U.S. patent application number 16/607170 was filed with the patent office on 2020-12-10 for raw material for cold-rolled stainless steel sheet and method for manufacturing the same.
This patent application is currently assigned to JFE Steel Corporation. The applicant listed for this patent is JFE Steel Corporation. Invention is credited to Mitsuyuki Fujisawa, Tomohiro Ishii, Shuji Nishida, Masataka Yoshino.
Application Number | 20200385834 16/607170 |
Document ID | / |
Family ID | 1000005058457 |
Filed Date | 2020-12-10 |
United States Patent
Application |
20200385834 |
Kind Code |
A1 |
Nishida; Shuji ; et
al. |
December 10, 2020 |
RAW MATERIAL FOR COLD-ROLLED STAINLESS STEEL SHEET AND METHOD FOR
MANUFACTURING THE SAME
Abstract
A raw material for a steel sheet, the raw material being
suitable for manufacturing a cold-rolled ferritic stainless steel
sheet having excellent corrosion resistance, formability, and
ridging resistance, and a manufacturing method therefor are
provided. A raw material for a cold-rolled stainless steel sheet
has a chemical composition containing, in terms of mass %, C: 0.005
to 0.030%, Si: 0.05 to 1.00%, Mn: 0.05 to 1.00%, P: 0.040% or less,
S: 0.030% or less, Al: 0.001 to 0.150%, Cr: 10.8 to 14.4%, Ni: 0.01
to 2.50%, and N: 0.005 to 0.060%, with the balance being Fe and
incidental impurities, in which the raw material has a structure
containing 10 to 90% of a martensite phase in terms of area ratio
with the balance being a ferrite phase.
Inventors: |
Nishida; Shuji; (Chiyoda-ku,
Tokyo, JP) ; Ishii; Tomohiro; (Chiyoda-ku, Tokyo,
JP) ; Yoshino; Masataka; (Chiyoda-ku, Tokyo, JP)
; Fujisawa; Mitsuyuki; (Chiyoda-ku, Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE Steel Corporation |
Tokyo |
|
JP |
|
|
Assignee: |
JFE Steel Corporation
Tokyo
JP
|
Family ID: |
1000005058457 |
Appl. No.: |
16/607170 |
Filed: |
April 13, 2018 |
PCT Filed: |
April 13, 2018 |
PCT NO: |
PCT/JP2018/015579 |
371 Date: |
October 22, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/001 20130101;
C22C 38/14 20130101; C22C 38/60 20130101; C22C 38/005 20130101;
C21D 2211/005 20130101; C21D 8/0273 20130101; C22C 38/08 20130101;
C22C 38/10 20130101; C22C 38/04 20130101; C21D 9/46 20130101; C22C
38/002 20130101; C22C 38/16 20130101; C22C 38/02 20130101; C22C
38/12 20130101; C21D 2211/008 20130101; C21D 8/0226 20130101; C22C
38/008 20130101; C21D 8/0236 20130101 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C21D 8/02 20060101 C21D008/02; C22C 38/00 20060101
C22C038/00; C22C 38/02 20060101 C22C038/02; C22C 38/04 20060101
C22C038/04; C22C 38/08 20060101 C22C038/08; C22C 38/10 20060101
C22C038/10; C22C 38/12 20060101 C22C038/12; C22C 38/14 20060101
C22C038/14; C22C 38/16 20060101 C22C038/16; C22C 38/60 20060101
C22C038/60 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 25, 2017 |
JP |
2017-086010 |
Mar 6, 2018 |
JP |
2018-039385 |
Claims
1. A raw material for a cold-rolled stainless steel sheet, the raw
material having a chemical composition containing, in terms of mass
%, C: 0.005 to 0.030%, Si: 0.05 to 1.00%, Mn: 0.05 to 1.00%, P:
0.040% or less, S: 0.030% or less, Al: 0.001 to 0.150%, Cr: 10.8 to
14.4%, Ni: 0.01 to 2.50%, and N: 0.005 to 0.060%, with the balance
being Fe and incidental impurities, wherein the raw material has a
structure containing 10 to 90% of a martensite phase in terms of
area ratio with the balance being a ferrite phase.
2. The raw material for a cold-rolled stainless steel sheet
according to claim 1, wherein the chemical composition further
contains, in terms of mass %, one or two or more selected from Co:
0.01 to 0.50%, Cu: 0.01 to 0.80%, Mo: 0.01 to 0.30%, and W: 0.01 to
0.50%.
3. The raw material for a cold-rolled stainless steel sheet
according to claim 1, wherein the chemical composition further
contains, in terms of mass %, one or two or more selected from Ti:
0.01 to 0.30%, V: 0.01 to 0.10%, Zr: 0.01 to 0.10%, and Nb: 0.01 to
0.30%; and a value of formula (1) below is 0.0 or less:
54.times.(Ti+V+Zr+Nb)-5.times.Mn-19.times.Ni+1.0 formula (1) where,
in formula (1) above, respective element symbols represent contents
(mass %) of respective elements, or represent 0 when corresponding
elements are not contained.
4. The raw material for a cold-rolled stainless steel sheet
according to claim 2, wherein the chemical composition further
contains, in terms of mass %, one or two or more selected from Ti:
0.01 to 0.30%, V: 0.01 to 0.10%, Zr: 0.01 to 0.10%, and Nb: 0.01 to
0.30%; and a value of formula (1) below is 0.0 or less:
54.times.(Ti+V+Zr+Nb)-5.times.Mn-19.times.Ni+1.0 formula (1) where,
in formula (1) above, respective element symbols represent contents
(mass %) of respective elements, or represent 0 when corresponding
elements are not contained.
5. The raw material for a cold-rolled stainless steel sheet
according to claim 1, wherein the chemical composition further
contains, in terms of mass %, one or two or more selected from B:
0.0003 to 0.0030%, Mg: 0.0005 to 0.0100%, Ca: 0.0003 to 0.0030%, Y:
0.01 to 0.20%, and REM (rare earth metal): 0.001 to 0.100%.
6. The raw material for a cold-rolled stainless steel sheet
according to claim 2, wherein the chemical composition further
contains, in terms of mass %, one or two or more selected from B:
0.0003 to 0.0030%, Mg: 0.0005 to 0.0100%, Ca: 0.0003 to 0.0030%, Y:
0.01 to 0.20%, and REM (rare earth metal): 0.001 to 0.100%.
7. The raw material for a cold-rolled stainless steel sheet
according to claim 3, wherein the chemical composition further
contains, in terms of mass %, one or two or more selected from B:
0.0003 to 0.0030%, Mg: 0.0005 to 0.0100%, Ca: 0.0003 to 0.0030%, Y:
0.01 to 0.20%, and REM (rare earth metal): 0.001 to 0.100%.
8. The raw material for a cold-rolled stainless steel sheet
according to claim 4, wherein the chemical composition further
contains, in terms of mass %, one or two or more selected from B:
0.0003 to 0.0030%, Mg: 0.0005 to 0.0100%, Ca: 0.0003 to 0.0030%, Y:
0.01 to 0.20%, and REM (rare earth metal): 0.001 to 0.100%.
9. The raw material for a cold-rolled stainless steel sheet
according to claim 1, wherein the chemical composition further
contains, in terms of mass %, one or two selected from Sn: 0.001 to
0.500% and Sb: 0.001 to 0.500%.
10. The raw material for a cold-rolled stainless steel sheet
according to claim 2, wherein the chemical composition further
contains, in terms of mass %, one or two selected from Sn: 0.001 to
0.500% and Sb: 0.001 to 0.500%.
11. The raw material for a cold-rolled stainless steel sheet
according to claim 3, wherein the chemical composition further
contains, in terms of mass %, one or two selected from Sn: 0.001 to
0.500% and Sb: 0.001 to 0.500%.
12. The raw material for a cold-rolled stainless steel sheet
according to claim 4, wherein the chemical composition further
contains, in terms of mass %, one or two selected from Sn: 0.001 to
0.500% and Sb: 0.001 to 0.500%.
13. The raw material for a cold-rolled stainless steel sheet
according to claim 5, wherein the chemical composition further
contains, in terms of mass %, one or two selected from Sn: 0.001 to
0.500% and Sb: 0.001 to 0.500%.
14. The raw material for a cold-rolled stainless steel sheet
according to claim 6, wherein the chemical composition further
contains, in terms of mass %, one or two selected from Sn: 0.001 to
0.500% and Sb: 0.001 to 0.500%.
15. The raw material for a cold-rolled stainless steel sheet
according to claim 7, wherein the chemical composition further
contains, in terms of mass %, one or two selected from Sn: 0.001 to
0.500% and Sb: 0.001 to 0.500%.
16. The raw material for a cold-rolled stainless steel sheet
according to claim 8, wherein the chemical composition further
contains, in terms of mass %, one or two selected from Sn: 0.001 to
0.500% and Sb: 0.001 to 0.500%.
17. A method for manufacturing the raw material for a cold-rolled
stainless steel sheet according to claim 1, the method comprising:
hot-rolling a steel slab having the chemical composition to prepare
a hot-rolled sheet, and performing hot-rolled sheet annealing that
involves holding the hot-rolled sheet in a temperature range of
900.degree. C. or more and 1100.degree. C. or less for 5 seconds to
15 minutes.
18. A method for manufacturing the raw material for a cold-rolled
stainless steel sheet according to claim 16, the method comprising:
hot-rolling a steel slab having the chemical composition to prepare
a hot-rolled sheet, and performing hot-rolled sheet annealing that
involves holding the hot-rolled sheet in a temperature range of
900.degree. C. or more and 1100.degree. C. or less for 5 seconds to
15 minutes.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is the U.S. National Phase application of
PCT/JP2018/015579, filed Apr. 13, 2018, which claims priority to
Japanese Patent Application 2017-086010, filed Apr. 25, 2017, and
Japanese Patent Application 2018-039385, filed Mar. 6, 2018 the
disclosures of these applications being incorporated herein by
reference in their entireties for all purposes.
