CZ9602010A3 - Cold rolled reinforcing bars and process for producing thereof - 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

Technical field

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to reinforcing steel, cold rolled and formed in the form of a circular cross-section, comprising circumferentially rolled depressions arranged in two to six, in particular three longitudinal rows, evenly distributed over the circumference of the reinforcing rod. The surface area of the reinforcing bar is formed by circular arcs with different radii of curvature, axially symmetrical to the depression.

BACKGROUND OF THE INVENTION

Reinforcing steels or prestressing steels for reinforcing reinforced concrete or prestressed concrete have ribs on their surface to achieve the necessary cohesion between steel and concrete. In this way, the transfer of forces between the concrete and the steel and vice versa is ensured in such a way that short anchor or transfer lengths of the rods are achieved.

A number of technical conditions for the production of reinforcing steels have gradually changed over the course of development, especially for cold formed steels, for example for reinforcing mats or reinforcing steel supplied in coils. This also includes new knowledge of non-linear dimensioning of reinforced concrete structures. Coupling of reinforcing steels has so far been considered only in the elastic area of the steel deformation line. The use of non-linear dimensioning also affects the coupling in the plastic region of the steel (DE-A1-40 11 486). In this case, it has been shown that too hard a coupling formed by ribs on the reinforcing bars leads to cracks. Therefore, it was necessary to look for the interaction of steel with concrete, which the existing building regulations allow for the utility state, but which is softer in the plastic area of the deformation line of reinforcing steel.

Much of the reinforcing steel is produced by hot or cold forming in the form of reinforcing wires or bars, supplied in the form of coils, from which shaped reinforcing bars or reinforcing mats are then prepared. In order to bring the supplied reinforcing steel into a shape in which it can serve as reinforcement, it must be straightened using suitable machines. Ribbed steels have a non-circular cross-section along their entire length and their ribs are removed to some extent after the straightening process. The ribs formed on the perimeter of the reinforcing bar are also a source of considerable noise during straightening. Consequently, in the case of reinforcing steel produced in coils, it is necessary to substantially improve the properties of the steels, which are important for straightening and reducing the level of noise during straightening.

In the ribs of reinforcing steel there is a considerable part of the weight of the rod. To create ribs, it is necessary to achieve up to 25% deformation, which itself requires considerable energy consumption. In view of the need to reduce energy consumption, efforts should be made to reduce the energy requirement for rib forming.

The high degree of deformation required in cold forming reduces the default values of the ductility parameters Agt (elongation at maximum stress before breaking) and Rm / Re (tensile strength / yield strength). The required nominal values (see ENV 10080) are therefore hardly achievable.

Welding of reinforcing steels takes place in a machine that performs up to 120 bars per minute. Achieving constant good quality welded joints is only possible if the non-circularity of the welded elements caused by the circumferential ribs is kept as low as possible. This is especially true for welding double rods. For this reason, it is still sought to create reinforcing bars whose shape is as close as possible to smooth bars of circular cross-section.

This results in the main tasks for creating the surface of reinforcing steels.

During production:

low forming costs, more favorable material forming of cold forming to achieve lower energy consumption and wear of the rolling equipment when shaping the surface of the reinforcing bar.

During further processing:

good leveling at low noise emissions, improved welding capability. Eliminates the possibility of damaging the surface of reinforcing steel and reduces wear on straightening tools.

For use as reinforcement:

sufficient coupling with concrete in the usable state and enabling activated steel to be stretched until cracks in the concrete (plastic joint), high values of Agt and Rm / Re parameters occur. Low notch effect and thus high fatigue limit at oscillating load.

Tests have shown that these stated objectives are best achieved in concrete steels having the features set forth in the preamble of claim 1 and in the introduction to the description. Reinforcing steel of this kind is described in DE-AS 10 84 464.

This document discloses a reinforcing wire or bar, in particular for prestressed concrete, in which surface recesses have recurring recesses on both sides, these recesses having an elliptical shape and occupy approximately half the circumference of the bar. The depressions are separated in the area of their shorter axes by narrow bumps which are arranged obliquely to the longitudinal axis of the beam. These bumps, like the slanted ribs, are formed by milling in a shaped indentation cylinder, and the material of the formed strand is pressed into them during rolling.

