US20170306434A1 - High-carbon steel sheet and method of manufacturing the same - Google Patents

High-carbon steel sheet and method of manufacturing the same Download PDF

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US20170306434A1
US20170306434A1 US15/513,130 US201415513130A US2017306434A1 US 20170306434 A1 US20170306434 A1 US 20170306434A1 US 201415513130 A US201415513130 A US 201415513130A US 2017306434 A1 US2017306434 A1 US 2017306434A1
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steel sheet
content
cementite
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Kengo Takeda
Toshimasa Tomokiyo
Yasushi Tsukano
Takashi Aramaki
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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 CORPORATION reassignment NIPPON STEEL CORPORATION CHANGE OF NAME (SEE DOCUMENT FOR DETAILS). Assignors: NIPPON STEEL & SUMITOMO METAL CORPORATION
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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    • C21D6/00—Heat treatment of ferrous alloys
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    • C21D6/00—Heat treatment of ferrous alloys
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    • C21D6/00—Heat treatment of ferrous alloys
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    • 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
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
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    • 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
    • C—CHEMISTRY; METALLURGY
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • 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
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • 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
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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    • 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
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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    • 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
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
    • C22C38/40—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
    • 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
    • C23G1/081—Iron or steel solutions containing H2SO4
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/003—Cementite
    • C—CHEMISTRY; METALLURGY
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite

Definitions

  • the present invention relates to a high-carbon steel sheet with improved formability and a method of manufacturing the same.
  • a high-carbon steel sheet is used for various steel products, which are a driving system component for automobile such as a chain, a gear and a clutch, a saw, a knife, and others.
  • a driving system component for automobile such as a chain, a gear and a clutch, a saw, a knife, and others.
  • forming and heat treatments of a high-carbon steel sheet are performed.
  • a strength of a high-carbon steel sheet is higher than that of a mild steel sheet, and therefore a metal mold used for forming of a high-carbon steel sheet is more easily worn than a metal mold used for forming of a mild steel sheet. Further, a high-carbon steel sheet cracks more easily than a mild steel sheet during forming.
  • Patent Literatures 1 to 5 For suppressing the wearing of a metal mold, improving lubricity on a surface of a high-carbon steel sheet is effective, and for suppressing the cracking during forming, softening of a high-carbon steel sheet is effective.
  • Patent Literatures 1 to 5 Some techniques have been proposed aiming at an improvement in lubricity and softening.
  • Patent Literature 6 Although a carbon steel sheet aiming at an improvement in punchability has been described in Patent Literature 6 and a high-carbon steel sheet aiming at an improvement in formability has been described in Patent Literature 7, it is not possible for them to obtain sufficient formability.
  • Patent Literature 1 Japanese Laid-open Patent Publication No. 2010-174252
  • Patent literature 2 Japanese Laid-open Patent Publication No, 2009-215612
  • Patent Literature 3 Japanese Laid-open Patent Publication No. 2011-168842
  • Patent Literature 4 Japanese Laid-open Patent Publication No. 2010-255066
  • Patent Literature 5 Japanese Laid-open Patent Publication No, 2000-34542
  • Patent Literature 6 Japanese Laid-open Patent Publication No. 2000-265240
  • Patent Literature 7 Japanese Laid-open Patent Publication No. 10-147816
  • An object of the present invention is to provide a high-carbon steel sheet capable of obtaining excellent formability while avoiding a significant increase in cost, and a method of manufacturing the same.
  • the present inventors conducted earnest studies repeatedly to solve the above-described problem, and consequently found out that it is important that a high-carbon steel sheet contains a specific amount of B, that a coefficient of micro-friction of ferrite on a surface is a specific one, and that form of cementite is a specific one. Further, it was also found out that, in order to manufacture such a high-carbon steel sheet, it is important to perform hot-rolling and annealing under specific conditions while assuming hot-rolling and annealing as what is called a consecutive process. Then, the inventors of the present application devised the following various aspects of the invention based on these findings.
  • a high-carbon steel sheet including:
  • V 0.000% to 0.500%
  • Ta 0.000% to 0.500%
  • a coefficient of micro-friction of ferrite on a surface of the steel sheet is less than 0.5.
  • V 0.001% to 0.500%
  • Ta 0.001% to 0.500%
  • La 0.001% to 0.500%
  • a method of manufacturing a high-carbon steel sheet including:
  • the slab including a chemical composition represented by, in mass %:
  • V 0.000% to 0.500%
  • Ta 0.000% to 0.500%
  • the slab is heated at a temperature of 1000° C. or more and less than 1150° C.
  • a finish rolling temperature is 830° C. or more and 950° C. or less
  • a coiling temperature is 450° C. or more and 700° C. or less
  • the annealing comprises:
  • V 0.001% to 0.500%
  • Ta 0.001% to 0.500%
  • La 0.001% to 0.500%
  • a B content, a coefficient of micro-friction of ferrite on a surface and others are appropriate, thereby making it possible to obtain excellent formability while avoiding a significant increase in cost.
