EP0738361B1 - Cold-rolled reinforcing steel and process for its manufacture - Google Patents

Abstract

In a reinforcing steel that has indentations (4) rolled into a steel rod of approximately circular cross section, the indentations are formed as the reinforcing steel is unwound through circular arcs of different radii set axially symmetrical relative to the indentation. The indentations are rolled into the surface of the rod at a constant depth and have steep flanks. This ensures sufficient bonding despite a lower deformation effort, providing for better ductility parameters. Furthermore, the coiled reinforcing steel has better directivity and improved suitability for welding in the manufacture of welded wire mesh. A process for manufacturing such a reinforcing steel comprises at least two deformation steps.

Description

Reinforcing steel (prestressing steel) for the reinforcement of steel (prestressed) concrete has ribs in order to achieve a sufficient bond between the steel and the concrete. This ensures the power transmission between the steel and the concrete and vice versa so that short anchoring or transmission lengths are achieved in the state of use.

In the course of developments, a number of conditions have arisen for the production of reinforcing bars, in particular cold-formed reinforcing bars, such as those e.g. used for reinforcing steel mesh or reinforcing steel in the ring. These include e.g. new insights into non-linear design methods in reinforced concrete construction. The combination of reinforcing steel has so far only been considered in the elastic range of the stress-strain line of the steel. The use of non-linear design also includes the composite effect in the plastic area of steel (DE-A1-4011486). It has been shown here that a bond that is too "hard" on ribs on the concrete crack releases too little steel expansion. The aim is to achieve a bond that allows the previous structural regulations in use, but is softer in the plastic area of the stress-strain line of the reinforcing steel.

A large part of the reinforcing steel is produced by hot or cold working as reinforcing steel in the ring and processed into steel bars or reinforcing steel mesh. In order to shape the steel into a form that can serve as reinforcement, it must be straightened using suitable machines. Ribbed steels are always out of round. Furthermore, the ribs are usually heavily removed during the straightening process. The ribs on the circumference of the bar also ignite considerable noise during the straightening process. In the case of reinforcing steels that are manufactured in the ring, the aim is to improve the straightening properties and reduce the noise during straightening.

A considerable part of the mass is in the ribs of the reinforcing steel. To produce the ribs, up to 25% deformation is necessary, which alone means a large amount of energy and therefore consume a lot of energy. With a view to saving energy, there is therefore a need to reduce the energy to be generated to produce the ribs.

The high degrees of deformation just mentioned significantly reduce the initial values of the ductility parameters Agt (elongation at maximum load) and Rm / Re (tensile strength / yield strength) during cold forming. The desirable nominal values (see ENV 10080) are therefore difficult to achieve.

Reinforcing steel mesh is welded in systems that run at up to 120 cycles / min. Achieving consistently good welded joints is only possible if the out-of-roundness caused by the ribs is kept as small as possible. This applies especially to the welding of double bars. In view of this, rods with a surface that come as close as possible to smooth round steel should be aimed for.

This results in the following objectives for the surface design of reinforcing steel.

In the preparation of:
Low deformation effort, material-technically more favorable application of cold forming and thus lower energy consumption and roller wear when generating the surface shape.

For further processing:
Good straightening ability with low noise emissions, improved weldability. Avoiding surface damage to the reinforcing steel and reducing wear on the straightening tool.

For use as reinforcement:
Sufficient bond in use condition and enabling activatable steel expansion at the concrete crack (plastic joint), high values of Agt and Rm / Re. Low notch effect and therefore high fatigue strength.

Studies have shown that a reinforcing steel according to the preamble of claim 1 is best suited with regard to these objectives.

A reinforcing steel of this type is known from DE-AS 10 84 464.

This document describes a reinforcement wire or rod, in particular for prestressed concrete, the surface of which has regularly repeated recesses on both sides, the recesses being elliptical and taking up almost half the circumference of the rod. The depressions are separated in the area of their short axis by narrow ridges that are oblique to the rod axis. Similar to bevelled ribs, these beads are formed by milling in the embossing roll, into which the material is displaced during rolling.

GB-A-1 334 757 (= FR-A-2 127 426) describes a reinforcing steel of the type specified in the preamble of claim 1, in which the depressions are elliptical, the longitudinal axes of the ellipses being inclined at an angle of 30 to Can be arranged 70 ° relative to the rod axis. In order to improve the bending properties of the bars, the flanks of the depressions are formed symmetrically and the flank angle is limited to 50 °.

