Technical Field
[0001] The present invention relates to a ferritic stainless steel sheet excellent in heat
resistance which is optimal for use in particular for an exhaust system member in
which high temperature strength or oxidation resistance is required.
Background Art
[0002] An exhaust manifold, front pipe, center pipe, or other exhaust system member of an
automobile carries high temperature exhaust gas which is exhausted from the engine.
For this reason, the material forming the exhaust system member requires oxidation
resistance, high temperature strength, heat fatigue characteristics, and other various
characteristics.
[0003] In the past, cast iron was generally used for automobile exhaust members, but from
the viewpoints of meeting tougher exhaust gas restrictions, improving engine performance,
reducing the weight of the chassis, etc., exhaust manifolds made of stainless steel
are being used.
[0004] The exhaust gas temperature differs depending on the car model or engine structure,
but usually is 600 to 800°C or so. A material which has an excellent high temperature
strength and oxidation resistance in an environment of long term use in such a temperature
region has been demanded.
[0005] Among stainless steels, austenitic stainless steel is excellent in heat resistance
and workability. However, austenitic stainless steel has a large coefficient of heat
expansion, so when applied to a member like an exhaust manifold which is repeatedly
subjected to heating and cooling, thermal fatigue failure easily occurs.
[0006] Ferritic stainless steel is smaller in coefficient of heat expansion compared with
austenitic stainless steel, so is better in heat fatigue characteristics and scale
peeling resistance. It does not contain Ni, so is also lower in material cost compared
with austenitic stainless steel and is being generally used.
[0007] Ferritic stainless steel is lower than high temperature strength compared with austenitic
stainless steel. For this reason, the art of improving the high temperature strength
has been developed. For example, there are SUS430J1 (Nb steel), Nb-Si steel, and SUS444
(Nb-Mo steel). In each case, Nb is added. These use solution strengthening or precipitation
strengthening by Nb for raising the high temperature strength.
[0008] If adding Nb, the recrystallization temperature of stainless steel becomes higher,
so when producing steel sheet, it is necessary to raise the annealing temperature.
[0009] Stainless steel becomes harder due to the addition of Nb, so after hot rolling, it
is necessary to anneal the hot rolled sheet to soften it, then cold roll it.
[0010] Due to the Nb precipitates formed in the hot rolling process, the toughness falls
and cracking or fracture sometimes occurs in the production process.
[0011] In Nb steel, the product sheet easily becomes harder and easily falls in elongation
and, furthermore, the r-value, an indicator of deep drawability, is low. This is because
due to the presence of the solute Nb and precipitated Nb, the hardening at ordinary
temperature and growth of the recrystallized texture are suppressed. For this reason,
the press formability when forming exhaust parts falls and the freedom of shape becomes
smaller.
[0012] In the above way, Nb steel is inferior in productivity, manufacturability, and workability
of the steel sheet. Nb is high in alloy cost, so addition of Nb also results in higher
production costs.
[0013] Furthermore, the Mo which is added to SUS444 is high in alloy cost, so the part costs
remarkably rises.
[0014] If it were possible to increase the high temperature strength by an additive element
other than Nb, it is possible to suppress the amount of addition of Nb, so it becomes
possible to provide heat resistant ferritic stainless steel sheet which is low in
cost and excellent in workability.
[0015] As an additive element other than Nb and Mo which contributes to improvement of high
temperature strength, there is Cu.
[0016] PLT 1 discloses addition of 0.5% or less of Cu for improvement of the low temperature
toughness. This is not addition of Cu from the viewpoint of heat resistance.
[0017] PLT 2 discloses the art for utilizing the action of steel in improving the corrosion
resistance and weatherability. This is not addition of Cu from the viewpoint of heat
resistance.
[0018] PLT's 3 to 6 discloses the art of precipitation hardening by Cu precipitates to improve
the high temperature strength in a 600°C or 700 to 800°C temperature region. However,
these are all composite addition with Nb, are inferior in manufacturability and workability,
and are high in cost.
