U.S. patent application number 13/273760 was filed with the patent office on 2012-04-19 for automobile column and method for producing a hot-formed and press-hardened automobile column.
This patent application is currently assigned to Benteler Automobiltechnik GmbH. Invention is credited to JOHANNES BOKE, JAN DINGEMANS, MARKUS PELLMANN, ANDREAS ZIMMERMANN.
Application Number | 20120091758 13/273760 |
Document ID | / |
Family ID | 45347055 |
Filed Date | 2012-04-19 |
United States Patent
Application |
20120091758 |
Kind Code |
A1 |
ZIMMERMANN; ANDREAS ; et
al. |
April 19, 2012 |
AUTOMOBILE COLUMN AND METHOD FOR PRODUCING A HOT-FORMED AND
PRESS-HARDENED AUTOMOBILE COLUMN
Abstract
An automobile column and to a method for producing an automobile
column are disclosed. The automobile column has a region of a first
type and a region of a second type which have mutually different
strengths. A transition region having a width of less than 50 mm is
formed between the two regions. The automobile column has in the
region of the first type a bainitic structure and in the region of
the second type a martensitic structure.
Inventors: |
ZIMMERMANN; ANDREAS;
(Bielefeld, DE) ; DINGEMANS; JAN; (Paderborn,
DE) ; PELLMANN; MARKUS; (Sassenberg, DE) ;
BOKE; JOHANNES; (Blomberg, DE) |
Assignee: |
Benteler Automobiltechnik
GmbH
Paderborn
DE
|
Family ID: |
45347055 |
Appl. No.: |
13/273760 |
Filed: |
October 14, 2011 |
Current U.S.
Class: |
296/193.06 ;
148/643 |
Current CPC
Class: |
C22C 1/02 20130101; B62D
25/08 20130101; C22C 38/06 20130101; C21D 2221/00 20130101; C21D
1/673 20130101; B62D 25/04 20130101; C22C 38/02 20130101; C21D
2211/002 20130101; C21D 8/005 20130101; C21D 1/20 20130101; C21D
2211/008 20130101; C22C 38/14 20130101; C22C 38/32 20130101; C21D
8/04 20130101; C21D 8/0205 20130101; C22C 38/04 20130101 |
Class at
Publication: |
296/193.06 ;
148/643 |
International
Class: |
B62D 25/04 20060101
B62D025/04; C21D 9/00 20060101 C21D009/00; C21D 8/00 20060101
C21D008/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 15, 2010 |
DE |
10 2010 048 209.9 |
Feb 23, 2011 |
EP |
11 155 681.7 |
Claims
1. An automobile column comprising: at least two regions of
different strength produced by hot-forming and press-hardening,
wherein a region of a first type has after press-hardening a
substantially bainitic structure and a region of a second type has
after press-hardening a substantially martensitic structure, and a
transition region between the region of the first type and the
region of second type being smaller than 80 mm.
2. The automobile column of claim 1, wherein the transition region
is smaller than 50 mm.
3. The automobile column of claim 1, wherein the transition region
is smaller than 30 mm.
4. The automobile column of claim 1, wherein the transition region
is smaller than 20 mm.
5. The automobile column of claim 1, wherein the substantially
martensitic structure of the region of the second type comprises
additional structure components in a concentration of less than
50%.
6. The automobile column of claim 1, wherein the substantially
martensitic structure of the region of the second type comprises
additional structure components in a concentration of less than
30%.
7. The automobile column of claim 1, wherein the substantially
martensitic structure of the region of the second type comprises
additional structure components in a concentration of less than
15%.
8. The automobile column of claim 5, wherein the additional
structure component comprises bainite.
9. The automobile column of claim 1, wherein the substantially
bainitic structure of the region of the first type comprises
additional structure components in a concentration of less than
50%.
10. The automobile column of claim 1, wherein the substantially
bainitic structure of the region of the first type comprises
additional structure components in a concentration of less than
30%.
11. The automobile column of claim 1, wherein the substantially
bainitic structure of the region of the first type comprises
additional structure components in a concentration of less than
15%.
12. The automobile column of claim 1, wherein the region of the
first type is at least partially enclosed by the region of the
second type.
13. The automobile column of claim 12, wherein the region of the
first type is completely enclosed by the region of the second
type.
14. The automobile column of claim 1, wherein the region of the
first type is spot-shaped with a diameter of less than 40 mm.
