U.S. patent application number 10/593257 was filed with the patent office on 2007-10-04 for forged or stamped average or small size mechanical part.
This patent application is currently assigned to MITTAL STEEL GANDRANGE. Invention is credited to Mario Confente, Marie-Therese Perrot-Simonetta.
Application Number | 20070227634 10/593257 |
Document ID | / |
Family ID | 38557092 |
Filed Date | 2007-10-04 |
United States Patent
Application |
20070227634 |
Kind Code |
A1 |
Perrot-Simonetta; Marie-Therese ;
et al. |
October 4, 2007 |
Forged or Stamped Average or Small Size Mechanical Part
Abstract
The inventive average or small size mechanical steel part is
produced by hot forging or cold stamping i.e. by plastic processing
of a long ferrous semi-product which is obtainable by continuous
casting and hot rolling in the austenitic phase and, afterwards is
shaped by plastic deformation and heat treated in order to obtain a
metallographic structure substentially containing an acicular
ferrite at least in mechanical toughness and fatigue stress areas.
The composition of said steel, apart from iron and inevitable
residual impurities resulting from steel production, corresponds at
least to the following analysis: 0.2-0.5% C, 0.5-2.0% Mn, 0.05-0.5%
V, 0.6 1.5% Si, 0.05 1.0% Cr, 0.01-0.5% Mo, 0.02-0.10 S, preferably
from 0.01 to 0.02% Ti and/or up to 0.20% Al, and from 5 to 30 ppm
of Ca.
Inventors: |
Perrot-Simonetta;
Marie-Therese; (Montrequienne, FR) ; Confente;
Mario; (Plappeville, FR) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
MITTAL STEEL GANDRANGE
SITE INDUSTRIEL DE GANDRANGE
GANDRANGE
FR
F-57175
|
Family ID: |
38557092 |
Appl. No.: |
10/593257 |
Filed: |
March 16, 2005 |
PCT Filed: |
March 16, 2005 |
PCT NO: |
PCT/FR05/00646 |
371 Date: |
September 18, 2006 |
Current U.S.
Class: |
148/595 ;
148/334 |
Current CPC
Class: |
C22C 38/32 20130101;
C22C 38/04 20130101; C22C 38/24 20130101; C22C 38/22 20130101; C21D
8/005 20130101; C22C 38/02 20130101 |
Class at
Publication: |
148/595 ;
148/334 |
International
Class: |
C21D 8/00 20060101
C21D008/00; C22C 38/22 20060101 C22C038/22 |
Claims
1. A mechanical part made of steel derived from the hot forging or
the cold pressing thereof, of medium or small size, and resulting
from plastic transformation of a long siderurgical semiproduct,
which the steel of which it is composed has a composition that,
besides iron and the inevitable residual impurities resulting from
processing of the steel, corresponds at least to the following
analysis, given in weight percentages: 0.2.ltoreq.C.ltoreq.0.5,
0.5.ltoreq.Mn.ltoreq.2.0, 0.05.ltoreq.V.ltoreq.0.5,
0.6.ltoreq.Si.ltoreq.1.5, 0.05.ltoreq.Cr.ltoreq.1.0,
0.01.ltoreq.Mo.ltoreq.0.5, and 0.02.ltoreq.S.ltoreq.0.10, and
optionally up to 50 ppm of boron, wherein the said part is obtained
from a long semiproduct derived from continuous casting and
hot-rolling in the austenitic area, then formed by plastic
deformation and treated thermally in order to obtain a
metallographic structure containing essentially acicular ferrite at
least in the zones of mechanical stressing in tenacity and
fatigue.
2. The mechanical part according to claim 1, wherein the steel
further comprises from 0.01 to 0.02% titanium and/or up to 0.20%
aluminum.
3. The mechanical part according to claim 1, wherein the steel
further comprises between 5 and 30 ppm of calcium.
4. A steel for the manufacture of a mechanical part by plastic
deformation, wherein, besides the inevitable residual impurities
resulting from processing of the steel, its chemical composition
comprises at least, expressed in weight content:
0.2.ltoreq.C.ltoreq.0.5, 0.5.ltoreq.Mn.ltoreq.2.0,
0.05.ltoreq.V.ltoreq.0.5, 0.6.ltoreq.Si.ltoreq.1.5,
0.05.ltoreq.Cr.ltoreq.10, 0.01.ltoreq.Mo.ltoreq.0.5, and
0.02.ltoreq.S.ltoreq.0.10, and optionally up to 50 ppm of B,
wherein the metallographic microstructure that the steel will have,
once the part is implemented, is essentially composed of acicular
ferrite at least in the zones of the part subjected to mechanical
stressing in tenacity and fatigue.
