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 invention relates to a wire rod for an ultra-high strength spring, a steel wire, and a method for manufacturing the same, and more particularly, to a wire rod for an ultra-high strength spring having excellent workability, a steel wire, and a method for manufacturing the same.
• TM tissue steel for automobiles has been reviewed, but automobile suspension springs have difficult management standards, are difficult to manufacture, and are expensive, so they are difficult to apply to motorcycle suspension springs. .
• motorcycle suspension springs require higher workability when processing the springs because the suspension size is smaller than that of automobiles.
• motorcycle suspension spring is used with a relatively thin diameter, it is difficult to decarburize and control the low temperature structure. Therefore, there is a need for a new high-strength suspension spring that can be used in a motorcycle suspension.
• the present invention has been devised to solve the above problems, and an object of the present invention is to provide a wire rod for an ultra-high strength spring having excellent workability, a steel wire, and a manufacturing method thereof.
• the wire rod for an ultra-high strength spring is, by weight, C: 0.55 to 0.65%, Si: 0.5 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.3 to 0.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, Nb: more than 0%, 0.04% or less, the remainder including Fe and unavoidable impurities, the following The value of Equation (1) may be 0.77 or more and 0.83 or less.
• C, Mn, Cr, and Si mean the content (wt%) of each element.
• the sum of the area fractions of bainite and martensite having a hardness of 400 Hv or more on a cross section perpendicular to the longitudinal direction may be 1% or less.
• the thickness of the ferrite decarburized layer may be 1 ⁇ m or less.
• the average size of the ferrite grains may be 10 ⁇ m or less.
• each ultra-high strength spring wire 1000 pieces/mm 2 or more of Nb-based carbides having a size of 20 nm or less may be distributed.
• the tensile strength may be 1200 MPa or less.
• the method for manufacturing a wire for an ultra-high strength spring according to an example of the present invention is by weight %, C: 0.55 to 0.65%, Si: 0.5 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.3 to 0.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, Nb: more than 0%, 0.04% or less, remainder Fe and unavoidable impurities Including, homogenizing heat treatment of an ingot having a value of 0.77 or more and 0.83 or less of the following formula (1) within 180 minutes at a heating temperature of 900 to 1100 °C, a finishing rolling temperature of 730 to Ae3 °C, and 3 °C It may include the step of cooling at a cooling rate of °C / s or less.
• C, Mn, Cr, and Si mean the content (wt%) of each element.
• the deformation amount in the wire rod rolling step may be 0.3 to 2.0.
• the average size of the austenite grains before the finish rolling in the wire rod rolling step may be 5 to 15 ⁇ m.
• the steel wire for ultra-high strength spring according to an example of the present invention is, by weight, C: 0.55 to 0.65%, Si: 0.5 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.3 to 0.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, Nb: more than 0%, 0.04% or less, the remainder contains Fe and unavoidable impurities, , the value of the following formula (1) is 0.77 or more and 0.83 or less, and may include 90% or more of tempered martensite as an area fraction.
• C, Mn, Cr, and Si mean the content (wt%) of each element.
• the Nb-based carbide having a size of 20 nm or less may be distributed over 1000 pieces/mm 2 .
• the prior austenite average grain size may be 10 ⁇ m or less.
• the wire diameter may be 15mm or less.
• the strength may be 1700 MPa or more.
• the reduction in section may be 35% or more.
• the method for manufacturing an ultra-high strength spring steel wire according to an example of the present invention is, by weight, C: 0.55 to 0.65%, Si: 0.5 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.3 to 0.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, Nb: more than 0%, 0.04% or less, remainder Fe and unavoidable impurities
• the step of drawing a wire rod having a value of 0.77 or more and 0.83 or less of the following formula (1), heating at 900 to 1000 ° C, water cooling at high pressure, tempering at 400 to 500 ° C, and water cooling may include steps.
• C, Mn, Cr, and Si mean the content (wt%) of each element.
• the heating step may include heating to 900 to 1000° C. within 10 seconds and then maintaining for 5 to 60 seconds.
