KR102362665B1 - Wire rod, high strength steel wire and method for manufacturing thereof - Google Patents

Abstract

The present invention relates to a wire rod, a high-strength steel wire, and a manufacturing method thereof.
The wire rod according to an embodiment of the present invention, by weight, C: 0.90 to 1.15%, Si: 0.5 to 1.0%, Mn: 0.07% or less (excluding 0%), Cr: 0.7 to 1.2%, B : 0.002 to 0.005%, P: 0.02% or less (excluding 0%), S: 0.02% or less (excluding 0%), remainder Fe and unavoidable impurities.

Description

Wire rod, high-strength steel wire, and manufacturing method thereof

The present invention relates to a wire rod, a high-strength steel wire, and a manufacturing method thereof. More specifically, it relates to a high-strength steel wire having excellent torsional properties applicable to tire cords, etc., a wire rod for wire drawing for producing a steel wire, and a method for manufacturing the same.

Typically, tire cords used as reinforcing materials for tire carcasses and belts are manufactured using the following method.

After casting molten steel having a carbon strength in the eutectoid-hypereutectoid range into bloom, rolling the bloom into a billet, homogenizing the billet in a heating furnace to homogenize the structure into an austenite single phase, and then rolling the wire rod Through the down-sizing to a small size, the wire rod having a fully pearlite structure is manufactured by cooling it at an appropriate rate on the stelmore cooling zone so that pro-eutectoid ferrite or pro-eutectoid cementite is not formed. After that, the customer is drawn according to the purpose and use. In general, heat treatment to give ductility to the material is included in some cases, usually twice, and the final steel wire is obtained through dry and wet drawing. , after which the stranding operation of twisting the same steel wires is additionally included.

In general, products such as tire cords require excellent strength and torsional properties. This is to meet the purpose for which they are used, since in the case of tire cords, the cord structure is simplified as the strength increases, and this makes it possible to reduce the weight of the tire itself in addition to the weight of the reinforcement material. Strengthening of steel wire is achieved by increasing the strength of the wire rod or heat-treated material according to the E-F empirical formula proposed in 1960, or by refining the spacing between the pearlite layers and increasing the amount of wire drawing. Another desirable feature is good torsional properties. This is because delamination does not occur during the twisting process of twisting the wire rod to manufacture the final product, and disconnection does not occur only when the number of twists is high.

In order to increase the strength of the product, 1) increase the strength of the wire rod or heat treatment material by adding C, Cr, Si or increasing the amount of alloy while fixing the amount of wire drawing, 2) increasing the amount of wire drawing while fixing the amount of alloy, 3) alloy Three methods are being used: increasing the amount and increasing the amount of wire drawing. Each method has advantages and disadvantages, but if optimization is not performed, only the strength is increased and the torsional properties are not good. This is because, in addition to pearlite, low-temperature structures such as proeutectoid cementite and martensite and coarse secondary ferrite are generated, and the draw limit is also lowered due to the decrease in ductility according to the increase in strength.

For products with higher strength and lighter weight than the final steel wire strength of commercialized products so far, it is necessary to develop high-grade tire cord products.

An object of the present invention is to provide a wire rod, a high-strength steel wire, and a manufacturing method thereof. More specifically, it is an object of the present invention to provide a high-strength steel wire with excellent torsional properties applicable to tire cords, etc., a wire rod for wire drawing for manufacturing a steel wire, and a method for manufacturing the same.

The wire rod according to an embodiment of the present invention, by weight, C: 0.9 to 1.15%, Si: 0.5 to 1.0%, Mn: 0.07% or less (excluding 0%), Cr: 0.7 to 1.2%, B : 0.002 to 0.005%, P: 0.02% or less (excluding 0%), S: 0.02% or less (excluding 0%), remainder Fe and unavoidable impurities.

The wire rod may further include Ti: 0.01 to 0.03% and N: 0.008% or less (excluding 0%).

The wire rod may be a wire rod for drawing.

In the method of manufacturing a wire rod according to an embodiment of the present invention, in weight%, C: 0.9 to 1.15%, Si: 0.5 to 1.0%, Mn: 0.07% or less (excluding 0%), Cr: 0.7 to 1.2 %, B: 0.002 to 0.005%, P: 0.02% or less (excluding 0%), S: 0.02% or less (excluding 0%), the remainder of heating the steel piece containing Fe and unavoidable impurities; rolling the heated slab; and cooling the rolled steel piece.

