JPH07300653A - High strength steel bar excellent in delayed fracture resistance and its production - Google Patents

Abstract

PURPOSE:To increase the aspect ratio of old austenite grains, to refine the carbide grains precipitated in crystalline grain boundaries, and to provide superior delayed fracture resistance and high strength by subjecting a steel of specific composition to controlled rolling and then applying direct quenching and tempering treatment to the resulting rolled stock. CONSTITUTION:This steel has a composition consisting of, by weight ratio, 0.15-0.50% C, 0.1-2.0% Si, 0.05-2.0% Mn, <=0.015% P, <=0.02% S, 0.005-0.1% Al, and the balance Fe with inevitable impurities. Further, the steel has a structure consisting of tempered martensite and having >=2 length-to-width ratio, that is, aspect ratio of old austenite grains, <=0.2mum size of carbides in grain boundaries, and >=145kgf/mm<2> strength. Therefore, the steel is applied with a draft of 30% at 700-850 deg.C at the time of hot rolling to form a martensitic structure, then immediately subjected, to temp. rising at >=50 deg.C/sec heating rate, held at 350-500 deg.C for 10-50sec, and tempered.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is applied to the production of high-strength steel rods having excellent delayed fracture resistance. Specifically, J
IS G 3109 Class D No. 1 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 PC piles are used to prevent cracks in the piles.
PC steel rod is used as a reinforcing material to secure bending strength. In the process of manufacturing a PC pile, first, a rebar cage is formed from a PC steel rod, and a tensile strength of 70 is applied to the PC steel rod.
% Tensile stress, the cage is placed in a mold, concrete raw material is put into the mold, and centrifugal molding is performed. Here, the PC steel rod loaded with tensile stress has an action of preventing cracking of concrete by giving compressive stress to concrete. Since the PC steel rod has a high strength of 145 kgf / mm ² or more and is used in a state of being loaded with a tension of 70% of the strength in concrete, delayed fracture is likely to occur. Therefore, a steel rod having excellent delayed fracture resistance is required.

Delayed fracture is dominated by grain boundary fracture, so
Measures for refining old austenite grains (Japanese Patent Application Laid-Open No. 5-17
1356), and measures for increasing the tempering temperature (JP-A-5-117811). Also,
In order to suppress the penetration of hydrogen, which causes delayed fracture, it has also been attempted to improve the corrosion resistance by adding an alloying element (JP-A-2-240236 and JP-A-2-24036).
No. 240237 and Japanese Patent Laid-Open No. 2-240244). There is also a proposal that high-temperature tempering by adding alloy elements improves delayed fracture resistance.

[0004]

However, none of the above-mentioned methods is sufficient for suppressing the intergranular fracture, which is a characteristic of delayed fracture, and is not perfect for improving delayed fracture resistance.
Therefore, the present invention has been made in view of the above circumstances, and its purpose is to provide a high strength steel bar with low delayed fracture susceptibility by controlling the aspect ratio of old austenite grains and the size of grain boundary carbides. It is the one to be provided.

[0005]

The present inventors have developed a delayed fracture test method capable of simulating the delayed fracture phenomenon of a high strength steel and evaluating the delayed fracture characteristics by the amount of hydrogen required for fracture. From the test results using this method, it was found that the hydrogen rate required for delayed fracture can be increased by controlling the heating rate, holding temperature, and holding time during tempering in the heat treatment of steel bars, that is, the delayed fracture resistance can be improved. Thus, the present invention has been completed.

The gist of the matter is that C: 0.15 to 0.15.
0.50%, Si: 0.1-2.0%, Mn: 0.05
.About.2.0%, P: 0.015% or less, S: 0.02% or less, Al: 0.005 to 0.1%, and if necessary, Cr: 0.1 to 3.0%. , Mo: 0.05-1.
2%, Ni: 0.05 to 2.0%, V: 0.10 to 0.
50%, Ti: 0.005 to 0.10%, Nb: 0.0
It contains one or more of 05 to 0.10%,
The balance is steel consisting of Fe and unavoidable impurities, consisting of a tempered martensite structure, the aspect ratio of the former austenite grains is 2 or more, and the size of the carbide of the former austenite grain boundaries is 0.05 to 0.2 μm, And a strength of 145 kgf / mm ² or more, which is a high-strength steel rod excellent in delayed fracture resistance, and a temperature of 700 ° C. or more 85
After a step of giving a rolling reduction of 30% or more in a temperature range of 0 ° C. or less, immediately after water cooling to form a martensite structure,
The temperature is raised at a heating rate of 50 ° C./sec or more to 350 ° C. to 5
A method for producing a high-strength steel rod excellent in delayed fracture resistance, characterized by holding in a temperature range of 00 ° C for 10 to 50 seconds and tempering.

