TECHNICAL FIELD
[0001] The present invention relates to a ferritic stainless steel and a ferritic stainless
steel for automobile exhaust gas passage members.
BACKGROUND ART
[0002] Ferritic stainless steels are used in heat-resistant applications where thermal distortion
is problematic, because they have a lower thermal expansion coefficient, better thermal
fatigue characteristics and better high-temperature oxidation characteristics as compared
with austenitic stainless steels. Typical applications of the ferritic stainless steels
include automobile exhaust gas passage members such as exhaust manifolds, front pipes,
outer cylinders of catalyst supports, center pipes, mufflers and tail pipes.
[0003] Recent automobile engines tend to increase a temperature of an exhaust gas in order
to improve an exhaust gas purification efficiency and an output, and particularly
high heat resistance (high-temperature strength, high-temperature oxidation resistance)
is required for members close to the engine, such as an exhaust manifold, a front
pipe, and an outer cylinder of a catalyst support. Also, recently, a shape of the
exhaust gas passage member tends to be complicated. In particular, the exhaust manifold
and the outer cylinder of the catalyst support are formed into complex shapes by various
methods such as mechanical press molding, servo press molding, spinning, and hydroforming.
The complicated shape leads to concentration of thermal strain at one point due to
the start and stop of the engine so that thermal fatigue failure tends to take place,
as well as leads to a local increase in a temperature of the material so that abnormal
oxidation also tends to take place. Therefore, the heat resistance cannot be sacrificed
in order to improve formability.
[0004] SUH409L and SUS430J1L are known as ferritic stainless steels having high heat resistance.
SUH409L has good processability and is often used for exhaust gas passage members.
However, in view of the heat resistance level, it is not preferable to apply it to
applications where the temperature of the material is more than 800 °C. On the other
hand, SUS430J1L has good heat resistance, which can be used at 900 °C. However, since
it is hard, any application may be difficult in terms of processability. Therefore,
the following ferritic stainless steels have been developed.
[0005] Patent Document 1 proposes a technique for improving processability by not adding
Nb to a steel composition based on SUS 429 and for suppressing deterioration of thermal
fatigue characteristics by adding Cu to the steel composition. However, the maintenance
in a Cu precipitation temperature range for a long period of time leads to coarse
precipitates of Cu due to agglomeration of the precipitates, resulting in a decreased
effect of improving the high-temperature strength. Therefore, the thermal fatigue
characteristics of the ferritic stainless steel may be degraded.
[0006] Patent Document 2 propose a technique for improving thermal fatigue characteristics
by adding Nb and Cu to a steel composition based on SUS 429, and for leaving martensite
in a slab by increasing ymax to improve toughness of the slab. However, since the
ferritic stainless steel has the increased ymax, the martensitic phase may be formed
when heated at an elevated temperature as in welding, whereby the thermal fatigue
characteristics may be degraded.
CITATION LIST
Patent Documents
[0007]
Patent Document 1: Japanese Patent Application Publication No. 2012-188748 A
Patent Document 2: Japanese Patent Application Publication No. 2012-007195 A
SUMMARY OF INVENTION
Technical Problem
[0008] As described above, the ferritic stainless steels used for applications such as automobile
exhaust gas passage members require improved processability that can be processed
into complex shapes by various forming methods and can contribute to an increase in
design freedom of the member. Further, since the ferritic stainless steels used for
applications such as automobile exhaust gas passage members are required to have good
thermal fatigue characteristics and good oxidation characteristics even at elevated
temperature, it is not desirable that the heat resistance is decreased. However, as
can be seen from the patent documents as described above, any ferritic stainless steel
that simultaneously achieves improved processability and improved heat resistance
has not been provided at this time.
[0009] In addition, there is a method of reducing Cr and Si for a decrease in alloying which
is a general means, as a means for improving the processability. However, in this
method, the ymax increases, so that a martensitic phase is easily formed when used
at elevated temperature and the thermal fatigue characteristics are degraded. Further,
when Cr and Si are reduced, the high-temperature oxidation characteristics are also
reduced.
[0010] Further, there is a method of decreasing a slab heating temperature in order to increase
strain during hot rolling, as a general means for improving the processability. However.
in this case, it is known that the quality of a surface is deteriorated. Moreover,
its cause and countermeasure are not specified.
[0011] An object of the present invention is to provide a ferritic stainless steel and a
ferritic stainless steel for automobile exhaust gas passage members, which have improved
processability and improved heat resistance and also has good surface quality.
