Note: Descriptions are shown in the official language in which they were submitted.
CA 02686071 2010-03-18
HOT-FOR1VIING STEEL ALLOY
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0002] The present invention relates to a hot-forming steel alloy with high
toughness
and at the same time a great hardening depth and/or improved martensitic
through-
hardening capability with a thermal quenching and tempering of products such
as, for
example, die-casting dies or extrusion dies and the like.
2. Discussion of Background Information
[0003] A thermal quenching and tempering of a part, e.g., of hot-forming
steel, to
adjust a high material hardness at operating temperatures of the part up to
about 550 C
and more, essentially means heating the material to a temperature at which it
has a cubic
face-centered atomic structure or an austenitic structure, followed by a
forced cooling to
obtain a martensitic structure and a subsequent tempering treatment,
optionally multiple
times, at temperatures of generally more than about 500 C. During the
tempering, on the
one hand the stresses in the material formed during the cooling and structural
transformation are reduced at least in part, and on the other hand the
material hardness is
increased or a so-called secondary increase in hardness is achieved due to
carbide
precipitates.
[0004] A transformation of an austenitic structure into a martensitic
structure, as one
skilled in the art is aware, calls for a minimum cooling rate of the material,
because this
transformation takes place as a diffusionless flip-over process of the atomic
structure due
to a markedly high subcooling. Lower cooling rates lead to the formation of a
bainite or
pearlite structure.
[0005] The properties of a material depend on the chemical composition thereof
and on
the microstructure thereof adjusted by a thermal treatment and produce
therefrom a
specific property profile of a part.
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[0006] In other words: the chemical composition of a material and the
intensity of the
cooling or the heat dissipation from the surface during the hardening of the
part
determine the microstructure in the region of the surface and, due to the
rewarming from
the interior of the part, the microstructural development depending on the
distance from
the part surface. The respective local fine structure determines the material
properties of
the thermally quenched and tempered material locally present.
[0007] For reasons of increasingly economic production of the products, hot-
forming
materials for die cast molds and the like are subject to increasing stresses
through
shortened press sequence times and increased casting pressures. Furthermore,
complex
geometries of the mold cavities are provided to an increasing extent, so that
much higher
total stresses of the material are present overall. These total stresses can
cause tool failure
due to stress cracks, fire cracks, coarse fracture, corrosion and erosion, so
that materials
with a high hardness and strength as well as high toughness and ductility at
the same time
are required. However, these required properties depend on the chemical
composition of
the alloy and the tempered properties of the same resulting therefrom.
[0008] Cr-Mo-V steels have long been used for hot-forming tools, wherein
the steel
types X38 CrMoV 51 and X38 CrMoV 53 according to DIN steel iron list material
no.
1.2343 and material no. 1.2367, as also given in the list, are "highly
resistant to
tempering" and suitable for "tools with large dimensions."
[0009] Material no. 1.2343 is used for "highly stressed tools, dies and
presses."
[0010] The above materials have a high hardening depth and a deep-reaching
tempering
quality to required hardness values between 50 and 55 HRC. However, their
toughness
properties are low, which can be a disadvantage for the wearing qualities of
die casting
molds.
[0011] With a material no. 1.2343, a considerable increase in the material
toughness
after a quenching and tempering treatment can be achieved by a reduction of
the provided
silicon content from 0.90 to 1.20% by weight to a concentration of about 0.2%
by weight,
but high cooling rates during hardening are necessary for this, which often
cannot be
achieved.
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[0012] It would be advantageous to have available a hot-forming steel alloy of
the type
mentioned at the outset which forms a largely complete martensitic
microstructure during
a forced cooling from the austenite range even at low cooling rates, after
which a high
hardness and improved toughness of the material are achieved through a
targeted
tempering treatment.
SUMMARY OF THE INVENTION
[0013] The present invention provides hot-forming steel alloy. The alloy
comprises, in
the following elements in % by weight, based on the total weight of the alloy:
Carbon (C) from about 0.35 to about 0.42
Silicon (Si) from about 0.15 to about 0.29
=
Manganese (Mn) from about 0.40 to about 0.70
Chromium (Cr) from about 4.70 to about 5.45
Molybdenum (Mo) from about 1.50 to about 1.95
Vanadium (V) from about 0.40 to about 0.75
Nitrogen (N) from about 0.011 to about 0.016,
remainder iron and impurity elements.
[0014] In one aspect of the alloy of the present invention, the maximum
concentrations
of one or more impurity elements present therein may be as follows:
Phosphorus (P) not more than about 0.005
Sulfur (S) not more than about 0.003
Nickel (Ni) not more than about 0.10
Tungsten (W) not more than about 0.10
Copper (Cu) not more than about 0.10
Cobalt (Co) not more than about 0.10
Titanium (Ti) not more than about 0.008
Niobium (Nb) not more than about 0.03
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Oxygen (0) not more than about 0.003
Boron (B) not more than about 0.001
Arsenic (As) not more than about 0.01
Tin (Sn) not more than about 0.0025
Antimony (Sb) not more than about 0.01
Zinc (Zn) not more than about 0.001
Calcium (Ca) not more than about 0.0002
Magnesium (Mg) not more than about 0.0002.
