US20160326608A1 - Hot-formed member and method of manufacturing same - Google Patents

Hot-formed member and method of manufacturing same Download PDF

Info

Publication number
US20160326608A1
US20160326608A1 US15/109,582 US201415109582A US2016326608A1 US 20160326608 A1 US20160326608 A1 US 20160326608A1 US 201415109582 A US201415109582 A US 201415109582A US 2016326608 A1 US2016326608 A1 US 2016326608A1
Authority
US
United States
Prior art keywords
hot
steel sheet
formed member
ferrite
temperature
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.)
Abandoned
Application number
US15/109,582
Other languages
English (en)
Inventor
Koutarou Hayashi
Toshinobu Nishibata
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.)
Nippon Steel Corp
Original Assignee
Nippon Steel and Sumitomo Metal Corp
Priority date (The priority date 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 date listed.)
Filing date
Publication date
Application filed by Nippon Steel and Sumitomo Metal Corp filed Critical Nippon Steel and Sumitomo Metal Corp
Assigned to NIPPON STEEL & SUMITOMO METAL CORPORATION reassignment NIPPON STEEL & SUMITOMO METAL CORPORATION ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: HAYASHI, KOUTAROU, NISHIBATA, TOSHINOBU
Publication of US20160326608A1 publication Critical patent/US20160326608A1/en
Assigned to NIPPON STEEL CORPORATION reassignment NIPPON STEEL CORPORATION CHANGE OF NAME (SEE DOCUMENT FOR DETAILS). Assignors: NIPPON STEEL & SUMITOMO METAL CORPORATION
Abandoned legal-status Critical Current

