WO2023223078A1 - Tôle d'acier martensitique et son procédé de fabrication - Google Patents

Tôle d'acier martensitique et son procédé de fabrication Download PDF

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Publication number
WO2023223078A1
WO2023223078A1 PCT/IB2022/054657 IB2022054657W WO2023223078A1 WO 2023223078 A1 WO2023223078 A1 WO 2023223078A1 IB 2022054657 W IB2022054657 W IB 2022054657W WO 2023223078 A1 WO2023223078 A1 WO 2023223078A1
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Prior art keywords
steel sheet
temperature
anyone
cooling
rolled steel
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PCT/IB2022/054657
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English (en)
Inventor
Vincent LHOIST
Julien LAMOUCHE
Quentin MILLEE
Matthieu SIEBENTRITT
Elena Uta
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Arcelormittal
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Priority to PCT/IB2022/054657 priority Critical patent/WO2023223078A1/fr
Publication of WO2023223078A1 publication Critical patent/WO2023223078A1/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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
    • C21D1/22—Martempering
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—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
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—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
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese

Definitions

  • the present invention relates to a method of manufacturing of a martensitic steel suitable for automotive industry , yellow goods and green goods. More particularly related to Martensitic steels having tensile strength 1180 MPa or more.
  • Automotive parts are required to satisfy two inconsistent necessities, viz. ease of forming and strength but in recent years a third requirement of improvement in fuel consumption is also bestowed upon automobiles in view of global environment concerns.
  • automotive parts must be made of material having high formability in order that to fit in the criteria of ease of fit in the intricate automobile assembly and at same time have to improve strength for vehicle crashworthiness and durability while reducing weight of vehicle to improve fuel efficiency.
  • the steel sheet of WO2017/065371 is manufactured through the steps of: rapidly heating a material steel sheet for 3 to 60 seconds to an Ac3 transformation point or higher and maintaining the material steel sheet, the material steel sheet containing 0.08 to 0.30 wt% of C, 0.01 to 2.0 wt% of Si, 0.30 to 3.0 wt% of Mn, 0.05 wt% or less of P and 0.05 wt% or less of S and the remainder being Fe and other unavoidable impurities; rapidly cooling the heated steel sheet to 100°C/s or
  • WO2017/065371 do not mention the tensile strength of 1180 MPa or more in both rolling and transverse direction. Further hole expansion ratio of the steel of WO2017/065371 is not more than 40% even after having a tempered martensite single phase structure.
  • WO2010/036028 relates to a hot dip galvanized steel sheet and a manufacturing method thereof.
  • the hot dip galvanize steel sheet includes a steel sheet including a martensitic structure as a matrix, and a hot dip galvanized layer formed on the steel sheet.
  • the steel sheet includes C of 0.05 wt % to 0.30 wt %, Mn of 0.5 wt % to 3.5 wt %, Si of 0.1 wt % to 0.8 wt %, Al of 0.01 wt % to 1.5 wt %, Cr of 0.01 wt % to 1.5 wt %, Mo of 0.01 wt % to 1.5 wt %, Ti of 0.001 wt % to 0.10 wt %, N of 5 ppm to 120 ppm, B of 3 ppm to 80 ppm, an impurity, and the remainder of Fe.
  • the steel of WO2010/036028 does not mentions hole expansion ratio.
  • the purpose of the present invention is to solve these problems by making available cold- rolled martensitic steel sheets that simultaneously have: - an ultimate tensile strength of 1180MPa or more in both transversal direction as well as rolling direction and preferably more than 1220MPa in both transversal direction as well as rolling direction - a yield strength from 900 MPa to 1180MPa in both transversal direction as well as rolling direction and preferably from 950 MPa to 1180 MPa in both transversal direction as well as rolling direction - a total elongation of more than 3% in both transversal direction as well as rolling direction - a hole expansion ratio of more than 40% and preferably above 50%
  • such steel can also have a good suitability for forming, for rolling with good weldability and coatability.
  • the chemical composition of the cold rolled martensitic steel comprises of the following elements: Carbon is present in the steel of present invention is from 0.08% to 0.14%. Carbon is an element necessary for increasing the strength of the Steel of present invention by producing a low-temperature transformation phases such as Martensite, Therefore, Carbon plays two pivotal roles, one is to increase the strength. But Carbon content less than 0.08% will not be able to impart the tensile strength to the steel of present invention.
  • a preferable content for the present invention may be kept from 0.09% to 0.13 and more preferably from 0.1% to 0.12%.
  • Manganese content of the steel of present invention is from 1.95% to 2.6%. This element is gammagenous. Manganese provides solid solution strengthening and suppresses the ferritic transformation temperature and reduces ferritic transformation rate hence assist in the formation of martensite. An amount of at least 1.95% is required to impart strength as well as to assist the formation of Martensite. But when Manganese content is more than 2.6% it produces adverse effects such as it retards transformation of Austenite to Martensite during cooling after annealing.
  • Manganese content of above 2.6% can get excessively segregated in the steel during solidification and homogeneity inside the material is impaired which can cause surface cracks during a hot working process.
  • the preferred limit for the presence of Manganese is from 2% to 2.5% and more preferably from 2% to 2.4% and even more preferably from 2% to 2.3%.
  • Silicon content of the steel of present invention is from 0.1% to 0.8%. Silicon is an element that contributes to increasing the strength by solid solution strengthening. Silicon is a constituent that can retard the precipitation of carbides during cooling after annealing, therefore, Silicon promotes formation of Martensite.