On the other hand, the high-strength steel wire according to an embodiment of the present invention, by weight, C: 0.9 to 1.15%, Si: 0.5 to 1.0%, Mn: 0.07% or less (excluding 0%), Cr: 0.7 to 1.2 %, B: 0.002 to 0.005%, P: 0.02% or less (excluding 0%), S: 0.02% or less (excluding 0%), remainder Fe and unavoidable impurities.

The high-strength steel wire may further include Ti: 0.01 to 0.03% and N: 0.008% or less (excluding 0%).

The microstructure of the high-strength steel wire may include 18 area% or less of intergranular ferrite.

The method of manufacturing a high-strength steel wire according to an embodiment of the present invention, by weight, C: 0.9 to 1.15%, Si: 0.5 to 1.0%, Mn: 0.07% or less (excluding 0%), Cr: 0.7 to 1.2%, B: 0.002 to 0.005%, P: 0.02% or less (excluding 0%), S: 0.02% or less (excluding 0%), the remainder of heating the steel piece containing Fe and unavoidable impurities; rolling the heated slab; manufacturing a wire rod by cooling the rolled steel piece; and drawing the wire rod; heat-treating the fresh wire rod; includes

heat-treating the fresh wire rod; The microstructure of the later steel piece may be such that, based on the same C content, the grain boundary ferrite is reduced by 85% or less in terms of area fraction than when the B content does not fall within the corresponding range.

heat-treating the fresh wire rod; The heat treatment of may be an LP heat treatment.

heat-treating the fresh wire rod; Thereafter, wet drawing of the heat-treated wire rod may be further included.

According to one embodiment of the present invention, it is possible to solve the processing breakage that occurred in large amount due to the decarburization formation due to the use of high Si, and thereby, it is possible to increase the product price competitiveness by improving the productivity of the customer.

In this specification, in order to clearly explain the present invention, parts irrelevant to the description are omitted, and the same reference numerals are given to the same or similar components throughout the specification.

In this specification, the terminology used is for the purpose of referring to specific embodiments only, and is not intended to limit the present invention. As used herein, the singular forms also include the plural forms unless the phrases clearly indicate the opposite. The meaning of "comprising," as used herein, specifies a particular characteristic, region, integer, step, operation, element and/or component, and includes the presence or absence of another characteristic, region, integer, step, operation, element and/or component. It does not exclude additions.

Although not defined otherwise, all terms including technical terms and scientific terms used herein have the same meaning as those commonly understood by those of ordinary skill in the art to which the present invention belongs. Commonly used terms defined in the dictionary are additionally interpreted as having a meaning consistent with the related technical literature and the presently disclosed content, and unless defined, are not interpreted in an ideal or very formal meaning.

In addition, unless otherwise specified, % means weight %, and 1 ppm is 0.0001 weight %.

The wire rod according to an embodiment of the present invention, by weight, C: 0.9 to 1.15%, Si: 0.5 to 1.0%, Mn: 0.07% or less (excluding 0%), Cr: 0.7 to 1.2%, B : 0.002 to 0.005%, P: 0.02% or less (excluding 0%), S: 0.02% or less (excluding 0%), remainder Fe and unavoidable impurities.

More specifically, Ti: 0.01 to 0.03% and N: 0.008% or less (excluding 0%) may be further included.

In addition, the wire rod may be a wire rod for drawing.

Hereinafter, the composition of each component is demonstrated.

Carbon (C): 0.9 to 1.15% by weight

C is a major element for securing strength. C, a substitutional element, exists mainly as cementite. It is known that as the C content increases, the cementite fraction increases, and the strength also increases. In particular, it is reported that the strength of 80 MPa increases whenever 0.1 wt % is added. For the above effect, a high content of carbon is preferable, and when too little is added, it is difficult to secure the strength targeted in the present invention. On the other hand, if too much is added, brittle fracture may occur due to problems such as deterioration of freshness due to the formation of central segregation or the formation of grain boundaries of proeutectoid cementite. More specifically, it may be 0.9 to 1.1 wt%, more specifically 0.9 to 1.05 wt%.

