Sulphur treatment of magnesium-contaminated Fe-Cr-Al alloy for improved oxide whisker growth

Abstract: A method for producing aluminium-containing ferritic stainless steel from a magnesium-contaminated melt is disclosed, such that the steel is suitable for growing oxide whiskers on a surface thereof, so as to cover said surface. The method comprises adding sulphur to the contaminated melt in an amount sufficient to increase the sulphur content to not less than 1.3 times the magnesium content, whereupon the sulphur reacts with the magnesium to render the magnesium inert and thus prevent it from inhibiting oxide whisker growth. The method also preferably comprises an addition of titanium, zirconium or hafnium in amounts sufficient to react with any residual sulphur present in the melt in excess of the amount required for complete stoichiometric reaction with the magnesium present in the melt, thereby mitigating the adverse effect of free sulphur in the steel on oxide adhesion to the steel.

Description: This invention relates to formulation of an aluminium-containing ferritic stainless steel that is oxidizable to produce a protective surface layer characterized by multitudinous oxide whisker formations. More particularly, this invention relates to formulation of such a steel from a magnesium-contaminated melt as specified in the preamble of claim 1, for example as disclosed in U.S. patent No.4,588,449. Aluminium-containing ferritic stainless steel is particularly useful for high-temperature applications, for example, as a substrate in an automotive catalytic converter. A typical steel comprises about 15 to 25 weight percent chromium, about 3 to 6 weight percent aluminium and the balance mainly iron. When exposed to oxygen at elevated temperatures, the steel forms a surface alumina layer that protects the underlying metal against further corrosion. The alloy may contain a minor addition of yttrium or rare earth metal, such as cerium or lanthanum, to promote oxide adhesion and thereby improve high temperature corrosion resistance. It is also known to add titanium, zirconium or hafnium to refine grain size, improve workability, counter-act undesirably high carbon content and increase high-temperature strength. In contrast to the relatively smooth surface of the oxide layer that typically is formed on such a stainless steel, it is known to oxidize the steel surface under conditions that produce a layer comprising oxide whiskers of a configuration suitable to promote the bonding thereof to an applied coating. U.S. Patent Nos. 4,331,631 and 4,318,828 describe oxidation treatments of foil formed of such a iron-chromium-aluminium alloy that produce oxide whiskers that substantially cover the foil surface. It has been found that the presence of magnesium impurity in amounts as little as 0.002 weight percent noticeably inhibits growth of the desired whiskers. U.S. Patent No. 4,588,449 describes a treatment for growing oxide whiskers on magnesium-contaminated foil that comprises sustained heating to sublime the magnesium present. Whilst this treatment has been generally successful for reclaiming contaminated foil for oxide whisker growth, it nevertheless requires a prolonged and expensive high-temperature treatment. Another typical contaminant in commercial aluminium-containing ferritic stainless steel foil is sulphur. In the absence of a suitable getter, most notably yttrium or rare earth elements, sulphur tends to reduce oxide adhesion and thereby diminish corrosion protection. It has now been found that, in the absence of yttrium or a rare earth getter, sulphur reacts with the magnesium impurity, and further that the magnesium sulphide product does not inhibit formation of an adherent whisker layer. However, the sulphur impurity typically found in commercial steel is of the order of 0.003 weight percent and is insufficient to react with all the magnesium present, which is typically of the order of 0.01 weight percent. At the same time, it is necessary to protect the steel formulation from the unwanted effects of free sulphur. A method of producing an aluminium-containing stainless steel from a magnesium-contaminated melt according to the present invention is characterised by the features specified in the characterising portion of claim 1. A steel according to the invention is defined in claim 9 and a method of producing a steel foil of said steel is defined in claim 13. Preferred embodiments are shown in claims 2 - 8 and 10 - 12. Therefore, it is an object of this invention to provide a method for producing an aluminium-containing stainless steel foil that is contaminated by a magnesium impurity sufficient to inhibit oxide whisker formation thereon, which method renders the magnesium present ineffective and thereby permits subsequent growth of oxide whiskers on a surface of said foil. More particularly, it is an object of this invention to provide a method for producing an aluminium-containing ferritic stainless steel from a melt contaminated with magnesium, which method includes adding sulphur in an amount stoichiometrically sufficient to react with all magnesium impurity present and thereby to prevent the