Description

Technical Field

[0001 ] The invention relates to a steel strand comprising a steel core wire surrounded by steel layer wires that are twisted around this steel core wire.

[0002] The invention also relates to the use of such a steel strand as a pretensioned strand or as a post-tensioned strand.

Background Art

[0003] A steel strand comprising a steel core wire surrounded by steel layer wires that are twisted around this steel core wire is a flexible and robust tension member that finds many applications.

[0004] The steel core wire and the steel layer wires usually have a circular crosssection. When a closed layer of steel layer wires is formed around the steel core wire, i.e. a layer where all steel layer wires touch adjacent steel layer wires, interstices are formed between the steel core wire and the steel layer wires. Humid environments or moisture already present in these interstices may cause condensation and lead ultimately to corrosion and a premature end of life of the steel strand. Compacting the steel strand may reduce the size of the interstices but does not take the interstices completely away. When used as a pre-tensioned steel strand or as a post-tensioned steel strand, the steel strand is subjected to tensile stresses and the mentioned potential corrosion failure problem is even more present.

[0005] The steel core wire and the steel layer wires may be provided with a corrosion resistant coating to increase the lifetime of the steel strand.

[0006] An example of a corrosion resistant coating is a polymer coating. If of a sufficient thickness, the steel layer wires exercise a pressure on the polymer coating so that it starts to flow around the steel layer wires and fill the interstices. However, this polymer coating is not resistant against a downstream heat treatment of the rope. Another drawback is that the rope with a polymer inside cannot be installed and used in warm circumstances.

[0007] Another corrosion resistant coating is a metallic coating such as a zinc or a zinc alloy coating applied by means of a hot dip operation or electroplating operation. While increasing the lifetime of the steel strand, these metallic coatings are not thick enough to fill the interstices.

Disclosure of Invention

[0008] It is a general object of the invention to avoid or mitigate the drawbacks of the prior art.

[0009] It is a particular object of the invention to provide a steel strand that can operate under tensile forces and that has an increased lifetime.

[0010] It is another object of the invention to fill in a sustainable way the interstices between the steel core wire and the steel layer wires.

[0011 ] According to a first aspect of the invention, there is provided a steel strand. The steel strand comprises a steel core wire. The steel core wire is surrounded by steel layer wires that are twisted around the steel core wire. The steel core wire is covered with a corrosion resistant core coating provided by strip cladding or by metal extrusion. The steel layer wires are covered with a corrosion resistant layer coating provided by a hot dip operation or by an electroplating or chemical plating process. The steel strand is compacted so that the steel layer wires have a non-circular cross-section and that said corrosion resistant core coating fills the interstices between the steel core wire and the steel layer wires.

[0012] Briefly stated, the steel strand is provided with a two corrosion resistant coatings. A relatively thick metallic coating around the steel core wire and relatively thin metallic coatings around the steel layer wires. [0013] A first way to provide the relatively thick core coating is by strip cladding. According to the technique of strip cladding, a strip of a corrosion resistant metal and of predetermined and desired thickness can be formed into a tube form. The width of this strip is somewhat greater or equal to the circumference of the steel wire to be covered. The strip is closed in a tube and welded on or around the steel wire. The steel wire with the strip around is then drawn until its final steel core wire diameter.

[0014] A second way to provide the relatively thick core coating is by means of metal extrusion. According to the technique of metal extrusion, corrosion resistant metal material is pushed through a die of the desired crosssection around the steel wire.

[0015] A hot dip operation or an electroplating or chemical plating process are the ways apply the relatively thin layer corrosion resistant coating on the steel layer wires.

[0016] When finally compacting the steel strand, the metal of the thick core coating will start to flow somewhat around the steel layer wires and fill the interstices between the steel core wire and the steel layer wires. In contrast with the polymer material, this metal material is able to resist high temperatures. The steel strand with the metal material inside can be subjected to downstream heat treatments, such as a stress-relieving treatment at about 350 °C to 420 °C. Moreover, the steel strand can be installed and used in warm circumstances.

[0017] Preferably the thick corrosion resistant core coating is of aluminium or of an aluminium alloy.

[0018] Most preferably, the thick corrosion resistant coating has a weight ranging from 400 g/m ² to 2500 g/m ² , preferably from 450 g/m ² to 2250 g/m ² , where the smaller weights correspond to the smaller diameters and the higher weights correspond to the greater diameters.

A steel strand with an outer diameter of 7.0 mm may have a weight of the thick corrosion resistant coating of 486 g/m ² . A steel strand with an outer diameter of 15.2 mm may have a weight of the thick corrosion resistant coating of 810 g/m ² .

A steel strand with an outer diameter of 18.0 mm may have a weight of the thick corrosion resistant coating of 2119 g/m ² .

[0019] Preferably the thin corrosion resistant layer coating is of zinc or of a zinc alloy.

[0020] A zinc aluminium coating has a better overall corrosion resistance than zinc. In contrast with zinc, the zinc aluminium coating is temperature resistant. Still in contrast with zinc, there is no flaking with the zinc aluminium alloy when exposed to high temperatures.

