Classifications

• • 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
  • 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
  • C22C38/02—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

Definitions

• the invention relates to the manufacture of steel sheets, more particularly "TRIP” ("Transformation Induced Plasticity”) steels, ie having a plasticity induced by an allotropic transformation.
• TRIP Transformation Induced Plasticity
• TRIP steel cold-rolled sheet is produced by reheating during annealing in a field where austenitization occurs partially, followed by rapid cooling to avoid the formation of perlite and subsequent maintenance. isothermal in the bainitic domain: a part of the austenite is transformed into bainite, another part is stabilized by the increase of the carbon content of the islands of residual austenite. Thus, the initial presence of ductile residual austenite is associated with high deformability. Under the effect of a subsequent deformation, for example during stamping, the residual austenite of a TRIP steel part gradually changes to martensite, which results in a significant hardening. A steel exhibiting a TRIP behavior thus makes it possible to guarantee an important ability to deform and a high mechanical resistance, these two properties being usually antagonistic. This combination provides a high energy absorption potential, a quality typically sought in the automotive industry for shock-resistant parts.
• the present invention aims to solve the problems mentioned above.
• the subject of the invention is a composition for the manufacture of steel having a TRIP behavior, comprising, the contents being expressed by weight: 0.08% ⁇ C ⁇ 0.23%, 1% ⁇ Mn ⁇ 2 %, 1 ⁇ If ⁇ 2%, Al ⁇ 0.030%, 0.1% ⁇ V ⁇ 0.25%, Ti ⁇ 0.010%, S ⁇ 0.015%, P ⁇ 0.1%, 0.004% ⁇ N ⁇ 0.012% , and optionally one or more elements selected from: Nb ⁇ 0.1%, Mo ⁇ 0.5%, Cr ⁇ 0.3%, the remainder of the composition consisting of iron and unavoidable impurities resulting from the development.
• the carbon content is such that: 0.08% ⁇ C ⁇ 0.13%.
• the carbon content is such that: 0.13% ⁇ C ⁇ 0.18%.
• the carbon content is such that: 0.18% ⁇ C ⁇ 0.23%.
• the manganese content is such that: 1.4% ⁇ Mn ⁇ 1.8%.
• the manganese content satisfies: 1.5% ⁇ Mn ⁇ 1.7%.
• the silicon content is such that: 1.4% ⁇ Si ⁇ 1.7%.
• the aluminum content satisfies: Al ⁇ 0.015%.
• the vanadium content is such that: 0.12% ⁇ V ⁇ 0.15%.
• the titanium content is such that: Ti ⁇ 0.005%.
• the invention also relates to a steel sheet of the above composition, the microstructure of which consists of ferrite, bainite, residual austenite, and possibly martensite.
• the microstructure of the steel comprises a residual austenite content of between 8 and 20%.
• the microstructure of the steel preferably comprises a martensite content of less than 2%.
• the average size of the residual austenite islands is less than or equal to 2 microns.
• the average size of the residual austenite islands is preferably less than or equal to 1 micrometer.
• the temperature T fl at the end of the hot rolling, the speed V r of the cooling, the temperature T bob of the winding are chosen such that the microstructure of the steel comprises a residual austenite content of between 8 and 20%. .
• the temperature T fl of the hot rolling end, the cooling speed V r , the winding temperature T bob are chosen such that the microstructure of the steel comprises a martensite content of less than 2%.
• the temperature T fl of the hot rolling end, the cooling speed V r , the winding temperature T bob are chosen such that the average size of the residual austenite islands is less than or equal to 2 microns. and very preferably less than 1 micrometer.
• the winding temperature T bob is less than 400 ° C.
• the subject of the invention is also a process for manufacturing a cold-rolled sheet exhibiting a TRIP behavior, according to which a hot-rolled steel sheet manufactured according to any of the processes described above is supplied, the sheet, the sheet is cold-rolled, the sheet is subjected to a thermal annealing treatment, the heat treatment comprising a heating phase at a heating rate V cm , a holding phase at a holding temperature T m during a holding time t m , followed by a cooling phase at a cooling rate V rm when the temperature is lower than Ar3, followed by a holding phase at a holding temperature T ' m during a holding time t m , the parameters V cm , T m , t m , V rm , T ' m , t' m being chosen such that the microstructure of said steel is made of ferrite, bainite, residual austenite, and possibly martensite.
• the parameters V cm , T m , t m , V rm , T ' m , t' m are chosen such that the microstructure of the steel comprises a residual austenite content of between 8 and 20%. .
