JP2015160999A - Rebar - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength reinforcement excellent in a delayed fracture characteristic while having a high strength property with tensile strength of 1600 MPa.SOLUTION: There is provided a high strength reinforcement consisting of steel having C content in a zone from a surface to the depth of at least 10 μm of 0.01 mass% or less and the steel has a two phase structure of ferrite in a surface layer and martensite in a core part, a fraction of ferrite in all structure is less than 5% and tensile strength is 1600 MPa or more.

Description

  The present invention relates to a reinforcing bar used for, for example, a shear reinforcing bar having a tensile strength of 1600 MPa or more used for a reinforced concrete structure.

  For example, in a reinforced concrete structure, a shear reinforcing bar is used as a reinforcing material in order to prevent its collapse. In reinforced concrete structures using shear reinforcement, when the reinforced concrete structure undergoes shear deformation, the shear reinforcement is stretched and plastically deformed, so that the deformation energy of the reinforced concrete structure is absorbed by the shear reinforcement and the reinforced concrete structure Collapse is prevented.

  The shear reinforcement which has been used so far has a tensile strength of about 1200 MPa. However, in recent years, there is a need for slimming and lightening the cross section of a reinforced concrete structure, or increasing the height of a reinforced concrete structure, and development of ultra-high strength concrete is rapidly progressing. Along with this, in order to balance the increase in concrete strength, it is necessary to increase the strength of the shear reinforcement.

  However, in general, when the strength of a steel material is increased, the sensitivity to delayed fracture becomes sharper. In particular, when hydrogen generated on the steel surface due to corrosion of the steel material in concrete penetrates into the steel material, the stress concentration portion Prone to delayed destruction due to hydrogen accumulation. Therefore, when the strength of the shear reinforcement is increased, it is assumed that the hydrogen generated due to the corrosion of the surface by chloride ions existing in the concrete increases especially in the reinforcing bars embedded in concrete. There is concern about the deterioration of delayed fracture characteristics.

Thus, several proposals have been made to overcome the above problems.
For example, Patent Document 1 discloses ferrite on the surface of a steel material by controlling the content of C, Si, and Mn in the steel material to an appropriate range and controlling the cooling condition after heating the steel material to the austenite region. A heat treatment method for securing a decarburized phase of 0.12 mm or more and making the inside a ferrite pearlite structure is disclosed. However, when the ferrite decarburized phase is 0.12 mm or more, it is difficult to ensure the strength. Further, the internal structure is a ferrite / pearlite structure, and it is difficult to obtain a high-strength reinforcing bar.

  In Patent Document 2, the content of C, Si, Mn, Ni and Al in the steel wire is optimized, the ferrite decarburized layer on the surface of the steel wire is controlled to 0.12 mm or more, and the inside is ferrite / pearlite structure or spherical A steel wire rod excellent in delayed fracture characteristics controlled to a cementitized cementite structure is disclosed. However, as described above, when the ferrite decarburization is 0.12 mm or more, it is difficult to ensure the strength. Further, since the internal structure is a ferrite / pearlite structure or a spheroidized cementite structure, it is difficult to obtain a high-strength reinforcing bar.

Japanese Patent No. 3156166 JP-A-6-306540

  As described above, with the development of ultra-high-strength concrete, it is necessary to increase the strength of the reinforcing bars. However, if the strength of the reinforcing bars is increased, the susceptibility to delayed fracture increases and delayed fracture tends to occur. It was an issue that had to be done.

  The present invention has been made to solve such a problem, and an object of the present invention is to provide a reinforcing bar having high strength characteristics with a tensile strength of 1600 MPa or more and excellent delayed fracture resistance.