presence of free magnesium that would otherwise inhibit the formation of oxide whiskers during subsequent oxidation of the steel. The treatment is necessarily carried out in the absence of any addition of yttrium or rare earth metals. In one aspect of this invention, the treatment comprises a further addition of one or more elements of Group IVB of the Periodic Table, as illustrated in Webster's Third New International Dictionary (1961), page 1680, i.e., titanium, zirconium or hafnium in amounts sufficient to react with any residual free sulphur remaining that might otherwise reduce adherence of the oxide layer. It is a further object of this invention to provide an aluminium-containing ferritic stainless steel containing magnesium impurity in an amount otherwise sufficient to inhibit oxide whisker growth, but further including a sulphur concentration adjusted to prevent the existence of free magnesium in the steel and thereby to permit oxide whisker growth on the steel. In one aspect of this invention, the steel further includes an element from Group IVB of the Periodic Table to react with excess sulphur and thereby promote adhesion of a subsequently formed oxide layer thereon. It is a still further object of this invention to provide a method for growing oxide whiskers on the surface of aluminium-containing ferritic stainless steel foil prepared from a magnesium-contaminated melt, which includes adding sulphur to the melt in an amount effective to react with all the magnesium present and thereby to eliminate the detrimental effects of free magnesium on whisker growth, and which optionally includes an addition of a metal from Group IVB of the Periodic Table to the melt to react with any excess sulphur present and thereby to promote the adhesion of oxide whiskers subsequently grown on the foil. In accordance with a preferred embodiment of the invention, these and other objects are accomplished by an aluminium-containing ferritic stainless steel that is formulated to include a sulphur addition to react with magnesium present in the melt as an impurity to produce magnesium sulphide, MgS. Thus, a melt is initially prepared comprising, as major constituents, aluminium, chromium and iron. A preferred melt comprises between about 3 and 6 weight percent aluminium, between about 15 and 25 weight percent chromium and the balance substantially iron, but containing impurities, including magnesium and sulphur impurities. The melt is analyzed to determine magnesium and sulphur concentrations. Sulphur is added, preferably as iron sulphide, to increase the concentration to an amount sufficient for stoichiometric reaction with the entire concentration of magnesium present. Except where otherwise noted herein, concentration is designated by weight percent. Thus, following the sulphur addition, the sulphur concentration is greater than or equal to 1.3 times the concentration of the magnesium present. For a typical Fe-Cr-Al melt, the amount of sulphur to be added is at least 0.01 weight percent. Because unreacted sulphur reduces oxide adhesion, it is desired to minimize the sulphur addition. Nevertheless, the sulphur addition preferably includes a small excess over the theoretical stoichiometric minimum of 1.3 times the magnesium concentration in order to assure complete magnesium reaction. In a further aspect of this invention, the excess sulphur is neutralized by an addition of an element from Group IVB of the Periodic Table, that is titanium, zirconium or hafnium. These Group IVB elements not only react with excess sulphur, but also react with carbon, nitrogen and oxygen impurities in the melt. Thus, an optimum addition of one or more of the Group IVB elements provides sufficient of the elements to react with carbon, nitrogen and oxygen, as well as the excess sulphur, present in the melt and may be calculated in accordance with the following Equation 1: % Ti ÷ [3.99(% C)+3.42(% N)+2.99(% O)+1.49(% S excess )] + % Zr ÷ [7.60(% C)+6.52(% N)+2.85(% O)+1.90(% S excess )] + % Hf ÷ [14.9(% C)+12.8(% N)+5.60(% O)+2.79(% S excess )] > 1 wherein % refers to weight percent and S excess is the amount of sulphur in excess of the stoichiometric amount needed for magnesium reaction and is calculated by subtracting 1.3 times the magnesium concentration from the adjusted sulphur concentration. A portion of the carbide, nitride, oxide and sulphide reaction products with Group IVB elements may separate from the melt to form a dross. Residual reaction products in the metal, particularly carbides and nitrides of the Group IVB elements added thereto, serve as grain refiners to enhance strength and increase ductility of the steel product. A small excess of the Group IVB elements over the stoichiometric amount needed for reaction with the various impurities and excess sulphur present is preferred to assure complete reaction and is not considered detrimental to the steel produced. The steel produced in accordance with this invention does not include any addition of yttrium or of rare earth metals, such as cerium and lanthanum, in order to achieve superior high-temperature oxidation resistance and to form the desired oxide whisker topography