JPH10251801A - High strength pc steel bar and production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a high strength PC steel bar which can be spot-welded and has a good delayed fracture resistance and uniform extending characteristic by making substantially lamellar structure of martensite and ferrite oriented in the longitudinal direction of the PC steel bar. SOLUTION: As the components the objective steel containes 0.2-0.4% C, <=3.0% Si, 0.2-2.0% Mn, 0.005-0.1% Al by wt. and the balance Fe which inevitable impurities and further, Cr, Mo, Ni, Cu, W and B are contained to a specific contents to improve hardenability and delayer fracture resistant characteristic. Further, V, Nb, Ti and Ta are contained to specific contents to generate carbon-nitride in the ferritite, in the lemellar structure of the martensite and ferrite to increase yield strength and improve relaxation characteristic. The steel having such components, is heated in the temp. range of Ac1 -Ac3 and cooled at >=20 deg.C/sec cooling speed after executing hot-working at >=20% rolling reduction ratio to produce the PC steel bar.

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 widely used as a reinforcing material for poles, piles, and prestressed concrete structures such as buildings and bridges. Saga 1230
TECHNICAL FIELD The present invention relates to a high-strength PC steel bar which is not less than MPa and has excellent delayed fracture characteristics and uniform elongation, and a method for producing the same.

[0002]

2. Description of the Related Art PC steel, which is widely used as a reinforcing material for prestressed concrete structures such as poles, piles, buildings, bridges, etc., is usually a PC steel wire, a PC steel strand, and a PC steel rod. . The PC steel wire is manufactured by performing a patenting process and then drawing.

On the other hand, PC steel bars are, for example,
No. 684, the C content is 0.25
After hot rolling using ~ 0.35% medium carbon steel, quenching
It is manufactured by tempering. Although the strength of the PC steel wire is higher than that of the PC steel bar, there is a disadvantage that spot welding cannot be performed due to the high C content.

[0004] On the other hand, although the spot weldability of a PC steel rod is better than that of a PC steel wire, it is referred to in “Prestressed Concrete Design and Construction Standards and Explanation” (edited by the Architectural Institute of Japan,
As described on pages 43 to 45 of Maruzen, 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 weld is rapidly cooled to have a structure mainly composed of martensite, and the spot weld tends to suffer from delayed fracture. There is a point. Furthermore, recently, as described in Japanese Patent Application Laid-Open No. 7-3396, the development of a PC steel rod having excellent uniform elongation has been demanded in order to improve bending toughness for applications such as piles. .

As a conventional finding for improving the delayed fracture characteristics of PC steel bars, for example, Japanese Patent Publication No. 5-59967 proposes that it is effective to reduce the contents of P and S. Certainly, low P and low S are effective for delayed fracture, but the P and S contents of the current PC steel rods are already around 0.01%. Although it is possible to further reduce the contents of P and S, there is a disadvantage that the production cost is increased. In Japanese Patent Publication No. 5-41684, Si, Mn
It has been proposed to regulate the content and to perform bending or drawing in a tempering step after quenching. Japanese Patent Application Laid-Open No. 5-7963 proposes that a resin coating layer is provided around a spot weld between a PC steel rod and an iron wire to reduce the susceptibility to delayed fracture. Further, Japanese Patent Application Laid-Open No. 6-212346 proposes a means for improving delayed fracture characteristics by adding an alloy element such as Ni. However, none of the proposals has led to a significant improvement in delayed fracture characteristics.

On the other hand, as a conventional finding to improve the uniform elongation of a PC steel bar, for example, Japanese Patent Application Laid-Open No. 7-3396 proposes a method in which the tempering temperature is higher than 500 ° C. Has a drawback that spot welding cannot be performed because of its high value of 0.45% or more. Further, Japanese Unexamined Patent Publication No.
JP-A-158010 states that the addition of a large amount of Si and Al is effective for improving uniform elongation. However, when Si and Al are added in large amounts, there are disadvantages in that the delayed fracture characteristics of the spot welds are reduced and the manufacturing cost is increased.

