Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
  • C25F1/00—Electrolytic cleaning, degreasing, pickling or descaling
  • C25F1/02—Pickling; Descaling
  • C25F1/04—Pickling; Descaling in solution
  • C25F1/06—Iron or steel
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
• • 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
  • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum

Definitions

• Austenitic stainless steel strip with a bright surface finish and excellent mechanical characteristics.
• the present invention relates to an austenitic stainless steel strip, having an elastic limit Rp o , 2 greater than or equal to 600 MPa, a higher tensile strength Rm greater than or equal to 800 MPa, an elongation A 80 greater than or equal to 40% and a shiny surface appearance of annealing-gloss type.
• the invention also relates to a continuous manufacturing process for this austenitic stainless steel strip. Because of their very good cold forming capacity characterized by high mechanical strength and ductility, good weldability and excellent corrosion resistance, austenitic stainless steels are used in a wide range of end applications such as for example the manufacture of mechanical parts, cooking utensils, and tubes.
• the strip of austenitic stainless steel is subjected to a heat treatment and a final etching which, according to the conditions of implementation, gives it either a surface appearance having a high gloss, interesting by example for platerie, an aspect of matte surface interesting for the manufacture of building facades.
• the brightness corresponds to the measurement of the reflectivity of the surface.
• the gloss is measured according to an angle of illumination of the surface of 60 °, according to the international standard ISO7668 (1986).
• the term "glossy surface appearance” means a surface having a gloss measured at 60 ° greater than 50, and a dull surface appearance, a surface having a gloss measured at 60 ° less than 20.
• the austenitic stainless steel strip is cold-rolled beforehand with rolls which impart a glossy surface appearance to the strip.
• the cold rolled strip is then degreased and rinsed, and then undergoes a heat treatment in a vertical furnace called "bright annealing" in which there is a reducing atmosphere.
• the strip travels in the oven consisting of a enclosure completely isolated from the outside atmosphere, comprising three zones in which a neutral or reducing gas circulates.
• This gas> is chosen for example from hydrogen, nitrogen or a mixture of hydrogen and nitrogen (HNX gas), and has a dew point between -60 and -45 ° C.
• the strip is first heated in the first zone of the oven at a temperature of between 1050 and 1150 ° C, and at a heating rate of 30 to 60 ° C / sec. Then, it is maintained at this temperature in the second zone of the oven for a time sufficient to allow the recrystallization of the steel and the restoration of the mechanical properties. Finally, it is cooled in the third zone of the oven to a temperature of the order of 150 ° C to prevent re-oxidation of the surface of the strip with the oxygen of the air, when the band leaves the oven enclosure.
• the glossy surface appearance imparted to the strip during cold rolling is maintained, because the oxide film which formed during the annealing is very thin, of a thickness of the order of 10 angstrom.
• an austenitic stainless steel strip having a glossy surface appearance is to subject the strip to a final treatment of the annealing-pickling type, which gives it an annealed-stripped surface appearance; to say a matte surface appearance, then to proceed to an operation of either polishing the surface of the strip, or skin-pass of the strip.
• the procedure is as follows. The previously cold-rolled strip undergoes a continuous annealing at a temperature of the order of 1100 ° C., for about 1 minute, in an oven where the thermal energy is generated by combustion of hydrocarbons, the arrival of which is regulated. air to the burner so as to obtain an oxidizing atmosphere.
• the strip is excluded to subject the strip to a reducing atmosphere, that is to say an atmosphere containing an excess of hydrocarbons, to avoid degradation of the corrosion resistance of the strip by re-carburation of steel by hydrocarbons.
• the annealed strip then undergoes air cooling and / or forced cooling by spraying water outside the oven.
• etching capable of removing the thick oxide layer, of the order of 0.1 to 0.3 ⁇ m, which formed on the surface of the strip during annealing in the oven.