following oxidation. These agents react with sulphur preferentially to the desired magnesium and sulphur reaction, thereby releasing magnesium to inhibit whisker growth. Following formulation of the melt including adjustment of the sulphur concentration and addition of the Group IVB elements to remove any excess sulphur present, the melt is cast and formed, for example, by peeling or cold-rolling, into the desired steel stock, such as a thin foil for manufacturing a catalytic converter substrate. The steel is oxidized in accordance with a process described in U.S. Patent No. 4,331,631 or U.S. Patent No. 4,318,828, to produce an adherent, protective layer composed of oxide whiskers. It is found that the steel produced in accordance with this invention grows multitudinous, densely-packed oxide whiskers, despite the magnesium contamination, and further forms a tightly adherent and protective layer, despite the sulphur addition. This is accomplished without the expense of a yttrium or rare earth addition, as is common in current steel products to promote adequate oxidation resistance, and achieves the desired properties using a minimal addition of less expensive Group IVB elements. In a preferred embodiment of this invention, an aluminium-containing stainless steel is produced by an argon-oxygen decarburization (AOD) process from a magnesium-contaminated melt. A master alloy of about 20 parts by weight chromium and 75 parts by weight iron is melted in a suitable foundry vessel and treated by bubbling nitrogen or argon gas containing oxygen through the melt for decarburization. Thereafter, residual oxygen is removed by adding ferrosilicon to the melt whilst continuing to stir the melt by bubbling oxygen-free argon gas through the melt. About 5 parts by weight aluminium is added to the melt to complete the desired formulation of the major metal constituents present in the steel. In accordance with this invention, prior to casting the steel, a sample of the melt is analyzed for magnesium, sulphur, oxygen, carbon and nitrogen impurities. As used herein, carbon is considered an impurity because of the low level desired in the steel. Magnesium concentration is suitably determined by spark emission spectroscopy wherein visible light from an electrical spark between the sample and an inert counter-electrode is analyzed for a wavelength associated with magnesium, whereupon the intensity of light at the wavelength is indicative of the magnesium concentration. For illustration purposes, the magnesium concentration is about 0.01 weight percent, typical of commercial iron-chromium-aluminium steel. Carbon and sulphur concentrations are determined by combustion infra-red absorption spectroscopy wherein a sample of the alloy is heated in a ceramic crucible in the presence of a tin catalyst and oxygen gas, and the effluent gas is analyzed by infra-red spectrometry for the presence of carbon dioxide and sulphur dioxide. The sulphur concentration in this example is about 0.005 weight percent. The carbon concentration is about 0.01 weight percent. Oxygen and nitrogen are determined by a procedure similar to the combustion spectroscopic procedure for carbon and sulphur, but wherein the sample is heated in the presence of carbon and in contact with inert helium atmosphere, whereupon gaseous nitrogen and carbon monoxide are evolved. The carbon monoxide is catalyzed to carbon dioxide and analyzed by infra-red spectrometry. Nitrogen is determined by electrical conductivity. The nitrogen concentration in this example is about 0.01 weight percent. The oxygen concentration in this example is about 0.002 weight percent. Following analysis, the sulphur concentration of the melt is adjusted for stoichiometric reaction with magnesium. Complete reaction requires a sulphur concentration that is equal to or greater than 1.3 times the magnesium concentration present. For the melt in this example containing 0.01 percent magnesium, the required sulphur concentration is 0.013 percent. Since the initial sulphur concentration is 0.005 percent, an additional 0.008 percent sulphur is required. Sulphur is added in the form of a master compound, iron sulphide. In this example, iron sulphide is added to increase the sulphur concentration by 0.015 percent to produce an adjusted total sulphur concentration of 0.02 weight percent, resulting in an excess of 0.007 weight percent sulphur. The excess sulphur is provided to assure complete reaction of the magnesium present. Also, in this example, a titanium addition of about 0.3 weight percent is made to react with the excess sulphur, forming titanium sulphide to eliminate free sulphur from the steel that would otherwise reduce oxide adhesion. The titanium also reacts with carbon to form titanium carbide, with nitrogen to form titanium nitride, and with oxygen to form titanium oxide. The titanium addition is calculated to provide more than the minimum by Equation 1 to provide an excess to assure complete reaction of the undesired elements present. A portion of the titanium reaction products may separate as dross on the melt surface. Following the sulphur adjustment and the titanium addition, the melt surface