EP2733228B1 - Hot press formed member and method for manufacturing the member - Google Patents
Hot press formed member and method for manufacturing the member Download PDFInfo
- Publication number
- EP2733228B1 EP2733228B1 EP11869574.1A EP11869574A EP2733228B1 EP 2733228 B1 EP2733228 B1 EP 2733228B1 EP 11869574 A EP11869574 A EP 11869574A EP 2733228 B1 EP2733228 B1 EP 2733228B1
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- EP
- European Patent Office
- Prior art keywords
- steel sheet
- hot
- less
- excluding
- steel
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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- 238000000034 method Methods 0.000 title claims description 51
- 238000004519 manufacturing process Methods 0.000 title claims description 34
- 229910000831 Steel Inorganic materials 0.000 claims description 184
- 239000010959 steel Substances 0.000 claims description 184
- 229910001566 austenite Inorganic materials 0.000 claims description 35
- 238000001816 cooling Methods 0.000 claims description 31
- 229910052799 carbon Inorganic materials 0.000 claims description 30
- 238000010438 heat treatment Methods 0.000 claims description 29
- 230000000717 retained effect Effects 0.000 claims description 28
- 229910052710 silicon Inorganic materials 0.000 claims description 23
- 229910001563 bainite Inorganic materials 0.000 claims description 21
- 229910052782 aluminium Inorganic materials 0.000 claims description 18
- 229910052757 nitrogen Inorganic materials 0.000 claims description 18
- 229910052717 sulfur Inorganic materials 0.000 claims description 18
- 229910052748 manganese Inorganic materials 0.000 claims description 16
- 239000012535 impurity Substances 0.000 claims description 13
- 229910052750 molybdenum Inorganic materials 0.000 claims description 11
- 239000010960 cold rolled steel Substances 0.000 claims description 10
- 230000009977 dual effect Effects 0.000 claims description 10
- 229910052758 niobium Inorganic materials 0.000 claims description 10
- 229910052720 vanadium Inorganic materials 0.000 claims description 8
- 229910052804 chromium Inorganic materials 0.000 claims description 7
- 229910052759 nickel Inorganic materials 0.000 claims description 7
- 229910052719 titanium Inorganic materials 0.000 claims description 7
- 238000007747 plating Methods 0.000 claims description 6
- 230000009466 transformation Effects 0.000 description 26
- 230000000052 comparative effect Effects 0.000 description 21
- 229910000734 martensite Inorganic materials 0.000 description 20
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 18
- 239000010955 niobium Substances 0.000 description 18
- 239000011572 manganese Substances 0.000 description 16
- PXHVJJICTQNCMI-UHFFFAOYSA-N nickel Substances [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 description 15
- XEEYBQQBJWHFJM-UHFFFAOYSA-N iron Substances [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 13
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 12
- 239000011651 chromium Substances 0.000 description 12
- 239000010936 titanium Substances 0.000 description 12
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 description 11
- 238000005098 hot rolling Methods 0.000 description 11
- 239000010703 silicon Substances 0.000 description 11
- 230000015572 biosynthetic process Effects 0.000 description 9
- 229910000859 α-Fe Inorganic materials 0.000 description 9
- 239000012467 final product Substances 0.000 description 8
- ZOXJGFHDIHLPTG-UHFFFAOYSA-N Boron Chemical compound [B] ZOXJGFHDIHLPTG-UHFFFAOYSA-N 0.000 description 7
- 229910052796 boron Inorganic materials 0.000 description 7
- 230000000694 effects Effects 0.000 description 7
- 229910001562 pearlite Inorganic materials 0.000 description 7
- NINIDFKCEFEMDL-UHFFFAOYSA-N Sulfur Chemical compound [S] NINIDFKCEFEMDL-UHFFFAOYSA-N 0.000 description 6
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 6
- 238000000137 annealing Methods 0.000 description 6
- GUCVJGMIXFAOAE-UHFFFAOYSA-N niobium atom Chemical compound [Nb] GUCVJGMIXFAOAE-UHFFFAOYSA-N 0.000 description 6
- 239000011593 sulfur Substances 0.000 description 6
- PWHULOQIROXLJO-UHFFFAOYSA-N Manganese Chemical compound [Mn] PWHULOQIROXLJO-UHFFFAOYSA-N 0.000 description 4
- 238000005097 cold rolling Methods 0.000 description 4
- 238000005246 galvanizing Methods 0.000 description 4
- 239000000203 mixture Substances 0.000 description 4
- 239000000047 product Substances 0.000 description 4
- 238000005096 rolling process Methods 0.000 description 4
- LEONUFNNVUYDNQ-UHFFFAOYSA-N vanadium atom Chemical compound [V] LEONUFNNVUYDNQ-UHFFFAOYSA-N 0.000 description 4
- VYZAMTAEIAYCRO-UHFFFAOYSA-N Chromium Chemical compound [Cr] VYZAMTAEIAYCRO-UHFFFAOYSA-N 0.000 description 3
- ZOKXTWBITQBERF-UHFFFAOYSA-N Molybdenum Chemical compound [Mo] ZOKXTWBITQBERF-UHFFFAOYSA-N 0.000 description 3
- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 description 3
- 238000007598 dipping method Methods 0.000 description 3
- 239000011733 molybdenum Substances 0.000 description 3
- 238000005554 pickling Methods 0.000 description 3
- 101710178035 Chorismate synthase 2 Proteins 0.000 description 2
- 101710152694 Cysteine synthase 2 Proteins 0.000 description 2
- 229910001335 Galvanized steel Inorganic materials 0.000 description 2
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 2
- 230000033228 biological regulation Effects 0.000 description 2
- 229910001567 cementite Inorganic materials 0.000 description 2
- 239000013078 crystal Substances 0.000 description 2
- 230000003111 delayed effect Effects 0.000 description 2
- 230000006866 deterioration Effects 0.000 description 2
- 238000009713 electroplating Methods 0.000 description 2
- 239000008397 galvanized steel Substances 0.000 description 2
- 239000011159 matrix material Substances 0.000 description 2
- 229910052760 oxygen Inorganic materials 0.000 description 2
- 239000001301 oxygen Substances 0.000 description 2
- 239000002244 precipitate Substances 0.000 description 2
- 238000007670 refining Methods 0.000 description 2
- 230000000087 stabilizing effect Effects 0.000 description 2
- 239000000126 substance Substances 0.000 description 2
- 238000010301 surface-oxidation reaction Methods 0.000 description 2
- 208000016261 weight loss Diseases 0.000 description 2
- 229910000680 Aluminized steel Inorganic materials 0.000 description 1
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 1
- 238000005275 alloying Methods 0.000 description 1
- 238000005269 aluminizing Methods 0.000 description 1
- UFGZSIPAQKLCGR-UHFFFAOYSA-N chromium carbide Chemical compound [Cr]#C[Cr]C#[Cr] UFGZSIPAQKLCGR-UHFFFAOYSA-N 0.000 description 1
- 150000001875 compounds Chemical class 0.000 description 1
- 238000009749 continuous casting Methods 0.000 description 1
- 230000007797 corrosion Effects 0.000 description 1
- 238000005260 corrosion Methods 0.000 description 1
- 230000001934 delay Effects 0.000 description 1
- 230000002542 deteriorative effect Effects 0.000 description 1
- 238000009792 diffusion process Methods 0.000 description 1
- 239000000446 fuel Substances 0.000 description 1
- 238000005244 galvannealing Methods 0.000 description 1
- 238000002844 melting Methods 0.000 description 1
- 230000008018 melting Effects 0.000 description 1
- 150000001247 metal acetylides Chemical class 0.000 description 1
- 238000000465 moulding Methods 0.000 description 1
- 230000003647 oxidation Effects 0.000 description 1
- 238000007254 oxidation reaction Methods 0.000 description 1
- 238000001556 precipitation Methods 0.000 description 1
- 238000010791 quenching Methods 0.000 description 1
- 230000000171 quenching effect Effects 0.000 description 1
- 238000001953 recrystallisation Methods 0.000 description 1
- 230000009467 reduction Effects 0.000 description 1
- 238000003303 reheating Methods 0.000 description 1
- 238000007711 solidification Methods 0.000 description 1
- 230000008023 solidification Effects 0.000 description 1
- 238000009628 steelmaking Methods 0.000 description 1
- 229910003470 tongbaite Inorganic materials 0.000 description 1
- 239000013585 weight reducing agent Substances 0.000 description 1
Images
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- 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
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21B—ROLLING OF METAL
- B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
- B21B1/02—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling heavy work, e.g. ingots, slabs, blooms, or billets, in which the cross-sectional form is unimportant ; Rolling combined with forging or pressing
- B21B1/026—Rolling
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES OR PROFILES, OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
- B21C47/00—Winding-up, coiling or winding-off metal wire, metal band or other flexible metal material characterised by features relevant to metal processing only
- B21C47/26—Special arrangements with regard to simultaneous or subsequent treatment of the material
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- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
- C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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- 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
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- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C22C—ALLOYS
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- 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
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- 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
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- 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
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- 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
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- 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
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- 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
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- C23C2/12—Aluminium or alloys based thereon
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- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
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- C21D2211/002—Bainite
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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 by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
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- 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
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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 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
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/12—All metal or with adjacent metals
- Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
- Y10T428/12736—Al-base component
- Y10T428/1275—Next to Group VIII or IB metal-base component
- Y10T428/12757—Fe
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/12—All metal or with adjacent metals
- Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
- Y10T428/12771—Transition metal-base component
- Y10T428/12785—Group IIB metal-base component
- Y10T428/12792—Zn-base component
- Y10T428/12799—Next to Fe-base component [e.g., galvanized]
Definitions
- the present disclosure relates to a hot-press formed member , and a method for manufacturing such a member.
