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
• • 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
• • 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
  • C21D8/065—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires of ferrous 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
  • 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
  • C21D9/02—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for springs
• • 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
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • 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
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing 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
  • 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
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing 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
  • C22C38/10—Ferrous alloys, e.g. steel alloys containing cobalt
• • 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
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • 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
  • C22C38/16—Ferrous alloys, e.g. steel alloys containing 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
• • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/004—Dispersions; Precipitations
• • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/009—Pearlite
• • 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
  • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length

Definitions

• the present invention relates to a wire rod and a steel wire using the same each of which is used for or as prestressing steel wires and wire ropes.
• Prestressed concrete (hereinafter also referred to as "PC") is well known as a way to strengthen such concrete members.
• PC Prestressed concrete
• compression stress is applied to material concrete by using steel wires.
• Such a steel wire for PC namely, a prestressing steel wire (PC steel wire)
• PC steel wire when having a higher strength, can contribute more satisfactorily to a higher strength and a lighter weight of PC.
• a prestressing strand including 7 wires with a diameter of 15.2 mm and having a maximum force of about 261 kN, as prescribed in Japanese Industrial Standard (JIS) G3536.
• the delayed fracture is a phenomenon where, when a steel is used for a long time under the application of a stress, hydrogen migrated into the steel accumulates typically in a fine flaw in the steel surface, causes a microstructure around the flaw to become brittle, and thereby induces brittle fracture.
• the prestressing steel wires are used while being always tensed and may possibly undergo delayed fracture. To prevent this, strict specifications are prescribed on them.
• the prestressing steel wires are well known to become more susceptible to delayed fracture with an increasing strength. Demands are therefore made to develop steels that can less suffer from delayed fracture even when having a higher strength.
• Patent Literature 1 discloses a technique of improving delayed fracture resistance of a prestressing steel wire having a carbon content of 0.6% to 1.1%. According to the technique, a wire rod after wire drawing is subjected to blueing at a temperature of 450°C or higher to spheroidize plate-like cementite to thereby improve the delayed fracture resistance.
• the technique in Patent Literature 1 causes the steel wire to have a low strength due to the spheroidization of plate-like cementite, thereby has limitations in strength improvement, and disadvantageously fails to help the steel wire to have a wire strength of 2000 MPa or more.
• Patent Literature 2 discloses a technique of improving delayed fracture resistance of a prestressing steel wire having a carbon content of 0.6% to 1.3%. The improvement is achieved by imparting a compressive residual stress to a surface layer of the steel wire and thereby forming a deformed pearlite in the surface layer.
• the technique in Patent Literature 2 is applied to prestressing steel wires having a wire strength of up to about 1600 MPa and may probably fail to sufficiently ensure resistance to delayed fracture that is caused by hydrogen diffusion at higher wire strengths of typically 2000 MPa or more.
• Patent Literature 3 discloses a technique of improving delayed fracture resistance in a tempered martensite phase of not a prestressing steel wire, but a bearing steel having a carbon content of 0.65% to 1.20%. The improvement is achieved by dispersing particles typically of Ti- or Al-containing nitrides having a particle size of 50 to 300 nm in an amount at a predetermined or higher so as to trap hydrogen.
• hydrogen behaves differently in different microstructures, and dimensions, amounts, and other factors of precipitates acting as appropriate trapping sites also differ in different microstructures. This impedes the application of the technique in Patent Literature 3 typically to a prestressing steel wire without modification, where the prestressing steel wire includes a pearlite as a main phase.
• a manufacturing process for such a bearing steel performs a quenching-tempering treatment after wire drawing; whereas a manufacturing process for a prestressing steel wire performs wire drawing after a patenting treatment.
• the two manufacturing processes significantly differ from each other and employ different procedures to control precipitation typically of nitrides.
• EP 1 897 964 A1 discloses a high strength wire rod excelling in wire drawing performance and a process for producing the same.