is skimmed to remove the dross. The melt is cast into a billet, hot-rolled and finally cold-rolled to produce a foil about 0.05 millimetre thick. The foil surface is cleaned and oxidized to produce an oxide whisker layer. This is accomplished by heating the foil initially for 10 seconds at 900°C in a nitrogen atmosphere containing 5 to 1,000 parts per million oxygen and thereafter oxidizing the foil in air at 925°C for 16 hours. In the described embodiment, representative values for typical magnesium, sulphur, oxygen, nitrogen and carbon levels in commercial iron-chromium-aluminium melt were selected to illustrate sulphur and titanium additions in accordance with this invention. In production, the levels of contaminants varies from heat to heat. Magnesium impurity in commercial chromium-aluminium steel is typically between about 0.005 and 0.015 weight percent and may occasionally be up to 0.02 weight percent or more. A typical range for sulphur impurity is between about 0.001 and 0.005 weight percent. Oxygen impurity typically varies between about 0.001 and 0.030 weight percent. Nitrogen impurity typically varies between about 0.003 and 0.030 weight percent. Carbon impurity typically varies between about 0.010 and 0.040 weight percent. Also, the composition of a particular melt may vary over time because of separation into dross or contamination from the foundry vessel or atmosphere in contact therewith. Table 1 shows compositions of several melts of iron-chromium-aluminium alloy prepared to further illustrate this invention. In each example, the melt was cast, cold-rolled into a foil, and oxidized by the described process for growing oxide whiskers. The oxidized surface was visually inspected and also examined using an electron microscope. In each of Examples 1 to 4, the sulphur content was greater than 1.3 times the magnesium concentration, in accordance with this invention. The Group IVB element(s) concentration was sufficient to react with excess sulphur as well as carbon, nitrogen and oxygen present in the melt, in accordance with Equation 1. In each of the Examples 1 to 4, the oxidized, cold-rolled foil surface was substantially covered by tightly adherent oxide whiskers having a high aspect ratio and well-suited for promoting the adhesion thereto of an applied coating. This was accomplished despite the presence of magnesium and with sulphur concentrations greater than typical impurity level and achievable by sulphur addition in accordance with this invention. For purposes of comparison, and in contrast to Examples 1 to 4, Example 5 contains sulphur in significantly less quantity than that required for complete magnesium reaction, resulting in the presence of free magnesium within the steel. The oxidized foil did not form large oxide whiskers having a high aspect ratio and covering the surface, as in Examples 1 to 4, which result is attributed to the presence of the free magnesium within the steel. In Example 6, the sulphur concentration was greater than 1.3 times the magnesium concentration so as to prevent the presence of free magnesium within the steel in accordance with this invention. But, in the absence of Group IVB element addition, the excess of sulphur resulted in the presence of free sulphur within the steel. The oxidized foil was covered by densely-spaced, high-aspect oxide whiskers, but the oxide did not exhibit the desired tight adhesion, which was attributed to the presence of unreacted sulphur within the steel. Thus, the addition of sulphur to a Fe-Cr-Al melt in an amount sufficient to stoichiometrically react with the magnesium present has been demonstrated to mitigate the detrimental effect of magnesium upon subsequent oxide whisker growth. The reaction of magnesium and sulphur need not be completed in the melt or at the time of casting, but rather may take place by solid state diffusion within the foil during treatment of the foil to grow the oxide whiskers thereon. Although a precise addition is calculable to provide stoichiometric proportions of sulphur and magnesium, in practice it is preferred to add excess sulphur to assure reaction with all the magnesium present in the steel. In general, an excess sulphur quantity of 20 percent to 50 percent, or between 1.6 and 1.8 times the magnesium concentration, more than the stoichiometric concentration required, is believed to provide an adequate excess. Also, an addition of Group IVB metal element(s) has also been shown to scavenge excess sulphur from the steel to mitigate the adverse effects of free sulphur on oxide adhesion. An excess of Group IVB elements over the stoichiometric requirement is preferred. Thus, an addition of at least 0.3 weight percent titanium is generally sufficient to assure complete sulphide, carbide, oxide, and nitride formation. Similarly, an addition of at least 0.4 percent zirconium is sufficient to assure the desired reactions. An addition of at least 0.7 percent hafnium is also generally sufficient. Whilst this invention has been described in terms of certain embodiments thereof, it will be appreciated that the scope of the invention is to be considered limited only by the scope of the following claims.

