Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • 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
  • B21B15/00—Arrangements for performing additional metal-working operations specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills
  • B21B15/0035—Forging or pressing devices as units
• • 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
  • C21D1/84—Controlled slow cooling
• • 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
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
• • 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
  • C21D7/00—Modifying the physical properties of iron or steel by deformation
  • C21D7/02—Modifying the physical properties of iron or steel by deformation by cold working
  • C21D7/10—Modifying the physical properties of iron or steel by deformation by cold working of the whole cross-section, e.g. of concrete reinforcing bars
• • 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
  • 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
• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/06—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • 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
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • 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
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • 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
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • 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
  • C21D2211/001—Austenite

Definitions

• the present invention relates to a non-heat-treated steel wire rod with excellent cold forgeability and a method of manufacturing the same, and more particularly, to a non-heat-treated steel wire rod capable of achieving excellent strength and toughness without heat treatment, and a method of manufacturing the same.
• Heat-treated steel wire rods which are quenched and tempered in the austenite region after cold forging to enhance material strength and toughness, are commonly used for the mechanical structure of vehicle parts.
• a typical heat-treated steel wire rod is manufactured through a raw material manufacturing step S10, and a parts processing step S20 for processing the manufactured wire rod into parts.
• the raw material manufacturing step S10 includes a steelmaking and continuous casting step S11, a bloom reheating step S12, a billet rolling step S13, a wire rod rolling step S14, and a wire rod manufacturing step S15.
• the wire rod manufactured through the above-mentioned steps is post-processed according to product specifications to ultimately manufacture non-heat-treated wire rod products.
• the post-processing refers to the parts processing step S20, and the parts processing step S20 includes a cold drawing step S21, a spheroidization step S22, a cold forging step S23, a quenching/tempering step S24, a processing step S25, and a product manufacturing step S26.
• a non-heat-treated steel wire rod is a steel produced by omitting heat treatment during the above-mentioned post-processing.
• the spheroidization step S22 to be performed after the cold drawing step S21, and the quenching/tempering step S24 to be performed after the cold forging step S23 may be omitted.
• tensile strength and impact toughness similar to those of the heat-treated steel may be achieved without the above-mentioned heat treatment processes.
• process simplification and cost savings may be enabled to achieve economic feasibility, and heat treatment defects and warping may be prevented to produce products that require straightness.
• non-heat-treated steel wire rod does not undergo heat treatment during the parts manufacturing process and thus the mechanical properties of the raw material affect the final parts properties, variations in mechanical properties may pose a significant risk.
• the related art documents include Japanese Patent Registration No. 7044197 and Korean Patent Registration No. 10-1262462 .
• the present invention provides a non-heat-treated steel wire rod with excellent cold forgeability, which achieves excellent strength and toughness without heat treatment and exhibits no variation in tensile strength, and a method of manufacturing the same.
• the above description is an example, and the scope of the present invention is not limited thereto.
• a non-heat-treated steel wire rod with excellent cold forgeability including carbon (C): approximately 0.20 wt% to 0.40 wt%, silicon (Si): approximately 0.10 wt% to 0.30 wt%, manganese (Mn): approximately 1.30 wt% to 1.60 wt%, phosphorus (P): more than 0 wt% and up to approximately 0.05 wt%, sulfur (S): more than 0 wt% and up to approximately 0.05 wt%, chromium (Cr): approximately 0.02 wt% to 0.30 wt%, nickel (Ni): approximately 0.02 wt% to 0.30 wt%, molybdenum (Mo): approximately 0.02 wt% to 0.30 wt%, vanadium (V): approximately 0.01 wt% to 0.15 wt%, niobium (Nb
• the non-heat-treated steel wire rod may include a composite precipitate with a diameter of approximately 50 nm or less, and the composite precipitate may be a composite precipitate including one or more selected from Nb, V, Ti, and Al, or one or more selected from composite precipitates including TiN, VC, VN, NbC, NbN, AIN, and BN.
• the composite precipitate may have an austenite grain size of number 10 or above.
