U.S.C. 119(a)-(d), the contents of
which are incorporated herein by reference in its entirety as if
fully set forth herein.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a method of producing
press-hardened structural parts, in particular for a body or
components of motor vehicles.
[0003] The following discussion of related art is provided to
assist the reader in understanding the advantages of the invention,
and is not to be construed as an admission that this related art is
prior art to this invention.
[0004] To decrease the total weight and to reduce fuel consumption
of motor vehicles, the so called New Steel Body concept has been
developed in recent years, involving lightweight high-strength
steel construction that allows designers to save weight by reducing
the sheet metal thickness and thus the total weight of the vehicle.
Such vehicle components of high strength steel normally undergo
heat treatment and press-hardening.
[0005] U.S. Pat. No. 5,972,134 discloses a process for the
manufacture of a metallic molded structural part for motor vehicle
components such as door impact girders or bumpers with areas with a
higher ductility in relation to the rest of the structural
component part. For this purpose, partial areas of a plate are
initially heated to a temperature of 900.degree. C. within a period
of less than 30 seconds. Subsequently, the thermal-treated plate is
shaped in a pressing tool to form the molded structural part and is
heat-treated in the pressing tool. This conventional process
suffers shortcomings in particular, when narrow and especially
linear areas are subjected to heat treatment. Temperatures reaching
up to 900.degree. C. cause warping and deformation of the
structural part and may cause the heat-treated area of the
structural part to harden again because the steel microstructure
depending on the composition is not only tempered starting from a
temperature of 723.degree. C. but is also partly austenitized.
Further, this conventional process renders a non-warping securement
of the structural part difficult because of the use of a conveyor
device.
[0006] Materials used during heat treatment and press-hardening and
the resultant structural part have high strengths of 700 MPa and
more during shaping. At such strengths, the risk of a delayed
cracking and the possibility of unpredictable failure of the
structural parts rises in view of the presence of hydrogen which
can be absorbed during production (metallurgical hydrogen) or as a
result of surface treatment or when the material corrodes.
[0007] A delayed cracking may also be encountered after undergoing
a plastic deformation during which the surface is activated. (Hard)
cut edges of high or super high strength structures that are
exposed to a corrosive environment in the presence of atomic
hydrogen tend to brittle in the area of the cut edges as a result
of inherent stress and thus tend to encounter a delayed cracking.
When heat treated and press-hardened motor vehicle components such
as B pillars for example are involved, such a hydrogen
embrittlement of trimmed edges may lead to reduced energy
absorption in the event of a crash.
[0008] It would therefore be desirable and advantageous to address
these problems and to obviate other prior art shortcomings.
SUMMARY OF THE INVENTION
[0009] According to one aspect of the present invention, a method
includes the steps of subjecting a blank of unhardened steel sheet
in a press tool to a hot forming and press-hardening process to
produce a structural part, heat treating at least one linear zone
of the structural part, and subjecting the structural part after
the heat treating step to a bending or cutting operation along the
linear zone.
[0010] In accordance with the present invention, the mechanical
properties of a press-hardened structural part is optimized in a
narrow linear zone through heat treatment in order to permit
subsequent bending or cutting operations to be executed simpler and
more cost-efficient. The structural part that has been completely
through hardened is hereby heat treated along a linear zone
commensurate with a later bending line or trimming line after being
hot formed. As a result, strength is reduced in the linear zone and
ductility is increased. The hot formed and press-hardened
structural part can then in an optimum manner undergo a bending or
cutting operation along the linear zone. The processing steps are
thus efficient, cost saving, and used processing machines are
subject to less wear. In addition, the method according to the
present invention results in a resistance to a delayed cracking of
the structural parts.
[0011] Critical parameters that promote delayed cracking include
i.a. local accumulation of hydrogen atoms and the presence of
residual or internal stress. Thus, the present invention proposes
to reduce the strength of edges of the structural part, especially
the cut edges, and to increase ductility. By targeting the heat
treatment, e.g. inductive heat treatment, along the trimming line
after the hot forming process, mechanical properties of the fully
through hardened structural part are locally optimized along the
cut edges and/or in the regions adjacent to the cut edges so that
the tendency to embrittlement and cracking are minimized or even
eliminated.
