BACKGROUND OF THE INVENTION
Field of the Invention:
[0001] This invention relates to a ferritic steel sheet concurrently improved in formability
such as deep drawability, stretch formability and the like, high-temperature strength,
high-temperature oxidation resistance, and low-temperature toughness, particularly
a steel sheet that, being usable in a 800 - 900 °C high-temperature atmosphere, is
suitable for utilization in automobile engine exhaust gas passage components.
Background Art:
[0002] Since ferritic stainless steels have a smaller thermal expansion coefficient than
austenitic stainless steels and are excellent in thermal fatigue property and high-temperature
oxidation property, they are used as heat-resistant materials in applications where
thermal strain is an issue. Typical applications include automobile engine exhaust
gas passage components such as exhaust manifolds, front pipes, catalyst carrier outer
cylinders, center pipes, mufflers and tailpipes.
[0003] A trend seen in recent automobile engines is to increase exhaust gas temperature
in order to improve exhaust gas purification efficiency and output. This has increased
the need for high heat resistance (high-temperature strength and high-temperature
oxidation resistance) particularly in the exhaust manifold, front pipe, outer cylinder
of the catalyst carrier and other components near the engine. A tendency for the shape
of exhaust gas passage components to become more complicated has also recently emerged.
This is especially notable in the exhaust manifold and outer cylinder of the catalyst
carrier, which are being formed in complex configurations by various methods, including
mechanical pressing, servo pressing, spinning and hydroforming. It is therefore not
sufficient for the materials used in these components to be good merely in stretching
elongation and bendability. They are now also required to be excellent in formability
such as typically deep drawability and stretch formability, and the in-plane anisotropy
of their workability has to be small. In addition, owing to the fact that consideration
needs to be given to prevention of ductile fructuer and brittle cracking during secondary
and tertiary machining, they must also be excellent in low-temperature toughness.
Moreover, heat resistance cannot be sacrificed in the interest of improving formability
and low-temperature toughness, because greater shape complexity increases the likelihood
of thermal fatigue fracture occurring owing to concentration of thermal strain at
a single location during engine starting and stopping and also boosts vulnerability
to abnormal oxidation caused by the material temperature rising locally.
[0004] SUH409L and SUS430J1L are known as ferritic stainless steels having high heat resistance.
SUH409L is commonly used as an exhaust gas passage component material because of its
good workability and low-temperature toughness. However, the level of its heat resistance
makes it unsuitable for applications in which the material temperature exceeds 800
°C. It is also lacks sufficient deep drawability for application to components with
complicated shapes. SUS430J1L has excellent heat resistance that makes it usable at
900 °C. But it is hard and poor in formability.
[0005] In light of the foregoing, the following heat-resistant ferritic steels have been
developed.
[0006] Patent Reference 1 among the references listed below teaches a ferritic heat-resistant
stainless steel with a Cr level of 17.0 - 25.0%. This steel is added with Mo and Cu
in combination to improve high-temperature strength and with Mn to suppress scale
spalling. Degradation of impact value by Mo is overcome to some degree by combined
addition of Cu and Ni. However, the steel's formability is not adequate to cope with
the needs of complexly shaped exhaust gas passage components. And its high Cr content
makes it disadvantageous from the cost aspect.
[0007] Patent Reference 2 teaches a 13% Cr ferritic stainless steel that exhibits heat resistance
at least as good as an 18% Cr ferritic stainless steel and also has improved high-temperature
salt corrosion property. In this steel, high-temperature strength is increased by
ensuring the presence of solid solution Nb, high-temperature oxidation property is
improved by liberal addition of Mn and Si, and NaCl-induced hat corrosion resistance
is improved by the Si. As no particular consideration is given to improvement of formability
and low-temperature toughness, however, the steel cannot adequately respond to the
recent harsh requirements mentioned earlier.
[0008] Patent Reference 3 teaches a Nb-containing heat-resistant ferritic stainless steel
with a Cr level of 11.0 - 15.5% that is aimed at improving high-temperature oxidation
resistance and scale adherence. These properties are markedly improved by strictly
constraining Mn/Sn within the range of 0.7 - 1.5. This Patent Reference also teaches
improvement of low-temperature toughness and workability by Cu addition. Regarding
workability, for example, it presents data showing that no cracking occurred in a
180-degree bending test. In light of the fact that demands regarding the shape of
exhaust gas passage components are becoming more and more challenging, however, the
materials used in these components have come to require excellent formability that
is compatible with various forming methods (discussed later). Regarding this point,
the steel of Patent Reference 3 pays no attention to deep drawability and other drawability-related
stretch formability and, as such, cannot be considered capable of responding to today's
severe requirements. In addition, its Cr content of 11.0% or greater is on the level
required in a stainless steel, which is inconsistent with the desire to reduce cost
by lowering the Cr content in exhaust gas passage components that do not necessarily
require use of stainless steels.
[0009] Patent Reference 4 teaches a ferritic stainless steel for exhaust manifolds that
contains Cr at 11 - 14%. This is a steel enhanced in high-temperature strength by
positive addition of Si to a Nb-containing steel. Its high temperature can be considered
to be the same as the steel of Patent Reference 3. As it does not give consideration
to improving formability and low-temperature toughness beyond the level of the prior
art, however, it is incapable of thoroughly responding to the harsh demands placed
on steels in recent years. It also needs further reduction in Cr level.
[0010] Patent Reference 5 teaches a ferritic heat-resistant steel for engine exhaust gas
passage components having a Cr content of 8.0 - 10.0%. This is a steel that achieves
better heat resistance than SUH409L while also reducing cost through low Cr content.
