KR20220169248A - Wire rod and steel wire wire with improved strength and softening resistance, and manufacturing method there of - Google Patents

Abstract

Disclosed are a wire rod wire with improved strength and softening resistance, a steel wire, and a manufacturing method thereof. According to an embodiment, the disclosed wire rod with improved strength and softening resistance comprises 0.8 to 1.1 wt% of C, 0.1 to 0.3 wt% of Si, 0.2 to 0.8 wt% of Mn, 0.4 to 1.0 wt% of Cr, 0.5 to 1.0 wt% of Mo, and the balance Fe and other unavoidable impurities, and the pro-eutectoid cementite structure having a maximum thickness of 20 μm or more at the austenite grain boundary is 10% or less in area fraction.

Description

Wire rod, steel wire with improved strength and softening resistance, and manufacturing method thereof

The present invention relates to a wire rod, a steel wire having improved strength and softening resistance, and a method for manufacturing the same, and more particularly, to a wire rod, a steel wire having excellent strength and improved softening resistance by controlling alloy composition and precipitates, and a method for manufacturing the same. .

PC (Pre-stressed Concrete) steel wire, which is commonly used for external wall reinforcement such as liquefied natural gas storage tanks and silos, uses carbon steel having a carbon content of 0.7% by weight or more. On the other hand, the steel wire used for storage structure reinforcement has a yield strength of about 1,580 MPa, and is manufactured using a steel wire having a value of about 2,360 MPa based on tensile strength.

The strengthening of products is continuously progressing around the world. The reason is that the higher the strength, the lighter the product can be, and the shorter the construction period and the lower the manufacturing cost.

As a method for increasing the strength of PC steel wire, there is a method of increasing the strength of the initial material or improving the strength through heat treatment, and in terms of structure, there is a method of securing high strength by forming fine crystal grains. That is, as high as the strength of the initial material is, high strength can be secured in the final steel wire, and as the crystal grains are finer, strength is secured and at the same time, wire-drawing performance can be greatly improved by suppressing cementite fragmentation during wire-drawing.

Methods for controlling the strength and grain size of the initial material include a method of adding alloying elements such as C and Cr and a method of controlling process factors such as microstructure formed during heat treatment and conditions of the wire drawing process.

However, the method of adding alloying elements has an excellent effect of increasing strength, but it can be said that controlling process factors is more effective because heat treatment temperature, holding time, and wire drawing condition changes must be considered when the amount of alloy is changed. In particular, it is effective to increase the amount of wire drawing among process factors. However, since the amount of wire drawing is highly dependent on the properties of the material and high wire drawing processing technology is required, technological reinforcement is required.

On the other hand, as the size of large buildings for future cities increases in the Middle East and Europe, the demand for PC steel stranded wire having excellent high temperature softening resistance is increasing as resistance to fire is required.

One of the effective methods for improving softening resistance at high temperatures is to precipitate fine carbonitrides. This is because it is possible to secure high-temperature strength by forming fine carbonitride precipitates at high temperature through the addition of Cr, Mo, Nb, V, etc., thereby pinning dislocations in ferrite grains.

In the case of bolts fastening existing buildings, carbonitride precipitates have been precipitated and developed as fireproof bolts. However, since the development of PC steel strands for fire resistance through precipitate control is insufficient, it is necessary to develop steel materials having a novel alloy composition.

Korean Patent Publication No. 10-2011-0014853 (published on February 14, 2011)

In order to solve the above-described problems, the present invention optimizes the component ranges of C, Cr, and Mo and controls the microstructure and precipitates, thereby securing high yield strength and improving softening resistance. It is intended to provide a manufacturing method.

The wire rod having improved strength and softening resistance according to the present invention for achieving the above object contains, in weight percent, C: 0.8 to 1.1%, Si: 0.1 to 0.3%, Mn: 0.2 to 0.8%, Cr: 0.4 to 0.4%. 1.0%, Mo: 0.5 to 1.0%, including the rest of Fe and other unavoidable impurities, and a pro-eutectoid cementite structure having a maximum thickness of 20 μm or more at the austenite grain boundary may be 10% or less in area fraction.

Here, when the diameter of the wire rod is D, the maximum content of carbon (C _max ) and the average content of carbon (C ₀ ) may satisfy the following equation (1).

Equation (1): C _max /C ₀ ≤1.25

Here, the wire rod may have a bainite structure including carbides having an average particle diameter of 1 μm or more in an area fraction of 90% or more in a region where the carbon content of the surface layer of the wire rod exceeds 0.90% by weight.

Here, the wire rod may have a yield strength of 1000 MPa or more.

Here, the wire rod may have an elongation of 12% or more.

