Note: Descriptions are shown in the official language in which they were submitted.
CA 02128019 1998-07-14
HIGH-STRENGTH STEEL PARTS AND METHOD OF MAKING SAME
Field of the Invention
The present invention relates to a method of making
high-strength steel parts and the parts produced thereby, and
more particularly it relates to a method in which a blank of
high-strength steel is warm formed into a desired geometric
configuration while maintaining the high-strength properties of
the steel blank.
Background of the Invention
Steel parts have heretofore been formed using cold
forging or hot forging techniques which are well known in the
art. In hot forging parts from steel, the material is initially
heated to about 2000~F and higher. At these hot forging
temperatures, substantial scale and decarburization of the steel
~093/1~233 2 ~ ~ t 1 PCT/US93/~519
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occurs. Since the scale and decarburized surfaces
must be removed to obtain the finished part, known hot
forging techniques result in the waste of a certain
amount of material; furthermore, such techniques are
5 costly due to increased processing steps required to
remove the scale and the higher energy consumption
because of the high temperatures. On the other hand,
there are drawbacks to cold forming parts as well.
Since the part is formed at or about room temperature,
the reshaping or forming steps require substantially
higher forces. This frequently necessitates a series
of cold forming steps in which the material is formed
into the desired part gradually. This increases die
wear and noise associated with such processes.
Furthermore, if the material is worked to a
substantial degree over a series of forming stages,
the strength of the part is increased and the part
must therefore be annealed between successive cold
forming operations to relieve internal stress, which
adds to the time and cost of such processes.
- To avoid the above drawbacks, warm forging
may be utilized to form parts from materials at an
intermediate temperature which is high enough to
reduce the strength of the material and thereby
facilitate forming, and yet is below the hot forging
temperature at which scaling and decarburization
occurs. One such warm forming method is disclosed in
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U.S. Pat. No. 3,557,587. Certain other patents disclose
processes which include rolling and extruding steps carried out
at "warm" temperatures so as to avoid the drawbacks of
decarburization and scaling and/or to impart or improve desired
metallurgical and mechanical properties to the steel. See U.S.
Pat. Nos. 2,767,836; 2,767,837; 2,880,855; 3,076,361; 3,573,999,
and "Warm Working of Steel", Gokyu, et al, translation of the
Japanese Institute of Metal, 1968, Volume 9, Supplement, Pages
177-181 .
Additionally, there are other known methods for bending
or forging steel bars, rods or billets to form a desired product
which methods include a warm forming or warm forging step. See
U.S. Pat. Nos. 2,953,794; 3,720,087; 3,877,821; 4,312,210;
4,317,355; 4,608,851 and 4,805,437. No representation is made
that any of the above cited references fairly represent the prior
art or that such references are the most material references.
U.S. Patent Nos. 3,001,897 and 3,904,445 both describe
methods of forming a blank of high strength steel for subsequent
use to form parts. The former Patent describes a method in which
steel is first cold reduced and then further reduced to the final
gauge at a warm temperature. The latter Patent describes a
method in which steel is processed into bar stock by hot
reduction to the final gauge, rapid cooling and cold finishing.
The stock is then stress relieved at a warm temperature.
Abstract No. 101411, Chemical Abstracts Vol. 95, No.
11, (Japanese Patent Application No. 56-4612) describes a method
of forming bolts from hardened steel rods containing 0.15-0.4
carbon in which a rod is hot-rolled, water cooled, cold-drawn,
~ CA 02128019 1998-07-14
heated to 550~C, hot shaped to bolts and then water cooled. The
tensile strength of the products is at least 100 kg/mm2.
U.S. Patent No. 3,959,999 describes a method of
producing long articles such as wire, strip and rolled sections,
from hot rolled carbon steel by preforming alternate cold and
warm plastic deformations of a rod at least twice in succession.
U.S. Patent No. 3,720,087 describes a process for
bending steel bars or rods to effect a desired curvature or
straightness at a temperature in the range 150~C to 480~C. The
strength properties of the bars or rods are retained provided the
steel is one which strain or work hardens.
