JP7091163B2 - PC steel rod - Google Patents

Description

The present invention relates to PC steel rods, and more particularly to PC steel rods for concrete piles.

Concrete piles are used as the foundation of buildings. A concrete pile is a structure that receives a vertical load. The concrete pile is buried underground to support the building. The concrete pile after burial is mainly subjected to vertical load as described above. Therefore, the concrete pile is required to have excellent strength (tensile strength TS and proof stress YP).

Prestressed concrete (PC) is used in concrete piles to increase strength. Prestressed concrete is concrete to which compressive stress is applied in advance in the longitudinal direction. Prestressed concrete is manufactured by the following method. The steel material is pulled in the longitudinal direction, and the concrete is poured while being tense. After casting, the concrete is solidified. After the concrete solidifies, the tension of the steel material is released. When the tension is released, the steel material elastically contracts. As a result, compressive stress is applied in the longitudinal direction of the concrete, and prestressed concrete is formed.

The steel material used for such prestressed concrete is called a PC steel rod. The mechanical properties of PC steel rods are standardized in JIS G 3137 (2008). Among the standardized PC steel rods, the highest strength class D is specified to have a tensile strength TS of 1420 MPa or more, a proof stress YP of 1275 MPa or more, and a total elongation of 5.0% or more.

By the way, the concrete pile receives a bending load during transportation and installation work. The bending load creates the risk of cracking or breaking the concrete pile. If the uniform elongation of the PC steel rod in the concrete pile is high, it is possible to suppress breakage due to bending load. Therefore, the PC steel rod is required to have sufficient uniform elongation as well as strength (tensile strength TS, proof stress YP) and total elongation.

PC steel rods for meeting these demands are JP-A-11-158582 (Patent Document 1), JP-A-2003-268494 (Patent Document 2), and JP-A-2001-294980 (Patent Document 3). Has been proposed to.

The PC steel rod described in Patent Document 1 contains 0.2 to 1% of C as a chemical composition, and has a martensite structure and an average diameter of carbides of 0.1 μm or less. The PC steel rod of Patent Document 1 is manufactured by the following method. After drawing the rolled wire into a deformed round bar, quenching and tempering are performed. The heating method of the quenching treatment is, for example, high frequency induction heating, and the tempering is carried out, for example, at 600 ° C. or higher.

The PC steel rod described in Patent Document 2 has C: 0.15 to 0.29% by mass, Si: 0.8 to 2.0% by mass, Mn: 0.8 to 2.0% by mass, and Al: 0. It contains .005 to 0.050% by mass, Nb: 0.005 to 0.150% by mass, N: 0.0030 to 0.0150% by mass, and further, Cr: 0.05 to 2.00% by mass, Mo. : 0.05 to 1.00% by mass, V: 0.05 to 1.00% by mass, one or more kinds (Cr + Mo + V) ≧ 0.5% by mass, the balance is Fe and unavoidable impurities It is a steel material obtained by hot rolling a PC steel material made of the above material, cooling it at a cooling rate of 0.2 ° C./sec or more to form a martensite structure, and tempering it in the range of 350 to 650 ° C. Patent Document 2 describes that the PC steel rod has a tensile strength of 1420 N / mm ² or more, a 0.2% proof stress of 1275 N / mm ² or more, and a uniform elongation of 5% or more.

The chemical composition of the PC steel rod described in Patent Document 3 contains C, Si, Mn, P, S, and Al, and may contain Mo, Ti, and B as optional elements, and the balance is Fe and impurities. Consists of. The PC steel rod of Patent Document 3 is manufactured by the following method. A steel material having the above chemical composition is hot-rolled in a single-phase region of austenite or in a two-phase region of austenite and ferrite. The steel material after hot rolling is hardened by induction heating (induction hardening). After that, the steel material is reheated by high frequency induction heating or direct energization heating to perform tempering.

Japanese Unexamined Patent Publication No. 11-15582 Japanese Unexamined Patent Application Publication No. 2003-268493 Japanese Unexamined Patent Publication No. 2001-294980

It is described that all of the PC steel rods described in Patent Documents 1 to 3 have tensile strength TS, proof stress YP and total elongation according to the above JIS standard Class D, and are also excellent in uniform elongation. ing.

However, in the PC steel rod disclosed in Patent Document 1, the rolled wire is reheated by high-frequency heating or other method to perform quenching treatment (hereinafter referred to as offline quenching), so that the number of steps increases and the manufacturing cost increases. Will be higher.

In Patent Document 2, since quenching is carried out in the cooling process from hot rolling (hereinafter, defined as direct quenching), the process cost can be suppressed. However, it contains Nb as an alloying element and contains Cr, Mo, and V in a total amount of 0.5% or more. Although these alloying elements make it easier to obtain the above mechanical properties, these alloying elements are expensive. Therefore, the cost of the material is high. In particular, Cr has a smaller effect of maintaining strength after tempering than Mo and V. Therefore, when only Cr is contained without containing Mo and V, the Cr content is 0.9% or more. In this case, the raw material cost increases.

The PC steel rod disclosed in Patent Document 3 is composed of ferrite and martensite, and the average area ratio of ferrite is as high as 20 to 40% in order to obtain uniform elongation. However, Patent Document 3 defines the particle size of ferrite and martensite to be 5 μm or less, and in order to obtain such a structure, a special rolling and cooling process for high-speed cooling in direct quenching is required. I need. Therefore, the process cost increases in spite of the direct quenching.

