US20240052467A1 - High-strength wire rod for cold heading with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics, heat-treated component, and method for manufacturing same - Google Patents

High-strength wire rod for cold heading with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics, heat-treated component, and method for manufacturing same Download PDF

Info

Publication number
US20240052467A1
US20240052467A1 US18/267,224 US202118267224A US2024052467A1 US 20240052467 A1 US20240052467 A1 US 20240052467A1 US 202118267224 A US202118267224 A US 202118267224A US 2024052467 A1 US2024052467 A1 US 2024052467A1
Authority
US
United States
Prior art keywords
resistance
hydrogen
heat treatment
wire rod
delayed fracture
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
US18/267,224
Inventor
Byung-In Jung
Hanhwi KIM
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Posco Holdings Inc
Original Assignee
Posco Co Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Posco Co Ltd filed Critical Posco Co Ltd
Assigned to POSCO CO., LTD reassignment POSCO CO., LTD ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: JUNG, Byung-in, KIM, Hanhwi
Publication of US20240052467A1 publication Critical patent/US20240052467A1/en
Pending legal-status Critical Current

Links

Images

Classifications

    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D6/00Heat treatment of ferrous alloys
    • C21D6/002Heat treatment of ferrous alloys containing Cr
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D1/00General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18Hardening; Quenching with or without subsequent tempering
    • C21D1/19Hardening; Quenching with or without subsequent tempering by interrupted quenching
    • C21D1/22Martempering
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D1/00General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18Hardening; Quenching with or without subsequent tempering
    • C21D1/25Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D1/00General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/26Methods of annealing
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D1/00General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/26Methods of annealing
    • C21D1/32Soft annealing, e.g. spheroidising
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D1/00General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/56General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
    • C21D1/58Oils
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D6/00Heat treatment of ferrous alloys
    • C21D6/005Heat treatment of ferrous alloys containing Mn
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D6/00Heat treatment of ferrous alloys
    • C21D6/02Hardening by precipitation
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D7/00Modifying the physical properties of iron or steel by deformation
    • C21D7/02Modifying the physical properties of iron or steel by deformation by cold working
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D7/00Modifying the physical properties of iron or steel by deformation
    • C21D7/02Modifying the physical properties of iron or steel by deformation by cold working
    • C21D7/10Modifying the physical properties of iron or steel by deformation by cold working of the whole cross-section, e.g. of concrete reinforcing bars
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D8/00Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/06Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
    • C21D8/065Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires of ferrous alloys
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D9/00Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/0075Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for rods of limited length
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D9/00Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/0093Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for screws; for bolts
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D9/00Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/52Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D9/00Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/52Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
    • C21D9/525Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length for wire, for rods
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/001Ferrous alloys, e.g. steel alloys containing N
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/02Ferrous alloys, e.g. steel alloys containing silicon
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/04Ferrous alloys, e.g. steel alloys containing manganese
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/06Ferrous alloys, e.g. steel alloys containing aluminium
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D2211/00Microstructure comprising significant phases
    • C21D2211/001Austenite
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D2211/00Microstructure comprising significant phases
    • C21D2211/002Bainite
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D2211/00Microstructure comprising significant phases
    • C21D2211/008Martensite
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D2211/00Microstructure comprising significant phases
    • C21D2211/009Pearlite