  • Silicon is also a ferrite former and also increases the Ac3 transformation point which will push the annealing temperature to higher temperature ranges that is why the content of Silicon is kept at a maximum of 0.8%. Silicon content above 0.8% can also temper embrittlement and in addition silicon also impairs the coatability.
  • the preferred limit for the presence of Silicon is from 0.15% to 0.7 % and more preferably from 0.2% and 0.6%.
  • the content of the Aluminum is from 0.01% to 0.1%. in the present invention Aluminum removes Oxygen existing in molten steel to prevent Oxygen from forming a gas phase during solidification process. Aluminum also fixes Nitrogen in the steel to form Aluminum nitride to reduce the size of the grains.
  • Titanium is added to the Steel of present invention from 0.001 % to 0.1%. It forms Titanium-nitrides appearing during solidification of the cast product. The amount of Titanium is so limited to 0.1% to avoid the formation of coarse Titanium-nitrides detrimental for formability. In case the Titanium content below 0.001% does not impart any effect on the steel of present invention.
  • Boron is an essential optional element for the steel of present invention and may be present from 0.0001% to 0.05%. Boron forms boro-nitirides and impart additional strength to steel of present invention when added in an amount of at least 0.0001%.
  • Sulfur is not an essential element but may be contained as an impurity in steel and from point of view of the present invention the Sulfur content is preferably as low as possible but 0.09% or less from the viewpoint of manufacturing cost. Further if higher Sulfur is present in steel it combines to form Sulfides especially with Manganese and reduces its beneficial impact on the present invention.
  • Phosphorus constituent of the Steel of present invention is from 0% to 0.09%, Phosphorus reduces the spot weldability and the hot ductility, particularly due to its tendency to segregate at the grain boundaries or co-segregate with Manganese. For these reasons, its content is limited to 0.09 % and preferably lower than 0.06%.
  • Chromium is an optional element that may constitutes of steel from 0% to 1%. Chromium provide strength to the steel by solid solution strengthening and a minimum of 0.1% is required to impart the strength but when used above 1% impairs surface finish of steel. The preferred limit for the presence of Chromium is from 0.1% to 0.5%.
  • Molybdenum is an optional element that constitutes 0% to 0.4% of the Steel of present invention; Molybdenum plays an effective role in improving hardenability and hardness, delays the appearance of Bainite hence promote the formation of Martensite, in particular when added in an amount of at least 0.001% or even of at least 0.002%. However, the addition of Molybdenum excessively increases the cost of the addition of alloy elements, so that for economic reasons its content is limited to 0.4%. Niobium is present in the Steel of present invention from 0% to 0.1% and suitable for forming carbo-nitrides to impart strength of the Steel of present invention by precipitation hardening.
  • Niobium will also impact the size of microstructural components through its precipitation as carbo-nitrides and by retarding the recrystallization during heating process. Thus, finer microstructure formed at the end of the holding temperature and as a consequence after the complete annealing will lead to the hardening of the product.
  • Niobium content above 0.1% is not economically interesting as a saturation effect of its influence is observed this means that additional amount of Niobium does not result in any strength improvement of the product.
  • Vanadium is effective in enhancing the strength of steel by forming carbides or carbo- nitrides and the upper limit is 0.1% from economic points of view.
  • Nickel may be added as an optional element in an amount of 0% to 1% to increase the strength of the steel present invention and to improve its toughness.
  • a minimum of 0.01% is preferred to get such effects.
  • Nickel causes ductility deterioration.
  • Copper may be added as an optional element in an amount of 0% to 1% to increase the strength of the of Steel of present invention and to improve its corrosion resistance.
  • a minimum of 0.01% is preferred to get such effects.
  • Calcium can be added to the steel of present invention in an among from 0.001% to 0.01%%. Calcium is added to steel of present invention as an optional element especially during the inclusion treatment. Calcium contributes towards the refining of the Steel by binding the detrimental Sulfur content in globular form thereby retarding the harmful effect of Sulfur.
  • Sn , Pb or Sb can be added individually or in combination in the following proportions: Sn ⁇ 0.1%, Pb ⁇ 0.1% and Sb ⁇ 0.1%. Up to the maximum content levels indicated, these elements make it possible to refine the grain during solidification.
  • the remainder of the composition of the steel consists of iron and inevitable impurities resulting from processing.
  • the microstructure of the martensitic steel sheet will now be described in details, all percentages being in area fraction. Martensite constitutes at least 92% of the microstructure by area fraction.
  • the martensite of the present invention can comprise both fresh and tempered martensite.
  • fresh martensite is an optional microconstituent which is preferably limited in the steel at an amount of from 0% to 4%, preferably from 0 to 2% and even better equal to 0%.
  • Fresh martensite may form during cooling after tempering. Tempered martensite is formed from the martensite which forms during the second step of cooling after annealing and particularly after below Ms temperature and more particularly from Ms-10°C to 20°C.Such martensite is then tempered during the holding at a tempering temperature Ttemper from 150°C to 300°C.
  • the martensite of the present invention imparts ductility and strength to such steel.
  • the content of martensite is from 93% to 99% and more preferably from 94% to 98%.
  • the cumulated amount of ferrite and bainite represents from 1% to 8% of the microstructure.
  • the cumulative presence of bainite and ferrite does not affect adversely to the present invention till 8% but above 8% the mechanical properties may get impacted adversely.
  • the preferred limit for the cumulative presence ferrite and bainite is kept from 1% to 7% and more preferably from 1% to 6%.
  • Bainite forms during the reheating before tempering.
