U.S. patent application number 13/166942 was filed with the patent office on 2011-12-29 for tailored properties by post hot forming processing.
This patent application is currently assigned to MAGNA INTERNATIONAL INC.. Invention is credited to Nicholas Adam, Pascal Charest, Jaswinder Pal Singh, Alexander Zak.
Application Number | 20110315281 13/166942 |
Document ID | / |
Family ID | 45351389 |
Filed Date | 2011-12-29 |
United States Patent
Application |
20110315281 |
Kind Code |
A1 |
Charest; Pascal ; et
al. |
December 29, 2011 |
Tailored Properties By Post Hot Forming Processing
Abstract
A method of forming a product from an initial blank comprises
subjecting the initial blank to a hot forming and press hardening
operation to form the product with substantially a uniform first
tensile strength. Subsequently, the product is subjected to post
hot-forming processing in which a first region of the product is
heated selectively to above a known temperature, using one of
conduction heating, resistance heating, and induction heating. The
first region is then cooled, such that the first region attains a
second tensile strength that is substantially less than the first
tensile strength.
Inventors: |
Charest; Pascal; (Caledon,
CA) ; Adam; Nicholas; (Trenton, CA) ; Singh;
Jaswinder Pal; (Sterling Heights, MI) ; Zak;
Alexander; (Moedling, AT) |
Assignee: |
MAGNA INTERNATIONAL INC.
Aurora
CA
|
Family ID: |
45351389 |
Appl. No.: |
13/166942 |
Filed: |
June 23, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61358174 |
Jun 24, 2010 |
|
|
|
Current U.S.
Class: |
148/567 ;
148/714 |
Current CPC
Class: |
C21D 2221/00 20130101;
C22F 1/00 20130101; C21D 1/34 20130101; B62D 25/04 20130101; B62D
1/195 20130101; C21D 8/00 20130101; C21D 2211/008 20130101 |
Class at
Publication: |
148/567 ;
148/714 |
International
Class: |
C22F 1/00 20060101
C22F001/00 |
Claims
1. A method of forming a product from an initial blank, comprising:
subjecting the initial blank to a hot forming and press hardening
operation to form the product with substantially a uniform first
tensile strength; and subsequently, subjecting the product to post
hot-forming processing, comprising selectively heating a first
region of the product to above a known temperature, while
simultaneously maintaining below the known temperature a second
region of the product that is adjacent to the first region, and
then cooling the first region such that the first region attains a
second tensile strength that is substantially less than the first
tensile strength.
2. A method according to claim 1, wherein the post hot-forming
processing comprises using conduction heating to heat the first
region of the product to above the known temperature.
3. A method according to claim 2, wherein the conduction heating is
performed using a heated die including at least one of an upper
heated die half and a lower heated die half.
4. A method according to claim 2, wherein the conduction heating is
performed using at least one heated plate.
5. A method according to claim 2, wherein the conduction heating is
performed using a heated fluid.
6. A method according to claim 2, wherein the conduction heating is
performed using a heated fluid and a contacting medium.
7. A method according to claim 6, wherein the contacting medium is
selected from the group consisting of sand, salt and ceramic.
8. A method according to claim 1, wherein the post hot-forming
processing comprises using resistance heating to heat the first
region of the product to above the known temperature.
9. A method according to claim 1, wherein the post hot-forming
processing comprises using induction heating to heat the first
region of the product to above the known temperature.
10. A method according to claim 9, wherein the induction heating is
performed using a pair of induction coils disposed one each along
opposite sides of the first region.
11. A method according to claim 9, wherein the induction heating is
performed using a single induction coil.
12. A method according to claim 9, wherein the induction heating is
performed using a pair of induction plates disposed one each along
opposite sides of the first region.
13. A method according to claim 1, wherein the initial blank is
heated to an austenitization temperature during the hot forming and
press hardening operation, and wherein the known temperature is
substantially lower than the austenitization temperature.
14. A method according to claim 1, comprising protecting the second
region from being heated to substantially the known
temperature.
15. A method according to claim 1, wherein the known temperature is
between approximately 370.degree. C. and approximately 800.degree.
C.
16. A method according to claim 1, wherein the known temperature is
between approximately 400.degree. C. and approximately 700.degree.
C.
17. A method according to claim 1, wherein the initial blank is
fabricated from a press hardenable steel alloy material.
