Thin strip casting of carbon steels

A substantially carbon free (e.g. 50-80 ppm carbon max.) iron base melt is strip cast to provide a cast strip having a low strength, high ductility, essentially ferrite matrix substantially free of hardening acicular ferrite, bainite and martensite phases. The strip strength may be enhanced by subjecting the strip to a carburizing or nitriding treatment either directly after casting or after casting followed by cold rolling and annealing.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the continuous casting of thin carbon steel strip 
and, more particularly, to such casting of a liquid steel containing 
carbon in a critical maximum amount of about 60 parts per million (ppm) 
(0.006 weight percent) to produce a product of low strength and high 
ductility which later may be strengthened, as by carburizing or nitriding 
the cast strip. 
2. Description of the Prior Art 
Continuous casting of carbon steels in the form of slabs having a thickness 
in the range, e.g., of 8 to 10 inches, at high casting speeds, e.g., 30 to 
80 inches per minute (ipm), has become very common in the steelmaking art, 
and today still is the conventional way to cast carbon steels. Such thick 
slab casting technology is well established for nearly all ranges of 
carbon level, including ultra-low (0.005% max.) carbon interstitial free 
steels, suitable for a wide variety of applications. Such technology 
includes the casting of very low carbon steels having relatively low 
strength and high ductility. An example is the use of such compositions in 
the manufacture of enameling steels, such as disclosed in Japanese patent 
numbers 60-110,845 and 60-221,520. To similar effect is U.S. Pat. No. 
5,460,665 disclosing the manufacture of a conventionally cast, hot rolled, 
cold rolled and annealed sheet of steel having an ultra-low carbon content 
of 0.004% maximum. As disclosed in the latter patent, the manufacture of 
sheets or strip of such steels may involve post-casting processing, such 
as hot rolling, pickling, cold rolling, and recrystallization annealing. 
Recently, there has been a trend, especially in the mini-mill sector, to 
cast thinner slabs (e.g. 2 to 4 inches thick) and at higher casting 
speeds, and the technology has been developed to produce steels with all 
ranges of carbon common to thick slab cast steels. This trend has further 
developed production of even thinner cast products. For example, Japanese 
patent number 61-133,324 shows the use of low carbon (up to 0.007%) steel 
in the production of thin steel ingots reduced by rolling to a thickness 
below 50 mm. Similarly, U.S. Pat. No. 4,586,966 discloses the production 
by continuous casting of thin (e.g. 10-40 mm) cast plate of low carbon 
(0.001-0.015%) steel which is directly cold rolled and annealed. 
In the manufacture of the above-mentioned products, it is known to add 
certain carbide, nitride and sulfide formers, such as titanium, niobium, 
vanadium, zirconium, boron, etc. to affect the properties of the cast and 
processed steel, e.g. by forming strengthening particulates of such 
elements. For example, the low carbon, slab-cast enamelling steel of 
Japanese Patent No. 60-110,845, mentioned above, contains 0.05-0.12% 
titanium in order to improve the steel surface, enhance press formability 
and avoid fish scaling. The above-mentioned U.S. Pat. No. 4,586,966 adds 
titanium, niobium or zirconium to the 0.0010 to 0.015%C steel in order to 
remove nitrogen as nitrides of these additive elements. U.S. Pat. No. 
5,578,143 is directed to the continuous slab casting of interstitial free 
(IF) steels of low carbon content (up to 0.005% in the base metal, and 
0.01-0.08% in a surface layer) and with the addition of at least one of 
titanium, niobium or zirconium to combine with the carbon and nitrogen as 
carbides, nitrides, or carbonitrides, of the respective additives. 
It is also known in the art to strengthen conventionally cast low carbon 
steels by carburizing or nitriding them, generally to form a hard outer 
layer or case on the steel. These processes may proceed by known means 
such as liquid or, more commonly, gas carburizing, e.g. in a natural gas 
atmosphere, or by nitriding, e.g. in an ammonia-containing gas atmosphere 
as described in U.S. Pat. No. 3,928,087, or U.S. patent application Ser. 
