U.S. patent application number 13/273826 was filed with the patent office on 2012-12-20 for side rail and method for producing a hot-formed and press-hardened side rail.
This patent application is currently assigned to Benteler Automobiltechnik GmbH. Invention is credited to JOHANNES BOKE, JAN DINGEMANS, MARKUS PELLMANN, ANDREAS ZIMMERMANN.
Application Number | 20120318415 13/273826 |
Document ID | / |
Family ID | 45347055 |
Filed Date | 2012-12-20 |
United States Patent
Application |
20120318415 |
Kind Code |
A1 |
ZIMMERMANN; ANDREAS ; et
al. |
December 20, 2012 |
SIDE RAIL AND METHOD FOR PRODUCING A HOT-FORMED AND PRESS-HARDENED
SIDE RAIL
Abstract
A side rail and to a method for producing a side rail are
disclosed. The side rail 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 side rail 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.: |
13/273826 |
Filed: |
October 14, 2011 |
Current U.S.
Class: |
148/653 ;
148/320 |
Current CPC
Class: |
C22C 38/04 20130101;
C21D 1/20 20130101; C21D 8/0205 20130101; C22C 38/06 20130101; C22C
1/02 20130101; C21D 1/673 20130101; C21D 8/04 20130101; C21D
2221/00 20130101; C22C 38/14 20130101; C22C 38/32 20130101; C21D
8/005 20130101; C21D 2211/002 20130101; B62D 25/04 20130101; C22C
38/02 20130101; B62D 25/08 20130101; C21D 2211/008 20130101 |
Class at
Publication: |
148/653 ;
148/320 |
International
Class: |
C21D 8/00 20060101
C21D008/00; C22C 38/00 20060101 C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 15, 2010 |
DE |
10 2010 048 209.9 |
Feb 23, 2011 |
EP |
11 155 717.9 |
Claims
1. A side rail 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 side rail of claim 1, wherein the transition region is
smaller than 50 mm.
3. The side rail of claim 1, wherein the transition region is
smaller than 30 mm.
4. The side rail of claim 1, wherein the transition region is
smaller than 20 mm.
5. The side rail 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 side rail 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 side rail 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 side rail of claim 5, wherein the additional structure
component comprises bainite.
9. The side rail 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 side rail 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 side rail 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 side rail of claim 1, wherein the region of the first type
is at least partially enclosed by the region of the second
type.
13. The side rail of claim 12, wherein the region of the first type
is completely enclosed by the region of the second type.
14. The side rail of claim 1, wherein the region of the first type
is spot-shaped with a diameter of less than 40 mm.
15. The side rail of claim 1, wherein the region of the first type
is spot-shaped with a diameter of less than 20 mm.
16. The side rail of claim 1, wherein the region of the first type
is spot-shaped with a diameter of less than 10 mm.
17. The side rail of claim 1, wherein the region of the first type
is constructed as a coupling location for coupling additional
components to the side rail.
18. The side rail of claim 1, wherein the region of the first type
is formed in regions of the side rail which are subject to strong
deformations in a crash or which are configured to dissipate crash
energy through deformations.
19. The side rail 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 side rail of claim 1, further comprising a passage or an
edge, or both, in the region of the first type after
hot-forming.
21. The side rail of claim 1, wherein the region of the first type
has a stretchability A50 between 10% and 30%.
22. The side rail of claim 1, wherein the region of the first type
has a stretchability A50 between 12% and 20%.
23. The side rail of claim 1, wherein the region of the first type
has a stretchability A50 between 12% and 16%.
24. The side rail of claim 1, wherein the region of the first type
has a stretchability A50 between 14% and 16%.
25. The side rail of claim 1, wherein the region of the first type
has a tensile strength between 500 and 1000 N/mm.sup.2.
26. The side rail of claim 1, wherein the region of the first type
has a tensile strength between 550 and 800 N/mm.sup.2.
27. The side rail 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 side rail 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 side rail 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 side rail of claim 1, wherein the region of the second type
has a strength of more than 1000 N/mm.sup.2.
31. The side rail of claim 1, wherein the region of the second type
has a strength of more than 1200 N/mm.sup.2.
32. The side rail of claim 1, wherein the region of the second type
has a strength of more than 1400 N/mm.sup.2.
33. The side rail of claim 1, wherein region of the first type has
a yield strength between 200 and 800 N/mm.sup.2.
34. The side rail of claim 1, wherein region of the first type has
a yield strength between 250 and 600 N/mm.sup.2.
35. The side rail of claim 1, wherein region of the first type has
a yield strength between 250 and 500 N/mm.sup.2.
36. The side rail of claim 1, wherein region of the first type has
a yield strength between 300 and 500 N/mm.sup.2.
37. The side rail of claim 1, wherein the side rail is manufactured
from a Tailor Welded Blank or a Tailor Rolled Blank.
38. A method for producing a hot-formed and press-hardened side
rail having at least two regions of different hardness, the 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 side rail.