Links

Images

Classifications

    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • 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
    • B21D37/16—Heating or cooling
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
    • 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—Final recrystallisation annealing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • 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
    • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • 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
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/26—After-treatment
    • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/26—After-treatment
    • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
    • C23C2/29—Cooling or quenching
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/62—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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a hot-formed member for use in machine structural components such as structural components for the body of an automobile, and a method of manufacturing the hot-formed member. Specifically, the invention relates to a hot-formed member which has excellent ductility and impact resistance while having a tensile strength of 900 MPa or more, and a method of manufacturing the hot-formed member.
  • a member having a complicated shape can be formed with high shape accuracy from a high-strength steel sheet in a method called hot pressing for press-forming a heated steel sheet.
  • hot pressing for press-forming a heated steel sheet.
  • the hot pressing method by heating the steel sheet in an austenite single phase region before the pressing, and rapidly cooling (hardened) the steel sheet in a die after the pressing, it is possible to achieve an increase in the strength of the member due to martensitic transformation.
  • the hot pressing method is an excellent forming method capable of simultaneously securing an increase in the strength of a member and the formability of a steel sheet.
  • Patent Document 2 discloses a pre-press quenching method in which a steel sheet is previously formed into a predetermined shape at room temperature, and a member obtained is heated to an austenite region and rapidly cooled in a die to achieve an increase in the strength of the member.
  • the pre-press quenching method which is an aspect of hot pressing; the deformation of the member due to distortion by heating can be suppressed by restraining the member with the die.
  • the pre-press quenching method is an excellent forming method capable of increasing the strength of the member and of obtaining high shape accuracy.
  • Patent Documents 1 and 2 in which the metallographic microstructure is substantially a martensite single phase, it is not possible to meet such a demand.
  • Patent Document 3 discloses a member which is regarded to be excellent in ductility and have a dual phase microstructure including ferrite and martensite by heating a steel sheet having the C content limited to 0.1% or less in an austenite single phase region and by performing hot pressing. In this manner, when the steel sheet is heated in an austenite single phase region, the member has a uniform metallographic microstructure.
  • the C content is limited to 0.1% or less in the member described in Patent Document 3, and thus the tensile strength of the member is at most 700 MPa. Therefore, the member does not have a sufficient strength for contributing to a reduction in the weight of an automobile.
  • Patent Document 4 discloses a member which has a tensile strength of 980 MPa or more and excellent ductility and which has a dual phase microstructure, specifically, a two phase microstructure of ferrite and martensite by heating a steel sheet having a large amount of Cr added thereto to an austenite single phase region, to transform a part of the austenite into ferrite before or after pressing.
  • carbide such as cementite and M 23 C 6 formed in the steel is poorly solid-soluble during heating, and thus it is necessary to perform heating for a long period of time to secure stable mechanical properties.
  • the method described therein is a method which significantly impairs productivity in addition to an increase in the cost for the manufacturing of the hot-formed member, and is not suitable as a mass production technology.
  • Patent Documents 5 to 7 disclose members which has a high strength and excellent ductility with a two phase microstructure of ferrite and martensite and in which each of the ferrite and the martensite has an average grain size of 7 ⁇ m or less by heating a cold-rolled steel sheet having an average grain size (average grain size of ferrite, or in the case in which a second phase is further included, average grain size of ferrite and the second phase) of 15 ⁇ m or less to form a two phase microstructure of ferrite and austenite, pressing the cold rolled steel sheet while maintaining the microstructure, and rapidly cooling the cold rolled steel sheet in a die.
  • an average grain size average grain size of ferrite, or in the case in which a second phase is further included, average grain size of ferrite and the second phase
  • Patent Document 1 British Patent Publication No. 1490535
  • Patent Document 2 Japanese Unexamined Patent Application, First Publication No. H10-96031
  • Patent Document 3 Published Japanese Translation No. 2010-521584 of the PCT International Publication
  • Patent Document 4 Japanese Unexamined Patent Application, First Publication No. 2010-131672
  • Patent Document 5 Japanese Unexamined Patent Application, First Publication No. 2010-65293
  • Patent Document 6 Japanese Unexamined Patent Application, First Publication No. 2010-65292
  • Patent Document 7 Japanese Unexamined Patent Application, First Publication No. 2010-65295
  • the metallographic microstructure of the steel sheet subjected to hot pressing has an influence on the metallographic microstructure of the hot-formed member.
  • making a fine metallographic microstructure is an important microstructure control method contributing to an improvement in the ductility of the member.
  • the inventors have newly found that the impact resistance of the member can be improved by refining and uniformizing the microstructure of the steel sheet to be subjected to hot forming.
  • the inventors have further found that in order to refine and uniformize the microstructure of the steel sheet to be subjected to hot forming, it is required that the hot-rolled steel sheet is cold-rolled and is annealed at a recrystallization temperature.
  • the annealing temperature is controlled to be near an Ac 1 temperature in order to refine the microstructure of the steel sheet to be subjected to hot pressing.
  • a large amount of unrecrystallized ferrite remains in the steel sheet subjected to hot pressing under such manufacturing conditions. Since the unrecrystallized ferrite is not recrystallized even when heated in a two phase temperature range in which ferrite and austenite coexist, the microstructure after hot pressing is extremely non-uniform.
  • the methods disclosed in Patent Documents 5 to 7 also assume that the steel sheet contains a large amount of Ti.
  • Patent Documents 5 to 7 Since Ti acts to prevent the ferrite from being recrystallized, there is a concern that a large amount of unrecrystallized ferrite may remain when a large amount of Ti is contained.
  • the technologies disclosed in Patent Documents 5 to 7 do not suggest any ideas for controlling the unrecrystallized ferrite. Accordingly, in the methods disclosed in Patent Documents 5 to 7, although the microstructure of the hot-formed member can be made fine and the ductility thereof can be improved, the impact resistance thereof is significantly insufficient.
  • a specific object of the present invention is to provide a hot-formed member having excellent ductility and impact resistance and a tensile strength of 900 MPa or more, that could not be mass-produced in the past as described above, and a method of manufacturing the hot-formed member.
  • the inventors have conducted an intensive study to improve the ductility and the impact resistance of a hot-formed member having a tensile strength of 900 MPa or more, and as a result, obtained novel knowledge that the ductility and the impact resistance of a hot-formed member are improved by (1) setting the Ti content in the hot-formed member to a limited range and (2) making the metallographic microstructure of the hot-formed member as a fine and uniform metallographic microstructure consisting of ferrite and martensite.
  • a hot-formed member having such a metallographic microstructure is achieved by using a steel sheet having the above-described chemical composition and having a fine and uniform metallographic microstructure as a steel sheet to be subjected to hot forming, and by adjusting the heat treatment conditions in the hot forming.
  • the present invention is contrived based on such knowledge, and the gist thereof is as follows.
  • a hot-formed member according to an aspect of the present invention has a chemical composition containing, by mass %: 0.10% to 0.40% of C; 0% to 2.0% of Si; 1.0% to 3.0% of Mn; 0.05% or less of P; 0.01% or less of S; 0.001% to 1.0% of sol.
  • the hot-formed member has a metallographic microstructure which has, in terms of area %, 10% to 90% of a ferrite, 0% to 2.0% of an unrecrystallized ferrite, 10% to 90% of a martensite is, in which a total area ratio of the ferrite and the martensite is 90% to 100%, and in which an average grain size of the ferrite is 0.5 ⁇ m to 5.0 ⁇ m, and a tensile strength is
  • the chemical composition may contain, by mass %, one or two or more selected from the group consisting of 0.003% to 0.4% of Nb, 0.003% to 0.4% of V, 0.005% to 1.0% of Cr, 0.005% to 1.0% of Mo, 0.005% to 1.0% of Cu, and 0.005% to 1.0% of Ni
  • the chemical composition may contain, by mass %, one or two or more selected from the group consisting of 0.0003% to 0.01% of Ca, 0.0003% to 0.01% of Mg, 0.0003% to 0.01% of REM, and 0.0003% to 0.01% of Zr.