FR-A-1 207 928 discloses a reinforcing steel in which a surface shape with a smooth surface is aimed at in order to improve the weldability when manufacturing reinforcing steel mats. Three or more rows of shallow depressions of low constant depth are rolled into a rectangular or parallelogram-like shape in a reinforcing steel of round cross-section.

From FR-A-1 202 576 a method for producing a ribbed reinforcing steel has become known, in which a round wire rod is converted in a cold drawing process with a cross-section reduction of approximately 20% into a round steel of reduced diameter. The round steel is then rolled into a cross section in the form of a triangle with rounded corners, and indentations are rolled into the rounded corners to form the ribs. The cold deformation for producing the ribs is of the order of 20%.

The invention has for its object to form the surface shape of a reinforcing steel according to the preamble of claim 1 so that despite a lower deformation effort, the better ductility parameters Agt and Rm / Re, a sufficient bond is guaranteed. If plastic deformations of the reinforcing steel are included (non-linear design), a soft bond should large relative shifts between steel and concrete allows. At the same time, the surface shape of the reinforcing steel should, if possible, be adjusted to that of a round rod with a smooth surface with a view to good straightening ability and improved weldability. With a view to a high fatigue strength, notch stresses should be kept as low as possible. Furthermore, a method for producing a reinforcing steel with the properties mentioned is to be specified.

The reinforcing steel according to the invention is characterized by the features of claim 1. Advantageous embodiments of this reinforcing steel can be found in claims 2 to 12. The method according to the invention is characterized by the features of claim 13. Advantageous refinements of the method can be found in the remaining claims.

While the known reinforcing steel according to DE-AS 10 84 464, which is particularly intended for prestressed concrete, strives for a uniformly high tensile strength and a high hat strength - this is due to the strong cold deformation of a cold-drawn smooth round wire and the formation of beads by displacing it outwards Material reached - in the reinforcing steel according to the invention, the deformation is small and thus the ductility is kept large by rolling in only shallow depressions. With the exception of the rolled-in depressions, the smooth surface of the round rod is retained, i.e. Bead formation is avoided with a view to improving the leveling ability and to enable a "soft" bond when using the reinforcing steel.

A graduated "failure mode" is achieved through the formation of the depressions with different depths and through the formation of projections or depressions in the bottom surface of the depressions. Before the concrete bracket shears off on a large recess, it shears off on a small recess, thereby opening up an expansion path. Similarly, if a protrusion or trough is formed in the bottom surface of a recess. This mechanism enables larger ones Relative shifts between steel and concrete. The reinforcing steel according to the invention is therefore particularly suitable as a reinforcement element for reinforced concrete components that are dimensioned using local plastic deformations of the reinforcement.

Since the boundary line of the depressions is formed from arcs and straight sections in which notch stresses can be concentrated are avoided, and the flanks of the depressions are rounded and merge into the floor surfaces, so that stresses on the reinforcing steel that affect the fatigue strength are largely avoided.

Due to the shape of a depression approximated to an ellipse, the surface portions of the smooth surface are enlarged compared to that of the depressions in comparison to, for example, a round shape of the depression with a given composite effect, whereby straightening ability and weldability are improved. In addition, with a view to good directivity, the depressions of adjacent longitudinal rows should be offset from one another in the longitudinal direction of the rod.

With regard to the desired ductility parameters Agt and Rm / Re, not only is the overall cross-sectional reduction when rolling in the depressions kept low, but a multi-stage process is used to produce the rolled reinforcing steel. In this process, a wire rod is rolled into a round bar in a first cold-forming process with a cross-sectional reduction of 8 to 15%, and the recesses are rolled in a further cold-forming process with a cross-sectional reduction of 2 to 7%. Instead of the first deformation process, two or even three cold deformation processes, each with a correspondingly smaller cross-sectional reduction, can be provided, in which the rod is brought to a round cross-section before the depressions are rolled in.

