U.S. patent application number 12/396966 was filed with the patent office on 2009-07-02 for ferritic stainless steel sheet excellent in press formability and secondary formability and its manufacturing method.
This patent application is currently assigned to Nisshin Steel Co., Ltd.. Invention is credited to Yasutoshi Hideshima, Naoto Hiramatsu, Kouki Tomimura.
Application Number | 20090165905 12/396966 |
Document ID | / |
Family ID | 33157175 |
Filed Date | 2009-07-02 |
United States Patent
Application |
20090165905 |
Kind Code |
A1 |
Hideshima; Yasutoshi ; et
al. |
July 2, 2009 |
Ferritic Stainless Steel Sheet Excellent in Press Formability and
Secondary Formability and its Manufacturing Method
Abstract
A ferritic stainless steel sheet has a composition of C up to
0.02 mass %, Si up to 0.8 mass %, Mn up to 1.5 mass %, P up to
0.050 mass %, S up to 0.01 mass %, 8.0-35.0 mass % of Cr, N up to
0.05 mass %, 0.05-0.40 mass % of Ti and 0.10-0.50 mass % of Nb with
a product of (% Ti % N) less than 0.005. Precipitates of 0.15 .mu.m
or more in particle size except TiN are distributed in a steel
matrix at a rate of 5000-50000/mm.sup.2. The steel sheet is
manufactured by hot-rolling a slab at a finish-temperature of
800.degree. C. or lower, annealing the hot-rolled steel sheet at
450-1080.degree. C., cold-rolling the hot-rolled steel sheet in
accompaniment with intermediate-annealing at a temperature within a
range of from (a recrystallization-finishing temperature
-100.degree. C.) to (a recrystallization-finishing temperature) and
then finish-annealing the cold-rolled steel sheet at 1080.degree.
C. or lower. The ferritic stainless steel sheet is press-formed
with high dimensional accuracy and excellent secondary formability
due to controlled distribution of the precipitates.
Inventors: |
Hideshima; Yasutoshi;
(Yamaguchi, JP) ; Tomimura; Kouki; (Shunan-shi,
JP) ; Hiramatsu; Naoto; (Shunan-shi, JP) |
Correspondence
Address: |
THE WEBB LAW FIRM, P.C.
700 KOPPERS BUILDING, 436 SEVENTH AVENUE
PITTSBURGH
PA
15219
US
|
Assignee: |
Nisshin Steel Co., Ltd.
Tokyo
JP
|
Family ID: |
33157175 |
Appl. No.: |
12/396966 |
Filed: |
March 3, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10860349 |
Jun 3, 2004 |
|
|
|
12396966 |
|
|
|
|
Current U.S.
Class: |
148/609 ;
148/326; 148/328; 148/621; 148/645 |
Current CPC
Class: |
C22C 38/04 20130101;
C21D 8/0468 20130101; C22C 38/28 20130101; C21D 8/0463 20130101;
C22C 38/02 20130101; C21D 8/0473 20130101; C21D 8/0405 20130101;
C22C 38/26 20130101 |
Class at
Publication: |
148/609 ;
148/326; 148/328; 148/621; 148/645 |
International
Class: |
C21D 8/02 20060101
C21D008/02; C22C 38/18 20060101 C22C038/18; C22C 38/44 20060101
C22C038/44 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 4, 2003 |
JP |
2003159275 |
Claims
1. A ferritic stainless steel sheet excellent in press formability
and secondary formability, which has: a composition consisting 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.times.% N) less than 0.005, wherein the
metallurgical structure contains precipitates of 0.15 .mu.m or more
in particle size other than TiN being distributed at a rate of
5000-50000/mm.sup.2 in a steel matrix.
2. A method of manufacturing a ferritic stainless steel sheet
excellent in press formability and secondary formability, which
comprises the steps of: providing a slab of a ferritic stainless
steel having the composition consisting 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.times.% N) less than 0.005; hot-rolling the slab at a
finish-temperature of 800.degree. C. or lower; annealing the
hot-rolled steel sheet at a temperature within a range of
450-1080.degree. C.; cold-rolling the annealed steel sheet in
accompaniment with at least one intermediate-annealing at a
temperature within a range of from (a recrystallization-finishing
temperature -100.degree. C.) to (a recrystallization-finishing
temperature); and then finish-annealing the cold-rolled steel sheet
at a temperature of 1080.degree. C. or lower.
