Note: Descriptions are shown in the official language in which they were submitted.
~~5636~
1
HEAT RESISTING FERRITIC STAINLESS STEEL EXCELLENT IN LOW
TEMPERATURE TOUGHNESS, WELDABILITY AND HEAT RESISTANCE
Field of the Invention
The present invention relates to a heat resisting
ferritic stainless steel excellent in low temperature
toughness, weldability and heat resistance. The stain-
less steel according to the invention is suitable for use
in composing a part of an exhaust gas path-way of an au-
tomobile, especially, a path-way from an engine to a con-
verter, which is exposed to high temperatures.
Background of the Invention and Prior Art
In recent years, air pollution caused by an automo-
bile exhaust gas has become a serious problem and NOx,
HC, C0, etc. of the exhaust gas have been restricted in
quantities from a view point of preventing environmental
pollution. The restriction is now getting more and more
severe in consideration of acidic rain and others.
Therefore, it is necessary to further improve an effi-
2 0 ciency of the exhaust gas purification.
On the other hand, a recent increasing demand for a
more powerful and capable engine tends to rise up the ex-
haust gas temperature. Under the circumstances, parts of
2 5 an exhaust gas system are exposed to higher temperatures
2~~636~
2
while driving the engine. Particularly, parts between
the engine and a converter of exhaust gas purifying in-
struments, for example, an exhaust manifold, dual tube
and the like, cannot help being exposed to still higher
temperatures. In addition, these parts undergo not only
changes in mechanical stress due to oscillation caused by
driving the engine and running of the automobile, but
also changes in temperature due to heating and cooling
cycles depending upon patterns of driving and, in some
1~ cases, to freezing in cold areas. Thus, the parts are
exposed to mechanically and thermally severe conditions.
As long as a heat resisting steel, for example, a
stainless steel is applied as a material for the produc-
tion of these parts, heat resistivity, of course, is ex-
cellent. However, because of weld-joints (the pipe used
for these parts is usually made by weld and is often
jointed to other parts by weld), the material must be ex-
cellent in weldability and in mechanical workability.
2 0 Therefore, it is important that the material used for
this purpose must be not only corrosion resistant which
is the fundamental property of a stainless steel but also
heat resistant, tough at low temperature, weldable and
workable.
zo~~~~2
3
SUS304, a typical austinitic stainless steel, has
been considered as a favorable material for use for the
above-mentioned purpose because of its excellent worka-
bility and favorable weldability. However, since an aus-
tinitic stainless steel has a large thermal expansion co-
efficient, fears are entertained for a thermal fatigue
cracking caused by a thermal stress which comes about in
the repeated heating and cooling. In addition, because
of a large difference in thermal expansion between an
austinitic stainless steel and its surface oxide, the ox-
ide layer tends to splinter off from the surface of the
steel. For these reasons, a nickel base alloy repre-
sented by Inconel 600 is used in some parts as the path-
way material for an exhaust gas of an automobile. This
alloy is promising for the reasons that its thermal ex-
pansion coefficient is small whereby the oxide layer is
tight adhesive to the surface and, in consequence, it is
excellent in high temperature oxidation resistance as
well as high temperature strength. However, this alloy
2 0 is very expensive so that it is not extensively used.
On the other hand, when compared with the austinitic
stainless steel, a ferritic stainless steel is cheap and,
in addition, excellent in thermal fatigue properties be-
t 5 cause of its small thermal expansion coefficient, so that
it is considered suitable for use in parts which are
20~~~0~
4
subjected to cyclic variation of temperature such as
heating and cooling. Type 409 or SUS430, a representa-
tive of the ferritic stainless steel, is going on to use
in part of an automobile exhaust gas path-way . However,
these materials have a property that the strength goes
sharply down as the temperature 900 °C. and higher, and
in consequence, give rise to problems of which one is fa-
tigue cracking due to insufficient strength and the other
is abnormal oxidation when conditions go beyond the limit
1~ of oxidation resistivity. A counter action to these
problems may be possible by means of addition of various
alloying elements, which improve high temperature
strength, or by means of increasing a chromium content to
improve oxidation resistance. However, such means of ad-
dition of alloying elements or increase of chromium con-
tent make, in general, not only impact toughness of the
steel weaken steeply but also weldability and workability
get worse remarkably.
