U.S. patent application number 16/609872 was filed with the patent office on 2020-02-27 for martensitically hardenable steel and use thereof, in particular for producing a screw.
The applicant listed for this patent is HILTI AKTIENGESELLSCHAFT. Invention is credited to Michael BISCHOF, Roland SCHNEIDER, Alexander TOMANDL.
Application Number | 20200063231 16/609872 |
Document ID | / |
Family ID | 59298183 |
Filed Date | 2020-02-27 |
![](/patent/app/20200063231/US20200063231A1-20200227-D00001.png)
United States Patent
Application |
20200063231 |
Kind Code |
A1 |
SCHNEIDER; Roland ; et
al. |
February 27, 2020 |
MARTENSITICALLY HARDENABLE STEEL AND USE THEREOF, IN PARTICULAR FOR
PRODUCING A SCREW
Abstract
A steel comprising 0.07 to 0.14 wt. % of carbon, 13 to 15 wt. %
of chromium, 1.3 to 1.7 wt. % of molybdenum, 1.5 to 2.0 wt. % of
nickel and 1.0 to 1.5 wt. % of manganese and use of the steel for
producing screws is provided.
Inventors: |
SCHNEIDER; Roland; (Schlins,
AT) ; BISCHOF; Michael; (Bregenz, AT) ;
TOMANDL; Alexander; (Feldkirch, AT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
HILTI AKTIENGESELLSCHAFT |
Schaan |
|
LI |
|
|
Family ID: |
59298183 |
Appl. No.: |
16/609872 |
Filed: |
June 20, 2018 |
PCT Filed: |
June 20, 2018 |
PCT NO: |
PCT/EP2018/066405 |
371 Date: |
October 31, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 8/0273 20130101;
C21D 1/74 20130101; C21D 9/0093 20130101; C23C 8/32 20130101; C22C
38/44 20130101; C23C 8/26 20130101; C21D 8/0257 20130101; C22C
38/04 20130101; C21D 1/25 20130101; C21D 6/04 20130101; C21D 1/06
20130101 |
International
Class: |
C21D 9/00 20060101
C21D009/00; C23C 8/32 20060101 C23C008/32; C21D 6/04 20060101
C21D006/04; C21D 1/06 20060101 C21D001/06; C21D 8/02 20060101
C21D008/02; C22C 38/04 20060101 C22C038/04; C22C 38/44 20060101
C22C038/44 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 26, 2017 |
EP |
17177789.9 |
Claims
1. A method for producing a shaped body, the method comprising
producing a blank comprising a steel with 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.
2. The method according to claim 1, including case-hardening the
blank with nitrogen from the gas phase.
3. The method according to claim 2, including the case-hardening
the blank with nitrogen from the gas phase in combination with
carburizing the blank with carbon from the gas phase.
4. The method according to claim 2, including processing the blank,
wherein case-hardening the blank is performed subsequent to
processing the blank.
5. The method according to claim 1, including deep-freezing the
blank at temperatures below minus 80.degree. C. and subsequently
annealing the blank at temperatures between 150.degree. C. and
500.degree. C.
6. The method according to claim 1, wherein the shaped body is a
screw shape.
7. The method according to claim 6, including local induction
hardening of a tip portion of the screw shape.
8. A steel comprising 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.
9. A screw produced using the steel of claim 8.
10. A screw at least partially comprising a steel obtainable in the
method according to claim 1.
11. A screw according to claim 10, having a ratio of an outside
diameter (d) of a thread of the screw to a thread pitch (p) of the
thread is in the range of 1 to 2.
12. The method according to claim 2, comprising case-hardening the
blank with nitrogen from the gas phase at temperatures between
1,000.degree. C. and 1,150.degree. C.
13. The method according to claim 2, comprising case-hardening the
blank with nitrogen from the gas phase at a nitrogen partial
pressure between 0.05 and 0.3 bar.
14. The method according to claim 12, comprising case-hardening the
blank with nitrogen from the gas phase at a nitrogen partial
pressure between 0.05 and 0.3 bar.
15. The method according to claim 3, including processing the
blank, wherein case-hardening the blank is performed subsequent to
processing the blank.
16. The method according claim 2, including deep-freezing the blank
at temperatures below minus 80.degree. C. and subsequently
annealing the blank at temperatures between 150.degree. C. and
500.degree..
17. The method according claim 3, including deep-freezing the blank
at temperatures below minus 80.degree. C. and subsequently
annealing the blank at temperatures between 150.degree. C. and
500.degree..
