JPH10287957A - High strength pc steel bar and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a high strength PC steel bar capable of spot welding and excellent in uniform elongation as well as in immunity to delayed fracture by providing a steel with a lamellar structure composed essentially of martensite and ferrite. SOLUTION: In the lamellar structure composed essentially of tempered martensite and ferrite, immunity to delayed fracture can be improved to a greater extent and strength can be increased by precipitation strengthening and also relaxation characteristic can be improved when the carbonitrides of one or more elements among V, Nb, Ti, and Ta are incorporated into ferrite. Further, the composition of the steel consists of, by weight, 0.2-0.4% C, <=3.0% Si, 0.2-2.0% Mn, 0.005-0.1% Al, and the balance Fe with inevitable impurities. At the time of manufacture, the steel with the above composition is heated to a temp. range between Ac1 and Ac3 , cooled at >=20 deg.C/sec cooling rate, and then tempered at 200 to 600 deg.C.

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, as described in Japanese Patent Publication No. 5-41684, for example, a PC steel rod has a C content of 0.25 to 0.3.
It is manufactured by performing quenching and tempering after hot rolling using 5% medium carbon steel. 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. On the other hand, the spot weldability of PC steel rod is better than that of PC steel wire,
As described on pages 43-45 of the same commentary (edited by the Architectural Institute of Japan, Maruzen), the strength was 1275 MPa (130
kgf / mm ² ) for high-strength PC steel bars
The delayed fracture characteristics are inferior to the C 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 there is a problem that delayed fracture easily occurs in the spot weld. . Furthermore, recently, Japanese Patent Application Laid-Open
As described in Japanese Patent No. 3396, development of a PC steel rod having excellent uniform elongation is required 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 discloses 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, it is proposed to control the contents of Si and Mn and to perform a bending process or a drawing process during a tempering process after a quenching process. 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 for improving the uniform elongation of a PC steel bar, for example, Japanese Patent Application Laid-Open No. 7-3396 proposes a method in which a tempering temperature is set to a high temperature of 500 ° C. or more. However, there is a disadvantage that spot welding cannot be performed because the ratio is as high as 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, in 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 that drastically improves both of these problems. There was a limit in manufacturing PC steel bars.

[0006]

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.

[0007]

Means for Solving the Problems The present inventors first made various strength levels of P produced by quenching and tempering.
The delayed fracture behavior was analyzed in detail using a C steel rod. It has already been clarified that delayed fracture is caused by hydrogen in steel materials. Therefore, in this analysis, regarding the delayed fracture characteristics, the "critical diffusible hydrogen amount" where delayed fracture does not occur
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. 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 studies from the viewpoint of microstructure to improve the critical diffusible hydrogen content of the high-strength PC steel rod, that is, to improve delayed fracture characteristics and uniform elongation, martensite or tempered martensite was obtained. If the formation of a layered structure composed of ferrite and martensite or tempered martensite with carbonitride precipitated ferrite and a layered structure of
A completely new finding was obtained that the critical diffusible hydrogen content is high even in a high strength region exceeding a, and the delayed fracture characteristics are greatly improved. In addition, it has been found that uniform elongation is remarkably improved by controlling such a layered structure. Furthermore, the layered structure composed of martensite or tempered martensite and ferrite and the relationship between carbonitride precipitated in ferrite and delayed fracture characteristics and uniform elongation are analyzed in detail, and the optimum manufacturing conditions are established. It is the invention.

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

(1) A high-strength PC steel rod characterized in that the steel structure substantially comprises a layered structure of martensite and ferrite.

(2) A high-strength PC steel rod characterized in that the steel structure substantially comprises a layered structure of tempered martensite and ferrite.

(3) The structure of the steel is substantially composed of a layered region of tempered martensite and ferrite, and one or more carbonitrides of V, Nb, Ti, and Ta are precipitated in the ferrite. A high-strength PC steel rod characterized in that:

(4) By weight%, C: 0.2 to 0.4% Si: 3.0% or less Mn: 0.2 to 2.0% Al: 0.005 to 0.1% Wherein the balance comprises Fe and unavoidable impurities, and the high-strength PC according to the above (1) or (2)
Steel bar.

(5) By weight%, C: 0.2 to 0.4% Si: 3.0% or less Mn: 0.2 to 2.0% Al: 0.005 to 0.1% , 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: High strength PC according to the above (1) or (2), containing one or more of 0.0003 to 0.005%, with the balance being Fe and unavoidable impurities. Steel bar.

