Note: Descriptions are shown in the official language in which they were submitted.
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A THIN CAST STRIP PRODUCT WITH MICROALLOY
ADDITIONS, AND METHOD FOR MAKING THE SAME
[0001]
BACKGROUND AND SUMMARY
[0002] This invention relates to making of high strength thin cast strip,
and the
method for making such cast strip by a twin roll caster.
[0003] In a twin roll caster, molten metal is introduced between a pair of
counter--
rotated, internally cooled casting rolls so that metal shells solidify on the
moving roll
surfaces, and are brought together at the nip between them to produce a
solidified strip
product, delivered downwardly from the nip between the casting rolls. The term
"nip" is used
herein to refer to the general region at which the casting rolls are closest
together. The
molten metal is poured from a ladle through a metal delivery system comprised
of a tundish
and a core nozzle located above the nip to form a casting pool of molten
metal, supported on the
casting surfaces of the rolls above the nip and extending along the length of
the nip. This
casting pool is usually confined between refractory side plates or dams held
in sliding
engagement with the end surfaces of the rolls so as to dam the two ends of the
casting pool
against outflow.
[0004] In the past, high-strength low-carbon thin strip with yield
strengths of 413
MPa (60 ksi) and higher, in strip thicknesses less than 3.0 mm, have been made
by recovery
annealing of cold rolled strip. Cold rolling was required to produce the
desired thickness. The
cold roll strip was then recovery annealed to improve the ductility without
significantly
reducing the strength. However, the final ductility of the resulting strip
still was relatively
low and the strip would not achieve total elongation levels over 6%, which is
required for
structural steels by some building codes for structural components. Such
recovery annealed
cold rolled, low-carbon steel was generally suitable only for simple forming
operations, e.g.,
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roll forming and bending. To produce this steel strip with higher ductility
was not technically
feasible in these final strip thicknesses using the cold rolled and recovery
annealed
manufacturing route.
[0005] In the
past, high strength, steel has been made by microalloying with elements
such as niobium, vanadium, titanium or molybdenum, and hot rolling to achieve
the desired
thickness and strength level. Such microalloying required expensive and high
levels of
niobium, vanadium, titanium or molybdenum and resulted in formation of a
bainite-ferrite
microstructure typically with 10 to 20% bainite. See U.S. Patent No.
6,488,790. Alternately,
the microstructure could be ferrite with 10-20% pearlite. Hot rolling the
strip resulted in the
partial precipitation of these alloying elements. As a result, relatively high
alloying levels of
the Nb, V, Ti or Mo elements were required to provide enough age hardening of
the
predominately ferritic transformed microstructure to achieve the required
strength levels.
These high microalloying levels significantly raised the hot rolling loads
needed and
restricted the thickness range of the hot rolled strip that could be
economically and practically
produced. Such alloyed high strength strip could be directly used for
galvanizing after
pickling for the thicker end of the product range greater than 3 mm in
thickness.
[0006] However,
making of high strength, steel strip less than 3 mm in thickness with
additions of Nb, V, Ti or Mo to the base steel chemistry was very difficult,
particularly for
wide strip due to the high rolling loads, and not always commercially
feasible. In the past,
large additions of these elements were needed for strengthening the steel, and
in addition,
caused reductions in elongation properties of the steel. High strength
microalloyed hot rolled
strips in the past were relatively inefficient in providing strength,
relatively expensive, and
often required compensating additions of other alloying elements.
[0007]
Additionally, cold rolling was generally required for lower thicknesses of
strip; however, the high strength of the hot rolled strip made such cold
rolling difficult
because of the high cold roll loadings required to reduce the thickness of the
strip. These high
alloying levels also considerably raised the recrystallization annealing
temperature needed,
requiring expensive to build and operate annealing lines capable of achieving
the high
annealing temperature needed for full recrystallization annealing of the cold
rolled strip.
[0008] In
short, the application of previously known microalloying practices with Nb,
V, Ti or Mo elements to produce high strength thin strip could not be
commercially produced
economically because of the high alloying costs, relative inefficiency of
element additions,
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difficulties with high rolling loads in hot rolling and cold rolling, and the
high
recrystallization annealing temperatures required.
[0009] A steel product is disclosed comprising, by weight, less than 0.25%
carbon,
between 0.20 and 2.0% manganese, between 0.05 and 0.50% silicon, less than
0.01%
aluminum, and niobium between about 0.01% and about 0.20% and having a
majority of the
microstructure comprised of bainite and acicular ferrite and having more than
70% niobium
in solid solution. Alternately, the niobium may be less than 0.1%. The steel
product may
further comprise at least one element selected from the group consisting of
molybdenum
between about 0.05% and about 0.50%, vanadium between about 0.01% and about
0.20%,
and a mixture thereof.
[0010] The steel product may have a yield strength of at least 340 MPa, and
may have
a tensile strength of at least 410 MPa. The steel product may have a yield
strength of at least
485 MPa and a tensile strength of at least of at least 520 MPa. The steel
product has a total
elongation of at least 6%. Alternately, the total elongation may be at least
10%.
[0011] The steel product may be a thin cast steel strip. Optionally, the
thin cast steel
strip may have fine oxide particles of silicon and iron distributed through
the steel
microstructure having an average particle size less than 50 nanometers
[0012] The steel product may be a thin cast steel strip of less than 3 mm
in thickness.
The thin cast steel strip may have a thickness of less than 2.5 mm.
Alternately, the thin cast
steel strip may have a thickness of less than 2.0 mm. In yet another
alternative, the thin cast
steel strip may have a thickness in the range from about 0.5 mm to about 2 mm.
[0013] The hot rolled steel product of less than 3 millimeters thickness is
also
disclosed comprised, by weight, of less than 0.25% carbon, between 0.20 and
2.0%
manganese, between 0.05% and 0.50% silicon, less than 0.01% aluminum, and
niobium
between about 0.01% and about 0.20%, and have a majority of the microstructure
comprised
of bainite and acicular ferrite and capable of providing a yield strength of
at least 410 MPa
with a reduction of between 20% and 40%. The steel product may have a yield
strength of at
least 485 MPa and a tensile strength of at least of at least 520 MPa.
Alternately, the niobium
may be less than 0.1%.
[0014] Optionally, the hot rolled steel product may have fine oxide
particles of silicon
and iron distributed through the steel microstructure having an average
particle size less than
50 nanometers.
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[0015] The hot
rolled steel product has a total elongation of at least 6%. Alternately,
the total elongation may be at least 10%. The hot rolled steel product may
have a thickness of
less than 2.5 mm. Alternately, the hot rolled steel product may have a
thickness of less than
2.0 mm. In yet another alternative, the hot rolled steel product may have a
thickness in the
range from about 0.5 mm to about 2 mm.
[0016] Also
disclosed is a coiled steel product comprised, by weight, of less than
0.25% carbon, between 0.20 and 2.0% manganese, between 0.05 and 0.50% silicon,
less than
0.01% aluminum, and at least one element selected from the group consisting of
niobium
between about 0.01% and about 0.20%, vanadium between about 0.01% and about
0.20%,
and a mixture thereof, and having more than 70% niobium and/or vanadium in
solid solution
after coiling and cooling. Alternately, the niobium may be less than 0.1%.