• the pickling is generally carried out in several pickling tanks containing acid solutions capable of removing this oxide layer, such as a mixture of nitric acid and hydrofluoric acid.
• the strip is subjected to either a skin-pass operation or a polishing operation until the desired glossy surface appearance is obtained.
• the skin-pass is made with so-called mirror-polishing cylinders, ie cylinders having an arithmetic average roughness Ra of between 0.05 and 0.08 ⁇ m which give the steel strip a surface appearance. gloss.
• the austenitic stainless steel strips obtained according to these two processes have insufficient mechanical characteristics, since their elastic limit Rp 0i2 is between 250 and 350 MPa, and their load at break Rm is between 600 and 700 MPa, A ⁇ o elongation of between 50 and 60%.
• the operation of skin-pass or polishing is an additional step.
• the polishing operation is a long and delicate operation.
• the present invention therefore aims to avoid the disadvantages of the processes of the prior art, and to provide a method for conferring on a strip austenitic stainless steel treated in an oven with hydrocarbon combustion, a brightening surface appearance, an elastic limit Rp o , 2 of 600 MPa and a tensile strength Rm of 800 MPa associated with an Aso elongation greater than or equal to 40%.
• the subject of the invention is an austenitic stainless steel strip having an elastic limit Rp o , 2 greater than or equal to 600 MPa, a tensile strength Rm greater than or equal to 800 MPa, an elongation A 8 o greater than or equal to 40%, the composition of which in% by weight is: 0,025 ⁇ C ⁇ 0,15%
• the strip of steel according to the invention also advantageously has a surface whose arithmetic average roughness is less than or equal to 0.08 ⁇ m, which gives the strip a smooth surface and therefore an even brighter surface appearance.
• the invention also relates to a continuous manufacturing process for this austenitic stainless steel strip.
• an austenitic stainless steel strip In order to obtain an austenitic stainless steel strip according to the invention, it is first necessary to form and then flow in the form of a slab an austenitic stainless steel which comprises the following elements;
• Carbon having a content of between 0.025 and 0.15% by weight. Carbon promotes the formation of austenite, and controls the amount and hardness of deformation martensite. In addition, its solid solution solution hardens the steel and increases its mechanical strength.
• the carbon content is less than 0.025%, the steel becomes unstable and a lot of martensite is formed, resulting in insufficient A 8 O elongation.
• the carbon content is greater than 0.15%, the steel becomes stable, the formation of deformation martensite is insufficient and the steel no longer has enough energy to recrystallize.
• the minimum annealing temperature for initiating recrystallization is high and the size of the austenite grains becomes too large to achieve high mechanical characteristics.
• even higher carbon contents favor the formation of chromium carbides at the grain boundaries during subsequent heat treatments and thus increase the risk of intergranular corrosion.
• silicon having a content of between 0.20 and 1.0% by weight. Silicon is used as the deoxidizer of liquid steel, and it participates in hardening in solid solution. Its content is limited to 1.0% by weight because it has a tendency to disturb the manufacturing process of the steel strip by posing problems of segregation during slab casting of the steel. - Manganese at a content between 0.50 and 2.0% by weight.
• Manganese promotes the formation of austenite. If the manganese content is greater than 2.0%, the austenite being too stable, the formation of deformation martensite is insufficient and this does not occur. does not achieve the required yield point levels. However, if the manganese content is less than 0.50%, the deoxidation of the steel is insufficient.
• chromium at a content of between 16.0 and 20.0%. Chromium promotes the formation of deformation martensite, and is an essential element in giving steel good corrosion resistance. If the chromium content is greater than 20.0%, too much deformation martensite is generated, which makes it necessary to increase the content of the elements favoring the formation of austenite such as carbon, nitrogen, nickel and manganese. If the chromium content is less than 16.0%, the corrosion resistance of the steel is insufficient.