FIELD OF THE INVENTION
[0002] The present invention relates to a raw material for a
cold-rolled stainless steel sheet, the raw material being suitable
for manufacturing a cold-rolled ferritic stainless steel sheet
having excellent corrosion resistance, formability, and ridging
resistance, and to a method for manufacturing the raw material.
BACKGROUND OF THE INVENTION
[0003] Ferritic stainless steel sheets are low-cost, excellent
price-stable material compared to austenitic stainless steel sheets
since the Ni content is not high, and have been used in various
applications, such as building materials, transportation equipment,
and home electric appliances, due to excellent corrosion
resistance. In particular, unlike austenitic stainless steel
sheets, ferritic stainless steel sheets have magnetism, and, thus,
are increasingly used in cooking tools, which are available for
induction heating (IH) systems. Cooking tools such as pots are
mostly formed by bulging. Thus, sufficient elongation is necessary
to obtain a desired shape.
[0004] Meanwhile, ferritic stainless steel sheets have a problem in
that, during forming, surface irregularities (ridging) that
deteriorate appearance frequently occur on the surfaces. The
surface appearance determines commercial value of cooking tools,
therefore if ridging occurs on their surface, a polishing step for
removing the irregularities must be performed after forming. In
other words, there is a problem in that occurrence of extensive
ridging increases the manufacturing cost. In general, extensive
ridging tends to appear when larger strain is applied to the
ferritic stainless steel sheet, in other words, when severe working
is performed.
[0005] In recent years, shapes of home cooking tools have become
increasingly diverse, and thus ferritic stainless steel sheets that
can be subjected to severer working are in demand. In other words,
ferritic stainless steel sheets with higher elongation are
desirable.
However, it is also desirable to decrease the manufacturing cost of
home cooking tools. In other words, ferritic stainless steel sheets
in which ridging that causes the increase in manufacturing cost, is
decreased are desired. In response to these requests, there is a
demand for a ferritic stainless steel sheet that has higher
elongation and reduces ridging sufficiently even if strain larger
than conventional one is applied.
[0006] Regarding the aforementioned problem, for example, Patent
Literature 1 discloses a ferritic stainless steel sheet having
excellent formability, characterized in containing, in terms of
mass %, C: 0.02 to 0.06%, Si: 1.0% or less, Mn: 1.0% or less, P:
0.05% or less, S: 0.01% or less, Al: 0.005% or less, Ti: 0.005% or
less, Cr: 11 to 30%, and Ni: 0.7% or less, and satisfying
0.06.ltoreq.(C+N).ltoreq.0.12, 1.ltoreq.N/C, and
1.5.times.10.sup.-3.ltoreq.(V.times.N).ltoreq.1.5.times.10.sup.-2
(C, N, and V respectively represents contents of the elements in
mass %).
[0007] Patent Literature 2 discloses a method for manufacturing a
ferritic stainless steel sheet having excellent ridging resistance
and formability, characterized in that a hot-rolled sheet of a
ferritic stainless steel sheet containing, in terms of weight %,
0.15% or less of C and 13 to 25% of Cr is annealed for 10 minutes
or less in a range of 930 to 990.degree. C. where austenite and
ferrite phases coexist so as to form a two-phase structure of a
martensite phase and a ferrite phase, the resulting annealed sheet
is cold-rolled, and the resulting cold-rolled sheet is annealed in
a range of 750 to 860.degree. C.
[0008] Patent Literature 3 discloses a raw material for a
cold-rolled stainless steel sheet, containing, in terms of mass %,
C: 0.007 to 0.05%, Si: 0.02 to 0.50%, Mn: 0.05 to 1.0%, P: 0.04% or
less, S: 0.01% or less, Cr: 15.5 to 18.0%, Al: 0.001 to 0.10%, and
N: 0.01 to 0.06% with the balance being Fe and incidental
impurities, in which the raw material has a metal structure
containing, in terms of area ratio, 10 to 60% of a martensite phase
and the balance being a ferrite phase, the martensite phase has a
hardness of HV500 or less.
PATENT LITERATURE
[0009] PTL 1: Japanese Patent No. 3584881
[0010] PTL 2: Japanese Examined Patent Application Publication No.
47-1878
[0011] PTL 3: International Publication No. 2015/111403
SUMMARY OF THE INVENTION
[0012] In the invention disclosed in Patent Literature 1, ridging
evaluation is carried out on a test piece subjected to a prestrain
of 20%, and ridging that occurs due to severer working is not
sufficiently evaluated. The inventors of the present invention
prepared various kinds of steel sheets by methods described in
Patent Literature 1, and the ridging height that occurred when a
prestrain of 23% was applied was evaluated by the method described
below. However, none of the steel sheets exhibited excellent
ridging resistance.
[0013] In the invention disclosed in Patent Literature 2, the
prestrain applied to evaluate ridging is not described. The
inventors of the present invention prepared various kinds of steel
sheets by methods described in Patent Literature 2, and the ridging
height that occurred when a prestrain of 23% was applied was
evaluated by the ridging evaluation method described below. As a
result, none of the steel sheets exhibited excellent ridging
resistance. In addition, in aspects of this invention, the shape of
the test piece used for evaluating elongation is not described. It
is a well-known fact that the value of elongation obtained changes
depending on the shape of the test piece used for evaluation.
The inventors of the present invention prepared various kinds of
steel sheets by methods described in Patent Literature 2, and the
elongation after fracture of the steel sheets was evaluated by the
tensile test method described below. As a result, none of the steel
sheets exhibited excellent formability.
[0014] The inventors of the present invention prepared various
kinds of steel sheets by methods described in Patent Literature 3,
and the elongation after fracture of the steel sheets was evaluated
by the tensile test method described below. As a result, none of
the steel sheets exhibited excellent formability.
[0015] Aspects of the present invention have been developed under
the current circumstances described above, and an object thereof is
to provide a raw material for a cold-rolled stainless steel sheet,
the raw material being suitable for manufacturing a cold-rolled
ferritic stainless steel sheet having excellent corrosion
resistance, formability, and ridging resistance, and a method for
manufacturing the raw material.
[0016] Here, "excellent corrosion resistance" means that the rust
area ratio measured by the method described below is 30% or less.
Preferably, the rust area ratio is 20% or less. The corrosion test
for evaluating the corrosion resistance is carried out in
accordance with JASO M609-91. First, in the testing method, a test
piece is polished with an emery paper to #600, washed with water,
and ultrasonically degreased in ethanol for 5 minutes.
Subsequently, a three-cycle corrosion test is carried out, each
cycle consisting of salt spraying (5 mass % aqueous NaCl solution,
35.degree. C.) 2 h.fwdarw.drying (60.degree. C., relative humidity:
40%) 4 h.fwdarw.wetting (50.degree. C., relative humidity: 95% or
more) 2 h. After the test, the appearance of the corroded surface
is photographed, and a 30 mm.times.30 mm region at the center of
the test piece in the photographed image is subjected to image
analysis to calculate the rust area ratio.
[0017] Furthermore, "excellent formability" means that the
elongation after fracture of the steel sheet measured by the method
described below is 28% or more. More preferably, the elongation
after fracture is 32% or more. In order to evaluate the elongation
after fracture, first, JIS No. 13B tensile test pieces are taken in
accordance with JIS Z 2241 such that longitudinal directions
thereof are, respectively, the rolling direction (L direction), a
direction 45 degrees with respect to the rolling direction (D
direction), and a direction 90 degrees with respect to the rolling
direction (C direction). Subsequently, a tensile test is carried
out in accordance with JIS Z 2241, and the elongation after
fracture (El) is measured for each test piece. The three-direction
average ((L+2D+C)/4, where L, D, and C respectively represent
elongation after fracture (%) in the respective directions) of the
obtained elongation after fracture is calculated, and is determined
to be the elongation after fracture of the steel sheet.
[0018] Furthermore, "excellent ridging resistance" means that the
ridging height of the steel sheet surface measured by the method
described below is 3.0 .mu.m or less. More preferably, the ridging
height is 2.5 .mu.m or less. Yet more preferably, the ridging
height is 2.0 .mu.m or less. To measure the ridging height of the
steel sheet surface, first, a JIS No. 5 tensile test piece is taken
in a direction parallel to the rolling direction. Next, after the
surface of the test piece is polished with a #600 emery paper, a
tensile strain of 23% is applied. Next, the surface profile is
measured with a laser displacement meter in a direction 90 degrees
with respect to the rolling direction on a polished surface of the
parallel portion of the test piece. The measurement length is 16 mm
per line, and the height is measured with 0.05 mm increments. In
addition, the line interval is set to 0.1 mm, and a total of fifty
lines are measured. The obtained profile data of each line is
smoothed and subjected to a waviness removal process by using a
Hanning window function-type finite impulse response (FIR) bandpass
filter with a high-cut filter wavelength of 0.8 mm and a low-cut
filter wavelength of 8 mm. Subsequently, on the basis of the
processed profile data of each line, the data corresponding to 2 mm
portions at both ends of each line is eliminated, and the
arithmetic mean waviness Wa prescribed in JIS B 0601 (2001) is
measured for each line. The average value of the values of the
arithmetic mean waviness, Wa, of fifty lines is the ridging height
of the steel sheet surface.
[0019] Note that, in the ridging resistance evaluation of the
related art, test pieces subjected to a 15% or 20% tensile strain
are mostly used. However, an assumption according to aspects of the
present invention is that the steel sheet is formed into a shape
more complex than that in the related art. Thus, the tensile strain
applied to the test pieces is set to 23% for evaluation under the
assumption that the steel sheet is formed more severely, in other
words, is subjected to higher strain than in the related art.
[0020] To address the problems described above, the inventors of
the present invention have investigated a raw material for a
cold-rolled stainless steel sheet, the raw material being suitable
for manufacturing a cold-rolled ferritic stainless steel sheet
having excellent corrosion resistance, formability, and ridging
resistance, and a method for manufacturing the raw material. As a
result, the following was found.
[0021] A cold-rolled ferritic stainless steel sheet having
excellent formability and ridging resistance is obtained by using a
raw material for a cold-rolled stainless steel sheet prepared by
hot-rolling and then annealing a ferritic stainless steel with an
appropriate chemical composition in a preferable temperature region
that constitutes a ferrite-austenite two-phase region before
cold-rolling, and then cold-rolling this raw material and annealing
the resulting cold-rolled sheet.
