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 sheet made of austenitic stainless steel having high mechanical properties and especially a very advantageous combination of mechanical strength and uniform elongation.
• austenitic stainless steels denoted according to the EN 10088-1 standard by the reference 1.4318 are known in which the composition contains (in contents 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 owing to the formation of martensite during deformation at room temperature.
• the object of the invention is therefore to provide hot-rolled sheets of austenitic stainless steel having mechanical properties superior or equivalent to those of grades of the 1.4318 type mentioned above, which are inexpensive to manufacture and are insensitive to the appearance of vermicular defects.
• the object of the invention is also to provide hot-rolled sheets made of austenitic stainless steel having a product P greater than 21000 MPa. %, which may be combined with a yield strength R p0.2 of greater than 650 MPa, or else of a uniform elongation of greater than 45%.
• the subject of the invention is a hot-rolled sheet made of austenitic stainless steel, the product P (R p0.2 (MPa) ⁇ uniform elongation (%) of which is greater than 21000 MPa. % and the chemical composition of which 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:
• Nb/8+0.1% ⁇ N ⁇ Nb/8+0.12% optionally: 0.0005% ⁇ B ⁇ 0.0025%, Mo ⁇ 0.6%, the balance of the composition consisting of iron and inevitable impurities resulting from the smelting.
• the niobium and nitrogen contents of the steel 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, the yield strength R p0.2 of which is greater than 650 MPa, characterized in that the mean austenitic grain size of the steel is less than 6 microns, in that the non-recrystallized surface fraction is between 30 and 70% and in that the niobium is completely in the form of precipitates.
• the subject of the invention is also hot-rolled sheet made of austenitic stainless steel according to any one of the above features, the uniform elongation of which is greater than 45%, characterized in that the niobium is not completely precipitated.
• the subject of the invention is also a process for manufacturing a hot-rolled sheet made of austenitic stainless steel, the yield strength R p0.2 of which is greater than 650 MPa, in which: a semi-finished product made of steel having the composition according to any one of the above compositions is supplied; then said semi-finished product is reheated to a temperature of between 1250° C. and 1320° C.; and then said semi-finished product is rolled with an end-of-rolling temperature below 990° C. and a cumulative reduction ratio E on the last two finishing stands of greater than 30%.
• a semi-finished product made of steel having the composition above, containing 0.20% ⁇ Nb ⁇ 0.31%, 0.12% ⁇ N ⁇ 0.16%, is supplied and then said semi-finished product is rolled with an end-of-rolling temperature below 970° C.
• the subject of the invention is also a process for manufacturing a hot-rolled sheet made of austenitic stainless steel, the uniform elongation of which is greater than 45%, in which: a semi-finished product made of steel having the composition according to any one of the above compositions is supplied; then said semi-finished product is reheated to a temperature of between 1250° C. and 1320° C.; and then said semi-finished product is rolled with an end-of-rolling temperature above 1000° C.
• the subject of the invention is also a process for manufacturing a hot-rolled sheet made of austenitic stainless steel, the product P (R p0.2 (MPa) ⁇ uniform elongation (%)) of which is greater than 21000 MPa. %, in which: a semi-finished product made of a steel having the composition according to any one of the above compositions is supplied; then said semi-finished product is reheated to a temperature of between 1250° C. and 1320° C.; and then said semi-finished product is hot-rolled.
• the subject of the invention is also the use of a hot-roiled sheet made of stainless steel according to any one of the above features or manufactured by any one of the above processes, for the manufacture of structural components in the automotive field.
• the carbon content must be equal to or less than 0.030% so as to avoid the risk of sensitivity to intergranular corrosion.
• the carbon content must be equal to or greater than 0.015%.
• Manganese like silicon, is an element known for its deoxidizing properties in its liquid state and for increasing the hot ductility, in particular by being combined with sulphur. Moreover, at ambient temperature, manganese promotes stability of the austenitic phase and reduces the stacking fault energy. It also increases the solubility of nitrogen. These favourable effects are obtained inexpensively when the manganese content is between 0.5 and 2%.
• silicon is an element usually added for the purpose of deoxidizing the liquid steel. Silicon also increases the yield strength and the tensile strength, by solid-solution hardening or by its action on the content of ferrite ⁇ . However, above 2%, the weldability and hot ductility are reduced. Chromium is an element well known for increasing the oxidation resistance and corrosion resistance in aqueous medium. This effect is obtained satisfactorily when its content is between 16.5% and 18%.
• Nickel is an essential element for ensuring sufficient stability of the austenitic structure of the steel at ambient temperature.
• the optimum content must be determined in relation to other elements of the composition promoting alpha-phase formation, such as chromium, or those promoting gamma-phase formation, such as carbon and nitrogen. Its effect on the stability of the structure is sufficient when its content is equal to or greater than 6%. Above 7%, the production cost increases excessively because of the expense of this addition element.
• Molybdenum enables the pitting resistance to be increased.
• an addition of molybdenum in an amount ranging up to 0.6% may be carried out.
• Boron is used to improve the forgibility of the steel.
