U.S. patent number 3,615,926 [Application Number 04/819,756] was granted by the patent office on 1971-10-26 for quench system.
This patent grant is currently assigned to Inland Steel Company. Invention is credited to Harold L. Taylor.
United States Patent |
3,615,926 |
Taylor |
October 26, 1971 |
QUENCH SYSTEM
Abstract
Low-carbon unalloyed steel strip is heated to a temperature at
least above the A.sub.1 point, preferably above the A.sub.3 point,
and is quenched to transform substantially all the austenite to
martensite. Quenching is accomplished by passing the heated strip
through an elongated restricted quench channel having high-velocity
quench liquid flowing through the channel, either concurrently or
countercurrently. The temperature and flow velocity of the quench
liquid are regulated in the channel so as to provide an initial
high rate of heat withdrawal from the strip and a subsequent lower
rate of heat withdrawal during the time the strip is quenched
through the temperature range of martensite formation, thereby
effecting tempering of the martensite.
Inventors: |
Taylor; Harold L. (Hammond,
IN) |
Assignee: |
Inland Steel Company (Chicago,
IL)
|
Family
ID: |
25228968 |
Appl.
No.: |
04/819,756 |
Filed: |
April 28, 1969 |
Current U.S.
Class: |
148/661; 148/657;
148/663; 148/320; 148/664 |
Current CPC
Class: |
C21D
9/52 (20130101); C21D 1/64 (20130101) |
Current International
Class: |
C21D
1/62 (20060101); C21D 9/52 (20060101); C21D
1/64 (20060101); C21d 001/18 () |
Field of
Search: |
;148/143,145,153,36,155,156,157 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Dean; Richard O.
Claims
I claim:
1. In the method of quenching a strip of plain carbon steel having
a carbon content of from about 0.03 weight percent to about 0.25
weight percent and a manganese content of from about 0.20 weight
percent to about 0.60 weight percent wherein the strip is heated to
a temperature above the A.sub.1 critical point so as to at least
partially austenitize the steel and thereafter the heated strip is
passed through an elongated restricted quench channel and quench
liquid is also passed through said channel for quenching the strip
at a rate in excess of the critical cooling rate so as to transform
substantially all of the austenite to martensite; the improvement
which comprises controlling the quench conditions in one portion of
said channel adjacent the strip entry end thereof so as to obtain a
relatively high heat withdrawal rate during the initial portion of
the quench period and thereby rapidly lowering the strip
temperature to the range of martensite formation, and controlling
the quench conditions in the remaining portion of said channel so
as to obtain a relatively lower heat withdrawl rate during
quenching through the temperature range of martensite formation,
thereby effecting tempering of the martensite.
2. The method of claim 1 further characterized in that said quench
conditions are controlled by regulating the temperature and flow
velocity of the quench liquid in the respective portions of the
quench channel.
3. The method of claim 1 further characterized in that the heat
withdrawal rate in said one portion of said channel is from about 1
.times. 10.sup.6 to about 3.times.10.sup.6 B.t.v./sg. ft./hr. and
the heat withdrawal rate in said remaining portion of said channel
is from about 0.25.times.10.sup.6 to about 0.5.times.10.sup.6
B.t.u./sq. ft./hr.
4. The method of claim 1 further characterized in that the time for
lowering the temperature of the heated strip to the range of
martensite formation is from about 0.35 sec. to about 1.33 sec.,
and the time for quenching the strip through the temperature range
of martensite formation is from about 1.24 sec. to about 3.6
sec.
5. The method of claim 1 further characterized in that the quench
liquid passed through said one portion of said channel is at a
lower temperature than the quench liquid passed through said
remaining portion of said channel.
6. The method of claim 5 further characterized in that said lower
temperature is from about 35.degree. F. to about 90.degree. F. and
the quench liquid passed through said remaining portion of said
channel is at a temperature of from about 120.degree. F. to about
200.degree. F.
