Note: Descriptions are shown in the official language in which they were submitted.
CA 02916155 2015-12-18
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,
Wear-resistant, at least partly uncoated steel part
The invention relates to a wear-resistant, at least partly uncoated steel part
consisting
of a hardenable steel grade which has been produced from a semifinished part
by hot
forming and/or hardening. In addition, the invention relates to a process for
producing a wear-resistant, at least partly uncoated processing, conveying
and/or
crushing means of agricultural machines, conveying machines, mining machines
or
building machines from a semifinished part, in which the semifinished part is
heated
to a temperature above the Ac1 transformation temperature and is subsequently
hot
formed and/or hardened.
Wear-resistant, at least partly uncoated steel parts which have to have high
strengths
and at the same time are subjected to abrasive forces are required, for
example, for
the production of agricultural machines, in particular plows, and also for
buckets of a
dredge or conveying screws for abrasive materials, for example the conveying
screw
of a concrete mixer. In order to achieve the necessary high strengths in the
abovementioned applications, the parts are preferably subjected to hot forming
in
which the semifinished parts from which the steel parts are produced are
firstly
heated to a temperature above the Acl transformation temperature point, so
that
transformation hardening of the microstructure is effected by hot forming and
subsequent hardening, i.e. rapid cooling, and a material having a martensitic
microstructure is formed. The martensitic microstructure has a significantly
greater
hardness but also a significantly greater mechanical strength, for example
tensile
strength. Corresponding steel parts are known, for example, from the German
patent
DE 10 2010 050 499 B3. The German patent describes a process for producing
dredger buckets, concrete mixer conveying screws, conveying screw blades or
other
transport blades of conveying plants, in which the components are hot formed
and
press hardened.
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However, it has been found that the components produced in this way have
problems
in respect of the wear resistance despite the hardening process during
production,
especially on contact with abrasive materials.
The German first publication DE 10 2010 017 354 A1 is concerned with the
problem
of hot forming of zinc-plated flat steel products to produce high-strength or
very high-
strength steel components. When the melting point of the metal of the
protective
coating is exceeded, there is a risk of "liquid metal embrittlement" which is
caused by
penetration of the molten metal of the coating into the notches or cracks
arising in
forming of the flat steel product. The liquid metal which has penetrated into
the steel
substrate deposits at grain boundaries and there reduces the maximum tensile
or
compressive stress which can be withstood. As a solution, the patent
publication
offers nitriding of the outer layer regions, so as to produce finely
structured outer
layer regions.
The present invention is, in contrast, concerned with the problem that hot-
formed
and/or hardened steel parts do not have the desired wear resistance in the
uncoated
regions and are therefore not optimally suited for use as conveying means, for
example on contact with abrasive materials. It is therefore an object of the
present
invention to propose at least partly uncoated steel parts having improved
suitability
for use with abrasive materials. In addition an inexpensive production process
for
corresponding steel parts should be proposed.
The object indicated is achieved, for a steel part, the steel part at least
partially having
a surface region which has been hardened to a depth of not more than 100 p.m,
preferably to a depth of up to 40 pm, by surface hardening before hot forming
and/or
hardening.
It has been found that the heating of the semifinished parts for production of
the steel
parts to a temperature above the Ac1 transformation temperature or above the
Ac3
temperature before hot forming and/or hardening leads to decarburization of
regions
close to the surface, so that the carbon content of these regions is
significantly lower
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than the carbon content of the base material. As a result, the region close to
the
surface up to a depth of 100 In, in particular the region up to a depth of 40
p.m,
cannot be hardened to the required degree during hot forming and/or hardening.
However, it has been found that at least partial surface hardening of the
uncoated
regions of the semifinished parts before hot forming and/or hardening to give
the
steel part leads to both the surface region and the base material having very
high
hardness despite the decarburization of the regions close to the surface as a
result of
the high temperatures during hot forming or hardening. This provides a steel
part of
which at least partially has a surface region which has been hardened to a
depth of
preferably 100 pm or in the region down to a depth of 40 pm and is therefore
significantly more wear resistant than the at least partly uncoated steel
parts known
hitherto.
In a first embodiment, the hardened surface region of the steel part is
hardened by
carburization or nitriding. Both processes offer the opportunity of hardening
regions
close to the surface of the steel part in a targeted manner before hot forming
or
hardening. In addition, nitriding has the advantage that the hardness is not
reduced
during hot forming. In the case of carburization, the carbon content in the
surface
regions is increased but decreases again due to hot forming.
