US20180044754A1 - Steel sheet for hot stamping and method for producing steel sheet for hot stamping, and hot stamp formed body - Google Patents

Steel sheet for hot stamping and method for producing steel sheet for hot stamping, and hot stamp formed body Download PDF

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Publication number
US20180044754A1
US20180044754A1 US15/559,731 US201615559731A US2018044754A1 US 20180044754 A1 US20180044754 A1 US 20180044754A1 US 201615559731 A US201615559731 A US 201615559731A US 2018044754 A1 US2018044754 A1 US 2018044754A1
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steel sheet
hot
mass
hot stamping
steel
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Inventor
Masafumi Azuma
Yoshihiro Suwa
Yusuke Kondo
Koichi Sato
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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Assigned to NIPPON STEEL & SUMITOMO METAL CORPORATION reassignment NIPPON STEEL & SUMITOMO METAL CORPORATION ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: AZUMA, MASAFUMI, KONDO, YUSUKE, SATO, KOICHI, SUWA, Yoshihiro
Publication of US20180044754A1 publication Critical patent/US20180044754A1/en
Assigned to NIPPON STEEL CORPORATION reassignment NIPPON STEEL CORPORATION CHANGE OF NAME (SEE DOCUMENT FOR DETAILS). Assignors: NIPPON STEEL & SUMITOMO METAL CORPORATION
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/02—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling heavy work, e.g. ingots, slabs, blooms, or billets, in which the cross-sectional form is unimportant ; Rolling combined with forging or pressing
    • B21B1/04—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling heavy work, e.g. ingots, slabs, blooms, or billets, in which the cross-sectional form is unimportant ; Rolling combined with forging or pressing in a continuous process
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/46—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling metal immediately subsequent to continuous casting
    • B21B1/463—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling metal immediately subsequent to continuous casting in a continuous process, i.e. the cast not being cut before rolling
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B45/00—Devices for surface or other treatment of work, specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills
    • B21B45/02—Devices for surface or other treatment of work, specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills for lubricating, cooling, or cleaning
    • B21B45/0239—Lubricating
    • B21B45/0242—Lubricants
    • 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
    • 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
    • B21D22/20—Deep-drawing
    • 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
    • B21D24/00—Special deep-drawing arrangements in, or in connection with, presses
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0278—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular surface treatment
    • 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
    • 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
    • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
    • 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
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • 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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • 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
    • C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
    • C23F11/00—Inhibiting corrosion of metallic material by applying inhibitors to the surface in danger of corrosion or adding them to the corrosive agent
    • 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
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
    • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
    • C23G1/08—Iron or steel
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/02—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling heavy work, e.g. ingots, slabs, blooms, or billets, in which the cross-sectional form is unimportant ; Rolling combined with forging or pressing
    • B21B2001/028—Slabs

Definitions

  • the present invention relates to a steel sheet for hot stamping excellent in scale adhesion at the time of hot stamping and a method for producing the steel sheet for hot stamping, and a hot stamp formed body that is a formed body of the steel sheet for hot stamping.
  • Weight reduction of the members such as door guard bars and side members of automobiles are being studied to cope with the recent trend of improvement in fuel efficiency, and in terms of a material, increase in strength of a steel sheet is promoted from the viewpoint of strength and crash safety that should be ensured even when the thickness is reduced.
  • strength means both tensile strength and yield strength.
  • formability of a material deteriorates as the strength increases, and therefore in order to realize reduction in weight of the above described members, it is necessary to produce a steel sheet that satisfies both formability and high strength.
  • TRIP Treatment Induced Plasticity
  • Patent Literature 1 and Patent Literature 2 As a method for obtaining high formability simultaneously with high strength, there are TRIP (TRansformation Induced Plasticity) steels taking advantage of martensitic transformation of retained austenite that are described in Patent Literature 1 and Patent Literature 2, and application of TRIP steels has been expanding in recent years.
  • TRIP steels In the steel, however, although deep drawability and elongation are improved at the time of forming, due to a high steel sheet strength, it has a problem of low shape fixability of a member after press forming.
  • Patent Literature 3 In order to form a high strength steel sheet, which is inferior in formability, with good shape fixability, there is a method called hot press that is described in Patent Literature 3 and Patent Literature 4.
  • the method performs forming at a temperature of 200° C. to about 500° C. at which the steel sheet strength reduces.
  • the method has problems in that even when the forming temperature is increased, the steel sheet strength may still be high in some cases and thus forming is difficult, and in that the steel sheet strength after forming is reduced by heating, and thus predetermined strength cannot be obtained in some cases.
  • the steel sheet which is taken out is exposed to the atmosphere, and thus the technique has a problem of unavoidable formation of scale.
  • the method for electrically heating a steel sheet in the atmosphere has been developed. At the time of heating in the atmosphere, avoidance of oxidation of the steel sheet is difficult, and thus a problem of die wear due to loose scale at the time of hot stamping easily becomes evident. As a result, regular repair of the die is essential.
  • the present invention has an object to provide a steel sheet for hot stamping that is excellent in scale adhesion at the time of hot stamping, without an occurrence of adhesion of a molten metal to a die, a method for manufacturing the steel sheet for hot stamping, and a hot stamp formed body.