[0019] The prior art for improvement of the high temperature strength by addition of Cu
utilizes Cu precipitates. When Cu precipitates are exposed to a high temperature for
a long time, aggregation and combination of the precipitates rapidly occur, so the
precipitation strengthening ability remarkably falls.
[0020] When subjected to a heat cycle accompanying startup and stopping of an engine such
as like in an exhaust manifold, long term use results in a remarkable drop in high
temperature strength and the danger of thermal fatigue failure.
[0021] In particular, in the case of a composition of ingredients in which a large amount
of Nb is added, at the time of high temperature heating, Cu precipitates form at the
interface of the coarse precipitates (Fe
2Nb) called "Laves phases" and the base phase, so the effect of precipitation strengthening
by the Cu precipitates cannot be obtained.
[0022] PLT 6 discloses the art of using composite addition of Nb-Cu-B to cause precipitation
of fine Cu. However, even with this method, composite precipitation with the Laves
phases cannot be avoided. Further, it adds a fine amount of Mo, so the workability
becomes inferior and the cost becomes high.
[0023] In the above way, there are examples of causing the precipitation of Cu for improving
the high temperature strength, but in the prior art, fine precipitation of Cu has
not yet been achieved. The results were insufficient from the viewpoints of workability
and cost as well.
[0024] Further, as steel containing B, PLT's 7 to 9 disclose ferritic stainless steel excellent
in high temperature characteristics. These all add B for improving the workability
and do not disclose addition from the viewpoint of the heat resistance. PLT 10 discloses
a ferritic stainless steel, which has a composition containing, by weight, <0.02%
C, 0.01-1.00% Si, 0.01-1.00% Mn, 7.0-15.0% Cr, 0.02-0.25% Ti, 0.0002-0.01% B, and
<0.02% N.
[0025] PLT11 discloses a ferritic stainless steel sheet comprising (by mass): C: 0.01% or
less, N: 0.02% or less, Si: 0.05-1%, Mn: 0.1-2%, Cr: 10-30%, Mo: 0.1-1%, Cu: 1-2%,
Nb: 0.2-0.7%, Ti: 0.01-0.3% and B: 0.0002-0.0050%, with the remainder being Fe and
unavoidable impurities.
Citation List
Patent Literature
[0026]
PLT 1: Japanese Patent Publication (A) No. 2006-37176
PLT 2: Japanese Patent No. 3446667
PLT 3: WO2003/004714
PLT 4: Japanese Patent No. 3468156
PLT 5: Japanese Patent No. 3397167
PLT 6: Japanese Patent Publication (A) No. 2008-240143
PLT 7: Japanese Patent Publication (A) No. 9-279312
PLT 8: Japanese Patent Publication (A) No. 2000-169943
PLT 9: Japanese Patent Publication (A) No. 10-204590
PLT 10: Japanese Patent Publication (A) No. 6-306552
PLT 11: Japanese Patent Publication (A) No. 2008-240143
Summary of Invention
Technical Problem
[0027] The present invention has as its object the inexpensive provision of ferritic stainless
steel excellent in heat resistance and workability which may be used in particular
under a heat environment of a maximum temperature of the exhaust gas of 600 to 800°C.
Solution to Problem
[0028] In the present invention, the object is to improve the high temperature characteristics
of ferritic stainless steel sheet by adding Cu in steel ingredients not including
Nb and causing the fine dispersion of Cu precipitates.
[0029] Therefore, the inventors focused on the use of composite addition of Ti-Cu-B and
resultant increased fineness of precipitates.
[0030] The inventors investigated in detail the strength at 500°C to 800°C or so and the
ordinary temperature ductility of steel to which no Nb is added (or a small amount
is added) and obtained the following discoveries.
[0031] In the case of Cu steel, in the range of 500°C to 800°C or so, a large amount of
Cu precipitate forms, so the method of controlling the form of the precipitates is
effective for improving the high temperature strength.