[0017]
Optionally, the coiled steel product may have fine oxide particles of silicon
and iron distributed through the steel microstructure having an average
particle size less than
50 nanometers.
[0018] The
coiled steel product may have a yield strength of at least 340 MPa, and
may have a tensile strength of at least 410 MPa. The coiled steel product has
a thickness of
less than 3.0 mm. The steel product may have a yield strength of at least 485
MPa and a
tensile strength of at least of at least 520 MPa.
[0019]
Alternately, the coiled steel product has a thickness of less than 2.5 mm.
Alternately, the coiled steel product may have a thickness of less than 2.0
mm. In yet another
alternative, the coiled steel product may have a thickness in the range from
about 0.5 mm to
about 2 mm. The coiled steel product has a total elongation of at least 6%.
Alternately, the
total elongation may be at least 10%.
[0020] An age
hardened steel product is also disclosed comprising, by weight, less
than 0.25% carbon, between 0.20 and 2.0% manganese, between 0.05 and 0.50%
silicon, less
than 0.01% aluminum, at least one element from the group consisting of niobium
between
about 0.01% and about 0.20%, vanadium between about 0.01% and about 0.20%, and
a
mixture thereof, and having a majority of the microstructure comprised of
bainite and
acicular ferrite and having an increase in elongation and an increase in yield
strength after
age hardening. Alternately, the niobium may be less than 0.1%.
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[0021] The age
hardened steel product may comprise, in addition, fine oxide particles
of silicon and iron distributed through the steel microstructure having an
average particle size
less than 50 nanometers.
[0022] The
steel product may have a yield strength of at least 340 MPa, or at least
380 MPa, or at least 410 MPa, or at least 450 MPa, or at least 500 MPa, or at
least 550 MPa,
or at least 600 MPa, or at least 650 MPa, as desired. The steel product may
have a tensile
strength of at least 410 MPa, or at least 450 MPa, or at least 500 MPa, or at
least 550 MPa, or
at least 600 MPa, or at least 650 MPa, or at least 700 MPa, as desired. The
age hardened steel
product has a thickness of less than 3.0 mm. Alternately, the age hardened
steel product has a
thickness of less than 2.5 mm. Alternately, the age hardened steel product may
have a
thickness of less than 2.0 mm. In yet another alternative, the age hardened
steel product may
have a thickness in the range from about 0.5 mm to about 2 mm. The age
hardened steel
product has a total elongation of at least 6%. Alternately, the total
elongation may be at least
10%.
[0023] A steel
product comprising, by weight, less than 0.25% carbon, between 0.20
and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.01% aluminum,
and at
least one element selected from the group consisting of niobium between about
0.01% and
about 0.20% and vanadium between about 0.01% and about 0.20%, and having a
majority of
the microstructure comprised of bainite and acicular ferrite and comprising
fine oxide
particles of silicon and iron distributed through the steel microstructure
having an average
particle size less than 50 nanometers. Alternately, the niobium may be less
than 0.1%.
Optionally, the steel product may comprise molybdenum between about 0.05% and
0.50%.
[0024] The
steel product may have a yield strength of at least 340 MPa, and may have
a tensile strength of at least 410 MPa. The steel product may have a yield
strength of at least
485 MPa and a tensile strength of at least of at least 520 MPa. The steel
product has a total
elongation of at least 6%. Alternately, the total elongation may be at least
10%.
[0025] An age
hardened steel product comprising, by weight, less than 0.25% carbon,
between 0.20 and 2.0% manganese, between 0.05 and 0.50% silicon, less than
0.01%
aluminum, and niobium between about 0.01% and about 0.20%, and having a
majority of the
microstructure comprised of bainite and acicular ferrite and having niobium
carbonitride
particles with an average particle size of less than 10 nanometers.
Carbonitride particles, in
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the present specification and appended claims, includes carbides, nitrides,
carbonitrides, and
combinations thereof. Alternately, the niobium may be less than 0.1%.
[0026] The age
hardened steel product may have substantially no niobium
carbonitride particles greater than 50 nanometers. The age hardened steel
product may have a
yield strength of at least 340 MPa, and may have a tensile strength of at
least 410 MPa. The
age hardened steel product has a total elongation of at least 6%. Alternately,
the total
elongation may be at least 10%.
[0027] A method
is disclosed for preparing coiled thin cast steel strip comprising the
steps of:
assembling internally a cooled roll caster having laterally positioned casting
rolls
forming a nip between them, and forming a casting pool of molten steel
supported on the casting rolls above the nip and confined adjacent the ends of
the casting rolls by side dams,
counter rotating the casting rolls to solidify metal shells on the casting
rolls as the
casting rolls move through the casting pool, and
forming from the metal shells downwardly through the nip between the casting
rolls a
steel strip, and
cooling the steel strip at a rate of at least 10 C per second to provide a
composition
comprising by weight, less than 0.25% carbon, between 0.20 and 2.0%
manganese, between 0.05 and 0.50% silicon, less than 0.01% aluminum, and
at least one element selected from the group consisting of niobium between
about 0.01% and about 0.20%, vanadium between about 0.01% and about
0.20%, and a mixture thereof, and having a majority of the microstructure
comprised of bainite and acicular ferrite and having more than 70% niobium
and/or vanadium in solid solution.
[0028] The
method may provide in the steel strip as coiled fine oxide particles of
silicon and iron distributed through the steel microstructure having an
average particle size
less than 50 nanometers. Further, the method may comprise the steps of hot
rolling the steel
strip, and coiling the hot rolled steel strip at a temperature between about
450 and 700 C.
Alternately, the coiling of the hot rolled steel strip may be at a temperature
less than 650 C.
[0029] The
method may further comprise the step of age hardening the steel strip to
increase the tensile strength at a temperature of at least 550 C.
Alternately, the age hardening
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may occur at a temperature between 625 C and 800 C. In yet another
alternate, the age
hardening may occur at a temperature between 650 C and 750 C.
[0030] Also
disclosed is a method of preparing a thin cast steel strip comprised the
steps of:
assembling internally a cooled roll caster having laterally positioned casting
rolls
forming a nip between them, and forming a casting pool of molten steel
supported on the casting rolls above the nip and confined adjacent the ends of
the casting rolls by side dams,
counter rotating the casting rolls to solidify metal shells on the casting
rolls as the
casting rolls move through the casting pool, and
forming steel strip from the metal shells cast downwardly through the nip
between the
casting rolls, and
cooling the steel strip at a rate of at least 10 C per second to provide a
composition
comprising by weight, less than 0.25% carbon, less than 0.01% aluminum, and
at least one element from the group consisting of niobium between about
0.01% and about 0.20%, vanadium between about 0.01% and about 0.20%,
and a mixture thereof, and having a majority of the microstructure comprised
of bainite and acicular ferrite and having more than 70% niobium and/or
vanadium in solid solution,
age hardening the steel strip at a temperature between 625 C and 800 C.
[0031] The
method may further comprise the step of age hardening the steel strip to
increase the tensile strength. Alternately, the age hardening may occur at a
temperature
between 650 C and 750 C.
[0032] The
method may provide the age hardened steel strip having niobium
carbonitride particles with an average particle size of less than 10
nanometers. Alternately,
the age hardened steel strip has substantially no niobium carbonitride
particles greater than 50
nanometers.
