JP2003129177A - High-strength pc steel bar superior in delayed fracture resistance, and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a PC (prestressed concrete) steel bar superior in delayed fracture characteristics, especially a PC steel bar having both superior delayed fracture characteristics and strength of 1,300 MPa or more. SOLUTION: The high-strength PC steel bar superior in delayed fracture resistance comprises, by mass%, 0.55-1.2% C, 0.01-3% Si, 0.1-2% Mn, and further, one or more of 0.05-1.5% Cr, 0.003-0.1% Al, 0.001-0.05% Nb, 0.01-4% Cu, 0.01-4% Ni, 0.0001-0.005% B, 0.01-1.5% V, and 0.01-3% Mo, and the balance Fe with unavoidable impurities, and has a bainite structure in the layer from the surface to a depth of at least 50 μm or deeper.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material having excellent delayed fracture resistance, particularly a PC steel rod having a tensile strength of 1300 MPa or more and excellent delayed fracture resistance, and a method for producing the same.

[0002]

2. Description of the Related Art High strength pearlite steel, which is widely used for PC steel rods, bridge wires and rails, has a tensile strength of 1
It is known that the risk of delayed fracture increases when the pressure exceeds 300 MPa, and the range of use is limited.

As a method for improving the delayed fracture resistance of pearlite steel, for example, Japanese Patent Laid-Open No. 2000-37332,
JP-A-2000-337333 and JP-A-2000-3
In Japanese Patent No. 37334, it is proposed that hard drawing of pearlite steel is effective for improving delayed fracture resistance. Certainly, when ferritic steel is subjected to hard wire drawing, the delayed fracture resistance is improved, but there is a problem that high wire drawing cost increases, and the shape and size are also limited because hard wire drawing is required. .

[0004]

SUMMARY OF THE INVENTION The present invention has been made in consideration of the above circumstances, and the problem to be solved by the present invention is to provide a high strength PC steel bar excellent in delayed fracture characteristics, particularly Good delayed fracture characteristics and strength of 1300M
The purpose is to provide a PC steel rod mainly composed of a pearlite structure of Pa or more.

[0005]

In order to solve the above problems, the present inventors have reached the conclusion that a PC steel bar excellent in delayed fracture characteristics can be realized by optimally selecting the steel material composition and microstructure. It is the invention.

The gist of the present invention is as follows.

(1)% by mass, C: 0.55 to 1.2
%, The area ratio of bainite is 30% or more up to a depth of at least 50 μm from the surface, and the area ratio of pearlite is 70% or more in the inside thereof. Steel rod.

(2) The maximum hardness of bainite at a depth of at least 50 μm from the surface is 350-Vickers hardness.
The high-strength PC steel rod excellent in delayed fracture resistance as described in (1) above, which is 450.

(3) In mass%, Si: 0.01 to 3%, Mn: 0.1 to 2% are further contained, and Cr: 0.05 to 1.5%, Al: 0. 003 to 0.1%, Nb: 0.001 to 0.05%, Cu: 0.01 to 4%, Ni: 0.01 to 4%, B: 0.0001 to 0.005%, Ti: 0 0.005% or less, V: 0.01 to 1.5%, Mo: 0.01 to 3%, and one or more of them are contained, and the balance is Fe and inevitable impurities. A high-strength PC steel rod excellent in delayed fracture resistance as described in (1) or (2).

(4) A high-strength PC steel excellent in delayed fracture resistance as set forth in any one of (1) to (3) above, wherein the critical diffusible hydrogen content is 0.2 ppm or more. rod.

(5) A method for producing a high-strength PC steel rod excellent in delayed fracture resistance as set forth in any one of (1) to (4) above, which comprises (1) or (3) above. A steel material having the described components is cooled at a cooling rate of 1 to 10 ° C./sec from Ac ₃ or higher to Ms to 550 ° C. after completion of rolling or after reheating, and is excellent in delayed fracture resistance. Strength P
C Steel bar manufacturing method.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have made detailed analyzes of delayed fracture behavior using PC steel rods with various strength levels that have been subjected to accelerated cooling as-rolled and after rolling. It is already clear that delayed fracture is caused by diffusible hydrogen that penetrates into the steel from the external environment and can diffuse in the steel at room temperature.

