U.S. patent application number 09/967105 was filed with the patent office on 2002-04-18 for method of producing steel strip.
Invention is credited to Blejde, Walter, Mahapatra, Rama, Mukunthan, Kannappar, Strezov, Lazar.
Application Number | 20020043304 09/967105 |
Document ID | / |
Family ID | 3824520 |
Filed Date | 2002-04-18 |
United States Patent
Application |
20020043304 |
Kind Code |
A1 |
Strezov, Lazar ; et
al. |
April 18, 2002 |
Method of producing steel strip
Abstract
Steel strips and methods for producing steel strips are
provided. In an illustrated embodiment, a method for producing
steel strips includes continuously casting molten steel into a
strip, said molten steel comprising a concentration of residuals of
2.0 wt % or less is selected with regard to the microstructure of
the finished strip to provide a desired yield strength; and cooling
the strip to transform the strip from austenite to ferrite in the
temperature range of 850.degree. C. to 400.degree. C. Cast steel
with improved yield strength properties is produced by such
method.
Inventors: |
Strezov, Lazar; (Adamstown
Heights, AU) ; Mukunthan, Kannappar; (Rankin Park,
AU) ; Blejde, Walter; (Brownsburg, IN) ;
Mahapatra, Rama; (Indianapolis, IN) |
Correspondence
Address: |
BARNES & THORNBURG
11 South Meridian Street
Indianapolis
IN
46204
US
|
Family ID: |
3824520 |
Appl. No.: |
09/967105 |
Filed: |
September 28, 2001 |
Current U.S.
Class: |
148/320 ;
148/541 |
Current CPC
Class: |
B22D 11/0622 20130101;
C21D 9/573 20130101; C21D 8/0215 20130101; C21D 1/18 20130101; C21D
8/0226 20130101; B22D 11/225 20130101 |
Class at
Publication: |
148/320 ;
148/541 |
International
Class: |
C22C 038/00; C21D
001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 2, 2000 |
AU |
PR0460 |
Claims
The claims defining the invention are as follows:
1. A method of producing steel strip comprising the steps of: (a)
continuously casting molten steel into a strip including austenite
grains, said molten steel comprising a concentration of residuals
selected with regard to the microstructure of the finished strip to
provide a desired yield strength; and (b) cooling the cast strip to
transform austenite grains in the strip to ferrite in a temperature
range between 850.degree. C. and 400.degree. C.
2. The method of claim 1 wherein the total amount of the residuals
is 2.0 wt % or less.
3. The method of claim 1 wherein the total amount of the residuals
is 1.2 wt % or less.
4. The method of claim 1 wherein the cast strip produced in step
(a) has a thickness of no more than 2 mm.
5. The method of claim 1 wherein the cast strip produced in step
(a) includes austenite grains that are columnar.
6. The method of claim 1 wherein the steel is low carbon steel.
7. The method of claim 6 wherein the low carbon steel is a
silicon/manganese killed steel.
8. The method in claim 7 wherein the silicon/manganese killed steel
includes, by wt %:
3 Carbon 0.02-0.08% Manganese 0.30-0.80% Silicon 0.10-0.40% Sulphur
0.002-0.05% Aluminium less than 0.01%
9. The method of claim 1 wherein the low carbon steel is an
aluminum killed steel.
10. The method described in claim 9 wherein the aluminum killed low
carbon steel has the following composition by weight:
4 Carbon 0.02-0.08% Manganese 0.40% max Silicon 0.05% max Sulphur
0.002-0.05% Aluminum 0.05% max
11. The method of claim 1 wherein the continuous caster is a twin
roll caster.
12. The method of claim 1 further comprising the step of in-line
hot rolling the casted strip of step (a) prior to step (b).
13. The method of claim 1 wherein step (b) includes cooling the
casted strip to transform the strip from austenite to ferrite in a
temperature range between 850.degree. C. and 400.degree. C. at a
selected cooling rate of at least 0.01.degree. C./sec to produce a
microstructure that provides required yield strength of the casted
strip, the microstructure being selected from a group consisting
of: (i) predominantly polygonal ferrite; (ii) a mixture of
polygonal ferrite and low temperature transformation products; and
(iii) predominantly low temperature transformation products.
14. The method of claim 13 wherein the cooling rate is selected so
that the microstructure is either (ii) a mixture of polygonal
ferrite and low temperature transformation products; or (iii)
predominantly low temperature transformation products.
