U.S. patent application number 15/824533 was filed with the patent office on 2018-05-31 for press hardened steel with increased toughness and method for production.
The applicant listed for this patent is AK Steel Properties, Inc.. Invention is credited to Erasmus Amoateng, John Andrew Roubidoux.
Application Number | 20180147614 15/824533 |
Document ID | / |
Family ID | 60813975 |
Filed Date | 2018-05-31 |
United States Patent
Application |
20180147614 |
Kind Code |
A1 |
Roubidoux; John Andrew ; et
al. |
May 31, 2018 |
PRESS HARDENED STEEL WITH INCREASED TOUGHNESS AND METHOD FOR
PRODUCTION
Abstract
A method for processing a press hardenable steel includes first
heating a slab of the press hardenable steel. The slab is heated to
a re-heat furnace temperature of approximately 2300.degree. F. The
slab is subjected to rolling into a steel sheet having a
predetermined thickness. The temperature of the slab during rolling
corresponds to a rolling temperature that is greater than or equal
to 1600.degree. F. The steel sheet is coiled. The temperature of
the steel sheet during coiling corresponds to a coiling temperature
of approximately 1050.degree. F.
Inventors: |
Roubidoux; John Andrew;
(Cody, WY) ; Amoateng; Erasmus; (Franklin,
OH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
AK Steel Properties, Inc. |
Middletown |
OH |
US |
|
|
Family ID: |
60813975 |
Appl. No.: |
15/824533 |
Filed: |
November 28, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62426788 |
Nov 28, 2016 |
|
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|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 8/0294 20130101;
C21D 6/008 20130101; C22C 38/54 20130101; C22C 38/26 20130101; C22C
38/48 20130101; C21D 6/005 20130101; C21D 9/48 20130101; C22C 38/28
20130101; C22C 38/32 20130101; C22C 38/04 20130101; C21D 9/46
20130101; B21B 2001/028 20130101; C22C 38/002 20130101; C22C 38/14
20130101; C22C 38/38 20130101; C21D 8/0205 20130101; C22C 38/08
20130101; C21D 8/0405 20130101; C22C 38/02 20130101; C22C 38/44
20130101; C22C 38/06 20130101; C22C 38/12 20130101; C22C 38/22
20130101; B21B 1/02 20130101; C21D 1/18 20130101; C21D 6/002
20130101; C21D 8/0226 20130101 |
International
Class: |
B21B 1/02 20060101
B21B001/02; C22C 38/32 20060101 C22C038/32; C22C 38/26 20060101
C22C038/26; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C21D 9/46 20060101 C21D009/46; C21D 8/02 20060101
C21D008/02; C21D 6/00 20060101 C21D006/00; C21D 1/18 20060101
C21D001/18 |
Claims
1. A method for processing a press hardenable steel, the method
comprising: (a) heating a slab of the press hardenable steel to a
re-heat furnace temperature of approximately 2300.degree. F.; (b)
rolling the slab into a steel sheet having a predetermined
thickness, wherein the temperature of the slab during rolling
corresponds to a rolling temperature that is greater than or equal
to about 1600.degree. F. (871.degree. C.); and (c) coiling the
steel sheet, wherein the temperature of the steel sheet during
coiling corresponds to a coiling temperature of approximately
1050.degree. F.
2. The method of claim 1, wherein the rolling step includes a rough
rolling operation and a finish rolling operation.
3. The method of claim 2, wherein the temperature of the slab
during the rough rolling operation is greater than or equal to
2000.degree. F.
4. The method of claim 2, wherein the temperature of the slab
during the finish rolling operation is greater than or equal to
about 1600.degree. F. (871.degree. C.).
5. The method of claim 2, wherein the temperature of the slab
during the rough rolling operation is approximately 2000.degree.
F.
6. The method of claim 2, wherein the temperature of the slab
during the finish rolling operation is approximately 1600.degree.
F. to 1700.degree. F.
7. The method of claim 1, further comprising hot stamping at least
a portion of the steel sheet after coiling the steel sheet.
8. The method of claim 1, further comprising cooling the press
hardenable steel from the re-heat furnace temperature to the
rolling temperature at a first cooling rate, and cooling the press
hardenable steel from the rolling temperature to the coiling
temperature at a second cooling rate, wherein the second cooling
rate is greater than the first cooling rate.