This reference further teaches that Cu is effective for improving both low-temperature
toughness and workability. Regarding workability, it was for example found to possess
ductility on a par with SUH409L in tensile tests at room temperature. As it is not
aimed at improvement of the in-plane anisotropy of ductility or deep drawability,
however, it leaves unresolved the problem of imparting formability thoroughly matched
to the needs of various forming methods (discussed later). Nor does it offer a method
for consistently imparting excellent low-temperature toughness. Patent Reference 5
can therefore not be viewed as sufficiently responding to the recent severe requirements
with respect to exhaust gas passage components.
[0011] Patent References 6 and 7 teach ferritic steels with a Cr content of 10 to less than
15% that improve the corrosion resistance against condensed moisture needed by mufflers
and other low-temperature components and also the high-temperature strength needed
by exhaust manifolds and other high-temperature components. But they merely assess
workability in terms of proof stress, while offering nothing specific with regard
to high-temperature oxidation resistance. Patent References 6 and 7 are not directed
to the goal of concurrently and consistently improving high-temperature oxidation
resistance and formability with good reproducibility, and are silent regarding methods
for achieving this objective. From the viewpoint of fabrication into exhaust gas passage
components of various complex shapes, therefore, the steels taught by Patent References
6 and 7 cannot be considered steels that fully meet all formability requirements.
Patent Reference 1
JP-A-Hei 3(1991)-274245 (p3, upper right column, line 1 - p4, upper right column,
line 9)
Patent Reference 2
JP-A-Hei 5(1993)-125491 (paragraphs 0012 - 0016)
Patent Reference 3
JP-A-Hei 7(1995)-11394 (paragraphs 0014 - 0021, 0028, 0029, Table 6, Figure 1)
Patent Reference 4
JP-A-Hei 7(1995)-145453 (paragraphs 0011 - 0021)
Patent Reference 5
JP-A-Hei 10(1998)-147848 (paragraphs 0003 - 0005, 0014)
Patent Reference 6
JP-A-Hei 10(1998)-204590 (paragraphs 0026 - 0036, 0072)
Patent Reference 7
JP-A-Hei 10(1998)-204591 (paragraphs 0028 - 0037, 0074)
[0012] As explained in the foregoing, steel sheet for automobile exhaust gas passage components
is now being required to contribute to greater component design freedom by offering
excellent formability that enables fabrication into complicated shapes by a diversity
of forming methods. But this need should best be met while maintaining high-temperature
strength and high-temperature oxidation resistance at 800 - 900 °C on a par with SUS430J1L,
and also ensuring excellent low-temperature toughness. As can be seen from the aforesaid
Patent References, however, no steel sheet has yet been developed that is concurrently
improved to a high degree in all of formability, high-temperature strength, high-temperature
oxidation resistance, and low-temperature toughness.
[0013] An object of the present invention is to provide a new ferritic heat-resistant steel
that concurrently offers excellent formability enabling ready application to complexly
configured automobile exhaust gas passage components, excellent high-temperature strength
and high-temperature oxidation resistance enabling it to withstanding use at 900 °C
and excellent low-temperature toughness having an energy transition temperature of
minus 50 °C or lower, and that is lowered in cost by reducing Cr content to below
11 mass percent.
SUMMARY OF THE INVENTION
[0014] The inventors carried out a study to determine why excellent formability, high-temperature
strength, high-temperature oxidation resistance, and low-temperature toughness have
not yet been concurrently achieved in a steel sheet. Based on our findings, we concluded
that a major cause is the fact that no means has been found for concurrently establishing
with high stability and good reproducibility the properties of formability and high-temperature
oxidation resistance among the foregoing properties. Then, in an ensuing in-depth
study, the inventors discovered that when the austenite balance is adjusted in the
manner of Equation (3) set out below, then, as shown by Equation (2) set out below,
a region of Si and Cr content exists in which both excellent formability and excellent
high-temperature oxidation resistance can be concurrently established.
[0015] Moreover, in evaluating workability into complexly configured exhaust gas passage
components, the property of deep drawability among the different aspects of formability
cannot be disregarded. It was found that an effective way to improve the deep drawability
of a heat resistance ferritic steel added with Nb is to add Ti in combination with
the Nb. The inventors further learned that deep drawability (average plastic strain
ratio r
AV) and the in-plane anisotropy thereof (plastic anisotropy Δr) can be improved by partially
recrystallizing the hot-rolled sheet.
[0016] It should be noted, however, that Ti addition degrades low-temperature toughness.
It was found that combined addition of Cu and B more effectively improved the low-temperature
toughness than did addition of Cu alone. 0018 When the amount of added Cu was progressively
increased, however, an abnormal oxidation-inducing phenomenon was observed to appear
abruptly. Further, an appropriate range of Cu enabling concurrent improvement of low-temperature
toughness and high-temperature oxidation resistance was discovered.
[0017] The present invention was accomplished based on the foregoing findings.
[0018] Specifically, the aforesaid object is achieved by a ferritic steel sheet concurrently
improved in formability, high-temperature oxidation resistance, high-temperature strength,
and low-temperature toughness comprising, in mass percent, C : not more than 0.02%,
Si : 0.7 - 1.1%, Mn : not more than 0.8%, Ni : not more than 0.5%, Cr : 8.0 to less
than 11.0%, N : not more than 0.02%, Nb : 0.10 - 0.50%, Ti: 0.07- 0.25%, Cu: 0.02-
0.5%, B: 0.0005- 0.02%, V: 0 (no addition) - 0.20%, preferably 0.01 - 0.20%, one or
both of Ca and Mg : 0 (no addition) - 0.01% in total, preferably 0.0003 - 0.01% in
total, one or more elements among Y and rare earth elements : 0 (no addition) - 0.20%
in total, preferably 0.01 - 0.20% in total, and the balance of Fe and unavoidable
impurities, and having a chemical composition satisfying all of Equations (1) - (3):
[0019] The steel sheet may further include Mo : not more than 0.50% and Al : not more than
0.10%.
