Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
  • C21D8/065—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires of ferrous alloys
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
  • B21B1/16—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling wire rods, bars, merchant bars, rounds wire or material of like small cross-section
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
• • 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
  • 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
• • 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
  • 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
  • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
  • C21D1/22—Martempering
• • 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
  • 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")
• • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/34—Methods of heating
  • C21D1/42—Induction heating
• • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/56—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
  • C21D1/60—Aqueous agents
• • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/002—Heat treatment of ferrous alloys containing Cr
• • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
• • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/008—Heat treatment of ferrous alloys containing Si
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/02—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for springs
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
  • C21D9/525—Heat 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
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—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
  • 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
  • C21D2211/002—Bainite
• • 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
  • C21D2211/008—Martensite

Definitions

• the present disclosure relates to a wire rod for an ultra-high strength spring, a steel wire and a manufacturing method thereof, and more particularly, to a wire rod for an ultra-high strength spring having excellent processibility, a steel wire and a manufacturing method thereof.
• suspensions of motorcycles are smaller than those of automobiles, springs for motorcycles require relatively higher processibility while processing the springs.
• suspension springs for motorcycles have relatively smaller diameters, it is difficult to control decarburization and a low-temperature structure. Therefore, there is a need to develop new high-strength suspension springs applicable to suspensions of motorcycles.
• a wire rod for an ultra-high strength spring having excellent processibility, a steel wire and a manufacturing method thereof.
• a wire rod for an ultra-high strength spring including, in percent by weight (wt%), 0.55 to 0.65% of C, 0.5 to 0.9% of Si, 0.3 to 0.8% of Mn, 0.3 to 0.6% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.005% or less of N, more than 0% and 0.04% or less of Nb, and the remainder being Fe and unavoidable impurities, and satisfying a value of Formula (1) below being 0.77 or more and 0.83 or less: C + 1 / 6 * Mn + 1 / 5 * Cr + 1 / 24 * Si wherein in Formula (1), C, Mn, Cr, and Si represent the content (wt%) of each element.
• a sum of area fractions of bainite and martensite having a hardness may be 400 Hv or more is 1% or less.
• a ferrite decarburized layer may have a thickness of 1 ⁇ m or less.
• an average grain size of ferrite may be 10 ⁇ m or less.
• an Nb-based carbide having a size of 20 nm or less may be distributed at a density of 1000 grains/mm 2 or more.
• a tensile strength may be 1200 MPa or less.
• a method for manufacturing a wire rod for an ultra-high strength spring including: homogenization heat-treating an ingot including, in percent by weight (wt%), 0.55 to 0.65% of C, 0.5 to 0.9% of Si, 0.3 to 0.8% of Mn, 0.3 to 0.6% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.005% or less of N, more than 0% and 0.04% or less of Nb, and the remainder being Fe and unavoidable impurities, and satisfying a value of Formula (1) below being 0.77 or more and 0.83 or less, at a heating temperature of 900 to 1100°C within 180 minutes; wire rod rolling the ingot at a finish rolling temperature of 730 to Ae3°C; and cooling the wire rod at a cooling rate of 3°C/s or less: C + 1 / 6 * Mn + 1 / 5 * Cr + 1 / 24 * Si where
• a strain in the wire rod rolling may be from 0.3 to 2.0.
• an average grain size of austenite before finish rolling in the wire rod rolling may be from 5 to 15 ⁇ m.
• a steel wire for an ultra-high strength spring including, in percent by weight (wt%), 0.55 to 0.65% of C, 0.5 to 0.9% of Si, 0.3 to 0.8% of Mn, 0.3 to 0.6% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.005% or less of N, more than 0% and 0.04% or less of Nb, and the remainder being Fe and unavoidable impurities, and satisfying a value of Formula (1) below being 0.77 or more and 0.83 or less, wherein the steel wire includes a tempered martensite in an area fraction of 90% or more: 0.77 ⁇ C + 1 / 6 * Mn + 1 / 5 * Cr + 1 / 24 * Si ⁇ 0.83 wherein in Formula (1), C, Mn, Cr, and Si represent the content (wt%) of each element.
• an Nb-based carbide having a size of 20 nm or less may be distributed at a density of 1000 grains/mm 2 or more.