Fig. 1

die Abwicklung eines erfindungsgemäßen Betonstahls mit drei Längsreihen von ellipsenförmigen Vertiefungen,

Fig. 2

den Schnitt II-II von Fig. 2 in vergrößerter Darstellung,

Fig. 3

eine Längsreihe von zur Stabachse schräg angeordneten ellipsenförmigen Vertiefungen in einer Darstellung entsprechend Fig. 1,

Fig. 4

ein Diagramm das die Verbundeigenschaften eines erfindungsgemäßen Betonstahls im Vergleich zu denen eines bekannten kaltgerippten Betonstahls darstellt,

Fig. 5

einen der Fig. 2 entsprechenden Querschnitt eines weiteren Ausführungsbeispiels, und

Fig. 6 und 7

den Fig. 2 und 3 entsprechende Ansichten eines weiteren Betonstahls.

The invention is explained in more detail by means of exemplary embodiments with reference to seven figures. Show it

Fig. 1

the development of a reinforcing steel according to the invention with three longitudinal rows of elliptical depressions,

Fig. 2

the section II-II of Fig. 2 in an enlarged view,

Fig. 3

2 shows a longitudinal row of elliptical depressions arranged obliquely to the rod axis in a representation corresponding to FIG. 1,

Fig. 4

2 shows a diagram which shows the composite properties of a reinforcing steel according to the invention in comparison with those of a known cold-ribbed reinforcing steel,

Fig. 5

2 corresponding cross section of a further embodiment, and

6 and 7

2 and 3 corresponding views of another reinforcing steel.

The section of a reinforcing steel shown in the development in FIG. 1 has three longitudinal rows 1, 2, 3 of depressions 4. The longitudinal rows are evenly distributed over the circumference of the bar. The depressions of adjacent longitudinal rows are offset from one another in the longitudinal direction of the rod. With three longitudinal rows, the degree of displacement corresponds to approximately one third of the distance between two adjacent depressions in a longitudinal row. With the exception of the depressions 4, the rod surface 5 is smooth, i.e. it corresponds to that of a smooth round rod.

In the development of the reinforcing steel shown in FIG. 1, the boundary line of a depression is formed by circular arcs 6 and 7 which have different radii and are each arranged axially symmetrically with respect to the depression 4. The arcs 6 with a smaller radius are symmetrical to the axis of symmetry 8 and the arcs with a larger radius are arranged symmetrically to the axis of symmetry 9. The axis of symmetry 9 the circular arc 7 with a larger radius runs in the exemplary embodiment according to FIG. 1 at 90 ° to the rod axis, ie transverse to the rod axis, the axis of symmetry 8 runs parallel to the rod axis.

D ≈

0,75.d_s

B ≈

0,72.d_s

s ≈

0,25.d_s

b ≈

0,80 d_s

t ≈

0,06·d_s

wobei

• d_s den Nenndurchmesser des Stabes,
• D die Erstreckung der Vertiefung in Umfangsrichtung des Stabes,
• B die Erstreckung der Vertiefung in Längsrichtung des Stabes, gemessen in der Mitte der Quererstreckung D_,
• s den Abstand zwischen den Begrenzungslinien benachbarter Vertiefungen in Längsrichtung des Stabes, gemessen in der Mitte der Quererstreckung D,
• b den Abstand zwischen den Begrenzungslinien benachbarter Längsreihen von Vertiefungen in Querrichtung des Stabes, und
• t die Tiefe der Vertiefung

bedeuten.The size and spacing of the depressions 4 are determined as follows in the exemplary embodiment according to FIG. 1:

D ≈

0.75.d _s

B ≈

0.72.d _s

s ≈

0.25.d _s

b ≈

0.80 d _s

t ≈

0.06 · d _s

in which

• d _s the nominal diameter of the rod,
• D the extent of the depression in the circumferential direction of the rod,
• B the extent of the depression in the longitudinal direction of the rod, measured in the middle of the transverse extent D _,
• s the distance between the boundary lines of adjacent depressions in the longitudinal direction of the rod, measured in the middle of the transverse extent D,
• b the distance between the boundary lines of adjacent longitudinal rows of depressions in the transverse direction of the rod, and
• t the depth of the depression

mean.

With the dimensions given, the proportion of the total area of the depressions to the total area of the rod is approximately 40%.

2 shows the cross section of a depression in the longitudinal direction of the rod. From this it can be seen that the depression is flat, rolled into the rod surface 5 at a constant depth t and is delimited by steep flanks 10. The flanks 10 merge into the bottom surface 12 via roundings 11 with a small radius.

