High strength low carbon steel wire rods and method of producing them

Abstract: High strength low carbon steel wire rods of enhanced cold drawing property have a composite structure in which an acicular low temperature transformation phase comprising a martensite, bainite and/or the mixed structure thereof which comprises, by weight %, C : 0.02 - 0.30%, Si: less than 2.5%, Mn: less than 2.5% and the balance of iron and inevitable impurities and incidental constituents and which may partially contain retained auste­ nite, is uniformly dispersed at the volume ratio of from 10 to 70% in the ferrite phase, and in which the weight of (C + N) in solution in the ferrite phase is less than 40 ppm.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention is concerned with high strength low carbon steel wire rods with enhanced cold drawing properties and a method of producing them. This invention further relates to a method of producing ultra-fine steel wires using the high strength low carbon steel wire rods of the invention and also to brass plated ultra-fine steel wires. Description of the Prior Art Steel wires drawn from steel wire rods into diameters of from several millimeters to several tens of micrometers have been used, depending on their diameters, in various applications such as PC wires, various kinds of spring wires, rope wires, tyre bead wires, tyre cord wires, high pressure hose wires, switching wires, corona wires and dot printer wires. Ultra-fine steel wires have usually been produced from rolled wire rods of about 5.5 mm diameter composed of high carbon steels by way of several cold drawing steps while preventing a reduction in the toughness of the drawn wire rods at each drawing step by the application of a patenting treatment several times in the course of production. A number of production steps are accordingly required and the production cost is inevitably increased. On the other hand, it is also possible to draw ultra-fine wires by intense work from steel wire rods made of pure iron or low carbon ferrite-pearlite steels. However, the strength of the ultra-fine wires as the final product is low since the strength is less increased in the drawing work. Thus, even in the case of drawn wires subjected to intense work at 95 - 99% rate, the strength is only from 686465.5 kPa to 1274864.5 kPa (70 to 130 kgf/mm²) and no high strength greater than 1667130.5 kPa (170 kgf/mm²) can be attained. Further, even with a drawing work at higher than a 99% rate, the strength is still lover than 1863263.5 kPa (190 kgf/mm²). Wire rods having a tempered martensite structure which are prepared by the heat treatment of hardening and tempering are also known. However, since no desirable workability can be obtained for the wire rods only by the hardening, the workability has been obtained by significantly reducing the strength of the wire rods by the tempering treatment and, accordingly, strong and ductile steel wires cannot be obtained. Moreover, the wire rods in the state as hardened suffer from surface cracking in the pickling step which is applied as the pretreatment to the drawing, and also suffer from an inevitable insufficiency in their ductility. The present inventors have sought to obtain high strength and high ductility steel wire rods instead of conventional ferrite-pearlite wire-rods, pearlite wire rods and tempered martensite wire rods. They have found that steel wire rods having composite structures in which a fine low temperature transformation phase comprising an acicular bainite, martensite and/or mixed structure thereof which comprises predetermined chemical compositions and may partially contain retained austenite is uniformly dispersed in a ferrite phase have excellent intense workability. The inventors have already filed a U.S. patent application based on such findings (as Ser. No. 686884) which has now been patented as U.S. patent No. 4578124. However, the inventors have also found that even the steel wire rods having such an excellent cold drawing property show degradation in their ductility and may sometimes be disconnected when drawn at a drawing speed higher than 20 m/min. Such a degradation in ductility is a problem characteristic to composite structures in general, not being restricted only to the acicular structure, when the steel wire rods are subjected to quenching before drawing. Specifically, upon such high speed drawing, the ductility is degraded even in the steel wire rod having a metal structure excellent in its cold drawing property due to the temperature rise during drawing work because of the high aging effect. In addition, an effect of hydrogen tends to be developed when the strength of the drawn wire rod is increased by the drawing work and the tensile strength is increased to greater than about 1470997.5 kPa (150 kgf/mm²). The effect of hydrogen is particularly significant in the case where the strength is greater than about 1961330.0 kPa (200 kgf/mm²). For instance, Figure 1 of the accompanying drawings shows the tensile strength and the reduction of area at break of a drawn wire obtained from a high strength wire rod of 7.5 mm diameter having a mixed structure comprising 8% ferrite and 92% martensite prepared by rolling and then directly hardening the steel material represented by the reference R2 and having chemical compositions shown in Table 1, at a drawing speed of 1 m/min or 50 m/min. That is, a high strength and high ductility drawn wire having a strength greater than 1961330.0 kPa (200 kgf/mm²) can be obtained at a working rate of 70 to 80% in the case of using the drawing speed of 1 m/min. However, since the ductility begins to be degraded in the drawn wire at about 50% working rate in the case of the drawing speed of 50 m/min, it is difficult to obtain a high ductility drawn wire with a strength greater than 1961330.0 kPa (200 kgf/mm²). Further, steel materials represented by the steel No. A and having the chemical compositions shown in Table 1 are rolled into wire rods, followed by direct hardening to obtain a wire rod of 5.5 mm diameter having a structure mainly composed of martensite, which are re-heated into a ferrite-austenite 2-phase region followed by water cooling to obtain an intensely workable wire rod having a mixed structure, in which fine acicular martensite is uniformly dispersed by 21% volume ratio into the ferrite phase. Then, the wire rod is drawn at a low speed or drawn at a speed of 30 - 530 m/min. As shown by the result in Figure 2, a high strength drawn wire having a tensile strength greater than 3138128.0 kPa (320 kgf/mm²) can be obtained at 99.9% working rate in the case of the drawing speed of 1 m/min, but it is diffiuclt to obtain a drawn wire having a tensile strength greater than 1961330.0 kPa (200 kgf/mm²) in the case of continuous drawing at a speed of 30-530 m/min since the ductility begins to be degraded from about 95% working rate. SUMMARY OF THE INVENTION In view of the above, the present inventors have sought to overcome the foregoing problems and, as a result, have found that drawn steel wires having stably high ductility can be obtained irrespective of the wire drawing speed, by a method of producing steel wire rod of a composite structure having a low temperature transformation phase comprising martensite, bainite and/or a mixed structure thereof, which may contain austenite, by the rolling of steels having predetermined chemical compositions into wire rods or by re-heating the wire rods followed by cooling, wherein dehydrogenation is applied to the wire rods under a predetermined condition in the above-mentioned cooling step, thereby restricting the weight of (C+N) solid-solubilized into the ferrite phase in the metal texture of the wire rods to less than 40 ppm, which enables to maintain the excellent workability inherent to such a structure. It has further been found that the high ductility drawn wires can also be obtained stably irrespective of the drawing speed by producing the wire rods of the composite structure as described above and then applying an over-aging treatment under a predetermined condition. Furthermore, the present inventors have found that steel wire rods more excellent in the intense workability can be obtained by re-heating the wire rods having the foregoing composite structure, followed by cooling to transform the low temperature transformation phase into a fine acicular structure and then applying the dehydrogenation or overaging treatment to these wire rods. Accordingly, a primary object of this invention is to provide high strength steel wire rods excellent in the cold drawing property, as well as a method of producing them, particularly, high strength steel wire rods excellent in the cold drawing property capable of providing high strength and high ductility drawn wires having a tensile strength greater than 1470997.5 kPa (150 kgf/mm²), preferably greater than 1961330.0 kPa (200 kgf/mm²), as well as a method of producing them by drawing the wire rods at a drawing speed higher than 20 m/min and with a total reduction of area greater than 30%. Furthermore, the present inventors have found that ultra-fine steel wires having higher strength and higher ductility can be obtained by applying, to the wire rods of the aforementioned composite structure for use in cold wire drawing, a heat treatment comprising heating to a temperature lower than the recrystallization point and subsequent cooling in the course of the cold drawing and further applying the drawing work. In the case of the production of ultra-fine steel wires with a diameter