GB-PS 1 334 757 discloses reinforcing steel which also has the features mentioned at the beginning of the description and in the preamble of claim 1 having circumferential elliptical depressions on its circumference, wherein the long axis of the ellipse lies at an angle of 30 ° to 70 ° to the longitudinal axis rod. In order to improve the bending properties of the concrete reinforcement in the form of a reinforcing bar, the sides of the depressions are formed symmetrically and their apex angle is limited to 50 °.

FR-A-1 207 928 discloses a concrete reinforcement in which the surface area of its reinforcing bars is formed in a smooth mold to improve weldability in the manufacture of reinforcing mats. At least three rows of through-recesses having a constant shallow depth and a rectangular or trapezoidal shape are formed by rolling in the reinforcing bars of this concrete reinforcement having a circular cross-section.

FR-A-1 202 576 discloses a method for manufacturing finned reinforcing bars in which a cold-rolled round wire is reduced by a cross-sectional reduction of about 20% into a reduced-section circular bar. Then, the round rod is rolled into a triangular cross-section with rounded corners and pressed into the rounded corners to form the ribs. The cold deformation for the production of ribs is about 20%.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a reinforcing steel surface according to the generic features of claim 1 which, at a low cost of wire forming, would allow better ductility parameters Agt and Rm / Re to be achieved and ensure sufficient steel to concrete cohesion. It is also to be achieved by including plastic deformation of reinforcing steel (non-linear dimensioning) of a softer anchoring of the steel in the concrete, which allows relatively large relative displacements between steel and concrete. In addition, due to the need for good comparability and improved weldability of the rods, a relatively smooth surface of the rod with a circular cross-section should be achieved. To achieve long reinforcement under dynamic stress, the notch stresses should be kept as low as possible. The invention is also intended to provide a method for producing this reinforcing steel.

reinforcing life

SUMMARY OF THE INVENTION

These objects are solved by the reinforcing steel according to the invention, which is based on the fact that the depressions are rolled into the surface of the reinforcing bar with a constant depth t and are circumferentially circumscribed by steep sides. Further advantageous embodiments of the concrete reinforcement according to the invention are described in claims 2 to 12.

The principle of the process according to the invention consists in the first rolling of the wire rod in at least one first cold forming process, in which the cross-sectional area is reduced by 8% to 20%, into a reinforcing bar with a circular cross-section into which In the cold forming process, the depressions roll at the same time reducing the cross-sectional area by 2% to 7%. Advantageous embodiments of the method are set forth in the remaining claims.

While in the known reinforcing steel according to DE-AS 10 84 464, intended especially for prestressed concrete structures, the basic aim was to achieve high tensile strength and high cohesion with concrete, which was achieved by a significant cold deformation of smooth drawn round wire and ribs. by extruding the material outward during cold processing, in the reinforcing steel according to the invention, the deformation of the reinforcing bar is relatively small, since only flat depressions are rolled into its surface so that the ductility remains high. With the exception of shaped flat depressions, the surface of the round bar remains smooth, that is, the bead formation has been eliminated to improve the conditions for straightening bars and wires and to provide a soft anchorage of the steel reinforcing bars to the concrete structure.

By providing depressions with different depths and also by forming protrusions or grooves in the bottom surface of the depressions, a graded failure mode is achieved. It is to be understood that, before the large concrete brackets formed by the liquid-penetrated concrete into larger depressions are crushed in the reinforced concrete structure, the small concrete brackets with a smaller cross-section are first crushed, thereby opening the way for further elongation. A similar situation is in another preferred embodiment in which protrusions or grooves are formed in the bottom of the depressions. This mechanism allows greater relative displacements, the reinforcing steel according to the invention is therefore particularly suitable for reinforcing elements of reinforced concrete structures which are dimensioned using local plastic deformations of the reinforcement.

Since the contour line of the depressions is formed by circular arcs, thereby removing straight sections where the stresses in the notch could concentrate, and because the sides of the depressions pass into their bottom rounded transition sections, these notch stresses, which significantly affect the durability of the reinforcement under dynamic load, significantly reduced.