  • the chemical composition may contain, by mass %, 0.0003% to 0.01% of B.
  • the chemical composition may contain, by mass %, 0.0003% to 0.01% of Bi.
  • a method of manufacturing a hot-formed member according to another aspect of the present invention includes: heating a base steel sheet having the same chemical composition as that of the hot-formed member according to any one of (1) to (5) and a metallographic microstructure in which the amount of an unrecrystallized ferrite is 0 area % to 2.0 area % and an average grain size of a ferrite is 0.5 am to 7.0 am to a temperature range of 720° C. to lower than an Ac 3 temperature; then maintaining a temperature of the base steel sheet for 1 minute to 20 minutes in the temperature range of 720° C. to lower than the Ac 3 temperature; then hot-forming the base steel sheet; and then cooling the base steel sheet under the conditions in which an average cooling rate is 20° C./sec to 500° C./sec in a temperature range of 600° C. to 150° C.
  • a method of manufacturing a hot-formed member includes: heating a base steel sheet having the same chemical composition as that of the hot-formed member according to any one of (1) to (5) and a metallographic microstructure in which an unrecrystallized ferrite is more than 2.0 area % and an average grain size of a ferrite is 0.5 ⁇ m to 7.0 ⁇ m to a temperature range of an Ac 3 temperature to the Ac 3 temperature+100° C.; then maintaining a temperature of the base steel sheet for 30 seconds or longer and shorter than 20 minutes in the temperature range of the Ac 3 temperature to the Ac 3 temperature+100° C.; then hot-forming the base steel sheet; and then cooling the base steel sheet under the conditions in which an average cooling rate is 3° C./sec to 20° C./sec in a temperature range of the Ac 3 temperature to 600° C.
  • the base steel sheet may be one selected from the group consisting of a cold-rolled steel sheet, a hot-dip galvanized steel sheet, and a galvannealed steel sheet.
  • FIG. 1 is a flowchart illustrating a manufacturing method according to the present invention.
  • hot-formed member and a method of manufacturing the hot-formed member according to an embodiment of the present invention, which have been achieved based on the above-described knowledge, will be described.
  • hot forming hot pressing that is a specific aspect thereof will be used as an example for description.
  • forming methods other than hot pressing for example, roll forming and the like may be employed as a hot forming method.
  • the tensile strength of the hot-formed member according to this embodiment is 900 MPa to 1800 MPa. In order to reduce the weights of machine structural components of automobiles and the like, the tensile strength of the material thereof is required to be set to 900 MPa or more.
  • the tensile strength in order to prevent brittle fracture of a steel sheet from occurring, it is necessary to set the tensile strength to 1800 MPa or less. In this embodiment, such a tensile strength is achieved by appropriately controlling the amount of various alloy elements such as C and the manufacturing method.
  • the chemical composition of a hot-formed member according to an embodiment of the present invention is specified as described above.
  • the symbol “%” indicating the amount of each alloy element means “mass %” unless specifically noted. Since the chemical composition of steel does not change even when the steel is subjected to hot forming, the amount of the respective elements in a base steel sheet before hot forming is the same as that in a hot-formed member after hot forming.
  • the C content is a very important element which increases hardenability of steel and has the strongest influence on the strength after hardening.
  • the C content is set to 0.10% or more.
  • the C content is preferably 0.11% or more.
  • the C content is set to 0.40% or less.
  • the C content is preferably set to 0.28% or less from the viewpoint of weldability.
  • the hot-formed member according to this embodiment does not essentially contain Si. Accordingly, the lower limit of the Si content is 0%.
  • Si is an element which acts to increase the strength after hardening while not deteriorating or improving ductility.
  • the Si content may be set to 0.001% or more.
  • the Si content is set to 0.05% or more, ductility is further improved. Accordingly, the Si content is preferably set to 0.05% or more.
  • the Si content is more than 2.0%, the effects obtained by the above-described actions are saturated and economic disadvantages are caused. Moreover, surface properties significantly deteriorate. Accordingly, the Si content is set to 2.0% or less, and is preferably 1.5% or less.
  • Mn is a very effective element for increasing hardenability of steel and stably securing the strength after hardening.
  • the Mn content is set to 1.0% or more.
  • the Mn content is 1.6% or more, a tensile strength of 980 MPa or more can be secured after hardening. Therefore, the Mn content is preferably set to 1.6% or more.
  • the Mn content is more than 3.0%, the hot-formed member has a non-uniform metallographic microstructure, and impact resistance significantly deteriorates.
  • the Mn content is set to 3.0% or less.
  • productivity in a later hot forming process is improved.
  • the Mn content is preferably set to 2.4% or less.
  • P is an impurity inevitably contained in steel.
  • P acts to increase the strength of the hot-formed member by solid-solution strengthening
  • P may be actively contained.
  • the P content is set to 0.05% or less.
  • the P content is preferably set to 0.02% or less.
  • the P content is preferably set to 0.003% or more.
  • the lower limit of the P content is 0%.
  • the S content is an impurity contained in steel.
  • the S content is preferably as small as possible in order to improve weldability.
  • the S content is set to 0.01% or less.
  • the S content is preferably set to 0.003% or less, and more preferably 0.0015% or less. Since the S content is preferably as small as possible, it is not necessary to specify the lower limit of the S content. That is, the lower limit of the S content is 0%.
  • sol. Al indicates solid solution Al existing in a solid solution state in steel.
  • Al is an element acting to deoxidize steel, and is an element which prevents a carbonitride forming element such as Ti from being oxidized to promote the formation of carbonitride. Due to these actions, generation of surface defects on the steel can be suppressed, and the yield of the steel can be improved.
  • the sol. Al content is less than 0.001%, it is difficult to obtain the above-described actions. Accordingly, the sol. Al content is set to 0.001% or more. In order to more securely obtain the above-described actions, the sol. Al content is preferably set to 0.015% or more.
  • the sol. Al content is more than 1.0%, weldability of the hot-formed member significantly decreases, and the amount of oxide-based inclusions increases in the hot-formed member, whereby surface properties of the hot-formed member significantly deteriorate. Accordingly, the sol. Al content is set to 1.0% or less. In order to more securely avoid the above-described phenomenon, the sol. Al content is preferably 0.080% or less.
  • Ti is an important element in this embodiment. Since fine precipitates that are Ti carbide, Ti nitride, and/or Ti carbonitride are formed in the hot-formed member when Ti is contained, the metallographic microstructure after hardening can be made fine, and whereby the ductility of the hot-formed member is significantly improved. When the Ti content is less than 0.050%, the metallographic microstructure after hardening is not made fine, and thus the ductility cannot be improved. Accordingly, the Ti content is set to 0.050% or more. The Ti content is preferably 0.070% or more. When the Ti content is more than 0.30%, coarse carbonitride is formed during casting and during hot rolling, and thus the impact resistance of the hot-formed member significantly deteriorates. Accordingly, the Ti content is set to 0.30% or less. The Ti content is preferably 0.25% or less, and more preferably 0.20% or less.
  • the N is an impurity contained in steel.
  • the N content is preferably as small as possible.
  • the N content is set to 0.01% or less.
  • the N content is preferably 0.006% or less. Since the N content is preferably as small as possible, it is not necessary to specify the lower limit of the N content. That is, the lower limit of the N content is 0%.
  • the chemical composition of the hot-formed member according to this embodiment includes the balance of Fe and impurities.
  • the impurities mean components that are incorporated from raw materials such as ore or scrap, or due to various factors in the manufacturing process in the industrial manufacturing of steel, and are permitted within a range in which this embodiment is not affected.
  • the hot-formed member according to this embodiment may further contain, as arbitrary components, elements described below. Since necessary characteristics for solving the problems can be obtained even when the arbitrary elements to be described below are not contained in the hot-formed member, it is not necessary to limit the lower limit of the amount of the arbitrary elements. That is, the lower limit of the amount of each arbitrary element is 0%.
  • All these elements are effective elements for increasing hardenability of steel and stably securing the strength of the hot-formed member after hardening. Accordingly, one or two or more of these elements may be contained in the hot-formed member.
  • Nb and V are contained in an amount of more than 0.4%, respectively, it is difficult to perform hot rolling and cold rolling in the manufacturing process.
  • Nb and V are contained in an amount of more than 0.4%, respectively, the hot-formed member after hardening is likely to have a non-uniform microstructure, and thus the impact resistance of the hot-formed member significantly deteriorates.