[0022] Specifically, in the steel chemical composition, the C
content is set to 0.030% or less, the Cr content is set to 14.4% or
less, and the N content is set to 0.060% or less. A steel ingot
having the aforementioned composition is hot-rolled, and the hot
rolled sheet is annealed at 900 to 1100.degree. C., which is the
ferrite-austenite two-phase region. In this hot-rolled sheet
annealing, the steel composition is adjusted so that the area ratio
of the austenite phase is 10 to 90%. Within the steel chemical
composition range according to aspects of the present invention,
nearly all of this austenite phase is transformed into a martensite
phase during the cooling process after the hot-rolled sheet
annealing. Such a hot-rolled and annealed sheet (raw material for a
cold-rolled steel sheet) having a martensite phase is then
cold-rolled so as to destroy colonies (crystal grain groups having
similar crystal orientations), that cause ridging, and efficiently
apply the rolling strain to the ferrite/martensite grain
boundaries. Since the rolling strain is efficiently applied as
described above and since the Cr content, the C content, and the N
content in the steel are sufficiently low, recrystallization is
accelerated during the cold-rolled sheet annealing. By the
recrystallization accelerating effect, the cold-rolled sheet is
recrystallized sufficiently in the temperature range of 780 to
830.degree. C., which is a ferrite single phase region, and a
cold-rolled and annealed sheet (cold-rolled ferritic stainless
steel sheet) having excellent formability is obtained. Furthermore,
by the colony destroying effect described above, the cold-rolled
and annealed sheet exhibits excellent ridging resistance.
[0023] Aspects of the present invention are based on the
aforementioned findings, and are summarized as follows.
[1] A raw material for a cold-rolled stainless steel sheet, the raw
material having a chemical composition containing, in terms of mass
%,
C: 0.005 to 0.030%,
Si: 0.05 to 1.00%,
Mn: 0.05 to 1.00%,
[0024] P: 0.040% or less, S: 0.030% or less,
Al: 0.001 to 0.150%,
Cr: 10.8 to 14.4%,
Ni: 0.01 to 2.50%, and
N: 0.005 to 0.060%,
[0025] with the balance being Fe and incidental impurities, in
which the raw material has a structure containing 10 to 90% of a
martensite phase in terms of area ratio with the balance being a
ferrite phase. [2] The raw material for a cold-rolled stainless
steel sheet described in [1], in which the chemical composition
further contains, in terms of mass %, one or two or more selected
from
Co: 0.01 to 0.50%,
Cu: 0.01 to 0.80%,
Mo: 0.01 to 0.30%, and
W: 0.01 to 0.50%.
[0026] [3] The raw material for a cold-rolled stainless steel sheet
described in [1] or [2], in which the chemical composition further
contains, in terms of mass %, one or two or more selected from
Ti: 0.01 to 0.30%,
V: 0.01 to 0.10%,
Zr: 0.01 to 0.10%, and
Nb: 0.01 to 0.30%; and
[0027] a value of formula (1) below is 0.0 or less:
54.times.(Ti+V+Zr+Nb)-5.times.Mn-19.times.Ni+1.0 formula (1)
where, in formula (1) above, respective element symbols represent
contents (mass %) of respective elements, or represent 0 when
corresponding elements are not contained. [4] The raw material for
a cold-rolled stainless steel sheet described in any one of [1] to
[3], in which the chemical composition further contains, in terms
of mass %, one or two or more selected from
B: 0.0003 to 0.0030%,
Mg: 0.0005 to 0.0100%,
Ca: 0.0003 to 0.0030%,
Y: 0.01 to 0.20%, and
[0028] REM (rare earth metal): 0.001 to 0.100%. [5] The raw
material for a cold-rolled stainless steel sheet described in any
one of [1] to [4], in which the chemical composition further
contains, in terms of mass %, one or two selected from
Sn: 0.001 to 0.500% and
Sb: 0.001 to 0.500%.
[0029] [6] A method for manufacturing the raw material for a
cold-rolled stainless steel sheet described in any one of [1] to
[5], the method including:
[0030] hot-rolling a steel slab having the chemical composition to
prepare a hot-rolled sheet, and performing hot-rolled sheet
annealing that involves holding the hot-rolled sheet in a
temperature range of 900.degree. C. or more and 1100.degree. C. or
less for 5 seconds to 15 minutes.
[0031] Aspects of the present invention provide a raw material for
a cold-rolled stainless steel sheet, the raw material being
suitable for manufacturing a cold-rolled ferritic stainless steel
sheet having excellent corrosion resistance, formability, and
ridging resistance, and a method for manufacturing the raw
material.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0032] Embodiments of the present invention will now be
specifically described.
[0033] A raw material for a cold-rolled stainless steel sheet
according to aspects of the present invention has a chemical
composition containing, in terms of mass %, C: 0.005 to 0.030%, Si:
0.05 to 1.00%, Mn: 0.05 to 1.00%, P: 0.040% or less, S: 0.030% or
less, Al: 0.001 to 0.150%, Cr: 10.8 to 14.4%, Ni: 0.01 to 2.50%,
and N: 0.005 to 0.060%, with the balance being Fe and incidental
impurities, and has a structure containing 10 to 90% of a
martensite phase on an area ratio basis with the balance being a
ferrite phase. By using the raw material for a cold-rolled
stainless steel sheet according to aspects of the present
invention, a cold-rolled ferritic stainless steel sheet having
excellent corrosion resistance, formability, and ridging resistance
can be manufactured.
[0034] First, the reasons for limiting the chemical composition to
the aforementioned ranges in accordance with aspects of the present
invention are described. Note that % indicating the unit of the
content of a composition means mass % unless otherwise noted.
[0035] C: 0.005 to 0.030%
[0036] Carbon (C) is an element effective for increasing the
strength of the steel. Furthermore, C is an element that improves
ridging resistance since it promotes formation of the austenite
phase during the hot-rolled sheet annealing. This effect is
obtained at a C content of 0.005% or more. However, at a C content
exceeding 0.030%, formability deteriorates due to an increase in
the hardness of the steel. Thus, the C content is set to 0.005 to
0.030%. The C content is preferably 0.007% or more and more
preferably 0.010% or more. The C content is preferably 0.020% or
less and more preferably 0.015% or less.
[0037] Si: 0.05 to 1.00%
[0038] Silicon (Si) is an element useful as a deoxidant. This
effect is obtained at a Si content of 0.05% or more. However, at a
Si content exceeding 1.00%, formability deteriorates due to an
increase in the hardness of the steel. Furthermore, since the
amount of the austenite phase formed during the hot-rolled sheet
annealing decreases, the ridging resistance deteriorates. Thus, the
Si content is set to 0.05 to 1.00%. The Si content is preferably
0.07% or more, more preferably 0.10% or more, and yet more
preferably 0.20% or more. The Si content is preferably 0.50% or
less, more preferably less than 0.40%, and yet more preferably less
than 0.30%.
[0039] Mn: 0.05 to 1.00%
[0040] Manganese (Mn) has a deoxidizing effect. Furthermore, Mn is
an element that improves the ridging resistance since it promotes
formation of the austenite phase during the hot-rolled sheet
annealing. This effect is obtained at a Mn content of 0.05% or
more. However, at a Mn content exceeding 1.00%, precipitation and
coarsening of MnS are accelerated, and corrosion resistance
deteriorates since MnS serves as a starting point of rust
generation. Thus, the Mn content is set to 0.05 to 1.00%. The Mn
content is preferably 0.10% or more and more preferably 0.15% or
more. The Mn content is preferably 0.80% or less and more
preferably 0.60% or less.
[0041] P: 0.040% or less
[0042] Phosphorus (P) is an element that deteriorates corrosion
resistance. Moreover, P segregates in crystal grain boundaries and
deteriorates hot workability. Thus, the P content is preferably as
low as possible, and is set to 0.040% or less. Preferably, the P
content is 0.030% or less.
[0043] S: 0.030% or less
[0044] Sulfur (S) forms a precipitate, MnS, with Mn. Since this MnS
serves as a starting point of corrosion pitting, corrosion
resistance deteriorates. Thus, the S content is preferably as low
as possible, and is set to 0.030% or less. Preferably, the S
content is 0.020% or less.
[0045] Al: 0.001 to 0.150%
[0046] Aluminum (Al) is an element effective for deoxidation. This
effect is obtained at an Al content of 0.001% or more. However, at
an Al content exceeding 0.150%, the formability deteriorates due to
an increase in the hardness of the steel. Thus, the Al content is
set to 0.001 to 0.150%. The Al content is preferably 0.005% or more
and more preferably 0.010% or more. The Al content is preferably
0.100% or less and more preferably 0.050% or less.
[0047] Cr: 10.8 to 14.4%
[0048] Chromium (Cr) is an element that improves corrosion
resistance by forming passive film. At a Cr content less than
10.8%, sufficient corrosion resistance is not obtained. Meanwhile,
at a Cr content exceeding 14.4%, since the austenite phase is not
formed sufficiently in the steel during the hot-rolled sheet
annealing process, the ridging resistance is deteriorated, and the
formability deteriorates due to an increase in hardness of the
steel. Thus, the Cr content is set to 10.8 to 14.4%. The Cr content
is preferably 11.0% or more, more preferably 11.5% or more, and yet
more preferably 12.0% or more. The Cr content is preferably 14.0%
or less, more preferably 13.5% or less, and yet more preferably
13.0% or less.
