Patent Application
20080318083
The present invention relates to an austenitic steel and to a method for producing the same and to the use of the steel.
The strength of austenitic steels is particularly enhanced by interstitially dissolved atoms of the elements carbon and nitrogen. To dissolve the volatile element nitrogen in the melt, chromium and manganese are above all added to the alloy for reducing nitrogen activity. While chromium alone prompts the formation of ferrite, an austenitic structure can be adjusted with manganese by solution annealing and can be stabilized by quenching in water up to room temperature. The influence of carbon and nitrogen is illustrated by way of an iron alloy having 18% by mass of chromium and 18% by mass of manganese in
As can be seen from
As shown in
Different approaches are known for overcoming this limit. One approach refers to the simultaneous use of chromium and manganese, (Cr+Mn) approach. The content of the solubility-promoting elements chromium and manganese is here raised to such an extent that up to 1% by mass of nitrogen can be dissolved under atmospheric pressure in the melt and in the austenite. Reference is here made to steel A in the subsequent Table 1. To avoid nitride precipitations, the solution annealing temperature must be raised to about 1150° C. A further drawback is the limitation of the forging temperature range and the risk of edge cracks during hot forming.
Another approach comprises the simultaneous addition of carbon and nitrogen, (C+N) approach, as is e.g. indicated in B. D. Shanina, V. G. Gavriljuk, H. Berns, F. Schmalt: Steel research 73 (2002)3, pages 105-113. The increase in the concentration of free electrodes in the austenite lattice by simultaneous dissolution of carbon and nitrogen is here exploited. This stabilizes the austenite, i.e. the range of solubility is increased for interstitial elements. Since the nitrogen is partly replaced by carbon, its outgassing from the melt can be avoided in the case of a reduced chromium and manganese content as is required according to the (Cr+Mn) approach. So far a CrMn steel with a (C+N) content of about 0.8% by mass has been molten according to the (C+N) approach under atmospheric pressure; cf. steel B of the subsequent Table 1. Steels C and D according to the following Table 1 must also be assigned to this group.
Among the open-melted steels having a high interstitial content it is not possible to find CrNi steels because nickel, just like silicon, reduces the solubility for carbon and nitrogen. The Rp0.2 yield strength of the standard steel of this group X5CrNi18-10 is about 220 MPa. The known chromium-manganese steels achieve more than twice the value. In addition they have a high true break strength R, which is due to a strong work hardening with a correspondingly large uniform elongation Ag. This work hardening ability is also the reason for the high wear resistance of said high-strength austenitic steels.
Further known corrosion-resistant austenitic steels shall briefly be mentioned in the following:
A known chromium-manganese steel is e.g. described in CH 202283. The chromium-manganese steel comprises 0.01-1.5% carbon, 5-25% chromium and 10-35% manganese, and a nitrogen content of 0.07-0.7%. However, it becomes apparent from the enclosed table that according to this disclosure both carbon and nitrogen are rather used in the lower range of the indicated amount and that adequately good results are already achieved thereby.
Furthermore, U.S. Pat. No. 4,493,733 discloses a corrosion-resistant non-magnetic steel comprising 0.4% or less of carbon, 0.3-1% nitrogen, 12-20% chromium, 13-25% manganese and less than 2% silicon. Furthermore, the steel according to the indicated composition may contain up to 5% molybdenum. In this instance, too, it becomes particularly apparent from the table that a carbon content that is as low as possible is preferred for achieving good properties of the finished steel.
A further austenitic corrosion-resistant alloy is known from EP 0875591, said alloy being particularly used for articles and components that get into contact with living beings at least in part. The alloy comprises 11-24% by wt. of Cr, 5-26% by wt. of Mn, 2.5-6% by wt. of Mo, 0.1-0.9% by wt. of C, and 0.2-2% by wt. of N. Special emphasis is placed on increased carbon contents and is based on the finding that carbon in solid solution enhances the resistance to crevice corrosion of austenitic stainless steels in acid chloride solutions.
Furthermore, DE 19513407 refers to the use of an austenitic steel alloy for articles compatible with the skin, the steel alloy comprising up to 0.3% by mass of carbon, 2-26% by mass of manganese, 11-24% by mass of chromium, more than 2.5-5% by mass of molybdenum, and more than 0.55-1.2% by mass of nitrogen, the balance being iron and unavoidable impurities. It is here stated with respect to the carbon amount that even slightly increased carbon contents adversely affect the resistance to corrosion or to stress corrosion cracking, and the carbon content should therefore be as small as possible, preferably less than 0.1% by mass.