18. A method of forming a product from an initial blank,
comprising: heating the initial blank to an austenitizing
temperature; hot-shaping the initial blank in a cooled pair of dies
to form the product; cooling the product during a first period of
time, using a rate of cooling that is sufficiently rapid to support
formation of a martensitic structure within substantially the
entire product; subjecting a first portion of the product to post
hot-forming processing, comprising selectively heating the first
portion of the product to a known temperature that is less than the
austenitizing temperature, while simultaneously maintaining below
the known temperature a second portion of the product that is
adjacent to the first portion; and, cooling the product such that
tempered martensite is formed within the first portion of the
product while at the same time the second portion of the product
remains substantially free of tempered martensite.
19. A method of forming a product from an initial blank,
comprising: providing the initial blank; heating the initial blank
to the austenite state; hot-stamping the initial blank in a cooled
pair of dies to form the product; hardening substantially the
entire product while it is still inside the pair of dies by cooling
the product using a rate of cooling that is sufficiently fast to
form a martensitic structure; using conduction heating, heating a
first portion of the product to at least a predetermined first
temperature, while at the same time maintaining a second portion of
the product below a predetermined second temperature that is lower
than the first temperature; and, cooling the product such that
tempered martensite is formed within the first portion of the
product while at the same time the second portion of the product
remains substantially free of tempered martensite, wherein
subsequent to cooling, a tensile strength of the first portion of
the product is less than a tensile strength of the second portion
of the product.
20. A method of forming a product from an initial blank,
comprising: providing the initial blank; heating the initial blank
to the austenite state; hot-stamping the initial blank in a cooled
pair of dies to form the product; hardening substantially the
entire product while it is still inside the pair of dies by cooling
the product using a rate of cooling that is sufficiently fast to
form a martensitic structure; using resistance heating, heating a
first portion of the product to at least a predetermined first
temperature, while at the same time maintaining a second portion of
the product below a predetermined second temperature that is lower
than the first temperature; and, cooling the product such that
tempered martensite is formed within the first portion of the
product while at the same time the second portion of the product
remains substantially free of tempered martensite, wherein
subsequent to cooling, a tensile strength of the first portion of
the product is less than a tensile strength of the second portion
of the product.
21. A method of forming a product from an initial blank,
comprising: providing the initial blank; heating the initial blank
to the austenite state; hot-stamping the initial blank in a cooled
pair of dies to form the product; hardening substantially the
entire product while it is still inside the pair of dies by cooling
the product using a rate of cooling that is sufficiently fast to
form a martensitic structure; using induction heating, heating a
first portion of the product to at least a predetermined first
temperature, while at the same time maintaining a second portion of
the product below a predetermined second temperature that is lower
than the first temperature; and, cooling the product such that
tempered martensite is formed within the first portion of the
product while at the same time the second portion of the product
remains substantially free of tempered martensite, wherein
subsequent to cooling, a tensile strength of the first portion of
the product is less than a tensile strength of the second portion
of the product.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of U.S. provisional
application Ser. No. 61/358,174 filed Jun. 24, 2010, the entire
disclosure of the application being considered part of the
disclosure of this application, and hereby incorporated by
reference.
FIELD OF THE INVENTION
[0002] The instant invention relates generally to metallic products
having tailored properties, and more particularly to a method of
producing regions of reduced hardness and reduced strength in
products via post hot-forming processing.
BACKGROUND OF THE INVENTION
[0003] In the field of vehicle construction, more and more vehicle
parts that are made of high-strength and ultra-high-strength steel
are being employed in order to satisfy criteria for lightweight
construction. This applies to car body construction where, in order
to meet weight goals and safety requirements, for instance
structural and/or safety elements such as door intrusion beams, A
and B columns, bumpers, side rails and cross rails are increasingly
being produced from UHSS (Ultra High Strength Steel), thermo-shaped
and press-hardened steel having tensile strengths greater than 1000
MPa.
[0004] In different applications of motor vehicle engineering,
shaped parts are to have high strength in certain regions while in
other regions they are to have higher ductility relative thereto.
"Tailoring" the properties of shaped parts in this way facilitates
subsequent forming operations, such as for instance trimming or
perforating the part, and results in regions that can convert crash
energy into deformation by crumpling.
[0005] It is known to treat a part using heat treatments such that
local regions have higher strength or higher ductility. Lundstrom
discloses one such approach in U.S. Pat. No. 5,916,389, wherein a
sheet of hardenable steel is heated to an austenitization
temperature and then pressed between cooled die halves in order to
form a shaped part having a desired profile. Sections of the die
halves that are adjacent to portions of the part that are to have
higher ductility in the finished product are adapted to prevent
rapid cooling, such that hardening does not occur within these
portions to the same extent that it occurs within other portions of
the finished product. Unfortunately, the die halves must be
specially made for each part, which is both laborious and costly.
In addition, special effort is required in order to minimize the
extent of formation of transition regions between the different
portions, since typically these transition regions exhibit
properties that are less well defined than the properties of the
rest of the finished product.
