KR102222579B1 - Wire rod excellent in stress corrosion resistance for prestressed concrete steel wire, steel wire and manufacturing method thereof - Google Patents

Abstract

Disclosed is a wire rod capable of manufacturing an ultra-high strength PC steel wire capable of securing excellent stress corrosion resistance by adding Cr, Ni, and Cu in a specific ratio to a high-carbon hypereutectoid pearlite structure, and a method of manufacturing the same.
PC steel wire having excellent stress corrosion resistance according to an embodiment of the present invention, by weight, C: 0.9 to 1.1%, Si: 0.8 to 1.5%, Mn: 0.3 to 0.6%, Cr: 0.2 to 0.8%, Ni : 0.16 to 0.96%, Cu: 0.16 to 0.96%, the remaining Fe and inevitable impurities are included, and the weight ratio of Cr, Ni and Cu satisfies the following formula (1).
(1) Cr:Ni:Cu = 1:(0.8~1.2):(0.8~1.2)

Description

Wire rod for PC steel wire with excellent stress corrosion resistance, steel wire and their manufacturing method {WIRE ROD EXCELLENT IN STRESS CORROSION RESISTANCE FOR PRESTRESSED CONCRETE STEEL WIRE, STEEL WIRE AND MANUFACTURING METHOD THEREOF}

The present invention is an ultra-high-strength PC steel wire capable of securing excellent stress corrosion resistance, suppressing surface decarburization during LP heat treatment by adding Cr, Ni, and Cu in a specific ratio to a high-carbon hypereutectoid pearlite structure, And it relates to a wire rod capable of manufacturing a PC steel wire and a method of manufacturing the same.

PC steel wire is a steel wire manufactured to have a certain level of tensile strength through wire drawing after being as-rolled using hard steel wire or controlled to a complete pearlite microstructure through patterning heat treatment. Because it is used by making and using it, it must be able to withstand without breakage when it is given more than a certain number of twists. Even in the case of fracture due to torsion, fracture should occur in the form of the fracture surface at right angles to the length of the steel wire.

The tensile strength of the commonly used PC steel wire is 1,600 ~ 2,000 MPa, and it is used up to the hard steel wire standard SWRH62 ~ SWRH82, and the size and wire diameter of the wire rod are determined according to the required wire diameter, strength, and wire cutting amount of the final product. The alloy composition of hard steel wire is composed of C, Si, and Mn, and the cooling rate is controlled or constant temperature transformation heat treatment is applied after rolling the wire so that the microstructure becomes a complete pearlite structure. Even if it is controlled with a complete pearlite structure, some cornerstone ferrite and cornerstone cementite exist.

Existing studies on the stress corrosion resistance properties of PC steel wires have not been largely done, because when using a full pearlite microstructure, the stress corrosion properties did not become a problem as long as the tensile strength and torsional properties were secured. However, as the concrete structure became larger, the tensile strength of the PC steel wire exceeded 2,000 MPa, and along with the deterioration of the stress corrosion characteristics due to the increase in strength, a liquid admixture was used to develop the strength quickly after pouring the concrete to reduce the construction period of the concrete structure. I'm doing it. However, since the ammonium thiocyanate (NH ₄ SCN) component added to the admixture accelerates fracture due to stress corrosion of steel, it is necessary to develop a material for PC steel wire with improved stress corrosion resistance.

The present invention is to provide a PC steel wire and a wire for PC steel wire, and a method of manufacturing the same, with improved tensile strength of the final product of 2,000 MPa or more, and _{improved stress corrosion resistance even in an atmosphere of wire drawing and ammonium thiocyanate (NH 4 SCN) aqueous solution.} do.

The wire rod for a PC steel wire having excellent stress corrosion resistance according to an embodiment of the present invention is, by weight, C: 0.9 to 1.1%, Si: 0.8 to 1.5%, Mn: 0.3 to 0.6%, Cr: 0.2 to 0.8%, Ni: 0.16 to 0.96%, Cu: 0.16 to 0.96%, P: 0.015% or less, S: 0.015% or less, the remaining Fe and inevitable impurities are included, and the weight ratio of Cr, Ni and Cu satisfies the following formula (1) do.

