WO2023048467A1 - Tôle d'acier à haute résistance et épaisse dotée d'excellentes capacités d'expansion de trou et de ductilité et son procédé de fabrication - Google Patents

Tôle d'acier à haute résistance et épaisse dotée d'excellentes capacités d'expansion de trou et de ductilité et son procédé de fabrication Download PDF

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WO2023048467A1
WO2023048467A1 PCT/KR2022/014116 KR2022014116W WO2023048467A1 WO 2023048467 A1 WO2023048467 A1 WO 2023048467A1 KR 2022014116 W KR2022014116 W KR 2022014116W WO 2023048467 A1 WO2023048467 A1 WO 2023048467A1
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steel sheet
less
excluding
ductility
strength
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Korean (ko)
Inventor
조경래
김성규
박준호
한상호
김정훈
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Posco Holdings Inc
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Posco Co Ltd
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Priority to CN202280065119.XA priority Critical patent/CN118103542A/zh
Priority to EP22873172.5A priority patent/EP4411013A4/fr
Priority to MX2024003632A priority patent/MX2024003632A/es
Priority to US18/694,803 priority patent/US20240401164A1/en
Priority to JP2024518912A priority patent/JP2024535916A/ja
Publication of WO2023048467A1 publication Critical patent/WO2023048467A1/fr
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys 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
    • 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
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES, PROFILES OR LIKE SEMI-MANUFACTURED PRODUCTS 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
    • C21D1/185—Hardening; Quenching with or without subsequent tempering from an intercritical temperature
    • 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
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
    • C21D1/22—Martempering
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys 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
    • 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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • 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
    • 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 invention relates to a steel suitable as a material for automobiles, and more particularly, to a high-strength thick steel sheet having excellent hole expandability and ductility and a manufacturing method thereof.
  • high-strength steel with excellent strength is used as a material for structural members such as members, seat rails, and pillars to improve the impact resistance of the vehicle body.
  • the higher the strength of the steel the more advantageous it is to absorb impact energy.
  • the higher the strength the lower the elongation, thereby reducing the molding processability.
  • the yield strength is excessively high, the flow of material from the mold during molding is reduced, resulting in poor moldability and an increase in manufacturing cost.
  • hole expandability is required for smooth molding, but high-strength steel has low hole expandability, resulting in cracks and cracks during molding. I have a problem with the same glitch. As such, if the hole expandability is poor, there is a concern that the safety of the occupant may be threatened as the part is easily destroyed due to cracks occurring in the part forming part when the car crashes. In addition, as the standard for occupant safety increases, the adoption of thick material materials to secure rigidity is steadily increasing, centering on some automobile companies.
  • high-strength steels used as materials for automobiles include Dual Phase Steel (DP Steel), Transformation Induced Plasticity Steel (TRIP Steel), and Complex Phase Steel (CP Steel). steel), ferrite-bainite steel (Ferrite Bainite steel, FB steel), etc.
  • DP steel an ultra-high-strength steel, has a low yield ratio of approximately 0.5 to 0.6, so it is easy to process and has the advantage of having a high elongation next to TRIP steel. Accordingly, it is mainly applied to door outers, seat rails, seat belts, suspensions, arms, wheel disks, and the like.
  • TRIP steel is characterized by excellent formability (high ductility) by having a yield ratio in the range of 0.57 to 0.67, and is therefore suitable for parts requiring high formability such as members, roofs, seat belts, and bumper rails.
  • CP steel is applied to side panels and underbody stiffeners due to its low yield ratio, high elongation and bending workability, and FB steel has excellent hole expandability and is mainly applied to suspension lower arms or wheel discs.
  • DP steel is mainly composed of ferrite with excellent ductility and hard phases with high strength (martensite phase, bainite phase), and a small amount of retained austenite may be present.
  • This DP steel has excellent characteristics such as low yield strength, high tensile strength, low yield ratio (Yield Ratio, YR), high work hardening rate, high ductility, continuous yield behavior, room temperature aging resistance, baking hardenability, and the like.
