Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE INVENTIO~:
This invention relates to a method for treating low
carbon nitrogenized steel to p~oduce a material of suitable
ductility having a high yield strength similar to high strength
low alloy steels (HSL~) and being readily producible in
markedly thinner gauges.
The need to reduce the weight of the automobile has
become increasingly urgent in recent years with the need to
- otherwise accommodate weight increases due to additions of
safety and emission control devices and to improve engine
performance and fuel economy. These considerations have
prompted interest in automobile structural materials having a
higher strength-to-weight ratio.
One group of such materials presently being considered
is the family of the aforementioned high strength low alloy
steels with yield strengths in the neighborhood of 80,000 psi.
These steels offer an attractive combination of increased
strength and acceptable formability.
The high yield strength of the HSLA steel is developed
through a controlled combination of grain refinement,
precipitation hardening, and solid solution strengthening which
results from the addition of titanium, vanadium, or niobium to
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the basic low carbon steel chemistry, and from the carefully
controlled cooling in the hot strip mill in which such
; steels are produced. Often rare earth alloying elements
are added to control the shape of the inclusions and hence
to improve the steel's formability. Unlike other steels
the 80 KSI HSLA steels cannot be produced in gauges thinner
than .070 inch due to limitations in the present commercial
rolling equipment. Generally, thinner steel gauges of lesser
strength may be formed by cold rolling. The requirements of
the strengthening mechanism of present HSLA steels as above
indicated make them unavailable in cold rolled or thinner
conditions with suitable ductility to form au-tomotive
components with strengths of about 80,000 psi.
The term "low carbon steel" as used herein is a steel
containing up to 0.25% carbon and only residual amounts of
elements other than those required for deoxidation, ,
particularly silicon 0.6% or less, and manganese 1.65% or
less. The term "nitrogenized steel" as used herein is a
"low carbon steel" containing nitrogen preferably in the
range of 80 to 200 parts per million or 0.008 to 0.02% by
weight.
SUMMARY OF THE IMVE~TIO~:
High strength steels in gauges of .050 inch or less are
highly desirable for automotive application for weight
reduction objectives and it is the basic object of this
invention to provide a method whereby a cold rolled nitrogenized
low carbon steel is strengthened to about an 80,000 psi yield
strength or more and is not subject to the minimum gauge -~
limitations of the HSLA steels.
In general, the method is applicable to an aging
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nitrogenized low carbon cold rolled steel and comprises a
first heat treatment followed by a stamping or forming step
(prestrain) and then by a second heat treatment. The total
added strength provided by the method is the sum of added
strength provided by microstructural changes induced
metallurgically ~ the first heat treatment, a strength
increment due to cold working the steel which involves forming
the steel in the high work hardening rate condition created
by the first heat treatment and finally the strain age
strengthening increment caused by subjecting the part to the ~ -
second heat treatment.
The first heat treatment is central to the invention
because it produces a pronounced increase in work hardening
rate and the ultimate or tensile strength. This heat treatment,
accordingly, provides the basis for increasing the final
strength of the nitrogenized low carbon steel to 80,000 psi
or more. Furthermore, the first heat treatment provides a
decreased yield strength during forming, a high tensile to
yield strength ratio and a good "n" value (slope of the natural ~;~
log true stress vs. natural log true strain plot) which thus
provides the steel with a degree of formability as good or
better than the best HSLA steel commercially available today.
In general the first heat treatment consists of rapidly ~-;
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heating a sheet of an aging nitrogenized low carbon steel to a g
temperature in the alpha plus gamma region of the phase
diagram for 30 to 60 seconds and quenching. The sheet is then `
press ~ormed to obtain a minimum prestrain of at least 2%.
The sheet is then aged at room temperature or at paint-curing
temperatures to develop a final strength of about 80,000 psi. `~
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DESCRIPTION OF T~E DRAWINGS:
FIGURE 1 is a stress-strain curve of the nitrogenized
steel identified as AAN showing the effect of the first heat
treatment of the method of this invention. A curve for an
HSLA steel is shown for comparison.
