Note: Descriptions are shown in the official language in which they were submitted.
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HIGH STRENGTH STEEL PRODUCTS AND
ANNEALING PROCESSES FOR MAKING THE SAME
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S. Patent Application
No.
15/591,344, filed May 10, 2017, now U.S. Patent No. 10,385,419, which claims
priority to U.S.
Provisional Application No. 62/334,189 filed May 10, 2016, and U.S.
Provisional Application
No. 62/396,602 filed September 19, 2016. This application is also a
continuation-in-part of U.S.
Patent Application No. 16/459,757, filed July 2, 2019, which is a continuation
of U.S. Patent
Application No. 15/591,344. All of the foregoing applications are incorporated
herein by
reference.
FIELD OF THE INVENTION
[0002] The present invention relates to high strength steel products having
favorable
properties, and to annealing processes for making such products.
BACKGROUND INFORMATION
[0003] Over past several years, the worldwide steel industry has focused on
development
of a third generation of advanced high strength steel (AHSS) for the
automotive market. These
Generation 3 steels have a favorable balance of tensile strength and
elongation, typically in a
UTS=TE range of about 20,000 MPa% or greater. However, the steel industry has
had a difficult
time commercializing Generation 3 AHSS as the majority of approaches require
high alloy
content, e.g., typically greater than 4 weight percent manganese, which
results in difficulties
when manufacturing such steels with conventional steel production equipment.
Additionally,
currently available AHSS has been difficult to weld by techniques such as spot
welding, have
been difficult to coat with zinc-based galvanic coatings, and have been
difficult to manufacture
into the thin gauge sheet needed for wide scale application.
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SUMMARY OF THE INVENTION
[0004] The present invention provides steel sheet products having controlled
compositions that are subjected to two-step annealing processes to produce
sheet products having
desirable microstructures and favorable mechanical properties such as high
strength and ultra-
high formability. The steel sheet products may be cold rolled or hot rolled.
Steels processed in
accordance with the present invention exhibit favorable combined ultimate
tensile strength and
total elongation (UTS=TE) properties, e.g., of greater than 25,000 MPa-% when
tested using
standard subsize ASTM or full size JIS tensile testing procedures. In
addition, steels produced in
accordance with the present invention exhibit favorable combinations of TE and
hole expansion,
i.e., both global formability and local formability are good. Steels with
these properties fall into
the category of Generation 3 advanced high strength steels, and are highly
desired by various
industries including automobile manufacturers.
[0005] An aspect of the present invention is to provide a high strength rolled
steel sheet
product comprising from 0.12 to 0.5 weight percent C, from 1 to 3 weight
percent Mn, and from
0.8 to 3 weight percent of a combination of Si and Al, wherein the steel sheet
product has been
subjected to a two-step annealing process, comprises ferrite and substantially
equiaxed retained
austenite grains having an average aspect ratio of less than 3:1, and has a
combination of
ultimate tensile strength and total elongation UTS=TE of greater than 25,000
MPa%.
[0006] Another aspect of the present invention is to provide a method of
producing a
high strength rolled steel sheet product comprising from 0.12 to 0.5 weight
percent C, from 1 to
3 weight percent manganese, and from 0.8 to 3 weight percent of a combination
of Si and Al.
The method comprises subjecting the steel sheet product to a first step
annealing process to
achieve a predominantly martensitic microstructure, and subjecting the steel
sheet product to a
second step process comprising soaking the sheet product in an intercritical
regime at a
temperature of from 720 to 850 C, followed by holding the sheet product at a
temperature of
from 370 to 445 C.
[0007] These and other aspects of the present invention will be more apparent
from the
following description.
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BRIEF DESCRIPTION OF THE DRAWINGS
[0008] Fig. 1 includes plots of temperature versus time illustrating a two-
step annealing
process in accordance with an embodiment of the present invention.
[0009] Fig. 2 includes plots of temperature versus time illustrating a two-
step annealing
process in accordance with another embodiment of the present invention.
[0010] Fig. 3 is a plot of temperature versus time illustrating a two-step
annealing
process that combines the two-step thermal process with an optional zinc-based
hot-dipped
coating operation in a single production facility.
[0011] Fig. 4 is a plot of temperature versus time for a second step of an
annealing
process defining soaking and holding zones in the thermal cycle in accordance
with an
embodiment of the invention.
[0012] Figs. 5 and 6 are electron backscatter diffraction (EBSD)
photomicrographs
illustrating the microstructure of a high strength steel sheet product in
accordance with an
embodiment of the invention.