  • high-strength steel with high hole expandability can be manufactured by controlling the fraction, recrystallization degree, and distribution uniformity of each phase.
  • DP steel for automobiles is manufactured as a final product through an annealing process after manufacturing a slab through a steelmaking and casting process, then obtaining a hot-rolled coil by performing [heating-rough rolling-finish hot rolling] on the slab.
  • the annealing process is a process mainly performed in the manufacture of cold-rolled steel sheet, and the cold-rolled steel sheet is pickled to remove the surface scale of the hot-rolled coil, cold-rolled at a constant reduction rate at room temperature, and then annealed and necessary. It is manufactured through an additional temper rolling process according to.
  • Cold-rolled steel sheets (cold-rolled products) obtained by cold rolling are in a very hardened state and are not suitable for manufacturing parts that require workability. can make it
  • the steel sheet (cold-rolled material) is heated to approximately 650 to 850° C. in a heating furnace and maintained for a predetermined time to lower hardness and improve workability through recrystallization and phase transformation.
  • the steel sheet that has not undergone the annealing process has high hardness, particularly surface hardness, and lacks workability
  • the steel sheet subjected to the annealing process has a recrystallized structure, so that hardness, yield point, and tensile strength are lowered to improve workability.
  • ferrite is completely recrystallized in the heating process during continuous annealing to produce equiaxed crystals, so that austenite is created and grown in equiaxed crystals in the subsequent process, so that the grain size is reduced. It is advantageous to form a small, uniform austenite phase.
  • the thickness of the hot rolled steel since the thickness of the hot rolled steel must be relatively thick in order to secure a certain reduction ratio, there is a problem in that the load during subsequent cold rolling is large and the operability is lowered.
  • the reduction ratio is low during manufacturing of thick materials, the non-uniformity of the structure due to non-recrystallization of ferrite during annealing increases and the yield strength increases, and the cold rolling directionality is maintained in the microstructure, thereby reducing workability. Therefore, in the case of a thick material material, since the material deviation in the thickness direction is inevitably large due to dimensional characteristics, a technology for homogenizing the material as much as possible is required to improve processability and physical properties.
  • Patent Document 1 discloses that it is possible to secure hole expandability and elongation by forming fine precipitates using Ti, Mo, etc., and configuring them into ferrite, bainite, and martensite phases as microstructures.
  • Patent Document 1 Korean Patent Publication No. 10-2021-0095156
  • One aspect of the present invention provides a high-strength thick steel sheet having a low yield ratio, high strength, and excellent formability such as hole expandability through improvement of ductility and a method for manufacturing the same, as a material suitable for automotive structural members, etc. It's what you want to do.
  • a high-strength thick steel sheet with excellent hole expandability and ductility, including 10-30% area fraction of ferrite, 10-25% of recrystallized ferrite bridge, 20-30% of bainite and the remainder martensite in microstructure do.
  • Another aspect of the present invention preparing a steel slab; heating the steel slab in a temperature range of 1100 to 1300 °C; manufacturing a hot-rolled steel sheet by hot-rolling the heated steel slab; winding the hot-rolled steel sheet in a temperature range of 400 to 700° C.; cooling the hot-rolled steel sheet to room temperature after the winding; manufacturing a cold-rolled steel sheet by cold-rolling the cooled hot-rolled steel sheet at a cold rolling reduction ratio of 55 to 80%; Continuously annealing the cold-rolled steel sheet; Primary cooling at an average cooling rate of 1 to 10 ° C / s to a temperature range of 650 to 700 ° C after the continuous annealing; And secondary cooling at an average cooling rate of 5 to 50 ° C / s to a temperature range of 450 to 500 ° C after the first cooling,
  • the continuous annealing is performed in a facility equipped with a heating zone, a soaking zone, and a cooling zone, and hole expandability and ductility, characterized in that the cold-rolled steel sheet goes through a recrystallization zone maintained at 600 ⁇ 700 °C for 1 to 3 minutes when the temperature is raised on the heating zone
  • a method for manufacturing an excellent high-strength thick steel sheet is provided.