• the average size of the austenite grains after the heating step may be 10 ⁇ m or less.
• the tempering step may be heated to 400 to 500° C. within 10 seconds and then maintained within 30 seconds.
• a wire rod for an ultra-high strength spring in which surface decarburization and formation of a low-temperature structure are suppressed by using a low C eq and low Si alloy composition.
• the steel wire for ultra-high strength spring according to the present invention has a wire diameter of 15 mm or less and has a small diameter suitable as a steel wire for a motorcycle suspension spring.
• the steel wire for ultra-high strength spring according to the present invention has a low C eq and low Si alloy composition by utilizing induction heating and water cooling, the strength is 1700 MPa or more, and it is possible to secure the ultra-high strength properties required for a motorcycle suspension spring.
• the steel wire for ultra-high-strength spring according to the present invention can secure high ductility with a cross-sectional reduction ratio (RA) of 35% or more through grain refinement, and thus can be cold-formed at room temperature to manufacture a motorcycle suspension spring.
• RA cross-sectional reduction ratio
• the wire rod for ultra-high strength spring is, by weight, C: 0.55 to 0.65%, Si: 0.5 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.3 to 0.6%, P: 0.015% or less , S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, Nb: more than 0%, 0.04% or less, the remainder includes Fe and unavoidable impurities, and the value of the following formula (1) is 0.77 or more , 0.83 or less.
• C, Mn, Cr, and Si mean the content (wt%) of each element.
• the inventors of the present invention have derived an optimal alloy composition of low C eq and low Si, which is easy to suppress surface decarburization and low-temperature structure formation, in order to provide a wire and steel wire for an ultra-high strength spring having excellent workability.
• the ultra-high strength spring may be manufactured by cold forming the steel wire disclosed herein at room temperature, and the steel wire may be manufactured by drawing the wire rod disclosed herein.
• the wire rod for ultra-high strength spring is, by weight, C: 0.55 to 0.65%, Si: 0.5 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.3 to 0.6%, P: 0.015% or less , S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, Nb: more than 0%, 0.04% or less, the remainder may include Fe and unavoidable impurities.
• Carbon is an element added to secure product strength.
• the carbon content is less than 0.55% by weight, the desired strength and low C eq cannot be obtained. Accordingly, when the steel is cooled, the martensitic structure is not completely formed, so it may be difficult to secure strength, and even if an intact martensitic structure is formed, it may be difficult to secure the desired strength.
• the carbon content exceeds 0.65% by weight, impact properties may be deteriorated and quenching cracks may occur during water cooling. Accordingly, according to the present invention, the carbon content can be controlled to 0.55 to 0.65 wt%.
• Silicon is used for deoxidation of steel and is an element advantageous for securing strength through solid solution strengthening.
• silicon may be added in an amount of 0.5 wt% or more.
• the upper limit thereof may be limited to 0.9% by weight in consideration of this because there is a difficulty in processing the material.
• the present invention suppresses surface decarburization by using a low Si alloy design in which silicon is controlled to 0.9 wt% or less, and secures sufficient workability.
• Manganese is a hardenability improving element, and is one of the essential elements for forming a high-strength tempered martensite steel. In order to secure strength, in the present invention, manganese may be added in an amount of 0.3% by weight or more. However, if the manganese content is excessive in tempered martensitic steel, the upper limit of the manganese content may be limited to 0.8% by weight because toughness is lowered.
• Chromium is effective in improving hardenability together with manganese and improves corrosion resistance of steel.
• chromium may be added in an amount of 0.3% by weight or more.
• chromium is a relatively expensive element compared to silicon and manganese, and since it increases C eq , its upper limit in the present invention may be limited to 0.6 wt %.
• phosphorus is an element that segregates at grain boundaries to reduce toughness and reduces resistance to delayed hydrogen fracture, it is preferable to exclude it as much as possible from steel.
• the upper limit may be limited to 0.015% by weight.
• the upper limit may be limited to 0.01% by weight.
• the upper limit may be limited to 0.01% by weight.
