JP2013081982A - Extra-fine steel wire having excellent delamination-resistance characteristics and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the delamination resistance characteristics in an extra-fine steel wire which is obtained by executing the cold wire drawing of a high-carbon steel wire having a pearlite structure, and has the tensile strength of ≥3,000 MPa, and has a substantially circular section with its wire diameter being 50-380 μm.SOLUTION: Copper plating or brass plating is provided on a surface of an extra-fine steel wire. A projection part is formed, in which a boundary line between the steel wire of the cross section of the extra-fine steel wire and the plating enters inwardly in a projection shape from a substantially circular shape of the basic cross section of the steel wire. The depth of the projection part is ≤1.0 μm. The maximum length of cracks present in the projection part of the plating is ≤0.8 μm, and the angle formed between the crack propagating direction and the radial direction of the cross section of the steel wire is ≥35°.

Description

  The present invention relates to high-strength ultrafine steel wires such as tires, belt cords, high-pressure hoses, etc., steel cords used for reinforcing rubber and organic materials.

  In general, ultra-fine steel wires are subjected to hot rolling, adjusted and cooled high carbon wire with a diameter of about 4.0 to 5.5 mm, and intermediate wire drawing and dry primary wire drawing according to the ductility deterioration of the steel wire. Repeatedly, after final patenting with a wire diameter corresponding to the target tensile strength, copper plating or brass plating treatment is performed, and wet drawing is performed.

  In order to use the ultra-fine steel wire thus manufactured as a reinforcing material for tires, belt cords, high-pressure hoses, etc., stranded wire processing is usually performed. At the time of stranded wire processing, a plurality of ultra fine steel wires are twisted together at a high speed, so that the individual ultra fine steel wires are required to have ductility to withstand disconnection.

In response to such demands, the development of extra fine steel wires has been made. For example, in Japanese Patent Laid-Open No. 60-204865, for steel wires having a tensile strength of 2450 N / mm ² or more and a diameter of 0.5 mm or less, the Mn content is defined to suppress the formation of supercooled structures during patenting. And the technique which improves the intensity | strength and tough ductility of a wire rod by prescribing | regulating content of C, Si, and Mn, and aims at reduction of a strand wire break is disclosed.

  However, in recent years, in response to demands for weight reduction and high performance of tires, steel cords have been rapidly increased in tension, and those with a tensile strength exceeding 3000 MPa are becoming mainstream. When the tensile strength of a steel wire increases, ductility generally decreases, vertical cracks called delamination occur, and there is a tendency that breakage tends to occur during stranded wire processing.

  Therefore, in recent years, various developments have been made as described below in order to obtain ductile steel with a high strength exceeding 3000 MPa in tensile strength and to obtain an ultrafine steel wire that is difficult to break.

  In Patent Document 1, the upper limit of the heating temperature at the time of patenting is defined, and the time when the surface layer temperature of the wire becomes lower than the internal temperature after the start of the cooling stage and before the pearlite transformation is set, and the average pearlite nodule size of the surface layer is A technique for improving the twisting characteristics of a steel wire by making the structure smaller by 0.3 μm or more than the inside is proposed.

  In patent document 2, in the forced cooling stage from the austenitizing temperature at the time of patenting, after cooling once to 500 to 560 ° C., the pearlite transformation is performed after reheating, whereby the cross section of the wire drawing material (hereinafter, Longitudinal cracks (delamination) in high-strength materials by controlling the product (Da x Db) of the major axis length (Da) and minor axis length (Db) of ferrite grains in C section) ) Has been proposed.

  In Patent Literature 3, the upper limit of the maximum void diameter contained in the steel wire is defined in relation to the shear yield stress of the steel wire, and the upper limit value of the tensile residual stress value of the surface layer and the variation in the circumferential direction thereof are defined. Therefore, we have proposed a technology for manufacturing steel wires with excellent delamination resistance.

  Patent Document 4 specifies the strength of the final patenting material, the value of (area of pro-eutectoid ferrite / area of cementite), and the wire drawing method after patenting. We have proposed a technology that suppresses delamination by keeping the value below the wire diameter.

  In general, aging of steel wires during or after wire drawing is considered to have an adverse effect on ductility. Methods for suppressing heat generation during wire drawing as much as possible, Many proposals have been made to reduce the coefficient of friction.

For example, in Patent Document 5, by performing wire drawing in a wet lubricant at a temperature of 5 ° C. or lower, the heat generation during wire drawing is suppressed, thereby suppressing the aging of the steel wire and improving the twist value of the steel wire. There has been proposed a method of improving the delamination resistance characteristics.
Patent Document 6 proposes to suppress heat generation during wire drawing by setting the coefficient of friction between the die and the steel wire to less than 0.07.