7. The method of claim 1 further characterized in that said channel
is disposed vertically and said strip and said quench liquid are
passed downwardly through said channel in concurrent relation.
8. The method of claim 1 further characterized in that said channel
is disposed vertically and said strip is passed downwardly through
said channel and said quench liquid is passed upwardly through said
channel in countercurrent relation.
9. The method of claim 1 further characterized in that one portion
of said quench liquid is introduced into one end of said channel
and another portion of said quench liquid is introduced into said
channel at a submerged location within said channel adjacent to but
spaced from the strip entry end of the channel.
10. The method of claim 9 further characterized in that said other
portion of said quench liquid is directed in the form of liquid
sheets substantially perpendicularly against the opposite surfaces
of the strip and extending uniformly across the width of the
strip.
11. The method of claim 9 further characterized in that said strip
and said quench liquid are passed through said channel in
concurrent relation, and said other portion of said quench liquid
has a higher temperature than said one portion of said quench
liquid.
12. The method of claim 11 further characterized in that said
quench liquid comprises an aqueous liquid, said one portion of said
quench liquid being at a temperature of from about 35.degree. F. to
about 90.degree. F. and said other portion of said quench liquid
being at a higher temperature within the range of from about
120.degree. F. to about 200.degree. F.
13. The method of claim 9 further characterized in that said strip
and said quench liquid are passed through said channel in
countercurrent relation, and said one portion of said quench liquid
has a higher temperature than said other portion of said quench
liquid.
14. The method of claim 13 further characterized in that said
quench liquid comprises an aqueous liquid, said other portion of
said quench liquid being at a temperature of from about 35.degree.
F. to about 90.degree. F. and said one portion of said quench
liquid being at a higher temperature within the range of from about
120.degree. F. to about 200.degree. F.
15. In the method of quenching a strip of plain carbon steel having
a carbon content of from about 0.03 weight percent to about 0.25
weight percent and a manganese content of from about 0.20 weight
percent to about 0.60 weight percent wherein the strip is heated to
a temperature above the A.sub.1 critical point so as to at least
partially austenitize the steel and thereafter the heated strip is
passed through an elongated restricted quench channel and quench
liquid is also passed through said channel for quenching the strip
at a rate in excess of the critical cooling rate so as to transform
substantially all of the austenite to martensite, one portion of
said quench liquid being introduced into one end of said channel
and another portion of said quench liquid being introduced into
said channel at a submerged location within said channel adjacent
to but spaced from the strip entry end of the channel; the
improvement which comprises:
regulating the temperature and flow velocity of the quench liquid
in one portion of said channel adjacent the strip entry and thereof
so as to obtain during the initial portion of the quench period a
relatively high heat withdrawal rate which is sufficient to rapidly
lower the strip temperature to the martensite start temperature;
and
regulating the temperature and flow velocity of the quench liquid
in the remaining portion of said channel so as to obtain, during
quenching from the martensite start temperature to the martensite
finish temperature, a heat withdrawal rate which is less than the
heat withdrawal rate in said one portion of said channel, thereby
effecting increased tempering of the martensite.
16. The method of claim 15 further characterized in that said strip
and said quench liquid are passed through said channel in
concurrent relation, and said other portion of said quench liquid
has a higher temperature than said one portion of said quench
liquid.
17. The method of claim 16 further characterized in that said
quench liquid comprises an aqueous liquid, said one portion of said
quench liquid being at a temperature of about 35.degree. F. to
about 90.degree. F. and said other portion of said quench liquid
being at a higher temperature within the range of from about
120.degree. F. to about 200.degree. F.
18. The method of claim 15 further characterized is that said strip
and said quench liquid are passed through said channel in
countercurrent relation, and said one portion of said quench liquid
has a higher temperature than said other portion of said quench
liquid.
19. The method of claim 18 further characterized in that said
quench liquid comprises an aqueous liquid, said other portion of
said quench liquid being at a temperature of from about 35.degree.