of several tens of micrometers from wire rods of the aforementioned composite structure by cold drawing with a total reduction of area greater than 99.0%, optimally 99.9%, the strength of the intermediate drawn wire and that of the finally obtained ultra-fine steel wire are substantially determined solely by the strength of the wire rods having the composite structure. Accordingly, wire drawing is normally applied to wire materials of unnecessarily high strength and its repetition reduces die life or damages the ductility of the wire product. In particular, if the strength of the drawn wire rods exceeds 2941995.0 kPa (300 kgf/mm²), the die life is remarkably reduced. The present inventors have found that the strength of the drawn wire rods can be adjusted to a desired value by means of a heat treatment comprising heating to a temperature lower than the recrystallization point and subsequent cooling one or more times in the course of the drawing work when producing ultra-fine steel wires from wire rods having the composite structure described above by cold wire drawing, particularly, at a total reduction of area greater than 99.9%, as well as that ultra-fine steel wires having a final strength greater than 2941995.0 kPa (300 kgf/mm²) can be obtained while preventing any reduction in die life by controlling the strength of the drawn wire material by the heat treatment. Accordingly, a secondary object of this invention is to provide high strength and high ductility ultra-fine steel wires from low carbon steel wire rods having a predetermined composite structure, as well as a method of producing ultra-fine steel wires of improved strength, particularly, in the case of producing ultra-fine steel wires by drawing with a total reduction of area greater than 90%, and a method of producing ultra-fine steel wires without reducing the die life by applying drawing while controlling the strength of the intermediate drawn wires at a total reduction of area greater than 99%. Further, the wire rods having the above-mentioned composite structure can also be applied to the production of steel wires having brass-plated layers at the surface for use in tyre cord wires, high pressure hose wires, etc. Since these brass-plated ultra-fine steel wires have usually been produced by preparing ultra-fine steel wires of a predetermined diameter by several steps of cold drawing works while applying patenting treatment several times in the course of the drawing work to rolled high carbon steel ware rods of 5.5 mm diameter for preventing the reduction in the toughness of the drawn wire material on every drawing work and then applying brass plating thereto, a number of production steps are required and the production cost is inevitably increased. Since the lubricating treatment has usually been conducted by means of phosphate coating in the continuous cold drawing for the wire rods in the above application use, lubrication for the drawing work becomes difficult along with the increase in the working rate, and no ultra-fine steel wires with uniform surface property can be obtained due to the insufficient lubricating performance in the case of applying continuous cold wire drawing at the reduction of area greater than 90 %, preferably, 98 %. This is attributable to the fact that non-uniform deformed layers are formed at the outermost surface of the drawn rods where the drawn rods and dies are in contact upon continuous wire drawing. Since such uniform deformed layers grow and develop on every die, they become remarkable as the rate of working is increased in which the non-uniform deformed layers are extended to such a degree as to damage the ductility of the drawn wires. In the conventional high carbon steel wire rod, since the patenting treatment is applied in the course of the working the non-uniform deformed layers are not accumulated and extended, due to the insufficiency in the intense workability in the wire rod material. More specifically, if the lubricating performance is worsened during drawing, since metal-to-metal contact is introduced between the drawn wire rod and the dies, the surface of the drawn wire rod is made smooth, so that the powdery lubricant becomes less depositing on the drawn wire rod, thereby reducing the amount of lubricant introduced into the dies. The amount of the lubricant deposited on the drawn wire rod is an index representing the lubricating performance, which is made smaller as the die angle is made larger or the drawing speed is made faster. Further, the deposition amount of the lubricant is significantly reduced as the number of dies, that is, the number of repeating passes, is increased. Figure 13 illustrates the change in the deposition amount of the lubricant depending on the increase in the number of passes for the drawing wires regarding the conventional wire rods of high carbon steels subjected to lead patenting (LP) and wire rods having the composite structure with the intense workability described above. As shown by the curves II and III, when the wire rods of the foregoing composite structure are subjected to continuous cold drawing at a total reduction of area greater than 90 %, since the number of passes for the wires is increased and the amount of the lubricant is remarkably decreased along with the increased number of the passes, the cold drawing inevitably suffers from poor lubricancy and, as a result, the ductility of the drawn wires is degraded. The present inventors have found, for the method of producing brass-plated ultra-fine steel wires by using the wire rods of the composite structure having the intense workability, that brass-plated ultra-fine steel wires of high strength, and high ductility can directly be obtained without requiring heat treatment such as patenting in the course of the drawing, by applying brass-plating before or during the continuous cold wire drawing for the wire rods of the composite structure and utilizing the lubricating effect of the plated layer. In view of another aspect, the ultra-fine steel wires brass-plated at the surface have been produced by applying patenting treatment during wire drawing of the wire rods or applying brass-plating to the drawn wires after the drawing. While, on the other hand, according to this invention, brass plating is applied before or during the drawing work, whereby continuous drawing can be carried out with ease at the reduction of area greater than 98 % and, preferably, greater than 99 % due to the lubricating effect of the plating and brass-plated ultra-fine steel wires can be obtained without requiring patenting or like other heat treatment. Moreover, since the ductility is improved and the homogenization of the plated layer is enhanced by the intense work after the plating for the brass-plated ultra-fine steel wires obtained in such a method, the close bondability with rubber can significantly be improved. Accordingly, the third object of this invention is to provide brass-plated ultra-fine steel wires and a method of producing them and, particularly, brass-plated ultra-fine steel wires prepared from low carbon steel wire rods having a predetermined structure by applying continuous cold wire drawing after the brass-plating, whereby the ductility is improved and the close bondability with rubber is outstandingly excellent due to the unified and homogenized plated layer. The present invention relates to a high strength high ductility low carbon steel wire rod with an enhanced cold drawing property, the steel wire having a composite structure in which an acicular low temperature transformation phase comprising a martensite, bainite and/or the mixed structure thereof that comprises, by weight %, C : 0.02 - 0.30 %, Si : less than 2.5 %, Mn : less than 2.5 %, and the balance being iron, inevitable impurities and incidental constituents, and which may partially contain retained austenite, is uniformly dispersed at a volume ratio of from 10 to 70% in the ferrite phase and the weight of (C+N) in solution in the ferrite phase is less than 40 ppm. Further, the method of producing high strength low carbon steel wire rods excellent in the cold drawing property for attaining the first object of this invention comprises a production process of wire rods having a composite structure in which a low temperature transformation phase comprising a martensite, bainite and/or the mixed structure thereof that may partially contain retained austenite is finely dispersed in the ferrite phase, by rolling steel materials containing, on a weight basis, C : less than 0.4% Si : less than 2% and Mn : less than 2.5%, into wire rods or re-heating the wire rods followed by cooling, wherein the volume ratio of said low temperature transformation ratio is set within a range from 10 to 95%, and the average cooling rate in a temperature range from 550 to 200°C is set to lower than 40°C/sec upon cooling said wire rods. Explanation will at first be made of the chemical compositions in this invention. C has to be added at least by 0.02% in order to provide hot-rolled wire rods prepared from steel pieces with a predetermined composite structure and with a required strength. However, the upper limit for the addition amount is set to 0.30 % since excess addition will degrade the ductility of the low temperature transformation phase comprising martensite, bainite and/or the mixed structure thereof (hereinafter sometimessimply referred to as the secondary phase). Si is effective as an element for reinforcing the ferrite phase but the upper limit for the addition amount is set to 2.5 %, preferably, 1.5 % since addition in excess of 2.5 % will remarkably shift the transformation temperature toward the high temperature side and tends to cause decarbonization at the surface of the wire rods. Mn is added for reinforcing the wire rods, improving the hardening property of the secondary phase and making the configuration, preferably, acicular, but the upper limit for the addition amount of Mn is set to 2.5 % since the effect will be saturated if it is added in excess of 2.5 %. While on the other hand, since insufficient addition provides no substantial effect, Mn is added preferably by more than 0.3 %. In this invention, at least one of elements selected from Nb, V and Ti can be added further for making the metal structure of the wire rods finer. For making the structure finer, it is required to add any of the elements by more than 0.005 %. However, since the effect is saturated, if added in excess, and it is economically disadvantageous as well, the upper limit is set to 0.2 % for Nb and 0.3 % for V and Ti,respectively. Description will now be made of the elements inevitably or optimally contained in the wire rods of this invention. S is preferably added by less than 0.005 % for decreasing the amount of MnS in the wire rod, by which the ductility of the wire rod can be improved. Further, it is preferably set to less than 0.003 % in order to improve the hydrogen-resistant property. P is added preferably such that the content is less than 0.01 % since it is an element for causing remarkable grain boundary segregation. N is an element most likely to develop aging if present in a solid-solubilized state. Accordingly, it is added, preferably, by less than 0.004 % and, particularly desirably, by less than 0.002 % since it is aged during working to hinder the workability and, further, aged even after the working to degrade the ductility of the ultra-fine wires obtained by the drawing. Al forms oxide type inclusions, which are less deformable and hence may hinder the workability of the wire rod, by which breakings tend to be caused starting from the inclusions during drawing of the wire rod. Accordingly, the Al content is usually less than 0.01 % and, particularly preferably, less than 0.003 %. Further, if the Si/Al ratio in the wire rod is increased, the amount of silicate type inclusions is increased and, if the Al amount is smaller, the amount of the silicate type inclusions is increased particularly remarkably to degrade the drawing property of the wire rod, as well as degrade the fatigue property of the drawn wire obtained by drawing. Accordingly, the Si/Al ratio is set to less than 400 and, particularly preferably, less than 250 in this invention. Furthermore, the Si/Mn ratio is preferably set to less than 0.7 and, particularly desirably, less than 0.4 in this invention, because, if the Si/Mn ratio exceeds 0.7, the composition and the configuration of the inclusions are varied to degrade the drawing property of the wire rod due to the dispersion and the distribution of the inclusions. On the other hand, it is also desirable to adjust the configuration of the MnS inclusions by adding rare earth elements such as Ca and Ce. Furthermore, solid-solubilized C and N can be fixed by adding Al including Nb, V and Ti as described above. Further, depending on the application use of the ultra-fine wires according to this invention, it is also possible to properly add Cr, Cu and/or Mo by less than 1.0 % respectively, Ni by less than 6 %, Al and/or P by less than 0.1 % respectively and B by less than 0.02 %. In addition, it is essential for the wire rods according to this invention that the weight of (C+N) solid-solubilized in the ferrite phase be less than 40 ppm. That is, drawn wires having stabilized high ductility can be obtained according to this invention irrespective of the drawing speed by setting the weight of (C+N) solid-solibulized in the ferrite phase to less than 40 ppm. If the weight of (C+N) exceeds 40 ppm, the ductility of the drawn wire is degraded and it becomes difficult to obtain high strength drawn wires with a tensile strength greater than 1961330.0 kPa (200 kgf/mm²) as the working rate is increased. As has been described above, since dehydrogenation or overaging is applied under a predetermined condition to the wire rod excellent in the cold drawing property to suppress the (C+N) amount in the ferrite phase to less than a predetermined value according to this invention, the excellent drawing property of the low carbon steel wire rods can be retained and, accordingly, highly ductile wire rods can be obtained irrespective of the drawing speed, which of course cause no disconnection even during or upon high speed drawing. Particularly, drawn wires having a strength greater than 1470997.5 kPa (150 kgf/mm²) and high ductility can be obtained stably by the wire rod according to this invention at a drawing speed higher than 20 m/min and at a total reduction of area greater than 30 %. Explanation will be made of the structure of the wire rods according to this invention and the method of producing them. This invention provides a method of producing wire rods having a composite structure in which a low temperature transformation phase comprising a martensite, bainite and/or the mixed structure thereof that may partially contain retained austenite is uniformly dispersed in the ferrite phase by rolling steel materials containing the chemical compositions as described above into wire rods, or by heating them again followed by cooling, wherein the volume ratio of the low temperature transformation phase is set within a range from 10 to 95 % and the average cooling rate in a temperature range from 550 to 200°C is set to less than 40°C/sec upon cooling the above-mentioned wire rod. At first, according to this invention, a wire rod having a composite structure in which a low temperature transformation phase comprising a martensite, bainite and/or the mixed structure thereof which may partially contain retained austenite is uniformly dispersed in the ferrite phase is obtained from steel pieces having the predetermined chemical compositions described above. The method of obtaining a wire rod having such a mixed structure is described in U.S. Patent No.4578124 as cited above. Specifically, for making the secondary phase in the wire rod (low temperature transformation phase) into a fine acicular structure, heat treatment under a predetermined condition is applied to the hot-rolled wire rod having the predetermined composition as described above prior to the heating to a temperature region Ac1 - Ac3 thereby transforming the structure into a bainite, martensite and/or fine mixed structure thereof which may partially contain retained austenite and in which the grain size of the former austenite is less than 35 µm and, preferably, less than 20 microns(hereinafter sometimes referred to simply as a pre-structure). By rendering the pre-structure thus finer, the final structure can be made finer to improve the ductility and the toughness of the wire rod of the composite structure, thereby providing them with a desired strength. For adjusting the grain size of the austenite to less than 35 µm, it is necessary to apply hot working to steel pieces obtained by ingotting or continuous casting at a reduction of area greater than 30 % within a temperature range where the recrystallization or the grain growth of austenite proceeds extremely slowly, that is, within the temperature range lower than 980°C and higher than the Ar3 point, because austenite tends to recrystallize or cause grain growth if the hot working temperature exceeds 980°C and it is impossible to make the grain size of the austenite finer if the reduction of area is lower than 30 %. Furthermore, it is required to control the temperature for the final working pass to below 900°C in order to obtain fine austenite grains of about 10 to 20 µm, and it is necessary to maintain the final working step at a strain rate of greater than 300/sec in order to obtain ultra-fine grains of about 5 - 10 µm, in addition to the working conditions described above. While it is also possible to obtain a desired configuration by applying cold working after the hot working as described above for controlling the grain size of the former austenite, the working rate for the cold work should be up to 40 %. If cold working greater than 40 % is applied to the pre-structure, martensite recrystallizes upon heating to the temperature region Ac1 - Ac3 as described later, failing to obtain a desired final structure. The pre-structure of the bainite, martensite and/or the mixed structure thereof can be formed by the following methods. In the first method, a desired pre-structure is obtained during the rolling step, in which the steel piece is rolled under control or hot-rolled followed by accelerated cooling. It is necessary to set the cooling rate at more than 5°C/sec, because the usual ferrite-pearlite structure will result if the cooling rate is lower than the above-mentioned level. In the second method of obtaining the pre-structure, the rolled steel material is again subjected to a heat treatment, in which steels are heated to the austenite region above the Ac3 point followed by controlled cooling. In this method, it is also desired to control the heating temperature in a range of Ac3 ∼ Ac3 + 100°C in the same manner as referred to for the first method. In this way, where the rolled steel materials in which the structure before heating to the region Ac1 - Ac3 is a low temperature transformation phase comprising a