Classifications

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  • 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
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/001—Continuous casting of metals, i.e. casting in indefinite lengths of specific alloys
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/005—Continuous casting of metals, i.e. casting in indefinite lengths of wire
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/26—Methods of annealing
  • C21D1/32—Soft annealing, e.g. spheroidising
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/34—Methods of heating
  • C21D1/44—Methods of heating in heat-treatment baths
  • C21D1/48—Metal baths
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  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/002—Heat treatment of ferrous alloys containing Cr
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  • C21D6/00—Heat treatment of ferrous alloys
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  • C21D7/00—Modifying the physical properties of iron or steel by deformation
  • C21D7/13—Modifying the physical properties of iron or steel by deformation by hot working
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  • 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
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
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  • 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
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  • 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
  • C21D9/54—Furnaces for treating strips or wire
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  • 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
  • C21D9/54—Furnaces for treating strips or wire
  • C21D9/56—Continuous furnaces for strip or wire
  • C21D9/573—Continuous furnaces for strip or wire with cooling
  • C21D9/5732—Continuous furnaces for strip or wire with cooling of wires; of rods
• • 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
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  • 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
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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  • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
  • C21D1/76—Adjusting the composition of the atmosphere
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  • 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

Definitions

• the present disclosure relates to a wire rod and a steel wire for a spring, a spring with improved strength and fatigue limit, and a method for manufacturing the same, and more particularly, to a wire rod and a steel wire for a spring and a spring having an ultra-high strength at a level of 2,200 MPa and excellent workability, allowing easy nitriding at a high temperature, and having improved nitriding properties and fatigue limit, and a method for manufacturing the same.
• springs used in transmissions and engine valves of vehicles are also required to have high strength.
• wire diameter decreases to increase sensitivity to inclusions, thereby reducing fatigue limit. That is, there are limits to increase fatigue limit by increasing strength.
• spring manufacturers attempted to increase fatigue limit of materials for springs by increasing surface hardness while maintaining strength by nitriding.
• nitriding for other parts is generally performed at a temperature above 500°C
• nitriding for steels for springs is performed at a temperature of 420 to 460°C to prevent a decrease in strength and for a long time over 10 hours to obtain a sufficient nitrogen penetration depth.
• a tempering heat treatment temperature of common steels for springs is 450°C is below, heat treatment for a long time at a temperature of 420 to 450°C may decrease strength of most of the springs, and thus a highly alloying material including an element capable of improving softening resistance by forming a carbide should be used.
• a carbide-forming element such as Mo and V
• a decrease in strength may be inhibited during nitriding, but a low-temperature structure may be formed by central segregated region and a problem of decreasing a reduction of area may be caused.
• PAGS prior austenite grain size
• spring manufacturers require to shorten a processing time of nitriding by performing nitriding at a temperature as high as possible to shorten a nitriding time and also requires a high strength wire rod not causing a problem in productivity in the field.
• Patent Document 0001 Korean Patent Laid-open Publication No. 10-2000-0043776 (Published on July 15, 2000 )
• a wire rod, a steel wire, and a spring each having excellent strength and workability, allowing easy nitriding at a high temperature, and having improved nitriding properties and fatigue limit, and a method for manufacturing the same.
• a wire rod for a spring with improved strength and fatigue limit includes, in percent by weight (wt%), 0.6 to 0.7% of C, 2.0 to 2.5% of Si, 0.2 to 0.7% of Mn, 0.9 to 1.5% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.25% or less of Mo, 0.25% or less of W, 0.05% to 0.2% of V, 0.05% or less of Nb, and the balance of Fe and unavoidable impurities, wherein Mn+Cr ⁇ 1.8% is satisfied, 0.05 at% ⁇ Mo+W ⁇ 0.15 at% is satisfied, a proportion (wt%) of an area satisfying one or more of C > 0.85%, Si > 3.0%, Mn > 0.8%, and Cr > 2.0% is 10% or less within an area of 1 mm 2 of a central region of a cross-section perpendicular to a lengthwise direction.
• the wire rod may include, in an area fraction, 80% or more of a pearlite structure and the balance of a bainite structure or a martensite structure.
• a prior austenite average grain size may be 20 ⁇ m or less.
• the number of a carbonitride having a maximum diameter of 15 ⁇ m or more distributed in a cross-section parallel to a lengthwise direction within a surface depth of 1 mm may be less than 2 per cm 2 .
• a tensile strength may be 1,400 MPa or less, and a reduction of area may be 35% or more.