[0013] Fig. 7 is an optical photomicrograph of a steel sheet product subjected
to the
thermal process shown in Fig. 1, showing darker ferrite grains and lighter
austenite grains.
[0014] Fig. 8 is a bar graph illustrating aspect ratios of the austenite
grains shown in Fig.
7.
[0015] Figs. 9 and 10 are graphs for a high strength steel sheet product
showing austenite
and ferrite grain size distributions in accordance with an embodiment of the
invention.
[0016] Figs. 11 is EBSD photomicrograph illustrating the microstructure of a
high
strength steel sheet product processed as shown in Fig. 1.
[0017] Figs. 12 and 13 are EBSD photomicrographs showing steel sheet products
processed as shown in Fig. 2.
[0018] Fig. 14 is an EBSD photomicrograph of a steel sheet product processed
as shown
in Fig. 3.
[0019] Fig. 15 is a graph of total elongation vs. ultimate tensile strength
for high strength
steel sheet products of the present invention in comparison with other steel
sheet products
processed outside the scope of the present invention.
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[0020] Fig. 16 is a graph of total elongation vs. ultimate tensile strength
for high strength
steel products produced in mill trials in accordance with embodiments of the
present invention.
[0021] Fig. 17 is a plot of temperature versus time for cold rolled and hot
rolled
substrates processed with a thermal cycle in accordance with an embodiment of
the invention.
[0022] Fig. 18 is an EB SD photomicrograph of a cold rolled steel sheet
substrate
subjected to the thermal process shown in Fig. 17 showing darker ferrite
grains and lighter
retained austenite grains.
[0023] Fig. 19 is an EB SD photomicrograph of a hot rolled steel sheet
substrate subjected
to the thermal process shown in Fig. 17 showing darker ferrite grains and
lighter retained
austenite grains.
DETAILED DESCRIPTION
[0024] The high strength steel sheet products of the present invention have
controlled
compositions that, in combination with controlled annealing processes, produce
desirable
microstructures and favorable mechanical properties including high strengths
and ultra-high
formabilities. In certain embodiments, the steel composition may include
carbon, manganese
and silicon, along with any other suitable alloying additions known to those
skilled in the art.
Examples of steel compositions including ranges of C, Mn, Si, Al, Ti, and Nb
are listed in
Table 1 below.
Table 1
Steel Compositions (wt. %)
Example C Mn Si Al Si + AL Ti Nb
A 0.12-0.5 1-3 0-2 0-2 0.8-3 0-
0.05 0-0.05
0.15-0.4 1.3-2.5 0.2-1.8 0-1.5 0.9-2.5 0-0.03 0-0.03
0.17-
1.5-2.3 0.4-1.5 0-1 1-2 0-0.02 0-0.02
0.35
[0025] In addition to the amounts of C, Mn, Si, Al, Ti and Nb listed in Table
1, the steel
compositions may include minor or impurity amounts of other elements, such as
0.015 max S,
0.03 max P, 0.2 max Cu, 0.02 max Ni, 0.2 max Cr, 0.2 max Mo, 0.1 max Sn, 0.015
max N, 0.1
max V, and 0.004 max B. As used herein the term "substantially free", when
referring to the
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composition of the steel sheet product, means that a particular element or
material is not
purposefully added to the composition, and is only present as an impurity or
in trace amounts.
[0026] In the steel sheet products of the present invention, C provides
increased strength
and promotes the formation of retained austenite. Mn provides hardening and
acts as a solid
solution strengthener. Si inhibits iron carbide precipitation during heat
treatment, and increases
austenite retention. Al inhibits iron carbide precipitation during heat
treatment, and increases
austenite retention. Ti and Nb may act as a strength-enhancing grain refiners.
[0027] In certain embodiments, Al may be present in an amount of at least 0.1
weight
percent or at least 0.2 weight percent. For example, Al may be present in an
amount of from 0.5
to 1.2 weight percent, or from 0.7 to 1.1 weight percent, in certain
embodiments. Alternatively,
the steel sheet product may be substantially free of Al.
[0028] Steel sheet products having compositions as described above are
subjected to a
two-stage annealing process, as more fully described below. The resultant
sheet products have
been found to possess favorable mechanical properties including desirable
ultimate tensile
strengths, high elongations, high lambda values, high bendability and high
yield ratios
(YS/UTS).
[0029] In certain embodiments, the ultimate tensile strength (UTS) of the
steel sheet
products range from 700 to 1,100 MPa or more. In certain embodiments, the
steel sheet product
has an ultimate tensile strength of greater than 700 MPa, for example, from
720 to 1,100 MPa, or
from 750 to 1,050 MPa.
