EP1832667A1 - Procédé de fabrication de tôles d'acier à très hautes caractéristiques de résistance, de ductilité et de tenacité, et tôles ainsi produites - Google Patents

Procédé de fabrication de tôles d'acier à très hautes caractéristiques de résistance, de ductilité et de tenacité, et tôles ainsi produites Download PDF

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Publication number
EP1832667A1
EP1832667A1 EP06290386A EP06290386A EP1832667A1 EP 1832667 A1 EP1832667 A1 EP 1832667A1 EP 06290386 A EP06290386 A EP 06290386A EP 06290386 A EP06290386 A EP 06290386A EP 1832667 A1 EP1832667 A1 EP 1832667A1
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EP
European Patent Office
Prior art keywords
steel
temperature
composition
steel sheet
weight
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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EP06290386A
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German (de)
English (en)
French (fr)
Inventor
Sébastien Allain
Audrey Couturier
Thierry Iung
Christine Colin
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ArcelorMittal France SA
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Arcelor France SA
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Publication date
Application filed by Arcelor France SA filed Critical Arcelor France SA
Priority to EP06290386A priority Critical patent/EP1832667A1/fr
Priority to CN2007800159016A priority patent/CN101437975B/zh
Priority to CA2645059A priority patent/CA2645059C/fr
Priority to MX2008011274A priority patent/MX2008011274A/es
Priority to US12/281,839 priority patent/US9856548B2/en
Priority to PL07730968T priority patent/PL1994192T3/pl
Priority to KR1020087024511A priority patent/KR101073425B1/ko
Priority to RU2008139605/02A priority patent/RU2397268C2/ru
Priority to PCT/FR2007/000256 priority patent/WO2007101921A1/fr
Priority to AT07730968T priority patent/ATE455875T1/de
Priority to BRPI0708649A priority patent/BRPI0708649B1/pt
Priority to EP07730968A priority patent/EP1994192B1/fr
Priority to JP2008557785A priority patent/JP5055300B2/ja
Priority to UAA200811832A priority patent/UA92075C2/uk
Priority to DE602007004454T priority patent/DE602007004454D1/de
Priority to ES07730968T priority patent/ES2339292T3/es
Publication of EP1832667A1 publication Critical patent/EP1832667A1/fr
Priority to ZA200807519A priority patent/ZA200807519B/xx
Priority to MA31209A priority patent/MA30261B1/fr
Priority to US15/711,335 priority patent/US10370746B2/en
Withdrawn legal-status Critical Current

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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
    • 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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/32—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for gear wheels, worm wheels, or the like
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—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
    • C22C38/18—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
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
    • C22C38/18—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
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the invention relates to the manufacture of hot rolled sheets of so-called "multiphase" steels, simultaneously having a very high strength and a deformation capacity for carrying out cold forming operations.
  • the invention more specifically relates to predominantly bainitic microstructure steels having a strength greater than 1200 MPa and a yield / strength ratio lower than 0.75.
  • the automotive sector and the general industry are particularly fields of application for these hot-rolled steel sheets.
  • multiphase steels have been developed. mainly bainitic structure: in the automotive industry or in the general industry, these steels are used with advantage for structural parts such as bumper rails, uprights, various reinforcements, wear parts resistant to abrasion.
  • the ability to shape these parts requires simultaneously sufficient elongation, greater than 10% and a ratio (yield strength / resistance) not too high so as to have a sufficient plasticity reserve.
  • the patent US 6,364,968 describes the manufacture of niobium or titanium microalloyed hot-rolled sheets with a resistance greater than 780 MPa of bainitic or bainitomensitic structure comprising at least 90% of bainite with a grain size of less than 3 micrometers: the examples in the patent show that the resistance obtained hardly exceeds 1200 MPa, together with a ratio Re / R m greater than 0.75. It is also noted that the carbides present in this type of very predominantly bainitic structure lead to mechanical damage in case of stress, for example in hole expansion tests.
  • the patent US 4,472,208 also describes the manufacture of titanium microalloyed hot-rolled steel sheet with a predominantly bainitic structure, comprising at least 10% of ferrite, and preferably 20 to 50% of ferrite, and a precipitation of titanium carbides TiC. Due to the large amount of ferrite, however, the strength of the grades made according to this invention is less than 1000 MPa, which may be insufficient for some applications.