[0020] Each element symbol in Equations (1) - (3) is replaced by a value representing the
content of the element in mass percent. In Equation (3), symbols of elements not contained
are replaced by zero.
[0021] In the present invention, the aforesaid steel sheet may have a metallic structure
obtained by cold rolling and annealing a partially recrystallized hot-rolled sheet.
A "partially recrystallized hot-rolled sheet" as termed here means a hot-rolled sheet
10 - 90 vol% of whose structure is accounted for by recrystallized grains and the
remainder of which is accounted for by un-recrystallized grains. The amount of recrystallized
grains present can be ascertained by observing a cross-section of the hot-rolled sheet
with an optical microscope. By "hot-rolled sheet" is meant the steel sheet that has
been subjected to hot rolling and may have been subjected to heat treatment after
hot rolling but has not be subjected to cold rolling. The final metallic structure
obtained by conducting cold rolling and annealing is totally recrystallized.
[0022] In the present invention, further, the aforesaid steel sheet may have a metallic
structure obtained by cold rolling and annealing a tatally recrystallized hot-rolled
sheet. A "tatally recrystallized hot-rolled sheet" as termed here means a hot-rolled
sheet more than 90 vol% of whose structure is accounted for by recrystallized grains.
[0023] The steel sheet provided by the present invention is particularly one used as fabricated
into an automobile engine exhaust gas passage component.
BRIEF EXPLANATION OF THE DRAWINGS
[0024]
FIG. 1 is a graph showing how Ti content and difference between partial and complete
recrystallization after hot rolling affected r value (rD) at 45 degrees to the rolling direction in ferritic steels whose basic composition
was 10 Cr - 0.9 Si - 0.3 Nb - 0.1 V - 0.1 Cu.
FIG. 2 is a graph showing how Cu content affected energy transition temperature and
amount of oxidation increase after 900 °C x 200 hour heating in the atmosphere in
ferritic steels whose basic composition was 10 Cr - 0.9 Si - 0.3 Nb - 0.1 V - 0.001
B.
FIG. 3 is a graph showing how Cr content and Si content affected high-temperature
oxidation resistance and formability in ferritic steels whose basic composition was
8 to 14 Cr - 0.5 to 1.0 Si - 0.3 Nb - 0.1 Ti - 0.1 V - 0.1 Cu.
FIG. 4 is a graph showing how elongation at 45 degrees to the rolling direction in
a room-temperature tensile test varied with AM value (AM = 420 C - 11.5 Si + 7 Mn
+ 23 Ni - 11.5 Cr - 12 Mo + 9 Cu - 49 Ti - 25 (Nb + V) - 52 Al + 470 N + 189) in ferritic
steels whose basic composition was 8 to 14 Cr - 0.5 to 1.0 Si - 0.3 Nb - 0.1 Ti -
0.1 V - 0.1 Cu and that satisfied Equations (1) and (2).
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0025] FIG. 1 shows how Ti content and difference between partial and complete recrystallization
after hot rolling affected r value (r
D) at 45 degrees to the rolling direction in ferritic steels whose basic composition
was 10 Cr- 0.9 Si - 0.3 Nb - 0.1 V - 0.1 Cu. The partially recrystallized hot-rolled
sheets were 4.0 mm-thick hot-rolled sheets heat treated at 700 - 1000 °C for 1 minute
to have a structure 10 - 90 vol% of which was accounted for by recrystallized grains.
The totally recrystallized hot-rolled sheets were 4.0 mm-thick hot-rolled sheets heat
treated at about 1050 °C for 1 minute. The hot-rolled sheets were cold rolled to 2.0
mm and totally recrystallized by annealing at 1050 °C, whereafter tensile test pieces
were cut from them. As can be seen in FIG. 1, the r
D value rose sharply when Ti was added to a content of 0.07 mass% or more. Moreover,
the r
D value was improved markedly over the full range of Ti content by partially recrystallizing
the steel sheets after hot rolling.
[0026] While it is not altogether clear what caused these improvements, it is likely that
Ti, whose carbonitride producing capability is stronger than that of Nb, fixed C and
N, and since this reduced solid solution C and N, the purity of the matrix was increased
to a high level that promoted development of (111) plane aggregate texture favorable
for improvement of workability during recrystallization at final annealing. This effect
is thought to manifest itself when the Ti content reaches 0.07 mass% or more. On the
other hand, it is likely that partial recrystallization of the hot-rolled sheet uniformly
produced fine Nb-Ti precipitates that suppressed development of (100) plane aggregate
texture thought to be detrimental to workability improvement and promoted development
of (111) plane aggregate texture.
[0027] FIG. 2 shows how Cu content affected energy transition temperature and amount of
oxidation increase after 900 °C x 200 hour heating in the atmosphere in ferritic steels
whose basic composition was 10 Cr- 0.9 Si-0.3 Nb - 0.1 V - 0.001 B. The specimens
used were totally recrystallized steel sheets obtained by cold rolling partially crystallized
4.0 mm-thick hot-rolled sheets to a thickness of 2.0 mm and then finally annealing
them at 1050 °C. The energy transition temperature was ascertained by a Charpy impact
test. No. 5 test pieces (2 mm width) were taken in accordance with JIS Z 2202 so that
the impact direction would be parallel to the rolling direction, the test was conducted
at minus 100 to plus 25 °C in accordance with JIS Z 2242, and the energy transition
temperature was determined from the relationship between the test temperature and
the absorbed energy. Oxidation amount increase was determined in accordance with JIS
Z 2281 by measuring the weight increase of the test piece when it was maintained at
900 °C in the atmosphere continuously for 200 hours. As can be seen form FIG. 2, in
a ferritic steel containing an appropriate amount of B, slight addition of Cu at around
0.02 mass% effectively produced an improvement in low-temperature toughness. However,
it was newly discovered that when the amount added exceeds 0.5 mass%, the oxidation
resistance degenerates rapidly.
