STEEL AND PROCESSING METHOD FOR THE MANUFACTURE OF HIGH STRENGTH, FRACTURE-SPLITTABLE MACHINERY COMPONENTS

Abrégé français

L'invention concerne un acier et un procédé de transformation pour produire des pièces mécaniques pouvant être séparées par rupture présentant une résistance accrue, ces pièces étant constituées d'au moins deux parties pouvant être séparées par rupture. L'invention se caractérise en ce que la composition chimique de l'acier est la suivante (en pourcentage en poids) : 0,40% <= C <= 0,60%; 0,20% <= Si <= 1,00%; 0,50% <= Mn <= 1,50%; 0% <= Cr <= 1,00%; 0% <= Ni <= 0,50%; 0% <= Mo <= 0,20%; 0% <= Nb <= 0,050%; 0% <= V <= 0,30%; 0% <= AI <= 0,05%; 0,005% <= N <= 0,020%, le reste étant constitué de fer et des impuretés et résidus issus de la production de l'acier.

Abrégé anglais

The invention relates to a steel and a processing method for higher-strength fracture-splittable machine components that are composed of at least two fracture-splittable parts. Said steel and method are characterized in that the chemical composition of the steel (expressed in percent by weight) is as follows: 0.40% <= C <= 0.60%; 0.20% <= Si <= 1.00%; 0.50% <= Mn <= 1.50%; 0% <= Cr <= 1.00%; 0% <= Ni <= 0.50%; 0% <= Mo <= 0.20%; 0% <= Nb <= 0.050%; 0% <= V <= 0.30%; 0% <= Al <= 0.05%; 0.005% <= N <= 0.020%, the rest being composed of iron and smelting-related impurities and residual matter.

Revendications

4
claims
1. A steel and the processing method for the manufacture of high strength,
fracture-splittable machinery components, characterised in that they
consist of at least two fracture-splittable parts, characterised in that their
chemical composition has the following components in percentages by
weight:
0.40% .ltoreq. C .ltoreq. 0.60%
0.20% .ltoreq. Si .ltoreq. 1.00%
0.50% .ltoreq. Mn .ltoreq. 1.50%
0% .ltoreq. Cr.ltoreq. 1.00%
0% .ltoreq. Ni .ltoreq. 0.50%
0% .ltoreq. Mo .ltoreq. 0.20%
0% .ltoreq. Nb .ltoreq. 0.050%
0% .ltoreq. V .ltoreq. 0.30%
0% .ltoreq. Al .ltoreq. 0.05%
0.005% .ltoreq. N .ltoreq. 0.020%
whereby the remainder consists of iron and impurities from the melting
process and residues.
2. A steel in accordance with claim 1, characterised in that its chemical
composition is the same except that:
0.10% .ltoreq. V .ltoreq. 0.20%
3. A steel in accordance with claim 1 or 2, characterised in that its chemical
composition is the same except that:
0.020% .ltoreq. Nb .ltoreq. 0.030%

4. A steel in accordance one of claims 1 to 3, characterised in that its
chemical composition is the same except that:
0.010% .ltoreq. N .ltoreq. 0.020%
5. A steel in accordance one of claims 1 to 4, characterised in that its
chemical composition is the same except that:
0.45% .ltoreq. C .ltoreq. 0.55%
0.50% .ltoreq. Si .ltoreq. 0.70%
0.90% .ltoreq. Mn .ltoreq. 1.10%
0.10% .ltoreq. Cr .ltoreq. 0.40%
0.10% .ltoreq. Ni .ltoreq. 0.30%
0.10% .ltoreq. V .ltoreq. 0.20%
0.010% .ltoreq. Al .ltoreq. 0.020%
0.020% .ltoreq. Nb .ltoreq. 0.030%
0.010% .ltoreq. N .ltoreq. 0.020%
6. A steel in accordance one of claims 1 to 4, characterised in that its
chemical composition is the same except that:
0.45% .ltoreq. C .ltoreq. 0.55%
0.50% .ltoreq. Si .ltoreq. 0.70%
0.90% .ltoreq. Mn .ltoreq. 1.10%
0.10% .ltoreq. Cr .ltoreq. 0.40%
0.10% .ltoreq. Ni .ltoreq. 0.30%
0.10% .ltoreq. V .ltoreq. 0.20%
0.010% .ltoreq. Al .ltoreq. 0.020%
0.020% .ltoreq. Nb .ltoreq. 0.030%
0.010% .ltoreq. N .ltoreq. 0.020%
0.020% .ltoreq. Ti .ltoreq. 0.030%

6
7. A steel in accordance one of claims 1 to 4, characterised in that its
chemical composition is the same except that:
0.45% .ltoreq. C .ltoreq. 0.55%
0.50% .ltoreq. Si .ltoreq. 0.70%
0.90% .ltoreq. Mn .ltoreq. 1.10%
0.30% .ltoreq. Cr .ltoreq. 0.40%
0% .ltoreq. Ni .ltoreq. 0.20%
0.10% .ltoreq. V .ltoreq. 0.20%
0.010% .ltoreq. Al .ltoreq. 0.020%
0.020% .ltoreq. Nb .ltoreq. 0.030%
0.015% .ltoreq. N .ltoreq. 0.020%
8. The use of a steel in accordance one of claims 1 to 7 to manufacture
fracture-splittable components used in the construction of vehicles,
whereby they have a predominantly pearlitic structure from the
precipitation of special carbides after forging and controlled cooling.
9. A component in accordance with claim 8, characterised in that the
apparent limit of elasticity is over 750 N/mm2 after cooling down from the
forming temperature.
10. A component in accordance with claim 9, characterised in that the tensile
strength falls between 950 N/mm2 and 1200 N/mm2 after cooling down
from the forming temperature.
11. A component in accordance with claim 10, characterised in that the
elongation to fracture exceeds 10% after cooling down from the forming
temperature.
12. A component in accordance with claim 11, characterised in that the
reduction in area at fracture exceeds 25% after cooling down from the
forming temperature.