9. The method of claim 1, wherein the step of cooling the press
hardenable steel from the rolling temperature to the coiling
temperature is performed using a run-out table accelerated cooling
method.
10. The method of claim 1, wherein the press hardenable steel has a
composition comprising: 0.10 to 0.50% Carbon; 0.00 to 0.005% Boron;
0.0 to 0.50% Chromium; 0.75 to 3.0% Manganese; 0.090% or less
Niobium; 0.02 to 1.50% Silicon; 0.0 to 0.80% Aluminum; 0.0 to
0.060% Titanium; 0.0 to 0.50% Molybdenum; 0.0 to 0.60% Nickel; and
the balance including iron and impurities.
Description
PRIORITY
[0001] This application claims priority to U.S. Provisional
application Ser. No. 62/426,788 filed Nov. 28, 2016, entitled
"Press Hardened Steel with Increased Toughness and Method for
Production;" the disclosure of which is incorporated by reference
herein.
BACKGROUND
[0002] The present application relates to an improvement in press
hardened steels, hot press forming steels, hot stamping steels, or
any other steel that is heated to an austenitization temperature
and formed and quenched in a stamping die to achieve desired
mechanical properties in the final part. These types of steels are
also sometimes referred to as "heat treatable boron-containing
steels." In this application, they will all be referred to as
"press hardened steels."
[0003] Press hardened steels are primarily used as structural
members in automobiles where high strength, low weight, and
improved intrusion resistance are desired by automobile
manufacturers. A common structural member where press hardened
steels are employed in the automobile structure is the
B-pillar.
[0004] Current industrial processing of press hardened steel
involves heating a blank (piece of steel sheet) to a temperature
greater than the A.sub.3 temperature (the austenitization
temperature), typically in the range 900-950.degree. C., holding
the material at that temperature for a certain duration, placing
the austenitized blank into a hot stamping die, forming the blank
to the desired shape, and quenching the material in the die to a
low temperature such that martensite is formed. The end result is a
material with a high ultimate tensile strength and a fully
martensitic microstructure.
[0005] The as-quenched microstructure of prior art press hardened
steel is fully martensitic. Conventional press hardened steels have
ultimate tensile strengths of approximately 1500 MPa and total
elongations on the order of 6%.
SUMMARY
[0006] The steels and methods of the present application improve
upon currently available press hardened steel alloys by using
chemistry and processing to achieve higher residual toughness in
the press hardened condition. Residual toughness refers to the
toughness the material has in the press hardened condition.
[0007] The strength-ductility property of embodiments of the
present steel alloys include ultimate tensile strengths greater
than or equal to 1100 MPa and elongations of approximately 8%.
DESCRIPTION OF DRAWINGS
[0008] FIG. 1 shows a thermal profile and processing schematic for
embodiments of the present alloys.
[0009] FIG. 2 shows another thermal profile and processing
schematic for embodiments of the present alloys.
[0010] FIG. 3 shows a plot of stress-strain curves for composition
4310, with results from a first pre-processing method shown in
solid-line form and results from a second pre-processing method
shown in dashed-line form.
[0011] FIG. 4 shows a plot of stress-strain curves for composition
4311, with results from the first pre-processing method shown in
solid-line form and results from the second pre-processing method
shown in dashed-line form.
[0012] FIG. 5 shows a plot of stress-strain curves for composition
4312, with results from the first pre-processing method shown in
solid-line form and results from the second pre-processing method
shown in dashed-line form.
[0013] FIG. 6 shows a plot of stress-strain curves for composition
4313, with results from the first pre-processing method shown in
solid-line form and results from the second pre-processing method
shown in dashed-line form.
[0014] FIG. 7 shows the results of a double edge-notch tensile test
for embodiments of the present alloys after being subjected to the
second pre-processing method.
[0015] FIG. 8 shows the results of a double edge-notch tensile test
for embodiments of the present alloys after being subjected to the
first pre-processing method.
[0016] FIG. 9 shows the results of strain energy computations for
embodiments of the present alloys plotted as a function of niobium
concentration.
[0017] FIG. 10 shows a photomicrograph of composition 4310 after
being subjected to the first pre-processing method.
[0018] FIG. 11 shows a photomicrograph of composition 4310 after
being subject to the second pre-processing method.
[0019] FIG. 12 shows a photomicrograph of composition 4311 after
being subjected to the first pre-processing method.
