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
  • C21D6/00—Heat treatment of ferrous alloys
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • 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
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
• • 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

Definitions

• the invention relates to the manufacture of hot and cold rolled sheets of austenitic iron-carbon-manganese steels having very high mechanical characteristics, and in particular mechanical strength.
• patent FR 2 829 775 discloses austenitic alloys having as main elements: iron-carbon (up to 2%) manganese (between 10 and 40%) capable of being hot-rolled or cold, with a resistance likely to exceed 1200 MPa.
• the object of the invention is therefore to provide a hot-rolled or cold-rolled steel sheet or product of economical manufacture, having a resistance greater than 900 MPa, an elongation at break greater than 50%, particularly suitable for cold forming and having a very high resistance to delayed cracking, without the particular need for a specific heat treatment for degassing.
• the subject of the invention is an austenitic iron-carbon-manganese steel sheet, the chemical composition of which comprises the contents being expressed by weight: 0.45% ⁇ C ⁇ 0.75%, 15% ⁇ Mn ⁇ 26%, Si ⁇ 3%, Al ⁇ 0.050%, S ⁇ 0.030%, P ⁇ 0.080%, N ⁇ 0.1% at least one metal element selected from vanadium, titanium, niobium, chromium, molybdenum: 0.050% ⁇ V ⁇ 0.50%, 0.040% ⁇ Ti ⁇ 0.50%, 0.070% ⁇ Nb ⁇ 0, 50%, 0.070% ⁇ Cr ⁇ 2%, 0.14% ⁇ Mo ⁇ 2% and optionally one or more elements selected from 0.0005% ⁇ B ⁇ 0.003%, Ni ⁇ 1%, Cu ⁇ 5%, the rest of the composition consisting of iron and unavoidable impurities resulting from the production, the amount of metal elements in the form of carbides, nitrides or carbonitrides precipitated being:
• the composition of the steel comprises: 0.50% ⁇ C ⁇ 0.70%
• the composition of the steel comprises: 17% ⁇ Mn ⁇
• the composition of the steel comprises 0.070% ⁇ V ⁇ 0.40%, the amount of vanadium in the form of carbides, nitrides or carbonitrides precipitated being 0.070% ⁇ V p ⁇ 0.140%
• the composition of the steel comprises 0.060% ⁇ Ti ⁇ 0.40%, the amount of titanium in the form of carbides, nitrides or carbonitrides precipitated being: 0.060% ⁇ Ti p ⁇ 0.110%
• the composition of the steel steel advantageously comprises 0.090% ⁇ Nb ⁇ 0.40%, the amount of niobium in the form of carbides, nitrides or carbonitrides precipitated being: 0.090% ⁇ Nb p ⁇ 0.200%
• the composition of the steel comprises 0.20% ⁇ Cr ⁇ 1.8%, the amount of chromium in the form of precipitated carbides being 0.20% ⁇ Cr p ⁇ 0.5%
• the composition of the steel comprises 0.20% ⁇ Mo ⁇ 1.8%, the amount of molybdenum in the form of precipitated carbides being 0.20% ⁇ Mo p ⁇ 0.35%
• the average size of the precipitates is between 5 and 25 nanometers, and more preferably between 7 and 20 nanometers
• At least 75% of the population of said precipitates is located in intragranular position
• the invention also relates to a method of manufacturing a sheet metal cold-rolled austenitic iron-carbon-manganese steel according to which is supplied a steel whose chemical composition comprises, the contents being expressed by weight: 0.45% ⁇ C ⁇ 0.75%, 15% ⁇ Mn ⁇ 26%, If ⁇ 3%, Al ⁇ 0.050%, S ⁇ 0.030%, P ⁇ 0.080%, N ⁇ 0.1%, at least one metal element selected from vanadium, titanium, niobium, chromium, molybdenum: 0.050 % ⁇ V ⁇ 0.50%, 0.040% ⁇ Ti ⁇ 0.50%, 0.070% ⁇ Nb ⁇ 0.50%, 0.070% ⁇ Cr ⁇ 2%, 0.14% ⁇ Mo ⁇ 2%, and optional one or more elements selected from 0.0005% ⁇ B ⁇ 0.003%, Ni ⁇ 1%, Cu ⁇ 5%, the remainder of the composition consisting of iron and unavoidable impurities resulting from the elaboration, it is proceeded to the
• the parameters Vc, Tm, tm, Vr, Ty, t u are adjusted so that the average size of the carbide, nitride or carbonitride precipitates after the annealing is between 5 and 25 nanometers, and preferentially between 7 and 20 nanometers.
