EP1990431A1 - Method of manufacturing annealed, very high-resistance, cold-laminated steel sheets, and sheets produced thereby - Google Patents

Method of manufacturing annealed, very high-resistance, cold-laminated steel sheets, and sheets produced thereby Download PDF

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Publication number
EP1990431A1
EP1990431A1 EP07290598A EP07290598A EP1990431A1 EP 1990431 A1 EP1990431 A1 EP 1990431A1 EP 07290598 A EP07290598 A EP 07290598A EP 07290598 A EP07290598 A EP 07290598A EP 1990431 A1 EP1990431 A1 EP 1990431A1
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EP
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Prior art keywords
steel
sheet
rolled
temperature
cold
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EP07290598A
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German (de)
French (fr)
Inventor
Javier Gilotin
Antoin Moulin
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ArcelorMittal France SA
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ArcelorMittal France SA
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Application filed by ArcelorMittal France SA filed Critical ArcelorMittal France SA
Priority to EP07290598A priority Critical patent/EP1990431A1/en
Priority to ES08805523T priority patent/ES2655476T5/en
Priority to HUE08805523A priority patent/HUE035549T2/en
Priority to BRPI0821572-3A priority patent/BRPI0821572B1/en
Priority to JP2010506964A priority patent/JP5398701B2/en
Priority to EP08805523.1A priority patent/EP2155915B2/en
Priority to CA2686940A priority patent/CA2686940C/en
Priority to KR1020097023517A priority patent/KR101523395B1/en
Priority to US12/599,166 priority patent/US20100307644A1/en
Priority to MX2009011927A priority patent/MX2009011927A/en
Priority to PL08805523.1T priority patent/PL2155915T5/en
Priority to RU2009145940/02A priority patent/RU2437945C2/en
Priority to CN2008800153809A priority patent/CN101765668B/en
Priority to PCT/FR2008/000609 priority patent/WO2008145871A2/en
Priority to ARP080101971A priority patent/AR066508A1/en
Publication of EP1990431A1 publication Critical patent/EP1990431A1/en
Priority to ZA200907430A priority patent/ZA200907430B/en
Priority to MA32328A priority patent/MA31555B1/en
Priority to US15/243,610 priority patent/US10612106B2/en
Priority to US16/592,341 priority patent/US11414722B2/en
Priority to US17/575,300 priority patent/US20220136078A1/en
Withdrawn legal-status Critical Current

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    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D9/00Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B22CASTING; POWDER METALLURGY
    • B22DCASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00Continuous casting of metals, i.e. casting in indefinite lengths
    • B22D11/001Continuous casting of metals, i.e. casting in indefinite lengths of specific alloys
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D8/00Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D8/00Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D8/00Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226Hot rolling
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D8/00Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236Cold rolling
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D9/00Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/52Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/001Ferrous alloys, e.g. steel alloys containing N
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/002Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/02Ferrous alloys, e.g. steel alloys containing silicon
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/04Ferrous alloys, e.g. steel alloys containing manganese
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/06Ferrous alloys, e.g. steel alloys containing aluminium
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/12Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/14Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32Ferrous alloys, e.g. steel alloys containing chromium with boron
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • CCHEMISTRY; METALLURGY
    • C23COATING 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
    • C23GCLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00Cleaning or pickling metallic material with solutions or molten salts
    • CCHEMISTRY; METALLURGY
    • C23COATING 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
    • C23GCLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00Cleaning or pickling metallic material with solutions or molten salts
    • C23G1/02Cleaning or pickling metallic material with solutions or molten salts with acid solutions
    • C23G1/08Iron or steel

Definitions

  • the invention relates to the manufacture of cold-rolled and annealed thin sheets of steels having a strength greater than 1200 MPa and an elongation at break greater than 8%.
  • the automotive sector and the general industry are notably fields of application for these steel sheets.
  • steels having a "TRIP" (Transformation Induced Plasticity) behavior with very advantageous combinations of properties (resistance-ability to deformation) have been developed: these properties are related to the structure of these structures.
  • multiphase steels have a predominantly bainitic structure.
  • multi-phase steel sheets of medium thickness are used with advantage for structural parts such as bumper crosspieces, uprights, various reinforcements.
  • the patent EP1559798 discloses the manufacture of steels of composition: 0.10-0.25% C, 1.0-2.0% Si, 1.5-3% Mn, the microstructure consisting of at least 60% bainitic ferrite and at least 5% residual austenite, the polygonal ferrite being less than 20%.
  • the exemplary embodiments presented in this document show that the resistance does not exceed 1200 MPa.
  • the patent EP 1589126 also discloses the manufacture of cold-rolled thin sheet, the product of which (resistance x elongation) is greater than 20000 MPa%.
  • the composition of the steels contains: 0.10-0.28% C, 1.0-2.0% Si, 1-3% Mn, less than 0.10% Nb.
  • the structure consists of more than 50% bainitic ferrite, 5 to 20% residual austenite, and less than 30% of polygonal ferrite. Again, the examples presented show that the resistance is still below 1200 MPa.
  • the present invention aims to solve the problems mentioned above. It aims to provide a cold rolled and annealed thin steel sheet having a mechanical strength greater than 1200 MPa together with an elongation greater than 8% rupture and good cold forming ability. The invention also aims at providing a steel that is not very sensitive to damage during cutting by a mechanical method.
  • the invention aims to provide a method of manufacturing thin sheets, small variations in the parameters do not lead to significant changes in the microstructure or mechanical properties.
  • the invention also aims to provide a sheet of steel easily fabricated by cold rolling, that is to say whose hardness after the hot rolling step is limited so that the rolling forces remain moderate during of the cold rolling step.
  • the subject of the invention is a cold-rolled and annealed steel sheet with a resistance greater than 1200 MPa, the composition of which comprises the contents being expressed by weight: 0.10% ⁇ C ⁇ 0.25% , 1% ⁇ Mn ⁇ 3%, Al ⁇ 0.010%, Si ⁇ 2.990%, S ⁇ 0.015%, P ⁇ 0.1%, N ⁇ 0.008%, with the proviso that 1% ⁇ Si + Al ⁇ 3%, the composition optionally comprising: 0.05% ⁇ V ⁇ 0.15%, B ⁇ 0.005%, Mo ⁇ 0.25%, Cr ⁇ 1.65%, with the proviso that Cr + (3 ⁇ Mo) ⁇ 0.3%, Ti in an amount such that Ti / N ⁇ 4 and Ti ⁇ 0.040%, the remainder of the composition consisting of iron and unavoidable impurities resulting from the elaboration, the microstructure of said steel comprising 15 to 90% of bainite, the balance consisting of martensite and residual austenite.
  • the subject of the invention is also a steel sheet of the above composition, with a resistance greater than 1400 MPa, with an elongation at break greater than 8%, characterized in that it contains: Mo ⁇ 0.25%, Cr ⁇ 1.65%, it being understood that Cr + (3 x Mo) ⁇ 0.3%, the microstructure of the steel comprising 45 to 65% of bainite, the remainder being islands of martensite and residual austenite L
  • Another subject of the invention is a steel sheet of the above composition, with a resistance greater than 1600 MPa, with an elongation at break greater than 8%, characterized in that it contains: Mo ⁇ 0.25%, Cr ⁇ 1.65%, it being understood that: Cr + (3 x Mo) ⁇ 0.3%, the microstructure of the steel comprising 15 to 45% of bainite, the remainder consisting of martensite and residual austenite.
  • the composition comprises: 0.19% ⁇ C ⁇ 0.23% According to a preferred mode, the composition comprises: 1.5% ⁇ Mn ⁇ 2.5% Preferably, the composition comprises: 1.2% ⁇ Si ⁇ 1.8% Preferably, the composition comprises: 1.2% ⁇ AI ⁇ 1.8% According to one particular embodiment, the composition comprises: 0.05% ⁇ V ⁇ 0.15% 0.004 ⁇ N ⁇ 0.008%. In a preferred embodiment, the composition comprises: 0.0005 ⁇ B ⁇ 0.003%.
  • the average size of the islands of martensite and residual austenite is less than 1 micrometer, the average distance between the islands being less than 6 microns.
  • a semi-finished product is cast from this steel, then the semi-finished product is heated to a temperature above 1150 ° C. and the semi-finished product is hot-rolled to obtain a hot-rolled sheet.
  • the sheet is reeled and stripped and then cold rolled with a reduction ratio of between 30 and 80% so as to obtain a cold-rolled sheet.
  • the cold-rolled sheet is heated at a speed V c of between 5 and 15 ° C./s up to a temperature T 1 of between Ac 3 and Ac 3 + 20 ° C., for a time t 1 of between 50 and 150 seconds. cools the sheet at a speed V R1 greater than 40 ° C / s to a temperature T 2 between (M s -30 ° C and M s + 30 ° C).
  • the sheet is maintained at said temperature T 2 for a time t 2 of between 150 and 350 seconds and then cooling is carried out at a speed V R2 of less than 30 ° C./s up to room temperature.
  • the subject of the invention is also a process for manufacturing a cold-rolled steel sheet with a resistance greater than 1200 MPa and an elongation at break greater than 8%, according to which a steel with a composition of 0.10 is supplied.
  • a semifinished product is cast from this steel, the semi-finished product is heated to a temperature above 1150 ° C., and then the semi-finished product is hot-rolled to obtain a hot-rolled sheet.
  • the sheet is reeled, the latter is scoured, then the sheet is cold-rolled with a reduction ratio of between 30 and 80% so as to obtain a cold-rolled sheet.
  • the cold-rolled sheet is heated to a speed V c of between 5 and 15 ° C / s to a temperature T 1 between Ac3 and Ac3 + 20 ° C, for a time t 1 between 50 and 150s and then cooled at a speed V R1 greater than 25 ° C / s up to a temperature T 2 between B s and (M s - 20 ° C)
  • the sheet is maintained at the temperature T 2 for a time t 2 of between 150 and 350 s and then cooling is carried out at a lower speed V R2 at 30 ° C / s to room temperature.
  • the invention also relates to the use of a cold rolled steel sheet annealed in one of the above modes, or manufactured by a method according to one of the above modes, for the manufacture structural parts or reinforcement elements, in the automotive field.
  • the inventors have demonstrated that the above problems were solved when the annealed cold-rolled thin steel sheet exhibited a bainitic microstructure, with in addition islands of martensite and residual austenite, or "M-A" islands.
  • M-A martensite and residual austenite
  • carbon plays a very important role in the formation of the microstructure and in the mechanical properties: in combination with other elements of the composition (Cr, Mo, Mn) and with the heat treatment of annealing after cold rolling, it increases the quenchability and makes it possible to obtain a bainitic transformation.
  • the carbon contents according to the invention also lead to the formation of islands of martensite and residual austenite whose quantity, morphology, composition make it possible to obtain the properties referred to above.
  • Carbon also retards the formation of pro-eutectoid ferrite after thermal annealing treatment after cold rolling: otherwise, the presence of this phase of low hardness would cause excessive local damage at the interface with the matrix whose hardness is higher. The presence of proeutectoid ferrite resulting from the annealing must therefore be avoided in order to obtain high levels of mechanical strength.
  • the carbon content is between 0.10 and 0.25% by weight: Below 0.10%, sufficient strength can not be obtained and the stability of the residual austenite is not not satisfactory. Beyond 0.25%, the weldability is reduced due to the formation of quenching microstructures in the heat-affected zone.
  • the carbon content is between 0.19 and 0.23%: within this range, the weldability is very satisfactory, and the quantity, the stability and the morphology of the islets MA are particularly adapted to obtain a favorable pair of mechanical properties (resistance-elongation)
  • an addition of manganese makes it possible to avoid the formation of proeutectoid ferrite during cooling after annealing after cold rolling.
  • Manganese also helps to deoxidize steel during liquid phase processing.
  • the addition of manganese also contributes to effective solid solution hardening and increased strength.
  • the manganese is between 1.5 and 2.5% so that these effects are obtained, and without risk of formation of harmful band structure.
  • Silicon and aluminum play an important role together according to the invention.
  • Silicon delays the precipitation of cementite during cooling from the austenite after annealing.
  • An addition of silicon according to the invention thus contributes to stabilizing a sufficient amount of residual austenite in the form of islets which subsequently and progressively transform into martensite under the effect of a deformation. Another part of the austenite is transformed directly into martensite during cooling after annealing.
  • Aluminum is a very effective element for the deoxidation of steel. As such, its content is greater than or equal to 0.010%. Like silicon, it stabilizes residual austenite.
  • the silicon content is preferably between 1.2 and 1.8% to stabilize a sufficient amount of residual austenite and to avoid intergranular oxidation during the hot winding step preceding the cold rolling. This also avoids the formation of strongly adherent oxides and the possible appearance of surface defects leading in particular to a lack of wettability in dip galvanizing operations.
  • the aluminum content is preferably between 1.2 and 1.8%.
  • the effects of aluminum are indeed similar to those described above for silicon, but the risk of occurrence of superficial defects is however less.
  • the steels according to the invention optionally comprise molybdenum and / or chromium: molybdenum increases quenchability, avoids the formation of pro-eutectoid ferrite and effectively refines the bainitic microstructure. However, a content greater than 0.25% by weight increases the risk of forming a predominantly martensitic microstructure to the detriment of bainite formation.
  • Chromium also helps to prevent the formation of pro-eutectoid ferrite and the refinement of the bainitic microstructure. Beyond 1.65%, the risk of obtaining a predominantly martensitic structure is important.
  • the chromium and molybdenum contents are such that: Cr + (3x Mo) ⁇ 0.3%.
  • the coefficients of chromium and molybdenum in this relationship reflect their influence on the quenchability, in particular the respective ability of these elements to avoid the formation of pro-eutectoid ferrite under the particular cooling conditions of the invention.
  • the steel may comprise very low or zero molybdenum and chromium contents, ie contents of less than 0.005% by weight for these two elements, and 0% boron.
  • the phosphorus content is limited to 0.1% so as to maintain sufficient hot ductility.
  • the nitrogen content is limited to 0.008% to avoid possible aging.
  • the steel according to the invention optionally contains vanadium in an amount of between 0.05 and 0.15%.
  • vanadium in an amount of between 0.05 and 0.15%.
  • the nitrogen content is between 0.004 and 0.008%, the precipitation of vanadium can occur during annealing after cold rolling in the form of fine carbonitrides which give additional hardening.
  • the steel may optionally comprise boron in an amount of less than or equal to 0.005%.
  • the steel preferentially contains between 0.0005 and 0.003% of boron, which contributes to the suppression of pro-eutectoid ferrite in the presence of chromium and / or molybdenum.
  • the addition of boron in the amount mentioned above makes it possible to obtain a resistance greater than 1400 MPa.
  • the steel may optionally comprise titanium in an amount such as Ti / N ⁇ 4 and Ti ⁇ 0.040%, which allows the formation of titanium carbonitrides and increases the hardening.
  • the rest of the composition consists of unavoidable impurities resulting from the elaboration.
  • the contents of these impurities, such as Sn, Sb, As, are less than 0.005%.
  • the microstructure of the steel is composed of 65 to 90% of bainite, these contents referring to surface percentages, the balance consists of islands of martensite and residual austenite (islets of MA compounds)
  • This bainitic structure which does not contain proeutectoid ferrite of low hardness, has an elongation capacity greater than 10%.
  • the M-A islands regularly dispersed in the matrix have an average size of less than 1 micrometer.
  • the figure 1 shows an example of microstructure of a steel sheet according to the invention.
  • the morphology of the islets MA was revealed by means of appropriate chemical reagents: after attack, the islets MA appear in white on a bainitic matrix more or less dark. Some small islands are located between the slats of bainite ferrite. The islands are observed at magnitudes ranging from 500 to 1500x on a statistically representative surface and the average size of the islands as well as the average distance between these islets is measured using an image analysis software. In the case of figure 1 , the surface percentage of the islets is 12% and the average size of the islets MA is less than 1 micrometer.
  • the microstructure is composed of 45 to 65% of bainite, the balance consisting of islands of martensite and residual austenite.
  • the microstructure is composed of 15 to 45% of bainite, the balance being consisting of martensite and residual austenite.
  • the cast semifinished products are first brought to a temperature higher than 1150 ° C. to reach at any point a temperature favorable to the high deformations which the steel will undergo during rolling.
  • the hot rolling step of these semi-finished products starting at more than 1150 ° C. can be done directly after casting. that an intermediate heating step is not necessary in this case.
  • the semi-finished product is hot-rolled.
  • An advantage of the invention is that the final characteristics and the microstructure of the cold-rolled and annealed sheet are relatively independent of the end-of-rolling temperature and the cooling after hot rolling.
  • the sheet is then reeled hot.
  • the winding temperature is preferably less than 550 ° C to limit the hardness of the hot-rolled sheet and intergranular oxidation at the surface. Too much hardness of the hot-rolled sheet leads to excessive forces during subsequent cold rolling and possibly to edge defects.
  • the hot-rolled sheet is then etched according to a method known per se in order to give the latter a surface condition suitable for cold. This is done by reducing the thickness of the hot-rolled sheet by 30 to 80%.
  • microstructural constituents measured by quantitative microscopy were also reported: surface fraction of bainite, martensite and residual austenite.
  • the tensile mechanical properties obtained were given in Table 3 below.
  • the Re / Rm ratio was also indicated.
  • the breaking energy at -40 ° C was determined from Charpy V type resilience specimens reduced to 1.4 mm thickness. Damage related to a cut (for example, shearing or punching) has also been evaluated which could possibly reduce the capacity for subsequent deformation of a cut piece. For this purpose, 20 ⁇ 80 mm 2 specimens were cut by shearing. Some of these specimens were then polished at the edges. The specimens were coated with photodeposited grids and then subjected to uniaxial traction until rupture.
  • the sheets of composition according to the invention and manufactured according to the conditions of the invention (I1-a, I2-ab, 13-a, I4, I5) have a combination particularly advantageous mechanical properties: on the one hand a mechanical strength greater than 1200 MPa, on the other hand an elongation at break always greater than or equal to 10%.
  • the steels according to the invention also have a Charpy V fracture energy at -40 ° C. of greater than 40 Joules / cm 2 . This allows the manufacture of parts resistant to the sudden propagation of a fault especially in case of dynamic stresses.
  • the microstructures of the steels with a minimum strength of 1200 MPa and a minimum breaking elongation of 10% according to the invention comprise a bainite content of between 65 and 90%, the balance consisting of MA islands.
  • the figure 1 thus presents the microstructure of the steel sheet 13a comprising 88% bainite and 12% islets MA, revealed by a LePera reagent attack.
  • the figure 2 presents this microstructure revealed by a Nital attack.
  • the steels according to the invention have a bainite content of between 45 and 65%, the balance being MA islands.
  • the steels according to the invention have a bainite content of between 15 and 35%, the balance being martensite and residual austenite.
  • the steel sheets according to the invention have an island size of less than 1 micrometer MA, the inter-island distance being less than 6 micrometers.
  • the steels according to the invention also have good resistance to damage in case of cutting since the damage factor ⁇ is limited to -23%.
  • a steel sheet that does not have these characteristics (R5) may have a 43% damage factor.
  • the sheets according to the invention have good hole expansion capability.
  • the steels according to the invention also have good weldability: for welding parameters adapted to the thicknesses mentioned above, the welded joints are free of cold or hot cracks.
  • the steel sheets I1-b and 11-c were annealed at a temperature T 1 too low, the austenitic transformation is not complete. As a result, the microstructure contains proeutectoid ferrite (40% for I1b, 20% for I1-c) and an excessive content of MA islands. The mechanical strength is then reduced by the presence of proeutectoid ferrite.
  • the holding temperature T 2 is greater than Ms + 30 ° C: the bainitic transformation which occurs at a higher temperature gives rise to a coarser structure and leads to insufficient mechanical strength.
  • the cooling rate V R1 after annealing is not sufficient, the microstructure formed is more heterogeneous and the elongation at break is reduced to less than 10%.
  • the holding temperature T 2 is less than Ms-20 ° C: consequently, the cooling V R1 causes the appearance of a bainite formed at low temperature and martensite, associated with insufficient elongation .
  • the steel R1 has an insufficient (silicon + aluminum) content, the holding temperature T 2 is less than Ms-20 ° C. Due to the insufficient content of (Si + Al), the amount of islets MA formed is insufficient to obtain a resistance greater than or equal to 1200 MPa.
  • R2 and R3 steels have insufficient carbon, manganese, silicon + aluminum contents.
  • the amount of MA compounds formed is less than 10%.
  • the annealing temperature T 1 lower than A c3 leads to an excessive content of proeutectoid ferrite and cementite, and insufficient strength.
  • the steel R4 has an insufficient content of (Si + Al)
  • the cooling rate V R1 is in particular too low.
  • the enrichment of carbon austenite during cooling is then insufficient to allow the formation of martensite and to obtain the strength and elongation properties of the invention.
  • Steel R5 also has an insufficient content of (Si + Al)
  • the insufficiently fast cooling rate after annealing leads to excessive proeutectoid ferrite content and insufficient mechanical strength.
  • the invention allows the manufacture of steel sheets combining a very high strength and high ductility.
  • the steel sheets according to the invention are used profitably for the manufacture of structural parts or reinforcement elements in the automotive field and general industry.

