U.S. patent application number 12/272207 was filed with the patent office on 2009-05-21 for low surface roughness cast strip and method and apparatus for making the same.
This patent application is currently assigned to Nucor Corporation. Invention is credited to Walter N. Blejde, Jay Jon Ondrovic.
Application Number | 20090126896 12/272207 |
Document ID | / |
Family ID | 38436867 |
Filed Date | 2009-05-21 |
United States Patent
Application |
20090126896 |
Kind Code |
A1 |
Blejde; Walter N. ; et
al. |
May 21, 2009 |
LOW SURFACE ROUGHNESS CAST STRIP AND METHOD AND APPARATUS FOR
MAKING THE SAME
Abstract
A thin cast strip is formed having at least one microstructure
selected from the group consisting of polygonal ferrite, acicular
ferrite, Widmanstatten, bainite and martinsite, a surface roughness
of less than 1.5 microns Ra and a scale thickness of less than
about 10 microns by applying a mixture of water and oil on the work
rolls of the hot rolling mill, passing the thin cast strip at a
temperature of less than 1100.degree. C. through the hot rolling
mill while the mixture of oil and water is applied to the work
rolls, and shrouding the thin cast strip from the casting rolls
through the hot rolling mill in an atmosphere of less than 5%
oxygen to form the thin cast strip.
Inventors: |
Blejde; Walter N.;
(Brownsburg, IN) ; Ondrovic; Jay Jon; (Brownsburg,
IN) |
Correspondence
Address: |
HAHN LOESER & PARKS, LLP
One GOJO Plaza, Suite 300
AKRON
OH
44311-1076
US
|
Assignee: |
Nucor Corporation
Charlotte
CT
|
Family ID: |
38436867 |
Appl. No.: |
12/272207 |
Filed: |
November 17, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11362682 |
Feb 27, 2006 |
|
|
|
12272207 |
|
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|
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Current U.S.
Class: |
164/463 |
Current CPC
Class: |
B21B 27/10 20130101;
B21B 2261/14 20130101; B21B 2027/103 20130101; B21B 1/463 20130101;
B21B 1/46 20130101 |
Class at
Publication: |
164/463 |
International
Class: |
B22D 11/06 20060101
B22D011/06 |
Claims
1. A method of producing a thin cast steel strip having a surface
roughness of less than 1.5 microns Ra, the method comprising: a.
introducing molten steel between a pair of casting rolls having a
nip there between; b. counter-rotating the casting rolls to form
solidified metal shells on the surfaces of the casting rolls and to
form an intermediate cast steel strip through the nip between the
casting rolls from the solidified shells; c. introducing the
intermediate cast steel strip to a hot rolling mill having back-up
rolls and work rolls with work surfaces forming a gap between the
work rolls through which the intermediate cast steel strip is
rolled; d. applying a mixture of oil and water directly on the
back-up rolls of the hot rolling mill, the mixture of oil and water
being transferred from the back-up rolls to the work rolls through
engagement of the back-up rolls and the work rolls; and e. rolling
the intermediate cast steel strip between the work rolls having the
mixture of oil and water thereon to produce the thin cast steel
strip.
2. The method of claim 1, where the rate of production of the thin
cast steel strip is above 80 meters per minute.
3. The method of claim 1, where the intermediate cast steel strip
is rolled through the hot rolling mill at a temperature below
1100.degree. C.
4. The method of claim 1, where the intermediate cast steel strip
is rolled through the hot rolling mill at a temperature below
1050.degree. C.
5. The method of claim 1, where the intermediate cast steel strip
is rolled through the hot rolling mill at a temperature below
900.degree. C.
6. The method of claim 1, where the mixture of oil and water is
applied directly on the back-up rolls at a rate between 10 and 30
gallons per minute.
7. The method of claim 1, where the surface roughness of the thin
cast steel strip is below 1.0 microns Ra.
8. The method of claim 7, where the rate of production of the thin
cast steel strip is above 80 meters per minute.
9. The method of claim 7, where the intermediate cast steel strip
is rolled through the hot rolling mill at a temperature below
1100.degree. C.