15. A cast steel strip produced by the steps of: (a) continuously
casting molten steel into a strip including austenite grains, said
molten steel comprising a concentration of residuals selected with
regard to the microstructure of the finished strip to provide a
desired yield strength; and (b) cooling the cast strip to transform
the austenite grains in the strip to ferrite in a temperature range
between 850.degree. C. and 400.degree. C.
16. The cast steel strip of claim 15 wherein the total amount of
the residuals is 2.0 wt % or less.
17. The cast steel strip of claim 15 wherein the total amount of
the residuals is 1.2 wt % or less.
18. The cast steel strip of claim 15 wherein the cast strip
produced in step (a) includes austenite grains that are
columnar.
19. The cast steel strip of claim 15 wherein the steel is low
carbon steel.
20. The cast steel strip of claim 19 wherein the low carbon steel
is a silicon/manganese killed steel.
21. The cast steel strip of claim 20 wherein the silicon/manganese
killed steel includes, by wt %:
5 Carbon 0.02-0.08% Manganese 0.30-0.80% Silicon 0.10-0.40% Sulphur
0.002-0.05% Aluminum less than 0.01%
22. The cast steel strip of claim 19 wherein the low carbon steel
is an aluminum killed steel.
23. The cast steel strip described in claim 22 wherein the aluminum
killed low carbon steel has the following composition by
weight:
6 Carbon 0.02-0.08% Manganese 0.40% max Silicon 0.05% max Sulphur
0.002-0.05% Aluminum 0.05% max
24. The cast steel strip of claim 15 further comprising the step of
in-line hot rolling the casted strip of step (a) prior to step
(b).
25. The cast steel strip of claim 15 wherein step (b) includes
cooling the casted strip to transform the strip from austenite to
ferrite at a selected cooling rate of at least 0.01.degree. C./sec
to produce a microstructure that provides required yield strength
of the casted strip, the microstructure being selected from a group
consisting of: (i) predominantly polygonal ferrite; (ii) a mixture
of polygonal ferrite and low temperature transformation products;
and (iii) predominantly low temperature transformation
products.
26. The cast strip of claim 25 wherein the cooling rate is selected
so that the microstructure is either (ii) a mixture of polygonal
ferrite and low temperature transformation products; or (iii)
predominantly low temperature transformation products.
Description
[0001] This application claims priority to Australian Provisional
Patent Application No. PR0460, filed Oct. 2, 2000.
BACKGROUND AND SUMMARY OF THE INVENTION
[0002] The present invention relates to a method of producing steel
strip and the cast strip produced according to the method.
[0003] In particular, the present invention relates to producing
steel strip in a continuous strip caster.
[0004] The term "strip" as used in the specification is to be
understood to mean a product of 5 mm thickness or less.
[0005] The applicants have carried out extensive research and
development work in the field of casting steel strip in a
continuous strip caster in the form of a twin roll caster.
[0006] In general terms, casting steel strip continuously in a twin
roll caster involves introducing molten steel between a pair of
contra-rotated horizontal casting rolls which are internally water
cooled so that metal shells solidify on the moving rolls surfaces
and are brought together at the nip between them to produce a
solidified strip delivered downwardly from the nip between the
rolls, the term "nip" being used to refer to the general region at
which the rolls are closest together. The molten metal may be
poured from a ladle into a smaller vessel from which it flows
through a metal delivery nozzle located above the nip so as to
direct it into the nip between the rolls, so forming a casting pool
of molten metal supported on the casting surfaces of the rolls
immediately above the nip and extending along the length of the
nip. This casting pool is usually confined between side plates or
dams held in sliding engagement with end surfaces of the rolls so
as to dam the two ends of the casting pool against outflow,
although alternative means such as electromagnetic barriers have
also been proposed. The casting of steel strip in twin roll casters
of this kind is for example described in U.S. Pat. Nos. 5,184,668,
5,277,243 and 5,934,359.
[0007] The concentration of residuals in the steel composition can
have a significant effect on the finished microstructure, and in
turn affect yield strength properties of cast strip. In particular,
higher concentrations of residuals make it possible to use lower
cooling rates to transform the strip from austenite to ferrite in a
temperature range between 850.degree. C. and 400.degree. C. to
produce microstructures in cast strip that provide high yield
strengths. It is understood that the transformation temperature
range is within the range between 850.degree. C. and 400.degree. C.
and not that entire temperature range. The precise transformation
temperature range will vary with the chemistry of the steel
composition and processing characteristics.