  • the steel sheet of the present invention with improved formability can prevent processing defects such as cracks or wrinkles during press forming, it has an effect of being suitably applied to parts such as structures requiring processing into complex shapes. Furthermore, it is also effective in manufacturing a material with improved collision resistance so that defects such as cracks are not easily formed when a vehicle to which such a part is applied is unavoidably collided.
  • FIG. 1 shows a thermal history and a phase transformation history during continuous annealing according to an embodiment of the present invention.
  • the dotted line indicates the thermal history during conventional continuous annealing
  • the solid line indicates the thermal history during continuous annealing according to the present invention.
  • Figure 2 shows a void (void) forming mechanism in the tissue, (b) shows an interface reinforcing mechanism in the tissue of the inventive example according to an embodiment of the present invention.
  • FIG. 3 and 4 show microstructure photographs of a comparative example (FIG. 3) and an inventive example (FIG. 4) according to an embodiment of the present invention (arrows in FIG. 4 indicate a recrystallized ferrite bridge structure) ).
  • the inventors of the present invention conducted in-depth research to develop a material having a level of moldability that can be suitably used for parts requiring processing into complex shapes among automotive materials.
  • the present inventors derive a structure that can increase crack resistance between hard phases in a thick steel sheet for automobiles, which has a relatively low cold reduction ratio, and at the same time, to prevent generation and propagation of voids. It was confirmed that the objective could be achieved through the miniaturization of the advantageous hard phase and the control of the crystal grain shape, and the present invention was completed.
  • the present invention introduces a recrystallized ferrite bridge having a structure connecting the hard phases to each other so that the unidirectionality of the hard phases is eliminated, and has technical significance in optimizing the alloy composition and manufacturing conditions in forming such a structure.
  • the content of each element is based on weight, and the ratio of tissue is based on area.
  • Carbon (C) is an important element added for solid solution strengthening, and this C contributes to improving the strength of steel by forming fine precipitates in combination with precipitated elements.
  • the C may be included in 0.05 ⁇ 0.12%. More advantageously, it may be included at 0.06% or more, and may be included at 0.10% or less.
  • Manganese (Mn) is an element that is advantageous for preventing hot brittleness due to the formation of FeS by precipitating sulfur (S) in steel as MnS, and for solid solution strengthening of steel.
  • Mn-Band Mn oxide band
  • the Mn may be included in 2.0 ⁇ 3.0%. More advantageously, 2.2% or more and 2.8% or less may be included.
  • Silicon (Si) is a ferrite stabilizing element, and is advantageous in securing a target level of ferrite fraction by accelerating ferrite transformation. In addition, it is effective in increasing the strength of ferrite because of its good solid solution strengthening ability, and is a useful element in securing strength without reducing the ductility of steel.
  • the Si may be included in an amount of 0.5% or less, and 0% may be excluded. More advantageously, it may be included at 0.1% or more.
  • Chromium (Cr) is an element that exerts a hardenability effect during cooling to facilitate the formation of a bainite phase, suppresses the formation of a martensite phase during annealing heat treatment, and contributes to strength improvement by forming fine carbides.
  • the Cr acts as a ferrite stabilizing element during heating to delay the austenite phase transformation reaction, and as the phase transformation starts at a higher temperature, only recrystallization occurs during heating (Trex ⁇ A1) for a long time. As a result, a recrystallized ferrite bridge structure can be secured.
  • the Cr may be included in an amount of 1.0% or less, and 0% may be excluded. More advantageously, it may contain 0.01% or more.
  • Niobium (Nb) is an element that segregates at austenite grain boundaries to suppress coarsening of austenite crystal grains during annealing heat treatment and contributes to strength improvement by forming fine carbides.