In a further embodiment, after hot forming and/or hardening the hardened
surface
region of the steel part preferably has at least the hardness of the base
material of the
steel part located under the surface region.
The wear resistance of the steel part can preferably also be improved by the
hardness
of the surface region of the steel part being greater than the hardness of the
base
material. It has been found that, in particular, the hardness of the surface
regions is
responsible for the wear resistance of the steel part on contact with highly
abrasive
materials, so that a very wear-resistant steel part can be produced even when
using a
somewhat softer base material.
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Consequently, the steel part is, according to a further embodiment of the
steel part,
configured for use as processing, conveying and/or crushing means in
agricultural
machines, conveying machines, mining machines or building machines, with at
least
the regions of the steel part which are subjected to abrasive forces being
surface-
hardened.
In addition, manganese-boron steels, dual-phase steels or transformation
induced
plasticity (TRIP) steels, in which particularly pronounced martensite
formation or
transformation of residual austenitic components into martensite makes an
increase
in the hardnesses possible, are also particularly advantageous.
In a further embodiment of the steel part, the surface region of the steel
part which
has been hardened before hot forming and/or hardening has, at least in
regions, a
hardness of from 400 to 700 HV. These values are generally achieved only by
very
high-strength steel grades after hot forming or hardening in the base
material. The
surface hardening before hot forming or hardening offers, in particular, the
opportunity of providing the starting material for production of the steel
components
on a coil.
According to further teaching of the present invention, the abovementioned
object is
achieved by a process for producing a wear-resistant, at least partly uncoated
steel
part for processing, conveying and/or crushing means of agricultural machines,
conveying machines, mining machines or building machines from a semifinished
part,
in which the semifinished part is heated, at least in regions, to a
temperature above
the Ac1 transformation temperature and is subsequently hot formed and/or
hardened, in that the semifinished part at least partially is subjected to
surface
hardening in which a surface region is hardened to a depth of not more than
100 p.m
before hot forming and/or hardening. Preference is given to hardening a
surface
region having a depth of up to 40 [im, in which decarburization processes
usually take
place during hot forming. The depth of the surface region which is to be
hardened is
controlled by the duration of the hardening treatment. It has been found, in
particular,
that despite heating to a temperature above the Ac1 transformation temperature
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point, the surface-hardened regions of the steel part remain stable in respect
of the
surface hardness, so that high surface hardness can be achieved after hot
forming
and/or hardening. This leads to the steel parts of processing, conveying
and/or
crushing means of agricultural machines, conveying machines, mining machines
or
5 building machines which are in contact with abrasive materials displaying
reduced
wear.
The hardening of the surface regions before hot forming or before hardening
makes it
possible to carry out the surface hardening on coilable materials, i.e. on
steel strip, so
that particularly economical production of wear-resistant, at least partly
uncoated
steel parts from semifinished parts is made possible. In a preferred
embodiment of the
process, hardening of the surface region is effected by nitriding or by
carburization.
Both processes make it possible to provide a higher hardness in the surface
region,
which after hot forming and/or after hardening make a higher wear resistance
of the
surface of the hot-formed or hardened steel part possible.
The surface hardening is, in a further embodiment, particularly preferably
carried out
by a heat treatment in a heat treatment atmosphere comprising up to 25% by
volume
of H2, 0.1-10% by volume of NH3, H20 and a balance N2 and also unavoidable
impurities at a holding temperature of from 600 C to 900 C. The dew point of
the heat
treatment atmosphere is preferably in the range from -50 C to -5 C, so that
the effect
of atmospheric moisture on the hardening process is reduced. In addition,
preference
is given to a maximum of 10% by volume of H2 and a maximum of 5% by volume of
NH3 being permitted and the dew point being set to a dew point temperature of
from
-40 C to -15 C at a temperature of from 680 to 840 C. The latter process
parameters
gave improved and more uniform surface hardening.
The depth of the surface hardening can be set via the time for which the
holding
temperature is maintained. The time for which the semifinished part has the
holding
temperature during surface hardening is preferably set to from 5 s to 600 s,
preferably
from 30 s to 120s.
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The surface hardening is preferably carried out in a continuous hardening
furnace, so
that, for example, a strip-like semifinished part, i.e. a coilable
semifinished part, is also
surface-hardened and can be fed to the further hot forming and/or press
hardening
steps. However, surface hardening in a chamber furnace is also conceivable.
