JP3348188B2 - High-strength PC steel rod and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a PC steel rod which is widely used as a reinforcing material for prestressed concrete structures such as poles, piles, buildings, bridges, etc., and has particularly good spot weldability. Strength 1450MP
The present invention relates to a high-strength PC steel rod excellent in delayed fracture characteristics as described above and a method for producing the same.

[0002]

2. Description of the Related Art PC steel materials widely used as reinforcements for prestressed concrete structures such as poles, piles, buildings, bridges, etc. are usually PC steel wires and PC steel strands specified in JIS G3536, and JIS. G31
PC steel bars specified in No. 09 are used. The material used for the PC steel wire is a piano wire material conforming to JIS G3502, and is manufactured by performing a patenting process and then drawing. On the other hand, a PC steel rod is manufactured by quenching and tempering using medium carbon steel having a C content of 0.25 to 0.35% as described in, for example, Japanese Patent Publication No. 5-41684. Have been.

[0003] Although the strength of a PC steel wire is higher than that of a PC steel rod, there is a disadvantage that spot welding cannot be performed due to a high C content. On the other hand, the spot weldability of the PC steel rod is better than that of the PC steel wire, but it is described in “Prestressed Concrete Design and Construction Standards and Explanations” (edited by the Architectural Institute of Japan, Maruzen), pp. 43-45. As described above, a high-strength PC steel rod having a strength exceeding 1275 MPa (130 kgf / mm ² ) has inferior delayed fracture characteristics as compared with a PC steel wire. Further, as described in Japanese Patent Publication No. 5-59967, the spot welded portion is rapidly cooled, so that it has a structure mainly composed of martensite. As a result, delayed fracture easily occurs in the spot welded portion.

As a conventional finding for improving delayed fracture characteristics of PC steel rods, for example, Japanese Patent Publication No. 5-59967 proposes that it is effective to reduce the P and S contents. Certainly, low P and low S are effective against delayed fracture, but the P and S contents of the current PC steel rods are already around 0.01%, and are specified in JIS G3109. In fact, it is at a level lower than the amount. Although it is possible to further reduce the P and S contents, the production cost increases. In addition, Japanese Patent Publication 5-41684
Japanese Patent Laid-Open Publication No. 2000-133873 proposes that the content of Si and Mn be regulated and that, after quenching, bending or drawing be performed in a tempering step. However, no mention is made of spot weldability and delayed fracture characteristics of spot welds.

On the other hand, as a technique for improving the spot weldability, for example, Japanese Patent Application Laid-Open No. Hei 4-247825 discloses a technique that regulates the amount of steel components and limits the hot rolling conditions.
A method of manufacturing a wire for C steel wire is proposed. However, since the C content is limited to 0.2% or less, high strength PC
Manufacture of steel wire is difficult. As described above, in the conventional technique, there is a limit in manufacturing a high-strength PC steel rod having good spot weldability and excellent delayed fracture characteristics.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above situation, and has been developed to provide a high-strength PC steel rod having a good spot weldability and a good delayed fracture characteristic and a strength of 1450 MPa or more. It is an object of the present invention to provide a manufacturing method thereof.

[0007]

Means for Solving the Problems The present inventors first made various strength levels of P produced by quenching and tempering.
The delayed fracture behavior was analyzed in detail using a C steel rod. It is already clear that delayed fracture has occurred due to hydrogen in steel. Therefore, the delayed fracture characteristics were evaluated by obtaining the “critical diffusible hydrogen amount” at which delayed fracture did not occur. In this method, various levels of diffusible hydrogen are contained by electrolytic hydrogen charging, and then Cd plating is performed to prevent hydrogen from leaking from the sample to the atmosphere during the delayed fracture test, and then the air is charged in the atmosphere. A predetermined load is applied to evaluate the diffusible hydrogen amount at which delayed fracture does not occur.