[0008] There is provided a method of producing steel strip which
includes the steps of:
[0009] (a) continuously casting molten steel into a strip including
austenite grains, said molten steel comprising a concentration of
residuals in the steel composition selected with regard to the
microstructure of the strip that is required to provide required
mechanical properties; and
[0010] (b) cooling the cast strip to transform the austenite grains
in the strip to ferrite in a temperature range between 850.degree.
C. and 400.degree. C.
[0011] The continuous caster may be a twin roll caster.
[0012] The term "residuals" covers levels of elements, such as
copper, tin, zinc, nickel, chromium, and molybdenum, that are
included in relatively small amounts, and are usually as a
consequence of standard steel making. By way of example, the
elements may be present as a result of using scrap steel to produce
steel.
[0013] In one embodiment, the total amount of the residuals is 1.2
wt % or less. These residuals may be up to 2.0 wt % where harder
steel strip is desired with yield strengths up to and in excess of
700 MPa. This weight percent is the total weight percent in the
steel strip including the residuals from scrap steel and steel
processing.
[0014] In one embodiment, the cast strip produced in step (a) may
have a thickness of no more than 2 mm.
[0015] In one embodiment, the cast strip produced in step (a) may
include austenite grains that are columnar.
[0016] The steel may be low carbon steel. The term "low carbon
steel" is understood to be mean steel of the following composition,
in wt %:
[0017] C: 0.02-0.08
[0018] Si: 0.5 or less;
[0019] Mn: 1.0 or less;
[0020] residuals: 1.2 or less; and
[0021] Fe: balance.
[0022] The low carbon steel may be silicon/manganese killed and may
have the following composition by weight:
1 Carbon 0.02-0.08% Manganese 0.30-0.80% Silicon 0.10-0.40% Sulphur
0.002-0.05% Aluminum less than 0.01%
[0023] The low carbon steel may be aluminum killed and may have the
following composition by weight:
2 Carbon 0.02-0.08% Manganese 0.40% max Silicon 0.05% max Sulphur
0.002-0.05% Aluminum 0.05% max
[0024] The aluminum killed steel may be calcium treated.
[0025] The method may further include the step of in-line hot
rolling the cast strip after step (a) and prior to step (b).
[0026] Step (b) may include cooling the strip to transform the
strip from the austenite to ferrite at a selected cooling rate of
at least 0.01.degree. C./sec, and usually at least 0.1.degree.
C./sec, to produce a microstructure that provides required yield
strength properties of the cast strip, the microstructure being
selected from a group that includes microstructures that are:
[0027] (i) predominantly polygonal ferrite;
[0028] (ii) a mixture of polygonal ferrite and low temperature
transformation products; and
[0029] (iii) predominantly low temperature transformation
products.
[0030] It is understood that most embodiments of the present
invention will have microstructures of types (ii) and (iii).
[0031] The term "low temperature transformation products" includes
Widmanstatten ferrite, acicular ferrite, bainite, and
martensite.
[0032] In order that the invention may be more fully explained, an
example will be described with reference to the accompanying
drawings, in which:
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIG. 1 illustrates a strip casting installation
incorporating an in-line hot rolling mill and coiler;
[0034] FIG. 2 illustrates details of the twin roll strip caster;
and
[0035] FIG. 3 illustrates the effect of residuals on yield strength
of cast strip.
DETAILED DESCRIPTION OF THE INVENTION
[0036] The following description is in the context of continuous
casting steel strip using a twin roll caster. The present invention
is not limited to the use of twin roll casters and extends to other
types of continuous strip casters.
[0037] FIG. 1 illustrates successive parts of a production line
whereby steel strip can be produced in accordance with the present
invention. FIGS. 1 and 2 illustrate a twin roll caster denoted
generally as 11 which produces a cast steel strip 12 that passes in
a transit path 10 across a guide table 13 to a pinch roll stand 14
comprising pinch rolls 14A. Immediately after exiting the pinch
roll stand 14, the strip passes into a hot rolling mill 16
comprising a pair of reduction rolls 16A and backing rolls 16B by
in which it is hot rolled to reduce its thickness. The rolled strip
passes onto a run-out table 17 on which it may be force cooled by
water jets 18 and through a pinch roll stand 20 comprising a pair
of pinch rolls 20A, and thence to a coiler 19.
