Continuous wire-drawing method for steel wire

Abstract

To reduce, when producing a high-tensile-strength steel wire using a straight-type continuous wire-drawing machine, strain age hardening (embrittlement) caused by temperature rise of the steel wire, support high-speed wire drawing, and improve line set.SOLUTION: In wire drawing, at least in the drawing passes other than the final pass, a hole die or a roller die is used, and the cross-sectional shape of a workpiece material at a die exit is formed not as a circle but as an elliptical shape having a straight portion or as a rounded rectangular shape. By winding and taking up the workpiece material with the straight portion thereof in contact with a drawing capstan, rolling (twisting) of the workpiece material is restrained, and contact cooling by the drawing capstan is enhanced, thereby suppressing age hardening. The action of back tension also reduces processing heat generation. The restraint of twisting improves line set.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a continuous wire drawing method for drawing a high carbon steel wire rod to produce a high tensile strength steel wire. [Background technology]

[0002] Steel wire is made by drawing wire rod through a hole die and processing it to the desired dimensions and mechanical properties. This process requires multiple passes, and a continuous wire drawing machine (hereinafter referred to as a continuous drawing machine) is often used, in which the wire passes through a series of single-head drawing machines without stopping. One problem with continuous wire drawing machines is that the processing temperature tends to be high, and another problem is that the bending and twisting tendency of the steel wire is inferior to that of single-head wire drawing machines.

[0003] Regarding the former problem, the temperature of the steel wire rises due to the heat generated during drawing. The heated steel wire gradually cools as it comes into contact with the drawing kettle around which it is wound, but in the continuous drawing machine, as it reaches the next pass during this cooling process, the temperature before processing rises compared to the previous pass, and the temperature after processing also rises accordingly. The repeated accumulation of temperature rises causes hardening and embrittlement of the steel wire (called strain aging embrittlement), which is separate from work hardening. In particular, high-tensile steel wire tends to lack ductility and toughness due to its strength, and its large yield strength also causes large heat generation and temperature rise during processing, which makes it prone to embrittlement. Various measures have been taken to address this quality issue, both in terms of strengthening cooling and suppressing heat generation.

[0004] Usually, measures such as strengthening the water cooling inside the drawing kettle or increasing the number of times the wire is wound into the kettle are adopted to increase the cooling time. There are two methods for winding the wire into the drawing kettle. One is the wire storage method, in which the wire is stacked and wound into the kettle to create a sufficient amount of storage, resulting in a long cooling time and easy control of the wire drawing temperature. It is also easy to adjust the speed between kettles. This method is often used for products where quality is not a strict requirement.

[0005] The other is a straight-through type, in which the steel wire is wound in an aligned manner around the kettle and moves between the kettle sections under constant tension. The reverse tension reduces the die surface pressure, which is good for the die life, and it is less likely to twist, resulting in better wire shape than the wire storage type, less likely to be scratched, and has stable lubrication, all of which are advantageous for quality. On the other hand, there are problems such as wire breakage due to improper tension control (for this reason, reverse tension is kept relatively low), insufficient cooling (measures taken in terms of work include reducing the wire drawing speed and increasing the number of windings, and in terms of equipment, using larger kettle sections), and the complex control mechanism required to accurately adjust the speed between kettle sections. The present invention is directed to the straight-through type, which is advantageous for quality.

[0006] On the other hand, to suppress heat generation, the degree of processing per pass (area reduction rate) is reduced, but an increase in the number of passes can result in a shortage of kettles. Increasing the tensile strength of the product requires an increase in the degree of processing, so the number of passes must be increased. For this reason, the latest continuous stretching machines tend to have an increasing number of connected kettles, which places a heavy burden on equipment costs. The use of high-performance lubricants (reducing heat generation through low friction) has some effect, but it is not sufficient. The reality is that there are no effective measures. The mistaken idea of ​​lowering the wire drawing speed is often used, but although it is effective in cooling, it is ineffective in preventing temperature rise.

[0007] Patent Document 1 discloses that embrittlement of steel wires is prevented by controlling the wire drawing temperature below a certain level. Methods of controlling the temperature to a predetermined level include suppressing temperature rise by decreasing the area reduction rate, strengthening cooling, slowing down the wire drawing, etc., all of which are well known and in use.

[0008] Patent Document 2 discloses a new mechanism that makes it easy to adjust tension and speed in a straight-line drawing machine. Although the mechanism improves workability of wire drawing due to the stable reverse tension, it does not suggest anything beyond conventional knowledge about the significance of reverse tension.

