U.S. patent application number 14/383434 was filed with the patent office on 2015-01-22 for heat-resistant cold rolled ferritic stainless steel sheet, hot rolled ferritic stainless steel sheet for cold rolling raw material, and methods for producing same.
This patent application is currently assigned to Nippon Steel & Sumikin Stainless Steel Corporation. The applicant listed for this patent is Nippon Steel & Sumikin Stainless Steel Corporation. Invention is credited to Fumio Fudanoki, Junichi Hamada, Yoshiharu Inoue, Tadashi Komori, Yuji Koyama, Naoto Ono, Toshio Tanoue.
Application Number | 20150020933 14/383434 |
Document ID | / |
Family ID | 49260066 |
Filed Date | 2015-01-22 |
United States Patent
Application |
20150020933 |
Kind Code |
A1 |
Hamada; Junichi ; et
al. |
January 22, 2015 |
HEAT-RESISTANT COLD ROLLED FERRITIC STAINLESS STEEL SHEET, HOT
ROLLED FERRITIC STAINLESS STEEL SHEET FOR COLD ROLLING RAW
MATERIAL, AND METHODS FOR PRODUCING SAME
Abstract
Provided is a heat-resistant cold rolled ferritic stainless
steel sheet containing, in terms of mass %, 0.02% or less of C,
0.1% to 1.0% of Si, greater than 0.6% to 1.5% of Mn, 0.01% to 0.05%
of P, 0.0001% to 0.0100% of S, 13.0% to 20.0% of Cr, 0.1% to 3.0%
of Mo, 0.005% to 0.20% of Ti, 0.3% to 1.0% of Nb, 0.0002% to
0.0050% of B, 0.005% to 0.50% of Al, and 0.02% or less of N, with
the balance being Fe and inevitable impurities, in which
{111}-oriented grains are present at an area ratio of 20% or
greater in a region from a surface layer to t/4 (t is a sheet
thickness), {111}-oriented grains are present at an area ratio of
40% or greater in a region from t/4 to t/2, and {011}-oriented
grains are present at an area ratio of 15% or less in the entire
region in a thickness direction.
Inventors: |
Hamada; Junichi;
(Hikari-shi, JP) ; Koyama; Yuji; (Kita-kyushu-shi,
JP) ; Inoue; Yoshiharu; (Kita-kyushu-shi, JP)
; Komori; Tadashi; (Hikari-shi, JP) ; Fudanoki;
Fumio; (Nagoya-shi, JP) ; Tanoue; Toshio;
(Yokohama-shi, JP) ; Ono; Naoto; (Nagoya-shi,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Nippon Steel & Sumikin Stainless Steel Corporation |
Tokyo |
|
JP |
|
|
Assignee: |
Nippon Steel & Sumikin
Stainless Steel Corporation
Tokyo
JP
|
Family ID: |
49260066 |
Appl. No.: |
14/383434 |
Filed: |
March 26, 2013 |
PCT Filed: |
March 26, 2013 |
PCT NO: |
PCT/JP2013/058856 |
371 Date: |
September 5, 2014 |
Current U.S.
Class: |
148/603 ;
148/325; 148/602; 148/610 |
Current CPC
Class: |
C21D 8/0236 20130101;
C22C 38/22 20130101; C22C 38/32 20130101; C22C 38/44 20130101; C22C
38/42 20130101; C22C 38/001 20130101; C22C 38/06 20130101; C21D
9/46 20130101; C22C 38/04 20130101; C22C 38/26 20130101; C22C 38/48
20130101; C21D 7/13 20130101; C22C 38/20 20130101; C22C 38/52
20130101; C22C 38/008 20130101; C21D 6/002 20130101; C21D 2201/05
20130101; C22C 38/002 20130101; C22C 38/54 20130101; C22C 38/28
20130101; C21D 2211/005 20130101; C22C 38/40 20130101; C22C 38/02
20130101; C22C 38/50 20130101; C21D 8/0263 20130101; C21D 7/02
20130101; C22C 38/004 20130101 |
Class at
Publication: |
148/603 ;
148/602; 148/610; 148/325 |
International
Class: |
C21D 8/02 20060101
C21D008/02; C22C 38/54 20060101 C22C038/54; C22C 38/52 20060101
C22C038/52; C22C 38/50 20060101 C22C038/50; C22C 38/48 20060101
C22C038/48; C22C 38/44 20060101 C22C038/44; C22C 38/42 20060101
C22C038/42; C22C 38/32 20060101 C22C038/32; C22C 38/28 20060101
C22C038/28; C22C 38/26 20060101 C22C038/26; C22C 38/22 20060101
C22C038/22; C22C 38/20 20060101 C22C038/20; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00; C21D 9/46 20060101
C21D009/46 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 30, 2012 |
JP |
2012-081998 |
Claims
1. A heat-resistant cold rolled ferritic stainless steel sheet
comprising, in terms of mass %, 0.02% or less of C, 0.1% to 1.0% of
Si, greater than 0.6% to 1.5% of Mn, 0.01% to 0.05% of P, 0.0001%
to 0.0100% of S, 13.0% to 20.0% of Cr, 0.1% to 3.0% of Mo, 0.005%
to 0.20% of Ti, 0.30% to 1.0% of Nb, 0.0002% to 0.0050% of B,
0.005% to 0.50% of Al, and 0.02% or less of N, with the balance
being Fe and inevitable impurities, wherein when a sheet thickness
is represented by t, {111}-oriented grains are present at an area
ratio of 20% or greater in a region from a surface layer to t/4,
{111}-oriented grains are present at an area ratio of 40% or
greater in a region from t/4 to t/2, and {011}-oriented grains are
present at an area ratio of 15% or less in the entire region in a
thickness direction.
