Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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HIGH STRENGTH STEEL PRODUCT WITH IMPROVED FORMABILITY AND
STEEL MANUFACTURING PROCESS
FIELD OF THE INVENTION
The present invention relates to high strength steel products, and more
particularly
to high strength low alloy (HSLA) flat rolled steel products having high yield
strength
and high formability. The invention also relates to manufacturing processes
for
producing flat rolled steel products having high yield strength and high
formability.
BACKGROUND OF THE INVENTION
Most HSLA steels are produced in conventional processes where molten steel
from
a basic oxygen furnace (BOF) or an electric arc furnace (EAF) is cast, cooled,
reheated and reduced in thickness while still hot in a rolling mill. The
rolling mill
reduces the thickness of the slab to produce thin gauge steel sheet or strip
material
having high strength characteristics. Some HSLA steels are produced by modern
thin-slab or medium-slab casting processes in which slabs of steel, still hot
from the
caster, are transferred directly to a reheating or equalizing furnace prior to
thickness
reduction in the hot rolling mill.
HSLA steel products are commonly used for automotive and other applications
where high strength and reduced weight are required. Such applications also
require material having good formability to allow it to be shaped into parts.
Due to the steel microstructure and metallurgical transformations taking place
in the
material during hot rolling, reducing the gauge of the material also causes
the
material to become harder. As the hardness increases, further thickness
reduction
by rolling becomes more difficult, and the rolling mill must operate with
increasing
power levels to reduce the material thickness to the desired level at a
particular
width. Due to the high power required to reduce the thickness, higher strength
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HSLA sheet or strip material, typically having a strength above about 350 MPa,
is
only available in limited widths.
As the strength of the material is increased through rolling, the subsequent
formability of the material in service is reduced. This makes shaping of the
material
more difficult. Thus, rolling the HSLA material to light gauges interferes
with the
ability to shape the material, limiting its utility for many applications
requiring high
strength, light weight and good formability, such as automotive applications.
Therefore, there is a need for HSLA steel products having high strength, thin
gauge
and acceptable formability.
SUMMARY OF THE INVENTION
In one aspect, the present invention provides a process for producing a steel
product comprised of high strength, low alloy steel containing a hardness-
promoting
microalloy and having a yield strength of at least about 100 ksi, the process
comprising: (a) casting molten steel to form a solid, as-cast product having a
thickness, the as-cast product comprising austenite; (b) transferring the as-
cast
product to a first rolling apparatus, wherein a temperature of the as-cast
product as
it enters the first rolling temperature is greater than a recrystallization
stop
temperature of the austenite; (c) conducting a first reduction step in the
first rolling
apparatus to reduce the thickness of the as-cast product by a first amount,
thereby
producing a first thickness-reduced product, wherein a temperature of the as-
cast
product entering the first rolling apparatus and a temperature of the first
thickness-
reduced product exiting the first rolling apparatus are above the
recrystallization
stop temperature; (d) holding the first thickness-reduced product at a
temperature
above the recrystallization stop temperature for a time sufficient to permit
substantially complete recrystallization of the austenite and thereby reduce a
grain
size of the austenite; (e) transferring the first thickness-reduced product to
a second
rolling apparatus; (f) conducting a second reduction step in the second
rolling
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apparatus to reduce the thickness of the first thickness-reduced product by a
second amount, thereby producing a second thickness-reduced product, wherein a
temperature of the first thickness-reduced product entering the second rolling
apparatus and a temperature of the second thickness-reduced product exiting
the
second rolling apparatus are above a phase transformation temperature at which
austenite is transformed to ferrite; (g) cooling the second thickness-reduced
product
to below the phase transformation temperature, thereby producing a cooled
product; and (h) conducting a third reduction step in a third rolling
apparatus to
reduce the thickness of the cooled product by a third amount, thereby
producing the
steel product having a yield strength of at least about 100 ksi.
In another aspect, the present invention provides a process for producing a
high
strength, formable steel product having a yield strength of at least 100 ksi,
comprising: (a) providing a first steel product comprised of high strength,
low alloy
steel containing a hardness-promoting microalloy, the first steel product
having a
yield strength of at least about 70 ksi and less than 100 ksi, the first steel
product
having a formability, as measured by n-value, within a range from about 0.1 to
about 0.16; and (b) cold rolling the first steel product to reduce its
thickness and
increase the yield strength to at least I O0ksi, while maintaining sufficient
formability
such that the high strength, formable steel product can withstand a
longitudinal or
transverse 180 bend of less than 1.0 times its thickness.