[0028] The reasons for the foregoing observations has not yet been fully ascertained, it
is likely regarding low-temperature toughness that occurrence of twin crystal, a cause
of low-temperature brittleness, was suppressed, while it is likely regarding occurrence
of abnormal oxidation that destabilization of the matrix phase balance caused by Cr
and Si oxidation was aggravated by Cu.
[0029] FIG. 3 shows how Cr content and Si content affected high-temperature oxidation resistance
and formability in ferritic steels whose basic composition was 8 to 14 Cr - 0.5 to
1.0 Si - 0.3 Nb - 0.1 Ti - 0.1 V - 0.1 Cu. The specimens were prepared by the process
explained regarding FIG. 2. The 0.2% proof stress at 45 degrees to the rolling direction
determined in a room-temperature tensile test was used as an index of formability.
When this value exceeded 300 MPa, it was judged that as a material for exhaust gas
passage components the steel was basically lacking in formability capable of meeting
the needs of various forming methods. As can be seen from FIG. 3, low content of Cr
and Si resulted in occurrence of abnormal oxidation during 900 °C x 100 hour heating
in the atmosphere. On the other hand, formability deteriorated with increasing Cr
and Si content. There was, however, found to be a Cr-Si combination region in which
satisfactory high-temperature oxidation resistance at 900 °C and satisfactory formability
can both be obtained. The existence of such a region was not known heretofore. Therefore,
despite the development of various ferritic heat-resistant steels, all were inferior
in either high-temperature oxidation resistance or formability and no steel emerged
that could satisfy both requirements reliably and with good reproducibility.
[0030] The region in which excellent high-temperature oxidation resistance and formability
can both be obtained is that in which the blank circle plots are present in FIG. 3.
This region is delimited by Equations (1) and (2):
[0031] FIG. 4 shows how elongation in a room-temperature tensile test varied with AM value
(AM = 420 C - 11.5 Si + 7 Mn + 23 Ni - 11.5 Cr - 12 Mo + 9 Cu - 49 Ti - 25 (Nb + V)
- 52 Al + 470 N + 189) in ferritic steels whose basic composition was 8 to14 Cr -
0.5 to 1.0 Si - 0.3 Nb - 0.1 Ti - 0.1 V - 0.1 Cu and that satisfied Equations (1)
and (2). AM value represents the balance between ferrite phase and austenite phase.
As can be seen from FIG. 4, high ductility was obtained only in the region of an AM
value not more than 70 and degenerated precipitously when AM exceeded 70. Thus formability
and high-temperature oxidation resistance are concurrently improved only when Equation
(3) is satisfied in addition to Equations (1) and (2):
[0032] Features defining the present invention will now be explained.
[0033] C and N are generally effective for improving creep strength, creep rupture strength
and other high-temperature strength properties. In a ferritic steel, however, low-temperature
toughness is degraded by a high content of C and N. This makes it necessary to increase
the amount of added Nb and Ti in order to stabilize C and N as carbonitrides. The
result is higher cost. On the other hand, an attempt to markedly lower C and N content
makes steelmaking more onerous, which also increases cost. Through various studies
it was found that in the present invention a content of up to 0.02 mass% is permissible
for both C and N. It should be noted, however, that when the amount of added Ti and
Nb is appropriately set, particularly good formability and heat resistance are obtained
when the amount of (C + N) is in the range of 0.01 - 0.02 mass%. The total content
of C and N combined is therefore preferably 0.01 - 0.02 mass%..
[0034] Si and Cr are both very effective for improving high-temperature oxidation property
but they also harden the steel. In order to establish both excellent formability and
excellent high-temperature oxidation resistance, the Si and Cr contents need to be
controlled to within the range satisfying Equations (1) and (2), as explained earlier
with reference to FIG. 3. In addition to being controlled based on these equations,
however, upper and lower limits of Si and Cr content are further defined from the
standpoint of ensuring good corrosion resistance and low-temperature toughness. To
wit, the minimum required level of corrosion resistance exemplified by SUH409L cannot
be achieved when the Si and Cr contents are too small, whereas the low-temperature
toughness level of the SUH409L steel cannot be realized when their contents are too
high. Si content is therefore defined as 0.7 - 1.1 mass%. A more preferable range
of Si content is 0.8 - 1.0 mass%. Cr content is defined as 8.0 to less than 11.0%.
A more preferable range of Cr content is 9.0 to less than 11.0% and a still more preferable
range of Cr content is 9.0 to less than 10.0%.
[0035] Mn hardens the steel and degrades its low-temperature toughness and formability when
added in excess. Particularly in the composition system of the present invention,
excessive addition of Mn is liable to adversely affect high-temperature oxidation
resistance by causing generation of austenitic phase during hot use. The upper limit
of Mn content is therefore defined as 0.8 mass%. In the composition system of the
present invention, particularly when excellent scale adherence at the 900 °C level
is required, Mn is preferably added to within the content range of 0.2 - 0.8 mass%.
[0036] Ni is effective for improving low-temperature toughness but hardens the steel and
degrades its formability when added in excess. Moreover, in the composition system
of the present invention, excessive addition of Ni is, like excessive addition of
Mn, liable to degrade high-temperature oxidation resistance by causing generation
of austenitic phase during hot use. The upper limit of Ni content is therefore defined
as 0.5 mass%.