[0012] After hot forming and press-hardening, the structural part
has a tensile strength of at least 700 MPa. Currently preferred is
a tensile strength of 1,000 MPa to 1,500 MPa.
[0013] According to another advantageous feature of the present
invention, the ductility in the linear zone can be increased by at
least 5% in relation to a region of the structural part that is
adjacent to the linear zone and does not undergo a heat
treatment.
[0014] The hot formed and press-hardened structural part is
suitably clamped and fixed in position during heat treatment in a
holding device. The heat treatment can hereby be carried out within
the holding device in accordance with a temperature-time diagram
that is specific for the structural part at hand. The linear zone
of the structural part can be subjected to heat treatment at a
temperature between 100.degree. C. and 700.degree. C. over a time
period of less than or equal to (.ltoreq.)30 seconds. As a result,
ductility is locally increased in the linear zone and the strength
reduced.
[0015] According to another advantageous feature of the present
invention, the heat treating step can be executed inductively. As
an alternative, the heat treating step may also be executed through
infrared radiation. Inductive heat treatment is applicable for very
narrow linear zones of the structural part. A very small and fine
transition zone can hereby be created between regions of high
strength and high hardness and regions of less strength and less
hardness.
[0016] According to another advantageous feature of the present
invention, the structural part can be clamped in the holding device
until the structural part has cooled down to a temperature level
that does not cause distortion. As a result, the structural part
does not encounter or encounters only insignificant distortion
which is within permissible dimensional tolerances after the heat
treatment. The holding device with integrated heating unit, for
example integrated inductor, thus forms an important aspect of the
present invention.
[0017] According to another advantageous feature of the present
invention, at least some areas of the structural part can be
maintained in the holding device under elastic tension while the
linear zone undergoes heat treatment. As a result, the structural
part can be further shaped with a desired geometry. In this way, by
superimposing forced bending, tensile and pressure forces and heat
stress, the properties of the structural part can be tailored in
the region undergoing heat treatment.
[0018] According to another advantageous feature of the present
invention, the structural part can be secured in the holding device
in such a way that geometrical changes of the structural part are
compensated during the heat treating step so that the structural
part has a desired geometry after undergoing heat treatment and/or
removal from the holding device. There is thus no need to
necessarily correspond the contour of the holding device to the
desired end geometry of the structural part. Geometric changes of
the structural part are taken into account during heat treatment
and utilized during configuration of the structural part and
production of the linear zone that has changed material
microstructure.
[0019] As described above, the hardened structural part is heated
in the holding device for a time period of 30 seconds to a
temperature between 100.degree. C. and 700.degree. C. Suitably, the
heat treatment is executed over a time period of less than 10
seconds. Currently preferred is a time period of 2 seconds. As the
temperature for heat treatment is below 700.degree. C.,
re-hardening of the heat treated zone is avoided.
[0020] According to another advantageous feature of the present
invention, the structural part may be provided with a surface
coating. The surface coating may be applied onto the blank before
undergoing heat treatment and press-hardening or after subjecting
the linear zone to heat treatment. Examples of a pre-coated blank
as starting material include hot dip aluminized blank, a blank
coated with an aluminum-silicon alloy, or a blank coated with a
zinc or aluminum-zinc alloy. A steel blank used as starting
material is thus provided with a protection against corrosion
already before hot forming and press-hardening. The protection
against corrosion may be based on a light metal alloy.