[0033] The
method may provide in the steel strip as coiled fine oxide particles of
silicon and iron distributed through the steel microstructure having an
average particle size
less than 50 nanometers. Further, the method may comprise the steps of hot
rolling the steel
strip, and coiling the hot rolled steel strip at a temperature less than 700
C. Alternately, the
coiling of the hot rolled steel strip may be at a temperature less than 650
C.
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[0034] The method of preparing a thin cast steel strip may comprise the
steps of:
assembling internally a cooled roll caster having laterally positioned casting
rolls
forming a nip between them, and forming a casting pool of molten steel
supported on the casting rolls above the nip and confined adjacent the ends of
the casting rolls by side dams,
counter rotating the casting rolls to solidify metal shells on the casting
rolls as the
casting rolls move through the casting pool; and
forming from the metal shells downwardly through the nip between the casting
rolls a
steel strip; and
cooling the steel strip at a rate of at least 10 C per second to provide a
composition
comprising by weight, less than 0.25% carbon, between 0.20 and 2.0%
manganese, between 0.05 and 0.50% silicon, less than 0.01% aluminum, and
at least one element from the group consisting of niobium between about
0.01% and about 0.20%, vanadium between about 0.01% and about 0.20%,
and a mixture thereof, and having a majority of the microstructure comprised
of bainite and acicular ferrite,
age hardening the steel strip at a temperature between 625 C and 800 C and
having
an increase in elongation and an increase in yield strength after age
hardening.
[0035] The method may provide in the steel strip as coiled fine oxide
particles of
silicon and iron distributed through the steel microstructure having an
average particle size
less than 50 nanometers. Further, the method may provide the age hardened
steel strip having
niobium carbonitride particles with an average particle size of less than 10
nanometers.
Alternately, the age hardened steel strip has substantially no niobium
carbonitride particles
greater than 50 nanometers.
[0036] The method may comprise the steps of hot rolling the steel strip,
and coiling
the hot rolled steel strip at a temperature less than 750 C. Alternately, the
coiling of the hot
rolled steel strip may be at a temperature less than 700 C.
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] In order that the invention may be described in more detail, some
illustrative
examples will be given with reference to the accompanying drawings in which:
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[0038] FIG. 1 illustrates a strip casting installation incorporating an in-
line hot rolling
mill and coiler;
[0039] FIG. 2 illustrates details of the twin roll strip caster;
[0040] FIG. 3 illustrates the effect of coiling temperature on strip yield
strength with
and without niobium or vanadium additions;
[0041] FIG. 4a is an optical micrograph of a niobium steel strip;
[0042] FIG. 4b is an optical micrograph of a standard UCS SS Grade 380
steel strip;
[0043] FIG. 5 is graph showing the effect of post coil age hardening on
yield strength
of the present steel strip;
[0044] FIG. 6 is a graph showing the effect of post coiling simulated age
hardening
cycle on yield and tensile strength of the present steel strip,
[0045] FIG. 7 is a graph showing the effect of hot rolling reduction on the
yield
strength; and
[0046] FIG. 8 is a graph showing the effect of yield strength on
elongation;
[0047] FIG. 9 is a graph showing the effect of niobium amount on the yield
strength
at low levels of niobium;
[0048] FIG. 10a shows micrographs of the microstructure of a first sample
of 0.065%
niobium steel after hot rolling;
[0049] FIG. 10b shows micrographs of the microstructure of a second sample
of
0.065% niobium steel after hot rolling;
[0050] FIG. 11 is a graph showing the effect of niobium amount on the yield
strength;
[0051] FIG. 12 is a graph showing the effect of coiling temperature on the
yield
strength;
[0052] FIG. 13 is a graph showing the effect of coiling temperature on the
yield
strength at low niobium levels;
[0053] FIG. 14 is a graph showing the effect of heat treating conditions on
the yield
strength;
[0054] FIG. 15 is a graph showing the effect of age hardening heat treating
temperature on the yield strength of 0.026% niobium steel;
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[0055] FIG. 16 is a graph showing the effect of peak aging temperature on
the yield
strength of 0.065% niobium steel;
[0056] FIG. 17 is a graph showing the effect of peak aging temperature and
hold time
on the yield strength of 0.065% niobium steel;
[0057] FIG. 18 is a graph showing the effect of peak aging temperature and
hold time
on the yield strength for 0.084% niobium steel;
[0058] FIG. 19 is a graph showing the effect of yield strength on
elongation before
and after age hardening;
[0059] FIG. 20 is a graph showing heat treating results continuous
annealing;
[0060] FIG. 21 is a graph showing age hardened condition;
[0061] FIG. 22 is a graph showing the effect of temperature and time on
hardness;
[0062] FIG. 23 is a graph showing the effect of heat treating on the yield
strength for
the present vanadium steel; and
[0063] FIG. 24 is a graph showing the effect of hot rolling reduction on
the yield
strength for the present vanadium steel.
DETAILED DESCRIPTION OF THE DRAWINGS
[0064] The following description of the embodiments is in the context of
high
strength thin cast strip with microalloy additions made by continuous casting
steel strip
using a twin roll caster.
[0065] FIG. 1 illustrates successive parts of strip caster for continuously
casting steel
strip. FIGS. 1 and 2 illustrate a twin roll caster 11 that continuously
produces a cast steel strip
12, which passes in a transit path 10 across a guide table 13 to a pinch roll
stand 14 having
pinch rolls 14A. Immediately after exiting the pinch roll stand 14, the strip
passes into a hot
rolling mill 16 having a pair of reduction rolls 16A and backing rolls 16B
where the cast strip
is hot rolled to reduce a desired thickness. The hot rolled strip passes onto
a run-out table 17
where the strip may be cooled by convection and contact with water supplied
via water jets
18 (or other suitable means) and by radiation. The rolled and cooled strip is
then passes
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through a pinch roll stand 20 comprising a pair of pinch rolls 20A and then to
a coiler 19.
Final cooling of the cast strip takes place after coiling.
[0066] As shown
in FIG. 2, twin roll caster 11 comprises a main machine frame 21,
which supports a pair of laterally positioned casting rolls 22 having casting
surfaces 22A.
Molten metal is supplied during a casting operation from a ladle (not shown)
to a tundish 23,
through a refractory shroud 24 to a distributor or moveable tundish 25, and
then from the
distributor 25 through a metal delivery nozzle 26 between the casting rolls 22
above the nip
27. The molten metal delivered between the casting rolls 22 forms a casting
pool 30 above
the nip. The casting pool 30 is restrained at the ends of the casting rolls by
a pair of side
closure dams or plates 28, which are pushed against the ends of the casting
rolls by a pair of
thrusters (not shown) including hydraulic cylinder units (not shown) connected
to the side
plate holders. The upper surface of casting pool 30 (generally referred to as
the "meniscus"
level) usually rises above the lower end of the delivery nozzle so that the
lower end of the
delivery nozzle is immersed within the casting pool 30. Casting rolls 22 are
internally water
cooled so that shells solidify on the moving roller surfaces as they pass
through the casting
pool, and are brought together at the nip 27 between them to produce the cast
strip 12, which
is delivered downwardly from the nip between the casting rolls.