martensite, bainite and/or the mixed structure thereof which may contain retained austenite is heated to the repion Ac1 - Ac3 instead of the conventional ferrite - pearlite structure, a great amount of initial austenite grains are formed around the retained austenite or cementite present at the lath boundary in the low temperature transformation phase as preferential nuclei and they grow along this boundary. Then, martensite or bainite transformed from the austenite is made acicular by the cooling under a predetermined condition so as to be well-matched with the surrounding ferrite phase, by which the grains in the secondary phase are made much finer as compared with the conventional ferrite pearlite pre-structure. Accordingly, it is important to determine the heating and cooling conditions to the Ac1 - Ac3 region. That is, the secondary phase becomes bulky or bulky grains are mixed in the secondary phase depending on the conditions to impair the intense workability. Referring more specifically, since the adverse transformation upon heating the pre-structure comprising a fine bainite, martensite and/or the mixed structure thereof to the austenite region is started by the formation of bulky austenite from the former austenite grain boundary and by the formation of acicular austenite within the grains up to about 20 % of the austenite ratio, a structure in which the acicular and bulky low temperature transformation phase is dispersed in the ferrite is obtained by quenching from this state at a cooling rate, for example, greater than 150 - 200°C/sec. Accordingly, as the former austenite grains are finer, the bulky austenite is produced at a higher frequency. When the austenization further proceeds to greater than 40 %, since the acicular austenite grains are joined with each other into bulky austenite, if they are quenched from this state, a mixed structure comprising ferrite and coarse bulky low temperature transformation phase is formed. Further, if the austenization proceeds to greater than about 90 %, since the bulky austenite grains are joined to each other and grow to complete the austenization, if they are quenched from this state, a structure mainly composed of a low temperature transformation phase is obtained. In this invention, the volume ratio of the secondary phase in the ferrite phase is from 10 to 70%. When the volume ratio of the secondary phase lies within the latter range, the secondary phase grains are acicular and the average grain size thereof is less than 3 µm, whereby the thus obtained wire rods have excellent intense workability due to a characteristic composite structure not known in the prior art. On the other hand, if the volume ratio of the secondary phase is outside the above range, the bulky secondary phase tends to be mixed into the final structure even if the cooling is conducted under the conditions described above. The cooling is stopped at a temperature from ambient temperature to 500°C, because the bainite, martensite and/or the mixed structure thereof as the low temperature transformation phase can be obtained, and the thus formed secondary phase can also be tempered by retarding the cooling rate or stopping the cooling within the above-mentioned temperature range. For obtaining a desired composite structure, it is also possible to formulate such a structure in the course of the wire drawing in addition to the method of previously forming the composite structure before wire drawing described above. That is, it is possible to use, as the wire rods, those having a composite structure in which a low temperature transformation phase comprising fine acicular martensite, bainite and/or the mixed structure thereof is uniformly dispersed in the ferrite phase or those having a fine ferrite-pearlite structure, and to apply the steps of drawing such wire rods to intermediate wire rods of diameter from 3.5 to 0.5 mm, applying heat treatment to the intermediate wire rods under a predetermined condition, thereby obtaining intermediate wire rods of a composite structure in which fine low temperature transformation phase comprising an acicular martensite, bainite and/or the mixed structure thereof is uniformly dispersed in the ferrite phase, and then applying cold drawing for the intermediate wire rods of the composite structure by way of cold wire drawing into ultra-fine wires of diameter from 150 to 20 µm. The conditions for the heat treatment for producing the wire rod having the predetermined composite structure as described above and for producing the intermediate wire rod of the composite structure as described above are substantially identical. However, it is necessary that the rod diameter be less than 3.5 mm for making the intermediate wire rod of the composite structure in order to provide the intermediate wire rod with the intense workability. On the other hand, the cost of the heat treatment is increased for making the composite structure if the diameter of the intermediate wire rod is too small. Accordingly, the intermediate wire rod is prepared by drawing the starting wire rod into a diameter of from 0.5 to 3.5 mm in this invention. A particularly preferred diameter for the intermediate wire rod is within a range from 0.8 to 3.0 mm. The 0.8 mm diameter is the lower limit for the drawing work capable of drawing the ferrite-pearlite structure. Then, the volume ratio of the low temperature transformation phase in the wire rod is set within a range from 10 to 70 % and, preferably, from 20 to 50 % in this invention. The strength of the obtained wire rod is poor if the volume ratio of the low temperature transformation phase is lower than 10 %. On the other hand, if the ratio exceeds 70 %, the workability is poor although a high strength is obtained. Further, in this invention, it is preferred that the ratio between the C content (wt%) in the steel of the obtained wire rod and the volume ratio of the low temperature transformation phase in the metal structure of the obtained wire rod is less than 0.005. If the value exceeds 0.005, the ductility of the secondary phase itself may be reduced. In the conventional method, no high strength wire rod can be obtained since the concentration of the C content in the residual austenite is accelerated during cooling after heating to the ferrite - austenite region and the hard secondary phase is uniformly dispersed in a small amount. In the method of producing the high strength low carbon steel wire rods according to this invention, the average cooling rate within a temperature range from 550 to 200°C during the cooling is set to below 40°C/sec. If the average cooling rate exceeds 40°C/sec, dehydrogenation for the wire rod is insufficient, making it difficult to obtain wire rods excellent in the high speed wire drawing property. The average cooling rate particularly preferred in view of the practical use usually ranges from 1 to 30 °C/sec. The method according to this invention as described above also comprises a procedure of maintaining the wire rod for a period greater than 5 sec within a temperature range from 550°C to 200°C in the course of the cooling. In the method according to this invention, it is particularly preferred that the low temperature transformation phase in the metal structure of the wire rod be of a fine acicular form and uniformly dispersed and distributed in the ferrite phase. The wire rod having such a composite structure can be obtained, for example, by preparing a wire rod having the composite structure from the steel pieces having the chemical compositions as described above, heating the wire rod to a temperature region Ac1 - Ac3 for austenization to proceed, cooling the thus obtained wire rod at an average cooling rate of 40°C/sec to obtain a wire rod having the composite structure, re-heating the wire rod for more than 5 sec. within a temperature range of from 200 to 600°C, and then applying an overaging treatment. A heating temperature outside the above-mentioned range is not suitable for the overaging treatment. Further, a treatment time shorter than 5 sec lacks effectiveness of the overaging, failing to yield the desired wire rod. As has been described above according to this invention, since wire rods having an excellent cold drawing property are applied with dehydrogenation or overaging treatment under a predetermined condition, an excellent wire drawing property can be retained therein and there is no worry of disconnection even upon high speed drawing, and high strength and high ductility ultra-fine steel wires can be obtained by such high speed drawing. Thus, according to this invention, it is possible to produce high strength and high ductility ultra-fine steel wires having a strength greater than 1470997.5 kPa (150 kgf/mm²) and, preferably, greater than 1961330.0 kPa (200 kgf/mm²), at a drawing speed higher than 20 m/min and at a total reduction of area greater than 30 %. The method of producing high strength and high ductility ultra-fine wires for attaining the second object of this invention comprises cold drawing a wire rod having a composite structure, in which an acicular low temperature transformation phase comprising acicular martensite, bainite and/or the mixed structure thereof that comprises, by weight %, C : 0.01 - 0.30%, Si : 1.5%, Mn : 0.3 - 2.5 %, and the balance of iron and inevitable impurities and incidental constituents, is uniformly dispersed in the ferrite phase at a volume ratio to the ferrite phase of 10 to 70% and at a total reduction of area greater than 90%, wherein heat treatment is applied to the drawn wire