|Title of Invention||
COLD-FORMABLE CHROME STEEL
|Abstract||A cold-formable, corrosion-resistant chrome steel includes, by weight percent, 14% to 20% chromium, 0.005% to 0.05% carbon, up to 0.01 % nitrogen, 0.2% to 0.6% silicon, 0.3% to 1.0% manganese, 0.1% to 1.0% molybdenum, up to 0.8% nickel, 0.2% to 1.0% copper, 0.15% to 0.65% sulfur, as well as separately or in combination 0.01% to 0.1% lead, 0.01% to 0.5% bismuth, 0.01% to 0.1% arsenic, 0.01% to 0.1% antimony, 0.005% to 0.08% of each of vanadium, titanium, niobium, and zirconium, 0.02% to 0.2% of each of selenium and tellurium, the remainder iron and incidental smelting-related impurities.|
|Full Text||COLD-FORMABLE CHROME STEEL CROSS-REFERENCES TO RELATED APPLICATIONS
 This application claims the priority of German Patent Applications, Serial Nos. 10 2004 015 992.0-24, filed April 1, 2004, and 10 2004 063 161.1, filed December 29, 2004, pursuant to 35 U.S.C. 119(a)-(d).
BACKGROUND OF THE INVENTION
 The present invention relates to a cold-formable chrome steel with a ferritic structure.
 Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
 Without the implementation of special alloying procedures, cold- formable and corrosion-resistant ferritic chrome steels have poor machining properties, mostly due to sticking and welding that occurs during machining in the region of sharp tool edges. The cutting edge can then become jagged and can splinter, the tool may wear poorly, and the surface quality of the machined workpieces may be poor.
 Sticking and welding may also be detrimental when using stamping and forming tools, because these processes occur predominantly in the region of high surface pressure, thus diminishing the surface quality of the machined workpieces and shortening the service life of the tools. In addition to an adequate machining and processing ability, the steels should also have a certain minimum rigidity that is only achievable by incorporating in the alloy certain additives that, like titanium, vanadium, niobium, zirconium, and molybdenum, form carbides and carbo-nitrides. These are present in the structure as hard precipitate phases with a low solubility and tend to build up locally in the structure, forming agglomerates, clusters or cellular structures.
 This increases the risk that during micro-machining, for example when drilling bore holes, grooves and recesses with small to extremely small dimensions, the tool, for example a drill, runs off center, caused by the local concentration of hard precipitate phases, thus causing substantial deviations in the final dimensions. This is caused by the fact that the machining tools, for example a small diameter drill, tend to migrate away from areas with greater hardness or greater carbide concentration. Even the use of micro-tools or drills made of high-grade hard metals, for example with a diameter of less than 0.8 mm, cannot prevent tool runoff, because the tool is diverted from the predetermined machining direction by regions of high concentration of structural carbide components.
 Steels of the afore-described type are known in the art. They have excellent magnetizability, like the soft-magnetic chrome steel described in U.S. patent 4,714,542, which includes up to 0.03% carbon, up to 0.40 to 1.10% silicon, up to 0.50% manganese, 9.0 to 19% chromium, up to 2.5% molybdenum, up to 0.5% nickel, up to 0.5% copper, 0.02 o 0.25% titanium, 0.010 to 0.030% sulfur, up to 0.03% nitrogen, 0.31 to 0.60% aluminum, 0.10 to 0.30% lead, and 0.02 to 0.10% zirconium. The steel is rust-free and cold-formable, and can be employed in the fabrication of cores for solenoid valves, electromagnetic couplings or housings for electronic injection systems for internal combustion engines.
 Another soft-magnetic rust-free chrome steel with up to 0.05% carbon,
up to 6% silicon, 11 to 20% chromium, up to 5% aluminum, 0.03 to 0.40% lead, 0.001 to 0.009% calcium, and 0.01 to 0.30% tellurium is disclosed in U.S. Pat. No. 3,925,063. This steel can be easily machined due to the presence of lead, calcium and tellurium.
However, the relatively high silicon, aluminum and titanium content in the steel produces hard oxide inclusions which causes severe wear during precision machining. A relatively high lead concentration of 0.03 to 0.40% is incorporated to neutralize this effect. Disadvantageously however, lead has a very low melting point and therefore does not form stable compounds or precipitates. Lead also has an extremely inhomogeneous distribution in the structure.