2. The heat-resistant cold rolled ferritic stainless steel sheet
according to claim 1, further comprising, in terms of mass %, one
or more selected from 0.4% to 2.0% of Cu, 0.1% to 2.0% of Ni, 0.1%
to 3.0% of W, 0.05% to 0.30% of Zr, 0.05% to 0.50% of Sn, 0.05% to
0.50% of Co, and 0.0002% to 0.0100% of Mg.
3. A hot rolled ferritic stainless steel sheet for a material for
cold rolling for manufacturing the heat-resistant cold rolled
ferritic stainless steel sheet according to claim 1, wherein when a
sheet thickness is represented by t', a microstructure in a region
from t'/2 to t'/4 is a non-recrystallized microstructure.
4. A method for producing the hot rolled ferritic steel sheet for a
material for cold rolling according to claim 3, the method
comprising: performing hot rolling under conditions where a heating
temperature of a slab is set to be in a range of 1200.degree. C. to
1300.degree. C. and a finishing temperature is set to be in a range
of 800.degree. C. to 950.degree. C. so as to form a hot rolled
sheet; coiling the hot rolled sheet at a coiling temperature of
500.degree. C. or lower; and then, annealing the hot rolled sheet
at a temperature of 925.degree. C. to 1000.degree. C.
5. A method for producing the heat-resistant cold rolled ferritic
stainless steel sheet according to claim 1, the method comprising:
subjecting a hot rolled ferritic steel sheet for a material for
cold rolling in which when a sheet thickness is represented by t',
a microstructure in a region from t'/2 to t'/4 is a
non-recrystallized microstructure to cold rolling at a rolling
reduction ratio of 60% or greater so as to form a cold rolled
sheet; and annealing the cold rolled sheet at a temperature of
1000.degree. C. to 1100.degree. C.
6. The method for producing the heat-resistant cold rolled ferritic
stainless steel sheet according to claim 5, the method further
comprising: performing hot rolling under conditions where a heating
temperature of a slab is set to be in a range of 1200.degree. C. to
1300.degree. C. and a finishing temperature is set to be in a range
of 800.degree. C. to 950.degree. C. so as to form a hot rolled
sheet; coiling the hot rolled sheet at a coiling temperature of
500.degree. C. or lower; and annealing the hot rolled sheet at a
temperature of 925.degree. C. to 1000.degree. C. so as to produce
the hot rolled ferritic steel sheet for a material for cold
rolling.
7. A hot rolled ferritic stainless steel sheet for a material for
cold rolling for manufacturing the heat-resistant cold rolled
ferritic stainless steel sheet according to claim 2, wherein when a
sheet thickness is represented by t', a microstructure in a region
from t'/2 to t'/4 is a non-recrystallized microstructure.
8. A method for producing the hot rolled ferritic steel sheet for a
material for cold rolling according to claim 7, the method
comprising: performing hot rolling under conditions where a heating
temperature of a slab is set to be in a range of 1200.degree. C. to
1300.degree. C. and a finishing temperature is set to be in a range
of 800.degree. C. to 950.degree. C. so as to form a hot rolled
sheet; coiling the hot rolled sheet at a coiling temperature of
500.degree. C. or lower; and then, annealing the hot rolled sheet
at a temperature of 925.degree. C. to 1000.degree. C.
9. A method for producing the heat-resistant cold rolled ferritic
stainless steel sheet according to claim 2, the method comprising:
subjecting a hot rolled ferritic steel sheet for a material for
cold rolling in which when a sheet thickness is represented by t',
a microstructure in a region from t'/2 to t'/4 is a
non-recrystallized microstructure to cold rolling at a rolling
reduction ratio of 60% or greater so as to form a cold rolled
sheet; and annealing the cold rolled sheet at a temperature of
1000.degree. C. to 1100.degree. C.
