Note: Descriptions are shown in the official language in which they were submitted.
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Martensitically hardenable steel and use thereof, in particular for producing
a screw
The invention relates to a method for producing a shaped body, in particular a
hardened shaped
body, from a blank, a steel, its use for producing a screw, and a screw.
Desirable for concrete screws and self-tapping screws used in steel-metal
applications are
thaterial concepts with high surface hardness (at least in some areas of the
screw), high core
toughness, high resistance to general corrosion, pitting corrosion, as well as
chloride or
hydrogen-induced embrittlement and good plastic formability. According to EP
2204244 Al and
the concrete screws which are offered under the name "Hilti HUS-HR", this is
achieved by
carbide cutting elements, which are welded in the tip region of the screw on
the screw thread,
with the screw otherwise made of austenitic stainless A4 steel, comparable to
1.4401. However,
such screws can be relatively expensive to manufacture.
In the field of stainless self-tapping screws for steel-metal applications, so-
called bimetallic
screws are known, which are characterized in that a bolt made of hardenable
carbon steel is
welded on the screw body, i.e. on the head and the shaft, which are made of
austenitic stainless
A2 or A4 steel (comparable to 1.4301 or 1.4401). This bolt is hardened after
welding and forming
by means of a local heat treatment. Even such screws can be relatively
expensive to
manufacture.
DE4033706 Al describes a heat treatment method for increasing the corrosion
resistance of a
hardened edge layer of near net-shaped components made of martensitic
stainless steels with
less than 0.4 wt% carbon, by diffusion of 0.2 to 0.8 wt% nitrogen into the
edge layer. However,
an application in screws is not taught by DE4033706A1 and, more particularly,
DE4033706A1 is
not concerned with tuning the chemical composition of the steel and the heat
treatment
parameters for use in screws.
DE19626833A1 describes a method for producing a highly corrosion-resistant
martensitic edge
layer over a ferritic-martensitic core in components made of stainless steel.
The chemical
composition of the steel is limited so that there is a ferritic-martensitic
structure and, after case-
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hardening at a temperature of 1,050 C - 1,150 C with nitrogen, the ferrite
content in the core is
between 40 and 90 vol%, and the core hardness is less than 300 HV30. Once
again, an
application of the method for screws is not taught.
DE1 0201 3 10801 8 A 1 describes a screw made of a stainless steel, for
example, 1.41 13,
wherein the steel is substantially free of nickel, wherein, due to a nitriding
heat treatment at
1,000-1,200 C, an edge layer has a dissolved nitrogen content which is
elevated compared to
the rest of the structure, and wherein the screw in the edge layer has a
martensitic structure and
otherwise a ferritic structure.
WO1 4040995 A 1 describes a method for producing a self-tapping concrete screw
in which a
blank of a martensitically hardenable steel, in particular with a carbon
content less than 0.07%, is
hardened at a temperature greater than 900 C in a nitrogenous gas atmosphere.
TVV201418549 A describes a screw with a steel comprising 0.26 to 0.40% carbon,
12 to 14%
. .
chromium, 0 to 0.6% nickel, and 0 to 1% manganese.
The summary retrievable under the link:
https://online.unileoben.ac.atimu_online/wbAbs.showThesis?pThesisNr=61746&pOrg
Nr=1
indicates that martensitic steels, edge-nitrided with the high temperature gas-
nitriding process
and comprising 14% chromium, may be particularly suitable for use as a
material for a fastening
element.
The object of the invention is to provide a method for producing a shaped
body, in particular a
screw shape, from a steel blank and a corresponding steel, with which at
particularly low
production costs and with a high product reliability, especially for screw
applications, a
particularly advantageous combination of surface hardness, core toughness and
corrosion
resistance, resistance to hydrogen- and chloride-induced embrittlement and
workability, in
particular formability, can be realized. Another object of the invention is to
provide a use of such
a steel and a screw comprising such a steel, with which the aforementioned
advantages can be
implemented.
The object is achieved according to the invention by a method having the
features of claim 1, a
steel having the features of claim 8, the use thereof for producing a screw
according to claim 9,
and a screw according to claim 10. Preferred embodiments are indicated in the
dependent claims.