  • Nb may be included in an amount of 0.1% or less, and 0% may be excluded.
  • Titanium (Ti) is an element that forms fine carbides and contributes to securing yield strength and tensile strength.
  • Ti has an effect of precipitating N in steel as TiN to suppress the formation of AlN by Al inevitably present in steel, thereby reducing the possibility of cracking during continuous casting.
  • Ti may be included in an amount of 0.1% or less, and 0% may be excluded.
  • Boron (B) is an element that retards the transformation of austenite into pearlite during cooling after annealing heat treatment, but when its content exceeds 0.003%, B is excessively concentrated on the surface and may cause deterioration of plating adhesion. .
  • B may be included in an amount of 0.003% or less, and 0% may be excluded in consideration of an unavoidably added level.
  • Aluminum (sol.Al) is an element added for the grain size refinement effect and deoxidation of steel, and if the content is less than 0.02%, aluminum killed steel cannot be manufactured in a stable state. On the other hand, if the content exceeds 0.05%, the crystal grains are refined and the strength is improved, but the excessive formation of inclusions during steel casting operation increases the risk of surface defects of the coated steel sheet.
  • the sol.Al may be included in an amount of 0.02 to 0.05%.
  • Phosphorus (P) is a substitutional element having the greatest solid-solution strengthening effect, and is an element that is advantageous for improving in-plane anisotropy and securing strength without significantly deteriorating formability.
  • P Phosphorus
  • the content of P can be controlled to 0.05% or less, and 0% can be excluded in consideration of the level that is unavoidably added.
  • S Sulfur
  • S is an element that is unavoidably added as an impurity element in steel, and since it inhibits ductility, it is desirable to manage its content as low as possible.
  • S since S has a problem of increasing the possibility of generating red heat brittleness, it is preferable to control the content to 0.01% or less.
  • 0% can be excluded.
  • Nitrogen (N) is a solid solution strengthening element, but when its content exceeds 0.01%, the risk of brittleness increases, and there is a risk of impairing performance quality by combining with Al in steel to excessively precipitate AlN.
  • N may be included in an amount of 0.01% or less, and 0% may be excluded in consideration of an inevitably added level.
  • the remaining component of the present invention is iron (Fe).
  • Fe iron
  • the steel sheet of the present invention having the above-described alloy composition has a microstructure of soft phase ferrite and hard phase bainite and martensite phases, and a recrystallized ferrite bridge formed by connecting the hard phase ( bridge) organization.
  • the biggest change in the microstructure due to the generation of the recrystallized ferrite bridge is the loss of the rolling direction of the existing ferrite and the high degree of connection around the hard phase.
  • the generation position of reverse transformation austenite is reduced, and the generation of austenite at high temperature is delayed, so that a secondary phase of a smaller size after cooling can be generated.
  • the non-recrystallized ferrite region remains as an irregular rough interface as an elongated structure with a rolling direction, and the recrystallized ferrite bridge grain boundary is characterized by having a polygonal smooth interface.
  • the method for determining the recrystallized ferrite bridge can be classified, for example, by EBSD crystal orientation (electron backscatter diffraction orientation), or an aqueous hydrogen peroxide solution (ex. distilled water 140ml, hydrogen peroxide 100ml, oxalic acid 4g, sulfuric acid 2ml, hydrofluoric acid 1.5ml) It can be optically distinguished by etching.
  • EBSD crystal orientation electron backscatter diffraction orientation
  • aqueous hydrogen peroxide solution ex. distilled water 140ml, hydrogen peroxide 100ml, oxalic acid 4g, sulfuric acid 2ml, hydrofluoric acid 1.5ml
  • the steel sheet of the present invention includes a ferrite phase in an area fraction of 10 to 30%, a recrystallized ferrite bridge phase in an area fraction of 10 to 25%, and a hard phase of 20 to 30% bainite and the remainder martensite phase.
  • a trace amount of retained austenite phase may be included.