• an addition of boron in an amount of between 0.0005 and 0.0025% may be carried out. An addition with a greater amount would critically reduce the burning temperature.
• Sulphur is an element that particularly degrades the hot forgibility and the corrosion resistance—its content must be kept equal to or less than 0.015%. Phosphorus likewise degrades the hot ductility—its content must less than 0.045% in order to obtain satisfactory results.
• Aluminium is a powerful agent for deoxidizing the liquid metal.
• an optimum effect is obtained when its content is equal to or less than 0.050%.
• Niobium and nitrogen are important elements of the invention for the purpose of manufacturing austenitic stainless steels having high mechanical properties.
• Niobium retards recrystallization during hot rolling—for a given end-of-hot-rolling temperature, its addition results in a higher work-hardening factor being maintained (the hot rolling is said to be “work hardening”), thus increasing the tensile strength of this steel. It is generally used like Ti to combat the formation of chromium carbides (EN 1.4580 and EN 1.4550 Nb stabilized austenitic stainless steels). Finally, it may lead to the formation of an intermetallic phase giving an improvement in hot creep resistance.
• Nitrogen is an element hardening in interstitial solid solution, which most particularly increases the yield strength in this regard. It is also known, in solid solution, as a powerful stabilizer for the austenitic phase and as a retarder for the precipitation of chromium carbides Cr 23 C 6 . The solubility of nitrogen during solidification goes through a maximum—too high a content results in the formation of volume defects in the metal.
• the possibility of reducing 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 equal to or less than 0.16%.
• a sufficient amount of niobium must be present so as to obtain a hardening effect and to retard the recrystallization. This amount must be adapted so as to obtain an NbN solvus above the end-of-rolling temperature in order to obtain precipitation at the end of hot rolling.
• the niobium and nitrogen contents according to the invention enable substantial precipitation of NbN after hot rolling to be obtained.
• niobium preferably 0.20 to 0.31% niobium
• nitrogen 0.12 to 0.16%
• the niobium and nitrogen contents being such that: Nb/8+0.1% ⁇ N ⁇ Nb/8+0.12%, makes it possible to obtain an advantageous yield strength/elongation combination, the product P of which is greater than 21000 MPa. %.
• the remainder of the composition consists of inevitable impurities resulting from the smelting, such as for example Sn or Pb.
• the manufacturing process according to the invention is implemented as follows:
• a steel having a composition explained above is smelted. This smelting may be followed by the steel being cast into ingots or, in the most general case, cast continuously, for example in the form of slabs ranging from 150 to 250 mm in thickness. The casting may also be carried out in the form of thin slabs a few tens of millimetres in thickness between steel counter rotating rolls.
• These cast semi-finished products are firstly heated to a temperature between 1250 and 1320° C.
• the purpose of the 1250° C. temperature is to dissolve any niobium-based precipitates (nitrides and carbonitrides). However, the temperature must be below 1320° C.
• the rolling is generally carried out on a continuous hot-rolling mill comprising in particular roughing stands and finishing stands. It has been demonstrated that a particularly high yield strength of R p0.2 is obtained by especially controlling the reduction ratio in the last two finishing stands: if the thickness of the sheet entering the penultimate finishing stand is denoted by e N-2 and the thickness of the sheet exiting the last finishing stand is denoted by e N , the cumulative reduction ratio over the last two finishing stands is defined by:
• the semi-finished steel products were reheated at 1280° C. for 30 minutes.
• a hot-rolling operation was then carried out by varying the end-of-rolling temperature between 900 and 1100° C. and the cumulative reduction ratio ⁇ , so as to reach a final thickness of 3 mm.
• Steel sheets I1-1, I1-2, I1-3, etc. denote sheets obtained from the same semi-finished product I1 rolled under different conditions.
• the microstructure of the steel obtained was characterized by measuring in particular the surface fraction of recrystallized austenitic phase, the fraction of precipitated niobium relative to the total niobium and the average grain size. In the case of an incompletely recrystallized structure, the latter measurement was carried out on the recrystallized part of the structure.
• the tensile mechanical properties were also determined, in particular the yield strength R p0.2 and the uniform elongation.
• the possible presence of local deformation during the tensile trial was also recorded. It is known that the presence of such a local deformation is associated with the appearance of vermicular defects during forming operations.
• This table also shows that, when the non-recrystallized fraction is between 30 and 70% and when the average grain size is less than 6 microns, the yield strength R p0.2 is greater than 650 MPa (trials I1-1, I1-2, I2-1, I2-2). Moreover, when the non-recrystallized fraction is greater than 70%, the elongation tends to be reduced.
• the stress-strain curves of the steels according to the invention show no plateau indicating local deformation, whatever the hot-rolling conditions, in contrast with the reference steel that exhibits local deformation whenever it is partially recrystallized (trials R-1, R-2, R-3). This point is particularly advantageous for the forming operation, by ensuring that there are no vermicular defects.
• the hot-rolled steel sheets according to the invention will be advantageously used for applications requiring good formability and high corrosion resistance.
• their advantages will be profitably enjoyed for the economic manufacture of structural components.

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