WO2011055919A2

WO2011055919A2 - Wire rod for drawing having excellent drawability, super-high-strength steel wire and manufacturing method thereof

Info

Publication number: WO2011055919A2
Application number: PCT/KR2010/007279
Authority: WO
Inventors: Hyun-Jin Kim; Soo-Dong Park; Byong-Seol Seog; Choong-Yeol Lee
Original assignee: Posco
Priority date: 2009-11-03
Filing date: 2010-10-22
Publication date: 2011-05-12
Also published as: KR20110048744A; JP5521052B2; WO2011055919A3; JP2013510234A; CN102575312A; KR101309881B1

Abstract

A wire rod having excellent drawability includes, by wt%, 0.8-1.0% C, 0.3-0.7% Mn, 0.2-0.6% Cr and a balance of Fe and inevitable impurities. A method for manufacturing a super-high-strength steel wire includes: a first heat-treatment step of heating a wire rod having the above steel composition, and maintaining the heated wire rod at 1100~1200℃; a second heat-treatment step of maintaining the heated wire rod at 900~1000℃; a step of subjecting the wire rod of the second heat-treatment step to lead patenting at 540~640℃; and a step of drawing the lead-patented wire rod.

Description

WIRE ROD FOR DRAWING HAVING EXCELLENT DRAWABILITY, SUPER-HIGH-STRENGTH STEEL WIRE AND MANUFACTURING METHOD THEREOF

The present invention relates to a wire rod for drawing which is used for large-diameter high-strength steel wire, a steel wire and a manufacturing method thereof, more particularly, to a wire rod for drawing having excellent drawability, which can ensure super-high-strength and torsion characteristics without adding Si, a super-high-strength steel wire and a manufacturing method thereof.

In recent years, as industrialization has advanced, sea-crossing bridges connecting mainland areas with islands have been actively constructed in order to increase the efficiency of the utilization of a nation s national territory. Such bridges are super-long-span bridges having a center span longer than 2km. In super-long-span bridges, large-diameter high-strength steel wires are used to support loads. Also, as continental-shelf oil fields have been gradually exhausted, work on exploring or developing oil fields in deeper sea areas has been conducted. In such an enterprise, large-diameter high-strength steel wires are also used.

Typical examples of large-diameter high-strength steel wires include PC steel wires that are used to reinforce concrete in cable or tunnel construction in suspension bridges or cable-stayed bridges, cables for large-sized buildings or structures, and anchor ropes for supporting offshore oil fields or various structures. Also, in order to satisfy various requirements in a wide range of industries, steel wires need to have high strength. In addition, when steel wires are actually applied to bridges or buildings, they are used in the form of bundles by twisting several steel wire strands together, and thus they also need to have excellent torsion characteristics.

The strength of steel wires is ensured by the strength of the steel material prior to wire drawing and by the steel strength that is increased by work hardening during wire drawing. As is known in the art, the strength of steel wires shows a value relative to ductility, and thus, when the strength of the steel material prior to wire drawing is high, the wire drawing limit is decreased, so that the amount of work hardening is relatively small. On the other hand, when the strength of steel wires is low, the amount of work hardening is relatively large, because the drawing limit thereof can be increased. In addition, as the amount of work hardening of the steel becomes larger, the ductility of the steel material is rapidly reduced, so that the torsion characteristics thereof are deteriorated.

Accordingly, in the prior art, in order not to deteriorate the torsion characteristics thereof, steel wires were manufactured in such a way to maximize the strength of the steel material prior to wire drawing, rather than to increase the strength thereof by wire drawing. Generally, the strength of steel wires for drawing has been maximized through the solid solution strengthening effect of Si which is contained in a given amount or more in the steel wires. However, when such steel wires are drawn, the ductility of the steel wires is reduced, thus deteriorating the torsion characteristics thereof. In addition, because the drawing limit of wire rods is low, wire rods for drawing corresponding to the diameters of steel wires that are final products should be manufactured separately, thus reducing the productivity thereof.