FIG. 1 shows an example in which the relationship between the amount of diffusible hydrogen and the rupture time until delayed fracture is analyzed. As the amount of diffusible hydrogen contained in the sample decreases, the time until delayed fracture increases, and when the amount of diffusible hydrogen is less than a certain value, delayed fracture does not occur. This amount of hydrogen is defined as “critical diffusible hydrogen amount”. The higher the critical diffusible hydrogen content, the better the delayed fracture resistance properties of the steel material, which is a value specific to the steel material determined by the composition of the steel material and the manufacturing conditions such as heat treatment. The amount of diffusible hydrogen in a sample can be easily measured by gas chromatography.

FIG. 2 shows an example of an analysis of the relationship between the strength and the critical diffusible hydrogen content of a PC steel rod manufactured by quenching and tempering, which is a conventional method. As the strength increases, the critical diffusible hydrogen amount at which delayed fracture does not occur begins to decrease at 1500 MPa
It was found that the strength was significantly reduced in the strength region exceeding, and that delayed fracture occurred with a very small amount of diffusible hydrogen. Further, as a result of observing the fracture surface of the sample in which delayed fracture occurred, it was found that the former austenite grain boundary was a grain boundary crack regardless of the strength of the PC steel rod. When the P and S contents, which are known as grain boundary segregation elements, are reduced to 0.005%, the critical diffusible hydrogen content increases and the delayed fracture characteristics improve.
In the high-strength region exceeding a, the improvement margin was small.

Therefore, in order to increase the critical diffusible hydrogen content of the high-strength PC steel rod, that is, to improve delayed fracture characteristics,
Further studies were conducted on the austenitic crystal grain size, the composition of the steel material, and the effects of the heat treatment conditions. As a result, it has been found that even if any of the above factors is largely changed, it is not possible to significantly improve delayed fracture characteristics. This means
This shows that there is a limit to the improvement of delayed fracture characteristics in the means for increasing the strength by heat treatment. Since delayed fracture is caused by grain boundary cracking of the former austenite grain boundary, it has been concluded that it is important to prevent the occurrence of grain boundary cracking in order to achieve a significant improvement in delayed fracture characteristics.

Therefore, as a result of various studies on means for preventing the prior austenite grain boundary cracking, the <110> texture was formed in a region extending at least 5% of the radius in the axial center direction from the surface of the PC steel rod. If it is formed, 1
It has been found that the prior austenite grain boundary cracking can be prevented even in a high strength region exceeding 500 MPa. That is, <11
0> We found a completely new finding that steel with bainite structure having texture does not cause the prior austenite grain boundary cracking, so that the critical diffusible hydrogen content is greatly increased and the delayed fracture resistance is significantly improved. It is. Further, it has been clarified that cold drawing is an extremely effective means for providing a <110> texture.

However, steel having a high bainite structure has a drawback that wire breakage is liable to occur during wire drawing due to poor drawability. Further, when the area reduction rate of the wire drawing process is increased to increase the strength, the elongation is significantly reduced. Therefore, in order to improve these properties, the controlling factors of the drawability of bainite structure were analyzed in detail. As a result, it has been clarified that the drawability of bainite steel is mainly governed by the austenite grain size, and it has been found that if the above factors are optimally selected, the drawability is improved.

Further, as a result of studying means for improving the elongation of the drawn steel, it was found that the elongation can be improved by performing an optimal heat treatment after the drawing. Further, in order to improve the spot weldability of a drawn wire having a bainite structure, the component content of the steel is set to P _CM =
It is clear that the regulation should be limited to 0.15 to 0.45%. Based on the above study results, if the steel composition, heat treatment conditions, strength of bainite before wire drawing, and the total area reduction rate of wire drawing are optimally selected, high weldability with excellent spot weldability and delayed fracture characteristics can be achieved. The inventors have concluded that a high-strength PC steel bar can be realized, and have made the present invention. The present invention has been made based on the above findings, and the gist thereof is as follows.

