EP4265763A1 - Hochfestes stahlblech mit hervorragender bearbeitbarkeit und verfahren zur herstellung davon - Google Patents

Hochfestes stahlblech mit hervorragender bearbeitbarkeit und verfahren zur herstellung davon Download PDF

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Publication number
EP4265763A1
EP4265763A1 EP21906929.1A EP21906929A EP4265763A1 EP 4265763 A1 EP4265763 A1 EP 4265763A1 EP 21906929 A EP21906929 A EP 21906929A EP 4265763 A1 EP4265763 A1 EP 4265763A1
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Prior art keywords
steel sheet
less
balance
tensile strength
temperature
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EP21906929.1A
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English (en)
French (fr)
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Jae-Hoon Lee
Sang-Ho Han
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Posco Holdings Inc
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Posco Co Ltd
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Publication of EP4265763A1 publication Critical patent/EP4265763A1/de
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Definitions

  • Patent Documents 1 and 2 As a technique for further improving the workability of a steel sheet, a method of utilizing tempered martensite is disclosed in Patent Documents 1 and 2. Since tempered martensite made by tempering hard martensite is softened martensite, there is a difference in strength between tempered martensite and existing untempered martensite (fresh martensite). Therefore, when fresh martensite is suppressed and tempered martensite is formed, the workability may increase.
  • a balance (B TE ) of tensile strength and elongation expressed by the following relational expression 2 satisfies 3.0*10 6 to 6.2*10 6 (MPa 2 % 1/2 )
  • a balance (B TH ) of tensile strength and a hole expansion ratio expressed by the following relational 3 expression satisfies 6.0*10 6 to 11.5*10 6 (MPa 2 % 1/2 )
  • a yield ratio evaluation index (I YR ) expressed by the following relational expression 4 satisfies 0.15 to 0.42:
  • B TE Tensile Strength TS , MPa 2 * Elongation El , % 1 / 2 ;
  • B TH Tensile Strength TS , MPa 2 * Hole Expansion Ratio HER , % 1 / 2 ;
  • I YR 1 ⁇ Yield Ratio YR .
  • a steel sheet that may be used for automobile parts and the like, and to a steel sheet having an excellent balance of tensile strength and ductility, an excellent balance of tensile strength and hole expansion ratio, and an excellent yield ratio evaluation index, and a method for manufacturing the same.
  • the present disclosure relates to a high strength steel sheet having excellent workability and a method for manufacturing the same, and exemplary embodiments of the present disclosure will hereinafter be described. Exemplary embodiments of the present disclosure may be modified into various forms, and it is not to be interpreted that the scope of the present disclosure is limited to exemplary embodiments described below. The present exemplary embodiments are provided in order to further describe the present disclosure in detail to those skilled in the art to which the present disclosure pertains.
  • the inventors of the present disclosure recognized that in a boron(B)-added transformation-induced plasticity (TRIP) steel comprising bainite, tempered martensite, fresh martensite and retained austenite, when the fractions of the tempered martensite, the fresh martensite, and the retained austenite are controlled to be within certain ranges, the contents of the boron (B) contained in the tempered martensite and the fresh martensite are controlled to be within certain ranges, and a shape and a size of the retained austenite are controlled to be within certain ranges, it is possible to simultaneously secure an excellent balance of tensile strength and ductility, an excellent balance of tensile strength and a hole expansion ratio, and an excellent yield ratio evaluation index. Based thereon, the present inventors have conceived of the present disclosure by devising a method such that excellent strength, excellent yield ratio, excellent ductility, and an excellent hole expansion ratio may be simultaneously provided.
  • TRIP transformation-induced plasticity
  • Carbon (C) is an unavoidable element for securing strength of a steel sheet, and is also an element for stabilizing retained austenite that contributes to the improvement in ductility of the steel sheet. Accordingly, in the present disclosure, 0.1% or more of carbon (C) may be added in order to achieve such an effect.
  • a preferable content of carbon (C) may be greater than 0.1%, may be 0.11% or more, and may be 0.12% or more.
  • an upper limit of the content of carbon (C) of the present disclosure may be limited to 0.25%.
  • the content of carbon (C) may be 0.24% or less, and a more preferable content of carbon (C) may be 0.23% or less.
  • Sulfur (S) is an element which is contained as an impurity to form MnS in a steel sheet and deteriorate ductility. Therefore, it is preferable that the content of sulfur (S) is 0.03% or less.
  • the steel sheet of the present disclosure has an alloy composition that may be additionally included in addition to the above-described alloy components, which will be described in detail below.
