KR920010159B1 - Production of wire rod for high-strength spiral hoop and high-strength spiral hoop - Google Patents

Abstract

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Description

Manufacturing method of high strength spiral rebar wire and high strength spiral rebar

1 is a planar cross-sectional view comparing the reinforcement state of the rectangular high-strength spiral reinforcement made of a wire rod according to the present invention and the rectangular high-strength spiral reinforcement made of a high-strength wire rod according to the conventional method.

Figure 2 is a front view of an embodiment wire reheating line according to the present invention method.

3a to h are diagrams showing the hardness test results of each specimen in each (Experimental Example 1).

4A and 4B are diagrams showing a part of the tensile test results in (Experimental Example 1).

5 is a diagram showing the mechanical properties of each specimen obtained in (Experimental Example 1) by A and B.

6A is a front view of the bending machine in (Experimental Example 2).

Figure 6b is a front view of the specimen after the bending process.

7 is a front view of a tensile tester in (Experimental Example 2).

8A and 8B are diagrams showing the strength ratios for the base metals according to the bending angles of the specimens after bending, for steel grades A and B, respectively.

Figure 9a is a plan sectional view showing the reinforcement state of the spiral reinforcement made of structural steel wire.

Figure 9b is a planar cross-sectional view showing the problems present in the conventional rectangular high strength spiral rebar.

* Explanation of symbols for the main parts of the drawings

1: payoff stand 2: textile machine

3: hydration device 3a: heating device

3b: cooling jacket 4: tempering device

4a: heating coil 4b: cooling jacket

5: winding stand 6, 9: jig

7: pressure bending roll 8, 81: chuck

The present invention relates to a method for producing a high strength spiral reinforcing wire rod and a high strength spiral reinforcing bar used for shear reinforcement of girders, columns, etc. to be made of reinforced concrete.

Recently, when constructing girders, columns, etc. made of reinforced concrete, if the high-strength spiral reinforcing bar is formed in the form of spiral steel for spiral reinforcement, high strength wire reinforcing the steel bar equivalent to PC bar standard, The improved strength has attracted the attention of the industry and tends to spread rapidly.

In this case, the spiral reinforcing bar made of a wire rod having a yield strength of 130 Kgf / m 2 or more has a remarkable effect of dramatically improving the shear longitudinal strength, thus reducing the reinforcing bar ratio, and thus the weight of the spiral reinforcing bar. There are various effects, such as reducing the pressure and making the reinforcement work easier.

In the case of manufacturing high strength spiral reinforcing bars, there are methods of increasing the strength after bending the wire rod and bending processing after the wire is continuously subjected to the high-strengthening process. It is manufactured by the method.

In the case of high strength by continuously annealing the wire, it is necessary to continuously quench and temper the material steel wire, and to quench and harden the wire directly in the final stage of hot wire rolling. Tempering may occur.

As described above, bending processing for forming a spiral shape after increasing the strength of the wire rod involves considerable difficulty because of its strength.

For example, when forming a high strength wire rod having a yield point strength of 130 Kgf / m2 or more with a square spiral reinforcement, or bending a terminal even in a circular spiral reinforcement, the bending process is not only difficult. There is a concern that the strength of the wire rod may decrease due to plastic working. For this reason, the spiral bar is subjected to cold bending, and the bending guide diameter is 5d or more (3d or more in the conventional tensile strength of 30-40 Kgf / m2 spiral bar). By doing so, consideration is taken not to hinder as much as possible the improvement of the discontinued national power as described above.

For example, the helical reinforcing bar is wound and disposed in a spiral shape while contacting the outer circumference of each of the four main bars for forming parallel sides of girders, columns, and the like, as is well known.

In this case, in the conventional helical rebar made of structural steel wire having a strength of 30 to 40 Kgf / mm2, the bending process is easy and there is no fear of fear of the decrease in strength due to the bending process. Bending is performed at the inside diameter of the bending of 3d or more), and as shown in FIG. 9a, the spiral reinforcing bar F " is circumscribed to each of the main roots MM constituting each side, and is connected almost linearly. Could install as

On the other hand, in the case of the rectangular spiral reinforcing bars made of high-strength wire, as described above, the bending angle is not allowed to be bent in 3d in the case of the conventional tensile strength of 30 to 40 Kgf / mm 2, and as shown in FIG. 9B. The high-strength spiral reinforcing bars F 'are circumscribed in contact with each of the main roots MM constituting each side, and are installed in a state in which they are expanded slightly outward from a straight line connecting the two marked with broken lines.