Abstract

A high-strength, cold rolled, annealed steel sheet (I) has composition (by weight, excluding iron, processing impurities and various optional components) 0.10-0.25% carbon, 1-3% manganese, at least 0.010% aluminum, up to 2.990% silicon (provided that Si + Al is 1-3%), up to 0.015% sulfur, up to 0.1% phosphorus and up to 0.008% nitrogen. The microstructure is 15-90% bainite, the remainder being martensite and residual austenite. A cold rolled, annealed steel sheet (I), with strength more than 1200 MPa, has composition (by weight, excluding iron and processing impurities) (i) 0.10-0.25% carbon, 1-3% manganese, at least 0.010% aluminum, up to 2.990% silicon (provided that Si + Al is 1-3%), up to 0.015% sulfur, up to 0.1% phosphorus and up to 0.008% nitrogen and optionally (ii) 0.05-0.15% vanadium, up to 0.25% molybdenum, up to 1.65% chromium (provided that Cr + (3 x Mo) is at least 0.3%) and up to 0.040% titanium (provided that Ti/N is at least 4). The microstructure is 15-90% bainite, the remainder being martensite and residual austenite. Independent claims are included for: (1) the production of (I) with elongation at break more than 8% from a steel as above, by (A) casting a semi-finished product from the steel; (B) heating to more than 1150[deg] C; (C) rolling to give a hot-rolled sheet; (D) coiling the sheet; (E) cleaning the sheet; (F) cold rolling the sheet at a degree of reduction of 30-80%; and (G) reheating the sheet at 5-15[deg] C per second to a temperature of Ac3 to Ac3 plus 20[deg] C, maintaining the temperature for 50-150 seconds, cooling at more than 25[deg] C per second to a temperature between B s and M s minus 20[deg] C, maintaining this temperature for 150-350 seconds and cooling at less than 30[deg] C per second to ambient temperature; and (2) the production of (I) with elongation at break more than 10% by a variant of the process, involving steps (A) - (G) as above except that (1) the steel contains more than 0.005% molybdenum, more than 0.005% chromium and no boron and consists of 65-90% bainite, the remainder being islets of martensite and residual austenite, and (2) in the reheating step (G) the sheet is cooled from the temperature of Ac3 to Ac3 plus 20[deg] C at more than 40[deg] C per second to a temperature between M s plus 30[deg] C and M s minus 30[deg] C (the holding time at this temperature and further cooling being as above).

Description

L'invention concerne la fabrication de tôles minces laminées à froid et recuites d'aciers présentant une résistance supérieure à 1200 MPa et un allongement à rupture supérieur à 8%. Le secteur automobile et l'industrie générale constituent notamment des domaines d'application de ces tôles d'aciers.The invention relates to the manufacture of cold-rolled and annealed thin sheets of steels having a strength greater than 1200 MPa and an elongation at break greater than 8%. The automotive sector and the general industry are notably fields of application for these steel sheets.

Il existe en particulier dans l'industrie automobile un besoin continu d'allègement des véhicules et d'accroissement de la sécurité. On a proposé successivement différentes familles d'aciers pour répondre à ce besoin de résistance accrue : on a tout d'abord proposé des aciers comportant des éléments de micro-alliage. Leur durcissement est dû à la précipitation de ces éléments et à l'affinement de la taille de grains. On a ensuite assisté au développement d'aciers « Dual-Phase » où la présence de martensite, constituant d'une grande dureté, au sein d'une matrice ferritique plus douce, permet d'obtenir une résistance supérieure à 450MPa associée à une bonne aptitude au formage à froid.In particular, there is a continuing need in the automotive industry for lighter vehicles and increased safety. Different families of steels have been successively proposed to meet this need for increased strength: first, steels comprising micro-alloy elements have been proposed. Their hardening is due to the precipitation of these elements and the refinement of the grain size. We then witnessed the development of "Dual-Phase" steels where the presence of martensite, constituting a great hardness, in a softer ferritic matrix, allows to obtain a resistance greater than 450MPa associated with good cold forming ability.

Afin d'accroître encore la résistance, on a développé des aciers présentant un comportement « TRIP » (Transformation Induced Plasticity ») avec des combinaisons de propriétés (résistance-aptitude à la déformation) très avantageuses : ces propriétés sont liées à la structure de ces aciers constituée d'une matrice ferritique comportant de la bainite et de l'austénite résiduelle. La présence de ce dernier constituant confère une ductilité élevée à une tôle non déformée. Sous l'effet d'une déformation ultérieure, par exemple lors d'une sollicitation uniaxiale, l'austénite résiduelle d'une pièce en acier TRIP se transforme progressivement en martensite, ce qui se traduit par une consolidation importante et retarde l'apparition d'une déformation localisée.In order to further increase the resistance, steels having a "TRIP" (Transformation Induced Plasticity) behavior with very advantageous combinations of properties (resistance-ability to deformation) have been developed: these properties are related to the structure of these structures. steels consisting of a ferritic matrix comprising bainite and residual austenite. The presence of the latter component gives a high ductility to a non-deformed sheet. Under the effect of a subsequent deformation, for example during a uniaxial loading, the residual austenite of a TRIP steel part gradually changes to martensite, which results in a significant consolidation and delays the appearance of 'localized deformation.

Des tôles d'aciers Dual Phase ou TRIP ont été proposées, avec un niveau de résistance maximal de l'ordre de 1000MPa. L'obtention de niveaux de résistance significativement supérieurs, par exemple 1200-1400MPa se heurte à différentes difficultés :

  • L'accroissement de résistance mécanique nécessite une analyse chimique nettement plus chargée en éléments d'alliage, au détriment de l'aptitude au soudage de ces aciers.
  • On observe un accroissement de la différence de dureté entre la matrice ferritique et les constituants durcissants : ceci a pour conséquence une concentration locale des contraintes et des déformations et un endommagement plus précoce, comme en témoigne la baisse de l'allongement réparti
  • On observe également un accroissement de la fraction des constituants durcissants au sein de la matrice ferritique : dans ce cas, les îlots, initialement isolés et de petite taille lorsque la résistance est faible, deviennent progressivement connexes et forment des constituants de grande taille qui favorisent là encore un endommagement précoce.
Dual Phase or TRIP steel plates have been proposed, with a maximum resistance level of the order of 1000 MPa. Obtaining significantly higher resistance levels, for example 1200-1400MPa, faces various difficulties:
  • The increase in mechanical strength requires a much more heavily loaded chemical analysis of alloying elements, to the detriment of the weldability of these steels.
  • An increase in the hardness difference between the ferritic matrix and the hardening constituents is observed: this results in a local concentration of stresses and deformations and an earlier damage, as evidenced by the decrease in the distributed elongation.
  • There is also an increase in the fraction of hardening components within the ferritic matrix: in this case, the islets, initially isolated and small when the resistance is low, become progressively connected and form large constituents which favor there still early damage.

