WO2005091308A1

WO2005091308A1 - Cable with steel core with increased yield strength for aluminum conductor

Info

Publication number: WO2005091308A1
Application number: PCT/EP2005/050303
Authority: WO
Inventors: Ludo Adriaensen; Danny Gonnissen; Flip Verhoeven
Original assignee: Nv Bekaert Sa
Priority date: 2004-03-15
Filing date: 2005-01-25
Publication date: 2005-09-29

Abstract

A steel core wire (18) for aluminum or aluminum alloy conductor (10) is coated with a zinc aluminum alloy coating (20). The core wire (18) has been aged so that the yield strength Rp0,2 is at least 1200 MPa at room temperature in order to give the conductor sufficient strength to operate in temperatures ranging up to 300°C. Advantages are a reduced number of processing steps to manufacture, a wider range of temperature to operate in, and a higher corrosion resistance.

Description

CABLE WITH STEEL CORE WITH INCREASED YIELD STRENGTH FOR ALUMINIUM CONDUCTOR

Field of t he i nventio n . The present invention relates to a steel core wire for alum inum or 5 aluminum alloy conductors. I n a second instance, the invention relates to an alum inum conductor comp rising one or more of these steel core wires. I n a third instance, the invention relates to a method of m anufacturing such a steel core wire.

10 Backg ro u nd of the inventio n . I n the past electricity production was strongly localized in the area where it was consumed. I n contrast herewith and relating to the opening of the previously protected markets, the current trend is to buy electricity where it is cheapest. This trend increases the am ou nt 15 of electricity transport over large distances, by using the existing electricity networks. The result is that the installed capacity of high- voltage power lines is getting insufficient. Moreover, economical and ecological objections prevent the building of new lines in a lot of places. The only remaining solution is to increase significantly the 20 capacity of existing lines. A way to achieve this is to force m ore electrical current to flow through the existing lines. As heat generation increases quadratic with the current, a lot m ore heat is generated. The nom inal operating tem perature rises from 50 °C to 200 °C and up to 300 °C. -Existing power grids equipped with 25 traditional ACSR conductors, i.e. aluminu m conductor steel reinforced, are not suitable for operating at these temperatures. The traditional ACSR conductors comprise galvanized steel core wires and alum inum conductors. Both the steel core wires and the alum inum conductors m echanically support the ACSR conductor. With rising tem peratures, 30 the alum inum conductors completely loose their mechanical strength. I n addition hereto, the zinc of the galvanized steel wires diffuses and forms a brittle iron- zinc layer and causes flaking. The result is a steep decrease in corrosion resistance. Moreover, the hard particles between the conductor wires may induce fretting, with a risk for a 35 sudden fracture. As a consequence, there is a need for higher strength conductors, which can operate in a wide tem perature range without loosing their strength. JP- A- 04- 236742 (Nippon Steel) has recognized this need and discloses a high- strength steel wire for ACSR. The steel com prises between 0.75 and 1 .0 % C, between 0.1 5 and 1 .30 % Si, between 0.30 and 1 .0 % Mn. The steel wire is process hardened by drawing and is thereafter coated with a zinc alum inum alloy by means of a hot dip method at about 450 °C. A substantial part of the high strength obtained as a result of the drawing gets lost during the hot dip operation. I n order to recuperate this loss in tensile strength, the coated steel wire is drawn after coating until an area reduction ranging from 20% to 80% is obtained. After this drawing operation, the coated and drawn steel wire is subjected to a so- called blueing treatment at temperatures in the range of 300 °C to 370 °C in order to restore the ductility, and m ore particularly the elongation, which was lost due to the final drawing steps. The solution provided in JP-A- 04- 236742 requires two additional processing steps after the hot dip coating operation : a num ber of final drawing steps and a heat treatm ent. The drawing of steel wires coated with zinc alum inum , although perfectly possible from a technical point of view, causes a high degree of die wear due to the hard alum inum oxide parts present in the coating. As a result, this drawing is done at a low speed and requires more expensive dies or m ore m aintenance costs. It is an uneconomical operation.

Su m m ary of t he invent io n . It is an object of the present invention to avoid the drawbacks of the prior art.

It is also an object of the present invention to provide a steel core wire for alum inum conductors m ade in an alternative way.

It is another obj ect of the present invention to provide an alternative steel core wire with increased strength at higher tem peratures.