Further, when the concrete pile is manufactured, a heat treatment is performed at the time of placing the concrete in order to promote the solidification of the concrete. However, Patent Documents 1 to 3 do not describe the mechanical properties of the PC steel rod after the heat treatment at the time of placing concrete.

An object of the present invention is to provide a PC steel rod which suppresses manufacturing cost and raw material cost, has high tensile strength, proof stress and total elongation, and has high uniform elongation after heat treatment at the time of placing concrete. be.

The PC steel rod according to the present invention has a chemical composition of% by mass, C: 0.21 to 0.29%, Si: 0.80 to 1.40%, Mn: 1.00 to 2.00%, Cr. : 0.20 to 0.60%, P: 0.030% or less, S: 0.030% or less, Ni: 0 to 0.50%, Cu: 0 to 0.50%, Ti: 0 to 0. It contains 100%, Al: 0 to 0.100%, and B: 0 to 0.0060%, and the balance consists of Fe and impurities. In the microstructure, the area ratio of martensite is 70.0% or more. The area ratio of baynite is 5.0% or more, the total area ratio of proeutectoid ferrite and pearlite is 5.0% or less (including 0.0%), and the tensile strength TS is 1420 MPa or more. The withstand strength YP is 1275 MPa or more, the total elongation is 5.0% or more, and the uniform elongation after holding at 180 ° C. for 3 hours or the uniform elongation after holding at 80 ° C. for 3 hours is 2. It is 0% or more.

The PC steel rod according to the present invention has high tensile strength, proof stress and total elongation while suppressing manufacturing cost and raw material cost, and has high uniform elongation after heat treatment at the time of placing concrete.

The present inventors have a mechanical property of class D in JIS G 3137 (2008) (tensile strength TS is 1420 MPa or more, proof stress YP is 1275 MPa or more, total elongation is 5.0% or more, while suppressing manufacturing cost and raw material cost. ), And a PC steel rod that can suppress bending and breakage of the concrete pile even if it receives a bending load during transportation or installation work (hereinafter referred to as transportation, etc.) was investigated and examined. .. Here, the proof stress YP means the nominal stress when 0.2% plastic elongation occurs.

First, as a measure for reducing the manufacturing cost, the present inventors examined the following matters. In offline quenching, in which the steel material after hot rolling is cooled and then reheated to perform quenching, it is necessary to reheat the steel material before quenching. On the other hand, the quenching method for performing quenching on a steel material in a high temperature state after hot rolling is defined as direct quenching. In the case of direct quenching, unlike offline quenching, the step of reheating the steel material at room temperature can be omitted. Therefore, when direct quenching is adopted, the manufacturing cost can be reduced as compared with offline quenching.

Further, as a measure for reducing the raw material cost, the present inventors examined the following matters. In order to reduce the raw material cost, it is effective to suppress the content of alloying elements such as Cr, Mo, V and Nb. However, when direct quenching is carried out, it is necessary to contain a certain amount of alloying elements in order to satisfy the mechanical properties of Class D of the above JIS standard. Therefore, the present inventors have investigated the chemical composition of the PC steel rod manufactured by direct quenching to satisfy the mechanical properties of Class D of the above JIS standard. As a result, the mass% of the PC steel rod was C: 0.21 to 0.29%, Si: 0.80 to 1.40%, Mn: 1.00 to 2.00%, Cr: 0.20. If it contains ~ 0.60%, P: 0.030% or less, S: 0.030% or less, and the manufacturing conditions of the manufacturing process including direct quenching are appropriately controlled, the mechanical properties of Class D are satisfied. I found that.

By the way, in a concrete pile in which a PC steel rod is used, as described above, it is necessary to suppress bending and breakage caused by a bending load applied to the concrete pile during transportation and the like. Therefore, the PC steel rod is required to have sufficient uniform elongation so that the concrete pile can be prevented from bending or breaking during transportation or the like.

When concrete piles are manufactured using PC steel rods, heat treatment is performed at the time of concrete placement in order to promote solidification of concrete. There are two types of heat treatment during concrete placement: steam curing and autoclave curing.

Steam curing is carried out by the following method. The concrete on which the PC steel rods are placed is held for 3 to 5 hours in an environment where the vapor pressure is 1 atm (0.1 MPa) at a temperature of 65 to 80 ° C. In addition, autoclave curing is carried out by the following method. The concrete on which the PC steel rods are arranged is held for 3 to 5 hours in an environment where the vapor pressure is 10 to 11 atm (1.0 to 1.1 MPa) at a temperature of 180 to 190 ° C. The above curing promotes the solidification of concrete.

In consideration of the above, what is required of the PC steel rod is not a sufficient uniform elongation before the heat treatment at the time of concrete placing, but a sufficient uniform elongation after the heat treatment at the time of concrete placing. Therefore, the present inventors first investigate various PC steel rods in order to investigate the uniform elongation of the PC steel rods that can suppress bending and breakage when a bending load is applied to the concrete pile during transportation or the like. Was used to evaluate the prototype. As a result, it was found that if the uniform elongation of the PC steel rod after the heat treatment at the time of placing concrete is 2.0% or more, the bending and breaking of the concrete pile can be suppressed during transportation and the like.

Based on the above findings, the present inventors assume heat treatment at the time of placing concrete, and the uniform elongation after performing either of the two curings on the PC steel rod is 2.0% or more. If so, it was considered that the bending and breaking of the concrete pile could be sufficiently suppressed.