• an average grain size of spherical austenite may be 10 ⁇ m or less.
• a wire diameter may be 15 mm or less.
• a strength may be 1700 MPa or more.
• a reduction in area may be 35% or more.
• a method for manufacturing a steel wire for an ultra-high strength spring including: drawing a wire rode including, in percent by weight (wt%), 0.55 to 0.65% of C, 0.5 to 0.9% of Si, 0.3 to 0.8% of Mn, 0.3 to 0.6% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.005% or less of N, more than 0% and 0.04% or less of Nb, and the remainder being Fe and unavoidable impurities, and satisfying a value of Formula (1) below being 0.77 or more and 0.83 or less, heating the wire rod at a temperature of 900 to 1000°C, water quenching the wire rod at a high pressure, tempering the wire rod at a temperature of 400 to 500°C, and water quenching the wire rod: C + 1 / 6 * Mn + 1 / 5 * Cr + 1 / 24 * Si wherein in Formula (1), C, M
• the heating step may include heating the wire rod to a temperature of 900 to 1000°C within 10 seconds and maintaining the temperature for 5 to 60 seconds.
• an average grain size of spherical austenite after the heating step may be 10 ⁇ m or less.
• the tempering step may include heating the wire rod to a temperature of 400 to 500°C within 10 seconds and maintaining the temperature within 30 seconds.
• a wire rod for an ultra-high strength spring in which surface decarburization and formation of a low-temperature structure are inhibited by using an alloy composition having a low C eq and a low Si content, may be provided.
• a wire rod for an ultra-high strength spring in which grain size is reduced by using an Nb-based carbide and controlling rolling may be provided.
• a steel wire for an ultra-high strength spring according to the present disclosure has a small wire diameter of 15 mm or less which is suitable for suspension springs for motorcycles.
• the steel wire for an ultra-high strength spring according to the present disclosure may have a strength of 1700 MPa or more by induction heat treatment and water quenching, although the alloy composition has a low C eq and a low Si content, thereby having an ultra-high strength required for suspension springs of motorcycles.
• the steel wire for an ultra-high strength spring according to the present disclosure may have a high ductility with a reduction in area (RA) of 35% or more by grain refinement, and thus the steel wire may be cold-rolled at room temperature to be manufactured into suspension springs for motorcycles.
• RA reduction in area
• a wire rod for an ultra-high strength spring includes, in percent by weight (wt%), 0.55 to 0.65% of C, 0.5 to 0.9% of Si, 0.3 to 0.8% of Mn, 0.3 to 0.6% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.005% or less of N, more than 0% and 0.04% or less of Nb, and the remainder being Fe and unavoidable impurities, wherein a value of Formula (1) below is 0.77 or more and 0.83 or less: C + 1 / 6 * Mn + 1 / 5 * Cr + 1 / 24 * Si wherein in Formula (1), C, Mn, Cr, and Si represent the content (wt%) of each element.
• the present inventors have found an optimal alloy composition having a low C eq and a low Si content and efficient for inhibiting surface decarburization and formation of a low-temperature structure to provide a wire rod and a steel wire for an ultra-high strength spring having excellent processibility.
• An ultra-high strength spring may be manufactured by cold forming the steel wire disclosed in this specification at room temperature and the steel wire may be manufactured by drawing the wire rod disclosed in this specification.
• the wire rod for an ultra-high strength spring may include, in percent by weight (wt%), 0.55 to 0.65% of C, 0.5 to 0.9% of Si, 0.3 to 0.8% of Mn, 0.3 to 0.6% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.005% or less of N, more than 0% and 0.04% or less of Nb, and the remainder being Fe and unavoidable impurities.
• C is an element added to obtain strength of products.
• the C content is less than 0.55%, a target strength and a low carbon equivalent (Ceq) cannot be obtained. Accordingly, a martensite structure is not completely formed during cooling, and thus it difficult to obtain strength. Even when the martensite structure is formed, it may be difficult to obtain the target strength.
• the C content exceeds 0.65%, impact resistance may deteriorate and quenching cracks may occur during water quenching. Therefore, the C content may be controlled from 0.55 to 0.65 wt%.
