Alloy steel for roll caster shell

Alloy steel having particular utility for roll caster shells used in the direct casting of molten aluminum to sheet, consisting essentially of from 0.50% to 0.60% carbon, about 0.40% to about 1.0% manganese, 0.10% to 0.30% silicon, about 0.02% maximum phosphorus, about 0.02% maximum sulfur, 0.40% to 0.90% nickel, 1.50% to 3.00% chromium, 0.80% to 1.20% molybdenum, 0.30% to 0.50% vanadium, and balance essentially iron. Caster shells fabricated from the steel exhibit markedly longer service life.

BACKGROUND OF THE INVENTION 
This invention relates to an alloy steel for fabrication into roll caster 
shells used in the direct casting of molten aluminum to sheet, and to a 
roll caster shell made therefrom. The steel of this invention exhibits 
markedly longer service life than the standard prior art steel used for 
this purpose. 
Molten aluminum is direct cast at a temperature of about 675.degree. C. 
(1250.degree. F.) to sheet thickness between pairs of water cooled roll 
caster shells. After a number of hours of operation conventional roll 
shell surfaces develop heat checks or cracks which gradually penetrate 
deeper into the shell, causing marks on the cast strip and eventually 
breaking the shell. Periodic shutdowns are thus necessary in order to 
remachine the shell surfaces and eliminate the cracks. Each remachining 
may remove about 15% of the thickness of the shell, and eventually the 
shells must be scrapped after production of about 5 to 8 million pounds of 
aluminum, under present practice. Typically a shell made from the standard 
alloy steel operates for about 300 to about 500 hours before the first 
surface remachining (representing about 1 million pounds of aluminum), 
which reduces the shell to about 85% of its original thickness. In 
contrast to this, the steel of the present invention can operate for about 
1500 to 2000 hours (representing about 4.5 million pounds of aluminum) 
before the first remachining, and requires reduction only to about 95% of 
its original thickness. At least about 10 million pounds of sheet aluminum 
can be produced with the roll caster shells of the present invention 
before scrapping thereof. 
The standard alloy steel now used for roll caster shells comprises, in 
weight percent, from 0.53% to 0.58% carbon, 0.45% to 0.65% manganese, 
0.20% to 0.30% silicon, about 0.02% maximum phosphorus, about 0.02% 
maximum sulfur, 0.40% to 0.50% nickel, 1.0% to 1.2% chromium, 0.45% to 
0.55% molybdenum, 0.10% to 0.15% vanadium and balance essentially iron. 
The standard aluminum die casting die steel now used, designated as H-13, 
comprises, in weight percent, from 0.30% to 0.40% carbon, 0.20% to 0.40% 
manganese, 0.80% to 1.20% silicon, 4.75% to 5.50% chromium, 1.25% to 1.75% 
molybdenum, 0.80% to 1.20% vanadium, and balance essentially iron. This 
alloy can be used for roll caster shells, but it is expensive and can 
present processing difficulties. 
Literature references relating to thermal fatigue, thermal cracking, high 
temperature alloys and alloying elements are as follows: 
Glenny, E., "Thermal Fatigue," Metallurgical Reviews, 1961, Vol. 6, No. 24. 
Sobolev, N. D. and Egoror, V. I., "Thermal Fatigue and Thermal Shock," 
Strength and Deformation in Non-uniform Temperature Fields, Edited by Ya. 
B. Fridman, Consultants Bureau, New York, 1964. 
High Temperature High Strength Alloys, AISI Publication, No. 601, New York, 
February, 1963. 
Benedyk, J. C. et. al., "Thermal Fatigue Behavior of Die Materials for Al 
Die Casting," Paper 111, 6th SDCE International Die Casting Congress, 
Cleveland, Ohio, Nov. 16-19, 1970. 
Young, W., "Are You Getting Maximum Performance From Your Die Casting 
Dies?" ASTME Paper, CM 68-587, 1968. 
Northcott, L., and Baron, H. G., "The Craze-Cracking of Metals," Jnl. of 
ISI, Dec. 1956. 
Glenny, E., "Thermal Fatigue Resistance of Martensitic Steels," Jnl. of 
Matls., JMLSA, Vol. 4, No. 1, March 1969. 
Rostoker, W., "Thermal Fatigue Resistance of Martensitic Steels," Jnl. of 
Matls., JMLSA, Vol. 4, No. 1, March 1969. 