As described above, according to the prior art, in a high-strength PC steel rod capable of spot welding, not only the individual problems of delayed fracture characteristics and uniform elongation, but also a high-strength steel rod which drastically improves both of these problems. There was a limit in manufacturing PC steel bars.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and is capable of performing spot welding, has a delayed fracture characteristic, and has a uniform strength of 1230M.
It is an object of the present invention to realize a high-strength PC steel bar of Pa or more and to provide a method of manufacturing the same.

[0010]

Means for Solving the Problems The present inventors first analyzed in detail the delayed fracture behavior using PC steel rods of various strength levels manufactured by quenching and tempering after rolling. It has already been clarified that delayed fracture is caused by hydrogen in steel. Therefore, in this analysis, delayed fracture characteristics indicate that "critical diffusible hydrogen amount"
Was evaluated. In this method, various levels of diffusible hydrogen are contained in a sample 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,
A predetermined load is applied in the atmosphere, and the amount of diffusible hydrogen at which delayed fracture does not occur is evaluated.

FIG. 1 shows an example in which the relationship between the amount of diffusible hydrogen and the rupture time until delayed fracture is analyzed. It can be seen that 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 manufacturing conditions such as heat treatment. The amount of diffusible hydrogen in a sample can be easily measured by gas chromatography.

As a result of various investigations from the viewpoint of microstructure to improve the critical diffusible hydrogen content of the high-strength PC steel rod, that is, to improve the delayed fracture characteristics and uniform elongation, the length of the PC steel rod was determined. If a layered structure composed of martensite or tempered martensite and ferrite oriented in the direction (L direction) is formed, even in a high-strength region exceeding 1230 MPa, the amount of critical diffusible hydrogen is high, and the delayed fracture characteristics are significantly increased. We have found a completely new finding of improvement. In addition, it has been found that uniform elongation is remarkably improved by controlling such a layered structure. Furthermore, the relationship between the layered structure of martensite or tempered martensite oriented in the longitudinal direction of PC steel rod and ferrite, delayed fracture characteristics and uniform elongation was analyzed in detail, and the optimum manufacturing conditions were established. It was made.

The present invention has been made based on the above findings, and its gist is as follows.

(1) A high-strength PC steel rod comprising a layered structure of substantially martensite and ferrite oriented in the longitudinal direction of the PC steel rod.

(2) A high-strength PC steel rod comprising a layered structure of substantially tempered martensite and ferrite oriented in the longitudinal direction of the PC steel rod.

(3) C: 0.2-0.4% by weight,
Si: 3.0% or less, Mn: 0.2 to 2.0%, Al:
The high-strength PC steel rod according to the above (1) or (2), containing 0.005 to 0.1%, with the balance being Fe and unavoidable impurities.

(4) By weight%, C: 0.2-0.4%,
Si: 3.0% or less Mn: 0.2 to 2.0%, Al:
0.005 to 0.1%, and (A) Cr: 0.05 to 2.0%, Mo: 0.05 to 1.0
%, Ni: 0.05-5.0%, Cu: 0.05-1.
0% W: 0.05-0.5%, B: 0.0003-0.
One or more of 0050%, or (B) V: 0.05 to 0.3%, Nb: 0.005 to 0.1
%, Ti: 0.005 to 0.05%, Ta: 0.005
The above (1) or the above (1), wherein the composition contains one or more of one or more of (A) and (B) in an amount of 0.5% or more, and the balance consists of Fe and unavoidable impurities. (2) The high-strength PC steel rod according to (2).

(5) In weight%, C: 0.2-0.4%,
Si: 3.0% or less Mn: 0.2 to 2.0%, Al:
A steel containing 0.005 to 0.1%, the balance being Fe and unavoidable impurities, is heated to a temperature range of Ac ₁ to Ac ₃ ,
A high-strength PC characterized in that after hot working at a rolling reduction of 20% or more, it is cooled at a cooling rate of 20 ° C./sec or more.
Manufacturing method of steel bars.

(6) C: 0.2-0.4% by weight,
Si: 3.0% or less, Mn: 0.2 to 2.0%, Al:
A steel containing 0.005 to 0.1%, the balance being Fe and unavoidable impurities, is heated to a temperature range of Ac ₁ to Ac ₃ ,
Production of a high-strength PC steel bar characterized by performing hot working at a rolling reduction of 20% or more, cooling at a cooling rate of 20 ° C / sec or more, and then tempering in a temperature range of 200 to 600 ° C. Method.

