CA1041323A - Oxidation-resistant low alloy steel and article - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A low alloy steel for use as a substrate for aluminum or aluminum alloy coatings, the steel containing from 0.1% to 0.13% carbon, from 0.5% to 3% chromium, from 0.8% to 3% aluminum, from 0.4% to 1.5% silicon, from 0.1%
to 0.6% manganese, from 0.1% to 1% titanium and remainder substantially iron. The steel has good oxidation resis-tance at elevated temperature, good weldability and forma-bility, thereby enhancing its utility for fabrication into welded and wrought coated products, such as space heaters, automotive exhaust systems, e.g., catalytic converters, mufflers, and the like.

Description

~0~3~Z~
This lnvention relates to a chromium-aluminum-sllicon-titanium steel of low alloy content for use as a substrate for aluminum or aluminum alloy coatings, and wrought coated products thereof havinÆ good resistance to oxidation at elevated temperatures up to about 1700F
(925C) and resistance against attack by hydrocarbon combus-tion products at elevated temperature, together with high strength.
Increasingly stringent requirements for antipol-lution controls on motor vehicles has created a need for relatively low cost alloys which will be oxidation resistant at elevated temperatures, for use in automotive exhaust sys-tems, such as catalytic converters, mufflers, and llke articles.
Aluminum coated carbon steel has proved to be not completely satisfactory for some hlgh temperature appli-cations. The automotive industry has substituted stainless ' .
steels such as Armco 409 (containing 0.05% carbon, 11%
chromium,traces of aluminum, residual nickel, 0.5% titanium and remainder iron) and other stainless steels containing 11% or more chromium. The cost of such steel is high, thus makin~ it undesirable for proposed use in automotive exhaust systems, such as catalytlc converters, and the lllce. More-over, although this stainless steel has fair oxidation re-25 slstance at elevated temperature, and good formability, it ;~
does not adequately wlthstand attack by molt-en salts and hydrocarbon combustlon products at elevated temperature.
The provision of an aluminum coating on such a steel has c/e~?ark . .

~04~323 been found to result ln a product having the deslred proper-ties, but this solution obviously adds even ~reater cost.
United States Patent 3,698,964, issued October 17, 1972, to E. .J. Caule et al, discloses an iron base al-loy with good oxidation resistance at tem~eratures Or about 700 to 800 C (about 1300 to 1475 F). The alloy o~ thls patent contains up to 2~ carbon, 1% to 5~ chromium, 1% to 4% aluminum, and/or 1~ to 4% silicon, up to 1.5% man-ganese, up to 2% copper, up to 0.20% total of nickel, molyb-denum, vanadium and other alloyin'g elements. Pre~erably, a combinatlon of 2% chromium and 3~ alumlnum, or 3% chromlum and 2% silicon, are used, with manganese ~referably up to 0.2%, copper not more than 0.5%, carbon not more than 1% and most prererably rrom 0.01% to 0.2'5%. ;
The high carbon content of the steel of the Caule et al patent results in a brittle structure havln~ very li- ;~
mited cold workability, poor welding characteristics, and poor mechanical properties generally. The optional presence o~ ~
molybdenum and vanadlum adversely afrects oxidatlon scale ~' ' resistance, as well as adding to the cost. The harmful effect o~ molybdenum on oxidation resistance is reported in "Stain-less And Heat Resistin~ Steels", by Colombler and Hochmann, p. 330, St. Martin's Press, Mew York, N.Y. (1968). Moreover, '' copper, nlckel,and other austenite stabilizers in amounts ~reater than typical re'sidual contents of about 0.2~ each are ! ;' ~ '' undesirable since it can cause a phase chanGe, with a conse- ' quent change in volume, on heatln~, and cooling. This volume ~'~
change results in s~alling a~ter cyclic heating and coolin~.
-~ 2 8~dr 6~
Unlted States Patent ~ issued March 4, 1958, to E~M. Herzog, discloses a steel, and a heat treatment :.. .. .. ~, , , .... ~ . . . ................ . . .... .
, . , , , . . . . ~

