US3909250A - Oxidation-resistant ferrous alloy - Google Patents

Abstract

A low alloy steel for use as a substrate for aluminum or aluminum alloy coatings, the steel containing 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, from 0.1% to 1% titanium and remainder substantially iron. The steel has good oxidation resistance at elevated temperature, good weldability and formability, thereby enhancing its utility for fabrication into a variety of wrought coated products.

Description

United States Patent [1 1 Jasper [4 1 Sept. 30, 1975 [73] Assignee: Armco Steel Corporation,

Middletown, Ohio [22] Filed: May 15, 1974 [211 App]. N0.: 470,175

[52] US. Cl. 75/124; 75/126 D; 75/126 F [51] Int. Cl. C22C 38/06; C22C 38/26;

C22C 38/28 [58] Field of Search 75/124, 126 D, 126 F [56] References Cited UNITED STATES PATENTS 11/1956 Herzog 75/124 X 3/1958 l-lerzog 75/124 X 10/1972 Caule et a1 148/315 Primary Examiner-C. Lovell Assistant Examiner-Arthur J. Steiner Attorney, Agent, or Firm-Melville, Strasser, Foster & Hoffman 5 7 ABSTRACT A low alloy steel for use as a substrate for aluminum or aluminurrf alloy coatings, the steel containing 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, from 0.1% to 1% titanium and remainder substantially iron. The steel has good oxidation resistance at elevated temperature, good weldability and formability, thereby enhancing its utility for fabrication into a variety of wrought coated products.

3 Claims, No Drawings OXIDATION-RESISTANT FERROUS ALLOY This is a division of application Ser. No. 373,278, filed June 25, 1973.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a chromium-aluminumsilicon-titanium steel of low alloy content'for use as a substrate for aluminum or aluminum alloy coatings, and wrought coated products thereof havinggood resistance to oxidation at elevated temperatures up to about 1,700F, and resistance against attack by hydrocarbon combustion products at elevated temperature, together with high strength.

2. Description of the Prior Art Increasingly stringent requirements for antipollution 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 systems, such as catalytic converters, mufflers, and like articles.

Aluminum coated carbon steel has proved to be not completely satisfactory for some high temperature applications. 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 1 1% or more chromium. The cost of such steel is high, thus making it undesirable for proposed use in automotive exhaust systems, such as catalytic converters, and the like. Moreover, although this stainless steel has fair oxidation resistance at elevated temperature, and good formability, it does not adequately withstand attack by molten salts and hydrocarbon combustion products at elevated temperature. The provision of an aluminum coating on such a steel has been found to result in a product having the desired properties, but this solution obviously adds even greater cost.

U.S. Pat. No. 3,698,964, issued October 17, 1972, to E. J. Caule et a1, discloses an iron base alloy with good oxidation resistance at temperatures of about 700 to 800 C (about l,300 to 1,475 F). The alloy of this patent contains up to 2% carbon, 1 to 5% chromium, l to 4% aluminum, and/or 1 to 4% silicon, up to 1.5% manganese, up to 2% copper, up to 0.20% total of nickel, molybdenum, vanadium and other alloyingelements. Preferably, a combination of 2% chromium and 3% aluminum, or 3% chromium and 2% silicon, are used, with manganese preferably up to 0.2%, copper not more than 0.5%, carbon not more than 1% and most preferably from 0.01 to 0.25%.

The high carbon content of the steel of the Caule et al patent results in a brittle structure having very limited cold workability, poor welding characteristics, and poor mechanical properties generally. The optional presence of molybdenum and vanadium adversely affects oxidation scale resistance, as well as adding to the cost. The harmful effect of molybdenum on oxidation resistance is reported in Stainless And Heat Resisting Steels," by Colombier and Hochmann, p. 330, St. Mar tins Press, New York, N.Y. (1968). Moreover, copper, nickel, and other austenite stabilizers in amounts greater than typical residual contents of about 0.2% each are undesirable since it can cause a phase change, with a consequent change in volume, on heating and cooling. This volume change results in cyclic heating and cooling.