The delayed fracture resistance was evaluated by determining the amount of critical diffusible hydrogen in which delayed fracture did not occur. In this method, various amounts of diffusible hydrogen are contained by electrolytic hydrogen charging, and then Cd plating is performed to suppress hydrogen in the sample from being released into the atmosphere during the delayed fracture test, and then the atmosphere Among them, the amount of diffusible hydrogen at which a prescribed load is applied and delayed fracture does not occur is evaluated.

The hydrogen release profile shown in FIG. 3 is a standard steel release profile, and the diffusible hydrogen content is the diffusible hydrogen content obtained by integrating the peak area observed at 100 ° C.

FIG. 4 shows an example of analysis of the relationship between the amount of diffusible hydrogen and the fracture time until delayed fracture occurs.
The smaller the amount of diffusible hydrogen contained in the sample, the longer the time until delayed fracture occurs, and the delayed fracture does not occur when the amount of diffusible hydrogen is below a certain value. The upper limit amount of diffusible hydrogen at which this delayed fracture does not occur is defined as "limit diffusible hydrogen amount". The higher the critical diffusible hydrogen content, the better the delayed fracture property.

The amount of diffusible hydrogen can be measured by temperature rising analysis using a gas chromatograph.

Therefore, as a result of a detailed study to improve the delayed fracture characteristics of PC steel rods, the critical diffusivity was improved by forming a bainite-based structure excellent in delayed fracture resistance near the surface layer which is the starting point of delayed fracture. The conditions for increasing the hydrogen content and improving the delayed fracture resistance were established.

First, the reasons for limiting the components of the present invention will be explained.

C: C is an important element for the present invention, and is an element essential for securing the strength of steel and forming a pearlite structure. However, if it is less than 0.55%, it is difficult to form a pearlite structure. If it exceeds 1.2%, the ductility is remarkably deteriorated, so the content is limited to 0.55 to 1.2%.

In the present invention, in addition to the above components, it is preferable to add the following elements, if necessary.

Si: Si has the effect of suppressing the formation of pro-eutectoid cementite and improving the hardenability. Further, it has a solid solution in ferrite between pearlite lamellas, and has the effect of increasing the strength by solid solution hardening. However, if it is less than 0.01%, this strengthening effect cannot be obtained, and if it exceeds 3%, the ductility is remarkably reduced, so the range is limited to 0.01-3%.

Mn: Mn is an element effective not only for deoxidizing and desulfurizing but also for improving hardenability, but if it is less than 0.1%, the above effect cannot be obtained. If it exceeds 2%, a supercooled structure such as martensite is generated in the segregated portion of Mn and the ductility is deteriorated.
Limited to 2%.

Cr: Cr has the effect of reducing the lamellar spacing of pearlite to improve the strength. But 0.0
If it is less than 5%, the above effect cannot be obtained, and if it exceeds 1.5%, the transformation end time becomes too long, and a supercooled structure such as martensite is formed to lower the ductility.
The range was limited to 1.5%.

Al: Al has an effect of preventing coarsening of austenite grains by deoxidizing and forming AlN, but if it is less than 0.003%, the above effect is not exhibited and 0.1% is used. If it exceeds, the effect will be saturated, so 0.0
It was limited to the range of 03-0.1%. Nb: Nb forms carbonitrides and improves delayed fracture characteristics, but if it is less than 0.001%, the above effect is insufficient, and if it exceeds 0.05%, this effect saturates. It was limited to the range of 0.001 to 0.05%.