[0067] The twin
roll caster may be of the kind that is illustrated and described in
some detail in U.S. Patent. Nos. 5,184,668 and 5,277,243 or U.S. Patent. No.
5,488,988.
Reference may be made to those patents for appropriate construction details of
a twin roll
caster appropriate for use in an embodiment of the present invention.
[0068] A high
strength thin cast strip product can be produced using the twin roll
caster that overcomes the shortcomings of conventional light gauge steel
products and
produces a high strength, light gauge, steel strip product. The invention
utilizes the elements
including niobium (Nb), vanadium (V), titanium (Ti), or molybdenum (Mo), or a
combination thereof.
[0069]
Microalloying elements in steel are commonly taken to refer to the elements
titanium niobium, and vanadium. These elements were usually added in the past
in levels
below 0.1%, but in some cases levels as high as 0.2%. These elements are
capable of exerting
strong effects on the steel microstructure and properties via a combination of
hardenability,
grain refining and strengthening effects (in the past as carbonitride
formers). Molybdenum
has not normally been regarded as a microalloying element since on its own it
is a relatively
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weak carbonitride former, but may be effective in the present circumstances
and may form
complex carbonitride particles along with niobium and vanadium. Carbonitride
formation is
inhibited in the hot rolled strip with these elements as explained below.
[0070] The high
strength thin cast strip product combines several attributes to achieve
a high strength light gauge cast strip product by microalloying with these
elements. Strip
thicknesses may be less than 3 mm, less than 2.5 mm, or less than 2.0 mm, and
may be in a
range of 0.5 mm to 2.0 mm. The cast strip is produced by hot rolling without
the need for
cold rolling to further reduce the strip to the desired thickness. Thus, the
high strength thin
cast strip product overlaps both the light gauge hot rolled thickness ranges
and the cold rolled
thickness ranges desired. The strip may be cooled at a rate of 10 C per
second and above,
and still form a microstructure that is a majority and typically predominantly
bainite and
acicular ferrite.
[0071] The
benefits achieved through the preparation of such a high strength thin cast
strip product are in contrast to the production of previous conventionally
produced
microalloyed steels that result in relatively high alloy costs, inefficiencies
in microalloying,
difficulties in hot and cold rolling, and difficulties in recrystallization
annealing since
conventional continuous galvanizing and annealing lines are not capable of
providing the
high annealing temperatures needed. Moreover, the relatively poor ductility
exhibited with
strip made by the cold rolled and recovery annealed manufacturing route is
overcome.
[0072] In
previous conventionally produced microalloyed steels, elements such as
niobium and vanadium could not remain in solid solution through
solidification, hot rolling,
coiling and cooling. The niobium and vanadium diffused through the
microstructure forming
carbonitride particles at various stages of the hot coil manufacturing
process. Carbonitride
particles, in the present specification and appended claims, includes
carbides, nitrides,
carbonitrides, and combinations thereof. The formation and growth of carbon
and nitrogen
particles in the hot slab and subsequent coiling of previous conventionally
produced
microalloyed steels further reduced the grain size of austenite in the hot
slab, decreasing the
hardenability of the steel. In these prior steels, the effect of particles in
the hot slab had to be
overcome by increasing the amount of microalloying elements, reheating the
cast slabs to
higher temperatures, and lowering carbon content.
[0073] In
contrast to the previous conventionally produced steels, the present high
strength thin cast steel strip product was produced comprising, by weight,
less than 0.25%
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carbon, between 0.20 and 2.00% manganese, between 0.05 and 0.50% silicon, less
than
0.06% aluminum, and at least one element selected from the group consisting of
titanium
between about 0.01% and about 0.20%, niobium between about 0.01% and about
0.20%,
molybdenum between about 0.05% and about 0.50%, and vanadium between about
0.01%
and about 0.20%, and having a microstructure comprising a majority bainite.
The steel
product may further comprising fine oxide particles of silicon and iron
distributed through the
steel microstructure having an average particle size less than 50 nanometers.
The steel
product may further comprise a more even distribution of microalloys through
the
microstructure than previously produced with conventional slab cast product.
[0074]
Alternately, the high strength thin cast steel strip product may comprise, by
weight, less than 0.25% carbon, between 0.20 and 2.0% manganese, between 0.05
and 0.50%
silicon, less than 0.01% aluminum, and niobium between about 0.01% and about
0.20%, and
having a majority of the microstructure comprising bainite and acicular
ferrite and having
more than 70% soluble niobium.
[0075] In
another alternate, a coiled steel product may comprise, by weight, less than
0.25% carbon, between 0.20 and 2.0% manganese, between 0.05 and 0.50% silicon,
less than
0.01% aluminum, and at least one element selected from the group consisting of
niobium
between about 0.01% and about 0.20% and vanadium between about 0.01% and about
0.20%, and a combination thereof, and having more than 70% soluble niobium and
vanadium, as selected, after coiling and cooling. The coiled high strength
thin cast steel strip
product may have more than 70% soluble niobium and vanadium, as selected,
particularly
after hot rolling reduction and subsequent coiling and before age hardening.
The
microstructure may be a mixture of bainite and acicular ferrite. Alternately,
the
microstructure of the hot rolled and subsequently coiled and cooled steel may
comprise
bainite and acicular ferrite with more than 80% niobium and/or vanadium
remaining in solid
solution, and alternately may have more than 90% remaining in solid solution.
[0076]
Alternatively or in addition, the steel product may have a total elongation
greater than 6% or greater than 10%. The steel product may have a yield
strength of at least
340 MPa (about 49 ksi) or a tensile strength of at least 410 MPa, or both,
exhibiting
satisfactory ductility. The relationship between yield strength and total
elongation in the hot
rolled product is shown in FIG. 8.
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[0077] After
hot rolling the hot rolled steel strip may be coiled at a temperature in the
range from about 500-700 C. The thin cast steel strip may also be further
processed by age
hardening the steel strip to increase the tensile strength at a temperature of
at least 550 C.
The age hardening may occur at a temperature between 550 C and 800 C, or
between 625
C and 750 C, or between 675 C and 750 C. Conventional furnaces of
continuous
galvanizing or annealing lines are thus capable of providing the age hardening
temperatures
needed to harden the microalloyed cast strip product.
[0078] For
example, a steel composition was prepared by making a steel composition
of a 0.026% niobium, 0.04% by weight carbon, 0.85% by weight manganese, 0.25%
by
weight silicon that has been cast by a thin cast strip process. The strip was
cast at 1.7 mm
thick and inline hot rolled to a range of strip thickness from 1.5 mm to 1.1
mm using a twin
roll caster as illustrated in FIGS. 1 and 2. The strip was coiled at coiling
temperatures of 590-
620 C (1094-1148 F).
[0079] As shown
in FIG. 3, the yield and tensile strength levels achieved in the
present cast strip are compared to the yield and tensile strength levels
achievable in the base,
non-microalloyed, cast strip steel composition over a range of coiling
temperatures. It can be
seen that the niobium steel strip achieved yield strengths in the range of 420-
440 MPa (about
61-64 ksi) and tensile strengths of about 510 MPa (about 74 ksi). The present
cast strip
product is compared to C-Mn-Si base steel compositions processed with the same
coiling
temperature as the microalloyed steel, with the niobium steel producing
substantially higher
strength levels. The compared base steel strip had to be coiled at very low
temperatures to
approach comparable strength levels to the cast niobium steel product. The
cast niobium steel
product did not need to be coiled at low coiling temperatures to achieve its
strengthening
potential with the hot rolling. Moreover, the yield and tensile strength
levels for the cast
niobium steel was not significantly affected by the degree of inline hot
rolling with a
reduction of at least 19% to 37% as shown in FIG. 7.