• Si is used for deoxidization of steels and is also effective for enhancing strength via solid solution strengthening. Si may be added in an amount of 0.5 wt% or more to obtain strength in the present disclosure. However, an excess of Si may cause surface decarbonization and make it difficult to process materials, and thus an upper limit thereof may be controlled to 0.9 wt% in consideration thereof. As described above, according to the present disclosure, surface decarburization is inhibited and sufficient processibility is obtained using a low Si alloy designed to control the Si content to 0.9 wt% or less.
• Manganese enhances hardenability as an essential element for forming a steel having a high-strength tempered martensite structure.
• manganese may be added in an amount of 0.3 wt% or more in the present disclosure.
• an upper limit of the Mn content may be controlled to 0.8 wt%.
• Chromium is effective for enhancing hardenability together with manganese and enhances corrosion resistance of a steel.
• chromium may be added in an amount of 0.3 wt% or more.
• an upper limit of the Cr content may be controlled to 0.6 wt%.
• an upper limit thereof may be controlled to 0.015 wt%.
• an upper limit of the S content may be controlled to 0.01 wt% in the present disclosure.
• Aluminum as a powerful deoxidizing element, may increase purity by removing oxygen from a steel.
• addition of Al causes formation of Al 2 O 3 , thereby deteriorating fatigue resistance. Therefore, an upper limit of the Al content may be controlled to 0.01 wt%.
• Nitrogen binds to aluminum or vanadium contained in a steel to form coarse AlN or VN precipitates that are not melted during heat treatment. Therefore, an upper limit of the N content may be controlled to 0.005%.
• Niobium as an element binding to carbon contained in a steel to form an Nb-based carbide, decreases grain size, thereby improving processibility.
• the Nb content may be greater than 0 wt% in the present disclosure.
• niobium may be added in an amount of 0.04 wt% or less. More preferably, niobium may be added in an amount of 0.02 wt% or less to improve processibility.
• the Nb-based carbide formed by adding Nb may be distributed in structures of the wire rod and the steel wire for an ultra-high strength spring according to the present disclosure.
• the size of the formed Nb-based carbide may be 20 nm or less. When the size of the formed Nb-based carbide is greater than 20 nm, there is a possibility that processibility may deteriorate.
• it is preferable that the Nb-based carbide is uniformly distributed at a density of 1000 grains/mm 2 or more. When the Nb-based carbide is distributed at a density less than 1000 grains/mm 2 , there may be a possibility that grains are not sufficiently refined.
• Nb may be contained at 10 at% or more.
• 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.
• those impurities in addition to the above-described alloy components are 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.
• a value of Formula (1) 0.77 or more and 0.83 or less
• the C eq value is controlled to inhibit surface decarburization and formation of a low-temperature structure which are easily occurring during cooling after wire rod rolling.
• the C eq value may be represented by Formula (1) below.
• the value of Formula (1) is controlled to 0.77 or more and 0.83 or less to inhibit surface decarburization and formation of a low-temperature structure.
• C, Mn, Cr, and Si represent the content (wt%) of each element.
• the wire rod for an ultra-high strength spring according to the present disclosure is manufactured by homogenization heat-treating an ingot having the above-described alloy composition and satisfying the range of the value of Formula (1), wire rod rolling the ingot, and cooling the wire rod.
• each step of the manufacturing process will be described.
• the homogenization heat treating step may be performed in a heating furnace at a heating temperature of 900 to 1100°C within 180 minutes.
• a finish rolling temperature of the wire rod rolling step may be from 730 to Ae3°C.
• a main structure of the wire rod is transformed from austenite into ferrite.
• a main structure of the wire rod before finish rolling is austenite and a main structure of the wire rod after the finish rolling is ferrite.
• a strain of the wire rod rolling may be from 0.3 to 2.0.
• reduction rate is a value obtained by (A-Ai)/A ⁇ 100 wherein A is an area of a cross-section of a wire rod perpendicular to the longitudinal direction before rolling the wire rod, and A 1 is an area of a cross-section of the wire rod perpendicular to the longitudinal direction after rolling the wire rod.
• the strain is less than 0.3 during the wire rod rolling, it is difficult to obtain sufficient grain refinement.
• the strain exceeds 2.0, a manufacturing process is not appropriately performing due to too much processing amount. Therefore, according to the present disclosure, it is preferable to control the strain from 0.3 to 2.0.