A method according to the invention is used to produce a shaped body,
preferably a screw shape,
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and uses a blank comprising a steel with a weight fraction of 0.07 to 0.14 wt%
carbon, 13 to 15
wt% chromium, 1.3 to 1.7 wt% molybdenum, 1.5 to 2.0 wt% nickel and 1.0 to 1.5
wt%
manganese. In addition, the steel may have other admixtures customary in
steel, for example
vanadium (in particular <0.2 wt%), niobium (in particular <0.2 wt%), titanium
(in particular
<0.2 wt%) and/or silicon (in particular <0.5 wt%). The remainder is iron with
unavoidable
impurities, for example sulfur and/or phosphorus, in particular <0.02% by
weight in each case.
Statements to the effect of "wt%" can be understood in the usual way as
percentages by weight.
In particular, the steel may be referred to as a martensitically hardenable
stainless steel.
Preferably, the steel has a weight proportion of 0.08 to 0.12% carbon by
weight.
The invention is based on the recognition that stainless martensitic steels
can be promising
candidates to meet the in part conflicting requirements of steel that may
arise when it is used in
screws, especially in self-tapping screws. In order to meet the diverse
requirements of high
surface hardness and sufficient hardness penetration depth, good corrosion
resistance, good
toughness and high resistance to chloride- or hydrogen-induced embrittlement,
the chemical
composition of the steel (in particular with regard to the alloy constituents
carbon, chromium,
molybdenum, nickel and manganese) and the multi-stage heat treatment required
for the
adjustment of the property profile consisting of high-temperature gas-
nitriding, gas phase
quenching, low-temperature cooling, annealing and optional local induction
hardening must be
carefully coordinated. Previous concepts according to the prior art are often
based on a
combination of a chemical composition of the stainless martensitic steel and a
heat treatment
that is rather unfavorable for a screw, so that often not all properties
required for a screw ban be
sufficiently met at the same time.
Normally, the achievable hardness of martensitic stainless steels primarily
depends on the
carbon content, as in classic tempered steels. As the carbon content
increases, the hardness
increases, but with higher sensitivity to hydrogen embrittlement and less
toughness. For this
reason, steels comparable to 1.4108 are generally unsuitable for use in screws
under corrosive
conditions. Such nitrogen-alloyed steels comprising a carbon content of 0.25-
0.35 wt% have a
sufficiently high surface hardness and a good resistance to pitting corrosion,
but are regularly
very brittle and relatively sensitive to hydrogen embrittlement.
In soft-martensitic steels, the carbon content is lowered (<0.1wt% C) and
replaced by austenite-
stabilizing nickel. As a result, an extreme toughness, but at the same time a
slightly lower
hardness compared with martensites with higher carbon content, is achieved as
a rule. The steel
grade 1.4313 (X3CrNiMo13-4) is a representative of the soft-martensitic steel
group. In addition,
this type of stainless martensitic steels is often suitable for use in screws
only to a limited extent
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at best, because the steels often have only a relatively low surface hardness
and often have a
corrosion resistance that is sill too low for a screw application.
Stainless martensitic steels offer the option of setting the required
combination of high edge
hardness with simultaneously lower core hardness (equivalent to good toughness
and high
resistance to chloride- or hydrogen-induced embrittlement) and good corrosion
resistance by
case-hardening with nitrogen instead of carbon. Nitrogen, which is dissolved
in the component
during a case-hardening process of this kind, increases the surface hardness,
the corrosion
resistance, and the compressive residual stress of the edge layer. Due to the
nitrogen dissolved
in the surface layer of the component, this heat treatment method is also
referred to as "solution
nitriding".
The invention includes a steel, the chemical composition of which is based on
a combination of
the alloying constituents carbon (0.07-0.14% by
weight, preferably
008-0.12% by weight), chromium (13-15% by weight), molybdenum (1.3 - 1.7% by
weight),
nickel (1.5 2.0
% by weight) and manganese (1.0 to 1.5%, preferably 1.2% by weight).
In the context of the invention, it has been recognized that with such a
steel, in particular in
connection with a heat treatment matched to the steel (preferably a multi-
stage heat treatment) a
particularly advantageous property profile for screws can be obtained, but in
principle also for
other components. As explained in detail below, this may be due in particular
to austenitization
with structure stabilization by delta ferrite content during the heat
treatment. In particular, a
property profile could be realized, which is characterized by a good
formability of the blank, a
high surface hardness of 580 HVO.3 or higher, a maximum core hardness of 450
HVO.3 or less, a
high resistance to general corrosion and pitting corrosion in the core
(represented by a PRE
index of 17 or higher) and in the periphery (represented by a PRE index of 23
or higher), high
core toughness (particularly as a result of a combination of low carbon
content and stable delta
ferrite, whereby increasing coarse grain growth is suppressed in the heat
treatment) and a high
resistance to chloride- or hydrogen-induced embrittlement. Specifically, the
steel may be
advantageous in particular in the following respect:
1) Due to the carbon content of the steel, the core hardness is 450 HVO.3
or less.