  In order to solve the above-mentioned problems, the inventors manufactured high-strength reinforcing bars in which the amount of C, the steel structure, and the structure fraction in the region from the surface to a depth of at least 10 μm were changed, and intensively investigated the delayed fracture characteristics. As a result, the surface side of the reinforcing bar has a ferrite single-phase structure (hereinafter also referred to as surface ferrite), and the C content in the region from the reinforcing bar surface to a predetermined depth is reduced, thereby generating hydrogen generated on the reinforcing bar surface. It was found that the amount of hydrogen penetrating into the reinforcing bar can be reduced by reducing the amount. Furthermore, if the core part of the reinforcing bar has a martensite single-phase structure (hereinafter also referred to as core martensite) and the fraction of ferrite with respect to the entire structure (surface ferrite + core martensite) is less than 5% by area ratio. Further, it was found that the tensile strength of 1600 MPa or more can be secured, and the effect of reducing the intrusion hydrogen by the surface layer ferrite is maintained. In this way, the C content in the region from the surface to a depth of at least 10 μm is set to 0.01 mass% or less, the steel structure is composed of ferrite in the surface layer and martensite in the core, and the ferrite fraction in the entire structure is 5%. If satisfying the condition of less than, it was found that the amount of diffusible hydrogen permeating into the rebar can be reduced, and even if the steel has a high strength with a tensile strength of 1600 MPa or more, good delayed fracture characteristics can be imparted. It came to complete.

That is, the gist configuration of the present invention is as follows.
The C content in the region from the surface to a depth of at least 10 μm is 0.01 mass% or less, and the steel has a two-phase structure of ferrite in the surface layer and martensite in the core. Rebar with a fraction of less than 5% and a tensile strength of 1600 MPa or more.

  Here, the ferrite fraction is an area ratio {ferrite area / (ferrite area + martensite area)} in a cross section perpendicular to the length direction of the reinforcing bar.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide the reinforcement which has the delayed fracture characteristic which was excellent in strength compared with the conventional reinforcement. The reinforcing bar of the present invention has a tensile strength of 1600 MPa or more, has excellent delayed fracture characteristics, and contributes to slimming and weight reduction of the reinforced concrete structure and increasing the height of the reinforced concrete structure. , Industrially beneficial effects are brought about.

It is a figure which shows the shape of the test piece with which it uses for the evaluation test of a delayed fracture characteristic. It is a figure which shows the sampling position of a test piece. It is a graph which shows the relationship between the amount of diffusible hydrogen, and a fracture | rupture time.

Hereinafter, the C content in a predetermined region and the steel structure will be specifically described for the reinforcing bar of the present invention.
[C content in region from surface to depth of at least 10 μm: 0.01 mass% or less]
When the C content in the region from the surface of the reinforcing bar to a depth of at least 10 μm (hereinafter also referred to as a low C region) exceeds 0.01 mass%, the amount of solute C increases, and further, the bainite structure and / or the martensite structure Generation increases susceptibility to delayed fracture. In addition, when the C content in the low C region exceeds 0.01 mass%, the amount of hydrogen generated along with the corrosion of the rebar surface increases, and the amount of hydrogen entering the rebar that causes delayed fracture also increases. .
In addition, since it is difficult for C content of a low C area | region to completely make C content 0 mass%, it is preferable to set it as 0.001 mass% or more.

  Here, the region where the C content is 0.01 mass% or less is at least 10 μm deep from the surface, that is, the thickness of the low C region is 10 μm or more, when the thickness of the low C region is less than 10 μm, Because the effect of reducing the amount of hydrogen generated due to corrosion becomes insufficient, the amount of hydrogen entering the steel increases, hydrogen tends to accumulate in the hard martensite structure inside the steel, and delayed fracture is promoted. It is. The thickness of the low C region is preferably 15 μm or more.

  In order to obtain strength as a high-strength reinforcing bar, specifically, a tensile strength of 1600 MPa or more, the thickness of the region where the C content is 0.01 mass% or less is preferably 100 μm or less. Because, when the thickness of the region where the C content is 0.01 mass% or less exceeds 100 μm, the force applied to the surface ferrite increases, and the surface ferrite is cracked at the time of tension, and then the crack progresses immediately, Tensile strength decreases.