• Grain refinement may be obtained by wire rod rolling under the above-described conditions.
• an average grain size of austenite before finish rolling may be from 5 to 15 ⁇ m.
• an average grain size of ferrite in a final wire rod structure after subsequent finish rolling and cooling processes may also be reduced.
• the cooling step may be performed by cooling the wire rod at a cooling rate of 3°C/s or less.
• the cooling rate exceeds 3°C/s, it is difficult to inhibit formation of the low-temperature structure.
• the wire rod for an ultra-high strength spring including the above-described alloy composition and manufactured by the above-described manufacturing method according to an embodiment may include pearlite and ferrite as microstructures, e.g., 60% or more of pearlite in an area fraction and the remainder of ferrite according to an embodiment.
• the wire rod for an ultra-high strength spring may hardly include a low-temperature structure on the cross-section perpendicular to the longitudinal direction.
• a sum of area fractions of bainite and martensite having a hardness of 400 Hv or more may be 1% or less.
• the low-temperature structure refers to bainite and martensite in the present disclosure.
• the wire rod for an ultra-high strength spring of the present disclosure may have sufficient processibility by inhibiting formation of the low-temperature structure.
• the surface decarburization phenomenon may be inhibited by using the above-described alloy composition having a low C eq and a low Si content and satisfying the range of the value of Formula (1).
• a ferrite decarburized layer of the wire rod may have a thickness of 1 ⁇ m or less.
• ferrite grains may be reduced in size by using the Nb-based carbide and controlling rolling.
• an average grain size of ferrite may be 10 ⁇ m or less.
• the wire rod for an ultra-high strength spring according to the present disclosure may have sufficient processibility by grain refinement.
• the wire rod for an ultra-high strength spring may have a tensile strength is 1200 MPa or less.
• a steel wire for an ultra-high strength spring includes, in percent by weight (wt%), 0.55 to 0.65% of C, 0.5 to 0.9% of Si, 0.3 to 0.8% of Mn, 0.3 to 0.6% of Cr, 0.015% or less of P, 0.01% or less of S, 0.01% or less of Al, 0.005% or less of N, more than 0% and 0.04% or less of Nb, and the remainder being Fe and unavoidable impurities, wherein a value of Formula (1) below is 0.77 or more and 0.83 or less, and the steel wire includes a tempered martensite in an area fraction of 90% or more.
• the steel wire for an ultra-high strength spring according to the present disclosure is manufactured by drawing a wire rod including the above-described alloy composition and satisfying the range of the value of Formula (1), heating the wire rod, water quenching the wire rod at a high pressure, tempering the wire rod, and water quenching the wire rod.
• a wire rod including the above-described alloy composition and satisfying the range of the value of Formula (1) heating the wire rod, water quenching the wire rod at a high pressure, tempering the wire rod, and water quenching the wire rod.
• a target ultra-high strength may be obtained by using the above-described alloy composition having a low C eq and a low Si content and satisfying the range of the value of Formula (1) via induction heat treatment and water quenching while reducing the contents of alloying elements, compared to suspension springs for automobiles.
• the drawing step of the present disclosure may be performed by drawing the wire rod including the above-described alloy composition and satisfying the range of the value of Formula (1) to a wire diameter of 15 mm or less applicable to suspension springs of motorcycles.
• the heating step for QT heat treatment may be performed by heating the drawn steel wire to a quenching temperature of 900 to 1000°C within 10 seconds and maintaining the temperature for 5 to 60 seconds, thereby transforming the structure of the steel wire into austenite.
• a quenching temperature of 900 to 1000°C exceeds 10 seconds, it is difficult to obtain desired physical properties since grains grow.
• the maintaining time is less than 5 seconds, the pearlite structure may not be transformed into austenite.
• the maintaining time exceeds 60 seconds, coarse grains may be formed. Therefore, it is preferable to control the maintaining time from 5 to 60 seconds.
• the average grain size of austenite of the austenized steel wire may be reduced to 10 ⁇ m or less.
• grains of the final steel wire for an ultra-high strength spring that are manufactured by subsequent water quenching at a high pressure, tempering, and water quenching may also be controlled to be fine. Accordingly, the steel wire for an ultra-high strength spring according to the present disclosure has excellent processability due to fine grains and may be manufactured into suspension springs of motorcycles as being cold-formed at room temperature.