  • the present inventors have studied earnestly on methods to solve the above described problems.
  • 0.50 mass % to 3.00 mass % of Si is contained in the steel sheet, the amount of rust inhibiting oil that is applied to the steel sheet is set to be within a range of 50 mg/m 2 to 1500 mg/m 2 , and surface roughness of the steel sheet is set as Rz>2.5 ⁇ m.
  • an S content included in the rust inhibiting oil is preferably set at 5 mass % or less.
  • enclosures in the coating oil concentrate into an interface between a base iron and scale, and thereby deteriorate scale adhesion.
  • the present invention is made based on the above described knowledge, and the gist of the present invention is as follows.
  • Si 0.50% to 3.00%
  • N 0.0100% or less
  • Nb 0% to 0.100%
  • V 0% to 0.100%
  • an amount of S contained in the coating oil which is applied onto the steel sheet is 5% or less in mass %.
  • composition of the steel sheet contains, in mass %
  • V 0.005% to 0.100%
  • composition of the steel sheet contains, in mass %
  • V 0% to 0.100%
  • a rust inhibiting oil remaining amount on a steel sheet surface is limited to 50 mg/m 2 to 1500 mg/m 2 .
  • an amount of S in the rust inhibiting oil that is applied to the steel sheet is 5% or less in mass %.
  • composition of the slab contains, in mass %
  • V 0.005% to 0.100%
  • composition of the slab contains, in mass %
  • V 0% to 0.100%
  • a thickness of the scale is 10 ⁇ m or less.
  • composition of the hot stamp formed body contains, in mass %
  • V 0.005% to 0.100%
  • composition of the hot stamp formed body contains, in mass %
  • the steel sheet for hot stamping excellent in scale adhesion at the time of hot stamping, in which adhesion of a molten metal to the die does not occur the method for producing the steel sheet for hot stamping and the hot stamp formed body can be provided.
  • FIG. 1 is a diagram illustrating a relationship between a coating oil amount on a steel sheet and surface roughness Rz of the steel sheet.
  • FIG. 2 is a diagram for explaining that when an S concentration in coating oil increases, scale easily detaches.
  • FIG. 3 is a diagram illustrating a relationship between a pickling time period and the surface roughness Rz of the steel sheet.
  • FIG. 4A is a photograph showing a microstructure of a surface layer of a hot-rolled steel sheet before pickling.
  • FIG. 4B is a photograph showing the surface layer microstructure after pickling.
  • FIG. 5 is a diagram illustrating a relationship between an coating oil amount and a thickness of scale.
  • FIG. 6A is a photograph showing a section of a hot stamp formed body surface of an example of the present invention.
  • FIG. 6B is a photograph showing a section of a hot stamp formed body surface of a comparative example.
  • FIG. 7 is a diagram for explaining that when the surface roughness Rz before hot stamp thermal treatment is less than 2.5, a number density of irregularities after hot stamp thermal treatment is less than 3.
  • a steel sheet for hot stamping of the present invention contains from 0.5 mass % to 3.0 mass % of Si in the steel sheet, an amount of rust inhibiting oil applied to the steel sheet is in a range of 50 mg/m 2 to 1500 mg/m 2 , and surface roughness of the steel sheet is Rz>2.5 ⁇ m. It is preferable that an S content contained in the rust inhibiting oil be 5 mass % or less.
  • the present inventors have investigated the surface properties of the steel sheets, and influences of various kinds of treatment. As a result, the present inventors have found that although the steel sheets after degreasing show excellent scale adhesion, scale adhesion significantly deteriorates after rust inhibiting oil is applied.
  • the present inventors investigated the relationship between scale adhesion and rust inhibiting oil in more detail it has been found that when an amount of S contained as impurities in the rust inhibiting oil increases, scale tends to detach easily. It is conceivable that S in the rust inhibiting oil has an influence on scale adhesion, although the detailed reason is unclear.
  • rust inhibiting oil such as mineral oil
  • a pickled hot-rolled steel sheet for hot stamping and a cold-rolled steel sheet for hot stamping after cold rolling or annealing in order to restrain rust from occurring in the period from production to use.
  • a steel sheet after pickling has been generally coated with oil of more than 1500 mg/m 2 , assuming that the period from delivery to a customer to use is long.
  • the present inventors investigated the influence of the coating oil amount for the purpose of making scale adhesion and rust inhibition properties compatible, the present inventors have found that as illustrated in FIG. 1 , scale adhesion is enhanced by strictly controlling the range of the coating oil amount and the surface roughness of a steel sheet.
  • the effect is exhibited by setting the coating oil amount at 50 mg/m 2 to 1500 mg/m 2 .
  • a lower limit of the coating oil amount is set at 50 mg/m 2 , because it is difficult to ensure excellent rust inhibition properties with the coating oil amount less than the coating oil amount of 50 mg/m 2 .
  • the lower limit of the coating oil amount is preferably 100 mg/m 2 or more, and more preferably 200 mg/m 2 or more.
  • An upper limit is set at 1500 mg/m 2 to obtain an effect of excellent scale adhesion.
