[0001] This invention relates to the production of rim-stabilized steel ingots.
[0002] United States Patent No. 3,754,591 is directed to a method for making rim-stabilized
steel ingots whereby a rimming-type steel is teemed into an ingot mold until the mold
is about 80 to 95% full, whereupon teeming is interrupted to allow a rimming action
in the mold for a period of from 1/2 to 15 minutes. Thereafter, teeming is continued
until the mold is full. After teeming is commenced following the rimming action, sufficient
molten aluminum is injected into the teem stream to completely kill the molten core,
such that all the total required aluminum is added before teeming is complete.
[0003] The practice as disclosed in that patent provides considerable advantages over more
conventional procedures utilizing solid forms of aluminum. Specifically, the use of
molten aluminum as disclosed provides a higher aluminum recovery, better and more
uniform aluminum distribution, fewer stringer-type alumina inclusions, a higher incidence
of a smaller dispersed, globular type (which is more desirable) of inclusions, and
an absence of porosity at the rim-core interface. The superiority in using molten
aluminum instead of solid aluminum to kill the core of the steel is associated with
the fact that molten aluminum dissolves almost instantaneously in molten steel, thus
minimizing the harmful effect of the difference in the densities between the two metals.
Because of these advantages, the resulting sheet steel products produced from such
ingots are indeed superior to prior art rim-stabilized products in having exceptional
drawability and good surface characteristics. In the few years since its development,
that process has gained considerable commercial success, as more and more tons are
produced by that process by various steel producers, for a progressively wider variety
of applications.
[0004] The progressively increasing use of the above- described process however, has been
accompanied by increasing problems. Specifically, since the above process requires
a complete killing of the ingot core, the ingots must be handled like any killed steel
ingot. That is, unlike rimmed or semi-killed steel ingots, killed steel ingots must
be allowed to solidify completely before they are moved in order to avoid segregation
problems. Such ingots therefore are colder when stripped and placed into a soaking
pit, and accordingly must soak for a significantly longer period of time in order
to reach a uniform hot working temperature. This not only increases the cost of production,
but when significant tonnages are produced, can cause serious bottle-neck problems.
Indeed, some mills have such limited soaking pit capacities that they cannot sustain
high production rates of rim-stabilized steels.
[0005] While it is of course recognized that the above problems, characteristic of killed-steel
production, can be alleviated by producing more rimmed or semi-killed steel ingots,
the different types of teeming practices produce different steel characteristics,
none of which are suitable for all applications. Indeed, killed steels or rim-stabilized
core killed steels are far superior insofar as deep-drawing and other cold formability
properties (r
m, Δr, n) are concerned. Furthermore, the rim-stabilized steels core-killed with molten
aluminum pursuant to the process disclosed in the above-mentioned patent have surface
qualities and deep-drawability superior to most killed steels produced by techniques
prior thereto. Therefore, while one might reasonably assume that the above-mentioned
patented process could be utilized to produce a rim-stabilized steel ingot having
a semi-killed core, sheet products produced therefrom should be quite inferior insofar
as cold-forming characteristics are concerned.
[0006] This invention is predicated on our development of a process for making rim-stabilized
steel ingots whereby the ingot core is semi-killed rather than killed, with a carefully
controlled addition of molten aluminum. While this process provides the obvious advantages
of permitting the steel ingots to be handled while the interior portion is still molten
to thereby shorten soaking time and increase production capacity, it also provides
the unexpected advantage in producing a steel which, unlike conventional semi-killed
steels, is characterized by cold-forming characteristics for some applications to
be as good as those steels produced pursuant to the patented process wherein the core
is fully killed primarily because of a high incidence of smaller, finely dispersed,
globular-type of inclusions which are more desirable than the long stringer-type of
alumina inclusions formed when solid aluminum additions are made. However, it is recognized
that such steels with semi-killed cores will not have the grain texture equivalent
to steels having larger molten aluminum additions to provide the excellent deep-drawing
properties required for all cold-forming applications. In addition, production techniques
can be simplified because hot-tops or side boards are completely unnecessary, and
ingot-to- slab yields are improved.
[0007] According to the present invention, there is provided a method of producing rim-stabilized
steel ingots, comprising teeming a rimming steel into an ingot mold until the mold
is 80 to 95% full, permitting the teemed steel to rim for a period of 1/2 to 15 minutes,
recommencing teeming to substantially fill the ingot mold and adding molten aluminum
during said recommenced teeming, said addition of molten aluminum being commenced
after recommencement of teeming and terminated before termination of recommenced teeming,
and said m6lten aluminum being added only in an amount as will give rise to a dissolved
aluminum content in the ingot core of 0.002 to 0.008% and a dissolved oxygen content
in the ingot core of 35 to 75 ppm.
[0008] According to the preferred embodiment of this invention, the step-by-step production
sequence is substantially the same as that described in United States Patent No. 3,754,591.