:lV413~3 thereror whereln a mlcro-structure ls ~roduced havlng reslstance to stress-corrosion cracking in wet hvdro~en sulride atmospheres. The steel of this patent contains from o.o8% to 0.20~ carbon, from o.60% to 5.0~ chromium, from 0.15% to 1.20% aluminum, from 0.30% to 1.20% man~a-nese, rrom 0.10% to 0.50~ sllicon, u~ to 0.50% mol~bdenum, up to 1.0% vanadium, up to 1.0% titanium and remainder lron. A heat treatment at 740 to 780 C, a second heat treatment at about 970 to 1080 C followed by a water-quench and a tempering treatment at about 625~ to 670 C, are stated to result in the desired micro-structure and an ultimate tensile strength Or at least 95 ksi. Oxidation reslstance at elevated temperature is not contemplated in this patent, and the composition would not lnherently pro-duce a steel having thls propert~. Moreover, the ~resence Or molybdenum and vanadium is deleterious ~or reasons set forth above~
Other patents disclosing low alloysteels con- -talning chromium,~luminum, and/or silicon in varying a-mounts include United States Patents: 3,431,101; 2,835,570;
and 2,770,563.
None of the above patents discloses a low alloy steel having a completely ferritic structure withln the con-templated operating temperature ran~e, and exhlbiting in combination good oxidation resistance at elevated temperature, good resistance a~ainst attack by hydrocarbon combustlon products, good weldabllity and rormabilit~ and relativel~
high strength. Hence, there still exists a great need for a low-cost alloy having the ahove comhination of pro~erties for fabricatloninto coated welded and ~!rought products such . ,. ~ ,. . .
.

1(~4~ 3 as space heaters, automotive exhaust systems, e.g., catalytic converters and mufflers, and the like.
~ ccording to the invention there is provided an alloy steel having good oxidation resistance at elevated temperature, good weldability and good formability, consisting essentially of, by weight percent, from 0.01% to 0.13% carbon~ from 0.5% to 3% chromium, from 0.8% to 3% aluminum, from 0.4%
to 1.5% silicon, from 0.1% to 0.6% manganese, a maximum of about 0.05% ;~
molybdenum, a maximum of about 0.05% vanadium, less than about 0.2% copper, ;~

less than about 0.2% nickel, an element chosen from the group consisting of ~ ; :
titanium, columbium, zirconium, and mixtures thereof, titanium when present being from 0.1% to 1% columbium ~hen present being from 0.1% to 1.5%
zirconium when present being from 0.1% to 1.5%, and mixtures when present being from 0.1% to 1.5% total, and remainder iron except for incidental impurities.
The steel of the present invention preferably contains chromium, `
aluminum, silicon and titanium in a total amount of less than about 5% and provides in sheet form a substrate for aluminum or aluminum alloy coatings, the coated sheet being readily formable into wrought articles having good ;~ `~
oxidation resistance at elevated temperature, good resistance against attack by hydrocarbon combustion products at elevated temperature, and relatively high retained strength at elevated temperature.
The carbon, chromium, aluminum, silicon and titanium percentage ranges are critical and departure therefrom results in loss of one or more -~
of the above properties. Control of the critically low molybdenum, vanadium, copper, nickel and other austenite stabilizer contents is also essential.
: . .
Carbon is essential in an amount of at least 0.01% in order to provide the necessary strength in the steel. More than 0.13% carbon cannot `
be tolerated because of its adverse effect upon the weldability, formability and general mechanical properties of the steel, and because it is a strong austenite former. 5 ~L04~323 At least 0.5% chromium in combination with at least 0~8% aluminum and 0.4% silicon is necessary in order to provide good oxidation resistance.
A maximum of 3% chromium should be observed in order to mirimize cost and avoid processing difficulties.
At least 0.8% aluminum is necessary not onl;y for oxidation resis-tance at elevated temperature but also to provide adequate tensile strength.
More than 3% aluminum results in a loss of formability and workability.
At leas* 0.4% silicon is essential since it co-operates with the chromium and aluminum to impart oxidation resistance. However, a maximum of 1.5% silicon should be observed since amounts in excess thereof also result in loss of formability and workability.
An amount of titanium,columbium or zirconium of at least 0.1% is necessary in order to impart good weldability to the steel. Moreover~ excess titanium over that needed to stabilize carbon has been found to improve the oxidation resistance at elevated temperature. This excess can be slight in view of the high cost of titanium and of the relatively low residual sulfur, nitrogen and oxygen contents of the steel of the invention. Preferably the titanium content is 8 times the carbon content, and a maximum of about 1%
titanium should thus be observed at the carbon levels contemplated herein.
Since it is known that columbium and/or zirconium generally function in an ;
equivalent manner in stainless steels, it is considered within the scope of ~
the invention to substitute columbium and/or zirconium in whole or in part ~-for titanium. Such substitution would generally be on a stoichiometric basis, advantageously with a minimum weight ratio of columbium or zirconium to carbon of 8:1, preferably at least `

.. ,.,.
~
~ ''`~ .

