WO2014034070A1

WO2014034070A1 - Steel for reinforcing bars, and reinforcing bar

Info

Publication number: WO2014034070A1
Application number: PCT/JP2013/004997
Authority: WO
Inventors: 稔本庄; 清史上井; 遠藤　茂
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2012-08-31
Filing date: 2013-08-23
Publication date: 2014-03-06
Also published as: IN2015DN00727A; JPWO2014034070A1; KR20150034799A; KR101631522B1; JP5741773B2; CN104603310A; CN104603310B

Abstract

Provided is a steel for reinforcing bars, with which a reinforcing bar having high strength and excellent bending workability can be produced with excellent drawability. This steel for reinforcing bars has a component composition which contains from 0.37% by mass to 0.50% by mass (inclusive) of C, from 1.75% by mass to 2.30% by mass (inclusive) of Si, from 0.2% by mass to 1.0% by mass (inclusive) of Mn, from 0.01% by mass to 1.2% by mass (inclusive) of Cr, from 0.05% by mass to 1.0% by mass (inclusive) of Mo, 0.025% by mass or less of P, 0.025% by mass or less of S and 0.0015% by mass or less of O, and wherein C, Si and Cr are added so as to satisfy a predetermined relation.

Description

Steel and rebar for rebar

The present invention relates to, for example, a high-strength reinforcing bar used as a shear reinforcement used in a reinforced concrete structure and a steel for reinforcing steel used as a material thereof.

For example, reinforced concrete structures such as buildings and apartment houses use shear reinforcement as a reinforcing material to prevent their collapse. In a reinforced concrete structure that uses shear reinforcement, when the reinforced concrete structure undergoes shear deformation, the shear reinforcement stretches and plastically deforms, so that the deformation energy of the reinforced concrete structure is absorbed by the shear reinforcement and the reinforced concrete structure Collapse is prevented.

Conventionally, a shear reinforcing bar having a tensile strength of about 1200 MPa is generally used. However, in recent years, there is a need for slimming, lightening, and high-rise sections of reinforced concrete structures, and for this reason, development of ultra-high-strength concrete is rapidly progressing. If the strength of the concrete increases, it will be necessary to increase the strength of the shear reinforcement in order to balance it.

By the way, the shear reinforcement bar functions as a reinforcing material by wrapping it around the main bar that prevents the shaft that supports the structure of the building from bending, such as concrete columns and beams, for example. The cross section is shaped into a coil shape such as a circle or a square. Since these cross-sectional shapes are given by bending, bending workability is required for the molding. Therefore, if the shear reinforcement has excellent elongation characteristics, bending work becomes easy, which is a great advantage in terms of workability. However, as described above, if the strength of the shear reinforcement is increased in accordance with the increase in the strength of the concrete, there are newly concerns about problems such as breakage during bending.

Now, to increase the strength of the shear reinforcement, it is necessary to increase the amount of addition of alloy elements including C, Si, and Mn. However, the shear reinforcing bars are generally manufactured by drawing steel for reinforcing bars and then performing heat treatment. Therefore, when the addition amount of the alloy element is increased, the hardness of the steel for reinforcing bars is increased, the wire is disconnected when the steel for reinforcing bars is drawn, and the productivity is lowered. Therefore, in order to achieve high strength, it was necessary to avoid a decrease in the drawability of the steel for reinforcing bars.

Thus, several proposals have been made to overcome the above problems.
First, Patent Document 1 discloses a heat treatment method in which C, Si, and Mn are controlled within an appropriate range, and cooling conditions are controlled to ensure a surface decarburization of 0.12 mm or more. However, when the ferrite decarburization is 0.12 mm or more, it is difficult to ensure the strength when a high-strength reinforcing bar is manufactured. Therefore, it is necessary to increase the strength other than the ferrite decarburized layer, and it is necessary to add an alloy. Therefore, the hardness of the material for reinforcing bars is increased, and the drawability is lowered. Moreover, after manufacturing a high-strength reinforcing bar, the toughness of the reinforcing bar is lowered and the bending workability is lowered. Further, in order to make the ferrite decarburized layer 0.12 mm or more, it is necessary to maintain a temperature range of 830 to 900 ° C. for at least 30 seconds, depending on the amount of Si to be added, and a temperature of 600 to 700 ° C. Since it is preferable to hold in the range for 1 hour or more, the efficiency at the time of operation falls, and an extra operation cost starts.