• a steel is procured whose chemical composition includes 0.050% ⁇ V ⁇ 0.50%, is laminated to the hot semi-finished product to a temperature of the upper end lamination or equal to 950 0 C, the sheet is reeled at a temperature below 500 ° C., the sheet is cold-rolled with a reduction ratio greater than 30%, an annealing heat treatment is carried out with a heating rate Vc of between 2 and 10 ° C / s, at a temperature Tm between 700 and 870 0 C for a time between 30 and 180 s, and the sheet is cooled at a speed between 10 and 50 ° C / s.
• the heating rate Vc is preferably between 3 and 7 ° C./s.
• the holding temperature Tm is between 720
• the casting of the semi-finished product is advantageously carried out in the form of casting slabs or thin strips between counter-rotating steel rolls.
• the invention also relates to the use of an austenitic steel sheet described above or manufactured by a method described above, for the manufacture of structural parts, reinforcing elements or external parts. , in the automotive field.
• the carbon content is between 0.50 and 0.70% by weight so as to obtain sufficient strength combined with optimum precipitation of carbides or carbonitrides.
• Manganese is also an essential element for increasing strength, increasing stacking fault energy and stabilizing the austenitic phase. If its content is less than 15%, there is a risk of formation of martensitic phases which significantly reduce the ability to deform. On the other hand, when the manganese content is greater than 26%, the ductility at room temperature is degraded. In addition, for questions of cost, it is not desirable that the manganese content be high.
• the manganese content is between 17 and 24% so as to optimize the stacking fault energy and to avoid the formation of martensite under the effect of a deformation. Moreover, when the manganese content is greater than 24%, the mode of deformation by twinning is less favored compared to the sliding mode of perfect dislocations.
• Aluminum is a very effective element for the deoxidation of steel. Like carbon, it increases the stacking fault energy. However, its excessive presence in steels with a high manganese content has a disadvantage: in fact, manganese increases the solubility of nitrogen in the liquid iron.
• the nitrogen content must be less than or equal to 0.1% in order to prevent this precipitation and the formation of volume defects (blowholes) during solidification.
• the nitrogen content in the presence of elements capable of precipitating in the form of nitrides, such as vanadium, niobium or titanium, the nitrogen content must not exceed 0.1% otherwise the risk of obtaining an ineffective coarse precipitation will be observed. with respect to the trapping of hydrogen.
• Silicon is also an effective element for deoxidizing steel as well as for hardening in the solid phase. However, beyond a content of 3%, it decreases the elongation, tends to form undesirable oxides during certain assembly processes and must therefore be kept below this limit. Sulfur and phosphorus are impurities that weaken the grain boundaries.
• boron may be added in an amount of from 0.0005 to 0.003%. This element segregates at the austenitic grain boundaries and reinforces their cohesion. Below 0.0005%, this effect is not obtained. Above 0.003%, boron precipitates as borocarbons, and the effect is saturated.
• Nickel can be used as an option to increase the strength of the steel by hardening in solid solution. Nickel contributes to a high elongation break and increases in particular the toughness. However, it is also desirable for cost issues to limit the nickel content to a maximum content of less than or equal to 1%.
• addition of copper to a content of less than or equal to 5% is a means of hardening the steel by precipitation of metallic copper.
• copper is responsible for the appearance of surface defects hot sheet.
• the metal elements capable of forming precipitates such as vanadium, titanium, niobium, chromium, molybdenum, play an important role in the context of the invention.
• the quantity of metal elements in the form of precipitates is greater than or equal to a critical content, depending on the nature of the precipitates.
• the quantity of metal elements in the form of precipitates of carbides, nitrides, or carbonitrides is designated by V p , Ti p , Nb p , respectively for vanadium, titanium and niobium, and Cr p , Mo p for chromium and molybdenum carbides.