Accordingly, steel wires having all excellent strength and torsion characteristics are required. For this purpose, there are required studies on methods that ensure strength by increasing the limit of wire drawing but do not deteriorate torsion characteristics.

An aspect of the present invention provides a wire rod having excellent drawability, a super-high-strength steel wire having excellent tensile strength and torsion characteristics, and a manufacturing method thereof.

According to an aspect of the present invention, there is provided a wire rod for drawing having excellent drawability, the wire rod including, by wt%, 0.8-1.0% C, 0.3-0.7% Mn, 0.2-0.6% Cr and a balance of Fe and inevitable impurities.

The wire rod may have a fine pearlite structure formed from prior austenite having a particle size of 100㎛ or larger.

The pearlite structure may have a lamellar spacing of 100㎚ or less, wherein the deviation of the lamellar spacing is 50㎚ or less.

According to another aspect of the present invention, there is provided a super-high-strength steel wire manufactured by drawing the above wire rod at a reduction rate per pass of 30% or less and a total reduction rate of 85% or more.

The steel wire may be a super-high-strength steel wire having a tensile strength of 2000MPa or higher.

The steel wire may have a rectangular fracture shape when fractured by twisting and has a twist number of 20 times/100D (D: steel wire diameter) or more.

According to another aspect of the present invention, there is provided a method for manufacturing a super-high-strength steel wire, the method including: a first heat-treatment step of heating a wire rod including, by wt%, 0.8-1.0% C, 0.3-0.7% Mn, 0.2-0.6% Cr and a balance of Fe and inevitable impurities, and maintaining the heated wire rod at 1100~1200℃; a second heat-treatment step of maintaining the heated wire rod at 900~1000℃; a step of subjecting the wire rod of the second heat-treatment step to lead patenting at 540~640℃; and a step of drawing the lead-patented wire rod.

The first heat treatment step may be maintained for 5 minutes or more.

By the first heat-treatment step, the wire rod may include prior austenite having a particle size of 100㎛ or larger.

The drawing step may be carried out at a reduction rate per pass of 30% or less and a total reduction rate of 85% or more. Also, it may be carried out at a drawing strain of 1.0-3.0%.

According to the present invention, there can be provided a super-high-strength steel wire having a tensile strength of 2000 MPa or higher and excellent torsion characteristics. Also, steel wires having various diameters can be provided using materials having the same diameters as those of the steel wires.

FIG. 1 is a graphic diagram showing the distribution in lamellar spacing in the pearlite of Inventive Example 1 and Comparative Example 1.

FIG. 2 is a set of TTT curves showing the transformation completion time at each temperature, obtained by controlling the particle sizes of prior austenite of samples having an alloy component of 0.92C-0.5Mn-0.6Cr to 44.9㎛ (a) and 110.6㎛ (b), respectively, and then subjecting the samples to constant-temperature transformation in a lead bath of 540~640℃.

The strength of steel wires can be ensured by the strength of the steel material prior to wire drawing and by the strength caused by work hardening resulting from wire drawing. In the conventional steel wires containing Si, the Si content is distributed in the ferrite structure to exert a solid solution-strengthening effect, thereby increasing the strength of the steel material prior to wire drawing. However, such wire rods have high strength, but the ductility thereof is low, and for this reason, the amount of drawing thereof during wire drawing is reduced and the torsion characteristics thereof are also insufficient. To solve such shortcomings, according to the present invention, a wire rod for drawing containing no Si is used to provide a super-high-strength steel wire. Because this wire rod for drawing contains no Si, it cannot obtain a solid solution-strength effect caused by Si. However, a super-high-strength steel wire having improved strength and torsion characteristics after drawing can be provided by increasing the austenizing temperature of the wire rod to enlarge the average particle size of the prior austenite so as to delay the transformation of the pearlite thereof, thereby obtaining a fine and uniform pearlite which increases the limit of amount of wire drawing of the wire rod.

As used herein, the term "wire rod" refers to a lead-patented state, and the term "steel wire" refers to a state obtained by drawing the wire rod.