• the parameters V cm , T m , t m , V rm , T ' m , t' m are chosen such that the microstructure of the steel comprises less than 2% of martensite.
• the parameters V cm , T m , t m , V rm , T ' m , t' m are chosen such that the average size of the residual austenite islands is less than 2 micrometers, very preferably less than at 1 micrometer.
• the subject of the invention is also a process for manufacturing a cold-rolled sheet exhibiting a TRIP behavior, according to which the sheet is subjected to an annealing heat treatment, the heat treatment comprising a heating phase at a speed V cm. greater than or equal to 2 ° C / s, a holding phase at a holding temperature T m between Ac1 and
• the holding temperature T m is preferably between 770 and 815 ° C.
• the invention also relates to the use of a steel sheet having a TRIP behavior, according to one of the variants described above, or manufactured by one of the processes described above, for the manufacture of parts of structure or reinforcement elements in the automotive field.
• a bainitic transformation takes place from an austenitic structure formed at high temperature , and slats of bainitic ferrite are formed. Given the much lower solubility of carbon in ferrite compared to austenite, the carbon of the austenite is rejected between the slats. Thanks to certain alloying elements of the steel composition according to the invention, in particular silicon and manganese, the precipitation of carbides, in particular of cementite, intervenes very little. Thus, the austenite interlatte is progressively enriched in carbon without the precipitation of carbides intervening.
• the carbon content is between 0.08 and 0.23% by weight.
• the carbon content is in a first range of 0.08 to 0.13% by weight.
• the carbon content is greater than 0.13% and is less than or equal to 0.18% by weight.
• the carbon content is in a third preferred range, where it is greater than 0.18 and less than or equal to 0.23% by weight.
• the minimum carbon content of each of the three preferred ranges makes it possible to obtain a minimum strength of 600 MPa, 800 MPa and 950 MPa on cold-rolled and annealed sheets, respectively at each of the three preferred ranges. beaches above.
• the maximum carbon content of each of three ranges makes it possible to guarantee satisfactory weldability, particularly in spot welding, if the level of resistance obtained in these three preferred ranges is taken into account.
• an addition of manganese contributes to reducing the martensitic transformation start temperature Ms and stabilizing the austenite.
• This addition of manganese also contributes to an effective hardening in solid solution and thus to obtaining increased strength.
• the manganese is preferably between 1.4 and 1.8% by weight: a satisfactory curing and an increase in the stability of the austenite are combined in this way without, however, excessively increasing the quenchability in the welded joints.
• the manganese content is between 1.5 and 1.7% by weight. In this way, the effects sought above are obtained without risk of formation of a harmful band structure that would come from a possible segregation of manganese during solidification.
• silicon inhibits the precipitation of cementite during cooling from austenite by considerably retarding the growth of carbides: this is due to the fact that the solubility of silicon in cementite is very low. low and that this element increases the carbon activity in the austenite. In this way, a possible cementite seed forming will be surrounded by a silicon-rich austenitic zone which has been rejected at the precipitate-matrix interface.
• This silicon-enriched austenite is also richer in carbon and the growth of cementite is slowed down because of the small diffusion resulting from the reduced carbon gradient between the cementite and the surrounding austenitic zone.
• This addition of silicon thus contributes to stabilizing a sufficient amount of residual austenite to obtain a TRIP effect.
• this addition of silicon makes it possible to increase the resistance thanks to hardening in solid solution.
• an excessive addition of silicon causes the formation of strongly adherent oxides, which are difficult to eliminate during a stripping operation, and the possible appearance of surface defects due in particular to a lack of wettability in dip galvanizing operations.
• the silicon content is preferably between 1.4 and 1.7% by weight.
• Aluminum is a very effective element for the deoxidation of steel. Like silicon, it is very slightly soluble in cementite and could be used as such to prevent the precipitation of cementite during maintenance at a bainitic transformation temperature and stabilize the residual austenite.
• the aluminum content is less than or equal to 0.030% by weight: in fact, as will be seen below, a very effective hardening is obtained by means of a precipitation of vanadium carbonitrides: when the aluminum content is greater than 0.030%, there is a risk of precipitation of aluminum nitride which reduces the amount of nitrogen likely to precipitate with vanadium.
• this amount is less than or equal to 0.015% by weight, any risk of precipitation of aluminum nitride is discarded and the full effect of hardening by the precipitation of vanadium carbonitrides is obtained.
• the titanium content is less than or equal to 0.010% by weight in order not to precipitate a significant amount of nitrogen in the form of titanium nitrides or carbonitrides.