in the course of wire drawing at a temperature lower than the recrystallizing point and, further, applying wire drawing. According to the method of this invention, ultra-fine steel wires of improved strength are produced from wire rods of the composite structure in which a low temperature transformation phase containing the chemical compositions as described above and comprising an acicular martensite, bainite and/or the mixed structure thereof is uniformly dispersed in the ferrite phase, by cold drawing them at the total reduction of area greater than 90%, wherein a heat treatment is applied to the wire under drawing in the course of drawing at a temperature lower than the recrystallization point and further applying wire drawing. Particularly, it provides a method of producing high strength and ductility ultra-fine steel wires with a strength greater than 2941995.0 kPa (300 kgf/mm²) by applying cold wire drawing at the total reduction of area greater than 99%, wherein the heat treatment is applied to the drawn material in the course of the wire drawing at a temperature lower than the recrystallization point, while adjusting the strength of the drawn wire rod, thereby preventing a reduction in die life. In the method according to this invention, the heat treatment as described above means heating to such a temperature and time as not to destroy the structural flow formed with the ferrite-martensite two-phase extended in the working direction, and the heating temperature usually ranges from 200 to 700°C and, preferably, from 300 to 600°C while depending on the heating time. Generally, in the wire rods, each of the phases in the structure is extended in the working direction by the wire drawing to form a so-called structural flow, as well as dislocation microstructures being formed in each of the phases, and the strength of the drawn wire is increased depending on these changes. In the method according to this invention, the microstructure is partially recovered and slight precipitation of elements such as C and N occurs in each of the phases by applying heating to the structural flow to such an extent as not to destroy the structural flow in the course of the drawing. Accordingly, upon further applying cold drawing to the drawn wire subjected to such heat treatment, new dislocation microstructures are formed and developed around the precipitates present in the microstructures. On the other hand, since the structural flow develops on every drawing step succeeding the previous wire drawing, the working limit for the wire rod is improved and, accordingly, the strength of the drawn wire can also be enhanced. Accordingly, a minimum degree for the wire drawing is defined for forming and developing the structural flow and the dislocation microstructures due to the wire drawing before heat treatment. Further, a minimum degree of wire drawing is defined after the heat treatment so as to form and develop new microstructures. According to the study of the present inventors, both of the minimum degress of working as described above are substantially from 50 to 80%. Further, since the strength after the heat treatment and the work hardening ratio by the subsequent working are changed depending on the extent of the recovery of the dislocation microstructures and the precipitation of elements such as C and N in the heat treatment, it is preferred to optimally set the temperature and the time for the heat treatment depending on the purpose. It has been known to heat drawn wires worked to their working limit at a temperature higher than the recrystallization point, thereby eliminating the worked structure and recovering the state before the working, and then to apply drawing work again. However, the heat treatment in this case is a so-called annealing, whereas the heat treatment in the method according to this invention is heating to a temperature lower than the recrystallization point and, accordingly, it is different from the conventional annealing treatment. If the temperature for heat treatment is higher than the recrystallization point in the method according to this invention, the strength after the heat treatment is reduced, by which the strength cannot be improved even applying the cold working again subsequently and only the drawing work can be conducted. According to the method of this invention, the strength of the finally obtained ultra-fine steel wires can be improved or high strength and high ductility ultra-fine steel wires with a strength greater than 2941995.0 kPa (300 kgf/mm²) can be produced while controlling the tensile strength upon manufacturing ultra-fine steel wires by applying intense working for wire rods having a predetermined composite structure, by applying a heat treatment comprising heating to a temperature lower than the recrystallization point and subsequent cooling during wire drawing. Further, ultra-fine wires with diameters below 50 µm have previously been difficult to produce using conventional high carbon steel wire rods even if patenting treatment and wire drawing are applied several times. The method of producing ultra-fine steel wires for attaining the third object of this invention comprises a method of producing ultra-fine steel wires by applying a continuous cold wire drawing to wire rods having a composite structure, in which an acicular low temperature transformation phase mainly comprising an acicular martensite, bainite and/or the mixed structure thereof that comprises C : 0.02 - 0.30 %, Si : less than 2.5 %, Mn : less than 2.5 % and the balance of iron and inevitable impurities and incidental constituents, is uniformly dispersed in the ferrite phase at a volume ratio from 10 to 70%, wherein plating is applied before or during the wire drawing step. The brass-plated ultra-fine steel wires for attaining the third object of this invention have a chemical composition comprising by weight %: C : 0.02 - 0.30 %, Si : less than 2.5%, Mn : less than 2.5%, and the balance of iron and inevitable impurities and incidental constituents and also contain a brass-plated layer comprising: Cu : 40 - 65 %, Zn : 35 - 60 %, and the balance of inevitable impurities and incidental constituents. According to this invention, plated ultra-fine steel wires with high strength and high ductility can be obtained by applying plating to the wire rod before or during wire drawing, and then applying continuous cold wire drawing at a working rate of greater than 90 % and, preferably, greater than 98 %, thereby obtaining preferable lubricating performance for the plated layer. Particularly, ultra-fine steel wires with high strength and high ductility that are not known in the prior art can be attained by the cold wire drawing at a working rate greater than 98 % in the case of setting the volume ratio of the low temperature transformation product to 15 - 40 % and the average grain size to less than 3 µm. In this invention, the plating treatment means to deposit highly ductile plated layers onto the wire rod by means of electrical plating, chemical plating, molten plating or the like. There is no particular restriction on the plating composition and the composition can include, for example, Cu, Cu alloys, Al and Al alloys. Further, plating deposits may be in the form of a single layer or a plurality of layers, which can be homogenized subsequently. In this invention, the composition for the brass plating lies within a range of Cu 40 - 70 % and Zn 60 - 30 %. In the conventional method of producing surface-plated ultra-fine steel wires by applying plating after the drawing of the wire rod, the composition for the brass-plating usually contains Cu 60 - 70 % and Zn 40 - 30 %. It has been considered that, if Zn is used in a greater amount, the quality of the plated ultra-fine steel wires will be degraded due to the poor ductility of the plated layer. However, in the method according to this invention, if the Zn amount is increased to such a range as Cu 40 - 65 % and Zn 60 - 35 %, the plated layer exhibits a preferable lubricating effect for the wire drawing upon applying intense working utilizing the layer as a lubricant to ensure excellent continuous cold drawing properties while preventing the formation of irregular layers on the surface of the drawn wire upon wire drawing, although the reason therefor has not yet been clear at present, as well as the ductility of the thus obtained drawn wire being unexpectedly improved and, further, surface-plated ultra-fine steel wires having a uniform and homogenous plating layer can be obtained. Particularly, the surface brass-plated ultra-fine steel wires according to this invention in which the amount of Zn is increased have a remarkably improved close bondability with rubber as compared with conventional surface-plated ultra-fine steel wires. In this invention, the plating has to be deposited in such an amount as to be capable of yielding an uniform plating thickness after the intense drawing work and, preferably, it is about from 1 to 15 g per 1 kg of the wire rod although depending on the diameter of the ultra-fine steel wires. Particularly, in the intense drawing of greater than 98 %, the property of the plating layer itself, for example, uniform and homogenous property can be improved extremely by maintaining the amount of the plated layer within a range from 0.2 to 1.0 % by weight based on the finally obtained ultra-fine steel wires. In this invention, it is desirable to set the approaching angle of the drawing dies to 4 - 15° in the drawing work for the wire rod after the plating and the