• the steel comprises one or more metal elements chosen from: vanadium, in an amount of between 0.050 and 0.50% by weight, and whose quantity V p in the form of precipitates is between 0.030% and 0.150 % in weight.
• vanadium content is between 0.070% and 0.40%, the amount V p being between 0.070% and 0.140% by weight.
• titanium in an amount Ti of between 0.040 and 0.50% by weight, the amount Ti p in the form of precipitates being between 0.030% and 0.130%.
• the titanium content is between 0.060% and 0.40%, the amount Ti p being between 0.060% and 0.10% by weight.
• the niobium content is between 0.090% and 0.40%, the amount Nb p being between 0.090% and 0.200% by weight - chromium, in an amount of between 0.070% and 2% by weight, the amount Cr p in the form of precipitates being between 0.070% and 0.6%.
• the chromium content is between 0.20% and 1.8%, the amount Cr p being between 0.20 and 0.5% - Molybdenum, in an amount between 0.14 and 2% weight, the amount Mo p in the form of precipitates is between 0.14 and
• the molybdenum content is between 0.20 and 1.8%, the amount Mo p being between 0.20 and 0.35%.
• the minimum value expressed for these various elements corresponds to a quantity of addition necessary to form precipitates taking into account the thermal cycles of manufacture. A preferred minimum content (for example 0.070% for vanadium) is recommended, so as to obtain a larger quantity of precipitates.
• the maximum value expressed for these various elements corresponds to excessive precipitation, or in an inappropriate form, deteriorating the mechanical properties, or to an uneconomic implementation of the invention. A preferred maximum content (for example of 0.40% for vanadium) is recommended, so as to optimize the addition of the element.
• the minimum value of metallic elements in the form of precipitates corresponds to a quantity of precipitates for very effectively reducing the sensitivity to delayed cracking.
• a preferred minimum amount (for example 0.070% in the case of vanadium) is recommended, so as to obtain a particularly high resistance to delayed cracking.
• the maximum value of metallic elements in the form of precipitates marks a deterioration of the ductility or the tenacity, the rupture starting on the precipitates. Furthermore, beyond this maximum value, intense precipitation occurs, which can prevent total recrystallization during continuous annealing thermal treatments after cold rolling.
• a preferred maximum content in the form of precipitates (for example 0.140% for vanadium) is recommended, so that the ductility is preserved as much as possible and that the precipitation obtained is compatible with the recrystallization under the usual annealing conditions. recrystallization.
• the inventors have demonstrated that a too large average size of precipitates reduces the efficiency of trapping.
• mean size of precipitates is the size that can be measured, for example, from replicates with extraction, followed by observations by transmission electron microscopy: the diameter is measured (in the case of spherical or quasi-spherical precipitates) or the largest length (in the case of irregularly shaped precipitates) of each precipitate, then establishes a histogram of size distribution of these precipitates, the average of which is calculated from the count of a statistically representative number of particles. Beyond an average size of 25 nanometers, the efficiency of hydrogen scavenging decreases due to the decrease in the interface between precipitates and matrix. At a given precipitate amount, an average size of precipitates exceeding 25 nanometers also decreases the density of precipitates present, thereby excessively increasing the inter-site trapping distance. The trapping interfacial surface for hydrogen is also reduced.
• the average size of precipitates is less than 20 nanometers in order to trap the largest amount of hydrogen possible.
• the average particle size is less than 5 nanometers, the precipitates will tend to form coherently with the matrix, thus reducing the trapping ability.
• the difficulty of controlling these very fine precipitates is also increased.
• These difficulties are optimally avoided when the average size of precipitates is greater than 7 nanometers.
• This average value can integrate the presence of many very fine precipitates, whose size is of the order of one nanometer.
• the inventors have also demonstrated that the precipitates are advantageously located in the intragranular position in order to reduce the sensitivity to delayed cracking: in fact, when at least 75% of the population of precipitates is located in the intragranular position, the distribution of hydrogen possibly present is more homogeneous, without accumulation at the austenitic grain boundaries which are potential sites of embrittlement.
