WO2022086050A1 - 연성이 우수한 초고강도 강판 및 그 제조방법 - Google Patents

연성이 우수한 초고강도 강판 및 그 제조방법 Download PDF

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WO2022086050A1
WO2022086050A1 PCT/KR2021/014215 KR2021014215W WO2022086050A1 WO 2022086050 A1 WO2022086050 A1 WO 2022086050A1 KR 2021014215 W KR2021014215 W KR 2021014215W WO 2022086050 A1 WO2022086050 A1 WO 2022086050A1
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steel sheet
less
hot
cold
ultra
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PCT/KR2021/014215
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English (en)
French (fr)
Korean (ko)
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류주현
안연상
최강현
최을용
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주식회사 포스코
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Priority to EP21883121.2A priority Critical patent/EP4234750A1/de
Priority to US18/029,865 priority patent/US20230357881A1/en
Priority to JP2023524378A priority patent/JP2023547102A/ja
Priority to CN202180072362.XA priority patent/CN116507753A/zh
Publication of WO2022086050A1 publication Critical patent/WO2022086050A1/ko

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    • 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
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • 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
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    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
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    • 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
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    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
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    • C21D6/00—Heat treatment of ferrous alloys
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—Final recrystallisation annealing
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0278—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular surface treatment
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • 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
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    • 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
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
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    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
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    • C21D2211/00—Microstructure comprising significant phases
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    • C21D2211/00—Microstructure comprising significant phases
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    • C21D2211/00—Microstructure comprising significant phases
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    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
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    • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath

Definitions

  • the present invention relates to a steel sheet suitable as a material for automobiles, and more particularly, to an ultra-high strength steel sheet having excellent ductility.
  • Transformation steel is a ferrite-martensitic dual phase (DP) steel in which a hard martensite phase is formed on a ferrite matrix, TRIP (Transformation Induced Plasticity) steel using transformation-induced plasticity of retained austenite, ferrite and hard It is classified into CP (Complexed Phase) steel composed of a bainite or martensitic structure, and each of these steels has different mechanical properties, ie, tensile strength and elongation level, depending on the type and fraction of the mother phase and the second phase.
  • DP ferrite-martensitic dual phase
  • TRIP Transformation Induced Plasticity steel using transformation-induced plasticity of retained austenite
  • ferrite and hard It is classified into CP (Complexed Phase) steel composed of a bainite or martensitic structure, and each of these steels has different mechanical properties, ie, tensile strength and elongation level, depending on the type and fraction of the mother phase and the second phase
  • TRIP steel containing a large amount of retained austenite phase has the highest balance (TS ⁇ El) value between tensile strength and elongation.
  • Patent Document 1 discloses a steel capable of securing a tensile strength of 780 MPa or higher, in which the product of tensile strength and elongation is 21000 MPa% or more, including about 10% of the retained austenite phase in addition to ferrite and martensite. .
  • the steel contains a large amount of carbon (C) of about 0.2% and silicon (Si) of about 1.5% or more, there is a concern that spot weldability and hot-dip galvanizing properties are inferior.
  • the annealing is performed twice to realize high physical properties, there is a problem in that the manufacturing cost of the steel sheet increases.
  • Patent Document 2 the content of Si is lowered to 1% level in order to secure good plating properties and spot weldability, and it is composed of martensite, bainite and ferrite without including the retained austenite phase as a microstructure, so that the tensile strength of 980 MPa or more Disclosed is a technology capable of securing strength and elongation of 15% or more.
  • high-strength steel with excellent yield strength is being adopted for structural members such as members, seat rails, and pillars in order to improve the impact resistance of the car body.
  • the steel has a yield strength of 700 MPa or less, so there is a limit to the subject of application.
  • Patent Document 1 Korean Patent Publication No. 2015-0130612
  • Patent Document 2 Korean Patent Publication No. 2013-0106142
  • One aspect of the present invention is to provide a steel sheet suitable for structural members of automobiles and the like, which is excellent in tensile strength as well as yield strength, and has improved ductility, and a method of manufacturing the same.
  • the subject of the present invention is not limited to the above.