[0009] Patent Document 3 discloses that in order to suppress embrittlement due to strain aging in a straight-line continuous drawing machine, the axial inclination of the diameter of the drawing kettle (making the kettle shape a truncated cone) is appropriately set to improve contact between the steel wire and the kettle, preventing lap winding, thereby maintaining regular winding and preventing a decrease in contact cooling, i.e., increasing cooling capacity. This method is somewhat effective for cooling, but has no effect on temperature rise.

[0010] Patent Document 4 discloses a method for reducing the drawing force by setting the reverse tension higher than conventional methods, thereby suppressing heat generation and temperature rise, and for stabilizing this. While this method is expected to be quite effective, precise control of excessive reverse tension is somewhat difficult. When the reverse tension exceeds the limit, a break with a conical fracture surface occurs.

[0011] Patent Document 5 discloses detailed research into the processing method and decline in ductility and toughness during the production of ultra-high tensile strength steel wire, and solutions to these problems. According to this research, when steel wire exceeding ordinary strength is produced by drawing using a hole die (hereinafter referred to as wire drawing), strength is obtained through work hardening, but ductility and toughness in terms of elongation, reduction, twisting, etc. decline. The decline is due to the influence of heat generation and temperature rise in the steel wire caused by processing, which leads to strain aging embrittlement. Changing the processing method to rolling or roller die drawing reduces friction, which is a part of the heat generation, and thus suppresses embrittlement accordingly. It is disclosed that the majority of the product is obtained by this processing method, and the remainder is obtained by wire drawing.

[0012] The problems with this method are discussed below. When rolling is incorporated into wire drawing, the cost efficiency of the equipment is low. This is because the cross-sectional area of ​​steel wire is small, so production efficiency is not high. On the other hand, the equipment cost of the rolling mill is several times that of the wire drawing machine. In the case of roller dies, the hole die is simply replaced with a roller block, so the problem of the equipment cost of rolling is solved. The problem arises when increasing the wire drawing speed to improve efficiency. In a typical continuous wire drawing machine, the steel wire is wound around a drawing kettle and cooled while it is held there, so that by the time of the next processing pass, the temperature has dropped to a level suitable for cold processing. Increasing the speed reduces the cooling time, which undesirably increases the processing temperature for the next pass. In the case of ultra-high tensile wire, processing heat is large in proportion to the strength, and cooling tends to be insufficient. Increasing the wire drawing speed is an extremely difficult task.

[0013] Regarding the "wire curl" of steel wire, which was raised as another problem with wire drawing machines, bending and twisting deformations are always produced during winding and reeling when drawing with a wire drawing machine. It is inevitable that wire curls will occur. With piano wire, ring diameter and ring misalignment (rise) are the areas that need to be managed. Variations in the length direction are also a problem. Managing this problem depends heavily on the skills of the wire drawing worker, and adjustment work has a negative impact on productivity. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] Published Patent Publication 2007-29965 [Patent Document 2] Unexamined Patent Publication No. 8-24939 [Patent Document 3] Unexamined Patent Publication No. Heisei 7-124628 [Patent Document 4] Patent No. 6106814 [Patent Document 5] Unexamined Patent Publication No. 63-4016 Summary of the Invention [Problem to be solved by the invention]

[0015] When continuously drawing high-tensile steel wires such as piano wire and PC steel wire, a straight-line continuous drawing machine is usually used, which is advantageous for quality. However, due to the high tensile strength, the temperature rise of the steel wire is large, and as a result, strain aging embrittlement is likely to occur. The wire drawing temperature must be controlled below a specified value, but in reality, this is maintained by suppressing the wire drawing speed, which causes problems in production efficiency. Countermeasures require both strengthening the cooling capacity of the wire drawing machine and suppressing heat generation during drawing. The recent trend toward higher tensile strength products has made this problem even more difficult. The adjustment work required to stabilize the wire curl that tends to occur in the stretching machine also contributes to a decline in production efficiency.

[0016] The present invention aims to solve the above problems by preventing embrittlement due to an increase in wire drawing temperature even with normal cooling capacity, normal area reduction, and increased wire drawing speed, and by stabilizing the bending and twisting tendency of the product. To achieve this, the problem to be solved is to provide a method that minimizes heat generation during drawing, strengthens cooling until the subsequent passes, and makes the twisting action acting on the steel wire constant. [Means for solving the problem]

[0017] The first invention of the present application is a continuous wire drawing method for high tensile strength steel wire, characterized in that, when producing high tensile strength steel wire by a straight-line continuous wire drawing machine in which reverse tension is applied, a hole die or a roller die is used in at least the remaining drawing passes excluding the final pass, so that the cross-sectional shape of the workpiece at the die outlet is not circular but elliptical with straight portions or rectangular with rounded corners, and the straight portions are wound and taken up in contact with the drawing kettle, thereby restricting twisting of the workpiece and enhancing contact cooling by the drawing kettle.

