Low alloy steel for caster shell applications

A low-alloy steel suitable for use in caster shells for continuous aluminum casting operations. It has a lower carbon and chromium content than prior art steels, and exhibits high yield strength at elevated temperatures, excellent toughness over the entire temperature range of aluminum casting, and decreased heat checking.

TECHNICAL FIELD 
The present invention relates to a low-alloy steel for use in caster shells 
in continuous aluminum casting processes. The steel of the present 
invention resists cracking and exhibits high strength and high toughness 
at temperatures from 40.degree. to 650.degree. C. 
BACKGROUND ART 
During continuous casting of aluminum, the solidification and formation of 
the strip occur in contact with forged caster shells having an outer 
diameter of about 0.5-1.0 meters (21-40 inches). The casting speed is 
nominally about 1 RPM. During operation the caster shells are water 
cooled. The maximum temperature is approximately 575.degree. C. to about 
650.degree. C., while the minimum temperature is as low as about 
40.degree. C. Thus, the surface of the shell is exposed to a temperature 
variation of about 600.degree. C. with a cycle time of about 1 minute. The 
rapid thermal cycling has the consequence that extensive thermal fatigue 
cracking or "heat checking", takes place. The resulting crack pattern of 
the caster shell is imparted to the aluminum strip, thus producing 
unacceptable aluminum product. In operation, the caster shell must be 
disassembled and the crack pattern removed by machining. Extensive or 
repeated heat checking reduces the service life of the caster shell and 
increases the down time and the operating cost of the casting apparatus. 
U.S. Pat. No. 4,409,027 issued to Cordea, et al. on Oct. 11, 1983 discloses 
a steel for use in caster shells for continuous aluminum casting. The 
steel described therein has a carbon content of about 0.53% to 0.58% and a 
chromium content of about 1.5% to 3.0%. While this steel is commonly 
currently used for caster shells, it is still prone to heat checking. 
Literature references to thermal fatigue, thermal cracking, high 
temperature alloys and alloying elements include: 
Nes, E. and Fartum, P., "Thermal Fatigue of Caster Shell Steels", 
Scandinavian Journal of Metallurgy, 1983, Vol. 12, pp. 107-111. 
Sandstrom, R., Samuelsson, A., Larsson, L., and Lundberg, L., "Crack 
Initiation and Growth during Thermal Fatigue of Aluminum Caster Shells", 
Scandinavian Journal of Metallurgy, 1983, Vol. 12, pp. 99-105. 
Chavanne-Ketin, "Shells and Roll Cores for Aluminium Continuous Casters", 
pp. 1-19. 
DISCLOSURE OF THE INVENTION 
The steel of the present invention is suitable for use in caster shells for 
continuous aluminum casting. It is a low alloy steel consisting 
essentially of, in weight percent, from about 0.30% to about 0.35% carbon, 
about 0.30% to about 0.60% manganese, about 0.015% maximum phosphorus, 
about 0.010% maximum sulfur, about 0.25% to about 0.40% silicon, about 
0.30% to about 0.60% nickel, about 1.25% to about 1.5% chromium, about 
0.90% to about 1.2% molybdenum, about 0.25% to about 0.35% vanadium, about 
0.40% to about 0.60% tungsten, about 0.001% to about 0.003% boron, about 
0.010% to about 0.015% nitrogen, with the balance being essentially iron. 
The steel exhibits a 0.2% yield strength of at least 60 ksi (415 MPa) and 
an elongation in 5.08 cm of at least 25% at 650.degree. C. after forging 
and heat treating. The steel also exhibits a charpy impact value of at 
least 20 ft-lb (27.5 joules) at 40.degree. C. and 50 ft-lb (68.5 joules) 
at 650.degree. C. Finally, the steel has significant resistance to heat 
cracking in the temperature range of about 40.degree. to about 650.degree. 
C. 
The preferred steel of the present invention consists essentially of, in 
weight percent, about 0.32% carbon, about 0.50% manganese, about 0.015% 
maximum phosphorus, about 0.010% maximum sulfur, about 0.30% silicon, 
about 0.50% nickel, about 1.35% chromium, about 1.00% molybdenum, about 
0.30% vanadium, about 0.50% tungsten, about 0.002% boron, about 0.012% 
nitrogen, with the balance being essentially iron. 
The steel of the present invention may be fashioned into a 
cylindrically-shaped caster shell and used in continuous aluminum casting. 
It exhibits the yield strength, elongation, and charpy impact values 
described above. Further, it has significant resistance to heat cracking 
during thermal cycling in the temperature range of about 40.degree. to 
about 650.degree. C.