Definitions

  • the present disclosure relates to a high-strength wire rod for cold heading with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics, a heat-treated component, and a method for manufacturing the same. More particularly, it relates to a high-strength wire rod for cold heading with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics, which is applicable to a high-strength bolt, etc., a heat-treated component, and a method for manufacturing the same.
  • wire rods for cold heading are classified into process-eliminated wire rods for cold heading with heat treatment and machining processes eliminated and high-strength wire rods for cold heading that allow weight reduction of components.
  • the high-strength wire rod for cold heading is manufactured by cold-heating a wire rod after spheroidization heat treatment, and then it is prepared into a heat-treated component such as a mechanical structure, an automobile part, etc. through quenching and tempering.
  • the metal structure of a general wire rod is mainly composed of pearlite, and there is an inconvenience in that heat treatment for a long time is required to dissolve cementite during austenitization heat treatment.
  • a tempered martensite microstructure is formed when austenitization heat treatment is performed. It is difficult to use the tempered martensite microstructure because it is very sensitive to resistance of hydrogen-delayed fracture at 1300 MPa or higher.
  • the present disclosure is directed to providing a high-strength wire rod for cold heading with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics, a heat-treated component, and a method for manufacturing the same.
  • the present disclosure provides a high-strength wire rod for cold heading with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics, which comprises, by wt %, 0.3-0.6% of C, 0.05-0.3% of Si, 0.2-1.0% of Mn, 0.5-2.0% of Cr, 0.5-2.0% of Mo, 0.02-0.05% of Al, 0.01-0.03% of N, and Fe and other impurities as the balance, and has a microstructure comprising, by area fraction, 80% or more of bainite, 1-15% of pearlite and 0.1-2% of martensite, and comprises 2 ⁇ 10 19 /m 3 or more of aluminum nitride having a diameter of 5-50 nm.
  • the high-strength wire rod may have a prior austenite average grain size of 10 ⁇ m or smaller.
  • the martensite may be comprised of 60% or higher in the prior austenite grain boundary.
  • the present disclosure also provides a method for manufacturing a high-strength wire rod for cold heading with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics, which comprises: heating a billet comprising, by wt %, 0.3-0.6% of C, 0.05-0.3% of Si, 0.2-1.0% of Mn, 0.5-2.0% of Cr, 0.5-2.0% of Mo, 0.02-0.05% of Al, 0.01-0.03% of N and Fe and other impurities as the balance at 1000-1200° C., conducting hot rolling at a finish hot rolling temperature of 750-950° C., and conducting cooling at a cooling rate of 0.2-1.0° C./s, wherein the cooled wire rod has a microstructure comprising, by area fraction, 80% or more of bainite, 1-15% of pearlite and 0.1-2% of martensite, and comprises 2 ⁇ 10 19 /m 3 or more of aluminum nitride having a diameter of 5-50 nm.
  • the present disclosure also provides a heat-treated component with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics, which comprises, by wt %, 0.3-0.6% of C, 0.05-0.3% of Si, 0.2-1.0% of Mn, 0.5-2.0% of Cr, 0.5-2.0% of Mo, 0.02-0.05% of Al, 0.01-0.03% of N and Fe and other impurities as the balance, and has a microstructure comprising, by area fraction, 90% or more of tempered martensite, and comprises 2 ⁇ 10 19 /m 3 or more of aluminum nitride having a diameter of 5-50 nm.
  • the heat-treated component may have a prior austenite average grain size of 5 ⁇ m or smaller.
  • the heat-treated component with superior resistance of hydrogen-delayed fracture characteristics may have a tensile strength of 1400 MPa or higher and an impact toughness of 60 J or higher.
  • the present disclosure also provides a method for manufacturing a heat-treated component with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics, which comprises: preparing a wire rod comprising, by wt %, 0.3-0.6% of C, 0.05-0.3% of Si, 0.2-1.0% of Mn, 0.5-2.0% of Cr, 0.5-2.0% of Mo, 0.02-0.05% of Al, 0.01-0.03% of N and Fe and other impurities as the balance and having a microstructure comprising, by area fraction, 80% or more of bainite, 1-15% of pearlite and 0.1-2% of martensite, wherein the martensite is comprised of 60% or higher in the prior austenite grain into a steel wire by conducting spheroidization heat treatment and drawing once or more times, preparing the prepared steel wire into a component by conducting cold heading, heating the prepared component at 800-900° C. for 1,000-2,000 seconds, quenching the heated component at 50-150° C., and tempering the quenched component at 500-600° C.
  • the microstructure comprises, by area fraction, 80% or more of bainite, 1-15% of pearlite and 0.1-2% of martensite, austenitization heat treatment can be conducted quickly and, thus, the energy consumed for heat treatment can be reduced.
  • the wire rod since the wire rod has a microstructure and fine carbide is distributed therein, resistance of hydrogen-delayed fracture resistance can be improved.
  • FIG. 1 shows the tensile strength of examples and comparative examples.
  • FIG. 2 shows the impact toughness of examples and comparative examples.
  • the present specification discloses a high-strength wire rod for cold heading with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics, which comprises, by wt %, 0.3-0.6% of C, 0.05-0.3% of Si, 0.2-1.0% of Mn, 0.5-2.0% of Cr, 0.5-2.0% of Mo, 0.02-0.05% of Al, 0.01-0.03% of N and Fe and other impurities as the balance, and has a microstructure comprising, by area fraction, 80% or more of bainite, 1-15% of pearlite and 0.1-2% of martensite and comprises 2 ⁇ 10 19 /m 3 or more of aluminum nitride having a diameter of 5-50 nm.
  • a high-strength wire rod for cold heading with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics comprises, by wt %, 0.3-0.6% of C, 0.05-0.3% of Si, 0.2-1.0% of Mn, 0.5-2.0% of Cr, 0.5-2.0% of Mo, 0.02-0.05% of Al, 0.01-0.03% of N and Fe and other impurities as the balance.
  • the reason why the alloy composition is limited will be described in detail.
  • the composition means wt % unless specified otherwise.
  • C is an element added to ensure the strength of a product. If the content of C is less than 0.3%, it is difficult to ensure the target strength, and it is not easy to ensure sufficient hardenability after final Q/T (quenching/tempering) heat treatment. On the contrary, if the content of C exceeds 0.6%, fatigue life is reduced due to excessive generation of carbide. Accordingly, in the present disclosure, the upper limit of the C content is limited to 0.6%.
  • Si an element that is used not only for deoxidization of steel but also for ensuring strength through solid solution strengthening.
  • Si is added in an amount of 0.05% or more for ensuring deoxidization and strength. But, if the content is excessive, it is difficult to process complex parts such as a bolt because of unsatisfactory cold heading property. Accordingly, in the present disclosure, the upper limit of the Si content is limited to 0.3%.
  • Mn is an element which is advantageous in ensuring strength by improving the hardenability of parts, increases rollability and reduces brittleness. It is added in an amount of 0.2% or more in order to ensure sufficient strength. But, if the content is excessive, a hard tissue may be formed easily during cooling after hot rolling and fatigue property may be deteriorated due to the generation a large amount of MnS inclusions. Accordingly, in the present disclosure, the upper limit of the Mn content is limited to 1.0%.
  • Cr is an element which is effective for improving hardenability together with Mn and improves the corrosion resistance. If the Cr content is less than 0.5%, enough corrosion resistance cannot be ensured. On the other hand, if the content is excessive, there are problems that impact toughness is decreased and coarse carbide with poor resistance of hydrogen-delayed fracture resistance is formed. Accordingly, in the present disclosure, the upper limit of the Cr content is limited to 2.0%.
  • Mo is an element which improves hardenability through precipitation hardening by precipitation of fine carbide and solid solution hardening.
  • the improvement of hardenability by Mo is more effective as compared to Mn or Cr. If the Mo content is less than 0.5%, it is not easy to ensure strength because fine carbide is not precipitated sufficiently during Q/T heat treatment. On the other hand, if the content is excessive, the shape of the part is distorted after quenching due to excessively high hardenability, requiring an additional process for correction or resulting in microcrack defects in the parts. Accordingly, in the present disclosure, the upper limit of the Mo content is limited to 2.0%.
  • Al is an element widely used as a deoxidizing agent in steelmaking processes. Al reacts with N to form aluminum nitride (AlN) and refines austenite grains. If the Al content is less than 0.02%, grain refinement is not easy because the amount of the nitrogen compound is insufficient. On the other hand, if the content is excessive, the occurrence of defects may be intensified due to excessive formation of non-metallic inclusions such as alumina. Accordingly, in the present disclosure, the upper limit of the Al content is limited to 0.05%.