  • chromium (Cr) and molybdenum (Mo) are elements that not only suppress austenite decomposition during alloying treatment, but also stabilize austenite like manganese (Mn), in the present disclosure, one or more of chromium (Cr) and molybdenum (Mo) may be added in order to achieve such an effect.
  • the present disclosure may limit the contents of chromium (Cr) and molybdenum (Mo) to 3.0% or less, respectively.
  • antimony (Sb) and tin (Sn) are elements that improve plating wettability and plating adhesion of a steel sheet
  • one or more of antimony (Sb) and tin (Sn) may be added in order to achieve such an effect.
  • the present disclosure may limit the contents of antimony (Sb) and tin (Sn) to 0.5% or less, respectively.
  • Y yttrium
  • Hf hafnium
  • yttrium (Y) and hafnium (Hf) are elements that improve corrosion resistance of a steel sheet
  • one or more of the yttrium (Y) and hafnium (Hf) may be added in order to achieve such an effect.
  • the present disclosure may limit the contents of yttrium (Y) and hafnium (Hf) to 0.2% or less, respectively.
  • the present disclosure limits the ratio of the content of boron (B) contained in the fresh martensite to the content of boron (B) contained in the tempered martensite to be in a range of 0.03 to 0.55 as shown in relational expression 1, thereby securing a balance (TS 2 *EL 1/2 ) of tensile strength and elongation, a balance (TS 2 *HER 1/2 ) of tensile strength and a hole expansion ratio, and a yield ratio evaluation index (1-YR) targeted thereby.
  • a method for manufacturing a high strength steel sheet having excellent workability may comprises: heating (primarily heating) a cold-rolled steel sheet having a predetermined alloy composition to a temperature of 700°C at an average heating rate of 5°C/s or more, heating (secondarily heating) the primarily heated steel sheet to a temperature within a range of Ac3 to 920°C at an average heating rate of 5°C/s or less, and then maintaining (primarily maintaining) the secondarily heated steel sheet for 50 to 1200 seconds; cooling (primarily cooling) the primarily maintained steel sheet to a temperature within a range of 200 to 400°C at an average cooling rate of 1°C/s or more; heating (tertiarily heating) the primarily cooled steel sheet to a temperature within a range of 350 to 550°C at an average heating rate of 5°C/s or more, and then maintaining (secondarily maintaining) the tertiarily heated steel sheet for 50 seconds or more; and cooling (secondarily cooling) the secondarily maintained steel sheet to
  • the cold-rolled steel sheet is heated (primarily heated) to a temperature of 700°C at an average heating rate of 5°C/s or more, is heated (secondarily heated) to a temperature within a range of Ac3 to 920°C at an average heating rate of 5°C/s or less, and then is maintained (primarily maintained) for 50 to 1200 seconds.
  • tempered martensite When the primary cooling stop temperature is lower than 200°C, tempered martensite is excessively formed, and retained austenite becomes insufficient, whereby a balance (TS 2 *EL 1/2 ) of tensile strength and elongation and a balance (TS 2 *HER 1/2 ) of tensile strength and a hole expansion ratio of the steel sheet may be lowered.
  • the primary cooling stop temperature exceeds 400°C, bainite is excessively formed, and tempered martensite becomes insufficient, whereby a balance (TS 2 *EL 1/2 ) of tensile strength and elongation and a balance (TS 2 *HER 1/2 ) of tensile strength and a hole expansion ratio of the steel sheet may be lowered.
  • Tensile strength (TS) and elongation (El) were evaluated through a tensile test, and the tensile strength (TS) and the elongation (El) were measured by evaluating the specimens collected in accordance with JIS No. 5 standard based on a 90° direction with respect to a rolling direction of a rolled sheet.
  • the secondary average heating rate was more than 5°C/s, so that lump austenite was formed, and boron (B) was not concentrated into tempered martensite.
  • [B] FM /[B] TM was more than 0.55
  • a yield ratio evaluation index (I YR ) was more than 0.42
  • a balance (B TE ) of tensile strength and elongation was less than 3.0*10 6
  • a balance (B TH ) of tensile strength and a hole expansion ratio was less than 6.0*10 6 , in specimen 3.
  • the primary cooling stop temperature was more than 400°C, so that a fraction of bainite was excessive, and a fraction of tempered martensite was insufficient.
  • a balance (B TE ) of tensile strength and elongation was less than 3.0*10 6
  • a balance (B TH ) of tensile strength and a hole expansion ratio was less than 6.0*10 6 , in specimen 7.
  • specimen 9 the secondary maintaining temperature was more than 550°C, so that a fraction of retained austenite was insufficient. As a result, a balance (B TE ) of tensile strength and elongation was less than 3.0*10 6 , in specimen 9.
  • the secondary maintaining time was less than 50 seconds, so that a fraction of tempered martensite was excessive, and a fraction of retained austenite was insufficient.
  • a balance (B TE ) of tensile strength and elongation was less than 3.0*10 6
  • a balance (B TH ) of tensile strength and a hole expansion ratio was less than 6.0*10 6 , in specimen 10.

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EP21906929.1A 2020-12-17 2021-12-01 Hochfestes stahlblech mit hervorragender bearbeitbarkeit und verfahren zur herstellung davon Pending EP4265763A1 (de)

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