  • the subject of the present invention will be understood from the overall content of the present specification, and those of ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the additional subject of the present invention.
  • Another aspect of the present invention comprises the steps of: preparing a steel slab satisfying the above-described alloy composition and Relations 1 to 3; heating the steel slab in a temperature range of 1050 to 1300 °C; manufacturing a hot-rolled steel sheet by hot-rolling the heated steel slab in a temperature range of 800 to 1000°C; winding the hot-rolled steel sheet in a temperature range of 400 to 700°C; manufacturing a cold-rolled steel sheet by cold-rolling the wound hot-rolled steel sheet at a total reduction ratio of 20 to 70%; annealing the cold-rolled steel sheet in a temperature range of 800 to 900°C; cooling the continuously annealed cold-rolled steel sheet to a temperature range of 250 to 400°C; and reheating and maintaining the cooled cold-rolled steel sheet,
  • the reheating and maintaining step provides a method of manufacturing an ultra-high strength steel sheet having excellent ductility, which is performed for 0.1 to 60 minutes in a temperature range of the cooled temperature +50 ° C. to the cooled temperature + 200 ° C. or less.
  • the steel sheet of the present invention it is possible to provide a steel sheet having excellent yield strength as well as tensile strength and improved ductility, and the steel sheet of the present invention has the advantage of guaranteeing formability and collision stability required for a steel sheet for cold forming.
  • FIG. 1 shows a photograph of the microstructure of the inventive steel according to an embodiment of the present invention measured by SEM.
  • FIG. 2 is a photograph showing the microstructure of comparative steel according to an embodiment of the present invention measured by SEM.
  • the inventors of the present invention studied in depth to provide a steel sheet applicable to structural members that require processing into complex shapes by ensuring formability and collision stability with excellent yield strength as well as tensile strength and ductility as an automobile material. .
  • the present invention provides a method for properly dispersing a soft phase and a hard phase by controlling the content relationship of specific elements among alloy components and optimizing the process conditions of a steel sheet manufactured through a series of processes.
  • a steel sheet having a composite structure is provided.
  • the ultra-high strength steel sheet having excellent ductility is, by weight, carbon (C): 0.1 to 0.2%, silicon (Si): 0.1 to 1.0%, manganese (Mn): 2.0 to 3.0%, aluminum ( Al): 1.0% or less (excluding 0%), Chromium (Cr): 1.0% or less, Molybdenum (Mo): 0.5% or less, Titanium (Ti): 0.1% or less, Niobium (Nb): 0.1% or less, Antimony ( Sb): 0.1% or less (excluding 0%), phosphorus (P): 0.05% or less, sulfur (S): 0.02% or less, nitrogen (N): 0.02% or less.
  • the content of each element is based on the weight, and the ratio of the tissue is based on the area.
  • Carbon (C) is an element that greatly contributes to strengthening the strength of the steel sheet, and the C is precipitated in the grains of the steel sheet to induce solid solution strengthening, and promotes the formation of martensite in the steel to strengthen the steel.
  • C is an austenite stabilizing element, and plays an important role in forming retained austenite. Specifically, as the amount of carbon (C) dissolved in the austenite increases, the austenite stability increases, and the fraction of austenite in the steel increases. This induces an increase in the fraction of martensite formed due to the transformation of austenite, thereby improving the strength of the steel sheet, and some austenite remains at room temperature and remains as retained austenite.
  • C can be added in an amount of 0.1% or more, but when the content exceeds 0.2%, the fraction of the martensite phase is excessively increased, and the fraction of the ferrite phase having relatively excellent elongation and shock absorption energy decreases. do. For this reason, the ductility of a steel plate decreases, and becomes a cause which increases the possibility of occurrence of brittleness.
  • the C may be included in an amount of 0.1 to 0.2%, more advantageously 0.12% or more and 0.18% or less.
  • Silicon (Si) is an element contributing to stabilization of retained austenite by suppressing precipitation of carbides in ferrite and inducing diffusion of carbon in ferrite into austenite.