10. The method of claim 7, where the intermediate cast steel strip
is rolled through the hot rolling mill at a temperature below
1050.degree. C.
11. The method of claim 7, where the intermediate cast steel strip
is rolled through the hot rolling mill at a temperature below
900.degree. C.
12. The method of claim 7, where the mixture of oil and water is
applied directly on the back-up rolls at a rate between 10 and 30
gallons per minute.
13. The method of claim 1, where the surface roughness of the thin
cast steel strip is below 0.7 microns Ra.
14. The method of claim 13, where the rate of production of the
thin cast steel strip is above 80 meters per minute.
15. The method of claim 13, where the intermediate cast steel strip
is rolled through the hot rolling mill at a temperature below
1100.degree. C.
16. The method of claim 13, where the intermediate cast steel strip
is rolled through the hot rolling mill at a temperature below
1050.degree. C.
17. The method of claim 13, where the intermediate cast steel strip
is rolled through the hot rolling mill at a temperature below
900.degree. C.
18. The method of claim 13, where the mixture of oil and water is
applied directly on the back-up rolls at a rate between 10 and 30
gallons per minute.
19. The method of claim 1, where the surface roughness of the thin
cast steel strip is below 0.5 microns Ra.
20. The method of claim 19, where the rate of production of the
thin cast steel strip is above 80 meters per minute.
21. The method of claim 19, where the intermediate cast steel strip
is rolled through the hot rolling mill at a temperature below
1100.degree. C.
22. The method of claim 19, where the intermediate cast steel strip
is rolled through the hot rolling mill at a temperature below
1050.degree. C.
23. The method of claim 19, where the intermediate cast steel strip
is rolled through the hot rolling mill at a temperature below
900.degree. C.
24. The method of claim 19, where the mixture of oil and water is
applied directly on the back-up rolls at a rate between 10 and 30
gallons per minute.
25. The method of claim 1, where the mixture of oil and water is
applied directly on the back-up rolls using a plurality of spray
nozzles positioned upstream of the work rolls.
26. The method of claim 1, where the mixture of oil and water is
applied directly on downstream surfaces of the back-up rolls.
27. The method of claim 1, where the mixture of oil and water is
applied directly on upstream surfaces of the back-up rolls.
28. The method of claim 1, where the mixture of oil and water is
less than 5% oil.
29. The method of claim 1, where the thin cast steel strip has a
surface scale thickness of less than about 10 microns.
30. The method of claim 1, where the thin cast steel strip has a
surface scale thickness of less than about 7 microns.
31. The method of claim 1, where the thin cast steel strip has a
surface scale thickness of less than about 4 microns.
32. The method of claim 1, where the thin cast steel strip has at
least one microstructure selected from the group consisting of
polygonal ferrite, acicular ferrite, Widmanstatten, bainite and
martensite.
33. The method of claim 1, further comprising shrouding the
intermediate cast steel strip from the casting rolls through the
hot rolling mill in an atmosphere containing less than 5% oxygen.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/362,682 filed Feb. 27, 2006. The disclosure
of which is incorporated herein by reference.
BACKGROUND AND SUMMARY OF THE INVENTION
[0002] This invention relates to cast strip made by a twin roll
caster, and the method and apparatus for making such cast
strip.
[0003] In a twin roll caster, molten metal is introduced between a
pair of counter-rotated horizontal casting rolls which are cooled
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.
[0004] The term "nip" is used herein to refer to the general region
at which the casting rolls are closest together. The molten metal
may be 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.
[0005] When casting steel strip in a twin roll caster, the strip
leaves the nip at very high temperatures on the order of
1400.degree. C. or higher. If exposed to normal atmosphere, it
would suffer very rapid scaling due to oxidation at such high
temperatures. Therefore, a sealed enclosure is provided beneath the
casting rolls to receive the hot strip and through which the strip
passes on the way from the strip caster, the enclosure containing
an atmosphere which inhibits oxidation of the strip. The oxidation
inhibiting atmosphere may be created by injecting a non-oxidizing
gas, for example, an inert gas such as argon or nitrogen, or
combustion exhaust gases which may be reducing gases.
