Note: Descriptions are shown in the official language in which they were submitted.
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This application relates to a method of
treating steel in a continuous process. This
application is a divisional application of Canadian
Application Serial No. 563,296, filed April 5, 1988.
- There exists today a group of steels which
are characterized by among other things enhanced
mechanical properties including higher yield strengths
and tensile strengths than plain carbon structural
steels. These are known as high-strength, low-alloy
(HSLA) steels. Different types of HSLA steels are
available, some of which are carbon-manganese steels
and others of which are microalloyed by additions of
such elements as niobium, vanadium, and titanium to
achieve enhanced mechanical properties. The original
demand for HSLA steels arose from the need to obtain
improved strength-to-weight ratios to reduce dead
weight in transportation equipment. In addition to
the original uses, HSLA steels are used today in a
wide range of applications including vehicles, con-
struction machinery, materials-handling equipment,
bridges and buildings.
Commercial HSLA steels typically have
minimum yield strengths of 275 to 345 MPa and minimum
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tensile strengths of 410 to 480 MPa. The mechanical
- properties and other characteristics of HSLA steels
are set forth in standard specifications such as
Society of Automotive Engineers (SAE) J410c. Micro-
alloyed HSLA steels have even higher strengths on the
order of minimum yield strengths of 345 to 550 MPa and
minimum tensile strengths of 450 to 655 MPa. These
steels use additions of alloying elements such as
niobium, vanadium, titanium, zirconium and rare earth
elements in concentrations generally below 0.10 to
0.15% to achieve higher strength levels. Heat treat-
ment is not involved because the properties of micro-
alloyed HSLA steels result from controlled rolling on
continuous hot strip mills.
One grade of high-strength low-alloy steel
under SAE J410c is grade 950 A,B,C,D, which is charac-
terized by a minimum yield strength (0.2% offset) of
345 MPa, minimum tensile strength of 480 MPa, and
minimum elongation (5 cm specimen) of 22%. This
material exhibits its mechanical properties as hot
rolled, and when later cold reduced to sheet thick-
ness, is subjected to a low temperature recovery
anneal for an extended period of time to maintain the
as-rolled mechanical properties.
Another grade of microalloyed, high-strength,
low-alloy steel under SAE J410c is grade 970X, which
is characterized by a minimum yield strength (0.2%
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offset) of 480 MPa, minimum tensile strength of 585
MPa, and minimum elongation (5 cm specimen) of 14~.
As stated, this material exhibits its mechanical
properties as hot rolled. When later cold reduced to
sheet thickness, these steels are also subjected to a
low temperature recovery anneal for an extended period
of time to maintain the controlled rolled mechanical
properties. In addition to the increased cost because
of the addition of microalloying elements, this
recovery anneal is disadvantageous because of either
the extended times required for box annealing or the
enormous investment required for equipment for con-
tinuous annealing.
There thus exists today a need for steels
possessing the desired combination of strength and
ductility required for HSLA steel applications but
which can be produced economically from cold reduced
sheet stock without the need for extended recovery
annealing. Moreover, there exists a need for such
steels wherein the higher mechanical properties,
particularly yield strength and tensile strength, are
achieved without the intentional inclusion of micro-
alloyinq agents such as niobium, titanium and vanadium,
which otherwise would add significantly to the cost of
the steel.
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Summary Of The Invention
It is among the principal objectives of this
invention to provide a method for treating cold
reduced steel compositions characterized by a rela-
tively low carbon content and the absence of expensivemicroalloying agents which nevertheless exhibit in the
treated condition mechanical properties, i.e., yield
strength, tensile strength, and elongation, meeting
the specifications for microalloyed HSLA steels, for
example, grades 950 A,B,C,D and 970X of SAE J410c.
Moreover, it is among the principal objectives of this
invention to provide such a method for producing cold
reduced steels having the uniformly higher mechanical
properties of the microalloyed HSLA steels which can
be produced in a continuous process at relatively high
speed and very economically.
To these ends, the present invention is
directed to a non-microalloyed low carbon manganese
steel compositions and to a heat treatment method
therefor. One steel composition included within this
invention has a carbon content ranging from .04 to
.15% by weight carbon and 0.25 to 0.70~ by weight
manganese. Microalloying elements such as niobium,
titanium and vanadium are not added to the steel
composition to achieve enhanced mechanical properties.
The steel, which is cold reduced to a desired sheet
thickness, e.g., in the range of 0.078 to 0.236 mm, is
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passed continuously through three heating stages. The
first stage is a preheating stage wherein the tempera-
ture of the cold rolled sheet is raised to a tempera-
ture in the range of about 700F to 1000F. The steel
is then heated to a temperature in the range of 1625F
to 1725F, quenched at a temperature in the range
650F to 750F, and then cooled to room temperature.
The heat treatment is carried
out continuously at a line speed in the range of 50 to
30~ feet/minute whereby a continuous length of steel
strip of desired gauge and width is passed continuously
and sequentially through the three heating stages.
