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EP4428262A1 - Hochfestes stahlblech mit hervorragender kollisionssicherheit und formbarkeit sowie verfahren zur herstellung davon - Google Patents

Hochfestes stahlblech mit hervorragender kollisionssicherheit und formbarkeit sowie verfahren zur herstellung davon Download PDF

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Publication number
EP4428262A1
EP4428262A1 EP22890363.9A EP22890363A EP4428262A1 EP 4428262 A1 EP4428262 A1 EP 4428262A1 EP 22890363 A EP22890363 A EP 22890363A EP 4428262 A1 EP4428262 A1 EP 4428262A1
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EP
European Patent Office
Prior art keywords
steel sheet
less
cooling
formability
strength
Prior art date
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Pending
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EP22890363.9A
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English (en)
French (fr)
Inventor
Yeon-Sang Ahn
Joo-Hyun Ryu
Eul-Yong Choi
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Posco Holdings Inc
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Posco Co Ltd
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Publication of EP4428262A1 publication Critical patent/EP4428262A1/de
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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
    • 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
    • C22C38/18—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
    • 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
    • C21D8/0247—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
    • 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
    • C22C38/22—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
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • C—CHEMISTRY; METALLURGY
    • 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
    • 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
    • C21D2211/008—Martensite

Definitions

  • the present disclosure relates to a high-strength steel sheet used for a vehicle structure member, and more specifically, to a high-strength steel sheet having excellent crashworthiness and formability, and a method for manufacturing the same.
  • high-strength vehicle materials may be classified into precipitation reinforced steel, bake hardened steel, solid solution strengthened steel, and transformation reinforced steel.
  • the transformation reinforced steel includes Dual Phase Steel (DP steel), Complex Phase Steel (CP steel), or Transformation Induced Plasticity Steel (TRIP steel).
  • DP steel Dual Phase Steel
  • CP steel Complex Phase Steel
  • TRIP steel Transformation Induced Plasticity Steel
  • ASS Advanced High Strength Steel
  • DP steel is a steel that secures high strength by finely and homogeneously dispersing a hard martensite in a soft ferrite
  • CP steel is a steel that includes two or three phases of a ferrite, a martensite, and a bainite, and adds precipitation hardening elements such as Ti and Nb to improve strength
  • TRIP steel is a steel that includes fine and homogeneously dispersed retained austenite, and may ensure high strength and high ductility by causing a retained austenite phase to be transformed into a martensite during room temperature processing.
  • ductility decreases as the strength of the steel sheet increases, which may cause a problem of deteriorating molding processability, and thus, the development of a material capable of supplementing the problem is required. In other words, it may be essential to develop a material with high yield strength and excellent ductility to simultaneously secure collision stability and component formability.
  • the processed components have a shear surface therein or at an edge thereof, and accordingly, when the Hall Expansion Station (HER) is excellent, the components may be molded without defects such as cracks in the shear surface, and even in the event of collisions, collision performance may be improved through impact absorption.
  • HER Hall Expansion Station
  • a steel sheet in which a microstructure has a tempered martensite phase may be manufactured through a tempering process after forming a martensite by performing cracking in an annealing process and then depositing in water.
  • Patent Document 1 which is a prior art related to the aforementioned technology, discloses martensite steel with a volume ratio of 80 to 97% of martensite, obtained by continuously annealing a steel material containing 0.18% or more of carbon (C) and then cooling the steel material to room temperature, and then treating over-aging on the steel material at a temperature of 120 to 300°C for 1 to 15 minutes.
  • C carbon
  • the yield ratio is very high, but the shape quality of a coil may be deteriorated due to temperature variations in a width direction and a length direction, resulting in problems such as an occurrence of cracks and workability reduction occur during molding processing.
  • Patent Document 2 relates to a steel sheet formed of a complex tissue mainly composed of martensite, and discloses a method of manufacturing a high-tensile steel sheet in which fine precipitated copper particles having a particle diameter of 1to 100 nm are dispersed inside the tissue to improve processability.
  • fine precipitated copper particles having a particle diameter of 1to 100 nm are dispersed inside the tissue to improve processability.
  • hot shortness due to Cu may occur, which may excessively increase manufacturing costs.
  • Patent Document 3 relates to a precipitation-reinforced steel sheet containing 2 to 10% of pearlite with ferrite as a base structure, which adds carbon/nitride forming elements such as Nb, Ti, V, and the like, thereby improving strength by precipitation reinforcement and grain refinement.