10. The method for producing the heat-resistant cold rolled
ferritic stainless steel sheet according to claim 9, the method
further comprising: performing hot rolling under conditions where a
heating temperature of a slab is set to be in a range of
1200.degree. C. to 1300.degree. C. and a finishing temperature is
set to be in a range of 800.degree. C. to 950.degree. C. so as to
form a hot rolled sheet; coiling the hot rolled sheet at a coiling
temperature of 500.degree. C. or lower; and annealing the hot
rolled sheet at a temperature of 925.degree. C. to 1000.degree. C.
so as to produce the hot rolled ferritic steel sheet for a material
for cold rolling.
Description
TECHNICAL FIELD
[0001] The present invention relates to a heat-resistant cold
rolled ferritic stainless steel sheet which is appropriate for use
especially in exhaust system members of vehicles and the like
requiring a high temperature strength and oxidation resistance and
has excellent workability, a hot rolled ferritic stainless steel
sheet for a material for cold rolling (cold rolling raw material),
and methods for producing the same.
[0002] The present application claims priority on Japanese Patent
Application No. 2012-081998 filed on Mar. 30, 2012, the content of
which is incorporated herein by reference.
BACKGROUND ART
[0003] Heat-resistant steel containing Cr is used in exhaust system
members such as exhaust manifolds and mufflers of vehicles, since
the members require a high temperature strength and oxidation
resistance. Since these exhaust system members may be manufactured
through press working from a steel sheet or through various
formings after pipe working of a steel sheet, cold rolled steel
sheets as a raw material are required to have formability.
[0004] With an increase in the temperature of exhaust gas, the
operating temperature of the members is also increased every year,
and it is necessary to increase the high temperature strength and
the like by increasing the added amounts of alloys such as Cr, Mo,
and Nb. However, in the case where the amounts of added elements
are increased, the workability of a raw material steel sheet is
decreased in a simple manufacturing method, and thus press forming
may not be performed on members having a complicated shape.
[0005] In order to improve the Lankford value (r-value) which is an
index of workability of ferritic stainless steel sheets, it is
effective to increase a cold rolling reduction ratio. However,
since a relatively thick cold rolled steel sheet (approximately 1.5
mm to 2.5 mm) is used as a raw material for the above-described
exhaust system members, there is a problem in that a sufficient
cold rolling reduction ratio cannot be secured in the current
manufacturing process in which the raw material thickness is
regulated to a certain degree when performing cold rolling.
[0006] In order to solve this problem, a component and a
manufacturing method have been devised for improving an r-value
which is an index of press formability without deteriorating high
temperature characteristics,
[0007] Patent Document 1 discloses component adjustment to improve
workability of conventional heat-resistant ferritic stainless steel
sheets, but with this, a problem occurs such as press cracking in
thick materials having a relatively low cold rolling reduction
ratio.
[0008] In Patent Document 2, in order to improve the r-value, the
most appropriate annealing temperature of a hot rolled sheet is
specified based on a relationship of an annealing temperature of a
hot rolled sheet to a hot finish rolling start temperature, a hot
finish rolling end temperature, and an Nb content. However, with
this, sufficient workability may not be obtained according to
influences of other elements (C, N, Cr, Mo, and the like) involved
especially in Nb-based precipitates.
[0009] Patent Document 3 discloses a method of subjecting a hot
rolled sheet to aging for 1 hour or longer, but in this case, there
is a disadvantage in that the industrial manufacturing efficiency
is greatly reduced.
[0010] Patent Document 4 discloses a technology for obtaining a
Cr-containing heat-resistant steel sheet having a high r-value in
which conditions for hot rolling and annealing of a hot rolled
sheet are specified to control the crystal orientation of a center
layer in a sheet thickness direction. However, since the r-value is
not determined only with the crystal orientation of the center
layer in a sheet thickness direction of the product, sufficient
workability may not be obtained. In addition, since a heating
temperature of a slab in hot rolling is in a range of 1,000.degree.
C. to 1,150.degree. C. which is low, there is a problem such as
surface scratches.
[0011] Patent Document 5 discloses a technology of specifying the
crystal orientation in a region from an outermost layer to a depth
of one quarter of a sheet thickness in a ferritic stainless steel
sheet for an exhaust component having excellent workability. This
technique increases the r-value and total elongation in a direction
at an angle of 45.degree. with respect to a rolling direction, and
the technique is characterized in that annealing of a hot rolled
sheet is omitted in the manufacturing method. However, in the case
where only the r-value in the direction at an angle of 45.degree.
is increased, press formability is not satisfied, and in the case
where annealing of a hot rolled sheet is omitted, surface defects
which are called ridging cause a problem in press working and there
is a problem in manufacturability such as surface scratches.