[Steel structure: It has a two-phase structure of ferrite in the surface layer and martensite in the core, and the ferrite fraction in the entire structure is less than 5%]
The steel has a two-phase structure of ferrite on the surface layer and martensite that forms the core part inside. The martensite phase is useful for increasing the strength of steel, but if the surface layer of the reinforcing bar has a martensite structure, delayed fracture susceptibility increases and delayed fracture resistance decreases. Therefore, it is important that the surface layer of the reinforcing bar has a ferrite single-phase structure with low delayed fracture susceptibility and a martensite single-phase structure other than the core, that is, the surface layer, to ensure strength. On the other hand, when the ferrite fraction in the entire structure is 5% or more, the ferrite fraction having a lower strength than martensite becomes large and it becomes difficult to secure a desired strength. Less than 5%.
From the above, the steel structure was composed of a ferrite in the surface layer and martensite in the core, and a two-phase structure in which the ferrite fraction in the entire structure was less than 5%.

Tensile strength: 1600MPa or more Reinforcing bars cannot be used to increase the strength of concrete if the tensile strength of the reinforcing bars is less than 1600MPa. Preferably, it is 1650 MPa or more.

  These rebars are made from steel melted in a converter or vacuum melted into steel ingots, slabs, blooms, billets, etc., and heated to hot rolling or hot forging. Then, after pickling, removing the scale and adjusting to a predetermined thickness by wire drawing, it is manufactured by heating / holding, quenching, or further tempering.

  Here, if the martensite structure is used at the time of melting, the tensile strength can be 1600 MPa or more. Preferably, a material having a steel composition that can have a tensile strength of 1650 MPa or more is melted if a martensite structure is formed in consideration of the strength reduction due to the formation of surface ferrite.

Next, by decarburizing the surface layer during the heating and holding described above, the C content in the region from the surface to a depth of at least 10 μm is adjusted to 0.01 mass% or less. That is, by heating and holding in a decarburizing atmosphere (air, N ₂ atmosphere, etc.), the C content in the region from the surface to a depth of at least 10 μm is set to 0.01 mass% or less. At this time, the heating temperature and the holding time are appropriately adjusted so that the C content of the core portion inside the low C region is maintained at the time of melting. The conditions of heating temperature and holding time for decarburizing the C content in the region from the surface to a depth of at least 10 μm to 0.01 mass% or less vary depending on the steel composition of the material, so it is optimal for the steel composition What is necessary is just to obtain | require an appropriate condition beforehand. And after heating and holding, the core part has a martensite structure and the surface layer has a ferrite structure by performing a quenching treatment. Similarly, the ferrite fraction with respect to the entire structure can be adjusted by adjusting the surface depth at which decarburization occurs by controlling the heating temperature and holding time during heat treatment.

  In addition, the thickness of the ferrite structure of the surface layer is larger than the thickness of the low C region where the C content is 0.01 mass% or less. This is because the C content is more than 0.01 mass%, the C content is smaller than the core, and the region that does not become martensite and becomes ferrite even when quenched is inside the low C region (the core side). ).

  Although the reinforcing bars thus obtained can be manufactured at low cost, they have excellent delayed fracture characteristics despite their high strength, and are applicable to shear reinforcements such as high-rise apartment buildings that require high strength of 1600 MPa or more. Is possible.

The evaluation of the delayed fracture characteristics of a reinforcing bar is most preferably performed by actually manufacturing a reinforcing bar and using it for a reinforced concrete structure. However, since this method takes time, this example was evaluated as follows.
That is, after melting and casting steel having the component composition shown in Table 1, it was formed into a round bar having a diameter of 60 mm by hot forging. A test piece was collected from the round bar according to the specifications shown in FIG. As shown in FIG. 2, the test piece was collected so that the 1/4 depth position (1 / 4D position) of the round bar diameter D from the surface of the round bar 2 was the axial center of the test piece 1. . The collected test pieces were heated and held at the heating temperature shown in Table 2 and the in-furnace time at 850 ° C. or higher, and then quenched under oil cooling at 60 ° C. Thereafter, tempering at the heating temperature and holding time shown in Table 2 was performed. The heating and holding here simulates heating during quenching applied to a high-strength reinforcing bar. Next, after pickling the test piece after tempering and descaling, the C content, tensile strength and diffusible hydrogen content of the surface layer were measured under the following conditions, and delayed fracture characteristics were evaluated. Furthermore, the steel structure was observed.