  • the recrystallized ferrite bridge phase is an advantageous structure for suppressing the propagation of voids and voids generated along grain boundaries of the hard phase by eliminating the unidirectionality of the hard phase, and is different from conventional polygonal ferrite. is a distinct organization.
  • the recrystallized ferrite bridge is a structure distinct from general recrystallized ferrite, and is relatively coarse, preferably having an average size of 1 to 6 ⁇ m based on an equivalent circular diameter. If the size of the recrystallized ferrite bridge phase is less than 1 ⁇ m, it is difficult to eliminate the orientation of the hard phase, so the desired effect cannot be obtained. On the other hand, if the size exceeds 6 ⁇ m, the structure becomes excessively coarse, resulting in Physical properties may be impaired.
  • the structure (b) intended in the present invention is formed as a structure that connects the hard phases while removing the unidirectionality of the hard phase, it is difficult for voids to coalesce along grain boundaries, resulting in cracks, It has the effect of greatly suppressing its propagation.
  • the fraction of the recrystallized ferrite bridge phase is excessively high, the fraction of the hard phase is relatively low, making it impossible to secure a target level of strength. In consideration of this, 25% or less of the recrystallized ferrite bridge phase may be included. On the other hand, if the fraction is less than 10%, the above-mentioned effects (elimination of unidirectionality of the hard phase, suppression of void propagation, etc.) cannot be sufficiently obtained, resulting in inferior hole expandability.
  • the fraction of the ferrite phase is less than 10%, it is disadvantageous to secure the ductility of the steel. On the other hand, if the fraction exceeds 30%, the fraction of the hard phase is relatively low, making it difficult to secure the target level of strength.
  • the fraction of the bainite phase is less than 20%, it is difficult to secure strength, and there is a problem in that the hardness difference between the soft phase and the martensite phase increases. On the other hand, when the fraction exceeds 30%, the fraction of the soft phase is lowered, making it difficult to secure ductility.
  • the fraction of the martensite phase is not specifically limited, but the area fraction is 15% or more to secure ultra-high strength of 980 MPa or more in tensile strength. can include However, when the fraction of the martensite phase exceeds 60%, ductility is lowered, making it difficult to secure a target level of moldability.
  • the fraction of the retained austenite phase does not exceed 3%, and even if the fraction is 0%, it is revealed that there is no difficulty in securing intended physical properties.
  • the high-strength thick steel sheet of the present invention having the above-described microstructure has a tensile strength of 980 MPa or more, a yield strength of 550 to 700 MPa, and an elongation (total elongation) of 14% or more, so that it may have high strength and high ductility.
  • the steel sheet has a hole expansion ratio (HER) of 30% or more, so that resistance to cracks that may occur during processing and impact fracture resistance are excellent.
  • HER hole expansion ratio
  • the high-strength thick steel sheet of the present invention may have a thickness of 1 to 3 mm, more preferably 1.5 to 2.5 mm.
  • the present invention can manufacture a desired steel sheet through the processes of [steel slab heating - hot rolling - winding - cold rolling - continuous annealing], and each process will be described in detail below.
  • This process is performed in order to smoothly perform the subsequent hot rolling process and sufficiently obtain target physical properties of the steel sheet.
  • the conditions of such a heating process there is no particular restriction on the conditions of such a heating process, and it may be a normal condition.
  • the heating process may be performed in a temperature range of 1100 to 1300 °C.
  • the steel slab heated according to the above may be hot-rolled to produce a hot-rolled steel sheet, and at this time, finish hot-rolling may be performed at an outlet temperature of more than Ar3 and less than 1000 ° C.
  • the finish hot rolling may be performed in a temperature range of 760 ⁇ 940 °C.
  • the hot-rolled steel sheet manufactured according to the above may be wound into a coil shape.
  • the winding may be performed in a temperature range of 400 to 700 °C. If the coiling temperature is less than 400 ° C, martensite or recrystallized ferrite bridge phase is excessively formed, resulting in an excessive increase in strength of the hot-rolled steel sheet, which may cause problems such as shape defects due to load during subsequent cold rolling. there is. On the other hand, when the coiling temperature exceeds 700 ° C., the surface scale increases and the pickling property deteriorates.