  • FIG. 1 is a chart illustrating a relationship between a coefficient of micro-friction of ferrite and a B content
  • FIG. 2 is a chart illustrating a relationship between a coefficient of micro-friction of ferrite and a number of pressing until a flaw occurs;
  • FIG. 3A is a micrograph showing a surface of a high-carbon steel sheet before measuring a coefficient of micro-friction
  • FIG. 3B is a micrograph showing the surface of the high-carbon steel sheet after measuring the coefficient of micro-friction
  • FIG. 4 is a schematic diagram illustrating changes in temperature from hot-rolling to cooling
  • FIG. 5A is a schematic diagram illustrating a structure at time t A ;
  • FIG. 5B is a schematic diagram illustrating a structure at time t B ;
  • FIG. 5C is a schematic diagram illustrating a structure at time t C ;
  • FIG. 5D is a schematic diagram illustrating a structure at time t D ;
  • FIG. 5E is a schematic diagram illustrating a structure at time t E ;
  • FIG. 6A is a schematic diagram illustrating a structure when a slab heating temperature is high than 1150° C.
  • FIG. 6B is a schematic diagram illustrating a structure when the slab heating temperature is lower than 1000° C.
  • FIG. 6C is a schematic diagram illustrating a structure when an annealing retention temperature is lower than 730° C.
  • FIG. 6D is a schematic diagram illustrating a structure when the annealing retention temperature is higher than 770° C. or an annealing retention is longer than 60 hours;
  • FIG. 6E is a schematic diagram illustrating a structure when the annealing retention is shorter than 3 hours
  • FIG. 6F is a schematic diagram illustrating a structure when a cooling rate is less than 1° C./hr;
  • FIG. 6G is a schematic diagram illustrating a structure when the cooling rate is greater than 60° C./hr.
  • FIG. 7 is a chart illustrating a relationship between a coefficient of micro-friction of ferrite and a B content for a part of inventive examples in a first experiment or a third experiment.
  • the high-carbon steel sheet according to the embodiment and the slab used for manufacturing the same include a chemical composition represented by C: 0.30% to 0.70%, Si: 0.07% to 1.00%, Mn: 0.20% to 3.00%, Ti: 0.010% to 0.500%, Cr: 0.01% to 1.50%, B: 0.0004% to 0.0035%, P: 0.025% or less, Al: 0.100% or less, S: 0.0100% or less, N: 0.010% or less, Cu: 0.500% or less, Nb: 0.000% to 0.500%, Mo: 0.000% to 0.500%, V: 0.000% to 0.500%, W: 0.000% to 0.500%, Ta: 0.000% to 0.500%, Ni: 0.000% to 0.500%, Mg: 0.000% to 0.500%, Ca: 0.000% to 0.500%, Y: 0.000% to 0.500%, Zr: 0.000% to 0.500%, La: 0.000% to 0.500%, Ce: 0.000% to 0.500%, and balance:
  • the impurities ones contained in raw materials such as ore and scrap, and ones contained during a manufacturing process are exemplified.
  • Sn, Sb or As or any combination thereof may mix in by 0.003% or more. If the content is 0.03% or less, none of them hinder the effect of the embodiment, and thus may be tolerated as impurities.
  • O may be tolerated as an impurity up to 0.0025%. O forms oxide, and when oxides aggregate and become coarse, sufficient formability is not obtained. Therefore, the O content is the lower the better. However, it is technically difficult to decrease the O content to less than 0.0001%.
  • the C content is less than 0.30%, the amount of cementite is insufficient, resulting in that sufficient lubricity cannot be obtained and adhesion to a metal mold occurs during forming.
  • the C content is 0.30% or more, and preferably 0.35% or more.
  • the C content is greater than 0.70%, the amount of cementite is excessive, resulting in that a crack originating from the cementite occurs easily during forming.
  • the C content is 0.70% or less, and preferably 0.65% or less.
  • Si operates as a deoxidizes, and is effective for suppressing excessive coarsening of cementite during annealing.
  • the Si content is less than 0.07%, the effect by the above-described operation cannot be obtained sufficiently.
  • the Si content is 0.07% or more, and preferably 0.10% or more.
  • the Si content is greater than 1.00%, the ductility of ferrite is low and a crack originating from transgranular fracture of ferrite occurs easily during forming.
  • the Si content is 1.00% or less, and preferably 0.80% or less.
  • Mn is important for controlling pearlite transformation.
  • the Mn content is less than 0.20%, the effect by the above-described operation cannot be obtained sufficiently. That is, when the Mn content is less than 0.20%, pearlite transformation occurs in cooling after dual-phase annealing and a spheroidized ratio of cementite becomes insufficient.
  • the Mn content is 0.20% or more, and preferably 0.25% or more.
  • the Mn content is greater than 3.00%, the ductility of ferrite is low and a crack originating from transgranular fracture of ferrite occurs easily during forming.
  • the Mn content is 3.00% or less, and preferably 2.00% or less.
  • Ti forms a nitride in molten steel, and effective for preventing formation of BN.
  • the Ti content is 0.010% or more, and preferably 0.040% or more.
  • the Ti content is greater than 0.500%, a crack originating from a coarse oxide of Ti occurs easily during forming. This is because during continuous casting, coarse oxides of Ti are formed to get involved inside the slab.
  • the Ti content is 0.500% or less, and preferably 0.450% or less.
  • Cr has a high affinity with N, effective for suppressing formation of BN, and effective also for controlling pearlite transformation.
  • the Cr content is 0.01% or more, and preferably 0.05% or more.
  • the Cr content is 1.50% or less, and preferably 0.90% or less.
  • B lowers the coefficient of micro-friction of ferrite on the surface of the high-carbon steel sheet.
  • B segregates to and concentrates at an interface between ferrite and cementite during later-described annealing and suppresses peeling at the interface during forming, and B is also effective for preventing a crack.
  • the B content is less than 0.0004%, the effect by the above-described operation cannot be obtained sufficiently.