• a method of manufacturing a non-heat-treated steel wire rod with excellent cold forgeability including forming a billet by reheating a steel material including carbon (C): approximately 0.20 wt% to 0.40 wt%, silicon (Si): approximately 0.10 wt% to 0.30 wt%, manganese (Mn): approximately 1.30 wt% to 1.60 wt%, phosphorus (P): more than 0 wt% and up to approximately 0.05 wt%, sulfur (S): more than 0 wt% and up to approximately 0.05 wt%, chromium (Cr): approximately 0.02 wt% to 0.30 wt%, nickel (Ni): approximately 0.02 wt% to 0.30 wt%, molybdenum (Mo): approximately 0.02 wt% to 0.30 wt%, vanadium (V): approximately 0.01 wt% to 0.15 w
• C carbon
• Si silicon
• Mn manganes
• the temperature region above the A3 transformation point may be approximately 750 °C to 900 °C.
• cooling may be performed at a cooling rate of approximately 2 °C/s or less after preforming the rolling.
• a drawing reduction rate may be approximately 30% to 50%.
• a speed of a conveyor for transporting the wire rod after being rolled may be controlled to approximately 0.2 m/s to 0.7 m/s to control an overlap density of the wire rod transported by the conveyor.
• the wire rod manufactured after the cold forging may satisfy a tensile strength of approximately 900 MPa or more.
• a non-heat-treated steel wire rod with excellent cold forgeability which achieves excellent strength and toughness without heat treatment and exhibits no variation in tensile strength, may be manufactured.
• the above effects of the present invention are examples, and the scope of the present invention is not limited thereto.
• the present invention relates to a method of manufacturing a non-heat-treated steel wire rod with excellent cold forgeability by controlling precision rolling conditions and a cooling rate during wire rod manufacturing, and the non-heat-treated steel wire rod with excellent cold forgeability will be described first before describing the manufacturing method.
• the non-heat-treated steel wire rod with excellent cold forgeability includes carbon (C): approximately 0.20 wt% to 0.40 wt%, silicon (Si): approximately 0.10 wt% to 0.30 wt%, manganese (Mn): approximately 1.30 wt% to 1.60 wt%, phosphorus (P): more than 0 wt% and up to approximately 0.05 wt%, sulfur (S): more than 0 wt% and up to approximately 0.05 wt%, chromium (Cr): approximately 0.02 wt% to 0.30 wt%, nickel (Ni): approximately 0.02 wt% to 0.30 wt%, molybdenum (Mo): approximately 0.02 wt% to 0.30 wt%, vanadium (V): approximately 0.01 wt% to 0.15 wt%, niobium (Nb): approximately 0.01 wt% to 0.05 wt%
• the functions and contents of the components included in the non-heat-treated steel wire rod according to the present invention are as follows.
• the unit for the contents of the constituent elements is wt%.
• C is an element that forms an Nb- or V-based precipitate and dissolves in the matrix to increase the strength of steel. Although approximately 0.20 wt% or more of C is required to ensure sufficient strength of steel, when the content of C increases to more than approximately 0.40 wt%, the increase in strength leads to significant decreases in toughness and ductility. Therefore, C is added at approximately 0.20 wt% to 0.40 wt% to ensure excellent strength and toughness of non-heat-treated steel.
• Si is an element that contributes to high strength and is useful for enhancing fatigue deformation resistance by increasing softening resistance.
• Si may significantly deteriorate the cold forgeability of non-heat-treated steel for which spheroidization is omitted.
• Si is added at approximately 0.10 wt% to 0.30 wt% based on the total weight of non-heat-treated steel according to an embodiment of the present invention.
• Mn is an element that is useful for deoxidizing steel, and dissolves in the matrix to effectively increase and ensure the strength of non-heat-treated steel. Mn lowers the transformation point to contribute to intermediate pearlite refinement and enhance toughness. When the content of Mn is less than approximately 1.30 wt% based on the total weight of non-heat-treated steel according to an embodiment of the present invention, sufficient strength may not be ensured, and when the content of Mn is greater than approximately 1.60 wt%, the increase in strength may result in a decrease in toughness.
• P is an element that easily segregates to impair the toughness of steel, and has an excellent solid solution strengthening effect to enhance the strength of steel with only a small amount added.
• P may be added at a content ratio of more than 0 wt% and not more than approximately 0.05 wt% based on the total weight of non-heat-treated steel according to an embodiment of the present invention. When the content of P is greater than approximately 0.05 wt%, the toughness of steel may decrease.
