Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF T~E INVENTION
This invention relates to the production of small
diameter austenitic stainless steel products of the type
commonly used in the cold heading trade. As herein
employed, the term "small diameter" de~ines products
ranging in diameter from about 4.0 to 5.5 mm~
A combination of problems, including lack of
stiffness, unacceptable tolerances, surface scratching in
the laying heads, etc. has heretofore made it impossible
to consistently roll such products as rods on a rod mill.
Thus, such products have been conventionally produced as
heat treated drawn wires, for which a price premium is
paid.
SUMMARY OF THE PRESENT INVENTION
A basic objective of the present invention is to
overcome to above-mentioned problems, thereby making it
possible to consistently produce small diameter austenitic
stainless steel products as rods on a rod mill, at a
considerable cost saving as compared to conventional wire
drawing techniques.
In a preferred embodiment of the invention to be
described hereinafter in more detail, this is accomplished
by continuously hot rolling a process section, for example
a reheated billet, into a small diameter rod by passing the
~5 same through a succession of roughing and intermediate stands
and then through a finishing block. The process section is
water cooled prior to its entry into as well as during its
passage through the finishing block. The resulting reduction
in rod surface temperature to about 900C is sufficient to
advantageously increase the stiffness of the rod emerging
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from the Einishing block. As herein employed, the term
"about", when used in connection with temperatures, defines a
range of + 50C.
After being allowed to substantially equalize to a
bulk temperature of about 950C, the thus stiffened rod is
passed through a sizing mill which imparts to the rod a
tolerance of about + .04 mm. As herein employed, the term
"bulk temperature" means the equalized temperature between the
core end surface of the rod, and the term "sizing mill" means
one or more roll passes wherein the total reduction imparted
to the rod is 10% or less. The rod exiting from the sizing
mill is then subje~ted to additional water cooling prior to
being directed through a laying head where it is formed into a
continuous series of rings. This additional water cooling
will chill the surface of the rod to a temperature of about
650C. This surface chilling enables the rod to resist
scratching during passage through the laying head.
The rings formed by the laying head are received on
a conveyor along which they are transported in a mutually
offset relationship. While moving along the conveyor, the
offset rings are sequentially reheated to an elevated bulk
temperature of about 1100C. Minimum energy is required to
reheat the rod due to the fact that the preceding water quench
only affected the surface of the rod, with the core portion
remaining at a fairly elevated temperature. After reheating,
the rings are water quenched at a rate of between about 200 to
800C/sec. to a surface temperature of about 300C, with an
accompanying decrease in core temperature to about 750C. Air
cooling of the rings then continues at a more gradual rate
while drying the same. The rings are then collected from the
delivery end of the conveyor into cylindrical coils.
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This combination of steps will enable small
diameter, close tolerance, scratch-free, heat treated stain-
less steel rod to be rolled on a rolling mill at significantly~
reduced costs as compared to conventional wire drawing
methods. For example, it is conservatively estimated that,
with the cost of all other things being equal, e.g., raw
materials, fuel and labor, the above-describe~ process will
make it possible to achieve approximately a S0% cost savings
per ton as compared to the production of conventional heat
treated drawn wires.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure lA is an illustration of one portion of a
rod mill in accordance with the present invention, including
the billet reheat furnace and the roughing and intermediate
mill stan~s; and
Figure lB is a graph illustrating surface, core
and bulk temperatures of a rod being processed through the
remaining portion of the same rod mill, with the mill com-
ponents being shown along the horizontal axis of the graph,
and with the vertical axis of the graph being incrementally
subdivided into C.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENT
It will be understood that the components in the
illustrated rod mill are well known to those skilled in the
art. Consequently, they have been shown in diagrammatric
form, since the invention resides not in the specific form of
such components, but rather in the method or process of
operating them in combination.
Referring initially to Figure lA, one portion of a
rod mill is shown comprising a furnace 2 in which process
sections such as discrete billets are reheated to a rolling
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temperature of above about 1040 to 1260C. The billets are
extracted from the furnace by conventional means (not shown)
and are continuously rolled along the mill pass line 4 through
a series of roughing stands 6, and then through a series of
intermediate st2nds 8. Thereafter, and with reference to
Figure lB, the semifinished product continues through a
finishing block lO.
Although the successive work roll pairs of the
finishing block have been illustrated horizontally, those
skilled in the art will appreciate that in actual practice,
the roll axes of successive roll pairs are offset by 90 so
as to eliminate any twisting of the product as it progresses
through the finishing block. A typical finishing block of
this type is shown, for example, in U. S. Patent No. RE
28,107.
In accordance with the present invention, the
finishing block lO has been modified to incorporate water
cooling nozzles between the successive roll pairs. As
. schematically depicted by the arrows in the drawing, these
nozzles apply water to the surface of the product as it passes
through the finishing block.
The finishing block 10 is preceded by a water box
12 which also can be of conventional design, having a
succession of water nozzles through which the product is
directed after leaving the last roll stand 8 of the preceding
intermediate train. ~gain, as schematically depicted by the
arrows in the drawing, the water nozzles of cooling box 12
apply cooling water to the surface of the product passing
therethrough.
The finishing block lO is followed by a sizing
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mill 14 which in turn is followed by another water box 16.