5. The steel according to claim 4, wherein, in order to protect the
vanadium, the steel further comprises from 0.01 to 0.02% titanium
and/or up to 0.20% aluminum.
6. The steel according to claim 4, further comprising between 5 and
30 ppm of calcium.
7. A process for the manufacture of a mechanical part made of
steel, wherein, for the purpose of obtaining acicular ferrite at
least locally on the part, the process comprises the following
stages: providing a continuous casting billet made of steel with a
composition according to claim 4, which is hot-rolled at a
temperature in excess of 1000.degree. C. into a bar or wire before
being cooled to room temperature after rolling; subjecting the wire
to a controlled cooling prior to formation into rings to obtain a
metallographic structure composed essentially of acicular ferrite,
which wire then is cut into pieces and cold-pressed into a finished
part ready for use; and cooling the bar naturally in the rolling
heat prior to cutting the bar into pieces which then are hot-forged
into a rough shape of a part that is cooled by controlled cooling
to obtain a structure essentially composed of acicular ferrite at
least in the stressed zones of the part, which rough shape then is
machined, as need be, to the desired dimensions to make it into a
finished part ready for use.
8. The process according to claim 7, wherein the controlled cooling
is a natural cooling to room temperature.
9. The process according to claim 7, wherein the controlled cooling
is a forced cooling ensuring a surface cooling speed of
approximately 0.5 to 15.degree. C./s.
10. A long, medium carbon siderurgical semiproduct, intended to be
transformed by forge or by press into a mechanical part with high
characteristics, of small size or of medium size, wherein, in order
that the part may have a metallographic microstructure essentially
composed of acicular ferrite at least in the zones of the part
subjected to mechanical stressing in tenacity and fatigue, the
steel that constitutes the part corresponds at least to the
following analysis, given in weight percentages:
0.2.ltoreq.C.ltoreq.0.5, 0.5.ltoreq.Mn.ltoreq.2.0,
0.05.ltoreq.V.ltoreq.0.5, 0.6.ltoreq.Si.ltoreq.1.5,
0.05.ltoreq.Cr.ltoreq.1.0, 0.01.ltoreq.Mo.ltoreq.0.5. and
0.02.ltoreq.S.ltoreq.0.10, and optionally up to 50 ppm of boron,
wherein the metallographic microstructure that it will have after
transformation will be essentially composed of acicular ferrite at
least in the zones of the part subjected to mechanical stressing in
tenacity and fatigue.
Description
[0001] The invention relates to mechanical parts of medium or small
size made of medium carbon micro-alloyed steel, such as wheel hubs,
connecting rods or swivels for an automobile, or other similar
mechanical parts obtained through hot or cold plastic deformation
of a long siderurgical semiproduct and for which there are sought,
above all, properties of resistance to fatigue and of tenacity. By
medium or small size, there is understood here parts the diameter
of which does not exceed approximately 80 mm.
[0002] In order to produce such parts, it is known to make use of
steels specially alloyed to obtain a metallographic structure of a
bainitic or essentially bainitic type. By "essentially," there
usually must be understood 80% and more by volume of the bainitic
structure at the place on the part where this structure is
sought.
[0003] Their manufacture in fact requires being able to withstand
significant modifications in form without breaking or cracking,
while in the end affording a good resistance to fragile breaking
(tenacity) and fatigue in view of the cycles of repetitive stresses
to which the parts are subjected in use, as well as to impacts
(high resilience). Furthermore, these steels must afford good
machinability characteristics, in order to permit a precise final
dimensioning by machining of the part ready for use required in a
number of applications.
[0004] The manufacturing process usually can comprise an operation
of cold (press or forge) or hot (forge) plastic deformation, the
choice of the hot or cold method often being made according to the
final size of the parts. In all cases, this operation will be
performed on pieces of steel cut up into bars deriving from long,
continuously cast hot-rolled siderurgical semiproducts. When the
plastic deformation is performed "hot," the pieces of steel are
reheated beforehand to a temperature of approximately 1000 to
1200.degree. C., then hot-formed in the forge. The parts obtained
then are cooled and treated thermally by hardening and tempering.
When the plastic deformation is performed "cold," the pieces are
cold-formed in the press, possibly after having undergone a
globularization annealing. The parts obtained then are treated
thermally by hardening and tempering.
