Invar type alloy wire and manufacturing method thereof

Abstract: An invar type alloy wire has areal ratio of carbide existing at grain boundary of the wire in processed final wire size of at most 4 %, or average grain size in transverse direction within a range of 1 to 5 µm, and hence it has superior twisting property.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an invar type alloy wire and, more specifically, to an invar type alloy wire excellent in toughness, strength and low thermal expansion property which can be used preferably as strands for overhead conductor cables. Description of the Related Art An invar alloy having the composition of Fe-36 wt% Ni has been known as an alloy having low thermal expansion property, which is used for precision parts, for example. Meanwhile, in order to increase transmission capacity of an aluminum cable steel reinforced (ACSR) as overhead conductor cable, a method of reducing slack of conductor cable caused by increase in temperature during power transmission has been studied. One method has been known in which an alloy wire having low thermal expansion property is used as a steel core to reduce slacking. An invar type alloy wire such as disclosed in Japanese Patent Laying-Open No. 55-119156 has been developed as such alloy wire having low thermal expansion property. The alloy wire developed in accordance with Japanese Patent Laying-Open No. 55-119156 is a hard material and exhibits tensile strength of 120 kg/mm 2 . However, it exhibits low toughness property stability such as turns of twisting after it has been finally subjected to zinc or zinc alloy plating, thereby reducing production yield of conductor cables. Zinc alloy plating or the like is applied to improve corrosion resistance of the conductor cable. However, an intermetallic compound formed at the interface with the plating tends to lower the twisting property of the alloy wire. Therefore, an object of the present invention is to improve toughness of a conventionally used invar type alloy wire having high strength, and, more particularly, to improve twisting property of the wire in the final wire size. Document EP 0 723 030 A1 which has the same priority date as this application discloses a Fe-Ni-based alloy wire and a method of manufacturing such a wire. The wire shown in this citation is prepared by processing the material in which the areal percentages of the precipitations at the grain boundaries is up to 2% at finishing hot wire rolling. Moreover, the average crystal grain size of the wire in the longitudinal direction is in the range of 5-70 µ m at finishing the hot wire rolling. SUMMARY OF THE INVENTION The invar type alloy wire in accordance with one aspect of the present invention contains Fe and Ni as main alloy elements, and the wire has high toughness, high strength and low thermal expansion property, wherein the average grain size in transverse direction of said wire in a final wire size is within a range of 1 to 5 µ m; wherein the areal ratio of precipitates existing at grain boundary of said wire in said final wire size is at most 4 % and wherein impurity contained in said alloy is at most 0.01 percent in weight of P, at most 0.004 percent by weight of S, at most 0.005 percent of weight of O and at most 0.008 percent by weight of N. Due to the fact that the areal ratio of the precipitates existing at the grain boundary of the wire in the processed final wire size is at most 4%, the twisting property of the wire is improved. Especially when the areal ratio of the precipitates existing at the grain boundary of the wire in the final wire size is at most 2%, twisting property and reliability of the wire can be remarkably improved. In the invar type alloy wire in accordance with the present invention, the average grain size in the transverse direction of the wire in the processed final wire size is 1 to 5 µ m, and hence twisting property of the wire is improved. Especially when the average grain size in the transverse direction of the wire in the final wire size is within 1.5 to 4 µ m, the twisting property and the reliability of the wire can significantly be improved. The method of manufacturing an invar type alloy wire in accordance with the present invention, comprises the steps of: preparing an invar type alloy containing, as main elements, Fe and Ni, wherein impurity contained in said alloy is at most 0.01 percent in weight of P, at most 0.004 percent by weight of S, at most 0.005 percent of weight of O and at most 0.008 percent by weight of N; performing hot working and heat treatment in combination to set areal ratio of precipitates existing at grain boundary of said alloy to be at most 2 % and to set average grain size in longitudinal direction of a rod shape processed by said hot working and heat treatment to be within a range of 5 to 40 µ m ; and therafter, performing cold working and heat treatment in combination to set areal ratio of precipitates existing at the grain boundary of said alloy in final wire size of said wire to be at most 4 % and to set average grain size in transverse direction of said wire in said final wire size to be within a range of 1 to 5 µ m. Such an invar type alloy wire has superior twisting property. Especially when the areal ratio of the precipitates existing at the grain boundary of the alloy is set