PRIOR ART DOCUMENTS
Patent Documents
[0012] Patent Document 1: Japanese Unexamined Patent Application,
First Publication No. H9-279312
[0013] Patent Document 2: Japanese Unexamined Patent Application,
First Publication No. 2002-30346
[0014] Patent Document 3: Japanese Unexamined Patent Application,
First Publication No. H8-199235
[0015] Patent Document 4: PCT International Publication No.
WO2004/53171
[0016] Patent Document 5: Japanese Unexamined Patent Application,
First Publication No. 2006-233278
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0017] An object of the invention is to solve problems of known
technologies and to provide a heat-resistant cold rolled ferritic
stainless steel sheet which has excellent workability, a hot rolled
ferritic stainless steel sheet for a material for cold rolling, and
methods for producing the same.
Means for Solving the Problems
[0018] In order to solve the problems, the inventors of the
invention have conducted an intensive study on steel composition
and on structures and precipitates in manufacturing processes,
i.e., a hot rolling process and a cold rolling process with regard
to an improvement in workability of a heat-resistant cold rolled
ferritic stainless steel sheet, especially in r-value.
[0019] The features of the invention for solving the problems are
as follows.
[0020] According to a first aspect of the invention, there is
provided a heat-resistant cold rolled ferritic stainless steel
sheet containing, in terms of mass %, 0.02% or less of C, 0.1% to
1.0% of Si, greater than 0.6% to 1.5% of Mn, 0.01% to 0.05% of P,
0.0001% to 0.0100% of S, 13.0% to 20.0% of Cr, 0.1% to 3.0% of Mo,
0.005% to 0.20% of Ti, 0.30% to 1.0% of Nb, 0.0002% to 0.0050% of
B, 0.005% to 0.50% of Al, and 0.02% or less of N, with the balance
being Fe and inevitable impurities, wherein when a sheet thickness
is represented by t, {111}-oriented grains are present at an area
ratio of 20% or greater in a region from a surface layer to t/4,
{111}-oriented grains are present at an area ratio of 40% or
greater in a region from t/4 to t/2, and {011}-oriented grains are
present at an area ratio of 15% or less in the entire region in a
thickness direction.
[0021] The cold rolled stainless steel sheet has excellent
workability. In the first aspect, the region from the surface layer
to t/4 is a region ranging from the surface of the steel sheet to a
depth of t/4, and the region from t/4 to t/2 is a region ranging
from a depth of t/4 to the center in the sheet thickness
direction.
[0022] According to a second aspect of the invention, there is
provided the heat-resistant cold rolled ferritic stainless steel
sheet according to the first aspect, which further contains, in
terms of mass %, one or more selected from 0.4% to 2.0% of Cu, 0.1%
to 2.0% of Ni, 0.1% to 3.0% of W, 0.05% to 0.30% of Zr, 0.05% to
0.50% of Sn, 0.05% to 0.50% of Co, and 0.0002% to 0.0100% of
Mg.
[0023] According to a third aspect of the invention, there is
provided a hot rolled ferritic stainless steel sheet for a material
for cold rolling for manufacturing the heat-resistant cold rolled
ferritic stainless steel sheet according to the first or second
aspect, wherein when a sheet thickness is represented by t', a
microstructure in a region from t'/2 to t'/4 is a
non-recrystallized microstructure.
[0024] The region from t'/2 to t'/4 is a region ranging from a
depth of t'/4 to the center in the sheet thickness direction. The
hot rolled ferritic stainless steel sheet according to the third
aspect has substantially the same composition as the heat-resistant
cold rolled ferritic stainless steel sheet according to the first
or second aspect.
[0025] According to a fourth aspect of the invention, there is
provided a method for producing the hot rolled ferritic steel sheet
for a material for cold rolling according to the third aspect,
including: performing hot rolling under conditions where a heating
temperature of a slab (semi-finished product) is set to be in a
range of 1200.degree. C. to 1300.degree. C. and a finishing
temperature is set to be in a range of 800.degree. C. to
950.degree. C. so as to form a hot rolled sheet; coiling the hot
rolled sheet at a coiling temperature of 500.degree. C. or lower;
and then annealing the hot rolled sheet at a temperature of
925.degree. C. to 1000.degree. C.
[0026] In the fourth aspect, as the slab which is a raw material of
the steel sheet, a slab having substantially the same composition
as the steel sheet according to the first or second aspect is
used.
[0027] According to a fifth aspect of the invention, there is
provided a method for producing the heat-resistant cold rolled
ferritic stainless steel sheet according to the first or second
aspect, including: subjecting a hot rolled ferritic steel sheet for
a material for cold rolling in which when a sheet thickness is
represented by t', a microstructure in a region from t'/2 to t'/4
is a non-recrystallized microstructure to cold rolling at a rolling
reduction ratio of 60% or greater so as to form a cold rolled
sheet; and then annealing the cold rolled sheet at a temperature of
1000.degree. C. to 1100.degree. C.