  • N is an element used for grain refinement instead of the expensive alloying element. N reacts with Al to form aluminum nitride (AlN) and refines austenite grains. If the content of N is less than 0.01%, grain refinement is not easy because the amount of the nitrogen compound is insufficient. On the other hand, if the content is excessive, dislocation and deposition occur during cold heading due to heading heat, resulting in decreased mold life due to fixation of free nitrogen and increased deformation strength. Accordingly, in the present disclosure, the upper limit of the N content is limited to 0.03%.
  • the remaining component is iron (Fe). But, the mixing of unwanted impurities from the raw materials or the surrounding environment cannot be excluded. The impurities will not be described in detail because they are known to those skilled in the art.
  • V vanadium
  • CHQ column heading quality
  • the wire rod for cold heading has a microstructure comprising, by area fraction, 80% or more of bainite, 1-15% of pearlite and 0.1-2% of martensite.
  • the heat treatment time for dissolving cementite during austenitization heat treatment can be reduced.
  • the microstructure may contain 2 ⁇ 10 19 /m 3 or more of aluminum nitride having a diameter of 5-50 nm and may have a prior austenite average grain size of 10 ⁇ m or smaller.
  • austenite grains can be refined and resistance of hydrogen-delayed fracture resistance can be improved.
  • the prior austenite grain boundary of the wire rod refers to the grain boundary of the austenite structure of the wire rod after winding and before cooling.
  • 60% or more of martensite may be comprised in the prior austenite grain boundary.
  • a tensile strength of 1400 MPa or higher and an impact toughness of 60 J or higher can be ensured.
  • the inventors of the present disclosure have found out that the strength and resistance of hydrogen-delayed fracture resistance of the wire rod for cold heading can be improved further when the contents of C, Cr and Mo satisfy a specific condition, and have derived the following relation.
  • the wire rod for cold heading may satisfy the following formula (1) while satisfying the above-described alloy composition.
  • C, Cr and Mo mean the wt % of each element. If there is an element other than C, Cr and Mo, 0 is allocated for the element.
  • fine carbide that can trap diffusible hydrogen is necessary.
  • the fine carbides that can trap hydrogen comprise CrC and MoC carbides having Cr and Mo as main components, respectively. Only when a certain number of the fine carbides is ensured, a strength of 1400 MPa or higher can be ensured at the tempering temperature of 500-600° C. and the effect of hydrogen trapping can be maximized. Considering this, by controlling the alloy composition to satisfy the formula (1), the strength and resistance of hydrogen-delayed fracture resistance of the heat-treated component at the high tempering temperature (500-600° C.) can be improved.
  • the method for manufacturing a high-strength wire rod for cold heading with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics may comprise a step of heating a billet satisfying the composition described above, a step of preparing the heated billet into a wire rod, and a step of cooling the wire rod.
  • the billet may satisfy the composition described above and may be heated at 1000-1200° C.
  • the billet may satisfy the formula (1).
  • the heated billet may be prepared into a wire rod by finish hot-rolling at 750-950° C. and then winding.
  • the wire rod In the step of cooling the wire rod, the wire rod may be cooled at a cooling rate of 0.2-1.0° C./s such that the average austenite grain size after winding is 10 ⁇ m or smaller.
  • the cooling may be performed by air cooling, although not being specially limited thereto.
  • the cooled wire rod may have a microstructure comprising, by area fraction, 80% or more of bainite, 1-15% of pearlite and 0.1-2% of martensite, and the aerial ratio of the martensite formed in the prior austenite grain boundary may be 60% or higher.
  • the prior austenite grain boundary refers to the grain boundary of the austenite structure of the wire rod after winding and before cooling.
  • the microstructure of the cooled wire rod may contain 2 ⁇ 10 19 /m 3 or more of aluminum nitride having a diameter of 5-50 nm.
  • the method for manufacturing a high-strength heat-treated component with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics may comprise a step of lowering strength by spheroidization heat-treating the cooled wire rod, a step of preparing the wire rod into a component through cold heading, a step of heating the component, a step of quenching the heated component, and a step of tempering the quenched component.
  • drawing may be performed at least once.
  • the cooled wire rod may be prepared into a steel wire by conducting spheroidization heat treatment and drawing at least once.
  • the spheroidization heat treatment is performed appropriately to process the steel before drawing, and the drawing may be performed appropriately in consideration of the drawing limit.
  • the wire rod may be prepared into a steel wire that can be prepared into a component of a complicated shape through spheroidization heat treatment and drawing.
  • the steel wire may be prepared into a component through cold heading.
  • the component may be, for example, a screw, a bolt, etc.
  • the bolt may have a body diameter of 12-30 mm.
  • the component may be heated at high temperature.
  • the carbide precipitated during the rolling of the wire rod is redissolved.
  • the component may be heated such that the alloy component has a uniform composition and has an average austenite grain size of 5 ⁇ m or smaller.
  • the component may be heated at 800-900° C. for 1000-2000 seconds.
  • the heated component may be quenched to 50-150° C.
  • the quenching may be performed by immersing the heated component in an oil at 50-150° C., although not being specially limited thereto.
  • the final microstructure of the heat-treated component is controlled to tempered martensite.
  • the tempering step may be performed by tempering at 500-600° C. The tempering may be performed for 3000-10000 seconds.
  • the heat-treated component with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics prepared by the method described above may contain, by wt %, 0.3-0.6% of C, 0.05-0.3% of Si, 0.2-1.0% of Mn, 0.5-2.0% of Cr, 0.5-2.0% of Mo, 0.02-0.05% of Al, 0.01-0.03% of N and Fe and other impurities as the balance, and the microstructure may contain, by area fraction, 90% or more of tempered martensite and may contain 2 ⁇ 10 19 /m 3 or more of aluminum nitride having a diameter of 5-50 nm.
  • the prior austenite average grain size may be 5 ⁇ m or smaller.
  • the prior austenite grain boundary refers to the grain boundary of the austenite structure of the wire rod after winding and before cooling.
  • the tensile strength may be 1400 MPa or higher
  • the impact toughness may be 60 J or higher.
  • the final component with a body diameter of 12-30 mm may have a tensile strength of 1400 MPa or higher and an impact toughness of 60 J or higher.
  • the heat-treated component satisfying the alloy composition described above may satisfy the following formula (1).
  • the restriction in the formula (1) will not be described here because it was described earlier.
  • C, Cr and Mo mean the wt % of each element.
  • a billet having the composition described in Table 1 was heated to 1000-1200° C., finish-rolled at 750-950° C., and then wound at 730-900° C. After the winding, the wire rod was cooled at a cooling rate of 0.2-1° C./s. After the cooling was completed, the wire rod had a microstructure comprising, by area fraction, 80% or more of bainite, 1-15% of pearlite and 0.1-2% of martensite, and the ratio of martensite formed in the prior austenite grain boundary was 60% or higher. In addition, it comprised 2 ⁇ 10 19 /m 3 or more of aluminum nitride having a diameter of 5-50 nm.
  • the values of the formula (1) were 6.65 or higher, the grain boundary martensite ratio was 60% or higher, and the number of aluminum nitride with a size of 5-50 nm was 2 ⁇ 10 19 /m 3 or more.
  • the values of the formula (1) were lower than 6.65, the grain boundary martensite ratio was lower than 60%, the number of aluminum nitride with a size of 5-50 nm was smaller than 2 ⁇ 10 19 /m 3 , or the alloy composition was outside the range of 0.3-0.6% of C, 0.05-0.3% of Si, 0.2-1.0% of Mn, 0.5-2.0% of Cr, 0.5-2.0% of Mo, 0.02-0.05% of Al and 0.01-0.03% of N.
  • the hot-rolled wire rod having the composition described in Table 1 was processed into a cylindrical sample with a diameter of 25 mm, heated at 860° C. for 1,500 seconds, quenched by immersing in an oil at 100° C., and then tempering at 500-600° C. for 5,000 seconds. Then, after processing into a test sample according to ASTM E8 and ASTM E23, tensile test and impact test were performed. The result of tensile test and impact test is shown in FIG. 1 and FIG. 2 . All the examples exhibited a tensile strength of 1,400 MPa or higher and an impact toughness of 60 J or higher at the tempering temperature of 500-600° C., whereas the comparative examples showed poor tensile strength and impact toughness.
  • the present disclosure may provide a high-strength wire rod for cold heading with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics, a heat-treated component, and a method for manufacturing the same.

