JP3153071B2 - High-strength steel rod excellent in delayed fracture resistance 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 high-strength steel rod excellent in delayed fracture resistance and a method for producing the same. Specifically, JIS G 3109 D class 1 No. SBPD
It relates to improvement of delayed fracture resistance of PC steel rod equivalent to 130/145.

[0002]

2. Description of the Related Art Prevention of pile cracks in PC piles,
PC steel bars are used as reinforcements to ensure bending strength. In the manufacturing process of the PC pile, first, a reinforcing rod cage is formed by using a PC steel rod, and the PC steel rod has a tensile strength of 70%.
% Tensile stress is applied, the basket is placed in a mold, the concrete raw material is put in the mold, and the basket is manufactured by centrifugal molding. Here, the PC steel bar to which the tensile stress is applied has an action of preventing the concrete from cracking by applying a compressive stress to the concrete. Since the PC steel bar has a high strength of 145 kgf / mm ² or more and is used in a state where a tension of 70% of the strength is applied in the concrete, delayed fracture is highly likely to occur. For this reason, a steel rod having excellent delayed fracture resistance is required.

[0003] In the delayed fracture, grain boundary fracture is dominant.
Countermeasures for refining old austenite grains (JP-A-5-17
No. 1356) and measures to increase the tempering temperature (Japanese Patent Laid-Open No. 5-117811). Also,
In order to suppress the intrusion of hydrogen that causes delayed fracture, attempts have been made to improve the corrosion resistance by adding an alloying element (JP-A-2-240236 and JP-A-2-240236).
240237, JP-A-2-240244). Further, there is a proposal that high-temperature tempering by adding an alloy element improves delayed fracture resistance.

[0004]

However, none of the above-mentioned methods sufficiently suppresses grain boundary fracture, which is a feature of delayed fracture, and cannot be said to be perfect for improving delayed fracture resistance. The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a high-strength steel bar having low delayed fracture susceptibility by controlling the size of grain boundary carbide.

[0005]

Means for Solving the Problems The present inventors have developed a delayed fracture test method which can simulate the delayed fracture phenomenon of a high-strength steel material and can evaluate the delayed fracture characteristic with the amount of hydrogen required for fracture. From the test results using this method, it was found that the amount of hydrogen required for delayed fracture can be increased by controlling the heating rate, holding temperature, and holding time during tempering in heat treatment of steel bars, that is, delayed fracture resistance can be improved. As a result, the present invention has been completed.

[0006] The summary is that C: 0.15
0.50%, Si: 0.1 to 2.0%, Mn: 0.05
2.0%, P: 0.015% or less, S: 0.02% or less, Al: 0.005 to 0.1%, and Cr: 0.1 to 3.0% as required. , Mo: 0.05-1.
2%, Ni: 0.05-2.0%, V: 0.10-0.
50%, Ti: 0.005 to 0.10%, Nb: 0.0
Containing one or more of 0.05 to 0.10%,
The balance is steel consisting of Fe and unavoidable impurities, has a tempered martensite structure, has a prior austenite grain size number of 10 or more according to JIS G 0551, and has a carbide size of 0.2 μm in the prior austenite grain boundary.
A high-strength steel rod excellent in delayed fracture resistance, characterized by having a strength of not less than 145 kgf / mm ² , and a steel of the above-mentioned composition being hot-rolled and immediately heated to 350 ° C. 50 ° C / sec in a temperature range of ~ 500 ° C
The temperature was raised at the above heating rate, tempered by holding for 10 to 50 seconds, and the prior austenite grain size according to JIS G 0551.
Number 10 or more and the former austenite grain boundary
The size of the carbide is 0.2 μm or less and the strength is 1
Tempered martensite set with 45 kgf / mm ² or more
This is a method for producing a high-strength steel rod excellent in delayed fracture resistance, characterized by obtaining a steel rod made of a woven material .

[0007]

This delayed fracture test method is based on the cantilever type test shown in FIG. 2 after charging hydrogen to a test material consisting of a PC steel rod having an annular notch in the shape of FIG. A constant load tensile stress of 70% of the tensile strength is applied by a machine, and the time required for breaking is measured. On the other hand, a cathode material is charged under the same conditions to a specimen having the same shape, and the amount of hydrogen charged to the specimen is measured by gas chromatography. At this time, the measurement is performed by heating at a heating rate of 100 ° C./hour, and two peaks appear in the hydrogen release profile as shown in FIG. Since the peak on the low temperature side shows the amount of hydrogen that can diffuse at room temperature, this is defined as the amount of diffusible hydrogen.