15. The automobile column of claim 1, wherein the region of the
first type is spot-shaped with a diameter of less than 20 mm.
16. The automobile column of claim 1, wherein the region of the
first type is spot-shaped with a diameter of less than 10 mm.
17. The automobile column of claim 1, wherein the region of the
first type is constructed as a flange, a connecting flange or an
outside edge of the automobile column, or a combination
thereof.
18. The automobile column of claim 1, wherein the region of the
first type is formed in regions of the automobile column which are
subject to strong deformations in a crash or which are configured
to dissipate crash energy through deformations.
19. The automobile column of claim 1, wherein the region of the
first type has an increased wall thickness in relation to the
region of the second type.
20. The automobile column of claim 1, further comprising a passage
or an edge, or both, in the region of the first type after
hot-forming.
21. The automobile column of claim 1, wherein the region of the
first type has a stretchability A50 between 10% and 30%.
22. The automobile column of claim 1, wherein the region of the
first type has a stretchability A50 between 12% and 20%.
23. The automobile column of claim 1, wherein the region of the
first type has a stretchability A50 between 12% and 16%.
24. The automobile column of claim 1, wherein the region of the
first type has a stretchability A50 between 14% and 16%.
25. The automobile column of claim 1, wherein the region of the
first type has a tensile strength between 500 and 1000
N/mm.sup.2.
26. The automobile column of claim 1, wherein the region of the
first type has a tensile strength between 550 and 800
N/mm.sup.2.
27. The automobile column of claim 1, wherein a yield strength or a
tensile strength decreases or increases in the transition region
with a gradient of more than 100 N/mm.sup.2 per 10 mm.
28. The automobile column of claim 1, wherein a yield strength or a
tensile strength decreases or increases in the transition region
with a gradient of more than 200 N/mm.sup.2 per 10 mm.
29. The automobile column of claim 1, wherein a yield strength or a
tensile strength decreases or increases in the transition region
with a gradient of more than 400 N/mm.sup.2 per 10 mm.
30. The automobile column of claim 1, wherein the region of the
second type has a strength of more than 1000 N/mm.sup.2.
31. The automobile column of claim 1, wherein the region of the
second type has a strength of more than 1200 N/mm.sup.2.
32. The automobile column of claim 1, wherein the region of the
second type has a strength of more than 1400 N/mm.sup.2.
33. The automobile column of claim 1, wherein region of the first
type has a yield strength between 200 and 800 N/mm.sup.2.
34. The automobile column of claim 1, wherein region of the first
type has a yield strength between 250 and 600 N/mm.sup.2.
35. The automobile column of claim 1, wherein region of the first
type has a yield strength between 250 and 500 N/mm.sup.2.
36. The automobile column of claim 1, wherein region of the first
type has a yield strength between 300 and 500 N/mm.sup.2.
37. The automobile column of claim 1, wherein the automobile column
is manufactured from a Tailor Welded Blank or a Tailor Rolled
Blank.
38. A method for producing a hot-formed and press-hardened
automobile column having at least two regions of different
hardness, said method comprising the steps of: providing a
hardenable metal plate or semi-finished product and heating the
hardenable metal plate or semi-finished product to at least an
austenizing temperature, intermediately cooling a region of a first
type of the metal plate or semi-finished product with a cooldown
speed selected to be greater than a lower critical cooldown speed
of a material of the metal plate or semi-finished product, and
hot-forming and press-hardening the metal plate or semi-finished
product in a press-hardening tool to form the automobile
column.
39. The method of claim 38, wherein a region of a second type is
held above the austenizing temperature until the region of a second
type is transported into the press-hardening tool.
40. The method of claim 38, wherein the cooldown speed during
intermediate cooling of the region of the first type is selected
such that a bainitic structure is obtained.
41. The method of claim 40, wherein the region of the first type is
cooled to a cooling temperature between 600 and 400.degree. C.
42. The method of claim 41, wherein the region of the first type is
cooled to a cooling temperature of about 500.degree. C.
43. The method of claim 40, wherein the region of the first type is
held at the cooling temperature for a predetermined time.
44. The method of claim 40, wherein the region of the first type is
held at the cooling temperature isothermally.
45. The method of claim 38, further comprising the step of
quenching the region of the first type in the press-hardening tool
from a bainitic structure transformation stage, whereby a mixed
structure of martensite and bainite, or a mixed structure of
martensite, bainite and at least one of ferrite and perlite, is
adjusted in the region of the first type.