[0030] In certain embodiments, the steel sheet products have a total
elongation (TE)
typically greater than 22 percent, for example, greater than 27 percent, or
greater than 33 percent.
For example, the steel sheet product may have a total elongation of at least
20% or at least 25%
or at least 27%, e.g., from 22 to 45%, or from 25 to 40%.
[0031] The steel sheet products may possess lambda (X) values as measured by a
standard hole expansion test typically greater than 20 percent, for example,
greater than 25
percent, or greater than 30 percent, or greater than 35 percent. The whole
expansion ratio or
lambda may be greater than 20%, for example, from 22 to 80%, or from 25 to
60%.
[0032] In certain embodiments, increased values of both total elongation (TE)
and hole
expansion (X) result in steel sheet products exhibiting good global
formability and local
formability.
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[0033] Strength elongation balances (UTS=TE) of greater than 25,000 are
observed for
the present steel sheet products, making them fall into the category of
Generation 3 steels highly
desired by industries such as the auto industry. In certain embodiments UTS=TE
values may be
greater than 27,000, or greater than 30,000, or greater than 35,000.
[0034] In accordance with certain embodiments of the invention, the final
microstructure
of the steel sheet products may primarily comprise ferrite, e.g., at least 50%
up to 80% or higher,
with lesser amounts of retained austenite, e.g., from 5 to 25%, and minor
amounts of fresh
martensite, e.g., from 0 to 10% or 15%. The amounts of ferrite, austenite and
martensite may be
determined by standard EBSD techniques. Alternatively, retained austenite
content may be
determined by magnetic saturation methods. Unless otherwise specified herein,
the volume
percent of retained austenite is determined by the EB SD technique.
[0035] In certain embodiments, the retained austenite comprises from 1 to 25
volume
percent, for example, from 5 to 20 volume percent. The amount of fresh
martensite may
comprise less than 15 volume percent, or less than 10 volume percent, or less
than 5 volume
percent. In certain embodiments, the steel sheet product is substantially free
of fresh martensite.
It has been found that when fresh martensite amounts are greater than 15%,
hole expansion
values decrease significantly, e.g., local formability is significantly
decreased.
[0036] At least a portion of the ferrite may be formed during the heating
section, as
described below, by recrystallization and/or tempering of martensite, or
during the cooling and
holding section of the second annealing process by austenite decomposition.
Some of the ferrite
may be considered bainitic ferrite. The ferrite, austenite and martensite
phases are fine grained,
e.g., having average grain sizes of less than 10 microns, for example, less
than 5 microns, or less
than 3 microns. For example, ferrite grain size may range from less than 10
microns, for
example, less than 8 microns, or less than 6 microns. Average austenite grain
size may range
from less than 2 microns, for example, less than 1 micron, or less than 0.5
micron. Martensite
grain size, when present, may range from less than 10 microns, for example,
less than 8 microns,
or less than 6 microns.
[0037] The austenite grains may be substantially equiaxed, e.g., having
average aspect
ratios less than 3:1 or less than 2:1, for example, about 1:1. It has been
found that amounts of
retained austenite below about 5% results in significantly decreased total
elongations (TE). It
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has further been found that amounts of retained austenite above 25% can only
be obtained at
very high carbon levels, which results in poor weldability.
[0038] In certain embodiments of the invention, a two-step annealing process
is used to
produce advanced high strength steel products with favorable mechanical
properties, such as
those described above. Within each of the first and second annealing steps,
multiple
methodologies for undertaking the heat treatment may be used. Examples of two-
step annealing
processes are shown in Figs. 1-3 and described below. Fig. 1 represents a
continuous annealing
line (CAL) followed by a continuous annealing line (CAL) production route.
Fig. 2 represents a
CAL plus continuous galvanizing line (CGL) production route. Fig. 3 represents
a specially
designed line allowing for both CAL + CAL or CAL + CGL steps to take place in
a single
facility. While a direct-fired furnace (DFF) followed by a radiant tube (RT)
furnace embodiment
is shown in Fig. 3, other embodiments such as all radiant tube, electric
radiant heating, and the
like may be used to achieve the desired thermal cycles.