  • the coiled hot-rolled steel sheet it is preferable to cool the coiled hot-rolled steel sheet to room temperature at an average cooling rate of 0.1° C./s or less (excluding 0° C./s).
  • the rolled hot-rolled steel sheet may be cooled after passing through processes such as transfer and stacking, and it should be noted that the process prior to cooling is not limited thereto.
  • the hot-rolled steel sheet wound according to the above may be cold-rolled to produce a cold-rolled steel sheet, and in the present invention, the cold rolling may be performed at a cold rolling reduction of 55 to 80%.
  • the present invention can further promote recrystallization of ferrite in the heating section during the subsequent continuous annealing process in a state where an appropriate level of cold rolling reduction is applied during cold rolling, and from this, the formation of fine ferrite is induced to form austenite at the ferrite grain boundary. It can also be formed small and uniformly.
  • the reduction ratio during cold rolling is less than 55%, ferrite recrystallization is delayed, making it difficult to obtain a fine and uniform austenite phase.
  • the yield strength is excessively reduced due to excessive recrystallization, thereby reducing the target level of strength. will not be able to obtain More advantageously, it can be performed at 78% or less.
  • the cold rolling reduction rate can be implemented with a high rolling capacity (eg, 5000KN/mm level) of a ZRM facility from a hot-rolled material having a certain thickness, as well as for repeated rolling by utilizing a reversing mill.
  • a process of achieving a target reduction ratio may also be included.
  • the material after hot rolling may have a thickness of 4 to 8 mm, and when the thickness of the material after hot rolling is 6 mm or more, a cold rolling process using a reversing mill may be performed.
  • the reversing mill is a type of rolling mill generally used for rolling thin materials, and refers to a rolling mill that rolls while reciprocating the material between a pair of rolls, and one way can be set to one pass when reciprocating the material .
  • the hot-rolled steel sheet may be pickled before the cold rolling, and the pickling process may be performed in a conventional manner.
  • the continuous annealing treatment may be performed in, for example, a continuous annealing furnace (CAL).
  • CAL continuous annealing furnace
  • a continuous annealing furnace may consist of [heating zone - soaking zone - cooling zone (slow cooling zone and rapid cooling zone) - (overaging zone, if necessary)]. After that, it is heated to a specific temperature in the heating zone, and after reaching the target temperature, it goes through a process of holding in the soaking zone for a certain period of time.
  • the temperature of the heating zone and the soaking zone can be equally controlled, which means that the end temperature of the heating zone and the starting temperature of the soaking zone are equally controlled (FIG. 1).
  • the temperature of the heating zone and soaking zone may be controlled to 790 to 850 ° C.
  • the temperature of the heating zone is less than 790 ° C, it is impossible to apply sufficient heat input for recrystallization, whereas if the temperature exceeds 850 ° C, productivity is reduced and an austenite phase is excessively formed, resulting in a hard phase after subsequent cooling The fraction of is greatly increased, and there is a concern that the ductility of the steel is inferior.
  • the temperature of the soaking zone is less than 790 ° C, excessive cooling is required at the end temperature of the heating zone, which is economically unfavorable and there is a risk that the amount of heat for recrystallization may be insufficient.
  • the temperature exceeds 850 ° C. the austenite fraction becomes excessive, and there is a concern that formability may decrease due to an increase in the hard phase during cooling.
  • the hard phase fraction in the final structure is increased to increase the yield strength, and at the same time, the hole expansion rate is improved by reducing the hardness difference between phases through the introduction of bainite.
  • the present invention is characterized by inducing the generation of a recrystallized ferrite bridge (bridge) by causing sufficient recrystallization in the annealing process.
