Classifications

• • 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 / resistance ratio of less than 0.75.
• the automobile sector and the general industry are notably fields of application of these hot-rolled steel sheets. In the automotive industry, there is a continuing need for vehicle lightening and increased safety. This is how we proposed several families of steels offering different levels of resistance:
• the residual austenite is stabilized by the addition of silicon or aluminum, these elements delaying the precipitation of carbides in the austenite and in the bainite.
• the presence of residual austenite gives high ductility to an undeformed sheet. Under the effect of a subsequent deformation, for example during uniaxial loading, the residual austenite of a TRIP steel part is transformed progressively in martensite, which results in a significant consolidation and delays the appearance of a necking.
• 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 exemplary embodiments in the patent show that the resistance obtained barely exceeds 1200 MPa, together with a Re / R m ratio 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.
• US Pat. No. 4,472,208 also describes the production 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, as well as a precipitation of carbides. titanium 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.
• Patent JP2004332100 describes the manufacture of hot-rolled sheet with a resistance greater than 800 MPa 1 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. 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 object of the invention is also to provide a steel that is not very sensitive to damage when it is cut by a mechanical method.
• It 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 more than 28 Joules at 20 ° C. is sought.
• It also aims at having a steel having good weldability by means of the usual assembly processes in a thickness range from 1 to more than 30 millimeters, especially during spot or arc resistance welding, particularly in MAG ("Metal Active Gas”) welding.
• the invention also aims to provide a steel whose composition does not include expensive micro-alloy elements 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 1 with 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%,
• 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% bainite, residual austenite in quantity greater than or equal to 5%, and of marten $ ite in a quantity greater than or equal to
• the carbon content of the steel sheet is such that:
• the carbon content is such that: 0.15% ⁇ C ⁇
• the carbon content is such that: 0.17% ⁇ C ⁇
• the carbon content is such that: 0.22% ⁇ C ⁇ 0.25%
• the composition of the steel comprises: 1%
• the composition of the steel is such that: 1.5% ⁇ Mn
• the composition of the steel comprises: 2.3% ⁇ Mn ⁇ 3%
• the composition of the steel comprises; 1, 2% ⁇ If ⁇
• the composition of the steel comprises: 1.2% ⁇ AI ⁇ 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 being less than or equal to
• the subject of the invention is also a steel sheet comprising residual martensite-austenite islands, whose NMA number per unit area, of residual martensite-austenite islands whose maximum size L max is greater than 2 micrometers and whose elongation factor is less than
• the subject of the invention is also a process for producing a hot-rolled steel sheet with a resistance greater than 1200 MPa, a Re / Rm ratio of less than 0.75 and an elongation at break greater than 10%, depending on which :
• the half-product is brought to a temperature greater than 115 ° C.
• the semi-finished product is hot-rolled in a temperature range where the structure of the steel is entirely austenitic
• the sheet is cooled from the temperature T F R with a secondary cooling rate V ' R between 0.08 D C / min and 600 ⁇ C / min to the ambient temperature,
• the temperature B's being equal to Bs + 60 ° C when the speed VR is greater than 2 ° C / min and less than or equal to 60 ° C / min
• the subject of the invention is also a process for producing a hot-rolled steel sheet with a resistance greater than 1200 MPa, a Re / Rm ratio of less than 0.75 and an elongation at break greater than 10%, depending on which: - one supplies a steel of composition above,
• the half-product is carried at a temperature above 1150 ° C. and is hot rolled in a temperature range where the microstructure of the steel is entirely austenitic, then
• the sheet thus obtained is cooled from a temperature T D R located above Ar3 to an intermediate temperature Ti with a cooling rate Vm greater than or equal to 7 (TC / s, the temperature T 1 being lower or equal to 650 p C, then
• the plate is cooled from the temperature T to a temperature TFR, the temperature T FR being between B's and M s + 50 ° C, B's denoting a temperature defined relative to the start temperature Bs bainitic transformation, and M 5 denoting the martensitic transformation start temperature, such that the cooling rate between the temperature TDR and the temperature T FR Is between 20 and 90 ⁇ C / s, then
• the sheet is cooled from the temperature TFR with a secondary cooling rate V R of between 0.08 ° C./min and 60 ° C./min until the ambient temperature,
• the temperature B 1 S being equal to Bs when the speed V ' R is between 0, Q8 and 2 ° C / min
• the temperature B 1 S being equal to Bs + 60 ° C when the speed VR is greater than 2 ° C / min and less than or equal to 600 ° C / min
• the invention also relates to a method for manufacturing a hot-rolled steel sheet according to which
• the semi-finished product is hot-rolled in a temperature range where the structure of the steel is entirely austenitic
• the primary cooling start temperature TDR above Ar3 the primary cooling end temperature TFR, the primary cooling speed VR between T D R and T F R, and the secondary cooling speed V R are adjusted.