In yet another aspect, the present invention provides steel products comprised
of
high strength, low alloy steel containing a hardness-promoting microalloy and
having yield strength of at least about 100 ksi, produced according to the
processes
of the invention.
In yet another aspect, the present invention provides a flat rolled, high
strength,
formable steel product having a yield strength of at least about 100 ksi and
having
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sufficient formability such that it can withstand a longitudinal or transverse
180
bend of less than 1.0 times its thickness, the steel product being comprised
of a
high strength, low alloy steel containing a vanadium-nitride alloy.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described, by way of example only, with reference to
the
accompanying drawings, in which:
Figure 1 is a schematic diagram illustrating the process and apparatus
according to
the invention;
Figure 2 is a graph of yield strength against thickness of HSLA steel produced
according to the present invention;
Figure 3 is a graph of n-value (formability) against thickness of HSLA steel
produced according to the present invention;
Figure 4 is a photograph of a first steel sample according to the invention
having
undergone longitudinal (L) and transverse (T) bending tests; and
Figure 5 is a photograph of a second steel sample according to the invention
having
undergone longitudinal (L) and transverse (T) bending tests.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The process according to the present invention preferably utilizes many of the
same
process steps and apparatus as modern thin slab and medium slab processes for
producing flat rolled steel products. Typical processes of this type utilize a
furnace
to produce molten steel, at least a portion of which may comprise scrap
material.
The molten steel is cast, preferably on a continuous basis, to produce a slab
having
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a thickness of from about 30 to about 200 mm. According to the present
invention,
it is preferred that the hot as-cast slab is directly charged into a reheating
or
equalizing furnace to prevent excessive cooling. However, the process of the
invention is also compatible with processes in which the as-cast slab is
allowed to
cool before further processing.
A preferred process and apparatus according to the present invention are
schematically illustrated in Figure 1. As in known thin slab and medium slab
casting
processes, molten steel 10 is produced in a furnace (riot shown) which may
preferably comprise a BOF or an EAF. The molten steel 10 is withdrawn from the
furnace and is transferred to a ladle 12, also known as a ladle metallurgy
station
(LMS), where alloy elements may be added to the molten steel 10. The molten
steel 10 is transferred from the ladle 12 to a tundish 14. The tundish 14 has
a
nozzle 16 through which the molten steel 10 flows into a water-cooled mold 20
which preferably comprises a continuous casting mold. The steel solidifies in
the
mold 20 to form an as-cast steel product 22 which, as shown in Figure 1,
preferably
comprises a continuous sheet or strip of steel which is shaped and guided
along a
path by rollers 24.
In most known thin slab and medium slab casting processes, the thickness of
the
as-cast product is from about 30 to about 200 mm, typically in the range of
from
about 30 to 80 mm, and more typically from 50 to 75 mm. Even more typically,
the
thickness of the as-cast product is no greater than 50 mm so that the as-cast
material can be directly accepted by a hot rolling strip mill. In the process
of the
present invention, the thickness of the as-cast product is preferably in the
range
from about 70 mm to about 80 mm, more preferably about 70 mm to about 75 mm,
and even more preferably about 72 mm.
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In terms of composition, the steel preferably comprises a high strength low
alloy
(HSLA) steel composition which includes a hardness-promoting microalloy.
Preferably, the microalloy is a vanadium-nitride (V-N) alloy having a
composition
which is the same as or similar to the V-N alloy steel compositions set out in
Table 1
of Glodowski, "Vanadium Microalloying in Steel Sheet, Strip and Plate
Products",
pages 145 to 157, Use of Vanadium in Steel, A Selection of Papers Presented at
the Vanitec International Symposium, Beijing, China, 13-14 October, 2001,
published by Vanitec, Vanadium international Technical Committee, Westerham,
Kent, England, 2002, preferably those having a yield strength of about 550 MPa
or
greater. The Glodowski paper is incorporated herein by reference in its
entirety.
Most preferably, the nitrogen is present in a sub-stoichiometric amount
relative to
the vanadium (i.e. mole ratio of V:N >1:1; weight percent ratio V:N > 3.6:1).