Silicon (Si): 0.5 to 1.0 wt%

Si is a ferrite solid solution strengthening element. It is dissolved in ferrite, and in pearlite, it is an element segregated in ferrite and at the interface between cementite and ferrite. It has been reported that the yield strength of 14 to 16 MPa is increased whenever Si is added by 0.1 wt%. Since it is a ferrite strengthening element, it is an element that can improve the strength of the heat treatment material, but has little effect on the work hardening rate during wire drawing. The steel wire according to an embodiment of the present invention is a steel grade developed with the concept of greatly increasing the tensile strength during heat treatment and reducing the processing amount at the same time. Therefore, in order to secure the target strength in the final steel wire, an appropriate amount of Si is preferable, and when the amount of Si is too large, the scale peelability is not effective, and it must be appropriately controlled. More specifically, it may be 0.65 to 0.9 wt%.

Chromium (Cr): 0.7 to 1.2 wt%

Cr is a ferrite stabilizing element, which greatly increases the strength. In addition, since it is a substitution-type element that can be easily located at a general site in cementite, it is easily substituted with Fe, thereby reducing the thickness of cementite and improving the work hardening rate. If chromium is added too little, it is difficult to achieve this goal, and if too much chromium is added, it may cause breakage during processing due to the formation of coarse carbide. More specifically, it may be 0.7 to 1.0 wt%.

Manganese (Mn): 0.07 wt% or less

Mn is an element that can be substituted with Fe in cementite, but since the bonding force of Mn-C is relatively small compared to that of Fe-C, it promotes the decomposition of cementite during wire drawing, which deteriorates the drawability. In addition, it combines with S contained in molten steel to form ductile inclusions such as MnS, which causes breakage during wire drawing or stranding under severe conditions. Since Mn may be included as an impurity even if it is not intentionally added, it is important to manage its upper limit.

Boron (B): 0.001 to 0.005 wt%

B serves to suppress the formation of grain boundary ferrite that occurs during heat treatment. C diffusion to the outside occurs naturally to maintain concentration equilibrium, and when grain boundary ferrite is formed along the austenite interface, delamination occurs in which cracks propagate in the longitudinal direction during final wet drawing or when torsional stress is applied. make it This decarburization occurs when the wire rod is subjected to heat treatment, particularly LP heat treatment. This is a problem in the steel wire for tire cords of 4.0 GPa or higher using high carbon, high silicon steel. In order to suppress this problem, B is added in the present invention. Although the exact mechanism of B suppresses the decarburization phenomenon and suppresses the formation of intergranular ferrite during wire heat treatment is currently unknown, what can be confirmed experimentally is that the scale formed on the surface when B-added steel is hot-rolled and then cooled. that there is a thickening layer. In general, the scale formed in the base material is FeO, and Fe ₃ O ₄ , Fe ₂ O ₃ are formed toward the outer layer portion with more oxygen and bonds. The B-enriched layer is formed on FeO, and the bonding strength with the base material is increased by forming the Fe-OB layer. This can be interpreted as preventing the entry of external oxygen into the interior or the diffusion of internal Fe ions to the outside. If B is added too little, the effect is hardly observed, and if B is added too much, the grain boundaries may be embrittled by forming Fe ₂₃ (C,B) ₆ grain boundaries, so it is desirable to control the appropriate amount.

Titanium (Ti): 0.01 to 0.03 wt%

Ti is an element having good bonding strength with N, and may be added to suppress the formation of BN. If N is added too little, it is difficult to control N, and if too much is added, coarse crystals may be formed, so it is preferable to control an appropriate amount.

Nitrogen (N): 0.008 wt% or less

N is not only an element that causes age hardening by being fixed to dislocations formed on the ferrite matrix during wire drawing, but also inhibits the effect of B by forming BN in grains in B-added steel when it is excessively contained. Since the formation of BN is easy, it is preferable to add less.

Phosphorus (P) and sulfur (S): 0.020 wt% or less each

P and S are impurities, and the lower the content, the better. In addition, when the content of P and S is increased, the ductility of the material is reduced and the wire-drawing property is deteriorated. Therefore, it is important to control the upper limits of the P and S contents.

Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only intended to illustrate the present invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

On the other hand, in the method of manufacturing a wire rod according to an embodiment of the present invention, by weight%, C: 0.9 to 1.15%, Si: 0.5 to 1.0%, Mn: 0.07% or less (excluding 0%), Cr: 0.7 to 1.2%, B: 0.002 to 0.005%, P: 0.02% or less (excluding 0%), S: 0.02% or less (excluding 0%), remainder Fe and unavoidable impurities. ; rolling the heated slab; and cooling the rolled steel piece.

The composition of the steel piece is as mentioned above, and each step will be described.

heating the slab

The step of heating the slab may be maintained for 60 minutes to 90 minutes in a heating furnace of 1,000 ° C. to 1,250 ° C. More specifically, it may be heating to 1,100 ℃ to 1,250 ℃. By heating the steel piece in the above temperature range, the austenite single phase can be maintained, coarsening of the austenite grains can be prevented, and the remaining segregation, carbides and inclusions can be effectively dissolved. If the heating temperature of the steel piece is too low, it may be difficult to obtain the effect by heating. On the other hand, if it is too high, the austenite grains become very coarse, making it difficult to secure a high strength and high toughness wire rod. Here, the slab refers to all semi-finished products such as blooms and billets that can be manufactured as wire rods.

rolling the heated slab

It is preferable to manufacture a wire rod by rolling a heated steel piece. The rolling may be hot rolling, and the hot rolling temperature may be 900°C to 1,050°C. More specifically, it may be 950°C to 1050°C.

When hot rolling is performed at too low a temperature, there is a problem in that the rolling load becomes large and the life of the roll becomes short. On the other hand, when it is performed at too high a temperature, there is a problem in that the austenite grains are coarse and ductility is reduced.

After the step of rolling, it may include an L/H step, and the L/H temperature may be 890°C to 920°C. If the coiling temperature is too low, the formation of proeutectoid cementite in the grain boundary may decrease, and if it is too high, austenite grain coarsening may appear, so it is preferable to control it below that.

cooling the rolled slab

After performing hot rolling, a step of cooling may be followed. More preferably, it is preferable to perform cooling in Stelmor. There is an effect of securing a homogeneous tissue by performing cooling in the stelmor (Stelmor).

Rapid cooling at 30°C/s to 40°C/s from 450°C to 500°C in the cooling zone, then holding at the corresponding holding temperature for 10 seconds to 20 seconds, and then cooling to 200°C at 10°C/s . By cooling at the above rate, proeutectoid cementite and pearlite are not formed. In addition, if the cooling rate is too slow, there is a problem in that proeutectoid cementite and pearlite are formed, and if it is too fast, it is impossible in the stelmo phase.

The tensile strength of the wire rod manufactured in this way may be 1,336 MPa or more, and the cross-sectional reduction rate may be 28% or more.

On the other hand, the high-strength steel wire according to an embodiment of the present invention, by weight, C: 0.9 to 1.15%, Si: 0.5 to 1.0%, Mn: 0.07% or less (excluding 0%), Cr: 0.7 to 1.2 %, B: 0.002 to 0.005%, P: 0.02% or less (excluding 0%), S: 0.02% or less (excluding 0%), remainder Fe and unavoidable impurities.

The high-strength steel wire may further include Ti: 0.01 to 0.03% and N: 0.008% or less (excluding 0%).

The microstructure of the high-strength steel wire may contain 18 area% or less of intergranular ferrite. More specifically, it may be a structure in which grain boundary ferrite of 18 area% or less is elongated in the longitudinal direction. More specifically, the thickness of the grain boundary ferrite may be 20 nm or less. More specifically, 17 area% or less of intergranular ferrite may be included.

The high-strength steel wire may have a tensile strength of 3,800 MPa or more, and the number of twists may be 60 or more when a load of tensile strength x cross-sectional area x 0.008 on the basis of 200D is applied.