to be at most 1 % by the combination of hot working and heat treatment, the areal ratio of the precipitates existing at the grain boundary of the alloy in the final wire size can be easily set to be at most 2 % by the subsequent combination of cold working and heat treatment, and hence an invar type alloy wire having significantly improved twisting property can be provided. DESCRIPTION OF THE PREFERRED EMBODIMENTS A conventionally used invar type alloy wire having high strength contains, as main elements, Fe and Ni, and Co may partially substitute for Ni. Generally, such an invar type alloy wire having high strength contains at least one of Mo, Cr, C, W, Nb, Ti, V, Si or the like as strengthing element, and in addition, at least one of Mn, Al, Mg, Ti, Ca or the like as a deoxidizer. The inventors performed various investigations to eliminate instabilizing factors related to the toughness of such invar type alloy wire having high strength. As a result, it was found that crystal grain size of the wire, the amount of precipitates at the grain boundary and amount of specific impurity elements have significant effects on the toughness of the wire. It was also found that there is a preferable method of processing and heat treatment for controlling the grain size and the amount of precipitates at the grain boundary. The precipitates at the grain boundary here are often carbide. In order to reduce precipitates at the grain boundary in the final wire size of the wire, any of the following methods may be used: a method in which during hot rolling, cooling is started from solid solution temperature (cooling from the solid solution temperature may be considered to be a kind of heat treatment); a method in which solution heat treatment is performed prior to hot rolling; and a method in which solution heat treatment is performed after hot rolling. No matter which of the methods is employed, the smaller the amount of precipitates at the grain boundary after the combined hot working and heat treatment, the smaller the amount of precipitates at the grain boundary precipitated during subsequent cold working and heat treatment, and therefore the smaller the amount of precipitates existing at the grain boundary in the wire in the final wire size. Element C Mn Ni Co Cr Mo P S O N Fe Wt % 0.25 0.30 35.0 3.01 0.98 2.01 0.002 0.001 0.0015 0.0013 remaining part As an example, an invar type alloy having such a composition as shown in Table 1 was dissolved and cast. Sample Rolling Start Temperature (°C) Cooling Rate (°C/sec) Areal Ratio of Precipitates at Grain Boundary (%) Average Grain size in Longitudinal Direction (µm) A 1200 10 0.2 22 B 1150 8 0.6 14 C 1100 7 0.9 5 D 1250 3 3.8 75 E 1200 5 2.8 59 Table 2 shows the influence of the temperature at which rolling is started and the rate of cooling during rolling until the temperature reaches 600°C when the invar type alloy shown in Table 1 is subjected to hot rolling, on the average grain size in the longitudinal direction of the rod after rolling and on the areal ratio of the precipitates at the grain boundary. For measuring the areal ratio of the precipitates at the grain boundary, the rolled rod was cut along the longitudinal direction, the cut surface is polished and etched for 40 seconds by using 5 % nital solution, and the surface was photographed with the magnification of 4000 by using a scanning type electron microscope. The microphotograph was processed by an automatic image processing apparatus, the areal ratio of the precipitates existing at the grain boundary was calculated, and the average grain size in the longitudinal direction was calculated. As is apparent from Table 2, the average grain sizes in the longitudinal direction of samples A, B and C of which rate of cooling during hot working was relatively fast are within the range of 5 to 40 µm, and the areal ratio of the precipitates at the grain boundary is at most 2.0 %. Meanwhile, the grain sizes of samples D and E of which cooling rate during hot working was slow are far greater than 40 µm, and the areal ratio of precipitates at the grain boundary exceeds 2.0 %. Referring to Table 2, a billet having a square cross section of about 120 x 120 mm 2 was passed through a plurality of shaping rolls and rolled to be a rod having a circular cross section of about 12 mm in diameter. Thereafter, all the samples A to E shown in Table 2 were subjected to first cold working, first heat treatment, scraping, second heat treatment and second cold working. As the first cold working, drawing with the degree of processing of about 30 % was performed by using a plurality of dies. The first heat treatment was performed in a non-oxidizing atmosphere such as in a decomposed ammonia gas containing 75 % by volume of H 2 and 25 % by volume of N 2 , at 650°C for 10 hours. The samples softened by the first heat treatment were peeled by scraping dies, and then subjected to second heat treatment under the same condition as the first heat treatment. The samples softened by the second heat treatment were drawn to have the diameter of about 2 to 5 mm with the degree of processing of about 85 %, by passing through a plurality of dies, and thereafter the samples were dipped in Zn-5 wt % Al alloy melt. The average grain size in the transverse direction, areal ratio of precipitates at the grain boundary