JP6135553B2 - Reinforcing bar and method for manufacturing the same - Google Patents

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 and a method for manufacturing the same.

  For example, in a reinforced concrete structure, a shear reinforcing bar is used as a reinforcing material in order to prevent its 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.

  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.

  In order 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 reinforcement is 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, and the strands are broken when the steel for reinforcing bars is drawn, so that the productivity is lowered. For this reason, there is a concern that the wire drawability deteriorates when the strength is increased.

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, increasing the strength of shear reinforcement has been an issue. However, if an alloying element is added considering only the strengthening of the shear reinforcement, the wire drawability deteriorates, making it difficult to manufacture the reinforcement, and the hardness of the steel for high-strength reinforcement increases, so that delayed fracture susceptibility Increases and the delayed fracture characteristics deteriorate. In addition, depending on the techniques of Patent Document 1 and Patent Document 2, although delayed fracture resistance is considered, it is difficult to obtain a steel having a tensile strength of 1600 MPa or more.

  The present invention has been made to solve such problems, has excellent wire drawing workability at the time of rebar manufacturing, has high strength characteristics with a tensile strength of 1600 MPa or more, and excellent delayed fracture resistance. An object of the present invention is to provide a high-strength reinforcing bar with its manufacturing method.

  In order to solve the above problems, the inventors changed the amount of addition of C, Si, Mn, Cr and Mo, and further changed the amount of C, the steel structure and the fraction of the region from the surface to a depth of at least 10 μm. We made high strength reinforcing bars and changed them to investigate the delayed fracture characteristics.

  As a result, the addition amount of C, Si, Mn, Cr and Mo is optimized, the A value calculated from the C, Si and Cr contents in the steel, and the C and Si contents in the steel. By controlling the B value and the C content of the surface layer within an appropriate range, it has been found that good drawability and delayed fracture characteristics can be imparted as a high-strength reinforcing bar, and the present invention has been completed.

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 0.05% by mass or less,
P: 0.025 mass% or less,
S: 0.025 mass% or less,
Mo: 0.05 mass% or more and 1.0 mass% or less and O: 0.0015 mass% or less, A value calculated by the following formula (1) is 770 or more and 850 or less, B value calculated by the following formula (2) is 0.40 or more And the balance is inevitable impurities and Fe component composition, and further, the C content in the region from the surface to a depth of at least 10 μm is 0.01% by mass or less, and the steel is composed of ferrite and core of the surface layer. Reinforcing bars that have a two-phase structure with a martensite and have a ferrite fraction of less than 5% in the entire structure and a tensile strength of 1600 MPa or more.
A = α + β + γ (1)
Where α = −334 × [C] ² + 806 × [C] +291
β = 24 × [Si] ² + 67 × [Si]
γ = −4 × [Cr] ² + 23 × [Cr] −5
B = [Si] / (10 x [C]) (2)
However, [] is the content of the component in parentheses (% by mass)

  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.

2. The component composition further comprises:
Al: 0.01 mass% or more and 0.50 mass% or less,
Cu: 0.005 mass% to 1.0 mass% and
Ni: The reinforcing bar as described in 1 above, containing one or more selected from 0.005% by mass to 2.0% by mass.

3. The component composition further includes:
W: 0.001% to 2.0% by mass,
Nb: 0.001 mass% or more and 0.1 mass% or less,
Ti: 0.001 mass% or more and 0.2 mass% or less, and V: 0.002 mass% or more and 0.5 mass% or less, 1 type selected from 2 or more types, or 2 or more types characterized by the above-mentioned.

4). The component composition further includes:
B: The reinforcing bar according to any one of 1 to 3 above, which contains 0.0002 mass% or more and 0.005 mass% or less.

5. The steel slab having the composition described in any one of 1 to 4 above is heated in a temperature range of 850 ° C. to 1050 ° C. for 100 minutes to 300 minutes in a decarburizing atmosphere, and then hot-rolled, A method for manufacturing a reinforcing bar, comprising performing washing, wire drawing, quenching and tempering.