Hereinafter, the components of the present invention will be described.

C (carbon): 0.8-1.0wt%

C is an essential element that is added to the steel material in order to ensure the strength of the steel material. If the content of C is less than 0.8wt%, the fraction of cementite in the pearlite tissue will be relatively low, and thus the minimum strength required in the steel cannot be ensured. However, if the content of C is more than 1.0wt%, proeutectoid cementite can be produced in the wire rod during lead patenting to significantly reduce the drawability of the wire rod. For these reasons, the content of C may be limited to 0.8-1.0wt%.

Mn (manganese): 0.3-0.7wt%

Mn is an element advantageous for ensuring the strength of steel by increasing the hardenability of steel when contained in steel. If the content of Mn is less than 0.3wt%, it will be difficult to obtain sufficient strength and hardenability required in the steel, and if it is more than 0.7wt%, the austenite-to-pearlite transformation will be significantly delayed, so that the steel is water-cooled before the transformation is completed, whereby martensite is undesirably produced. For these reasons, the content of Mn may be limited to 0.3-0.7wt%.

Cr (chromium): 0.2-0.6wt%

Cr is an element that is effective in strengthening a solid solution, stabilizing cementite and improving oxidation resistance and is also useful for making the pearlite lamellar spacing fine. If the content of Cr is less than 0.2wt%, the effect of making the pearlite lamellar spacing fine will be insufficient and it will be difficult to achieve the effect of stabilizing cementite. On the other hand, if the content of Cr is more than 0.6wt%, it will increase the nose temperature on the time-temperature-transformation curve (TTT curve) and make the shape of cementite in the pearlite structure non-uniform, thus making it difficult to obtain a fine and uniform pearlite. For these reasons, the content of Cr may be limited to 0.2-0.6wt%.

In the steel composition of the present invention, the remaining component is iron (Fe). In addition, unintended impurities cannot be excluded, because they may be inevitably incorporated into the steel from the raw materials or the surrounding environment during conventional steel manufacturing processes. These impurities can be understood by any person skilled in the field of steel manufacturing, and thus all the impurities will not be specifically described herein.

However, among the impurities, oxygen (O), phosphorus (P) and sulfur (S) will be briefly described below, because they are impurities that are frequently mentioned.

O (oxygen): 0.0015wt% or less

The content of O is limited to 0.0015wt% or less. If the content of O is more than 0.0015wt%, oxide-based nonmetallic inclusions become coarse, thus reducing the drawability of the steel.

P: 0.02wt% or less

P is an element that is inevitably contained in the steel during the manufacturing of the steel. Because P is segregated in the grain boundary to reduce the toughness of the steel, the content thereof may be controlled to the lowest possible level. Although it is theoretically advantageous to limit the content of P to 0%, P is inevitably added to the steel during the manufacturing process. For this reason, it is important to control the upper limit of the content of P. In the present invention, the upper limit of the content of P may be 0.02wt%.

S: 0.02wt% or less

S is an element that is inevitably contained in the steel during the manufacturing of the steel. It is a low-boiling-point element and is segregated in the grain boundary to reduce the toughness of the steel. Also, it can form sulfides that have an adverse effect on the drawability of the steel. For these reasons, the content of S may be controlled to the lowest possible level. Although it is theoretically advantageous to limit the content of S to 0%, P is inevitably added to the steel during the manufacturing process. For this reason, it is important to control the upper limit of the content of S. In the present invention, the upper limit of the content of S may be 0.02wt%.

In the present invention, the steel composition preferably contains no Si. Even when Si is contained as an impurity in an amount of 0.1wt% or less, the strength and torsion characteristics of the steel wire, which are sought in the present invention, can be ensured. As described above, Si is distributed in the ferrite structure to reduce the ductility of the ferrite structure, thus reducing the drawability of the steel. For this reason, the steel wire contains no Si, such that the drawability thereof can be significantly increased. However, the decrease in the strength of the steel wire, which occurs because the steel wire contains no Si, can be complemented using work hardening through wire drawing as described below. Although a high degree of work hardening occurs in the steel wire of the present invention, the ductility of the steel wire is ensured so that the torsion characteristics thereof are good, because the steel wire contains no Si.