F. to about 90.degree. F. and said one portion of said quench
liquid being at a higher temperature within the range of from about
120.degree. F. to about 200.degree. F.
Description
This invention relates to a novel method for continuously quenching
metal strip and more particularly for quenching low-carbon
unalloyed steel strip to obtain a microstructure which is at least
partially martensitic and preferably fully martensitic.
The principal constituents of steel which determine its properties
are ferrite and cementite. At a relatively high temperature, which
is dependent upon the carbon content, steel exists in the form
known as austenite which is a solid solution of carbon or cementite
in ferrite. When steel is cooled slowly from a high temperature at
which austenite is stable, the ferrite and cementite precipitate
together in a characteristic lamellar structure known as pearlite.
However, dependent upon the rate of quenching and other factors,
the transformation from austenite to pearlite proceeds through a
series of different microstructures. The low-temperature
transformation product in the transformation of austenite upon
cooling is martensite which is a body-centered tetragonal structure
in which the carbon atoms are thoroughly dispersed. Martensitic
steels are characterized by high tensile and yield strengths.
Plain carbon steels of relatively high carbon content and certain
alloy steels, particularly those containing hardenability agents
such as boron, are more easily quenched to a martensitic structure,
but the plain carbon steels of relatively low carbon content are
considerably more difficult to quench to martensite. As will be
understood from the customary isothermal transformation diagrams,
plain carbon steels of low carbon content (0.03-0.25 weight
percent) require extremely rapid quenching in order to transform
substantially all the austenite to martensite. In general, it may
be stated that low carbon steel strip must be quenched from the
austenitizing temperature to below the temperature for the start of
martensite formation in about 0.1 to about 0.8 seconds. In
addition, the quenching must be accomplished uniformly so as to
obtain a uniform microstructure and so as to avoid excessive
warpage or distortion of the strip.
In my prior U.S. Pat. No. 3,410,734, granted Nov. 12, 1968, I have
described and claimed a channel quench system which is particularly
adapted for the continuous quenching of low-carbon steel strip to
obtain a fully or partially martensitic microstructure. In such
system, it is found that in most cases a certain amount of in situ
tempering or self-tempering of the martensite can occur during the
quenching of the strip because the temperature range of martensite
formation is relatively high for plain carbon steels of low-carbon
content (0.03 to 0.25 weight percent). Consequently, in spite of
the rapidity of the quench provided by such system, a significant
degree of tempering can take place during the short time required
to cool the strip from the martensite transformation temperature
range to ambient temperature.
However, it is desirable to be able to regulate the extent of
tempering of the martensite rather than to be dependent solely upon
the uncontrolled self-tempering effect which occurs as described
above. By modifying the operation of a channel quench system, such
as the one disclosed in my aforementioned patent, it is possible to
provide selective control over the heat withdrawal rate in
different portions of the quench system and thereby achieve a
controlled degree of tempering. Thus, within limits, it is possible
to tailor the quench action to the chemistry of the steel and other
process variables so as to obtain varying combinations of physical
properties in the end product.
Accordingly, the broad object of the invention is to provide a
novel and improved quench system for making low-carbon martensitic
steel strip.
A further object of the invention is to provide a novel and
improved method for quenching low-carbon steel strip in a channel
quench system so as to obtain at least a partially martensitic
microstructure with a desired degree of tempering of the
martensite.
Another object of the invention is to provide a novel and improved
quench method for making low-carbon martensitic steel strip
characterized by the provision of different heat withdrawal rates
in different portions of the quench system so as to control the
extent of tempering of the martensite.
An additional object of the invention is to provide a novel and
improved method for increasing the extend of tempering of
martensite during the quenching of low-carbon steel strip to form
martensitic steel strip.
Other objects and advantages of the invention will become apparent
from the subsequent detailed description taken in conjunction with
the accompanying drawing, wherein:
FIG. 1 is a schematic vertical sectional view of one embodiment of
a quench apparatus for the production of martensitic steel strip in
accordance with the present invention; and
FIG. 2 is a similar view of another embodiment of the quench
apparatus.
