Note: Descriptions are shown in the official language in which they were submitted.
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METHODS OF PRODUCING CONTINUOUSLY CAST HOT ROLLED
HIGH STRENGTH STEEL SHEET PRODUCTS
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S. Patent Application
No.
16/544,127 filed August 19, 2019, which 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 also
claims priority
to U.S. Provisional Application No. 62/844,301 filed May 7, 2019. All of the
foregoing
applications are incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to methods of providing continuously cast
hot
rolled high strength steel sheet products, including quenching finish rolled
steel sheets on a
hot strip mill to produce a predominantly martensitic microstructure, followed
by soaking at a
temperature in an intercritical regime and holding at a lower temperature to
produce a
microstructure that is predominantly ferrite and retained austenite.
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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[0004] U. S . Patent No. 10,385,419 and U.S. Application Publication No.
US 2020/0040422, which are incorporated herein by reference, disclose
annealing processes
for making high strength steel products.
SUMMARY OF THE INVENTION
[0005] The present invention provides methods of producing continuously cast
hot
rolled steel sheet products, including continuously casting a steel slab and
then hot rolling
with finish rolling on a hot strip mill, quenching on the hot strip mill to
form a predominantly
martensitic microstructure, and performing a thermal cycling step including
soaking at an
intercritical temperature followed by holding at a lower temperature. The
thermal cycling
step can take place on a continuous galvanize line or continuous annealing
line. The resultant
hot rolled steel sheet products have a microstructure comprising ferrite and
retained austenite.
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-%. Steels with these properties may fall into the category of Generation 3
advanced
high strength steels, and are highly desired by various industries including
automobile
manufacturers.
[0006] An aspect of the present invention is to provide a method of producing
a high
strength continuously cast hot rolled steel sheet product, the method
comprising:
continuously casting a steel slab comprising from 0.15 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; hot
rolling the continuously cast steel slab including a finish rolling step on a
hot strip mill with a
finish rolling temperature of at least 820 C to form a hot rolled steel sheet
product on the hot
strip mill; quenching the hot rolled steel sheet product on the hot strip mill
to form a
predominantly martensitic microstructure; subjecting the quenched hot rolled
steel sheet
product to a thermal cycling step 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 360 to 445 C; and quenching the thermally cycled sheet
product to room
temperature, wherein the steel sheet product comprises ferrite and retained
austenite grains,
and has a combination of ultimate tensile strength and total elongation UTS=TE
of greater
than 25,000 MPa%.
[0007] Another aspect of the present invention is to provide a high strength
continuously cast hot rolled steel sheet product produced by the method
described above.
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[0008] These and other aspects of the present invention will be more apparent
from
the following description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] Fig. 1 includes plots of temperature versus time illustrating a two-
step
annealing process.
[0010] Fig. 2 includes plots of temperature versus time illustrating a two-
step
annealing process.
[0011] 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.
[0012] 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.
[0013] Figs. 5 and 6 are electron backscatter diffraction (EBSD)
photomicrographs
illustrating the microstructure of a high strength steel sheet product.
[0014] 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.
[0015] Fig. 8 is a bar graph illustrating aspect ratios of the austenite
grains shown in
Fig. 7.
[0016] Figs. 9 and 10 are graphs for a high strength steel sheet product
showing
austenite and ferrite grain size distributions.
[0017] Figs. 11 is EBSD photomicrograph illustrating the microstructure of a
high
strength steel sheet product processed as shown in Fig. 1.
[0018] Figs. 12 and 13 are EBSD photomicrographs showing steel sheet products
processed as shown in Fig. 2.
[0019] Fig. 14 is an EBSD photomicrograph of a steel sheet product processed
as
shown in Fig. 3.
[0020] Fig. 15 is a graph of total elongation vs. ultimate tensile strength
for high
strength steel sheet products in comparison with other steel sheet products
processed outside
the scope of the present invention.
[0021] Fig. 16 is a graph of total elongation vs. ultimate tensile strength
for high
strength steel products produced in mill trials.
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[0022] Fig. 17 is a plot of temperature versus time for cold rolled and hot
rolled
substrates processed with a thermal cycle.
[0023] Fig. 18 is an EBSD 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.
[0024] Fig. 19 is an EBSD 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.
[0025] Fig. 20 is a plot of temperature versus time illustrating the first
step of a two-
step thermal treatment process in which a hot rolled sheet is quenched on the
runout table to
form martensite in accordance with an embodiment of the present invention.
[0026] Fig. 21 is a plot of temperature versus time illustrating the second
step of a
two- step thermal treatment process in accordance with an embodiment of the
present
invention in which the quench annealed sheet of Fig. 17 is subjected to a
thermal cycling
step.