(7) By weight%, C: 0.2-0.4%,
Si: 3.0% or less, Mn: 0.2 to 2.0%, Al:
A steel containing 0.005 to 0.1%, the balance being Fe and unavoidable impurities, is heated to a temperature range of Ac ₁ to Ac ₃ ,
After hot working at a rolling reduction of 20% or more, it is cooled at a cooling rate of 20 ° C./sec or more, and then plastic strain of 0.3% or more is applied, and subsequently fired in a temperature range of 200 to 600 ° C. A method for manufacturing a high-strength PC steel bar, characterized by returning.

(8) By weight%, C: 0.2-0.4%,
Si: 3.0% or less, Mn: 0.2 to 2.0%, Al:
A steel containing 0.005 to 0.1%, the balance being Fe and unavoidable impurities, is heated to a temperature range of Ac ₁ to Ac ₃ ,
After hot working at a rolling reduction of 20% or more, cooling at a cooling rate of 20 ° C / sec or more, and then heating to a temperature range of 200 to 600 ° C to give a plastic strain of 0.3% or more. A method for producing a high-strength PC steel rod, characterized by the following.

(9) By weight%, (A) Cr: 0.05-2.0%, Mo: 0.05-1.0
%, Ni: 0.05-5.0%, Cu: 0.05-1.
0%, W: 0.05-0.5%, B: 0.0003-
One or more of 0.0050%, or (B) V: 0.05 to 0.3%, Nb: 0.005 to 0.1
%, Ti: 0.005 to 0.05%, Ta: 0.005
(5), (6), (7), and (8), wherein the composition contains one or more of one or more of the groups (A) and (B). )) The method for producing a high-strength PC steel rod according to any one of the above.

[0023]

Next, an embodiment of the present invention will be described.

First, the reason for limiting the microstructure, which is the most important point for improving the delayed fracture characteristics and the uniform elongation of the high-strength PC steel bar aimed at in the present invention, will be described. FIG. 2 shows an example of a scanning electron micrograph of the layered structure of martensite or tempered martensite and ferrite oriented in the longitudinal direction (L direction) of the PC steel bar defined in the present invention.

FIG. 2 shows the case of martensite and ferrite. The martensite and ferrite have a fine layered structure, and the longitudinal direction (L direction) of the PC steel rod.
It turns out that it is orientated. When this structure is tempered, it becomes a layered structure of tempered martensite and ferrite. By controlling the fine layered structure of martensite or tempered martensite and ferrite, both delayed fracture characteristics and uniform elongation can be simultaneously improved.

FIGS. 3 and 4 show an example of an analysis of the effects of the texture on the delayed fracture characteristics and uniform elongation. PC
It can be seen that by forming a layered structure of martensite or tempered martensite and ferrite in the longitudinal direction of the steel rod, the critical diffusible hydrogen content is high and uniform elongation is greatly improved. On the other hand, the single structure of tempered martensite by the conventional quenching / tempering method has low critical diffusible hydrogen content and low uniform elongation. In addition, in order to significantly improve the strength and delayed fracture characteristics and uniform elongation, in the layered structure of martensite or tempered martensite and ferrite, the average interval between martensites or tempered martensite is 10 μm or less. Preferred and more preferred conditions are 5 μm or less.
The average interval can be measured by a cutting method in a direction perpendicular to the longitudinal direction (L direction) in the photograph shown in FIG.

Further, even if retained austenite or a small amount of bainite or pearlite is present in martensite or tempered martensite having a layered structure, there is no problem with uniform elongation and delayed fracture characteristics. The layered structure of martensite or tempered martensite and ferrite does not have to be formed over the entire cross section of the PC steel bar, but may be formed from the surface layer of the PC steel bar in a region at least 15% of the radius of the PC steel bar. For example, there is an effect of improving delayed fracture characteristics and uniform elongation.