Step 1
[0039] The goal of the first step of the annealing process is to achieve a
martensitic
microstructure in the cold rolled or hot rolled steel sheet product. In the
first annealing stage of
the first step, an annealing temperature above the A3 temperature may
typically be used, for
example, an annealing temperature of at least 820 C may be used. In certain
embodiments, the
first stage annealing temperature may typically range from 830 to 980 C, for
example, from 830
to 940 C, or from 840 to 930 C, or from 860 to 925 C. In certain embodiments,
the peak
annealing temperature may be typically held for at least 20 seconds, for
example, from 20 to 500
seconds, or from 30 to 200 seconds. Heating may be accomplished by
conventional techniques
such as a non-oxidizing or oxidizing direct-fired furnace (DFF), oxygen-
enriched DFI, induction,
gas radiant tube heating, electric radiant heating, and the like. Examples of
heating systems that
may be adapted for use in the processes of the present invention are disclosed
in U.S. Patent Nos.
5,798,007; 7,368,689; 8,425,225; and 8,845,324, U.S. Patent Application No.
2009/0158975, and
Published PCT Application No. WO/2015083047, assigned to Fives Stein.
Additional examples
of heating systems that may be adapted for use in the processes of the present
invention include
U.S. Patent No. 7,384,489 assigned to Dreyer International, and U.S. Patent
No. 9,096,918
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assigned to Nippon Steel and Sumitomo Metal Corporation. Any other suitable
known types of
heating systems and processes may be adapted for use in Step 1 and Step 2.
[0040] In the first stage, after the peak annealing temperature is reached and
held for the
desired period of time, the cold rolled or hot rolled steel sheet is quenched
to room temperature,
or to a controlled temperature above room temperature, as more fully described
below. The
quench temperature may not necessarily be room temperature but should be below
the martensite
start temperature (Ms), and preferably below the martensite finish temperature
(MF), to form a
microstructure of predominantly martensite. In certain embodiments, between
the first step
process and the second step process, the steel sheet product may be cooled to
a temperature
below 300 C, for example, below 200 C.
[0041] Quenching may be accomplished by conventional techniques such as water
quenching, submerged knife/nozzle water quenching, gas cooling, rapid cooling
using a
combination of cold, warm or hot water and gas, water solution cooling, other
liquid or gas fluid
cooling, chilled roll quench, water mist spray, wet flash cooling, non-
oxidizing wet flash cooling,
and the like. A quench rate of from 30 to 2,000 C/sec may typically be used.
[0042] Various types of cooling and quenching systems and processes known to
those
skilled in the art may be adapted for use in the processes of the present
invention. Suitable
cooling/quenching systems and processes conventionally used on a commercial
basis may
include water quench, water mist cooling, dry flash and wet flash, oxidizing
and non-oxidizing
cooling, alkane fluid to gas phase change cooling, hot water quenching,
including two-step water
quenching, roll quenching, high percentage hydrogen or helium gas jet cooling,
and the like. For
example, dry flash and/or wet flash oxidizing and non-oxidizing
cooling/quenching such as
disclosed in published PCT Application No. W02015/083047 to Fives Stein may be
used. Other
Fives Stein patent documents describing cooling/quenching systems and
processes that may be
adapted for use in the processes of the present invention include U.S. Patent
Nos. 6,464,808B2;
6,547,898B2; and 8,918,199B2, and U.S. Patent Application Publication Nos.
U52009/0158975A1; US2009/0315228A1; and U52011/0266725A1. Other examples of
cooling/quenching systems and processes that may be adapted for use in the
processes of the
present invention include those disclosed in U.S. Patent Nos. 8,359,894B2;
8,844,462B2; and
7,384,489B2, and U.S. Patent Application Publication Nos. 2002/0017747A1 and
2014/0083572A1.
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[0043] In certain embodiments, after the first-stage peak annealing
temperature is
reached and the steel is quenched to form martensite, the martensite can be
optionally tempered
to soften the steel somewhat to make further processing more feasible.
Tempering takes place by
raising the temperature of the steel in the range of room temperature to about
500 C and holding
for up to 600 seconds. If tempering is utilized, the tempering temperature may
be held constant,
or may be varied within this preferred range.
[0044] After tempering, the temperature is ramped down to room temperature.
The rate
of such ramp-down may typically range from 1 to 40 C/sec, for example, from 2
to 20 C/sec. In
the case of a single pass facility furnace, as in Fig. 3, tempering may not be
necessary.
Step 2
[0045] The second step of the annealing process may include a first stage that
is
conducted at relatively high annealing temperature, and a second stage that is
conducted at
relatively low temperature. These stages are defined as the "soaking" and
"holding" zones of the
second anneal, as described in Fig. 4. The temperatures are controlled in
order to promote the
formation of the desired microstructure in the final product.