(1) C: 0.1 to 0.4% by weight,
Si: 0.05 to 2.0%, Mn: 0.2 to 2.0%,
Al: 0.005 to 0.1%, P: 0.015% or less,
S: contains 0.015% or less, or further contains T
i: 0.005 to 0.05%, B: 0.0003 to 0.
0050%, Cr: 0.1 to 2.0%, Mo: 0.05
0.5%, Ni: 0.1-5.0%, Cu: 0.05
-0.5%, V: 0.05-0.5%, Nb: 0.00
Together comprising one or more _{5~0.1%, P CM (%)} = C + Si / 30 + (Mn + Cr + Cu) / 20 + Ni / 60 + Mo /
P _CM represented by 15 + V / 10 + 5B is in the range of 0.15 to 0.45%,
The remainder is composed of Fe and unavoidable impurities, has a bainite structure, and has a <110> texture in a region extending at least 5% of the radius in the axial center direction from the surface layer, and has a strength of 1450 MPa or more. A high-strength PC steel bar characterized by the following:

(2) A steel rod or wire having the above chemical composition is
After heating to a temperature range of c _{3 to} Ac ₃ + 200 ° C., 25
A bainite structure having a strength of 800 MPa or more is formed by quenching to a temperature range of 0 to 500 ° C. and isothermal transformation, and then wire drawing is performed with a total area reduction of 20% or more.
A high-strength PC steel rod which is heat-treated so as to satisfy a relationship of 000 ≧ T × (20 + log t) ≧ 11000 (T: heating temperature expressed in absolute temperature, t: heating time (hr)). Manufacturing method.

Hereinafter, the present invention will be described in detail. First, the high-strength PC steel rod in the present invention means a steel having a strength of 1450 MPa or more and excellent in ductility, delayed fracture properties, spot weldability, and relaxation properties required for the PC steel rod. ing. Described below reasons for limiting components and P _CM of steel to which the present invention. C: C is an indispensable element for achieving high strength of the PC steel bar, but if it is less than 0.1%, the required strength is not obtained in the bainite structure, while if it exceeds 0.4%, spot welding is performed. Therefore, the content was limited to the range of 0.1 to 0.4%. Si: Si has an effect of improving relaxation properties and increasing strength by a solid solution hardening action.
If it is less than 0.05%, the above effect cannot be exerted.
Even if it exceeds 0%, an effect commensurate with the added amount cannot be expected, so the range is limited to 0.05 to 2.0%.

Mn: Mn is an element effective not only for deoxidation and desulfurization but also for enhancing the hardenability for obtaining a bainite structure. On the other hand, if it exceeds 2.0%, the spot weldability deteriorates, so the content is limited to the range of 0.2 to 2.0%. Al: Al has the effect of preventing the austenite grains from coarsening by forming AlN during deoxidation and heat treatment, and also has the effect of fixing N and securing solid solution B effective for hardenability. When the content is less than 0.005%, these effects are not exhibited, and when the content exceeds 0.1%, the effect is saturated. Therefore, the content is limited to the range of 0.005 to 0.1%.

P: P segregates at the austenite grain boundary and is set to 0.015% or less in order to reduce delayed fracture characteristics. Preferably, it is 0.010% or less. S: S is also segregated at austenite grain boundaries similarly to P, and is set to 0.015% or less in order to deteriorate delayed fracture characteristics.
Preferably, it is 0.010% or less. The above are the basic components of the steel targeted by the present invention. In the present invention, the steel further contains Ti: 0.005 to 0.05% and B: 0.
0003-0.0050%, Cr: 0.1-2.0%,
Mo: 0.05 to 0.5%, Ni: 0.1 to 5.0%,
Cu: 0.05-0.5%, V: 0.05-0.5%,
Nb: One or two or more of 0.005 to 0.1% can be contained.

