KR101631522B1 - Steel for reinforcing bars, and reinforcing bar - Google Patents

Abstract

Provided is a steel for reinforcing bars, which makes it possible to manufacture reinforcing bars having high strength and excellent bending workability under excellent drawing quality. C: not less than 0.37 mass% and not more than 0.50 mass%, Si: not less than 1.75 mass% and not more than 2.30 mass%, Mn: not less than 0.2 mass% and not more than 1.0 mass%, Cr: not less than 0.01 mass% and not more than 1.2 mass% C, Si and Cr satisfy a predetermined relationship in a composition of not more than 1.0 mass%, not more than 0.025 mass% of P, not more than 0.025 mass% of S and not more than 0.0015 mass% of O.

Description

STEEL FOR REINFORCING BARS AND REINFORCING BAR

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reinforcing bar used as a shear reinforcing bar used in, for example, a reinforced concrete structure, and a steel for reinforcing bars used as a material of the reinforcing bar.

For example, in a reinforced concrete structure such as a building or apartment house, a shear reinforcement is used as a reinforcing material in order to prevent the concrete from collapsing. In a reinforced concrete structure using a shear reinforcement, when the shear deformation of the reinforced concrete structure occurs, the deformation energy of the reinforced concrete structure is absorbed by the shear reinforcement to prevent the collapse of the reinforced concrete structure due to plastic deformation of the shear reinforcement do.

Conventionally, a shear reinforcement is generally used that has a tensile strength of about 1,200 MPa. However, in recent years, there has been a demand for slimmer, lighter, and higher-layered structures of reinforced concrete structures, and the development of ultra-high strength concrete is proceeding rapidly. If the strength of the concrete rises, it is also necessary to strengthen the shear reinforcement to balance it.

However, the shear reinforcement function as a reinforcement in the form of, for example, a concrete column or a beam, which is tied around a main reinforcement to prevent the axis supporting the structure of the building from bending, Sectional shape corresponding to the cross-sectional shape is molded and processed into a coil shape such as a circular shape or a square shape. Since these sectional shapes are imparted by bending, bending workability is required in the molding. Therefore, when the shear reinforcement is excellent in stretchability, bending is facilitated, which is a great advantage in terms of workability. However, as described above, if the strength of the shear reinforcement is increased with the increase in the strength of the concrete, a problem such as breakage at the time of bending is newly worried.

However, in order to increase the strength of the shear reinforcement, it is necessary to increase the addition amount of alloying elements including C, Si, and Mn. However, the shear reinforcement is generally manufactured by drawing a steel for reinforcing bars and then performing heat treatment. Therefore, if the addition amount of the alloying element is increased, the hardness of the steel for reinforcing bars is increased, and the wire rod is broken when the steel for reinforcing bars is pulled out, resulting in deterioration of the production. Therefore, in order to realize high strength, it has been necessary to avoid a decrease in the wire drawability of steel for reinforcing bars.

Thus, in order to overcome the above problem, several proposals have been made.

First, Patent Document 1 discloses a heat treatment method in which C, Si, and Mn are controlled in an appropriate range and the cooling conditions are controlled to secure ferrite decarburization of 0.12 mm or more on the surface. However, if the ferrite decarburization is 0.12 mm or more, it becomes difficult to secure strength in the case of producing a high-strength steel bar. Therefore, it is necessary to increase the strength of the steel other than the ferrite decarburized layer. As a result, the hardness of the material for reinforcing bars increases, resulting in lowering of the freshness. Further, after manufacturing the high-strength steel bar, the toughness of the steel bar is lowered and the bending workability is lowered. In order to make the ferrite decarburized layer at least 0.12 mm thick, it is necessary to maintain the temperature range of 830 to 900 占 폚 for at least 30 seconds and maintain the temperature within the range of 600 to 700 占 폚 for 1 hour or more It is desirable to maintain and maintain the efficiency of the operation, and the operation cost is increased.

Patent Document 2 proposes a method of optimizing the amount of C, Si, Mn, Ni and Al, controlling the ferrite decarburization layer of the surface of the steel material to 0.12 mm or more, and forming a ferrite-pearlite structure, It is disclosed that a steel wire rod having excellent delayed fracture characteristics is obtained by controlling it with a spheroidal cementite structure. However, as described above, if the ferrite decarburization is 0.12 mm or more, it becomes difficult to secure strength in the case of producing a high strength steel bar. Therefore, it is necessary to increase the strength of the steel other than the ferrite decarburized layer. As a result, the hardness of the material for reinforcing bars is increased and the freshness is lowered, or the toughness of the reinforcing bars is lowered after the production of the high strength reinforcing bars, and the bending workability is lowered. Further, after the wire rolling, it is necessary to perform heat treatment on-line or off-line, and the extra operation cost such as reheating is increased.

