JPH07268545A - High strength pc steel bar and its production - Google Patents

Abstract

PURPOSE:To produce a high strength PC steel bar excellent in delayed fracture properties and good in spot weldability and to provide a method for producing the same. CONSTITUTION:A high strength PC steel bar having a compsn. contg. 0.1 to 0.4% C, 0.05 to 2.0% Si, 0.2 to 2.0% Mn, 0.005 to 0.1% Al, <=0.015% P an <=0.015% S, furthermore contg., at need, one or >= two kinds among Ti, B, Cr, Mo, Ni, Cu, V and Nb, in which PCM is limited to the range of 0.15 to 0.45%, and the balance Fe with inevitable impurities, moreover having a bainitic structure, furthermore, in which a <110> texture is contaied in a range over at least 5% of the radius in the axial center direction from the surface layer and having >=1450MPa strength is produced; where PCM(%)=C+Si/30+(Mn+Cr+Cu)/20+Ni/60+Mo/15+V/10+5B.

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 and buildings, bridges, etc., and particularly has good spot weldability. Strength is 1450MP
The present invention relates to a high-strength PC steel rod excellent in delayed fracture characteristics as described above and a method for manufacturing the same.

[0002]

2. Description of the Related Art PC steel widely used as a reinforcing material for prestressed concrete structures such as poles, piles and buildings, bridges is generally PC steel wire and PC steel stranded wire specified in JIS G3536, and JIS steel. G31
The PC steel rod specified in 09 is 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, the 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. Has been done.

Although the strength of PC steel wire is higher than that of PC steel rod, it has a drawback that spot welding cannot be performed because of its high C content. On the other hand, although the spot weldability of PC steel rod is better than that of PC steel wire, it is described on pages 43 to 45 of "Prestressed concrete design and construction standard / commentary" (edited by the Architectural Institute of Japan, Maruzen). As described above, the high-strength PC steel bar whose strength exceeds 1275 MPa (130 kgf / mm ² ) is inferior in delayed fracture property to PC steel wire. Further, as described in Japanese Examined Patent Publication No. 5-59967, the spot welded portion is rapidly cooled, so that the structure mainly has martensite. As a result, delayed fracture easily occurs in the spot welded portion.

As a conventional finding for improving the 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, lowering P and S is effective for delayed fracture, but the P and S contents of the current PC steel bar are already around 0.01%, which is specified by JIS G3109. Actually, it is at a level lower than the amount that is used. Although it is possible to further reduce the P and S contents, the manufacturing cost increases. In addition, Japanese Examined Patent Publication 5-41684
In the publication, it is proposed that the Si and Mn contents are regulated and that after the quenching treatment, bending or drawing is performed in the tempering process. However, no mention is made of spot weldability and delayed fracture characteristics of spot welds.

On the other hand, as a technique for improving spot weldability, for example, in Japanese Unexamined Patent Publication No. 4-247825, P is used to regulate the amount of steel components and to limit hot rolling conditions.
A method for manufacturing a wire for C steel wire is proposed. However, since the C content is limited to 0.2% or less, high strength PC
Steel wire is difficult to manufacture. As described above, the conventional technique has a limit in producing 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 provides a high-strength PC steel bar having a good spot weldability and a delayed fracture property of 1450 MPa or more. It is an object of the present invention to realize and provide a manufacturing method thereof.

[0007]

The inventors of the present invention firstly prepared P of various strength levels manufactured by quenching and tempering.
The delayed fracture behavior was analyzed in detail using a C steel rod. It is already clear that delayed fracture is caused by hydrogen in steel. Therefore, the delayed fracture characteristics were evaluated by determining the "limit diffusible hydrogen content" at which delayed fracture does not occur. In this method, various amounts of diffusible hydrogen are contained by electrolytic hydrogen charging, and then Cd plating is applied to prevent hydrogen from being released from the sample into the atmosphere during the delayed fracture test, and then in the atmosphere. Is to evaluate the amount of diffusible hydrogen at which a predetermined load is applied and delayed fracture does not occur.

FIG. 1 shows an example of analysis of the relationship between the amount of diffusible hydrogen and the fracture time until delayed fracture. As the amount of diffusible hydrogen contained in the sample decreases, the time until delayed fracture becomes longer, and delayed fracture does not occur when the amount of diffusible hydrogen is below a certain value. This amount of hydrogen is defined as the “limit diffusible hydrogen amount”. The higher the critical diffusible hydrogen content is, the better the delayed fracture resistance of the steel material is, which is a value peculiar 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 the sample can be easily measured with a gas chromatograph.