  • Si in an amount of 0.1% or more, but when the content exceeds 1.0%, Si oxide is formed on the steel surface, thereby inhibiting the effect of hot-dip plating and chemical conversion coating there is
  • the Si may be included in an amount of 0.1 to 1.0%, more advantageously 0.2% or more, and even more advantageously 0.4% or more. On the other hand, the Si may be included in 0.9% or less more advantageously.
  • Manganese (Mn) may act as an austenite stabilizing element similarly to C above. Specifically, the Mn may contribute to increasing the fraction of martensite in the steel by reducing the critical cooling rate at which martensite is formed in the composite steel.
  • Mn-Band a striped band
  • the Mn may be included in an amount of 2.0 to 3.0%, and more advantageously may be included in an amount of 2.2% or more and 2.8% or less.
  • Aluminum (Al) is an element added for deoxidation of steel, and is a ferrite stabilizing element similar to Si.
  • the Al is effective in improving martensite hardenability by distributing carbon in ferrite to austenite, and is a useful element for improving ductility of steel sheet by effectively suppressing precipitation of carbides in bainite when maintained in the bainite region am.
  • the Al may be included in an amount of 1.0% or less, and 0% is excluded. More advantageously, the Al may include 0.01% or more.
  • Al means soluble aluminum (Sol.Al).
  • Chromium (Cr) is an element added to improve hardenability of steel and secure high strength, and plays an important role in forming martensite.
  • it is advantageous for the manufacture of composite structure steel having high ductility by minimizing the decrease in elongation compared to the increase in strength.
  • the Cr may be included in an amount of 1.0% or less, and even if the Cr is not intentionally added, there is no difficulty in securing the intended physical properties.
  • Molybdenum (Mo) is an element that forms carbides in steel, and can contribute to improving yield strength and tensile strength by combining with Ti and Nb in steel to form fine carbides in steel.
  • Mo Molybdenum
  • the Mo may be included in an amount of 0.5% or less, and even if the Mo is not intentionally added, there is no difficulty in securing the intended physical properties.
  • Titanium (Ti) forms fine carbides in steel like Mo, and can contribute to securing yield strength and tensile strength of steel.
  • Ti may inhibit the N contained in the steel from being precipitated as TiN by forming a nitride so that the N is combined with Al and precipitated as AlN, which has the effect of reducing the risk of cracks occurring in the casting process.
  • the Ti may be included in an amount of 0.1% or less, and even if the Ti is not intentionally added, there is no difficulty in securing the intended physical properties.
  • Niobium (Nb) is segregated at the austenite grain boundary to suppress coarsening of austenite grains during annealing heat treatment, and may contribute to increasing the strength of the steel sheet by precipitating fine carbides on the grains.
  • the Nb may be included in an amount of 0.1% or less, and even if the Nb is not intentionally added, there is no difficulty in securing the intended physical properties.
  • Antimony (Sb) is distributed at grain boundaries and delays diffusion of oxidizing elements such as Mn, Si, and Al through grain boundaries in steel to suppress surface thickening of oxides, and coarsening of surface thickenings due to temperature rise and changes in hot rolling process has a beneficial effect in suppressing
  • the Sb may be included in an amount of 0.1% or less, and 0% is excluded. More advantageously, the Sb may be included in an amount of 0.01% or more.
  • Phosphorus (P) segregates at grain boundaries and is a major cause of occurrence of temper brittlement, and has a problem of impairing weldability and toughness. Accordingly, it is advantageous to control the content of P as low as possible to be as close to 0% as possible, but it is inevitably contained in the steel manufacturing process. Therefore, it is effective to manage the upper limit.
  • the P may be limited to 0.05% or less, and more advantageously may be limited to 0.03% or less. However, it should be noted that 0% can be excluded in consideration of the unavoidably added level.
  • S is an impurity that is unavoidably contained in steel together with the above-described P, and has a problem of impairing the ductility and weldability of the steel sheet. Accordingly, it is advantageous to control the S content as low as possible to be as close to 0% as possible, but it is effective to manage the upper limit in consideration of the cost and time consumed in the process for reducing the S content.
  • the S may be limited to 0.02% or less, and more advantageously may be limited to 0.01% or less. However, it should be noted that 0% can be excluded in consideration of the unavoidably added level.