[0080] The
hardenability of the present steels is shown in FIG. 9. As shown in FIG. 9,
a niobium level of as little as 0.007% was effective in increasing the
strength of the final
strip, and yield strength levels of over 380 MPa were achieved with niobium
levels greater
than about 0.01%. Note that niobium levels less than about 0.005% may be
considered
residual. Thus even very small additions of microalloying elements can be
effective in
substantial strengthening.
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[0081] The high
strengths were achieved by utilizing the niobium microalloying
addition to increase the hardenability of the steel through the suppression of
the formation of
proeutectic ferrite. FIG. 4b shows that proeutectic ferrite formed along the
prior austenite
grain boundaries (allotriomorphic ferrite) in the base steel, but it was not
present in the
niobium steel shown in FIG. 4a. The hardenability effects of the niobium
addition suppressed
the ferrite transformation, hence enabling the stronger bainitic and acicular
ferrite
microstructure to be produced while using conventional cooling rates during
cooling and
higher coiling temperatures. The final microstructure of the present niobium
steels comprises
mostly a combination of bainite and acicular ferrite. The base steel shown in
FIG. 4b was
cooled to a relatively low coiling temperature, less than 500 C, a cooling
condition known to
suppress ferrite formation at the austenite grain boundaries.
[0082] The
effect of hot reduction on yield strength is reduced in the present niobium
steel. In previous C-Mn products, there is typically a decrease in strength
with increasing hot
reduction. In contrast, as shown in FIG. 7, the effect of hot reduction on
yield strength is
significantly reduced in the present steel product. In this experiment, the
coiling temperature
was kept constant, and covering the range of hot rolling reductions up to at
least 40%
represented the strip thickness range of 1.0mm to 1.5mm. Unlike the non-
microalloyed base
steel, the strength levels of the niobium microalloyed steels of the present
disclosure in the
as-hot rolled cast strip product are relatively insensitive to the degree of
hot rolled reduction
for reductions up to at least 40%. Further, these high strength levels were
achieved using
conventional coiling temperatures in the range of 550 C to 650 C, as shown
in FIG. 3.
[0083] To
investigate this effect further, the austenite grain size was measured at each
thickness in the 0.026 Nb steel. Where the base steel tended to be fully
recrystallized above
about 25% hot reduction, the 0.026 Nb steel showed only limited
recrystallization even at
40% reduction. This indicates that the niobium in solid solution reduced the
effect of hot
reduction on the strength properties by suppressing static recrystallization
of the deformed
austenite after hot rolling. This is shown in FIG. 10, where it can be seen
that the austenite
grains have been elongated by the hot rolling reduction without
recrystallising into finer
grains. Finer grains increase the austenite grain boundary area, thereby
reducing the steel
hardenability. However, while recrystallization to a finer austenite grain
size was suppressed,
such high hot rolling reductions are known to raise the ferrite transformation
start
temperature. In addition, high hot rolling reduction can induce local high
strain regions
within the austenite grains, usually referred to as shear bands, which can act
as intragranular
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nucleation sites for ferrite nucleation. In the present steels, the
hardenability effect of the
niobium was sufficient to suppress the formation of ferrite within the
deformed austenite
grains, which resulted in strength levels that were largely insensitive to the
degree of hot
rolling.
[0084] The thin
cast strip niobium steel product had consistent yield and tensile
strength levels over the range of hot rolling applied, and capable of
providing a yield strength
of at least 410 MPa with a reduction of between 20% and 40%. The prior
austenite grain size
was determined for each strip thickness. The austenite grain size measurements
indicated that
only very limited recrystallization had occurred at high hot rolling
reductions, whereas in the
comparable base steel strip, the microstructure almost fully recrystallized at
hot rolling
reductions over about 25%. The addition of niobium to the cast steel strip
suppressed the
recrystallization of the coarse as-cast austenite grain size during the hot
rolling process, and
resulted in the hardenability of the steel being retained after hot rolling
and retention of
niobium in solution.
[0085] The
higher strength of the present steel strip after hot rolling was mostly due
to the microstructure formed. As shown in FIG. 4a, the microstructure of the
cast niobium
steel was comprised of a majority if not mostly bainite for all strip
thicknesses. In contrast, as
shown in FIG. 4b, the comparable non-microalloyed steel achieved similar
strength by
coiling at a low coiling temperature and had a microstructure comprising
mostly acicular
ferrite with some grain boundary ferrite. The addition of niobium to the steel
strip provided
an increase in the hardenability of the steel and suppressed the formation of
the grain
boundary ferrite and promoted the bainitic microstructure, even at
considerably higher coiling
temperatures.
[0086] The
yield and tensile strength results from the trial steels, shown in Table 2
below, in the as-hot rolled condition are summarized in FIG. 11. The strength
level increases
with increasing niobium content, with yield strength of at least 340 MPa, with
levels up to
about 500 MPa in the as-hot rolled condition. The tensile strength may be at
least 410 MPa.
The initial rapid increase in strength is attributed to the suppression of
proeutectic ferrite
formation and the promotion of bainite and acicular ferrite, while the
subsequent
strengthening can be attributed to continued microstructural refinement and
possibly solid
solution hardening from niobium retained in solid solution.
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[0087] In
addition, transmission electron microscopy (TEM) examination did not
reveal any substantial niobium precipitation in the as hot rolled cast strip.
This indicates that
the niobium had been retained in solid solution and that the strengthening
produced was
mainly attributed to the enhanced hardenability effect of the niobium
resulting in the
formation of a majority and likely predominantly bainitic microstructure. The
hardenability
of the cast steel strip is also believed to be enhanced by the retention of
coarse austenite grain
produced during formation of the cast strip. The transformation to bainite,
rather than ferrite,
is believed to be a major factor in suppressing the precipitation of the
microalloy addition of
niobium in the thin cast strip during cooling of the coil from the coiling
temperature.
[0088] The
transmission electron microscopy (TEM) examination may be used to
determine the size, identity and volume fraction of niobium carbonitride
particles present in
the steel. The absence of any niobium carbonitride particles upon TEM
examination
supported the view that the observed strength was largely attributable to the
microstructure
being largely bainite rather than ferrite. The subsequent observed
strengthening increment
arising from an age hardening heat treatment therefore leads to the conclusion
that niobium
had been substantially in solution in the hot rolled strip. After determining
the volume
fraction of carbonitride particles in the microstructure using TEM analysis,
the amount of
microalloy element in solid solution can be concluded.
[0089] Thin
foils or carbon replicates may be evaluated by TEM in determining the
amount of the present carbonitride particles. In our analysis, a JEOL 2010
transmission
electron microscope was used. However, from our experience with this
instrument, Nb
particles below 4 nanometers may not be resolvable in heavily dislocated
ferrite.
