EP4534719A1 - Hot stamping part - Google Patents

Hot stamping part Download PDF

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Publication number
EP4534719A1
EP4534719A1 EP22945039.0A EP22945039A EP4534719A1 EP 4534719 A1 EP4534719 A1 EP 4534719A1 EP 22945039 A EP22945039 A EP 22945039A EP 4534719 A1 EP4534719 A1 EP 4534719A1
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EP
European Patent Office
Prior art keywords
hot stamping
amount
less
stamping part
steel plate
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.)
Pending
Application number
EP22945039.0A
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German (de)
French (fr)
Inventor
Byung Gil Yoo
Seok Hyeon Kang
Je Woo Soo KIM
Seong Kyung Han
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Hyundai Steel Co
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Hyundai Steel Co
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Publication date
Application filed by Hyundai Steel Co filed Critical Hyundai Steel Co
Publication of EP4534719A1 publication Critical patent/EP4534719A1/en
Pending legal-status Critical Current

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D37/00—Tools as parts of machines covered by this subclass
    • B21D37/01—Selection of materials
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D22/00—Shaping without cutting, by stamping, spinning, or deep-drawing
    • B21D22/02—Stamping using rigid devices or tools
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D22/00—Shaping without cutting, by stamping, spinning, or deep-drawing
    • B21D22/02—Stamping using rigid devices or tools
    • B21D22/022—Stamping using rigid devices or tools by heating the blank or stamping associated with heat treatment
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/62—Quenching devices
    • C21D1/673—Quenching devices for die quenching
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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
    • C21D7/00—Modifying the physical properties of iron or steel by deformation
    • C21D7/13—Modifying the physical properties of iron or steel by deformation by hot working
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • 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
    • C21D9/0068—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for particular articles not mentioned below
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—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
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/004—Dispersions; Precipitations
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite
    • 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
    • C21D2211/009—Pearlite

Definitions

  • Embodiment s of the present disclosure relate to a hot stamping part, and more particularly, to a hot stamping part of which a molded part after hot stamping exhibits excellent mechanical properties, such as high strength and high toughness.
  • high strength steel is applied to parts for vehicles. Meanwhile, high strength steel may obtain high strength properties compared with weight, but as strength increases, press formability deteriorates, causing a material to break during processing or causing a springback phenomenon and thus making it difficult to form products with complex and precise shapes.
  • Hot stamping is a method to improve these problems, and as interest in the method increases, research on materials for hot stamping is also actively being conducted.
  • the hot stamping method is a forming technique for manufacturing a high-strength part by heating a steel plate for hot stamping to a high temperature and then rapidly cooling the steel plate while forming the steel plate in a press mold.
  • boron steel 22MnB5 containing carbon (C), and manganese (Mn), boron (B), or the like, as an element for improving heat treatment performance, is used as a typical example of a steel plate for hot stamping.
  • embodiments of the present disclosure are directed to provide a hot stamping part of which a molded part after hot stamping exhibits excellent mechanical properties, such as high strength and high toughness.
  • a hot stamping part of which a molded part after hot stamping exhibits excellent mechanical properties such as high strength and high toughness.
  • a steel plate for hot stamping of which a molded part after hot stamping exhibits excellent mechanical properties, such as high strength, high toughness, and the like, and a manufacturing method thereof, may be implemented. According to an embodiment of the present disclosure, the scope of the disclosure is not limited by the above effects.
  • an average particle size of the precipitates may be 10 nm or less.
  • the average number of the precipitates per unit area may be 10,000/100 ⁇ m 2 to 35,000/100 ⁇ m 2 .
  • an average gap between the precipitates may be 0.1 nm to 100 nm.
  • the hot stamping part may satisfy a bending angle of 50° or more.
  • Silicon (Si) operates as a ferrite stabilizing element in a steel plate. Silicon, as a solid-solution strengthening element, increases strength of a steel plate, and restricts formation of low-temperature carbide so as to increase carbon thickening in austenite. Furthermore, silicon is a core element in hot-rolling, cold-rolling, and hot-press structure homogenization and ferrite fine dispersion. Silicon operates as a martensite strength inhomogeneous control element to improve crashworthiness. The silicon may be included in an amount of 0.02 to 2.0 wt% to the total weight of a steel plate.
  • the silicon content is less than 0.02 wt%, it may be difficult to obtain the effect described above, and cementite formation and coarseness may occur in martensite structure of a molded part after hot stamping.
  • the silicon content exceeds 2.0 wt%, load on hot rolling and cold rolling may increase, and the plating properties of a steel plate may deteriorate.
  • the pearlite region may be formed within the hot stamping steel plate with a minimum content.
  • Phosphorus (P) is an element that contributes to strength improvement.
  • the phosphorus may be included in an amount of greater than 0 to 0.03 wt% or less to the total weight of a steel plate, to prevent deterioration of toughness of a steel plate.
  • an iron phosphide compound is formed so that toughness and weldability deteriorate, and cracks may be generated in a steel plate during a manufacturing process.
  • S is an element that contributes to improvement of machinability.
  • the sulfur may be included in an amount of greater than 0 to 0.008 wt% or less to the total weight of a steel plate.
  • the sulfur content exceeds 0.008 wt%, hot machinability, weldability, and shock-resistant properties may deteriorate, and due to generation of large inclusions, surface defects, such as cracks and the like, may be generated.
  • Chromium (Cr) is added for the purpose of increasing hardenability and strength during heat treatment. Chromium enables grain refinement and strength securement through precipitation hardening.
  • the chromium may be included in an amount of 0.05 wt% to 0.9 wt% to the total weight of a steel plate. When the chromium content is less than 0.05 wt%, precipitation hardening effect may be reduced. In contrast, when the chromium content exceeds 0.9 wt%, Cr-based precipitates and matrix solid content increase so that toughness is reduced, and production costs may increase due to an increased cost.
  • Boron (B) which secures a martensite structure by restricting ferrite, pearlite, and bainite transformation, is added for the purpose of obtaining hardenability and strength during heat treatment. Furthermore, boron is segregated at grain boundaries and lowers grain boundary energy so as to increase hardenability, and has a grain refinement effect by increasing the austenite grain growth temperature.
  • the boron may be included in an amount of 0.0005 wt% to 0.01 wt% to the total weight of a steel plate. When the boron is included in the range described above, occurrence of hard phase grain boundary brittleness may be prevented, and high toughness and bendability may be secured. When the boron content is less than 0.0005 wt%, the hardenability effect is insufficient.
  • Calcium (Ca) may be added for control of precipitates. Calcium has a high bonding strength with sulfur so as to form CaS precipitates, which may suppress the generation of MnS that impedes weldability.
  • the calcium may be included in an amount of 0.00001 wt% to 0.006 wt% to the total weight of a steel plate. When the calcium content is less than 0.00001 wt%, a MnS control effect deteriorates. When the calcium content exceeds 0.006 wt%, continuous casting properties may deteriorate.
  • Titanium (Ti) may effectively contribute to grain refinement by forming precipitates at high temperature.
  • the titanium may be included in an amount of 0.001 wt% to 0.095 wt%, in particular 0.005 wt% to 0.06 wt%, to the total weight of a steel plate.
  • the titanium is included in the content range, continuous casting defects and precipitate coarseness may be prevented, the physical properties of structural steel may be easily secured, and defects, such as crack generation and the like, on a surface of structural steel may be prevented.
  • the titanium content falls below the lower limit, the effect may not be appropriately achieved.