The fine structure of the wire rod according to the present invention includes a pearlite structure formed from prior austenite having a particle size of 100㎛ or larger. The pearlite structure is formed according to a manufacturing method as described below. In addition, the pearlite structure may have a lamellar spacing of 100㎚ or less, wherein the deviation of the lamellar spacing is preferably 50㎚ or less. For this reason, the present invention can provide a wire rod that has excellent drawability due to the fine pearlite structure, even though it contains no Si.

Hereinafter, the manufacturing method according to the present invention will be described.

The present invention provides a method for manufacturing a super-high-strength steel wire, the method including: a first heat-treatment step of heating a wire rod including, by wt%, 0.8-1.0% C, 0.3-0.7% Mn, 0.2-0.6% Cr and a balance of Fe and inevitable impurities, and maintaining the heated wire rod at 1100~1200℃; a second heat-treatment step of maintaining the heated wire rod at 900~1000℃; a step of subjecting the wire rod of the second heat-treatment step to lead patenting at 540~640℃; and a step of drawing the lead-patented wire rod.

FIG. 2 is a set of TTT curves showing the transformation completion time at each temperature, obtained by subjecting each wire rod to constant-temperature transformation in a lead bath of 540~640℃, in a case in which the particle size of prior austenite was controlled to 44.9㎛ by a conventional heat-treatment method (FIG. 2(a)), and a case in which the particle size of prior austenite was controlled to 110.6㎛ by the heat-treatment method of the present invention (FIG. 2(b)). The eutectoid temperature of a given component is 733℃, as calculated using Thermocalc, a thermodynamic calculation program. When the degree of undercooling of pearlite transformation is calculated from the eutectoid temperature, it can be seen that the difference in undercooling degree between the surface and center of the wire rod is smaller in the case in which the particle size of prior austenite is 110㎛ than in the case in which the particle size is to 44.9㎛. Namely, the difference in temperature between the surface and center of the wire rod is smaller in the case in which the prior austenite particles became coarse. In the present invention, a method callable of coarsening prior austenite particles will be described below.

Heat-treatment steps

In the present invention, two heat-treatment steps are carried out immediately before lead patenting. First, a first heat-treatment step may be carried out in which the wire rod is heated (austenized) to 1100~1200℃ and maintained at that temperature. Herein, the maintenance time may be 5 minutes or more. The average particle size of the prior austenite particles can be coarsened to 100㎛ or larger by increasing the austenizing temperature of the wire rod and maintaining the wire rod at that temperature for 5 minutes or more. However, in view of the functioning of processing equipment and economical factors, the austenizing temperature may be limited to 1200℃, and the upper limit of the maintenance time may also be suitably limited. In addition, the upper limit of the particle size of the prior austenite may also be determined depending on the temperature and time ranges.

Because the cooling rates of the surface and center of the wire rod after the first heat-treatment step differ from each other, the wire rod may be subjected to a second heat-treatment step at 900~1000℃ in order to make the cooling rates equal to each other. After the first heat-treatment step, the wire rod may be cooled by any cooling method and may be air cooled. When the cooling rates of the surface and center of the wire rod are maintained at the same level, the pearlite transformations at the surface and center of the wire rod are initiated at substantially the same temperature, and thus the difference in fine structure between the surface and the center can be minimized, thereby ensuring a uniform fine structure.

Also, in general ferrite structures, when the particle size of prior austenite is increased, the size of ferrite grains will also be increased, and thus both the strength and ductility of the ferrite structures will be reduced. However, in the case of pearlite structures, the lamellar spacing of the pearlite structure has no connection with the particle size of prior austenite and is determined only the undercooling degree of the pearlite structure, and thus, the undercooling degree is a major microstructural factor determining the strength and ductility of the pearlite structure. For this reason, the pearlite structure needs to be subjected to lead patenting (heat treatment).