Claims: A method of producing an aluminium-containing stainless steel from a magnesium-contaminated melt formed of an iron-chromium-aluminium alloy containing between 3 and 6 weight percent aluminium, between 15 and 25 weight percent chromium, one or more of Ti, Zr and Hf traces of carbon, nitrogen, oxygen, magnesium and sulphur, optionally sulfur and the balance iron, and comprising a surface suitable for oxidation to form thereon an adherent oxide layer by whisker formations, which method comprises treating the magnesium present in the magnesium-contaminated melt to prevent that magnesium from inhibiting subsequent production of said oxide whisker formations, characterised in that said magnesium-contaminated melt excludes any yttrium and rare earth metals therein; and said method comprises the steps of analyzing a sample of said melt of the iron-chromium-aluminium alloy to determine both the magnesium and sulphur concentrations in the melt; adding sulphur to said melt in an amount sufficient to increase the sulphur concentration therein to at least 1.3 times the magnesium concentration therein; reacting the added sulphur with the magnesium present in the melt; adding to the melt one or more metals selected from the group consisting of titanium, zirconium and hafnium, in an amount sufficient to react with any residual sulphur present in the melt in excess of the sulphur concentration required for complete stoichiometric reaction with the magnesium present in the melt; and then solidifying the resulting melt to form said steel. A method according to claim 1, in which the sulphur concentration in the melt following the addition thereof is between 1.6 and 1.8 times the magnesium concentration in the melt. A method according to claim 1, in which the amount of sulphur added to the melt is at least 0.01 weight percent of the melt. A method according to claim 1, in which the magnesium concentration in the melt prior to the sulphur addition is of the order of 0.01 weight percent. A method according to claim 1, in which the concentrations of titanium, zirconium or hafnium to be added to the melt are determined in accordance with the equation: % Ti ÷ [3.99(% C)+3.42(% N)+2.99(% O)+1.49(% S excess )] + % Zr ÷ [7.60(% C)+6.52(% N)+2.85(% O)+1.90(% S excess )] + % Hf ÷ [14.9(% C)+12.8(% N)+5.60(% O)+2.79(% S excess )] ≧ 1, wherein S excess is the amount of sulphur present in the melt in excess of the stoichiometric amount needed for reaction with the magnesium present in the melt. A method according to claim 5, in which the metal addition comprises at least 0.3 weight percent titanium. A method according to claim 5, in which the metal addition comprises at least 0.4 weight percent zirconium. A method according to claim 5, in which the metal addition comprises at least 0.7 weight percent hafnium. A magnesium-contaminated aluminium-containing ferritic stainless steel which, on the oxidation of a surface thereof, produces an alumina layer thereon containing oxide whisker formations, said steel being composed of an iron-base alloy containing between 3 and 6 weight percent aluminium and between 15 and 25 weight percent chromium, one or more of Ti, Zr and Hf and traces of carbon, nitrogen, oxygen, magnesium and sulphur, optionally sulfur and the balance iron characterised in that said steel is free of yttrium and rare earth metals; the sulphur concentration in the steel is at least 1.3 times the magnesium concentration in the steel; and said steel further contains one or more metals selected from the group consisting of titanium, zirconium, and hafnium, in an amount effective to react with the sulphur present in the steel that is in excess of the sulphur concentration required for complete stoichiometric reaction with the magnesium present in the steel. A magnesium-contaminated aluminium-containing ferritic stainless steel according to claim 9, in which the magnesium concentration is at least 0.01 weight percent. A magnesium-contaminated aluminium-containing ferritic stainless steel according to claim 9, in which the sulphur concentration is at least 0.013 weight percent. A magnesium-contaminated aluminium-containing ferritic stainless steel according to claim 9, in which the titanium, zirconium and hafnium concentrations are in accordance with the equation: % Ti ÷ [3.99(% C)+3.42(% N)+2.99(% O)+1.49(% S excess )] + % Zr ÷ [7.60(% C)+6.52(% N)+2.85(% O)+1.90(% S excess )] + % Hf ÷ [14.9(% C)+12.8(% N)+5.60(% O)+2.79(% S excess )] ≧1, wherein S excess is the amount of sulphur present in the melt in excess of the stoichiometric amount needed for reaction with the magnesium present in the melt. A method of producing an aluminium-containing stainless steel foil having oxide whiskers formed thereon, which method comprises rolling a billet of a magnesium-contaminated aluminium-containing ferritic stainless steel according to any one of claims 9 to 12 into a foil having a surface, and oxidizing the foil surface by heating the foil initially for 10 seconds at 900°C in a nitrogen atmosphere containing 5 to 1,000 parts per million oxygen and thereafter oxidizing the foil in air at 925°C for 16 hours, so as to form an oxide layer thereon containing multitudinous oxide whisker formations.

Inventors: SIGLER DAVID R, SIGLER, DAVID R.
Applicants: GEN MOTORS CORP, GENERAL MOTORS CORPORATION
Application year: 1991
Main classification: C21C7
IPC codes: C23C8, C21C7, C22C33