WO2023135550A1 - Acier micro-allié à faible teneur en carbone laminé à froid et son procédé de fabrication - Google Patents

Acier micro-allié à faible teneur en carbone laminé à froid et son procédé de fabrication Download PDF

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WO2023135550A1
WO2023135550A1 PCT/IB2023/050288 IB2023050288W WO2023135550A1 WO 2023135550 A1 WO2023135550 A1 WO 2023135550A1 IB 2023050288 W IB2023050288 W IB 2023050288W WO 2023135550 A1 WO2023135550 A1 WO 2023135550A1
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cold rolled
low carbon
carbon micro
less
alloyed steel
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PCT/IB2023/050288
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English (en)
Inventor
Alok Kumar Singh
Somjeet Biswas
Basudev Bhattacharya
Devesh Kumar Chouhan
Monojit DUTTA
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Tata Steel Limited
Indian Institute Of Technology, Kharagpur
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Publication of WO2023135550A1 publication Critical patent/WO2023135550A1/fr

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    • 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
    • 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
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
    • C21D8/0426—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
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
    • C21D8/0436—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
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
    • C21D8/0473—Final recrystallisation annealing
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a cold rolled low carbon micro-alloyed steel and more particularly, to the cold rolled low carbon micro-alloyed steel having excellent deep drawing and hydroforming characteristics, and method of manufacturing thereof.
  • Metals such as steel, aluminum are widely employed for applications such as, automobile parts, construction materials etc. Sheets and strips of steel compositions have been used in forming body structural members and body panels for automotive vehicles.
  • AHSS advanced high strength steels
  • dual -phase steels are important members as they provide high strength and high strain hardening ability at a very nominal cost.
  • Lean alloying and simple thermo-mechanical treatments have been used in these kinds of steels.
  • DP steels are produced industrially by hot rolling and/or cold rolling, followed by Intercritical annealing (IA).
  • the other unconventional methods of production of DP steels include severe plastic deformation techniques such as equal channel angular pressing (ECAP) followed by Intercritical annealing.
  • ECAP equal channel angular pressing
  • thermomechanical processing (TMP) parameters optimization are very important to be carried out in a DP to obtain a combination of high strength, high n, high normal anisotropy, and lower planer anisotropy for the best sheet metal formability.
  • Another objective of the present invention is to obtain the combination of high r (Normal Anisotropy), high strength and ductility simultaneously in steel with minimum possible C and alloying additions ( ⁇ 3%), for usage in automobiles.
  • Another objective of the present invention is to provide a new cold-rolled (CR) fine grained low carbon micro-alloyed dual-phase steels having very high ductility, strength, high strain hardening exponent, and isotropy combination.
  • Another objective of the present invention is to provide a new DP steel with an ultimate tensile strength (UTS) in the range of 624 — 1301MPa, ductility in the range of 19—48%, n in the range of 0.17 — 0.30 and r in the ranges of 1.07- 1.85 in a combination of low Ar.
  • UTS ultimate tensile strength
  • a cold rolled low carbon microalloyed steel having excellent deep drawing and hydroforming characteristics comprising the following composition expressed in weight %: Carbon (C): 0.04% - 0.08%, Manganese (Mn): 1.2% - 1.5%, Sulphur (S): 0.003% - 0.004%, Phosphorus (P): 0.014% or less, Nitrogen (N): 70 ppm or less, Silicon (Si): 0.25% - 0.5%, Aluminium (Al): 0.02% - 0.04%, Chromium (Cr): 0.5 - 0.75%, Molybdenum (Mo): 0.002% or less, Copper (Cu): 0.009% or less, Vanadium (V): 0.001% or less, Titanium (Ti): 0.01% or less, Niobium (Nb): 0.02% or less, Calcium (Ca): 0.003% or less, Boron (B): 0.003% -
  • the cold rolled low carbon micro-alloyed steel comprises the composition expressed in weight %: Carbon (C): 0.053%, Manganese (Mn): 1.32%, Sulphur (S): 0.003%, Phosphorus (P): 0.014%, Nitrogen (N): 60 ppm, Silicon (Si): 0.35%, Aluminium (Al): 0.037%, Chromium (Cr): 0.61%, Molybdenum (Mo): 0.001%, Copper (Cu): 0.0083%, Vanadium (V): 0.001%, Niobium (Nb): 0.015%, Titanium (Ti): 0.001%, Calcium (Ca): 0.0016%, Boron (B): 0.005%, Nickel (Ni): 0.019%, and the balance being Iron (Fe) and unavoidable impurities.