  • the B content is 0.0004% or more, and preferably 0.0008% or more.
  • the B content is greater than 0.0035%, a crack originating from boride such as carbide of Fe and B occurs easily during forming.
  • the B content is 0.0035% or less, and preferably 0.0030% or less.
  • FIG. 1 is a chart illustrating a relationship between a coefficient of micro-friction of ferrite and a B content.
  • the coefficient of micro-friction of ferrite is significantly low as compared to the case when it is less than 0.0004%. It may be inferred that the reason why wearing of a metal mold can be suppressed as a coefficient of micro-friction of ferrite is lower is because a hard film of B is formed on a surface of a high-carbon steel sheet, as will be described later.
  • the operation that B segregated to and concentrated at an interface between ferrite and cementite improves strength of the interface, suppresses cracking of a high-carbon steel sheet, and suppresses wearing of a metal mold caused by cracking is also a reason for the above.
  • P is not an essential element and is contained as an impurity in the steel sheet, for example. P strongly segregates to the interface between ferrite and cementite, and thereby the segregation of B to the interface is hindered and peeling at the interface is caused. Therefore, the P content is the smaller the better. When the P content is greater than 0.025%, adverse effects are particularly prominent. Thus, the P content is 0.025% or less. Decreasing the P content takes refining cost, and it requires a considerable refining cost to decrease the P content to less than 0.0001%. Thus, the P content may be 0.0001% or more.
  • Al operates as a deoxidizer in steelmaking and is effective for fixing N, but is not an essential element of the high-carbon steel sheet and is contained as an impurity in the steel sheet, for example.
  • the Al content is set to 0.100% or less.
  • fixation of N sometimes may be insufficient.
  • the Al content may be 0.001% or more.
  • S is not an essential element and is contained as an impurity in the steel sheet, for example.
  • S forms coarse non-metal inclusions such as MnS to impair formability. Therefore, the S content is the smaller the better.
  • the S content is greater than 0.0100%, adverse effects are particularly prominent. Thus, the S content is 0.0100% or less. Decreasing the S content takes refining cost, and it requires a considerable refining cost to decrease the S content to less than 0.0001%. Thus, the S content may be 0.0001% or more.
  • N is not an essential element and is contained as an impurity in the steel sheet, for example. N lowers an amount of solid-solution B due to formation of BN so as to cause adhesion to the metal mold, cracking during forming, and the like. Therefore, the N content is the smaller the better. When the N content is greater than 0.010%, adverse effects are particularly prominent. Thus, the N content is set to 0.010% or less. Decreasing the N content takes refining cost, and it requires a considerable refining cost to decrease the N content to less than 0.001%. Thus, the N content may be 0.001% or more.
  • Cu is not an essential element and is mixed from scrap or the like to be contained as an impurity in the steel sheet, for example.
  • Cu causes an increase in strength and brittleness in hot working. Therefore, the Cu content is the smaller the better.
  • the Cu content is greater than 0.500%, adverse effects are particularly prominent.
  • the Cu content is 0.500% or less. Decreasing the Cu content takes refining cost, and it requires a considerable refining cost to decrease the Cu content to less than 0.001%. Thus, the Cu content may be 0.001% or more.
  • Nb, Mo, V, W, Ta, Ni, Mg, Ca, Y, Zr, La, and Ce are not essential elements, and are optional elements that may be appropriately contained in the high-carbon steel sheet and the slab up to a specific amount.
  • Nb forms a nitride and is effective for suppressing formation of BN.
  • Nb may be contained.
  • the Nb content is greater than 0.500%, the ductility of ferrite is low to make it impossible to obtain sufficient formability.
  • the Nb content is 0.500% or less.
  • the Nb content is preferably 0.001% or more.
  • Mo is effective for improving hardenability.
  • Mo may be contained.
  • the Mo content is greater than 0.500%, the ductility of ferrite is low to make it impossible to obtain sufficient formability.
  • the Mo content is 0.500% or less.
  • the Mo content is preferably 0.001% or more.
  • V forms a nitride and is effective for suppressing formation of BN similarly to Nb.
  • V may be contained.
  • the V content is greater than 0.500%, the ductility of ferrite is low to make it impossible to obtain sufficient formability.
  • the V content is 0.500% or less.
  • the V content is preferably 0.001% or more.
  • W is effective for improving hardenability similarly to Mo.
  • W may be contained.
  • the W content is 0.500% or less.
  • the W content is preferably 0.001% or more.
  • Ta forms a nitride and is effective for suppressing formation of BN similarly to Nb and V.
  • Ta may be contained.
  • the Ta content is greater than 0.500%, the ductility of ferrite is low to make it impossible to obtain sufficient formability.
  • the Ta content is 0.500% or less.
  • the Ta content is preferably 0.001% or more.
  • Ni is effective for improving toughness and improving hardenability.
  • Ni may be contained.
  • the Ni content is greater than 0.500%, the coefficient of micro-friction of ferrite is high to cause adhesion to the metal mold easily.
  • the Ni content is 0.500% or less.
  • the Ni content is preferably 0.001% or more.
  • Mg is effective for controlling the form of sulfide.
  • Mg may be contained.
  • Mg forms oxide easily, and when the Mq content is greater than 0.500%, sufficient formability cannot be obtained due to a crack originating from the oxide.
  • the Mg content is 0.500% or less.
  • the Mg content is preferably 0.001% or more.
  • Ca is effective for controlling the form of sulfide similarly to Mg.
  • Ca may be contained.