• nickel at a content of between 6.0 and 12.0%. Nickel stabilizes the austenite and promotes re-passivation. If the nickel content is less than 6.0%, the corrosion resistance of the steel is insufficient. If the nickel content is greater than 12.0%, the austenite over-stabilizes, there is not enough deformation martensite, and the mechanical characteristics of the steel are insufficient. molybdenum at a content of less than or equal to 3.0%. Molybdenum promotes the formation of deformation martensite and increases corrosion resistance, especially when combined with nitrogen. Beyond a content of 3.0%, the corrosion resistance of the steel will not be improved. nitrogen at a content of between 0.030 and 0.160%.
• Nitrogen promotes the formation of austenite, delays the precipitation of carbides, stabilizes austenite, and improves formability. In addition, it plays a role in the adjustment of grain size in the structure. However, if it is added at a level greater than 0.160%, it may deteriorate the hot ductility of the steel.
• Phosphorus is a segregating element. It promotes hardening in solid solution of steel, however its content should be limited to 0.50
• composition may optionally include:
• vanadium at a content of between 0, 10 and 0.50%. Vanadium promotes the weldability of steel, and inhibits the growth of austenite grains in the heat affected zone. Above 0.50%, vanadium does not contribute to the improvement of the weldability, and below 0.10%, the weldability of the steel is insufficient.
• niobium at a content of between 0.03 and 0.50%. Niobium promotes the weldability of steel, however beyond 0.50%, it degrades the hot formability of the steel strip. - with a total niobium and vanadium content of between 0.10 and 0.50% to guarantee the weldability of steel without any adverse effect on hot ductility.
• the remainder of the composition is iron and other elements usually expected to be found as impurities resulting from the processing of stainless steel, in proportions that do not affect the properties sought.
• the slab After being cast, the slab is hot rolled in a strip train to form a hot rolled strip which is annealed and optionally etched.
• the hot-rolled strip then undergoes various treatments, so as to obtain a strip having both excellent characteristics mechanical and surface appearance briliant, and this without resorting either to annealing in a briquetting annealing oven, nor to a final polishing of the surface of the strip or a final skin-pass operation.
• the installation used to manufacture the strip according to the invention comprises a cold rolling device of strips, consisting of a band train comprising working rolls between which scrolls the austenitic stainless steel strip of composition according to the invention.
• the working rolls have an arithmetic mean roughness Ra of less than or equal to 0.15 ⁇ m, and preferably less than or equal to 0.10 ⁇ m.
• the diameter of the working rolls of the belt gear is between 50 and 100 mm, to minimize the rolling forces for the high reduction rates, that is to say from 75% reduction.
• the band train not only reduces the thickness of the band, but also to help crush the asperities from the previously hot rolled strip.
• the installation comprises a hydrocarbon combustion furnace comprising an open enclosure through which the band passes, and means for introducing a gas mixture of hydrocarbon and air.
• the open enclosure comprises, in the running direction of the band shown, two successive zones, a first heating zone and a second temperature maintenance.
• the first heating zone is equipped with powerful heating means (not shown) capable of rapidly heating the strip at a heating rate V1, up to a holding temperature T1.
• the strip is maintained at this temperature T1 in the second zone, during a holding time M, and then cooled at a speed V2 in a cooling zone located just after the exit of the oven.
• the installation comprises a pickling device which comprises at least one acid-resistant pickling tank, and containing a pickling solution.
• the previously hot rolled austenitic steel strip is cold rolled at room temperature, with a reduction ratio of between 55 and 85%.
• a cold-rolled strip having a thickness of between 0.6 and 2 mm is thus obtained.
• the cold rolling operation at a reduction rate of between 55 and 85%, it forms between 50 and 90% by volume of deformation martensite ⁇ '.
• the deformation martensite ⁇ ' is observed by micrography and its volume fraction can be measured by X-ray diffraction or magnetic induction measurement (ferromagnetic phase).