• An object of the present invention is to provide a wire rod including a pearlite as a main phase, where the wire rod less suffers from reduction in delayed fracture resistance even when having a high strength and is usable typically as a high strength prestressing steel wire and wire rope that have such delayed fracture resistance as to meet building standards.
• the present inventors made investigations on inclusions having a hydrogen trap effect in a wire rod including a pearlite as a main phase. As a result, they have found that it is important to ensure AlN particles in an amount at a predetermined level or higher and to ensure, of such AlN particles, AlN particles having a size of 10 to 20 ⁇ m in an amount at a predetermined level or higher.
• the present invention provides, in an aspect, a wire rod including C in a content of 0.8% to 1.2% (in mass percent, hereinafter the same for chemical composition); Si in a content of 0.1% to 2.0%; Mn in a content of 0.1% to 2.0%; N in a content of 0.002% to 0.010%; Al in a content of 0.04% to 0.15%; P in a content of 0.02% or less (including 0%); and S in a content of 0.02% or less (including 0%), with the remainder being iron and inevitable impurities, in which the Al content and N content meet a condition specified by Expression (1) given as follows: Al ⁇ ⁇ 2.1 ⁇ 10 ⁇ N + 0.255 where [Al] and [N] are contents (in mass percent) of Al and N, respectively; the wire rod has a microstructure including 95 percent by area or more of a pearlite; the wire rod has a content of AlN of 0.005% or more; and a percentage of AlN particles having a diameter d GM of 10
• the wire rod may further contain any of (a) at least one element selected from the group consisting of Cr in a content of 1.0% or less (excluding 0%), Ni in a content of 1.0% or less (excluding 0%), Co in a content of 1.0% or less (excluding 0%), Mo in a content of 1.0% or less (excluding 0%), and Cu in a content of 0.5% or less (excluding 0%); and (b) at least one element selected from the group consisting of B in a content of 0.005% or less (excluding 0%), Nb in a content of 0.5% or less (excluding 0%), and V in a content of 0.5% or less (excluding 0%).
• the present invention adjusts the Al and N contents appropriately and controls the total content of AlN particles and the content of AlN particles having a predetermined size (d GM of 10 to 20 ⁇ m) appropriately.
• the present invention can therefore provide a wire rod having excellent delayed fracture resistance.
• the present invention in the preferred embodiment, adjusts a solute nitrogen content at a predetermined level or higher and can thereby help the steel wire to have better twisting properties.
• the present inventors After intensive investigations, the present inventors have found that, in a wire rod including a pearlite as a main phase, it is effective to ensure AlN (particles) in a predetermined content as a hydrogen trap site and to ensure AlN particles having a size of 10 to 20 ⁇ m in a content at a predetermined level or higher.
• the AlN content is specified to 0.005% or more, because the wire rod offers an increasing hydrogen trap effect with an increasing AlN content.
• the AlN content is preferably 0.006% or more, more preferably 0.007% or more, and particularly preferably 0.01% or more. Though not critical, the upper limit of the AlN content is generally about 0.04%.
• a extreme value distribution of maximum values is herein employed as an index for ensuring AlN particles having a size of 10 to 20 ⁇ m in a number at a predetermined level or higher.
• a geometrical mean (ab) 1/2 of the length "a” and the thickness "b" of an AlN particle is employed as a size of the AlN particle and is indicated as d GM ( ⁇ m).
• d GM ⁇ m
• the term 'length "a"" of an AlN particle refers to the length (dimension) of the AlN particle in the wire rod longitudinal direction
• the term “thickness "b”" of the AlN particle refers to a dimension of the AlN particle in a direction perpendicular to the wire rod longitudinal direction.
• extreme value distribution of maximum values of d GM refers to a distribution determined by measuring a maximum d GM (max) among d GM values of AlN particles present in a predetermined area; repeating this procedure on two or more view fields; and subjecting the measured two or more d GM (max) values to a statistical processing.