Bain, E. C., and Paxton, H. W., Alloying Elements in Steel, ASM Publ., 
1966. 
Archer, R. S., et. al., Molybdenum, Steels-Irons-Alloys, Climax Molybdenum 
Co. Publ., N.Y., 1962. 
Vanadium, Steels and Irons, Vanadium Corp. of America, N.Y., 1937. 
SUMMARY OF THE INVENTION 
During the direct casting of molten aluminum to sheet thickness on pairs of 
water cooled roll caster shells, rapid heating and cooling of the shell 
surfaces occur. As a result, significant stresses can be developed. If 
these stresses exceed the yield strength and are tensile in nature, heat 
checks or cracks are developed on the shell surfaces. It is generally 
agreed by those skilled in the art that heat checks of this type are 
formed by a thermal fatigue mechanism when cyclic yielding or plastic flow 
of the material occurs. 
On start-up the surface of the roll caster shell is believed to be in 
tension because of the residual tensile stresses resulting from heat 
treatment and the shrink fit on the core. Although the magnitude of such 
stress is not known, it is assumed to be elastic, i.e. below the yield 
strength and hence insufficient to cause plastic damage. During the rapid 
heat-up in the casting operation, the surface of the shell attempts to 
expand but is held back by the bulk of the shell material below the 
surface. If the temperature differential is sufficiently great, the shell 
surface can exceed the yield strength in compression, and localized 
"buckling" of the material can occur. Upon cooling the surface will tend 
to contract back to its original dimensions but is restrained from doing 
so because of the compressive plastic deformation which occurred 
previously. With the drop in temperature, the tensile stress increases 
rapidly, exceeding the yield strength, and plastic flow occurs, probably 
in the valleys of the buckled areas. The plastic deformation per cycle 
contributes to the development of thermal fatigue cracks. 
The total thermal strain (.epsilon..sub.t) is derived from the equation 
EQU .epsilon..sub.t =.alpha..DELTA.T 
where .alpha. is the coefficient of thermal expansion and .DELTA.T is 
1150.degree. F. (maximum roll surface temperature of 1250.degree. F. and 
minimum roll surface temperature of 100.degree. F.). 
The total strain is assumed to be composed of elastic and plastic 
components. The elastic component can be represented by the elevated 
temperature strength in tension divided by the elastic modulus. The 
plastic component of the total strain can be determined if an accurate 
estimate of the elastic component can be made. Such calculations are very 
useful because they provide a quantitative measure of physical properties 
necessary to reduce the onset of thermal fatigue and heat checking. 
It has been recognized in the prior art that minimizing heat checking is 
empirically dependent upon control of such properties as coefficient of 
thermal expansion, thermal conductivity, elevated temperature yield 
strength, elevated temperature ductility and elevated temperature modulus 
of elasticity. More specifically, it has been believed that optimum 
results are obtained with a low coefficient of thermal expansion, high 
thermal conductivity, high elevated temperature yield strength, high 
elevated temperature ductility and low elevated temperature modulus of 
elasticity. Unfortunately, no known alloy system exhibits this combination 
of properties, and attempts to improve one of the properties in a 
particular alloy system usually results in the sacrifice of another. For 
example, an increase in yield strength typically results in a decrease in 
ductility in a steel alloy. Substitution of a copper base alloy would 
result in much higher thermal conductivity and a lower modulus of 
elasticity (both of which are desirable), but the coefficient of thermal 
expansion is high and the yield strength is low. Similar problems arise 
with respect to austentitic stainless steels. 
The literature has reported that AISI Type 347 austenitic stainless steel 
and A-286 (an austenitic precipitation hardening steel) spall severely 
when exposed to molten aluminum less than 1000 times. A nickel base alloy 
(Waspaloy) was also reported to exhibit similar behavior. A molybdenum 
base alloy designated as TZM exhibited excellent properties but is 
prohibitive in cost. 
A low coefficient of thermal expansion, a low modulus of elasticity at 
elevated temperature and a high yield strength are known to be beneficial, 
because the total thermal strain is reduced and a greater proportion of 
the strain occurs in the elastic range of the alloy. However, once plastic 
flow occurs, thermal fatigue damage is initiated, and thereafter the roll 
shell life is dependent on the inherent ductility of the material, its 
resistance to a potentially corrosive environment, and any superimposed 
mechanical fatigue cycle. 