As indicated above, semifinished parts such as manganese-boron steels, dual-
phase
steels and TRIP steels firstly display a particularly high strength increase
during hot
forming or during hardening and secondly provide the opportunity of bringing
the
regions close to the surface to identical hardness in the range from 400 to
700 HV by
nitriding. As a result, steel parts which are very wear-resistant and have
particularly
high strengths can be produced inexpensively.
In the following, the invention will be illustrated with the aid of examples
in
conjunction with the drawing. In the drawing,
Fig. 1 schematically shows an example of the process for producing a
wear-
resistant, at least partly uncoated steel part,
Fig. 2 shows the layer structure of the semifinished part or steel
part treated
as per the example in Fig. 1 in a schematic illustration,
Fig. 3, 4 shows examples of a steel part for agricultural machines and
conveying
machines and
Fig. 5 shows a graph of the hardness profile as a function of the distance
from
the surface for two examples and a comparative example.
Fig. 1 firstly shows, very schematically, an example of the production of a
wear-
resistant, at least partly uncoated steel part in a schematic illustration.
The
semifinished part 1, which consists of a steel, for example a manganese-boron
steel,
dual-phase steel or TRIP steel, is firstly fed to surface hardening 2. If a
strip-like
semifinished part is reeled off a coil la and fed to surface hardening 2, it
is, for
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example, advantageous to carry out surface hardening, for example in the case
of
nitriding, in a continuous hardening furnace at the end of which, for example,
the
strip-like semifinished part 1, now provided with a hardened surface, can be
wound
up on a coil (not shown). The surface-hardened strip-like semifinished part is
cut to
length and fed to hot forming and/or hardening 3, so that process step 3 can
produce
a formed, at least partly uncoated steel part 4 which is suitable for
processing,
conveying and/or crushing means of agricultural machines, conveying machines,
mining machines or building machines. Firstly, the steel part 4 produced in
this way
characterizes high strength values owing to the hot forming and/or hardening
step.
Secondly, the surface region of the steel part also has an increased hardening
due to
the nitriding of the surface which has taken place before hot forming and/or
before
hardening. As indicated above, the process of the invention enables the
decarburization of the surface regions, which takes place to a depth of 100
p.m, to be
countered by the surface region being surface-hardened to a depth of 100 pm or
in a
region down to a depth of 40 p.m. The surface hardening is preferably carried
out by
nitriding. However, carburization of the surface region is also conceivable.
The surface hardening in process step 2 is preferably carried out by means of
a heat
treatment in a heat treatment atmosphere comprising up to 25% by volume of H2,
0.1-10% by volume of NH3,1120 and balance N2 and also unavoidable impurities
at a
holding temperature of from 600 C to 900 C. Reduction of the hydrogen
concentration to a maximum of 10% by volume or limiting of the NH3
concentration to
a maximum of 5% by volume also leads to a further improvement of the nitriding
result.
The depth of the surface hardening can be set via the duration of the surface
hardening, for example at a holding temperature of from 5 s to 600 s. The
surface is
preferably nitrided at a holding temperature of from 30 s to 120 s, with the
temperature being from 680 C to 840 C. Carrying out the surface hardening
before
hot forming or hardening has the advantage that a heat treatment process can
be
carried out significantly more efficiently using a, for example, strip-like
semifinished
part in a continuous hardening furnace or a plate in a continuous hardening
furnace
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than when using formed steel parts which have different shapes and different
geometries. The quality of the surface hardening can likewise be ensured more
easily
by the use of strip-like semifinished parts or semifinished parts configured
as a blank.
Fig. 2 then schematically shows a cross section of the semifinished part at
three
different points in time during the process. At first, the semifinished part 1
has a more
or less homogeneous, for example ferritic microstructure la corresponding to
the
production process, which is determined by the combination of production
process
and steel composition. As a result of the surface hardening, the surface
region lb is
hardened by inward diffusion of nitrogen in the case of nitriding or carbon in
the case
of carburization, with the microstructure changing there. The thickness of the
surface
region lb depends on the duration of the heat treatment. The surface region is
usually
up to a maximum of 100 gm in which the hardness of the semifinished part is
altered.