(1) Cr:Ni:Cu = 1:(0.8~1.2):(0.8~1.2)

Further, according to an embodiment of the present invention, the tensile strength may be 1,300 to 1,600 MPa at a wire diameter of 8 to 15 mm.

In addition, according to an embodiment of the present invention, when a tensile test is performed in an equilibrium state after hydrogen is released, the sectional shrinkage may be 15 to 25%.

The method of manufacturing a wire rod for a PC steel wire having excellent stress corrosion resistance according to an embodiment of the present invention is, by weight, C: 0.9 to 1.1%, Si: 0.8 to 1.5%, Mn: 0.3 to 0.6%, Cr: 0.2 to Heating a billet containing 0.8%, Ni: 0.16 to 0.96%, Cu: 0.16 to 0.96%, P: 0.015% or less, S: 0.015% or less, remaining Fe and inevitable impurities at 800 to 1,200°C; Rolling a wire rod to a wire diameter of 8 to 15 mm; Winding at a winding temperature of 850 to 950°C; And cooling to suppress the formation of cornerstone cementite and martensite by blowing air within 2 to 5 seconds after the winding.

In addition, according to an embodiment of the present invention, the weight ratio of Cr, Ni, and Cu of the billet may satisfy Equation (1) below.

(1) Cr:Ni:Cu = 1:(0.8~1.2):(0.8~1.2)

Further, according to an embodiment of the present invention, the cooling may include cooling to 650 to 750°C at a rate of 5 to 15°C/s; Cooling to 400 to 500°C at a rate of 5°C/s or less; And air-cooling to room temperature.

The heat-treated wire rod for PC steel wire having excellent stress corrosion resistance according to an embodiment of the present invention is, by weight, C: 0.9 to 1.1%, Si: 0.8 to 1.5%, Mn: 0.3 to 0.6%, Cr: 0.2 to 0.8% , Ni: 0.16 to 0.96%, Cu: 0.16 to 0.96%, P: 0.015% or less, S: 0.015% or less, the remaining Fe and inevitable impurities are included, and the weight ratio of Cr, Ni and Cu is expressed by the following formula (1). Satisfies.

(1) Cr:Ni:Cu = 1:(0.8~1.2):(0.8~1.2)

In addition, according to an embodiment of the present invention, the matrix structure is pearlite, and the area fraction of ferrite in the cross section of the surface layer from the surface to a depth of 50 μm may be 30% or less.

The method of manufacturing a heat-treated wire rod for a PC steel wire having excellent stress corrosion resistance according to an embodiment of the present invention is, by weight, C: 0.9 to 1.1%, Si: 0.8 to 1.5%, Mn: 0.3 to 0.6%, Cr: 0.2 To 0.8%, Ni: 0.16 to 0.96%, Cu: 0.16 to 0.96%, P: 0.015% or less, S: 0.015% or less, the remaining Fe and inevitable impurities are included, and the weight ratio of Cr, Ni and Cu is shown in the following formula (1 Maintaining a wire rod satisfying the) at 950 to 1,050° C. for 2 to 5 minutes; Cooling to 550 to 650° C. at a rate of 20° C./s or more; And performing constant temperature heat treatment at 550 to 650° C. for 3 to 5 minutes.

(1) Cr:Ni:Cu = 1:(0.8~1.2):(0.8~1.2)

PC steel wire having excellent stress corrosion resistance according to an embodiment of the present invention, by weight, C: 0.9 to 1.1%, Si: 0.8 to 1.5%, Mn: 0.3 to 0.6%, Cr: 0.2 to 0.8%, Ni : 0.16 to 0.96%, Cu: 0.16 to 0.96%, P: 0.015% or less, S: 0.015% or less, the remaining Fe and inevitable impurities are included, and the weight ratio of Cr, Ni and Cu satisfies the following formula (1) .

(1) Cr:Ni:Cu = 1:(0.8~1.2):(0.8~1.2)

In addition, according to an embodiment of the present invention, the tensile strength of the PC steel wire may be 2,000 MPa or more.

In addition, according to an embodiment of the present invention, _{after immersion in a 20% NH 4} SCN solution at 50° C. for 4 hours, the amount of hydrogen increase in the steel may be 2.0 ppm or less.

In addition, according to an embodiment of the present invention, _{when immersed in a 20% NH 4} SCN solution at 50° C. and a load of 80% of the tensile strength is applied, the minimum value of the breaking time may exceed 120 minutes and the median value may exceed 300 minutes. .