Les possibilités d'obtenir simultanément de très hauts niveaux de résistance et certaines autres propriétés d'usage au moyen d'aciers TRIP ou à microstructure Dual Phase, semblent ainsi limitées. Pour atteindre une résistance encore plus élevée, c'est à dire un niveau supérieur à 800-1000 MPa, on a développé des aciers dits « multiphasés » à structure majoritairement bainitique. Dans l'industrie automobile ou dans l'industrie générale, des tôles d'aciers multiphasés de moyenne épaisseur sont utilisées avec profit pour des pièces structurales telles que traverses de pare-chocs, montants, renforts divers.The possibilities of simultaneously obtaining very high levels of resistance and certain other properties of use using TRIP or Dual Phase microstructure steels thus seem limited. To achieve an even higher resistance, ie a level greater than 800-1000 MPa, so-called "multiphase" steels have a predominantly bainitic structure. In the automotive industry or in the general industry, multi-phase steel sheets of medium thickness are used with advantage for structural parts such as bumper crosspieces, uprights, various reinforcements.

En particulier, dans le domaine des tôles d'acier multiphasés laminées à froid de plus de 980MPa, le brevet EP1559798 décrit la fabrication d'aciers de composition : 0,10-0,25% C, 1,0-2,0% Si, 1,5-3%Mn, la microstructure étant constituée d'au moins 60% de ferrite bainitique et d'au moins 5% d'austénite résiduelle, la ferrite polygonale étant inférieure à 20%. Les exemples de réalisation présentés dans ce document montrent que la résistance ne dépasse pas 1200MPa.In particular, in the field of cold-rolled multiphase steel sheets over 980 MPa, the patent EP1559798 discloses the manufacture of steels of composition: 0.10-0.25% C, 1.0-2.0% Si, 1.5-3% Mn, the microstructure consisting of at least 60% bainitic ferrite and at least 5% residual austenite, the polygonal ferrite being less than 20%. The exemplary embodiments presented in this document show that the resistance does not exceed 1200 MPa.

Le brevet EP 1589126 décrit également la fabrication de tôles minces laminées à froid, dont le produit (résistance x allongement) est supérieur à 20000 MPa%. La composition des aciers contient : 0,10-0,28%C, 1,0-2,0%Si, 1-3%Mn, moins de 0,10%Nb. La structure est constituée de plus de 50% de ferrite bainitique, de 5 à 20% d'austénite résiduelle, et de moins de 30% de ferrite polygonale. Là encore, les exemples présentés montrent que la résistance est encore inférieure à 1200MPa.The patent EP 1589126 also discloses the manufacture of cold-rolled thin sheet, the product of which (resistance x elongation) is greater than 20000 MPa%. The composition of the steels contains: 0.10-0.28% C, 1.0-2.0% Si, 1-3% Mn, less than 0.10% Nb. The structure consists of more than 50% bainitic ferrite, 5 to 20% residual austenite, and less than 30% of polygonal ferrite. Again, the examples presented show that the resistance is still below 1200 MPa.

La présente invention vise à résoudre les problèmes évoqués ci-dessus. Elle vise à mettre à disposition une tôle d'acier mince laminée à froid et recuite présentant une résistance mécanique supérieure à 1200 MPa conjointement avec un allongement à rupture supérieur à 8% et une bonne aptitude au formage à froid. L'invention vise également à mettre à disposition un acier peu sensible à l'endommagement lors de la découpe par un procédé mécanique.The present invention aims to solve the problems mentioned above. It aims to provide a cold rolled and annealed thin steel sheet having a mechanical strength greater than 1200 MPa together with an elongation greater than 8% rupture and good cold forming ability. The invention also aims at providing a steel that is not very sensitive to damage during cutting by a mechanical method.

Par ailleurs, l'invention vise à mettre à disposition un procédé de fabrication de tôles minces dont de faibles variations des paramètres n'entraînent pas de modifications importantes de la microstructure ou des propriétés mécaniques. L'invention vise également à mettre à disposition une tôle d'acier aisément fabricable par laminage à froid, c'est à dire dont la dureté après l'étape de laminage à chaud est limitée de telle sorte que les efforts de laminage restent modérés lors de l'étape de laminage à froid.Furthermore, the invention aims to provide a method of manufacturing thin sheets, small variations in the parameters do not lead to significant changes in the microstructure or mechanical properties. The invention also aims to provide a sheet of steel easily fabricated by cold rolling, that is to say whose hardness after the hot rolling step is limited so that the rolling forces remain moderate during of the cold rolling step.

Elle vise également à disposer d'une tôle d'acier mince apte au dépôt éventuel d'un revêtement métallique selon les procédés usuels.It also aims to have a thin steel sheet suitable for the possible deposit of a metal coating according to the usual methods.

Elle vise également à disposer d'une tôle d'acier peu sensible à un endommagement par découpe.It also aims to have a steel sheet insensitive to damage by cutting.

Elle vise encore à disposer d'un acier présentant une bonne aptitude au soudage au moyen des procédés d'assemblage usuels tels que le soudage par résistance par points.It also aims to have a steel having good weldability by means of conventional assembly methods such as spot resistance welding.

Dans ce but, l'invention a pour objet une tôle d'acier laminée à froid et recuite de résistance supérieure à 1200 MPa, dont la composition comprend, les teneurs étant exprimées en poids : 0,10% ≤ C ≤ 0,25%, 1%≤ Mn ≤ 3%, Al ≥ 0,010 %, Si≤2,990%, S ≤ 0,015%, P≤ 0,1%, N≤0,008%, étant entendu que 1% ≤Si+Al ≤3%, la composition comprenant éventuellement : 0,05% ≤ V ≤ 0,15%, B≤0,005%, Mo ≤ 0,25%, Cr ≤ 1,65%, étant entendu que Cr+(3 x Mo) ≥0,3%, Ti en quantité telle que Ti/N≥4 et que Ti≤0,040%, le reste de la composition étant constitué de fer et d'impuretés inévitables résultant de l'élaboration, la microstructure dudit acier comprenant 15 à 90% de bainite, le solde étant constitué de martensite et d'austénite résiduelle.For this purpose, the subject of the invention is a cold-rolled and annealed steel sheet with a resistance greater than 1200 MPa, the composition of which comprises the contents being expressed by weight: 0.10% ≤ C ≤ 0.25% , 1% ≤ Mn ≤ 3%, Al ≥ 0.010%, Si≤2.990%, S ≤0.015%, P≤0.1%, N≤0.008%, with the proviso that 1% ≤Si + Al ≤3%, the composition optionally comprising: 0.05% ≤ V ≤ 0.15%, B≤0.005%, Mo ≤ 0.25%, Cr ≤ 1.65%, with the proviso that Cr + (3 × Mo) ≥0.3%, Ti in an amount such that Ti / N≥4 and Ti≤0.040%, the remainder of the composition consisting of iron and unavoidable impurities resulting from the elaboration, the microstructure of said steel comprising 15 to 90% of bainite, the balance consisting of martensite and residual austenite.

L'invention a également pour objet une tôle d'acier de composition ci-dessus, d'allongement à rupture supérieur à 10%, caractérisée en ce que Mo< 0,005%, Cr<0,005%, B=0, la microstructure de l'acier comprenant 65 à 90% de bainite, le solde étant constitué d'îlots de martensite et d'austénite résiduelle
L'invention a également pour objet une tôle d'acier de composition ci-dessus, caractérisée en ce qu'elle contient :Mo ≤ 0,25%, Cr ≤ 1,65%, étant entendu que Cr+(3 x Mo) ≥0,3%, B=0, la microstructure de l'acier comprenant 65 à 90% de bainite, le solde étant constitué d'îlots de martensite et d'austénite résiduelle
The subject of the invention is also a sheet of steel with the above composition, with an elongation at break greater than 10%, characterized in that Mo <0.005%, Cr <0.005%, B = 0, the microstructure of the steel comprising 65 to 90% bainite, the balance consisting of islands of martensite and residual austenite
The invention also relates to a steel sheet of the above composition, characterized in that it contains: Mo ≤ 0.25%, Cr ≤ 1.65%, it being understood that Cr + (3 × Mo) ≥ 0.3%, B = 0, the microstructure of the steel comprising 65 to 90% of bainite, the remainder being islands of martensite and residual austenite

L'invention a encore pour objet une tôle d'acier de composition ci-dessus, de résistance supérieure à 1400MPa, d'allongement à rupture supérieur à 8%, caractérisée en ce qu'elle contient : Mo ≤ 0,25%, Cr ≤ 1,65%, étant entendu que Cr+(3 x Mo) ≥0,3%, la microstructure de l'acier comprenant 45 à 65% de bainite, le solde étant constitué d'îlots de martensite et d'austénite résiduelle L'invention a également pour objet une tôle d'acier de composition ci-dessus, de résistance supérieure à 1600MPa, d'allongement à rupture supérieur à 8%, caractérisée en ce qu'elle contient: Mo ≤ 0,25%, Cr ≤ 1,65%, étant entendu que : Cr+(3 x Mo) ≥0,3%, la microstructure de l'acier comprenant 15 à 45% de bainite, le solde étant constitué de martensite et d'austénite résiduelle.
Selon un mode particulier, la composition comprend : 0,19% ≤ C ≤ 0,23%
Selon un mode préféré, la composition comprend : 1,5% ≤Mn ≤ 2,5%
Préférentiellement, la composition comprend : 1,2% ≤Si ≤ 1,8%
A titre préféré, la composition comprend : 1,2% ≤AI ≤ 1,8%
Selon un mode particulier, la composition comprend : 0,05% ≤ V ≤ 0,15% 0,004 ≤N ≤ 0,008%.
Selon un mode préféré, la composition comprend : 0,0005≤ B ≤ 0,003%.
The subject of the invention is also a steel sheet of the above composition, with a resistance greater than 1400 MPa, with an elongation at break greater than 8%, characterized in that it contains: Mo ≤ 0.25%, Cr ≤ 1.65%, it being understood that Cr + (3 x Mo) ≥0.3%, the microstructure of the steel comprising 45 to 65% of bainite, the remainder being islands of martensite and residual austenite L Another subject of the invention is a steel sheet of the above composition, with a resistance greater than 1600 MPa, with an elongation at break greater than 8%, characterized in that it contains: Mo ≤ 0.25%, Cr ≤ 1.65%, it being understood that: Cr + (3 x Mo) ≥0.3%, the microstructure of the steel comprising 15 to 45% of bainite, the remainder consisting of martensite and residual austenite.
According to a particular mode, the composition comprises: 0.19% ≤ C ≤ 0.23%
According to a preferred mode, the composition comprises: 1.5% ≤Mn ≤ 2.5%
Preferably, the composition comprises: 1.2% ≤Si ≤ 1.8%
Preferably, the composition comprises: 1.2% ≤ AI ≤ 1.8%
According to one particular embodiment, the composition comprises: 0.05% ≤ V ≤ 0.15% 0.004 ≤ N ≤ 0.008%.
In a preferred embodiment, the composition comprises: 0.0005 ≤ B ≤ 0.003%.

Préférentiellement, la taille moyenne des îlots de martensite et d'austénite résiduelle est inférieure à 1 micromètre, la distance moyenne entre les îlots étant inférieure à 6 micromètres.Preferably, the average size of the islands of martensite and residual austenite is less than 1 micrometer, the average distance between the islands being less than 6 microns.

L'invention a également pour objet un procédé de fabrication d'une tôle d'acier laminée à froid de résistance supérieure à 1200 MPa, d'allongement à rupture supérieur à 10%, selon lequel on approvisionne un acier de composition : 0,10% ≤ C ≤ 0,25%, 1%≤ Mn ≤ 3%, AI ≥ 0,010 %, Si≤2,990%, étant entendu que :1% ≤Si+AI ≤3%, S ≤ 0,015%, P≤ 0,1%, N≤0,008%, Mo<0,005%, Cr<0,005%, B=0, la composition comprenant éventuellement : 0,05% ≤ V ≤ 0,15%, Ti en quantité telle que Ti/N≥4 et que Ti≤0,040%. On procède à la coulée d'un demi-produit à partir de cet acier, puis on porte le demi-produit à une température supérieure à 1150°C et on lamine à chaud le demi-produit pour obtenir une tôle laminée à chaud. On bobine et on décape la tôle, puis on lamine à froid celle-ci avec un taux de réduction compris entre 30 et 80% de façon à obtenir une tôle laminée à froid. On réchauffe la tôle laminée à froid à une vitesse Vc comprise entre 5 et 15°C/s jusqu'à une température T1 comprise entre Ac3 et Ac3+20°C, pendant un temps t1 compris entre 50 et 150s puis on refroidit la tôle à une vitesse VR1 supérieure à 40°C/s jusqu'à une température T2 comprise entre (Ms-30°C et Ms+30°C). On maintient la tôle à ladite température T2 pendant un temps t2 compris entre 150 et 350s puis on effectue un refroidissement à une vitesse VR2 inférieure à 30°C /s jusqu'à la température ambiante.The subject of the invention is also a process for manufacturing a cold-rolled steel sheet with a resistance greater than 1200 MPa, with an elongation at break greater than 10%, according to which a composition steel is supplied: 0.10 % ≤ C ≤ 0.25%, 1% ≤ Mn ≤ 3%, AI ≥ 0.010%, Si≤2.990%, provided that: 1% ≤Si + AI ≤3%, S ≤ 0.015%, P≤ 0, 1%, N≤0.008%, Mo <0.005%, Cr <0.005%, B = 0, the composition optionally comprising: 0.05% ≤V ≤ 0.15%, Ti in an amount such that Ti / N≥4 and than Ti≤0.040%. A semi-finished product is cast from this steel, then the semi-finished product is heated to a temperature above 1150 ° C. and the semi-finished product is hot-rolled to obtain a hot-rolled sheet. The sheet is reeled and stripped and then cold rolled with a reduction ratio of between 30 and 80% so as to obtain a cold-rolled sheet. The cold-rolled sheet is heated at a speed V c of between 5 and 15 ° C./s up to a temperature T 1 of between Ac 3 and Ac 3 + 20 ° C., for a time t 1 of between 50 and 150 seconds. cools the sheet at a speed V R1 greater than 40 ° C / s to a temperature T 2 between (M s -30 ° C and M s + 30 ° C). The sheet is maintained at said temperature T 2 for a time t 2 of between 150 and 350 seconds and then cooling is carried out at a speed V R2 of less than 30 ° C./s up to room temperature.