- the invention is of benefit for manufacturing high-strength and high-ductility products suitable for impact members and crashworthy members of automobiles through a hot press forming process.
- AHSS high strength steel
- DP dual phase
- TRIP transformation induced plasticity
- Such steel sheets have a tensile strength of about 500 MPa to 1,000 MPa which may be insufficient to satisfy current rigidity and crash safety requirements while allowing for the lightening of automobiles.
- a steel forming method known as hot press forming has been commercialized to overcome such limitations and realize ultra high-strength automotive components.
- the hot press forming method after blanking, a steel sheet is subjected to heating to an Ac 3 temperature or higher for transformation into austenite, extracting, press forming, and die quenching, so as to form a martensite or mixed martensite-bainite microstructure.
- Ultra high-strength members having a tensile strength of 1 GPa or greater and high dimensional precision owing to high-temperature forming can be obtained using the hot press forming method.
- a hot press forming method of the related art is suitable for satisfying rigidity and crash safety requirements while lightening automotive components, final products have an elongation of 10% or less. That is, final products have a very low level of ductility.
- components manufactured by a hot press forming method may be used as impact members in automobiles, but may not be suitable for use as crashworthy members that absorb crash energy to protect vehicle occupants in a crash.
- KR20100091243 A discloses a high-strength part and process for producing the same.
- EP2719786 A1 is a prior art document falling under Art. 54 (3) EPC and discloses a hot press molded article, method for producing same, and thin steel sheet for hot press molding.
- aspects of the present invention may provide a hot-press formed member having high strength and high ductility and methods for manufacturing the member.
- a steel sheet for hot press forming may include, by wt%, C: 0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al: 0.01% to 2.0%, S: 0.015% or less, N: 0.01% or less, and the balance of Fe and inevitable impurities.
- a method for manufacturing a steel sheet for hot press forming may include: heating a steel slab to a temperature within a range of 1100°C to 1300°C, the steel slab including, by wt%, C: 0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al: 0.01% to 2.0%, S: 0.015% or less, N: 0.01% or less, and the balance of Fe and inevitable impurities; performing a finish hot-rolling process at a temperature within a range of an Ar 3 transformation point to 950°C to form a steel sheet; and coiling the steel sheet at a temperature within a range of M s to 720°C.
- a hot-press formed member consist of, by wt%, C: 0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al: 0.01% to 2.0%, S: 0.015% or less, N: 0.01% or less, and the balance of Fe and inevitable impurities, optionally further consisting of at least one selected from the group consisting of Mo: 0.5% or less, excluding 0%, Cr: 1.5% or less, excluding 0%, Ni: 0.5% or less, excluding 0%, Nb: 0.005% to 0.1%, and V: 0.005% to 0.1% optionally further consisting of a combination of B: 0.005% or less, excluding 0%, and Ti: 0.06% or less, excluding 0%, wherein the hot-press formed member has a dual phase microstructure formed by bainite and retained austenite.
- a method for manufacturing a hot-press formed member having a dual phase microstructure formed by bainite and retained austenite includes: heating a steel sheet to a temperature equal to or higher than Ac 3 , the steel sheet consisting of, by wt%, C: 0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al: 0.01% to 2.0%, S: 0.015% or less, N: 0.01% or less, and the balance of Fe and inevitable impurities, optionally further consisting of at least one selected from the group consisting of Mo: 0.5% or less, excluding 0%, Cr: 1.5% or less, excluding 0%, Ni: 0.5% or less, excluding 0%, Nb: 0.005% to 0.1%, and V: 0.005% to 0.1%, optionally further consisting of a combination of B: 0.005% or less, excluding 0%, and Ti: 0.06% or less, excluding 0%; hot-press forming the heated steel sheet; cooling the
- a high-strength, high-ductility steel sheet for hot press forming Disclosed herein is a high-strength, high-ductility steel sheet for hot press forming.
- the present disclosure also provides a member formed using the steel sheet and having a dual phase microstructure constituted by bainite and retained austenite and a TS(MPa) ⁇ El(%) value of 25,000 MPa% or greater. Since the member has high ductility as well as high strength, the member may be usefully used as a crashworthy member of an automobile.
- Embodiments and examples of the present invention provide a method for manufacturing a formed member having a high degree of ductility as well as high strength for use as a crashworthy member of an automobile.
- a steel sheet disclosed herein has a high degree of ductility for use in manufacturing such a formed member.
- the present disclosure provides four categories: a steel sheet for hot press forming having a high degree of ductility, a method for manufacturing the steel sheet, a hot-press formed member, and a method for manufacturing the hot-press formed member.
- the steel sheet for hot press forming has a high degree of ductility as well as a high degree of strength so that a member formed of the steel sheet through a hot press forming process may have a high degree of ductility and a high degree of strength.
- the steel sheet for hot press forming includes, by wt%, C: 0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al: 0.01% to 2.0%, S: 0.015% or less, N: 0.01% or less, and the balance of Fe and inevitable impurities.
- Carbon (C) is an element included in the steel sheet to enhance the strength thereof. Furthermore, in the embodiment of the present disclosure, carbon (C) is diffused into retained austenite by elements such as silicon (Si) to stabilize the retained austenite and thus to prevent transformation to martensite.
- the steel sheet for hot press forming may include 0.3 wt% to 1.0 wt% of carbon (C). If the carbon content is less than 0.3%, the amount of retained austenite is low after forming, and thus it may be difficult to guarantee both strength and ductility. If the carbon content is greater than 1.0%, bainite transformation is markedly slowed, and the formation of pearlite is facilitated, thereby deteriorating properties of the steel sheet.
- Manganese (Mn) is included in the steel sheet to prevent red shortness caused by FeS formed by sulfur (S) inevitably included in the steel sheet during a manufacturing process.
- the content of manganese (M) may be within the range of 0.01% to 4.0%. If the content of manganese (M) is less than 0.01%, red shortness may be caused by FeS. If the content of manganese (M) is greater than 4.0%, bainite transformation may be slowed to increase the time required for a heat treatment in a hot press forming process. As a result, the productivity of the hot press forming process may be lowered, and the manufacturing cost of the steel sheet may be increased.
- Silicon (Si) is an element included in the steel sheet to guarantee the ductility of a final product. Silicon (Si) facilitates ferrite transformation and diffuses carbon (C) into retained austenite to stabilize the retained austenite by an increased amount of carbon (C) in the retained austenite, thereby preventing transformation to martensite.
- the content of silicon (Si) may be within the range of 1.0 wt% to 2.0 wt%. If the content of silicon (Si) is less than 1.0%, the effect of stabilizing retained austenite may be poor. If the content of silicon (Si) is greater than 2.0%, the rolling characteristics of the steel sheet may be deteriorated. For example, the steel sheet may be cracked during a rolling process. Therefore, the upper limit of the content of silicon (Si) is set as 2.0%.
- Aluminum (Al) removes oxygen from the steel sheet to prevent the inclusion of nonmetallic substances therein during solidification.
- aluminum (Al) facilitates the diffusion of carbon (C) into retained austenite to stabilize the retained austenite.
- the content of aluminum (Al) may be within the range of 0.01% to 2.0%. If the content of aluminum (Al) is less than 0.01%, oxygen may be insufficiently removed, and thus it may be difficult to prevent the inclusion of nonmetallic substances. If the content of aluminum (Al) is greater than 2.0%, the unit cost of steel making may be increased.
- Sulfur (S) is an element inevitably included in the steel sheet during a manufacturing process thereof. Sulfur (S) combines with iron (Fe) to form FeS causing red shortness. Therefore, it may be necessary to keep the content of sulfur (S) as low as possible. For example, the content of sulfur (S) may be limited to 0.015% or less.
- Nitrogen (N) is an element inevitably included in the steel sheet during a manufacturing process. The content of nitrogen (N) may be kept as low as possible. For example, the content of nitrogen (N) may be limited to 0.01% or less.
- the steel sheet for hot press forming may further include at least one element selected from the group consisting of Mo: 0.5% or less (excluding 0%), Cr: 1.5% or less (excluding 0%), Ni: 0.5% or less (excluding 0%), Nb: 0.005% to 0.1%, and V: 0.005% to 0.1%.
- Molybdenum (Mo) may be added to the steel sheet to suppress the formation of pearlite. Since molybdenum (Mo) is relatively expensive and may increase the manufacturing cost of the steel sheet, 0.5 wt% or less of molybdenum (Mo) may be added.
- Chromium (Cr) may be added to the steel sheet to suppress the formation of ferrite and expand bainite transformation. If the content of chromium (Cr) is greater than 1.5 wt%, chromium carbide may be formed to lower the amount of dissolved carbon (C). Therefore, 1.5 wt% or less of chromium (Cr) may be added.
- Nickel (Ni) may be added to increase the faction of austenite and the hardenability of the steel sheet. Since nickel (Ni) is expensive and increases the manufacturing cost of the steel sheet, 0.5 wt% or less of nickel (Ni) may be added.