[0020] FIG. 13 shows a photomicrograph of composition 4311 after
being subjected to the second pre-processing method.
[0021] FIG. 14 shows a photomicrograph of composition 4312 after
being subjected to the first pre-processing method.
[0022] FIG. 15 shows a photomicrograph of composition 4312 after
being subjected to the second pre-processing method.
[0023] FIG. 16 shows a photomicrograph of composition 4313 after
being subjected to the first pre-processing method.
[0024] FIG. 17 shows a photomicrograph of composition 4313 after
being subjected to the second pre-processing method.
DETAILED DESCRIPTION
[0025] Press hardened steels are generally desirable for their high
strength characteristics. In practice, this permits manufacturers
to produce components having greater strength and less weight
relative to components produced of non-press hardened steels. These
high strength characteristics are generally achieved through
formation of a predominately martensitic microstructure. In
particular, during a hot stamping process associated with a press
hardened steel blank, the blank is first subjected to an
austenitization heat treatment. During this heat treatment, the
temperature of the blank is raised to greater than the A.sub.3
temperature for the particular composition of the blank to thereby
transform the microstructure of the blank into predominately
austenite.
[0026] Once the austenitization heat treatment is complete, the
blank is stamped into a predetermined shape using an internally
cooled die set. In addition to shaping the blank, the stamping
process also has the effect of rapidly cooling the blank below the
martensite start temperature (M.sub.s). As a consequence, the
predominately austenitic microstructure of the blank is transformed
to a microstructure of predominantly martensite. Because martensite
is generally characterized as a strong and hard microstructure, the
stamping process generally results in a final part having high
strength and high hardness.
[0027] Although a high strength of the final hot stamped part is
generally desirable for a wide variety of applications, in some
circumstances additional toughness may be desirable. For instance,
as described above, hot stamping generally results in a final part
with high strength and high hardness. With high levels of hardness,
the final part generally has relatively low ductility and thus
relatively low toughness. Thus, in some circumstances it may be
desirable to have a press hardened steel having the high strength
characteristics of a conventional press hardened steel, but with
improved residual toughness characteristics.
[0028] Prior to the hot stamping process described above, press
hardened steels are subjected to a variety of pre-processing steps.
FIG. 1 shows a conventional pre-processing method (10).
Pre-processing method includes subjecting a steel sheet to a
plurality of pre-processing steps (20, 30, 40, 50). These steps
(20, 30, 40, 50) are generally performed prior to hot stamping and
prior to formation of press hardened steel blanks for the final hot
stamping process. Generally, these steps (20, 30, 40, 50) are
performed on sheet material in a continuous rolling mill. For
instance, the press hardened steel initially begins as an as-cast
slab comprising a predetermined composition. The slab then enters a
re-heat furnace (20) and is subjected to a re-heat temperature of
approximately 2300.degree. F. (1260.degree. C.).
[0029] Once the slab is elevated to the re-heat temperature via the
re-heat furnace (20), the slab is subjected to rough rolling (30)
and then finishing rolling (40). These rolling steps progressively
reduce the thickness of the slab to a final sheet thickness. During
the rolling process, the temperature of the slab continuously
decreases from the initial 2300.degree. F. (1260.degree. C.)
re-heat temperature to a roughing temperature associated with rough
rolling (30). In some examples the roughing temperature is
approximately 2000.degree. F. (1093.degree. C.). During finishing
rolling (40), the slab is subject to a finishing temperature of
approximately 1600.degree. F. (871.degree. C.). As the temperature
decreases, the slab is subjected to rolling operations that
progressively reduce the thickness of the slab by relatively large
amounts during rough rolling (30) to relatively small amounts
during finishing rolling (40).
[0030] From the initial re-heat temperature associated with the
re-heat furnace (20) to the temperature associated with finishing
rolling (40), the temperature of the slab decreases at a relatively
constant rolling cooling rate (12).
[0031] After completion of rolling, the press hardened steel
material is in a steel sheet form. In the steel sheet form, the
steel sheet is subject to coiling (50). Coiling (50) can be
performed at a coiling temperature of approximately 1200.degree. F.
(649.degree. C.). In some examples, coiling (50) can begin
immediately after finishing (40). Thus, in some examples coiling
(50) may begin at temperatures above 1600.degree. F. (871.degree.