Japanese Patent Application Laid-Open No. 11-241280 Japanese Patent Laid-Open No. 11-199978 JP 2001-279380 A JP 2001-279281 A JP 2002-28716 A JP-A-11-309509

The present inventors examined application of the above-described techniques to a high-strength material exceeding 3000 MPa. By adjusting the steel structure and suppressing the residual tensile stress on the surface, it was not always possible to obtain a clear effect of suppressing delamination during twisting.
The present invention has been made in view of such circumstances, and an object of the present invention is to suppress the delamination of the steel wire starting from the surface and to supply an ultrathin steel wire having high ductility. It is.

The ultra fine steel wire is manufactured by applying the copper plating or the brass plating to the surface of the steel wire after patenting and pickling as described above, and then wet-drawing it. The present inventors investigated the form of the interface between the plating and the steel wire base material, which has not been noticed in the past, and examined the relationship with the delamination.
As a result, it has been found that there are many places where the plating penetrates into the inside of the steel wire, the existence form of the protrusion has a great influence on the delamination resistance characteristics, and further, as a result of examining the protrusion in detail, The present invention has been achieved to solve the above-mentioned problems, and the gist of the present invention is as follows.

(1) C: 0.75 to 1.10%, Si: 0.5 to 2.0%, Mn: 0.2 to 2.0%, cold drawn steel wire having a pearlite structure An ultrafine steel wire having a circular cross section with a tensile strength of 3000 MPa or more and a wire diameter of 50 to 380 μm obtained by processing,
The surface of the steel wire has copper plating or brass plating, and the boundary between the steel wire base material and the plating in the cross section of the ultra fine steel wire protrudes inward from the outer circumference circle of the basic ultra fine steel wire cross section. The maximum depth of the plating protrusion formed thereby is 1.0 μm or less, the maximum length of the crack existing in the protrusion is 0.8 μm or less, and An ultra fine steel wire with excellent delamination resistance, characterized in that the angle formed between the direction of progress and the radial direction of the cross section of the ultra fine steel wire is 35 ° or more.
(2) In the ultra fine steel wire according to (1) above, an average number per unit circumferential length of the protrusions present along the circumferential direction of the cross-sectional surface of the steel wire is 0.5 piece / μm or less An ultra-fine steel wire with excellent delamination resistance characteristics.
(3) C: 0.75 to 1.1%, Si: 0.5 to 2.0%, Mn: 0.2 to 2.0%, and a steel wire having a pearlite structure is primarily drawn. In the method for producing an ultrafine steel wire having excellent delamination resistance as described in (1) or (2) above, which is subjected to final patenting, pickling, copper plating or brass plating, and wet wire drawing.
The temperature in the furnace at the time of reheating at the time of the final patenting is controlled to 800 to 1050 ° C., the heating time is controlled to 10 minutes or less in the in-furnace time, and the pickling after the final patenting is 15 to 30% by mass. In this case, hydrochloric acid having a temperature of 20 to 45 ° C. is used and the pickling time is 120 minutes or less, and the maximum value (Rmax) max of the surface roughness Rmax of the steel wire after the pickling is 4.5 μm or less. A method for producing an ultra fine steel wire, characterized in that the thickness of copper or brass plating before wet drawing is 1 to 10 μm.

  As in the present invention, by controlling the protrusion of copper plating or brass plating on the surface of the steel wire and the form of cracks therein, it has an ultrafine high ductility while having a tensile strength of 3000 MPa or more. Since it can be made of steel wire and the occurrence of delamination during twisted wire processing can be suppressed and the occurrence of disconnection can be reduced, the effect is extremely large in industry.

It is the figure which showed roughly the state of plating of the C cross section of an ultra fine steel wire. It is a figure which shows typically the biting state of plating of the C cross section of an ultra fine steel wire. It is a figure which shows an example of the torque curve obtained by the evaluation test apparatus of delamination resistance. It is a figure which shows typically the surface state of the steel wire after pickling. It is a figure which shows typically the state of the longitudinal cross-section of the steel wire after pickling and plating. It is a figure which shows typically the outline | summary of the evaluation test apparatus of delamination resistance.

