JP2002069580A - High strength rolled pc steel bar and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength rolled PC steel bar having YS (0.2% proof stress) of >=1,200 MPa, TS of >=1,400 MPa and elongation of >=4.5%. SOLUTION: This steel bar consists of steel containing, by mass, 0.8 to 1.3% C, in which the area ratio of pearlite is >=80%, and further, whose surface is subjected to indenting and has YS (0.2% proof stress) of >=1,200 MPa, TS of <=1,400 MPa and elongation of >=4.5%. The steel bar moreover contains 0.10 to 2.5% Si and 0.25 to 2.0% Mn and suitably contains one or more kinds of Al, Ti, Ca, rare earth metals, V and Nb and/or one or more kinds of B, Cr, Cu, Ni and Mo. A wire rod having the composition is isothermally transformed at 450 to 650 deg.C, is subjected to indent machining, is imparted with strain of 1 to 4% thereto, and then is subjected to blueing treatment at 200 to 500 deg.C for 5 to 600 sec, thereby obtaining the objective steel bar.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a PC (prestressed concrete) steel rod used for a concrete pole, a concrete pile, and the like, and a method of manufacturing the same.

[0002]

2. Description of the Related Art Concrete poles and concrete piles are provided with a compressive force applied to concrete to improve rigidity and bending strength and to prevent cracking, thereby increasing the strength of concrete poles and concrete piles themselves.
There are C pole and PC pile. And these are manufactured as follows.

[0003] First, a mild steel wire is spirally wound around a PC steel material arranged in parallel on a circumference (hereinafter referred to as "spiral muscle").
Next, the intersection between the PC steel material and the helical muscle is fixed, and a cylindrical cage piece-type reinforcement (hereinafter, referred to as “reinforcement”) is manufactured.
Next, the reinforcing body is introduced into a mold, and P
Both ends of the C steel material are fixed, and tension is applied with a stress of about 70% of the tensile strength. Next, concrete is poured into the formwork, and after the concrete is solidified, the tensile stress is released from the PC steel. By this release, a compressive force is applied to the concrete, and a PC pole or a PC pile is manufactured.

[0004] As a PC steel material used for such a concrete structure, PC steel specified in JIS G3137 is used.
A steel bar and a PC steel wire specified in JIS G3536 can be cited as typical examples. The PC steel bar is manufactured by quenching and tempering a steel bar that is hot-rolled and air-cooled. By this quenching and tempering, JISG3137 (D
The seed (TS) has a TS (tensile strength) of 1420 MPa or more.

For example, Japanese Unexamined Patent Publication (Kokai) No. 3-151445 discloses that in order to improve spot weldability and relaxation characteristics, a PC steel rod containing reduced Si and added with Mo is subjected to quenching and tempering to obtain a TS of 1420 MPa or more. High strength P
It is disclosed to manufacture C steel bars. in this way,
Normally, quenching and tempering is applied to the PC steel rod. The structure of the PC steel rod becomes a tempered martensite structure by the quenching and tempering, so that required uniform elongation and delayed fracture resistance characteristics are secured. It is difficult. For example, "Iron and steel vol.
81 (1995) .P1625 ”, 1400 M
In a PC steel rod having a tempered martensite structure of Pa or more, delayed fracture resistance deteriorates.

On the other hand, as a PC steel rod having a structure other than tempered martensite, there has been provided a rolled PC steel rod obtained by subjecting a hot-rolled material to cold working and then performing a bluing treatment. Regarding this rolled PC steel bar, "Prestressed concrete vol.13 (1971) p.52" is subjected to stretching and bluing treatment on a wire rod obtained by hot-rolling a slab to obtain a PC having a TS of 1200 MPa or less. It is disclosed that steel bars can be manufactured.

[0007] Rolled PC steel bars have excellent features such as high uniform elongation, but have YS (yield strength) of 1100.
Actually, the strength is not sufficiently increased. For this reason, there is a need for a PC steel rod having higher strength and excellent delayed fracture resistance.

[0008]

Accordingly, the present invention provides a high-strength steel having a tensile strength (TS) of 1400 MPa or more,
And, a high-strength high-strength rolled PC steel bar, and the PC steel bar,
It is an object of the present invention to provide a manufacturing method for manufacturing at low cost by aging treatment such as bluing and heat stretching, without passing through a drawing process usually adopted after patting after hot rolling. .