18. The method according claim 4, including deep-freezing the blank
at temperatures below minus 80.degree. C. and subsequently
annealing the blank at temperatures between 150.degree. C. and
500.degree..
19. The method according to claim 2, wherein the shaped body is a
screw shape.
20. The screw according to claim 11, wherein the ratio of the
outside diameter of the thread to the thread pitch is in the range
of 1.2 to 1.45.
Description
[0001] 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.
[0002] Desirable for concrete screws and self-tapping screws used
in steel-metal applications are material 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 A1 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.
[0003] 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.
[0004] DE4033706 A1 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.
[0005] 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-hardening at a
temperature of 1,050.degree. C.-1,150.degree. 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.
[0006] DE1 0201 3 10801 8 A1 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.degree. 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.
[0007] WO1 4040995 A1 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.degree. C. in
a nitrogenous gas atmosphere.
[0008] TW201418549 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.
[0009] The summary retrievable under the link:
https://online.unileoben.ac.at/mu_online/wbAbs.showThesis?pThesisNr=61746-
&pOrgNr=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.
[0010] 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.
[0011] 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.
[0012] A method according to the invention is used to produce a
shaped body, preferably a screw shape, 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.
[0013] 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 can be sufficiently met at the
same time.
[0014] 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.
[0015] In soft-martensitic steels, the carbon content is lowered
(<0.1 wt % 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 uitable for use
in screws only to a limited extent 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.
[0016] 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".
[0017] 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 0.08-0.12% 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).
[0018] 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 HV0.3 or
higher, a maximum core hardness of 450 HV0.3 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:
[0019] Due to the carbon content of the steel, the core hardness is
450 HV0.3 or less.
[0020] Due to the carbon, chromium, molybdenum, nickel and
manganese content, the following will result:
[0021] 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.
[0022] The steel has good processability into semi-finished forms
such as rolled wire or drawn bare wire. Both rolled and bare wire
have good cold workability, preferably represented by a yield
strength R.sub.p 0.2<650 N/mm.sup.2, which may be particularly
advantageous for the production of screws in a cold-forming
process, preferably with a rolling process.
[0023] For case-hardening in the temperature range between
1,000.degree. C. and 1,150.degree. C., preferably between
1,030.degree. C. and 1,100.degree. 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%.
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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.degree.
C. and 1,150.degree. C., more preferably between 1,030.degree. C.
and 1,100.degree. 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.degree. 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 HV0.3 or higher with a limit hardness of 550 HV0.3
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 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.
[0028] 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).
[0029] 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 product
properties.
[0030] The step of case-hardening may expediently be followed by a
step of deep-freezing the blank, preferably at temperatures below
minus 80.degree. C., particularly preferably at a temperature of
minus 150.degree. C., and then annealing the blank, preferably at
temperatures between 150.degree. C. and 500.degree. C.,
particularly preferably at temperatures between 200.degree. C. and
250.degree. 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.
[0031] 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.
[0032] 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.
[0033] 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 HV0.3
without compromising toughness in failure-critical areas of the
screw, such as in the head, underhead and/or shaft area.
[0034] 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.
[0035] 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.
[0036] 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 1 to 2, in
particular in the range of 1.2 to 1.45. These are typical thread
dimensions 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.
[0037] 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.
[0038] 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.
[0039] 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, in
which:
[0040] FIG. 1 shows a schematic flow diagram of a production method
according to the invention; and
[0041] FIG. 2 shows a schematic representation of a screw according
to the invention, produced in a production method according to the
invention.
[0042] 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.
[0043] 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.
[0044] The blank formed as a screw shape is then hardened in step 3
at a temperature greater than 900.degree. C., especially between
1,000.degree. C. and 1,150.degree. C., more preferably between
1,030.degree. C. and 1,100.degree. 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.
[0045] Subsequently, the blank formed as a screw shape is quenched
in step 4, in particular gas-quenched.
[0046] In the subsequent step 5, a deep-freeze treatment of the
form of a screw blank follows at temperatures below minus
80.degree., for example at minus 150.degree. C.
[0047] Finally, the blank formed as a screw shape is annealed in
step 6, preferably in a temperature range between 150.degree. C.
and 500.degree. C., more preferably between 200.degree. C. and
250.degree. C., and/or for a holding time between 1 hour and 5
hours.
[0048] 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.
[0049] An exemplary embodiment of a screw according to the
invention, which is formed as a concrete screw, is shown in FIG.
2.
[0050] 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.
[0051] 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 a hole in an attachment 53 which is
secured to the substrate 50 by the rotary drive 15 formed as a
screw head.
* * * * *
References