3. The method as claimed in claim 2 wherein, the metallurgical
structure contains precipitates of 0.15 .mu.m or more in particle
size other than TiN being distributed at a rate of
5000-50000/mm.sup.2 in a steel matrix.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of co-pending patent
application Ser. No. 10/860,349 filed Jun. 3, 2004, which claims
priority to Japanese Patent Application No. 2003-159275 filed Apr.
6, 2003.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] 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.
[0004] 2. Prior Art
[0005] 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 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
SUMMARY OF THE INVENTION
[0010] 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.
[0011] 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 .mu.m or more in particle size
except TiN at a rate of 5000-50000/mm.sup.2.
[0012] The ferritic stainless steel sheet is manufactured as
follows:
[0013] 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.degree. C. or lower and annealed at
450-1080.degree. 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.degree. C.) to (a
recrystallization-finishing temperature). The cold-rolled steel
sheet is finally subjected to finish-annealing at a temperature of
1080.degree. C. or lower.
[0014] 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.
BRIEF DESCRIPTION OF THE DRAWING
[0015] FIG. 1 is a view for explaining circularity of a steel
sheet, which is cylindrically formed by a multiplaten press.
DETAILED DESCRIPTION OF THE INVENTION
[0016] 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.
[0017] The effects of shape and distribution of the precipitates on
press formability and dimensional accuracy may be explained as
follows:
[0018] 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.
[0019] 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 .mu.m or more in particle size except TiN at a
rate of 5000-50000/mm.sup.2, as recognized in the later
examples.
[0020] 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 apexes
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.
[0021] The inventive ferritic stainless steel sheet contains
alloying components at predetermined ratios, as follows:
[0022] [0.02 mass % or less of C]
[0023] 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.
[0024] [0.8 mass % or less of Si]
[0025] 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.
[0026] [1.5 mass % or less of Mn]
[0027] 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.
[0028] [0.050 mass % or less of P]
[0029] P is harmful on hot-workability, so that its upper limit is
determined at 0.050 mass %.
[0030] [0.01 mass % or less of S]
[0031] 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.
[0032] [8.0-35.0 mass % of Cr]
[0033] 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.
[0034] [0.05 mass % or less of N]
[0035] 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.
[0036] [0.05-0.40 mass % of Ti]
[0037] 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 steel cost
and induces surface defects originated in titanium inclusions.
[0038] [0.10-0.50 mass % of Nb]
[0039] 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 .mu.m or more in particle size except TiN. The
niobium inclusions are probably composed of carbides and
Fe.sub.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.
[0040] [0.50 mass % or less of Ni]
[0041] Ni is an optional element for toughness of a hot-rolled
steel sheet and corrosion-resistance. But excess addition of Ni
raises material cost and hardens a steel sheet, so that an upper
limit of a Ni content is determined at 0.5 mass %.
[0042] [3.0 mass % or less of Mo]
[0043] Mo is an optional element for corrosion-resistance, but
excess Mo above 3.0 mass % is unfavorable for hot-workability.
[0044] [2.0 mass % or less of Cu]
[0045] Cu is an optional element, which is often included in a
stainless steel made 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.
[0046] [0.3 mass % or less each of V and Zr]
[0047] 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 %.
[0048] [0.3 mass % or less of Al]
[0049] 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.
[0050] [0.0100 mass % or less of B]
[0051] B is an optional element, which stabilizes N and improves
corrosion-resistance and formability of a stainless steel. The
effects of B is apparently noted at 0.0010 mass % or more, but
excess B above 0.0100 mass % is disadvantageous for hot-workability
and weldability.
[0052] 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 have 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.
[0053] [(% Ti % N)<0.005]
[0054] 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 .mu.m or More in
Particle Size Except TiN at 5000-50000/mm.sup.2]
[0055] Carbide and nitride precipitates of 0.15 .mu.m 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.
[0056] 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 apexes and act
as a starting point of micro-cracks. Distribution of the
precipitates of 0.15 .mu.m or more in particle size except TiN at a
rate of 5000-50000/mm.sup.2 assures a pinning action effective for
press formability and dimensional accuracy of a press-formed steel
piece, as noted in the later examples.
[0057] 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 .mu.m or more, and become bigger as an
increase of the particle size. However, coarse precipitates above
1.0 .mu.m 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.sup.2 or more,
but excess distribution of the precipitates above 50000/mm.sup.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.