L'invention a également pour objet un procédé de fabrication d'une tôle d'acier laminée à froid de résistance supérieure à 1200 MPa, d'allongement à rupture supérieur à 8%, selon lequel on approvisionne un acier de composition :0,10% ≤ C ≤ 0,25%, 1%≤ Mn ≤ 3% , Al ≥ 0,010 %, Si≤2,990%, étant entendu que 1% ≤Si+Al ≤3%, S ≤ 0,015%, P≤ 0,1%, N≤0,008%, Mo ≤ 0,25%, Cr ≤ 1,65%, étant entendu que Cr+(3 x Mo) ≥0,3%, éventuellement 0,05% ≤ V ≤ 0,15%, B≤0,005%, Ti en quantité telle que Ti/N≥4 et que Ti≤0,040%. On procède à la coulée d'un demi-produit à partir de cet acier, on porte le demi-produit à une température supérieure à 1150°C, puis on lamine à chaud le demi-produit pour obtenir une tôle laminée à chaud. On bobine la tôle, on décape celle-ci, puis on lamine à froid la tôle avec un taux de réduction compris entre 30 et 80% de façon à obtenir une tôle laminée à froid. On réchauffe la tôle laminée à froid à une vitesse Vc comprise entre 5 et 15°C/s jusqu'à une température T1 comprise entre Ac3 et Ac3+20°C, pendant un temps t1 compris entre 50 et 150s puis on refroidit celle-ci à une vitesse VR1 supérieure à 25°C/s jusqu'à une température T2 comprise entre Bs et (Ms - 20°C) On maintient la tôle à la température T2 pendant un temps t2 compris entre 150 et 350s puis on effectue un refroidissement à une vitesse VR2 inférieure à 30°C /s jusqu'à la température ambiante.The subject of the invention is also a process for manufacturing a cold-rolled steel sheet with a resistance greater than 1200 MPa and an elongation at break greater than 8%, according to which a steel with a composition of 0.10 is supplied. % ≤ C ≤ 0.25%, 1% ≤ Mn ≤ 3%, Al ≥ 0.010%, Si≤2.990%, provided that 1% ≤Si + Al ≤3%, S ≤ 0.015%, P≤ 0.1 %, N≤0.008%, Mo ≤ 0.25%, Cr ≤ 1.65%, with the proviso that Cr + (3 x Mo) ≥0.3%, optionally 0.05% ≤ V ≤ 0.15%, B ≤0.005%, Ti in an amount such that Ti / N≥4 and Ti≤0.040%. A semifinished product is cast from this steel, the semi-finished product is heated to a temperature above 1150 ° C., and then the semi-finished product is hot-rolled to obtain a hot-rolled sheet. The sheet is reeled, the latter is scoured, then the sheet is cold-rolled with a reduction ratio of between 30 and 80% so as to obtain a cold-rolled sheet. The cold-rolled sheet is heated to a speed V c of between 5 and 15 ° C / s to a temperature T 1 between Ac3 and Ac3 + 20 ° C, for a time t 1 between 50 and 150s and then cooled at a speed V R1 greater than 25 ° C / s up to a temperature T 2 between B s and (M s - 20 ° C) The sheet is maintained at the temperature T 2 for a time t 2 of between 150 and 350 s and then cooling is carried out at a lower speed V R2 at 30 ° C / s to room temperature.

L'invention a également pour objet l'utilisation d'une tôle d'acier laminée à froid et recuite selon l'un des modes ci-dessus, ou fabriquée par un procédé selon l'un des modes ci-dessus, pour la fabrication de pièces de structure ou d'éléments de renfort, dans le domaine automobile.The invention also relates to the use of a cold rolled steel sheet annealed in one of the above modes, or manufactured by a method according to one of the above modes, for the manufacture structural parts or reinforcement elements, in the automotive field.

D'autres caractéristiques et avantages de l'invention apparaîtront au cours de la description ci-dessous, donnée à titre d'exemple et faite en référence aux figures annexées ci-jointes :

  • La figure 1 présente un exemple de structure d'une tôle d'acier selon l'invention, la structure étant révélée par réactif LePera.
  • La figure 2 présente un exemple de structure d'une tôle d'acier selon l'invention, la structure étant révélée par réactif Nital.
Other features and advantages of the invention will become apparent from the description below, given by way of example and with reference to the appended figures attached hereto:
  • The figure 1 shows an example of structure of a steel sheet according to the invention, the structure being revealed by LePera reagent.
  • The figure 2 shows an example of structure of a steel sheet according to the invention, the structure being revealed by Nital reagent.

Les inventeurs ont mis en évidence que des problèmes ci-dessus étaient résolus lorsque la tôle d'acier mince laminée à froid et recuite présentait une microstructure bainitique, avec en complément des îlots de martensite et d'austénite résiduelle, ou îlots « M-A ». Pour les aciers dont la résistance est la plus élevée, supérieure à 1600MPa, la microstructure comporte une quantité plus importante de martensite et d'austénite résiduelle.The inventors have demonstrated that the above problems were solved when the annealed cold-rolled thin steel sheet exhibited a bainitic microstructure, with in addition islands of martensite and residual austenite, or "M-A" islands. For steels with the highest strength greater than 1600 MPa, the microstructure contains a greater amount of martensite and residual austenite.

En ce qui concerne la composition chimique de l'acier, le carbone joue un rôle très important sur la formation de la microstructure et sur les propriétés mécaniques : en liaison d'autres éléments de la composition (Cr, Mo, Mn) et avec le traitement thermique de recuit après laminage à froid, il augmente la trempabilité et permet d'obtenir une transformation bainitique. Les teneurs en carbone selon l'invention conduisent également à la formation d'îlots de martensite et d'austénite résiduelle dont la quantité, la morphologie, la composition permettent d'obtenir les propriétés visées ci-dessus.With regard to the chemical composition of steel, carbon plays a very important role in the formation of the microstructure and in the mechanical properties: in combination with other elements of the composition (Cr, Mo, Mn) and with the heat treatment of annealing after cold rolling, it increases the quenchability and makes it possible to obtain a bainitic transformation. The carbon contents according to the invention also lead to the formation of islands of martensite and residual austenite whose quantity, morphology, composition make it possible to obtain the properties referred to above.

Le carbone retarde également la formation de la ferrite pro-eutectoïde après traitement thermique de recuit après laminage à froid: dans le cas contraire, la présence de cette phase de faible dureté provoquerait un endommagement local trop important à l'interface avec la matrice dont la dureté est plus élevée. La présence de ferrite proeutectoïde issue du recuit doit donc être évitée pour obtenir des niveaux élevés de résistance mécanique.Carbon also retards the formation of pro-eutectoid ferrite after thermal annealing treatment after cold rolling: otherwise, the presence of this phase of low hardness would cause excessive local damage at the interface with the matrix whose hardness is higher. The presence of proeutectoid ferrite resulting from the annealing must therefore be avoided in order to obtain high levels of mechanical strength.

Selon l'invention, la teneur en carbone est comprise entre 0,10 et 0,25% en poids : Au dessous de 0,10%, une résistance suffisante ne peut pas être obtenue et la stabilité de l'austénite résiduelle n'est pas satisfaisante. Au delà de 0,25%, la soudabilité est réduite en raison de la formation de microstructures de trempe dans la Zone Affectée par la Chaleur.According to the invention, the carbon content is between 0.10 and 0.25% by weight: Below 0.10%, sufficient strength can not be obtained and the stability of the residual austenite is not not satisfactory. Beyond 0.25%, the weldability is reduced due to the formation of quenching microstructures in the heat-affected zone.

Selon un mode préféré, la teneur en carbone est comprise entre 0,19 et 0,23% : au sein de cette plage, la soudabilité est très satisfaisante, et la quantité, la stabilité et la morphologie des îlots M-A sont particulièrement adaptées pour obtenir un couple favorable de propriétés mécaniques (résistance-allongement)According to a preferred mode, the carbon content is between 0.19 and 0.23%: within this range, the weldability is very satisfactory, and the quantity, the stability and the morphology of the islets MA are particularly adapted to obtain a favorable pair of mechanical properties (resistance-elongation)

En quantité comprise entre 1 et 3% en poids, une addition de manganèse, élément à caractère gammagène, permet d'éviter la formation de ferrite proeutectoïde lors du refroidissement au recuit après laminage à froid. Le manganèse contribue également à désoxyder l'acier lors de l'élaboration en phase liquide. L'addition de manganèse participe également à un durcissement efficace en solution solide et à l'obtention d'une résistance accrue. Préférentiellement, le manganèse est compris entre 1,5 et 2,5% de façon à ce que ces effets soient obtenus, et ce sans risque de formation de structure en bandes néfaste.In an amount of between 1 and 3% by weight, an addition of manganese, a gamma-type element, makes it possible to avoid the formation of proeutectoid ferrite during cooling after annealing after cold rolling. Manganese also helps to deoxidize steel during liquid phase processing. The addition of manganese also contributes to effective solid solution hardening and increased strength. Preferably, the manganese is between 1.5 and 2.5% so that these effects are obtained, and without risk of formation of harmful band structure.

Le silicium et l'aluminium jouent de façon conjointe un rôle important selon l'invention.Silicon and aluminum play an important role together according to the invention.

Le silicium retarde la précipitation de la cémentite lors du refroidissement à partir de l'austénite après recuit. Une addition de silicium selon l'invention contribue donc à stabiliser une quantité suffisante d'austénite résiduelle sous forme d'îlots qui se transforment ultérieurement et progressivement en martensite sous l'effet d'une déformation. Une autre partie de l'austénite se transforme directement en martensite lors du refroidissement après recuit.Silicon delays the precipitation of cementite during cooling from the austenite after annealing. An addition of silicon according to the invention thus contributes to stabilizing a sufficient amount of residual austenite in the form of islets which subsequently and progressively transform into martensite under the effect of a deformation. Another part of the austenite is transformed directly into martensite during cooling after annealing.

L'aluminium est un élément très efficace pour la désoxydation de l'acier. A ce titre, sa teneur est supérieure ou égale à 0,010%. Comme le silicium, il stabilise l'austénite résiduelle.Aluminum is a very effective element for the deoxidation of steel. As such, its content is greater than or equal to 0.010%. Like silicon, it stabilizes residual austenite.

Les effets de l'aluminium et du silicium sur la stabilisation de l'austénite sont voisins ; lorsque les teneurs en silicium et en aluminium sont telles que: 1%≤Si+Al≤3%, une stabilisation satisfaisante de l'austénite est obtenue, ce qui permet de former les microstructures recherchées tout en conservant des propriétés d'usage satisfaisantes. Compte tenu du fait que la teneur minimale en aluminium est de 0,010%, la teneur en silicium est inférieure ou égale à 2,990%.The effects of aluminum and silicon on the stabilization of austenite are similar; when the silicon and aluminum contents are such that: 1% ≤Si + Al≤3%, a satisfactory stabilization of the austenite is obtained, which makes it possible to form the desired microstructures while retaining satisfactory use properties. Given that the minimum aluminum content is 0.010%, the silicon content is less than or equal to 2.990%.

La teneur en silicium est de préférence comprise entre 1,2 et 1,8% pour stabiliser une quantité d'austénite résiduelle suffisante et pour éviter une oxydation intergranulaire lors de l'étape de bobinage à chaud précédant le laminage à froid. On évite aussi de la sorte la formation d'oxydes fortement adhérents et l'apparition éventuelle de défauts de surface conduisant notamment à un manque de mouillabilité dans les opérations de galvanisation au trempé.The silicon content is preferably between 1.2 and 1.8% to stabilize a sufficient amount of residual austenite and to avoid intergranular oxidation during the hot winding step preceding the cold rolling. This also avoids the formation of strongly adherent oxides and the possible appearance of surface defects leading in particular to a lack of wettability in dip galvanizing operations.

Ces effets sont également obtenus lorsque la teneur en aluminium est de préférence comprise entre 1,2 et 1,8%. A teneur équivalente, les effets de l'aluminium sont en effet semblables à ceux exposés ci-dessus pour le silicium, mais le risque d'apparition de défauts superficiels est cependant moindre.These effects are also obtained when the aluminum content is preferably between 1.2 and 1.8%. At equivalent content, the effects of aluminum are indeed similar to those described above for silicon, but the risk of occurrence of superficial defects is however less.