• Nitrogen combines with aluminum or vanadium in steel to form coarse AlN or VN precipitates that are not dissolved during heat treatment. Accordingly, in the present invention, the upper limit may be limited to 0.005% by weight or less.
• Niobium is an element that combines with carbon in steel to form Nb-based carbide, and improves workability by refining crystal grains.
• niobium may be added in an amount exceeding 0 wt%.
• 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 in terms of improving processability.
• the Nb-based carbide formed by adding niobium may be distributed in the structure of the ultra-high strength spring wire rod and steel wire according to the present invention.
• the size of the formed Nb-based carbide is preferably 20 nm or less. This is because, on the contrary, when the size of the Nb-based carbide exceeds 20 nm, the workability may be deteriorated.
• it is preferable that the Nb-based carbide is evenly distributed at 1000 pieces/mm 2 or more. This is because, when the Nb-based carbide is distributed in less than 1000 pieces/mm 2 , there is a risk that the crystal grains may not be sufficiently refined.
• Nb may be included in an amount of 10at% or more.
• the remaining component of the present invention is iron (Fe).
• Fe iron
• the impurities are known to any person skilled in the art of a conventional manufacturing process, all details thereof are not specifically mentioned in the present specification.
• the reason for limiting the alloy composition of the wire rod described above is the same as the reason for limiting the alloy composition of the steel wire, and for convenience, the reason for limiting the alloy composition of the steel wire will be omitted.
• the alloy composition of the wire rod and the steel wire of the present invention may further limit the relationship between them as follows, in addition to limiting the content of each alloying element to the above-described conditions.
• the present invention controls the C eq value to suppress surface decarburization and the formation of low-temperature structures, which are likely to occur during cooling after rolling the wire rod.
• the C eq value can be expressed by Equation (1) below, and in the present invention, the value of Equation (1) is controlled to be 0.77 or more and 0.83 or less in order to suppress surface decarburization and formation of low-temperature tissue.
• C, Mn, Cr, and Si mean the content (% by weight) of each element.
• Equation (1) When the value of Equation (1) exceeds 0.83, surface decarburization occurs, and there is a possibility that a low-temperature structure may be formed. On the other hand, if the value of Equation (1) is less than 0.77, it is difficult to secure the target strength.
• the wire for ultra-high strength spring according to the present invention is manufactured by subjecting an ingot satisfying the above-described alloy composition and Equation (1) value range to homogenization heat treatment, rolling the wire rod, and then cooling.
• each manufacturing step will be described.
• the homogenizing heat treatment may be performed within 180 minutes at a heating temperature of 900 to 1100° C. in a heating furnace.
• the finish rolling temperature of the wire rod rolling step may be 730 to Ae3 °C.
• finish rolling is performed under the above temperature range of 730 to Ae3°C, the main structure of the wire is transformed from austenite to ferrite.
• the main structure of the wire rod before finish rolling is austenite, and the main structure of the wire rod after finish rolling is ferrite.
• the deformation amount of the wire rolling may be 0.3 to 2.0.
• the amount of deformation is expressed by the following equation.
• the area reduction ratio is a value calculated as (AA 1 )/A * 100 when A is the cross-sectional area perpendicular to the longitudinal direction of the wire before rolling the wire, and A 1 is the cross-sectional area perpendicular to the longitudinal direction of the wire after rolling. .
• the deformation amount during wire rod rolling is less than 0.3, it is difficult to sufficiently refine the crystal grains, and when the deformation amount exceeds 2.0, the processing amount is too high, which causes difficulty in the production process. Therefore, according to the present invention, it is preferable that the deformation amount is controlled to 0.3 to 2.0.
• the average size of the austenite grains before the finish rolling may be 5 to 15 ⁇ m.
• the average size of the ferrite grains of the final wire rod structure that has undergone the subsequent finish rolling and cooling processes can also be refined.
• the cooling step in the present invention may cool the wire rod at a cooling rate of 3° C./s or less. When the cooling rate exceeds 3°C/s, it is difficult to suppress the formation of low-temperature tissues.