• the water quenching step performed at a high pressure is a step of transforming the main structure of the steel wire from austenite into martensite and may be performed at a high pressure enough to removing a boiling film of the austenitized steel wire in the previous step.
• the target strength cannot be obtained due to a low C eq and a low Si content.
• the high pressure enough to remove the boiling film is used during water quenching, the probability of occurrence of quenching cracks increases, and thus it is preferable to perform water quenching at a temperature as high as possible.
• the surface of the steel wire may be sufficiently hardened by rapidly cooling using water in this step after induction heating to the quenching temperature in the above-described heating step.
• the cooling rate according to an embodiment during the water quenching may be 100°C/s or more.
• the tempering step is a step of heating martensite, as a main structure of the water-quenched steel wire, into a tampered martensite.
• the tempering step may be performed by heating the wire rod to a temperature of 400 to 500°C within 10 seconds and maintaining the temperature for 30 seconds.
• the tempering temperature is less than 400°C, toughness cannot be obtained so that processibility deteriorates and the risk of damage to products increases.
• the tempering temperature exceeds 500°C, strength may deteriorate. Therefore, the tempering temperature is controlled to the above-described temperature range.
• the heating to the above-described temperature range is not performed within 10 seconds during tempering, coarse carbides are formed, thereby deteriorating toughness. Thus, it is preferable to rapidly heat within 10 seconds.
• the tempered steel wire is water-quenched to room temperature.
• the steel wire for springs including the above-described alloy composition, satisfying the range of the value of Formula (1), and manufactured under the above-described manufacturing conditions, may include a tempered martensite in an area fraction of 90% or more.
• an Nb-based carbide having a size of 20 nm or less may be distributed at a density of 1000 grains/mm 2 or more.
• an average grain size of spherical austenite may be 10 ⁇ m or less.
• the spherical austenite refers to an austenite structure of the steel wire after the step of heating the drawn steel wire of the present disclosure for QT heat treatment.
• the steel wire for an ultra-high strength spring has a wire diameter of 15 mm or less, which is suitable for a steel wire for suspension springs for motorcycles.
• the steel wire for an ultra-high strength spring may have a strength of 1700 MPa or more, which is an ultra-high strength required for suspension springs of motorcycles.
• the steel wire for an ultra-high strength spring according to an embodiment of the present disclosure may have a reduction in area (RA) of 35% or more, which is high ductility, and thus may be manufactured into suspension spring of motorcycles by cold-rolling at room temperature.
• austenite grains may be reduced in size before finish rolling of the wire rod rolling by adding Nb and thus the reduction in area (RA) may further be increased.
• the steel wire for an ultra-high strength spring according to an embodiment of the present disclosure may have a reduction in area (RA) of 45% or more.
• the results of Table 2 below show physical properties of wire rods prepared according to the above-described process.
• the area fraction of the low-temperature structure of Table 2 indicates a sum of area fractions of bainite and martensite on the cross-section of the wire rod perpendicular to the longitudinal direction.
• the AGS of Table 2 refers to an average grain size of austenite before finish rolling during the wire rod rolling step and was measured according to the ASTM E112 standard.
• the thickness of the ferrite decarburized layer indicates a thickness of a layer formed only of ferrite on the surface of a steel after the wire rod rolling by decarburization, and the thickness of the total decarburized layer is measured a vertical distance from the surface of the decarburized layer to a point where a concentration of carbon is the same as that of carbon of a matrix.
• the wire rod of Table 2 was drawn to a steel wire having a diameter of 10 mm, heated, and water-quenched at a high pressure. After the high-pressure water quenching, the steel wire was tempered and water-quenched to prepare a final steel wire for an ultra-high strength spring.
• the heating temperature in Table 3 indicates a temperature at which the steel wire was heated after drawing, and the tempering temperature indicates a temperature at which the steel wire is tempered after the high-pressure water quenching.
• RA represents a reduction in area.
• the wire rod for an ultra-high strength spring according to the present disclosure may be applicable to suspension springs of various means of transportation such as automobiles and motorcycles or to springs used in various industrial fields.

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• 2019
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• 2020
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