Les aciers selon l'invention comportent éventuellement du molybdène et/ou du chrome : le molybdène augmente la trempabilité, évite la formation de ferrite pro-eutectoïde et affine efficacement la microstructure bainitique. Cependant, une teneur supérieure à 0,25% en poids augmente le risque de former une microstructure majoritairement martensitique au détriment de la formation de bainite.The steels according to the invention optionally comprise molybdenum and / or chromium: molybdenum increases quenchability, avoids the formation of pro-eutectoid ferrite and effectively refines the bainitic microstructure. However, a content greater than 0.25% by weight increases the risk of forming a predominantly martensitic microstructure to the detriment of bainite formation.

Le chrome contribue également à éviter la formation de ferrite pro-eutectoïde et à l'affinement de la microstructure bainitique. Au delà de 1,65%, le risque d'obtenir une structure majoritairement martensitique est important.Chromium also helps to prevent the formation of pro-eutectoid ferrite and the refinement of the bainitic microstructure. Beyond 1.65%, the risk of obtaining a predominantly martensitic structure is important.

Comparé au molybdène, son effet est cependant moins marqué ; selon l'invention, les teneurs en chrome et en molybdène sont telles que : Cr+(3 x Mo) ≥0,3%. Les coefficients du chrome et du molybdène dans cette relation traduisent leur influence sur la trempabilité, en particulier l'aptitude respective de ces éléments à éviter la formation de ferrite pro-eutectoïde dans les conditions de refroidissement particulières de l'invention.Compared to molybdenum, its effect is however less marked; according to the invention, the chromium and molybdenum contents are such that: Cr + (3x Mo) ≥0.3%. The coefficients of chromium and molybdenum in this relationship reflect their influence on the quenchability, in particular the respective ability of these elements to avoid the formation of pro-eutectoid ferrite under the particular cooling conditions of the invention.

Selon un mode économique de l'invention, l'acier peut comporter des teneurs en molybdène et en chrome très faibles ou nulles, c'est à dire des teneurs inférieures à 0,005% en poids pour ces deux éléments, et 0% de bore.According to an economic mode of the invention, the steel may comprise very low or zero molybdenum and chromium contents, ie contents of less than 0.005% by weight for these two elements, and 0% boron.

Pour obtenir une résistance supérieure à 1400MPa, l'addition de chrome et/ou de molybdène est requise, dans des quantités mentionnées ci-dessus.To obtain a strength greater than 1400 MPa, the addition of chromium and / or molybdenum is required, in amounts mentioned above.

Lorsque la teneur en soufre est supérieure à 0,015%, l'aptitude à la mise en forme est réduite en raison de la présence excessive de sulfures de manganèse.When the sulfur content is greater than 0.015%, the shaping ability is reduced due to the excessive presence of manganese sulphides.

La teneur en phosphore est limitée à 0,1 % de façon à maintenir une ductilité à chaud suffisante.The phosphorus content is limited to 0.1% so as to maintain sufficient hot ductility.

La teneur en azote est limitée à 0,008% pour éviter un vieillissement éventuel.The nitrogen content is limited to 0.008% to avoid possible aging.

L'acier selon l'invention contient éventuellement du vanadium en quantité comprise entre 0,05 et 0,15%. En particulier, lorsque la teneur en azote est comprise conjointement entre 0,004 et 0,008%, la précipitation du vanadium peut intervenir lors du recuit après laminage à froid sous forme de fins carbonitrures qui confèrent un durcissement supplémentaire.The steel according to the invention optionally contains vanadium in an amount of between 0.05 and 0.15%. In particular, when the nitrogen content is between 0.004 and 0.008%, the precipitation of vanadium can occur during annealing after cold rolling in the form of fine carbonitrides which give additional hardening.

L'acier peut éventuellement comprendre du bore en quantité inférieure ou égale à 0,005%. Selon un mode préféré, l'acier contient préférentiellement entre 0,0005 et 0,003% de bore, ce qui contribue à la suppression de la ferrite pro-eutectoïde en présence de chrome et/ou de molybdène. En complément des autres éléments d'addition, l'ajout de bore en quantité mentionnée ci-dessus permet d'obtenir une résistance supérieure à 1400 MPa.The steel may optionally comprise boron in an amount of less than or equal to 0.005%. In a preferred embodiment, the steel preferentially contains between 0.0005 and 0.003% of boron, which contributes to the suppression of pro-eutectoid ferrite in the presence of chromium and / or molybdenum. In addition to the other addition elements, the addition of boron in the amount mentioned above makes it possible to obtain a resistance greater than 1400 MPa.

L'acier peut éventuellement comprendre du titane en quantité telle que Ti/N≥4 et que Ti≤0,040%, ce qui permet la formation de carbonitrures de titane et augmente le durcissement.The steel may optionally comprise titanium in an amount such as Ti / N≥4 and Ti≤0.040%, which allows the formation of titanium carbonitrides and increases the hardening.

Le reste de la composition est constitué d'impuretés inévitables résultant de l'élaboration. Les teneurs de ces impuretés, telles que Sn, Sb, As, sont inférieures à 0,005%.The rest of the composition consists of unavoidable impurities resulting from the elaboration. The contents of these impurities, such as Sn, Sb, As, are less than 0.005%.

Selon un mode de réalisation de l'invention destiné à la fabrication de tôles d'acier de résistance supérieure à 1200MPa, la microstructure de l'acier est composée de 65 à 90% de bainite, ces teneurs se référant à des pourcentages surfaciques, le solde est constitué d'îlots de martensite et d'austénite résiduelle (îlots de composés M-A)According to one embodiment of the invention intended for the manufacture of steel sheets with a resistance greater than 1200 MPa, the microstructure of the steel is composed of 65 to 90% of bainite, these contents referring to surface percentages, the balance consists of islands of martensite and residual austenite (islets of MA compounds)

Cette structure en majorité bainitique, ne comportant pas de ferrite proeutectoïde de faible dureté, présente une capacité d'allongement à rupture supérieure à 10%.This bainitic structure, which does not contain proeutectoid ferrite of low hardness, has an elongation capacity greater than 10%.

Selon l'invention, les îlots M-A régulièrement dispersés dans la matrice ont une taille moyenne inférieure à 1 micromètre.According to the invention, the M-A islands regularly dispersed in the matrix have an average size of less than 1 micrometer.

La figure 1 présente un exemple de microstructure d'une tôle d'acier selon l'invention. La morphologie des îlots M-A a été révélée au moyen de réactifs chimiques appropriés : après attaque, les îlots M-A apparaissent en blanc sur une matrice bainitique plus ou moins sombre. Certains îlots de petite taille sont localisés entre les lattes de ferrite bainitique. On observe les îlots à des grandissements allant de 500 à 1500x environ sur une surface statistiquement représentative et on mesure grâce à un logiciel d'analyse d'images la taille moyenne des îlots ainsi que la distance moyenne entre ces îlots. Dans le cas de la figure 1, le pourcentage surfacique des îlots est de 12% et la taille moyenne des îlots M-A est inférieure à 1 micromètre.The figure 1 shows an example of microstructure of a steel sheet according to the invention. The morphology of the islets MA was revealed by means of appropriate chemical reagents: after attack, the islets MA appear in white on a bainitic matrix more or less dark. Some small islands are located between the slats of bainite ferrite. The islands are observed at magnitudes ranging from 500 to 1500x on a statistically representative surface and the average size of the islands as well as the average distance between these islets is measured using an image analysis software. In the case of figure 1 , the surface percentage of the islets is 12% and the average size of the islets MA is less than 1 micrometer.

On a mis en évidence qu'une morphologie spécifique des îlots M-A était à rechercher particulièrement : lorsque la taille moyenne des îlots est inférieure à 1 micromètre et lorsque la distance moyenne entre ces îlots est inférieure à 6 micromètres, on obtient simultanément les effets suivants :

  • un endommagement limité en raison de l'absence d'amorçage de la rupture sur des îlots M-A de grande taille
  • un durcissement significatif en raison de la proximité de nombreux constituants M-A de faible taille
It has been demonstrated that a specific morphology of the MA islets is to be particularly sought: when the average size of the islets is less than 1 micrometer and when the average distance between these islets is less than 6 micrometers, the following effects are simultaneously obtained:
  • limited damage due to lack of breakout initiation on large MA islands
  • a significant hardening due to the proximity of many small MA constituents

Selon un autre mode de réalisation de l'invention destiné à la fabrication de tôles d'acier de résistance supérieure à 1400MPa et d'allongement à rupture supérieur à 8%, la microstructure est composée de 45 à 65% de bainite, le solde étant constitué d'îlots de martensite et d'austénite résiduelle.According to another embodiment of the invention intended for the manufacture of steel sheets with a strength greater than 1400 MPa and elongation at break greater than 8%, the microstructure is composed of 45 to 65% of bainite, the balance consisting of islands of martensite and residual austenite.

Selon un autre mode de réalisation de l'invention destiné à la fabrication de tôles d'acier de résistance supérieure à 1600MPa et d'allongement à rupture supérieur à 8%, la microstructure est composée de 15 à 45% de bainite, le solde étant constitué de martensite et d'austénite résiduelle.According to another embodiment of the invention intended for the manufacture of steel sheets with a resistance greater than 1600 MPa and an elongation at break greater than 8%, the microstructure is composed of 15 to 45% of bainite, the balance being consisting of martensite and residual austenite.

La mise en oeuvre du procédé de fabrication d'une tôle mince laminée à froid et recuite selon l'invention est la suivante :

  • On approvisionne un acier de composition selon l'invention
  • On procède à la coulée d'un demi-produit à partir de cet acier. Cette coulée peut être réalisée en lingots ou en continu sous forme de brames d'épaisseur de l'ordre de 200mm. On peut également effectuer la coulée sous forme de brames minces de quelques dizaines de millimètres d'épaisseur, ou de bandes minces, entre cylindres d'acier contra-rotatifs.
The implementation of the method for manufacturing a cold rolled and annealed thin sheet according to the invention is as follows:
  • A composition steel is supplied according to the invention
  • A semi-finished product is cast from this steel. This casting can be carried out in ingots or continuously in the form of slabs of the order of 200mm thickness. The casting can also be carried out in the form of thin slabs of a few tens of millimeters thick, or thin strips, between contra-rotating steel rolls.

Les demi-produits coulés sont tout d'abord portés à une température supérieure à 1150°C pour atteindre en tout point une température favorable aux déformations élevées que va subir l'acier lors du laminage.The cast semifinished products are first brought to a temperature higher than 1150 ° C. to reach at any point a temperature favorable to the high deformations which the steel will undergo during rolling.

Naturellement, dans le cas d'une coulée directe de brames minces ou de bandes minces entre cylindres contra-rotatifs, l'étape de laminage à chaud de ces demi-produits débutant à plus de 1150°C peut se faire directement après coulée si bien qu'une étape de réchauffage intermédiaire n'est pas nécessaire dans ce cas.Naturally, in the case of a direct casting of thin slabs or thin strips between contra-rotating rolls, the hot rolling step of these semi-finished products starting at more than 1150 ° C. can be done directly after casting. that an intermediate heating step is not necessary in this case.

On lamine à chaud le demi-produit. Un avantage de l'invention est que les caractéristiques finales et la microstructure de la tôle laminée à froid et recuite sont relativement peu dépendantes de la température de fin de laminage et du refroidissement suivant le laminage à chaud.The semi-finished product is hot-rolled. An advantage of the invention is that the final characteristics and the microstructure of the cold-rolled and annealed sheet are relatively independent of the end-of-rolling temperature and the cooling after hot rolling.

On bobine ensuite la tôle à chaud. La température de bobinage est préférentiellement inférieure à 550°C pour limiter la dureté de la tôle laminée à chaud et l'oxydation intergranulaire en surface. Une dureté trop importante de la tôle laminée à chaud conduit à des efforts excessifs lors du laminage ultérieur à froid ainsi éventuellement qu'à des défauts en rives.The sheet is then reeled hot. The winding temperature is preferably less than 550 ° C to limit the hardness of the hot-rolled sheet and intergranular oxidation at the surface. Too much hardness of the hot-rolled sheet leads to excessive forces during subsequent cold rolling and possibly to edge defects.

On décape ensuite la tôle laminée à chaud selon un procédé connu en lui-même de façon à conférer à celle-ci un état de surface propre au laminage à froid. Ce dernier est effectué en réduisant l'épaisseur de la tôle laminée à chaud de 30 à 80%.The hot-rolled sheet is then etched according to a method known per se in order to give the latter a surface condition suitable for cold. This is done by reducing the thickness of the hot-rolled sheet by 30 to 80%.

On effectue ensuite un traitement thermique de recuit, préférentiellement par un recuit en continu, qui comporte les phases suivantes :

  • Une phase de chauffage avec une vitesse Vc comprise entre 5 et 15°C/s. jusqu'à une température T1 Lorsque Vc est supérieure à 15°C/s, la recristallisation de la tôle écrouie par le laminage à froid peut ne pas être totale. Une valeur minimale de 5°C/s est requise pour la productivité.
    La température T1 est comprise entre Ac3 et Ac3+20°C, la température Ac3 correspondant à la transformation totale en austénite lors du chauffage. Ac3 dépend de la composition de l'acier et de la vitesse de chauffage et peut être déterminée par exemple par dilatométrie. L'austénitisation totale permet de limiter la formation ultérieure de ferrite proeutectoïde. Il est important que la température T1 soit inférieure à Ac3+20°C dans le but d'éviter un grossissement exagéré du grain austénitique. Au sein de cette plage (Ac3 - Ac3+20°C), les caractéristiques du produit final sont peu sensibles à une variation de température T1.
  • Un maintien à la température T1 pendant un temps t1 compris entre 50s et 150s. Cette étape conduit à une homogénéisation de l'austénite.
An annealing heat treatment is then carried out, preferably by continuous annealing, which comprises the following phases:
  • A heating phase with a speed V c of between 5 and 15 ° C / s. up to a temperature T 1 When V c is greater than 15 ° C / s, the recrystallization of the cold-worked sheet by the cold rolling may not be complete. A minimum value of 5 ° C / s is required for productivity.
    The temperature T 1 is between A c3 and A c3 + 20 ° C, the temperature A c3 corresponding to the total conversion to austenite during heating. A c3 depends on the composition of the steel and the heating rate and can be determined for example by dilatometry. Total austenitization limits the subsequent formation of proeutectoid ferrite. It is important that the temperature T 1 be less than A c3 + 20 ° C in order to avoid exaggerated magnification of the austenitic grain. Within this range (A c3 - A c3 + 20 ° C), the characteristics of the final product are insensitive to a temperature variation T 1 .
  • A maintenance temperature T 1 for a time t 1 between 50s and 150s. This step leads to a homogenization of the austenite.

