Steel cord with reduced residual torsions

Abstract

A steel cord for reinforcing a breaker or belt ply in a rubber tire having a core group and a sheath group. The core group consists of two to four core steel filaments with a first diameter dc and the sheath group consists of one to six sheath steel filaments with a second diameter ds. The ratio dc/ds of the first diameter dc to the second diameter ds ranges from 1.10 to 1.70. The two core steel filaments are untwisted or have a twisting step greater than 300 mm. The sheath group is twisted around the core group with a cord twisting step in a cord twisting direction. The ratio of the difference in residual torsions of the core group and the sheath group to the difference in saturation level between the core group and the sheath group ranges from 0.10 to 0.65, preferably from 0.10 to 0.60.

Claims

1. A steel cord adapted to reinforce a breaker or belt ply in a rubber tire, said steel cord comprising a core group and a sheath group, said core group consisting of two to four core steel filaments with a first diameter d.sub.c, said sheath group consisting of one to six sheath steel filaments with a second diameter d.sub.s, the ratio d.sub.c/d.sub.s of said first diameter d.sub.c to said second diameter d.sub.s ranging from 1.10 to 1.70, said core steel filaments being untwisted or having a twist pitch of greater than 300 mm, said sheath group being twisted around said core group with a cord twisting step in a cord twisting direction, wherein the ratio of the absolute value of the difference in residual torsions between the core group and the sheath group to the absolute value of the difference in saturation level between the core group and the sheath group ranges from 0.15 to 0.65.

2. The steel cord according to claim 1, wherein the amount of residual torsions of said core group is different from the amount of residual torsions of said sheath group.

3. The steel cord according to claim 1, wherein each sheath filament is twisted by itself.

4. The steel cord according to claim 1, wherein said one to six sheath steel filaments are twisted around each other with said cord twisting step and in said cord twisting direction.

5. The steel cord according to claim 1, wherein each of said core steel filaments has a wave height h.sub.c ranging from 2.2xd.sub.c to 2.7xd.sub.c.

6. The steel cord according to claim 1, wherein each of said sheath filaments has a wave height h.sub.s ranging from 2.2xd.sub.s to 3.9xd.sub.s.

7. The steel cord according to claim 1, wherein said steel cord has no flare.

8. The steel cord according to claim 1, wherein said steel cord has a tensile strength exceeding 2500 MPa.

9. A rubber ply comprising a plurality of steel cords according to claim 1, said steel cords being arranged in parallel next to each other, said rubber ply having a tip rise being lower than 30 mm.

10. The steel cord according to claim 1, wherein the ratio of the absolute value of the difference in residual torsions ranges from 0.25 to 0.50.

11. The steel cord according to claim 1, wherein said steel cord has a tensile strength exceeding 2700 MPa.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic drawing of the equipment and process for making a steel cord according to the first aspect of the invention;

(2) FIG. 2a shows torsion diagrams of a core steel filament and a sheath steel filament in a double twister followed by a single false twister;

(3) FIG. 2b shows torsion diagrams of a core steel filament and a sheath steel filament in a double twister followed by a double false twister;

(4) FIG. 3 illustrates the influence of a double false twister on tip rise of a rubber ply;

(5) FIG. 4a, FIG. 4b, FIG. 4c and FIG. 4d show cross-sections of a steel cord according to the first aspect of the invention;

(6) FIG. 5 shows a longitudinal view of a steel cord according to a first aspect of the invention;

(7) FIG. 6 shows a rubber ply.

DETAILED DESCRIPTION OF THE INVENTION

(8) A steel cord according to the first aspect of the invention may be made in the following way.

(9) Starting material may be a steel wire rod with a minimum carbon content of 0.65%, e.g. a minimum carbon content of 0.75%, a manganese content ranging from 0.40% to 0.70%, a silicon content ranging from 0.15% to 0.30%, a maximum sulfur content of 0.03%, a maximum phosphorus content of 0.30%, all percentages being percentages by weight. Micro-alloying elements such as chromium and copper, with percentages going from 0.10% up to 0.40% are not excluded, but are not needed.

