[0001] The present invention relates to a ferritic stainless steel sheet, which can be press-formed
to a predetermined profile without defects such as poor circularity and torsion and
then secondary-formed to a final profile with good hot-extruding, and also relates
to a method of manufacturing thereof.
[0002] Ferritic stainless steels, represented by SUS430 or SUS430LX, have been used so far
in various fields, e.g. durable consumers' goods, due to good corrosion-resistance
and cheapness compared with austenitic stainless steels, which contain Ni as an expensive
element. Conditions for press-forming a ferritic stainless steel sheet to a product
profile become severe and severe as development of its application. A press-formed
steel sheet is often secondarily formed for extrusion of a hole, for instance. In
response to development of the application, there is a strong demand for provision
of a new ferritic stainless steel sheet, which is fairly excellent in formability
compared with conventional ferritic stainless steel sheets and so formed to a product
profile without defects even under severe conditions.
[0003] There are many reports on formability of ferritic stainless steel sheets. A representative
improvement is addition both of Ti and Nb for stabilization of dissolved C and N as
carbonitrides. Furthermore, JP2000-192199A teaches distribution of magnesium inclusions,
which are effective on anti-ridging property, in a ferritic stainless steel containing
both of Ti and Nb. JP8-26436B teaches combination of hot-rolling conditions, which
are designed for improvement of Lankford value (r) as an index of formability, with
addition of Ti and Nb.
[0004] Shape-freezability and secondary formability of a primary-formed steel sheet, which
will be formed to a final profile, are also important factors as well as to Lankford
value (r) and anti-ridging property.
[0005] A ferritic stainless steel sheet is generally inferior in formability to an austenitic
stainless steel sheet. Especially, it significantly reduces thickness in a primary-formed
state, and thickness reduction is anisotropic. Consequently, dimensional accuracy
such as circularity becomes worse as severer forming conditions, when the ferritic
stainless steel sheet is press-formed to a cylindrical profile. Thickness deviation
in the primary-formed state leads to serious degradation of secondary formability
such as hole-extruding.
[0006] In the case where a ferritic stainless steel sheet maintains high dimensional accuracy
(e.g. circularity, straightness and anti-torsion) as well as secondary formability
in a press-formed state, a cheap ferritic stainless steel sheet can be used as parts
or members, instead of an expensive austenitic stainless steel sheet, which have been
unavoidably used in view of severe forming conditions.
[0007] The present invention aims at provision of a ferritic stainless steel sheet improved
in dimensional accuracy and secondary formability in a press-formed state by controlling
particle size and distribution of precipitates, which are dispersed in a steel matrix.
[0008] The present invention proposes a new ferritic stainless steel sheet, which consists
of 0.02 mass % or less of C, 0.8 mass % or less of Si, 1.5 mass % or less of Mn, 0.050
mass % or less of P, 0.01 mass % or less of S, 8.0-35.0 mass % of Cr, 0.05 mass %
or less of N, 0.05-0.40 mass % of Ti, 0.10-0.50 mass % of Nb, optionally one or more
selected from the group consisting of 0.5 mass % or less of Ni, 3.0 mass % or less
of Mo, 2.0 mass % or less of Cu, 0.3 mass % or less of V, 0.3 mass % or less of Zr,
0.3 mass % or less of Al and 0.0100 mass % or less of B, and the balance being Fe
except inevitable impurities with a product of (%Ti × %N) less than 0.005. Its metallurgical
structure is defined by distribution of precipitates of 0.15 µm or more in particle
size except TiN at a rate of 5000-50000/mm
2.
[0009] The ferritic stainless steel sheet is manufactured as follows:
[0010] A molten steel with a predetermined composition is cast to a slab. The slab is hot-rolled
to a steel sheet at a finish-temperature of 800°C or lower and annealed at 450-1080°C.
The annealed hot-rolled steel sheet is pickled and cold-rolled in accompaniment with
at least one intermediate-annealing within a temperature range of from (a recrystallization-finishing
temperature - 100°C) to (a recrystallization-finishing temperature). The cold-rolled
steel sheet is finally subjected to finish-annealing at a temperature of 1080°C or
lower.
