Description

Technical Field

[0001 ] The invention relates to a steel cord without flare and to a method of

making such a steel cord.

Background Art

[0002] Flare is the phenomenon that after cutting of a steel cord spreading of the filament ends or strand ends is observed. Due to the springy nature of high-carbon steel filaments, residual torsions either on cord level or on filament level or on both are present and cause the spreading if no precautions are taken.

[0003] The spreading of the filament ends or strand ends makes pulling some of the steel cords through inserts or small holes of installations difficult or even impossible. As a consequence, some cord ends require a lot manual labour before being suitable for further processing.

[0004] Applying a false twisting operation may avoid or reduce flare for some steel cord types, but not for all steel cord types.

[0005] In a lot of practices the steel cord ends are welded to assure that the

filaments or strands stick together and do not spread. This welding operation, however, sometimes results in a local thickening of the steel cord ends. And this local thickening may make the insertion of the steel cord in the inserts so difficult that the welded end must be cut away, which may lead - again - to spreading of the freed steel filaments or steel strands. The risk for local thickening is particularly high for small steel cords since the electric power exercised by the welding equipment may result in a large heat affected zone.

Disclosure of Invention

[0006] It is a general object of the invention to avoid the problems of the prior art.

[0007] It is a particular object of the invention to avoid flare at a steel cord end.

[0008] It is another object of the invention to avoid welding of the steel cord ends. [0009] It is yet another object of the invention to provide a robust and simple solution to the flare problem.

[0010] According to a first aspect of the invention, there is provided a steel cord having two ends and comprising steel filaments. At least some of the steel filaments are twisted along the length of the steel cord - except for a local region at at least one of the two ends - with a cord lay length L _c . The steel cord has been additionally cold twisted, i.e. cold over-twisted, in a local region at at least one of the two ends in the same twisting direction of the filaments with the cord lay length L _c , so that locally the steel filaments with an original lay length L _c have a lay length that is less than 60%, e.g. less than 45%, e.g. less than 30%, e.g. less than 20% of the cord lay length L _c in order to avoid flare.

[001 1 ] The terms "a local region at at least one of the two ends" means that the local region comprises the end of the steel cord or is very close to the steel cord end.

The terms "cold twisted" or "cold over-twisted" means that the additional twisting is done at ambient temperature, without extra heat treatment such as welding, glueing, or stress-relieving.

[0012] With respect to the upper limit 60% of the range of the local lay length, it is a good practice to stop over-twisting once flare is avoided. There is no need to over-twist more.

[0013] With respect to the lower limits 20% of the cord lay length L _c , care must be taken that no cord defects occur during over-twisting and that no cord diameter increase occurs.

[0014] The invention has the advantage that no other material needs to be added and that welding and any other heat treatment is be avoided. The invention is a simple measure.

[0015] The over-twisted steel cord end according to the invention has also shown to be more robust than a welded or burnt steel cord end. Indeed, in case the welded or burnt steel cord end hits a hard material or a spool, one or more individual filaments may become loose due to the brittleness introduced during with welding. This is not the case with an over-twisted steel cord end.

[0016] Prior art documents WO 2013/079404 A1 , WO 03/100164 and US 5 238 177 A all disclose metal ropes or cords with a local region where the lay length has been expressly decreased. However, this local decrease of lay length is always combined with a heat treatment such as welding.

[0017] According to a preferable embodiment, the local region has a length that is at least one time, preferably two or three or more times the cord lay length l_c. Usually the length of the local region corresponds to a few centimetre.

[0018] The invention is particularly relevant for steel cords comprising filaments that have a different torsion saturation level. The torsion saturation level of a steel filament is the level at which the residual torsions no longer increase despite the increasing number of applied torsions. The torsion saturation level of a steel filament is dependent upon the filament diameter, the tensile strength and the filament material, i.e. the steel composition. These steel cords often have flare, if no appropriate measures are taken.

[0019] So preferable embodiments of the invention are:

- steel cords with steel filaments with a filament diameter di and steel filaments with a filament diameter 02, where di is substantially different from 02,

- steel cords with steel filaments with a first tensile strength and steel filaments with a second tensile strength, where the first tensile strength is different from the second tensile strength; examples of tensile strength levels are normal tensile strength, high-tensile strength, super-high-tensile strength and ultra-high-tensile strength;

for a filament diameter range between 0.18 mm and 0.28 mm:

- a normal tensile strength has values up to 2400 MPa,

- a high-tensile strength has values ranging from 2400 MPa to 3200 MPa,

- a super-high-tensile strength has values ranging from 3200 MPa to 3800 MPa, - an ultra-high-tensile strength has values ranging from 3800 MPa to 4500 MPa;

- steel cords with steel filaments with a first steel composition and steel filaments with a second steel composition, where the first steel

composition is different from the second steel composition; examples can be a steel composition for normal tensile strength with typically a carbon content of about 0.70 weight per cent and a steel composition for higher tensile strengths with typically a carbon content of 0.80 weight per cent and higher and possibly with micro alloying elements such as chromium in amounts greater than 0.20 weight per cent.