In
addition to vanadium and nitrogen, the steel composition may also contain one
or
more other elements selected from the group comprising carbon, manganese,
silicon, molybdenum, niobium, and aluminum. In a particularly preferred
embodiment of the invention, the steel composition according to the invention
comprises up to about 0.080 wt% carbon, from about 1.00 to about 1.65 wt%
manganese, from about 0.01 to about 0.40 wt% silicon, from about 0.07 to about
0.13 wt% vanadium, from about 0.015 to about 0.025 wt% nitrogen and about
0.008
wt% molybdenum or niobium. In an example of a composition having an
acceptable V:N ratio, the nitrogen content is about 0.020 wt% and the vanadium
content is about 0.10 to about 0.12 wt%.
In terms of microstructure, the as-cast steel product 22 is comprised of a
mixed
austenite structure comprised of grains having a wide range of grain sizes,
ranging
roughly from about 100 pm to about 1,000 pm. The austenite grains in the
surface
regions of the as-cast product 22 tend to be larger columnar grains while
those in
the interior of the as-cast product tend to be smaller particles with a more
spherical
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shape. The grains of the as-cast product are subjected to refinement as
described
below in order to provide a fine grain structure throughout the product and to
attenuate variations in grain size and structure, thereby contributing to the
high
strength and formability of the final product.
As mentioned above, in conventional processes the as-cast slab is cast, cooled
and
reheated prior to entering the strip mill. In order to minimize use of energy
to reheat
the slab, the as-cast steel product in the process of the invention is
preferably not
permitted to cool to ambient temperature after emerging from the continuous
casting mould 20. Preferably, the as-cast product is directly charged into an
equalization or reheating furnace 25 which causes retention of the coarse as-
cast
microstructure. The temperature of the as-cast steel product 22 as it enters
the
furnace 25 is greater than the recrystallization stop temperature, preferably
greater
than about 1020 C, more preferably in the range from about 1020 to about 1200
C,
and even more preferably from about 1050 to about 1200 C.
The temperature inside the equalizing furnace 25 is sufficient to maintain the
temperature of the as-cast product above the recrystallization stop
temperature,
preferably above about 1020 C, more preferably in the range from about 1020 to
about 1200 C, and even more preferably from about 1050 to about 1200 C. This
temperature is sufficiently high to prevent significant precipitation of V-N
particles in
the steel, and to permit recrystallization of austenite, which occurs in
subsequent
process steps. It will, however, be appreciated that the process according to
the
invention includes embodiments in which the as-cast slab is cast, cooled and
reheated as in conventional processes.
In most known thin-slab and medium-slab casting processes, the as-cast product
is
transferred from the equalization furnace directly to a hot rolling strip mill
in which
the product is reduced to its final thickness dimension. In a typical process,
the strip
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mill may reduce the thickness of the steel product from about 50 mm to below
1.5
mm. The strip mill typically comprises about five or six rolling stands which
are
closely coupled together, with a typical interpass time of from about 0.3 to 6
seconds.
In contrast, according to the present invention, the as-cast product 22 is
transferred
directly from the equalization furnace 25 to a rougher 26, also referred to
herein as
a roughing mill. In the rougher 26, the thickness of the as-cast product 22 is
reduced, preferably in one pass, by an amount of from about 40 to about 60% of
the
thickness of the as-cast product, thereby producing a rough-reduced product
28.
For example, where the thickness of the as-cast product is 75 mm, the rougher
reduces the thickness of the product to the range of about 30 to 45 mm. The
rougher 26 is preferably in close proximity to the equalization furnace 25, so
that the
as-cast product 22 is not significantly cooled prior to entering the rougher
26.
Accordingly, the temperature of the as-cast steel product 22 as it enters the
rougher
26 (the "rougher entry temperature") is above the recrystallization stop
temperature,
preferably above about 1020 C, more preferably in the range of about 1020 to
about 1200 C, and even more preferably about 1050 to about 1200 C.
During the roughing operation, the columnar and mixed grains in the as-cast
austenite structure are flattened and elongated. Deformation of the austenite
grains
under selected temperature conditions and for selected periods of time, as in
the
present invention, causes recrystallization of the austenite and results in
reduction
of austenite grain size as well as attenuation of variations in the grain size
and
shape.
Thus, the rougher entry temperature and the temperature of the rough-reduced
steel product 28 as it exits the rougher 26 (the "rougher exit temperature")
must be
sufficiently high to permit recrystallization of the austenite to occur. Most
preferably,
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the rougher entry temperature and the rougher exit temperature are greater
than the
recrystallization stop temperature so as to promote recrystallization of the
austenite.