2) Due to the carbon, chromium, molybdenum, nickel and manganese content,
the following
will result:
a) A relatively high PRE Index (Pitting Resistance Equivalent), preferably
17 or higher, can
be achieved, but without leaving the state range of a martensitic or
martensitic-ferritic structure.
b) The steel has good processability into semi-finished forms such as
rolled wire or drawn
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bare wire. Both rolled and bare wire have good cold workability, preferably
represented by a yield
strength Rp 0.2 <650 N/mm2, which may be particularly advantageous for the
production of
screws in a cold-forming process, preferably with a rolling process.
c) For case-hardening in the temperature range between 1,000 C and 1,150 C,
preferably
between 1,030 C and 1,100 C, a predominantly austenitic structure (preferably
between 70%
and 95%) with a small proportion of delta ferrite particles may occur in the
core region of the
blank with a ferrite delta in the amount of 5% - 30%, wherein this delta
ferrite content of 5% -
30% can have a stabilizing effect on the structure at said temperatures and
thereby counteract
grain coarsening, which can have an advantageous effect on the toughness
properties. The delta
ferrite content may be deliberately limited to a maximum of 30%, since at
higher delta ferrite
levels toughness could decrease again. The austenitic proportion of 70% -95%
has high carbon
solubility, effectively counteracting the formation of chromium carbides and
the associated
relatively high susceptibility to intergranular corrosion. Preferably, a delta
ferrite content between
10% and 15% can be provided, corresponding to an austenite content between 90%
and 85%.
d) When case-hardening in the aforementioned temperature range with
diffusion of nitrogen,
a mainly austenitic structure which has a high solubility of carbon and
nitrogen can occur in the
edge zone of the blank, so that the formation of chromium carbides or chromium
nitrides and the
associated relatively high susceptibility to intercrystalline corrosion is
efficiently counteracted.
e) After the heat treatment, a mainly martensitic structure with a small
proportion of delta
ferrite in the amount of 5% - 30% (preferably 10% - 15%) can be present in the
core region of the
blank. The delta ferrite content may be deliberately limited to a maximum of
30%, since at higher
delta ferrite levels toughness could decrease again.
After the heat treatment, a mainly martensitic structure can be present in the
edge zone
of the blank, so that a high degree of hardening can be achieved.
Advantageously, in the method according to the invention, the step of case-
hardening the blank
with nitrogen from the gas phase is provided, preferably at temperatures
between 1,000 C and
1,150 C, more preferably between 1,030 C and 1,100 C, and/or a nitrogen
partial pressure
between 0.05 bar and 0.3 bar, particularly preferably between 0.10 bar and
0.20 bar, preferably
following the step of processing the blank. Through such case-hardening with
nitrogen (alone or,
as explained below, optionally in combination with carbon), the boundary zone
of the blank can
be selectively modified in a particularly advantageous manner for screw
application. In particular,
during case-hardening between 1,000 and 1,150 C, nitrogen can be dissolved in
the boundary
zone of the then austenitic basic structure. In connection with the steel
according to the invention,
such a case-hardening made it possible to achieve a surface hardness of 580
HVO.3 or higher
with a limit hardness of 550 HVO.3 at a distance from the surface of 0.15-0.30
mm (which can be
particularly advantageous for concrete screws) or 0.1 - 0.15 mm (which can be
particularly
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advantageous for self-tapping screws). This, in turn, can provide good
resistance to thread wear,
even when furrowing in concrete and rebars, which in turn allows for a high
bearing capacity of
the screw. In addition, the dissolved nitrogen can increase the PRE index in
the boundary zone
locally to 23 or higher and thereby significantly improve the resistance to
pitting corrosion,
preferably to a level comparable to a 1.4401 steel. In particular, the
electrochemical parameter of
the "breakthrough potential" can be brought to a level comparable to a 1.4401
steel. The reason
for the upper limit of the nitrogen partial pressure of 0.3 bar or 0.20 bar is
that the formation of
chromium-containing and/or nitrogen-containing precipitates can thereby be
efficiently
counteracted, which is advantageous in terms of corrosion resistance. The
reason for the lower
limit of the nitrogen partial pressure of 0.05 bar or 0.10 bar is that a
significant effect of the
nitrogen occurs only after reaching this pressure. The atmosphere provided in
the step of case-
hardening may be pure nitrogen or a gas mixture which has equivalent nitrogen
activity-at the
given temperatures. In particular, a pure nitrogen atmosphere may be provided,
provided that the
process is carried out in a low-pressure furnace. In an atmospheric pressure
furnace, dilution
could be carried out with noble gases, for example.
