METHOD FOR THE MANUFACTURE OF STEEL PRODUCTS OF A PRECIPITATION HARDENED MARTENSITIC STEEL, STEEL PRODUCTS OBTAINED WITH SUCH METHOD AND USE OF SAID STEEL PRODUCTS

Abstract: A method for the manufacture of steel products and products thus produced, wherein steel is subjected to isothermal martensitic formation and precipitation hardening in a martensitic structure subsequent to soft annealing and shaping. The method steps include shaping followed by solution annealing between 1050 DEG C and 1200 DEG C, quenching from the solution annealing temperature with a quenching speed of at least 5 DEG C per second to a temperature below 500 DEG C, subjecting said steel to an isothermal martensitic transformation and subsequently hardening the steel at a temperature between 450 DEG C and 550 DEG C to precipitate particles in said martensitic structure.

Description: METHOD FOR THE MANUFACTURE OF STEEL PRODUCTS OF A PRECIPITATION HARDENED MARTENSITIC STEEL, STEEL PRODUCTS OBTAINED WITH SUCH METHOD AND USE OF SAID STEEL PRODUCTS FIELD OF THE INVENTION 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. BACKGROUND OF THE INVENTION In the discussion of the state of the art that follows, reference is made to certain structures and/or methods. However, the following references should not necessarily be construed as an admission that these structures and/or methods constitute prior art. Applicant explicitly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art against the present invention. In published international patent application WO93/07303, a method of manufacture of the above mentioned kind has been described wherein the transformation into the martensitic structure is accomplished by air cooling after annealing in the austenitic region or by cold working. Air cooling after annealing normally results in the so- called athermal kinetic mode of martensite transformation. The formation of air cooling induced martensite is suppressed by alloying elements like nickel, titanium and aluminum, which are used for precipitation of hardenable steel. It may be that at relatively high concentrations of such alloying elements the austenite is stabilized such that the martensitic transformation start temperature becomes impracticably low. SUMMARY OF THE INVENTION It is an object of the invention to offer a method for the manufacture of steel products, steel products so manufactured and the use of said steel products whereby a practical optimum is achieved between ductility, strength, wear and corrosion resistance, homogeneity of martensite distribution and a practical level of the martensite transformation temperature. 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. BRIEF DESCRIPTION OF THE DRAWING FIGURE The objects and advantages of the invention will become apparent from the following detailed description of preferred embodiments thereof in connection with the accompanying drawing. Figure 1 is a temperature profile in time of the heat treatment and processing method of the present invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 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 combination of an isothermal martensitic transformation and precipitation hardening is known (See Scripta Metallurgica et Materialia, 1995, Vol. 33, No. 9, pp. 1367-1373). However, a method of manufacture of the above-mentioned kind which allows a steel product to be formed into a relatively complicated shape by deformation whilst achieving an optimum between ductility, strength, wear and corrosion resistance and homogeneity of martensite distribution is not disclosed. It is a further object of the invention to provide a very efficient method for the manufacture of steel products with a homogeneous distribution of martensite and precipitates. Accordingly, 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 %. Generally, 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. The invention will be elucidated further by the use of practical examples: Example 1 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. Before rolling, the steel is reheated to a temperature of 1150°C to 1250°C and soaked at this temperature for at least 1 hour in order to give the material an austenitic structure and sufficient ductility to be hot rolled. Reheating to a temperature of 1150°C to 1250°C 2 is followed by hot rolling 3. 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, however, 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. To ensure a homogeneous martensite transformation throughout the product and sufficient hardenability of the formed martensite by precipitation hardening, 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. In order to achieve an optimal isothermal martensitic transformation 10, the martensitic transformation 10 should be carried out at a temperature between -30°C and -50°C for at least one hour. More preferably, 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. After quenching 9, 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): C+N < 0.05 Cr 12.00 Mn 0.30 Fe balance Ni 9.00 Mo 4.00 Ti 0.90 Al 0.30 Si 0.15 Cu 2.00 Example 2 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. Depending on the required accuracy of the diaphragm plate spring dimensions, 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. Although the present invention has been described in connection with preferred embodiments thereof, it will be appreciated by those skilled in the art that additions, deletions, modifications, and substitutions not specifically described may be made without department from the spirit and scope of the invention as defined in the appended claims.

Claims: CLAIMS 1. A method for the manufacture of a steel product comprising the steps of: (a) subjecting the steel to precipitation hardening in a martensitic structure subsequent to soft annealing; (b) shaping said steel, followed by solution annealing between a temperature of 1050°C and 1200°C and for a time period of between 5 and 30 minutes; (c) quenching the steel from the solution annealing temperature to a temperature below 500°C with a quenching rate of at least 5°C per second, said quenched steel being subjected to an isothermal martensitic transformation; and (d) hardening at a temperature between 450°C and 550°C for at least 3 minutes to cause particles to precipitate out from solution into the martensitic structure. 2. The method according to claim 1, wherein the quenched steel is subjected to an isothermal martensitic transformation at a temperature between -30°C and -50°C for at least one hour. 3. The method according to claim 1, wherein, between the solution annealing and the quenching of the steel, the steel is subjected to a sensitizing procedure between 850°C and 950°C for at least 5 minutes thereby optimizing initiation of the isothermal martensitic transformation. 4. The method according to claim 3, wherein the sensitizing procedure homogeneously nucleates martensite during the isothermal martensitic transformation. 5. The method according to claim 1, wherein the steel comprises chromium (Cr) in a weight percentage between 10 % and 14 %. 6. A steel product manufactured by the method of claim 1 having a homogenous hardness of at least 450 HV. 7. The steel product of claim 6, wherein the steel product is a cap of an electric rotary shaver. 8. The steel product of claim 6, wherein the steel product is a cutter of an electric rotary shaver. 9. The steel product of claim 6, wherein the steel product is a cutter in a domestic appliance. 10. The steel product of claim 6, wherein the steel product is a knife in a domestic appliance. 11. The steel product of claim 6, wherein the steel product is a spring in a domestic appliance. 12. The steel product of claim 6, wherein the steel product is a medical instrument. 13. The steel product of claim 6, wherein the steel product is a dental instrument. 14. The steel product of claim 6, wherein the steel product is a diaphragm plate spring in a fluid valve.

Inventors: NILSSON JAN-OLOF, OLSSON SOEREN, SJOERD BLAAUW HUBERT, VAN DER GRIJN ADRIAAN, NILSSON, JAN-OLOF, OLSSON, SOEREN, SJOERD BLAAUW, HUBERT, VAN DER GRIJN, ADRIAAN
Applicants: KONINKL PHILIPS ELECTRONICS NV, SANDVIK AB, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2000
Main classification: C21D6
IPC codes: C21D6, C21D9, C22C38