  • the patent JP2004332100 describes the manufacture of hot-rolled sheet with a resistance greater than 800 MPa, with a predominantly bainitic structure, containing less than 3% of residual austenite. In order to obtain high values of resistance, however, expensive additions of niobium must be made.
  • the patent JP2004190063 discloses the manufacture of high strength hot rolled steel sheet having a strength-elongation product of greater than 20000 MPa%, and containing austenite. These steels, however, contain expensive additions of copper, in relation to the sulfur content.
  • the present invention aims to solve the problems mentioned above. It aims at providing a hot-rolled steel having a mechanical strength greater than 1200 MPa together with good cold formability, a Re / R m ratio of less than 0.75, an elongation at break greater than 10%.
  • the invention also aims at providing a steel that is not very sensitive to damage during cutting by a mechanical method.
  • the invention also aims to have a steel with good toughness so as to withstand the sudden propagation of a defect, especially in case of dynamic solicitation.
  • a Charpy V energy of rupture higher than 28 Joules is sought at 20 ° C.
  • It also aims at having a steel having good weldability by means of the usual assembly processes in a thickness range ranging from 1 to more than 30 millimeters, in particular during resistance welding or spot welding. arc, in particular in MAG welding ("Metal Active Gas").
  • the invention also aims to provide a steel whose composition does not include expensive micro-alloy element such as titanium, niobium or vanadium. In this way, the manufacturing cost is lowered and the thermomechanical manufacturing diagrams are simplified. It is also intended to provide a steel having a very high fatigue endurance limit.
  • the invention further aims to provide a manufacturing method in which small variations in the parameters do not lead to significant changes in the microstructure or mechanical properties.
  • the subject of the invention is a hot-rolled steel sheet with a resistance greater than 1200 MPa, a Re / R m ratio of less than 0.75 and an elongation at break greater than 10%, the composition of which contains, the contents being expressed by weight: 0.10% ⁇ C ⁇ 0.25%, 1% ⁇ Mn ⁇ 3%, Al ⁇ 0.015%, Si ⁇ 1.985%, Mo ⁇ 0.30%, Cr ⁇ 1, 5%, S ⁇ 0.015%, P ⁇ 0.1%, Co ⁇ 1.5%, B ⁇ 0.005%, with the proviso that 1% ⁇ Si + Al ⁇ 2%, Cr + (3 x Mo) ⁇ 0.3 %, the remainder of the composition consisting of iron and unavoidable impurities resulting from the development, the microstructure of the steel consisting of at least 75% of bainite, residual austenite in quantity greater than or equal to 5%, and martensite in an amount greater than or equal to 2%.
  • the carbon content of the steel sheet is such that: 0.10% ⁇ C ⁇ 0.15%.
  • the carbon content is such that: 0.15% ⁇ C ⁇ 0.17%.
  • the carbon content is such that: 0.17% ⁇ C ⁇ 0.22%.
  • the carbon content is such that: 0.22% ⁇ C ⁇ 0.25%
  • the composition of the steel comprises: 1% ⁇ Mn ⁇ 1.5%.
  • the composition of the steel is such that: 1.5% ⁇ Mn ⁇ 2.3%.
  • the composition of the steel comprises: 2.3% ⁇ Mn ⁇ 3%
  • the composition of the steel comprises: 1.2% ⁇ Si ⁇ 1.8%.
  • the composition of the steel comprises: 1.2% ⁇ Al ⁇ 1.8%.
  • the composition of the steel is such that: Mo ⁇ 0.010%.
  • the invention also relates to a steel sheet whose carbon content of the residual austenite is greater than 1% by weight.
  • the subject of the invention is also a steel sheet, comprising carbides between the bainite slats, the number N of interlayer carbides greater than 0.1 micrometers per unit area is less than or equal to 50000 / mm 2 .
  • the invention also relates to a steel sheet comprising residual martensite-austenite islands, the number of which is N MA per unit area, of residual martensite-austenite islands whose maximum size L max is greater than 2 micrometers and whose elongation factor The max The min is less than 4, is less than 14000 / mm 2 .
  • the invention also relates to a manufacturing method according to which the primary cooling start temperature T DR is set above Ar3, the primary cooling end temperature T FR , the primary cooling rate V R between T DR and T FR , and the secondary cooling rate V ' R , such that the carbon content of the residual austenite is greater than 1% by weight.