Due to the shape of the depressions approaching the shape of the ellipse, for example, the circular shape of the depressions, given the coupling effect, increases the surface area of the flat portion of the smooth surface compared to the total area of the depression so that improved properties important for straightening processes and weldability of the reinforcement. In addition, due to the good conditions for straightening the rods, the depressions in the adjacent longitudinal rows are arranged offset from one another.

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In order to achieve more advantageous ductility parameters Agt and Rm / Re, it is advantageous that not only is the cross-sectional reduction of the bar rolled at a low level while the recesses are rolled, but at the same time the reinforcing steel is rolled by a multi-stage forming process. In this process, the precursor material in the form of wire rod in the first cold forming operation is rolled to form a reinforcing bar with a circular cross-section and a reduction of the cross-sectional area by 8% to 25% and depressions are rolled into the bar surface in the second operation. reduction of the cross-sectional area by 2% to 7% is achieved. The first operation may be carried out in two to three sub-forming stages with correspondingly smaller cross-section reductions, which form a reinforcing bar with a circular cross-section before the depressions are rolled.

Overview of the drawings

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a depicted surface of a steel reinforcing bar provided with three longitudinal rows of elliptical depressions; FIG. 2 is a longitudinal section of a portion of the reinforcing bar shown on an enlarged scale taken along II-II in FIG. Fig. 1 is a view of a portion of a series of elliptical depressions inclined at an angle to the bar axis and having the same shape as in Fig. 1, Fig. 4 a diagram of the coupling properties of a reinforcing bar according to the invention compared to known cold formed reinforcing steels; Fig. 5 is a cross-sectional view of a portion of another exemplary reinforcing bar similar to that of Fig. 2; Fig. 6 is a cross-sectional view of a portion of a third exemplary rod corresponding to that of Fig. 2; 3.

DETAILED DESCRIPTION OF THE INVENTION

The steel reinforcing bar of the invention, one section of which is shown in view of its developed surface, is provided with three longitudinal rows 1, 2, 2 of depressions 4. The longitudinal rows 1, 2, 2 are equally spaced along the circumference of the reinforcing bar. The depressions 4 in the adjoining longitudinal rows are offset in the longitudinal direction relative to each other, the extent of this offset corresponding to approximately one third of the spacing of the adjacent depressions 4 in three longitudinal rows. Except for these depressions 4, the surface 5 of the reinforcing bar is completely smooth. corresponds to the surface of a smooth bar with a circular cross-section.

The contour line of the depressions 4 in the example of FIG. 1 is formed by circular arcs 6, 7 which have different radius of curvature and are arranged axially symmetrically in relation to the depressions 4. The smaller circular arcs 6 with a smaller radius of curvature are located symmetrically to the first axis of symmetry 8 and the larger circular arcs 7 with a larger radius of curvature are positioned symmetrically to the second axis of symmetry 9. The second axis of symmetry 9 of the larger circular arcs 7 with greater radius of curvature runs in the example shown in Fig. 1 at 90 ° to the longitudinal axis of the reinforcing bar, i.e. perpendicular to the reinforcing bar, and the axis of symmetry 2 is parallel to the longitudinal axis of the reinforcing bar.

The dimensions and spacings of the depressions 4 in the exemplary embodiment of the reinforcing bar according to FIG. 1 are as follows:

D 0,7 0.75 d _s B 0,7 0.72 d _s s 0,2 0.25 d _s b 0 0.80 d _s t 0,0 0.06 dg, where dg is the nominal diameter of the reinforcing bar,

D is the dimension of the recesses 4 in the circumferential direction of the reinforcing bar,

B is the dimension of the depressions 4 in the longitudinal direction of the reinforcing bar, measured in the middle of the transverse dimension, s is the distance between the contour lines of adjacent depressions 4. in the longitudinal direction of the reinforcing bar measured in the middle of the transverse dimension of the reinforcing bar and t is the depth of depressions 4.

With these dimensions, the proportion of the total area of depressions 4 in the total area of the reinforcing bar is about 40%.

FIG. 2 shows a section through one of the depressions 4 taken in the longitudinal direction of the reinforcing bar. It can be seen from this illustration that the depression 4 has a flat bottom, is rolled with the same depth t in the surface 5 of the reinforcing bar and is circumferentially limited by steep flanks 10. These flanks 10 pass rounded transitions 11 with small radius of curvature into the bottom 12.