  • the titanium content exceeds the upper limit precipitates become coarse so that an elongation rate and bendability may be dropped.
  • Niobium (Nb) and vanadium (V) may increase strength and toughness according to a decrease in the martensite packet size.
  • the niobium and vanadium may each be included in an amount of 0.005 wt% to 0.06 wt% to the total weight of a steel plate.
  • the niobium is included in the above range, in hot rolling and cold rolling processes, grain refinement effect of a steel plate is excellent, during steel making/continuous casting, occurrence of cracks in a slab and brittle fracture of products may be prevented, and generation of coarse precipitates in steel making may be minimized.
  • the niobium content is less than 0.005 wt%, the effect may not be appropriately achieved.
  • niobium content exceeds 0.06 wt%
  • strength and toughness do not improve further with increasing niobium content, and the niobium exists as a state employed in ferrite so that there is a risk that impact toughness may be rather reduced.
  • Vanadium may also have a tendency similar to the niobium described above.
  • the shape of a microstructure formed in a steel plate may be controlled.
  • the microstructure may include, for example, an area where a pearlite structure is locally accumulated (hereinafter, referred to as the pearlite region).
  • the area where a pearlite structure is accumulated affects a size of a grain and fraction coarseness after hot stamping, which may deteriorate hydrogen embrittlement and bending angle (e.g., V-bending angle) performance of a hot stamping part after hot stamping.
  • the microstructure of a steel plate before hot stamping may include ferrite and pearlite.
  • a steel plate may include ferrite: 50-99% and pearlite: 0.1-50% at an area fraction.
  • a steel plate may include other inevitable structures.
  • a steel plate may include other inevitable structures of 0% or more and less than 5%.
  • an average grain size of ferrite included in a steel plate before hot stamping may be controlled to satisfy a range of 2 ⁇ m or more and 10 ⁇ m or less.
  • Carbon (C) and/or manganese (Mn) may be segregated in the pearlite, and thus, the microstructure of a steel plate for hot stamping may include pearlite with relatively high carbon content and/or manganese content. Furthermore, the pearlite with relatively high carbon content and/or manganese content is locally accumulated in a steel plate so as to form a pearlite region.
  • the steel plate before hot stamping may be controlled such that the size, density, and area fraction of a pearlite region that a steel plate for hot stamping has satisfy preset conditions, while including carbon and manganese as much as the content optimized as described above. Accordingly, the mechanical properties, such as tensile strength, yield strength, bending properties, an elongation rate, and the like, of a molded part after hot stamping may be controlled.
  • the tensile strength of a hot stamping part formed by hot stamping a steel plate may satisfy a range of 1,700 MPa or more, in particular 1,760 MPa or more and 1,950 MPa or less. Furthermore, the yield strength of a hot stamping part may satisfy a range of 1,150 MPa or more, in particular 1,200 MPa or more and 1,350 MPa or less. Furthermore, a hot stamping part may satisfy a bending angle of 50° or more, and may have an elongation rate of 5% or more.
  • the "bending angle” may mean a V-bending angle in a rolling direction (a rolling direction, RD).
  • the degree of influence on the mechanical properties of a hot stamping part after hot stamping may vary depending on the content of carbon (C) and the content of manganese (Mn) included in the pearlite accumulated in a pearlite region in a steel plate before hot stamping.
  • C carbon
  • Mn manganese
  • what affects the mechanical properties of a hot stamping part is an area where pearlite including carbon of 0.27 wt% or more and manganese of 1.0 wt% or more is locally concentrated.
  • the effect of an area where pearlite having a carbon content of less than 0.27 wt% or a manganese content of less than 1.0 wt% is locally concentrated area on the mechanical properties of a hot stamping part is minimal.
  • a steel plate for hot stamping is controlled such that the size, density, and area fraction of an area where pearlite including carbon of 0.27 wt% or more and manganese of 1.0 wt% or more is locally concentrated satisfy preset conditions.
  • the steel plate for hot stamping may include a pearlite region in which pearlite including carbon (C) of 0.27 to 0.70 wt% and/or manganese (Mn) of 1.0 to 5.0 wt% locally accumulated.
  • the size, shape, and area fraction of the pearlite region may be controlled to satisfy preset conditions.
  • the average length of the pearlite region may be controlled to satisfy a range of 0.01 ⁇ m or more and 500 ⁇ m or less, in particular 0.1 ⁇ m or more and 100 ⁇ m or less.
  • the average thickness of the pearlite region may be controlled to satisfy a range of 0.01 ⁇ m or more and 30 ⁇ m or less.
  • an average gap between pearlite regions may be controlled to be 0.01 ⁇ m or more and 10 ⁇ m or less.
  • an area fraction of a pearlite region in a steel plate for hot stamping may be controlled to satisfy a range of 0.1% or more and 15% or less.
  • the pearlite region may contain an area fraction of 50% or more pearlite and 5% or less ferrite. Furthermore, selectively, a low temperature phase structure, such as precipitate, martensite, and/or bainite, and the like may be included up to 5%.
  • FIG. 2 is an image showing the structure of a hot stamping part formed by hot stamping the steel plate according to FIG. 1 .
  • the hot stamping part may include microstructure including austenite grains.
  • a steel plate may include an area fraction of 70% or more martensite phase, the austenite grains may be generally distributed in the martensite phase.
  • an average size of austenite grains in the hot stamping part may be about 15 ⁇ m or less, in particular 13 ⁇ m or less.
  • AGS austenite grain size
  • the austenite grain size may be controlled through an element that forms precipitate within a steel plate.
  • the steel plate may further include vanadium (V) other than niobium (Nb), titanium (Ti), and molybdenum (Mo).
  • specimen 1 and specimen 2 according to comparative examples are formed such that a fraction of an austenite grain size of about 20 ⁇ m or more exceeds 10%. Specimen 1 and specimen 2 according to comparative examples have fracture occurring during hydrogen embrittlement evaluation, and thus, it can be seen that specimen 1 and specimen 2 according to comparative examples fall short of the performance to be implemented by the present disclosure.
  • specimen 3 and specimen 4 according to the present embodiment are formed such that a fraction of an austenite grain size of about 20 ⁇ m or more is 10% or less. Specimen 3 and specimen 4 according to the present embodiment do not have fracture occurring during hydrogen embrittlement evaluation, and thus, it can be seen that the performance to be implemented by the present disclosure is satisfied.
  • the average size of austenite grains in the hot stamping part is about 15 ⁇ m or less, and furthermore, the specimen is formed such that a fraction of an austenite grain size of about 20 ⁇ m or more is 10% or less, hydrogen embrittlement and bending angle performance may be satisfied.
  • Specimen 3 and specimen 4 according to the present embodiment may be formed such that the average size of austenite grains in the hot stamping part is 13 ⁇ m or less.
  • the hot stamping part may include precipitates including at least one of at least one of niobium (Nb), titanium (Ti), molybdenum (Mo), and vanadium (V).
  • Niobium (Nb), titanium (Ti), molybdenum (Mo), and vanadium (V) included in a steel plate are carbide generating elements that contribute to the formation of precipitates. Titanium (Ti), niobium (Nb), and molybdenum (Mo) form carbon (C)-based precipitates, and thus, strength, hydrogen embrittlement, and bendability of a hot stamping part may be secured.
  • the elements may function as a hydrogen trap site effective for improving delayed fracture resistance.
  • the precipitates may be distributed within a steel plate to trap hydrogen.
