JP5315790B2 - High strength PC steel wire with excellent delayed fracture resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength PC steel wire having &ge;2,000 MPa strength and especially excellent in a delayed crack resistance characteristic with respect to the PC steel wire to be used for a prestressed concrete or the like, and to provide a manufacturing method therefor. <P>SOLUTION: As for the high strength DC steel wire excellent in the delayed crack resistance characteristic and the manufacturing method therefor, the PC steel wire contains, by mass%, 0.9-1.2% C, 0.01-1.5% Si, 0.2-1.5% Mn, 0.001-0.05% Al, 0.0005-0.010% N and the balance Fe with inevitable impurities, is composed of &ge;90% wire-drawing pearlite and &le;10% ferrite and bainite structures. and has &ge;2,000 MPa tensile strength. When the wire diameter of the PC steel wire is defined as D, the ratio (Hv surface/Hv inner part)of the surface layer Hv hardness (Hv surface) of 0.1D range (surface layer part) from the surface of the PC steel wire to the inner part Hv hardness (Hv inner part) of the inside range (inner part) from the surface layer part, is &le;1.1. Further, it is possible to contain one or more kinds among Cr, Mo, V, Ni, Cu, B and/or one or more kinds among Ti, Nb, Zr. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a PC steel wire and a PC steel stranded wire used for prestressed concrete and the like, and particularly to a high strength PC steel wire having excellent delayed fracture resistance and a strength of 2000 MPa or more.

  PC steel wire is manufactured through a pearlite transformation treatment (patenting treatment) of a high carbon steel wire such as a piano wire, followed by a drawing process followed by a blueing process in which heating at about 300 ° C. is performed. With the reduction in cost of prestressed concrete and the practical application of high-strength concrete, there is an increasing demand for high-strength PC steel wires.

  Although the strength of PC steel wire can be increased by increasing the strength after pearlite transformation and increasing the wire drawing strain, there is a problem that the delayed fracture resistance and ductility deteriorate. Conventionally, high carbon steel wires such as PC steel wires have been known as materials with superior delayed fracture resistance compared to tempered martensitic steels. Delayed fracture characteristics are reduced, and the risk of delayed fracture is greatly increased.

  On the other hand, as a conventional technique for improving the delayed fracture resistance of PC steel wire, Patent Document 1 discloses a technique for applying compressive residual stress to the surface portion of PC steel wire, and Patent Documents 2 and 3 describe the structure of PC steel wire. A technique that defines the shape of plate-like cementite as a factor, Patent Document 4, proposes a structure control technique for PC steel wires or PC steel bars. Although these techniques are effective in improving the delayed fracture resistance of PC steel wire, as is clear from the examples of the above patent documents, the strength of PC steel wire is less than 2000 MPa, and more than 2000 MPa. No mention is made of delayed fracture resistance of high strength PC steel wires. Thus, in the prior art, there was a limit to improving the delayed fracture resistance of a high strength PC steel wire having a strength of 2000 MPa or more.

JP 2004-131797 JP 2004-360005 A Japanese Patent Laid-Open No. 2004-323870 Japanese Patent Laid-Open No. 11-229090

  The present invention has been made in view of the above situation, and in a high-strength PC steel wire having a strength of 2000 MPa or more, it has excellent delayed fracture resistance, and has an extremely low risk of causing delayed fracture even in an actual environment. An object is to provide a high-strength PC steel wire of 2000 MPa or more and a manufacturing method thereof.

