Low-resistivity gradient crystal wire and preparation method thereof

Abstract

The invention discloses a low-resistivity gradient crystal wire and a preparation method thereof. The high-frequency induction heating device is adopted for conducting continuous controllable heat treatment on the wire, by setting parameters such as the diameter of an induction coil, the turn-to-turn distance, the power frequency, the current and the wire inlet rate, the section of the wire obtains a non-evenly-distributed temperature field through the induction heating skin effect, and if residual stress exists on the wire, the temperature field is not evenly distributed. The surface layer temperature can be controlled to be higher than the recrystallization temperature, and short-time recrystallization growth of crystal grains is realized; if no residual stress exists in the lead, the surface temperature can be controlled to be close to but lower than the recrystallization temperature, and growth of crystal grains is achieved by increasing the migration rate of the crystal boundary interface. The core temperature is lower than the surface temperature, the grain size change is not obvious, and gradient structure distribution of grains from the surface to the core is formed. In the alternating current transmission process, large grains on the surface layer can effectively reduce the obstruction of grain boundaries to current, the resistivity of the wire is reduced, and meanwhile, the high plasticity of the wire is reserved.

Description

Technical Field

[0001] The invention belongs to the technical field of electric wire materials, and in particular relates to a low resistivity gradient crystal wire and a preparation method thereof. Background Art

[0002] China has a vast territory, and energy resources in different regions are unevenly distributed. Some areas with rich power generation resources may be far away from areas with concentrated electricity demand. Using AC transmission technology to achieve cross-regional and long-distance power transmission can achieve optimal resource allocation and improve energy utilization efficiency. Although AC transmission has been widely used, the skin effect in the transmission process cannot be avoided. The skin effect refers to the phenomenon that the current density on the wire always appears as a large surface and a small core during AC transmission, which will cause serious problems such as overheating of the wire surface, reduced transmission efficiency, and affect electromagnetic compatibility. At present, there are two main solutions to the skin effect. One is to divide the wire into many fine twisted or braided wires, each of which is insulated, thereby reducing the skin effect, but this method is complicated to process and increases the cost. Another method is to use a hollow conductor so that the current flows mainly in the outer shell of the conductor, but the strength of this structure is worse than that of a solid conductor, and the cost may also increase. Summary of the invention

[0003] The present invention provides a low-resistivity gradient crystal conductor and a preparation method thereof, so as to solve the problems of heat loss and low efficiency caused by the skin effect of the conductor during AC power transmission.

[0004] The technical solution of the present invention is as follows:

[0005] In a first aspect, the present invention provides a low resistivity gradient crystal wire, wherein starting from the center of the wire circle, along the cross-sectional direction and axial direction of the wire, from inside to outside, the grain size changes gradually from fine grains to coarse grains.

[0006] Preferably, the conductor is divided into a surface tissue and a core tissue along its cross-section, the interface between the surface tissue and the core tissue is defined by a circle, the center of the circle is the center of the conductor, and the radius of the circle where the core tissue is located is 60-80% of the radius of the conductor.

[0007] Preferably, the grain size of the surface structure is 400-800 μm, and the grain size of the core structure is 50-100 μm.

[0008] In a second aspect, the present invention provides a method for preparing a low resistivity gradient crystal wire, comprising the following steps:

[0009] (1) Determine the recrystallization temperature of the wire according to the wire material, adjust the power supply current and frequency of the induction heating device, and determine the wire feed speed, coil diameter and turn spacing. If the wire has residual stress, make the surface temperature of the wire higher than the recrystallization temperature by 50 to 100 K. If the wire has no residual stress, make the surface temperature of the wire lower than the recrystallization temperature by 0 to 50 K.

[0010] (2) Fix the inlet and outlet ends of the conductor, and the traction power is driven by the motor. After the induction heating device is stabilized for 5 minutes, the conductor starts to enter the wire from the inlet drum, and the wire is taken up by the take-up drum after passing through the induction coil;

[0011] (3) Repeat step (2) until the grain size of the surface structure of the wire is 400-800 μm and the grain size of the core structure is 50-100 μm.