  • the precipitates provide a trap site to hydrogen introduced into the interior of a steel plate before hot stamping, and thus, hydrogen delayed fracture properties of the hot stamping part may be improved.
  • the precipitation behavior of the precipitates may be measured by a method of analyzing a transmission electron microscopy (TEM) image.
  • TEM images for certain regions as many as a preset number may be obtained with respect to specimen, precipitates may be extracted from the obtained images through an image analysis program and the like, and the number of precipitates, an average distance between precipitates, diameters of precipitates, and the like may be measured for the extracted precipitates.
  • niobium (Nb), titanium (Ti), and vanadium (V) are each included exceeding 0.06 wt%, or molybdenum (Mo) is included to be less than 0.05 wt%, the average particle size of precipitates exceeds 10 nm so that the probability of hydrogen embrittlement occurring in hot stamping part may increase.
  • the component (that is, an average component) of precipitates may include 50 wt% or less of titanium (Ti) and 30 wt% or more of molybdenum (Mo).
  • the component of precipitates satisfies 50 wt% or less of titanium (Ti) and 30 wt% or more of molybdenum (Mo).
  • the component of precipitates includes titanium (Ti) exceeding 50 wt%, or molybdenum (Mo) to be less than 30 wt%, the coarseness of precipitates occurs, and thus, designed strength may not be secured.
  • a gap between adjacent precipitates may be controlled to satisfy a preset range.
  • the "average distance” may be measured through a mean free path of precipitates.
  • the average distance between precipitates may be calculated using a particle area fraction and the number of particles per unit length.
  • a method of measuring the precipitation behavior of the precipitates is not limited to the example described above, and various methods may be employed therefor.
  • the average distance between precipitates may be 0.1 nm or more 100 nm or less, in particular 0.1 nm or more and 50 nm or less, or 0.1 nm or more 10 nm or less.
  • the average distance between microprecipitates is less than 0.1 nm, formability to bendability may deteriorate.
  • the average distance between microprecipitates exceeds 100 nm, strength may deteriorate.
  • the average number of precipitates per unit area may be controlled to satisfy preset conditions.
  • refinement of precipitates may be carried out.
  • the average number of precipitates per unit area may be 10,000/100 ⁇ m 2 to 35,000/100 ⁇ m 2 .
  • coarseness of precipitates is carried out.
  • the average number of precipitates per unit area may be formed to be less than 10,000/100 ⁇ m 2 .
  • Table 1 shows the composition of a steel plate for hot stamping
  • Table 2 shows measurement values for evaluation of an austenite grain size and hydrogen embrittlement of specimens corresponding to a hot stamping part.
  • Comparative Example 1 to Comparative Example 12 are specimens that do not satisfy the value of [Inequality 1] among the composition of Table 1, and Embodiment 1 to Embodiment 8 are specimens corresponding to a hot stamping part formed by hot stamping a steel plate having such a composition as Table 1.
  • An evaluation for hydrogen embrittlement of each specimen employs the ASTM G39-99 reference (4-point bending test) test method.
  • a specimen is loaded in a 4-point bending tester, and a yield strength (YP) of stress 100% is applied thereto.
  • the specimen is dipped in 0.1 N HCl of an aqueous solution for 100 hours, and then, it is measured whether a crack, that is, fracture, has occurred in a surface of the specimen.
  • Comparative Example 13 to Comparative Example 16 are specimens that satisfy the composition of Table 1, but do not satisfy the conditions of precipitate and the like due to a difference in the process control conditions.
  • Comparative Example 13 to Comparative Example 16 a fraction of an austenite grain size of 10 ⁇ m or more is formed to exceed 67%, and a fraction of an austenite grain size of 20 ⁇ m or more is formed to exceed 10%.
  • fracture occurs in the hydrogen embrittlement evaluation so that the design conditions of the present disclosure are unsatisfied.

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Abstract

According to an aspect of the present disclosure, provided is a hot stamping part including a steel plate that includes carbon (C) in an amount of 0.26 to 0.40 wt%, silicon (Si) in an amount of 0.02 to 2.0 wt%, manganese (Mn) in an amount of 0.3 to 1.60 wt%, phosphorus (P) in an amount of 0.03 wt% or less, sulfur (S) in an amount of 0.008 wt% or less, chromium (Cr) in an amount of 0.05 to 0.90 wt%, boron (B) in an amount of 0.0005 to 0.01 wt%, molybdenum (Mo) in an amount of 0.05 to 0.2 wt%, titanium (Ti) in an amount of 0.001 to 0.095 wt%, niobium (Nb) in an amount of 0.001 to 0.095 wt%, vanadium (V) in an amount of 0.001 to 0.095 wt%, the balance of iron (Fe), and other inevitable impurities, the hot stamping part having tensile strength of 1,700 MPa or more and yield strength of 1,150 MPa or more, wherein the hot stamping part includes microstructure including austenite grains and carbon-based precipitates including at least one of niobium (Nb), titanium (Ti), molybdenum (Mo), and vanadium (V), and an average size of the austenite grains is 15 µm or less.

Description

    Technical Field
  • Embodiment s of the present disclosure relate to a hot stamping part, and more particularly, to a hot stamping part of which a molded part after hot stamping exhibits excellent mechanical properties, such as high strength and high toughness.
    • Background Art
  • For lightness and stability, high strength steel is applied to parts for vehicles. Meanwhile, high strength steel may obtain high strength properties compared with weight, but as strength increases, press formability deteriorates, causing a material to break during processing or causing a springback phenomenon and thus making it difficult to form products with complex and precise shapes.
  • Hot stamping is a method to improve these problems, and as interest in the method increases, research on materials for hot stamping is also actively being conducted. For example, as disclosed in Korean Patent Publication No. 10-2017-0076009 , the hot stamping method is a forming technique for manufacturing a high-strength part by heating a steel plate for hot stamping to a high temperature and then rapidly cooling the steel plate while forming the steel plate in a press mold.
  • Furthermore, as disclosed in Korean Patent Publication No. 10-2019-0095858 , so-called boron steel (22MnB5) containing carbon (C), and manganese (Mn), boron (B), or the like, as an element for improving heat treatment performance, is used as a typical example of a steel plate for hot stamping.
    • Disclosure of Invention Technical Problem
  • However, in the hot stamping part according to the related art, due to a difference in strength between regions generated by components and microstructure included in a steel plate for hot stamping, there is a problem that mechanical properties, such as tensile strength, bending properties, or the like, of a molded part after hot stamping deteriorate.
  • To solve various problems including the above problem, embodiments of the present disclosure are directed to provide a hot stamping part of which a molded part after hot stamping exhibits excellent mechanical properties, such as high strength and high toughness. However, such an objective is an example, and the scope of the present disclosure is not limited thereby.
    • Solution to Problem
  • According to an aspect of the disclosure, provided is a hot stamping part including a steel plate that includes carbon (C) in an amount of 0.26 to 0.40 wt%, silicon (Si) in an amount of 0.02 to 2.0 wt%, manganese (Mn) in an amount of 0.3 to 1.60 wt%, phosphorus (P) in an amount of 0.03 wt% or less, sulfur (S) in an amount of 0.008 wt% or less, chromium (Cr) in an amount of 0.05 to 0.90 wt%, boron (B) in an amount of 0.0005 to 0.01 wt%, molybdenum (Mo) in an amount of 0.05 to 0.2 wt%, titanium (Ti) in an amount of 0.001 to 0.095 wt%, niobium (Nb) in an amount of 0.001 to 0.095 wt%, vanadium (V) in an amount of 0.001 to 0.095 wt%, the balance of iron (Fe), and other inevitable impurities, the hot stamping part having tensile strength of 1,700 MPa or more and yield strength of 1,150 MPa or more, wherein the hot stamping part includes microstructure including austenite grains and carbon-based precipitates including at least one of niobium (Nb), titanium (Ti), molybdenum (Mo), and vanadium (V), and an average size of the austenite grains is 15 µm or less.