C.) and decrease to the coiling temperature of approximately
1200.degree. F. (649.degree. C.).
[0032] Prior to coiling (50), the steel sheet can be cooled to the
coiling temperature at one or more different cooling rates (14, 16)
as shown in FIG. 1. For instance, at a first cooling rate (14) or
second cooling rates (16), the steel sheet is cooled relatively
slowly at between about 18.degree. F./second and about 20.degree.
F./second.
[0033] At the conclusion of coiling (50), the coiled steel sheet is
permitted to cool to ambient or room temperature. The coiled steel
sheet is then subsequently formed into blanks of steel material for
press hardening. The blanks can then be subjected to the hot
stamping process described above.
[0034] As described above, in some circumstances it may be
desirable to increase the toughness of press hardened steel parts.
In some circumstances, toughness can be improved by refining the
grain size of the press hardened steel material by modifying
certain parameters of the pre-processing steps described above.
[0035] FIG. 2 shows modified pre-processing method (100). As with
pre-processing method (10) described above, pre-processing method
(100) of the present example includes a series of pre-processing
steps (120, 130, 140, 150). As similarly described above, these
steps (120, 130, 140, 150) are generally performed prior to hot
stamping and prior to formation of press hardened steel blanks for
the final hot stamping process. Generally, these steps (120, 130,
140, 150) are performed on sheet material in a continuous rolling
mill. For instance, the press hardened steel initially begins as an
as-cast slab comprising a predetermined composition. The slab then
enters a re-heat furnace (120), where the slab is subjected to a
re-heat temperature. Like with the reheat temperature described
above with respect to re-heat furnace (20), the reheat temperature
in the present example is approximately 2300.degree. F.
(1260.degree. C.).
[0036] Once the slab is elevated to the re-heat temperature of
re-heat furnace (120), the slab is subjected to rough rolling (130)
and then finishing rolling (140). This progressively reduces the
thickness of the slab to a final sheet thickness. As an example,
during the rolling process, the temperature of the slab
continuously decreases from the initial 2300.degree. F.
(1260.degree. C.) re-heat temperature of the re-heat furnace (120)
to a roughing temperature of approximately 2000.degree. F.
(1093.degree. C.) associated with rough rolling (130). Next, the
slab is further reduced to a finishing temperature of approximately
1600.degree. F. (871.degree. C.) associated with finishing rolling
(140). Unlike finishing rolling (40) in the conventional
pre-processing method (10) described above, finishing rolling (140)
in the present example is performed at a relatively lower
temperature. As will be described in greater detail below, this
relatively lower temperature can lead to increased grain refinement
when performed in connection with a modified coiling temperature.
As the temperature decreases, the slab is subjected to rolling
operations that reduce the thickness of the slab by relatively
large amounts during rough rolling (130) to relatively small
amounts during finishing rolling (140).
[0037] From the initial re-heat temperature associated with the
re-heat furnace (120) to the temperature associated with finishing
rolling (140), the temperature of the slab decreases at a
relatively constant rolling cooling rate (112). This cooling rate
is similar to the rolling cooling rate (12) of the prior
process.
[0038] After completion of rolling, the press hardened steel
material is in a steel sheet form. In the steel sheet form, the
steel sheet is subject to coiling (150). Coiling (150) can be
performed at a coiling temperature of approximately 1050.degree. F.
(566.degree. C.). In some examples, coiling (150) can begin
immediately after finishing (140). Thus, in some examples coiling
(150) may begin at approximately 1600.degree. F. (871.degree. C.)
and decrease to the coiling temperature of approximately
1050.degree. F. (566.degree. C.). Alternatively, in some examples
coiling (150) can be delayed until the steel sheet reaches the
coiling temperature of approximately 1050.degree. F. (566.degree.
C.). Once the coiling temperature is reached (150), the steel sheet
may be held isothermally for the entirety of coiling (150).
Preferably, the finishing (140) is performed at the finishing
temperature of about 1600.degree. F. (871.degree. C.), the steel
sheet is lowered to the coiling temperature of 1050.degree. F.
(566.degree. C.), and coiling (150) is performed while the steel
sheet is held at the coiling temperature.
[0039] Regardless of how the coiling temperature is reached, it
should be understood that the coiling temperature of approximately
1050.degree. F. (566.degree. C.) is generally low relative to the
coiling temperatures described above with respect to conventional
pre-processing method (10). As will be understood, this reduced
coiling temperature can generally result in improved grain
refinement of the steel sheet that can lead to increased residual
toughness in a final work product after hot stamping.