The present invention is effective in improving the delayed fracture resistance of high-strength PC steel wire of 2000 MPa or more, and further in improving ductility, and reducing the Hv hardness of the surface portion of the PC steel wire is effective. In order to achieve both resistance and delayed fracture resistance, the ratio of the surface layer Hv hardness (Hv table) to the internal Hv hardness (within Hv) (within Hv table / Hv) must be controlled to 1.1 or less. Has been made based on the knowledge that is extremely effective, and the gist thereof is as follows.
(1) By mass%, C: 0.9-1.2%, Si: 0.01-1.5%, Mn: 0.2-1.5%, Al: 0.001-0.05%, N: 0.0005-0.010%, the balance being Fe and inevitable PC steel wire consisting of mechanical impurities, 90% or more of drawn pearlite, 10% or less of ferrite and bainite structure, and having a tensile strength of 2000 MPa or more. The ratio of the surface layer Hv hardness (Hv table) in the 0.1D region (surface layer part) to the inner Hv hardness (inside Hv) in the region (inside) inside the surface layer part from the surface of the PC steel wire High-strength PC steel wire with excellent delayed fracture resistance, characterized in that (in Hv table / Hv) is 1.1 or less.
(2) In mass%, Cr: 0.01-2.0%, Mo: 0.01-0.5%, V: 0.01-0.5%, Ni: 0.01-1.0%, Cu: 0.01-0.5%, B: 0.0001-0.005% The high strength PC steel wire excellent in delayed fracture resistance according to (1), characterized by containing one or more of the above.
(3) by mass%, further containing one or more of Ti: 0.001 to 0.05%, Nb: 0.001 to 0.07%, Zr: 0.001 to 0.07% (1) or (2 ) High strength PC steel wire with excellent delayed fracture resistance .

  According to the present invention, it is possible to provide a high-strength steel wire having excellent delayed fracture resistance and ductility with a strength of 2000 MPa or more and a method for producing the same.

  First, in order to improve the delayed fracture resistance of high strength PC steel wire, the present inventors, steel material composition, pearlite transformation treatment conditions, wire drawing conditions, brewing conditions, the influence of PC steel wire hardness distribution, etc. We investigated in detail. As a result, in order to improve the delayed fracture resistance of high-strength PC steel wire, the surface Hv hardness (Hv table) in the region (surface layer part) 0.1D from the surface in the PC steel wire with a wire diameter of D It has been found that it is important to control the internal Hv hardness (inside Hv) ratio (inside Hv / Hv, hereinafter referred to as Hv hardness ratio) of the inner region (inside) of the surface layer portion to 1.1 or less. Furthermore, the manufacturing method which makes Hv hardness ratio 1.1 or less, ie, the technique of the optimal pearlite transformation process and bluing process, was established.

  The high-strength PC steel wire excellent in delayed fracture resistance of the present invention has an Hv hardness ratio, that is, when the PC steel wire diameter is D, the Hv hardness in the region of 0.1D (surface layer part) from the surface. The greatest feature is that the ratio of Hv hardness (within Hv) (Hv table / within Hv, Hv hardness ratio) between the (Hv table) and the inner region (inside) of the surface layer portion is 1.1 or less. The reason for this limitation will be described below.

  FIG. 1 is an example of a cross-sectional Hv hardness distribution in a PC steel wire having a strength of 2000 MPa or more. Usually, as the strength of PC steel wire increases, the amount of heat generated during wire drawing increases. As a result, age hardening due to processing heat generation becomes remarkable as in example A (● mark) in FIG. 1, and in particular, the increase in the hardness of the surface layer portion of the PC steel wire becomes remarkable. It was found that the delayed fracture resistance and ductility of the PC steel wire with the hardness distribution as in example A deteriorated. On the other hand, it was found that the delayed fracture resistance and ductility are extremely effective even with the same strength in the PC steel wire in which the increase in Hv hardness of the surface layer portion as in Example B (□ mark) is suppressed. Delayed fracture resistance tends to deteriorate as the strength of the steel increases.On the other hand, cracking of delayed fracture occurs near the surface of the steel wire, so the Hv hardness of the surface of the steel wire is large in the delayed fracture resistance. It is estimated that it has an influence. Here, when the Hv hardness ratio exceeds 1.1, the delayed fracture resistance and ductility deteriorate, so the upper limit of the Hv hardness ratio is limited to 1.1 or less. From the viewpoint of further improving the delayed fracture resistance of the high strength PC steel wire, a more preferable Hv hardness ratio is 0.95 or less. Hv hardness ratio in the present invention is a hardness test under the condition that the load is 50 gf (0.49 N), the hardness of 10 points or more is measured on both the surface layer and inside, and the ratio of the average values is obtained. It is.

