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 mechanical strength and very advantageous distributed elongation.
• the parts are made from sheets of thickness ranging from 1 to 3 mm. For some parts, however, we would like to simultaneously have a higher corrosion resistance combined with a large capacity of deformation so
• austenitic stainless steels are commonly used because of their excellent resistance to corrosion and their mechanical characteristics, in particular their high ductility.
• known austenitic stainless steels designated according to EN 10088-1 0 by the reference 1.4318 whose composition 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.
• yield strength Rp o , 2 yield strength corresponding to 0.2% elongation
• A> 45% Rm (maximum strength)> 700 MPa.
• the product P then reaches about 18,000 MPa. These characteristics are satisfactory for some applications. They remain nevertheless insufficient in the case where one wishes high resistance in service, for example for a gain in relief, and a great aptitude for prior shaping operations.
• An alternative method to cold rolling hardening is hot rolling work at a sufficiently low temperature. 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 Rpo, 2 greater than 650 MPa, or a distributed elongation greater than 45%.
• the subject of the invention is a hot-rolled sheet of austenitic stainless steel whose product P (Rpo, 2 (MPa) x extended elongation (%)) is greater than 21000 MPa.% And whose chemical composition comprises , the contents being expressed by weight: 0.015% ⁇ C ⁇ 0.030% 0.5% ⁇ Mn ⁇ 2% Si ⁇ 2%, 16.5% ⁇ Cr ⁇ 18%, 6% ⁇ Ni ⁇ 7%, S ⁇ 0.015 %, P ⁇ 0.045%, Al ⁇ 0.050%, 0.15% ⁇ Nb ⁇ 0.31%, 0.12% ⁇ N ⁇ 0.16%, the Nb and N contents being such that:
• the niobium and nitrogen contents of the steel are such that: 0.20% ⁇ Nb ⁇ 0.31%, 0.12% ⁇ N ⁇ 0.16%.
• the invention also relates to a hot-rolled sheet made of austenitic stainless steel according to any one of the above compositions, whose yield strength Rp o , 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 of manufacturing a hot rolled sheet of austenitic stainless steel whose yield strength Rp o , 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., and then the semi-finished product is rolled out with a rolling end temperature of less than 990 ° C. and a cumulative reduction rate ⁇ on the last two finishing cages, greater than 30%.
• a semi-finished product of steel 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 0 C and 132O 0 C, then the semi-finished product is rolled with a temperature of end of rolling greater than 1000 0 C.
• the invention also relates to a method for manufacturing a hot-rolled sheet of austenitic stainless steel whose product P (Rpo, 2 (MPa) x distributed elongation (%)) is greater than 21000 MPa.%, According to which supplying a semi-finished product of steel according to any one of the above compositions ⁇ and then heating the semi-finished product to a temperature of between 1250 ° C. and 1320 ° C., and the half product.
• 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 room temperature, it promotes the stability of the austenitic phase and reduces 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 the 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 maintain a higher rate of work hardening (it is called hot rolling "hardening"), thus increasing the strength of the 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. The joint addition of niobium and nitrogen for curing is unusual in austenitic stainless steels.
• stainless steels the composition of which is close to that of the 1.4318 steels mentioned above, advantageously benefit from a particular joint addition of niobium and nitrogen, optimized for to obtain certain mechanical properties under specific conditions that will be exposed:
• 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.
• the nitrogen content must be less than or equal to 0.16%.
• 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.
• niobium and nitrogen 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 semifinished 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 less than 1320 ° C or it may 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 harmful for hot shaping.
• the step of hot rolling of these semi-products starting at a temperature below 1250 ° C. can be done directly after casting so that one step intermediate heating is not necessary in this case.
• the rolling is generally carried out on a continuous hot stream comprising in particular roughing cages and finishing cages. It has been demonstrated that a particularly high yield strength Rp o , 2 is obtained by controlling in particular the reduction ratio in the two last finishing stands: if e N- 2 denotes the thickness of the sheet at the entrance of the penultimate finishing cage, and by e N the thickness of the sheet at the exit of the last finishing cage, the cumulative reduction ratio is defined on the two last finishing stands by: ⁇ _ _ j t 2. - N 5 e
• end of rolling temperature is less than 990 ° C. and that the cumulative reduction rate ⁇ is greater than 30%, the yield strength Rpo, 2 of the final product obtained was greater than 650 MPa, the Nb then being totally under form of precipitates.
• this minimum value of 650 MPa is obtained when the end-of-rolling temperature is less than 970 ° C. C and ⁇ greater than 30%.
• Table 1 Composition of steels (percentage by weight)
• the semi-finished steel products were heated at 1280 ° C. for 30 minutes. Hot rolling was then carried out by varying the end-of-rolling temperature between 900 and 1100 ° C. as well as the cumulative reduction ratio ⁇ , to reach a final thickness of 3 mm.
• the sheets 11-1, 11-2, 11-3 ... designate sheets from the same half-product 11, laminated under different conditions.
• the microstructure of the steel obtained was characterized by measuring in particular the recrystallized austenitic phase surface fraction, the fraction of niobium precipitated relative 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 Rpo, 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.
• hot-rolled steel sheets according to the invention will be advantageously used for applications requiring good shaping and a high resistance to corrosion.
• their advantages will be exploited for the economic manufacture of structural elements.

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