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

Patent Document 4 discloses a method for producing a high strength steel for reinforcing bars having a large yield and elongation by controlling the composition of components and the rolling method. However, when the tensile strength is 100 kg / mm 2 or less and the strength is increased, the yield elongation is lowered, so that the bending workability may be lowered.

Further, Patent Document 5 discloses a method for producing a steel for high-strength reinforcing bars having a large yield elongation by controlling the composition of components and the rolling method. As described above, since the yield strength is lowered when the strength is increased, the bending workability may also be lowered.

Japanese Patent Publication No. 3156166 Japanese Unexamined Patent Application Publication No. 6-306540 Japanese Laid-Open Patent Publication No. 2012-67363 Japanese Patent Application Laid-Open No. 4-173922 Japanese Laid-Open Patent Publication No. 9-324215

As described above, with the development of ultrahigh strength concrete, it has been a problem to increase the strength of the shear reinforcement to be applied to a reinforced concrete structure made of such concrete. However, in order to increase the strength of the shear reinforcement, it is necessary to add an alloy element. When the alloying element is added, the hardness of the steel for reinforcing bars is increased. Therefore, there has been a problem that the wire rod is broken at the time of drawing at the time of producing the reinforcing bars and the drawability is lowered. In addition, it was also necessary to prevent the deterioration of the bending workability accompanying the high strength.

An object of the present invention is to provide a steel for reinforcing bars capable of producing a reinforcing bar having high strength and excellent bending workability under excellent drawability.

In order to solve the above problems, the inventors of the present invention fabricated a steel for high-strength reinforcing bars in which the addition amounts of C, Si, Mn, Cr and Mo were variously varied, and found that the freshness and the hardness (hardness of the material before drawing) Yes. The tensile strength and the bending workability of the high-strength reinforcing bars produced by drawing the high-strength reinforcing bar steel and subjecting the heat-treated reinforcing bars to the heat treatment were examined.

As a result, in addition to optimizing the addition amounts of C, Si, Mn, Cr, and Mo, it is necessary to regulate the amount of addition of C, Si, and Cr under a predetermined relationship to obtain a high strength and good bending workability Which is indispensable for providing steel for reinforcing steel, which can be a reinforcing steel or a material thereof, and has completed the present invention.

That is, the structure of the present invention is as follows.

1. C: not less than 0.37 mass% and not more than 0.50 mass%

Si: 1.75 mass% or more and 2.30 mass% or less,

Mn: 0.2 mass% or more and 1.0 mass% or less,

Cr: 0.01% by mass or more and 1.2% by mass or less

Mo: 0.05 mass% or more and 1.0 mass% or less,

P: 0.025 mass% or less,

0.025 mass% or less of S 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 River:

group

A = alpha + beta + gamma ... (One)

Here,? = -334 x [C] ² + 806 x [C] + 291

? = 24 x [Si] ² + 67 x [Si]

γ = -4 × [Cr] ² + 23 × [Cr] -5

B = [Si] / (10 x [C]) ... (2)

Note that the above [] indicates the content (mass%) of the component in the parentheses.

2. The composition according to claim 1,

Al: 0.50 mass% or less,

Cu: 1.0 mass% or less and

Ni: 2.0 mass% or less

(1) The steel for reinforcing bars according to (1) above, which contains at least one selected from the group consisting of iron and iron.

3. The composition of claim 1,

W: 2.0 mass% or less,

Nb: 0.1 mass% or less,

Ti: 0.2 mass% or less and

V: not more than 0.5% by mass

(1) or (2) above.

4. The composition of claim 1,

The steel for reinforcing bars according to any one of (1) to (3) above, wherein B contains 0.005 mass% or less.

5. The steel for reinforcing bars according to any one of 1 to 4 above, wherein the hardness in the thickness region of at least 20 mu m from the surface is equal to or less than HV250 and the hardness in the depth region of 1/4 of the diameter of the steel from the surface is equal to or lower than HRC40.

6. A reinforcing steel having the composition described in any one of 1 to 4 above and having a hardness of HV 300 or less in a thickness region of at least 10 mu m from the surface.

According to the present invention, it becomes possible to stably produce a steel for reinforcing bars capable of producing a reinforcing bar having a high degree of drawability and high strength. Reinforcing bars made from steel for steel reinforcing steel or steel reinforcing steel produced through drawing and heat treatment have a tensile strength of 1600 MPa or more and have excellent bending workability so that the slim, And contributes industrially advantageous effects.