(10) The wire rod is firstly cleaned by mechanical descaling and/or by chemical pickling in a H.sub.2SO.sub.4 or HCl 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.

(11) At this first intermediate diameter d.sub.1, 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 C.-650 C. The steel wire is then ready for further mechanical deformation.

(12) Thereafter the steel wire is further dry drawn from the first intermediate diameter d.sub.1 until a second intermediate diameter d.sub.2 in a second number of diameter reduction steps. The second diameter d.sub.2 typically ranges from 1.0 mm to 2.5 mm.

(13) At this second intermediate diameter d.sub.2, 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 C. to 650 C. to allow for transformation to pearlite.

(14) If the total reduction in the first and second dry drawing step is not too big a direct drawing operation can be done from wire rod till diameter d.sub.2.

(15) 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.

(16) 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 0.65 percent by weight (e.g. above 0.75 percent by weight), with a tensile strength typically above 2000 MPa (e.g. above 2500 MPa) and adapted for the reinforcement of elastomer products.

(17) For the manufacture of a steel cord according to the present invention two different steel filament diameters are required, e.g. 0.16, 0.17 or 0.20 mm steel filaments and 0.22, 0.24 and 0.265 mm steel filaments.

(18) FIG. 1 gives an overview of an equipment 100 which may be used to make a steel cord according to the invention.

(19) Starting from the left side of FIG. 1, three core steel filaments 102 with a filament diameter of d.sub.c are drawn from two supply spools 104 and guided to a double-twister or buncher 106. After passing a first stationary guiding pulley 108 the three core steel filaments 102 receive a first twist in the Z-direction due to the rotation direction 109 of a first flyer 110. Just before going over stationary reversing pulley 112, the three core steel filaments 102 receive a second twist in the Z-direction. The thus twisted core steel filaments 102 are then guided to an assembly point 113.

(20) Three sheath steel filaments 116 with a filament diameter of d.sub.s are drawn from three supply spools 118 which are located in a stationary cradle (not shown) inside the double-twister 106. The three sheath steel filaments 116 are brought together with the three twisted core steel filaments 102 at the assembly point 113. At the level of the second stationary reversing pulley 114, both the core steel filaments 102 and the sheath steel filaments 116 receive a twist in the S-direction. This means that the three core steel filaments 102 are partially untwisted (from 2xZ-twists to one Z-twist) while the sheath steel filaments 116 are twisted. The assembly of two core steel filaments 102 and three sheath steel filaments 116 is guided over a second flyer 120 to a second stationary guiding pulley 122. At the level of the second stationary guiding pulley 122 the assembly receives a second twist in the S-direction. This means that the three core steel filaments 102 are now completely untwisted (from one Z-twist to zero) and that the three sheath steel filaments 116 have now been twisted twice in S-direction.

(21) The resulting product leaving the double-twister 106 is a steel cord with a core group and a sheath group. The core group consists of three untwisted core steel filaments 102. The sheath group has three S-twisted sheath steel filaments 116. The sheath group is twisted in S-direction around the core group. This is a complete steel cord but not yet with all the features according to the invention.

(22) The steel cord leaves the double-twister 106 and is led through a first false twister 124 which rotates in a direction 126 opposite to the rotation direction of the double-twister 106. The effect of this first false twister 124 will be explained with reference to FIG. 2a and FIG. 2b.

(23) Subsequently the steel cord is also led to a second false twister 128 which rotates in a direction 130 opposite to the rotation direction of the first double-twister 124. The effect of this second false twister 128 will be explained with reference to FIG. 2b.

(24) Finally a steel cord 132 possessing all the features of a steel cord according to the invention leaves the second false twister 128 and is wound upon a cord spool 134.

(25) FIG. 1 also shows various positions a-b-c-d-e-f-g-h along the path followed by either the core steel filaments 102 and the sheath steel filaments 116 or both.

(26) FIG. 2a and FIG. 2b show torsion diagrams with mention of: a-b-c-d-e-f-g-h: this corresponds each time with the torsion level of a core steel filament at the various positions a-b-c-d-e-f-g-h in FIG. 1; a-b-c-d-e-f-g-h: this corresponds each time with the torsion level of a sheath steel filament at the various positions a-b-c-d-e-f-g-h in FIG. 1.