The ultra fine steel wire is manufactured by applying the copper plating or the brass plating to the surface of the steel wire after patenting and pickling as described above, and then wet-drawing it. The present inventors investigated the form of the interface between the plating and the steel wire base material, which has not been noticed in the past, and examined the relationship with the delamination.
In the examination, the heating conditions at the time of patenting and the thickness of the brass plating were changed to produce a large number of ultrafine steel wires, samples were cut out from each steel wire, and the cross section (C cross section) was taken with an electron microscope ( SEM) was observed in detail. Moreover, the delamination resistance evaluation test of the produced ultrafine steel wire was carried out, the delamination resistance of each steel wire was evaluated, and the relationship between the cross-sectional shape of the plating and the delamination resistance characteristics was investigated.

In the observation of the interface between the ultrafine steel wire plating and the steel wire preform, as shown schematically in the enlarged surface (a) of FIG. 1, the boundary between the steel wire preform and the plating 1 in the cross section of the ultrafine steel wire is There are many places where protrusions 3 are formed by plating into the inside of the steel wire base material 2 rather than the outer peripheral circle of the basic ultrafine steel wire cross section, and are formed toward the inside of the steel wire base material 2. It was found to exist. In addition, since the plating 1 remains in a state where the plating 1 remains very thin in other portions, the protrusions 3 formed by the plating are clearly distinguishable.
Further, when the projection 3 was observed in detail, as shown in FIG. 2, some of the projections 3 have cracks 4 from the surface to the inside of the projections. As shown in FIGS. 2 (a) and 2 (b), it has been found that there are those extending from the surface toward the center of the steel wire and those extending obliquely from the surface as shown in FIG. 2 (c). It was.

Therefore, the relationship between the shape of the protrusion and the crack existing in the protrusion and delamination was further investigated.
Regarding the shape of the protrusions and cracks, the length of L (μm) was observed along the circumferential direction of the sample in the C cross section of the sample obtained from the produced ultrafine steel wire, and the number of protrusions in the range of the length L n was counted to obtain an average number n / L of protrusions, and the depth D of the protrusions was measured to determine the maximum depth in the protrusions. When a crack exists in the protrusion, the crack length S and the crack folding angle θ were measured.
Note that the crack length S is the length in the crack propagation direction, that is, the length of the line connecting the starting point (width center) and the ending point of the crack, and the crack folding angle θ is As shown to (d), it is set as the angle between the line 5 which goes to the propagation direction of a crack, and the line 6 which goes to a radial direction.

  Next, the delamination resistance evaluation test of another sample was carried out, and from the obtained torque curve, the ultrafine steel wire was judged to be sufficiently ductile without causing delamination (evaluation ○), It was classified into two types (devaluation x) in which delamination occurred and it was judged that there was almost no ductility.

  The shape of the protrusion obtained from the SEM observation and the shape of the crack existing in the protrusion were examined in relation to the evaluation result of the delamination resistance. As a result, the maximum depth of the protrusion is 1.0 μm or less as a condition for obtaining the evaluation ◯, and when the crack exists in the protrusion, the length is 0.8 μm or less, and the steel wire The angle formed by the radial direction of the cross section needs to be 35 ° or more, and more preferably, per unit peripheral length of protrusions of copper or brass plating protrusions counted along the circumferential direction of the cross section surface. The condition that the average number is 0.5 pieces / μm or less was obtained.

Here, the evaluation test and the evaluation of the delamination resistance were performed as follows.
In the evaluation test, both ends of the sample 21 obtained from the produced ultrafine steel wire are held by the chucks 22 and 23 of the delamination resistance evaluation test apparatus 20 shown in FIG. 6, and one chuck 23 is fixed so as not to rotate. Then, the chuck 22 at the other end was rotated at a constant speed of 10 to 200 revolutions / minute, the steel wire was twisted and continued until breaking.

  Since the steel wire to be evaluated in the present invention is very thin, a knot is formed by sagging during twisting, and it may break before the normal characteristics of the steel wire are evaluated from that point. Therefore, the sample 21 of the ultrafine steel wire under test is applied with a tension by a load 24 that is 1% of the tensile breaking load measured in advance through a fixed chuck 23, and the chuck 22 at the other end is attached in this state. Try to rotate.

  The chuck of the test apparatus is equipped with an encoder for measuring the rotation angle, a load cell for measuring torque, and an extensometer. The torque curve (horizontal axis: rotation angle [rad], vertical axis) recorded in the control apparatus 25 in each test. From the shaft: torque [N · m]), the delamination resistance of the steel wire was evaluated as follows.