BRIEF DESCRIPTION OF THE DRAWING
[0021] Other features and advantages of the present invention will
be more readily apparent upon reading the following description of
currently preferred exemplified embodiments of the invention with
reference to the accompanying drawing, in which:
[0022] FIG. 1 is a perspective illustration of an inductor for use
in the production of a structural part in accordance with to the
present invention;
[0023] FIG. 2 is a perspective illustration of a holding device
with integrated inductor for use in the production of a structural
part in accordance with to the present invention;
[0024] FIG. 3 is an enlarged detailed view of an area encircled in
FIG. 2 and marked "A";
[0025] FIG. 4 is a perspective illustration of a detail of the
holding device, depicting the disposition of the inductor; and
[0026] FIG. 5a is a schematic illustration of a detail of a
structural part;
[0027] FIG. 5b is a schematic illustration of a side area of the
structural part with bent longitudinal edge; and
[0028] FIG. 5c is a schematic illustration of the side area of the
structural part with trimmed longitudinal edge.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0029] Throughout all the figures, same or corresponding elements
may generally be indicated by same reference numerals. These
depicted embodiments are to be understood as illustrative of the
invention and not as limiting in any way. It should also be
understood that the figures are not necessarily to scale and that
the embodiments are sometimes illustrated by graphic symbols,
phantom lines, diagrammatic representations and fragmentary views.
In certain instances, details which are not necessary for an
understanding of the present invention or which render other
details difficult to perceive may have been omitted.
[0030] Turning now to the drawing, and in particular to FIG. 1,
there is shown a perspective illustration of an inductor, generally
designated by reference numeral 1, for use in the production of a
structural part 5 (FIGS. 5a, 5b) in accordance with the present
invention. Although the use of an inductor is described here by way
of example, it is of course also conceivable to use infrared
radiation for example as an alternative.
[0031] The inductor 1 includes a narrow square copper tube 2 which
has three sides facing away from the surface of the structural part
being treated and surrounded by concentrator sheets 3, 4,
respectively. In practical application, the copper tube 2 may have
a width of 4 millimeters and a height of 8 millimeters. The
concentrator sheets 3, 4 extend over the entire length of the
copper tube 2 and have a thickness of 4 millimeters. The inductor 1
has a longitudinal dimension which is a multiple of the width
thereof, and may be configured with several curves in the course of
its longitudinal dimension. The geometric contour and curved
profile is suited to the heat treatment being carried out on the
structural part 5, as will be described in greater detail
hereinafter,
[0032] FIGS. 2 and 3 show the inductor 1 integrated in a holding
device 6. The holding device 6 essentially includes a top tool 7
and a bottom tool 8. Both, top and bottom tools 7, 8 are made of
light metal, such as aluminum or aluminum alloy. Aluminum or
aluminum alloy have the benefit that the material is heat
conducting.
[0033] Production of a structural part 5 of complex configuration,
for use in a body or as component of a motor vehicle, is carried
out as follows. A blank of non-hardened and hot formable steel
sheet is heated to a temperature above the austenitizing
temperature and hot formed in a press tool into the structural part
5 which is clamped in the press tool and undergoes press-hardening.
The structural part 5 has a tensile strength of at least 700 MPa.
Currently preferred is a structural part 5 after undergoing hot
forming and press-hardening with a tensile strength in a range
between 1,000 MPa and 1,500 MPa. The thus press-hardened structural
part 5 is removed from the press tool and a targeted narrow linear
zone B of the structural part 5 is then subjected to a heat
treatment. Although the disclosure relates to the presence of one
linear zone B, it will be understood by persons skilled in the art
that this is done by way of example only, as the structural part
may, of course, have several linear zones that are subjected
locally to heat treatment in order to reduce strength and increase
ductility.
[0034] The linear zone B corresponds hereby to the course of a
later bending line or trimming line of the structural part 5, as
will be described hereinafter with reference to FIGS. 5a and
5b.