No. 08/773,205, filed Dec. 23, 1996 and assigned to the assignee hereof, 
which application is incorporated herein and made a part hereof by this 
reference. 
A third technique of continuous casting of carbon steels is currently being 
developed; that is, strip casting at low product thicknesses, e.g. about 
0.1 inch or less, and at very high casting speeds, e.g. about 1000-6000 
inches per minute (ipm). Examples of thin strip casting include U.S. Pat. 
No. 5,484,009 disclosing a casting method and apparatus wherein liquid 
steel is partially cooled by a rotating casting roll, leaving an upper 
surface of the cast strip in liquid form which subsequently is solidified. 
U.S. Pat. No. 5,520,243 discloses metal strip casting wherein quality of 
the cast strip is a function of the roughness of the casting and cooling 
roll, and the metal is vibrated during casting, providing possible thicker 
strip with higher K value. 
Metallurgically, strip casting of carbon steels is very different from 
conventional thick slab casting or even thin slab or plate casting, in 
that the cooling rates to which the strip cast steel is subjected are much 
higher, e.g. on the order of 2000.degree. C. per second, and rates as high 
as 10,000.degree. C./second may be involved. Such extremely high cooling 
rates are required in strip casting to be sure that the strip, or at least 
a substantial part of the thickness thereof, is solidified before leaving 
the mold or cooling roll surface at the extremely high casting speed 
necessary for practical commercial production justifying the capital 
investment and maintaining a competitive operating cost. The metallurgical 
structure produced in carbon steels is very dependent on the cooling rate 
during casting. Too high a cooling rate will produce undesirable phases 
such as acicular ferrite, bainite, or martensite, as exemplified in FIG. 1 
below. These phases are much higher in strength and lower in ductility 
than the typical ferrite structure produced with lower cooling rates for 
conventional thick slab or thin slab casting. These latter cooling rates 
are sufficiently low that these undesirable phases are not present in 
sufficient quantity to adversely affect the strength or ductility of the 
cast products. On the other hand, the high casting speeds and resulting 
required high quenching rates inherently associated with thin strip 
casting produce a cast strip with the undesirable properties, such as high 
hardness and brittleness, resulting from such unavoidable metallurgical 
structure. Coiling of such hard, brittle strip may result in strip 
cracking problems. It has been suggested that "the unique metallurgical 
structure of acicular ferrite, bainite and martensite found in thin strip 
cast products is a challenging starting point for subsequent 
thermomechanical processing of such cast strip in order to convert the 
cast microstructure to an acceptable condition having better mechanical 
properties". (AISI Strip Casting Update: July 1997) Such additional, 
post-casting processing might include high temperature anneals, e.g. 
austenitization followed by slow cooling--which could cause scaling 
problems--and then pickling. Thus even if the postulated thermomechanical 
processing of thin cast steel strip successfully changes the undesirable 
cast phases to acceptable ones, the achievement likely will be at the 
price of further processing yield losses and costs. 
SUMMARY OF THE INVENTION 
This invention is based on the finding that the undesirable hardening and 
embrittling acicular ferrite, bainite and martensite phases produced by 
the very high quench rates of thin strip casting of carbon steel can be 
substantially avoided, and low strength, ductile steel can be produced, by 
strip casting substantially carbon-free iron, such as an ultra-low carbon 
content steel having carbon below about 80 ppm, that is, in the region of 
solid solution of carbon in alpha iron, denoted as "X" in the well-known 
iron-carbon equilibrium diagram (FIG. 2 as appears in Metal Progress Data 
Sheet, November, 1946, Page 970), preferably 60 parts per million or less, 
especially about 50 ppm max. Reduction of amounts of hardening bainite and 
martensite with decreasing carbon content at various cooling rates is 
illustrated in the continuous cooling transformation diagrams of FIGS. 3A, 
4A and 5A, as published in 1978 by British Steel Corporation; and 
corresponding decrease of as-cooled hardness is shown in the corresponding 
prior art diagrams of FIGS. 3B, 4B and 5B. Thus-produced steel strip has a 
ferritic microstructure, substantially free of hardening acicular ferrite, 
bainite and martensite. Except for a finer grain structure, it is similar 
to conventionally thick or thin slab cast and slower cooled carbon steel, 
is relatively soft and ductile, and thereafter may be subjected to a 
post-casting treatment, such as carburizing or nitriding, for example, if 
higher strengths or lower ductilities are required.