[C content of surface layer]
The C content of the surface layer is obtained by cutting out the central part of the parallel part of the test piece shown in FIG. 1 and embedding it in a resin so that the cross section perpendicular to the axis of the test piece becomes the observation surface. Hereinafter, it was measured using EPMA). The measurement conditions of EPMA are: beam diameter: 5 μmφ, acceleration voltage: 20 kV, current: 4 × 10 ⁻⁷ A, line analysis from the surface to a depth of 1 mm, and every depth from the surface (5 μm pitch) The amount of C was measured. And the value of the depth from the surface of the area | region where C content will be 0.01 mass% or less was calculated | required.

[Tissue observation]
The structure is examined by using the above-mentioned test piece whose C concentration is measured, corroding with 3% nital after mirror polishing, and then observing with an optical microscope 500 times, and the ferrite structure and martensite in the cross section of the test piece. Obtain the area of the structure, respectively. F = (Ferrite area / Cross-sectional area of test piece) × 100 (%)
However, the ferrite fraction F was determined according to the cross-sectional area of the test piece: ferrite area + martensite area.

[Tensile test]
The tensile strength was evaluated by conducting a tensile test on the test piece of FIG. 1 according to JIS Z2241 at a tensile speed of 5 mm / min.

[Delayed fracture characteristics]
In order to investigate the delayed fracture characteristics as a high-strength reinforcing bar, the FIP test was conducted using the test piece shown in FIG. The FIP test was conducted in accordance with JSCE S 1201: 2012 (Corrosion Protection Association). That is, when immersed in a 20% ammonium thiocyanate (NH ₄ SCN) aqueous solution at 50 ° C and loaded with a test load that becomes 70% of the tensile strength, if it does not break even after 100 hours of test time, delayed fracture The property was defined as good.

[Amount of diffusible hydrogen]
The amount of diffusible hydrogen was determined by cutting a 10 mm long sample from the fractured surface of the fractured test piece and the unbroken test piece from the parallel part after fracture or completion of the test in the FIP test described above. The temperature was increased at a rate of temperature increase of 200 ° C / hour with GC7000T manufactured by Science Lab. The amount of hydrogen released up to 350 ° C was defined as the amount of diffusible hydrogen, and the amount of diffusible hydrogen was measured.

  Table 2 shows the depth, the ferrite fraction, the amount of diffusible hydrogen, the rupture time in the FIP test, and the tensile strength in the region where the C content existing in the surface layer is 0.01 mass% or less. In all examples, the steel structure of the surface layer was a ferrite single phase, and the core structure was a martensite single phase. Steel Nos. 1-3, 6, 7, 10, and 12 (all invention examples) satisfying the requirement that the amount of C of the present invention is 0.01 mass% or less and the ferrite fraction is less than 5% are FIP. No breakage was observed in the test. On the other hand, it can be seen that Steel Nos. 4, 5, and 8 (Comparative Examples), which are outside the scope of the present invention, are prematurely broken in the FIP test, and the delayed fracture characteristics are reduced. Steel Nos. 9 and 11 have a long heating and holding time, the surface decarburization amount or the ferrite fraction increases, and the delayed fracture property is good, but the tensile strength is out of the range of the present invention. Recognize. In addition, about the result shown in Table 2, the graph which showed the relationship between the amount of diffusible hydrogen and fracture | rupture time is shown in FIG. In the examples of the present invention, the excellent delayed fracture resistance is attributed to the fact that the amount of diffusible hydrogen is a low value of 3 mass ppm.

1 Test piece 2 Round bar

Claims (1)

  The C content in the region from the surface to a depth of at least 10 μm is 0.01 mass% or less, and the steel has a two-phase structure of ferrite in the surface layer and martensite in the core, and the ferrite content in the entire structure Rebar with a fraction of less than 5% and a tensile strength of 1600 MPa or more.

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