(Mode for carrying out the invention)

Next, the composition of the reinforcing bars of the present invention and the manufacturing conditions thereof will be described.

C: 0.37 mass% or more and 0.50 mass% or less

C is an indispensable element in order to secure the required strength. When it is less than 0.37 mass%, it is difficult to secure a predetermined strength. In order to secure a predetermined strength, a large amount of alloying element is required to be added, , It is 0.37 mass% or more. On the other hand, the addition of more than 0.50 mass% is 0.50 mass% or less in that the strength is greatly increased, and further, the strength of the reinforcing bar is increased more than necessary and the bending workability is lowered. 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 enhancing solid solution strengthening or tempering softening resistance as a deoxidizing agent and is also a ferrite decarburization promoting element. Therefore, to secure a surface layer region of HV250 or lower as described later It is also useful. Therefore, in the present invention, it is added in an amount of 1.75 mass% or more. However, the addition of more than 2.30 mass% causes ductility to be lowered, and cracking occurs in the material during casting, so that it is necessary to care for the material and increase the manufacturing cost. Therefore, the upper limit of Si is set to 2.30 mass%. More preferably, it is in the range of 1.75 to 2.25% by mass.

Mn: 0.2% by mass or more and 1.0% by mass or less

Mn is added in an amount of 0.2 mass% or more to improve the quenching property of the steel. However, the addition of more than 1.0% by mass causes a decrease in the strength of the steel. Therefore, the upper limit of Mn is set to 1.0% by mass. More preferably, it is in the range of 0.25 to 1.0 mass%. domain

Cr: 0.01% by mass or more and 1.20% by mass or less

Cr is an element that increases the strength by improving the quenching of the steel. Therefore, 0.01 mass% or more is added. On the other hand, the addition of more than 1.20 mass% leads to a higher strength of the steel, resulting in lowering the freshness at the time of drawing and the bending workability as a high-strength reinforcing bar. In view of the above, the amount of Cr is set to 0.01 mass% or more and 1.20 mass% or less. More preferably, it is in the range of 0.01 to 1.00 mass%.

Mo: 0.05 mass% or more and 1.0 mass% or less

Mo is an element that increases the strength by increasing the quenching of the steel. Therefore, 0.05 mass% or more is added. On the other hand, the addition of more than 1.0% by mass causes the steel to have a high strength, resulting in the drawability at the time of drawing and the lowering of the bending workability as a high strength reinforcing bar. In view of the above, the amount of Mo is set to 0.05 mass% or more and 1.0 mass% or less. More preferably, it is in the range of 0.05 to 0.5 mass%.

P: not more than 0.025 mass%

S: 0.025 mass% or less

P and S are regulated to 0.025 mass% or less, respectively, in that they cause segregation in the grain boundaries and deterioration of the toughness of the base steel of the steel. In particular, since S is present in the steel as MnS, there is a possibility that MnS becomes a starting point at the time of bending and tends to cause cracking. Therefore, it is necessary to suppress as much as possible, preferably to 0.015 mass% or less Do. In addition, it is industrially preferable that P and S are contained in an amount of 0.0002 mass% or more, respectively, in order to make P and S less than 0.0002 mass% each, since high cost is required.

O: 0.0015 mass% or less

O is combined with Si or Al to form hard oxide-based nonmetallic inclusions, which may be a starting point at the time of bending and tends to cause cracking. Therefore, in the present invention, 0.0015 mass % Is allowed. Further, it is industrially preferable that O is contained in an amount of 0.0005 mass% or more in order to make O less than 0.0005 mass% requires high cost.

A value (expression (1)): 770 to 850

The A value is an index for obtaining good strength, freshness and bending workability. When the A value is less than 770, the bending workability is good, but it becomes difficult to secure the strength of the reinforcing bar. 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, resulting in breakage at the time of drawing, and the drawability is rather lowered. Further, the bending workability as a reinforcing bar deteriorates. Therefore, in the present invention, the A value is 770 or more and 850 or less. More preferably, it is in the range of 770 to 849 or less.

B value (expression (2)): 0.40 or more

The B value is an index for obtaining good freshness. By setting the B value to 0.40 or more, it is possible to ensure good ductility in steel for reinforcing bars and good bending workability in reinforcing bars. The reason why the B value is 0.40 or more is that the freshness and the bending workability are improved because the decarburization layer is formed in the surface layer region of the steel for reinforcing bars or in the surface layer region of the reinforcing bars, And the workability is improved. Specifically, as will be described later in detail, the B value is set to be not less than 0.40 so that the hardness of the steel for reinforcing bars at least 20 μm or more from the surface is not more than HV250, can do. By setting the B value to 0.40 or more, the hardness of the reinforcing bars in the thickness region of at least 10 mu m or more from the surface can be made HV300 or less, and the bending workability can be improved.