[0032] Specifically, by composite addition of Ti and Cu and, furthermore, addition of B,
the Cu precipitates uniformly finely form and it becomes possible to utilize precipitation
strengthening and suppress a drop in strength due to aging heat treatment. This is
effective for the durable stability of parts such- as exhaust members which are repeatedly
subjected to a heat cycle and are used over long periods of time.
[0033] Even when adding Cu to Nb steel, the Cu precipitates form and act to strengthen the
steel, but simultaneously precipitates of Fe and Nb called the Laves phases (Fe
2Nb) are formed. In Mo steel as well, precipitates of Fe and Mo (Fe
2Mo) are similarly formed.
[0034] In this case, Cu compositely precipitates at the interface between the coarse Laves
phases and base phase, so the result does not become fine precipitation. Further,
depending on the temperature conditions, the Cu precipitates rapidly coarsen along
with the elapse of time.
[0035] In the case of such a mode of precipitation, the effect of the precipitation strengthening
falls, so sufficient high temperature strength cannot be obtained and the durability
becomes lower.
[0036] Based on the above discovery, the inventors caused the sole formation of fine Cu
precipitates so as to obtain the effect of precipitation strengthening and suppressed
the coarsening of Cu precipitates by using the art of fine precipitation where composite
precipitation of the Laves phases and Cu does not occur so as to provide ferritic
stainless steel sheet which is inexpensive and exhibits heat resistance.
[0037] The gist of the present invention is as follows:
- (1) Ferritic stainless steel sheet excellent in heat resistance and workability which
contains, by mass%,
C: 0.02% or less,
N: 0.02% or less,
Si: 0.2 to 1.0%,
Mn: 2% or less,
Cr: 10 to 20%,
Cu: 0.4 to 3%,
Ti: 0.01 to 0.5%,
B: 0.0002 to 0.0015%, optionally one or more of:
Nb: 0.01 to 0.3%,
Mo: 0.01 to 0.3%,
Al: 2.5% or less,
V: 1% or less,
Zr: 1% or less, and
Sn: 1% or less, and has a balance of Fe and unavoidable impurities, wherein the total
amount of Nb and Mo is less than 0.2%.
- (2) Method of production of ferritic stainless steel sheet excellent in heat resistance
and workability characterized by hot rolling ferritic stainless steel which has a
composition of ingredients of (1) so as to obtain a hot rolled sheet, then, omitting
annealing of the hot rolled sheet, pickling the hot rolled sheet, cold rolling it
by rolling rolls of a diameter of 400 mm or more, then performing final annealing
at 850 to 970°C.
Advantageous Effect of Invention
[0038] According to the present invention, even if not adding a large amount of Nb, ferritic
stainless steel sheet excellent in high temperature strength and workability is obtained.
The ferritic stainless steel sheet of the present invention gives a large effect in
environmental protection and lowering cost of parts upon application in particular
to an exhaust system member of an automobile etc.
Brief Description of Drawings
[0039] FIG. 1 is a view showing the 0.2% yield strength in a high temperature tensile test
of invention steel and comparative steel.
Description of Embodiments
[0040] Below, the reason for limitation of the composition of ingredients of the ferritic
stainless steel sheet of the present invention will be explained. Steel with no provision
of a lower limit is included in the scope of the present invention down to the level
of unavoidable impurities.
[0041] C causes deterioration of the formability and corrosion resistance and causes a drop
in the high temperature strength, so the smaller the content, the better. For this
reason, the content of C is made 0.02% or less, more preferably 0.009% or less. The
lower limit of the content of C is not particularly provided, but excessive reduction
leads to an increase in the refining cost, so the content is preferably made 0.001%
or more.
[0042] N, like C, causes deterioration of the formability and corrosion resistance and causes
a drop in the high temperature strength, so the smaller the content, the better. For
this reason, the content of N is made 0.02% or less, more preferably 0.015% or less.