[0015] In yet
another aspect of the alloy of the present invention, the alloy may
comprise from about 0.37 % to about 0.40 % by weight of C and/or
from about 0.16 % to about 0.28 % by weight, e.g., from about 0.18 % to about
0.25 %
by weight of Si and/or
from about 0.45 % to about 0.60 % by weight, e.g., from about 0.50 % to about
0.58 %
by weight of Mn and/or from about 4.80 % to about 5.20 % by weight, e.g., from
about
4.90 % to about 5.10 % by weight of Cr and/or
from about 1.55 % to about 1.90 % by weight, e.g., from about 1.65 % to about
1.80 %
by weight of Mo and/or
from about 0.45 % to about 0.70 % by weight, e.g., from about 0.52 % to about
0.60 %
by weight of V and/or
from about 0.012 % to about 0.015 % by weight of N.
[0016] In a
further aspect, the alloy of the present invention may comprise, in % by
weight:
Carbon (C) from about 0.37 to about 0.40
Silicon (Si) from about 0.16 to about 0.28
Manganese (Mn) from about 0.45 to about 0.60
Chromium (Cr) from about 4.80 to about 5.20
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Molybdenum (Mo) from about 1.55 to about 1.90
Vanadium (V) from about 0.45 to about 0.70
Nitrogen (N) from about 0.012 to about 0.015.
[0017] In a still further aspect, the alloy may comprise, in % by weight:
Carbon (C) from about 0.37 to about 0.40
Silicon (Si) from about 0.18 to about 0.25
Manganese (Mn) from about 0.50 to about 0.58
Chromium (Cr) from about 4.90 to about 5.10
Molybdenum (Mo) from about 1.65 to about 1.80
Vanadium (V) from about 0.52 to about 0.60
Nitrogen (N) from about 0.012 to about 0.015.
[0018] The present invention also provides a part which comprises the alloy
of the
present invention as set forth above (including the various aspects thereof).
The part may,
for example, be a die-casting die or an extruder, or a part of a die-casting
die or an
extruder.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] The present invention is further described in the detailed
description which
follows, in reference to the drawings by way of non-limiting examples of
exemplary
embodiments of the present invention, and wherein:
Fig. 1 is a graph showing impact strength values of tested materials after a
thermal
quenching and tempering as a function of the cooling parameters in the
hardening
treatment.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
[0020] The particulars shown herein are by way of example and for purposes
of
illustrative discussion of the embodiments of the present invention only and
are presented
in the cause of providing what is believed to be the most useful and readily
understood
description of the principles and conceptual aspects of the present invention.
In this
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regard, no attempt is made to show structural details of the present invention
in more
detail than is necessary for the fundamental understanding of the present
invention, the
description taken with the drawings making apparent to those skilled in the
art how the
several forms of the present invention may be embodied in practice.
100211 As set forth above, the present invention provides a hot-working
steel alloy
which comprises the alloying elements in the following concentrations in % by
weight,
based on the total weight of the alloy:
Carbon (C) from about 0.35 to about 0.42
Silicon (Si) from about 0.15 to about 0.29
Manganese (Mn) from about 0.40 to about 0.70
Chromium (Cr) from about 4.70 to about 5.45
Molybdenum (Mo) from about 1.50 to about 1.95
Vanadium (V) from about 0.40 to about 0.75
Nitrogen (N) from about 0.011 to about 0.016,
remainder iron (Fe) and impurity elements.
100221 On of the advantages of the alloy according to the present invention
may be seen
essentially in that the elements overall, in particular the elements silicon,
molybdenum,
vanadium and nitrogen, are coordinated with one another in terms of
transformation
kinetics so that a desired strength and hardness with a high toughness of the
material can
be achieved with a thermal quenching and tempering with a reduced cooling rate
during
hardening.
100231 It is thus possible either to achieve greater penetration depths of a
martensitic
hardness microstructure that is favorable for the mechanical properties of the
part with a
given cooling rate or advantageously to use a lower cooling rate during a
hardening and
to thus minimize the hardening strains in a die-casting die, which often is
provided with
an engraving or with a negative mold of the cast part. This is particularly
important
because a so-called vacuum hardening of molded parts is being used to an
increasing
extent, wherein a heating in vacuum also takes place for reasons of avoiding
oxidations
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and decarburization of the processed surface of the workpiece or the mold
during an
austenitization, after which a forced cooling is carried out with a nitrogen
gas flow. For
this type of hardening of a part, an alloy with the chemical composition
according to the
present invention has proven to be particularly useful.