[0020] The invention is particularly appropriate for small steel cords, i.e. steel cords with a limited number of steel filaments, e.g. between two and nine, e.g. between two and six and with filament diameters below 0.40 mm, e.g. below 0.30 mm.

[0021 ] The invention has proved to be particularly useful for a steel cord with a core group and a sheath group. The core group comprises core steel filaments. The sheath group comprises sheath steel filaments. The core steel filaments are untwisted or have a lay length greater than 300 mm - except in the local region. The sheath group and the core group are twisted around each other with a cord lay length - except in said local region. This cord lay length is much smaller than 300 mm, e.g. smaller than 100 mm.

The core steel filaments may have a diameter di and the sheath filaments a diameter 02, where di is substantially different from 02. For example, di is greater than 02.

The core group may have two to four steel filaments.

The sheath group may have one to six steel filaments.

[0022] According to a second aspect of the invention, there is provided a spool filled with a steel cord according to the first aspect of the invention. The local region with the over-twisting is situated at full spool. So only one steel cord end, namely the steel cord end at full spool (not the steel cord end at empty spool) needs to be over-twisted. The reason is that the steel cord end at full spool is the one which is first unwound and led to inserts.

[0023] According to a third aspect of the invention, there is provided a method of manufacturing a steel cord according to the first aspect. The method comprises the following steps:

- twisting at least some of the steel filaments along the length of the steel cord with a cord lay length L _c ;

- determining a local region at at least one of the ends;

- additionally twisting the steel cord in said local region to further locally decrease the lay length in order to avoid flare; this additional twisting is done in a cold state, i.e. without heat treatment such as welding, stress- relieving, glueing, soldering, ...

Brief Description of Figures in the Drawings

[0024] Figure 1 illustrates the prior art practice of welding.

[0025] Figure 2 illustrates the invention on particular steel cord.

[0026] Figure 3 illustrates the general principle of the invention.

[0027] Mode(s) for Carrying Out the Invention

Figure 1 is a longitudinal view of a steel cord 10 with a core group of three steel filaments 12 with a diameter of 0.265 mm and a sheath group of three steel filaments 14 with a diameter of 0.17 mm. The lay length Lc of the cord is 14 mm. According to the prior art practice, one end 16 of the steel cord 10 has been burnt to weld the individual ends of the steel filaments 12, 14 together to avoid spread.

[0028] Figure 2 is a longitudinal view of a steel cord 20 with a core group of three steel filaments 22 with a filament diameter of 0.265 mm and a sheath group of three steel filaments 24 with a filament diameter 0.17 mm. The cord lay length is 14 mm. According to the invention, one end of the steel cord 20 has been over-twisted over a local region 26 of about 3 cm to have locally a lay length of only 3.5 mm. Other specific examples are:

1 ) 2+2x0.32 High Tensile - lay length 16 mm

length local region 3 cm

local lay length 3.5 mm

2) 2x0.25 Ultra High Tensile - lay length 14 mm

length local region 3.2 cm

local lay length 4.8 mm

3) 3x0.30 High Tensile - lay length 16 mm

length local region 3cm

local lay length 3.5 mm

[0029] In practice and in general one tries to limit the degree of over-twisting:

Over-twisting is done until flare is no longer present thereby avoiding introducing cord defects or increase of cord diameter.

[0030] Other similar cord constructions are:

- 2x0.24 + 1 x0.20

- 4x0.20 + 6x0.16

- 2x0.22 + 3x0.16

[0031 ] Figure 3 illustrates the general principle of the invention.

[0032] Figure 3 is a longitudinal view of a steel cord 30 with steel filaments 32.

The lay length of the steel cord is L _c . At one end, over a local region /, the lay length has been reduced to L _e to avoid flare.

[0033] Other steel constructions for which the invention is particularly suitable are:

- 5x0.20

- 3x0.24

- 2x0.30

- 0.22 + 6x0.20

[0034] The steel composition of steel cords adapted for the reinforcement of rubber products such as tyres is along the following lines: a carbon content (% C) ranging from 0.60% to 1 .20%, e.g. 0.80% to 1 .1 %;

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 sulfur content (% S) below 0.03%, e.g. below 0.01 %;

a phosphorus content (% P) below 0.03%, e.g. below 0.01 %,

all mentioned percentages being percentages by weight. Steel filaments adapted for the reinforcement of tyres typically have filaments with a final diameter ranging from 0.05 mm to 0.60 mm, e.g. from 0.10 mm to 0.40 mm. Examples of filament diameters are 0.10 mm, 0.12 mm, 0.15 mm, 0.175 mm, 0.18 mm, 0.20 mm, 0.22 mm, 0.245 mm, 0.28 mm, 0.30 mm, 0.32 mm, 0.35 mm, 0.38 mm, 0.40 mm.

Steel cords adapted for the reinforcement of rubber usually have a coating which promotes the adhesion with rubber such as a brass coating or a ternary alloy coating.