The lower limit of the content of N is not particularly provided, but excessive reduction
leads to an increase in the refining cost, so the content is preferably made 0.003%
or more.
[0043] Si is an element which is useful as a deoxidizing agent and an element which improves
the high temperature strength and oxidation resistance. The high temperature strength
up to 800°C or so is improved along with an increase in the amount of Si. To obtain
this effect, the content of Si is made 0.2% or more. Excessive addition of Si lowers
the ordinary temperature ductility, so the upper limit of the content of Si is made
1.0%. Also considering the oxidation resistance, the content of Si is 0.2 to 1.0%.
[0044] Mn is an element which is added as a deoxidizing agent and contributes to the rise
in the high temperature strength in the medium temperature region of 600 to 800°C
or so. Further, during long term use, it forms Mn-based oxides at the surface layer
and thereby contributes to improvement of scale adhesion and suppression of abnormal
oxidation. If the content of Mn is over 2%, the ordinary temperature ductility falls
and, furthermore, MnS is formed and thereby the corrosion resistance falls, so the
upper limit of content of Mn is made 2%. If considering the high temperature ductility
and scale adhesion, the content of Mn is preferably 0.1 to 1.0%.
[0045] Cr is an element essential in the present invention for securing oxidation resistance
and corrosion resistance. If the content of Cr is less than 10%, that effect cannot
be obtained. If the content of Cr is over 20%, a drop in workability and deterioration
of the toughness are caused. For this reason, the content of Cr is made 10 to 20%.
If considering the manufacturability and the high temperature ductility, 10 to 18%
is preferable.
[0046] Cu in particular is an element which is effective for improvement of the high temperature
strength in the medium temperature region of 600 to 800°C or so. This is due to the
precipitation strengthening due to the Cu precipitates in the medium temperature region.
[0047] FIG. 1 shows the 0.2% yield strength in a high temperature tensile test of steels
of the present invention (steel A, steel B, and steel C) and comparative steels (SUH409L
and Nb-Si steel).
[0048] The composition of ingredients of the steel A is 0.005%C-0.007%N-0.41%Si-0.45%Mn-10.5%Cr-1.25%Cu-0.15%Ti-0.0009%B.
[0049] The composition of ingredients of the steel B is 0.006%C-0.009%N-0.88%Si-0.31%Mn-13.9%Cr-1.42%Cu-0.11%.Ti-0.0005%B.
[0050] The composition of ingredients of the steel C is 0.004%C-0.011%N-0.11%Si-0.13%Mn-17.5%Cr-1.36%Cu-0.19%Ti-0.0004%B.
[0051] The comparative steels are steels which are used for general purposes.
[0052] The composition of ingredients of the SUH409L is 0.005%C-0.007%N-0.35%Si-0.50%Mn-10.5%Cr-0.15%Ti.
[0053] The composition of ingredients of the Nb-Si steel is 0.006%C-0.009%N-0.90%Si-0.35%Mn-13.8%Cr-0.45%Nb.
[0054] The high temperature tensile test was performed based on JISG0567 by running a tensile
test in the rolling direction and measuring the 0.2% yield strength.
[0055] From the results of the test, it is learned that the steel A, steel B, and steel
C, regardless as to the fact that Nb is not added, have high temperature strengths
higher than SUH409L and Nb-Si steel in all temperature regions.
[0056] The steel of the present invention is high in strength in the 600°C or so temperature
region and is particularly effective when used in an environment with a low exhaust
gas temperature. The steel of the present invention can be applied even in an environment
less than 600°C.
[0057] In the present invention, considering the high temperature yield strength of the
Nb-Si steel being used for general purposes, a 600°C yield strength of 150 MPa or
more and a 800°C yield strength of 30 MPa or more are made the required properties
of the high temperature strength.
