JP3457498B2 - High-strength PC steel bar and method of manufacturing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a PC steel rod which is widely used as a reinforcing material for prestressed concrete structures such as poles, piles and buildings, bridges, etc. 1230
The present invention relates to a high-strength PC steel bar excellent in delayed fracture property and uniform elongation, which is not less than MPa, and a manufacturing method thereof.

[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 and bridges, is usually PC steel wire, PC steel stranded wire, or PC steel rod. . The PC steel wire is manufactured by performing a patenting treatment and then drawing. On the other hand, the PC steel bar has a C content of 0.25 to 0.3 as described in, for example, Japanese Patent Publication No. 5-41684.
It is manufactured by hot rolling using 5% medium carbon steel, followed by quenching and tempering. Although the strength of PC steel wire is higher than that of PC steel bar, it has the drawback that spot welding cannot be performed because of the high C content. On the other hand, the spot weldability of PC steel rods is better than that of PC steel wires, but the "prestressed concrete design and construction standards-
As described on pages 43 to 45 of “Commentary” (edited by Architectural Institute of Japan, Maruzen), the strength is 1275 MPa (130
High-strength PC steel rods that exceed kgf / mm ² ) have P
Delayed fracture characteristics are inferior to C steel wire. Further, as described in Japanese Patent Publication No. 5-59967, since the spot weld is rapidly cooled, it has a structure mainly composed of martensite, and there is a problem that delayed fracture is likely to occur in the spot weld. . Furthermore, recently, Japanese Unexamined Patent Publication No.
As described in JP-A-3396, there is a demand for the development of a PC steel rod having excellent uniform elongation in order to improve bending toughness for applications such as piles.

As a conventional finding for improving the delayed fracture property of PC steel rods, for example, Japanese Patent Publication No. 5-59967 discloses that it is effective to reduce the P and S contents. Certainly, lowering P and lowering S is effective for delayed fracture, but the P and S contents of the current PC steel bar are already around 0.01%. Although it is possible to further reduce the P and S contents, there is a drawback that the manufacturing cost becomes high. Japanese Examined Patent Publication (Kokoku) No. 5-41684 proposes to control the Si and Mn contents and to perform bending or drawing in the tempering process after the quenching process. Further, Japanese Patent Laid-Open No. 5-7963 proposes to provide a resin coating layer around the spot welded portion between the PC steel rod and the iron wire to reduce the susceptibility to delayed fracture. Further, Japanese Patent Laid-Open No. 6-212346 proposes a means for improving delayed fracture characteristics by adding an alloying element such as Ni. However, none of the proposals has significantly improved delayed fracture characteristics.

On the other hand, as a conventional finding for improving the uniform elongation of PC steel rods, for example, Japanese Patent Laid-Open No. 7-3396 proposes a method of performing tempering at a high temperature of 500 ° C. or higher. Has a defect that spot welding cannot be performed because of a high value of 0.45% or more. In addition, JP-A-8-
Japanese Patent No. 158010 describes that addition of a large amount of Si and Al is effective for improving uniform elongation. However, if Si and Al are added in a large amount, there is a drawback that the delayed fracture property of the spot welded portion is deteriorated and the manufacturing cost is increased.

As described above, according to the prior art, in the high strength PC steel bar capable of spot welding, not only the individual problems of delayed fracture property and uniform elongation but also the high strength of which both are drastically improved. There were limits to the production of PC steel rods.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and is capable of spot welding, has a delayed fracture characteristic and a good strength of uniform elongation 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 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 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 content" at which delayed fracture does not occur
It was evaluated by determining. In this method, various amounts of diffusible hydrogen are contained in a sample by electrolytic hydrogen charging, and then Cd plating is performed to prevent hydrogen from being released from the sample to the atmosphere during a delayed fracture test, and then,
This is to evaluate the amount of diffusible hydrogen at which a prescribed load is applied in the atmosphere and delayed fracture does not occur. FIG. 1 shows an example of the 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.