46. The method of claim 38, further comprising the step of holding
the region of the first type isothermally so as to form a
substantially pure bainitic structure by press-hardening.
47. The method of claim 38, wherein the cooldown speed of the
intermediate cooling is selected to be greater than an upper
critical cooling-down speed.
48. The method of claim 38, wherein the intermediate cooling of the
region of the first type is performed in the press-hardening
tool.
49. The method of claim 48, wherein the intermediate cooling of the
region of the first type is performed by using cooling plates
arranged in the press-hardening tool.
50. The method of claim 38, wherein the metal plate is pre-formed
into a semi-finished product while cold before being heated to at
least the austenizing temperature.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application claims the priority of German Patent
Application, Serial No. 10 2010 048 209.9, filed Oct. 15, 2010, and
of European Patent Application Serial No. 11 155 681.7, filed Feb.
23, 2011 pursuant to 35 U.S.C. 119(a)-(d), the content of which is
incorporated herein by reference in its entirety as if fully set
forth herein.
[0002] This is one of two applications both filed on the same day.
Both applications deal with related inventions. They are commonly
owned and have the same inventive entity. Both applications are
unique, but incorporate the other by reference. Accordingly, the
following U.S. patent application is hereby expressly incorporated
by reference: "SIDE RAIL AND METHOD FOR PRODUCING A HOT-FORMED AND
PRESS-HARDENED SIDE RAIL".
BACKGROUND OF THE INVENTION
[0003] The present invention relates to an automobile column,
produced by hot-forming and press hardening. The present invention
also relates to a method for producing an automobile column by
hot-forming and press hardening.
[0004] 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.
[0005] The requirements profile for vehicle safety increases in the
automotive industry due to regulatory and manufacturer-specific
guidelines. At the same time, the automobile manufacturers strive
to reduce the weight of the automobile bodies in order to minimize
fuel consumption and CO.sub.2 emission. This creates a divergence
between low weight and high bending and torsion strength and high
crash safety.
[0006] According to one approach, for example light-metal
materials, in particular aluminum alloys, or bodies in hybrid
construction, for example made of metallic alloys and fiber
composite material or plastics, can be used. However, the
aforementioned approaches are both associated with high material
costs, which in turn increases the vehicle production costs of
models produced in large quantities.
[0007] However, a metallic alloy, in particular steel, still
remains the preferred material for constructing the body, in
particular the raw body. Due to consequent improvements, steel is
still viewed as a high-tech material which due to different
processing approaches represents a good compromise between
favorable manufacturability, excellent crash safety and long
service life.
[0008] Heat-treatment is according to the state-of-the-art
typically performed in a temperature range between 320.degree. C.
and 400.degree. C. and hardly changes the material properties and
the strength values adjusted in the hot-forming and trans-hardening
process. At the same time, however, the ductility of the material
is increased so as to allow superior fold formation in a crash.
[0009] However, the additional heat-posttreatment once more
increases the production costs due to significantly higher tooling
costs up to the start of the series production.
[0010] It would therefore be desirable and advantageous to obviate
prior art shortcomings and to provide an improved automobile
component and a method for its manufacture, which has lower
manufacturing costs compared to the state-of-the-art, while
simultaneously allowing precise adjustment of material properties
inside the component.
SUMMARY OF THE INVENTION
[0011] According to one aspect of the present invention, an
automobile column with at least two regions of different strength
is produced by hot-forming and press-hardening, wherein a region of
a first type has after press-hardening a substantially bainitic
structure and a region of a second type has after press-hardening a
substantially martensitic structure, and a transition region
between the region of the first type and the region of second type
being smaller than 80 mm.
[0012] According to one advantageous feature of the present
invention, the transition region may be smaller than 50 mm,
preferably smaller than 30 mm and more preferably smaller than 20
mm. Because the transition region is very small, the component can
within the context of the invention specifically adjusted in a
single production step, namely the production method itself, so
that the required crash properties can be reliably implemented with
the current manufacturing tolerances, while simultaneously having
improved manufacturability.
[0013] An automobile column can, for example, be formed as an
A-column, a B-column, a C-column, or a D-column. A softer structure
with high elongation at break or ductility may be required for
attachment in the region of the attachment points to a roof beam or
a rocker panel, thus preventing crack-off in a crash while
simultaneously adjusting a targeted fold structure.