• a method for manufacturing a wire rod for a spring with improved strength and fatigue limit includes: preparing a bloom by continuously casting a molten steel including, in percent by weight (wt%), 0.6 to 0.7% of C, 2.0 to 2.5% of Si, 0.2 to 0.7% of Mn, 0.9 to 1.5% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.25% or less of Mo, 0.25% or less of W, 0.05% to 0.2% of V, 0.05% or less of Nb, and the balance of Fe and unavoidable impurities; heating the bloom at a temperature of 1,200°C or above and rolling the bloom to prepare a billet; heating the billet at a temperature of 1,030°C or above and rolling the billet at a temperature of 1,000°C below to prepare a wire rod; coiling the rolled wire rod at a temperature of 800 to 900°C; and cooling the coiled wire rod
• the continuously casting process may include performing soft reduction with a total rolling reduction 20 mm or more.
• the soft reduction may be performed to allow each roll to roll by reducing 4 mm or less and may have a cumulative rolling reduction of 60% or more at a solidification fraction of 0.6 or more.
• a steel wire for a spring with improved strength and fatigue limit includes, in percent by weight (wt%), 0.6 to 0.7% of C, 2.0 to 2.5% of Si, 0.2 to 0.7% of Mn, 0.9 to 1.5% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.25% or less of Mo, 0.25% or less of W, 0.05% to 0.2% of V, 0.05% or less of Nb, and the balance of Fe and unavoidable impurities, wherein Mn+Cr ⁇ 1.8% is satisfied, 0.05 at% ⁇ Mo+W ⁇ 0.15 at% is satisfied, and the steel wire includes, in an area fraction, 85% or more of a tempered martensite structure and the balance of an austenite structure.
• a prior austenite average grain size may be 15 ⁇ m or less.
• the number of a carbonitride having a maximum diameter of 15 ⁇ m or more distributed in a cross-section parallel to the lengthwise direction within a surface depth of 1 mm may be less than 2 per cm 2 .
• the number of carbides in an area of 100 ⁇ m 2 may be from 10 to 50, the maximum diameter of the carbide may be from 5 to 50 nm, and a content of V or Nb may be 10 at% or more.2
• a tensile strength may be 2,100 MPa or more, and a reduction of area may be 45% or more.
• a method for manufacturing a steel wire for a spring with improved strength and fatigue limit includes: performing LA heat treatment on the wire rod; drawing the LP heat-treated wire rod to prepare a steel wire; and performing QT heat treatment on the steel wire, wherein the LP heat treatment includes: a primary austenizing process of heating to a temperature of 950 to 1100°C within 3 minutes and maintaining for 3 minutes or less; and a process of passing the primarily austenized wire rod through a Pb bath at a temperature of 650 to 700°C within 3 minutes.
• a pearlite transformation completion time may be less than 130 seconds.
• the method may further include performing LA heat treatment on the wire rod before the LP heat treatment, wherein the LA heat treatment may further include performing heat treatment at a temperature of 650 to 750°C; and performing acid pickling.
• the QT heat treatment may include a secondary austenizing process of heating to a temperature of 900 to 1000°C within 3 minutes and maintaining for 3 minutes or less; and a primary oil quenching process performed at 70°C or below; a tempering process of heating to a temperature of 450 to 550°C within 3 minutes and maintaining for 3 minutes or less; and a secondary oil quenching process performed at 70°C or below.
• a spring with improved strength and fatigue limit may include, in percent by weight (wt%), 0.6 to 0.7% of C, 2.0 to 2.5% of Si, 0.2 to 0.7% of Mn, 0.9 to 1.5% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.25% or less of Mo, 0.25% or less of W, 0.05% to 0.2% of V, 0.05% or less of Nb, and the balance of Fe and unavoidable impurities, wherein Mn+Cr ⁇ 1.8% and 0.05 at% ⁇ Mo+W ⁇ 0.15 at% are satisfied, and a fatigue limit which withstands repeated stress more than 10 million times is 700 MPa or more.
• a method for manufacturing a spring with improved strength and fatigue limit includes: cold forming the steel wire according to an embodiment of the present disclosure in a spring form; performing stress-relieving heat treatment on the formed spring; and nitriding the resultant.
• the fatigue limit may increase by 10% or more after nitriding.
• a wire rod, a steel wire, and a spring capable of inhibiting formation of a low-temperature structure at a central region by reducing central segregation, and obtaining an excellent reduction of area and a tensile strength of 2,200 MPa or more, and a method for manufacturing the same.