  • the steel sheet has a good hole expansion ratio, but has a limitation to increasing tensile strength, and also has high yield strength and low ductility, resulting in an occurrence of cracks during press molding.
  • Patent Document 4 discloses a method of manufacturing a cold-rolled steel sheet with excellent plate shape after continuous annealing and securing high strength and high ductility at the same time by utilizing a tempered martensite, but since a carbon content is as high as 0.2% or more, the weldability may be degraded, and dent defects in a furnace may occur due to a large amount of Si.
  • An aspect of the present disclosure is to provide a steel sheet used for a vehicle structure member, that is, a steel sheet having excellent strength as well as ductility and having improve crashworthiness and formability, and a method for manufacturing the same.
  • the aspect of the present disclosure is not limited to the above.
  • the aspect of the present disclosure may be understood from overall contents of the present specification, and those of ordinary skill in the technical field to which the present disclosure pertains will have no difficulty in understanding an additional aspect of the present disclosure.
  • a high-strength steel sheet having excellent crashworthiness and formability may include: by wt%, carbon (C): 0.06 to 0.2%, silicon (Si): 0.4 to 1.4%, manganese (Mn): 1.8 to 3.0%, aluminum acid (Sol.Al): 1.0% or less, molybdenum (Mo): 0.4% or less, chromium (Cr): 1.0% or less, antimony (Sb): 0.06% or less, boron (B): 0.01% or less, phosphorus (P): 0.1% or less, sulfur (S): 0.01% or less, and a balance of Fe and unavoidable impurity elements, wherein the C, Si and Al satisfy the following Relational Expression 1,
  • a microstructure includes 40 to 80% of a sum of tempered martensite and bainite phases as an area fraction, 3 to 15% of a retained austenite phase, and a balance of ferrite and fresh martensite, and in the retained austenite phase, a share (A TM+B /A T ) of retained austenite (A TM+B ) adjacent to tempered martensite and bainite among a total retained austenite fraction (A T ) is 90% or more, 8 ⁇ C + 1.1 ⁇ Si + 0.8 ⁇ Al ⁇ 1.7 (In the relational expression 1, each element denotes a weight content).
  • a method for manufacturing a high-strength steel sheet having excellent crashworthiness and formability may include: heating a steel slab in a temperature range of 1050 to 1250°C, the steel slab including, by wt%, carbon (C): 0.06 to 0.2%, silicon (Si): 0.4 to 1.4%, manganese (Mn): 1.8 to 3.0%, aluminum acid (Sol.Al): 1.0% or less, molybdenum (Mo): 0.4% or less, chromium (Cr): 1.0% or less, antimony (Sb): 0.06% or less, boron (B): 0.01% or less, phosphorus (P): 0.1% or less, sulfur (S): 0.01% or less, and a balance of Fe, and unavoidable impurity elements, the C, Si and Al satisfying the following Relational Expression 1; manufacturing a hot-rolled steel sheet by finish hot rolling the reheated steel slab at a temperature range of a finish hot
  • a steel sheet having high strength and excellent ductility may be provided. Specifically, since the steel sheet of the present disclosure has a higher yield ratio than that of conventional DP steel, the steel sheet has an excellent hole expansion ratio, thereby having excellent impact resistance and formability.
  • the steel sheet of the present disclosure may be suitably applied as a material for a vehicle structure member requiring processing into a complex shape.
  • the inventors of the present disclosure studied deeply to provide a high-strength steel sheet having improved crashworthiness and formability by increasing a yield ratio (YR) and a hole expansion ratio, as compared to conventional DP steel, while satisfying high ductility, which is a characteristic of the conventional DP steel.
  • a steel sheet may have an advantageous structure for securing target physical properties, from which it is possible to provide a steel sheet suitable for a vehicle structure member that require processing into a complex shape, and have completed the present disclosure.
  • a high-strength steel sheet having excellent crashworthiness and formability may include: by wt%, carbon (C): 0.06 to 0.2%, silicon (Si): 0.4 to 1.4%, manganese (Mn): 1.8 to 3.0%, aluminum acid (Sol.Al): 1.0% or less, molybdenum (Mo): 0.4% or less, chromium (Cr): 1.0% or less, antimony (Sb): 0.06% or less, boron (B): 0.01% or less, phosphorus (P): 0.1% or less, sulfur (S): 0.01% or less.