[0006] In another approach, a shaped product having substantially
uniform hardness is produced using conventional hot forming and
press hardening techniques, followed by separate additional heat
treatment processing of the product to form regions of lower
tensile strength therein. For instance, in United States patent
application Publication 2010/0086803, Patberg discloses a method of
forming mild zones along a bend edge of a hot formed and press
hardened component. In particular, a laser beam is used to heat a
narrow region of the component along the bend edge. Additionally,
Patberg suggests that the heat that is produced by welding may
result in the formation of mild zones adjacent to a weld joint.
Unfortunately, the use of highly specialized laser equipment adds
to the cost and complexity of manufacturing the component. In
addition, this approach is not well suited either to batch
processing or to applications requiring the formation of
substantial regions having reduced tensile strength within the
component.
[0007] It would be advantageous to provide a method that overcomes
at least some of the above-mentioned limitations of the prior
art.
SUMMARY OF EMBODIMENTS OF THE INVENTION
[0008] According to at least one embodiment of the instant
invention, a method of forming a product from an initial blank is
provided in which the initial blank is subjected firstly to a hot
forming and press hardening operation, so as to form the product
with substantially a uniform first tensile strength. In particular,
the initial blank is heated to a temperature above its transition
temperature Ac3, which is defined as the temperature at which
transformation of ferrite into austenite is completed upon heating.
By way of a specific and non-limiting example, the initial blank is
heated to approximately 950.degree. C. The heated initial blank is
inserted into a cooled press having a pair of die halves, which is
used for both hot forming and hardening the product. The press is
subsequently closed, thereby deforming the initial blank such that
it conforms to contours that are defined along facing surfaces of
the die halves. Deformation and concomitant rapid cooling of the
initial blank within the die halves produces the product, in which
the austenite structure has been transformed into martensite
structure. The tensile strength and hardness of the product is
substantially uniform throughout. Optionally, the die halves are
not cooled, provided that a suitably rapid cooling rate of the
product can still be achieved to form the martensite structure.
[0009] Subsequently, in a second thermal treatment step, a first
region of the product is heated in a selective fashion to a known
temperature that is lower than the transition temperature Ac3. By
way of a specific and non-limiting example, the first region of the
product is heated to a temperature between approximately
370.degree. C. and 800.degree. C. In an embodiment, the first
region of the product is heated to a temperature within the range
of temperatures between about 400.degree. C. and 700.degree. C. By
way of several specific and non-limiting examples, the first region
of the product is heated using one of conduction heating,
resistance heating, and induction heating. The first region is then
cooled in such a way that the first region attains a second tensile
strength that is substantially less than the first tensile
strength. The second thermal treatment at a temperature between
approximately 370.degree. C. and 800.degree. C. results in a
tempered martensite composition within the first region.
[0010] Optionally, the conduction heating may be performed using
heating plates, such as for instance a heated die, or using a
heated fluid and a suitable contacting medium, such as for instance
hot compressed air and one of sand, ceramic, salt, etc. Optionally,
the induction heating is performed using one or more induction
coils, or alternatively the induction heating is performed using
induction plates.
[0011] According to an embodiment of the instant invention, the
first region of the product is gas-cooled subsequent to the second
thermal treatment step. Optionally, the first region of the product
is cooled using another suitable cooling technique, such as for
instance one of gas-blasting, fluidized bed cooling, die cooling,
water/mist cooling, and cooling with the use of cooling
fans/jets.
[0012] According to an embodiment, other regions of the product are
cooled or at least insulated from being heated to the known
temperature as a result of the one of conduction heating,
resistance heating and induction heating. For instance, the product
is gripped using a cooled collar that surrounds a second region of
the product that is adjacent to the first region. Alternatively,
the second region of the product is protected from being heated
using a curtain of a cooled gas, such as for instance air, or by
spraying or misting the second region with a suitable cooling
liquid, such as for instance water.
[0013] Intermediate processing, such as for instance forming and/or
cutting and/or perforating, etc., optionally is performed
subsequent to the hot stamping and press hardening steps but prior
to the post hot-forming processing. Optionally, forming and/or
cutting and/or perforating, etc. is performed subsequent to the
post hot-forming processing.
[0014] According to another embodiment of the instant invention,
the hot forming and press hardening steps are omitted. By way of a
specific and non-limiting example, the product is roll formed from
a coil of Ultra High Strength Steel and subjected subsequently to
post-forming thermal treatment by the one of conduction heating,
resistance heating and induction heating as described above. In
particular, products may be formed having a geometry that is not
sufficiently complex so as to require the use of hot forming and
press hardening techniques.