FIG. 4 shows a graph of the rupture time thus obtained and the amount of diffusible hydrogen at that time. From this figure, the amount of hydrogen Hc that does not break even after 100 hours from the loading is determined, and this is defined as the critical diffusible hydrogen amount, and the delayed fracture resistance of the steel material is determined based on the magnitude of the critical amount of hydrogen. As a result of investigating the occurrence and propagation of delayed fracture cracks in PC steel rods using this delayed fracture test method, the following was found. That is, the steel is subjected to normal hot rolling, and then water-cooled to form a martensite structure, and then heated at a heating rate of 50 ° C / sec or more, preferably 100 ° C / sec or more, to 350 ° C.
10 to 50 seconds, preferably 1 to 500 ° C.
Tempering while holding for a time of 0 to 20 seconds does not have enough time for the carbide to precipitate on the prior austenite grain boundaries, so that the grain size of the grain boundary carbides is 0.1 mm by ordinary tempering.
A tempered martensitic structure having fine grain boundary carbides of about 4 μm or less and 0.2 μm or less and 0.1 μm or less under the above preferable conditions is obtained. In steels with a structure characterized by such fine grain boundary carbides, it is difficult for the crack to propagate along the old austenite grain boundaries, and thus requires a large amount of diffusible hydrogen for grain boundary fracture. That is, they have found that the delayed fracture resistance can be greatly improved.

As a result of these experiments and studies, the present invention has been completed, and the range of the alloy composition of the high-strength steel rod according to the present invention is determined for the following reasons. C is required to be 0.15% or more in order to obtain high strength by quenching and tempering, but if it exceeds 0.5%, it is an element that deteriorates toughness and delayed fracture resistance. 15% or more and 0.50% or less. Si is an element necessary for increasing deoxidation and strength of steel, and is added in an amount of 0.1% or more.
If it exceeds 2.0%, it will cause embrittlement, so it was made 0.1% or more and 2.0% or less. Mn is required to be 0.05% or more in order to ensure the deoxidation and hardenability of steel. If it exceeds 2.0%, Mn segregates at the grain boundary during heating in the austenite region, embrittles the grain boundary, and has a delayed fracture resistance. Since it is an element that deteriorates, the content is set to 0.05% or more and 2.0% or less.

P is effective as a hardenable element,
Since the element segregates microscopically during solidification and segregates at the grain boundary during heating in the austenite region, embrittles the grain boundary and deteriorates delayed fracture resistance, the content is 0.015% or less. Although S is an unavoidable impurity, it is segregated at the grain boundary during heating in the austenite region, embrittles the grain boundary, and deteriorates the delayed fracture resistance, so that the content of S is set to 0.02% or less. Al is an effective element for deoxidizing steel, so
It is required to be at least 05%, but if it exceeds 0.1%, toughness is deteriorated.

[0011] Further, an alloying element affecting the delayed fracture resistance,
Examination of the effect of the tempering temperature revealed that the addition of Cr, Mo, Ni, V, Ti, and Nb was more effective than conventional high-strength steel bars. Therefore, one or more of these elements may be contained as necessary.

[0012] In order to obtain the hardenability of steel, Cr is 0.1%.
% Is necessary, but if it is too large, it is an element that causes deterioration of toughness and cold workability, so that the content was made 3.0% or less.
Mo has a tempering softening resistance to obtain the hardenability of steel, and has a stable temperature of 145 kg at a tempering temperature of 400 ° C. or more.
It is an element necessary for 0.05% or more to obtain a tensile load of f / mm ² or more. If it exceeds 1.2%, its effect is saturated and the cost is increased. % Or less. Ni is required to be 0.05% or more to improve toughness and delayed fracture resistance.
If it exceeds 2.0%, the effect is not saturated but rather increases the cost.

V, Ti, and Nb are elements that contribute to the refinement of crystal grains and have a high affinity for hydrogen and are effective in improving delayed fracture resistance by suppressing the diffusion and accumulation of hydrogen in steel. Therefore, V: 0.10% or more and Ti:
0.005% or more, Nb: 0.005% or more is required. However, if the content is too large, the effect saturates and the element is rather deteriorated in toughness. Therefore, V: 0.50% or less, Ti: 0.10% or less, and Nb: 0.10% or less, respectively.

The prior austenite grain size is JIS G 0
Fine particles having a particle size number of 10 or more according to 551 have a large critical diffusible hydrogen content, and the delayed fracture characteristics are improved. When the particle size number is less than 10, the critical diffusible hydrogen amount is small and the delayed fracture characteristics deteriorate, so the particle size number is set to 10 or more. Next, the reason for limiting the size of the grain boundary carbide, which is the most important point for the improvement of the delayed fracture characteristics of the high-strength PC steel rod intended in the present invention, will be described. FIG. 5 shows an example in which the effect of the grain boundary carbide size on the critical diffusible hydrogen content of a PC steel rod having a tempered martensite structure is analyzed. If the grain boundary carbide size is 0.2 μm or more, the effect of improving the critical diffusible hydrogen amount is small, that is, the effect of improving the delayed fracture characteristics is small, so the grain boundary carbide size is limited to 0.2 μm or less.