A method of manufacturing a PC steel wire having excellent stress corrosion resistance according to an embodiment of the present invention includes the steps of maintaining the wire rod for PC steel wire at 950 to 1,050°C for 2 to 5 minutes; Cooling to 550 to 650° C. at a rate of 20° C./s or more and performing constant temperature heat treatment for 3 to 5 minutes; It may include a; step of drawing at a reduction rate of 15 to 25% per pass.

The wire rod for PC steel wire according to the present invention and the PC steel wire manufactured using the same, suppresses decarburization of the surface layer during LP heat treatment, improves freshness during wire drawing, and has excellent stress corrosion resistance even in the atmosphere of _{ammonium thiocyanate (NH 4 SCN) aqueous solution.} Can have characteristics.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following examples are presented in order to sufficiently convey the spirit of the present invention to those of ordinary skill in the art to which the present invention pertains. The present invention is not limited only to the examples presented here, but may be embodied in other forms. In the drawings, in order to clarify the present invention, portions not related to the description may be omitted, and the size of components may be slightly exaggerated to aid understanding.

In addition, when a part "includes" a certain component, it means that other components may be further included rather than excluding other components unless specifically stated to the contrary.

Singular expressions include plural expressions, unless the context clearly makes exceptions.

PC steel wire is a high-strength reinforcement material that is reinforced and inserted into concrete to compensate for the shear strength and tensile strength, which are weak points of high-strength concrete. Because it cures the steel wire, when the tensile force is removed from the steel wire, it naturally imparts a compressive force to the concrete, which can greatly improve the tensile strength and shear strength of the concrete structure. In order to apply compressive stress while in direct contact with concrete, a certain level of tensile force is always applied to the steel wire itself, so it is exposed to a stress corrosion environment along with moisture that has penetrated through the concrete.

Conventionally, rather than improving the original stress corrosion resistance characteristics of the material from a metallurgical point of view, mainly from a metallurgical point of view, such as a technology to relieve residual stress remaining on the surface of a steel wire after wire drawing, a surface treatment, and a change in the structure of a strand, the modification or mechanical point of view limited to the surface Improvements have been made in the direction of solving the stress concentration in Stress corrosion means fracture in a much shorter time compared to the service life of steel, which is expected due to normal corrosion, deterioration, fatigue, etc., when all three factors of "stress + corrosive environment + material" are met. It does not occur even if only one of the elements is removed. In other words, since it occurs only when the material that reacts to the stress and the atmosphere that can cause corrosion to the steel material and the material that reacts to it is used, the stress applied to the material is reduced by adjusting the residual stress on the surface of the steel wire. Improvements have been made in the direction of treating special components on the surface so that corrosion does not occur easily in the environment. However, this technology only improves the stress corrosion resistance by focusing on the surface portion of the steel wire, but has a weakness in that resistance to the stress corrosion phenomenon rapidly decreases when the surface portion is removed due to friction or abrasion. Therefore, improving the stress corrosion resistance of the steel itself is the only way to ensure the safety of large concrete structures.

The inventors have confirmed that when Cr, Ni, and Cu components are added to the PC steel wire product and the weight ratio is kept constant, the stress corrosion resistance characteristics are rapidly improved. This is because the improvement of the mechanical properties through the addition of Cr and the improvement of the corrosion resistance through the addition of Ni and Cu produce a synergistic effect. In particular, by adding Ni and Cu together, decarburization is suppressed and the grain boundary non-equilibrium microstructure of the cornerstone cementite or cornerstone Precipitation of ferrite can be suppressed, so that the wire drawing processability can be improved as well.

The wire rod for a PC steel wire having excellent stress corrosion resistance according to an embodiment of the present invention is, by weight, C: 0.9 to 1.1%, Si: 0.8 to 1.5%, Mn: 0.3 to 0.6%, Cr: 0.2 to 0.8%, Ni: 0.16 to 0.96%, Cu: 0.16 to 0.96%, P: 0.015% or less, S: 0.015% or less, the remaining Fe and inevitable impurities are included, and the weight ratio of Cr, Ni and Cu satisfies the following formula (1) do.