[0049] Ni: 0.01 to 2.50%
[0050] Nickel (Ni) is an element that suppresses active dissolution
in a low pH environment. In a so-called crevice structure in which
steel sheets are overlapped each other, a low pH environment, that
easily causes corrosion, is sometimes formed. Furthermore, in cases
other than the crevice structure formed between the steel sheets as
described above, an aqueous solution containing chloride ions, that
cause rusting of steel sheets, may become condense on the steel
sheets, salt may precipitate from the aqueous solution, and a
crevice structure may be formed between the precipitated salt and
the steel sheet such that a low pH environment that easily causes
corrosion is formed. Ni suppresses progress of corrosion in such
environments, and improves corrosion resistance of the steel. In
other words, Ni is highly effective for improving the crevice
corrosion resistance, suppresses progress of corrosion in an active
dissolution state markedly, and thereby improves corrosion
resistance. Furthermore, Ni is an element that improves ridging
resistance since it promotes formation of the austenite phase
during the hot-rolled sheet annealing.
[0051] This effect is obtained at a Ni content of 0.01% or more.
However, at a Ni content exceeding 2.50%, formability deteriorates
due to an increase in the hardness of the steel. Thus, the Ni
content is set to 0.01 to 2.50%. The Ni content is preferably 0.03%
or more, more preferably 0.05% or more, and yet more preferably
0.10% or more. The Ni content is preferably 1.20% or less, more
preferably 0.80% or less, and yet more preferably 0.25% or
less.
[0052] N: 0.005 to 0.060%
[0053] Nitrogen (N) is an element effective for increasing the
strength of the steel. Furthermore, nitrogen is an element that
improves ridging resistance since it promotes formation of the
austenite phase during the hot-rolled sheet annealing. This effect
is obtained at a N content of 0.005% or more. However, at a N
content exceeding 0.060%, formability of the steel deteriorates due
to an increase in the hardness of the steel. Thus, the N content is
set to 0.005 to 0.060%. The N content is preferably 0.007% or more
and more preferably 0.010% or more. The N content is preferably
0.020% or less and more preferably 0.015% or less.
[0054] The balance other than the elements described above is Fe
and incidental impurities. Representative examples of the
incidental impurities include O (oxygen), Zn, Ga, Ge, As, Ag, In,
Hf, Ta, Re, Os, Ir, Pt, Au, and Pb. Among these elements, O
(oxygen) can be contained in an amount of 0.02% or less. A total of
0.1% or less of other elements can be contained.
[0055] In accordance with aspects of the present invention, the
following elements may be contained as appropriate in addition to
the basic components described above.
[0056] Co: 0.01 to 0.50%
[0057] Cobalt (Co) is an element that improves crevice corrosion
resistance of stainless steel. However, excessively containing Co
results in saturated effects and deterioration of the workability.
Thus, if Co is to be contained, the Co content is preferably 0.01
to 0.50%. The Co content is more preferably 0.30% or less and yet
more preferably 0.10% or less.
[0058] Cu: 0.01 to 0.80%
[0059] Copper (Cu) is an element that improves corrosion resistance
by strengthening the passive film. However, excessively containing
Cu results in saturated effects and deterioration of the
workability; furthermore, s-Cu tends to precipitate and the
corrosion resistance is deteriorated. Thus, if Cu is to be
contained, the Cu content is preferably 0.01 to 0.80%. The Cu
content is more preferably 0.15% or more and yet more preferably
0.40% or more. The Cu content is more preferably 0.60% or less and
yet more preferably 0.45% or less.
[0060] Mo: 0.01 to 0.30%
[0061] Molybdenum (Mo) has an effect of improving crevice corrosion
resistance of stainless steel. However, excessively containing Mo
results in saturated effects and deterioration of the workability.
Thus, if Mo is to be contained, the Mo content is preferably 0.01
to 0.30%. The Mo content is more preferably 0.20% or less and yet
more preferably 0.10% or less.
[0062] W: 0.01 to 0.50%
[0063] Tungsten (W) is an element that improves crevice corrosion
resistance of stainless steel. However, excessively containing W
results in saturated effects and deterioration of the workability.
Thus, if W is to be contained, the W content is preferably 0.01 to
0.50%. The W content is more preferably 0.03% or more and yet more
preferably 0.05% or more. The W content is more preferably 0.30% or
less and yet more preferably 0.10% or less.
[0064] Ti: 0.01 to 0.30%
[0065] Titanium (Ti) is an element that has an effect of improving
formability of the cold-rolled and annealed sheet since its
precipitation as carbides or nitrides during hot rolling due to its
high affinity to C and N decreases the amounts of dissolved C and
dissolved N in the base metal. Meanwhile, excessively containing Ti
deteriorates the ridging resistance since it suppresses formation
of the austenite phase during the hot-rolled sheet annealing. Thus,
if Ti is to be contained, the Ti content is preferably 0.01 to
0.30%. More preferably, the Ti content is 0.02% or more. The Ti
content is more preferably 0.10% or less and yet more preferably
0.08% or less.
[0066] V: 0.01 to 0.10%
[0067] Vanadium (V) is an element that has an effect of improving
formability of the cold-rolled and annealed sheet since its
precipitation as carbides or nitrides during hot rolling due to its
high affinity to C and N decreases the amounts of dissolved C and
dissolved N in the base metal. Meanwhile, excessively containing V
deteriorates the ridging resistance since it suppresses formation
of the austenite phase during the hot-rolled sheet annealing. Thus,
if V is to be contained, the V content is preferably 0.01 to 0.10%.
The V content is more preferably 0.02% or more and yet more
preferably 0.03% or more. The V content is more preferably 0.08% or
less and yet more preferably 0.05% or less.
[0068] Zr: 0.01 to 0.10%
[0069] Zirconium (Zr) is an element that has an effect of improving
formability of the cold-rolled and annealed sheet since its
precipitation as carbides or nitrides during hot rolling due to its
high affinity to C and N decreases the amounts of dissolved C and
dissolved N in the base metal. Meanwhile, excessively containing Zr
deteriorates the ridging resistance since it suppresses formation
of the austenite phase during the hot-rolled sheet annealing. Thus,
if Zr is to be contained, the Zr content is preferably 0.01 to
0.10%. The Zr content is more preferably 0.02% or more and yet more
preferably 0.03% or more. The Zr content is more preferably 0.08%
or less and yet more preferably 0.05% or less.
[0070] Nb: 0.01 to 0.30%
[0071] Niobium (Nb) is an element that has an effect of improving
formability of the cold-rolled and annealed sheet since its
precipitation as carbides or nitrides during hot rolling due to its
high affinity to C and N decreases the amounts of dissolved C and
dissolved N in the base metal. Meanwhile, excessively containing Nb
deteriorates the ridging resistance since it suppresses formation
of the austenite phase during the hot-rolled sheet annealing. Thus,
if Nb is to be contained, the Nb content is preferably 0.01 to
0.30%. More preferably, the Nb content is 0.02% or more. The Nb
content is more preferably 0.10% or less and yet more preferably
0.08% or less.
[0072] When one or two or more selected from Ti, V, Zr, and Nb is
contained, the value of formula (1) below is 0.0 or less.
54.times.(Ti+V+Zr+Nb)-5.times.Mn-19.times.Ni+1.0 formula (1)
In formula (1), respective element symbols represent contents (mass
%) of respective elements, or represent 0 when corresponding
elements are not contained.
[0073] In embodying the present invention, when one or two or more
selected from Ti, V, Zr, and Nb is contained, the contents of the
respective elements must satisfy the aforementioned ranges and the
value of formula (1) above must be 0.0 or less in order to obtain
excellent ridging resistance.
[0074] As mentioned above, Ti, V, Zr, and Nb have an effect of
suppressing formation of the austenite phase during the hot-rolled
sheet annealing process. Meanwhile, even when these elements are
contained, by sufficiently increasing the contents of Mn and Ni
that promote formation of the austenite phase, a sufficient amount
of austenite phase can be formed in the steel during the hot-rolled
sheet annealing process.
[0075] In other words, when one or two or more selected from Ti, V,
Zr, and Nb is contained, the steel composition is adjusted so that
the value of formula (1) is 0.0 or less. In this manner, it becomes
possible to form a sufficient amount of austenite phase in the
hot-rolled sheet during the hot-rolled sheet annealing and thus a
sufficient amount of martensite phase can exist in the hot-rolled
and annealed sheet. Thus, colonies can be sufficiently destroyed in
the cold-rolling process, and excellent ridging resistance can be
given to the cold-rolled and annealed sheet. However, when the
value of formula (1) exceeds 0.0, a sufficient amount of austenite
phase is not formed in the hot-rolled sheet during the hot-rolled
sheet annealing, the hot-rolled and annealed sheet does not include
a sufficient amount of martensite phase, destruction of colonies
becomes insufficient during the cold rolling process, and the
ridging resistance of the cold-rolled and annealed sheet
deteriorates.
[0076] B: 0.0003 to 0.0030%
[0077] Boron (B) is an element effective for preventing
low-temperature secondary work embrittlement. However, excessively
containing B results in deterioration of hot workability. Thus, if
B is to be contained, the B content is preferably 0.0003 to
0.0030%. More preferably, the B content is 0.0005% or more. More
preferably, the B content is 0.0020% or less.
[0078] Mg: 0.0005 to 0.0100%
[0079] Magnesium (Mg) acts as a deoxidant by forming Mg oxides with
Al in molten steel. However, excessively containing Mg results in
deterioration of toughness of the steel and decreases the
productivity. Thus, if Mg is to be contained, the Mg content is
preferably 0.0005 to 0.0100%. More preferably, the Mg content is
0.0010% or more. The Mg content is more preferably 0.0050% or less
and yet more preferably 0.0030% or less.
[0080] Ca: 0.0003 to 0.0030%
[0081] Calcium (Ca) is an element that improves hot workability.
However, excessively containing Ca results in deterioration of
toughness of the steel, decreases the productivity, and,
furthermore, deteriorates corrosion resistance due to precipitation
of CaS. Thus, if Ca is to be contained, the Ca content is
preferably 0.0003 to 0.0030%. More preferably, the Ca content is
0.0010% or more. More preferably, the Ca content is 0.0020% or
less.
[0082] Y: 0.01 to 0.20%
[0083] Yttrium (Y) is an element that decreases the viscosity of
the molten steel and improves cleanliness. However, excessively
containing Y results in saturated effects and deterioration of the
workability. Thus, if Y is to be contained, the Y content is
preferably 0.01 to 0.20%. More preferably, the Y content is 0.10%
or less.