U18. Pat. No. 2,825,669, issued Mar. 4, 1958, to E. M. Herzog, discloses a steel, and a heat treatment therefor wherein a micro-structure is produced having resistance to stress-corrosion cracking in wet hydrogen sulfide atmospheres. The steel of this patent contains from 0.08 to 0.20% carbon, from 0.60 to 5.0% chromium, from 0.15 to 1.20% aluminum, from 0.30 to 1.20% manganese, from 0.10 to 0.50% silicon, up to 0.50% molybdenum, up to 1.0% vanadium, up to 1.0% titanium and remainder iron. A heat treatment at 740 to 780 C, a second heat treatment at about 970 to l,080 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 of atleast ksi. Oxidation resistance at elevated temperature is not contemplated in this patent, and the composition would not inherently produce a steel having this property. Moreover, the presence of molybdenum and vanadium is deleterious for reasons set forth above.

Other patents disclosing low alloy steels containing chromium, aluminum, and/or silicon in varying amounts include US. Pat. Nos. 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 within the contemplated operating temperature range, and exhibiting in combination good oxidation resistance at elevated temperature, good resistance against attack by hydrocarbon combustion products, good weldability and formability and relatively high strength. Hence, there still exists a great need for a low-cost alloy having the above combination of properties for fabrication into coated welded and wrought products such as space heaters, automotive exhaust systems, e.g., catalytic converters and mufflers, and the like.

, SUMMARY The present invention provides a low alloy steel containing chromium, aluminum, silicon and titanium (preferablywith a total alloy content of less than about 5%) which 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. In its broad'composition ranges, the steel of the invention consists essentially of from about 0.01% to about 0.13% carbon, from about 0.5% to about 3% chromium, from about 0.8% to about 3% aluminum, from about 0.4% to about 1.5% silicon, from about 0.1% to about 0.6% manganese, from about 0.1% to about 1% titanium, and remainder iron except for incidental impurities. Molybdenum and vanadium are restricted to a maximum of about 0.05% each, and copper, nickel and other austenite stabilizers to less than about 0.2% each. i

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.

spalling after Carbon is essential in an amount of at least about 0.01% in order to provide the necessary strength in the steel. More than about 0.13% carbon cannot be tolerated because of its adverse effect upon the weldability, formability and and general mechanical properties of the steel, and because it is a strong austenite former.

At least 0.5% chromium in combination with at least about 0.8% aluminum and about 0.4% silicon is necessary in order to provide good oxidation resistance. A maximum of 3% chromium should be observed in order to minimize cost and avoid processing difficulties.

- At least about 0.8% aluminum is necessary not only for oxidation resistance at elevated temperature but also to provide adequate tensile strength. More than 3% aluminum results in a loss of formability and workability.

At least about 0.4% silicon is essential since it cooperates with the chromium and aluminum to impart oxidation resistance. However, a maximum of about 1.5% silicon should be observed since amounts in excess thereof also result in loss of formability and workability.

Titanium is essential in an amountof at least about 0.1% 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 be on a stoichiometric basis, with a minimum weight ratio of columbium or zirconium to carbon of 8:1, preferably at least about 10:1. Columbium and/or zirconium would thus range from about 0.10% to about 1.5% if substituted for titanium.

Impurities at residual levels normal for ferritic stainless steels can be tolerated in the steel of the invention. More specifically, a maximum of about 0.03% sulfur 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 at a maximum of less than 0.2% each for reasons set forth above.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 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 about 0.04 to about 0.06% Chromium about 1.7 to about 2.1 Aluminum about 1.7 to about 2.0

Silicon about 0.6 to about 0.9 7: Manganese about 0.2 to about 0.4 70 Titanium about 0.1 to about 0.6

the titanium being about 8 times the carbon content and remainder iron except for incidental impurities.

In order to investigate the effect of the relative proportions of chromium, aluminum, silicon and titanium onthe 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 1 below.

Steels of the invention also containing 0.1% Cu, 0.04% Mo, and 0.03% V.

The above materials were hot rolled from 2100 F (ll49 C) from 1 inch by 3 inch ingots to 0.1 inch thickness. Samples were annealed at 1,700 F (927 C) for 10 minutes, descaled and cold rolled to 0.05 inch thickness. It should be recognized that annealing the hot rolled material is optional. Tensile strengths were determined on the cold rolled samples at this stage while the remainder of the cold rolled strip was an- 'nealed at 1600 F (871 C) for 6 minutes and pickled. This material was tested for the remaining mechanical properties reported below in Table I].