[0037] Nb is very effective for improving high-temperature strength. Since Ti is added in
the present invention, substantially no Nb is fixed to C and N, so that essentially
all added Nb can be considered to work effectively toward enhancing high-temperature
strength. This effect manifests itself at a content of not less than 0.10 mass%. On
the other hand, excessive Nb addition degrades formability and low-temperature toughness.
The Nb content is therefore defined as 0.10 - 0.50 mass%. In order to obtain still
higher formability and high-temperature strength, a Nb content in the range of 0.10
- 0.40 mass% is preferable.
[0038] Ti fixes C and N and is generally known to improve grain boundary corrosion resistance.
In this invention, however, it is a very important element for improving formability
(particularly deep drawability). The formability improving effect of Ti appears conspicuously
at content of not less than 0.07 mass% (see FIG. 1). However, excessive Ti addition
degrades toughness and adversely affects product surface properties. Ti content is
therefore defined as 0.07 - 0.25 mass%. For obtaining a high level of high-temperature
strength, Ti is preferably added to satisfy Ti ≥ 6 (C + N). In order to obtain a product
with surface properties as good as or better than SUH409L, Ti is preferably added
to a content of not more than 0.20 mass%.
[0039] Mo is effective for increasing high-temperature strength but makes the steel brittle
when present at a high content. In addition, Mo is very expensive. While adequate
heat resistance can be secured without Mo addition by optimizing the contents of other
constituent elements, Mo addition is advantageous in that it increases the freedom
of composition design. When Mo is incorporated, its content is preferably not more
than 0.50 mass%. When heat resistance is a bigger concern than cost, Mo can be added
in excess of 0.5 mass% but should not be added in excess of 3.0 mass%, the level beyond
which an extreme decline in low-temperature toughness occurs.
[0040] Cu improves low-temperature toughness. In order to markedly improve low-temperature
toughness to the level required in exhaust gas passage components, however, it is
important to incorporate not less than 0.02 mass% of Cu in combination with B (discussed
later). When the Cu content exceeds 0.5 mass%, however, high-temperature oxidation
resistance degenerates sharply (see FIG. 2). Cu content is therefore defined as 0.02
- 0.5 mass% in the present invention.
[0041] V, like Nb and Ti, is a carbonitride-forming element that is effective for improving
grain boundary corrosion resistance and the toughness of sites affected by welding
heat. Moreover, like Nb, V contributes to high-temperature strength improvement in
the solid solution state. This effect is particularly pronounced when V is present
together with Nb. In addition, V is thought to be effective for improving high-temperature
oxidation resistance. However, a V content in excess of 0.20 mass% degrades workability
and low-temperature toughness. When V is added, therefore, its content must be kept
to not more than 0.20 mass%. For thoroughly obtaining the foregoing effects of V,
it is preferably added in the range of 0.01 - 0.20 mass%.
[0042] Al is highly effective for improving high-temperature oxidation resistance. However,
the composition according to the present invention is designed to enable excellent
high-temperature oxidation resistance even without incorporation of Al. Excessive
Al addition degrades formability, weldability and low-temperature toughness. Moreover,
deoxidation by Al is not particularly necessary because the present invention calls
for addition of Ti and Si. When Al is incorporated, it must be added at no more than
0.10 mass%. When adding Al in a case where formability, weldability and low-temperature
toughness are particularly important, the Al content is preferably restricted to not
more than 0.07 mass%.
[0043] B suppresses low-temperature brittleness and secondary work brittleness in a ferritic
steel also containing Nb and Ti. This effect was found to be pronounced when B is
added in combination with Cu. In order to thoroughly improve low-temperature toughness,
B needs to be added at not less than 0.0005 mass%. On the other hand, excessive B
addition beyond 0.02 mass% leads to generation of borides that degrade formability
and degrade rather than improve low-temperature toughness. In the present invention,
B is incorporated at 0.0005 - 0.02 mass% together with Cu at 0.02 - 0.5 mass%.
[0044] Ca and Mg have strong binding force with S and therefore reduce the amount of MnS
generation to improve corrosion resistance. In addition, Ca and Mg are elements that
in themselves effectively work to improve high-temperature oxidation resistance. When
importance is attached to corrosion resistance and high-temperature oxidation resistance,
therefore, these elements can be added as required. However, addition in large amounts
increases inclusions that degrade low-temperature toughness and formability. When
one or both of Ca and Mg are added, therefore, the combined content thereof needs
to be held to not more than 0.01 mass%. In order to bring out the effect of Ca and
Mg addition strongly, the total of the Ca and Mg contents should preferably made 0.003
- 0.01 mass%.
[0045] Y and REMs (rare earth elements) such as La and Ce stabilize the chromium oxide coating
that forms on the steel surface and, by enhancing the adherence between the steel
matrix and the oxide coating, manifestly improve the high-temperature oxidation resistance
of the steel sheet. When high-temperature oxidation resistance is a major concern,
therefore, these elements can be added as required. However, addition in large amounts
not only degrades formability and low-temperature toughness but also promotes generation
of inclusions that may become starting points of abnormal oxidation, meaning that
high-temperature oxidation resistance is degraded rather than improved. Therefore,
when one or more elements selected from among Y and rare earth elements are added,
the combined amount thereof must be made not more than 0.20 mass%. For maximizing
the effect of Y and REM addition, one or more elements selected from among these elements
should preferably be added to a combined total of 0.01 - 0.20 mass%.
[0046] As additional elements, one or more of Zr, Hf, Ta, W, Re and Co can be included for
their ability to improve high-temperature strength. Since excessive addition of these
elements hardens the steel, however, they must, when incorporated, be added to a combined
content of not more than 3.0 mass%. The preferable amount is not more than 0.5 mass%
in total.