• the percentage of AlN particles having a d GM (max) of 10 to 20 ⁇ m is 50% or more (in number percent) in the extreme value distribution in the embodiment of the present invention. If AlN particles having a size d GM greater than 20 ⁇ m is present in a large number percent, AlN particles are present in a smaller total number and may fail to exhibit the hydrogen trap effect sufficiently.
• AlN particles having a size d GM less than 10 ⁇ m exhibit a lower hydrogen trap effect.
• AlN particles that are effective for hydrogen trap can therefore be ensured sufficiently by controlling AlN particles having a d GM (max) of 10 to 20 ⁇ m to be present in a number percent of 50% or more in the extreme value distribution.
• the wire rod according to the embodiment of the present invention includes a pearlite that occupies 95 percent by area or more of the main phase.
• the area percentage of the pearlite is preferably 97% or more, and more preferably 100%.
• Carbon (C) content 0.8% to 1.2%
• Carbon (C) element effectively contributes to a higher strength.
• the wire rod and a steel wire after cold working have higher strengths with an increasing carbon content.
• the carbon content is therefore specified to 0.8% or more.
• the carbon content is preferably 0.85% or more, and more preferably 0.90% or more.
• carbon if present in an excessively high content, may cause aging embrittlement during cold wire drawing, thereby cause the steel wire to have inferior toughness, and disadvantageously invite cracking during stranding.
• the carbon content is specified to 1.2% or less.
• the carbon content is preferably 1.1% or less, and more preferably 1.05% or less.
• Silicon (Si) content 0.1% to 2.0%
• Silicon (Si) element not only acts as a deoxidizer, but also effectively has actions of helping the wire rod to have a higher strength and to offer better relaxation properties.
• silicon element When hot dip galvanizing is applied to the wire rod, silicon element also offers an action of suppressing strength reduction occurring upon galvanizing.
• the Si content is specified to 0.1% or more.
• the Si content is preferably 0.2% or more, and more preferably 0.4% or more.
• Si if present in an excessively high content, may cause the wire rod to have inferior cold wire drawability and to suffer from a higher breakage ratio.
• the Si content is specified to 2.0% or less.
• the Si content is preferably 1.8% or less, and more preferably 1.5% or less.
• Manganese (Mn) content 0.1% to 2.0%
• Manganese (Mn) element not only act as a deoxidizer as with Si, but also particularly has an action of fixing sulfur (S) in the steel as MnS and helping the steel to have better toughness and ductility. To exhibit these actions effectively, the Mn content is specified to 0.1% or more. The Mn content is preferably 0.15% or more, and more preferably 0.2% or more. However, manganese element is readily segregated and, if added in excess, may cause the formation of supercooled phases such as martensite because of excessively increased hardenability of a region where Mn is segregated. To prevent this, the Mn content is specified to 2.0% or less. The Mn content is preferably 1.8% or less, and more preferably 1.5% or less.
• Nitrogen (N) content 0.002% to 0.010%
• Nitrogen (N) element is important for the formation of AlN that features the embodiment of the present invention and is contained in a content of 0.002% or more.
• the nitrogen content is preferably 0.0025% or more, more preferably 0.0030% or more, and particularly preferably 0.0040% or more.
• nitrogen if added in excess, may cause the wire rod to have inferior twisting properties due to an increased solute nitrogen content. This is because nitrogen dissolves as an interstitial element in the steel as with carbon and causes embrittlement due to strain aging. To prevent this, the nitrogen content is specified to 0.010% or less.
• the nitrogen content is preferably 0.0090% or less, and more preferably 0.0080% or less.
• Solute nitrogen causes inferior twisting properties and is preferably minimized in amount.
• the solute nitrogen content is therefore preferably 0.003% or less, more preferably 0.002% or less, and furthermore preferably 0.001% or less.
