JPH07268547A - 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. 0.15 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 and <=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 0.20 to 0.45%, and the balance Fe with inevitable impurities, moreover, the ratio of tempered martensite is limited to 60 to 90% by volume fraction, and the balance Fe with inevitable impurities, furthermore having a <110> texture in the range over at least 5% of the radius in an axial center direction from the surface layer an 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]

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 constructions, bridges, etc. And the strength is 1450
The present invention relates to a high-strength PC steel rod excellent in delayed fracture property of MPa or more and a manufacturing method thereof.

[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 is mainly composed of 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 Patent Publication No. 5-4168
Japanese Patent Laid-Open No. 4 proposes to control the Si and Mn contents and to perform bending or drawing in the tempering process after the quenching process. However, spot weldability,
The delayed fracture characteristics of spot welds are not mentioned.

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. Also,
As a result of observing the fracture surface of the sample in which the delayed fracture occurred, it was found that the former 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 region over MPa, the margin for improvement 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
0> Steel consisting of ferrite with tempered structure and tempered martensite structure does not cause old austenite intergranular cracking, so the amount of critical diffusible hydrogen increases significantly and delayed fracture resistance is greatly improved. We have found such findings. It was also clarified that cold wire drawing is an extremely effective means for imparting a <110> texture.

However, since the steel having a composite structure of ferrite and martensite has a low strength and a low wire-drawing hardening property, the total area reduction rate of the wire-drawing is increased in order to achieve a strength of 1450 MPa or more. Need to let. When the area reduction rate is increased to increase the strength, the elongation is remarkably reduced and the wire breakage easily occurs. Therefore, as a result of a detailed analysis in order to improve these properties, it was found that the wire drawing workability of steel composed of a composite structure of ferrite and martensite is important for the prestructural morphology and heat treatment conditions. It has been found that the wire drawing workability is improved by the optimum selection.

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. Moreover, in order to secure high strength and good spot weldability, the component content of the steel material is P _CM = 0.20 to
It was clarified that the regulation should be 0.45%. Based on the above examination results, if the steel composition, prestructural morphology, heat treatment conditions for obtaining a composite structure of ferrite and martensite, and the total area reduction rate of wire drawing are optimally selected, spot weldability and delayed fracture The present invention has been made upon the conclusion that a high-strength PC steel bar excellent in properties can be realized.

The present invention has been made based on the above findings, and the gist thereof is as follows. (1) C: 0.15 to 0.40% by weight, Si: 0.
05-2.0%, Mn: 0.2-2.0%, Al: 0.0
05-0.1%, P: 0.015% or less, S: 0.01
5% or less, or further Ti: 0.005
~ 0.05%, B: 0.0003 to 0.0050%, C
r: 0.1 to 2.0%, Mo: 0.05 to 0.5%, N
i: 0.1 to 5.0%, Cu: 0.05 to 0.5%,
V: 0.05 to 0.5%, Nb: 0.005 to 0.1%
P _CM (%) = C + Si / 30 with one or more of
+ (Mn + Cr + Cu) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B, P _CM is in the range of 0.20 to 0.45%, the balance is Fe and inevitable impurities, and tempered martensite in volume fraction. Is 60 to 90% and the balance is ferrite, and in the region extending at least 5% of the radius in the axial center direction from the surface layer <11
0> High-strength PC steel rod having a texture and a strength of 1450 MPa or more.

(2) A steel rod or steel wire having the above chemical composition is made into martensite, tempered martensite or bainite structure, and then Ac _{1 to} A at a heating rate of 20 ° C./sec or more.
By heating in the temperature range of c ₃ and quenching, a martensite structure having a volume fraction of 60 to 90% and a balance of ferrite is formed, and then wire drawing is performed at a total area reduction rate of 50% or more. , Then 14000 ≧ T × (20 + lo
g t) ≧ 11000 (T: heating temperature expressed in absolute temperature, t: heating time (hr)) A heat treatment is carried out so as to satisfy the requirement, and a high-strength PC steel bar manufacturing method is characterized.