  • the upper limit of the coating oil amount is set at 1500 mg/m 2 because when the coating oil amount exceeds 1500 mg/m 2 , scale adhesion deteriorates.
  • the upper limit is preferably 1000 mg/m 2 , is more preferably 900 mg/m 2 , and far more preferably is 800 mg/m 2 . Further, coated oil on the steel sheet surface burns at the time of heating, and therefore becomes the cause of generating soot. From this, a smaller coating oil amount is more preferable.
  • Scale adhesion illustrated in FIG. 1 was evaluated by a hot shallow drawing test in a cylindrical die of ⁇ 70 mm and a depth of 20 mm. After a steel sheet was heated to a temperature range of 800° C. to 1100° C. at 50° C./s in an electrical heater, and was retained for 0 seconds to 120 seconds, energization was stopped, the steel sheet was cooled to 650° C. by standing to cool, and hot shallow drawing was performed in the above described die.
  • specimens after forming were visually observed, and specimens in which an area where scale was detached accounted for 5% or less were determined as having good (circle) scale adhesion, specimens in which the area where scale was detached accounted for 5 to 15% were determined as poor (triangle), and specimens in which the area where scale was detached accounted for more than 15% were determined as very poor (X).
  • the specimens in which the area where scale was detached accounted for 5% or less were determined as within the range of the present invention.
  • scale adhesion without particularly limiting the heating method.
  • conditions of any of a heating furnace, far-infrared rays, near-infrared rays and electrical heating may be adopted.
  • more excellent scale adhesion can be obtained by thinning scale by controlling the atmosphere in the heating furnace and restraining oxidation of the steel sheet.
  • a shallow drawing test temperature may be in any temperature region as long as a steel sheet can be processed, but in general, a steel sheet for hot stamping has high strength and excellent shape fixability by processing in an austenite region and subsequent die hardening. From this, characteristics evaluation was carried out by hot shallow drawing at 650° C. exceeding Ar3.
  • oil coating method electrostatic oil coating, spray, a roll coater and the like are generally used, but the oil coating method is not limited as long as the coating oil amount can be ensured.
  • NOX-RUST530F made by PARKER INDUSTRIES, INC.
  • the kind of oil is not limited.
  • the coating oil amount may be measured by any method as long as the coating oil amount can be measured, the present inventors measured the coating oil amount by the following method.
  • the steel sheet coated with rust inhibiting oil was cut into 150 mm square first, and thereafter, a tape was applied so that a 100 mm by 100 mm region is exposed. Subsequently, the weights of the coating oil and the steel sheet to which seal was carried out (including the weight of the tape) were measured in advance. Subsequently, degreasing was performed by wiping off the rust inhibiting oil on the steel sheet surface with cloth containing acetone, the weight of the degreased steel sheet was measured, the weights before and after degreasing were compared, and thereby the coating oil amount per unit area was calculated. Measurement was carried out at three spots in each of the steel sheets, and an average value of the attached amounts was determined as a coating oil attaching amount of each of the steel sheets.
  • the present inventors investigated the relationship between the S content in the coating oil and a scale detached area ratio as illustrated in FIG. 2 , the present inventors have found that as the S content in the coating oil becomes smaller, the scale adhesion increases, and especially when the S content in the coating oil is 5 mass % or less, the scale detached area becomes substantially 0%. It is conceivable that while the oil contained in the rust inhibiting oil is burned and eliminated during heating, S contained as an impurity remains on the steel sheet surface to concentrate into scale, and thereby deteriorates scale adhesion, although detailed mechanism is unclear. Hence, it is preferable to reduce the content of S contained in the rust inhibiting oil.
  • the S content is preferably mass % or less, and is more preferably 3 mass % or less.
  • the surface roughness of the steel sheet will be described next.
  • the surface roughness of the steel sheet needs to satisfy Rz>2.5 ⁇ m.
  • a result obtained by investigating a relationship between the surface roughness Rz of the steel sheet and scale adhesion is as illustrated in FIG. 1 described above.
  • the effect is generally referred to as an anchor effect.
  • scale that is generated at the time of heating in the present steel sheet is thin.
  • the surface roughness of the steel sheet before hot stamping needs to satisfy Rz>2.5 ⁇ m.
  • Rz2.5 ⁇ m the surface roughness of the steel sheet is small, and the anchor effect is insufficient, and thus excellent scale adhesion at the time of hot stamping cannot be ensured.
  • the effect of the excellent scale adhesion of the present invention can be obtained without particularly providing the upper limit, if scale adhesion is excessively increased, it becomes difficult to remove scale in a downstream process such as shot blast, for example.
  • Rz ⁇ 7.0 ⁇ m It is more preferable to set Rz ⁇ 7.0 ⁇ m.
  • Rz8.0 ⁇ m it is possible to ensure excellent scale adhesion that is the effect of the present invention.
  • the interface between the base iron and the scale becomes flat by excessive oxidation. As a result, the irregularities in the interface between the scale and the base iron are eliminated, and the effect of the excellent scale adhesion that is the effect of the present invention is not exhibited.
  • a contact surface roughness measuring instrument SURFCOM2000DX/SD3 made by TOKYO SEIMITSU CO., LTD
  • the steel sheet for hot stamping of the present invention ensures scale adhesion by control of the irregularities in the interface between the scale and the base iron.