[0011] The hot-rolled steel sheet may be box-annealed for a predetermined time period of
one hour or shorter. The inter-mediate annealing and the finish-annealing may be performed
in a continuous annealing furnace for one minute or shorter.
[0012] Fig. 1 is a view for explaining circularity of a steel sheet, which is cylindrically
formed by a multiplaten press.
[0013] The inventors have researched and examined on manufacturing conditions of ferritic
stainless steel sheets for improvement of dimensional accuracy (e.g. circularity,
straightness and torsion) from various aspects, and discovered that circularity and
secondary formability of a press-formed steel sheet are fairly affected by shape and
distribution of TiN and other precipitates in an annealed state. The inventors presume
on the basis of the discovery that objective properties are imparted to a ferritic
stainless steel sheet by properly controlling shape and distribution of the precipitates.
Formation of the precipitates to the shape and distribution suitable for the purpose
is realized by adding both of Ti and Nb to a ferritic stainless steel at a value more
than a stoichiometric ratio for stabilization of C and N as carbonitrides and subjecting
the ferritic stainless steel to optimum thermo-mechanical treatment.
[0014] The effects of shape and distribution of the precipitates on press formability and
dimensional accuracy may be explained as follows:
[0015] C and N in a ferritic stainless steel are mostly precipitated as carbonitrides by
addition of Ti and Nb. The precipitated carbonitrides except TiN are substantially
reformed to very fine particles in the manufacturing process of from annealing a hot-rolled
steel sheet through cold-rolling to finish-annealing. The fine particles allow predominant
growth of recrystallized grains with a certain orientation without pinning action,
when the manufactured steel sheet is annealed for recrystallization, resulting in
formation of an anisotropic grain-mixed structure. The anisotropy causes concentration
of strains along a certain direction during primary-forming a steel sheet and so worsens
press formability and dimensional accuracy of the steel sheet.
[0016] The pinning action during recrystallization annealing is expected by distribution
of precipitates having a particle size bigger than a certain value. The pinning action
suppresses orientative grain growth or growth to coarse grains, so as to improve isotropy
and dimensional accuracy of a press-formed steel sheet. The effects of the pinning
action on press formability and dimensional accuracy are typically noted by distribution
of precipitates of 0.15 µm or more in particle size except TiN at a rate of 5000-50000/mm
2, as recognized in the later examples.
[0017] Among the precipitates, TiN is disadvantageous for press formability and dimensional
accuracy. In fact, a steel sheet, which has a product of (%Ti ×%N) above 0.005, is
cracked in a press-formed state. Coarse TiN particles, which have grown to a cubic
shape, are observed at starting points of cracks. The observation result means that
strains concentrate at cubic a pices during press-forming and induce micro-cracks.
Concentration of strains and formation of micro-cracks around the TiN particles are
also unfavorable for hole-extruding in a secondary-forming step.
[0018] The inventive ferritic stainless steel sheet contains alloying components at predetermined
ratios, as follows:
[0.02 mass % or less of C]
[0019] C is converted to carbides effective for random growth of recrystallized ferritic
grains in a finish-annealing step, but degrades formability of a steel sheet due to
its hardening effect. Precipitation of carbides also cause inferior corrosion-resistance.
In this regard, a C content is controlled at a lowest possible level, i.e. 0.02 mass
% or less for formability and corrosion-resistance. The C content is preferably reduced
to 0.015 mass % or less for improvement of secondary formability. However, reduction
of the C content to an extremely lower level necessitates a long-term refining operation
and raises a steel-manufacturing cost. Therefore, a lower limit of the C content is
preferably determined at 0.001 mass %. Definition of the lower limit also assures
the effect of carbides on random growth of recrystallized ferritic grains in a finish-annealing
step.
[0.8 mass % or less of Si]
[0020] Si is an alloying component, which is added as a deoxidizing agent during a steel-making
process, but has a strong solid-solution hardening effect. Excess Si above 0.8 mass
% unfavorably hardens a steel sheet, resulting in poor ductility. An upper limit of
a Si content is preferably determined at 0.5 mass % for ductility and secondary formability.
