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
  • 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
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • 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
  • C21D1/26—Methods of annealing
  • C21D1/28—Normalising
• • 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
  • C21D6/001—Heat treatment of ferrous alloys containing 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
  • C21D7/13—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 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
• • 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
• • 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
  • C21D9/54—Furnaces for treating strips or wire
  • C21D9/56—Continuous furnaces for strip or wire
  • C21D9/573—Continuous furnaces for strip or wire with cooling
  • C21D9/5732—Continuous furnaces for strip or wire with cooling of wires; of rods
• • C—CHEMISTRY; METALLURGY
  • 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
  • C21D9/54—Furnaces for treating strips or wire
  • C21D9/56—Continuous furnaces for strip or wire
  • C21D9/573—Continuous furnaces for strip or wire with cooling
  • C21D9/5735—Details
• • 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
  • C22C38/06—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
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
• • 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
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • 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
• • 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
  • C21D2211/005—Ferrite
• • 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 high-strength wire rod, superior in impact toughness, and a manufacturing method therefor, and more particularly, to a high-strength wire rod having superior impact toughness, which can preferably be used as a material for industrial machines or automobiles exposed to various external load environments, and a manufacturing method therefor.
• a wire rod having a ferrite or perlite structure is limited in terms of securing excellent strength and impact toughness.
• a material having the above-described structure may have high impact toughness, but strength thereof may be relatively low. High strength may be obtained when cold drawing is performed to increase strength, but there may be a disadvantage in which impact toughness may rapidly decrease in proportion to an increase in strength.
• a bainite structure or a tempered martensite structure is used.
• a bainite structure may be obtained through a constant temperature transformation heat treatment using a hot-rolled steel material
• a tempered martensite structure may be obtained through a quenching and tempering heat treatment.
• it is difficult to stably obtain such structures simply by using general hot rolling and continuous cooling processes it is necessary to perform an additional heat treatment process described above using a hot-rolled steel material.
• An aspect of the present disclosure is to provide a high-strength wire rod having superior impact toughness and a manufacturing method therefor without an additional heat treatment process.
• a high-strength wire rod including, by wt %, less than 0.05% of C, excluding 0%, 0.05% or less of Si, excluding 0%, 3.0 to 4.0% of Mn, 0.020% or less of P, 0.020% or less of S, 1.0 to 3.0% of Ni, 0.0010 to 0.0030% of B, 0.010 to 0.030% of Ti, less than 0.0030% of N, 0.010 to 0.050% of Al, and the balance of Fe and inevitable impurities, and a microstructure includes 3 area % or less of a martensite-austenite (MA) constituent, including 0 area %, 2 area % or less of proeutectoid ferrite, including 0 area %, and 95 area % or higher of bainitic ferrite, including 100 area %.
• MA martensite-austenite
• a method of manufacturing a high-strength wire rod including reheating steel comprising, by wt %, less than 0.05% of C, excluding 0%, 0.05% or less of Si, excluding 0%, 3.0 to 4.0% of Mn, 0.020% or less of P, 0.020% or less of S, 1.0 to 3.0% of Ni, 0.0010 to 0.0030% of B, 0.010 to 0.030% of Ti, less than 0.0030% of N, 0.010 to 0.050% of Al, and a balance of Fe and inevitable impurities; obtaining a wire rod by hot-rolling the reheated steel; primarily cooling the wire rod at a speed of 10 to 20° C./sec to a temperature within a range of Bs° C.
• a wire rod of the present disclosure may have superior strength and impact toughness, and may accordingly be used as a material for industrial machines or automobiles exposed to various external load environments.
• a wire rod of the present disclosure may secure superior strength and impact toughness without an additional heat treatment process, and may thus be advantageous in an economical sense.
• Carbon (C) may be soluble to steel or may exist as carbide or cementite, and may improve strength of a wire rod.
• carbon is not intentionally added. Even if carbon is not included, there may be no difficulty in securing properties. However, considering a content of inevitably added carbon, 0% may be excluded.
• a content of carbon may be controlled to be less than 0.05%.
• Si 0.05% or less (excluding 0%)
• Silicon (Si) is known as a deoxidation element along with aluminum, and is known as an element which may be soluble to ferrite and may be effective for increasing strength by strengthening solid solution of a steel material.