In Patent Document 2, the amount of C, Si, Mn, Ni, and Al is optimized, the ferrite decarburized layer on the steel surface layer is controlled to 0.12 mm or more, and the inside is controlled to a ferrite pearlite structure or a spheroidized cementite structure. Thus, it is disclosed that a steel wire having excellent delayed fracture characteristics is obtained. However, as described above, when the ferrite decarburization is 0.12 mm or more, it is difficult to ensure the strength when a high-strength reinforcing bar is manufactured, so it is necessary to increase the strength other than the ferrite decarburized layer, and an alloy addition is necessary. Become. For this reason, the hardness of the material for the reinforcing bars is increased and the wire drawing property is decreased, or the toughness of the reinforcing bars is decreased after manufacturing the high-strength reinforcing bars, and the bending workability is decreased. In addition, after the wire rod rolling, it is necessary to perform heat treatment online or offline, which increases extra operation costs such as reheating.

Here, Patent Document 3 discloses a high-strength reinforcing bar in which the component composition and the ferrite area ratio and the total area ratio of ferrite and pearlite structure are controlled. However, when the pearlite structure is generated, the toughness is lowered, so that the bending workability may be lowered.

Patent Document 4 discloses a method for producing high-strength steel with high yield elongation by controlling the component composition and rolling method. However, the tensile strength is 100 kg / mm ² or less, and when the strength is increased, the yield elongation decreases, so the bending workability may decrease.

Furthermore, Patent Document 5 discloses a method for producing a high-strength steel with high yield elongation by controlling the component composition and the rolling method. As described above, since the yield elongation decreases when the strength is increased, the bending workability may also decrease.

Japanese Patent No. 3156166 JP-A-6-306540 JP 2012-67363 A JP-A-4-173922 Japanese Patent Laid-Open No. 9-324215

As described above, with the development of ultra-high-strength concrete, there has been a problem of increasing the strength of the shear reinforcement to be applied to reinforced concrete structures made of such concrete. However, in order to increase the strength of the shear reinforcement, it is necessary to add an alloy element. When alloying elements are added, the hardness of the steel for reinforcing bars increases, so that there is a problem that the wire rod is disconnected at the time of drawing processing at the time of reinforcing bar manufacturing, and the drawability is lowered. In addition, it is naturally necessary to prevent a decrease in bending workability due to the increase in strength.

The present invention has been made to solve such a problem, and provides a steel for reinforcing bars that can produce a reinforcing bar having high strength and excellent bending workability under excellent wire drawing. With the goal.

In order to solve the above-mentioned problems, the inventors manufactured high-strength steel for reinforcing bars with various amounts of addition of C, Si, Mn, Cr and Mo, and made the wire drawing property and hardness of the steel for reinforcing steel (drawing). Hard investigation of the hardness of the previous material. The steel for high-strength reinforcing steel was drawn and subjected to heat treatment, and the tensile strength and bending workability of the high-strength reinforcing steel produced by conducting heat treatment were intensively investigated.
As a result, in addition to optimizing the amount of addition of C, Si, Mn, Cr and Mo, it is possible to regulate the amount of addition of C, Si and Cr under a predetermined relationship. It has been found that it is indispensable for the provision of steel for reinforcing bars that can be used as a reinforcing bar or a material thereof having high bending strength while ensuring high strength, and has completed the present invention.

That is, the gist configuration of the present invention is as follows.
1. C: 0.37 mass% or more and 0.50 mass% or less,
Si: 1.75% by mass to 2.30% by mass,
Mn: 0.2% by mass or more and 1.0% by mass or less,
Cr: 0.01% by mass or more and 1.2% by mass or less Mo: 0.05% by mass or more and 1.0% by mass or less,
P: 0.025 mass% or less,
S: 0.025 mass% or less and O: 0.0015 mass% or less, A value according to the following (1) is 770 or more and 850 or less, B value according to the following formula (2) is 0.40 or more, and the balance of inevitable impurities and Fe Steel for reinforcing steel with component composition.
A = α + β + γ (1)
Here, α = −334 × [C] ² + 806 × [C] +291
β = 24 × [Si] ² + 67 × [Si]
γ = −4 × [Cr] ² + 23 × [Cr] −5
B = [Si] / [10 × C] (2)
However, the above [] is the content (mass%) of the component in parentheses.

2. The component composition further comprises:
Al: 0.50 mass% or less,
2. The steel for reinforcing bars as described in 1 above, containing one or more selected from Cu: 1.0% by mass or less and Ni: 2.0% by mass or less.

3. The component composition further includes:
W: 2.0 mass% or less,
Nb: 0.1% by mass or less,
The steel for reinforcing steel as described in 1 or 2 above, containing one or more selected from Ti: 0.2% by mass or less and V: 0.5% by mass or less.

4). The component composition further includes:
B: The steel for reinforcing steel as described in any one of 1 to 3 above, containing 0.005% by mass or less.

5. 5. The steel for reinforcing bars as described in any one of 1 to 4 above, wherein the hardness in the thickness region of at least 20 μm from the surface is HV250 or less, and the hardness in the depth region ¼ of the diameter of the steel material from the surface is HRC40 or less. .