[0038] As shown in FIG. 2, twin roll caster 11 comprises a main
machine frame 21 which supports a pair of parallel casting rolls 22
having a casting surfaces 22A. Molten metal is supplied during a
casting operation from a ladle (not shown) to a tundish 23, through
a refractory shroud 24 to a distributor 25 and thence through a
metal delivery nozzle 26 into the nip 27 between the casting rolls
22. Molten metal thus delivered to the nip 27 forms a pool 30 above
the nip and this pool is confined at the ends of the rolls by a
pair of side closure dams or plates 28 which are applied to the
ends of the rolls by a pair of thrusters (not shown) comprising
hydraulic cylinder units connected to the side plate holders. The
upper surface of pool 30 (generally referred to as the "meniscus"
level) may rise above the lower end of the delivery nozzle so that
the lower end of the delivery nozzle is immersed within this
pool.
[0039] Casting rolls 22 are water cooled so that shells solidify on
the moving roll surfaces and are brought together at the nip 27
between them to produce the solidified strip 12 which is delivered
downwardly from the nip between the rolls.
[0040] The twin roll caster may be of the kind which is illustrated
and described in some detail in U.S. Pat. Nos. 5,184,668 and
5,277,243 or U.S. Pat. No. 5,488,988 and reference may be made to
those patents for appropriate constructional details which form no
part of the present invention.
[0041] Typically, the strip passing from the twin roll caster will
be of the order of 1400.degree. C. and the temperature of the strip
presented to the hot rolling mill may be about 900-1100.degree. C.
The strip may have a width in the range of 0.9 m to 2.0 m and a
thickness in the range of 0.7 mm to 2.0 mm. The strip speed may be
in the order of 1.0 m/sec.
[0042] The cooling rate in transforming the strip from austenite to
ferrite in a temperature range between 850.degree. C. and
400.degree. C. is selected to be at least 0.01.degree. C./sec,
preferably at least 0.1.degree. C./sec, and may be in excess of
100.degree. C./sec. With such cooling rates for low carbon steel it
is possible to produce cast strip having microstructures
including:
[0043] (i) predominantly polygonal ferrite;
[0044] (ii) a mixture of polygonal ferrite and low temperature
transformation products, such as a acicular ferrite, Widmanstatten
ferrite, and bainite; and
[0045] (iii) predominantly low temperature transformation
products.
[0046] It is understood that most embodiments of the present
invention will have microstructures of types (ii) and (iii).
[0047] In the case of low carbon steels, such a range of
microstructures can produce yield strengths in excess of 450
MPa.
[0048] The concentration of residuals in the steel is selected
having regard to the finished microstructure of the cast strip that
is required to provide required mechanical properties for the
strip.
[0049] The present disclosure is based on experimental work that
has found the presence of a high amount of residuals (0.2% Cr, 0.2%
Ni, 0.2% Mo, 0.4% Cu, 0.2% Sn) has produced a strip with improved
microstructure.
[0050] The experimental findings include that the austenite
microstructure of strip cast at 75 m/min was similar to the
microstructure of strip without residuals. However, when the cast
strip with residuals was subjected to a standard cooling rate of
10-15.degree. C./sec, the resultant finished microstructure was
very different from that of the cast strip without residuals cooled
at the same rate.
[0051] The observed microstructure of cooled cast strip with
residuals was predominantly bainitic with only a thin band of grain
boundary ferrite appearing along the prior austenite grain
boundaries, indicating a severely suppressed ferrite transformation
caused by the presence of residuals. The mechanical properties of
the resultant product are very desirable, with typical values of
540 MPa yield strength, 650 MPa tensile strength and 15% total
elongation. Such values could be achieved in the past by
microalloying which added considerable cost to the production of
the cast strip.
[0052] The effect of residuals was to enhance the proportion of low
temperature transformation products (particularly the bainites) by
lowering austenite to ferrite transformation temperatures and
slowing the kinetics of polygonal ferrite formation.
[0053] One, but not the only one, of the important consequences of
this finding is that an increase in the concentration of residuals
effects a reduction in the cooling rate that is required to
transform austenite to ferrite to form a required microstructure to
provide high yield strengths.
[0054] Although the invention has been illustrated and described in
detail in the foregoing drawings and description with reference to
several embodiments, it should be understood that the description
is illustrative and not restrictive in character, and that the
invention is not limited to the disclosed embodiments. Rather, the
present invention covers all variations, modifications and
equivalent structures that come within the scope and spirit of the
invention. Additional features of the invention will become
apparent to those skilled in the art upon consideration of the
detailed description, which exemplifies the best mode of carrying
out the invention as presently perceived. Many modifications may be
made to the present invention as described above without departing
from the spirit and scope of the invention.
* * * * *