(1) Cr:Ni:Cu = 1:(0.8~1.2):(0.8~1.2)

First, the reason for the numerical limitation of the content of the alloy component element in the embodiment of the present invention will be described. Hereinafter, unless otherwise specified, the unit is% by weight.

The content of C is 0.9 to 1.1%.

C is an element that can most effectively increase the strength of the material, and it is known that the strength of about 100 MPa can be improved when C is increased by 0.1% in pearlite steel. If the C content is less than 0.9%, it is not possible to secure a high strength of 2,000 MPa or more after wire drawing, and if it exceeds 1.1%, it is preferable to add less than that because it cannot suppress the occurrence of cornerstone cementite during wire rod manufacturing.

The content of Si is 0.8 to 1.5%.

Si is mostly dissolved in ferrite during austenite-to-pearlite transformation, and it is hardly distributed in cementite, and its diffusion rate is slower than that of C, so when a large amount of Si is solid-solution, the overall transformation of pearlite is slowed. For this reason, it is an effective element to increase the strength because it has the effect of minimizing the layer gap of pearlite and basically shows a solid solution strengthening effect while being dissolved in ferrite. When Si is less than 0.8%, the solid solution strengthening effect is insufficient and it is difficult to achieve high strength, and when it exceeds 1.5%, it is preferable to add less than that because the wire drawing property is insufficient as the ferrite is strengthened.

The content of Mn is 0.3 to 0.6%.

Mn does not have a significant effect of increasing the strength in the complete pearlite steel, but is added to maintain the hardenability at an appropriate level depending on the cooling performance of the wire. If Mn is less than 0.3%, it is difficult to see the effect of quenching, and if it exceeds 0.6%, since it is a high carbon steel, a martensite structure can be formed in the segregation part along with C, so it is preferable to add less than that.

The content of Cr is 0.2 to 0.8%.

Cr is a very effective element in improving the mechanical properties of pearlite, such as increasing tensile strength and improving wire drawing, because it makes the gap between the lamellar layers of pearlite fine. When the Cr content is less than 0.2%, it is difficult to clearly show the effect of minimizing the interlayer spacing of the lamellar, and the tensile strength increases as the amount of addition increases, but when it exceeds 0.8%, the hardenability increases excessively and the cementite growth of pearlite does not occur smoothly. Therefore, it is preferable to add less than that, since mechanical properties may be deteriorated.

The content of Ni and Cu is 0.16 to 0.96%, respectively.

Ni hardly contributes to the increase in tensile strength of pearlite, but it is an element that improves the plastic deformability of cementite and increases the ductility of the steel wire after wire drawing. It is also known as an effective element in improving the corrosion resistance of pearlite steel together with Cu. In the present invention, by adding Ni and Cu in a combination ratio with Cr in a certain ratio as described later, decarburization of the microstructure of the surface layer is suppressed, and both corrosion properties and stress corrosion resistance properties of steel can be improved.

The weight ratio of Ni and Cu to the Cr content satisfies a ratio of 0.8 to 1.2 as shown in the following formula (1).

(1) Cr:Ni:Cu = 1:(0.8~1.2):(0.8~1.2)

As described above, the weight ratio of Cr, Ni, and Cu is very important in order to improve wire drawing and stress corrosion resistance.The Cr content can be controlled up to 0.2 to 0.8% depending on the tensile strength of the final product to be controlled, but Ni The content of and Cu should be added in a ratio of 1:(0.8~1.2) by the amount of Cr to be added. If the weight ratio is out of this, the surface decarburization is deepened or the stress corrosion resistance is deteriorated. Therefore, the amount of Ni and Cu added should always be added in a ratio of (0.8~1.2) with respect to the added amount of Cr. That is, if the amount of Cr is 0.3%, the amount of Ni and Cu must be controlled in the range of 0.24 to 0.36%. Ni and Cu must be added together, and the ratio of each to Cr can be adjusted, but there is no need to control a specific ratio between Ni and Cu.

By heating the billet satisfying the above-described alloy component system and formula (1)-rolling wire-winding-cooling, it is possible to manufacture a wire rod for PC steel wire having excellent stress corrosion resistance.

