JP3370582B2 - High uniform elongation Low yield ratio PC steel bar - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high uniform elongation low yield ratio PC steel rod used for PC structures such as PC poles and PC piles. [0002] PC steel bars are high-strength members used for prestressed concrete. And such a PC
Steel bars are JIS G3109 "PC steel bars" and JIS
In G3137 “Small diameter deformed PC steel bar”, tensile strength, proof stress, elongation, and relaxation value are standardized. If the tensile strength is 1420 N / mm ² or more, JI
Class D of SG3137 (TS ≧ 1420 N / mm ² , YS
≧ 1275 N / mm ² ). However, there is no standardized uniform elongation or yield ratio. [0003] Incidentally, a prestressed concrete structure sometimes undergoes bending failure during an earthquake, and it is desired to increase bending toughness in order to prevent the failure. For that purpose, it is known that the use of a high uniform elongation PC steel rod is effective. Recently, it has been found that a PC steel rod also needs to have a low yield ratio in order to make full use of high uniform elongation characteristics in a concrete structure. 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. Several techniques for improving ductility such as elongation at break, uniform elongation, and yield ratio of PC steel bars have been proposed. Here, the ductility of steel is substantially controlled by the structure determined by its chemical composition and heat treatment. Japanese Patent Application Laid-Open No. Sho 62-50408 discloses a structure factor that substantially controls the ductility. Kaihei 2-107
743 and Japanese Patent Application Laid-Open No. 3-28351. Of these, JP-A-62-50
No. 408 discloses that a wire rod in which low-temperature transformation products (including residual austenite) such as bainite or martensite are uniformly dispersed in ferrite is subjected to light working during or after cold working. It is to obtain ductility. JP-A-2-107743 discloses that fine ferrite of 3 μm or less is dispersed in martensite at a volume fraction of 0.5 to 5% by utilizing large rolling. Japanese Patent Application Laid-Open No. 3-28351 discloses a tempered martensite structure in which the grain size of martensite packets or prior austenite is 5 μm or less. [0005] However, in the above-mentioned technology, attention is paid to a structure for improving elongation at break and uniform elongation in particular among ductility, and to a structure for obtaining a low yield ratio. Absent. Further, even if attention is paid to a structure that improves uniform elongation, a high uniform elongation sufficient to improve the bending toughness of the prestressed concrete structure has not been obtained. That is, in Japanese Patent Application Laid-Open No. Sho 62-50408, the elongation at break is at most 3.6% when the tensile strength is of the order of 2000 N / mm ² , and there is no mention of uniform elongation and yield ratio. In JP-A-2-107743, the uniform elongation is at most 4.4% when the tensile strength is 1200 to 1900 N / mm ² . JP-A-3-28
No. 351, the uniform elongation is at most 5.5% at a tensile strength of 1500 N / mm ² . These two publications do not mention the yield ratio at all. [0007] The object of the present invention, in view of such circumstances, D
P with a revolutionary high uniform elongation and low yield ratio of 8% or more uniform elongation and a yield ratio of 0.9 or less at a tensile strength of 1420 N / mm ² or more, which is a kind of standard strength level.
To provide a C steel rod. Means for Solving the Problems In order to solve the above problems and achieve the object, the present invention uses the following means. (1) The steel rod of the present invention is a steel containing C: 0.2 to 1% by weight, having a structure mainly composed of martensite, having a prior austenite average particle size of 50 μm or more, and tempering. The precipitated carbide in martensite is in the form of fine particles having an average diameter of 0.1 μm or less , and
A high uniform elongation low yield ratio PC steel bar having a tensile strength of at least mm ² . DETAILED DESCRIPTION OF THE INVENTION The present inventors have eagerly studied the effects of microstructural factors on uniform elongation and yield ratio of a PC steel rod having a tensile strength of 1420 N / mm ² or more in order to solve the above problems. Repeated research. As a result, in order to realize a high uniform elongation and a low yield ratio, the structure in which the prior austenite grains are coarse and the carbides precipitated in the martensite during the tempering process are fine grains, The finding that it is the most effective was obtained. Based on such findings, the present inventors have proposed a steel-based martensite-based structure with a specified C content.
And prior austenite grain size, 0.1 [mu] m or less the shape and size of the carbide precipitates in the tempered martensite with an average diameter of
By specifying it as fine granules , a tensile strength of 1420N
/ Mm ² or more high uniform elongation low yield ratio PC steel bar,
The present invention has been completed. [0011] That is, the present invention limits the steel composition and structure to the following ranges to provide a D-class strength level,
That is, at a tensile strength of 1420 N / mm ² or more,
It is possible to provide a PC steel bar having an epoch-making high uniform elongation and low yield ratio of 8% or more of uniform elongation and 0.9 or less of yield ratio. The reasons for adding the components, the reasons for limiting the components, and the reasons for limiting the structure of the present invention will be described below. C: 0.2 to 1% C is an element necessary for enhancing hardenability and increasing strength, but if it is less than 0.2%, the required strength cannot be obtained. On the other hand, if it exceeds 1%, precipitated carbides after tempering become coarse, and uniform elongation is deteriorated. Therefore, the C content is 0.2-1%
It is. In the present invention, elements other than C are not particularly limited. By adjusting the quenching / tempering temperature, the austenite particle size of 50 μm or more is 0.1%.