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 bar, but if less than 0.15%, the strength of the composite structure consisting of ferrite and martensite is too low,
Since it is difficult to finally raise the strength of the PC steel bar to 1450 MPa or more, the lower limit was set to 0.15%. on the other hand,
If the C content exceeds 0.40%, the spot weldability deteriorates significantly, so the upper limit was limited to 0.40%. 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 improving the hardenability during heat treatment, but if it is less than 0.2%, the above effect cannot be obtained. On the other hand, if the content exceeds 2.0%, the spot weldability deteriorates, so the content is limited to the range of 0.2 to 2.0%. Al: Al forms AlN during deoxidation and heat treatment, thereby having an effect of preventing coarsening of crystal grains and 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 0.0
It was limited to the range of 05 to 0.1%.

P: P is set to 0.015% or less in order to segregate at the 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: 0.005-0.
05%, B: 0.0003 to 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: 0.005-0.1% of 1
It is possible to contain one kind or two or more kinds.

Ti: Ti, like Al, also has the effect of preventing coarsening of crystal grains by forming TiN during deoxidation and heat treatment, and the effect of fixing N and securing a solid solution B effective for hardenability. However, if less than 0.005%, these effects are not exhibited, and if more than 0.05%, the effects are saturated, 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 in the heat treatment step after wire drawing, but if it is less than 0.1%, its effect cannot be sufficiently exerted, while 2. If it exceeds 0%, spot weldability and wire drawability deteriorate, so the content is limited to 0.1 to 2.0%. 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 in order to improve the ductility which deteriorates with increasing strength and the hardenability during heat treatment to increase the tensile strength.
If it is less than 5.0%, the effect is small, while if it exceeds 5.0%, the effect corresponding to the added amount cannot be exhibited, so
Limited to the range of%. Cu: Cu is an element effective for increasing the tempering softening resistance, but if it is less than 0.05%, the effect cannot be exhibited and
If it exceeds 5%, the hot workability deteriorates, so 0.05 to
Limited to 0.5%.

V: V has the effect of forming carbonitrides during the quenching treatment to refine the crystal grains and increase the relaxation value, but 0.05%
If it is less than 0.5%, the effect of the above-mentioned action cannot be obtained. On the other hand, if it exceeds 0.5%, the effect is saturated, so the content is limited to 0.05 to 0.5%. Nb: Nb is also an element effective for refining crystal grains by forming carbonitrides similarly to V.
If it is less than 005%, the effect is insufficient, while 0.1
%, This effect is saturated, so 0.005 to 0.
Limited to 1%. Although N is not particularly limited, Ti, A
0.003 to 0.01 because the austenite grains have a grain refining effect by forming a nitride of 1, V, and Nb.
5% 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, when the amount of alloying elements is reduced to make P _CM less than 0.20%, spot weldability is improved, but hardenability is deteriorated, so that it becomes difficult to obtain martensite with a fraction of 60 to 90%. , The lower limit was limited to 0.20%.

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 of the present invention, will be described. Fig. 3 shows P consisting of ferrite and tempered martensite structure.
An example of analyzing the influence of the texture on the critical diffusible hydrogen content of the C steel rod will be shown. Critical diffusible hydrogen content of PC steel bar having <110> texture (indicated by the present invention example in the figure)
It can be seen that is at a much higher level than that of a PC steel rod having no texture (indicated by a comparative example in the figure), that is, a PC steel rod manufactured by conventional quenching and tempering.

FIG. 4 shows the critical diffusible hydrogen content and <11
0> An example of analysis of the relationship of the ratio of the radius to the depth in the axial center direction from the surface layer of the PC steel rod in which the texture is generated is shown. It can be seen that when the <110> texture generation 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 the region extending over 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. Also, PC
The distribution of texture from the surface layer of the 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,
Using steel of martensite or tempered martensite or bainite structure, after forming a martensite and ferrite structure of a predetermined fraction, cold wire drawing, then further heat treatment, The reasons for limiting these manufacturing conditions are described below.