[0047] The content of P, S, O, Zn, Sn, Pb and other common impurity elements is preferably
reduced to the lowest level possible in order to ensure good formability and low-temperature
toughness. Specifically, these elements should, at the most lenient, be restricted
to P : not more than 0.04 mass%, S : not more than 0.03 mass%, O : not more than 0.02
mass%, Zn : not more than 0.10 mass%, Sn : not more than 0.10 mass%, and Pb: not more
than 0.10 mass%. At the actual production site, more severe restrictions can be imposed
in accordance with the product quality desired.
[0048] As explained earlier, Equations (1) - (3) define the composition range required for
concurrent improvement of formability and high-temperature oxidation resistance. Although
no particular lower limit is specified for the value (AM value) of the left side of
Eqution (3), a steel with a low AM value ordinarily contains liberal amounts of ferrite
generating elements like Si, Cr, Mo, Ti, Nb, V and Al. When large amounts of these
elements are contained, formability and low-temperature toughness degenerate. Studies
showed that it is preferable to regulate the constituents so that the AM value is
40 or higher.
[0049] Satisfying the aforesaid chemical composition concurrently improves formability,
high-temperature oxidation resistance, high-temperature strength and low-temperature
toughness.
[0050] In order to further improve the formability realized in this way, it is highly effective
to subject the hot-rolled sheet to partial recrystallization treatment followed by
cold rolling and annealing. Specifically, the r value, an index of deep drawability,
can be markedly improved by the steps of making a hot-rolled sheet 10 - 90 vol% of
whose structure is accounted for by recrystallized grains and the remainder of which
is accounted for by un-recrystallized grains, cold rolling the hot-rolled sheet, and
totally recrystallizing it by annealing (see FIG. 1). The steel sheet having the metallic
structure obtained in this manner possesses formability fully capable of responding
to the increasingly severe shape requirements of today's exhaust gas passage components.
[0051] Partial recrystallization of the hot-rolled sheet can be carried out directly during
hot rolling process or by heating conducted between hot rolling and cold rolling.
[0052] Partial recrystallization during hot rolling can, for instance, be conducted by hot
rolling in the temperature range of 950 - 1250 °C, coiling, and cooling in the coiled
state. Optimum conditions can be selected in accordance with the facility specifications
and the hot-rolling pass schedule. Partial recrystallization by heating after hot
rolling can be conducted, for example, by heating the steel sheet cooled after hot
rolling in the temperature range of 850 - 1000 °C. The heating can be carried out
at any stage before cold rolling.
[0053] The hot-rolled sheet partially recrystallized by one of the foregoing methods is
then totally recrystallized by annealing. The cold rolling is conducted at a reduction
in the range of, for example, 30 - 90%. When the steel sheet is to be used in an automobile
exhaust gas passage component, the final sheet thickness is adjusted to, for example,
about 0.4 - 1.2 mm. The annealing temperature is preferably in the range of, for instance,
950 - 1150 °C. The ferritic steel sheet obtained is excellent in formability and low-temperature
toughness and these properties are retained even after fabrication into welded steel
tube.
[0054] In case an article having been formed should have a beautiful surface appearance
and make much of good looking outer surface, it is preferable to use the totally recrystallized
hot-rolled sheet to form the article. The totally recrystallized hot-rolled sheet
can be obtained by subjecting the hot-rolled sheet into a heat treatment at the temperatures
between 950 and 1100 °C
Example 1
[0055] Ferritic steels having the chemical compositions shown in Tables 1 and 2 were made
using a high-frequency vacuum melting furnace and cast into 30-Kg ingots. The ingots
were hot-forged and then hot-rolled into 4.0 mm hot-rolled sheets. The hot rolling
was conducted at a hot-rolling temperature of 700 - 1250 °C and a draft (rolling reduction)
per pass of about 30%. Each hot-rolled sheet was water cooled and then held at 900
- 1000 °C for 1 minute. The cross-sectional metallic structure of the hot-rolled sheet
was observed with an optical microscope. Recrystallized grains were found to account
for 10 - 90 vol.% of every specimen, the balance being un-recrystallized structure.
It was thus ascertained that partial recrystallization had been achieved. The partially
recrystallized hot-rolled sheets were cold rolled to a thickness of 2 mm and thereafter
totally recrystallized by annealing for 1 minute at 1050 °C to afford cold-rolled
annealed sheets. Nos. 1-21 in Table 1 are ferritic steels satisfying the chemical
composition defined by the present invention. Nos. 22 - 31 in Table 2 are comparative
steels not meeting compositional requirements of the present invention. Among the
comparative steels, No. 22 corresponds to SUH409Land No. 23 to SUS430J1L.
[0056] A test piece cut from each cold-rolled annealed sheet was subjected to a tensile
test, a Charpy impact test, a high-temperature tensile test, and a high-temperature
oxidation test.
[0057] Formability was evaluated based on the 0.2% strength, breaking extension and plastic
strain ratio determined by the tensile test. No. 13B test pieces (prescribed by JIS
Z 2201) cut from each steel sheet specimen in directions parallel, 45 degrees and
90 degrees to the rolling direction were used as the tensile test pieces. 0.2% strength
and breaking extension were determined by subjecting the test piece taken 45 degrees
with respect to the rolling direction to the tests prescribed by JIS Z 2241. Plastic
strain ratio was determined in accordance with JIS Z 2254 using the test pieces taken
in all three of the aforesaid directions. More specifically, the plastic strain ratio
in each direction was calculated from the ratio between the lateral strain and thickness
direction strain under application of a 15% uniaxial tensile pre-strain, and the average
plastic strain ratio r
AV and in-plane plastic anisotropy Δr were determined in accordance with the following
equations:
where
r
L = Plastic strain ratio parallel to rolling direction
r
D = Plastic strain ratio 45 degrees to rolling direction
r
T = Plastic strain ratio 90 degrees to rolling direction.