In the layered structure of martensite or tempered martensite, the ferrite fraction is 10%.
The preferred condition is in the range of ~ 70%. This is because if the ferrite fraction is less than 10%, a significant improvement in uniform elongation cannot be expected, while if it exceeds 70%, it is difficult to achieve a high strength of 1230 MPa or more. A more preferable range of the ferrite fraction is 20 to 60% in that the strength of the PC steel rod and the improvement of the delayed fracture characteristic and the uniform elongation are simultaneously achieved.

Next, the reasons for limiting the components of the steel to which the present invention is applied will be described.

C: C is an essential element for securing the strength of the PC steel rod, but if it is less than 0.2%, the desired high strength PC steel rod cannot be obtained. If it exceeds, the spot weldability deteriorates remarkably, so the range is limited to the range of 0.2 to 0.4%.

Si: Si has an effect of improving relaxation properties and increasing strength by a solid solution hardening action. Even if it exceeds 3%, an effect commensurate with the added amount cannot be expected, and the delayed fracture characteristics of the spot welded portion are reduced.
It was restricted to a range of 3% or less. The preferred range is 0.2 to
2.5%.

Mn: Mn is not only necessary for deoxidation and desulfurization but is also an effective element for enhancing hardenability, but if it is less than 0.2%, the above effects cannot be obtained. If it exceeds 2.0%, the spot weldability deteriorates.
It was limited to the range of 0.2 to 2.0%.

Al: Al has the effect of preventing the austenite grains from becoming coarser by deoxidizing and forming AlN, but if it is less than 0.005%, these effects are not exhibited, and 0.1% If the value exceeds 0.1, the effect is saturated.
It was limited to the range of 005 to 0.1%.

The above are the basic components of the steel to be used in the present invention. In the present invention, (A) Cr: 0.05-2.0%, Mo: 0.05-1. 0
%, Ni: 0.05-5.0%, Cu: 0.05-1.
0%, W: 0.05-0.5%, B: 0.0003-
One or more of 0.0050%, or (B) V: 0.05 to 0.3%, Nb: 0.005 to 0.1
%, Ti: 0.005 to 0.05%, Ta: 0.005
０．５0.5% of one or more of one or both of the groups (A) and (B) can be included. (A)
The elements of the group are mainly added for the purpose of improving the hardenability and the delayed fracture characteristics. On the other hand, the elements of the group (B) mainly produce carbonitrides in ferrite in the layered structure of martensite or tempered martensite and ferrite to refine crystal grains and increase yield strength and relaxation properties. It is added for the purpose of improving.

Cr: Cr is an element effective for improving hardenability and delayed fracture characteristics and increasing softening resistance at the time of tempering. However, if it is less than 0.05%, its effect cannot be sufficiently exhibited. On the other hand, if it exceeds 2.0%, the spot weldability deteriorates, so the content is limited to 0.05 to 2.0%.

Mo: Mo is an element that improves hardenability and delayed fracture characteristics like Cr, has strong tempering softening resistance, is effective for increasing tensile strength after heat treatment, and further improves relaxation characteristics. It also has an effect,
If the content is less than 0.05%, the effect is small. On the other hand, if the content exceeds 1.0%, the effect is saturated.
Limited to 1.0% range.

Ni: Ni is added in order to enhance hardenability and delayed fracture characteristics. If Ni is less than 0.05%, its effect is small, while if it exceeds 5.0%, the effect corresponding to the added amount is exhibited. Since it cannot be performed, it was limited to the range of 0.05 to 5.0%.

Cu: Cu is an element effective for improving the hardenability and the strength of tempered martensite.
If it is less than 5%, the effect cannot be exhibited, and if it exceeds 1.0%, the hot workability deteriorates, so the content is limited to 0.05 to 1.0%.

B: B has the effect of improving delayed fracture characteristics and also has the effect of significantly increasing hardenability. However, if B is less than 0.0003%, the above effect is not exhibited, and Even if it exceeds 0.0050%, the effect will be saturated, so that it will be 0.
0003 to 0.0050%.

V: V has the effect of reducing the crystal grain size and increasing the yield strength and improving the relaxation characteristics by forming carbonitrides. On the other hand, if the content exceeds 0.3%, the effect is saturated, so that the content is limited to 0.05 to 0.3%.