Also, the rougher entry temperature and the rougher exit temperature are
sufficiently high to prevent significant precipitation of the microalloy
during the
roughing stage. Preferably, the rougher entry and exit temperatures are above
the
recrystallization stop temperature, preferably above about 1020 C and more
preferably in the range from about 1020 to about 1200 C. Even more preferably,
the rougher entry temperature is from about 1050 to about 1200 C and the
rougher
exit temperature is from about 1020 to about 1150 C.
In addition to proper temperature control during the roughing stage, the
inventors
have found that it is important to carefully control the temperature of the
rough-
reduced product 28 after it exits the rougher 26. Specifically, the rough-
reduced
material 28 is preferably held at a temperature high enough and for a time
sufficient
to permit substantially complete recrystallization of the austenite grains,
preferably
such that at least about 90 percent of the austenite grains are within about
100 to
about 400 pm in size. The recrystallized austenite grains tend to be round and
have
an attenuated variation in structure as compared to the as-cast product.
Preferably, the rough-reduced product 28 is held at a temperature greater than
the
recrystallization stop temperature of the austenite, preferably above about
1020 C,
more preferably in the range from about 1020 to about 1200 C, and even more
preferably from about 1020 C to about 1150 C. Preferably, the rough-reduced
product 28 is held at this temperature for a time of from about 10 to about 30
seconds, more preferably from about 15 to about 25 seconds. During this time,
the
relatively coarse austenite grains of mixed shape and size, which have been
flattened and elongated in the rougher 26, recrystallize to the smaller, more
regular
grain size and shape mentioned above.
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In order to ensure that the temperature of the rough-reduced product 28 is
maintained at a suitable level during recrystallization, the rough-reduced
product 28
preferably exits the rougher 26 and is transferred directly to a heating
apparatus
such as a second furnace (not shown) or a heated run-out table 30 having a
temperature sufficient to maintain the temperature of the rough-reduced
product 28
above the recrystallization stop temperature, preferably above about 1020 C,
more
preferably in the range from about 1020 to about 1200 C, and even more
preferably
from about 1020 to about 1150 C.
After the recrystalization step, the rough-reduced product 28 is transferred
to a
second rolling apparatus, preferably a hot rolling strip mill 32, for further
thickness
reduction. Preferably, the strip mill 32 is in close proximity to the heated
run-off
table 30 so that the temperature of the rough-reduced product 28 entering the
strip
mill 32 is substantially the same as the temperature at which the austenite
was
recrystallized, i.e. above the recrystallization stop temperature, preferably
above
about 1020 C, more preferably in the range from about 1020 to about 1200 C,
and
even more preferably from about 1020 to about 1150 C. In other words, the
temperature of the rough-reduced product 28 entering strip mill 32 is
preferably
greater than the recrystallization stop temperature and is greater than a
temperature
at which significant precipitation of microalloy will occur in the strip mill
32.
Furthermore, the temperature of the rough-reduced material 28 is sufficiently
high
so that the temperature of the hot rolled product 46 exiting the rolling mill
is greater
than a temperature at which austenite is transformed to ferrite and is greater
than a
temperature at which significant precipitation of the microalloy will occur.
Preferably, the temperature of the hot rolled product 46 exiting the rolling
mill is
greater than about 820 C, more preferably in the range from about 820 C to
about
950 C. Therefore, the rough-reduced product 28 remains in the austenitic state
during the entire rolling operation and the microalloy essentially remains in
solution
during the entire rolling operation. Furthermore, the rough-reduced product 28
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entering the strip mill 32 is at a temperature sufficient for further
recrystallization to
occur as it passes through the strip mill, resulting in further grain
refinement.
The strip mill 32 itself is of conventional form, comprising a plurality of
rolling stands
in which the thickness of the rough-reduced product is progressively reduced
to
produce the hot rolled product 46 having a thickness of from about 1 mm to
about 6
mm, usually from about 1 mm to about 2 mm. Preferably, the strip mill 32
comprises from four to six stands, and the preferred strip mill schematically
shown
in the drawings comprises a total of five stands 34, 36, 38, 40 and 42. The
time
interval between adjacent rolling stands, also referred to as the "interpass
time" is
preferably from about 0.3 to about 6 seconds. It will be appreciated that the
thickness reduction achieved in the strip mill (measured as a fraction of the
thickness of the hot rolled product 46) may preferably be greater than the
thickness
reduction achieved in the rougher (measured as a fraction of the thickness of
the
as-cast product 22). However, the thickness reduction (measured in mm) is
typically, but not necessarily, greater in the rougher than in the strip mill.
