The present invention relates to a steel cord. The steel cord is a multi-strand steel cord, i.e. a steel cord comprising more than one strand, and each strand comprises more than one steel filament or a single strand or layered steel cord. The invention also relates to various uses of the steel cord.
U.S. Pat. No. 4,651,513 describes a steel cord for reinforcing rubber products comprising two or more successive wire layers of which an inward layer comprises wires coated a corrosion resistant coating and the outer surface layer comprises wires with a rubber adherable coating such as brass. The referenced corrosion resistant coatings in this application are zinc or a zinc binary or ternary alloy that comprises at least 50 wt % zinc. These coatings are alternatives i.e. the application does not mention that the coating can consist of zinc together with a zinc-alloy on the same wire.
EP-B1-1280958 discloses a steel cord adapted for the reinforcement of thermoplastic elastomers. The steel cord is a multi-strand steel cord. At least some the steel filaments have a zinc-iron alloy layer and on top of this zinc-iron alloy layer a separate layer of mainly zinc. The thickness of the separate top layer of zinc—not including the alloy layer—is smaller than two micrometer. This intermediate layer of a zinc-iron alloy and a relatively thin top layer of a zinc layer are obtained by means of a hot dip operation. The steel filaments are dipped into a bath of molten zinc. Instead of leaving the bath vertically, the filaments leave the bath under a small angle with respect to a horizontal line and a great amount of zinc is wiped off mechanically.
As mentioned in EP-B1-1280958, the resulting steel cords with such steel filaments have several advantages. First of all, due to the thin zinc layer, there are only a small number of separate zinc particles and less zinc dust is created in the processing of the steel cords. The reduction of zinc particles and of zinc dust increases the adhesion level. Secondly, due to the zinc-steel alloy layer, corrosion resistance is still much better than in the case of steel filaments which have been coated with zinc by means of an electrolytic deposition method. Thirdly, due to the zinc layer and zinc-iron alloy layer being also thinner, the level of fatigue resistance has significantly increased. Steel cords according to EP-B1-1280958 have given satisfactory results not only on a lab scale but on a wide scale in various industrial applications.
This wide commercial use, however, has also highlighted some points which are open for improvement.
First of all, although very thin, there is still zinc at the surface and zinc is known as difficult to twist in a downstream operation. Either the speed of twisting is seriously reduced, or lubrication becomes unavoidable. After the twisting process, the added lubricants need to be removed, since the presence of these lubricants would be at the detriment of the adhesion level in a polymer or elastomer matrix. Experience has shown, however, that complete removal of the lubricants is costly and time consuming.
Secondly, the presence of zinc at the surface, may lead to processability problems at the customer. An example is the extrusion of polymer strips around steel cords, if the steel cords have to pass through small openings before entering an extrusion apparatus, the steel cords rub against the wall of the openings; zinc becomes loose, heaps up locally and eventually blocks the whole processing. As will be described hereunder, the strips may show dark spots indicating the presence of zinc dust or may even loose their planar character. In extreme cases the steel cords were broken due to zinc dust blocking the extrusion dies.
It is a general aspect of the present invention to avoid the drawbacks of the prior art.
It is a first particular aspect of the present invention to facilitate the drawing of coated steel filaments.
It is a second particular aspect of the present invention to increase the level of adhesion.
It is a third particular aspect of the present invention to increase the processability of the steel cords.
Viewed from a first and broad perspective, the present invention provides a steel cord. The steel cord comprises more than one steel filament. At least some of the steel filaments have a zinc-iron alloy layer and, possibly, a zinc cover partially covering the zinc-iron alloy layer. Preferably the zinc cover is present in valleys in the zinc-iron alloy layer. The invention is featured by this zinc-iron alloy layer occupying more than fifty percent of the total volume of zinc-iron layer and zinc cover the steel filaments.
In a preferably embodiment of the invention, the zinc-iron alloy layer occupies more than 60%, e.g. more than 75%, e.g. more than 90%, e.g. more than 95% of the total volume of zinc-iron alloy and zinc cover. In other terms, the zinc-iron alloy layer occupies the majority of the volume of the coating.