Ti: Ti has a T
The formation of iN has the effect of preventing the austenite grains from coarsening and also has the effect of fixing N and securing solid solution B effective for quenchability. However, if it is less than 0.005%, these effects are exhibited. However, the effect is saturated even if it exceeds 0.05%, so that it was limited to the range of 0.005 to 0.05%. B: B remarkably enhances hardenability by segregating at austenite grain boundaries during heat treatment, and also improves delayed fracture characteristics by reducing the amount of P and S grain boundary segregation that tends to segregate at austenite grain boundaries. 0.0003%
If it is less than 0.0050%, the above effect is not exhibited, and if it exceeds 0.0050%, the effect is saturated, so that it is 0.0003 to 0.0050.
%. Cr: Cr is an element effective for improving the hardenability and increasing the softening resistance of the bainite structure in the heat treatment step after wire drawing, but if it is less than 0.1%, the effect cannot be sufficiently exhibited. On the other hand, if it exceeds 2.0%, spot weldability,
Since the drawability deteriorates, the content is limited to 0.1 to 2.0%. Mo: Mo, like Cr, has a strong tempering softening resistance and is an effective element for increasing the tensile strength after heat treatment, and has an effect of further improving relaxation properties, but less than 0.05% Is less effective, while 0.5
If it exceeds 0%, the spot weldability and the wire drawability deteriorate, so the content was limited to 0.05 to 0.50%.

Ni: Ni is added in order to improve ductility, which deteriorates with increasing strength, and to improve the hardenability during heat treatment to increase the tensile strength of the bainite structure, but less than 0.1%. Is less effective, while 5.
Even if it exceeds 0%, the effect corresponding to the added amount cannot be exerted. Therefore, the content is limited to the range of 0.1 to 5.0%. Cu: Cu is an element effective for increasing the softening resistance of the bainite structure. However, if it is less than 0.05%, the effect cannot be exhibited, and if it exceeds 0.5%, the hot workability deteriorates.
Limited to 0.05-0.5%.

V: V has the effect of reducing the austenite grains and increasing the relaxation value by forming carbonitride during the quenching treatment.
If it is less than 0.05%, the above-mentioned effect cannot be obtained.
Limited to 5%. Nb: Like Nb, Nb is also an effective element for refining austenite grains by forming carbonitrides, but if it is less than 0.005%, its effect is insufficient.
On the other hand, if it exceeds 0.1%, this effect is saturated, so that 0.0%
Limited to 05-0.1%. Although N is not particularly limited, the generation of nitrides of Ti, Al, V, and Nb has an effect of refining austenite grains.
A preferred range is 3 to 0.015%. _{P CM: P CM (%)} = C + Si / 30 + (Mn + Cr + Cu) / 20 + Ni / 60 + Mo /
The P _CM represented by 15 + V / 10 + 5B is an index indicating the spot weldability means as spot weldability this value is low is good. When P _CM exceeds 0.45%, the spot weld spot weldability is deteriorated ductility becomes intense martensite structure low, easily broken at the spot weld. The upper limit was set to 0.45% because the elongation during the tensile test was reduced and the delayed fracture characteristics were further deteriorated. Meanwhile, it becomes difficult bainite structure of a given strength can be obtained to decrease the hardenability of which improved spot weldability be less than 0.15% of P _CM reduces the amount alloying elements,
The lower limit was limited to 0.15%.

Next, the reason why the <110> texture of the PC steel rod is limited, which is the most important point for improving the delayed fracture characteristics of the high-strength PC steel rod intended in the present invention, will be described. FIG. 3 shows an example of analyzing the effect of texture on the critical diffusible hydrogen content of a PC steel rod having a bainite structure. <110> The critical diffusible hydrogen content of a PC steel rod having a texture (represented in the example of the present invention in the figure) is a PC steel rod having no texture (represented in the comparative example of the figure), that is, the conventional hardening. -It turns out that it is at a much higher level than the PC steel bar manufactured by tempering.

FIG. 4 shows that the strength is between 1500 and 1550 MP.
Analysis of the relationship between the critical diffusible hydrogen content and the ratio of the radius to the depth in the axial center direction from the surface of the PC steel rod where <110> texture is generated, using the PC steel rod composed of bainite structure adjusted to a An example is shown below. If the <110> texture formation region is less than 5% of the axial direction of the surface of the PC steel bar relative to the axial center, the effect of improving the critical diffusible hydrogen amount is small, that is, the effect of improving delayed fracture characteristics is small.