  • Ca forms oxide easily, and when the Ca content is greater than 0.500%, sufficient formability cannot be obtained due to a crack originating from the oxide.
  • the Ca content is 0.500% or less.
  • the Ca content is preferably 0.001% or more.
  • Y is effective for controlling the form of sulfide similarly to Mg and Ca.
  • Y may be contained.
  • Y forms oxide easily, and when the Y content is greater than 0.500%, sufficient formability cannot be obtained due to a crack originating from the oxide.
  • the Y content is 0.500% or less.
  • the Y content is preferably 0.001% or more.
  • Zr is effective for controlling the form of sulfide similarly to Mg, Ca, and Y.
  • Zr may be contained.
  • Zn forms oxide easily, and when the Zr content is greater than 0.500%, sufficient formability cannot be obtained due to a crack originating from the oxide.
  • the Zr content is 0.500% or less.
  • the Zr content is preferably 0.001% or more.
  • La is effective for controlling the form of sulfide similarly to Mg, Ca, Y, and Zr.
  • La may be contained.
  • La forms oxide easily, and when the La content is greater than 0.500%, sufficient formability cannot be obtained due to a crack originating from the oxide.
  • the La content is 0.500% or less.
  • the La content is preferably 0.001% or more.
  • Ce is effective for controlling the form of sulfide similarly to Mg, Ca, Y, Zr, and La. Thus, Ce may be contained. However, Ce forms oxide easily, and when the Ce content is greater than 0.500%, sufficient formability cannot be obtained due to a crack originating from the oxide. Thus, the Ce content is 0.500% or less. In order to securely obtain the effect by the above-described operation, the Ce content is preferably 0.001% or more.
  • Nb, Mo, V, W, Ta, Ni, Mg, Ca, Y, Zr, La and Ce are optional elements, and it is preferred that “Nb: 0.001% to 0.500%,” “Mo: 0.001% to 0.500%,” “V: 0.001% to 0.500%,” “W: 0.001% to 0.500%,” “Ta: 0.001% to 0.500%,” “Ni: 0.001% to 0.500%,” “Mg: 0.001% to 0.500%,” “Ca: 0.001% to 0.500%,” “Y: 0.001% to 0.500%,” “Zr: 0.001% to 0.500%,” “La: 0.001% to 0.500%,” or “Ce: 0.001% to 0.500%,” or any combination thereof be satisfied.
  • the coefficient of micro-friction of ferrite on the surface of the high-carbon steel sheet according to the embodiment is less than 0.5.
  • the coefficient of micro-friction of ferrite on the surface closely relates to adhesion of the high-carbon steel sheet to the metal mold during forming.
  • the coefficient of micro-friction of ferrite is 0.5 or more, micro-adhesion occurs between the high-carbon steel sheet and the metal mold during forming using the metal mold.
  • the coefficient of micro-friction of ferrite is less than 0.5. From the viewpoint of formability, the coefficient of micro-friction is the lower the better.
  • the coefficient of micro-friction often tends to be 0.35 or more, though it depends on a method of manufacturing the high-carbon steel sheet and others.
  • FIG. 2 is a chart illustrating a relationship between a coefficient of micro-friction of ferrite and a number of pressing (shots) until a flaw occurs on a metal mold or a high-carbon steel sheet in punch forming of high-carbon steel sheets. As illustrated in FIG. 2 , when the coefficient of micro-friction is less than 0.5, the number of pressing until a flaw occurs is significantly high as compared to the case when it is 0.5 or more.
  • a coefficient of micro-friction may be measured using a nanoindenter. That is, a kinetic friction force F to occur when a diamond indenter loads a normal load P of 10 ⁇ N onto a surface of a high-carbon steel sheet and is moved horizontally is obtained. A moving speed then is 1 ⁇ m/second, for example.
  • a coefficient of micro-friction ⁇ (kinetic friction coefficient) is calculated by Expression (1) below. “TI-900 TriboIndenter” made by Omicron, Inc. may be used as a nanoindenter, for example.
  • FIG. 3A is a micrograph showing a surface of a high-carbon steel sheet before measuring a coefficient of micro-friction
  • FIG. 3B is a micrograph showing the surface of the high-carbon steel sheet after measuring the coefficient of micro-friction.
  • FIG. 3A and FIG. 3B each show an example of a 10 ⁇ m ⁇ 10 ⁇ m visual field.
  • ferrite 31 and cementite 32 exist in the visual field example.
  • measurement flaws 33 caused by horizontal movement of the diamond indenter exist after the measurement.
  • the coefficient of micro-friction of cementite was 0.4 or less.
  • the high-carbon steel sheet according to the embodiment includes a structure represented by a spheroidized ratio of cementite: 80% or more and an average diameter of cementite: 0.3 ⁇ m to 2.2 ⁇ m.
  • the spheroidized ratio of cementite is 80% or more, and preferably 85% or more. From the viewpoint of formability, the spheroidized ratio of cementite is preferred to be as higher as possible, and may be 100%. However, when the spheroidized ratio of cementite is attempted to become 100%, productivity could decrease, and the spheroidized ratio of cementite is preferably 80% or more and less than 100% from the viewpoint of productivity.
  • the average diameter of cementite closely relates to the degree of the stress concentration to cementite.
  • the average diameter of cementite is less than 0.3 ⁇ m, an Orowan loop is formed by dislocation occurred during forming with respect to cementite, and thereby a dislocation density in the vicinity of cementite increases and voids occur.