approaching angle is more desirably set to 4 - 8° in the initial half of the wire drawing at the total working rate of about 80 % after plating and the drawn wire strength of less than 1176798 kPa (120 kgf/mm²). In this way, uniform working for the plated layer is facilitated and irregularlity of the plated layer can be prevented. Furthermore, by the method according to this invention, ultra-fine steel wires having higher final strength can be obtained upon producing such wires by applying continuous cold wire drawing to the wire rods of the composite structure as described above at a total reduction rate of greater than 90 %, by applying a heat treatment comprising heating to a temperature lower than the recrystallization point during drawing and subsequent cooling, since the increase in the strength relative to the reduction of area is greater as compared with the case of applying no such heat treatment. In the case where molten plating is employed in the plating treatment for the method according to this invention, the heat treatment as described above can be carried out simultaneously by adjusting the plating composition to have a desirable melting point. That is, the plating bath can be utilized as the heating bath and/or cooling bath in the heat treatment. In the method according to this invention, the heat treatment as described above means such heating at such a temperature and within a time as not to destroy the structural flow formed with the ferrite and martensite two phases extended in the working direction, and the heating temperature usually ranges from 200 to 700°C and, preferably, from 300 to 600°C while depending on the heating time. Generally, in the wire rods, each of the phases in the structure is extended in the working direction by the wire drawing to form a so-called structural flow, as well as dislocation microstructures being formed in each of the phases, and the strength of the drawn wire rod is increased due to these changes. In the method according to this invention, the microstructure is partially recovered and slight precipitation of elements such as C and N occurs in each of the phases by applying heating to such an extent as not to destroy the structural flow in the course of the drawing. Accordingly, upon further applying cold drawing to the drawn wire subjected to such heat treatment, new microstructures are formed and developed around the precipitates present in the microstructures. While on the other hand, since the structural flow develops on every drawing step succeeding to the previous wire drawing, the working limit for the wire rod is improved and, accordingly, the strength of the drawn wire rod can also be enchanced. Accordingly, a minimum degree of wire drawing is defined for forming and developing the structural flow and microstructures in the wire drawing before heat treatment, while a minimum degree of wire drawing is defined after the heat treatment so as to form and develop new microstructures in the drawing work. According to the study of the present inventors, both of the minimum degrees of working as described above are substantially from 50 to 80 %. Further, since the strength after the heat treatment and the work hardening ratio by the subsequent working are changed depending on the extent of recovery of the dislocation microstructures and the precipitation of elements such as C and N in the heat treatment, it is preferred to optimally set the temperature and the time for the heat treatment depending on the purpose. It has been known to heat drawn wires worked to their working limit to a temperature higher than the recrystallization point, thereby eliminating worked structure and recovering the state before the working and then applying the drawing work again. However, the heat treatment in this case is a so-called annealing treatment, whereas the heat treatment in the method according to this invention is the heating to a temperature lower than the recrystallization point and, accordingly, it is different from the conventional annealing treatment. If the temperature for the heat treatment is higher than the recrystallization point in the method according to this invention, the strength after the heat treatment is reduced, by which the strength cannot be improved even when applying cold working again subsequently and only the drawing work can be conducted. Upon producing ultra-fine steel wires by applying intense cold working to wire rods having a predetermined composite structure, according to this invention, wire rods can be cold-drawn while ensuring a preferred cold drawing property by applying plating treatment before or during the wire drawing and utilizing the lubricating effect of the plated layer, and ultra-fine steel wires having an uniform and homogenous plated layer and of improved ductility can be obtained in this way. Further, the strength of the finally obtained ultra-fine steel wires can be improved by applying a heat treatment comprising heating to a temperature lower than the recrystallization point and subsequent cooling during the wire drawing work. Further, the surface brass-plated ultra-fine steel wires according to this invention are highly excellent in the close bondability with rubber since the brass-plating containing Zn in a greater amount than usual is made uniform and homogenized due to the intense work to the wire rods. Furthermore, the strength of the finally obtained ultra-fine steel wires can be improved by applying heat treatment comprising heating to a temperature lower than the recrystallization point and subsequently by cooling in the course of the wire drawing step. DESCRIPTION OF THE ACCOMPANYING DRAWINGS These and other objects, as well as advantageous features of this invention will become apparent by reading the following description of preferred embodiments of this invention in conjunction with the accompanying drawings, wherein: Figure 1 is a graph showing the relationship between the drawing speed and the tensile strength and reduction of area at break in high strength wire rods comprising a composite structure having a low temperature transformation phase; Figure 2 is a graph showing the relationship between the drawing speed and the tensile strength and reduction of area at break in high strength and high ductility wire rods comprising a fine acicular low temperature transformation phase ; Figures 3 and 4 are graphs showing the drawing strain in the wire rod and the tensile strength and the reduction of area at break of the drawn wire obtained by the method according to this invention relative to different drawing speeds ; Figures 5 and 6 are graphs showing the drawing strain upon high speed drawing and the tensile strength and the reduction of area at break of the thus obtained drawn wire with respect to the drawn wire by the method according to this invention and the drawn wire of a comparative example; Figure 7 is a graph showing the relationship of the configuration of the low temperature transformation phase and the volume ratio thereof in the ferrite phase; relative to the heating temperature and the average cooling rate when the steels having the composition as defined in this invention are heated to the Ac1 - Ac3 region, followed by cooling at comparison cooling rates. Figure 8 is a graph showing the relationship between the volume ratio of the secondary phase and the configuration and average grain size in the secondary phase; Figure 9 is a graph showing the relationship among the drawing strain, temperature for the heat treatment and the tensile strength for the drawn wire thus obtained when the wire rod of a composite structure is heat treated in accordance with the method of this invention; Figure 10 is a graph showing the relationship among the drawing strain, the diameter of the intermediate drawn wire and the tensile strength of the thus obtained drawn wire when the wire rod of the composite structure of a predetermined diameter is heat-treated in accordance with the method of this invention; Figure 11 is a graph showing the heat resistivity of the ultra-fine steel wires according to this invention ; Figure 12 is a graph showing the relationship between the drawing strain and the tensile strength of the drawn wire rod upon drawing the wire rod of the composite structure by the method according to this invention; and Figure 13 is a graph showing the relationship between the reduction of area and the deposition amount of the lubricant in the case of subjecting a conventional high carbon steel and a wire rod of composite structure used in this invention, respectively, to dry continuous wire drawing. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention will now be explained specifically referring to examples. Example 1 Steels represented by reference R1 having a chemical composition as shown in Table 1 were rolled into a wire rod of 10 mm diameter and subjected to controlled cooling, at an average cooling rate of 2°C/sec at a temperature within a range from 550 to 200°C by a Stelmor cooling, thereby producing a wire rod of a composite structure in which martensite was uniformly dispersed in ferrite at a volume ratio of 16 %. Further, steels represented by reference R2 were rolled into a wire rod of 5.5 mm diameter and directly hardened, thereby producing a wire rod of a composite structure in which martensite was uniformly dispersed in ferrite at a volume ratio of 70 %. Then, the thus obtained wire rods were subjected to overaging treatment at 330°C for 5 minutes. The results for the measurement of weight of solid solubilized (C+N) based on the internal friction in these wire rods are shown in Table 1 below. Each of the thus obtained wire rods was subjected to wire drawing after pickling and lubricating treatment. As shown by the result of Figure 3, the wire rod corresponding to the steels R1 shows no degradation in the ductility of the drawn wire depending on the drawing rate. Further, as shown in Figure 4, a high strength and high ductility drawn wire with a tensile strength greater than 1961330.0 kPa (2000 kgf/mm²) could be produced by drawing the wire rod corresponding to steels R2 at a drawing rate of 50 m/min. Example 2 Steels A and B having the chemical compositions shown in Table 1 were respectively rolled into wire rods of 5.5 mm diameter and directly hardened to form a structure mainly composed of martensite. Then, the wire rods were re-heated to a ferrite-austenite two phase region, followed by cooling into an acicular low temperature transformation phase. The volume ratio of the low temperature transformation phase was 20 % for the wire rod prepared from steels A and 25 % for the wire rods prepared from steels B. The results of the measurement for the weight of the solid-solubilized (C+N) due to the internal friction in these wire rods are shown in Table 1. Then, these wire rods A and B were re-heated followed by cooling, in which wire rods obtained by cooling with water from the re-heated temperature 800°C are respectively referred to as comparative wire rods A1 and B1 (the average cooling rate within a range from 550 to 200°C is 115°C/sec), while the wire rods obtained by controlled cooling from about 550°C in the course of water cooling with respect to the wire rod A is referred to as the wire rod A2 according to this invention (average cooling rate was 25°C/sec at a temperature from 550 to 200°C). In the same way, the wire rod obtained by water cooling the wire rod B from 800°C and then interrupting the cooling for 10 sec at about 350°C is referred to as the wire rod B2 according to this invention. The aging change in the ductility after the heat treatment to the cold wire drawing for each of the wire rods was evaluated by the reduction of area at break (%), which is shown in Table 2. Degradation in the ductility with the lapse of time after the heat treatment is remarkable both in the wire rods A1 and B1 as comparative wire rods and the degradation in the ductility due to pickling was also remarkable. That is, it may be understood that these wire rods have high hydrogen sensitivity. Then, drawing results for the comparative wire rod A1 and the wire rod A2 of the invention are shown in Figure 5. While both of the wire rods had metal structures excellent in the intense cold drawing property, degradation in the ductility was observed at the drawing strain greater than about 3 in the course of the high speed drawing for A1. While on the other hand, wire drawing at the drawing strain greater than 6 was possible even under high speed drawing for A2 and high strength and high ductility drawn wire having a tensile strength of 2451662.50 kPa (250 kgf/mm²) could be obtained. Further, although both of the comparative wire rod B1 and the wire rod B2 of the invention had metal structures excellent in the intense cold drawing property, degradation in the ductility resulted in the wire rod B1 in the state as water cooled in the course of the high speed drawing and high strength and high ductility drawn wire having a tensile strength of greater than 1961330.0 kPa (200 kgf/mm²) could not be obtained as shown in Figure 6. In addition, drawing work at the drawing strain greater than 5 was difficult. Comparative Example 1 (Production and properties of wire rods of composite structure) Steels A and B having chemical compositions as shown in Table 3 were rolled followed by water cooling to form fine martensite pre-structures, which are respectively referred to as A1 and B1. As a comparison, steels A were rolled followed by air cooling to form a ferrite-pearlite pre-structure, which is referred to as A2. The former austenite grain size was less than 20 µm in either of the cases. Then, A1 and B1 were heated and maintained for three minutes within the Ac1 - Ac3 region so as to have different austenizing ratios and they were cooled to room temperature at various average cooling rates. Figure 7 shows the configuration and the volume ratio of the grains in the secondary phase relative to the heating temperature and the cooling rate. The solid line represents an uniform mixed structure of ferrite and secondary acicular phase, while the broken line shows the mixed structure of ferrite and secondary bulky phase, or a mixed structure of ferrite and acicular or bulky secondary phase. When cooling at an average cooling rate of 125°C/sec or 80°C/sec, the configuration of the secondary phase of the rolled wire rode was acicular and the structure was composed of the secondary phase uniformly dispersed in the ferrite phase. The volume ratio of the secondary phase was substantially constant irrespective of the heating temperature. On the other hand, if the average cooling rate was higher than 170°C/sec, the configuration of the secondary phase was bulky or a mixture of bulky and acicular grains and the secondary phase ratio was increased as the heating temperature was higher. Figure 8 shows the relationship between the volume ratio of the secondary phase and the calculated average grain size of the secondary phase grains contained in the final structure with respect to the steels A1 and B1 as the martensite pre-structure, as well as the steels A2 and B2 as the ferrite - pearlite pre-structure, respectively. In this case, the calculated average grain size means the average diameter when the area is converted into that of a circle for any of the configurations. While the size of the secondary phase grains was enlarged along with the increase in the volume ratio of the secondary phase for any of the rolled wire rods, the size of the grains obtained from the martensite pre-structure was much smaller as compared with that obtained from the ferrite - pearlite pre-structure for the identical secondary phase ratio. That is, even for the steel pieces having an identical composition, the size of the grains in the secondary phase could be made extremely finer by conditioning the pre-structure from the ferrite-pearlite to martensite structure. Although the ductility in the rolled wire rods could significantly be improved by making the secondary phase grains finer, it did not always lead to the improvement in the intense workability. That is, when the secondary phase volume ratio was set to a range from 15 to 40 %, the secondary phase became predominantly acicular, the secondary phase was composed of fine acicular grains with the calculated average grain size of less than 3 µm and, further, the fine acicular secondary phase was uniformly dispersed and distributed into ferrite, whereby excellent intense workability was attained. Of course, the foregoing situation is also applicable to the case where the secondary phase comprises acicular bainite, or the structure in admixture with martensite. Then, Table 4 shows the conditions for heating and cooling, the final structures and the mechanical properties for the rolled wire rods A1 and A2. It is apparent that the wire rods represented by steels Nos 3, 4, 5 and 6 prepared by heating the wire rod A1 in which the pre-structure comprises fine martensite to the Ac1 - Ac3 region such that the austenizing ratio is more than 20 %, followed by cooling at 125°C/sec have a composite structure in which fine acicular martensite (secondary phase) is uniformly mixed and dispersed in the ferrite phase at a volume ratio in a range from 15 to 40 % and are outstandingly excellent in the balance between the strength and the ductility. On the other hand, the rolled wire rod A2 having the ferrite-pearlite pre-structure formed the steels Nos 10, 11 or 12, in which the secondary phase was in a bulky form irrespective of the heating and cooling conditions, any of which was poor in the balance between the strength and ductility. Furthermore, even if the pre-structure was composed of martensite, steels Nos 1 and 2 were in the fine mixture of ferrite and bulky and acicular martensite since the cooling rate after heating to the Ac1 - Ac3 region was too low for steel No.1 and since the austenizing ratio upon heating to the Ac1 - Ac3 region is 16 % for steel No.2 ; accordingly, they were inferior to the steel materials according to this invention although excellent over steels Nos 10 - 12 described above in the balance between the strength and the ductility. Wire rods of 6.4 mm diameter having different secondary phase configurations were subjected to intense cold drawing. Table 5 shows the properties after the drawing work. From the wire rod of the steels No. 1, a wire rod of 2 mm diameter with a tensile strength of 8825998.5 kPa (90 kgf/mm²) and reduction of area at break of 58% could be obtained at the working rate of 90 %, while a wire rod of 0.7 mm diameter of a further higher strength could be obtained at the working rate of 98 %. On the other hand, for the comparative steel wire rod of the steel number 2 having the bulky secondary phase, the ductility was rapidly degraded with the increase of the working rate and disconnection resulted at a working rate of about 90 %. The comparative wire rod of steel No. 3 had a structure finer than that of steel No. 2, and although it was excellent over steel No. 2 in view of the intense workability, the degradation in the