Therefore, the uniform elongation of the PC steel rod after performing the following two concrete casting simulated heat treatments simulating the concrete placing heat treatment (steam curing and autoclave curing) corresponds to the uniform elongation after the concrete placing heat treatment. It was used for evaluation.
Steam curing simulated heat treatment: Holds at 80 ° C for 3 hours in the atmosphere Autoclave curing simulated heat treatment: Holds at 180 ° C for 3 hours in the atmosphere Specifically, the concrete casting simulated heat treatment (steam curing simulated heat treatment) is applied to a PC steel rod. After performing either heat treatment or autoclave curing simulated heat treatment), a tensile test conforming to JIS Z 2241 (2011) was performed at room temperature (25 ° C) to create a stress-strain curve, and the created stress was created. -The uniform elongation obtained from the strain curve was defined as the uniform elongation after the simulated heat treatment for concrete placement.

Using the uniform elongation evaluation by the concrete casting simulated heat treatment, the present inventors suppress the manufacturing cost and the raw material cost, and satisfy the mechanical properties of the D type (tensile strength TS, proof stress YP, total elongation). Further studies were conducted on PC steel rods having a uniform elongation of 2.0% or more after simulated heat treatment for concrete placement.

As described above, by containing 0.20 to 0.60% of Cr, the strength of the above type D (tensile strength TS and proof stress YP) is satisfied even if direct quenching is performed. However, in this case, if the microstructure of the PC steel rod is substantially a martensite single phase, the ductility is lowered. Therefore, the total elongation and the uniform elongation after the simulated concrete casting heat treatment are lowered, and further, cracks may occur at the time of straightening after direct quenching. On the other hand, if the proportions of proeutectoid ferrite and pearlite in the microstructure of the PC steel rod increase, the above-mentioned class D strengths (tensile strength TS and proof stress YP) cannot be satisfied.

Therefore, the present inventors considered including bainite in the microstructure of PC steel rods. Bainite ductility is higher than martensite. Therefore, it is possible to suppress cracking during straightening and increase uniform elongation. Moreover, the strength of bainite is higher than that of proeutectoid ferrite and pearlite. Therefore, if an appropriate amount of bainite is contained in the microstructure of the PC steel rod, the strength of the above D type can be satisfied and the uniform elongation after the simulated heat treatment for concrete placement can be enhanced. As a result of further studies by the present inventors, the microstructure of the PC steel rod having the above chemical composition contains martensite having an area ratio of 70.0% or more and bainite having an area ratio of 5.0% or more. If the area ratio of the ferrite and pearlite is 5.0% or less (including 0.0%), the strength of the above D type is satisfied, and the uniform elongation after the concrete casting simulated heat treatment is 2.0%. I found that it was the above.

The PC steel rod according to the present invention completed based on the above findings has a chemical composition of 0.21 to 0.29% by mass, C: 0.21 to 0.29%, Si: 0.80 to 1.40%, Mn: 1. 00 to 2.00%, Cr: 0.20 to 0.60%, P: 0.030% or less, S: 0.030% or less, Ni: 0 to 0.50%, Cu: 0 to 0.50 %, Ti: 0 to 0.100%, Al: 0 to 0.100%, and B: 0 to 0.0060%, and the balance consists of Fe and impurities. The area ratio is 70.0% or more, the area ratio of baynite is 5.0% or more, and the total area ratio of proeutectoid ferrite and pearlite is 5.0% or less (including 0.0%). The tensile strength TS is 1420 MPa or more, the proof stress YP is 1275 MPa or more, the total elongation is 5.0% or more, and uniform elongation after holding at 180 ° C. for 3 hours or holding at 80 ° C. for 3 hours. The subsequent uniform elongation is 2.0% or more.

The chemical composition is Ni: 0.10 to 0.50%, Cu: 0.05 to 0.50%, Ti: 0.005 to 0.100%, Al: 0.005 to 0.100%, and , B: One or more selected from the group consisting of 0.0010 to 0.0060%.

Hereinafter, the PC steel rod according to the present invention will be described in detail. Unless otherwise specified, "%" for an element means mass%.

[Chemical composition]
The chemical composition of the PC steel rod according to the present invention contains the following elements.

C: 0.21 to 0.29%
Carbon (C) increases the strength of steel. If the C content is less than 0.21%, the tensile strength TS and the proof stress YP conforming to the D type in JIS G 3137 (2008) cannot be obtained. On the other hand, if the C content exceeds 0.29%, the strength after quenching becomes too high. In this case, it is not possible to satisfy the total elongation according to the D type in the above JIS standard. Therefore, the C content is 0.21 to 0.29%. The lower limit of the C content is preferably 0.22%, more preferably 0.23%. The preferred upper limit of the C content is 0.28%, more preferably 0.27%.

Si: 0.80 to 1.40%
Silicon (Si) suppresses temper softening of steel. Due to the inclusion of Si, it is possible to increase the total elongation while maintaining a high proof stress YP even when tempering at a high temperature is performed. If the Si content is less than 0.80%, the above effect cannot be obtained. On the other hand, if the Si content exceeds 1.40%, the ductility of the steel is lowered, and sufficient uniform elongation after total elongation and / or concrete casting simulated heat treatment according to the above JIS standard Class D can be obtained. I can't. Therefore, the Si content is 0.80 to 1.40%. The lower limit of the Si content is preferably 0.90%, more preferably 1.00%. The preferred upper limit of the Si content is 1.20%.

