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EP4357476A1 - Ultra high strength steel sheet having high yield ratio and excellent bendability and method of manufacturing same - Google Patents

Ultra high strength steel sheet having high yield ratio and excellent bendability and method of manufacturing same Download PDF

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Publication number
EP4357476A1
EP4357476A1 EP22825376.1A EP22825376A EP4357476A1 EP 4357476 A1 EP4357476 A1 EP 4357476A1 EP 22825376 A EP22825376 A EP 22825376A EP 4357476 A1 EP4357476 A1 EP 4357476A1
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Prior art keywords
steel sheet
less
temperature
ceq
cold
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German (de)
English (en)
French (fr)
Inventor
Sang-Hyun Kim
Min-Seo KOO
Eun-Young Kim
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Posco Holdings Inc
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Posco Co Ltd
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Publication of EP4357476A1 publication Critical patent/EP4357476A1/en
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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
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES OR PROFILES, OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
    • B21C47/00—Winding-up, coiling or winding-off metal wire, metal band or other flexible metal material characterised by features relevant to metal processing only
    • B21C47/02—Winding-up or coiling
    • 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
    • C21D1/84—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
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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
    • 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
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips 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
    • 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
    • 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
    • C21D8/02—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
    • C21D8/0236—Cold rolling
    • 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
    • C21D8/02—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
    • 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
    • C21D8/02—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
    • 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
    • C21D9/46—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
    • 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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • 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
    • 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
    • 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
    • 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
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • 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
    • C22C38/38—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
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
    • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
    • 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
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
    • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
    • C23G1/08—Iron or steel
    • 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 an ultra-high strength steel sheet having a high yield ratio, and excellent bendability, and a method of manufacturing the same.
  • an ultra-high strength steel sheet may be manufactured by a tempering method, and in this case, the yield ratio is very high, but a problem in which shape quality of a coil deteriorates may occur due to temperature deviation thereof in width and length directions, and a problem such as material defects, workability deterioration, and the like, depending on parts, when processing roll-forming parts, may occur.
  • elongation of the steel sheet generally decreases as the strength of the steel sheet increases, there may be a problem in that forming processability deteriorates, the application thereof as a material for cold stamping may be limited.
  • a hot press forming (HPF) method in which a material is formed at a high temperature at which forming is relatively easily performed, and then required strength is secured through water cooling between a die and the material is being developed. Since it is possible to secure high strength compared to the same thickness, the HPF method is widely used in manufacturing parts, but there is a problem in application thereof due to excessive equipment investment and increase in process costs, so it is necessary to develop a material for cold stamping. Therefore, it is required to develop a cold-rolled steel sheet suitable for use as a material for cold stamping, having high strength and a high yield ratio, and excellent bending properties in order to secure good crash performance.
  • An aspect of the present disclosure is to provide an ultra-high strength steel sheet having a high yield ratio having excellent bending properties and a method of manufacturing the same.
  • a steel sheet including, by weight: carbon (C): 0.1 to 0.3%, manganese (Mn): 1.0 to 2.3°, silicon (Si): 0.05 to 1.0%, phosphorous (P): 0.1% or less, sulfur (S): 0.03% or less, aluminum (Al): 0.01 to 0.5%, with a balance of Fe and inevitable impurities,
  • the steel sheet may further include two or more of chromium (Cr): 0.01 to 0.2%, molybdenum (Mo): 0.01 to 0.2%, and boron (B): 0.005% or less (excluding 0%).
  • Cr chromium
  • Mo molybdenum
  • B boron
  • the steel sheet may further include one or more of titanium (Ti): 0.1% or less (excluding 0%) and niobium (Nb): 0.1% or less (excluding 0%).
  • the steel sheet may include 99 area% or more of martensite or tempered martensite as a microstructure.
  • the steel sheet may have a tensile strength of 1300 MPa or more, and a bending property (R/t) of less than 4, where R is a minimum bending radius at which cracks do not occur in a bent portion after a 90° bending test, and t is a thickness of the steel sheet.
  • a method of manufacturing a steel sheet including operations of: preparing a cold-rolled steel sheet including, by weight: carbon (C): 0.1 to 0.3%, manganese (Mn): 1.0 to 2.3%, silicon (Si): 0.05 to 1.0%, phosphorous (P): 0.1% or less, sulfur (S): 0.03% or less, aluminum (Al): 0.01 to 0.5%, with a balance of Fe and inevitable impurities, wherein an R value defined by the following Relational Expression 1 is 0.12 to 0.27;
  • the cold-rolled steel sheet may further include two or more of chromium (Cr): 0.01 to 0.2%, molybdenum (Mo): 0.01 to 0.2%, and boron (B): 0.005% or less (excluding 0%).