  • the average diameter of cementite is 0.3 ⁇ m or more, and preferably 0.5 ⁇ m or more.
  • the average diameter of cementite is greater than 2.2 ⁇ m, dislocations occurred during forming are accumulated in large amounts, local stress concentration is generated and a crack occurs.
  • the average diameter of cementite is 2.2 ⁇ m or less, and preferably 2.0 ⁇ m or less.
  • the spheroidized ratio and the average diameter of cementite may be measured by structure observation using a scanning electron microscope.
  • an observation surface is mirror finished by wet polishing with an emery paper and polishing with diamond abrasive grains having a size of 1 ⁇ m, then the observation surface is etched with an etching solution of 3 vol % of nitric acid and 97 vol % of alcohol.
  • An observation magnification is between 3000 times to 10000 times, for example, 10000 times, 16 visual fields where 500 or more grains of cementite exist on the observation surface are selected, and structure images of them are taken. Then, an area of each cementite in the structure image is measured by using image processing software.
  • Win ROOF made by MITANI Corporation may be used as an image processing software, for example. Any cementite grain having an area of 0.01 ⁇ m 2 or less is excluded from the target of evaluation in order to suppress an influence of measurement error by noise in the measuring. Then, the average area of cementite as an evaluation target is obtained, and the diameter of a circle with which this average area can be obtained is obtained, thereby taking this diameter as the average diameter of cementite.
  • the average area of cementite is a value obtained by dividing the total area of cementite as the evaluation target by the number of grains of cementite in question.
  • any cementite having a ratio of major axis length to minor axis length of 3 or more is assumed as an acicular cementite
  • any cementite having the ratio of less than 3 is assumed as a spherical cementite grain
  • a value obtained by dividing the number of spherical cementite by the number of all cementite is taken as the spheroidized ratio of cementite.
  • the manufacturing method includes hot-rolling of a slab including the above chemical composition so as to obtain a hot-rolled steel sheet, pickling of the hot-rolled steel sheet, and thereafter annealing of the hot-rolled steel sheet.
  • the slab is heated at a temperature of 1000° C. or more and less than 1150° C.
  • a finish rolling temperature is 830° C. or more and 950° C. or less
  • a coiling temperature is 450° C. or more and 700° C. or less.
  • the hot-rolled steel sheet is retained at a temperature of 730° C. or more and 770° C.
  • An atmosphere of the annealing may be one containing hydrogen by 75 vol % or more at a temperature higher than 400° C., for example, but is not limited to that.
  • FIG. 4 is a schematic diagram illustrating changes in temperature.
  • FIG. 5A to FIG. 55 are schematic diagrams illustrating changes in structure.
  • hot-rolling S 1 includes slab heating S 11 , finish rolling S 12 , and coiling S 13
  • annealing S 3 includes high-temperature retention S 31 and cooling S 32 .
  • Pickling S 2 is performed between the hot-rolling S 1 and the annealing S 3 , and after cooling S 4 is performed the annealing S 3 .
  • B atoms 13 segregate to an interface between austenite 12 and austenite 12 , as illustrated in FIG. 5A .
  • the structure of the steel sheet contains ferrite 11 and the austenite 12 , as illustrated in FIG. 5B .
  • the B atoms 13 segregate to an interface between the ferrite 11 and the austenite 12 .
  • Some of the B atoms 13 are present also on a surface 15 of the steel sheet, and the B atoms 13 present on the surface of the steel sheet are bonded to each other by covalent bonding 14 .
  • cementite is also contained in the structure of the steel sheet and some of the B atoms 13 segregate also to an interface between the ferrite 11 and the cementite.
  • the ratio of the ferrite 11 increases and the ratio of the austenite 12 decreases as compared to the structure illustrated in FIG. 5B , as illustrated in FIG. 5C , and the interface between these two phases moves due to the increasing and decreasing or the ratios.
  • the B atoms 13 present on the surface of the steel sheet increase with the movement of the interface.
  • the ratio of the ferrite 11 increases, the ratio of the austenite 12 decreases, and the B atoms 13 present on the surface of the steel sheet increase as compared to the structure illustrated in FIG. 5C , as illustrated in FIG. 5D .
  • the austenite 12 disappears and the surface 15 of the steel sheet is covered with many of the B atoms 13 , as illustrated in FIG. 5E . Since the B atoms 13 are bonded to each other by the covalent bonding 14 , they are crystallized.
  • the structure illustrated in FIG. 5E does not change also during the cooling S 4 , and is maintained even when the temperature of the steel sheet has reached room temperature, for example, a temperature of less than 600° C.
  • the surface 15 of the steel sheet is covered with many of the B atoms 13 bonded to each other by the covalent bonding 14 , and thereby the coefficient of micro-friction of ferrite on the surface 15 can be less than 0.5.
  • the slab heating temperature is 1150° C. or less, and preferably 1140° C. or less.
  • the slab heating temperature is lower than 1000° C., micro-segregation and/or macro-segregation formed during casting cannot be eliminated, and as illustrated in FIG.
  • the slab heating temperature is 1000° C. or more, and preferably 1030° C. or more.
  • the finish rolling temperature is higher than 950° C.
  • coarse scales are generated until completion of coiling on a run out table (ROT), for example.
  • the coarse scales can be removed by pickling, but traces of large irregularities are left, resulting in that adhesion to the metal mold sometimes occurs during forming due to the traces. Further, when coarse scales are generated, irregular flaw is caused on the surface of the steel sheet in the coiling, resulting in that due to the flaw, adhesion to the metal mold sometimes occurs during forming.
