KR20110048744A

KR20110048744A - Wire rod for drawing with excellent drawability, ultra high strength steel wire and manufacturing method of the same

Info

Publication number: KR20110048744A
Application number: KR1020090105444A
Authority: KR
Inventors: 김현진; 박수동; 석병설; 이충열
Original assignee: 주식회사 포스코
Priority date: 2009-11-03
Filing date: 2009-11-03
Publication date: 2011-05-12
Also published as: WO2011055919A3; JP5521052B2; CN102575312A; JP2013510234A; WO2011055919A2; KR101309881B1

Abstract

PURPOSE: Wire with excellent drawing performance and ultra high strength steel wire are intended to provide tensile strength of over 2000MPa and an excellent twisting property. CONSTITUTION: Wire with excellent drawing performance consists of C of 0.8~1.0weight%, Mn of 0.3~0.7weight%, Cr of 0.2~0.6weight%, Fe and inevitable impurities. The wire comprises fine pearlite formed from guostenite of over 100μm in grain size. The lamellar spacing of the pearlite is below 100nm and the deviation of the lamellar spacing is below 50nm.

Description

Wire rod for drawing with excellent drawability, ultra high strength steel wire and manufacturing method of the same}

The present invention relates to a wire rod for use in large diameter high strength steel wire, a steel wire and a manufacturing method thereof, and more particularly, a wire rod for wire drawing, ultra-high drawability, which can secure ultra high strength and torsion characteristics without adding Si. It relates to a high strength steel wire and a method of manufacturing the same.

In recent years, as industrialization is advanced, the construction of offshore bridges connecting the inland and the islands is increasing to increase the efficiency of land use. These bridges are ultra long span bridges with a center span of more than 2 km. In the case of ultra long span bridges, large diameter high strength steel wires are used to support the load. In addition, as the continental shelf oil field is gradually depleted, work is being made to explore or develop oil fields in deeper oceans, and large diameter high-strength steel wire is also used for such work.

Representative examples include PC steel wire used for concrete reinforcement for suspension bridges, cable-stayed bridges, and tunnel construction, cables for large buildings and structures, anchor ropes for supporting offshore oil fields and various structures. And, in order to meet the various needs of the entire industry, it is required to increase the strength of the steel wire. In addition, when the steel wire is actually applied to bridges or buildings, it is required to have excellent twisting characteristics because it is applied in the form of a bundle made of multiple strands.

The strength of the steel wire is ensured by the strength of the material before the wire drawing and by the increase in work hardening due to the wire drawing. Since the strength of steel wire usually shows a value relative to ductility, when the strength of the material before wire drawing is high, the limit of drawing work that can be given becomes small, and the amount of work hardening is relatively small. On the contrary, when the strength of steel wire is low, It is known that the amount of work hardening is relatively high because it can be given a lot. In addition, when the amount of work hardening increases, the ductility of the material is sharply lowered, and the torsional property becomes worse.

Therefore, conventionally, the steel wire was manufactured to secure the maximum strength of the material before the wire processing, rather than improving the strength according to the wire processing, so as not to reduce the torsion characteristics. In general, Si has been included in a certain amount or more to secure the maximum strength of the wire for wire drawing through the solid-solution strengthening effect by Si. However, when the wire is drawn, there is a problem in that the ductility of the steel wire is deteriorated and the torsion characteristic is lowered. In addition, since the limit of the amount of wire processing is small, each of the wire rods corresponding to the wire diameter of the final product of the steel wire has to be manufactured, thereby reducing productivity.

Therefore, there is a demand for a steel wire that can secure both excellent strength and torsional characteristics. To this end, research has been conducted on a method of securing strength by increasing the limit of the amount of drawn processing, but not reducing the torsional characteristics.

The present invention is to provide a wire rod for excellent wire workability, an ultra high strength steel wire excellent in tensile strength and torsional properties and a manufacturing method thereof.

The present invention, in one embodiment, by weight percent, C: 0.8 ~ 1.0%, Mn: 0.3 ~ 0.7%, Cr: 0.2 ~ 0.6%, remainder Fe and other excellent wire drawing for excellent wire workability including other unavoidable impurities to provide.

