WO2016083091A1

WO2016083091A1 - Off-the-road steel cord

Info

Publication number: WO2016083091A1
Application number: PCT/EP2015/075783
Authority: WO
Inventors: Hongzhen Zhu; Tao Huang; Yuping Wang; Qingyuan WEI
Original assignee: Nv Bekaert Sa
Priority date: 2014-11-27
Filing date: 2015-11-05
Publication date: 2016-06-02
Also published as: CN107075797B; CN107075797A

Abstract

This invention relates to a steel cord adapted for the reinforcement of rubber product. The steel cord (30) comprising a core strand (32) and three or more outer strands (34) twisted around said core strand (32). The outer strands (34) are multi- layer strands comprising multiple steel filaments arranged in an outer strand core (36) and at least one outer strand layer (38, 40), the steel filaments in the outer strands have a round or substantially round cross-section. At least one outer strand (34) comprises flattened portions (52) and non-flattened portions (54) along its length. The flattened portions create spacing S between adjacent outer strands (34) and gaps G between adjacent filaments in the outer strand (34), which improves rubber penetration and elongation of the steel cord (30), and finally improves the impact resistance of the steel cord (30). Besides, the flattened portions (52) limit the flare of the steel cord.

Description

Off-the-road steel cord

Description

Technical Field

[1 ] The invention relates to a steel cord adapted to reinforce rubber products, more specifically for heavy duty tires such as the off-the-road tires and earthmover tires.

Background Art

[2] The large off-the-road pneumatic tires used in heavy construction and

earthmoving operations have operating loads and inflation pressures much higher than conventional trucks and lightweight vehicles. Therefore, both the carcass ply and belt ply of off-the-road tires exhibit tremendous load-carrying capacity and need particular reinforcing cords.

[3] JP10131066A discloses a 7x7 cord to meet this load-carrying requirement.

JP2006104636A further discloses a 1 x(3+9)+6x(3+9) cord wherein the twisting direction of the strands is the same as the twisting direction of the cord.

[4] Besides, another requirement of the off-the-road tire is insuring adequate rubber penetration into the cords, which is achieved during the

manufacture of the belt layers and in subsequent tire vulcanization.

Coupled to this better rubber flow is a desire for higher steel mass and improved wire cut resistance to improve the overall durability of the tire. A further requirement for steel cord reinforcing off-the-road tires is impact resistance capacity, because the surface of off-the-road is not as smooth as that of paved highway. Improved impact resistance capacity not only prolongs the lifetime of the tire but also makes drivers feel more

comfortable when travelling on a bumpy surface.

[5] US2004/0020578A1 discloses a 7x7 cord with different filament diameters to increase the spacing between filaments, which allows better rubber penetration for corrosion resistance and superior cut resistance. However, the increased void area in the cord reduces the area of load-bearing steel filaments, which undermines the load-carrying capacity of the cord.

[6] WO2010/060878A1 discloses a 7x7 cord wherein the outer strands are crimped into wavy form to ensure the spacing between outer strands for rubber penetration. This solution also increases the cord diameter and undermines the load-carrying capacity of the cord.

[7] Besides, to limit the fretting between filaments in the strands and to

improvement efficiency of strand manufacturing, the layers of the strands are twisted in the same direction. But the same twist direction in the strands may result steel cord flare, wherein the filament ends or the strand ends unravel and spread at the cut end of the steel cord.

Disclosure of Invention

[8] The primary object of the invention is to provide a multi-strand steel cord with improved rubber penetration and impact resistance coupled with a maximum load-carrying capacity.

[9] The other object of the invention is to provide a multi-strand steel cord with limited flare at cut end coupled with limited fretting and high manufacturing efficiency.

[10] According to the invention, a steel cord adapted for the reinforcement of rubber product comprises a core strand and three or more outer strands twisted around the core strand. The outer strands are multi-layer strands comprising multiple steel filaments arranged in an outer strand core and at least one outer strand layer. The steel filaments of the outer strand have a round cross-section (the substantially round cross-section which is caused by the damage from the manufacturing process is also included). At least one outer strand comprises flattened and non-flattened portions along its length. At the flattened portion, spacing between adjacent outer strands and the gaps among the steel filaments of the outer strand are created for rubber penetration. This is done without decreasing the amount of steel cross-section. Moreover, the deformation of the strand improves the elongation of the steel cord, and the deformation is very stable. Last but not least, the flattened portions on the steel strands also regulate the free rotation of the strands at the cut ends and limit the flare of the steel cord. Since flare is expressed as the unravelled length in millimetres of filament end or strand end at the cut end, the flare of steel cord is the unravelled length of strand at the cut end while the flare of stand is the unravelled length of filament end at the cut end. The flare of the steel cord may less than the twist pitch of the steel cord, and preferably, the flare of the steel cord may less than 0.3 times the twist pitch of the steel cord.