property after the working was remarkable as compared with that of steel No. 1. As shown in Table 3, the comparative steel B and steel C having a chemical composition as defined in this invention were formed into wire rods of 5.5 mm diameter having a uniform fine composite structure comprising ferrite and acicular martensite according to this invention, which are referred to as B1 and C1 respectively. Table 6 shows the mechanical properties of wire rods B1 and C1 and the mechanical properties of drawn wire material worked into ultra-fine steel wires of a diameter below 1.0 mm. Both of the wire rods B1 and C1 had high ductility and could be intensely worked at 99.9 % rate, and the thus obtained wire rods also had high strength and high ductility. Table 4 also shows the mechanical properties of wire rod C1 after drawing at a working rate of 97 % into a drawn wire (0.95 mm diameter) and then annealing at a low temperature from 300 to 400°C. It is apparent that the ductility of the wire rods was improved due to the annealing at low temperature. Reduction in the strength is not recognized. Accordingly, the ductility of the wire material can be improved by the heat treatment of annealing at low temperature and, further, the ductility of the obtained drawn wire can further be improved by combining the annealing at low temperature with the step in the course of the drawing of the wire material. Example 3 (Production of ultra-fine steel wires) Steel pieces A and B having the chemical compositions shown in Table 7 were hot rolled into wire rods of 5.5 mm diameter, rolled and then cooled with water. The rolled wire rods were heated to 810°C, cooled in water into martensite and thereby formed into wire rods A and B having a mixed structure of the secondary phase mainly composed of martensite and ferrite. The wire rod A was subjected to pickling and brass-plating, then drawn into 0.96 mm diameter, subjected to a heat treatment to a predetermined temperature and further drawn to a diameter of 0.30 mm. For the comparison, the wire rod A was subjected to pickling and brass-plating, and then drawn into 0.30 mm diameter without applying heating treatment in the course of the wire drawing. Figure 9 shows the drawing strain after the heat treatment and tensile strength of the obtained ultra-fine steel wires. It is apparent that the strength was remarkably increased due to the drawing after the heat treatment. Next, the wire rod B was subjected to pickling and lubrication, then drawn into diameters of 0.96 mm, 1.20 mm, 1.50 mm and 1.80 mm, applied with brass-plating respectively, and then subjected to a heat treatment of heating to a temperature of 500°C for one minute, followed by cooling and then further drawn respectively into ultra-fine steel wires of 0.25 mm diameter. For the comparison, the result of drawing the wire rod B of 5.5 mm diameter with no heat treatment is shown by the dotted line. The work hardening rate was apparently increased by the heat treatment, and, according to the method of this invention, the strength of the ultra-fine steel wires was significantly improved by about 490332.5 kPa (50 kgf/mm²). Figure 11 shows the heat resistance of ultra-fine steel wires of 0.25 mm diameter which were the final drawn wire material obtained as described above, and the reduction in the strength due to the temperature was low in the steel wires according to this invention. On the other hand, the reduction in the strength was remarkable in the comparative steel wires described above. Example 4 (Production of ultra-fine steel wires) Steels C having the chemical compositions shown in Table 7 were hot rolled into a wire rod of 5.5 mm diameter, and then rolled followed by cooling in oil. The rolled wire rod was heated to 810°C, cooled with water into martensite thereby to produce a wire rod having a mixed structure comprising a secondary phase mainly composed of martensite and ferrite (the composition of Steel C is shown in Table 7). In the course of drawing the wire rod C into ultra-fine steel wires of 0.06 mm diameter (total reduction of area 99.99%), the rod was once drawn into a wire rod of 0.58 mm and 0.15 mm diameter and subjected to heat treatments as shown in Figure 12. Figure 12 shows the relationship between the drawing strain and the tensile strength of the obtained drawn wire. That is, according to this invention, high strength and high ductility ultra-fine steel wire having a final strength greater than 300 kgf/mm² could be obtained while adjusting the strength of the drawn wire rod in the course of the drawing to less than 300 kgf/mm² and improving the life of the drawing dies as shown in the drawing. For the comparison, the wire rod C was drawn to 0.15 mm diameter without applying heat treatment in the course of the step. As shown in the figure together with the result, it is apparent that the strength was remarkably increased along with the wire drawing and an unfavourable effect was given on the die life and on the characteristics of the drawn wire rod. Example 5 Steels represented by the reference A in Table 8 were hot rolled into wire rods of 5.5 mm diameter, cooled with water into structures mainly composed of martensite, heated to 820°C and cooled at a rate of 15°C/sec after the heating in the heat treatment to prepare a mixed structure of ferrite and acicular martensite, which is referred to as A1. Table 9 shows the mechanical properties of drawn wires obtained by pickling the wire rods A1 of 5.5 mm diameter, applying brass plating of Cu and applying continuous cold wire drawing at the total reduction of area at 97 %. Table 10 also shows the mechanical properties of drawn wires prepared by pickling the same wire rods A1, applying conventional lubricating treatment of phosphate coating and then applying continuous cold wire drawing together for the comparison. The wire rods A1 applied with lubricating treatment by ordinary phosphate coating as the pretreatment to the wire drawing contained less deposition amount and result in poor lubricaney. While on the other hand, in the case of applying brass-plating before the wire drawing, undesired effect on the drawn wire could be avoided due to the lubricancy of the plating present at the surface of the drawn wire, for example, if the amount of powdery lubricant introduced upon wire drawing work was insufficient, as seen in the drawn wire from the wire rod A1. That is according to this invention, the lubricanting property upon wire drawing was improved due to the brass-plating before the wire drawing.

Claims: A high strength high ductility low carbon steel wire rod with an enhanced cold drawing property said steel wire rod having a composite structure in which an acicular low temperature transformation phase comprising a martensite, bainite and/or the mixed structure thereof which comprises, by weight %, C : 0.02 - 0.30%, Si : less than 2.5%, and Mn : less than 2.5%, the balance being iron, inevitable impurities and incidental constituents, and which may partially contain retained austenite, is uniformly dispersed at a volume ratio of from 10 to 70% in a ferrite phase, characterised in that the weight of (C+N) in solution in the ferrite phase is less than 40 ppm. A steel wire rod as defined in Claim 1, wherein the structure comprises, by weight %, Al : less than 0.01%, P : less than 0.01%, S : less than 0.005%, N : less than 0.004%, and in which the ratio Si/Al is less than 400 and Si/Mn is less than 0.7. A method of producing a high strength high ductility low carbon steel wire rod as defined in claim 1 or 2, characterised in that the wire rod is subjected to a cooling treatment, either during production or after reheating, within a temperature range of from 550 to 200°C at an average cooling rate of below 40°C/sec. A method according to Claim 3, further characterised in that, the steel wire rod is subjected to an annealing or tempering step by heating for more than five seconds within a temperature range of from 600 to 200°C. A method of producing high strength and high ductility ultra-fine steel wires, characterised in that a steel wire rod having a composite structure as defined in Claim 1 or Claim 2, and obtainable by a method according to Claim 3 or 4, is subjected to cold drawing at a total reduction of area greater than 90%, in that a heat treatment is applied at a temperature lower than the recrystallization point to the drawn wire rod in the course of the drawing, and in that subsequent drawing is further applied. A method of producing ultra-fine steel wires characterized in that a steel wire rod having a composite structure as defined in Claim 1 or Claim 2, and obtainable by a method according to Claim 3 or 4, is subjected to continuous cold drawing at a reduction of area greater than 90%, and in that plating is applied before and during the wire drawing. A brass-plated ultra-fine steel wire obtainable from a brass-plated steel wire rod with an inner steel composite structure as defined in Claim 1 or 2, and produced by a method according to Claim 3 or 4, and having an outer brass-plated layer comprising: Cu : 40 - 65%, and Zn : 35 - 60%, the balance being inevitable impurities and incidental constituents, by continuous cold drawing at a reduction of area greater than 90%.

Inventors: HOSOGI YAHUHIRO, KATOH TAKEHIKO, KATSUMATA MASAAKI, YUTORI TOSHIAKI, HOSOGI, YAHUHIRO, KATOH, TAKEHIKO, KATSUMATA, MASAAKI, YUTORI, TOSHIAKI
Applicants: KOBE STEEL LTD, KABUSHIKI KAISHA KOBE SEIKO SHO ALSO KNOWN AS KOBE STEEL LTD.
Application year: 1986
Main classification: C21D1
IPC codes: C21D8