[1.5 mass % or less of Mn]
[0021] Mn does not harden a steel sheet so much due to its solid-solution hardening effect
weaker than Si. However, excess Mn above 1.5 mass % causes discharge of manganese
fumes during a steel-making process, resulting in poor productivity.
[0.050 mass % or less of P]
[0022] P is harmful on hot-workability, so that its upper limit is determined at 0.050 mass
%.
[0.01 mass % or less of S]
[0023] S is a harmful element, which segregates at grain boundaries and embrittles the grain
boundaries. Such defects can be suppressed by controlling a S content to 0.01 mass
% or less.
[8.0-35.0 mass % of Cr]
[0024] A Cr content is controlled to 8.0 mass % or more for assuring corrosion-resistance
of a stainless steel. However, toughness and formability of the stainless steel becomes
worse as an increase of the Cr content, so that an upper limit of the Cr content is
determined at 35.0 mass %. The Cr content is preferably controlled to 20.0 mass %
or less for further improvement of ductility and secondary formability.
[0.05 mass % or less of N]
[0025] N is converted to nitrides effective for random growth of recrystallized ferritic
grains in a finish-annealing step, but has a hardening effect. Since excess N degrades
ductility of a steel sheet, a N content is controlled to a lowest possible level,
i.e. 0.05 mass % or less. The N content is preferably controlled to 0.02 mass % or
less for further improvement of ductility and secondary formability. However, reduction
of the N content to an extremely lower level necessitates a long-term refining operation
and raises a steel-manufacturing cost. Therefore, a lower limit of the N content is
preferably determined at 0.001 mass %. Definition of the lower limit also assures
the effect of nitrides on random growth of recrystallized ferritic grains in a finish-annealing
step.
[0.05-0.40 mass % of Ti]
[0026] Ti is an alloying component, which stabilizes C and N as carbonitrides for formability
and corrosion-resistance. Such effect is apparently noted at a Ti content of 0.05
mass % or more. However, excess Ti above 0.40 mass % leads to rising of a steel cost
and induces surface defects originated in titanium inclusions.
[0.10-0.50 mass % of Nb]
[0027] Nb, which has the same effect as Ti on stabilization of C and N, is an essential
component for precipitation of niobium inclusions of 0.15 µm or more in particle size
except TiN. The niobium inclusions are probably composed of carbides and Fe
2Nb. A Nb content of 0.10 mass % or more is necessary for precipitation of such niobium
inclusions. However, excess Nb above 0.50 mass % causes exaggerated precipitation
and unfavorably raises a recrystallizing temperature of a ferritic stainless steel.
[0.50 mass % or less of Ni]
[0028] Ni is an optional element for toughness of a hot-rolled steel sheet and corrosion-resistance.
But excess addition of Ni raises a material cost and hardens a steel sheet, so that
an upper limit of a Ni content is determined at 0.5 mass %.
[3.0 mass % or less of Mo]
[0029] Mo is an optional element for corrosion-resistance, but excess Mo above 3.0 mass
% is unfavorable for hot-workability.
[2.0 mass % or less of Cu]
[0030] Cu is an optional element, which is often included in a stainless steel from scraps
during a steel-making process. Since excess Cu causes poor toughness and degradation
of hot-workability, a Cu content is controlled to 2.0 mass % at most.
[0.3 mass % or less each of V and Zr]
[0031] V and Zr are optional elements. V fixes free C as carbide in a steel matrix for formability,
while Zr captures free O for formability and toughness. However, excess addition of
V or Zr is necessarily avoided for productivity. In this sense, an upper limit each
of V and Zr is determined at 0.3 mass %.
[0.3 mass % or less of Al]
[0032] Al is an optional element, which is added as a deoxidizing agent in a steel-making
process. However, excess Al above 0.3 mass % causes an increase of nonmetallic inclusions,
resulting in poor toughness and surface defects.
