Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR PROCESSING ADVANCED
HIGH STRENGTH STEEL
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This PCT International Patent Application claims the
benefit of and priority
to U.S. Provisional Patent Application Serial No. 63/026,230 filed on May 18,
2020, titled
"Method For Processing Advanced High Strength Steel," the entire disclosure of
which is
hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0002] The present invention relates to processing steel, a
method of manufacturing
a component formed of steel, and components formed of steel, such as energy
absorbing
components for vehicle applications.
2. Related Art
100031 This section provides background information related
to the present
disclosure which is not necessarily prior art.
[0004] Energy absorbing components, such as structural
components for vehicle
applications, are oftentimes formed of steel. Energy absorption is the product
of strength
and ductility, and manufacturability requires good formability and
weldability. Thus, the
energy absorbing components formed of steel preferably have a good combination
of
strength, ductility, weldability, and formability.
[0005] There are several types of steel materials used to
manufacture energy
absorbing components. Many traditional energy absorbing components are formed
of a
steel material referred to as boron steel. A traditional process of forming a
component from
boron steel includes heating a sheet formed of the boron steel to a defined
elevated
temperature and for a time period that enables the formation of a face-
centered cubic
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crystallographic phase referred to as austenite. The austenitic steel sheet is
then transferred
to a temperature-controlled steel die. A hydraulic press forms the component
and achieves
a desired profile. The hydraulic press applies the force required to form the
desired profile
and controls the rate of heat transfer, to achieve the desired cooling rate.
The cooling rate
and alloy composition of the boron steel causes a phase transformation of the
low strength
austenite to either a high strength martensite phase or pearlitic
microstructure. The critical
cooling rate is based on the alloy composition. The combination of alloy
composition and
cooling rates imposed by conventional hot stamp processing of boron steel does
not result in
retained austenite.
100061 Emerging energy absorbing components are currently
comprised of bainitic
quenched and partitioned steels referred to as bainitic GEN3 steels. The GEN3
steels are a
commercially available series of advanced high strength steel (AHSS) which
have a high
strength and ductility, which is associated with the bainitic microstructure.
There are
various grades of GEN3 steels, based on alloy composition and thermal
processing. The
transformation of austenite to bainite in the steel is typically accomplished
at the rolling mill
and is referred to as a quench and partition heat treatment. The components
formed of the
GEN3 steels are formed at room temperature.
[0007] There are advantages and disadvantages associated
with the boron steel and
the GEN3 steels described above.
[0008] For example, the hot stamped boron steel exhibits a
higher energy absorption
characteristic than the GEN 3 steel. The forming tonnage required to form the
boron steel at
an elevated temperature is lower than that required for the GEN3 steel at room
temperature.
In addition, the cost of a boron steel sheet is less than a GEN3 steel sheet.
[0009] However, post-formed hot stamped boron steel has a
relatively low ductility,
which limits commercial applications to crash-formed strength-based bending
applications,
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which do not include flange design features. The flange design features
increase design
efficiency and facilitate attachment to other components. The post-formed
strength and
ductility characteristics of components formed of the hot stamped boron steel
necessitate the
use of lasers to trim the stamped components. The processing and manufacturing
costs of
the hot stamped boron steel components are high due to the capital costs,
operating costs,
and floor space allocation associated with blank preheat furnaces, hydraulic
presses, and
laser trim equipment typically used to manufacture the components. The
manufacturing and
processing costs are greater than those associated with the GEN3 steels, due
to the increased
capital and operating cost associated with inline solution heat treat of the
boron steel sheet
prior to the forming operation, use a hydraulic press capable of stopping at
the bottom to
achieve the required transformation cooling rate, and the laser-based trim
processes required
to trim the stamped components formed of the boron steel. In addition, the
post-formed
microstructure of the conventional hot stamped boron steel typically includes
martensite,
but does not include retained austenite. Thus, the boron steel components lack
a post-
forming work hardening response associated with the transformation of retained
austenite in
the post-formed matrix.