  • the invention also relates to a method according to which the primary cooling start temperature T DR is set above Ar3, the primary cooling end temperature T FR , the primary cooling rate V R between T DR and T FR , and the secondary cooling rate V ' R such that the number of interlayer carbides larger than 0.1 micrometers per unit area is less than or equal to 50000 / mm 2 .
  • the invention also relates to a method according to which the primary cooling start temperature T DR is set above Ar3, the primary cooling end temperature T FR , the primary cooling rate V R between T DR and T FR , and the secondary cooling rate V ' R , such that the number N MA per unit area, residual martensite-austenite islands whose maximum size L max is greater than 2 micrometers and whose elongation factor The max The min is less than 4, ie less than 14000 / mm 2 .
  • the invention also relates to the use of a hot-rolled steel sheet according to the characteristics described above, or manufactured by a method according to one of the above modes, for the manufacture of structural parts. or reinforcing elements, in the automotive field.
  • the invention also relates to the use of a hot-rolled steel sheet according to the characteristics described above, or manufactured by a method according to one of the above modes, for the manufacture of reinforcements and parts. structure for general industry, and abrasion resistance parts.
  • a steel containing about 0.2% C and 1.5% Mn is converted, during a cooling from the austenite, bainite composed of ferrite slats and carbides.
  • the microstructure may contain a greater or lesser amount of pro-eutectoid ferrite formed at a relatively high temperature.
  • the flow limit of this component is low, so that it is not possible to obtain a very high level of resistance when this constituent is present.
  • the steels according to the invention do not include pro-eutectoid ferrite. In this way, the mechanical strength is significantly increased beyond 1200 MPa.
  • the precipitation of interlayer carbides is also delayed, the microstructure then consists of bainite, residual austenite, and martensite resulting from the transformation of the austenite.
  • the structure also has an appearance of thin bainitic packs (a package designating a set of parallel slats within the same austenitic former grain) whose strength and ductility are superior to those of polygonal ferrite.
  • the size of the bainite slats is of the order of a few hundred nanometers, the size of the batten packets, of the order of a few micrometers.
  • carbon plays a very important role in the formation of the microstructure and in the mechanical properties: From an austenitic structure formed at high temperature after rolling of a hot sheet a bainitic transformation occurs, and bainitic ferrite slats are initially formed within a matrix still predominantly austenitic. Due to the solubility Very lower carbon in ferrite compared to that in austenite, the carbon is rejected between slats. Thanks to certain alloying elements present in the compositions according to the invention, in particular thanks to the combined additions of silicon and aluminum, the precipitation of carbides, in particular cementite, occurs in a very limited manner.
  • the untransformed austenite interlayer is progressively enriched in carbon substantially without significant precipitation of carbides intervening at the austenite-bainite interface.
  • This enrichment is such that the austenite is stabilized, that is to say that the martensitic transformation of most of this austenite practically does not occur during cooling to room temperature.
  • a limited amount of martensite appears as islets, contributing to increased resistance.
  • Carbon also delays the formation of pro-eutectoid ferrite, the presence of which must be avoided 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 by the formation of low-tenacity microstructures in the heat-affected zone or in the melted zone under autogenous welding conditions.
  • the carbon content is between 0.10 and 0.15%: within this range, the weldability is very satisfactory and the toughness obtained is particularly high. Continuous casting is particularly easy because of a favorable solidification mode.
  • the carbon content is greater than 0.15% and less than or equal to 0.17%: within this range, the weldability is satisfactory and the toughness obtained is high.
  • the carbon content is greater than 0.17% and less than or equal to 0.22%: this range of compositions optimally combines strength properties on the one hand, ductility, toughness and weldability on the other hand.
  • the carbon content is greater than 0.22% and less than or equal to 0.25%: in this way the highest levels of mechanical strength are obtained at the cost of a slight decrease in toughness. .
  • an addition of manganese stabilizes the austenite by lowering the transformation temperature Ar 3.
  • 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 and 1.5%: in this way a satisfactory curing is combined without risk of formation of harmful band structure.
  • the manganese content is greater than 1.5% and less than or equal to 2.3%. In this way, the effects sought above are obtained without, however, excessively increasing the quenchability in the welded joints.
  • the manganese is greater than 2.3% and less than or equal to 3%.
  • Aluminum is a very effective element for the deoxidation of steel. As such, its content is greater than or equal to 0.015%. Like silicon, it is very slightly soluble in cementite and stabilizes the residual austenite.