[0028] In the fifth aspect, as the hot rolled steel sheet which is
a raw material of the cold rolled steel sheet, a steel sheet having
substantially the same composition as the cold rolled steel sheet
according to the first or second aspect is used.
[0029] The method for producing the heat-resistant cold rolled
ferritic stainless steel sheet may include a process of
manufacturing the hot rolled ferritic steel sheet for a material
for cold rolling. That is, according to a sixth aspect of the
invention, there is provided the method for producing the
heat-resistant cold rolled ferritic stainless steel sheet according
to the fifth aspect, including: performing hot rolling under
conditions where a heating temperature of a slab is set to be in a
range of 1200.degree. C. to 1300.degree. C. and a finishing
temperature is set to be in a range of 800.degree. C. to
950.degree. C. so as to form a hot rolled sheet; coiling the hot
rolled sheet at a coiling temperature of 500.degree. C. or lower;
and then annealing the hot rolled sheet at a temperature of
925.degree. C. to 1000.degree. C. so as to produce the hot rolled
ferritic steel sheet for a material for cold rolling. In that case,
as the slab which is a raw material of the steel sheet, a slab
having substantially the same composition as the steel sheet
according to the first or second aspect is used.
Effects of the Invention
[0030] As described above, according to the invention, in a
heat-resistant cold rolled ferritic stainless steel sheet, it is
possible to secure a high r-value by specifying a component
composition of steel together with optimizing conditions of a hot
rolling process and a cold rolling process, and controlling
microstructures of respective regions in a sheet thickness
direction.
[0031] Particularly, when a coiling temperature and an annealing
temperature of a hot rolled sheet are strictly specified in the hot
rolling process, and the steel microstructure before the cold
rolling process is set to be a non-recrystallized microstructure in
which recrystallization is suppressed together with a {111} texture
allowed to remain, a lot of crystal grains having a {111} direction
which effectively act for an improvement in the r-value can be
generated in the subsequent cold rolling and annealing processes;
and thereby, a recrystallized microstructure which is advantageous
for workability can be obtained.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] FIG. 1 is a graph showing a relationship between an area
ratio of {111}-oriented grains in a region from a surface layer to
t/4 (t: sheet thickness) and an average r-value in a cold rolled
ferritic stainless steel sheet of an embodiment.
[0033] FIG. 2 is a graph showing a relationship between an area
ratio of {111}-oriented grains in a region from t/4 to t/2 (t:
sheet thickness) and an average r-value in the cold rolled ferritic
stainless steel sheet of the embodiment.
[0034] FIG. 3 is a graph showing a relationship between an area
ratio of {011}-oriented grains in the entire region in a sheet
thickness direction and an average r-value in the cold rolled
ferritic stainless steel sheet of the embodiment.
[0035] FIG. 4 is a graph showing a relationship between an
annealing temperature T1of a hot rolled sheet and an average
r-value of a cold rolled ferritic stainless steel sheet (product
sheet) of the embodiment.
EMBODIMENTS OF THE INVENTION
Cold Rolled Ferritic Stainless Steel Sheet
[0036] Hereinafter, a cold rolled ferritic stainless steel sheet of
an embodiment will be described in detail.
[0037] A cold rolled ferritic stainless steel sheet of this
embodiment contains, in terms of mass %, 0.02% or less of C, 0.1%
to 1.0% of Si, greater than 0.6% to 1.5% of Mn, 0.01% to 0.05% of
P, 0.0001% to 0.0100% of S, 13.0% to 20.0% of Cr, 0.1% to 3.0% of
Mo, 0.005% to 0.20% of Ti, 0.30% to 1.0% of Nb, 0.0002% to 0.0050%
of B, 0.005% to 0.50% of Al, and 0.02% or less of N, with the
balance being Fe and inevitable impurities. When a sheet thickness
is represented by t, crystal grains having a {111} orientation are
present at an area ratio of 20% or greater in a region from a
surface layer to t/4, crystal grains having a {111} orientation are
present at an area ratio of 40% or greater in a region from t/4 to
t/2, and crystal grains having a {011} orientation are present at
an area ratio of 15% or less in the entire region in a thickness
direction.
[0038] Here, the region from the surface layer to t/4 is a region
ranging from the surface of the steel sheet to a depth of t/4, and
the region from t/4 to t/2 is a region ranging from a depth of t/4
to the center in the sheet thickness direction.