2 0 Any stainless steel that is in conformity with the
above-mentioned conditions becoming more and more severe
according to the demands for a more powerful and capable
engine and for the progress of a purification efficiency
of an exhaust gas is not come out yet. In other words, a
2 S material which is economical and satisfies simultaneously
various demands for properties such as high temperature
205636
s
strength, oxidation resistance, heat resistance, tough-
ness, weldability and workability is not yet obtainable
from austinitic or the ferritic stainless steels nowa-
days. If a ferritic stainless steel retaining the previ-
ously stated desirable properties inherent to the fer-
ritic stainless steel, and having improved heat resistiv-
ity and high temperature strength and, in addition, being
excellent in productivity, workability, weldability and
low temperature toughness comes to be obtainable, it may
be said that such a material is very promising for the
particular use mentioned above.
JP A 64-8254 discloses a ferritic stainless steel
for the like use, but is completely silent with respect
to low temperature toughness. JP B 59-52226 and 61-44121
disclose to improve a ferritic stainless steel in its
rust development due to chlorine ion and its acid resis-
tivity by adding copper and nickel while extremely lower-
ing S, but teach nothing about high temperature strength,
2 0 heat resistance, weldability and low temperature tough-
ness.
Object of the Invention
Accordingly, an object of the invention is to pro-
vide a ferritic stainless steel having properties which
2 S simultaneously meet the above-mentioned many severe con-
ditions required for a material of an automobile exhaust
_ 246362
6
gas path-way, particularly, of a part between an engine
and a converter where the material is exposed to high
temperatures. Another object of the invention is to im-
prove low temperature toughness, which is an inherent de-
fect of ferritic stainless steels. A further object of
the invention is the provision of a heat resistive fer-
ritic stainless steel which does not suffer from a prob-
lem of high temperature cracking of weld heat-affected
zone.
Summary of the Invention
The invention provides a heat resisting ferritic
stainless steel excellent in low temperature toughness,
weldability, and heat resistance which comprises, by
1 S weight,
up to 0.03 0 of C,
from 0.1 to 0.8 % of Si,
from 0.6 to 2.0 % of Mn,
up to 0.006 0 of S,
2 ~ up to 4 0 of Ni,
from 17.0 to 25.0 0 of Cr,
from 0.2 to 0.8 °s of Nb,
from 1.0 to 4.5 0 of Mo,
from 0.1 to 2.5 0 of Cu, and
2 S up to 0 . 03 0 of N,
20~6~6~
the balance being Fe and unavoidable impurities, wherein
the alloying elements are further adjusted so that the
ratio of Mno/So is not less than 200, [Nb] defined by the
equation:
[Nb] - Nbo - 8 (Co + No)
is not less than 0.2, and (Nio + Cuo) is not more than 4.
The invention further provides a heat resisting fer-
ritic stainless steel excellent in low temperature tough-
ness, weldability and heat resistance which comprises, in
addition to the elements of the above-mentioned steel,
one or more of:
up to 0.5 0 Al,
of
up to 0.6 0 Ti,
of
1 $ up to 0.5 % V,
of
up to 1.0 0 Zr,
of
up to 1.5 0 W,
of
up to 0.01% B, and
of
up to 0.1 % REM.
of
Brief Description of the Drawings
Fig. 1 shows a relationship between molybdenum con-
tent and tensile strength at the indicated elevated tem-
~~~~3~
s
peratures obtained by the elevated temperature tensile
test noted below;
Fig. 2 shows a relationship between manganese con-
s tent and amount of scale which has splintered off after
the elevated temperature oxidation test noted below;
Fig. 3 shows a relationship between Mn/S and criti-
cal strain obtained by the weld high temperature cracking
test noted below; and
Fig. 4 shows a relationship between copper content
and Charpy impact strength obtained by the Charpy impact
test at the indicated temperatures. The invention is
1S based on the results shown in these figures.
