Classifications

• • 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
  • C22C33/00—Making ferrous alloys
  • C22C33/02—Making ferrous alloys by powder metallurgy
  • C22C33/0257—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements
  • C22C33/0278—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements with at least one alloying element having a minimum content above 5%
  • C22C33/0285—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements with at least one alloying element having a minimum content above 5% with Cr, Co, or Ni having a minimum content higher than 5%
• • 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
  • C22C38/04—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
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
• • G—PHYSICS
  • G04—HOROLOGY
  • G04B—MECHANICALLY-DRIVEN CLOCKS OR WATCHES; MECHANICAL PARTS OF CLOCKS OR WATCHES IN GENERAL; TIME PIECES USING THE POSITION OF THE SUN, MOON OR STARS
  • G04B37/00—Cases
  • G04B37/22—Materials or processes of manufacturing pocket watch or wrist watch cases
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F2998/00—Supplementary information concerning processes or compositions relating to powder metallurgy
  • B22F2998/10—Processes characterised by the sequence of their steps
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F2999/00—Aspects linked to processes or compositions used in powder metallurgy

Definitions

• the present invention relates to the field of stainless high-alloy steels used in the watch industry.
• Watch cases are usually made by punching them out of sheets and plates. In order to achieve their desired final shape, depending on the type of housing, they have to be severely cold compressed and annealed depending on the height of the housing. The same applies to the production of profiles for bracelets by cold rolling.
• the work hardening behavior is of crucial importance for the cold massive forming.
• steel is best suited for this type of forming, which is the least strain hardened at low yield strengths with increasing degree of deformation.
• ferritic stainless steels behave similarly to unalloyed steels during massive forming.
• the polishability of a steel ie its suitability for producing a highly polished surface, for example for housings, is another requirement for a watch steel. This requirement is only met to a limited extent by the austenitic steel No. 1.4435 used in the watch industry today.
• Ferritic steels such as steel No. 1.4521 are even more difficult to polish than austenitic steels: the ferritic steels are stabilized almost exclusively with titanium or Nb in order to prevent the precipitation of Cr carbides at the grain boundaries.
• titanium or niobium carbides of high hardness are eliminated, which destroy the polishability of the ferritic steel.
• the precipitated carbide particles in the order of 5-10 ⁇ m are not removed during mechanical polishing and protrude from the surrounding, better polished surface as so-called craters. So-called polishing tails are formed, ie deposits of polishing paste in the polishing shadow of the carbide particles can be perceived by the naked eye as extremely annoying.
• the stabilization of the steel structure by addition of Ti or Nb is therefore not applicable to a polishable ferritic chrome steel due to the negative influence on the polishability. Without Ti or Nb stabilizing the structure, the elimination of chromium carbides at the grain boundaries takes place so quickly due to the diffusion rate, which is 2 orders of magnitude higher for ferritic steels, that it cannot be prevented by rapid quenching from the solution annealing temperature.
• the chrome carbides also form hard inclusions, which in turn destroy the polishability of a steel.
• the polishability of a steel is also decisively influenced by the grain size. Coarse-grained steels cause a so-called "orange peel" during polishing, which is unacceptable for polished surfaces. The reason for this is the different properties of the randomly arranged grains (crystals) in the different directions. If the grain size measured in accordance with ASTM E 112 falls below the value 4 (> 80 ⁇ m), the human eye can recognize the crystal surfaces that have been removed differently during the polishing process as point-like particles and an image of an "orange peel" is formed.
• Drilling and milling work also requires good machinability of the alloy.
• Good corrosion resistance, especially in saline media, is another main requirement for watch steel.
• Wristwatches have direct skin contact and are particularly at risk of corrosion due to the aggressive body sweat.
• the degree of purity of a steel has a major impact on corrosion resistance.