  • the silicon content is between 1.2 and 1.8%: in this way, the precipitation of carbides is avoided and excellent weldability is obtained; there is no cracking in MAG welding, with sufficient latitude in terms of welding parameters. Spot resistance welds are also free from defects. Moreover, since silicon stabilizes the ferritic phase, an amount of less than or equal to 1.8% makes it possible to avoid the formation of undesirable pro-eutectoid ferrite. An excessive addition of silicon also causes 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.
  • these effects are obtained when the aluminum content is between 1.2 and 1.8%.
  • the effects of aluminum are indeed very similar to those noted above for silicon.
  • the risk of occurrence of superficial defects is however reduced.
  • Molybdenum retards bainitic transformation contributes to hardening by solid solution and also refines the size of the bainitic slats formed.
  • the molybdenum content is less than or equal to 0.3% to prevent the excessive formation of quenching structures.
  • chromium has an effect substantially similar to molybdenum since it also helps to prevent the formation of proeutectoid ferrite and the hardening and refinement of the bainitic microstructure.
  • the contents of chromium and molybdenum are such that: Cr + (3 ⁇ Mo) ⁇ 0.3%.
  • coefficients of chromium and molybdenum in this relation reflect the respective greater or lesser ability of these two elements to retard the ferritic transformation: when the above inequality is satisfied, the formation of pro-eutectoid ferrite is avoided in the specific cooling conditions according to the invention.
  • molybdenum is an expensive element: the inventors have demonstrated that it is possible to manufacture a steel particularly economically by limiting the molybdenum content to 0.010% and compensating for this reduction by adding chromium to respect the relationship. : Cr + (3 x Mo) ⁇ 0.3%.
  • Phosphorus is a known element to segregate at grain boundaries. Its content must be limited to 0.1% in order to maintain sufficient hot ductility. The sulfur and phosphorus limitations also make it possible to obtain good weldability in spot welding.
  • the steel may also comprise cobalt: in an amount of less than or equal to 1.5%, this hardening element makes it possible to increase the carbon content in the residual austenite. The quantity must also be limited for reasons of cost.
  • the steel may also include boron in an amount less than or equal to 0.005%. Such addition increases quenchability and contributes to the removal of pro-eutectoid ferrite. It also allows to increase the levels of resistance.
  • the rest of the composition consists of unavoidable impurities resulting from the preparation, such as, for example, nitrogen.
  • the microstructure of the steel consists of at least 75% of bainite, of residual austenite in an amount greater than or equal to 5%, and of martensite in an amount of greater than or equal to 2%, these contents being referring to surface percentages.
  • This bainitic majority structure without proeutectoid ferrite, gives a very good resistance to further mechanical damage.
  • the microstructure of the hot-rolled sheet according to the invention contains a quantity greater than or equal to 5% of residual austenite, which is preferred rich in carbon, stabilized at ambient temperature, in particular by the additions of silicon and aluminum.
  • the residual austenite is in the form of islands and interlayer films in the bainite, ranging from a few hundredths of a micrometer to a few micrometers.
  • the residual austenite of a piece made from the hot-rolled sheet gradually turns into martensite.
  • the latter has a very high hardness because of its high carbon content. This transformation results in an important consolidation and repels the appearance of the necking.
  • a residual austenite amount of less than 5% does not allow interlayer films to significantly increase the resistance to damage.
  • the carbon content of the residual austenite is greater than 1% in order to reduce the formation of carbides and to obtain residual austenite sufficiently stable at ambient temperature.
  • FIG. 2 shows an example of a microstructure of a steel sheet according to the invention:
  • the residual austenite A in surface content here equal to 7%, appears in white, in the form of islands or films.
  • Martensite M in area content here equal to 15%, is here in the form of very dark constituent on a bainitic matrix B appearing in gray.
  • the local carbon content and thus the local hardenability may vary. Residual austenite is then locally associated with martensite within these islets, which are referred to as "MA" islands, associating Martensite and residual Austenite.
  • MA residual austenite
  • the morphology of the islets MA can be revealed by means of appropriate chemical reagents known per se: after chemical attack, the MA islands appear for example in white on a bainitic matrix more or less dark. These islets are observed by light microscopy at magnitudes ranging from 500 to 1500x on a surface that has a statistically representative population.