[0058] In the above way, high temperature strength becomes higher due to the precipitation
strengthening due to the formation of Cu precipitate.
[0059] To obtain this effect, it is necessary to make the content of Cu 0.4% or more.
[0060] In the present invention, coarsening of the Cu precipitate due to the composition
precipitation with the Laves phases is suppressed. Due to the composite addition with
Ti or B, fine Cu precipitate is formed.
[0061] If the content of Cu exceeds 3%, the ordinary temperature ductility and oxidation
resistance deteriorate. Further, the edge cracking in the hot rolling process becomes
remarkable and the manufacturability deteriorates. For this reason, the upper limit
of the content of Cu is made 3%. If considering the manufacturability, scale adhesion,
weldability, etc., the content of Cu is preferably 0.5 to 2.5%.
[0062] Ti is an element which bonds with C, N, and S and improves the corrosion resistance,
grain boundary corrosion resistance, ordinary temperature ductility, and deep drawability.
Further, by addition of a suitable quantity in composition addition with Cu, it gives
rise to uniform formation of the Cu precipitates and improves the high temperature
strength and heat fatigue characteristics.
[0063] This action is believed to be because the clusters of Ti in the crystal grains or
the fine precipitates of Ti form sites for formation of Cu precipitates and suppress
coarse formation of Cu at the grain boundaries.
[0064] Furthermore, if adding Ti, a recrystallized texture easily grows at the time of recrystallization
annealing after cold rolling, so the r-value is improved and the press formability
is remarkably improved.
[0065] TO obtain these effects, the content of Ti is made 0.01% or more. If the content
of Ti is over 0.5%, the amount of solute Ti increases, the ordinary temperature ductility
falls, further, coarse Ti-based precipitates are formed and become starting points
of cracking at the time of hole enlargement, and the press formability deteriorates.
Furthermore, the oxidation resistance deteriorates. For this reason, the content of
Ti is made 0.5% or less. If considering the formation of surface flaws or toughness,
the content of Ti is preferably 0.05 to 0.3%.
[0066] B is an element which improves the secondary workability at the time of press-forming
a product. In the present invention, by composite addition with Ti-Cu, Cu precipitates
finely form and the high temperature strength is improved.
[0067] In general, B easily forms (Fe,Cr)
23(C,B)
6 or Cr
2B at a high temperature. However, in Ti-Cu composite steel, it is learned that these
precipitates do not form and Cu precipitates can be made to finely form.
[0068] Cu precipitates usually form extremely finely at the initial stage of precipitation.
The effect of improvement of strength is large, but coarsening occurs due to aging
heat treatment and the strength greatly falls after aging. However, by adding B, coarsening
of the Cu precipitates is suppressed and the stability of strength becomes higher
at the time of use.
[0069] The mechanism of the effect of addition of B on increasing the fineness of Cu precipitates
and suppressing coarsening is not clear, but it is believed that B precipitates at
the grain boundaries and thereby suppresses the grain boundary formation and coarsening
of the Cu precipitates and causes fine precipitation of Cu in the grains.
[0070] To obtain these effects, the content of B is made 0.0002% or more. If the content
of B is over 0.0030%, the steel hardens, the grain boundary corrosion and oxidation
resistance deteriorate, and, furthermore, weld cracking easily occurs. For this reason,
the content of B is made 0.0002 to 0.0015%. If considering the corrosion resistance
and production costs, 0.0003 to 0.0015% is preferable.
[0071] In addition to the above elements, in accordance with need, Nb, Mo, Al, V, and Zr
may be added.
[0072] Nb may be added in accordance with need so as to improve the high temperature strength
or heat fatigue characteristics. If the content of Nb is less than 0.01%, the effect
of addition cannot be obtained. If adding Nb, formation of Laves phases occurs and
the effect of precipitation of Cu on precipitation strengthening is suppressed, so
addition of a large amount is not preferred. Further, the workability is inhibited
and the elongation at break at ordinary temperature deteriorates. Therefore, the upper
limit of the content of Nb is made 0.3%. From the viewpoint of productivity and manufacturability,
the content of Nb is preferably 0.01 to 0.2%.