The inventors of the present invention conducted various experiments to evaluate the influence of the composition of the steel for reinforcing steel, particularly the A value and the B value, on the freshness and the bending workability. The representative two experimental results will be described in detail below.

[Experiment 1]

With respect to the A value and the B value, the inventors fabricated a steel for reinforcing bars in which the composition of the components, the A value and the B value were changed, and investigated the hardness thereof. Further, the reinforcing bars were produced by performing a drawing process, a quenching-tempering process on the steel for reinforcing bars, and the freshness at the drawing process was examined. In addition, with respect to the obtained reinforcing bars, Strength, surface hardness and bending workability, and further investigation of the structure. The tensile strength, surface hardness and bending workability were measured by a test method described later. Table 1 shows the composition of the components, Table 2 shows the range of HV250 or less, the hardness, the tensile strength as the reinforcing bars, and the evaluation results of the bending workability, respectively.

The manufacturing conditions are as follows.

First, a steel ingot made by vacuum melting furnace was heated at room temperature to the heating temperature shown in Table 2 at a heating rate shown in Table 2, and hot rolling was performed. The production conditions after hot rolling were the same. That is, hot rolling was performed at 850 占 폚 and then cooled to 1 占 폚 / s to obtain a wire having a diameter of 13.5 mm, which was used as a steel for reinforcing bars. The hardness of the cross section perpendicular to the longitudinal direction of the obtained wire rod was measured. The hardness was measured by a test method described later.

Next, reinforcing bars were produced using the steel for reinforcing bars as a material. The manufacturing conditions of the reinforcing bars were the same. That is, the manufacturing conditions are as follows.

13.5 mm of wire rod was drawn at 12.6 mm and then quenched and tempered by heating at 1000 캜 and then cooling at 60 캜 with oil and heating at 350 캜 and water cooling. The freshness was judged by whether or not the element wire was broken at the time of the drawing process, and it was judged that the drawing had good drawing characteristics if the wire was not broken.

The quenching-tempered wire rod was processed into a parallel portion 1/4 in. Tensile test piece described in ASTM E8. Further, the bending workability was cut into a length of 500 mm, and then the test was carried out by the following test method.

In Table 2, the hardness at HV250 or lower of the steel for reinforcing bars, the hardness at 1/4 deep portion (1/4 D portion) from the surface of wire rod diameter D, the drawability at the time of drawing work, the tensile strength as bending strength, and the bending workability were 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 understood that the above-described characteristics are good. In view of the above, it has been found that when the value of A is adjusted to 770 or more and 850 or less and the value of B is adjusted to 0.40 or more, high strength and freshness are improved.

[Experiment 2]

With respect to the A value and the B value, the inventors fabricated reinforcing bars from steel for reinforcing bars in which the composition of components, the A value and the B value were changed, and examined the tensile strength, surface hardness and bending workability, and further the structure. The tensile strength, surface hardness and bending workability were measured according to the following test methods. In addition, the organization examined the structure of the structure and the core (1 / 2D part: D of the diameter of the reinforcing bar) in the range of HV300 or less. Table 3 shows the composition of the components, Table 4 shows the tensile strength, HV300 range and evaluation results of the bending workability.

Here, in the evaluation, the production conditions of the reinforcing bar were the same. That is, the manufacturing conditions are as follows.

First, a steel ingot obtained by dissolving in a vacuum melting furnace was heated to 1100 占 폚 and subjected to hot extend forging to obtain a round bar having a diameter of 11.5 mm. The obtained round bar was processed into a tensile test piece of 1/4 in. In parallel section described in ASTM E8, and subjected to quenching-tempering treatment. The heating temperature, the holding and holding time of the quenching process, the tempering temperature and the holding time of the tempering process are shown in Table 4. The bending workability was evaluated by a quenching-tempering treatment under the above-described conditions after cutting the round bar after the above-described hot rolling to a length of 500 mm, and the test was performed by a test method described later.

Table 4 shows evaluation results of tensile strength, HV 300 or less, and bending workability. As shown in this table, when the A value and the B value are both controlled within the range of the present invention, it can be understood that good tensile strength, HV 300 or less, and bending workability are obtained. In view of the above, it has been found that when the A value is adjusted to be in the range of 770 to 850 and the B value is set to the range of 0.40 or more, high strength and bending workability are improved when made into reinforcing bars.