• a wire rod, steel wire, and a spring having improved nitriding properties and fatigue limit by controlling the grain size and the number of precipitates, and a method for manufacturing the same.
• a wire rod for a spring with improved strength and fatigue limit includes, in percent by weight (wt%), 0.6 to 0.7% of C, 2.0 to 2.5% of Si, 0.2 to 0.7% of Mn, 0.9 to 1.5% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.25% or less of Mo, 0.25% or less of W, 0.05% to 0.2% of V, 0.05% or less of Nb, and the balance of Fe and unavoidable impurities, wherein Mn+Cr ⁇ 1.8% and 0.05 at% ⁇ Mo+W ⁇ 0.15 at% are satisfied, and a proportion (wt%) of an area satisfying one or more of C > 0.85%, Si > 3.0%, Mn > 0.8%, and Cr > 2.0% is 10% or less within an area of 1 mm 2 of a central region of a cross-section perpendicular to a lengthwise direction.
• a wire rod for a spring with improved strength and fatigue limit includes, in percent by weight (wt%), 0.6 to 0.7% of C, 2.0 to 2.5% of Si, 0.2 to 0.7% of Mn, 0.9 to 1.5% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.25% or less of Mo, 0.25% or less of W, 0.05% to 0.2% of V, 0.05% or less of Nb, and the balance of Fe and unavoidable impurities.
• a content of C is from 0.6 to 0.7%.
• C is an element increasing strength of a material and may be added in an amount of 0.6% or more to obtain sufficient strength of the material.
• an excess of C may cause a significant deterioration in impact properties after quenching & tempering (QT) heat treatment and an increase in the possibility of formation of a low-temperature structure during a manufacturing process of wire rods, thereby deteriorating the quality of the wire rods.
• QT quenching & tempering
• the C content is excessive, a heat treatment time of LP heat treatment, one of steel wire-manufacturing processes, significantly increases to reduce productivity.
• an upper limit of the C content may be controlled to 0.7%.
• a content of Si is from 2.0 to 2.5%.
• Si used for deoxidization of steels is also effective for obtaining strength by solid solution strengthening, and may be added in an amount of 2.0% or more to inhibit a decrease in strength during nitriding and to improve deformation resistance of a spring.
• an excess of Si may cause surface decarburization and deterioration of workability of a material.
• an upper limit of the Si content may be controlled to 2.5%.
• a content of Mn is from 0.2 to 0.7%.
• Mn as a hardenability-enhancing element, may be added in an amount of 0.2% or more to obtain hardenability of a material, form a high strength tempered martensite structure, and make S harmless by fixing S.
• an excess of Mn may cause deterioration of quality due to segregation.
• an upper limit of the Mn content may be controlled to 0.7%.
• a content of Cr is from 0.9 to 1.5%.
• Cr is a hardenability-enhancing element together with Mn and may be added in an amount of 0.9% or more to enhance softening resistance of a steel.
• an excess of Cr may cause a significant decrease in toughness of a steel wire and promote formation of a low-temperature structure while cooling a wire rod.
• an upper limit of the Cr content may be controlled to 1.5%.
• a content of P is 0.015% or less.
• P is an element segregated in grain boundaries resulting in deterioration of toughness and deterioration of hydrogen delayed fracture resistance of materials, and thus it is desirable to remove P from steel materials.
• an upper limit of the P content may be controlled to 0.015%.
• a content of S is 0.01% or less.
• S may be segregated in grain boundaries resulting in deterioration of toughness and deterioration of hydrogen delayed fracture resistance of materials by forming MnS.
• an upper limit of the S content may be controlled to 0.01%.
• a content of Al is 0.01% or less.
• Al as a powerful deoxidizing element, increases purity by removing oxygen from a steel, Al 2 O 3 inclusions may be formed thereby, resulting in a decrease in fatigue resistance.
• an upper limit of the Al content may be controlled to 0.01%.
• a content of N is 0.01% or less.
• N is an impurity
• N binds to Al or V to form crude AlN or VN precipitates that do not melt during heat treatment.
• an upper limit of the N content may be controlled to 0.01%.
• a content of Mo is 0.25% or less.
• Mo is an element improving softening resistance and forming a carbide with V to improve strength during temperature.
• Mo forms a MC carbide and maintain strength of a material even after a heat treatment for a long time.
• an excess of Mo inhibits formation of a pearlite structure, and thus quality of the wire rod may deteriorate due to formation of a low-temperature structure after rolling the wire rod.