Billets for wire rod rolling can be manufactured through steel plate rolling after bloom casting, or directly cast into billets. The manufactured billets are heated and extracted in a wire heating furnace to a temperature range of 800 to 1,200°C depending on the cross-sectional size of the billet. Then, it goes through wire rod rolling. The wire diameter of the final wire rod product ranges from 8 to 15 mm, and the wire rod can be rolled up to the final wire diameter range.

Since it is a high carbon steel, it is important to control the winding temperature and cooling schedule to control the cornerstone cementite and martensite in the manufacture of wire rods. In order to suppress the formation of cornerstone cementite, the winding temperature is controlled in the range of 850 to 950°C, and cooling through blowing should be started within 2 to 5 seconds after winding. After the start of cooling in the coiling temperature range, it must be cooled at a cooling rate of 5 to 15°C/s up to the temperature range of 650 to 750°C to suppress the formation of cornerstone cementite as much as possible, and then 5°C/s or less until the temperature range of 400 to 500°C. The formation of martensite should be suppressed by controlling the cooling rate of. After that, it can be air-cooled up to room temperature without special control.

The wire rod thus manufactured may have a tensile strength in the range of 1,300 to 1,600 MPa depending on the alloy component and the wire diameter. In addition, in order to eliminate the deterioration of ductility due to hydrogen introduced during the manufacturing process after the wire rod is manufactured, the time required for hydrogen to be released must be left at room temperature, and 5 to 20 days of standing time is required depending on the alloy composition and wire diameter. . With the passage of time, the rate of hydrogen evolution decreases, and the sectional shrinkage of the wire rod may reach 15 to 25% during a tensile test after hydrogen evolution reaches equilibrium.

In order to manufacture an ultra-high strength PC steel wire with a final strength of 2,000 MPa or more using the wire rod, a patterning heat treatment should be applied. After maintaining the wire rod in a temperature range of 950 to 1,050°C for 2 to 5 minutes, cooling at a cooling rate of 20°C/s or higher to a temperature range of 550 to 650°C can be performed for 3 to 5 minutes. have. At this time, the faster the cooling rate is, the better, and when the transformation to the pearlite structure is completed after maintaining a constant temperature, it must be washed with water to remove foreign substances on the surface.

By this patterning heat treatment, the heat-treated wire rod increases the tensile strength by 50 to 150 MPa compared to the conventional wire rod state, reduces the variation in tensile strength existing in the longitudinal direction, and has a microstructure and mechanical properties suitable for wire drawing.

On the other hand, in the wire rod subjected to patterning heat treatment by controlling the weight ratio of Cr, Ni, and Cu according to Equation (1), decarburization of the surface layer portion is suppressed. In the present invention, the surface layer portion is defined to a depth of 50 μm from the surface, and the surface layer portion of the cross-section of the heat-treated wire rod may have an area fraction of ferrite generated by decarburization of 30% or less.

Subsequently, the wire rod subjected to the patterning heat treatment is continuously drawn at a reduction rate of 15 to 25% per pass, thereby manufacturing a PC steel wire having a final target wire diameter. For example, a wire diameter of a commercially available PC steel wire may be 5.32 mm.

The PC steel wire having excellent stress corrosion resistance according to an embodiment of the present invention may have a tensile strength of 2,000 MPa or more in a total wire drawing amount of 1.7 or more in strain, and the stress corrosion breaking time is _{immersed in a 20% NH 4} SCN aqueous solution at 50°C. Therefore, when a load is applied with 80% of the tensile strength, the breaking time can exceed the minimum value of 2 hours (120 minutes) and the median value of 5 hours (300 minutes). In addition, after immersion in a 20% NH ₄ SCN aqueous solution at 50° C. for 4 hours, the amount of hydrogen increase in the steel may be 2.0 ppm or less.

Hereinafter, it will be described in more detail through a preferred embodiment of the present invention.

Example

In this embodiment, when Cr, Ni, and Cu are added at a certain ratio and when they are not, the difference in the microstructure of the surface layer after the same LP heat treatment and the final product after wire drawing are treated with NH ₄ SCN having a temperature of 50°C and a concentration of 20%. The excellent stress corrosion resistance properties of the examples of the present invention were confirmed through the change of the hydrogen content in the steel when immersed in the solution for 4 hours, and the change in the breaking time during the stress corrosion test that finally gives 80% of the tensile strength in the same solution. I wanted to see it.

