U.S. patent number RE31,306 [Application Number 06/260,904] was granted by the patent office on 1983-07-12 for cold rolled, ductile, high strength steel strip and sheet and method therefor.
This patent grant is currently assigned to Armco Inc.. Invention is credited to James A. Elias, deceased, John R. Newby, Marvin B. Pierson.
United States Patent |
RE31,306 |
Elias, deceased , et
al. |
July 12, 1983 |
Cold rolled, ductile, high strength steel strip and sheet and
method therefor
Abstract
Cold reduced, annealed steel strip and sheet stock having 0.2%
yield strength of 45 to 65 ksi with an elongation of at least 25%,
or having a yield strength of at least 90 ksi with an elongation of
at least 10%. A low carbon steel (0.02-0.10% C) typical of rimmed
or drawing steel analysis is preferably vacuum degassed, and 0.02%
to 0.18% columbium is added. The casting is hot rolled, coiled not
higher than 1300.degree. F., cold reduced 40% to 70%, and annealed
at low temperature for a time sufficient to restore desired
ductility without substantially decreasing yield strength.
Inventors: |
Elias, deceased; James A. (late
of Middletown, OH), Newby; John R. (Middletown, OH),
Pierson; Marvin B. (Middletown, OH) |
Assignee: |
Armco Inc. (Middletown,
OH)
|
Family
ID: |
26948256 |
Appl.
No.: |
06/260,904 |
Filed: |
May 6, 1981 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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554158 |
Feb 28, 1975 |
3963531 |
|
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Reissue of: |
674862 |
Apr 8, 1976 |
04067754 |
Jan 10, 1978 |
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Current U.S.
Class: |
148/531; 148/533;
148/603 |
Current CPC
Class: |
C21D
8/0236 (20130101); C22C 38/12 (20130101); C21D
8/0226 (20130101); C21D 8/0278 (20130101); C21D
8/0273 (20130101) |
Current International
Class: |
C22C
38/12 (20060101); C21D 8/02 (20060101); C21D
007/02 (); C21D 007/14 () |
Field of
Search: |
;148/12F,12C,12D |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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48-79722 |
|
Oct 1973 |
|
JP |
|
48-81721 |
|
Nov 1973 |
|
JP |
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Primary Examiner: Stallard; W.
Attorney, Agent or Firm: Frost & Jacobs
Parent Case Text
This is a division of application Ser. No. 554,158 filed Feb. 28,
1975, now U.S. Pat. No. 3,963,531.
Claims
We claim:
1. A method of producing cold reduced low carbon steel strip and
sheet stock having a 0.2% offset yield strength of at least 90 ksi
and an elongation in two inches of greater than 10% in the annealed
condition, comprising the steps of providing a vacuum degassed,
fully killed low carbon steel casting consisting essentially of, by
weight percent, from 0.02% to about 0.10% carbon, about 0.1% to
0.9% manganese, 0.02% to about 0.18% columbium, residual
phosphorus, sulfur, silicon, oxygen and nitrogen, about 0.01% to
about 0.08% aluminum, and balance essentially iron except for
incidental impurities, the columbium being substantially completely
combined with carbon, hot rolling to intermediate gauge, coiling at
a temperature .[.not higher than.]. .Iadd.of about 1000.degree. to
.Iaddend.about 1300.degree. F., removing hot mill scale, cold
reducing to final gauge with a reduction in thickness of 40% to
70%, and annealing at a temperature and for a time sufficient to
recover ductility but not recrystallize whereby to obtain an
elongation of greater than 10% with a yield strength of at least 90
ksi.
2. The method claimed in claim 1, .[.wherein coiling is effected
within the range of about 1000.degree. to 1300.degree. F., and.].
wherein said annealing is conducted within the range of about
1100.degree. to about 1300.degree. F. with a time of about 7
minutes to 24 hours, said time being inversely proportional to said
temperature.
3. The method claimed in claim 2, wherein said steel consists
essentially of from about 0.03% to about 0.05% carbon, about 0.3%
to about 0.6% manganese, about 0.04% to about 0.12% columbium,
about 0.006% to about 0.01% phosphorus, about 0.01% to about 0.017%
sulfur, about 0.004% maximum nitrogen, about 0.03% to about 0.05%
aluminum, about 0.01% maximum oxygen, about 0.1% maximum silicon,
and balance essentially iron.
