Ferritic stainless steel and processing therefor

Careful control of chemistry, and in particular niobium, and of annealing temperatures provides a ferritic stainless steel of improved creep strength. Annealing is performed at a temperature of at least 1900.degree. F., and in certain embodiments, at a temperature no higher than 1990.degree. F.

The present invention relates to a ferritic stainless steel and the 
manufacture thereof. 
The lower coefficient of thermal expansion of ferritic stainless steels, in 
comparison to austenitic stainless steels, renders them attractive for 
elevated temperature applications such as exhaust pollution control 
systems and various heat transfer devices. Detracting from their 
attractiveness is the fact that their creep strength is generally not 
equal to that of the austenitic steels. 
Through the present invention there is provided a ferritic stainless steel 
of improved creep strength and a process for providing the steel. Niobium 
is added to a ferritic stainless steel melt in specific well defined 
amounts. The melt is subsequently cast, worked and annealed at a 
temperature of at least 1900.degree. F. 
U.S. Pat. No. 4,087,287 describes a niobium bearing ferritic stainless 
steel of improved creep strength, but yet one which is dissimilar to that 
of the subject invention. Among other differences in chemistry, niobium is 
not controlled within the tight limits of the subject invention. 
Processing is also dissimilar from that of the subject invention. 
An article entitled, "Elevated Temperature Mechanical Properties and Cyclic 
Oxidation Resistance of Several Wrought Ferritic Stainless Steels", by J. 
D. Whittenberger, R. E. Oldrieve and C. P. Blankenship discusses creep 
properties for ferritic stainless steels. The article appeared in the 
November 1978 issue of Metals Technology, pages 365-371. It does not 
disclose the niobium-bearing steel of the subject invention. Moreover, it 
discloses a maximum annealing temperature of 1285.degree. K. (1825.degree. 
F.), whereas the minimum annealing temperature for the subject invention 
is 1900.degree. F. 
A third reference, U.S. Pat. No. 4,059,440, discloses a niobium-bearing 
ferritic stainless steel, but not one within the limits of the subject 
invention. U.S. Pat. No. 4,059,440 is not at all concerned with creep 
strength. No reference to an anneal at a temperature of at least 
1900.degree. F. is found therein. 
It is accordingly an object of the present invention to provide an improved 
ferritic stainless steel and a process for the manufacture thereof. 
By carefully controlling chemistry, and in particular niobium, and by 
controlling processing to include an anneal at a temperature of at least 
1900.degree. F., the present invention provides a ferritic stainless steel 
of improved creep strength and a process for producing it. The present 
invention provides an 11 to 20% chromium ferritic stainless steel 
characterized by a creep life to one percent elongation at 1600.degree. F. 
under a load of 1200 pounds per square inch, of at least 160 hours and 
preferably at least 250 hours. 
Processing for the subject invention comprises the steps of: preparing a 
steel melt containing, by weight, up to 0.1% carbon, up to 0.05% nitrogen, 
from 11 to 20% chromium, up to 5% aluminum, up to 5% molybdenum, up to 
1.5% manganese, up to 1.5% silicon, up to 0.5% nickel, up to 0.5% copper, 
up to 0.6% titanium and from 0.63 to 1.15% effective niobium (discussed 
hereinbelow); casting the steel; working the steel; and annealing the 
steel at a temperature of at least 1900.degree. F. Part of the niobium may 
be replaced by tantalum so as to provide an effective niobium and tantalum 
content in accordance with the following equation: 
##EQU1## 
Effective niobium and tantalum are computed, in accordance with the 
following: 
##EQU2## 
If A is positive or zero: Then Effective Nb content=weight % Nb 
Effective Ta content=weight % Ta 
If A is negative: 
Then When Ta is absent 
Effective Nb content= 
##EQU3## 
When Nb and Ta are present together 
##EQU4## 
Then if B is positive or zero: Effective Nb content=B 
Effective Ta content=weight % Ta 
If B is negative: 
Effective Nb content=0 
Effective Ta content= 
##EQU5## 
Tantalum which may be present as an impurity in niobium is not, in the 
absence of specific tantalum additions, taken into account in determining 
effective niobium and tantalum contents. The effective tantalum content is 
usually less than four times the effective niobium content. 