(27) FIG. 2a shows the torsion curve 200 of a core steel filament 102 being double-twisted and going through a single false twister 124 and the torsion curve 202 of sheath steel filament 116 being double-twisted and going through a single false twister 124.

(28) The abscissa shows the applied torsions (number of revolutions per meter): S in the right direction, Z in the left direction.

(29) The ordinate shows the residual torsions (number of revolutions per meter): Z in direction upwards, S in direction downwards.

(30) Dash line 204 shows the torsion saturation level (number of revolutions per meter) of a core steel filament 102.

(31) Dot and dash line 206 shows the torsion saturation level (number of revolutions per meter) of a sheath steel filament 116.

(32) The torsion saturation level 204 of a core steel filament is lower than the torsion saturation level 206 of a sheath steel filament, since the core steel filament is thicker and reaches quicker the plastic deformation zone.

(33) Still referring to FIG. 2a, and following torsion curve 200, a core steel filament 102 receives a first Z-twist at position a and a second Z-twist at position b. At position c the core steel filament 102 is partially untwisted because of a first S-twist. At position d, the core steel filament leaves the double-twister untwisted, i.e. with zero applied twists, because of a second S-twist. The core steel filament 102 is then sent to a false twister 124, where it receives first twists in S-directionpoint eand immediately thereafter twists in Z-direction to arrive at point f, with zero applied twists but with +3 residual revolutions per meter.

(34) Still referring only to FIG. 2a, and following torsion curve 202, sheath steel filament 116 receives a first S-twist at c and a second S-twist at d when leaving the double-twister 106. Sheath steel filament 116 is then guided through false twister 124 where it receives first additional twists in S-directionpoint eand immediately thereafter twists in Z-direction to arrive at point f, with a number of applied torsions corresponding to the desire lay length or cord twisting step and with 4.5 residual revolutions per meter.

(35) With one false twister 124, the difference in residual torsions between a core steel filament 102 and a sheath steel filament 116 is 7.5 residual revolutions per meter.

(36) This high difference in residual torsions per meter causes instability in the steel cord and requires a high deformation degree of the core steel filaments in order to anchor the steel cord in a rubber ply and to prevent tip rise of a rubber ply reinforced with this steel cord.

(37) The improvement of the invention is explained with reference to FIG. 2b.

(38) Curve 208-210 is the torsion curve of a core steel filament 102. Part 208 is the part with only one false twister 124, the dash part 210 is the part with an additional second false twister 128.

(39) Core steel filament 102 receives a first Z-twist at position a and a second Z-twist at position b. At position c the core steel filament 102 is partially untwisted because of a first S-twist. At position d, the core steel filament leaves the double-twister untwisted, i.e. with zero applied twists, because of a second S-twist. The core steel filament 102 is then sent to a first false twister 124, where it receives first twists in S-directionpoint eand immediately thereafter a first series of twists in Z-direction because of first false twister 124 and a second series of twists in Z-direction because of second false twister 128points f-g. Finally the second series of twists in Z-direction are compensated by twists in S-direction (action of second false twister 128) to arrive at point h with zero applied twists andonly +1.8 residual revolutions per meter.

(40) Curve 212-214 is the torsion curve of a sheath steel filament 116. Part 212 is the part with only one false twister 124, the dash part 214 is the part with an additional second false twister 128.

(41) Sheath steel filament 116 receives a first S-twist at c and a second S-twist at d when leaving the double-twister 106. Sheath steel filament 116 is then guided to false twister 124 where it receives first additional twists in S-directionpoint e. Thereafter, sheath steel filament 116 receives a first series of Z-twists (action of first false twister 124) and a second series of Z-twists (action of second false twister 128)points f-g. Finally the second series of Z-twists are compensated by a series of S-twists (action of second false twister 128) to arrive at point h, with a number of applied twists corresponding to the desired lay length or cord twisting step and with 2.5 residual revolutions per meter.

(42) The number of residual torsions is determined per group, i.e. the number of residual torsions is determined for the core group as a whole andseparatelyfor the sheath group as a whole.