[0040] Prior to coiling (150), the steel sheet can be cooled to the
coiling temperature at a cooling rate (114) as shown in FIG. 2. In
the present example, the cooling rate (114) is between about
35.degree. F./second and about 50.degree. F./second.
[0041] Unlike cooling rates (14, 16) described above, cooling rate
(114) in the present example is generally relatively fast. This
relatively fast cooling rate can be achieved using a run-out-table
accelerated cooling method. As will be understood, this relatively
fast cooling rate (114) can generally lead to increased grain
refinement and associated improved residual toughness in a final
work product after hot stamping.
[0042] At the conclusion of coiling (150), the coiled steel sheet
is permitted to cool to ambient or room temperature. The coiled
steel sheet is then subsequently formed into blanks of steel
material for press hardening. The blanks can then be subjected to
the hot stamping process described above.
[0043] As described above, the pre-processing methods (10, 100) can
be performed using an as-cast slab comprising a predetermined
composition. It should be understood that the particular
composition of the slab can be varied such that a variety of
compositions can be used with the methods (10, 100) described
above. As will be described in greater detail below, various
elements can be added to the slab to influence numerous
metallurgical properties of the final work product.
[0044] Carbon is added to reduce the martensite start temperature,
provide solid solution strengthening, and to increase the
hardenability of the steel. Carbon is an austenite stabilizer. In
certain embodiments, carbon can be present in concentrations of
0.1-0.5 mass %; in other embodiments, carbon can be present in
concentrations of 0.2-0.30 mass %.
[0045] Manganese is added to reduce the martensite start
temperature, provide solid solution strengthening, and to increase
the hardenability of the steel. Manganese is an austenite
stabilizer. In certain embodiments, manganese can be present in
concentrations of 0.75-3.0 mass %; in other embodiments, manganese
can be present in concentrations of 1.15-1.25 mass %.
[0046] Silicon is added to provide solid solution strengthening.
Silicon is a ferrite stabilizer. In certain embodiments, silicon
can be present in concentrations of 0.02-1.5 mass %; in other
embodiments, silicon can be present in concentrations of 0.15-0.30
mass %.
[0047] Aluminum is added for deoxidation during steelmaking and to
provide solid solution strengthening. Aluminum is a ferrite
stabilizer. In certain embodiments, aluminum can be present in
concentrations of 0.0-0.8 mass %; in other embodiments, aluminum
can be present in concentrations of 0.02-0.15 mass %. In other
embodiments, aluminum is entirely optional and can be therefore
omitted or limited to an impurity element in some embodiments.
[0048] Titanium is added to getter nitrogen. In certain
embodiments, titanium can be present in concentrations of 0.0-0.060
mass %; in other embodiments, titanium can be present in
concentrations of a maximum of 0.045 mass %. In other embodiments,
titanium is entirely optional and can be therefore omitted or
limited to an impurity element in some embodiments.
[0049] Molybdenum is added to provide solid solution strengthening
and to increase the hardenability of the steel. In certain
embodiments, molybdenum can be present in concentrations of 0-0.5
mass %; in other embodiments, molybdenum can be present in
concentrations of 0-0.3 mass %. In other embodiments, molybdenum is
entirely optional and can be therefore omitted or limited to an
impurity element in some embodiments.
[0050] Chromium is added to reduce the martensite start
temperature, provide solid solution strengthening, and increase the
hardenability of the steel. Chromium is a ferrite stabilizer. In
certain embodiments, chromium can be present in concentrations of
0-0.5 mass %; in other embodiments, chromium can be present in
concentrations of 0.15-0.25 mass %.
[0051] Boron is added to increase the hardenability of the steel.
In certain embodiments, boron can be present in concentrations of
0-0.005 mass %; in other embodiments, boron can be present in
concentrations of 0.003-0.005 mass %.
[0052] Nickel is added to provide solid solution strengthening and
reduce the martensite start temperature. Nickel is an austenite
stabilizer. In certain embodiments, nickel can be present in
concentrations of 0.0-0.6 mass %; in other embodiments, nickel can
be present in concentrations of 0.02-0.3 mass %. In still other
embodiments, nickel is entirely optional and can be therefore
omitted or limited to an impurity element in some embodiments.