(6) By weight%, C: 0.2 to 0.4% Si: 3.0% or less Mn: 0.2 to 2.0% Al: 0.005 to 0.1% And V: 0.05 to 0.3% Nb: 0.005 to 0.1% Ti: 0.005 to 0.05% Ta: 0.005 to 0.5% (1) and (2), wherein the balance is Fe and unavoidable impurities.
Or the high-strength PC steel rod according to (3).

(7) By weight%, C: 0.2 to 0.4% Si: 3.0% or less Mn: 0.2 to 2.0% Al: 0.005 to 0.1% , 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: contains one or more of 0.0003 to 0.005%, and V: 0.05 to 0.3% Nb: 0.005 to 0.1% Ti: 0.005 to (1), (2) wherein one or more of 0.05% Ta: 0.005 to 0.5% is contained, and the balance is Fe and unavoidable impurities.
Or the high-strength PC steel rod according to (3).

(8) C: 0.2 to 0.4% by weight Si: 3.0% or less Mn: 0.2 to 2.0% Al: 0.005 to 0.1% with the balance being the balance Of steel consisting of Fe and unavoidable impurities
c₁~ Ac _ThreeAfter heating to the temperature range, cooling at 20 ° C / sec or more
Of high-strength PC steel bars characterized by cooling at a high speed
Method.

(9) By weight%, C: 0.2 to 0.4% Si: 3.0% or less Mn: 0.2 to 2.0% Al: 0.005 to 0.1% A steel whose balance is composed of Fe and unavoidable impurities
c₁~ Ac _ThreeAfter heating to the temperature range, cooling at 20 ° C / sec or more
Cooling at a speed and then in a temperature range of 200-600 ° C
A method for producing a high-strength PC steel bar, characterized by tempering.

(10) By weight%, C: 0.2 to 0.4% Si: 3.0% or less Mn: 0.2 to 2.0% Al: 0.005 to 0.1% A steel whose balance is composed of Fe and unavoidable impurities
c₁~ Ac _ThreeAfter heating to the temperature range, cooling at 20 ° C / sec or more
Cool at speed, then apply plastic strain of 0.3% or more
And then tempered in the temperature range of 200 to 600 ° C.
A method for producing a high-strength PC steel bar, characterized by the following.

(11) In weight%, C: 0.2 to 0.4% Si: 3.0% or less Mn: 0.2 to 2.0% Al: 0.005 to 0.1% A steel whose balance is composed of Fe and unavoidable impurities
c₁~ Ac _ThreeAfter heating to the temperature range, cooling at 20 ° C / sec or more
Cooling at a speed, then to a temperature range of 200-600 ° C
It is characterized by applying plastic strain of 0.3% or more by heating
Of producing high-strength PC steel bars.

(12) Cr: 0.05 to 2.0% Mo: 0.05 to 1.0% Ni: 0.05 to 5.0% Cu: 0.05 to 1.0% by weight% (8), (9), (10) or (11), wherein one or more of W: 0.05 to 0.5% B: 0.0003 to 0.005% are contained. )).

(13) By weight%, V: 0.05-0.3% Nb: 0.005-0.1% Ti: 0.005-0.05% Ta: 0.005-0.5% The method for producing a high-strength PC steel rod according to the above (8), (9), (10) or (11), comprising one or more of the following.

(14) Cr: 0.05-2.0% Mo: 0.05-1.0% Ni: 0.05-5.0% Cu: 0.05-1.0% by weight% W: 0.05 to 0.5% B: One or more of 0.0003 to 0.005%, and V: 0.05 to 0.3% Nb: 0.005 to 0.5% (8), (9), (10) or (1) wherein one or more of 1% Ti: 0.005 to 0.05% Ta: 0.005 to 0.5% is contained. (11) The method for producing a high-strength PC steel rod according to (11).

[0024]

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 uniform elongation of the high-strength PC steel rod intended in the present invention, is described.

FIG. 2 shows an example of a scanning electron micrograph of the layered structure of martensite or tempered martensite and ferrite of the present invention. FIG. 2 shows the case of martensite and ferrite, and it can be seen that martensite and ferrite have a fine layered structure. 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 analysis of the influence of the microstructure on the delayed fracture characteristics and uniform elongation. It can be seen that by forming a layered structure of martensite or tempered martensite and ferrite, 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 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 martensite or tempered martensite is preferably 10 μm or less, More preferable conditions are 5 μm or less. The average interval can be measured by a cutting method. Further, even if residual 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. In the layered structure of martensite or tempered martensite, the ferrite fraction is 10 to 10.
A preferred range is 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. In order to simultaneously enhance the strength and delayed fracture characteristics of PC steel bars and improve uniform elongation, a more preferable range of the ferrite fraction is as follows.
20 to 60%.