[0009]

Means for Solving the Problems The present inventors have studied the structure and composition of eutectoid steel or hypereutectoid steel in order to solve the above-mentioned problems. For example, in the case of 0.8% C steel, in order to make the TS have an appropriate strength, a pearlite structure of 80% or more is required to be subjected to isothermal transformation. However, TS is 140
Even if it is 0 MPa or more, it is difficult to make YS 1200 MPa or more. For this purpose, it has been found that heat stretching is performed to improve YS.

On the other hand, it is well known that indentation is performed to improve the adhesion to concrete. PC steel wire is indented before stretching. However, they do not always match the stretching conditions of the present invention. Therefore, the material characteristics of the indented material were investigated. As a result, they have found that the material properties such as delayed fracture do not deteriorate.

The present invention solves the above problems based on the above findings, and the gist is as follows. (1) It is composed of steel having a mass percentage of C: 0.8 to 1.3%, a pearlite area ratio of 80% or more, and a surface indented, and YS (0. 2% proof stress) 1200 MPa or more, TS 1400 MPa or more,
A high-strength rolled PC steel bar having an elongation of 4.5% or more.

(2) The steel further comprises Si:
0.10 to 2.5% and Mn: 0.25 to 2.0
%, The high-strength rolled PC steel bar according to the above (1), (3) The steel further contains, by mass%, Al: 0.05% or less, Ti: 0.005 to 0.05%, Ca: 0.000
5 to 0.005%, REM: 0.0005 to 0.005
%, V: 0.002 to 0.5%, and Nb: 0.00
The high-strength rolled PC steel bar according to the above (1) or (2), comprising at least one of 5 to 0.1%.

(4) The steel further comprises, by mass%, B:
0.0005 to 0.01%, Cr: 0.05 to 2.0
%, Cu: 0.05-1.0%, Ni: 0.05-1.
The high-strength rolled PC steel bar according to the above (1), (2) or (3), containing one or more of 0% and Mo: 0.05 to 0.50%.

(5) A steel slab having a chemical composition relating to the high-strength rolled PC steel bar according to (1), (2), (3) or (4) is heated to an austenite region and then hot-rolled. The wire is then subjected to a constant temperature transformation at a temperature of 450 to 650 ° C., thereafter, the wire is indented, a strain of 1 to 4% is given, and then a wire of 200 to 500
A method for producing a high-strength rolled PC steel bar, comprising performing a bluing treatment at a temperature of 5C for a retention time of 5 to 600 seconds.

(6) A steel slab having the chemical composition relating to the high-strength rolled PC steel bar according to (1), (2), (3) or (4) is heated to an austenite region and then hot-rolled. The wire is then subjected to isothermal transformation at a temperature of 450 to 650 ° C., then indented to the wire, and further heat stretched at a temperature of 200 to 500 ° C. and a tensile strain of 0.5 to 6%. A method for producing a high-strength rolled PC steel bar, characterized in that:

(7) The wire rod having the chemical composition relating to the high-strength rolled PC steel bar according to the above (1), (2), (3) or (4) is reheated to an austenite region and then cooled.
Next, the wire is subjected to isothermal transformation at a temperature of 450 to 650 ° C., and thereafter, the wire is indented, and a strain of 1 to 4% is given.
A method for producing a high-strength rolled PC steel bar, comprising performing a bluing treatment with a retention time of 600 seconds.

(8) The wire rod having the chemical composition relating to the high-strength rolled PC steel rod according to the above (1), (2), (3) or (4) is reheated to an austenite region and then cooled.
Next, a constant temperature transformation is performed at a temperature of 450 to 650 ° C., and then the wire is indented.
A high-strength rolling P characterized by being subjected to heat stretching at a temperature of 00 ° C. and a tensile strain of 0.5 to 6%.
Manufacturing method of C steel rod.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the chemical composition of the high-strength rolled PC steel of the present invention (steel of the present invention) will be described.
C is an important and economical element for securing TS and YS, but TS: 140 required as a PC steel rod.
In order to obtain 0 MPa or more and YS: 1200 MPa or more, at least 0.8% or more is required, and if it is less than 0.8%, the required strength cannot be obtained. Desirably, 0.85% or more is required. On the other hand, C is 1.3%
If it exceeds, network cementite or coarse cementite precipitates at the grain boundaries, and the ductility is significantly reduced. For this reason, the amount of C added is set to 0.8 to 1.3%.