[0053] Niobium is added to provide improved grain refinement.
Niobium can also increase hardness and strength. In certain
embodiments, niobium can be present in concentrations of 0-0.090
mass %.
Example 1
[0054] A plurality of alloy compositions shown in Table 1 were
prepared using standard steel making processes, except as noted
below.
TABLE-US-00001 TABLE 1 Composition range. Compositions are in mass
pct. C B Cr Mn Nb Si 4310 0.21 0.003 0.21 1.18 0.000 0.24 4311 0.21
0.0029 0.19 1.19 0.029 0.24 4312 0.21 0.0029 0.20 1.20 0.043 0.24
4313 0.22 0.003 0.19 1.20 0.052 0.25
Example 2
[0055] Composition 4310 of Table 1 in Example 1 was subjected to
both pre-processing methods (10, 100) described above. The steel
underwent simulated hot stamping. The steel was heated to
approximately 930.degree. C. for 5 min and then quenched in
water-cooled copper dies. Samples subjected to each pre-processing
method (10, 100) plus simulated hot stamping were then subjected to
tensile testing to generate stress-strain curves. The resulting
stress-strain curves are shown in FIG. 3 with pre-processing method
(10) shown in solid-line form and pre-processing method (100) shown
in dashed-line form.
[0056] As can be seen in FIG. 3, the samples subjected to
pre-processing method (100) generally resulted in improved residual
toughness relative to samples subjected to pre-processing method
(10). Photomicrographs were prepared for each sample in the
hot-rolled condition prior to the simulated hot stamping and are
shown in FIGS. 10 and 11, with FIG. 10 corresponding to
pre-processing method (10) and FIG. 11 corresponding to
pre-processing method (100). As can be seen, pre-processing method
(100) generally resulted in a more refined grain structure relative
to the grain structure produced from pre-processing method (10). As
a consequence of this, improved residual toughness was observed in
FIG. 3.
Example 3
[0057] Composition 4311 of Table 1 in Example 1 was subjected to
both pre-processing methods (10, 100) described above. The steel
underwent simulated hot stamping. The steel was heated to
approximately 930.degree. C. for 5 min and then quenched in
water-cooled copper dies. Samples subjected to each pre-processing
method (10, 100) plus simulated hot stamping were then subjected to
tensile testing to generate stress-strain curves. The resulting
stress-strain curves are shown in FIG. 4 with pre-processing method
(10) shown in solid-line form and pre-processing method (100) shown
in dashed-line form.
[0058] As can be seen in FIG. 4, the samples subjected to
pre-processing method (100) generally resulted in improved residual
toughness relative to samples subjected to pre-processing method
(10). Photomicrographs were prepared for each sample in the
hot-rolled condition prior to the simulated hot stamping and are
shown in FIGS. 12 and 13, with FIG. 12 corresponding to
pre-processing method (10) and FIG. 13 corresponding to
pre-processing method (100). As can be seen, pre-processing method
(100) generally resulted in a more refined grain structure relative
to the grain structure produced from pre-processing method (10). As
a consequence of this, improved residual toughness was observed in
FIG. 4.
Example 4
[0059] Composition 4312 of Table 1 in Example 1 was subjected to
both pre-processing methods (10, 100) described above. The steel
underwent simulated hot stamping. The steel was heated to
approximately 930.degree. C. for 5 min and then quenched in
water-cooled copper dies. Samples subjected to each pre-processing
method (10, 100) plus simulated hot stamping were then subjected to
tensile testing to generate stress-strain curves. The resulting
stress-strain curves are shown in FIG. 5 with pre-processing method
(10) shown in solid-line form and pre-processing method (100) shown
in dashed-line form.
[0060] As can be seen in FIG. 5, the samples subjected to
pre-processing method (100) generally resulted in improved residual
toughness relative to samples subjected to pre-processing method
(10). Photomicrographs were prepared for each sample in the
hot-rolled condition prior to the simulated hot stamping and are
shown in FIGS. 14 and 15, with FIG. 14 corresponding to
pre-processing method (10) and FIG. 15 corresponding to
pre-processing method (100). As can be seen, pre-processing method
(100) generally resulted in a more refined grain structure relative
to the grain structure produced from pre-processing method (10). As
a consequence of this, improved residual toughness was observed in
FIG. 5.