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
European Patent Application Serial No. 11 155 717.9, 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: "AUTOMOBILE COLUMN AND METHOD FOR PRODUCING A
HOT-FORMED AND PRESS-HARDENED AUTOMOBILE COLUMN".
BACKGROUND OF THE INVENTION
[0003] The present invention relates to a side rail, produced by
hot-forming and press hardening. The present invention also relates
to a method for producing a side rail 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 side rail 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 within the side rail.
SUMMARY OF THE INVENTION
[0011] According to one aspect of the present invention, a side
rail 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 is smaller than 50 mm, preferably
smaller than 30 mm and still more preferably smaller than 20 mm.
Because the transition region is very small, the component can
within the context of the invention be specifically adjusted in a
single production step, namely the production process itself, so
that the required crash properties can be reliably implemented with
the current manufacturing tolerances, while simultaneously having
improved manufacturability.
[0013] According to another advantageous feature of the present
invention, a side rail with advantageous material properties can be
attained in previously defined regions of the first and second type
by a reliable process and a specific design. During the hot-forming
and press-hardening of a metal plate or of a preform or semi
finished product made of high-strength hardenable steel, regions of
the first type are intermediately cooled, so that regions of the
first type and regions of the second type with different strengths,
hardness and ductility properties can be intentionally adjusted. A
material structure having a more ductile tendency is produced in
the regions of the first type, as compared to the regions of the
second type. The transition region between both regions tends to
have clearly defined edges. This significantly relaxes or even
entirely eliminates production tolerances. Regions with ductile
material properties are produced by the substantially bainitic
structure in the region of the first type.
[0014] An intentional deformation is favored in the regions of the
first type, so that folds or a compressions can be formed in a
crash, without causing cracks or detachment. The energy absorption
capacity of the side rail according to the invention is hereby
increased while retaining its high stiffness. A high degree of
energy is then absorbed in an automobile equipped with the side
rail according to the invention by converting kinetic energy from
the impact into deformation energy, while retaining the high
stiffness of the rest of the automobile body.
[0015] Moreover, the side rail may be used as engine support or may
be employed in the area of the luggage compartment, where a higher
energy absorption capacity may be required than in the region of
the passenger compartment itself. A sequence of regions of the
first type and regions of the second type may be produced along the
length of the side rail according to the invention in the direction
of the vehicle by specifically creating regions of the first type
and regions of the second type. The side rail can the
advantageously collapse in a crash like an accordion.
[0016] The side rail according to the invention may be arranged,
for example, in an automobile body transversely at the front and/or
rear side to intentionally absorb an impacting object or another
automobile. The side rail should absorb the body colliding with the
automobile or the stationary body hit by the automobile such that
the side rail is minimally deformed for dissipating the energy and
a deliberate intrusion of a body into the automobile itself is
prevented. With the side rail according to the invention, the
overall energy absorption capability is increased, which in turn
increases the overall energy absorption capacity of the automobile
body, while at the same time providing a high stiffness. In
addition, material savings can be achieved in the regions of higher
ductility because of, for example, thinner wall thicknesses, which
in turn reduce the overall weight of the automobile body. A high
degree of energy is then absorbed in an automobile equipped with
the side rail according to the invention in that kinetic energy of
the impact is converted into deformation energy, while the
stiffness of the passenger compartment and hence of the body
remains high or is even increased.
[0017] According to another advantageous feature of the present
invention, a side rail according to the invention may also prevent
unintentional buckling in regions which are intentionally formed as
regions of the second type after hard-forming and press-hardening
according to the invention. The high hardness of the regions of the
second type therefore prevents an undesirable deformation in
certain regions.
[0018] 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 side
rail, i.e. the regions of the second type, have a substantially
martensitic structure with particularly high strength values, so
that the minimally required strength and crash characteristic of
the component is adequately attained.
[0019] The side rail 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.
[0020] According to another advantageous feature of the present
invention, the region of the second type, which has a martensitic
structure as the major component of the structure, includes other
structures in concentrations of more than 50%, in particular more
of than 80%, preferably of more than 90%, and particularly
preferred situations of more than 95%.
[0021] The high torsional stiffness and bending stiffness due to
the martensitic structure guarantees the elevated hardness of the
side rail according to the invention, which preserves the integrity
of the body and of passenger compartment as much as possible and
thus protects the vehicle occupants.
[0022] 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.
[0023] According to another advantageous feature of the present
invention, the region of the first type may have as the primary
structure component bainite, wherein additional structure
components with less than 50%, preferably less than 30%, and in
particular less than 15% may be present. 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.
[0024] According to another 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 may be completely enclosed by the region of the
second type. The region of the first type is preferably completely
enclosed by the region of the second type in the region of the
attachment points for, for example, crash boxes. 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, for example in form of an
undesired rated breakpoint, is produced. The region of the first
type is also ductile so as to prevent crack formation as much as
possible. The ductility of the region of the first type also
largely prevents components attached to the side rail or other
couples components from being torn off, for example in an offset
crash.