[0039] The crystal grains having a {111} orientation
({111}-oriented grains) are crystal grains in which the sheet
surface (the surface of the steel sheet) and the {111} plane are in
parallel. The crystal grains having a {011} orientation
({011}-oriented grains) are crystal grains in which the sheet
surface and the {011} plane are in parallel. The area ratio can be
indicated as an area ratio of {111}-oriented grains and as an area
ratio of {011}-oriented grains in a plane which is perpendicular to
the sheet surface and in parallel to a rolling direction. The area
ratio can be obtained by, for example, measuring a crystal
orientation distribution in a cross-section of the steel sheet
through an electron backscatter diffraction image method.
[0040] Hereinafter, the reasons for restricting the steel
composition of the cold rolled ferritic stainless steel sheet of
the invention will be described. Unless specifically noted, the
symbol % means mass % in association with the composition.
[0041] Carbon (C): 0.02% or less in terms of mass %
[0042] C deteriorates workability, corrosion resistance, and
oxidation resistance.
[0043] Therefore, the smaller the content thereof is, the better it
is. Accordingly, the upper limit is set to 0.02%. However, since an
excessive reduction leads to an increase in refining costs, the
lower limit is preferably set to 0.001%. The content of C is
desirably in a range of 0.002% to 0.01% in consideration of
manufacturing costs and corrosion resistance.
[0044] Silicon (Si): 0.1% to 1.0% in terms of mass %
[0045] Si may be added as a deoxidation element and is an element
which improves oxidation resistance and high temperature strength
of steel. In addition, Si is an element which promotes
precipitation of Laves phases. Accordingly, the addition thereof in
an amount of 0.1% or greater causes precipitation of coarse Laves
phases during annealing of a hot rolled sheet, and contributes to
the development of {111}-oriented grains and the suppression of
{011}-oriented grains during annealing of a cold rolled sheet and
an improvement in the r-value. Since an excessive addition reduces
room temperature ductility and thus deteriorates workability, the
upper limit is set to 1.0%. The content of Si is desirably in a
range of 0.2% to 0.5% in consideration of material quality and
oxidation characteristics.
[0046] Manganese (Mn): greater than 0.6% to 1.5% in terms of mass
%
[0047] Mn forms MnCr.sub.2O.sub.4 and MnO at a high temperature,
and Mn improves scale adhesion. Since these effects are exhibited
in the case where the amount of Mn is greater than 0.6%, the lower
limit is set to be in a range of greater than 0.6%. Meanwhile,
since a mass gain due to oxidation is increased, breakaway
(abnormal oxidation) easily occurs in the case where Mn is added in
an amount of greater than 1.5%. In exhaust gas components such as
exhaust manifolds, in the case where scale peeling and breakaway
occur, problems may occur in subsequent components such as a
catalyst, a muffler, or reliability of a structure may be reduced
due to a reduction in the sheet thickness. Furthermore, the content
of Mn is desirably in a range of 0.7% to 1.1% in consideration of
workability and manufacturability.
[0048] Phosphorus (P): 0.01% to 0.05% in terms of mass %
[0049] P is a solid solution strengthening element as is the case
with Si, but P is an element which is harmful to corrosion
resistance and toughness of steel. Accordingly, the smaller the
content thereof is, the better it is in view of material quality.
Thus, the upper limit is set to 0.05%. However, since an excessive
reduction leads to an increase in refining costs, the lower limit
is set to 0.01%. The content of P is desirably in a range of 0.015%
to 0.025% in consideration of manufacturing costs and oxidation
resistance.
[0050] Sulfur (S): 0.0001% to 0.0100% in terms of mass %
[0051] The smaller the amount of S is, the better it is from the
viewpoint of material quality, corrosion resistance, and oxidation
resistance. Accordingly, the upper limit is set to 0.0100%.
Particularly, an excessive addition of S leads to generation of a
compound with Ti, and thus recrystallization and grain growth of
the hot rolled and annealed sheet are promoted. Thus,
non-recrystallized microstructure cannot be secured in the hot
rolled steel sheet; and as a result, the r-value is deteriorated.
However, since an excessive reduction leads to an increase in
refining costs, the lower limit is set to 0.0001%. The content of S
is desirably in a range of 0.0010% to 0.0050% in consideration of
manufacturing costs and corrosion resistance.
[0052] Chromium (Cr): 13.0% to 20.0% in terms of mass %
[0053] Cr is required to be added in an amount of 13% or greater in
order to improve a high temperature strength and oxidation
resistance. However, in the case where Cr is added in an amount of
20% or greater, the manufacturability of the steel sheet
deteriorates due to a deterioration in toughness, and the material
quality also deteriorates. Accordingly, the amount of Cr is set to
be in the range of 13.0% to 20.0%. The content of Cr is desirably
in a range of 15.0% to 19.0% from the viewpoint of costs and
corrosion resistance.