  Next, the reasons for limiting the components of the steel that is the subject of the present invention will be described. Hereinafter, mass% is simply expressed as%.

  C is an indispensable element for achieving high strength of PC steel wire, but if it is less than 0.9%, it is difficult to increase the strength after pearlite transformation treatment. Even if measures are taken, it is difficult to realize a high strength PC steel wire of 2000 MPa or more, which is the object of the present invention, so the lower limit of C is limited to 0.9%. On the other hand, even if added over 1.2%, the above effect is saturated, and pro-eutectoid cementite that deteriorates wire drawing workability and delayed fracture resistance during pearlite transformation treatment is likely to precipitate, so the upper limit is 1.2% Restricted to.

  Si has the effect of increasing the strength after the pearlite transformation treatment by the solid solution strengthening effect, and also has the effect of preventing the strength reduction of the PC steel wire during the high temperature blueing treatment above 450 ° C. When the addition amount of Si is less than 0.01%, the above-mentioned effect cannot be exhibited.On the other hand, even if it exceeds 1.5%, an effect commensurate with the addition amount cannot be expected, and further, decarburization of the surface portion of the PC steel wire increases. Limited to 1.5% range. From the viewpoint of increasing the strength of the PC steel wire, the preferable Si addition range is 0.5 to 1.5%.

  Mn is an element effective not only for deoxidation and desulfurization but also for increasing the strength after pearlite transformation treatment. However, if the amount of Mn added is less than 0.2%, the above effect cannot be obtained.On the other hand, even if added over 1.5%, an effect commensurate with the amount added cannot be obtained, so the range is limited to 0.2 to 1.5%. .

  Al has the effect of preventing austenite grains from coarsening by forming AlN during heating in deoxidation and pearlite transformation treatment. However, when the addition amount of Al is less than 0.001%, these effects are insufficient, and even if added over 0.05%, the effect is saturated, so the content is limited to a range of 0.001 to 0.05%.

  N has an effect of making the austenite grains finer by forming nitrides of Al, Ti, Nb, and Zr. When the amount of N added is less than 0.0005%, this effect is slightly insufficient. When it exceeds 0.010%, the ductility of the PC steel wire is lowered, so the content is limited to the range of 0.0005 to 0.010%.

  The above is the basic component of the high strength PC steel wire excellent in delayed fracture resistance of the present invention.In the present invention, the strength after pearlite transformation treatment is increased, the work hardening rate in wire drawing is increased, the high temperature From the viewpoint of preventing strength reduction at the time of blueing treatment and improving delayed fracture resistance, one or more of Cr, Mo, V, Ni, Cu, B is used from the viewpoint of austenite grain refinement, One or more of Ti, Nb and Zr can be contained.

  Cr has an effect of increasing the strength after the pearlite transformation treatment. In addition, since it has the effect of increasing the work hardening rate during wire drawing, it is possible to achieve high strength of PC steel wire even if there is little wire drawing distortion. There is also the effect. Furthermore, it has the effect of preventing strength reduction during high temperature bluing treatment, and is an effective element for improving delayed fracture resistance. If the amount of Cr added is less than 0.01%, the above effect is somewhat insufficient, and even if added over 2.0%, the effect is saturated, and the pearlite transformation finish time during pearlite transformation treatment is prolonged, producing Therefore, it is desirable that the content be in the range of 0.01 to 2.0%. From the viewpoint of the above action, the preferable Cr addition amount is 0.1 to 1.0%.

