KR101518602B1 - Method for manufacturing high-strength drawn wire having excellent twist property - Google Patents

Abstract

Provided is a method to manufacture a high-strength steel wire with excellent torsional rigidity. The manufacturing method of the present invention includes a step of preparing a wire comprising: 0.80-1.0 wt% of C, 0.5-1.5 wt% of Si, 0.3-0.7 wt% of Mn, 0.4-0.8 wt% of Cr, 0.015 wt% or less of P, 0.015 wt% or less of S, 0.03-0.05 wt% of Sol.Al, 0.002-0.008 wt% of N, and the remainder consisting of Fe and inevitable impurities; and a step of manufacturing the final steel wire by additionally reducing a cross section of the steel wire through a swaging process after dry-wire drawing the prepared steel wire.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a high strength steel wire having excellent twist characteristics,

The present invention relates to a method of manufacturing a high strength steel wire having excellent torsional characteristics, and more particularly, to a method of manufacturing a high strength steel wire which improves a torsional characteristic by reducing a surface residual stress of a steel wire generated in a drawing process.

Wire ropes used in wire ropes, etc. are reduced in diameter step by step during passing through dies of various sizes from the first wire rod to the final product, and then drawn (drawn and drawn) into the final product.

The steel wire is a high strength steel wire having a carbon content of 0.7% or more and a tensile strength of 1600 Mpa or more at a diameter of 3 to 7 mm. The main characteristics of the steel wire are strength and torsion characteristics, and other functions such as fatigue and stress relaxation are also required depending on the case.

The high strength of the steel wire is required because the strength of the steel wire is increased by 100Mpa to increase the weight by 10% or reduce the diameter of the rope. Finally, it is very effective in the usage of concrete and simplification of the structure. I can get it. However, when the strength is increased to some extent, delamination occurs in which a crack is formed perpendicularly along the longitudinal direction (i.e., axial direction) of the drawing material due to the characteristic of the steel wire made of pearlite material .

Up to now, the high strength of the steel wire has been achieved through microfabrication and increased throughput. One of the ways to reduce pearlite interlayer spacing is to use alloying elements. A representative alloying element is C. When C is added at 0.1 wt%, the tensile strength of 80 MPa can be improved. In addition, it is known that Cr improves by about 40 MPa. However, the addition of the alloying element causes problems such as grain boundary formation of crushed stone cementite, formation of Cr-based carbide and increase of completion time, and the optimization for the operating condition should be reset.

In addition, an increase in the strength depending on the amount of drawing processing is also an effective method. As the drawing is applied to the wire, the randomly formed pearlite is aligned in the longitudinal direction (drawing amount (e): 0.5 level), and the index tends to increase exponentially. However, there is a limitation in increasing the strength through the drawing, because the reduction of the cross-sectional reduction rate occurs due to the freshness. That is, there is a time point at which the ductility greatly decreases, and at this time, the above-described delamination occurs (in other words, referred to as the freshness limit). Therefore, it can be said that the steel wire or rope which is currently commercialized is produced as a product after the drawing is finished at the stage before the drawing limit. Therefore, the strengthening of the steel wire should not be achieved through the control of the main factors, but rather through the combined action of various factors.

The steel wire is also required to have excellent torsion characteristics because it involves twisting ropes. In the case where the torsional characteristics are poor, if the crack progresses in the longitudinal direction at the initial stage of the twist test of the steel wire, breakage occurs in the twisted wire. Therefore, although it varies depending on the strength class, a certain number of torsional rotations is required within a range in which delamination does not occur.

Factors affecting torsional properties include extrinsic factors such as intrinsic factor and surface flaws, such as pearlite interlayer spacing, cementite thickness and fraction, secondary ferrite, cubic cementite and ferrite, intergrown carbide However, the main factor that dominates the torsional characteristics has not been identified so far. This means that the factors affecting the torsional characteristics are not only affected by one factor but also acting in a complex manner.

The residual stress is also a factor that greatly dominates the torsional characteristic. This is because, unlike extrusion and the like, when the steel wire is manufactured by drawing, the tensile residual stress is formed on the surface though the strength is high. In the drawing process, when the total amount of reduction is high, the tensile residual stress is more than 1,000 MPa, which causes the difference between the center and the center to cause irregularity of the material, and the tensile residual stress on the surface serves as a starting point for frequent cracking Therefore, the torsional characteristic is not good. Therefore, by controlling the tensile residual stress existing on the surface of the steel wire, the twist characteristic of the steel wire can be improved.