Mn: 1.00 to 2.00%
Manganese (Mn) enhances the hardenability of steel and enhances the strength of steel. If the Mn content is less than 1.00%, the above effect cannot be obtained. On the other hand, if the Mn content exceeds 2.00%, segregation is likely to occur and coarse MnS is generated. Coarse MnS reduces the toughness and fatigue strength of PC steel bars. Therefore, the Mn content is 1.00 to 2.00%. The preferred lower limit of the Mn content is 1.20%, more preferably 1.40%. The preferred upper limit of the Mn content is 1.80%.

Cr: 0.20 to 0.60%
Chromium (Cr) enhances the hardenability of steel and enhances the strength of steel. Cr further increases the hardness during tempering. Cr further enhances the total elongation and the uniform elongation after the concrete casting simulated heat treatment. If the Cr content is less than 0.20%, the above effect cannot be obtained. On the other hand, if the Cr content exceeds 0.60%, the cost of the material increases. If the Cr content exceeds 0.60%, the strength of the steel after quenching becomes too high, and the total elongation of the PC steel rod and / or the uniform elongation after the simulated concrete casting heat treatment is lowered. Therefore, the Cr content is 0.20 to 0.60%. The lower limit of the Cr content is preferably 0.30%, more preferably 0.40%. The preferred upper limit of the Cr content is 0.55%.

P: 0.030% or less Phosphorus (P) is an impurity. P embrittles the steel. In particular, P segregated at the old austenite grain boundaries lowers the impact value of the steel and further causes delayed fracture due to the intrusion of hydrogen. Therefore, the P content is 0.030% or less. The preferred upper limit of the P content is less than 0.030%, more preferably 0.015%. It is preferable that the P content is as low as possible.

S: 0.030% or less Sulfur (S) is an impurity. S embrittles the steel. S further combines with Mn to form MnS. Since the coarse MnS serves as a fracture starting point, the fracture characteristics of the steel deteriorate. MnS can also be a source of corrosion. Therefore, the corrosion resistance of the steel is lowered. Therefore, the S content is 0.030% or less. The preferred upper limit of the S content is less than 0.030%, more preferably 0.015%. The lower the S content, the better.

The balance of the chemical composition of the PC steel rod according to the present invention consists of Fe and impurities. Here, the impurities are mixed from ore, scrap, manufacturing environment, etc. as a raw material when the PC steel rod of the present invention is industrially manufactured, and have an adverse effect on the PC steel rod of the present invention. It means what is allowed within the range that does not give.

[About arbitrary elements]
The chemical composition of the above-mentioned PC steel rod may further contain one or more selected from the group consisting of Ni and Cu instead of a part of Fe. Both Ni and Cu are optional elements and enhance the corrosion resistance and toughness of steel.

Ni: 0 to 0.50%
Nickel (Ni) is an optional element and may not be contained. When contained, Ni enhances the corrosion resistance and toughness of steel. If even a small amount of Ni is contained, the above effect can be obtained to some extent. However, if the Ni content exceeds 0.50%, the above effect is saturated. If the Ni content exceeds 0.50%, the raw material cost is further increased and the manufacturability is also lowered. Therefore, the Ni content is 0 to 0.50%. The preferable lower limit of the Ni content for more effectively obtaining the above effect is 0.10%, and more preferably 0.15%. The preferred upper limit of the Ni content is 0.40%, more preferably 0.25%.

Cu: 0 to 0.50%
Copper (Cu) is an optional element and may not be contained. When contained, Cu enhances the corrosion resistance and toughness of steel, similar to Ni. If even a small amount of Cu is contained, the above effect can be obtained to some extent. However, if the Cu content exceeds 0.50%, the above effect is saturated. If the Cu content exceeds 0.50%, the raw material cost is further increased and the manufacturability is also lowered. Therefore, the Cu content is 0 to 0.50%. The preferable lower limit of the Cu content for more effectively obtaining the above effect is 0.05%, and more preferably 0.10%. The preferred upper limit of the Cu content is 0.30%, more preferably 0.20%.

The chemical composition of the above-mentioned PC steel rod may further contain Ti instead of a part of Fe.

Ti: 0 to 0.100%
Titanium (Ti) is an optional element and may not be contained. When contained, Ti combines with C or N to form precipitated particles of Ti carbides, Ti nitrides and the like. These precipitated particles function as pinning particles in austenite and refine the austenite particles. If even a small amount of Ti is contained, the above effect can be obtained to some extent. However, if the Ti content exceeds 0.100%, the precipitated particles are coarsened and serve as a fracture starting point. In this case, the ductility and toughness of the PC steel rod are reduced. Therefore, the Ti content is 0 to 0.100%. The preferable lower limit of the Ti content for more effectively obtaining the above effect is 0.005%, and more preferably 0.010%. The preferred upper limit of the Ti content is 0.070%, more preferably 0.040%.

The chemical composition of the above-mentioned PC steel rod may further contain Al instead of a part of Fe.

Al: 0 to 0.100%
Aluminum (Al) is an optional element and may not be contained. When contained, Al deoxidizes the steel. If Al is contained even in a small amount, the above effect can be obtained to some extent. However, if the Al content exceeds 0.100%, the oxide-based inclusions become coarse and become a fracture starting point. In this case, the ductility and toughness of the PC steel rod are reduced. Therefore, the Al content is 0 to 0.100%. The preferable lower limit of the Al content for more effectively obtaining the above effect is 0.005%, and more preferably 0.010%. The preferred upper limit of the Al content is 0.070%, more preferably 0.040%.