Example 5
[0061] Composition 4313 of Table 1 in Example 1 was subjected to
both pre-processing methods (10, 100) described above. The steel
underwent simulated hot stamping. The steel was heated to
approximately 930.degree. C. for 5 min and then quenched in
water-cooled copper dies. Samples subjected to each pre-processing
method (10, 100) plus simulated hot stamping were then subjected to
tensile testing to generate stress-strain curves. The resulting
stress-strain curves are shown in FIG. 6 with pre-processing method
(10) shown in solid-line form and pre-processing method (100) shown
in dashed-line form.
[0062] As can be seen in FIG. 6, the samples subjected to
pre-processing method (100) generally resulted in improved residual
toughness relative to samples subjected to pre-processing method
(10). Photomicrographs were prepared for each sample in the
hot-rolled condition prior to the simulated hot stamping and are
shown in FIGS. 16 and 17, with FIG. 16 corresponding to
pre-processing method (10) and FIG. 17 corresponding to
pre-processing method (100). As can be seen, pre-processing method
(100) generally resulted in a more refined grain structure relative
to the grain structure produced from pre-processing method (10). As
a consequence of this, improved residual toughness or retained
ductility was observed in FIG. 6.
Example 6
[0063] Toughness for samples having each composition identified in
Table 1 of Example 1, above, was evaluated further using
double-edge-notch tensile tests. A sample for each composition
(e.g., 4310, 4311, 4312, 4313) was subject to each pre-processing
method (10, 100) described above. Steels then underwent a simulated
press hardening procedure in which they were austenitized at
approximately 930.degree. C. for 300 s and then quenched in flat,
water-cooled dies. Double-edge notched tensile tests were then
performed. Plots were then prepared of the resulting data for each
composition as shown in FIGS. 7 and 8. For instance, FIG. 7 shows
the results for each sample subjected to pre-processing method
(100). FIG. 8 shows the results for each sample subjected to
pre-processing method (10). For both FIGS. 7 and 8, the data for
each composition is identifiable by symbols. For instance, circles
correspond to composition 4310, triangles correspond to composition
4311, stars correspond to composition 4312, and crosses correspond
to composition 4313.
[0064] As can be seen in FIGS. 7 and 8, materials subjected to
pre-processing method (100) exhibited a higher peak load/force
prior to fracture in compassion to the materials subject to
pre-processing method (10). Thus, FIGS. 7 and 8 are indicative of
pre-processing method (100) resulting in increased toughness or
retained ductility.
Example 7
[0065] The data discussed above with respect to Example 6 was
analyzed further. In particular, integration of the area under the
force-displacement curves shown in FIGS. 7 and 8 can be used to
obtain a value of strain energy. Strain energy is considered a
measure of material toughness. Accordingly, a measure of material
toughness for each sample discussed above with respect to Example 6
was generated.
[0066] The resulting strain energy for each sample was then plotted
as a function of niobium concentration in the corresponding
composition for each sample. The resulting plot is shown in FIG. 9.
Unlike FIGS. 7 and 8 discussed above, FIG. 9 utilizes a different
symbolic scheme to identify the correspondence between specific
data points and composition. For instance, in FIG. 9, circles
correspond to composition 4310, crosses correspond to composition
4311, triangles correspond to composition 4312, and squares
correspond to composition 4313. In addition, because the results
for samples subjected to each pre-processing method (10, 100) are
included in a single plot, FIG. 9 depicts a comparison of samples
subjected to pre-processing method (10) and samples subjected to
pre-processing method (100). In each case, the steels underwent
simulated hot stamping prior to testing. In FIG. 9, solid symbols
represent processing method (10) and open symbols represent
processing method (100).
[0067] As can be seen in FIG. 9, samples subjected to
pre-processing method (100) generally resulted in increased strain
energy and therefore increased toughness. In addition, some
increase in toughness was observed in response to a composition
with increased niobium. For instance, composition 4313 included the
highest niobium concentrations and also included the highest strain
energy or toughness measurements.
Example 8
[0068] A press hardenable steel comprising by total mass percentage
of the steel:
[0069] wherein said steel is subject to the following processing:
[0070] (a) heating a slab of the press hardenable steel to a
re-heat furnace temperature of approximately 2300.degree. F.;
[0071] (b) rolling the slab into a steel sheet having a
predetermined thickness, wherein the temperature of the slab during
rolling corresponds to a rolling temperature that is greater than
or equal to about 1600.degree. F. (871.degree. C.); and [0072] (c)
coiling the steel sheet, wherein the temperature of the steel sheet
during coiling corresponds to a coiling temperature of
approximately 1050.degree. F.