[0008] As a result of various studies from the viewpoint of the microstructure, a means for increasing the critical diffusible hydrogen content of a high-strength PC steel bar, that is, delayed fracture characteristics and uniform elongation, has been found to be martensite or tempered martensite. 1230MP if a layered structure composed of and ferrite is formed, or if a layered structure of ferrite and martensite or tempered martensite in which carbonitride is deposited is formed.
Even in a high-strength region exceeding a, the critical diffusible hydrogen content was high, and a completely new finding was obtained that the delayed fracture characteristics were significantly improved. Moreover, it was found that uniform elongation was remarkably improved by controlling such a layered structure. Furthermore, the relationship between the layered structure consisting of martensite or tempered martensite and ferrite, the carbonitride precipitated in ferrite, delayed fracture properties, and uniform elongation is analyzed in detail, and the optimum manufacturing conditions are established by establishing the optimum manufacturing conditions. It is an invention.

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

(1) % by mass, C: 0.2 to 0.4%, Si: 3.0% or less, Mn: 0.2 to 2.0%, Al: 0.005 to 0.1% And the balance consists of Fe and unavoidable impurities, and the steel structure consists essentially of a layered structure of martensite and ferrite (excluding those oriented in the longitudinal direction of the PC steel bar).
Ri, high-strength PC steel rod average spacing between martensite characterized der Rukoto below 10 [mu] m.

(2) In mass%, C: 0.2 to 0.4%, Si: 3.0% or less, Mn: 0.2 to 2.0%, Al: 0.005 to 0.1% Containing Fe, the balance consisting of Fe and unavoidable impurities, and the structure of the steel being substantially a layered structure of tempered martensite and ferrite (excluding those oriented in the longitudinal direction of the PC steel bar)
The high-strength PC steel rod is characterized by having an average spacing between martensites of 10 μm or less.

(3) % by mass, C: 0.2 to 0.4%, Si: 3.0% or less, Mn: 0.2 to 2.0%, Al: 0.005 to 0.1% 0.05 to 0.3%, Nb:: more V contains, the, 0.005~0.1%, Ti: 0.005~0.05% , Ta: the 0.005 to 0.5% Contains 1 or 2 or more, with the balance being Fe and unavoidable
Impurities consists, the structure of the steel consists essentially tempered martensite and ferrite lamellar structure, martensite
A high-strength PC steel rod, characterized in that the average distance between the toes is 10 μm or less, and one or more carbonitrides of V, Nb, Ti, and Ta are precipitated in the ferrite.

(4) In mass%, Cr: 0.05-2.0%, Mo : 0.05-1.0%, Ni: 0.05-5.0%, Cu: 0.05- 1.0%, W: 0.05 to 0.5%, B: 0.0003 to 0.005% , one or more kinds are contained, and the balance is Fe and inevitable impurities. The high-strength PC steel rod according to (1) or (2) above.

(5) In mass%, V: 0.05 to 0.3%, Nb: 0.005 to 0.1%, Ti: 0.005 to 0.05 %, Ta: 0.005 to 0 The high-strength PC steel rod according to the above (1) or (2), characterized in that it contains 0.5% of one kind or two or more kinds, and the balance is Fe and inevitable impurities.

(6) In mass%, Cr: 0.05-2.0%, Mo : 0.05-1.0%, Ni: 0.05-5.0%, Cu: 0.05- 1.0%, W: 0.05 to 0.5%, B: 0.0003 to 0.005% , one or more kinds are contained, and the balance is Fe and inevitable impurities. The high-strength PC steel rod according to (3) above.

(7) The ferrite fraction is in the range of 10 to 70%.
Characterized 囲Dea Rukoto have among (1) to (6)
A high-strength PC steel bar described in some cases .

(8) In mass% , 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 containing Al and the balance Fe and unavoidable impurities
After heating to a temperature range of _C1 to A _C3, the method of producing a high strength PC steel bar, characterized in that the cooling at a cooling rate of more than 20 ° C. / sec.

(9) After the manufacturing method described in (8 ) above ,
A method for producing a high-strength PC steel rod, which comprises tempering in a temperature range of 200 to 600 ° C.

(10)Of the manufacturing method described in (8) above.
Then, a plastic strain of 0.3% or more is applied, and then 200%
High strength characterized by tempering in the temperature range of ~ 600 ℃
Degree PC steel rod manufacturing method.

(11)Of the manufacturing method described in (8) above.
After that, heat to a temperature range of 200 to 600 ° C and 0.3% or more
High-strength PC steel rod characterized by imparting plastic strain of
Manufacturing method.

(12) In mass%, Cr: 0.05 to 2.0% , Mo: 0.05 to 1.0% , Ni: 0.05 to 5.0% , Cu: 0.05 to 1 0.0% , W: 0.05 to 0.5% , B: 0.0003 to 0.005%, one kind or two or more kinds of the above (8) to (11) . High strength P described in any one of
C Steel bar manufacturing method.