[0084] Rare earth metal (REM): 0.001 to 0.100%
[0085] Rare earth metals (REM: elements of atomic numbers 57 to 71
such as La, Ce, and Nd) are elements that improve high-temperature
oxidation resistance. However, excessively containing REM results
in saturated effects, causes surface defects during hot-rolling,
and decreases productivity. Thus, if REM is to be contained, the
REM content is preferably 0.001 to 0.100%. More preferably, the REM
content is 0.005% or more. More preferably, the REM content is
0.05% or less.
[0086] Sn: 0.001 to 0.500%
[0087] Tin (Sn) is effective for improving ridging resistance by
promoting formation of the deformation band during rolling.
However, excessively containing Sn results in saturated effects and
deterioration of the formability. Thus, if Sn is to be contained,
the Sn content is preferably 0.001 to 0.500%. More preferably, the
Sn content is 0.003% or more. More preferably, the Sn content is
0.200% or less.
[0088] Sb: 0.001 to 0.500%
[0089] Antimony (Sb) is effective for improving ridging resistance
by promoting formation of the deformation band during rolling.
However, excessively containing Sb results in saturated effects and
deterioration of the formability. Thus, if Sb is to be contained,
the Sb content is preferably 0.001 to 0.500%. More preferably, the
Sb content is 0.003% or more. More preferably, the Sb content is
0.200% or less.
[0090] Structure containing 10 to 90% of martensite phase in terms
of area ratio with the balance being ferrite phase
[0091] In accordance with aspects of the present invention, it is
important that a particular amount of a martensite phase be present
in the structure. In accordance with aspects of the present
invention, a particular amount of an austenite phase is formed in
the steel by performing hot-rolled sheet annealing. Nearly all of
the austenite phase turns into a martensite phase when cooled after
the hot-rolled sheet annealing. Due to the presence of the
martensite phase, the colonies are destroyed in the cool-rolling
process, and the ridging resistance of the cold-rolled and annealed
sheet is improved.
[0092] This effect is obtained when the area ratio of the
martensite phase after the hot-rolled sheet annealing is 10% or
more. Meanwhile, when the area ratio of the martensite phase
exceeds 90%, the hot-rolled and annealed sheet becomes hard, the
rolling load in the cold-rolling step increases, edge cracking and
sheet shape defects occur, and the productivity is decreased. Thus,
the area ratio of the martensite phase is set to 10 to 90%. The
area ratio of the martensite phase is preferably 15% or more and
more preferably 20% or more. The area fraction of the martensite
phase is preferably 70% or less and more preferably 50% or
less.
[0093] The method for measuring the area ratio of the martensite
phase in accordance with aspects of the present invention is as
follows. First, a test piece for structure observation is taken
from near the width center portion of a raw material for a
cold-rolled steel sheet, and after a section taken in the rolling
direction is mirror-polished, the test piece is corroded (etched)
with a Murakami reagent (8 mass % KOH-8 mass %
[K.sub.3Fe(CN).sub.6] aqueous solution). By using an optical
microscope, setting a portion of 1.0 mm depth from the surface
layer to the center of the view areas, ten view areas are
photographed at a magnification of 400. The obtained structure
images are binarized by image analysis, and then one of the values
is deemed as the martensite phase and the other as the ferrite
phase to identify and separate the martensite phase and the ferrite
phase. Then the area ratio of the martensite phase is measured. The
measurements results of a total of ten view areas are averaged, and
the calculated average is used as the area fraction of the
martensite phase.
[0094] Next, a preferable method for manufacturing a raw material
for a cold-rolled stainless steel sheet according to aspects of the
present invention is described. A steel having the above-described
chemical composition is melted by a known method that uses a
converter, an electric furnace, a vacuum melting furnace, or the
like, and prepared into a steel (steel slab) by a continuous
casting method or an ingoting-slabbing method. After this slab is
heated to 1000.degree. C. or more and 1200.degree. C. or less, the
heated slab is hot-rolled to a sheet thickness of 2.0 to 6.0 mm
under the condition that the finishing temperature is 700.degree.
C. or more and 1000.degree. C. or less.
[0095] Next, the hot-rolled sheet is subjected to hot-rolled sheet
annealing that involves holding the hot-rolled sheet in a
temperature range of 900.degree. C. or more and 1100.degree. C. or
less, which is the ferrite-austenite two-phase region, for 5
seconds to 15 minutes. The hot-rolled sheet annealing is an
extremely important step for obtaining the structure according to
aspects of the present invention.
[0096] At a hot-rolled sheet annealing temperature less than
900.degree. C., annealing is performed in the ferrite single phase
region or a temperature region close thereto, and as a result a
sufficient amount of austenite phase is not formed in the
hot-rolled sheet. At a hot-rolled sheet annealing temperature
exceeding 1100.degree. C. also, annealing is performed in the
ferrite single phase region or a temperature region close thereto,
and as a result a sufficient amount of austenite phase is not
formed in the hot-rolled sheet.
In addition, when the holding time during hot-rolled sheet
annealing is less than 5 seconds, a sufficient amount of austenite
phase is not formed in the hot-rolled sheet during the hot-rolled
sheet annealing. In contrast, when the holding time in the
hot-rolled sheet annealing exceeds 15 minutes, the crystal grains
coarsen during the hot-rolled sheet annealing, which results in
coarsening of crystal grains of a cold-rolled and annealed sheet
obtained by subsequent cold-rolling and annealing for manufacturing
the cold-rolled steel sheet. Such a structure causes surface
roughening known as orange peel, which is different from ridging,
during forming. Thus, in accordance with aspects of the present
invention, hot-rolled sheet annealing that involves holding a
hot-rolled sheet in a temperature range of 900.degree. C. or more
and 1100.degree. C. or less for 5 seconds to 15 minutes is
performed to obtain a hot-rolled and annealed sheet. The hot-rolled
sheet annealing is preferably performed in a temperature range of
950.degree. C. or more. The hot-rolled sheet annealing is
preferably performed in a temperature range of 1050.degree. C. or
less. The hot-rolled sheet annealing preferably involves holding
the sheet in the aforementioned temperature range for 20 seconds or
more. The hot-rolled sheet annealing preferably involves holding
the sheet in the aforementioned temperature range for 1 minute or
less.
[0097] The prepared hot-rolled and annealed sheet (raw material for
a cold-rolled stainless steel sheet) may be subsequently
pickled.
[0098] An example of the method for manufacturing a ferritic
stainless steel sheet from a hot-rolled and annealed sheet (raw
material for a cold-rolled stainless steel sheet) is a method that
involves cold-rolling the raw material for a cold-rolled stainless
steel sheet so as to prepare a cold-rolled sheet, and then
annealing the cold-rolled sheet to prepare a cold-rolled and
annealed sheet. The cold-rolled and annealed sheet can be further
pickled in a pickling line to remove the scale. The cold-rolled,
annealed, and pickled sheet from which scale is removed may be
subjected to skinpass rolling. The cold rolling conditions are not
particularly limited, and a common method may be employed. For
example, in cold-rolling, the total rolling reduction can be 40 to
90%. The cold-rolled sheet annealing is preferably a process of
holding the cold-rolled sheet in a temperature range of 780.degree.
C. or more and 830.degree. C. or less for 5 seconds to 5 minutes.
When the cold-rolled sheet annealing temperature is 780.degree. C.
or more, less unrecrystallized structure remains in the
manufactured cold-rolled ferritic stainless steel sheet, which can
improves formability further. When the cold-rolled sheet annealing
temperature is 830.degree. C. or less, since existence of the
martensite phase in the structure after annealing can be suppressed
by suppressing formation of the austenite phase in the steel during
annealing, formability can be further improved. Moreover, when the
holding time in cold-rolled sheet annealing is 5 seconds or more,
since the martensite phase contained in the cold-rolled sheet can
be sufficiently decomposed during annealing and existence of the
martensite phase in the structure after annealing can be
suppressed, formability can be further improved. When the holding
time in cold-rolled sheet annealing is 5 minutes or less,
coarsening of the crystal grains during the cold-rolled sheet
annealing can be suppressed with result that it becomes easier to
suppress surface roughening known as orange peel, which is
different from ridging, during forming of the manufactured
cold-rolled ferritic stainless steel sheet. Note that the
cold-rolled sheet annealing is preferably performed in a continuous
annealing line.
Example 1
[0099] Each of ferritic stainless steels having chemical
compositions (the balance being Fe and incidental impurities)
indicated in Nos. 1-1 to 1-3 in Table 1 was prepared into a 100 kg
steel ingot, and then hot-rolled under heating at a temperature of
1050.degree. C. so as to obtain a hot-rolled sheet having a
thickness of 4.0 mm.
[0100] Each of the hot-rolled sheets was divided into five, and
four of these were annealed in air for 20 seconds at respective
temperatures of 830 to 1200.degree. C. indicated in Table 1, and
top and bottom surfaces were ground to remove scale to prepare a
raw material for a cold-rolled stainless steel sheet. Each of the
raw materials for cold-rolled steel sheets was halved by shearing
at the longitudinal center portion, one half was used for
evaluation described below, and the other half was prepared into a
cold-rolled, annealed, and pickled sheet through the steps
described below.
[0101] The remaining one of the divided pieces of each hot-rolled
sheet was annealed in an air atmosphere at 800.degree. C. for 8
hours, and top and bottom surfaces were ground to remove scale to
prepare a raw material for a cold-rolled stainless steel sheet.
Each of the raw materials for cold-rolled steel sheets was halved
by shearing at the longitudinal center portion, one, half was used
for evaluation described below, and the other half was prepared
into a cold-rolled, annealed, and pickled sheet through the
processes described below.
[0102] Each of the obtained raw materials for cold-rolled steel
sheets was cold-rolled to prepare a cold-rolled sheet having a
thickness of 1.0 mm. The obtained cold-rolled sheets were annealed
in an air atmosphere at 800.degree. C. for 20 seconds to obtain
cold-rolled and annealed sheets. The cold-rolled and annealed
sheets were pickled by a common method so as to obtain cold-rolled,
annealed, and pickled ferritic stainless steel sheets.