[0015] Optionally, the initial blank is formed from either a coated
material or an uncoated material.
[0016] In accordance with an aspect of an embodiment of the
invention there is provided a method of forming a product from an
initial blank, comprising: subjecting the initial blank to a hot
forming and press hardening operation to form the product with
substantially a uniform first tensile strength; and subsequently,
subjecting the product to post hot-forming processing, comprising
selectively heating a first region of the product to above a known
temperature, while simultaneously maintaining below the known
temperature a second region of the product that is adjacent to the
first region, and then cooling the first region such that the first
region attains a second tensile strength that is substantially less
than the first tensile strength.
[0017] In accordance with an aspect of an embodiment of the
invention there is provided a method of forming a product from an
initial blank, comprising: heating the initial blank to an
austenitizing temperature; hot-shaping the initial blank in a
cooled pair of dies to form the product; cooling the product during
a first period of time, using a rate of cooling that is
sufficiently rapid to support formation of a martensitic structure
within substantially the entire product; subjecting a first portion
of the product to post hot-forming processing, comprising
selectively heating the first portion of the product to a known
temperature that is less than the austenitizing temperature, while
simultaneously maintaining below the known temperature a second
portion of the product that is adjacent to the first portion; and,
cooling the product such that tempered martensite is formed within
the first portion of the product while at the same time the second
portion of the product remains substantially free of tempered
martensite.
[0018] In accordance with an aspect of an embodiment of the
invention there is provided a method of forming a product from an
initial blank, comprising: providing the initial blank; heating the
initial blank to the austenite state; hot-shaping the initial blank
in a cooled pair of dies to form the product; hardening
substantially the entire product while it is still inside the pair
of dies by cooling the product using a rate of cooling that is
sufficiently fast to form a martensitic structure; using conduction
heating, heating a first portion of the product to at least a
predetermined first temperature, while at the same time maintaining
a second portion of the product below a predetermined second
temperature that is lower than the first temperature; and, cooling
the product such that tempered martensite is formed within the
first portion of the product while at the same time the second
portion of the product remains substantially free of tempered
martensite, wherein subsequent to cooling, a tensile strength of
the first portion of the product is less than a tensile strength of
the second portion of the product.
[0019] In accordance with an aspect of an embodiment of the
invention there is provided a method of forming a product from an
initial blank, comprising: providing the initial blank; heating the
initial blank to the austenite state; hot-shaping the initial blank
in a cooled pair of dies to form the product; hardening
substantially the entire product while it is still inside the pair
of dies by cooling the product using a rate of cooling that is
sufficiently fast to form a martensitic structure; using resistance
heating, heating a first portion of the product to at least a
predetermined first temperature, while at the same time maintaining
a second portion of the product below a predetermined second
temperature that is lower than the first temperature; and, cooling
the product such that tempered martensite is formed within the
first portion of the product while at the same time the second
portion of the product remains substantially free of tempered
martensite, wherein subsequent to cooling, a tensile strength of
the first portion of the product is less than a tensile strength of
the second portion of the product.
[0020] In accordance with an aspect of an embodiment of the
invention there is provided a method of forming a product from an
initial blank, comprising: providing the initial blank; heating the
initial blank to the austenite state; hot-shaping the initial blank
in a cooled pair of dies to form the product; hardening
substantially the entire product while it is still inside the pair
of dies by cooling the product using a rate of cooling that is
sufficiently fast to form a martensitic structure; using induction
heating, heating a first portion of the product to at least a
predetermined first temperature, while at the same time maintaining
a second portion of the product below a predetermined second
temperature that is lower than the first temperature; and, cooling
the product such that tempered martensite is formed within the
first portion of the product while at the same time the second
portion of the product remains substantially free of tempered
martensite, wherein subsequent to cooling, a tensile strength of
the first portion of the product is less than a tensile strength of
the second portion of the product.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] Exemplary embodiments of the invention will now be described
in conjunction with the following drawings, in which:
[0022] FIG. 1 is a schematic diagram of a thermoforming line for a
steel component, including post hot-forming processing, according
to an embodiment of the instant invention;
[0023] FIG. 2 is a top view depicting a B column having
substantially uniform tensile strength, as formed by a conventional
hot forming process;
[0024] FIG. 3 is a top view depicting the B column of FIG. 2, with
a first region thereof being subjected to post hot-forming