DETAILED DESCRIPTION OF THE INVENTION 
Low carbon interstitial free steels are known and commercially produced by 
conventional thick and thin slab casting and applied to a wide range of 
applications. Examples of such steels of relatively low strength, e.g. 
about 20-26 ksi off-set yield strength, 40 ksi or greater ultimate tensile 
strength, n-value of about 0.220-0.260, and r.sub.m value of about 
1.8-2.2, are set out in Table I, wherein r.sub.m is the mean plastic 
anisotropy, which is calculated from the Lankford value measured in the 
longitudinal, transverse, and diagonal directions of the sheet, and 
defines drawability, i.e. resistance to thinning in a tensile test; and n 
is a work hardening exponent measuring the slope of the log stress vs. log 
strain curve in the region of uniform plastic strain. 
TABLE I 
______________________________________ 
Element Steel IA.sup.(2) 
Steel IB.sup.(2) 
Steel IC.sup.(1) 
______________________________________ 
Carbon 0.005 max 0.003 max 0.005 max 
Manganese 0.264 max 0.204 max 0.254 max 
0.095 min 0.146 min 0.095 min 
Phosphorous 0.020 max 0.015 max 0.020 max 
Sulfur 0.012 max 0.009 max 0.012 max 
Silicon 0.030 max 0.020 max 0.030 max 
copper 0.100 max 0.060 max 0.100 max 
Nickel 0.100 max 0.040 max 0.100 max 
Chromium 0.100 max 0.060 max 0.100 max 
Molybdenum 0.030 max 0.020 max 0.030 max 
Tin 0.030 max 0.020 max 0.030 max 
Aluminum 0.055 max 0.054 max 0.055 max 
0.020 min 0.020 min 0.020 min 
Nitrogen 0.006 max 0.003 max 0.006 max 
Niobium 0.045 max 0.035 max 0.004 max 
0.025 min 0.025 min 
Vanadium 0.008 max 0.008 max 0.004 max 
Boron 0.0007 max 0.007 max 0.007 max 
Titanium.sup.(1) 
0.040 max 0.040 max 0.080 max 
0.020 min 0.020 min 0.050 min 
Antimony 0.010 max 0.010 max 0.010 max 
______________________________________ 
.sup.(1) Ti.sub.min = (4 .times. C) + (1.5 .times. S) + (3.42 .times. N) 
.sup.(2) Ti = 3.42N + 1.5S and Nb = 7.74C 
The steel compositions set out in Table II are representative of 
commercially-produced higher strength interstitial free steels. 
TABLE II 
__________________________________________________________________________ 
Steel Number 
Element IIA IIB IIC IID IIE IIF 
__________________________________________________________________________ 
Carbon 
max. 
0.003 
0.005 
0.005 
0.005 
0.005 
0.005 
Manganese 0.25/ 
0.10/ 
0.10/ 
0.18/ 
0.25/ 
0.25/ 
0.35 0.25 
0.25 0.33 
0.35 0.35 
Phosphorous 
0.03/ 
0.025/ 
0.025/ 
0.04/ 
0.04/ 
0.035/ 
0.05 0.040 
0.040 
0.06 
0.06 0.055 
Sulfur 
max 0.012 
0.012 
0.012 
0.012 
0.012 
0.012 
Silicon 
max 0.035 
0.035 
0.035 
0.035 
0.035 
0.035 
Aluminum 0.02/ 
0.02/ 
0.02/ 
0.02/ 
0.02/ 
0.02/ 
0.05 0.05 
0.05 0.05 
0.05 0.05 
Nitrogen 
max 0.003 
0.006 
0.006 
0.006 
0.006 
0.006 
Titanium 0.01/ 
0.02/ 
0.02/ 
0.02/ 
0.02/ 
0.02/ 
0.02 0.04 
0.04 0.04 
0.04 0.04 
Niobium 
max 0.03 0.025/ 
0.025/ 
0.025/ 
0.025/ 
0.025/ 
0.04 0.045 
0.045 
0.045 
0.045 
0.045 
Boron 0.0006/ 
-- 0.0006/ 
0.0006/ 
-- 0.0006/ 
0.0012 0.012 
0.012 0.012 
__________________________________________________________________________ 
The yield strengths of these higher strength, conventionally cast carbon 
steels of Table II are about 25-35 ksi, the tensile strengths are about 
50+ ksi, the n-values are about 0.180-0.230 and the r.sub.m -values are 
about 1.4-1.8. 