By the way, when constructing girders, pillars, etc. made of reinforced concrete, the reinforcing bars packed in the concrete have a concrete covering (thickness of the concrete covering the surface of the rebar) of 3 cm or more in terms of corrosion protection. Is required by design.

Therefore, girders, columns, and the like, which are made of reinforced concrete constructed using straight high-strength spiral reinforcement, as described above, are installed in a state in which they are expanded from a straight line connecting the main MM to the outside as compared with the case of using a conventional helix reinforcement. It is easy to increase the finished dimension by a considerable amount, and in particular, the wire rod is inevitably larger in the rectangular high strength spiral reinforcing bar with a yield strength of 130 Kgf / mm2 or more.

In order to satisfy the requirements of corrosion resistance, girders, columns, etc., which have a large finished size, do not contribute to the improvement of strength in the strength calculation, but increase the use of concrete, exterior materials, paints, etc. Since it is to increase the solution, a solution was sought.

The present invention has been made to solve the above problems in high-strength spiral reinforcing rods, which are essential for shear reinforcement of girders, columns, and the like, which are made of reinforced concrete. It is an object of FIG. 1 to provide a method for producing a high strength spiral reinforcing wire rod which is easy to bend while maintaining.

In addition, the present invention is a high-strength spiral reinforcing bar made of a wire of 130Kgf / mm2 or more yield point strength that can be dramatically improved by the shear ultimate strength, such as girders, pillars, etc. made of reinforced concrete, the yield point strength of the wire including the bending part 130Kgf It provides high strength spiral reinforcing bars with the bending inside diameter of the bending processing part as small as possible, especially in the case of square high strength spiral reinforcing bars, columns, columns, etc. to be made of reinforced concrete in addition to the above. The second object is to reduce the size and achieve the reduction of the material and the weight reduction.

[Configuration of the First Invention]

The gist of the first invention of the present application is (1) when the wire is continuously heat treated to increase the strength, (2) a high frequency induction heating means is used for the final heating to complete the wire to a predetermined yield point strength, and (3 And (4) by heating the wire rod surface to a sufficiently high temperature in a very short time, and (4) quenching the wire rod after the heating, compared to the usual case in which the final heating is completed at the predetermined yield point strength. High strength spiral reinforcing steel, characterized in that the surface layer portion of the wire rod is less than or equal to the hardness required to maintain the predetermined yield point strength, and the center portion is more than the hardness required to maintain the predetermined yield point strength. It is a manufacturing method of a molten iron material.

The first invention is described below.

In increasing the strength by continuously heat treating the wire, it is well known that the final heating temperature determines the finished yield point strength of the wire.

Usually, the heating means for final heating includes induction heating, direct current resistance heating, or radiant heat. In the case of any heating means, the final heating temperature depends on the chemical composition of the wire rod, and according to the target yield yield strength, it is set in advance in the experiment or rule of thumb, respectively. It cools and completes to a predetermined yield point strength.

On the other hand, in the first invention of the present application, heating in hardening may use a desired heating means, but the final heating means is limited to high frequency induction heating and far exceeds the heating temperature for completing each of the usual predetermined yield point strengths. When the wire surface is heated up to a high temperature in a very short time, and the heating is completely conducted to the wire center and cooled before being averaged, for example, when the target yield yield strength of the wire is set to 100 Kgf / mm2, Even if the yield point strength is softened to about 80Kgf / mm2, the yield point strength at the center is maintained to be 120Kgf / mm2, so that the average yield point strength of the wire cross section is 100Kgf / mm2 or more. It is characterized by.

In this case, the final heating refers to the continuous hardening of the raw material wire, the tempering heating in the case of tempering, the hardening of the wire directly in the final process of hot wire rolling, and the reheating of the hardening wire continuously to the predetermined yield point. In the case of strength, the reheating is indicated.

In addition, it is well known to use a high frequency induction heating means for tempering heating in the case of continuously heat-treating the wire, but the conventional method is to position as one heating means for the wire rod cross section to a predetermined uniform temperature. It is not a usage with the above characteristics as in the present invention. Since an advantage contrasts with the conventional usage and the usage of this invention in the experiment example mentioned later, it will be understood clearly.