One presently preferred steel composition is
a steel having about 0.10 to 0.15~ by weight carbon
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and about 0.25 to 0.70% by weight manganese, the
balance being iron and the normal residuals from
deoxidation. When treated in accordance with the
first heat treatment schedule described above, the
treated steel exceeds the minimum yield strength of
345 MPa, minimum tensile strength of 480 MPa, and
minimum elongation of 22% specified for grade 950
A,B,C,D of SAE J410c specifications. Another lower
carbon composition containing from about 0.04 to 0.07%
by weight carbon and about 0.25 to 0.40% by weight
manganese when treated by the method of this invention
exhibits a minimum yield strength of 345 MPa, minimum
tensile strength of 410 MPa, and a relatively high
elongation of 28~.
The method of this invention for treating
steels having the relatively low carbon and the
manganese content recited and the absence of micro-
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alloying agents results in a cold reduced product
having mechanical properties meeting or exceeding some
existing HSLA steel specifications for microalloy
steels. The present invention is thus characterized
by the higher mechanical properties of some of the
commercial microalloyed high-strength low-alloy steels
but obtainable in a non-microalloyed, cold reduced low
carbon steel and by the economies inherent in the
absence of microalloying agents, and the continuous
process for the treatment of a cold reduced product.
Brief Description Of The Drawings
Fig. 1 is a schematic illustration of the
treatment process.
Detailed Description Of The Preferred Mode
The carbon-manganese steel compositions
treated by the method of this invention contain in one
case from about 0.04 to 0.15% by weight carbon and
0.25 to 0.70% by weight manganese and in another case
from about 0.11 to 0.18% by weight carbon and 1.20 to
1.40% by weight manganese. The steel is killed, pref-
erably, aluminum killed and continuously cast, to
achieve uniformity of mechanical properties. As a
result, the composition can contain residual silicon
and aluminum from the deoxidation process. The steel
may also be a silicon killed or semi-killed steel.
Referring to Fig. 1, hot rolled coils of
steel, which may be pickled and oiled, are cold
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reduced through a series of cold rolling passes to a
sheet 10 having a desired reduced thickness, for
example, on the order of 0.078 to 0.236 mm. The cold
rolled and reduced sheet 10 is then passed over roller
11 and down into a preheating bath 12 which may be a
bath of molten lead maintained at a temperature in the
range of 700 to 1000F. The lead bath may be heated
by any of a number of means, e.g., natural gas or
electricity. Alternatively to a lead bath, other
media capable of providing a liquid bath having a
temperature in the range of 700 to 1000F may~be used.
The material then passes upwardly out of the bath and
over an elevated roller 14. The material then passes
down into a second molten lead bath 16 which is the
quench bath.
In the heating stage, the material is heated
to a temperature in the range of 1625 to 1725F
depending on composition. In the quench stage, the
material is quenched at a temperature in the range of
650 to 950F depending on composition. That is, the
lower manganese composition is heated in the range of
1625 to 1725F and quenched in the range of 650 to
750F while the higher manganese composition is heated
in the range of 1500 to 1575F and quenched in the
range of 800 to 950F. Heating of the material in
the heating stage is accomplished by resistance
heating. That is, the preheat bath 12 and the quench
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g
bath 16 are maintained at a potential of about 90
volts and current of 8000 amperes with the quench bath
being grounded. As a consequence, the sheet material
10 passing between the preheat bath and the quench
bath shunts the current and is thereby resistance
heated. The length of material passing through the
heating stage, current, and travel speed are con-
trolled to subject the material in the heating stage
to the desired treatment temperature. A protective
atmosphere is maintained in the heating stage by
enveloping the sheet material 10 in an atmosphere
housing 18 which is flushed with a protective exo-
thermic gas. The gas prevents the sheet material from
oxidizing as it passes from the preheat bath 12 to the
quench bath 16. Alternatively to resistance heating,
the material 10 may be heated by other heating means
such as induction, infrared, and gas heating.
The quench bath 16 is also a lead bath which
can be heated by such means as electric immersion
heaters or radiant gas tubes to the desired tempera-
ture. After quenching, the material then passes out
of the quench bath 16 and vertically upward over a
roller 20 and through a charcoal chute 22 which
contains ignited charcoal designed to prevent the lead
from being dragged out of the quench bath on the sheet
material. The sheet material which is now at a
temperature of about 500F is then passed through a
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downstream water tank or water spray (not shown) to
bring its temperature down to about 150F. However,
all of the transformation of the steel is completed by
the time the material leaves the quench bath 16.
After cooling, the material may be coiled for shipment
or subsequently processed by known techniques or
combination of known techniques, e.g., acid and/or
abrasive cleaning, painting, plating, flattening,
tension leveling, and the like.