  • Cr chromium
  • Mo molybdenum
  • B boron
  • the cold-rolled steel sheet may further include one or more of titanium (Ti): 0.1% or less (excluding 0%) and niobium (Nb): 0.1% or less (excluding 0%).
  • the operation of preparing the cold-rolled steel sheet may include operations of:
  • An operation of pickling the cooled and wound steel sheet with hydrochloric acid may be further included.
  • a steel sheet having high strength and a high yield ratio, and excellent bending properties and a method of manufacturing the same may be provided.
  • a steel sheet that can be applied as a body-in-white (BIW) structural member and a method of manufacturing the same may be provided.
  • BAW body-in-white
  • FIG. 1 (a) and (b) are SEM microstructure pictures (x10.000) of Inventive Example 15 and Comparative Example 21 according to an embodiment of the present disclosure.
  • an alloy composition and processing conditions were optimized.
  • the present inventor has confirmed that a content of component elements such as C, Mn, Si, P, and S was strictly controlled, conditions of secondary cooling and reheating and overaging processes during continuous annealing were optimized, so that bending properties and high strength may be secured while securing basic welding properties, thereby completing the present disclosure.
  • % indicating a content of each element is based on weight.
  • steel may include by weight, carbon (C): 0.1 to 0.3%, manganese (Mn): 1.0 to 2.3%, silicon (Si): 0.05 to 1.0%, phosphorus (P): 0.1% or less, sulfur (S): 0.03% or less, aluminum (Al): 0.01 to 0.5%, with a balance of Fe and inevitable impurities.
  • Carbon (C) is an interstitial solid-solution element, and is the most effective and important element in improving strength of steel, and is an element that should be added in order to secure strength of martensitic steel.
  • carbon (C) is preferably added in an amount of 0.1% or more, more preferably 0.12% or more.
  • a content of C exceeds 0.3%, the martensite strength may be increased, but carbides may be easily generated and coarsened during a continuous annealing process, so that ductility may be reduced and bending properties may be inferior.
  • Manganese (Mn) is an element that is easy to secure final martensite by inhibiting ferrite formation and promoting austenite formation in a composite structure steel.
  • a content of manganese (Mn) exceeds 2.3%, manganese (Mn) is segregated in a thickness direction and it is easy to form a manganese (Mn) band in a slab, so there is a problem in that occurrence of defects increases during a rolling process along with continuous casting cracks. Therefore, manganese (Mn) may be included more preferably in an amount of 2.1% or less.
  • a lower limit thereof may be limited to 1.0%. A more preferable lower limit thereof may be 1.4%.
  • silicon (Si) serves to suppress carbide generation and control a size of carbides in reheating and overaging operations after cooling in a martensitic steel
  • a lower limit of silicon (Si) may be limited to 0.05%. More preferably, silicon (Si) may be included in an amount of 0.09% or more.
  • silicon (Si) is a ferrite stabilizing element, and when a content of silicon (Si) exceeds 1.0%, ferrite may be generated during cooling in a continuous annealing furnace, which may weaken the strength.
  • an upper limit of silicon (Si) may be limited to 1.0%. More preferably, the upper limit thereof may be limited to 0.6%.
  • Phosphorus (P) 0.1% or less
  • Phosphorus (P) is an impurity element included in steel, and a content of 0% is excluded in consideration of a case where P is inevitably included during a manufacturing process.
  • an upper limit of P may be limited to 0.1%.
  • a more preferable upper limit of P may be 0.03%.
  • S Sulfur
  • S is an impurity which is unavoidably included in steel, and is an element impairing ductility and weldability of a steel sheet, so it is preferable to keep a content of S as low as possible. Therefore, it is preferable to limit the content of S to 0.03% or less. More preferably, the content of S may be limited to 0.005% or less. Meanwhile, 0% is excluded in consideration of a case inevitably included during a manufacturing process.
  • Aluminum (Al) may be added to remove oxygen, and like Si, is an element stabilizing ferrite.
  • Al is a component capable of improving hardenability of final martensitic steel by increasing a content of C in austenite, it is preferable to add 0.01% or more of a content of Al.
  • ferrite may be generated during cooling in a continuous annealing furnace, which may weaken the strength.