DETAILED DESCRIPTION
[0027] 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 0.15-1 0.5-4 0-2 0-2 0.8-3 0-0.05 0-
0.05
0.2-0.4 1.3-2.5 0.2-1.8 0-1.5 0.9-2.5 0-0.03
0-0.03
0.17-0.35 1.5-2.3 0.4-1.6 0-1 1-2 0-0.02 0-
0.02
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[0028] 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 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.
[0029] 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.
[0030] 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.
[0031] 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).
[0032] 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.
[0033] 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%.
[0034] The steel sheet products may possess lambda (2) 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%.
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[0035] In certain embodiments, increased values of both total elongation (TE)
and
hole expansion (2\,) result in steel sheet products exhibiting good global
formability and local
formability.
[0036] 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 26,000, or greater than 27,000, or greater than
30,000.
[0037] 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 EBSD
technique.
[0038] 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.
[0039] At least a portion of the ferrite may be formed during the soaking or
heating
section, as described below, by recrystallization and/or tempering of
martensite, or during the
cooling and holding section of the thermal cycling 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.
[0040] 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
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amounts of retained austenite below about 5% results in significantly
decreased total
elongations (TE). It 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.
[0041] In certain embodiments of the invention, a two-step thermal treatment
process
is used to produce advanced high strength steel products with favorable
mechanical
properties, such as those described above. The first step is performed on a
hot strip mill after
a steel slab has been continuously cast and hot rolled including a finish
rolling step on the hot
strip mill. The finish rolled steel sheet is quenched on the hot strip mill to
form a
predominantly martensitic microstructure, and the second step includes soaking
the steel
sheet in an intercritical regime followed by holding at a lower temperature.
The second step
can be performed on a continuous annealing line or continuous galvanizing
line. The final
steel sheet product comprises ferrite and retained austenite, i.e., having a
microstructure that
is predominantly ferrite and retained austenite.
[0042] In certain embodiments, the steel sheet products may be produced on a
combined continuous casting and hot rolling line in which molten steel is
continuously cast to
form a slab of steel that may be subjected to initial rough hot rolling to
reduce thickness,
followed by final hot rolling to further reduce thickness. The continuous
casting/hot rolling
line may comprise an endless strip production (ESP) process or the like. After
final hot
rolling, the sheet may be fed to a runout cooling table where it may be
subjected to a
quenching process to a temperature below MF, e.g., using a thermomechanical
schedule as
shown in Fig. 20. The resultant quenched hot rolled sheet may then be
subjected to the
second step of the process, e.g., by adopting a two-stage thermal cycle as
shown in Fig. 21.
[0043] In certain embodiments, Step 1 of the two-step process may be performed
using continuous casting and hot rolling equipment and processes as disclosed
in Arvedi U.S.
Patent Nos. 5,329,688; 5,497,821; 6,125,916; 7,343,961; 7,832,460; 7,967,056;
8,025,092;
8,162,032; 8,257,647; and 9,186,721, which are incorporated herein by
reference.
[0044] Within each of the first and second annealing or thermal cycling steps,
multiple methodologies for undertaking the heat treatment may be used.
Examples of two-
step annealing processes as described in U.S. Patent No. 10,385,419 and U.S.
Application
Publication No. US 2020/0040422 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
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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
[0045] The goal of the first step of the process is to achieve a predominantly
martensitic microstructure in the continuously cast and hot rolled steel sheet
product. In the
first step, the hot rolled steel sheet has a finish rolling temperature above
the A3, for example,
a finish rolling temperature of at least 820 C may be provided on the hot
strip mill prior to
quenching. In certain embodiments, the quench 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.
[0046] The continuously cast and finish 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.
[0047] Quenching to below MF (typically below 250 C) 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. For example,
water quenching
may be used in Step 1 as is typically used to cool the strip after finish
rolling on the runout
table of a conventional hot strip mill, CSP mill, or ESP mill. A quench rate
of from 30 to
1,000 C/sec may typically be used.
[0048] 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
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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.
[0049] In certain embodiments, after 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 may take place in a continuous annealing
process by
raising the temperature of the steel in the range of room temperature to about
500 C, e.g.,
from 30 to 500 C, and holding for up to 600 seconds, e.g., from 1 to 600
seconds.
Tempering may also take place using a batch annealing process, where the same
temperature
could be reached over a period of several days. For example, the intermediate
batch
tempering may be performed at a temperature of from 125 to 500 C for up to
seven days. If
tempering is utilized, the tempering temperature may be held constant, or may
be varied
within this preferred range.
[0050] After tempering, the temperature may be ramped down to room
temperature.
The rate of such ramp-down may typically range up to 40 C/sec, for example,
from 0.1 to
20 C/sec.
Step 2
[0051] The second step of the thermal treatment process may include a first
soak
stage that is conducted at relatively high soaking temperature, and a second
hold stage that is
conducted at relatively low temperature. These stages may be defined as the
"soaking" and
"holding" zones, as described in Figs. 4 and 21. The temperatures are
controlled in order to
promote the formation of the desired microstructure in the final product.