[Effect of First Invention]

The first invention of the present application softens the wire surface layer portion where the plastic strain at the bending process is maximum, and the center of the wire rod having the small plastic strain is made to have a hardness equal to or greater than the predetermined yield point strength, and as a result, while maintaining the predetermined yield point strength as an average of the entire cross section. In addition, it has the effect of finishing with a wire rod which is extremely excellent in bending workability.

[Configuration of Second Invention]

The gist of the second invention of the present invention is (1) a high-strength spiral reinforcing bar formed by forming a heat-treated wire rod into a spiral shape, and (2) the used wire rod has a high hardness at the center portion, a low hardness at the surface layer portion, and a bending processed portion at a predetermined curvature. The high strength spiral reinforcing bar is characterized in that the overall cross-sectional average yield point strength of the wire rod is maintained at 130 Kgf / mm 2 or more.

The second invention is described below.

As described above, in the high-strength spiral reinforcing bar made of a wire rod made of high strength at a yield point strength of 130 Kgf / mm 2 or more, bending processing is particularly difficult, and considering the reduction in strength due to bending processing, 5d or more of the inside diameter of the bending wire It is regulated that the current bending diameter of each spiral reinforcing bar of 7.4 ~ 13.0mm wire rod or the terminal bending part of circular spiral reinforcing bar has a bending inner diameter of 5d or more.

Conventional high-strength spiral reinforcing bars using wire rods having a high yield strength of 130 Kgf / mm2 or more are disclosed in the examples described later. However, when the bending inner diameter is bent to 4d in a manner that ignores the above restrictions, the bending process is performed. It is difficult to reliably secure the yield point strength of the portion to 130 Kgf / mm 2 or more, and when bending processing is performed in 3d, the yield point strength of the bending portion becomes 130 Kgf / mm 2 or less. The same can be said for the terminal bent portion of the circular high strength spiral rebar.

In the second invention, the high-strength spiral reinforcing rod made of the high-strength wire obtained in accordance with the first invention has a predetermined curvature, for example, a bending inside diameter of 4d or less of the wire diameter. It is characterized in that the overall yield average yield point strength including 130Kgf / ㎡ or more.

[Action of the second invention]

The second invention of the present application acts to ensure that the high strength spiral reinforcement reliably exerts a balanced shearing endurance with a yield point strength of 130 Kgf / mm 2 or more, and in addition to the above-described guaranteeing function for the square high strength spiral reinforcing bar, There is an action that does not inflate the reinforcement state between the main roots forming the side of the pillar to the outside.

The action on the reinforcement state between the main roots will be specified in specific values.

For example, according to the conventional method, a wire rod including a conventional high strength spiral reinforcing bar bent at 5.4 d by increasing the yield point strength to 130 Kgf / mm2 or more according to the conventional method, and a wire rod having an angled portion bent at 4 d according to the second invention of the present invention. Compared to the case where the square high strength spiral reinforcing bars having the overall cross-sectional average yield strength of at least 130 Kgf / mm2 are reinforced around the main group. FIG. 1 shows the conventional products installed in the main root group M such as girders and columns as dashed line F ', and the present invention is indicated by solid line F, but 0.14d on one side and 0.28d on both sides opposing each other, that is, planarly facing bar It is possible to narrow the gap so that it is slightly smaller than 4 mm, and as a result, the long cubic girder, the column, and the like can be thinly finished, so that the volume and weight reduction amount is large.

Experimental Example 1

The present inventors have conducted a number of experiments in the course of completing the present invention, some of which are described below.

Experiment Description: This example is a case where the high strength is applied by continuously hardening and tempering a material steel wire, and two kinds of steel wires having different chemical constituents are treated in a heat treatment line. The hardening in the heat treatment was heated to a different temperature corresponding to the chemical composition of each of the raw material steel wires, followed by quenching and hardening.

In the tempering heating by the high frequency induction heating means positioned as a feature of the first invention of the present application, the heating coil is applied to the wire rod traveling at a constant feed speed in producing specimens for each steel wire rod. The heating energy is the same for all specimens, but the pitch is changed to change the number of windings of the wound coils. As a result, the present invention method allows the heating energy application time to be shorter than in the case of the conventional method. A plurality of specimens with different heating temperatures and heating times were prepared. The reason for the constant heating energy is that consideration is given to completing each specimen with the same target tensile strength as later. In addition, the target tensile strength maintains the later target yield point strength in the high-strength spiral rebar of the second invention manufactured using the test specimen of the present invention, and the target yield point strength is equivalent to the mechanical properties specified in the PC steel bar standard. It is because it tries to complete it ideally. The test request is specified below.