[0.0100 mass % or less of B]
[0033] B is an optional element, which stabilizes N and improves corrosion-resistance and
formability of a stainless steel. The effects of B are apparently noted at 0.0010
mass % or more, but excess B above 0.0100 mass % is disadvantageous for hot-workability
and weldability.
[0034] Ca, Mg, Co, REM (rare earth metals), etc. other than the above elements may be included
from scraps during steel-making. Such elements do not put significant effects on circularity
of a deeply-drawn steel sheet or dimensional accuracy of a press-formed steel sheet,
unless they are included at extraordinary ratios.
[(%Ti×%N)<0.005]
[0035] TiN grows to coarse particles or forms clusters as an increase of (%Ti × %N). The
coarse TiN particles or cluster promotes accumulation of strains during primary-forming,
resulting in formation of micro-cracks at an early stage of a drawing step. Such harmful
effects of the coarse TiN particles or cluster are eliminated by control of (%Ti×%N)
to a value less than 0.005, as recognized in the later examples.
[A distribution rate of precipitates of 0.15 µm or more in particle size except TiN
at 5000-50000/mm2]
[0036] Carbide and nitride precipitates of 0.15 µm or more in particle size have a pinning
action to suppress orientative grain growth and also growth to coarse grains, so as
to improve isotropy of a stainless steel sheet, circularity in a cylindrically-drawn
state and dimensional accuracy in a press-formed state.
[0037] Precipitates are carbides and nitrides of Ti and Nb, Laves phase and mixtures thereof.
TiN particles, which are precipitated in a cubic shape, are excluded from the precipitates
effective for press formability and dimensional accuracy, since the cubic TiN particles
are likely to concentrate strains at their apices and act as a starting point of micro-cracks.
Distribution of the precipitates of 0.15 µm or more in particle size except TiN at
a rate of 5000-50000/mm
2 assures a pinning action effective for press formability and dimensional accuracy
of a press-formed steel piece, as noted in the later examples.
[0038] The effects of the precipitates on press formability and dimensional accuracy of
a press-formed steel piece are noted at a particle size of 0.15 µm or more, and become
bigger as an increase of the particle size. However, coarse precipitates above 1.0
µm in particle size are not unfavorable, since the coarse particles promote accumulation
of strains and formation of micro-cracks during press-forming, resulting in poor shape-freezability
The pinning action of the precipitates is apparently noted at a distribution rate
of 5000/mm
2 or more, but excess distribution of the precipitates above 50000/mm
2 rather degrades ductility and deep-drawability of a steel sheet. The excess distribution
unfavorably raises a recrystallizing temperature of the steel sheet, so that the steel
sheet is hardly annealed to a recrystallized state.
[0039] Manufacturing conditions, which are necessary to control shape and distribution of
precipitates, will be understood from the following explanation.
[Hot-rolling at a finish-temperature of 800°C or lower]
[0040] A ferritic stainless steel sheet is hot-rolled at a relatively lower finish-temperature
in order to induce nuclear site for precipitates, which will be distributed in a finish-annealed
steel sheet. Boundaries of ferritic grains and internal strains in a hot-rolled state
serve as the nuclear site. A finish-temperature of hot-rolling is determined at 800°C
or lower in order to induce the nuclear sites as many as possible.
[Annealing a hot-rolled steel sheet at 450-1080°C]
[0041] Precipitates in a hot-rolled steel sheet are moderated to a shape suitable for controlling
the precipitates, which will be distributed in a finish-annealed steel sheet, to 0.15
µm or more in particle size, by annealing the hot-rolled steel sheet at 450-1080°C.
If the annealing temperature is lower than 450°C, effective precipitates are scarcely
formed. If the hot-rolled steel sheet is heated at a temperature above 1080°C on the
contrary, the precipitates except TiN are unpreferably re-dissolved in a steel matrix.