  According to the present invention, it is possible to provide a reinforcing bar having a delayed fracture characteristic that is excellent in strength compared to a conventional reinforcing bar. The rebar of the present invention has a tensile strength of 1600 MPa or more, and has excellent delayed fracture characteristics. This contributes to an industrially beneficial effect.

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.

First, the reinforcing bars of the present invention will be described in order from the component composition.
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, the addition of more than 0.50% by mass increases the strength of the reinforcing bars and causes a reduction in wire drawing. From the above, the C content is set to 0.37 mass% or more and 0.50 mass% or less. Preferably, it is 0.37 mass% or more and 0.49 mass% or less.

Si: 1.75% by mass to 2.30% by mass
Si, as a deoxidizer, is an element effective in increasing the strength of steel by further improving solid solution strengthening and temper softening resistance. Furthermore, since it is a ferrite decarburization accelerating element, it is added at 1.75% by mass or more in the present invention. On the other hand, the addition exceeding 2.30% by mass leads to an increase in strength of the reinforcing bar core part, so that the drawability is lowered. Further, Si promotes decarburization in a heat treatment in which the C content on the reinforcing steel surface side described later is 0.01% by mass or less, and if the Si content is too high, the structure fraction of the ferrite phase becomes 5 as described later. Control to make it less than% becomes difficult. In particular, when the Si content exceeds 2.30% by mass, the control range of the heat treatment conditions for appropriately allowing the ferrite content to be present in the surface layer becomes narrow, and it becomes difficult to ensure the tensile strength. From the above, the Si content is 1.75 mass% or more and 2.30 mass% or less. Preferably, it is 1.80 mass% or more and 2.30 mass% or less.

Mn: 0.2% by mass or more and 1.0% by mass or less
Mn is added in an amount of 0.2% by mass or more in order to improve the hardenability of the steel. However, the addition of Mn exceeding 1.0% by mass increases the strength of the steel and leads to a decrease in drawability. Therefore, the upper limit of Mn is 1.0% by mass. From the above, the amount of Mn is set to 0.2% by mass or more and 1.0% by mass or less. Preferably, it is 0.32 mass% or more and 1.0 mass% or less.

P: 0.025% by mass or less S: 0.025% by mass or less P and S segregate at the prior austenite grain boundaries, resulting in a decrease in wire drawing. From the above, it is preferable to reduce these elements as much as possible. Therefore, both P and S are 0.025 mass% or less.

Cr: 0.01% to 0.05% by mass
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, when Cr is added in excess of 0.05% by mass, decarburization is suppressed in the heat treatment in which the C content of the reinforcing steel surface layer described later is 0.01% by mass or less, that is, the decrease in the C content of the surface layer is suppressed. It becomes difficult to reduce the C content to 0.01% by mass or less, which leads to an increase in delayed fracture sensitivity. From the above, the Cr content is 0.01 mass% or more and 0.05 mass% or less.

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, thereby causing a reduction in wire drawing. From the above, the amount of Mo is set to 0.05% by mass or more and 1.0% by mass or less. Preferably, it is 0.05 mass% or more and 0.95 mass% or less.

O: 0.0015% by mass or less O is bonded to Si or Al to form a hard oxide-based non-metallic inclusion, and it is likely that disconnection is likely to occur at the time of drawing starting from that, so it is as low as possible. Although it is better, in the present invention, 0.0015% by mass is allowed.

The content of the elements described above is an essential requirement in the component composition of the reinforcing bar of the present invention, but the present invention can further contain the following elements in addition to the above components.
Al: 0.01 mass% or more and 0.50 mass% or less, Cu: 0.005 mass% or more and 1.0 mass% or less, and Ni: 0.005 mass% or more and 2.0 mass% or less, ie, Cu or Ni is hardenability. It is an element that increases the strength after tempering and can be selected and 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 that causes an increase in cost, and the oxide in the steel increases and the drawability is lowered. Therefore, Al is preferably added with an upper limit of 0.50% by mass.

