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
  • C21D7/00—Modifying the physical properties of iron or steel by deformation
  • C21D7/13—Modifying the physical properties of iron or steel by deformation by hot working
• • 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

Definitions

• the present invention relates to the manufacture of hot-rolled sheets of austenitic stainless steels having high mechanical characteristics, and in particular a combination of very advantageous mechanical strength and distributed elongation.
• known austenitic stainless steels designated according to the standards EN 10088-1 by the reference 1.4318, the composition of which contains (content expressed by weight): C ⁇ 0.030%, Si ⁇ 1.00%, Mn ⁇ 2.00% , P ⁇ 0.045%, S ⁇ 0.015%, Cr: 16.50 to 18.50%, Ni: 6.00 to 8.00%, N: 0.10 to 0.20%.
• These steels have high mechanical properties due to the formation of martensite during deformation at room temperature.
• the typical mechanical properties of these steels in the annealed state are the following: yield strength Rp 0.2 (yield strength corresponding to 0.2% elongation): 300-400 MPa, distributed elongation: A ⁇ 45%, Rm (maximum resistance) ⁇ 700 MPa.
• the product P then reaches about 18,000 MPa.
• This method confers a better compromise elongation - resistance, but has the major disadvantage of leading to localizations of the deformation during shaping, resulting in vermiculures. To avoid these vermiculures on standard steel 1.4318 not recrystallized after hot rolling, it is necessary to anneal after hot rolling.
• the object of the invention is therefore to have hot-rolled sheets of austenitic stainless steel with mechanical characteristics greater than or equivalent to those of the grades of the type 1.4318 presented above, which are inexpensive to manufacture, and which have no sensitivity to appearance of vermiculures.
• Another object of the invention is to provide hot-rolled sheets of austenitic stainless steel having a product P greater than 21000 MPa.%, Which can be associated with a yield strength R P0.2 greater than 650 MPa, or else with a distributed elongation greater than 45%.
• the subject of the invention is a hot-rolled sheet made of austenitic stainless steel whose chemical composition comprises, the contents being expressed by weight: 0.015% ⁇ C ⁇ 0.030%, 0.5% ⁇ Mn ⁇ 2%, If ⁇ 2%, 16.5% ⁇ Cr ⁇ 18%, 6% ⁇ Ni ⁇ 7%, S ⁇ 0.015%, P ⁇ 0.045%, Al ⁇ 0.050%, 0.005% ⁇ Nib ⁇ 0.40%, 0, 10% ⁇ N ⁇ 0.17%, the contents of Nb and N being such that: Nb / 8 + 0.1% ⁇ N ⁇ Nb / 8 + 0.12%, and, optionally: 0.0005 % ⁇ B ⁇ 0.0025%, Mo ⁇ 0.6%, the remainder of the composition consisting of iron and unavoidable impurities resulting from the preparation.
• the chemical composition of the steel comprises, the contents being expressed by weight: 0.10% ⁇ Nb ⁇ 0.31%, 0.11 ⁇ N ⁇ 0.16%
• the invention also relates to a hot-rolled sheet of austenitic stainless steel according to the above chemical composition, whose product P (Rp 0.2 (MPa) x distributed elongation (%)) is greater than 21000 MPa. , characterized in that the niobium and nitrogen contents of the steel, expressed by weight, are such that: 0.15% ⁇ Nb ⁇ 0.31%, 0.12% ⁇ N ⁇ 0.16%. According to a preferred embodiment, the niobium and nitrogen contents of the steel, expressed by weight, are such that: 0.20% ⁇ Nb ⁇ 0.31%, 0.12% ⁇ N ⁇ 0.16%.
• the subject of the invention is also a hot-rolled sheet made of austenitic stainless steel according to any one of the above compositions, whose yield strength Rp 0.2 is greater than 650 MPa, characterized in that the average size The austenitic grain of the steel is less than 6 microns, the non-recrystallized surface fraction is between 30 and 70%, and the niobium is completely in the form of precipitates.
• the invention also relates to a hot-rolled sheet of austenitic stainless steel according to any one of the above characteristics, the distributed elongation of which is greater than 45%, characterized in that the niobium is not totally precipitated. .
• the invention also relates to a method for manufacturing a hot-rolled sheet of austenitic stainless steel whose yield strength Rp 0.2 is greater than 650 MPa, according to which a semi-finished steel product is supplied. according to any one of the above compositions, then the semi-finished product is heated to a temperature of between 1250 ° C and 1320 ° C, then the semi-finished product is rolled out with a rolling end temperature below 990 ° C and a cumulative reduction rate ⁇ on the last two finishing cages, greater than 30%.