[0014] However, within the context of the invention, specific
regions with a higher ductility may be required in the region of
door hinges and other attachment points to prevent cracking in a
crash. For example, also when additional components or
reinforcement patches are joined with rivets, regions with high
ductility can be specified for the automobile column according to
the invention with high accuracy during production, so that rivets
or other attachment points are prevented to the greatest possible
extent from detaching in these regions in a crash.
[0015] Weakening of the material caused by vibrations under high
permanent load and/or vibrations in conjunction with a high
stiffness is also prevented. The remaining components of the
automobile column, i.e. the regions of the second type, have a
substantially martensitic structure with particularly high strength
values, so that the minimally required strength of the component is
adequately attained.
[0016] The automobile column produced according to the invention
can thus be produced more cost-effectively compared to components
produced with conventional production methods, because only a
single reforming and press-hardening process is required for
adjusting the most important required properties of the components.
The adjustment by way of a substantially martensitic structure and
an intermediate structure which is substantially defined by a
bainitic structure, allows a particularly error-free, specific
adjustment of the required material properties in clearly defined
regions of the component.
[0017] According to one advantageous feature of the present
invention, in the region of the second type having a martensitic
structure as the major component of the structure, the martensitic
fraction of the structure may be higher than 50%, in particular
higher than 80%, preferably higher than 90% and even more preferred
higher than 95%.
[0018] The high torsional stiffness and bending stiffness of the
martensitic structure guarantees the elevated hardness of the
automobile column according to the invention, which preserves the
integrity of the passenger compartment as much as possible in the
event of a flip-over or other crash effects and also protects the
vehicle occupants as best as possible.
[0019] According to another advantageous feature of the present
invention, bainite may be present as an additional structure
component in the region of the second type.
[0020] According to one advantageous feature of the present
invention, the region of the first type may have as the primary
structure component bainite, wherein additional structure
components may be present in concentrations of less than 50%,
preferably less than 30%, and in particular less than 15%. For
example, a mixed structure of bainite, with ferrite and/or perlite
may be present. Optionally, within the context of the invention,
martensite may also be present as a component of the structure in
the region of the first type.
[0021] For applications, for example, in a B-column of an
automobile, the attachment region is arranged in the foot or roof
region, meaning to the rocker panel or roof beam, adjacent to the
region of second type with a substantially parallel transition
boundary.
[0022] According to one advantageous feature of the present
invention, the region of the first type may be at least partially
enclosed by the region of the second type; preferably, the region
of the first type is completely enclosed by the region of the
second type. For example, the region of the first type may be
completely enclosed by the region of the second type in the region
of the attachment points for automobile doors. Due to the
particularly small transition region according to the invention,
the stiffness in the direction of the component remains unchanged,
so that essentially no weakened location is produced, for example
in form of an undesired rated breakpoint. The region of the first
type is also in ductile form, preventing cracking as much as
possible. The ductility of the region of the first type also
largely prevents door hinges or door locks from being torn off, for
example in a side crash.
[0023] According to one advantageous feature of the present
invention, the region of the first type may be spot-shaped, for
example with a diameter of less than 40 mm, in particular of less
than 20 mm and particularly preferred of less than 10 mm.
[0024] According to one advantageous feature of the present
invention, a passage may be produced in the region of the first
type. This means the passage may be formed simultaneously during
the reforming process and/or press-hardening process; in a
particularly preferred embodiment, the passage can also be created
after the end of the press-hardening process. Due to the higher
ductility, tool wear of a punching or stamping tool is reduced, or
the passage can only be produced by this process without crack
formation.
[0025] According to one advantageous feature of the present
invention, marginal regions, in particular recesses and flanges,
may be formed as regions of the first type, wherein cracks
originating from the edge can be effectively prevented. Also
regions subjected to mechanical processing after press-hardening,
such as re-orientations, can advantageously be implemented as
regions of the first type.
[0026] According to one advantageous feature of the present
invention, the region of the first type can also be provided as a
region for producing cutting edges. This provides an initial
material characteristic which is gentle on the cutting or
separation tool to advantageously allow cold cutting after
hot-forming and press hardening, for example with simple cutting
and/or separation methods. Further machining of the component, for
example by cutting, is here particularly gentle, precise and
cost-effective while maintaining the required tight tolerances. In
particular, the need for an expensive laser cutting of the
otherwise hard edge of the component can be eliminated. To this
end, a circumferential, narrow region of the second type can
advantageously be formed proximate to the edge contour. The risk of
a delayed formation of cracks, caused by local stress in the hard
structure, is at the same time significantly reduced.