Si forms a solid solution in ferrite (ferrite ground in pearlite), and has a remarkable solid solution strengthening action, whereby YS is formed.
Is an element that improves the In order to obtain this improvement effect, it is necessary to add at least 0.10% or more. On the other hand, if the added amount of Si exceeds 2.5%, the strength becomes too high and the ductility decreases. For this reason, the upper limit of the amount of added Si is set to 2.5%.

Mn is an element effective for enhancing the hardenability to increase the strength and for making the structure in the cross section of the steel rod uniform. In order to obtain these effects, it is necessary to add at least 0.25% or more. On the other hand, when Mn is excessively added, micro-martensite that reduces ductility is likely to be formed in the central segregation portion. In order to suppress the formation of micromartensite, it is necessary to lengthen the isothermal transformation time, but this is not practical. For this reason, the upper limit of the amount of added Mn is set to 2.0%, and Mn is added.
The added amount is 0.25 to 2.0%.

P is an element that segregates at the grain boundaries and easily causes grain boundary embrittlement, so it must be reduced to 0.03% or less. In the steel of the present invention, P is an impurity element,
It is desirable to reduce as much as possible. S, like P, is an element that segregates at grain boundaries and easily causes grain boundary embrittlement.
It is necessary to reduce it to 0.03% or less. In the steel of the present invention, S is an impurity element like P, and it is desirable to reduce it as much as possible.

Next, selected elements contained in the steel of the present invention will be described. Mainly, Al, Ti, Ca, REM, Nb, and
One or two or more kinds of V paste are added. Also, mainly to improve the strength of the rolled PC steel bar, B, C
One or more of r, Cu, Ni, and Mo are added.

Al is an element added to reduce the γ grain size during heat treatment due to the pinning effect of fine Al ₂ O ₃ or AlN precipitates. However, if it exceeds 0.05%, coarse Al ₂ O ₃ is generated, and ductility is reduced. Therefore, the upper limit of the amount of Al added is set to 0.05%. Ti is an oxide such as TiO ₂ or T such as TiN or TiC.
i An element added to refine the γ grain size during heat treatment due to the pinning effect of the precipitate. In order to obtain this effect, 0.005% or more must be added. However, if it exceeds 0.05%, a large amount of coarse TiN precipitates and deteriorates ductility. For this reason, the upper limit of the amount of Ti added is set to 0.05%.

Ca is an element effective for reducing the γ grain size during the heat treatment by generating CaS (O). 0.0
Less than 005% has no effect, so 0.0005%
The lower limit of the amount of Ca added. However, if added in excess of 0.005%, the cleanliness is reduced, and Ca inclusions are coarsened and ductility is reduced.
And

REM, like Ca, is also an effective element for reducing the γ grain size during heat treatment. 0.0005%
If less than 0.005%, there is no effect, so 0.0005% is made the lower limit of the REM addition amount. However, if it is added in excess of 0.005%, the cleanliness decreases, the inclusions including REM become coarse, and the ductility decreases, so the upper limit is 0.005%.
%.

V is an element for precipitating carbonitrides, making γ grains finer, and improving strength and ductility. V is
It becomes a trap site for hydrogen that has entered steel and is also an element that improves delayed fracture characteristics. To obtain these effects, 0.002% or more must be added. However, a large amount of the additive saturates the effect and becomes economically disadvantageous.
The upper limit is set to 0.5%.

Nb is an element that refines austenite grains by the pinning effect of Nb precipitates and improves the ductility of rolled PC steel bars. Therefore, it is necessary to add 0.005% or more. However, even if it is added in a large amount, the effect is saturated and it is economically disadvantageous, so the upper limit is 0.1%. For this reason, the Nb addition amount is set to 0.005 to 0.1%.