• the solute nitrogen content may be controlled typically by adjusting the contents of nitride-forming elements such as Al, B, and Nb; and the nitrogen content.
• Aluminum (Al) content 0.04% to 0.15% and [Al] ⁇ -2.1 ⁇ 10 ⁇ [N]+0.255
• Aluminum (Al) element acts as a deoxidizer and is important herein, because aluminum is combined with nitrogen to form AlN, thereby traps hydrogen, and helps the wire rod to have better delayed fracture resistance.
• the aluminum nitride AlN also effectively contributes to grain refinement by a pinning effect.
• the Al content is specified to 0.04% or more.
• the Al content is preferably 0.05% or more, and more preferably 0.055% or more.
• Al if present in an excessively high content particularly in a range of high nitrogen contents, may form coarse AlN particles, and this may reduce the hydrogen trap effect of AlN.
• the Al content is specified to 0.15% in terms of its upper limit and is adapted to meet a condition specified by Expression (1) given as follows. [Math. 1] Al ⁇ ⁇ 2.1 ⁇ 10 ⁇ N + 0.255
• Expression (1) [Al] and [N] denote contents (in mass percent) of Al and N, respectively.
• Expression (1) is an expression that has been derived from many experimental examples in which delayed fracture resistance was examined at varying nitrogen contents and aluminum contents.
• the upper limit of the Al content is more strictly controlled in a range of high nitrogen contents so as to suppress the formation of coarse AlN particles.
• the Al content is preferably 0.14% or less, and more preferably 0.12% or less in terms of its upper limit.
• Phosphorus (P) content 0.02% or less
• Phosphorus (P) element is segregated at a prior austenite grain boundary, makes the grain boundary brittle, and causes the wire rod to have inferior fatigue properties.
• the phosphorus content is preferably minimized and is specified herein to 0.02% or less.
• the phosphorus content is preferably 0.015% or less, and more preferably 0.010% or less.
• S Sulfur (S) content: 0.02% or less
• S Sulfur element is segregated at a prior austenite grain boundary, makes the grain boundary brittle, and causes the wire rod to have inferior fatigue properties, as with phosphorus.
• the sulfur content is preferably minimized and is herein specified to 0.02% or less.
• the sulfur content is preferably 0.015% or less, and more preferably 0.010% or less.
• the wire rod according to the embodiment of the present invention has a basic chemical composition as above, with the remainder substantially being iron.
• inevitable impurities are naturally acceptable, where the inevitable impurities are brought into the steel under conditions typically of raw materials, facility materials, and manufacturing facilities and are contained in the steel.
• the wire rod according to the embodiment of the present invention may further contain any of elements as follows.
• Chromium (Cr) element has actions of reducing lamellar spacing of pearlite and helping the wire rod to have a higher strength and better toughness.
• the Cr content is preferably 0.05% or more, more preferably 0.1% or more, and furthermore preferably 0.2% or more.
• Cr if present in an excessively high content, may cause the wire rod to have higher hardenability and thereby increase the risk of the formation of a supercooled phase during hot rolling.
• the Cr content is preferably 1.0% or less, more preferably 0.6% or less, and furthermore preferably 0.5% or less.
• Nickel (Ni) element helps the steel wire after wire drawing to have better toughness.
• the Ni content is preferably 0.05% or more, more preferably 0.1% or more, and furthermore preferably 0.2% or more.
• Ni, if added in excess, may exhibit saturated effects, thus being economically useless.
• the Ni content is preferably 1.0% or less, more preferably 0.7% or less, and furthermore preferably 0.6% or less.
• Co Cobalt element has actions of reducing pro-eutectoid cementite (particularly at a high carbon content) and helping the wire rod to more readily control its microstructure to be a homogeneous pearlite.
• the Co content is preferably 0.05% or more, more preferably 0.1% or more, and furthermore preferably 0.2% or more.
• Co if added in excess, may exhibit saturated effects, thus being economically useless.