In the layered structure of tempered martensite and ferrite, when one or more carbon nitrides of V, Nb, Ti, and Ta are precipitated in the ferrite, delayed fracture occurs as shown in FIG. The characteristics are further improved. Furthermore,
Precipitation strengthening has the effect of increasing strength and at the same time improving relaxation characteristics.

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, while 0.4% 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 the effect of improving the relaxation properties and increasing the strength by the 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 coarse by deoxidizing and forming AlN. However, 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 targeted by the present invention. In the present invention, (A) Cr: 0.05 to 2.0% Mo: 0.05 to 1.0 % Ni: 0.05 to 5.0% Cu: 0.05 to 1.0% W: 0.05 to 0.5% B: 0.0003 to 0.005% (B) V: 0.05 to 0.3% Nb: 0.005 to 0.1% Ti: 0.005 to 0.05% Ta: 0.005 to 0.5% (A) and / or (B). The elements of group (A) are added mainly for the purpose of improving hardenability and delayed fracture characteristics. On the other hand, the elements of the group (B) are added for the purpose of precipitating carbonitride in the ferrite as described above, improving the delayed fracture and relaxation properties, increasing the strength, and further refining the crystal grains. Is what you do.

Cr: Cr is an element effective for improving hardenability and delayed fracture characteristics and increasing softening resistance during tempering, but if it is less than 0.05%, its effect cannot be fully 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 which improves hardenability and delayed fracture characteristics like Cr, has a strong tempering softening resistance and 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 the 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 increasing 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 improving hardenability. However, if B is less than 0.0003%, the above effect is not exhibited, and Even if it exceeds 0.005%, the effect is saturated, so that 0.0
003 to 0.005%.

V: V forms carbonitride by
It has the effect of improving the delayed fracture characteristics and increasing the yield strength and the relaxation characteristics.
If it is less than 5%, the above effect cannot be obtained, while 0.3%
Exceeds 0.05%, the effect is saturated.

Nb: Nb also produces carbonitrides like V, thereby improving delayed fracture characteristics, increasing yield strength and improving relaxation characteristics. If less than 0.005%, the above effect is insufficient,
On the other hand, if it exceeds 0.1%, this effect is saturated, so that 0.0%
Limited to 05-0.1%.

Ti: Ti is an element effective for improving delayed fracture characteristics and increasing yield strength and relaxation characteristics by deoxidizing and forming carbonitrides. Effect is not exhibited, and even if it exceeds 0.05%, the effect is saturated.
It was limited to the range of 0.05 to 0.05%.

Ta: Like V, Ta also produces carbonitrides, thereby improving delayed fracture characteristics, increasing yield strength, and improving relaxation characteristics. The effect is not exhibited, and the effect is saturated even if added over 0.5%.
It was limited to 0.005 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 according to the present invention,
Ac₁~ Ac _ThreeAfter heating to the temperature range of
Cool at a cooling rate of at least
Tempering or cooling in the temperature range of 200-600 ° C
Later, a plastic strain of 0.3% or more is applied, and
Tempering in the temperature range of 600 ° C. or 2 after cooling
0.3% or more plasticity by heating to the temperature range of 00-600 ° C
This is to impart distortion. Below are the reasons for limiting the manufacturing conditions
State.

The steel material has two phases of ferrite and austenite
In this case, the heating temperature is Ac₁
Less than or Ac _ThreeCrosses the
Or tempered martensite and ferrite layered set
Since weaving cannot be obtained, the heating temperature range is set to Ac₁~ Ac _ThreeLimited to
Specified. Note that (Ac₁+10) to (Ac _Three-10) ° C
The range is a more preferable condition. The heating time depends on the heating method
There is no particular limitation because it differs depending on the
This is a good condition. This means that less than 2 seconds
This is because there is a possibility that the men- tite may remain. Also,
Blow with martensite or tempered martensite
In order to refine the layered structure of the
Is martensite or tempered martensite or
Is based on bainite or perlite.
It is desirable to adjust the ferrite fraction at this time.
Is a condition that is preferably less than 10%. What
As a means to control the structure before heating, after hot rolling
Adjustment of the cooling rate of
-Adjustment of the cooling rate after the
It does not matter if it is shifted.