One of the prior art attempts to improve the physical properties of the steel wire by controlling the surface residual stress of the steel wire is disclosed in Korean Patent Publication No. 2011-0020256 entitled "High Strength Ultrafine Steel Wire and Its Manufacturing Method" (Patent Document 1) have. According to this production method, the steel wire contains chemical components of C: 0.7 to 1.2 mass%, Si: 0.05 to 2.0 mass% and Mn: 0.2 to 2.0 mass%, the balance being Fe and inevitable impurities, The steel wire has a pearlite structure and an average C concentration of the ferrite phase center portion of the outermost layer of the steel wire is 0.2 mass% or less and a residual compressive stress in the steel wire longitudinal direction of the outermost layer is 600 MPa or more.

However, in the above manufacturing method, only the residual compressive stress at the outermost layer of the steel wire is set to a specific value or higher by a method similar to the skin pass, so that the balance between the strength and ductility of the superfine steel wire is improved. And it is not intended to improve the physical properties of the steel wire.

Korean Patent Laid-Open Publication No. 2011-0020256: High Strength Ultrafine Steel Wire and Method for Manufacturing the Same (Applicant: Shin-Nippon Seitetsu Kabushikikaisha), 2011. 03. 02. Open

The present invention has been developed through new attempts different from those of the prior art. The present invention reduces the surface residual stress existing in the longitudinal direction and the center direction of the steel wire by adding the swaging process in the dry drawing process of the wire, And a method for manufacturing a high strength steel wire capable of improving the properties of the steel wire.

According to an aspect of the present invention,

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet comprises, by weight, 0.80 to 1.0% of C, 0.5 to 1.5% of Si, 0.3 to 0.7% of Mn, 0.4 to 0.8% of Cr, 0.015% or less of P, 0.05%, N: 0.002 to 0.008%, the balance Fe and other unavoidable impurities; The present invention also relates to a method for manufacturing a high strength steel wire having excellent torsional characteristics, comprising the steps of: (a) drying the prepared wire rod and then further reducing its cross section through a swaging process to produce a final steel wire;

At this time, it is preferable that the wire rod has a pearlite structure in which 95% or more of the microstructure is in an area fraction, and less than 5% is a non-pearlite structure including pro-eutectoid ferrite and cobalt cementite.

The wire rod preferably has a tensile strength of 1400 MPa or more and a section reduction ratio (RA) of 10% or more.

In addition, it is preferable that the dry drawing process is such that the wire material is drawn to a reduction amount of 70 to 80%, and the swaging process is further configured to further reduce the cross section of the dry drawn wire material to 30 to 60% Do.

The difference in the longitudinal residual stress between the surface portion and the center portion of the high strength steel wire is preferably 550 MPa or less.

It is preferable that the high-strength steel wire is twisted 15 times or more.

According to the manufacturing method of the high strength steel wire excellent in the twisting property of the present invention having the above-described structure, the twisting property can be greatly improved without lowering the strength of the steel wire.

Further, even if the same amount of reduction is applied to the wire rod, it is possible to obtain a steel wire having excellent torsional characteristics, so that the drawing limit of the steel wire can be increased. As a result, since the occurrence of delamination of the steel wire can be suppressed, the strength of the steel wire can be increased.

FIG. 1 is a view illustrating a dry drawing and a swaging working method according to an embodiment of the present invention.

Hereinafter, the technical construction according to the present invention will be described in more detail with reference to various embodiments and the accompanying drawings.

First, in the present invention, in the present invention, it is preferable that the steel sheet contains 0.80 to 1.0% of C, 0.5 to 1.5% of Si, 0.3 to 0.7% of Mn, 0.4 to 0.8% of Cr, 0.015% or less of P, 0.03 to 0.05% of Al, 0.002 to 0.008% of N, and the balance Fe and other unavoidable impurities. Hereinafter, the critical significance of the characteristics and the composition range of each alloy element will be briefly described.

C (carbon): 0.80 to 1.0 wt%

C is a main element for securing strength, and when C is added, it mostly exists in the form of cementite. Increasing the carbon content has the effect of increasing the fraction of cementite and decreasing the pearlite interlayer spacing, which increases the strength. It is reported that the strength of 80 MPa is increased when the carbon content is increased by 0.1 wt%. If the carbon content is less than 0.8% by weight, it is difficult to secure the strength required in the present invention. If the carbon content is more than 1.0% by weight, a hornblende cementite is formed in the grain boundary, and when 0.1% by weight is added, precipitation and solid solution strengthening effect of 80 MPa is not generated. Therefore, the content of C is preferably limited to 0.8 to 1.0% by weight.