[0103] The raw materials for cold-rolled stainless steel sheets and
the cold-rolled, annealed, and pickled ferritic stainless steel
sheets obtained under the aforementioned manufacturing conditions
were subjected to the following evaluations.
[0104] First, a test piece for structure observation was taken from
near the width center portion of a raw material for a cold-rolled
steel sheet, and after a section taken in the rolling direction was
mirror-polished, the test piece was corroded (etched) with a
Murakami reagent (8 mass % KOH-8 mass % [K.sub.3Fe(CN).sub.6]
aqueous solution). By using an optical microscope, setting a
portion of 1.0 mm depth from the surface layer to the center of the
view areas, ten view areas were photographed at a magnification of
400. The obtained structure images were binarized by image analysis
to identify and separate the martensite phase and the ferrite
phase, and the area ratio of the martensite phase was measured. The
measurements results of a total of ten view areas were averaged,
and the calculated average was used as the area ratio of the
martensite phase.
[0105] <Corrosion Resistance>
[0106] From each of the manufactured cold-rolled, annealed, and
pickled sheets, a 80 mm (length).times.60 mm (width) steel sheet
was cut out by shearing, the surface thereof was polished with an
emery polishing paper to #600, and, after washing with water, the
steel sheet was ultrasonically degreased for 5 minutes in ethanol
to obtain a test piece. A corrosion test according to JASO M609-91
was performed on the obtained test piece to evaluate corrosion
resistance. After end portions and the rear surface of a test piece
were covered with a vinyl tape, the test piece was placed in a
tester with a slope of 60.degree. and with the lengthwise direction
being set in the vertical direction. A three-cycle corrosion test
was carried out, each cycle consisting of salt spraying (5 mass %
aqueous NaCl solution, 35.degree. C.) 2 h.fwdarw.drying (60.degree.
C., relative humidity: 40%) 4 h.fwdarw.wetting (50.degree. C.,
relative humidity: 95% or more) 2 h. After the test, the appearance
of the corroded surface was photographed, and a 30 mm.times.30 mm
region at the center of the test piece in the photographed image
was subjected to image analysis to calculate the rust area ratio.
Samples with a rust area ratio of 20% or less were evaluated as
".largecircle." (pass, excellent), samples with a rust area ratio
exceeding 20% but not exceeding 30% were evaluated as
".quadrature." (pass), and samples with a rust area ratio exceeding
30% were evaluated as ".tangle-solidup." (fail).
[0107] <Formability>
[0108] From each of the cold-rolled, annealed, and pickled sheets
manufactured as above, a JIS No. 13B tensile test piece was taken
in accordance with JIS Z 2241 such that longitudinal directions
thereof were, respectively, the rolling direction (L direction), a
direction 45 degrees with respect to the rolling direction (D
direction), and a direction 90 degrees with respect to the rolling
direction (C direction), and a tensile test was performed at room
temperature according to the same standard to evaluate the
formability. Samples having a three-direction average ((L+2D+C)/4
where L, D, and C represent elongations after fracture (%) of
respective directions) of total elongation after fracture (%) of
32% or more were evaluated as ".largecircle." (pass, excellent),
samples with an average of less than 32% but not less than 28% were
evaluated as ".quadrature." (pass), and samples with an average of
less than 28% were evaluated as ".tangle-solidup." (fail).
[0109] <Ridging Resistance>
[0110] Furthermore, from each of the cold-rolled, annealed, and
pickled sheets manufactured as above, a JIS No. 5 test piece
specified in JIS Z 2241 was taken so that the rolling direction was
the longitudinal direction of the test piece, and, after the
surface thereof was polished with a #600 emery paper, a tensile
test was performed in accordance with the same standard to apply a
tensile strain of 23%. Subsequently, the surface profile was
measured with a laser displacement meter in a direction 90 degrees
with respect to the rolling direction on a polished surface at the
center of the parallel portion of the test piece. The measurement
length was 16 mm per line, and the height was measured with 0.05 mm
increments. The obtained profile data was smoothed and subjected to
a waviness removal process by using a Hanning window function-type
finite impulse response (FIR) bandpass filter with a high-cut
filter wavelength of 0.8 mm and a low-cut filter wavelength of 8
mm. Subsequently, on the basis of the processed profile data of
each line, the data corresponding to 2 mm portions at both ends of
each lines was eliminated, and the arithmetic mean waviness, Wa,
specified in JIS B 0601 (2001) was measured for each line. Note
that the line interval was set to 0.1 mm, and a total of fifty
lines were measured. The average of the values of the arithmetic
mean waviness, Wa, of fifty lines was used as the ridging height of
the steel sheet surface, and the ridging resistance was evaluated.
The case in which the ridging height was 2.0 .mu.m or less was
evaluated as ".diamond." (pass, particularly excellent), the case
in which the ridging height was more than 2.0 .mu.m but not more
than 2.5 .mu.m was evaluated as ".largecircle." (pass, excellent),
the case in which the ridging height was more than 2.5 .mu.m but
not more than 3.0 .mu.m was evaluated as ".quadrature." (pass), and
the case in which the ridging height was more than 3.0 .mu.m was
evaluated as ".tangle-solidup." (fail).
[0111] The obtained results are indicated in Table 1. The
cold-rolled, annealed, and pickled sheets prepared from the raw
materials for cold-rolled steel sheets having a martensite phase
area ratio within the range of the present invention, in other
words, the raw materials for cold-rolled steel sheets of the
invention examples, were evaluated as ".largecircle." or
".quadrature." for corrosion resistance, ".largecircle." for
formability, and ".diamond." or ".largecircle." for ridging
resistance, indicating excellent corrosion resistance as well as
excellent formability and ridging resistance.
[0112] The cold-rolled, annealed, and pickled sheets prepared from
the raw materials for cold-rolled steel sheets having a martensite
phase area ratio outside the range of the present invention, in
other words, the raw materials for cold-rolled steel sheets of the
comparative examples, were evaluated as ".tangle-solidup." for
ridging resistance. These cold-rolled, annealed, and pickled sheets
exhibited poor ridging resistance because the amount of the
martensite phase contained in the raw materials for cold-rolled
steel sheets were insufficient and the colonies were not
satisfactorily destroyed by cold rolling.
TABLE-US-00001 TABLE 1 Hot-rolled sheet annealing Chemical
composition (mass %) conditions Test Other Temperature No. C Si Mn
P S Al Cr Ni N elements (.degree. C.) 1-1 0.008 0.37 0.20 0.016
0.005 0.016 13.3 0.25 0.010 -- 800 850 900 1000 1150 1-2 0.015 0.15
0.23 0.019 0.003 0.042 12.4 0.82 0.005 -- 800 830 900 1050 1200 1-3
0.019 0.43 0.52 0.024 0.006 0.003 11.5 0.12 0.013 -- 800 840 900
1100 1200 Martensite Hot-rolled area fraction sheet (%) of raw
Evaluation results of cold-rolled, annealing material for annealed,
and pickled sheet Test conditions cold-rolled Corrosion Ridging No.
Time steel sheet resistance Formability resistance Remarks 1-1 8 hr
0 .largecircle. .largecircle. .tangle-solidup. Comparative Example
20 S 7 .largecircle. .largecircle. .tangle-solidup. Comparative
Example 20 S 28 .largecircle. .largecircle. .largecircle. Invention
Example 20 S 35 .largecircle. .largecircle. .largecircle. Invention
Example 20 S 0 .largecircle. .largecircle. .tangle-solidup.
Comparative Example 1-2 8 hr 0 .largecircle. .largecircle.
.tangle-solidup. Comparative Example 20 S 6 .largecircle.
.largecircle. .tangle-solidup. Comparative Example 20 S 87
.largecircle. .largecircle. .diamond. Invention Example 20 S 80
.largecircle. .largecircle. .diamond. Invention Example 20 S 8
.largecircle. .largecircle. .tangle-solidup. Comparative Example
1-3 8 hr 0 .quadrature. .largecircle. .tangle-solidup. Comparative
Example 20 S 9 .quadrature. .largecircle. .tangle-solidup.
Comparative Example 20 S 84 .quadrature. .largecircle. .diamond.
Invention Example 20 S 70 .quadrature. .largecircle. .diamond.
Invention Example 20 S 3 .quadrature. .largecircle.
.tangle-solidup. Comparative Example * The balance other than the
above-described chemical composition is Fe and incidental
impurities. *[Hot-rolled sheet annealing time] In examples
involving 800.degree. C., annealing was performed for 8 hours in a
batch annealing furnace, and in examples not involving 800.degree.
C., annealing was performed for 20 seconds in a continuous
annealing furnace. *[Corrosion resistance] After three corrosion
test cycles, samples with a rust area ratio of 20% or less were
evaluated as ".largecircle." (pass, excellent), samples with a rust
area ratio exceeding 20% but not exceeding 30% were evaluated as
".quadrature." (pass), and samples with a rust area ratio exceeding
30% were evaluated as ".tangle-solidup." (fail). *[Formability] A
tensile test was performed at room temperature, and samples having
a three-direction average of total elongation after fracture (%) of
32% or more were evaluated as ".largecircle." (pass, excellent),
samples with an average of less than 32% but not less than 28% were
evaluated as ".quadrature." (pass), and samples with an average of
less than 28% were evaluated as ".tangle-solidup." (fail).
*[Ridging resistance] After 23% tensile strain was applied, the
case in which the ridging height on the surface of the center of
the parallel portion of a test specimen was 2.0 .mu.m or less was
evaluated as ".largecircle." (pass, particularly excellent), the
case in which the ridging height was more than 2.0 .mu. m but not
more than 2.5 .mu.m was evaluated as ".largecircle." (pass,
excellent), the case in which the ridging height was more than 2.5
.mu.m but not more than 3.0 .mu.m was evaluated as ".quadrature."
(pass), and the case in which the ridging height was more than 3.0
.mu.m was evaluated as ".tangle-solidup." (fail). * Underlines
indicate items outside the scope of the present invention.
