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
  • 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
  • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/02—Hardening by precipitation
• • 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
  • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
• • 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
  • C22C38/10—Ferrous alloys, e.g. steel alloys containing cobalt
  • C22C38/105—Ferrous alloys, e.g. steel alloys containing cobalt containing Co and Ni
• • 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
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • 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
  • C21D2201/00—Treatment for obtaining particular effects
• • 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
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0236—Cold rolling
• • 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
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0273—Final recrystallisation annealing

Definitions

• the present invention relates to a maraging steel particularly suitable for the manufacture of parts requiring very good resistance to fatigue.
• Many parts are made from maraging steel strips containing, in% by weight, about 18% of nickel, 9% of cobalt, 5% of molybdenum, 0.5% of titanium and 0.1% of aluminum, treated to have an elastic limit greater than 1800 MPa. These strips are obtained by hot rolling and cold rolling. The strips or the pieces cut from the strips are then hardened by a heat treatment to harden around 500 ° C.
• the parts are optionally nitrided on the surface to improve their resistance to fatigue. However, the fatigue strength of these parts is insufficient.
• maraging steels having different chemical compositions and mechanical characteristics, such as maraging steels containing 18% of nickel, 12% of cobalt, 4% of molybdenum, 1.6% titanium and 0.2% aluminum, or maraging steels containing 18% nickel, 3% molybdenum, 1.4% titanium and 0.1% aluminum, or maraging steels containing 13% chromium, 8% nickel, 2% molybdenum and 1% aluminum.
• maraging steels having different chemical compositions and mechanical characteristics, such as maraging steels containing 18% of nickel, 12% of cobalt, 4% of molybdenum, 1.6% titanium and 0.2% aluminum, or maraging steels containing 18% nickel, 3% molybdenum, 1.4% titanium and 0.1% aluminum, or maraging steels containing 13% chromium, 8% nickel, 2% molybdenum and 1% aluminum.
• the object of the present invention is to remedy this drawback and to propose a band or a piece of maraging steel having improved resistance to fatigue.
• the subject of the invention is a method for manufacturing a strip or a part cut from a cold rolled maraging steel strip. According to this method, before carrying out the hardening heat treatment, the strip or the part is subjected to cold plastic deformation with a rate of work hardening greater than 30% and the strip or the part is subjected to annealing. recrystallization so as to obtain a fine grain of ASTM index greater than 8.
• the chemical composition of the steel comprises, by weight:
• the strip or the part is subjected to cold rolling with a reduction rate of between 1% and 10%.
• the maraging steel is remelted under vacuum by the VAR process or remelted a first time under vacuum by the VAR process or under electroconductive slag by the process ESR and remelted a second time under vacuum by the VAR process.
• the invention also relates to a strip or part, of thickness less than 1 mm, made of maraging steel having a fine grain of ASTM index greater than 8 and a yield strength after hardening greater than 1850 MPa.
• the strip or the part thus obtained can be used for the manufacture of parts such as belts. These parts are hardened by a hardening treatment between 450 and 550 ° C for 1 to 10 hours which can be followed by nitriding on the surface.
• a cold rolled strip of maraging steel is produced by targeting a carbon of less than 0.005% and then by deoxidizing with aluminum.
• the steel thus produced is cast in the form of reflow electrodes.
• These electrodes are either remelted under vacuum (VAR process, “Vacuum Arc Remelting” known in itself) to form ingots or slabs, or remelted a first time under vacuum (VAR) or under electroconductive slag (ESR process, “Electro Slag Remelting ", known in itself) to form second electrodes which are themselves remelted under vacuum (VAR) to form ingots or slabs.
• VAR process remelted under vacuum
• ESR process electroconductive slag Remelting
• the ingots or slabs are then hot rolled after reheating to around 1200 ° C, and for example between 1150 ° C and 1250 ° C to obtain hot rolled strips a few millimeters thick, and for example about 4 , 5 mm thick.
• the hot-rolled strips are pickled and then cold-rolled with one or more recrystallization anneals to obtain cold-rolled strips of thickness less than 1 mm, for example 0.4 mm or 0.2 mm thick .
• the last intermediate recrystallization annealing treatment is carried out at a thickness such that the cold-rolled strip has a work hardening rate greater than 30% and better still greater than 40%.
• the strip thus hardened is annealed, for example in a passage oven, to obtain a fine grain of ASTM index greater than 8 (corresponding to an average grain diameter less than 20 microns), and better still greater than 10 (corresponding to a diameter grain size less than 10 microns); the grain size being determined according to ASTM E112.