Si (silicon): 0.5 to 1.5 wt%

Si is dissolved in a ferrite matrix to increase the strength by solid solution strengthening effect. It is known as an element having a very low solubility in cementite, in ferrite grain, ferrite / cementite grain boundary. The wire rope is immersed at a temperature of 400 占 폚 or more because of the hot-dip plating process, in which the strength deterioration occurs. At this time, Si plays a role of suppressing the change to spherical cementite. When the amount is less than 0.5% by weight, the effect of preventing the decrease in strength is insignificant. When the amount is more than 1.5% by weight, a problem of scale peeling occurs. Therefore, the content of Si is preferably limited to 0.5 to 1.5% by weight.

Mn (manganese): 0.3 to 0.7 wt%

Mn is an austenite stabilizing element and added for securing entrapmentability. When Mn is added, it is combined with S to form an MnS inclusion, which is a soft inclusion but may cause a break in drawing. If less than 0.3% by weight is added, it is difficult to secure the entrapment property, and a quartzite cementite is formed in the grain boundary. When the amount exceeds 0.7% by weight, the center segregation becomes severe. Therefore, the content of Mn is preferably limited to 0.3 to 0.7% by weight.

Cr (Cr): 0.4 to 0.8 wt%

Cr is an element which greatly increases the strength after C and V. Generally, when 0.1 wt% is added, tensile strength is increased by 40 MPa. (Fe, Cr) ₃ C, Cr ₃ C, etc. because it is easy to substitute with Fe in cementite. When Cr is added in an amount of 0.4 wt% or more, it enhances bonding force with carbon in the cementite to inhibit the decomposition of cementite in the wire and suppress the occurrence of room temperature aging. However, when a large amount of Cr is added, there is a disadvantage in that the productivity is lowered because the transformation end time is prolonged. Therefore, it is preferable to limit Cr to 0.8 wt% or less.

P (phosphorus), S (sulfur): 0.015 wt% or less

The lower the content of P and S is, the better the impurity content is. However, if the content is too restrictive, the cost of removing impurities in the steelmaking process increases. In the present invention, P and S are controlled to 0.015 wt% or less from the viewpoint of securing ductility as in the conventional steel wire.

Sol.Al (aluminum): 0.03 to 0.05 wt%

When Al is present, fine AlN precipitates are formed in the austenite grain boundaries by bonding with N, which prevents austenite grains from becoming large during high-temperature heating and high-temperature rolling. When the amount is less than 0.03 wt%, the effect of AGS pinning by AlN is insignificant, and when it is added in an amount exceeding 0.05 wt%, hard nonuniform alumina-based nonmetallic inclusions are formed to deteriorate soft deterioration and drawability. Therefore, the content of Sol.Al is preferably controlled to 0.03 to 0.05% by weight.

N (nitrogen): 0.002 to 0.008 wt%

N is an element which is adhered to a potential formed on a ferrite base during drawing to cause age hardening. Other effects such as B and the like are produced to nitride the austenite grains. Also, N in the steel is combined with Al to form AlN, which is pinned to the austenite grain boundaries during heating and rolling, and has the effect of making the austenite grains finer. In order to obtain the austenite grain refining effect described above, the N content should be 0.002 wt% or more. However, in consideration of the adverse effect of the physical properties of steel, N does not usually exceed 0.008% by weight.

On the other hand, it is preferable that the high carbon wire material according to the present invention has a non-pearlite structure including at least 95% of pearlite and 5% or less of pro-eutectoid ferrite, cobalt cementite and bainite Do. The microstructure of the wire is controlled to have a pearlite structure with an area fraction of 95% or more. Unlike ferrites and other dual phases, the pearlite structure exhibits an exponential increase in strength It is because it is visible.

In addition, it is preferable that the high carbon wire material to be the base material of dry drawing and swaging according to the present invention has a tensile strength of 1400 MPa or more and a section reduction ratio (RA) of 10% or more. The high carbon wire material having the physical properties described above can easily obtain an effect of reducing the residual stress on the surface portion and the center portion of the present invention through dry drawing and swaging, which will be described later.

Next, in the present invention, the wire rod prepared as described above is dried fresh, and its cross section is further reduced through a swaging process to produce a final steel wire.

FIG. 1 schematically shows a dry drawing and a swaging working process according to an embodiment of the present invention.

The dry drawing apparatus of the present invention is provided with one or more dies (20, 21) having a gradually decreasing diameter between the take-up bobbin and the take-up bobbin so as to reduce the diameter of the wire. Between the dies 20 and 21, there is provided one or more drums 10, 11 and 12 for temporarily winding the wire-drawn wire to adjust the tension of the entire machining step.