Example 2
[0113] Raw materials for cold-rolled steel sheets and cold-rolled,
annealed, and pickled sheets having the chemical compositions
indicated in Nos. 2-1 to 2-57 in Tables 2-1 and 2-2 were
manufactured under the conditions indicated in Example 1. However,
for the hot-rolled sheet annealing conditions, annealing was
performed in an air atmosphere at 1000.degree. C. for 20 seconds.
These raw materials for cold-rolled steel sheets and the
cold-rolled, annealed, and pickled sheets were subjected to the
tests indicated in Example 1, and the martensite phase area ratio
in the structure of the raw material for a cold-rolled steel sheet,
corrosion resistance, formability, and ridging resistance of the
cold-rolled, annealed, and pickled sheet were evaluated.
[0114] The obtained results are indicated in Tables 2-1 and
2-2.
TABLE-US-00002 TABLE 2-1 Chemical composition (mass %) Other Test
No. C Si Mn P S Al Cr Ni N elements 2-1 0.005 0.37 0.24 0.017 0.005
0.016 11.1 0.21 0.008 -- 2-2 0.020 0.45 0.54 0.022 0.006 0.002 11.5
0.09 0.011 -- 2-3 0.007 0.31 0.20 0.018 0.007 0.011 11.7 0.15 0.010
-- 2-4 0.012 0.36 0.17 0.016 0.007 0.007 12.5 0.16 0.006 -- 2-5
0.014 0.31 0.21 0.021 0.007 0.025 13.2 0.18 0.005 -- 2-6 0.015 0.35
0.15 0.019 0.004 0.038 14.4 0.18 0.011 -- 2-7 0.005 0.34 0.16 0.020
0.004 0.019 12.9 0.03 0.008 -- 2-8 0.007 0.26 0.15 0.020 0.007
0.001 12.7 0.07 0.008 -- 2-9 0.014 0.30 0.20 0.015 0.006 0.028 12.5
0.12 0.010 -- 2-10 0.005 0.22 0.19 0.020 0.006 0.030 12.9 0.77
0.007 -- 2-11 0.014 0.31 0.19 0.018 0.005 0.014 12.6 2.46 0.005 --
2-12 0.018 0.39 0.24 0.017 0.006 0.030 13.5 0.21 0.013 -- 2-13
0.029 0.29 0.18 0.023 0.007 0.026 12.9 0.17 0.014 -- 2-14 0.015
0.37 0.24 0.023 0.004 0.018 13.2 0.19 0.019 -- 2-15 0.006 0.22 0.22
0.024 0.004 0.024 12.5 0.17 0.057 -- 2-16 0.006 0.07 0.18 0.020
0.007 0.029 12.9 0.23 0.008 -- 2-17 0.006 0.48 0.21 0.021 0.004
0.003 13.1 0.10 0.009 -- 2-18 0.013 0.95 0.18 0.024 0.005 0.019
12.8 0.20 0.014 -- 2-19 0.009 0.30 0.08 0.015 0.005 0.031 13.4 0.20
0.007 -- 2-20 0.012 0.29 0.76 0.024 0.006 0.021 12.7 0.22 0.008 --
2-21 0.013 0.35 0.96 0.023 0.006 0.014 12.6 0.16 0.007 -- 2-22
0.012 0.32 0.23 0.019 0.006 0.024 13.3 0.24 0.013 Cu: 0.42 2-23
0.005 0.29 0.16 0.016 0.005 0.025 13.4 0.20 0.008 Mo: 0.06 2-24
0.007 0.38 0.22 0.024 0.008 0.005 13.5 0.24 0.006 Ti: 0.04 2-25
0.012 0.33 0.17 0.024 0.004 0.007 12.9 0.22 0.013 Nb: 0.05 2-26
0.008 0.28 0.21 0.021 0.006 0.015 12.9 0.15 0.010 Sn: 0.005 2-27
0.009 0.29 0.23 0.022 0.006 0.018 12.6 0.24 0.009 Co: 0.03, W: 0.07
2-28 0.009 0.25 0.17 0.015 0.005 0.016 12.6 0.21 0.006 V: 0.04, Mg:
0.0021 2-29 0.005 0.22 0.21 0.023 0.008 0.010 12.9 0.11 0.005 Sn:
0.008, Sb: 0.010 2-30 0.008 0.33 0.25 0.023 0.006 0.024 12.9 0.18
0.006 Cu: 0.22, Y: 0.04, La: 0.05 2-31 0.011 0.24 0.16 0.015 0.006
0.037 12.9 0.11 0.015 Ca: 0.0014, Ce: 0.02, Sn: 0.121 2-32 0.007
0.31 0.17 0.025 0.005 0.003 12.5 0.24 0.015 Mo: 0.15, Zr: 0.05, Sb:
0.248 2-33 0.010 0.31 0.23 0.019 0.005 0.026 12.9 0.17 0.014 W:
0.18, B: 0.0011, Sn: 0.195 2-34 0.007 0.29 0.18 0.022 0.005 0.037
12.8 0.21 0.009 Ti: 0.03, Nb: 0.04 Martensite area fraction (%) of
raw Evaluation results of cold-rolled, material for annealed, and
pickled sheet Formula cold-rolled Corrosion Ridging Test No. (1)
steel sheet resistance Formability resistance Remarks 2-1 -- 85
.quadrature. .largecircle. .diamond. Invention Example 2-2 -- 88
.quadrature. .largecircle. .diamond. Invention Example 2-3 -- 66
.quadrature. .largecircle. .diamond. Invention Example 2-4 -- 40
.largecircle. .largecircle. .diamond. Invention Example 2-5 -- 27
.largecircle. .largecircle. .largecircle. Invention Example 2-6 --
15 .largecircle. .quadrature. .quadrature. Invention Example 2-7 --
25 .largecircle. .largecircle. .diamond. Invention Example 2-8 --
36 .largecircle. .largecircle. .diamond. Invention Example 2-9 --
56 .largecircle. .largecircle. .diamond. Invention Example 2-10 --
68 .largecircle. .largecircle. .diamond. Invention Example 2-11 --
88 .largecircle. .largecircle. .diamond. Invention Example 2-12 --
33 .largecircle. .largecircle. .largecircle. Invention Example 2-13
-- 56 .largecircle. .largecircle. .diamond. Invention Example 2-14
-- 39 .largecircle. .largecircle. .largecircle. Invention Example
2-15 -- 84 .largecircle. .largecircle. .diamond. Invention Example
2-16 -- 30 .largecircle. .largecircle. .diamond. Invention Example
2-17 -- 21 .largecircle. .largecircle. .largecircle. Invention
Example 2-18 -- 31 .largecircle. .largecircle. .diamond. Invention
Example 2-19 -- 16 .largecircle. .largecircle. .largecircle.
Invention Example 2-20 -- 57 .largecircle. .largecircle. .diamond.
Invention Example 2-21 -- 64 .largecircle. .largecircle. .diamond.
Invention Example 2-22 -- 35 .largecircle. .largecircle.
.largecircle. Invention Example 2-23 -- 26 .largecircle.
.largecircle. .largecircle. Invention Example 2-24 -2.5 28
.largecircle. .largecircle. .largecircle. Invention Example 2-25
-1.3 39 .largecircle. .largecircle. .diamond. Invention Example
2-26 -- 32 .largecircle. .largecircle. .diamond. Invention Example
2-27 -- 35 .largecircle. .largecircle. .diamond. Invention Example
2-28 -1.7 32 .largecircle. .largecircle. .diamond. Invention
Example 2-29 -- 27 .largecircle. .largecircle. .diamond. Invention
Example 2-30 -- 30 .largecircle. .largecircle. .diamond. Invention
Example 2-31 -- 36 .largecircle. .largecircle. .diamond. Invention
Example 2-32 -1.7 31 .largecircle. .largecircle. .diamond.
Invention Example 2-33 -- 34 .largecircle. .largecircle. .diamond.
Invention Example 2-34 -0.1 30 .largecircle. .largecircle.
.diamond. Invention Example * The balance other than the
above-described chemical composition is Fe and incidental
impurities. *[Corrosion resistance] After three corrosion test
cycles, samples with a rust area ratio of 20% or less were
evaluated as ".largecircle." (pass, excellent), samples with a rust
area ratio exceeding 20% but not exceeding 30% were evaluated as
".quadrature." (pass), and samples with a rust area ratio exceeding
30% were evaluated as ".tangle-solidup." (fail). *[Formability] A
tensile test was performed at room temperature, and samples having
a three-direction average of total elongation after fracture (%) of
32% or more were evaluated as ".largecircle." (pass, excellent),
samples with an average of less than 32% but not less than 28% were
evaluated as ".quadrature." (pass), and samples with an average of
less than 28% were evaluated as ".tangle-solidup." (fail).
*[Ridging resistance] After 23% tensile strain was applied, the
case in which the ridging height on the surface of the center of
the parallel portion of a test specimen was 2.0 .mu.m or less was
evaluated as ".diamond." (pass, particularly excellent), the case
in which the ridging height was more than 2.0 .mu. m but not more
than 2.5 .mu.m was evaluated as ".largecircle." (pass, excellent),
the case in which the ridging height was more than 2.5 .mu.m but
not more than 3.0 .mu.m was evaluated as ".quadrature." (pass), and
the case in which the ridging height was more than 3.0 .mu.m was
evaluated as ".tangle-solidup." (fail). *[Formula (1)] 54 .times.
(Ti + V + Zr + Nb) - 5 .times. Mn - 19 .times. Ni + 1.0 where the
respective element symbols represent the contents (mass %) of the
respective elements and represent 0 when the corresponding elements
are not contained. * Underlines indicate items outside the scope of
the present invention.