  • Nitrogen (N) may combine with Al in steel to form an alumina-based non-metallic inclusion of AlN.
  • the AlN decreases the playing quality and increases the brittleness of the steel sheet to increase the risk of fracture defects.
  • the N may be limited to 0.02% or less, and more advantageously may be limited to 0.01% or less. However, 0% may be excluded in consideration of the unavoidable inflow level.
  • the remaining component of the present invention is iron (Fe).
  • Fe iron
  • the content relationship between specific elements in the steel preferably satisfies all of the following Relations 1 to 3.
  • the above Relations 1 and 2 are component relations derived by quantifying the degree of contribution to strengthening the yield strength and tensile strength of the steel sheet by controlling the microstructure phase fraction constituting the steel sheet and improving the solid solution strengthening effect. .
  • C has a relatively large coefficient compared to Si and Mn, and this is because C is dissolved in the grains of the steel sheet and greatly contributes to strength improvement.
  • Si has a relatively small coefficient compared to C, which is due to a smaller effect of contributing to solid solution strengthening than C.
  • Al has a negative coefficient value, which contributes to solid solution strengthening, but causes a decrease in strength by remaining dual phase region ferrite during annealing, or promoting ferrite transformation during subsequent cooling This is due to the greater effect of
  • Cr and Mo are representative hardenability elements, and since they suppress ferrite transformation during cooling after annealing, they have an effect of improving strength, and are expressed as positive values.
  • Ti and Nb are elements contributing to the improvement of strength by forming fine carbides, they may have positive coefficient values in the strength relational expression according to the component elements. However, while the fine carbide is formed, the amount of solid-solution carbon is reduced, so that the solid-solution strengthening effect of carbon is reduced. Accordingly, Ti and Nb have a positive coefficient value when the precipitation strengthening effect is dominant due to their addition, whereas it can be expressed as a negative coefficient value when the solid solution strengthening effect of carbon by the precipitation of carbide is dominant. .
  • Equation 3 is a component relational expression derived by quantifying the degree of contribution to improving the elongation of the steel sheet as well as the improvement of the solid solution strengthening effect by specific elements.
  • Equation 3 since it contributes to securing retained austenite as well as to the effect of improving strength by solid solution strengthening, Equation 3 also has a positive coefficient value.
  • the steel sheet of the present invention having the above-mentioned alloy composition system includes a fine structure in which a soft phase and a hard phase are appropriately dispersed, especially ferrite having an area fraction of 3 to 20%, retained austenite of 1 to 10%, and bay of 1 to 30%. It is characterized by containing nite, 30-70% tempered martensite and the remainder fresh martensite.
  • the ferrite is an allotrope of iron (Fe) having a body-centered cubic structure (BCC), and has a soft structure unlike martensite and bainite. Accordingly, the elongation is higher than that of the bainite and martensite phases, and there is an advantage in that the shock absorption energy is excellent.
  • the fraction of ferrite exceeds 20%, the soft tissue in the steel sheet is excessively formed to promote plastic deformation, which causes a decrease in the yield strength of the steel sheet.
  • the fraction of the ferrite is less than 3%, there is a problem in that the elongation of the steel sheet is reduced and the formability is deteriorated.
  • the ferrite may be included in an area fraction of 3 to 20%, and more advantageously in an area fraction of 5 to 15%.
  • the retained austenite cannot be transformed into martensite or bainite in a series of heat treatment processes (corresponding to the [annealing-cooling-reheating and maintenance] process in the present invention) during the manufacturing process of the steel sheet, but austenite remaining in the steel It refers to the knight structure, and serves to control the balance between the strength and elongation of the steel sheet.
  • the elongation decreases and formability decreases, and when the elongation of the steel sheet increases, the strength decreases, making it difficult to secure the physical properties required as a structural member, but the retained austenite phase is the tensile strength of the steel sheet ( TS) x elongation (El) value is increased, so it is useful for improving the balance between strength and elongation.
  • TS tensile strength of the steel sheet
  • El elongation
  • the residual austenite phase may be included in an area fraction of 1% or more, but when the fraction exceeds 10%, the sensitivity of liquid metal embrittlement increases and the spot weldability is inferior.