Alternatively, the enclosure may be sealed against ingress of
oxygen containing atmosphere during operation of the strip caster.
The oxygen content of the atmosphere within the enclosure is then
reduced during an initial phase of casting by allowing oxidation of
the strip to extract oxygen from the sealed enclosure as disclosed
in U.S. Pat. Nos. 5,762,126 and 5,960,855.
[0006] Cast strip produced by twin roll caster is generally hot
rolled in a hot rolling mill after the strip emerges from the
caster to shape the thin strip. It is generally understood that to
cast strip with acceptable shape, the hot rolling mill is used in
connection with a twin roll caster is to provide the desired
cross-sectional profile to the cast strip. Still, a surface
roughness of 6 to 8 microns Ra with surface micro-cracking was
common for cast strip emerging from the hot rolling mill while
casting at a standard casting speed of 80 m/min and 16% reduction
of the strip by the hot rolling mill. FIG. 1 is a micrograph
showing typical surface roughness of such cast strip emerging from
a hot rolling mill in-line with a twin roll caster. With the
direction of rolling from left to right, the micrograph shows
pronounced lapping on the strip surface (20 to 30 .mu.m deep). The
reason or reasons for this surface roughness may be shearing at the
strip surface caused by welding of the strip to the work roll
surface, imprinting of the texture of the work roll surface onto
the surface of the strip, and/or other factors. Moreover,
micro-cracking on the surface of the cast strip was a problem. It
was possible to reduce the casting speed and the heat rate of the
strip to reduce microcracking, but it was uneconomical to reproduce
these conditions during production.
[0007] The microstructure of hot strip mill products is essentially
100% equiaxed ferrite. However, in making a cast strip with a twin
roll caster, previous experience was that microstructure was coarse
grains of polygonal ferrite, acicular ferrite, and Widmanstatten.
It was typical that the microstructure was 30-60% polygonal
ferrite, 70-40% Widmanstatten and acicular ferrite. With this
microstructure, the typical surface roughness was 4-7 microns
Ra.
[0008] A thin cast strip is provided having at least one
microstructure selected from the group consisting of polygonal
ferrite, acicular ferrite, Widmanstatten, bainite and martinsite, a
surface roughness of less than 1.5 microns Ra and a scale thickness
of less than about 10 microns made by the steps comprising:
[0009] a. assembling a twin roll caster having laterally positioned
caster rolls forming a nip between them and a hot rolling mill
having work rolls and back-up rolls adjacent the twin roll
caster,
[0010] b. forming a thin cast strip from the nip between the
casting rolls of the twin roll caster,
[0011] c. applying a mixture of water and oil on the work rolls of
the hot rolling mill,
[0012] d. passing the thin cast strip at a temperature of less than
1100.degree. C. through the hot rolling mill while the mixture of
oil and water is applied to the work rolls, and
[0013] e. shrouding the thin cast strip from the casting rolls
through the hot rolling mill in an atmosphere of less than 5%
oxygen forming a cast strip having at least one microstructure
selected from the group consisting of polygonal ferrite, acicular
ferrite, Widmanstatten, bainite and martinsite, a surface roughness
of less than 1.5 microns Ra, and a scale thickness of less than
about 10 microns.
[0014] A thin cast strip may have a surface roughness less than 1.0
microns Ra or less than 0.7 or 0.5 microns Ra. The thin cast strip
may have a scale thickness less than 7 or 4 microns. The cast strip
may be passed through the hot rolling mill at a temperature less
than 1050.degree. C. while a mixture of oil and water is applied to
the work rolls. The mixture of oil and water may be applied by
spraying the work rolls, or may be applied to the back-up rolls.