Viewed from a second more detailed perspective, the present invention provides a steel cord. The steel cord comprises more than one steel filament. At least some of the steel filaments have a zinc-iron alloy layer and, possibly, a partial zinc cover above the zinc-iron alloy layer. The invention has the feature that the free surface of the zinc-iron alloy layer occupies more than fifty percent of the outer surface of said steel filaments. With ‘free surface of the zinc-iron alloy layer’ is meant that part of the surface of the filament where the zinc-iron layer is accessible from the outside of the filament i.e. is substantially uncovered or is visible from the outside. In a preferably embodiment of the invention, the free surface of the zinc-iron alloy layer occupies more than 60%, e.g. more than 75%, e.g. more than 90%, e.g. more than 95% of the outer surface of said filaments. It follows that the ‘pure’ zinc is only present in a minority of valleys, the majority of the outer surface of the filaments showing an iron-zinc alloy layer.
The measurement of volume occupied and surface exposed is done by means of the standard techniques of the metallurgist. To this end a filament is embedded in an epoxy matrix. A cross section substantially perpendicular to the axis of the filament is made and the section is carefully polished. By means of nital (a solution of about 2% nitric acid in alcohol that is well known to the metallurgist) the surface is slightly etched. After proper cleaning the section is observed under the optical microscope equipped with a suitable CCD camera that is connected to a computer for further numerical processing of the frames. The difference between steel, zinc-iron alloy layer and pure zinc can be clearly discerned after choosing the appropriate magnification and can be selected from the frame by the software. The ratio of pure zinc volume over total volume of pure zinc and zinc-iron alloy can be determined by calculating the ratio of the surface area in cross section of the pure zinc to the total surface area in cross section of the pure zinc together with the zinc-iron alloy layer. As no variations in zinc coating are generally observed in the longitudinal direction along the filament (given is method of production, see further), this ratio is only subject to minute variation in the length.
In the same manner the free surface of zinc-iron alloy can be measured by identifying on the frame those line sections that delineate the transition from the alloy layer to the epoxy, summing the line sections and dividing them by the overall length of the epoxy wire transition.
When the coating comes very thin, the frame analysis procedure can equally well be based on a Scanning Electron Microscope (SEM) picture in the same manner. The SEM allows for easy elemental analysis and the different layers (zinc-iron vs. pure zinc) can be discriminated in this way.
The avoidance of a continuous layer of zinc at the outer surface and the presence of iron-zinc alloy at the outer surface, offers a lot of advantages to the steel cord.
The reduced amount of ‘pure’ zinc together with the presence of a hard zinc-iron alloy layer at the surface result in a further reduction in the amount of zinc dust and zinc particles. Hence, the adhesion anchorage in a polymer or elastomer matrix can be further increased.
Another advantageous result is that processability problems, such as clogging of the extrusion dies or dark spots in extrusion strips are avoided or at least further reduced. It is hereby understood that the iron-zinc alloy layer at the surface adheres very well to the steel core of the steel filament and does not lead to zinc dust or zinc particles.
Viewed from a third perspective, the invention provides various uses or applications of the steel cord.
The steel cord may be used as an elevator rope. The steel cord may also be used as a window elevator rope. These ropes may be coated by means of a polymer or elastomer.
The steel cord may be used as a reinforcement of a thermoplastic elastomer or polymer, a vulcanisable rubber or a thermoset. The final product may then be a strip, a flexible pipes, a hose or a tire. The steel cord may be used as a reinforcement of concrete or for retrofitting of existing concrete structures.
The invention will now be described into more detail with reference to the accompanying drawings wherein
a gives a cross-section of a steel filament together with an enlarged view of part of this cross-section;
b gives an enlarged top view of a steel filament;
a and
a gives a cross-section of a steel filament 10 used in a steel cord according to the invention.
The manufacturing process of a steel filament with a cross-section as illustrated in
The steel filaments are made from wire rod with a steel composition which is along the following lines: a carbon content ranging from 0.30% to 1.15%, a manganese content ranging from 0.10% to 1.10%, a silicon content ranging from 0.10% to 0.90%, sulfur and phosphorous contents being limited to 0.15%, preferably to 0.10% or even lower; additional micro-alloying elements such as chromium (up to 0.20%-0.40%), copper (up to 0.20%) and vanadium (up to 0.30%) may be added.
The steel rod is cold drawn to the desired filament diameters. The subsequent cold drawing steps may be alternated by one or more suitable thermal treatments such as patenting, in order to allow for further drawing.