• the reduction ratio is between 70 and 85%, so as to obtain a band having a smooth surface topography, ie an arithmetic average roughness Ra of between 0.07 and 0.12 ⁇ m, free micro-defects of shot blasting craters and / or etched grain boundaries.
• This also makes it possible to store sufficient plastic deformation energy to promote faster recrystallization at low temperatures.
• Applicants wish to emphasize that obtaining a glossy surface appearance not by a conventional bright annealing process, but by an oxidative annealing process followed by stripping was contrary to the initial expectations of the inventors, which provided, according to their theory, to obtain a strip having a matt low gloss surface appearance characteristic of steels annealed in a hydrocarbon combustion furnace.
• the inventors thought that, according to their theory, the limitation of the growth of the grain size in volume, obtained by controlled recrystallization of austenitic stainless steel, while increasing the surface density of chemically etched grain boundaries, would favor the diffuse reflection of the light on the surface and thus obtaining a matt and non-glossy surface.
• the inventors have demonstrated that when the strip is cold rolled with a sufficiently high reduction ratio, and with work rollers having an arithmetic mean roughness Ra less than or equal to 0.15 ⁇ m, then is subject to partial recrystallization annealing at a temperature of the order of 800 0 C, in a hydrocarbon combustion furnace, to form an oxide layer sufficiently thin to be easily removed by acid etching, without the grain boundaries are attacked, then the strip has both excellent mechanical characteristics and a glossy, gloss-like surface finish. Under the conditions of the invention, that is to say in the absence of the attack of the grain boundaries of the steel, the average arithmetic roughness Ra transferred to the strip by the working rolls during the operation of Cold rolling is very little degraded.
• the working rolls have an arithmetic average roughness of less than or equal to 0.15 ⁇ m, and preferably less than 0.10 ⁇ m.
• the brightness measured in the context of the present invention corresponds to the measurement of the reflectivity of the surface and is measured according to an illumination angle of 60 °, according to the international standard ISO 7668 (1986).
• the cold-rolled strip is then passed through the open chamber of the hydrocarbon combustion furnace, inside which there is an oxidizing atmosphere with respect to the iron, in order for it to undergo a heat treatment. consisting of a partial recrystallization annealing of the steel, followed by forced cooling.
• the atmosphere in the furnace is composed of a gaseous mixture of air and at least one hydrocarbon in an air / hydrocarbon volume ratio of between 1.1 and 1.5, the gas mixture comprising in addition 3 to 8 % by volume of oxygen.
• the furnace atmosphere is preferably a gaseous mixture of air and hydrocarbon in an air / hydrocarbon volume ratio of between 1.1 and 1.5, the gaseous mixture further comprising 3 to 8% by volume of oxygen.
• the at least one hydrocarbon is chosen from natural gas, butane and methane. Natural gas is preferentially chosen because of its low cost and ease of transportation.
• the atmosphere in the annealing furnace is too oxidizing and the formed oxide layer is so thick that to eliminate it, aggressive pickling solutions will have to be used. that will attack the grain boundaries.
• the air / hydrocarbon volume ratio is less than 1.1, the atmosphere in the annealing furnace is too reducing. Therefore, the re-carburization of the steel by the hydrocarbons can not be avoided, and the corrosion resistance of the steel will be degraded.
• the heat treatment is adjusted so as to obtain a steel strip whose recrystallized volume fraction is between 60 and 75%.
• the non-recrystallized volume fraction (measured by micrographic observation and image analysis) is greater than 40%, the microstructure of the steel induces mechanical properties that are too high, and the elongation A 80 of the strip is less than at 40%.
• the non-recrystallized volume fraction is less than 25%, the mechanical characteristics such as the elastic limit Rpo, 2> will be insufficient.
• the partial recrystallization annealing is carried out at a speed V1 of between 10 and 80 ° C./s, a temperature T of between 800 and 950 ° C. and a holding time M of between 10 and 100 seconds, advantageously between 60 and 80 seconds.