This oval shape of the depressions 4 arranged perpendicular to the longitudinal axis of the reinforcing bar has a sufficient anchoring and coupling effect despite its flat design. Creating the surface of a reinforcing bar according to the invention is more advantageous than the surface area of known bars provided with rolled ribs or protrusions for dimensioning concrete structures using local plastic deformation of the reinforcement, i.e. its articulation, because the shaping of the surface of the reinforcing bar according to the invention is softer anchoring possible.

FIG. 3 shows a portion of a series of depressions formed on the circumference of a reinforcing bar of another exemplary embodiment.

In this case, the oval depressions are arranged obliquely to the longitudinal axis of the reinforcing bar. The angle α between the axis of symmetry 9 and the longitudinal axis of the reinforcing bar should be between 60 ° and 90 °. The angle α = 90 ° corresponds to the example of Fig. 1.

In view of the rules for determining the size and spacing of the depressions in this example, it should be explained that B is the dimension of the depressions in the longitudinal direction of the reinforcing bar measured at the center of dimension D, i.e. along the centreline of the respective longitudinal row of depressions. applies to the value s, which is equal to the distance between the contour lines of two adjacent depressions 4 in the longitudinal direction of the reinforcing bar.

The method for producing the described reinforcing bar according to the invention differs from the known manufacturing processes in that, in the first cold forming operation, it is rolled out of a round bar by cold rolling, and in the second cold forming operation, roll flat recesses. The first forming process, which may optionally be divided into two or three partial forming operations, serves primarily to increase the strength of the reinforcing bar. Depending on the type of starting material, which can be rolled bars of 380 to 420 BSTM quality, the necessary deformation of the cross-section of the rod is between 10% and 20 liters, which can be reduced by dividing the process into two or three sub-operations. When three partial forming operations are carried out, it is possible by the method according to the invention to produce rods with a precise circular cross-section in the desired dimensions. In the multi-stage cold forming process in the production of round bars, the reduction in cross-section should be greater than in the subsequent steps of manufacturing reinforcing steel with a circular cross-section.

Since the cross-section of a circular rod with a circular cross-section is firstly reduced by a cold treatment, peak stresses can be eliminated during this forming. In the subsequent cold forming process, the rolling of the flat depressions reduces the reduction of the cross-sectional area to 7%, in particular 5%, and during punching of these only flat depressions 4, the cold-processed steel is subjected to relatively uniform stresses. Thus, the total deformation including the rolling of the depressions 4 proceeds uniformly and continuously in two or more steps, so that excellent ductility parameters Agt and Rm / Re can be achieved in conjunction with the surface geometry of the reinforcing bar. This, in conjunction with the softer anchoring achievable by the selected surface area geometry of the reinforcing bar, leads to considerable advantages when using the reinforcing bar using local plastic deformation (articulation) for reinforcement.

The reinforcing bars in the form of reinforcing bars according to the invention are primarily intended for the production of reinforcing mats for concrete structures. A minimum yield point of 500 N / mm ² is prescribed for this purpose. In particular, the starting material for the production of reinforcing steel is rolled bars or wires with a yield point of 380 to 420 N / mm ² and analytical values in mass amounts

carbon: 0.04 to 0.14 % manganese: 0.35 to 0.70 % silicon: 0.20 to 0.30 %.

Specific examples of the process according to the invention will be described in the following.

In a method for producing reinforcing bars in the form of reinforcing bars, in a first forming process, a round wire having a diameter of 8.0 mm is rolled from a round wire having a diameter of 7.5 mm. The reduction in cross-sectional area is thus 11%.

Values of wire used as starting material: nominal diameter 8.0 mm ovality 7.85 to 8.17 tensile strength 421 Nn / mm ² elongation at break (A10) 34% chemical composition: C: 0.07, MnO, 61, Si: 0.20, P: 0.016, Si: 0.037, Cu: 0.26, Cr: 0.1, Ni: 0.14, Mo: 0.02, N: 0.009

Three parallel rows of depressions according to FIG. 3, which run parallel to the longitudinal axis of the wire, are rolled into a round wire with a cross section in the second forming process, the depressions being 0.6 mm deep. The angle α clamped by the long axes of the elliptical or oval depressions with the longitudinal axis of the wire is 60%. The internal tension was not mechanically removed from the wire. The following strength values were achieved by this procedure:

elongation at break: 508 - 536 N / mm ² tensile strength: 1,07 - 1,10 elongation at break at maximum load (Agt): 4,7 - 7,1%

The ranges given are the results of several tests.