[0058] 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.degree. C. or
Lower]
[0059] 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.degree. C. or lower in order to
induce the nuclear sites as many as possible.
[Annealing a Hot-Rolled Steel Sheet at 450-1080.degree. C.]
[0060] 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 .mu.m or more
in particle size, by annealing the hot-rolled steel sheet at
450-1080.degree. C. If the annealing temperature is lower than
450.degree. C., effective precipitates are scarcely formed. If the
hot-rolled steel sheet is heated at a temperature above
1080.degree. C. on the contrary, the precipitated except TiN are
unpreferably re-dissolved in a steel matrix.
[0061] 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.degree. C.) to (a Recrystallizing
Temperature)]
[0062] 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.degree. C.) to (a recrystallizing
temperature).
[0063] 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.degree. C. or Lower]
[0064] A rolling texture is eliminated by finish-annealing. But, a
heating temperature above 1080.degree. 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.
[0065] The finish-annealing is completed in one minute, accounting
faculty of a conventional continuous annealing furnace.
[0066] 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
[0067] 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.
[0068] 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.
[0069] Table 1 shows the Nb, Ti and N contents together with a
product of (% Ti % N) and a recrystallization-finishing
temperature.
TABLE-US-00001 TABLE 1 Nb, Ti and N contents (mass %) together with
(% Ti % N) and a recrystallization-finishing temperature T.sub.rf
(.degree. C.) Steel No. Nb Ti N % Ti % N T.sub.rf 1 0.2 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 0.02
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 The
underlined figures are values out of the present invention.
[0070] Each slab was hot-rolled to thickness of 4 mm at a
finish-temperature of 750.degree. C.
[0071] The hot-rolled steel sheets Nos. 1-7 were annealed at
800.degree. 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.degree. C.) for 60
seconds. The cold-rolled steel sheets were finish-annealed at
1000.degree. C. for 60 seconds.
[0072] 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.
TABLE-US-00002 TABLE 2 Conditions of annealing hot-rolled steel
sheets, intermediate-annealing and finish-annealing annealing
Anneal- hot-rolled intermediate- finish- Steel ing steel sheets
annealing annealing No. No. (.degree. C.) (seconds) (.degree. C.)
(seconds) (.degree. C.) (seconds) 8 Y1 1090 60 950 60 950 60 Y2 700
60 850 60 950 60 Y3 700 28800 850 60 950 60 9 Y4 1100 60 950 60
1000 60 Y5 700 60 1000 60 1000 60 Y6 700 60 900 60 1000 60 Y7 750
1500 900 60 1000 60 Y8 700 3000 900 60 1000 60 Y9 700 28800 900 60
1000 60
[Distribution Rate and Shape of Precipitates]
[0073] 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]
[0074] 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 5 mm 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]
[0075] 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:
[0076] 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.
[0077] 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 .mu.m
in particle size except TiN were distributed in a steel matrix only
a few.
[0078] 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.
[0079] 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.
[0080] 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.
[0081] The above results prove that dimensional accuracy and
secondary formability of a press-formed steel sheet depend on
distribution of precipitates of 0.15 .mu.m 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.sup.2 is effective for dimensional accuracy and
secondary formability.
TABLE-US-00003 TABLE 3 Circularity and secondary formability in
relation with distribution of precipitates except TiN Number of
Sample Steel Annealing precipitates* Secondary No. No. No.
(/mm.sup.2) Circularity 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 .mu.m
or more in particle size except TiN.
Example 2
[0082] 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.
[0083] 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.
