JPH10265904A - Production of pc steel bar having high uniform elongation, low yield ratio, and excellent high temperature relaxation characteristic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a PC steel bar having a high uniform elongation and low yield ratio, excellent in relaxation characteristic at a high temp., and having tensile strength of a specific value or above by hardening a steel, having a specific composition where large amounts of C and (Si+Al) are added, from a high temp. in a specific range. SOLUTION: A steel, which has a composition containing, by weight, 0.5-0.7% C, 0.2-2.0% Mn, and 2.0-5.0% of (Si+Al) and also containing, if necessary, one or more kinds among 0.01-0.1% Nb, 0.01-0.05% Ti, 0.0003-0.005% B, 0.2-1.0% Ni, 0.2-1.0% Cu, 0.1-1.0% Cr, and 0.05-0.5% Mo, is hardened from 1160-1300 deg.C and then tempered. By this procedure, the desired PC steel bar, having >=1080 N/mm<2> tensile strength, can be obtained. Although C contributes greatly to the reduction of low yield ratio and the improvement of uniform elongation under the condition that the amount of (Si+Al) is large, such a remarkable improvement in ductility cannot be expected when the content of C is below the lower limit, and, when it exceeds the upper limit, ductility is deteriorated. Although (Si+Al) can controbute greatly toward improving uniform elongation, the effect is reduced when the content of (Si+Al) is below the lower limit, and workability is deteriorated when it exceeds the upper limit.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a P which is used for PC structures such as PC poles and PC piles and has a high uniform elongation, a low yield ratio and excellent high temperature relaxation characteristics.
The present invention relates to a method for manufacturing a C steel rod.

[0002]

2. Description of the Related Art PC steel bars are high-strength members used for prestressed concrete. And such a P
C steel bars are JIS G 3109 “PC steel bars” and J
The tensile strength, proof stress, elongation, and relaxation value of ISG 3137 “Small Diameter PC Steel Bar” are standardized. Specifically, JIS G 3109 and JIS
G 3137 Class B (tensile strength ≧ 1080)
N / mm ^2, yield strength ≧ 930N / mm ²⁾ and Class C (tensile strength ≧ 1230N / mm ^2, yield strength ≧ 1080N / mm
² ), Class D (tensile strength ≧ 1420 N / mm ² , yield strength ≧ 1275 N / mm ² ) is standardized in JIS G 3137. However, there is no standardization of uniform elongation, yield ratio and high temperature relaxation.

[0003] Incidentally, a prestressed concrete structure may cause a bending failure during an earthquake, and in order to prevent the failure, it is effective to increase the bending toughness by using a high uniform elongation PC steel rod. Are known. Also,
Recently, it has been found that PC steel rods also need to have a low yield ratio in order to fully utilize the high uniform elongation characteristics in concrete structures. If the yield ratio is high even if the uniform elongation is excellent, if the concrete cracks, the PC steel bar at that site is locally deformed, leading to breakage. That is, from the viewpoint of earthquake resistance, the PC steel rod is required to have a low yield ratio and a high uniform elongation. These PC steel bars may be used together with lateral restraint of concrete.

In order to shorten the curing time, concrete curing at the time of manufacturing a prestressed concrete pile or pole is often performed by autoclave curing at 180 to 200 ° C. in an atmosphere of about 10 atm. However, the autoclave curing requires PC in concrete.
There is a disadvantage that relaxation of the steel bar increases,
There is a demand for a PC steel rod having a small relaxation at such a high temperature.

Improvement techniques have been established for the individual properties of uniform elongation and high-temperature relaxation, but a PC steel rod excellent in both uniform elongation and high-temperature relaxation properties has not yet been established.