L'étape suivante du procédé dépend de la teneur en chrome et en molybdène de l'acier :

  • Lorsque l'acier ne comporte pratiquement pas de chrome, de molybdène et de bore, c'est à dire lorsque Cr<0,005%, Mo<0,005%, B=0%, on effectue un refroidissement avec une vitesse VR1 supérieure à 40°C/s jusqu'à une température T2 comprise entre Ms-30°C et Ms+30°C. Un maintien est réalisé à cette température T2 pendant un temps t2 compris entre 150 et 350s. Ms désigne la température de début de transformation martensitique. Cette température dépend de la composition de l'acier mis en oeuvre et peut être déterminée par exemple par dilatométrie. Ces conditions permettent d'éviter la formation de ferrite proeutectoïde lors du refroidissement. On obtient également dans ces conditions une transformation bainitique de la plus grande partie de l'austénite. La fraction restante est transformée en martensite ou est éventuellement stabilisée sous forme d'austénite résiduelle.
  • Lorsque l'acier comporte une teneur en chrome et en molybdène telles que Mo ≤ 0,25%, Cr ≤ 1,65%, et Cr+(3 x Mo) ≥0,3%, on effectue un refroidissement avec une vitesse VR1 supérieure à 25°C/s jusqu'à une température T2 comprise entre (Bs et Ms-20°C) Un maintien est réalisé à cette température T2 pendant un temps t2 compris entre 150 et 350s. Bs désigne la température de début de transformation bainitique. Ces conditions permettent d'obtenir les mêmes caractéristiques microstructurales que ci-dessus. L'addition de chrome et/ou de molybdène permet en particulier de garantir que la formation de ferrite proeutectoïde n'intervient pas. Dans les limites de vitesse de refroidissement VR1 selon l'invention, les caractéristiques finales du produit sont relativement peu sensibles à une variation de cette vitesse VR1.
  • L'étape suivante du procédé est identique, que le produit comporte ou non du chrome et/ou du molybdène : on effectue un refroidissement à une vitesse VR2 inférieure à 30°C /s jusqu'à la température ambiante. En particulier, lorsque la température T2 est peu élevée au sein des plages selon l'invention, le refroidissement à une vitesse VR2 inférieure à 30°C /s provoque un revenu des îlots de martensite nouvellement formée, ce qui est favorable en termes de propriétés d'usage.
The next step in the process depends on the chromium and molybdenum content of the steel:
  • When the steel has practically no chromium, molybdenum and boron, that is to say when Cr <0.005%, Mo <0.005%, B = 0%, cooling is carried out with a speed V R1 greater than 40 ° C / s up to a temperature T 2 between M s -30 ° C and M s + 30 ° C. Hold is performed at this temperature T 2 for a time t 2 between 150 and 350s. M s denotes the martensitic transformation start temperature. This temperature depends on the composition of the steel used and can be determined for example by dilatometry. These conditions prevent the formation of proeutectoid ferrite during cooling. In these conditions, a bainitic transformation of most of the austenite is also obtained. The The remaining fraction is converted to martensite or is optionally stabilized as residual austenite.
  • When the steel has a content of chromium and molybdenum such that Mo ≤ 0.25%, Cr ≤ 1.65%, and Cr + (3 x Mo) ≥0.3%, cooling is carried out with a speed V R1 greater than 25 ° C / s to a temperature T 2 between (B s and M s -20 ° C) Hold is performed at this temperature T 2 for a time t 2 between 150 and 350s. B s denotes the bainitic transformation start temperature. These conditions make it possible to obtain the same microstructural characteristics as above. The addition of chromium and / or molybdenum makes it possible in particular to ensure that the formation of proeutectoid ferrite does not occur. Within the cooling speed limits V R1 according to the invention, the final characteristics of the product are relatively insensitive to a variation of this speed V R1 .
  • The next step of the process is identical, whether or not the product comprises chromium and / or molybdenum: cooling is carried out at a rate V R2 of less than 30 ° C./s up to room temperature. In particular, when the temperature T 2 is low within the ranges according to the invention, cooling at a speed V R2 of less than 30 ° C./s causes a return of the islands of newly formed martensite, which is favorable in terms of of use properties.

Exemple :Example:

On a élaboré des aciers dont la composition figure au tableau ci-dessous, exprimée en pourcentage pondéral. Outre les aciers 1-1 à 1-5 ayant servi à la fabrication de tôles selon l'invention, on a indiqué à titre de comparaison la composition d'aciers R-1 à R-5 ayant servi à la fabrication de tôles de référence. Tableau 1 Compositions d'aciers (% poids). I= Selon l'invention. R= référence Valeurs soulignées : Non conforme à l'invention. Acier C
(%)
Mn
(%)
Si
(%)
Al
(%)
Si+Al
(%)
Mo
(%)
Cr
(%)
Cr+(3xMo)
(%)
S
(%)
P
(%)
V
(%)
Ti
(%)
B
(%)
N
(%)
I-1 0,19 2 1,5 0,040 1,54 - - - 0,003 0,015 - - - 0,004 I-2 0,2 2 1,5 0,040 1,54 0,25 - 0,75 0,003 0,015 - - - 0,004 I-3 0,19 2 1,5 0,040 1,54 0,14 0,34 0,76 0,003 0,015 - - - 0,004 I-4 0,2 2 1,5 0,040 1,54 0,25 - 0,75 0,003 0,015 - 0,020 0,0038 0,004 I-5 0,2 2 1,5 0,040 1,54 0,25 - 0,75 0,003 0,015 0,15 0,020 0,0038 0,004 R-1 0,110 2,2 0,347 0,031 0.378 0,13 0,4 0,79 0,003 0,015 - 0,027 - 0004 R-2 0.038 0.212 0,036 0,053 0.089 11 0,21 3,51 0,003 0,015 - 0,002 - 0,004 R-3 0.035 0.21 0,035 0,054 0.089 0.5 0,034 1,534 0,003 0,015 - 0,002 - 0,004 R-4 0,19 1,3 0,25 0,040 0.29 - 0,18 0,18 0,003 0,015 - 0,003 0,006 R-5 0,148 1,925 0,214 0,024 0.238 - 0,19 0,19 0,002 0,012 - 0,024 - 0,005
Steels have been developed, the composition of which is given in the table below, expressed in percentage by weight. In addition to the steels 1-1 to 1-5 used for the production of sheets according to the invention, the composition of steels R-1 to R-5 used for the manufacture of reference sheets has been indicated for comparison purposes. . Table 1 Compositions of steel (% by weight). I = according to the invention. R = reference Underlined values: Not in accordance with the invention. Steel VS
(%)
mn
(%)
Yes
(%)
al
(%)
Si + Al
(%)
MB
(%)
Cr
(%)
Cr + (3xMo)
(%)
S
(%)
P
(%)
V
(%)
Ti
(%)
B
(%)
NOT
(%)
I-1 0.19 2 1.5 0,040 1.54 - - - 0,003 0,015 - - - 0,004 I-2 0.2 2 1.5 0,040 1.54 0.25 - 0.75 0,003 0,015 - - - 0,004 I-3 0.19 2 1.5 0,040 1.54 0.14 0.34 0.76 0,003 0,015 - - - 0,004 I-4 0.2 2 1.5 0,040 1.54 0.25 - 0.75 0,003 0,015 - 0,020 0.0038 0,004 I-5 0.2 2 1.5 0,040 1.54 0.25 - 0.75 0,003 0,015 0.15 0,020 0.0038 0,004 R-1 0.110 2.2 0.347 0.031 0378 0.13 0.4 0.79 0,003 0,015 - 0,027 - 0004 R-2 0038 0212 0,036 0.053 0089 11 0.21 3.51 0,003 0,015 - 0,002 - 0,004 R-3 0035 0.21 0,035 0,054 0089 0.5 0,034 1,534 0,003 0,015 - 0,002 - 0,004 R-4 0.19 1.3 0.25 0,040 0.29 - 0.18 0.18 0,003 0,015 - 0,003 0.006 R-5 0,148 1,925 0.214 0,024 0238 - 0.19 0.19 0,002 0.012 - 0,024 - 0.005

Des demi-produits correspondant aux compositions ci-dessus ont été réchauffés à 1200°C, laminés à chaud jusqu'à une épaisseur de 3 mm et bobinés à une température inférieure à 550°C. Les tôles ont été ensuite laminées à froid jusqu'à une épaisseur de 0,9 mm soit un taux de réduction de 70%. A partir d'une même composition, certains aciers ont fait l'objet de différentes conditions de fabrication. Les références I1-a, I1-b et I1-c, I1-d désignent par exemple quatre tôles d'aciers fabriquées selon des conditions différentes à partir de la composition d'acier I1. Le tableau 2 indique les conditions de fabrication des tôles recuites après laminage à froid. La vitesse de réchauffage Vc est de 10°C/s dans tous les cas.Semi-products corresponding to the above compositions were heated to 1200 ° C, hot rolled to a thickness of 3 mm and coiled at a temperature below 550 ° C. The sheets were then cold rolled to a thickness of 0.9 mm or a reduction rate of 70%. From the same composition, some steels have been subject to different manufacturing conditions. The references I1-a, I1-b and I1-c, I1-d designate for example four steel sheets manufactured under different conditions from the steel composition I1. Table 2 shows the manufacturing conditions of the annealed sheets after cold rolling. The heating rate V c is 10 ° C / s in all cases.

Les températures de transformation Ac3, Bs et Ms ont été également portées au tableau 2.The transformation temperatures A c3 , B s and M s were also reported in Table 2.

On a également indiqué les différents constituants microstructuraux mesurés par microscopie quantitative : fraction surfacique de bainite, martensite et d'austénite résiduelle.The different microstructural constituents measured by quantitative microscopy were also reported: surface fraction of bainite, martensite and residual austenite.

Les îlots M-A ont été mis en évidence par le réactif de LePera. Leur morphologie a été examinée au moyen d'un logiciel d'analyse d'images Scion®. Tableau 2 : Conditions de fabrication et microstructure des tôles laminées à chaud obtenues. I= Selon l'invention. R= référence Valeurs soulignées : Non conformes à l'invention. Tôle d'acier T1
(°C)
Ac3
(°C)
t1
(s)
VR1
(°C/s)
T2
(°C)
Bs
(°C)
Ms
(°C)
t2
(s)
VR2
(°C/s)
I1-a 850 830 100 54 350 600 380 200 15 I1-b 800 830 100 54 400 600 380 200 15 I1-c 825 830 100 54 400 600 380 200 15 I1-d 850 830 100 54 450 600 380 200 15 I2-a 850 830 100 54 400 575 375 200 15 I2-b 850 830 120 54 400 575 375 240 15 I2-c 850 830 95 22 400 575 375 200 5 I3-a 850 830 100 54 400 565 395 200 15 I3-b 850 830 100 65 350 565 395 200 15 I4 850 830 100 54 400 575 375 200 15 I5 850 830 100 54 400 575 375 200 15 R1 850 845 100 54 400 520 425 200 15 R2 800 930 60 20 460 695 510 20 15 R3 800 915 60 20 460 760 520 20 15 R4 850 845 300 20 460 650 425 20 15 R5 800 900 60 20 460 605 425 60 20
The MA islets have been highlighted by LePera reagent. Their morphology was examined using Scion® image analysis software. Table 2: Manufacturing conditions and microstructure of the hot-rolled sheets obtained. I = according to the invention. R = reference Underlined values: Not according to the invention. Galvanised steel T 1
(° C)
ac3
(° C)
t 1
(S)
V R1
(° C / s)
T 2
(° C)
B s
(° C)
M s
(° C)
t 2
(S)
R2
(° C / s)
I1-a 850 830 100 54 350 600 380 200 15 I1-b 800 830 100 54 400 600 380 200 15 I1-c 825 830 100 54 400 600 380 200 15 I1-d 850 830 100 54 450 600 380 200 15 I2-a 850 830 100 54 400 575 375 200 15 I2-b 850 830 120 54 400 575 375 240 15 I2-c 850 830 95 22 400 575 375 200 5 I3-a 850 830 100 54 400 565 395 200 15 I3-b 850 830 100 65 350 565 395 200 15 I4 850 830 100 54 400 575 375 200 15 I5 850 830 100 54 400 575 375 200 15 R1 850 845 100 54 400 520 425 200 15 R2 800 930 60 20 460 695 510 20 15 R3 800 915 60 20 460 760 520 20 15 R4 850 845 300 20 460 650 425 20 15 R5 800 900 60 20 460 605 425 60 20

Les propriétés mécaniques de traction obtenues (limite d'élasticité Re, résistance Rm, allongement uniforme Au, allongement à rupture At) ont été portées au tableau 3 ci-dessous. Le rapport Re/Rm a été également indiqué. Dans certains cas on a déterminé l'énergie de rupture à -40°C à partir d'éprouvettes de résilience du type Charpy V d'épaisseur réduite à 1,4mm. On a également évalué l'endommagement lié à une découpe (cisaillage ou poinçonnage par exemple) qui pourrait éventuellement diminuer les capacités de déformation ultérieure d'une pièce découpée. Dans ce but, on a découpé par cisaillage des éprouvettes de dimension 20 x 80 mm2. Une partie de ces éprouvettes a été ensuite soumise à un polissage des bords. Les éprouvettes ont été revêtues de grilles photodéposées puis soumises à une traction uniaxiale jusqu'à rupture. Les valeurs des déformations principales ε1 parallèles au sens de la sollicitation ont été mesurées au plus près de l'amorçage de la rupture à partir des grilles déformées. Cette mesure a été effectuée sur les éprouvettes à bords découpés mécaniquement et sur les éprouvettes à bords polis. La sensibilité à la découpe est évaluée par le facteur d'endommagement : Δ = ε1(bords découpés)-ε1(bords polis)/ ε1(bords polis).The tensile mechanical properties obtained (yield strength Re, resistance Rm, uniform elongation Au, elongation at break At) were given in Table 3 below. The Re / Rm ratio was also indicated. In some cases, the breaking energy at -40 ° C was determined from Charpy V type resilience specimens reduced to 1.4 mm thickness. Damage related to a cut (for example, shearing or punching) has also been evaluated which could possibly reduce the capacity for subsequent deformation of a cut piece. For this purpose, 20 × 80 mm 2 specimens were cut by shearing. Some of these specimens were then polished at the edges. The specimens were coated with photodeposited grids and then subjected to uniaxial traction until rupture. The values of the principal deformations ε 1 parallel to the direction of the stress have been measured as close as possible to the initiation of the rupture from the deformed grids. This measurement was carried out on the specimens with mechanically cut edges and on the test pieces with polished edges. The sensitivity to cutting is evaluated by the damage factor: Δ = ε 1 (cut edges) -ε 1 (polished edges) / ε 1 (polished edges).