FIG. 2 shows an example of analysis of the relationship between the strength and the limit diffusible hydrogen content of a PC steel rod manufactured by the conventional method of quenching and tempering. The limit diffusible hydrogen content that does not cause delayed fracture begins to decrease with increasing strength, 1500 MPa
It became clear that in the strength range over 10%, the fracture rate was remarkably reduced, and delayed fracture occurred with an extremely 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 austenite grain boundaries were intergranular cracks regardless of the strength of the PC steel bar. 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 range 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 the delayed fracture property,
Further studies were conducted on the effects of austenite grain size, steel composition, heat treatment conditions, and the like. As a result, it was found that even if any of the above factors was changed to a large extent, the delayed fracture characteristics could not be significantly improved. This is
It has been shown that there is a limit to the improvement of the delayed fracture property by means of strengthening by heat treatment. Since delayed fracture is a grain boundary crack at the former austenite grain boundary, it was concluded that it is important to prevent the occurrence of grain boundary crack in order to achieve a significant improvement in delayed fracture properties.

Then, as a result of further studies on means for preventing the former austenite grain boundary cracking, as a result, a <110> texture was formed in a region extending at least 5% of the radius in the axial center direction from the surface layer of the PC steel bar. If formed, 1
It was discovered that the former austenite intergranular cracks can be prevented even in a high strength region exceeding 500 MPa. That is, <11
In the steel of bainite structure having 0> texture, old austenite intergranular cracks do not occur, so that the amount of critical diffusible hydrogen greatly increases and the delayed fracture resistance is remarkably improved. Of. As a method of imparting a <110> texture, it was clarified that cold wire drawing is an extremely effective means.

However, the steel having a high strength of bainite structure has a poor wire-drawing workability, so that it has a drawback that wire breakage easily occurs during wire-drawing. Further, when the area reduction rate of wire drawing is increased and the strength is increased, the elongation is remarkably reduced. Therefore, in order to improve these properties, the factors governing the wire drawability of the bainite structure were analyzed in detail. As a result, it was clarified that the wire drawability of bainitic steel was mainly controlled by the austenite grain size, and it was found that the wire drawability was improved by optimally selecting the above factors.

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 optimum heat treatment after the drawing work. Further, in order to improve the spot weldability of a wire drawing material having a bainite structure, the component content of the steel material is P _CM =
It was clarified that it should be restricted to 0.15 to 0.45%. Based on the above study results, if the steel material composition, heat treatment conditions, strength of bainite before wire drawing, and total area reduction rate of wire drawing are optimally selected, spot weldability and delayed fracture characteristics are excellent. The present inventors have reached the conclusion that a high strength PC steel bar can be realized. The present invention has been made on the basis of 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: 0.005-0.1%, P: 0.015% or less,
S: contains 0.015% or less, or further T
i: 0.005-0.05%, B: 0.0003-0.
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 to 0.5%, Nb: 0.00
5 to 0.1% of 1 type or 2 types or more, and 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 balance consists 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. High-strength PC steel bar characterized by.

(2) A steel bar or steel 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 obtained by rapid cooling to a temperature range of 0 to 500 ° C and isothermal transformation, and then wire drawing is performed at a total surface reduction rate of 20% or more.
A high-strength PC steel bar characterized by being heat-treated so as to satisfy the relationship of 000 ≧ T × (20 + log t) ≧ 11000 (T: heating temperature expressed in absolute temperature, t: heating time (hr)). Manufacturing method.