• an excess of Mo inhibits perlite transformation during LP heat treatment before drawing to increase a pearlite transformation time, resulting in a significant decrease in productivity.
• an upper limit of the Mo content may be controlled to 0.25%.
• a content of W is 0.25% or less.
• W as an element improving softening resistance together with Mo among the materials for nitriding, forms a MC carbide to maintain strength of a material even after heat treatment for a long time.
• an excess of W may inhibit formation of pearlite and promote formation of a low-temperature structure in the wire rod.
• an upper limit of the W content may be controlled to 0.25%.
• a content of V is from 0.05 to 0.2%.
• V as an element improving softening resistance, together with Mo, among the materials for nitriding, forms a carbide to increase strength during tempering and may maintain strength even after nitriding is performed for a long time.
• V unlike Mo and W, has a high solid solution temperature of a carbide serving to maintain a prior austenite grain size.
• a thermostatic transformation time may be reduced during LP heat treatment, and productivity may be improved during a steel wire manufacturing process, and thus V may be added in an amount of 0.05% or more.
• the V content is excessive, a crude carbonitride may be formed during a wire rod producing process, and temperature should be raised by heating while rolling the wire rod. In consideration thereof, an upper limit of the V content may be controlled to 0.2%.
• a content of Nb is 0.05% or less.
• Nb as a carbonitride-forming element, has a higher solid solution temperature than that of V to have superior effects on controlling the prior austenite grain size to V.
• an upper limit of the Nb content may be controlled to 0.05%, and the addition of Nb may be omitted in the case where the prior austenite grain size is controlled during the manufacturing process.
• the remaining component of the composition of the present disclosure is iron (Fe).
• the composition may include unintended impurities inevitably incorporated from raw materials or surrounding environments, and thus addition of other alloy components is not excluded.
• the impurities are not specifically mentioned in the present disclosure, as they are known to any person skilled in the art of manufacturing.
• the wire rod with improved strength and fatigue limit according to an embodiment of the present disclosure may satisfy, in percent by weight (wt%), Mn+Cr ⁇ 1.8%.
• a low-temperature structure such as bainite or martensite may be formed during a process of cooling the wire rod, and a pearlite transformation completion time may increase during LP heat treatment.
• carbon equivalent (Ceq) significantly increases to limit the amounts of W and Mo, and thus a decrease in strength of a material may be prevented during nitriding.
• the pearlite transformation time increases failing to obtain a complete pearlite structure during the process of cooling the wire rod and the LP heat treatment time increases to cause a decrease in productivity.
• the wire rod with improved strength and fatigue limit according to an embodiment of the present disclosure may satisfy 0.05 at% ⁇ Mo+W ⁇ 0.15 at%.
• at% refers to atomic weight percent.
• the pearlite transformation completion time during the lead patenting (LP) heat treatment may be less than 130 seconds.
• the LP heat treatment process may include a process of heating at a temperature of 950 to 1100°C and rapidly cooling to a temperature of 650 to 750°C. If the pearlite transformation completion time exceeds 130 seconds during the LP heat treatment, a problem of decreasing productivity may occur.
• the wire rod with improved strength and fatigue limit according to an embodiment of the present disclosure may include a pearlite structure in an area fraction of 80% or more.
• the wire rod with improved strength and fatigue limit may have a prior austenite average grain size of 20 ⁇ m or less.
• the prior austenite average grain size exceeds 20 ⁇ m, the time for the LP heat treatment process increases and a problem of deteriorating workability of the wire rod may occur.
• a proportion (wt%) of an area satisfying one or more of C > 0.85%, Si > 3.0%, Mn > 0.8%, and Cr > 2.0% may be 10% or less within an area of 1 mm 2 of a central region of a cross-section perpendicular to a lengthwise direction.
• the number of carbonitrides having a maximum diameter of 15 ⁇ m or more distributed in a cross-section parallel to the lengthwise direction within a surface depth of 1 mm may be less than 2 per cm 2 .
• the number of carbonitrides having a maximum diameter of 15 ⁇ m or more existing in a cross-section parallel to the lengthwise direction within a surface depth of 1 mm may be less than 2 per cm 2 .
• the wire rod with improved strength and fatigue limit according to an embodiment of the present disclosure may have a tensile strength of 1,400 MPa or less and a reduction of area (RA) of 35% or more.
