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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/02—Hardening by precipitation
• • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
• • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/26—Methods of annealing
• • 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
  • C21D6/002—Heat treatment of ferrous alloys containing Cr
• • 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
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
• • 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
  • C21D9/02—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for springs
• • 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
  • C21D9/18—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for knives, scythes, scissors, or like hand cutting tools
• • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite
• • 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
  • C21D6/04—Hardening by cooling below 0 degrees Celsius

Definitions

• the present invention relates to a method for the manufacture of a steel product wherein the steel is subjected to isothermal martensite formation and precipitation hardening in a martensitic structure subsequent to soft annealing and is shaping.
• the invention also relates to a steel product obtained with such method and to the use of said steel product.
• One aspect of the present invention is a method for the manufacture of a steel product comprising the steps of subjecting the steel to isothermal martensite formation and precipitation hardening in a martensitic structure subsequent to soft annealing.
• the steel is then shaped into the desired form subsequent to soft annealing, followed by solution annealing at a temperature between 1050°C and 1200°C and for a time period of between 5 and 30 minutes. From the annealing temperature, the steel is quenched at a rate of at least 5°C per second to a temperature below 500°C. The quenched steel being subjected to an isothermal martensitic transformation. Precipitation hardening of the steel is then accomplished at a temperature of between 450°C and 550°C for at least 3 minutes to cause precipitation of particles in the martensitic structure.
• Figure 1 is a temperature profile in time of the heat treatment and processing method of the present invention.
• a method for the manufacture of a steel product according to the invention is characterized by shaping of the steel followed by solution annealing between 1050°C and 1200°C for between 5 and 30 minutes, after which the steel is quenched from the solution annealing temperature to a temperature below 500°C with a quenching speed of at least 5°C per second.
• the quenched steel is then subjected to an isothermal martensitic transformation and is subsequently strengthened by being held at a temperature between 450°C and 550°C for at least 3 minutes to precipitate particles in the martensitic structure.
• a method for the manufacture of steel products according to the invention is further characterized by subjecting the quenched steel to an isothermal martensitic transformation by holding the steel at a temperature between -30°C and - 50°C for at least one hour.
• a method for the manufacture of steel products according to the invention is still further characterized by a sensitizing procedure in which the steel is held at a temperature between 850°C and 950°C for at least 5 minutes so as to allow initiation of the martensitic transformation to become optimal.
• the sensitizing procedure occurs between solution annealing and quenching the steel.
• a steel subjected to a sensitizing procedure alleviates thermo-mechanical stresses which would otherwise build up internally in the steel product. The reduced internal thermo-mechanical stresses enable the manufacture of a steel product with a very accurate size and which is stable in use.
• a further object of the invention is to provide a method of manufacture of a steel product exhibiting a combination of superior strength, corrosion resistance and ductility.
• Such a method is further characterized in that the steel comprises chromium (Cr) in a weight percentage between 10 % and 14 %.
• Cr chromium
• martensitic steels with a low weight percentage of carbon so-called maraging steels, may be with or without chromium.
• Corrosion resistant maraging steels comprise a weight percentage of chromium between 10.5 and 18 %.
• a particular type of maraging steel which may be obtained by the method according to the invention, contains in weight percentage 10-14 % Cr, 7-10 % Ni, 3-6 % Mo, 0-9 % Co, 0.5-4 % Cu, 0.05-0.5 % Al, 0.4-1.4 % Ti and less than 0.03 % C and N.
• a steel material suitable for use with the present invention with the above mentioned composition was produced as a strip material from a full scale seven ton melt in a high frequency furnace and then subjected to rolling.
• the solidification process after melting 1 is shown in figure 1 in which the temperature profile over time is indicated by a solid line. Solidification of the melt leads to crystallization of Ti (C, N), thereby binding the free carbon and free nitrogen. The binding of free nitrogen is important because the free nitrogen would otherwise suppress the isothermal martensitic transformation.
• Hot rolling 3 produces a material in a strip shape with a suitable grain size and evenly distributed intermetallic particles.
• Scale oxide layers formed during soaking and hot rolling has to be removed by pickling and/or grinding before the material can be cold rolled to final dimensions.