The chemical composition of the above-mentioned PC steel rod may further contain B instead of a part of Fe.

B: 0 to 0.0060%
Boron (B) is an optional element and may not be contained. When contained, B enhances the hardenability of steel and enhances the tensile strength TS and proof stress YP of the PC steel rod. If B is contained even in a small amount, the above effect can be obtained to some extent. However, if the B content exceeds 0.0060%, coarse precipitates (Fe ₂₃ (CB), BN, etc.) are formed, and the ductility and toughness of the PC steel rod are lowered. Therefore, the B content is 0 to 0.0060%. The preferable lower limit of the B content for more effectively obtaining the above effect is 0.0010%, and more preferably 0.0015%. The preferred upper limit of the B content is 0.0050%, more preferably 0.0040%.

[Micro organization]
In the microstructure of the PC steel rod of the present invention, the area ratio of martensite is 70.0% or more, and the area ratio of bainite is 5.0% or more. The total area ratio of proeutectoid ferrite and pearlite is 5.0% or less (including 0.0%). The total area ratio of proeutectoid ferrite and pearlite may be 0.0%. That is, the microstructure (matrix) of the PC steel rod of the present invention may be composed of martensite and bainite.

If the area ratio of martensite is less than 70.0%, the tensile strength TS and the proof stress YP conforming to the above JIS standard Class D cannot be obtained. Further, if the total area of the proeutectoid ferrite and pearlite is higher than 5.0%, the tensile strength TS and the proof stress YP conforming to the JIS standard D type cannot be obtained.

If the area ratio of bainite is less than 5.0%, the uniform elongation after the simulated concrete casting heat treatment is less than 2.0%, and sufficient uniform elongation cannot be obtained. Further, when direct quenching is carried out in the manufacturing process described later, the area ratio of martensite in the microstructure becomes excessively high, and the steel material cracks or breaks in the straightening process. There is.

In the microstructure of PC steel rods, the martensite area ratio is 70.0% or more, the bainite area ratio is 5.0% or more, and the total area ratio of proeutectoid ferrite and pearlite is 5.0% or less (0.0). If it is (including%), the mechanical properties of Class D of the above JIS standard are satisfied, and the uniform elongation after the simulated heat treatment for placing concrete is 2.0% or more.

In the above microstructure, the preferable lower limit of the area ratio of bainite is 10.0%. The preferred upper limit of the area ratio of bainite is 25.0%. The preferred upper limit of the total area of proeutectoid ferrite and pearlite is 2.0%.

[Measurement method of area ratio of each phase in microstructure]
The area ratio of each phase (martensite, bainite, proeutectoid ferrite, pearlite) in the microstructure can be measured by the following method.

A sample including the R / 2 position (the position that bisects the line segment (radius) connecting the center and the surface of the PC steel rod) is taken in the cross section perpendicular to the longitudinal direction of the PC steel rod. Of the sample, the surface including the R / 2 position (referred to as the observation surface) is mechanically polished. The mechanically polished observation surface is etched with 3% nital (an ethanol solution containing 3% nitric acid by mass). Of the etched observation surfaces, an arbitrary 5 visual fields (the area of each visual field is 100 μm × 80 μm) are observed with a 1000 times optical microscope.

Each phase (martensite, bainite, proeutectoid ferrite, pearlite) is identified for each field of view with an optical microscope, and the total area (μm ² ) of each phase in each field is determined. Identification of each phase can be made based on the presence or absence of carbides, carbide morphology, and tissue morphology. The total area of martensite (μm ² ), the total area of bainite (μm ² ), the total area of proeutectoid ferrite (μm ² ), and the total area of pearlite (μm ² ) in all five fields are determined. The ratio of the total area of each obtained phase to the total area of the five visual fields is defined as the area ratio (%) of each phase.

[Mechanical characteristics]
[Tensile strength TS, proof stress YP and total elongation]
The PC steel rod according to the present invention has the above chemical composition and microstructure, and further has the following mechanical properties.
Tensile strength TS: 1420 MPa or more Strength YP: 1275 MPa or more Total elongation: 5.0% or more

All of these mechanical properties satisfy the mechanical properties of Class D specified in JIS G 3137 (2008).

[Uniform elongation after simulated heat treatment for concrete placement]
Further, the PC steel rod according to the present invention has a uniform elongation of 2.0% or more after the simulated heat treatment for placing concrete. The concrete casting simulated heat treatment is one of the autoclave curing simulated heat treatment and the steam curing simulated heat treatment defined below.

The autoclave curing simulated heat treatment is carried out by the following method. The PC steel rod is placed in the atmosphere at a temperature of 180 ° C. for 3 hours.

The steam curing simulated heat treatment is carried out by the following method. The PC steel rod is placed in the atmosphere at a temperature of 80 ° C. for 3 hours.

The concrete casting simulated heat treatment may be either the autoclave curing simulated heat treatment or the steam curing simulated heat treatment. The uniform elongation after the simulated heat treatment for concrete placement is obtained by the following method. A tensile test piece parallel to the longitudinal direction of the PC steel rod is prepared with the R / 2 position of the cross section of the PC steel rod (cross section perpendicular to the longitudinal direction of the PC steel rod) after the simulated heat treatment for concrete placement as the central axis. do. The prepared tensile test piece was subjected to a tensile test in accordance with JIS Z 2241 (2011) Annex D-specified No. 2 test piece at room temperature (25 ° C) and in the atmosphere, and stress-strain. Get a curve. If the PC steel rod is indented and the cross section is not round, the stress is calculated using the nominal diameter of the PC steel rod as a standard. From the obtained stress-strain curve, the uniform elongation (%) is obtained.