The mixture of oil and water may be less than 5% oil to form a thin
cast strip with a low surface roughness of less than 1.5 microns
Ra.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The operation of an illustrative twin roll casting plant in
accordance with the present invention is described with reference
to the accompanying drawings, in which:
[0016] FIG. 1 is a micrograph showing typical surface roughness of
a cast strip after hot rolling;
[0017] FIG. 2 is a schematic illustrating a thin strip casting
plant having a hot rolling mill for controlling the shape of cast
strip;
[0018] FIG. 3 is an enlarged cut-away side view of the caster of
the thin strip casting plant of FIG. 2;
[0019] FIG. 4 is a schematic diagram showing a system for the
application of an oil and water mixture to the rolls of a hot
rolling mill; and
[0020] FIG. 5 is a diagram showing the Average Surface Roughness
for Thin Cast Steel Strip, Sequence 2613, made using o the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0021] The illustrated casting and rolling installation comprises a
twin-roll caster denoted generally by 11 which produces thin cast
steel strip 12 which passes into a transient path across a guide
table 13 to a pinch roll stand 14. After exiting the pinch roll
stand 14, thin cast strip 12 passes into and through hot rolling
mill 15 comprised of back-up rolls 16 and upper and lower work
rolls 16A and 16B, where the thickness of the strip reduced. The
strip 12, upon exiting the rolling mill 15, passes onto a run out
table 17 where it may be forced cooled by water jets 18, and then
through pinch roll stand 20 comprising a pair of pinch rolls 20A
and to a coiler 19.
[0022] Twin-roll caster 11 comprises a main machine frame which
supports a pair of laterally positioned casting rolls 22 having
casting surfaces 22A and forming a nip between them. Molten metal
is supplied during a casting campaign from a ladle (not shown) to a
tundish 23, through a refractory shroud to a removable tundish 25
(also called distributor vessel or transition piece), and then
through a metal delivery nozzle 28 (also called a core nozzle)
between the casting rolls 22 above the nip.
[0023] Molten steel is introduced into removable tundish 25 from
tundish 23 via an outlet of the refractory shroud. The tundish 23
is fitted with a stopper rod and a slide gate valve (not shown) to
selectively open and close the outlet of the shroud and effectively
control the flow of molten metal from the tundish 23 to the caster.
The molten metal flows from removable tundish 25 through an outlet
and optionally to and through the delivery nozzle 28.
[0024] Molten metal thus delivered to the casting rolls 22 forms a
casting pool above nip supported by casting roll surfaces 22A. This
casting pool is confined at the ends of the rolls by a pair of side
dams or plates, which are applied to the ends of the rolls by a
pair of thrusters (not shown) comprising hydraulic cylinder units
connected to the side dams. The upper surface of the casting pool
(generally referred to as the "meniscus" level) may rise above the
lower end of the delivery nozzle 28 so that the lower end of the
deliver nozzle is immersed within the casting pool.
[0025] Casting rolls 22 are internally water cooled by coolant
supply (not shown) and driven in counter rotational direction by
drives (not shown) so that shells solidify on the moving casting
roll surfaces and are brought together at the nip to produce the
thin cast strip 12, which is delivered downwardly from the nip
between the casting rolls.
[0026] Below the twin roll caster 11, the cast steel strip 12
passes within a sealed enclosure 10 to the guide table 13, which
guides the strip to a pinch roll stand 14 through which it exits
sealed enclosure 10. The seal of the enclosure 10 may not be
complete, but is appropriate to allow control of the atmosphere
within the enclosure and access of oxygen to the cast strip within
the enclosure as hereinafter described. After exiting the sealed
enclosure 10, the strip may pass through further sealed enclosures
after the pinch roll stand 14, including the hot rolling mill
15.
[0027] Enclosure 10 is formed by a number of separate wall sections
that fit together at various seal connections to form a continuous
enclosure wall. These sections comprise a first wall section 41 at
the twin roll caster to enclose the casting rolls 22, and a wall
enclosure 42 extending downwardly beneath first wall section 41 to
form an opening that is in sealing engagement with the upper edges
of a scrap box receptacle 40. A seal 43 between the scrap box
receptacle 40 and the enclosure wall 42 may be formed by a knife
and sand seal around the opening in enclosure wall 42, which can be
established and broken by vertical movement of the scrap box
receptacle 40 relative to enclosure wall 42. Seal 43 is formed by
raising the scrap box receptacle 40 to cause the knife flange to
penetrate the sand in the channel to establish the seal.