  • a content of each element is based on weight, and a ratio of a structure is based on area.
  • Carbon (C) is a greatly important element added to strengthen a transformation structure.
  • the C promotes the high-strength of steel and promotes a formation of martensite in complex structure steel.
  • the content of C increases, the amount of martensite increases.
  • C may be included in an amount of 0.06 to 0.20%, and more preferably 0.08% or more and 0.18% or less.
  • the Si may be included in an amount of 0.4 to 1.4%, and more preferably may be 0.5% or more and 1.2% or less.
  • Manganese (Mn) is an effective element for micronizing particles without decreasing ductility, and strengthening steel as well as preventing hot brittleness caused by FeS generation by completely precipitating sulfur (S) in steel as MnS.
  • the Mn reduces critical cooling rate at which a martensite phase is obtained in composite phase steel, thereby facilitating the formation of martensite.
  • Mn-Band manganese oxide band
  • the Mn may be included in an amount of 1.8 to 3.0%, and more preferably 2.0% or more and 2.9% or less.
  • Acid-Soluble Aluminum (Sol.Al): 1.0% or less
  • Acid-soluble aluminum is an element added for refining a particle size of the steel and deoxidation, and is a ferrite stabilizing element similar to Si.
  • the Al is an element useful for improving hardenability of martensite by distributing carbon in ferrite to austenite. Furthermore, the Al may effectively suppress the precipitation of carbides in bainite when maintained in a bainite area during annealing to promote C concentration with untransformed austenite and delay martensite transformation during cold rapid cooling, and may improve the ductility of a steel sheet by generating a retained austenite phase.
  • the Al may be included in an amount of 1.0% or less, excluding 0%. More preferably, the Al may be included in an amount of 0.01% or more.
  • Molybdenum is an element delaying the transformation of austenite into pearlite and improving the miniaturization and strength of ferrite.
  • the Mo has an advantage of improving the hardenability of the steel and controlling a yield ratio by finely forming martensite on a grain boundary.
  • the Mo is an expensive element, and as a content thereof increase, the manufacturing costs thereof increases, which is economically disadvantageous.
  • the Mo may be added in a maximum amount of 0.4%.
  • content of Mo exceeds 0.4%, alloy costs are rapidly increased, leading to a decrease in economic feasibility, and also, an effect refining crystal grains and an effect of solid solution strengthening, leading to a decrease in the ductility of the steel.
  • the content of the Mo may 0.4% or less. According to the present disclosure, even if the Mo is not added, there is no difficulty in securing intended microstructure and physical properties, and thus, the content of the Mo may be 0%. However, when the Mo is added, the Mo may be more advantageously included in the amount of 0.01% or more.
  • Chromium is an element added to improve the hardenability of steel and secure high strength, and plays an important role in the formation of martensite. Furthermore, the Cr is advantageous for manufacturing highly flexible complex phase steel by minimizing the decrease in an elongation ratio as compared to an increase in strength.
  • the content of the Cr may be 1.0% or less.
  • the Cr even if the Cr is not added, there is no difficulty in ensuring intended microstructure and physical properties, and thus, the content of Cr may be 0%.
  • the Cr when the Cr is added, the Cr may be more advantageously included in the amount of 0.1% or more.
  • Antimony (Sb) is distributed in a grain boundary to delay a diffusion of oxidizing elements such as Mn, Si, and Al through the grain boundary, thus suppressing a surface concentration of an oxide. Furthermore, the Sb has an excellent effect on suppressing the coarsening of surface agglomerates due to temperature increase and hot-rolled process change. When the content of Sb exceeds 0.06%, the above-described effects may be saturated, manufacturing costs may be increased, and processability may be degraded.
  • the content of the Sb may be 0.06% or less. According to the present disclosure, even if the Sb is not added, there is no difficulty in ensuring intended microstructure, physical properties, and the like, and thus, the content of Sb may be 0%. However, when the Sb is added, the Sb may be more advantageously included in an amount of 0.01% or more.
  • B Boron
  • B is an element delaying the transformation of austenite into pearlite in a cooling process during annealing, and is a hardenability element inhibiting ferrite formation and promoting martensite formation.
  • the content of B exceeds 0.01%, the B may be excessively enriched on a surface of steel, which may degrade plating adhesiveness.