  • the cold rolled low carbon micro-alloyed steel exhibits a tensile strength of at least 600 MPa. In an embodiment, the cold rolled low carbon micro-alloyed steel exhibits tensile strength ranging from about 600 MPa to 1301 MPa. In an embodiment, the cold rolled low carbon micro-alloyed steel exhibits tensile strength ranging from about 624 MPa to 1301 MPa.
  • the cold rolled low carbon micro-alloyed steel exhibits a ductility of at least 19%. In an embodiment, the cold rolled low carbon microalloyed steel exhibits a ductility ranging from 19% to 48%. [0018] In an embodiment, the cold rolled low carbon micro-alloyed steel exhibits strain hardening exponent (n) ranging from 0.10 to 0.3. In an embodiment, the cold rolled low carbon micro-alloyed steel exhibits the Lankford parameter of normal anisotropy (r) ranging from 1.07 to 1.85.
  • the cold rolled low carbon micro-alloyed steel exhibits a unique combination of high r (Normal Anisotropy), high strength and ductility.
  • a minimal carbon of about 0.053 wt.% and microalloying kept at a minimum of ⁇ 2.56% of the cold rolled low carbon microalloyed steel provides good weldability with minimum segregation effects.
  • a method of manufacturing the cold rolled low carbon micro-alloyed steel sheet or strip or blank comprising producing a hot rolled steel sheet by hot rolling a slab having chemical composition expressed in weight %: Carbon (C): 0.04% - 0.08%, Manganese (Mn): 1.2% - 1.5%, Sulphur (S): 0.003% - 0.004%, Phosphorus (P): 0.014% or less, Nitrogen (N): 70 ppm or less, Silicon (Si): 0.25% - 0.5%, Aluminium (Al): 0.02% - 0.04%, Chromium (Cr): 0.5 - 0.75%, Molybdenum (Mo): 0.002% or less, Copper (Cu): 0.009% or less, Vanadium (V): 0.001% or less, Titanium (Ti): 0.01% or less, Niobium (Nb): 0.02% or less, Calcium (Ca): 0.003% or less
  • the method also comprises cooling the hot rolled (HR) steel sheet till a coiling temperature (TCT) in the range 560 - 600°C is reached and coiling thereafter.
  • the method further comprises normalizing the hot rolled (HR) steel sheet at temperature ranging between 800°C - 1000°C for a time duration of 5 - 100 minutes.
  • the method comprising cold rolling the hot rolled (HR) and normalized steel sheet to obtain a cold rolled (CR) steel sheet.
  • the method further comprising heating the cold rolled (CR) steel sheet to a first predetermined temperature ranging between inter-critical temperature (between Acl and Ac3) of 750- 920 °C and soaking at the first predetermined temperature for a first predetermined time of 5-30 minutes.
  • the method comprising quenching the cold rolled (CR) and heated steel sheet to a third predetermined temperature in a bath at a third cooling rate to obtain the cold rolled low carbon micro-alloyed steel sheet or strip or blank having a fine grain ferrite-martensite dual phase structure including, by volume, between 46% - 85 % of ferrite, and between 15% to 54% of martensite.
  • the hot rolled (HR) and normalized steel sheet is cold rolled in five consecutive passes with rolling reduction per pass in the range of 25- 34%, and with a total accumulated von-Mises strain of 2. 1 to obtain the cold rolled (CR) steel sheet.
  • the third predetermined temperature is below 40°C and the third cooling rate is between 70 — 120°C/s.
  • the third predetermined temperature is room temperature and the third cooling rate of 100 °C/s.
  • the first predetermined temperature is 750 °C and first predetermined time of 5 minutes.
  • the cold rolled low carbon micro-alloyed steel manufactured by keeping the first predetermined temperature at 750 °C and first predetermined time at 5 minutes exhibits a Ferrite-0.15Martensite dual-phase microstructure with an average ferrite grain size of 6.4 + 2.2 [im and avg. Martensite island size of 2.9 + 0.78/im, 13.8% CSL boundary and 86.2% RHAGB boundaries in the microstructure, an UTS of 624MPa with 48% ductility, yield strength of 401 MPa, a strain hardening exponent (n) of 0.30, isotropic properties of f of 1.80 with a low Ar of -0.13, a toughness of 260 J/m3.
  • the first predetermined temperature is 800 °C and first predetermined time of 5 minutes.