More specifically, the wire rods A are rolled and wound on the primary drum 10 and then passed through the primary dies 20 while being subjected to primary drawing to decrease the diameter. The primary drawn wire (B) is wound and unwound on the secondary drum (11) and then subjected to secondary drafting while passing through the secondary dies (21). The secondary wire 7 is wound and wound on the third drum 12 and then transferred to the swaging device 30, which is a main component of the present invention.

Swaging is a method of processing a material using a plurality of cast iron having a certain shape. The swaging device 30 according to the present invention has a total of four rotors 31 as a mold iron as shown in FIG. The four rotors 31 are further rotated in a predetermined direction to further reduce the diameters of the wire material C subjected to the secondary drawing. To this end, the circular cross section formed by the four rotors 31 is configured such that the cross-sectional area D2 of the trailing end of the wire rod is smaller than the cross-sectional area D1 of the leading end of the wire rod.

The present inventors have found that the above swaging process reduces the tensile residual stress that exists in the longitudinal direction on the surface of the wire rather than the previous dry drawing process. This is because the plurality of dies 20, 21 used for dry drawing has a high contact angle with respect to the surface of the wire rod during processing, so that stress applied to the same deformation is high, It is judged that the stress applied to the surface of the wire rod is low even if the same amount of reduction is applied because the wire rod 31 contacts the wire rod over a relatively large area.

Although the present invention is not limited to the specific process conditions of the dry drawing and swaging processes, the dry drawing process may be carried out in such a manner that the wire material is drawn at a reduced amount of 70 to 80% It is preferable to further reduce the cross section by 60% reduction amount.

The dry drawing process is a main machining step for manufacturing a steel wire by a process of reducing the diameter by using a plurality of dies 20 and 21 shown in FIG. 1, using wire rods and LP heat-treated wire rods. Therefore, in the manufacture of the high-strength steel wire having excellent torsional characteristics according to the present invention, most of the machining for reducing the diameter is preferably performed through the dry drawing.

However, when the cross-sectional reduction rate of the wire rod is less than 70% due to the dry drawing process, the amount of wire drawing is too small to obtain a sufficient tensile strength required for the wire rope, and in addition, The machining efficiency of the tool is deteriorated. On the other hand, if the cross-sectional reduction ratio of the wire rod exceeds 80% due to dry drawing, the difference in residual stress between the surface portion and the center portion of the steel wire becomes too large, It is difficult to obtain the effect of reducing the residual stress difference between the surface portion and the center portion of the substrate.

The swaging step is a main processing step according to the present invention in which the dry drawn wire is further reduced in diameter by using the plurality of rotating bodies 31 shown in Fig.

However, if the cross-sectional reduction rate due to the swaging process of the dry-drawn wire is less than 30%, the total amount of processing (dry drawing + swaging) becomes too small to obtain sufficient strength. On the other hand, if the cross-sectional reduction rate due to the swaging process exceeds 60%, the amount of reduction of the material due to the swaging process becomes too large, so that sufficient effect of reducing the residual surface stress can not be obtained sufficiently.

On the other hand, it is preferable that the longitudinal residual stress present on the surface portion of the dry drawn wire is reduced by 60% or more through the swaging process. It is preferable that the difference in the residual stress in the longitudinal direction between the surface portion and the center portion of the high strength steel wire is 550 MPa or less. As described above, when the difference in residual stress between the surface portion and the center portion of the steel wire due to the decrease in the tensile residual stress in the longitudinal direction existing in the surface portion of the steel wire becomes smaller than the reference value, balance between strength and ductility of the steel wire is improved, The number of torsions is improved to 15 times or more. As a result, the wire rope made of the high-strength steel wire according to the present invention can prevent the occurrence of breakage in the wire rope in the twisting process because cracks do not progress in the longitudinal direction.

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

(Example)

In the present invention, a steel comprising 0.98% of C, 1.3% of Si, 0.5% of Mn, 0.6% of Cr, 0.015% or less of P and S, 0.04% of Sol.Al and 0.05% And then rolled in the usual manner through billet-ingot welding. The wire rod rolling conditions include a heating furnace of 950 ° C, a hot rolling temperature of 900 ° C, a L / H (Laying Head) temperature of 880 ° C, and a cooling rate of 10 ° C / s. The microstructure of the wire rods subjected to the wire rolling is composed of 95% or more of pearlite in an area fraction, and the non-pearlite structure including a cornerstone cementite is 5% or less. In this case, the tensile strength of the wire is 1412 MPa, and the reduction ratio (RA) is 12%.