[0027] According to one advantageous feature of the present
invention, the region of the first type may have a stretchability
A50 between 10 and 30%, preferably between 14 and 20%. This ensures
sufficiently high strength, with simultaneously adequate ductility,
thereby preventing the formation of cracks and hence individual
structural automobile components to be torn off in a crash.
[0028] According to one advantageous feature of the present
invention, the region of the first type may have a tensile strength
between 500 and 1000 N/mm.sup.2, preferably between 550 and 800
N/mm.sup.2. The region of the first type may have an elongation
limit between 200 and 800 N/mm.sup.2, preferably between 250 and
600 N/mm.sup.2, particularly preferred between 250 and 500
N/mm.sup.2, and especially preferred between 300 and 500
N/mm.sup.2.
[0029] According to one advantageous feature of the present
invention, between the region of the second type and the region of
the first type, the elongation limit and/or the tensile strength
may be formed with a decreasing or increasing gradient of more than
100 N/mm.sup.2, preferably more than 200 N/mm.sup.2, and in
particular more than 400 N/mm.sup.2 per 10 mm. This means that the
elongation limit and/or the tensile strength in the region of the
first type may increase by more than 100 N/mm.sup.2 per 10 mm in
the direction of the region of the second type.
[0030] According to one advantageous feature of the present
invention, the region of the second type may have a strength of
more than 1000 N/mm.sup.2, in particular more than 1200 N/mm.sup.2,
and particular preferred more than 1400 N/mm.sup.2.
[0031] According to another aspect of the present invention, a
method according to the invention for producing a hot-formed and
press-hardened automobile column, wherein the automobile column has
at least two regions of different strength, includes the following
method steps: [0032] providing a hardenable metal plate or
semi-finished product and heating the hardenable metal plate or
semi-finished product to at least an austenizing temperature,
[0033] intermediately cooling a region of a first type of the metal
plate or semi-finished product with a cooldown speed selected to be
greater than a lower critical cooldown speed of a material of the
metal plate or semi-finished product, and [0034] hot-forming and
press-hardening the metal plate or semi-finished product in a
press-hardening tool to form the automobile column.
[0035] According to one advantageous feature of the present
invention, an intermediate stage structure may be adjusted under
time control and/or temperature control. The intermediate stage
structure may be adjusted, in particular, in the region of the
first type of the metal plate by intermediate cooling. The cooldown
speed of the intermediate cooling may be selected within the
context of the invention so as to be above the lower critical
cooldown speed of the bainite formation of the material of the
metal plate. The cooldown speed may also be greater than the lower
critical cooldown speed of the bainite formation. In particular,
those regions are cooled which are designed to be soft after
press-hardening, i.e., they have a higher ductility.
[0036] According to one advantageous feature of the present
invention, the component may also be preformed to a semi-finished
product while cold. The component is then at least partially
preformed from a hardenable metal plate. Preferably, the preforming
step matches at least 80% of the final shape of the component.
Following the cold preforming process, which can be carried out,
for example, at room temperature, a heating step to at least the
austenizing temperature, i.e. to above the AC3 temperature, may be
performed. Thereafter, a region of the first type is at least
partially intermediately cooled, followed by additional steps of
the method according to the invention.
[0037] The cooldown process of the intermediate cooling is
performed after the hardenable metal plate is heated to the
austenizing temperature, but may also be performed within the
context of the invention before or during the hot-forming and
press-hardening process. In particular, if the cooldown process of
the intermediate cooling is performed during press-hardening,
suitable means are provided in the pressing tool capable of
performing a corresponding cooldown as well as corresponding
cooldown speeds.
[0038] If the intermediate cooling takes place before hot-forming
and press-hardening, then this is associated with a production line
with corresponding intermediate transfers of the metal plate that
was heated above the austenizing temperature.
[0039] The cooldown itself may be performed, for example, by free
or forced convection, with cooling rollers, two-sided or one-sided
annealing plates with an insulated abutment or by applying cooling
media, such as water, or with other suitable cooling devices. The
cooldown can hereby be performed in a fixedly installed
intermediate station as well as in a cooling unit which moves
commensurate with the production cycle. Preferably, a cooldown
speed for the intermediate cooling is between 200 Kelvin per second
and 5 Kelvin per second. Particularly preferred is a cooldown speed
of 50 Kelvin per second. The cooldown is hereby preferably
performed immediately after removal from the furnace. In this way,
strength values in the first regions between 550 and 900 MPa are
adjusted. Preferably, strength values of substantially 700 MPa are
adjusted.