FIG. 3 shows an example of a torque curve obtained by the evaluation test. In the obtained torque curve, a sudden load drop as shown in FIG. Determine if there is any.
The sudden torque drop in the torque curve corresponds to the occurrence of steel vertical cracks (delamination). If this is observed, it is judged that the steel wire has almost no ductility (evaluation x).
On the other hand, as shown in FIG. 4 (a), when a sudden torque drop is not observed before breaking, the steel wire does not break starting from the surface, and finally has a stable ductility. It shows plastic deformation until failure, and it is judged that the steel wire has sufficient ductility (evaluation ○).

  Even when a sudden torque drop is not recognized, serrations (unevenness) may be recognized in the obtained torque curve. It has been found that surface cracks are generated on the surface of the steel wire at the timing when the load drops at the torque curve. Cracks on the surface of the steel wire cause a ductile deterioration because the steel wire propagates in the longitudinal direction from the starting point during the processing of the steel wire such as a stranded wire and breaks. Therefore, the ductility evaluation was set to (x) even when serration was observed on the torque curve even if a sudden load drop (delamination) did not occur.

  The present invention has been made on the basis of the above examination results, and hereinafter, the requirements for the high carbon ultrafine steel wire excellent in the delamination resistance characteristic defined in the present invention will be sequentially described.

The present invention uses a steel wire material containing C: 0.75 to 1.10%, Si: 0.5 to 2.0%, Mn: 0.2 to 2.0% and having a pearlite structure as a material. To obtain extra fine steel wire.
The reason why the steel components to be used in the present invention are so limited is as follows. In addition, all% of the component shown below is the mass%.

  C: C has the effect of increasing the tensile strength after the patenting treatment and increasing the drawing work hardening rate, and can increase the tensile strength of the steel wire with less drawing distortion. If it is less than 0.75%, even if an alloying element is added, the tensile strength after the patenting treatment is low, and the drawing work hardening rate is also small, so a high strength ultra fine steel wire with a tensile strength exceeding 3000 MPa can be obtained. Absent. On the other hand, if it exceeds 1.10%, pro-eutectoid cementite precipitates at the austenite grain boundary during the patenting process and wire drawing workability deteriorates, and breakage is likely to occur in the wire drawing process. .75 to 1.10%.

  Si: Si is an element that is necessary for strengthening ferrite in pearlite and for deoxidizing steel, and is extremely effective for suppressing a decrease in strength due to heat. If it is less than 0.5%, the above effect cannot be expected. On the other hand, if it exceeds 2.0%, surface decarburization is likely to occur in the hot rolling process, so the Si range is set to 0.5 to 2.0%.

  Mn: Mn is not only necessary for deoxidation and desulfurization, but also an element effective for improving the hardenability of steel and increasing the tensile strength after patenting treatment, but less than 0.2% Then, the above effect cannot be obtained. On the other hand, if it exceeds 1.0%, the above effect is saturated, and the processing time for completing the pearlite transformation during the patenting process becomes too long and the productivity is lowered. -1.0%.

  The ultra fine steel wire of the present invention is based on a composition containing the above elements with the balance being Fe and inevitable impurities, and with this composition, the tensile strength is set to 3000 MPa or more without adding other elements. However, Cr: 0.5% or less, Ni: 0.5% or less, V: 0.5% or less, Co: 0.8% for the purpose of further improving mechanical properties such as strength, toughness and ductility. Even if 5% or less, Cu: 0.2% or less, Mo: 0.2% or less, W: 0.2% or less, or 2 or more types (however, 2% or less in total) are contained in the present invention The effect of is not disturbed.

In the present invention, the tensile strength of the ultra fine steel wire is intended for steel having a tensile strength of 3000 MPa or more. Since the effect of was not clear, it was excluded.
The wire diameter of 50 to 380 μm is based on the wire diameter required for steel cords used for reinforcing rubber and organic materials, such as tires, belt cords, and high-pressure hoses.

  Next, in the present invention, the reason why the projections formed by the penetration of the ultrathin steel wire cross section (C cross section) surface layer brass or copper plating into the steel wire base material observed by the above method will be described below. .

In addition, in order to observe in detail the penetration of the surface layer plating in the C cross section of the ultrafine steel wire, it is necessary to polish so that the plating portion of the surface layer corner portion of the sample does not sag and prepare a sample for SEM observation. is there.
As a method therefor, there can be used a method of using the gold when the sample is molded with a resin and mechanical polishing, or a method using ion beam etching (see, for example, JP 2009-25133 A).

In the mechanical polishing method, a steel wire that is equal to or harder than the steel wire is placed in close contact with the steel wire around the sample steel wire and embedded in a resin mold, and the end surface of the steel wire is applied to the gold wire. At the same time, appropriate emery paper polishing, buffing, and the like are performed.
In the ion beam etching method, the shield plate is placed in close contact with the steel wire so that the tip of the steel wire protrudes, and the protruding portion of the steel wire is irradiated with the ion beam to scrape off the tip of the shield plate. To.