processing, according to an embodiment of the instant
invention;
[0025] FIG. 4 is a top view depicting the B column of FIG. 2,
having substantially two regions of different tensile strength due
to post hot-forming processing, according to an embodiment of the
instant invention;
[0026] FIG. 5a is a simplified side view of a system for post
hot-forming processing by conduction heating using heated plates,
according to an embodiment of the instant invention;
[0027] FIG. 5b is a simplified diagram depicting a first region of
a product disposed between the pair of conduction heating plates of
FIG. 5a;
[0028] FIG. 6a is a simplified perspective view showing a product
being subjected to conduction heating using a heated fluid and a
suitable contacting medium, according to an embodiment of the
instant invention;
[0029] FIG. 6b is a partial cut-away view showing the product being
subjected to conduction heating by the heated fluid and the
suitable contacting medium of FIG. 6a;
[0030] FIG. 7 is a simplified diagram showing a first region of a
product being subjected to resistance heating, according to an
embodiment of the instant invention;
[0031] FIG. 8a is a simplified diagram showing a product being
subjected to induction heating using coils that are disposed along
opposite sides of a first region of the product, according to an
embodiment of the instant invention;
[0032] FIG. 8b is a simplified diagram showing a product being
subjected to induction heating using a coil that encircles a first
region of the product, according to an embodiment of the instant
invention;
[0033] FIG. 9a is a simplified perspective view showing a first
region of a product being subjected to induction heating using
induction plates, according to an embodiment of the instant
invention;
[0034] FIG. 9b is a simplified side view showing the product of
FIG. 9a being subjected to induction heating using induction
plates, according to an embodiment of the instant invention;
[0035] FIG. 10 is a simplified flow diagram of a method according
to an embodiment of the instant invention;
[0036] FIG. 11 is a simplified flow diagram of another method
according to an embodiment of the instant invention; and
[0037] FIG. 12 is a simplified flow diagram of yet another method
according to an embodiment of the instant invention.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0038] The following description is presented to enable a person
skilled in the art to make and use the invention, and is provided
in the context of a particular application and its requirements.
Various modifications to the disclosed embodiments will be readily
apparent to those skilled in the art, and the general principles
defined herein may be applied to other embodiments and applications
without departing from the scope of the invention. Thus, the
present invention is not intended to be limited to the embodiments
disclosed, but is to be accorded the widest scope consistent with
the principles and features disclosed herein.
[0039] Referring to FIG. 1, shown is a schematic diagram of a
thermoforming line for a steel component, according to an
embodiment of the instant invention. By way of a specific and
non-limiting example, the component that is produced in FIG. 1 is a
B column for an automobile. Of course, other types of components
may be produced in a similar fashion, and the specific example of
the B column is provided merely for illustrative purposes and in
order to facilitate a better understanding of the embodiments of
the instant invention.
[0040] An initial blank 100 is provided. For instance, the initial
blank 100 is stamped from a sheet of hardenable steel, such as
Usibor.RTM. 1500P, Usibor.RTM. 1500, another suitable boron steel
or any suitable hot stamp press hardened material. Optionally, the
initial blank 100 is pre-shaped specifically for producing a B
column, such as for instance by an additional cutting step or an
additional cold forming step (not shown in FIG. 1). The entire
initial blank 100 is then heated in an oven 102 to a temperature
above the Ac3 temperature. By way of a specific and non-limiting
example, the oven 102 is a roller-hearth or a batch style oven.
Once the initial blank 100 is in the austenite state it is
transferred rapidly to a die set shown generally at 104, the die
set 104 having an upper die half 106 and a lower die half 108. The
die set 104 optionally is cooled in order to ensure that a
sufficiently rapid cooling rate of the initial blank 100 is
achieved, such that martensite is formed. By way of a specific and
non-limiting example, channels are defined through the upper die
half 106 and through the lower die half 108 for flowing a cooling
fluid, such as for instance water, oil, saline, etc., through the
die halves to achieve the rapid cooling rate of the product that is
being formed from the initial blank 100. For instance, a typical
cooling rate is in the range of about 30.degree. C./second to about
100.degree. C./second. The product is held inside the die set
during cooling, so as to maintain the desired shape of the product
while it is being cooled and hardened. After being removed from the
die set 104, the product (shown at 110) is further cooled to about
room temperature, or at least to a temperature between about
20.degree. C. and about 250.degree. C. At this stage, the product
110 has substantially a uniform martensite structure.
[0041] After the product 110 has cooled to the desired temperature,
it is subjected to a post hot-forming process 112. The post
hot-forming process 112 includes heating a first region 120 of the
product 110, which is to have reduced tensile strength in the
finished product (i.e., "a soft zone"), to a known temperature. The
known temperature is in the range between about 370.degree. C. and
about 800.degree. C., and in particular between about 400.degree.