 The German laid-open application 101 43 390 A1 describes a con¬formable corrosion-resistant ferritic chrome steel with the 0.005% to 0.01% carbon, 0.2% to 1.2% silicon, 0.4% to 2.0% manganese, 8% to 20% chromium, 0.1% to 1.2% molybdenum, 0.01% to 0.5% nickel, 0.5% to 2.0% copper, 0.001% to 0.6% bismuth, 0.002% to 0.1% vanadium, 0.002% to 0.1% titanium, 0.002% to 0.1% niobium, 0.15% to 0.8% sulfur, and 0.001% to 0.08% nitrogen, remainder iron, including smelting-related impurities. This chrome steel, due to its excellent machinability, in particular its excellent metal-cutting properties, excellent wear resistance and surface quality, is a suitable material for precision-mechanical applications and precision devices, in particular for spinnerets and spray nozzles, as well as for writing utensils, jewel stylus and print heads.
 It would therefore be desirable and advantageous to produce a ferritic chrome steel that can not only be cut without causing sticking and welding, but which can also be micro-machined with a precisely maintained directional accuracy.
SUMMARY OF THE INVENTION
 According to one aspect of the present invention, a. chrome steel alloy according includes by weight percent 14% to 20% chromium, 0.005% to 0.05% carbon, up to 0.01% nitrogen, 0.2% to 0.6% silicon, 0.3% to 1.0% manganese, 0.1% to 1.0% molybdenum, up to 0.8% nickel, 0.2% to 1.0% copper, 0.02% to 0.2% selenium, and further at least one of 0.01% to 0.1% lead, 0.01% to 0.5% bismuth, 0.01% to 0.1% arsenic, 0.01% to 0.1% antimony, 0.005% to 0.08% vanadium, 0.005% to 0.08% titanium, 0.005% to 0.08% niobium, 0.005% to 0.08% zirconium, 0.15% to 0.65% sulfur, up to 0.20% tellurium, the remainder iron and incidental smelting-related impurities.
 According to one advantageous composition, the chrome steel alloy may include by weight percent 14% to 18% chromium, 0.01% to 0.03% carbon, up to 0.01% nitrogen, 0.03% to 0.5% silicon, 0.4% to 0.7% manganese, 0.1% to 0.6% molybdenum, up to 0.5% nickel, 0.2% to 0.6% copper, 0.02% to 0.2% selenium, and further at least one of 0.01% to 0.05% lead, 0.01% to 0.3% bismuth, 0.01% to 0.05% arsenic, 0.01% to 0.05% antimony, 0.005% to 0.08% vanadium, 0.005% to 0.08% titanium, 0.005% to 0.08% niobium, 0.005% to 0.08% zirconium, 0.15% to 0.65% sulfur, 0.01% to 0.2% tellurium, the remainder iron and incidental smelting-related impurities.
 The material properties can be optimized, if the composition of the steel alloy satisfies at least one of the following conditions:
 K1 = (%Ti + %V + %Nb + %Zr) / (%C) = 3 to 12
nozzles, tips for writing implements, injection nozzles for chemical and electronic devices, spinnerets, as well as other articles of small dimensions and/or recesses, in particular bore holes.
BRIEF DESCRIPTION OF THE DRAWING
 Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which:
 FIG. 1 shows the heat of formation of exemplary metal sulfides and selenides;
 FIG. 2 shows a schematic diagram of the concentration of an element in a precipitate;
FIG. 3 shows a schematic diagram of the concentration of an element and a corresponding lubricant zone;
 FIG. 4 shows a micrograph of a chrome steel alloy with precipitates;
 FIG. 5 shows schematically temperature curves during annealing and cool-down;
 FIG. 6 illustrates probing of a drilled hole with a test pin;
 FIG. 7 shows a micrograph of a smooth bore hole; and
 FIG. 8 shows a micrograph of a bore hole with jagged edges. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 Throughout all the Figures, same or corresponding elements are generally indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the drawings are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted.
 The mechanical properties of the steel of the invention are significantly affected not only by the presence of certain precipitate phases, but even more so by their physical properties and distribution in the structure. The structure therefore includes metal sulfides as well as metal selenides, which in turn interact with carbides and thio-carbides to improve the chip breaking characteristic. With the invention, certain alloy elements are set free in the region near the precipitates by rearrangement and exchange interactions so as to surround the hard precipitates with a lubricant zone of consisting of metals and/or metal compounds which then act as lubricant zones and improve the machining properties.