The present invention represents a discovery that an increase in elevated 
temperature yield strength of about 50% to 100% over that of the standard 
roll caster shell alloy steel composition surprisingly results in as much 
as a three-fold increase in service life, if the elevated temperature 
ductility is retained, despite the facts that the coefficient of thermal 
expansion and elevated temperature modulus of elasticity are not 
relatively low, and that the thermal conductivity is relatively low. It 
has been accepted in the prior art that the coefficient of thermal 
expansion and elevated temperature modulus of elasticity should be low and 
the thermal conductivity should be high, in order to minimize heat 
checking. 
The increasing yield strength of the steel of the present invention thus 
unexpectedly overcomes or compensates for supposed deficiencies in other 
properties considered by those skilled in the art to be controlling in 
minimizing heat checking. 
It is an object of the invention to provide an alloy steel having a minimum 
of expensive alloying elements which can be fabricated in conventional 
manner into roll caster shells having superior resistance to thermal 
fatigue. 
According to the present invention, there is provided a ferritic alloy 
steel providing long service life in a roll caster shell used in the 
continuous casting of the molten aluminum, consisting essentially of, in 
weight percent, from 0.50% to 0.60% carbon, about 0.40% to about 1.0% 
manganese, 0.10% to 0.30% silicon, about 0.02% maximum phosphorus, about 
0.02% maximum sulfur, 0.40% to 0.90% nickel, 1.50% to 3.00% chromium, 
0.80% to 1.20% molybdenum, 0.30% to 0.50% vanadium, and balance 
essentially iron.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, a representative type of vertical caster for molten 
aluminum is shown. Molten aluminum is maintained at a constant level in a 
headbox (not shown) positioned in such manner that the molten metal, 
indicated at 10, flows by gravity into a distribution box indicated at 11 
in which it is directed upwardly through a lip assembly 12 into a freezing 
zone 13. A pair of water-cooled rolls, indicated generally at 14, is 
provided which are driven in counter-rotating directions as shown by 
arrows in FIG. 1. The bite of rolls 14 is slightly above the freezing zone 
13, so that the molten aluminum solidifies just before reaching the bite 
of the rolls and is hot rolled as it passes therebetween. 
Each caster roll 14 comprises a roll shell 15 which is a forged, heat 
treated hollow cylinder fabricated from alloy steel. A core 16 is provided 
on which the outer shell 15 is mounted by a shrink fit, i.e. the shell is 
heated, causing it to expand, and is slipped over the core. The shell then 
shrinks upon cooling to fit tightly around the core. An axial drive shaft 
is indicated at 17 which is provided with cooling water conduits indicated 
at 18. A plurality of radial tubes indicated at 19 is provided 
communicating with conduits 18 to conduct cooling water outwardly to the 
inner surface of the shell 15. 
A caster of the type described above is known in the art, and the present 
invention relates only to the outer roll caster shell 15 thereof. 
Caster shells for this application are typically electric furnace, air 
melted heats. They are cast into ingots, soaked at temperatures of about 
1225.degree. C., pierced and open die forged on a mandrel to tubes 600 mm 
to 1000 mm in diameter. They are subsequently austenitized at 870.degree. 
C., followed by an oil quench and tempering to the desired strength level. 
The steel of the present invention is processed in this manner. 
As indicated hereinabove the ferritic alloy steel of the present invention 
has been found to prolong the service life of roll caster shells of the 
type illustrated in FIG. 1 by at least 100% and as much as 300% under 
optimum conditions, by reason of the relatively high yield strength and 
ductility of the steel at elevated temperature. 
The percentage ranges of carbon, silicon, chromium, molybdenum and 
vanadium, and the balancing therebetween are critical, and departure 
therefrom results in a loss of the combination of high yield strength and 
ductility at elevated temperature. To a lesser extent the nickel range is 
also critical. 
Carbon is present in order to provide hardness and strength necessary for 
thermal fatigue resistance at high temperatures. A minimum of 0.50% is 
necessary for this purpose, while a maximum of 0.60% is observed in order 
to avoid thermal cycling above and below the A.sub.3 temperature. It has 
been found that if the range of temperature fluctuations includes a phase 
change, an increase in the carbon content can increase the degree of 
thermal fatigue cracking. Carbon is maintained within the range of 0.50% 
to 0.60%, and preferably from about 0.53 to about 0.58%, in order to 
ensure adequate strength which is believed to result from its distribution 
in the microstructure of the steel and resistance to localized softening. 