It is still an obj ect of the present invention to provide a steel core wire for alum inum conductors, which may operate in a wide temperature range. According to a first aspect of the present invention, there is provided a steel core wire for alum inum or aluminum alloy conductor. The core wire is coated with a zinc aluminum alloy coating. The invention core wire is characterized in that it has been aged so that the yield strength R_p0]2 is at least 1 200 MPa, , e.g. at least 1230 MPa, e.g. at least 1275 MPa at room temperature. This minimum level is required in order to give the conductor sufficient strength to operate in temperatures ranging up to 300 °C.

The core wire has a m inim um tensile strength R-, of 1500 MPa, preferably of 1590 MPa, e.g. of 1650 MPa.

The yield strength R-,_0|2 is the strength at 0.2 % perm anent elongation. The yield strength R_p0,2 is directly related to one of the two design parameters for electrical conductors, namely the force F required to keep the conductor in the air. For reason of com pleteness and clarity, the second design parameter is the sag of the conductor. The maxim um allowable force for a conductor - safe operation limit - corresponds typically with one third of the yield strength. The higher the yield strength, the higher the safe operation lim it.

Due to the ageing treatm ent applied on the invention steel core, the yield strength at room tem perature has increased with 10 to 1 5 % . As the yield strength unavoidably decreases with increasing temperatures, this increase of 1 0 to 1 5 % thanks to ageing, allows the steel core and the conductor to operate in wider temperature ranges up to 300 °C.

The ratio of yield strength R-_{0 2} to the tensile strength R,-, is preferably at least 0.87, e.g . at least 0.88, e.g. at least 0.89, at room tem perature (25 °C) .

The difference between the tensile strength R-, and the yield strength Rp₀,2 is preferably at least 300 MPa at room temperature (25 °C) . The ageing treatment applied to the coated steel core constitutes only one additional processing step. The ageing treatm ent can be done at a temperature ranging from 150 ^CC to maxim um 300 °C and during a tim e period of about 2 hours. The aged and coated steel core does not need any further drawing . The purpose of this ageing treatm ent is to increase the yield in order to obtain better stress- strain characteristics for use with conductors.

The ageing treatment step according to the present invention is an explicit and additional step and must be distinguished from the ageing phenomenon naturally occurring in a steel wire at room tem perature. This natural ageing does not increase the yield strength to the same degree, as does the ageing treatment at elevated tem peratures.

The ageing treatment according to the present invention m ust also be distinguished from the blueing treatment me ntioned in JP- A- 04- 236742, in that the blueing treatment aims at restoring ductility and elongation, which is the opposite what happens during the ageing treatm ent. During the ageing treatment, ductility will go down.

The steel co re has a steel com position along the following lines : a carbon content ranging from 0.40 per cent to 1 .20 per cent, a silicon com position ranging from 0.1 0 per cent to 1 .50 per cent, e.g. from 0.15 per cent to 0.40 per cent, a m anganese composition ranging from 0.1 0 per cent to 2.0 per cent. Possibly, chromium may be present in amounts ranging from 0.1 0 per cent to 0.40 per cent.

The zinc aluminum coating on the steel core has an alum inum content ranging from 2 per cent to 12 per cent, with a preferable composition arou nd the eutectoid position : Al about 5 per cent. The zinc alloy coating further has 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 im purities. The zinc alu minum coating has a better overall corrosion resistance than zinc. I n contrast with zinc, the zinc alum inum coating is temperature resistant and withstands the pre-annealing process of ACSS (cfr. Infra). Still in contrast with zinc, there is no flaking with the zinc aluminum alloy when exposed to high temperatures.

According to a second aspect of the invention, there is provided an aluminum conductor, which comprises one or more core wires according to the first aspect of the present invention. The aluminum conductor may be a T-ACSR conductor, an ACSS conductor or an ACSR conductor. ACSS stands for aluminum conductor steel supported. An ACSS conductor is similar to an ACSR conductor from a geometric point of view. In contrast to an ACSR conductor, the ACSS is annealed in the factory to permanently soften the hard-drawn aluminum wires. In all operating conditions the steel core wires only support the ACSS. T-ACSR stands for temperature resistant aluminum conductor steel reinforced. Here again, the conductor is similar to an ACSR, except that the aluminum is alloyed with zirconium. Zr prevents the mechanical properties of the drawn aluminum to change irreversibly at elevated temperatures.

According to a third aspect of the invention, there is provided a method of manufacturing a steel core wire for an aluminum or aluminum alloy conductor. The method comprises the steps of : a) drawing a steel wire rod to a steel wire ; b) coating the drawn steel wire with a zinc aluminum alloy coating ; c) ageing the coated steel wire.

Brief description of the drawings.