• the Co content is preferably 1.0% or less, more preferably 0.8% or less, and furthermore preferably 0.6% or less.
• Mo Molybdenum element helps the steel wire to have better corrosion resistance.
• the Mo content is preferably 0.05% or more, and more preferably 0.1% or more.
• Mo if present in an excessively high content, may cause the formation of a supercooled phase more readily during hot rolling and cause the wire rod to have inferior ductility.
• the Mo content is preferably 1.0% or less, more preferably 0.5% or less, and furthermore preferably 0.3% or less.
• Copper (Cu) element helps the steel wire to have better corrosion resistance.
• the Cu content is preferably 0.05% or more, and more preferably 0.08% or more.
• Cu if present in an excessively high content, may react with sulfur to be segregate as CuS in a grain boundary region and thereby cause a flaw to be generated during the wire rod manufacturing.
• the Cu content is preferably 0.5% or less, more preferably 0.2% or less, and furthermore preferably 0.18% or less.
• Boron (B) element has actions of preventing the formation of pro-eutectoid ferrite and pro-eutectoid cementite and helping the wire rod to readily control its microstructure to be a homogeneous pearlite. Boron also has actions of fixing, as boron nitride (BN), excess solute nitrogen after the precipitation of AlN, suppressing strain aging caused by the solute nitrogen, and helping the wire rod to have better toughness. In addition, solute boron itself has an action of helping the wire rod to have better toughness. To exhibit such actions effectively, the boron content is preferably 0.0003% or more, more preferably 0.0005% or more, and furthermore preferably 0.001% or more.
• boron if present in an excessively high content, may cause the precipitation of a compound with iron, i.e., an Fe-B compound such as FeB 2 and cause cracks upon hot rolling.
• the boron content is preferably 0.005% or less, more preferably 0.004% or less, and furthermore preferably 0.003% or less.
• Niobium (Nb) element forms a nitride with excess solute nitrogen after the precipitation of AlN and contributes to grain refinement.
• the element also advantageously fixes solute nitrogen and thereby suppresses aging embrittlement.
• the Nb content is preferably 0.01% or more, more preferably 0.03% or more, and furthermore preferably 0.05% or more.
• Nb if present in an excessively high content, may exhibit saturated effects, thus being economically useless.
• the Nb content is preferably 0.5% or less, more preferably 0.4% or less, and furthermore preferably 0.2% or less.
• Vanadium (V) element forms a nitride with excess solute nitrogen after the precipitation of AlN and contributes to grain refinement, as with Nb.
• vanadium also fixes solute nitrogen and thereby suppresses aging embrittlement.
• the vanadium content is preferably 0.01% or more, more preferably 0.02% or more, and furthermore preferably 0.03% or more.
• vanadium if present in an excessively high content, may exhibit saturated effects, thus being economically useless.
• the vanadium content is preferably 0.5% or less, more preferably 0.4% or less, and furthermore preferably 0.2% or less.
• a regular wire rod (referring to one before cold wire drawing) can be manufactured generally by preparing a steel ingot having appropriately controlled chemical compositions by ingot-making, and subjecting the ingot to blooming and hot rolling (where necessary, further to a patenting treatment).
• the wire rod according to the embodiment of the present invention is intended to control the content and particle size distribution of AlN particles appropriately, where the particle size distribution is controlled so that the percentage of AlN particles having a size d GM of 10 to 20 ⁇ m be 50% or more (in number percent) in the d GM extreme value distribution of maximum values of AlN particles.
• AlN begins to be precipitated at about 1300°C or lower, precipitated in a larger amount with a falling temperature, and completely precipitated at about 900°C.
• blooming and hot rolling processes significantly affect the precipitation behavior of AlN because the steel is exposed to temperatures within the above-mentioned ranges in these processes. Accordingly, blooming and hot rolling conditions should be appropriately controlled In general, cooling after blooming is performed at a low cooling rate and thereby often causes precipitated AlN particle to coarsen. In contrast, cooling after hot rolling is performed at a relatively high cooling rate and thereby allows precipitated AlN particles to be fine.