  Mo increases the hardenability, thereby increasing the strength after the pearlite transformation treatment and preventing the strength from being lowered during the blueing treatment. If Mo is less than 0.01%, the above effect cannot be expected. On the other hand, even if added over 0.5%, the effect is saturated, so a range of 0.01 to 0.5% is desirable.

  V has the effect of increasing the strength after the pearlite transformation treatment by precipitation strengthening, and also has the effect of preventing the strength reduction of the high temperature bluing treatment. Furthermore, it is an effective element for improving the delayed fracture resistance of high strength PC steel wires. If the addition amount of V is less than 0.01%, the above effect is slightly insufficient, and even if added over 0.5%, the effect is saturated, and the pearlite transformation treatment time becomes long. Is desirable.

  Ni has an effect of improving hardenability and is an element effective in increasing the strength after pearlite transformation treatment. It is also an effective element for improving delayed fracture resistance because it has the effect of suppressing the amount of hydrogen entering the PC steel wire from the environment in prestressed concrete. If the addition amount of Ni is less than 0.01%, the above effect cannot be sufficiently exerted, and even if it is added in excess of 1.0%, the effect is saturated. Therefore, the range of 0.01 to 1.0% is desirable.

  Cu has an effect of improving hardenability and is an effective element for increasing the strength after pearlite transformation. Furthermore, like Ni, it has the effect of suppressing the amount of hydrogen entering the PC steel wire from the environment in prestressed concrete, so it is an effective element for improving delayed fracture resistance. In addition, the effect of suppressing intrusion hydrogen is further improved by adding Ni and Cu in combination. If the addition amount of Cu is less than 0.01%, the above effect cannot be sufficiently exerted, and even if added over 0.5%, the effect is saturated. Therefore, the range of 0.01 to 0.5% is desirable.

  B has an effect of improving hardenability and is an element effective for increasing the strength after the pearlite transformation treatment. If the addition amount of B is less than 0.0001%, the above effect cannot be exerted, and if adding over 0.005%, a large amount of B compound is easily generated and the ductility of the PC steel wire is lowered, so the range is 0.0001 to 0.005%. It is desirable.

  Ti has the effect of preventing coarsening of austenite grains by deoxidizing and forming Ti carbonitride. As a result, the wire drawing workability after the pearlite transformation treatment is improved and the element is effective for increasing the ductility of the PC steel wire. However, if the amount of Ti added is less than 0.001%, these effects are slightly insufficient, and if added over 0.05%, coarse Ti carbonitride tends to be formed, and wire drawing workability and delayed fracture resistance are reduced. Therefore, it is desirable to set it as 0.001 to 0.05% of range.

  Nb is an element having an effect similar to that of Ti, and is an effective element for refining austenite grains by generating Nb carbonitride. In addition, this effect is effective in improving wire drawing workability and ductility. However, if the amount of Nb added is less than 0.001%, the above effect is somewhat insufficient. On the other hand, if it exceeds 0.07%, this effect is saturated, so it is desirable that the content be in the range of 0.001 to 0.07%.

  Zr is an element having the same effect as Ti and Nb. The effect of refining austenite grains by carbonitride of Zr has the effect of improving the drawing workability after pearlite transformation and the ductility of PC steel wire. If the amount of Zr added is less than 0.001%, the above effect cannot be sufficiently exerted. On the other hand, even if added over 0.07%, the effect is saturated, so it is desirable that the content be in the range of 0.001 to 0.07%.

  P and S are inevitable impurities, and although not particularly limited, 0.02% or less is each preferable range from the viewpoint of improving delayed fracture resistance of a high-strength PC steel wire.