A consideration of the mechanical properties reported in Table 11 indicates that the steels of the invention have ultimate tensile strengths equivalent to those of comparable prior art alloys but have improved elongation. This is believed to result from the relatively low carbon contents (ranging from about 0.03% to about 0.07%). The yield strength and tensile strengths of the alloys containing about 1% aluminum were about 5 ksi and 4 ksi, respectively, less than those containing 2% aluminum. Of greater significance is the comparison with the 1% chromium alloy (Sample Code 15) also having low aluminum and silicon contents. It will be noted that the yield strength of the 1% chromium alloy was about 12 ksi lower and the tensile strength about 8 k'si lower than the steels of the invention containing from about 1.7% 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 combination of mechanical properties, it is thus apparent that the carbon content should not exceed TABLE II 0.2% Olsen Sample YS UTS 71 Elong. Hardness Cup Test Code (ksi) (ksi) in 2" (Rockwell B) Heightlnches Steels of the present invention Samples of the steels of Table l in the cold rolled, an nealed and pickled condition were surface ground, and a full penetration autogenous GTA weld was run down the longitudinal axis of the strip of each sample. 180 bend and Olsen cup test specimens were cut from the samples and tested with both root and face side in tension. These tests are reported in Table III.

From the data in Table 111 it is evident that optimum as-welded ductility is exhibited with about 1% aluminum and chromium in excess of 1 At the 2% aluminum level better results apparently are obtained with chromium at about 2%.

TABLE 111 pickled condition, both on coated and uncoated specimens. For the coated specimens, a pure aluminum coating was applied by hot-dipping using a Lundin flux, details of which are disclosed in US. Pat. Nos. 2,686,354 and 2,686,355. Coating weight was about one-half ounce per square foot of sheet (total coating weight on both surfaces). For purposes of comparison, oxidation tests were also run on plain carbon steel coated with pure aluminum and with aluminum alloy containing up to 10% silicon, uncoated Armco Type 409 stainless steel, and an uncoated commercial alloy containing 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 l,600 F, and 1,700 F, respectively. These tests are reported in Table IV below.

Since still air tests are not necessarily definitive, further specimens of coated and uncoated materials were subjected to cyclic testing, using a cycle of 25 minutes in and 5 minutes out of the furnace for a total of As Welded Properties Sample 180 Bend Test Olsen Cup Test Code Parallel to Weld Heightlnches Root in Tension Face in Tension .12" diam. Flat .12" diam. Flat Root in Tension Face in Tension 15 P,F P,F P,P P,P .310, .290 .340, .360 41* P,P P,P P,F F,F .300, .300 .300, .305 42* P,P P,P P,P P,F .225, .400 .350, .340 43* P,P P,P P,P P,F .370, .370 .350, .390 61* PP P.P P.P P,P .405, .390 .380, .400 62* P,P P,P P,P P,P .380, .315 .370, .310 P,P P,P .065 86* P,P P,P .360

P Pass F Fail Steels of the Invention Duplicate tests in each condition Oxidation resistance tests were conducted on the samples of Table I in the cold rolled, annealed and 130-135 cycles. These results are reported in Table V below.

TABLE IV Oxidation Tests Hours Still Air TABLE IVContinued Oxidation Tests 100 Hours Still Air 1600 F 1700" F Sample Weight Weight Increase Increase Code mg/in rng/in 42 uncoated 6.7 1 1.3 43 uncoated 4.1 7.8 61 uncoated 12.8 14.2 62 uncoated 6.3 19.3 85 uncoated 18.1 25.0 86 uncoated 2.1 3.9 Al coated 30.2 41 Al coated 12.5 11.1 42 Al coated 16.2 10.0 43 Al coated 13.6 10.0 61 Al coated 14.8 13.6 62 Al coated 16.6 9.8 86 Al coated 6.1 6.2

TABLE V superior to other samples in the coated condition despite the fact that it was subjected tocyclic test temper- Oxidation Tests atures 100 F. higher than any of the other materials Sample Cyclic 25,5 w Inch tested. In the uncoated condition it was at least equiva- Code Conditions ease m /in lent to other uncoated samples.