[0007]

The delayed fracture test method is the cantilever type test shown in FIG. 2 in the atmosphere after hydrogen is charged by the constant current cathodic charge to the specimen made of PC steel rod having the annular notch of the shape shown in FIG. With a machine, a constant load tensile stress of 70% of the tensile strength is applied, and the time required for breaking is measured. On the other hand, a sample material having the same shape as this is subjected to cathode charging under the same conditions, and the amount of hydrogen charged in the sample material is measured by a gas chromatograph method. At this time, measurement is performed by heating at a temperature rising rate of 100 ° C./hour, but 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.

The breaking time thus obtained and the amount of diffusible hydrogen at that time are plotted in a graph as shown in FIG. From this figure, the hydrogen amount Hc that does not fracture even after 100 hours from loading is determined, and this is defined as the limit diffusible hydrogen amount, and the delayed fracture resistance property of the steel material is judged by this amount. As a result of investigating the occurrence and propagation phenomenon of the delayed fracture crack in the PC steel rod by this delayed fracture test method, the following was found. That is, the rolling finishing temperature is 700 ° C. or higher and 850 ° C. which is a non-recrystallization temperature range.
C. or less, 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 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. . 50 ° C / sec or more, preferably 100 ° C / sec
By heating at the above heating rate and holding in a temperature range of 350 ° C. to 500 ° C. for 10 seconds to 50 seconds, preferably 10 to 20 seconds and tempering, carbides precipitated on the former austenite grain boundaries are Since there is not enough time to grow, the grain boundary carbide size is about 0.4 μm in ordinary tempering, while the grain boundary carbide size is 0.2 μm or less, and a minute grain boundary of 0.1 μm or less under the above preferable conditions. A tempered martensitic structure with carbides is obtained. In a steel having a structure characterized by such fine grain boundary carbides, it is difficult for cracks to propagate along the former austenite grain boundary, and therefore a large amount of diffusible hydrogen is required 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 examinations, the present invention has been completed, and the range of alloy components of the high strength steel rod according to the present invention is determined for the following reason. 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, and therefore, is 0.1. It was set to 15% or more and 0.50% or less. Si is an element necessary for increasing the deoxidation and strength of steel, and is added in an amount of 0.1% or more.
If it exceeds 2.0%, it causes embrittlement, so the content was made 0.1% to 2.0%. Mn is required to be 0.05% or more in order to secure the deoxidation and hardenability of steel, and if it exceeds 2.0%, it segregates to the grain boundaries during heating in the austenite region and embrittles the grain boundaries, and also provides delayed fracture resistance. Since it is an element that deteriorates, it is set to 0.05% to 2.0%.

Although P is effective as a hardenable element,
Since it is an element that causes micro-segregation during solidification and further segregates at grain boundaries during heating in the austenite region to embrittle the grain boundaries and deteriorate delayed fracture resistance, the content was made 0.015% or less. Although S is an unavoidable impurity, it is an element that segregates at the grain boundaries during heating in the austenite region, embrittles the grain boundaries, and deteriorates the delayed fracture resistance. Since Al is an element effective for deoxidizing steel, 0.0
It is necessary to be 05% or more, but if it exceeds 0.1%, the toughness is deteriorated, so the content was made 0.005% or more and 0.1% or less.

Further, alloying elements which affect delayed fracture resistance,
When the influence of the tempering temperature was investigated, it was found that the addition of Cr, Mo, Ni, V, Ti and Nb was more effective than the conventional high strength steel bar. Therefore, if necessary, one kind or two or more kinds of these elements are contained.

Cr is 0.1 in order to obtain hardenability of steel.
% Or more, but if it is too much, it is an element that causes deterioration of toughness and cold workability, so the content was made 3.0% or less.
Mo has a tempering softening resistance in order to obtain the hardenability of steel, and is stable at a tempering temperature of 400 ° C. or higher and is stable at 145 kg.
It is an element that is required to be 0.05% or more to obtain a tensile load of f / mm ² or more, and when it exceeds 1.2%, its effect is saturated and the cost increases, so 0.05% or more 1.2 % Or less. Ni is required to be 0.05% or more in order to improve toughness and delayed fracture resistance,
If it exceeds 2.0%, the effect is saturated and rather the cost is increased, so the content is made 0.05% or more and 2.0% or less.

V, Ti, and Nb are elements effective for improving the delayed fracture resistance by contributing to the refinement of crystal grains and having a high affinity with hydrogen, and suppressing the diffusion and accumulation of hydrogen in steel. Therefore, V: 0.10% or more and Ti:
0.005% or more and Nb: 0.005% or more are required. However, if it is too large, the effect is saturated and rather it is an element that deteriorates toughness, so V: 0.50% or less, Ti: 0.10% or less, and Nb: 0.10% or less, respectively.