  • the cold rolled low carbon micro-alloyed steel manufactured by keeping the first predetermined temperature at 800 °C and first predetermined time at 5 minutes exhibits a Ferrite-0.31Martensite dual-phase microstructure with an average ferrite grain size of 6.8 + 2.6 rrn and avg. Martensite island size of 3.0 + 0.99/im, 18.4% CSL boundary and 81.6% RHAGB boundaries in the microstructure, a UTS of 850MPa with 31% ductility, a yield strength of 464 MPa, a strain hardening exponent (n) of 0.20, isotropic properties of f of 1.85 with a low Ar of 0.21, a toughness of 233 J/m3.
  • the first predetermined temperature is 800 °C and first predetermined time of 30 minutes.
  • the cold rolled low carbon micro-alloyed steel manufactured by keeping the first predetermined temperature at 800 °C and first predetermined time at 30 minutes exhibits a Ferrite-0.36Martensite dual-phase microstructure with an average ferrite grain size of 7.2 + 2.5 [im and avg. Martensite island size of 3.7 + 0.78/im, 16.2% CSL boundary and 83.8% RHAGB boundaries in the microstructure, a UTS of 760MPa with 41% ductility, a yield strength of 398 MPa, a strain hardening exponent (n) of 0.24, isotropic properties of r of 1.1 with a low Ar of 0.13, a toughness of 276 J/m3.
  • the first predetermined temperature is 840 °C and first predetermined time of 5 minutes.
  • the cold rolled low carbon micro-alloyed steel manufactured by keeping the first predetermined temperature at 840 °C and first predetermined time at 5 minutes exhibits a Ferrite 0.39Martensite dual-phase microstructure with an average ferrite grain size of 6.0 + 2.6 rrn and avg. Martensite island size of 3.8 + 1.2/im, 13.1% CSL boundary and 86.9% RHAGB boundaries in the microstructure, a UTS of 959MPa with 30% ductility, a yield strength of 554 MPa, a strain hardening exponent (n) of 0.17, isotropic properties of r of 1.07 with a low Ar of 0.12, a toughness of 253 J/m3.
  • a component is produced from the cold rolled low carbon micro-alloyed steel.
  • the component is used in structural as well as automotive applications.
  • Figure la illustrates a preferred example of production line for manufacturing cold rolled low carbon micro-alloyed steel, according to an embodiment of the present invention
  • Figure lb illustrates a flowchart of method for manufacturing cold rolled low carbon micro-alloyed steel, according to an embodiment of the present invention
  • the present disclosure provides a cold rolled low carbon micro-alloyed steel having very high ductility, strength, high strain hardening exponent, and isotropy combination.
  • the cold rolled low carbon micro-alloyed steel is used to produce components which may used in structural as well as automotive applications.
  • the cold rolled low carbon micro-alloyed steel having excellent deep drawing and hydroforming characteristics comprises the following composition expressed in weight %: Carbon (C): 0.04% - 0.08%, Manganese (Mn): 1.2% - 1.5%, Sulphur (S): 0.003% - 0.004%, Phosphorus (P): 0.014% or less, Nitrogen (N): 70 ppm or less, Silicon (Si): 0.25% - 0.5%, Aluminium (Al): 0.02% - 0.04%, Chromium (Cr): 0.5 - 0.75%, Molybdenum (Mo): 0.002% or less, Copper (Cu): 0.009% or less, Vanadium (V): 0.001% or less, Titanium (Ti): 0.01% or less, Niobium (Nb): 0.02% or less, Calcium (Ca): 0.003% or less, Boron (B): 0.003% - 0.005%, Nickel (Ni): 0.02% or less, and
  • the low carbon micro-alloyed steel comprises the composition expressed in weight %: as shown in Table 1.
  • the cold rolled low carbon micro-alloyed steel comprises a fine gram ferrite-martensite dual phase structure including, by volume, between 46% - 85 % of ferrite, and between 15% - 54% of martensite.
  • the cold rolled low carbon micro-alloyed steel exhibits a tensile strength of at least 600 MPa. More preferably, the low carbon micro-alloyed steel exhibits tensile strength ranging from about 600 MPa to 1301 MPa. In the preferred embodiment, the low carbon micro-alloyed steel exhibits tensile strength ranging from about 624 MPa to 1301 MPa.