It suffices if fine and granular carbides of not more than μm can be precipitated. The prior austenite grain size greatly affects the yield ratio. If the prior austenite average particle size is less than 50 μm, the martensite packet (particle size factor in martensite), which is a transformed structure, becomes fine, and the yield ratio does not significantly decrease. Therefore, the prior austenite average particle size is 50 μm or more. Here, the former austenite is a structure (structure at a high temperature) before being transformed into martensite by quenching, and the former austenite grain size is the grain size. Since the prior austenite particle size does not change when measured after quenching and tempering, it can be measured even in the final product. The carbide form in tempered martensite is
It has a significant effect on uniform elongation through the easiness of void generation or the effect on work hardening behavior. If the carbide form is rod-shaped or plate-shaped, the carbide itself is easily broken, and the stress concentration at the tip is large and voids are easily generated, so that the uniform elongation is reduced. Therefore, the carbide form needs to be granular. Even if the carbide form is granular, work hardening is difficult if the average diameter exceeds 0.1 μm, so the size of the granular carbide is 0.1 μm or less. In the present invention, the production conditions are not particularly limited. That is, the method of smelting steel, the method of rolling (working) at the time of manufacturing a PC steel bar, and the method of heat treatment may be any conditions that are generally employed. The prior austenite grain size of 50 μm or more can be realized by increasing the quenching temperature, but the addition of Si is effective for tempering at a high temperature of 600 ° C. or more to precipitate fine and granular carbides of 0.1 μm or less. It is. For example, steel having a content of Si of 1 to 4% is 1050 to 1
If quenched from 300 ° C. and tempered at 600 ° C. or more and less than Ac ₁ point, a structure in which fine and granular carbides having a prior austenite grain size of 50 μm or more and 0.1 μm or less can be obtained. EXAMPLE A steel having the composition shown in Table 1 was used as a test steel, rolled into a round bar having a diameter of 8 mm, drawn out into a round bar having a diameter of 7.1 mm, and then quenched and tempered by high-frequency heating. . The heat treatment was performed under the conditions shown in Table 2 (Nos. 1-4:
Example of the present invention, 5 to 10: Comparative Example). The heat treatment is not limited to high frequency heating. Here, the tempering was performed so as to obtain a strength level of Class D standard in JIS G3137. Thereafter, they were subjected to a tensile test. The uniform elongation was measured by marking 2d (d is a diameter) on the test material and measuring 10d as a gauge length at a distance of 5d or more from the fractured portion. Table 3 shows the results of the tensile test. The old austenite grain size shown in Table 2 was obtained by corroding the old austenite grain boundary of the PC steel rod obtained in this embodiment by adding sodium dodecylbenzenesulfonate and ferric chloride to an aqueous picric acid solution. It appeared in a liquid and was examined using an optical microscope, and the form and size of carbide in the tempered martensite were examined using a transmission electron microscope by an extraction residue method. Old austenite grain size is 5 times at 100 times
Observation of the visual field, image analysis and calculation as an average value,
The diameter of the carbide in the tempered martensite is 89000
Observation was performed at five times in five fields, and this was image-analyzed and calculated as an average value. As shown in Table 3, the present invention example No. 1 to N
o. 4 is uniform elongation at a strength level of Class D standard 8
% And a yield ratio of 0.9 or less, both of which have been confirmed to be significantly superior to those of conventional steel. In order to significantly improve uniform elongation, a tempering temperature of 60
Although it is desirable that the temperature is 0 ° C. or higher, if the steel according to the present invention (Si: 1 to 4% is added), it is tempered at a high temperature of 600 ° C. or higher to obtain a D class strength, It can be seen that uniform elongation was easily ensured. On the other hand, in Comparative Example No. Sample No. 5 used the test steel E having a small C content, and thus did not have a predetermined strength by heat treatment. Comparative Example No. Sample No. 6 used test steel F having a large C content, so that the precipitation carbides were coarsened by tempering to obtain a predetermined strength, and uniform elongation was not significantly improved. Comparative Example No. In Sample No. 7, since the test steel G containing few elements that finely granulate precipitated carbides such as Si was used, the shape of the precipitated carbides became acicular and uniform elongation was not significantly improved. Comparative Example No. Nos. 8 and 9 are examples of the present invention. Although the same test steels A and B as in Nos. 1 and 2 were used, the quenching temperature was low and the prior austenite grain size was fine, so that the yield ratio was high. Comparative Example N
o. No. 10 of the present invention. Although the same test steel C as in No. 3 was used, the tempering temperature was set higher than necessary.
The precipitated carbides became coarse and uniform elongation was not significantly improved. [Table 1] [Table 2] [Table 3] As described above, according to the present invention,
Specified steel composition and carbide in tempered martensite
By specifying the morphology as fine grains having an average diameter of 0.1 μm or less , a high uniform elongation and low yield ratio PC steel rod having a tensile strength of 1420 N / mm ² or more is provided. This is extremely useful in industry, such as dramatically increasing the earthquake resistance of the prestressed concrete structure.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Shigeru Mizoguchi 5983 Tamura, Hiratsuka-shi, Kanagawa Prefecture High-frequency thermal smelting company Shonan Plant (72) Inventor Eiji Yamashita 5983 Tamura, Hiratsuka-shi, Kanagawa Prefecture High-frequency thermal smelting company Shonan business In-house (72) Inventor Kazushi Nitta 5893 Tamura, Hiratsuka-shi, Kanagawa Pref. High-frequency thermal smelting company Shonan Plant (58) Field surveyed (Int. Cl. ⁷ , DB name)

Claims (1)

(57) [Claims 1] In a steel containing 0.2% to 1% by weight of C, the steel has a structure mainly composed of martensite, and its prior austenite average particle size is 50 μm. or more, and tempering carbide precipitates form in the martensite is less fine particulate average diameter 0.1 [mu] m, high uniform elongation low yield ratio PC steel rod and having a 1420N / mm ² tensile strength of not less than .