• the microstructure of the steel consists of at least 75% bainite, residual austenite in an amount greater than or equal to 5%, and martensite in a quantity greater than or equal to at 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 F R, the primary cooling rate V R between T D R and TFR, and the secondary cooling rate VR, so 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 TDR above Ar3, the primary cooling end temperature T F R, the primary cooling rate Vp between TDR and T F R are adjusted. , and the secondary cooling rate V ' R so that the number of interlayer carbides greater than 0.1 micrometers per unit area is less than or equal to 50000 / mrn 2 .
• the subject of the invention is also a method according to which the primary cooling start temperature T D R above Ar3, the primary cooling end temperature T FR , the primary cooling rate V R between T D R are adjusted. and T FR , and the secondary cooling rate VR, such that the NMA number per unit area, residual martensite-austenite islands whose maximum size L max is greater than 2 micrometers and whose elongation factor - is less than 4, less than 14000 / mrn 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.
• FIG. 1 shows a schematic description of an embodiment of the manufacturing method according to the invention, in connection with a transformation diagram from the austenite.
• FIG. 2 shows an exemplary microstructure of a steel sheet according to the invention.
• 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 bundles of slats, 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 Taustenite, 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 any risk of damaging band structure formation.
• 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. It has been shown that the effects of aluminum and silicon on the stabilization of austenite are very similar: When the silicon and aluminum contents are such that: 1% ⁇ Si + AI ⁇ 2%, a stabilization satisfactory austenite is obtained, which allows to form the desired microstructures while retaining satisfactory use properties. Given that the minimum aluminum content is 0.015%, the silicon content is less than or equal to 1.985%.
• 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. Preferentially, 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 a substantially similar effect to molybdenum since it also helps to prevent the formation of proutectoid 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 relationship reflect the greater or lesser ability of these two elements to retard 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 MB) ⁇ 0.3%.
• 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.
• 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.
• islets MA were to be particularly sought.
• the morphology of the islets MA can be revealed by means of appropriate chemical reagents known per se: after chemical attack, the islets MA 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 1500 ° approximately on a surface that has a statistically representative population. It determines, for example by means of a known image analysis software itself, such as for example the Noesis ® software the company Noesis, the maximum size L my ⁇ and minimum L min of each of the islands. The ratio between the maximum and minimum size characterizes the elongation factor of an island given.
• a particularly high ductility is obtained by reducing the NMA number of MA islands whose maximum length L max is greater than 2 micrometers and whose elongation factor is less than 4. These large and large islands are prime areas of priming during a subsequent mechanical solicitation. According to the invention, the number of NMA islands 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 graduation greater 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 method of manufacturing a hot-rolled sheet according to the invention is as follows: - A steel of composition according to the invention is supplied - A semi-product is cast from this steel. This casting may be carried out in ingots, or continuously in the form of slabs of thickness of the order of 200 mm. It is also possible to cast in the form of slabs of a few tens of millimeters thick, or thin strips, between contra-rotating steel cylinders.
• the cast half-products are first brought to a temperature above 115O 0 C to reach at any point a temperature favorable to the high deformations that will undergo the steel during rolling.
• the hot rolling step of these semi-finished products starting at more than 115 ° C. can be done directly after casting so well. that an intermediate heating step is not necessary in this case.
• thermomechanical manufacturing diagram 1 shows a thermomechanical manufacturing diagram 1 according to the invention, as well as a transformation diagram indicating the areas of ferritic transformation 2 bai ⁇ itic 3 and martensitic 4.
• Controlled cooling is then performed, starting at a TDR temperature, located above Ar3 (ferritic transformation start temperature from austenite) and ending at a temperature T F R (end-of-cooling temperature).
• T D R temperature
• T F R end-of-cooling temperature
• the speed VR is between 50 and 90 ° C./s: When the cooling rate is lower than 50 ° C./s, pro-eutectoid ferrite is formed which is harmful for obtaining high characteristics of resistance. According to the invention, this avoids ferritic transformation from austenite.
• the cooling range of the invention is advantageous from an industrial point of view because it is not necessary to cool the sheet quickly after the hot rolling, for example at a speed of about 200 ù C / s, which avoids the need for expensive specific installations.
• the cooling rate range according to the invention can be obtained by spraying water or air-water mixture, depending on the thickness of the sheet.
• the method can also be implemented according to the following variant: From the TDR temperature, a rapid cooling is carried out up to a temperature Ti less than or equal to 650 p C. The speed V R i of this rapid cooling is greater at 7O 0 CVs.