• the object of the invention is to simultaneously dispose of steels with very high mechanical characteristics and insensitive to delayed fracture.
• the steel should be completely recrystallized after the annealing cycle. Too early precipitation, for example at the stage of casting, hot rolling or winding, will be a potential brake on recrystallization and may harden the metal and increase the hot or cold rolling forces. It will also be less effective because it will intervene significantly on the austenitic grain boundaries. The size of these precipitates formed at high temperature will be larger, often greater than 25 nanometers. The inventors have shown that vanadium additions are particularly desirable insofar as the precipitation of this element hardly occurs during hot rolling or winding.
• the pre-existing adjustments of hot and cold rolling forces are not to be modified and all the vanadium is available for a very fine and homogeneous precipitation during the subsequent annealing cycle after cold rolling.
• the precipitation takes place in the form of VC and in the form of nanometric VN or V (CN) homogeneously distributed, the vast majority of the precipitates being located in the intragranular position, ie in the most desirable form for the entrapment of the nanoparticles. 'hydrogen.
• this fine precipitation limits the growth of the grain, a finer austenitic grain size can thus be obtained after annealing.
• a steel is produced whose composition comprises: 0.45% ⁇ C ⁇ 0.75% 15% ⁇ Mn ⁇ 26%, Si ⁇ 3%, Al ⁇ 0.050%, S ⁇ 0.030, P ⁇ 0.080%, N ⁇ 0.1%, one or more elements selected from 0.050% ⁇ V ⁇ 0.50%, 0.040% ⁇ Ti ⁇ 0.50%, 0.070% ⁇ Nb ⁇ 0.50%, 0.070% ⁇ Cr ⁇ 2%, 0.14% ⁇ Mo ⁇ 2%, and optionally one or more elements selected from 0.0005% ⁇ B ⁇ 0.003%, Ni ⁇ 1%, Cu ⁇ 5%, the rest being iron and unavoidable impurities from the elaboration.
• This development can be followed by casting in ingots, or continuously in the form of slabs of thickness of the order of 200 mm. It is also possible to advantageously perform the casting in the form of thin slabs, a few tens of millimeters thick, or thin strips of a few millimeters.
• certain addition elements according to the invention such as titanium or niobium are present, the casting in the form of thin products will lead more particularly to a precipitation of nitrides or very thin and thermally stable carbonitrides, the presence of which reduces sensitivity to delayed cracking.
• These cast semi-finished products are first brought to a temperature of between 1100 and 1300 ° C. This is intended to achieve at all points the temperature ranges favorable to the high deformations which the steel will undergo during rolling.
• the reheating temperature must not be greater than 1300 0 C, otherwise it will be too close to the solidus temperature that could be reached in possible zones enriched locally with manganese and / or carbon and cause a passage local by a liquid state that would be harmful for hot shaping.
• the hot rolling step of these semi-products starting between 1300 and 1000 0 C can be done directly after casting without going through the intermediate heating step.
• the semi-finished product is hot-rolled, for example to obtain a thickness of hot rolled strip 2 to 5 millimeters thick, or even 1 to 5 mm in the case of semi-finished product from a thin slab casting. , or 0.5 to 3 mm in the case of a casting of thin strips.
• the low aluminum content of the steel according to the invention makes it possible to avoid excessive precipitation of AlN which would adversely affect the hot deformability during rolling.
• the end-of-lamination temperature In order to avoid any problem of cracking due to lack of ductility, the end-of-lamination temperature must be greater than or equal to 890 ° C.
• the strip After rolling, the strip must be wound at a temperature such that a precipitation of carbides, essentially intergranular cementitious (Fe, Mn) 3 C), does not occur significantly, which would lead to a reduction of certain mechanical properties. This is obtained when the winding temperature is less than 580 ° C.
• the elaboration conditions will also be chosen so that the product obtained is completely recrystallized. We can then proceed to a subsequent cold rolling followed by annealing. This additional step makes it possible to obtain a grain size smaller than that obtained on hot strip and thus to higher strength properties. It must naturally be implemented if one seeks to obtain products of thinner thickness, ranging for example from 0.2 mm to a few mm thick.