Silicon is generally present in steel since it is used as a primary 
deoxidizer. Although silicon is soluble in ferrite in amounts up to about 
1% and provides increased strength, silicon is restricted to a maximum of 
0.30%, and preferably about 0.20%, in the steel of the present invention 
in order to avoid a decrease in plasticity and the possibility of silicate 
inclusions at grain boundaries. 
Chromium is essential in order to confer strength and oxidation resistance 
at elevated temperature. Its carbide-forming tendency increases the 
elevated temperature strength of carbon steels up through about 
700.degree. C. Chromium also has a tendency to raise the eutectoid 
temperature, thereby stabilizing the ferrite to higher temperatures. In 
the steel of the invention the above benefits are obtained by chromium 
additions within the range of 1.50% to 3.00%. An increase above 3.00% is 
undesirable since it would tend to reduce ductility and increase the cost. 
Preferably chromium ranges from about 1.90% to about 2.30%. 
Molybdenum is also a strong carbide-forming element and high temperature 
strength is increased thereby. Molybdenum raises the eutectoid temperature 
and counteracts temper embrittlement during heat treatment of the steel. 
For these reasons a minimum of 0.80% molybdenum is required, and a maximum 
of 1.20% is observed since a loss in impact toughness may occur at higher 
levels. A molybdenum range of about 0.90% to about 1.10% is preferred. 
Vanadium is also a carbide former and is added for the purpose of 
increasing the elevated temperature strength. Vanadium in solution has a 
marked effect on hardenability, and a mininum of 0.30% is required for 
these reasons. A maximum of 0.50% and preferably about 0.35% is observed 
since levels in excess thereof may adversely affect impact toughness. 
Nickel is required within the range of 0.40% to 0.90% in order to promote 
toughness, thereby balancing the tendency of chromium, molybdenum and 
vanadium toward decreasing toughness. A maximum of 0.90% is desired in 
order to avoid the tendency of nickel to retain austenite after quenching 
and to minimize cost. A range of about 0.45% to about 0.55% nickel is 
preferred. 
A manganese range of about 0.40% to about 1.00% is desirable for 
hardenability, formation of manganese sulfides and deoxidation. Levels 
above 1.00% would tend to increase the tendency toward heat checking or 
cracking because of the austenite stabilizing tendency of manganese after 
heat treatment and quenching, and a maximum of 0.70% is preferred. 
Phosphorus and sulfur are ordinarily present as residual elements, and each 
should be restricted to a maximum of about 0.02% in order to avoid 
embrittlement and an increased tendency toward formation of heat checks or 
cracks. 
Other carbide-forming elements such as tungsten, columbium and titanium 
could be added to the steel of the present invention as partial 
substitutes for molybdenum or vanadium in amounts not exceeding about 0.2% 
each. As is the case with molybdenum and vanadium excessive amounts of 
tungsten, columbium and/or titanium would adversely affect ductility and 
impact toughness. 
A preferred steel of the invention thus consists essentially of, in weight 
percent, from about 0.53% to about 0.58% carbon, about 0.40% to about 
0.70% manganese, 0.10% to about 0.20% silicon, about 0.02% maximum 
phosphorus, about 0.02% maximum sulfur, about 0.45% to about 0.55% nickel, 
about 1.90% to about 2.30% chromium, about 0.90% to about 1.10% 
molybdenum, 0.30% to about 0.35% vanadium, and balance essentially iron. 
It will be understood that any one or more of the preferred ranges above 
can be used with any one or more of the broad ranges for the remaining 
elements indicated above. 
If the stresses are greater than the yield strength and tensile in nature, 
heat checks or cracks will be produced by a thermal fatigue mechanism when 
cyclic yielding or plastic flow occurs. A hysteresis loop can be plotted 
representing the accumulation of plastic damage during each cycle for the 
circumferential stresses perpendicular to the longitudinal cracks in the 
roll shell surface. In prior art chromium-molybdenum steels the number of 
cycles to failure is about 10.sup.4 if the extent of plastic deformation 
per cycle is about 0.01 inch per inch or slightly less. 
In the equation set forth above for total thermal strain (.epsilon..sub.t), 
wherein the total strain is assumed to be the sum of elastic and plastic 
components. 
##EQU1## 
where .theta.y is the yield strength in tension and E is the elastic 
modulus. Thus, the elastic component of the strain is represented by the 
yield strength in tension divided by the elastic modulus. In the case of a 
steel having a yield strength of 200,000 psi at room temperature 
##EQU2## 
If the yield strength decreases by 50% at elevated temperature (such as 
650.degree. C.) 