• a method for manufacturing a wire rod for a spring with improved strength and fatigue limit includes: preparing a bloom by continuously casting a molten steel including, in percent by weight (wt%), 0.6 to 0.7% of C, 2.0 to 2.5% of Si, 0.2 to 0.7% of Mn, 0.9 to 1.5% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.25% or less of Mo, 0.25% or less of W, 0.05% to 0.2% of V, 0.05% or less of Nb, and the balance of Fe and unavoidable impurities; heating the bloom at a temperature of 1,200°C or above and rolling the bloom to prepare a billet; heating the billet at a temperature of 1,030°C or above and rolling the billet at a temperature of 1,000°C below to prepare a wire rod; coiling the rolled wire rod at a temperature of 800 to 900°C; and cooling the coiled wire rod at a seed
• the continuously casting process may include performing soft reduction with a total rolling reduction 20 mm or more.
• a method of casting a slab having a unsolidified layer in the final solidification stage in a continuous casting machine while gradually compressing the slab with a total rolling reduction and at a compressing rate approximately corresponding to a sum of the amount of solidification shrinkage and the amount of thermal shrinkage by passing the slab through a collection of reduction rolls is referred to as soft reduction.
• the total rolling reduction refers to an amount of rolling reduction from the start to the end of the compression.
• the total rolling reduction is less than 20 mm, it is difficult to obtain a segregation-removing effect by soft reduction, and thus the total rolling reduction of the soft reduction may be controlled to 20 mm or more to minimize segregation of the wire rod.
• the soft reduction may be performed such that each roll reduces 4 mm or less, and a cumulative rolling reduction is 60% or more at a solidification fraction of 0.6 or more.
• the solidification fraction refers to a ratio of a weight of sold-phase molten steel to a total weight of the entire molten steel.
• a Mold Electro Magnetic Stirrer (Mold-EMS) and a Strand-EMS may be set according to conditions for conventional springs or arbitrarily set depending on equipment.
• the internal carbonitride may be minimized by heating the prepared bloom at a temperature of 1,200°C or above and rolling the heated blood to a billet.
• the billet may be heat-treated at a temperature of 1,030°C or above and rolled at a temperature of 1,000°C or below to prepare a wire rod.
• the component V in the material does not sufficiently melt failing to form a solid solution of the carbide, thereby causing a problem of deterioration in softening resistance in a final product.
• the rolling of the billet to a wire rod may be performed at a temperature of 1000°C or below to perform the coiling at a temperature of 900°C or below.
• the rolled wire rod may be coiled at a temperature of 800 to 900°C.
• the process of coiling the rolled wire rod may be performed at a temperature of 800 to 900°C.
• the coiled wire rod may be cooled at a rate of 0.5 to 2°C/s.
• steels for spring for nitriding include a lot of highly alloying elements, and thus it is necessary to inhibit formation of a low-temperature structure. If the coiled wire rod is cooled at a rate less than 0.5°C/s, decarburization may occur. On the contrary, if the cooling speed exceeds 2°C/s, a material may break by a low-temperature structure.
• a steel wire for a spring with improved strength and fatigue limit may include, in percent by weight (wt%), 0.6 to 0.7% of C, 2.0 to 2.5% of Si, 0.2 to 0.7% of Mn, 0.9 to 1.5% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.25% or less of Mo, 0.25% or less of W, 0.05% to 0.2% of V, 0.05% or less of Nb, and the balance of Fe and unavoidable impurities.
• the steel wire for a spring with improved strength and fatigue limit according to an embodiment of the present disclosure may satisfy Mn+Cr ⁇ 1.8%.
• the steel wire for a spring with improved strength and fatigue limit according to an embodiment of the present disclosure may satisfy 0.05 at% ⁇ Mo+W ⁇ 0.15 at%.
• the steel wire for a spring with improved strength and fatigue limit may include, in area fraction, 85% or more of a tempered martensite structure and the balance of an austenite structure.
• the steel wire for a spring with improved strength and fatigue limit may have a prior austenite average grain size of 15 ⁇ m or less.
• a proportion (wt%) of an area satisfying one or more of C > 0.85%, Si > 3.0%, Mn > 0.8%, and Cr > 2.0% may be 10% or less within an area of 1 mm 2 of a central region of a cross-section perpendicular to a lengthwise direction.
• the above-described proportion (wt%) of the area exceeds 10%, deterioration of quality of a material such as formation of a low-temperature structure due to central segregation may be caused, and deterioration of workability may be caused thereby increasing a frequency of breakage while processing a spring.
• the above-described proportion (wt%) of the area exceeds 10%, the carbide effect may decrease due to concentration of carbide forming elements at the center.