In the method for producing a high-strength PC steel rod according to the present invention,
Immediately after ordinary hot rolling, the steel is quenched to form a martensite structure, and then tempered at a predetermined heating rate, temperature, and holding time. Next, reasons for limiting the tempering conditions will be described. Heating rate: When the heating rate is less than 50 ° C./sec, the lower limit of the heating rate is set to 50 because the grain boundary carbides precipitate and become coarse in the course of raising the temperature, and it is difficult to stably obtain fine grain boundary carbides. ° C / sec. At a heating temperature of less than 350 ° C., grain boundary embrittlement becomes remarkable and delayed fracture characteristics deteriorate. If the temperature exceeds 500 ° C., the strength is reduced, and it is difficult to stably obtain fine grain boundary carbides.
It was as follows. Holding time: If the holding time exceeds 50 seconds, the carbides become coarse. On the other hand, in a short time of less than 10 seconds, it is difficult to obtain a uniform tempered martensite structure. Therefore 1
It was limited to 0 to 50 seconds.

[0016]

EXAMPLES The chemical composition of the test steel is shown in Table 1, and the heat treatment conditions, yield point, tensile strength, elongation, and critical diffusible hydrogen content obtained by the delayed fracture test described above are shown in Table 2. (1) to (1)
9) is in accordance with the components and heat treatment conditions of the steel for high-strength steel bars of the present invention, and (20) , (22), (23), (2)
5) to (43) are comparative steels. Immediately after hot rolling to a diameter of 7.4 mm, water cooling was performed, and thereafter, tempering was performed in a high-frequency heating furnace. The heat treatment conditions are JIS G 3109 D
The test was performed so that the tensile strength was equivalent to that of seed No. 1 SBPD 130/145.

[0017]

[Table 1]

[0018]

[Table 2]

According to this table, (1) to (19) in the range of the composition, tempering temperature and tempering speed of the present invention are comparative materials (20) , (22), (23) and (25) .
It is clear that the critical hydrogen content is higher than that of (43) and that delayed fracture is less likely to occur, and that the comparative material has little added element and does not show its effect.

[0020]

According to the present invention, by controlling the size of the grain boundary carbide, a PC steel rod having a tensile strength equivalent to JIS G 3109 D type 1 SBPD 130/145 and excellent in delayed fracture resistance is manufactured. it can.

[Brief description of the drawings]

FIG. 1 is a plan view of a test piece used for a delayed fracture test of a steel material.

FIG. 2 is an explanatory diagram of a delayed fracture test apparatus.

Fig. 3 Hydrogen release profile of hydrogen content analysis

FIG. 4 is an explanatory diagram of a critical diffusible hydrogen amount.

FIG. 5 is a graph showing the effect of grain boundary carbide size on critical diffusible hydrogen content.

[Explanation of symbols]

 1 test piece 2 balance weight 3 fulcrum

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-62-267420 (JP, A) JP-A-57-198211 (JP, A) JP-A-47-22813 (JP, A) 35066 (JP, B2) (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 1/00-38/60 C21D 8/00-8/10

Claims (4)

(57) [Claims] C: 0.15 to 0.50% Si: 0.1 to 2.0% Mn: 0.05 to 2.0% P: 0.015% or less S: 0.02% by weight % Or less Al: 0.005 to 0.1%, with the balance being Fe and unavoidable impurities, having a tempered martensitic structure, JIS G
The former austenite grain size number according to 0551 is 10 or more, the size of carbide at the former austenite grain boundary is 0.2 μm or less, and the strength is 145 kgf / mm.
A high-strength steel rod with excellent delayed fracture resistance, characterized by being ² or more. 2. In addition to the steel component of claim 1, further comprises a weight%
Cr: 0.1 to 3.0% Mo: 0.05 to 1.2% Ni: 0.05 to 2.0% V: 0.10 to 0.50% Ti: 0.005 to 0.10 % Nb: 0.005 to 0.10%, and at least one of them is contained. The high-strength steel rod having excellent delayed fracture resistance according to claim 1. 3. C: 0.15 to 0.50% by weight% Si: 0.1 to 2.0% Mn: 0.05 to 2.0% P: 0.015% or less S: 0.02 % Or less Al: 0.005 to 0.1%, with the balance being Fe and unavoidable impurities, water-cooled immediately after normal hot rolling to form a martensitic structure, and then immediately 350 ° C to The temperature is raised to a temperature range of 500 ° C. at a heating rate of 50 ° C./sec or more,
Tempering by holding for ~ 50 seconds , according to JIS G 0551
Old austenite particle size number is 10 or more and old
The size of carbide at austenite grain boundary is 0.2μm or less
With a strength of 145 kgf / mm ² or more
A method for producing a high-strength steel rod having excellent delayed fracture resistance, characterized by obtaining a steel rod having a returned martensite structure . 4. In addition to the steel component according to claim 3, further by weight%
Cr: 0.1 to 3.0% Mo: 0.05 to 1.2% Ni: 0.05 to 2.0% V: 0.10 to 0.50% Ti: 0.005 to 0.10 The method for producing a high-strength steel rod excellent in delayed fracture resistance according to claim 3, wherein one or more of Nb: 0.005 to 0.10% are contained.