* Material Steel Wire:

o Chemical Constituents: Class 2 steels A and B shown in Table 1

o Diameter of wire rod: Steel grades A and B were used to draw a diameter of 12mm to 11mm.

* Heat treatment line: The heat treatment line shown in FIG. 2 was used. In the drawings, reference numeral 1 denotes a pay-off stand, reference numeral 2 denotes a weaving machine, reference numeral 3 denotes a hardening device comprising a heating device 3a and a cooling jacket 3b, and reference numeral 4 denotes a high frequency induction heating means. The tempering device 5 consisting of the coil 4a and the cooling jacket 4b is a winding stand. In addition, in the heat treatment line, induction heating means was used as the heating device 3a in the hide device 3.

* Conditions for the production of specimens:

o Wire feed rate… 220mm / sec (same for all specimens)

o Hardening heating temperature: as indicated in the second table.

o Tempering: In tempering the wire rod after hydration, the number of turns indicated in the second table passes through different heating coils individually, changing the heating time and the heating temperature of the wire rod surface (but the applied heating energy is the same). After tempering heating, it was quenched and the specimen (a0) according to the conventional method for the emphasis A and the specimens (a1) to (a3) according to the present invention for the emphasis A, and the specimen (b0) and the pattern according to the conventional method for the steel grade B. The specimens (b1) to (b3) according to the invention method were produced.

Period: The interval between the heating coil 4a and the cooling jacket 4b is constant, and the interval is the distance at which the specimens a0 and b0 to be tempered and heated in accordance with the conventional method are brought to a full cross-sectional uniform temperature state and quenched. The contrast with the conventional method is aimed at.

o Tempering heating power source: same for all specimens.

Print… 200kw

frequency… 140KHz

o Target tensile strength of finished wire rod 150Kgf / ㎡

o Finish target yield strength of spiral reinforcing bars manufactured using specimen wires. 130Kgf / ㎡ or more

(The yield point strength specified as SBPR 130/145 or SBPD 130/145 of PC steel bar JIS G3109 standard, hereinafter referred to as 130Kgf / mm2 class.)

The heat treated specimens obtained according to the above conditions were submitted to various adequacy tests.

Hardness test: The hardness of the cut section of each specimen was measured using a microbeakers hardness tester. The measurement results are shown in FIGS. 3a to h.

Test results: For each specimen, the correlation between the maximum principal hardness, softening layer depth (distance from the surface below MHv 440 or less), and central hardness in the second table including test results were compared and examined. The specimens (a0) and (b0) were completed at approximately the same hardness over the entire cross section, whereas the specimens (a1) to (a3) and (b1) to (b3) each had a low surface hardness. The lower the surface hardness and the deeper the softened layer, the higher the hardness of the central portion of the specimen (a0) (b0) or higher according to the conventional method. It was expected to be completed with tensile strength.

* Mechanical property test: Each specimen was subjected to a tensile test according to the test method specified in the JIS standard to investigate the mechanical properties. Tensile test results of specimens (a2) and (b2), which are a part of them, are shown in FIGS. 4a and (b), and tensile strength, yield point strength, yield ratio, and breakage of each specimen obtained from the test results in FIG. Elongation and contraction are shown as a diagram summarizing steel grades A and B.

o Test results: It was confirmed from the second table of test specimens written for each specimen that each specimen was completed with approximately the target tensile strength. Also, compared to the specimens (a0) and (b0) according to the conventional method, even if the specimens (a3) and (b3) according to the present invention have a slight decrease in the yield point strength, they are of course higher than the target yield point strength, In the reduction, it became clear that it was almost the same as the conventional method specimen. In addition, between steel grades A and B, a difference of about 4% occurs in the yield ratio due to the difference in hardening heating temperature, and a difference in tempering heating elevated temperature occurs due to the difference in C content. It turned out.

From the above test results, it is demonstrated that even if each specimen according to the present invention method softens to the surface layer, the mechanical properties prescribed by the above-described standard 130 Kgf / mm 2 class are sufficiently exceeded.