  • All these elements are elements contributing to control of inclusions, particularly, fine dispersion of inclusions, and acting to increase low-temperature toughness of the hot-formed member. Accordingly, one or two or more of these elements may be contained. However, when any of these elements is contained in an amount of more than 0.01%, surface properties of the hot-formed member may deteriorate. Accordingly, in a case in which the elements are contained, the amount of each element is set as described above. In order to more securely obtain the effects caused by the above-described actions, the amount of each element to be added is preferably set to 0.0003% or more.
  • the term “REM” indicates total 17 elements including Sc, Y, and lanthanoid, and the “REM content” means the total amount of these 17 elements.
  • the REM is industrially added in the form of misch metal.
  • B is an element acting to increase low-temperature toughness of the hot-formed member. Accordingly, B may be contained in the hot-formed member. However, when B is contained in an amount of more than 0.01%, the hot workability of a base steel sheet deteriorates, and it becomes difficult to perform hot rolling. Accordingly, in a case in which B is contained in the hot-formed member, the B content is set to 0.01% or less. In order to more securely obtain the effects caused by the above-described actions, the B content is preferably set to 0.0003% or more.
  • Bi is an element uniformizing the metallographic microstructure of the hot-formed member and acting to increase impact resistance of the hot-formed member. Accordingly, Bi may be contained in the hot-formed member. However, when Bi is contained in an amount of more than 0.01%, hot workability of a base steel sheet deteriorates, and it becomes difficult to perform hot rolling. Accordingly, in the case in which Bi is contained in the hot-formed member, the Bi content is set to 0.01% or less. In order to more securely obtain the effects caused by the above-described actions, the Bi content is preferably set to 0.0003% or more.
  • the configuration of the metallographic microstructure to be described below is the configuration at approximately 1 ⁇ 2t to 1 ⁇ 4t of a sheet thickness, and not in a central segregation portion.
  • the central segregation portion may have a different metallographic microstructure from a representative metallographic microstructure of steel.
  • the central segregation portion is a very small region with respect to the entire sheet thickness, and has little influence on the mechanical properties of the steel. That is, the metallographic microstructure of the central segregation portion does not represent the metallographic microstructure of the steel.
  • the metallographic microstructure of the hot-formed member according to this embodiment is specified at approximately 1 ⁇ 2t to 1 ⁇ 4t of the sheet thickness and not in the central segregation portion.
  • the “position at 1 ⁇ 2t” indicates a position at a depth of 1 ⁇ 2 of a thickness t of the member from a surface of the hot-formed member
  • the “position at 1 ⁇ 4t” indicates a position at a depth of 1 ⁇ 4 of the thickness t of the member from the surface of the hot-formed member.
  • unrecrystallized ferrite ferrite plastically deformed by rolling, elongated in a rolling direction, and then remaining without being recrystallized.
  • ferrite ferrite other than the unrecrystallized ferrite
  • normal ferrite ferrite other than the unrecrystallized ferrite
  • the normal ferrite includes recrystallized ferrite generated by recrystallization and transformed ferrite generated by phase transformation and the like.
  • the unrecrystallized ferrite In grains of the unrecrystallized ferrite, crystal orientation continuously changes due to the plastic deformation by rolling.
  • the crystal orientation in grains of the normal ferrite is almost uniform, and crystal orientations between the normal ferrite grains adjacent to each other are different from each other. Due to such a difference, the unrecrystallized ferrite has higher hardness than the normal ferrite. Since the unrecrystallized ferrite has a shape elongated in the rolling direction, the unrecrystallized ferrite and the normal ferrite can be discriminated from each other by observing the metallographic microstructures with a microscope.
  • the unrecrystallized ferrite and the normal ferrite are different in the state of the crystal orientation, the unrecrystallized ferrite and the normal ferrite can be discriminated from each other by analyzing crystal orientation measurement data of an electron back scattering pattern (EBSP) of the metallographic microstructure through a kernel average misorientation method (KAM method).
  • EBSP electron back scattering pattern
  • KAM method kernel average misorientation method
  • ferrite with an aspect ratio of 4 or more is the unrecrystallized ferrite
  • ferrite with an aspect ratio of less than 4 is normal ferrite.
  • the area ratio of the ferrite is set to 10% or more.
  • the area ratio of the ferrite is more than 90%, the area ratio of the martensite is less than 10%, and as will be described later, it is difficult to secure a tensile strength of 900 MPa or more after hardening. Accordingly, the area ratio of the ferrite is set to 90% or less.
  • the ratio between the ferrite and the martensite is not particularly limited as long as the area ratio of the ferrite is within the above-described range. However, the ferrite is preferably 25% to 85%, and the martensite is preferably 15% to 75%.
  • the area ratio of the unrecrystallized ferrite is set to 2.0% or less (including 0%).
  • the strength of the hot-formed member after hardening can be increased by forming the martensite in the metallographic microstructure of the hot-formed member.
  • the area ratio of the martensite is set to 10% or more.
  • the area ratio of the martensite is more than 90%, the area ratio of the ferrite (recrystallized ferrite) is less than 10%, and as described above, the ductility cannot be improved. Accordingly, the area ratio of the martensite is set to 90% or less.
  • the hot-formed member according to this embodiment has a metallographic microstructure mainly including ferrite and martensite.
  • a metallographic microstructure mainly including ferrite and martensite.
  • one or two or more of bainite, residual austenite, cementite, and pearlite may be incorporated in the metallographic microstructure as a phase or microstructure other than the ferrite and the martensite.
  • the area ratio of the phase or metallographic microstructure other than the ferrite and the martensite is more than 10%, target mechanical properties may not be obtained due to the influence of these phases or metallographic microstructures. Accordingly, the area ratio of the phase or microstructure other than the ferrite and the martensite is less than 10%.
  • the total area ratio of the ferrite and the martensite is set to 90% or more. Since it is not necessary to specify the upper limit of the total area ratio of the ferrite and the martensite, the upper limit of the total area ratio of the ferrite and the martensite is 100%.
  • test pieces are prepared from the hot-formed member in a direction in which a base steel sheet that is a raw material of the hot-formed member is rolled and in a direction perpendicular to the rolling direction.
  • metallographic microstructures of a cross-section in the rolling direction and a cross-section perpendicular to the rolling direction in the test piece are photographed by an electron microscope.
  • Electron micrographs of regions of 800 ⁇ m ⁇ 800 ⁇ m (800-square- ⁇ m regions) obtained as described above are analyzed to calculate area ratios of unrecrystallized ferrite, ferrite, and martensite.
  • ferrite grains and martensite grains can be easily discriminated from surrounding microstructures.
  • ferrite grains can be discriminated from unrecrystallized ferrite grains by calculating aspect ratios of grains from shapes of the grains, by recognizing ferrite grains having an aspect ratio of 4 or more as unrecrystallized ferrite grains, and by recognizing ferrite grains having an aspect ratio of less than 4 as ferrite grains.
  • the average grain size of the ferrite is set to 5.0 ⁇ m or less in order to secure good ductility and impact resistance while maintaining a tensile strength to 900 MPa or more. Since the average grain size of the ferrite is preferably as small as possible, it is not necessary to specify the lower limit of the average grain size of the ferrite. However, the substantial lower limit of the average grain size of the ferrite is about 0.5 ⁇ m in consideration of a manufacturing equipment capacity.
  • the hot-formed member represents a member hot-formed from a base steel sheet, and includes, for example, a steel member formed by hot pressing.
  • Representative hot-formed members include components for the body structure of an automobile such as a door guard bar and a bumper reinforcement, and hot-formed steel pipes for building construction.
  • the configuration of the metallographic microstructure to be described below is the configuration at about 1 ⁇ 2t to 1 ⁇ 4t of a sheet thickness, and not in a central segregation portion.
  • the central segregation portion may have a different metallographic microstructure from a representative metallographic microstructure of steel.
  • the central segregation portion is a very small region with respect to the entire sheet thickness, and has little influence on the mechanical properties of the steel. That is, the metallographic microstructure of the central segregation portion does not represent the metallographic microstructure of the steel.
  • the metallographic microstructure of the hot-formed member according to this embodiment is specified at about 1 ⁇ 2t to 1 ⁇ 4t of the sheet thickness and not in the central segregation portion.
  • a hot-formed member after hardening is required to have a metallographic microstructure (final metallographic microstructure) in which, in terms of area %, 10% to 90% of ferrite, 0% to 2.0% of unrecrystallized ferrite, and 10% to 90% of martensite are included, the total area ratio of the ferrite and the martensite is 90% or more, and the average grain size of the ferrite is 5.0 ⁇ m or less.