• blooming may be performed at a heating temperature of 1230°C to 1280°C and a cooling rate of 0.2°C/second or more. Blooming, when performed at a high heating temperature and at a high cooling rate, can prevent precipitation and coarsening of AlN particles. For this reason, the blooming temperature is preferably 1230°C or higher, and more preferably 1240°C or higher. In contrast, blooming, if performed at an excessively high heating temperature, may cause quenching cracks. To prevent this, the blooming temperature is preferably 1280°C or lower, and more preferably 1270°C or lower in terms of its upper limit.
• the cooling rate is preferably 0.2°C/second or more, more preferably 0.4°C/second or more, and furthermore preferably 0.5°C/second or more.
• the cooling rate is not limited in its upper limit, but is typically 1.5°C/second or less, and preferably 1.2°C/second or less.
• a billet obtained by blooming is hot-rolled, cooled down to 850°C to 950°C typically by water cooling, and placed in the form of a coil.
• Fine AlN particles (having a d GM of 10 to 20 ⁇ m) can be precipitated by placing the coil-form wire rod at a relatively low temperature.
• the placing temperature is preferably 950°C or lower, more preferably 940°C or lower, and furthermore preferably 920°C or lower.
• placing, if performed at an excessively low temperature may cause very fine AlN particles to be precipitated in a large number, where such very fine AlN particles do not contribute to hydrogen trap.
• the placing temperature is preferably 850°C or higher, more preferably 870°C or higher, and furthermore preferably 890°C or higher.
• the content and particle size distribution of AlN particles may be not appropriately controllable typically when at least part of the blooming and hot rolling conditions does not meet the above-specified conditions. In this case, it is also effective to perform a patenting treatment in an appropriate temperature range after hot rolling.
• the patenting treatment is preferably performed at a re-heating temperature of 880°C to 1000°C and a patenting temperature of 530°C to 620°C. If the work after hot rolling has a low AlN content, the re-heating temperature may be set to be relatively low (e.g., about 880°C to about 940°C) so as to increase the amount of AlN precipitation.
• the re-heating temperature may be set to be relatively high (e.g., 940°C to 1000°C) so as to allow the coarsened AlN particles to be once dissolved in the steel and to be precipitated again.
• the wire rod according to the embodiment of the present invention sufficiently includes AlN particles capable of effectively acting as hydrogen trap sites, can thereby give steel wires such as wire ropes and prestressing steel wires having excellent delayed fracture resistance, and are useful
• the present invention in another aspect, also includes such steel wires.
• Group D and Group DS inclusions as specified in JIS G0551 were regarded as AlN particles, and the geometrical mean (ab) 1/2 of the length (a) and thickness (b) of each AlN particle was employed as the size of the AlN particle.
• the above-obtained wire rod coils were subjected to wire drawing to give steel wires, and the tensile strength (wire strength) of each steel wire was measured.
• the steel wires were further subjected to stranding and hot stretching to give strands having strand diameters and strand structures as given in Table 2, and the rope strength, delayed fracture resistance, and twisting properties of each strand were measured. The results are indicated in Table 3.
• the tensile strength of each steel wire was measured according to JIS Z2241.
• the maximum force of a sample in a tensile test was measured according to JIS G3536.
• the delayed fracture property was measured in the following manner. Each of twelve (12) samples was immersed in a 20 percent by mass ammonium thiocyanate solution at 50°C under a load of 0.8 p.u according to the description in Literature 1 (fib Bulletin No. 30: Acceptance of stay cable systems using prestressing steels, January 2005), and a time period until the sample was broken was measured.
• the term "0.8 p.u” refers to a load of 80% of a breaking load.