  • the maximum size L max and minimum L min of each of the islands The ratio between maximum and minimum size
  • the max The min characterizes the elongation factor of a given island.
  • a particularly high ductility is obtained by reducing the number N MA of islets MA whose maximum length L max is greater than 2 micrometers and whose elongation factor is less than 4. These islands are massive and large. size are revealed priming areas privileged during a subsequent mechanical solicitation.
  • the number of islets N MA per unit area must be less than 14000 / mm 2 .
  • the structure of the steels according to the invention also contains, in addition to the bainite and the residual austenite, martensite in an amount greater than or equal to 2%: this characteristic allows additional hardening which makes it possible to obtain superior mechanical strength. at 1200 MPa.
  • the number of carbides located in position interlatts is limited. These carbides can be observed for example in optical microscopy at a magnification higher than or equal to 1000x. It has been demonstrated that N, the number of interlayer carbides greater than 0.1 micrometers per unit area, should be less than 50000 / mm 2 , otherwise the damage becomes excessive in case of subsequent solicitation, for example during hole expansion tests. In addition, the excessive presence of carbides can cause early initiation of fracture and reduced toughness.
  • 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 in a temperature range where the structure of the steel is totally austenitic up to an end-of-rolling temperature T FL , with reference to the appended FIG.
  • This figure shows a thermomechanical manufacturing diagram 1 according to the invention, as well as a transformation diagram indicating the zones of ferritic transformation 2 bainitic 3 and martensitic 4.
  • Controlled cooling is then performed, starting at a temperature T DR , located above Ar3 (ferritic transformation start temperature from austenite) and ending at a temperature T FR (end-of-cooling temperature). Between T DR and T FR is equal to V R.
  • This cooling and the associated speed V R are called primary.
  • the speed V R is between 50 and 90 ° C / s: When the cooling rate is lower than 50 ° C / s, pro-eutectoid ferrite is formed which is detrimental to obtain high characteristics. of resistance. According to the invention, the ferritic transformation is thus avoided from the austenite.
  • the cooling range according to the invention is advantageous from an industrial point of view since it is not necessary to cool the sheet very rapidly after hot rolling, for example at a speed of the order of 200 ° C. / s, which avoids the need for expensive specific installations.
  • the range of cooling speed according to the invention can be obtained by spraying water or air-water mixture, depending on the thickness of the sheet.
  • the first case corresponds to the manufacture of thin sheets of thickness, up to about 15mm, hot-wound, and thus cooled slowly after the winding operation.
  • the second case corresponds to the manufacture of sheets of greater thickness non-hot rolled: according to the thickness of the sheets, the cooling rates greater than 2 ° C / min and less than or equal to 600 ° C / min correspond to a slightly accelerated cooling or cooling to air.
  • the process has a low sensitivity to a variation of the manufacturing parameters.
  • the secondary cooling associated with a temperature T FR between B ' S and M S + 50 ° C makes it possible to control the bainitic transformation from austenite, to locally enrich this austenite so as to stabilize it, and to obtain a ratio (bainite / residual austenite / martensite) appropriate.
  • the primary speed V R between Ar3 and Ar1 the end of cooling temperature T FR , the secondary cooling rate V ' R , so that the microstructure of the steel consists of at least 75% of bainite, residual austenite in an amount greater than or equal to 5%, and martensite in an amount greater than or equal to 2%.
  • These parameters can also be adjusted to obtain a particular morphology and nature of the MA islands, in particular chosen so that the number N MA of islands of martensite-residual austenite whose size is greater than 2 micrometers and whose elongation factor is less than 4, ie less than 14000 / mm 2 .
  • the parameters can also be adjusted so that the carbon content of the residual austenite is greater than 1% by weight.
  • the parameters V R , T FR , V ' R can also be adjusted so that the number N of bainitic carbides of size greater than 0.1 micrometer per unit area is less than or equal to 50000 / mm 2 .
  • the steel sheets I-1a to c, I-4, I-5a and b, R-6 have a thickness of 12mm, the other sheets of 3mm.
  • the MA islets have been highlighted by Klemm's reagent. Their morphology was examined using image analysis software to determine the parameter N MA . In some cases, the presence of carbides greater than 0.1 micron in the bainitic phase was investigated by Nital etching and observation at high magnification. The number N (/ mm 2 ) of interlayer carbides larger than 0.1 micrometer was determined.
  • the KCV fracture energy at 20 ° C was determined from V resilient test pieces.