The steel is annealed at a temperature of at least 1900.degree. F. so as to 
improve its creep strength. The annealing time is usually for a period of 
from 10 seconds to 10 minutes. Longer annealing times can be uneconomical, 
and in addition, can adversely affect grain size. Grain size control is 
significant in those instances where the steel is to be cold formed. Steel 
which is to be cold formed should be characterized by a structure wherein 
substantially all of the grains are about ASTM No. 5 or finer. As 
excessive grain growth can occur at higher temperatures, a particular 
embodiment of the subject invention is dependent upon a maximum annealing 
temperature of 1990.degree. F. 
The alloy of the subject invention is a ferritic stainless steel which 
consists essentially of, by weight, up to 0.1% carbon, up to 0.05% 
nitrogen, from 11 to 20% chromium, up to 5% aluminum, up to 5% molybdenum, 
up to 1.5% manganese, up to 1.5% silicon, up to 0.5% nickel, up to 0.5% 
copper, up to 0.6% titanium, and niobium and tantalum in accordance with 
the following: 
(a) 0.63 to 1.15% effective niobium, in the absence of tantalum. 
(b) effective niobium and tantalum in accordance with the equation 
##EQU6## 
when both niobium and tantalum are present, balance essentially iron. As 
described hereinabove, effective niobium and tantalum are computed, in 
accordance with the following: 
##EQU7## 
If A is positive or zero: Then Effective Nb content=weight % Nb 
Effective Ta content=weight % Ta 
If A is negative: 
Then When Ta is absent 
Effective Nb content= 
##EQU8## 
When Nb and Ta are present together 
##EQU9## 
Then if B is positive or zero: Effective Nb content=B 
Effective Ta content=weight % Ta 
If B is negative: 
Effective Nb content=0 
Effective Ta content= 
##EQU10## 
Carbon and nitrogen are preferably maintained at maximum levels of 0.03%. 
At least 11% chromium is required to provide sufficient oxidation 
resistance for use at elevated temperatures. Chromium is kept at or below 
20% to restrict the formation of embrittling sigma phase at elevated 
temperatures. Up to 5% aluminum may be added to improve the steel's 
oxidation resistance. When added, additions are generally of from 0.5 to 
4.5%. Molybdenum may be added to improve the alloy's creep strength. 
Additions are generally less than 2.5% as molybdenum can cause 
catastrophic oxidation. Titanium may be added to affect stabilization of 
carbon and nitrogen as is known to those skilled in the art. Niobium (with 
or without tantalum) in critical effective amounts greater than that 
required for stabilization, has been found to provide an increase in 
elevated temperature creep life values. Some niobium and/or tantalum may 
act as a stabilizer in lieu of titanium, without materially affecting the 
equations discussed hereinabove. Manganese, silicon, copper and nickel may 
be present within the ranges set forth hereinabove, for reasons well known 
to those skilled in the art. 
The ferritic stainless steel of the subject invention is characterized by a 
creep life to one percent elongation at 1600.degree. F. under a load of 
1200 pounds per square inch, of at least 160 hours and preferably at least 
250 hours. A particular embodiment thereof, is as discussed hereinabove, 
characterized by a structure wherein substantially all of the grains are 
about ASTM No. 5 or finer.

The following examples are illustrative of several aspects of the 
invention. 
EXAMPLE I 
Samples from two heats (Heats A and B) were hot rolled, cold rolled to a 
thickness of 0.05 inch and annealed at temperatures at 1997.degree. and 
2045.degree. F. The chemistry of the heats appears hereinbelow in Table I. 
TABLE I 
__________________________________________________________________________ 
Composition (wt. %) 
Heat 
C N Cr Al Mo Mn Si Ni Ti Nb Fe 
__________________________________________________________________________ 
A 0.017 
0.009 
11.50 
0.021 
0.01 
0.39 
0.43 
0.23 
0.14 
0.74 
Bal. 