• S Sulfur (S): more than 0 wt% and up to approximately 0.05 wt%
• S is an element that impairs workability and material properties. S segregates at the grain boundaries to impair the ductility of steel, and forms sulfides which are the primary cause of the deterioration in delayed fracture resistance and stress relaxation properties. Therefore, S may be added at a content ratio of more than 0 wt% and not more than approximately 0.05 wt% based on the total weight of non-heat-treated steel according to an embodiment of the present invention. When the content of S is greater than approximately 0.05 wt%, martensite grain boundaries may fracture, hot workability may decrease, and surface defects such as cracks may occur due to the formation of coarse inclusions.
• Cr not only increases the strength of steel but also enhances hardenability and strength as a ferrite stabilizing element.
• Cr may be added at a ratio of approximately 0.02 wt% to 0.30 wt% based on the total weight of non-heat-treated steel according to an embodiment of the present invention.
• the strength enhancement effect may be insufficient
• the content of Cr is greater than approximately 0.30 wt%, the improved hardenability may lead to the formation of low-temperature structures during cooling, and cold drawability and forgeability may decrease.
• Ni contributes to increasing hardenability and enhancing toughness.
• Ni may be added at approximately 0.02 wt% to 0.30 wt% based on the total weight of non-heat-treated steel according to an embodiment of the present invention. When the content of Ni is less than approximately 0.02 wt%, the effect of Ni addition may be insignificant, and when the content of Ni is greater than approximately 0.30 wt%, the costs may increase.
• Mo contributes to enhancing strength and toughness.
• Mo may be added at approximately 0.02 wt% to 0.30 wt% based on the total weight of non-heat-treated steel according to an embodiment of the present invention. When the content of Mo is less than approximately 0.02 wt%, the effect of Mo addition may be insignificant, and when the content of Mo is greater than approximately 0.30 wt%, hardness may increase to reduce workability, and the manufacturing costs of non-heat-treated steel may increase significantly.
• Vanadium (V) approximately 0.01 wt% to 0.15 wt%
• V is an element that reacts with C and N to form an NbV (carbide/nitride) composite precipitate and contributes to strength enhancement through precipitation hardening.
• V acts as a ferrite nucleation site during wire rod rolling to increase the fraction of ferrite and enhance strength and toughness.
• V may be added at approximately 0.01 wt% to 0.15 wt% based on the total weight of non-heat-treated steel according to an embodiment of the present invention. When the content of V is less than approximately 0.01 wt%, the effect of V addition is insufficient, and when the content of V is greater than approximately 0.15 wt%, the precipitation hardening effect is insignificant and ineffective.
• Nb is an element that reacts with C and N to form Nb (carbide/nitride).
• the Nb-based precipitate is an element that enables precipitation hardening and prevents grain boundary coarsening of steel.
• Nb acts as a ferrite nucleation site during wire rod rolling to increase the fraction of ferrite and enhance strength and toughness.
• the increase in Nb raises the solutioning temperature, and Nb which does not dissolve during raw material rolling forms a coarse precipitate.
• the formation of the coarse precipitate does not effectively impede the movement of dislocations, and thus the effect on fatigue life improvement is insignificant.
• Nb may be added at approximately 0.01 wt% to 0.05 wt% based on the total weight of non-heat-treated steel according to an embodiment of the present invention.
• Nb and V may be approximately 0.02 wt% to 0.2 wt%.
• Nb and V are elements that form carbides or nitrides. Although the individual content ranges of these elements are also important, because the two are interchangeable and excessive addition may lead to the formation of a coarse precipitate, the total amount of both elements needs to be appropriately controlled in the present invention.
• Al primarily acts as a deoxidizer, and combines with oxygen in steel to form oxides. Al remaining after reacting with oxygen combines with nitrogen to form AlN.
• AIN prevents grain boundary coarsening to enhance the toughness of the product.
• Al may be added at approximately 0.005 wt% to 0.060 wt% based on the total weight of non-heat-treated steel according to an embodiment of the present invention. When the content of Al is less than approximately 0.005 wt%, Al is insufficient to form AIN, and when the content of Al is greater than approximately 0.060 wt%, a deterioration in mechanical properties may occur.