FI&URE 2 is a time temperature curve generally depicting
the three steps of the invention.
FIGURE 3 is a true stress-prestrain curve for the AA~
nitrogenized steel showing the dependence of the aged yield
strength on the prestrain step of the invention.
DESCRIPTION OF THE PREFERRED EMBODIME~T:
The invention will now be described in terms of specific
embodiments. A commercial nitrogenized low carbon steel is
selected which is basically an SAE lOlO mild steel with about
four times the nitrogen content of the standard SAE lO10 steel.
Such steel is currently available, for example, from Inland
Steel Company, Great Lakes Steel Company, and Bethlehem ;~
Steel Company. These steels have yield strengths in the range
of 45,000 and 55,000 psi and are intended for applications
where aging at paint curing temperatures (212 - 392 F.) can
be used to develop final yield strengths of 60,000 psi through
strain aging with a 2% plastic prestrain. The material is
relatively fine grained with a ferritic grain size of ASTM 9-11
developed through controlled processing rather than by
alloying. One sample AAO has a composition % by weight of
silicon less than .005, sulphur .027, manganese .55, nickel .02,
chromium .006, cobalt less than .01, titanium less than .005,
aluminum .004, copper .072, phosphorous .003, carbon .13, `~
oxygen .0238, nitrogen .0134 and the balance iron. A second
sample AA~ consists of silicon less than .005, sulphur .012,
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manganese .5, nickel .02, chromium .006, cobalt less than .01,
titanium less than .005, aluminum .005, copper .066, phosphorous
.003, carbon .13, oxygen .0220, and nitrogen .0138.
As previously indicated, yield strengths of 80,000 psi
or more can be developed in a stamping by means of the method
~ of this invention which consists of the following three
essential steps as illustrated in FIGURE 2: . ;
~ 1. A rapid conditioning heat treatment applied to the
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steel prior to forming comprising heating the sheet rapidly
10 to a temperature between its eutectoid temperature to about : .
1400F. for at least three seconds and then quenching;
2. A prestrain step in which the material is plastically :`:
deformed by stamping or the like to a strain level of 2% or
more, preferably within about 5 days; and
3. An aging cycle to obtain the final desired strength, :
for example, at room temperature for 2 to 3 weeks or at about ..
400 F. for 10 to 60 minutes. ~ -.
The conditioning first heat treatment is essentially a
solution heat treatment designed to take advantage of the -
20 large increase in the solubility of carbon and nitrogen in :
ferrite near the eutectoid temperatures and to produce a i :
controlled volume fraction or martensite or bainite upon
quenching. This is readily accomplished by induction heating ;
the steel rapidly as above indicated or into its alpha plus : .
gamma phase region and near the critical Al temperature and .
holding the steel at this temperature for at least 3 seconds ` :
and then quenching in brine, oil, or water. :Q~', ' ,'
EX~MPLE I. The sample AAN was induction heated to a f
temperature of about 1319F. and held at this temperature for . ~
30 a total time of 10 seconds including heating and soaking time, ~ ;
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and immediately quenching in brine. As shown by the tensile
stress strain curves of FIGURE 1, the yield stress is
decreased about 30O/o after treatment. The rate of work
hardening and ultimate strength are markedly increased with
tensile to yield ratios of two to one or greater and "n"
values near .19 and total tensile elongations of about 20%.
These values are typical of nitrogenized low carbon steels ,
treated in this fashion, indicating superior formability
for this steel after the first heat treatment, as compared
to HSLA steels. A tensile curve for Van-80, a typical MSLA
steel in the 80,000 psi yield range is shown for comparison. ~
Final strength improvements are obtained by aging the -
treated material at room or elevated temperatures. For
example, specimens yield at about 80,000 psi when deformed
about 2% and aged for 12 days at room temperatures or for 10
minutes at about 400F. Treated specimens yield at about
100,000 psi when deformed lO~/o before aging 12 days at room
temperature or ten minutes at about 400 F.