The sheet material continuously passes
through the preheat, heat and quench stages. ,Typical
line speeds are on the order of 15 to 100 meters per
minute. The preheat, heat, and quench stages are
approximately 3 to 8 meters long. As a consequence,
the material is heated or quenched very rapidly in
each stage on the order of only 6-15 seconds, for
example, at a line speed of 30 meters per minute.
Representative equipment for accomplishing
such heating is disclosed in United States Patent Nos.
2,224,988 and 2,304,225 to Wood et al. Again, heating
and quenching media other than molten lead can be used
for both the preheat and quench baths.
It is believed that the relatively short
cycle times in the preheat, heat, and quench stages
result in grain refinement and consequently increased
strength. That is, in the preheat and heat stages,
the strain introduced into the material from cold
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rolling causes recrystallization of the ferrite to a
fine grain structure. The short cycle times limit
grain growth keeping the grain size small, typically
under 0.01 mm and frequently 0.003 to 0.004 mm and
finer. In addition, small amounts of austenite form
at the grain boundaries on heating and act to pin the
grain boundaries against movement again serving to
limit gain growth and resulting in higher strength
levels. At the same time, the carbides in the pearlite
are spheroidized and imperfections removed increasing
the ductility of the steel. During the quench, the
carbides precipitate introducing ductility and removing
the potential for subsequent strain aging.
Example I
Using the equipment described in Fig. 1,
5 cm wide by 0.11 cm thick steel strip cold reduced
from 0.2 cm material was heat treated. The steel was
aluminum killed for uniformity of properties and the
composition contained 0.10% carbon, 0.40% manganese,
0.012% silicon and 0.057% aluminum, the silicon and
aluminum components being residuals from the
deoxidation of the steel before casting. The strip
material traveled at a rate of 33 meters per minute.
The length of the strip under the lead in the preheat
bath was 3 meters, in the quench bath 6 meters, and in
the heating stage 7.3 meters. Roller 14 was 2.4
meters above the lead baths. An optical pyrometer was
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used to measure strip temperature. The treatment
schedule and resulting mechanical properties are set
forth in Table I.
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TABLE I
SampleStrip Quench Tensile Yield % Elongation
CodePreheat F Temp- F Temp- F Strength (MPa) Strength (MPa) (5 cm gauge) YS/TS Hardness
1-0860 1725 690 505.4 455.1 26.7 .90 86
2-0 775 1670 720 503.3 456.5 26.7 .91 86
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As may be seen from Table I, the mechanical
properties resulting from the treatment process
exceeded the minimum mechanical properties specified
for grade 950 A,B,C,D (345 MPa yield strength, 480 MPa
tensile strength, 22% elongation).
A second, similar steel composition was run
using the same process conditions. This composition
comprised 0.04/0.06% carbon and 0.25/0.35% manganese.
The treatment schedule and resulting mechanical
properties are set forth in Table II.
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TABLE II
Sample Strip Quench Tensile Yield % Elongation
CodePreheat F Temp- F Temp- F Strength (MPa) Strength (MPa) (5 cm gauge) YS/TS Hardness
1-0860 1725 690 417.2 371.6 28.7 .89 75
2-0775 1670 720 413.7 264.7 28.0 .88 73
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This material although lower in tensile
strength than the previous example was characterized
by excellent degree of elongation and thus would be
expected to have a high degree of formability.
Example II
Using the equipment described in Fig. 1,
5 cm wide by 0.11 cm thick steel strip cold reduced
from 0.2 cm material was heat treated. The steel was
aluminum killed for uniformity of properties and the
composition contained 0.14% carbon, 1.33% manganese,
0.22% silicon and 0.019% aluminum, the silicon and
aluminum components being residuals from the
deoxidation of the steel before casting. The strip
material traveled at a rate of 33 meters per minute.
The length of the strip under the lead in the preheat
bath was 3 meters, in the quench bath 6 meters, and in
the heating stage 7.3 meters. Roller 14 was 2.4
meters above the lead baths. An optical pyrometer was
used to measure strip temperature. The treatment
schedule and resulting mechanical properties are set
forth in Table III.
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TABLE III
SampleStrip Quench Tensile Yield % Elongat~on
CodePreheat F Temp. F Temp- F Strength (MPa) Strength (~a) (5 cm gauge) YS/TS Hardness
3-M795 1535 855 639.1 559.9 18.7 .88 95
4-M 820 1500 950 593.0 524.0 22.0 .88 92
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As may be seen from Table III, the mechanical
properties resulting from the treatment process
exceeded the minimum mechanical properties specified
for grade 970X (480 MPa yield strength, 585 MPà
tensile strength, 14~ elongation). Both samples
exhibited excellent ductility in combination with the
higher strength levels.
The method of the present invention is
applicable to a range of steel compositions within the
compositional limits set forth above. As the preceding
specific example shows, the treatment method provides
low carbon high manganese cold reduced steels with the
desired combination of strength and ductility charac-
terizing commercial microalloyed and hot rolled
high-strength low-alloy steels.
Thus having described the invention, what is
claimed is:
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