[0035] The heat treatment takes place in the holding device 6 in
which the hot formed and press-hardened structural part 5 is either
received and clamped entirely in a form-fitting manner or by linear
support surfaces 9, 10, shown in FIG. 3. The support surfaces 9, 10
extend along the structural part contour on both sides in
correspondence to the course of the linear zone B and inductor 1,
i.e. the bending or trimming line. In the illustrated example, the
inductor 1 is integrated in an oblong hole 11 of the holding device
6, as shown in FIG. 4. The inductor 1 is suitably integrated in the
upper tool 7, although it is, of course, conceivable to integrate
the inductor in the lower tool 8. The inductor 1 with its square
copper tube 2 and the surrounding concentrator sheets 3, 4 is
arranged in the holding device 6 such as to have a same distance to
the surface of the structural part 5 over the entire length.
Currently preferred is a distance of 2 millimeters.
[0036] As the structural part 5 is securely clamped in the holding
device 6, the linear zone B is subjected to a heat treatment. The
heat treatment is performed inductively by means of the inductor 1.
The linear zone B of the hardened structural part 5 is hereby
heated to a temperature between 100.degree. C. and 700.degree. C.
for a time period of less than or equal to 30 seconds. Currently
preferred is a time period of 2 seconds. The heat treatment of the
linear zone B increases its ductility and reduces its strength. The
holding device 6 retains the shape of the inductively heated
structural part 5, normally maximal 30 seconds, until the
temperature is lowered to a level that does not cause warping or
distortion of the structural part 5.
[0037] The heat treatment and the resultant change in material
properties can be carried out in a very precise manner and tailored
to a very small or narrow but relative long linear region (zone B)
with fine transitions. The tailored heat treatment prevents the
drawbacks of hydrogen embrittlement, in particular hydrogen-induced
cracking. A localized accumulation of hydrogen atoms and the
presence of residual or internal tensions are reduced by the heat
treatment to uncritical levels.
[0038] FIGS. 5a, 5b, 5c show schematically a hot formed and
press-hardened structural part 5. FIG. 5a shows the
three-dimensional structural part 5 after the linear zone B
underwent heat treatment to alter its material property. The
targeted heat treatment at a temperature between 100.degree. C. and
700.degree. C. for a time period of 30 seconds increases ductility
and reduces strength.
[0039] Following the heat treatment in the holding device 6, a
bending operation and/or trimming operation is carried out on the
structural part 5 along the linear zone B. FIG. 5b shows a
right-hand side area 12 of the structural part 5 with bent
longitudinal edge 13. The ductile strength-reducing linear zone B
extends at the outer free end 14 of the bent side area 12 along the
curved region of the longitudinal edge over the length of the
structural part 5.
[0040] FIG. 5c shows the side area 12 of the structural part 5 with
trimmed edge 15. The ductile strength-reducing linear zone B
extends at the outer trimmed edge 15 over the length of the
structural part 5. The cutting operation has been executed within
the linear zone B. The side area 12 is further bent in the linear
zone B.
[0041] Although not shown in detail, the structural part 5 can be
provided with a surface coating. Application of the surface coating
may be carried out after heat treatment and following bending or
trimming operations or also prior thereto.
[0042] It is also conceivable to use a pre-coated blank as starting
material for producing a press-hardened structural part 5. Heat
treatment of a linear zone B can then be carried out, as described
above, on the press-hardened structural part produced from the
pre-coated blank.
[0043] It is further possible to maintain at least some areas of
the structural part 5 in the holding device 6 under elastic tension
during heat treatment. The structural part 5 is hereby fixed in the
holding device 6 in a way as to take into account geometrical
changes of the structural part 5 as a result of the heat treatment
so that the structural part 5 has the desired geometry after heat
treatment and/or removal from the holding device 6.
[0044] While the invention has been illustrated and described in
connection with currently preferred embodiments shown and described
in detail, it is not intended to be limited to the details shown
since various modifications and structural changes may be made
without departing in any way from the spirit and scope of the
present invention. The embodiments were chosen and described in
order to explain the principles of the invention and practical
application to thereby enable a person skilled in the art to best
utilize the invention and various embodiments with various
modifications as are suited to the particular use contemplated.
[0045] What is claimed as new and desired to be protected by
Letters Patent is set forth in the appended claims and includes
equivalents of the elements recited therein:
* * * * *