(13) % by mass, V: 0.05 to 0.3% , Nb: 0.005 to 0.1% , Ti: 0.005 to 0.05% , Ta: 0.005 to 0% 0.5% of 1 type (s) or 2 or more types, The high strength P as described in any one of said (8) - (11) characterized by the above-mentioned.
C Steel bar manufacturing method.

(14) In mass% , Cr: 0.05 to 2.0% , Mo: 0.05 to 1.0% , Ni: 0.05 to 5.0% , Cu: 0.05 to 1 0.0% , W: 0.05 to 0.5% , B: 0.0003 to 0.005%, one or more kinds are contained, and further , V: 0.05 to 0.3% , Nb. : 0.005 to 0.1% , Ti: 0.005 to 0.05% , Ta: 0.005 to 0.5%, and one or more of the above (8). ~ High strength P according to any one of (11)
C Steel bar manufacturing method.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION Next, embodiments of the present invention will be described. First, the reason for limiting the structure of the structure, which is the most important point for the delayed fracture property and the improvement of uniform elongation of the high-strength PC steel rod aimed at by the present invention, will be described.

An example of a scanning electron micrograph of the layered structure of martensite or tempered martensite and ferrite of the present invention is shown in FIG. FIG. 2 shows the case of martensite and ferrite, but 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 improved at the same time. An example of analysis of the influence of the structure morphology on delayed fracture characteristics and uniform elongation is shown in FIGS. 3 and 4. It can be seen that the limiting diffusible hydrogen content is high and the uniform elongation is significantly improved by using the layered structure of martensite or tempered martensite and ferrite. On the other hand, the single structure of tempered martensite obtained by the conventional quenching and tempering method has low critical diffusible hydrogen content and low uniform elongation. In order to have a high strength and to significantly improve delayed fracture characteristics and uniform elongation, in the layered structure of martensite or tempered martensite and ferrite, the average spacing between martensites or tempered martensite is preferably 10 μm or less, More preferable condition is 5 μm or less. The average interval can be measured by the 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 properties. Further, in the layered structure of martensite or tempered martensite, the ferrite fraction is 10 to
The preferable 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 becomes difficult to achieve a high strength of 1230 MPa or more. From the viewpoint of simultaneously achieving high strength and delayed fracture properties of PC steel rods and improvement of uniform elongation, the range of more preferable ferrite fraction is
20-60%.

In the layered structure of tempered martensite and ferrite, when one or more carbonitrides 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 improving relaxation characteristics.

Next, the reasons for limiting the components of the steel targeted by the present invention will be described.

C: C is an essential element for ensuring 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% is used. If it exceeds the range, the spot weldability deteriorates significantly, so the range was limited to 0.2 to 0.4%.

Si: Si has the effect of improving the relaxation characteristics and the strength of the solid solution hardening effect. Even if it exceeds 3%, the effect commensurate with the added amount cannot be expected, and the delayed fracture property of the spot welded part deteriorates.
The range was limited to 3% or less. The preferred range is 0.2 to
It is 2.5%.

Mn: Mn is an element effective not only for deoxidizing and desulfurizing but also for improving hardenability, but if it is less than 0.2%, the above effect cannot be obtained. If it exceeds 2.0%, spot weldability deteriorates.
The range was limited to 0.2 to 2.0%.

Al: Al has an effect of preventing coarsening of austenite grains by deoxidizing and forming AlN, but if it is less than 0.005%, these effects are not exhibited, and 0.1%. The effect is saturated even if it exceeds 0.
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, however, (A) Cr: 0.05 to 2.0% Mo: 0.05 to 1.0 is further added to this steel. % Ni: 0.05 to 5.0% Cu: 0.05 to 1.0% W: 0.05 to 0.5% B: 0.0003 to 0.005% One or more kinds, or (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% One or more kinds. One or both of the groups (A) and (B) can be included. 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 carbonitrides in ferrite as described above, improving delayed fracture characteristics and relaxation characteristics, increasing strength, and further refining crystal grains. To do.