B 0.02 
0.027 
19.10 
0.020 
0.028 
0.42 
0.55 
0.32 
0.26 
0.68 
Bal. 
__________________________________________________________________________ 
The samples were tested for creep life to one percent elongation at 
1600.degree. F. under a load of 1200 pounds per square inch. The test 
results appear hereinbelow in Table II. 
TABLE II 
______________________________________ 
EFFECTIVE 
ANNEALING NIOBIUM 
HEAT TEMPERATURE (.degree.F.) 
LIFE (hours) 
(wt. %) 
______________________________________ 
A 1997 165 0.74 
A 2045 282 0.74 
B 1997 255 0.68 
B 2045 395 0.68 
______________________________________ 
From Table II, it is noted that all of the samples had a creep life to one 
percent elongation at 1600.degree. F. under a load of 1200 pounds per 
square inch in excess of 160 hours. Significantly, each was processed 
within the limits of the subject invention. All had an effective niobium 
content within the 0.63 to 1.15% range discussed hereinabove, and all were 
annealed at a temperature in excess of 1900.degree. F. It is also noted 
that 75% of the samples had a creep life in excess of 250 hours. 
EXAMPLE II 
Samples from three heats (Heats C, D and E) were hot rolled, cold rolled to 
a thickness of 0.05 inch and annealed at temperatures of 1950.degree. and 
2064.degree. F. The chemistry of the heats appears hereinbelow in Table 
III. 
TABLE III 
__________________________________________________________________________ 
Composition (wt. %) 
Heat 
C N Cr Al Mo Mn Si Ni Ti Nb Fe 
__________________________________________________________________________ 
C 0.028 
0.011 
16.19 
0.029 
0.031 
0.39 
0.41 
0.27 
0.36 
0.42 
Bal. 
D 0.029 
0.015 
16.27 
0.025 
0.031 
0.39 
0.39 
0.27 
0.32 
0.61 
Bal. 
E 0.025 
0.012 
14.34 
0.002 
0.001 
0.37 
0.38 
0.25 
0.001 
0.65 
Bal. 
__________________________________________________________________________ 
The samples were tested for creep life to one percent elongation at 
1600.degree. F. under a load of 1200 pounds per square inch. The test 
results appear hereinbelow in Table IV. 
TABLE IV 
______________________________________ 
EFFECTIVE 
ANNEALING NIOBIUM 
HEAT TEMPERATURE (.degree.F.) 
LIFE (hours) 
(wt. %) 
______________________________________ 
C 1950 60 0.42 
C 2064 13 0.42 
D 1950 130 0.61 
D 2064 65 0.61 
E 1950 148 0.38 
E 2064 67 0.38 
______________________________________ 
From Table IV, it is noted that none of the samples had a creep life to one 
percent elongation at 1600.degree. F. under a load of 1200 pounds per 
square inch of 160 hours. None of the samples were processed in accordance 
with the subject invention, despite the fact that they were annealed at 
temperatures in excess of 1900.degree. F. Not one of them had an effective 
niobium content as high as 0.63%. With regard thereto, it is noted that 
Heat E had a niobium content of 0.65%, but an effective niobium content of 
only 0.38%. 
EXAMPLE III 
Samples from a niobium-free, high titanium heat (Heat F) were hot rolled, 
cold rolled to a thickness of 0.05 inch and annealed at temperatures of 
1938.degree. and 2000.degree. F. The chemistry of the heat appears 
hereinbelow in Table V. 
TABLE V 
__________________________________________________________________________ 
Composition (wt. %) 
Heat 
C N Cr Al Mo Mn Si Ni Ti Nb Fe 
__________________________________________________________________________ 
F 0.015 
0.012 
11.62 
0.026 
0.024 
0.39 
0.43 
0.15 
0.62 
&lt;0.01 
Bal. 
__________________________________________________________________________ 
The samples were tested for creep life to one percent at 1600.degree. F. 
under a load of 1200 pounds per square inch. The test results appear 
hereinbelow in Table VI. 
TABLE VI 
______________________________________ 
EFFECTIVE 
ANNEALING NIOBIUM 
HEAT TEMPERATURE (.degree.F.) 