If the cooling rate after heating is less than 20 ° C./sec, a large amount of ferrite, pearlite, and bainite will be formed during cooling and the strength is likely to decrease. Limited to seconds. 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 steels containing V, Nb, Ti, and Ta, tempering treatment produces carbonitrides of these alloying elements in ferrite, thereby improving delayed fracture characteristics in addition to relaxation characteristics. At this time, if the temperature is lower than 200 ° C., the effect of the tempering is small.
It was limited to 00-600 ° 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, after cooling, 0.3%
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, Ac₁~ Ac _ThreeHeating to the temperature range
Before heating, after heating / cooling or after tempering
Light wire drawing or plasticity for purposes such as shaping and deforming
Delayed fracture characteristics and remarkable deterioration of uniform elongation even after processing
There is no restriction.

[0049]

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

The steel having the chemical composition shown in Table 1 was hot-pressed.
Adjustment of cooling rate after rolling or after austenitizing
After quenching and tempering into various structural forms,
c₁~ Ac _ThreeHeating to the temperature range of
Then, it was cooled. Tempering is 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.
Table 2 shows the results.

Test No. 2 in Table 2 1 to 16 are examples of the present invention,
Test No. 3 in Table 3 17 to 26 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 is 1230 MPa or more, and a fine layered structure of martensite or tempered martensite and ferrite, or tempered martensite and carbonitride are formed. By forming a fine layered structure of the precipitated ferrite, a PC steel rod having a higher critical diffusible hydrogen content, superior delayed fracture characteristics, and high uniform elongation than conventional PC steel rods 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 a normal quenching and tempering treatment, and the structure is an example of a single structure of tempered martensite. For this reason, the critical diffusible hydrogen content is low, the delayed fracture characteristic of the spot weld is poor, and the uniform elongation is low.

The comparative example No. 20 and 21 are examples where the heating temperature is inappropriate. That is, No. In No. 20, since the heating temperature exceeded Ac3, a single phase of austenite was formed, and a single structure of tempered martensite was obtained, so that uniform elongation was low and delayed fracture characteristics were not improved. In addition, No. 21 has a heating temperature of Ac1 or lower,
A high temperature tempered martensite structure in which cementite is spheroidized, and high strength has not been achieved.

In addition, in Comparative Example No. Sample No. 22 is an example in which a large amount of ferrite and pearlite were generated during cooling because the cooling rate after heating was too low, and the strength was reduced.

Further, in Comparative Example No. 23-26
Steel is an inappropriate example. That is, No. In No. 23, the C content was too high. No. 25 has high Mn content and N
o. Sample No. 26 is an example in which the Si content was too high, cracks were generated in the spot welds, and the delayed fracture characteristics of the welds were significantly deteriorated. No. In No. 24, the intended strength could not be achieved because the C content was too low.

[0057]

[Table 1]

[0058]

[Table 2]

[0059]

[Table 3]

[0060]

As is clear from the above examples, the present invention provides a fine layered structure of martensite or tempered martensite and ferrite, or a fine layered structure of tempered martensite and ferrite in which carbonitride is precipitated. By doing so, it is possible to significantly improve the delayed fracture characteristics and uniform elongation of the high strength PC steel rod capable of spot welding, and by optimally selecting the chemical composition of the steel, heat treatment conditions, etc. The production 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 of a metal structure showing an example of a layered structure of martensite and ferrite.

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 (14)