Pour certaines tôles, on a également évalué l'endommagement au voisinage de bords découpés à partir d'échantillons de 105x105mm2 comportant un trou d'un diamètre initial de 10mm. On mesure l'augmentation relative du diamètre du trou après introduction d'un poinçon conique jusqu'à ce qu'une fissure apparaisse.

Figure imgb0001
For some sheets, damage was also evaluated in the vicinity of edges cut from samples of 105x105mm 2 having a hole with an initial diameter of 10mm. The relative increase in the diameter of the hole is measured after introducing a conical punch until a crack appears.
Figure imgb0001

Les tôles de composition conforme à l'invention et fabriquées selon les conditions de l'invention (I1-a, I2-a-b, 13-a, I4, I5) présentent une combinaison de propriétés mécaniques particulièrement avantageuse : d'une part une résistance mécanique supérieure à 1200 MPa, d'autre part un allongement à rupture toujours supérieur ou égal à 10%. Les aciers selon l'invention présentent également une énergie de rupture Charpy V à -40°C supérieure à 40 Joules/cm2. Ceci permet la fabrication de pièces résistant à la propagation brutale d'un défaut notamment en cas de sollicitations dynamiques. Les microstructures des aciers avec une résistance minimale de 1200MPa et un allongement à rupture minimal de 10% selon l'invention comportent une teneur en bainite comprise entre 65 et 90%, le solde étant constitué d'îlots MA. La figure 1 présente ainsi la microstructure de la tôle d'acier 13a comportant 88% de bainite et 12% d'îlots M-A, révélée par une attaque au réactif LePera. La figure 2 présente cette microstructure révélée par une attaque Nital. Dans le cas d'aciers présentant une résistance minimale de 1400MPa et un allongement à rupture minimal de 8%, les aciers selon l'invention présentent une teneur en bainite comprise entre 45 et 65%, le solde étant des îlots M-A. Dans le cas d'aciers présentant une résistance minimale de 1600MPa et un allongement à rupture minimal de 8%, les aciers selon l'invention présentent une teneur en bainite comprise entre 15 et 35%, le solde étant de la martensite et de l'austénite résiduelle. Les tôles d'acier selon l'invention présentent une taille d'îlots M-A inférieure à 1 micromètre, la distance inter-îlots étant inférieure à 6 micromètres.The sheets of composition according to the invention and manufactured according to the conditions of the invention (I1-a, I2-ab, 13-a, I4, I5) have a combination particularly advantageous mechanical properties: on the one hand a mechanical strength greater than 1200 MPa, on the other hand an elongation at break always greater than or equal to 10%. The steels according to the invention also have a Charpy V fracture energy at -40 ° C. of greater than 40 Joules / cm 2 . This allows the manufacture of parts resistant to the sudden propagation of a fault especially in case of dynamic stresses. The microstructures of the steels with a minimum strength of 1200 MPa and a minimum breaking elongation of 10% according to the invention comprise a bainite content of between 65 and 90%, the balance consisting of MA islands. The figure 1 thus presents the microstructure of the steel sheet 13a comprising 88% bainite and 12% islets MA, revealed by a LePera reagent attack. The figure 2 presents this microstructure revealed by a Nital attack. In the case of steels having a minimum strength of 1400 MPa and a minimum breaking elongation of 8%, the steels according to the invention have a bainite content of between 45 and 65%, the balance being MA islands. In the case of steels having a minimum strength of 1600 MPa and a minimum breaking elongation of 8%, the steels according to the invention have a bainite content of between 15 and 35%, the balance being martensite and residual austenite. The steel sheets according to the invention have an island size of less than 1 micrometer MA, the inter-island distance being less than 6 micrometers.

Les aciers selon l'invention présentent également une bonne résistance à l'endommagement en cas de découpe puisque le facteur d'endommagement Δ est limité à -23%. Une tôle d'acier ne présentant pas ces caractéristiques (R5) peut présenter un facteur endommagement de 43%. Les tôles selon l'invention présentent ont une bonne aptitude à l'expansion de trou.The steels according to the invention also have good resistance to damage in case of cutting since the damage factor Δ is limited to -23%. A steel sheet that does not have these characteristics (R5) may have a 43% damage factor. The sheets according to the invention have good hole expansion capability.

Les aciers selon l'invention présentent également une bonne aptitude au soudage homogène : pour des paramètres de soudage adaptés aux épaisseurs rapportés ci-dessus, les joints soudés sont exempts de fissures à froid ou à chaud.The steels according to the invention also have good weldability: for welding parameters adapted to the thicknesses mentioned above, the welded joints are free of cold or hot cracks.

Les tôles d'acier I1-b et 11-c ont été recuites à une température T1 trop faible, la transformation austénitique n'est pas complète. En conséquence la microstructure comporte de la ferrite proeutectoïde (40% pour I1b, 20% pour I1-c) et une teneur excessive en îlots M-A. La résistance mécanique est alors diminuée par la présence de ferrite proeutectoïde.The steel sheets I1-b and 11-c were annealed at a temperature T 1 too low, the austenitic transformation is not complete. As a result, the microstructure contains proeutectoid ferrite (40% for I1b, 20% for I1-c) and an excessive content of MA islands. The mechanical strength is then reduced by the presence of proeutectoid ferrite.

Pour la tôle d'acier I1-d, la température de maintien T2 est supérieure à Ms+30°C : la transformation bainitique qui intervient à plus haute température donne naissance à une structure plus grossière et conduit à une résistance mécanique insuffisante.For the steel sheet I1-d, the holding temperature T 2 is greater than Ms + 30 ° C: the bainitic transformation which occurs at a higher temperature gives rise to a coarser structure and leads to insufficient mechanical strength.

Pour la tôle d'acier I-2c, la vitesse de refroidissement VR1 après recuit n'est pas suffisante, la microstructure formée est plus hétérogène et l'allongement à rupture est réduit au dessous de 10%.For the steel sheet I-2c, the cooling rate V R1 after annealing is not sufficient, the microstructure formed is more heterogeneous and the elongation at break is reduced to less than 10%.

Pour la tôle I-3b, la température de maintien T2 est inférieure à Ms-20°C : en conséquence, le refroidissement VR1 provoque l'apparition d'une bainite formée à basse température et de martensite, associées à un allongement insuffisant.For the sheet I-3b, the holding temperature T 2 is less than Ms-20 ° C: consequently, the cooling V R1 causes the appearance of a bainite formed at low temperature and martensite, associated with insufficient elongation .

L'acier R1 a une teneur en (silicium+aluminium) insuffisante, la température de maintien T2 est inférieure à Ms-20°C. En raison de la teneur insuffisante en (Si+Al), la quantité d'îlots M-A formée est insuffisante pour obtenir une résistance supérieure ou égale à 1200MPa.The steel R1 has an insufficient (silicon + aluminum) content, the holding temperature T 2 is less than Ms-20 ° C. Due to the insufficient content of (Si + Al), the amount of islets MA formed is insufficient to obtain a resistance greater than or equal to 1200 MPa.

Les aciers R2 et R3 ont des teneurs en carbone, manganèse, silicium+aluminium, insuffisantes. La quantité de composés M-A formés est inférieure à 10%. En outre, la température de recuit T1 inférieure à Ac3 conduit à une teneur excessive en ferrite proeutectoïde et en cémentite, et à une résistance insuffisante.R2 and R3 steels have insufficient carbon, manganese, silicon + aluminum contents. The amount of MA compounds formed is less than 10%. In addition, the annealing temperature T 1 lower than A c3 leads to an excessive content of proeutectoid ferrite and cementite, and insufficient strength.

L'acier R4 a une teneur insuffisante en (Si+Al) La vitesse de refroidissement VR1 est notamment trop faible. L'enrichissement de l'austénite en carbone au refroidissement est alors insuffisant pour permettre la formation de martensite et pour obtenir les propriétés de résistance et d'allongement visées par l'invention.The steel R4 has an insufficient content of (Si + Al) The cooling rate V R1 is in particular too low. The enrichment of carbon austenite during cooling is then insufficient to allow the formation of martensite and to obtain the strength and elongation properties of the invention.

L'acier R5 présente également une teneur insuffisante en (Si+Al) La vitesse de refroidissement insuffisamment rapide après le recuit conduit à une teneur excessive en ferrite proeutectoïde et à une résistance mécanique insuffisante.Steel R5 also has an insufficient content of (Si + Al) The insufficiently fast cooling rate after annealing leads to excessive proeutectoid ferrite content and insufficient mechanical strength.

Ainsi, l'invention permet la fabrication de tôles d'aciers alliant une très haute résistance et une ductilité élevée. Les tôles d'aciers selon l'invention sont utilisées avec profit pour la fabrication de pièces de structure ou d'éléments de renfort dans le domaine automobile et de l'industrie générale.Thus, the invention allows the manufacture of steel sheets combining a very high strength and high ductility. The steel sheets according to the invention are used profitably for the manufacture of structural parts or reinforcement elements in the automotive field and general industry.

Claims (15)