  • the content of B may be 0.01% or less. According to the present disclosure, even if the B is not added, there is no difficulty in ensuring intended microstructure and physical properties, and thus, the content of B may be 0%. However, when the B is added, the B may be more advantageously included in an amount of 0.0005% or more.
  • Phosphorus (P) 0.1% or less
  • Phosphorus (P) is a substituted element having a large solid solution strengthening effect and is the most advantageous element for improving in-plane anisotropy and securing strength without significantly impairing formability.
  • P Phosphorus
  • the P may be included in an amount of 0.1% or less, and the amount of 0% may be excluded in consideration of a level inevitably added during a steel manufacturing process.
  • S Sulfur
  • S is an impurity inevitably added to steel and is an element that inhibits ductility and weldability, and thus, it may be advantageous to manage the content of S to be as low as possible. Specifically, since there is a high possibility of generating hot brittleness, it may be preferable to control the content thereof to 0.01% or less. However, 0% may be excluded in consideration of the level inevitably added during a steel manufacturing process.
  • the steel sheet of the present disclosure may further include at least one of Ti and Nb for the purpose of further improving the mechanical properties of the steel sheet.
  • Titanium (Ti) and niobium (Nb) are effective elements for increasing the strength of steel and refining crystal grains by forming nano-precipitation hardening. When these elements are added, they may be coupled to carbon to form very fine nano-precipitation hardening, and the nano-precipitation hardening may serve to strengthen a matrix structure and reduce a hardness difference between phases.
  • each of them may be included in an amount of 0.05% or less.
  • the remaining component of the present disclosure is iron (Fe).
  • Fe iron
  • impurities that are not intended from raw materials or the surrounding environments may inevitably be mixed in the normal manufacturing process, the impurities may not be excluded. Since these impurities are known to those skilled in the steel manufacturing field, not all of these impurities are specifically mentioned in this specification.
  • a relationship of the content of C, Si and Al may satisfy Relational Expression 1 below. 8 ⁇ C + 1.1 ⁇ Si + 0.8 ⁇ Al ⁇ 1.7 (In the relational expression 1, each element denotes a weight content.)
  • Si and Al in steel are ferrite stabilizing elements promoting ferrite transformation, and contributing to the formation of retained austenite and martensite by encouraging C concentration to untransformed austenite.
  • C is also an element contributing to the formation and fraction adjustment of martensite by promoting the C concentration to untransformed austenite.
  • R relational Expression 1 when a value of R relational Expression 1 is controlled to be 1.7 or more, the fraction of retained austenite that can contribute to ductility may be secured, from which it is possible to improve the ductility and formability of the steel sheet.
  • Relational Expression 1 above when Relational Expression 1 above is not satisfied, the retained austenite fraction is insufficient, and a distribution of the generated retained austenite is not uniform, which may make it difficult to secure ductility and formability.
  • the present disclosure may distribute retained austenite produced by optimizing the steel sheet manufacturing process together with the above-described alloy component system finely around a hard phase, thus mitigating local stress concentration, from which it may be possible to improve ductility thereof and to secure excellent formability.
  • the steel sheet of the present disclosure may include 40 to 80% of tempered martensite and bainite phases as a total area fraction, 3 to 15% of a retained austenite phase, and a balance of ferrite and fresh martensite , as a microstructure.
  • the tempered martensite and bainite structures assist in forming retained austenite in addition to an effect contributing to strength.
  • Specificall y when Si and Al are added to steel, by delaying the precipitation of carbides during bainite transformation, carbon (C) is accumulated into untransformed austenite around bainite, thereby lowering a martensite transformation temperature below room temperature. In this case, retained austenite may be secured at room temperature.
  • carbon (C) introduced in the martensite moves to surrounding untransformed austenite and is then accumulated, thereby lowering the martensite transformation temperature below room temperature, as well as securing retained austenite at room temperature.
  • the steel sheet of the present disclosure may include 3 to 15% of a retained austenite phase.
  • a retained austenite phase By securing 3% or more of the retained austenite phase, it is advantageous to secure ductility of the steel sheet by causing transformation induced plasticity during molding.
  • the steel sheet tends to be vulnerable to liquid metal brittleness (LME) during point welding to assemble plated steel sheets into vehicle components.
  • the retained austenite phase may be included in an amount of 15% or less.
  • the retained austenite phase is characterized in that a share (A TM+B /A T ) of retained austenite (A TM+B ) adjacent to tempered martensite and bainite among a total retained austenite fraction (A T ) is more than 90%.
