STEEL WIRE WITH IMPROVED DRAWABILITY, AND MANUFACTURING METHOD THEREFOR

Abstract

A steel wire with improved drawability, and a manufacturing method therefor are disclosed. A steel wire according to the present invention comprises, by wt %, 0.52-0.69% of C, 0.3-0.8% of Mn, 0.1-0.5% of Si, and the balance of Fe and inevitable impurities, wherein the carbon content of cementite in pearlite is 7 at. % or more, and the following formula (1) is satsified. [TS]+exp(ε)*10<1500  (1) In Expression (1), [TS] means the tensile strength (MPa) of a wire before drawing, and e means draw strain.

Description

TECHNICAL FIELD

The present invention relates to a steel wire with improved drawability and a method for manufacturing the same, and more specifically, to a steel wire without delamination and with improved torsion properties and drawability, and a method for manufacturing the same.

BACKGROUND ART

In general, various methods can be used to obtain a high-strength steel wire.

First, a method of increasing the strength of the material itself may be used. That is, as one of the methods for obtaining a high-strength steel wire, a method of increasing the strength of the material itself by adding a large amount of a reinforcing element that increases the strength of the steel may be used. A typical example of such a reinforcing element is carbon. When a carbon content is increased, a fraction of cementite, which is a hard phase, inside a wire rod increases and the lamellar spacing of a pearlite structure becomes denser, so that the strength of the material is improved. Techniques for adding various alloying elements in addition to carbon have been proposed.

Alternatively, strength can be improved by increasing the drawing strain of the steel wire. At this time, the drawing strain of the material is closely related to the ductility of the material, and it is advantageous to improve the strength as the material itself is easily processed without disconnection during drawing.

When a large amount of alloying elements are simply added to improve the strength of the wire rod, problems such as disconnection may occur due to poor ductility of the wire rod in a subsequent manufacturing process of the steel wire after rolling the wire rod. Therefore, the most economical method among methods for increasing the strength of the steel wire is a method of reducing the amount of alloying elements and increasing the drawing strain.

However, delamination usually occurs when the drawing strain increases, and when delamination occurs during a torsion test of a steel wire, it is regarded as a defect, and the maximum drawing strain at which delamination does not occur is defined as a drawing limit.

The occurrence of delamination is related to cementite decomposition, and when cementite decomposition occurs due to an increase in drawing strain, carbon from cementite is released into ferrite, thereby rapidly reducing the plastic deformability of ferrite and generating cracks. This acts as a major obstacle to the high strength of the steel wire.

Therefore, a high-strength steel wire capable of suppressing the occurrence of delamination while increasing the drawing strain is required.

DISCLOSURE

Technical Problem

The present invention is directed to providing a steel wire with improved drawability by controlling the strength of a wire rod and slowing down a decomposition rate of cementite, and a method for manufacturing the same.

Technical Solution

A steel wire having improved drawability according to one embodiment of the present invention includes, in weight percent, C: 0.52 to 0.69%, Mn: 0.3 to 0.8%, Si: 0.1 to 0.5%, a balance of Fe, and unavoidable impurities, has a carbon content of cementite in pearlite of 7 at % or more, and satisfies the following Expression (1).

TS+exp(ε)*10<1500  (1)

In Expression (1), TS denotes a tensile strength (MPa) of a wire rod before drawing, and ε denotes a drawing strain.

A tensile strength of a wire rod may be 700 to 1,000 MPa.

The torsion number of the steel wire may be 30 or more.

Delamination may not occur in the steel wire at a drawing strain of 4.02 or less.

A method for manufacturing a steel wire for a spring with improved fatigue life and toughness according to one embodiment of the present invention includes: hot-rolling a billet including, in weight percent, C: 0.52 to 0.69%, Mn: 0.3 to 0.8%, Si: 0.1 to 0.5%, a balance of Fe, and unavoidable impurities to obtain a wire rod; cooling the hot-rolled wire rod at a cooling rate of 3 to 20° C./s; and obtaining a steel wire by drawing the cooled wire rod to satisfy the following Expression (1).