 Precipitates of sulfides, selenides or tellurides or mixtures thereof, but also precipitates resulting from rearrangement or exchange reactions with carbides, are produced at different temperatures in the solid phase of the steel alloy. When the melt cools down, so-called primary precipitates are formed which subsequently grow and coarsen. According to the invention, certain elements, such as lead and/or bismuth and/or arsenic and/or antimony and/or vanadium, titanium, niobium, as well as zirconium, are combined with the precipitate formers carbon, nitrogen, sulfur, selenium and tellurium, producing a large number of possible reactions that can prevent the detrimental growth of these primary precipitates.
 Turning now to the drawing, and in particular to FIG. 1, there is shown a diagram with exemplary heat of formation values for important sulfides and selenides which are significant for the invention. Precipitates are formed only if the thermodynamic conditions are favorable, with the heat of formation being an important predictor. Because all these metal compounds have a negative heat of formation, thermodynamically stable precipitates can form. A more negative heat of formation of a certain precipitate indicates that this precipitate is more likely to form.
In the steel alloy of the invention, the non-metallic precipitate formers carbon, sulfur, selenium, tellurium and optionally nitrogen, are only present in low concentrations so as to prevent supersaturation, because otherwise rapidly growing coarse precipitates could form, which would be difficult to reduce in grain size or completely dissolve. A low carbon concentration appears to be of particular significance for moving the reaction equilibrium to promote formation of sub-stoichiometric carbides.
 Because the precipitates mainly form during cooling, diffusion effects (solid state diffusion in steel alloys) play an important role during the formation and growth of the precipitates. In general, elements with a small atomic mass diffuse more easily and faster than heavy atoms. Carbide and nitride precipitates, also referred to as so-called primary precipitates, are therefore readily generated in steel alloys. Sulfides and/or selenides and other precipitates, such as thio-carbides and thio-carbo-selenides, are only formed after precipitation of the primary precipitates.
 Sub-stoichiometric carbon-deficient primary carbides can be produced due to the low carbon concentration. This carbon deficiency is compensated through diffusion of carbon only after an extended period of time; carbon can also be partially replaced by sulfur or selenium.
 The sub-stoichiometric primary carbides are produced, for example, according to the equation
Me1 + xC -> Me1Cx (1)
wherein Me1 refers to the elements titanium, vanadium, niobium and zirconium, and x is the stoichiometric factor. However, these elements can also react with nitrogen, sulfur and selenium (tellurium), forming thio-carbides, thio-selenides or thio-carbo-selenides. Sub-stoichiometric precipitates therefore remain active after these compounds have been formed.
 The composition of the primary carbides (or primary precipitates) of the Me1-metals can vary over a wide range without adversely affecting the lattice structure of the precipitates. It is known from published references that, for example, titanium carbide forms stable alloys over a wide range from TiCo.22 to TiCi.o- For example, for a stoichiometric factor of for example x = 0.5, the equation 1 for titanium could be written as:
Ti + 0.5 C TiCo.s (1a)
 Due to their position in the periodic system, sulfur, selenium and also tellurium show similar reactions, which is also evident from the thermodynamic numbers listed in Table I. The elements copper, lead, arsenic, antimony and manganese are important for forming precipitates by reacting with sulfur, selenium and tellurium; they have to be differentiated from the Me1-metals and will subsequently be referred to as Me"-metals.
 Typical reaction equations with sulfur and selenium include: Me" + S —► Me"S (2)
Me" + Se —► Me"Se (3)
 Unlike the Me1 elements, they do not form carbides, carbo-nitrides or thio-carbides.
 All precipitates typically form so-called depletion zones in their immediate vicinity, which are produced when from the matrix those elements are removed by diffusion that are required for producing a precipitate and incorporated in the precipitate. This results in a concentration dependence of the elements depicted in the diagrams of FIGS. 2 and 3.
 FIG. 2 shows schematically the spatial distribution of the concentration of an element in a precipitate 1. The element has an average concentration c'M in the matrix which increases to a concentration c'A in the precipitate. A depletion zone with width D forms around the precipitate, which itself has a size R. FIG. 3 shows again the concentration c1 of an element in a precipitate, wherein this time the precipitate is surrounded by a lubricant zone 2 with a concentration c2 of the lubricant.
Because these depletion zones hinder the desired rearrangement and exchange reactions between the precipitates, the invention recommends specific measures for minimizing the depletion zones. These measures include, in combination, cold-forming and heat treatment which cause rearrangement and exchange reactions between primary and secondary precipitates.