The method of manufacturing a high-strength steel wire according to an embodiment of the present invention, by weight, C: 0.9 to 1.15%, Si: 0.5 to 1.0%, Mn: 0.07% or less (excluding 0%), Cr: 0.7 to 1.2%, B: 0.002 to 0.005%, P: 0.02% or less (excluding 0%), S: 0.02% or less (excluding 0%), the remainder of heating the steel piece containing Fe and unavoidable impurities; rolling the heated slab; manufacturing a wire rod by cooling the rolled steel piece; and drawing the wire rod; heat-treating the fresh wire rod; includes

The drawing of the wire rod may be dry drawing.

heat-treating the fresh wire rod; The microstructure of the later steel piece may be such that, based on the same C content, the grain boundary ferrite is reduced by 85% or less in terms of area fraction than when the B content does not fall within the corresponding range. More specifically, it may be reduced by 40% or more and 85% or less.

heat-treating the fresh wire rod; The heat treatment of may be an LP heat treatment.

heat-treating the fresh wire rod; Thereafter, wet drawing of the heat-treated wire rod may be further included.

Hereinafter, the microstructure of the heat treatment material and high-strength steel wire according to an embodiment of the present invention will be described again.

The microstructure of the heat treatment material subjected to the wire rod heat treatment according to an embodiment of the present invention may contain 18 area% or less of intergranular ferrite. On the other hand, the microstructure of the heat treatment material, that is, the high-strength steel wire manufactured by wet drawing of the heat-treated wire rod may contain 18 area% or less of intergranular ferrite, like the microstructure of the heat treatment material. More specifically, it may be a structure in which grain boundary ferrite of 18 area% or less is elongated in the longitudinal direction. More specifically, the thickness of the grain boundary ferrite may be 20 nm or less.

Hereinafter, the present invention will be described in more detail through examples. However, these examples are only for illustrating the present invention, and the present invention is not limited thereto.

[Example]

In the present invention, an experiment was conducted to manufacture a steel wire having excellent strength and torsional properties of the final steel wire and having a small fraction of intergranular ferrite area after the wire rod heat treatment.

In the present invention, molten steel having an alloy composition as shown in Table 1 is cast and continuously cast to produce a bloom of 400x500 mm ² , and then a billet of 160x160 mm ² is prepared through steel slab rolling, and then heated in a heating furnace at 1,020° C. for 100 minutes After holding for a while, the hot rolling temperature was set to 980℃, and after hot rolling, the L/H temperature was changed to 890℃, and then cooled to 480℃ at 35℃/s, held at that temperature for 18 seconds, and then 200℃ The wire rod was manufactured by cooling at 12°C/s until

After the wire rod was dry-fresh at a general working temperature (25° C.), it was quenched with a lead to obtain an LP (Lead Patenting) heat-treated material. Thereafter, a steel wire was manufactured by drawing to a diameter of 0.2 mm through wet drawing.

The tensile strength (TS) and the reduction in area (RA) of the wire rod and the LP heat-treated material were measured, the intergranular ferrite area fraction of the LP heat-treated material was measured, and the tensile strength (TS) and torsional properties of the steel wire were measured and shown in Table 2 below. indicated.

Alloy composition, weight (%) C Si Cr Mn B Ti N P S Comparative Example 1 1.00 0.80 0.78 0.068 - - - 0.0140 0.0150 Invention Example 1 1.02 0.80 0.72 0.068 0.001 - - 0.0150 0.0100 Invention example 2 1.01 0.82 0.80 0.069 0.002 - - 0.0100 0.0120 Invention example 3 1.02 0.83 0.81 0.069 0.003 - - 0.0110 0.0130 Comparative Example 2 1.01 0.81 0.79 0.068 0.0053 - - 0.0100 0.0100 Invention Example 4 1.02 0.82 0.79 0.069 0.002 0.02 0.005 0.0120 0.0110 Invention Example 5 1.01 0.80 0.80 0.067 0.002 0.01 0.005 0.0090 0.0140 Comparative Example 3 0.92 0.80 0.81 0.069 - - - 0.0100 0.0140 Invention example 6 0.91 0.81 0.81 0.067 0.002 - - 0.0120 0.0120 Invention Example 7 0.93 0.80 0.81 0.069 0.003 - - 0.0090 0.0150 Invention Example 8 0.91 0.83 0.80 0.068 0.004 - - 0.0100 0.0130 Invention Example 9 0.92 0.82 0.82 0.069 0.002 0.02 0.005 0.0120 0.0110 Comparative Example 4 0.80 0.81 0.81 0.070 0.019 - - 0.0110 0.0110 Reference Example 1 1.06 0.82 0.81 0.070 0.002 - - 0.0110 0.0100 Comparative Example 5 1.16 0.82 0.80 0.070 0.0020 - - 0.0120 0.0100 Reference Example 2 1.02 0.60 0.81 0.070 0.0021 - - 0.010 0.012 Reference example 3 1.00 1.00 0.82 0.070 0.0022 - - 0.011 0.010 Comparative Example 6 1.02 1.10 0.80 0.071 0.0021 - - 0.010 0.011 Comparative Example 7 1.01 0.80 1.30 0.072 0.0019 - - 0.009 0.01