In order to determine the properties important for the consistency of the reinforcing steel thus produced with concrete, plucking tests were carried out to detect anchoring in the concrete body provided above and correspondingly fixed to the areas. In addition, comparative tests were carried out with the test rods at the positions determined by the anchor or coupler at the same time as conventional bars on which slanted ribs were formed by cold forming arranged in three longitudinal rows.

Test parameters:

beam position: lower and upper bearing direction of concreting perpendicular to the longitudinal axis of the beam concrete cover: 1.75 x beam diameter concrete quality: B 25

In the graph shown in FIG. 4 are the results obtained for a beam according to the invention are identified _in I or I _a, where indi ™ CIE u denotes the curve valid for the lower rod and the indicia relates hotního rod. Values valid for the comparative beam are similarly labeled II _U , II _O. ^{On the} horizontal axis, the pull-out path is plotted, measured at the unstretched end of the rod, and the corresponding coupling stresses are plotted on the ordinate.

The comparison of the curves shows that in the area of smaller relative displacements (e < 0.1 mm), i.e. in the utility state, the coupling stress lies in the usual re-dispersion region. However, the maximum displacements at maximum loads are 0.2 or 0.35 mm for a ribbed rod and 0.33 to 0.9 mm for a rod according to the invention.

The rod according to the invention thus has approximately the same cohesion values between concrete and steel in the area of use, but allows considerably greater displacements.

A further improvement with respect to the so-called soft anchoring can be achieved if additional measures are implemented which implement a gradual failure mode. These measures are illustrated in Figures 5, 6 and 7.

In the reinforcing steel shown in FIG. 5 in a sectional view corresponding to that of FIG. 2, i.e. in a longitudinal section through the center of the depressions, the depressions are formed with different depths t1, t2. Under stress, the concrete bracket is first sheared in the region of the smaller recess with a smaller depth t1 and only then the concrete brackets are sheared in the deeper recesses with a greater depth t2. This results in a higher elongation value and a softer anchorage of the rod in the concrete. and

A graded failure mode is also achieved when the depressions 4 have different transverse dimensions D related to similar dimensions as in Fig. 3 and measured perpendicular to the longitudinal axis of the reinforcement bar between two tangents to the contour line of the depressions 4 parallel to the longitudinal axis of the reinforcement rod.

The same effects are achieved if a projection 13 is formed in at least a portion of the bottom surface 12 of the depression 4. Instead of these projections 13, a circumferential groove can also be formed in the bottom surface 12.

Claims (15)