Expediently, it may be provided that the case-hardening of the blank with
nitrogen from the gas
phase occurs in combination with a carburizing of the blank with carbon from
the gas phase.
Thus, in addition to an increase in the nitrogen content, it is additionally
possible to provide an
increase in the carbon content as a result of carbon being diffused from the
gas phase. This
embodiment is based on the recognition that, with simultaneous availability of
carbon and
nitrogen, the solubility of both elements can be increased simultaneously,
wherein, with a higher
nitrogen content and simultaneous avoidance of carbides and nitrides, a
further advantageous
increase in hardness and corrosion resistance can be achieved. In order to
perform case-
hardening of the blank with nitrogen from the gas phase in combination with
carburizing the blank
with carbon from the gas phase, gaseous nitrogenous and carbonaceous media,
for example,
may be introduced into the process chamber separately and alternately.
Alternatively, a gas
mixture that provides both carbon and nitrogen can be used (for example,
ethyne, C2H2, along
with N2).
In particular, the method may comprise the step of "processing the blank". In
this step, the blank
can be made into the shape of the shaped body. The processing may include, for
example,
forming a thread on the blank. In particular, the processing of the blank may
include a non-cutting
forming, in particular a cold-forming, preferably a rolling, of the blank. It
is particularly preferred
that the step of case-hardening of the blank takes place subsequent to the
step of processing the
blank. Accordingly, the blank is processed before hardening. The timing of the
case-hardening
subsequent to the processing can simplify the processing and ensure
particularly homogeneous
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product properties.
The step of case-hardening may expediently be followed by a step of deep-
freezing the blank,
preferably at temperatures below minus 80 C, particularly preferably at a
temperature of minus
150 C, and then annealing the blank, preferably at temperatures between 150 C
and 500 C,
particularly preferably at temperatures between 200 C and 250 C, and/or for
hold times between
1 hour and 5 hours. As a result, an even higher hardness and/or toughness can
be set without
reducing the corrosion resistance.
The blank is expediently in the form of wire at the beginning of the process,
i.e. it is a wire-
shaped semi-finished product, which can further reduce the effort.
As already mentioned several times, the invention is particularly suitable for
the production of
screws. It is therefore particularly preferred that the shaped body is a screw
shape, preferably
with a screw shaft and a thread arranged on the screw shaft. The screw shape
can form part of a
finished screw or preferably the entire screw at the end of the method, which
is to say that
preferably a monolithic screw is provided.
In particular, a local induction hardening of a tip region of the screw shape
and, preferably,
thereafter a deep-freezing of the screw shape can be provided. Inductive
hardening at the screw
tip can provide a localized increase in hardness to 580-700 HVO.3 without
compromising
toughness in failure-critical areas of the screw, such as in the head,
underhead and/or shaft area.
The invention also relates to the aforementioned steel as such, namely a steel
with 0.07 to
0.14% carbon by weight, preferably 0.08 to 0.12% carbon by weight, 13 to 15%
chromium by
weight, 1.3 to 1.7% molybdenum by weight, 1.5 to 2.0% nickel by weight and 1.0
to 1.5%,
preferably 1.2% manganese by weight.
The invention also relates to the use of a steel according to the invention
for producing a screw
and/or a screw which at least partially comprises a steel according to the
invention and/or which
is obtainable, and in particular is obtained, in a method according to the
invention. The screw
may preferably be a self-tapping screw. It can for example be a concrete
screw, that is a screw
for cutting into concrete, or a self-tapping screw for metal sheets.
Preferably, the screw is a
monolithic screw.
The ratio of the outer diameter of a thread of the screw to the thread pitch
of the thread may be in
the range of Ito 2, in particular in the range of 1.2 to 1.45. These are
typical thread dimensions
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for screws intended for self-tapping screwing into mineral substrates such as
concrete. Thread
pitch is understood in particular to mean the axial distance of successive
turns of a thread.