[0090] For thin
foil analysis, a foil is prepared. The foil is cut and ground to a
thickness of 0.1 mm. The sample is then thinned to electron transparency by
electro-polishing
using a 5% perchloric acid, 95% acetic acid electrolyte in a Tenupole-2
electro-polishing
unit. The sample can then be directly transferred to the TEM.
[0091] For
carbon replication, a desired sample may be prepared by etching a
polished sample in Nital (a solution of alcohol and nitric acid) after
etching, coating the
samples with carbon, and then scoring the carbon coating into appropriate
dimensions (for
example 2 mm square) for TEM analysis. After scoring, carbon replicas may be
liberated
from the sample by dissolving the ferrite matrix in 3% Nital. The carbon
replica samples are
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collected on 3mm diameter support grids, then repeatedly washed in
ethanol/water solutions.
The carbon extraction replica with the supporting grid can then be transferred
to the TEM.
[0092] An
additional factor believed to account for the absence of niobium
carbonitride particles in the hot rolled cast strip relates to the nature of
the dispersion of
niobium with the rapid solidification of the strip during its formation by the
method of
continuously making cast strip described. In previously made microalloyed high
strength
strip, relatively long time intervals were involved in the solidification with
slab cooling, slab
reheating and thermo-mechanical processing that permitted opportunities for
pre-clustering
and/or solid state precipitation of carbonitride particles such as
(Nb,V,Ti,Mo)(CN), that
enabled the kinetics for subsequent precipitation through the stages of the
manufacturing
process. In the present process described, where the cast strip is
continuously formed from a
casting pool between casting rolls, the extremely rapid initial solidification
in forming the
cast strip (in about 160 microseconds) is believed to inhibit pre-clustering
and/or solid state
precipitation of carbonitride particles, and in turn, slow and reduce the
kinetics for
precipitation of the microalloys in subsequent processing including rolling
and coiling
operations. This means that the microalloys of Nb, V, Ti, and Mo are
relatively more evenly
distributed in the austenite and ferrite phases, than in thin steel strip
previously made by
conventional slab casting and processing.
[0093] Atom
probe analysis of niobium cast strip made by forming from a casting
pool between casting rolls as above described has verified the more even
distribution of
microalloys (indicating reduced pre-clustering and/or solid state
precipitation) in both the as
cast and the hot rolled strip when coiled at about 650 C or lower. This more
even
distribution of elements is believed to be inhibiting the formation of
carbonitrides in the
coiling operation under conditions where fine coherent precipitation of such
elements
occurred in previous conventionally made and processed microalloyed slab cast
steel. The
reduction or absence of pre-clustering and/or solid state formation of
carbonitrides in the
microalloyed cast strip made by twin roll casting also slows the kinetics of
formation of
carbonitrides during subsequent thermo-mechanical processing such as
annealing. This then
permits the opportunity for age hardening at temperatures higher than those
where the
particles in previously conventionally processed strip lost their
strengthening capacity
through coarsening (Ostwald ripening) mechanisms.
[0094] With an
age hardening heat treatment, higher tensile strength was found to be
achievable. For example, with a 0.026% niobium addition, an increase of at
least a 35 MPa
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(about 5 ksi) increase in yield strength from 410 to 450 MPa (about 60-65 ksi)
was observed.
With a 0.05% niobium addition, it is contemplated that with a age hardening,
an increase of
at least 10 ksi is expected, and a with 0.1% niobium addition, it is
contemplated that with a
age hardening, an increase of at least 20 ksi is expected. The microstructure
of the present age
hardened steel product may have niobium carbonitride particles with an average
particle size
of 10 nanometers and less. The microstructure of the age hardened steel
product may have
substantially no niobium carbonitride particles greater than 50 nanometers.
[0095]
Laboratory ageing heat treatments were conducted on samples of 0.026 %
niobium steels at various temperatures and times to induce action of the
niobium, that was
believed retained in solid solution in the hot rolled strip. As shown in FIG.
5, ageing heat
treatments produced a significant increase in strength, with yield strengths
of about 480MPa
(about 70 ksi). This confirmed that the niobium was retained in a solid
solution and was
available to provide age hardening on subsequent ageing, for example, through
the use of an
annealing furnace on continuous galvanizing lines or by using a continuous
annealing line.
Accordingly, short time age hardening is carried out to simulate the ageing
potential from
processing the niobium microalloyed cast steel product through an annealing
furnace attached
to continuous galvanizing line or conventional continuous annealing line. In
the latter case
the age hardened high strength strip product maybe subsequently galvanized,
painted or
utilized uncoated.
[0096] The
results, as shown in FIG. 6, clearly show that for a peak processing
temperature of 700 C (1292 F), significant strengthening was realized, with
strength levels
approaching that achieved for the longer times at lower temperatures. The
tensile properties
of the niobium thin cast steel product after the short time ageing treatment
using a peak
temperature of 700 C (1292 F) are given in Table 1. Besides the high
strength of the cast
strip product, the ductility and formability is satisfactory for structural
quality products. The
cast strip product produced is a thin, high strength strip product for
structural applications
through the use of niobium microalloying. It is contemplated that higher
microalloying levels
would realize even higher yield strengths, potentially well in excess of 550
MPa (about 80
ksi).
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TABLE 1
Strip Yield Strength, Tensile Strength, Total
YS/TS 'n' Value 'r' Value
Thickness, mm MPa MPa Elongation, %
1.1 477 563 18 0.85 0.12 0.90
[0097] Recently, in addition to producing the 0.026 wt% niobium steel,
steels with
niobium additions of 0.014 wt% and 0.065 wt% have been successfully produced
via the
present process. Heat compositions are shown below in Table 2.
TABLE 2
Steel C (wt%) Mn
(wt%) Si (wt%) Nb (wt%) V (wt%) N (wt%)
A 0.032 0.72 0.18 0.014 <0.003 0.0078
B 0.029 0.73 0.18 0.024 <0.003
0.0063
C 0.038 0.87 0.24 0.026 <0.003 0.0076
D 0.032 0.85 0.21 0.041 <0.003
0.0065
E 0.031 0.74 0.16 0.059 <0.003
0.0085
F 0.030 0.86 0.26 0.065 <0.003 0.0072
G 0.028 0.82 0.19 0.084 <0.003
0.0085
H 0.026 0.90 0.21 <0.003 0.042
0.0070
Base Steel 0.035 0.85 0.27 <0.003 <0.003 0.0060
[0098] The yield strengths achieved for steel C and steel F are shown in
FIG. 12, and
the yield strength results for the 0.014% Nb heat, steel A, produced with a
lower Mn content,
are presented in FIG. 13. The niobium additions increased the yield strength
at all coiling
temperatures relative to the base steel composition. The yield strength
increased by about 70
to 100 MPa (10 to 15 ksi) for the 0.014% Nb and 0.026 Nb additions, and by
about 140 to
175 MPa (20-25 ksi) for the 0.065 Nb addition. From FIG. 12 it can be seen
that the 0.026%
Nb steel achieved higher yield strengths than the 0.8 Mn base steel for
similar coiling
temperatures, and comparable yield strengths to when the 0.8 Mn base steel was
coiled a low
temperatures. Alternatively, the strengths achieved in the 0.8 Mn base steel
at low coiling
temperatures (about 500 C) can be achieved at higher coiling temperatures
(about 600 C)
with this Nb addition.