  • AlN formation may cause casting cracks in a slab, and there is a problem of inhibiting hot rolling properties, and an upper limit of Al may be limited to 0.5%.
  • the steel of the present disclosure may include remaining iron (Fe) and unavoidable impurities in addition to the above-described composition. Since unavoidable impurities may be unintentionally incorporated in a common manufacturing process, the component may not be excluded. Since these impurities are known to any person skilled in the common manufacturing process, the entire contents thereof are not particularly mentioned in the present specification.
  • steel may further include two or more of chromium (Cr): 0.01 to 0.2%, molybdenum (Mo) : 0.01 to 0.2%, and boron (B) : 0.005% or less (excluding 0%).
  • Cr chromium
  • Mo molybdenum
  • B boron
  • Chromium (Cr) is a component added to improve hardenability and secure high strength of steel, and is useful in manufacturing ultra-high strength steel having pure martensite by suppressing bainite generation. Therefore, it is preferable to add chromium (Cr) in an amount of 0.01% or more in order to secure the above-described effect.
  • Cr chromium
  • an upper limit of Cr may be limited to 0.2%, more preferably 0.1%.
  • Molybdenum is an element improving hardenability of steel, and is preferably added in an amount of 0.01% or more to obtain a hardenability effect.
  • Mo Molybdenum
  • an upper limit of Mo to 0.2%, and more preferably to 0.1%.
  • Boron (B) is an element which suppresses transformation of austenite into ferrite during a continuous annealing process, and is an element which is effective in improving hardenability of martensite, such as Cr, and Mo, even when added in a very small amount thereof.
  • a content of boron (B) exceeds 0.005%, an Fe 23 (B,C) 6 precipitated phase precipitates at an austenite grain boundary, thereby promoting ferrite formation, so it is preferable to limit an upper limit of B to 0.005%.
  • Steel according to an aspect of the present disclosure may further include one or more of titanium (Ti): 0.1% or less (excluding 0%), niobium (Nb): 0.1% or less (excluding 0%).
  • Ti titanium
  • Nb niobium
  • Titanium (Ti) is an element for forming fine carbides, thereby contributing to securing yield strength and tensile strength.
  • titanium (Ti) is scavenged by precipitating N in steel as TiN, and to this end, it is preferable to add 48/14*[N] or more in a chemical equivalent, and when B is added, to maximize an addition effect thereof, it is preferable to add titanium (Ti).
  • a content of titanium (Ti) exceeds 0.1%, coarse carbides may be precipitated, strength and elongation may be reduced by reducing an amount of carbon in steel, and nozzle clogging may be caused during casting, so it is preferable to limit an upper limit of Ti to 0.1%.
  • Niobium (Nb) is an element which is segregated at austenite grain boundaries to suppress coarsening of austenite crystal grains during an annealing heat treatment, and to contribute to increase strength by forming fine carbides.
  • Nb niobium
  • an upper limit of Nb may be preferably limited to 0.1%.
  • steel may have an R value, defined in the following Relational Expression 1 may be 0.12 to 0.27.
  • Relational Expression 1 is a complex relational expression of Ceq1 and Ceq2 representing welding properties according to the content of respective elements, and when the R value of Relational Expression 1 is 0.12 to 0.27, physical properties including welding properties, targeted by the present disclosure may be secured.
  • % representing a fraction of the microstructure is based on an area unless otherwise specified.
  • the steel according to an aspect of the present disclosure may include martensite or tempered martensite by an amount of 99 area% or more as a microstructure, and the number of carbides per 1 ⁇ m 2 area may be 40 or less, and an average length of a major axis of the carbide may be 300 nm or less.
  • martensite or tempered martensite may be included as a microstructure in order to secure a cold-rolled steel sheet having high strength and a high yield ratio, and it is preferable to add the same by an amount of 99% or more to secure a high strength level of 1.3G-level or higher.
  • the number of carbides in order to secure excellent bending properties, it is preferable to control the number of carbides to 40 or less, more preferably 35 or less.
  • an average length of a major axis of the carbide may be preferably 300 nm or less, more preferably 200 nm or less.
  • the number of carbides of the present disclosure represents a n average of the number of carbides in a 1 ⁇ m 2 region (average of 10 regions) in a x10,000 SEM image, and a length of the major axis of is shown by measuring x30,000 to x100,000 images on a TEM bright field.