• silicon is not intentionally added. Even if silicon is not included, there may be no difficulty in securing property. However, considering a content of inevitably added carbon, 0% may be excluded.
• a content of silicon may be controlled to be 0.05% or less to secure excellent impact toughness.
• Manganese (Mn) may increase strength of a steel material, and may improve hardenability such that manganese may allow a low temperature structure, such as bainite or martensite, to be easily be formed at a relatively wide range of cooling speeds.
• a content of manganese is less than 3.0%, hardenability may not be sufficient such that it may be difficult to stably secure a low temperature structure through consecutive cooling processes after a hot-rolling process.
• a content of manganese exceeds 4.0%, hardenability may excessively increase such that a martensite structure may be obtained even during air-cooling, and thus, the content may not be preferable.
• Phosphorus (P) may be one of impurities inevitably added in steel. Phosphorus may be segregated into a grain boundary, and may degrade toughness of steel, and may decrease delayed fracture resistance. Thus, it may be preferable to not include phosphorus as possible. In the present disclosure, an upper limit content of phosphorus may be controlled to be 0.020%.
• S may be one of impurities inevitably added to steel. Similarly to phosphorus, sulfur may be segregated into a grain boundary and may degrade toughness, and may also form low melting point sulfide, which may adversely affect a hot-rolling process. Thus, it may be preferable to not include sulfur as possible. In the present disclosure, an upper limit content of sulfur may be controlled to be 0.020%.
• Nickel (Ni) may work as an element which may improve hardenability along with manganese. Accordingly, nickel may decrease the formation of a martensite-austenite (M/A) constituent. When a content of nickel is less than 1.0%, hardenability may not be sufficient such that the effect of preventing the formation of a martensite-austenite constituent may be insignificant. When a content of nickel exceeds 3.0%, hardenability may excessively increase such that a martensite structure may be obtained, and thus, the content may not be preferable. A more preferable content of nickel may be 1.2 to 2.8%.
• Boron (B) is an element which may improve hardenability. Boron may be dispersed to an austenite grain boundary and may prevent the formation of ferrite during cooling, and may allow bainite or martensite to be easily formed. However, when a content of boron is less than 0.0010%, the effect of addition of boron may not be expected, and when a content of boron exceeds 0.0030%, a further increase of the effect may not be expected, and grain boundary strength may degrade due to precipitation of boron-based nitride in a grain boundary, which may degrade hot-forming processability.
• Titanium (Ti) may have great reactivity with nitrogen such that titanium may form nitride earlier than other elements.
• TiN When TiN is formed by adding titanium, and most of nitrogen of steel is exhausted, titanium may prevent precipitation of BN and may allow boron to exist in a soluble state such that the effect of increase of hardenability may be obtained.
• a content of titanium is less than 0.010%, the effect of addition of titanium may be insignificant, and when a content of titanium exceeds 0.030%, coarse nitride may be formed, which may degrade mechanical properties.
• nitrogen (N) may be maintained in a soluble state along with boron, and to greatly generate the effect of improving hardenability, nitrogen should not be added as possible.
• Nitrogen should also be limited to not allow a martensite-austenite constituent to be easily formed in bainite transformation. In the present disclosure, a content of nitrogen may be controlled to be less than 0.0030%.
• Aluminum is a strong deoxidation element, and may increase purity by removing oxygen of steel. Aluminum may also be combined with solute nitrogen of steel and may form AlN, which may improve impact toughness. Thus, in the present disclosure, aluminum may be actively added. When a content of aluminum is less than 0.010%, it may be difficult to expect the effect of addition of aluminum, and when a content thereof exceeds 0.050%, a large amount of alumina inclusion may be created, which may greatly degrade mechanical properties.
• a remainder other than the above-described composition is Fe.
• impurities may not be excluded.
• a person skilled in the art may be aware of the impurities, and thus, the descriptions of the impurities may not be particularly provided in the present disclosure.
• carbon may deteriorate impact toughness by forming cementite or a martensite-austenite (M/A) constituent
• silicon may deteriorate impact toughness as silicon may be soluble to steel or may allow a martensite-austenite (M/A) constituent to be easily formed.
• Nickel may prevent the formation of a martensite-austenite (M/A) constituent by improving hardenability.