6). A rebar having the component composition according to any one of 1 to 4 and having a hardness in a thickness region of at least 10 μm from the surface of HV300 or less.

According to the present invention, it is possible to stably manufacture steel for reinforcing bars that has excellent wire drawing properties and can manufacture high-strength reinforcing bars. Reinforcing bars using this steel for reinforcing steel, or reinforcing steel manufactured by drawing and heat treatment from steel for reinforcing steel have a tensile strength of 1600 MPa or more and excellent bending workability. It greatly contributes to slim, light weight and high-rise, and brings about an industrially beneficial effect.

Next, the component composition of the reinforcing bar of the present invention and the production conditions will be described.
C: 0.37% by mass or more and 0.50% by mass or less C is an essential element for ensuring the necessary strength, and if it is less than 0.37% by mass, it is difficult to ensure the predetermined strength, and in order to ensure the predetermined strength, an alloy It is necessary to add a large amount of element, which causes an increase in alloy cost. On the other hand, addition exceeding 0.50% by mass significantly increases the strength, which in turn increases the strength of the reinforcing bar more than necessary, leading to a decrease in bending workability. More preferably, it is in the range of 0.37 to 0.45 mass%.

Si: 1.75 mass% or more and 2.30 mass% or less Si is an element that increases the strength of steel by improving solid solution strengthening and temper softening resistance as a deoxidizer, and also a ferrite decarburization promoting element. As will be described later, it is also useful for securing a surface layer region of HV250 or less. Therefore, in this invention, it adds at 1.75 mass% or more. However, addition exceeding 2.30% by mass lowers the ductility and causes cracks in the material during casting, necessitating care of the material, leading to an increase in manufacturing costs. Therefore, the upper limit of Si is 2.30% by mass. More preferably, it is in the range of 1.75 to 2.25% by mass.

Mn: 0.2 mass% or more and 1.0 mass% or less Mn is added at 0.2 mass% or more in order to improve the hardenability of the steel. However, addition exceeding 1.0% by mass reduces the strength of the steel. Therefore, the upper limit of Mn is 1.0% by mass. More preferably, it is in the range of 0.25 to 1.0% by mass.

Cr: 0.01% by mass or more and 1.20% by mass or less Cr is an element that improves the hardenability of the steel and increases the strength. Therefore, 0.01 mass% or more is added. On the other hand, addition exceeding 1.20% by mass increases the strength of the steel on the contrary, which leads to a reduction in the wire drawability during drawing and the bending workability as a high strength rebar. From the above, the Cr content is set to 0.01% by mass or more and 1.20% by mass or less. More preferably, it is in the range of 0.01 to 1.00% by mass.

Mo: 0.05 mass% or more and 1.0 mass% or less Mo is an element that improves the hardenability of the steel and increases the strength. Therefore, 0.05 mass% or more is added. On the other hand, addition exceeding 1.0% by mass increases the strength of the steel on the contrary, so that the drawability at the time of drawing and the bending workability as a high-strength reinforcing bar are reduced. From the above, the amount of Mo is set to 0.05% by mass or more and 1.0% by mass or less. More preferably, it is in the range of 0.05 to 0.5% by mass.

P: 0.025% by mass or less S: 0.025% by mass or less P and S are segregated at grain boundaries to cause a reduction in the base metal toughness of the steel. In particular, since S is present in steel as MnS, MnS may be the starting point during bending, and cracks are likely to occur. Therefore, it is necessary to suppress as much as possible. Is preferred. In addition, since it will require high cost to make P and S each less than 0.0002 mass%, it is preferable industrially that P and S contain 0.0002 mass% or more, respectively.

O: 0.0015% by mass or less O is bonded to Si or Al to form hard oxide-based non-metallic inclusions, which may be the starting point during bending and may easily crack. However, in the present invention, up to 0.0015% by mass is allowed. In addition, since it will require high cost to make O less than 0.0005 mass%, it is preferable industrially that O is contained 0.0005 mass% or more.

A value (the above formula (1)): 770 or more and 850 or less The A value is an index for obtaining good strength, drawability and bending workability. If the A value is less than 770, the bending workability is good, but it is difficult to ensure the strength of the reinforcing bars. On the other hand, when the A value exceeds 850, good strength can be obtained, but the hardness of the steel for reinforcing bars is increased, causing breakage at the time of drawing, and the drawability is lowered. Furthermore, since the bending workability as a reinforcing bar is lowered, the A value is set to 770 or more and 850 or less in the present invention. More preferably, it is the range of 770 or more and 849 or less.