TABLE-US-00003 TABLE 2-2 Chemical composition (mass %) Other Test
No. C Si Mn P S Al Cr Ni N elements 2-35 0.012 0.33 0.22 0.017
0.005 0.026 10.6 0.18 0.008 -- 2-36 0.005 0.22 0.18 0.016 0.004
0.025 15.5 0.23 0.005 -- 2-37 0.005 0.28 0.23 0.022 0.006 0.031
12.7 -- 0.010 -- 2-38 0.006 0.28 0.17 0.016 0.006 0.007 12.8 2.63
0.013 -- 2-39 0.002 0.32 0.20 0.017 0.005 0.009 13.5 0.15 0.013 --
2-40 0.033 0.36 0.16 0.019 0.004 0.010 13.5 0.24 0.006 -- 2-41
0.007 0.24 0.15 0.022 0.007 0.017 13.4 0.19 0.003 -- 2-42 0.012
0.31 0.16 0.016 0.004 0.015 12.8 0.23 0.065 -- 2-43 0.011 1.14 0.20
0.019 0.004 0.024 12.8 0.15 0.009 -- 2-44 0.013 0.27 0.18 0.022
0.006 0.012 16.5 0.17 0.010 -- 2-45 0.006 0.46 0.06 0.021 0.005
0.024 10.8 0.82 0.007 -- 2-46 0.008 0.23 0.43 0.025 0.002 0.023
11.0 0.78 0.008 Ti: 0.27 2-47 0.006 0.35 0.99 0.022 0.003 0.024
11.7 0.39 0.007 Nb: 0.21 2-48 0.012 0.25 0.38 0.018 0.004 0.017
11.4 0.42 0.021 V: 0.06, Zr: 0.08 2-49 0.012 0.39 0.79 0.022 0.006
0.026 12.7 0.48 0.009 Mo: 0.23, Ti: 0.22, Y: 0.08, Sb: 0.031 2-50
0.007 0.42 0.18 0.020 0.008 0.038 13.3 0.13 0.008 Cu: 0.04, Mo:
0.02, V: 0.03 2-51 0.005 0.25 0.45 0.018 0.005 0.037 11.2 0.81
0.008 Cu: 0.02, Mo: 0.03, V: 0.04, Ti: 0.25 2-52 0.011 0.28 0.49
0.024 0.004 0.022 13.3 0.93 0.012 Ti: 0.34 2-53 0.007 0.24 0.13
0.023 0.006 0.046 12.4 0.27 0.006 V: 0.09 2-54 0.006 0.08 0.19
0.020 0.007 0.028 11.3 0.17 0.007 Nb: 0.03, Zr: 0.03 2-55 0.005
0.44 0.34 0.021 0.001 0.057 10.7 0.07 0.010 Ti: 0.18, V: 0.05 2-56
0.007 0.11 0.14 0.023 0.002 0.043 11.3 0.08 0.010 Ti: 0.16, V: 0.05
2-57 0.009 0.38 0.56 0.015 0.005 0.005 13.3 0.88 0.011 Nb: 0.33
Martensite area fraction (%) of raw Evaluation results of
cold-rolled, material for annealed, and pickled sheet Formula
cold-rolled Corrosion Ridging Test No. (1) steel sheet resistance
Formability resistance Remarks 2-35 -- 100 .tangle-solidup.
.largecircle. .diamond. Comparative Example 2-36 -- 0 .largecircle.
.quadrature. .tangle-solidup. Comparative Example 2-37 -- 35
.tangle-solidup. .largecircle. .diamond. Comparative Example 2-38
-- 96 .largecircle. .tangle-solidup. .diamond. Comparative Example
2-39 -- 9 .largecircle. .largecircle. .tangle-solidup. Comparative
Example 2-40 -- 39 .largecircle. .tangle-solidup. .diamond.
Comparative Example 2-41 -- 8 .largecircle. .largecircle.
.tangle-solidup. Comparative Example 2-42 -- 83 .largecircle.
.tangle-solidup. .diamond. Comparative Example 2-43 -- 6
.largecircle. .tangle-solidup. .tangle-solidup. Comparative Example
2-44 -- 9 .largecircle. .quadrature. .tangle-solidup. Comparative
Example 2-45 -- 85 .largecircle. .largecircle. .diamond. Invention
Example 2-46 -1.4 64 .largecircle. .largecircle. .diamond.
Invention Example 2-47 0.0 12 .largecircle. .largecircle.
.largecircle. Invention Example 2-48 -1.3 47 .largecircle.
.largecircle. .diamond. Invention Example 2-49 -0.2 27
.largecircle. .largecircle. .largecircle. Invention Example 2-50
-0.8 35 .largecircle. .largecircle. .diamond. Invention Example
2-51 -1.0 43 .quadrature. .largecircle. .diamond. Invention Example
2-52 -0.8 2 .largecircle. .largecircle. .tangle-solidup.
Comparative Example 2-53 0.1 8 .largecircle. .largecircle.
.tangle-solidup. Comparative Example 2-54 0.1 7 .largecircle.
.largecircle. .tangle-solidup. Comparative Example 2-55 10.4 9
.tangle-solidup. .largecircle. .tangle-solidup. Comparative Example
2-56 10.1 5 .largecircle. .largecircle. .tangle-solidup.
Comparative Example 2-57 -0.7 7 .largecircle. .largecircle.
.tangle-solidup. Comparative Example * The balance other than the
above-described chemical composition is Fe and incidental
impurities. *[Corrosion resistance] After three corrosion test
cycles, samples with a rust area ratio of 20% or less were
evaluated as ".largecircle." (pass, excellent), samples with a rust
area ratio exceeding 20% but not exceeding 30% were evaluated as
".quadrature." (pass), and samples with a rust area ratio exceeding
30% were evaluated as ".tangle-solidup." (fail). *[Formability] A
tensile test was performed at room temperature, and samples having
a three-direction average of total elongation after fracture (%) of
32% or more were evaluated as ".largecircle." (pass, excellent),
samples with an average of less than 32% but not less than 28% were
evaluated as ".quadrature." (pass), and samples with an average of
less than 28% were evaluated as ".tangle-solidup." (fail).
*[Ridging resistance] After 23% tensile strain was applied, the
case in which the ridging height on the surface of the center of
the parallel portion of a test specimen was 2.0 .mu.m or less was
evaluated as ".diamond." (pass, particularly excellent), the case
in which the ridging height was more than 2.0 .mu. m but not more
than 2.5 .mu.m was evaluated as ".largecircle." (pass, excellent),
the case in which the ridging height was more than 2.5 .mu.m but
not more than 3.0 .mu.m was evaluated as ".quadrature." (pass), and
the case in which the ridging height was more than 3.0 .mu.m was
evaluated as ".tangle-solidup." (fail). *[Formula (1)] 54 .times.
(Ti + V + Zr + Nb) - 5 .times. Mn - 19 .times. Ni + 1.0 where the
respective element symbols represent the contents (mass %) of the
respective elements, and represent 0 when the corresponding
elements are not contained. * Underlines indicate items outside the
scope of the present invention.
[0115] The cold-rolled, annealed, and pickled sheets prepared from
the raw materials for cold-rolled steel sheets of the invention
examples were evaluated as ".largecircle." or ".quadrature." for
corrosion resistance, ".largecircle." or ".quadrature." for
formability, and ".diamond.", ".largecircle.", or ".quadrature."
for ridging resistance, indicating excellent corrosion resistance
as well as excellent formability and ridging resistance.
[0116] Test No. 2-35 had poor corrosion resistance since it was
prepared from a raw material for a cold-rolled steel sheet of a
comparative example in which the Cr content was lower than the
component range of the present invention.
[0117] Test No. 2-36 had poor ridging resistance since it was
prepared from a raw material for a cold-rolled steel sheet of a
comparative example in which the Cr content was higher than the
component range of the present invention.
[0118] Test No. 2-37 had poor corrosion resistance since it was
prepared from a raw material for a cold-rolled steel sheet of a
comparative example in which the Ni content was lower than the
component range of the present invention.
[0119] Test No. 2-38 had poor formability since it was prepared
from a raw material for a cold-rolled steel sheet of a comparative
example in which the Ni content was higher than the component range
of the present invention.
[0120] Test Nos. 2-39 and 2-41 had poor ridging resistance since
they were prepared from raw materials for cold-rolled steel sheets
of comparative examples in which the C content and the N content,
respectively, were lower than the component ranges of the present
invention.
[0121] Test Nos. 2-40 and 2-42 had poor formability since they were
prepared from raw materials for cold-rolled steel sheets of
comparative examples in which the C content and the N content,
respectively, were higher than the component ranges of the present
invention.
[0122] Test No. 2-43 had poor formability and ridging resistance
since it was prepared from a raw material for a cold-rolled steel
sheet of a comparative example in which the Si content was higher
than the component range of the present invention.
[0123] Test No. 2-44 had poor ridging resistance since it was
prepared from a raw material for a cold-rolled steel sheet of a
comparative example in which the Cr content was higher than the
component range of the present invention.
[0124] Test No. 2-52 had poor ridging resistance since it was
prepared from a raw material for a cold-rolled steel sheet of a
comparative example in which the Ti content was higher than the
component range of the present invention.
[0125] Test Nos. 2-53, 2-54, and 2-56 had poor ridging resistance
since they were prepared from raw materials for cold-rolled steel
sheets of comparative examples in which the value of formula (1)
exceeded 0.0.
[0126] Test No. 2-55 had poor corrosion resistance and ridging
resistance since it was prepared from a raw material for a
cold-rolled steel sheet of a comparative example in which the Cr
content was lower than the component range of the present invention
and the value of formula (1) exceeded 0.0.
[0127] Test No. 2-57 had poor ridging resistance since it was
prepared from a raw material for a cold-rolled steel sheet of a
comparative example in which the Nb content was higher than the
component range of the present invention.
INDUSTRIAL APPLICABILITY
[0128] A raw material for a cold-rolled stainless steel sheet
according to aspects of the present invention is preferable for
manufacturing a cold-rolled ferritic stainless steel sheet having
excellent corrosion resistance, formability, and ridging
resistance. Since a cold-rolled ferritic stainless steel sheet
manufactured from a raw material for a cold-rolled stainless steel
sheet of the present invention has excellent corrosion resistance,
formability, and ridging resistance, it can be used in home cooking
tolls, parts of home electric appliances, parts of office and
stationery supplies, parts of automobile interiors, pipes for
automobile exhaust, building materials, and the like.
* * * * *