First, the reason for using martensite, tempered martensite, or steel having a bainite structure is to obtain a structure having good wire drawability when forming a composite structure composed of martensite and ferrite. That is,
If a large amount of lumpy martensite is present, wire drawing workability deteriorates, and wire breakage easily occurs in the wire drawing process. In order to improve the wire drawing workability, it is necessary to make the morphology of martensite as lumpy as possible.

[0028] When martensite or tempered martensite or bainite structure steel is used, the formation of massive martensite is prevented and it tends to become acicular, so the steel is limited to the above structure. Further, from the viewpoint of improving the wire drawing workability, it is preferable to use steel having the above-mentioned structure with a grain size of 15 μm or less. The reasons for limiting the heat treatment conditions for obtaining the composite structure of martensite and ferrite are as follows.

Heating rate: If the heating rate is less than 20 ° C./sec, the rate of lumpy martensite is increased and the wire drawing workability is deteriorated. Therefore, the lower limit of the heating rate was set to 20 ° C./sec. Heating temperature: When the heating temperature exceeds Ac ₃ , a single-phase structure of martensite is formed, and when the heating temperature is less than Ac ₁ , martensite is not generated, and therefore the heating temperature is Ac ₁ to Ac.
Limited to a range of ₃ . It should be noted, Ac ₃ -10 ℃ ~Ac ₃ -
A temperature range of 50 ° C. is a preferable condition for obtaining a martensite structure fraction of 60 to 90%. After heating, the steel made of ferrite and martensite can be formed by quenching, for example, water cooling. The heat treatment may be any of ordinary furnace heating, salt bath, high frequency heating and the like, but salt bath and high frequency heating are preferred heat treatments because a large heating rate can be obtained.

Martensite Microstructure Fraction: When the martensite fraction is less than 60%, it is difficult to finally achieve a high strength PC steel bar of 1450 MPa or more, while 9
If it exceeds 0%, wire drawing workability deteriorates and wire breakage easily occurs, so the martensite structure fraction was limited to 60 to 90%. The balance consists of ferrite. Even if retained austenite is contained in the martensite structure, there is no problem with wire drawing workability.

Total area reduction rate: If the total area reduction rate of wire drawing is less than 50%, it is difficult to finally make the strength of the PC steel bar 1450 MPa or more, so the lower limit was made 50%. Further, if the total area reduction ratio is 50% or more, the <110> texture can be surely formed in the region extending over 5% of the radius from the surface layer in the axial center direction.

Heat treatment conditions after wire drawing: If steel made of ferrite and martensite structure is cold drawn, the elongation and relaxation properties are poor. Therefore, heat treatment is performed for the purpose of improving these properties. is there. Further, the martensite contained in the fraction of 60 to 90% by this heat treatment becomes a tempered martensite structure. Heat treatment parameter defined by heat treatment temperature and heat treatment time: T × (20 + log t) is 11000 to 1400
Within the range of 0, the elongation of the PC steel bar having a strength of 1450 MPa or more can be 5% or more and the relaxation value can be 1.5% or less. Here, T is absolute temperature, and t is time (hr).

If 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 if it exceeds 14000,
Since it becomes difficult to reduce the relaxation value to 1.5% or less and the strength tends to be less than 1450 MPa, the heat treatment parameter defined by the formula of T × (20 + log t) has an upper limit of 14000 and a lower limit of 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 lower is preferable.

[0034]

EXAMPLE A test material having the chemical composition shown in Table 1 was hot-rolled to have a predetermined wire diameter, and then subjected to high-frequency heating or heat treatment with a salt bath to have a predetermined structure (martensite, grain size of 5 to 12 μm). Tempered martensite, bainite) and further heat-treated by high-frequency heating to obtain a steel having a composite structure composed of ferrite and martensite 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. Using the above sample, the mechanical properties,
Table 2 shows the results of evaluation of spot weldability, texture, delayed fracture property and relaxation value.