B value (the above formula (2)): 0.40 or more The B value is an index for obtaining good wire drawing. By setting this B value to 0.40 or more, it is possible to ensure good wire drawing in steel for reinforcing bars and good bending workability in reinforcing steel. The reason why the drawability and bending workability are improved by setting the B value to 0.40 or more is that a decarburized layer is formed in the surface layer region of the steel for reinforcing bars or the surface layer region of the reinforcing bars, and the hardness of this surface layer region is This is because the processability is lowered and the processability is improved. Specifically, as will be described in detail in the experimental results to be described later, by setting the B value to 0.40 or more, in the steel for reinforcing bars, the hardness in the thickness region of at least 20 μm or more from the surface becomes HV250 or less, and the drawability Can be good. Further, by setting the B value to 0.40 or more, the hardness of the region having a thickness of at least 10 μm or more from the surface of the reinforcing bar can be made HV300 or less, and the bending workability can be improved.

The inventors evaluated the effects of the component composition of steel for reinforcing steel, particularly the above-described A value and B value, on the drawability and bending workability in various experiments. Two typical experimental results are described in detail below.
[Experiment 1]
With respect to the above A value and B value, the inventors made steel for reinforcing bars in which the component composition, the A value, and the B value were changed, and investigated the hardness thereof. Furthermore, the steel for reinforcing bars is manufactured by drawing, quenching and tempering, and the wire drawing property at the time of drawing is investigated, and the tensile strength, The surface hardness and bending workability as well as the structure were investigated. In addition, the tensile strength, surface layer hardness, and bending workability were measured by the test method mentioned later. Table 1 shows the component composition, and Table 2 shows the evaluation results of the range of HV250 or less, the hardness, the tensile strength as the reinforcing bar, and the bending workability of the reinforcing steel.

The manufacturing conditions are as follows.
First, the steel ingot melted by vacuum melting was heated from room temperature to the heating temperature shown in Table 2 at the heating rate shown in Table 2, and then hot rolled. The manufacturing conditions after hot rolling were the same. That is, after hot rolling was completed at 850 ° C., it was cooled at 1 ° C./s to obtain a wire having a diameter of 13.5 mm, which was used as a steel for reinforcing steel. The hardness of the cross section perpendicular to the longitudinal direction of the obtained wire was measured. The hardness was measured by the test method described later.

Next, using this steel for reinforcing bars as a raw material, a reinforcing bar was manufactured. The manufacturing conditions for the reinforcing bars were the same. That is, the manufacturing conditions are as follows.
A 13.5 mm wire was drawn to 12.6 mm, and then quenched and tempered by heating to 1000 ° C., cooling with 60 ° C. oil, heating to 350 ° C., and water cooling. The drawability was judged by whether or not the wire was broken during the drawing process, and if it was not broken, it was judged that the wire had good drawability.
The wire rod after quenching and tempering was processed into a tensile test piece having a parallel portion of 1/4 in. Described in ASTM E8. The bending workability was tested by the test method described later after cutting to 500 mm length.

Table 2 shows the range of steel for reinforcing steels in the range of HV250 or less, the hardness in the depth region (1 / 4D part) from the surface of the wire diameter D, the drawability at the time of drawing, the tensile strength as a reinforcing bar, Bending workability was shown. As shown in this table, when both the A value and the B value are controlled within the range of the present invention, it can be seen that the above-described characteristics are good. From the above, it was found that by adjusting the A value to the range of 770 to 850 and the B value to the range of 0.40 or more, the drawability is improved while maintaining high strength.

[Experiment 2]
Regarding the above A value and B value, the inventors prepared reinforcing bars from steel for reinforcing bars in which the component composition and the A value and B value were changed, and the tensile strength, surface hardness and bending workability, Investigated the organization. In addition, the tensile strength, surface layer hardness, and bending workability were measured according to the test method mentioned later. Moreover, the structure | tissue in the range below HV300 and the structure | tissue of a core part (1 / 2D part: D is the diameter of a reinforcing bar) were investigated. Table 3 shows the component composition, and Table 4 shows the tensile strength, the range of HV300 or less, and the bending workability evaluation results.

Here, in the evaluation, the manufacturing conditions of the reinforcing bars were the same. That is, the manufacturing conditions are as follows.
First, a steel ingot melted by vacuum melting was heated to 1100 ° C. and then hot forged into a round bar having a diameter of 11.5 mm. The obtained round bar was processed into a tensile test piece having a parallel part of 1/4 in. Described in ASTM E8 and subjected to quenching and tempering treatment. Table 4 shows the heating temperature and holding time of the quenching treatment, and the tempering temperature and holding time of the tempering treatment. For bending workability, the round bar after hot forging was cut into a length of 500 mm, and then quenched and tempered under the above-described conditions, and the test was performed by the test method described later.