• this treatment has the effect of restoring the ductility and to obtain a precipitation according to the invention.
• This annealing preferably carried out continuously, comprises the following sucessive steps:
• a heating phase characterized by a heating rate Vc; a holding phase at a temperature Tm during a holding time tm;
• a holding phase at a temperature Tu during a holding time t u Before the optional phase of maintaining the temperature Tu, the product can optionally be cooled to room temperature.
• This phase of maintaining the temperature You can possibly be carried out within a separate device, for example a furnace for the static annealing of steel coils.
• the precise choice of the parameters Vc, Tm, tm, Vr, Tu, t u is usually carried out in such a way that the desired mechanical properties are obtained, in particular thanks to a complete recrystallization.
• a steel of composition 0.45% ⁇ C ⁇ 0.75%, 15% ⁇ Mn ⁇ 26%, Si ⁇ 3%, Al ⁇ 0.050%, S ⁇ 0.030%, P ⁇ 0.080%, N ⁇ 0.1%, 0.050% ⁇ V ⁇ 0.50%, and optionally one or more elements selected from 0.0005% ⁇ B ⁇ 0.003%, Ni ⁇ 1%, Cu ⁇ 5%, optimally a steel sheet according to the invention by casting a half-product, bringing it to a temperature between 1100 and 1300 0 C, by hot rolling this half-product to a temperature of end of rolling greater than or equal to 950 0 C, then conducting a winding at a temperature below 500 0 C.
• the sheet is cold rolled with a reduction rate greater than 30% (the reduction ratio being defined by: (thickness of the sheet before cold rolling - thickness of the sheet after cold rolling) / (thickness of the front plate
• the rate of 30% corresponds to a minimum deformation so as to obtain a recrystallization.
• An annealing heat treatment is then carried out with a heating rate Vc of between 2 and 10 ° C./s (preferably between 3 and 7 ° C.). ° C / s), at a temperature Tm between 700 and 870 ° C (preferably between 720 and 850 ° C) for a time between 30 and 180s and the sheet will be cooled at a speed between 10 and 50 ° C / s
• Table 1 Composition of steels 11-2: according to the invention.
• R1-3 Reference to Table 1
• Semi-finished products of these steels were heated to 118O 0 C, hot rolled to a temperature of 950 0 C to bring them to a thickness of 3mm and then wound at a temperature of 500 ° C.
• the steel sheets thus obtained were then cold-rolled with a reduction rate of 50% up to a thickness of 1.5 mm, and then annealed under the conditions presented in Table 2.
• the quantity of metallic elements was determined. precipitated in the form of carbides, nitrides or carbonitrides, in these different sheets by chemical extraction and selective dosing. Given the compositions and the manufacturing conditions, these potential precipitates are here based on vanadium, mainly vanadium carbonitrides.
• the amount of vanadium V p in the form of precipitates was reported in Table 2 as well as the average size of the precipitates measured from extracted replicas observed by transmission electron microscopy.
• Table 3 shows the mechanical tensile properties: strength and elongation at break, obtained under these conditions.
• circular blanks with a diameter of 55 mm were cut in the cold-rolled and annealed sheets. These blanks were then embossed by swallowing in the form of flat-bottomed cups (swift shrinkage tests) using a 33mm diameter punch.
• the factor ⁇ characterizing the severity of the test is 1.66.
• the possible presence of micro-cracks was then noted either immediately after shaping, or after a waiting period of 3 months, thus characterizing a possible sensitivity to delayed cracking. The results of these observations were also reported in Table 3.
• Steels 11 and 12 according to the invention comprise precipitates of suitable size and nature. These are located at more than 75% in intragranular position. These steels combine excellent mechanical characteristics (resistance greater than 1000 MPa, elongation greater than 55% and a high resistance to delayed fracture. This last property is obtained even without specific heat treatment of degassing.
• the hot-rolled or cold-rolled sheets according to the invention are advantageously used in the automobile industry in the form of structural parts, reinforcing elements or external parts which, because of their very high strength and their high ductility, contribute to a very effective reduction of vehicle weight while increasing safety in case of impact.

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• Treatment Of Steel In Its Molten State (AREA)
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• 2005
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