• a semi-finished steel product of the above composition containing 0.20% ⁇ Nb ⁇ 0.31%, 0.12% ⁇ N ⁇ 0.16% is supplied, then the half produced with an end-of-lamination temperature below 970 ° C.
• the invention also relates to a method for manufacturing a hot-rolled sheet of austenitic stainless steel, the distributed elongation of which is greater than 45%, according to which a semi-finished product of steel is supplied according to one of any of the above compositions, then the semi-finished product is heated to a temperature between 1250 ° C and 1320 ° C, and then the semi-finished product is rolled out with a rolling end temperature above 1000 ° C.
• the invention also relates to a method for manufacturing a hot-rolled sheet of austenitic stainless steel whose product P (R P0.2 (MPa) x distributed elongation (%)) is greater than 21000 MPa.%,
• a semi-finished steel product composition according to a composition above comprising 0.10% ⁇ Nb ⁇ 0.31%, 0.11% ⁇ N ⁇ 0.16%, or 0.20% ⁇ Nb ⁇ 0.31%, 0.12% ⁇ N ⁇ 0.16%, then the semi-finished product is heated to a temperature of between 1250 ° C and 1320 ° C, and then the semi-finished product is hot-rolled.
• Another object of the invention is the use of a stainless steel hot rolled sheet according to any of the above features, or manufactured by any of the above methods, for the manufacture of elements in the automotive field.
• the carbon content must be less than or equal to 0.030% in order to avoid the risk of sensitization to intergranular corrosion. In order to obtain a yield strength greater than 650 MPa, the carbon content must be greater than or equal to 0.015%.
• Manganese like silicon, is an element known for its deoxidative properties in the liquid state and to increase the hot ductility, especially by combining with sulfur. On the other hand, at ambient, it promotes the stability of the austenitic phase and decreases the stacking fault energy. It also increases the solubility of nitrogen. These favorable effects are obtained economically when the manganese content is between 0.5 and 2%.
• silicon is an element usually added for the purpose of deoxidizing liquid steel. Silicon also increases the yield strength and the resistance, by hardening in solid solution or by its action on the ⁇ ferrite content. However, beyond 2%, the weldability and hot ductility are reduced.
• Chromium is a well-known element for increasing resistance to oxidation and corrosion in aqueous media. This effect is satisfactorily obtained when its content is between 16.5% and 18%.
• Nickel is an indispensable element to ensure sufficient stability of the austenitic structure of steel at room temperature.
• the optimum content should be determined in relation to other elements of the alphagene composition such as chromium, or those with a gamma-like character such as carbon and nitrogen. Its effect on the stability of the structure is sufficient when its content is greater than or equal to 6%. Above 7%, the cost of production increases excessively because of the high cost of this element of addition.
• Molybdenum increases the resistance to pitting.
• molybdenum addition up to 0.6% can be carried out.
• Boron improves the forgeability of steel.
• boron in an amount between 0.0005 and 0.0025% can be carried out. Addition in greater quantity would critically decrease the burn temperature.
• Sulfur is an element that particularly degrades hot forgeability and corrosion resistance, its content must be maintained less than or equal to 0.015%.
• Phosphorus likewise degrades hot ductility, its content must be less than 0.045% to obtain satisfactory results.
• Aluminum is a powerful deoxidation agent for the liquid metal. In combination with the silicon and manganese contents mentioned above, an optimal effect is obtained when its content is less than or equal to 0.050%.
• Niobium and nitrogen are important elements of the invention for the manufacture of austenitic stainless steels with high mechanical properties.
• Niobium retards recrystallization during hot rolling: for a given hot rolling end temperature, its addition leads to maintaining a higher rate of work hardening (referred to as "hot rolling"), thereby increasing the resistance steel. It is generally used as Ti to combat the formation of chromium carbides (austenitic stainless steels stabilized with EN 1.4580 and EN 1.4550). Finally, it can lead to intermetallic phase formation conferring an improvement in creep resistance.
• Nitrogen is a hardening element in interstitial solid solution, which particularly increases the yield strength in this respect. It is also known, in solid solution, as a powerful stabilizer of the austenitic phase and as a retarder of the precipitation of Cr 23 C 6 chromium carbides. The solubility of the nitrogen during the solidification knows a maximum. Too high a content leads to the formation of volume defects in the metal.
• NbN nitrides which occurs at the end of hot rolling reduces the amount of nitrogen in solid solution.
• the preceding relation (1) ensures that as much solid solution nitrogen remains after complete precipitation of all available niobium as in 1.4318 (N ⁇ 0.1%). This makes it possible to obtain the same metastability of the austenite at room temperature.
• the possibility of decreasing the Ni content by increasing the N content is limited by the solubility limit of nitrogen in the steel during solidification. For the contents of Cr, Mn and Ni steels according to the invention, the nitrogen content must be less than or equal to 0.17%.