In addition, the method for manufacturing a wire rod with improved strength and softening resistance according to the present invention for achieving the above object is C: 0.8 to 1.1%, Si: 0.1 to 0.3%, Mn: 0.2 to 0.8%, in weight% , Cr: 0.4 to 1.0%, Mo: 0.5 to 1.0%, maintaining the billet containing the remainder Fe and other unavoidable impurities in a heating furnace at 1000 to 1200 ° C. for 90 to 120 minutes; manufacturing a wire rod by rolling the billet; Winding the prepared wire at 900 to 980 ° C; first cooling the wound wire from a winding temperature to 500 °C at a cooling rate of 10 to 15 °C/s; and secondly cooling the cooled wire rod from 500°C to 300°C at a cooling rate of 0.1°C/s or less.

Here, the rolling may include finishing rolling at 1000 to 1200 ° C.

In addition, the steel wire with improved strength and softening resistance according to the present invention for achieving the above object, in weight%, C: 0.8 to 1.1%, Si: 0.1 to 0.3%, Mn: 0.2 to 0.8%, Cr: 0.4 to 1.0%, Mo: 0.5 to 1.0%, including remaining Fe and other unavoidable impurities, maintained at 600 ° C for 30 minutes and then quenched, MC in the 500 nm ² region of the cross section perpendicular to the longitudinal direction of the wire rod The number of carbides (where M is Cr or Mo) may be 10 or more.

Here, the steel wire may have a yield strength of 780 MPa or more after being maintained at 600° C. for 30 minutes.

Here, after the steel wire is maintained at 600° C. for 30 minutes, the ratio of yield strength to tensile strength may be 0.35 or more.

Here, the steel wire may have a yield strength of 2200 MPa or more.

Here, the number of twisting times in which delamination does not occur in the steel wire based on 100D may be 12 or more.

Here, when the steel wire is drawn, the number of disconnections per ton may be 2 or less.

In addition, the method for manufacturing a steel wire having improved strength and softening resistance according to the present invention for achieving the above object includes drawing the wire rod.

According to one aspect of the present invention, by controlling the microstructure and precipitates by optimizing the composition range and manufacturing process of C, Cr, Mo, a wire rod, steel wire with improved softening resistance and a method for manufacturing the same while securing high yield strength can provide

1 is a continuous cooling transformation (CCT) graph of Example 1 calculated with the J-Mat commercial program.
2 is an equilibrium diagram of Example 1 calculated using the Thermo-Cale program.
3 is a graph showing the change in yield strength according to temperature when Example 1 and Comparative Example 8 were maintained for 30 minutes at each temperature and then quenched.

Preferred embodiments of the present invention are described below. However, the embodiments of the present invention can be modified in many different forms, and the technical spirit of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

Terms used in this application are only used to describe specific examples. Therefore, for example, expressions in the singular number include plural expressions unless the context clearly requires them to be singular. In addition, the terms "include" or "have" used in this application are used to clearly indicate that the features, steps, functions, components, or combinations thereof described in the specification exist, but other features It should be noted that it is not intended to be used to preliminarily exclude the presence of any steps, functions, components, or combinations thereof.

Meanwhile, unless otherwise defined, all terms used in this specification should be regarded as having the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention belongs. Accordingly, certain terms should not be interpreted in an overly idealistic or formal sense unless clearly defined herein. For example, in this specification, a singular expression includes a plurality of expressions unless there is a clear exception from the context.

In addition, "about", "substantially", etc. in this specification are used at or in the sense of or close to the value when manufacturing and material tolerances inherent in the stated meaning are presented, and are accurate to aid in understanding the present invention. or absolute numbers are used to prevent unfair use by unscrupulous infringers of the stated disclosure.

The wire rod having improved strength and softening resistance according to an embodiment of the present invention contains, in weight percent, C: 0.8 to 1.1%, Si: 0.1 to 0.3%, Mn: 0.2 to 0.8%, Cr: 0.4 to 1.0%, Mo : 0.5 to 1.0%, including the remainder Fe and other unavoidable impurities.

Here, the wire rod having improved strength and softening resistance may further include N: 0.01% or less, P: 0.035% or less, S: 0.035% or less, and Al: 0.02 to 0.05%, in weight%.

Hereinafter, the reason for limiting the composition range of each alloy element will be described. Hereinafter, unless otherwise specified, units are % by weight.

The content of C is 0.8 to 1.1%.

C (carbon) increases the fraction of cementite constituting pearlite and reduces the interlayer spacing of pearlite. C is a solid solution strengthening element that increases the strength by about 80 MPa when 0.1% by weight is added, and may be added in an amount of 0.8% or more to secure the strength of the material. However, when the content is excessive, there is a problem of causing the formation of central segregation and causing disconnection during wire drawing. Considering this, the upper limit of the C content may be limited to 1.1%.

The content of Si is 0.1 to 0.3%.

