Note: Descriptions are shown in the official language in which they were submitted.
93~;Z
The invention relates to weldable concrete reinforcing
steel having a carbon content of less than 0.25%. The
invention also relates to a process for the formation of
such a concrete reinforcing steel.
It is known to use steel with a chemical composition
of 0.35 to 0.45~ carbon, up to 1.3~ manganese, 0.2 to
0.36 silicon and the usual impurities, as very hard
concrete reinforcing steel. The formation of such steel
is very inexpensive, since carbon, manganese and silicon
are used as the principal hardening agents. ~owever, the
shaping ability of this steel is relatively low, and in
particular the weldability is low.
Concrete reinforcing steels are also known which have
a lower carbon content (max 0.28%) and a silicon content
of 0.5%, a manganese content of at most 1.6% and, in
addition the usual impurities, a copper content of at
least 0.2% (ASTM designation : A 440-74, page 336).
Such steel is however subjected to cold forming. A
particular disadvantage of this weldable reinforcing
steel is thereby produced, in that it registers a reduc-
tion of yield point or tensile strength after a short
age-hardening at temperatures between room temperature
and 800C. Such temperatures however are produced during
welding as well as during warm bending of the reinforcing
steel on the building site.
A reinforcing steel with desirable properties, which
does not have these disadvantages, is described in DE-OS
24 26 920. By a special cooling procedure, reinforcing
steel rods are prepared which consist of a more finely-
grained micro-structure. At the periphery of the rods,
the steel consists of strong tempered martensite-bainite,
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and of ferrite-bainite to ~errite-perlite including
b~ ite at the centre of the rods. Thereby the proportion
of perlite with bainite~ expressed as a percentage, should
be greater than the proportion of ferrite.
Such a steel exhibits the required welding proper-
ties as well as sufficiently high tensile strength and
sufficiently great yield point values. It has however
come to light that the elongation at rupture of the known
steel requires improvement. It tends to form cracks and
therefore tends not to have an optimal fatigue performance.
The basis of the present invention is therefore to
provide a steel having the desirable properties of known
steel which however is less susceptible to cracking and
which consequently has better values of breaking strain.
According to the invention there is provided weldable
reinforcing steel having a carbon content of less than
0.25~, a yield point~ o 2 of at least 500 N/mm and
a tensile strength of at least 550 N/mm , and which
consists of a conc~entric central zone and surface layer,
the central zone being formed from a pure perlite-ferrite
mixture, in which the ferrite portion is between 20~ and
80~, and the central zone having no intermediate layer
bordering the surface layer, the latter being formed from
pure martensite.
The steel requires no additional treatment, e.g.
cold-forming, patenting (heat treatment for wire) or
surface tempering and has a yield point ~ o 2 of at least
500N/mm and a tensile strength of at least 550 N/mm2.
The steel according to the invention is consequently
characterized by a pure concentric two layer construction
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in which both layers are completely free of bainite. The
steel according to the invention shows good welding pro-
perties and the mechanical properties required by DIN-Norm
4~8 as well as a substantially improved elongation at
rupture, whereby it is substantially unsusceptible to
crack formation and shows an improved fatigue performance.
In a preferred form the reinforcing steel in the
central zone has ferrite and perlite in approximately
equal proportions. It has been found that the reinforcing
steel according to the invention is very well suited for
ribbed reinforcing rods since both layers adapt themselves
to the rib form so that the ribs have the same mechanical
properties as unribbed rods.
Furthermore, if the proportion of the surface layer
advantageously forms 20%, preferably 33%, of the cross
sectional surface area of the rod.
The steel according to the invention has the addi-
tional advantage that it is quite inexpensive and can
be formed quickly on a wire rolling mill. The process
according to the invention for the formation of the
inventive reinforcing steel is characterized by the
following process steps:
a) The reinforcing steel is prepared on a wire rolling
mill
b) after the preparation step the rolled stock is
subjected to an intensive, preferably multi-stage cooling
c) by the cooling, the surface of the rolled stock is
cooled below the martensite initiating temperature
d) the cooling proceeds with such an intensity that
an equalization temperature between the centre and the
surface is achieved before a transformation to bainite,
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ferrite or perlite commences and that the equalization
temperature lies in the temperature region in which the
earliest possible transformation of the austenite into
ferrite and perlite can take place.
e) after the e~ualization temperature is achieved, the
temperature is held approximately constant until the end
of the perlite transformation and the rolled stock is then
subjected to a slow cooling.