For this reason, the region where the <110> texture is generated is limited to a region extending at least 5% of the radius in the axial center direction from the surface layer. In order to obtain a PC steel rod having even higher strength and excellent delayed fracture characteristics, the region where the <110> texture is generated is preferably 10% or more of the radius. The texture can be easily obtained by measuring a pole figure by X-rays. The distribution of the texture from the surface of the PC steel rod in the axial center direction can be determined by measuring a pole figure by X-rays at each point in the depth direction after chemical polishing or electrolytic polishing.

In the method for producing a high-strength PC steel rod according to the present invention,
After a bainite structure having a predetermined composition and strength is obtained, wire drawing is performed cold, and then heat treatment is further performed. Next, reasons for limiting the manufacturing conditions will be described.
First, regarding the strength of bainite, if the strength is less than 800 MPa, a large area reduction rate is required to increase the strength by wire drawing, and the lower limit is 800 MPa because it is not economical.
a. The upper limit of the strength is not particularly limited, but is 1700MP.
If a exceeds 1,700 MPa
a The following strength is preferred. In addition, even if the residual austenite, ferrite, pearlite, and martensite structures having a volume fraction of 10% or less are mixed in the bainite structure, there is no problem with the drawability.

The reasons for limiting the heat treatment conditions for obtaining the bainite structure are as follows. Heating temperature; Heating temperature is A
completely not austenitized less than c _3, whereas, A
If the temperature exceeds c ₃ + 200 ° C., the austenite grains become coarse and the austenite particle size easily exceeds 15 μm. Therefore, the heating temperature range is limited to Ac ₃ to Ac ₃ + 200 ° C. When the austenite particle size exceeds 15 μm, the wire drawing workability of the bainite structure is deteriorated, and disconnection is likely to occur. The austenite particle size is preferably 15 μm or less, more preferably 10 μm or less. Under the conditions of the components of the present invention and heat treatment, an austenite particle size of 15 μm or less can be obtained.

Bainite transformation temperature: If the temperature for obtaining a bainite structure is less than 250 ° C., it takes a long time to complete transformation and it is not economical.
If the temperature exceeds 500 ° C., the strength of the bainite structure decreases, and
The upper limit temperature is 500 ° C because it tends to be less than 00MPa
Limited to. The heat treatment may be performed by any method such as ordinary furnace heating, a salt bath, and high-frequency heating. However, in order to improve the drawability of the bainite structure, a salt having a heating rate of 30 ° C./sec or more is obtained. Bath and high frequency heating are preferred.

If the total area reduction rate of wire drawing is less than 20%, it may be difficult to finally increase the strength of the PC steel rod to 1450 MPa or more, and <110>
The texture is at least 5% of the radius in the axial center direction from the surface layer
Therefore, the lower limit is set to 20% because it is difficult to form the film over a region extending over the region and improvement in delayed fracture characteristics cannot be expected. The upper limit of the total area reduction rate is not particularly limited because it varies depending on the strength of the steel before wire drawing. If it exceeds 90%, however, the strength becomes too high and disconnection easily occurs, so the upper limit is preferably 90%.

Heat treatment conditions after wire drawing: If the bainite-structured steel is cold drawn, it has poor elongation and relaxation properties, so heat treatment is performed to improve these properties. Heat treatment parameter defined by heat treatment temperature and heat treatment time: If T × (20 + log t) is in the range of 11000 to 14000, the strength is 145.
The elongation of the PC steel bar of 0 MPa or more can be made 5% or more, and the relaxation value can be made 1.5% or less. Where T
Is an absolute temperature, and t is time (hr).