 Already generated precipitates are then dissolved and new precipitates are formed; however, copper can also be set free that acts in the vicinity of the primary precipitates as a lubricant. Because rearrangement reactions take place predominantly during the cooling cycle, the precipitates are necessarily very fine-grained. Sufficient time should be allocated for rearrangement reactions, because the material transport that plays a role in the rearrangement reactions occurs by diffusion. Advantageously, a slow cool-down and/or soaking times at 700 to 500 °C and/or a subsequent heat treatment can be implemented.
 The rearrangement and exchange reactions between sub- stoichiometric carbide Me1-precipitates and one or more sulfide and/or selenides precipitates presumably take place by release of the elements.
 An exemplary reaction of a sub-stoichiometric precipitate with a sulfide (in this case copper sulfide) could be written for TiC0.5 as:
4 TiCo.5 + 2 CuS Ti4C2S2 + 2 Cu (4)
 Because the sulfur of the copper sulfide reaches the lattice of the thio-carbide (Ti4C2S2) through diffusion, copper is released that precipitates in the immediate vicinity of the hard titanium carbo-sulfide precipitate. The released elements, in this case copper, acts as a lubricant during machining. Similar reactions also take place between the other Me1 precipitates and Me"-sulfides or -selenides (for example, with precipitates of manganese and lead).
 Dissolution reactions according to equation 4 are important, because they advantageously dissolve or etch coarse or linearly arrayed Me"- sulfides (for example manganese sulfide), forming new, extremely fine microscopic precipitates according to equation 4. The chrome steel of the invention therefore has a structure with a large number of fine precipitates (FIG. 4).
 Advantageously, according to the afore-described reaction equations, the following conditions should exist to facilitate sufficiently fast and unconstrained re-dissolution and release reactions:
The length of the diffusion paths between the different precipitates should be as short as possible to achieve fast reaction times; The number and/or size of the depletion zones in the regions near the precipitates should be reduced to enhance the reactivity of the precipitates;
The effect of the reaction temperatures and reaction times should be adjusted so that the reactions, for example according to equation 3, occur over a short time.
 According to the invention, the steel should therefore be initially subjected to one or more severe deformations to introduce dislocations and to better mix the components of the structure. At the same time, the separation between the precipitates is advantageously changed and the size of the depletion zones is reduced. The severe deformations also shorten the diffusion paths, which again significantly increases the reactivity.
 In order to enable the re-dissolution and release reactions to take place with sufficient speed, the preferably cold-formed steel is annealed at temperatures from Ti = 750 °C to T2 = 1080 °C (see FIG. 5). In this range, the re-dissolution and release reactions take place under formation of new precipitates, possibly having a new composition, similar to the reaction described in equation 4 above. According to the invention, a final annealing step can be performed at temperatures not exceeding 450 °C in order to solidify released lubricant metals or newly formed very fine precipitates, to harden in the steel matrix, to reduce strain, and to adjust the hardness or stability of the steel alloy. The hardness can progressively decrease during the final annealing step, if the temperature is above approximately 350 °C, which suggests a loss of cohesion of the matrix.
 Preferably, after at least one cold-forming step with a deformation of more than 65%, the steel is annealed for 30 to 60 minutes at a temperature of 750 °C to 1080 °C (curve 3) and thereafter controllably cooled down for 30 to 180 minutes to a temperature T2 from 500 °C to 700 °C, while supplying a small amount of energy (FIG. 5). The precipitates produced during the annealing are thereby stabilized by controlled diffusion. Advantageously, the steel is held steady at least once at one or more intermediate temperatures of, for example, 680 °C during the cooling step by briefly supplying more heat (FIG. 5, equation 4).
The invention will now be described in more detail with reference to certain illustrated embodiments.
 Table I lists the composition of four exemplary alloys E1 to E4 according to the invention and of eight comparative alloys V1 to V8. Table II lists the corresponding K1, K2, and K3 values as well as the results of the machining tests. BV represents a characteristic value for the drilling path, BG for the burr width, and BWG a characteristic value for the surface quality.
 After an etching step, a bare wire having the composition E2 with a diameter of 6 mm was initially subjected to a 3-stage cold-forming process producing a total deformation of 85%. The wire was then annealed in an inert gas atmosphere for 30 minutes at a temperature T-i = 840 °C (see FIG. 5, curve 3) and thereafter controllably cooled down over 120 minutes to a temperature of T2 = 600 °C. During the cool-down step, an-intermediate 15 minute intermediate heating step was applied twice at respective temperatures of 760 °C and 680 °C, while maintaining a constant temperature, to attain a stepped cool-down for stabilizing the precipitates (see FIG. 5, curve 4a).