• the titanium content is preferably less than or equal to 0.005% by weight. Such a titanium content then makes it possible to avoid the precipitation of (Ti, V) N on hot-rolled sheets.
• Vanadium and nitrogen are important elements of the invention: The inventors have demonstrated that, when these elements are present in defined amounts according to the invention, they precipitate in the form of very fine vanadium carbonitrides associated with hardening. important. When the vanadium content is less than 0.1% by weight or when the nitrogen content is less than 0.004% by weight, the precipitation of vanadium carbonitrides is limited and curing is insufficient. When the vanadium content is greater than 0.25% by weight or when the nitrogen content is greater than 0.012% by weight, the precipitation occurs at an early stage after hot rolling in the form of coarser precipitates.
• the uniform or breaking elongation is particularly increased.
• sulfur tends to precipitate excessively in the form of manganese sulfides which greatly reduce the formability.
• Phosphorus is a known element to segregate at grain boundaries. Its content shall be limited to 0.1% by weight so as to maintain sufficient hot ductility and to promote breakage by peeling during tensile-shear tests carried out on spot-welded joints.
• elements such as chromium and molybdenum that delay bainitic transformation and promote hardening by solid solution, can be added in amounts of less than or equal to 0.3 or 0.5% by weight, respectively.
• Niobium may also optionally be added in an amount of less than or equal to 0.1% by weight so as to increase the resistance by additional precipitation of carbonitrides.
• the cast semifinished products are first brought to a temperature above 1200 ° C. in order to attain at all points a temperature favorable to the high deformations which the steel will undergo during rolling as well as to avoid at this stage of manufacture the presence of vanadium carbonitrides.
• the hot rolling step of these semi-finished products starting at more than 1200 ° C. can be done directly after casting. that an intermediate reheating step is not then necessary.
• this minimum temperature of 1200 ° C. also makes it possible to carry out hot rolling in a fully austenitic phase under satisfactory conditions on a continuous train of hot rolling.
• the semi-finished product is hot-rolled up to a rolling end temperature T f equal to or greater than 900 ° C.
• T f rolling end temperature
• the rolling is entirely carried out in the austenitic phase where the solubility of the vanadium carbonitrides is greater and where the probability of precipitation of V (CN) is the lowest.
• the sheet thus obtained is then cooled with a cooling rate V r greater than or equal to 20 ° C./s in order to avoid a precipitation of the vanadium carbonitrides in the ferrite.
• This cooling can be carried out for example by means of spraying water on the sheet.
• the sheet obtained is then reeled at a temperature of less than or equal to 450 ° C.
• the quasi-isothermal maintenance associated with this winding leads to the formation of a microstructure consisting of bainite, ferrite, residual austenite, possibly a small amount of martensite, as well as a hardening precipitation of vanadium carbonitrides.
• the winding temperature is less than or equal to 400 ° C, the total elongation and the distributed elongation are increased.
• T fl end temperature of hot rolling, the cooling rate V r and the winding bob temperature T such that the microstructure comprises a residual austenite content of between 8 and 20%:
• the amount residual austenite is less than 8%, a sufficient TRIP effect can not be demonstrated during mechanical tests: in particular, it is demonstrated during tensile tests that the coefficient of hardening n is less than 0 , 2 and decreases rapidly with the deformation ⁇ .
• the residual austenite is progressively transformed into martensite during the deformation, n is greater than 0.2 and the necking appears for larger deformations.
• the residual austenite content is greater than 20%, the residual austenite formed under these conditions has a carbon content relatively weak and destabilizes too easily during a subsequent phase of deformation or cooling.
• the parameters T f , V r , T bob will preferably be chosen so that the microstructure of the hot-rolled steel sheet contains less than 2% of martensite.
• the elongation is reduced as well as the absorption energy related to the area under the traction curve ( ⁇ - ⁇ ).
• ⁇ - ⁇ absorption energy related to the area under the traction curve
• the excessive presence of martensite leads to a mechanical behavior approaching that of a Dual-Phase steel with an initial value of the coefficient of hardening n high decreasing when the rate of deformation increases.
• the microstructure does not contain martensite.
• the parameters T f , V r , T bob will also preferably be chosen so that the average size of the residual austenite islands of the microstructure is less than or equal to 2 micrometers. Indeed, when the austenite is transformed into martensite under the influence of the lowering of the temperature or of a deformation, the islands of martensite of average size superior to 2 micrometers play a preferential role for the damage as a result of decohesion with the matrix.