[0018] The second is a continuous wire drawing method for high tensile strength steel wire as described in the first invention, characterized in that the workpiece is immediately water-cooled after passing through the final pass, and that a rolling constraint roller is provided to press down on the aligned windings on the drawing kettle and constrain the rolling of the workpiece.

[0019] The third is a continuous wire drawing method for a high tensile strength steel wire as described in the first or second invention, characterized in that the magnitude of the reverse tension is set to 40% or more of the yield strength of the workpiece before drawing, and the temperature of the workpiece immediately after drawing is kept to 145°C or less. Here, "immediately after being pulled out" refers to the lowest winding portion. [Effects of the Invention]

[0020] The first effect of the present invention is that, while conventionally, a workpiece heated by drawing is gradually cooled by contact with the drawing kettle, in the present invention, the cross section of the workpiece has a straight section rather than a circle, so the shape of the contact area with the drawing kettle expands from a linear shape to a planar shape, and the cooling capacity increases several times. Even if the wiredrawing speed is increased, the temperature before the next pass is maintained within the specified range, contributing to improved production efficiency. The application of roller dies reduces some of the friction between the die and the workpiece, lowering the temperature rise and making it easier to increase the wire speed.

[0021] Second, the final pass is the most sensitive to strengthening and embrittlement. Strengthening induces embrittlement. In many cases, strain aging hardening due to temperature rise is the main cause, but in the present invention, the temperature is kept low just before and just after processing, preventing embrittlement of the finished steel wire.

[0022] Third, the workpiece is subjected to bending and twisting on the drawing kettle, but with the present invention, the workpiece contacts the kettle with a flat surface and does not roll. Twisting occurs only to a certain extent due to winding and winding up, eliminating fluctuations in twisting along the length. This stabilizes the shape of the steel wire. This reduces the work required to adjust the wire shape. [Brief explanation of the drawings]

[0023] [Figure 1] 1 is a schematic view of a continuous wire drawing machine for carrying out the continuous wire drawing method of the present invention. [Figure 2] This shows how to form a flat surface by single-stage roller die drawing. [Figure 3] This shows a method for forming a flat surface by drawing with a set of two roller dies. [Figure 4]Examples of pass schedules using one or two roller dies are shown below. [Figure 5] An example of forming a flat surface by hole die drawing is shown. [Figure 6] This shows the state of the workpiece being wound around the drawing furnace and in contact with it. [Figure 7] The cooling of the workpiece in the drawing furnace and the value of the clarified pseudo-heat transfer coefficient are shown. [Figure 8] FIG. 10 is a diagram illustrating the state in which the workpiece is wound up on the drawing hook. DETAILED DESCRIPTION OF THE INVENTION

[0024] The following describes an embodiment with reference to the drawings. Figure 1 shows the schematic structure of a linear extension machine to which the present invention is applied. In the figure, the number of drawing hooks is three, but normally there are seven to ten. The steel wire material 1 is fed to the No. 1 roller die 3, wound around the No. 1 drawing kettle 4, and undergoes the drawing process. At this time, the brake roll 2 applies reverse tension to resist the drawing. The workpiece 1 forms an aligned winding 5 in the drawing kettle, and after a predetermined number of windings, it goes straight to the No. 2 roller die 6 and the No. 2 drawing kettle 7, where it similarly undergoes the drawing process. The No. 1 drawing kettle functions as a brake. After the aligned winding 5 (from the start of winding 8 to the end of winding 9), it heads to the next pass. The final pass uses a circular hole die 10, and a powerful water cooling device 11 is installed immediately after the hole die 10 to cool the hot surface of the workpiece 1, after which it is wound around the drawing kettle. The number of windings can be kept to a minimum. At this time, the aligned windings on the kettle are pressed against rolling constraint rollers 12 to restrict rolling, and the workpiece 1 moves upward in a parallel motion only by sliding. The workpiece 1 is then formed into a coil by a winder 13.