[TS]+exp(ε)*10<1500  (1)

In Expression (1), [TS] denotes a tensile strength (MPa) of a wire rod before drawing, and ε denotes a drawing strain.

During drawing, delamination may not occur at a drawing strain of 4.02 or less.

Advantageous Effects

In a steel wire according to an embodiment of the present invention, the occurrence of delamination can be suppressed and torsion properties and a drawing limit can be increased even during drawing with a high drawing strain by controlling the strength of a wire rod and slowing down a decomposition rate of cementite.

Specifically, the steel wire according to the embodiment of the present invention can secure a torsion number of 27 or more even at a drawing strain of 4.02, and since delamination does not occur during drawing, a drawing limit can be increased and, ultimately, a steel wire having ultra-high strength can be provided.

[Modes of the Invention]

A steel wire having improved drawability according to an embodiment of the present invention includes, in weight percent, C: 0.52 to 0.69%, Mn: 0.3 to 0.8%, Si: 0.1 to 0.5%, a balance of Fe, and unavoidable impurities, has a carbon content of cementite in pearlite of 7 at % or more, and satisfies the following Expression (1).

TS+exp(ε)*10<1500  (1)

In the above Expression (1), TS denotes a tensile strength (MPa) of a wire rod before drawing, and a denotes a drawing strain.

This specification does not describe all elements of the embodiments, and general content or overlapping content between the embodiments in the technical field to which the present invention pertains will be omitted.

In addition, when a portion is said to “comprise” a component, this means that the component may further comprise other components, rather than excluding other components, unless specifically stated to the contrary.

Singular expressions include plural expressions unless the context clearly dictates otherwise.

Hereinafter, the present invention will be described in more detail.

A steel wire having improved drawability according to an embodiment of the present invention includes, in weight percent, C: 0.52 to 0.69%, Mn: 0.3 to 0.8%, Si: 0.1 to 0.5%, a balance of Fe and unavoidable impurities, has a carbon content of cementite in pearlite of 7 at % or more after drawing, and satisfies the following Expression (1).

[TS]+exp(ε)*10<1500  (1)

In Expression (1), [TS] denotes a tensile strength (MPa) of the wire rod before drawing, and a denotes the drawing strain.

Hereinafter, the reason for limiting a numerical value of an alloy component element content in an embodiment of the present invention will be described. Hereinafter, unless otherwise specified, units are % by weight.

A content of C is 0.52 to 0.69%.

C is an element added to improve the strength of the steel wire. When a C content is less than 0.52%, a strength improvement effect is not sufficient, and when the C content exceeds 0.69%, the strength of the steel can be secured but ductility is reduced, so that in the present invention, it is preferable to control the C content to 0.52 to 0.69%.

A content of Mn is 0.3 to 0.8%.

Mn is an effective element for increasing hardenability. When an Mn content is less than 0.3%, the above-mentioned effect cannot be sufficiently obtained, and when the Mn content exceeds 0.8%, centerline segregation may occur, and the possibility of causing a low-temperature structure is very high, so that it is preferable to control the Mn content to 0.3 to 0.8% in the present invention.

A content of Si is 0.1 to 0.5%.

Si is an effective element for increasing the cleanliness of a wire rod and strengthening steel by being employed in ferrite, which is a base structure, to improve strength. When a Si content is less than 0.1%, the above-mentioned effect cannot be obtained, and when the Si content is more than 0.5%, if the amount of drawing is large, it may be a crack propagation path, and ductility is rapidly reduced to deteriorate drawability, so that the Si content is preferably controlled to 0.1 to 0.5%.

In addition to the above composition, the remainder is Fe, and includes other impurities inevitably mixed in manufacturing processes. In the present invention, the addition of other alloying elements other than the above-mentioned alloy composition is not excluded.

The steel wire having improved drawability according to one embodiment of the present invention may have a carbon content of cementite in perlite of 7 at % or more after drawing.