  • the cold rolled low carbon micro-alloyed steel exhibits yield strength of at least 350 MPa. More preferably, the cold rolled low carbon micro-alloyed steel exhibits yield strength ranging from about 350 to 770 MPa. In the preferred embodiment, the cold rolled low carbon micro-alloyed steel exhibits yield strength ranging from about 398 MPa to 770 MPa.
  • the cold rolled low carbon micro-alloyed steel exhibits a ductility of at least 19%. In the preferred embodiment, the cold rolled low carbon micro-alloyed steel exhibits a ductility ranging from 19 % to 48 %.
  • the cold rolled low carbon micro-alloyed steel exhibits strain hardening exponent (n) ranging from 0.10 to 0.3.
  • the hot rolled or cold rolled low carbon micro-alloyed steel exhibits the Lankford parameter of normal anisotropy (r) ranging from 1.07 to 1.85.
  • the cold rolled low carbon micro-alloyed steel exhibits a unique combination of high f (Normal Anisotropy), high strength and ductility.
  • the minimal carbon of about 0.053 wt.% and microalloying kept at a minimum of -2.56% of the cold rolled low carbon micro-alloyed steel provides good weldability with minimum segregation effects.
  • the method (100) initiates at step (102).
  • the method (100) comprises producing a hot rolled steel sheet by hot rolling a slab having chemical composition expressed in weight %: Carbon (C): 0.04% - 0.08%, Manganese (Mn): 1.2% - 1.5%, Sulphur (S): 0.003% - 0.004%, Phosphorus (P): 0.014% or less, Nitrogen (N): 70 ppm or less, Silicon (Si): 0.25% - 0.5%, Aluminium (Al): 0.02% - 0.04%, Chromium (Cr): 0.5 - 0.75%, Molybdenum (Mo): 0.002% or less, Copper (Cu): 0.009% or less, Vanadium (V): 0.001% or less, Titanium (Ti):
  • the steel having a composition expressed in weight %: Carbon (C): 0.053%, Manganese (Mn): 1.32%, Sulphur (S): 0.003%, Phosphorus (P): 0.014%, Nitrogen (N): 60 ppm, Silicon (Si): 0.35%, Aluminium (Al): 0.037%, Chromium (Cr): 0.61%, Molybdenum (Mo): 0.001%, Copper (Cu): 0.0083%, Vanadium (V): 0.001%, Niobium (Nb): 0.015%, Titanium (Ti): 0.001%, Calcium (Ca): 0.0016%, Boron (B): 0.005%, Nickel (Ni): 0.019%, and the balance being Iron (Fe) and unavoidable impurities is used.
  • a steel slab cast in a conventional continuous caster or in a thin-slab caster is rough-rolled in the roughing stands above the austenite recrystallization temperature and then subsequently hot-rolled or finish rolled below the recrystallization temperature.
  • the hot rolling process may be carried out by passing the steel through a pair of rolls and rolling may be carried out for at least five times to reduce the thickness of the hot rolled (HR) steel sheet to about 2 - 5 mm.
  • the method (100) comprises cooling the hot rolled (HR) steel sheet till a coiling temperature (TCT) in the range 560 to 600°C is reached and coiling thereafter.
  • TCT coiling temperature
  • the method ( 100) comprises normalizing the hot rolled (HR) steel sheet at temperature ranging between 800°C - 1000°C for a time duration of 5 - 100 minutes.
  • the hot rolled (HR) steel sheet is normalized at temperature of 900°C for a duration of 5 minutes.
  • the method ( 100) comprises cold rolling the hot rolled (HR) and normalized steel sheet in five consecutive passes with a total von-Mises strain of 2.1 (equivalent to -80% CR thickness reduction) to obtain a cold rolled (CR) steel sheet.
  • the method (100) comprises heating the cold rolled (CR) steel sheet to a first predetermined temperature ranging between inter-critical temperature (between Aci and Acs) of 750 - 920°C and soaking at the first predetermined temperature of 750 - 920° C for a first predetermined time of 5-30 minutes.
  • the cold rolled (CR) steel sheet is heated to the first predetermined temperature of 840°C and soaked at the first predetermined temperature of 840°C for the first predetermined time of 5 minutes.
  • the first predetermined temperature is 750 °C and first predetermined time of 5 minutes.
  • the first predetermined temperature is 800 °C and first predetermined time of 5 minutes.
  • the first predetermined temperature is 800 °C and first predetermined time of 30 minutes.
  • the first predetermined temperature is 920 °C and first predetermined time of 30 minutes.