In a preferred embodiment the rolled stock is coiled
i~nediately after t~e cooling step and cooled in the air
in its coiled condition. Thereby the isothermal trans-
formation strived for in the invention as well as the
annealing of the martensite in the peripheral zone is
achieved directly from the heat generated during rolling,
i.e. without an additional step.
The process according to the invention makes possible
a quick and dependable preparation of the inventive
reinforcing steel, without great expenditure. In a
surprisingly simple way, reinforcing steel is produced on
a wire rolling mlll and so handled that the properties so
long sought after can be achieved during the preparation
without great expense.
In a preferred embodiment of the process of the
invention, normal steel having a thickness of up to 13mm
is used for the preparation of the reinforcing steel.
In normal steel, the sum of all the alloying elements
(manganese, silicon, sulfur and the like) is below 1.7%.
This normal steel is especially inexpensive and is
suitable for the preparation of a steel according to the
invention of high quality using normal water cooling if
the thickness of the reinforcing steel is below 13mm.
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It is advantageous to use a normal steel with
a di.ameter of > 13mm which is micro-alloyed with a
proportion of micro-alloying elements up to 0.08%, in
order to be able to achieve the cooling without too much
expenditure.
Alternatively, for diameters between 13 and 25mm an
alloy steel can be used. In such a steel the sum of the
alloying elements is between l.7% and 3%. For diameters
of more than 25rnm the alloy steel must be mixed with
micro-alloying elements and indeed with a proportion of
micro-alloying elements up to 0.03~.
These directions regarding the use of alloys depend
upon the knowledge that the transformation of austenite to
ferrite, perlite or bainite is postponed to a later time
through the use of alloyed steel and/or micro-alloyed
steel.
It appears that the process of the invention is
economical if the first step of the cooling is terminated
within 0.2 seconds~
Further explanation of the reinforcing steel and of
the process according to the invention are given in the
following description. Reference is made therein to the
accompanying drawings, in which:
Figure l is a photograph of a cross-section of a known
reinforcing steel according to DE-OS 24 26 920;
Figures 2a to 2d are photomicrographs at 500x magni-
fication of the structure of the known reinforcing steel;
Figure 3, which appears on the same sheet of drawings
as Figure l, is a photograph of the cross-section of a
reinforcing steel according to the invention;
Figures 4a and 4b are photo-micrographs at 500x
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magnification of both structures of the reinforcing
steel according to the invention;
Figure ~ is a diagram to explain the controlled
coo:ling according to the process of the invention;
Figure 6 is a table to explain the cooling of steels
of various diameters and their cooling behaviour;
Figure 7 is a time-temperature-transformation-graph
of normal steel with a low carbon content; and
Figure 3 is a time-temperatuare-transformation-graph
of an alloy steel with a low carbon content.
Figures 1 and 2 are photographs of the reinforcing
steels of DE-OS 24 26 920. It can clearly be seen from
Figure 1 that the reinforcing steel has at least four
concentric layers within its cross section. The outermost
layer consists oE annealed martensite-bainite, the inner
side of which is bordered by a bainitic intermediate
layer. Then there follows a ring-like ferrite-bainite
layer, while the centre consists essentially of ferrite
and perlite.
These four stràta are shown in the micrographs of
Figures 2a to 2d at 500 times magniEication. The finely
striated outer layer of annealed martensite-bainite is
clearly differentiated from the bainitic intermediate
layer of Figure 2b. As can be seen from Figure 2c the
bordering ferrite-bainite layer has a larger structure.
The structure of the central zone is shown in Figure 2d
in which the dark flecks of perlite and the light flecks
of ferrite can be seen.
Figures 3 and 4 are corresponding micrographs of a
reinforcing steel of the invention. This steel is formed
of only two layers. The outer layer consists of pure
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annealed martensite and borders directly onto a central
layer which consists of a pure perlite-ferrite structure.
This is particularly clear from Figures 4a and 4b, in
which Figure 4a shows the surface layer of annealed
martensite and Figure 4b shows the sudden transformation
of the annealed martensite structure into the clearly
different ferrite-perlite structure. The micrographs
of Figure 4 also have a magnification of 500 times.
The strictly two layer structure of the steels of the
invention gives the pre~iously unexpected advantageous
properties that were explained above.