Si (silicon) is a ferrite hardening element, and can be added in an amount of 0.1% or more to remove oxygen in molten steel and improve strength. However, when the content is excessive, there is a problem in that scale exfoliation property is deteriorated due to the formation of Fe ₂ sSiO ₄ having excellent bonding strength with the parent material. Considering this, the upper limit of the Si content may be limited to 0.3%.

The content of Mn is 0.2 to 0.8%.

Mn (manganese) is an austenite stabilizing element, and may be added in an amount of 0.2% or more to improve hardenability and suppress formation of proeutectoid cementite at grain boundaries. However, when the content is excessive, there is a problem in that MnS inclusions are formed by combining with S, and the possibility of disconnection during wire drawing increases due to manganese segregation in the center. Considering this, the upper limit of the Mn content may be limited to 0.8%.

The content of Cr is 0.4 to 1.0%.

Cr (chrome) can increase the strength of products because it reduces the size of crystal grains that can hinder the movement of dislocations such as bainite and pearlite. In addition, Cr is a major element that forms Cr-based carbides when maintained at high temperatures, and may be added in an amount of 0.4% or more to secure softening resistance at a temperature of 600 ° C. However, when the content is excessive, martensite is formed in the central segregation zone, resulting in poor wire-drawing properties, and the effect on target strength and softening resistance is saturated. Considering this, the upper limit of the Cr content may be limited to 1.0%.

The content of Mo is 0.5 to 1.0%.

Mo (molybdenum) is an element that forms MoC carbide to improve the strength of the material due to the precipitation hardening effect and improves the work hardening rate during drawing by pinning dislocations. In addition, Mo is an element that improves softening resistance at high temperatures along with Cr, and may be added in an amount of 0.5% or more to prevent a decrease in yield strength at a temperature of 600 ° C. However, if the content is excessive, a martensite phase is formed in the cooling zone, so that the target level of the bainite structure in the present invention cannot be secured. In addition, Mo is an expensive element, and when its content is excessive, a manufacturing cost is increased and productivity is lowered. Considering this, the upper limit of the Mo content may be limited to 1.0%.

The content of N is 0.01% or less.

N (nitrogen) is an element with excellent strength increase by solid solution strengthening. However, when the content is excessive, the aging strength increases due to the combination of dislocation and N, resulting in poor ductility of the material and an increase in manufacturing cost. Considering this, the upper limit of the N content may be limited to 0.01%.

The content of P is 0.035% or less.

P (phosphorus) is usually difficult to avoid the central segregation zone when the C content in steel is high, and may be segregated at grain boundaries or formed as FeP at grain boundaries, causing disconnection during wire drawing. Considering this, the upper limit of the P content may be limited to 0.035%.

The content of S is 0.035% or less.

S (sulfur) has a problem of reducing processability by forming MnS inclusions at grain boundaries when contained in large amounts. Considering this, the upper limit of the S content may be limited to 0.035%.

The content of Al is 0.02 to 0.05%.

Al (aluminum) is an element that reacts easily with oxygen, and when the Si content is low, it can be added in an amount of 0.02% or more to control inclusions by using it in the deoxidation reaction of steelmaking. However, when the content is excessive, nozzle clogging due to inclusions occurs during playing, and in particular, coarse aluminum inclusions such as Al ₂ O ₃ are formed, causing disconnection during wire drawing. Considering this, the upper limit of the Al content may be limited to 0.05%.

In addition to the above composition, the remaining components are iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in a normal manufacturing process, this cannot be excluded. Since these impurities are known to anyone skilled in the ordinary manufacturing process, not all of them are specifically mentioned in this specification.

In addition, the wire rod according to an embodiment of the present invention may include 80% or more of pearlite structure and the remaining bainite structure or martensite structure in area fraction. At this time, the pro-eutectoid cementite structure having a maximum thickness of 20 μm or more at the austenite grain boundary may be 10% or less in terms of area fraction.

When the area fraction of the pro-eutectoid cementite structure having a maximum thickness of 20 μm or more at the austenite grain boundary exceeds 10%, cracks are likely to occur during wire drawing, resulting in deterioration in wire drawing performance.

In addition, in the wire rod according to an embodiment of the present invention, the maximum carbon content (C _max ) and the average carbon content (C ₀ ) are measured in a region from the center of the cross section perpendicular to the longitudinal direction to 0.25D in the wire surface direction. The ratio may satisfy Equation (1) below. Here, the D means the diameter.

Equation (1): C _max /C ₀ ≤1.25

The ratio of the maximum content of carbon (C _max ) and the average content of carbon (C ₀ ) in the wire rod is a factor affecting ~. In the region from the center of the cross section perpendicular to the longitudinal direction to 0.25D in the direction of the wire surface, when the ratio of the maximum carbon content (C _max ) to the average carbon content (C ₀ ) exceeds 1.25, the center C segregation increases. As a result, workability is inferior, and wire breakage may occur during wire drawing.