4. The method claimed in claim 2, wherein said coiling is effected
at about 1100.degree. F., wherein said cold reducing involves a
reduction in thickness of 45% to 55%, and wherein said annealing is
an open coil anneal conducted at about 1100.degree. F. with a time
at temperature of about 1/2 hour.
5. The method claimed in claim 1, .[.wherein coiling is effected
within the range of about 1000.degree. to 1300.degree. F., and.].
wherein said annealing is a continuous anneal at a peak strip
temperature not exceeding 1300.degree. F. for about 7 to 10
minutes.
6. A method of producing hot dipped metallic coated low carbon
steel strip and sheet stock having a 0.2% offset yield strength of
at least 90 ksi and an elongation in two inches of greater than 10%
in the annealed condition, comprising the steps of providing a
vacuum degassed, fully killed, low carbon steel casting consisting
essentially of, by weight percent, from 0.02% to about 0.10%
carbon, about 0.1% to 0.9% manganese, 0.02% to about 0.18%
columbium, residual phosphorus, sulfur, silicon, oxygen and
nitrogen, about 0.01% to about 0.08% aluminum, and balance
essentially iron except for incidental impurities, the columbium
being substantially completely combined with carbon, hot rolling to
intermediate gauge, coiling at a temperature .[.not higher than.].
.Iadd.of about 1000.degree. to .Iaddend.1300.degree. F., removing
hot mill scale, cold reducing the final gauge with a reduction in
thickness of 40% to 70%, subjecting the cold reduced steel to wet
chemical cleaning, heating in a hydrogen-inert gas atmosphere to a
temperature .Iadd.and for a time sufficient to recover ductility
but not recrystallize whereby to obtain an elongation of greater
than 10% with a yield strength of at least 90 ksi, cooling said
strip to a temperature .Iaddend.about equal to that of the molten
coating metal bath, said coating metal being chosen from the group
consisting of aluminum, zinc, alloys of aluminum, alloys of zinc,
and terne, passing said steel through said bath of molten coating
metal, and solidifying said coating metal.
7. A method of producing hot dip metallic coated low carbon steel
strip and sheet stock having a 0.2% offset yield strength of at
least 90 ksi and an elongation in two inches of greater than 10% in
the annealed condition, comprising the steps of providing a vacuum
degassed, fully killed low carbon steel casting consisting
essentially of, by weight percent, from 0.02% to about 0.10%
carbon, about 0.1% to 0.9% manganese, 0.02% to about 0.18%
columbium, residual phosphorus, sulfur, silicon, oxygen and
nitrogen, about 0.01% to about 0.08% aluminum, and balance
essentially iron except for incidental impurities, the columbium
being substantially completely combined with carbon, hot rolling to
intermediate gauge, coiling at a temperature .[.not higher than.].
.Iadd.of about 1000.degree. to .Iaddend.1300.degree. F., removing
hot mill scale, cold reducing to final gauge with a reduction in
thickness of 40% to 70%, heating the cold reduced steel in strip
form in a continuous furnace to a temperature sufficient to remove
oil and related surface contaminants, heating in a hydrogen-inert
gas atmosphere capable of reducing residual surface oxide wherein
said strip is brought to a temperature and for a time sufficient to
recover ductility but not recrystallize whereby to obtain an
elongation of greater than 10% with a yield strength of at least 90
ksi, cooling said strip approximately to the temperature of the
molten coating metal bath, said coating metal being chosen from the
group consisting of aluminum, zinc, alloys of aluminum, alloys of
zinc, and terne, passing said strip through said molten coating
metal bath, and solidifying said coating metal.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to cold reduced, low carbon, low alloy steel
strip and sheet having high yield strength in combination with
ductility higher than that previously attainable and to a method
for production thereof. More specifically, the present invention
provides cold rolled steel strip and sheet stock having a 0.2%
yield strength of at least 90 ksi with an elongation in 2 inches of
at least 10%, or a cold rolled strip and sheet stock having a 0.2%
yield strength of 45 to 65 ksi with an elongation in 2 inches of at
least 25%, the composition for each embodiment being substantially
the same. The invention further relates to a metallic coated
product having a steel substrate exhibiting the above
properties.
2. Description of the Prior Art
High strength cold rolled steel has generally been produced
previously by either of two approaches. One approach is to make
relatively large additions of strengthening elements such as
manganese (greater than 1%) and silicon (greater than 0.3%) to a
steel containing more than 0.1% carbon, together with lesser
additions of other strengthening alloying elements such as
titanium, columbium, zirconium, and vanadium. Annealing of such
steel produces high yield strengths by precipitation hardening.