Landscapes

  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Manufacturing & Machinery (AREA)
  • Oil, Petroleum & Natural Gas (AREA)
  • Heat Treatment Of Steel (AREA)
  • Heat Treatment Of Strip Materials And Filament Materials (AREA)

Abstract

Disclosed in the present specification are: a high-strength wire rod for cold heading, having superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics, the rod being applicable to a bolt, etc.; a heat-treated component; and a method for manufacturing the same. According to an exemplary embodiment, the high-strength wire rod for cold heading with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics comprises, by wt %, 0.3-0.6% of C, 0.05-0.3% of Si, 0.2-1.0% of Mn, 0.5-2.0% of Cr, 0.5-2.0% of Mo, 0.02-0.05% of Al, 0.01-0.03% of N, and Fe and other impurities as the balance, and has a microstructure comprising, by area fraction, 80% or more of bainite, 1-15% of pearlite and 0.1-2% of martensite, and comprises 2×1019/m3 or more of aluminum nitride having a diameter of 5-50 nm.

Description

    TECHNICAL FIELD
  • The present disclosure relates to a high-strength wire rod for cold heading with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics, a heat-treated component, and a method for manufacturing the same. More particularly, it relates to a high-strength wire rod for cold heading with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics, which is applicable to a high-strength bolt, etc., a heat-treated component, and a method for manufacturing the same.
  • BACKGROUND ART
  • In general, wire rods for cold heading are classified into process-eliminated wire rods for cold heading with heat treatment and machining processes eliminated and high-strength wire rods for cold heading that allow weight reduction of components.
  • The high-strength wire rod for cold heading is manufactured by cold-heating a wire rod after spheroidization heat treatment, and then it is prepared into a heat-treated component such as a mechanical structure, an automobile part, etc. through quenching and tempering.
  • However, the metal structure of a general wire rod is mainly composed of pearlite, and there is an inconvenience in that heat treatment for a long time is required to dissolve cementite during austenitization heat treatment.
  • In addition, a tempered martensite microstructure is formed when austenitization heat treatment is performed. It is difficult to use the tempered martensite microstructure because it is very sensitive to resistance of hydrogen-delayed fracture at 1300 MPa or higher.
  • Accordingly, it is necessary to develop a high-strength wire rod for cold heading with superior resistance of hydrogen-delayed fracture characteristics at 1300 MPa or higher.
  • DISCLOSURE Technical Problem
  • The present disclosure is directed to providing a high-strength wire rod for cold heading with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics, a heat-treated component, and a method for manufacturing the same.
  • Technical Solution
  • The present disclosure provides a high-strength wire rod for cold heading with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics, which comprises, by wt %, 0.3-0.6% of C, 0.05-0.3% of Si, 0.2-1.0% of Mn, 0.5-2.0% of Cr, 0.5-2.0% of Mo, 0.02-0.05% of Al, 0.01-0.03% of N, and Fe and other impurities as the balance, and has a microstructure comprising, by area fraction, 80% or more of bainite, 1-15% of pearlite and 0.1-2% of martensite, and comprises 2×1019/m3 or more of aluminum nitride having a diameter of 5-50 nm.
  • The high-strength wire rod may have a prior austenite average grain size of 10 μm or smaller.
  • The martensite may be comprised of 60% or higher in the prior austenite grain boundary.
  • The present disclosure also provides a method for manufacturing a high-strength wire rod for cold heading with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics, which comprises: heating a billet comprising, by wt %, 0.3-0.6% of C, 0.05-0.3% of Si, 0.2-1.0% of Mn, 0.5-2.0% of Cr, 0.5-2.0% of Mo, 0.02-0.05% of Al, 0.01-0.03% of N and Fe and other impurities as the balance at 1000-1200° C., conducting hot rolling at a finish hot rolling temperature of 750-950° C., and conducting cooling at a cooling rate of 0.2-1.0° C./s, wherein the cooled wire rod has a microstructure comprising, by area fraction, 80% or more of bainite, 1-15% of pearlite and 0.1-2% of martensite, and comprises 2×1019/m3 or more of aluminum nitride having a diameter of 5-50 nm.
  • The present disclosure also provides a heat-treated component with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics, which comprises, by wt %, 0.3-0.6% of C, 0.05-0.3% of Si, 0.2-1.0% of Mn, 0.5-2.0% of Cr, 0.5-2.0% of Mo, 0.02-0.05% of Al, 0.01-0.03% of N and Fe and other impurities as the balance, and has a microstructure comprising, by area fraction, 90% or more of tempered martensite, and comprises 2×1019/m3 or more of aluminum nitride having a diameter of 5-50 nm.
  • The heat-treated component may have a prior austenite average grain size of 5 μm or smaller.
  • In addition, the heat-treated component with superior resistance of hydrogen-delayed fracture characteristics may have a tensile strength of 1400 MPa or higher and an impact toughness of 60 J or higher.
  • The present disclosure also provides a method for manufacturing a heat-treated component with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics, which comprises: preparing a wire rod comprising, by wt %, 0.3-0.6% of C, 0.05-0.3% of Si, 0.2-1.0% of Mn, 0.5-2.0% of Cr, 0.5-2.0% of Mo, 0.02-0.05% of Al, 0.01-0.03% of N and Fe and other impurities as the balance and having a microstructure comprising, by area fraction, 80% or more of bainite, 1-15% of pearlite and 0.1-2% of martensite, wherein the martensite is comprised of 60% or higher in the prior austenite grain into a steel wire by conducting spheroidization heat treatment and drawing once or more times, preparing the prepared steel wire into a component by conducting cold heading, heating the prepared component at 800-900° C. for 1,000-2,000 seconds, quenching the heated component at 50-150° C., and tempering the quenched component at 500-600° C. for 3,000-10,000 seconds.
  • Advantageous Effects
  • According to an exemplary embodiment of the present disclosure, since the microstructure comprises, by area fraction, 80% or more of bainite, 1-15% of pearlite and 0.1-2% of martensite, austenitization heat treatment can be conducted quickly and, thus, the energy consumed for heat treatment can be reduced.
  • According to an exemplary embodiment of the present disclosure, since the wire rod has a microstructure and fine carbide is distributed therein, resistance of hydrogen-delayed fracture resistance can be improved.
  • BRIEF DESCRIPTION OF DRAWINGS
  • FIG. 1 shows the tensile strength of examples and comparative examples.
  • FIG. 2 shows the impact toughness of examples and comparative examples.
  • BEST MODE
  • The present specification discloses a high-strength wire rod for cold heading with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics, which comprises, by wt %, 0.3-0.6% of C, 0.05-0.3% of Si, 0.2-1.0% of Mn, 0.5-2.0% of Cr, 0.5-2.0% of Mo, 0.02-0.05% of Al, 0.01-0.03% of N and Fe and other impurities as the balance, and has a microstructure comprising, by area fraction, 80% or more of bainite, 1-15% of pearlite and 0.1-2% of martensite and comprises 2×1019/m3 or more of aluminum nitride having a diameter of 5-50 nm.
  • MODE FOR INVENTION
  • Hereinafter, specific exemplary embodiments of the present disclosure are described. However, the exemplary embodiments of the present disclosure may be modified in various forms and the technical idea of the present disclosure is not limited to the embodiments described below. In addition, the exemplary embodiments of the present disclosure are provided to more completely explain the present disclosure to those having ordinary knowledge in the art.
