Improvements in and relating to hafnium-containing alloy steels

Description

[0001] This invention relates to hafnium-containing alloy steels treated with rare-earth additions to enhance the physical properties of the alloy. The invention also concerns the production of foil manufactured from such alloys and especially foils for use as, for example, substrate material in catalytic converters employed to clean the emissions of vehicle exhausts.

[0002] It is known that the oxidation resistance of steels and particularly their resistance to oxidation in the presence of sulphur and oxygen can be greatly enhanced by small additions of hafnium. Thus European Patent 35369 discloses a ferritic alloy containing between 0.05 and 1.00% hafnium. It has now been found, however, that the high temperature properties of alloy steels can be significantly enhanced by treatment with rare earth additions to give in the final steel specified contents of cerium and/or lanthanum.

[0003] The high temperature properties achieved by such additions are comparable to those to be found in yttrium-­containing steels such as Fecralloy; however, alloy steels produced in accordance with the present invention are less costly and more easily produced because, in their production no expensive and time-consuming vacuum induction melt is required. Also the yield of hafnium has been found to approach 100% as compared with a yield of around 50% for yttrium.

[0004] There exists a considerable body of research into the reactive element affect on the adhesion of Al₂ O₃ scales to high temperature alloys. Without reactive metal additions alumina scales tend to spall from the underlying metal due to the action of oxide growth and thermal cycling stresses. However, small additions of certain reactive elements result in large improvements in scale adherence. The Applicants have found that a combination of hafnium and cerium is particularly effective in this respect.

[0005] As mentioned previously, steels in accordance with this invention can advantageously be used in the production of foils for use as substrate material in catalytic converters for cleaning the emissions of vehicle exhausts. The service conditions of such substrate material are onerous and include thermal cycling up to 800°C with occasional excursions up to temperatures in excess of 1100°C in addition to exposure to highly corrosive atmospheres. In particular, stringent requirements are laid down by the end-users regarding the maximum possible weight gain achieved after the foil substrate has been subjected to an oxidation generating atmosphere at a high temperature for a considerable length of time. One such criterion specifies a maximum weight gain of 6% after oxidation for times exceeding 300 hours at 1100°C in air. Other criteria are set down regarding the physical properties of the substrate material.

[0006] The present invention sets out inter alia to provide a steel from which such foils can be produced.

[0007] According to the present invention in one aspect, there is provided a steel alloy of composition containing by weight:
10 to 25% chromium;
1 to 10% aluminium;
0.5% maximum carbon;
up to 3% silicon;
up to 2% manganese;
0.010% maximum sulphur;
up to 5% nickel;
up to 2% titanium;
from 0.01 to 1% hafnium;
0.01 to 0.10% zirconium;
up to 0.05% nitrogen;
up to 1% molybdenum;
up to 1% copper;
0.1 max phosphorous;
rare earth additions to give a cerium and/or lanthanum content of between 0.01 to 0.20%;
the balance being iron and incidental amounts of impurities.

[0008] A preferred composition contains from 18 to 25% chromium, from 5.0 to 6.5% aluminium, and a rare earth addition to give from 0.02 to 0.15% cerium and/or lanthanum.

[0009] The percentage of nickel is chosen so that its presence does not produce significant amounts of a second phase taking in account the amounts chosen for each of the other ingredients of the alloy. Preferably, the amount of nickel does not exceed 0.5%.

[0010] The rare earth additions may take the form of Misch metal.

[0011] According to the present invention in another aspect, there is provided a foil for use, for example, in calatytic converters, the foil having a thickness within the range of 45 to 55 microns (or µm) and comprising a ferrous alloy of a composition by weight percent including:-
C 0.01 to 0.10; Hf 0.01 to 1.00; Cr 18.00 to 25.00; Al 4.00 to 6.00; and Ce and/or La 0.01 to 0.20.

[0012] Alloys in accordance with the invention are preferably produced by a route which includes melting a suitable feedstock within an induction furnace or an electric arc furnace; subjecting the melt or an ingot produced therefrom to secondary refining; and rolling the ingot to gauge (e.g. 0.05mm foil) for high temperature service. One example of a product of an alloy steel is a catalytic converter substrate material used at temperatures of up to 1250°C for cleaning the exhausts of vehicles.