In addition, in the wire rod according to an embodiment of the present invention, the bainite structure including carbide having an average particle diameter of 1 μm or more may have an area fraction of 90% or more in a region where the carbon content of the surface layer of the wire rod exceeds 0.90% by weight.

In a region where the carbon content of the surface layer of the wire rod exceeds 0.90% by weight, when the area fraction of the bainite structure including carbides having an average particle diameter of 1 μm or more is less than 90%, a problem in which the yield strength is lowered may occur.

In addition, the wire rod according to an embodiment of the present invention may have a yield strength of 1000 MPa or more.

In addition, the wire rod according to an embodiment of the present invention may have an elongation of 12% or more.

Next, a method for manufacturing a wire rod having improved strength and softening resistance according to an embodiment of the present invention will be described.

In the manufacturing method of a wire rod according to an embodiment of the present invention, in weight%, C: 0.8 to 1.1%, Si: 0.1 to 0.3%, Mn: 0.2 to 0.8%, Cr: 0.4 to 1.0%, Mo: 0.5 to 0.5% maintaining a billet containing 1.0% of Fe and other unavoidable impurities in a heating furnace at 1000 to 1200° C. for 90 to 120 minutes; manufacturing a wire rod by rolling the billet; Winding the prepared wire at 900 to 980 ° C; first cooling the wound wire to 500° C. at a cooling rate of 10 to 15° C./s; and secondly cooling the cooled wire rod to 300°C at a cooling rate of 0.1°C/s or less.

Here, the billet may further include N: 0.01% or less, P: 0.035% or less, S: 0.035% or less, and Al: 0.02 to 0.05%, by weight%.

The reason for limiting the composition range of each alloy element is as described above, and each manufacturing step will be described in detail below.

First, after preparing a billet having the above-described components, a heating step of homogenizing the austenite single phase is performed.

The heating step may be performed at a temperature of 1000° C. or more in consideration of a charging time, securing a temperature range for subsequent wire rod rolling, securing a microstructure of the billet as an austenite single phase. However, when the temperature in the heating step is excessive, the surface quality is deteriorated due to scale generation and decarburization, and a load may be applied to the heating furnace. In consideration of this, the upper limit of the heating temperature may be limited to 1200°C.

In addition, the heating step may be performed for 90 minutes to secure the austenite phase in the center of the wire rod and sufficiently dissolve the solid solution strengthening element for securing strength. However, when the heating time is excessively long, coarse crystal grains may be formed, which may cause a problem in that sufficient strength cannot be secured. In consideration of this, the upper limit of the heating time may be limited to 120 minutes.

The heated billet can then be rolled under conventional rolling conditions. That is, the wire rod may be manufactured by sequentially performing hot rolling consisting of rough rolling, intermediate rough rolling/finish rolling, and finish rolling on the heated billet.

In this case, the rolling may include finishing rolling at 1000 to 1200 ° C. Here, it is preferable to control the temperature as a front end temperature of finish rolling. Considering that the temperature of the material is high after finishing rolling before finish rolling, it is more preferable to control the temperature at the inlet side of the front end of finish rolling.

When the finish rolling temperature is less than 1000 ° C, a large roll load may occur during finish rolling, and as the time for C to diffuse is reduced, the amount of C segregation in the center of the wire rod may increase. As a result, when the finish rolling temperature is less than 1000° C., the ratio of the maximum carbon content (C _max ) to the average carbon content (C ₀ ) of the material may exceed 1.25.

On the other hand, when the finish rolling temperature exceeds 1200°C, dynamic strain aging of the wire rod is accelerated during processing due to heat generated during processing, and static strain aging is promoted after processing. can cause layer separation.

Subsequently, the manufactured wire rod may be wound in a temperature range of 900 to 980 °C.

At this time, when the coiling temperature is less than 900 ° C., the scale thickness is thin, which is disadvantageous in terms of peelability. On the other hand, when the coiling temperature exceeds 980° C., the coiling shape is not suitable and scale may be excessively formed during cooling.

Meanwhile, in the coiled wire rod, the ratio of the maximum carbon content (C _max ) and the average carbon content (C ₀ ) in the region from the center of the cross section perpendicular to the longitudinal direction to 0.25D in the wire rod surface direction is expressed by the following formula (2 ) can be satisfied. Here, D means the diameter of the wire rod.

Equation (2): 1≤C _max /C ₀ ≤1.1

Subsequently, a first cooling step may be performed in which the coiled wire rod is cooled from the coiling temperature to 500° C. at a cooling rate of 10 to 15° C./s.