It is preferable that the said wire rod contains fine pearlite formed from the old austenite with a particle size of 100 micrometers or more.

The lamellar spacing of the pearlite is 100 nm or less, and the deviation is preferably 50 nm or less.

In another embodiment, the present invention provides an ultra-high strength steel wire manufactured by drawing the wire at a reduction rate of 30% or less per pass and a reduction rate of 85% or more.

Preferably, the steel wire is an ultra high strength steel wire having a tensile strength of 2000 MPa or more.

When the steel wire is twisted at the time of the torsional fracture, the fractured form is the rectangular fractured form, and the number of twists is preferably 20 times / 100D (D: diameter) or more.

In another embodiment, the present invention provides a weight percent of 1100-1200 by heating a wire containing C: 0.8-1.0%, Mn: 0.3-0.7%, Cr: 0.2-0.6%, balance Fe and other unavoidable impurities. A first heat treatment step maintained at ℃; A second heat treatment step of maintaining the heated wire at 900 to 1000 ° C; Performing a lead patterning heat treatment on the wire maintained at the temperature at 540˜640 ° C .; And it provides a method for producing an ultra-high strength steel wire comprising the step of drawing the wire patterned heat treatment wire.

The first heat treatment step is preferably maintained for at least 5 minutes.

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

The drawing step is preferably performed at 30% or less reduction rate per pass and 85% or more in total reduction rate. Moreover, it is preferable that it is 1.0 to 3.0% of drawing processing strain ((epsilon)).

Through the present invention, it is possible to provide an ultra high strength steel wire having a tensile strength of 2000 MPa or more and excellent torsion characteristics. In addition, it is possible to provide a steel wire of various wire diameter as the material of the same wire diameter.

The strength of steel wire can be secured through the strength of the material before wire drawing and the work hardening according to wire drawing. In the conventional steel wire containing Si, the Si is distributed in the ferrite to create a solid solution effect. The strength of the material before processing increases. However, these wire rods are high in strength but low in ductility, and thus, the amount of processing is reduced and the torsion characteristics are not good when drawing. The present inventors intend to provide an ultra high strength steel wire by using a wire for drawing that does not add Si to improve this. Since it does not contain Si, the wire rod for drawing cannot obtain the solid solution strengthening effect by Si. However, by increasing the austenizing temperature, the average austenite average particle size is increased, thereby slowing the transformation of pearlite to obtain fine and uniform pearlite, thereby increasing the limit of the amount of fresh working, thereby improving the strength and torsional characteristics after processing. Can provide ultra high strength steel wire.

In the present invention, the wire rod refers to a state subjected to lead patterning heat treatment, and the steel wire refers to a state after the wire is fresh.

Hereinafter, the component system of this invention is demonstrated.

C (carbon): 0.8-1.0 wt%

C is an essential element added to secure the strength of the material. When the content of C is less than 0.8 wt%, the cementite fraction is relatively small in the pearlite structure, and thus the minimum strength required for the steel cannot be obtained. However, if the content of C exceeds 1.0 wt%, lead cementation may lead to the formation of cementite cementite inside the wire rod during lead patenting heat treatment, thereby significantly reducing the freshness. Therefore, the content of C is preferably limited to 0.8 to 1.0% by weight.

Mn (manganese): 0.3-0.7 wt%

Mn is an element that is advantageous in securing strength by improving the hardenability of steel when present in steel. If the Mn content is less than 0.3% by weight, it is difficult to obtain required strength and quenchability.On the contrary, when the Mn content exceeds 0.7% by weight, the transformation from austenite to pearlite is remarkably delayed and the transformation is completed. There is a problem that the martensite is produced by being water-cooled before being. Therefore, the content of Mn is preferably limited to 0.3 to 0.7% by weight.

Cr (chrome): 0.2-0.6 wt%

Cr is an effective element for solid solution strengthening, cementite stabilization and oxidation resistance, and is also a useful element for refining pearlite spacing. When the content of Cr is less than 0.2% by weight, the effect of sufficiently minimizing the laminar spacing of pearlite is reduced and it is difficult to expect the stabilization effect of cementite. However, when the Cr content exceeds 0.6% by weight, the nose temperature is excessively increased on the TTT curve (time-temperature-transformation curve) and the shape of cementite in pearlite is increased. Heterogeneous makes it difficult to obtain fine and homogeneous pearlite. Therefore, the content of Cr is preferably limited to 0.2 to 0.6% by weight.