- Niobium (Nb) may be added to improve the strength, grain refining characteristics, and ductility of the steel sheet. During reheating, niobium (Nb) suppresses grain growth, and during cooling, niobium (Nb) delays transformation of austenite into ferrite. 0.005 wt% to 0.1 wt% of niobium (Nb) may be added. If the content of niobium (Nb) is less than 0.005%, it is difficult to assure the effect of grain refinement, and if the content of niobium (Nb) is greater than 0.1%, carbonitrides may excessively precipitate to cause delayed fractures in the steel sheet or decrease the workability of the steel sheet.
- Vanadium (V) may be added to improve the strength, grain refining characteristics, and hardenability of the steel sheet. 0.005 wt% to 0.1 wt% of vanadium (V) may be added. If the content of vanadium (V) is less than 0.005%, such effects may not be obtained, and if the content of vanadium (V) is greater than 0.1%, carbonitrides may excessively precipitate to cause delayed fractures in the steel sheet or decrease the workability of the steel sheet.
- the steel sheet for hot press forming may further include B: 0.005% or less (excluding 0%) and Ti: 0.06% or less (excluding 0%).
- Boron (B) may be added to suppress the formation of ferrite. If the content of boron (B) is greater than 0.005 wt%, boron (B) may combine with iron (Fe) or carbon (C) to form a compound facilitating the formation of ferrite. Therefore, 0.005 wt% of less of boron (B) may be added.
- Titanium (Ti) may be added to maximize the effect of boron (B). Titanium (Ti) combines with nitrogen (N) existing as an impurity in the steel sheet to form TiN, and thus boron (B) may not combine with nitrogen (N). Therefore, the formation of ferrite may be suppressed by boron (B). This effect may be assured by adding 0.06 wt% or less of titanium (Ti).
- the steel sheet may be a hot-rolled or cold-rolled steel sheet.
- the steel sheet may be a cold-rolled steel sheet coated with a plating layer for improving corrosion resistance and suppressing the formation of a surface oxide layer.
- the steel sheet for hot press forming has high strength and high ductility owing to the above-described composition, the steel sheet may be usefully used to manufacture hot-press formed members (described later) having high strength and ductility.
- the method for manufacturing a steel sheet for hot press forming includes: heating a steel slab to a temperature within a range of 1100°C to 1300°C, the steel slab including, by wt%, C: 0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al: 0.01% to 2.0%, S: 0.015% or less, N: 0.01% or less, and the balance of Fe and inevitable impurities; performing a finish hot-rolling process at a temperature within a range of Ar 3 transformation point to 950°C to form a steel sheet; and coiling the steel sheet at a temperature within a range of M s to 720°C.
- the continuous-casting structure of the steel slab may be insufficiently uniformized, and it may be difficult to assure a finish rolling temperature. If the steel slab is heated to a temperature greater than 1300°C, the size of crystal grains and the possibility of surface oxidation may increase to deteriorate the strength and surface properties of the steel slab. Therefore, the steel slab may be heated to a temperature within a range of 1100°C to 1300°C.
- the finish hot-rolling temperature is lower than Ar 3 transformation point, dual phase rolling may occur to result in hot-rolling mixed grain sizes, and if the finish hot-rolling temperature is higher than 950°C, crystal grains may be coarsened and surface oxidation may occur during the finish hot-rolling process. Therefore, the finish hot-rolling temperature may be within the range of the Ar 3 transformation point to 950°C.
- the coiling temperature is lower than M s , austenite may transform to martensite to decrease the ductility of the steel sheet and thus to make it difficult to perform a hot coiling process on the steel sheet. If the coiling temperature is higher than 720°C, a thick surface oxide layer may be formed on the steel sheet together with internal oxidation in the steel sheet. Therefore, the coiling temperature may be within the range of M s to 720°C.
- the method for manufacturing a steel sheet for hot press forming may include: heating a steel slab to a temperature within a range of 1100°C to 1300°C, the steel slab including, by wt%, C: 0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al: 0.01% to 2.0%, S: 0.015% or less, N: 0.01% or less, and the balance of Fe and inevitable impurities; performing a finish hot-rolling process at a temperature within a range of Ar 3 transformation point to 950°C to form a steel sheet; coiling the steel sheet at a temperature within a range of M s to 720°C; pickling the steel sheet; cold-rolling the steel sheet; continuously annealing the steel sheet at a temperature within a range of 750°C to 900°C; and overaging the steel sheet at a temperature within a range of M s to 550°C.
- the pickling of the steel sheet is performed to remove surface oxides formed during the heating and finish hot-rolling processes. Thereafter, the cold-rolling process is performed. If the continuous annealing temperature for the cold-rolled steel sheet is lower than 750°C, recrystallization may occur insufficiently, and thus a desired degree of workability of the steel sheet may not be obtained. If the continuous annealing temperature is higher than 900°C, it may difficult to heat the steel sheet to the continuous annealing temperature due to the limitation of heating equipment. In addition, if the overaging temperature is lower than M s , martensite may be formed to excessively increase the strength of the steel sheet and negatively affect the ductility of the steel sheet.
- the overaging temperature is higher than 550°C, the processability of the steel sheet may be lowered due to roll surface deterioration in an annealing furnace, and intended carbide precipitation and bainite transformation may not occur in the overaging process.
- the method for manufacturing a steel sheet for hot press forming may include: heating a steel slab to a temperature range of 1100°C to 1300°C, the steel slab including, by wt%, C: 0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al: 0.01% to 2.0%, S: 0.015% or less, N: 0.01% or less, and the balance of Fe and inevitable impurities; performing a finish hot-rolling process at a temperature within a range of Ar 3 transformation point to 950°C to form a steel sheet; coiling the steel sheet at a temperature within a range of M s to 720°C; pickling the steel sheet; cold-rolling the steel sheet; continuously annealing the steel sheet at a temperature within a range of 750°C to 900°C; overaging the steel sheet at a temperature within a range of M s to 550°C; and plating the overaged steel sheet by any one of hot-dip galvanizing, galvannea
- a hot-dip galvanized steel sheet may be manufactured by dipping a cold-rolling steel sheet in a galvanizing bath.
- a galvannealed steel sheet may be manufactured by dipping a cold-rolled steel sheet in a plating bath and performing an alloying heat-treatment process on the steel sheet.
- An electro-galvanized steel sheet may be manufactured by performing an electro galvanizing process or a Zn-Fe electroplating process on a cold-rolled steel sheet in a continuous electroplating line.
- a hot-dip aluminized steel sheet may be manufactured by heating a cold-rolled steel sheet, dipping the steel sheet in an aluminum plating bath, and cooling the steel sheet at room temperature at a cooling rate of 5°C/sec to 15°C/sec.
- the steel slab may further include at least one selected from the group consisting of Mo: 0.5% or less (excluding 0%), Cr: 1.5% or less (excluding 0%), Ni: 0.5% or less (excluding 0%), Nb: 0.005% to 0.1%, and V: 0.005% to 0.1%.
- the steel slab may further include B: 0.005% or less (excluding 0%) and Ti: 0.06% or less (excluding 0%).
- the hot-press formed member has high ductility and high strength.
- the hot-press formed member includes, by wt%, C: 0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al: 0.01% to 2.0%, S: 0.015% or less, N: 0.01% or less, and the balance of Fe and inevitable impurities.
- the hot-press formed member may have a microstructure formed of bainite and retained austenite without martensite.
- the hot-press formed member may further include at least one selected from the group consisting of Mo: 0.5% or less (excluding 0%), Cr: 1.5% or less (excluding 0%), Ni: 0.5% or less (excluding 0%), Nb: 0.005% to 0.1%, and V: 0.005% to 0.1%.
- the hot-press formed member may further include B: 0.005% or less (excluding 0%) and Ti: 0.06% or less (excluding 0%).
- Hot-press formed members of the related art are manufactured to have ultra high strength, and thus martensite is requisitely formed therein.
- martensite lowers the ductility of such hot-press formed members and thus makes such hot-press formed members unsuitable to be used as crashworthy members of automobiles. Therefore, in the embodiment of the present disclosure, the formation of martensite in the hot-press formed member is suppressed, and the amount of retained austenite is increased.
- the hot-press formed member has dual phases: bainite and retained austenite.
- the hot-press formed member having the above-mentioned composition and microstructure has good strength-ductility balance.
- TS ⁇ El of the hot-press formed member may be 25,000 or greater so as to be used as a crashworthy member of an automobile as well as being used as an impact member, where TS denotes tensile strength [MPa] and El denotes elongation [%].
- the method is for performing a hot press forming process on the above-described steel sheet to provide an ultra high-strength automotive component having high ductility.
- the method includes: heating a steel sheet to a temperature equal to or higher than Ac 3 , the steel sheet including, by wt%, C: 0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al: 0.01% to 2.0%, S: 0.015% or less, N: 0.01% or less, and the balance of Fe and inevitable impurities; hot-press forming the heated steel sheet; cooling the hot-press formed steel sheet to a temperature range of M s to 550°C at a rate of 20°C/sec or higher; and heat-treating the cooled steel sheet in a heating furnace heated at a temperature within a range of M s to 550°C.
- the steel sheet may further include at least one selected from the group consisting of Mo: 0.5% or less (excluding 0%), Cr: 1.5% or less (excluding 0%), Ni: 0.5% or less (excluding 0%), Nb: 0.005% to 0.1%, and V: 0.005% to 0.1%.
- the steel sheet may further include B: 0.005% or less (excluding 0%) and Ti: 0.06% or less (excluding 0%).