The present invention will be described in detail below. First, the high-strength PC steel rod in the present invention means a steel having a strength of 1450 MPa or more and excellent ductility, delayed fracture properties, spot weldability, and relaxation properties required for PC steel rods. ing. Next, the constituents of the steel to which the present invention is applied and the reasons for limiting P _CM will be described. C: C is an essential element for achieving high strength of the PC steel rod, but if it is less than 0.1%, the required strength cannot be obtained in the bainite structure, while if it exceeds 0.4%, spot welding is performed. Since the properties are remarkably deteriorated, the range is limited to 0.1 to 0.4%. Si: Si has an effect of improving relaxation characteristics and an effect of increasing strength by a solid solution hardening effect.
If it is less than 0.05%, the above effect cannot be exhibited, while 2.
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, but if it is less than 0.2%, the above effect is obtained. However, if it exceeds 2.0%, the spot weldability deteriorates. Therefore, the range is limited to 0.2 to 2.0%. Al: Al has an effect of preventing coarsening of austenite grains by forming AlN during deoxidation and heat treatment, and also has an effect of fixing N and securing a solid solution B effective for hardenability. If it is less than 0.005%, these effects are not exhibited, and if it exceeds 0.1%, the effect is saturated, so the range is limited to 0.005 to 0.1%.

P: P is set to 0.015% or less in order to segregate at austenite grain boundaries and deteriorate delayed fracture characteristics. Preferably it is 0.010% or less. S: S is also set to 0.015% or less in the same manner as P in order to segregate in the austenite grain boundaries and deteriorate the 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, however, Ti: 0.005 to 0.05% and 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: 0.005-0.1% of 1 type (s) or 2 or more types can be included.

Ti: Ti is T during deoxidation and heat treatment.
By forming iN, it has an effect of preventing coarsening of austenite grains and also an effect of fixing N to secure a solid solution B effective for hardenability, but if it is less than 0.005%, these effects are exhibited. However, the effect is saturated even if it exceeds 0.05%, so the range is limited to 0.005 to 0.05%. B: B segregates at the austenite grain boundaries during heat treatment to remarkably enhance hardenability, and also reduces the amount of grain boundary segregation of P and S, which tend to segregate at the austenite grain boundaries, thereby improving delayed fracture characteristics. 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 0.0003 to 0.0050.
Limited to%. 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%, its effect cannot be sufficiently exhibited. On the other hand, if it exceeds 2.0%, spot weldability,
It is limited to 0.1 to 2.0% because the drawability deteriorates. Mo: Mo is an element that has strong temper softening resistance similar to Cr and is effective for increasing the tensile strength after heat treatment, and also has the effect of improving relaxation characteristics, but less than 0.05% Is less effective, while 0.5
If it exceeds 0%, spot weldability and wire drawing workability deteriorate, so the content was limited to 0.05 to 0.50%.

Ni: Ni is added to improve the 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%. Has less effect, while 5.
Even if it exceeds 0%, the effect corresponding to the added amount cannot be exhibited, so the range is limited to 0.1 to 5.0%. Cu: Cu is an element effective for increasing the softening resistance of the bainite structure, but if it is less than 0.05%, the effect cannot be exhibited, and if it exceeds 0.5%, the hot workability deteriorates.
It was limited to 0.05-0.5%.

V: V has the effect of refining the austenite grains and increasing the relaxation value by forming carbonitrides during the quenching treatment.
If it is less than 0.05%, the effect of the above-mentioned action cannot be obtained, while if it exceeds 0.5%, the effect is saturated, so 0.05 to 0.
Limited to 5%. Nb: Nb is also an element effective for refining austenite grains by forming carbonitrides like V, 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 0.0
It was limited to 05 to 0.1%. Although N is not particularly limited, it has a fine graining effect on austenite grains by forming a nitride of Ti, Al, V, and Nb, and therefore 0.00
3 to 0.015% is a preferable range. P _CM : P _CM (%) = C + Si / 30 + (Mn + Cr + Cu) / 20 + Ni / 60 + Mo /
P _CM represented by 15 + V / 10 + 5B is an index showing the spot weldability, and the lower this value, the better the spot weldability. If P _CM exceeds 0.45%, the spot weld becomes a martensite structure with low ductility and high strength, the spot weldability deteriorates, and the spot weld easily breaks. Further, the elongation in the tensile test is lowered and the delayed fracture property is also deteriorated, so the upper limit was made 0.45%. On the other hand, if the amount of alloying elements is decreased to make P _CM less than 0.15%, spot weldability is improved, but hardenability is deteriorated, and it becomes difficult to obtain a bainite structure having a predetermined strength.
The lower limit was limited to 0.15%.