Nb: Like V, Nb also has the effect of forming carbonitrides to refine crystal grains, increase yield strength and improve relaxation characteristics.
If the content is less than 0.005%, the above effect is insufficient.
Limited to ~ 0.1%.

Ti: Ti is an element that prevents de-oxidation and formation of carbonitride to prevent crystal grains from coarsening, and is effective in increasing yield strength and improving relaxation characteristics, but less than 0.005% In these cases, these effects are not exhibited, and even if the content exceeds 0.05%, the effects are saturated.
It was limited to the range of 005 to 0.05%.

Ta: Ta also has the effect of reducing the crystal grain size by producing carbonitrides as in V, increasing the yield strength and improving the relaxation properties. The effect is not exhibited,
If the content exceeds 0.5%, the effect is saturated.
005 to 0.5%.

Although P and S are not particularly limited,
From the viewpoint of improving the delayed fracture characteristics of the PC steel rod, the respective ranges are preferably 0.015% or less. In addition, since N has the effect of reducing the crystal grain size, increasing the yield strength, and improving the relaxation properties by forming nitrides of Al, V, Nb, and Ti, N is preferably 0.002 to 0.015%. Range.

In the method for producing a high-strength PC steel rod of the present invention,
The steel of the above component is heated to a temperature range of Ac ₁ to Ac ₃ and is heated to 20%
After performing the hot working at the above reduction rate, it is cooled at a cooling rate of 20 ° C./sec or more, or 200 to 60 after cooling.
Tempered in a temperature range of 0 ° C or 0.3% after cooling
The above plastic strain is applied and tempered in a temperature range of 200 to 600 ° C. or 200 to 600 ° C. after cooling.
Heating to a temperature range of ° C. gives plastic strain of 0.3% or more. The reasons for limiting the manufacturing conditions are described below.

The steel is heated to a two-phase region of ferrite and austenite. At this time, the heating temperature is Ac ₁
Exceeding the less than or Ac _3, since the final martensite or tempered martensite and ferrite lamellar structure is not obtained, the heating temperature range is limited to Ac ₁ to Ac _3. Incidentally, it is more preferable condition range _{℃ (Ac 1 +10) ~ (} Ac 3 -10). In order to refine the layered structure of martensite or tempered martensite and ferrite, the steel before heating may be adjusted to a structure mainly composed of martensite or tempered martensite or bainite or pearlite. Desirably, the ferrite fraction at this time is more preferably less than 10%. Means for controlling the structure before heating includes adjusting the cooling rate after hot rolling, or adjusting the cooling rate after ordinary quenching / tempering or austenitizing treatment, and any method may be used.

The rolling reduction (cross-sectional reduction ratio) of hot working is 20
%, The layered structure oriented in the longitudinal direction of the PC steel bar cannot be obtained, so the lower limit of the rolling reduction is limited to 20%.
From the viewpoint of fine layered structure, a rolling reduction of 30% or more is a preferable condition. Further, when using a steel having a normal ferrite-pearlite structure, it is preferable to perform hot working at a rolling reduction of 50% or more in order to realize a fine layered structure. The hot working may be performed by any method.

If the cooling rate after the hot working is less than 20 ° C./sec, a large amount of ferrite, pearlite, and bainite are formed during cooling, and the possibility of lowering the strength is increased.
The lower limit of the cooling rate was limited to 20 ° C / sec. From the viewpoint of minimizing ferrite, pearlite, and bainite which are easily generated during cooling, a preferable cooling rate is 50 ° C./sec or more. The cooling end temperature is not higher than the martensite transformation temperature (not higher than the Ms point).

Tempering after cooling is performed to increase yield strength and improve relaxation characteristics.
In particular, in the case of steel containing V, Nb, Ti, and Ta, the tempering treatment produces carbonitrides of these alloying elements in ferrite, so that the yield strength is increased and the effect of improving relaxation properties is obtained. At this time, if the temperature is lower than 200 ° C., the effect of the tempering is small. On the other hand, if the temperature exceeds 600 ° C., the strength decreases and it becomes difficult to produce a high-strength PC steel rod. Therefore, the tempering temperature range is limited to 200 to 600 ° C. From the viewpoint of improving delayed fracture characteristics, the heating rate during tempering is preferably 20 ° C./sec or more.