The first cooling step may be cooled to 500 ° C., which is a temperature of A1 or less, in order to suppress the formation of proeutectoid cementite. At this time, when the cooling rate is less than 10 ° C / s, the pro-eutectoid cementite structure having a maximum thickness of 20 μm or more at the austenite grain boundary exceeds 10% in area fraction, and wire breakage may occur during wire drawing. When the cooling rate exceeds 15° C./s, there arises a problem of cost for an additional process.

Subsequently, a second cooling step of extremely slowly cooling the first cooled wire rod from 500° C. to 300° C. may be performed.

When the cooling rate exceeds 0.1° C./s in the second cooling step, martensite is formed in an amount of 10% or more, resulting in poor workability, and thus disconnection may occur during wire drawing. In consideration of this, the cooling rate of the second cooling step may be limited to 0.1° C./s or less.

Thereafter, a steel wire may be manufactured by drawing the wire rod after descaling. At this time, the fresh processing may proceed with a total freshness reduction of 87% or more.

Next, a steel wire having improved strength and softening resistance according to an embodiment of the present invention will be described.

In the steel wire according to one embodiment of the present invention, in weight%, C: 0.8 to 1.1%, Si: 0.1 to 0.3%, Mn: 0.2 to 0.8%, Cr: 0.4 to 1.0%, Mo: 0.5 to 1.0%, It contains the remaining Fe and other unavoidable impurities.

Here, the steel wire having improved strength and softening resistance may further include, in weight%, N: 0.01% or less, P: 0.035% or less, S: 0.035% or less, and Al: 0.02 to 0.05%.

The reason for limiting the composition range of each alloy element is as described above.

In addition, when the steel wire according to one embodiment of the present invention is quenched after being maintained at 600° C. for 30 minutes, the number of MC carbides in the 500 nm ² region of the cross section perpendicular to the longitudinal direction of the wire rod is 10 regardless of its size. There may be more than one.

Here, M means Cr or Mo, and the number of carbides means the number of MC carbides measured regardless of sizes such as average particle diameter and maximum diameter. The quenching may be performed by conventional methods such as water cooling, oil cooling, air cooling, and furnace cooling.

When the number of MC carbides is less than 10 in the 500 nm ² region of the cross section perpendicular to the longitudinal direction of the wire, resistance to softening at high temperatures cannot be secured. On the other hand, when the number of MC carbides is 10 or more, the decrease in yield strength at high temperatures can be suppressed.

As the MC carbide is formed in the steel wire according to an embodiment of the present invention, it is possible to secure a yield strength of 780 MPa or more by suppressing a decrease in yield strength even after holding at 600° C. for 30 minutes.

In addition, the steel wire according to one embodiment of the present invention, as a result of suppressing the decrease in yield strength even after holding at 600 ° C. for 30 minutes, the ratio of yield strength to tensile strength may be 0.35 or more.

In addition, the steel wire according to an embodiment of the present invention may have a yield strength of 2200 MPa or more.

In addition, the steel wire according to an embodiment of the present invention, based on 100D, the number of times of twisting without delamination may be 12 or more.

In addition, in the case of the steel wire according to an embodiment of the present invention, the number of wire breakage per ton may be 2 or less during drawing.

The wire rod according to one embodiment of the present invention can be used for manufacturing high-strength PC steel wire, stranded wire, PC steel stranded wire, and the like. In addition, the steel wire according to an embodiment of the present invention can be manufactured through current heat treatment facilities without LP heat treatment. In addition, it is possible to secure the effect of resistance to fire by preventing the yield strength from lowering at high temperatures while securing strength.

Hereinafter, the present invention will be described in more detail through examples. However, the description of these examples is only for exemplifying the practice of the present invention, and the present invention is not limited by the description of these examples. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

{Example}

Steels satisfying the various alloy compositions shown in Table 1 below were manufactured in the course, and then cast under normal conditions to produce 160 * 160 mm ² continuous billet materials. Thereafter, the prepared billet was maintained at a heating furnace temperature of 1,150 ° C. for 95 minutes for the purpose of securing the austenite phase and strength.

weight% C Si Mn Cr Mo Example 1 0.98 0.2 0.6 0.6 0.6 Example 2 1.08 0.2 0.6 0.6 0.6 Example 3 0.98 0.2 0.6 0.9 0.6 Example 4 0.98 0.2 0.6 0.6 0.9 Comparative Example 1 0.98 0.2 0.6 0.6 0.6 Comparative Example 2 0.98 0.2 0.6 0.6 0.6 Comparative Example 3 0.98 0.2 0.6 0.6 0.6 Comparative Example 4 0.98 0.2 0.6 0.6 0.6 Comparative Example 5 1.12 0.2 0.6 0.6 0.6 Comparative Example 6 0.98 0.2 0.6 1.2 0.6 Comparative Example 7 0.98 0.2 0.6 0.6 1.1 Comparative Example 8 0.98 0.2 0.6 0.2 0.2

Next, according to the finish rolling temperature conditions in Table 2 below, rolling was performed to manufacture wire rods. The finish rolling temperature in Table 2 below means the temperature at the front end of finish rolling after finishing rolling.

