US20100307644A1 - Process for manufacturing cold-rolled and annealed steel sheet with a very high strength, and sheet thus produced - Google Patents

Process for manufacturing cold-rolled and annealed steel sheet with a very high strength, and sheet thus produced Download PDF

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US20100307644A1
US20100307644A1 US12/599,166 US59916608A US2010307644A1 US 20100307644 A1 US20100307644 A1 US 20100307644A1 US 59916608 A US59916608 A US 59916608A US 2010307644 A1 US2010307644 A1 US 2010307644A1
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steel
sheet
rolled
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composition
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Javier Gil Otin
Antoine Moulin
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ArcelorMittal France SA
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
    • B22D11/001—Continuous casting of metals, i.e. casting in indefinite lengths of specific alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
    • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
    • C23G1/08—Iron or steel

Definitions

  • the invention relates to the manufacture of thin cold-rolled and annealed steel sheet having a strength greater than 1200 MPa and an elongation at break greater than 8%.
  • the automotive sector and general industry particularly constitute fields of application of such steel sheet.
  • Dual-phase or TRIP steel sheets have been proposed with a maximum strength level of the order to 1000 MPa.
  • a maximum strength level of the order to 1000 MPa.
  • significantly higher strength levels for example 1200 ⁇ 1400 MPa, various difficulties arise:
  • multiphase steels having a predominantly bainitic structure have been developed.
  • multiphase steel sheet of moderate thickness is used to advantage for structural parts such as fender cross-members, pillars and various reinforcements.
  • patent EP 1 559 798 discloses the manufacture of steels having the composition: 0.10 ⁇ 0.25% C; 1.0 ⁇ 2.0% Si; and 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 strength does not exceed 1200 MPa.
  • Patent EP 1 589 126 also discloses the manufacture of thin cold-rolled sheet, the strength ⁇ elongation product of which is greater than 20000 MPa %.
  • the composition of the steels contains: 0.10 ⁇ 0.28% C; 1.0-2.0% Si; 1 ⁇ 3% Mn; and less than 0.10% Nb.
  • the structure consists of more than 50% bainitic ferrite, 5 to 20% residual austenite and less than 30% polygonal ferrite.
  • the embodiments presented show that the strength is still less than 1200 MPa.
  • the object of the present invention is to solve the abovementioned problems. Its aim is to provide a cold-rolled and annealed steel sheet having a strength greater than 1200 MPa together with an elongation at break greater than 8% and good cold formability. Another aim of the invention is to provide a steel that is largely insensitive to damage when being cut by a mechanical process.
  • the aim of the invention is to provide a process for manufacturing thin sheet in which slight variations of the parameters do not result in substantial modifications to the microstructure or the mechanical properties.
  • the aim of the invention is also to provide a steel sheet that can be easily manufactured by cold rolling, that is to say the hardness of which after the hot-rolling step is limited in such a way that the rolling forces remain modest during the cold-rolling step.
  • the aim of the invention is also to provide a thin steel sheet suitable for the optional deposition of a metal coating using standard processes.
  • the aim of the invention is also to provide a steel sheet that is largely insensitive to damage by cutting and is capable of hole expansion.
  • the aim of the invention is also to provide a steel exhibiting good weldability by means of standard assembly processes such as spot resistance welding.
  • one subject of the invention is a cold-rolled and annealed steel sheet with a strength 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%, it being understood that 1% ⁇ Si+Al ⁇ 3%, the composition optionally comprising: 0.05% ⁇ V ⁇ 0.15%, B ⁇ 0.005%, Mo ⁇ 0.25%, Cr ⁇ 1.65%, it being understood that Cr+3Mo ⁇ 0.3%, Ti in an amount such that Ti/N ⁇ 4 and Ti ⁇ 0.040%, the balance of the composition consisting of iron and inevitable impurities resulting from the smelting, the microstructure of said steel comprising 15 to 90% bainite, the remainder consisting of martensite and residual austenite.
  • Yet another subject of the invention is a steel sheet of the above composition, with a strength greater than 1400 MPa and an elongation at break greater than 8%, characterized in that it contains: Mo ⁇ 0.25%, Cr ⁇ 1.65%, it being understood that Cr+3Mo ⁇ 0.3%, the microstructure of the steel comprising 45 to 65% bainite, the remainder consisting of islands of martensite and residual austenite.