Lead patenting step

The wire rod that has been subjected to the first and second heat-treatment steps is subjected to lead patenting. Herein, the lead patenting step is preferably carried at a temperature of 540~640℃, and more preferably 580~600℃. When the wire rod is subjected to constant-temperature transformation in the above temperature range, a fine pearlite structure can be obtained. The lamellar spacing of the pearlite structure is less than 100㎚, and the deviation of the lamellar spacing can be controlled to 50㎚ or less.

Drawing step

The lead-patented wire rod is drawn. This drawing step may be carried out at a reduction rate per pass of 30% or less and a total reduction rate of 85% or more. Also, because the limit of wire drawing is sufficiently ensured, steel wires having various diameters can be manufactured at various reduction ratios using materials having the same diameters as those of the steel wires. The wire drawing strain (ε) of the wire rod may be 1.0-3.0%.

The steel wire manufactured according to the above-described manufacturing method may have a tensile strength of 2000MPa or more. Also, it may have a twist number of 25 times/100D (D: wire diameter) or more and show a rectangular fracture shape when fractured by twisting. Fracture failures are caused by various factors, but if the wire rod is not suitable for wire drawing, various microstructural failures, including spiral, shear, conical and torn shapes, will appear. In the present invention, because the internal and external structures of the wire rod before drawing are uniform, a steel wire having a large number of twists can be obtained. Also, the fracture shape in the steel wire is a normal state and is perpendicular to the length direction of the steel wire.

Hereinafter, the present invention will be described in detail with reference to examples.

(Examples)

An ingot having the components (P, S and O are omitted) shown in Table 1 below was cast into a billet which was then rolled into a sheet. The sheet was cut into wire rods having a diameter of 13Φ㎜. In Inventive Examples 1 and 2, the wire rod was heated to 1100℃, maintained at that temperature for 10 minutes, air-cooled to 1000℃, and then lead-patented at 590℃ for 5 minutes. In Comparative Example 1, the wire rod was heated to 1000℃, maintained at that temperature for 10 minutes, and lead-patented at 590℃ for 5 minutes. The tensile strengths of the wire rods were measured, and the results of the measurement are shown in Table 2 below. The wire rods were drawn to have diameters of 7.44Φ㎜(reduction rate: 67.2%), 5.95Φ㎜(reduction rate: 79.1%), 5.32Φ㎜(reduction rate: 83.3%), 4.92Φ㎜(reduction rate: 85.7%), 4.40Φ㎜(reduction rate: 88.5%) and 3.96Φ㎜(reduction rate: 90.7%), and the tensile strength and twist number (fracture shape) of each of the drawn wire rods were measured, and the results of the measurement are shown in Table 2 below. Also, the lamellar spacing of each of Inventive Example 1 and Comparative Example 1 was measured, and the measurement results are shown in FIG. 1 as a graph that can compare the sizes of the spacing.

Table 1

	C (wt%)	Si (wt%)	Mn (wt%)	Cr (wt%)
Inventive Example 1	0.92	0.0	0.5	0.3
Inventive Example 2	0.92	0.0	0.5	0.6
Comparative Example 1	0.92	1.3	0.5	0.3

As can be seen in Table 1 above, Comparative Example 1 contained Si in an amount of 1.3wt% which was higher than the upper limit of the content range defined in the present invention. Inventive Examples 1 and 2 satisfied all the components and contents limited by the present invention.