Example 9
[0073] A press hardenable steel of Example 8 or any one of the
following Examples, comprising by total mass percentage of the
steel:
[0074] 0.10 to 0.50% Carbon;
[0075] 0.00 to 0.005% Boron;
[0076] 0.0 to 0.50% Chromium;
[0077] 0.75 to 3.0% Manganese;
[0078] 0.090% or less Niobium;
[0079] 0.02 to 1.50% Silicon;
[0080] 0.0 to 0.8% Aluminum;
[0081] 0.0 to 0.060% Titanium;
[0082] 0.0 to 0.50% Molybdenum;
[0083] 0.0 to 0.6% Nickel; and [0084] the balance including iron
and impurities,
Example 10
[0085] A press hardenable steel of Example 8 or 9 or any one of the
following Examples, comprising 0.2-0.3 mass % carbon.
Example 11
[0086] A press hardenable steel of any one of Examples 8 through 10
or any one of the following Examples, comprising 1.15-1.25 mass %
manganese.
Example 12
[0087] A press hardenable steel of any one of Examples 8 through 11
or any one of the following Examples, comprising 0.15-0.30 mass %
silicon.
Example 13
[0088] A press hardenable steel of any one of Examples 8 through 12
or any one of the following Examples, comprising 0.02-0.15 mass %
aluminum.
Example 14
[0089] A press hardenable steel of any one of Examples 8 through 13
or any one of the following Examples, comprising a maximum of 0.045
mass % titanium.
Example 15
[0090] A press hardenable steel of any one of Examples 8 through 14
or any one of the following Examples, comprising 0-0.30 mass %
molybdenum.
Example 16
[0091] A press hardenable steel of any one of Examples 8 through 15
or any one of the following Examples, comprising 0.15-0.25 mass %
chromium.
Example 17
[0092] A press hardenable steel of any one of Examples 8 through 16
or any one of the following Examples, comprising 0.003-0.005 mass %
boron.
Example 18
[0093] A press hardenable steel of any one of Examples 8 through 17
or any one of the following Examples, comprising 0.02-0.3 mass %
nickel.
Example 19
[0094] A press hardenable steel of any one of Examples 8 through 18
or any one of the following Examples, comprising 0-1.0 mass %
molybdenum.
Example 20
[0095] A press hardenable steel of any one of Examples 8 through 19
or any one of the following Examples, wherein the rolling step
includes a rough rolling operation and a finish rolling
operation.
Example 21
[0096] A press hardenable steel of any one of Examples 8 through 20
or any one of the following Examples, wherein the temperature of
the slab during the rough rolling operation is greater than or
equal to 2000.degree. F.
Example 22
[0097] A press hardenable steel of any one of Examples 8 through 21
or any one of the following Examples, wherein the temperature of
the slab during the finish rolling operation is greater than or
equal to about 1600.degree. F. (871.degree. C.).
Example 23
[0098] A press hardenable steel of any one of Examples 8 through 22
or any one of the following Examples, further comprising the step
of hot stamping at least a portion of the steel sheet after coiling
the steel sheet.
Example 24
[0099] A press hardenable steel of any one of Examples 8 through 23
or any one of the following Examples, further comprising the step
of cooling the press hardenable steel from the re-heat furnace
temperature to the rolling temperature at a first cooling rate, and
cooling the press hardenable steel from the rolling temperature to
the coiling temperature at a second cooling rate, wherein the
second cooling rate is greater than the first cooling rate.
Example 25
[0100] A press hardenable steel of any one of Examples 8 through 24
or any of the following Examples, wherein the step of cooling the
press hardenable steel from the rolling temperature to the coiling
temperature is performed using a run-out table accelerated cooling
method.
Example 26
[0101] A press hardenable steel of any one of Examples 8 through 25
or the following Example, wherein the temperature of the slab
during the rough rolling operation is approximately 2000.degree.
F.
Example 27
[0102] A press hardenable steel of any one of Examples 8 through
26, wherein the temperature of the slab during the finish rolling
operation is approximately 1600.degree. F. to 1700.degree. F.
* * * * *