This invention relates to steels suitable for
manufacturing cold forgeable objects with good
machinability, in particular objects having geometrical
forms which would normally cause special difficulties in
cold forging.
Steels having good cold forgeability are known, for
example those manufactured according to the cold forging
steel standard ISO 4954:1993 such as CC11X or CC11A and
CC21K or CC21A. However, steels of this type cannot be
subjected to heat treatment, and the hardening achievable by
cold working of such steels is limited, which places limits
on the kind of objects which may be manufactured from them.
Cold working affects the lattice structure of the material.
It gives rise to lattice defects, e.g. dislocations, and
this raises the strength of the material and reduces its
ductility. The strength of a cold worked object may vary in
different parts of the object, since it is dependent on the
degree of cold working. Wear resistance cannot be improved
by mere cold working, and fatigue strength of an object
hardened by cold working usually remains weak.
It is known to manufacture gears in part by cold
forging. However, toothing of the gears is usually
achieved by machining, which breaks the flow pattern of the
material and weakens the mechanical properties of the gear.
Gears and other cold forged objects cannot usually be
manufactured solely by cold forging, as it is almost always
necessary to carry out machining after cold forging.
Machining of cold forged objects is problematic because of
the microstructural changes and internal stresses caused by
cold working. These problems manifest themselves, for
example, as long chips and built-up edges, which impair the
quality of the machined surface of the object. Though it is
technically possible to carry out toothing by cold forging,
the types of steels presently used for gears are not
suitable for cold forging, mainly due to their strength and
hardness.
The invention seeks to provide an improved cold
forgeable steel from which high quality cold forged objects
may be manufactured economically and simply. The invention
thus provides a steel according to claim 1, and also extends
to a billet according to claim 7, an object according to
claim 8, and a method according to claim 9.
The essence of the invention is to combine good cold
forgeability, great final strength, good ductility,
toughness and good machinability, which is achieved by a
correctly balanced composition of the steel. Cold
forgeability is usually weakened by components which raise
the strength of the steel. The composition of the steel is
optimized by keeping the carbon, manganese, silicon and
phosphorus content low and by alloying the steel with
chromium, sulphur and possibly also boron. Carbon, silicon
and manganese, in particular, raise the strength of the
steel, influencing the microstructure of the steel after
annealing. Manganese also raises the strength of the steel
by influencing the microstructure of the steel after hot
rolling. The final strength of the object can be improved
by inclusion of chromium and boron, without weakening its
cold forgeability. A steel with sulphur may be hot short,
if the manganese content is too low. Hot shortage gives
rise to internal defects in the cast billet in continuous
casting, which in the cold forging stage may cause cracking
of the worked object. The characterising proportion of
manganese and sulphur according to the invention is
important, because manganese advantageously influences
castability, reducing the risk of internal cracking, and
sulphur improves the machining properties of the object by
lessening the amount of long chips. Therefore, the steel
should preferably contain at least 0.015 percent by weight
of sulphur.
The cold forgeable steel billet is preferably
manufactured of a material whose carbon content is at most
0.12, preferably at most 0.09 percent by weight. A greater
carbon content may cause an excessive rise in the strength
in the annealed steel, which weakens cold forgeability. On
the other hand a smaller carbon content lowers the strength
of hardened steel, which for many applications is not
desirable.
The manufacturing of steel becomes more difficult when
the silicon content is lowered, but on the other hand
silicon weakens the cold forgeability of the steel.
Therefore it is recommended that the silicon content is at
most 0.25 percent by weight.
The preferable value for the manganese content is
greater than 0.6 percent by weight. For cold forging the
lower the manganese content the better, but a manganese
content which is too low makes the manufacturing of the
steel difficult, for example by giving rise to internal
defects, such as heat cracks, in the casting phase. A
manganese content that is too high increases the segregation
tendency of manganese and other elements, e.g. carbon and
sulphur, during the solidification of the steel.
Cold forgeability decreases with increasing phosphorus
content, but on the other hand the manufacturing of the
steel becomes more difficult with a low phosphorus content.
The maximum amounts according to the invention are intended
to minimise the disadvantages caused by phosphorus.
Chromium is only slightly or not at all disadvantageous
to cold forgeability. Its purpose is to give the steel a
sufficient hardenability. The hardenability may also be
improved by manganese, but manganese is much more
disadvantageous for cold forgeability. In steels according
to the invention the chromium content of 1.0 to 2.0 percent
by weight has an almost insignificant effect on the cold
forgeability. Usually, a sufficiently good hardenability is
achieved when the chromium content is at most 1.4 percent by
weight.
A small boron content, about 0.004 percent by weight,
gives the steel hardenability. Boron does not weaken cold
forgeability.
An addition of calcium improves machinability by
modifying the composition and morphology of the non-metallic
enclosures (sulphides and oxides) in the steel, but this
improvement only occurs if the steel does not contain
titanium. However, titanium is often added to protect boron
against nitrogen, in which case calcium does not have an
improving effect on machinability. Conventional steel
manufacturing usually provides some calcium in the steel,
generally about up to 0.0015 percent by weight.
One advantage of steel composed according to the
invention is that a billet of the steel in a hot rolled
condition may have a tensile strength of at most about 550
N/mm2 and a hardness of at most about 160 HB, before cold
forging. If the steel is in an annealed condition, the
corresponding values are: tensile strength at most about 450
N/mm2 and hardness at most about 135 HB before cold forging.
These values are advantageous for cold forgeability.