##EQU3## 
Where .epsilon..sub.t is equal to or greater than twice the elastic strain 
(.epsilon..sub.Elastic), to account for the compression and tension 
portion of the elastic reaction, then plastic flow is possible in both the 
compression and tension ends of the cycle. With a 50% decrease in yield 
strength to 100,000 psi, 2.times..epsilon..sub.Elastic 
=2.times.4.16.times.10.sup.-3 =8.32.times.10.sup.-3 in/in. then the 
plastic component becomes .epsilon..sub.Plastic =.epsilon..sub.t 
-2.times..epsilon..sub.Elastic =8.0.times.10.sup.-4 in/in. 
Consequently, a plastic flow of about 0.001 inch per inch per cycle is 
possible, which would indicate a potential exhaustion of plasticity and 
failure in 10.sup.4 to 10.sup.5 cycles. 
Although not intending to be bound by theory, the above calculations 
support applicants' belief that the high elevated temperature yield 
strength of the steel of the invention causes a much greater percentage of 
the thermal expansion and contraction to occur in the elastic region. This 
minimizes the plastic reaction and results in much greater resistance to 
heat checking. Maintenance of the high ductility of the steel at a higher 
yield strength insures a regarded crack growth rate once heat checking 
does occur. 
Compositions of two conventional steels now used for roll caster shells and 
two preferred steels of the invention are set forth in Table I. The 
elevated temperature mechanical properties (at 650.degree. C.) of the 
steels of Table I are compared in Table II. All samples were heat treated 
by austenitizing at about 870.degree. C., oil quenched and tempered. It is 
evident that the yield strengths of alloys 3 and 4 (steels of the 
invention) ranged from about 50% to about 100% higher than those of the 
prior art steels, yet the ductility of the steels of the invention, as 
measured by percent elongation in 5 cm and percent reduction in area, was 
at least equal to that of the prior art steels. The variation in yield & 
tensile strengths of steels 3 and 4 at elevated temperature is within the 
range normally to be expected. A 50% to 100% increase in yield strength at 
650.degree. C. can result in at least a 100% increase in service life, as 
shown by the graph of FIG. 2 from which it is evident that a shell 
fabricated from a prior art steel has been remachined several times down 
to 50% of its original thickness between 1500 hours and 2000 hours of 
casting operation. In contrast to this, experimental shells fabricated 
from a steel of the present invention have been remachined only to about 
85% of original thickness at 3500 hours of casting operation. Further 
remachining down to the same level of 50% of original shell thickness (as 
in the case of the prior art steel) would be expected to permit at least 
about 5000 hours of casting operation before scrapping, which would 
represent at least about 10 million pounds of sheet aluminum production. 
It is therefore believed to be demonstrated that the ferritic alloy steel 
of the present invention provides a relatively low cost product which can 
be fabricated in conventional manner by forging and heat treatment into a 
tubular roll caster shell which will result in at least a 100% and as much 
as a 300% increase in service life. 
Modifications may be made in the invention without departing from the 
spirit and scope thereof, and it will be understood that all matter 
described herein is to be interpreted as illustrative and not as a 
limitation. 
TABLE I 
______________________________________ 
Compositions in Weight Percent 
Alloy C Mn Si P S Ni Cr Mo V 
______________________________________ 
1-Prior 
0.55 0.61 0.23 -- -- 0.47 1.12 0.42 0.13 
Art 
2-Prior 
0.51 0.56 0.32 0.004 
0.003 
0.45 1.13 0.54 0.16 
Art 
3-Pres. 
0.57 0.49 0.13 0.014 
0.015 
0.47 2.03 1.00 0.33 
Inv. 
4-Pres. 
0.54 0.54 0.13 0.014 
0.013 
0.48 2.13 1.01 0.34 
Inv. 
______________________________________ 
TABLE II 
______________________________________ 
Mechanical Properties at 650.degree. C. 
0.2% Yield Tensile 
Strength Strength % Elong. 
Alloy (ksi) (MPa) (ksi) 
(MPa) in 5 cm 
% R.A. 
______________________________________ 
1-Prior Art 
31.9 220 49.4 341 40 94 
2-Prior Art 
28.2 195 47.0 324 55 98 
3-Pres. Inv. 
68.8 475 89.0 614 47 94 
4-Pres. Inv. 
45.8 316 67.6 466 52 95 
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