wire rod heat-treated material final liner The tensile strength
(MPa) RA
(%) The tensile strength
(MPa) RA
(%) Intergranular ferrite area fraction
(%) Fraction ratio of each invention example to the fraction of grain boundary ferrite of Comparative Example 1 The tensile strength
(MPa) torsion
(episode) Comparative Example 1 1,415 30 1,674 35 21 One 4,420 68 Invention Example 1 1,412 29 1,659 34 17 0.810 4,415 70 Invention example 2 1,417 31 1,651 37 13 0.619 4,427 74 Invention example 3 1,409 29 1,671 34 10 0.476 4,424 71 Comparative Example 2 1,401 24 1,580 23 22 1.048 monorail - Invention Example 4 1,412 28 1,657 34 15 0.714 4,430 71 Invention Example 5 1,420 31 1,654 33 16 0.762 4,418 69 The tensile strength
(MPa) RA
(%) The tensile strength
(MPa) RA
(%) Intergranular ferrite area fraction (%) Fraction ratio of each invention example to the fraction of grain boundary ferrite of Comparative Example 3 The tensile strength
(MPa) torsion
(episode) Comparative Example 3 1,335 34 1,530 38 19 One 3,870 62 Invention example 6 1,340 33 1,510 37 14 0.737 3,890 67 Invention Example 7 1,338 31 1,528 36 12 0.632 3,840 70 Invention Example 8 1,350 34 1,541 38 11 0.579 3,900 69 Invention Example 9 1,338 32 1,518 37 12 0.632 3,870 67 Comparative Example 4 1,228 46 1,378 52 One 0.048 3,900 82 Reference Example 1 1,455 28 1585 34 12 0.571 4,435 72 Comparative Example 5 1,492 20 1592 28 12 0.571 monorail 0 Reference Example 2 1,318 37 1,533 38 12 0.571 4,301 77 Reference example 3 1,345 36 1,573 37 12 0.571 4,353 62 Comparative Example 6 1,387 36 1,617 36 17 0.810 monorail 0 Comparative Example 7 1,597 25 1,797 34 21 1.105 monorail 0

Wire rod strength comparison

Comparison of the strength of the wire rod by the addition of B is as follows.

The tensile strength of Comparative Example 1 without B added was 1,415 MPa, and RA was 30%. On the other hand, Inventive Examples 1 to 3 in which B is added and Ti and N are not added, and Inventive Examples 4 to 5 in which B, Ti and N are added have similar physical properties. In view of this, it was determined that BN, Fe ₂₃ (C,B) ₆ and the like were not formed at the grain boundary according to the addition of B.

In addition, the tensile strength of Comparative Example 3 in which B was not added was 1,335 MPa. On the other hand, Inventive Examples 6 to 9 are steels containing 0.001 to 0.003 wt% of B in 0.92% C, and have similar physical properties to the wire rod of Comparative Example 3. In view of this, it can be seen that the addition of B does not affect the physical properties of the wire rod, and it can be seen that the aforementioned BN and the like are not formed at the grain boundary.

Comparison of microstructure of heat treated material

In the microstructure observed on the surface after the LP heat treatment for Comparative Example 1, it was confirmed that ferrite was formed along the austenite grain boundary in the surface portion. It was measured by area fraction, and it was confirmed that about 21% was formed when B was not added. If grain boundary ferrite is suppressed, the number of twists in the product can be improved, and since the disconnection rate is also lowered, productivity is expected to be greatly improved.

In the microstructure observed on the surface after the LP heat treatment for Inventive Example 2, it was confirmed that about 13% of the grain boundary ferrite was formed differently from the case of Comparative Example 1. That is, it was confirmed that the grain boundary ferrite decreased by the addition of B.