[0028] This seal 43 can be broken by lowering the scrap box
receptacle 40 from its operative position, preparatory to movement
away from the caster to a scrap discharge position (not shown).
Scrap box receptacle 40 is mounted on a carriage 45 fitted with
wheels 46 which run on rails 47, whereby the scrap box receptacle
can be moved to the scrap discharge position. Carriage 45 is fitted
with a set of powered screw jacks 51 operable to lift the scrap box
receptacle 40 from a lowered position, where it is spaced from the
enclosure wall 42, to a raised position where the knife flange
penetrates the sand to form seal 43 between the two.
[0029] Sealed enclosure 10 further may have a third wall section
disposed 61 about the guide table 13 and connected to the frame of
pinch roll stand 14, which includes a pair of pinch rolls 50. The
third wall section disposed 61 of enclosure 10 is sealed by sliding
seals.
[0030] Most of the enclosure wall sections 41, 42 and 61 may be
lined with fire brick. Also, scrap box receptacle 40 may be lined
either with fire brick or with a castable refractory lining. In
this way, the complete enclosure 10 is sealed prior to a casting
operation, thereby limiting access of oxygen to thin cast strip 12,
as it passes from the casting rolls 22 through the pinch roll stand
14 and the hot rolling mill 15. Initially the strip can take up all
of the oxygen from enclosure 10 space by forming heavy scale on an
initial section of the strip. However, the sealing enclosure 10
limits ingress of oxygen into the enclosure from the surrounding
atmosphere to below the amount of oxygen that could be taken up by
the strip. Thus, after an initial start-up period, the oxygen
content in the enclosure 10 will remain depleted so limiting the
availability of oxygen for oxidation of the strip 12. In this way,
the formation of scale is controlled to a thickness less than 10
microns without the need to continuously feed a reducing or
non-oxidizing gas into the enclosure. Of course, a reducing or
non-oxidizing gas may be fed through the enclosure walls. However,
in order to avoid the heavy scaling during the start-up period, the
enclosure 10 can be purged immediately prior to the commencement of
casting so as to reduce the initial oxygen level within enclosure
10, thereby reducing the time period for the oxygen level to
stabilize in the enclosure as a result of the interaction of the
oxygen in oxidizing the strip passing through it. Thus,
illustratively, the enclosure may conveniently be purged with, for
example, nitrogen gas. It has been found that reduction of the
initial oxygen content to levels of between 5% will limit the
scaling of the strip at the exit from the enclosure 10 to about 10
microns to 17 microns even during the initial start-up phase. In an
embodiment of the present invention, the thin cast steel strip has
a scale thickness less than about 10 microns, or the scale
thickness may be less than 7 or 4 microns, during continuous
casting.
[0031] At the start of a casting campaign, a short length of
imperfect strip is produced as the casting conditions stabilize.
After continuous casting is established, the casting rolls 22 are
moved apart slightly and then brought together again to cause this
lead end of the strip to break away in the manner described in
Australian Patent 646,981 and U.S. Pat. No. 5,287,912, to form a
clean head end of the following thin cast strip 12. The imperfect
material drops into scrap box receptacle 40 located beneath caster
11, and at this time swinging apron 38, which normally hangs
downwardly from a pivot 39 to one side of the caster as shown in
FIG. 3, is swung across the caster outlet to guide the clean end of
thin cast strip 12 onto the guide table 13 where the strip is fed
to the pinch roll stand 14. Apron 38 is then retracted back to its
hanging position as shown in FIG. 3 to allow the strip 12 to hang
in a loop 36 beneath the caster as shown in FIGS. 2 and 3 before
the strip passes onto the guide table 13. The guide table 13
comprises a series of strip support rolls 37 to support the strip
before it passes to the pinch roll stand 14. The rolls 37 are
disposed in an array extending from the pinch roll stand 14
backwardly beneath the caster and curve downwardly to smoothly
receive and guide the strip from the loop 36.
