JP3153072B2 - 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 is applied to the production of a high-strength steel rod excellent in delayed fracture resistance. Specifically, J
IS G 3109 D class 1 No. SBPD 130 /
The present invention relates to improvement of delayed fracture resistance of a PC steel rod equivalent to 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 methods is sufficient to suppress the intergranular fracture, which is a feature of delayed fracture, and cannot be said to be perfect for improving delayed fracture resistance.
Therefore, the present invention has been made in view of the above circumstances, the purpose of which is to control the aspect ratio of old austenite grains, and the size of grain boundary carbides, to provide a high-strength steel bar with low delayed fracture susceptibility. It is something to offer.

[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 an aspect ratio of prior austenite grains of 2 or more, and has a carbide size of 0.05 to 0.2 μm at the prior austenite grain boundaries, It is a high-strength steel rod excellent in delayed fracture resistance characterized by having a strength of 145 kgf / mm ² or more.
After passing through a step of giving a rolling reduction of 30% or more in the non-recrystallization temperature range of 0 ° C. or less, immediately cooling with water to obtain a martensite structure, and then raising the temperature at a heating rate of 50 ° C./sec or more,
This is a method for producing a high-strength steel rod excellent in delayed fracture resistance, characterized in that it is tempered while being held in a temperature range of 0 ° C to 500 ° C for 10 to 50 seconds.

[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 rolling finish temperature is set to 700 ° C. or more 850 which is the non-recrystallization temperature range.
° C or less, and the rolling reduction in this temperature range is 30% or more, preferably 50% or more, and the cooling temperature is 150 ° C / sec.
By cooling in the above range, the prior austenite crystal grains from the surface layer to a depth of at least 0.5 mm or more are elongated, and a martensite structure having an aspect ratio of 2 or more and 4 or more under the above preferable conditions is obtained. . This is 50 ° C./sec or more, preferably 100 ° C./sec.
By increasing the temperature at the above heating rate and holding and tempering in a temperature range of 350 ° C. to 500 ° C. for 10 seconds to 50 seconds, preferably 10 to 20 seconds, carbides precipitated on the prior austenite grain boundaries are reduced. Since there is not enough time for the growth, the grain size carbide is about 0.4 μm by ordinary tempering, but 0.2 μm or less, and 0.1 μm or less under the above preferable conditions. A tempered martensitic structure with carbides 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.

Next, the most important points for improving the delayed fracture characteristics of the high-strength PC steel bar aimed at by the present invention, the aspect ratio of the prior austenite grains and the reason for limiting the grain size carbide size will be described. FIG. 5 shows an example in which the influence of the aspect ratio of the prior austenite grains on the critical diffusion hydrogen amount of the PC steel rod having the tempered martensite structure is analyzed. When the aspect ratio is less than 2, the effect of improving the amount of critical diffusible hydrogen is small, that is, the effect of improving delayed fracture characteristics is small, so the aspect ratio is limited to 2 or more.

FIG. 6 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 exceeds 0.2 μm, the effect of improving the critical diffusible hydrogen amount is small. That is, since the effect of improving delayed fracture characteristics is small, the grain boundary carbide size is limited to 0.2 μm or less. FIG. 7 shows an example of analyzing the effect of these two parameters on the critical diffusion hydrogen amount. The critical diffusible hydrogen amount can be increased by setting the aspect ratio to 2 or more and setting the grain boundary carbide size to 0.2 μm or less. Therefore, the aspect ratio is 2 or more and the grain boundary carbide size is 0.2 μm.
m or less.

In the method for producing a high-strength PC steel rod according to the present invention,
After performing hot rolling under predetermined conditions, immediately after quenching to a martensite structure, tempering is performed at a predetermined heating rate, temperature, and holding time. Next, reasons for limiting the manufacturing 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. Rolling temperature; When the rolling temperature exceeds 850 ° C., recrystallization during rolling becomes remarkable, and it is difficult to obtain a martensite structure having an aspect ratio of 2 or more. On the other hand, when the rolling temperature is 70
If the temperature is lower than 0 ° C., it is not possible to secure a sufficient rolling reduction for obtaining a structure having a predetermined aspect ratio. Therefore, the rolling temperature was limited to 700 ° C. or more and 850 ° C. Reduction ratio: To obtain a martensite structure having an aspect ratio of 2 or more, a reduction ratio of 30% or more is necessary, so the reduction ratio was limited to 30% or more. 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.

[0017]

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 above-mentioned delayed fracture test are shown in Table 2. (1)-(2
0) is in accordance with the components and heat treatment conditions of the steel for high-strength steel bars of the present invention, and ( 22 ) to (43) are comparative steels.
Immediately after rolling to a diameter of 7.4 mm under predetermined conditions, the steel was cooled with water and then tempered in a high-frequency heating furnace. The heat treatment was performed so that the tensile strength was equivalent to JIS D type 1 SBPD 130/145.

[0018]

[Table 1]

[0019]

[Table 2]

According to the table, (1) to (20) in the range of the composition, the tempering temperature and the tempering speed of the present invention have a higher critical hydrogen content than the comparative materials ( 22 ) to (43). It is clear that delayed fracture is difficult, or that the comparative material has little added element and does not show its effect.

[0021]

According to the present invention, by controlling the aspect ratio and the size of the grain boundary carbide, JIS G 310 can be obtained.
9 D class No. 1 Produces a PC steel rod having tensile strength equivalent to SBPD 130/145 and excellent in delayed fracture resistance.

[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 the aspect ratio of prior austenite grains on the critical diffusible hydrogen content.

FIG. 6 is a graph showing the effect of grain boundary carbide size on critical diffusible hydrogen content.

FIG. 7 is a graph showing the effect of the aspect ratio of prior austenite grains and the grain boundary carbide size on the critical diffusible hydrogen content.

[Explanation of symbols]

 1 test piece 2 balance weight 3 fulcrum

Continuation of front page (56) References JP-A-62-267420 (JP, A) JP-A-57-198211 (JP, A) JP-A-47-22813 (JP, A) (58) Fields investigated (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 martensite structure, and having a ratio of the length and width of prior austenite grains (hereinafter referred to as aspect ratio). ) Is 2
And the size of the carbide at the grain boundary is 0.2 μm
A high-strength steel rod excellent in delayed fracture resistance, characterized by having a strength of 145 kgf / mm 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%, the balance being 30% or more in a non-recrystallization temperature range of 700 ° C. to 850 ° C. when hot-rolling steel consisting of Fe and unavoidable impurities. After passing through a step of giving a rolling reduction, after forming a martensite structure, 50 ° C./sec.
A method for producing a high-strength steel rod excellent in delayed fracture resistance, characterized in that the temperature is raised at the above heating rate, and the steel is tempered while being kept in a temperature range of 350 ° C to 500 ° C for 10 to 50 seconds. 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.