  The high-strength PC steel wire having excellent delayed fracture resistance according to the present invention comprises 90% or more drawn pearlite, 10% or less ferrite and bainite structure. In high-strength PC steel wires with a strength of 2000 MPa or more, if the fraction of non-pearlite structure such as ferrite and bainite increases, delayed fracture resistance deteriorates and it is difficult to ensure ductility. The fraction of 90% or more is necessary. A more preferable fraction of the drawn pearlite structure is 95% or more. The structure fraction of the drawn pearlite is a value obtained by taking a photograph of 10 fields of view or more with a scanning electron microscope at a magnification of 5000, measuring the area fraction of the drawn pearlite structure by image processing, and calculating the average value thereof. It is.

Further, the delayed fracture characteristics of the PC steel wire in the present invention are evaluated under the following conditions. After dipping the PC steel wire in a 20% NH ₄ SCN solution at 50 ° C, a static load of 70% of the tensile breaking load of the PC steel wire is applied, and the breaking time is obtained. During the delayed fracture test, the NH ₄ SCN solution temperature is controlled at 50 ° C. The test time was 50 hours at the maximum, and if the fracture time was 50 hours or more, the delayed fracture resistance was evaluated as good.

  Next, the manufacturing method of this invention is demonstrated. The PC steel wire of the present invention is manufactured by a process of pearlite transformation treatment (patenting treatment), wire drawing, and bluing treatment.

  In the pearlite transformation treatment, reheating or rapid cooling is performed after hot rolling of the wire, and the pearlite transformation is held in a temperature range where pearlite transformation is performed.

  In the case of reheating, if the heating temperature is less than 900 ° C, austenitization is somewhat insufficient, while if it exceeds 1100 ° C, austenite grains become coarse and wire drawing workability decreases. Restricted to 900-1100 ° C. A more preferable condition is 900 to 1000 ° C. from the viewpoint of austenite grain refinement. After reheating, the normal manufacturing method is to quench and hold in the temperature range of 550 to 600 ° C and perform pearlite transformation treatment, but in the present invention, it is held at 600 to 650 ° C and subjected to partial pearlite transformation treatment. Subsequently, it is characterized in that the pearlite transformation is completed at 540 to less than 600 ° C. The reason for this is that in the first stage pearlite transformation treatment, the surface layer portion has a pearlite structure with a rough lamellar spacing, and in the second stage pearlite transformation, the inside has a fine pearlite structure.

  If the first stage pearlite transformation temperature is less than 600 ° C, it is difficult to make the Hv hardness ratio of the PC steel wire after wire drawing and brewing 1.1 or less, and if it exceeds 650 ° C, the lamellar spacing is coarse. Therefore, the range of the first stage pearlite transformation treatment temperature is limited to 600 to 650 ° C., because it becomes difficult to reduce the wire drawing workability and increase the strength of 2000 MPa or more. Moreover, it is a preferable condition that the pearlite transformation fraction is 10 to 50% in the first stage pearlite transformation treatment.

  If the pearlite transformation temperature of the second stage is less than 540 ° C, bainite that deteriorates the wire drawing workability and delayed fracture resistance tends to occur, while at 600 ° C or more, the lamellar spacing becomes coarse. Since it was difficult to increase the strength of the steel wire, it was limited to less than 540 to 600 ° C.

  The holding time for performing the pearlite transformation varies depending on the austenite grain size after reheating, the chemical composition, the pearlite transformation temperature, and the type of heat treatment furnace such as a lead bath furnace, a salt bath furnace, or a fluidized bed furnace. Furthermore, the austenite particle size varies depending on the chemical component, the reheating temperature, and the retention time. For this reason, the retention time of the pearlite transformation treatment is not particularly limited, but from the viewpoint of controlling the pearlite transformation fraction to 10 to 50%, 5 to 30 seconds is a preferable range in the first stage pearlite transformation treatment. The holding time of the second stage pearlite transformation treatment is preferably in the range of 10 to 120 seconds from the viewpoint of completing the pearlite transformation.