[0046] In the first annealing stage of the second step, a soaking zone
temperature
between Ai and A3 may be used, for example, an annealing temperature of at
least 720 C may be
used. In certain embodiments, the soaking zone temperature may typically range
from 720 to
850 C, for example, from 760 to 825 C. In certain embodiments, the peak
annealing
temperature may be typically held for at least 15 seconds, for example, from
20 to 300 seconds,
or from 30 to 150 seconds.
[0047] During the first stage of the second step, the soaking zone temperature
may be
achieved by heating the steel from a relatively low temperature below Ms,
e.g., room
temperature, at an average rate of from 0.5 to 50 C/sec, for example, from
about 2 to 20 C/sec.
In certain embodiments, the ramp-up may take from 25 to 800 seconds, for
example, from 100 to
500 seconds. The first stage heating of the second step may be accomplished by
any suitable
heating system or process, such as using radiant heating, induction heating,
direct fired furnace
heating and the like.
[0048] After the soaking zone temperature is reached and held for the desired
period of
time, the steel may be cooled to a controlled temperature above room
temperature to the holding
zone. In certain embodiments, the steel sheet product is maintained at a
temperature above
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300 C between the second step soaking process and the second step holding
process. Cooling
from the soaking to holding zone may be accomplished by conventional
techniques such as water
cooling, gas cooling and the like. An average cooling rate of from 5 to 400
C/sec may typically
be used. Any suitable types of cooling and quenching systems may be adapted
for use in cooling
from the soaking temperature to the holding temperature, including those
described above.
[0049] In accordance with embodiments of the invention, the holding zone step
is carried
out at a typical temperature of from 360 to 445 C, for example, from 370 to
440 C. The holding
zone may be held for up to 800 seconds, for example, from 30 to 600 seconds.
[0050] The holding zone temperature may be held constant, or may be varied
somewhat
within the preferred temperature range. After holding, the steel may be
reheated, such as by
induction or other heating method, to enter a hot-dip coating pot at the
proper temperature for
good coating results, if the steel is to be hot-dip coated.
[0051] In certain embodiments, after the holding zone temperature has been
maintained
for a desired period of time, the temperature may be ramped down to room
temperature. Such a
ramp-down may typically take from 10 to 1,000 seconds, for example, from about
20 to 500
seconds. The rate of such ramp-down may typically range from 1 to 1,000 C/sec,
for example,
from 2 to 20 C/sec.
[0052] In accordance with certain embodiments, one or both of the first-step
and second-
step annealing processes may be performed on a continuous annealing line
(CAL). After going
through a CAL+CAL process, the steel may be electrogalvanized to produce a
zinc based coated
product.
[0053] In certain embodiments, the annealed steel sheet is hot-dip galvanized
at the end
of the holding zone. Galvanizing temperatures may typically range from 440 to
480 C, for
example, from 450 to 470 C. In certain embodiments, the galvanizing step may
be performed as
part of the second-step annealing process on a continuous galvanizing line
(CGL), e.g., as shown
in Fig. 2. This CAL + CGL process can be used to produce both a zinc-based or
zinc alloy-based
hot-dip galvanized product or reheated after coating to produce an iron-zinc
galvanneal type
coated product. An optional nickel-based coating step can take place between
the CAL and CGL
steps in the process to improve zinc coating properties. The use of a
continuous galvanizing line
in the second step increases the production efficiency of producing a coated
GEN3 product
versus using a CAL + CAL + EG route.
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[0054] A galvanized product or zinc-based alloy hot-dip coated product can
also be made
on a specially designed CGL in which the two-step annealing can take place in
a single line, as
shown in Fig. 3. Galvannealing can also be an option in this case.
Furthermore, a single
production facility can also be specially designed and built to combine the
two-step thermal
process to produce uncoated Generation 3 steels as defined in the invention.
[0055] The following examples are intended to illustrate various aspects of
the present
invention, and are not intended to limit the scope of the invention.
Example 1
[0056] A cold rolled steel sheet having a composition as listed in Table 2,
Sample No. 1,
was subjected to a two-step annealing process as illustrated in Fig. 1. The
microstructure of the
resultant product is shown in Figs. 5 and 6. EBSD techniques using commercial
EDAX
orientation imaging microscopy software show the dark ferrite grains and light
austenite grains
in Fig. 5.
Example 2
[0057] A cold rolled steel sheet having a composition as listed in Table 2,
Sample No. 2,
was subjected to a two-step annealing process as illustrated in Fig. 1. The
microstructure of the
resultant product is shown in Fig. 11. Mechanical properties of Sample No. 2
are listed in Table
2. Grain size distributions of austenite and ferrite are shown in Figs. 9 and
10, respectively. The
average austenite grain size is less than 1 micron and the average ferrite
grain size is less than 10
microns.