  • Another subject of the invention is a steel sheet of the above composition, with a strength greater than 1600 MPa and an elongation at break greater than 8%, characterized in that it contains: Mo ⁇ 0.25%, Cr ⁇ 1.65%, it being understood that Cr+3Mo ⁇ 0.3%, the microstructure of the steel comprising 15 to 45% bainite, the remainder consisting of martensite and residual austenite.
  • the composition comprises: 0.19% ⁇ C ⁇ 0.23%
  • the composition comprises: 1.5% ⁇ Mn ⁇ 2.5%
  • the composition comprises: 1.2% ⁇ Si ⁇ 1.8%
  • the composition comprises: 1.2% ⁇ Al ⁇ 1.5%
  • the composition comprises 0.05% ⁇ V ⁇ 0.15% 0 . 004 ⁇ N ⁇ 0.008%.
  • the composition comprises: 0.12% ⁇ V ⁇ 0.15%
  • the composition comprises: 0.0005 ⁇ B ⁇ 0.003%.
  • the average size of the islands of martensite and residual austenite is less than 1 micron, the average distance between the islands being less than 6 microns.
  • a semifinished product is cast from this steel; then the semifinished product is brought to a temperature greater than 1150° C.
  • the semifinished product is hot-rolled so as to obtain a hot-rolled sheet.
  • the sheet is coiled and pickled; then the latter 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 reheated at a rate V c between 5 and 15° C./s up to a temperature T 1 between Ac3 and Ac3+20° C., and held there for a time t 1 between 50 and 150 s, then the sheet is cooled at a rate V R1 greater than 40° C./s but below 100° C./s down 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 between 150 and 350 s and then it is cooled at a rate V R2 of less than 30° C./s down to the ambient temperature.
  • Another subject of the invention is a process for manufacturing a cold-rolled steel sheet with a strength greater than 1200 MPa and an elongation at break greater than 8%, in which a steel is provided having a composition: 0.10% ⁇ C ⁇ 0.25%; 1% ⁇ Mn ⁇ 3%; Al ⁇ 0.010%; Si ⁇ 2.990%, it being understood that 1% ⁇ Si+Al ⁇ 3%; S ⁇ 0.015%; P ⁇ 0.1%; N ⁇ 0.008%; Mo ⁇ 0.25%; Cr ⁇ 1.65%, it being understood that Cr+3Mo ⁇ 0.3%, optionally 0.05% ⁇ V ⁇ 0.15%, B ⁇ 0.005% and Ti in an amount such that Ti/N ⁇ 4 and Ti ⁇ 0.040%.
  • a semifinished product is cast from this steel; then the semifinished product is brought to a temperature greater than 1150° C.; then the semifinished product is hot-rolled so as to obtain a hot-rolled sheet.
  • the sheet is coiled; then the latter is pickled; 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 reheated at a rate V c between 5 and 15° C./s up to a temperature T 1 between Ac3 and Ac3+20° C., and held there for a time t 1 between 50 and 150 s, then the latter is cooled at a rate V R1 greater than 25° C./s but below 100° C./s down 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 between 150 and 350 s and then it is cooled at a rate V R2 of less than 30° C./s down to the ambient temperature.
  • the temperature T 1 is preferably between Ac3+10° C. and Ac3+20° C.
  • Another subject of the invention is the use of a cold-rolled and annealed steel sheet according to one of the above embodiments, or manufactured by a process according to one of the above embodiments, for the manufacture of structural parts or reinforcing elements in the automotive field.
  • FIG. 1 shows an example of the structure of a steel sheet according to the invention, the structure being revealed by the LePera etchant;
  • FIG. 2 shows an example of the structure of a steel sheet according to the invention, the structure being revealed by the Nital etchant.
  • the inventors have demonstrated that the above problems are solved when the cold-rolled and annealed thin steel sheet has a bainitic microstructure, complemented with 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 conjunction with other elements (Cr, Mo, Mn) of the composition and with the annealing heat treatment after cold rolling, carbon increases the hardenability and makes it possible to obtain a bainitic transformation.
  • the carbon contents according to the invention also result in the formation of islands of martensite and residual austenite, the quantity, the morphology and the composition of which enable the above-mentioned properties to be obtained.