Protrusions due to plating bite observed as described above are considered to be formed as follows.
As shown in FIG. 4, the surface of the steel wire has an isotropic shape due to non-uniform surface oxidation depth formed by heating during patenting and non-uniform dissolution by acid to remove scale. The unevenness 7 is generated. Since the copper or brass plating 1 is electrically formed on the irregularities after final patenting, the surface of the plating material before wet wire drawing has a substantially isotropic shape as shown in FIG. It has a surface with irregularities 7.

  As described above, the steel wire that has been repeatedly subjected to intermediate patenting and pickling has irregularities with an isotropic shape that has no directionality due to heat treatment and pickling. In order to extend the wire surface in the longitudinal direction and reduce the diameter, the surface is compressed in the circumferential direction. Therefore, the isotropic unevenness originally formed on the surface is elongated in the longitudinal direction together with the wire drawing to form a long and narrow groove shape (see FIG. 5B). At the same time, brass or copper plating, which is softer than steel wire, is pushed into the depth direction (R (radius) direction) of the steel wire, so it can be broken in a grooved bite depending on how it is deformed. It is considered that a protrusion is generated and becomes a protrusion.

  The brass or copper plating pushed into the surface of the steel wire has a high probability of acting as a starting point of fracture depending on the form. When the depth of the projection of the plating exceeds 1.0 μm in the observation of the cross-sectional surface of the C cross section, a crack is generated starting from the tip and propagates easily in the longitudinal direction (longitudinal direction of the steel wire). Often leads to breakage. Therefore, in the present invention, the upper limit of the depth of the plating protrusion is defined as 1.0 μm. Furthermore, by setting the depth of the plating protrusion to 0.5 μm or less, the generation of cracks from the plating protrusion can be more stably suppressed, so the thickness is desirably 0.5 μm or less.

The average number of copper or brass plating protrusions counted along the circumferential direction of the surface of the cross section of the steel wire is preferably 0.5 / μm or less. If the number of protrusion-like intrusions exceeds 0.5 / μm, cracks propagate inside the steel wire starting from the tip and easily propagate in the longitudinal direction of the steel wire. More often leads to breakage.
In addition, the protrusion used as the object at the time of counting the number is assumed to protrude 0.1 μm or more to the steel wire base material side inside the outer peripheral circle of the basic ultra fine steel wire C cross section.
Further, as described above, the number n of protrusions is counted for a length L of 20 μm along the circumferential direction of the surface of the steel wire, and the average number of protrusions is determined as n / L.

  As described above, the surface plating is pushed into the steel core of the steel wire, and cracks may exist in the protrusions after wet drawing when the plating is folded. When the crack length is 0.8 μm or more, the stress concentration at the tip becomes very high, so that the crack propagates to the steel wire side with the tip as the starting point. In many cases, the steel wire easily propagates in the vertical direction and breaks the steel wire. For this reason, in the present invention, the length of the crack caused by the folding existing from the beginning in the plating protrusion is defined to be 0.8 μm or less. By setting the crack length to 0.3 μm or less, propagation to the steel wire can be more stably suppressed, and therefore, it is more preferably 0.3 μm or less.

  In addition, when the angle of the crack propagation direction with respect to the radial direction of the steel wire (folding angle θ) is less than 35 °, the crack propagates inside the steel wire starting from the tip, and in the longitudinal direction of the steel wire Often propagates easily to break the steel wire. For this reason, in the present invention, the angle formed by the crack existing from the beginning in the plating projection with the radial direction of the steel wire is defined as 35 ° or more. If this angle is set to 60 ° or more, the propagation of cracks to the steel wire can be more stably suppressed. Therefore, the angle is more preferably 60 ° or more.

Next, a method for producing an ultrafine steel wire excellent in delamination resistance characteristics of the present invention will be described.
The ultra fine steel wire of the present invention uses a high carbon wire with a diameter of about 4.0 to 5.5 mm, which is adjusted and cooled by blast after hot rolling, as a material, and is subjected to wire drawing as in the past. Manufacture.
In primary wire drawing, depending on the ductility deterioration of the steel wire due to work hardening, intermediate patenting and scale removal by pickling are performed to finish the wire diameter according to the target tensile strength, and the final Ting. After final patenting, the oxide scale is removed by pickling and copper plating or brass plating is performed in an aqueous solution. In the case of brass plating, zinc plating is further performed on the copper plating, and copper and zinc are alloyed by diffusion by heat of about 500 ° C. to make a brass. After the plating treatment, a die is dipped in a wet lubricant and drawn to obtain an ultrafine steel wire having a predetermined strength and wire diameter.