  • Steel according to an aspect of the present disclosure may be manufactured by heat treatment, primary cooling, secondary cooling, and reheating and overaging of a cold-rolled steel sheet satisfying the alloy composition described above.
  • a cold-rolled steel sheet satisfying the alloy composition of the present disclosure may be prepared.
  • the cold-rolled steel sheet of the present disclosure may be manufactured under common processing conditions, and may be manufactured by reheating, hot rolling, cooling, winding, and cold rolling a steel slab, preferably under conditions described below.
  • a steel slab satisfying the above-described alloy composition of the present disclosure may be reheated to a temperature within a range of 1100 to 1300°C.
  • Reheating may be performed to smoothly perform a subsequent hot rolling process, and may be performed to sufficient secure physical properties, targeted by the present disclosure.
  • a reheating temperature is lower than 1100°C, there may be a problem in that hot rolling load increases rapidly.
  • the reheating temperature is higher than 1300°C, an amount of surface scales increases, reducing yield of a material and causing surface defects, which may adversely affect the final quality.
  • the reheated steel slab may be hot rolled to a finish hot rolling temperature of Ar3 or higher.
  • finish hot rolling temperature may be limited to Ar3 (a temperature at which ferrite begins to appear during austenite cooling) or higher, which is because ferrite and austenite two-phase or ferrite reverse rolling may be performed at a temperature of Ar3 or lower to form a mixed structure, and there is a concern of malfunction due to fluctuations in hot rolling load.
  • Ar3 a temperature at which ferrite begins to appear during austenite cooling
  • the hot-rolled steel sheet may be cooled to a temperature within a range of 700°C or lower, and then wound.
  • a winding temperature exceeds 700°C, an oxide may be excessively formed on a surface of the steel sheet, which may cause defects.
  • an oxide layer formed on a surface of the wound steel sheet may be removed by a pickling process prior to cold rolling, which is a subsequent process.
  • the cooled and wound steel sheet may be cold rolled at a reduction ratio of 30 to 80%.
  • the reduction ratio of cold rolling is less than 30%, it may be difficult to secure a target thickness, and there may be a concern in that austenite formation and final physical properties may be affected during annealing heat treatment due to remaining hot-rolled crystal grains.
  • the reduction ratio exceeds 80%, there may be a problem in which material deviation of the final steel sheet due to an uneven rolling reduction rate in length and width directions from work hardening, and it may be difficult to secure a target thickness due to a rolling load.
  • the cold-rolled steel sheet may be heat treated at a temperature of Ac3 or higher for 30 seconds or more.
  • heat treatment may be performed to secure an austenite fraction of 100% through austenite single phase annealing.
  • austenite fraction 100% through the heat treatment, it is possible to prevent a decrease in strength due to ferrite formation during annealing.
  • Ac 3 910 ⁇ 203 ⁇ C ⁇ 15.2 Ni + 44.7 Si + 104 V + 31.5 Mo + 13.1 W where [C], [Ni], [Si], [V], [Mo], and [W] are weight percent (%) of respective elements.
  • primary cooling may be performed at an average cooling rate of 1 to 10°C/s to a temperature within a range of 500 to 750°C.
  • a cooling rate when a cooling rate is less than 1°C/s or less, it may be difficult to secure a target strength due to formation of ferrite during cooling.
  • the cooling rate exceeds 10°C/s, during secondary cooling, the average cooling rate may be deteriorated and a fraction of other low-temperature transformation phases, other than martensite may increase, making it difficult to finally secure the target strength.
  • phase such as ferrite, bainite, or the like, may be formed and there may be a concern that the strength is deteriorated, and when the temperature exceeds 750°C, there may be a problem in an actual production line.
  • the primarily-cooled steel sheet may be secondarily cooled at an average cooling rate of 20 to 80°C/s to a temperature of Ms-190°C or lower.
  • Mf martensite transformation finish temperature
  • a secondary cooling end temperature is limited to a temperature of Ms-190°C or lower in order to secure an effect of increasing yield strength by carbide precipitation during subsequent tempering.
  • a tempering temperature when a tempering temperature is increased, bendability may be deteriorated, it is intended to secure bending properties by limiting the secondary cooling end temperature to enable sufficient tempering without raising the tempering temperature too much.
  • the tempering temperature exceeds a temperature of Ms-190°C, it may be difficult to realize desired physical properties since a fraction of martensite or martensite is not sufficiently secured.