  • Carbon also retards the formation of proeutectoid ferrite after the annealing heat treatment following the cold rolling: otherwise, the presence of this low-hardness phase would result in excessively large amounts of local damage at the interface with the matrix, the hardness of which is higher. To achieve high strength levels, the presence of proeutectoid ferrite resulting from the annealing must therefore be avoided.
  • the carbon content is between 0.10 and 0.25% by weight. Below 0.10%, sufficient strength cannot be obtained and the stability of the residual austenite is unsatisfactory. Above 0.25%, the weldability is reduced because of the formation of quench microstructures in the heat-affected zone.
  • the carbon content is between 0.19 and 0.23%.
  • the weldability is very satisfactory and the quantity, the stability and the morphology of the M-A islands are particularly suitable for obtaining a favorable pair of mechanical properties, namely strength/elongation.
  • an addition of manganese which is an element promoting formation of the gamma-phase, prevents the formation of proeutectoid ferrite upon cooling after the annealing that follows the cold rolling.
  • Manganese also contributes to deoxidizing the steel during smelting in the liquid phase.
  • the addition of manganese also contributes to effective solid-solution hardening and to the achievement of a higher strength.
  • the manganese content is between 1.5 and 2.5% so that its effects are obtained, but without the risk of forming a deleterious banded structure.
  • An addition of silicon according to the invention therefore helps to stabilize a sufficient amount of residual austenite in the form of islands, which subsequently and progressively are transformed to martensite under the effect of a deformation. Another portion of the austenite is transformed directly to martensite upon cooling after annealing.
  • Aluminum is a very effective element for deoxidizing the steel. In this regard, its content is equal to or greater than 0.010%. Like silicon, it stabilizes the residual austenite.
  • the effects of aluminum and silicon on the stabilization of the austenite are similar.
  • the silicon and aluminum contents are such that 1% ⁇ Si+Al ⁇ 3%, satisfactory stabilization of the austenite is obtained, thereby making it possible to form the desired microstructures while still maintaining satisfactory usage properties.
  • the minimum aluminum content is 0.010%, the silicon content does not exceed 2.990%.
  • the silicon content is between 1.2 and 1.8% for stabilizing a sufficient amount of residual austenite and to prevent integranular oxidation during the hot-coiling step that precedes the cold rolling. In this way, the formation of highly adherent oxides is avoided, as is any appearance of surface defects that would result in particular in a lack of wettability in hot-dip galvanizing operations.
  • the aluminum content is preferably between 1.2 and 1.8%.
  • the effects of the aluminum are similar to those explained above in the case of silicon, but the risk of surface defects appearing is however less.
  • the steels according to the invention optionally contain molybdenum and/or chromium.
  • Molybdenum increases the hardenability, prevents the formation of proeutectoid ferrite and effectively refines the bainitic microstructure.
  • a content greater than 0.25% by weight increases the risk of forming a predominantly martensitic microstructure to the detriment of the formation of bainite.
  • Chromium also contributes to preventing the formation of proeutectoid ferrite and to the refinement of the bainitic microstructure. Above 1.65%, the risk of obtaining a predominantly martensitic structure is high.
  • the chromium and molybdenum contents are such that Cr+3Mo ⁇ 0.3%.
  • the chromium and molybdenum factors in this relationship reflect their influence on the hardenability, in particular the respective capability of these elements to prevent the formation of proeutectoid ferrite under the particular cooling conditions of the invention.
  • the steel may have very low or zero molybdenum and chromium contents, that is to say contents below 0.005% by weight for these two elements, and 0% boron.
  • the phosphorus content is limited to 0.1% so as to maintain a sufficient hot ductility.
  • the nitrogen content is limited to 0.008% so as to avoid any ageing.
  • the steel according to the invention optionally contains vanadium in an amount between 0.05 and 0.15%.
  • the nitrogen content is between 0.004 and 0.008%, precipitation of the vanadium in the form of fine carbonitrides may occur during the annealing that follows cold rolling, these carbonitrides providing additional hardening.
  • the vanadium content is between 0.12 and 0.15% by weight, the uniform elongation or the elongation at break is particularly increased.
  • the steel may optionally contain boron in an amount not exceeding 0.005%.
  • the steel preferably contains between 0.0005 and 0.003% boron, thereby helping to suppress the proeutectoid ferrite in the presence of chromium and/or molybdenum.
  • boron added in the amount mentioned above, makes it possible to obtain a strength greater than 1400 MPa.