A method of making high-strength steel parts, in
accordance with the invention, comprises providing a blank of
high-strength steel material having a tensile strength of at
least 800 MPa (120,000 psi) and a yield strength of at least 600
MPa (90,000 psi) and warm forming the blank at a temperature
between 150~C (300~F) to 650~C (1200~F) to provide a part having
a desired geometric configuration, whereby the mechanical
properties of tensile strength and yield strength of the part are
greater than the blank wherein the high-strength steel material
comprises, by weight percent:
carbon 0.30 to 0.65
manganese 0.30 to 2.5
vanadium up to 0.35
iron balance.
There is thus provided, which has heretofore been
lacking, a method of making a high-strength steel part from a
blank of steel possessing desired high-strength properties, which
- CA 02128019 1998-07-14
method includes a warm forming step whereby the blank is formed
into a desired part and whereby the mechanical properties of the
part are greater than those originally possessed by the blank,
and in which the part is, preferably, produced without additional
strengthening processing steps to impart mechanical strength
properties to the steel part.
The present invention is directed to a method of making
high-strength steel parts from blanks of high-strength steel
material having a tensile strength of at least about 800 MPa
(120,000 psi) and a yield strength of at least about 600 MPa
(90, 000 psi) .
In one of its aspects, the present invention provides
a method of making high-strength steel parts from blanks of high-
strength steel material by warm forming the blank to provide a
part having a desired geometric configuration, whereby the
mechanical properties of tensile strength and yield strength of
the part are greater than the blank.
The present invention also provides a method of making
high-strength steel parts including warm forming a blank of high-
strength steel material whereby the mechanical properties oftensile strength and yield strength of the part are substantially
the same as or greater than the blank and wherein the part, with
the desired mechanical properties of tensile strength and yield
strength, is produced without further strengthening processing
steps. High-strength steel parts are produced in accordance with
the method of this invention.
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The principles of this invention, its objectives and
advantages will be further understood with reference to the
following detailed description.
Detailed Description of the Invention
The method of the present invention is useful for
producing a wide variety of high-strength steel parts including
various types of bolts (U-bolts, eye-bolts, ~-bolts, hex-head
bolts, square-head bolts, etc.), axles, cam shafts, screws, sway
bars and other parts susceptible to forming by the warm forging
or forming process disclosed herein.
In a preferred embodiment, the method for making a
high-strength steel part includes providing a blank of high-
strength steel material having a tensile strength of at least
about 800 MPa (120,000 psi), and preferably at least about 1025
MPa (150,000 psi), and a yield strength of at least about 600 MPa
(90,000 psi), and preferably at least about 900 MPa (130,000
psi). In one form, the high-strength steel
material utilized as the blank has been hot reduced and cold
drawn to provide the blank having the mechanical properties of
tensile strength and yield strength stated above.
The high-strength steel material may be exemplified by
the following composition, by weight percent:
carbon about 0.30 to about 0.65
manganese about 0.30 to about 2.5
vanadium up to about 0.35
iron balance.
In a more preferred form, the high-strength steel material has
the following composition, by weight percent:
CA 02128019 1998-07-14
carbon about 0.50 to about 0.55
manganese about 1.20 to about 1.65
vanadium about 0.03 to about 0.15
iron balance.
The blank, having a composition and mechanical
properties of tensile strength and yield strength as given above,
is thereafter warm formed at a temperature of about 150~C (300~F)
to about 650~C (1200~F) to provide a part having a desired
geometric configuration, whereby the mechanical properties of
tensile strength and yield strength of the part are greater than
the blank. The temperature at which the part is formed is
related to the chemical composition of the steel material used.
The formed part, with the mechanical properties of tensile
strength and yield strength given, is produced without any
further strengthening processing steps subsequent to the warm
forming thereof.