The results of the above tests can be summarized as follows. By regulating the composition of the components, particularly the A value and the B value in the above-mentioned range, it is possible to obtain a steel having a hardness of HV250 or less, It can be seen that the hardness of 1/4 deep region (1 / 4D portion) is not more than HRC40 and that the hardness of the thickness region of at least 10 mu m from the surface after quenching is HV300 or less. The hardness of the surface layer is preferably regulated as described above. The reason for this is as follows.

When the hardness in the thickness region of at least 20 mu m from the surface is less than HV250

It is preferable that the hardness in the thickness region of at least 20 mu m from the surface is HV250 or less. When the hardness of this region exceeds HV250, the hardness of the steel for high strength steel becomes high and the drawability is lowered. It is easy to do.

When the thickness is less than 20 占 퐉, the high-ductility region from the surface becomes smaller, and breakage of the wire becomes more likely to occur at the time of drawing. On the other hand, if the region is expanded from the surface to a depth of more than 100 mu m, the strength as a high-strength reinforcing bar is lowered. Therefore, in order to increase the tensile strength to 1600 MPa or more as a whole, further strengthening of the deep portion is required. The drawability at the time of drawing and the bending workability as the high-strength reinforcing bar tend to decrease. Therefore, a region from the surface to a depth of 100 mu m is sufficient for the region to be HV250 or less.

Here, to a hardness of at least 20㎛ thickness region of from the surface to less than HV250, it is suitable to heat the steel in the atmosphere at a heating rate of 30 ℃ / min or less to the Ac ₃ point or higher. This process can be performed in a material heating process for hot rolling, which will be described later.

The hardness at the depth of 1/4 of the diameter of the steel from the surface is less than HRC40

The reason why the hardness at the depth of 1/4 of the diameter of the steel material from the surface is preferably HRC 40 or less is that when this area exceeds HRC 40, the area to be machined becomes hard at the time of drawing processing and the life of the die becomes short, This is because disconnection easily occurs. Here, the depth region of 1/4 is a portion (1/4 D portion) where the distance from the steel material surface is 1/4 of the diameter D of the steel material.

Further, in order to make the hardness of this region to be HRC 40 or less, martensite structure should not be generated in the steel structure. Specifically, the steel wire rod for reinforcing bars is produced by hot rolling, It is preferable to terminate at least _three points of Ar, and then to set the cooling rate to at least 700 DEG C at 2 DEG C / s or less.

In the reinforcing bars after the quenching and tempering treatment, the hardness in the thickness region of at least 10 mu m from the surface is not more than HV300

The reason why it is preferable that the hardness in the thickness region of at least 10 mu m from the surface is HV300 or less is that if the hardness of this region exceeds HV300, the hardness becomes high and the ductility is lowered so that cracks are likely to occur in the surface layer of the reinforcing bar at the time of bending . Therefore, in the present invention, it is preferable that the hardness of the surface layer region is HV 300 or less. The reason why the thickness is in the range from the surface to the depth of at least 10 mu m from the surface layer region is that when the thickness is less than 10 mu m, the high-softness region becomes small and cracks tend to occur in the surface layer of the reinforcing bar at the time of bending. In view of the above, it is preferable that the hardness in the thickness region of at least 10 mu m from the surface is HV300 or less.

Here, to the 10㎛ hardness of at least the thickness from the surface area to less than HV300, it is suitable to heat the Ac ₃ point or higher in the atmosphere in the river. This process can be performed in a quenching process to be described later.

It is sufficient that the depth from the surface of the region to be HV300 or less is from 10 mu m to 150 mu m. That is, when the depth region from the surface of HV300 or less is more than 150 mu m, the strength of the reinforcing bar is lowered. Therefore, the depth from the surface of the region to be HV300 or less is preferably 150 mu m or less. Further, it is preferable that the region to be HV300 or less is a ferrite single-phase structure. This is because stress concentration occurs in the surface layer at the time of bending, but if the region of HV300 or less is a single-phase ferrite structure, stress concentration is relaxed due to the high ductility of ferrite and bending workability is better. Further, since bainite structure has some degree of ductility, it may be incorporated into the structure within a range that can satisfy HV300 or less. On the other hand, the deep section of the reinforcing bars is preferably a martensite structure in order to secure the strength as a reinforcing bar.

The steel for reinforcing bars of the present invention may contain the following components in addition to the above components in order to increase the strength or to improve the bending workability in the reinforcing bars.

0.5% by mass or less of Al, 1.0% by mass or less of Cu and 2.0% by mass or less of Ni

Cu and Ni are elements that increase quenching property or strength after tempering, and can be added selectively. In order to obtain such an effect, Cu and Ni are preferably added in an amount of 0.005 mass% or more. However, when 1.0 mass% of Cu and more than 2.0 mass% of Ni are added, the cost of alloy is rather increased, so it is preferable to add 1.0 mass% of Cu and 2.0 mass% of Ni as the upper limit.