• an alloy of steel having the composition range described above it may be preferably to control contents of Mn, Ti, N, and B to satisfy Relational Expression 2 as below.
• a more preferable range of Relational Expression 2 may be 10.0 or higher, and an even more preferable range may be 12.0 or higher.
• manganese may increase hardenability and may allow bainitic ferrite to be easily formed even when a cooling speed is relatively low.
• titanium may be combined with nitrogen and may from nitride, and may allow boron to be sufficiently soluble to steel such that titanium may prevent the formation of ferrite and may allow bainitic ferrite to be easily formed.
• the inventors conducted researches and experiments and have founded that, when contents of manganese, titanium, boron, and nitrogen satisfy Relational Expression 2, a wire rod having a bainitic ferrite structure with excellent strength and impact toughness may be provided.
• the wire rod of the present disclosure may include 3 area % or less of a martensite-austenite (MA) constituent, including 0 area %, 2 area % or less of proeutectoid ferrite, including 0 area %, and 95 area % or higher of bainitic ferrite, including 100 area %.
• the wire rod of the present disclosure may have bainitic ferrite as a main structure, and may include a martensite-austenite (MA) constituent and proeutectoid ferrite as second phases, and area fractions of the elements may be limited to be less than 3% and 2%, respectively.
• Bainite may be denoted by various terms depending on a content of carbon or morphology.
• bainite may be denoted as upper/lower bainite in medium carbon (approximately 0.2 to 0.45 wt %) or higher, and may be denoted as bainitic ferrite, acicular ferrite, granular ferrite, and the like, in a low carbon range, 0.2% or less.
• the wire rod of the present disclosure may have a bainitic ferrite structure among the structures.
• the wire rod of the present disclosure has a bainitic ferrite structure as a main structure, excellent strength and impact toughness may be secured at the same time. If general ferrite is a main structure, not bainitic ferrite, it may be advantageous in terms of impact toughness, but degradation of strength may not be prevented, and thus, it may not be preferable.
• an area fraction of a martensite-austenite constituent may be more advantageous in terms of strength of a wire rod, but impact toughness may be deteriorated. Considering the issue above, it may be preferable to control an area fraction of a martensite-austenite constituent to be low as possible. As described above, in the present disclosure, an area fraction of a martensite-austenite constituent may be controlled to be 3% or less.
• Proeutectoid ferrite may be formed along a prior austenite grain boundary and may greatly deteriorate impact toughness. Thus, it may also be preferable to control an area fraction of proeutectoid ferrite to be low. As described above, in the present disclosure, an area fraction of proeutectoid ferrite may be controlled to be 2% or less.
• a grain size of a martensite-austenite constituent is 5 ⁇ m or less (excluding 0 ⁇ m). If a grain size exceeds 5 ⁇ m an area of an interfacial surface in contact with a bainitic ferrite matrix may increase, which may deteriorate impact toughness.
• a grain size may refer to an equivalent circular diameter of each of particles detected by observing a cross-sectional surface of the wire rod.
• the high-strength wire rod may be manufactured by various methods, and the manufacturing method is not particularly limited. However, as a preferable example embodiment, the high-strength wire rod may be manufactured by the method as described below.
• a steel material having the above-described composition system may be prepared, and may be reheated.
• a form of the steel material may not be particularly limited, and generally, the steel material may have a bloom or billet form.
• a preferable range of a reheating temperature may be 950 to 1050° C.
• the temperature range may be determined as above to prevent coarsening of a grain by performing the reheating of the steel material at a relatively low temperature.
• a wire rod may be obtained by finish-hot-rolling the reheated steel material.
• a preferable range of the finish-hot-rolling temperature may be 750 to 850° C.
• the temperature range may be determined as above to improve impact toughness by refining an austenite grain through a sufficient low temperature rolling process and consequently obtaining a fine bainite structure after phase transformation.
• the wire rod may be primarily cooled at a speed of 10 to 20° C./sec to a temperature within a range of Bs° C. to Bs+50° C.
• Bs may refer to a temperature from which bainite phase transformation starts on a consecutive cooling curve.
• the wire rod may be cooled at a relatively high speed to right before bainite phase transformation such that proeutectoid ferrite formed along an austenite grain boundary may be actively prevented.