Cr: Cr is an effective element for improving the hardenability and delayed fracture characteristics and increasing the softening resistance during tempering treatment, but if it is less than 0.05%, its effect cannot be sufficiently exhibited. On the other hand, if the content 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 similarly to Cr, has a strong temper softening resistance, and is effective for increasing tensile strength after heat treatment, and further improves relaxation characteristics. It also has an effect,
If it is less than 0.05%, its effect is small. On the other hand, if it exceeds 1.0%, its effect is saturated.
The range was limited to 1.0%.

Ni: Ni is added in order to enhance the hardenability and delayed fracture characteristics. If it is less than 0.05%, its effect is small, while if it exceeds 5.0%, the effect of matching the added amount is exhibited. Since it cannot be done, 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, but 0.0
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 an effect of improving delayed fracture characteristics and also has an effect of remarkably increasing hardenability, but if B is less than 0.0003%, the above effect is not exhibited and 0 The effect is saturated even if it exceeds 0.005%, so 0.0
It was limited to 003 to 0.005%.

V: V is due to the formation of carbonitride,
It has the effect of improving delayed fracture properties and increasing yield strength and relaxation properties,
If it is less than 5%, the effect of the above action cannot be obtained, while if it is 0.3%.
Since the effect is saturated even if it exceeds, the content is limited to 0.05 to 0.3%.

Nb: Nb also produces carbonitrides in the same manner as V, and thereby has the effects of improving delayed fracture characteristics and increasing yield strength and 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 0.0
It was limited to 05 to 0.1%.

Ti: Ti is an element that is effective in improving delayed fracture properties and increasing yield strength and relaxation properties by deoxidizing and forming carbonitrides. Effect is not exhibited, and the effect is saturated even if it exceeds 0.05%, so 0.0
It was limited to the range of 05 to 0.05%.

Ta: Ta also has the effect of improving delayed fracture properties and increasing yield strength and relaxation properties by forming carbonitrides like V, but if it is less than 0.005%, The effect is not exhibited, and the effect is saturated even if added over 0.5%,
It was limited to 0.005-0.5%.

Although P and S are not particularly limited,
From the viewpoint of improving the delayed fracture characteristics of PC steel rods, 0.015% or less is a preferable range. Further, N has the effect of refining crystal grains, increasing yield strength, and improving relaxation characteristics by forming nitrides of Al, V, Nb, and Ti, so 0.002 to 0.015% is preferable. It is a range.

In the method for producing a high strength PC steel rod of the present invention,
Ac steel with the above composition₁~ Ac ₃After heating to the temperature range of 20
Cool at a cooling rate of ℃ / sec or more, or after cooling
Tempering or cooling in the temperature range of 200-600 ℃
Afterwards, a plastic strain of 0.3% or more is applied, and then 200-
Tempering in the temperature range of 600 ° C or 2 after cooling
Plasticity of 0.3% or more when heated in the temperature range of 00-600 ℃
It gives a distortion. Reasons for limiting manufacturing conditions
State.

The steel material has two phases of ferrite and austenite.
The heating temperature is Ac.₁
Less than or Ac ₃When it exceeds the
Or tempered martensite and ferrite layered set
Weaving temperature range is Ac₁~ Ac ₃Limited to
Decided In addition, (Ac₁+10) to (Ac ₃-10) ° C
The range is a more preferable condition. The heating time is the heating method
There is no particular limitation because it depends on the setting, but 2 seconds or more is preferable.
It is a good condition. This is a large amount of security if it is less than 2 seconds.
This is because mentite may remain. Also,
Martensite or tempered martensite and blowjob
Steel before heating to refine the layered structure of iron
Martensite or tempered martensite or
Is mainly composed of bainite or perlite.
It is desirable to adjust the ferrite fraction at this time.
Is a condition that is preferably less than 10%. Na
As a means to control the structure before heating, after hot rolling
Adjustment of the cooling rate, or normal quenching / tempering,
ー Adjusting the cooling rate after the austenitizing process, etc.
It does not matter if the method is offset.

If the cooling rate after heating is less than 20 ° C./sec, a large amount of ferrite, pearlite, and bainite are generated during cooling, and the strength is likely to decrease, so the lower limit of the cooling rate is 20 ° C./second. Limited to seconds. From the viewpoint of preventing ferrite, pearlite, and bainite that are likely to be generated during cooling as much as possible, the preferable cooling rate is 50 ° C./second or more. The cooling end temperature is equal to or lower than the martensitic transformation temperature (Ms point or lower).