Solution to Problem
[0012] In a ferritic stainless steel, the decreasing of Cr and Si to improve the processability
leads to an increase in ymax and tends to generate a martensitic phase, so that the
thermal fatigue characteristics are deteriorated. Therefore, as a result of studying
a relationship between the ymax and the martensitic phase formation/thermal fatigue
characteristics, the present inventors have found that if the ymax is 55 or less,
no martensitic phase is generated and the thermal fatigue characteristics are not
affected.
[0013] Further, when the slab heating temperature is lowered during hot rolling to improve
the processability, the surface quality is degraded. Therefore, the present inventors
have focused on a formed state of oxide scales in the case where the slab heating
temperature is decreased, and have made various studies. As a result, the present
inventors have found that local generation of oxide scales based on Fe rather than
uniform generation during heating of the slab is one of causes of the deterioration
of the surface quality. The local generation of the oxide scales based on Fe would
allow surface defects to occur due to the contact of thin portions of the oxide scales
based on Fe with a roll of a hot rolling mill. Therefore, as a result of intensive
studies, the present inventors have found that Si and Cr greatly affect the local
formation of the oxide scales in the case where the slab heating temperature during
hot rolling is decreased. Then, the present inventors have found that by controlling
amounts of Si and Cr to be added, the oxide scales based on Fe are uniformly generated
even if the slab heating temperature is decreased, so that the surface quality during
hot rolling can be improved.
[0014] Thus, the present invention relates to a ferritic stainless steel containing 0.03%
by mass or less of C; from 0.1 to 0.8% by mass of Si; 1.0% by mass or less of Mn;
0.04% by mass or less of P; 0.01% by mass or less of S; 0.5% by mass or less of Ni;
from 12.0 to 15.0% by mass of Cr; 0.03% by mass or less of N; from 0.1 to 0.5% by
mass of Nb; from 0.8 to 1.5% by mass of Cu; and 0.1% by mass or less of Al, the balance
being Fe and unavoidable impurities, the ferritic stainless steel having a ymax of
55 or less, as represented by the following equation (1):
in which C, Si, Mn, Ni, Cr, N, Cu and Al mean % by mass of the corresponding elements.
[0015] Further, the present invention relates to a ferritic stainless steel for automobile
exhaust gas passage members, the ferritic stainless steel containing 0.03% by mass
or less of C; from 0.1 to 0.8% by mass of Si; 1.0% by mass or less of Mn; 0.04% by
mass or less of P; 0.01% by mass or less of S; 0.5% by mass or less of Ni; from 12.0
to 15.0% by mass of Cr; 0.03% by mass or less of N; from 0.1 to 0.5% by mass of Nb;
from 0.8 to 1.5% by mass of Cu; and 0.1% by mass or less of Al, the balance being
Fe and unavoidable impurities, the ferritic stainless steel having a ymax of 55 or
less, as represented by the following equation (1):
in which C, Si, Mn, Ni, Cr, N, Cu and Al mean % by mass of the corresponding elements.
Advantageous Effects of Invention
[0016] According to the present invention, it is possible to provide a ferritic stainless
steel and a ferritic stainless steel for automobile exhaust gas passage members, which
have improved processability and improved heat resistance, as well as good surface
quality.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0017] A ferritic stainless steel according to the present invention contains C, Si, Mn,
P, S, Ni, Cr, N, Nb, Cu and Al, and the balance is Fe and unavoidable impurities.
Further, the ferritic stainless steel may further contain one or more selected from
the group consisting of Ti, Mo, V, Zr, W, Co and B as optional components.
[0018] Here, in the present specification, a content of an element without definition of
a lower limit indicates that the element can be contained up to an unavoidable impurity
level. Reasons for limitation of each element are described below.
[0019] C and N are generally regarded as elements effective for improving high-temperature
strength, such as creep strength. However, when excessive amounts of C and N are contained,
a martensitic phase tends to be generated, so that thermal fatigue characteristics,
oxidation characteristics and processability are degraded. For a steel composition
containing Nb as an element for fixing C and N as carbonitrides, an appropriate amount
of Nb for C and N concentrations is required, so costs of the ferritic stainless steel
are increased. On the other hand, if C and N are to be significantly decreased, the
burden on steelmaking will be excessive and cost will increase. For those reasons,
in the present invention, both of C and N are limited to 0.03% by mass or less. It
should be noted that in view of the oxidation characteristics and the processability,
each of C and N is preferably 0.015% by mass or less.