[0025] Figure 2 shows the structure of a single roller die. The deformation is the same as in rolling, and the grooves are those of a flat roll 21 or a light box 23. After the second drawing vessel 7, the cross-sectional shape after deformation becomes a rectangle 22, 24 with rounded corners. When using roller dies to achieve the same degree of reduction (area reduction rate) as with hole dies, the rolling reaction force is large, so the bearings and other components must have sufficient rigidity. Figure 3 shows a two-stage roller die structure. The purpose of using two to four stages is to reduce the rolling reaction force and to restore the cross-sectional shape, usually round → oval → round. In the present invention, a square with rounded corners is repeated. As described below, an oval with a flat portion → round may also be repeated.

[0026] Figure 4 shows the pass schedule for continuous drawing using roller dies. A is an example of a cross-sectional shape when using a single-stage roller die, and B is when using a two-stage roller die. The final pass is a perfect circle, just like in normal wire drawing. Figure 5 shows the deformation of the cross section of the workpiece when drawing is performed using a hole die. A circular cross section of wire rod 51 is processed into an oval 52 with a flat section. The flat section can be located on either the long side (A) or the short side (B). It is safe to continue the deformation in a similar shape (52 ⇒ 53, 54 ⇒ 55). In any case, in the present invention, it is an essential requirement that the cross-sectional shape after processing, except for the final pass, is not a conventional circle or oval, but that a straight line portion (flat surface) is formed on part of the outer periphery.

[0027] The formation of a flat surface has two advantages in terms of plastic processing. First, it significantly increases the contact area between the workpiece and the drawing vessel. Figure 6 is a longitudinal cross-sectional view of the center of a drawing hob, showing the contact state between the hob and the aligned windings 62, 63, and 64 of the workpiece wound around the drawing hob 61. In the conventional circular cross-sectional case (A), the contact is linear from a geometrical perspective, but elastically there is a slight width due to some deformation. Contact conduction heat transfer occurs through this small contact surface. In the present inventions (B, C), the flat surface comes into contact with the drawing vessel, so the contact area is greatly enlarged, and conductive heat transfer is promoted.

[0028] The results of the investigation and analysis of the degree of contact heat transfer are shown in Figure 7. The upper half of the figure shows the analysis method. The figure on the bottom left shows primary data obtained by measuring the temperature distribution in an operating drawing furnace using an infrared temperature camera. The data is displayed in color. The figure on the bottom right is a cooling diagram created based on the temperature distribution diagram. Heat transfer between the drawing kettle and the steel wire is essentially conductive, but to avoid complication of analysis, the actual measurements are converted to convective heat transfer to calculate the pseudo heat transfer coefficient. As shown in the figure, the value of the pseudo heat transfer coefficient α in the current continuous drawing machine is 50 to 150. (kcal / m 2 h℃). In the case of a similar process, press quenching, a cooling capacity that results in "burning" is obtained, and the α value exceeds 1000. The α value of the present invention is between the above two, and is estimated to be several times higher than the current value.

[0029] The second purpose of forming a flat surface is to stabilize the shape imperfections of the resulting steel wire ring (commonly known as wire curl, which consists of twisting and bending). Poor twisting reduces the accuracy of subsequent forming processes (e.g., spring processing). Although the straight-line continuous stretching machine is better at eliminating twisting than the wire storage type, it is still insufficient. Figure 8 is a diagram explaining the behavior of a workpiece 82 wound around a drawing shuttle 81. The workpiece 82 is wound around the collar of the drawing shuttle 81, forming an aligned winding, and is wound up one piece at a time with each rotation. The winding itself is a constant parallel movement, but a closer look reveals that each workpiece has two elements: sliding 84 and rolling 83 on the shuttle surface.

[0030] The drawing hook 81 is cylindrical in shape, with a slightly smaller diameter at the top, and it is well known that the inclination is related to sliding and rolling, but observation reveals that there is a great deal of variation. In the present invention, due to the flat contact, the workpiece 82 is prevented from rolling 83 on the drawing hook 81, and only sliding 84 occurs. The torsional strain caused by winding is maintained constant. Therefore, the torsional tendency of the workpiece 82 is highly stable.

[0031] Most steel wire products have a circular cross section. The final pass of the continuous drawing machine requires a die with a precise circular hole. This makes it impossible to utilize the cooling enhancement and twisting constraint effects of the new flat surface formation. To address this issue, the present invention adopts the following measures.

