Note: Descriptions are shown in the official language in which they were submitted.
~ 3 70(~0~
BAC~CG~OUND OF THE INVENTION
l'his inven-tion relates to rimmed steel products
and methodsoEprc,du,~il-lg thcm, being pxoduct: resulting
from rolling operations, i.e. hot rolling and for presently
preferred uses, also cold rolling with appropriate anneal-
ing. The invention is more particularly related to rimmed
steel products of the stated characterj which are essentially
non-aging, the latter term being used herein as equivalent
to retarded aging r in the sense of aging being so retarded,
e.g. under expected temperatures of use r as to have no
significantly adverse effect on the properties of the steel
or articles formed from it over indefinite periods of time~
It has long been known that rimmed steel, although
having many virtues in the areas of ingo-t metal yield,
1~ superior surface, and good qualities of forming, e.g. draw-
ing, bending and the like, and also articles formed from
such steel, are susceptible to so-called aging which affects
the mechanical properties in a manner which may be undesir-
able for some uses. It has also long been known that
; 20 addition of vanadium, i.e. to the molten steel in the ladle
before pouring ingotst has the ef~ect of retarding aging in
-the ultimate products, resulting in what have been called
non-aging rimming steels; in such case, the vanadium has been
stated to combine with nitrogen while permitting a mild
rimminy action. Addition o~ boron in like manner has been
known to have similar effect in retarding againg.
It is found, nevertheless, that the non-aging,
vanadium-added or boron-added steels as just described,
althollgh purportedly of rimmed character, are deficient
especially in that the surface zone is by no means equiva-
~ :~ 7(~0~
t lent to that of ordin~ry or plain carbon~ rimmed steel,The clear t clean surface of normal rimmed steel i.s extremely
important, for example in such uses as automotive body stock,
where a formed surface free from irregularities is re~uired;
the tendency of ~he previous vanadium-added (or boron-added)
steels is to e~hibi~ rela-tively poor surfaces, unsuitable for
the purposes. A special requirement is for a rimmed steel
sheet ~hich in cold-rolled, annealed, and temper rolled
condition exhibits little or no Luders strain (with charac-
teristic surface lines) and which after forming, and after
an interval that normally involves aging, exhibits noappearance of Luders strain, returned. Hence, there has
remained a need for an essentially non-aging rimmed steel
having the advantages of such steel, in production and use,
and having excellent surface characteristics especially in
the cold-rolled, annealed and temper rolled condition.
SUMMARY OF THE INVENTION
The invention is predicated on the finding that
significant improvement in rimmed steel products as defined
above is attainabIe by providing a cast ingot ln which there
is an outer layer or skin of fully rimmed steel, of normal
composition for rimming, surrounding an ultimately olidified
core of the same composition but including a significant con-
tent of vanadium or boron (or both) for retarding aging.
Thus, the basic composition may be 0.04 to 0.12% (most
usually 0.07 to 0.11%) carbon, 0.2 to 0.8% manganese, 0.035
max.. % sulfur, balance iron and incidentals; all percentages
herein are in weight percent. The new products are made by
a special procedure, as follows: Molten steel, as of the
above base composition, is poured into an ingot mold up to
~ 1 ~QOO~
1 a filling level of ~0% or more, e.g. between 80~ and 95%
and preferably around 85% to 90~. Teeming is interrupted
and the effervesc~nl rimrnin~s action which characterizes
this basic melt is allowed to proceed, as for at least one
minute, preferably from two to five minutes or even more
(depending on conditions, or on the desired outer layer),
until a shell or skin next to the mold wall has solidified,
as to a thickness of 0.75 -to 4 inches, Then pouring
is resumed from the ladle, i.e. metal of the same composi;
tion, but with simultaneous addition of vanadium or boron,
for example by feeding small particles of ferro-vanadium
~e.g. 35 to 80% V) or ferro-boron, into the stream of molten
metal or into the molten core, to provide a content of O.Ol
to 0.2% V in the core, or up to 0.05~ B.