Due to the oval shape of the recesses, which is transverse to the rod axis, an adequate composite effect can be achieved despite the flat design of the recesses. The surface design of the reinforcing steel according to the invention is for design methods in which locally plastic deformations of the reinforcement, i.e. a joint rotation is used, more suitable than rods with ribs or ridges applied to the surface, since a "soft" bond is made possible by the surface design according to the invention.

3 shows a section of a row of ribs of a further embodiment of a reinforcing steel according to the invention. Here oval depressions are arranged obliquely to the rod axis. The angle relative to the rod axis is labeled α and should be in the range between 60 ° and 90 °. An angle α = 90 ° corresponds to the embodiment according to FIG. 1.

In view of the above design rule for the size and spacing of the depressions, it should be noted that B means the extent of the depression in the longitudinal direction of the rod, measured in the middle of the transverse extent D, i.e. measured along the center line of the relevant longitudinal row of depressions. The same applies to the parameter s, i.e. the distance between the boundary lines of adjacent depressions in the longitudinal direction of the rod.

The method according to the invention for producing the reinforcing steel described differs from the known methods in that a round wire rod is first cold-rolled into a round bar in a first cold-forming process and the shallow depressions are rolled in in a further cold-forming process. The first cold forming process, which can also be divided into two or three cold forming processes, primarily serves to increase the strength of the reinforcing steel. Depending on the starting material - this can be wire rods 380 to 420 of BSTM quality - a cross-sectional deformation of 10 to 20% will be required here, which can be reduced accordingly when dividing into 2 or 3 cold-forming processes. If 3rd Cold forming processes are provided, these enable the production of an exact round cross section of the desired dimension. In the case of several cold-forming processes for the production of the round steel, the reduction in cross-section in the first deformation step should be greater than in the subsequent steps for the production of the round steel.

Since the cross-section of the round wire rod is initially evenly reduced by a cold path, stress peaks can be avoided in this deformation process. In the final cold-forming process when rolling in the shallow depressions, the reduction in cross-section is limited to 7%, preferably 5%, the steel being stressed relatively evenly in this cold-forming operation by stamping only shallow depressions. The total deformation, including the rolling in of the depressions, is carried out evenly and gently in two or more steps, which, in conjunction with the surface geometry of the reinforcing steel, enables excellent ductility parameters Agt and Rm / Re to be achieved. In conjunction with the softer bond that can be achieved through the surface geometry of the reinforcing steel, this leads to a particular suitability when using the reinforcing steel using local plastic deformations (joint rotation) of the reinforcement.

Kohlenstoff:

0,04 bis 0,14 Gew %

Mangan:

0,35 bis 0,70 Gew %

Silicium:

0,20 bis 0,30 Gew %.

The reinforcing steel or concrete wire according to the invention is primarily intended for the production of reinforcing steel mats. A minimum yield strength of 500 N / mm ^{2 is} required for this. Particularly suitable as the starting material for the production of the reinforcing steel are wire rods with a yield strength of 380 to 420 N / mm ² with analysis values of

Carbon:

0.04 to 0.14% by weight

Manganese:

0.35 to 0.70% by weight

Silicon:

0.20 to 0.30% by weight.

A specific embodiment is described below.

The process according to the invention was carried out in a first shaping process a smooth round wire rod with a diameter of 8.0 mm is rolled into a round wire with a diameter of 7.5 mm. The cross-sectional reduction was 11%.

Nenndurchmesser:

8,0 mm

Ovalität:

7,85 bis 8,17

Zugfestigkeit:

421 N/mm²

Bruchdehnung (A10):

34 %

Chem. Zusammensetzung:

C: 0,07; Mn: 0,61; Si: 0,20; P: 0,016; Si: 0,037; Cu: 0,26; Cr: 0,11; Ni: 0,14; Mo: 0,02; N: 0,009;

Information on the wire rod used as the starting material:

Nominal diameter:

8.0 mm

Ovality:

7.85 to 8.17

Tensile strenght:

421 N / mm ²

Elongation at break (A10):

34%

Chem. Composition:

C: 0.07; Mn: 0.61; Si: 0.20; P: 0.016; Si: 0.037; Cu: 0.26; Cr: 0.11; Ni: 0.14; Mo: 0.02; N: 0.009;