Table 4 shows the evaluation results of tensile strength, range of HV300 or less, and bending workability. As shown in this table, when both the A value and the B value are controlled within the range of the present invention, it can be seen that good tensile strength, a range of HV300 or less and bending workability can be obtained. From the above, it was found that by adjusting the A value to 770 or more and 850 or less and the B value to 0.40 or more, the bending workability is improved while the strength is increased when the reinforcing bar is used.

By summing up the above experimental results, by limiting the component composition, particularly the A value and B value, to the above-mentioned ranges, the hardness of the steel at least 20 μm from the surface in the steel for reinforcing bars is HV250 or less, and from the surface to the steel material The hardness of the depth region (1 / 4D part) of 1/4 of the diameter is HRC40 or less, and the hardness of the thickness region at least 10 μm from the surface after quenching and tempering is HV300 or less. Recognize. Note that the hardness of the surface layer is preferably regulated as described above. The reason for this is as follows.

The hardness of the thickness region of at least 20μm from the surface is HV250 or less The reason why the hardness of the thickness region of at least 20μm from the surface is preferably HV250 or less is that if the hardness of this region exceeds HV250, high strength steel for rebar This is because the hardness of the wire becomes high and the drawability is lowered, and the wire breakage tends to occur during the drawing process.
Also, the reason why the thickness is in the range of at least 20 μm from the surface is that when the thickness is less than 20 μm, the high ductility region from the surface becomes small, and on the contrary, the strands are easily broken during the drawing process. On the other hand, if the region expands to a depth of more than 100 μm from the surface, the strength as a high-strength reinforcing bar will decrease, so that the overall strength of the core will be further increased to achieve a tensile strength of 1600 MPa or more. On the other hand, the drawability at the time of drawing and the bending workability as a high-strength reinforcing bar tend to be lowered. Therefore, the region below HV250 is sufficient up to a depth of 100 μm from the surface.
Here, in order to set the hardness in the thickness region of at least 20 μm from the surface to HV250 or less, it is preferable that the steel is heated in the atmosphere to Ac ₃ point or more at a heating rate of 30 ° C./min or less. This step can be performed in a raw material heating step for hot rolling described later.

Hardness in the depth region of 1/4 of the steel diameter from the surface is HRC40 or less The reason why the hardness in the depth region of 1/4 of the steel diameter from the surface is preferably HRC40 or less is that this region is HRC40 If the temperature is too high, the region subjected to the processing during the drawing process becomes hard, the die life is shortened, and the material is easily broken. Here, the 1/4 depth region refers to a portion (1 / 4D portion) whose distance from the steel surface is 1/4 of the diameter D of the steel material.

In order to make the hardness of this region HRC40 or less, it is only necessary to avoid the formation of a martensite structure in the steel structure. Specifically, a steel wire rod for rebar is manufactured by hot rolling, The rolling is preferably finished at Ar ₃ points or more, and then the cooling rate to at least 700 ° C. is preferably 2 ° C./s or less.

The hardness of the rebar after quenching and tempering is HV300 or less in the thickness area of at least 10 μm from the surface. The reason why the hardness of the thickness area of at least 10 μm from the surface is preferably HV300 or less is that the hardness in this area exceeds HV300 When it becomes, since hardness will become high and ductility will fall, it will become easy to produce a crack in a reinforcing steel surface layer at the time of bending. Therefore, in the present invention, the hardness of the surface layer region is preferably HV300 or less. Further, the reason why the surface layer region is set to a range from the surface to a depth of at least 10 μm is that when the thickness is less than 10 μm, the high ductility region becomes small, and cracks tend to occur in the reinforcing steel surface layer during bending. From the above, it is preferable that the hardness of the thickness region of at least 10 μm from the surface is HV300 or less.
Here, in order to set the hardness in the thickness region of 10 μm or more from the surface to HV300 or less, it is preferable to heat the steel to Ac ₃ points or more in the atmosphere. This step can be performed in a quenching step described later.

It should be noted that it is sufficient that the region to be HV300 or less is 10 μm to 150 μm in depth from the surface. That is, when the depth region from the surface to be HV300 or less exceeds 150 μm, the strength of the reinforcing bar is decreased, and therefore the depth from the surface to the region to be HV300 or less is preferably 150 μm or less. Further, the region of HV300 or less is preferably a ferrite single phase structure. This is because stress concentration occurs in the surface layer during bending, but if the region below HV300 is a ferrite single-phase structure, stress concentration is relaxed by the high ductility of ferrite and bending workability is improved. Since the bainite structure also has a certain degree of ductility, it may be mixed in the structure within a range that can satisfy HV300 or less. On the other hand, the core structure of the reinforcing bar is preferably a martensite structure in order to ensure the strength as a reinforcing bar.