Table 1 below shows the alloy components of the present invention examples and comparative examples.

division C Si Mn Cr Ni Cu Ni/Cr
(Cr:Ni) Cu/Cr
(Cr:Cu) Invention Example 1 0.91 1.32 0.49 0.30 0.28 0.29 0.933 0.967 Invention Example 2 1.02 1.01 0.48 0.36 0.38 0.36 1.056 1.0 Inventive Example 3 1.09 0.83 0.51 0.61 0.55 0.56 0.902 0.918 Comparative Example 1 0.91 1.31 0.49 0.60 - 0.51 0 0.85 Comparative Example 2 0.91 1.29 0.48 0.31 0.29 - 0.935 0 Comparative Example 3 0.92 1.30 0.50 0.30 0.15 0.17 0.5 0.567 Comparative Example 4 0.91 1.31 0.50 0.29 0.49 0.50 1.69 1.724 Comparative Example 5 0.92 1.32 0.48 0.60 0.59 0.20 0.983 0.333 Comparative Example 6 0.93 1.28 0.49 0.59 0.20 0.61 0.339 1.034 Comparative Example 7 0.92 1.29 0.50 0.30 - - 0 0 Comparative Example 8 0.98 1.30 0.50 0.59 - - 0 0

In Table 2, the invention examples and comparative examples having the above alloying components were manufactured with a wire rod having a wire diameter of 13.0 mm, and then austenitized at 1,000°C for 10 minutes and then immersed in a solder bath at 600°C for 4 minutes. The depth of the layer and the fraction of the completely ferrite-like structure on the cross-section of the surface layer from the surface to the depth of 50 μm were shown.

division Surface decarburization depth Area fraction of ferrite in the surface layer Invention Example 1 100㎛ 21% Invention Example 2 90㎛ 25% Inventive Example 3 70㎛ 18% Comparative Example 1 410㎛ 91% Comparative Example 2 120㎛ 58% Comparative Example 3 110㎛ 27% Comparative Example 4 110㎛ 21% Comparative Example 5 110㎛ 31% Comparative Example 6 350㎛ 88% Comparative Example 7 120㎛ 78% Comparative Example 8 90㎛ 43%

In Table 2, the characteristic part is that when Ni and Cr are added together, decarburization occurs along the grain boundaries, and it is observed that pearlite and grain boundary ferrite are mixed. There is no significant change in depth, but the surface layer is completely decarburized and the area where only ferrite is present gradually decreases.

On the other hand, in Comparative Example 7 in which Ni and Cu were not added together, in particular Comparative Example 1 in which only Cu was added, and in Comparative Example 6 in which a relatively large amount of Cu was added, the thickness of the decarburized layer was generally increased and the surface portion of the ferrite It can be seen that the area ratio also rises sharply, resulting in poor decarburization quality.

Table 3 shows the heat-treated wire rods of the Inventive Examples and Comparative Examples in the steel after 4 hours by immersing _{in a 20% NH 4} SCN aqueous solution stipulated in ASTM E-45 at a temperature of 50° C. and the same as in the stress corrosion test environment after wire drawing. It showed a change in hydrogen concentration. Before immersion in NH ₄ SCN aqueous solution means as-drawn state.

division Before immersion
Hydrogen concentration After 4 hours immersion
Hydrogen concentration Out of the river
Hydrogen increase Invention Example 1 0.3 ppm 2.2 ppm 1.9 ppm Invention Example 2 0.4 ppm 2.3 ppm 1.9 ppm Inventive Example 3 0.4 ppm 2.1 ppm 1.7 ppm Comparative Example 1 0.5 ppm 3.3 ppm 2.8 ppm Comparative Example 2 0.4 ppm 3.2 ppm 2.9 ppm Comparative Example 3 0.3 ppm 3.1 ppm 2.8 ppm Comparative Example 4 0.4 ppm 3.0 ppm 2.6 ppm Comparative Example 5 0.3 ppm 3.0 ppm 2.7 ppm Comparative Example 6 0.5 ppm 3.3 ppm 2.8 ppm Comparative Example 7 0.4 ppm 3.2 ppm 2.8 ppm Comparative Example 8 0.4 ppm 3.2 ppm 2.8 ppm

Since hydrogen in the steel acts as a key cause of stress corrosion failure, the more hydrogen in the steel increases during the same time under a corrosive environment, the more the stress corrosion resistance becomes inferior. The characteristic of hydrogen increase in steel is that only one peak is observed between 350 and 450°C when analyzing the behavior of the hydrogen released while increasing the temperature of the sample in the state before immersion in the corrosion solution after wire drawing. It shows hydrogen that is fixed to many potentials generated during wire drawing. When the test sample is immersed in the corrosion solution, a new peak is observed around 250°C. This indicates that the degree of trapping is weaker than the potential, that is, the amount of diffusible hydrogen that can easily move when a load is applied to the steel material is greatly increased.