  In order to prevent the penetration of the plating into the steel wire and suppress the formation of the protrusion and the occurrence of cracks in the protrusion, the surface properties of the steel wire before plating are smoothed. From the viewpoint that it is necessary to suppress the penetration of the plating into the steel wire, the heating conditions at the final patenting and the subsequent pickling conditions were investigated.

  As a result, if the maximum value (Rmax) max of the surface roughness Rmax measured in the circumferential direction of the steel wire is about 4.5 μm or less, the surface roughness of the steel wire ground iron after the pickling is as described above. It was found that the requirements of protrusions such as can be satisfied. In order to make the surface roughness so, the furnace temperature at the time of reheating at the time of final patenting is controlled to 800 to 1050 ° C., and the heating time is controlled to 10 minutes or less in the in-furnace time. It has been found that the pickling after the casting may be carried out using hydrochloric acid having a concentration of 15 to 30% by mass and a temperature of 20 to 45 ° C. under a condition that the pickling time is 120 minutes or less.

Here, the surface roughness in the present invention of the steel wire (patenting material) obtained by pickling after the final patenting is defined as follows.
The C cross-section surface layer of the patenting material is observed by SEM in the same manner as for the ultrafine steel wire. The observation of the unevenness on the surface is performed at each of the eight equally spaced positions on the surface of the C cross section. Observation is performed with the standard length of each field of view as 20 μm, and the surface roughness Rmax (the maximum and minimum difference) is measured in accordance with JIS B0601 (2001). The maximum value (maximum height difference) of the Rmax values of the respective visual fields at the eight equal positions was defined as (Rmax) max, which was used as a reference for evaluating the surface roughness of the patenting material.

Next, the reason for determining the final patenting and pickling conditions as described above will be described.
The furnace atmosphere temperature at the time of reheating the final patenting is set to 800 ° C. to 1050 ° C. When the temperature is less than 800 ° C., the steel material cannot be sufficiently austenitized within an industrially profitable time. When the temperature exceeds 1050 ° C., the unevenness of the surface of the steel material due to surface oxidation becomes large, and the subsequent wet wire drawing deepens the penetration of the plating into the steel wire, and the crack propagates to the steel wire. It tends to happen.

  The heating time during reheating is 10 minutes or less. When heating is performed for more than 10 minutes, the unevenness of the surface of the steel material due to surface oxidation becomes large, and subsequent wet drawing deepens the penetration of the plating into the steel wire, and into the steel wire during stranded wire processing, etc. Propagation of cracks is likely to occur.

  The acid used for pickling after the final patenting is hydrochloric acid. When an inexpensive industrially usable acid other than hydrochloric acid, such as sulfuric acid, is used, deep corrosion pits are often formed on the surface, and the maximum value (Rmax) of the surface roughness Rmax of the steel wire after pickling. ) Max often exceeds 4.5 μm. When (Rmax) max exceeds 4.5, the penetration of the plating into the steel wire side becomes deeper in the subsequent wet drawing, and crack propagation to the steel wire is likely to occur during stranded wire processing. Therefore, the acid used was limited to hydrochloric acid.

  Regarding the concentration and temperature of hydrochloric acid in pickling, when pickling is performed at a hydrochloric acid concentration of less than 20% and at a temperature of less than 20 ° C, the scale dissolution rate by the acid is not sufficient, and the operation is within an industrially profitable time. I can't. Therefore, the hydrochloric acid concentration was set to 20% or higher, and the temperature was set to 20 ° C. or higher.

  In addition, when pickling at a hydrochloric acid concentration exceeding 30% and a temperature exceeding 45 ° C., deep corrosion pits are often formed in either case, and (Rmax) max often exceeds 4.5 μm. Become. If the (Rmax) max of the patenting material exceeds 4.5 μm, the subsequent wet drawing will deepen the penetration of the plating into the steel wire, causing crack propagation to the steel wire during wire processing. It becomes easy. Therefore, the hydrochloric acid concentration is 30% or less and the temperature is 45 ° C. or less. Furthermore, when pickling is performed for a time exceeding 120 minutes, the (Rmax) max often exceeds 4.5 μm, as in the case where the hydrochloric acid concentration and the hydrochloric acid temperature are too high. It was as follows.