• Coarse and cellularly arranged non-metallic inclusions mean a weak point on the surface where the pitting can begin and then continue unhindered. For this reason, steels are often used in technology according to the electro-slag remelting process (ESR process), which reduces the non-metallic, corrosion-demanding particle sizes by approx. 2 units according to DIN 65602 leads, remelted.
• ESR process electro-slag remelting process
• steel alloys can be controlled in their chemical and mechanical properties by alloying metallic and non-metallic elements.
• Molybdenum increases the corrosion resistance and the resistance to pitting corrosion in the presence of halide ions.
• Silicon is more likely to be regarded as an undesirable impurity in polishable steels because it forms hard oxide inclusions.
• silicon is a desirable alloying element if the alloy is to be soft magnetic.
• N Nitrogen improves corrosion resistance. Since the yield point and the tendency to hardening are increased by the addition of N, the N content is usually limited to 0.2%. In austenitic steels, N additives should significantly delay the start of M 23 C 6 precipitation (PR Levey, PR, van Bennekom, A., Corrosion 51, 911-921 (1995)). On the other hand, the presence of nitrogen is disruptive if an alloy with soft magnetic properties is desired (see, for example, "Ullmann's Encyclopedia of Industrial Chemistry” Fifth Edition, Volume A16, page 26, left column, 2nd section).
• carbon promotes the hardness of a steel, but on the other hand it is very strong
• the presence of carbon is also very troublesome when an alloy with soft magnetic properties is desired (see, for example, "Ullmann's Encyclopedia of Industrial Chemistry” Fifth Edition, Volume A16, page 26, left column, 2nd section).
• Ni-containing alloys typically 30 to 80 percent by weight
• a ferritic steel due to its property as an austenite former.
• allergic reactions to Ni-containing alloys have developed into a serious medical problem in industrialized countries. In Europe, for example, more than 20% of young women and 6% of young men suffer from a nickel allergy. This is important for the cases of wristwatches, since they lie directly on the skin.
• the two-dimensional structure diagram of the chrome-nickel steels allows a rough estimate of which structure (austenite, ⁇ -ferrite, martensite or mixtures thereof) depending on the Cr content (plotted on the x-axis in the diagram) and the Ni content (plotted on the y-axis in the diagram).
• This structure diagram can be expanded by taking additional elements into account; however, the additional elements are only taken into account summarily and as an estimate in the form of additional nickel or chromium equivalents.
• the Schaeffler diagram A Schaeffler: MS Thesis, Univ. Of Wisconsin, June 1944; AL Schaeffler, The Welding Journal 26/10, 601-620 (1947); AL Schaeffler, Metal Progress vol.
• a rudimentary estimate of the resistance of a Cr / Mo steel to pitting corrosion can also be obtained from a two-dimensional diagram (Grafen, H., Chem. Ing. Techn. 54, p. 108-119 (1982)).
• Y-axis dependency of the limit potential for the start of pitting corrosion
• X-axis Cr content
• the molybdenum content is determined in the form of chromium equivalents (ibid., And Lorenz, K., Medawar, G.,
• Thyssen Research 1, p. 97-108 (1969)) is also taken into account. An approximately linear correlation between the limit potential and the Cr (Mo) content is observed. This However, the diagram does not take any other alloy elements into account, and it does not allow any conclusions to be drawn as to whether it is a ferritic alloy or its machinability, polishability and magnetic properties.
• the effective sum WS which is defined as follows:
• Table 1 gives an overview of eight previously known concrete steels (indicated by their material numbers) and their contents of important alloying elements in percentages by weight. To the knowledge of the applicant, steel No. 1.4521 specified there is not watch steel.
• the object of the present invention is to provide a polishable ferritic steel which has soft magnetic properties, in which the risk of polishing errors is minimized, which has mechanical properties comparable to steel No. 1.4521 and which has the same or improved corrosion resistance as steel No. 1.4435 regarding pitting and crevice corrosion.