    • Advantageous Effects of Invention
  • According to an embodiment of the present disclosure configured as above, a hot stamping part of which a molded part after hot stamping exhibits excellent mechanical properties, such as high strength, high toughness, and the like, may be implemented.
  • In detail, by adjusting a difference in strength between regions of a steel plate for hot stamping through control of the properties of components, microstructure, and precipitates included in the steel plate for hot stamping, a steel plate for hot stamping of which a molded part after hot stamping exhibits excellent mechanical properties, such as high strength, high toughness, and the like, and a manufacturing method thereof, may be implemented. According to an embodiment of the present disclosure, the scope of the disclosure is not limited by the above effects.
    • Brief Description of Drawings
    • FIG. 1 is an image showing a portion of microstructure of a steel plate before hot stamping, according to an embodiment of the present disclosure.
    • FIG. 2 is an image showing the structure of a hot stamping part formed by hot stamping the steel plate according to FIG. 1.
    • FIGS. 3 and 4 are graphs showing the measurements of austenite grain sizes and fractions of a hot stamping part, according to embodiments of the present disclosure and comparative examples.
    Best Mode for Carrying out the Invention
  • According to the present embodiment, a fraction of the austenite grains having a size of 10 µm or more may be 67% or less.
  • According to the present embodiment, a fraction of the austenite grains having a size of 20 µm or more may be 10% or less.
  • According to the present embodiment, when contents of titanium (Ti), niobium (Nb), vanadium (V), and molybdenum (Mo) included in the steel plate may be represented by [Ti], [Nb], [V], and [Mo] in wt%, respectively, [Inequality 1] below may be satisfied, 0.015 ≤ 0.25 Ti + Nb + V + 0.25 Mo ≤ 0.060 unit : wt % .
    Figure imgb0001
  • According to the present embodiment, the amount of activated hydrogen of the hot stamping part may be 0.6 wppm or less.
  • According to the present embodiment, an average particle size of the precipitates may be 10 nm or less.
  • According to the present embodiment, the precipitates may include 50 wt% or less of titanium (Ti) and 30 wt% or more of molybdenum (Mo).
  • According to the present embodiment, the average number of the precipitates per unit area may be 10,000/100 µm2 to 35,000/100 µm2.
  • According to the present embodiment, an average gap between the precipitates may be 0.1 nm to 100 nm.
  • According to the present embodiment, the hot stamping part may satisfy a bending angle of 50° or more.
  • Other aspects, features, and advantages than those described above will become apparent from the following drawings, claims, and detailed description of the disclosure
    • Mode for the Invention
  • Various modifications may be applied to the present embodiments, and particular embodiments will be illustrated in the drawings and described in the detailed description section. The effect and features of the present embodiments, and a method to achieve the same, will be clearer referring to the detailed descriptions below with the drawings. However, the present embodiments may be implemented in various forms, not by being limited to the embodiments presented below.
  • In the following embodiment, it will be understood that although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These elements are only used to distinguish one element from another.
  • In the following embodiment, as used herein, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.
  • In the following embodiment, it will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated components or elements, but do not preclude the presence or addition of one or more other components or elements thereof.
  • Sizes of elements in the drawings may be exaggerated for convenience of explanation. For example, since sizes and thicknesses of elements in the drawings are arbitrarily illustrated for convenience of explanation, the following embodiments are not limited thereto.
  • When a certain embodiment may be implemented differently, a specific process order may be performed differently from the described order. For example, two consecutively described processes may be performed substantially at the same time or performed in an order opposite to the described order.
  • In the specification, the expression such as "A and/or B" may include A, B, or A and B. Also, the expression such as "at least one of A and B" may include A, B, or A and B.
  • Hereinafter, embodiments will be described in detail with reference to the accompanying drawings, and in the description with reference to the drawings, the same or corresponding constituents are indicated by the same reference numerals and redundant descriptions thereof are omitted.
  • FIG. 1 is an image showing a portion of microstructure of a steel plate before hot stamping, according to an embodiment of the present disclosure.
  • In detail, FIG. 1 illustrates a steel plate for hot stamping which is controlled and manufactured such that the content of a material forming a steel plate for hot stamping, a configuration of microstructure of a steel plate for hot stamping, and a process condition for manufacturing a steel plate for hot stamping satisfy preset conditions.
  • The steel plate according to the present embodiment may be a steel plate manufactured by performing a hot rolling process and/or a cold rolling process on a slab cast to contain a predetermined content of a certain alloy element.
  • The steel plate may include carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), chromium (Cr), boron (B), calcium (Ca), molybdenum (Mo), titanium (Ti), niobium (Nb), vanadium (V), the balance of iron (Fe), and other inevitable impurities. In an embodiment, the steel plate for hot stamping may include carbon (C) in an amount of 0.26 to 0.40 wt%, silicon (Si) in an amount of 0.02 to 2.0 wt%, manganese (Mn) in an amount of 0.3 to 1.60 wt%, phosphorus (P) in an amount of 0.03 wt% or less, sulfur (S) in an amount of 0.008 wt% or less, chromium (Cr) in an amount of 0.05 to 0.90 wt%, boron (B) in an amount of 0.0005 to 0.01 wt%, molybdenum (Mo) in an amount of 0.05 to 0.2 wt%, titanium (Ti) in an amount of 0.001 to 0.095 wt%, niobium (Nb) in an amount of 0.001 to 0.095 wt%, vanadium (V) in an amount of 0.001 to 0.095 wt%, the balance of iron (Fe), and other inevitable impurities. Furthermore, selectively, the steel plate for hot stamping may further include calcium (Ca) in an amount of 0.00001 to 0.0060 wt%.
  • Carbon (C) operates as an austenite stabilizing element in a steel plate. Carbon is a major element in determining strength and hardness of a steel plate, and may be added for the purpose of increasing hardenability and strength during heat treatment. The carbon may be included in an amount of 0.26 to 0.40 wt% to the total weight of a steel plate. When the carbon content is less than 0.26 wt%, it is difficult to obtain a hard phase (e.g., martensite, etc.) so that it may be difficult to satisfy the mechanical strength of a molded part after hot stamping. In contrast, the carbon content, which exceeds 0.40 wt%, may cause deterioration of machinability of a steel plate or deterioration of bending performance of a molded part after hot stamping.
  • Silicon (Si) operates as a ferrite stabilizing element in a steel plate. Silicon, as a solid-solution strengthening element, increases strength of a steel plate, and restricts formation of low-temperature carbide so as to increase carbon thickening in austenite. Furthermore, silicon is a core element in hot-rolling, cold-rolling, and hot-press structure homogenization and ferrite fine dispersion. Silicon operates as a martensite strength inhomogeneous control element to improve crashworthiness. The silicon may be included in an amount of 0.02 to 2.0 wt% to the total weight of a steel plate. When the silicon content is less than 0.02 wt%, it may be difficult to obtain the effect described above, and cementite formation and coarseness may occur in martensite structure of a molded part after hot stamping. In contrast, when the silicon content exceeds 2.0 wt%, load on hot rolling and cold rolling may increase, and the plating properties of a steel plate may deteriorate.