C. and about 700.degree. C.
[0042] Referring still to FIG. 1, the first region 120 of the
product 110 is heated using one of conduction heating, resistance
heating, and induction heating. Heating by conduction heating is
described in greater detail with reference to FIGS. 5a, 5b, 6a and
6b, heating by resistance heating is described in greater detail
with reference to FIG. 7, and heating by induction heating is
described in greater detail with reference to FIGS. 8a, 8b, 9a and
9b. The first region 120 is heated to the known temperature between
about 370.degree. C. and about 800.degree. C., and in particular
between about 400.degree. C. and about 700.degree. C. Optionally, a
second region 122 adjacent to the first region 120 is cooled and/or
insulated, so as to prevent substantial heating of the second
region 122 during the post hot-forming processing. For instance,
the second region 122, which is to have high tensile strength in
the finished product, is protected from being heated using a
curtain of cooled gas (such as for instance cooled air) or by
spraying or misting with a cooling liquid (such as for instance
water).
[0043] At the end of the post hot-forming processing, the first
region 120 of the product 110 is cooled to room temperature.
According to one embodiment of the instant invention, the first
region 120 is gas-cooled. Optionally, a fixture is used to maintain
the dimensions of the product 110 during the cooling process.
Further optionally, the first region 120 is cooled using another
suitable cooling technique, such as for instance one of
gas-blasting, fluidized bed cooling, die cooling, water/mist
cooling, and cooling with the use of cooling fans/jets, etc.
Optionally, additional not illustrated post processing is performed
subsequent to cooling, such as for instance trimming or
perforating, etc. Optionally, the additional not illustrated post
processing is performed subsequent to the hot forming and press
hardening steps, but prior to the post hot-forming processing.
[0044] The post hot-forming processing results in the formation of
tempered martensite within the first region 120 of the product 110,
due to the metal within that region being reheated to between about
370.degree. C. and about 800.degree. C. and then cooled. On the
other hand, the second region 122 of the product 110 is not
reheated and cooled in this way, such that tempered martensite is
not formed within the second region 122. Instead, the original
martensite structure that is formed during the hot forming and
press hardening operation is retained within the second region 122
in the finished product. Of course, a transition zone (not
illustrated) of finite width exists along the "boundary" 124
between the first region 120 and the second region 122. The tensile
strength of the product 110 within the transition zone is
intermediate the tensile strength within the first region 120 and
the tensile strength within the second region 122.
[0045] Optionally, additional cycles of heating the product 110,
followed by cooling the product 110, are performed in order to form
additional "soft zones" within different regions of the product
110. Alternatively, two or more non-contiguous regions of the
product 110 are heated at the same time, such that two or more
"soft zones" are formed in a single pass.
[0046] Referring now to FIG. 2, shown is the product 110 that is
obtained at point "A" of the thermoforming line of FIG. 1. The
product 110 at point "A" is substantially uniformly of martensite
structure. Referring now to FIG. 3, shown is the product 110
corresponding to point "B" of the thermoforming line of FIG. 1. The
structure of the product 110 at point "B" is also substantially
uniformly of martensite structure, but the first region 120 of the
product 110 is being subjected to post hot-forming processing. FIG.
4 illustrates that two regions of substantially different tensile
strength are obtained at point "C" of the thermoforming line of
FIG. 1, following the completion of post hot-forming processing of
the product 110. More particularly, subsequent to being reheated
and then cooled to room temperature, the second region 122 on one
side of boundary 124 retains the original martensite structure,
whereas tempered martensite has been formed within the first region
120 on the other side of the boundary 124. As discussed supra a
transition zone (not illustrated) of finite width exists along the
"boundary" 124 between the first region 120 and the second region
122.
[0047] The first region 120 of the product 110 may be heated by one
of conduction heating, resistance heating, and induction heating.
Each heating method will now be described in greater detail,
below.
[0048] Referring to FIG. 5a, shown is a simplified side view of a
system 500 for post hot-forming processing by conduction heating
using heated plates. The system 500 comprises a press 502 having an
upper heated plate 504 and a lower heated plate 506. A product 508
that is to be subjected to post hot-forming processing is
positioned in the press, such that a first region thereof is
disposed between the upper heated plate 504 and the lower heated
plate 506. This positioning is illustrated more clearly in FIG. 5b,
which shows a first region 508a of the product 508 disposed in a
stacked-arrangement between the upper and lower heated plates 504
and 506. A second region 508b, which is adjacent to the first
region 508a, is positioned externally with respect to the heated
plates 504/506.