[0010]
As indicated above, the quenched and partitioned GEN3 steels, comprised of
a combination of bainite and retained austenite, have improved formability and
ductility
relative to martensitic hot stamped steel enabling the ability to form flange
features to
increase the design efficiency of the component. The quenched and partitioned
GEN3
steels also have the advantage of reduced processing costs, relative to the
hot stamped boron
steel. The reduced processing costs are typically associated with processing
at room
temperature, reduced cycle time related to the use of a higher speed
mechanical press,
avoidance of dwell time associated with transformation cooling, and feasible
secondary
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operations (restrike, trim, flange and pierce) which are performed in-line to
the forming
operation.
[0011] However, the GEN3 steels are typically more costly
than the boron steel.
The post-formed dimensional repeatability of the GEN3 steel stamped components
is low
relative to the hot stamped boron steel and other high strength steel alloys
stamped at room
temperature. The reduced dimensional repeatability is related to spring back.
The post-
formed total energy absorption characteristics of the GEN3 steel is lower
relative to boron
steel. The strength of the GEN3 steel during the forming operation is high
relative to the
hot stamped boron steel, which limits the size (area) or number of GEN3 steel
parts formed
for a given press tonnage. Increased press tonnage is required relative to the
hot stamped
boron steel. In addition, bainitic GEN3 steel does not exhibit a work
hardening
characteristic due to the lack of retained austenite after the forming
operation.
[0012] A commercially available series of GEN3 steel is an
austenitic advanced
high strength steel (AHSS) referred to as austenitic GEN3 TRIP steel. TRIP
steels leverage
the strength and ductility associated with the transformation to austenite to
martensite
(known as the TRIP effect) to enhance formability and strength
characteristics, as a result of
strain imposed during the forming process.
[0013] There is a continuing desire to further develop and
refine steels used to form
energy absorbing components, such as those used in vehicle applications.
Objectives
include increasing product design efficiency by enabling the capability to
form flange
features; avoiding cost by enabling inline restriking, trimming, flanging, and
piercing
operations; avoiding cost associated with secondary laser processing
operations; improving
post-formed dimensional repeatability, associated with springback of GEN3
steel stamped
components; avoiding cost by cycle time reduction; avoiding capital cost
associated with
use of a mechanical press compared to a hydraulic press; avoiding cost by
cycle time
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reduction, specifically by eliminating cooling rate-dependent transformation
events;
reducing capital equipment, operational costs, and floorspace required by
preheat ovens,
presses, and secondary trim operations; and increasing energy absorption
associated with
TRIP enabled work hardening and plasticity characteristics during a strain
induced crash
event.
SUMMARY
[0014] One aspect of the invention provides a method for
processing steel material,
such as material used to form an energy absorbing component for a vehicle. The
method
comprises heating a steel material to a temperature above an upper critical
temperature
(Ac3) of the steel material. The steel material has a microstructure which
includes ferrite
and bainite, and the heating step includes converting a portion of the ferrite
and bainite to
austenite. The method further includes forming the steel material into a
component after the
steel material is heated to the temperature above the upper critical
temperature (Ac3) of the
steel material. The steel material is cooled during the forming step, and a
portion of the
austenite transforms to martensite and bainite during the forming step.
[0015] Another aspect of the invention provides a component
formed of the steel
material, for example an energy absorbing component for a vehicle. The steel
material
includes iron in an amount of 91.95 to 98.55 wt. %, carbon in an amount of
0.15 to 0.3 wt.
%, manganese in an amount of 1.5 to 2.5 wt. %, silicon in an amount of 0.6 to
1.6 wt. %,
chromium in an amount of 0.55 to 0.65 wt. %, copper in an amount of 0.0 to 1.0
wt. %,
nickel in an amount of 0.0 to 1.0 wt. % and aluminum in an amount of 0.0 to
1.0 wt. %,
based on the total weight of the steel material. The steel material also
includes bainite and
martensite.