One or two of W: 0.001% to 2.0% by mass, Nb: 0.001% to 0.1% by mass, Ti: 0.001% to 0.2% by mass, and V: 0.002% to 0.5% by mass More than seeds W, Nb, Ti and V are all elements that increase the hardenability and strength of the 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 carbides / nitrides / carbonitrides are formed in the steel, and the drawability is lowered. Therefore, it is preferable to add Nb, Ti, and V 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 is lowered because the strength is increased. Therefore, it is preferable to add W with an upper limit of 2.0% by mass.

B: 0.0002 mass% or more and 0.005 mass% or less B is an element that increases the strength of the steel after tempering by increasing the hardenability, and can be contained if necessary. In order to acquire the said effect, adding at 0.0002 mass% or more is preferable. However, if added over 0.005% by mass, BN tends to precipitate at the prior austenite grain boundaries, and on the contrary, the drawability is lowered. Therefore, it is preferable to add B in the range of 0.0002 to 0.005 mass%.

  The balance other than the elements described above is Fe and inevitable impurities. Moreover, the component composition of the reinforcing bar of the present invention requires that the content of each element be in the above-described range, and the following A value and B value be adjusted to the following ranges.

A value 770 or more and 850 or less The A value calculated by the following equation (1) is an index for obtaining good strength. If the A value is less than 770, 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 strength of the reinforcing bars increases and the drawability decreases. Therefore, in the present invention, the A value is set to 770 or more and 850 or less.
A = α + β + γ (1)
Where α = −334 × [C] ² + 806 × [C] +291
β = 24 × [Si] ² + 67 × [Si]
γ = −4 × [Cr] ² + 23 × [Cr] −5
However, [] is the content of the component in parentheses (% by mass)

B value: 0.40 or more The B value calculated by the following equation (2) is an index for controlling the C amount of the surface layer. When the B value is less than 0.40, as will be described later, the heat treatment for securing at least 10 μm or more in the region where the C content is 0.01% by mass or less takes a long time, leading to a decrease in productivity. Moreover, wire drawing property falls. From the above, the B value is 0.40 or more.
B = [Si] / (10 x [C]) (2)
However, [] is the content of the component in parentheses (% by mass)

  Next, the C content of the surface layer of the reinforcing bar of the present invention and the steel structure will be described. The inventors simulated a reinforcing bar to produce a wire material in which the component composition and the A value and the B value described above were changed, and investigated its tensile strength, wire drawability, and delayed fracture characteristics.

The evaluation of the delayed fracture characteristics of the reinforcing bars is most preferably performed by actually manufacturing the reinforcing bars and using them for the reinforced concrete structures. However, since this method takes time, the evaluation was performed as follows in the present invention.
That is, steel having the component composition shown in Table 1 was melt cast and made into billets, and then heated at the heating temperature shown in Table 2 in an air atmosphere, and a wire rod having a diameter of 8 mm was manufactured by hot rolling. Table 2 shows the in-furnace time in the temperature range of 850 ° C. or higher during heating. Thereafter, the steel sheet was drawn to a diameter of 6.0 mm, heated to 1000 ° C. so that the in-furnace time in the temperature range of 850 ° C. or higher was 30 seconds, cooled with water, heated at 350 ° C., and then cooled with water. The obtained wire was subjected to a tensile test and a delayed fracture test by the method described later, and the amount of diffusible hydrogen was measured. Further, a sample having a length of 5 mm was taken from the obtained wire, and the C content measurement and the structure observation of the surface layer on the surface perpendicular to the rolling direction (surface having a diameter of 6.0 mm) were performed under the conditions described later. It was.

  Table 2 shows the depth from the surface, tensile strength, amount of diffusible hydrogen, and delayed fracture test results in the region where the C content of the high-strength reinforcing bar is 0.01% by mass or less. First, in all the steels, the steel structure of the surface layer was ferrite, and the structure of the core part was martensite. As shown in Table 1, Cr: 0.05% by mass or less, 770 ≦ A value ≦ 850 and B value ≧ 0.40 are satisfied, and as shown in Table 2, the C content is 0.01% by mass or less. When the depth from the surface satisfies 10 μm or more and the ferrite fraction <5%, it was found that the above-described tensile strength and delayed fracture resistance were good.