  • the scale can be scale mainly composed of an Si oxide, Fe 3 O 4 , Fe 2 O 3 and FeO.
  • An Si oxide exists in the interface between base iron and iron scale (FeO, Fe 2 O 3 , Fe 2 O 3 ), and thereby controls a thickness of the iron scale.
  • the scale needs to contain an Si oxide. Since the main object is to control the thickness of the iron oxide, even if the Si oxide is very thin, it is sufficient if the Si oxide exists, and even with 1 nm, the Si oxide exhibits the effect.
  • Composition analysis of the scale of the formed body was carried out by X-ray diffraction by cutting out the sheet from a bottom of the cylindrical portion of a shallow drawn specimen piece. From a peak intensity ratio of the respective oxides, volume ratios of the respective Fe oxides were measured. The Si oxide existed very thinly, and the volume ratio was less than 1%, and thus quantitative evaluation in X-ray diffraction was difficult. However, it is possible to confirm that an Si oxide exists in the interface between the scale and the base iron by line analysis of EPMA (Electron Probe Micro Analyzer).
  • EPMA Electro Probe Micro Analyzer
  • the thickness of the scale is preferably 10 ⁇ m or less.
  • the thickness of the scale is 10 ⁇ m or less.
  • the thickness of the scale is more preferably 7 ⁇ m or less, and is more preferably 5 ⁇ m or less.
  • the thickness of the scale is achieved by controlling the coating oil amount within the predetermined range simultaneously with controlling the Si content of the steel sheet within a predetermined range.
  • FIG. 5 illustrates a relationship between the coating oil amount and the scale thickness.
  • FIG. 6A shows a photograph of an interface between a base iron and scale of a formed body excellent in scale adhesion
  • FIG. 6B shows a photograph of an interface between a base iron and scale inferior in scale adhesion. Since the irregularities contribute to enhancement in scale adhesion at the time of hot stamping, and thus excellent scale adhesion can be ensured by controlling the irregularities within the above described range. Irregularities of less than 0.2 ⁇ m provide an insufficient anchor effect, and provide inferior scale adhesion.
  • irregularities of 8.0 ⁇ m or more scale adhesion is so strong that scale is difficult to remove in the subsequent scale removal process, for example, by shot blast or wet blast, and therefore it is preferable to make the irregularities in the interface between scale and the base iron 8.0 ⁇ m or less.
  • the irregularities are more preferably 6.0 ⁇ m or less, and more preferably 4.0 ⁇ m or less. Note that even if the irregularities exceed 8.0 ⁇ m, excellent scale adhesiveness that is the effect of the present invention can be ensured.
  • the number of irregularities of 0.2 ⁇ m to 8.0 ⁇ m per 100 ⁇ m is less than three, an improvement effect of scale adhesion is not sufficient, and thus the number of irregularities per 100 ⁇ m is set at three or more. It is possible to ensure excellent scale adhesion which is the effect of the present invention without particularly setting an upper limit of the number of irregularities per 100 ⁇ m.
  • the irregularities of the formed body are correlated with the surface roughness Rz of the steel sheet as illustrated in FIG. 7 , and are controllable by setting the steel sheet surface roughness as Rz>2.5 ⁇ m.
  • C represents an element that is contained to enhance the strength of the steel sheet. If a C content is less than 0.100%, tensile strength of 1180 MPa or more cannot be ensured, and a formed body with high strength which is the object of hot stamp cannot be ensured. When the C content exceeds 0.600%, weldability and processibility become insufficient, and thus the C content is set at 0.100% to 0.600%.
  • the C content is preferably 0.100% to 0.550%, and is more preferably 0.150% to 0.500%. However, if the strength of the formed body is not required, excellent scale adhesion can be ensured even if the C content is less than 0.150%.
  • Si enhances scale adhesion by controlling the scale composition at the time of hot stamping, and therefore Si is an essential element. If the Si content is less than 0.50%, the thickness of Fe scale cannot be controlled, and excellent scale adhesion cannot be ensured. Consequently, it is necessary to set the Si content at 0.50% or more. Further, when application to a member which is difficult to form at the time of hot stamping is considered, it is preferable to increase the Si content. Accordingly, the Si content is preferably 0.70% or more, and is more preferably 0.90% or more. Meanwhile, Si increases an Ae3 point, and the heating temperature necessary to make martensite a main phase, and thus if the Si is excessively contained, productivity and economic efficiency are reduced. Hence, an upper limit of the Si content is set as 3.00%. The upper limit of the Si content is preferably 2.5%, and the upper limit is more preferably 2.0%. However, it is possible to ensure excellent scale adhesion excepting productivity and economic efficiency.
  • Mn delays ferrite transformation in a cooling process at the time of hot stamping, and makes a hot stamp formed body into a structure having a martensite main phase, and thus it is necessary to contain 1.20% or more of Mn. If the Mn content is less than 1.20%, martensite cannot be made a main phase, and it is difficult to ensure high strength which is an object of the hot stamp formed body, and thus a lower limit of the Mn content is set as 1.20%. However, if the strength of the formed body is not required, excellent scale adhesion can be ensured even if the Mn content is less than 1.20%.