The blank of high-strength steel material having a
tensile strength of at least about 800 MPa (120,000 psi) and a
yield strength of at least about 600 MPa (90,000 psi), which is
used as the starting piece in the method of the present invention
is produced by any suitable method known in the art. One such
method is disclosed in U.S. Patent No. 3,904,445 which discloses
a processing sequence to produce a high-strength
steel bar stock of the type particularly useful for producing
threaded fasteners, including U-bolts. In the described process,
the bar stock produced has a fine grained structure between about
ASTM No. 5-8. In the disclosed process, a steel, having a
chemistry falling within certain disclosed ranges, is subjected
CA 02128019 1998-07-14
to a standard hot reducing operation to within 10~-15~ of final
gauge. The hot reduced bar stock is then cut or severed into
individual lengths for rapid air cooling. Thereafter the
individual lengths of hot reduced bar stock are subjected to a
cold finishing to final gauge. The final step is a controlled
stress relieving step to increase the mechanical strength
properties. This stress relieving step comprises heating the
lengths of bar stock to between about 260-450~C (500-850~F) for
about 1 hour, but may or may not be necessary. Thus, such bar
stock, with and without further stress relieving may be used to
form the starting blank material of high-strength steel.
The following examples illustrate the practice of the
present invention to produce a U-bolt from high-strength steel
bar stock produced in accordance with thé method disclosed in
U.S. Pat. No. 3,904,445, described above.
ExamPle 1
High-strength steel bar stock of Grade 8 strength
steel, having a diameter of 2.1 cm (0.825"), is cut to lengths
of approximately 90 cm (36.0"). This stock has a tensile
strength of at least about 1025 MPa (150,000 psi) and a yield
strength of at least about 900 MPa (130,000 psi). Both ends of
the bar stock segments are threaded using known threading
processes to provide 6.4 cm (2~") threaded sections at either end
thereof. The bar stock segments are then heated to approximately
450~C (850~F) and a medial section of each bar stock segment is
flattened using a mechanical forging press applying approximately
100 MPa (1,000 tons) of pressure. The flattened section of the
CA 02128019 1998-07-14
bar stock is approximately 1.1 cm (7/16") thick and 3.2 cm (1~
wide. Thereafter, the bar stock segments are bent at the
location of the flattened section to form the U-bolt product.
The flattening step provides elongation to the bar stock segment
such that the total length of the finished U-bolt product is
somewhat in excess of 90 cm (36"). The finished U-bolt product
has the desired mechanical properties of tensile strength and
yield strength originally possessed by the bar stock and
therefore requires no further strengthening processing steps.
~ 9
Examples 2-6
Five high-strength cold drawn C-1541 steel bars
having diameters of about 18.3 mm (O.72") were cut to
lengths of approximately 46 cm (18'l). These bars were
turned down to diameters of about 1.25 cm (0.5") and tested
for mechanical properties reported in Table I. For
instance, these bars had tensile strengths of about 960 MPa
(140,000 psi) and yield strengths of about 900 MPa
(130,000 psi) as set forth in Table I.
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Examples 7-11
Five high-strength cold drawn C-1541 steel bars
having diameters of about 18.3 mm (0.72") were cut to
lengths of approximately 46.0 cm (18") from the same length
of bar stock used to make bars of Examples 2-6. The bars
were then heated to approximately 270~C (550~F) and a
medial section of each bar was flattened in a fashion
similar to Example 1 using a mechanical forging press
applying approximately 100 MPa (1,000 tons) of pressure.
The flattened bars were then turned down to a diameter of
about 1.25 cm (0.5") and tested for the properties listed
in Table II.
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The finished bars of Table II had desired mechanical
properties greater than those originally possessed by the
bars of Table I. For example, the tensile strengths
increased from about 960 MPa (140,000 psi) to about 1100
MPa (160,000 psi) and the yield strengths increased from
about 890-905 MPa (130-132,000 psi) to about 1015-1025 MPa
(148-150,000 psi).
Thus, the method provides for warm forming a blank
into a part whereby the mechanical properties of tensile
and yield strengths may be greater than the blank.