• the number of carbonitrides having a maximum diameter of 15 ⁇ m or more distributed in a cross-section parallel to the lengthwise direction within a surface depth of 1 mm may be less than 2 per cm 2 .
• the number of carbonitrides having a maximum diameter of 15 ⁇ m or more may be less than 2 per cm 2 in a cross-section parallel to the lengthwise direction within a surface depth of 1 mm.
• the number of carbides within an area of 100 ⁇ m 2 may be from 10 to 50, the maximum diameter of the carbides may be from 5 to 50 nm, and the content of V or Nb may be 10 at% or more.
• the number of carbides having a maximum diameter of 5 to 50 nm is less than 10, it is difficult to control the prior austenite grain size. On the contrary, if the number of carbides having a maximum diameter of 5 to 50 nm is greater than 50, the carbides with 5 nm or less used for precipitation hardening decreases, thereby decreasing a tensile strength of the steel wire.
• the steel wire for a spring with improved strength and fatigue limit may have a tensile strength of 2,100 MPa or more and a reduction of area (RA) of 45% or more.
• a method for manufacturing a steel wire for a spring according to an embodiment of the present disclosure includes: performing LA heat treatment on the wire rod according to an embodiment of the present disclosure; performing LP heat treatment; and drawing the wire rod to prepare a steel wire; and performing QT heat treatment on the steel wire.
• the wire rod according to an embodiment of the present disclosure may be subjected to a low temperature annealing (LA) at a temperature of 650 to 750°C.
• LA low temperature annealing
• the carbide coarsens making it difficult to control the carbide during a subsequent process and thus the LA heat treatment may be performed within 2 hours.
• the strength of the wire rod may decrease to 1,200 MPa or less. If required, the LA heat treatment process may be omitted.
• the LA heat-treated wire rod is acid-pickled and lead patenting (LP) heat treatment may be performed.
• the LP heat treatment may include a primary austenizing process of heating to a temperature of 950 to 1 100°C within 3 minutes and maintaining for 3 minutes or less; and a process of passing the primarily austenized wire rod through a Pb bath at a temperature of 650 to 700°C within 3 minutes.
• an austenite structure may be obtained and the carbide coarsened in the LA process may form a solid solution again.
• the primarily austenized wire rod may be isothermally transformed via rapid cooling by passing through a Pb bath at a temperature of 650 to 750°C within 3 minutes, and a pearlite structure may be obtained. If the Pb bath temperature is below 650°C, a low-temperature structure may be formed. On the contrary, if the Pb bath temperature is above 750°C, the carbide coarsens and strength may decrease.
• LP heat-treated wire rod may be drawn to prepare a steel wire.
• the prepared steel wire may have a wire diameter of 5 mm.
• the LP heat treatment may be performed again to control the wire diameter of the steel wire to 2 mm or less.
• the prepared steel wire may be subjected to QT heat treatment process to obtain a tempered martensite structure.
• the QT heat treatment may include a secondary austenizing process of heating to a temperature of 900 to 1000°C within 3 minutes and maintaining for 3 minutes or less; and a primary oil quenching process performed at 70°C or below; a tempering process of heating to a temperature of 450 to 550°C within 3 minutes and maintaining for 3 minutes or less; and a secondary oil quenching process performed at 70°C or below.
• the austenizing temperature may be from 900 to 1000°C such that the fine carbides precipitated during the LP heat treatment are maintained.
• the austenizing process may be performed for 6 minutes or less in the QT heat treatment.
• the tempering temperature is below 450°C in the QT heat treatment, the nitriding temperature is lowered, formation of additional carbides cannot be induced, and toughness may deteriorate. On the contrary, if the tempering temperature exceeds 550°C in the QT heat treatment, a sufficient strength cannot be obtained.
• a spring with improved strength and fatigue limit includes, in percent by weight (wt%), 0.6 to 0.7% of C, 2.0 to 2.5% of Si, 0.2 to 0.7% of Mn, 0.9 to 1.5% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.25% or less of Mo, 0.25% or less of W, 0.05% to 0.2% of V, 0.05% or less of Nb, and the balance of Fe and unavoidable impurities, satisfies Mn+Cr ⁇ 1.8%, and satisfies 0.05 at% ⁇ Mo+W ⁇ 0.15 at%.
• a fatigue limit increases by 10% or more after nitriding.
• the fatigue limit refers to a limit withstanding repeated loads more than 10 million times during a fatigue test after designing a spring.
• the spring according to an embodiment of the present disclosure may have a fatigue limit of 700 MPa or more which withstands repeated stress more than 10 million times.