  • the Mn content is preferably within a range of 1.50% to 3.50%, and is more preferably within a range of 2.00% to 3.00%.
  • Ti is an element that combines with N to form TiN, and thereby restrains B from being a nitride to enhance hardenability. The effect becomes remarkable when a Ti content is 0.005% or more, and thus the Ti content is set as 0.005% or more. However, when the Ti content exceeds 0.100%, a Ti carbide is formed, an amount of C that contributes to strengthening martensite is reduced, and reduction in strength is caused, and thus an upper limit of the Ti content is set as 0.100%.
  • the C content is preferably within a range of 0.005% to 0.080%, and is more preferably within a range of 0.005% to 0.060%.
  • B enhances hardenability at the time of hot stamping, and contributes to making a main phase of martensite.
  • the effect is remarkable when a B content is 0.0005% or more, and thus it is necessary to set the B content at 0.0005% or more.
  • the B content exceeds 0.0100%, the effect is saturated, an iron boride is precipitated, and the effect of hardenability of B is lost, and thus an upper limit of the B content is set at 0.0100%.
  • the B content is preferably within a range of 0.0005% to 0.0080%, and is more preferably within a range of 0.0005% to 0.0050%.
  • an upper limit of a P content is set at 0.100%.
  • a more preferable upper limit is 0.050%.
  • an upper limit of an S content is set at 0.0100%.
  • a lower limit of the S content is set at 0.0001% because it is economically disadvantageous from the viewpoint of productivity and cost of dephosphorization.
  • the S content is preferably within a range of 0.0001% to 0.0070%, and is more preferably within a range of 0.0003% to 0.0050%.
  • Al acts as a deoxidizer, and thus an Al content is set as 0.005% or more.
  • an Al content is set as 0.005% or more.
  • the Al content is less than 0.005%, a sufficient deoxidization effect cannot be obtained, and a large amount of enclosure (oxide) exist in the steel sheet. These enclosures become starting points of destruction at the time of hot stamping, and the causes of breakage, and therefore are not preferable.
  • the effect becomes remarkable when the Al content reaches 0.005% or more, and thus it is necessary to set the Al content at 0.005% or more.
  • the Al content exceeds 1.000%, the Ac3 point is increased and a heating temperature at the time of hot stamping is increased.
  • hot stamp is a technique of obtaining a formed body with high strength having a complicated shape by heating a steel sheet to an austenite single phase region, and subjecting the steel sheet to hot die press excellent in formability, and rapidly cooling by using a die.
  • the Ac3 point is significantly increased, increase in the heating temperature required for austenite single phase region heating is caused, and productivity is reduced. Consequently, it is necessary to set an upper limit of the Al content at 1.000%.
  • the Al content is preferably within a range of 0.005% to 0.500%, and is more preferably within a range of 0.005% to 0.300%.
  • N is an element that forms coarse nitrides and deteriorates bendability and hole-expandability.
  • an N content exceeds 0.0100%, bendability and hole-expandability are significantly deteriorated, and thus an upper limit of the N content is set at 0.0100%.
  • the N content is preferably 0.0070 or less, and is more preferably 0.0050% or less.
  • the N content is more preferably 0.0005% or more.
  • 0 forms an oxide and exists as an enclosure.
  • the steel sheet of the present invention further contains the following elements in accordance with necessity.
  • Ni, Cu, Cr and Mo are elements that contribute to increase in strength by enhancing hardenability at the time of hot stamping, and making a main phase of martensite.
  • the effect becomes remarkable by containing 0.01% or more of each one kind or two or more kinds selected from a group consisting of Ni, Cu, Cr and Mo, and thus contents of the elements are preferably 0.01% respectively.
  • contents of the elements are preferably set at 2.00%.
  • V 0.005 to 0.100%
  • Nb, V and W are elements that strengthen fine grains by inhibiting growth of austenite at the time of hot stamping, and contribute to increase in strength and enhancement in tenacity.
  • one kind or two or more kinds selected from a group consisting of these elements may be contained.
  • the effect becomes more remarkable when 0.005% or more of each of the elements are contained, and thus it is preferable that 0.005% or more of each of the elements be contained.
  • Nb, V and W carbides are formed, an amount of C that contributes to strengthening martensite is reduced, and reduction in strength is caused.
  • Each of the elements is preferably in a range of 0.005% to 0.090%.
  • 0.0003% to 0.0300% of one kind or two or more kinds selected from a group consisting of REM, Ca, Ce and Mg may be further contained in total.
  • REM, Ca, Ce and Mg are elements that enhance strength and contribute to improvement of the material.
  • the total of one kind or two or more kinds selected from the group consisting of REM, Ca, Ce and Mg is less than 0.0003%, a sufficient effect cannot be obtained, and thus it is preferable to set a lower limit of the total at 0.0003%.
  • the total of one kind or two or more kinds selected from the group consisting of REM, Ca, Ce and Mg exceeds 0.0300%, castability and hot workability are likely to be deteriorated, and thus it is preferable to set an upper limit of the total at 0.0300%.