In the steel wire before drawing, the carbon content of cementite (Fe₃C) in pearlite is 25 at %, but the carbon content of cementite in pearlite becomes lower than 25 at % through the drawing process. When the carbon content in cementite is less than 7 at %, delamination may occur, so that the carbon content in cementite is limited to 7 at % or more.

The steel wire with improved drawability according to one embodiment of the present invention satisfies the following Expression (1).

[TS]+exp(ε)*10<1500  (1)

In Expression (1), [TS] denotes a tensile strength (MPa) of a wire rod before drawing, and a denotes a drawing strain.

The drawing strain (ε) is expressed as 21n(di/df). Here, di denotes an initial diameter of the steel wire before drawing, and df denotes a diameter of the steel wire after drawing.

The present invention can slow down a decomposition rate of cementite and suppress the occurrence of delamination by controlling the tensile strength and drawing strain of the wire rod so that the value of Expression (1) is less than 1,500. In general, when excessive drawing strain is applied in the drawing process, carbon present in cementite is released into ferrite, which rapidly reduces the plastic deformability of ferrite, to cause cracks and increase the possibility of delamination, but when the tensile strength of the wire rod is reduced to satisfy Expression (1), the cementite decomposition rate can slow down even at a high drawing strain, and thus delamination can be suppressed.

Specifically, the case where the drawing strain is 3.0 or more is considered excessive deformation when drawing is performed, but in the present invention, even when a severe drawing strain of 4.02 is applied during drawing, it is possible to provide a steel wire having a torsion number of 27 or more and having ultra-high strength. The ultra-high strength steel wire with an increased drawing limit of the present invention can be applied to products such as tire cord, saw wire, wire rope, piano wire, and steel wire for bridges.

The tensile strength of the wire rod according to the present invention may be in the range of 700 to 1,000 MPa.

Next, a method for manufacturing a steel wire with improved drawability according to one embodiment of the present invention will be described.

A method for manufacturing a steel wire with improved drawability according to one embodiment of the present invention includes: hot-rolling a billet including, in weight percent, C: 0.52 to 0.69%, Mn: 0.3 to 0.8%, Si: 0.1 to 0.5%, a balance of Fe, and unavoidable impurities to obtain a wire rod; cooling the hot-rolled wire rod at a cooling rate of 3 to 20° C./s; and obtaining a steel wire by drawing the cooled wire rod to satisfy the following Expression (1).

[TS]+exp(ε)*10<1500  (1)

In Expression (1), [TS] denotes a tensile strength (MPa) of a wire rod before drawing, and ε denotes a drawing strain.

The steel wire with improved drawability according to the present invention can be manufactured by hot-rolling a billet including the alloy composition described above to obtain a wire rod, cooling the hot-rolled wire rod, and drawing the cooled wire rod to satisfy Expression (1).

At this time, the billet can be hot-rolled at a temperature range of 800° C. to 1200° C. after heating to a temperature range of 900 to 1100° C.

At this time, cooling of the wire rod may be performed at a cooling rate of 3 to 20° C./s. When the cooling rate is less than 3° C./s, there is a problem that segmented and coarse pearlite is generated, and when the cooling rate exceeds 20° C./s, there is a problem in that a low temperature structure such as martensite is caused. Therefore, in the present invention, it is preferable to control the cooling rate of the wire rod to 3 to 20° C./s.

Subsequently, the cooled wire rod may be drawn to satisfy the above-described Expression (1).

Hereinafter, the present invention will be described in more detail with reference to preferred examples. However, the following examples are only examples for explaining the present invention in detail, and do not limit the scope of the present invention.

EXAMPLES

After hot-rolling a billet having an alloy composition and a component range including, in weight percent, C: 0.57 to 0.67%, Mn: 0.3 to 0.8%, Si: 1.0 to 1.6%, a balance of Fe, and other unavoidable impurities, cooling was performed at a cooling rate of 17° C./s in Inventive Examples 1 to 3, and cooling at a cooling rate of 5° C./s in Inventive Examples 5 and 6 and Comparative Example 1 to obtain wire rods, and then for Inventive Examples 1 to 5 and Comparative Example 1 in Table 1 below, steel wires were manufactured by drawing according to conditions of a carbon content of cementite, a wire rod strength, and a drawing strain in Table 1 below. Table 1 below shows a torsion number of each steel wire manufactured as described above and whether delamination occurred.