  • the metallographic microstructure of a base steel sheet (also called “start steel sheet”) before being subjected to hot press forming is previously adjusted to a predetermined state, and hot pressing is performed under predetermined hot press forming conditions.
  • a steel sheet having the same chemical composition as that of the above-described hot-formed member and having a metallographic microstructure in which unrecrystallized ferrite is 0 area % to 2.0 area % and the average grain size of the ferrite is 0.5 ⁇ m to 7.0 ⁇ m is prepared as a base steel sheet.
  • a base steel sheet in which the amount of unrecrystallized ferrite is 2.0 area % or less is obtained by, for example, performing a recrystallization annealing treatment on a steel sheet after cold rolling for a sufficient period of time.
  • a cold-rolled steel sheet, a hot-dip galvanized cold-rolled steel sheet, and a galvannealed steel sheet having a metallographic microstructure in which unrecrystallized ferrite is 2.0 area % and the average grain size of ferrite is 0.5 ⁇ m to 7.0 ⁇ m can be manufactured by, for example, annealing a cold-rolled steel sheet in a temperature range of (Ac 3 temperature—20° C.) or higher.
  • the steel sheet for hot pressing prepared in this manner which is a base steel sheet having the same chemical composition as that of the above-described hot-formed member and having a metallographic microstructure in which unrecrystallized ferrite is 2.0 area % or less and the average grain size of the ferrite is 0.5 ⁇ m to 7.0 ⁇ m, is subjected to hot press forming in accordance with the following conditions. Since the area ratio of the unrecrystallized ferrite of the base steel sheet is limited to 2.0 area % or less, the metallographic microstructure of the hot-formed member does not become a non-uniform microstructure.
  • the metallographic microstructure of the base steel sheet is a fine microstructure, the ductility and impact resistance of the hot-formed member can be significantly improved by the manufacturing method according to this embodiment.
  • the unrecrystallized ferrite is preferably as small as possible, and thus the lower limit of the unrecrystallized ferrite is substantially 0%.
  • the area ratio of each metallographic microstructure of the above-described base steel sheet can be obtained through the same method as the method of obtaining an area ratio of each metallographic microstructure of the hot-formed member.
  • the base steel sheet prepared as described above is heated to a temperature range of 720° C. to lower than the Ac 3 temperature in a heating process.
  • the temperature of this base steel sheet is maintained for 1 minute to 20 minutes in a temperature range of 720° C. to lower than the Ac 3 temperature in a holding process, and then the steel sheet is hot-pressed in a hot forming process.
  • the steel sheet is cooled under the conditions in which the average cooling rate is 20° C./sec to 500° C./sec in a temperature range of 600° C. to 150° C.
  • the base steel sheet can be processed for a short period of time without being heated in an austenite single phase region.
  • a cold-rolled steel sheet or a hot-dip galvanized cold-rolled steel sheet having the same chemical composition as that of the hot-formed steel sheet and having a metallographic microstructure in which unrecrystallized ferrite is 2.0 area % or less and the average grain size of the ferrite is 0.5 ⁇ m to 7.0 ⁇ m can be used.
  • the chemical composition of the base steel sheet is specified as described above, and particularly, C, Mn, and Ti are specified within specific ranges, respectively. Accordingly, the above-described base steel sheet can be easily obtained by sufficiently performing recrystallization annealing under normal conditions.
  • the cold-rolled steel sheet and the hot-dip galvanized cold-rolled steel sheet having the above-described metallographic microstructure can be manufactured by, for example, annealing in a temperature range of (Ac 3 temperature—20° C.) or higher.
  • Heating Temperature of Base Steel Sheet Temperature Range of 720° C. to Lower than Ac 3 Temperature
  • the base steel sheet In a process of heating the base steel sheet in the hot forming process, the base steel sheet is heated to a temperature range of 720° C. to lower than the Ac 3 temperature (° C.). In a process of holding the base steel sheet, the temperature of the base steel sheet is maintained for 1 minute to 20 minutes in the above-described temperature range, that is, in a temperature range of 720° C. to lower than the Ac 3 temperature.
  • the Ac 3 temperature is a temperature specified by the following Formula (i) obtained by an experiment, and in the case in which steel is heated in a temperature range of the Ac 3 temperature or higher, the metallographic microstructure of the steel is an austenite single phase.
  • the chemical symbol in the above formula indicates the amount (unit: mass %) of each element in the chemical composition of the steel sheet.
  • “sol. Al” indicates a concentration (unit: mass %) of solid solution Al.
  • the metallographic microstructure of the base steel sheet is a microstructure close to a ferrite single phase, and it is difficult to secure a tensile strength of 900 MPa or more after hardening. Accordingly, the heating temperature and the holding temperature are set to 720° C. or higher.
  • the heating temperature in the heating process and the holding temperature in the holding process are equal to or higher than the Ac 3 temperature
  • the metallographic microstructure of the hot-formed member after hardening is a martensite single phase, and the ductility of the hot-formed member significantly deteriorates. Accordingly, the heating temperature and the holding temperature are lower than the Ac 3 temperature.
  • the holding time in the holding process is shorter than 1 minute, undissolved carbide such as cementite remains in the hot-formed member, and the impact resistance of the hot-formed member deteriorates. Accordingly, the holding time is set to 1 minute or longer.
  • the holding time is set to 20 minutes or shorter.
  • the rate is preferably set to 0.2° C./sec to 100° C./sec.
  • the average heating rate is easily controlled in the case in which the heating is performed using a normal furnace.
  • high-frequency heating or the like it is possible to control the heating temperature with high accuracy even when the heating is performed at a heating rate of higher than 100° C./sec.
  • the cooling in a temperature range of 600° C. to 150° C. is performed such that diffusive transformation does not occur.
  • the average cooling rate in the above temperature range is lower than 20° C./sec, bainitic transformation excessively proceeds, the area ratio of martensite that is a phase (strengthening phase) strengthening the strength of the hot-formed member cannot be secured, and thus it is difficult to secure a tensile strength of 900 MPa or more after hardening.
  • the average cooling rate in the above temperature range is set to 20° C./sec or higher. It is difficult to increase the average cooling rate in the above temperature range to be higher than 500° C./sec using normal equipment. Accordingly, the average cooling rate in the above temperature range is set to 500° C./sec or lower.
  • the average cooling rate in the above temperature range is preferably 200° C./sec or lower.
  • cooling is achieved when a die having a room temperature or a temperature of about several tens of ° C. immediately before hot pressing takes heat from the hot-formed member.
  • a heat capacity of the die may be changed by changing a size of the die.
  • the cooling rate can also be changed by changing the material of the die to dissimilar metal (for example, copper).
  • the cooling rate can also be changed by using a fluid cooling-type die and by changing the flow rate of a coolant.
  • the cooling rate can also be changed by using a die in which several grooves are previously cut and by passing a coolant (water or gas) in the groove during pressing.
  • the cooling rate can also be changed by separating a die from the hot-formed member by operating a press machine during pressing and by passing a coolant therebetween. Furthermore, the cooling rate can also be changed by changing the contact area between a die and the steel sheet (hot-formed member) by changing a clearance of the die. In view of the above facts, the following methods can be considered to be methods of changing the cooling rate at about 600° C.
  • the hot-formed member is moved to a die having a different heat capacity or a die in a room temperature state to change the cooling rate;
  • a hot-formed member having a predetermined metallographic microstructure can be obtained through the above-described method.
  • a hot-formed member having a predetermined metallographic microstructure can be obtained through the following method.
  • a steel sheet having the same chemical composition as that of the above-described hot-formed member and having a metallographic microstructure in which the average grain size of ferrite is 7.0 ⁇ m or less and the amount of unrecrystallized ferrite is more than 2.0 area % is prepared as a base steel sheet.
  • a cold-rolled steel sheet, a hot-dip galvanized cold-rolled steel sheet, and a galvannealed steel sheet having a metallographic microstructure in which the average grain size of ferrite is 7.0 ⁇ m or less and the amount of unrecrystallized ferrite is more than 2.0 area % can be manufactured by, for example, annealing a cold-rolled steel sheet in a temperature range of lower than (Ac 3 temperature—20° C.).
  • the base steel sheet prepared as described above is hot-pressed after being held for 30 seconds or longer and shorter than 20 minutes in a temperature range of the Ac 3 temperature to the Ac 3 temperature+100° C., and is cooled at an average cooling rate of 3° C./sec to 20° C./sec in a temperature range of the Ac 3 temperature to 600° C.