  • the steel of present invention contains 1 to 3% of bainite. Bainite can impart formability to the steel but when present in a too big amount, it may adversely impact the tensile strength of the steel. Ferrite may form during the first step of cooling after annealing but is not required as a microstructural constituent.
  • Ferrite formation must be kept as low as possible and preferably less than 2% or even less than 1%.
  • Residual Austenite is an optional microstructure that can be present from 0% to 2% in the steel.
  • the microstructure of the cold rolled martensitic steel sheet is free from microstructural components such as pearlite and cementite.
  • the steel according to the invention can be manufactured by any suitable methods. It is however preferable to use the method according to the invention that will be detailed, as a non-limitative example. Such preferred method consists in providing a semi-finished casting of steel with a chemical composition of the prime steel according to the invention.
  • the casting can be done either into ingots or continuously in form of thin slabs or thin strips, i.e. with a thickness ranging from approximately 220mm for slabs up to several tens of millimeters for thin strip.
  • a slab having the chemical composition according to the invention is manufactured by continuous casting wherein the slab optionally underwent a direct soft reduction during the continuous casting process to avoid central segregation and to ensure a ratio of local Carbon to nominal Carbon kept below 1.10.
  • the slab provided by continuous casting process can be used directly at a high temperature after the continuous casting or may be first cooled to room temperature and then reheated for hot rolling. The temperature of the slab, which is subjected to hot rolling, must be at least 1000° C and must be below 1280°C.
  • the temperature of the slab In case the temperature of the slab is lower than 1280° C, excessive load is imposed on a rolling mill and, further, the temperature of the steel may decrease to a Ferrite transformation temperature during finishing rolling, whereby the steel will be rolled in a state in which transformed Ferrite contained in the structure. Therefore, the temperature of the slab must be high enough so that hot rolling should be completed in the temperature range of Ac3 to Ac3+100°C. Reheating at temperatures above 1280°C must be avoided because they are industrially expensive.
  • the sheet obtained in this manner is then cooled at a cooling rate of at least 20°C/s to the coiling temperature which must be below 650°C. Preferably, the cooling rate will be less than or equal to 200° C/s.
  • the hot rolled steel sheet is then coiled at a coiling temperature below 650°C to avoid ovalization and preferably from 475°C to 625°C to avoid scale formation, with an even preferred range for such coiling temperature from 500°C to 625°C.
  • the coiled hot rolled steel sheet is then cooled down to room temperature before subjecting it to optional hot band annealing.
  • the hot rolled steel sheet may be subjected to an optional scale removal step to remove the scale formed during the hot rolling before optional hot band annealing.
  • the hot rolled sheet may then have subjected to an optional hot band annealing.
  • such hot band annealing is performed at temperatures from 400°C to 750°C, preferably for at least 12 hours and not more than 96 hours, the temperature preferably remaining below 750°C to avoid transforming partially the hot-rolled microstructure and, therefore, possibly losing the microstructure homogeneity.
  • an optional scale removal step of this hot rolled steel sheet may be performed through, for example, pickling of such sheet.
  • the hot rolled steel sheet thus obtained preferably has a thickness from 0.5mm to 10mm and more preferably from 1 mm to 8 mm and even more preferably from 1 mm to 6 mm.
  • This hot rolled steel sheet may optionally be send directly for the heat treatment to manufacture an optional Hot rolled heat treated martensitic steel suitable for yellow and green goods.
  • the heat treatment process for both the Hot rolled steel sheet and cold rolled steel sheet are identical.
  • This hot rolled steel sheet is then subjected to cold rolling to obtain a cold rolled steel sheet with a thickness reduction from 35 to 90%.
  • the cold rolled steel sheet is then heated in a two steps heating process wherein the first step of heating starts from room temperature, the cold rolled steel sheet being heated, at a heating rate HR1 of at least 10°C/s, to a temperature HT1 which is in a range from 410°C to 750°C.
  • the heating rate HR1 for such first step of heating is at least 12°C/s and more preferably at least 13°C/s.
  • the preferred HT1 temperature for such first step is from 425°C to 725°C and more preferably from 435°C to 690°C.
  • the cold rolled steel sheet is heated from HT1 to an annealing temperature Tsoak which is from Ac3 to Ac3 + 100°C, preferably from Ac3 +10°C to Ac3 + 100°C, at a heating rate HR2 which is from 0.5°C/s to 50°C/s.
  • the cold rolled steel sheet is held at Tsoak during 10 seconds to 500 seconds to ensure a complete recrystallization and full transformation to austenite of the strongly work hardened initial structure.
  • the cold rolled steel sheet is then cooled in a two steps cooling process wherein the first step of cooling starts from Tsoak, the cold rolled steel sheet being cooled down, at a cooling rate CR1 from 15°C/s to 150°C/s, to a temperature T1 which is in a range from 620°C to 750°C.
  • the cooling rate CR1 for such first step of cooling is from 20°C/s to 120°C/s.
  • the preferred T1 temperature for such first step is from 630°C to 725°C.
  • the cold rolled steel sheet is cooled down from T1 to a temperature T2 which is from Ms-10°C to 20°C, at a cooling rate CR2 of at least 50°C/s.
  • T2 a temperature
  • the cooling rate CR2 for the second step of cooling is at least 100°C/s and more preferably at least 150°C/s.
  • the preferred T2 temperature for such second step is from Ms-50°C to 20°C.
  • the preferred temperature range for tempering is from 200°C to 300°C and the preferred duration for holding at Ttemper is from 200 s to 600 s. Then, the cold rolled steel sheet is cooled down to room temperature to obtain a cold rolled martensitic steel.
  • the cold rolled martensitic steel sheet of the present invention may optionally be coated with zinc or zinc alloys, or with aluminum or aluminum alloys to improve its corrosion resistance.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Sheet Steel (AREA)