[0040] According to one advantageous feature of the present
invention, a region of the second type is held above the
austenizing temperature, wherein the region of the second type may
be any region of the metal plate that is not occupied by the region
of the first type. This means that after the metal plate is heated
to at least in the austenizing temperature, a corresponding
temperature above the austenizing temperature is maintained. This
may be done actively by using external heat sources, or passively
by employing suitable insulation. A temperature above the
temperature AC1 may also be maintained. Although a certain loss in
strength may occur compared to forming from AC3, this is
noncritical in most situations.
[0041] When employing external heat sources, the temperature may be
held in the region of the second type, in particular with infrared
lamps, heating coils, pore burners, insulation plates or similar
external heat sources. Within the context of the invention, a
temperature significantly above the austenizing temperature may be
selected, wherein the time after the heat-up to above the
austenizing temperature has ended to the start of the
press-hardening process and the accompanying cooldown are matched
to one another such that the region of the second type is at the
start of the press-hardening process still at a temperature which
is at least above the austenizing temperature.
[0042] According to one advantageous feature of the present
invention, the cooldown speed during intermediate cooling of the
region of the first type may be selected so that a bainitic
structure is obtained; preferably, the material is cooled down to a
temperature between 700 and 400.degree. C., preferably 650 to
450.degree. C., and in particular to 650 to 500.degree. C. With
cooldown speeds that are greater than the lower critical cooldown
speed of the respective employed material, but which stop above the
martensitic start temperature, the so-called bainite formation
occurs during isothermal holding of the cooldown temperature, also
known as intermediate structure or as intermediate stage.
[0043] Unlike with conventional methods, where perlite or ferrite
is formed, with perlite being formed mainly directly from the
austenite by diffusion, the diffusion of carbon in the austenite is
significantly hindered in the intermediate stage of the bainite as
a result of the more rapid cooldown. Small austenite regions,
mostly originating at grain boundaries, are transformed during
bainite formation into a distorted alpha lattice. Because the
diffusion velocity in the alpha lattice is significantly greater
than in the gamma lattice, small cementite grains precipitate in
these alpha mixed crystals which are oversaturated with carbon,
which become finer with faster cooldown. This produces a
substantially needle-like bainitic structure. This also produces a
grainy structure of the carbides caused by the increasing hardness
which increases with the grain fineness. A further difference is
made in the bainite structure between an upper intermediate stage,
in which the carbides are combined for increased incursion, and a
lower intermediate stage, in which the carbides are very finely
distributed.
[0044] According to one advantageous feature of the present
invention, the region of the first type may be maintained at the
cooldown temperature of the intermediate cooling for a
predetermined time; preferably, the temperature is held
substantially isothermal. With this approach, the respective
required or desired strength values of the bainitic intermediate
structure can be adjusted exactly as a function of time. The
intermediate cooling in this embodiment takes place substantially
to a temperature where the material structure in the region of the
first type has been transformed into austenite, or occurs directly
into the intermediate structure. From this cooling temperature, the
material structure is further transformed by isothermal holding for
a specified time. The material is then transformed from an
austenitic structure to a bainitic structure. If the material is
cooled directly into the intermediate stage by selecting the
cooldown speed, then a mixed structure between austenite and
bainite are already adjusted. By holding at the cooldown
temperature, holding is performed for a predetermined time in a
purely bainitic structural transformation range. The longer the
region of the first type is held at the temperature, the greater
becomes the bainitic component of the structure.
[0045] According to one advantageous feature of the present
invention, the intermediate structure range cooled to the cooldown
temperature is further quenched from the bainitic structural
transformation stage in the press-hardening tool itself, so that a
mixed structure of martensite and bainite is adjusted in the region
of the first type. By quenching the region of the first type, where
the structure has an intermediate stage, the residual austenite
fractions are transformed to martensite fractions during
press-hardening. As a result, a martensite-bainite mixed structure
is produced in the regions of the first type. The fractions of the
bainite in relation to martensite depend again from the duration
during which the first region is held in the intermediate stage,
before the press-hardening process begins.
[0046] According to one advantageous feature of the present
invention, the region of the first type may be held isothermally
during a certain time interval so as to transform the region of the
first type is completely into bainite. This produces a material
structure with a higher strength compared to a ferritic-perlitic
structure. In particular, a perlitic structure is hereby
intentionally avoided, which would reduce the ductility.