[0012] Preferably, the coil diameter is 1.5-5 times the wire diameter, and the turn spacing should be as small as possible to improve the electromagnetic induction efficiency of the coil. To ensure sufficient heat treatment time, the turn spacing is 1-5 mm.

[0013] Preferably, the wire is made of copper or aluminum.

[0014] Preferably, the line feed rate is 1-20 cm / s, the induced current is 50-500 A, and the frequency is 10 kHz-50 kHz.

[0015] Compared with the prior art, the present invention has the following significant advantages:

[0016] (1) The present invention is universally applicable to the initial state of the conductor, and a gradient structure can be prepared regardless of whether the conductor surface has residual stress.

[0017] (2) The prepared large / small grain gradient structure not only reduces the resistivity of the wire (≤0.01690Ω·mm 2 / m), and can also simultaneously improve the strength and plasticity of the wire, thereby reducing the working deformation of the wire, reducing the maintenance and replacement costs of the wire, and improving the safety performance of the wire.

[0018] (3) The preparation process of the present invention is simple, rapid, efficient, and low-cost, and can continuously prepare low-resistivity gradient crystal wires, which has universal applicability and promotion value.

[0019] The present invention is described in detail below in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a current density distribution diagram during the continuous preparation process of the low resistivity gradient crystal wire described in Example 1.

[0021] Figure 2This is a temperature distribution diagram during the continuous preparation process of the low resistivity gradient crystal wire described in Example 1.

[0022] Figure 3 This is a schematic diagram of the large / small grain gradient distribution of the cross-section of the low resistivity gradient crystal wire described in Example 1.

[0023] Figure 4 This is a temperature distribution diagram of the continuous preparation process of the low resistivity gradient crystal wire in Example 2 when the frequency is 500 Hz.

[0024] Figure 5 This is a scanned image of the cross section of the aluminum wire prepared in Example 3. DETAILED DESCRIPTION

[0025] The present invention heats the wire with an induction heating device, determines the coil diameter and the spacing between turns, controls the induced current, frequency and the speed at which the wire travels, and utilizes the skin effect during the induction heating process to achieve the preparation of the gradient structure wire described in the present invention, which significantly reduces the resistivity of the wire. The present invention selects appropriate process parameters according to the initial state and recrystallization temperature of the wire. If the wire has undergone a drawing process but has not been annealed, its surface residual stress is relatively large, and the surface temperature can be controlled to be 50 to 100K higher than the recrystallization temperature to achieve short-term recrystallization and growth of the grains. If the wire has undergone a drawing process but has been fully annealed, there is no residual stress, and the surface temperature can be controlled to be close to but lower than the recrystallization temperature (0 to 50K). The growth of the grains is achieved by accelerating the migration rate of the peripheral grain boundary interface, so that along the cross-sectional direction and axial direction of the wire, from the inside to the outside, the grain size changes from fine grains to coarse grains in a gradient, and its cross-sectional schematic diagram is as shown in FIG. Figure 3 shown.

[0026] Example 1

[0027] (1) The conductor is selected as a copper conductor (10 mm, resistivity 0.01851 Ω·mm 2 / m), the wire has been drawn but not annealed, and a large amount of residual stress has accumulated on the surface. According to the data, the recrystallization temperature of the copper wire is about 573K. According to the Comsol simulation software, the electric field and thermal field coupled during induction heating are simulated, and the power supply current of the induction heating device is adjusted to 100A and the frequency is 30kHz. The wire feed rate is determined to be 3cm / s, the coil size is 20mm, and the coil turn spacing is 2mm. The current density distribution in the wire is obtained as shown in the figure Figure 1 As shown in Figure 2, the temperature field in the wire is as follows: Figure 2 As shown, the surface temperature is about 620K, which is greater than 573K, while the internal temperature is about 400K, which is significantly lower than 573K;

[0028] (2) Fix the inlet and outlet ends of the conductor, and the traction power is driven by the motor. After the induction heating device is stable for 5 minutes, the conductor starts to be fed from the inlet reel at 3 cm / s. After the conductor passes through the induction coil, the reel completes the reel reeling operation;

[0029] (3) In order to further grow the surface grains, step (2) was repeated three times to complete the continuous preparation of the low resistivity gradient crystal wire. After testing, the resistivity of the prepared low resistivity gradient crystal wire was 0.01690 Ω·mm 2 / m.