• a test sample having a minimum rupture time of 2 hours or longer and a median rupture time of 5 hours or longer was accepted herein.
• Test Nos. 1 to 3, 5, 9, 10, and 13 to 20 had chemical compositions, microstructures, AlN contents, and AlN distributions respectively meeting the conditions specified in the present invention, thereby achieved a wire strength of 2000 MPa or more (preferably 2100 MPa or more), offered such a high strand strength as to meet a criterion prescribed in JIS G3536, still had good delayed fracture resistance, and could give high-strength strands that are practically workable.
• these test samples also met the condition for solute nitrogen content and thereby offered excellent twisting properties.
• Test Nos. 15 to 18 were samples particularly having a reduced solute nitrogen content and thereby offered very excellent twisting properties; whereas Test No. 9 had a highest solute nitrogen content and offered a smallest number of twisting among the samples according to the embodiment of the present invention.
• Test Nos. 10, 15, and 17 underwent hot rolling performed at a placing temperature out of the range of preferred condition, but underwent an appropriate patenting treatment thereafter, and could give wire rods meeting the conditions specified in the present invention.
• Test Nos. 4, 6 to 8, 11, 12, and 21 to 27 were samples that failed to meet any of the conditions specified in the present invention or were manufactured under a condition not meeting the manufacturing conditions required for obtaining steels according to the embodiment of the present invention.
• Test No. 4 underwent blooming performed at a low heating temperature; and Test No. 6 underwent cooling performed at a low cooling rate after blooming. These samples each suffered from precipitation of coarse AlN particles, had an AlN particle size distribution not meeting the condition specified in the present invention, and offered inferior delayed fracture resistance.
• Test No. 7 underwent placing performed at an excessively high temperature after hot rolling, suffered from insufficient precipitation of AlN particles during placing, had an AlN content and an AlN particle size distribution both not meeting the conditions specified in the present invention, and offered inferior delayed fracture resistance.
• Test No. 8 underwent placing performed at an excessively low temperature after hot rolling, suffered from excessive refinement of AlN particles, thereby had an AlN particle size distribution not meeting the condition specified in the present invention, and offered inferior delayed fracture resistance.
• Test No. 11 underwent blooming performed at an excessively high heating temperature and suffered from quenching cracks.
• Test No. 12 underwent a patenting treatment performed at an excessively low temperature, thereby had a microstructure including a mixture (P+B) of bainite (B) and pearlite (P) phases, and offered inferior wire drawability.
• This sample had a bainite fraction of about 20 percent by area.
• Test No. 21 was a sample having an excessively high carbon content, underwent significant aging embrittlement during wire drawing, and suffered from numerous breaks.
• Test No. 22 was a sample having an excessively low carbon content and failed to offer a strength corresponding to strand B type as prescribed in JIS G3536.
• Test No. 23 was a sample having an excessively low Al content, failed to include AlN particles in a sufficient amount, and offered inferior delayed fracture resistance.
• Test No. 24 was a sample having a nitrogen content within the range specified in the present invention but being relatively low and having an excessively high Al content, suffered from the formation of Al-containing oxides in a large amount, and suffered from numerous breaks upon wire drawing.
• Test No. 25 was a sample having an excessively low nitrogen content, failed to include AlN particles in a sufficient amount, had an AlN particle size distribution not meeting the conditions specified in the present invention, and offered inferior delayed fracture resistance.
• Test No. 26 was a sample having an excessively high nitrogen content, suffered from the precipitation of coarse AlN particles, and thereby offered inferior delayed fracture resistance.
• Test No. 26 had a solute nitrogen content not meeting the preferred condition in the present invention and had a smallest number of twisting among the entire test samples.
• Test No. 27 was a sample having a nitrogen content within the range specified in the present invention, but being relatively high, and having an Al content not meeting the condition specified by Expression (1), suffered from the precipitation of coarse AlN particles, and offered inferior delayed fracture resistance.

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