Another approach is to produce a high strength steel containing
carbon and nitrogen (together with small amounts of strengthening
alloying elements) and subject the steel to special annealing
treatments which results in an only partially recrystallized
microstructure.
In both the above approaches, high strength is achieved only at the
sacrifice of ductility and formability.
U.S. Pat. No. 3,761,324, issued Sept. 25, 1973, to J. A. Elias and
R. E. Hook, disclose hot rolled and cold rolled strip and sheet
material having a wide range of mechanical properties. In this low
carbon steel (maximum carbon content 0.015%), columbium is added in
excess of the amount required to combine with all the carbon and
free nitrogen, so that uncombined columbium is present. This patent
contains a recognition that columbium retards the recrystallization
rate, thereby making possible the production of high strength
hot-dip metallic coated products. However, at the maximum yield
strength of 90 ksi for the steel of that invention, the elongation
is less than 10%.
U.S. Pat. No. 3,671,334, issued June 20, 1972, to J. H. Bucher et
al, discloses a renitrogenized columbium-bearing steel, and cold
rolled, strain-aged articles formed therefrom having a yield
strength of 70 to 90 ksi. The process of this patent involves a
cold reduction of at least 50%, annealing to restore ductility with
a consequent reduction in yield strength to about 50 to 55 ksi,
pre-straining and heat treating to obtain 70 to 90 ksi yield
strength by precipitation hardening. Forming into articles follows
the anneal to restore ductility and precedes the precipitation
hardening heat treatment. Percent elongation values of about 20%
maximum were obtained at a yield strength of about 70 ksi.
It is evident from the above background of the prior art that there
is not now available a low carbon steel which can be cold reduced
to obtain high yield strength and retain sufficient ductility to
permit forming into articles of final use without subsequent
strain-aging and precipitation hardening.
SUMMARY
It is a principal object of the invention to provide a low carbon,
low alloy steel which in cold reduced and annealed condition
exhibits a yield strength ranging from 45 to 65 ksi or at least 90
ksi, and sufficient ductility to permit bending and forming
operations.
It is a further object of the invention to provide cold rolled, low
carbon strip and sheet stock which can be metallic coated with
aluminum, zinc, or alloys thereof, while maintaining a high yield
strength.
Cold reduced, low carbon steel strip and sheet stock according to
the present invention consists essentially of, by weight percent,
from 0.02% to about 0.10% carbon, about 0.1% to about 0.9%
manganese, 0.02% to about 0.18% columbium, residual phosphorus,
sulfur, oxygen and nitrogen, up to about 0.1% silicon, about 0.01%
to about 0.08% aluminum, and balance essentially iron except for
incidental impurities. The columbium is substantially completely
combined with carbon at room temperature.
The method of the invention comprises the steps of providing a
vacuum degassed, fully killed, low carbon steel casting having the
above composition, hot rolling to intermediate gauge, coiling at a
temperature not higher than about 1300.degree. F., removing hot
mill scale, cold reducing to final gauge with a reduction in
thickness of 40% to 70%, and annealing at a temperature and for a
time sufficient to restore ductility adequate to permit bending and
forming without substantial decrease in yield strength.
Steel processed in accordance with one embodiment of the invention
is preferably coiled after hot rolling at about 1000.degree. to
about 1300.degree. F., and annealed after cold rolling under
conditions which result in substantially recovered but
unrecrystallized strip and sheet stock having a yield strength of
at least 90 ksi and a percent elongation of at least 10%.
According to another embodiment, steel of the invention is
preferably coiled at about 1000.degree. to about 1300.degree. F.,
and annealed after cold rolling under conditions which result in
fully recrystallized strip and sheet stock having a yield strength
of 45 to 65 ksi and a percent elongation of at least 25%.
BRIEF DESCRIPTION OF THE DRAWINGS
Reference is made to the accompanying drawings wherein FIGS. 1-3
are graphic representations of yield strengths vs. annealing
temperatures of steels processed in accordance with the invention;
and FIG. 4 is a graphic representation of percent elongation vs.
annealing temperature of steels processed both within and outside
the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A steel having a composition typical of low carbon rimmed or
drawing steel may be melted in an open hearth, basic oxygen furnace
of electric furnace. Such a steel, which may be partially
deoxidized with aluminum or silicon, is then preferably vacuum
degassed to a carbon content ranging between 0.02% and about 0.10%,
and sufficient aluminum (or equivalent nitride former) is added to
combine completely with the residual nitrogen which typically will
be up to about 0.004%. Columbium is then added, either during the
degassing or in the ladle or mold, with proper distribution means.