[0042] The annealing is completed in one hour for properly controlling distribution number
of precipitates without growth to coarse particles. [Intermediate-annealing at a temperature
within a range of from (a recrystallizing temperature - 100°C) to (a recrystallizing
temperature)]
[0043] During cold-rolling, a steel sheet is annealed at a relatively lower temperature
in order to inhibit re-dissolution of the precipitates, which have been formed by
annealing the hot-rolled steel sheet. A temperature for intermediate-annealing just
below a recrystallization-finishing temperature is preferable for relief of stress,
which is introduced into the steel sheet by cold-rolling. The steel sheet can be softened
without re-dissolution of precipitates regardless somewhat remains of rolling texture,
which is not recrystallized yet, as far as the annealing temperature is held within
a range of from (a recrystallizing temperature - 100°C) to (a recrystallizing temperature).
[0044] The intermediate annealing period is completed in one minute in order to avoid re-dissolution
of the precipitates, accounting faculty of a conventional continuous annealing furnace.
[Finish-annealing at a temperature of 1080°C or lower]
[0045] A rolling texture is eliminated by finish-annealing. But, a heating temperature above
1080°C is not only disadvantageous for mass-productivity but also promotes re-dissolution
of the precipitates and growth to coarse grains, resulting in poor toughness.
[0046] The finish-annealing is completed in one minute, accounting faculty of a conventional
continuous annealing furnace.
[0047] The other features of the present invention will be apparently understood from the
following examples, although the scope of the present invention is not restricted
by these examples.
Example 1 (Fundamental Experiments)
[0048] The inventors have investigated effects of TiN, which is often precipitated in a
ferritic stainless steel matrix, as well as shape effects of precipitates on dimensional
accuracy and secondary formability of a press-formed steel piece under the following
conditions.
[0049] Several steels were melted in an experimental furnace and cast to slabs, wherein
each steel was adjusted to a composition of 0.007 mass % C, 0.40 mass % Si, 0.25 mass
% Mn, 0.030 mass % P, 0.0005 mass % S, 0.05 mass % Cu, 16.50 mass % Cr, 0.04 mass
% Al except Fe and inevitable impurities with the provision that Nb, Ti and N contents
were varied within ranges of 0.02-0.30 mass %, 0.05-0.30 mass % and 0.005-0.035 mass
%, respectively.
[0050] Table 1 shows the Nb, Ti and N contents together with a product of (%Ti×%N) and a
recrystallization-finishing temperature.
Table 1:
Nb, Ti and N contents (mass %) together with (%Ti×%N) and a recrystallization-finishing
temperature Trf (°C) |
Steel No. |
Nb |
Ti |
N |
%Ti×%N |
Trf |
1 |
|
0.06 |
0.005 |
0.0003 |
910 |
2 |
|
0.06 |
0.035 |
0.0021 |
900 |
3 |
|
0.2 |
0.01 |
0.0020 |
930 |
4 |
0.2 |
0.2 |
0.02 |
0.0040 |
940 |
5 |
|
0.3 |
0.01 |
0.0030 |
955 |
6 |
|
0.3 |
0.02 |
0.0060 |
950 |
7 |
|
0.3 |
0.035 |
0.0105 |
940 |
8 |
0.02 |
0.2 |
0.01 |
0.0020 |
910 |
9 |
0.3 |
0.2 |
0.01 |
0.0020 |
960 |
[0051] The underlined figures are values out of the present invention.
[0052] Each slab was hot-rolled to thickness of 4 mm at a finish-temperature of 750°C.
[0053] The hot-rolled steel sheets Nos. 1-7 were annealed at 800°C for 60 seconds, pickled
and then cold-rolled to thickness of 2 mm. The steel sheets were further cold-rolled
to final thickness of 0.5 mm, in accompaniment with intermediate-annealing at a temperature
of (a recrystallization-finishing temperature -50°C) for 60 seconds. The cold-rolled
steel sheets were finish-annealed at 1000°C for 60 seconds.
[0054] The hot-rolled steel sheets Nos. 8 and 9 were annealed, pickled and then cold-rolled
to thickness of 2 mm. The steel sheets were intermediately annealed and further cold-rolled
to final thickness of 0.5 mm. The cold-rolled steel sheets were subjected to finish-annealing.
Table 2 shows conditions of annealing the hot-rolled steel sheets, intermediate-annealing
and finish-annealing.