On the other hand, several methods have been proposed for improving both uniform elongation and high-temperature relaxation properties. The method disclosed in Japanese Patent Publication No. 3-79410 is a method of imparting a small bending strain of 2% or less at the time of tempering, and the method disclosed in Japanese Patent Application Laid-Open No. 3-28351 is processing at 900 to 400 ° C. or plastic working. A method in which the temperature is increased while adding the material to reverse transformation to obtain an ultrafine structure;
Japanese Patent No. 120738 discloses a method of performing cooling at the time of straightening using a wet linear machine. JP-A-8-158010 discloses a method in which Si + Al is added in a large amount of 2% to 5%.

[0007]

However, the above-mentioned Tokuhei 3
-79410, JP-A-3-28351, JP-A-58
The methods disclosed in the respective publications of -1203838 require a step of imparting distortion, and have the disadvantage of low productivity. The method disclosed in Japanese Patent Application Laid-Open No. 8-158010 is excellent in productivity, is excellent in uniform elongation and high-temperature relaxation properties, but is 1450 N /
The uniform elongation is about 5% at the mm ² class strength level, and it cannot be said that a drastic improvement is realized as compared with the current 3%. Further, there is a problem that the yield ratio is as high as 0.95 or more, and the high uniform elongation property may not be actually utilized in the concrete structure. Also, in these publications,
In any case, there is no mention of a technique for realizing a low yield ratio.

The present invention has been made in view of the above circumstances, and can be manufactured by an ordinary process without requiring a strain imparting process, and has excellent uniformity, a low yield ratio, and excellent relaxation characteristics at high temperatures. Tensile strength 1080 N / mm
^An object of the present invention is to provide a method for manufacturing ^two or more PC steel bars.

[0009]

Means for Solving the Problems As a result of repeated studies to solve the above-mentioned problems, the present inventors have found that in order to achieve high uniform elongation, a low yield ratio, and excellent high-temperature relaxation characteristics, It has been found that it is effective to add a large amount of C and Si + Al and quench them from a high temperature.

The present invention has been made based on such findings, and firstly, as a chemical component, C:
0.5% to 0.7%, Mn: 0.2% to 2.0%, steel containing Si and Al to satisfy 2.0% ≦ Si + Al ≦ 5.0% from 1160 ° C. to 1300 ° C. Quenching,
This is a method for producing a PC steel rod having a tensile strength of 1080 N / mm ² or more, a high uniform elongation, a low yield ratio, and excellent high-temperature relaxation characteristics, which is characterized by tempering.

Secondly, C: 0.5% by weight as a chemical component.
To 0.7%, Mn: 0.2 to 2.0%, Si and Al
2.0% ≦ Si + Al ≦ 5.0%, Nb: 0.01-0.1%, Ti: 0.0
1 to 0.05%, B: 0.0003 to 0.005%, N
i: 0.2 to 1.0%, Cr: 0.1 to 1.0%, M
o: steel containing at least one of 0.05 to 0.5% and Cu: 0.2 to 1.0% is 1160 ° C. to 1300
108, characterized by quenching from ℃ and then tempering.
This is a method for producing a PC steel rod having a tensile strength of 0 N / mm ² or more, high uniform elongation, a low yield ratio, and excellent high-temperature relaxation properties.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the components of the present invention and the heat treatment conditions will be described below. C is an element necessary for enhancing hardenability and increasing strength. It is also an element that raises the tempering temperature, and greatly contributes to lowering the yield ratio and improving uniform elongation under the condition that a large amount of Si + Al is added. If it is less than 0.5 wt%, such a dramatic improvement in ductility cannot be expected. On the other hand, if it exceeds 0.7 wt%, the ductility is rather deteriorated. Therefore, the amount of C is 0.5w
At least t% and at most 0.7 wt%.

[0013] Si and Al are used as deoxidizers,
In addition to being effective for relaxation properties,
It greatly contributes to the improvement of uniform elongation. However, Si + Al
If the value is less than 2.0 wt%, the effect is small. On the other hand, if the value exceeds 5.0 wt%, the relaxation characteristics deteriorate. Therefore, the value of Si + Al is set to 2.0 wt% or less.
5.0 wt%.