Further, Al is useful as a deoxidizing agent and is further added at a content of 0.01% by mass or more, since it is an effective element for maintaining the strength by inhibiting the growth of austenite grains at the time of quenching. However, even if it is added in an amount exceeding 0.50% by mass, the effect becomes saturated, which causes a disadvantage that the cost is increased, and the oxide in the steel is increased, which is a starting point in bending processing and the bending workability is lowered. Therefore, it is preferable to add Al at an upper limit of 0.50 mass%.

W: 2.0 mass% or less, Nb: 0.1 mass% or less, Ti: 0.2 mass% or less, and V: 0.5 mass%

W, Nb, Ti, and V are elements that increase the quenching property or the strength of the steel after tempering, and can be selectively added according to the required strength. In order to obtain such an effect, W, Nb and Ti are preferably added in an amount of 0.001% by mass or more, and V is added in an amount of 0.002% by mass or more. However, when V is added in an amount exceeding 0.5 mass%, Nb in an amount of 0.1 mass%, and Ti in an amount of more than 0.2 mass%, a large amount of hard carbides, nitrides and carbonitrides are produced in the steel, And the bending workability is lowered. Nb, Ti and V are preferably added in the upper limits of the above values. When W is added in an amount exceeding 2.0 mass%, the drawability is lowered due to the higher strength, the bending workability is lowered, and the alloy cost is increased. Therefore, W is preferably added in an amount of 2.0% by mass as an upper limit.

B: 0.005 mass% or less

B is an element that increases the strength of the steel after tempering by the increase in quenching property and can be contained as required. In order to obtain the above effect, it is preferable to add it at 0.0002 mass% or more. However, when it is added in an amount exceeding 0.005 mass%, the bending workability is deteriorated. Therefore, B is preferably added in the range of 0.0002 to 0.005 mass%.

The steel ingot having the above-mentioned composition can be used either as a solvent by a converter or by a vacuum solvent. The materials such as steel ingots, slabs, blooms or billets are heated and hot-rolled, pickled and scaled off, fresh, adjusted to a predetermined thickness, subjected to heat treatment, It is provided in the river.

In the reinforcing bars produced by using the steel for reinforcing bars of the present invention, it is preferable that the reinforcing bars have the above composition and the tensile strength is 1,600 MPa or more. That is, when the tensile strength is less than 1600 MPa, it is impossible to cope with the increase in the strength of reinforced concrete.

Here, to a tensile strength of more than 1600㎫, after the steel has a composition wherein component a hot rolling, a steel reinforcement for removal of the scale, and then the fresh and adjusted to a predetermined thickness of, heated to Ac ₃ point or higher in the air (Cooling rate is 60 占 폚 / s or more), and then tempering is preferably performed in a temperature range of 100 to 600 占 폚. That is, the tempering temperature is preferably 100 占 폚 or higher from the viewpoint of ensuring the bending workability, and is preferably 600 占 폚 or lower from the viewpoint of securing a tensile strength of 1600 MPa or higher.

The reinforcing bars produced through the above-described manufacturing process become the tempering martensite at the deep portion. Further, since the surface layer has a decarburized layer even after freshness and decarburization is further promoted by quenching-tempering, the surface layer portion has a lower hardness than the deep portion. That is, the metal structure in the reinforcing bars of the present invention is a mixed structure of the ferrite single phase structure or the ferrite and bainite in the above-mentioned surface layer inversion, and the structure in the radial direction becomes the tempering martensite. As a result, it becomes a high-strength reinforcing bar having excellent bending workability.

The reinforcing bar obtained in this way can be applied to a shear reinforcement such as a high-rise apartment building having high strength, excellent bending workability and high strength of 1600 MPa or more, though it can be produced at low cost.

Example 1

The steel was melted in accordance with the composition shown in Table 5 and continuously cast to form billets. The billets were then heated in accordance with the heating rate and the heating temperature shown in Table 6, and the hot rolling was carried out with the rolling finish temperature being Ar ₃ point or higher, Thereafter, the cooling rate to at least 700 캜 was set to 2 캜 / s or less as shown in Table 6, and a wire rod having a diameter D of 13.5 mm was produced. Thereafter, a sample having a diameter of 13.5 mm and a length of 5 mm was taken from the tip, middle, and end of the wire rod and the surface of the surface perpendicular to the rolling direction (surface with a diameter of 13.5 mm) The hardness of the surface layer, the texture of the surface layer, the hardness of 1 / 4D, and the texture were measured and observed under the conditions described below.