  • the steel sheets I-1 to I-7 according to the invention have a particularly advantageous combination of mechanical properties: on the one hand a mechanical strength greater than 1200 MPa, on the other hand an elongation at break greater than 10%. and a ratio Re / Rm of less than 0.75 ensuring good formability.
  • the steels according to the invention also have a Charpy V breakdown energy at room temperature of greater than 28 Joules. This high tenacity allows the manufacture of parts
  • the steel sheets I-1 to I-7 according to the invention have a particularly advantageous combination of mechanical properties: on the one hand a mechanical strength greater than 1200 MPa, on the other hand an elongation at break greater than 10%. and a ratio Re / Rm of less than 0.75 ensuring good formability.
  • the steels according to the invention also have a Charpy V breakdown energy at room temperature of greater than 28 Joules. This high tenacity allows the manufacture of parts resistant to the sudden propagation of a fault especially in case of dynamic stresses.
  • the microstructures of steels according to the invention have a number of islands N MA less than 14,000 / mm 2.
  • the steel sheets I-2a and I-5a have a small surface proportion of large and large islets of MA, respectively 10500 and 13600 compounds per mm 2 .
  • the steels according to the invention also have good resistance to damage in the event of cutting, since the damage factor ⁇ is limited to -12 or -13%.
  • R-1 steel is deficient in chromium and / or molybdenum.
  • the cooling conditions relating to the steels R-1 to R-3 (V R too high, T FR too low) are not suitable for the formation of a fine bainitic structure and an unstable residual austenite.
  • the absence of martensite does not allow sufficient hardening, the resistance is significantly lower than 1200 MPa and the ratio Re / R m is excessive.
  • R-6 steel has an excessive carbon content.
  • the steel sheet R-6 therefore has insufficient resistance to the sudden propagation of a defect since its Charpy V fracture energy at 20 ° C is much lower than 28 Joules.
  • Steel sheets R-7a and R7-b also have an excessive carbon content.
  • the transition temperature at the 28 Joule level estimated from specimens of reduced thickness, is higher than the ambient temperature, testifying to poor toughness. Welding ability is reduced. It will be noted that, despite their higher carbon content, these steel sheets do not have a greater mechanical strength than that of the steels of the invention.
  • the R-9 steel sheet was cooled at an excessive speed until the cooling end temperature was too low. As a result, the structure is almost completely martensitic and the elongation at break is insufficient.
  • the invention enables the manufacture of bainitic matrix steel sheets without the addition of expensive microalloy elements. These combine very high strength and high ductility. Thanks to their high strength, these steel sheets are suitable for the manufacture of elements undergoing cyclic mechanical stresses.
  • 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.

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EP06290386A 2006-03-07 2006-03-07 Procédé de fabrication de tôles d'acier à très hautes caractéristiques de résistance, de ductilité et de tenacité, et tôles ainsi produites Withdrawn EP1832667A1 (fr)

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EP06290386A EP1832667A1 (fr) 2006-03-07 2006-03-07 Procédé de fabrication de tôles d'acier à très hautes caractéristiques de résistance, de ductilité et de tenacité, et tôles ainsi produites
AT07730968T ATE455875T1 (de) 2006-03-07 2007-02-14 Verfahren zur herstellung eines stahlblechs mit sehr hohen festigkeits-, biegbarkeits- und härteeigenschaften und auf diese weise produziertes blech
BRPI0708649A BRPI0708649B1 (pt) 2006-03-07 2007-02-14 Chapa de aço laminada a quente tendo limite de resistência à tração e características de ductilidade e tenacidade muito altas e processo para fabricação de chapa de aço laminada a quente
MX2008011274A MX2008011274A (es) 2006-03-07 2007-02-14 Proceso para la manufacturacion de chapas de acero que tienen caracteristicas de resistencia, ductilidad y tenacidad muy altas y chapas producidas de esta manera.