B is an element that improves the hardenability and increases the strength of the rolled PC steel bar. B is also an element that segregates preferentially at the grain boundaries and suppresses the degradation of delayed fracture through a grain boundary cleaning effect of suppressing grain boundary segregation such as P, S, and Mn. For this reason, the lower limit of the amount of B added is set to 0.0005%. However, if it exceeds 0.01%, Fe ₂₃ B precipitates and the delayed fracture resistance deteriorates. Therefore, the amount of B added is set to 0.0005 to 0.01%.

Cr is an element that increases the strength by solid solution strengthening, improves the hardenability, reduces the lamella spacing of pearlite, and increases the strength. If less than 0.05%, this effect is insufficient. However, when added in excess of 2.0%, the strength becomes too high, and the ductility decreases. For this reason,
The upper limit is set to 2.0%. Cu is an element added for improving hardenability. Cu is also an element that generates stable corrosion products, suppresses intrusion of hydrogen, and improves delayed fracture. To achieve this effect, 0.05%
The above addition amount is necessary. However, if added in excess of 1.0%, hot cracking tends to occur during rolling, so the upper limit is made 1.0%.

Ni, like Cu, is an element added to improve the hardenability. Ni is also an element that generates stable corrosion products, suppresses intrusion of hydrogen, and improves delayed fracture. Further, Ni is an element that also has an effect of suppressing Cu embrittlement. In order to improve the delayed fracture resistance, an addition amount of 0.05% or more is required. However, even if it is added in excess of 1.0%, the effect saturates and becomes economically disadvantageous, so the upper limit is made 1.0%.

Mo is an element effective for improving relaxation characteristics. In order to increase the strength of steel, it is necessary to add at least 0.05% or more. However, if the addition exceeds 0.50%, the production of ferrite is suppressed, so the upper limit is made 0.50%.
Therefore, the amount of Mo added is set to 0.10 to 0.50%.

The chemical components of steel have been described above. In order to secure YS of 1200 MPa or more in a rolled PC steel bar, it is necessary to set T
S needs to be at least 1400 MPa or more. In consideration of this, it is necessary to determine the amount of C, the combination and addition amount of other strengthening elements, and configure the chemical composition of the steel.

Next, the manufacturing method for manufacturing the high-strength rolled PC steel bar of the present invention (the manufacturing method of the present invention) will be described.
A feature of the production method of the present invention is that YS is 1200 MPa or more in a pearlite structure in a state where strong processing such as drawing is not performed. That is, the pearlite structure in the steel of the present invention has high ductility.

As described above, even if the DLP wire is a hypereutectoid steel having a TS of 1400 MPa or more, except for the case where the carbon content is increased to 1.0% or more, the YLP remains unchanged.
It is difficult to set S to 1200 MPa or more. In the wire rod with DLP applied to actual hypereutectoid steel, TS was 14
In the case of more than 00MPa, the elongation is as low as about 7%.
When stretching and bluing treatments are performed, ductility may be reduced due to age hardening.

In order to solve the above-mentioned problems, the inventors of the present invention attempted to increase the YS and the elongation at break after performing a blueing treatment or a heat stretching treatment after stretching without performing wire drawing. Was examined. Then, by performing stretching + bluing treatment or heat stretching treatment on a pearlite structure steel having a TS of 1400 MPa or more, it is possible to increase YS by about 100 MPa or more and suppress a decrease in elongation to about 2% or less. I found something to come.

As described above, even in a steel having a C content of 0.8% or more, YS: 1200 MPa or more, TS: 1400
It has become possible to easily secure an elongation of at least 4.5 MPa. Next, the structure and manufacturing conditions will be described in detail. The feature of the steel of the present invention is that it has a pearlite-based structure and a TS of 1400 MPa or more. For this purpose, it is necessary that the above-mentioned chemical components are satisfied and the pearlite structure is present in an amount of 80% or more. In the production method of the present invention, after hot rolling of steel or after reheating of wire, , 450-650 ° C. If the pearlite structure is less than 80%, a desired strength (TS) cannot be obtained.