In the present invention, 0.3% is added to the PC steel rod after cooling.
The above plastic strain may be applied and then tempered, or at the time of tempering, plastic strain of 0.3% or more may be applied. The purpose is to increase the yield strength and improve the relaxation properties. If it is less than 0.3%, the above effect is small, so the lower limit is limited to 0.3%. The means for imparting plastic strain may be any method such as tensile strain or strain due to straightening.

In the present invention, even if light wire drawing or plastic working is performed for the purpose of wire diameter adjustment, deforming, etc. before hot working, after hot working, after cooling, or after tempering, uniform delay fracture characteristics and uniform There is no noticeable degradation in elongation and there is no restriction.

[0052]

[Table 1]

[0053]

[Table 2]

[0054]

[Table 3]

[0055]

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

A steel material having the chemical composition shown in Table 1 was adjusted to a cooling rate after hot rolling or quenched and tempered after austenitizing treatment to obtain various microstructures.
It was heated to a temperature range of c _{1 to} Ac ₃ to perform hot working, and then cooled. Tempering was performed by high frequency heating, and plastic strain was applied using a tensile tester.

Using the above samples, the morphology, mechanical properties, delayed fracture properties, and relaxation properties were evaluated. 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 test was performed according to JIS G 3137.
Tables 2 and 3 show these results.

Test No. 2 in Table 2 1 to 16 are examples of the present invention,
No. 3 in Table 3. 17 to 24 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 rod was 12%.
By having a fine layered structure of martensite or tempered martensite and ferrite in the longitudinal direction of the PC steel rod at the same time as 30 MPa or more, the critical diffusible hydrogen content is higher than that of the conventional PC steel rod and the delayed fracture characteristics Thus, a PC steel rod having excellent uniformity and high uniform elongation has been realized. The relaxation is also at the same level as the conventional PC steel bar.

On the other hand, the comparative example No. 17, 1
8 and 19 are all manufactured by a conventional manufacturing method. That is, it is manufactured by normal quenching and tempering, and the structure is an example of tempered martensite. For this reason, the critical diffusible hydrogen content is low, and the delayed fracture characteristic of the spot weld is inferior to the example of the present invention. Also, the uniform elongation is low.

In Comparative Example No. 20 and 21 are examples in which the heating temperature during hot working is inappropriate. No. 20 is
This is an example in which the heating temperature exceeds Ac _3, and a single phase of austenite was formed, and the layered structure of tempered martensite and ferrite could not be achieved. Not. In addition, No.
Sample _No. 21 has a pearlite structure spherical with ferrite because the heating temperature is Ac ₁ or lower, and high strength cannot be achieved.

Further, in Comparative Example No. 22 is an example in which the rolling reduction of the hot working is inappropriate. That is, since the rolling reduction is too small, a layered structure of tempered martensite and ferrite oriented in the longitudinal direction of the PC steel rod is not formed, and the effect of improving delayed fracture characteristics and uniform elongation is small.

No. No. 23 is an example in which the cooling rate after hot working was too low, so that a large amount of ferrite and pearlite were generated during cooling, and the strength was reduced.

Further, in Comparative Example No. 24 is steel C
This is an example in which the content is too high, the spot weldability is deteriorated, and a crack is generated in the spot weld. As a result, the critical diffusible hydrogen amount is significantly reduced, and the delayed fracture characteristic of the spot weld is deteriorated.

[0064]

As is clear from the above embodiments, the present invention enables spot welding by forming a fine layered structure of martensite or tempered martensite and ferrite in the longitudinal direction of a PC steel rod. In addition to making it possible to significantly improve the delayed fracture characteristics and uniform elongation of high-strength PC steel rods, and by optimally selecting the chemical composition of steel, hot working conditions, heat treatment conditions, etc. The method has been established, and the industrial effect is extremely remarkable.

[Brief description of the drawings]

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

FIG. 2 is a scanning electron micrograph showing an example of a layered structure of martensite and ferrite oriented in a longitudinal direction of a PC steel rod.

FIG. 3 is a diagram showing an example of a relationship between delayed fracture characteristics and a microstructure of a PC steel rod.