LIFE (hours) 
(wt. %) 
______________________________________ 
F 1938 21 0 
F 2000 13 0 
______________________________________ 
From Table VI, it is evident that titanium does not improve creep life as 
does niobium. The longest creep life to one percent elongation at 
1600.degree. F. under a load of 1200 pounds per square inch is 21 hours, 
despite the fact that the titanium content is 0.62%. On the other hand, 
niobium-bearing heats A and B with respective titanium contents of 0.14 
and 0.26%, have creep life values in excess of 160 hours (see Example I.). 
EXAMPLE IV 
Samples from four heats (Heats G, H, I and J) were hot rolled, cold rolled 
to a thickness of 0.05 inch and annealed at temperatures of 1913.degree. 
and 2064.degree. F. The chemistry of the heats appears hereinbelow in 
Table VII. 
TABLE VII 
__________________________________________________________________________ 
Composition (wt. %) 
Heat 
C N Cr Al Mo Mn Si Ni Ti Nb Fe 
__________________________________________________________________________ 
G 0.030 
0.015 
16.16 
0.026 
0.031 
0.38 
0.39 
0.27 
0.30 
0.80 
Bal. 
H 0.026 
0.011 
16.11 
0.032 
0.041 
0.37 
0.38 
0.26 
0.36 
1.00 
Bal. 
I 0.027 
0.011 
16.03 
0.024 
0.041 
0.37 
0.38 
0.26 
0.35 
1.20 
Bal. 
J 0.028 
0.011 
16.01 
0.022 
0.040 
0.37 
0.38 
0.26 
0.33 
1.40 
Bal. 
__________________________________________________________________________ 
The samples were tested for creep life to one percent elongation at 
1600.degree. F. under a load of 1200 pounds per square inch. The test 
results appear hereinbelow in Table VIII. 
TABLE VIII 
______________________________________ 
EFFECTIVE 
ANNEALING NIOBIUM 
HEAT TEMPERATURE (.degree.F.) 
LIFE (hours) 
(wt. %) 
______________________________________ 
G 1913 222 0.80 
G 2064 158 0.80 
H 1913 230 1.00 
H 2064 272 1.00 
I 1913 69 1.20 
I 2064 56 1.20 
J 1913 21 1.40 
J 2064 36 1.40 
______________________________________ 
From Table VIII, it is noted that the samples from Heats G and H had a 
creep life to one percent at 1600.degree. F. under a load of 1200 pounds 
per square inch about or in excess of 160 hours and that the samples from 
Heats I and J had a creep life of a substantially shorter duration. 
Significantly, the samples from Heats G and H were processed in accordance 
with the subject invention, whereas those from Heats I and J were not. The 
samples from Heats G and H had an effective niobium content below 1.15%, 
whereas those from Heats I and J had an effective niobium content in 
excess of 1.15%. Alloys within the subject invention have an effective 
niobium content of from 0.63 to 1.15%. 
EXAMPLE V 
Samples from Heats A through J were hot rolled, cold rolled to a thickness 
of 0.05 inch and annealed at temperatures of from 1852.degree. to 
1870.degree. F. The samples were subsequently tested for creep life to one 
percent elongation at 1600.degree. F. under a load of 1200 pounds per 
square inch. The test results appear hereinbelow in Table IX. 
TABLE IX 
______________________________________ 
EFFECTIVE 
ANNEALING NIOBIUM 
HEAT TEMPERATURE (.degree.F.) 
LIFE (hours) 
(wt. %) 
______________________________________ 
A 1870 40 0.74 
B 1870 131 0.68 
C 1866 33 0.42 
D 1866 148 0.61 
E 1866 107 0.38 
F 1852 25 0 
G 1866 107 0.80 
H 1866 113 1.00 
I 1866 51 1.20 
J 1866 23 1.40 
______________________________________ 
From Table IX, it is noted that none of the samples had a creep life to one 
percent elongation at 1600.degree. F. under a load of 1200 pounds per 
square inch of 160 hours. None of the samples were processed in accordance 
with the subject invention, despite the fact that some of them had an 
effective niobium content of from 0.63 to 1.15%. Not one of them was 
annealed at a temperature of at least 1900.degree. F. 