The operation of back~filling the mold, i.e. com-
pleting the filling in the manner stated while adding
vanadium or boron (or both) causes the additional material
to mix into the molten core of the partly cast ingot,
whereby the completed ingot has a vanadium-added or boron-
added core surrounded by an outer layer of plain rimmed
steel which becomes essentially pure ferrite. The core re-
tains some rimming action even after back-filllng, and in
consequence, the total solidification practice may be
essentially as for normal rimming steel, with relatively
short, so-called track time, being the interval needed
between teeminy and the time for moving the filled ingot
mold from the melt shop to the soaking pit area, in contrast
to the requirements of aluminum killed steel which is the
usual non-aging alternative. The ingot yield of metal is
~ also, as is characteristic of rimmed steel, greater than
~ ~ 7~00~
1 for AK steels, so that for these reasons, production capacity
is increased and likewise shipping capability, in comparison
with aluminum killed metal inrespect to which the present
products have many of the same properties and uses, as will
be apparent hereinbelow.
Ingots so produced are processed by conventional
operations appropriate to plain rimmed steel, i.e. rolled
to slabs~ hot rolled to strip (e.g. ~.06 to 0.5 inch thick),
pickled, cold rolled to the desired final gauge, then
annealed (continuously or as coils) at suitable temperature
under protective atmosphere, and if desired, temper rolled.
The steel is also eminently useful in as-hot-rolled form
(such as strip), e.g. hot rolled black or hot rolled pickled.
Each product derived from the ingots so produced,
whether hot rolled or cold rolled, has a thin skin at its
principal surface (i.e. consisting of both sides of the strip)
which is pLain rimmed steel lacking aging-retardant elements
and is thus essentia]ly ferrite with a clear, clean surface,
normal for such steel. At the same time, the body or core
of the product contains vanadium or boron, and provldes an
essentially non-aging steel article, but lacking the surface
problems of previous vanadium-added or boron-added, rimmed
steels. Moreover, in the pouring operation described above,
the rimming action (e.g. for several minutes) prior to the
~5 addition of vanadium or boron allows the nitrogen level in
the molten portion of the ingot to stabilize; hence, better
control of the stabilizing action of the additive is
achieved, i.e. more control of the V- or B-nitrogen reaction,
in contrast to addition of V or B at the outset. As stated,
the products have essentially the same freedom from aging as
Q ~
aluminum-killed steels, in that by tests sufficient to
demonstrate absence of aging, there is no material change
in properties over extended periods of time (e.g. 30 days
at room temperature) or upon so--called artificial aging
(e.g. 1 or 2 hours at 212F), while the products have
greatly superior surface properties as compared with pre-
vious vanadium-added and boron-added rimmed steels and
especially as compared with aluminum-killed steels tha-t
are notorious for inclusions at or near the surface. It
has also been discovered that the new products, especially
where vanadium is added in suitable amount, and for example
when converted to the cold-rolled and annealed state, have
unexpectedly higher yield strength than ordinary, vanadium-
free, rimmed steels of otherwise identical composition.
The present invention is directed to a rimmed
steel product which is produced by rolling and which is
essentially non-aging. It consists of a skin of rimmed
steel which is essentially ferrite, over the principal
surface area of the product, and a core of substantially
non-aging rimmed steel beneath the skin. The steel of the
skin consists essentially of 0.04 to 0.12% C, 0.2 to 0.8%
Mn, and 0.035 max. % S, balance iron and incidental elements.
The core consists essentially of 0.04 to 0.12% C, 0.2 to
0.8~ Mn, 0.035 max. % S, together with aging-retardant
element or elements selected from the group consisting of
0.075 to 0.2% V, 0.009 to 0.04% B, or 0.03 to 0.2% V and
0.005 to 0.04% B. The core also contains one or more
elements from the group consisting of 0 to 0.01% Cb, 0 to
0.15% Si, 0 to 0.01% Ti, 0 to 0.01% Zr, and 0 to 0.01% A1,
balance iron and incidental elements.