EXAMPLE II. The steel is heated by submersion in a
liquid salt bath at about 1380F. for a total of 35 seconds,
and immediately quenched in brine. Again the yield stress
is decreased and the work hardening rate, the tensile strength,
and the tensile strength to yield strength ratio are
increased. The treatment conditions for this method of
heating were selected to give a steel of comparable ductility
to that of Example I. Although the increase in work hardening
rate is not as great (providing an ultimate strength of
78,500 psi as compared to 91,000 psi for Example I), it is
sufficient to give a yield strength after 2% prestrain and
aging of 80,000 psi.
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EX~MPLE III. The steel is treated by submersion in a
liquid salt bath at 1380F. for a total of 80 seconds, and
immediately quenched in oil. Again the property changes out-
lined above were noted. The decrease in tensile ductility is
not as great as in Examples I and II with a total elongation
of about 25% being typical (as compared to about 20D/o of the
previous Examples). As before, additional strengthening is
obtained by aging after forming. Similar results were obtained
using the AA0 steel.
As is apparent from the foregoing, realization of the
80,000 psi yield strength is dependent on the selection of a
suitable heating temperature and the duration of the heat
treatment in the alpha plus gamma region, the amount of
prestrain and the aging treatment. ~-
It has been determined that a 2% prestrain, as shown by . `
the broken line in FIGURE 3, is a practical lower limit for
automotive applications. To achieve a final yield strength ~ `
of 80,000 psi it has been found that-a heating duration of
at least three seconds by induction heating at a temperature
of about 1380F. for the nitrogenized low carbon steel is
operative to form austenite and to solubilize sufficient carbon
in the ferrite and to, on quenching, convert sufficient
austenite to martensite or bainite to develop the 80,000 psi
yield strength on aging. The maximum duration of the heat ;
treatment is determined by the ductility requirements in the
automotive applications. Such applications typically require
a tensile elongation of about 18% which limits the heat
treatment duration at 1380F. to about 60 seconds.
Adjustments in the duration of heating and in the temperatures
selected for the first heat treatment can be used to produce
a relatively broad spectrum of mechanical properties in a
treated sheet. As shown in Example I J a single rapid heat
treatment can be used to convert a nitrogenized low carbon
steel into a product with a strength level, after forming and
aging, comparable to the present family of HSLA steels. Because
this invention is relatively simple to implement and is not
subject to the minimum gauge restriction of the HSLA steel,
it is viewed as an attractive alternative thereto.
In the practical application of this invention, the
nitrogenized cold rolled steel of suitable gauge in the
neighborhood of 0.05 inch is provided at the automobile
component stamping site. A sheet of suitable size is preferably
induction heated to a temperature for a time as above
indicated and quenched. The sheet is then placed in a
stamping die and formed to a desired configuration with a -
prestrain of at least 2%. Thereafter, the part is aged at
room temperature or while it is taken through a painting cycle
to develop the desired final strength. Accordingly, the
process readily leads itself to the conventional operations -
presently in practice at automobile component stamping plants.
As previously stated, the first heat treatment requires
a temperature within the alpha plus gamma region. It is
preferred to select a temperature just above the eutectic
temperature since higher temperatures require lesser time
periods and are thus more difficult to control and hence achieve
product consistency. It is to be understood that the
solubilizing effect is greater at the higher temperatures so
that both temperature and heating duration must be controlled
to obtain the desired yield strength and ductility. It has `
been found that with present control equipment an upper .
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heating temperature of about 1~00F. is practical and
preferred.
:: In summary, the principle advantages for the nitrogenized
low carbon steel treated in accordance with this invention as
compared to HSLA steel are that it is less expensive, has
equivalent or better formability with yield strength of 80,000
psi or more obtained by aging with a 2% minimum prestrain, no
minimum gauge restrictions, lower press loads required for
forming, superior residual ductility after strain aging, and .
the ability to form exterior appearance parts of moderate
complexity since the initial heat treatment of this invention . .
suppresses stretcher strain formation during forming. .
The method has also been found to markedly improve the :
strength of low carbon steels generally, and although this ~`
invention has been described in terms of specific embodiments ::
it is obvious that other forms may be adopted within the scope .
o~ this invention.
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