 After the controlled cool-down, the wire was cooled in air (see FIG. 5, upper curve 5) without supplying additional energy and thereafter sized, which resulted in a deformation of 15%. Sizing was followed by a 15 minute final annealing or tempering at 340 °C. The wire had an excellent machinability with micro-tools.
connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.
 What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims and includes equivalents of the elements recited therein:
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What is claimed is:
1. A chrome steel alloy having a composition comprising, by weight percent, 14% to 20% chromium, 0.005% to 0.05% carbon, up to 0.01% nitrogen, 0.2% to 0.6% silicon, 0.3% to 1.0% manganese, 0.1% to 1.0% molybdenum, up to 0.8% nickel, 0.2% to 1.0% copper, 0.02% to 0.2% selenium and in addition, separately or in combination 0.01% to 0.1% lead, 0.01% to 0.5% bismuth, 0.01% to 0.1% arsenic, 0.01% to 0.1% antimony, 0.005% to 0.08% vanadium, 0.005% to 0.08% titanium, 0.005% to 0.08% niobium, 0.005% to 0.08% zirconium, 0.15% to 0.65% sulfur,
up to 0.20% tellurium,
the remainder iron and incidental smelting-related impurities.
2. The chrome steel alloy of claim 1, having, by weight percent, 14% to 18% chromium, 0.01% to 0.03% carbon, up to 0.01% nitrogen, 0.03% to 0.5% silicon, 0.4% to 0.7% manganese, 0.1% to 0.6% molybdenum, up to 0.5% nickel, 0.2% to 0.6% copper, 0.02% to 0.2% selenium, and in addition, separately or in combination 0.01% to 0.05% lead, 0.01% to 0.3% bismuth, 0.01% to 0.05% arsenic, 0.01% to 0.05% antimony, 0.005% to 0.08% vanadium, 0.005% to 0.08% titanium, 0.005% to 0.08% niobium, 0.005% to 0.08% zirconium, 0.15% to 0.65% sulfur,
0.01% to 0.2% tellurium,
the remainder iron and incidental smelting-related impurities.
3. The chrome steel alloy of claim 1, satisfying the following condition
K1 = (%Ti + %V + %Nb + %Zr) / %C = 3 to 12.
4. The chrome steel alloy of claim 1, satisfying the following condition
K2 = (%S + 3%Se + 3%Te) / 10 (%C + + %N) = 1.5 to 3.5.
5. The chrome steel alloy of claim 1, satisfying the following condition
K3 = %S / (%S+ % Se + %Te) = 0.68 to 0.98.
6. A method of thermal treatment of a cold-formed chrome steel alloy according to claim 1, comprising the steps of:
cold-forming the steel at least once for a total deformation from 65% to 90%; and
annealing the steel for 30 to 60 minutes at a temperature from 750 to 1080°C.
7. The method of claim 6, further comprising the step of cooling the steel from the annealing temperature down to a temperature of 700 °C at small energy supply over a period of 30 to 180 minutes.
8. The method of claim 7, wherein the steel is cooled down to a temperature of 500 °C.
9. The method of claim 7, further comprising the step of maintaining the temperature of the steel during the cool-down at least once for 10 to 30 minutes at an approximately constant temperature value.
10. The method of claim 9, wherein the steel is maintained at a constant temperature of 680 °C.
11. The method of claim 7, wherein in a final processing step, the steel is heated up to a maximum temperature of 450 °C for at least 30 minutes.
12. A method of using a chrome steel alloy according to claim 1 for producing an article to be machined with a cutting tool.
13. The method of claim 12, wherein the cutting tool includes a micro-cutting tool.
14. A method of using a chrome steel alloy according to claim 1 for producing an article selected from the group consisting of printer nozzles, writing stylus, injection nozzles for chemical and electronic devices, spinnerets, and articles of small size with or without recesses.
15. An article for industrial use having a feature size of 0.6 mm or less and being made from a chromium steel alloy having a composition according to
|Indian Patent Application Number||140/CHE/2005|
|PG Journal Number||44/2012|
|Date of Filing||18-Feb-2005|
|Name of Patentee||STAHLWERK ERGSTE WESTIG GMBH|
|Applicant Address||LETMATHER STRASSE 69,58212 SCHWERTE|
|PCT International Classification Number||C22C38/54|
|PCT International Application Number||N/A|
|PCT International Filing date|