The molten steel may either be teemed into ingot molds or
continuously cast.
The minimum carbon and columbium contents of the steel must be
considered critical. The maximum columbium addition must be
restricted to a level which, for a given carbon content, will
result in substantially no uncombined columbium, as determined by
analysis at room temperature. In other words, the columbium content
will not exceed about 7.75 times the carbon content.
Since nitrogen is substantially completely combined with aluminum,
or other nitride formers, the formation of columbium nitrides or
carbonitrides is minimized, and the columbium is substantially
completely combined as columbium carbide.
In the product and process of the present invention, it has been
found that carbon at lower levels has an effect in strengthening
the steel. More specifically, in the range of about 0.01% to about
0.025% carbon a strengthening effect is obtained. At carbon levels
above about 0.025%, however, carbon contributes nothing further in
strengthening steel (as shown in FIG. 2) within the yield strength
range of the invention, and further strengthening becomes an almost
linear function of the columbium content. Accordingly, the maximum
carbon content of 0.10% is not considered critical, although it is
preferred that carbon be varied directly in proportion to columbium
so as to provide up to 0.025% uncombined carbon (i.e., in excess of
that combined with columbium).
Although the composition is not otherwise considered critical
except for the above discussed relationship between the carbon and
columbium contents, nevertheless optimum properties are achieved
with the following preferred composition, in weight percent:
______________________________________ carbon 0.03-0.05% aluminum
0.03-0.05% manganese 0.3-0.6% nitrogen 0.004% max. columbium
0.04-0.12% oxygen 0.01% max. phosphorus 0.006-0.01% silicon 0.1%
max. sulfur 0.01-0.017% iron balance
______________________________________
Manganese is purposefully added to prevent hot shortness and to
increase the tensile strength. An addition of from 0.1% to 0.9%,
and preferably from 0.3% to 0.6% by weight, is adequate for these
purposes.
The preferred phosphorus, sulfur, nitrogen and oxygen ranges set
forth above are typical of residual values which are attained in a
vacuum degassed low alloy steel. Silicon will also be present in
residual amounts unless purposefully added (in amounts up to 0.1%)
as a deoxidant.
Zirconium is known to possess the same effect as columbium in
increasing the recrystallization temperature of low carbon steels,
and hence it is within the scope of the present invention to
substitute stoichiometrically equivalent amounts of zirconium in
place of columbium, at least in part.
Titanium may be substituted in place of aluminum as a nitride
former in stoichiometrically equivalent amounts, but it should be
recognized that titanium does not have the same effect as columbium
and zirconium in increasing the recrystallization temperature.
Hence, titanium is not a substitute for columbium in the steel of
the present invention.
Silicon may be substituted for aluminum, as a deoxidant, and if
this is done, preferably enough titanium is added to combine with
the residual nitrogen in the melt.
Rare earth metals or mischmetal may be added for sulfide shape
control where optimum transverse mechanical properties are
desired.
From the processing standpoint, it has been found that the hot
rolling finishing temperature has little or no effect on properties
so long as a finishing temperature of about 1650.degree. F. is not
exceeded. Accordingly, conventional finishing temperatures within
the range of about 1550.degree. to 1650.degree. F. may be
practiced. Coiling temperature cannot exceed about 1300.degree. F.
and preferably should not exceed about 1200.degree. F.
Ductility of the final product has been found to be independent of
the finishing and coiling temperatures.
The amount of cold reduction must be at least 40% but may not
exceed about 70%. Preferably cold rolling will be carried out with
a reduction in thickness of 45% to 55%, in one or more stages. If
the maximum of 70% reduction in thickness is necessary for certain
final products, a longer or higher temperature anneal may be needed
in order to restore ductility to the desired values, as shown in
FIG. 1 and in Table II. For a given yield strength, greater
ductility is obtained at 50% cold reduction than at 60%.