Steels according to the invention are especially
suitable for heat treatment in order to raise the final
strength of the manufactured object. The manufactured
object may be through hardened, or case hardened by
carburizing hardening or nitrocarburizing hardening after
cold forging. Surface hardening, for example by induction,
flame, laser or electron beam welding methods may also be
carried out.
Through hardening gives high yield strength and high
fatigue strength. Case hardening and surface hardening are
also advantageous for improving fatigue strength.
Case hardening improves both bending fatigue strength
(which, in the case of a gearwheel, has an effect on, for
example, the dynamic strength of the root of the gearwheel),
and rolling contact fatigue strength (which has an effect on
the dynamic strength of the tooth side of the gearwheel).
Surface hardening gives a better fatigue strength than
through hardening, but a weaker rolling contact fatigue
strength when compared to case hardening.
Case hardening of steels according to the invention by
carburizing or nitrocarburizing gives good wear resistance,
because these hardening methods give a high surface
hardness, of at least 700 HV. With surface hardening, the
HV.
surface hardness usually remains at a level of around 500
The hardening method used will depend on the intended
use of the object.
Quenching in relation to hardening is usually done in
water, oil or polymeric emulsion, gas (e.g. nitrogen or
argon), or in a fluidized bed. Due to their low carbon
content, steels according to the invention may be quenched
in water without risk of cracking, which is advantageous
since water is cheap, environmental friendly, readily
available and does not contaminate the manufactured object.
However, water may cause dimensional changes, which may be
avoided by carrying out quenching by using one of the other
quenching means.
Conventionally, tempering is used for making the steel
more ductile or tough. As a result of the low carbon
content of steels according to the invention, tempering is
usually not necessary, which lowers the manufacturing costs,
increases throughput and reduces the risk of faults during
manufacturing.
Furthermore, steels according to the invention do not
always have to be annealed, which is usually necessary for
cold forging. Due to the good cold forgeability of the
steels of the invention, several cold forging stages can be
carried out without interstage annealings, which
substantially reduces the manufacturing costs. Moreover,
the high ductility of the steels of the invention means that
even relatively slim workpieces such as steering racks are
well able to withstand the cold straightening caused by
distortion due to hardening.
Hot rolled steel according to the invention, for
example in the form of bars, threads or pipes, may readily
be cold forged to produce substantially rotationally
symmetric objects, such as axles, gearwheels, valve lifters,
and the like. After hot rolling, and before cold forging,
the steel billet is cooled. Cooling may be carried out by
known techniques in a cooling bed or by means of retarded
cooling, for example by preventing convection or using a
cooling tunnel. By slowing down the transfer of heat from
the steel billet, the steel is given a lower hardness and
better cold forgeability, without substantial additional
cost. The hardness of a steel that has only been hot rolled
may in some cases be too high for achieving desired cold
forgeability.
For objects in which forging is especially difficult,
for example those having a small radius of curvature or a
complex form, annealing may advantageously be carried out
prior to cold forging. Such objects include, for example,
gears, where the toothing is also made by cold forging.
Three main methods may be used in annealing, namely
supercritical annealing (spheriodizing), isothermal
perlitizing or subcritical (isothermal) annealing.
Supercritical or subcritical annealing is usually used for
maximizing cold forgeability. Isothermal perlitizing is a
short annealing which usually does not weaken machinability
in the same way as other annealing methods.
The methods described above are applied to the billet
and/or manufactured object with the aim of achieving a
sufficiently high final strength. By using water quenching,
for instance, a yield strength of about 800 N/mm2 and an
impact strength, measured by a V-notched bar (KV + 20°C),
of about 50 J may be reached when the carbon content of the
steel is more than 0.05 percent by weight and the thickness
of the object at most 80 mm. When the carbon content rises
above 0.1 percent by weight, the yield strength of the
object may be about 900 N/mm2.
Set out below, by way of example, are the compositions
in wt% (in addition to iron and incidental impurities) of
three steels manufactured according to the invention,
together with various measured mechanical properties of the
steels, mainly relating to their ductility. Steels 1 and 3
were tested in a hot rolled condition, whereas steel 2 was
tested in both a hot rolled condition and with subsequent
annealing.
Steel | 1 | 2 | 3 |
C | 0.06 | 0.08 | 0.12 |
Si | 0.33 | 0.30 | 0.09 |
Mn | 1.00 | 0.92 | 0.87 |
Cr | 1.23 | 1.17 | 1.32 |
Mo | 0.11 | 0.10 | 0.03 |
S | 0.082 | 0.030 | 0.017 |
Al | 0.02 | 0.04 | 0.03 |
B | - | 0.004 | 0.004 |
Ti | - | 0.02 | 0.02 |
Ca | 0.001 | 0.001 | 0.001 |
P | 0.018 | 0.011 | 0.016 |
Steel | 1 | 2 | 2 | 3 |
| Hot rolled | Hot rolled | Annealed | Hot rolled |
Yield stress (Re) N/mm2 | 220 | 270 | 250 | 300 |
Tensile Strength (Rm) N/mm2 | 420 | 480 | 410 | 490 |
Elongation (A5) % | 39 | 30 | 34 | 34 |
Reduction of area (Z) % | 75 | 62 | 76 | 76 |
Hardness (HB) | 130 | 140 | 125 | 150 |
The invention is not limited to the embodiments
disclosed, but several variations thereof are feasible,
including variations which have features equivalent to, but
not necessarily within the literal meaning of, features in
any of the attached claims.