Type 409 stainless 135 cycles 1500F 118 15 uncoated 135 cycles 1500"F 666 41 uncoated 135 cycles 1500F 315 42 uncoated] 135 cycles 1500F 270 43 uncoated 135 cycles 1500F 250' 61 uncoated 135 cycles 1500F 382 62 uncoated 135 cycles 1500F 376 85 uncoated 132 cycles 1500F 198 86 uncoated 132 cycles 1500F l 17 15 Al coated 135 cycles 1500F 37.2 41 Al coated 135 cycles 1500F 17.4 42 Al coated 136 cycles 1500F 15.0 43 Al coated 135 cycles 1500F 12.3 61 A1 coated 135 cycles 1500F 14.3 62'Al coated 135 cycles 1500F 18.2 86 A1 coated 130 cycles 1600F 10.2

The oxidation tests indicate that all the steels of the present invention exhibited good scaling resistance both in still air and cyclic tests without coatings. With aluminum coatings, the steels of the invention are superior to Armco Type 409 in uncoated condition. It should further be noted that the still air tests on aluminum and aluminum alloy coated plain carbon steel base metal or substrate showed this material to be completely unacceptable for oxidation resistance at elevated temperatures of the order of l,500 l,700 F, because of blistering and war-page.

Optimum oxidation resistance is achieved in a steel of the invention containing about 2% chromium, about 2% aluminum, about 1% silicon and about 0.5% titanium (with titanium about 8 times the carbon content). Sample Code 86, having this approximate analysis, was

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An alloy having good oxidation resistance at elevated temperature, good weldability and good formability, consisting essentially of, by weight percent, from about 0.04 to about 0.06% carbon, from about 1.7 to about 2.1% chromium, from about 1.7 to about 2.0% aluminum, from about 0.6 to about 0.9% silicon, from'about 0.2 to about 0.4% manganese, from about 0.1 to about 0.6% titanium, with the titanium content being at least about 8 times the carbon content, and remainder iron except for incidental impurities.

2. The alloy claimed in claim 1, including a maximum of about 0.05% m0lybdenum, a maximum of about 0.05% vanadium, and less than about 0.2% copper.

3. An alloy'having' good oxidation resistance at elevated temperature, good weldability and formability, consisting essentially of, by weight percent, from about 0.04 to about 0.06% carbon, from about 1.7 to about 2.1% chromium, from about 1.7 to about 2.0% aluminum, from about 0.6 to about 0.9% silicon, from about 0.2 to about 0.4% manganese, from about 0.1 to about 1.5% of an element chosen from the group consisting of columbium, zirconium, and mixtures thereof, with the content of said element beingat least about 10 times the carbon content, and remainder iron except for incidental impurities

Claims (3)

1. AN ALLOY HAVING GOOD OXIDATION RESISTANCE AT ELEVATED TEMPERATURE, GOOD WELDABILITY AND GOOD FORMALITY, CONSISTING ESSENTIALLY OF, BY WEIGHT PERCENT, FROM ABOUT 0.04 TO ABOUT 0.06% CARBON, FROM ABOUT 1.7 TO ABOUT 2.1% CHROMIUM FROM ABOUT 1.7 TO ABOUT 2.0% ALUMINUM, FROM ABOUT 0.6 TO ABOUT 0.9% SILICON, FROM ABOUT 0.2 TO ABOUT 0.4% MANGANESE, FROM ABOUT 0.1 TO ABOUT 0.6% TITANIUM, WITH THE TITANIUM CONTENT BEING AT LEAST ABOUT 8 TIMES THE CARBON CONTANT, AND REMAINDER IRON EXCEPT FOR INCIDENTAL IMPURITIES.

2. The alloy claimed in claim 1, including a maximum of about 0.05% molybdenum, a maximum of about 0.05% vanadium, and less than about 0.2% copper.

3. An alloy having good oxidation resistance at elevated temperature, good weldability and formability, consisting essentially of, by weight percent, from about 0.04 to about 0.06% carbon, from about 1.7 to about 2.1% chromium, from about 1.7 to about 2.0% aluminum, from about 0.6 to about 0.9% silicon, from about 0.2 to about 0.4% manganese, from about 0.1 to about 1.5% of an element chosen from the group consisting of columbium, zirconium, and mixtures thereof, with the content of said element being at least about 10 times the carbon content, and remainder iron except for incidental impurities.