  • a cold-rolled steel sheet or a hot-dip galvanized cold-rolled steel sheet having the same chemical composition as that of the hot-formed member and having a metallographic microstructure in which the average grain size of ferrite is 7.0 ⁇ m or less and the amount of unrecrystallized ferrite is more than 2.0 area % can be used.
  • Heating Temperature of Base Steel Sheet Temperature Range of Ac 3 Temperature to Ac 3 Temperature+100° C.
  • the steel sheet to be subjected to hot pressing is heated by being held for 30 seconds or longer and shorter than 20 minutes in a temperature range of the Ac 3 temperature (° C.), which is specified by the above-described Experimental Formula (i), to the Ac 3 temperature+100° C.
  • the holding temperature is set to the Ac 3 temperature or higher.
  • the holding temperature is set to the Ac 3 temperature+100° C. or lower.
  • the holding time when the holding time is shorter than 30 seconds, the strength of the base steel sheet largely scatters. Since conditions under which such a phenomenon occurs are not suitable for mass production, the holding time is set to 30 seconds or longer. On the other hand, when the holding time is 20 minutes or longer, austenite grains excessively grow, and the metallographic microstructure becomes non-uniform, and thus, the impact resistance of the hot-formed member significantly decreases. Accordingly, the holding time is set to be shorter than 20 minutes.
  • the heating rate up to a temperature range of the Ac 3 temperature to the Ac 3 temperature+100° C. is preferably 0.2° C./sec to 100° C./sec.
  • the average heating rate is easily controlled in the case in which the heating is performed using a normal furnace.
  • high-frequency heating or the like it is possible to control the heating temperature with high accuracy even when the heating is performed at a heating rate of higher than 100° C./sec.
  • the cooling in a temperature range of the Ac 3 temperature to 600° C. is performed such that the average cooling rate is 3° C./sec to 20° C./sec.
  • the average cooling rate in the above temperature range is set to 3° C./sec or higher.
  • the average cooling rate in the above temperature range is set to 20° C./sec or lower.
  • the average cooling rate in a temperature range of lower than 600° C. is set to 20° C./sec to 500° C./sec.
  • the aspect of the forming in the hot pressing method is not particularly limited.
  • the aspect of the forming include bending, drawing, stretch forming, hole expansion forming, and flanging.
  • a preferred form may be appropriately selected among the above-described forms of the forming in accordance with the kind and the shape of a target hot-formed member.
  • the material of a base steel sheet to be subjected to the hot pressing method in this embodiment include a cold-rolled steel sheet, a hot-dip galvanized steel sheet, and a galvannealed steel sheet.
  • the hot-formed member include a door guard bar and a bumper reinforcement which are reinforcing components for an automobile.
  • the hot-formed member is a bumper reinforcement
  • the above-described base steel sheet which is a galvannealed steel sheet having a predetermined length is prepared, and under the above-described conditions, working such as bending may be sequentially performed in a die.
  • the hot-formed member according to this embodiment is characterized by excellent ductility and excellent impact resistance.
  • the hot-formed member according to this embodiment preferably has such ductility that a total elongation in a tension test is 10% or more.
  • the hot-formed member according to this embodiment preferably has such impact resistance that an impact value in a Charpy test at 0° C. is 20 J/cm 2 or more.
  • the hot-formed member having such mechanical properties is realized by satisfying the above-described specifications related to the chemical composition and the metallographic microstructure.
  • a shot blasting treatment is generally performed on the hot-formed member for the purpose of removing scale.
  • This shot blasting treatment is effective for introducing a compressive stress to a surface of a member to be treated. Accordingly, performing the shot blasting treatment on the hot-formed member has advantages in that delayed fracture is limited in the hot-formed member and the fatigue strength of the hot-formed member is improved.
  • the base steel sheet is as soft as possible, and the base steel sheet has high ductility.
  • the tensile strength of the base steel sheet is desirably 800 MPa or less.
  • the hot forming has been described using hot pressing which is a specific aspect thereof, but the manufacturing method according to this embodiment is not limited to the hot press forming. Similarly to hot pressing, the manufacturing method according to this embodiment can be applied to all hot forming processes having a means which cools a steel sheet simultaneously or immediately after forming. Examples of the hot forming include roll forming.
  • Base steel sheets (sheet thickness t: 1.2 mm) having chemical compositions shown in Table 1 and metallographic microstructures and tensile strengths shown in Table 2, respectively, were subjected to hot pressing.
  • base steel sheets are steel sheets (cold-rolled steel sheets in Table 2) manufactured by subjecting slabs manufactured in a laboratory to hot rolling, cold rolling, and recrystallization annealing. Using a plating simulator, some steel sheets were subjected to a hot dip galvanizing treatment (plating adhesion amount per side: 60 g/m 2 ) and an galvannealed treatment (plating adhesion amount per side: 60 g/m 2 , Fe content in coating film: 15 mass %). These are hot-dip galvanized cold-rolled steel sheets and galvannealed steel sheets in Table 2. Steel sheets as cold-rolled, and not subjected to recrystallization annealing (full hard in Table 2), were also used as base steel sheets.
  • the “average heating rate” in Table 3 indicates an average of heating rates in a temperature range of a room temperature to 720° C.
  • the “holding time” in Table 3 indicates a period of time in which the steel is held in a temperature range of 720° C. or higher.
  • the “cooling rate*1” in Table 3 indicates an average cooling rate from an Ac 3 temperature to 600° C. in the case in which the heating temperature is the Ac 3 temperature or higher, and indicates an average cooling rate from the heating temperature to 600° C.
  • the “cooling rate*2” is an average cooling rate in a temperature range of 600° C. to 150° C.
  • the steel sheets obtained under the various manufacturing conditions were subjected to a tensile test, a Charpy test, and metallographic microstructure observation.
  • the steel sheet members prepared in this example are not subjected to hot pressing by a die, but undergo the same thermal history as a hot-formed member. Therefore, the steel sheets have substantially the same mechanical properties as the hot-formed member having the same thermal history.
  • a JIS No. 5 tensile test piece of which the longitudinal direction was perpendicular to a rolling direction was prepared from each steel sheet, and a tensile strength (TS) and a total elongation (EL) were measured. Samples having a TS of 900 MPa or more and an EL of 10% or more were determined to be accepted.
  • Test pieces were prepared from the base steel sheets and the heat-treated steel sheets in a rolling direction of the base steel sheet and the heat-treated steel sheet and in a direction perpendicular to the rolling direction.
  • metallographic microstructures of a cross-section in the rolling direction and a cross-section perpendicular to the rolling direction in the test piece were photographed by an electron microscope. Electron micrographs of regions of 800 ⁇ m ⁇ 800 ⁇ m obtained as described above were analyzed to calculate area ratios of unrecrystallized ferrite, ferrite, and martensite.
  • Sample No. 3 had poor ductility and poor impact resistance since the average grain size of ferrite of the base steel sheet was out of the range specified in the present invention.
  • Sample No. 13 had poor ductility and poor impact resistance since the manufacturing method specified in the present invention was not applied.
  • Sample No. 14 had poor ductility since the manufacturing method specified in the present invention was not applied.
  • Sample Nos. 9 and 26 had poor impact resistance since the chemical composition was out of the range specified in the present invention.
  • Sample Nos. 10 and 17 could not obtain a target tensile strength since the manufacturing conditions were out of the range specified in the present invention, and thus a desired microstructure could not be obtained.
  • Sample No. 18 had poor ductility since the manufacturing conditions were out of the range specified in the present invention, and thus a desired microstructure could not be obtained.
  • Sample Nos. 28 and 32 could not obtain a target tensile strength since the chemical composition was out of the range specified in the present invention.
  • Sample No. 34 had poor ductility since the chemical composition was out of the range specified in the present invention, and thus a desired microstructure could not be obtained.
  • Sample No. 8 had poor ductility since the heating temperature was higher than the Ac 3 temperature.
  • Sample No. 12 could not obtain a target tensile strength since the heating temperature was lower than 720° C.
  • surface scale was generated beyond the allowable extent since the holding time was out of the range specified in the present invention.
  • Sample No. 30 could not obtain target impact resistance since the holding time at 750° C. or higher was out of the range specified in the present invention.
  • a total area ratio of ferrite and martensite of the hot-formed member was out of the range specified in the present invention, and the tensile strength was insufficient since the cooling rate at a temperature lower than 600° C. was out of the range specified in the present invention.