  • the present invention may introduce a recrystallization zone that maintains the cold-rolled steel sheet at an intermediate temperature for a certain period of time when the temperature is raised to the temperature range of the heating zone, more preferably a process of maintaining the temperature range of 600 ⁇ 700 °C for 1 to 3 minutes It is preferable to go through (dotted line graph in FIG. 2).
  • the temperature of the recrystallization zone is less than 600° C. or the holding time is less than 1 minute, recrystallization of ferrite is not sufficient, so a recrystallized ferrite bridge phase cannot be formed in a targeted fraction.
  • the temperature exceeds 700 ° C or the holding time exceeds 3 minutes recrystallization becomes excessive, and there is a risk of deterioration in physical properties due to strength reduction and grain coarsening.
  • the present invention improves workability by enhancing crack toughness, that is, crack resistance, while maintaining strength by introducing a recrystallized ferrite bridge phase along with an appropriate fraction of the hard phase and soft phase into the final microstructure through the recrystallization zone process. effect can be obtained.
  • the step-by-step cooling may consist of primary cooling - secondary cooling, and specifically, after primary cooling at an average cooling rate of 1 to 10 ° C / s to a temperature range of 650 to 700 ° C after the continuous annealing, Secondary cooling may be performed at an average cooling rate of 5 to 50° C./s up to a temperature range of 450 to 500° C.
  • the end temperature of the primary cooling is less than 650 ° C, the diffusion activity of carbon is low due to the too low temperature, and the carbon concentration in ferrite increases, while the yield ratio increases due to excessive hard phase fraction as the carbon concentration in austenite decreases. This increases the tendency of cracking during machining.
  • the cooling rate between the soaking zone and the cooling zone becomes too high, causing a problem that the shape of the plate becomes non-uniform.
  • the end temperature exceeds 700° C., there is a disadvantage in that an excessively high cooling rate is required during subsequent cooling (secondary cooling).
  • the primary cooling may be performed at an average cooling rate of 1° C./s or more.
  • the average cooling rate during the secondary cooling is less than 5 ° C / s, there is a risk that the fraction of the soft phase will be excessive, whereas if it exceeds 50 ° C / s, there is a possibility that the hard phase will be insufficient.
  • the overaging treatment is a process of holding for a predetermined time after the secondary cooling end temperature, and has an effect of improving shape quality by performing uniform heat treatment in the width direction and length direction of the coil. To this end, the overaging treatment may be performed for 200 to 800 seconds.
  • the temperature may be the same as the end temperature of the secondary cooling, may be within the range of the end temperature of the secondary cooling, or may be performed at a lower temperature. It is revealed that more advantageously, it can be carried out in a temperature range of 300 to 450 ° C.
  • the high-strength thick steel sheet of the present invention manufactured as described above has a microstructure composed of a hard phase and a soft phase.
  • the site may have a uniformly distributed texture.
  • the thick steel sheet of the present invention has a high tensile strength of 980 MPa or more, it is possible to ensure excellent formability such as hole expandability by securing a low yield ratio and high ductility.
  • each steel slab was heated at 1200 ° C. for 1 hour, and then hot-rolled hot-rolled at a finish rolling temperature of 880 to 920 ° C. to prepare a hot-rolled steel sheet. Thereafter, each hot-rolled steel sheet was wound at 650° C. and then cooled to room temperature at a cooling rate of 0.1° C./s. Thereafter, the rolled hot-rolled steel sheet was subjected to cold rolling and continuous annealing under the conditions shown in Table 2 below, and then, after gradual cooling (1st-2nd), overaging was performed at 360°C for 520 seconds, resulting in a final product with a thickness of 1.8mm. A steel plate was prepared.
  • the first cooling was performed at an average cooling rate of 3°C/s and the second cooling was performed at an average cooling rate of 20°C/s.
  • the tensile test for each test piece was performed at a strain rate of 0.01/s after taking a JIS No. 5 size tensile test piece in the direction perpendicular to the rolling direction.