Cu: Cu improves the hardenability and enhances the strength due to the precipitation effect, but if it is less than 0.01%, the above effect cannot be obtained, and if it exceeds 4%, grain boundary embrittlement occurs and delay resistance is delayed. In order to deteriorate the fracture characteristics, the range is limited to 0.01 to 4%.

Ni: Ni has an effect of improving ductility, but if it is less than 0.01%, the above effect cannot be obtained, and it is 4%.
If it exceeds 0.1%, the transformation end time becomes too long, and a supercooled structure such as martensite is generated to lower the ductility, as in the case of Cr. Therefore, the content is limited to the range of 0.01 to 4%.

B: B has the effect of suppressing grain boundary fracture and improving delayed fracture characteristics. Further, B segregates at the austenite grain boundaries to remarkably enhance the hardenability,
If it is less than 0.0001%, the above effect cannot be obtained, and
Even if it exceeds 005%, the effect is saturated, so 0.0001
The range is limited to 0.005%.

V: V is an element that improves the hardenability and forms carbides or nitrides or their composite precipitates, and these precipitates serve as hydrogen trap sites to improve delayed fracture characteristics. However, if it is less than 0.01%, the effect of this hydrogen trap is hardly obtained, and
Even if it exceeds 5%, the effect corresponding to the added amount cannot be obtained, so the range is set to 0.01 to 1.5%.

Mo: Mo is an element that improves the hardenability similarly to V, and forms a carbide or a nitride or a composite precipitate thereof, which serves as a hydrogen trap site, and the delayed fracture property is improved. But 0.01%
If it is less than the above, the effect of the hydrogen trap is hardly obtained, and if it exceeds 3%, the effect corresponding to the added amount cannot be obtained, so the range is set to 0.01 to 3%.

Impurities P, S and N are not particularly limited, but from the viewpoint of improving delayed fracture characteristics, 0.015% or less is a preferable range.

Next, the structure of the present invention will be described.

In the C section perpendicular to the longitudinal direction of the steel bar,
When a bainite structure is formed in the surface layer, delayed fracture characteristics are improved. If the depth is less than 50 μm from the surface layer, there is no effect of improving the delayed fracture property, so the depth of the surface bainite was limited to at least 50 μm from the surface layer. Even if the bainite structure has a depth of more than 500 μm, the effect of improving the delayed fracture property is saturated and the strength is reduced.
It is preferably 500 μm or less. The area ratio of bainite is set to 30% or more in order to improve delayed fracture characteristics. The surface layer structure may be made of bainite at 100%, but the balance is 1% of ferrite or pearlite in order to secure fracture toughness.
The area ratio of pearlite is 7 in a tissue containing two or more species.
It is preferably 0% or more from the viewpoint of ensuring surface hardness.

The area ratio of bainite is from the surface layer to 500 μm.
The area between m is continuously photographed at 100 to 500 times with an optical microscope, the continuous structure of the surface layer to 500 μm is observed in 5 to 10 fields of view, and it is defined as the value obtained by the area ratio.

In order to secure the strength of the inside, it is necessary that the area ratio of pearlite is 70% or more. Although pearlite may be 100% in the interior, it is preferable that the remaining structure contains 30% or less of one or two kinds of ferrite and bainite in terms of delayed fracture characteristics and fracture toughness.

The area ratio of ferrite and pearlite is defined as a value measured by the same method as the area ratio of bainite.

Further, by making the surface layer a bainite structure and adjusting the strength level in this bainite structure to 450 or less by Vickers hardness, the delayed fracture property is further improved, so that the maximum hardness in the bainite structure of the surface layer is improved. Is preferably 450 or less in Vickers hardness. On the other hand, it is preferably 350 or more in order to secure fatigue characteristics.

The maximum hardness of bainite is defined as the average value when the sample is measured at 5 to 20 points with a Vickers hardness of 100 to 1000 g.

Next, the reason for limiting the limit diffusible hydrogen amount will be described.