If the copper plating or brass plating thickness before wet wire drawing is thick, it affects the formation of protrusions. When the plating thickness is less than 1 μm, the lubrication is lost during wet wire drawing, the heat generation amount is increased, and the ductility of the steel wire is deteriorated. Further, when the plating thickness exceeds 10 μm, the soft plating greatly bites into the steel wire side in an irregular shape, and crack propagation to the steel wire is more likely to occur during wire processing. From the above, the plating thickness before wet drawing was set to 1 to 10 μm.
The plating thickness can be adjusted by the current density and energization time during the electroplating of copper and zinc.

  The conditions of wet drawing are not particularly specified, but the approach angle of the die used for drawing is 6 to 14 ° in all angles, and the wet drawing of each stage after the final patenting is performed. The area reduction is up to 10% in the first half of the total area reduction. Each area reduction is 14% to 25%, and the latter 90% is less than 30% in each area. It is preferable to perform a double die skin pass with a reduction in area of 1 to 5%.

  When the die approach angle used for wet wire drawing is less than 6 °, the contact length between the die and the steel wire becomes longer, the pulling force increases and the heat generated by friction also increases. It leads to deterioration. In addition, when the die approach angle exceeds 14 ° in all angles, the surface layer and the center portion are not processed uniformly, and the ductility of the steel wire is deteriorated. It tends to occur.

Regarding the area reduction rate of each stage of wet wire drawing, if the first 10% of the total area reduction area of wet wire drawing is less than 14% or more than 25% of each area reduction area, it will be a high material. Since the carbon steel wire is not sufficiently stretched in the longitudinal direction, cracks are likely to occur in the central portion of the steel wire, and the ductility of the steel wire is deteriorated.
In addition, for each step reduction rate of the remaining 90% of the total area reduction rate, when each step reduction rate exceeds 30%, the plastic processing of the surface layer and the central portion becomes uneven, and a large residual in the tensile direction remains on the surface. Stress is generated and the ductility of the steel wire is deteriorated.

  Finally, a double die skin pass is performed with the final 1-3 stages of wire drawing, but even if a double die skin pass of less than 1% or more than 5% is performed at the final wire drawing stage, a large pulling direction has occurred up to the previous stage. The residual stress cannot be relaxed.

  The present invention is configured as described above, and examples thereof will be shown below. In addition, this Example is an example for demonstrating the feasibility and effect of this invention concretely, This invention is not limited to this.

As the material for the ultrafine steel wire, A (JIS SWRA72), B (SWRA82), and C (92C material) with increased C content were used as the material. Table 1 shows the steel components of the material (the balance is Fe and inevitable impurities).
First, after removing the mill scale of the hot-rolled wire with hydrochloric acid, the lubricant undercoating was performed. Dry wire drawing before patenting was performed at a drawing speed of 50 m / min using a die schedule with an approach angle of 14 ° in full angle and a step reduction rate of 20%. A lubricant mainly composed of metal soap such as calcium stearate and sodium stearate was used. At the time of wire drawing, the winding drum and the die box were cooled with circulating water so that the wire temperature did not exceed 100 ° C.

  Subsequently, the heating temperature and the heating time were changed as shown in Table 2 to perform patenting, and after pickling, plating was performed. Pickling is performed using hydrochloric acid or sulfuric acid under the conditions shown in Table 2, and the surface roughness Rmax of the steel wire after pickling is measured in the range of 20 μm in length in the circumferential direction, and the result is as follows. The maximum value was determined as (Rmax) max. The surface roughness Rmax was determined according to JIS B0601 (2001).

All plating was performed wet, first Cu plating and then Zn plating. The current density was set so that the plating thickness was approximately 6: 3 Cu: Zn by weight. After plating Cu and Zn, induction heating was performed on the line, diffusion heat treatment at 500 ° C. × 1 sec was performed, brassing was performed, and the oxide film on the surface was removed with nitric acid.
Wet wire drawing after plating was performed in a state where the die was immersed in a lubricant in which a solid powder lubricant was dispersed with a surfactant in water. In all steps, the die approach angle was 14 ° in all angles, and each step area reduction rate was 20% in all steps. Thereby, an ultrafine steel wire having a diameter of 0.2 to 0.35 mm was obtained.

A sample was cut out from each obtained ultrafine steel wire, its tensile strength was measured, and an evaluation test for delamination resistance was performed. The evaluation was performed as described above using the obtained torque curve.
Moreover, the sample for observation of C cross section was created from another sample. The end face of the sample was adjusted by a method using ion beam etching.
SEM observation of a range of length L = 20 μm along the surface circumferential direction of the end face of each sample, counting the number n of protrusions, obtaining the average number n / L (/ μm) of protrusions, and the depth of the protrusions Further, when a crack was present in the protrusion, the length of the crack (μm) and the crack folding angle (°) were measured. The depth of protrusion and the length of protrusion showed the maximum length. The crack folding angle was the smallest among the cracks.
The obtained measurement results are shown in Table 2.