  • the steel may optionally contain titanium in an amount such that Ti/N ⁇ 4 and Ti ⁇ 0.040%. This enables titanium carbonitrides to be formed and increases the hardening.
  • the balance of the composition consists of inevitable impurities resulting from the smelting.
  • the contents of these impurities, such as Sn, Sb and As, are less than 0.005%.
  • the microstructure of the steel is composed of 65 to 90% bainite, these contents referring to percentages per unit area, the remainder consisting of islands of martensite and residual austenite (islands of M-A compounds).
  • This structure is predominantly bainitic, containing no low-hardness proeutectoid ferrite, and has an elongation at break greater than 10%.
  • the M-A islands uniformly dispersed in the matrix have an average size of less than 1 micron.
  • FIG. 1 shows an example of the microstructure of a steel sheet according to the invention.
  • the morphology of the M-A islands was revealed by means of appropriate chemical etchants: after etching, the M-A islands appear as white on a relatively dark bainite matrix. Some of the small islands are localized between the bainitic ferrite laths. The islands are observed at magnifications ranging from about 500 ⁇ to 1500 ⁇ on a statistically representative area and the average size of the islands and the average distance between these islands are measured using image analysis software. In the case of FIG. 1 , the percentage of islands per unit area is 12% and the average size of the M-A islands is less than 1 micron.
  • the microstructure is composed of 45 to 65% bainite, the remainder consisting of islands of martensite and residual austenite.
  • the microstructure is composed of 15 to 45% bainite, the remainder consisting of martensite and residual austenite.
  • the casting may be carried out to form ingots or continuously to form slabs with a thickness of around 200 mm.
  • the casting may also be carried out to form thin slabs with a thickness of a few tens of millimeters, or to form thin strip between steel counter-rotating rolls.
  • the cast semifinished products are firstly heated to a temperature above 1150° C. so as to achieve, at all points, a temperature favorable for the high deformation that the steel undergoes during rolling.
  • the step of hot rolling these semifinished products starting at most at 1150° C. may be carried out directly after casting, so that an intermediate reheating step is in this case unnecessary;
  • the next step of the process depends on the chromium and molybdenum contents of the steel:
  • microstructural constituents measured by quantitative microscopy, namely fractions per unit area of bainite, martensite and residual austenite.
  • the M-A islands were revealed by the LePera etchant. Their morphology was examined using Scion® image analysis software.
  • the fracture energy at ⁇ 40° C. was determined on toughness specimens of the Charpy V type with a thickness reduced to 1.4 mm.
  • the damage associated with cutting (for example shearing or punching), which could possibly reduce the subsequent deformability of a cut part, was also evaluated.
  • the damage near the cut edges on specimens measuring 105 ⁇ 105 mm 2 having a hole with an initial diameter of 10 mm was also evaluated.
  • the relative increase in the diameter of the hole after introducing a conical punch was measured until cracking occurred.
  • the sheets of composition according to the invention and manufactured according to the conditions of the invention have a particularly advantageous combination of mechanical properties: on the one hand, a strength greater than 1200 MPa and, 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 fraction energy at ⁇ 40° C. of greater than 40 joules/cm 2 . This allows the manufacture of parts that are resistant to the sudden propagation of a fault, especially in the case of dynamic stressing.
  • the microstructures of the steels with a minimum strength of 1200 MPa and a minimum elongation at break of 10% according to the invention have a bainite content between 65 and 90%, the remainder consisting of M-A islands.
  • FIG. 1 thus shows the microstructure of the steel sheet 13 a comprising 88% bainite and 12% M-A islands, this micro-structure being revealed by etching with the LePera etchant.
  • FIG. 2 shows this microstructure revealed by a Nital etchant.
  • the steels according to the invention have a bainite content of between 45 and 65%, the remainder being M-A islands.
  • the steels according to the invention have a bainite content of between 15 and 35%, the remainder being martensite and residual austenite.
  • the steel sheets according to the invention have an M-A island size of less than 1 micron, the inter-island distance being less than 6 microns.
  • the steels according to the invention also have good resistance to damage in the case of cutting, since the damage factor ⁇ is limited to ⁇ 23%.
  • a steel sheet (R5) not having these features may have a damage factor of 43%.
  • the sheets according to the invention exhibit good hole expansion capability.
  • the steels according to the invention also have good homogeneous weldability: for welding parameters suitable for the thicknesses indicated above, the welded joints are free of cold or hot cracks.