US12/281,839 US9856548B2 (en) 2006-03-07 2007-02-14 Process for manufacturing steel sheet having very high strength, ductility and toughness characteristics, and sheet thus produced
PL07730968T PL1994192T3 (pl) 2006-03-07 2007-02-14 Sposób wytwarzania arkusza blachy o bardzo wysokiej wytrzymałości, ciągliwości i udarności oraz arkusz wytworzony tym sposobem
KR1020087024511A KR101073425B1 (ko) 2006-03-07 2007-02-14 매우 높은 강도, 연성 및 인성을 갖는 강 시트 제조 방법 및 이 방법으로 제조된 강 시트
RU2008139605/02A RU2397268C2 (ru) 2006-03-07 2007-02-14 Способ производства стального листа с очень высокими характеристиками прочности на разрыв, пластичности и ударной прочности и изготовленный по способу лист
PCT/FR2007/000256 WO2007101921A1 (fr) 2006-03-07 2007-02-14 Procede de fabrication de tôles d1acier a tres hautes caracteristiques de resistance, de ductilite et de tenacite, et tôles ainsi produites
CN2007800159016A CN101437975B (zh) 2006-03-07 2007-02-14 制造具有极高强度、延展性和韧性特征的钢板的方法以及由此生产的板材
CA2645059A CA2645059C (fr) 2006-03-07 2007-02-14 Procede de fabrication de toles d'acier a tres hautes caracteristiques de resistance, de ductilite et de tenacite, et toles ainsi produites
EP07730968A EP1994192B1 (fr) 2006-03-07 2007-02-14 Procédé de fabrication de tôles d'acier à tres hautes caracteristiques de resistance, de ductilite et de tenacite, et tôles ainsi produites
JP2008557785A JP5055300B2 (ja) 2006-03-07 2007-02-14 極めて高い強度、延性および靭性特性を有する鋼板を製造するための方法、ならびにこのように製造された板
UAA200811832A UA92075C2 (uk) 2006-03-07 2007-02-14 Сталевий лист з високими показниками міцності, пластичності і твердості, спосіб його виготовлення (варіанти) та застосування (варіанти)
DE602007004454T DE602007004454D1 (de) 2006-03-07 2007-02-14 Verfahren zur herstellung eines stahlblechs mit sehr hohen festigkeits-, biegbarkeits- und härteeigenschaften und auf diese weise produziertes blech
ES07730968T ES2339292T3 (es) 2006-03-07 2007-02-14 Procedimiento de fabricacion de chapas de acero con caracteristicas muy elevadas, de resistencia, ductilidad y tenacidad, y chapas asi producidas.
ZA200807519A ZA200807519B (en) 2006-03-07 2008-08-28 Process for manufacturing steel sheet having very high tensile strength, ductility and toughness characteristics, and sheet thus produced
MA31209A MA30261B1 (fr) 2006-03-07 2008-09-05 Procede de fabrication de tole d'acier a tres hautes caracteristiques de resistance, de ductilite et de tenacite, et tole ainsi produites.
US15/711,335 US10370746B2 (en) 2006-03-07 2017-09-21 Process for manufacturing steel sheet

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EP07730968A Active EP1994192B1 (fr) 2006-03-07 2007-02-14 Procédé de fabrication de tôles d'acier à tres hautes caracteristiques de resistance, de ductilite et de tenacite, et tôles ainsi produites

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WO2020079096A1 (en) * 2018-10-19 2020-04-23 Tata Steel Nederland Technology B.V. Hot rolled steel sheet with ultra-high strength and improved formability and method for producing the same

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KR101073425B1 (ko) 2011-10-17
PL1994192T3 (pl) 2010-06-30
CA2645059C (fr) 2012-04-24
RU2008139605A (ru) 2010-04-20
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JP5055300B2 (ja) 2012-10-24
MA30261B1 (fr) 2009-03-02
ES2339292T3 (es) 2010-05-18
DE602007004454D1 (de) 2010-03-11
JP2009529098A (ja) 2009-08-13
US20180010220A1 (en) 2018-01-11
EP1994192B1 (fr) 2010-01-20
WO2007101921A1 (fr) 2007-09-13
EP1994192A1 (fr) 2008-11-26
CA2645059A1 (fr) 2007-09-13
ZA200807519B (en) 2009-05-27
BRPI0708649A2 (pt) 2011-06-07
BRPI0708649B1 (pt) 2015-09-29
KR20080106337A (ko) 2008-12-04
MX2008011274A (es) 2008-09-12
US20090107588A1 (en) 2009-04-30
US9856548B2 (en) 2018-01-02
CN101437975A (zh) 2009-05-20
CN101437975B (zh) 2011-06-01
US10370746B2 (en) 2019-08-06
RU2397268C2 (ru) 2010-08-20
ATE455875T1 (de) 2010-02-15

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