Furthermore, the steel for reinforcing bars of the present invention can contain the following components in addition to the above components in order to increase the strength and improve the bending workability of the reinforcing bars.
One or more of Al: 0.50% by mass or less, Cu: 1.0% by mass or less, and Ni: 2.0% by mass or less Cu and Ni are elements that increase hardenability and strength after tempering, and are selected. Can be added. In order to obtain such an effect, Cu and Ni are preferably added at 0.005 mass% or more. However, if Cu is added in an amount of 1.0% by mass and Ni is added in an amount exceeding 2.0% by mass, the alloy cost is increased. Therefore, it is preferable to add Cu at an upper limit of 1.0% by mass and Ni at an upper limit of 2.0% by mass.
Further, Al is useful as a deoxidizer, and is an element effective for maintaining strength by suppressing the growth of austenite grains during quenching. Therefore, Al is preferably added in an amount of 0.01% by mass or more. However, even if added over 0.50% by mass, the effect is saturated, resulting in a disadvantage incurring a cost increase, and the oxide in the steel is increased, which becomes a starting point during bending and the bending workability is lowered. Therefore, Al is preferably added with an upper limit of 0.50% by mass.

W: 2.0% by mass or less, Nb: 0.1% by mass or less, Ti: 0.2% by mass or less, and V: 0.5% by mass or less W, Nb, Ti and V are all hardenable It is an element that enhances the strength of steel after tempering, and can be selected and added according to the required strength. In order to obtain such an effect, it is preferable to add 0.001% by mass or more for W, Nb, and Ti and 0.002% by mass or more for V, respectively. However, when V is added in an amount of 0.5% by mass, Nb is added in an amount of more than 0.1% by mass and Ti is added in an amount of more than 0.2% by mass, a large amount of hard carbide / nitride / carbonitride is generated in the steel, and cracking occurs during bending. It tends to be a starting point and causes a decrease in bending workability. Nb, Ti and V are preferably added with the above values as upper limits. On the other hand, when W is added in excess of 2.0% by mass, the wire drawing property and bending workability are lowered due to the increase in strength, and the alloy cost is increased. Therefore, it is preferable to add W with an upper limit of 2.0% by mass.

B: 0.005% by mass or less B is an element that increases the strength of the steel after tempering by increasing hardenability, and can be contained as necessary. In order to acquire the said effect, adding at 0.0002 mass% or more is preferable. However, when it exceeds 0.005 mass%, bending workability will deteriorate. Therefore, B is preferably added in the range of 0.0002 to 0.005 mass%.

The steel ingot having the above component composition can be used either by melting by a converter or by vacuum melting. And steel ingots, slabs, blooms, billets and other materials are heated and hot rolled, pickled and scaled, drawn, adjusted to a predetermined thickness, heat treated, and rebar Used for industrial steel.

The reinforcing bars manufactured using the steel for reinforcing bars of the present invention preferably have the above component composition and have a tensile strength of 1600 MPa or more. In other words, if the tensile strength is less than 1600 MPa, it is not possible to cope with the increase in the concrete strength of the rebar, so 1600 MPa or more is necessary.

Here, in order to set the tensile strength to 1600 MPa or more, after hot-rolling the steel having the above composition, the steel for reinforcing bars from which the scale has been removed is then drawn and adjusted to a predetermined thickness in the atmosphere. It is preferable to heat to Ac ₃ point or higher and quench (cooling rate is 60 ° C./s or higher), and then temper in a temperature range of 100 to 600 ° C. That is, the tempering temperature is preferably 100 ° C. or higher from the viewpoint of ensuring bending workability, and preferably 600 ° C. or lower from the viewpoint of ensuring a tensile strength of 1600 MPa or higher.

The rebar manufactured through the above manufacturing process has a tempered martensite at the core. In addition, even after wire drawing, the surface layer portion has a decarburized layer, and further, decarburization is promoted by quenching and tempering, so that the surface layer portion has lower hardness than the core portion. That is, in the metal structure in the reinforcing bar of the present invention, the surface layer region described above is a ferrite single phase structure or a mixed structure of ferrite and bainite, and the structure inside the radial direction is tempered martensite. As a result, a high-strength reinforcing bar having excellent bending workability is obtained.

Although the reinforcing bars thus obtained can be manufactured at low cost, they have excellent bending workability 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.

Steel is melted in accordance with the composition shown in Table 5, continuously cast into billets, heated according to the heating rate and heating temperature shown in Table 6, and subjected to hot rolling at a rolling end temperature of Ar ₃ or higher. Thereafter, the wire D having a diameter D of 13.5 mm was manufactured by cooling at a cooling rate of at least 700 ° C. at 2 ° C./s or less as shown in Table 6. Thereafter, a sample having a diameter of 13.5 mm and a length of 5 mm was taken from the tip, middle, and tail ends of the wire, and the surface hardness of the surface perpendicular to the rolling direction (surface with a diameter of 13.5 mm), surface structure, 1 / The hardness and structure of 4D were measured and observed under the conditions described later.