• the wire rod for ultra-high strength spring according to the present invention manufactured by the alloy composition and manufacturing method described above may include pearlite and ferrite as a microstructure, and according to an example, it includes 60% or more of pearlite and the remaining ferrite as an area fraction. can do.
• the wire rod for an ultra-high strength spring according to an embodiment of the present invention may hardly include a low-temperature structure on a cross-section perpendicular to the longitudinal direction of the wire rod.
• the sum of the area fractions of bainite and martensite having a hardness of 400 Hv or more on a cross-section (C-section) perpendicular to the longitudinal direction may be 1% or less.
• the low-temperature structure means bainite and martensite.
• the wire rod for ultra-high strength spring of the present invention can secure sufficient workability by suppressing the formation of a low-temperature structure.
• the thickness of the ferrite decarburized layer of the wire rod may be 1 ⁇ m or less.
• the present invention it is possible to refine the ferrite grains through Nb-based carbide and controlled rolling.
• the average size of the ferrite grains of the wire rod according to an embodiment of the present invention may be 10 ⁇ m or less.
• the wire rod for ultra-high strength spring of the present invention can secure sufficient workability by refining crystal grains.
• the wire rod for ultra-high strength spring may have a tensile strength of 1200 MPa or less.
• the steel wire for ultra-high strength spring is, by weight, C: 0.55 to 0.65%, Si: 0.5 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.3 to 0.6%, P: 0.015% or less , S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, Nb: more than 0%, 0.04% or less, the remainder includes Fe and unavoidable impurities, and the value of formula (1) is 0.77 or more, 0.83 or less, and may include 90% or more of tempered martensite as an area fraction.
• the reason for limiting the alloy composition of the steel wire and the value range of Equation (1) is the same as the reason for limiting the alloy composition of the wire rod and the value range of Equation (1) described above, so the description thereof will be omitted for convenience.
• the steel wire for ultra-high strength spring according to the present invention is manufactured by drawing, heating, water cooling at high pressure, and then tempering and then water cooling a wire rod satisfying the above-described alloy composition and Equation (1) value range.
• each manufacturing step will be described.
• the means for heating to the quenching temperature and the means for tempering upon heating utilize induction heating to sufficiently harden the surface during subsequent water cooling by rapid heating.
• the present invention uses induction heating and water cooling in the low C eq and low Si alloy compositions that satisfy the above-described alloy composition and formula (1) value range, thereby lowering the content of alloying elements compared to automobile suspension springs, and high strength can be obtained.
• a wire that satisfies the above-described alloy composition and value range of Equation (1) can be drawn to a wire diameter of 15 mm or less that can be applied to a motorcycle suspension spring to manufacture a steel wire.
• the fresh steel wire is heated to the quenching temperature of 900 to 1000° C. within 10 seconds and then maintained for 5 to 60 seconds to austenitize the structure of the steel wire.
• the heating time to the target temperature of 900 to 1000°C exceeds 10 seconds, crystal grains grow and it is difficult to secure desired physical properties. If the holding time is less than 5 seconds, the pearlite structure may not be transformed into austenite, and if it exceeds 60 seconds, the crystal grains may be coarsened, so the holding time is preferably controlled to 5 to 60 seconds.
• the steel wire for ultra-high strength spring according to the present invention has fine grains and excellent workability, and can be cold-formed at room temperature to be manufactured as a motorcycle suspension spring.
• the step of water cooling at high pressure is a step of transforming the main structure of the steel wire from austenite to martensite, and water cooling can be performed at a high pressure enough to remove the boiling film of the austenitized steel wire of the previous step.
• water cooling can be performed at a high pressure enough to remove the boiling film of the austenitized steel wire of the previous step.
• the desired strength cannot be secured due to the low C eq and low Si alloy composition.
• the high pressure is not high enough to remove the boiling film during water cooling, the possibility of cracking during quenching increases. Therefore, it is preferable to perform high-pressure water cooling at a pressure as high as possible during water cooling.
• the surface of the steel wire can be sufficiently hardened by rapidly heating with induction heating to the quenching temperature in the above-described heating step, followed by rapid cooling with water in this step.