Tôle d'acier laminée à froid et recuite de résistance supérieure à 1200 MPa, dont la composition comprend, les teneurs étant exprimées en poids : 0,10% ≤ C ≤ 0,25% 1%≤ Mn ≤3% Al ≥0,010 % Si≤2,990% S ≤ 0,015% P≤0,1% N≤0,008% étant entendu que 1% ≤Si+Al ≤3%, la composition comprenant éventuellement : 0,05% ≤ V ≤ 0,15% B≤0,005% Mo ≤ 0,25% Cr ≤ 1,65% étant entendu que Cr+(3 x Mo) ≥0,3%, Ti en quantité telle que Ti/N≥4 et que Ti≤0,040%, le reste de la composition étant constitué de fer et d'impuretés inévitables résultant de l'élaboration, la microstructure dudit acier comprenant 15 à 90% de bainite, le solde étant constitué de martensite et d'austénite résiduelleCold-rolled and annealed steel sheet with a resistance greater than 1200 MPa, the composition of which comprises the contents being expressed by weight: 0.10% ≤ C ≤ 0.25% 1% ≤ Mn ≤3% Al ≥0.010% Si≤2,990% S ≤ 0.015% P≤0,1% N≤0,008% Being heard that 1% ≤Si + Al ≤3%, the composition optionally comprising: 0.05% ≤ V ≤ 0.15% B≤0,005% Mo ≤ 0.25% Cr ≤ 1.65% Being heard that Cr + (3 x Mo) ≥0.3%, Ti in an amount such that Ti / N≥4 and Ti≤0.040%, the remainder of the composition consisting of iron and unavoidable impurities resulting from the preparation, the microstructure of said steel comprising 15 to 90% of bainite, the remainder consisting of martensite and residual austenite Tôle d'acier selon la revendication 1, d'allongement à rupture supérieur à 10%, caractérisée en ce que
Mo<0,005%
Cr<0,005%
B=0%
la microstructure dudit acier comprenant 65 à 90% de bainite, le solde étant constitué d'îlots de martensite et d'austénite résiduelle
Steel sheet according to claim 1, having a tensile elongation greater than 10%, characterized in that
Mo <0.005%
Cr <0.005%
B = 0%
the microstructure of said steel comprising 65 to 90% of bainite, the remainder being islands of martensite and residual austenite
Tôle d'acier selon la revendication 1, caractérisée en ce qu'elle contient
Mo ≤ 0,25%
Cr ≤ 1,65%
étant entendu que
Cr+(3 x Mo) ≥0,3%,
B=0%
la microstructure dudit acier comprenant 65 à 90% de bainite, le solde étant constitué d'îlots de martensite et d'austénite résiduelle
Sheet steel according to Claim 1, characterized in that it contains
Mo ≤ 0.25%
Cr ≤ 1.65%
Being heard that
Cr + (3 x Mo) ≥0.3%,
B = 0%
the microstructure of said steel comprising 65 to 90% of bainite, the remainder being islands of martensite and residual austenite
Tôle d'acier selon la revendication 1, de résistance supérieure à 1400MPa, d'allongement à rupture supérieur à 8%, caractérisée en ce qu'elle contient
Mo ≤ 0,25%
Cr ≤ 1.65%
étant entendu que
Cr+(3 x Mo) ≥0,3%,
la microstructure dudit acier comprenant 45 à 65% de bainite, le solde étant constitué d'îlots de martensite et d'austénite résiduelle
Steel sheet according to Claim 1, with a resistance greater than 1400 MPa, elongation at break greater than 8%, characterized in that it contains
Mo ≤ 0.25%
Cr ≤ 1.65%
Being heard that
Cr + (3 x Mo) ≥0.3%,
the microstructure of said steel comprising 45 to 65% of bainite, the remainder being islands of martensite and residual austenite
Tôle d'acier selon la revendication 1, de résistance supérieure à 1600MPa, d'allongement à rupture supérieur à 8%, caractérisée en ce qu'elle contient
Mo ≤ 0,25%
Cr ≤ 1,65%
étant entendu que
Cr+(3 x Mo) ≥0,3%,
la microstructure dudit acier comprenant 15 à 45% de bainite, le solde étant constitué de martensite et d'austénite résiduelle
Steel sheet according to Claim 1, with a resistance greater than 1600 MPa, elongation at break greater than 8%, characterized in that it contains
Mo ≤ 0.25%
Cr ≤ 1.65%
Being heard that
Cr + (3 x Mo) ≥0.3%,
the microstructure of said steel comprising 15 to 45% of bainite, the remainder consisting of martensite and residual austenite
Tôle d'acier selon l'une quelconque des revendications 1 à 5 caractérisée en ce que la composition dudit acier contient, la teneur étant exprimée en poids : 0,19% ≤ C ≤ 0,23% Steel sheet according to any one of claims 1 to 5 characterized in that the composition of said steel contains, the content being expressed by weight: 0.19% ≤ C ≤ 0.23% Tôle d'acier selon l'une quelconque des revendications 1 à 6, caractérisée en ce que la composition dudit acier contient, la teneur étant exprimée en poids : 1,5% ≤Mn ≤ 2,5% Steel sheet according to any one of claims 1 to 6, characterized in that the composition of said steel contains, the content being expressed by weight: 1.5% ≤Mn ≤ 2.5% Tôle d'acier selon l'une quelconque des revendications 1 à 7, caractérisée en ce que la composition dudit acier contient, la teneur étant exprimée en poids : 1,2% ≤Si ≤ 1,8% Steel sheet according to any one of claims 1 to 7, characterized in that the composition of said steel contains, the content being expressed by weight: 1.2% ≤Si ≤ 1.8% Tôle d'acier selon l'une quelconque des revendications 1 à 8, caractérisée en ce que la composition dudit acier contient, la teneur étant exprimée en poids : 1,2% ≤Al ≤ 1,8% Steel sheet according to any one of claims 1 to 8, characterized in that the composition of said steel contains, the content being expressed by weight: 1.2% ≤Al ≤ 1.8% Tôle d'acier selon l'une quelconque des revendications 1 à 9, caractérisée en ce que la composition dudit acier contient, la teneur étant exprimée en poids: 0,05% ≤ V ≤ 0,15% 0,004 ≤N ≤ 0,008% Steel sheet according to any one of claims 1 to 9, characterized in that the composition of said steel contains, the content being expressed by weight: 0.05% ≤ V ≤ 0.15% 0.004 ≤N ≤ 0.008% Tôle d'acier selon l'une quelconque des revendications 1, 4 ou 5, caractérisée en ce que la composition dudit acier contient, la teneur étant exprimée en poids : 0,0005≤ B ≤ 0,003% Steel sheet according to any one of claims 1, 4 or 5, characterized in that the composition of said steel contains, the content being expressed by weight: 0.0005≤ B ≤ 0.003% Tôle d'acier selon l'une quelconque des revendications 1 à 10, caractérisé en ce que la taille moyenne desdits îlots de martensite et d'austénite résiduelle est inférieure à 1 micromètre, la distance moyenne entre lesdits îlots étant inférieure à 6 micromètresSteel sheet according to any one of claims 1 to 10, characterized in that the average size of said islands of martensite and residual austenite is less than 1 micrometer, the average distance between said islands being less than 6 micrometers Procédé de fabrication d'une tôle d'acier laminée à froid de résistance supérieure à 1200 MPa, d'allongement à rupture supérieur à 10%, selon lequel : - on approvisionne un acier de composition selon la revendication 2, puis - on procède à la coulée d'un demi-produit à partir de cet acier, puis - on porte ledit demi-produit à une température supérieure à 1150°C, puis - on lamine à chaud ledit demi-produit pour obtenir une tôle laminée à chaud, puis - on bobine ladite tôle, puis - on décape ladite tôle laminée à chaud, puis - on lamine à froid ladite tôle avec un taux de réduction compris entre 30 et 80% de façon à obtenir une tôle laminée à froid, puis - on réchauffe ladite tôle laminée à froid à une vitesse Vc comprise entre 5 et 15°C/s jusqu'à une température T1 comprise entre Ac3 et Ac3+20°C, pendant un temps t1 compris entre 50 et 150s puis on refroidit ladite tôle à une vitesse VR1 supérieure à 40°C/s jusqu'à une température T2 comprise entre (Ms-30°C et Ms+30°C), on maintient ladite tôle à ladite température T2 pendant un temps t2 compris entre 150 et 350s puis on effectue un refroidissement à une vitesse VR2 inférieure à 30°C /s jusqu'à la température ambiante A process for manufacturing a cold-rolled steel sheet with a resistance greater than 1200 MPa, with a tensile elongation greater than 10%, wherein: a composition steel is supplied according to claim 2, and then - the casting of a half-product from this steel, then said half-product is brought to a temperature above 1150 ° C., and then said half-product is hot-rolled to obtain a hot-rolled sheet, and then said sheet is reeled, then said hot-rolled sheet is de-scoured and then said sheet is cold-rolled with a reduction ratio of between 30 and 80% so as to obtain a cold-rolled sheet, and then said cold-rolled sheet is heated at a speed V c of between 5 and 15 ° C./s up to a temperature T 1 between Ac 3 and Ac 3 + 20 ° C., for a time t 1 of between 50 and 150 s then said sheet is cooled at a speed V R1 greater than 40 ° C / s to a temperature T 2 between (M s -30 ° C and M s + 30 ° C), said sheet is maintained at said temperature T 2 for a time t 2 of between 150 and 350 seconds and then cooling is carried out at a speed V R2 of less than 30 ° C./s up to room temperature Procédé de fabrication d'une tôle d'acier laminée à froid de résistance supérieure à 1200 MPa, d'allongement à rupture supérieur à 8%, selon lequel : - on approvisionne un acier de composition selon l'une quelconque des revendications 1, ou 3 à 5, les teneurs en Mo et en Cr étant telles que Mo ≤ 0,25%, Cr ≤ 1,65%, étant entendu que : Cr+(3 x Mo) ≥0,3%, puis - on procède à la coulée d'un demi-produit à partir de cet acier, puis - on porte ledit demi-produit à une température supérieure à 1150°C, puis - on lamine à chaud ledit demi-produit pour obtenir une tôle laminée à chaud, puis - on bobine ladite tôle, puis - on décape ladite tôle laminée à chaud, puis - on lamine à froid ladite tôle avec un taux de réduction compris entre 30 et 80% de façon à obtenir une tôle laminée à froid, puis - on réchauffe ladite tôle laminée à froid à une vitesse Vc comprise entre 5 et 15°C/s jusqu'à une température T1 comprise entre Ac3 et Ac3+20°C, pendant un temps t1 compris entre 50 et 150s puis on refroidit ladite tôle à une vitesse VR1 supérieure à 25°C/s jusqu'à une température T2 comprise entre Bs et (Ms - 20°C), on maintient ladite tôle à ladite température T2 pendant un temps t2 compris entre 150 et 350s puis on effectue un refroidissement à une vitesse VR2 inférieure à 30°C /s jusqu'à la température ambiante Process for the production of a cold-rolled steel sheet with a resistance greater than 1200 MPa and an elongation at break greater than 8%, according to which: a steel of composition according to any one of Claims 1 or 3 to 5 is supplied, the contents in Mo and Cr being such that Mo ≤ 0.25%, Cr ≤ 1.65%, with the proviso that: Cr + (3 x Mo) ≥0.3%, then - the casting of a half-product from this steel, then said half-product is brought to a temperature above 1150 ° C., and then said half-product is hot-rolled to obtain a hot-rolled sheet, and then said sheet is reeled, then said hot-rolled sheet is de-scoured and then said sheet is cold-rolled with a reduction ratio of between 30 and 80% so as to obtain a cold-rolled sheet, and then said cold-rolled sheet is heated at a speed V c of between 5 and 15 ° C./s up to a temperature T 1 between Ac 3 and Ac 3 + 20 ° C., for a time t 1 of between 50 and 150 s then said sheet is cooled at a speed V R1 greater than 25 ° C / s to a temperature T 2 between B s and (M s - 20 ° C), said sheet is maintained at said temperature T 2 for a time t 2 between 150 and 350 seconds and then cooling is carried out at a speed V R2 of less than 30 ° C./s up to room temperature Utilisation d'une tôle d'acier laminée à froid et recuite selon l'une quelconque des revendications 1 à 12, ou fabriquée par un procédé selon l'une quelconque des revendications 13 ou 14 pour la fabrication de pièces de structure ou d'éléments de renfort, dans le domaine automobileUse of a cold rolled and annealed steel sheet according to any one of claims 1 to 12, or manufactured by a method according to any one of claims 13 or 14 for the manufacture of structural parts or elements reinforcement, in the automotive field
EP07290598A 2007-05-11 2007-05-11 Method of manufacturing annealed, very high-resistance, cold-laminated steel sheets, and sheets produced thereby Withdrawn EP1990431A1 (en)

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EP07290598A EP1990431A1 (en) 2007-05-11 2007-05-11 Method of manufacturing annealed, very high-resistance, cold-laminated steel sheets, and sheets produced thereby
PCT/FR2008/000609 WO2008145871A2 (en) 2007-05-11 2008-04-28 Process for manufacturing cold-rolled and annealed steel sheets with very high strength, and sheets thus produced
PL08805523.1T PL2155915T5 (en) 2007-05-11 2008-04-28 Process for manufacturing cold-rolled and annealed steel sheets with very high strength, and sheets thus produced
CN2008800153809A CN101765668B (en) 2007-05-11 2008-04-28 Process for manufacturing cold-rolled and annealed steel sheets with very high strength, and sheets thus produced
BRPI0821572-3A BRPI0821572B1 (en) 2007-05-11 2008-04-28 Cold-annealed and cold-rolled steel sheet and cold-rolled and annealed-sheet steelmaking process
JP2010506964A JP5398701B2 (en) 2007-05-11 2008-04-28 Process for producing cold-rolled annealed steel sheet having extremely high strength and board produced thereby
EP08805523.1A EP2155915B2 (en) 2007-05-11 2008-04-28 Process for manufacturing cold-rolled and annealed steel sheets with very high strength, and sheets thus produced
CA2686940A CA2686940C (en) 2007-05-11 2008-04-28 Process for manufacturing cold-rolled and annealed steel sheets with very high strength, and sheets thus produced
KR1020097023517A KR101523395B1 (en) 2007-05-11 2008-04-28 Process for manufacturing cold-rolled and annealed steel sheets with very high strength, and sheets thus produced
US12/599,166 US20100307644A1 (en) 2007-05-11 2008-04-28 Process for manufacturing cold-rolled and annealed steel sheet with a very high strength, and sheet thus produced
MX2009011927A MX2009011927A (en) 2007-05-11 2008-04-28 Process for manufacturing cold-rolled and annealed steel sheets with very high strength, and sheets thus produced.
ES08805523T ES2655476T5 (en) 2007-05-11 2008-04-28 Method of manufacturing cold-rolled and annealed steel sheets with very high strength, and sheets produced in such a way
RU2009145940/02A RU2437945C2 (en) 2007-05-11 2008-04-28 Procedure for fabrication of high strength cold-rolled and annealed steel sheets and sheets manufactured by this procedure
HUE08805523A HUE035549T2 (en) 2007-05-11 2008-04-28 Process for manufacturing cold-rolled and annealed steel sheets with very high strength, and sheets thus produced
ARP080101971A AR066508A1 (en) 2007-05-11 2008-05-09 A PROCESS FOR MANUFACTURING A COLD RECOGNIZED AND COLDED STEEL SHEET, WITH A VERY HIGH RESISTANCE, AND A SHEET PRODUCED THIS WAY
ZA200907430A ZA200907430B (en) 2007-05-11 2009-10-23 Process for manufacturing cold-rolled and annealed steel sheet with a very high strength, and sheet thus produced
MA32328A MA31555B1 (en) 2007-05-11 2009-11-03 PROCESS FOR MANUFACTURING COLD ROLLED AND RECOVERED STEEL SHEETS WITH HIGH RESISTANCE AND SHEETS THUS PRODUCED
US15/243,610 US10612106B2 (en) 2007-05-11 2016-08-22 Process for manufacturing cold-rolled and annealed steel sheet with a very high strength, and sheet thus produced
US16/592,341 US11414722B2 (en) 2007-05-11 2019-10-03 Process for manufacturing cold-rolled and annealed steel sheet with a very high strength, and sheet thus produced
US17/575,300 US20220136078A1 (en) 2007-05-11 2022-01-13 Process for manufacturing cold-rolled and annealed steel sheet with a very high strength, and sheet thus produced

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