~04~3~3 about 10:1. Columbi~D and/or zirconium would thlls range from 0.10% ~o 1.5% if substituted for titanium~
Impurities at residual levels normal for ferritic stainless steels can be tolerated in the steel of the inv~ntion.
More specifically, a maximum of about 0.03% sulEur and a maximum -of about 0.04% phosphorus do not adversely affect the properties of the steel. Molybdenum and vanadium are undesirable in the steel of the invention as explained above, and are maintained at the minimum practicable levels. Copper and nickel are maintained 10at a maximum of less than 0.2% each for reasons set forth above. ;
While the desirable novel combination of properties `~
is achieved in a steel having the broad composition ranges hereinabove set forth, optimum properties are obtained in a steel having the following preferred analysis by weight percent:
Carbon 0.04% to 0.06% ~;
Chromium ~ 1.7% to 2.1%
Aluminum 1.7% to 2.0%
Silicon 0.6% to 0.9%
Manganese 0.2% to 0.4%

Titanium 0.1% to 0.6%
and remainder iron except or incidental impurities.
In order to investigate the effect of the relative proportions of chromium, aluminum, silicon and titanium on the properties of the steel, a series of experimental heats was prepared and tested. For purposes of comparison, tests were also conducted on plain carbon steel coated with aluminum and aluminum alloys containing up to 10% silicon, and on Armco Type 409 stainless steel. The compositions of the experimental heats are set forth in Table I below.
,:

7 ~

: : ~: :. - :,. . :: :. . : : .. : , ::: ........ : : : ~ : :
.. . - . ~- ..

1C~4~3~3 TABLE I

Sample Code C Cr Al Si Mn Tl 0.048 1.0 <0.1 0.2 ~.~ o.4 41* o.o33 '5 1.7 0.8 0.4 0.4 42~ O.Q33 l.o 1.7 o.7 o.4 o.4 43* 0.034 1.5 1.7 0.7 0.4 0.4 61* 0.037 l.o l.o o.7 o,4 0.3 62* o.o38 1.7 .9 o.8 0.4 o.3 0-054 2.5 1.9 0.47 0.21 0 86~ 0.073 2.0 1.92 0.81 0.32 0.48 Steels of the invention - also containin~ 0.1% Cu, 0. 04 Mo, and 0.03% V.

The above materials were hot rolled rrom 21nO ~
(1149~ C) from 1 lnch by 3 inch ingots to 0.1 inch thlckness.
Samples were annealed at 1700 F (927 C) for 10 minutes, descaled and cold rolled to 0.05 lnoh thickness. It should ~ -be recognized that annealin~ the hot rolled material is option- '' al. Tensile stren~ths were determined on the cold rolled sam-.
ples at this stage while the remainder of the cold rolled stri~
was annealed at 1600 F (871 C) for 6 minutes and pickled.
This material was tested for the remalnin~ mechanical properties reported below in Table II.
A consideration of the mechanical properties ..
reported in Table II lndicates that the steels of the inven- ~-tion have ultimate tensile strengths equivalent to those o~
comparable prior art allo~s but have imDroved elon~ation.
Thls is believed to result from the relatively low carbon contents (rangin~ from about 0.03% to about 0.07~). The yield stren~th and tensile stren~ths o~ ~he alloys containin~ about ;;~
8 ~

, . ~ ' ~ . , , :. . ' !

~4~Z3 ; ~
l~ aluminum were about 5 ksi and 4 ksi, respectively, less than those containln~ 2~ alu~inum. Of greater si~nificance is the comparison with the 1% chromium alloy (Sample Code 15) also havlng low aluminumand silicon contents. It will be noted that the yield ~trength of the 1% ch:romium alloy was :. :
about 12 ksi lower and the tensile strength about 8 ksi lower ~ -than the steels of the invention containing from about l.7%
to about 2% aluminum with chromium ranging from 0.5% to 2%.
.
At the same time the elongation values of these steels of the invention were about equivalent to that of Sample Code 15. For an optimum comblnation of mechanical propertles, it is thus apparent that the carbon content should not ex~
ceed about 0.06%, that aluminum should ran~e from about l.7 to about ?% in combination with at least about l.7%
chromium and at least about 0.6% silicon.
, TABLE II