  • Meanwhile, when silicon (Si) is added appropriately in an amount of 0.3 wt% or more, by suppressing excessive formation of a pearlite region where pearlite is accumulated in the hot stamping steel plate, the pearlite region may be formed within the hot stamping steel plate with a minimum content.
  • Manganese (Mn) may operate as an austenite stabilizing element in a steel plate. Manganese is added for the purpose of increasing hardenability and strength during heat treatment. The manganese may be included in an amount of 0.3 to 1.60 wt% to the total weight of a steel plate. When the manganese content is less than 0.3 wt%, a hardenability effect is insufficient so that a hard phase fraction in a molded part after hot stamping may be insufficient due to insufficient hardenability. In contrast, when the manganese content exceeds 1.60 wt%, an area of concentrated manganese-segregated pearlite occurs so that ductility and toughness may deteriorate, which cause deterioration of the bending performance of a molded part after hot stamping and generation of inhomogeneous microstructure.
  • Phosphorus (P) is an element that contributes to strength improvement. The phosphorus may be included in an amount of greater than 0 to 0.03 wt% or less to the total weight of a steel plate, to prevent deterioration of toughness of a steel plate. When the phosphorus content exceeds 0.03 wt%, an iron phosphide compound is formed so that toughness and weldability deteriorate, and cracks may be generated in a steel plate during a manufacturing process.
  • Sulfur (S) is an element that contributes to improvement of machinability. The sulfur may be included in an amount of greater than 0 to 0.008 wt% or less to the total weight of a steel plate. When the sulfur content exceeds 0.008 wt%, hot machinability, weldability, and shock-resistant properties may deteriorate, and due to generation of large inclusions, surface defects, such as cracks and the like, may be generated.
  • Chromium (Cr) is added for the purpose of increasing hardenability and strength during heat treatment. Chromium enables grain refinement and strength securement through precipitation hardening. The chromium may be included in an amount of 0.05 wt% to 0.9 wt% to the total weight of a steel plate. When the chromium content is less than 0.05 wt%, precipitation hardening effect may be reduced. In contrast, when the chromium content exceeds 0.9 wt%, Cr-based precipitates and matrix solid content increase so that toughness is reduced, and production costs may increase due to an increased cost.
  • Boron (B), which secures a martensite structure by restricting ferrite, pearlite, and bainite transformation, is added for the purpose of obtaining hardenability and strength during heat treatment. Furthermore, boron is segregated at grain boundaries and lowers grain boundary energy so as to increase hardenability, and has a grain refinement effect by increasing the austenite grain growth temperature. The boron may be included in an amount of 0.0005 wt% to 0.01 wt% to the total weight of a steel plate. When the boron is included in the range described above, occurrence of hard phase grain boundary brittleness may be prevented, and high toughness and bendability may be secured. When the boron content is less than 0.0005 wt%, the hardenability effect is insufficient. In contrast, when the boron content exceeds 0.01 wt%, solubility is low so that the boron is easily precipitated at grain boundaries according to heat treatment conditions, which causes deterioration of hardenability or high temperature embrittlement, and as hard phase grain boundary brittleness occurs, toughness and bendability may deteriorate.
  • Calcium (Ca) may be added for control of precipitates. Calcium has a high bonding strength with sulfur so as to form CaS precipitates, which may suppress the generation of MnS that impedes weldability. The calcium may be included in an amount of 0.00001 wt% to 0.006 wt% to the total weight of a steel plate. When the calcium content is less than 0.00001 wt%, a MnS control effect deteriorates. When the calcium content exceeds 0.006 wt%, continuous casting properties may deteriorate.
  • Titanium (Ti) may effectively contribute to grain refinement by forming precipitates at high temperature. The titanium may be included in an amount of 0.001 wt% to 0.095 wt%, in particular 0.005 wt% to 0.06 wt%, to the total weight of a steel plate. When the titanium is included in the content range, continuous casting defects and precipitate coarseness may be prevented, the physical properties of structural steel may be easily secured, and defects, such as crack generation and the like, on a surface of structural steel may be prevented. When the titanium content falls below the lower limit, the effect may not be appropriately achieved. In contrast, when the titanium content exceeds the upper limit, precipitates become coarse so that an elongation rate and bendability may be dropped.
  • Niobium (Nb) and vanadium (V) may increase strength and toughness according to a decrease in the martensite packet size. The niobium and vanadium may each be included in an amount of 0.005 wt% to 0.06 wt% to the total weight of a steel plate. When the niobium is included in the above range, in hot rolling and cold rolling processes, grain refinement effect of a steel plate is excellent, during steel making/continuous casting, occurrence of cracks in a slab and brittle fracture of products may be prevented, and generation of coarse precipitates in steel making may be minimized. When the niobium content is less than 0.005 wt%, the effect may not be appropriately achieved. In contrast, when the niobium content exceeds 0.06 wt%, strength and toughness do not improve further with increasing niobium content, and the niobium exists as a state employed in ferrite so that there is a risk that impact toughness may be rather reduced. Vanadium may also have a tendency similar to the niobium described above.
  • Molybdenum (Mo), as a substitution element, improves strength of steel with a solid strengthening effect. The molybdenum may be added for the purpose of reducing precipitate coarseness and increasing hardenability. Furthermore, the molybdenum (Mo) may serve to increase hardenability of steel. The molybdenum may be included in an amount of 0.05 wt% to 0.2 wt% to the total weight of a steel plate. When the molybdenum content is less than 0.05 wt%, the effect may be appropriately achieved. In contrast, when the molybdenum content exceeds 0.2 wt%, there is a risk of lowering of rolling productivity and an elongation rate, and only the manufacturing costs are raised without an additional effect.
  • The titanium (Ti), niobium (Nb), vanadium (V), and molybdenum (Mo) described above may be used as elements to control the formation of precipitates in a molded part after hot stamping. In an embodiment, a steel plate may appropriately include titanium (Ti), niobium (Nb), and vanadium (V) each in an amount of 0.005 to 0.06 wt%. Accordingly, when titanium (Ti), niobium (Nb), and vanadium (V) are each included in an amount of 0.005 to 0.06 wt%, and molybdenum (Mo) is included in an amount of 0.05 to 0.2 wt%, a microstructure area in a steel plate before hot stamping may be easily controlled, and furthermore, conditions that are easy to control precipitates in a hot stamping part after hot stamping may be satisfied.
  • In an embodiment, when the contents of titanium (Ti), niobium (Nb), vanadium (V), and molybdenum (Mo) included in a steel plate are represented by [Ti], [Nb], [V], and [Mo] in wt%, respectively, the following Inequality 1 may be satisfied. 0.015 ≤ 0.25 Ti + Nb + V + 0.25 Mo ≤ 0.060 unit : wt %
    Figure imgb0002
  • Accordingly, the shape of a microstructure formed in a steel plate may be controlled. The microstructure may include, for example, an area where a pearlite structure is locally accumulated (hereinafter, referred to as the pearlite region). The area where a pearlite structure is accumulated affects a size of a grain and fraction coarseness after hot stamping, which may deteriorate hydrogen embrittlement and bending angle (e.g., V-bending angle) performance of a hot stamping part after hot stamping. Accordingly, in a steel plate before hot stamping, when the contents of titanium (Ti), niobium (Nb), vanadium (V), and molybdenum (Mo) satisfy the [Inequality 1], the grain refinement of a hot stamping part after hot stamping may be easily controlled, and thus, hydrogen embrittlement and bending angle performance may be secured.