[0058] The microstructure includes about 80 volume percent ferrite with an
average grain
size of about 5 microns, about 10 volume percent retained austenite having
substantially
equiaxed grains and an average grain size of about 0.5 micron, and about 10
volume percent
fresh martensite with an average grain size of about 5 microns. Mechanical
properties of Sample
No. 1 are listed in Table 2 below.
Example 3
[0059] A cold rolled steel sheet having a composition as listed in Table 2,
Sample No. 3,
was subjected to a two-step annealing process as illustrated in Fig. 2. The
microstructure of the
resultant product is shown in Figs. 12 and 13. In Fig. 13, austenite is light
in color and ferrite is
dark in color. Mechanical properties of Sample No. 3 are listed in Table 2.
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Example 4
[0060] A cold rolled steel sheet having a composition as listed in Table 2,
Sample No. 4,
was subjected to a two-step annealing process as illustrated in Fig. 3. The
microstructure of the
resultant product is shown in Figure 14. In Fig. 14, austenite is light in
color and ferrite is dark
in color. Mechanical properties of Sample No. 4 are listed in Table 2.
Example 5
[0061] A cold rolled steel sheet having a composition as listed in Table 2,
Sample No. 5,
was subjected to a two-step annealing process as illustrated in Fig. 1.
Mechanical properties of
Sample No. 5 are listed in Table 2.
Example 6
[0062] A cold rolled steel sheet having a composition as listed in Table 2,
Sample No. 6,
was subjected to a two-step annealing process as illustrated in Fig. 1.
Mechanical properties of
Sample No. 6 are listed in Table 2. Fig. 7 is an optical image showing the
microstructure of the
steel shown in Fig. 2, Sample No. 6, which was subjected to the two-step
annealing process
shown in Fig. 1. In Fig. 7, the dark regions of the photomicrograph are
ferrite grains, while the
light regions are austenite grains. Fig. 8 is a graph illustrating the aspect
ratios of the austenite
grains shown in Fig. 7. The optical image of Fig. 7 was used to determine the
aspect ratios of the
austenite grains using image analysis with commercially available software.
Fig. 7 shows that
the average aspect ratio is less than 3:1 for the austenite grains.
Example 7
[0063] A cold rolled steel sheet having a composition as listed in Table 2,
Sample No. 7,
was subjected to a two-step annealing process as illustrated in Fig. 2.
Mechanical properties of
Sample No. 7 are listed in Table 2.
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Example 8
[0064] A cold rolled steel sheet having a composition as listed in Table 2,
Sample No. 8,
was subjected to a two-step annealing process as illustrated in Fig. 3.
Mechanical properties of
Sample No. 8 are listed in Table 2.
[0065] The steels in Examples 1-8 exhibited UTS levels in the range of 700 to
1,100
MPa.
Comparative Examples 1-4
[0066] Cold rolled steel sheets having compositions as listed in Table 2,
Sample
Nos. C1-C4, were subjected to a two-step annealing process as illustrated in
Fig. 1. Mechanical
properties of Sample Nos. C1-C4 are listed in Table 2. The steels in
Comparative Examples 1-4
exhibited UTS levels less than 700 MPa.
Comparative Examples 5-8
[0067] Cold rolled steel sheets having compositions as listed in Table 2,
Sample
Nos. C5-C8, were subjected to a two-step annealing process as illustrated in
Fig. 1. Mechanical
properties of Sample Nos. C5-C8 are listed in Table 2. The steels in
Comparative Examples 5-8
exhibited UTS levels greater than 1,100 MPa.
Comparative Examples 9-11
[0068] Cold rolled steel sheets having compositions as listed in Table 2,
Sample
Nos. C9-C11, were subjected to a two-step annealing process similar to that
illustrated in Fig. 1,
except the soaking or holding temperature in the second anneal were outside
the preferred ranges
of the invention. Mechanical properties of Sample Nos. C9-C11 are listed in
Table 2.
Comparative Example 12
[0069] A cold rolled sheet having a composition listed in Table 2, Sample No.
C12, was
subjected to a two-step annealing process similar to that illustrated in Fig.
2, except the holding
zone temperature in the second anneal was outside the preferred range of the
invention.
Mechanical properties of Sample No. C12 are listed in Table 2.