[0005] It is recalled that in service, these parts ordinarily are
subjected to variable, even cyclic, mechanical stressing, which
generates a significant fatigue effect. Steel fatigue is expressed
by the occurrence of microfissures that propagate until breaking,
even if the stress is lower than the tensile strength or the limit
of elasticity of the metal that constitutes the part. Nowadays it
is estimated that fatigue is responsible for nearly 90% of the
breaking of mechanical parts in service. Likewise, the impacts that
a mechanical part may undergo in service bring about the occurrence
of microfissures that can cause the part to break prematurely if
particular attention is not given to the resilience properties of
the metal that constitutes it.
[0006] Now, the bainitic structure of the steel ordinarily appears
in the form of parallel laths that consequently present few
obstacles to the propagation of microfissures. This structure,
although sought for its properties of mechanical resistance and
ductility, does not necessarily afford a satisfactory tenacity or
resistance to fatigue.
[0007] It is known, for example through document EP 0 787 812, to
improve the fatigue resistance of forged parts by virtue of the
presence of residual austenite within the bainite, obtained by
means of an appropriate controlled cooling combined with the choice
of a grade of steel the composition of which was specially enriched
with silicon.
[0008] The purposes of the invention is to contribute another
solution to the improvement of the fatigue resistance and tenacity
of forged or pressed mechanical parts that preserves their high
mechanical characteristics, for example of resistance, ductility
and resilience.
[0009] To this end, the invention has as its purpose a mechanical
part made of steel deriving from the hot forge or the cold press,
of medium or small size, resulting from the plastic deformation of
a long siderurgical semiproduct, characterized in that the steel of
which it is composed has a composition which, besides iron and the
inevitable residual impurities resulting from the processing of
steel, corresponds at least to the following analysis, given in
weight percentages: [0010] 0.2.ltoreq.C.ltoreq.0.5 [0011]
0.5.ltoreq.Mn.ltoreq.2.0 [0012] 0.05.ltoreq.V.ltoreq.0.5 [0013]
0.6.ltoreq.Si.ltoreq.1.5 [0014] 0.05.ltoreq.Cr.ltoreq.1.0 [0015]
0.01.ltoreq.Mo.ltoreq.0.5 [0016] 0.02.ltoreq.S.ltoreq.0.10 [0017]
and possibly up to 50 ppm boron and in that the said part is
obtained from a long semiproduct deriving from continuous casting
and hot-rolled in the austenitic area, then formed by plastic
deformation and treated thermally to obtain a metallographic
structure containing essentially acicular ferrite, at least in the
zones of mechanical stressing in tenacity and fatigue.
[0018] By "essentially" there is understood here at least 50% and
preferably 60%, or even advantageously 80% and more of acicular
ferrite by volume.
[0019] The invention further has as its purpose a steel for the
manufacture of a mechanical part by plastic deformation
characterized in that, besides the inevitable residual impurities
resulting from the processing of steel, its chemical composition
includes at least, expressed in weight content: [0020]
0.2.ltoreq.C.ltoreq.0.5 [0021] 0.5.ltoreq.Mn.ltoreq.2.0 [0022]
0.05.ltoreq.V.ltoreq.0.5 [0023] 0.6.ltoreq.Si.ltoreq.1.5 [0024]
0.05.ltoreq.Cr.ltoreq.1.0 [0025] 0.01.ltoreq.Mo.ltoreq.0.5 [0026]
0.02.ltoreq.S.ltoreq.0.10 [0027] and possibly up to 50 ppm of B and
in that the metallographic microstructure that it will have, once
the said part is implemented, is composed essentially of acicular
ferrite at least in the zones of the part subjected to mechanical
stressing in tenacity and fatigue.
[0028] With regard to both the mechanical part and the grade of
steel defined hereinabove, in order to facilitate the obtaining of
acicular ferrite, the steel furthermore includes preferably 5 to 30
ppm of Ca, and/or 0.01 to 0.02% Ti, with possibly up to 0.2%
Al.