  • The terms used in this application are used only to describe specific examples. Therefore, for example, singular expressions include plural expressions unless the context clearly indicates otherwise. In addition, the terms such as “include”, “have”, etc. used in this application are used to clearly indicate that the features, steps, functions, components or combinations thereof described in the specification exist, but do not preclude the presence of other features, steps, functions, components or combinations thereof.
  • Meanwhile, unless defined otherwise, all the terms used in the present specification should be regarded as having the same meanings commonly understood by those having ordinary knowledge in the technical field to which the present disclosure belongs. Therefore, unless explicitly defined in the present specification, certain terms should not be interpreted in an overly idealistic or formal sense. For example, in the present specification, singular expressions include plural expressions unless the context clearly indicates otherwise.
  • In addition, the expressions such as “about”, “substantially”, etc. in the present specification are used to indicate specific or similar numerical values within given tolerances. They are used to help understanding the present disclosure or prevent undue exploitation of the disclosure by unscrupulous infringers.
  • A high-strength wire rod for cold heading with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics according to the present disclosure comprises, by wt %, 0.3-0.6% of C, 0.05-0.3% of Si, 0.2-1.0% of Mn, 0.5-2.0% of Cr, 0.5-2.0% of Mo, 0.02-0.05% of Al, 0.01-0.03% of N and Fe and other impurities as the balance.
  • The reason why the alloy composition is limited will be described in detail. The composition means wt % unless specified otherwise.
  • Carbon (C): 0.3-0.6 wt %
  • C is an element added to ensure the strength of a product. If the content of C is less than 0.3%, it is difficult to ensure the target strength, and it is not easy to ensure sufficient hardenability after final Q/T (quenching/tempering) heat treatment. On the contrary, if the content of C exceeds 0.6%, fatigue life is reduced due to excessive generation of carbide. Accordingly, in the present disclosure, the upper limit of the C content is limited to 0.6%.
  • Silicon (Si): 0.05-0.3 wt %
  • Si an element that is used not only for deoxidization of steel but also for ensuring strength through solid solution strengthening. In the present disclosure, Si is added in an amount of 0.05% or more for ensuring deoxidization and strength. But, if the content is excessive, it is difficult to process complex parts such as a bolt because of unsatisfactory cold heading property. Accordingly, in the present disclosure, the upper limit of the Si content is limited to 0.3%.
  • Manganese (Mn): 0.2-1.0 wt %
  • Mn is an element which is advantageous in ensuring strength by improving the hardenability of parts, increases rollability and reduces brittleness. It is added in an amount of 0.2% or more in order to ensure sufficient strength. But, if the content is excessive, a hard tissue may be formed easily during cooling after hot rolling and fatigue property may be deteriorated due to the generation a large amount of MnS inclusions. Accordingly, in the present disclosure, the upper limit of the Mn content is limited to 1.0%.
  • Chromium (Cr): 0.5-2.0 wt %
  • Cr is an element which is effective for improving hardenability together with Mn and improves the corrosion resistance. If the Cr content is less than 0.5%, enough corrosion resistance cannot be ensured. On the other hand, if the content is excessive, there are problems that impact toughness is decreased and coarse carbide with poor resistance of hydrogen-delayed fracture resistance is formed. Accordingly, in the present disclosure, the upper limit of the Cr content is limited to 2.0%.
  • Molybdenum (Mo): 0.5-2.0 wt %
  • Mo is an element which improves hardenability through precipitation hardening by precipitation of fine carbide and solid solution hardening. The improvement of hardenability by Mo is more effective as compared to Mn or Cr. If the Mo content is less than 0.5%, it is not easy to ensure strength because fine carbide is not precipitated sufficiently during Q/T heat treatment. On the other hand, if the content is excessive, the shape of the part is distorted after quenching due to excessively high hardenability, requiring an additional process for correction or resulting in microcrack defects in the parts. Accordingly, in the present disclosure, the upper limit of the Mo content is limited to 2.0%.
  • Aluminum (Al): 0.02-0.05 w %
  • Al is an element widely used as a deoxidizing agent in steelmaking processes. Al reacts with N to form aluminum nitride (AlN) and refines austenite grains. If the Al content is less than 0.02%, grain refinement is not easy because the amount of the nitrogen compound is insufficient. On the other hand, if the content is excessive, the occurrence of defects may be intensified due to excessive formation of non-metallic inclusions such as alumina. Accordingly, in the present disclosure, the upper limit of the Al content is limited to 0.05%.
  • Nitrogen (N): 0.01-0.03 w %
  • N is an element used for grain refinement instead of the expensive alloying element. N reacts with Al to form aluminum nitride (AlN) and refines austenite grains. If the content of N is less than 0.01%, grain refinement is not easy because the amount of the nitrogen compound is insufficient. On the other hand, if the content is excessive, dislocation and deposition occur during cold heading due to heading heat, resulting in decreased mold life due to fixation of free nitrogen and increased deformation strength. Accordingly, in the present disclosure, the upper limit of the N content is limited to 0.03%.
  • The remaining component is iron (Fe). But, the mixing of unwanted impurities from the raw materials or the surrounding environment cannot be excluded. The impurities will not be described in detail because they are known to those skilled in the art.
  • In addition, because coarse carbonitride strongly traps hydrogen and can cause hydrogen embrittlement, it is necessary to prevent its formation as much as possible. For reference, vanadium (V), which is frequently added to high-strength CHQ (cold heading quality) steel with a tensile strength of 1400 MPa or higher, can form coarse carbide having poor resistance of hydrogen-delayed fracture resistance. In the present disclosure, V is not added so that no undissolved coarse carbide remains after Q/T heat treatment even when a large part with a body diameter of 16-30 mm is manufactured. Through this, resistance of hydrogen-delayed fracture resistance can be ensured.
  • The wire rod for cold heading according to an exemplary embodiment of the present disclosure has a microstructure comprising, by area fraction, 80% or more of bainite, 1-15% of pearlite and 0.1-2% of martensite. When the wire rod has such a microstructure, the heat treatment time for dissolving cementite during austenitization heat treatment can be reduced.
  • In addition, the microstructure may contain 2×1019/m3 or more of aluminum nitride having a diameter of 5-50 nm and may have a prior austenite average grain size of 10 μm or smaller. When 2×1019/m3 or more of aluminum nitride having a diameter of 5-50 nm is comprised, austenite grains can be refined and resistance of hydrogen-delayed fracture resistance can be improved. Here, the prior austenite grain boundary of the wire rod refers to the grain boundary of the austenite structure of the wire rod after winding and before cooling.
  • In addition, 60% or more of martensite may be comprised in the prior austenite grain boundary. When 60% or more of martensite is comprised in the prior austenite grain boundary, a tensile strength of 1400 MPa or higher and an impact toughness of 60 J or higher can be ensured.
  • The inventors of the present disclosure have found out that the strength and resistance of hydrogen-delayed fracture resistance of the wire rod for cold heading can be improved further when the contents of C, Cr and Mo satisfy a specific condition, and have derived the following relation. In an exemplary embodiment of the present disclosure, the wire rod for cold heading may satisfy the following formula (1) while satisfying the above-described alloy composition.