Thus, the present inventors conducted the following experiments to investigate how the softened surface layer of each specimen according to the present invention contributes to the workability during bending, and to what extent plastic deformation caused by bending reduces the mechanical properties. It was done.

Experimental Example 2

* Bending process test: Each of the specimens (a0) to (b3) obtained according to the above (Test Example: 1) was attached to a 180 ° bending process to investigate the difficulty.

Bending process test method: The bending machine shown in Fig. 6a was used. The bending machine is provided with a jig 6 made of a fixed shaft member, a pressure bending roll 7 for planetary movement in the direction of the arrow while maintaining an interval corresponding to the outer circumference of the jig 6 and the wire diameter. Then, one end of the specimen indicated as the TP of a predetermined length is gripped by the chuck 8 and processed, and the center portion of the specimen TP is formed on the periphery of the jig 6 by the planetary motion of the bending roll 7 with respect to the jig 6. According to the bending process is possible. Thus, the specimens a0 and (a3) were bent as shown in FIG. 6B using the jig 6 having the inside diameters of 5d, 4d and 3d with respect to the diameter of the specimen TP of 11 mm. .

Test result: Even in the case of obtaining any bending angle, the conventional method specimens (a0) and (b0) have a high degree of processing difficulty, especially in the case of 3d, whereas the specimens of the present invention can be bent very easily. there was. In the case of the present invention, when bending with the same bending inner diameter, the deeper the depth of the softening layer, the easier the bending process, or the spring back becomes larger as the bending inner diameter becomes smaller from 5d to 4d and from 4d to 3d. In the case of 3d machining, it was found to be slightly more difficult than in the case of 4d machining.

Tensile Test of Bending Processed Parts: Each of the specimens (a0) to (b3) subjected to bending process was attached to a tensile test to investigate the mechanical properties after plastic working.

Test Method: The tensile tester shown in FIG. 7 was used. In the same drawing, reference numeral 81 denotes a chuck for holding both ends of the specimen, and reference numeral 9 denotes a jig for tensile testing adapted to each of the bending inner diameters 5d, 4d, and 3d. Since the load P is applied to each of both ends of the specimen TP, the bending 2 of the specimen TP abutting on the outer circumference of the jig 9 is loaded with the load 2P.

o Test results: Table 3 shows the breaking loads of specimens (a0) to (b3), and Figures 8a and (b) show the breaking loads indicated by specimens for each steel grade A and B, respectively. In the relationship between the tempering heating temperature and the breaking load of the base material (pre-bending specimen) (wherein, after the bending, the load (2P) is loaded, calculated as the tensile load X2 in Table 2), and the plastic working The intensity ratio is shown as a percentage of the decrease in strength, and the fourth table is a list thereof. From the above test results, it is natural that the strength decreases due to plastic working, but the reduction rate is the largest in the specimens according to the conventional method, and the reduction ratio of the specimens according to the present invention method is surprisingly small. In a2), (a3), and steel grade B, it was confirmed that, like the specimen (b3), the minimum heating time conditions were optimal for preventing the strength decrease by plastic working.

Comparison with standard values: Tensile test data of the specimens (a0) to (b3) after bending were used to compare the yield point strength of the standard 130 Kgf / mm2 class. The percentage of yield point strengths of public bodies (a0) to (b3) calculated using the yield point strength of 130 Kgf / mm2 standard as 100 is shown as the fifth table (in this case, however, similarly to the case where the strength ratio is calculated). , As standard value X2).

o Test results: In the table, the specimens (a0) and (b0) of conventional products after bending are above the standard value when the inside diameter of bending is 4d, but below the standard value when 3d is used. On the contrary, the specimens of the present invention are both standard values and above standard values even when the bending inner diameter is 4d and 3d. However, supposing that 4 to 5% of the above standard value ensures the stability in productivity, the bending processing to the bending inner diameter 4d is the same for specimens (a1), (a2), (a3) and (b1). The treatment under tempering heating conditions in (b2), (b2) and (b3) is optimal in maintaining the mechanical strength as well as the ease of bending processing, and the bending processing to the bending inner diameter 3d is slightly unsatisfactory in the case of steel wire A. In the case of steel grade B, it can be said that tempering heating conditions applied to the specimen (b3) are preferable in maintaining mechanical strength.