The present invention is also directed to an
essentially non-aging rimmed steel product, produced by
7~
~ 1 7(~
rolling, which consists of an essentially ferrite skin of
rimmed steel over the principal surface area of the product,
and a core of substantially non-aging rimmed steel beneath
the skin, wherein the skin consists essentially of 0.05 to
0.12% C, 0.2 to 0.8% Mn, and 0.035 max. % S, balance iron
and incidental elements, and the core consists essentially
of 0.05 to 0.12% C, 0.2 to 0.8% Mn, 0.035 max. % S, together
with aging-retardant element or elements selected from
the group consisting of 0.075 to 0.2% V, 0.009 to 0.04%
B or 0.03 to 0.2% V and 0.005 to 0.04% B.
Another aspect of the present invention is a
method of making an essentially non-aging rolled product
of rimmed steel. The method comprises the following steps:
pouring an ingot mold 80% to 95% full of molten steel con-
sisting essentially of 0.04 to 0.12% C, 0.2 to 0.8% Mn,
0.035 max. % S, balance iron and incidental elements;
allowing this filling to undergo rimming action while a
shell of rimmed steel solidifies next to the mold surround-
ing a still-molten core; and then completing pouring of
the molten steel into the ingot mold while adding to the
molten steel in the mold sufficient material to provide in
the finished core steel having the aforesaid composition
together with aging-retardant element or elements selected
from the group consisting of 0.075 to 0.2% V, 0.009 to
0.04% B, or 0.03 to 0.2% V and 0.005 to 0.04% B, and also
together with 0 to 0.01% Cb, 0 to 0~15% Si, 0 to 0.01% Ti,
0 to 0.01% Zr and 0 to 0.01% Al; and after solidification
of the ingot, converting the same by rolling to a rolled
product having a skin of rimmed steel which is essentially
ferrite, over the principal surface area of the product.
Further details and examples of the inventlon
are set forth hereinbelow.
- 5a -
1 1 7()(~0~
DETAILED DESCRIPTION
In partlcular, the invention is carried out by
preparing a melt of rimming steel, e.g~ 0.07 to 0.11% C,
0.2 to 0.8% Mn, 0.035 mc~x. % S, in a suitable furnace such
as a so-called basic oxygen type furnace or one employing
a similar process. Such melt is used in the firs-t stage
of pouring each ingot, the steel in the ladle being teemed
into the ingot mold un-til the latter is, for instance, 85%
to 90% full; the flow is then interrupted. While similar
partial filling is performed in one or more further ingot
molds, the first poured steel, which has been undergoing
rimming action, freezes against the walls of khe mold, thus
yielding a solidified shell or skin. Such action may require
from two to five minutes or more, possibly up to 15 mimltes
, . . ... .
- 5b -
f~.
~1.,
~ 1 700~
1 depending on th'ckness desired for the solid ri~ned zone.
This step is then immediately followed by final filling
(back-filling) with further steel from the ladle (which is
brought back for the purpose), while at the same time add-
ing the element vanadium, e.g. as ferro-vanadium, or boron
(e.g. ferro-boron),and in the manner described above. ~s
will be understood, the ladle can then be moved along for
similar back-filling of further partly filled mold or molds
that may be waiting.
The added vanadium is found useful for retarded
aging in amounts of 0.01 to 0.2% in the ingot core, prefer-
able at least 0.04~ to take advantage of the discovery of
improved yield strength in the ultimate cold rolled product.
Alternatively, boron, considered the e~uivalent of vanadium
for retarded aging, but somewhat more economical, can be
similarly added to achieve 0~004 to 0.04~ B in the ingot
core. Most conveniently, the addition is made with solid
particles of ferro-vanadium, or vanadium alone, or ferro-
boron, or boron alone, for example not larger than about
1/4 inch, which are fed directly into the stream of molten
steel fàlling from the bottom of the ladle into the mold.