[0029] The invention further has as its purpose a process for
manufacture of such a mechanical part made of steel characterized
in that, with the goal of obtaining acicular ferrite at least
locally on the said piece, it comprises the following stages:
[0030] there is supplied a continuous-casting billet made of steel
of a composition in accordance with the analysis given hereinabove,
that is hot-rolled at a temperature in excess of 1000.degree. C.
into a bar or wire before being cooled to room temperature after
rolling;
[0031] the wire being subjected to a controlled cooling prior to
its formation into a ring for obtaining a metallographic structure
composed essentially of acicular ferrite, which wire then is cut
into pieces and cold-pressed into a finished part ready for
use;
[0032] the bar itself being cooled naturally in the rolling heat
prior to its cutting into pieces which then are hot-forged into a
rough shape of a part which is cooled by controlled cooling for
obtaining a structure essentially composed of acicular ferrite at
least in the stressed zones of the part, which rough shape then is
machined, as need be, to the desired dimensions to make it into a
finished part ready for use.
[0033] In a variation, the controlled cooling is a natural cooling
to room temperature. In practice, in fact, it happens that the
forged parts are stored immediately in bulk in buckets, on top of
each other. The parts located on the top of the pile are going to
cool more rapidly than those located underneath. A controlled
cooling of each piece, therefore, is not sought at this stage,
since they usually then are going to be treated thermally
anyway.
[0034] On the other hand, in the process according to the
invention, the parts certainly may cool naturally (that is, without
blowing of air), but this cooling nonetheless must be controlled in
order to ensure the formation of acicular ferrite. This control of
the cooling may be accomplished, for example, by depositing the
parts one by one, apart from each other, directly after the forge
operation, on a conveyor belt that transports them to the receiving
area of the works with a view to their storage prior to
shipment.
[0035] According to a preferred variation of the invention,
however, the controlled cooling is a forced cooling, for example
with blown air, ensuring a surface cooling speed of approximately
0.5 to 15.degree. C./s.
[0036] It is recalled that vocabulary practices in the siderurgical
trade provide that rolled products with diameters ranging up to
approximately 30 mm in diameter (which frequently are packaged in
the form of rings) are referred to as "wire," and those rolled
products starting from 18 mm in diameter and which are delivered
straight after cutting lengthwise at the rolls outlet are referred
to as "bars."
[0037] Finally, the invention has as its purpose a long, medium
carbon siderurgical semiproduct, intended to be transformed by
forge or cold press into a mechanical part with high
characteristics, of small size or medium size, characterized in
that the steel that constitutes it corresponds to the following
analysis, given in weight percentages: [0038]
0.2.ltoreq.C.ltoreq.0.5 [0039] 0.5.ltoreq.Mn.ltoreq.2.0 [0040]
0.05.ltoreq.V.ltoreq.0.5 [0041] 0.6.ltoreq.Si.ltoreq.1.5 [0042]
0.05.ltoreq.Cr.ltoreq.1.0 [0043] 0.01.ltoreq.Mo.ltoreq.0.5 [0044]
0.02.ltoreq.S.ltoreq.0.10 [0045] and possibly up to 50 ppm of boron
and in that the metallographic microstructure that it will have
after transformation will be composed essentially of acicular
ferrite at least in the zones of the part subjected to mechanical
stressing in tenacity and fatigue.
[0046] As undoubtedly will have been understood, the invention in
fact consists in proposing the manufacture of a tough, resilient
mechanical part endowed with a microstructure essentially composed
of acicular ferrite at least in the zones of the part mechanically
stressed in fatigue, from a medium carbon steel combined, in the
analysis brackets given in these elements, with manganese (itself
also gammagenic) for resistance to breaking, and micro-alloyed with
vanadium supported by sulfur in order to promote the development of
acicular ferrite and combined, firstly, with molybdenum in order to
improve resilience and to harden the ferrite even more than the
vanadium alone, secondly, with chromium in order to facilitate the
effectiveness of the controlled cooling at the time of the
transformation operation, and thirdly, with silicon, itself
alphagenic, to increase resilience, but also to favor the
precipitation at the grain joints of a ferrite which will prevent
the bainite from invading everything and thus will allow the
acicular ferrite to appear in order to take its rightful place.
[0047] It should be recalled here that acicular ferrite is a
metallographic constituent known in siderurgy. It already is used,
for example, as shown in EP-A No. 0288054, to facilitate the
process for manufacture of fine-grained sheets for low-temperature
use (offshore, etc.) by eliminating the intermediate reheating
state between casting and hot-rolling.
[0048] Likewise, as shown in U.S. Pat. No. 6,669,789, it is known
to make use, aside from the customary polygonal ferrite-perlite, of
an acicular ferrite structure (that forms on the carbides) for the
manufacture of titanium steel sheet with high resistance and ample
elongation in order to limit the austenitic grain size starting
from hot-rolled thin slabs.