  • the method (100) comprises quenching the cold rolled (CR) and heated steel sheet to a third predetermined temperature in a bath at a third cooling rate to obtain cold rolled low carbon micro-alloyed steel sheet or strip or blank having a fine grain ferrite -martensite dual phase structure including, by volume, between 46% - 85 % of ferrite, and between 15% to 54% of martensite.
  • the cold rolled (CR) and heated steel sheet is quenched to the third predetermined temperature below 40°C in either a water bath or an oil bath at third cooling rate of 70 — 120°C/s to obtain the cold rolled low carbon micro-alloyed steel sheet or strip or blank having a fine grain ferrite -martensite dual phase structure including, by volume, between 46% - 85 % of ferrite, and between 15% to 54% of martensite.
  • the cold rolled (CR) and heated steel sheet is quenched to room temperature in water bath at 100°C/s third cooling rate.
  • the hot rolled (HR) and normalized steel sheet is cold rolled in five consecutive passes with rolling reduction per pass in the range of 25- 34%, and with a total accumulated von-Mises strain of 2. 1 to obtain the cold rolled (CR) steel sheet.
  • Table 3 provides Cold rolling and Intercritical heat treatment schedules that have been used to produce the disclosed steels. A high rolling reduction of ⁇ 25- 34% per pass was carried out during the present investigations.
  • a hot rolled steel sheet of ⁇ 6mm thickness was used in normalized condition ( 1193 K" (900 ° C) for 5 mins followed by air cooling) for the CR proce ss .
  • Intercritical heat treatment was carried out by heating the sample at 750°C, 800°C, 840°C and 920°C, holding there for 5 minutes, followed by room temperature water quenching. Another sample at 800°C was held for 30 minutes, followed by room temperature water quenching.
  • the cold rolled low carbon micro-alloyed steel manufactured by keeping the first predetermined temperature at 750 °C and first predetermined time at 5 minutes exhibits a Ferrite-0.15Martensite dual-phase microstructure with an average ferrite grain size of 6.4 + 2.2 rrn and avg. Martensite island size of 2.9 + 0.78/im, 13.8% CSL boundary and 86.2% RHAGB boundaries in the microstructure, an UTS of 624MPa with 48% ductility, yield strength of 401 MPa, a strain hardening exponent (n) of 0.30, isotropic properties of r of 1.80 with a low Ar of -0.13, a toughness of 260 J/m3.
  • the cold rolled low carbon micro-alloyed steel manufactured by keeping the first predetermined temperature at 800 °C and first predetermined time at 5 minutes exhibits a Femte-0.31Martensite dual-phase microstructure with an average ferrite grain size of 6.8 + 2.6 rrn and avg. Martensite island size of 3.0 + 0.99/im, 18.4% CSL boundary and 81.6% RHAGB boundaries in the microstructure, a UTS of 850MPa with 31% ductility, a yield strength of 464 MPa, a strain hardening exponent (n) of 0.20, isotropic properties of f of 1.85 with a low Ar of 0.21, a toughness of 233 J/m3.
  • the cold rolled low carbon micro-alloyed steel manufactured by keeping the first predetermined temperature at 800 °C and first predetermined time at 30 minutes exhibits a Ferrite-0.36Martensite dual-phase microstructure with an average ferrite grain size of 7.2 + 2.5 rm and avg. Martensite island size of 3.7 + 0.78/im, 16.2% CSL boundary and 83.8% RHAGB boundaries in the microstructure, a UTS of 760MPa with 41% ductility, a yield strength of 398 MPa, a strain hardening exponent (n) of 0.24, isotropic properties of f of 1.1 with a low Ar of 0.13, a toughness of 276 J/m3.
  • the cold rolled low carbon micro-alloyed steel manufactured by keeping the first predetermined temperature at 840 °C and first predetermined time at 5 minutes exhibits a Ferrite 0.39Martensite dual-phase microstructure with an average ferrite grain size of 6.0 + 2.6 rrn and avg. Martensite island size of 3.8 + 1.2/im, 13.1% CSL boundary and 86.9% RHAGB boundaries in the microstructure, a UTS of 959MPa with 30% ductility, a yield strength of 554 MPa, a strain hardening exponent (n) of 0.17, isotropic properties of r of 1.07 with a low Ar of 0.12, a toughness of 253 J/m3.