• a preferable temperature range of Bs may be 600 to 650° C.
• the primarily cooled wire rod may be secondarily cooled at a speed of 2 to 5° C./sec to a temperature within a range of Bf ⁇ 50° C. to Bf° C., and may be air-cooled.
• Bf may refer to a temperature at which bainite phase transformation terminates on a consecutive cooling curve.
• the secondary cooling termination temperature exceeds Bf° C., it may be difficult to secure a sufficient amount of bainitic ferrite structure, and when the temperature is less than Bf ⁇ 50° C., it may be easy to handle the steel material as the steel material is sufficiently cooled, but productivity may degrade.
• the secondary cooling speed is less than 2° C./sec, proeutectoid ferrite may be greatly formed, and when the speed exceeds 5° C./sec, martensite may be formed in the steel, which may deteriorate strength and impact toughness.
• Molten steel having an alloy composition indicated in Table 1 below was casted, and the molten steel was reheated at 1000° C., was wire-rod rolled to a diameter of 15 mm (a finish hot-rolling temperature: 750° C.), was primarily and secondarily cooled under conditions indicated in Table 2, and was air-cooled at 350° C. or less, a Bf temperature, thereby manufacturing a wire rod.
• Bf a bainite phase transformation terminating temperature, was measured using a dilatometer. Bf varied depending on a chemical composition, and had a range of approximately 350 to 400° C.
• a microstructure of the wire rod manufactured as above was analyzed and the result is listed in Table 2, and tensile strength and impact toughness were measured and the result is listed in Table 2.
• An area fraction and a grain size of a martensite-austenite (MA) constituent of the microstructure of the wire rod were measured using an image analyzer.
• a room temperature tensile test was conducted at a crosshead speed of 0.9 mm/min until a yield point and at 6 mm/min thereafter. Also, in an impact test, a curvature of an edge of a striker which applied impact to samples was 2 mm, and the impact test was carried out at a room temperature using an impact tester having a test capacity of 500 J.
• samples 1 to 5 satisfying both of the alloy composition and process conditions suggested in the present disclosure had excellent tensile strength and impact toughness, which are, 600 MPa or higher of tensile strength and 200 J or higher of impact toughness.
• sample 7 a content of carbon of sample 7 exceeded the range suggested in the present disclosure. Accordingly, tensile strength was excellent, but impact toughness was deteriorated. That is because carbon was solute on MA such that a stable MA phase was formed.
• a content of silicon of sample 8 exceeded the range suggested in the present disclosure.
• a content of silicon was increased similarly to carbon, solution amount increased in a matrix such that the solution strengthening effect appeared, and MA phase also increased.
• tensile strength was excellent, impact toughness was degraded.
• an alloy composition of sample 10 satisfied the range suggested in the present disclosure, but the composition relational expression (Relational Expression 1) and a speed of the secondary cooling in the manufacturing process exceeded the range suggested in the present disclosure. Accordingly, as a martensite-austenite constituent and martensite were formed, tensile strength was excellent, but impact toughness was deteriorated.
• sample 11 an alloy composition of sample 11 satisfied the range suggested in the present disclosure, but a speed of the secondary cooling was below the range suggested in the present disclosure. Accordingly, as ferrite was formed, tensile strength was deteriorated.
• sample 12 a content of titanium of sample 12 was below the range suggested in the present disclosure. As the amount of solute boron decreased, hardenability decreased, and when the cooling speed was also low, the mount of proeutectoid ferrite precipitation increased such that tensile strength was degraded.
• samples 13 and 14 contents of manganese and nickel of samples 13 and 14 exceeded the ranges suggested in the present disclosure.
• hardenability was excessively increased, even when the steel was cooled at the cooling speed suggested in the present disclosure, martensite was created such that strength increased, but impact toughness was deteriorated.

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• 2016
  • 2016-12-13 KR KR1020160169308A patent/KR101879068B1/ko active IP Right Grant
• 2017
  • 2017-11-23 US US16/468,115 patent/US20200071792A1/en not_active Abandoned
  • 2017-11-23 CN CN201780076488.8A patent/CN110062813B/zh active Active
  • 2017-11-23 JP JP2019531080A patent/JP6806905B2/ja active Active
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