  • being adjacent to the tempered martensite and the bainite may refer to the periphery of their phases, more specifically an interface area of these phases.
  • austenite phases may be finely and evenly distributed around the tempered martensite and the bainite, thereby improving ductility through an effect of relieving local stress concentration, and accordingly, excellent formability without cracks when forming components may be secured.
  • the tempered martensite phase of the total fraction may be included in an amount of 25 to 65%.
  • ferrite and fresh martensite phases may be included in addition to the above-described tempered martensite, bainite, and retained austenite phases.
  • a ferrite phase may be included in an amount of 40% or less, and a fresh martensite phase may be included in an amount of 20% or less.
  • the amount of 0% of the ferrite phase and the fresh martensite phase are excluded.
  • the fraction of the ferrite phase exceeds 40%, not only cannot a target level of strength be secured, but it may be difficult to improve a yield ratio. Furthermore, when the fraction of the fresh martensite exceeds 20%, the fraction of the retained austenite phase may decrease to reduce the ductility, and the formability thereof may not be secured.
  • the steel sheet of the present disclosure which has the above-described alloy component system and microstructure, not only has high strength having a tensile strength of 980 MPa or more, but also has a yield ratio of 0.6 to 0.9, an elongation ratio of 10% or more, and a hole expansion ratio of 20% or more.
  • the yield ratio is less than 0.6, the hole expansion ratio is degraded, but when the yield ratio exceeds 0.9, the ductility decreases.
  • the steel sheet of the present disclosure may provide a steel sheet having both high yield ratio and high ductility as a relationship between the yield ratio, the elongation ratio, and the tensile strength satisfies the following Relational Expression 2.
  • the high yield ratio of the steel sheet may contribute to the improvement of stability during a vehicle collision due to excellent crashworthiness of the material, and the high ductility may prevent processing defects such as cracks and wrinkles that occur during press processing into components, thereby securing excellent formability.
  • the steel sheet of the present disclosure having the mechanical properties as described above may prevent processing defects such as cracks and wrinkles when processing into components, the steel sheet may be used in various ways for a vehicle structure member.
  • the steel sheep may contribute to improving the safety of structural components and vehicles by delaying an occurrence of crashworthiness and cracks during a collision.
  • the steel sheet of the present disclosure may be a cold-rolled steel sheet, and may be a hot-dip galvanized steel sheet including a zinc-based plating layer on at least one surface of the cold-rolled steel sheet, and an alloyed hot-dip galvanized steel sheet alloyed with the hot-dip galvanized steel sheet.
  • the zinc-based plating layer may be a zinc plating layer mainly containing zinc and a zinc alloy plating layer containing aluminum and/or magnesium other than zinc.
  • the present disclosure may manufacture a desired steel sheet through a process of [steel slab reheating - hot rolling - coiling - cooling - cold rolling - continuous annealing - cooling - reheating and maintaining], and then further processes such as [hot dip galvanizing - alloying heat treatment] may be performed.
  • the steel slab may be heated. This process is performed to smoothly perform a subsequent hot rolling process and to sufficiently obtain properties of a desired steel sheet.
  • the heating process may be performed in a temperature range of 1050 to 1250°C.
  • the heating temperature is less than 1050°C, friction between the steel sheet and a rolling mill increases, which may rapidly increase a load applied to a roller during hot rolling.
  • the temperature exceeds 1250°C, not only does energy costs required for temperature rise increase, but also the amount of surface scale increases, leading to material loss.
  • the heating process may be performed in a temperature range of 1050 to 1250°C.
  • a hot-rolled steel sheet may be manufactured by finishing hot rolling the steel slab heated according to the aforementioned process at a Ar3 transformation point or higher, and in this case, a temperature on an outlet side may satisfy Ar3 to Ar3+50°C.
  • finishing hot rolling may be performed in a temperature range of 800 to 1000°C.
  • the hot-rolled steel sheet produced according to the aforementioned process may be coiled, and in this case, the coiling may be performed in a temperature range of 400 to 700°C.
  • the strength of the hot-rolled steel sheet may be excessively increased, which may result in a rolling load during subsequent cold rolling. Furthermore, it takes excessive costs and time to cool the hot-rolled steel sheet to the coiling temperature, causing an increase in process costs. On the other hand, when the temperature exceeds 700°C, excessive scale may occur on a surface of the hot-rolled steel sheet, which is likely to cause surface defects, and this may cause the deterioration of plating.