[0073] Mo is an element which further improves the high temperature strength and heat fatigue
characteristics. If the content of Mo is less than 0.01%, the effect of addition cannot
be obtained. If adding Mo, formation of the Laves phases occurs, the effect of precipitation
of Cu on precipitation strengthening is suppressed, and, further, the ordinary temperature
ductility falls, so addition of a large amount is not preferred. For this reason,
the content of Mo is made 0.3% or less. The more preferable content of Mo is 0.01%
to 0.2%.
[0074] When simultaneously adding Nb and Mo, the workability sometimes falls. For this reason,
the total of the contents of Nb and Mo is less than 0.2%.
[0075] Al is an element which is added in accordance with need as a deoxidizing element
or for improving the oxidation resistance. Furthermore, it is useful for improving
the strength at 600 to 700°C as a solution strengthening element. To stably obtain
this effect, the content of Al is preferably made 0.01% or more. If excessively adding
Al, the steel hardens, uniform elongation remarkably falls, and, furthermore, the
toughness remarkably falls. For this reason, the upper limit of the content of Al
is made 2.5%. If considering the occurrence of surface flaws and weldability and manufacturability,
the content of Al is preferably 0.01 to 2.0%.
[0076] V forms fine carbonitrides and contributes to the improvement of the high temperature
strength by the precipitation strengthening action, so is an element which is added
in accordance with need: To stably obtain this effect, the content of V is preferably
made 0.01% or more. If the content of V is over 1%, the precipitates coarsen, the
high temperature strength falls, and the heat fatigue life falls. For this reason,
the upper limit of content of V is made 1%. If considering the production cost and
manufacturability, the content of V is preferably 0.08 to 0.5%.
[0077] Zr is carbonitride-forming element. It contributes to the improvement of the high
temperature strength and improvement of the oxidation resistance by the increase of
the amounts of solute Ti and Nb. To stably obtain this effect, the content of Zr is
preferably made 0.2% or more. If the content of Zr is over 1%, the manufacturability
remarkably deteriorates. For this reason, the upper limit of the content of Zr is
made 1%. If considering the cost and surface quality, 0.2 to 0.6% is preferable.
[0078] Sn is an element which is large in atomic radius and which is effective for solution
strengthening. It does not cause major deterioration of the mechanical properties
at ordinary temperature, so is an element which is added according to need. To stably
obtain this effect, the content of Sn is preferably made 0.1% or more. If the content
of Sn is over 1%, the manufacturability and the weldability remarkably deteriorate.
For this reason, the upper limit of the content of Sn is made 1%. If considering the
oxidation resistance etc., the content of Sn is preferably 0.2 to 0.5%.
[0079] The steel of the present invention either does not have any Nb and Mo added or contains
them in low concentrations and thereby secures high temperature strength. As a result,
it is possible to realize improvement of the ordinary temperature elongation.
[0080] Next, the method of production of the steel sheet of the present invention sheet
will be explained. The process of production of the steel sheet of the present invention
is comprised of the steps of steelmaking, hot rolling, pickling, cold rolling, annealing,
and pickling.
[0081] In steelmaking, the method of smelting steel containing the above essential ingredients
and ingredients which are added in accordance with need in a converter, then performing
secondary refining is preferable. The smelted molten steel is made into a slab by
continuous casting or another known casting method.
[0082] The slab is heated to a predetermined temperature and is hot rolled to a predetermined
sheet thickness by continuous rolling. The cold rolling of stainless steel sheet is
usually reverse rolling by a Sendimir rolling mill with a roll diameter of 60 to 100
mm or so or one-directional rolling by a tandem rolling mill with a roll diameter
of 400 mm or more. In each case, the slab is rolled by several passes.