The process for the preparation of the reinforcing
steel of the invention is explained in further detail with
reference to Fiyures 5 to 8. Figure 5 shows a diagram in
which the cooling of a reinforcing steel is represented,
which initially has a temperature of about 850C in the
cooling operation and then undergoes a three step water
cooling. The steel is immediately coiled after the
termination of the cooling period and is further cooled
in air in the form of a coil. The coiled rolled goods
undergoes an isothermal transformation in the coil,
whereby the ferrite in the central zone is transformed
into ferrite and perlite and the martensite of the surface
layer is annealed by the energy of transformation thus
liberated. This is explained in further detail below.
Figure 5 shows in the left part the slow cooling of the
rolled goods during the running through of the finished
stage. The rolled goods are introduced into the cooling
operation at t:h~ int of time identified as to and they
remain approximately 0.15 seconds in the first cooling
step. The third cooling step is of approximately 0.35
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seconds duration.
The rolled goods are divided into concentric rings
in Figure 5 for the illustration of the temperature
development over the cross-section o the wire. The
outer surface is identified as 1 and the middle point
is identified as 4. The ring identified as 2 extends
about half a diameter and the ring identified as 3 has
a diameter that corresponds to a quarter of the diameter
of the wire diameter. The ring identified as la has a
radius of about 9/11 of the radius (R) of the rolled goods
and it approximately identifies the boundary between the
martensite layer and the central zone.
The temperature changes of the rings during the
cooling can be seen from the curves of Fig. 5 identified
by corresponding numerals 1, la, 2, 3 and 4. The outer
ring is cooled below the martensite producing temperature
Ms so that an outer layer of martensite forms between
rings 1 and la. Since the central region is naturally noL
so rapidly cooled, the martensite layer between rings 1
and la is further heated up by the heat present in the
central zone, whereby on the one hand the martensite is
annealed and on the other hand an equalization tempera-
ture TA is reached. The achievement of the equalization
temperature is equivalent to the fact that the rolled
goods have an equal temperature over the whole of the
cross section after the cooling. This temperature is now
maintained until the transformation of the austenite to
ferrite and perlite is complete. Then a continued cooling
may take place.
The equali~ation temperature TA is so chosen that
during the isothermal transformation the bainite region
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is not entered. Moreover, it desirably lies in a region
in which the earliest possible transformation of the
austenite into ferrite can take place. This assures that
the transformation of austenite into ferrite and perlite
takes place in the shortest possible time and that it does
not degenerate to a very lengthy process.
It is clear from Fig. 5, that according to the
in~ention the formation of bainite is prevented in that
the equalization temperature is achieved before a trans-
formation to ferrite can take place, and beyond that thetransformation ensues isothermally so that during the
cooling the bainite region is not entered.
The transformation curves chosen in Figure 5 cor-
respond to the usual time-temperature-transformation-
graphs wherein the ferrite formation region is represented
by F, the perlite formation region is represented by P,
the bainite formation region is indicated by B and the
martensite formation temperature is represented by Ms.
Austenite which is cooled below the martensite formation
temperature is transformed immediately to martensite.
The table of Figure 6 shows, in a worked example, the
possible arrangement oE the cooling for various steel
diameters from 5.5 to 30mm. In this example, a normal
alloy steel in which the sum of the alloying elements
does not exceed a proportion of 1.7%, is cooled from an
initial temperature of 850C.
From this it is clear tl~at the first cooling step
lasts no longer than 0.2 seconds. While a single cooling
step is sufficient for a diameter of 5.5mm, up to eight
cooling steps can be considered for greater diameters.
The total cooling is therefore complete at the latest
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after three seconds. In the next column, the time
required to reach the equalization temperature is given.
In this column the rein~orcing steels are divided into
three groups 1, II and III according to diameter. The
first group includes the diameters from 5.5 to 13mm, the
second group from 13 to 25mm and the third group from 25
to 30mm.
The equalization temperature for the first group is
reached within two seconds. The equalization temperature
for the second group is reached within 10 seconds, and in
the third group within 14 seconds. These associations
have an important meaning for the a2plicability of the
water cooling, which is explained in more detail below.
In the further columns of Figure 6, the temperatures
of the central zone at the end of each cooling step are
given Eor the various diameters. By "central zone", here
we mean the diameter r=o. Furthermore, the equalization
temperature achieved is given for each wire diameter.