• a strength change before and after nitriding is 15% or less, and a nitriding temperature may be 430°C or above.
• a method for manufacturing a spring with improved strength and fatigue limit according to an embodiment of the present disclosure includes: cold forming the steel wire according to an embodiment of the present disclosure in a spring form; performing stress-relieving heat treatment on the formed spring; and nitriding the resultant.
• the fatigue limit of the steel wire according to an embodiment of the present disclosure may be improved by performing nitriding before shot peening in the spring-manufacturing process.
• nitriding temperature if the nitriding temperature is too low, nitrogen cannot appropriately penetrate into the surface. If the nitriding temperature is too high, hardness of the central region of the material decreases and a desired strength of the material cannot be obtained.
• the nitriding process may be performed at a temperature of 420 to 450°C for 10 hours or more.
• Steel materials including various compositions of alloying elements shown in Table 1 below were continuously cast with a total soft reduction of 10 to 25 mm to prepare blooms.
• the prepared blooms were subjected to heat treatment at a temperature of 1,200°C for homogenization and heat treatment at a temperature of 1050°C, and then hot rolled to a final wire diameter of 6.5 mm while cooling to 850°C to prepare wire rods having a final wire diameter of 6.5 mm. Then, the hot-rolled wire rods were coiled at a temperature of 800 to 900°C and cooled at a rate of 1°C/s.
• Table 2 below shows at% contents of W+Mo and total soft reduction of the examples and comparative examples.
• the segregation areas of Table 2 below were derived by analyzing 1 mm 2 of a central region of a cross-section perpendicular to a lengthwise direction of the prepared wire rod.
• the 'C segregation area' of Table 2 refers to a proportion of an area satisfying C > 0.85 wt% within an area of 1 mm 2 of a central region of a cross-section perpendicular to a lengthwise direction.
• the 'Si segregation area' refers to a proportion of an area satisfying Si > 3.0 wt% within an area of 1 mm 2 of a central region of a cross-section perpendicular to a lengthwise direction.
• the 'Mn segregation area' refers to a proportion of an area satisfying Mn > 0.8 wt% within an area of 1 mm 2 of a central region of a cross-section perpendicular to a lengthwise direction.
• the 'Cr segregation area' refers to a proportion of an area satisfying Cr > 2.0 wt% within an area of 1 mm 2 of a central region of a cross-section perpendicular to a lengthwise direction.
• the segregation area was measured by using an Electron Probe X-ray Micro Analyzer, EPMA (Model No. E MPA-1600).
• Example 1 0.11 25 mm ⁇ 1% 2.5 ⁇ 1% 2.5 ⁇ 7%
• Example 2 0.08 25 mm ⁇ 1% 4.3 ⁇ 1% 2.3 ⁇ 8.6%
• Comparative Example 1 0.11 10 mm 5.5 11.2 3.1% 10.2 30%
• Comparative Example 2 0.08 25 mm ⁇ 1% 4.5 ⁇ 1% 2.2 ⁇ 8.7%
• Comparative Example 3 0.17 25 mm ⁇ 1% 3.2 ⁇ 1% 3.4 ⁇ 8.6%
• Comparative Example 4 0.11 25 mm ⁇ 1% 4.2 ⁇ 1% 2.4 ⁇ 8.6%
• Table 3 below shows tensile strength, reduction of area (RA), central low-temperature structure, prior austenite average grain size, pearlite structure, and the number of carbonitrides of the prepared wire rods.
• the prior austenite average grain size, the pearlite structure, and the number of carbonitrides were measured by using a scanning electron microscope (SEM) (Model No. JEOL, JSM-6610LV).
• the 'O' of Table 3 indicates a case in which an area fraction of the low-temperature structure exceeded 20%, and the 'X' indicates a case in which an area fraction of the low-temperature structure is not more than 20%.
• the pearlite structure of Table 3 below refers to the number of samples in which an area fraction of the pearlite structure was 80% or more in a microstructure of a cross-section perpendicular to the lengthwise direction of each sample.
• Examples 1 and 2 a low-temperature structure was not formed in central areas, and the prior austenite average grain size was not more than 20 ⁇ m.
• 6 or more samples exhibited 80% or more of the pearlite structure according to Examples 1 and 2, and the tensile strength was not more than 1400 MPa indicating excellent workability.
• a carbonitride was not formed on the surfaces in Examples 1 and 2.
• Comparative Example 1 the tensile strength exceeded 1400 MPa, the reduction of area less than 35% exhibited inferior workability, and the low-temperature structure was formed in the central area.