As a result of evaluating the delayed fracture of the steel materials having the above-mentioned components, the surface layer which is the starting point of the delayed fracture has a structure mainly composed of bainite, and more preferably the strength level is adjusted, whereby the critical diffusible hydrogen content is remarkably improved. Especially 1300MPa
With the above high-strength PC steel bars, due to the above effects, 0.2p
The limit diffusible hydrogen content can be pm or more, preferably 0.5 ppm or more. Next, the manufacturing conditions of the present invention will be described.

After the hot rolling is completed, or by accelerated heating from the temperature in the austenite region by reheating, the surface layer becomes a structure mainly composed of bainite, and the inside is a structure mainly composed of pearlite or ferrite / pearlite due to reheat during bainite transformation. become. The reheating temperature is not particularly specified, but it is preferably 750 ° C. or higher from the viewpoint of austenite formation and 1050 ° C. or lower from the viewpoint of preventing austenite grain coarsening. When the cooling start temperature is lower than Ac _3, the structure is mainly composed of low-strength ferrite / pearlite, bainite is not formed, and the delayed fracture property is deteriorated. Therefore, the cooling start temperature is limited to Ac ₃ or higher. On the other hand, the upper limit of the cooling start temperature is not specified, but from the relationship of productivity of accelerated cooling time,
Ac ₃ + 300 ° C. or lower is desirable.

Ac ₃ is defined as a value obtained by a small thermal expansion type transformation measuring device (Formaster).

The accelerated cooling stop temperature is limited to Ms to 550 ° C., since martensite is formed below the Ms point and bainite is not formed above 550 ° C. If the cooling rate is lower than 1 ° C / sec, bainite will not be formed and 10
If it is faster than ° C / sec, a bainite-based structure is generated on the entire cross section, so the rate was limited to 1-10 ° C / sec.

[0043]

EXAMPLES The effects of the present invention will be described more specifically below with reference to examples.

1200 steels having the chemical compositions shown in Table 1 were prepared.
After heating to ℃, by hot rolling to produce a steel rod φ7mm, 95
It was heated to 0 ° C. and cooled under the cooling conditions shown in Table 2. Table 2 shows the mechanical properties and the delayed fracture characteristics evaluated by the delayed fracture tester shown in FIG. 2 using the delayed fracture test piece shown in FIG.

The delayed fracture characteristics were evaluated by the amount of diffusible hydrogen which does not cause delayed fracture after 100 hours shown in FIG. 4, that is, the amount of limit diffusible hydrogen, and the load was applied under the condition of 90% of the tensile strength. For hydrogen charging, the hydrogen level was changed by changing the charging current using the electrolytic hydrogen charging method.

Steel types A to F in Tables 1 and 2 and symbols 1 to 6
Is a steel of the present invention, steel types G to L, and symbols 7 to 17 are comparative steels.

Each of the steels of the present invention has a tensile strength of 1300.
In addition to being at least MPa, the depth of the bainite layer from the surface layer is at least 50 μm, the maximum hardness of bainite is 450 or less in Vickers hardness, and the critical diffusible hydrogen content in the delayed fracture test is at least 0.2 ppm. .