The quench system of the present invention is illustrated in the
drawing as embodied in a continuous heat-treating and quenching
line for making martensitic steel strip. The resulting martensitic
steel strip may be used as such or it may be tin plated,
galvanized, or aluminum coated.
The steel strip starting material is plain carbon steel having the
following composition range (weight percent): carbon 0.03-0.25,
manganese 0.20-0.60, phosphorus 0.05 max., sulfur 0.03 max., and
the balance iron with residual elements in the usual amounts.
Preferably, the steel strip starting material is in work-hardened
or as-cold reduced condition, and although the gauge of the strip
will usually and preferably be within the range of from about 0.002
to about 0.050 inch, the invention in its broadest aspect is also
applicable to steel strip having a thickness as low as about 0.0002
inch and as high as about 0.100 inch.
As seen in FIG. 1, steel strip 10 is fed downwardly through a
furnace, shown fragmentarily at 15, after first passing through
conventional cleaning and rinsing steps (not shown) in which the
residual rolling oil is removed. In the furnace 15 the steel strip
is heated to a uniform temperature above the A.sub.1 critical point
so that the steel is at least partially austenitized. This
temperature may range from about 1,330.degree. F. to as high as
about 2,100.degree. F., dependent upon the carbon content, but from
a practical standpoint effective results may be obtained within the
range of from about 1,330.degree. F. to about 1,750.degree. F. In
order to obtain a fully martensitic product the steel strip must be
heated above the A.sub.3 critical point, i.e. to a temperature
within the range of from about 1,525.degree. F. to about
2,100.degree. F. and particularly within the range of from about
1,525.degree. F. to about 1,750.degree. F. Immediately upon leaving
the furnace 15 the heated strip passes into the quench system,
designated generally at 20, where the strip is rapidly quenched to
ambient or room temperature so as to obtain at least a partially
martensitic microstructure. Preferably, the strip is quenched at a
rate in excess of the critical cooling rate so that substantially
all of the austenite is transformed to martensite.
The quench system 20 comprises a tank 30 provided with a strip exit
chute 31 and containing a sinker roll 32. Extending upwardly from
the top wall of the tank 30 is an elongated conduit 34 of
rectangular cross section which provides a restricted quench
channel 35. A trough 36 having a rectangular cross section
surrounds the upper end of the conduit 34. A rectangular baffle 37
depends from an inwardly extending peripheral flange 33 on the
trough 36, the baffle 37 surrounding the conduit 34 and terminating
above the bottom of the trough 36. Extending downwardly from the
outlet end of the furnace 15 is a tubular connecting or seal
section 38 the lower end of which extends into the trough 36 below
the upper end of the conduit 34 and disposed between the conduit 34
and the baffle 37. Water (or other quench liquid) is supplied to
the trough 36 by inlet pipes 39, the water flowing downwardly and
thence upwardly around the baffle 37 and then overflowing the open
upper end of the conduit 34, as indicated by the arrows. The quench
water and the strip 10 pass concurrently downwardly through the
channel 35 into the tank 30. The strip 10 passes around the sinker
roll 32 and then outwardly through the chute 31. The quench water
flows from the tank 30 upwardly through the chute 31 and is
discharged through a side outlet 39' .
Since the level of the water overflowing the upper end of the
conduit 34 is above the lower end of the seal section 38, it will
be seen that the section 38 is sealed by the water in the trough 36
so as to prevent infiltration of air into the furnace 15. If
desired, a reducing or other nonoxidizing gas may be supplied to
the section 38 (by means not shown) for passage upwardly through
the furnace 15, thereby preventing oxidation of the strip. A
plurality of view ports 40 are provided in the tubular section 38
and the conduit 34 to permit observation of the quench action.