[0021 ] A zinc aluminium coating may have an aluminium content ranging from 2 per cent by weight to 12 per cent by weight, e.g. ranging from 3 % to 11 %.

[0022] One preferable composition lies around the eutectoid position: Aluminium about 5 per cent. The zinc alloy coating may further have a wetting agent such as lanthanum or cerium in an amount less than 0.1 per cent of the zinc alloy. The remainder of the coating is zinc and unavoidable impurities.

[0023] Another preferable composition contains about 10% aluminium. This increased amount of aluminium provides a better corrosion protection then the eutectoid composition with about 5% of aluminium.

[0024] A highly preferable composition in the context of the present invention is a zinc aluminium alloy that comprises 2 % to 10 % aluminium and 0.2 % to 3.0 % magnesium, the remainder being zinc. An example is 5% Al, 0.5 % Mg and the rest being Zn.

[0025] Other elements such as silicon (Si) may be added to the zinc aluminium alloy coatings mentioned above. [0026] The thin corrosion resistant layer coating may have a weight ranging from 150 g/m ² to 450 g/m ² . Here again, the smaller weights correspond to the smaller diameters and the higher weights correspond to the greater diameters.

[0027] The steel strand may have an outer diameter ranging from 6.0 mm to 25.0 mm, preferably from 6.5 mm to 20.0 mm.

[0028] According to a second aspect of the invention, the steel strand can be used as a pre-tensioned strand or as a post-tensioned strand.

[0029] The steel strand can be used in elevated transportation systems, in ACSR power cables, as guardrail, stay cable strand, umbilical or pre-cast concrete reinforcing elements.

Brief Description of Figure in the Drawing

[0030] FIGURE 1 shows a cross-section of a steel strand according to a first aspect of the present invention.

Mode(s) for Carrying Out the Invention

[0031] FIGURE 1 shows a cross-section of a steel strand 10. The steel strand 10 has a steel core wire 12 and six steel layer wires 14 that are twisted around the steel core wire 12. The steel core wire 12 is provided with an aluminium strip clad core coating 16. The steel layer wires 14 are provided with a zinc aluminium magnesium layer coating 18.

[0032] The steel strand 10 is compacted so that the steel layer wires 14 obtain a trapezium-like cross-section. The steel layer wires 14 form a closed layer around the steel core wire 12 and push the aluminium strip clad core coating 16 to flow and fill the interstices between the steel core wire 12 and the steel layer wires 14. [0033] The aluminium strip clad core coating 16 fills all interstices and holes inside the steel strand and prevents moisture from penetrating inside the steel strand 10. At the same time the aluminium strip clad core coating 16 gives corrosion resistance to steel core wire 12.

[0034] The zinc aluminium magnesium layer coating 18 gives corrosion resistance to the steel layer wires 14.

[0035] A steel strand according to the invention can be made along following lines.

[0036] The composition of the starting material (wire rod) is usually a microalloyed high-carbon composition as follows: (all percentages being percentages by weight):

- a carbon content (% C) ranging from 0.50% to 1 .20%, e.g. 0.60% to 1.0%;

- a manganese content (% Mn) ranging from 0.10% to 1.0%, e.g. from 0.20% to 0.80%;

- a silicon content (% Si) ranging from 0.10% to 1 .50%, e.g. from 0.15% to 0.70%;

- a sulphur content (% S) below 0.03%, e.g. below 0.01%;

- a phosporus content (% P) below 0.03%, e.g. below 0.01 %. Micro-alloying elements such as chromium, vanadium, aluminium or nickel may or may not be present in amounts up to 0.50%.

[0037] Starting from wire rod, the steel core wire is drawn until an intermediate steel wire diameter. An aluminium strip is formed and welded or extruded around the steel wire and the thus formed composite is further drawn until a final diameter.

[0038] Starting also from wire rod, the steel layer wires are drawn until an intermediate steel wire diameter. The steel layer wires are then guided through a hot dip bath of zinc-aluminium-magnesium. The thus coated steel layer wires are then further drawn until their final wire diameter. [0039] The steel layer wires are twisted around the steel core wire with the aluminium strip to form a non-compacted steel strand. Thereafter, the steel strand is finally subjected to a compacting treatment.

[0040] Examples of a 1 + 6 steel strand (6 steel layer wires):

- diameters of the steel core wire resp. (steel only): 2.64 mm, 5.60 mm, 6.50 mm;

- diameters of the steel core wire with aluminium strip: 3.00 mm, 6.35 mm, 8.07 mm;

- diameters of the coated and drawn steel layer wires: 2.53 mm, 5.60 mm, 6.46 mm;

- diameter of the compacted steel strand: 7.0 mm, 15.2 mm, 18.0 mm.

[0041] The compacted steel rope according to the invention is preferably subjected to a stress-relieving treatment in order to lower the relaxation and to improve the straightness. A suitable temperature for carrying out such a stress-relieving treatment is 380 °C.

[0042] The compacted steel rope according to the invention can obtain following tensile strength grades: 1700 MPa, 1860 MPa, 1960 MPa and 2060 MPa.

List of Reference Numbers

10 Steel strand

12 Steel core wire

14 Steel layer wire

16 Thick corrosion resistant core coating

18 Thin corrosion resistant layer coating