[0054] Molybdenum (Mo): 0.1% to 3.0% in terms of mass %
[0055] Mo improves corrosion resistance, and also leads to an
improvement in high temperature strength and thermal fatigue
characteristics of steel by a solid-solubilized Mo. Since these
effects are exhibited in the case where the amount of Mo is in a
range of 0.1% or greater, the lower limit is set to 0.1%. However,
an excessive addition leads to a deterioration in toughness and a
reduction in elongation. In addition, too many Laves phases are
generated in the annealing process of a hot rolled sheet and in the
annealing process of a cold rolled sheet, and thus {011}-oriented
grains are easily generated and the r-value is reduced. Moreover,
since oxidation resistance deteriorates in the case where Mo is
added in an amount greater than 3.0%, the upper limit is set to
3.0%. The content of Mo is desirably in a range of 1.5% to 1.8% in
consideration of high temperature characteristics after exposure to
a high temperature for a long period of time, especially, a high
temperature strength, thermal fatigue characteristics, and
high-temperature and high-cycle fatigue characteristics and in
consideration of manufacturing costs and manufacturability.
[0056] Titanium (Ti): 0.005% to 0.20% in terms of mass %
[0057] Ti is an element which is added to further improve corrosion
resistance, intergranular corrosion resistance, and deep
drawability by bonding to C, N, and S. Particularly, since
development of {111} crystal orientation which improves the r-value
is exhibited in the case where Ti is added in an amount of 0.005%
or greater, the lower limit is set to 0.005%. Since toughness and
secondary workability deteriorate in the case where Ti is added in
an amount of 0.20% or greater, the upper limit is set to 0.2%. The
content of Ti is desirably in a range of 0.06% to 0.15% in
consideration of manufacturing costs, surface scratches, and scale
spallability.
[0058] Niobium (Nb): 0.30% to 1.0% in terms of mass %
[0059] Nb improves a high temperature strength and high temperature
fatigue characteristics due to solid solution strengthening and
precipitation strengthening, and thus Nb is an essential element.
In addition, Nb fixes C and N as carbonitrides to develop a
recrystallization texture of the cold rolled steel sheet (product
sheet), Nb forms an intermetallic compound of Fe and Nb, which is
called as Laves phase, Nb has an influence on the formation of the
recrystallization texture according to a volume fraction and a size
thereof, and thus Nb contributes to an improvement in the
r-value.
[0060] Since these actions are exhibited in the case where the
addition amount of Nb is in a range of 0.30% or greater, the lower
limit is set to 0.30%. Since an excessive addition of Nb leads to
hardening, and thus this results in a reduction in room temperature
ductility, the upper limit is set to 1.0%. The content of Nb is
desirably in a range of 0.40% to 0.60% in consideration of costs
and manufacturability.
[0061] Nitrogen (N): 0.02% or less in terms of mass %
[0062] N deteriorates workability and oxidation resistance of steel
as is the case with C. Thus, the smaller the content thereof is,
the better it is. Therefore, the upper limit is set to 0.02%.
However, since an excessive reduction leads to an increase in
refining costs, the content of N is desirably in a range of 0.005%
to 0.015% in consideration of costs.
[0063] Boron (B): 0.0002% to 0.0050% in terms of mass %
[0064] B is an element which improves secondary workability during
press working of the product and improves a high temperature
strength in an intermediate temperature range. Since these effects
are exhibited in the case where the addition amount of B is in a
range of 0.0002% or greater, the lower limit is set to 0.0002%. In
the case where B is added in an amount greater than 0.0050%, a B
compound such as Cr.sub.2B and the like is generated, intergranular
corrosion and fatigue characteristics are deteriorated,
{011}-oriented grains are increased, and thus the r-value is
reduced. Therefore, the upper limit is set to 0.0050%. Furthermore,
the content of B is desirably in a range of 0.0003% to 0.0020% in
consideration of weldability and manufacturability.
[0065] Aluminum (Al): 0.005% to 0.50% in terms of mass %
[0066] Al may be added as a deoxidation element and Al improves a
high temperature strength and oxidation resistance of steel. Since
the actions thereof are exhibited in the case where the amount of
Al is in a range of 0.005% or greater, the lower limit is set to
0.005%. In the case where Al is added in an amount greater than
0.50%, elongation of stainless steel is reduced, weldability and
surface quality are deteriorated, generation of {011}-oriented
grains is promoted by an Al oxide, and the r-value of the steel
sheet is reduced, and thus the upper limit is set to 0.50%. The
content of Al is desirably in a range of 0.01% to 0.15% in
consideration of refining costs.