As a result of the experiment, the hydrogen content of all the samples of alloy components before immersion in the corrosion solution was 0.3 to 0.5 ppm, which was similar, but after immersion in the corrosion solution for 4 hours, Cr:Ni:Cu = 1:(0.8~1.2):(0.8~1.2) It can be seen that only in the invention examples introduced at a weight ratio, the increase in the amount of diffusible hydrogen in the steel was controlled to 2.0 ppm or less, and in all the other comparative examples, an increase in the hydrogen content exceeding 2.0 ppm was observed. This increase in diffusible hydrogen could predict that the stress corrosion resistance properties of the comparative examples would be relatively inferior to those of the invention examples.

Table 4 shows the minimum and intermediate values of the results obtained by repeating the breaking time 7 times for each alloy component when 80% of the tensile strength is applied in the same corrosive environment as the hydrogen content evaluation. I got it. In addition, the tensile strength of the PC steel wire with a final wire diameter of 5.32 mm is also shown.

division Breaking time (min) The tensile strength
(MPa) #One #2 #3 #4 #5 #6 #7 Invention Example 1 145 187 242 343 447 554 590 2,101 Invention Example 2 160 192 250 430 501 556 602 2,178 Inventive Example 3 148 188 240 366 475 526 588 2,199 Comparative Example 1 64 121 199 255 266 325 414 2,180 Comparative Example 2 111 161 233 277 289 425 543 2,078 Comparative Example 3 118 178 235 305 421 469 533 2,056 Comparative Example 4 82 139 190 301 423 459 521 2,099 Comparative Example 5 123 173 211 278 388 423 515 2,177 Comparative Example 6 118 170 226 252 272 347 432 2,134 Comparative Example 7 70 98 132 218 267 379 435 2,056 Comparative Example 8 55 74 111 207 309 363 424 2,240

In all component systems, the tensile strength exceeded 2,000 MPa after 13㎜ → 5.32㎜ wire drawing was secured. In the case of Inventive Examples 1 to 3, the minimum value of 2 hours (120 minutes) and the median value of 5 hours (300 minutes) were exceeded. It appeared within 5 hours. In addition, even in the case of the longest time, it can be seen that the inventive examples showed a breaking time close to 10 hours, but the comparative examples showed a difference such that the longest breaking time was less than 9 hours. In particular, in the case of Comparative Examples 7 and 8 in which both Ni and Cu, which are anticorrosive elements, are not added, they show the most heat resistant stress corrosion rupture time, and show a greater gap than the difference shown in the behavior of hydrogen. It is presumed to be due to the notch effect in which the stress is concentrated because it is very rough.

As described above, although exemplary embodiments of the present invention have been described, the present invention is not limited thereto, and those of ordinary skill in the art are within the scope of not departing from the concept and scope of the following claims. It will be appreciated that various modifications and variations are possible.

Claims (14)