  • The microstructure of a steel plate before hot stamping may include ferrite and pearlite. In an embodiment, a steel plate may include ferrite: 50-99% and pearlite: 0.1-50% at an area fraction. Furthermore, a steel plate may include other inevitable structures. For example, a steel plate may include other inevitable structures of 0% or more and less than 5%. Meanwhile, in an embodiment, an average grain size of ferrite included in a steel plate before hot stamping may be controlled to satisfy a range of 2 µm or more and 10 µm or less.
  • Carbon (C) and/or manganese (Mn) may be segregated in the pearlite, and thus, the microstructure of a steel plate for hot stamping may include pearlite with relatively high carbon content and/or manganese content. Furthermore, the pearlite with relatively high carbon content and/or manganese content is locally accumulated in a steel plate so as to form a pearlite region.
  • The steel plate before hot stamping according to an embodiment of the present disclosure may be controlled such that the size, density, and area fraction of a pearlite region that a steel plate for hot stamping has satisfy preset conditions, while including carbon and manganese as much as the content optimized as described above. Accordingly, the mechanical properties, such as tensile strength, yield strength, bending properties, an elongation rate, and the like, of a molded part after hot stamping may be controlled.
  • The tensile strength of a hot stamping part formed by hot stamping a steel plate may satisfy a range of 1,700 MPa or more, in particular 1,760 MPa or more and 1,950 MPa or less. Furthermore, the yield strength of a hot stamping part may satisfy a range of 1,150 MPa or more, in particular 1,200 MPa or more and 1,350 MPa or less. Furthermore, a hot stamping part may satisfy a bending angle of 50° or more, and may have an elongation rate of 5% or more. Here, the "bending angle" may mean a V-bending angle in a rolling direction (a rolling direction, RD).
  • The degree of influence on the mechanical properties of a hot stamping part after hot stamping may vary depending on the content of carbon (C) and the content of manganese (Mn) included in the pearlite accumulated in a pearlite region in a steel plate before hot stamping. In detail, what affects the mechanical properties of a hot stamping part is an area where pearlite including carbon of 0.27 wt% or more and manganese of 1.0 wt% or more is locally concentrated. In contrast, the effect of an area where pearlite having a carbon content of less than 0.27 wt% or a manganese content of less than 1.0 wt% is locally concentrated area on the mechanical properties of a hot stamping part is minimal. Accordingly, a steel plate for hot stamping according to an embodiment of the present disclosure is controlled such that the size, density, and area fraction of an area where pearlite including carbon of 0.27 wt% or more and manganese of 1.0 wt% or more is locally concentrated satisfy preset conditions.
  • The steel plate for hot stamping according to an embodiment of the present disclosure may include a pearlite region in which pearlite including carbon (C) of 0.27 to 0.70 wt% and/or manganese (Mn) of 1.0 to 5.0 wt% locally accumulated. The size, shape, and area fraction of the pearlite region may be controlled to satisfy preset conditions.
  • In an embodiment, the average length of the pearlite region may be controlled to satisfy a range of 0.01 µm or more and 500 µm or less, in particular 0.1 µm or more and 100 µm or less. Furthermore, the average thickness of the pearlite region may be controlled to satisfy a range of 0.01 µm or more and 30 µm or less. Furthermore, an average gap between pearlite regions may be controlled to be 0.01 µm or more and 10 µm or less.
  • In an embodiment, an area fraction of a pearlite region in a steel plate for hot stamping may be controlled to satisfy a range of 0.1% or more and 15% or less.
  • The pearlite region may contain an area fraction of 50% or more pearlite and 5% or less ferrite. Furthermore, selectively, a low temperature phase structure, such as precipitate, martensite, and/or bainite, and the like may be included up to 5%.
  • FIG. 2 is an image showing the structure of a hot stamping part formed by hot stamping the steel plate according to FIG. 1.
  • The hot stamping part may include martensite, bainite, ferrite, and/or austenite. A ratio of microstructure and an average grain size of the microstructure of the hot stamping part may be controlled to satisfy preset conditions. Accordingly, the mechanical properties of the hot stamping part, such as tensile strength, yield strength, bending properties, an elongation rate, and the like, may be controlled. For example, the tensile strength of the hot stamping part may satisfy a range of 1,700 MPa or more, in particular 1,760 MPa or more and 1,950 MPa or less. Furthermore, the yield strength of the hot stamping part may satisfy a range of 1,150 MPa or more, in particular 1,200 MPa or more and 1,350 MPa or less. Furthermore, the hot stamping part may satisfy a bending angle of 50° or more, and may have an elongation rate of 5% or more.
  • In an embodiment, the microstructure of the hot stamping part may include 70% or more martensite, 30% or less bainite and ferrite, 5% or less residual carbide, and retained austenite.
  • In an embodiment, the microstructure included in the hot stamping part may be refined. In detail, the average grain size of the microstructure included in the hot stamping part may be controlled to satisfy a range of 15 µm or less, in particular 2 µm or more and 15 µm or less.
  • Referring to FIG. 2, the hot stamping part according to an embodiment of the present disclosure may include microstructure including austenite grains. A steel plate may include an area fraction of 70% or more martensite phase, the austenite grains may be generally distributed in the martensite phase.
  • In an embodiment, an average size of austenite grains in the hot stamping part may be about 15 µm or less, in particular 13 µm or less. When the average size of austenite grains exceeds 15 µm, fracture may occur during hydrogen embrittlement evaluation. By controlling an austenite grain size (AGS) to be below a certain level in the hot stamping part, sensitivity to hydrogen embrittlement may be reduced. The austenite grain size may be controlled through an element that forms precipitate within a steel plate. As an example, for a steel plate including niobium (Nb), titanium (Ti), and molybdenum (Mo), the refinement of austenite grains may be easily implemented within a hot stamping part after hot stamping. Furthermore, the steel plate may further include vanadium (V) other than niobium (Nb), titanium (Ti), and molybdenum (Mo).
  • FIGS. 3 and 4 are graphs showing the measurements of austenite grain sizes and fractions of a hot stamping part, according to embodiments of the present disclosure and comparative examples.
  • In detail, FIG. 3 is a graph of measuring a fraction of an austenite grain size of about 10 µm or more of the hot stamping part according to the present embodiment, and FIG. 4 is a graph of measuring a fraction of an austenite grain size of about 20 µm or more of the hot stamping part according to the present embodiment. Specimen ① and specimen ② are according to comparative examples, and specimen ③ and specimen ④ are according to embodiments of the present disclosure. Specimen ① and specimen ② according to comparative examples have fracture occurring during hydrogen embrittlement evaluation, whereas specimen ③ and specimen ④ according to the present embodiment do not have fracture occurring during hydrogen embrittlement evaluation, which satisfies the performance to be implemented by the present disclosure.
  • In an embodiment, a fraction of an austenite grain size of about 10 µm or more of the hot stamping part may be 67% or less, in particular 65% or less.