[1 1] The flattened portions and non-flattened portions present along the length of the steel strand, i.e. the outer strand. Both the "flattened portion" and "non-flattened protion" are sections or segments of the steel strand but with a pre-determined length. The flattened portion is formed by giving a flattening operation to a segment of the steel strand with a certain length, and this means a length of steel strand is flattened, thereby, the cross- section of the flattened portion of the steel strand is no longer round but shaped. When observing the cross-section of the flattened portion of the steel strand, the arrangement of the steel filaments is changed from round or substantially round to non-round, i.e. polygonal shape. However, the steel filaments of the flattened portion of the steel strand are not deformed but still have a round or substantially round cross-section, and the steel strand is still straight or substantially straight but with flattened portion and non-flattened portion. The deformation extent of the cross-section of the flattened protion is depending on the force of flattening operation. "Non- flatteded portion" is opposite to the flattened portion, it is a section of original steel strand without any further deformation including flattenning operation, this means the cross-section of the non-flattened portion is round or substantailly round.

[12] The combination of flatteded portion and non-flattened portion contribute to a stale structure of steel cord. The shape of flatted portion is well maintained due to the existence of the non-flattened portion. [13] Preferably, the flattened portion and non-flattened portion may be located on the outer strand at regular intervals. Thereby the deformation of flattened portion is very stable and maintained well.

[14] At the flattened portion of steel strand, the cross-section of the flattened portion of the steel strand has a major diameter D and a minor diamter d, as the cross-section of the steel strand is not round. Preferably, the ratio between the major diameter D and the minor diameter d of the cross- section of the flattened portion ranges from 1 .2 to2. The major diameter is a diameter of the minimum circumscribed circle (circumcircle) to the cross- section of the flattened portion, and the minor diameter is a diameter of the maximum inscribed circle (incircle) to the cross-section of the flatteded portion.

[15] Preferably, micro-gaps between filaments may be present in the outer strand for rubber penetration.

[16] Preferably, all the outer strands comprise flattened and non-flattened

portions. Preferably, the number of the outer strands ranges from three to nine.

[17] Preferably, the elongation of the steel cord at 20% breaking load may

more than 0.45%, or more than 0.50%. The maximum limit of the elongation of the steel cord at 20% breaking load is around 1 %, because too much deformation on the outer strand may increase the elongation but may reduce the tensile strength of the steel cord.

[18] Preferably, the outer strand may be a 1 +6+12 strand, a 3+9+15 strand, a 4+9+14 strand, or a 3+8+13 strand. Comparatively, 4+9+14 and 3+8+13 strand will have a better rubber penetration than 1 +6+12 and 3+9+15 strand, because there are gaps between the filaments of the strand.

Preferably, in the flattened portion of the outer strand, the rubber penetration rate is higher than 90% or even up to 100%, as the gaps between the steel filaments can be sufficient big.

[19] A steel cord according to the invention may be used as reinforcement for an off-the-road tire, e.g. in the belt ply or the carcass ply of the off-the-road tire.

Brief Description of Figures in the Drawings

[20] The invention will now be described into more detail with reference to the accompanying drawings.

[21 ] Figure 1 shows a cross-sectional view of a 7x27+1 steel cord according to the prior art.

[22] Figure 2 shows a cross-sectional view of a 7x27+1 steel cord according to the present invention.

[23] Figure 3 schematically shows how to determine the major diameter D and minor diameter d of the cross-section of a flattened portion.

[24] Figure 4a and Figure 4b schematically show methods to make flattened portion and non-flattened portion on the strand at intervals.

[25] Figure 5 schematically shows the load-elongation curves of steel cords according to prior arts and present invention respectively.

Mode(s) for Carrying Out the Invention

[26] A typical steel composition for tire cord has a minimum carbon content of 0.65%, a manganese content ranging from 0.40% to 0.70%, a silicon content ranging from 0.15% to 0.30%, a maximum sulphur content of 0.03%, a maximum phosphorus content of 0.30%, all percentages being percentages by weight. There are only traces of copper, nickel and / or chromium. A typical steel tire cord composition for high-tensile steel cord has a minimum carbon content of between 0.80 and 0.85 weight %. To further improve the tensile strength of the steel filaments, the minimum carbon content may range between 0.85 and 0.90 weight %, or even between 0.90 and 0.95 weight %. Besides, other alloy ingredients may be added, for example Cr.

[27] The process to manufacture steel filaments for a steel cord always starts with a wire rod with above steel composition. The wire rod is firstly cleaned by mechanical descaling and / or by chemical pickling in a H2SO4 or HCI solution in order to remove the oxides present on the surface. The wire rod is then rinsed in water and is dried. The dried wire rod is then subjected to a first series of dry drawing operations in order to reduce the diameter until a first intermediate diameter.