[0020] Both of Si and Cr greatly affect the high-temperature oxidation characteristics and
the processability. Higher amounts of Si and Cr added provide better high-temperatue
oxidation characteristics, but decrease the processability. Further, although the
high-temperature oxidation characteristics are improved, the surface quality is deteriorated
when the slab heating temperature during hot rolling is decreased, because the oxide
scales based on Fe are locally generated without being uniformly generated. In order
to provide the surface quality, the addition range of Si and Cr should be strictly
limited. Therefore, in order to achieve all of the processability, high-temperature
oxidation resistance and surface quality during hot rolling, Si is limited to 0.1
to 0.8% by mass, and preferably 0.2 to 0.6% by mass. For the same reason, Cr is limited
to 12.0 to 15.0% by mass.
[0021] Mn is an alloy element that improves the high-temperature oxidation characteristics,
particularly scale strippability, of ferritic stainless steel, but an excessive addition
of Mn degrades the processability. Further, since Mn is also an austenite phase stabilizing
element, excessive addition of Mn to a steel type having a small amount of Cr added
facilitates the formation of the martensitic phase, resulting in deterioration of
the thermal fatigue characteristics and processability. Therefore, Mn is limited to
1.0% by mass or less, and preferably 0.8% by mass or less.
[0022] P and S adversely affect the high-temperature oxidation resistance and toughness
of a hot-rolled sheet, so it is preferable to reduce them as much as possible. Therefore,
P is limited to 0.04% by mass or less, and S is limited to 0.01% by mass or less.
[0023] Ni is an element effective for improvement of low-temperature toughness. However,
since Ni is an austenite phase stabilizing element, excessive addition of Ni to a
steel type having a low Cr content generates a martensitic phase as with Mn, thereby
reducing the thermal fatigue characteristics and processability. Further, since Ni
is expensive, excessive addition of Ni should be avoided. Therefore, the Ni content
is limited to 0.5% by mass or less. A lower limit of the Ni content is not particularly
limited, but it is preferably more than 0% by mass, and more preferably 0.01% by mass
or more.
[0024] Nb fixes C and N as carbonitrides, and the remaining solution Nb after fixing of
carbonitrides has an effect of increasing the high-temperature strength. However,
the addition of an excessive amount of Nb deteriorates the processability. Therefore,
the Nb content is limited to 0.1 to 0.5% by mass, and preferably 0.2 to 0.4% by mass.
[0025] Cu is an element that improves the high-temperature strength. In order to obtain
the required high-temperature strength, a Cu content of 0.8% by mass or more is required.
However, as the Cu content increases, the processability and the high-temperature
oxidation resistance are deteriorated. Therefore, the Cu content is limited to 0.8
to 1.5% by mass, and preferably 0.9 to 1.3% by mass.
[0026] Al is added as a deoxidizer during steel making, and also exhibits an effect of improving
the high-temperature oxidation resistance. However, excessive addition of Al lowers
surface properties and adversely affects the processability. Therefore, a lower Al
content is preferable, and it is limited to 0.1% by mass or less, and preferably 0.05%
by mass or less.
[0027] Ti is an element which fixes solution C and N in steel as carbonitrides to improve
ductility and processability. Further, Ti can also be expected to produce effects
of suppressing grain boundary precipitation of Cr carbides and improving corrosion
resistance. However, the addition of an excessive amount of Ti deteriorates the surface
properties of the steel material due to formation of TiN, which adversely affects
weldability and low-temperature toughness. Therefore, Ti may be optionally added in
an amount of 0.20% by mass or less, and preferably 0.1% by mass or less.
[0028] Mo, V, Zr, W and Co are elements that improve the high-temperature strength and thermal
fatigue resistance by solution strengthening or precipitation strengthening. However,
the addition of an excessive amount excessively hardens the steel material. Therefore,
each of Mo, Zr, W and Co may be optionally added in an amount of 0.5% by mass or less,
and V may be optionally added in an amount of 0.1% by mass or less.
[0029] B is an element that improves the secondary workability of steel and suppresses cracking
during multistage forming. However, excessive addition of B deteriorates the productivity
and weldability. Therefore, B may be optionally added in an amount of 0.01% by mass
or less.
[0030] Each of equations (1) and (2) represents ymax, which is an index for generation of
an austenitic phase. When the ymax is too high, the martensitic phase tends to be
formed, and when the martensitic phase is present, the thermal fatigue characteristics
are deteriorated. Therefore, in order not to form the martensitic phase, the ymax
is controlled to 55 or less. In addition, the equation (1) is ymax in the case where
Mo or Ti which is an optional component, is not contained, and the equation (2) is
ymax in the case where Mo or Ti which is an optional component is contained.