• Streckgrenze: 508 - 536 N/mm²
• $\frac{\text{Zugfestigkeit}}{\text{Streckgrenze}}$
  
  : 1,07 - 1,10
• Dehnung bei Höchstlast (Agt): 4,7 - 7,1 %

In a second deformation process, three rows of depressions corresponding to FIG. 3, parallel to the longitudinal axis of the wire, were rolled into the rolled wire, the depth of which was 0.6 mm. The angle α to the longitudinal axis was 60%. The wire was not relaxed mechanically. The following strength values resulted:

• Yield strength: 508 - 536 N / mm ²
• $\frac{\text{tensile strenght}}{\text{Stretch limit}}$
  
  : 1.07 - 1.10
• Elongation at maximum load (Agt): 4.7 - 7.1%

The values given above represent the range limits of several tests.

In order to investigate the bond properties of the reinforcing steel according to the invention produced in this way, pull-out tests were carried out from concrete bodies with test bars at the top and bottom in accordance with the defined bond areas. Comparative tests were also carried out with a conventional cold-ribbed wire that had three longitudinal rows of inclined ribs.

Test parameters:

Position of the steels:

on the bottom and on the top, direction of concreting across the bar axis

Concrete cover

1.75 bar diameter

Concrete quality

B 25

In the diagram according to FIG. 4, the result of the wire according to the invention is denoted by I _u and I _o, respectively, the indices u and o signifying below and above, respectively. The comparison wire is designated II _u or II _o . The extension path s, measured at the tension-free rod end, is plotted on the abscissa and the related bond stress is plotted on the ordinate.

gerippten Draht:

0,2 bzw. 0,35 mm, und beim

erfindungsgemäßen Draht:

0,33 bzw. 0,9 mm.

A comparison of the curves shows that in the area of small relative displacements (ε <0.1 mm), ie in the state of use, the related composite stresses are in the usual range of scatter. However, the maximum shifts at maximum load are at

ripped wire:

0.2 or 0.35 mm, and at

wire according to the invention:

0.33 or 0.9 mm.

The reinforcing steel according to the invention is thus characterized by a composite which is approximately the same size in use, but which permits considerably greater displacements.

A further improvement with regard to a "soft" bond can be achieved if additional measures are provided which implement a graded "failure mode". Such measures are shown in Fig. 5 and in Figs. 6 and 7.

5 - the illustration corresponds to that of FIG. 2, ie it is a longitudinal section of the rod through the center of the depressions - the depressions are formed at different depths t1 and t2. If there is a load, the concrete bracket is sheared off in the area of the small depression t2 before it Area of the large depressions t1 is sheared off. As a result, a greater elongation value is released and a softer bond is achieved.

A graded "failure mode" is also achieved if the recesses 4 have different transverse dimensions D (see FIG. 3), measured perpendicular to the longitudinal axis of the rod between the tangents parallel to the longitudinal axis at the boundary line of the relevant recess.

Furthermore, the same effect is achieved if, as shown in FIGS. 6 and 7, which correspond to FIGS. 2 and 3, at least some of the depressions have projections 13. Instead of the projections, troughs could also be provided within the bottom surface of the depressions.

Claims (15)