An iron-zinc alloy layer 14 can be obtained if, in contrast with an electrolytic deposition method of zinc, the steel wire is zinc coated by means of a hot dip operation. In a hot dip operation the steel wire travels through a bath of molten zinc and leaves the bath zinc coated.
The time of immersion and the temperature of the molten zinc determines the thickness of the iron-zinc alloy layer. The longer the immersion time or the higher the temperature of the molten zinc, the thicker the iron-zinc alloy layer 14.
In the context of the present invention the term ‘zinc’ refers to 100% pure zinc or to zinc alloys or zinc compositions with impurities or additional elements in such amounts that the creation and growth of a substantial iron-zinc alloy layer is not prevented.
As a first method of manufacturing and in analogy with EP-B1-1280958, the steel wire may leave the bath under a small angle with respect to a horizontal line and the leaving steel wire is wiped mechanically. In difference with EP-B1-1280958, however, the mechanical wiping is carried out twice in series.
Alternatively, as a second method of manufacturing, the mechanical wiping may be carried out under an increased pressure. This intense mechanical wiping reduces the amount of zinc 16.
As a third method of manufacturing, the cooling which is normally applied upon the wire leaving the zinc bath, is left out or is applied in a less intensive way, so that the growth of the iron-zinc alloy layer is not stopped immediately.
As a fourth method of manufacturing, the temperature of the zinc bath is increased in order to increase the speed of growth of the zinc-iron alloy layer.
The thus coated steel wire can be further drawn, e.g. by means of a cold drawing process, to the desired final diameter. The drawing smears out the zinc remaining and guarantees a longitudinally constant amount of zinc coating per unit of surface area.
Two or more filaments are then twisted into a steel cord, or in case of multi-strand steel cords, into a strand and two or more stands can be twisted into a final multi-strand steel cord. The twisting process can be done by means of tubular twisting machines or by means of double-twisting machines.
Possible configurations are:
7×7×0.175 10/14 SZ (i.e. all filaments have the same diameter)
and
d1+6×d2+6×(d2+6×d3) P1P2 SZ
with d1>1.05×d2 and
d2>1.05×d3,
where
Due to the difference in filament diameters, this latter configuration has the advantage of having both open strands and a more open steel cord. This openness is advantageous for mechanical anchoring of the steel cord in a matrix material such as a thermoplastic material or an elastomer.
Following examples are here given by way of illustration:
0.21+6×0.19+6×(0.19+6×0.175)
0.25+6×0.23+6×(0.23+6×0.21)
0.26+6×0.24+6×(0.24+6×0.22)
0.39+6×0.34+6×(0.34+6×0.30)
Another suitable construction has as general formula 19+8×7. Following examples are given by way of illustration:
(0.19+18×0.17)+8×(0.16+6×0.16) (compact core);
(0.19+18×0.17)+8×(0.17+6×0.155) (compact core);
(0.17+6×0.16+6×0.17+6×0.13)+8×(0.14+6×0.14) (Warrington core);
(0.17+6×0.16+6×0.17+6×0.13)+8×(0.15+6×0.14) (Warrington core);
(0.155+6×0.145+12×0.145)+8×(0.14+6×0.14).
a and
The adhesion level of an invention cord has been compared with the adhesion level of a prior art cord. Both cords are of the following formula: 7×3×0.15. The invention cord and the prior art cords are embedded in a polyurethane matrix over an embedment length of 25 mm. The pull-out force, i.e. the force needed to pull the steel cords out of the polyurethane matrix, is a measure for the adhesion level and is recorded. The following table mentions the relative values of these pull-out forces.
Steel cord 70 corresponds to formula d1+18×d2.
Number | Date | Country | Kind |
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07102605 | Feb 2007 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2008/051638 | 2/12/2008 | WO | 00 | 8/12/2009 |
Publishing Document | Publishing Date | Country | Kind |
---|---|---|---|
WO2008/101831 | 8/28/2008 | WO | A |
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4651513 | Dambre | Mar 1987 | A |
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5768874 | Bourgois et al. | Jun 1998 | A |
5784874 | Bruyneel et al. | Jul 1998 | A |
6920745 | Bruyneel et al. | Jul 2005 | B2 |
7191585 | Vanderbeken et al. | Mar 2007 | B2 |
Number | Date | Country |
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1 280 958 | Feb 2003 | EP |
Number | Date | Country | |
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20100031623 A1 | Feb 2010 | US |