• the annealing treatment intended to obtain a fine grain is carried out under a protective atmosphere by suitably adjusting the temperature and duration parameters. These parameters depend on the particular conditions for carrying out the heat treatment and those skilled in the art know how to determine these parameters in each particular case.
• the duration that is to say the residence time of any point of the strip in the oven
• the setpoint temperature oven is between 900 ° C and 1100 ° C
• the atmosphere of the oven can be argon with a dew temperature preferably below -50 ° C.
• the strip In order to improve the flatness of the strip and, if necessary, to perfect the martensitic transformation, the strip can, moreover, be subjected to a light cold rolling with a reduction rate of between 1% and 10%, which leads to a work hardening rate of the same value.
• the hardening treatment can also be carried out in a passage oven at a temperature between 600 ° C and 700 ° C for a time between 30 seconds and 3 minutes.
• the part can be hardened on the surface by a nitriding treatment carried out by keeping it for a few hours at around 500 ° C. in a reactive gas mixture rich in nitrogen.
• blanks of parts can be cut from cold-rolled strips of thickness greater than the final thickness desired for the parts. These blanks are shaped, possibly welded, then cold-rolled to the final thickness so as to have a work hardening rate greater than 30% or better still greater than 40%.
• the parts are then annealed under the same conditions as what has just been described, so as to obtain a fine grain of ASTM index greater than 8, or better still greater than 10, then subjected to a hardening treatment as indicated above. above.
• the yield strength obtained is high and the resistance to fatigue is excellent.
• Parts can also be produced by cutting, for example by chemical cutting, from hardened strips. The entire process, including the hardening heat treatment, is then applied to the strip.
• These parts are, for example, integrated circuit support grids.
• the maraging steel which it is preferable to use to obtain very good fatigue properties and an elastic limit greater than 1850 MPa, contains mainly, in% by weight:
• the nickel and molybdenum contents must be such that 20% ⁇ Ni + Mo ⁇ 27%, and preferably such that 22% ⁇ Ni + Mo ⁇ 25%.
• the cobalt and molybdenum contents In order to obtain an elastic limit, after thermal hardening treatment, greater than 1850 MPa, the cobalt and molybdenum contents must be such that Co x Mo> 50, and preferably such that Co x Mo> 70. Indeed , the higher this product, the higher the elastic limit. However, in order to obtain sufficient ductility, the cobalt and molybdenum contents must be such that Co x Mo ⁇ 200, and preferably such that Co x Mo ⁇ 120. These values correspond respectively to elastic limits less than around 3000 MPa and 2500 MPa. Molybdenum has a favorable effect on hardening by surface nitriding. To obtain good curing, the molybdenum content should preferably be greater than 4%, and better still greater than 6%. However, it is preferable for it to remain below 8% to limit the problems of segregation and to facilitate the hot transformation operations as well as to improve the ductility of the final product. Two preferred ranges of molybdenum contents can
• the following preferential composition domains can be defined for the main elements: 1) in order to obtain a yield strength greater than 1850 MPa and an average suitability for hardening by nitriding:
• the minimum content can be 0% or traces.
• the nitrogen and titanium contents must be such that: Ti x N ⁇ 2 x 10 "4 , or better, ⁇ 1 x 10 " 4 .
• maraging steel strips of composition have been produced:
• a first strip A given by way of example, was annealed in a hydrogen oven at 1020 ° C for 1 minute to obtain a fine grain of ASTM 11 index and then hardened by maintaining at 490 ° C for 3 hours.
• a second strip B given for comparison, was annealed in a passage oven at 1150 ° C for 1 minute to obtain a coarse grain of index ASTM 7 and then hardened by maintaining at 490 ° C for 3 hours. Comparative fatigue resistance tests were carried out with bands A and B by wavy traction, at 25 hertz, with a maximum stress of 750 MPa and a minimum stress of 75 MPa.
• the fatigue limit was greater than 8 ⁇ 10 8 cycles, while for band B, the fatigue limit was equal to 5 ⁇ 10 8 cycles.
• a strip of maraging steel was also produced containing 18% nickel, 9% cobalt, 5% molybdenum, 0.5% titanium and 0.1% aluminum. This strip was manufactured by the method according to the invention, the grain had an ASTM index of 10 and the elastic limit was 1910 MPa. The fatigue limit measured under the same test conditions as in the previous case was 2 x 10 8 cycles.
• These bands can advantageously be used to manufacture belts or any other product, such as grids supporting integrated circuits.
• transmission belts for internal combustion engines are made up of jumpers held by rings made up of narrow bands according to the invention and the two ends of which are welded. These belts have a lifespan more than ten times longer than the lifespan of identical belts but manufactured with strips of maraging steel in accordance with the prior art.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Physics & Mathematics (AREA)
• Heat Treatment Of Sheet Steel (AREA)
• Heat Treatment Of Articles (AREA)
• Manufacturing Of Steel Electrode Plates (AREA)
• Soft Magnetic Materials (AREA)

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