  • the finish rolling temperature is 950° C. or less, and preferably 940° C. or less.
  • the finish rolling temperature is lower than 830° C.
  • adhesiveness of scales generated until completion of coiling to the steel sheet is extremely high, thus making it difficult to remove the scales by pickling.
  • the scales may be removed by performing strong pickling, but the strong pickling makes the surface of the steel sheet rough, resulting in that adhesion to the metal mold sometimes occurs during forming.
  • the finish rolling temperature is lower than 830° C.
  • recrystallization of austenite is not completed by the coiling, so that anisotropy of the hot-rolled steel sheet increases. The anisotropy of the hot-rolled steel sheet is carried over even after annealing, and thus sufficient formability cannot be obtained.
  • the finish rolling temperature is 830° C. or more, and preferably 840° C. or more.
  • the coiling temperature is higher than 700° C.
  • coarse lamellar pearlite is formed in the hot-rolled steel sheet to hinder spheroidizing of cementite during annealing, resulting in that the spheroidized ratio of 80% or more cannot be obtained.
  • the coiling temperature is 700° C. or less.
  • the coiling temperature is higher than 570° C., coarse scales are generated until completion of coiling. Therefore, adhesion to the metal mold sometimes occurs during forming for a reason similar to the case where the finish rolling temperature is higher than 950° C.
  • the coiling temperature is preferably 570° C. or less, and further preferably 550° C. or less.
  • the coiling temperature is lower than 450° C.
  • adhesiveness of scales generated until completion of coiling to the steel sheet is extremely high, thus making it difficult to remove the scales by pickling.
  • the scales may be removed by performing strong pickling, but the strong pickling makes the surface of the steel sheet rough, resulting in that adhesion to the metal mold sometimes occurs during forming.
  • the coiling temperature is lower than 450° C.
  • the hot-rolled steel sheet becomes brittle and the hot-rolled steel sheet may crack when a coil is uncoiled in pickling, resulting in that a sufficient yield cannot be obtained.
  • the coiling temperature is 450° C. or more, and preferably 460° C. or more.
  • a rough-rolled bar may be heated near an inlet of a finishing mill in order to ensure qualities in a longitudinal direction and a width direction of a hot-rolled coil obtained by coiling (to reduce variation of quality or the like).
  • An apparatus to be used for the heating and a method of the heating are not limited in particular, but heating by high-frequency induction heating is desirably performed.
  • a preferred temperature range of the heated rough-rolled bar is between 850° C. and 1100° C. Temperatures less than 850° C.
  • the heating temperature is preferably 850° C. or more. Increasing the temperature of the rough-rolled bar to temperature higher than 1100° C. takes excessive time, and the productivity decreases. Therefore, if rough-rolled bar is heated, the heating temperature is preferably 1100° C. or less.
  • the austenite 12 is not formed sufficiently, and as illustrated in FIG. 6C , although a large number of interfaces between the ferrite 11 and the ferrite 11 exist, sites where the B atom 13 segregates are insufficient. Therefore, even though a process thereafter is performed appropriately, a good surface covered with crystals of B cannot be obtained, resulting in that the coefficient of micro-friction of ferrite on the surface cannot be less than 0.5.
  • the annealing retention temperature is 730° C. or more, and preferably 735° C. or more.
  • the annealing retention temperature is higher than 770° C., as illustrated in FIG. 6D , the B atoms 13 concentrate and coarse crystals of B are formed in the vicinity of the triple point of the ferrite 11 , the austenite 12 , and the surface of the steel sheet.
  • the annealing retention temperature is higher than 770° C., thermal expansion of the hot-rolled steel sheet coiled in a coil shape is large, and the hot-rolled steel sheet itself sometimes rubs together during annealing to cause abrasions on the surface. The appearance of the surface is impaired and the yield is decreased by the abrasions.
  • the annealing retention temperature is 770° C. or less, and preferably 765° C. or less.
  • the annealing retention time is less than 3 hours, as illustrated in FIG. 6E , the B atoms 13 do not sufficiently segregate to the interface between the ferrite 11 and the austenite 12 , and therefore, even though a process thereafter is performed appropriately, a good surface covered with crystals of B cannot be obtained, resulting in that the coefficient of micro-friction of ferrite on the surface cannot be less than 0.5.
  • the annealing retention time is less than 3 hours, cementite does not become coarse sufficiently, resulting in that the average diameter of cementite cannot be 0.3 ⁇ m or more.
  • the annealing retention time is 3 hours or more, and preferably 5 hours or more.
  • the annealing retention time is greater than 60 hours, the coefficient of micro-friction of ferrite on the surface cannot be less than 0.5 for a reason similar to the case where the annealing retention temperature is higher than 770° C. Further, when the annealing retention time is greater than 60 hours, cementite becomes coarse excessively, resulting in that the average diameter of cementite cannot be 2.2 ⁇ m or less. Thus, the annealing retention time is 60 hours or less, and preferably 40 hours or less.
  • the cooling rate down to 650° C. is less than 1° C./hr, as illustrated in FIG. 6F , crystals of B are formed excessively during cooling and the crystals of B form a projection on the surface of the high-carbon steel sheet. Once a projection is formed, the thickness of the film of the crystals of B varies greatly, resulting in that adhesion to the metal mold occurs during forming and a flaw occurs on the metal mold. Further, when the cooling rate down to 650° C. is less than 1° C./hr, sufficient productivity cannot be obtained. Thus, the cooling rate down to 650° C. is 1° C./hr or more, and preferably 2° C./hr or more. When the cooling rate down to 650° C.