(43) To determine the number of residual torsions per group a 4 meter length steel cord sample is taken. All residual cord torsions are first released. This 4 meter sample is fixed between two clamps which have an interdistance of 100 cm. The clamps have a rubber path in contact with the steel cord to avoid damage to the steel cord.

(44) The purpose is to determine the number of residual torsions over this 100 cm length.

(45) Outside the clamps, the steel cord is cut but leaving a length of about 10 cm. At one end, outside the clamps, the steel cord is plastically bent so that a length of about 5 cm points vertically upwards. The number of rotations of this bent part will indicate the number of residual torsions per meter.

(46) For the determination of the residual torsions of the core group, one end of the steel cord is unclamped. The sheath steel filaments are unravelled by means of a gripper until past the clamp, while the bent part of the core group is kept vertical. Thereafter the core group is clamped again and the sheath steel filaments are unravelled until the second clamp. Now one is ready to determine the residual torsions in revolutions per meter of the core group: the first clamp is released again while holding the bent part of the core group vertical and thereafter the bent part is released and its number of rotations is counted.

(47) For the determination of the residual torsions of the sheath group, one end of the steel cord is unclamped. The sheath steel filaments are unravelled by means of a gripper not only until past the first clamp but until the second clamp, while the gripper is kept horizontal so that the bent part of the sheath steel filaments is also kept stable. Once the unravelling has been done until the second clamp, one is ready to determine the residual torsions of the sheath group in revolutions per meter: the gripper releases the sheath group and the number of rotations of the bent part of the sheath group is counted.

(48) With two false twisters 124, 128 the difference in residual torsions between the core group and the sheath group has been reduced to 4.3 residual revolutions per meter. This is a much more stable cord without flare and causing no tip rise in a rubber ply without having to deform the core steel filaments heavily.

(49) FIG. 3 illustrates the influence of a double false twister on tip rise of a rubber ply. The abscissa axis gives the rotation speed of the second false twister 128 in percentage. The ordinate gives the tip rise T of a rubber ply reinforced with steel cords in millimetre. Curve 30 is for a wave height h.sub.c of the core steel filaments of 2.7xd.sub.c while curve 32 is for a wave height h.sub.c of the core steel filaments of 1.6xd.sub.c.

(50) As a matter of example, the tip rise T can be limited to 10 mm with a wave height h.sub.c of 2.7xd.sub.c and a rotation speed of 35%. Increasing the rotation speed to 75% may reduce the wave height h.sub.c to 0.36 mm without increase of tip rise T.

(51) FIG. 4a, FIG. 4b, FIG. 4c and FIG. 4d show various cross-sections of a steel cord 132 according to the first aspect of the invention.

(52) Referring to FIG. 4a, steel cord 132 has a core group of three parallel core steel filaments 102 each with a filament diameter d.sub.c. Steel cord 132 further has a sheath group of three twisted sheath steel filaments 116 each with a filament diameter d.sub.s. Due to the fact that the three core steel filaments 102 are untwisted the cord 132 has an oval cross-section with a major axis or major diameter D.sub.maj and a minor axis or minor diameter D.sub.min.

(53) FIG. 4b is a cross-section of the same steel cord 132 but at a distance of of a cord twisting step from the situation of FIG. 4a.

(54) FIG. 4c is a cross-section of the same steel cord 132 but at a distance of of a cord twisting step from the situation of FIG. 4a.

(55) FIG. 4d is a cross-section of the same steel cord 132 but at a distance of of a cord twisting step from the situation of FIG. 4a.

(56) As a result of the double-twisting process in the double-twister 106, the sheath steel filaments 116 are not only twisted around each other but each sheath steel filament 116, as such, also shows a twist in the same direction and to the same degree around its own longitudinal axis.

(57) FIG. 5 is a longitudinal view of a steel cord 132 according to the invention. The wave height h.sub.c of the core steel filaments 102 is the amplitude formed by the wave of the core steel filaments 102 including the diameter of the core steel filament(s).

(58) As has been explained hereabove, thanks to the action of the double false twister 128, the difference in residual torsions between the core steel filaments 102 and the sheath steel filaments 116 can be reduced. As a result of this reduction the wave height h.sub.c can also be reduced leading to a more stable and closed structure and without causing flare or tip rise.