  Moreover, when performing a pearlite transformation process immediately after hot rolling, it is necessary to make finishing rolling temperature into 700-950 degreeC. The reason for this is to reduce the hardenability by reducing the austenite grain size of the wire surface layer by lowering the finish rolling temperature. It is a preferable condition that the austenite grain size in the region of 1 mm from the wire surface layer is 10 μm or less. The wire material austenite grain size may be coarse. As a result, the structure of the surface portion of the wire after the pearlite transformation treatment becomes a rough pearlite structure, and finally the Hv hardness ratio of the PC steel wire can be made 1.1 or less. Here, if the finish rolling temperature is less than 700 ° C, the deformation resistance during hot rolling is too large and hot rolling is difficult, whereas if it exceeds 950 ° C, it becomes difficult to refine the austenite crystal grains in the surface layer. The finish rolling temperature range was limited to 700-950 ° C. Preferred conditions are 700-850 ° C.

After finish rolling, the hot-rolled wire is cooled to a temperature range of 500 to 600 ° C. and subjected to pearlite transformation treatment. Here, in cooling rate is less than 10 ° C. / sec coarse pearlite structure is likely to occur during cooling, for drawing workability is lowered, preferably the cooling rate is over 10 ° C. / second or more. In addition, when the pearlite transformation temperature is less than 500 ° C, the occurrence frequency of bainite structure that deteriorates wire drawing workability and delayed fracture resistance increases. On the other hand, when it exceeds 600 ° C, it becomes a coarse pearlite structure and increases the strength of PC steel wire. Therefore, the range of the pearlite transformation temperature was limited to 500 to 600 ° C. The reason why the optimum pearlite transformation temperature after hot rolling is different from the reheated pearlite transformation temperature is that, after hot rolling, the pearlite transformation treatment is performed not in a single wire but in a ring shape after winding.

  In the present invention, wire drawing is performed using the wire material that has been subjected to the above-described pearlite transformation treatment. The wire drawing distortion varies depending on the wire drawing conditions such as the strength after pearlite transformation, the component that changes the work hardening rate during wire drawing, the area reduction rate of each die, and the wire drawing speed. In the chemical composition and pearlite transformation treatment conditions, the true strain is in the range of 1.5 to 2.1. Here, the true strain of wire drawing is a value represented by 2 × ln (wire diameter before wire drawing / wire diameter after wire drawing) (ln represents a natural logarithm).

  In the present invention, the blueing process is performed after the wire drawing. There are two methods for the bluing process of the present invention. The bluing treatment at 250 ° C. to 450 ° C. is a commonly used bluing treatment, and the purpose is to improve the ductility and relaxation properties of the PC steel wire. When the temperature of bluing treatment is less than 250 ° C., the above effect cannot be exhibited. When the temperature exceeds 450 ° C., the amount of strength reduction after the bluing treatment increases, so the temperature is limited to 250 ° C. to 450 ° C. The bluing treatment time is not particularly limited because it varies depending on the heating method such as furnace heating, high frequency heating, salt bath heating, lead bath heating, etc., but 2 seconds to 2 minutes is a preferable condition.

  The other bluing treatment method is a method in which the bluing treatment is carried out at 250 to 450 ° C. after being kept in a temperature range of more than 450 ° C. to 650 ° C. for 2 to 30 seconds. This two-stage bluing process is a normal patenting process, that is, a pearlite transformation process is performed at 550-600 ° C after heating to 900-1100 ° C, or finish rolling at around 1000 ° C, which is normal by hot rolling. This is effective for improving delayed fracture resistance and ductility even when a pearlite transformation treatment is performed later and then wire drawing is performed. The first stage heating is aimed at reducing the Hv hardness of the PC steel wire surface layer, and the Hv hardness ratio is 1.1 or less. If the first stage heating is 450 ° C or less, it is difficult to reduce the Hv hardness ratio to 1.1 or less.On the other hand, if it exceeds 650 ° C, the amount of strength decrease becomes too large, and it is difficult to increase the strength to 2000 MPa or more. Therefore, it was limited to a temperature range of more than 450 ° C to 650 ° C. Further, when the heating time is less than 2 seconds, the above-described effects cannot be sufficiently exhibited. On the other hand, when the heating time is longer than 30 seconds, the strength is significantly reduced. The first stage heating method is preferably rapid heating because it is important to heat only the surface portion of the PC steel wire. Specifically, high-frequency heating, salt bath heating, and lead bath heating are preferable instead of furnace heating.

