KR20140009883A - Steel wire having excellent torsion property and method for manufacturing thereof - Google Patents

Abstract

The present invention relates to a steel wire including excellent torsional properties and a manufacturing method thereof. The present invention provides the steel wire including the excellent torsional properties while maintaining tensile strength by controlling heat treatment before LP heat treatment and reducing fluxes or by maintaining processes such as a process for changing a reduction rate. [Reference numerals] (AA) Temperature; (BB,DD) Perlite; (CC,EE) Ferrite; (FF) Time

Description

TECHNICAL FIELD [0001] The present invention relates to a steel wire having excellent torsional characteristics and a method of manufacturing the steel wire. [0002]

The present invention relates to a steel wire having excellent torsional characteristics and a method of manufacturing the same. More particularly, to a steel wire used for a tire cord and the like, and a manufacturing method thereof.

The tire cord refers to a reinforcing material or member that enters the inside of rubber to improve the durability, running property and stability of the tire in a vehicle such as an automobile. The tire cord material is mainly composed of synthetic fibers such as polyester and nylon, In recent years, high strength and excellent heat resistance are required, and steel cords are widely used.

The tire cord should have high strength and excellent torsional characteristics.

It is highly desirable that all the properties are excellent. However, since the twisted yarn is used as a cord or the like through a twisted process, the twist characteristic is a particularly important characteristic to be used as a tire cord.

The torsional characteristics are evaluated by the number of torsions until a delamination phenomenon in which vertical fracture occurs when a torsional stress is applied. In the case of a tire cord, 40 to 50 times or more torsions are required.

The strength, surface flaw and tensile residual stress of the steel wire are known to be factors affecting such torsional characteristics. Generally, when the strength is high, the ductility is decreased, and the final delamination is also affected, and when the strength is too high, delamination occurs. In addition, the surface flaw formed by the use of the defective die acts as a stress concentration spot at the time of applying the stress, and therefore, there is a problem in that initial fracture can be caused.

In addition, the tensile residual stress formed on the surface during drawing may affect the degree of delamination or twisting depending on the level of the value. As the tensile residual stress is lowered or the compressive residual stress is formed on the surface, .

As a method for lowering the tensile residual stress, there are various methods such as slowing the wire speed or changing the amount of reduction. However, such a method has a problem that the production efficiency is very low when manufacturing a steel wire.

The present invention provides a steel wire having excellent torsional characteristics while maintaining the tensile strength of the steel wire without changing the processes such as slowing down the ship speed or changing the amount of reduction by controlling the heat treatment before the LP heat treatment.

In one aspect of the present invention, the steel wire having excellent torsional characteristics comprises, by weight, 0.92 to 1.02% of C, 0.2 to 0.5% of Si, 0.2 to 0.5% of Mn, 0.1 to 0.5% of Cr, the balance Fe and other unavoidable impurities , And the maximum value of the tensile residual stress from the surface to the depth of 5 mu m in the center direction is 650 MPa or less, and the number of twisting times in which delamination does not occur is 65 or more times.

A method of manufacturing a steel wire having excellent torsional characteristics according to another aspect of the present invention is characterized by comprising, by weight percent, 0.92 to 1.02% of C, 0.2 to 0.5% of Si, 0.2 to 0.5% of Mn, 0.1 to 0.5% of Cr, And other unavoidable impurities at 1000 to 1100 캜, hot-rolling the heated strip at 900 to 1000 캜 to produce a wire rod, cooling the wire rod at a cooling rate of 15 to 25 캜 / sec And a step of drawing the cooled wire rod more than twice, and a heat treatment is performed to form a surface decarburization layer between the drawing and the drawing.

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. The various features of the present invention and the advantages and effects thereof will be more fully understood by reference to the following specific embodiments.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to obtain an effect of reducing the heat due to friction between the die and the material and increasing the torsional characteristics by appropriately controlling the heat treatment conditions of the steel wire such as a tire cord having a tensile strength of 3600 MPa or more.

Figure 1 is a graph of temperature versus time for the primary heat treatment of the present invention.