Steels such as those given in Tables I and II and, indeed, substantially 
pure iron with almost no carbon (e.g. C.sub.max =50 ppm) are useful in the 
practice of the present invention. Alloying elements such as manganese, 
silicon, phosphorous, etc. may be added to the iron base melt to provide 
additional strengthening in the higher strength steels, if desired. Such 
steels may be produced, for example, in a top- or bottom-blown oxygen 
furnace wherein the heat is blown to a low carbon level, e.g. about 0.03 
to 0.05 wt. %, with oxygen level at about 500-900 ppm. The heat is tapped 
open, with no killing, or perhaps an oxygen trim with aluminum may be used 
if the oxygen is too high; about 200-300 ppm oxygen is needed for the 
subsequent carbon/oxygen reaction. The molten steel then is transferred 
from the ladle to a degasser, such as an RH degasser, to conduct a vacuum 
carbon deoxidation (VCD) reaction to reduce carbon to the desired 
ultra-low level. Then the steel may be killed with a deoxidant, such as 
aluminum; then titanium, niobium or similar carbide and nitride formers 
may be added to provide a stabilized interstitial free steel substantially 
free of carbon in solution and with any remaining carbon present as 
carbides in a ferrite matrix. 
I have found that, even when strip cast at the necessary rapid cooling 
rates, these steels are ferritic, i.e. polygonal or equiaxed ferrite, 
similar to the structure of conventional slab cast steel. Such cast strip 
is free of the above-mentioned undesirable hardening phases and is soft 
and ductile, with mechanical properties similar to those of conventionally 
thick or thin slab cast products, and useful, in the as-cast condition, 
for many practical applications such as automotive body parts, appliances, 
enamelling, etc. Although such products may be subjected to further 
thermomechanical processing such as cold rolling and annealing, they 
provide, for the first time in the art, the possibility of practical 
application directly in the as cast condition. To broaden the possible 
field of applications, e.g. those requiring higher strength with similar 
or lower ductility, this invention includes subjecting the cast strip 
product to a strengthening carburizing or nitriding treatment. Because the 
strip, as cast, is very thin, e.g. 0.10 to 0.125 inch or less, it is 
possible, within practical time limits, to carburize or nitride the full 
thickness of the strip to provide uniform through thickness mechanical 
properties. If the steel, as cast, contains no carbide/nitride formers, 
such as titanium, niobium, zirconium, vanadium, boron, etc., on 
carburizing, the steel is strengthened mainly by free carbon in solution 
in the iron matrix. If carbide formers are present, particle strengthening 
may occur due to carbide precipitation. As above noted, the steel contains 
one or more of the aforesaid nitride formers when the steel is to be 
strengthened by nitriding, after which the thus-treated steel has a higher 
strength, e.g. yield strength of 45 ksi or more as a function of nitride 
particle hardening and, to a lesser extent, from the presence of excess 
soluble nitrogen, and r.sub.m -value at least up to 1.8, especially after 
cold rolling. Thus, to further improve r.sub.m -value and n-value, the 
as-cast strip may be subjected to further processing, as cold rolling 
prior to annealing, but an important object of the invention is to provide 
final products in the form of the as-cast steels, either as-is, or 
strengthened by carburizing or nitriding. 
In view of the above-mentioned major difficulties being encountered in the 
development of strip casting, this invention of casting an almost pure 
iron with almost no carbon, followed by a strengthening post-treatment 
such as carburizing or nitriding, provides, for the first time, an 
economical way to avoid those difficulties and to produce, by strip 
casting, a wide range of commercially useful products.