and various mechanical properties of the wires having final wire size obtained through these steps are as shown in Table 3. Referring to Table 3, samples 1A to 1E are the samples obtained from samples A to E of Table 2. The samples 1A to 1E all have similar tensile strength exceeding the target property of 120 kg/mm 2 . However, comparative examples 1D and 1E are inferior in twisting property and elongation as compared with samples 1A, 1B and 1C in accordance with the present invention. The twisting property is represented by the number of possible twisting (turns/100d) at about 60 rpm of a single wire having the length one hundred times the diameter d until it breaks. The reference character σ represents standard deviation of the turns of twisting of one hundred wires. The smaller the value σ, the higher the reliability, as the twisting property is stable. Referring to Table 3, the samples 1A and 1B of which areal ratio of precipitates at the grain boundary is at most 2.0 % and average grain size in the transverse direction is within the range of 1.5 to 4 µm have superior twisting property exceeding 100 turns, and they have high reliability as represented by the standard deviation σ of at most 10. The sample 1C of which areal ratio of the precipitates at the grain boundary exceeds 2 % but not higher than 4 % and the grain size in the cross sectional direction is within a range of 1 to 5 µm but not higher than 1.5 µm has slightly inferior twisting property as compared with samples 1A and 1B. However, it can still satisfy the target property in 3σ management. More specifically, for the sample 1C, 95-3σ=35 turns, and hence it satisfies the required property (≥ 16 turns/100d). Meanwhile, comparative examples 1D and 1E of which areal ratio of precipitates at the grain boundary exceeds 4 % and the grain size in the transverse direction exceeds 5 µm, the twisting property cannot satisfy the target property, in accordance with 3σ management. Further, in comparative examples 1D and 1E, though target elongation property (≥ 1.5 %) is satisfied, the elongation property is inferior to the samples 1A to 1C in accordance with the present invention. Especially, in comparative example 1D, breakage was observed during cold working. As already mentioned, samples 1A to 1E are obtained by performing same cold working and heat treatment on the hot worked samples A to E of Table 2. If would be understood that in order to obtain preferable twisting property, it is preferable that the areal ratio of the precipitates at the grain boundary in the rod after hot working is at most 2 % and the grain size in the longitudinal direction is within the range of 5 to 40 µm. Table 4 shows influence of the degree of processing of the first cold working and the temperature of immediately following first heat treatment on the areal ratio of the precipitates at the grain boundary and various mechanical properties of the wires in the final wire size. Referring to Table 4, first cold working with various degrees of processing and first heat treatment at various temperatures were performed on sample A of Table 2. The processes after the first heat treatment are the same as those described with reference to Table 3. Samples 1A to 7A all have similar tensile strength higher than the target property of 120 kg/mm 2 . However, while samples 1A to 6A belonging to the present invention of which areal ratio of the precipitates at the grain boundary in the final state is at most 4 % can satisfy the target value of the twisting property (≥ 16 turns/100d) even under 3σ management, the comparative example 7A of which areal ratio of the precipitates at the grain boundary in the final state exceeds 4 % cannot satisfy the target value of the twisting property under 3σ management (89-3σ=89-3x35<16 turns). Now, sample 4A is processed with the degree of processing of the first cold working being 80 %, exceeding 70 %, and hence the areal ratio of the precipitates at the grain boundary of the final state exceeds 2 %, though not higher than 4 %. Therefore, it is inferior to samples 1A to 3A in twisting property and elongation. In other words, the degree of processing of the first cold working should more preferably be at most 70 %. Further, for the sample 5A, temperature of the first heat treatment was 570°C, which was not higher than 600°C. Therefore, the amount of precipitates at the grain boundary was small. However, since the strain in the wire is not sufficiently removed, the turns of twisting vary as compared with samples 1A to 3A, and as a result, average turns of twisting is low and elongation is degraded. Namely, the temperature for the first heat treatment should more preferably be at least 600°C. Further, for the sample 6A, relatively large degree of processing of 70 % was set for the first cold working, and relatively high temperature of 700°C was set for the heat treatment. Therefore, the areal ratio of the precipitates at the grain boundary in the final state was larger as compared with samples 1A to 1C and, as a result, twisting property and elongation were degraded. Especially when the temperature for the first heat treatment exceeds 700°C, the areal ratio of the precipitates at the grain boundary in the final state exceeds 4 % as in comparative example 7A, and hence target twisting property (≥ 16 