Next, in order to evaluate the properties as a high-strength reinforcing bar, the obtained wire was drawn to 12.6 mm, then heated to 1000 ° C. in the atmosphere and held for 300 seconds to obtain an oil at 60 ° C. After cooling at 350 ° C., it was maintained for 30 seconds after being heated at 350 ° C., and then water-cooled and quenched and tempered. The wire drawability as a steel material for reinforcing bars was judged by whether or not the wire was broken during the drawing process, and if it was not broken, it was judged that the wire had good drawability.
Furthermore, the wire rod after quenching and tempering is processed into a tensile test piece having a parallel part of 1/4 in. Described in ASTM E8, and a tensile test is performed by a test method described later, and the surface layer hardness is measured using the test piece. And investigated the organization. Further, the bending test piece was obtained by cutting a wire having a diameter D of 11.5 mm into a length of 500 mm, and then performing quenching and tempering under the above-described heat treatment conditions.

[Surface hardness measurement]
The investigation of the hardness range below HV250 was conducted by collecting samples with a diameter of 13.5mm and a length of 5mm from the tip, middle and tail ends of the wire (steel for rebar) before drawing. This sample was embedded so that the test surface was a surface perpendicular to the rolling direction (diameter of 13.5 mm), mirror-polished, and then loaded with an Akashi micro hardness tester (HM-115) with a load of 10 gf and a depth of 10 μm. Measurement was carried out at a pitch, and the region where HV250 or less was obtained.

[1 / 4D part hardness measurement]
Using the sample whose surface hardness was measured as described above, measure the hardness of 1 / 4D part (D is the diameter of the wire) on the C scale, 4 points with Mitutoyo Rockwell Hardness Tester (ARK-600) to obtain the average. It was.

[Microstructure observation]
Using the samples subjected to the hardness measurement described above, the structure in the range of HV250 and the structure of the 1 / 4D part were observed after corrosion with 3% nital. Tissue observation was performed at a magnification of 500 times.

[Drawability]
The drawability was evaluated by the presence or absence of wire breakage when a steel for reinforcing steel with a diameter of 13.5 mm was drawn to 12.6 mm. The number of wire breaks indicates the number of wire breaks during the 200 m drawing process, and it was judged that the wire drawability was lowered when the wire breakage occurred even once.

[Bending test]
In order to investigate the bending characteristics as a reinforcing bar, the bending workability was evaluated by bending 180 ° with a bending diameter (4D) four times the diameter D from the wire after drawing and heat treatment. A penetration test was conducted on the wire after bending to investigate the presence of cracks.

[Tensile test]
In order to investigate the tensile properties of high-strength reinforcing bars, tensile test specimens with a diameter of 1/4 in. Of the parallel part described in ASTM E8 were collected from the wire after drawing and heat treatment. The test was carried out at a speed of 5 mm / min. In the present invention, it was evaluated that a high-strength reinforcing bar was obtained when the tensile strength was 1600 MPa or more.

[Measurement of surface hardness after quenching and tempering]
For the investigation of the range below HV300, heat treatment was performed with the above-mentioned tensile test piece, the central part of the parallel part was cut out, embedded in resin, mirror-polished, and a microhardness tester (HM-115) manufactured by Akashi Co., Ltd. , Measurement was performed at a 5 μm depth pitch in order from the surface with a load of 10 gf, and an area of HV300 or less was specified.

[Observation of structure after quenching and tempering]
The structure in the range of HV300 or less and the structure of the core part (1 / 2D part: D is the diameter of the wire) were observed as follows. That is, using the sample on which the hardness measurement described above was performed, after erosion with 3% nital, the structure in the range of HV300 or less and the structure of the 1 / 2D part structure were observed. Tissue observation was performed at a magnification of 500 times, and the tissue at each position was identified.

Table 6 shows the range of HV250 and below, the structure of the steel for reinforcing bars, the structure and hardness of the 1 / 4D part, the drawability at the time of drawing, and the wire rod after the drawing was prepared by quenching and tempering. Each evaluation result of surface layer hardness of a reinforcing bar, structure | tissue, tensile strength, and bending workability is shown. Steels of C-1 to 4, C-6 to 10, C-16 to 19, and C-22 to 24 that satisfy the component composition, A value, and B value of the present invention have good drawability at the time of drawing, It can be seen that the tensile strength and bending workability are good. On the other hand, even if the component composition is within the range of the present invention, the C-5 steel whose B value does not satisfy the range of the present invention cannot satisfy the range of the present invention because the range of HV250 or less does not satisfy the range of the present invention. It can be seen that the sex has decreased. In addition, C-11-15, C-20-21, and C-25-26 steels whose component composition does not meet the scope of the present invention are drawn at the time of drawing, tensile strength at high strength rebar, bending It can be seen that one of the sexes has declined.

