Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D3/00—Diffusion processes for extraction of non-metals; Furnaces therefor
  • C21D3/02—Extraction of non-metals
  • C21D3/04—Decarburising
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
  • C21D8/08—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires for concrete reinforcement
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • E—FIXED CONSTRUCTIONS
  • E04—BUILDING
  • E04C—STRUCTURAL ELEMENTS; BUILDING MATERIALS
  • E04C5/00—Reinforcing elements, e.g. for concrete; Auxiliary elements therefor
  • E04C5/01—Reinforcing elements of metal, e.g. with non-structural coatings

Definitions

• 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.
• reinforced concrete structures such as buildings and apartment houses use shear reinforcement as a reinforcing material to prevent their collapse.
• 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.
• a shear reinforcing bar having a tensile strength of about 1200 MPa is generally used.
• a shear reinforcing bar having a tensile strength of about 1200 MPa is generally used.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the toughness of the reinforcing bar is lowered and the bending workability is lowered.
• 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.
• a steel wire having excellent delayed fracture characteristics is obtained.
• 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.
• 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.
• the wire rod rolling it is necessary to perform heat treatment online or offline, which increases extra operation costs such as reheating.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the component composition further comprises: Al: 0.50 mass% or less, 2.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• S 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.
• P and S 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.
• up to 0.0015% by mass is allowed.
• O 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.
• the B value (the above formula (2)): 0.40 or more
• the B value is an index for obtaining good wire drawing.
• 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.
• the hardness in the thickness region of at least 20 ⁇ m 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 ⁇ m or more from the surface of the reinforcing bar can be made HV300 or less, and the bending workability can be improved.
• Table 1 shows the component composition
• 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.
• 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.
• 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.
• 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.
• the A value and the B value are controlled within the range of the
• 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.
• the hardness of the steel at least 20 ⁇ m 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 ⁇ m 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 20 ⁇ m from the surface is HV250 or less
• the reason why the hardness of the thickness region of at least 20 ⁇ m 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.
• the reason why the thickness is in the range of at least 20 ⁇ m from the surface is that when the thickness is less than 20 ⁇ m, the high ductility region from the surface becomes small, and on the contrary, the strands are easily broken during the drawing process.
• the region expands to a depth of more than 100 ⁇ m 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.
• 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 ⁇ m from the surface.
• 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.
• 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.
• 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 ⁇ m from the surface.
• the reason why the hardness of the thickness area of at least 10 ⁇ m 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.
• the reason why the surface layer region is set to a range from the surface to a depth of at least 10 ⁇ m is that when the thickness is less than 10 ⁇ m, 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 ⁇ m from the surface is HV300 or less.
• HV300 the hardness of the thickness region of at least 10 ⁇ m from the surface.
• the region to be HV300 or less is 10 ⁇ m to 150 ⁇ m in depth from the surface. That is, when the depth region from the surface to be HV300 or less exceeds 150 ⁇ m, 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 ⁇ m 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.
• 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.
• the core structure of the reinforcing bar is preferably a martensite structure in order to ensure the strength as a reinforcing bar.
• 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.
• Cu and Ni are elements that increase hardenability and strength after tempering, and are selected. Can be added.
• 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.
• 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
• 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.
• Nb is added in an amount of more than 0.1% by mass
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the wire rod after quenching and tempering is processed into a tensile test piece having a parallel part of 1/4 in.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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

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