[0030] Example 2

[0031] Other conditions not mentioned are consistent with those in Example 1. The frequency is changed to 500 Hz, and the temperature field in the conductor is as follows: Figure 4 As shown, both the surface and internal temperatures are greater than 573K.

[0032] Due to the low frequency, the skin effect is not obvious, and the heating temperature field is very uniform, resulting in the entire treated wire undergoing recrystallization and grain growth processes. The final wire cross-section is finally uniformly distributed coarse equiaxed crystals.

[0033] Example 3

[0034] Other conditions not mentioned are consistent with those in Example 1. The wire is changed to an aluminum wire (which has been drawn but fully annealed without residual stress and has a resistivity of 0.02756 Ω·mm 2 / m), and its recrystallization temperature is 473K. The power supply current is adjusted to 60A and the frequency is 50kHz. A gradient structure wire with large surface grains (about 500μm) and small core grains (about 100μm) is prepared. The core grains account for about 70%. Figure 5 As shown, after testing, the resistivity of the low resistivity gradient crystal wire is 0.02645Ω·mm 2 / m.

[0035] Example 4

[0036] Other conditions not mentioned are consistent with those in Example 1. The traction power is driven by a motor, and the feed rate is fixed at 15 cm / s. Since the feed rate is too fast, the surface temperature is much lower than the recrystallization temperature, and a gradient structure wire with large / small grains cannot be obtained. The cross section is all small equiaxed crystals. After testing, the resistivity of the obtained wire is 0.01845 Ω·mm 2 / m.

Claims

1. A low resistivity gradient crystal wire, characterized in that: Taking the center of the conductor as the starting point, along the cross-sectional direction and axial direction of the conductor, from inside to outside, the grain size changes gradually from fine grains to coarse grains.

2. The low resistivity gradient crystal wire according to claim 1, characterized in that: The conductor is divided into a surface tissue and a core tissue along its cross-sectional direction. The interface between the surface tissue and the core tissue is defined by a circle, the center of the circle is the center of the conductor, and the radius of the circle where the core tissue is located is 60-80% of the radius of the conductor.

3. The low resistivity gradient crystal wire according to claim 1, characterized in that: The grain size of the surface tissue is 400-800μm, and the grain size of the core tissue is 50-100μm.

4. A method for preparing a low resistivity gradient crystal wire, characterized in that: The following steps are involved: (1) Determine the recrystallization temperature of the wire according to the wire material, adjust the power supply current and frequency of the induction heating device, and determine the wire feed speed, coil diameter and turn spacing. If the wire has residual stress, make the surface temperature of the wire higher than the recrystallization temperature by 50~100K. If the wire has no residual stress, make the surface temperature of the wire lower than the recrystallization temperature by 0~50K. (2) Fix the inlet and outlet ends of the conductor, and the traction power is driven by the motor. After stabilizing the induction heating device for 5 minutes, the conductor starts to enter from the inlet reel, and the wire is taken up by the take-up reel after passing through the induction coil; (3) Repeat step (2) until the grain size of the surface structure of the conductor is 400-800 μm and the grain size of the core structure is 50-100 μm.

5. The method according to claim 4, characterized in that The coil diameter is 1.5-5 times the wire diameter, and the turn spacing is 1~5mm.

6. The method according to claim 4, characterized in that The conductor material is either copper or aluminum.

7. The method according to claim 4, characterized in that The line rate is 1-20cm / s, the induced current is 50-500A, and the frequency is 10kHz-50kHz.

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