Next, in order to evaluate the properties as the high-strength steel bar, the obtained wire rod was subjected to drawing at 12.6 mm, after which it was heated to 1000 캜 in the air and then held for 300 seconds, cooled in oil at 60 캜, ° C and kept for 30 seconds, and quenched and tempered by water cooling. The freshness of the steel material for reinforcing bars was judged by whether or not the wire was broken at the time of the drawing process, and it was judged that the wire had good drawability if the wire was not broken.

Further, the wire after quenching tempering was processed into a tensile test piece of 1/4 in. In parallel section described in ASTM E8, subjected to a tensile test by the following test method, and the surface hardness and texture I investigated. The bend test piece was cut into a wire having a diameter D of 11.5 mm to a length of 500 mm, and then subjected to quenching tempering under the above-described heat treatment conditions.

[Surface hardness measurement]

Irradiation in the hardness range of HV250 or less was carried out by sampling a sample having a diameter of 13.5 mm and a length of 5 mm from the tip, middle, and tip of a wire rod (steel for reinforcing bars) before drawing. This sample was embedded so that the tested surface was a surface perpendicular to the rolling direction (13.5 mm in diameter), mirror polished, and then immersed in a micro hardness tester (HM) manufactured by Akashi Co., -115) at a depth of 10 gf and a depth pitch of 10 mu m to obtain a region having HV250 or less.

[1 / 4D hardness measurement]

The hardness of the 1/4 D part (D: the diameter of the wire rod) was measured by a Rockwell hardness tester (ARK-600, manufactured by Mitsutoyo Co., Ltd.) Respectively.

[Observation of microstructure]

Using the sample subjected to the hardness measurement described above, a texture of 1 / 4D portion and a structure in a range of HV250 or lower after 3% or nital corrosion was observed. Tissue observation was performed at a magnification of 500 times.

[Freshness]

The freshness was evaluated based on the presence or absence of disconnection when the steel for reinforcing bars having a diameter of 13.5 mm was drawn to 12.6 mm. The number of times of disconnection indicates the number of times of disconnection at the time of 200 m drawing, and it was judged that the drawability was deteriorated when the disconnection occurred even once.

[Bending Test]

In order to investigate the bending property as a reinforcing bar, bending workability was evaluated by performing 180 ° bending from the wire material after drawing and heat treatment to a bending diameter (4D) of four times the diameter D. After the bending, the wire rod was subjected to a penetrant test to investigate the presence or absence of cracks.

[Tensile test]

In order to investigate the tensile properties as a high strength steel rod, a tensile test piece having a diameter of 1/4 in. Of the parallel portion described in ASTM E8 was taken from the drawn and heat treated wire, and the tensile test specimen having a rated inter-level distance of 25.4 mm and a tensile speed of 5 mm / Test was conducted. In the present invention, when the tensile strength is 1600 MPa or more, it is evaluated that high-strength reinforcing bars are obtained.

[Measurement of surface hardness after quenching tempering]

The irradiation in the range of HV 300 or less was carried out by heat treatment with the aforementioned tensile test piece and the central part of the parallel part was cut out and embedded in the resin. After the mirror polishing, using a micro hardness tester (HM-115) Measurement was made at a depth of 5 mu m in order from the surface with a load of 10 gf, and a region of HV300 or less was specified.

[Observation of tissue after quenching tempering]

The structure of the structure and the deep portion (1 / 2D portion: D: the diameter of the wire) in the range of HV300 or less was observed as follows. That is, the above-mentioned hardness-measured sample was used to observe the texture of the tissue in the range of HV300 or less and the tissue of the 1 / 2D portion after 3% or more of the corroded portion. Tissue observation was performed at a magnification of 500 times, and tissues were identified at respective positions.

The results are shown in Table 6. The results are shown in Table 6. The results are shown in Table 6. The results are shown in Table 6. The results are shown in Table 6, Tissue, tensile strength and bending workability. The steels C-1 to C-4, C-6 to C-10, C-16 to C-19 and C-22 to C-24 satisfying the component composition, A value and B value of the present invention, The tensile strength and the bending workability of the steel sheet are good. On the other hand, C-5 steels whose B value does not satisfy the range of the present invention, even if the composition of the components is within the scope of the present invention, have a range of HV250 or less that does not satisfy the range of the present invention, . The steels C-11 to C-15, C-20 to C-20 to C-21 to C-25 to C-26 that do not satisfy the composition ranges of the present invention are excellent in drawability at the time of drawing, tensile strength at high- Which is lower than the other.