The remaining component of the present invention is iron (Fe). However, in the usual steel manufacturing process, impurities which are not intended from raw materials or the surrounding environment may be inevitably mixed, and thus cannot be excluded. Since these impurities are known to those skilled in the art of ordinary steel manufacturing, not all of them are specifically mentioned herein.

However, since O (oxygen), P, and S are generally mentioned impurities, the following is briefly described.

O (oxygen): 0.0015 wt% or less

The content of O is limited to 0.0015% by weight or less, but when it exceeds 0.0015% by weight, oxide-based nonmetallic inclusions are formed coarsely to reduce the freshness.

P: 0.02 wt% or less

P is an element that is inevitably contained in the manufacturing process, P is segregated at the grain boundary and lowers the toughness, so it is preferable to control it as low as possible. In theory, it is advantageous to limit the content of P to 0%, but it is inevitably added during the manufacturing process. There is no choice but to. Therefore, it is important to manage the upper limit, the upper limit of the content of P in the present invention is preferably limited to 0.02% by weight.

S: 0.02 wt% or less

S is an element that is inevitably contained in the manufacturing process, it is preferable to suppress the content as much as possible because it may lower the toughness by forming a grain boundary segregation as a low melting point element and form an emulsion, which may have a detrimental effect on fresh workability. Theoretically, it is advantageous to limit the content of S to 0%, but it is inevitably added in the manufacturing process. Therefore, it is important to manage the upper limit, the upper limit of the content of S in the present invention is preferably limited to 0.02% by weight.

In this invention, it is preferable not to contain Si. However, even when Si is contained in an amount of 0.1 wt% or less, it is possible to secure the strength and torsion characteristics of the steel wire intended by the present invention. As described above, since Si is distributed in the ferrite to reduce the ductility of the ferrite, thereby reducing the amount of drawing, the steel wire of the present invention can significantly increase the amount of drawing by not containing Si. However, the strength degradation caused by not including Si may be compensated for by using the work hardening through the drawing process described below. Although the steel wire of this invention produces | generates work hardening largely, since it does not contain Si, the ductility of steel wire is ensured and the torsion characteristic is favorable.

The microstructure of the wire rod of the present invention is composed of a pearlite structure, the pearlite structure is a pearlite formed from austenite having a particle size of 100 μm or more, and is formed by a preferable manufacturing method described below. In addition, it is preferable that the lamellar spacing of the pearlite structure is 100 nm or less, and the deviation of the lamellar spacing is 50 nm or less. For this reason, even if it does not contain Si, it is possible to provide a wire for excellent wire drawing by fine pearlite.

Hereinafter, the manufacturing method of this invention is demonstrated.

The manufacturing method of the present invention comprises a first heat treatment step of heating at a wire satisfying the component system at 1100 ~ 1200 ℃; A second heat treatment step of maintaining the heated wire at 900 to 1000 ° C; Performing a lead patterning heat treatment on the wire maintained at the temperature at 540˜640 ° C .; And it provides a method for producing an ultra-high strength steel wire comprising the step of drawing the wire patterned heat treatment wire.

Figure 2 is a case of controlling the old austenite particle size to (a) 44.9㎛ by the conventional heat treatment method and (b) 110.6㎛ by the heat treatment method of the present invention, the wire rod in the 540 ~ 640 ℃ TTT curve showing transformation completion time for each temperature when soaking at constant temperature. The vacancy temperature of a given component was calculated using Thermocalc, the thermodynamic calculation program, and the result was calculated to be 733 ° C. It can be seen that the difference in subcooling is smaller. In other words, when the coarse austenite particles are coarsened, the temperature difference between the surface of the wire rod and the center portion is smaller. In the present invention, a description will be given of a method capable of coarsening old austenite particles through the following description.