[Distribution rate and shape of precipitates]
[0055] A test piece sampled from each annealed steel sheet was etched in a nonaqueous electrolyte
of 10% acetylacetone-1% tetramethyl ammonium chloride-methyl alcohol under a potentiostatic
condition and then observed by a scanning electron microscope to investigate distribution
of precipitates. A cross section in parallel to a rolling direction was inspected
at arbitrary 50 points, and maximum length of each precipitate was measured and evaluated
as particle size.
[Dimensional accuracy of a press-formed steel piece]
[0056] A blank sampled from each annealed steel sheet was press-formed to a cylindrical
profile (shown in Fig. 1) by a multiplaten press. Maximum and minimum radii of a cylindrical
part C at a position apart 5mm from a flanged part F were measured by a laser displacement
meter. A ratio of (the maximum diameter - the minimum diameter)/(the minimum diameter)
was calculated and regarded as circularity to evaluate dimensional accuracy of a press-formed
steel sheet.
[Secondary formability]
[0057] Another test piece was bulged to a height of 10 mm, using a punch of 103 mm in diameter
with a curvature radius of shoulder being 10 mm and a die of 105 mm in diameter with
a curvature radius of shoulder being 8 mm under the condition a flange of the test
piece was fixed with a bead. A blank of 92 mm in diameter was sampled from a bottom
of the bulged test piece, and a hole of 10 mm in diameter was formed at a center of
the blank with a clearance of 10%. The blank was then subjected to a secondary formability
test as follows:
[0058] The blank was held between a flathead punch of 40 mm in diameter with a curvature
radius of shoulder being 3 mm and a die of 42 mm in diameter with a curvature radius
of shoulder being 3 mm, in the manner that burrs around the hole faced to the die.
The hole was extruded by the punch until occurrence of cracks at its edge, while a
flange of the blank was fixed with a bead. A diameter of the hole was measured at
the crack initiation. A secondary-extruding ratio was calculated according to the
equation of a secondary-extruding ratio (%) = (the diameter of the extruded hole -
the diameter of the un-extruded hole)/(the diameter of the un-extruded hole) × 100.
[0059] Results are shown in Table 3. It is noted that steel sheets were cracked during press-forming
at a product of (%Ti×%N) above 0.005. Steel sheets with a Nb content less than 0.02
mass % were formed with poor circularity, regardless manufacturing conditions. Observation
results on any of cracked steel sheets and formed steel sheets with poor circularity
prove that precipitates of 0.15 µm in particle size except TiN were distributed in
a steel matrix only a few.
[0060] On the other hand, circularity was improved as an increase of number of the precipitates,
which were distributed in a steel matrix with a Nb content of 0.3 mass % or more,
in combination with conditions of thermo-mechanical treatment. However, excess distribution
of the precipitates was improper for the circularity.
[0061] Steel sheets with a product of (%Ti×%N) more than 0.005 were extremely inferior in
secondary formability. Poor secondary formability was also noted as for steel sheets
with 0.02 mass % of Nb.
[0062] Improvement of secondary formability (i.e. hole-extruding) was recognized as an increase
of number of the precipitates, which were distributed in a steel matrix with a Nb
content of 0.3 mass % or more. However, excess distribution of the precipitates was
improper for the secondary formability.
[0063] The above results prove that dimensional accuracy and secondary formability of a
press-formed steel sheet depend on distribution of precipitates of 0.15 µm or more
in particle size except TiN. That is, the optimum thermo-mechanical treatment for
controlled distribution of such precipitates at a rate of 5000-50000/mm
2 is effective for dimensional accuracy and secondary formability.