• the parameters T f , V r , T bob will be chosen even more particularly, so that the average size of the residual austenite islands of the microstructure is less than or equal to 1 micrometer in order to increase their stability. to limit the damage to the matrix-island interface and to push the necking towards higher deformation values.
• a hot-rolled sheet is first produced according to one of the variants which have been explained above. Indeed, the inventors have found that the microstructures and the mechanical properties obtained by the cold rolling and annealing manufacturing process which will be exposed, depend relatively little on the manufacturing conditions within the limits of the process variants set out above. , in particular variations of the winding temperature T bob . In this way, the method for manufacturing cold-rolled sheets has the advantage of being insensitive to accidental variations in the conditions for producing hot-rolled sheets.
• a winding temperature of less than or equal to 400 ° C. will be chosen so as to keep more vanadium in solid solution available for precipitation during the subsequent annealing of the cold-rolled sheet.
• the hot-rolled sheet is scraped by a method known per se so as to give it a surface state suitable for cold rolling.
• the latter is carried out under customary conditions, for example by reducing the thickness of the hot-rolled sheet by 30 to 75%.
• a rapid cooling is carried out at a speed V rm greater than 15 ° C / s when the temperature is lower than Ar3. Rapid cooling when the temperature is lower than Ar3 is important in order to limit the formation of ferrite before the bainitic transformation.
• This rapid cooling phase when the temperature is lower than Ar 3 may be preceded, if necessary, by a slower cooling phase from the temperature T m .
• the holding temperature T m is between 770 and 815 ° C: below 770 ° C, the recrystallization may be insufficient. Beyond 815 ° C, the fraction of intercritical austenite formed is too large and the hardening of ferrite by the precipitation of vanadium carbonitrides is less effective: in fact, the intercritical ferrite content is less and the total amount of vanadium precipitates, vanadium being rather soluble in the austenite. On the other hand, vanadium carbonitride precipitates that form are more likely to grow and coalesce at high temperatures.
• the sheet is subjected to an annealing heat treatment whose parameters V cm , T m , t m , V rm , T ' m , t' m are chosen so that the microstructure of the steel obtained consists of ferrite, bainite and residual austenite, possibly martensite.
• parameters will be chosen such that the residual austenite content is between 8 and 20%. These parameters will preferably be chosen such that the average size of the residual austenite islands is less than or equal to 2 micrometers, optimally less than or equal to 1 micrometer. These parameters will also be chosen so that the martensite content is less than 2%. Optimally, the microstructure does not include martensite.
• the sheets manufactured according to the invention have a very high strength, significantly higher than 800 MPa for a carbon content of about 0.22%.
• Their microstructure is composed of ferrite, bainite and residual austenite, as well as martensite in an amount of less than 2%.
• residual austenite content 10.8%
• the carbon concentration of the residual austenite islets is 1.36% by weight. This indicates that the austenite is sufficiently stable to obtain a TRIP effect as shown by the behavior observed during the tensile tests performed on these steel sheets.
• the reference steel sheet R1 of bainito-pearlitic structure with a very low residual austenite content, does not exhibit TRIP behavior. Its resistance is less than 800 MPa, a level significantly lower than that of the steels of the invention.
• I2 steel according to the invention also has excellent toughness since its ductile-brittle transition temperature is significantly lower (-35 ° C) than that of a reference steel (0 ° C).
• the resulting microstructure was observed after Klemm reagent attack highlighting residual austenite islands and the average size of these islets was measured using image analysis software.
• the average size of the islands is 1.1 microns.
• the general microstructure is thinner with an average island size of 0.7 micrometer.
• these islets have a more even character.
• these characteristics particularly reduce the stress concentrations at the matrix-island interface.
• the mechanical properties after cold rolling and annealing are as follows: Table 3: Mechanical tensile characteristics of cold-rolled and annealed sheets. Steel Holding temperature T m Re (MPa) Rm (MPa) At (%) I2 775 630 1000 25 795 658 980 28 815 650 938 26 R1 775 480 830 nd 795 480 820 30 815 470 820 30 na: Not determined.
• the steel I2 manufactured according to the invention has a resistance greater than 900 MPa. At a maintenance temperature T m comparable, its resistance is significantly increased compared to the reference steel.
• the cold-rolled and annealed steels according to the invention have mechanical properties which are not very sensitive to small variations in certain manufacturing parameters such as the winding temperature or the annealing temperature T m .
• the invention allows the manufacture of steels with a TRIP behavior with increased mechanical strength.
• Parts made from steel sheets according to the invention are used with advantage for the manufacture of structural parts or reinforcement elements in the automotive field.

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• 2005
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