[0049] An actuator, for instance a hydraulic cylinder 510, moves
the upper heated plate into and out of contact with the first
region 508a of the product 508, and additionally applies sufficient
pressure to ensure conduction of heat from the upper and lower
heated plates 504 and 506 into the first region 508a of the product
508. Optionally, the first region 508a is soaked for a
predetermined amount of time after it reaches a desired
temperature, prior to the upper heated plate 504 being withdrawn by
the action of the actuator 510. Further optionally, the system 500
includes a not illustrated cooling sub-system, which prevents
substantial heating of the second region 508b of the product
508.
[0050] The upper and lower heated plates 504 and 506 are fabricated
from a suitable, thermally conductive material, such as for
instance copper. Although FIGS. 5a and 5b show a product 508 with a
substantially flat second region 508b disposed between flat upper
and lower heated plates 504 and 506, it is to be understood that
optionally the upper and lower heated plates 504 and 506 are shaped
with contours matching any contours of the second region 508b of
the product 508. In other words, the upper and lower heated plates
504 and 506 may be upper and lower die halves that are similar to
the upper die half 106 and a lower die half 108, respectively, used
to hot form and press harden the product 110 as described with
reference to FIG. 1.
[0051] In order to provide improved clarity, FIG. 5a does not show
a power source or electrical wiring extending between the power
source and the upper and lower heated plates 504 and 506.
Additionally, suitable temperature controllers and/or temperature
sensors have been omitted from FIG. 5a. Of course, numerous
modifications of the system that is shown in FIG. 5a may be
envisaged, such as for instance replacing the hydraulic actuator
510 with another type of actuator, or moving the lower heated plate
506 instead of or in addition to moving the upper heated plate 504,
etc.
[0052] Referring now to FIG. 6a, shown is a simplified perspective
view of a product 600 that is being subjected to conduction heating
using a heated fluid and a suitable contacting medium 602. The
product 600 is partially immersed in the heated fluid/contacting
medium 602, which is contained within a containing vessel 604. The
fluid may be a gas, such as for instance hot compressed air, and
the contacting medium may be any suitable particulate such as for
instance sand, ceramic or salt. By way of a specific and
non-limiting example, the heated fluid/contacting medium 602 is set
to a desired temperature between about 370.degree. C. and about
800.degree. C., and in particular in the range of between about
400.degree. C. and about 700.degree. C.
[0053] FIG. 6b is a partial cut away view showing the product 600
being subjected to conduction heating using the heated
fluid/contacting medium 602 of FIG. 6a. In particular, a first
region 600a of the product 600 is immersed in the heated
fluid/contacting medium 602. The first region 600a of the product
600 is left immersed in the heated fluid/contacting medium 602
until the first region 600a reaches the desired temperature in the
range of between about 370.degree. C. to about 800.degree. C., and
may or may not be soaked at that temperature for a period of time.
Optionally, a second region 600b of the product 600 is protected
from being heated, for instance using a collar box that is arranged
around the second region 600b for cooling/insulating the second
region. Optionally, the second region 600b is protected from being
heated using a curtain of cooled gas (such as for instance cooled
air) or by spraying or misting with a cooling liquid (such as for
instance water).
[0054] The product 600 is subsequently removed from the heated
fluid/contacting medium 602, and the product 600 is cooled to room
temperature. According to one embodiment of the instant invention,
the first region 600a is gas-cooled. Optionally, a fixture is used
to maintain the dimensions of the product 600 during the cooling
process. Further optionally, the first region 600a is cooled using
another suitable cooling technique, such as for instance one of
gas-blasting, fluidized bed cooling, die cooling, water/mist
cooling, and cooling with the use of cooling fans/jets, etc.
[0055] In order to provide improved clarity, FIGS. 6a and 6b do not
show an inlet or an outlet of the containing vessel 604, which are
used when a flow of the heated fluid is being provided through the
containing vessel 604. Additionally, suitable temperature
controllers, temperature sensors, compressed air sources, etc. have
been omitted from FIGS. 6a and 6b.
[0056] Optionally, the post hot-forming processing is performed
using conduction heating in which the product is contacted with hot
oil or hot gas, or another suitable solid or fluid medium for
transferring heat selectively to the first region that is to become
a "soft zone" in the final product. Optionally, the post
hot-forming processing is performed using conduction heating in
which the product is contacted with one or more of a flame, plasma,
microwave radiation, infrared radiation, etc.
[0057] Referring now to FIG. 7, shown is a simplified diagram of a
product 700 being subjected to post hot-forming processing, in
which resistance heating is used to heat a first region 700a of the
product 700. A power source 702 is electrically coupled to the
first region 700a of the product 700 via contacts 704a and 704b and
conductors 706a and 706b. In the example that is shown in FIG. 7,
the contacts 704a and 704b are electrically conductive clamps.