[0049] The invention will be well understood and other aspects and
advantages will emerge more clearly in view of the detailed
description that follows, given by way of an embodiment
example.
[0050] There are produced by continuous casting in the steelworks
long semiproducts (billets or blooms) deriving from a steel having,
besides, iron, the following composition by weight content in
relation to the iron:
[0051] From 0.2 to 0.5% carbon. At these contents, the carbon makes
it possible to obtain good mechanical resistance characteristics.
In particular, the required resilience is ensured by the 0.2%
minimums. On the other hand, the content thereof should not be too
high (approximately 0.5% maximum), in order not to favor the
formation of bainite instead of the sought acicular ferrite.
[0052] From 0.5 to 2.0% manganese. Manganese ordinarily is used
here in order to increase the temperability of the steel with the
aforementioned carbon contents. However, the content thereof
preferably is less than 2.0% in order to avoid its segregation
which would impair the homogeneity of the structure.
[0053] From 0.05 to 0.5% vanadium. Vanadium favors the development
of acicular ferrite, as already stated, by making it possible to
increase the size of the bainitic areas and by shifting them to the
high temperatures. It also reduces the area of occurrence of
perlite ferrite.
[0054] From 0.02 to 0.10% sulfur. Sulfur not only improves the
machinability of the parts, but performs a function mainly sought
here in the mechanism of nucleation of the acicular ferrite. It has
been discovered, in fact, that it is the sulfurs, and not the
carbides as in the case of the document U.S. Pat. No. 6,669,789
mentioned above, which actually constitute essential anchoring
points on which the grains of acicular ferrite, the development of
which is going to be promoted by the vanadium, alloyed with
silicon, are going to form.
[0055] From 0.6 to 1.5% silicon. Silicon usually serves to
deoxidize the steel. Its content here, however, should remain below
1.5% in order not to weaken the steel. Here it performs an
essential function in the controlled growth of the bainitic area in
which the acicular ferrite is formed by precipitating the primary
ferrite at the grain joints, as already indicated, and thus
allowing the vanadium to promote the development of acicular
ferrite.
[0056] From 0.05 to 1.0% chromium. The chromium makes it possible
to adjust the temperability of the grade and thus to follow the
increase in size of the parts to be produced. It also acts with the
silicon in order to increase the range of presence of the acicular
ferrite.
[0057] From 0.01 to 0.5% molybdenum. Molybdenum contributes to the
obtaining of the final structure through an adjustment of the
temperability of the grade. In fact, if the content of tempering
elements is too low, a ferrito-perlitic structure will be obtained,
and conversely, an excessively tempering grade may lead to the
obtaining of martensite or residual austenite.
[0058] Optionally, but highly recommended in practice, from 0.01 to
0.02% titanium in order to protect the elements from nitrogen and
in particular to keep free vanadium in sufficient quantity, for
otherwise it might form precipitated nitrides too readily.
[0059] Likewise optionally, but ordinarily widely used in practice,
from 5 to 30 ppm of calcium in order to improve the castability of
the steel and its implementation. It facilitates the obtaining of
oxide inclusions which may enter into the mechanism of nucleation
of the acicular ferrite.
[0060] Possibly up to 50 ppm of boron that will act in synergy with
the molybdenum to broaden the bainitic area in which the acicular
ferrite is formed.
[0061] Possibly up to 0.2% aluminum for control of the austenitic
grain size, but it also will perform a function in the preservation
of vanadium.
[0062] This optimized composition makes it possible for the steel
to have, following a controlled cooling, a structure essentially
composed of acicular ferrite. By essentially there will be
understood an acicular ferrite content of more than 50% and
preferably more than 60%, and advantageously approximately 80% or
even more. Such a metallographic structure makes it possible for
the steel to have good mechanical characteristics of resistance,
hardness and ductility, but also an enhanced resistance to impacts
and to fatigue effect.
[0063] As is going to be seen, the acicular ferrite is obtained
before or after forming of the part, but in any case by means of a
controlled cooling of the steel.
[0064] In the first case, deformation is performed cold on a steel
already having a structure essentially composed of acicular
ferrite. There is provided a long semiproduct composed of a steel
with an analysis according to the invention, which is hot-rolled as
need be after a reheating above 1100.degree. C., in accordance with
customary hot-rolling practice, until obtaining of a rolled wire 10
mm in diameter, for example. The removal temperature of the wire is
on the order of 900 to 950.degree. C. The rolled wire obtained is
cooled with blown air in the rolling "heat" itself in the customary
manner ("Steelmor" process, for example). If its diameter so
permits, the wire also may be cooled naturally to the ambient
atmosphere.