  • the coiling process may be performed in a temperature range of 400 to 700°C.
  • the coiled hot-rolled steel sheet may be cool to room temperature at average cooling rate of 0.1°C/s or less (excluding 0°C/s).
  • the cooling may be performed at the average cooling rate of more advantageously 0.05°C/s or less, and more advantageously 0.015°C/s or less.
  • the cooling denotes average cooling rate.
  • the hot-rolled steel sheet coiled according to the aforementioned process may be manufactured as a cold-rolled steel sheet by cold rolling, and in this case, the cold rolling may be performed at a cold reduction ratio (total reduction ratio) of 30 to 80%.
  • the present disclosure may increase energy stored in the steel by controlling a cumulative reduction ratio of an initial stand, preferably stands 1 to 3, by 20% or more, during the cold rolling, thereby having an effect of acting as a driving force for promoting recrystallization of ferrite in a subsequent annealing process. For this reason, the present disclosure may impart an effect of lowering the fraction of microcrystalline ferrite in the steel.
  • the cumulative reduction ratio of the initial stands 1 to 3 is less than 20% during the cold rolling, or the cold reduction ratio (total reduction ratio) to a final stand is less than 30%, it may be difficult to secure a desired thickness and to correct a shape of the steel sheet.
  • the fraction of microcrystalline ferrite increases to reduce the ductility.
  • the cold reduction ratio to the final stand exceeds 80% during the cold rolling, there may be a problem in that the strength thereof increases, resulting in a roll load during cold rolling, and the possibility of cracks occurring at an edge portion of the steel sheet may increase.
  • the cold rolling may be performed using a rolling mill comprised of five or six stands, but the present disclosure is not limited thereto.
  • the cold-rolled steel sheet produced according to the aforementioned process may be continuously annealed.
  • the continuous annealing treatment may be performed, for example, in a continuous alloying molten plating furnace.
  • the continuous annealing operation is a process of forming ferrite and austenite phases simultaneously with recrystallization and decomposing carbons.
  • the continuous annealing treatment may be preferably performed in a temperature range of Ac1+30°C to Ac3+30°C, and more preferably, in a temperature range of 800 to 870°C.
  • productivity may be reduced, and due to high-temperature annealing, the formation of surface enriched materials is intensified by elements degrading the wettability of hot-dip galvanizing of Si, Mn, B and the like, and thus plating surface quality may not be ensured.
  • the continuously annealed cold-rolled steel sheet may be cooled step by step.
  • the cooling may be performed at average cooling rate of 10°C/s or less (excluding 0°C/s) by 450 to 700°C (the cooling at this time is referred to as primary cooling) and then performed at average cooling rate of 3°C/s or more by 250 to 500°C (the cooling at this time is referred to as secondary cooling).
  • an end temperature in a subsequent secondary cooling process may be controlled, thereby controlling a fraction of martensite and bainite generated in this case.
  • a martensite transformation starting temperature Ms
  • a bainite phase may be relatively advantageously formed, and for this purpose, it is advantageous to control the end temperature of the primary cooling to be higher.
  • the primary cooling when the subsequent secondary cooling is terminated below Ms, the primary cooling may be performed up to a temperature range of 450 to 600°C, and when the subsequent secondary cooling is terminated in the bainite temperature range, the primary cooling may be performed by a temperature range of 550 to 700°C.
  • the primary cooling may be performed at the average cooling rate of 1°C/s or more.
  • the second cooling may be performed, and in this case, the formation of a desired microstructure may be induced by controlling a cooling end temperature and cooling rate.
  • the martensite When cooling is performed below Ms during the secondary cooling, quenching martensite may be formed, and as the temperature decreases, the fraction of the quenching martensite increases, which may lead to an improvement in the strength of the steel sheet. Furthermore, in a subsequent heat treatment (i.e., a reheating process of the present disclosure), the martensite may be tempered into tempered martensite, and supersaturated carbon in the martensite may be distributed to surrounding untransformed austenite, thereby increasing the stability of retained austenite and improving ductility.
  • a subsequent heat treatment i.e., a reheating process of the present disclosure
  • a bainite fraction may be increased.