[0083] In the present invention, to raise the r-value indicator of the workability, the
slab is preferably cold rolled by a tandem rolling mill with roll diameters of 400
mm or more. If the roll diameters are 100 mm or less, a large amount of shear strain
is introduced near the surface layer at the time of cold rolling, <111> and <554>
crystal orientation growth is suppressed at the time of recrystallization annealing,
and improvement of the r-value becomes difficult. By cold rolling by large size rolls,
shear strain is suppressed, the above crystal orientation remarkably grows, and improvement
of the r-value is contributed to.
[0084] Tandem rolling is one-directional rolling and has fewer rolling passes compared with
Sendimir rolling, so is also better in productivity. If the reduction rate in the
cold rolling process is low, after annealing, a recrystallized microstructure will
not be obtained or excessive grain coarsening will occur causing degraded mechanical
properties, so the reduction rate at the cold rolling process is preferably 30% or
more.
[0085] It is possible to anneal the hot rolled sheet as is normally performed in the production
of ferritic stainless steel sheet, but from the viewpoint of the improvement of the
productivity, it is preferable not to anneal the hot rolled sheet.
[0086] Ordinary Nb steel gives a hard hot rolled sheet, so is annealed before cold rolling.
However, the present invention steel has no Nb added or has only a small amount added,
so the annealing of the hot rolled sheet can be omitted. As a result, the production
costs can be reduced. Furthermore, by omitting the annealing of the hot rolled sheet,
the texture after cold rolling and annealing develops and the press formability is
improved by improvement of the r-value and reduction of the anisotropy.
[0087] The other steps of the method of production are not particularly limited. The hot
rolling conditions, hot rolled sheet thickness, cold rolled sheet annealing temperature,
atmosphere, etc. may be suitably selected. It is also possible to impart temper rolling
and a tension leveler after cold rolling and annealing. Furthermore, the product sheet
thickness may also be selected in accordance with the required member thickness.
[0088] The steel of the present invention has no Nb added or is low in Nb content, so the
annealing temperature after cold rolling is made a low temperature of 850 to 970°C.
Due to this, the high temperature yield strength is improved compared to when the
annealing temperature is over 970°C.
EXAMPLES
[0089] Steel of each of the compositions of ingredients shown in Tables 1 and 2 was smelted
and cast into a slab. The slab was hot rolled to obtain a 5 mm thick hot rolled coil.
After that, the hot rolled coil was pickled without annealing, then was cold rolled
down to a 2 mm thickness and was annealed and pickled to obtain the product sheet.
[0090] In cold rolling, a rolling mill having large sized rolls (diameter 450 mm) was used
for one-directional multipass rolling. For comparison, a rolling mill having small
sized rolls (diameter 100 mm) was used for reverse type multipass rolling.
[0091] The annealing temperature of the cold rolled sheet was made 850 to 970°C so as to
make the crystal grain size number 6 to 8 or so. For the comparative examples in which
the Nb content is over the upper limit of the present invention, the annealing temperature
of the cold rolled sheet was made 1000 to 1050°C.
[0092] In the tables, Nos. 1 to 17 and 37 are invention steels, while Nos. 18 to 36 are
comparative steels. Comparative Steel No. 18 is SUH409L, while Nos. 19 and 20 are
steels with goods records of use as Nb-Si steels.
[0093] From the thus obtained product sheets, test pieces for high temperature tensile tests
were obtained, tensile tests were run at 600°C and 800°C, and the 0.2% yield strength
was measured (based on JISG0567). The tensile tests which gave a 600°C yield strength
of 150 MPa or more and a 800°C yield strength of 30 MPa or more were judged passing.
[0094] Further, as tests for the oxidation resistance, a continuous oxidation test was performed
in the atmosphere at 900°C for 200 hours to evaluate the occurrence of abnormal oxidation
(based on JISZ2281). As a result of the tests, test pieces with no abnormal oxidation
were judged passing.