[0013] The invention will now be described with reference to the following Example which specifically concerns the production of alloy steel foil for use as substrates in catalytic converters employed in vehicle exhaust systems.

[0014] In a typical production route, a charge of high purity iron and low carbon ferrochromium is melted down in a basic lined induction furnace, either in air or under a basic slag, and the appropriate additions of aluminium, ferro-titanium, hafnium and misch metal made, in that order, to the melt. The melt is subsequently cast into an AOD vessel and subsequently subjected to secondary refining.

[0015] In this Example, the alloy steel ingot had the following composition by weight per cent:
C 0.015; Si 0.45; Mn 0.29; Ni 0.24; Cr 20.80; Mo 0.02; S 0.001; P 0.023; Al 5.15; Cu 0.06; Zr 0.047; Hf 0.055; Ce 0.025; Ti 0.04; N 0.006; remainder Fe apart from incidental inclusions and impurities.

[0016] The ingot was hot charged and slabbed, the slab being rolled to hot band of approximately 3mm thickness, annealed, shot blasted and pickled, and then cold rolled to the final gauge using a sequence of cold rolling and annealing. Finally, the rolled material was processed to foil of thickness 52 µm.

[0017] A sample foil approximating to 25 mm x 25 mm was taken and its surface area and weight measured carefully after a thorough degreasing treatment consisting of ultrasonic agitation in chlorinated solvent, followed by forced drying in hot air. This oxidation sample was placed in a platinum crucible in a furnace set at 1100°C. The sample was positioned so as to allow free access of air to its surfaces, After 200 hours the sample was removed, weighed, examined visually and replaced for a further 24 hours. This sequence was repeated for up to 320 hours. Examination of the surface of the as-rolled and as-oxidised sample was carried out on the scanning electron microscope (SEM). Chemical analysis of the scale was performed on the SEM by energy dispersive analysis of x-rays (EDA).

[0018] The weight gain result for the sample was as follows:-

Alloy	Thickness	Exposure Time, h
		200		224		248
		Weight Gain		Weight Gain		Weight Gain
		mg/cm²	%	mg/cm²	%	mg/cm²	%
BF5CeHf	52 µm	0.638	3.65	0.689	3.94	0.732	4.18

Alloy	Thickness	Exposure Time, h
		272		296		320
		Weight Gain		Weight Gain		Weight Gain
		mg/cm²	%	mg/cm²	%	mg/cm²	%
BF5CeHf	52 µm	0.760	4.35	0.782	4.47	0.818	4.67

The weight gain result is also presented in graphical form in Figures 1 and 2.

[0019] The results show a weight gain of 4.18% after 248 hours exposure rising to a weight gain of 4.67% after 320 hours exposure. As will be appreciated, this weight gain is below the 6% specified by the end user.

[0020] The oxides formed on the sample after 200 hours exposure were very good, displaying the desired characteristics of compactness and uniformity. On further exposure, the oxide coating deteriorated gradually with small areas of more voluminous brown/black coloured oxides becoming apparent There was no visual indication that oxide spalling from the samples's surface had occurred. Oxide spalling would not be acceptable to the end user.

[0021] On inspection, two oxide types were clearly visible on the sample, these being firstly a compact, nodular and fine grained oxide, and secondly a voluminous oxide. The first oxide type was alumina-rich and showed variously sized nodules. These nodules varied in composition but generally had a high aluminium content with small amounts of other elements and occasional significantly higher levels of titanium and hafnium, e.g. 19% and 14% respectively. The large grained oxide was high in iron oxide, 93%, but also showed some chromium.

[0022] Examining the oxidation behaviour of the sample in the light of requirements set by the industry, it is clear that a slow growing compact oxide layer would give the lowest weight gain and thus satisfy the requirements set. Such an oxide coat was provided by the sample, and although a small proportion of the more voluminous oxide type was present, this oxide was also protective.

[0023] Thus, in so far as oxidation behaviour is concerned, the sample satisfied the criteria set for such by the end user.