• the annealing of the strip at a temperature T between 800 and 950 ° C. makes it possible to limit the diffusion of chromium at the grain boundaries, and consequently limits the attack of the grain boundaries during the subsequent chemical stripping of the strip, which promotes obtaining a glossy surface appearance.
• the steel When the temperature T is less than 800 ° C., the steel does not recrystallize sufficiently to obtain the desired mechanical properties. Indeed, the steel has an elastic limit Rp o , 2 greater than 600 MPa but an A ⁇ o elongation of less than 40% mediocre, which greatly limits its cold deformation capabilities.
• the stainless steel can only recrystallize during a holding time M very long which is not compatible with the industrial requirements.
• the austenite grains grow in favor of martensite, and the elastic limit Rp o , 2 is insufficient to give the stainless steel good mechanical properties.
• the recrystallized volume fraction of the strip will be less than 60%, and the elongation A 8 o of the strip is insufficient.
• the austenitic grains enlarge in favor of martensite, and the mechanical characteristics, such as the elastic limit Rpo, 2 become insufficient.
• the partially recrystallized steel strip then undergoes forced cooling at a speed V 2 of between 10 and 80 ° C / s, for example by blowing air or by blowing air under pressure and spraying water.
• V 2 a speed of between 10 and 80 ° C / s
• the yield point Rpo, 2 and the breaking load Rm increase.
• the strip When the strip is cooled, it is stripped with an acid pickling solution capable of completely removing said oxide layer depending on its thickness and nature, without attacking the grain boundaries of the steel.
• an acid pickling solution capable of completely removing said oxide layer depending on its thickness and nature, without attacking the grain boundaries of the steel.
• the strip undergoes a first electrolytic pickling in a bath containing sodium sulphate whose concentration is between 150 and 200 g / l, with a pH of less than 3, and with an amperage of between 5 and 12 kA.
• the strip according to the invention also has the following advantages: - Better resistance of the gloss after deformation than the strips of austenitic stainless steel having been annealed in a bright annealing furnace (standard 2RB). Indeed, the loss of the brightness of the strip according to the invention is only 30% after stamping, while it is 80% for the standard annealed-gloss tape. - Better resistance against intergranular corrosion than the austenitic stainless steel strips having undergone a standard annealing-pickling treatment (2D standard).
• austenitic stainless steel strips according to the invention have comparable weldability to standard annealed austenitic stainless steel or standard annealed-pickled stainless steel strips.
• an austenitic stainless steel strip according to the invention will be compared with, on the one hand, a austenitic stainless steel strip of the standard annealed-pickled type (2D standard), and on the other hand a standard annealing-austenitic stainless steel strip (standard 2RB).
• the brightness measurement is carried out with an illumination of 60 ° according to the international standard ISO 7668 (1986). Then, the drawability of these three types of tape, their loss of gloss after stamping, their scratch resistance, and finally their resistance to intergranular corrosion will be compared.
• AS33 steel is cast to form a slab that is hot rolled to a thickness of 4.5 mm. This slab is then cold-rolled with working rolls having an arithmetic average roughness Ra of 0.1 ⁇ m, with a reduction ratio of 82% so as to obtain in a passage a strip of 0.8 mm thickness.
• This cold-rolled strip is subjected to a partial recrystallization annealing of the steel in a combustion furnace, by heating it with a heating rate of 50 ° C./s up to a holding temperature of 820 ° C. and during a hold time of 50 seconds.
• the atmosphere in the furnace is a mixture of air and natural gas comprising an oxygen level of 4% by volume.
• the volume ratio of air to natural gas is 1.3.
• the web is then cooled at a cooling rate of 70 ° C / sec to room temperature.