turns/100d) cannot be satisfied. In other words, the temperature for the first heat treatment should preferably be in the range of 600°C to 700°C. Table 5 shows influence of hot working, cold working and heat treatment on the average grain size in the transverse direction of the wire and twisting property of the wire in the final wire size. The alphabets (A) to (E) appended to the sample numbers of Table 5 represent that the samples are obtained by performing first cold working, first heat treatment, scraping, second cold working and Zn-5 wt % Al alloy plating on the hot worked samples A to E of Table 2. For each sample of Table 5, the temperature for the first and second heat treatments before and after scraping is set to be the same temperature. As can be seen from Table 5, samples 11 to 14 belonging to the present invention of which average grain size in the transverse direction in the final wire size is within the range of 1 to 5 µm can satisfy the target value of twisting property (≥ 16 turns/100d) even at the 3σ management. By contrast, comparative examples 15 to 17 of which grain size in the transverse direction is out of the range of 1 to 5 µm cannot satisfy the target value of twisting property at 3σ management. Now, for sample 14, the degree of processing in the first cold working was 80 %, which is higher than 70 %, so that the grain size in the transverse direction at the final state exceeds 4 %, though not higher than 5 %, and twisting property is inferior to samples 11 to 13. Therefore, the degree of processing of the first cold working should desirably be at most 70 %. The small grain size in the transverse direction of comparative example 15 may be related to small grain size in the longitudinal direction of sample C in Table 2. As compared with samples A and B, sample C has relatively large areal ratio of the precipitates at the grain boundary, and areal ratio of the precipitates at the grain boundary at the final wire size of sample 15 was increased to 4.4 %, even though the temperatures for the first and second heat treatments were relatively low. Further, in comparative example 17 having very large grain size in the transverse direction, breakage was observed during the second cold working. Then, alloys such as shown in Table 6 were melt and cast in order to see the influence of impurity on the invar type alloy wire. Referring to Table 6, numerical values related to respective elements denote percentage by weight in the alloy. Sample Twisting Property (turns/100d) Tensile Strength (kg/mm 2 ) Elongation (%) 100 wires average σ Present Invention 21 125 5 127 2.5 22 122 6 127 2.5 23 115 11 127 2.3 Comparative Example 24 75 20 127 2.0 25 96 27 126 2.1 26 74 22 126 1.9 27 48 25 125 1.7 Target Property ≧ 16 (turns/100d) ≧ 120 ≧ 1.5 Table 7 shows various mechanical properties at the final wire size of the invar type alloy wires having such compositions as shown in Table 6. Of each of the samples having such compositions as shown in Table 6, a billet was heated to 1200°C, and thereafter cooled to about 600°C at the cooling rate of 10°C/sec, while it is rolled by shaping rolls. The obtained rolled rods were examined and it was found that every rod has the areal ratio of the precipitates at the grain boundary of about 0.2 % and average grain size of about 22 µm in the longitudinal direction. The rolled rods which were hot worked were all subjected to a first cold drawing of 22 %, scraping, heat treatment at 650°C for 10 hours, a second cold drawing of 86 % and plating with Zn-5 wt % Al alloy. Referring to Table 7, samples 21 to 27 all have similar tensile strength exceeding the target value of 120 kg/mm 2 . However, it is apparent that comparative examples 24 to 27 are inferior in twisting property and elongation as compared with the samples 21 to 23 belonging to the present invention. More specifically, samples 21 to 23 belonging to the present invention containing P of at most 0.01 percent by weight, S of at most 0.004 percent by weight, O of at most 0.005 percent by weight and N of at most 0.008 percent by weight have superior twisting property. Especially, samples 21 and 22 which include P of at most 0.005 percent by weight, S of at most 0.002 percent by weight O of at most 0.003 percent by weight and N of at most 0.006 percent by weight only as impurities have superior twisting property and stability (that is, small a). Comparative examples 24 to 27 all include at least one impurity of P exceeding 0.01 percent by weight, S exceeding 0.004 percent by weight O exceeding 0.005 percent by weight N exceeding 0.008 percent by weight, so that these examples are far inferior in twisting property to the samples 21 to 23 of the present invention, and target value (≥ 16 turns/100d) of the twisting property cannot be achieved. In any of the above described embodiments, various properties of the wire are hardly influenced when Mo of the alloy element is replaced by V, and hence V can be similarly used as Mo. As described above, according to the present invention, toughness, especially twisting property of an invar type alloy wire having high strength can be improved, and by using the same, production yield of overhead conductor cables can be improved. Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being limited only by the terms of the appended claims.