In the equations (1) and (2), C, Si, Mn, Ni, Cr, N, Cu and Al mean % by mass of the
corresponding elements.
[0031] A method for producing the ferritic stainless steel according to the present invention
is not particularly limited, and the ferritic stainless steel may be produced by carrying
out the steps of heating a slab cast by a certain method at a temperature of from
1000 to 1250 °C for 1 to 3 hours; subjecting the slab to hot rolling by a certain
method; annealing the slab at a temperature of from 900 to 1100 °C; washing the slab
with an acid and subjecting it to cold rolling by a certain method; and annealing
it at a temperature of from 900 to 1100 °C and washing it with an acid.
[0032] In the ferritic stainless steel of the present invention thus produced, even if the
slab heating temperature is decreased, the oxide scales based on Fe are uniformly
generated, and the surface quality during hot rolling is satisfactory. Moreover, the
ferritic stainless steel has improved processability and heat resistance. Therefore,
the ferritic stainless steel according to the present invention is suitable for heat
resistance, in particular for automobile exhaust gas passage members.
EXAMPLES
[0033] Hereinafter, the present invention will be more specifically described by Examples.
It should be noted that the present invention is not limited to these examples.
[0034] Various ferritic stainless steels having the steel compositions as shown in Table
1 were melted in a vacuum melting furnace and cast into 30 kg ingots. After heating
each ingot (slab) at 1100 °C for 2 h, the ingot was subjected to hot rolling, annealing,
cold rolling and finish annealing in this order to produce a cold-rolled and annealed
sheet having a thickness of 1.5 mm. The ingot was also forged and annealed to produce
a round bar annealed material. In the table below, Nos. 1 to 20 represent inventive
steels, and Nos. 21 to 30 represent comparative steels. Among them, No. 21 represents
the steel corresponding to Patent Document 1, and No. 22 represents the steel corresponding
to Patent Document 2.
[0035] A method of confirming a generated state of the oxide scales when the slab heating
temperature is decreased will be described.
[0036] Each ingot was cut into 5 mm t × 25 mm w × 35 mm L, and the surface was polished
with a # 120 polishing belt, and heated at 1000 °C for 2 h in an electric furnace
which reproduced similar oxygen content and water vapor content to the hot rolling
heating furnace. The generated state of oxide scales was then confirmed by cross-sectional
observation. Uniform generation of the oxide scales based on Fe was evaluated as good
(○: the same hereinafter), and local generation or no generation of the oxide scales
was evaluated as poor (×: the same hereinafter).
[0037] The cold-rolled and annealed sheet having a thickness of 1.5 mm was subjected to
a high-temperature oxidation test and processability evaluation.
[0038] For the high temperature oxidation test, each sample having a size of 25 mm × 35
mm was prepared, and a continuous oxidation test was carried out in an air atmosphere
in the electric furnace by heating the sample in the furnace at 875 °C for 200 h,
and the weight of the sample was then measured. Measurement results of an increase
in oxidation content were compared with the weight before the test, and a weight change
of 5 mg/cm
2 or less was evaluated as ○, and a weight change of more than 5 mg/cm
2 was evaluated as ×.
[0039] The processability evaluation was conducted in accordance with a normal temperature
tensile test. Each sample of JIS 13 B was prepared, and an elongation at breakage
in the rolling direction was measured. A sample with an elongation at breakage of
35% or more was evaluated as ○, and a sample with an elongation at breakage of less
than 35% was evaluated as x.
[0040] Each sample for a thermal fatigue test was prepared from the round bar annealed material
and subjected to a thermal fatigue test. Here, for the sample for the thermal fatigue
test, a round bar sample was used which was prepared by cutting the round bar annealed
material having a diameter of 10 mm and providing a notch of R = 2.83 mm at a central
portion between target positions so as to have a diameter of 7 mm (the length between
the target positions was 15 mm). In the thermal fatigue test, heating and cooling
were carried out in a range from the minimum temperature of 200 °C to the maximum
temperature of 750 °C in a high-frequency heating device at 3 °C/sec, and each of
retention times at the minimum and maximum temperatures was 30 seconds, which was
regarded as one cycle. Further, the thermal fatigue test was carried out at a restraint
rate of 25%. The number of cycles in which the maximum stress in each cycle was reduced
by 25% from a value in a steady state was regarded as the thermal fatigue life, a
thermal fatigue life of 1600 cycles or more were evaluated as ○, and a thermal fatigue
life of less than 1600 cycles was evaluated as x.