The inventors of the present invention have conducted research to improve productivity and ensure excellent torsional characteristics without slowing the line speed or changing the reduction amount as in the conventional method, The steel wire with improved torsional characteristics can be produced while maintaining the tensile strength of the steel wire, thereby leading to the present invention.

Hereinafter, a steel wire having excellent torsional characteristics, which is one aspect of the present invention, will be described in detail.

In one aspect of the present invention, the steel wire having excellent torsional characteristics comprises, by weight, 0.92 to 1.02% of C, 0.2 to 0.5% of Si, 0.2 to 0.5% of Mn, 0.1 to 0.5% of Cr, the balance Fe and other unavoidable impurities , And the maximum value of the tensile residual stress from the surface to the depth of 5 mu m in the center direction is 650 MPa or less, and the number of twisting times in which delamination does not occur is 65 or more times.

Carbon (C): 0.92 to 1.02 wt%

The carbon is the most economical element for improving the strength of the steel wire. When the content is less than 0.92 wt%, the effect of improving the strength is insufficient. When the content is more than 1.02 wt%, the strength is improved but the ductility is decreased. Therefore, the carbon content is preferably limited to 0.92 to 1.02% by weight.

Silicon (Si): 0.2 to 0.5 wt%

The silicon is a component that stabilizes the pearlite layer with the solubility enhancement effect to increase strength and ductility. If it is less than 0.2% by weight, the strength and ductility increasing effect can not be achieved. On the other hand, when the content exceeds 0.5% by weight, there is a problem that the ductility is drastically reduced to deteriorate the drawability. Therefore, it is preferable that the silicon is limited to 0.2 to 0.5 wt%.

Manganese (Mn): 0.2 to 0.5 wt%

The manganese is an effective element for increasing the incombustibility or an element having severe center segregation. If it is less than 0.2% by weight, the effect of addition can not be sufficiently achieved. On the other hand, if it exceeds 0.5% by weight, the possibility of inducing a cold tissue is very high. Therefore, the content of manganese is preferably limited to 0.2 to 0.5 wt%.

Cr (Cr): 0.1 to 0.5 wt%

The chromium has the effect of improving the strength and ductility by making the layered structure of pearlite fine. When the amount is less than 0.1% by weight, the effect of improving the strength and ductility is insufficient because of insufficient fine structure of the layered structure. When the amount exceeds 0.5% by weight, the rate of the constant temperature transformation is slowed and the productivity is deteriorated. Therefore, it is preferable that the chromium is limited to 0.1 to 0.5 wt%.

It is preferable that the maximum value of the tensile residual stress from the surface to the depth of 5 mu m in the center direction from the surface is 650 MPa or less and the number of twisting times in which delamination does not occur is 65 or more. At this time, the number of twistings means the number of twists per 200 x D length (D: diameter of the steel wire).

According to one aspect of the present invention, it is possible to provide a steel wire having excellent torsional characteristics by satisfying the above-described composition system and physical properties.

In addition, it is preferable that the microstructure of the steel wire is a dual phase composed of ferrite and pearlite. By having such an abnormal structure as described above, friction between the die and the material is reduced, so that the tensile residual stress can be reduced while maintaining the tensile strength of the steel wire, and the torsional characteristic is increased.

It is more preferable that the microstructure includes 5 to 15% of ferrite and 85 to 95% of pearlite in an area fraction%. If the ferrite content is less than 5%, it can not serve as a stress relaxation layer in friction with the die, and if the ferrite content exceeds 15%, the strength is lowered. Therefore, it is preferable that the ferrite and the pearlite are contained in an amount of 5 to 15% and 85 to 95%.

The steel wire proposed by the present invention preferably includes a surface decarburization layer of 0.002 to 0.008 mm. By including the surface decarburization layer of 0.002 to 0.008 mm, it is possible to reduce the heat due to the friction between the die and the material, thereby improving the torsional characteristics. When the surface decarburization layer is less than 0.002 mm, sufficient tensile residual stress reduction effect can not be secured. On the other hand, when it exceeds 0.008 mm, it is difficult to secure the intended strength of the present invention. Therefore, it is preferable to include a surface decarburization layer of 0.002 to 0.008 mm.