  • the steel sheets I1-b and I1-c were annealed at too low a temperature T 1 , the austenitic transformation not being complete. Consequently, the microstructure includes proeutectoid ferrite (40% in the case of I1b and 20% in the case of I1-c) and an excessive content of M-A islands. The strength is therefore reduced by the presence of proeutectoid ferrite.
  • the soak temperature T 2 is above M S +30° C.: the bainitic transformation that occurs at a higher temperature gives rise to a coarser structure and results in an insufficient strength.
  • the soak temperature T 2 is below M S ⁇ 20° C. Consequently, the cooling rate V R1 causes the appearance of bainite formed at low temperature and of martensite, these being associated with an insufficient elongation.
  • Steel R1 has an insufficient (silicon+aluminum) content and the soak temperature T 2 is below M S ⁇ 20° C. Because of the insufficient (Si+Al) content, the quantity of M-A islands formed is insufficient to obtain a strength equal to or greater than 1200 MPa.
  • Steels R2 and R3 have insufficient carbon, manganese and silicon+aluminum contents.
  • the amount of M-A compounds formed is less than 10%.
  • the annealing temperature T 1 below Ac3 results in an excessive content of both proeutectoid ferrite and cementite, and leads to an insufficient strength.
  • Steel R4 has an insufficient (Si+Al) content and the cooling rate V R1 is in particular too low.
  • the enrichment of the austenite with carbon upon cooling is therefore insufficient to allow the formation of martensite and to obtain the strength and elongation properties intended by the invention.
  • Steel R5 also has an insufficient (Si+Al) content.
  • the insufficiently rapid cooling rate after annealing results in an excessive content of proeutectoid ferrite and to an insufficient mechanical strength.
  • a steel sheet I2-d was manufactured according to a process having identical characteristics, with the exception of the temperature T 1 , which was 830° C., i.e. the temperature Ac3. In the case in which T 1 is equal to Ac3, the capability of conical hole expansion is 25%. When the temperature T 1 is equal to 850° C.
  • the invention allows the manufacture of steel sheets that combine very high strength with high ductility.
  • the steel sheets according to the invention are used to advantage for the manufacture of structural parts or reinforcing elements in the automotive and general industry fields.

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US11655516B2 (en) 2016-12-21 2023-05-23 Arcelormittal Tempered and coated steel sheet having excellent formability and a method of manufacturing the same
US12065724B2 (en) 2016-12-21 2024-08-20 Arcelormittal Tempered and coated steel sheet having excellent formability and a method of manufacturing the same
US11530461B2 (en) 2017-12-05 2022-12-20 Arcelormittal Cold rolled and annealed steel sheet and method of manufacturing the same

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EP2155915A2 (fr) 2010-02-24
ES2655476T3 (es) 2018-02-20
AR066508A1 (es) 2009-08-26
ZA200907430B (en) 2010-07-28
ES2655476T5 (es) 2022-09-29
RU2009145940A (ru) 2011-06-20
US20220136078A1 (en) 2022-05-05
KR101523395B1 (ko) 2015-05-27
WO2008145871A2 (fr) 2008-12-04
WO2008145871A8 (fr) 2019-09-06
CA2686940A1 (fr) 2008-12-04
BRPI0821572B1 (pt) 2019-10-01
PL2155915T3 (pl) 2018-03-30
US11414722B2 (en) 2022-08-16
MA31555B1 (fr) 2010-08-02
EP2155915B2 (fr) 2022-04-27
US10612106B2 (en) 2020-04-07
KR20100016438A (ko) 2010-02-12
MX2009011927A (es) 2009-11-18
PL2155915T5 (pl) 2022-09-05
US20160355900A1 (en) 2016-12-08
CN101765668B (zh) 2011-12-21
EP1990431A1 (fr) 2008-11-12
US20200032366A1 (en) 2020-01-30
BRPI0821572A2 (pt) 2015-06-16
JP2010526935A (ja) 2010-08-05
JP5398701B2 (ja) 2014-01-29
CN101765668A (zh) 2010-06-30
CA2686940C (fr) 2014-01-21
RU2437945C2 (ru) 2011-12-27
HUE035549T2 (en) 2018-05-28
WO2008145871A3 (fr) 2009-02-19
EP2155915B1 (fr) 2017-10-25

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