  In addition, steel with good delayed fracture resistance, that is, steel with a rupture time of more than 100 h has been found to have a diffusible hydrogen content of 3 mass ppm or less as shown in Table 2. It is considered that the delayed fracture resistance is improved while the strength is high because the amount of diffusible hydrogen is reduced.

  In addition, in order to evaluate the wire drawing property when manufacturing the reinforcing bars, the presence or absence of disconnection (number of times of disconnection) at the time of drawing is also shown in Table 2, but when the A value described above is more than 850, It can also be seen that disconnection occurs during the drawing process, that is, the drawability deteriorates. The details of the method for evaluating the presence or absence of disconnection are the same as the method for evaluating drawability described later.

The reason why the C content of the surface layer and the steel structure of the reinforcing steel are specified will be described below.
[C content in region from surface to depth of at least 10 μm: 0.01% by 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% by mass, the amount of solid solution C increases, and further, the bainite structure and / or the martensite structure Generation increases susceptibility to delayed fracture. Further, when the C content in the low C region exceeds 0.01% by mass, the amount of hydrogen generated along with the corrosion of the reinforcing bar surface increases, and the amount of hydrogen penetrating into the reinforcing bar causing delayed fracture also increases. .
In addition, since C content of a low C area | region cannot completely make C content 0 mass%, it is preferable to set it as 0.001 mass% or more.

  Here, the region in which the C content is 0.01% by mass or less from the surface to a depth of at least 10 μm, that is, the thickness of the low C region is 10 μm or more, is that 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% by mass or less is preferably 100 μm or less. Because, when the thickness of the region where the C content is 0.01% by 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 area fraction of ferrite 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, the C content in the low C region is adjusted to 0.01% by mass or less by causing decarburization of the surface layer during heating before hot rolling. That is, by heating and maintaining steel satisfying the above-described component composition in a decarburizing atmosphere (air, N ₂ atmosphere, etc.) as a heating condition, the C content in the low C region can be 0.01% by mass or less. . The conditions for the heating temperature and holding time for decarburizing the C content in this low C region to 0.01% by mass or less are 850 ° C. or higher in a decarburizing atmosphere if the above component composition is satisfied. What is necessary is just to hold | maintain for 100 to 300 minutes in the temperature range of 1050 degrees C or less.

  This is because when the holding temperature in the decarburizing atmosphere is lower than 850 ° C., decarburization does not occur so that the C content in the region from the surface to a depth of 10 μm becomes 0.01% by mass or less. On the other hand, when the holding temperature in the decarburizing atmosphere exceeds 1050 ° C., C equivalent to or more than the amount of C decarburized from the surface diffuses from the inside to the surface layer, so the C content in the region up to 10 μm depth is 0.01 Cannot be less than mass%.

  In addition, even if kept in a temperature range of 850 ° C. or more and 1050 ° C. or less in a decarburizing atmosphere, if the holding time is less than 100 minutes, the progress of decarburization becomes insufficient and the region from the surface to a depth of 10 μm is obtained. C content does not become 0.01 mass% or less.

Furthermore, the holding time in the temperature range of 850 ° C. or higher and 1050 ° C. or lower in a decarburizing atmosphere is 300 minutes or shorter. Due to the progress of decarburization during heating and holding in a decarburizing atmosphere, the C concentration in the surface layer portion becomes lower than the C concentration in the core portion, and the surface layer region having this low C concentration becomes a ferrite single layer after the above quenching. It becomes a phase structure, and the core part other than the surface layer region becomes a martensite single phase structure. Here, if the holding time exceeds 300 minutes, the surface layer region having a C concentration lower than that of the core portion becomes too thick, and the ferrite fraction does not become less than 5%.
For the above reasons, the heating and holding in the decarburizing atmosphere may be performed under the condition of holding in the temperature range of 850 ° C. to 1050 ° C. for 100 minutes to 300 minutes.

  Note that the heating and holding for adjusting the C content in the low C region to 0.01% by mass or less may be performed at the time of the quenching process, not at the time of heating before the hot rolling. However, what is necessary is just to hold | maintain for 100 minutes or more and 300 minutes or less in the temperature range of 850 degreeC or more and 1050 degrees C or less in a decarburization atmosphere.