The tensile strength of the steel wire is preferably 3600 MPa or more.

Hereinafter, a method for manufacturing a steel wire having excellent twist characteristics, which is another aspect of the present invention, will be described in detail.

A method of manufacturing a steel wire having excellent torsional characteristics according to another aspect of the present invention is characterized by comprising, by weight percent, 0.92 to 1.02% of C, 0.2 to 0.5% of Si, 0.2 to 0.5% of Mn, 0.1 to 0.5% of Cr, And other unavoidable impurities at 1000 to 1100 캜, hot-rolling the heated strip at 900 to 1000 캜 to produce a wire rod, cooling the wire rod at a cooling rate of 15 to 25 캜 / sec And a step of drawing the cooled wire rod more than twice, and a heat treatment is performed to form a surface decarburization layer between the drawing and the drawing.

Heating stage

The steel piece which satisfies the said component system is heated at 1000-1100 degreeC. By heating the steel strip in this temperature range, it is possible to maintain the austenite single phase, prevent the coarsening of the austenite grains, and effectively dissolve the remaining segregation, carbides and inclusions. When the heating temperature of the steel strip exceeds 1100 ° C, the austenite grains become very coarse and it is difficult to obtain a high strength and high toughness wire rod. On the other hand, if it is less than 1000 ° C, it may be difficult to obtain the above effect by heating. Here, the term "steel" refers to all semifinished products such as blooms and billets, which can be produced from wire rods.

Hot rolling

The hot-rolled steel strip as described above can be subjected to hot rolling. At this time, the rolling is preferably performed at 900 to 1000 占 폚. If the rolling temperature is lower than 900 ° C, it is difficult to obtain the intended microstructure of the present invention, and there is a possibility that soft ferrite is secured. On the other hand, when the rolling temperature exceeds 1000 캜, the size of the austenite grains may increase and the strength and ductility may be lowered.

Cooling stage

It is preferable to cool the hot-rolled wire as described above. At this time, cooling is preferably performed at a rate of 15 to 25 DEG C / second. When the temperature is less than 15 DEG C / sec, the formation of the corner stone cementite is actively performed, and it is difficult to obtain the microstructure to which the present invention is intended. On the other hand, when the temperature is higher than 25 ° C / second, a hard texture is formed to make it difficult to secure ductility. Therefore, it is preferable that the cooling rate is in the range of 15 to 25 DEG C / sec.

After the cooling step, the step of winding may be further included to facilitate the storage and movement of the cooled wire rod.

Freshness stage

It is preferable to perform the drawing twice as many times as possible through the heat treatment to the cooled wire as described above to reduce it to the final thickness.

As a preferred example of the drawing step proposed by the present invention, the step of first dry-drawing the cooled wire rod; Maintaining the first dry drawn wire at 740-780 ° C; Performing an LP heat treatment; And performing the second wet drawing of the LP heat treated steel wire.

As described above, the primary drawing is desirably dry drawn, and the secondary drawing is preferably wet drawing taking into account the plating process. It is also preferable that the primary drawing is made to have a diameter of 1 to 2 mm and the secondary drawing is made to have a diameter of 0.15 to 0.4 mm to meet the standard of the tire cord to be used at the end.

Also, by maintaining the primary dry-drawn steel wire at 740 to 780 ° C, the carbon of the surface portion is forcibly decarburized by an appropriate amount to form a soft ferrite layer on the surface, whereby the friction between the die and the material is reduced and the tensile residual stress .

Also, if the temperature is maintained at the above-mentioned temperature, it is preferable to maintain the temperature for 30 to 90 seconds. If it is maintained for less than 30 seconds, decarburization in the surface layer does not occur, and if it is maintained for more than 90 seconds, spheroidization of cementite occurs and cementite is broken.