  • the precipitation of carbides may be delayed due to the effects of Si and Al, and as carbons is distributed from the bainite to the surrounding untransformed austenite, the stability of the retained austenite may be increased and ductility may be improved.
  • the secondary cooling may be terminated at 400°C or less.
  • an upper limit of the average cooling rate is not particularly limited, and may be selected appropriately by a person skilled in the art in consideration of the specifications of the cooling facility.
  • the cooling may be performed at 100°C/s or less.
  • the secondary cooling may use a hydrogen cooling facility using hydrogen gas (H 2 gas).
  • H 2 gas hydrogen gas
  • the cooling may be performed using a hydrogen cooling facility to obtain the effect of suppressing surface oxidation that may occur during the secondary cooling.
  • the hydrogen cooling facility may be controlled by 60 to 70% hydrogen (H 2 ) and residual nitrogen (N 2 ).
  • a process of maintaining in the cooling temperature range for 30 seconds or more may be further performed.
  • a microstructure intended in the present disclosure may be formed through a process of reheating and maintaining a cold-rolled steel sheet that has been cooled step by step. Specifically, it may be desirable to undergo a process of reheating the secondary cooled cold-rolled steel sheet at a temperature of 490°C or less and maintaining for more than 30 seconds.
  • the quenching martensite produced during the previous cooling process may be transformed into tempered martensite, and may also be accompanied by bainite transformation.
  • potential may be fixed to the tempered martensite and bainite, which may increase the yield strength, and as a result, it may be possible to obtain a steel sheet having a yield ratio of 0.6 to 0.9.
  • carbides in the tempered martensite and bainite may be coarsened to decrease the strength thereof, and a carbon redistribution effect to untransformed austenite due to the formation of coarsened carbides may be reduced, resulting in a decrease in a retained austenite fraction, and thus, it is difficult to expect an improvement in ductility.
  • the reheating temperature may be limited to 490°C or less, and more advantageously, the reheating temperature may be 350°C or higher.
  • the cold-rolled steel sheet reheated below 490°C may be maintained for more than 30 seconds at that temperature so that the above-described effects are sufficiently realized.
  • the tempered martensite and bainite may be formed as a matrix structure in a microstructure, and a certain fraction of retained austenite may be finely and uniformly around the tempered martensite and bainite, thereby increasing a yield ratio and ductility compared to the conventional DP steel, and improving the formability for processing components of a steel sheet and the crashworthiness during a vehicle collision.
  • the steel sheet of the present disclosure which is precisely controlled, may secure ductility while maintaining a higher yield ratio than the conventional DP steel. As a result, it may be possible to provide a high-strength steel sheet having excellent ductility, hole expansion ratio, formability, and crashworthiness.
  • the present disclosure may provide a plated steel sheet by plating the cold-rolled steel sheet produced according to the aforementioned process.
  • a hot-dip galvanized steel sheet may be manufactured by immersing a steel sheet in a hot-dip galvanized steel sheet bath after reheating and maintaining processes.
  • the hot-dip galvanizing may be performed under normal conditions, but for example, the hot-dip galvanizing may be performed in a temperature range of 430 to 490°C.
  • a composition of the hot-dip galvanized bath during the hot-dip galvanizing is not particularly limited, and may be a pure zinc plating bath or a zinc-based alloy plating bath containing Si, Al, Mg, and the like.
  • an alloyed hot-dip galvanized steel sheet may be obtained by performing an alloying heat treatment on the hot-dip galvanized steel sheet.
  • the alloying heat treatment process condition is not particularly limited, and may be provided as a normal condition.
  • an alloying heat treatment process may be performed in a temperature range of 480 to 600°C.
  • final cooling and coarse rolling process may be performed after the hot-dip galvanizing or alloying heat treatment.
  • the hot-dip galvanized or alloyed heat-treated steel sheet may be finally cooled to further introduce fresh martensite.
  • the final cooling may be performed at cooling rate of 3°C/s or more at room temperature.
  • an upper limit of the cooling rate is not particularly limited, but the cooling may be performed at the cooling rate of 50°C/s or less to form a certain fraction of press martensite.
  • a reduction ratio thereof may be less than 2% (excluding 0%).
  • the reduction ratio is 2% or more, it may be advantageous in terms of potential formation, but side effects such as plate breakage may occur due to a limitation in equipment capability.