Detailed Description of the Invention
After many experimental researches to achieve the
object mentioned before, the inventors have been able to
2 0 obtain the following information.
Fig. 1 shows results of the tensile tests at the
indicated elevated temperatures carried out on materials
having a basic composition of Fe-18 o Cr-0.45 ~-Nb with
2 5 various Mo and Cu contents to examine effects of Mo and
Cu on high temperature tensile strength. As seen from
9
the figure, high temperature strength is improved by the
addition of molybdenum in an amount of 1 0 or more.
Furthermore, the conjoint addition of molybdenum and cop-
per is more effective than the addition of molybdenum
alone to improve high temperature strength.
Fig. 2 shows results of the oxidation tests at the
indicated elevated temperatures carried out on materials
having a basic composition of Fe-18 % Cr-0.45 o-Nb with
l~ various Mn contents. The oxidation was continued in air
for 100 hrs at 900 °C. or 1000 °C., and at the end of the
period an amount of scale which had splintered off was
measured. As seen from the figure, the scale splintering
was suppressed, irrespective of the oxidation temperature
tested, by the addition of at least about 0.6 % of man-
ganese. Thus, it can be understood that, as for the fer-
ritic stainless steel, manganese makes the limit of oxi-
dation resistivity to rise up.
2 0 Fig. 3 shows results of the weld high temperature
affected cracking test on materials having a basic compo-
sition of Fe-18 o Cr-0.45 %-Nb with appropriate Mo and Cu
contents whose effects are recognized as shown in Fig. 1
(3 o Mo and 0.5 o Cu) and with varied Mn and S contents
2 5 to examine effects of the ratio, Mn/S, on weld high tem-
perature affected cracking. The test was carried out as
20~63~
to
follows. The cold rolled and annealed plate of 1.2 mm in
thickness was cut into test pieces of 40 mm x 200 m. The
test pieces were TIG welded under various tensile
stresses imposed longitudinally. The minimum strain at
which cracking began to occur was determined, which is
referred to herein as the critical strain and is a mea-
sure of the susceptibility to the weld high temperature
affected cracking. It is revealed from Fig. 3 that if
the ratio, Mn/S, is 200 or higher, ferritic stainless
steels having conjointly incorporated with Mo and Cu have
an increased critical strain, and, in consequence, an im-
proved weldability. Thus, in order to overcome the weld
high temperature affected cracking it is effective to add
a proper amount of Mn rendering the ratio, Mn/S, not less
than 200.
Fig. 4 shows results of the Charpy impact test car-
ried out on materials having a basic composition of Fe-18
o Cr-0.45 o-Nb with varied Mo and Cu contents for examin-
2 0 ing effects of molybdenum and copper on toughness. The
impact value is lowered by the addition of molybdenum, as
is known in the art. However, Fig. 4 provides new infor-
mation that the reduction in the impact value due to Mo
may be compensated to some extent by conjoint addition of
2 5 Cu. Particularly, even with such a material as a steel
containing 4 ~ of molybdenum whose impact toughness is
20~63G j
11
remarkably low, the conjoint addition of copper improves
the impact value well enough. Furthermore, the conjoint
addition of nickel and molybdenum can also improve low
temperature impact toughness, as will be manifested in
Examples described later. The information of these facts
is of great importance, particularly for a material which
constitutes parts exposed to low temperature circumstance
in winter, for example, a manifold or dual tube which
suffer from mechanical vibration in addition to low tem-
perature when the engine starts, whereupon the material
will become usable even under further more severe condi-
tions expected in the future.