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BRIEF DESCRIPTION OF THE DRAWINGS
[0016] Figure 1 illustrates an energy absorbing component
formed of a steel
material according to an example embodiment;
[0017] Figure 2 illustrates a quench and partition process
wherein a steel material is
heated above the Ac3 temperature of the steel material, die quenched in a
heated die to a
temperature between the M, and Mf temperature of the steel material, and then
heated to an
elevated temperature to increase energy absorption.
[0018] Figure 3 illustrates a quench and temper process
wherein a steel material is
heated above the Ac3 temperature of the steel material, die quenched in a
steel die to a
temperature below the Ms and Mf temperatures of the steel material, and
reheated to an
elevated temperature to increase energy absorption.
[0019] Figure 4 is a table showing ultimate tensile strength
(TS), yield strength
(YS), and elongation (E) for a steel material in an as-received condition and
steel materials
processed according to example embodiments.
[0020] Figure 5 is a graph of phase distribution and
temperature for a steel material
according to an example embodiment.
DETAILED DESCRIPTION
[0021] One or more of the above objectives are achieved by
embodiments of the
invention. In general, the subject embodiments are directed to a method for
processing
advanced high strength steel (AHSS). However, the example embodiments are only
provided so that this disclosure will be thorough, and will fully convey the
scope to those
who are skilled in the art. Numerous specific details are set forth such as
examples of
specific components and methods, to provide a thorough understanding of
embodiments of
the present disclosure. It will be apparent to those skilled in the art that
specific details need
not be employed, that example embodiments may be embodied in many different
forms and
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that neither should be construed to limit the scope of the disclosure. In some
example
embodiments, well-known processes, well-known device structures, and/or well-
known
technologies are not described in detail.
[0022] According to example embodiments, the method includes
processing of the
advanced high strength steel (AHSS) referred to as bainitic GEN3 steel.
Bainitic GEN3
steel typically comprises iron in an amount of 91 95 to 98.55 wt %, carbon in
an amount of
0.15 to 0.3 wt. %, manganese in an amount of 1.5 to 2.5 wt. %, silicon in an
amount of 0.6
to1.6 wt. %, chromium in an amount of 0.55 to 0.65 wt. %, copper in an amount
of 0.0 to
1.0 wt. %, nickel in an amount of 0.0 to 1.0 wt. % and aluminum in an amount
of 0.0 to 1.0
wt. %, based on the total weight of the steel material. According to a
specific example, the
composition of the steel material, demonstrated at lab scale, comprises iron
in an amount of
96.03 wt. %, carbon in an amount of 0.22 wt. %, manganese in an amount of 2.35
wt. %,
silicon in an amount of 0.6 wt. %, and aluminum in an amount of 0.8 wt. %,
based on the
total weight of the steel. The microstructure of the steel includes bainite,
typically in an
amount of at least 75 vol. %, based on the total volume of the steel material.
The remainder
of the steel material includes ferrite. The process begins with a blank formed
of the steel
material, which is typically in the form of a sheet. The following example
embodiments
will refer to the steel sheet, however, the steel material could comprise
other shapes.
[0023] The bainitic steel material is heated to a
temperature above the upper critical
temperature (Ac3) of the steel material. The Ac3 temperature is defined at the
temperate at
which the ferrite and bainite phases of the steel material transform to
austenite. Thus,
during the heating step, a portion of the ferrite and bainite transform to
austenite. Typically,
for the GEN3 steel material, the temperature above the Ac3 temperature ranges
from 850 C
to 900 C. The Ac3 temperature for the bainitic GEN3 steel disclosed above is
850 C.
However, the Ac3 temperature varies by composition, and Ac3 kinetics are slow.
Heating
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above the Ac3 temperature reduces the time required to achieve a
microstructure which is
100% austenite.
[0024] The specific fraction of ferrite, bainite and
austenite in the steel material after
the heating step is dependent on a phase equilibrium at temperature for the
specific
composition of the steel material. The fraction of ferrite, bainite and
austenite in the steel
sheet is also dependent on the temperature history of the steel sheet prior to
forming and the
specific composition of the steel material.