• Cooling is then carried out to a TFR temperature so that the average cooling rate between T D R and TF R is between 20 and 90 ° C / s.
• This variant has the advantage of requiring slower cooling. on average between TDR and T FR than in the previous variant, subject to performing faster cooling at the speed V R- 1 from T D R to ensure the absence of proeutectoid ferrite.
• a slower, so-called secondary, cooling phase is started, which starts at a temperature T FR of between B ' s and M s + 50 ° C and which ends at room temperature.
• the secondary cooling rate is designated V ' R.
• Ms denotes the martensitic transformation start temperature.
• This temperature Bs can be determined experimentally or evaluated from the composition by means of formulas known per se.
• Figure 1 illustrates this first method of manufacture.
• 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 lower or equal to 600 ° C / min correspond to slightly accelerated cooling or air cooling.
• the carbon enrichment of the austenite is not sufficient: after complete cooling, carbides or islands of martensite are formed. In this way, it is possible to obtain a steel having a "dual-phase" structure but whose combination of properties (strength-ductility) is lower than that of the invention. These structures also have a greater sensitivity to damage than those of the invention.
• the cooling termination temperature is less than Ms + 50 û C, carbon enrichment of the austenite is excessive. Under certain industrial conditions, there is a risk of formation of a marked band structure and excessive martensitic transformation. Thus, under the conditions according to the invention, the process has a low sensitivity to a variation of the manufacturing parameters.
• the secondary cooling associated with a temperature T F R between B's and Mg + 50 ° C makes it possible to control the bainitic transformation from austenite, to locally enrich this austenite in order to stabilize it, and to obtain a ratio (bainite / residual austenite / martensite) appropriate.
• the cooling rate V R will be chosen so as to be as fast as possible to avoid a pearlitic transformation (which would lead to an insufficient residual austenite content) and ferritic while remaining within the control capabilities of an industrial line so as to obtain a microstructural homogeneity in the longitudinal and transverse direction of the hot-rolled sheet.
• the cooling rate V R must however be limited to avoid the formation of a heterogeneous microstructure in the thickness of the sheet.
• the cooling rate VR is essentially dependent on the production capacities of the industrial sites and the thickness of the sheets. - Independently of V ' R, T FR will be chosen sufficiently low so as to avoid a pearlitic transformation, which would result in an incomplete bainitic transformation and a residual austenite content of less than 5%,
• the temperature TFR will be chosen high enough to allow time for the bainitic transformation to take place above the martensitic domain. The formation of more than 20% of martensite is then avoided by passing too fast in the martensitic domain. This last transformation would occur at the expense of bainitic transformation and the stabilization of residual austenite.
• 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 s .
• These parameters can also be adjusted so that the carbon content of the residual austenite is greater than 1% by weight.
• a cooling rate V R will be chosen which is not too high so as to avoid excessive formation of coarse MA islands.
• the parameters V R , TVR, VR can also be adjusted so that the number N of bainitic carbides greater than 0.1 micrometer by unit area is less than or equal to 50000 / mm 2 .
• the steel sheets 1-1 a to c , 1-4, 1-5a and b, R-6, have a thickness of 12mm, the other sheets of 3mm.
• Table 2 also indicates the transformation temperatures B ' s and M s + 50 ° C calculated from the chemical compositions using the following expressions, the compositions being expressed in percentage by weight:
• B s (0 C) 830-270 (C) - 90 (Mn) - 37 (Ni) - 70 (Cr) - 83 (Mo) M 5 (0 C) - 561-474 (C) - 33 (Mn 17 (Ni) - 17 (Cr) - 21 (Mo)
• the various microstructural constituents measured by quantitative microscopy were also reported: surface fraction of bainite, residual austenite by X-ray diffraction or sigmametry, and martensite .
• 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 tensile mechanical properties obtained were given in Table 3 below.
• the Re / Rm ratio was also indicated.
• the KCV rupture energy has been determined at 20 ° C. from the V test specimens.
• the steel sheets 1-1 to I-9 according to the invention have a particularly advantageous combination of mechanical properties: on the one hand a mechanical strength greater than 1200 MPa 1 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 fracture energy at room temperature greater than 28 Joules. This high tenacity allows the manufacture of parts resistant to the sudden propagation of a defect especially in case of dynamic stresses.
• the microstructures of steels according to the invention have a number dilots NMA below 140Q0 / mm z.
• the steel sheets 1-2a and 1-5a have a low 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%.
• the steel R-1 has an insufficient content of chromium and / or molybdenum.
• the cooling conditions relating to steels R-1 to R-3 (V R too high, T F R too low) are not suitable for the formation of a fine bainitic structure. 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.
• the steel sheet R-6 consequently 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 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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