On the other hand, reference numeral 7 which is a comparative steel is C
This is an example in which the area ratio of the internal pearlite structure is low because the amount is low, and the tensile strength is low at less than 1300 MPa. Reference numeral 8 is an example in which the cooling start temperature was low and surface bainite was not formed, so that the critical diffusible hydrogen content was as low as less than 0.2 ppm and the delayed fracture characteristics were poor. Reference numeral 9 is an example in which the amount of C was large and the bainite hardness of the surface layer was more than Hv450, so that the critical diffusible hydrogen amount was less than 0.2 ppm and the delayed fracture characteristics were deteriorated. Reference numeral 10 is an example in which the critical fracture diffusivity was less than 0.2 ppm and the delayed fracture characteristics were poor because the cooling stop temperature was high and surface bainite was not formed. Reference numeral 11 has a high Si content of 3.10% and a Mn content of 0.0.
Since it is as low as 5%, the hardness in the bainite structure is increased, the toughness is lowered, and the delayed fracture property is deteriorated. Reference numeral 12 is an example in which the cooling rate is slow and the tensile strength is less than 1300 MPa, which is low in strength. Reference numeral 13 is Si 0.00
It is as low as 9% and Mn is as high as 2.1%, so that the hardness in the bainite structure is high, the fracture toughness value is low, and the delayed fracture property is poor. Reference numeral 14 is an example in which the delayed fracture property was deteriorated with the critical diffusible hydrogen amount of less than 0.2 ppm because martensite was generated in the surface layer with a high cooling rate. Reference numeral 15 has a V content of 1.58% and a Cr content of 1.61% and has a very high hardenability, so martensite is generated in the surface layer, and the critical diffusible hydrogen content is less than 0.2 ppm, which is a delayed fracture property. Is a bad example. Reference numeral 16 has a low cooling stop temperature and generates a martensite structure in the surface layer,
This is an example in which the delayed fracture characteristics are significantly deteriorated due to the low area ratio of the bainite structure in the surface layer. Reference numeral 17 indicates Mo is 3.
This is an example in which delayed fracture characteristics are poor because martensite is generated in the surface layer and the area ratio of bainite is low because Cu and Ni are high and the hardenability is very high in addition to as high as 05%.

[0049]

[Table 1]

[0050]

[Table 2]

[0051]

According to the present invention, a PC steel bar having a tensile strength of 1300 MPa or more and excellent resistance to delayed fracture can be manufactured by forming a bainite layer having excellent delayed fracture characteristics on the surface layer. .

[Brief description of 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 device.

FIG. 3: Hydrogen release profile of hydrogen analysis.

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

[Explanation of symbols]

1 test piece 2 balance weight 3 fulcrum

Continued front page    F-term (reference) 4K032 AA01 AA02 AA06 AA07 AA11                       AA12 AA14 AA15 AA16 AA19                       AA20 AA22 AA23 AA24 AA31                       AA32 AA36 BA02 CD02                 4K042 AA14 BA02 CA02 CA05 CA06                       CA08 CA09 CA10 CA13 DA01                       DE01

Claims (5)

[Claims] 1. The content of C: 0.55 to 1.2% by mass% is such that the area ratio of bainite is 30% or more up to a depth of at least 50 μm from the surface, and the area ratio of pearlite is 70% inside. %, A high strength PC steel bar with excellent delayed fracture resistance. 2. The maximum hardness of bainite at a depth of at least 50 μm from the surface is 350-45 in Vickers hardness.
The high-strength PC steel rod excellent in delayed fracture resistance according to claim 1, which is 0. 3. In mass%, further contains Si: 0.01 to 3%, Mn: 0.1 to 2%, and further Cr: 0.05 to 1.5%, Al: 0.003. -0.1%, Nb: 0.001-0.05%, Cu: 0.01-4%, Ni: 0.01-4%, B: 0.0001-0.005% V: 0.01 To 1.5%, Mo: 0.01 to 3%, one or more, and the balance consisting of Fe and unavoidable impurities, delayed fracture resistance according to claim 1 or 2. Excellent high strength PC steel rod. 4. A high-strength PC steel rod excellent in delayed fracture resistance as set forth in any one of claims 1 to 3, wherein the limit diffusible hydrogen content is 0.2 ppm or more. 5. A method for producing a high-strength PC steel rod excellent in delayed fracture resistance according to any one of claims 1 to 4, which comprises the components according to claim 1 or claim 3. After rolling the steel material or after reheating, the steel material is Ac ₃ or more and Ms to 5
A method for producing a high-strength PC steel rod excellent in delayed fracture resistance, characterized by cooling to 50 ° C at a cooling rate of 1 to 10 ° C / sec.