1. Cold-rolled reinforcing steel which comprises a steel rod of approximately circular cross-section into which recesses (4) are rolled, which are uniformly distributed over the rod circumference in the form of 2 to 6, preferably 3 longitudinal rows, and whereby the peripheral contour of each recess is in the shape of arcs (6,7) having various radii in the developed view of the reinforcing steel rod, the arcs being arranged axial symmetrically relative to the recess, characterised in that shallow recesses (4) are rolled into the surface (5), these being bordered by steep flanks (10), which form an angle (β) with the tangent of the rod surface at the peripheral contour (6,7) of 60° to 80° and that the depth (4) of the recesses is defined by a cross-section reduction of from 2 to at the most 7% when the recesses are rolled in to a round cross-section steel rod.
2. Reinforcing steel as claimed in claim 1, characterised in that the peripheral contour of a recess (4) is formed from two opposing arcs (7) of larger radius and two opposing arcs (6) of smaller radius which connect them together.
3. Reinforcing steel as claimed in claim 2, characterised in that the axis of symmetry(9) of the arcs (7) having the greater radius, extends at an angle of 60° to 90° with respect to the rod axis.
4. Reinforcing steel as claimed in any one of claims 1 to 3, characterised in that three longitudinal rows (1,2,3) of recesses (4) are provided.
5. Reinforcing steel as claimed in any one of the claims 1 to 4, characterised in that the recesses (4) of adjacent longitudinal rows (1/2, 2/3, 3/1) are offset from each other in the longitudinal direction of the rod.
6. Reinforcing steel as claimed in any one of the claims 1 to 5, characterised in that the proportion of the total surface area of the rod occupied by the total surface area of the recesses, in the developed view of the rod surface, lies between 20% and 50%.
7. Reinforcing steel as claimed in any one of the claims 1 to 6, characterised in that the recesses (4) exhibit various depths (t1, t2) and/or varying sizes of transverse spans (D), measured normal to the longitudinal axis between the tangent parallel to the longitudinal axis at the peripheral contour.
8. Reinforcing steel as claimed in any one of the claims 1 to 7, characterised in that the recesses (4) are provided with projections (13) and/or cavities in their base surface(12).
9. Reinforcing steel asa claimed in any one of the claims 1 to 8, characterised in that the size and spacing of the recesses in the case of a rod having two to six longitudinal rows of recesses, are specified as follows:

   b =   (0.15 to 0.45).d_s

   D =   (0.12 to 1.42).d_s for n = 2
   (0.60 to 0.90).d_s for n = 3
   (0.30 to 0.65).d_s for n = 4
   (0.10 to 0.35).d_s for n = 6

   B =   (0.3 to 0.85).d_s

   s=   (0.1 to 1.5). d_s

   t =   (0.025to 0.08).d_s

   where:

   d_s = nominal diameter of the rod

   D = span of the recess in the circumference direction of the rod, measured normal to the longitudinal axis between the tangents parallel to the longitudinal axis at the peripheral contour.

   B = the span of the recess in the longitudinal direction of the rod, measured in the centre of the transverse span D.

   s = the spacing between the peripheral contours of adjacent recesses in the longitudinal direction of the rod, measured in the centre of the transverse span D.

   b = the spacing between the peripheral contours of adjacent longitudinal rows of recesses in the transverse direction of the rod.

   t = the depth of the recess, and

   n = the number of longitudinal rows of recesses.
10. Reinforcing steel as claimed in claim 9, characterised in that the size and spacing of the recesses for n = 3 are specified as follows:

    D ≈   0.75.d_s

    B ≈   0.72.d_s

    s ≈   0.25.d_s

    b ≈   0.80.d_s

    t ≈   0.06.d_s
11. Reinforcing steel as claimed in any one of the claims 1 to 10, characterised in that the size and spacing of the recesses are selected, so that the degree of ribbed surface f_s of the reinforcing steel rod lies between 0.02 and 0.07.
12. Reinforcing steel as claimed in claim 11, characterised in that the degree of ribbed surface f_s lies between 0.02 and 0.045
13. Method of manufacturing cold-rolled reinforcing steel, as claimed in claims 1 to 12, in which a wire rod is rolled to a round rod in at least one initial cold-rolling operation with a cross-section reduction from 8 to 20% while in a final cold-rolling operation shallow recesses (4) are rolled in, each bordered by a steep flank (10), which forms an angle (β) of from 60° to 80° at its peripheral contour with the tangent to the rod surface, the recesses being distributed uniformly over the rod circumference in the form of 2 to 6, preferably 3 longitudinal rows, whereby the rolling in of the recesses (4) is effected with a cross-section reduction of from 2 to 7%, and the peripheral contour of each recess is formed by arcs having various radii in the developed view of the reinforcing steel rod, which are arranged axial symmetrically with respect to the recess.
14. Method as claimed in claim 13, characterised in that two or three cold-rolling operations are provided in place of the initial cold-rolling operation, in which the wire rod is rolled to a round rod with a total cross-section reduction of 20% maximum.
15. Method as claimed in claim 13 or 14, characterised in that the melt analysis of the wire rod includes:

    0.04 to 0.14 C

    0.35 to 0.70 Mn

    0.20 to 0.30 Si

    in addition to the usual alloying elements and impurities, the remainder iron.

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