[0032] The twin-roll caster may be of a kind which is illustrated
and described in detail in U.S. Pat. No. 5,184,668 and 5,277,243,
or U.S. Pat. No. 5,488,988. Reference may be made to these patents
for construction details, which are no part of the present
invention.
[0033] Pinch roll stand 14 comprises a pair of pinch rolls 50
reactive to tension applied by the hot rolling mill 15.
Accordingly, the strip is able to hang in the loop 36 as it passes
from the casting rolls 22 to the guide table 13 and into the pinch
roll stand 14. The pinch rolls 50 thus provides a tension barrier
between the freely hanging loop and tension on the strip downstream
of the processing line. The pinch rolls 50 also stabilize the
position of the strip on the feed table 13, feeding the strip into
hot rolling mill 15.
[0034] From the pinch roll stand 14, the thin cast strip 12 is
delivered to the hot rolling mill 15 comprised of upper work roll
16A and lower roll 16B. As shown in FIG. 4, a preferred embodiment
of the present invention comprises spraying a mixture of water and
oil on the downstream surfaces of back-up rolls 16. An oil
reservoir 100 is provided with a heater 101 to maintain the oil at
approximately 50.degree. C., but heating is not necessary. The
heated oil is transferred through oil transfer lines 103 by fixed
displacement pumps 102 to static mixers 104 where the heated oil is
mixed with water.
[0035] Water is supplied from a source 110 to water strip chilling
headers 111 and to mill rolls supply lines 112. A first portion of
the water is supplied to spray headers 18 to supply cooling water
to cool the hot strip 12 after exiting the hot rolling mill 15.
Typically, the water pressure is reduced through pressure regulator
113 to about 40 psi. Between about 10 and 30 gpm of water is
supplied to each static mixer 104 where the water is mixed with
about 4 gph of heated oil.
[0036] The mixed oil and water is then applied to the downstream
surfaces (the direction of travel of the thin cast steel strip 12
is shown by arrow 120) of back-up rolls 16 through oil-water
nozzles 71. Alternately, the oil-water mixture may be applied to
cast strip 12 in the roll bite area, may be applied to the upstream
surfaces of the back-up rolls 16 or to the work rolls 16A, 16B.
[0037] Preferably, the temperature of the thin cast steel strip 12
in the hot rolling mill 15 is less than 1100.degree. C., and more
preferably less than 1050.degree. C., and most preferably less Than
900.degree.C. Also, preferably, the temperature of the thin cast
steel strip in the hot rolling mill 15 is above 400.degree. C.
[0038] The static mixers 104 are standard conventionally available
devices. Other forms of mixers may be used provided they are
capable of good mixing of the oil and water.
[0039] In one embodiment, the oil-water mixture is delivered at
between 5 and 30 gpm at 40 psi to the back-up rolls 16. Typically
the oil-water mixture is delivered to the back up rolls in this
embodiment at about 10 to 20 gpm, with 15 gpm a reasonable setting.
The oil-water mixture may comprise less than 5% oil, and in one
embodiment comprises 4 parts oil and between 600 parts to 1800
parts water by volume. The oil may be less than 2% or 1% of the
mixture. The oil is provided to be mixed with the water generally
at less than 15 gph.
[0040] FIG. 5 shows the Average Surface Roughness (Ra) in microns
for thin cast strip steel strip 12 produced using the present
invention. As can be seen in FIG. 5, the Average Surface Roughness
is noticeably lower, about 0.66 to about 1.5 microns with the
addition of an oil-water mixture as described above.
[0041] In one embodiment, the present invention comprises producing
thin cast steel strip using the oil-water application described
above to produce thin cast steel strip at a rate above 80 meters
per minute.
[0042] While the invention has been described with reference to
certain embodiments, it will be understood by those skilled in the
art that various changes may be made and equivalents may be
substituted without departing from the scope of the invention. In
addition, many modifications may be made to adapt a particular
situation or material to the teachings of the invention without
departing from its scope. Therefore, it is intended that the
invention not be limited to the particular embodiments disclosed,
but that the invention will include all embodiments falling within
the scope of the appended claims.
* * * * *