[0047] According to one advantageous feature of the present
invention, the cooldown speed during intermediate cooling may be
selected to be above a critical cooldown speed of the employed
material. In this way, an austenitic region can be selectively
adjusted which is thereafter held, preferably isothermally, during
a predetermined time at a temperature level, so that the structural
transformation is specifically adjusted to be bainitic during the
holding time. Depending on the employed holding time, a partially
bainitic-austenitic structure or an exclusively bainitic structure
can be adjusted. If a bainitic-austenitic structure is adjusted,
this structure is transformed to a bainitic-martensitic structure
in the subsequent press-hardening process.
[0048] Within the context of the invention, holding is to be
understood as maintaining a substantially identical temperature
below the ferrite and perlite temperature, but above a martensite
start temperature, i.e. substantially below 700.degree. C., in
particular below 600.degree. C., particularly preferred below
550.degree. C. For example, when isothermally holding for a longer
time, the temperature may decrease from 500 to 400.degree. C.,
which however is still considered within the context of the
invention to be substantially isothermal. Particularly preferred,
the region of the first type is held isothermally during a time
interval from 1 second to 80 seconds. Particularly preferred, the
holding time is 15 seconds. However, these values are to be
selected depending on the employed material alloy.
[0049] According to one advantageous feature of the present
invention, the intermediate cooling of the region of the first type
may be performed in the press-hardening tool, preferably with
cooling plates arranged in the press-hardening tool. This reduces
the cycle times and also the production costs. In particular, an
automobile component having a region of different strength is
produced with only two tool steps. Initially, heat-up is performed
in a furnace system, followed by a combination of intermediate
cooling and hot-forming and press-hardening using only a single
tool.
[0050] A cooldown speed of at least 25 Kelvin per second may be
selected as the cooldown speed in the actual press-hardening
process. In one embodiment, the cooldown speed is more than 27
Kelvin per second. However, higher cooldown speeds may be selected
for the actual press-hardening process. In particular, the
press-hardening process is then performed both in the region of the
first type and in the region of the second type at the same
cooldown speed depending on the local temperature gradient between
press-hardening tool and the workpiece. Due to the different
temperatures at the start of the press-hardly process in both
regions, the cooldown speed may slightly diverge from the region of
the first type to the region of the second type.
[0051] In one embodiment, a hardenable steel categorized as
micro-alloyed heat-treated steel is used with the method according
to the invention. This steel includes in particular the following
alloy element in mass weight percent fractions:
TABLE-US-00001 carbon (C) 0.19 to 0.25 silicon (Si) 0.15 to 0.30
manganese (Mn) 1.10 to 1.40 phosphorus (P) 0 to 0.025 sulfur (S) 0
to 0.015 chromium (Cr) 0 to 0.35 molybdenum (Mo) 0 to 0.35 titanium
(Ti) 0.020 to 0.050 boron (B) 0.002 to 0.005 aluminum (Al) 0.02 to
0.06.
[0052] According to one advantageous feature of the present
invention, the intermediate cooling of the regions of the first
type may be performed with a tool having integrated cooling plates.
The cooling plates have here an intrinsic temperature of up to
600.degree. 0, which is still less below the AC3 temperature of
more than 900.degree. C. The region of the first type may be cooled
down with these cooling plates and then, if desired, held
isothermally for a certain time. For example, such cooling plates
can be brought to the respective required temperature with
electrical heater cartridges or by backside burner heating or with
thermal oils.
[0053] According to one advantageous feature of the present
invention, the intermediate cooling may also be performed with
substantially cold cooling plates. The cooling plates may then have
a temperature significantly below 400.degree. C., preferably
between -100.degree. C. and +100.degree. C., particularly preferred
between -10.degree. C. and +25.degree. C. However, an isothermal
holding time can only be attained with cold cooling plates a
limited way. In one embodiment, both versions of cooling plates may
be integrated, for example, in a hot-forming tool and pressing
tool, so that the entire process following the actual furnace
heating is performed in only a single tool. Within the context of
the invention, the cooling plates for performing the intermediate
cooling may also be housed in a separate tool, so that the process
takes place from a heat-up furnace via intermediate cooling to the
actual hot-forming in press-hardening tool. This embodiment has the
advantage that the separate tool can be designed substantially as a
flat tool with substantially flat heating and/or cooling
plates.