Next, the aspect ratio of the former austenite grains and the reasons for limiting the grain boundary carbide size, which are the most important points for improving the delayed fracture properties of high strength PC steel rods aimed at by the present invention, will be described. FIG. 5 shows an example of analysis of the effect 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. When the aspect ratio is less than 2, the effect of improving the limit diffusible hydrogen amount is small, that is, the effect of improving the delayed fracture property is small. Therefore, the aspect ratio is limited to 2 or more.

FIG. 6 shows an example of the analysis of the effect of grain boundary carbide size on the critical diffusible hydrogen content of a PC steel rod having a tempered martensite structure. 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. An example of analyzing the effect of these two parameters on the limit diffusion hydrogen amount is shown in FIG. The critical diffusible hydrogen content can be increased by setting the aspect ratio to 2 or more and 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 0.2μ.
It was limited to m or less.

In the method for producing a high strength PC steel rod of the present invention,
After 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, the reasons for limiting the manufacturing conditions will be described. . Heating rate; If the heating rate is less than 50 ° C./sec, grain boundary carbides precipitate and coarsen during the temperature rising process, and it is difficult to stably obtain fine grain boundary carbides. Therefore, the lower limit of the heating rate is 50. 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, the rolling temperature is 70
If it is less than 0 ° C., it is impossible to secure a sufficient reduction rate to obtain a structure having a predetermined aspect ratio. Therefore, the rolling temperature is limited to 700 ° C or higher and 850 ° C. Reduction ratio: Since a reduction ratio of 30% or more is required to obtain a martensite structure having an aspect ratio of 2 or more, the reduction ratio is limited to 30% or more. Heating temperature: If the temperature is lower than 350 ° C, embrittlement of grain boundaries becomes significant, and delayed fracture characteristics deteriorate. At temperatures above 500 ° C, the strength decreases and it is difficult to stably obtain fine grain boundary carbides.
Below. Retention time; When retained for more than 50 seconds, carbides coarsen. 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 seconds to 50 seconds.

[0017]

[Examples] Table 1 shows the chemical composition of the sample steel, and Table 2 shows the heat treatment conditions, the yield point, the tensile strength, the elongation, and the critical diffusible hydrogen content obtained by the delayed fracture test. (1) ~ (2
0) is in accordance with the components and heat treatment conditions of the steel for high-strength steel rod of the present invention, and (21) to (43) are comparative steels.
Immediately after rolling to a diameter of 7.4 mm under a predetermined condition, it was water-cooled and then tempered in a high-frequency heating furnace. The heat treatment conditions were such that the tensile strength was equivalent to JIS D type 1 SBPD 130/145.

[0018]

[Table 1]

[0019]

[Table 2]

From this table, (1) to (20) in the composition, tempering temperature, and tempering speed range of the present invention have a higher limit hydrogen content than the comparative materials (21) to (43). It is clear that delayed fracture is unlikely to occur, or that the comparative material has few added elements and its effect cannot be seen.

[0021]

According to the present invention, by controlling the aspect ratio and the size of grain boundary carbides, JIS G 310
9 Type D No. 1 SBPD 130/145 equivalent tensile strength and PC steel bar excellent in delayed fracture resistance are manufactured.

[Brief description of drawings]

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

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

FIG. 3 Hydrogen release profile of hydrogen content analysis

FIG. 4 is an explanatory diagram of the limit diffusible hydrogen amount.

FIG. 5 is a graph showing the effect of the aspect ratio of old austenite grains on the critical diffusible hydrogen content.

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

FIG. 7 is a graph showing the effect of the aspect ratio of old 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

Claims (4)

[Claims] 1. 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 % Or less Al: A steel containing 0.005 to 0.1% and the balance of Fe and unavoidable impurities. The steel has a tempered martensite structure and the ratio of the length to the width of the former austenite grains (hereinafter referred to as the aspect ratio). ) Is 2
And the size of carbides at grain boundaries is 0.2 μm
A high-strength steel rod excellent in delayed fracture resistance, characterized in that the strength is 145 kgf / mm ² or more. 2. In addition to the steel composition according to claim 1, 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. % Nb: 0.005 to 0.10%, and one or more kinds are contained, The high strength steel rod excellent in delayed fracture resistance according to claim 1. 3. By weight%, 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 % Or less Al: 0.005 to 0.1% is contained, and the balance of Fe and inevitable impurities is hot-rolled. When hot rolling, a reduction ratio of 30% or more in a temperature range of 700 ° C to 850 ° C is applied. After the giving process,
After forming the martensite structure, the temperature is raised at a heating rate of 50 ° C./sec or more, and the temperature is increased to 10 ° C. in the temperature range of 350 ° C. to 500 ° C.
A method for producing a high-strength steel rod excellent in delayed fracture resistance, characterized by holding for 50 seconds and tempering. 4. In addition to the steel composition 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. % Nb: 0.005 to 0.10%, and one or more kinds are contained, The method for producing a high strength steel rod excellent in delayed fracture resistance according to claim 3.