[0058] The Charpy impact test was conducted by the method explained with reference to FIG.
2. The energy transition temperature was determined and used as an index of low-temperature
toughness.
[0059] The high-temperature tensile test was conducted in accordance with JIS G 0657 using
the tensile test piece taken at 45 degrees. The 0.2% strength at 900 °C was determined
and used as an index of high-temperature strength.
[0060] The high-temperature oxidation test was conducted in accordance with JIS Z 2281 by
determining the amount of oxidation increase after heating at 900 °C for 200 hours
in the atmosphere. The result was used as an index of high-temperature oxidation resistance.
[0061] The results of the foregoing tests are shown in Table 3.
[0062] As can be seen from Table 3, the steels Nos. 1 - 21, examples of the present invention,
all had softness (0.2% proof stress) falling about midway between SUH409L (No. 22)
and SUS430J1L (No. 23) and ductility (elongation) similar to SUH409L. They were superior
to SUH409L and SUS430J1L in deep drawability, i.e., in average plastic strain ratio
r
AV and in-plane plastic anisotropy Δr. Their low-temperature toughness (energy transition
temperature) performance was also excellent, matching that of SUH409L. The invention
steels were clearly superior to SUH409L and substantially matched the performance
of SUS430J1L in 900 °C heat resistance (high-temperature strength and high-temperature
oxidation resistance). In short, the steels of the present invention achieved excellent
formability while also thoroughly maintaining high-temperature strength, high-temperature
oxidation resistance and low-temperature toughness.
[0063] In contrast, steel No. 22, a comparative example steel equivalent to SUH409L, was
inferior in heat resistance, and No. 23, equivalent to SUS430J1L, was hard and insufficient
in formability. Steels Nos. 24 and 25 are types that have actually been used in automobile
engine exhaust gas passage components. However, No. 24 was inferior in formability
and low-temperature toughness owing to the fact that, inter alia, it was not added
with Ti and had Si and Cr contents falling outside the ranges of the present invention,
while No. 25 was poor in formability, low-temperature toughness and high-temperature
oxidation resistance because it was high in C and Nb and had Si and Cr contents falling
outside the ranges of the present invention. Steel No. 26 was inferior in formability
and high-temperature oxidation resistance because its phase stability was on the austenitic
side. Steels No. 27 - 31 exhibited deficient low-temperature toughness because they
contained elements harmful to low-temperature toughness in amounts exceeding the ranges
specified by the present invention.
Example 2
[0064] The steels shown in Tables 1 and 2 as from No.1 to No.10 and from No.22 to 26 were
hot-rolled and then subjected to the heat treatment at temperatures between 950 and
1100 °C for 1 minute, thereby to obtain the hot rolled sheet having totally recrystallized
structure. The sheets obtained were cold rolled into 2.0 mm and thereafter totally
recrystallized by annealed at 1050 °C for 1 minute to afford cold rolled annealed
sheets.
[0065] As same as Example 1, a piece cut from each cold-rolled annealed sheet was subjected
to the test to evaluate 0.2% strength, breaking extension, plastic strain ratio and
in-plane anisotropy. Further, in order to evaluate a surface appearance after formed,
a piece cut from each cold-rolled annealed sheet was imposed 20 % of plastic strain
in direction parallel to the rolling direction and subjected to the test to evaluate
the surface roughness of the test piece surface in direction perpendicular to the
rolling direction by using a contact-type surface roughness meter, the surface roughness
being 10 points average roughness Rz in accordance with JIS B 0660. For comparative,
the surface roughness was tested as same above for the test pieces shown in Table
3 which were derived from the partially recrystallized hot-rolled sheets and the results
was shown in Table 4 as Comparative value of surface roughness.
[0066] In comparison with the invention examples between Table 4 and Table 3, it is seen
that the test pieces derived from totally recrystallized hot-rolled sheets, those
of Table 4, tend to have same or lower average plastic strain ratio and have slightly
increased in-plane anisotropy than those of Table 3 for partially recrystallized hot
rolled sheets. This seems to be based on the slight lowering of r value in direction
45 degree to the rolling direction in the case of totally recrystallized hot-rolled
sheets. On the other hand, it is apparent that the surface roughness after formed
is remarkably decreased when the totally recrystallized hot-rolled sheets are used.
This means that by adoption of a treatment for totally recrystallization in the hot-rolled
sheet, there is provided a steel sheet suitable for use in the article formed which
requires superior fine surface appearance. Comparative examples in Table 4 are basically
below in formability compared with invention examples.
[0067] Thus the present invention enables concurrent improvement of formability, high-temperature
strength, high-temperature oxidation resistance and low-temperature toughness in a
ferritic heat-resistant steel sheet. The ferritic steel sheet of the present invention
is particularly notable in that it offers excellent formability, specifically deep
drawability and isotropy thereof, capable of responding to the needs of a diversity
of forming methods. In this aspect, the steel sheet of the present invention is endowed
with new capabilities not envisioned by conventional ferritic heat-resistant steel
sheets. It also offers high-temperature strength, high-temperature oxidation resistance
and low-temperature toughness that achieve a performance level equal to or better
than the steel sheets currently used in exhaust gas passage components. While conventional
ferritic steel sheets have been incapable of concurrently achieving high levels of
formability, high-temperature strength, high-temperature oxidation resistance and
low-temperature toughness, the present invention concurrently achieves excellent performance
on all of these points at a Cr content of not more than 11%. As such, the present
invention enables application of ferritic heat-resistant steel to complicatedly shaped
exhaust gas passage components, helps to expand the degree of freedom in designing
such components, and makes a marked contribution to cost reduction.