BRIEF DESCRIPTION OF THE DRAWING
[0054] 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:
[0055] FIG. 1 shows a detail area of an automobile column according
to the invention with a region of a first type, a transition region
and a region of a second type;
[0056] FIG. 2 shows an automobile column according to the
invention; and
[0057] FIG. 3 shows a time-temperature diagram for carrying out a
process according to the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0058] 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.
[0059] Turning now to the drawing, and in particular to FIG. 1,
there is shown a detail of an automobile column 1. As can be seen,
a region of the second type 3 according to the invention is formed
in a region of the first type 2. A transition region 4 is arranged
between the region of the first type 2 and the region of the second
type 3. A material structure having a tendency to be ductile is
produced in the region of the first type 2, whereas a hard material
structure is produced in the region of the second type 3. Within
the context of the present invention, the transition region 4 has
essentially a width a which is quite small compared to the region
of the first type 2.
[0060] FIG. 2 shows an automobile column 1 in form of an A-column 5
of a automobile body which is not shown in detail. The A-column 5
has joining flanges 6 at its respective sides 5a, 5b, which have a
higher ductility than a center profile section 7. The A-column 5
has therefore, due to its center profile section 7, high strength
and hardness which guarantees the protection of the passenger
compartment in a crash, whereas the joining flanges 6 produce a
rather ductile material property compared to the center profile
section, so that components (not shown in detail) attached to the
joining flanges 6 remain connected with the A-column 5, and the
connections, indicated by the joining flanges 6, do not tear
off.
[0061] FIG. 3 shows a time-temperature diagram of an exemplary
steel, without limiting the field of the present invention. Several
structures are indicated which are formed in the material at
various cooldown speeds as a function of temperature. The lower
part of the FIG. shows the martensite formation. Above, in the
center region of the FIG., the bainite formation is shown, and
there above the perlite and/or ferrite formation.
[0062] In the illustrated exemplary embodiment, three different
curves for the different cooldown processes are shown. Curve K1
shows the course of the temperature for a first region according to
the invention, wherein this region is first heated to a temperature
above the AC3 temperature. From this temperature, the material is
cooled down to an intermediate temperature of about 520.degree. C.
with a cooldown speed which in this case is greater than the upper
critical cooldown speed oK for the bainite formation of the
illustrated material. When the cooldown temperature of the
intermediate cooling of about 520.degree. C. is reached, the first
region is held substantially isothermally at a temperature for the
time t1. The temperature thereby decreases from about 520.degree.
C. to about 480.degree. C. due to heat loss in form of, for
example, heat radiation, convection or heat conduction. An
austenitic structure is produced at the time Z1 of the intermediate
cooling, and a bainitic-austenitic mixed structure is produced at
the time P1, corresponding to the start of press-hardening in the
first embodiment.
[0063] In the first embodiment, quenching thereafter occurs in the
press-hardening process from the time .degree.P1, such that the
bainitic-austenitic mixed structure in the first region is
transformed to a bainitic-martensitic mixed structure. In parallel,
the second region according to the invention is quenched from a
temperature above AC3 by press-hardening, producing a martensitic
structure directly from an austenitic structure; however, this is
not illustrated in detail for sake of clarity.
[0064] The second embodiment of the method according to the
invention is illustrated with the cooldown sequence according to
curve 2 of the first region. The cooldown sequence of the curve 2
is similar to the cooldown sequence of the curve K1, wherein the
cooldown temperature is held for a longer time from a time Z2
(equal to Z1), so that the press-hardening process starts at a time
P2. The time interval t2 is therefore greater than t1. The
structure in the first region is completely transformed to bainite
at the time P2 and therefore does not undergo any further
structural transformation after the time P2 due to the cooldown
speed.
[0065] In a third embodiment according to the present invention, a
cooldown speed from a temperature above the AC3 temperature
according to curve 3 is selected, so that a transformation occurs
directly into the bainitic intermediate structure during the
cooldown process of the intermediate cooling. In the first region,
an austenitic-bainitic intermediate structure was adjusted, so that
when the press-hardening process starts at the time P3, this
bainitic-austenitic mixed structure in the first region is
transformed to a bainitic-martensitic mixed structure. In the
embodiments according to curves 2 and 3, the second region which
was held above the AC3 temperature during the intermediate cooling,
is in both cases transformed from the austenitic region directly to
martensite by the cooldown during the press-hardening process. In
the embodiment according to curve 3, the temperature is selected
according to the invention to be always greater than the lower
critical cooldown speeds uK of the corresponding employed
material.
[0066] 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.
* * * * *