- 13 -
Table 2
Sample YS UTS TE
Product Type Process C Mn Si Other
UTS=TE Lambda 0
No. (MPa)
(MPa) (%)
CR 780 GEN3 CAL +
1 0.22 1.5 1.5 657 831 39.2
32575 44
super elongation CAL
CAL +
oe
2 CR 980 GEN3 0.185 2.2 1.4 630 1030 25
25750 30
CAL
0.8 Al,
Hot-dipped 980 CAL +
3 0.22 2.25 0.45 0.02 Ti, 576 988
30.2 29838 20
GEN3 CGL
0.02Nb
Single
process in
Hot-dipped 980
4 GEN3 newly 0.185 2.2 1.4 580 998
29.1 29042
designed
CGL
CR 980 GEN3 CAL +
0.35 1.5 1.5 685 999 38.2 38161 25
Super elongation CAL
CAL +
6 CR 780 GEN3 0.175 1.8 1.5 630 840 33
27720 45
CAL
Hot-dipped 780 CAL +
7 0.2 2.3 0.4 1.0 Al 533 915
32.8 30012 30
GEN3 CGL
Single
process in
Hot-dipped 780
8 GEN3 newly 0.2 2.3 0.4 1.0 Al 589 865
34.4 29756
designed
CGL
1-d
oe
Table 2 (continued)
0
Sample Product YS UTS TE
t.)
Process C Mn Si Other
UTS=TE Lambda o
No. Type
(MPa) (MPa) (%) t.)
1-,
CAL +
'a
Cl Alloy 1 0.105 1.55 1.2 512
666 32.6 21712 67 c,.)
CAL
.6.
oe
CAL +
C2 Alloy 2 0.14 1.5 1.3 556
690 30.2 20838 59
CAL
CAL +
C3 Alloy 3 0.17 1.1 1.1 560
686 26.9 18453 53
CAL
CAL +
C4 Alloy 4 0.13 0.9 0.9 533
618 26.0 16068 81
CAL
CAL +
C5 Alloy 1 0.21 2.15 1.5
0.003 B 597 1125 17.2 19350 25
CAL
CAL +
P
C6 Alloy 2 0.2 2.2 1.5 0.2 Mo
585 1148 16.3 18712 13 o
CAL
1-, CAL +
,
N)
vi C7 Alloy 3 0.23 2.4 1.5
0.003 B 683 1231 16.2 19942 6.7 .
CAL
r.,
CAL +
r.,0
N)
C8 Alloy 4 0.19 2.64 2.0 635
1359 14.8 20113 3.4 ,1,
CAL
r.,
,
,
Alloy 1 - .
CAL +
C9 High Soak 0.18 2.2 1.34 693
1058 18.2 19256 25
CAL
(849 C)
Alloy 2 -
CAL +
C10 Low Hold 0.18 2.2 1.34 602
1035 21.2 21942 30
CAL
(350 C)
Alloy 3 -
CAL +
Iv
C11 High Hold 0.18 2.2 1.34 477
1059 19.7 20862 19 n
CAL
(450 C)
Alloy 4 -
cp
CAL +
t.)
C12 High Hold 0.22 2.4 0.4 0.8
Al 465 1012 23.0 23276 16.5
CGL
o
(471 C)
'a
.6.
c:
oe
.6.
-4
CA 03151124 2022-02-14
WO 2021/034851 PCT/US2020/046847
[0070] Fig. 15 plots the total elongation (TE) and ultimate tensile strength
(UTS) of
Samples 1-8 of Examples 1-8, as well as Samples C1-C12 of Comparative Examples
C1-C12. A
line corresponding to UTS=TE of 25,000 is roughly drawn in Fig. 15. As can be
seen, the high
strength steel sheet samples produced in accordance with the present invention
possess superior
combinations of strength and elongation versus the comparative samples, i.e.,
high total
elongation properties at high UTS levels are observed for the inventive
examples. The Sample 1
through Sample 8 steels fall into the category of Generation 3 advanced high
strength steels,
which are highly desirable for automotive and other industries.
Example 9
[0071] Mill trials were conducted for samples labeled Ml-M5 in Table 3 below
using
either the CAL+CAL or CAL+CGL process. For Samples Ml, M2 and M5, the CAL+CAL
processing times and temperatures shown in Fig. 1 were used. For Samples M3
and M4, the
CAL+CGL processing times and temperatures shown in Fig. 2 were used.
Table 3
Mill Trial Results
Mill Coat
YS UTS TE
Trial Process C Mn Si Other UTS
TE Lambda
(MPa) (MPa) (%)
No.