[0035]

[Table 1]

[0036]

[Table 2]

[0037]

[Table 3]

[0038]

[Table 4]

[0039]

[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
1,14,15,17,18,21,22,25,2
7, 28 and 29 are examples of the present invention, and the 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 which is a comparative example 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. Further, the comparative example No. 3 is also manufactured by the conventional quenching and tempering, and the amount of limit diffusible hydrogen is lower than that of the present invention example No. 4. Comparative example No. 5 is A
This is an example in which the heating rate in the c _{1 to} Ac ₃ temperature range is inappropriate.
That is, this is an example in which the heating rate is less than 20 ° C./sec, the amount of lumpy martensite produced increases, and wire breakage occurs during the wire drawing process.

Comparative examples Nos. 8, 13, 19, and 20 are examples in which the chemical composition of steel is inappropriate. That is, No. 13 is C
This is an example in which the strength did not reach 1450 MPa even when wire drawing was performed at a total area reduction rate of 90% because the amount was too low.
No. 8 has a P content of 20 has an S content of 0.0
This is an example in which the critical diffusible hydrogen content is low because the steel exceeds 15%. No. 19 is an example in which the P _CM exceeds 0.45%, the elongation is less than 5%, and the spot weldability and the critical diffusible hydrogen content are also low.

Comparative examples Nos. 10 and 12 are examples in which the martensite fraction is inappropriate. That is, No. 10 has a martensite fraction of less than 60% and therefore has a strength of 145
It has not reached 0MPa. On the other hand, No. 12 has a martensite fraction of more than 90%, which deteriorates the wire drawability.
This is an example in which a wire breakage occurred during wire drawing.

No. which is a comparative example. 24, the total reduction rate is 50
This is an example in which the strength of 1450 MPa or more was not finally obtained because it was less than%. No. which is a comparative example. 16, 2
Nos. 3 and 26 are examples of inappropriate heat treatment conditions after wire drawing. That is, No. In No. 26, 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
Since t) is less than 11,000, the elongation is low as less than 5% and the relaxation value is also deteriorated by more than 1.5%. No. No. 23 has a heat treatment parameter of 14000
And the strength is less than 1450 MPa,
The relaxation value is also over 1.5%. Further, No. 30, which is a comparative example, is a steel having a ferrite-pearlite structure, and therefore a large amount of massive martensite with poor wire drawability was generated in the heat treatment for obtaining a composite structure of ferrite and martensite. This is an example in which a wire breakage occurred in the wire drawing process.

[0046]

According to the present invention, the chemical composition of steel, heat treatment conditions, microstructure fraction, total area reduction rate of wire drawing, and heat treatment conditions after wire drawing are optimally selected, and a <110> texture is introduced. By doing so, the spot weldability is excellent, the strength with excellent delayed fracture characteristics is a high strength PC steel bar of 1450 MPa or more, and the manufacturing thereof is possible, and the industrial effect is extremely remarkable. .

[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.

Claims (3)

[Claims] 1. By weight%, C: 0.15 to 0.40%, Si: 0.05
-2.0%, Mn: 0.2-2.0%, Al: 0.00
5 to 0.1%, P: 0.015% or less, S: 0.01
It contains 5% or less and P _CM shown in the following formula is 0.2
It is in the range of 0 to 0.45%, the balance is Fe and unavoidable impurities, and the volume fraction of tempered martensite is 60 to 90%, and the balance is ferrite. A radius of at least 5 towards the center
%, It has a <110> texture and has a strength of 1450 MPa or more.
C steel rod. 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 ~
0.50%, Ni: 0.1-5.0%, Cu: 0.05-
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 steel wire having the chemical composition according to claim 1 or 2 is formed into martensite, tempered martensite or bainite structure, and then Ac ₁ to Ac at a heating rate of 20 ° C./sec or more. Heated to a temperature range of ₃ ,
By quenching, the martensite structure becomes 6 by volume fraction.
0 to 90%, the balance is made of ferrite,
Then, wire drawing is performed at a total surface reduction rate of 50% or more, and then a relationship of 14000 ≧ T × (20 + log t) ≧ 11000 (T: heating temperature displayed in absolute temperature, t: heating time (hr)) A method for producing a high-strength PC steel rod, characterized by performing heat treatment so as to satisfy the requirement.