If the heat treatment parameter is less than 11,000, it is difficult to reduce the elongation to 5% or more and the relaxation value to 1.5% or less, while if it exceeds 14000, it is difficult to reduce the relaxation value to 1.5% or less. And the strength tends to be less than 1450 MPa, so that the upper limit is set to 14000 and the lower limit is set to 1 in the heat treatment parameter defined by the formula of T × (20 + log t).
It was set to 1,000. The temperature is not particularly limited, but if the temperature is lower than 300 ° C., it takes too much time to set the heat treatment parameter in the above range, and the productivity is lowered.
It is preferably at least 00 ° C. If the upper limit temperature exceeds 550 ° C., the strength tends to decrease, so that it is preferably 550 ° C. or less.

[0031]

EXAMPLE A test material having the chemical composition shown in Table 1 was hot-rolled to a predetermined wire diameter, and then subjected to high-frequency heating and heat treatment in a salt bath to obtain a bainite structure adjusted to various strengths.
Thereafter, wire drawing was performed cold to a wire diameter of 7.4 mm, and then heat treatment was performed. Table 2 shows the results of evaluating the austenite particle size, mechanical properties, spot weldability, texture, delayed fracture characteristics, and relaxation value using the above samples.

[Table 1]

[0032]

[Table 2]

[0033]

[Table 3]

[0034]

[Table 4]

[0035]

[Table 5]

The spot weldability test was conducted using a PC steel rod and JIS.
G3532 SWM-B was used. After the cross-welding, a tensile test was performed on ten test pieces. When the breaking rate of the spot-welded portion was 50% or less, the spot weldability was determined to be good (shown by a mark ○). The delayed fracture characteristic was evaluated by using the sample subjected to spot welding and the critical diffusible hydrogen amount described above, and the load stress was evaluated under the condition of 80% of the tensile strength. The relaxation value was measured based on JIS G3109.

Test No. 2 in Table 2 2,4,6,7,9,1
3,14,17,18,19,20,21,24,2
5, 27 and 28 are examples of the present invention, and others are comparative examples.
As can be seen from the table, in all of the present invention examples, the tensile strength of the PC steel rod is 1450 MPa or more, and if the strength is the same, the critical diffusible hydrogen amount is higher than that of the conventional PC steel rod. Thus, a PC steel rod having excellent delayed fracture characteristics has been realized. Also, the spot weldability and relaxation value are satisfactory.

On the other hand, in Comparative Example No. 1 is steel type A
It is a PC steel rod manufactured by conventional quenching and tempering using No.1. Test No. which is an example of the present invention. Although the strength is almost the same as that of No. 2, since the <110> texture is not formed, the critical diffusible hydrogen content is low and the delayed fracture characteristics are inferior. Also, in Comparative Example No. No. 3 was also manufactured by conventional quenching and tempering, and the critical diffusible hydrogen content was No. 3 of the present invention. 4 compared to 4. Further, in Comparative Example No. 1
Since No. 5 is a bainite-transformed steel that is not subjected to wire drawing, the critical diffusible hydrogen content is low, and the relaxation value also exceeds 1.5%.

In the comparative example No. 8 is an example in which the heating temperature during the quenching process is inappropriate. That is, the heating temperature is Ac ₃ +2
This is an example in which the austenite particle size became 15 μm or more because the temperature exceeded 00 ° C., thereby deteriorating the wire drawing workability and causing breakage during wire drawing. No. of Comparative Example. 5,1
1, 12, 19, and 23 are examples in which the chemical composition of steel is inappropriate. That is, No. No. 11 is an example in which the amount of C was too low and the strength of the bainite structure was less than 800 MPa, and the strength did not reach 1450 MPa even when wire drawing was performed at a total area reduction of 90%. No. No. 5 shows that the amount of P is no. Sample No. 12 is an example in which the amount of S exceeds 0.015%, so that the amount of critical diffusible hydrogen is low. No. 19 and 23 also P _CM Any 0.45
%, The elongation is less than 5%, and the spot weldability and the critical diffusible hydrogen content are low.