It is the object of the present invention to provide a corrosion-resistant austenitic steel that is characterized by high resistance to corrosion and by particularly high strength and wear resistance.
This object is achieved by a stainless austenitic steel having the following composition, in % by mass: 16-21% chromium, 16-21% manganese, 0.5-2.0% molybdenum, a total of 0.80-1.1% carbon and nitrogen, and having a carbon/nitrogen ratio of 0.5-1.1, the balance being iron, and a total content of ≦2.5% of impurities caused by the melting process.
The steel according to the invention is distinguished by a particularly high strength and good corrosion resistance in very different environments and thus offers a great number of possible applications. Moreover, the steel can be produced at low costs, so that it is suited for very different uses, particularly also for applications where corresponding steels have so far not been used for reasons of costs.
The steel of the invention starts from the (C+N) approach, but extends said approach. For instance, the interstitial alloy content of the homogeneous austenite is set to 0.80-1.1% by mass of carbon and nitrogen to achieve a high degree of yield strength, break strength and wear resistance. According to the invention the carbon/nitrogen mass ratio is set to a range between 0.5 and 1.1 to permit melting of the steel under normal atmospheric pressure of about one bar and its hot forming within a wide temperature range of the homogeneous austenite.
In contrast to the known prior art, it is possible to dissolve a high interstitial content with open melting in the steel by observing a carbon/nitrogen ratio, thereby achieving excellent strength characteristics without the need for limiting the forging range or for raising the substituted alloy content, as is the case with steels that are melted under atmospheric pressure and are given a high strength solely by nitrogen. In addition, the drawback of a low resistance to corrosion of CrMn steels, as compared with CrNi steels, is already compensated by a small Mo addition which in combination with N ensures the resistance to corrosion as is required for the intended use.
According to a preferred embodiment of the invention the total content of carbon and nitrogen is 0.80-0.95% by mass. In other embodiments a total content of carbon and nitrogen of 0.95-1.1% by mass has turned out to be useful. Thanks to the adjustment of the total content of carbon and nitrogen, the yield strength can directly be varied and the composition of the steel can thus be adapted to the desired use.
According to a further preferred embodiment the content of molybdenum is 0.5-1.2% by mass. Workpieces made from a steel having the indicated molybdenum content have turned out to be particularly suited for an application in which the workpieces are subject to atmospheric corrosion.
Advantageously, the molybdenum content may amount to more than 1.2-2.0% by mass. A corresponding molybdenum content is particularly suited for workpieces made from steel, which during use are exposed to corrosion by halide ions.
According to a further preferred embodiment, it may be that the content of nickel as an impurity caused by the melting process is less than 0.2% by mass. Ac correspondingly produced steel can particularly be used for workpieces which are temporarily in contact with the human body.
Advantageously, the corrosion-resistant austenitic steel can be subjected to open melting, i.e. under normal atmospheric pressure of about 1 bar. Thanks to this open melting the production costs are inter alia reduced considerably.
According to a further preferred embodiment the 0.2 yield strength after the dissolution process can exceed 450 MPa and in another embodiment it can exceed 550 MPa. Hence, the steel can be adapted through the selected composition to the properties demanded for the desired future use.
Advantageously, the steel of the invention can be used for producing high-strength, stainless, wear-resistant and/or non-magnetizable workpieces.
Furthermore, the present invention provides a method for producing a corrosion-resistant austenitic steel having the above-mentioned composition, by melting under atmospheric pressure of about 1 bar and subsequent shaping.
Since the steel can be produced and processed in conventional method steps, no additional apparatus is here needed for producing the steel of the invention.
Advantageously, the shaping process is selected from the group consisting of casting, powder metallurgy, forming and welding. It becomes apparent that the most different shaping processes can be used for giving the steel the desired shape, so that it is here also possible to form the most different workpieces.
Advantageously, the steel can be applied as a layer onto a metallic substrate.
Furthermore, the present invention relates to the use of the steel of the invention as wear-resistant workpieces for obtaining and processing mineral articles and for using them up in building.
According to a further embodiment the steel may be used for non-magnetizable cap rings, which can be work-hardened, in electric generators.