• the task is covered by a steel alloy, based on the alloy, at most 1.00 percent by weight silicon, 18.0 to 22.0 percent by weight chromium, 1.80 to 2.50 percent by weight molybdenum, 0.01 to 0.10 percent by weight nitrogen, at most 0.01 percent by weight titanium, at most 0.01 percent by weight niobium, at most 0.01 percent by weight aluminum and the remainder essentially iron.
• a steel alloy based on the alloy, at most 1.00 percent by weight silicon, 18.0 to 22.0 percent by weight chromium, 1.80 to 2.50 percent by weight molybdenum, 0.01 to 0.10 percent by weight nitrogen, at most 0.01 percent by weight titanium, at most 0.01 percent by weight niobium, at most 0.01 percent by weight aluminum and the remainder essentially iron.
• Preferred variants result from the dependent claims.
• the steel alloys according to the invention are soft magnetic CrMoN steel alloys.
• FIG. 1 shows current density-potential curves of a) a steel alloy according to the invention, and b) of a previously known steel alloy No. 1.4435. Measurement conditions: 3.2% NaCl, pH 4.0, 40 ° C.
• X-axis potential in mV against saturated calomel electrode (SCE) as reference electrode;
• Y axis the logarithm of the measured current density.
• SCE saturated calomel electrode
• the potential value given in the two figures is the limit potential at which pitting corrosion (strongly increasing anodic current) starts.
• the term “high-alloy” has the meaning customary in the art, ie it denotes a steel in which the alloy elements are present in a total of 5 percent by weight or more.
• the term “ferritic” has the meaning that at least 98 percent by volume, preferably at least 99.5 percent by volume and particularly preferably 100 percent by volume of the iron present in the alloys according to the invention is present as ferrite, the determination being carried out metallographically.
• soft magnetic is used in the context of the present application for steel alloys according to the invention which bring about at least the same magnetic shielding as soft iron.
• the metallic alloy elements chromium and molybdenum can be added to the alloys according to the invention by alloying suitable amounts of the pure elements into a pig iron or a raw steel by conventional methods.
• chromium is present in 18.0 to 22.0 percent by weight, preferably in 19.5 to 20.5 percent by weight and particularly preferably in about 20 percent by weight, based on the finished alloy.
• molybdenum is about 1.80 to about 2.50 percent by weight, preferably about 1.90 to 2.10 percent by weight and particularly preferably about 2 percent by weight. Percentages based on the finished alloy.
• Nitrogen can be added by melting the steel alloy in a nitrogen atmosphere, by blowing nitrogen into the melt or by metering the addition of high-nitrogen master alloys.
• the nitrogen content is about 0.01 to 0.10 percent by weight, based on the alloy, more preferably about 0.05 to about 0.10 percent by weight and particularly preferably about 0.05 percent by weight.
• Silicon can be present as SiO 2 (for example from the above deoxidation) in the alloy. Its content can be reduced by mechanically moving or shaking the molten steel under protective gas. This coagulates the Si0 2 and increases due to the lower density on the slag surface.
• the silicon content according to the invention is at most about 1 percent by weight, preferably about 0.7 to 0.9 percent by weight and more preferably about 0.8 percent by weight, based on the alloy.
• carbon is noticeably present as an admixture in the pig iron (4 to 4.5%) and can then, as is customary in the art, by adding oxygen or suitable amounts of iron oxides to the steel melt (conversion of the carbon to
• Carbon monoxide can be reduced practically as desired. According to the invention, it is preferably present in a maximum of 0.025 percent by weight, particularly preferably in a maximum of 0.01 percent by weight, based on the alloy.
• Sulfur comes from the smelting process (iron ore content of 'iron sulfides) and is mainly found in pig iron. mainly as manganese sulfide.
• the alloys according to the invention it is preferably present in amounts of at most about 0.03 percent by weight, more preferably in at most 0.002 percent by weight.
• Sulfur desulfurization can be achieved with sulfur, for example, mixtures of CaO and metallic magnesium.