The annealing range of the cold rolled material has been found to
be critical. Either a continuous or an open coil anneal may be
practiced, although an open coil anneal is preferred for material
having a yield strength of at least 90 ksi and greater than 10%
elongation. An open coil or box anneal ranging from about
1100.degree. F. with a time at temperature up to 24 hours, to about
1200.degree. F. with a time at temperature of less than 1/2 hour,
has been found to be satisfactory. Preferably the open coil anneal
is conducted at 1100.degree. F. with a time at temperature of about
1/2 hour. Time at temperature will thus be generally inversely
proportional to the temperature. A continuous anneal with a peak
strip temperature not exceeding about 1300.degree. F. with a at
temperature time of about 7-10 minutes can also be practiced.
When conducted under the above described conditions, the cold
rolled strip and sheet stock will recover ductility to an
elongation value of greater than 10% while retaining a yield
strength of at least 90 ksi. The product has a substantially
unrecrystallized microstructure.
In producing cold rolled strip and sheet stock having a yield
strength of about 45 to 65 ksi and greater than 25% elongation, all
steps up through the cold rolling remain the same as those
described previously, and the broad composition remains the
same.
For this embodiment either continuous, open coil, or batch
annealing can be practiced although batch annealing is preferred.
When using batch annealing or open coil annealing, a temperature
range of about 1200.degree. to about 1400.degree. F. should be
observed. The annealing time will be inversely proportional to
temperature with a minimum of 4 hours required for 1200.degree. F.,
or a minimum of 1/2 hour above 1250.degree. F. If a continuous
anneal is practiced, a peak strip temperature of about 1500.degree.
to 1700.degree. F. with a furnace time of about 7 to 10 minutes has
been found to be satisfactory.
Under these conditions, the cold rolled strip and sheet stock is
fully recrystallized and has a yield strength of about 45 to 65
ksi, with greater than 25% elongation.
Although not wishing to be bound by theory, it is believed that the
addition of columbium increases the recrystallization temperature
of the steel without affecting the rate of recovery of ductility of
the cold rolled material by means of the low temperature anneal. In
addition, as pointed out previously, columbium increases the yield
strength of the steel above the initial increment of increase
attributable to the presence of carbon in amounts up to about
0.025%. Accordingly, by raising the recrystallization temperature,
a range of about 200 Fahrenheit Degrees is available within which
to carry out the anneal which results in recovery of ductility,
while still avoiding recrystallization and thereby retaining a
yield strength of at least about 90 ksi. The recovery rate is
relatively rapid within the temperature range of 1000.degree. to
1150.degree. F., but substantially no recrystallization occurs.
While the annealing time and temperature are sufficient for
complete recrystallization, the product will have a yield strength
between 45 and 65 ksi as indicated in FIGS. 2 and 4.
From the above description, it will be recognized by those skilled
in the art that the cold rolled strip and sheet stock can be
metallic coated by continuous processes of the so-called
out-of-line anneal or preanneal type without substantially changing
the mechanical properties. Such processes include, but are not
limited to, hot dip coating in molten metal, and electroplating
wherein the preliminary coating line treatment is usually wet
chemical cleaning. Preanneal dip coating processes may then
incorporate either strip fluxing or strip heating in a
hydrogen-inert gas atmosphere prior to coating and involve a
maximum in-line strip temperature approximately equal to molten
metal bath temperature, which is usually maintained about
50.degree. to 100.degree. F. above the melting point of the coating
metal. Metals which may be used for continuous preanneal dip
coating processes include aluminum, zinc, alloys of aluminum or
zinc, or terne. Metals commonly used for continuous strip
electroplating include zinc and terne.
It is a further feature of the invention that continuous heat
treatments for recovery of ductility or for recrystallization of
the cold rolled steel may be carried out as an integral part of a
so-called in-line anneal hot dip metallic coating process. Such
processes do not utilize chemical fluxes but are characterized by
furnace processing for surface preparation with simultaneous heat
treatment. Exemplary processes include, but are not limited to the
Sendzimir, the Armco-Selas, and the U.S. Steel processes. These
differ primarily in the manner of removal of residual cold rolling
mill oil and related surface contaminants. The Sendzimir process
employs strip heating to 700.degree.-900.degree. F. to form a light
surface oxide, the Armco-Selas process utilizes high intensity
direct fuel-fired heating to 1000.degree.-1400.degree. F. without
strip oxidation; the U.S. Steel method utilizes wet chemical
cleaning.