  • the retained austenite may be included in an area fraction of 1 to 10%, and more advantageously may be included in an area fraction of 3 to 9%.
  • the bainite may contribute to improving workability by reducing the strength difference between structures in the steel. That is, it serves to prevent cracks, defects and fractures in the steel sheet due to the difference in hardness between the ferrite and retained austenite phases having relatively low hardness and tempered martensite and fresh martensite having relatively high hardness.
  • the area fraction may be included in an area fraction of 1% or more, more advantageously, 5% or more.
  • the fraction exceeds 30%, the fraction of fresh martensite is reduced, so that it is difficult to secure a target level of strength.
  • the bainite may be included in an area fraction of 1 to 30%.
  • the tempered martensite refers to a tissue softened by tempering a martensite phase obtained by quenching austenite at a temperature of about 500°C.
  • the tempered martensite phase has a higher strength than the aforementioned structures, and thus greatly contributes to the improvement of the yield strength and tensile strength of the steel sheet.
  • carbon in the martensite obtained by quenching is distributed to surrounding austenite during the tempering process to increase the thermal stability of the austenite so that it can remain at room temperature, thereby improving the elongation of the steel sheet.
  • the tempered martensite phase in an area fraction of 30% or more.
  • the fraction exceeds 70%, there is a problem in that the fraction of the retained austenite phase is relatively reduced.
  • the tempered martensite may be included in an area fraction of 30 to 70%.
  • the remaining structures other than the ferrite, retained austenite, bainite, and tempered martensite phases may include a fresh martensite phase.
  • the fresh martensite phase is a structure obtained in the process of final cooling to room temperature, and has the highest strength, thereby greatly contributing to the improvement of the yield strength and tensile strength of the steel sheet.
  • the fraction of the fresh martensite phase is not particularly limited, but as an example, it can be included in an area fraction of 3% or more.
  • the steel sheet of the present invention has excellent tensile strength, yield strength, and elongation by properly forming a soft phase and a hard phase, and specifically, it can have a yield strength of 700 MPa or more, a tensile strength of 980 MPa or more, and an elongation of 13% or more. there is.
  • the steel sheet of the present invention may be a cold-rolled steel sheet, a hot-dip galvanized steel sheet including a zinc-based plating layer on at least one surface of the cold-rolled steel sheet, or an alloyed hot-dip galvanized steel sheet obtained by alloying the hot-dip galvanized steel sheet.
  • the zinc-based plating layer may be a zinc plating layer mainly containing zinc or a zinc alloy plating layer containing aluminum and/or magnesium in addition to zinc.
  • the present invention can manufacture a desired steel sheet through the process of [steel slab reheating - hot rolling - winding - cold rolling - continuous annealing - cooling - reheating and maintenance], and then [hot-dip galvanizing - alloying heat treatment] A further process can be performed.
  • the heating process may be performed in a temperature range of 1050 to 1300 °C.
  • the heating temperature is less than 1050° C.
  • the temperature exceeds 1300° C. not only the energy cost required for temperature increase increases, but also the amount of surface scale increases, which may lead to material loss.
  • the heating process may be performed in a temperature range of 1050 to 1300°C, and more advantageously in a temperature range of 1090 to 1250°C.
  • the steel slab heated according to the above may be hot-rolled to produce a hot-rolled steel sheet, and at this time, the finish hot rolling may be performed in a temperature range of 800 to 1000°C.
  • the effect of simultaneously improving the rigidity and formability of the steel sheet can be obtained.
  • the temperature is less than 800 °C
  • the load due to the rolling greatly increases. This causes the formation of excessive dislocations to cause the formation of coarse grains on the surface of the steel sheet in the subsequent winding or cold rolling process, thereby causing a decrease in strength.
  • the temperature exceeds 1000 °C
  • the size of the ferrite grains increase, there is a problem that the strength also decreases.
  • scale occurs on the surface of the hot-rolled steel sheet, which may cause surface defects and shorten the life of the rolling roll.
  • the finish hot rolling during the hot rolling may be performed in a temperature range of 800 to 1000°C, and more advantageously in a temperature range of 850 to 950°C.