Abstract

La présente invention concerne une tôle d'acier martensitique comprenant les éléments suivants : 0,08 % ≦ C ≦ 0,14 % ; 1,95 % ≦ Mn ≦ 2,6 % ; 0,1 % ≦ Si ≦ 0,8 % ; 0,01 % ≦ Al ≦ 0,1 % ; 0,001 % ≦ Ti ≦ 0,1 % ; 0,0001 % ≦ B ≦ 0,05 % ; 0 % ≦ S ≦ 0,09 % ; 0 % ≦ P ≦ 0,09 % ; 0 % ≦ N ≦ 0,09 % ; 0,1 % ≦ Cr ≦ 1 % ; 0 % ≦ Ni ≦ 1 % ; 0 % ≦ Cu ≦ 1 % ; 0 % ≦ Mo ≦ 0,4 % ; 0 % ≦ Nb ≦ 0,1 % ; 0 % ≦ V ≦ 0,1 % ; 0 % ≦ Sn ≦ 0,1 % ; 0 % ≦ Pb ≦ 0,1 % ; 0 % ≦ Sb ≦ 0,1 % ; 0,001 % ≦ Ca ≦ 0,01 % ; la composition restante étant constituée de fer et d'impuretés inévitables, la microstructure dudit acier comprenant, en pourcentage de surface, au moins 92 % de martensite, une proportion cumulée de ferrite et de bainite de 1 % à 8 %, et une proportion facultative d'austénite résiduelle de 0 % à 2 %.
PCT/IB2022/054657 2022-05-19 2022-05-19 Tôle d'acier martensitique et son procédé de fabrication WO2023223078A1 (fr)

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WO2020250098A1 (fr) * 2019-06-12 2020-12-17 Arcelormittal Acier martensitique laminé à froid et procédé d'acier martensitique s'y rapportant

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