[0099] Additionally, in contrast to previous conventionally produced
microalloyed
steel, we have found that the microalloy addition suppresses the formation of
carbonitride
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particles in the hot rolled and subsequently coiled and cooled steel. Instead,
the
microstructure of the hot rolled and subsequently coiled and cooled steel
comprises bainite
and acicular ferrite with more than 70% niobium and/or vanadium remaining in
solid
solution. Alternately, the microstructure of the hot rolled and subsequently
coiled and cooled
steel may comprise bainite and acicular ferrite with more than 80% niobium
and/or vanadium
remaining in solid solution, and alternately may have more than 90 % remaining
in solid
solution.
[00100] Thus, it
has been shown that the niobium cast strip results in light gauge, high
strength, steel product. The niobium addition firstly is capable of
suppressing the austenite
recrystallization during hot rolling, which enhances the hardenability of the
steel by retaining
the relatively coarse as cast austenite size. The niobium being retained in
solid solution in
austenite after hot rolling, directly increases the steel's hardenability,
which assists in
transforming the austenite to a final microstructure comprised mostly of
bainite, even at
relatively high coiling temperatures. The formation of a bainitic
microstructure promoted the
retention of the niobium addition in solid solution in the hot rolled strip.
[00101] Further
improvement in properties may be obtained by age hardening the
present steels. In previous microalloyed and non-microalloyed steels, an
increase in strength
could be obtained by age hardening, but in such prior steels, a decrease in
elongation occurs
with an increase in strength. We have found that both an increase in
elongation and an
increase in strength may be obtained by age hardening the present steels.
[00102] It was
determined that the retention of the microalloying elements such as
niobium and vanadium in solid solution by the prior processing conditions
provided
considerable hardenability for subsequent age hardening cycle. Such an age
hardening cycle
can be produced using a suitable continuous galvanizing line or continuous
annealing facility.
Hence a microalloyed steel strip made using a thin strip casting process,
combined with an
age hardening heat treatment provided by a suitable galvanizing line or
annealing line, is a
unique manufacturing path providing a unique strengthening approach for this
type of steel
product.
[00103]
Isothermal ageing treatments of the hot rolled 0.026% Nb cast strip material
were carried out for 20 minutes at 600 C and 650 C (1110 F and 1200 F),
inducing
formation of niobium carbonitrides, or Nb(C,N), as confirmed by TEM
examination. This
resulted in an increase in yield strength of the material, as shown in FIG.
14. Also, as shown
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in FIGS. 6 and 14, the thermal cycle of strip through the annealing section of
a galvanizing
line also induced a significant strength increase, approaching that achieved
with the
isothermal aging at lower temperatures.
[00104] The
increase in the hardenability provided by the microalloy addition through
the suppression of the ferrite transformation significantly lowers the
austenite decomposition
temperature into the bainite/acicular ferrite temperature range. This lower
transformation start
temperature provides the potential to retain the vast majority of the
microalloy addition in
solid solution by applying conventional run out table cooling rates and
appropriate coiling
temperatures.
[00105] The
microalloying elements, such as niobium and vanadium, in solid solution
are available for age hardening during a subsequent heat treatment to increase
strength.
Laboratory age hardening studies established that substantial strengthening
could be achieved
even with relatively short heat treatment cycles, such as available with
continuous annealing
lines and galvanizing lines. The results from laboratory simulated continuous
annealing
cycles applied to trial Steel C (0.026% Nb), Steel F (0.065% Nb), and Steel G
(0.084% Nb)
are shown in FIGS. 15, through 18.
[00106] The
results from full scale plant trials with steels B and F, using the heat
treatment conditions established from the laboratory study are given in FIGS.
20 and 21,
respectively. Substantial strength increases were achieved with steels B and
F. Yield strength
levels in excess of 450 MPa were recorded with the 0.024% Nb steel (steel B)
and yield
strengths over 550 MPa with the 0.065% Nb steel (steel F). Strength increase
from the age
hardening was in the order of 70 MPa (10 ksi) for the 0.024% Nb steel (steel
B) and up to
about 100 MPa (15 ksi) for the 0.065% Nb steel (steel F). It is contemplated
that the 0.065%
Nb steel may achieve yield strengths over 600 MPa in the age hardened
condition.
TABLE 3
Steel F Thickness Yield Strength Tensile Strength Elongation
mm MPa MPa %
Hot band 0.996 512 599 11.47
Galvanized 0.991 581 645 14.16
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[00107] Samples
of steel F were age hardened on using the age hardening conditions
found on a galvanizing line. As shown in TABLE 3, the age hardened steel had a
strength of
almost 70 MPa, and the elongation increased from 11.47% to 14.16%. The
relationship
between yield strength and total elongation for the presently disclosed
niobium steels in the
as hot rolled condition and in the age hardened and galvanized condition
(longitudinal test
direction) is shown in FIG. 19.
[00108] As shown
in FIG. 16, we have found that a 10 second hold cycle may be used
between about 675 C to 725 C to prevent overaging. However, the temperature
range is a
function of the holding time. Increasing the hold time to 20 seconds lowered
the temperature
range slightly, while for the zero hold time, the temperature range was
increased slightly, as
shown in FIG. 17. The age hardening temperature range may be between about 625
C and
800 C depending upon on the overall heat treatment cycle time, i.e. heating
rates, the
holding time, and cooling rates.
[00109] In the
case of longer time heat treatments, lower temperatures in the range of
500 C to 650 C may be used. From FIG. 6 it can be seen that a heat treatment
of 20 minutes
at 600 C produces similar strength levels as 10 seconds in a continuous
annealing cycle at
700 C. FIG. 22 shows results of laboratory heat treatments carried out for 20
and 120
minutes. The results show that substantial hardening was achieved for a heat
treatment of 120
minutes at 550 C, but the 120 minute aging at temperatures over about 650 C
reduced the
hardness of the steel. Longer heat treatment times could be used with full
coil annealing
processes, such as batch annealing in the temperature range of 500 C to 650
C, or other post
coiling cooling practices for the hot rolled coil, designed to precipitate the
retained niobium,
by controlled cooling through the temperature range 500 C to 650 C.
[00110]
Transmission electron microscopy (TEM) was carried out on samples of steels
C and F, which had been given a heat treatment of 60 minutes at 650 C. Fine
particles in the
size range of 4 to 15 nanometers were found. These fine particles were found
to include
niobium carbonitrides, indicating that the strengthening may be attributed to
age hardening
by fine niobium carbonitride particles.
[00111] The
microstructure of the age hardened microalloyed steel product may have
niobium carbonitride particles, with an average particle size of 10 nanometers
and less. The
microstructure of the age hardened steel product may have substantially no
niobium
carbonitride particles greater than 50 nanometers. Samples of the present
niobium steel were
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inspected using TEM evaluation, and portions of the microstructure had no
measurable
amount of niobium carbonitride particles.