[0095] For ordinary temperature workability, a JIS No. 13B test piece was prepared and a
tensile test was performed in a direction parallel to the rolling direction and the
elongation at break measured. If the elongation at break at ordinary temperature is
35% or more, working into complicated parts becomes possible, so an elongation at
break of 35% or more was judged passing.
[0096] The average r-value was calculated by obtaining a JIS No. 13B tensile test piece,
giving 15% strain in the rolling direction, direction 45° from the rolling direction,
and direction 90° from the rolling direction, then using formula (1) and formula (2).
where, W
0 is the sheet width before tension, W is the sheet width after tension, t
0 is the sheet thickness before tension, and "t" is the sheet thickness after tension.
where, r
0 is the r-value in the rolling direction, r
45 is the r-value in the direction 45° from the rolling direction, and r
90 is the r-value in the direction 90° from the rolling direction. If the average r-value
is 1.3 or more, working into complicated parts becomes possible, so an average r-value
of 1.3 or more is preferable.
[0097] The underlines in the compositions of ingredients in Tables 1 and 2 mean outside
the scope of the present invention. The underlines in the results of evaluation of
quality mean the tests were not passed.
[0098] From Tables 1 and 2, it is learned that the Steel Nos. 1 to 17 having the compositions
of ingredients prescribed in the present invention had high temperature yield strengths
at 600°C and 800°C which are higher than the comparative examples and are excellent
in oxidation resistance as well without abnormal oxidation at 900°C in the case of
production by an ordinary method as explained above.
[0099] Further, it is learned that the Steel Nos. 1 to 17 have high elongations at break
of 35% or more and workabilities better than the comparative steels in mechanical
properties at ordinary temperature.
[0100] Comparative Steel Nos. 18, 19, and 20 are existing steels, but have high temperature
strengths lower than the requested values. Comparative Steel Nos. 19 and 20 to which
Nb is added in excess also have low r-values.
[0101] Nos. 21 and 22 have C and N which are over the upper limits and are inferior in high
temperature strength, oxidation resistance, and workability.
[0102] No. 23 has excessively added Si and is inferior in workability.
[0103] No. 24 has excessively added Mn and is inferior in oxidation resistance and workability.
[0104] No. 25 has a small amount of Cr, so is low in high temperature strength and is also
inferior in oxidation resistance.
[0105] No. 26 has a small amount of Cu, so is low in 0.2% yield strengths at 600°C and 800°C.
[0106] No. 27 has an amount of Ti which is over the upper limit, so is inferior in oxidation
resistance and workability.
[0107] No. 28 has an amount of Ti which is less than the lower limit and has excessively
added Nb, so is low in ductility.
[0108] No. 29 has excessively added Nb, so is low in ductility and r-value.
[0109] No. 30 has B which is over the upper limit, so is low in oxidation resistance and
workability.
[0110] No. 31 has an amount of addition of B of 0.0001% or less than the lower limit, so
at 800°C the Cu precipitate coarsens, the effect of the precipitation strengthening
falls, and the yield strength is low.
[0111] Nos. 32 to 36 has Mo, Al, V, Zr, and Sn which are over the upper limit, the ordinary
temperature ductility is low, and working into a part is hindered.
[0112] In the invention examples, Nos. 1 to 17 which use large sized rolls for cold rolling
exhibited average r-values of 1.3 or more, that is, good values.
[0113] The Invention Steel No. 37 is excellent in high temperature yield strength and ordinary
temperature elongation at break. However, the cold rolling roll size is small, so
the r-value is a value lower than the preferable range.
Industrial Applicability
[0114] According to the present invention, it is possible to provide a stainless steel sheet
excellent in high temperature characteristics and workability without addition of
large amounts of expensive alloy elements such as Nb and Mo. In particular, by using
this for an exhaust member, the reduction of the parts costs, reduction of weight,
and resultant contributions to environment protection and social contributions are
remarkably great.