Subsequently, after winding the manufactured wire rod, first cooling was performed on the wound wire rod up to 500 ° C, and then second cooling was performed from 500 ° C to 300 ° C. Table 2 below shows the temperature of the winding process, the ratio of the maximum carbon content (C _max ) to the average carbon content (C ₀ ) of the coiled wire rod, and the cooling rates of the first cooling process and the second cooling process.

finish rolling temperature
(℃) winding temperature
(℃) Cooling rate up to 500℃
(℃/s) Cooling rate up to 300℃
(℃/s) Example 1 1,050 950 14 0.1 Example 2 1,050 950 14 0.1 Example 3 1,050 950 14 0.1 Example 4 1,050 950 14 0.1 Comparative Example 1 950 950 14 0.1 Comparative Example 2 900 950 14 0.1 Comparative Example 3 1,050 950 4 0.1 Comparative Example 4 1,050 950 14 5 Comparative Example 5 1,050 950 14 0.1 Comparative Example 6 1,050 950 14 0.1 Comparative Example 7 1,050 950 14 0.1 Comparative Example 8 1,050 950 14 0.1

After the cooling process was completed, the pro-eutectoid cementite fraction, C _max /C ₀ , bainite fraction, yield strength (YS) and elongation of the wire rod were measured and shown in Table 3 below.

The pro-eutectoid cementite fraction in Table 3 below means the area fraction of a pro-eutectoid cementite structure having a maximum thickness of 20 μm or more at the austenite grain boundary. The maximum thickness of the cementite structure was calculated from the following equation (3) after measuring the pearlite layer spacing of the cross section perpendicular to the longitudinal direction of the wire rod through a scanning electron microscope (SEM) having a model name S-3700N.

Equation (3): t _c ={S ₀ *0.15(wt% C)}/V _p

In Equation (3), t _c is the maximum thickness of the cementite structure, S ₀ is the interlayer spacing, V _p is the measured pearlite fraction, and wt% C is the carbon content of the specimen.

C _max /C ₀ in Table 3 below means the ratio of the maximum carbon content (C _max ) to the average carbon content (C ₀ ).

The bainite fraction in Table 3 below means the area fraction of the bainite structure including carbides having an average particle diameter of 1 μm or more in a region where the carbon content of the surface layer of the wire rod exceeds 0.90% by weight.

The bainite fraction was first measured in a region in which the carbon content exceeded 0.90% by weight. Thereafter, the area fraction of the bainite structure was measured using a scanning electron microscope at 8 random points having an area of 1 mm ² in the corresponding region, and an average value was calculated.

Wire rod cornerstone cementite fraction (%) wire rod
C _max /C ₀ wire rod
bainite fraction
(%) wire yield strength
(MPa) wire rod
elongation rate
(%) Example 1 9 1.17 91 1,080 14 Example 2 9 1.21 91 1,170 13 Example 3 10 1.22 90 1,140 13 Example 4 10 1.24 92 1,180 12 Comparative Example 1 9 1.51 89 1,014 10 Comparative Example 2 8 1.86 88 970 7 Comparative Example 3 16 1.17 89 1,069 4 Comparative Example 4 9 1.18 78 780 4 Comparative Example 5 9 1.38 87 1,200 10 Comparative Example 6 9 1.17 86 1,190 9 Comparative Example 7 10 1.24 84 1,240 4 Comparative Example 8 8 1.15 92 920 18

Referring to Table 1, based on the component composition of Example 1, Example 2 was further added with C. Meanwhile, based on the component composition of Example 1, Example 3 further added Cr, and Example 4 further added Mo.

Referring to Tables 2 and 3, Examples 1 to 4 satisfied the alloy composition and manufacturing process conditions controlled by the present invention, and as a result, the wire rod pro-eutectoid cementite structure was formed at 10% or less, and the wire rod bainite structure fraction was 90%. % or more were satisfied.

In addition, Examples 1 to 4 were performed at a finish rolling temperature of 1000 to 1200 ° C., and as a result, the time for C to diffuse was increased. Accordingly, in Examples 1 to 4, the amount of C segregation in the center of the wire rod decreased, and the C _max /C ₀ value satisfied 1.25 or less.

In addition, Examples 1 to 4 were able to secure a yield strength of 1000 MPa or more and an elongation of 12% or more at the same time.