These oil removal steps are followed by heating in similar
hydrogen-inert gas atmosphere furnaces capable of reducing residual
surface oxide wherein the strip is brought to the
1100.degree.-1150.degree. F. range required for recovery or to the
1600.degree. F. range (for continuous annealing for the fully
recrystallized product of the invention. Heating is followed by
furnace cooling approximately to bath temperature and hot dip
coating. Coating metals suitable for continuous in-line anneal hot
dip coating processes include aluminum, zinc, alloys of aluminum or
zinc, or terne.
In all the above-described processes the formation of an
interfacial alloy layer between the steel substrate and the coating
metal is substantially completely avoided.
The present invention thus provides a coated strip and sheet
product, having yield strengths ranging between 45 and 65 ksi with
elongation values greater than 25%, and yield strengths of at least
90 ksi with elongation values greater than 10%, comprising an outer
layer of aluminum, zinc, alloys of aluminum or zinc, or terne, and
an inner substrate or base of cold reduced steel strip and sheet
having the broad composition set forth above, with substantially no
interfacial alloy layer therebetween.
It has been found that the weldability of cold reduced strip and
sheet material of the present invention is excellent. The yield
strength remains substantially at its original value in the heat
affected zone of the weldment, although the ductility decreases in
the heat affected zone.
Several mill heats have been prepared and processed in accordance
with the invention and are set forth below as exemplary but
non-limiting embodiments.
EXAMPLE 1
A heat was melted and refined in a basic oxygen furnace, vacuum
degassed with aluminum and columbium (in the form of
ferrocolumbium) additions in the vacuum degasser, to provide a melt
having the following ladle analysis, in weight percent.
______________________________________ C 0.037% Mn 0.59 N 0.0036 S
0.010 P 0.000 Si 0.012 Cb 0.099 Al 0.047 Fe balance, except for
incidental impurities. ______________________________________
The melt was cast into ingots, solidified, reduced to slabs, and
hot rolled to 0.114-0.120 inch thicknesses. The hot rolling finish
temperature was 1600.degree. F., and the coiling temperature was
1200.degree. F.
After scale removal of the hot rolled material was cold rolled to
final thicknesses of 0.033, 0.036, and 0.052 inch, these cold
reductions ranging from 60% to 70%.
The sheet analysis was as follows, in weight percent:
______________________________________ C 0.040% Mn 0.60 N 0.0048 S
0.013 P 0.004 Si 0.010 O 0.0013 Cb 0.11 Al 0.048 Fe balance
______________________________________
Samples were subjected to various annealing treatments, as follows:
1100.degree. F. for 1/2 hour - open coil anneal for 90+ksi Y.S.,
10% min. Elong. 1200.degree. F. for 4 hours - open coil anneal for
45-65 ksi Y.S., 25% min. Elong.
EXAMPLE 2
Another heat was melted and vacuum degassed in the same manner as
Example 1 to obtain a melt having the following ladle analysis:
______________________________________ C 0.038% Mn 0.51 N 0.0028 S
0.012 P 0.006 Si 0.010 Cb 0.088 Al 0.078 Fe balance, except for
incidental impurites. ______________________________________
The melt was poured into ingots, rare earth metal silicide
additions were made to the ingots, and slabs were hot rolled to
several different gages ranging from 0.093 to 0.120 inch, with hot
rolling finish temperatures of 1600.degree.-1650.degree. F., and
coiling temperatures ranging from 1120.degree. to 1190.degree. F.
The rare earth metal addition was made for sulfide shape
control.
Cold rolling was carried out as follows:
0.046 inch--50%, reduction
0.036 inch--60% reduction
0.028 inch--70% reduction
The sheet analysis was as follows:
______________________________________ C 0.045% Cb 0.094% Mn 0.53
Al 0.070 N 0.0063 Ce 0.027 S 0.010 La 0.015 P 0.008 Fe balance O
0.009 ______________________________________
Annealing treatments were as follows:
Continuously annealed 8 minutes at 1300.degree., 1400.degree.,
1500.degree., 1600.degree. and 1700.degree. F.
Batch annealed at various temperatures from 1100.degree. to
1400.degree. F. for times ranging from 1/2 to 24 hours.