1. Ein ferritisches Stahlblech, das gleichzeitig eine verbesserte Formbarkeit, Hochtemperaturoxidationswiderstand,
Hochtemperaturfestigkeit und Niedrigtemperaturzähigkeit besitzt, wobei das Blech in
Massenprozent Folgendes aufweist:
C: nicht mehr als 0,02 %,
Si: 0,7 - 1,1 %,
Mn: nicht mehr als 0,8 %,
Ni: nicht mehr als 0,5 %
Cr: 8,0 bis weniger als 11,0 %,
N: nicht mehr als 0,02 %,
Nb: 0,10 - 0,50 %,
Ti: 0,07 - 0,25 %,
Cu: 0,02 - 0,5 %,
B: 0,0005 - 0,02 %,
V: 0 (keine Addition) - 0,20 %,
eines oder beide der folgenden Elemente: Ca und Mg: 0 (keine Addition) - 0,01 % gesamt,
eines oder mehrere der Elemente aus Y und seltenen Erdelementen: 0 (keine Addition
- 0,20 % gesamt, und
den Rest aus Fe und nicht vermeidbaren Verunreinigungen, und wobei das Blech eine
chemische Zusammensetzung besitzt, die alle der folgenden Gleichungen (1) - (3) erfüllt:
wobei der Stahl optional in Massenprozent Folgendes aufweist:
Mo: nicht mehr als 0,50 %, und
Al: nicht mehr als 0,10 %.
2. Stahlblech nach Anspruch 1, wobei der Gehalt von V 0,01 bis 0,20 % beträgt.
3. Stahlblech nach Anspruch 1 oder 2, wobei der Gehalt von einem oder beiden der Elemente
Ca und Mg 0,0003 - 0,01 % gesamt beträgt.
4. Stahlblech nach einem der vorhergehenden Ansprüche, wobei der Gehalt von einem oder
mehreren der Elemente Y und selten Erdelemente 0,01 - 0,20 % gesamt beträgt.
5. Stahlblech nach einem der vorhergehenden Ansprüche, das ferner auf Kosten von Fe in
Massenprozent eines oder mehrere der folgenden Elemente aufweist: Zr, Hf, Ta, W, Re
und Co, die, wenn sie aufgenommen sind, einen kombinierten Gehalt von nicht mehr als
3,0 Massenprozent aufweisen und vorzugsweise nicht mehr als 0,5 Massenprozent gesamt.
6. Stahlblech nach einem der Ansprüche 1 bis 5, das eine Metallstruktur besitzt, die
erhalten wird durch Kaltwalzen und Anlassen bzw. Annealen eines teilweise rekristallisierten
heißgewalzten Blechs.
7. Stahlblech nach einem der Ansprüche 1 bis 5, das eine metallische Struktur besitzt,
die erhalten wird durch Kaltwalzen und Anlassen bzw. Annealen eines vollständig rekristallisierten
heißgewalzten Blechs.
8. Stahlblech nach einem der Ansprüche 1 bis 7, das wie hergestellt in einem Automotor-Abgaspassagenbauteil
verwendet wird.
1. Tôle d'acier ferritique améliorée simultanément en ce qui concerne sa capacité de
formage, sa résistance à l'oxydation à haute température, sa dureté à haute température,
et sa ténacité à basse température comprenant, en pourcents en poids :
C : pas plus de 0,02 %,
Si : 0,7 à 1,1 %,
Mn : pas plus de 0,8 %,
Ni : pas plus de 0,5 %
Cr : 8,0 à moins de 11,0 %,
N : pas plus de 0,02 %,
Nb : de 0,10 à 0,50 %,
Ti : 0,07 à 0,25 %,
Cu : 0,02 à 0,5 %
B : 0,0005 à 0,02 %,
V : 0 (pas d'adition) à 0,20 %,
un ou plusieurs de Ca et Mg : 0 (pas d'adition) à 0,01 % au total,
un ou plusieurs éléments choisis parmi Y et des éléments de terre rare : 0 (pas d'adition)
à 0,20 % au total,
le reste en fer et en impuretés inévitables,
et ayant une composition chimique satisfaisant les équations (1) à (3)
cet acier comprenant optionnellement en pourcent en poids :
Mo : pas plus de 0,50 %, et
Al : pas plus de 0,10 %
2. Tôle d'acier selon la revendication 1, dans laquelle le contenu en V est de 0,01 à
0,20 %.
3. Tôle d'acier selon la revendication 1 ou 2, dans laquelle le contenu d'un ou plusieurs
de Ca et Mg est de 0,0003 à 0,01 % au total.
4. Tôle d'acier selon l'une quelconque des revendications précédentes, dans laquelle
le contenu d'un ou plusieurs éléments parmi Y et des éléments de terre rare est de
0,01 à 0,20 % au total.
5. Tôle d'acier selon l'une quelconque des revendications précédentes, comprenant en
outre, à la place d'une partie du fer, en pourcent en poids, un ou plusieurs de Zr,
Hf, Ta, W, Re et Co qui, quand ils sont incorporés, ont un contenu combiné non supérieur
à 3,0 % en poids, de préférence non supérieur à 0,5 % en poids au total.
6. Tôle d'acier selon l'une quelconque des revendications de 1 à 5, ayant une structure
métallique obtenue par laminage à froid et recuit d'une tôle laminée à froid partiellement
recristallisée.
7. Tôle d'acier selon l'une quelconque des revendications de 1 à 5, ayant une structure
métallique obtenue par laminage à froid et recuit d'une tôle laminée à chaud totalement
recristallisée.
8. Tôle d'acier selon l'une quelconque des revendications de 1 à 7, utilisée telle que
fabriquée dans un composant de passage de gaz d'échappement de moteur automobile.