According to the invention, a concrete substrate may be provided with a bore
into which a screw
according to the invention is screwed, wherein in the concrete substrate, a
negative form of the
cutting thread of the screw is formed. Accordingly, the screw is screwed into
the bore in the
concrete substrate in a self-tapping manner to form a counter-thread.
Preferably, the steel according to the invention contains 0.08 to 0.12 wt%
carbon, whereby even
more advantageous material properties can be achieved, especially with regard
to screw
applications.
Features are used which are explained in connection with the method according
to the invention,
the steel according to the invention, the use according to the invention, or
the screw according to
the invention, are not intended to be limited to this category but can also be
applied to the other
category, that is to say method, steel, use or screw.
The invention is explained in more detail below with reference to preferred
exemplary
embodiments, which are shown schematically in the accompanying figures,
wherein individual
features of the exemplary embodiments shown below can be basically realized
individually or in
any desired combination in the context of the invention. The figures show
schematic illustrations,
ifl which:
Fig. 1 shows a schematic flow diagram of a production method according to
the invention;
and
Fig. 2 shows a schematic representation of a screw according to the
invention, produced in
a production method according to the invention.
Fig. 1 schematically shows the sequence of steps of a possible embodiment of a
production
method according to the invention. First, in step 1, a blank, preferably wire-
shaped, made of a
steel containing 0.07 to 0.14% carbon by weight, preferably 0.08 to 0.12 wt%
carbon, 13 to 15
wt% chromium, 1.3 to 1.7 wt% molybdenum, 1.5 to 2.0 wt% nickel, and 1.0 to 1.5
wt%
manganese. In addition, the steel may have other admixtures customary in
steel, for example
vanadium (in particular <0.2% by weight), niobium (in particular <0.2% by
weight), titanium (in
particular <0.2% by weight) and/or silicon (in particular <0.5% by weight).
The remainder is iron
with unavoidable impurities, for example sulfur and/or phosphorus, in
particular <0.02% by
weight in each case.
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Then, the blank is processed in step 2, for example, formed, preferably
rolled, and the blank
thereby made into the shape of a shaped body, in particular in the form of a
screw shape with a
screw shaft 20 and a thread 21 arranged on the screw shaft 20. Optionally, the
screw shape may
also have a rotary drive 15, for example a screw head, arranged on the screw
shaft 20. In this
case, step 2 of the processing may include, in addition to rolling,
compression of the blank.
The blank formed as a screw shape is then hardened in step 3 at a temperature
greater than
900 C, especially between 1,000 C and 1,150 C, more preferably between 1,030 C
and 1,100 C,
in a 'nitrogenous gas atmosphere, wherein the nitrogen partial pressure of the
gas atmosphere is
preferably between 0.05 bar and 0.6 bar, more preferably less than 0.3 bar and
particularly
preferably less than 0.20 bar. Optionally, the gas atmosphere may also contain
carbon.
Subsequently, the blank formed as a screw shape is quenched in step 4, in
particular gas-
quenched.
In the subsequent step 5, a deep-freeze treatment of the form of a screw blank
follows at
temperatures below minus 80 , for example at minus 150 C.
Finally, the blank formed as a screw shape is annealed in step 6, preferably
in a temperature
range between 150 C and 500 C, more preferably between 200 C and 250 C,
'and/or for a
holding time between 1 hour and 5 hours.
Optionally, in a subsequent step 7, a local, preferably inductive, hardening
can be provided at a
tip region of the blank designed as a screw shape, and preferably a subsequent
deep-freezing of
the blank formed as a screw shape can be provided.
An exemplary embodiment of a screw according to the invention, which is formed
as a concrete
screw, is shown in Figure 2.
The screw 10 has a cylindrical screw shaft 20, at the end of which a hexagonal
screw head is
provided which forms a rotary drive 15. Along the screw shaft 20, a thread 21
formed as a cutting
thread extends with an outer diameter d and a pitch p. Optionally, a smaller-
diameter support
thread 28 may be provided on the screw shaft 20.
The screw shaft 20 of the screw is screwed into a bore in a mineral substrate
50, in particular in a
concrete substrate, wherein the thread 21 formed as a cutting thread has cut
open a
corresponding thread in the substrate 50 during screwing. The screw shaft 20
is guided through
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a hole in an attachment 53 which is secured to the substrate 50 by the rotary
drive 15 formed as
a screw head.