  • the cooling rate down to 650° C. is 60° C./hr or less, and 50° C./or less.
  • excellent lubricity can be obtained, and therefore it is possible to suppress adhesion of the high-carbon steel sheet to the metal mold and suppress wearing of the metal mold. Further, according to the embodiment, it is also possible to suppress cracking during forming.
  • hot-rolling of a slab (Steel type A to Y, BK) including a chemical composition listed in Table 1 was performed, thereby obtaining a hot-rolled steel sheet having a thickness of 4 mm.
  • the slab heating temperature was 1130° C.
  • the time thereof was 1 hour
  • the finish rolling temperature was 850° C.
  • the coiling temperature was 520° C.
  • cooling was performed down to a temperature of less than 60° C., and pickling using sulfuric acid was performed. Thereafter, annealing of the hot-rolled steel sheet was performed to then obtain a hot-rolled annealed steel sheet.
  • the hot-rolled steel sheet was retained for 15 hours at 750° C., and then was cooled down to 650° C. at a cooling rate of 30° C./hr. Subsequently, cooling was performed down to a temperature of less than 60° C. In this manner, various high-carbon steel sheets were manufactured.
  • Blank fields in Table 1 indicate that the content of the element is less than a detection limit, and the balance is Fe and impurities.
  • the Cr content of Steel type BK may be regarded as 0.00%.
  • An underline in Table 1 indicates that the numeric value is out of the range of the present invention.
  • adhesion suppressive performance evaluation of adhesion suppressive performance and evaluation of crack sensitivity of each of the high-carbon steel sheets were performed as formability evaluation.
  • a draw bead test was performed. That is, an indentation bead with a tip having a 20 mm radius R was pressed against the high-carbon steel sheet with a load of 10 kN and was pulled out. Then, presence or absence of adhesive matter on the tip of the indentation bead was observed, and one with presence of adhesive matter was evaluated as X and one with no presence was evaluated as ⁇ .
  • the presence of adhesive matter in this test indicates that in press forming with several thousands to several tens of thousands of shots, an adhesive matter occurs early on the metal mold to deteriorate formability.
  • a compression test was performed. That is, a cylindrical test piece having a 10 mm diameter and a 4 mm height was cut out from the high-carbon steel sheet so that a height direction of the test piece was parallel to a sheet thickness direction, and the test piece was compressed to 1 mm in height. Then, an appearance observation and a sectional structure observation were performed, and then one in which cracking appeared in the appearance during compression or after compression and one in which a crack of 1 mm or more was present in the sectional structure observation were evaluated as X, and one other than the above was evaluated as ⁇ . Results of them are also listed in Table 2.
  • Sample No. 1 to Sample No. 9 were each within the range of the present invention, thus being able to obtain good adhesion suppressive performance and crack sensitivity.
  • the C content of Steel type W was too high, and thus the amount of cementite was excessive and a crack originating from the cementite occurred during the compression test.
  • the Ti content of Steel type X was too low, and thus BN precipitated, the amount of solid-solution B was insufficient, the coefficient of micro-friction of ferrite was low, and adhesion and cracking during the compression test occurred.
  • the Ti content of Steel type Y was too high, and thus coarse oxides of Ti were formed and a crack originating from the coarse oxide of Ti occurred during the compression test.
  • the Cr content of Steel type BK was too low, and thus BN precipitated, the amount of solid-solution B was insufficient, the coefficient of micro-friction of ferrite was low, and adhesion to the metal mold occurred during forming.
  • hot-rolling of a slab (Steel type Z to BJ) including a chemical composition listed in Table 3 was performed, thereby obtaining a hot-rolled steel sheet having a thickness of 4 mm.
  • the slab heating temperature was 1130° C.
  • the time thereof was 1 hour
  • the finish rolling temperature was 850° C.
  • the coiling temperature was 520° C.
  • cooling was performed down to a temperature of less than 60° C.
  • pickling using sulfuric acid was performed.
  • annealing of the hot-rolled steel sheet was performed to then obtain a hot-rolled annealed steel sheet.
  • the hot-rolled steel sheet was retained for 15 hours at 750° C., and then was cooled down to 650° C. at a cooling rate of 30° C./hr. Subsequently, cooling was performed down to a temperature of less than 60° C. In this manner, various high-carbon steel sheets were manufactured.
  • Blank fields in Table 3 indicate that the content of the element is less than a detection limit, and the balance is Fe and impurities.
  • An underline in Table 3 indicates that the numeric value is out of the range of the present invention.
  • Samples No. 31 to No. 43 were each within the range of the present invention, thus being able to obtain good adhesion suppressive performance and crack sensitivity.
  • the Mn content of Steel type AS was too low, and thus pearlite transformation occurred during cooling in the annealing, the spheroidized ratio of cementite was low, and a crack originating from acicular cementite occurred during the compression test.
  • the Ce content of Steel type AT was too high, and thus coarse oxides of Ce were formed and a crack originating from the coarse oxide of Ce occurred during the compression test.
  • the B content of Steel type AU was too high, and thus boride was formed and a crack originating from the boride occurred during the compression test.
  • the Ni content of Steel type AV was too high, and thus the coefficient of micro-friction of ferrite was high and adhesion occurred.
  • the S content of Steel type BC was too high, and thus coarse sulfides being non-metal inclusions were formed and a crack originating from the coarse sulfide occurred during the compression test.