  • the cold rolled low carbon micro-alloyed steel manufactured by keeping the first predetermined temperature at 920 °C and first predetermined time at 5 minutes exhibits a Ferrite 0.54Martensite dual-phase microstructure with an average ferrite grain size of 6.1 + 1.3 rm and avg. Martensite lath spacing of 0.7 + 0.3 wn , 26.6% CSL boundary and 73.4% RHAGB boundaries in the microstructure, a UTS of 1301MPa with 19% ductility, a yield strength of 770 MPa, a strain hardening exponent (n) of 0.10, isotropic properties of r of 1.11 with a low Ar of -0.14, a toughness of 191 J/m 3 .
  • Table 4 provides mechanical properties of different types of the low carbon micro-alloyed steel produced.
  • Figures 3a-3c, 4a-4c, 5a-5c, and 6a-6c represent similar information for CR 800°C IHT 5 Min, CR 800°C IHT 30 Min, CR 840°C IHT 5 Min and CR 920°C IHT 5 Min steel samples, respectively.
  • CR 800°C IHT 5 min steel sample produced Ferritic-0.31 Martensite microstructure, with an average ferrite grain size of 6.8 + 2.6pm, martensite island size of 3 + 0.99pm and grain size distribution in the range of 0.5-9pm ( Figures 3a-3b).
  • CR 920°C IHT 5 min steel sample produced Femte 0.54Martensite dualphase microstructure with an average ferrite grain size of 6.1 + 1.3 [im and avg. Martensite lath spacing of 0.7 + 0.3 im , 26.6% CSL boundary and 73.4% RHAGB boundaries in the microstructure (Fig 6a-6c).
  • the CR 920°C IHT 5 min steel sample has high strength and can be used for applications such as tool steel etc.
  • the high r are mostly associated with the crystallographic texture i.e., formation of strong y-fiber texture in this steel ( Figures 2-4). Beyond 800°C IHT the intensity of y-fiber reduces ( Figure 5) and high r could not be achieved.
  • the present invention relates to the cold-rolled (CR) low carbon microalloyed (C-0.05 and total alloying ⁇ 3%) steels having improved strength-ductility balance and good formability to be used in automobile applications.
  • the produced cold rolled low carbon micro-alloyed steel exhibits very high ductility, strength, high strain hardening exponent, and isotropy combination.
  • the ductile, damage- tolerant cold rolled low carbon micro-alloyed steel with high UTS is suitable for secondary sheet metal forming operations, such as deep drawing.
  • Components made of the cold-rolled (CR) low carbon micro-alloyed steel of the present disclosure provides good crashworthiness and damage tolerance in the automobile sector.
  • the strength-ductility combination obtained in the cold-rolled (CR) low carbon micro-alloyed steel lies facilitates in overcoming the drawbacks, such as poor weldability, expensive alloying, alloy segregation and processing difficulties due to high alloying.
  • the most important feature of the present invention is that it could be easily incorporated into an existing industrial production line without any major modifications.
  • the method of manufacturing may include additional processes such as descaling, pickling which are well known in the prior art and thus will not be described herein detail.

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Abstract

La présente divulgation concerne un procédé (100) de fabrication d'un acier micro-allié à faible teneur en carbone laminé à froid ayant d'excellentes caractéristiques de thermoformage profond et d'hydroformage comprenant la composition suivante exprimée en % en poids : carbone (C) : 0,04% à 0,08 %, manganèse (Mn) : 1,2 % à 1,5 %, soufre (S) : 0,003 % à 0,004 %, phosphore (P) : 0,014 % ou moins, azote (N) : 70 ppm ou moins, silicium (Si) : 0,25% à 0,5%, aluminium (Al) : 0,02 % à 0,04%, chrome (Cr) : 0,5 à 0,75 %, molybdène (Mo) : 0,002 % ou moins, cuivre (Cu) : 0,009 % ou moins, vanadium (V) : 0,001 % ou moins, titane (Ti) : 0,01 % ou moins, niobium (Nb) : 0,02 % ou moins, calcium (Ca) : 0,003 % ou moins, bore (B) : 0,003% à 0,005%, nickel (Ni) : 0,02 % ou moins et le reste étant du fer (Fe) et des impuretés inévitables. L'acier est constitué d'une structure à double phase ferrite-martensite à grains fins comprenant, en volume, entre 46 % et 85 % de ferrite, et entre 15 % et 54 % de martensite.
PCT/IB2023/050288 2022-01-13 2023-01-12 Acier micro-allié à faible teneur en carbone laminé à froid et son procédé de fabrication WO2023135550A1 (fr)

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