[0025] Next, the steel sheet, which was previously heat
treated to a temperature
above the Ac3 temperature, typically 850 C to 900 C, is formed into a
component 10
having a desired shape. The forming step is preferably conducted in a
temperature
controlled steel die using a forming press, preferably a mechanical press. The
method also
includes cooling the steel material during and possibly after the forming
step. The
temperature of the steel die ranges from 100 C to 360 C while forming the
steel material
into the desired shape. The temperature of the steel material itself during
the forming step
ranges from 900 C to a temperature ranging between 100 C to 360 C.
[0026] A high fraction percentage of the austenite is
transformed to martensite and
bainite during the forming process, as a result of the rate of heat transfer
imposed by the
forming process. The transformation of the austenite to a combination of
bainite and/or
martensite during the forming step reduces the forming tonnage required,
improves
formability, reduces dimensional variance by improving dimensional
repeatability
associated with spring back, and increases the strength of the formed
component 10. An
example of the component 10 is shown in Figure 1. According to this example,
the
component 10 is a B-pillar between a passenger and driver door of a vehicle.
[0027] As indicated above, during and possibly after the
forming step, the method
preferably includes cooling the steel material and/or shaped component in the
die, for
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example by quenching. The steel material and/or component is cooled to a
temperature
below the Ms temperature. After cooling to a temperature below the Ms
temperature, the
method preferably includes heating or tempering the component to a temperature
above the
Ms temperature in the die. The Ms temperature is the temperature at which the
formation of
martensite in the steel material begins, and the Mf temperature is the
temperature at which
the formation of martensite in the steel material finishes. Regulating the
temperature of the
die during and after the forming step controls the amount of martensite,
bainite, and retained
austenite in the component and thus is able to tailor the energy absorption,
weldability,
and/or deformation characteristics in specific regions of the component.
100281 The cooling step typically includes forming retained
austenite in the
component. The retained austenite is maintained in a matrix of bainite and
martensite. For
example, greater than 0 and up to 15 volume % of the austenite present in the
steel material
prior to the forming step may be retained in the matrix of bainite and
martensite after the
cooling step. The percentage of retained austenite in the post-formed steel
sheet is
dependent on the temperature of the form die, cooling rate, strain imposed
during the
forming process and the specific steel composition.
[0029] The amount of retained austenite present in the
component after forming is
the result of diffusion-related transformation kinetics relative to the
martensite start
temperature (Ms) and martensite finish (Ms) temperature range. The Ms
temperature for the
steel composition disclosed above is approximately 350 C to 360 C and the Mt
temperature is approximately 135 C to 145 C. The percentage of retained
austenite in the
component ranges from 0% to 15% based on stability of the austenite during
cooling
determined by the cooling rate below the Ms temperature. Austenite stability
and the
relative percentage of bainite versus martensite present in the formed
component is
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determined by the cooling rate below the Ms temperature which is influenced by
the
temperature of the steel die used to form the component.
[0030] The method can further including heating or tempering
the steel component
after the cooling and forming steps.
[0031] According to one embodiment, the temperature of the
steel component in the
steel die is controlled and is kept at temperature between the Ms and Mf
temperatures of the
steel material after the forming step, and then the steel component is heated
to a temperature
above the Ms temperature for a defined period of time. Figure 2 illustrates a
quench and
partition process wherein the steel material is heated above the Ac3
temperature, die
quenched in a heated die to a temperature between the Ms and Mf temperatures,
which are
specific to the steel material composition, and then heated to an elevated
temperature to
increase energy absorption.
[0032] According to another embodiment, the temperature of
the steel component is
controlled and kept at a temperature below the Mf temperature prior to heating
the steel
component to a temperature above the Ms temperature for a defined period of
time. Figure
3 illustrates a quench and temper process wherein the steel material is heated
above the Ac3
temperature, die quenched in a steel die to a temperature below the Ms and Mf
temperatures,
which are specific to the steel material composition, and reheated to an
elevated temperature
to increase energy absorption. The cooling and reheating steps are conducted
to increase
energy absorbing properties of the steel component. Various other heating,
tempering,
quenching, partitioning, and/or austenitizing steps can be conducted on the
steel component
after the forming step to increase energy absorbing properties of the steel
component.