Based on the information noted above, the invention
provides a ferritic stainless steel having well-balanced
excellent properties as a whole, including high tempera-
ture strength, thermal fatigue resistance, oxidation re-
sistance and low temperature toughness.
2 0 The reasons for the restriction of each chemical
component in the steel according to the invention will
now be outlined.
C and N: C and N are, in general, important ele-
2 5 ments because of promoting high temperature strength, but
excessive amounts of them demote oxidation resistance,
~o~s~~~
12
workability and toughness. Besides above, C and N react
and form compounds with Nb, thereby lowering the effec-
tive Nb in the ferritic phase. Accordingly, it is favor-
able that C and N are small in quantities, so that they
should be controlled not more than 0.03 0, respectively.
Si: Si is an effective element to improve oxidation
resistance, but an excessive amount of Si renders the
steel hard, and, in consequence, adversely affects worka-
bility and toughness. Therefore, Si is controlled within
the range from 0.1 o to 0.8 0.
Mn: Mn reacts with S, which is harmful for weld high
temperature affected cracking, and fixes S in the form of
MnS, whereby S is removed or reduced in welded metal . It
has been found that if the relation, Mn/S > 200, is sat-
isfied, the effect is the same as that of S reduction.
On the other hand, the addition of at least 0.6 0 of Mn
improves adhesion of scale Therefore, Mn is controlled
2 0 in the range from 0.6 o to 2.0 0, while satisfying the
relation: Mn/S > 200.
S: As previously stated, since S is harmful to the
weld high temperature affected cracking, it is desirable
2 S that S is as small as possible in quantity. However, the
smaller S is, the more the cost is needed for the produc-
205636 ~
13
tion. Even if S remains up to 0.006 0, enough durability
to the weld high temperature affected cracking is held on
the steel of this invention due to the effect of Mn, so
that the upper limit of S is now set as 0.006 0.
Ni: As illustrated in Examples, Ni brings about a
favorable result of improving toughness like copper does.
However, an excessive of Ni gives rise to deposition of
an austenite phase at elevated temperatures, and follows
the increase of thermal expansion coefficient as well as
anxiety about the deterioration of thermal fatigue.
Therefore, in the case of the conjoint addition of Ni and
Cu according to the invention, the Cu being also an
austenite former, it has been found that (Ni + Cu) should
be not more than 4 %.
Cr: Cr is an indispensable element to improve cor-
rosion resistivity and oxidation resistivity. The reason
of limiting Cr as not less than 17 o is that the addition
2 0 of at least 17 0 of Cr is required to keep a desired
level of oxidation resistance at a temperature of at
least higher 900 °C. In this view, the more Cr is, the
better, but the addition of an excessive amount of Cr
renders the steel brittle, and deteriorates workability
2 5 due to increase in hardness. Accordingly, the upper
limit of Cr is now set as 25 0.
2o~s362
14
Nb: Nb is a necessary element to maintain high tem-
perature strength. Furthermore, Nb improves workability
and oxidation resistivity, and still brings about a fa-
vorable influence in the manufacture of pipe by a high
frequency welding method. The results of the tensile
test at elevated temperatures, shown in Table 2 later on,
reveal that at least 0.2 0 of Nb must be added to improve
high temperature strength. However, Nb reacts and forms
compounds with C and N, so that the Nb dissolved in the
steel decreases and its effect on high temperature
strength decreases also as far as the lower limit of Nb
is merely set as 0.2%. Therefore, Nb must meet the con-
dition that [Nb] expressed in the equation,
1 5 [Nb] - Nb a - 8 (C + N) o,
is at least 0.20. On the other hand, when Nb is added in
excess, welded parts become susceptible to high tempera-
ture affected cracking. The upper limit of Nb is now set
as 0.8 o so that sufficient high temperature strength may
2 0 be held and susceptibility to weld high temperature af-
fected cracking may not be influenced so much.