Example 2

The steel was melted in accordance with the composition shown in Table 7, cast into billets by continuous casting, heated according to the heating rate and the heating temperature shown in Table 8, and subjected to hot rolling at a rolling end temperature of Ar ₃ or higher, Thereafter, the cooling rate to at least 700 캜 was set to 2 캜 / s or less as shown in Table 8, and a wire rod having a diameter D of 13.5 mm was produced. Thereafter, a sample having a diameter of 13.5 mm and a length of 5 mm was taken from the tip, middle, and tip of the wire rod, and the hardness of the surface layer (the surface having a diameter of 13.5 mm) perpendicular to the rolling direction, The hardness and texture of 4D were measured and observed in the same manner as in Example 1 described above.

Next, in order to evaluate the properties as a high-strength steel bar, the obtained wire rod was subjected to drawing at 11.5 mm, and thereafter heated to a temperature in the range of Ac ₃ point to 1200 ° C in the atmosphere according to the conditions shown in Table 8 Thereafter, the resultant was cooled in an oil at 60 deg. C and then heated and stored in a temperature range of 100 deg. C or higher and 600 deg. C or lower, and then quenched and tempered by water cooling. The freshness was determined by whether or not the wire was broken at the time of the drawing process, and it was judged that the wire had good drawability if the wire was not broken.

The wire after quenching tempering was processed into a tensile test piece of 1/4 in. In parallel section described in ASTM E8, and tested in the same manner as in Example 1 described above.

The results are shown in Table 8. The results are shown in Table 8. The results are shown in Table 8. The results are shown in Table 8. The results are shown in Table 8. The results are shown in Table 8, Tissue, tensile strength and bending workability. The steels of D-1 to D-4, D-6 to D-10, D-16 to D-19 and D-22 to 24 satisfying the composition of the present invention and the A value and the B value show good bending workability have. On the other hand, the steel of D-5 in which the B value does not satisfy the range of the present invention, even if the composition of the component is within the scope of the present invention, is small in the range of HV300 or less. The steels of D-11 to D-15, D-20 to D-21 and D-25 to 26 in which the composition of the components do not satisfy the range of the present invention have a tensile strength, a range of HV300 or less, .

Claims (10)

C: 0.37 mass% or more and 0.50 mass% or less,
Si: 1.75 mass% or more and 2.30 mass% or less,
Mn: 0.2 mass% or more and 1.0 mass% or less,
Cr: 0.01% by mass or more and 1.2% by mass or less
Mo: 0.05 mass% or more and 1.0 mass% or less,
P: 0.025 mass% or less,
0.025 mass% or less of S and
O: 0.0015 mass% or less, and an A value according to the following (1) is 770 or more and 850 or less, a B value according to the following formula (2) is 0.40 or more, the balance being inevitable impurities and Fe,
A steel for reinforcing bars having a hardness of HV250 or less in a thickness region of at least 20 mu m from the surface and a hardness in a depth region of 1/4 of the diameter of the steel from the surface is not more than HRC40:
A = alpha + beta + gamma ... (One)
Here,? = -334 x [C] ² + 806 x [C] + 291
? = 24 x [Si] ² + 67 x [Si]
γ = -4 × [Cr] ² + 23 × [Cr] -5
B = [Si] / (10 x [C]) ... (2)
Note that the above [] indicates the content (mass%) of the component in the parentheses. The method according to claim 1,
The above-mentioned composition of the component further includes,
Al: 0.50 mass% or less,
Cu: 1.0 mass% or less and
Ni: 2.0 mass% or less
A steel for reinforcing steel containing at least one member selected from the group consisting of iron and steel. The method according to claim 1,
The composition of the components may further include,
W: 2.0 mass% or less,
Nb: 0.1 mass% or less,
Ti: 0.2 mass% or less and
V: not more than 0.5% by mass
A steel for reinforcing steel containing at least one member selected from the group consisting of iron and steel. 3. The method of claim 2,
The composition of the components may further include,
W: 2.0 mass% or less,
Nb: 0.1 mass% or less,
Ti: 0.2 mass% or less and
V: not more than 0.5% by mass
A steel for reinforcing steel containing at least one member selected from the group consisting of iron and steel. The method according to claim 1,
The composition of the components may further include,
B: 0.005 mass% or less. 3. The method of claim 2,
The composition of the components may further include,
B: 0.005 mass% or less. The method of claim 3,
The composition of the components may further include,
B: 0.005 mass% or less. 5. The method of claim 4,
The composition of the components may further include,
B: 0.005 mass% or less. delete 9. A reinforcing bar having a composition according to any one of claims 1 to 8 and having a hardness of HV 300 or less in a thickness region of at least 10 mu m from the surface.