Two types of steel wires were manufactured using the high carbon wires prepared as described above, and the differences and the mechanical characteristics of the steel wires were shown in Table 1. [

    Wire rod    LP line Gunned wire rod Sweatshirt     Final liner T / S
(MPa) RA (%) T / S
(MPa) RA (%) Reduction amount
(%) T / S
(MPa) Reduction amount
(%) Total reduction (%) T / S
(MPa) torsion
(time) Comparative Example 1 1,412 10 1,584 34 73.8 2,238 - 83.2 2,358 8 Inventory 1 1,412 10 1,584 34 73.8 2,238 36 83.2 2.301 15

Comparative Example 1 is a steel wire manufactured through a general process, that is, a process of "wire material-> LP heat treatment-> dry grinding-> completion". Example 1 is "wire material-> LP heat treatment-> -> Finished ".

Therefore, the properties of Comparative Example 1 and Inventive Example 1 are the same as those of the intermediate drawn material produced through LP heat treatment and dry drawing process. In other words, the tensile strength (T / S) of the LP line is 1584 MPa and the section reduction ratio (RA) is 34%, and the reduction amount thereof is 73.8%.

Comparative Example 1 was processed to a final reduction amount of 83.2% through dry drawing, and the tensile strength of the final steel wire was 2358 MPa. Experiments were conducted on the torsion test condition 100D (where D means the steel wire diameter). As a result, no delamination occurred and 8 torsions were obtained.

On the other hand, Inventive Example 1 uses the swaging device to reduce the amount of wire roughened to 36%. The tensile strength was 2300 MPa, which was 50 MPa lower than that of Comparative Example 1, which was applied only to dry drawing. However, when twisted under the same twist condition, delamination did not occur and the number of twists was improved by 15 cycles and 87%.

                           Residual stress Longitudinal direction-surface portion
(MPa) Lengthwise-center
(MPa) Longitudinal residual stress
Difference (surface part - center part) Comparative Example 1      +298       -425      723 Inventory 1      +103       -421      520

To investigate the reason for the improved torsional characteristics, the surface and center residual stress values were measured using the FIB-DIC method. The results are shown in Table 2.

As a result of measurement of the residual stress in the longitudinal direction of the surface portion, there is a tensile residual stress of + 298 MPa in the case of Comparative Example 1 which is dry-fresh only, while in the case of Example 1 in which swaging is further applied, there is a tensile residual stress of +103 MPa , The residual stress reduction effect was about 65%. The longitudinal compressive residual stresses at the center were measured in the half-cut state by mechanical polishing. As a result, compressive residual stresses of about 420 MPa were present at the central portion of Comparative Example 1 and Inventive Example 1, which are almost similar to each other. This means that even if the swaging process is applied, the residual stress in the center portion does not greatly affect the residual stress.

In Example 1, the difference in residual stress in the longitudinal direction between the surface portion and the center portion was 520 MPa, which was compared with Comparative Example 1, and it was confirmed that the difference was reduced by about 200 MPa as a result of applying the swaging according to the present invention.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. It will be possible.

10, 11, 12: Drums 20, 21: Dies
30: Swaging device 31: Rotor
A: Wire rod B: First processed wire rod
C: Secondary finished wire rod

Claims (6)

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet comprises, by weight, 0.80 to 1.0% of C, 0.5 to 1.5% of Si, 0.3 to 0.7% of Mn, 0.4 to 0.8% of Cr, 0.015% or less of P, 0.05%, N: 0.002 to 0.008%, the balance Fe and other unavoidable impurities; And
And a step of further reducing the cross-section through the swaging process to produce a final steel wire,
Wherein the dry drawing is performed at a reduction amount of 70 to 80%, and the swaging step further reduces the cross section of the dry drawn wire by a reduction amount of 30 to 60%. Gt;
The high strength steel wire according to claim 1, wherein the wire has a pearlite structure including 95% or more of pearlite in an area fraction of the microstructure and less than 5% of non-pearlite structure including protonic ferrite and cubic cementite. Way.
The method of manufacturing a high strength steel wire according to claim 1, wherein the wire rod has a tensile strength of 1400 MPa or more and a section reduction ratio (RA) of 10% or more.
delete The method of manufacturing a high strength steel wire according to claim 1, wherein the difference in longitudinal direction residual stress between the surface portion and the center portion of the high strength steel wire is 550 MPa or less.
The method of manufacturing a high strength steel wire according to claim 1, wherein the number of twisting of the high strength steel wire is 15 or more times.

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