  Although the reinforcing bars thus obtained can be manufactured at low cost, they have high strength but excellent wire drawing and delayed fracture characteristics, and require high strength of 1600 MPa or higher, such as high-rise apartments. Application to is possible.

  After steel having the composition shown in Table 3 is melt cast and made into billets, it is heated in the atmosphere at the heating temperature shown in Table 4 and the in-furnace time at 850 ° C. or higher, and then hot rolled. To produce a wire having a diameter of 8 mm. The obtained 8 mm diameter wire was descaled by pickling and then drawn to a diameter of 6.0 mm. The drawn wire is heated to 1000 ° C and then water-cooled so that the in-furnace time in the temperature range of 850 ° C and higher is 30 seconds, and then tempered by heating to 350 ° C and water-cooling. Was given. The wire drawability was judged to have been reduced if the wire was disconnected even once when the conditions described later were pulled out. A tensile test, a delayed fracture test, and the amount of diffusible hydrogen were measured on the obtained wire having a diameter of 6.0 mm by the methods described later. Further, a sample with a length of 5 mm was taken from the obtained wire rod with a diameter of 6.0 mm, and the amount of C and the structure of the surface layer on the surface perpendicular to the rolling direction (circular cross section with a diameter of 6.0 mm) were as described below. Measured and observed.

[Drawability]
The drawability was evaluated based on the presence or absence of wire breakage by drawing the above-mentioned 8 mm diameter wire to 6.0 mm. The number of wire breaks indicates the number of wire breaks during the 20 m drawing process, and it was judged that the wire drawability was lowered when the wire breakage occurred even once.

[Tensile test]
The tensile properties as high-strength reinforcing bars were tested according to JIS Z 2241 Annex D. The test piece was sampled from the quenched and tempered wire according to JIS Z 2241 Annex D D.2.3.1.2, and the test was conducted at a tensile speed of 5 mm / min. In the present invention, it was defined that a high-strength reinforcing bar was obtained when the tensile strength was 1600 MPa or more.

[Surface layer C content]
The C content of the surface layer was determined using an electron beam microanalyzer (hereinafter referred to as EPMA) using the test piece in the tensile test described above. EPMA measurement conditions were as follows: beam diameter: 5 μmφ, acceleration voltage: 20 kV, current: 4 × 10 ⁻⁷ A, line analysis was performed from the surface layer 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 becomes 0.01 mass% or less was calculated | required.

[Delayed fracture characteristics]
In order to investigate the delayed fracture characteristics as a high-strength reinforcing bar, the FIP test was conducted using the specimen 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, 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 ruptured or expired in the FIP test described above, and then the fractured specimen was cut from the fractured surface, and the unbroken specimen was cut from a parallel portion to a 10 mm long sample.・ Raise the temperature with a laboratory GC7000T at a rate of 200 ° C / hour and define the amount of hydrogen released up to 350 ° C as the amount of diffusible hydrogen. If the amount of diffusible hydrogen is 3.0 mass ppm or less, delayed fracture The property was defined as good.

[Tissue observation]
The structure is examined by using the test piece whose C concentration is measured as described above, after mirror polishing, corroding with 3% nital, and observing with an optical microscope 500 times, and the ferrite structure and martensite structure in the cross section of the test piece. Each area was determined, and the ferrite fraction was determined by calculating ferrite structure / (rebar cross-sectional area (= ferrite area + martensite area)) × 100 (%).

  Table 4 shows the results of the depth, ferrite fraction, diffusible hydrogen content, delayed fracture test results, tensile strength, and wire drawability in the region where the C content existing in the surface layer is 0.01% by mass or less. In all examples except B-35, the steel structure on the surface side was a ferrite single phase, and the inner core structure was a martensite single phase. B-1 to 4, B-6 to 10, B-16 to 19 satisfying the depth and ferrite fraction of the component composition, A value, B value, and C content of 0.01% by mass or less of the present invention It can be seen that the steel of B-22 has good tensile strength, delayed fracture characteristics, and wire drawing. On the other hand, even if the component composition is within the range of the present invention, the steel of B-5 whose B value does not satisfy the range of the present invention has a depth in the region where the C content is 0.01% by mass or less. It can be seen that the above-mentioned range (at least 10 μm) cannot be satisfied and the delayed fracture characteristics are deteriorated.