The tempering after cooling is carried out in order to increase the yield strength and improve the relaxation characteristics.
In particular, in the steel containing V, Nb, Ti, and Ta, carbonitrides of these alloying elements are generated in the ferrite by the tempering treatment, and the delayed fracture property is improved in addition to the relaxation property. At this time, if the temperature is less than 200 ° C., the effect of tempering is small, while if it exceeds 600 ° C., the strength decreases and it becomes difficult to manufacture a high-strength PC steel rod.
It was limited to 00 to 600 ° C. From the viewpoint of improving the delayed fracture property, the heating rate during tempering is preferably 20 ° C./sec or more.

In the present invention, the PC steel bar has a 0.3% content after cooling.
The above plastic strain may be applied and then tempered, or a plastic strain of 0.3% or more may be applied during tempering. The purpose is to increase the yield strength and improve relaxation characteristics. 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 and strain due to straightening.

In the present invention, Ac₁~ Ac ₃Heating to temperature range
Before adjusting, after heating / cooling or after tempering, wire diameter adjustment
Mild wire drawing or plasticity for purposes such as straightening and deforming
Even if processed, delayed fracture characteristics and uniform deterioration of remarkable elongation
There is no restriction.

[0049]

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

A steel material having the chemical composition shown in Table 1 was hot pressed.
After adjusting the cooling rate after rolling or after austenitizing treatment
After forming various structural morphologies by quenching and tempering of
c₁~ Ac ₃Heating to the temperature range of
Then, it cooled. Tempering is performed by high frequency heating, and plastic strain
Was applied using a tensile tester.

Using the above samples, the structure 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 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 test was performed by the method of JIS G3137.
The results are shown in Table 2.

Test No. 2 in Table 2 1 to 16 are examples of the present invention,
Test No. of 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 bar is 1230 MPa or more, and the fine layered structure of martensite or tempered martensite and ferrite, or the tempered martensite and carbonitride By forming a fine layered structure of the precipitated ferrite, a PC steel bar having a higher critical diffusible hydrogen content and excellent delayed fracture properties and a higher uniform elongation than the conventional PC steel bar has been realized. Moreover, relaxation is at the same level as conventional PC steel bars.

On the other hand, No. 17, 1
Nos. 8 and 19 are manufactured by the conventional manufacturing method. That is, it is manufactured by a normal quenching / tempering process, and is an example of a single structure of tempered martensite. Therefore, the critical diffusible hydrogen content is low, the delayed fracture characteristics of spot welds are poor, and the uniform elongation is also low.

No. 1 which is a comparative example. Nos. 20 and 21 are examples of inappropriate heating temperature. That is, No. In No. 20, since the heating temperature exceeded Ac3, it became a single phase of austenite, and because it had a single structure of tempered martensite, its uniform elongation was low and delayed fracture characteristics could not be improved. In addition, No. 21 has a heating temperature of less than Ac1,
High-temperature tempered martensite structure in which cementite is spheroidized and high strength cannot be achieved.

In addition, No. No. 22 is an example in which the cooling rate after heating was too low, so that a large amount of ferrite and pearlite were generated during the cooling and the strength decreased.

Furthermore, No. 1 which is a comparative example. 23 to 26 are
An example of inappropriate steel composition. That is, No. No. 23 has a too high C content, and No. 25 has a high Mn content and N
o. No. 26 is an example in which the Si content was too high, so that cracks were generated in the spot welds and the delayed fracture characteristics of the welds were significantly deteriorated. No. No. 24 cannot achieve the target strength because the C content is 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 ferrite in which tempered martensite and carbonitride are precipitated. By doing so, it is possible to greatly improve the delayed fracture characteristics and uniform elongation of the high strength PC steel bar capable of spot welding, and by optimally selecting the chemical composition of steel, heat treatment conditions, etc. The manufacturing method has been established, and the industrial effect is extremely remarkable.

[Brief description of 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 a relationship between uniform elongation and a microstructure of a PC steel rod.