(59) FIG. 6 shows a rubber ply 60 which has been reinforced with steel cords 132 and which has been cut to become part of a breaker or belt ply in a tyre. The rubber ply 60 does not exhibit tip rise, i.e. edge 62 is not lifted.

(60) Comparison of Prior Art Cords Versus Invention Cords

(61) TABLE-US-00001 2 0.24 + 4 0.20 + 2 0.22 + 3 0.265 + Cord 1 0.20 HT 6 0.16 ST 3 0.16 ST 3 0.17 UT m 2 4 2 3 n 1 6 3 3 dc (mm) 0.24 0.2 0.22 0.265 ds (mm) 0.2 0.16 0.16 0.17 dc/ds 1.2 1.3 1.4 1.6 Rm core group (MPa) 3320 3580 3540 3870 Rm sheath group (MPa) 3400 3660 3660 4060 SLc (revolutions/m) 38.4 49.7 44.7 40.6 SLs (revolutions/m) 47.2 63.5 63.5 66.3 | SLc SLs | 8.8 13.8 18.8 25.7 process No DFT DFT No DFT DFT No DFT DFT No DFT DFT RTc (revolutions/m) 1.4 0.7 4.3 2.1 4.1 2.1 2.7 1.3 RTs (revolutions/m) 5.7 2.8 6.9 3.3 9.7 4.9 15.9 7.7 | RTc RTs | 7.0 3.5 11.2 5.4 13.8 7.0 18.6 9.0 ratio p 0.8 0.4 0.81 0.39 0.73 0.37 0.72 0.35

(62) m: number of filaments in core group

(63) n: number of filaments in sheath group

(64) dc: diameter of core steel filaments

(65) ds: diameter of sheath steel filaments

(66) Rm: tensile strength of steel filaments

(67) No DFT: prior art process without double false twister

(68) DFT: invention process with double false twister

(69) Factor : depends upon tensile strength level

(70) Ratio : ratio of difference in torsion gap measured to difference in sauration level

(71) SLc: saturation level core group

(72) SLs: saturation level sheath group

(73) RTc: Residual torsions of core group

(74) RTs: residual torsions of sheath group

(75) HT: high-tensile strength

(76) ST: super-high-tensile strength

(77) UT: ultra-high-tensile strength

(78) A high-tensile (HT) strength means a steel filament with a tensile strength between 3800-2000d MPa and 4000-2000d MPa, where d is the filament diameter and is expressed in mm.

(79) A super-high-tensile (ST) strength means a steel filament with a tensile strength between 4000-2000d MPa and 4400-2000d MPa, where d is the filament diameter and is expressed in mm.

(80) An ultra-high-tensile (UT) strength means a steel filament with a tensile strength above 4400-2000d MPa.

LIST OF REFERENCE NUMBERS

(81) 100 equipment to make a steel cord according to the invention 102 core steel filament 104 supply spool of core steel filament 106 double-twister 108 stationary guiding pulley 109 rotating direction of double-twister 110 first flyer 112 first stationary reversing pulley 113 assembly point 114 second stationary reversing pulley 116 sheath steel filament 118 supply spool of sheath steel filament 120 second flyer 122 second stationary guiding pulley 124 first false twister 126 direction of rotation of first false twister 128 second false twister 130 direction of rotation of second false twister 132 steel cord 134 cord spool for winding steel cord 200 torsion curve of core steel filament with single false twister 202 torsion curve of sheath steel filament with single false twister 204 torsion saturation level of a sheath steel filament 208-210 torsion curve of core steel filament with double false twister 212-214 torsion curve of sheath steel filament with double false twister 30 curve of tip rise versus rotation speed of second false twister 32 curve of tip rise versus rotation speed of second false twister 60 rubber ply 62 edge of rubber ply a position at first stationary guiding pulley 108 b position at first stationary reversing pulley 112 c position at second stationary reversing pulley 114 d position at second stationary guiding pulley 120 e position before entry into first false twister 124 f position after leaving first false twister 124 g position before entry into second false twister 128 h position after leaving second false twister 128