Next, the reason for limiting the <110> texture of the PC steel rod, which is the most important point for improving the delayed fracture property of the high-strength PC steel rod aimed at by the present invention, will be described. FIG. 3 shows an example of analysis of the influence of the texture on the critical diffusible hydrogen content of the PC steel rod having the bainite structure. The critical diffusible hydrogen content (indicated by the present invention example in the figure) of the PC steel bar having a <110> texture is the PC steel bar having no texture (indicated by the comparative example in the figure), that is, the conventional quenching. -It can be seen that it is at a much higher level than the PC steel bar manufactured by tempering.

FIG. 4 shows the strength of 1500 to 1550MP.
Using a PC steel rod with a bainite structure adjusted to a, analyze the relationship between the critical diffusible hydrogen content and the ratio of the radius to the depth in the axial direction from the surface layer of the PC steel rod where the <110> texture is generated. An example is shown below. It can be seen that when the <110> texture formation region is less than 5% in the axial center direction from the surface layer of the PC steel bar, the effect of improving the limit diffusible hydrogen amount is small, that is, the effect of improving delayed fracture properties 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 bar having even higher strength and excellent delayed fracture properties, 10% or more of the radius is a preferable condition for the generation region of the <110> texture. The texture can be easily obtained by measuring the pole figure by X-ray. The distribution of texture from the surface layer of the PC steel rod in the axial center direction can be obtained by measuring the pole figure by X-ray at each point in the depth direction after chemical polishing or electrolytic polishing.

In the method for producing a high strength PC steel rod of the present invention,
After forming a bainite structure having a predetermined composition and strength, cold wire drawing is performed, and then heat treatment is further performed. Next, the 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 it is not economical, so the lower limit is 800 MPa.
I assumed a. The upper limit of strength is not particularly limited, but 1700MP
When it exceeds a, wire drawing workability deteriorates, so 1700MP
a The following strength is preferable. Even if 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 wire drawing workability.

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

Bainite transformation temperature: If the temperature for obtaining the bainite structure is less than 250 ° C., it takes time to complete the transformation and it is not economical, so the lower limit temperature is set to 250 ° C. or higher.
If it exceeds 500 ° C, the strength of bainite structure decreases and
The upper limit temperature is 500 ° C because it tends to be less than 00 MPa.
Limited to. The heat treatment may be performed by any of ordinary furnace heating, salt bath, high-frequency heating, etc., but in order to improve the wire drawing workability of the bainite structure, a heating rate of 30 ° C./sec or more is obtained. Bathing and high frequency heating are preferred.

Total area reduction rate: 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 bar to 1450 MPa or more, and <110>
At least 5% of the radius in the axial direction from the surface layer
The lower limit was set to 20% because it becomes difficult to form in the region over the range and improvement in delayed fracture properties cannot be expected. The upper limit of the total area reduction ratio varies depending on the strength of the steel before wire drawing, but is not particularly restricted, but if it exceeds 90%, the strength becomes too high and wire breakage easily occurs, so the upper limit is preferably 90%.

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

When the heat treatment parameter is less than 11000, it is difficult to make the elongation 5% or more and the relaxation value 1.5% or less, while when it exceeds 14000, it is difficult to make the relaxation value 1.5% or less. And the strength tends to be less than 1450 MPa, the upper limit of the heat treatment parameter defined by the formula T × (20 + log t) is 14000, and the lower limit is 1
It was set to 1000. 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 within the above range and the productivity is lowered.
It is preferably 00 ° C or higher. If the upper limit temperature exceeds 550 ° C, the strength tends to decrease, so 550 ° C or less is preferable.

[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 with a salt bath to obtain bainite structures adjusted to various strengths.
Then, wire drawing was performed to a wire diameter of 7.4 mm in a cold state, and then heat treatment was performed. Table 2 shows the results of evaluation of the austenite grain size, mechanical properties, spot weldability, texture, delayed fracture property, 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 carried out using PC steel rod and JIS
It was carried out using SWM-B of G3532. After the cross-welding, a tensile test was performed with 10 test pieces, and when the fracture rate of the spot-welded portion was 50% or less, the spot-weldability was good (indicated by a circle). The delayed fracture characteristic was evaluated by using the sample subjected to spot welding with the amount of limiting diffusible hydrogen described above, and the load stress was carried out under the condition of 80% of the tensile strength. The relaxation value was measured based on JIS G3109.