[Production method]
An example of the method for manufacturing a PC steel rod according to the present invention will be described. The manufacturing method described below is an example of the manufacturing method for the PC steel rod of the present invention. Therefore, PC steel rods having the above chemical composition, microstructure, and mechanical properties may be manufactured by other manufacturing methods.

An example of the method for manufacturing a PC steel rod of the present invention includes a hot rolling step, a straightening step, and a tempering step. Hereinafter, each step will be described.

[Hot rolling process]
A material having the above-mentioned chemical composition is prepared. The material is, for example, a slab (bloom, slab or billet) or an ingot of steel. The material is manufactured by the following method. A molten steel having the above chemical composition is produced. A slab is manufactured by a continuous casting method using molten steel. Alternatively, the ingot is manufactured by the ingot method using molten steel. The material is prepared by the above process.

A hot working process is carried out on the prepared material to manufacture an intermediate product of PC steel rods (hereinafter referred to as an intermediate steel material). In the hot working process, hot working is usually performed one or more times. The material is heated before performing each hot work. After that, hot working is performed on the material. Hot working is, for example, hot forging or hot rolling. When performing hot working multiple times, the initial hot working is, for example, a roughing step by slab rolling or hot forging, and the final hot working is, for example, a finish rolling step using a continuous rolling mill. be. The temperature of the material before the final hot working is, for example, 900 to 1200 ° C. In the hot rolling mill, horizontal stands having a pair of horizontal rolls and vertical stands having a pair of vertical rolls are alternately arranged in a row. The intermediate steel material produced by the hot working process is a wire rod having a circular cross section. The diameter of the intermediate steel material is, for example, 7.4 mm to 11.5 mm. Even if the roll surface of the roll surface after the final rolling stand of the continuous rolling mill used in the finish rolling process has grooves for forming irregularities (indents) for adhering concrete to the surface of the intermediate steel material. good.

In the hot working process, a Stealmore type cooling conveyor is arranged downstream of the final rolling stand. On the Stelmore type conveyor, the intermediate steel material is formed in a ring shape. The intermediate steel material formed in a ring shape immediately after rolling is water-cooled and quenched while being maintained at a high temperature. Specifically, the ring-shaped intermediate steel material is transported to a water-cooled tank, immersed, and quenched. As described above, in the present invention, the intermediate steel material after rolling is not once cooled to room temperature and then reheated to perform quenching, but the intermediate steel material after rolling is quenched (directly quenched) as it is.

The manufacturing conditions in the rolling process are as follows.

Winding temperature: 750-950 ° C
The take-up temperature means the temperature of the intermediate steel material at the position where the intermediate steel material is formed in a ring shape. When the winding temperature is less than 750 ° C., the area ratio of proeutectoid ferrite and pearlite increases in the microstructure of the PC steel rod and exceeds 5.0%. As a result, the JIS standard D type tensile strength TS and / or proof stress YP cannot be satisfied. On the other hand, when the winding temperature exceeds 950 ° C., the microstructure of the PC steel rod becomes substantially martensite single phase, and the area ratio of bainite becomes less than 5.0%. As a result, the total elongation and / or the uniform elongation becomes low, or cracks occur in the intermediate steel material in the straightening step after direct quenching. Therefore, the take-up temperature is 750 to 950 ° C.

Transport time: 5 to 40 seconds The transport time is the time from the winding temperature measurement point to the water cooling tank (that is, the time taken from the winding temperature measurement point to just before the start of quenching). .. The intermediate steel material is usually transported on a Stelmore type conveyor from the measurement point of the take-up temperature to the water cooling tank. The intermediate steel material being transported is air-cooled.

If the transport time is less than 5 seconds, that is, when the intermediate steel material is dropped directly into the water-cooled tank, the shape after ring formation in the water-cooled tank deteriorates. On the other hand, if the transport time exceeds 40 seconds, the quenching temperature becomes too low. In this case, the area ratio of proeutectoid ferrite and pearlite becomes large and exceeds 5.0%. As a result, the JIS standard D type tensile strength TS and / or proof stress YP cannot be satisfied. Therefore, the transport time is 5 to 40 seconds.

Cooling rate: 2.0 to 10.0 ° C / sec The cooling rate means the average value (° C / sec) of the cooling rate from the temperature immediately before the start of direct quenching to 200 ° C for the temperature of the intermediate steel material. ..

If the cooling rate is less than 2.0 ° C./sec, the total area ratio of proeutectoid ferrite and pearlite in the microstructure of the PC steel rod becomes too large and exceeds 5.0%. On the other hand, when the cooling rate exceeds 10.0 ° C./sec, the microstructure of the PC steel rod is substantially only martensite, and the area ratio of bainite is less than 5.0%. Therefore, the cooling rate is 2.0 to 10.0 ° C./sec.

[Correcting process]
The intermediate steel material after direct quenching is wound into a ring shape. Therefore, the intermediate steel material after direct quenching is rectified and tempered to obtain desired mechanical properties (tensile strength TS, proof stress YP and total elongation conforming to the above JIS standard Class D).