Heat treatment step

The present invention undergoes two heat treatment steps immediately before the lead patterning heat treatment step. First, it is preferable to go through a first heat treatment step of heating (Austenizing) and maintaining the wire rod to 1100 ~ 1200 ℃. At this time, the holding time is preferably 5 minutes or more. The austenizing temperature can be raised to at least 1100 ° C. and maintained for at least 5 minutes to coarsen the average particle size of the old austenite particles to at least 100 μm. However, the temperature may be limited to 1200 ° C. in consideration of the process facilities and economic conditions, and the upper limit of the holding time may be appropriately limited. In addition, the upper limit of the former austenite particle size may be limited within a range of temperature and time.

However, since the cooling rate of the surface portion and the center of the wire rod is different after the first heat treatment step, it is preferable to go through the second heat treatment step at 900 ~ 1000 ℃ to maintain the same. At this time, the wire rod can be cooled by any cooling method after the first heat treatment step, and air cooling or air cooling is preferable. If the cooling rate of the surface part and the center of the wire rod is kept the same, the pearlite transformation of the wire surface portion and the center portion starts at about the same temperature during the lead patterning heat treatment described below. Can be secured.

In addition, in the conventional ferrite structure, as the size of the old austenite grain grows, the ferrite grains also increase, so that both the strength and the ductility of the ferrite structure decrease. Lead-patterned heat treatment is required because it is dominated by, and is the most powerful microhistological factor governing the strength and ductility of pearlite tissue.

Lead Patterning Heat Treatment Step

The wires subjected to the first and second heat treatment steps are heat treated with lead patenting (LP). At this time, the temperature range in the lead patterning heat treatment step is preferably 540 ~ 640 ℃. However, the temperature range in the lead patterning heat treatment step is more preferably 580 ~ 600 ℃. When the wire rod is inverted at this temperature, a fine pearlite structure can be obtained, and the lamellar spacing of the pearlite is 100 nm or less and the standard deviation can be controlled to 50 nm or less.

Fresh stage

The lead-pattern heat-treated wire is drawn, preferably, the drawing step is performed at a reduction rate of 30% or less and a total reduction rate of 85% or more per pass. In addition, since the limit of the amount of fresh processing is secured enough, steel wires of various wire diameters can be manufactured by applying various reduction rates using the same wire diameter material. It is preferable that the amount of wire strain (ε) is 1.0 to 3.0%.

Steel wire manufactured by the above-described manufacturing method can ensure a tensile strength of 2000MPa or more. In addition, it is possible to secure the number of twist times of 25 times / 100D (D: wire diameter) or more, and the fracture shape at the time of torsional fracture shows a rectangular fracture shape. The causes of fracture surface defects appear in various combinations, but in the case of micro histologically unsuitable for fresh processing, spiral, shear, cone type, torn shapes, etc. In the present invention, the internal and external structure of the wire rod is uniform to obtain an excellent number of twists, and when broken, the fracture shape is a normal state and appears in a right angle to the longitudinal direction of the wire rod.

Hereinafter, the present invention will be described in detail through examples.

(Example)

The billet was cast and plate rolled into an ingot having a component system (omit P, S and O contents) shown in Table 1, and then cut into a wire having a 13 ㎜ wire diameter. Inventive Example 1 and Inventive Example 2 were heated to 1100 ℃ and maintained for 10 minutes, air-cooled to 1000 ℃ and then subjected to lead patterning heat treatment at 590 ℃ for 5 minutes. Comparative Example 1 was heated to 1000 ℃ and maintained for 10 minutes, and the lead patterning heat treatment at 590 ℃ for 5 minutes. The tensile strength of the wire rod (13Φ mm) was measured, and is shown in Table 2 below. The wire rod diameter was 7.44Φ mm (reduction rate: 67.2%), 5.95Φ mm (reduction rate: 79.1%), and 5.32Φ mm (reduction rate: 83.3). %), 4.92 Φ mm (reduction rate: 85.7%), 4.40 Φ mm (reduction rate: 88.5%), and 3.96 Φ mm (reduction rate: 90.7%), after each fresh tensile strength and torsion recovery (breaking form) The measurement is shown in Table 2 below. In addition, Fig. 1 shows a graph in which the layer spacings of the pearlite of Inventive Example 1 and Comparative Example 1 were measured and the size of the spacings could be compared.