- the steel sheet may be one of a hot-rolled steel sheet, a cold-rolled steel sheet, and a plated cold-rolled steel sheet coated with a plating layer.
- the heat-treating after the hot-press forming is controlled differently as compared with the case of the related art, so as to manufacture a hot-press formed member having a different microstructure for improving ductility as compared with a hot-press formed member of the related art. That is, in the related art, heat-treatment conditions are adjusted to form martensite as a main microstructure to finally obtain an ultra high-strength member.
- heat treatment conditions for forming a microstructure constituted by bainite and retained austenite without martensite.
- the steel sheet is heated to a temperature equal to Ac 3 or higher for transformation to austenite, and is then hot-press formed.
- a hot-press formed steel sheet is directly die-quenched to a temperature equal to or lower than M s so as to form martensite as a main microstructure in a final product and thus to enhance the strength of the final product.
- martensite is excluded from the microstructure of a final product so as to improve the ductility of the final product while maintaining the strength of the final product at a level suitable for weight reduction.
- the hot-press formed steel sheet instead of cooling the hot-press formed steel sheet directly to room temperature equal to or lower than M s , the hot-press formed steel sheet is cooled to a temperature range of M s to 550°C, and heat-treated in a heating furnace at a temperature within a range of M s to 550°C so as to cause the hot-press formed steel sheet to undergo transformation to bainite.
- the cooling rate is adjusted to be within the range of M s to 550°C to form a dual phase microstructure constituted by bainite and retained austenite.
- Fe 3 C carbide may not be formed because elements such as silicon (Si) are sufficiently included in the steel sheet to diffuse carbon (C) into the retained austenite. That is, carbon (C) does not form carbides but is dissolved in the retained austenite to stabilize the retained austenite and thus to lower M s . Therefore, in the next cooling process, transformation to martensite is suppressed. Therefore, in a final product, the retained austenite remains instead of undergoing transformation to martensite, thereby improving ductility.
- elements such as silicon (Si) are sufficiently included in the steel sheet to diffuse carbon (C) into the retained austenite. That is, carbon (C) does not form carbides but is dissolved in the retained austenite to stabilize the retained austenite and thus to lower M s . Therefore, in the next cooling process, transformation to martensite is suppressed. Therefore, in a final product, the retained austenite remains instead of undergoing transformation to martensite, thereby improving ductility.
- the cooling rate may be 20°C/sec or higher. If the cooling rate is lower than 20°C/sec, transformation to pearlite may easily occur to lower properties of a final product.
- bainite was formed at a cooling rate of 30°C/sec.
- FIGS. 2B and 2C a pearlite structure in which ferrite and Fe 3 C were layered was formed at a cooling rate of 5°C/sec.
- the above-described processes for manufacturing a hot-press formed member according to the embodiment of the present disclosure may be summarized as follows. First, a steel sheet is inserted in a heating furnace to heat the steel sheet to Ac 3 or higher for forming austenite, and then the heated steel sheet is hot-press formed. After the hot press forming, the steel sheet is cooled to a temperature range of M s to 550°C at a cooling rate of 20°C/sec or higher so as not to form pearlite, and is then heat-treated in a heating furnace at a temperature within a range of M s to 550°C. These processes are for transformation to bainite, and during the processes, carbon (C) diffuses into austenite to lower M s . Although a hot-press formed member manufactured through the above-described processes is cooled to room temperature without any controlling, transformation to martensite does not occur. That is, a dual phase microstructure constituted by bainite and retained austenite may be obtained.
- Steel ingots 90 mm in length and 175 mm in width having compositions shown in Table 1 were manufactured by vacuum melting, and were then re-heated at 1200°C for 1 hour. Thereafter, the steel ingots were hot-rolled to obtain steel sheets having a thickness of 3 mm. At that time, a finish hot-rolling temperature was Ar 3 or higher. Then, after cooling the steel sheets, the steel sheets were inserted into a heating furnace previously heated to 600°C and left in the heating furnace for 1 hour. Thereafter, the steel sheets were cooled in the heating furnace to simulate hot coiling. Next, the steel sheets were cold-rolled at a reduction ratio of 60% to a thickness of 1.2 mm and were annealed at 900°C.
- the 1.2 mm thickness steel sheets manufactured as described above were heated to a temperature of 900°C and maintained at the temperature for 30 seconds. Then, the steel sheets were cooled to cooling temperatures at a rate of 30°C/sec. Next, the steel sheets were inserted into a heating furnace and heat-treated in the heating furnace at the same temperatures as the cooling temperatures for 400 seconds to 10,800 seconds. Thereafter, the steel sheets were air-cooled. In this way, hot-press formed members were obtained.
- the process conditions and mechanical properties of the hot-press formed members are shown in Table 2 below.
- Comparative Steel 1 Since TS ⁇ El of Comparative Steel 1 cooled at a cooling rate of 400°C is 16,785 MPa%, Comparative Steel 1 is not suitable as a crashworthy member of an automobile. The reason for this may be that the insufficient content of carbon (C) led to failure in stabilizing retained austenite. In the case that the cooling rate was 250°C, Comparative Steel 1 was cooled to a temperature lower than M s to result in a large amount of transformation to martensite, and thus Comparative Steel 1 had high strength but low ductility. In this case, TS ⁇ El of Comparative Steel 1 is 9,066 MPa%, and Comparative Steel 1 is not suitable to form a crashworthy member of an automobile.
- Comparative Steel 2 The carbon (C) content and silicon (Si) content of Comparative Steel 2 are also not sufficient to stabilize retained austenite, and the cooling temperature of Comparative Steel 2 is equal to or lower than M s to result in transformation to martensite. Therefore, Comparative Steel 2 has low ductility, and TS ⁇ El thereof is low at 10,150 MPa%. Comparative Steel 3 also has an insufficient content of carbon (C), and the cooling temperature of Comparative Steel 3 is equal to or lower than M s . Therefore, TS ⁇ El of Comparative Steel 3 is low at 8,940 MPa%, and Comparative Steel 3 is not suitable to form a crashworthy member of an automobile.
- Comparative Steel 4 has a sufficient content of carbon (C), the silicon (Si) content of Comparative Steel 4 is not sufficient to fully diffuse carbon (C) into retained austenite. Therefore, although TS ⁇ El of Comparative Steel 4 is relatively high at 19,216 MPa% as compared with other comparative steels, TS ⁇ El of Comparative Steel 4 is not greater than 25,000 MPa%. That is, Comparative Steel 4 is not suitable for forming a crashworthy member of an automobile.
- Samples of Inventive Steel 7 having a composition within the range of the present disclosure were cooled at a cooling rate of 30°C/sec and at a cooling rate of 5°C/sec, respectively.
- TS ⁇ El of Inventive Steel 7 was high at 46,923 MPa% and suitable for a crashworthy member of an automobile.
- TS ⁇ El of Inventive Steel 7 was low at 12, 480 MPa% and not suitable for a crashworthy member of an automobile. The reason for this may be that the low cooling rate led to the formation of pearlite as shown in FIGS. 2A to 2C and deterioration of properties thereof.
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Description
- The present disclosure relates to a hot-press formed member , and a method for manufacturing such a member. The invention is of benefit for manufacturing high-strength and high-ductility products suitable for impact members and crashworthy members of automobiles through a hot press forming process.
- Recently, safety regulations for protecting automobile occupants and fuel efficiency regulations for protecting the environment have been greatly tightened, and social requirements for vehicle weight reductions have markedly increased. The use of high-strength steel sheets is necessary to reduce the weight of automotive parts while maintaining the rigidity thereof and the crash safety of automobiles.
- However, if steel sheets for automobiles are improved in strength, the yield strength thereof is inevitably increased, and the elongation thereof is reduced. These factors significantly lower the formability of such steel sheets. In addition, due to excessive spring-back in high-strength steel sheets, the dimensions of components formed of high-strength steel sheets may be varied after a forming process. That is, the shape fixability of components may be lowered.
- To address these limitations, advanced high strength steel (AHSS), such as dual phase (DP) steel in which martensite is included in a ferrite matrix to lower the yield ratio thereof and transformation induced plasticity (TRIP) steel in which bainite and retained austenite are included in a ferrite matrix to markedly increase the strength-elongation balance thereof, have been developed and commercialized.
- However, such steel sheets have a tensile strength of about 500 MPa to 1,000 MPa which may be insufficient to satisfy current rigidity and crash safety requirements while allowing for the lightening of automobiles.
- Therefore, a steel forming method known as hot press forming has been commercialized to overcome such limitations and realize ultra high-strength automotive components. In the hot press forming method, after blanking, a steel sheet is subjected to heating to an Ac3 temperature or higher for transformation into austenite, extracting, press forming, and die quenching, so as to form a martensite or mixed martensite-bainite microstructure. Ultra high-strength members having a tensile strength of 1 GPa or greater and high dimensional precision owing to high-temperature forming can be obtained using the hot press forming method.
- Although such a hot press forming method of the related art is suitable for satisfying rigidity and crash safety requirements while lightening automotive components, final products have an elongation of 10% or less. That is, final products have a very low level of ductility. In other words, components manufactured by a hot press forming method may be used as impact members in automobiles, but may not be suitable for use as crashworthy members that absorb crash energy to protect vehicle occupants in a crash.
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KR20100091243 A EP2719786 A1 is a prior art document falling under Art. 54 (3) EPC and discloses a hot press molded article, method for producing same, and thin steel sheet for hot press molding. - Therefore, to use hot-press formed members as crashworthy members of automobiles, research into members having a high degree of ductility after being hot-press formed and steel sheets for forming such members through a hot press forming process is required.