Mo: As already stated, the more addition of Mo make
high temperature strength to increase. Besides, Mo is
2 5 effective to improve high temperature oxidation resis-
20~63~ ~?
is
tance and corrosion resistivity. However, an excessive
addition of it makes low temperature toughness as well as
productivity and workability to decrease remarkably.
Therefore, Mo is restricted within the range from 1.0 0
s to 4.5 0, preferably from 2.0 o to 4.5 0, still more
preferably within the range of more than 2.5 o and up to
4.5 a.
Cu: As mentioned previously, Cu is an important
element of the steel according to the invention because
of its remarkable effectiveness on toughness. As shown
in Fig. 4, Cu is needed at least 0.1 o to achieve an ap-
preciable improvement to toughness, so that the lower
limit of Cu is now set as 0.1 0. On the contrary, the
is addition of an excessive amount of Cu renders the steel
hard and deteriorates its workability, in particular its
hot workability, so that the upper limit of Cu is now set
as 2.5 0.
2 0 A1: Al improves oxidation resistivity at elevated
temperatures, but the addition of an excessive amount of
A1 poses problems on productivity as well as weldability.
For this reason the upper limit of A1 is now set as 0.5
o.
2s
20563G~
16
Ti: Ti increases high temperature strength and im-
proves workability. Like aluminum, the addition of an
excessive amount of Ti, causes problems on productivity
and weldability, so that the upper limit of Ti is now set
as 0.5
V: Like Ti, V increases high temperature strength
and improves workability, but the addition of an exces-
sive amount of V invites reduction in strength.
Therefore, the upper limit of V is now set as 0.5 0.
Zr: Zr increases high temperature strength and im-
proves oxidation resistance at elevated temperatures.
However, the addition of an excessive amount of Zr in-
1$ vites reduction in strength. Therefore, the upper limit
of Zr is now set as 1.0 0.
W: Similar to Ti and V, W increases high tempera-
ture strength and improves workability, but the addition
2 0 of an excessive amount of W invites reduction in
strength, so that the upper limit of W is now set as 1.5
o.
B: B improves hot workability, high temperature
2 5 strength and even workability. However, the addition of
an excessive amount of B, adversely affects hot workabil-
~~9~63G,
17
ity, on the contrary, therefore the upper limit of B is
now set as 0.01 0.
REM: Even in small quantity the addition of rare-
earth metal improves hot-workability, oxidation resis-
tance, particularly, adhesion of scale. However, the ad-
dition of an excessive amount of REM adversely affects
hot workability on the contrary. Therefore, the upper
limit of REM is now set as 0.1 0.
Examples
Table 1 shows chemical components, in % by weight,
of the tested steels. Steels M1 to M21 are those in ac
cordance with the invention, while Steels M22 to M30 are
control steels. Each steel was made into a 30 kg ingot
and forged to a rod having a diameter of 25 mm, or to a
slab having a thickness of 25 mm. The rod was annealed
at a temperature of from 950 °C. to 1100 °C., and test
pieces for the high temperature tensile test in accor-
2 0 dance with JIS were prepared from the annealed rod. The
slab was cut into pieces, which were heated in a furnace,
took out from the furnace at a temperature of 1200 °C.,
hot rolled to plates having a thickness of 5 mm and an-
nealed at a temperature of from 950 °C. to 1100 °C.
2 5 Some of the annealed plates were as such worked to Charpy
impact test pieces having a thickness of 4.5 mm, while
2o~s~s
1~
the others were made to cold plates having a thickness of
2 mm of 1.2 mm by repeating cold rolling and annealing.
The 2 mm plates were subjected to the high temperature
oxidation test, while the 1.2 mm plates were subjected to
the high temperature affected weld cracking test.
Table 2 shows tensile strength at elevated tempera-
tures determined by the tensile test in accordance with
JIS, amount of scale which splinters off by the oxidation
test continued for 100 hours at 900 °C. and at 1000 °C.,
critical strain of weldment caused by the high tempera-
ture affected cracking test which is previously de-
scribed, and results of the Charpy impact test carried
out on V-notched Charpy impact testing pieces of a thick-
ness of 4.5 mm.