Submerged jet or spray units 41 are provided in the opposite walls
of the conduit 34 somewhat below the upper end of the conduit for
directing streams of liquid toward opposite sides of the strip 10
across the entire width thereof. In the embodiment shown in FIG. 1,
two superimposed units 41 are mounted at each side of the strip 10,
but any desired number of such units may be used. Each unit 41 is
supplied with water or other quench liquid by means of a supply
pipe 42.
As more fully described in my aforementioned prior patent, each
spray unit 41 comprises an elongated boxlike structure having the
liquid supply pipe 42 extending into an opening in its rear wall
and having an elongated discharge orifice 43 in its front wall
extending substantially the entire length of the unit 41 and across
the entire width of the strip 10. The interior of the enclosure is
provided with a pair of elongated oppositely disposed baffles 44
extending between the ends of the enclosure and dividing the same
into parallel interconnected chambers. The quench liquid in passing
from the inlet 42 to the orifice 43 follows a tortuous path in
which the direction of flow is reversed several times, thereby
insuring a uniform distribution of liquid across the entire length
of the orifice opening 43.
As will be evident from the drawing, the submerged spray units 41
are mounted in suitable openings in the walls of the conduit 34 so
that thin sheets or curtains of water are directed from the
orifices 43 substantially perpendicularly toward opposite sides of
the strip 10. In fluid kinematics terminology, the unitary sheet or
curtain of water from an elongated rectangular orifice, such as 43,
may be characterized as having two-dimensional flow, i.e. the flow
is identical in parallel planes so as to extend uniformly across
the width of the strip 10.
As the heated steel strip 10 moves downwardly from the furnace 15
it passes through the seal section 38 and enters the upper end of
the water-filled quench channel 35 where it is immediately immersed
in the downwardly flowing stream of water. In addition, the
submerged spray units 41 direct water streams against the strip in
a direction generally perpendicular to the path of movement of the
strip, thereby creating a high degree of turbulence in the
uppermost portion or strip entry end of the quench channel. The
dimensions of the conduit 34 are restricted so as to provide in the
channel 35 a high velocity of water flow relative to the strip 10
while at the same time allowing sufficient clearance to permit
passage of the strip 10 without scraping the walls of the
conduit.
As previously mentioned, uniformity of quenching is essential not
only for the sake of obtaining a strip having uniform
microstructure and uniform physical properties but also to avoid
warpage and distortion of the strip. Irregular vaporization of the
water or other quenching medium in contact with the strip can
result in substantial differentials in heat transfer rates between
portions of the strip surface in contact with liquid water and
other portions in contact with water vapor. These differentials
cause different rates of contraction in the steel strip and result
in quenching stresses and deformation.
However, in the quench system of the present invention the desired
uniformity of quenching is realized as a result of several
cooperating factors. The provision of the restricted quench channel
35 results in a high water velocity relative to the strip 10 which,
by way of example, may be on the order of 1 to 10 feet/second in a
direction generally parallel to the strip 10. Furthermore, the
submerged sprays 41 are designed so as to provide water streams in
a direction generally perpendicular to the strip 10 at relatively
low pressures and relatively high flow rates so as to create
substantial turbulence within the channel adjacent the strip entry
end thereof. For example, the water pressure in the sprays 41 may
be on the order of 20 to 30 p.s.i. at the inlets 42 and on the
order of 5 to 10 p.s.i. at the discharge orifices 43. Although the
submerged jets 41 are designed to create internal turbulence in the
quench channel, nevertheless, the surface of the liquid in the
channel 35 where the strip 10 first contacts the quench liquid is
maintained substantially smooth, nonsplashing, and nonturbulent so
that every point across the width of the strip makes initial
contact with the quench liquid at substantially the same time. The
low pressure of the high flow rate submerged jets 41 makes it
possible to provide the desired surface smoothness while at the
same time providing the required internal turbulence below the
liquid surface. Thus, a quenched strip of at least partially
martensitic structure is obtained which is either flat enough for
its intended use or can easily be rolled to flatness.