[Other Experimental Example]

According to the above (Experimental Example 1), tensile strengths of 120, 130, and 140 Kgf / mm 2 were respectively obtained according to the above (Experimental Example 1), and the bending was performed according to (Experimental Example 2). Tensile test results of the bending processing part performed on one specimen also showed that the local heating time conditions in which the imparting heating energy was constant were more effective in preventing the strength decrease by plastic working, as in the above (Experimental Example 2).

When the test results obtained from the above (Experimental Example 1) and (Experimental Example 2) and the test results obtained from the plurality of Experimental Examples are combined, the first invention of the present application corresponds to the diameter of the heating heating applied in the tempering heating. In addition, it sets the same as the conventional case corresponding to the completion target yield point strength, but by raising the surface of the study surface to extremely high temperature higher than the conventional one, the surface layer part has high hardness in the softening layer and the center, and the overall cross-sectional average yield point strength Since the wire rod was finished with a wire rod having a predetermined yield point strength, it was confirmed that the obtained wire rod was extremely abundant in bending workability while maintaining the predetermined yield point strength, and was an optimal method for producing a wire rod for high strength spiral rebar.

In addition, the test results demonstrated that the yield point strength of 130 Kgf / mm 2 or more was reliably ensured even in the high-strength spiral reinforcing bar provided as the second invention of the present application, which was bent to 4d or less.

The present invention also shows that the present invention can be carried out almost without being influenced by the wire steel grade.

[Application Example of the Invention]

In each of the above experimental examples, the raw material steel wire was hardened and tempered by a heat treatment line to increase the strength. However, in the present invention, the wire rod obtained by directly hydrating the wire rod in the final step of hot wire rolling is further reheated to obtain a predetermined yield point strength. Even in the case of the high-strength heat treatment step of completing the process, the reheating may be carried out under predetermined heating conditions in the same manner as in the experimental example, and the same effects and effects may be obtained.

As described above, the present invention only allows the final heating to be a predetermined condition, thereby making it possible to manufacture a high-strength spiral reinforcing wire rod that is easy to bend while maintaining a predetermined high yield point strength, thereby eliminating complaints during bending. In addition, high-strength spiral reinforcing bars made of high strength wire rods with a yield strength of 130 Kgf / mm2 or more, which can dramatically improve shear endurance of girders and columns made of reinforced concrete, are easily bent and formed at a bending angle of a predetermined curvature. In the case of circular spiral reinforcement, the adhesion to the main bar of the terminal bent portion is increased, and in the case of the rectangular spiral reinforcing bar, the beams made of reinforced concrete, such as girders and pillars, are made of straight rectus bars By reducing the finished outer dimension of the material, it is possible to reduce the material and light weight. In addition, the strength of the wire rod including the bending processing portion is assuredly ensured.

Since the effect acts synergistically and contributes to the increase and spread of the reliability of the high strength spiral rebar, the effect brought by the present invention is remarkable.

[Table 1]

[Table 2]

[Table 3]

[Table 4]

[Table 5]

Claims (4)

In the case where the wire is continuously heat treated to increase the strength, high frequency induction heating means is used for the final heating for completing the wire at a predetermined yield point strength, and the final heating is completed in the predetermined yield strength. By setting the temperature of the wire rod surface to a temperature that is extremely short and sufficiently high in time, and rapidly cooling the wire rod after the heating, the surface layer portion of the obtained wire rod is less than the hardness required to maintain the predetermined yield point strength, and the center portion maintains the predetermined yield point strength. A method of manufacturing a high strength spiral reinforcing wire rod, characterized in that the hardness of the wire rod is greater than or equal to the hardness required, and the average yield point strength of the entire cross section of the wire rod maintains a predetermined yield point strength. The manufacturing method of the high strength spiral reinforcing wire rod of Claim 1 whose final heating is said tempering heating at the time of hardening and tempering a raw material steel wire. The method for producing a high strength spiral reinforcing wire rod according to claim 1, wherein the final heating is a tempering heating performed on the wire rod after directly hardening the hot rolled wire rod. In the high-strength spiral reinforcing steel rod formed by forming a heat-treated wire rod into a spiral shape, the used wire rod has a high hardness at the center portion, a low hardness at the surface layer portion, and maintains the overall cross-sectional average yield point strength of 130 Kgf / m² or more including the bent portion at a predetermined curvature. High-strength spiral reinforcement, characterized in that.

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