  The second stage heating at 250 to 450 ° C. performed following the first stage heating is the same as the normal bluing treatment, and is for heating to the center of the PC steel wire. The reason for limitation is as described above.

  Hereinafter, the effect of the present invention will be described more specifically with reference to examples.

  Using a steel material having the chemical components shown in Table 1, the wire diameter was finished to 13 mm by hot rolling. When the pearlite transformation treatment was performed immediately after hot rolling, the finishing rolling temperature and the pearlite transformation temperature shown in Tables 2 and 3 were used. The pearlite transformation treatment was performed by dipping in a salt bath at a cooling rate of 10 ° C./second or more after hot rolling. In addition, the pearlite transformation treatment by reheating after hot rolling was performed under the conditions shown in Tables 2 and 3, and a lead bath was used. The immersion time in the salt bath and lead bath was 10 to 135 seconds. Thereafter, pickling and zinc phosphate coating treatment were performed, the wire diameter was finished to 5 mm by dry drawing, and blueing treatment was performed under the conditions shown in Tables 2 and 3. The blueing treatment was performed by high-frequency heating in both the first stage and the second stage.

Tensile strength after pearlite transformation and after bluing treatment was determined by performing a tensile test in accordance with JIS Z 2241. The hardness test was performed according to JIS Z 2244. For the Hv hardness ratio, test load is the surface hardness (Hv table) in the 0.1D area (surface layer) from the surface of the PC steel wire, and the internal hardness (inside Hv) of the area inside the PC steel wire surface (inside) 20 points were randomly measured under the condition of 50 gf (0.49 N), and the average value ratio (within Hv table / Hv) was obtained. The ductility of the PC steel wire after the blueing treatment was evaluated using a torsion test and evaluated by the number of times until breakage. The torsion test was performed under the condition that the holding interval was 500 mm. The number of times to break in the torsion test was evaluated as good (◯ mark) when it was 12 times or more, and evaluated as poor (x mark) when less than 12 times. Delayed fracture resistance of PC steel wire after bluing treatment is as follows: PC steel wire is immersed in 20% NH ₄ SCN solution at 50 ° C, and then a static load of 70% of the tensile breaking load of PC steel wire is applied. And it evaluated by the breaking time. The test time was 50 hours at maximum, and the delayed fracture resistance was evaluated as good when the fracture time was 50 hours or longer.

  In addition, as for the structure of the PC steel wire, 10 fields of view were photographed at a magnification of 5000 using a scanning electron microscope, the area fraction of the drawn pearlite structure was measured by image processing, and the average value was obtained. Except for Test No. 29 in Table 3, the area fraction of the drawn pearlite structure was 96 to 99%, which was good.

  Tables 2 and 3 show the results of the strength after pearlite transformation, the strength of the PC steel wire, the HV hardness ratio, the ductility, and the delayed fracture time. Test Nos. 1 to 25 in Table 2 are examples of the present invention, and Test Nos. 26 to 39 in Table 3 are comparative examples.

  As shown in Table 2, in all of the inventive examples, the Hv hardness ratio is in the optimum range, the strength of the PC steel wire is 2000 MPa or more, the ductility is good, and the fracture time of delayed fracture is 50. More than an hour, high strength PC steel wire with excellent delayed fracture resistance and ductility has been realized.