  Moreover, the steel of B-11-15, B-20-21, and B-25-29 whose component composition does not satisfy the scope of the present invention is reduced in any of tensile strength, wire drawability, and delayed fracture characteristics. I understand that. Further, it can be understood that the steel of B-24 whose ferrite fraction is outside the range of the present invention has a high ferrite fraction and a reduced tensile strength.

  Next, even if the heating and holding time (in-furnace time) at 850 ° C. or higher is 100 minutes or longer, the B-30 to 32 steel having a heating temperature higher than 1050 ° C. has a C content of 0.01% by mass. The depth of the following regions cannot satisfy the scope of the present invention, and the delayed fracture characteristics are deteriorated. The steel of B-34 having an in-furnace time of more than 300 minutes at 850 ° C. has an increased ferrite fraction and does not satisfy the condition of less than 5% ferrite fraction. B-35 steel with a heating temperature of less than 850 ° C. does not have a region with a C content of 0.01% by mass or less in the surface layer, and the steel structure is ferrite, pearlite, martensite in both the core and the surface layer. The tensile strength is low.

  Furthermore, in the steels of B-23 and B-33 whose heating and holding time (in-furnace time) at 850 ° C. or higher is less than 100 minutes, the depth of the region where the C content is 0.01% by mass or less is within the range of the present invention ( It can be seen that at least 10 μm) cannot be satisfied, and the delayed fracture characteristics are degraded. 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.

Claims (5)

C: 0.37 mass% or more and 0.50 mass% or less,
Si: 1.75% by mass to 2.30% by mass,
Mn: 0.20 % by mass or more and 1.0% by mass or less,
Cr: 0.01% by mass or more and 0.05% by mass or less,
P: 0.025 mass% or less,
S: 0.025 mass% or less,
Mo: 0.05 mass% or more and 0.95 mass% or less and O: 0.0015 mass% or less, A value calculated by the following formula (1) is 770 or more and 850 or less, B value calculated by the following formula (2) is 0.40 or more And the balance is inevitable impurities and Fe component composition, and further, the C content in the region from the surface to a depth of at least 10 μm is 0.01% by mass or less, and the steel is composed of ferrite and core of the surface layer. Reinforcing bars that have a two-phase structure with a martensite and have a ferrite fraction of less than 5% in the entire structure and a tensile strength of 1600 MPa or more.
A = α + β + γ (1)
Where α = −334 × [C] ² + 806 × [C] +291
β = 24 × [Si] ² + 67 × [Si]
γ = −4 × [Cr] ² + 23 × [Cr] −5
B = [Si] / (10 x [C]) (2)
However, [] is the content of the component in parentheses (% by mass) The component composition further comprises:
Al: 0.01 mass% or more and 0.50 mass% or less,
Cu: 0.005 mass% to 1.0 mass% and
The reinforcing bar according to claim 1, comprising Ni: one or more selected from 0.005% by mass to 2.0% by mass. The component composition further includes:
W: 0.001% to 2.0% by mass,
Nb: 0.001 mass% or more and 0.1 mass% or less,
The reinforcing bar according to claim 1, comprising one or more selected from Ti: 0.001% by mass to 0.2% by mass and V: 0.002% by mass to 0.5% by mass. The component composition further includes:
B: The reinforcing bar according to any one of claims 1 to 3, characterized by containing 0.0002 mass% or more and 0.005 mass% or less. A steel slab comprising the component composition according to any one of claims 1 to 4 is heated in a temperature range of 850 ° C to 1050 ° C in a decarburizing atmosphere for 100 minutes to 300 minutes, and then hot-rolled. Thereafter, pickling, wire drawing, quenching, and tempering are performed, and the steel is made of a steel having a C content of 0.01% by mass or less from the surface to a depth of at least 10 μm. Has a two-phase structure of ferrite in the surface layer and martensite in the core, the ferrite fraction in the entire structure is less than 5%, and the tensile strength is 1600 MPa or more .

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