Mechanical properties of cold rolled samples of the steel of
Examples 1 and 2 are set forth in Table I. It will be noted that in
all embodiments processed in accordance with the invention in which
the yield strength was at least 90 ksi the percent elongation
exceeded 10%, while in embodiments processed in accordance with the
invention in which the yield strength was 45 to 65 ksi, the percent
elongation exceeded 25%. In contrast to this, in Example 2, the
specimen continuously annealed at 1400.degree. F. for 8 minutes
exhibited a yield strength of 68.8 ksi and an elongation of 22%;
similarly, the specimen box annealed at 1200.degree. F. for 4 hours
showed 75.9 ksi yield strength and an elongation of 20%, thus
indicating a partially recrystallized product outside the scope of
the invention. Specimens from Example 2 open coil annealed at
1050.degree. F. for 1/2 hours, and box annealed at 1100.degree. F.
for 4 hours, respectively, exhibited elongations less than 10%
which represent incomplete recovery, and are also outside the scope
of the invention. The processing ranges of the invention will yield
a material with either a recovery anneal or fully recrystallized
anneal properties. Between these two conditions, however, the
partially recrystallized product is outside the scope of the
invention.
The data of Table I are represented graphically in FIG. 4 as a
function of percent elongation vs. annealing temperature with yield
strengths, times and types of anneals also being shown. It will be
apparent from Table I and FIG. 4 that a temperature range of from
about 1100.degree. F. with a time at temperature up to about 24
hours, to about 1300.degree. F. with a time at temperature of about
7 to 10 minutes, results in an unrecrystallized product having a
yield strength of at least 90 ksi and a percent elongation greater
than 10%. An open coil anneal at about 1100.degree. F. with a time
at temperature of about 1/2 hour is preferred.
A temperature range of from about 1200.degree. F. to about
1700.degree. F., with a time at temperature of at least about 4
hours at 1200.degree. F. to about 7 to 10 minutes at 1700.degree.
F., results in a recrystallized product having a yield strength of
about 45 to 65 ksi and a percent elongation greater than 25%. A
batch or box anneal at about 1400.degree. F. with a time at
temperature of about 4 hours is preferred.
TABLE I ______________________________________ Mechanical
Properties Steels Processed in Examples 1 & 2
______________________________________ Open Coil Annealing
Annealing Time strip At 0.2% % Elong. Example Temperature
Temperature YS(ksi) in 2" ______________________________________ 2
1050.degree. F. 1/2 hr 125.1 6* 1 1100.degree. F. 1/2 hr 101.5 18 2
1150.degree. F. 1/2 hr 120.0 12 2 1200.degree. 26 1/2 hr 94.0 17 1
1200.degree. F. 4 hr 54.0 30 ______________________________________
Batch Anneal Annealing Time strip At 0.2% % Elong. Example
Temperature Temperature YS(ksi) in 2"
______________________________________ 2 1100.degree. F. 4 hr 122.0
6* 2 1100.degree. F. 24 hr 94.6 12.5 2 1180.degree. F. 1/2 hr 119.9
12.5 2 1200.degree. F. 4 hr 75.9* 20 2 1200.degree. F. 24 hr 64.5
26.5 2 1250.degree. F. 1/2 hr 62.8 25.5 2 1300.degree. F. 4 hr 63.7
26 2 1300.degree. F. 24 hr 55.9 28 2 1400.degree. F. 4 hr 53.6 31 2
1400.degree. F. 24 hr 53.5 32
______________________________________ Continuous anneal Annealing
Furnace Furnace 0.2% % Elong. Example Temperature Time YS(ksi) in
2" ______________________________________ 2 1300.degree. F. 8 min.
95.4 16 2 1400.degree. F. 8 min. 68.8* 22 2 1500.degree. F. 8 min.
59.3 28 2 1600.degree. F. 8 min. 51.5 29.5 2 1700.degree. F. 8 min.
53.5 28.5 ______________________________________ *Outside the scope
of the invention
The graph of FIG. 1 illustrates the effect of annealing temperature
and time on yield strength of 50% and 70% cold reduced specimens of
Example 2. This indicates that a temperature up to about
1150.degree. F. would require in excess of 4 hours to reduce the
yield strength to less than 90 ksi, whereas 24 hours at about
1150.degree. F. reduces the yield strength to about 80 ksi. It is
therefore apparent that the recrystallization rate is slow within
the range of about 1100.degree. to about 1175.degree. F.; the
process of the invention can thus tolerate operating variables of
relatively large magnitude without adverse effect.
The effect of percent of cold reduction on yield strength and
ductility is shown by the test results summarized in Table II for
unrecrystallized material having a yield strength of at least 90
ksi, the test specimens having the composition of Example I above.