Table 3:
Circularity and secondary formability in relation with distribution of precipitates
except TiN |
Sample No. |
Steel No. |
Annealing No. |
Number of precipitates* precipitates* (/mm2) |
Circularity |
Secondary formability |
1 |
1 |
- |
12000 |
0.8 |
51 |
2 |
2 |
- |
11000 |
1.6 |
52 |
3 |
3 |
- |
12700 |
1.7 |
59 |
4 |
4 |
- |
14200 |
2.2 |
53 |
5 |
5 |
- |
13500 |
1.9 |
52 |
6 |
6 |
- |
12500 |
cracked |
23 |
7 |
7 |
- |
12300 |
cracked |
20 |
8 |
8 |
Y1 |
50 |
3.9 |
43 |
9 |
8 |
Y2 |
50 |
4.2 |
48 |
10 |
8 |
Y3 |
150 |
3.1 |
46 |
11 |
9 |
Y4 |
1000 |
3.2 |
42 |
12 |
9 |
Y5 |
1500 |
3.7 |
40 |
13 |
9 |
Y6 |
7000 |
2.2 |
62 |
14 |
9 |
Y7 |
32000 |
1.8 |
58 |
15 |
9 |
Y8 |
42000 |
1.9 |
52 |
16 |
9 |
Y9 |
80000 |
4.2 |
38 |
Precipitates* are of 0.15 µm or more in particle size except TiN. |
Example 2
[0064] Several stainless steels with compositions shown in Table 4 were melted in a vacuum
furnace and cast to slabs. Steels A-H belong to the present invention, while Steels
I-L do not satisfy the compositional definitions of the present invention.
[0065] Each slab was hot-rolled to thickness of 4.0 mm, annealed, pickled and cold-rolled
to thickness of 2 mm. The cold-rolled steel sheet was intermediately annealed, further
cold-rolled to final thickness of 0.5 mm and then finish-annealed. Table 5 shows conditions
of a finish-temperature of hot-rolling, annealing hot-rolled steel sheets, intermediate-annealing
and finish-annealing.
[0066] Each steel sheet was examined by the same way as Example 1, to investigate shape
and distribution of precipitates as well as dimensional accuracy and secondary formability
of a press-formed steel sheet.
[0067] Results shown in Table 6 prove that ferritic stainless steel sheets, wherein precipitates
of 0.15 µm or more in particle size except TiN were distributed in steel matrix at
a rate of 5000-50000/mm
2, were press-formed to a good profile with circularity of 2.5 % or less.
[0068] On the other hand, comparative steel sheets (Example Nos. A2, B2, C2 and D2), which
satisfied compositional conditions of the present invention but were manufactured
under improper conditions, had poor dimensional accuracy and secondary formability
in a press-formed state due to the metallurgical structure that distribution number
of the precipitates except TiN were out of 5000 - 50000/mm
2.
[0069] The steel sheet I was too hard due to excess C and cracked during press-forming.
The steel sheet K was too strong due to excess Nb and cracked during press-forming.
The steel sheet L with a product of (%Ti×%N) above 0.005 was also cracked during press-forming,
wherein the cracks were initiated near coarse TiN particles. The steel sheet J with
shortage of Nb was press-formed with poor circularity.
[0070] It is understood from the above comparison that ferritic stainless steel sheets can
be press-formed to objective profiles with high dimensional accuracy and excellent
secondary formability, by controlled distribution of precipitates of 0.15 µm or more
in particle size except TiN.
[0071] According to the present invention as the above, ferritic stainless steel sheets,
which can be press-formed with high dimensional accuracy and excellent secondary formability,
are provided by distribution of precipitates of 0.15 µm or more in particle size except
TiN at a rate of 5000 - 50000/mm
2 in a steel matrix with controlled composition. Shape and distribution of such precipitates
suitable for the purpose are realized by properly controlling a finish-temperature
of hot-rolling and conditions of heat-treatment for annealing a hot-rolled steel sheet,
intermediate-annealing during cold-rolling and finish-annealing a cold-rolled steel
sheet. The ferritic stainless steel sheets manufactured in this way are useful as
members or parts, which demand for strict dimensional precision, in various fields,
e.g. sealing members for organic electro-luminescent devices, precision pressed parts,
sinks, utensils, burners of stoves, oil filler tubes of fuel tanks, motor casings,
covers, caps of sensors, injector tubes, thermostat valves, bearing seals, flanges
and so on, instead of expensive austenitic stainless steel sheets.