In comparison, in Comparative Example 1, as a result of performing the rolling process at 950° C. where the finish rolling temperature is less than 1000° C., the C _max /C ₀ value was 1.51, exceeding 1.25. In addition, Comparative Example 1 had a bainite structure of less than 90% and had an inferior elongation of 10%.

In Comparative Example 2, as a result of performing the rolling process at a finish rolling temperature of 900° C. less than 1000° C., the C _max /C ₀ value significantly increased to 1.86. In addition, Comparative Example 2 had a Baini structure of less than 90%, and yield strength and elongation were inferior.

In Comparative Example 3, as a result of cooling at a cooling rate of 4 ° C / s less than 10 ° C / s from the coiling temperature to 500 ° C, the fraction of the pro-eutectoid cementite structure having a thickness of 20 or more at the grain boundary greatly increased from 10% to 16%. . Accordingly, Comparative Example 3 had an elongation of 4% and poor workability, and a bainite structure of less than 90% was formed.

Comparative Example 4 was cooled at a cooling rate of 5 ° C / s exceeding 0.1 ° C / s from 500 ° C to 300 ° C, and the fraction of the bainite structure was greatly reduced to 78%, making it unsuitable for production as a final product.

In Comparative Example 5, as C was added in excess of 1.1%, the value of Cmax/C ₀ increased to 1.38. In Comparative Example 5, the bainite structure was formed at less than 90%, and the elongation was inferior at 10%.

Comparative Example 6 had an elongation of 9%, poor workability, and a bainitic structure of 86%, which is less than 90%, as Cr was added in excess of 1.0% by 1.2%.

In Comparative Example 7, as 1.1% of Mo exceeding 1.0% was added, the elongation was 4%, which was poor in processability, and the bainitic structure was formed at 84%, which is less than 90%.

In Comparative Example 8, 0.2% of less than 0.4% of Cr was added, and 0.2% of less than 0.5% of Mo was added. Accordingly, in Comparative Example 8, a problem occurred in which the yield strength was greatly reduced to 920 MPa, which is less than 1000 MPa.

Then, after removing some of the scale present on the surface of the manufactured wire rod using a mechanical peeling method, wire drawing was performed with a total wire drawing reduction of 87%.

The total amount of reduction in freshness was calculated through Equation (4) below.

Equation (4): Total wire drawing reduction (%) = 100*[1-(Diameter after drawing / Diameter before drawing) ² ]

Table 4 below shows the results after measuring the yield strength, the number of twists, the number of disconnections per ton, and whether or not the number of MC carbides is 10 or more of the prepared steel wire.

In Table 4, to evaluate the torsion characteristics of the steel wire, a general-purpose torsion tester (back load: breaking stress x 0.2) was used by applying a load of tensile strength * cross-sectional area * 0.008 to the manufactured steel wire, and the torsion length was 100 * D (D: It was performed by measuring the number of twists based on the diameter of the steel wire).

In Table 4 below, the number of MC carbides (M is Cr or Mo) means the number of MC carbides measured regardless of size. The number of MC carbides was measured by using a scanning electron microscope at eight random points having an area of 500 nm ² on the surface layer of the wire rod, and then the average value was calculated. In Table 4 below, '○' means the case where the number of MC carbides is 10 or more, and 'X' means the case where the number of MC carbides is less than 10.

Total fresh reduction amount
(%) steel wire
yield strength
(MPa) steel wire
torsion
(episode) cutting wire
(number of times/ton) More than 10 MC carbides Example 1 87 2,210 16 0.4 ○ Example 2 87 2,300 15 0.5 ○ Example 3 87 2,280 14 0.7 ○ Example 4 87 2,290 12 0.9 ○ Comparative Example 1 87 1,896 11 2.5 ○ Comparative Example 2 87 1,859 11 3.8 ○ Comparative Example 3 87 2,150 8 4.2 ○ Comparative Example 4 87 1,805 One 20.1 ○ Comparative Example 5 87 2,300 4 19.2 ○ Comparative Example 6 87 2,310 6 17.4 ○ Comparative Example 7 87 2,340 2 22.7 ○ Comparative Example 8 87 2,080 18 0.1 X

Examples 1 to 4 were excellent in yield strength of 2000 MPa or more, and the number of MC carbides satisfied 10 or more. In addition, in Examples 1 to 4, the number of twists without delamination was 12 times or more, and the number of wire breakage per ton was 2 times or less during drawing, so the workability was excellent.

In comparison, Comparative Examples 1 and 2 were inferior in yield strength to less than 2000 MPa, the number of twists was less than 12 times, and the number of broken wires exceeded 2 times.

Comparative Example 3 secured a yield strength of 2000 MPa or more, but the number of torsion was 8 times less than 12 times, and the number of broken wires was 4.2 times more than 2 times, and the workability was poor.