Claims: An invar type alloy wire having high toughness, high strength and low thermal expansion property, containing, as main alloy element, Fe and Ni, wherein the average grain size in transverse direction of said wire in a final wire size is within a range of 1 to 5 µ m; wherein the areal ratio of precipitates existing at grain boundary of said wire in said final wire size is at most 4 % and wherein impurity contained in said alloy is at most 0.01 percent in weight of P, at most 0.004 percent by weight of S, at most 0.005 percent of weight of O and at most 0.008 percent by weight of N. The invar type alloy wire according to claim 1, wherein areal ratio of said precipitates is at most 2 %. The invar type alloy wire according to claim 1, wherein average grain size in transverse direction of said wire in said final wire size is within a range of 1.5 to 4 µ m. The invar type alloy wire according to claim 1, wherein part of Ni in said invar type alloy is replaced by Co. The invar type alloy wire according to claim 1, wherein said invar type alloy further contains, as strengthing element, at least one of Mo, Cr, C, W, Nb, Ti, V and Si. The invar type alloy wire according to claim 1, wherein said invar type alloy further contains, as deoxidizer, at least one of Mn, Al, Mg, Ti and Ca. The invar type alloy wire according to claim 1, wherein impurity contained in said alloy is at most 0.005 percent by weight of P, at most 0.002 percent by weight of S, at most 0.003 percent of weight of O and at most 0.006 percent by weight of N. A method of manufacturing an invar type alloy wire having high toughness, high strength and low thermal expansion property, comprising the steps of: preparing an invar type alloy containing, as main elements, Fe and Ni, wherein impurity contained in said alloy is at most 0.01 percent in weight of P, at most 0.004 percent by weight of S, at most 0.005 percent of weight of O and at most 0.008 percent by weight of N; performing hot working and heat treatment in combination to set areal ratio of precipitates existing at grain boundary of said alloy to be at most 2 % and to set average grain size in longitudinal direction of a rod shape processed by said hot working and heat treatment to be within a range of 5 to 40 µ m ; and therafter, performing cold working and heat treatment in combination to set areal ratio of precipitates existing at the grain boundary of said alloy in final wire size of said wire to be at most 4 % and to set average grain size in transverse direction of said wire in said final wire size to be within a range of 1 to 5 µm. The method of manufacturing an invar type alloy wire according to claim 8, wherein areal ratio of said precipitates after said hot working and heat treatment in combination is set to be at most 1 % and areal ratio of said precipitates in said final wire size is set to be at most 2 %. The method of manufacturing an invar type alloy wire according to claim 8, wherein said alloy is subjected to a first cold working at a degree of processing of at most 70% after said hot working, and thereafter subjected to at least one heat treatment at a temperature within a range of 600 to 700°C.

Inventors: KITAMURA SHINICHI, MIYAZAKI KENJI, SAITO TAKANOBU, YAHAGI SHINICHIRO, YOSHIDA ATSUSHI, KITAMURA, SHINICHI, MIYAZAKI, KENJI, SAITO, TAKANOBU, YAHAGI, SHINICHIRO, YOSHIDA, ATSUSHI
Applicants: DAIDO STEEL COMPANY LTD, SUMITOMO ELECTRIC INDUSTRIES, DAIDO STEEL COMPANY LIMITED, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1996
Main classification: C21D8
IPC codes: C22C38, C21D9, C21D6, C21D8