[0025] According to another advantageous feature of the present
invention, the region of the first type may be spot-shaped,
preferably with a diameter of less than 40 mm, in particular less
of than 20 mm and particularly preferred of less than 10 mm.
[0026] According to another 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 one
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.
[0027] According to another 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.
[0028] According to another advantageous feature of the present
invention, the region of the first type may 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.
[0029] According to another 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.
[0030] According to another 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 even more preferred between 300 and 500
N/mm.sup.2.
[0031] 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
increase by more than 100 N/mm.sup.2 per 10 mm in the direction of
the region of the second type.
[0032] According to another 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 of more than 1200
N/mm.sup.2, and preferably of more than 1400 N/mm.sup.2.
[0033] According to another aspect of the invention, a method for
producing a hot-formed and press-hardened side rail, wherein the
side rail has at least two regions of different strength, includes
the following method steps: [0034] 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, [0035] 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 [0036]
hot-forming and press-hardening the metal plate or semi-finished
product in a press-hardening tool to form the side rail.
[0037] With the method according to the 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.
[0038] 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, preforming
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, is 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.
[0039] 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.
[0040] If the intermediate cooling takes place before hot-forming
and press-hardening, then this may be associated with a production
line with corresponding intermediate transfers of the metal plate
that was heated above the austenizing temperature.
[0041] 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 between 550 and 900 MPa are adjusted in the first
regions. Preferably, strength values of substantially 700 MPa are
adjusted.
[0042] According to another advantageous feature of the present
invention, a region of the second type may be held above the
austenizing temperature, wherein the region of the second type may
be any region of the metal plate that is not taken up 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.
[0043] 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.
[0044] 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 650 to 500.degree. C. With
cooldown speeds that are greater than the lower critical cooldown
speed of the respective employed material, but 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.
[0045] 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.
[0046] According to one advantageous feature of the method 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 may be held
substantially isothermal. With this embodiment, 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.
[0047] According to another advantageous feature of the present
invention, the intermediate structure range cooled to the cooldown
temperature may be 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.
[0048] According to another 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.
[0049] According to another advantageous feature of the present
invention, the cooldown speed during intermediate cooling may be
selected to be above a critical cooldown speeds 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.
[0050] 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.
[0051] According to another advantageous feature of the method of
the present invention, the intermediate cooling of the region of
the first type may be performed in the press-hardening tool, for
example 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.
[0052] A cooldown speed of at least 25 Kelvin per second may be
selected as the cooldown speed in the actual press-hardening
process. In another embodiment, the cooldown speed may be selected
to be higher than 27 Kelvin per second. However, higher cooldown
speeds may be selected for the actual press-hardening process. In
particular, the press-hardening process may then be 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.
[0053] 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.
[0054] 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 may here have an intrinsic temperature of up to
600.degree. C., which is still less below the AC3 temperature of
more than 900.degree. C. The region of the first type can 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.
[0055] According to one advantageous feature of the present
invention, the intermediate cooling may also be performed with
substantially cold cooling plates. The cooling plates 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 performed 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
[0056] 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:
[0057] FIG. 1 shows a detail area of a side rail according to the
invention with a region of a first type, a transition region and a
region of a second type;
[0058] FIG. 2 shows a side rail according to the invention;
[0059] FIG. 3 shows a time-temperature diagram for carrying out a
process according to the invention; and
[0060] FIG. 4 shows a side rail assembly according to the
invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0061] 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.
[0062] Turning now to the drawing, and in particular to FIG. 1,
there is shown a detail of a side rail 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.
[0063] FIG. 2 shows a side rail 1. The side rail 1 has beads 5,
openings 6 and recesses 7. The side rail 1 according to the
invention also has joining flanges 8 disposed in its marginal
regions. The beads 5, openings 6, recesses 7 and joining flanges 8
are each implemented as regions of the first type, depending on the
requirements, whereas the remaining region of the side rail 1 is
implemented as a region of the second type.
[0064] 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 obtained 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.
[0065] 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 ti. 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.
[0066] In the first embodiment, quenching thereafter occurs in the
press-hardening process from the time 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.
[0067] 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.
[0068] 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.
[0069] FIG. 4 shows a side rail assembly 9 formed of a side rail 1
and a heat-treated component 10. The side rail 1 is here formed in
the center region as a region of the second type and in an outer
region as the region of the first type. The side rail 1 and the
component are coupled with one another at their corresponding
lateral regions by joining flanges 8. The joining flanges 8
themselves are here formed as regions of the first type with a
rather ductile material characteristic. In the event of a
deformation, for example in a crash, a basic stiffness is provided
by the side rail 1 itself. Detachment is prevented by the rather
ductile material characteristic. Both components are connected with
each other at the coupling locations 11.
[0070] 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.
* * * * *