Mn is a deoxidizer like Si, and
Like C, it is an element necessary for enhancing hardenability and improving strength. However, if the amount is less than 0.2 wt%, the effect is small, while if it exceeds 2.0 wt%, the workability decreases.
Therefore, the amount of Mn is set to 0.2 to 2.0 wt%.

Nb, Ti, B, Ni, Cr, Mo, Cu
Are elements that contribute to the improvement of hardenability, and one or more of them are added as necessary. However, when these amounts are too small, such effects cannot be obtained, and when they are too large, the effects are saturated. Therefore, Nb: 0.01 to 0.1%,
Ti: 0.01-0.05 wt%, B: 0.0003-
0.005 wt%, Ni: 0.2-1.0 wt%, C
r: 0.1 to 1.0 wt%, Mo: 0.05 to 0.5
%, Cu: 0.2 to 1.0 wt%.

[0016] The quenching temperature has a large effect on the yield ratio through a change in particle size. In addition, the uniform elongation is improved in each step. If the quenching temperature is less than 1160 ° C., the particle size is small, and the yield ratio does not decrease significantly.
60 ° C. or higher. If the temperature exceeds 1300 ° C., decarburization becomes remarkable, so the temperature is set to 1300 ° C. or less.

[0017]

【Example】

(Example 1) Steel composition shown in Table 1 as sample steels, rolled into a round bar having a diameter of 8 mm, pulling the profiled rod having a diameter of 7.1 mm, was subjected to quenching and tempering by subsequent high-frequency heating. Note that the heat treatment is not limited to high-frequency heating.
Here, the quenching heating temperature is 1160 ° C. to 1300 ° C. (example of the present invention), and the tempering is JIS G 3109 (JISG).
3137) to obtain an intensity level of the C class or D class standard. Thereafter, they were subjected to a tensile test. The uniform elongation was measured by marking 2d (d is the diameter) on the test material and measuring 10d as a gauge length at a distance of 5d or more from the fractured portion. Table 2 shows the results of the tensile test. The relaxation test was conducted at room temperature according to JIS G3
109 (same for JIS G 3137). However, the value of 1000 hours has been standardized since 1994, but this time, a simple method was used to evaluate the value of 10 hours before that. In addition, the test was performed at 180 ° C. at a high temperature, a stress of 70% of the standard tensile strength was applied, the sample was cooled by the heating pattern shown in FIG. 1, the change in load after 23 hours was measured, and the ratio to the initial load was evaluated. did. Table 2 shows the results.

As shown in Table 2, No. 1 to No. 13
All of the examples of the present invention have a uniform elongation of 10.0% or more, a yield ratio of 0.87 or less and a high-temperature relaxation of 8% or less at a strength level of Class C standard, and a uniform elongation of 8% at a strength level of Class D standard. As described above, a yield ratio of 0.90 or less and a high temperature relaxation of 8% or less have been achieved, and it has been confirmed that both properties are significantly superior to those of conventional steel. The tempering temperature is desirably 600 ° C. or higher in order to significantly improve the uniform elongation.
Tempered at a high temperature of 0 ° C or higher to obtain a class D standard strength.
It can be seen that a uniform elongation of at least% is easily secured.