• niobium must be present to achieve a hardening effect and delay recrystallization. This amount must be adapted to obtain a NbN solvus higher than the end of rolling temperature to obtain precipitation at the end of hot rolling.
• a niobium content ranging from 0.10 to 0.31% Nb will be chosen, together with a nitrogen content ranging from 0.11 to 0.16%, the contents of niobium and of nitrogen being such that: Nb / 8 + 0.1% ⁇ N ⁇ Nb / 8 + 0.12%.
• Nb / 8 + 0.1% ⁇ N ⁇ Nb / 8 + 0.12% the contents of niobium and nitrogen contents make it possible to obtain a high precipitation of NbN after hot rolling.
• the rest of the composition consists of unavoidable impurities resulting from the preparation, such as for example Sn or Pb.
• a steel is produced whose composition has been explained above. This development can be followed by casting in ingots or, in the most general case, continuously, for example in the form of slabs ranging from 150 to 250 mm thick. It is also possible to perform the casting in the form of thin slabs of a few tens of millimeters thick between contra-rotating steel rolls. These cast half-products are first brought to a temperature of between 1250 and 1320 ° C. The purpose of the 1250 ° C temperature is to dissolve any niobium-based precipitates (nitrides, carbonitrides).
• the temperature must be below 1320 ° C, otherwise it will be too close to the solidus temperature that could be reached in any segregated areas and cause a local start to pass through a liquid state that would be detrimental to the setting in hot form.
• the step of hot rolling of these semi-products starting at a temperature below 1250 ° C. can be done directly after casting so that a step intermediate heating is not necessary in this case.
• this minimum value of 650 MPa is obtained when the end-of-rolling temperature is less than 970 ° C. C and ⁇ greater than 30%.
• the semi-finished steel products were reheated at 1280 ° C for 30 minutes.
• a hot rolling was then carried out by varying the end of rolling temperature between 900 and 1100 ° C and the cumulative reduction rate ⁇ , to reach a final thickness of 3 mm.
• the sheets I1-1, I1-2, I1-3 ... designate sheets from the same half-product I1, laminated under different conditions.
• the microstructure of the steel obtained in in particular, measuring the recrystallized austenitic phase surface fraction, the niobium fraction precipitated with respect to the total niobium, and the average grain size. In the case of a structure not completely recrystallized, the latter measurement is performed on the recrystallized part of the structure.
• the mechanical tensile characteristics in particular the yield strength Rp 0.2 and the distributed elongation, were also determined.
• the presence of a localization of the deformation during the tensile test has also been noted. It is known that the presence of such a location is associated with the appearance of vermiculides during shaping operations.
• Table 2 Manufacturing conditions and microstructural and mechanical characteristics of hot-rolled sheet No. of test TFL (° C) ⁇ > 30% Average grain size less than 6 microns Non-recrystallized fraction between 30 and 70% Niobium totally precipitated Rp 0.2 (MPa) AT (%) Rp 0.2 x A (MPa%) Location of the deformation I1-1 905 Yes Yes Yes Yes 689 40 27628 No I1-2 935 Yes Yes Yes Yes 651 40 25520 No I1-3 1040 Yes No No ( ⁇ 30%) No 432 49 21340 No I1-4 1050 Yes No No ( ⁇ 30%) No 467 46 21715 No I2-1 930 Yes Yes Yes Yes Yes Yes 677 38 25997 No I2-2 965 Yes Yes Yes Yes Yes Yes 681 39 26559 No I2-3 980 No No Yes Yes 631 41 26186 No I2-4 1000 No Yes No ( ⁇ 30%) No 627 46 28277 No I2-5 1100 Yes No No ( ⁇ 30%) No 547 53 29100 No R-1 900 Yes - Yes - 702
• the steels I1 and I2 according to the invention have a combination Rp 0.2 x A (MPa.%) Greater than 21000 MPa.% Particularly advantageous while the reference steel R does not have such a combination. , whatever the rolling conditions.
• the tensile curves of the steels according to the invention do not show any bearing testifying to a location of the deformation and whatever the hot rolling conditions, unlike the reference steel which has a localization as soon as possible. when it is partially recrystallized (tests R-1, R-2, R-3).
• This point is particularly advantageous for shaping, ensuring the absence of vermiculures.
• hot-rolled steel sheets according to the invention will be advantageously used for applications requiring good shaping and a high resistance to corrosion. In the case of their use in the automobile industry, their advantages will be exploited for the economic manufacture of structural elements.

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• 2008
  • 2008-03-21 EP EP08290267A patent/EP2103705A1/fr not_active Withdrawn
• 2009
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  • 2009-03-03 US US12/922,786 patent/US20110061776A1/en not_active Abandoned
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