  • 7.2 C+Cr+2.7 Mo≥6.65  (1)
  • In the formula (1), C, Cr and Mo mean the wt % of each element. If there is an element other than C, Cr and Mo, 0 is allocated for the element.
  • In order to further improve resistance of hydrogen-delayed fracture resistance, fine carbide that can trap diffusible hydrogen is necessary. The fine carbides that can trap hydrogen comprise CrC and MoC carbides having Cr and Mo as main components, respectively. Only when a certain number of the fine carbides is ensured, a strength of 1400 MPa or higher can be ensured at the tempering temperature of 500-600° C. and the effect of hydrogen trapping can be maximized. Considering this, by controlling the alloy composition to satisfy the formula (1), the strength and resistance of hydrogen-delayed fracture resistance of the heat-treated component at the high tempering temperature (500-600° C.) can be improved.
  • Hereinafter, a method for manufacturing a high-strength wire rod for cold heading with superior resistance of hydrogen-delayed fracture characteristics according to the present disclosure will be described.
  • The method for manufacturing a high-strength wire rod for cold heading with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics according to an exemplary embodiment of the present disclosure may comprise a step of heating a billet satisfying the composition described above, a step of preparing the heated billet into a wire rod, and a step of cooling the wire rod.
  • In the step of heating the billet, the billet may satisfy the composition described above and may be heated at 1000-1200° C. In addition, the billet may satisfy the formula (1).
  • In the step of preparing the heated billet into a wire rod, the heated billet may be prepared into a wire rod by finish hot-rolling at 750-950° C. and then winding.
  • In the step of cooling the wire rod, the wire rod may be cooled at a cooling rate of 0.2-1.0° C./s such that the average austenite grain size after winding is 10 μm or smaller. The cooling may be performed by air cooling, although not being specially limited thereto.
  • The cooled wire rod may have a microstructure comprising, by area fraction, 80% or more of bainite, 1-15% of pearlite and 0.1-2% of martensite, and the aerial ratio of the martensite formed in the prior austenite grain boundary may be 60% or higher. Here, the prior austenite grain boundary refers to the grain boundary of the austenite structure of the wire rod after winding and before cooling. In addition, the microstructure of the cooled wire rod may contain 2×1019/m3 or more of aluminum nitride having a diameter of 5-50 nm.
  • Hereinafter, a method for manufacturing a high-strength heat-treated component with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics using the wire rod for cold heading described above will be described.
  • The method for manufacturing a high-strength heat-treated component with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics according to an exemplary embodiment of the present disclosure may comprise a step of lowering strength by spheroidization heat-treating the cooled wire rod, a step of preparing the wire rod into a component through cold heading, a step of heating the component, a step of quenching the heated component, and a step of tempering the quenched component. After the spheroidization heat treatment, drawing may be performed at least once. Hereinafter, each step will be described in detail.
  • The cooled wire rod may be prepared into a steel wire by conducting spheroidization heat treatment and drawing at least once. The spheroidization heat treatment is performed appropriately to process the steel before drawing, and the drawing may be performed appropriately in consideration of the drawing limit. According to the present disclosure, the wire rod may be prepared into a steel wire that can be prepared into a component of a complicated shape through spheroidization heat treatment and drawing.
  • The steel wire may be prepared into a component through cold heading. The component may be, for example, a screw, a bolt, etc. The bolt may have a body diameter of 12-30 mm.
  • Then, the component may be heated at high temperature. In the step of heating the component, the carbide precipitated during the rolling of the wire rod is redissolved. The component may be heated such that the alloy component has a uniform composition and has an average austenite grain size of 5 μm or smaller. In an exemplary embodiment, the component may be heated at 800-900° C. for 1000-2000 seconds.
  • In the step of quenching the heated component, the heated component may be quenched to 50-150° C. The quenching may be performed by immersing the heated component in an oil at 50-150° C., although not being specially limited thereto.
  • In the step of tempering the quenched component, the final microstructure of the heat-treated component is controlled to tempered martensite. In an exemplary embodiment, the tempering step may be performed by tempering at 500-600° C. The tempering may be performed for 3000-10000 seconds.
  • The heat-treated component with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics prepared by the method described above may contain, by wt %, 0.3-0.6% of C, 0.05-0.3% of Si, 0.2-1.0% of Mn, 0.5-2.0% of Cr, 0.5-2.0% of Mo, 0.02-0.05% of Al, 0.01-0.03% of N and Fe and other impurities as the balance, and the microstructure may contain, by area fraction, 90% or more of tempered martensite and may contain 2×1019/m3 or more of aluminum nitride having a diameter of 5-50 nm.
  • In addition, the prior austenite average grain size may be 5 μm or smaller. Here, the prior austenite grain boundary refers to the grain boundary of the austenite structure of the wire rod after winding and before cooling.
  • In addition, the tensile strength may be 1400 MPa or higher, and the impact toughness may be 60 J or higher. When the heat-treated component is a bolt, the final component with a body diameter of 12-30 mm may have a tensile strength of 1400 MPa or higher and an impact toughness of 60 J or higher.
  • In an exemplary embodiment of the present disclosure, the heat-treated component satisfying the alloy composition described above may satisfy the following formula (1). The restriction in the formula (1) will not be described here because it was described earlier.