BEST MODE FOR CARRYING OUT THE INVENTION 
It is generally accepted among those skilled in the art that an optimal 
steel for use in caster shells would have the following properties: (1) a 
low coefficient of thermal expansion, (2) a high thermal conductivity, (3) 
a high yield strength at elevated temperatures, (4) a high degree of 
ductility at elevated temperatures, and (5) a low modulus of elasticity at 
elevated temperatures. In practice however, it is has been difficult to 
produce a steel with all of these properties. Prior art attempts have 
focused primarily on obtaining a high yield strength at elevated 
temperatures. While important, high yield strength must be balanced by 
high temperature ductility and excellent toughness over the complete 
operating temperature range of an aluminum casting process in order to 
minimize heat checking. Moreover, the steel should be economical to 
produce and should be obtainable using common forging and heat treatment 
processes. 
The present invention discloses a low-alloy steel having a high yield 
strength at elevated temperatures, as well as ductility and excellent 
toughness throughout the entire operating temperature range of the casting 
process. Moreover, the steel of the present invention contains only a 
minimum of expensive alloying elements and can be fabricated using 
conventional forging and heat treating techniques well known in the art, 
including melting, secondary refining, forging, quenching and tempering. 
The composition of the present steel is shown in Table 1 below, along with 
the compositions of the two steels most commonly used for caster shells. 
Alloy 1 is the steel disclosed in U.S. Pat. No. 4,409,027 issued to Cordea 
and obtainable from Armco Inc. Alloy 2 is a steel obtainable from 
Chavanne-Ketin. Alloy 3 is the steel of the present invention. As can be 
seen from the table, both the carbon content and chromium content of the 
present steel are substantially lower than those of the prior art steels. 
In addition, the present steel includes a few elements not present in the 
prior art steels, including tungsten, boron and nitrogen. 
TABLE 1 
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Composition of Alloy Steels by Weight Percent 
Alloy 1 Alloy 2 Alloy 3 
Element (Cordea) (C-K) (Present Invention) 
______________________________________ 
Carbon .53-.58 .30-.35 .30-.35 
Manganese .40-1.00 .40-.60 .30-.60 
Silicon .10-.20 .25-.40 .25-.40 
Phosphorus 
.02 max .015 max .015 max 
Sulfur .02 max .010 max .010 max 
Nickel .45-.55 .20-.40 .30-.60 
Chromium 1.5-3.0 2.9-3.2 1.25-1.5 
Molybdenum 
.80-1.2 .90-1.1 .90-1.2 
Vanadium .30-.50 .15-.25 .25-.35 
Tungsten N/A N/A .40-.60 
Boron N/A N/A .001-.003 
Nitrogen N/A N/A .010-.015 
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A comparison of the tensile properties of the steel of the present 
invention with that of Cordea is shown in Table 2, below. As can be seen 
from the table, the steel of the present invention exhibits higher yield 
strength at elevated temperatures, comparable tensile strength at elevated 
temperatures, while maintaining satisfactory elongation and reduction of 
area values over the temperature range tested. 
TABLE 2 
______________________________________ 
Alloy 3 
Temperature Alloy 1 (Present 
Property .degree.C. (.degree.F.) 
(Cordea) Invention) 
______________________________________ 
0.2% Yield 20 1309 1102 
Strength (70) (190) (160) 
MPa (ksi) 
650 482 551 
(1200) (70) (80) 
Tensile 20 1447 1205 
Strength (70) (210) (175) 
MPa (ksi) 
650 620 620 
(1200) (90) (90) 
% Elongation 
20 12 15 
in 5 cm (2 in.) 
(70) 
650 35 25 
(1200) 
% Reduction 
20 16 45 
of Area (70) 
650 95 85 
(1200) 
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Finally, Table 3 is a comparison of the toughness (charpy impact value) of 
the Cordea steel and the steel of the present invention. As can be seen 
from the table, the toughness of the present steel is excellent throughout 
a broad temperature range and at elevated temperatures is markedly higher 
than the toughness of the Cordea steel. This is critical from a fracture 
mechanics viewpoint, since increased toughness retards crack propagation. 
TABLE 3 
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Toughness 
at varying Alloy 1 Alloy 3 
Temperature .degree.C. (.degree.F.) 
(Cordea) (Present Invention) 
______________________________________ 
Joules (ft-lb) 
0 (30) 18 (13) 18 (13) 
32 (90) 22 (16) 22 (16) 
65 (150) 27 (20) 30 (22) 
93 (200) 30 (22) 51 (37) 
150 (300) 34 (24) 64 (46) 
204 (400) 37 (27) 66 (48) 
315 (600) 38 (28) 71 (52) 
427 (800) 40 (29) 77 (56) 
540 (1000) 41 (30) 77 (56) 
______________________________________ 
The preferred composition of the present invention is as follows: about 
0.32% carbon, about 0.50% manganese, about 0.015% maximum phosphorus, 
about 0.010% maximum sulfur, about 0.30% silicon, about 0.50% nickel, 
about 1.35% chromium, about 1.00% molybdenum, about 0.30% vanadium, about 
0.50% tungsten, about 0.002% boron, about 0.012% nitrogen, with the 
balance being essentially iron. 
From the foregoing, it will be appreciated that, although embodiments of 
the invention have been described herein for purposes of illustration, 
various modifications may be made without deviating from the spirit and 
scope of the invention. Accordingly, the invention is not limited except 
as by the appended claims.