In Comparative Example No. No. 10 is an example in which the transformation temperature for obtaining a bainite structure exceeded 500 ° C., so that the strength of bainite was less than 800 MPa and did not reach the strength of 1450 MPa even after 90% wire drawing. No. of Comparative Example. 22 and 26 are <11 with respect to the radius because the total area reduction rate of cold drawing is less than 20%.
0> This is an example in which the texture formation depth is less than 5% and the effect of improving delayed fracture characteristics is small. No. of Comparative Example. 1
Nos. 6, 29 and 30 are examples in which the heat treatment conditions after the wire drawing are inappropriate. That is, No. In No. 29, since no heat treatment was performed after the wire drawing, the elongation was less than 5%, and the relaxation value exceeded 1.5%. No. 16 is T × (20
+ Log t) is less than 11000, so that elongation is 5%.
And the relaxation value is more than 1.5% and deteriorates. No. 30 is a heat treatment parameter of 14
Since it exceeds 000, the strength becomes less than 1450 MPa, and the relaxation value also exceeds 1.5%.

[0041]

According to the present invention, the chemical composition of the steel, the heat treatment conditions, the tensile strength after the heat treatment, the total area reduction rate of the wire drawing, the heat treatment conditions after the wire drawing are optimally selected, and the <110> set By introducing the microstructure, a high-strength PC steel rod having good spot weldability and excellent delayed fracture characteristics having a strength of 1450 MPa or more and its production have been made possible, and the industrial effect is extremely remarkable. There is something.

[Brief description of the drawings]

FIG. 1 is a chart showing an example of the relationship between the amount of diffusible hydrogen and delayed fracture time.

FIG. 2 is a table showing an example of the relationship between the strength of a PC steel rod manufactured by quenching and tempering and the amount of critical diffusible hydrogen.

FIG. 3 shows a PC steel bar of the present invention and a PC manufactured by a conventional method.
It is a chart which shows an example of the relationship between the intensity | strength of a steel rod and the critical diffusible hydrogen amount.

FIG. 4 is a chart showing an example of the effect of <110> texture on the critical diffusible hydrogen amount.

────────────────────────────────────────────────── ─── Continued on the front page (51) Int.Cl. ⁷ Identification code FI C22C 38/06 C22C 38/06 E02D 5/58 E02D 5/58 Z (56) References JP-A-7-224355 (JP, A JP-A-7-258797 (JP, A) JP-A-6-306543 (JP, A) JP-A-53-51121 (JP, A) JP-A-60-245722 (JP, A) (58) Field (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (3)

(57) [Claims] C: 0.1-0.4%, Si: 0.05-% by weight
2.0%, Mn: 0.2-2.0%, Al: 0.005-
0.1%, P: 0.015% or less, S: 0.015%
P _CM of the following formula as well as containing less 0.15
0.45%, the remainder is composed of Fe and unavoidable impurities, and has a bainite structure, and further has a <110> texture in a region extending at least 5% of the radius in the axial center direction from the surface layer. A high-strength PC steel rod having a strength of 1450 MPa or more. P _CM (%) = C
+ Si / 30 + (Mn + Cr + Cu) / 20 + Ni / 60 + Mo / 15 + V / 10
+ 5B 2. In% by weight, Ti: 0.005 to 0.05%, B: 0.0003
~ 0.0050%, Cr: 0.1 ~ 2.0%, Mo: 0.05 ~
0.5%, Ni: 0.1-5.0%, Cu: 0.05-
0.5%, V: 0.05-0.5%, Nb: 0.005-
2. The high-strength PC steel rod according to claim 1, comprising one or more kinds of 0.1%. 3. A steel rod or a steel wire having the chemical composition according to claim 1 is heated to a temperature range of Ac ₃ to Ac ₃ + 200 ° C., and then rapidly cooled to a temperature range of 250 to 500 ° C. By isothermal transformation, a bainite structure with a strength of 800 MPa or more is formed, then wire drawing is performed with a total area reduction of 20% or more, and then 14000 ≧ T × (20 + log
t) A method for producing a high-strength PC steel rod, wherein heat treatment is performed so as to satisfy a relationship of ≧ 11000 (T: heating temperature expressed in absolute temperature, t: heating time (hr)).