[0065] The rolled wire is delivered in ring form to the transformer
which is going to cut it into pieces of required length and subject
them to a cold press for obtaining of the desired parts. The final
mechanical characteristics are obtained naturally by the
cold-drawing resulting from forming.
[0066] In the second case, plastic deformation is performed "hot"
and the metallographic structure is obtained directly on the rough
forge shapes. There is provided a long semiproduct composed of a
steel with an analysis according to the invention, which is
hot-rolled until giving it a diameter of 35 mm, for example. After
possible cooling, which does not need to be controlled at this
level, the bar is positioned lengthwise and delivered to the smith
customer.
[0067] The bars then are cut into pieces. Each piece is brought to
a temperature of at least 1100.degree. C. by means of an induction
furnace. This heating also can be performed more classically, but
the heating conditions (heating time, speed, etc. . . . ) then must
be optimized in order to obtain a homogeneous austenitic structure
having a grain size favorable to the formation of acicular ferrite.
The austenitic grain size then is estimated at 80 .mu.m. The pieces
are subjected to a hot plastic deformation operation. Forging is
concluded at a temperature in excess of 1100.degree. C. The rough
shapes of parts obtained in this manner then undergo a forced
cooling to room temperature at a cooling speed ranging from
approximately 0.5 to 15.degree. C./s, depending on the diameter of
the part and the optimization of the steel composition. The part
also may be cooled in a natural but controlled manner by placing
the rough shapes at the forge outlet one by one on a conveyor belt,
for example. The part then is machined to conform to the final
intended dimensions. Instead of machining, the part possibly may be
subjected to a second plastic deformation. This additional
operation may be carried out cold without running the risk of
cracking the part because of the ductile nature imparted to the
steel by the microstructure. It is not necessary to implement a
thermal hardening and tempering in order to obtain the intended
mechanical characteristics.
[0068] The grade of steel according to the invention makes it
possible to obtain a part with metallographic structure essentially
composed of acicular ferrite. It has the mechanical characteristics
of resistance to breaking and hardness required for its usage
properties, and meets the requirements for machinability. In
addition, it has an increased tenacity by virtue of its very
structure, in which the entanglement of the laths serves as a
barrier to the occurrence and the propagation of cracks. This
increased tenacity in fact enables it, as a result, also to have a
better resistance to impacts and a better resistance to fatigue.
Furthermore, is also makes possible a second cold forming by press,
for example. The obtaining of acicular ferrite also make it
possible to increase the mechanical resistance of the grade through
the ample dispersal density of its laths.
[0069] Tests were conducted in the laboratories of the producer of
semiproducts for a forge deriving from continuous casting. A wheel
hub was forged there from a steel according to the invention, the
chemical composition of which, besides iron and the impurities
resulting from processing, correspond to the following analysis:
TABLE-US-00001 % C % Mn % V % Si % Cr % Mo ppm B % S ppm Ca % Ti %
Al 0.31 1.33 0.12 1.18 0.28 0.03 20 0.04 11 0.015 0.02
[0070] Prior to forging, this piece was heated to 1200.degree. C.
by induction. The end temperature of forging is 1100.degree. C.
After forging, the rough shape is cooled at a speed of 2.degree.
C./s directly in the heat. No other thermal treatment is
applied.
[0071] The structure obtained on this test hub is 80% acicular
ferrite; it also has the following mechanical characteristics:
TABLE-US-00002 Rm (MPa) Rp.sub.0.2 (MPa) Hardness (HV) A (%) Z (%)
1150 800 300 11 25
[0072] It is recalled that: [0073] Rm represents the resistance to
breaking corresponding to the maximal force before breaking with
reference to the initial section of the wire. [0074] Rp.sub.0.2
represents the conventional limit of elasticity corresponding to
the force with reference to the initial section of the wire
producing a plastic elongation of 0.2%. [0075] A represents the
breaking elongation. [0076] Z represents the area contraction
corresponding to the reduction of the wire section after
breaking.
[0077] It goes without saying that the invention could not be
limited to the example that has just been described, a wheel hub,
but that it extends to numerous variations or equivalents, in type
of parts and in size and dimension, insofar as the definition
thereof given in the attached claims is observed.
* * * * *