Olsen Sample 0. 2% YS UTS ~ Elon~. Hardness Cup Test Code (ksi) (ksi) in 2~ (Rockwell B) Hei~ht-Inches :, 23.65 53. g5 35.5 57.0 .390, .4~0 41* 36.40 61.85 36. o 73.0 400, .410 42* 35.65 62.15 34.0 73.0 . 410, .405 43* 35.80 62.55 35.5 73.0 .400, ~4~0 61* 29.95 58.25 37.0 68.o .380, .400 62* 30.45 58.60 34. 5 68.5 .3g5, .44 39.70 61. 25 20.5 73.0 .390 86~ 37.15 61.10 26.0 7~.0 .408 - -* Steels of the present invention .
Samples of the steels Or Table I ln the cold rolled, 30 annealed and Dickled condltion were surface ~,round, and a full : ~ . . -104i323 Denetration auto~enous ~TA weld was run down the lon~ltu-dlnal axis of the strlp of each samole. 180 bend and Olsen cu~ test specimens were cut from the sam~le~s and tested wlth hoth root and face side ln tension. These tests are reported in Table III.
From the data ln Table III it is evldent that optimum as-welded ductillt,y i3 exhlbited with about 1%
aluminum and chromium in excess Or 1%. At the 2% alumlnum level better results apParently are obtained with chromium at about 2%.
The necessity for the presence of titanium for ~ood weld properties is demonstrated b,y Samole Code 85, wherein a poor Olsen cup value was obtained. In addition, Sample Codes 85 and 86 were sub,~ected to a further bend test across the weld (not reported in Table III), and it was found that Sample Code 85 cracked at 90 across the weld, whereas Sample Code 86 passed 180 flat across the weld.

. ~.

, ,:

~. . .