From the results of the tensile test shown in Table
2, it is understood that the addition of Nb, Mo and Ni
increases high temperature strength and the conjoint ad-
2 0 dition of Mo and Cu further improves high temperature
strength. The results of the continuous high temperature
oxidation tests carried out at 900° C. and at 1000 °C.,
indicate that resistivity of scale splintering off in-
creases remarkably when Mn content exceeds 0.6 0. The
2 S critical strain caused by the test of high temperature
affected weld cracking is highly improved when the ratio,
zo~s~s
19
Mn/S, is 200 or higher. The results of the Charpy impact
test reveal that while impact toughness decreases by the
addition of Mo, it is improved by the addition of Cu, and
the same is true with the addition of Ni.
2~~~~~~
Zo
Table 1 Chemical Components (wt.%) of Tested Steels
Steel C Si Mn P S Ni Cr Nb Mo Cu N Other Mn/sNi+Cu[Nb]
M1 0.01120.950.810.0250.00310.3018.1.90.421.200.970.0128- 2791.130.23
M2 0.01180.400.700.0220.00290.2218.280.451.990.240.0113- 2910.960.27
M3 0.01900.250.630.0200.00300.2218.950.912.050.480.0107- 2100.700.21
M4 0.01210.251.920.0200.00350.2018.370.932.010.460.0113- 9060.660.29
M5 0.01060.900.790.0230.00330.2018.550.952. 0.490.0111- 2390. 0.28
93 69
M6 0.01060.370.780.0230.00280.2918.390.973.010.930.0113- 2791.170.29
M7 0.00970.430.790.0210.00270.2718.990.452.971.980.0103- 2932.250.29
M$ 0.01020.920.850.0200.00270.2218.420.962.952.990.010- 3152.660.29
M9 0.01360.980.690.0190.00261.4918.440.933.090.180.013- 2651.490.21
M100.01260.990.680.0170.00292.9818.570.933.020.190.011- 2832.980.29
A Mll0.01100.910.760.0230.00280.2718.310.963.920.520.010- 2710.790.28
M120.01080.920.760.0290.00290.2718.400.963.990.930.0109- 2621.200.29
M130.01140.380.730.0230.00270.2318.220.969.021.880.0112- 2702.110.28
M140.01050.920.790.0220.00280.2118.370.459.920.950.0109- 2821.160.28
M150.01070.390.920.0230.00390.2418.970.462.980.99O.O110A1:0.452360.730.29
M160.01160.420.790.0200.00280.2618.290.973.120.510.010Ti:0.172820.770.29
M170.01120.410.820.0220.00310.2218.360.993.060.500.0121V :0.262650.720.25
M180.01100.410.820.0220.00280.2618.370.963.060.96O.OlO12r:0.732930.720.29
M190.01020.380.850.0210.00330.2518.510.953.010.510.010W :0.892580.760.28
M200.00980.400.710.0210.00320.2018.900.982.990.990.0103B :0.0092220.690.32
M210.01250.410.760.0200.00280.2318.380.433.020.510.0105REM:0.052710.740.25
M220.01260.990.830.0260.00390.2017.950.460.180.130.0099- 2440.330.28
M230.00590.920.830.0210.00250.1918.370.900.220.990.0103- 3320.630.27
M240.01030.490.790.0220.00270.2917.230.910.250.890.0191- 2741.130.29
M250.00910.390.800.0190.00180.2318.370.99- - 0.0105- 9440.230.33
B M260.01200.250.390.0210.00230.2118.250.912.09- 0.0110- 1700.210.23
M270.01190.370.260.0230.00320.2218.350.932.090.920.010- 81 0.690.29
M280.01280. 0. 0.0290.00360.2018.990.052.060.350.0117- 1360.55-0.