• Cold rolling 4 gives the strip steel the final thickness without formation of oxide layers.
• Cold rolling 4 leads to strain induced martensitic transformations and, to ensure sufficient ductility to form a complicated product, the material has to be brought back into the austenitic condition by annealing 5.
• This annealing 5 is carried out in a continuous furnace at a temperature around 1050°C, to prevent the material from transforming to martensite before shaping of the product.
• the product is cold formed in the austenitic condition 6 leading to a partial transformation to strain induced martensite.
• the material has to be solution annealed 7 for 5 to 30 minutes at a temperature between 1050°C and 1200°C.
• Solution annealing 7 also causes alloying elements, such as Al, Cu, Mo and Ti to go into solution in the austenitic structure and reversion of strain induced martensite to austenite. These elements are used for precipitation hardening of the isothermal martensite at a later stage of the manufacture.
• the martensitic transformation 10 should be carried out at a temperature between -30°C and -50°C for at least one hour.
• the isothermal martensitic transformation 10 is preceded by a sensitizing process 8.
• the sensitizing process 8 is positioned between a solution annealing step 7 and a quenching step 9.
• the sensitizing process 8 occurs when the steel is held at a temperature between 850°C and 950°C for at least five minutes.
• the sensitizing process 8 causes destabilization of the austenitic structure of the steel material and so facilitates the subsequent isothermal martensitic transformation 10. It has been determined that during the sensitizing process 8, Mo and Ti are removed from the solution and it is believed that Mo concentrates along crystal boundaries. The behavior of Ti is not yet clear. Sensitization further ensures homogeneous nucleation of martensite during the isothermal martensitic transformation 10. Quenching 9 to room temperature or even lower prevents premature precipitation of essential intermetallic compounds in the austenite.
• the steel material is subjected to an isothermal martensitic transformation 10.
• This transformation is accomplished by holding the steel at a temperature of -30°C to -50°C for at least one hour.
• the result is a homogeneous martensitic structure with uniformly distributed retained austenite in a fine grain size.
• the isothermal martensitic transformation 10 is followed by a hardening procedure 11 during which intermetallic compounds precipitate solution in the martensitic structure.
• the steel product so treated will have a homogeneous hardness of more than 450 HV.
• a steel product which is obtained by the present method is homogenous and exhibits excellent properties with respect to wear, corrosion resistance, hardness and ductility. This unique combination of properties makes the strip steel product very attractive for shaver caps of electric rotary shavers, which are subjected to deep drawing during manufacture in order to obtain the necessary bowl shape. The same applies to the heavily deformed cutters of shavers, the strongly shaped knives of blenders and the strongly folded return springs for thermostats in irons.
• the chemical composition in weight percentages of a steel material which is very well suited to be subjected to the treatment method according to the present invention is as follows (so-called Sandvik 1RK91 steel):
• a steel material or product with the same chemical composition as in Example 1 may be produced as a diaphragm plate spring functioning as a return spring in a fluid valve.
• it may be allowed to have so-called retained austenite in the product after quenching 9.
• Solution treatment 7 is preferred followed by sensitizing 8 which causes destabilization of the austenite so that the later isothermal martensitic transformation 10 is facilitated.
• Diaphragm plate springs for many applications use complicated shapes which require strong deformations during forming. Such deformations cause strain induced martensite which has to be reversed into austenite by solution treatment 7.
• the method of the present invention is well suited to preparing the steel stock for this application.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Mechanical Engineering (AREA)
• Physics & Mathematics (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Heat Treatment Of Sheet Steel (AREA)
• Heat Treatment Of Steel (AREA)
• Heat Treatment Of Articles (AREA)
• Treatment Of Steel In Its Molten State (AREA)

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Publications (1)

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• 1999
  • 1999-08-23 SE SE9902977A patent/SE520169C2/sv not_active IP Right Cessation
• 2000
  • 2000-08-23 DE DE60019141T patent/DE60019141T2/de not_active Expired - Lifetime
  • 2000-08-23 JP JP2001518468A patent/JP5099865B2/ja not_active Expired - Lifetime
  • 2000-08-23 CN CNB008119341A patent/CN1140640C/zh not_active Expired - Lifetime
  • 2000-08-23 WO PCT/SE2000/001634 patent/WO2001014601A1/en not_active Ceased
  • 2000-08-23 AT AT00957215T patent/ATE292194T1/de not_active IP Right Cessation
  • 2000-08-23 EP EP00957215A patent/EP1216311B1/en not_active Expired - Lifetime
  • 2000-08-23 KR KR1020027002263A patent/KR100767834B1/ko not_active Expired - Fee Related
  • 2000-08-23 US US09/644,079 patent/US6531007B1/en not_active Expired - Lifetime
• 2002
  • 2002-12-23 US US10/325,730 patent/US20030094218A1/en not_active Abandoned

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