Any suitable quenching liquid may be used including water, brine or
other aqueous salt solution, oil, liquid nitrogen, etc. However,
for quenching a heated steel strip to convert austenite to
martensite, the preferred quenching media are water and aqueous
brine or other aqueous salt solutions. For the latter purpose, the
volume rate of flow of the quench liquid must be high enough to
achieve the cooling rate required to transform the austenite to
martensite, and the turbulence of the quench liquid relative to the
strip, particularly at the strip entry end of the quench channel,
must be great enough to prevent the accumulation of vapor film
which would lead to nonuniformity of quenching and consequent
distortion of the strip.
Typical line speeds may range from about 100 feet/minute to about
2,000 feet/minute dependent upon the gauge of the strip and the
carbon content. The temperature of the quench water is regulated,
as described below, and the steel strip will normally be cooled
from its austenitizing temperature range of
1,330.degree.-2,100.degree. F. (1,525-2,100.degree. F. in the case
of a fully martensitic product) to approximately the water
temperature before leaving the quench tank. If desired, the water
or other quench medium may be recirculated through a heat exchanger
for temperature control.
Assuming that the quenching operation has been carried out under
optimum conditions, as discussed above, so as to achieve uniformity
of quenching across the full width of the strip, the final quenched
product will have acceptable flatness for many end uses, as
mentioned above. However, the quenched strip can readily be rolled,
as on a temper mill, to provide adequate commercial flatness for
any desired end use. For example, successful flattening is usually
obtained by a single pass through a twin-stand four-high temper
mill, each stand having two work rolls and two backup rolls.
Because of the unusual hardness of a fully martensitic strip, the
work rolls may have a high degree of roughness without impairing
the surface of the strip, thereby providing adequate flattening in
a single pass.
Other means may also be employed for creating the necessary
turbulence in the quench channel 35 by causing high-velocity
transverse flow of coolant in addition to longitudinal flow of
coolant through the channel. For example, the quench channel may be
provided with transverse baffles extending inwardly from the outer
walls of the channel toward the strip so as to direct a portion of
the quench medium from its generally longitudinal path parallel
with the strip to a transverse path generally perpendicular to the
strip.
The currents or streams of water directed transversely or generally
perpendicularly against the strip by the submerged sprays 41, in
cooperation with the high relative velocity of the main flow of
water through the restricted quench channel 35 parallel to the
strip, cause sufficient turbulence in the quench medium to
eliminate steam pockets which would otherwise tend to form or
accumulate at the strip surface. Such vapor formation causes a
vapor barrier which results in nonuniform quenching, nonuniform
transformation to martensite, and consequent warping and distortion
of the strip.
In the quenching action which is accomplished in the channel 35,
the quench period may be considered as having three portions.
First, the temperature of the heated strip is lowered rapidly to
the temperature range of martensite formation, i.e. to the
martensite start temperature (M.sub.s). Next, the strip is cooled
through the temperature range of martensite formation, i.e. from
the martensite start temperature (M.sub.s) to the martensite finish
temperature (M.sub.f). Finally, the strip is cooled to ambient
temperature or the temperature of the quench liquid. Since drastic
quenching is required to effect transformation of austenite to
martensite in low-carbon steel, it has generally been thought that
the strip should be quenched as rapidly as possible from its
elevated temperature through the temperature range of martensite
formation. As heretofore indicated, however, a certain amount of in
situ or self-tempering of the martensite takes place in spite of
rapid quenching because of the fact that the martensite
transformation temperature is quite high for low-carbon steels. For
example, for plain carbon steel containing 0.03 weight percent
carbon and 0.40 weight percent manganese, the martensite start
temperature is estimated to be about 990.degree. F. and the
martensite finish temperature is estimated as about 610.degree. F.
For plain carbon steel of 0.25 weight percent carbon and 0.40
weight percent manganese the respective martensite start and finish
temperatures are estimated as about 805.degree. F. and about
470.degree. F.