• the strip undergoes a first electrolytic pickling in a bath containing sodium sulphate whose concentration is 175 g / l, pH 2, with an amperage of 9 kA, and for a duration of 15 s, then a second etching electrochemical in a bath containing nitric acid whose concentration is 100 g / l, pH 2, with an amperage of 9 kA, and for a period of 15 s.
• AS33 steel is cast to form a slab that is hot rolled to a thickness of 4.5 mm. This slab is then cold rolled with a reduction rate of 82% so as to obtain in one pass a 0.8 mm thick strip.
• This cold-rolled strip is subjected to a complete recrystallization annealing of the steel, in a combustion furnace, at a temperature of 1120 ° C. for a period of 50 seconds.
• the atmosphere in the furnace is a mixture of air and natural gas comprising an oxygen level of 4% by volume.
• the volume ratio air / natural gas is 1, 3.
• the web is then cooled at a cooling rate of 80 ° C / sec to room temperature.
• the strip is stripped to completely remove the 0.2 ⁇ m thick formed oxide layer in sodium sulfate and sulfuric acid baths.
• the band obtained undergoes no further treatment, nor polishing of the surface, nor skin pass.
• AS33 steel is cast to form a slab that is hot rolled to a thickness of 4.5 mm. This slab is then cold rolled with working rolls which give a glossy surface appearance to the strip, with a reduction rate of 82% so as to obtain in one pass a 0.8 mm thick strip.
• This cold-rolled strip is subjected to a complete recrystallization annealing of the steel, in a bright-annealing furnace inside which an atmosphere consisting of a gaseous mixture comprising 10% by volume of nitrogen and 90% prevails. by volume of hydrogen and having a dew point of -50 ° C., by heating it with a heating rate of 50 ° C./s, up to a holding temperature of 1100 ° C.
• the strip is cooled at a cooling rate of 60 ° C / s to room temperature.
• the band obtained undergoes no further treatment, nor polishing of the surface, nor skin pass.
• Table 2 shows the mechanical and appearance characteristics of these three types of tape.
• the strip according to the invention has, compared to standard 2D and standard 2RB strips, both a glossy surface appearance and good mechanical characteristics. It also has a superficial hardness greater than the two bands of the prior art.
• Blanks are cut in the steel strip according to the invention, in the standard 2RB band and in the 2D standard band. These blanks are then stamped in a stamping press conventionally comprising a punch, a die and a blank holder, to form buckets.
• the loss of gloss is low, and much lower than that observed for the standard 2B and standard 2RB strips.
• the scratch resistance tests are carried out on the steel strip according to the invention, and the standard strip 2RB according to the international standard ISO 1518 using a Clemen machine whose hemispherical steel hardened tip has a hardness of 1500 Hv, and a diameter of 1 mm.
• the tests consist in applying, with variable loads of 50 g, 200 g and 400 g, the hemispherical tip on the surface of the strip so as to create a scratch.
• the test results are summarized in Table 4.
• test results show that the steel strip according to the invention is more resistant to scratching than the standard 2RB strips, an order of magnitude of 40% on average, corresponding to the difference in relative surface hardness of the strip.
• the intergranular corrosion resistance test is carried out on samples taken from the steel strip according to the invention and from the standard 2D strip.
• This test is performed according to the NFA 05-159 standard. It involves immersing the sample in a boiling solution of sulfuric acid and copper sulphate for a period of 20 hours. The sample is then bent at 90 ° and the observation of the convex face of the latter, in comparison with a reference sample which has not been immersed in said solution, makes it possible to determine the degree of cracking in the extreme skin.
• a low resistance to intergranular corrosion is characterized by the presence of numerous cracks on the convex face of the folded sample.
• the intergranular corrosion resistance tests show that the austenitic stainless steel strip according to the invention is more resistant to intergranular corrosion than the standard 2D strip.

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  • 2006-04-06 ES ES06743665T patent/ES2317540T3/es active Active
• 2007
  • 2007-12-07 ZA ZA200711238A patent/ZA200711238B/xx unknown

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