In the isothermal transformation, if the holding temperature is lower than 450 ° C., a large amount of bainite is formed and pearlite 8
A tissue of 0% or more cannot be secured, and a desired TS cannot be obtained. On the other hand, if the holding temperature exceeds 650 ° C., the lamellar interval becomes coarse, and the strength of the pearlite structure decreases. Therefore, the holding temperature in the isothermal transformation is 450 to 6
50 ° C.

The characteristics of the production method of the present invention are as follows.
The point is that the wire is indented after the isothermal transformation at a temperature of 0 ° C. By forming indentations on the surface of the wire by indentation, the adhesion to concrete is significantly improved. In the manufacturing method of the present invention, the indentation may be in any mode.

In order to raise YS, the wire is stretched to a plastic region by stretching, and then subjected to a bluing treatment to remove the processing strain applied at this time. As described above, YS: 1200 MPa or more, elongation at break of 4.5
% Or more, the conditions relating to stretching and bluing are as follows.
To hold at a temperature of 00 ° C. for a time of 5 to 600 seconds.

If the stretching is less than 1%, YS cannot be increased. On the other hand, when the stretching exceeds 4%, a breaking elongation of 4.5% or more cannot be secured. If the heat treatment temperature is lower than 200 ° C., the diffusion of C is insufficient and dislocations are not fixed, so that YS cannot be increased by aging. On the other hand, if the heat treatment temperature exceeds 500 ° C., carbides become coarse and ductility decreases. For this reason, the heat treatment temperature of the bluing process is set to 200 to 500 ° C.

Even in the above-mentioned appropriate heat treatment temperature range,
If the processing time is not appropriate, it is difficult to secure desired YS and elongation. If the processing time is less than 5 seconds, the diffusion of C is insufficient, and aging cannot be increased due to aging. On the other hand, even if the processing is performed for more than 600 seconds, the effect of the aging is saturated, so the upper limit of the processing time is set to 600 seconds. In the manufacturing method of the present invention, a heat stretching treatment can be used instead of the stretching + bluing treatment. This heat-stretching treatment is performed on the wire rod to a degree of 0.1 mm.
This treatment is performed by giving a tensile strain of 5 to 6% and heating to a temperature of 200 to 500 ° C.

If the applied tensile strain is less than 0.5%, Y
S cannot be increased. On the other hand, if the tensile strain exceeds 6%, a breaking elongation of 4.5% or more cannot be secured. If the heat treatment temperature is lower than 200 ° C., the diffusion of C is insufficient and dislocations are not fixed, so that YS cannot be increased by aging. On the other hand, if the heat treatment temperature exceeds 500 ° C., carbides become coarse and ductility decreases. Therefore, the heat treatment temperature of the heat stretching treatment is 200 to 50.
0 ° C.

Under the above conditions, the production method of the present invention
In a high-strength rolled PC steel bar, YS: 1200 MPa or more, TS: 1400 MPa or more, hot, elongation of 4.5% or more can be secured. Further, in the production method of the present invention, the wire drawing step conventionally required is omitted, and heating (austenitizing) → hot rolling → isothermal transformation → cooling → indentation processing → stretching + bluing treatment; or , Heating (austenitizing) → hot rolling → isothermal transformation → cooling → indenting → heat stretching treatment, through which the high-strength rolled PC steel rod with remarkably excellent adhesion to concrete, It became possible to manufacture at low cost.

[0044]

Embodiments of the present invention will be described below. Table 1
The slab of the chemical component of the chemical component is heated and then hot-rolled, and DL
P was performed. Thereafter, the wire was stretched and then subjected to a bluing treatment. Table 2 shows the manufacturing conditions and material properties. The elongation in the tensile test was measured by a butt method. The area ratio of pearlite was determined by observation with an optical microscope. In the steels T1 to T12 of the present invention, TS is 140
0 MPa or more, YS is 1200 MPa or more, and E1 is 4.
5% or more was satisfied.