  • REM is an abbreviation of Rare Earth Metal, and refers to an element belonging to a lanthanoid system. In the present invention, REM is often added in misch metal, and besides Ce, elements of a lanthanoid system are sometimes contained in combination.
  • the effect of the present invention becomes apparent even when elements of a lanthanoid system other than La and Ce are contained as unavoidable impurities, and the effect of the present invention becomes apparent even when the other elements such as metals are contained as impurities.
  • the effect of the present invention can be exhibited by any of a pickled hot-rolled steel sheet, a cold-rolled steel sheet obtained by cold-rolling a hot-rolled steel sheet, or a cold-rolled steel sheet to which annealing is applied after cold rolling.
  • steel sheets are heated to an austenite region exceeding 800° C. at the time of hot stamping, and therefore exhibit performance as steel sheets for hot stamping having excellent scale adhesion that is the effect of the present invention without particularly limiting the microstructure.
  • the strength of the steel sheets is preferably as low as possible in order to reduce wear and tear of dies, cutting edges of cutters, or punching dies. Consequently, the microstructure of the steel sheet for hot stamping is preferably ferrite and pearlite structures, or a bainite structure and a structure obtained by tempering martensite.
  • the microstructure of the formed body preferably has a martensite main phase.
  • a volume ratio of martensite that is a main phase is preferably made 60% or more. Martensite may be subjected to tempering after hot stamping, and made tempered martensite. As the structure other than martensite, bainite, ferrite, pearlite, cementite and retained austenite may be contained. Further, even if the martensite volume rate is less than 60%, it is possible to ensure the excellent scale adhesion of the present invention.
  • the following methods are used in identification of the microstructures (tempered martensite, martensite, bainite, ferrite, pearlite, retained austenite and a remaining structure) composing the steel sheet structure, confirmation of existence positions, and measurement of area ratios.
  • the present inventors determined the sheet thickness section parallel with the rolling direction of the steel sheet as an observation surface, extracted a specimen, polished the observation surface, performed nital etching, observed a range of thickness of 1 ⁇ 8 to 3 ⁇ 8 with 1 ⁇ 4 of the sheet thickness as a center with a field emission scanning electron microscope (FE-SEM: Field Emission Scanning Electron Microscope), measured an area fraction, and the area fraction was taken as a volume fraction.
  • FE-SEM Field Emission Scanning Electron Microscope
  • a slab having the same component composition as the component composition of the aforementioned steel sheet is cast first.
  • a continuously cast slab the slab produced by a thin slab caster or the like can be used.
  • the method for manufacturing the steel sheet of the present invention is adapted to a process like continuous casting-direct rolling (CC-DR) that performs hot rolling immediately after casting.
  • the slab heating temperature is preferably set at 1100° C. or higher.
  • the slab heating temperature in a temperature region of lower than 1100° C. causes reduction in the finishing rolling temperature, and thus strength at the time of finishing rolling tends to be high. As a result, there is the possibility that rolling becomes difficult, a poor shape of the steel sheet after rolling is caused, and thus the slab heating temperature is preferably set at 1100° C. or higher.
  • the finishing rolling temperature is preferably set at the Ar3 transformation point or higher.
  • a rolling load becomes high, and there is the possibility that rolling becomes difficult, and a poor shape of the steel sheet after rolling is caused, and thus a lower limit of the finishing rolling temperature is preferably set at the Ar3 transformation point.
  • An upper limit of the finishing rolling temperature does not have to be particularly set, but if the finishing rolling temperature is set to be excessively high, the slab heating temperature has to be made excessively high in order to ensure the temperature, and thus the upper limit of the finishing rolling temperature is preferably 1100° C.
  • the coiling temperature is preferably set at 700° C. or lower.
  • the coiling temperature exceeds 700° C., the thickness of the oxides formed on the steel sheet surface is excessively increased, and the pickling property is deteriorated, and thus the coiling temperature higher than 700° C. is not preferable.
  • a lower limit of the coiling temperature is preferably set at 400° C.
  • the coiling temperature is lower than 400° C., the strength of the hot-rolled steel sheet extremely increases, and a sheet fracture and a poor shape at the time of cold rolling are easily caused, and thus the lower limit of the coiling temperature is preferably set at 400° C.
  • the hot-rolled steel sheet which is coiled is intended to be softened by heating the coiled hot-rolled steel sheet in the box type annealing furnace or the continuous annealing facility, the steel sheet may be coiled at a low temperature of lower than 400° C. Note that at the time of hot-rolling, rough-rolled sheets may be bonded to one another and finishing rolling may be continuously performed. Further, the rough-rolled sheet may be coiled temporarily.
  • pickling is applied to the hot-rolled steel sheet which is produced in this way for 30 seconds or more in an aqueous solution with an temperature of 80° C. to 100° C. in which a concentration of acid is 3 mass % to 20 mass % and an inhibitor is included.
  • pickling under the present conditions is extremely important, and in order to control the surface roughness Rz of the steel sheet to more than 2.5 ⁇ m, pickling under the above described conditions is necessary.
  • an aqueous solution of a hydrochloric acid, a sulfuric acid or the like as an acid is generally used, and an aqua regia or the like may be used.