The torsion number of the steel wire was determined by the number of turns until the steel wire broke during a torsion test applying transverse stress, and the presence or absence of delamination was indicated by checking whether a spiral fracture defect appeared at a wire fracture portion.

TABLE 1
			Carbon
	Wire rod		content of		Delamination	Value of
	strength	Drawing	cementite	Torsion	Occurred or	Expression
Classification	(MPa)	strain	(at %)	number	not	(1)
Inventive	856	3.57	12	36	Did not occur	1211
Example 1
Inventive	844	3.79	11	34	Did not occur	1287
Example 2
Inventive	848	4.02	9	30	Did not occur	1405
Example 3
Inventive	948	3.57	10	35	Did not occur	1303
Example 4
Inventive	950	3.79	9	33	Did not occur	1393
Example 5
Comparative	967	4.02	6	24	Occurred	1524
Example 1

As shown in Table 1, it can be confirmed that Inventive Examples 1 to 5 manufactured under the carbon content of cementite and drawing conditions consistent with the present invention not only had an excellent torsion number of 30 or more, but also delamination did not occur. Specifically, in the case of Comparative Example 1 in which the carbon content of cementite in perlite after drawing is 6 at %, and a value of Expression (1) exceeds 1,500, the torsion number was remarkably low at 24, and delamination occurred, but in the case of Inventive Examples 1 to 5 having a carbon content of cementite of 9 at % or more and a value of Expression (1) of less than 1,500, the torsion number was 30 or more and delamination did not occur. It can be confirmed that as the drawing strain increases, the possibility of delamination increases, but when comparing Inventive Example 3 and Comparative Example 1, even though Inventive Example 3 had the same drawing strain as Comparative Example 1, the tensile strength of a wire rod was reduced by 119 MPa, thereby securing a torsion number of 30 or more, and suppressing delamination.

In the foregoing, although exemplary embodiments of the present invention have been described, the present invention is not limited thereto, and those skilled in the art will be able to understand that various changes and modifications are possible without departing from the concept and scope of the claims described below.

INDUSTRIAL APPLICABILITY

According to one embodiment of the present invention, it is possible to provide a steel wire with improved drawability by controlling the strength of a wire rod and slowing down a decomposition rate of cementite, and a method for manufacturing the same.

Claims

1. A steel wire with improved drawability, which comprises, in weight percent, C: 0.52 to 0.69%, Mn: 0.3 to 0.8%, Si: 0.1 to 0.5%, a balance of Fe, and unavoidable impurities, has a carbon content of cementite in pearlite of 7 at % or more, and satisfies the following Expression (1),

TS+exp(ε)*10<1500  (1)

In Expression (1), TS denotes a tensile strength (MPa) of a wire rod before drawing, and ε denotes a drawing strain.

2. The steel wire of claim 1, wherein the wire rod has a tensile strength of 700 to 1,000 MPa before the drawing.

3. The steel wire of claim 1, wherein the torsion number of the steel wire is 27 or more.

4. The steel wire of claim 1, wherein delamination does not occur at a drawing strain of 4.02 or less.

5. A method for manufacturing a steel wire with improved drawability, comprising:

hot-rolling a billet comprising, in weight percent, C: 0.52 to 0.69%, Mn: 0.3 to 0.8%, Si: 0.1 to 0.5%, a balance of Fe, and unavoidable impurities to obtain a wire rod;

cooling the hot-rolled wire rod at a cooling rate of 3 to 20° C./s; and

obtaining a steel wire by drawing the cooled wire rod to satisfy the following Expression (1), [TS]+exp(ε)*10<1500  (1)

In Expression (1), [TS] denotes a tensile strength (MPa) of a wire rod before drawing, and ε denotes a drawing strain.

6. The method of claim 5, wherein delamination does not occur at a wire strain rate of 4.02 or less during the drawing.