  • Ms 539 ⁇ 423 C ⁇ 30.4 Mn ⁇ 16.1 Si ⁇ 59.9 P + 43.6 Al ⁇ 17.1 Ni ⁇ 12.1 Cr + 7.5 Mo where [C], [Mn], [Si], [P], [Al], [Ni], [Cr] and [Mo] are weight percent(%) of respective elements.
  • the secondarily-cooled steel sheet may be reheated and overaged by heating the steel sheet to a temperature within a range of greater than secondary cooling end temperature+30°C and less than 270°C and holding the same for 1 to 20 minutes.
  • a lower limit of the reheating temperature is limited to a temperature of 30°C or higher, compared to a secondary cooling end temperature. In this case, yield strength increases due to formed fine carbides, but when a reheating and overaging temperature is less than the secondary cooling end temperature + 30°C, it is difficult to obtain the desired effect.
  • the temperature is higher than 270°C, there may be a problem in that bending properties may be inferior due to coarsening of carbides.
  • the steel of the present disclosure manufactured as described above may have a tensile strength of 1300MPa or more, a yield ratio of exceeding 0.73, and a bending property (R/t) of less than 4, where R is a bending radius at which cracks do not occur in a bent portion after a 90° bending test, and t is a thickness of the steel sheet, and have excellent bending properties while having a high yield ratio.
  • a steel slab having the composition shown in Table 1 below was heated at 1100 to 1300 °C, finish hot-rolled at 850 to 950° C, which is a temperature of Ar3 or higher, wound at a temperature within a range of 400 to 700 °C, and a cold reduction rate of 45 to 65% was applied to manufacture a cold-rolled steel sheet.
  • primary and secondary cooling were performed under the conditions illustrated in Table 2 below. In this case, a first cooling rate was applied at 2 to 4 °C/ s, and a second cooling rate was applied at 25 to 60 °C/ s.
  • YS yield strength
  • TS tensile strength
  • YS/TS yield ratio
  • T-El total elongation
  • U-El uniform elongation
  • the bending properties (R/t) were measured by specimen-processing the same cold-rolled steel sheet into a width of 100 mm x length of 30 mm, and then performing a 90° bending test under a condition of a test speed of 100 mm/min, and then cracks in a bent portion were confirmed using a microscope, so that an R/t value was obtained by dividing a minimum bending radius (R) at which cracks did not occur by a thickness (t) of a test piece, and when the value thereof was less than 4, it was represented as 0, and when the value thereof was greater than or equal to 4, it was represented as X.
  • R/t minimum bending radius
  • Comparative Examples 1 to 9 illustrates examples in which a reheating step is not included, and quenching and tempering are included as essential processes in the present disclosure, but the above-described examples are examples in which aging is performed at a temperature during cooling without reheating. That is, in the above-described examples, martensitic hardenability may be deteriorated, and since there is no tempering process, the yield strength was very inferior, so that the desired strength may not be obtained.
  • Comparative Examples 10 to 21 were inferior in a yield ratio and bending properties, targeted by the present disclosure.
  • the yield strength cannot be sufficiently increased, and in examples not satisfying the upper limit temperature condition of less than 270°C, bending properties were not secured due to formation of coarse carbides.
  • Comparative Examples 22 and 23 illustrate examples satisfying all of the manufacturing conditions proposed in the present disclosure, but not satisfying the alloy composition proposed in the present invention. Therefore, in the above-described examples, not only did not satisfy the desired microstructure fraction, but also failed to secure the desired strength.
  • FIGS. 1 (a) and (b) are SEM microstructure pictures (x10.000) of Inventive Example 15 and Comparative Example 21 according to an embodiment of the present disclosure.
  • Both (a) and (b) of FIG. 1 illustrate tempered martensite as a microstructure, and it can be confirmed that a carbide in a form of rice grains was formed on the microstructure. Meanwhile, in the case of (b), it can be confirmed that the carbide per unit area was formed on the microstructure in excess of the range proposed in the present disclosure, and a size thereof was also excessively large.

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EP22825376.1A 2021-06-18 2022-06-17 Ultra high strength steel sheet having high yield ratio and excellent bendability and method of manufacturing same Pending EP4357476A1 (en)

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KR1020210079154A KR20220169497A (ko) 2021-06-18 2021-06-18 굽힘 특성이 우수한 고항복비 초고강도 강판 및 그 제조방법
PCT/KR2022/008630 WO2022265453A1 (ko) 2021-06-18 2022-06-17 굽힘 특성이 우수한 고항복비 초고강도 강판 및 그 제조방법

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