Table 2

Wire diameter	Total reduction rate(%)	Reduction rate per pass(%)	Drawing strain	InventiveExample		1		InventiveExample 2		ComparativeExample 1
Wire diameter	Total reduction rate(%)	Reduction rate per pass(%)	Drawing strain	TS (MPa)	Twist number (fracture shape)	TS (MPa)	Twist number (fracture shape)	TS (MPa)	Twist number (fracture shape)
13	0	0	0	1073	-	1101	-	1271	-
7.44	67.2	20.0	1.12	1601	32(normal)	1627	33(normal)	1627	32(normal)
5.95	79.1	20.0	1.56	1769	34(normal	1725	36(normal)	1754	34(normal)
5.32	83.3	20.1	1.79	1870	35(normal)	1931	35(normal)	1870	36(abnormal)
4.92	85.7	14.5	1.94	1903	31(normal)	1945	36(normal)	1917	36(abnormal)
4.40	88.5	20.0	2.17	2014	30(normal	2072	28(normal)	-	-
3.96	90.7	19.0	2.38	2051	26(normal)	2109	30(normal)	-	-

As can be seen in Table 2 above, Inventive Examples 1 and 2 contained no Si having a solid solution-strengthening effect, and the tensile strength of the wire rods after heat treatment but before drawing was lower than Comparative Example 1 by about 200MPa. However, because the lamellar spacing and lamellar spacing deviation of Inventive Examples 1 and 2 were small, the work hardening rate thereof at the initial stage of drawing was high, the wire rods having a diameter 7.44Φ㎜(reduction rate: 67.2%) or larger in Inventive Examples 1 and 2 could have substantially the same tensile as Comparative Example 1.

When the twist number and the change in the fracture shape were examined, Inventive Examples 1 and 2 all showed a good normal state (the fracture had a shape perpendicular to the length direction of the wire rod), but the wire rods of Comparative Example 1 having a diameter of 5.32Φ㎜(total reduction rate: 83:3%) or larger showed fracture failures. The reduction ratio of Inventive Examples 1 and 2 was increased by 12% (79% → 91%), but the strain of Inventive Examples 1 and 2 was increased by about 153% (1.56 → 2.41) as compared to that of Comparative Example 1.

As can be seen in FIG. 1, the lamellar spacing of Inventive Examples 1 and 2 was smaller than that of Comparative Example 1, and the internal and external deviation was also smaller.

While the present invention has been shown and described in connection with the exemplary embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

A wire rod for drawing having excellent drawability, the wire rod comprising, by wt%, 0.8-1.0% C, 0.3-0.7% Mn, 0.2-0.6% Cr and a balance of Fe and inevitable impurities.
The wire rod of claim 1, wherein the wire rod comprises a fine pearlite structure formed from prior austenite having a particle size of 100㎛ or larger.
The wire rod of claim 1, wherein the pearlite structure has a lamellar spacing of 100㎚ or less, wherein the deviation of the lamellar spacing is 50㎚ or less.
A super-high-strength steel wire manufactured by drawing the wire rod of any one of claims 1 to 3 at a reduction rate per pass of 30% or less and a total reduction rate of 85% or more.
The super-high-strength steel wire of claim 4, wherein the steel wire has a tensile strength of 2000MPa or higher.
The super-high-strength steel wire of claim 4, wherein the steel wire has a rectangular fracture shape when fractured by twisting and has a twist number of 20 times/100D (D: steel wire diameter) or more.
A method for manufacturing a super-high-strength steel wire, the method comprising:

a first heat-treatment step of heating a wire rod comprising, by wt%, 0.8-1.0% C, 0.3-0.7% Mn, 0.2-0.6% Cr and a balance of Fe and inevitable impurities, and maintaining the heated wire rod at 1100~1200℃;

a second heat-treatment step of maintaining the heated wire rod at 900~1000℃;

a step of subjecting the wire rod of the second heat-treatment step to lead patenting at 540~640℃; and

a step of drawing the lead-patented wire rod.
The method of claim 7, wherein the first heat-treatment step is maintained for 5 minutes or more.
The method of claim 7, wherein the wire rod include prior austenite having a particle size of 100㎛ or larger by the first heat-treatment step.
The method of claim 7, wherein the drawing step is carried out at a reduction rate per pass of 30% or less and a total reduction rate of 85% or more.
The method of claim 7, wherein the drawing step is carried out at a drawing strain (ε) of 1.0-3.0%.