The reason for~the above-mentioned division into three
diameter groups is clear from Figures 7 and 8. Figure 7
shows the time-temperature-transformation-diagram for
a low carbon (c _ 0.25%) normal steel. The earliest
possible transformation from austenite to ferrite is
possible in about two seconds at a temperature of about
500C. Corresponding to the teaching of the present
invention, the equalization temperature should be adjusted
to this point. From this it appears that normal steel up
to a diameter of 13mm can be treated by the water cooling
characterized in Figure 6. The equalization temperature
lies somewhat above 500C.
In comparison to this, Figure 8 shows the
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time-temperature-transformation-graph of a low carbon
stee] in which the sum of the alloying elements lies
between 1.7% and 3%. From this it is clear that the
earliest possible transformation of austenite to ferrite
is Eirst possible after an order of magnitude of 10
seconds. Further, it is to be noticed that the required
equalization temperature is substantially higher, since
the earliest possible transformation to ferrite takes
place at about 700C. By the addition of alloying
elements the time of the earliest possible transfor-
mation of austenite to ferrite can be protracted, so
that more time is available to reach the equalization
temperature.
A similar effect, namely the displacement of the
time point of the earliest possible transformation of
austenite to ferrite to a later time, can be achieved by
the addition o~ micro-alloying elements, e.g. niobium,
vanadium or molybdenum. In contrast to the use of alloy
steel (Figure 8), the transformation curve of Fig~re 7 is
entirely displaced by about a unit of ten to the right,
without otherwise chan~ing the position or form of the
transformation curve. Therefore the addition of micro-
alloying element~, in contrast to the addition of other
alloying elements, does not change the equalization
temperature.
; For the water cooling indicated in Figure 6 it is
necesarily, for the preparation of reinforcing steel with
diameters - 13mm, either to use an alloy steel (having
a sum total of alloying elements between 1.7% and 3%) or
a micro-alloyed steel (containing vanadium, niobium or
molybdenum up to 0.8%).
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At a diameter of > 25mm in an alloy steel the sum of
the alloying elements should be more than 3%. This is not
in general recommended, so that for these diameters addi-
tional micro-alloying or micro-alloying alone is provided.
Instead of the alteration of the alloy proportion
of the steel, a more intensive cooling can be effected,
so that the equalization temperature is reached more
quickly. Such a cooling is however very uneconomical.
From the graphs of Figure 7 and 8 it can be inferred
that the ratio of ferrite to perlite in the central zone
can be influenced by the choice of the equalization
temperature.
In the actual example described above, the cooling
of the rolled goods is commenced from an initial tempera-
ture 850C. Other temperatures are conceivable, but the
initial temperature must be high enough that the austenite
remains stable and low enough that the cooling of the
rolled goods within the required time is possible. This
means that, particularly for small diameters, a higher
initial temperature of the rolled goods can be tolerated.
All together the temperature of 850C has proved to be
particularly suitable for this purpose.
The isothermal transformation of austenite to ferrite
and perlite can be achieved by insertion into an oven
after the cooling operation. It is indeed very advan-
tageous to coil the uncut reinforcing steel coming from
the wire rolling mill immediately after its emergence from
the cooling operation. In the coil form the temperature
of the reinforcing steel does not decrease as quickly,
since the temperature is maintained by the liberation of
transformation heat and in coil form a reduced heat loss
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to the air takes place. Furthermore, this method makes
it possible to apply the cooling to the rapid Eormation
process, which is known in connection with wire rolling
mills but has not yet been used for the preparation of
concrete reinforcing steel.
Under similar conditions, the elongation at rupture of
a reinforcing steel prepared according to the teachings of
DE-OS 24 26 920 amounted to 5.2%, whereas the reinforcing
steel according to the invention amounted to 10.1%. This
gives an improvement with respect to crack formation
stability and creep resistance.
Under favourable conditions, the elongation at rupture
for the reinforcing steel according to the invention can
be increased even more. The average elongation at rupture
can then, for example, be between 13.9% and 17.4%, whereby
the value required by DIN 488/sheet l can be considerably
exceeded.
The invention has been described in detail above, but
should not be considered limited to such details. The
modifications that can be made by a person skilled in this
art after reading the above description fall within the
scope of the invention as defined by the following claims.
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