Advantageously, the steel of the invention can be used for non-magnetizable rolling bearings that can be work-hardened and used in the vicinity of strong magnetic fields.
According to a further advantageous embodiment the steel of the invention can be used for non-magnetizable frames or mounts of strong magnetic coils for absorbing the mechanical forces.
According to a still further embodiment, the inventive steel can be used by virtue of its high plastic forming capacity for components that consume the arising impact energy by plastic deformation. Corresponding components are particularly suited for use during collision of vehicles.
A preferred embodiment of the present invention will now be explained in more detail with reference to a drawing, in which:
a is a calculated phase diagram for a known steel having 18% by mass of Cr and 18% by mass of Mn, which is alloyed with carbon;
b is a calculated phase diagram for a known steel having 18% by mass of Cr and 18% by mass of Mn, which is alloyed with nitrogen;
a is a calculated phase diagram for a steel of the invention having 18% by mass of Cr and 18% by mass of Mn and also carbon and nitrogen, the carbon/nitrogen ratio being 1,
b is a calculated phase diagram for a steel of the invention having 18% by mass of Cr and 18% by mass of Mn, and also carbon and nitrogen, the carbon/nitrogen ratio being 0.7.
When the substituted alloying content is 16-21% by mass for chromium and for manganese, the necessary solubility for nitrogen is achieved and the austenite is stabilized. With 0.5-2% by mass of molybdenum the corrosion resistance (particularly to pitting corrosion by chloride ions) is improved, said resistance being normally lower for CrMn austenite than for CrNi austenite. A synergistic effect of N+Mo is here exploited, which yields a noticeable improvement already at 0.5% by mass of Mo. Molybdenum contents of more than 2% by mass narrow the forging range again and are therefore excluded.
The chemical composition of two variants I and II of the steel of the invention is shown in the following Table 2. Its fusion and casting into blocks is carried out in the open in air under atmospheric pressure of about 1 bar. The blocks were rolled in heat into steel bars without the occurrence of cracks or other flaws. The further hot forming by forging to smaller sample dimensions also took place without any flaws.
The further steels indicated in Table 2 are conventionally obtainable steels, i.e. steel E is a manganese hard steel X120Mn12 which is not resistant to corrosion, and steel 11 is a stainless CrNi steel X5CrN18-10.
The mechanical properties determined in the tensile test according to DIN EN 100021 at room temperature for the two steels of the invention shown in Table 2 are illustrated in Table 3 and are compared with those of the stainless austenitic standard steel (F)=X5CrN18-10 and of the wear-resistant manganese hard steel (E)=X120Mn12 which is austenitic but not corrosion-resistant. Steel B is a weakly corrosion-resistant test alloy. Variants I and II according to the invention are clearly superior to the comparative steels in terms of yield strength and tensile strength.
Variants I and II also remain non-magnetizable after plastic deformation in the impact wear test, which is expressed in the low relative magnetic permeability μrel=1.0012, which was measured with a commercially available permeability sensor provided for this purpose on the impact wear surface. For the manganese hard steel E, μrel=1.0025. The stainless standard steel achieves μrel=1.1 due to the formation of deformation martensite and is thus weakly magnetizable.
In the permanent immersion test according to DIN 50905 Parts 1 and 2, variants I and II of the invention were not attacked in an aqueous solution with 1% by mass of H2SO3 at pH=2 and room temperature for 120 h. Acid mine water in a mine was imitated with the test solution. By contrast, the manganese hard steel E that had so far been used showed a clear mass loss by corrosion, as follows from Table 4. Although the stainless standard steel F turns out to be resistant, it is not suited for operational use due to its low resistance to wear. The break-through potentials for beginning crevice corrosion according to Table 4 follow from the determination of current density-potential curves according to DIN 50918 in aqueous solution with 3% by mass of NaCl. They suggest that the resistance of variants I and II of the invention is superior to that of the standard steel in seawater.
Thanks to the expansion of the C+N approach the steel of the invention can be produced at low costs, i.e. open melting without pressure or powder metallurgy, and achieves an excellent combination of mechanical, chemical, tribological and physical properties. This yields, in particular, the following examples of use for the steel according to the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2004 043 134.5 | Sep 2004 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP05/08960 | 8/18/2005 | WO | 00 | 9/8/2008 |