[28] At this first intermediate diameter d1 , e.g. at about 3.0 to 3.5 mm, the dry drawn steel wire is subjected to a first intermediate heat treatment, called patenting. Patenting means first austenitizing until a temperature of about 1000 °C followed by a transformation phase from austenite to pearlite at a temperature of about 600 - 650 °C. The steel wire is then ready for further mechanical deformation.

[29] Thereafter the steel wire is further dry drawn from the first intermediate diameter d1 until a second intermediate diameter d2 in a second number of diameter reduction steps. The second diameter d2 typically ranges from 1 .0 mm to 2.5 mm.

[30] At this second intermediate diameter d2, the steel wire is subjected to a second patenting treatment, i.e. austenitizing again at a temperature of about 1000 °C and thereafter quenching at a temperature of 600 to 650 °C to allow for transformation to pearlite.

[31 ] If the total reduction in the first and 2nd dry drawing step is not too big a direct drawing operation can be done from wire rod till diameter d2.

[32] After this second patenting treatment the steel wire is usually provided with a brass coating: copper is plated on the steel wire and zinc is plated on the copper. A thermo-diffusion treatment is applied to form the brass coating.

[33] The brass-coated steel wire is then subjected to a final series of cross- section reductions by means of wet drawing machines. The final product is a steel filament with a carbon content above 0.60 per cent by weight, with a tensile strength typically above 2000 MPa and adapted for the reinforcement of elastomer products. [34] Besides, the brass-coating may contain other ingredients, for example Co, to form a ternary alloy coating comprising Cu, Zn and Co to further improve the adhesion between the steel filaments and the polymer matrix when the steel cord is embedded into a polymer matrix.

[35] Steel filaments adapted for the reinforcement of tires 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. The filaments can be further twisted together to form a strand with a cabling machine or a bunching machine. The strands can be further twisted together to form a multi-strand steel cord.

[36] Figure 1 shows a cross-sectional view of a 7x27+1 steel cord according to prior art. The steel cord 10 comprises a core strand 12 and six outer strands 14 twisted around the core strand 12. Both the core strand 12 and the outer strands 14 are 3+9+15 strands comprising 27 steel filaments arranged in an outer strand core 16 and two outer strand layers 18 and 20. The outer strand core 16 comprises three filaments, the first outer strand layer 18 comprises nine filaments, and the second outer strand layer 20 comprises fifteen filaments. The steel cord 10 further comprises a wrap filament 22 on the surface. Since all the strands 12 and 14 are twisted multi-layer strands, the cross-section of the strands is substantially round.

[37] Figure 2 shows a cross-sectional view of a 7x27+1 steel cord according to the present invention. The steel cord 30 comprises a core strand 32 and six outer strands 34, 34' twisted around the core strand 32. Both the core strand 32 and the outer strands 34, 34' are 3+9+15 strands comprising 27 steel filaments. The 27 steel filaments of an outer strand are arranged in an outer strand core 36 and two outer strand layers 38 and 40. The outer strand core 36 comprises three filaments, the first outer strand layer 38 comprises nine filaments, and the second outer strand layer 40 comprises fifteen filaments. The same is true for a core strand. The steel cord 30 further comprises a wrap filament 42 on the surface. The difference of outer strand 34 and outer strand 34' is: the outer strand 34' has the flattened portion, it is made by giving a flattening operation to the outer strand 34. One outer strand 34' comprises flattened and non-flattened portions along its length. The cross-section of a flattened portion is no longer round but a flattened profile. At the flattened portion, spacing S between adjacent outer strands 34 and 34' is created for rubber penetration. Furthermore the deformation of the strand improves the elongation of the steel cord. Besides, micro-gaps G between filaments are created in the outer strand 34' for rubber penetration. The rubber penetration of the flattened portion of the outer strand 34' is higher than 95%.

[38] Figure 3 schematically shows how to determine the major diameter D and minor diameter d of a cross-section of a flattened portion. Firstly, release the outer strand 34' comprising flattened portions from the steel cord 10, and embed the released outer strands 34' in a polymer matrix. Secondly, at the level of the flattened portion and preferably in the middle of the length where the flattening occurs, cut and polish the specimen, and take the photo of the cross-sectional view of the steel strand 34' with flattened portion under a micro-scope. Thirdly, draw the external common tangent line 42 for adjacent filaments on the second outer strand layer 40, and the common tangent lines 42 make a polygon 44 circumscribing the outer strand 34'. Fourthly, draw the minimum circumscribed circle 46 to polygon 44 to define the major diameter D, and draw the maximum inscribed circle 48 to polygon 44 to define the minor diameter d.