  • each heated slab was subjected to finished hot rolling at Ar3 to Ar3+50°C to manufacture a hot-rolled steel sheet. Thereafter, each hot-rolled steel sheet was coiled at 400 to 700°C and then cooled to room temperature at cooling rate of 0.1°C/s or less. Thereafter, the cooled hot-rolled steel sheet was subjected to cold rolling at a cold reduction ratio of 45 to 75%, thereby manufacturing a cold-rolled steel sheet.
  • the cold rolling was performed in a rolling mill comprised of six stands, and a cumulative reduction ratio of stands 1 to 3 was performed under the conditions illustrated in Table 2 below.
  • each cold-rolled steel sheet was continuously annealed under the conditions illustrated in Table 2 below, followed by step-by-step cooling (primary-secondary cooling and maintenance).
  • step-by-step cooling primary-secondary cooling and maintenance
  • the steel sheet was reheated at a temperature of 490°C or less, and then, a process of maintaining at that temperature was performed. The maintaining process was performed for 30 seconds after the secondary cooling.
  • the steel sheet was galvanized in a hot-dip galvanizing bath at 430 to 490°C and cooled to room temperature at cooling rate of 5°C/s, and was then subjected to temper rolling to less than 2%, thereby manufacturing a hot-dip galvanized steel sheet.
  • some steels were subjected to the galvanizing treatment, followed by alloying heat treatment.
  • each steel sheet manufactured according to the conditions described above were observed and illustrated in Table 3 below.
  • fractions of tempered martensite (TM), bainite (B), ferrite (F), fresh martensite (FM) and retained austenite (A) were measured using FE-SEM, an image analyzer, EBSD, and XRD, after Nital corrosion at 1/4t (t: steel plate thickness (unit mm)) point of a steel sheet.
  • t steel plate thickness (unit mm)
  • the hole expansion ratio (HER) was measured by pushing up a circular hole punched in a 10mm diameter according to the ISO 16630 procedure with a cone punch until cracks occurred in the specimen, and measuring a ratio of an initial hole diameter to a hole diameter after change, and was calculated using the following expression.
  • Hole Expansion Ratio HER , % D ⁇ D 0 / D 0 ⁇ 100 (Here, D means a hole diameter (mm) when the crack penetrates through the steel sheet in a thickness direction, and D 0 means an initial hole diameter (mm) .
  • a tempered martensite phase and a bainite phase were formed in a total of 40 to 80 area% as intended, and retained austenite phases were mainly formed around the tempered martensite phase and the bainite phase. Accordingly, a yield ratio thereof may satisfy 0.6 to 0.9 as well as a high strength of 980 MPa, and an elongation ratio of 10% or more and a hole expansion ratio of 20% or more may be secured.
  • the strength and ductility may be significantly improved at the same time, and specifically, by satisfying the value of Relational Expression 2, it may be possible to secure the crashworthiness and formability targeted in the present disclosure.
  • Comparative steels 1 to 5 that deviate from Relational Expression 1 proposed in the present disclosure and do not satisfy the manufacturing conditions, at least one physical property was degraded because an intended microstructure was not formed.
  • Comparative Steel 1 was unable to secure the target level of strength due to an excessive ferrite phase, and had a poor hole expansion ratio and deviated from Relational Expression 2, which made it impossible to secure crashworthiness and formability.
  • Comparative Steels 2 and 5 were excessively formed on a fresh martensite phase to secure high strength, whereas they deviated from Relational Expression 2, which made it impossible to secure crashworthiness and formability.
  • Comparative Steel 3 had a small ferrite phase, and the ductility thereof significantly decreased as the retained austenite phase was not formed around a hard phase.
  • Comparative Steel 4 the retained austenite phase was not formed around the hard phase to lead to a relatively low elongation ratio, it was impossible to secure crashworthiness and formability as the Comparative Steel 4 deviated from Relational Expression 2.
  • FIG. 1 illustrates a graph of a change in mechanical properties (Relational Expression 2) according to a value of Relational Expression 1.
  • Relational Expression 1 As illustrated in FIG. 1 , it may be confirmed that when the value of Relational Expression 1 proposed in the present disclosure satisfies 1.7 or more, a value of Relational Expression 2 may be ensured to be 9 or more.
  • FIG. 2 illustrates an image of a microstructure of Inventive Steel 4 measured by the EBSD.
  • the retained austenite phase is mainly formed around the tempered martensite phase and the bainite phase, and it may be seen that the ferrite phase and the fresh martensite phase are properly formed.

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