  • 7.2 C+Cr+2.7 Mo≥6.65  (1)
  • In the formula (1), C, Cr and Mo mean the wt % of each element.
  • Hereinafter, the present disclosure will be described more specifically through examples. However, the following examples are only for illustrating the present disclosure more specifically and the technical idea of the present disclosure is not limited by the examples.
  • EXAMPLES
  • A billet having the composition described in Table 1 was heated to 1000-1200° C., finish-rolled at 750-950° C., and then wound at 730-900° C. After the winding, the wire rod was cooled at a cooling rate of 0.2-1° C./s. After the cooling was completed, the wire rod had a microstructure comprising, by area fraction, 80% or more of bainite, 1-15% of pearlite and 0.1-2% of martensite, and the ratio of martensite formed in the prior austenite grain boundary was 60% or higher. In addition, it comprised 2×1019/m3 or more of aluminum nitride having a diameter of 5-50 nm.
  • In Table 1, ‘formula (1)’ was calculated by substituting the contents (wt %) of C, Cr and Mo in the ‘7.2 C+Cr+2.7 Mo’ of the formula (1). The AlN number indicates the number of aluminum nitride with a size of 5-50 nm.
  • TABLE 1
    Grain AIN
    Alloy composition (wt %) Formula boundary number
    Division C Si Mn Cr Mo Al N (1) martensite ratio (%) (/m3)
    Example 1 0.32 0.11 0.71 1.23 1.19 0.04 0.015 6.747 63 4.3 × 1019
    Example 2 0.41 0.11 0.62 1.01 1.12 0.03 0.016 6.986 62 3.4 × 1019
    Example 3 0.56 0.12 0.82 0.81 0.92 0.03 0.016 7.326 64 3.6 × 1019
    Example 4 0.42 0.10 0.72 1.52 0.95 0.03 0.017 7.109 63 5.0 × 1019
    Example 5 0.40 0.11 0.56 0.57 1.45 0.03 0.014 7.365 70 3.1 × 1019
    Comparative 0.32 0.13 0.69 1.25 0.85 0.03 0.016 5.849 65 3.6 × 1019
    Example 1
    Comparative 0.40 0.11 0.65 0.94 1.19 0.04 0.015 7.033 54 3.7 × 1019
    Example 2
    Comparative 0.54 0.11 0.75 0.83 0.94 0.03 0.009 7.256 68 1.9 × 1019
    Example 3
    Comparative 0.43 0.13 0.66 2.12 1.05 0.03 0.016 8.051 72 3.4 × 1019
    Example 4
    Comparative 0.41 0.10 0.76 0.77 2.21 0.04 0.015 9.689 74 3.7 × 1019
    Example 5
  • For the examples, the values of the formula (1) were 6.65 or higher, the grain boundary martensite ratio was 60% or higher, and the number of aluminum nitride with a size of 5-50 nm was 2×1019/m3 or more.
  • In contrast, for the comparative examples, the values of the formula (1) were lower than 6.65, the grain boundary martensite ratio was lower than 60%, the number of aluminum nitride with a size of 5-50 nm was smaller than 2×1019/m3, or the alloy composition was outside the range of 0.3-0.6% of C, 0.05-0.3% of Si, 0.2-1.0% of Mn, 0.5-2.0% of Cr, 0.5-2.0% of Mo, 0.02-0.05% of Al and 0.01-0.03% of N.
  • The hot-rolled wire rod having the composition described in Table 1 was processed into a cylindrical sample with a diameter of 25 mm, heated at 860° C. for 1,500 seconds, quenched by immersing in an oil at 100° C., and then tempering at 500-600° C. for 5,000 seconds. Then, after processing into a test sample according to ASTM E8 and ASTM E23, tensile test and impact test were performed. The result of tensile test and impact test is shown in FIG. 1 and FIG. 2 . All the examples exhibited a tensile strength of 1,400 MPa or higher and an impact toughness of 60 J or higher at the tempering temperature of 500-600° C., whereas the comparative examples showed poor tensile strength and impact toughness.
  • Although the exemplary embodiments of the present disclosure have been described above, the present disclosure is not limited thereto and those having ordinary knowledge in the art will understand that various changes and modifications can be made without departing from the concept and scope of the appended claims.
  • INDUSTRIAL APPLICABILITY
  • The present disclosure may provide a high-strength wire rod for cold heading with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics, a heat-treated component, and a method for manufacturing the same.

Claims (8)

1. A high-strength wire rod for cold heading with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics, which comprises, by wt %, 0.3-0.6% of C, 0.05-0.3% of Si, 0.2-1.0% of Mn, 0.5-2.0% of Cr, 0.5-2.0% of Mo, 0.02-0.05% of Al, 0.01-0.03% of N, and Fe and other impurities as the balance, and has a microstructure comprising, by area fraction, 80% or more of bainite, 1-15% of pearlite and 0.1-2% of martensite, and comprises 2×1019/m3 or more of aluminum nitride having a diameter of 5-50 nm.
2. The high-strength wire rod for cold heading with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics according to claim 1, which has a prior austenite average grain size of 10 μm or smaller.
3. The high-strength wire rod for cold heading with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics according to claim 1, wherein the martensite is comprised of 60% or higher in the prior austenite grain boundary.
4. A method for manufacturing a high-strength wire rod for cold heading with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics, comprising:
heating a billet comprising, by wt %, 0.3-0.6% of C, 0.05-0.3% of Si, 0.2-1.0% of Mn, 0.5-2.0% of Cr, 0.5-2.0% of Mo, 0.02-0.05% of Al, 0.01-0.03% of N and Fe and other impurities as the balance at 1000-1200° C.,
hot rolling at a finish hot rolling temperature of 750-950° C., and
cooling at a cooling rate of 0.2-1.0° C./s,
wherein the cooled wire rod has a microstructure comprising, by area fraction, 80% or more of bainite, 1-15% of pearlite and 0.1-2% of martensite, and comprises 2×1019/m3 or more of aluminum nitride having a diameter of 5-50 nm.
5. A heat-treated component with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics, which comprises, by wt %, 0.3-0.6% of C, 0.05-0.3% of Si, 0.2-1.0% of Mn, 0.5-2.0% of Cr, 0.5-2.0% of Mo, 0.02-0.05% of Al, 0.01-0.03% of N and Fe and other impurities as the balance, and has a microstructure comprising, by area fraction, 90% or more of tempered martensite, and comprises 2×1019/m3 or more of aluminum nitride having a diameter of 5-50 nm.
6. The heat-treated component with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics according to claim 5, which has a prior austenite average grain size of 5 μm or smaller.
7. The heat-treated component with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics according to claim 5, which has a tensile strength of 1400 MPa or higher and an impact toughness of 60 J or higher.
8. A method for manufacturing a heat-treated component with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics, comprising:
preparing a wire rod comprising, by wt %, 0.3-0.6% of C, 0.05-0.3% of Si, 0.2-1.0% of Mn, 0.5-2.0% of Cr, 0.5-2.0% of Mo, 0.02-0.05% of Al, 0.01-0.03% of N and Fe and other impurities as the balance and having a microstructure comprising, by area fraction, 80% or more of bainite, 1-15% of pearlite and 0.1-2% of martensite, wherein the martensite is comprised of 60% or higher in the prior austenite grain boundary into a steel wire by conducting spheroidization heat treatment and drawing once or more times,
preparing the prepared steel wire into a component by conducting cold heading,
heating the prepared component at 800-900° C. for 1,000-2,000 seconds,
quenching the heated component at 50-150° C., and
tempering the quenched component at 500-600° C. for 3,000-10,000 seconds.
US18/267,224 2020-12-14 2021-11-18 High-strength wire rod for cold heading with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics, heat-treated component, and method for manufacturing same Pending US20240052467A1 (en)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
KR1020200174141A KR102448754B1 (en) 2020-12-14 2020-12-14 High-strength wire rod with excellent heat treatment property and resistance of hydrogen delayed fracture, heat treatment parts using the same, and methods for manufacturing thereof
KR10-2020-0174141 2020-12-14
PCT/KR2021/016963 WO2022131589A1 (en) 2020-12-14 2021-11-18 High-strength wire rod for cold heading, having excellent heat treatment characteristics and hydrogen delayed fracture characteristics, heat treatment component, and manufacturing methods therefor