[Claims] 1. A high-strength steel comprising a steel having substantially a layered structure of martensite and ferrite.
C steel rod. 2. A high-strength PC steel rod characterized in that the steel structure substantially comprises a layered structure of tempered martensite and ferrite. 3. The steel structure is substantially composed of a layered region of tempered martensite and ferrite, and one or more carbon nitrides of V, Nb, Ti, and Ta are precipitated in the ferrite. A high-strength PC steel rod characterized in that: 4. The composition according to claim 1, wherein C: 0.2 to 0.4% Si: 3.0% or less Mn: 0.2 to 2.0% Al: 0.005 to 0.1% 3. The high-strength PC steel rod according to claim 1, wherein the balance comprises Fe and unavoidable impurities. 5. The composition according to claim 1, wherein C: 0.2 to 0.4% Si: 3.0% or less Mn: 0.2 to 2.0% Al: 0.005 to 0.1% Further, Cr: 0.05 to 2.0% Mo: 0.05 to 1.0% Ni: 0.05 to 5.0% Cu: 0.05 to 1.0% W: 0.05 to 0.5 The high-strength PC steel rod according to claim 1 or 2, wherein one or more kinds of B are contained, and the balance consists of Fe and unavoidable impurities. 6. In% by weight, C: 0.2 to 0.4% Si: 3.0% or less Mn: 0.2 to 2.0% Al: 0.005 to 0.1% V: 0.05 to 0.3% Nb: 0.005 to 0.1% Ti: 0.005 to 0.05% Ta: One or more of 0.005 to 0.5% The high-strength PC steel rod according to any one of claims 1, 2 and 3, wherein the balance consists of Fe and unavoidable impurities. 7. In% by weight, C: 0.2 to 0.4% Si: 3.0% or less Mn: 0.2 to 2.0% Al: 0.005 to 0.1% Further, Cr: 0.05 to 2.0% Mo: 0.05 to 1.0% Ni: 0.05 to 5.0% Cu: 0.05 to 1.0% W: 0.05 to 0.5 % B: contains one or more of 0.0003 to 0.005%, and V: 0.05 to 0.3% Nb: 0.005 to 0.1% Ti: 0.005 to 0% 5.05% Ta: 0.005 to 0.5%, and the balance is Fe and unavoidable impurities. A high-strength PC steel rod as described in Crab. 8. In% by weight, C: 0.2 to 0.4% Si: 3.0% or less Mn: 0.2 to 2.0% Al: 0.005 to 0.1%, with the balance being the balance A steel consisting of Fe and unavoidable impurities
c₁~ Ac _ThreeAfter heating to the temperature range, cooling at 20 ° C / sec or more
Of high-strength PC steel bars characterized by cooling at a high speed
Method. 9. In% by weight, C: 0.2 to 0.4% Si: 3.0% or less Mn: 0.2 to 2.0% Al: 0.005 to 0.1%, with the balance being the balance Of steel consisting of Fe and unavoidable impurities
c₁~ Ac _ThreeAfter heating to the temperature range, cooling at 20 ° C / sec or more
Cooling at a speed and then in a temperature range of 200-600 ° C
A method for producing a high-strength PC steel bar, characterized by tempering. 10. In% by weight, C: 0.2 to 0.4% Si: 3.0% or less Mn: 0.2 to 2.0% Al: 0.005 to 0.1%, with the balance being the balance Of steel consisting of Fe and unavoidable impurities
c₁~ Ac _ThreeAfter heating to the temperature range, cooling at 20 ° C / sec or more
Cool at speed, then apply plastic strain of 0.3% or more
And then tempered in the temperature range of 200 to 600 ° C.
A method for producing a high-strength PC steel bar, characterized by the following. 11. In% by weight, C: 0.2 to 0.4% Si: 3.0% or less Mn: 0.2 to 2.0% Al: 0.005 to 0.1% Of steel consisting of Fe and unavoidable impurities
c₁~ Ac _ThreeAfter heating to the temperature range, cooling at 20 ° C / sec or more
Cooling at a speed, then to a temperature range of 200-600 ° C
It is characterized by applying plastic strain of 0.3% or more by heating
Of producing high-strength PC steel bars. 12. In weight%, Cr: 0.05 to 2.0% Mo: 0.05 to 1.0% Ni: 0.05 to 5.0% Cu: 0.05 to 1.0% W : 0.05 to 0.5%, B: 0.0003 to 0.005%, and one or more of the following. Of producing high-strength PC steel bars. 13. In% by weight, V: 0.05 to 0.3% Nb: 0.005 to 0.1% Ti: 0.005 to 0.05% Ta: 0.005 to 0.5% The method for producing a high-strength PC steel rod according to any one of claims 8, 9, 10 and 11, wherein the method comprises one or more kinds. 14. Cr: 0.05 to 2.0% Mo: 0.05 to 1.0% Ni: 0.05 to 5.0% Cu: 0.05 to 1.0% W by weight% : 0.05 to 0.5% B: One or more of 0.0003 to 0.005% V: 0.05 to 0.3% Nb: 0.005 to 0.1 % Ti: 0.005 to 0.05% Ta: 0.005 to 0.5% 1 or 2 or more of the following: 3. The method for producing a high-strength PC steel rod according to item 1.