FIG. 4 is a diagram showing an example of the relationship between uniform elongation and the microstructure of a PC steel rod.

Claims (9)

[Claims] 1. A high-strength PC steel rod comprising a layered structure of substantially martensite and ferrite oriented in the longitudinal direction of the PC steel rod. 2. A high-strength PC steel rod comprising a layered structure of substantially tempered martensite and ferrite oriented in the longitudinal direction of the PC steel rod. 3. C: 0.2 to 0.4% by weight, S
i: 3.0% or less, Mn: 0.2 to 2.0%, Al:
The high-strength PC steel rod according to claim 1 or 2, comprising 0.005 to 0.1%, with the balance being Fe and unavoidable impurities. 4. C: 0.2 to 0.4% by weight, S
i: 3.0% or less, Mn: 0.2 to 2.0%, Al:
0.005 to 0.1%, and (A) Cr: 0.05 to 2.0%, Mo: 0.05 to 1.0
%, Ni: 0.05-5.0%, Cu: 0.05-1.
0%, W: 0.05-0.5%, B: 0.0003-
One or more of 0.0050%, or (B) V: 0.05 to 0.3%, Nb: 0.005 to 0.1
%, Ti: 0.005 to 0.05%, Ta: 0.005
The composition according to claim 1, wherein the composition contains at least 0.5% of one or more of one or both of the groups (A) and (B), with the balance being Fe and unavoidable impurities. 2. The high-strength PC steel rod according to 2. 5. C: 0.2 to 0.4% by weight, S
i: 3.0% or less, Mn: 0.2 to 2.0%, Al:
A steel containing 0.005 to 0.1%, the balance being Fe and unavoidable impurities, is heated to a temperature range of Ac ₁ to Ac ₃ ,
A high-strength PC characterized in that after hot working at a rolling reduction of 20% or more, it is cooled at a cooling rate of 20 ° C./sec or more.
Manufacturing method of steel bars. 6. C: 0.2-0.4% by weight, S
i: 3.0% or less, Mn: 0.2 to 2.0%, Al:
A steel containing 0.005 to 0.1%, the balance being Fe and unavoidable impurities, is heated to a temperature range of Ac ₁ to Ac ₃ ,
Production of a high-strength PC steel bar characterized by performing hot working at a rolling reduction of 20% or more, cooling at a cooling rate of 20 ° C / sec or more, and then tempering in a temperature range of 200 to 600 ° C. Method. 7. C: 0.2-0.4% by weight, S
i: 3.0% or less, Mn: 0.2 to 2.0%, Al:
A steel containing 0.005 to 0.1%, the balance being Fe and unavoidable impurities, is heated to a temperature range of Ac ₁ to Ac ₃ ,
After hot working at a rolling reduction of 20% or more, it is cooled at a cooling rate of 20 ° C./sec or more, and then plastic strain of 0.3% or more is applied, and subsequently fired in a temperature range of 200 to 600 ° C. A method for manufacturing a high-strength PC steel bar, characterized by returning. 8. In% by weight, C: 0.2-0.4%, S
i: 3.0% or less, Mn: 0.2 to 2.0%, Al:
A steel containing 0.005 to 0.1%, the balance being Fe and unavoidable impurities, is heated to a temperature range of Ac ₁ to Ac ₃ ,
After hot working at a rolling reduction of 20% or more, cooling at a cooling rate of 20 ° C / sec or more, and then heating to a temperature range of 200 to 600 ° C to give a plastic strain of 0.3% or more. A method for producing a high-strength PC steel rod, characterized by the following. 9. In weight%, (A) Cr: 0.05-2.0%, Mo: 0.05-1.0.
%, Ni: 0.05-5.0%, Cu: 0.05-1.
0%, W: 0.05-0.5%, B: 0.0003-
One or more of 0.0050%, or (B) V: 0.05 to 0.3%, Nb: 0.005 to 0.1
%, Ti: 0.005 to 0.05%, Ta: 0.005
The composition according to any one of claims 5, 6, 7, and 8, wherein the composition contains one or both of the groups (A) and (B) in an amount of 0.5% or more. A method for producing a high-strength PC steel rod according to the above description.