Publications (1)

Publication Number Publication Date
US20240052467A1 true US20240052467A1 (en) 2024-02-15

Family

ID=82057863

Family Applications (1)

Application Number Title Priority Date Filing Date
US18/267,224 Pending US20240052467A1 (en) 2020-12-14 2021-11-18 High-strength wire rod for cold heading with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics, heat-treated component, and method for manufacturing same

Country Status (5)

Country Link
US (1) US20240052467A1 (en)
EP (1) EP4261313A1 (en)
KR (1) KR102448754B1 (en)
CN (1) CN116724131A (en)
WO (1) WO2022131589A1 (en)

Family Cites Families (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP4629816B2 (en) * 1999-08-20 2011-02-09 株式会社神戸製鋼所 High strength bolt excellent in delayed fracture resistance and method for producing the same
JP4435953B2 (en) * 1999-12-24 2010-03-24 新日本製鐵株式会社 Bar wire for cold forging and its manufacturing method
KR101867677B1 (en) * 2016-07-22 2018-06-15 주식회사 포스코 Steel wire rod having enhanced delayed fracture resistance and method for manufacturing the same
KR102090227B1 (en) * 2017-12-20 2020-03-17 주식회사 포스코 High strength steel wire rod and high strength steel with excellent delay fracture resistance and manufacturing method thereof
KR102042061B1 (en) * 2017-12-21 2019-11-08 주식회사 포스코 High-strength wire rod and steel with excellent hydrogen retardation resistance and manufacturing the same

Also Published As

Publication number Publication date
WO2022131589A1 (en) 2022-06-23
EP4261313A1 (en) 2023-10-18
KR20220084557A (en) 2022-06-21
KR102448754B1 (en) 2022-09-30
CN116724131A (en) 2023-09-08

Similar Documents

Publication Publication Date Title
CA2341667C (en) Cold workable steel bar or wire and process
JP4381355B2 (en) Steel having excellent delayed fracture resistance and tensile strength of 1600 MPa class or more and method for producing the molded product thereof
JPH03188217A (en) Production of high carbon sheet
US11905571B2 (en) Wire rod for cold heading, processed product using same, and manufacturing methods therefor
JP2013227603A (en) High-strength hot-rolled steel sheet excellent in stretchability, hole expansibility and low-temperature toughness and manufacturing method therefor
JPH0441616A (en) Production of low-hardness water-resistant steel excellent in wear resistance and bendability
JP4057930B2 (en) Machine structural steel excellent in cold workability and method for producing the same
CN112877591A (en) High-strength and high-toughness steel for hardware tool and chain and manufacturing method thereof
CN108950150A (en) Manganese Q&P steel heat treatment process in superhigh intensity cold rolling based on complete austenitizing
JPH039168B2 (en)
JP3857835B2 (en) Steel for high strength bolt and method for producing high strength bolt
US20240052467A1 (en) High-strength wire rod for cold heading with superior heat treatment characteristics and resistance of hydrogen-delayed fracture characteristics, heat-treated component, and method for manufacturing same
US20230020467A1 (en) Wire rod and component, for cold forging, each having excellent delayed fracture resistance characteristics, and manufacturing methods therefor
KR102314433B1 (en) Wire rod for high strength cold head quality steel with excellent resistance to hydrogen embrittlement, and method for manufacturing thereof
KR20150001469A (en) High strength cold-rolled steel sheet and method of manufacturing the cold-rolled steel sheet
JPH11131187A (en) Rapidly graphitizable steel and its production
JP2017071859A (en) Non-heat-treated steel and method for producing the same
KR101140911B1 (en) Method for producing of V-Free microalloyed steel having equality quality of quenching and tempered alloy steel
JPH0813028A (en) Production of precipitation hardening steel material having high tensile strength and high toughness
KR100311791B1 (en) METHOD FOR MANUFACTURING QUENCHED AND TEMPERED STEEL WITH SUPERIOR TENSILE STRENGTH OF AROUND 600MPa AND IMPROVED TOUGHNESS IN WELDED PART
KR940007275B1 (en) Making mathod of steel wire rod
RU2813064C1 (en) Method for producing high-strength steel sheet
KR102448756B1 (en) High-strength wire rod with excellent resistance of hydrogen delayed fracture, heat treatment parts using the same, and methods for manufacturing thereof
CN114207168B (en) Wire rod and steel wire for high strength spring and method of manufacturing the same
KR20230048866A (en) Method and wire rod for cold forging having improved drilling properties

Legal Events

Date Code Title Description
AS Assignment

Owner name: POSCO CO., LTD, KOREA, REPUBLIC OF

Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:JUNG, BYUNG-IN;KIM, HANHWI;REEL/FRAME:063950/0827

Effective date: 20230608

STPP Information on status: patent application and granting procedure in general

Free format text: DOCKETED NEW CASE - READY FOR EXAMINATION