• the sulfur content can be at the upper limit of 0.03 percent by weight, preferably about 0.015 to 0.03 percent by weight, based on the alloy (so-called IMA grades), for which a regulated addition of sulfur can be made.
• a melt metallurgy with the addition of Ca-Si powder can be used for the production of these embodiments, which converts the hard aluminum oxide inclusions into relatively soft mixed oxides of the CaSiAl type and forms finely divided manganese sulfides, by means of which the chip is broken during mechanical processing and thus the tool life is extended.
• a controlled addition of sulfur lowers the corrosion resistance of these embodiments of the steel alloy according to the invention only slightly.
• Niobium is according to the invention . present in at most about 0.01 percent by weight, preferably at most about 0.005 percent by weight, based on the finished alloy. This content can be achieved by ensuring that suitable scrap is used when the steel alloy according to the invention is melted (avoiding steels containing niobium).
• manganese is preferred in at most about 1.00 percent by weight, more preferably at most about 0.40 percent by weight based on the finished alloy.
• Phosphorus originally comes from apatite or other phosphate-containing minerals that were present in iron ore.
• phosphate can be reduced to iron phosphide (mainly Fe 2 P) and as such can be found in pig iron or later steel.
• the preferably low phosphorus content according to the invention of at most 0.04 percent by weight and preferably at most 0.02 percent by weight can be reduced in the production of the alloys according to the invention as is customary in the art, for example by adding CaO when smelting the ore, as a result of which the phosphate-containing minerals be separated in the slag.
• the aluminum content according to the invention of at most about 0.01 percent by weight, preferably at most about 0.005 percent by weight, can be achieved if the deoxidation required in the melting process does not take place with aluminum but with silicon or in the AOD or VOD process (see below).
• Nickel is preferably present in at most 0.10 percent by weight, more preferably in at most 0.05 percent by weight, based on the finished alloy.
• Excess carbon, silicon and phosphorus are preferably removed at the same time as usual in the art by freshening with the addition of gaseous oxygen (conversion into oxides) and addition of CaO. Excess oxygen can then be removed as usual by using the Fresh in the form of VOD (Vacuum Oxygen Decarburization) or AOD (Argon Oxygen Decarburization) is carried out (removal of the excess oxygen by degassing in a vacuum or by blowing with argon).
• VOD Vauum Oxygen Decarburization
• AOD Aral Oxygen Decarburization
• the setting of the titanium content according to the invention of at most about 0.01 percent by weight, preferably at most about 0.005 percent by weight, particularly preferably at most about 0.002 percent by weight, can be achieved by controlled use of scrap (avoidance of Ti-containing scrap, for example of the Ti-containing known in Europe) Steel No.1.4571). As a further measure, Ti contamination in the lining of the converters used during the melting process can be avoided.
• the term "balance essentially iron” is intended to mean that the remaining percentages by weight of the alloy according to one of Claims 1 to 7, i.e. the percentages by weight, which are not contributed by elements mentioned by name in the corresponding claim, come almost exclusively from iron (typically at least 90 percent by weight, preferably at least 95 percent by weight and particularly preferably at least 99 of the rest or more).
• the elements of the rest other than iron should be selected in such a quantity and quantity that the finished steel alloy is ferritic according to the invention.
• the alloys according to the invention can be produced by customary processes. Reference is made, for example, to Chapter 2 in the section “Steels” of "Ullmann's Encyklopadie der Technischen Chemie” 4th edition, Verlag Chemie, and to the literature cited therein.
• refreshments are preferably carried out in series using the AOD and VOD processes, the VOD refining also being able to serve for nitriding at the same time.
• annealing is preferably carried out at temperatures of about 800 to 900 ° C., more preferably at about 850 ° C. during the thermoforming process in order to punctually enrich individual structural components and thus to avoid associated formation of inhomogeneities.
• the so-called "soaking" of the hot-rolled slabs or extended preheating times before hot-rolling are suitable for this.