7
13. A component in accordance with claim 12, characterised in that it is
fracture splittable.
14. A component in accordance with claim 13, characterised in that it is
suitable for induction hardening.
15. A component in accordance with claim 14, characterised in that the
mechanical properties can be adjusted both in the material prior to forging
as well as in the component through the use of thermomechanical
treatment.

Description

CA 02666677 2009-02-17
Georgsmarienhutte GmbH
Neue Huttenstr. 1
D-49124 Georgsmarienhutte
Steel and processinq method for the manufacture of high
strenqthi, fracture-splittable machirterY components.
The invention concerns a steel and the processing method for the manufacture
of high strength, fracture-splittable machinery components. This material was
developed, for example, for the manufacture of crack connecting rods.
The steel must be suitabie for forging or for other heat forming processes,
The
heat used in the forging is dissipated by controlled cooling to produce a
largely
pearlitic structure that has an apparent limit of elasticity in excess of 750
N/mm2,
a tensile strength between 1000 and 1200 N/mmz, elongation to fracture of over
10% and reduction in area at fracture of over 25%. Its fracture-splittability
is a
particularly important feature.
The desired properties can be obtained by intentionally creating a pearlitic
structure with the precipitation of special carbides (niobium and vanadium
carbide) and of manganese sulphides by an appropriately formulated chemical
composition, controlled temperature management during the heat forming when
producing the preliminary material as well as when forging the finished
components (thermomechanical treatment), and a suitable heat treatment after
completion of the forging or heat forming, as the case may be.
Up to now, examples of the steels used for this purpose are either mostly
eutectoid compositions with approximately 0.7% C, 0.5 to 0.9% Mn, 0.06 to
0,07% S and possibly 0.1 to 0.2% V (C70S6, 70MnVS4), or an average carbon
content of approximately 0.4%, about 1% Mn, 0,06 to 0.07% S and about 0.3%
V (36MnVS4) according to customers' technical specifications.

CA 02666677 2009-02-17
2
These steels have a predominantly pearlitic structure with vanadium carbides
and manganese sufphides and conform to requirements regarding mechanical
properties. The drawbacks in the various known material alternatives are that
the
versions used to this point need considerable resources in terms of expensive
and scarce alloying materials. Vanadium in particular is currently used
increasingly in the field of precipitation hardening ferritic-pearlitic steels
(AFP
steels), making vanadium an increasingly scarce commodity.
The aim of this invention is to avoid the cited disadvantages by proposing a
new
steel which has the required mechanical properties regarding strength (and
fatigue strength) and, additionally, good toughness characteristics in tensile
tests, combined at the same time with good splittability. The steel must also
be
easy to continuously cast and forge. Furthermore, the new steel must use fewer
resources when being produced than the known steels by partially substituting
the vanadium content with niobium using an appropriately adapted forming and
cooling strategy_
The high value for the apparent limit of elasticity is achieved, other than
with the
basic composition, by the precipitation of extremely finely dispersed carbides
of
special carbide formers niobium and vanadium, This requires a solution of the
present carbides before the final hot forming process as well as controlled
temperature management during the hot forming, followed by a final cooling off
stage. Finely dispersed precipitation can be achieved, in particular, by
having a
low final forming temperature, ending with slightly accelerated cooling. This
raises the apparent limit of elasticity in particular, thus improving the
elastic limit
ratio considerably.
The tensile strength value of the basic composition comprising 0.5% C, 0.6%
Si,
1.0% Mn, 0.23% Cr, 0.2% Ni and 0_14% V can be adjusted to the desired level
by slightly accelerated cooling after hot forming.
The toughness characteristic values are controlled, in particular, by
selective
alloying with 0.06 to 0.07% sulphur_ The carbon content and relatively high
nitrogen content also act positively in this regard.

CA 02666677 2009-02-17
3
A crystalline fracture or macroscopic deformation is essential for the
material to
split properly. This is achieved by designing the alloy with high contents of
carbon, nitrogen and sulphur, and comparatively low contents of chromium,
nickel and molybdenum.
According to the solution proposed, the invention concerns a steel for the
manufacture of fracture-spiittable components for the vehicle industry with
the
following chemical composition in percentages by weight:
0.4%<_C50.6%;0.2%5Si<_1.0%;0.5%<_Mn51.5%;0%SCr<_1.0%;
0%5NiS0.5%;0%5Mo50.2%;0%5Nb<0.05%;0%5V:5 0.3%;
0% S Al 5 0.05%; 0.005% 5 N 5 0.020%
whereby the remainder consists of iron and impurities from the melting
process.