Since the steels H1 to H3 are not suitable steel components, mechanical properties could not be secured. Steel H1 has a small C content and cannot obtain a predetermined strength. Steel H2 had a large C content and reduced ductility. Steel H3 is an example in which the amount of added Si is large and ductility is reduced. Steels H4 to H8 do not have proper production conditions, and cannot obtain material properties. Steel H4 had a low isothermal transformation temperature and did not have a pearlite fraction of 80% or more, and thus had a reduced strength. Further, in the case of steel H5, the strength is lowered because the isothermal transformation temperature is high. In steel H6, the amount of prestrain is small and Y
The predetermined strength of S cannot be obtained. This is an example in which the predetermined strength of YS cannot be obtained because steel H7 has a low bluing temperature and insufficient diffusion of C. Steel H8 had a high bluing temperature and reduced ductility due to age hardening. Steel H9 did not undergo indentation, and thus had poor concrete adhesion.

[0046]

[Table 1]

[0047]

[Table 2]

[0048]

According to the present invention, a rolled PC steel rod having high ductility and high strength can be manufactured and provided at low cost. Therefore, the present invention is industrially very useful.

 ──────────────────────────────────────────────────の Continued on front page (72) Inventor Yoshihiko Tamagawa 1 Kimitsu, Kimitsu City, Chiba Prefecture Nippon Steel Corporation (72) Inventor Hitoshi Saito 1 Kimitsu, Kimitsu City, Chiba Prefecture Nippon Steel Corporation F-term in Kimitsu Works (reference)

Claims (8)

[Claims] 1. A steel material containing 0.8% to 1.3% of C by mass%, having a pearlite area ratio of 80% or more, and further having a surface indented, YS ( 0.
A high-strength rolled PC steel bar having a 2% proof stress of 1200 MPa or more, a TS of 1400 MPa or more, and an elongation of 4.5% or more. 2. The steel according to claim 1, further comprising, by mass%, Si: 0.10 to 2.5% and Mn: 0.25 to 2.0%. A high-strength rolled PC steel bar as described. 3. The steel further contains, by mass%, Al: 0.05% or less, Ti: 0.005 to 0.05%, Ca: 0.0005 to 0.005%, REM: 0.0005. The high strength according to claim 1 or 2, further comprising at least one of the following:-0.005%, V: 0.002-0.5%, and Nb: 0.005-0.1%. Rolled PC steel bar. 4. The steel further contains, by mass%, B: 0.0005 to 0.01%, Cr: 0.05 to 2.0%, Cu: 0.05 to 1.0%, Ni: The high-strength rolled PC steel rod according to claim 1, 2 or 3, comprising one or more of 0.05 to 1.0% and Mo: 0.05 to 0.50%. 5. A steel slab having the chemical composition according to the high-strength rolled PC steel rod according to claim 1, 2, 3 or 4, which is heated to an austenite region, and then hot-rolled into a wire rod. A constant temperature transformation is performed at a temperature of 650 ° C., and thereafter, an indentation is performed on the wire, and a strain of 1 to 4% is given.
A method for producing a high-strength rolled PC steel bar, comprising performing a bluing treatment with a holding time of 00 seconds. 6. A high-strength rolled PC steel rod according to claim 1, 2, 3 or 4, wherein a steel slab having a chemical composition is heated to an austenite region, and then hot-rolled into a wire rod. A constant temperature transformation is performed at a temperature of 650 ° C., and then the wire is indented.
A high-strength rolled PC characterized by subjecting to heat stretching at a temperature of 0 ° C. and a tensile strain of 0.5 to 6%.
Manufacturing method of steel bars. 7. A high-strength rolled PC steel rod according to claim 1, 2, 3 or 4, wherein the wire having the chemical composition is reheated to an austenite region, cooled, and then cooled to 450 to 6%.
The wire is subjected to isothermal transformation at a temperature of 50 ° C., then indented to the wire, and further subjected to a strain of 1 to 4%, and then blued at a temperature of 200 to 500 ° C. for a holding time of 5 to 600 seconds. A method for producing a high-strength rolled PC steel bar, characterized by performing a treatment. 8. The high-strength rolled PC steel rod according to claim 1, 2, 3 or 4, wherein the wire having the chemical composition is reheated to an austenite region, cooled, and then cooled to 450 to 6%.
High temperature characterized by subjecting the wire to constant temperature transformation at a temperature of 50 ° C., thereafter indenting the wire, and further performing heat stretching at a temperature of 200 to 500 ° C. and a tensile strain of 0.5 to 6%. Manufacturing method of strength rolled PC steel bar.