As will be understood, techniques and devices are available
for such feeding; the operation can be timed to begin just
after the start oE back-filling and to end ~ust before the
latter is finished. The vanadium or boron addition can be
made in other ways, as directly into the molten core where
the teeming metal s-trikes, or alternatively by making wire
or rod of the ferro-alloy and pushing such into the molten
steel core. The chief aim is to get the addition made
not later than the end of back-fillingO
~.~7~
1 By way of one example, four experimental, commercial
size ingots were produced, in 67 inch by 88 inch high open
top molds eachhaving a capacity of 25 tons, using a
base heat of rimming steel of the composition 0.05 % C,
0.35 ~ Mn, 0.01~ % S, balance i~on and incidentals,
and following the procedure described above. Each mold was
first poured 85 % full, then allowed to rim for four
minutes; during back-filling, each mold was injected (in
the back-filling stream) with ferro-vanadium containing 40
lbs. of vanadium thereby adding 0.08% V to the core.
These ingots were fully processed, in conventional manner
for rimmed steel, through hot and cold rolling, to coils
of cold-rolled and annealed steel strip, 59.488 inches
wide and 0.031 inch thick. Tests throughout the length
o one coil showed highly satisfactory properties as rim~ed
steel, with the desired absence of aging and with excellent
surface~quality.
As further examples, several other ingots of commer-
cial size were produced in accordance with the invention, with
vanadium added during back-filling, the 85%-full ingots
being allowed first to rim for 4 to 5 minutes ~hile the
outer layer solidified. The cold rolled and annealed sheet
(stripj product in each case was desirably non-aging and had
very good surface quality, as required for automotive and
2~ like purposes.
In one of these instances, the base, rimming
~ composition was 0.11~ C, 0.35% Mn, 0.021% S, balance iron
and incidentals; the core additionally contained 0.075% V.
The mechanical properties of the cold rolled strip (0.050
inch thick) included the following:
700n6
1 Yield Strength Tensile Strength Elongation
(ksi) tksi) (%)
Longitudinal 37.l 52~7 37
Transverse 39.5 54.l 36
In the second of these instances, the base, rimming
composition was 0.08% C, 0.34% Mn, 0.025% S, balance iron
and incidentals; the core additionally contained 0.093% V.
The mechanicaI properties of the cold rolled strip (0.050
inch thick) included the following:
~ield Strength Tensile Strength Elongation
~ksi) -(ksi) (%)
Longitudinal 41.l ~ 52.5 38
Transverse 42.0 53.9 37
The mechanical properties noted in the above two
examples were remarkably good (being high yield strength,
w1th advantageous elongation) and are now attributed to the
; content of the vanadium;~it does not appear that previous vana-
dium-added, rimmed steels~(not produced according to this
: : : :
; ~invention) were observed to have any unusual properties
of this~sort, i.e. not1ceably different from ordinary rimmed
steel. Indeed, normal, vanadium-freej rimmed steels of the
~; base composit1on descr1bed;herein, in the cold rolled and
annealed condition, usually exhibit yield strength of less
than 30 ksi. To achieve yield strengths in the range of 30
to 50 ksi in the steels of the invention, vanadium contents
of 0.04 to 0.20~ are required in the core steel; for example,
to reach 50 ksi, at least 0.10% V is deemed requisite.
The result is of special importance for automotive applica-
tions of such steel, i~e. for components where thickness
can be reduced with a saving of weight while retaining
--8--
Q n ~
1 desired strength.
Although additions of boron have not, alone, been
observed to afford the strength-promoting effects of
vanadium, these elements (boron and vanadium), in their
respective stated concen-trations, are functional equivalents
for the basic purpose of the invention, namely to achieve
a substantially non-aging character for the steel products
having the excellent surface properties of ordinary rimmed
steel. The elements can also be used together (each within
the stated ranges) for some mutual enhancement of the non-
aging effects; each can then be said to protect the other,
i.e~ in the sense of allowing the other to be more effective
in its role of combining with and removing from solution
nitrogen present in the steel. For example, if boron is
considered the primary contributor to the retarding of
aging, vanadium can also simultaneously be added in amount
up to 0.05~ for protecting the boron as above, or in
amounts in the range greater than 0.04% to afford higher
tensile propertles as well. V and B are also both oxide formers.