- Aspects of the present invention may provide a hot-press formed member having high strength and high ductility and methods for manufacturing the member.
- According to an example of the present disclosure not specifically claimed herein, a steel sheet for hot press forming may include, by wt%, C: 0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al: 0.01% to 2.0%, S: 0.015% or less, N: 0.01% or less, and the balance of Fe and inevitable impurities.
- According to another example of the present disclosure also not specifically claimed herein, a method for manufacturing a steel sheet for hot press forming may include: heating a steel slab to a temperature within a range of 1100°C to 1300°C, the steel slab including, by wt%, C: 0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al: 0.01% to 2.0%, S: 0.015% or less, N: 0.01% or less, and the balance of Fe and inevitable impurities; performing a finish hot-rolling process at a temperature within a range of an Ar3 transformation point to 950°C to form a steel sheet; and coiling the steel sheet at a temperature within a range of Ms to 720°C.
- According to an aspect of the present invention, a hot-press formed member consist of, by wt%, C: 0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al: 0.01% to 2.0%, S: 0.015% or less, N: 0.01% or less, and the balance of Fe and inevitable impurities, optionally further consisting of at least one selected from the group consisting of Mo: 0.5% or less, excluding 0%, Cr: 1.5% or less, excluding 0%, Ni: 0.5% or less, excluding 0%, Nb: 0.005% to 0.1%, and V: 0.005% to 0.1% optionally further consisting of a combination of B: 0.005% or less, excluding 0%, and Ti: 0.06% or less, excluding 0%, wherein the hot-press formed member has a dual phase microstructure formed by bainite and retained austenite.
- According to another aspect of the present invention, a method for manufacturing a hot-press formed member having a dual phase microstructure formed by bainite and retained austenite, includes: heating a steel sheet to a temperature equal to or higher than Ac3, the steel sheet consisting of, by wt%, C: 0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al: 0.01% to 2.0%, S: 0.015% or less, N: 0.01% or less, and the balance of Fe and inevitable impurities, optionally further consisting of at least one selected from the group consisting of Mo: 0.5% or less, excluding 0%, Cr: 1.5% or less, excluding 0%, Ni: 0.5% or less, excluding 0%, Nb: 0.005% to 0.1%, and V: 0.005% to 0.1%, optionally further consisting of a combination of B: 0.005% or less, excluding 0%, and Ti: 0.06% or less, excluding 0%; hot-press forming the heated steel sheet; cooling the hot-press formed steel sheet to a temperature range of Ms to 550°C at a rate of 20°C/sec or higher; and heat-treating the cooled steel sheet at a temperature within a range of Ms to 550°C in a heating furnace.
- Disclosed herein is a high-strength, high-ductility steel sheet for hot press forming. The present disclosure also provides a member formed using the steel sheet and having a dual phase microstructure constituted by bainite and retained austenite and a TS(MPa)∗El(%) value of 25,000 MPa% or greater. Since the member has high ductility as well as high strength, the member may be usefully used as a crashworthy member of an automobile.
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FIG. 1 is a temperature-time graph illustrating manufacturing processes of a hot-press formed member according to an embodiment of the present disclosure. -
FIGS. 2A to 2C are images showing microstructures of hot-press formed members according to cooling rates after a forming process in a method for manufacturing a hot-press formed member, in whichFIG. 2A is the case of a cooling rate of 30°C/sec,FIG. 2B is the case of a cooling rate of 5°C/sec, andFIG. 2C is an enlarged image ofFIG. 2B . - Embodiments and examples of the present invention provide a method for manufacturing a formed member having a high degree of ductility as well as high strength for use as a crashworthy member of an automobile. A steel sheet disclosed herein has a high degree of ductility for use in manufacturing such a formed member. In detail, the present disclosure provides four categories: a steel sheet for hot press forming having a high degree of ductility, a method for manufacturing the steel sheet, a hot-press formed member, and a method for manufacturing the hot-press formed member.
- Hereinafter, a steel sheet for hot press forming will be described in detail.
- The steel sheet for hot press forming has a high degree of ductility as well as a high degree of strength so that a member formed of the steel sheet through a hot press forming process may have a high degree of ductility and a high degree of strength. The steel sheet for hot press forming includes, by wt%, C: 0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al: 0.01% to 2.0%, S: 0.015% or less, N: 0.01% or less, and the balance of Fe and inevitable impurities.
- Carbon (C) is an element included in the steel sheet to enhance the strength thereof. Furthermore, in the embodiment of the present disclosure, carbon (C) is diffused into retained austenite by elements such as silicon (Si) to stabilize the retained austenite and thus to prevent transformation to martensite. The steel sheet for hot press forming may include 0.3 wt% to 1.0 wt% of carbon (C). If the carbon content is less than 0.3%, the amount of retained austenite is low after forming, and thus it may be difficult to guarantee both strength and ductility. If the carbon content is greater than 1.0%, bainite transformation is markedly slowed, and the formation of pearlite is facilitated, thereby deteriorating properties of the steel sheet.
- Manganese (Mn) is included in the steel sheet to prevent red shortness caused by FeS formed by sulfur (S) inevitably included in the steel sheet during a manufacturing process. The content of manganese (M) may be within the range of 0.01% to 4.0%. If the content of manganese (M) is less than 0.01%, red shortness may be caused by FeS. If the content of manganese (M) is greater than 4.0%, bainite transformation may be slowed to increase the time required for a heat treatment in a hot press forming process. As a result, the productivity of the hot press forming process may be lowered, and the manufacturing cost of the steel sheet may be increased.
- Silicon (Si) is an element included in the steel sheet to guarantee the ductility of a final product. Silicon (Si) facilitates ferrite transformation and diffuses carbon (C) into retained austenite to stabilize the retained austenite by an increased amount of carbon (C) in the retained austenite, thereby preventing transformation to martensite. The content of silicon (Si) may be within the range of 1.0 wt% to 2.0 wt%. If the content of silicon (Si) is less than 1.0%, the effect of stabilizing retained austenite may be poor. If the content of silicon (Si) is greater than 2.0%, the rolling characteristics of the steel sheet may be deteriorated. For example, the steel sheet may be cracked during a rolling process. Therefore, the upper limit of the content of silicon (Si) is set as 2.0%.
- Aluminum (Al) removes oxygen from the steel sheet to prevent the inclusion of nonmetallic substances therein during solidification. In addition, like silicon (Si), aluminum (Al) facilitates the diffusion of carbon (C) into retained austenite to stabilize the retained austenite. The content of aluminum (Al) may be within the range of 0.01% to 2.0%. If the content of aluminum (Al) is less than 0.01%, oxygen may be insufficiently removed, and thus it may be difficult to prevent the inclusion of nonmetallic substances. If the content of aluminum (Al) is greater than 2.0%, the unit cost of steel making may be increased.
- Sulfur (S) is an element inevitably included in the steel sheet during a manufacturing process thereof. Sulfur (S) combines with iron (Fe) to form FeS causing red shortness. Therefore, it may be necessary to keep the content of sulfur (S) as low as possible. For example, the content of sulfur (S) may be limited to 0.015% or less. Nitrogen (N) is an element inevitably included in the steel sheet during a manufacturing process. The content of nitrogen (N) may be kept as low as possible. For example, the content of nitrogen (N) may be limited to 0.01% or less.
- In addition to the above-mentioned elements, the steel sheet for hot press forming may further include at least one element selected from the group consisting of Mo: 0.5% or less (excluding 0%), Cr: 1.5% or less (excluding 0%), Ni: 0.5% or less (excluding 0%), Nb: 0.005% to 0.1%, and V: 0.005% to 0.1%.
- Molybdenum (Mo) may be added to the steel sheet to suppress the formation of pearlite. Since molybdenum (Mo) is relatively expensive and may increase the manufacturing cost of the steel sheet, 0.5 wt% or less of molybdenum (Mo) may be added.
- Chromium (Cr) may be added to the steel sheet to suppress the formation of ferrite and expand bainite transformation. If the content of chromium (Cr) is greater than 1.5 wt%, chromium carbide may be formed to lower the amount of dissolved carbon (C). Therefore, 1.5 wt% or less of chromium (Cr) may be added.
- Nickel (Ni) may be added to increase the faction of austenite and the hardenability of the steel sheet. Since nickel (Ni) is expensive and increases the manufacturing cost of the steel sheet, 0.5 wt% or less of nickel (Ni) may be added.
- Niobium (Nb) may be added to improve the strength, grain refining characteristics, and ductility of the steel sheet. During reheating, niobium (Nb) suppresses grain growth, and during cooling, niobium (Nb) delays transformation of austenite into ferrite. 0.005 wt% to 0.1 wt% of niobium (Nb) may be added. If the content of niobium (Nb) is less than 0.005%, it is difficult to assure the effect of grain refinement, and if the content of niobium (Nb) is greater than 0.1%, carbonitrides may excessively precipitate to cause delayed fractures in the steel sheet or decrease the workability of the steel sheet.
- Vanadium (V) may be added to improve the strength, grain refining characteristics, and hardenability of the steel sheet. 0.005 wt% to 0.1 wt% of vanadium (V) may be added. If the content of vanadium (V) is less than 0.005%, such effects may not be obtained, and if the content of vanadium (V) is greater than 0.1%, carbonitrides may excessively precipitate to cause delayed fractures in the steel sheet or decrease the workability of the steel sheet.