In Comparative Example 4, the yield strength was inferior to 1805 MPa, which was less than 2000 MPa, and the number of twists was 1 time and the number of broken wires was 20.1 times.

Comparative Examples 5 to 7 secured a yield strength of 2000 MPa or more, but the number of twists was less than 12, and the number of broken wires exceeded 2, resulting in poor workability.

3 is a graph showing the change in yield strength according to temperature when Example 1 and Comparative Example 8 were maintained for 30 minutes at each temperature and then quenched.

Referring to FIG. 3, when maintained at a temperature of 0 to 500 ° C. for 30 minutes and then quenched, the yield strength reduction of Example 2 and Comparative Example 8 did not show a significant difference. However, in Comparative Example 8, less than 10 MC carbides were formed, so the yield strength of Comparative Example 8 was greatly reduced compared to Example 2 at a temperature of 600 ° C or higher.

In addition, in the case of Comparative Example 8, when quenched after maintaining at 600 ° C. for 30 minutes, the ratio of yield strength to tensile strength was 0.21. On the other hand, in the case of Example 1, when quenched after being maintained at 600 ° C. for 30 minutes, the ratio of yield strength to tensile strength was 0.35, which was 166% of Comparative Example 8.

According to the disclosed embodiment, it can be seen that by optimizing the alloy composition and manufacturing process, it is possible to secure excellent yield strength and elongation and at the same time suppress the decrease in yield strength at a high temperature of the level of 600 ° C.

Claims (13)

In weight percent, C: 0.8 to 1.1%, Si: 0.1 to 0.3%, Mn: 0.2 to 0.8%, Cr: 0.4 to 1.0%, Mo: 0.5 to 1.0%, the balance including Fe and other unavoidable impurities,
A wire rod with improved strength and softening resistance, in which the area fraction of a pro-eutectoid cementite structure with a maximum thickness of 20 μm or more at the austenite grain boundary is 10% or less. According to claim 1,
When the diameter of the wire is D, in the area from the center of the cross section perpendicular to the length direction to 0.25D in the direction of the wire surface,
A wire rod with improved strength and softening resistance, wherein the ratio of the maximum content of carbon (C _max ) and the average content of carbon (C ₀ ) satisfies the following formula (1).
Equation (1): C _max /C ₀ ≤1.25 According to claim 1,
In the region where the carbon content of the surface layer of the wire rod exceeds 0.90% by weight,
A wire rod with improved strength and softening resistance, wherein the bainite structure containing carbides having an average grain size of 1 μm or more is 90% or more in area fraction. According to claim 1,
A wire rod with improved strength and softening resistance with a yield strength of 1000 MPa or more. According to claim 1,
A wire rod with an elongation of 12% or more and improved strength and softening resistance. In weight percent, a billet containing C: 0.8 to 1.1%, Si: 0.1 to 0.3%, Mn: 0.2 to 0.8%, Cr: 0.4 to 1.0%, Mo: 0.5 to 1.0%, the balance Fe and other unavoidable impurities maintaining for 90 to 120 minutes in a heating furnace at 1000 to 1200 ° C;
manufacturing a wire rod by rolling the billet;
Winding the prepared wire at 900 to 980 ° C;
first cooling the wound wire from a winding temperature to 500 °C at a cooling rate of 10 to 15 °C/s; and
Second cooling the cooled wire rod from 500 ° C to 300 ° C at a cooling rate of 0.1 ° C / s or less; manufacturing method of a wire rod having improved strength and softening resistance. According to claim 6,
The rolling method of manufacturing a wire rod having improved strength and softening resistance, comprising the step of finishing rolling at 1000 to 1200 ° C. In weight percent, C: 0.8 to 1.1%, Si: 0.1 to 0.3%, Mn: 0.2 to 0.8%, Cr: 0.4 to 1.0%, Mo: 0.5 to 1.0%, the balance including Fe and other unavoidable impurities,
A steel wire with improved strength and softening resistance, wherein the number of MC carbides (where M is Cr or Mo) is 10 or more in the 500 nm ² area of the cross section perpendicular to the length direction of the steel wire. According to claim 8,
A steel wire with improved strength and softening resistance, having a yield strength of 780 MPa or more after holding at 600 ° C for 30 minutes. According to claim 8,
After holding at 600 ° C for 30 minutes,
A steel wire with improved strength and softening resistance with a ratio of yield strength to tensile strength of 0.35 or more. According to claim 8,
Steel wire with improved strength and softening resistance, with a yield strength of 2200 MPa or more. According to claim 8,
Based on 100D, a steel wire with improved strength and softening resistance, with more than 12 twists without delamination. A method for manufacturing a steel wire having improved strength and softening resistance, comprising: drawing the wire rod having improved strength and softening resistance according to any one of claims 1 to 5.

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