  Since Examples 1 to 9 of the present invention satisfy all the production conditions specified in the present invention, the maximum value (Rmax) max of the surface roughness Rmax after final patenting and pickling is 4.5 μm or less. It was. Therefore, the depth of plating on the surface of ultra fine steel wire after wet drawing, and the length, angle, and frequency of cracks in the projection can satisfy the conditions specified in the present invention, and the delamination resistance is excellent. It was.

  In Comparative Example 10, the atmospheric temperature during heating was high, and scale removal after patenting was performed with sulfuric acid, and (Rmax) max after patenting exceeded 4.5 μm. Since the surface properties of the steel wire do not satisfy the conditions specified in the present invention, the delamination resistance characteristics were inferior.

  In Comparative Example 11, the atmospheric temperature during heating was high, and scale removal after patenting was performed using sulfuric acid and over a long period of time. Therefore, (Rmax) max after patenting exceeded 4.5 μm, and thus wet stretching was performed. Since the surface properties of the ultra fine steel wire after the wire do not satisfy the conditions defined in the present invention, the delamination resistance property was inferior.

  In Comparative Examples 12, 14, 15, and 16, since the heating time was long, the unevenness of the steel material due to surface oxidation was large. Therefore, the (Rmax) max after patenting exceeded 4.5 μm, and therefore, the fineness after wet drawing was extremely fine. Since the surface properties of the steel wire do not satisfy the conditions specified in the present invention, the delamination resistance characteristics were inferior.

  In Comparative Example 13, since the atmospheric temperature during heating was high, (Rmax) max after patenting exceeded 4.5 μm, and therefore the surface properties of the ultrafine steel wire after wet drawing satisfied the conditions specified in the present invention. As a result, the delamination resistance was inferior.

DESCRIPTION OF SYMBOLS 1 Parts other than the projection of plating on the surface of an ultra fine steel wire 2 Base material part of an ultra fine steel wire 3 Protrusion of plating due to the plating biting into the steel wire base material part 4 Crack existing in the projection 10 The entire ultra fine steel wire 20 Delamination resistance evaluation test equipment θ Angle formed by crack propagation direction and radial direction of ultra fine steel wire cross section

Claims (3)

C: 0.75 to 1.10%, Si: 0.5 to 2.0%, Mn: 0.2 to 2.0%, and cold drawing a steel wire having a pearlite structure The obtained ultra-thin steel wire having a circular cross section with a tensile strength of 3000 MPa or more and a wire diameter of 50 to 380 μm,
The surface of the steel wire has copper plating or brass plating, and the boundary between the steel wire base material and the plating in the cross section of the ultra fine steel wire protrudes inward from the outer circumference circle of the basic ultra fine steel wire cross section. The maximum depth of the plating protrusion formed thereby is 1.0 μm or less, the maximum length of the crack existing in the protrusion is 0.8 μm or less, and An ultra fine steel wire with excellent delamination resistance, characterized in that the angle formed between the direction of progress and the radial direction of the cross section of the ultra fine steel wire is 35 ° or more.   The ultra fine steel wire according to claim 1, wherein an average number of the protrusions per unit circumferential length existing along the circumferential direction of the cross-sectional surface of the steel wire is 0.5 piece / μm or less. Wire drawing perlite extra fine steel wire. C: 0.75 to 1.10%, Si: 0.5 to 2.0%, Mn: 0.2 to 2.0%, and a steel wire having a pearlite structure is first drawn, and the final pattern In the method for producing an ultra fine steel wire excellent in delamination resistance according to claim 1 or 2, wherein the surface of the steel wire is subjected to copper plating or brass plating, and wet wire drawing is performed after pickling and pickling.
The temperature in the furnace at the time of reheating in the final patenting is controlled to 800 to 1050 ° C., the heating time is controlled to 10 minutes or less in the in-furnace time, and the pickling after the final patenting is performed at a concentration of 15 to 30 The maximum value (Rmax) max of the surface roughness Rmax of the steel wire after the pickling is 4.5 μm using hydrochloric acid having a mass% of 20 to 45 ° C. and a pickling time of 120 minutes or less. A method for producing an ultra fine steel wire, characterized in that the thickness of copper or brass plating before wet drawing is 1 to 10 μm.

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