Test No. of 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 each of the examples of the present invention, the tensile strength of the PC steel bar is 1450 MPa or more, and if the strength is the same, the critical diffusible hydrogen content is at a higher level than the conventional PC steel bar. A PC steel rod with excellent delayed fracture characteristics has been realized. The spot weldability and relaxation value are also perfect.

On the other hand, No. 1 is steel type A
It is a PC steel rod manufactured by conventional quenching and tempering. Test No. which is an example of the present invention Although the strength was almost the same as that of No. 2, the <110> texture was not formed, so the amount of critical diffusible hydrogen was low and the delayed fracture property was inferior. In addition, No. No. 3 which is manufactured by conventional quenching and tempering and has a critical diffusible hydrogen content of No. 3 of the present invention. Low compared to 4. Furthermore, No. which is a comparative example. 1
Since No. 5 is a steel that has not undergone wire drawing and is still in the bainite transformation, it has a low critical diffusible hydrogen content and a relaxation value of more than 1.5%.

No. which is a comparative example. No. 8 is an example in which the heating temperature during quenching is inappropriate. That is, the heating temperature is Ac ₃ +2
This is an example in which the austenite grain size becomes 15 μm or more because the temperature exceeds 00 ° C., which deteriorates the wire drawing workability and causes wire breakage during wire drawing. No. which is a 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 if wire drawing was performed at a total area reduction rate of 90%. Also No. No. 5 has a P content of No. No. 12 is an example in which the critical diffusible hydrogen content is low because each of the steels contains 0.015% S or more. No. Both 19 and 23 have a P _CM of 0.45
%, The elongation is less than 5%, and the spot weldability and the critical diffusible hydrogen content are also low.

No. which is a comparative example. No. 10 is an example in which the transformation temperature for obtaining the bainite structure exceeded 500 ° C., so that the strength of bainite was less than 800 MPa and the strength of 1450 MPa was not reached even after 90% wire drawing. No. which is a comparative example. Nos. 22 and 26 have a total area reduction rate of less than 20% during cold wire drawing, so <11
0> This is an example in which the formation depth of the texture is less than 5% and the effect of improving delayed fracture characteristics is small. No. which is a comparative example. 1
Nos. 6, 29 and 30 are examples of inappropriate heat treatment conditions after wire drawing. That is, No. In No. 29, the elongation was less than 5% and the relaxation value was more than 1.5% because the heat treatment after wire drawing was not performed. No. 16 is T × (20
+ Log t) is less than 11000, so the elongation is 5%
The relaxation value is less than 1.5% and deteriorates with the relaxation value exceeding 1.5%. No. 30 has 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 steel, the heat treatment conditions, the tensile strength after heat treatment, the total area reduction rate of wire drawing, and the heat treatment conditions after wire drawing are optimally selected, and the <110> aggregate is set. By introducing the structure, the spot weldability is good, and the strength with excellent delayed fracture characteristics is a high strength PC steel bar with a strength of 1450 MPa or more and the production thereof, and the industrial effect is extremely remarkable. There is something.

[Brief description of 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 is a PC steel rod of the present invention and a PC manufactured by a conventional method.
3 is a chart showing an example of the relationship between the strength of a steel rod and the amount of critical diffusible hydrogen.

FIG. 4 is a table showing an example of the influence of <110> texture on the critical diffusible hydrogen content.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C22C 38/06 E02D 5/58 Z

Claims (3)

[Claims] 1. By weight%, C: 0.1-0.4%, Si: 0.05-
2.0%, Mn: 0.2 to 2.0%, Al: 0.005
0.1%, P: 0.015% or less, S: 0.015%
It contains the following and P _CM shown in the following formula is 0.15 to
It is in the range of 0.45%, the balance is Fe and unavoidable impurities, and has a bainite structure, and also 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. 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 to 5.0%, Cu: 0.05 to
0.5%, V: 0.05 to 0.5%, Nb: 0.005
The high-strength PC steel rod according to claim 1, containing 0.1% of one or more kinds. 3. A steel rod or wire having the chemical composition according to claim 1 or 2 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 performing isothermal transformation, a bainite structure having a strength of 800 MPa or more is formed, and then wire drawing is performed with a total area reduction rate of 20% or more, and then 14000 ≧ T × (20 + log
t) ≧ 11000 (T: heating temperature expressed in absolute temperature, t: heating time (hr)) A heat treatment is performed to satisfy a high strength PC steel bar.