  • As illustrated in FIG. 3, specimen ① and specimen ② according to comparative examples are formed such that a fraction of an austenite grain size of about 10 µm or more exceeds 67%. Specimen ① and specimen ② according to comparative examples have fracture occurring during hydrogen embrittlement evaluation, and thus, it can be seen that specimen ① and specimen ② according to comparative examples fall short of the performance to be implemented by the present disclosure. In contrast, specimen ③ and specimen ④ according to the present embodiment are formed such that a fraction of an austenite grain size of about 10 µm or more is 67% or less. Specimen ③ and specimen ④ according to the present embodiment do not have fracture occurring during hydrogen embrittlement evaluation, and thus, it can be seen that the performance to be implemented by the present disclosure is satisfied. In other words, when the average size of austenite grains in the hot stamping part is about 15 µm or less, and furthermore, the specimen is formed such that a fraction of an austenite grain size of about 10 µm or more is 67% or less, hydrogen embrittlement and bending angle performance may be satisfied.
  • In an embodiment, a fraction of an austenite grain size of about 20 µm or more of the hot stamping part may be 10% or less, in particular 7% or less.
  • As illustrated in FIG. 4, specimen ① and specimen ② according to comparative examples are formed such that a fraction of an austenite grain size of about 20 µm or more exceeds 10%. Specimen ① and specimen ② according to comparative examples have fracture occurring during hydrogen embrittlement evaluation, and thus, it can be seen that specimen ① and specimen ② according to comparative examples fall short of the performance to be implemented by the present disclosure. In contrast, specimen ③ and specimen ④ according to the present embodiment are formed such that a fraction of an austenite grain size of about 20 µm or more is 10% or less. Specimen ③ and specimen ④ according to the present embodiment do not have fracture occurring during hydrogen embrittlement evaluation, and thus, it can be seen that the performance to be implemented by the present disclosure is satisfied. In other words, when the average size of austenite grains in the hot stamping part is about 15 µm or less, and furthermore, the specimen is formed such that a fraction of an austenite grain size of about 20 µm or more is 10% or less, hydrogen embrittlement and bending angle performance may be satisfied. Specimen ③ and specimen ④ according to the present embodiment may be formed such that the average size of austenite grains in the hot stamping part is 13 µm or less.
  • Meanwhile, the hot stamping part according to an embodiment of the present disclosure may include precipitates including at least one of at least one of niobium (Nb), titanium (Ti), molybdenum (Mo), and vanadium (V). Niobium (Nb), titanium (Ti), molybdenum (Mo), and vanadium (V) included in a steel plate are carbide generating elements that contribute to the formation of precipitates. Titanium (Ti), niobium (Nb), and molybdenum (Mo) form carbon (C)-based precipitates, and thus, strength, hydrogen embrittlement, and bendability of a hot stamping part may be secured. The elements may function as a hydrogen trap site effective for improving delayed fracture resistance. In other words, the precipitates may be distributed within a steel plate to trap hydrogen. In other words, the precipitates provide a trap site to hydrogen introduced into the interior of a steel plate before hot stamping, and thus, hydrogen delayed fracture properties of the hot stamping part may be improved.
  • As described above, niobium (Nb), titanium (Ti), and vanadium (V) may each be included in an amount of 0.005 to 0.06 wt%, and molybdenum (Mo) may be included in an amount of 0.05 to 0.2 wt%. In particular, niobium (Nb), titanium (Ti), molybdenum (Mo), and vanadium (V) may satisfy the [Inequality 1] described above. In this case, as described above, the average size of austenite grains in the hot stamping part is about 15 µm or less, a fraction of an austenite grain size of about 10 µm or more of the hot stamping part is 67% or less, and a fraction of an austenite grain size of about 20 µm or more of the hot stamping part may be formed to be 10% or less. Accordingly, hydrogen embrittlement and bending angle performance of the hot stamping part may be satisfied.
  • The precipitation behavior of the precipitates may be measured by a method of analyzing a transmission electron microscopy (TEM) image. In detail, TEM images for certain regions as many as a preset number may be obtained with respect to specimen, precipitates may be extracted from the obtained images through an image analysis program and the like, and the number of precipitates, an average distance between precipitates, diameters of precipitates, and the like may be measured for the extracted precipitates.
  • Furthermore, during measurement of the diameters of precipitates, considering non-uniformity in the form of precipitates, the shapes of precipitates are converted into circles to calculate the diameters of the precipitates. In detail, the area of the extracted precipitate may be measured using a unit pixel having a specific area, and the precipitate may be converted into a circle having the same area as the measured area so as to calculate the diameter of the precipitate.
  • In an embodiment, the average particle size (size, diameter) of precipitates may be controlled to satisfy preset conditions. In detail, the average particle size of precipitates formed in a hot stamping part may be 10 nm or less, in particular 1 nm or more and 6 nm or less. Furthermore, the amount of activated hydrogen of a hot stamping part including the precipitates may be 0.6 wppm or less.
  • As a comparative example, when niobium (Nb), titanium (Ti), and vanadium (V) are each included exceeding 0.06 wt%, or molybdenum (Mo) is included to be less than 0.05 wt%, the average particle size of precipitates exceeds 10 nm so that the probability of hydrogen embrittlement occurring in hot stamping part may increase.
  • In an embodiment, the component (that is, an average component) of precipitates may include 50 wt% or less of titanium (Ti) and 30 wt% or more of molybdenum (Mo). In order for the precipitates to satisfy the average particle size described above, the component of precipitates satisfies 50 wt% or less of titanium (Ti) and 30 wt% or more of molybdenum (Mo). As a comparative example, when the component of precipitates includes titanium (Ti) exceeding 50 wt%, or molybdenum (Mo) to be less than 30 wt%, the coarseness of precipitates occurs, and thus, designed strength may not be secured.
  • In an embodiment, a gap between adjacent precipitates, that is, an average distance, may be controlled to satisfy a preset range. Here, the "average distance" may be measured through a mean free path of precipitates. In detail, the average distance between precipitates may be calculated using a particle area fraction and the number of particles per unit length. However, a method of measuring the precipitation behavior of the precipitates is not limited to the example described above, and various methods may be employed therefor.
  • In detail, the average distance between precipitates may be 0.1 nm or more 100 nm or less, in particular 0.1 nm or more and 50 nm or less, or 0.1 nm or more 10 nm or less. When the average distance between microprecipitates is less than 0.1 nm, formability to bendability may deteriorate. In contrast, when the average distance between microprecipitates exceeds 100 nm, strength may deteriorate.
  • In an embodiment, the average number of precipitates per unit area may be controlled to satisfy preset conditions. In detail, when the [Inequality 1] described above is satisfied, refinement of precipitates may be carried out. In this case, the average number of precipitates per unit area may be 10,000/100 µm2 to 35,000/100 µm2. As a comparative example, when the [Inequality 1] described above is unsatisfied, coarseness of precipitates is carried out. In this case, the average number of precipitates per unit area may be formed to be less than 10,000/100 µm2.