After hot rolling, the product 46 is quickly cooled, preferably at a rate up
to about
70 C by water as shown at 48, to a temperature at which austenite is
transformed to
ferrite, and at which the microalloying elements precipitate. After cooling to
an
appropriate temperature, preferably less than about 820 C, more preferably in
the
range from ambient temperature to about 700 C, even more preferably in the
range
from about 550 C to about 700 C, the flat rolled product 50 is preferably
wound into
a coil 52 and allowed to cool to ambient temperature before further
processing. The
cooled (ambient temperature) product is referred to herein as the flat rolled
steel
product 50.
In most known thin-slab and medium-slab casting processes, the steel entering
the
strip mill retains the columnar and mixed grain structure of the as-cast slab.
Much
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of the recrystallization of the austenite in the prior art processes occurs
between the
first and second rolling stands in the strip mill. However, due to the
relatively short
interpass times in the strip mill, this amount of time is insufficient to
permit complete
recrystallization of the austenite. Thus, the austenitic grain structure of
the product
remains in a relatively variable state and does not achieve the same level of
refinement produced in the process of the present invention. As the product is
rolled it becomes stronger, making further thickness reduction difficult. On
known
thin-slab and medium-slab processes which do not utilize a rougher, the entire
thickness reduction from the as-cast product to the final product must be
accomplished in the strip mill. As the gauge is reduced, the power required to
achieve the final dimensions increases and as the mill works harder, it
becomes
more difficult to keep tolerances within acceptable limits.
In the process of the present invention, the added recrystallization step
provides the
rough-reduced steel product with increased grain refinement over the as-cast
product. It is known that grain refinement is a major strengthening mechanism
and
therefore the flat rolled steel product 50 has high strength, typically
exceeding 70ksi
and preferably having a strength of at least about 550 MPa (80ksi). In this
regard,
Figure 2 graphically illustrates a plot of yield strength against thickness
(gauge),
which shows that flat rolled steel product produced according to the invention
has
high yield strength, in excess of 80 ksi, typically 80 to 90 ksi, regardless
of the
gauge to which it is reduced. However, since there is little or no
precipitation of the
microalloy until after the material passes through the strip mill, the
material being
rolled is relatively "soft" as compared to known processes. Therefore, less
power is
required to roll the material in the strip mill 32 and there is a
corresponding
improvement in dimensional control. Since power required by the strip mill is
a
function of volume and cross-sectional area of the material being rolled, the
reduced
power demands of the process according to the invention also permits the
production of material having greater width dimensions than previously
possible.
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The inventors have also found that the flat rolled steel product 50 according
to the
invention possesses greater formability than materials produced by prior art
thin-
slab and medium-slab casting processes. As mentioned above, formability is
important in the production of shaped parts. Formability is represented by an
"n-
value" determined in accordance with ASTM A646 (00), Tensile Strain Hardening
Exponents (n-value) of Metallic Sheet Material, a longitudinal tensile test.
The
inventors have surprisingly found that the formability of the flat rolled
steel product
50 is essentially independent of the thickness to which the product is rolled
in the
strip mill 32. This is shown graphically in Figure 3, which comprises a plot
of the n-
value against thickness of the product. The n-values achieved according to the
method of the invention are preferably above about 0.1, more preferably in the
range from about 0.1 to about 0.16. Even more preferably, the n-values are
about
0.13. Thus, the formability of the steel is preserved independently of the
level of
thickness reduction in the strip mill, permitting the production of formable
high
strength steel in a wide range of gauges.
In the process according to the invention, the yield strength of the flat-
rolled steel
product 50 is increased from the 80 ksi range to about 100 ksi (690 MPa) or
higher.
This process involves the preparation of a high strength, formable flat rolled
product
50 by the process steps described above, and then further reducing the
thickness
(gauge) of the flat rolled product 50 by about an additional 2 to 20%, more
preferably by about an additional 5 to 20%, to produce a cold-rolled product
60.
Preferably, the further reduction in gauge is obtained by cold rolling the
flat rolled
product 50 in a cold rolling mill 54, preferably starting from ambient
temperature. As
shown in Figure 1, the flat rolled product 50, after cooling to a temperature
which is
at or near ambient temperature, is unwound from coil 52 and fed to the cold
rolling
mill 54. The cold rolling mill comprises one or more rolling stands 56, each
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comprising a pair of rollers, and may preferably comprise a reversing cold
mill. In
Figure 1, only a single rolling stand 56 is shown.
The number of passes and/or the number of rolling stands is selected to
achieve the
desired thickness and physical properties. in a preferred example where the
desired final thickness of the cold-rolled product 60 is from about 1.0 to
about 4 mm,
the thickness reduction can typically be obtained in one or two passes.