The only exception is that the molten aluminum addition is very carefully controlled
to a lesser amount as a function of the steel's oxygen content. The specific process
steps are as follows: (1) hot molten steel is teemed into an ingot mold at a normal
rate, i.e., 5 to 10 tons (4.5 to 9.1 tonnes) per minute, until the mold is about 80
to 95% full; (2) teeming is then stopped and the steel allowed to rim for a period
of 1/2 to 15 minutes and preferably 2 to 7 minutes; (3) teeming is thereafter resumed
to fill the remainder of the ingot mold; (4) during the step 3 teeming, the predetermined
amount of molten aluminum is added to the ingot mold, preferably by introducing the
molten aluminum into the teem stream such that the entire amount of aluminum is added
after teeming is resumed and before it is complete.
[0009] The above production sequence is substantially as disclosed and claimed in United
States Patent No. 3,754,591. As disclosed in that patent, it is essential that the
step 3 teeming should start before any molten aluminum is added, and should continue
after all the molten aluminum is added. This teeming overlap is necessary because
the teeming stream not only serves to carry the molten aluminum deep into the ingot
core, but also serves to divert the iron oxide scum on the upper surface of the molten
metal during step 3 thereby minimizing alumina formation. Preferably therefore, teeming
should be commenced two seconds before the addition of aluminum is started and should
continue for two seconds after all the aluminum has been added. Although such a two
second overlap is preferable, a one second overlap before and after will suffice.
[0010] Whereas the above-mentioned patent discloses the production of a rim-stabilized steel
ingot having a fully killed core, the crux of this invention resides in the careful
control of the amount of molten aluminum added so as to assure that the core is only
semi-killed as opposed to killed. To this end, the molten aluminum addition should
be limited so that the molten core of the ingot will contain from 35 to 75 ppm of
dissolved oxygen after the aluminum is added, and preferably 50 to 60 ppm. The actual
amount of molten aluminum added to the steel will depend upon the steel's carbon and
oxygen contents, other ingredients to some extent and the amount of rimming action
desired before the core is semi-killed. Most typically, the steels produced by the
patented process are utilized in cold-forming applications, and have carbon and dissolved
oxygen contents of 0.02 to 0.10% and 40 to 60 ppm respectively. Other alloy constituents
are seldom utilized, and rimming times of 2 to 7 minutes are typical. As suggested
in the above-mentioned patent, the amount of molten aluminum required to fully kill
the core for such steels is about 1.6 pounds of aluminum per ton of steel (0.8 kg/tonne).
In the process of this invention wherein the core is semi-killed, about 0.6 lb/ton
(0.3 kg/tonne) of molten aluminum may typically be added. The dissolved aluminum content
in the semi-killed core should range from 0.002 to 0.008%, whereas in the fully killed
core it would range from 0.025 to 0.065%.
[0011] It is well known in the prior art that sheet products produced from fully killed
steels have better cold forming properties than those produced from semi-killed steels.
Prior to this invention, semi-killed steels are rarely, if ever, used in the production
of low carbon (0.02 to 0.10%) cold-rolled sheets because of their inferior surface
and interior qualities when compared to rimmed or aluminum-killed steels. The most
surprising feature of the process of this invention is that sheet steel products rolled
from steel ingots produced pursuant hereto exhibit cold forming characteristics comparable
to those produced from rim-stabilized core-killed steels. A close examination of the
steel produced according to this invention reveals that the small molten aluminum
addition produces small, finely dispersed, globular type of manganese-aluminate and
alumina inclusions instead of the stringer type of inclusions, predominately alumina,
formed when solid aluminum additions are made.
[0012] In addition to the excellent cold forming characteristics, the microcleanliness of
steel produced pursuant to this invention is comparable to those steels produced according
to United States Patent No. 3,754,591. The table below provides the QTM (Quantitative
Television Microscopic) ratings for two commercial tinplate coils produced from ingots
teemed according to this invention. It can readily be seen that the QTM ratings are
comparable.
[0013] Production trial of these coils for D & I (drawn and ironed) can application demonstrated
that such steel . performed as well as did tinplate produced from ingots having a
fully killed cores that were killed with molten aluminum. D & I cans made on laboratory
facilities showed the steel to perform as well as did the steels having fully killed
cores.
1. A method of producing rim-stabilized steel ingots, comprising teeming a rimming
steel into an ingot mold until the mold is 80 to 95% full, permitting the teemed steel
to rim for a period of 1/2 to 15 minutes, commencing teeming to substantially fill
the ingot mold and adding molten aluminum during said recommenced teeming, said addition
of molten aluminum being commenced after recommencement of teeming and terminated
before termination of recommenced teeming, characterized by adding only enough molten
aluminum as will give rise to a dissolved aluminum content in the ingot core of 0.002
to 0.008% and a dissolved oxygen content in the ingot core of 35 to 75 ppm.
2. A method as claimed in claim 1, characterized by adding only enough molten aluminum
as will provide an oxygen content in the ingot core of 50 to 60 ppm.