  • In the following description, the present disclosure is described in detail through embodiments and comparative examples. However, the embodiments and comparative examples described below are to describe the present disclosure in more detail, and the scope of the present disclosure is not limited by the embodiments and comparative examples described below. The embodiments and comparative examples described below may be appropriately corrected and modified by a person skilled in the art within the scope of the present disclosure. [Table 1]
    C Si Mn P S Cr B Ca [Inequality 1] 0.25(Ti+Nb+V+ 0.25Mo)
    0.26-0.40 0.02-2.00 0.30-1.60 0.03 or less 0.008 or less 0.05-0.90 0.0005-0.01 0.00001 -0.0060 0.015-0.060
    [Table 2]
    Specimen Austenite Grain Size (AGS) (µm) After H/S Hot Stamping Part Hydrogen Embrittlement Evaluation
    Comparative Example 1 85 52 Fractured
    Comparative Example 2 83 53 Fractured
    Comparative Example 3 84 53 Fractured
    Comparative Example 4 83 52 Fractured
    Comparative Example 5 85 52 Fractured
    Comparative Example 6 84 52 Fractured
    Comparative Example 7 84 53 Fractured
    Comparative Example 8 85 57 Fractured
    Comparative Example 9 90 61 Fractured
    Comparative Example 10 89 62 Fractured
    Comparative Example 11 85 57 Fractured
    Comparative Example 12 84 58 Fractured
    Embodiment 1 65 9 Non-fractured
    Embodiment
    2 64 9 Non-fractured
    Embodiment
    3 66 10 Non-fractured
    Embodiment
    4 63 6 Non-fractured
    Embodiment
    5 65 8 Non-fractured
    Embodiment 6 64 8 Non-fractured
    Embodiment 7 64 7 Non-fractured
    Embodiment 8 65 8 Non-fractured
    Comparative Example 13 72 18 Fractured
    Comparative Example 14 70 15 Fractured
    Comparative Example 15 70 16 Fractured
    Comparative Example 16 71 17 Fractured
  • Table 1 shows the composition of a steel plate for hot stamping, and Table 2 shows measurement values for evaluation of an austenite grain size and hydrogen embrittlement of specimens corresponding to a hot stamping part. Comparative Example 1 to Comparative Example 12 are specimens that do not satisfy the value of [Inequality 1] among the composition of Table 1, and Embodiment 1 to Embodiment 8 are specimens corresponding to a hot stamping part formed by hot stamping a steel plate having such a composition as Table 1.
  • An evaluation for hydrogen embrittlement of each specimen employs the ASTM G39-99 reference (4-point bending test) test method. In detail, a specimen is loaded in a 4-point bending tester, and a yield strength (YP) of stress 100% is applied thereto. Next, the specimen is dipped in 0.1 N HCl of an aqueous solution for 100 hours, and then, it is measured whether a crack, that is, fracture, has occurred in a surface of the specimen.
  • Referring to Table 2, Comparative Example 1 to Comparative Example 12 are specimens of a hot stamping part manufactured from a steel plate having a value of [Inequality 1], that is, 0.25(Ti+Nb+V+0.25Mo), of the composition of Table 1, that is 0.013 wt%, which is out of a range of 0.015 to 0.060 wt%. Niobium (Nb), titanium (Ti), molybdenum (Mo), and vanadium (V) are carbide generating elements that contribute to the formation of precipitates, and Comparative Example 1 to Comparative Example 12, which do not satisfy precipitate control conditions, show a result of coarseness of the austenite grain size in a hot stamping part. Accordingly, it may be seen that, in Comparative Example 1 to Comparative Example 12, a fraction of an austenite grain size of 10 µm or more is formed to exceed 67% that is a reference value of the present disclosure. Furthermore, it may be seen that, in Comparative Example 1 to Comparative Example 12, a fraction of an austenite grain size of 20 µm or more is formed to be about 50% or more exceeding, by far, 10% that is a reference value of the present disclosure. As a result, it may be seen that, in Comparative Example 1 to Comparative Example 12, fracture occurs in the hydrogen embrittlement evaluation, and thus, the design conditions of the present disclosure are not satisfied.
  • Meanwhile, it may be seen that, in Embodiment 1 to Embodiment 8, which are embodiments of the present disclosure, while satisfying the composition of Table 1, a fraction of an austenite grain size of 10 µm or more is formed to be 67% or less, and a fraction of an austenite grain size of 20 µm or more is formed to be 10% or less. As a result, it may be seen that, in Embodiment 1 to Embodiment 8, no fracture occurs in the hydrogen embrittlement evaluation so that the design conditions of the present disclosure are satisfied.
  • In contrast, Comparative Example 13 to Comparative Example 16 are specimens that satisfy the composition of Table 1, but do not satisfy the conditions of precipitate and the like due to a difference in the process control conditions. In Comparative Example 13 to Comparative Example 16, a fraction of an austenite grain size of 10 µm or more is formed to exceed 67%, and a fraction of an austenite grain size of 20 µm or more is formed to exceed 10%. As a result, it may be seen that, in Comparative Example 13 to Comparative Example 16, fracture occurs in the hydrogen embrittlement evaluation so that the design conditions of the present disclosure are unsatisfied.
  • In the above, although embodiments have been described, these are merely examples, and those skilled in the art to which the present disclosure pertains could make various modifications and changes from these descriptions. Therefore, various changes in form and details may be made therein without departing from the spirit and scope of the disclosure as defined by the following claims.

Claims (10)

  1. A hot stamping part comprising a steel plate that includes carbon (C) in an amount of 0.26 to 0.40 wt%, silicon (Si) in an amount of 0.02 to 2.0 wt%, manganese (Mn) in an amount of 0.3 to 1.60 wt%, phosphorus (P) in an amount of 0.03 wt% or less, sulfur (S) in an amount of 0.008 wt% or less, chromium (Cr) in an amount of 0.05 to 0.90 wt%, boron (B) in an amount of 0.0005 to 0.01 wt%, molybdenum (Mo) in an amount of 0.05 to 0.2 wt%, titanium (Ti) in an amount of 0.001 to 0.095 wt%, niobium (Nb) in an amount of 0.001 to 0.095 wt%, vanadium (V) in an amount of 0.001 to 0.095 wt%, the balance of iron (Fe), and other inevitable impurities, the hot stamping part having tensile strength of 1,700 MPa or more and yield strength of 1,150 MPa or more,
    wherein the hot stamping part comprises microstructure including austenite grains and carbon-based precipitates including at least one of niobium (Nb), titanium (Ti), molybdenum (Mo), and vanadium (V), and
    an average size of the austenite grains is 15 µm or less.
  2. The hot stamping part of claim 1, wherein
    a fraction of the austenite grains having a size of 10 µm or more is 67% or less.
  3. The hot stamping part of claim 1, wherein
    a fraction of the austenite grains having a size of 20 µm or more is 10% or less.
  4. The hot stamping part of claim 1, wherein,
    when contents of titanium (Ti), niobium (Nb), vanadium (V), and molybdenum (Mo) included in the steel plate are represented by [Ti], [Nb], [V], and [Mo] in wt%, respectively, [Inequality 1] below is satisfied, 0.015 ≤ 0.25 Ti + Nb + V + 0.25 Mo ≤ 0.060 unit : wt % .
    Figure imgb0003
  5. The hot stamping part of claim 1, wherein
    an amount of activated hydrogen of the hot stamping part is 0.6 wppm or less.
  6. The hot stamping part of claim 1, wherein
    an average particle size of the precipitates is 10 nm or less.
  7. The hot stamping part of claim 1, wherein
    the precipitates include 50 wt% or less of titanium (Ti) and 30 wt% or more of molybdenum (Mo).
  8. The hot stamping part of claim 1, wherein
    an average number of the precipitates per unit area is 10,000/100 µm2 to 35,000/100 µm2.
  9. The hot stamping part of claim 1, wherein
    an average gap between the precipitates is 0.1 nm to 100 nm.
  10. The hot stamping part of claim 1,
    satisfying a bending angle of 50° or more.
EP22945039.0A 2022-05-31 2022-12-01 Hot stamping part Pending EP4534719A1 (en)

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