Landscapes

  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Oil, Petroleum & Natural Gas (AREA)
  • Heat Treatment Of Sheet Steel (AREA)
  • Heat Treatment Of Articles (AREA)
US15/109,582 2014-01-06 2014-01-06 Hot-formed member and method of manufacturing same Abandoned US20160326608A1 (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
PCT/JP2014/050019 WO2015102048A1 (fr) 2014-01-06 2014-01-06 Élément façonné à chaud et son procédé de fabrication

Publications (1)

Publication Number Publication Date
US20160326608A1 true US20160326608A1 (en) 2016-11-10

Family

ID=53493396

Family Applications (1)

Application Number Title Priority Date Filing Date
US15/109,582 Abandoned US20160326608A1 (en) 2014-01-06 2014-01-06 Hot-formed member and method of manufacturing same

Country Status (8)

Country Link
US (1) US20160326608A1 (fr)
EP (1) EP3093360A4 (fr)
JP (1) JP6229736B2 (fr)
KR (1) KR101849031B1 (fr)
CN (1) CN105899700B (fr)
CA (1) CA2935638C (fr)
MX (1) MX2016008808A (fr)
WO (1) WO2015102048A1 (fr)

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20180111179A1 (en) * 2016-10-21 2018-04-26 Ford Motor Company Method for monitoring quality of hot stamped components
US10625323B2 (en) 2016-02-19 2020-04-21 Ford Global Technologies, Llc Method for monitoring quality of hot stamped components
US20230047282A1 (en) * 2020-01-08 2023-02-16 Nippon Steel Corporation Steel sheet and method of manufacturing the same
US11913099B2 (en) * 2017-03-01 2024-02-27 Cleveland-Cliffs Steel Properties Inc. Press hardened steel with extremely high strength and method for production

Families Citing this family (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2015102051A1 (fr) 2014-01-06 2015-07-09 新日鐵住金株式会社 Élément formé à chaud et son procédé de fabrication
US10774405B2 (en) 2014-01-06 2020-09-15 Nippon Steel Corporation Steel and method of manufacturing the same
CN107099748B (zh) * 2017-04-28 2019-03-26 武汉钢铁有限公司 高温成型用超高强锌铝镁镀层钢板及其制造方法
CN107475623A (zh) * 2017-08-15 2017-12-15 苏州普热斯勒先进成型技术有限公司 一种热成形高强钢及其加工方法
CN110423953B (zh) * 2019-08-21 2020-12-01 马鞍山钢铁股份有限公司 一种抗拉强度1800MPa级以上的冷弯性能优良的热成形构件及其制备方法

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2010065292A (ja) * 2008-09-12 2010-03-25 Jfe Steel Corp 延性に優れたホットプレス部材、そのホットプレス部材用鋼板、およびそのホットプレス部材の製造方法
US20100108200A1 (en) * 2008-10-30 2010-05-06 Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel Ltd) High yield ratio and high-strength hot-dip galvanized steel sheet excellent in workability and production method thereof
US10273555B2 (en) * 2013-12-27 2019-04-30 Nippon Steel & Sumitomo Metal Corporation Hot-pressed steel sheet member
US10344351B2 (en) * 2013-12-20 2019-07-09 Nippon Steel & Sumitomo Metal Corporation Hot-pressed steel sheet member, method of manufacturing the same, and steel sheet for hot pressing