CAL + None
M1 0.22 1.4 1.4 627 810 38.6 31266
61
CAL
CAL + None
M2 0.185 2.2 1.4 624 1009 25 25255
38
CAL
0.8 Al, HDGI
CAL +
M3 0.23 2.4 0.4 0.02 Ti, 567 989 26.4
26110 24
CGL
0.02Nb
CAL + 0.8 Al, HDGI
M4 CGL 0.22 2.3 0.4 0.02 Ti, 655 941 30.9
29077 33
0.02Nb
CAL + None
M5 0.19 2.25 1.5 635 1048 25.3 26514 29
CAL
- 16 -
CA 03151124 2022-02-14
WO 2021/034851 PCT/US2020/046847
[0072] Fig. 16 shows the strength-elongation balance of the mill trial
materials, all
meeting a minimum UTS=TE of 25,000. The trial materials exhibited lambda
values greater than
20%.
Example 10
[0073] Cold rolled and hot rolled steel sheets having a composition of 0.23
weight
percent C, 2.3 weight percent Mn, 0.6 weight percent Si and 0.8 weight percent
Al corresponding
to Sample Nos. 9A-12B in Table 4 were subjected to a two-stage annealing
process as shown in
Fig. 17. In Table 4, cold rolled samples are listed as "CR" substrate types,
and hot rolled
samples are listed as "HR" substrate types. Mechanical properties of Sample
Nos. 9A-12B are
listed in Table 4. The hot rolled substrate samples showed comparable
exceptional YS, UTS, TE
and hole expansion properties as the cold rolled samples, showing that a hot
rolled substrate
processed directly into the two stage annealing process can produce Generation
3 AHSS
properties. Furthermore, as indicated in the EB SD phase maps shown in Figs.
18 and 19 in
which the retained austenite grains are lighter than the ferrite grains,
similar austenite contents,
distributions and morphologies are observed for the hot rolled material when
compared to the
cold rolled material. Fig. 18 shows the austenite content of cold rolled
sample 11A and Fig. 19
shows the austenite content of hot rolled sample 12A. A fine, predominantly
equiaxed
distribution of austenite is observed in both microstructures.
Table 4
Specimen Substrate Cycle Type YS (MPa) UTS TE (%) UTS=TE Hole
ID Type (MPa) Expansion
Ratio (%)
9A CR 427 C 706.32 973.1 31.39 30545.61 35
(800 F)
9B 712.02 959.2 34.41 33006.07
Hold - GI
10A HR 427 C 696.37 963.8 30.26 29164.59
46
(800 F)
10B 695.40 954.4 35.18 33575.79
Hold - GI
11A CR 443 C 618.23 1032.9 - - 31
(830 F)
11B 614.00 1030.4 28.04 28892.42
Hold - GI
12A HR 443 C 650.39 1002.2 27.21 27269.86
35
12B (830 F) 622.82 1007.6 27.75 27960.90
Hold - GI
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CA 03151124 2022-02-14
WO 2021/034851 PCT/US2020/046847
[0074] As used herein, "including," "containing" and like terms are understood
in the
context of this application to be synonymous with "comprising" and are
therefore open-ended
and do not exclude the presence of additional undescribed or unrecited
elements, materials,
phases or method steps. As used herein, "consisting of' is understood in the
context of this
application to exclude the presence of any unspecified element, material,
phase or method step.
As used herein, "consisting essentially of' is understood in the context of
this application to
include the specified elements, materials, phases, or method steps, where
applicable, and to also
include any unspecified elements, materials, phases, or method steps that do
not materially affect
the basic or novel characteristics of the invention.
[0075] Notwithstanding that the numerical ranges and parameters setting forth
the broad
scope of the invention are approximations, the numerical values set forth in
the specific
examples are reported as precisely as possible. Any numerical value, however,
inherently
contains certain errors necessarily resulting from the standard variation
found in their respective
testing measurements.
[0076] Also, it should be understood that any numerical range recited herein
is intended
to include all sub-ranges subsumed therein. For example, a range of "1 to 10"
is intended to
include all sub-ranges between (and including) the recited minimum value of 1
and the recited
maximum value of 10, that is, having a minimum value equal to or greater than
1 and a
maximum value of equal to or less than 10.
[0077] In this application, the use of the singular includes the plural
and plural
encompasses singular, unless specifically stated otherwise. In addition, in
this application, the
use of "or" means "and/or" unless specifically stated otherwise, even though
"and/or" may be
explicitly used in certain instances. In this application and the appended
claims, the articles "a,"
"an," and "the" include plural referents unless expressly and unequivocally
limited to one
referent.
[0078] Whereas particular embodiments of this invention have been described
above for
purposes of illustration, it will be evident to those skilled in the art that
numerous variations of
the details of the present invention may be made without departing from the
invention.
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