- In addition, the steel sheet for hot press forming may further include B: 0.005% or less (excluding 0%) and Ti: 0.06% or less (excluding 0%).
- Boron (B) may be added to suppress the formation of ferrite. If the content of boron (B) is greater than 0.005 wt%, boron (B) may combine with iron (Fe) or carbon (C) to form a compound facilitating the formation of ferrite. Therefore, 0.005 wt% of less of boron (B) may be added.
- Titanium (Ti) may be added to maximize the effect of boron (B). Titanium (Ti) combines with nitrogen (N) existing as an impurity in the steel sheet to form TiN, and thus boron (B) may not combine with nitrogen (N). Therefore, the formation of ferrite may be suppressed by boron (B). This effect may be assured by adding 0.06 wt% or less of titanium (Ti).
- The steel sheet may be a hot-rolled or cold-rolled steel sheet. For example, the steel sheet may be a cold-rolled steel sheet coated with a plating layer for improving corrosion resistance and suppressing the formation of a surface oxide layer.
- Since the steel sheet for hot press forming has high strength and high ductility owing to the above-described composition, the steel sheet may be usefully used to manufacture hot-press formed members (described later) having high strength and ductility.
- Hereafter, a method for manufacturing a steel sheet for hot press forming will be described in detail. This is an exemplary example for manufacturing a steel sheet suitable for manufacturing a hot-press formed member having improved ductility.
- The method for manufacturing a steel sheet for hot press forming includes: heating a steel slab to a temperature within a range of 1100°C to 1300°C, the steel slab including, by wt%, C: 0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al: 0.01% to 2.0%, S: 0.015% or less, N: 0.01% or less, and the balance of Fe and inevitable impurities; performing a finish hot-rolling process at a temperature within a range of Ar3 transformation point to 950°C to form a steel sheet; and coiling the steel sheet at a temperature within a range of Ms to 720°C.
- If the steel slab is heated to a temperature lower than 1100°C, the continuous-casting structure of the steel slab may be insufficiently uniformized, and it may be difficult to assure a finish rolling temperature. If the steel slab is heated to a temperature greater than 1300°C, the size of crystal grains and the possibility of surface oxidation may increase to deteriorate the strength and surface properties of the steel slab. Therefore, the steel slab may be heated to a temperature within a range of 1100°C to 1300°C. If the finish hot-rolling temperature is lower than Ar3 transformation point, dual phase rolling may occur to result in hot-rolling mixed grain sizes, and if the finish hot-rolling temperature is higher than 950°C, crystal grains may be coarsened and surface oxidation may occur during the finish hot-rolling process. Therefore, the finish hot-rolling temperature may be within the range of the Ar3 transformation point to 950°C. In addition, if the coiling temperature is lower than Ms, austenite may transform to martensite to decrease the ductility of the steel sheet and thus to make it difficult to perform a hot coiling process on the steel sheet. If the coiling temperature is higher than 720°C, a thick surface oxide layer may be formed on the steel sheet together with internal oxidation in the steel sheet. Therefore, the coiling temperature may be within the range of Ms to 720°C.
- The method for manufacturing a steel sheet for hot press forming may include: heating a steel slab to a temperature within a range of 1100°C to 1300°C, the steel slab including, by wt%, C: 0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al: 0.01% to 2.0%, S: 0.015% or less, N: 0.01% or less, and the balance of Fe and inevitable impurities; performing a finish hot-rolling process at a temperature within a range of Ar3 transformation point to 950°C to form a steel sheet; coiling the steel sheet at a temperature within a range of Ms to 720°C; pickling the steel sheet; cold-rolling the steel sheet; continuously annealing the steel sheet at a temperature within a range of 750°C to 900°C; and overaging the steel sheet at a temperature within a range of Ms to 550°C.
- The pickling of the steel sheet is performed to remove surface oxides formed during the heating and finish hot-rolling processes. Thereafter, the cold-rolling process is performed. If the continuous annealing temperature for the cold-rolled steel sheet is lower than 750°C, recrystallization may occur insufficiently, and thus a desired degree of workability of the steel sheet may not be obtained. If the continuous annealing temperature is higher than 900°C, it may difficult to heat the steel sheet to the continuous annealing temperature due to the limitation of heating equipment. In addition, if the overaging temperature is lower than Ms, martensite may be formed to excessively increase the strength of the steel sheet and negatively affect the ductility of the steel sheet. Therefore, before a hot press forming process, blanking may not be easily performed. If the overaging temperature is higher than 550°C, the processability of the steel sheet may be lowered due to roll surface deterioration in an annealing furnace, and intended carbide precipitation and bainite transformation may not occur in the overaging process.
- The method for manufacturing a steel sheet for hot press forming may include: heating a steel slab to a temperature range of 1100°C to 1300°C, the steel slab including, by wt%, C: 0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al: 0.01% to 2.0%, S: 0.015% or less, N: 0.01% or less, and the balance of Fe and inevitable impurities; performing a finish hot-rolling process at a temperature within a range of Ar3 transformation point to 950°C to form a steel sheet; coiling the steel sheet at a temperature within a range of Ms to 720°C; pickling the steel sheet; cold-rolling the steel sheet; continuously annealing the steel sheet at a temperature within a range of 750°C to 900°C; overaging the steel sheet at a temperature within a range of Ms to 550°C; and plating the overaged steel sheet by any one of hot-dip galvanizing, galvannealing, electro galvanizing, and hot-dip aluminizing.
- A hot-dip galvanized steel sheet may be manufactured by dipping a cold-rolling steel sheet in a galvanizing bath. A galvannealed steel sheet may be manufactured by dipping a cold-rolled steel sheet in a plating bath and performing an alloying heat-treatment process on the steel sheet. An electro-galvanized steel sheet may be manufactured by performing an electro galvanizing process or a Zn-Fe electroplating process on a cold-rolled steel sheet in a continuous electroplating line. A hot-dip aluminized steel sheet may be manufactured by heating a cold-rolled steel sheet, dipping the steel sheet in an aluminum plating bath, and cooling the steel sheet at room temperature at a cooling rate of 5°C/sec to 15°C/sec.
- The steel slab may further include at least one selected from the group consisting of Mo: 0.5% or less (excluding 0%), Cr: 1.5% or less (excluding 0%), Ni: 0.5% or less (excluding 0%), Nb: 0.005% to 0.1%, and V: 0.005% to 0.1%. In addition, the steel slab may further include B: 0.005% or less (excluding 0%) and Ti: 0.06% or less (excluding 0%).
- Hereinafter, a hot-press formed member will be described in detail according to an embodiment of the present invention.
- The hot-press formed member has high ductility and high strength. For this, the hot-press formed member includes, by wt%, C: 0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al: 0.01% to 2.0%, S: 0.015% or less, N: 0.01% or less, and the balance of Fe and inevitable impurities. The hot-press formed member may have a microstructure formed of bainite and retained austenite without martensite.
- The hot-press formed member may further include at least one selected from the group consisting of Mo: 0.5% or less (excluding 0%), Cr: 1.5% or less (excluding 0%), Ni: 0.5% or less (excluding 0%), Nb: 0.005% to 0.1%, and V: 0.005% to 0.1%. In addition, the hot-press formed member may further include B: 0.005% or less (excluding 0%) and Ti: 0.06% or less (excluding 0%).
- Hot-press formed members of the related art are manufactured to have ultra high strength, and thus martensite is requisitely formed therein. However, martensite lowers the ductility of such hot-press formed members and thus makes such hot-press formed members unsuitable to be used as crashworthy members of automobiles. Therefore, in the embodiment of the present disclosure, the formation of martensite in the hot-press formed member is suppressed, and the amount of retained austenite is increased. Thus, the hot-press formed member has dual phases: bainite and retained austenite.
- The hot-press formed member having the above-mentioned composition and microstructure has good strength-ductility balance. For example, TS∗El of the hot-press formed member may be 25,000 or greater so as to be used as a crashworthy member of an automobile as well as being used as an impact member, where TS denotes tensile strength [MPa] and El denotes elongation [%].
- Hereinafter, a method for manufacturing a hot-press formed member will be described in detail according to an embodiment of the present invention.
- The method is for performing a hot press forming process on the above-described steel sheet to provide an ultra high-strength automotive component having high ductility. For this, the method includes: heating a steel sheet to a temperature equal to or higher than Ac3, the steel sheet including, by wt%, C: 0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al: 0.01% to 2.0%, S: 0.015% or less, N: 0.01% or less, and the balance of Fe and inevitable impurities; hot-press forming the heated steel sheet; cooling the hot-press formed steel sheet to a temperature range of Ms to 550°C at a rate of 20°C/sec or higher; and heat-treating the cooled steel sheet in a heating furnace heated at a temperature within a range of Ms to 550°C.
- The steel sheet may further include at least one selected from the group consisting of Mo: 0.5% or less (excluding 0%), Cr: 1.5% or less (excluding 0%), Ni: 0.5% or less (excluding 0%), Nb: 0.005% to 0.1%, and V: 0.005% to 0.1%. In addition, the steel sheet may further include B: 0.005% or less (excluding 0%) and Ti: 0.06% or less (excluding 0%). The steel sheet may be one of a hot-rolled steel sheet, a cold-rolled steel sheet, and a plated cold-rolled steel sheet coated with a plating layer.