• the cooling rate during water cooling may be 100° C./s or more.
• the step of tempering in the present invention is a step of heating martensite, which is the main structure of the water-cooled steel wire, and tempering it with tempered martensite.
• the tempering step may be heated to 400 to 500° C. within 10 seconds and then maintained within 30 seconds. If the tempering temperature is less than 400 °C, toughness is not secured, so processing is difficult and the risk of product damage increases, and if it exceeds 500 °C, the strength is lowered, so the tempering temperature is limited to the above-mentioned temperature range. In addition, if the temperature is not heated within 10 seconds to the above-described temperature range during tempering, coarse carbides are formed and there is a risk that toughness may be deteriorated. Therefore, it is preferable to rapidly heat within 10 seconds.
• the tempered steel wire is water-cooled to room temperature.
• a steel wire for a spring that satisfies the above-described alloy composition and the value range of Equation (1), and manufactured according to the above-described manufacturing conditions, may contain 90% or more of tempered martensite as an area fraction.
• the steel wire for ultra-high strength spring according to an example of the present invention may have a prior austenite average grain size of 10 ⁇ m or less.
• the old austenite refers to the austenite structure of the steel wire after the heating step for QT heat treatment of the drawn steel wire of the present invention.
• the steel wire for ultra-high strength spring has a wire diameter of 15 mm or less, and has a narrow diameter suitable as a steel wire for a motorcycle suspension spring.
• the steel wire for an ultra-high strength spring according to an example of the present invention has a strength of 1700 MPa or more, thereby securing the ultra-high strength physical properties required for a motorcycle suspension spring.
• the steel wire for ultra-high strength spring according to an embodiment of the present invention can secure high ductility as a reduction in cross-section (RA) of 35% or more, and thus can be cold-formed at room temperature to be manufactured into a motorcycle suspension spring.
• RA reduction in cross-section
• the austenite grains before the finish rolling can be refined to further improve the reduction in area (RA).
• the steel wire for ultra-high strength spring according to a preferred embodiment of the present invention may have a reduction in area (RA) of 45% or more.
• the results in Table 2 below are results of measuring the physical properties of the wire rod manufactured by the above-described process.
• the low-temperature structure area fraction in Table 2 means the sum of the area fractions of bainite and martensite on a cross section perpendicular to the longitudinal direction of the wire rod.
• AGS in Table 2 means the average size of austenite grains before finish rolling in the wire rod rolling step, and was measured using ASTM E112 standard.
• the thickness of the ferrite decarburized layer is a measurement of the thickness of a layer made of only ferrite produced by decarburization on the steel surface after wire rod rolling. will be measured
• the wire rod in Table 2 was freshly made with a steel wire having a diameter of 10 mm, heated, and then subjected to high-pressure water cooling. After high-pressure water cooling, tempering was performed and general water cooling was performed to manufacture the final ultra-high strength spring steel wire. means one temperature. RA means the section reduction rate.
• Inventive Examples 1 and 2 satisfy the alloy composition, Equation (1) and manufacturing conditions of the present invention. As a result, the low-temperature structure of the wire and the formation of the ferrite decarburized layer are suppressed, and Nb is added to During rolling, the austenite grains before finish rolling were refined. In addition, as shown in Table 3, the tensile strength was 1700 MPa or more, and the reduction in area was 35% or more. On the other hand, in Comparative Example 1, the Si content was high, and the ferrite decarburized layer was thickly formed upon cooling. Comparative Example 2 did not secure the target strength of 1700 MPa or more because the value of Equation (1) was lower than 0.77. In Comparative Example 3, grain coarsening occurred because Nb was not added, and thus the average size of the target austenite grains could not be secured. Accordingly, the area reduction ratio (RA) was lower than that of the Nb-added material.
• RA area reduction ratio
• the wire rod for ultra-high strength spring according to the present invention can be applied as a suspension spring for automobiles, motorcycles, various means of transportation, or a spring used in various industrial fields.

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• 2019
  • 2019-12-20 KR KR1020190172003A patent/KR102326263B1/ko active IP Right Grant
• 2020
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