[0058] During use, electrical current is passed through the first
region 700a of the product 700 (as illustrated using dashed arrows
in FIG. 7), thereby heating the first region 700a of the product
700 to a desired temperature. By way of a specific and non-limiting
example, the first region 700a is heated to a desired temperature
between about 370.degree. C. and about 800.degree. C., and in
particular in the range of between about 400.degree. C. and about
700.degree. C. Optionally, the second region 700b is protected from
being heated, such as for instance by using a curtain of cooled gas
(e.g., cooled air) or by spraying or misting with a cooling liquid
(i.e., water).
[0059] Referring now to FIGS. 8a and 8b, shown are simplified
diagrams of a product 800 being subjected to post hot-forming
processing, in which induction heating is used to heat a first
portion 800a of the product 800. FIGS. 9a and 9b are simplified
diagrams of a product 900 being subjected to post hot-forming
processing, in which induction heating is used to heat a first
portion 900a of the product 900.
[0060] In general, induction heating is the process of heating an
electrically conducting object by electromagnetic induction, where
eddy currents are generated within the object and resistance leads
to Joule heating. Induction heating can produce high power
densities, which allow short interaction times to reach the desired
temperature. This gives tight control of the heating pattern, with
the pattern following the applied magnetic field quite closely and
allows reduced thermal distortion and damage. The depth of
induction heating can be controlled through the choice of
induction-frequency, power-density, and interaction time.
[0061] Referring now specifically to FIG. 8a, a system is shown for
heating the first portion 800a of the product 800 by induction
heating using a pair of induction coils 802 and 804. The induction
coils 802 and 804 are disposed, one each, along opposite sides of
the first portion 800a of the product 800.
[0062] Referring now to FIG. 8b, a system is shown for heating the
first portion 800a of the product 800 by induction heating using a
single induction coil 806. As is shown in FIG. 8b, the induction
coil 806 encircles the first portion 800a of the product 800.
[0063] Referring now to FIG. 9a, a system is shown for heating the
first portion 900a of the product 900 by induction heating using
induction plates 904 and 906.
[0064] FIG. 9b is a simplified side view showing the first portion
900a of the product 900 being heated by induction heating using the
induction plates 904 and 906 of the system of FIG. 9a.
[0065] Referring now to FIG. 10, shown is a simplified flow diagram
of a method according to an embodiment of the instant invention. At
1000 an initial blank is subjected to a hot forming and press
hardening operation to form a product with substantially a uniform
first tensile strength. At 1002 the product is subjected to post
hot-forming processing. The post hot-forming processing includes
selectively heating a first region of the product to above a known
temperature, while simultaneously maintaining below the known
temperature a second region of the product that is adjacent to the
first region, and then cooling the first region such that the first
region attains a second tensile strength that is substantially less
than the first tensile strength.
[0066] Referring now to 11, shown is a simplified flow diagram of a
method according to an embodiment of the instant invention. At 1100
the initial blank is heated to an austenitizing temperature. At
1102 the initial blank is hot-shaped in a cooled pair of dies to
form the product. At 1104 the product is cooled during a first
period of time, using a rate of cooling that is sufficiently rapid
to support formation of a martensitic structure within
substantially the entire product. At 1106 a first portion of the
product is subjected to post hot-forming processing, comprising
selectively heating the first portion of the product to a known
temperature that is less than the austenitizing temperature, while
simultaneously maintaining below the known temperature a second
portion of the product that is adjacent to the first portion. At
1108 the product is cooled such that tempered martensite is formed
within the first portion of the product while at the same time the
second portion of the product remains substantially free of
tempered martensite.
[0067] Referring now to FIG. 12, shown is a simplified flow diagram
of a method according to an embodiment of the instant invention.
The initial blank is provided at 1200. At 1202 the initial blank is
heated to the austenite state. At 1204 the initial blank is
hot-shaped in a cooled pair of dies so as to form the product. At
1206 the entire product is hardened, while it is still inside the
pair of dies, by cooling the product using a rate of cooling that
is sufficiently fast to form a martensitic structure. At 1208 one
of conduction heating, resistance heating and induction heating is
used to heat a first portion of the product to at least a
predetermined first temperature, while at the same time maintaining
a second portion of the product below a predetermined second
temperature that is lower than the first temperature. At 1210 the
product is cooled such that tempered martensite is formed within
the first portion of the product while at the same time the second
portion of the product remains substantially free of tempered
martensite. In a product that is formed according to the method
that is described with reference to FIG. 12, a tensile strength of
the first portion of the product is less than a tensile strength of
the second portion of the product.
[0068] Numerous other embodiments may be envisaged without
departing from the scope of the instant invention.
* * * * *