Comparison of properties of steel wire

The product of Comparative Example 1 had a tensile strength of 4,420 MPa and a torsion characteristic of 68 times. Compared with Inventive Examples 1 to 3, it can be seen that the number of twists is increased up to 6 times as the B content is increased from 0.001 wt% to 0.003 wt%.

In Comparative Example 2, the B content is more than 0.005%. Considering that processing breakage occurs when the B content exceeds 0.005%, it is considered preferable to add 0.005% or less of B.

Comparison according to addition of Ti and N

In Inventive Examples 4 and 5, the effect of B was observed when Ti and N were added. By forcibly removing N, which can be combined with B, a better effect of increasing torsional properties is expected, but the torsional properties are improved compared to Comparative Example 1. However, it was confirmed that there was no significant increase compared to the addition of B alone.

On the other hand, when Ti and N are added, crystal grain refinement may occur through the formation of TiN fine precipitates. Here, the grain means AGS, and the nucleated pearlite also means fine. When AGS is reduced, ductility is good, so it is advantageous in terms of drawing processability, and freshness can be increased. Since fine AGS in the wire rod can be made finer during LP heat treatment, the amount of wire drawing can be increased, or the strength of the final steel wire can be increased due to the fine pearlite formed here. However, if TiN is not well controlled, that is, if coarse TiN is formed by crystallization, disconnection may be induced during processing.

conclusion

Therefore, according to the results, the addition of B has little effect on the mechanical properties of the wire rod and heat-treated material, but since it suppresses the formation of surface-bound ferrite during heat treatment, the torsional properties of the steel wire secured after the final wet drawing are improved, suitable for this The content is 0.001% by weight to 0.003% by weight, and Ti and N have little effect compared to B alone, but it is judged that there is some effect.

In addition, when the amount of B added increases, grain boundary ferrite formed on the surface during heat treatment is reduced, and at this time, the area fraction can be reduced to a maximum of 10%, which is reduced to a level of 85% or less compared to the non-addition of B.

The present invention is not limited to the embodiments, but can be manufactured in various different forms, and those of ordinary skill in the art to which the present invention pertains can use other specific forms without changing the technical spirit or essential features of the present invention. It will be appreciated that this may be practiced. Therefore, it should be understood that the embodiments described above are illustrative in all respects and not restrictive.

Claims (11)

delete delete delete By weight%, C: 0.9 to 1.15%, Si: 0.5 to 1.0%, Mn: 0.07% or less (excluding 0%), Cr: 0.7 to 1.2%, B: 0.001 to 0.005%, P: 0.02% or less (excluding 0%), S: 0.02% or less (excluding 0%), heating the steel piece containing the remainder Fe and unavoidable impurities;
rolling the heated slab; and
Including; cooling the rolled steel piece;
Cooling the rolled steel piece;
A method for manufacturing a wire rod, which is rapidly cooled to 450°C to 500°C at 30°C/s to 40°C/s, and then cooled to 200°C at 10°C/s.
delete delete delete By weight%, C: 0.9 to 1.15%, Si: 0.5 to 1.0%, Mn: 0.07% or less (excluding 0%), Cr: 0.7 to 1.2%, B: 0.002 to 0.005%, P: 0.02% or less (excluding 0%), S: 0.02% or less (excluding 0%), heating the steel piece containing the remainder Fe and unavoidable impurities;
rolling the heated slab;
manufacturing a wire rod by cooling the rolled steel piece;
drawing the wire rod; and
heat-treating the fresh wire rod;
including,
Cooling the rolled steel piece;
A method for manufacturing a high-strength steel wire that is rapidly cooled to 450°C to 500°C at 30°C/s to 40°C/s, and then cooled to 200°C at 10°C/s.
9. The method of claim 8,
heat-treating the fresh wire rod; The microstructure of the subsequent steel piece, based on the same C content, than when the B content does not fall within the corresponding range, the intergranular ferrite is reduced by 85% or less in area fraction A method of manufacturing a high-strength steel wire.
9. The method of claim 8,
heat-treating the fresh wire rod; The heat treatment of the LP heat treatment is a high-strength steel wire manufacturing method.
9. The method of claim 8,
heat-treating the fresh wire rod; Since the,
High-strength steel wire manufacturing method further comprising; wet-drawing the heat-treated wire rod.