Instead of a
cold rolling mill 54, it may be preferred to cold roll the material in a
temper mill to
achieve the desired gauge reduction using multiple passes, if necessary. In
some
embodiments of the invention, the desired final thickness of the cold-rolled
product
60 may be in the range from about 1.0 to about 1.5 mm.
The inventors have found that the additional reduction step may produce a
corresponding decrease in formability of the cold rolled product 60 as
compared to
the flat rolled product 50. However, the inventors have found that the
formability of
the cold rolled product is still within acceptable limits for its intended end
uses.
Testing of steel samples according to the present invention has shown that
cold
rolling of the flat rolled steel product 50 simultaneously brings about an
increase in
strength and a decrease in formability. For example, where the strength of a
flat
rolled steel product 50 is increased from the range of about 80 to 90 ksi to
above
100 ksi by the process of the invention, the formability of the cold rolled
product 60
is such that it can withstand a longitudinal or transverse 1800 bend of less
than 0.5
T radius with no cracking in the longitudinal or transverse directions, where
T is the
thickness of the material. Shown in Figure 4 is a sample of 100 ksi cold
rolled
product 60 which has been bent 180 longitudinally (L) and transversely (T)
about a
0.3T radius without cracking in either direction.
...,...,..
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By further increasing the amount of cold reduction, the strength of the flat
rolled
steel product 50 can be increased from the range of about 80 ksi to 90 ksi to
at least
about 110 ksi, with a further decrease in formability. The inventors have
found that
110 ksi cold rolled product 60 is able to withstand a longitudinal or
transverse 180
bend of less than 1T radius with no cracking in the longitudinal or transverse
directions. Figure 5 illustrates a sample of 110 ksi cold rolled product 60
which has
been bent 180 longitudinally (L) and transversely (T) about a IT radius
without
cracking in either direction.
Preferably, oxide scale on the surface of the flat rolled product 50 is
removed prior
to the cold rolling step. The oxide scale, which may comprise iron oxides
Fe2O3,
Fe304 and FeO, is preferably removed by "pickling" the cold-rolled product,
i.e.
treating it with hot acid, preferably HCI, to dissolve and remove the oxide
scale. In
the preferred embodiment shown in Figure 1, the flat rolled product 50 is
passed
through at least one pickling tank 62 containing hot hydrochloric acid prior
to
entering the cold rolling mill 54.
In the prior art, steel having a strength level of 100 ksi is produced by
heavy alloying
of the hot rolled product, by recovery annealing or by heat treating to
achieve
microstructures other than ferrite/pearlite. Annealing is done to relieve the
work
hardening of the product through cold reduction and somewhat improves the
formability of the material. In the process of the present invention, the
yield strength
is significantly increased without an inhibiting reduction in formability, and
therefore
annealing is not required.
Once it emerges from the cold rolling mill 54, the high strength cold rolled
product
60 is preferably wound onto coils 64 for shipment to the end user.
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As stated throughout this application, the temperature of the steel product as
it
passes through the rougher and the strip mill is greater than the
recrystallization
stop temperature and above a temperature at which significant precipitation of
the
microalloy will occur. It will be appreciated that these temperatures are not
necessarily greater than the precipitation start temperature of the microalloy
which,
for vanadium nitride microalloys, is typically in the range from about 950 to
1110 C.
In fact, it has been found that there will be some microalloy precipitation at
even
higher temperatures. It will be appreciated that microalloy precipitation is a
solid
state reaction which is controlled by diffusion, and is therefore time-
dependent.
Therefore, even at temperatures below the precipitation start temperature,
there will
be little precipitation of microalloy until after the steel product exits the
strip mill. In
other words, the driving force for precipitation is small as the steel passes
through
the rougher and the strip mill at relatively high temperatures, and becomes
greater
as the steel is cooled to coiling temperatures, such that the precipitation is
driven to
completion.
The term "recrystallization stop temperature" as used herein is the
temperature
above which the austenite grains in the steel product reform, i.e.
recrystallize, into
lower energy configurations. The recrystallization stop temperature is
dependent on
the composition of the steel, and for preferred steel products of the type
described
and claimed in this application having vanadium nitride microalloys, the
recrystallization stop temperature is typically about 1020 C.
Although the invention has been described in connection with certain preferred
embodiments, it is not restricted thereto. Rather, the invention includes
within its
scope all embodiments which fall within the scope of the following claims.