[0024] A further significant increase in the toughness properties of the
tempered and
quenched material can be achieved when the hot-working steel alloy has the
follwoing
concentrations of one or all of the following impurity elements in % by
weight, based on
the total weight of the alloy:
Phosphorus (P) not higher than about 0.005
Sulfur (S) not higher than about 0.003
Nickel (Ni) not higher than about 0.10
Tungsten (W) not higher than about 0.10
Copper (Cu) not higher than about 0.10
Cobalt (Co) not higher than about 0.10
Titanium (Ti) not higher than about 0.008
Niobium (Nb) not higher than about 0.03
Oxygen (0) not higher than about 0.003
Boron (B) not higher than about 0.001
Arsenic (As) not higher than about 0.01
Tin (Sn) not higher than about 0.0025
Antimony (Sb) not higher than about 0.01
Zinc (Zn) not higher than about 0.001
Calcium (Ca) not higher than about 0.0002
Magnesium (Mg) not higher than about 0.0002.
100251 The above elements can form either precipitates or compounds which
are
enriched in particular at the grain boundaries and result in a leap-like
reduction of the
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toughness properties of the material once a concentration limit is reached or
they cause
grain boundary coatings, which likewise have an unfavorable effect.
[0026] Through a chemical composition of the material according to the
invention
adjusted within relatively narrow limits according to a preferred embodiment
of the same,
the hot-forming steel alloy may contain one or more of the alloying elements
in the
following concentrations in % by weight, based on the total weight of the
alloy:
Carbon (C) from about 0.37 to about 0.40
Silicon (Si) from about 0.16 to about 0.28, preferably from about 0.18 to
about
0.25
Manganese (Mn) from about 0.45 to about 0.60, preferably from about 0.50 to
about
0.58
Chromium (Cr) from about 4.80 to about 5.20, preferably from about 4.90 to
about
5.10
Molybdenum (Mo) from about 1.55 to about 1.90, preferably from about 1.65 to
about
1.80
Vanadium (V) from about 0.45 to about 0.70, preferably from about 0.52 to
about
0.60
Nitrogen (N) from about 0.012 to about 0.015,
remainder iron (Fe) and impurity elements.
[0027] By means of this alloy according to the invention, which makes
particular
demands on a smelting technology, it is possible to achieve high toughness
values of the
material even with low cooling rates in the thermal quenching and tempering
process
with high material hardnesses.
[0028] The invention is described in more detail below based on test results.
[0029] The test results are illustrated in Fig. 1.
[0030] Alloys with a chemical composition according to the invention and
according to
DIN material no. 1.2343 with standard-conforming and with reduced Si contents
and
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according to DIN material no. 1.2367, as given in Table I, were examined after
a thermal
quenching and tempering treatment to a material hardness of 44 HRC with
different
cooling parameters X during hardening. The value that characterizes the
parameter X is
calculated as follows:
Cooling parameter [X] corresponds to the time [in sec.] for a cooling from 800
C to
500 C divided by 100.
[0031] The alloying elements of the materials listed in Table I are
indicated below,
wherein the remainder represents the content of iron and accompanying elements
and
impurity elements.
TABLE 1
Alloy composition in % by weight
Material No. C Si Mn P S N Cr Mo Ni V
1.2343 0.39 1.11 0.41 0.021 0.023 - 5.28 1.26
0.21 0.38
1.2343 So 0.38 0.21 0.39 0.022 0.019 - 5.34 1.30
0.16 0.40
1.2367 0.38 0.40 0.47 0.029 0.021 - 5.00 2.98
0.20 0.61
W 350 0.39 0.19 0.51
0.004 0.001 0.013 4.91 1.69 0.06 0.53
[0032] Fig. 1 shows that with a cooling parameter up to approx. X = 12 the
material no.
1.2343 with Si contents reduced to approx. 0.20% by weight in a thermally
quenched and
tempered state to a material hardness of 44 HRC has the highest toughness
measured
according to Charpy V. However, with increasing cooling parameter X the
toughness
values subsequently drop sharply to a low level.
100331 The materials no. 1.2343 with standard-conforming Si contents and no.
1.2367
have a lower toughness with a quenched and tempered hardness of 44 HRC, but
have a
remarkable through-hardening capability, which is documented by only slightly
reduced
toughness values as a function of the cooling parameter.
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100341 Although at high cooling rates or in the range of the cooling parameter
A up to
13, a test alloy W 350 according to the invention shows slightly lower
toughness values
at room temperature in the state quenched and tempered to 44 HRC compared to
the So
material no. 1.2343 (Si 0.2% by
weight), the toughness of the material remains
essentially unchanged at superior high values even with reduced cooling rates
or higher
cooling parameters.
[0035] It is
noted that the foregoing examples have been provided merely for the
purpose of explanation and are in no way to be construed as limiting of the
present
invention. While the present invention has been described with reference to an
exemplary embodiment, it is understood that the words which have been used
herein are
words of description and illustration, rather than words of limitation.
Changes may be
made, within the purview of the appended claims, as presently stated and as
amended.
Although the present invention has been described herein with reference to
particular
means, materials and embodiments, the present invention is not intended to be
limited to
these particulars; rather, the present invention extends to all functionally
equivalent structures,
methods and uses, such as are within the scope of the appended claims.