The invention will now be described into more detail with reference to the accompanying drawings wherein FIGURE 1 illustrates the design parameters of an electrical conductor ; FIGURE 2 is a cross-section of an aluminum conductor. Description of the preferred embodiments of the invention. FIGURE 1 gives a schematical drawing of an electrical conductor 10 hung on two towers 12 and 14. The two most important design parameters are the sag S and the force F. The sag S is the distance between a horizontal line connecting the two tops of towers 12 and

14 and the lowest point of the conductor 10. The force F is the force required to keep the conductor in the air at a predetermined sag S. The smaller the sag S, the higher the required force F. The maximum allowable force for a conductor corresponds typically with one third of the yield strength : F= Ro₂/ 3

FIGURE 2 shows a cross- section of an ACSS conductor 10. Conductor

10 has a core steel strand 16 in a 1 + 6 construction : one core steel wire and six steel wires twisted around the core wire. Each steel wire has a steel core 18 and a zinc aluminum coating 20. A layer of rolled aluminum conductors 22 is wrapped around the core steel strand 16.

Additional layers of aluminum conductors may be present (not shown).

The wire diameter of the steel wires typically varies between 1.24 mm and 5.5 mm, e.g. between 2.2 mm and 5.0 mm. The weight of the zinc aluminum coating typically varies between 100 g/m² and 400 gtm², e.g. between 240 g/m² and 350 g/m².

1 ^st Exam pie A steel wire with a carbon content of 0.92% has been drawn to a diameter of 2.60 mm and coated with a zinc aluminum alloy until a total diameter of 2.65 mm. The wire has been stranded and has been aged during 12 hours at a temperature of 325 °C. The table hereunder summarizes the tensile strength FL, and the yield strength R₀₂ at various stages and at various temperatures. Table 1

Ex am ple 2 A steel wire with a carbon content of 0.92% has been drawn to a diameter of 2.72 mm and coated with a zinc aluminum alloy until a total diameter of 2.80 m m . The wire has been stranded and has been aged during 12 hours at a temperature of 325 °C. Table 2 hereunder sum m arizes the tensile strength FL, and the yield strength R_{p0 2} at various stages and at various tem peratures.

Table 2

Example 3 A steel wire with a carbon content of 0.80% has been drawn to a diameter of 2.73 mm and coated with a zinc aluminum alloy until a total diameter of 2.80 mm. The wire has been stranded and has been aged during 12 hours at a temperature of 325 ^CC. Table 3 hereunder summarizes the tensile strength R-, and the yield strength R-_oz at various stages and at various temperatures.

Table 3

Exam pie 4 A steel wire with a carbon content of 0.80% has been drawn to a diameter of 3.12 mm and coated with a zinc aluminum alloy until a total diameter of 3.20 mm. The wire has been stranded and has been aged during 12 hours at a temperature of 325 °C. Table 3 hereunder summarizes the tensile strength R^ and the yield strength R-₀₂ at various stages and at various temperatures. Table 4

Claims

CLAI MS

1 . A steel core wire for aluminum or alum inum alloy conductor, said core wire being coated with a zinc aluminum alloy coating, characterized in that said core wire has been aged so that the yield strength R _{0 2} is at least 1200 MPa at room temperature in order to give the conductor sufficient strength to operate in temperatures ranging up to 300 °C.

2. A core wire according to claim 1 , wherein said yield strength R_{p0 2} is at least 1 275 MPa.

3. A core wire according to any one of the preceding claims, wherein said core wire has a minim um tensile strength R-, of 1 590 MPa at room temperature.

4. A core wire according to any one of the preceding claims, said core wire having a tensile strength R,-, which is at least 300 MPa greater than its yield strength R_p0,z at 25 °C.

5. A core wire according to any one of the preceding claims, said core wire having a ratio yield strength R_p0,2 to tensile strength R„ at 25 °C, which is at least 0.87.

6. A core wire according to any one of the preceding claims, wherein said core wire has steel composition with a carbon content ranging from 0.40 to 1 .20 per cent, a silicon composition ranging from 0.10 to 1 .50 per cent, a m anganese com position ranging from 0.1 0 to 2.0 per cent .

7. A core wire according to claim 6, wherein said steel composition further has a chromium content ranging from 0.10 per cent to 0.40 per cent.

8. An aluminum conductor com prising one or m ore core wires according to any one of claims 1 to 7.

9. An alum inum conductor according to claim 8 wherein said conductor is selected from the group consisting of a T-ACSR conductor, an ACSS conductor and an ACSR conductor.

10. A method of manufacturing a core wire according to any one of claims 1 to 7, said method com prising the steps of : a) drawing a steel wire rod to a steel wire ; b) coating said drawn steel wire with a zinc alum inum alloy coating ; c) ageing said coated steel wire.