• Ti forms carbonitrides to cause precipitation hardening and enhance strength and toughness.
• Ti may be added at approximately 0.005 wt% to 0.020 wt% based on the total weight of non-heat-treated steel according to an embodiment of the present invention.
• the content of Ti is less than approximately 0.005 wt%, the effect of Ti addition is insufficient, and when the content of Ti is greater than approximately 0.02 wt%, the manufacturing costs increase rapidly, and coarse alloy carbides increase and act similarly to non-metal inclusions, thereby deteriorating the fatigue properties and the precipitation hardening effect.
• Cu is an element effective in increasing the strength and enhancing the toughness of steel.
• Cu may be added at approximately 0.01 wt% to 0.30 wt% based on the total weight of non-heat-treated steel according to an embodiment of the present invention.
• the content of Cu is less than approximately 0.01 wt%, the effect of Cu addition is insufficient, and when the content of Cu is greater than approximately 0.30 wt%, surface enrichment may be caused and cold drawability during a drawing process may decrease.
• B is an element that segregates at the grain boundaries and enhances the ductility and toughness of steel.
• B may be added at approximately 0.0001 wt% to 0.0020 wt% based on the total weight of non-heat-treated steel according to an embodiment of the present invention.
• the content of B is less than approximately 0.0001 wt%, the effect of B addition is insufficient, and when the content of B is greater than approximately 0.0020 wt%, the hardenability of steel may increase and low-temperature structures may be caused during rapid cooling.
• N combines with Al, V, and Nb to form nitrides such as AIN, VN, and NbN. These nitrides not only enable grain refinement through the grain boundary pinning effect, but also act as a ferrite nucleation site to increase the fraction of ferrite.
• N may be added at approximately 0.005 wt% to 0.015 wt% based on the total weight of non-heat-treated steel according to an embodiment of the present invention. When the content of N is less than approximately 0.005 wt%, the number of nitride particles formed may be insufficient, and when the content of N is greater than approximately 0.015 wt%, N may dissolve in the matrix to increase strength and decrease cold drawability.
• the remaining component of the present invention is iron (Fe).
• Fe iron
• impurities are known to any one of ordinary skill in the art and thus are not specifically described in this specification.
• the non-heat-treated steel wire rod with excellent cold forgeability and the above-mentioned composition may satisfy a tensile strength of approximately 900 MPa or more.
• the above-described non-heat-treated steel wire rod with excellent cold forgeability may include a composite precipitate with a diameter of 50 nm or less, and the composite precipitate may be a composite precipitate including one or more selected from Nb, V, Ti, and Al, or one or more selected from composite precipitates including TiN, VC, VN, NbC, NbN, AIN, and BN.
• the composite precipitate may have a diameter of approximately 50 nm or less.
• the grain refinement of the composite precipitate may be determined by measuring the austenite grain size (A, G, S).
• the austenite grain size may be number 10 or above.
• the grain refinement is a critical means of enhancing the strength and toughness of steel materials, and the principle thereof is based on the pinning effect by an MX precipitate, which inhibits grain growth. Therefore, the fine dispersion of the composite precipitate particles is crucial.
• the fine dispersion of the composite precipitate particles in the non-heat-treated steel wire rod of the present invention may be determined based on the number of composite precipitate particles per approximately 100 ⁇ m 2 . To achieve desired strength and toughness of steel materials, 1000 or more composite precipitate particles may be included per approximately 100 ⁇ m 2 .
• the above-described non-heat-treated steel wire rod according to an embodiment of the present invention may be manufactured as described below.
• FIG. 1 is a flowchart of a method of manufacturing a non-heat-treated steel wire rod, according to an embodiment of the present invention
• FIG. 2 is a schematic view specifically showing a wire rod rolling step shown in FIG. 1
• FIGS. 3 and 4 include schematic views of wire rods cooled on a cooling conveyor shown in FIG. 2 .
• the non-heat-treated steel wire rod manufacturing method may be divided into two steps.
• the method includes a raw material manufacturing step S10 for manufacturing a wire rod by a steelmaker, and a parts processing step S20 for manufacturing parts by processing the wire rod manufactured through the raw material manufacturing step.
• a heat-treated steel wire rod is manufactured by performing a raw material manufacturing step S10 and then a parts processing step S20 as shown in FIG. 5 .