~41323 o ~ -o U~ o o o o ~ .
~ ~ o ~ o~ o ,~
E~ ~ ~ ~ J ~r) I I
. ~ ....
~~~ A ~ ~ ~ . .
OO O O O O ' :`
30U~ L~OD ~ I I ' ;

Et C~ ~r, :
P.H . i, ~ I . .
~S ~ '~.' ~' ~.'', ~rl O , : ~
O ~: rl O O O O O U~ ' ' ` '' C: O~ O O ~ O~ ~1 1' 1 a~ ~ ~ ~ ~ ~ ~ I I .:
E-/ . . . . ;
O O U~ O L~ O U~' O
~t O C~ O C~
. ~ ~ ~ ~1 ~1 3 ~1 0 O . ~ . . ~ . .
O
~; . ', H 0 td . . . .
H Q. ~: ~1 h O ~ ~ ~ ~ P~
~ ~a E~ ',~

3 ~rl ~ 1 l ~a U) ~ l ~~ ' c~ ~ ~ al ~1 ,~
. ~ O .
J~
~ ~1 . .
a~ o , o :.
C ~ .1~ . :
o al ~d ¢
00 ~ S:~ ~1 O ~ ~4 P~ ~ P~ ~ ~ ~ ~ ~ O
U~ ~r/ O

E~ . ~:: S ~:
~ ~rl H
O~ S tq ~1 . ~ :.
o a~ :
' ' U~ ~ ' rt a) * * * ~ * *
C) ~1 ~ J J ~ I Q

, .

Oxidation resistance tests were conducted on the samples of Ta~ble I in the cold rolled, annealed and pickled condition, both on coated and uncoated specimens.
For the coated specimens, a pure aluminum coating was ap-5 plied by hot-dipping using a flux of ~luorides of zirconium ~-and/or titanium, details of whlch are disclose~ in United States of America Patents 2,6~6,354 and 2,686,355, issued August 17, 1954 to H. Lundin. Coating weight was about 1~2 ounce per square foot of sheet or 0.064 gram per square centimeter (total coatlng weight on both surfaces). For purposes of comparison, oxidation tests were also run on plain carbon steel coatèd with pure aluminum and with alu-minum alloy containing up to 10% sllicon, uncoated Armco Type 409 stainless steel, and an uncoated commercial alloy contalning 5% chromium, 0.5% molybdenum, 0.06% carbon, 0.35% --silicon, 0.4% manganese, residual aluminum and nickel, and balance substantially iron. The initial tests comprised 100 hours in still air at 1600F, and 1700F, respectively.
These tests are reported in Table IV below.
Since still air tests are not necessarily defi-nitive, further specimens of coated and uncoated materials were sub~ected to cyclic testing, using a cycle Or 25 minutes in and 5 minutes out o~ the rurnace for a total of 130-135 cycles. These results are reported in Table V below.
, :.

... . -~ .. . . . . .

~04~a3z3 ~;
-. ,.

~ o o oo o H~n co ~ ~ O 0~ :
o H ~:
O ,1 oO ~ t-- ~ O ~ 3 ~ U~
o ~ ~ .=r o L~ O t-- r~ ~1 ~ ~I ~ ' t--S br ~ ~~r O
~I bl~ E N
~rl ~:
'' ' .
.

h^ ~ : .
' , .. .
U~
h O E~
~rl ~7 , O
U~ C~
~' u~ 1_l h ~ ~ ~1 1~. Lt~ O u~ O
H ~ 0 1~ O
V~ O~1 ~ ~ ~r ,1 ~D ~ ~ J ~ ~o CO N
~1 O O ~ ~ N N Lr~ ~~ ~I ~ ~I
rl V~ \s~ r tr~ t_ ~ ~ ~ ~ . :.
E~ ~ o a) ~-1 g O ~
C . ' ': ~ ;
r-t Q~ .
Q~ ~

~: O ~ , p o u~
, R, ~ O O H
E~ cO) . h ~I ~1 0 * * * * * *

O O O O O O O O O O ;
O O
- + Q
¢ ¢ ~ =r 3 3~D ~D CO ) 1~ ' ' , h ~ .
i~, C) ~J N H O O ~o 0 (~J
o H ~
O ~ O ~1 0 0 ~ ~ ~O `
O J~ ~ t~ ,~
~.C bL .
,~ b.l E
:~

. . .
.~
~ ., irI I ) C~ C~ U~ ~ ~o' CO
o H ~:
O .rl Ll~ O ~) 3 ~O~O
O ~ ~ --I r~
,I bO i-. .. .

, .' .
.~.
~ .
@

Q~
i. O ~
~) . O
V~ ' ~ *

O O O O O O O J~ ~:
¢ Cl ~ ¢' Cl ¢ ~ , i ~,,.
=r 3 3 ~ ~ ~ *

.

3~3 TABL~ V

Oxl~ation Tests "
Cycllc 25/5 ;

Sample Wei~ht Increase Code Condltions m~/ln2.

T,ype 409 stainless 135 c,ycles - 1500F 118 15 uncoated 135 cycles - 1500F 666 41 uncoa~edx 135 cycles - 1500F 315 42 uncoated]* 135 cycles - 1$00F 270 43 uncoated9 135 c,vcles - 150nF 250 61 uncoated~ 135 cycles - 1500F 382 62 uncoated* 135 cycles - 1500F 376 85 uncoated 13Z c.ycles - 1500F 198 86 uncoated* 132 cycles - 1500F 117 ;~ :~
15 Al coated 135 cycles - 1500F 37.2 41 Al coated* 135 c~cles - 1500P 17.4 42 Al coated* 135 c,ycles - 1500F 15.0 43 Al coated* 135 c~cles - 1500F 12.3 61 Al coated* 135 cycles - 1500F 14.3 62 Al coated* 135 c~cles - 1500F 18.2 86 Al coa~ed* 130 cycles - 1600F 10.2 ' ~ .
* - Steels Or the invention - '` ' ' ' :, ,. . . ~ . .
:. . , . . ~ . . ~ ~

13~3 The oxldatlon tests lndicate that all the steels o~ the ~resent inventlon exhibited good scalin~ reslstance both in still air and cycllc tests without coatin~s. With aluminum coatlngs, the steels of the lnvention are superior to Armco Type 409 ln uncoated condltion. It should further be noted that the stlll air tests on aluminum and aluminum alloy coated plain carbon steel base metal or substrate showed this material to be completely unacce~table for oxi-datlon reslstance at elevated temperatures of the order of 1500 - 1700 F, because of blistering and warpa~e.
Optimum oxidation resistance ls achieved in a steel of the inventlon containing about 2~ chromium, about