Specifically, after direct quenching, the wound ring-shaped intermediate steel material is straightened by using a well-known piece straightening machine to form a straight line. The straightness of the intermediate steel material immediately after the correction is, for example, 15 mm / 2 m or less. For the purpose of softening the intermediate steel material, the intermediate steel material immediately before the baking may be heat-treated (baked) at about 100 ° C. to correct the intermediate steel material after baking. At the same time, a lubricating film treatment may be applied immediately before the intermediate steel material for the purpose of preventing the occurrence of flaws when the intermediate steel material is modified.

[Tempering process]
Tempering is carried out on the intermediate steel immediately after the correction. Tempering is carried out, for example, by using a high frequency heating device arranged on the same line as the piece straightening machine. The position of the high-frequency heating device used for tempering does not have to be on the same line as the piece straightening machine.

In tempering by high frequency heating, a preferable heating temperature (hereinafter referred to as tempering temperature) is 350 to 600 ° C. If the tempering temperature is less than 350 ° C., the tensile strength TS and the proof stress YP of the PC steel rod are too high, and the total elongation becomes low. On the other hand, if the tempering temperature exceeds 600 ° C., the tensile strength TS and the proof stress YP of the PC steel rod become too low, and the mechanical properties conforming to the above JIS standard Class D cannot be obtained. Therefore, the tempering temperature is 350 to 600 ° C.

In the above manufacturing method, the PC steel rod of the present invention can be manufactured by directly quenching without performing so-called offline quenching. Therefore, the PC steel rod of the present invention can suppress the manufacturing cost, and even if it does not contain a large amount of alloying elements, it has mechanical properties (tensile strength TS, proof stress YP, and strength YP, and mechanical properties conforming to the above JIS standard Class D). Can have full elongation). Furthermore, it has sufficient uniform elongation even after the simulated heat treatment for placing concrete. Therefore, even when the concrete pile manufactured by performing the concrete placing heat treatment using the PC steel rod of the present invention is transported, the concrete pile is not easily bent and is not easily damaged.

Hereinafter, examples of the present invention will be described. The present embodiment is an example for confirming the effect of the present invention. Therefore, the present invention is not limited to the present embodiment.

Blooms having the chemical compositions shown in Table 1 were produced.

Specifically, slabs (blooms) were produced by a continuous casting method using molten steel. Billet was produced by performing slab rolling on the produced bloom. The billet after lump-rolling was air-cooled to room temperature.

The billet was reheated and finish rolling was performed. A continuous rolling mill was used for finish rolling. The take-up temperature (° C) after finish rolling, the transport time (seconds), and the cooling rate during direct quenching (° C / sec) are as shown in Table 2.

The produced ring-shaped intermediate steel material was baked at 100 ° C. for 3 hours. Then, the scale on the surface of the intermediate steel material was removed by pickling, and a bonde film was applied to the surface. Then, the intermediate steel material was straightened using a piece straightening machine. The tempered intermediate steel material was tempered by high frequency heating. The tempering temperature was as shown in Table 2. A PC steel rod was manufactured by the above steps.

[Microstructure observation test]
A sample containing the R / 2 position was taken in a cross section perpendicular to the longitudinal direction of the PC steel rod. Of the sample, the surface including the R / 2 position (referred to as the observation surface) was mechanically polished. The mechanically polished observation surface was etched with 3% nital (an ethanol solution containing 3% nitric acid by mass). Of the etched observation surfaces, observation was performed with an optical microscope at a magnification of 1000 for any 5 visual fields (the area of each visual field is 100 μm × 80 μm). The phases (martensite, bainite, proeutectoid ferrite, pearlite) in each visual field were identified, and the total area (μm ² ) of each phase in each visual field was determined. The total area of martensite (μm ² ), the total area of bainite (μm ² ), and the total area of proeutectoid ferrite and pearlite (μm ² ) in all five fields were determined. The ratio of the total area of each obtained phase to the total area of the five visual fields was defined as the area ratio (%) of each phase. The measurement results are shown in Table 2.

[Tensile strength TS, proof stress YP and total elongation evaluation test]
Tensile test pieces were collected for the PC steel rods of each test number in accordance with JIS Z 2241 (2011) Annex D No. 2 test piece. In the tensile test piece, the rolled wire diameter was φ10.3 mm, the nominal diameter was φ10.0 mm, the distance between the gauge points was 80 mm, and the parallel portion length was 150 mm. Tensile tests were conducted in the air at room temperature (25 ° C) in accordance with JIS Z 2241 (2011) using tensile test pieces, and tensile strength TS (MPa), proof stress YP (MPa), and all. The growth (%) was calculated. The results obtained are shown in Table 2.

[Uniform elongation evaluation test after simulated heat treatment for concrete placement]
Tensile strength TS, proof stress YP, and tensile test pieces similar to those in the total elongation evaluation test were collected from the PC steel rods of each test number. The following concrete placement simulated heat treatment (curing) was carried out on the tensile test pieces at each test number.
Test numbers 2, 5-10, 21-23, 25, 27-29, 31, 33: Autoclave curing simulated heat treatment (described as "O curing" in Table 2)
Test numbers 1, 3, 4, 11, 13-15, 17-20: Steam curing simulated heat treatment (described as "J curing" in Table 2)

In the autoclave curing simulated heat treatment, the tensile test piece was held at 180 ° C. for 3 hours in the air. In the steam curing simulated heat treatment, the tensile test piece was held at 80 ° C. for 3 hours in the atmosphere.