[Table 2]
Table 2. Evaluation Test Results |
|
Formed State of Oxide Scales |
High-Temperature Oxidation characteristics (≤ 5mg/cm2) |
Elongation (≥ 35%) |
Thermal Fatigue Characteristics (≥ 1600 cycles) |
1 |
○ |
○(0.9mg/cm2) |
○(36%) |
○(1750 cycles) |
2 |
○ |
○(0.5mg/cm2) |
○(36%) |
○(1740 cycles) |
3 |
○ |
○(0.5mg/cm2) |
○(36%) |
○(1720 cycles) |
4 |
○ |
○(0.5mg/cm2) |
○(36%) |
○(1780 cycles) |
5 |
○ |
○(0.5mg/cm2) |
○(36%) |
○(1650 cycles) |
6 |
○ |
○(1.0mg/cm2) |
○(37%) |
○(1780 cycles) |
7 |
○ |
○(0.5mg/cm2) |
○(36%) |
○(1800 cycles) |
8 |
○ |
○(0.5mg/cm2) |
○(36%) |
○(1710 cycles) |
9 |
○ |
○(0.5mg/cm2) |
○(38%) |
○(1750 cycles) |
10 |
○ |
○(0.6mg/cm2) |
○(36%) |
○(1700 cycles) |
11 |
○ |
○(1.2mg/cm2) |
○(36%) |
○(1760 cycles) |
12 |
○ |
○(0.5mg/cm2) |
○(38%) |
○(1660 cycles) |
13 |
○ |
○(0.5mg/cm2) |
○(36%) |
○(1850 cycles) |
14 |
○ |
○(0.5mg/cm2) |
○(38%) |
○(1800 cycles) |
15 |
○ |
○(0.5mg/cm2) |
○(36%) |
○(1820 cycles) |
16 |
○ |
○(0.5mg/cm2) |
○(37%) |
○(1780 cycles) |
17 |
○ |
○(0.5mg/cm2) |
○(36%) |
○(1800 cycles) |
18 |
○ |
○(0.5mg/cm2) |
○(36%) |
○(1790 cycles) |
19 |
○ |
○(0.5mg/cm2) |
○(36%) |
○(1770 cycles) |
20 |
○ |
○(0.5mg/cm2) |
○(38%) |
○(1750 cycles) |
21 |
○ |
○(0.5mg/cm2) |
○(38%) |
×(1540 cycles) |
22 |
○ |
○(0.5mg/cm2) |
○(36%) |
×(1520 cycles) |
23 |
○ |
○(1.3mg/cm2) |
×(34%) |
×(1510 cycles) |
24 |
○ |
○(0.5mg/cm2) |
○(38%) |
×(1550 cycles) |
25 |
× |
○(0.4mg/cm2) |
×(33%) |
○(1880 cycles) |
26 |
○ |
○(13mg/cm2) |
×(33%) |
○(1750 cycles) |
27 |
○ |
×(7.5mg/cm2) |
○(37%) |
×(1450 cycles) |
28 |
× |
○(0.4mg/cm2) |
×(34%) |
×(1470 cycles) |
29 |
○ |
×(6.8mg/cm2) |
○(39%) |
○(1760 cycles) |
30 |
○ |
×(6.4mg/cm2) |
×(34%) |
○(1840 cycles) |
[0041] As can be seen from Table 2, all of the ferritic stainless steels according to Inventive
Examples had the improved generated state of oxide scales, the improved high-temperature
oxidation characteristics, the improved processability and the improved thermal fatigue
characteristics.
[0042] In contrast, the ferritic stainless steels according to Comparative Example 21 that
did not contain Nb, Comparative Example 24 where Nb was less than the lower limit,
and Comparative Example 28 where Cu was less than the lower limit had insufficient
high-temperature strength, so that the thermal fatigue characteristics were decreased.
Furthermore, the ferritic stainless steel according to Comparative Example 28 had
an excessive Cr content, so that the processability was deteriorated, as well as the
oxide scales based on Fe were non-uniformly generated during heating at 1000 °C for
2 h. Each of the ferritic stainless steels according to Comparative Examples 22 and
23 had the ymax more than the upper limit, so that the martensitic phase was easily
generated, and the thermal fatigue characteristics were deteriorated. Furthermore,
the ferritic stainless steel according to Comparative Example 23 had the higher content
of C, so that the processability was also insufficient.