2% aluminum, about 1% silicon and about 0.5% titanium (with titanium about 8 tlmes the carbon content). Sa~le Code 86, having thls approximate analysis, was superior to other sam-ples in the coated condltion des~lte the fact that lt was sub~ected to cyclic test temperatures 100 F degrees hi~her than any of the other materials tested. In the uncoated condition it was at least equivalent to uncoated Type 409 stainless steel and superior to the other uncoated samples.

:; .

...... .. . .. .. . . ..
. . .
... .... .. . .. . .. . : . . . .

Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An alloy steel having good oxidation resistance at elevated temperature, good weldability and good formability, consisting essentially of, by weight percent, from 0.01% to 0.13% carbon, from 0.5% to 3% chromium, from 0.8% to 3% aluminum, from 0.4% to 1.5% silicon, from 0.1% to 0.6%
manganese, a maximum of about 0.05% molybdenum, a maximum of about 0.05%
vanadium, less than about 0.2% copper, less than about 0.2% nickel, an ele-ment chosen from the group consisting of titanium, columbium, zirconium, and mixtures thereof, titanium when present being from 0.1% to 1%, columbium when present being from 0.1% to 1.5%, zirconium when present being from 0.1%
to 1.5%, and mixtures when present being from 0.1% to 1.5% total, and remain-der iron except for incidental impurities.

2. Alloy steel according to claim 1, consisting essentially of, by weight percent, from 0.04% to 0.06% carbon, from 1.7% to 2.1% chromium, from 1.7% to 2.0% aluminum, from 0.6% to 0.9% silicon, from 0.2% to 0.4% manganese, from 0.1% to 0.6% titanium, and remainder iron except for incidental impuri-ties.

3. Alloy steel according to claim 2 wherein columbium, zirconium, or a mixture thereof is substituted at least partially in place of titanium.

4. An aluminum or aluminum alloy coated article having good oxida-tion resistance at elevated temperature, good resistance against attack by hydrocarbon combustion products at elevated temperature, and high strength, said article having a substrate consisting essentially of, by weight percent, from 0.01% to 0.13% carbon, from 0.5% to 3% chromium, from 0.8% to 3% alumi-num, from 0.4% to 1.5% silicon, from 0.1% to 0.6% manganese, a maximum of about 0.05% molybdenum, a maximum of about 0.05% vanadium, less than about 0.2% copper, less than about 0.2% nickel, an element chosen from the group consisting of titanium, columbium, zirconium, and mixtures thereof, titanium when present being from 0.1% to 1%, columbium when present being from 0.1%

to 1.5%, zirconium when present being from 0.1% to 1.5%, and mixtures when present being from 0.1% to 1.5% total, and remainder iron except for incid-ental impurities.

5. Coated article according to claim 4 having a substrate consisting essentially of, by weight percent, from 0.04% to 0.06% carbon, from 1.7% to 2.1% chromium, from 1.7% to 2.0% aluminum, from 0.6% to 0.9% silicon, from 0.2% to 0.4% manganese, from 0.1% to 0.6% titanium, and remainder iron except for incidental impurities.

6. Article for high temperature applications having a coating of aluminum or aluminum alloys containing up to 10% silicon, the weight of said coating being about 1/2 ounce per square foot of sheet (0.064 gram per square centimeter), and a cold reduced and annealed sheet metal base consisting es-sentially of, by weight percent, from 0.01% to 0.13% carbon, from 0.5% to 3% chromium, from 0.8% to 3% aluminum, from 0.4% tol.5% silicon, from 0.1%
to 0.6% manganese, a maximum of about 0.05% molybdenum, a maximum of about 0.05% vanadium, less than about 0.2% copper, less than about 0.2% nickel, an element chosen from the group consisting of titanium, columbium, zirconium, and mixtures thereof, titanium when present being from 0.1% to 1%, columbium when present being from 0.1% to 1.5%, zirconium when present being from 0.1%
to 1.5%, and mixtures when present being from 0.1% to 1.5% total, and remain-der iron except for incidental impurities.

7. Article according to claim 6, wherein said sheet metal base consists essentially of, by weight percent, from 0.04% to 0.06% carbon, from 1.7% to 2.1% chromium, from 1.7% to 2.0% aluminum, from 0.6% to 0.9% silicon, from 0.2% to 0.4% manganese, from 0.1% to 0.6% titanium, and remainder iron except for incidental impurities, said articles having good oxidation resis-tance at elevated temperature, good resistance against attack by hydrocarbon combustion products at elevated temperature, and high strength.

8. Article according to claim 6 or claim 7, wherein columbium, zir-conium, or a mixture thereof is substituted at least partially for titanium in a stoichiometrically equivalent amount.