(57) Disclosed is a steel suitable for manufacturing cold-forgeable objects with good
machinability, in particular objects having a geometrical form causing special difficulties
in cold forging. In addition to iron and incidental impurities, the steel comprises
the following contents in percent by weight:
C from 0.04 to 0.14, preferably from 0.04 to 0.12
Si from 0.01 to 0.35, preferably from 0.01 to 0.25
Mn from 0.2 to 1.0, preferably greater than 0.6
Cr from 1.0 to 2.0, preferably from 1.0 to 1.4
Mo from 0 to 0.25
S from 0.015 to 0.10
Al from 0 to 0.1
B from o to 0.015, preferably about 0.004
Ti from 0 to 0.05
Ca from 0 to 0.01, preferably from 0.0015 to 0.01
P from 0 to 0.035, preferably from 0 to 0.0025,
wherein the ratio Mn/S is greater than 10, and preferably greater than 20. The steel
is hot rolled and, when necessary, also annealed before cold forging.
[0001] 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.
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] A small boron content, about 0.004 percent by weight, gives the steel hardenability.
Boron does not weaken cold forgeability.
[0012] 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.
[0013] 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/mm
2 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/mm
2 and hardness at most about 135 HB before cold forging. These values are advantageous
for cold forgeability.
[0014] 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.
[0015] Through hardening gives high yield strength and high fatigue strength. Case hardening
and surface hardening are also advantageous for improving fatigue strength.
[0016] 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).
[0017] Surface hardening gives a better fatigue strength than through hardening, but a weaker
rolling contact fatigue strength when compared to case hardening.
[0018] 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
[0019] The hardening method used will depend on the intended use of the object.
[0020] 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.
[0021] 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.
[0022] 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.
[0023] 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.
[0024] 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.
[0025] 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/mm
2 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/mm
2.
[0026] 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 |
[0027] 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.
1. A steel suitable for manufacturing cold-forgeable objects with good machinability,
characterised in that, in addition to iron and incidental impurities, the steel comprises
the following contents in percent by weight:
C from 0.04 to 0.14
Si from 0.01 to 0.35
Mn from 0.2 to 1.0
Cr from 1.0 to 2.0
Mo from 0 to 0.25
S from 0.015 to 0.10
Al from 0 to 0.1
B from 0 to 0.015
Ti from 0 to 0.05
Ca from 0 to 0.01
P from 0 to 0.035,
wherein the ratio Mn/S is greater than 10.
2. A steel according to claim 1, wherein the carbon content is in the range 0.04 to 0.12
percent by weight, the silicon content is in the range 0.01 to 0.25 percent by weight,
the manganese content is greater than 0.6 percent by weight, the chromium content
is in the range 1.0 to 1.4 percent by weight, the boron content is approximately 0.004
percent by weight, the calcium content is in the range 0.0015 to 0.01, and the phosphorus
content is not greater than 0.025 percent by weight.
3. A steel according to claim 1 or claim 2, wherein the ratio Mn/S is greater than 20.
4. A steel according to any of claims 1 to 3, wherein the carbon content of the steel
is at most 0.09 percent by weight.
5. A steel according to any preceding claim, further characterised in that, in its rolled
condition, the steel has a tensile strength of at most about 550 N/mm2, and a hardness of at most about 160 HB, before cold forging.
6. A steel according to claim 5, wherein, in an annealed state, the tensile strength
is at most about 450 N/mm2, and the hardness is at most about 135 HB, before cold forging.
7. A billet for use in the manufacture of a cold forgeable object with good machinability,
characterised in that the billet is composed of a steel according to any preceding
claim.
8. An object with good machinability manufactured by using cold forging, characterised
in that the object is composed of a steel according to any of claims 1 to 6.
9. A method of manufacturing a cold-forgeable object with good machinability, characterised
in that the method comprises the steps of: forming an object from at least one billet
composed of a steel according to any of claims 1 to 6; and hot rolling the steel.
10. A method according to claim 9, further comprising the step of annealing the steel
prior to cold forging.
11. A method according to claim 9 or claim 10, further comprising the step of through
hardening the object, or the step of hardening the object, after cold forging, ether
with or without tempering.