Front page continuation (58) Fields surveyed (Int.Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 8/08 C21D 9/00

Claims (14)

(57) [Claims] 1. In mass%, C: 0.2 to 0.4%, Si: 3.0% or less, Mn: 0.2 to 2.0%, Al: 0.005 to 0.1% Contained, the balance being Fe and unavoidable impurities, and the steel structure consisting essentially of a layered structure of martensite and ferrite (excluding those oriented in the longitudinal direction of the PC steel rod).
Ri, high-strength PC steel rod average spacing between martensite characterized der Rukoto below 10 [mu] m. 2. In mass%, C: 0.2 to 0.4%, Si: 3.0% or less, Mn: 0.2 to 2.0%, Al: 0.005 to 0.1% Included, the balance consisting of Fe and unavoidable impurities, and the structure of the steel is substantially a layered structure of tempered martensite and ferrite (excluding those oriented in the longitudinal direction of the PC steel bar).
And the average spacing between tempered martensite is 10μ.
A high-strength PC steel rod characterized by being m or less. In 3. A mass%, C: 0.2~0.4%, Si : 3.0% or less, Mn: 0.2~2.0%, Al: the 0.005% to 0.1% It contains further V: 0.05~0.3%, Nb: 0.005~0.1 %, Ti: 0.005~0.05%, Ta: 0.005~0.5% of one or Or contains two or more kinds, the balance is Fe and unavoidable
Impurities consists, the structure of the steel consists essentially tempered martensite and ferrite lamellar structure, tempered Marte
High-strength PC characterized in that the average spacing between the sites is 10 μm or less, and one or more carbonitrides of V, Nb, Ti, and Ta are precipitated in the ferrite.
Steel rod. 4. A further Cr: 0.05~2.0%, Mo: 0.05~1.0 %, Ni: 0.05~5.0%, Cu: 0.05~1.0%, W: 0.05 to 0.5% , B: 0.0003 to 0.005%, one or more kinds are contained, and the balance is Fe and inevitable impurities. The high-strength PC steel rod described in 2. 5. In mass%, V: 0.05-0.3% , Nb: 0.005-0.1% , Ti: 0.005-0.05% , Ta: 0.005-0. The high-strength PC steel rod according to claim 1 or 2, containing 0.5% of one kind or two or more kinds, and the balance being Fe and inevitable impurities. 6. mass%, further Cr: 0.05~2.0%, Mo: 0.05~1.0 %, Ni: 0.05~5.0%, Cu: 0.05~1 0.0% , W: 0.05 to 0.5% , B: 0.0003 to 0.005%, one kind or two or more kinds are contained, and the balance is Fe and inevitable impurities. The high-strength PC steel rod according to claim 3. 7. Ferrite fraction in the range of 10-70%
It exists in any one of Claims 1-5 characterized by the above-mentioned.
High strength PC steel rod. 8. In mass% , C: 0.2-0.4% , Si: 3.0% or less , Mn: 0.2-2.0% , Al: 0.005-0.1% A containing steel with the balance being Fe and unavoidable impurities
After heating to a temperature range of _C1 to A _C3, the method of producing a high strength PC steel bar, characterized in that the cooling at a cooling rate of more than 20 ° C. / sec. 9. After the manufacturing method according to claim 8 , 200 to
A method for producing a high-strength PC steel rod, which comprises tempering in a temperature range of 600 ° C. 10. After the manufacturing method according to claim 8 , 0.3
% Or more plastic strain is applied, and then 200-600 ℃
A method for producing a high-strength PC steel rod, characterized by tempering in the temperature range of. 11. After the manufacturing method according to claim 8 , 200
A method for producing a high-strength PC steel rod, which comprises heating to a temperature range of up to 600 ° C to impart a plastic strain of 0.3% or more. In 12. wt%, 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: 0.0003-0.005% 1 type or 2 types or more are contained, The inside of Claim 8, 9, 10 and 11 characterized by the above-mentioned. The method for manufacturing a high-strength PC steel rod according to any one of 1. 13. In mass%, V: 0.05 to 0.3% , Nb: 0.005 to 0.1% , Ti: 0.005 to 0.05% , Ta: 0.005 to 0. The method for producing a high-strength PC steel rod according to any one of claims 8, 9, 10 and 11, wherein 5% of 1 type or 2 types or more is contained. 14. mass%, Cr: 0.05~2.0%, Mo : 0.05~1.0%, Ni: 0.05~5.0%, Cu: 0.05~1. 0% , W: 0.05 to 0.5% , B: 0.0003 to 0.005%, one or more kinds are contained, and V: 0.05 to 0.3% , Nb: 0. .005~0.1%, Ti: 0.005~0.05%, Ta: 0.005~0.5% of one or claims 8, 9, characterized in that it contains two or more, The method for producing a high-strength PC steel rod according to any one of 10 and 11.