A stress-strain curve was obtained by conducting a tensile test in accordance with JIS Z 2241 at room temperature (25 ° C.) and in the air using a tensile test piece after a simulated heat treatment for placing concrete. From the obtained stress-strain curve, uniform elongation (%) was obtained. The results obtained are shown in Table 2.

[Evaluation results]
With reference to Table 2, the chemical compositions of Test Nos. 1-10 were appropriate and the production conditions were also appropriate. Therefore, in the microstructure of the manufactured PC steel rod, the area ratio of martensite was 70.0% or more, and the area ratio of bainite was 5.0% or more. Further, the total area ratio of proeutectoid ferrite and pearlite was 2.0% or less. As a result, the tensile strength TS was 1420 MPa or more, the proof stress YP was 1275 MPa or more, and the total elongation was 5.0% or more. Further, the uniform elongation after the simulated heat treatment for placing concrete was 2.0% or more.

On the other hand, in test number 11, the C content was too low. Therefore, the tensile strength TS was too low.

In test number 12, the C content was too high. Therefore, the bainite area ratio was less than 5.0%. As a result, cracks occurred in the steel material in the straightening process.

In test number 13, the Si content was too low. As a result, the tensile strength TS was too low.

In test number 14, the Si content was too high. As a result, the total growth was less than 5.0%.

In test number 15, the Cr content was too low. As a result, the uniform elongation after the simulated heat treatment for concrete placement was less than 2.0%.

In test number 16, the Cr content was too high. Therefore, the area ratio of bainite was less than 5.0%. As a result, cracks occurred in the steel material in the straightening process.

In test number 17, the P content was too high. Therefore, the total elongation was less than 5.0%, and the uniform elongation after the simulated concrete casting heat treatment was less than 2.0%.

In test number 18, the S content was too high. Therefore, the total elongation was less than 5.0%, and the uniform elongation after the simulated concrete casting heat treatment was less than 2.0%.

In Test Nos. 19 and 21, the tempering temperature was less than 350 ° C., although the chemical composition was appropriate. Therefore, the total elongation was less than 5.0%, and the uniform elongation after the concrete casting simulated heat treatment was less than 2.0%.

In test numbers 20 and 22, the tempering temperature exceeded 600 ° C., although the chemical composition was appropriate. Therefore, the tensile strength TS was less than 1420 MPa, and the proof stress YP was also less than 1275 MPa.

In test numbers 23 and 25, the chemical composition was appropriate, but the winding temperature was low. Therefore, the total area ratio of proeutectoid ferrite and pearlite exceeded 2.0%. As a result, the tensile strength TS was less than 1420 MPa.

In test numbers 24 and 26, the chemical composition was appropriate, but the take-up temperature was too high. Therefore, the area ratio of bainite was less than 5.0%. As a result, cracks occurred in the steel material in the straightening process.

In test numbers 27, 28 and 29, the chemical composition was appropriate, but the transport time was too long. Therefore, the total area ratio of proeutectoid ferrite and pearlite exceeded 2.0%. As a result, in the test numbers 27 and 29, the tensile strength TS was less than 1420 MPa and the proof stress YP was less than 1275 MPa. Further, in Test No. 28, the proof stress YP was less than 1275 MPa, and the uniform elongation after the simulated concrete casting heat treatment was less than 2.0%.

In test numbers 30 and 32, the chemical composition was appropriate, but the cooling rate was too fast. Therefore, the area ratio of bainite was less than 5.0%. As a result, cracks occurred in the steel material in the straightening process.

In test numbers 31 and 33, the chemical composition was appropriate, but the cooling rate was too slow. Therefore, the total area ratio of proeutectoid ferrite and pearlite exceeded 2.0%. As a result, the tensile strength TS was less than 1420 MPa and the proof stress YP was less than 1275 MPa.

The embodiment of the present invention has been described above. However, the embodiments described above are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-mentioned embodiment can be appropriately modified and carried out within a range not deviating from the gist thereof.

Claims (2)

The chemical composition is by mass%,
C: 0.21 to 0.29%,
Si: 0.80 to 1.40%,
Mn: 1.00 to 2.00%,
Cr: 0.20 to 0.60%,
P: 0.030% or less,
S: 0.030% or less,
Ni: 0 to 0.50%,
Cu: 0 to 0.50%,
Ti: 0 to 0.100%,
Al: 0 to 0.100% and
B: 0 to 0.0060%,
The balance consists of Fe and impurities.
In micro-organization
The area ratio of martensite is 70.0% or more,
The area ratio of bainite is 5.0% or more,
The total area ratio of proeutectoid ferrite and pearlite is 5.0% or less (including 0.0%).
The tensile strength TS is 1420 MPa or more, and the tensile strength TS is 1420 MPa or more.
The proof stress YP is 1275 MPa or more,
The total growth is 5.0% or more,
A PC steel rod having a uniform elongation after holding at 180 ° C. for 3 hours or a uniform elongation after holding at 80 ° C. for 3 hours at 2.0% or more. The PC steel rod according to claim 1.
The chemical composition is
Ni: 0.10 to 0.50%,
Cu: 0.05 to 0.50%,
Ti: 0.005 to 0.100%,
Al: 0.005 to 0.100%, and
B: 0.0010 to 0.0060%,
A PC steel rod containing one or more selected from the group consisting of.