  • the hot-rolled steel sheet manufactured according to the above can be wound, and in this case, it can be carried out in a temperature range of 400 to 700 °C.
  • the coiling temperature is less than 400 °C, the strength of the hot-rolled steel sheet is excessively increased, which may cause a rolling load during subsequent cold rolling. In addition, the cost and time for cooling the hot-rolled steel sheet to the coiling temperature are excessively required, which causes an increase in process cost. On the other hand, when the temperature exceeds 700° C., scale is excessively generated on the surface of the hot-rolled steel sheet, which is highly likely to cause surface defects and cause deterioration of plating properties.
  • the winding process may be performed in a temperature range of 400 to 700 °C, and more advantageously in a temperature range of 500 to 700 °C.
  • the wound hot-rolled steel sheet may be cooled to room temperature.
  • the cooling rate is not particularly limited, but may be performed by air cooling.
  • the hot-rolled steel sheet may be cold-rolled to manufacture a cold-rolled steel sheet, and in this case, it may be performed at a cold rolling reduction of 20 to 70%.
  • the cold rolling reduction is less than 20%, it is difficult to obtain a steel sheet having a target thickness, and it is difficult to correct the shape of the steel sheet.
  • it exceeds 70% there is a high possibility that cracks occur in the edge portion of the steel sheet, and there is a problem of bringing a cold rolling load.
  • coarse ferrite may be formed during subsequent continuous annealing due to an excessive load on the surface of the steel sheet.
  • the cold rolling can be performed at a cold reduction ratio of 20 to 70%, and more advantageously, it can be performed at a cold reduction ratio of 30 to 60%.
  • a pickling treatment may be performed on the hot-rolled steel sheet.
  • the pickling treatment is a process of removing the scale formed on the surface of the hot-rolled steel sheet using hydrochloric acid (HCl), etc., and may be performed under normal conditions, so the conditions are not particularly limited.
  • the cold-rolled steel sheet manufactured according to the above may be annealed, and as an example, a continuous annealing process may be performed, but the present invention is not limited thereto, and any known annealing method may be used.
  • the ferrite formed in the cold-rolled steel sheet can be recrystallized through the annealing process, and the fractions of ferrite and austenite in the steel can be adjusted.
  • the strength of the steel sheet manufactured after the final heat treatment (referred to as a reheating process to be described later) is determined by the fraction of each phase formed.
  • the higher the fraction of austenite the greater the fraction of martensite or bainite transformed from austenite.
  • the strength of the steel sheet tends to be improved.
  • the strength can be additionally controlled by a series of heat treatment conditions to be described later.
  • carbon (C) in the steel can be distributed through the annealing process, thereby increasing the amount of carbon (C) contained in austenite, and thus can have an austenite phase of up to 10 area% even at room temperature.
  • the annealing process may be performed in a temperature range of 800 to 900 °C.
  • the fraction of austenite formed through the annealing process is reduced, and there is a fear that the fraction of tempered martensite, bainite and fresh martensite formed during heat treatment to be described later may not be sufficient. This may cause the yield strength and tensile strength of the final steel sheet to be reduced.
  • the temperature exceeds 900° C. the fraction of austenite in the steel sheet becomes excessively high, and there is a problem in that some austenite is transformed into ferrite in the heat treatment process to be described later.
  • the carbon concentration of retained austenite is lowered, and there is a fear that mechanical stability may decrease, which causes a decrease in the elongation of the steel sheet.
  • moisture generated as Fe is oxidized in the steel reacts with Si, Mn, and Al in the steel to increase the possibility of forming an oxide film on the steel sheet.
  • the oxide film inhibits the wettability of Zn during hot-dip galvanizing, so that the surface quality of the steel sheet may be deteriorated.
  • the annealing process may be performed in a temperature range of 800 to 900°C, and more advantageously in a temperature range of 820 to 870°C.
  • the cold-rolled steel sheet having completed the annealing process may be cooled.
  • quenched martensite can be formed by cooling the annealed cold-rolled steel sheet, and for this purpose, the cooling is preferably performed below the martensite transformation initiation temperature (Ms). More preferably, it can be carried out up to a temperature range of 250 to 400°C.