Steel is melted in accordance with the composition shown in Table 7 and continuously cast into billets, followed by heating according to the heating rate and heating temperature shown in Table 8 and hot rolling with a rolling end temperature of Ar ₃ or higher. Thereafter, the wire D having a diameter D of 13.5 mm was manufactured by cooling at a cooling rate of at least 700 ° C. as shown in Table 8 at 2 ° C./s or less. Thereafter, a sample having a diameter of 13.5 mm and a length of 5 mm was taken from the tip, middle, and tail ends of the wire, and the surface hardness of the surface perpendicular to the rolling direction (surface with a diameter of 13.5 mm), surface structure, 1 / The hardness and structure of 4D were measured and observed in the same manner as in Example 1 above.

Next, in order to evaluate the characteristics as a high-strength reinforcing bar, the obtained wire was drawn to 11.5 mm, and then Ac ₃ points or higher and 1200 ° C or lower in the atmosphere according to the conditions shown in Table 8 After heating to a temperature range of 60 ° C., cooling in oil at 60 ° C., heating and holding at a temperature range of 100 ° C. to 600 ° C., cooling with water and quenching and tempering were performed. The drawability was judged by whether or not the wire was broken during the drawing process, and if it was not broken, it was judged that the wire had good drawability.
The wire rod after quenching and tempering was processed into a tensile test piece having a parallel portion of 1/4 in. Described in ASTM E8, and the test was performed in the same manner as in Example 1 described above.

Table 8 shows the steel for reinforcing steel, the range of HV250 or less and its structure, the structure and hardness of the 1 / 4D part, the drawability at the time of drawing, and the wire rod after the drawing was prepared by quenching and tempering. Each evaluation result of surface layer hardness of a reinforcing bar, structure | tissue, tensile strength, and bending workability is shown. Steels of D-1 to 4, D-6 to 10, D-16 to 19, and D-22 to 24 that satisfy the component composition of the present invention and the A value and the B value have good bending workability. I understand. On the other hand, even if the component composition is within the range of the present invention, the D-5 steel whose B value does not satisfy the range of the present invention has a low range of HV300 or less, resulting in a decrease in bending workability. I understand that. In addition, steels of D-11 to 15, D-20 to 21, and D-25 to 26 whose component composition does not meet the scope of the present invention are reduced in tensile strength, range of HV300 or less, or bending workability. You can see that

Claims

C: 0.37 mass% or more and 0.50 mass% or less,
Si: 1.75% by mass to 2.30% by mass,
Mn: 0.2% by mass or more and 1.0% by mass or less,
Cr: 0.01 mass% or more and 1.2 mass% or less,
Mo: 0.05 mass% or more and 1.0 mass% or less,
P: 0.025 mass% or less,
S: 0.025 mass% or less and O: 0.0015 mass% or less, A value according to the following (1) is 770 or more and 850 or less, B value according to the following formula (2) is 0.40 or more, and the balance of inevitable impurities and Fe Steel for reinforcing steel with component composition.
A = α + β + γ (1)
Here, α = −334 × [C] 2 + 806 × [C] +291
β = 24 × [Si] 2 + 67 × [Si]
γ = −4 × [Cr] 2 + 23 × [Cr] −5
B = [Si] / [10 × C] (2)
However, the above [] is the content (mass%) of the component in parentheses.
The component composition further comprises:
Al: 0.50 mass% or less,
The steel for reinforcing bars according to claim 1, comprising one or more selected from Cu: 1.0 mass% or less and Ni: 2.0 mass% or less.
The component composition further includes:
W: 2.0 mass% or less,
Nb: 0.1% by mass or less,
The steel for reinforcing bars according to claim 1 or 2, comprising one or more selected from Ti: 0.2 mass% or less and V: 0.5 mass% or less.
The component composition further includes:
B: Steel for reinforcing steel according to any one of claims 1 to 3, characterized by containing 0.005 mass% or less.
The reinforcing steel for reinforcing bars according to any one of claims 1 to 4, wherein the hardness in the thickness region of at least 20 µm from the surface is HV250 or less, and the hardness in the depth region of 1/4 of the diameter of the steel material from the surface is HRC40 or less. steel.
A reinforcing bar having the component composition according to any one of claims 1 to 4 and having a hardness in a thickness region of at least 10 μm from the surface of HV300 or less.