[0052] In the first soaking stage of the second step, a soaking zone
temperature in an
intercritical regime between At and A3 may be used, for example, a soak
temperature of at
least 720 C may be used. In certain embodiments, the soak temperature may
typically range
from 720 to 850 C, for example, from 760 to 825 C. In certain embodiments, the
peak
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annealing temperature may be typically held for at least 15 seconds, for
example, from 20 to
300 seconds, or from 30 to 150 seconds.
[0053] 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.
[0054] After the soak 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
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.
[0055] 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.
[0056] 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.
[0057] 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.
[0058] Examples of heating systems that may be adapted for use in the soak and
hold
thermal cycling step 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
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Published PCT Application No. WO/2015083047, assigned to Fives Stein.
Additional
examples of heating systems that may be adapted for use in the soak and hold
thermal cycling
step of the present invention include U.S. Patent No. 7,384,489 assigned to
Dreyer
International, and U.S. Patent No. 9,096,918 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.
[0059] In accordance with certain embodiments, the second-step thermal cycling
processes may be performed on a continuous annealing line (CAL). After going
through a
CAL process, the steel may be electrogalvanized to produce a zinc based coated
product.
[0060] 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 holding zone process on a continuous
galvanizing line
(CGL), e.g., as shown in Fig. 2. This 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
prior to the CGL step in the process to improve zinc coating properties. The
use of a
continuous galvanizing line in the second step may increase the efficiency of
producing
coated GEN3 products.
[0061] The following examples are not intended to limit the scope of the
invention.
Example 1
[0062] 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
[0063] 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,
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respectively. The average austenite grain size is less than 1 micron and the
average ferrite
grain size is less than 10 microns.
[0064] 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
[0065] 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.
Example 4
[0066] 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
[0067] 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
[0068] 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
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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
[0069] 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.
Example 8
[0070] 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.
[0071] The steels in Examples 1-8 exhibited UTS levels in the range of 700 to
1,100
MPa.
Comparative Examples 1-4
[0072] Cold rolled steel sheets having compositions as listed in Table 2,
Sample
Nos. Cl-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
[0073] 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
[0074] 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.
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Comparative Example 12
[0075] 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.
14
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 +
2 CR 980 GEN3 0.185 2.2 1.4 630 1030 25
25750 30
CAL
00
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
Table 2 (continued)
0
Sample Product YS UTS TE
t.)
Process C Mn Si Other
UTS=TE Lambda o
No. Type
(MPa) (MPa) (%) t.)
o
CAL +
Cl Alloy 1 0.105 1.55 1.2 512
666 32.6 21712 67 t.)
CAL
-4
.6.
CAL +
oe
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 +
.
c: C7 Alloy 3 0.23 2.4 1.5
0.003 B 683 1231 16.2 19942 6.7
CAL
r.,
CAL +
2
,
C8 Alloy 4 0.19 2.64 2.0 635
1359 14.8 20113 3.4 ' ,
CAL
,
,
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
1-,
-4
1-,
v:,
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[0076] 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
[0077] 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 lE
Trial Process C Mn Si Other UTS=TE Lambda
(MP) (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
+
M3 CALCGL 0.23 2.4 0.4 0.02 Ti, 567 989 26.4
26110 24
0.02Nb
0.8 Al, HDGI
+
M4 CALCGL 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
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[0078] 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
[0079] 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
10B (800 F) 695.40 954.4 35.18 33575.79
Hold - GI
11A CR 443 C 618.23 1032.9 - - 31
11B (830 F) 614.00 1030.4 28.04 28892.42
Hold - GI
12A HR 443 C 650.39 1002.2 27.21 27269.86
35
(830 F)
12B 622.82 1007.6 27.75 27960.90
Hold - GI
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Example 11
[0080] In accordance with an embodiment of the present invention, a hot rolled
steel
sheet having a composition of C = 0.29%, Mn = 1.85%, Si = 1.55%, Al = 0.04,
the balance
typical steel residuals, was subjected to a two-step process as illustrated in
Figs. 20 and 21. In
Step 1, martensite is formed on a hot strip mill by water quenching after
finish rolling using the
thermomechanical schedule shown in Fig. 20. An overageing step (not shown)
could optionally
be added at the end of this thermal cycle to soften and toughen the hot band
for subsequent
processing. In Step 2, a thermal cycle as shown in Fig. 21 is then performed.
The resultant
mechanical properties are: YS = 864 MPa; UTS = 864 MPa; Total Elongation =
31.2%; and
UTS x TE = 26957 MPa %.
[0081] 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.
[0082] 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.
[0083] 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.
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[0084] 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.
[0085] 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.