[00112] We
believe that the enhanced strength/elongation relationship in the present
age hardened steel may be due to portions of the microstructure being
substantially free of
particles greater than 5 nanometers in size, or "precipitate free zones," and
nano-clusters. The
development of precipitate free zones in the vicinity of grain boundaries may
influence the
strength and tensile elongation relationship by providing reduced hardness
regions adjacent to
grain boundaries. The relaxation of stress concentrations in precipitate free
zones has been
reported to enhance strength and elongation. The beneficial effects of
precipitate free zones
on elongation and strength may appear in circumstances where the precipitate
free zones are
narrow and the size of grain boundary precipitates is small.
[00113] In the
present steel, the element additions may provide for increased
elongation with increased strength after age hardening by producing smaller
precipitate free
zone width and smaller change in hardness than in conventionally produced
niobium steels.
Because of the more even dispersion of elements in rapidly solidified steels,
the kinetics of
age hardening can be retarded so as to effectively expand the time-temperature
window over
which the formation of nano-clusters can be stably controlled. The element
nano-clusters may
provide strengthening in the early stages of age hardening. Cluster
strengthening may be due
to the extra energy required for dislocations to cut the diffuse boundary of
the cluster of
solute species. The clusters may provide substantial strengthening without
reducing ductility
because their elastically soft boundaries do not severely inhibit dislocation
movement or
cause pile-ups in the way that normal second phase particles do.
[00114] In the
present steels, a more even distribution of elements remains in solid
solution during the rapid solidification of steel. In contrast to previous
conventionally
produced niobium and vanadium steels, the microstructure of the hot rolled and
subsequently
coiled and cooled steel comprises bainite and acicular ferrite with more than
70% niobium
and/or vanadium addition remaining in solid solution and substantially no
niobium
carbonitride particles greater than 50 nanometers. Alternately, the
microstructure of the hot
rolled and subsequently coiled and cooled steel may comprise bainite and
acicular ferrite with
more than 80% niobium and/or vanadium addition remaining in solid solution,
and
alternately may have more than 90% remaining in solid solution.
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[00115] The
elements remain trapped in solution in the hot rolled coil and do not
precipitate if the coiling temperature is below about 650 C. Formation is
effectively retarded
because the prior associations of atoms (such as in the form of particles)
that normally occur
in conventional slab casting and reheating for hot strip rolling are prevented
in the present
process. The observed increase in strength that occurs in the hot rolled coils
may thus be
largely attributable to hardenability and solid solution hardening effects.
[00116]
Formation of carbonitride particles can be activated during heat treatment.
Additionally, during age hardening, pre-precipitation clusters and finer
particles are stable
over an extended range of time and temperature because of the significant
amount of niobium
and/or vanadium in solid solution prior to age hardening. The precipitate free
zones that form
near grain boundaries as a normal precipitation phenomenon are narrower and
contain more
evenly dispersed nano-clusters and finer precipitates than for conventionally
produced steels.
Thus the hardness changes in the precipitate free zones relative to the grain
interior are
relatively small for the present steels. We believe that narrower precipitate
free zones and
small hardness changes across precipitate free zones reduce stress
concentrations in the
precipitate free zones reducing microcracking from preferential deformation in
the precipitate
free zones. We believe that the cluster strengthening may be characterized by
a strength
increase without a deterioration in ductility since dislocation pile-up does
not occur at
clusters. The combination of narrow precipitate free zones and cluster
strengthening
mechanisms is believed to lead to precipitate free zones of the present
steels. This results in
improved elongation because cracks are more difficult to initiate and less
constrained to the
grain boundary precipitate free zone region. Further, the nano-clusters may co-
exist with
distinct particles within the grain interior regions over a certain annealing
temperature/time
combinations.
[00117] An
annealing furnace may be used to perform the age hardening, which is not
a current strengthening approach for processing such products. The annealing
condition may
be a continuous annealing cycle with a peak temperature of at least 650 C and
less than 800
C and better 675 C to 750 C. Alternately, strengthening may be achieved in a
production
environment using a very short age hardening cycle available with conventional
annealing
furnaces incorporated in continuous galvanizing lines. The final strength
levels recorded in
the full scale plant trials were similar to that produced with the laboratory
heat treatments of
the respective steels.
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[00118] Similar results are contemplated with niobium between about 0.01%
and about
0.20%, as well as with titanium between about 0.01% and about 0.20%,
molybdenum
between about 0.05% and about 0.50%, and vanadium between about 0.01% and
about
0.20%.
[00119] The composition of the present steel utilizing vanadium is shown as
steel H in
Table 2. The yield strength of steel H is shown in FIG. 23. The vanadium steel
was produced
with two different coiling temperatures, and was subsequently aged for 20
minutes at 650 C
and 700 C to induce hardening by vanadium in solid solution. The results show
that
significant strengthening was achieved from these heat treatment conditions.
The
strengthening increment was slightly higher for the material produced with the
higher coiling
temperature, which may be due to the effects of opposing processes of
precipitation
hardening and microstructural softening. The strengthening increment realized
with the
material produced at the lower coiling temperature was of the same order of
that achieved
with the 0.026% Nb steel.
[00120] The yield strength of steel H in the as-hot rolled and galvanized
conditions are
presented in FIG. 24. FIGS. 23 and 24 indicate that the vanadium steel
achieved higher
strength levels than the plain carbon base steel, even though it was produced
using higher
coiling temperatures. In the samples shown in FIG. 24, the coiling temperature
of steel H was
570 C, and the base steel coiling temperature was less than 500 C.
[00121] Also shown in FIG. 24, a strength increase was realized in the
vanadium steel
from an age hardening using the annealing furnaces on a continuous galvanising
line, but the
strength increase was less than was realized from an equivalent niobium
content. The yield
strength of the sample in FIG. 24 on the galvanizing line was about 450 MPa in
the
galvanised condition, which is in the order achieved with the longer term
laboratory heat
treatments shown in FIG. 23. The strength of the vanadium steel may be more
sensitive to
coiling temperature than the niobium steels.
[00122] This thin cast strip enables production of new steel product types
including:
[00123] 1. A high
strength, light gauge, galvanized strip by utilizing a
microstructure that has bainite as the major constituent and age hardening
during the
galvanizing process. The annealing section of the galvanizing line can be used
to induce age
hardening of the niobium and/or vanadium of the thin cast strip that has been
hot rolled.
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CA 02686495 2014-06-12
[00124] 2. A high strength, light gauge, uncoated strip by utilizing a
microstructure that is majority bainite and age hardened during processing on
a continuous
annealing line. The high temperature furnace of the conventional continuous
annealing can be
used to induce activation of the niobium and vanadium elements retained in
solid solution by
the bainite microstructure after hot rolling of the thin cast strip.
[00125] 3. A high strength, light gauge, hot rolled cast strip product
where the
strength levels are insensitive to the degree of hot rolling reduction
applied. The bainitic
microstructure produces a relatively high strength product (YS > 380 MPa (-
55ksi)). The
suppression of austenite recrystallization during or after hot rolling can
provide final strength
levels insensitive to the degree of hot rolling reduction. The final strength
levels will be
consistent across a range of thicknesses that can be produced by a thin cast
strip process.
[00126] While the invention has been illustrated and described in detail in
the
foregoing drawings and description, the same is to be considered as
illustrative and not
restrictive in character, it being understood that only illustrative
embodiments thereof have
been shown and described. The scope of the claims should not be limited by the
embodiments
set forth in the examples, but should be given the broadest interpretation
consistent with the
description as a whole.
27