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)

Inventive Example 1 is characterized in that a steel sheet containing 0.92% of C, 0.3% of Mn, 0.2% of Si, 0.2% of Cr, balance of Fe and other unavoidable impurities is heated at 1100 占 폚 and hot rolling at 1000 占 폚 Followed by cooling at a cooling rate of 15 DEG C / second to produce a wire having a diameter of 5.5 mm.

Thereafter, primary dry drawing was carried out, and as shown in Fig. 1, the temperature was maintained at a temperature of 740 占 폚, which is the normal temperature of A1, for 1 minute. After the heat treatment as described above, the LP heat treatment, and then quenched and subjected to secondary wet drawing to prepare a steel wire having a final diameter of 0.18 mm Φ.

In Comparative Example 1, a steel sheet containing 0.92% of C, 0.3% of Mn, 0.2% of Si, 0.2% of Cr, and the balance of Fe and other unavoidable impurities was heated at 1100 ° C and hot rolling at 1000 ° C Followed by cooling at a cooling rate of 15 DEG C / second to produce a wire having a diameter of 5.5 mm.

Thereafter, after descaling by mechanical or chemical methods, the wire was drawn to have a diameter of 3 mmΦ using a dry lubricant, and then LP heat-treated, followed by wet drawing.

The tensile strength, residual stress, and torsional characteristics of Inventive Example 1 and Comparative Example 1 prepared as described above were evaluated and are shown in Table 1 below.

At this time, the residual stress was measured by FIB and the residual stress was measured by measuring the displacement occurring in the mark region by using commercially available software. Torsional characteristics were obtained by using a universal torsional tester (back load: x0.2) and the length of test specimen was 200D (D: diameter of steel wire).

division Microstructure type and fraction (%) Tensile strength of steel wire (MPa) Residual stress (MPa) Torsional properties Dil lamination occurs Torsion frequency Inventory 1 surface Ferrite 10% 3685 610 Never 75 inside Pearlite 90% Comparative Example 1 Perlite 100% 3692 1120 Never 52

As shown in Table 1, the tensile strengths of Inventive Example 1 and Comparative Example 1 are found to be similar.

In addition, the tensile residual stress of the steel wire in the case of LP heat treatment after maintaining for 1 minute at 740 ° C., which is directly above Al as in Inventive Example 1, compared to Comparative Example 1, as shown in Table 1, very low tensile residual It was confirmed to have a stress.

In addition, Inventive Example 1 has an abnormal structure of pearlite and ferrite, so that it was confirmed that the torsional characteristic was superior to Comparative Example 1.

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.

Claims (7)

And the balance Fe and other unavoidable impurities, and having a ratio of C: 0.92 to 1.02 Si: 0.2 to 0.5%, Mn: 0.2 to 0.5%, Cr: 0.1 to 0.5% A steel wire excellent in torsional characteristics having a maximum value of tensile residual stress of 650 MPa or less and a number of twist times of 65 or more without delamination.
The method of claim 1,
Wherein the microstructure contains 5 to 15% of ferrite and 85 to 95% of pearlite in an area percentage%.
The method of claim 1,
And the tensile stress of the steel wire is 3600 MPa or more.
The method of claim 1,
Wherein the steel wire comprises a surface decarburization layer of 0.002 to 0.008 mm.
A steel ingot having a composition comprising 0.92 to 1.02% of C, 0.2 to 0.5% of Si, 0.2 to 0.5% of Mn, 0.1 to 0.5% of Cr, balance Fe and other unavoidable impurities at a temperature of 1000 to 1100 캜 Heating;
Hot-rolling the heated billet at 900 to 1000 占 폚 to produce a wire rod;
Cooling the wire rod at a cooling rate of 15 to 25 DEG C / second; And
And a step of drawing the cooled wire rod more than twice,
And performing a heat treatment for forming a surface decarburization layer between the drawing and the drawing.
6. The method of claim 5,
The heat treatment may include maintaining the drawn steel wire at 740 to 780 占 폚; And
Method for producing a steel wire with excellent torsion characteristics including the LP heat treatment step.
The method according to claim 6,
Wherein the holding step is performed for 30 to 90 seconds.

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