• the parts processing step S20 includes a cold drawing step S21, a spheroidization step S22, a cold forging step S23, a quenching/tempering step S24, a processing step S25, and a product manufacturing step S26.
• the non-heat-treated steel wire rod may be a steel produced by omitting heat treatment during the above-mentioned parts processing step S20.
• the spheroidization step S22 to be performed after the cold drawing step S21, and the quenching/tempering step S24 to be performed after the cold forging step S23 may be omitted.
• non-heat-treated steel wire rod does not undergo the spheroidization step S22 and the quenching/tempering step S24, work hardening continuously occurs during the manufacture of the wire rod product. This increases the strength but reduces the ductility and toughness of the product. Therefore, additional control is required during the manufacturing process to ensure excellent strength and toughness without the above-mentioned heat treatment processes.
• the inventors of the present invention have optimized the process conditions for manufacturing the non-heat-treated steel wire rod, focusing on reducing the variation in mechanical properties during a wire rod rolling step S14.
• the non-heat-treated steel wire rod manufacturing method includes forming a billet by reheating a steel material with the above-mentioned composition, at approximately 1100 °C to 1350 °C; forming a wire rod by rolling the reheated billet; cold drawing the wire rod; and performing cold forging after the cold drawing.
• fine dispersion of the composite precipitate particles is crucial.
• the fine dispersion may be achieved through re-dissolution and re-precipitation of the composite precipitate within the matrix, which may be controlled through reheating before rolling a raw material bloom.
• the reheating is performed at a temperature below the liquidus temperature, approximately 1100 °C to 1350 °C, to dissolve the constituent elements of the composite precipitate as much as possible.
• the forming of the wire rod may include performing rolling while heating in a temperature region above the A3 transformation point, and the temperature region above the A3 transformation point may be approximately 750 °C to 900 °C. Cooling may be performed at a cooling rate of approximately 2 °C/s or less after preforming the rolling.
• the raw material structure is the same as the structure of the final product. Therefore, when the cooling rate after wire rod rolling is high, the transformation to low-temperature structures (e.g., bainite and martensite) may be promoted to cause problems in the parts manufacturing process.
• low-temperature structures e.g., bainite and martensite
• the microstructure in the wire rod may be controlled to include ferrite and pearlite.
• the area fraction of the ferrite may be approximately 30% to 60% and the area fraction of the pearlite may be the remaining fraction.
• the fraction refers to a ratio of area derived from a microstructural image of the steel material by using an image analyzer.
• the drawing reduction rate may be approximately 30% to 50%. Because spheroidization is omitted for non-heat-treated steel during the drawing process of the parts manufacturing process, work hardening continuously occurs. This may shorten the mold life during cold forging and negate the benefits of omitting heat treatment.
• cold drawing is performed at a reduction rate of approximately 30% to 50% by using the Bauschinger effect to increase the strength of the product and enhance the cold forgeability by minimizing work hardening.
• the Bauschinger effect is a phenomenon in which a metal becomes more susceptible to permanent deformation depending on the direction in which a load is applied, and refers to an effect in which, when a metal subjected to a load beyond its yield point is then exposed to a load in the opposite direction, its yield point is lowered and deformation occurs at a yield strength lower than the original yield strength.
• the non-heat-treated steel wire rod W of the present invention may be primarily processed in the form of a product through the wire rod rolling step S14.
• a raw material sequentially passes through a heating furnace 10, a roughing mill 20, a first water box 32, a precision mill 40, a second water box 34, a laying head 50, and a cooling conveyor 60 shown in FIG. 2 so as to be manufactured into the wire rod W, and then the manufactured wire rod W is finally put on a stand 70.
• the speed of the cooling conveyor 60 for cooling the wire rod W after being rolled is controlled.
• the wire rod W is dropped into a ring shape and transported by the cooling conveyor 60.
• the transported ring-shaped wire rod W has overlapping regions P, where several rings overlap each other on the cooling conveyor 60, depending on an overlap density of the wire rod W, resulting in a higher density at the overlapping regions P compared to a central region C.
• This causes a difference in cooling rate between the center and sides of the wire rod W during cooling. That is, the cooling rate at the overlapping regions P of the wire rod W is lower than that at the central region C of the wire rod W, and thus a uniform tensile strength across the entire wire rod W may not be obtained.