It will be noted that 40% cold reduction is necessary in order to
achieve the desired properties and that the ductility is decreased
with 60%-70% cold reduction, although such material can be brought
within the desired minimum of greater than 10% elongation by
annealing at somewhat higher temperature and/or for a longer time.
As expected, yield and tensile strengths increased with higher cold
reductions. The properties were also found to be relatively
independent of the coiling temperature, at least up to about
1300.degree. F.
TABLE II ______________________________________ 0.2% Y.S. % Elong.
Process Conditions % C.R. ksi T.S. in 2"
______________________________________ H.R. Finish 1600.degree. F.
40 99.9 103.0 14 Coil at 1100.degree. F. 50 105.4 106.3 11 O.C.
Anneal 1100.degree. F. 60 111.5 112.5 8 for 1/2 hr (lab. 70 111.1
111.1 6 simulated). H.R. Finish 1600.degree. F. 40 99.4 102.8 13
Coil at 1200.degree. F. 50 102.5 105.7 12 O.C. Anneal 1100.degree.
F. 60 109.1 110.3 12 for 1/2 hr (Lab. 70 110.1 110.1 6 simulated).
H.R. Finish 1600.degree. F. 40 97.6 100.5 11 Coil at 1300.degree.
F. 50 97.0 100.2 11 O.C. Anneal 1100.degree. F. 60 100.1 102.1 13
for 1/2 hr (lab. 70 102.5 102.5 8 simulated).
______________________________________
Similar tests were conducted on 45-65 ksi specimens of Example 2
above, coiled at 1100.degree.-1300.degree. F., cold reduced 40%,
50%, 60% and 70% and box annealed (lab. simulated) at 1250.degree.
F. for 4 hours. It was found that the different percentages of cold
reduction caused no differences in yield strength, tensile
strength, percent elongation and hardness. In other words, all
specimens were at substantially the same levels after
annealing.
The effect of variation in the carbon and columbium contents on
yield strength was also investigated. For these tests a series of
laboratory heats were prepared, adding a different columbium
content to each heat and casting 4 ingots from each heat, each
ingot being at a different carbon content.
The laboratory heats were vacuum melted, cast into ingots, hot
rolled to 0.10 inch, finishing at 1600.degree. F. and coiling at
1100.degree. F., cold rolled to 0.04 inch gage, a reduction of 60%,
and annealed under a variety of conditions. Specimens were
subjected to a simulated box anneal of 24 hours at 1100.degree.,
1200.degree., 1300.degree., 1400.degree., and 1500.degree. F., and
air cooled.
The cold rolled sheet compositions of the various samples were as
follows, in weight percent:
EXAMPLE 3
______________________________________ Ingot C Cb
______________________________________ 1 0.010% 0.057% 2 0.030%
0.057% 3 0.044% 0.057% 4 0.056% 0.057%
______________________________________
EXAMPLE 4
______________________________________ Ingot C Cb
______________________________________ 1 0.012% 0.063% 2 0.025%
0.063% 3 0.038% 0.063% 4 0.057% 0.063%
______________________________________
EXAMPLE 5
______________________________________ Ingot C Cb
______________________________________ 1 0.011% 0.099% 2 0.025%
0.099% 3 0.039% 0.099% 4 0.018% 0.099%
______________________________________
The chemistry on all three Examples above was 0.55% Mn, 0.002% N,
0.003% S, 0.0009% O and 0.02% Al.
Yield strengths of specimens of the above four ingots of Example 3
vs. annealing temperature are represented in the graph of FIG. 2 of
the drawings; (varying carbon and constant columbium contents),
FIG. 3 is a graph of similar plots of specimens of ingots of
Examples 3, 4 and 5.
It is evident from the plotted values at 1100.degree. and
1200.degree. F. that carbon contributes to the yield strength when
present in amounts up to about 0.025% but has less effect at higher
carbon levels. Of greater significance is the strengthening effect
resulting from progressively increased columbium contents (FIG. 3)
and the fact that carbon contents below 0.02% resulted in yield
strengths below 90 ksi at 1400.degree. F. annealing temperature
regardless of columbium content. This is particularly evident from
Example 5-1 wherein the carbon content of less than 0.02% and a
columbium:carbon ratio of greater than 7.75:1 (0.099% Cb and 0.011%
C) exhibited a yield strength of about 68 ksi at 1400.degree. F.
annealing temperature (24 hours).
Modifications may be made without departing from the scope of the
invention, and hence no limitations are to be inferred except
insofar as specifically set forth in the claims which follow.
* * * * *