division C (% by weight) Si (% by weight) Mn (% by weight) Cr (% by weight) 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

Comparative Example 1 contains 1.3 wt% of Si and exceeds the range defined by the present invention, and Inventive Example 1 and Inventive Example 2 satisfy all of the component systems defined in the present invention.

fairyland
(mm) Total reduction rate
(%) Reduction Rate Per Pass
(%) Fresh processing strain Inventive Example 1 Inventive Example 2 Comparative Example 1 TS
(MPa) Torsion recovery
(Break type) TS
(MPa) Torsion recovery
(Break type) TS
(MPa) Torsion recovery
(Break type) 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) - -

In the case of Inventive Example 1 and Inventive Example 2, since Si is not included, the tensile strength in the heat treatment state before drawing is lowered by about 200 MPa because of no solid solution strengthening effect by Si, but the lamellar spacing of pearlite is small and the variation is low. Because of its small size, the initial hardening rate is so large that the tensile strength of about 7.44 Φ mm of 67.2% of reduction rate can be obtained.

Judging from the number of twists and the change of the fracture surface shape, both of the invention examples 1 and 2 showed a good steady state (the fracture surface was perpendicular to the length of the wire rod), but in the case of Comparative Example 1, the total reduction ratio was 83.3%, It can be seen that fracture failure has been observed since the diameter of 5.32 mm. The reduction rate was increased by 79%-> 91% and increased by 12%, but when transformed into strain, 1.56-> 2.41 showed that Inventive Example 1 and Inventive Example 2 were improved by about 153% compared to Comparative Example 1. Can be.

1, it can be seen that the lamellar spacing of the pearlite of Inventive Example 1 is smaller than that of the pearlite lamellar spacing of Comparative Example 1, and the inner and outer deviations are also small.

1 is a graph showing the dispersion of the laminar spacing in pearlite of Inventive Example 1 and Comparative Example 1. FIG.

Figure 2 is a specimen with an alloying component of 0.92C-0.5Mn-0.6Cr when the thermoforming of the austenite grain size (a) 44.9㎛, (b) 110.6㎛ respectively and soaked in a 540 ~ 640 ℃ lead bath TTT curve showing transformation completion time for each temperature.

Claims

Wrought wire rod with excellent drawability including C: 0.8 ~ 1.0%, Mn: 0.3 ~ 0.7%, Cr: 0.2 ~ 0.6%, balance Fe and other unavoidable impurities.

The wire rod for wire drawing having excellent drawability according to claim 1, wherein the wire rod comprises fine pearlite formed from austenite having a particle size of 100 µm or more.

The wire rod of claim 2, wherein the lamellar spacing of the pearlite structure is 100 nm or less, and the deviation of the lamellar spacing is 50 nm or less.

An ultra-high strength steel wire manufactured by drawing the wire rod of any one of claims 1 to 3 with a reduction rate of 30% or less per pass and a total reduction rate of 85% or more.

The wire of claim 4, wherein the steel has a tensile strength of 2000 MPa or more.

The wire of claim 4, wherein the steel wire has a fracture shape at the time of torsional fracture, and has a twisting frequency of at least 20 times / 100D (D: diameter).

A first heat treatment step of heating a wire rod containing C: 0.8% to 1.0%, Mn: 0.3% to 0.7%, Cr: 0.2% to 0.6%, balance Fe, and other unavoidable impurities, by weight, at 1100 to 1200 ° C;

A second heat treatment step of maintaining the heated wire at 900 to 1000 ° C;

Performing a lead patterning heat treatment on the wire maintained at the temperature at 540˜640 ° C .; And

Ultra-high strength steel wire manufacturing method comprising the step of drawing the lead patterned heat treatment wire.

8. The method of claim 7, wherein the first heat treatment step is maintained for at least 5 minutes.

8. The method of claim 7, wherein the wire rod comprises the old austenite having a particle size of 100 µm or more by the first heat treatment step.

The method of manufacturing a super high strength steel wire according to claim 7, wherein the drawing step is performed at 30% or less reduction rate and 85% or more reduction rate per pass.

The method of manufacturing a super high strength steel wire according to claim 7, wherein the drawing step is performed at a drawing processing amount (ε) of 1.0 to 3.0%.