[39] Figure 4a and 4b schematically show the methods to make flattened

portion and non-flattened portion on the strand at intervals. The outer strand 34 passes through a pair of toothed wheels 50, and the teeth of the toothed wheels 50 create the flattened portions 52 on the outer strand 34, while the flattened portions 52 and non-flattened portions 54 are located on the outer strand 34' at intervals. Since the distance between the two toothed wheels is too bigger to crimp the strand into wavy form, the outer strand 34' remains straight but with flattened portions 52 and non-flattened portions 54 at intervals. In figure 4a the toothed wheels 50 are engaged, while in figure 4b the toothed wheels 50 are not engaged and the teeth of one toothed wheel 50 corresponds to those of the other toothed wheel 50. To make sure the match between the teeth of the two toothed wheels 50 in figure 4b, another engaged toothed wheel 56, shown by two circles, with bigger diameter may be installed on the same axis of the toothed wheels 50. The toothed wheels 50 are not driven by external means, but driven and rotated by the passing outer strand 34. Further, other devices deviating from the toothed wheels 50 as shown in figure 4a and 4b may also work. For example, a pair of eccentric rollers can also create flattened portions 52 and non-flattened portions 54 along the passing strand 34. One of the toothed wheels 50 or the eccentric rollers can be replaced with a roller, and the remaining toothed wheel 50 or the eccentric roller can create flattened portions 52 and non-flattened portions 54 along the passing strand 34 against the roller.

Below table shows the manufacturing data and mechanical properties of steel cords according to prior arts and present invention respectively.

Prior art Present invention steel cord 10 steel cord 30

Steel cord structure 7x27+1 7x27+1

Filament diameter(mm) 0.175 0.175

Core strand twist pitch (mm) 5/10/16 5/10/16

Core strand twist direction S/S/S/ S/S/S

Outer strand twist pitch (mm) 5/10/16 5/10/16

Outer strand twist direction z/z/z Z/Z/Z

Steel cord twist pitch (mm) 38 38

Steel cord twist direction S S

Wrap filament twist pitch (mm) 5 5

Wrap filament twist direction Z Z

Breaking load (N) 12124 1 1923

Elongation at 20% breaking load 0.354% 0.486% Figure 5 schematically shows the load-elongation curves of steel cords according to prior arts and present invention respectively. Curve 60 is the load-elongation curve for the 7x27+1 steel cord 30 according to present invention, while curve 62 is the load-elongation curve for the 7x27+1 steel cord 10 according to prior art. Compared with the reference cord 10, the steel cord 30 according to present invention has an improvement on elongation at 20% breaking load by 37%. Besides, steel cord according to present invention has improvement on impact resistance. In Figure 5, the area limited by the load-elongation curve, the horizontal axis E%, and the vertical line for load force at break, represents the energy absorbed by the steel cord. Comparatively, steel cord 30 according to present invention can absorb more energy than prior art steel cord 10.

Claims

1 . A steel cord adapted for the reinforcement of rubber products,

said steel cord comprising a core strand and three or more outer strands twisted around said core strand,

said outer strands are multi-layer strands comprising multiple steel filaments arranged in an outer strand core and at least one outer strand layer, said steel filaments of said outer strands have a round or substantially round cross- section, characterized in that

at least one outer strand comprises flattened and non-flattened portions along its length.

2. A steel cord according to claim 1 , wherein the flare of the steel cord is less than the twist pitch of the steel cord.

3. A steel cord according to claim 2, wherein the flare of the steel cord is 0.3

times the twist pitch of the steel cord.

4. A steel cord according to claim 1 , wherein said flattened portions and non- flattened portions are located on said outer strand at regular intervals.

5. A steel cord according to claim 1 , wherein the ratio between the major diameter D and the minor diameter d of the cross-sections of said flattened portions is between 1 .2 and 2.

6. A steel cord according to claim 1 , wherein micro-gaps between filaments are present in said at least one outer strand.

7. A steel cord according to any preceding claims, wherein all of said outer

strands comprise flattened and non-flattened portions along their lengths.

8. A steel cord according to claim 1 , wherein the elongation of the steel cord at 20% breaking load is more than 0.45%.

9. A steel cord according to claim 8, wherein the elongation of the steel cord at 20% breaking load is more than 0.50%.

10. A steel cord according to claim 1 , wherein the outer strands are 1 +6+12 strands.

1 1 .A steel cord according to claim 1 , wherein the outer strands are 3+9+15 strands.

12. A steel cord according to claim 1 , wherein the outer strands are 4+9+14 strands.

13. A steel cord according to claim 1 , wherein the outer strands are 3+8+13 strands.