The water box 16 is followed by a set of driven pinch rolls
18 which propel the product into and through a laying head 20.
The laying head forms the product into a continuous series of
rings 22 which are received on an open conveyor 24. The con-
veyor, which may be of any conventional chain or roller type,
transports the rings in a mutually offset relationship along a
path leading to a reforming station 26. As the offset rings
move along the conveyor path, they ~ass through a furnace 28
which may be of conventional design and heated by gas burners,
radiant heaters or the like. As the offset rings exit from
the furnace, they are quenched by water spray nozzles 30, and
then air cooled by means of a fan 32 acting through a plenum
34 underlying the conveyor. The rings are then collected from
the delivery end of the conveyor into coils at the reforming
station 26.
The operation of the foregoing installation will
now be described with reference to a typical example wherein
5.5 mmO diameter stainless steel rod is finish rolled at a
mill delivery speed of 80m./sec. As the product enters the
water box 12, it has a diameter of approximately 18mm, a
surface temperature o about 1140C, and it is travelling at a
speed of about 7~5m./sec. The water noz~les of the water box
12 operate to lower the surface temperature of the product to
about 925C, with an accompanying lowering of the core temper-
ature down to about 1120C. Thereafter, the surface and core
temperatures are allowed to substantially equalize to about
1037VC before the product enters the finishing block 10.
As the product progresses through the roll
passes of the finishing block, it experiences successive
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elongations accompanied by reductions in cross-sectional area.
In the diagrammatic representation of Figure lB, the finishing
block is illustrated as having three successive roll passes.
Those skilled in the art will appreciate, however, that in
actual practice the finishing block will normally include 8-10
roll passes. During this finish rolling, the water cooling
nozzles between the successive roll pairs of the finishing
block operate to intermittently lower the surface temperature
~f the product by increments averaging about 50C. ~owever,
because of the energy being imparted to the product during
finish rolling, the surface temperature rises between each
water application, and the core temperature only gradually
decreases with the net result being that as the rod emerges
from the finishing block, its surface temperature is about
930C, and its core temperature is about 1000C. The surface
and core temperatures then equalize to a bulk temperature of
about 960C as the product enters and passes through the
sizing mill 14. Since little if any reduction is being taken
in the sizing mill, the core and surface temperatures remain
relatively constant until the product enters the next water
box 16. At a bulk temperature of about 960C, the product has
sufficient inherent stiffness to enter and pass through both
the si~ing mill 14 and the water box 16 without cobbling or
breaking out.
The cooling in water box 16 rapidly and drastically
reduces the surface temperature of the product to about 660C,
with an accompanying relatively modest decrease in the core
temperature to about 940C. Thereafter, the surface and core
temperatures equalize to a bulk temperature about 870C as
the product continues through the driven pinch rolls 18
and the laying head 20. The reduced surface temperature
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allows the product to resist scratching or marking that might
otherwise result from frictional contact with the internal
guide surfaces of the laying head.
As previously indicated, the product rings 22 form-
ed by the laying head are received in an offset pattern on the
conveyor 24. As the offset rings move along the conveyor,
the~ enter the furnace 28 where they experience reheating to a
bulk temperature of about 1080C. Since the rings had rapidly
equalized to a bulk temperature of about 870C after the pre-
ceding water quench, minimum energy is consumed in reheating
the rings in the furnace 28. As the rings emerge from the
furnace, they are subjected to the water sprays 30 which
reduce their surface temperature at a rate of approximately
200C/sec to about 320C, with an accompanying decrease in
core temperature to just above 750C. As the rings emerge
from beneath the water noz~les 30, they pass over the plenum
34 which directs an upward flow of air from fan 32 through the
rings. This further reduces the surface temperature of the
rings to under 200C while also drying the rings. This
sequential reheating and rapid surface quenching is effective
in achieving the solid solution treatment required for stain-
less steel products. The rings are then accumulated into
coils at the reforming station 26. As an alternative, the
rings could be water cooled such that the core temperature is
brought down to about 300C. However, this would create a
substantial difference in temperature between the surface and
the core. Hence, the combination of water and air cooling is
preferred.
It thus will be seen that the successive steps of
the above-described process cooperate with one another to
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achieve the desired result. More particularly, the water
cooling in water box 12 and finishing block 10 increases the
stiffness of the small diameter product, thereby enabling the
product to pass through downstream equipment, including the
sizing mill 14 and the water box 16 without cobbling or
breaking out. The sizing mill imparts the required close
tolerance of about + 0.04 mm. to the rod. The additional
cooling in water box 16 chills the product surface, thereby
enabling the product to withstand surface scratching in the
laying head 20. The laying head cooperates with the conveyor
24 in arranging the product in a continuous series of offset
rings moving along the conveyor path. The furnace 28 and
water sprays 30 operate sequentially on the moving offset
rings to achieve a substantially uniform solid solution
treatment, with minimum energy consumption being involved in
the reheating cycle, and the fan 32 and plenum 34 operate to
further cool and dry the offset rings before they are finally
gathered into coil form.
The resulting product is a hot rolled, heat-
treated, scratch-free, close tolerance, small-diameter
stainless steel rod.
I claim:
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