  • the temperature of the aqueous solution is set at 80° C. to lower than 100° C., because with a temperature lower than 80° C., a reaction rate is low, and it takes a long time to bring the surface roughness of the hot-rolled steel sheet into a proper range. Meanwhile, heating at a temperature of 100° C. or higher is dangerous and is not preferable because the solution boils and splashes although the reaction of pickling has no problem.
  • the reason why the concentration of the acid is set at 3 mass % to 20 mass % is to control the surface roughness Rz of the hot-rolled steel sheet within the proper range.
  • the concentration of the acid is less than 3 mass %, it takes a long time to control the irregularities on the surface by pickling.
  • the concentration of the acid exceeds 20 mass %, a pickling tank is damaged significantly and facility management becomes difficult, and thus it is not preferable.
  • a preferable range of the concentration of the acid is a range of 5 mass % to 15 mass %.
  • the reason why the pickling time period is set at 30 seconds or more is to stably give predetermined irregularities (irregularities of Rz>2.5 ⁇ m) to the steel sheet surface by pickling.
  • the pickling tank is divided into a plurality of tanks, if a pickling time period of some of the pickling tanks or a total pickling time period satisfies the above described conditions, the surface roughness Rz of the hot-rolled steel sheet can be brought into the range of the present invention, even if concentrations or temperatures of the individual pickling tanks differ from one another. Further, pickling may be carried out by being divided into a plurality of times.
  • hydrochloric acid including an inhibitor was used, but the effect of the present invention can be obtained by using another acid such as hydrochloric acid using no inhibitor, a sulfuric acid, and a nitric acid, or a composite of these acids, as long as the surface roughness Rz can be controlled by pickling.
  • the irregularities formed by pickling of the hot-rolled steel sheet also remain even after temper rolling, cold rolling or annealing is carried out, and thus it is extremely important to control the pickling conditions, and give irregularities to the sheet surface after pickling. Consequently, temper rolling may be carried out to the hot-rolled steel sheet after pickling.
  • cold rolling is preferably performed with roll roughness Rz for cold rolling within a range of 1.0 ⁇ m to 20.0 ⁇ m, and the cold rolling roll also includes temper rolling roll.
  • Cold rolling is applied to the hot-rolled steel sheet pickled under the conditions as above at a draft of 30% to 80%, and the steel sheet may be passed through a continuous annealing facility.
  • a draft is less than 30%, it becomes difficult to keep the shape of the steel sheet flat, and ductility of the finished product deteriorates, and thus a lower limit of the draft is preferably set at 30%.
  • a rolling load becomes excessively large, and cold rolling becomes difficult, and thus an upper limit of the draft is preferably set at 80%.
  • the draft is more preferably 40% to 70%. The effect of the present invention becomes apparent even without particularly specifying the number of times of rolling pass and the draft of each pass, and thus it is not necessary to specify the number of times of rolling pass, and the draft at each pass.
  • the cold-rolled steel sheet may be passed through the continuous annealing line.
  • An object of the treatment is to soften the steel sheet which is highly strengthened by cold-rolling, and thus any conditions may be adopted as long as the condition is such that the steel sheet is softened.
  • the annealing temperature is in a range of 550° C. to 750° C.
  • dislocation introduced at the time of cold rolling is released by recovery, recrystalization, or phase transformation, and thus annealing is preferably performed in this temperature region.
  • the steel sheet for hot stamping excellent in scale adhesion of the present invention can be obtained.
  • oil coating is carried out.
  • electrostatic oiling, spray, a roll coater and the like are generally used, and as long as a coating oil amount in a range of 50 mg/m 2 to 1500 mg/m 2 can be ensured, the method is not limited.
  • coating of a predetermined amount of oil was carried out by an electrostatic oiling machine. Further, as long as the coating oil amount in the range of 50 mg/m 2 to 1500 mg/m 2 can be ensured, a rust inhibitor in an amount equal to or larger than the coating oil amount may be applied, and degreasing may be performed.
  • the excellent scale adhesion that is the effect of the present invention and a rust inhibition property can be made compatible without particularly limiting the hot stamping conditions.
  • compatibility of excellent performance of the tensile strength of 1180 MPa or more and productivity is achieved.
  • heating is preferably performed to a temperature region of 800° C. to 1100° C. at a heating rate of 2° C./second or more.
  • the heating rate is preferably 5° C./second or more, and is more preferably 10° C./second or more. Further, increase of the heating rate is also effective for the purpose of enhancing productivity.
  • the annealing temperature at the time of performing hot stamping is preferably within the range of 800° C. to 1100° C. By performing annealing in this temperature region, it is possible to make the structure into an austenite single phase structure, and the structure can be made into a structure having martensite as a main phase by cooling that is performed subsequently.
  • the annealing temperature at this time is lower than 800° C.
  • the structure at the time of annealing is made into a ferrite and austenite structures, the ferrite grows in the cooling process, the ferrite volume ratio exceeds 10%, and the tensile strength of the hot stamp formed body becomes lower than 1180 MPa. Consequently, a lower limit of the annealing temperature is preferably set at 800° C.
  • the annealing temperature exceeds 1100° C., not only the effect is saturated, but also the scale thickness is significantly increased, and there arises the fear that scale adhesion is reduced. Consequently, it is preferable to perform annealing at 1100° C. or lower.