On the other hand, in Comparative Example No. No. 14 has a C content of No. 14; In No. 16, since the amount of Si + Al is insufficient, the tempering temperature is low, the yield ratio is not remarkably reduced, and the uniform elongation is significantly deteriorated to 4 to 5% in the class D standard and around 7% in the class C standard. No. 15 and No. 17 has a high tempering temperature,
Although the yield ratio is low, the uniform elongation is slightly lower than that of the present invention because the C amount and the Mn amount exceed the appropriate amounts, respectively. %that's all,
The uniform elongation of 8% or more could not be achieved in the class D standard. N
o. In No. 18, although the chemical components were appropriate, the quenching temperature was as low as 1100 ° C., so that the yield ratio did not decrease and the uniform elongation did not achieve the target value. (Example 2) Steel composition shown in Table 1 as sample steels, rolled into a round bar having a diameter of 24 mm, pulling the profiled rod having a diameter of 23 mm, was subjected to quenching and tempering by subsequent high-frequency heating. Note that the heat treatment is not limited to high-frequency heating.
Here, the quenching heating temperature is 1160 ° C. to 1300 ° C. (example of the present invention), and the tempering is JIS G 3109 (JISG).
3137) to obtain the intensity level of the Class B standard. Thereafter, they were subjected to a tensile test and a relaxation test. The test method is the same as in Example 1.

Table 3 shows the heat treatment conditions and the obtained mechanical properties.
Shown in No. No. 1 to No. All of the 13 steels of the present invention have a uniform elongation of 11.0% or more at the strength level of Class B standard,
A yield ratio of 0.85 or less and a high-temperature relaxation of 8% or less have been achieved, and it has been confirmed that both properties are significantly superior to those of conventional steel. On the other hand, in Comparative Example No. 1
No. 4 has a C content of No. 4; In No. 16, since the amount of Si + Al was insufficient, the tempering temperature was lower than that of the example of the present invention, the yield ratio did not decrease remarkably as in the case of Example 1, and the uniform elongation was deteriorated to about 8% in the B class standard. No. 15 and No. 17
Have a high tempering temperature and a low yield ratio.
Since the amount and the amount of Mn exceeded the appropriate values, the uniform elongation was slightly lower than that of the examples of the present invention, and the uniform elongation of 11.0% or more could not be attained in the target class B standard. No. 18
In the same manner as in Example 1, despite the appropriate chemical components, the quenching temperature was as low as 1100 ° C., so that the yield ratio did not decrease and the uniform elongation could not achieve the target value.

[0021]

[Table 1]

[0022]

[Table 2]

[0023]

[Table 3]

[0024]

As described above, according to the method of the present invention, there is no need to add a step such as strain imparting, and it can be manufactured by a normal step, and has a high tensile strength of 1080 N / mm ² or more. It is possible to provide a PC steel rod which has excellent elongation, a low yield ratio, and excellent high-temperature relaxation characteristics. Therefore, the present invention is extremely useful in industry, such as greatly improving productivity and dramatically improving the reliability of a prestressed concrete structure.

[Brief description of the drawings]

FIG. 1 is a diagram showing a temperature cycle in a high-temperature relaxation test.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Shigeru Mizoguchi 5983 Tamura, Himuratsuka-shi, Kanagawa Pref. 72) Inventor Kazuki Nitta 5893 Tamura, Hiratsuka-shi, Kanagawa Prefecture

Claims (2)

[Claims] C: 0.5 to 0.7% by weight, Mn:
0.2-2.0%, Si and Al are 2.0% ≦ Si +
1160 steel contained to satisfy Al ≦ 5.0%
A method for producing a PC steel rod having a tensile strength of 1080 N / mm ² or more, a high uniform elongation, a low yield ratio, and excellent high-temperature relaxation properties, characterized by quenching from 1 ° C. to 1300 ° C. and then tempering. 2. C: 0.5 to 0.7% by weight%, Mn:
0.2-2.0%, Si and Al are 2.0% ≦ Si +
Al ≦ 5.0% is contained so as to further satisfy Nb:
0.01-0.1%, Ti: 0.01-0.05%,
B: 0.0003-0.005%, Ni: 0.2-1.
0%, Cr: 0.1-1.0%, Mo: 0.05-0.
5%, Cu: A steel containing at least one of 0.2 to 1.0% is quenched from 1160 ° C. to 1300 ° C., and then tempered, and has a tensile strength of 1080 N / mm ² or more. A method for producing a PC steel rod having high uniform elongation, a low yield ratio, and excellent high-temperature relaxation characteristics.