• the speed of the cooling conveyor 60 may be controlled to approximately 0.2 m/s to 0.7 m/s to control the overlap density of the wire rod W transported by the cooling conveyor 60.
• the overlap density of the wire rod W may be controlled uniformly and thus the material properties based on the cooling rate may also be controlled uniformly.
• the non-heat-treated steel wire rod of the present invention may satisfy a tensile strength of approximately 900 MPa or more after cold forging without performing heat treatment.
• test examples will now be described for better understanding of the present invention. However, the following test examples are merely to promote understanding of the present invention, and the present invention is not limited to thereto. The content not described herein may be sufficiently inferred by one of ordinary skill in the art, and therefore, a detailed description will not be provided.
• a raw material with the composition shown in Table 1 was heat-treated at approximately 1100 °C to 1350 °C, and rolled and then cooled under the conditions shown in Table 2, and the microstructure and material properties thereof are shown in Table 2.
• samples of Comparative Examples 1 and 2 of the present invention were manufactured using heat-treated steel and conventional non-heat-treated steel alloy compositions.
• Samples of Embodiment 2 and Comparative Examples 3 and 4 were manufactured using the same composition as Embodiment 1.
• FIG. 6 includes scanning electron microscopy (SEM) images (a low-resolution (bright-field (BF) mode) image (a), a high-resolution (BF mode) image (b), and a high-resolution (scanning transmission electron microscopy (STEM) mode) image (c)) of an MX precipitate of the sample of Embodiment 1 of the present invention
• FIG. 7 includes SEM images (a low-resolution (BF mode) image (a), a high-resolution (BF mode) image (b), and a high-resolution (STEM mode) image (c)) of an MX precipitate of the sample of Comparative Example 2.
• SEM scanning electron microscopy
• the samples of the comparative examples with a cooling rate higher than approximately 2 °C/s show the formation of low-temperature structures, and thus may not easily achieve an impact strength value of approximately 125 J/cm 2 .
• the samples of the comparative examples with a cooling rate lower than approximately 2 °C/s do not show the formation of low-temperature structures, but achieve an impact strength value lower than the target value.
• the samples of the embodiments of the present invention do not show the formation of low-temperature structures, exhibit similar hardness and tensile strength values, and achieve a high impact strength value of approximately 150 J/cm 2 or more.
• the sample of Embodiment 1 shows that the number of composite precipitate particles with an average size of approximately 20 nm in the wire rod W is 3000 or more per approximately 100 ⁇ m 2 .
• the sample of Comparative Example 2 shows that the number of composite precipitate particles with an average size of approximately 34 nm in the wire rod W is 100 or more per approximately 100 ⁇ m 2 .
• the greater the number of composite precipitate particles per an area of approximately 100 ⁇ m 2 the higher the dispersion of composite precipitate particles.
• the sample of Embodiment 1 exhibits a higher dispersion of composite precipitate particles compared to the sample of Comparative Example 2.
• the sample of Embodiment 1 exhibits higher values not only in hardness and tensile strength but also in impact strength compared to the sample of Comparative Example 2.
• the sample of Embodiment 1 of the present invention shows a tensile strength value of approximately mid-900 MPa or more at a cold drawing reduction rate of approximately 30% to 40%, and achieves a tensile strength of approximately 1000 MPa or more at a cold drawing reduction rate of approximately 50% without heat treatment.
• the compressive strength decreases as the drawing reduction rate increases, leading to an enhancement in cold forgeability. This result is attributed to the dislocations that pile up at the grain boundaries during drawing and slide under low stress.
• the compressive strength increases again at a cold drawing reduction rate of approximately 50%, and thus ferrite and pearlite grain refinement and an increase in total dislocation density based on deformation are expected.
• Wire rod samples produced using the same composition and process conditions as the sample of Embodiment 1 were cooled, and then an overlapping region tensile strength and a central region tensile strength of each sample were measured by controlling the conveyor speed under the conditions of Table 4, and variations in tensile strength were calculated and shown in Table 4.
• Embodiments 3, 4, and 5 exhibit a tensile strength variation within approximately 50 MPa and do not have any significant issues in terms of productivity.

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