- In the method for manufacturing a hot-press formed member according to the embodiment of the present invention, the heat-treating after the hot-press forming is controlled differently as compared with the case of the related art, so as to manufacture a hot-press formed member having a different microstructure for improving ductility as compared with a hot-press formed member of the related art. That is, in the related art, heat-treatment conditions are adjusted to form martensite as a main microstructure to finally obtain an ultra high-strength member. However, since such a technique of the related art is not suitable to manufacture a highly ductile member usable as a crashworthy member of an automobile, the inventors have suggested heat treatment conditions for forming a microstructure constituted by bainite and retained austenite without martensite.
- First, the steel sheet is heated to a temperature equal to Ac3 or higher for transformation to austenite, and is then hot-press formed.
- The heat-treatment conditions after the hot-press forming have a major effect on determining the microstructure of a product. In the related art, generally, a hot-press formed steel sheet is directly die-quenched to a temperature equal to or lower than Ms so as to form martensite as a main microstructure in a final product and thus to enhance the strength of the final product.
- However, in the embodiment of the present invention, martensite is excluded from the microstructure of a final product so as to improve the ductility of the final product while maintaining the strength of the final product at a level suitable for weight reduction. To this end, instead of cooling the hot-press formed steel sheet directly to room temperature equal to or lower than Ms, the hot-press formed steel sheet is cooled to a temperature range of Ms to 550°C, and heat-treated in a heating furnace at a temperature within a range of Ms to 550°C so as to cause the hot-press formed steel sheet to undergo transformation to bainite. If the steel sheet is cooled to a temperature equal to or lower than Ms, martensite may be formed to lower the ductility of the steel sheet, and if the steel sheet is cooled to a temperature higher than 550°C, pearlite may be formed to deteriorate properties of the steel sheet. Therefore, the cooling rate is adjusted to be within the range of Ms to 550°C to form a dual phase microstructure constituted by bainite and retained austenite.
- In the bainite formed as described above, Fe3C carbide may not be formed because elements such as silicon (Si) are sufficiently included in the steel sheet to diffuse carbon (C) into the retained austenite. That is, carbon (C) does not form carbides but is dissolved in the retained austenite to stabilize the retained austenite and thus to lower Ms. Therefore, in the next cooling process, transformation to martensite is suppressed. Therefore, in a final product, the retained austenite remains instead of undergoing transformation to martensite, thereby improving ductility.
- The cooling rate may be 20°C/sec or higher. If the cooling rate is lower than 20°C/sec, transformation to pearlite may easily occur to lower properties of a final product. Referring to
FIG. 2A , bainite was formed at a cooling rate of 30°C/sec. However, referring toFIGS. 2B and2C , a pearlite structure in which ferrite and Fe3C were layered was formed at a cooling rate of 5°C/sec. - For example, the above-described processes for manufacturing a hot-press formed member according to the embodiment of the present disclosure may be summarized as follows. First, a steel sheet is inserted in a heating furnace to heat the steel sheet to Ac3 or higher for forming austenite, and then the heated steel sheet is hot-press formed. After the hot press forming, the steel sheet is cooled to a temperature range of Ms to 550°C at a cooling rate of 20°C/sec or higher so as not to form pearlite, and is then heat-treated in a heating furnace at a temperature within a range of Ms to 550°C. These processes are for transformation to bainite, and during the processes, carbon (C) diffuses into austenite to lower Ms. Although a hot-press formed member manufactured through the above-described processes is cooled to room temperature without any controlling, transformation to martensite does not occur. That is, a dual phase microstructure constituted by bainite and retained austenite may be obtained.
- Hereinafter, the embodiments of the present invention will be described more specifically according to examples. The following examples are merely provided to allow for a clear understanding of the present disclosure, rather than to limit the scope thereof.
- Steel ingots 90 mm in length and 175 mm in width having compositions shown in Table 1 were manufactured by vacuum melting, and were then re-heated at 1200°C for 1 hour. Thereafter, the steel ingots were hot-rolled to obtain steel sheets having a thickness of 3 mm. At that time, a finish hot-rolling temperature was Ar3 or higher. Then, after cooling the steel sheets, the steel sheets were inserted into a heating furnace previously heated to 600°C and left in the heating furnace for 1 hour. Thereafter, the steel sheets were cooled in the heating furnace to simulate hot coiling. Next, the steel sheets were cold-rolled at a reduction ratio of 60% to a thickness of 1.2 mm and were annealed at 900°C. Then, the steel sheets were allowed to undergo bainite transformation at 400°C. In Table 1, the contents of elements are given in wt% except for the contents of sulfur (S) and nitrogen (N) given in ppm.
[Table 1] Steels C Si Mn Al Mo Cr Ni Ti B Nb V S N IS* 1 0.40 1.51 3.01 0.04 30 20 IS 2 0.63 1.49 0.72 0.50 20 20 IS 3 0.61 1.52 0.63 0.51 0.30 30 20 IS 4 0.61 1.50 0.65 0.50 0.015 30 20 IS 5 0.62 1.49 1.61 1.53 0.02 30 20 IS 6 0.60 1.50 2.91 0.04 0.25 1.20 20 20 IS 7 0.71 1.47 0.7 0.52 0.010 0.002 20 20 IS 8 0.68 1.48 0.71 0.04 0.24 30 20 IS 9 0.70 1.15 0.72 0.51 0.24 30 20 IS 10 0.71 1.15 0.71 0.04 0.24 0.010 0.002 30 20 IS 11 0.69 1.55 0.18 0.04 0.24 0.50 0.010 0.002 30 20 IS 12 0.82 1.49 0.51 0.54 30 20 IS 13 0.82 1.51 1.01 0.53 30 20 CS** 1 0.23 1.5 1.5 0.04 30 20 CS 2 0.20 0.5 1.5 0.03 30 20 CS 3 0.22 1.5 2 0.03 0.20 20 20 CS 4 0.68 0.42 0.70 0.52 20 20 *IS: Inventive Steel, **CS: Comparative Steel - To simulate a heat treatment in a heating furnace during a hot press forming process, the 1.2 mm thickness steel sheets manufactured as described above were heated to a temperature of 900°C and maintained at the temperature for 30 seconds. Then, the steel sheets were cooled to cooling temperatures at a rate of 30°C/sec. Next, the steel sheets were inserted into a heating furnace and heat-treated in the heating furnace at the same temperatures as the cooling temperatures for 400 seconds to 10,800 seconds. Thereafter, the steel sheets were air-cooled. In this way, hot-press formed members were obtained. The process conditions and mechanical properties of the hot-press formed members are shown in Table 2 below.
[Table 2] Steels Cooling Rate (°C/sec) Cooling Temperature (°C) Time (sec) YS (MPa) TS (MPa) El (%) TS∗El (MPa%) Ms (°C) Is* 1 30 400 3600 732 1265 28 35420 295 IS 2 30 400 3600 899 1187 39 46244 298 IS 3 30 400 3600 869 1196 37 44252 302 IS 4 30 400 3600 915 1289 35 45115 307 IS 5 30 400 3600 883 1185 36 42660 272 IS 6 30 400 10800 856 1420 26 36920 199 30 300 10800 985 1610 22 35420 199 IS 7 30 400 3600 900 1185 40 46923 273 5 400 3600 719 1128 11 12408 273 IS 8 30 400 600 816 1310 21 27510 280 IS 9 30 400 600 915 1240 29 35960 277 IS 10 30 400 600 845 1318 25 32950 270 IS 11 30 400 600 940 1288 26 33488 280 IS 12 30 400 3600 881 1229 35 43556 245 IS 13 30 400 3600 725 1306 39 50934 228 CS** 1 30 400 3600 640 1125 15 16875 399 30 250 3600 1295 1511 6 9066 399 CS 2 30 250 3600 1220 1450 7 10150 420 CS 3 30 250 3600 1280 1490 6 8940 384 CS 4 30 400 3600 870 1201 16 19216 295 *IS: Inventive Steel, **CS: Comparative Steel - Since TS∗El of Comparative Steel 1 cooled at a cooling rate of 400°C is 16,785 MPa%, Comparative Steel 1 is not suitable as a crashworthy member of an automobile. The reason for this may be that the insufficient content of carbon (C) led to failure in stabilizing retained austenite. In the case that the cooling rate was 250°C, Comparative Steel 1 was cooled to a temperature lower than Ms to result in a large amount of transformation to martensite, and thus Comparative Steel 1 had high strength but low ductility. In this case, TS∗El of Comparative Steel 1 is 9,066 MPa%, and Comparative Steel 1 is not suitable to form a crashworthy member of an automobile.
- The carbon (C) content and silicon (Si) content of Comparative Steel 2 are also not sufficient to stabilize retained austenite, and the cooling temperature of Comparative Steel 2 is equal to or lower than Ms to result in transformation to martensite. Therefore, Comparative Steel 2 has low ductility, and TS∗El thereof is low at 10,150 MPa%. Comparative Steel 3 also has an insufficient content of carbon (C), and the cooling temperature of Comparative Steel 3 is equal to or lower than Ms. Therefore, TS∗El of Comparative Steel 3 is low at 8,940 MPa%, and Comparative Steel 3 is not suitable to form a crashworthy member of an automobile.
- Although Comparative Steel 4 has a sufficient content of carbon (C), the silicon (Si) content of Comparative Steel 4 is not sufficient to fully diffuse carbon (C) into retained austenite. Therefore, although TS∗El of Comparative Steel 4 is relatively high at 19,216 MPa% as compared with other comparative steels, TS∗El of Comparative Steel 4 is not greater than 25,000 MPa%. That is, Comparative Steel 4 is not suitable for forming a crashworthy member of an automobile.