In% by weight, C: 0.9 to 1.1%, Si: 0.8 to 1.5%, Mn: 0.3 to 0.6%, Cr: 0.2 to 0.8%, Ni: 0.16 to 0.96%, Cu: 0.16 to 0.96%, P: 0.015% Hereinafter, S: 0.015% or less, containing the remaining Fe and inevitable impurities,
The weight ratio of Cr, Ni and Cu satisfies the following formula (1),
A wire rod for PC steel wire having excellent stress corrosion resistance with a tensile strength of 1,300 to 1,600 MPa at a wire diameter of 8 to 15 mm, and a cross-sectional shrinkage of 15 to 25% during a tensile test in an equilibrium state after release of hydrogen.
(1) Cr:Ni:Cu = 1:(0.8~1.2):(0.8~1.2) delete delete In% by weight, C: 0.9 to 1.1%, Si: 0.8 to 1.5%, Mn: 0.3 to 0.6%, Cr: 0.2 to 0.8%, Ni: 0.16 to 0.96%, Cu: 0.16 to 0.96%, P: 0.015% Hereinafter, S: 0.015% or less, containing the remaining Fe and inevitable impurities,
Heating a billet satisfying the following formula (1) at 800 to 1,200°C;
Rolling a wire rod to a wire diameter of 8 to 15 mm;
Winding at a winding temperature of 850 to 950°C; And
Cooling to suppress the formation of cornerstone cementite and martensite by performing blowing within 2 to 5 seconds after the winding; Method for producing a wire rod for PC steel wire having excellent stress corrosion resistance, including.
(1) Cr:Ni:Cu = 1:(0.8~1.2):(0.8~1.2) delete The method of claim 4,
The cooling step,
Cooling to 650 to 750°C at a rate of 5 to 15°C/s;
Cooling to 400 to 500°C at a rate of 5°C/s or less; And
Air-cooling to room temperature; a method of manufacturing a wire rod for a PC steel wire excellent in stress corrosion resistance including. In% by weight, C: 0.9 to 1.1%, Si: 0.8 to 1.5%, Mn: 0.3 to 0.6%, Cr: 0.2 to 0.8%, Ni: 0.16 to 0.96%, Cu: 0.16 to 0.96%, P: 0.015% Hereinafter, S: 0.015% or less, containing the remaining Fe and inevitable impurities,
The weight ratio of Cr, Ni and Cu satisfies the following formula (1),
The matrix structure is pearlite, and the heat-treated wire rod for PC steel wires with excellent stress corrosion resistance with a cross-sectional ferrite area fraction of 30% or less in the surface layer from the surface to a depth of 50㎛.
(1) Cr:Ni:Cu = 1:(0.8~1.2):(0.8~1.2) delete In% by weight, C: 0.9 to 1.1%, Si: 0.8 to 1.5%, Mn: 0.3 to 0.6%, Cr: 0.2 to 0.8%, Ni: 0.16 to 0.96%, Cu: 0.16 to 0.96%, P: 0.015% Hereinafter, S: maintaining a wire rod containing 0.015% or less, remaining Fe and unavoidable impurities, and having a weight ratio of Cr, Ni, and Cu satisfying the following formula (1) at 950 to 1,050° C. for 2 to 5 minutes;
Cooling to 550 to 650° C. at a rate of 20° C./s or more; And
A method of manufacturing a heat-treated wire rod for PC steel wire having excellent stress corrosion resistance, including a step of constant-temperature heat treatment at 550 to 650° C. for 3 to 5 minutes.
(1) Cr:Ni:Cu = 1:(0.8~1.2):(0.8~1.2) In% by weight, C: 0.9 to 1.1%, Si: 0.8 to 1.5%, Mn: 0.3 to 0.6%, Cr: 0.2 to 0.8%, Ni: 0.16 to 0.96%, Cu: 0.16 to 0.96%, P: 0.015% Hereinafter, S: 0.015% or less, containing the remaining Fe and inevitable impurities,
Cr, Ni, and 20% NH ₄ SCN solution and satisfy the following equation (1) the weight ratio of Cu, 4 hours after immersion in 20% NH ₄ SCN solution of 50 ℃, a 2.0 ppm of hydrogen increase in steel or less, 50 ℃ PC steel wire with excellent stress corrosion resistance, with the minimum value of breaking time exceeding 120 minutes and the median value exceeding 300 minutes when immersed in and applying a load of 80% of the tensile strength.
(1) Cr:Ni:Cu = 1:(0.8~1.2):(0.8~1.2) The method of claim 10,
PC steel wire with excellent stress corrosion resistance with a tensile strength of 2,000 MPa or more. delete delete Maintaining the wire rod for PC steel wire of claim 1 at 950 to 1,050° C. for 2 to 5 minutes;
Cooling to 550 to 650° C. at a rate of 20° C./s or more and performing constant temperature heat treatment for 3 to 5 minutes;
A method of manufacturing a PC steel wire having excellent stress corrosion resistance including; drawing at a reduction rate of 15 to 25% per pass.