  • the annealing temperature is more preferably in a range of 830° C. to 1050° C.
  • retention After heating, retention may be performed in the temperature region of 800° C. to 1100° C.
  • melting of carbides included in the steel sheet is possible, and contribution is made to increase in the strength of the steel sheet and enhancement in hardenability.
  • Retention includes residence, heating removal and cooling removal in the present temperature region. Since the object is to melt the carbides, the object is achieved as long as the residence time period in the present temperature region is ensured.
  • the limitation on the retention time period is not particularly provided, 1000 seconds is preferably set as an upper limit, because when the retention time period is 1000 seconds or more, the scale thickness becomes excessively large, and scale adhesion is deteriorated.
  • a temperature of 800° C. to 700° C. is preferably reduced at an average cooling rate of 5° C./second or more.
  • 700° C. is a die cooling start temperature
  • the reason why the temperature of 800° C. to 700° C. is reduced at 5° C./second or more is to avoid ferrite transformation, bainite transformation and pearlite transformation, and make the structure into a martensite main phase.
  • the cooling rate is less than 5° C./second, these soft structures are formed, and it is difficult to ensure the tensile strength of 1180 MPa or more. Meanwhile, the effect of the present invention is exhibited without particularly setting the upper limit of the cooling rate.
  • the effect of the present invention can be exhibited without particularly limiting the cooling method. That is, the effect of the present invention can be exhibited by either one of cooling using a die or die cooling using water cooling in combination.
  • the finished sheet thickness of the hot-rolled steel sheet provided for hot stamping as the hot-rolled steel sheet was made 1.6 mm.
  • the sheet thickness of the hot-rolled steel sheet provided for cold rolling was made 3.2 mm.
  • pickling was carried out under the conditions in Table 2 thereafter, and cold rolling was performed, the sheet thickness was made 50% (3.2 mm ⁇ 1.6 mm).
  • annealing was performed for some of the steel sheets in a continuous annealing facility, and the steel sheets were made into cold-rolled steel sheets.
  • NOX-RUST503F made by PARKER INDUSTRIES, INC.
  • NOX503F made by PARKER INDUSTRIES, INC.
  • the steel sheets were cut into a predetermined size, after which, electrical heating was performed to 900° C. at 50° C./second, retention for seconds at 900° C. was carried out, thereafter, standing to cool for 10 seconds was performed, and hardening was performed in the above described hot shallow drawing dies at a temperature of 650° C. or higher. Visual observation of the obtained hot stamp formed bodies was performed, and the steel sheets without detachment of scale were determined as the steel sheets excellent in scale adhesion.
  • *2 means that Mn was excessively high, many fractures occurred in casting and hot rolling time, and no hot-rolled steel sheet was able to be produced.
  • *3 means that at the time of hot stamping, a fracture with the enclosure as the starting point occurred, and the tensile test was not be able to be carried out with the formed
  • the tensile test pieces which were in conformity with JIS Z 2201 were extracted, the tensile test was performed in conformity with JIS Z 2241, and the maximum tensile strength was measured.
  • the formed bodies having the maximum tensile strength of 1180 MPa or more were determined as the formed bodies of the present invention.
  • Composition analyses of the scales of the formed bodies were carried out by X-ray diffraction by cutting out sheets from the bottoms of the cylindrical portions of the shallow drawing test pieces. From the peak strength ratios of the respective oxides, the volume ratios of the respective Fe oxides were measured. The Si oxides were present very thinly and the volume ratio was less than 1%, and thus quantitative evaluation by X-ray diffraction was difficult. However, it could be confirmed that the Si oxides were present in the interface between the scale and the base iron by the line analysis of EPMA.
  • the formed bodies satisfying the conditions of the present invention were able to make excellent rust inhibition properties and excellent scale adhesion compatible.
  • the formed bodies that do not satisfy the conditions of the invention were inferior in scale adhesion, or inferior in corrosion resistance.
  • the steel sheet excellent in scale adhesion at the time of hot stamping can be provided, the problems of wear and tear of the die at the time of hot stamping, plating adhesion to the die, and indentation flaws accompanying it can be solved, and thus the present invention can bring about significant enhancement in productivity, and has an industrially large value.

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EP3760755A4 (fr) * 2018-03-27 2021-10-27 Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.) Tôle d'acier pour estampage à chaud
CN113924379A (zh) * 2019-07-02 2022-01-11 日本制铁株式会社 热压用镀锌钢板、热压用镀锌钢板的制造方法和热压成形体
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CA2979978A1 (fr) 2016-10-06
JP6515356B2 (ja) 2019-05-22
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EP3278895A4 (fr) 2018-09-05
TW201702403A (zh) 2017-01-16
BR112017020165A2 (pt) 2018-06-05
MX2017012377A (es) 2017-12-14
WO2016158961A1 (fr) 2016-10-06
RU2683397C1 (ru) 2019-03-28
EP3278895A1 (fr) 2018-02-07
TWI597370B (zh) 2017-09-01
EP3278895B1 (fr) 2020-03-11
KR102000863B1 (ko) 2019-07-16

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