- Samples of Inventive Steel 7 having a composition within the range of the present disclosure were cooled at a cooling rate of 30°C/sec and at a cooling rate of 5°C/sec, respectively. In the case that the cooling rate was 30°C/sec, TS∗El of Inventive Steel 7 was high at 46,923 MPa% and suitable for a crashworthy member of an automobile. However, in the case that the cooling rate was 5°C/sec, TS∗El of Inventive Steel 7 was low at 12, 480 MPa% and not suitable for a crashworthy member of an automobile. The reason for this may be that the low cooling rate led to the formation of pearlite as shown in
FIGS. 2A to 2C and deterioration of properties thereof.
Claims (4)
- A hot-press formed member consisting of, by wt%, C: 0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al: 0.01% to 2.0%, S: 0.015% or less, N: 0.01% or less, and the balance of Fe and inevitable impurities, optionally further consisting of at least one selected from the group consisting of Mo: 0.5% or less, excluding 0%, Cr: 1.5 or less, excluding 0%, Ni: 0.5 or less, excluding 0%, Nb: 0.005% to 0.1%, and V: 0.005% to 0.1%, optionally further consisting of a combination of B: 0.005% or less, excluding 0%, and Ti: 0.06% or less, excluding 0%, wherein the hot-press formed member has a dual phase microstructure formed by bainite and retained austenite.
- The hot-press formed member of claim 1, wherein the hot-press formed member has a TS(MPa)∗El(%) value of 25,000 MPa% or greater, where TS denotes tensile strength in MPa and El denotes elongation in %.
- A method for manufacturing a hot-press formed member having a dual phase microstructure formed by bainite and retained austenite, the method comprising:heating a steel sheet to a temperature equal to or higher than Ac3, the steel sheet consisting of, by wt%, C: 0.3% to 1.0%, Mn: 0.01% to 4.0%, Si: 1.0% to 2.0%, Al: 0.01% to 2.0%, S: 0.015% or less, N: 0.01% or less, and the balance of Fe and inevitable impurities, optionally further consisting of at least one selected from the group consisting of Mo: 0.5% or less, excluding 0%, Cr: 1.5% or less, excluding 0%, Ni: 0.5% or less, excluding 0%, Nb: 0.005% to 0.1%, and V: 0.005% to 0.1%, optionally further consisting of a combination of B: 0.005% or less, excluding 0%, and Ti: 0.06% or less, excluding 0%;hot-press forming the heated steel sheet;cooling the hot-press formed steel sheet to a temperature range of Ms to 550°C at a rate of 20°C/sec or higher; andheat-treating the cooled steel sheet at a temperature within a range of Ms to 550°C in a heating furnace.
- The method of claim 3, wherein the steel sheet is one of a hot-rolled steel sheet, a cold-rolled steel sheet, and a plated cold-rolled steel sheet coated with a plating layer.
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PCT/KR2011/005242 WO2013012103A1 (en) | 2011-07-15 | 2011-07-15 | Hot press forming steel plate, formed member using same, and method for manufacturing the plate and member |
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EP2733228A1 EP2733228A1 (en) | 2014-05-21 |
EP2733228A4 EP2733228A4 (en) | 2015-08-12 |
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US (1) | US20140150930A1 (en) |
EP (1) | EP2733228B1 (en) |
JP (1) | JP2014520961A (en) |
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DE102013010946B3 (en) * | 2013-06-28 | 2014-12-31 | Daimler Ag | Method and plant for producing a press-hardened sheet steel component |
JP6327737B2 (en) * | 2013-07-09 | 2018-05-23 | 国立研究開発法人物質・材料研究機構 | Martensitic steel and manufacturing method thereof |
CN105518162B (en) * | 2013-09-10 | 2017-06-06 | 株式会社神户制钢所 | The manufacture method of stamping product and stamping product |
KR101569508B1 (en) | 2014-12-24 | 2015-11-17 | 주식회사 포스코 | Hot press formed parts having excellent bendability, and method for the same |
KR101665819B1 (en) * | 2014-12-24 | 2016-10-13 | 주식회사 포스코 | Steel material for heat treating, formed component having extra high strength and high fatigue resistance and method for manufacturing the formed component |
KR102096385B1 (en) | 2015-07-13 | 2020-04-02 | 제이에프이 스틸 가부시키가이샤 | Press forming method and method of manufacturing press-formed component |
US11993823B2 (en) | 2016-05-10 | 2024-05-28 | United States Steel Corporation | High strength annealed steel products and annealing processes for making the same |
US11560606B2 (en) | 2016-05-10 | 2023-01-24 | United States Steel Corporation | Methods of producing continuously cast hot rolled high strength steel sheet products |
BR112018073175B1 (en) | 2016-05-10 | 2022-08-16 | United States Steel Corporation | HIGH STRENGTH COLD-LAMINED SHEET STEEL PRODUCT, AND METHOD FOR PRODUCING A HIGH-STRENGTH COLD-LAMINED SHEET STEEL PRODUCT |
KR101858863B1 (en) | 2016-12-23 | 2018-05-17 | 주식회사 포스코 | Hot dip aluminum alloy plated steel material having excellent corrosion resistance and workability |
CN106947907B (en) * | 2017-03-03 | 2018-12-07 | 北京科技大学 | A kind of preparation method of graphitized free-machining steel high-speed rod |
KR102021200B1 (en) | 2017-06-27 | 2019-09-11 | 현대제철 주식회사 | Hot stamping product and method of manufacturing the same |
JP7319570B2 (en) * | 2020-01-09 | 2023-08-02 | 日本製鉄株式会社 | hot stamped body |
EP4089194A4 (en) * | 2020-01-09 | 2023-07-26 | Nippon Steel Corporation | Hot stamp molded body |
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EP2719786A1 (en) * | 2011-06-10 | 2014-04-16 | Kabushiki Kaisha Kobe Seiko Sho | Hot press molded article, method for producing same, and thin steel sheet for hot press molding |
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JP2718332B2 (en) * | 1992-09-29 | 1998-02-25 | 住友金属工業株式会社 | Method for producing high carbon steel strip with good formability |
US20040074529A1 (en) * | 2002-10-21 | 2004-04-22 | Robert Levy | Self-contained and ventilated temporary shelter |
PT1790422E (en) * | 2004-09-15 | 2012-05-25 | Nippon Steel Corp | Process for producing a high-strength part |
JP2006104546A (en) * | 2004-10-08 | 2006-04-20 | Nippon Steel Corp | High strength automobile member and hot pressing method |
JP4771745B2 (en) * | 2004-10-13 | 2011-09-14 | 新日本製鐵株式会社 | Steel material for high strength constant velocity joint intermediate shaft and high strength constant velocity joint intermediate shaft |
JP4495064B2 (en) * | 2005-10-24 | 2010-06-30 | 新日本製鐵株式会社 | Steel sheet for hot press |
JP4630188B2 (en) * | 2005-12-19 | 2011-02-09 | 株式会社神戸製鋼所 | Steel sheet for hot forming and hot-formed product excellent in joint strength and hot formability of spot welds |
JP5302009B2 (en) * | 2005-12-26 | 2013-10-02 | ポスコ | High carbon steel sheet with excellent formability and method for producing the same |
JP5292698B2 (en) * | 2006-03-28 | 2013-09-18 | Jfeスチール株式会社 | Extremely soft high carbon hot-rolled steel sheet and method for producing the same |
JP5176954B2 (en) * | 2006-05-10 | 2013-04-03 | 新日鐵住金株式会社 | Steel sheet for hot pressed steel sheet member and method for producing hot pressed steel sheet |
KR101010971B1 (en) * | 2008-03-24 | 2011-01-26 | 주식회사 포스코 | Steel sheet for forming having low temperature heat treatment property, method for manufacturing the same, method for manufacturing parts using the same and parts manufactured by the method |
JP5418047B2 (en) * | 2008-09-10 | 2014-02-19 | Jfeスチール株式会社 | High strength steel plate and manufacturing method thereof |
JP4766186B2 (en) * | 2009-08-21 | 2011-09-07 | Jfeスチール株式会社 | Hot pressed member, steel plate for hot pressed member, method for manufacturing hot pressed member |
US9068255B2 (en) * | 2009-12-29 | 2015-06-30 | Posco | Zinc-plated steel sheet for hot pressing having outstanding surface characteristics, hot-pressed moulded parts obtained using the same, and a production method for the same |
JP5327106B2 (en) * | 2010-03-09 | 2013-10-30 | Jfeスチール株式会社 | Press member and manufacturing method thereof |
JP5024407B2 (en) * | 2010-03-24 | 2012-09-12 | Jfeスチール株式会社 | Manufacturing method of ultra-high strength member |
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2011
- 2011-07-15 JP JP2014520100A patent/JP2014520961A/en active Pending
- 2011-07-15 US US14/232,784 patent/US20140150930A1/en not_active Abandoned
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CN103687973B (en) | 2016-08-31 |
EP2733228A1 (en) | 2014-05-21 |
CN103687973A (en) | 2014-03-26 |
WO2013012103A1 (en) | 2013-01-24 |
EP2733228A4 (en) | 2015-08-12 |
US20140150930A1 (en) | 2014-06-05 |
JP2014520961A (en) | 2014-08-25 |
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