  • the hole expansion ratio (HER, %) measurement test was performed according to the ISO16630 standard. Specifically, when a circular hole is punched in a test piece and then expanded using a conical punch, the hole enlargement until the crack generated at the edge of the hole penetrates in the thickness direction is expressed as a ratio to the initial hole. .
  • the specimen size was 120 mm ⁇ 120 mm
  • the clearance was 12%
  • the punching hole diameter was 10 mm
  • the punching holding load was 20 ton
  • the test speed was set to 12 mm/min.
  • the bainite phase corresponding to the hard phase of the tissue phase was picral (picral) etching
  • the martensite phase was observed through SEM at 2000 and 5000 magnifications after nital etching.
  • the respective fractions were measured using SEM and an image analyzer program after nital etching.
  • inventive examples 1 to 6 satisfying all of the steel alloy composition and manufacturing conditions, particularly the cold rolling and continuous annealing processes proposed in the present invention sufficiently recrystallize ferrite in the annealing process after cold rolling.
  • the hard phase was formed by being connected by a recrystallized ferrite bridge phase. Accordingly, while having high strength, it had a yield strength suitable for plate-shaped processing, and the elongation was excellent.
  • the hole expandability is excellent due to the distribution of the homogeneous hard phase, and thus it is possible to secure the target level of moldability.
  • Comparative Examples 5 and 10 had sufficient austenite stability to secure annealing temperature and strength for recrystallization driving, but recrystallization did not occur sufficiently due to insufficient reduction ratio, and as a result of not forming a uniform structure, the elongation was inferior, and the yield strength was relatively high.
  • Comparative Examples 11 to 14 having a very low secondary cooling temperature had excessively high yield strength, high risk of cracking during processing, and poor elongation due to the absence of a recrystallized ferrite bridge phase.
  • FIG. 3 shows pictures of microstructures of Comparative Examples 1, 4-5 and 9, and FIG. 4 shows pictures of microstructures of Inventive Examples 1, 3-4 and 6.
  • Comparative Examples 1, 4, and 9 the hard phase connection structure by the recrystallized ferrite bridge can hardly be confirmed because the recrystallization zone process is not introduced during the temperature increase process during continuous annealing.
  • Comparative Example 5 had a low recrystallization zone fraction due to insufficient reduction ratio, and formed a structure in which hard phases gathered together due to insufficient recrystallization due to low driving force, thereby forming a structure with low crack propagation resistance.

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Abstract

La présente invention concerne un acier approprié en tant que matériau pour des automobiles et, particulièrement, une tôle d'acier à haute résistance et épaisse présentant d'excellentes capacités d'expansion de trou et de ductilité, et son procédé de fabrication.
PCT/KR2022/014116 2021-09-27 2022-09-21 Tôle d'acier à haute résistance et épaisse dotée d'excellentes capacités d'expansion de trou et de ductilité et son procédé de fabrication Ceased WO2023048467A1 (fr)

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CN202280065119.XA CN118103542A (zh) 2021-09-27 2022-09-21 扩孔性和延展性优异的高强度厚钢板及其制造方法
EP22873172.5A EP4411013A4 (fr) 2021-09-27 2022-09-21 Tôle d'acier à haute résistance et épaisse dotée d'excellentes capacités d'expansion de trou et de ductilité et son procédé de fabrication
MX2024003632A MX2024003632A (es) 2021-09-27 2022-09-21 Lamina de acero de alta resistencia y gruesa que tiene excelente expansibilidad de agujero y ductilidad, y metodo de fabricacion para la misma.
US18/694,803 US20240401164A1 (en) 2021-09-27 2022-09-21 High-strength and thick steel sheet having excellent hole expandability and ductility, and manufacturing method therefor
JP2024518912A JP2024535916A (ja) 2021-09-27 2022-09-21 穴拡げ性及び延性に優れた高強度厚物鋼板及びその製造方法

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MX2024003632A (es) 2024-06-24
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KR20230045648A (ko) 2023-04-05
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