In accordance with the present invention, highly advantageous
results are obtained by selective control of the rate of quenching
or heat withdrawal in different portions of the quench period.
Thus, in the initial portion of the quench period the temperature
of the strip is lowered as rapidly as possible from its elevated
austenitizing temperature to the temperature range of martensite
formation, i.e. to the martensite start temperature. Thereafter,
however, the rate of quenching or heat withdrawal is substantially
diminished during quenching through the temperature range of
martensite formation, i.e. from the martensite start temperature to
the martensite finish temperature. As a result of this selective
variation or control, an increased tempering of the martensite is
obtained, as compared with a quench system relaying wholly on in
situ or self-tempering, so as to obtain a final product having
improved ductility for a given tensile strength level. For example,
increasing the quench time through the temperature range of
martensite formation from 0.3 second to 0.5 second can result in a
significant increase in the tempering effect on the martensitic
structure.
Although the invention, in its broadest aspect, is not limited to
any specific way of obtaining the desired selective control over
quench rate in the quench channel, a particularly convenient method
comprises regulating the temperature and flow velocity of the
quench liquid in the different portions of the quench channel. More
specifically, a portion of the quench liquid is introduced at one
end of the elongated quench channel and another portion of quench
liquid is introduced into the channel at a submerged location
within the channel and adjacent to but spaced from the strip entry
end of the channel. By maintaining a temperature differential
between the two portions of quench liquid, the desired control over
quench rate can be obtained.
Referring to the quench system shown in FIG. 1 wherein the strip
and the quench liquid move concurrently downwardly through the
quench channel, it will be seen that the strip 10 is first
contacted by the water which is introduced through the inlets 39 to
the trough 36 and overflows the upper end of the conduit 34.
Thereafter, the strip 10 is contacted by a mixture of the water
introduced through the lines 39 and the additional water introduced
through the submerged spray units 41. The desired difference in
quench rate is obtained by regulating the temperature of the
separate streams of water supplied through the inlets 39 and 42.
Since it is necessary to quench the strip initially as rapidly as
possible to the martensite start temperature, the water supplied
through the inlets 39 should be at a relatively low temperature
inasmuch as this water makes the initial contact with the strip 10.
Thereafter, in order to slow down the rate of heat withdrawal as
the strip is quenched through the temperature range of martensite
formation, the water supplied to the spray units 41 through the
lines 42 should be heated to a relatively higher temperature. The
respective temperatures of the two sources of water and the
relative volume flow rates from the two sources are such that the
temperature of the combined or commingled water streams is higher
than the temperature of the water supplied through the inlets 39,
thereby effecting heat withdrawal from the strip at a diminished
rate as the strip temperature is lowered through the martensite
transformation range.
As previously explained, a plurality of submerged spray units 41
are preferably provided in superimposed relation in the quench
conduit 34 at the opposite sides of the strip 10, and the
temperature of the water supplied through the spray units at
successive elevations may be adjusted as required to insure a
diminished rate of heat withdrawal only after the strip temperature
has been lowered to the martensite start temperature. For example,
in FIG. 1, it might be desirable to have the temperature of the
water introduced through the uppermost spray units 41 substantially
the same as the temperature of the water introduced through the
inlets 39, and higher temperature water would be introduced only
through the lowermost spray units 41. Also, instead of having fixed
spray units 41, a similar result could be obtained by providing
mechanical means for adjusting the elevations of the spray units 41
relative to the quench conduit 34. By either means, however, it is
possible to select the proper elevation for effecting a diminished
quench rate corresponding to the point at which the martensite
transformation begins. As will be understood, the latter location
will vary with strip gauge, line speed, and steel chemistry.