[0043] In the ferritic stainless steel according to Comparative Example 27, the Ni content
and ymax was more than the upper limits, so that the thermal fatigue characteristics
were deteriorated, as well as the Cr content was lower, so that the high-temperature
oxidation characteristics were also insufficient.
[0044] The ferritic stainless steel according to Comparative Example 25 had the higher content
of Si, so that the oxide scales based on Fe did not uniformly form during heating
at 1000 °C for 2 h, as well as it had the higher contents of Si and Nb, so that the
processability was also deteriorated.
[0045] The ferritic stainless steel according to Comparative Example 26 had the decreased
processability, because N and Al were excessive.
[0046] The ferritic stainless steel according to Comparative Example 29 had the decreased
high-temperature oxidation characteristics because the Si content was lower.
[0047] The ferritic stainless steel according to Comparative Example 30 had the decreased
processability as well as the decreased high-temperature oxidation characteristics,
because the Mn and Cu contents were excessive.
[0048] As described above, in all of the ferritic stainless steels according to Comparative
Examples, any of the formed state of the oxide scales, the high-temperature oxidation
characteristics, the processability, and the thermal fatigue characteristics was insufficient.
[0049] The present application claims priority based on Japanese Patent Application No.
2017-7842 filed on January 19, 2017, which is incorporated herein by reference in its entirety.
INDUSTRIAL APPLICABILITY
[0050] The ferritic stainless steel according to the present invention has improved surface
quality, high-temperature oxidation characteristics, processability and thermal fatigue
characteristics, and is suitable for use in exhaust gas flow passage members of various
internal combustion engines including automobiles, such as exhaust manifolds, front
pipes, center pipes, and outer cylinders of catalytic converters.
1. A ferritic stainless steel, containing:
0.03% by mass or less of C;
from 0.1 to 0.8% by mass of Si;
1.0% by mass or less of Mn;
0.04% by mass or less of P;
0.01% by mass or less of S;
0.5% by mass or less of Ni;
from 12.0 to 15.0% by mass of Cr;
0.03% by mass or less of N;
from 0.1 to 0.5% by mass of Nb;
from 0.8 to 1.5% by mass of Cu; and
0.1% by mass or less of Al, the balance being Fe and unavoidable impurities, the ferritic
stainless steel having a ymax of 55 or less, as represented by the following equation
(1):
in which C, Si, Mn, Ni, Cr, N, Cu and Al mean % by mass of the corresponding elements.
2. The ferritic stainless steel according to claim 1, further containing one or more
selected from the group consisting of Ti: 0.20% by mass or less; Mo: 0.5% by mass
or less; V: 0.1% by mass or less; Zr: 0.5% by mass or less; W: 0.5% by mass or less;
Co: 0.5% by mass or less; and B: 0.01% by mass or less, wherein the ferritic stainless
steel has a ymax of 55 or less, as represented by the following formula (2):
in which C, Si, Mn, Ni, Cr, N, Cu, Mo, Ti and Al mean % by mass of the corresponding
elements.
3. The ferritic stainless steel according to claim 1, wherein the ferritic stainless
steel is for heat resistance.
4. A ferritic stainless steel for automobile exhaust gas passage members, the ferritic
stainless steel containing:
0.03% by mass or less of C;
from 0.1 to 0.8% by mass of Si;
1.0% by mass or less of Mn;
0.04% by mass or less of P;
0.01% by mass or less of S;
0.5% by mass or less of Ni;
from 12.0 to 15.0% by mass of Cr;
0.03% by mass or less of N;
from 0.1 to 0.5% by mass of Nb;
from 0.8 to 1.5% by mass of Cu; and
0.1% by mass or less of Al, the balance being Fe and unavoidable impurities, the ferritic
stainless steel having a ymax of 55 or less, as represented by the following equation
(1):
in which C, Si, Mn, Ni, Cr, N, Cu and Al mean % by mass of the corresponding elements.
5. The ferritic stainless steel for automobile exhaust gas passage members according
to claim 4, further containing one or more selected from the group consisting of Ti:
0.20% by mass or less; Mo: 0.5% by mass or less; V: 0.1% by mass or less; Zr: 0.5%
by mass or less; W: 0.5% by mass or less; Co: 0.5% by mass or less; and B: 0.01% by
mass or less, wherein the ferritic stainless steel has a ymax of 55 or less, as represented
by the following formula (2):
in which C, Si, Mn, Ni, Cr, N, Cu, Mo, Ti and Al mean % by mass of the corresponding
elements.