  • Ms martensite transformation initiation temperature
  • cooling to the above-mentioned temperature range it can be carried out at an average cooling rate of 2 to 50 °C / s. If the cooling rate is less than 2°C/s, ferrite is further transformed during cooling to cause a decrease in strength, whereas if the cooling rate exceeds 50°C/s and is rapidly cooled, a cooling deviation for each position of the steel sheet occurs. There is a problem in that the shape of the steel sheet is inferior.
  • the cooling method is not particularly limited.
  • the cooling may be a single cooling method of cooling to the cooling termination temperature at the initially set cooling rate, and as another example, step-by-step cooling (step-by-cooling) in which slow cooling is performed up to a certain section and then gradual cooling is performed to the cooling termination temperature. step cooling) method, but is not limited thereto.
  • a process of maintaining the cooled temperature for a certain period of time may be performed, and in this process, an isothermal transformation phase is additionally introduced, thereby accelerating transformation of bainite in a subsequent process.
  • the holding step may be performed for 0.1 to 60 minutes.
  • the tempering treatment can be performed by maintaining the sheet for a certain period of time.
  • the quenched martensite phase formed in the cooling process is tempered and transformed into tempered martensite.
  • supersaturated carbon (C) in the quenched martensite is redistributed to surrounding austenite, or bainite transformation is induced to improve the stability of retained austenite, thereby improving elongation.
  • the reheating may be limited to be performed at the cooled temperature +200°C or less.
  • the holding time exceeds 60 minutes, ferrite and cementite in equilibrium are formed at the holding temperature, thereby reducing the strength of the steel sheet, and if it is less than 0.1 minutes, the intended effect cannot be obtained.
  • the steel sheet of the present invention is excellent in yield strength and tensile strength, and can have an effect of improved ductility.
  • the process of cooling to room temperature is not particularly limited, but may be performed by air cooling as an example. However, it will be obvious that it can be replaced by a known cooling method such as water cooling, oil cooling, or furnace cooling.
  • a plated steel sheet having a plating layer on at least one surface can be manufactured by plating the cold-rolled steel sheet that has completed the series of heat treatment processes according to the above as will be described later.
  • a hot-dip galvanized steel sheet may be manufactured by immersing the steel sheet manufactured through the above-described series of processes in a hot-dip galvanizing bath.
  • the hot-dip galvanizing may be performed under normal conditions, but may be performed, for example, in a temperature range of 430 to 490°C.
  • the composition of the hot-dip zinc-based plating bath is not particularly limited during the hot-dip galvanizing, and may be a pure zinc plating bath or a zinc-based alloy plating bath containing Si, Al, Mg, or the like.
  • an alloyed hot-dip galvanized steel sheet can be obtained by performing alloying heat treatment on the hot-dip galvanized steel sheet.
  • the alloying heat treatment process conditions are not particularly limited, and may be under normal conditions.
  • the alloying heat treatment process may be performed in a temperature range of 480 to 600°C.
  • yield strength YS
  • tensile strength TS
  • El elongation
  • the internal tissue was polished with a specimen, nital etched, and then the area of each phase was calculated using a scanning electron microscope (SEM).
  • Comparative Examples 1 and 2 which do not satisfy at least one of Relations 1 and 2 of the component relational equations proposed in the present invention, at least one of the yield strength and the tensile strength is not secured to the target level.
  • Comparative Example 7 which does not satisfy Relational Equation 3 among the component relational expressions, is significantly inferior in elongation.
  • the relational expression 1 characterized in the present invention contributes to the strengthening of the yield strength by the microstructure fraction of the steel sheet and the solid solution strengthening effect
  • the relation 2 contributes to the improvement of the tensile strength of the steel sheet
  • the relation 3 is to improve the ductility of the steel sheet. contribution has been proven.
  • Example 1 is a photograph showing the structure of Inventive Example 1, and it can be confirmed that ferrite, retained austenite, tempered martensite, and bainite were formed within the target fraction range, and fresh martensite phase was formed as the remainder structure. .

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