EXAMPLE VI 
Samples from Heats G, H and I were hot rolled, cold rolled to a thickness 
of 0.05 inch and annealed at temperatures of from 1852.degree. to 
2064.degree. F. The annealed samples were studied for grain size. The 
results appear hereinbelow in Table X. 
TABLE X 
______________________________________ 
ANNEALING ASTM GRAIN 
HEAT TEMPERATURE (.degree.F.) 
SIZE NO. 
______________________________________ 
G 1866 7-8 
G 1913 7-8 
G 1950 5-7 
G 2064 2-4 
H 1866 7-8 
H 1913 7-8 
H 1950 7-8 
H 2064 4-8 
I 1852 7-8 
I 1876 7-8 
I 1940 7-8 
I 1993 4-6 
______________________________________ 
From Table X, it is noted that samples annealed at a temperature in excess 
of 1990.degree. F. do not have a structure wherein substantially all of 
the grains are about ASTM No. 5 or finer, and that samples annealed at 
temperatures below 1990.degree. F. are so characterized. As discussed 
hereinabove, steel which is to be cold formed after annealing should not 
be annealed at a temperature above 1990.degree. F. Excessive grain growth, 
which is detrimental to cold formalibility, occurs at higher temperatures. 
EXAMPLE VII 
Samples from five heats (Heats A and K through N) were hot rolled, cold 
rolled to a thickness of 0.05 inch and annealed at temperatures of 
1950.degree. or 1997.degree. F. The chemistry of the heats appears 
hereinbelow in Table XI. 
TABLE XI 
__________________________________________________________________________ 
Composition (wt. %) 
Heat 
C N Cr Al Mo Mn Si Ni Ti Nb Fe 
__________________________________________________________________________ 
A 0.017 
0.009 
11.50 
0.021 
0.01 
0.39 
0.43 
0.23 
0.14 
0.74 
Bal. 
K 0.020 
0.015 
12.03 
1.36 
0.035 
0.30 
0.40 
0.20 
0.37 
0.73 
Bal. 
L 0.019 
0.011 
12.25 
1.93 
0.044 
0.36 
0.36 
0.26 
0.43 
0.80 
Bal. 
M 0.023 
0.011 
12.12 
2.88 
0.045 
0.36 
0.36 
0.26 
0.42 
0.80 
Bal. 
N 0.021 
0.011 
12.02 
3.93 
0.045 
0.36 
0.36 
0.26 
0.43 
0.80 
Bal. 
__________________________________________________________________________ 
The samples were tested for creep life to one percent elongation at 
1600.degree. F. under a load of 1200 pounds per square inch. The test 
results appear hereinbelow in Table XII. 
TABLE XII 
______________________________________ 
EFFECTIVE 
ANNEALING NIOBIUM 
HEAT TEMPERATURE (.degree.F.) 
LIFE (hours) 
(wt. %) 
______________________________________ 
A 1997 165 0.74 
K 1997 208 0.73 
L 1950 170 0.80 
M 1950 212 0.80 
N 1950 197 0.80 
______________________________________ 
From Table XII, it is noted that all of the samples had a creep life to one 
percent elongation at 1600.degree. F. under a load of 1200 pounds per 
square inch in excess of 160 hours. Significantly, each was processed 
within the limits of the subject invention. All had an effective niobium 
content within the 0.63 to 1.15% range discussed hereinabove, and all were 
annealed at a temperature in excess of 1900.degree. F. Heats K through N 
differ from Heat A in that they have varying amounts of aluminum. As 
discussed hereinabove, up to 5% aluminum may be added to the alloy of the 
subject invention, to improve its oxidation resistance. 
It will be apparent to those skilled in the art that the novel principles 
of the invention disclosed herein in connection with specific examples 
thereof will support various other modifications and applications of the 
same. It is accordingly desired that in construing the breadth of the 
appended claims they shall not be limited to the specific examples of the 
invention described herein.