Claims PATENTOVÉ Ρ 2; ——. * - < AT O· r— 30 O W O 2 O Xs < > • - <2 SA O O - ίβ n < • r- X CPH O <= □ r— ' ^e * • < ΓΠ » (YEAH* O OC < of O) “3 O zr. .M- Yb ‘ 1. Reinforcing steel, cold-rolled and formed in the form of a reinforcing bar of circular cross-section comprising circumferentially rolled depressions (4) arranged in two to six, in particular three longitudinal rows, distributed uniformly around the circumference of the reinforcing rod, the contour line of each depression ( 4) is formed in the developed surface of the reinforcing bar by circular arcs (6, 7) with different radii of curvature, placed axially symmetrical with respect to the depression (4), characterized in that the flat depressions (4) are rolled into the surface (5) and are circumscribed at their circumference by steep sides (10) forming an angle (B) of up to 60 ° to 80 ° on their contour line (6, 7) tangential to the surface of the reinforcing bar, the depth of the depressions (4) being determined by the maximum by reducing the cross-sectional area by 7% after the depressions (4) have been rolled into reinforcing steel with a circular cross-section. Concrete reinforcement according to claim 1, characterized in that the contour line of the depression (4) consists of two mutually opposed larger circular arcs (7) with a larger radius of curvature and two mutually opposed smaller circular arcs (6) with a smaller radius curvature. Concrete reinforcement according to claim 2, characterized in that the larger circular arc (7) has its axis of symmetry (9) oriented at an angle of 60 ° to 90 ° to the longitudinal axis of the reinforcing bar. Concrete reinforcement according to claims 1 to 3, characterized in that the depressions (4) are arranged in three longitudinal rows (1, 2, 3). Concrete reinforcement according to Claims 1 to 4, characterized in that the depressions (4) in the longitudinal rows (1/2, 2/3, 3/1) in the longitudinal direction are offset relative to one another in the longitudinal direction of the reinforcing bar. Concrete reinforcement according to claims 1 to 5, characterized in that in the developed surface of the reinforcing bar the proportion of the total area of the depressions (4) in the total surface of the reinforcing bar is between 20% and 50%. Concrete reinforcement according to claims 1 to 6, characterized in that the depressions (4) have different depths (t1, t2) and / or different transverse dimensions (D), measured in a direction perpendicular to the longitudinal axis of the beam between two tangents per contour. a line extending parallel to the longitudinal axis of the reinforcing bar. Concrete reinforcement according to claims 1 to 7, characterized in that the depressions (4) are protrusions (13) and / or grooves in their bottom surface (12). proclaimed in Concrete reinforcement according to claims 1 to 8, characterized in that the reinforcing bar with two to six longitudinal rows of depressions (4) comprises depressions (4) having the following dimensions and spacings: b = (0.15 to 0.45) .d _p D » (1.12 to 1.42) · dg, for n = 2, (0.60 to 0.90) .d _s , for n = 3, (0.30 to 0,65) .dg, for n = 4, (0.10 to 0.35) «dg, for n = 6, B = (0.30 to 0.85) .d _s , s - (0.10 to 1.50) .dg, t = (0,025 to 0,08) d _s , where d _s is the nominal diameter of the reinforcing bar, D is the dimension of the depression (4) measured in a direction perpendicular to the longitudinal axis of the beam between two tangents on the contour line, running parallel to the longitudinal axis of the reinforcing beam, B is the dimension of the depression (4) in the longitudinal direction of the reinforcement bar, measured at the center of dimension D, s is the distance between the contour lines of adjacent depressions in the longitudinal direction of the reinforcement bar, measured at the center of dimension D t is the depth of the depression (4) and n is the number of longitudinal rows (1, 2, 3) of the depressions (4). Concrete reinforcement according to claim 9, characterized in that the depressions (4) have the following dimensions and spacings on a reinforcing bar with three longitudinal rows: D «0,75.d _with B '0,72.d _with a" b _s 0.25 d "0.80 dg t« d _to 0.06. Concrete reinforcement according to claims 1 to 10, characterized in that the sizes and spacings of the depressions (4) are determined such that the reference rib surface f _{R of the} concrete reinforcement is between 0.02 and 0.07. Concrete reinforcement according to claim 11, characterized in that the reference rib surface f _{R of the} concrete reinforcement is between 0.02 and 0.045. Method for producing cold-rolled reinforcing concrete according to at least one of Claims 1 to 12, in which the rolled wire is rolled in at least one first cold-forming step with a reduction in the cross-sectional area of the rod by 8% to 20% to reinforcing bars. a bar with a circular cross-section into which in a further cold forming process the flat depressions (4), bounded on each circumference by steep sides (10), are arranged and arranged in two to six, in particular three longitudinal rows, distributed uniformly around the circumference of the bar, characterized by three (4), is carried out with a reduction of 7% and the contour line of each surface of the bar creates radii of curvature which are depressed axially symmetrically. The method according to claim 1, characterized in that the rolling of the cross-sectional area of the beam by 2 liters to the depressions (4) in the developed circular arcs with different Method according to claim 13, characterized in that the first cold forming process is divided into two or three partial forming operations, by means of which the wire rod is rolled, with a total reduction of the cross-sectional area up to a maximum of 20% per bar of circular cross-section. Method according to claim 13 or 14, characterized in that the fusion analysis of the wire rod comprises 0.04 to 0.14 C 0.35 to 0.70 Mn 0.20 to 0.30 Si as well as conventional alloying elements and impurities, the remainder being iron.