• Comparative Example 1 only 5 samples included 80% or more of the pearlite structure among the 8 samples, and 80% of more of the pearlite structure was not uniformly formed.
• Comparative Example 2 referring to the alloying elements of Table 1, because the V content was less than 0.15%, the prior austenite average grain size was 24 ⁇ m exceeding 20 ⁇ m indicating coarsening of grains.
• Comparative Example 3 because the tensile strength was 1510 MPa and the reduction of area was only 10%, workability was inferior and a low-temperature structure was formed in the central area. In addition, in Comparative Example 3, only 2 samples includes 80% or more of the pearlite structure among the 8 sample, indicating that the pearlite structure was not sufficiently formed.
• the samples of the examples and comparative examples were subjected to LA heat treatment at 720°C for 2 hours and acid pickling, and then LP heat treatment was performed.
• the LP heat treatment was performed by heating to a primary austenizing temperature within 3 minutes, and then proceeded under conditions shown in Table 4 below.
• Table 4 showed pearlite transformation time of LP heat treatment according to the examples and the comparative examples. The pearlite transformation time was measured by deriving a time-temperature-transformation (TTT) curve via a dilatometry experiment.
• TTT time-temperature-transformation
• the pearlite transformation time of Examples 1 and 2 was 110 seconds and 105 seconds, respectively less than 130 seconds, indicating excellent productivity. On the contrary, the pearlite transformation time of Comparative Example 3 was 130 seconds indicating inferior productivity to the extent that field production is difficult.
• the LP heat-treated materials of the examples and comparative examples were drawn to prepare steel wires having a wire diameter of 3 mm.
• the prepared steel wires were subjected to a secondary austenizing process and a primary quenching process, and then tempered and subjected to a secondary quenching process to obtain QT steel wires.
• the steel wires were heated to a secondary austenizing temperature within 3 minutes, and the primary and secondary quenching processes were performed in an oil at 60°C. The remaining process was performed under conditions of Table 5 below.
• Table 6 shows tensile strength, reduction of area (RA), and the number of carbides of the prepared QT steel wires.
• the number of carbides refers to the number of carbides having a maximum diameter of 5 to 50 nm and including the content of V or Nb is 10 at% or more in an area of 100 ⁇ m 2 .
• the number of carbides refers to an average of 8 values measured from 8 positions in an area of 100 ⁇ m 2 of the surface of the wire rod by using a transmission electron microscope (TEM) of FEI Tecnai OSIRIS.
• TEM transmission electron microscope
• Examples 1 and 2 exhibited 2200 MPa or more of excellent tensile strengths and 45% or more of reduction of area.
• the number of carbides of Examples 1 and 2 was from 10 to 50.
• Comparative Example 2 On the contrary, the reduction of area of Comparative Example 1 was only 32% and the number of carbides exceeded 50. According to Comparative Example 2, an inferior tensile strength not more than 2200 MPa was obtained, and the number of carbides was less than 10 causing a problem of difficulties in controlling the prior austenite average grain size. Comparative Example 4 exhibited an inferior tensile strength of 2200 MPa or less and the number of carbides exceeded 50.
• the QT steel wire was cold formed in a spring shape and the formed spring was heat treated and nitrided at a temperature of 420 to 450°C.
• Table 7 shows whether the spring breaks while forming the spring, fatigue limit values, and fatigue limit after nitriding.
• the fatigue limits before and after nitriding were measured under the conditions of a stress ratio R (tensile capacity/compression capacity) of -1 and a test speed of 30 to 60 Hz.
• the samples of Examples 1 and 2 did not break due to excellent workability and had excellent fatigue limits over 650 MPa before nitriding. In addition, the samples of Examples 1 and 2 had fatigue limits over 750 MPa after nitriding. Since the fatigue limit after nitriding was higher than that before nitriding by 10% or more, excellent nitriding properties were obtained.
• Comparative Examples 1 and 2 exhibited breakage due to inferior workability, and the fatigue limit after nitriding increased by less than 10% compared to that before nitriding.
• the composition of alloying elements and conditions of the manufacturing process by optimizing the composition of alloying elements and conditions of the manufacturing process, excellent tensile strength and reduction of area may be obtained and also nitriding properties and fatigue limit may be improved, and thus the spring may be applicable as a material of transmissions and engine valves of vehicles.
• a wire rod and a steel wire for a spring and a spring with improved strength and fatigue limit and a method for manufacturing the same may be provided.

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