TABLE-US-00004 TABLE 4 Chemical compositions (mass %) of ferritic
stainless steels Steel Kind C Si Mn P S Cr N Ti Nb Others TiN
T.sub.rf A 0.013 0.06 0.18 0.032 0.0002 13.93 0.007 0.18 0.13 Zr:
0.21 Al: 0.08 B 0.003 0.5 0.26 0.013 0.003 15.63 0.005 0.22 0.32 B:
0.0072 0.0011 970 Cu: 1.22 C 0.006 0.08 0.26 0.023 0.002 16.55
0.008 0.18 0.25 Al: 0.08 0.0014 960 B: 0.0015 D 0.008 0.08 0.36
0.022 0.001 17.3 0.009 0.14 0.25 Mo: 1.02 0.0013 960 E 0.01 0.2
0.52 0.024 0.003 22 0.011 0.31 0.24 Mo: 0.5 0.0034 990 F 0.008 0.16
0.36 0.009 0.004 9.8 0.009 0.2 0.22 0.0018 870 G 0.003 0.17 0.21
0.01 0.0005 32 0.005 0.12 0.2 V: 0.08 0.0006 1020 H 0.006 0.1 0.17
0.027 0.001 16.52 0.011 0.15 0.25 B: 0.0014 0.0017 950 Al: 0.016
Ni: 0.1 I 0.04 0.26 0.31 0.03 0.003 16.8 0.007 0.12 0.23 0.0008 950
J 0.008 0.2 0.31 0.02 0.002 18.3 0.008 0.1 0.04 0.0008 920 K 0.013
0.19 0.21 0.012 0.003 17.3 0.012 0.12 0.8 0.0014 1030 L 0.01 0.18
0.23 0.011 0.002 16.5 0.02 0.29 0.48 0.0058 1000 T.sub.rf is a
recrystallization-finishing temperature (.degree. C.).
TABLE-US-00005 TABLE 5 Finish-temperature of hot-rolling and
heat-treatment of steel sheets Steel Example A finish-
Annealing*.sup.1 Annealing*.sup.2 Annealing*.sup.3 Kind No.
temperature (.degree. C.) (.degree. C.) (sec.) (.degree. C.) (sec.)
(.degree. C.) (sec.) Note A A1 780 950 60 870 10 950 30 Inventive
Example A2 760 890 60 950 600 980 30 Comparative Example B B1 750
760 60 950 20 1000 5 Inventive Example B2 830 760 60 950 20 1020 5
Comparative Example C C1 750 490 3600 950 30 1000 60 Inventive
Example C2 760 490 7200 950 30 1000 60 Comparative Example D D1 790
550 600 900 60 1000 20 Inventive Example D2 740 550 600 900 60 1110
60 Comparative Example E E1 780 850 600 900 60 1040 60 Inventive
Example F F1 790 700 600 800 60 950 60 '' G G1 780 750 3600 1000 60
1000 60 '' H H1 760 700 60 900 60 1000 60 '' H2 760 700 60 1000 60
1000 60 Comparative Example H3 760 1100 60 900 60 1000 60 '' H4 760
700 60 900 60 1100 70 '' I I1 780 890 20 920 30 980 60 '' J J1 770
700 3600 880 60 980 60 '' K K1 760 800 600 950 60 1050 60 '' L L1
770 760 60 950 60 1050 60 '' Annealing*.sup.1 is heat-treatment to
a hot-rolled steel sheet. Annealing*.sup.2 is intermediate
heat-treatment during cold-rolling. Annealing*.sup.3 is final
heat-treatment to a cold-rolled steel sheet.
[0084] 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.
[0085] Results shown in Table 6 prove that ferritic stainless steel
sheets, wherein precipitates of 0.15 .mu.m or more in particle size
except TiN were distributed in steel matrix at a rate of
5000-50000/mm.sup.2, were press-formed to a good profile with
circularity of 2.5% or less.
[0086] 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.sup.2.
[0087] 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.
[0088] 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 .mu.m or more in
particle size except TiN.
TABLE-US-00006 TABLE 6 Circularity and secondary formability in
relation with distribution of precipitates Steel Example
Distribution number Circularity Secondary kind No. of precipitates*
(/mm.sup.2) (%) formability (%) Note A A1 7200 1.3 51 Inventive
Example A2 4500 2.7 48 Comparative Example B B1 12000 2.3 53
Inventive Example B2 4600 3.6 49 Comparative Example C C1 23000 2
53 Inventive Example C2 80000 2.9 44 Comparative Example D D1 16000
2.2 60 Inventive Example D2 320 3.6 32 Comparative Example E E1
8000 0.9 50 Inventive Example F F1 12000 0.8 51 '' G G1 23000 2.2
50 '' H H1 13000 0.8 68 '' H2 100 2.9 48 Comparative Example H3 50
3.2 44 '' H4 50 4 32 '' I I1 12000 cracked 12 '' J J1 130 3.3 34 ''
K K1 78000 cracked 22 '' L L1 13000 cracked 22 '' *Precipitates are
of 0.15 .mu.m or more in particle size except TiN.
[0089] 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 .mu.m or more in
particle size except TiN at a rate of 5000-50000/mm.sup.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 sheet
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.
* * * * *