Family Cites Families (18)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
SE435527B (sv) 1973-11-06 1984-10-01 Plannja Ab Forfarande for framstellning av en detalj av herdat stal
JP3407562B2 (ja) 1996-09-20 2003-05-19 住友金属工業株式会社 高炭素薄鋼板の製造方法および部品の製造方法
JP2001234282A (ja) * 2000-02-21 2001-08-28 Kawasaki Steel Corp 温間プレス成形性に優れた高張力熱延鋼板およびその製造方法
WO2008110670A1 (fr) 2007-03-14 2008-09-18 Arcelormittal France Acier pour formage a chaud ou trempe sous outil a ductilite amelioree
KR101010971B1 (ko) * 2008-03-24 2011-01-26 주식회사 포스코 저온 열처리 특성을 가지는 성형용 강판, 그 제조방법,이를 이용한 부품의 제조방법 및 제조된 부품
BRPI0909806B1 (pt) * 2008-03-27 2017-07-04 Nippon Steel & Sumitomo Metal Corporation Cold rolled sheet steel, galvanized sheet steel, hot dip galvanized sheet steel, and methods of producing the same
JP5347395B2 (ja) * 2008-09-12 2013-11-20 Ｊｆｅスチール株式会社 延性に優れたホットプレス部材、そのホットプレス部材用鋼板、およびそのホットプレス部材の製造方法
JP5347393B2 (ja) * 2008-09-12 2013-11-20 Ｊｆｅスチール株式会社 延性に優れたホットプレス部材、そのホットプレス部材用鋼板、およびそのホットプレス部材の製造方法
DE102008051992B4 (de) 2008-10-16 2011-03-24 Benteler Automobiltechnik Gmbh Verfahren zur Herstellung eines Werkstücks, Werkstück und Verwendung eines Werkstückes
JP5353642B2 (ja) * 2009-11-06 2013-11-27 新日鐵住金株式会社 熱処理用鋼板およびその製造方法
US20130095347A1 (en) * 2010-06-14 2013-04-18 Kaoru Kawasaki Hot-stamped steel, method of producing of steel sheet for hot stamping, and method of producing hot-stamped steel
JP2012082499A (ja) * 2010-10-14 2012-04-26 Sumitomo Metal Ind Ltd 溶融めっき鋼板およびその製造方法
US10030280B2 (en) * 2010-10-22 2018-07-24 Nippon Steel & Sumitomo Metal Corporation Steel sheet and method for manufacturing steel sheet
KR101253885B1 (ko) * 2010-12-27 2013-04-16 주식회사 포스코 연성이 우수한 성형 부재용 강판, 성형 부재 및 그 제조방법
JP5756773B2 (ja) * 2012-03-09 2015-07-29 株式会社神戸製鋼所 熱間プレス用鋼板およびプレス成形品、並びにプレス成形品の製造方法
JP5860333B2 (ja) * 2012-03-30 2016-02-16 株式会社神戸製鋼所 加工性に優れた高降伏比高強度冷延鋼板
JP5835621B2 (ja) * 2012-06-27 2015-12-24 新日鐵住金株式会社 熱間プレス鋼板部材およびその製造方法ならびに熱間プレス用鋼板
JP5630475B2 (ja) * 2012-07-17 2014-11-26 新日鐵住金株式会社 熱間成形鋼板部材およびその製造方法

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2010065292A (ja) * 2008-09-12 2010-03-25 Jfe Steel Corp 延性に優れたホットプレス部材、そのホットプレス部材用鋼板、およびそのホットプレス部材の製造方法
US20100108200A1 (en) * 2008-10-30 2010-05-06 Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel Ltd) High yield ratio and high-strength hot-dip galvanized steel sheet excellent in workability and production method thereof
US10344351B2 (en) * 2013-12-20 2019-07-09 Nippon Steel & Sumitomo Metal Corporation Hot-pressed steel sheet member, method of manufacturing the same, and steel sheet for hot pressing
US10273555B2 (en) * 2013-12-27 2019-04-30 Nippon Steel & Sumitomo Metal Corporation Hot-pressed steel sheet member

Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US10625323B2 (en) 2016-02-19 2020-04-21 Ford Global Technologies, Llc Method for monitoring quality of hot stamped components
US11554404B2 (en) 2016-02-19 2023-01-17 Ford Global Technologies, Llc Method for monitoring quality of hot stamped components
US20180111179A1 (en) * 2016-10-21 2018-04-26 Ford Motor Company Method for monitoring quality of hot stamped components
US11229935B2 (en) * 2016-10-21 2022-01-25 Ford Global Technologies, Llc Method for monitoring quality of hot stamped components
US11913099B2 (en) * 2017-03-01 2024-02-27 Cleveland-Cliffs Steel Properties Inc. Press hardened steel with extremely high strength and method for production
US20230047282A1 (en) * 2020-01-08 2023-02-16 Nippon Steel Corporation Steel sheet and method of manufacturing the same
US11866800B2 (en) * 2020-01-08 2024-01-09 Nippon Steel Corporation Steel sheet and method of manufacturing the same

Also Published As

Publication number Publication date
CN105899700B (zh) 2019-01-15
JP6229736B2 (ja) 2017-11-15
MX2016008808A (es) 2016-09-08
KR20160097348A (ko) 2016-08-17
CA2935638C (fr) 2019-08-27
KR101849031B1 (ko) 2018-04-13
EP3093360A4 (fr) 2018-03-07
JPWO2015102048A1 (ja) 2017-03-23
EP3093360A1 (fr) 2016-11-16
CN105899700A (zh) 2016-08-24
WO2015102048A1 (fr) 2015-07-09
CA2935638A1 (fr) 2015-07-09

Similar Documents

Publication Publication Date Title
CN110832098B (zh) 热轧钢板及其制造方法
US10711322B2 (en) Hot-pressed steel sheet member, method of manufacturing the same, and steel sheet for hot pressing
US10273555B2 (en) Hot-pressed steel sheet member
CA2935638C (fr) Element forme a chaud et procede de fabrication associe
US10266911B2 (en) Hot-formed member and manufacturing method of same
KR101533164B1 (ko) 핫스탬프 성형체의 제조 방법 및 핫스탬프 성형체
JP5630475B2 (ja) 熱間成形鋼板部材およびその製造方法
JP5803836B2 (ja) 熱間プレス鋼板部材、その製造方法と熱間プレス用鋼板
JP5585623B2 (ja) 熱間成形鋼板部材およびその製造方法
US10344351B2 (en) Hot-pressed steel sheet member, method of manufacturing the same, and steel sheet for hot pressing
KR20120068990A (ko) 성형성이 우수한 고강도 용융 아연 도금 강판 및 그 제조 방법
JP5835621B2 (ja) 熱間プレス鋼板部材およびその製造方法ならびに熱間プレス用鋼板
JP5857913B2 (ja) 熱間成形鋼板部材およびその製造方法ならびに熱間成形用鋼板

Legal Events

Date Code Title Description
AS Assignment

Owner name: NIPPON STEEL & SUMITOMO METAL CORPORATION, JAPAN

Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:HAYASHI, KOUTAROU;NISHIBATA, TOSHINOBU;REEL/FRAME:039071/0562

Effective date: 20160623

STPP Information on status: patent application and granting procedure in general

Free format text: FINAL REJECTION MAILED

AS Assignment

Owner name: NIPPON STEEL CORPORATION, JAPAN

Free format text: CHANGE OF NAME;ASSIGNOR:NIPPON STEEL & SUMITOMO METAL CORPORATION;REEL/FRAME:049257/0828

Effective date: 20190401

STPP Information on status: patent application and granting procedure in general

Free format text: DOCKETED NEW CASE - READY FOR EXAMINATION

STPP Information on status: patent application and granting procedure in general

Free format text: NON FINAL ACTION MAILED

STPP Information on status: patent application and granting procedure in general

Free format text: RESPONSE TO NON-FINAL OFFICE ACTION ENTERED AND FORWARDED TO EXAMINER

STCB Information on status: application discontinuation

Free format text: ABANDONED -- FAILURE TO RESPOND TO AN OFFICE ACTION