• the alloys according to the invention are preferably subjected to annealing at temperatures at 750 to 850 ° C., preferably about 800 ° C. for about 0.5 to 2 hours, and a subsequent water cooling. This takes place due to diffusion processes a concentration equalization of the chromium in the matrix takes place in the area of the finely dispersed chromium nitride particles. By optimizing the nitrogen content, however, the chromium nitride excretion can be largely suppressed.
• the steel alloys according to the invention can be polished reproducibly by means of the methods customary in the watch industry and would therefore be accepted as a raw material in the watch industry.
• the preferred in the present invention steel alloys, either dissolved or typically in the form of finely precipitated chromium nitrides, • senix in the groES of about 1 micron, is present and therefore does not have a negative impact on polishability.
• the steel alloys according to the invention typically have the following mechanical properties (sheet 6 mm thick, hot-rolled, annealed at 800 ° C. for 30 minutes, quenched in water):
• the alloys according to the invention are therefore comparable with the standard steel quality No. 1.4521.
• the content of associated oxides in the alloys according to the invention is correspondingly low. Due to the almost complete absence of titanium and niobium, the corresponding carbides are almost completely absent. On the other hand, the coordinated simultaneous addition of nitrogen together with the other alloy elements other than chromium means that no significant deposition of chromium carbides occurs and the alloy according to the invention is still ferritic despite the increased nitrogen content (austenite former).
• the overall degree of purity of the steel alloy according to the invention of non-metallic oxide or carbide inclusions is set so high that remelting according to the ESR method mentioned at the beginning is no longer necessary; however, the remelting can, if desired, be carried out in the steel alloys according to the invention.
• the steel alloys according to the invention are soft magnetic in the sense of the definition mentioned at the beginning.
• the alloys according to the invention can be used in the watchmaking industry for the production of magnetically shielding housing parts, for example for wristwatches or for other watches in which magnetic shielding of the movement is important.
• the steel alloys according to the invention in particular those of claim 7, are also suitable for the production of components for link bracelets.
• housing part encompasses the components normally used for the production of a watch housing, in particular a housing of a wristwatch, that is to say, for example, the housing base and the housing shell.
• housing part also includes the dial.
• housing part encompasses both the component as it occurs in the finished watch and any blank or semi-finished product thereof, which is further processed into the finished component by further processing with optional use of other materials or semi-finished products made from the alloy according to the invention or other materials become.
• Magnetically shielding watch housings according to the invention can consist of a housing base, a housing shell and a dial, all of which are made according to the invention Steel alloy are made.
• the steel alloys according to the invention can thus be used both as a material for the components and as a shielding cage against magnetic fields.
• the additional soft iron cage which is difficult to manufacture and which would have to be provided within the usual housing made of non-magnetic CrNi steel and which would lead to a higher height of the watch, can thus be omitted.
• the steel variant of the 1.4521 according to the invention is also outstandingly suitable for powder metallurgical production according to the MIM (Metal Injection Molding) process, in particular because the nitrogen content required according to the invention can be supplied without problems in the compacting process (sintering) under a nitrogen atmosphere.
• the MIM process is known per se in the technology of watchmaking.
• a steel alloy which contains the required elements in the final quantities (these would be the elements which are mentioned in one of claims 1 to 7 by name), but which at most is still deficient in nitrogen, is ground to powder and slurried with a liquid binder.
• This slurry is pressed, for example by means of an extruder, into a hollow mold, the hollow space of which has the shape of the housing part to be produced.
• the binder is then preferably evaporated off with a vacuum and the powder residue remaining in the hollow mold is sintered. If the alloy powder was deficient in nitrogen, a nitrogen atmosphere of a suitable pressure is created during the sintering step, so that the alloy still absorbs nitrogen during the sintering. Choosing the appropriate nitrogen pressure in the finished To achieve a nitrogen concentration in the housing part which is in accordance with the invention can be determined by series of tests.

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• 2003
  • 2003-03-30 US US10/529,325 patent/US20060130938A1/en not_active Abandoned
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