  In contrast, Nos. 26 and 27, which are comparative examples, are examples of inappropriate chemical components. That is, No. 26 is an example in which the strength of the PC steel wire does not reach 2000 MPa or more because the C content is less than the range of the present invention. No. 27 is an example in which pro-eutectoid cementite precipitated during the pearlite transformation because the C content was larger than the range of the present invention, and was broken in the wire drawing process.

  Comparative examples Nos. 28, 31, and 39 are examples in which a PC steel wire was manufactured using a conventional manufacturing method. No. 28 has a finish rolling temperature of 1060 ° C, and the austenite grain refinement of the wire surface layer is insufficient, so the Hv hardness ratio exceeds 1.1. As a result, good delayed fracture resistance and ductility are ensured. This is an example that could not be done. Nos. 31 and 39 are cases where pearlite transformation treatment was performed by reheating, and both are examples in which the Hv hardness ratio exceeded 1.1 and the delayed fracture resistance and ductility were reduced.

  Comparative examples No. 29, 30, 32, and 33 are all examples in which the pearlite transformation treatment conditions are inappropriate. That is, in No. 29, since the pearlite transformation temperature after hot rolling is too low, the microstructure fraction of the drawn pearlite is reduced to 88% due to the formation of bainite, and the Hv hardness ratio, delayed fracture resistance This is an example of poor ductility. No. 30 is an example in which undissolved carbides exist because the heating temperature at the time of reheating was too low, and as a result, the delayed fracture resistance and ductility deteriorated. No. 32 is an example where the second stage pearlite transformation temperature was too high, and No. 33 was the first stage pearlite transformation temperature too high, so the strength of the PC steel wire was less than 2000 MPa in both cases. .

  Furthermore, Nos. 34 to 38 of the comparative examples are examples in which the bluing treatment conditions after wire drawing are inappropriate. No. 34 is an example in which the Hv hardness ratio exceeded 1.1 because the first stage bluing temperature was too low, and the delayed fracture resistance and ductility were poor. On the other hand, No. 35 is an example in which the strength reduction amount increased and the strength did not reach 2000 MPa because the first stage bluing temperature was too high. No. 36 is an example where the second stage bluing temperature was too high, resulting in a large amount of strength reduction and high strength. No. 36 is an example where the Hv hardness ratio exceeded 1.1 because the bluing temperature in the second stage was too low, and the effect of improving delayed fracture resistance and ductility was small. No. 38 is an example in which the blueing treatment after wire drawing was not performed, and as a result, the Hv hardness ratio was high and the delayed fracture resistance was poor.

It is a figure which shows an example analyzed about the Hv hardness distribution of the cross section of a high strength PC steel wire.

Claims (3)

% By mass
C: 0.9-1.2%,
Si: 0.01-1.5%,
Mn: 0.2-1.5%
Al: 0.001 to 0.05%,
N: 0.0005-0.010%
PC steel wire comprising the balance of Fe and inevitable impurities, 90% or more of drawn pearlite, 10% or less of ferrite and bainite structure, and a tensile strength of 2000 MPa or more. When the wire diameter of the PC steel wire is D, the surface Hv hardness (Hv table) in the 0.1D region (surface layer portion) from the surface of the PC steel wire and the inside of the region (inside) inside the surface layer portion High strength PC steel wire with excellent delayed fracture resistance, characterized by Hv hardness (within Hv) ratio (Hv table / within Hv) of 1.1 or less. % By mass,
Cr: 0.01-2.0%,
Mo: 0.01-0.5%,
V: 0.01-0.5%
Ni: 0.01-1.0%,
Cu: 0.01-0.5%,
B: 0.0001-0.005%
2. The high-strength PC steel wire with excellent delayed fracture resistance according to claim 1, characterized by containing one or more of the following. % By mass,
Ti: 0.001 ~ 0.05%,
Nb: 0.001 to 0.07%,
Zr: 0.001 ~ 0.07%
The high-strength PC steel wire excellent in delayed fracture resistance according to claim 1 or 2, characterized by containing one or more of the following.

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