A preferred region, which is a compromise between sufficient surface hardening
and
duration of the heat treatment for surface hardening, has a thickness of from
20 to 40
gm. The duration of surface hardening, for example in nitriding, is then
preferably
from 30 s to 120 s. The microstructure of the material la remaining underneath
the
surface region lb remains essentially unchanged during the heat treatment.
In the hot forming step, the microstructure of the base material la is then
firstly
converted into austenite and, by means of hardening, later partially into
martensite. In
this way, high hardness and good mechanical strengths are achieved in the base
material lc. The surface region lb remains unchanged except for carburization
of
these layers. As a result of nitriding, the surface region can continue to
remain
hardened. In the case of targeted carburization of the surface region lb
instead of
nitriding, decarburization can be countered, so that an increase in the
hardness is also
achievable here. The formed steel part 4 thus has a hardened region lb and
also a
region lc which has been hardened by the hot forming and hardening.
Fig. 3 and 4 show typical fields of application for the wear-resistant, at
least partially
uncoated steel part in the form of a conveying screw 5 in Fig. 3 and a
plowshare 6 for
agricultural plows in Fig. 4. Both components are typical representatives of
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processing, conveying and/or crushing means which are used in agricultural
machines, conveying machines, mining machines or building machines, for
example
concrete mixers, and are exposed to highly abrasive materials. The use of hot
formed
and/or press hardened steel parts has hitherto not been very advantageous
because
of the increased susceptibility to wear. Due to the surface hardening of the
region
which is decarburized during hot forming and/or hardening the hot forming
steels
gain an enlarged range of uses.
Table 1
Measurement of HV Sample A Sample B
0.01 (1% NH3) (4% NH3)
depth gm
5 460 546
404 490
436 447
333 415
409 394
479 453
453 479
436 485
492 466
Table 1 shows measurements of the hardness of samples A and B which consist of
a
steel of grade 22MnB5. The samples A and B were subjected to surface nitriding
in a
heat treatment atmosphere comprising 1% by volume of NH3 or 4% by volume of
NH3
at 760 C and 90 s in each case. The surface nitriding was carried out at inter-
critical
temperatures (T > Ac1) since austenite can dissolve more nitrogen than
ferrite. The
samples were subsequently hot formed and hardened. Polished sections were made
from the hot formed or hardened steel parts and the hardness HV 0.01 (DIN EN
ISO
6507-1) was measured at a distance of 5 um from the surface. The microhardness
measurement on the samples as a function of the content of NH3 in the heat
treatment
atmosphere had a greater hardness at a higher NH3 content of the heat
treatment
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atmosphere at the same heat treatment parameters, i.e. hold time and hold
temperature.
The hardness of sample A firstly decreases from the value of 460 HV measured
at the
5 surface to a value of 333 HV at a depth of 20 pm. The hardness then
increases again to
a value of about 492 HV, which indicates that the decarburization of the base
material
ceases here. The uppermost region, in particular, from 5 to 15 pm was
significantly
hardened by the surface hardening. It can be seen from sample B that the
surface
hardening is more pronounced, both in terms of the amplitude and the depth of
10 hardening, at an increased NH3 content. This can be attributed to
greater diffusion of
nitrogen into the surface of the steel part taking place due to the higher NH3
concentration in the heat treatment atmosphere. The values for sample B start
at 546
at a depth of 5 pm and decrease to a value of 394 at a depth of 25 p.m. The
values
subsequently increase again to about 466 at a depth of 45 gm. It can clearly
be seen
that the surface is harder than the base material at a depth of 45 m.
A similar picture is shown by the measurements on two further examples shown
in
Fig. 5 compared to a comparative example. The comparative example illustrated
by a
dotted line displays a reduced hardness below 400 HV 1 (DIN EN ISO 6507-1) in
the
region of 5 to 35 pm. The reduction in the hardness compared to the base
material,
which is in the range from 450 HV 1 to 500 HV 1, is explained by
decarburization
during hot forming. The two comparative examples with two different nitriding
variants, once again 1% strength NH3 heat treatment atmosphere or 4% strength
NH3
heat treatment atmosphere, differ especially in this region close to the
surface, since
hardness of above 500 could be measured here. In this way, it is possible, in
the case
of wear-resistant, at least partly uncoated steel parts, to provide not only
the
particularly high tensile strength values of the hot formed and/or hardened
steel
parts but also a high wear resistance due to greater surface hardness in the
range
from, for example, 500 to 700 HV.
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