By way of example, for low-carbon steel strip having a thickness of
0.024 in., a carbon content of 0.03 to 0.25 weight percent, and a
manganese content of 0.20 to 0.60 weight percent, the heat
withdrawal rate in the initial portion of the quench period may be
from about 1.times.10.sup.6 to about 3.times.10.sup.6 B.t.u./sq.
ft./hr., and the diminished heat withdrawal rate when passing
through the martensite transformation temperature range may be from
about 0.25.times.10.sup.6 to about 0.5.times.10.sup.6 B.t.u./sq.
ft./hr. Such heat withdrawal rates may correspond to a time range
of from about 0.35 sec. to about 1.33 sec. for the initial portion
of the quench period and from about 1.24 sec. to about 3.6 sec.
during cooling through the martensite transformation range.
Obviously, the exact values will depend on the aforementioned
variables. As an example of the temperature differential between
the different portions of quench water in the FIG. 1 embodiment,
the relatively low-temperature water introduced through the inlet
39 may be from about 35.degree. F. to about 90.degree. F. and the
relatively higher temperature water introduced at one or more
elevations through the spray units 41 may be heated to a
temperature of from about 120.degree. F. to about 200.degree.
F.
In the FIG. 2 embodiment of the invention, the apparatus is
modified slightly to accommodate countercurrent movement of the
quench water relative to the downwardly moving strip. Thus, in FIG.
2 the trough 36 surrounding the upper end of the quench conduit 34
has been modified by the omission of the peripheral flange 33, the
depending baffle 37, and the water inlets 39. Instead, an upright
baffle or weir 50 extends upwardly from the bottom of the trough 36
in spaced relation between the outer walls of the trough and the
seal conduit 38. The upper end of the baffle 50 extends above the
lower end of the seal conduit 38. One portion of quench water is
introduced to the system through an inlet 51 pipe extending into
the tank 30 and passes upwardly through the quench channel 35 from
the lower end thereof. As the water approaches the upper end of the
quench channel 35 it is commingled with additional quench water
introduced through the spray units 41, and the commingled or
combined water stream overflows the upper end of the quench conduit
34 into the trough 36. The water then overflows the upper edge of
the baffle or weir 50 and is discharged from the trough 36 through
an outlet pipe 52.
In this embodiment of the invention the portion of the quench water
introduced through the inlet pipe 51 is heated to a relatively
higher temperature, whereas the water introduced through the spray
units 41 is at a relatively lower temperature. As a result, the
mixture of quench water streams which first contacts the downwardly
moving strip 10 has a lower temperature than the temperature of the
quench water at a lower point in the channel 35 below the spray
units 41, and the desired difference in heat withdrawal rates is
maintained in the manner previously described. As an example of the
temperature differential, the heated water introduced through the
line 51 may have a temperature of from about 120.degree. F. to
about 200.degree. F., and the lower temperature water introduced
through the lines 42 may be from about 35.degree. F. to about
90.degree. F.
In both systems of FIGS. 1 and 2, the variation in heat withdrawal
rate in the different portions of the quench channel is obtained by
regulating the temperature and flow velocity of the quench water.
In the concurrent flow system of FIG. 1, the maximum flow rate and
liquid velocity in the channel 35 is below the spray units 41, and
the temperature of the water at this point is greater than the
temperature of the water in the channel above the spray units 41 so
that the net result is a lower rate of heat withdrawal in the lower
portion of the quench channel. In the countercurrent flow system of
FIG. 2, the temperature of the water in the channel 35 below the
spray units 41 is again higher than the temperature of the water
above the spray units 41, but, in this case, the waterflow rate and
liquid velocity is greater in the upper portion of the quench
channel above the spray units 41. Thus, the net result in the FIG.
2 system is that a higher heat withdrawal rate is obtained in the
upper portion of the quench channel.
Although the invention has been described with reference to certain
specific embodiments, it should be understood that various
modifications and equivalents may be used without departing from
the scope of the invention as defined in the appended claims. In
addition, although the invention has been described with reference
to the continuous quenching of steel strip, it is also within the
scope of the invention to utilize separate sheets of steel.
* * * * *