[0024] From the Example, it will be seen that the 6% weight gain after 300 hours at 1100°C set for catalytic convertor substrate materials was met by the sample tested. Notably, the weight gain of the sample containing additions of hafnium and cerium was similar to that of the yttrium containing sample and the oxide adherence and appearance of oxides on these samples were similar, demonstrating that hafnium plus cerium could be used in place of yttrium additions. Tests have also shown that low chromium contents, e.g. 16% are to be avoided in such steels. The function of chromium is to act as a "getter" for oxygen. This allows time for aluminium diffusion to the metal surface to produce and maintain a slow growing, protective oxide layer. When this action does not occur base oxides are formed, which cause cracking and spalling of alumina.

[0025] In order to meet the specification set for the substrate material, it is apparent that a protective layer of alumina must be maintained for as long as possible. The weight gains occasioned at the exhaustion of the matrix aluminium have been calculated for a number of alloy steels. These calculations show that base oxides will begin to form at weight gains of about 4.7%, but more importantly, the calculations also show that this occurs much earlier in thinner foils.

[0026] Interestingly, it has been found that thinner gauge foils show a noticeable upturn in weight gains, reasonably close to the time t_b predicted above. Differences in the t_b values have been found to be pronounced and notably, the differences are still significant if the t_b values are calculated using the quoted range for the parabolic constants of alumina, instead of experimentally derived parabolic constants.

[0027] The most important factors influencing the onset of the breakaway stage are therefore the matrix chromium content, the foil thickness, the matrix aluminium content, the presence of surface defects and the adherence of the protective alumina layer, which is improved by active metal additions.

[0028] From the Example given above, it is apparent that foil made from alloy steels in accordance with the invention meet the industry's requirement of a maximum weight gain of 6% after oxidation for 300 h at 1100°C in air.

[0029] In the Example, the alloy examined, formed a compact, adherent alumina oxide layer under the stipulated oxidising conditions. Only after prolonged exposure did a small proportion of iron oxide form and it also appeared to be protective.

[0030] The fields of application of alloys produced in accordance with the invention are not limited to the specific Example given above but extend to other applications in which resistance to oxidation at high temperature is required.

Claims

1. A steel alloy characterised by a composition which contains by weight:
10 to 25% chromium;
1 to 10% aluminium;
0.5% maximum carbon;
up to 3% silicon;
up to 2% manganese;
0.010% maximum sulphur;
up to 5% nickel;
up to 2% titanium;
from 0.01 to 1% hafnium;
0.01 to 0.10% zirconium;
up to 0.05% nitrogen;
up to 1% molybdenum;
up to 1% copper;
0.1% max phosphorous;
and rare earth additions to give a cerium and/or lanthanum content of between 0.01 to 0.20%; the balance being iron and incidental amounts of impurities.

2. A steel alloy as claimed in Claim 1 characterised in that the composition contains by weight from 18 to 25% chromium, from 5.0 to 6.5% aluminium, 0.05 to 0.50% hafnium and a rare earth addition to give from 0.02 to 0.15% cerium and/or lanthanum.

3. A steel alloy as claimed in Claim 1 or Claim 2 characterised in that the amount of nickel does not exceed 0.5% by weight.

4. A steel alloy as claimed in any one of Claims 1 to 3 characterised in that the rare earth additions take the form of Misch metal.

5. A foil having a thickness within the range of 45 to 55 microns which is characterised in that it consists of a ferrous alloy of a composition by weight per cent including:-
C 0.01 to 0.10; Hf 0.01 to 1.00; Cr 18.00 to 25.00; Al 4.00 to 6.00; and Ce and/or La 0.01 to 0.20.

6. A method of producing an alloy as claimed in any one of Claims 1 to 5 which includes the steps of melting a suitable feedstock within a furnace; subjecting the melt or an ingot produced therefrom to secondary refining; and rolling the ingot to gauge.

7. A method as claimed in Claim 6 characterised in that the furnace is an induction or arc furnace.

8. A method as claimed in Claim 6 or Claim 7 characterised in that the ingot is rolled to a gauge of the order of 0.05mm.