[0033] The composition of the steel material of the finished
component still includes
iron in an amount of 91.95 to 98.55 wt. %, carbon in an amount of 0.15 to 0.3
wt. %,
manganese in an amount of 1.5 to 2.5 wt. %, silicon in an amount of 0.6 to 1.6
wt. %,
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chromium in an amount of 0.55 to 0.65 wt. %, copper in an amount of 0.0 to 1.0
wt. %,
nickel in an amount of 0.0 to 1.0 wt. % and aluminum in an amount of 0.0 to
1.0 wt. %,
based on the total weight of the steel material.
[0034] According to an example embodiment, the method
includes heating the steel
material to a temperature above the Ac3 temperature, preferably to a
temperature of 900 C.
The steel material is then cooled during the forming process in a steel die,
preferably
controlled to a temperature of 100 C to 350 C. The cooling rate of the steel
material below
the Ms temperature is greater than 10 C/second, preferably 50 C/second. The
formed
component is then reheated to a temperature above the Ms temperature,
preferably to a
temperature range of 360 C to 400 C.
[0035] Figure 4 is a table showing the ultimate tensile
strength (TS), yield strength
(YS), and elongation (E) for a steel component in the as-received condition;
and steel
components which have been austenized, quenched and partitioned; austenized
and
quenched; and austenized, quenched, and tempered according to example
embodiments.
Figure 5 includes a graph of phase distribution and temperature for a steel
material
according to an example embodiment.
[0036] The process can further include restriking, trimming,
flanging, and/or
piercing operations on the finished formed steel component. If the finished
formed
component is used in a vehicle application and includes a fraction percentage
of retained
austenite, then during a possible crash event the formed component is
subjected to strain
which transforms some of the retained austenite to martensite. The
transformation of the
retained austenite to martensite during the crash event increases strength and
energy
absorption characteristic of the component.
[0037] As indicated above, the process and finished
component formed by the
process described above provides numerous advantages. The transformation of
austenite to
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a combination of martensite, bainite and/or retained austenite addresses the
need to improve
dimensional repeatability, formability, forming tonnage requirements, and
energy
absorption characteristics, relative to GEN3 bainitic steel formed at room
temperature. The
transformation of austenite to a combination of martensite, bainite and
retained austenite
also addresses the need to reduce manufacturing costs, enables use of a
mechanical press,
and increases design efficiency relative to hot stamped boron steel
components. The
transformation of retained austenite to martensite during a strain event
imposed during a
crash addresses the need to improve energy absorption characteristics relative
to GEN3
bainitic steel.
100381 The steel component of the present disclosure also
provides enhanced
formability due to the presence of retained austenite and transformation of
the austenite to
martensite during the forming process. The dimensional characteristics
associated with the
steel component are also enhanced due to the presence of the retained
austenite and the
transformation of the austenite to martensite during the forming process. The
post-formed
energy absorption characteristics of the steel component are greater than GEN3
boron steel
due to the transformation of a portion of austenite to martensite during the
forming event
and the transformation of the retained austenite to martensite during a crash
event. The cost
associated with the manufacture of the steel component is less than boron
steel due to
reduced heating requirements and use of lower cost trimming methods. The
design
efficiency of the steel component is greater than hot stamped boron steel due
to the ability
to form flange features.
[0039] It should be appreciated that the foregoing
description of the embodiments
has been provided for purposes of illustration. In other words, the subject
disclosure it is
not intended to be exhaustive or to limit the disclosure. Individual elements
or features of a
particular embodiment are generally not limited to that particular embodiment,
but, where
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applicable, are interchangeable and can be used in a selected embodiment, even
if not
specifically shown or described. The same may also be varies in many ways.
Such
variations are not to be regarded as a departure from the disclosure, and all
such
modifications are intended to be included within the scope of disclosure.
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