Two further, experimental, commercial-size ingots
were produced following the procedure described above,
i.e. involving first partly pouring the open-top mold
(e~g. 90~) with plain rimming steel, then allowing the
rimming metal to stand while a skin solidifies, and finally
back-filling while adding further elements (in the teemed
stream) to complete the core composition as described
below.
To one of these ingots the injection contained
both ferro-boron and ferro-vanadium. After suitable hot
rolling, the metal from this ingot was converted to two
1 cold-rolled, annealed, and ternper rolled coilsA ~he co~p~-
sition of the core steel (considered in effect as that of
the strip) was 0.05 to 0.07% C, 0.31 to 0.37% Mn, 0.010~ P,
0.016 to 0.020~ S, 0.03 to 0.04% V and 0.005 to 0.010% B,
5 balance iron and incidentals. Mechanical properties were:
yield strength, 27 to 34 ksi; tensile strength, 44 to 51
ksi; elongation, 36 to 43%. After aging either at 5nO~F
for 15 seconds or at 212~F for 1 hour, the material exhi-
bited no Luders strain.
To the molten metal of the other of the ingots,
ferro-boron (alone) was injected during back-filling. From
the steel of this ingot, after hot rolling, a coil of cold-
rolled, annealed and temper rolled steel s~rip was produced,
having a skin of rimmed steel at its principal surface area
1~ and internally containing 0.068 to 0.070~ C, 0.28% Mn,
0.010% P, 0.016~ S and 0.009% B. Mechanical properties
were: yield strength, 29 to 31 ksi; tensile strength,
43 to 45 ksi; elongation, 40 to 43~. This cold-rolled
strip was substantially non-aging; after aging at 212F
for 1 hour, no Luders strain was obser~ed, and after aging
at 500~F for 15 seconds, inner laps of the coll exhibited
no Luders strain, while outer laps showed very slight but
acceptable Luders strain.
Many uses are envisaged for the non aging, good-
Z5 surface steel of the present invention, whether as-hot-rolled
or cold rolled, such as automotive bwnpers, suspension
slides for filing cabinets and other drawers, furniture,
dent-resistant automotive panels, and structural parts
for automotive and other purposes.
It is conceived that other elements may be added
--10--
~ ~ 7(~1)0~
1 to the molten steel core in the ingot mold, along with
the aging-retardant material ~anadium and/cr boron)
in at least a minimum amount of the latter for its aging-
retarding function. Thus, one such elernent is columbium,
to have similar effect against aging and in enhancement
of tensile properties and also to have a protective
function for boron, i.e. to constitute up to 0.0l~, e.g.
0.0C5 to 0.0l~ of the core. Another such element is
silicon, to impart solid solution strengthening to the
steel (or its equivalent for such purpose, phosphorus),
but very preferabIy in amounts insufficient to ha~e any
appreciable killing effect, i.e. up to 0.150%, e.g. 0.02
to 0.15% in the core. Other elements which can optionally
be added to the core during bac~-filling include titanium
(for strengthening and also to have protective effect,
e.g. or boron), in amounts up to 0.01%, e.g. 0.00l to 0.0l~
Ti, and likewise zirconium, up to 0.0l~, e.~. 0.00l to 0.0l~
zr,as well as aluminum in protective but non-killing amount,
up to 0.01%, e.g. 0.00l to 0.01% Al. In all cases, the
supplemental elemen~ or elements can be added in the m~nner
described above, as particles or small pieces of ferro-alloy
or where possible as the uncombined element or in multi-
element alloys with the vanadium or boron, or both, for
example being injected into the back-filling stream of
molten steel.
It is to be understood that the invention is not
limited to the specific compositions and procedural steps
hereinabove described, but may be carried out in other ways
without departure from its spirit.
--11--