  • the W content of Steel type BD was too high, and thus the ductility of ferrite was low and a crack originating from transgranular fracture of ferrite occurred during the compression test.
  • the Ti content of Steel type BE was too low, and thus EN precipitated, the amount of solid-solution B was insufficient, the coefficient of micro-friction of ferrite was low, and adhesion and cracking during the compression test occurred.
  • FIG. 1 illustrates the relationship between the coefficient of micro-friction of ferrite and the B content of Samples No. 1 to No. 25 and No. 31 to No. 67 except for Samples No. 11, No. 51, No. 53, and No. 62.
  • the coefficient of micro-friction of ferrite is significantly low as compared to the case when it is less than 0.0004%.
  • Samples No. 72, No. 74, No. 77 to No. 80, No. 82, No. 83, No. 85, and No. 88 to No. 92 were each within the range of the present invention, thus being able to obtain good adhesion suppressive performance and crack sensitivity.
  • Samples No. 103, No. 105, No. 106, No. 108 to No. 111, No. 114 to No. 117, and No. 120 to No. 122 were each also within the range of the present invention, thus being able to obtain good adhesion suppressive performance and crack sensitivity.
  • Samples No. 131, No. 133, No. 134, No. 136, No. 139, No. 141 to No. 143, No. 145, No. 147, No. 148, No. 151, and No. 152 were each also within the range of the present invention, thus being able to obtain good adhesion suppressive performance and crack sensitivity.
  • FIG. 7 illustrates the relationship between the coefficient of micro-friction of ferrite and the B content in the samples out of the examples in the first experiment or third experiment. As illustrated in FIG. 7 , when the B content is 0.0008% or more, the coefficient of micro-friction of ferrite is much lower as compared to the case when it is less than 0.0008%.
  • the present invention may be utilized in, for example, manufacturing industries and application industries of high-carbon steel sheets used for various steel products, such as a driving system component for automobile, a saw, a knife, and others.

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CN108193136B (zh) * 2018-02-09 2019-11-01 天津荣程联合钢铁集团有限公司 一种40Cr热轧圆钢及其生产方法
CN112575242B (zh) 2019-09-27 2022-06-24 宝山钢铁股份有限公司 一种合金结构用钢及其制造方法

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US20020153070A1 (en) * 2000-06-14 2002-10-24 Takaaki Toyooka High carbon steel pipe excellent in cold formability and high frequency hardenability and method for producing the same
US20060081314A1 (en) * 2003-04-16 2006-04-20 Jfe Steel Corporation Steel material with excellent rolling fatigue life and method of producing the same
JP2011208164A (ja) * 2010-03-26 2011-10-20 Nisshin Steel Co Ltd ボロン鋼圧延焼鈍鋼板およびその製造法

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JP2006336063A (ja) * 2005-06-01 2006-12-14 Nippon Steel & Sumikin Stainless Steel Corp 板巻加工性に優れたＣｒ含有鋼板
TWI362424B (en) * 2009-03-27 2012-04-21 Nippon Steel Corp Carbon steel sheet hiving high carburizing quenching property and manufacturing method thereof
JP5312230B2 (ja) * 2009-07-02 2013-10-09 新日鐵住金株式会社 打抜きカエリの小さい軟質高炭素鋼板及びその製造方法
JP5280324B2 (ja) * 2009-09-08 2013-09-04 日新製鋼株式会社 精密打抜き用高炭素鋼板
JP5695856B2 (ja) * 2010-07-20 2015-04-08 日機装株式会社 吸着材の充填方法および充填装置
JP5549640B2 (ja) 2011-05-18 2014-07-16 Ｊｆｅスチール株式会社 高炭素薄鋼板およびその製造方法
JP5594226B2 (ja) * 2011-05-18 2014-09-24 Ｊｆｅスチール株式会社 高炭素薄鋼板およびその製造方法
WO2013035848A1 (ja) * 2011-09-09 2013-03-14 新日鐵住金株式会社 中炭素鋼板、焼き入れ部材およびそれらの製造方法
US9840750B2 (en) * 2011-09-22 2017-12-12 Nippon Steel & Sumitomo Metal Corporation Medium carbon steel sheet for cold working and method for manufacturing the same

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US6027581A (en) * 1996-02-10 2000-02-22 Kawasaki Steel Corporation Cold rolled steel sheet and method of making
US20020153070A1 (en) * 2000-06-14 2002-10-24 Takaaki Toyooka High carbon steel pipe excellent in cold formability and high frequency hardenability and method for producing the same
US20060081314A1 (en) * 2003-04-16 2006-04-20 Jfe Steel Corporation Steel material with excellent rolling fatigue life and method of producing the same
JP2011208164A (ja) * 2010-03-26 2011-10-20 Nisshin Steel Co Ltd ボロン鋼圧延焼鈍鋼板およびその製造法

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EP3208357A1 (de) 2017-08-23
ES2807553T3 (es) 2021-02-23
MX2017004601A (es) 2017-07-10
WO2016059701A1 (ja) 2016-04-21
KR20170052681A (ko) 2017-05-12
PL3208357T3 (pl) 2020-11-02
EP3208357A4 (de) 2018-04-25
CN107075625B (zh) 2019-07-09
KR101919262B1 (ko) 2018-11-15
EP3208357B1 (de) 2020-05-13
BR112017007275A2 (pt) 2017-12-26
JP6388034B2 (ja) 2018-09-12
CN107075625A (zh) 2017-08-18

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