97 99 15
M290.01320.980.900.0210.00280.2318.930. 3.02- 0.0107- 1430. 0
19 66
M300.01260.500.980.0220.00350.251.8.760.979.01- 0.0108- 2800.250.28
Note: [Nb] = Nb~ - $(Ca + N~) A: According to the invention B: Control
20~6J~~
21
Table 2 Properties of Tested Steels
Amount Critical
Steel Tensile of Strain Charpy
strength scale upon impact
at splin- Weldingstrength
elevated tering (kg-m/cm2)
tem- after
peratures oxida-tion
(k test
/mm2) (mg/cmz)
700C 900C 900C 1000C( o) -25C 0C 25C
M1 21.7 4.2 0.07 0.12 4.7 18.9 20.2 24.2
M2 22.0 4.3 0.05 0.09 4.5 13.9 17.2 23.3
M3 22.2 4.4 0.04 0.08 4.0 19.0 21.7 27.6
M4 22.2 4.5 0.02 0.04 5.1 19.0 21.7 27.6
M5 22.4 4.6 0.01 0.03 3.9 10.3 11.0 18.9
M6 22.8 4.7 0.02 0.03 4.1 10.7 17.5 18.3
M7 23.1 4.8 0.01 0.04 4.4 6.4 13.6 16.9
M8 23.2 4.7 0.01 0.03 4.5 4.0 6.8 9.7
M9 22.5 4.8 0.01 0.04 4.1 5.9 13.9 17.8
M10 22.7 4.8 0.02 0.03 4.1 6.8 14.7 17.4
A M11 23.0 4.9 0.01 0.02 3.5 5.2 8.6 16.7
M12 23.3 5.0 0.01 0.02 3.7 7.1 14.9 16.3
M13 23.6 5.2 0.02 0.04 3.6 5.2 8.0 9.8
M14 23.4 5.1 0.01 0.03 3.7 6.2 9.7 12.3
M15 22.9 4.9 0.01 0.02 3.5 8.5 9.0 16.1
M16 21.9 4.7 0.02 0.03 4.3 9.2 10.7 17.2
M17 21.7 4.7 0.02 0.03 3.9 10.4 11.8 19.2
M18 21.9 4.8 0.01 0.03 4.3 10.2 13.1 19.7
M19 21.9 4.8 0.01 0.02 4.5 9.7 11.7 20.3
M20 21.8 4.7 0.01 0.02 3.7 10.1 10.9 19.1
M21 21.7 4.7 0.01 0.01 3.9 8.9 10.2 17.1
M22 19.4 3.1 0.10 0.22 3.9 15.6 21.1 25.5
M23 19.6 3.1 0.11 0.25 4.2 25.0 21.4 29.9
M24 20.0 3.2 0.11 0.28 4.4 18.1 19.3 23.2
M25 19.4 3.0 0.10 0.24 5.0 6.4 9.2 12.9
s M26 20.9 3.5 0.20 0.96 2.8 2.0 8.1 22.3
M27 19.1 2.9 0.32 1.32 2.0 17.9 20.5 22.3
M28 19.3 2.9 0.14 0.76 2.5 2.0 8.1 22.3
M29 22.3 4.6 0.16 0.66 1.9 1.9 6.0 6.7
M30 22.9 4.7 0.07 0.09 3.4 1.0 1.1 1.3
Note: A: According to the invention B: Control
20~6~6~
22
Having so described,
the invention has provided a heat resistive ferritic
stainless steel which achieves the above-mentioned object
$ and which has excellent high temperature strength, resis-
Lance to high temperature oxidation, resistance to high
temperature affected weld cracking, improved low tempera-
ture toughness, which is serious drawback of the ferritic
stainless steel. Accordingly, the novel and useful mate-
rial responsible to the progressive increase of power and
capability of the engine has now been offered for an au-
tomobile exhaust gas system, particularly, for a pipe be-
tween an engine and a converter, which pipe is prepared
by welding or jointed to other parts by welding.