[0032] As mentioned above, a water cooling device is installed immediately after the die in the final pass. It has long been known that water cooling is effective in maintaining the ductility and toughness of steel wire, but it has never been put into practice. The reason for this is that the powerful water jet strips away the lubrication from the surface, and the wetting of the workpiece by the splashed water induces abnormal friction, which causes seizure between the tool (die) and the material (workpiece) in the next pass, resulting in frequent cracks. In the present invention, this problem is solved because water cooling is performed after the final pass.

[0033] To address the problem of twisting during the final pass, constraining rollers 12 (Fig. 1) are provided to press down on the workpiece 82 above the wire drawing pot 81 during the final pass, thereby constraining the rolling. Although steel rollers are unstable due to friction, upstream water cooling makes it possible to use rubber rollers, which have enhanced friction like rubber tires.

[0034] As mentioned above, strengthening steel wire rods through wire drawing can be accompanied by embrittlement due to heat generated during processing. In the present invention, heat generation is quickly suppressed to prevent embrittlement and support an increase in wire drawing speed. The method described in Reference 4, namely, the use of reverse tension, suppresses heat generation itself. This method is also used in the present application to strengthen measures for both heat generation and cooling. Heat generation can be suppressed by strengthening and stabilizing reverse tension. The reverse tension can be set to 40% or more of the yield strength of the workpiece before drawing, and the temperature of the workpiece after drawing can be kept below 145°C. However, the above temperature is the temperature of the bottommost layer of the regular winding wound on the drawing hook. The basis for this is explained in detail in the cited reference 4. The first effect of the above invention (addition of reverse tension) is that the drawing temperature of the steel wire is controlled to a predetermined value (e.g., 145°C) or below, strain age hardening is suppressed, and it becomes easier to obtain a tough, high-tensile steel wire. The second effect is that since the temperature of the steel wire is kept below a predetermined value, there is no need to set the area reduction rate at a low level, which is a conventional method, and it becomes easier to increase the total area reduction rate even with existing wire drawing machines. [Example]

[0035] In continuous wire drawing using conventional hole dies, only the first die was changed from a circular cross section to an elliptical hole die with a parallel section, and the temperature just before the second die was measured. The wire diameter is 8.0 mm and the No. 1 die diameter is 7.2 mm. The cross section is an ellipse of 5.5mm x 8.1mm (short side R = 5.5 / 2mm) with a flat section width of 3.1mm. The reduction rate is the same as that of a circular cross section. Using the former conventional method as a comparison, the wire temperature dropped from approximately 130°C immediately after drawing to approximately 50°C in the kettle, but with the elliptical die, cooling proceeded to approximately 40°C, and it was confirmed that the conventional ductility and toughness were maintained even when the wiredrawing speed was increased. The aligned windings on the wiredrawing kettle moved parallel, and no rolling occurred. [Explanation of symbols]

[0036] 1: Workpiece 2: Brake roll 3: No. 1 roller die 4: No. 1 drawing hook 5: Linear winding 6: No. 2 roller die 7: No. 2 drawing hook 8: Start of winding 9: End of winding 10: Hole die 11; Water cooling device 12; Constraint roller 13 Winder 21; Roller die 22; Workpiece 23; Light box hole 24; Workpiece 31; Horizontal roller die 32; Workpiece 33; Vertical roller die 34; Workpiece 41; Wire rod 42, 43; Workpiece 44; Final cross section 51; Wire rod 52, 53, 54, 55; Workpiece 61; Drawing hook 62, 63, 64; Linear winding 81; Drawing hook 82; Workpiece 83; Rolling 84; Sliding

Claims

1. A continuous wire drawing method for high tensile strength steel wire, characterized in that, when producing high tensile strength steel wire by a straight-line continuous wire drawing machine in which reverse tension is applied, a hole die or a roller die is used in at least the remaining drawing passes excluding the final pass, so that the cross-sectional shape of the workpiece at the die outlet is not circular but elliptical with straight portions or rectangular with rounded corners, and the straight portions are wound and wound up in contact with the drawing kettle, thereby restricting twisting of the workpiece and enhancing contact cooling by the drawing kettle.

2. 2. A method for continuous drawing of high tensile strength steel wire according to claim 1, characterized in that the workpiece that has passed through the final pass is immediately water-cooled, and a rolling restraining roller is provided to press down on the aligned windings on the drawing kettle and restrain the rolling of the workpiece.

3. 3. A continuous wire drawing method for a high tensile strength steel wire according to claim 1 or 2, characterized in that the magnitude of the reverse tension is set to 40% or more of the yield strength of the workpiece before drawing, and the temperature of the workpiece immediately after drawing is suppressed to 145°C or less.

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