[0067] In addition, in this embodiment, the steel sheet preferably
further contains, in terms of mass %, one or more of 0.4% to 2.0%
of Cu, 0.1% to 2.0% of Ni, 0.1% to 3.0% of W, 0.05% to 0.30% of Zr,
0.05% to 0.50% of Sn, 0.05% to 0.50% of Co, and 0.0002% to 0.0100%
of Mg in addition to the above-described elements.
[0068] Copper (Cu): 0.4% to 2.0% in terms of mass %
[0069] Cu is an element which improves corrosion resistance of
stainless steel and increases a high temperature strength
especially in the intermediate temperature range by c-Cu
precipitation, and thus Cu is added to the steel material if
necessary. Since these effects are exhibited in the case where Cu
is added in an amount of 0.4% or greater, the lower limit is set to
0.4%. The addition thereof in an amount greater than 2.0% leads to
a deterioration in toughness of the steel material and an excessive
reduction in elongation. In addition, .epsilon.-Cu is excessively
precipitated during the course of hot rolling, {011}-oriented
grains are generated, and the r-value is reduced. Therefore, the
upper limit of the addition amount of Cu is set to 2.0%. The
content of Cu is desirably in a range of 0.5% to 1.5% in
consideration of oxidation resistance and manufacturability.
[0070] Nickel (Ni): 0.1% to 2.0% in terms of mass %
[0071] Ni is an element which improves toughness and corrosion
resistance, and thus Ni is added if necessary. Since the
contribution to the toughness is exhibited in the case where the
amount of Ni is 0.1% or greater, the lower limit is set to 0.1%. In
the case where Ni is added in an amount greater than 2.0%, an
austenite phase is generated and the r-value is reduced. Thus, the
upper limit is set to 2.0%. The content of Ni is desirably in a
range of 0.1% to 0.5% in consideration of costs.
[0072] Tungsten (W): 0.1% to 3.0% in terms of mass %
[0073] W is an element which is added if necessary in order to
increase a high temperature strength, and the actions thereof are
exhibited in the case where the amount of W is 0.1% or greater.
Therefore, the lower limit of the addition amount of W is set to
0.1%. However, an excessive addition leads to a deterioration in
toughness of the steel material and a reduction in elongation. In
addition, too many Laves phases are generated, and thus
{011}-oriented grains are easily generated and the r-value is
reduced. Accordingly, the upper limit is set to 3.0%. The content
of W is desirably in a range of 0.1% to 2.0% in consideration of
manufacturing costs and manufacturability.
[0074] Zirconium (Zr): 0.05% to 0.30% in terms of mass %
[0075] Zr is an element which improves oxidation resistance and is
added if necessary. Since the actions thereof are exhibited in the
case where the content of Zr is in a range of 0.05% or greater, the
lower limit is set to 0.05%. However, the addition in an amount
greater than 0.30% causes a significant deterioration in
manufacturability such as toughness and a pickling property, and
also causes coarsening of a compound of Zr, carbon, and nitrogen,
and thus the microstructure of the hot rolled and annealed sheet is
grain-coarsened and the r-value is reduced. Therefore, the upper
limit is set to 0.30%. The content of Zr is desirably in a range of
0.05% to 0.20% in consideration of manufacturing costs.
[0076] Tin (Sn): 0.05% to 0.50% in terms of mass %
[0077] Sn is an element which is added if necessary in order to
increase a high temperature strength by segregation in a grain
boundary. Since the actions thereof are exhibited in the case where
the content of Sn is in a range of 0.05% or greater, the lower
limit is set to 0.05%. However, the addition in an amount greater
than 0.5% causes generation of Sn segregation, and thus
{011}-oriented grains are generated in the segregation portion and
the r-value is reduced. Therefore, the upper limit is set to 0.50%.
The content of Sn is desirably in a range of 0.10% to 0.30% in
consideration of high temperature characteristics, manufacturing
costs, and toughness.
[0078] Cobalt (Co): 0.05% to 0.50% in terms of mass %
[0079] Co is an element which improves a high temperature strength
and is added in an amount of 0.05% or greater if necessary.
However, since an excessive addition deteriorates workability, the
upper limit is set to 0.50%. The content of Co is desirably in a
range of 0.05% to 0.30% in consideration of manufacturing
costs.
[0080] Magnesium (Mg): 0.0002% to 0.0100% in terms of mass %
[0081] Mg forms an Mg oxide together with Al in molten steel and Mg
acts as a deoxidizer. Moreover, the fine crystallized Mg oxide
serves as a nucleus and Nb-based precipitates or Ti-based
precipitates are finely precipitated. When these are finely
precipitated in the hot rolling process, the fine precipitates
suppress recrystallization and formation of {011}-oriented grains
in the hot rolling process and in the annealing process of a hot
rolled sheet, and the fine precipitates contributes to formation of
the non-recrystallized microstructure. Since this action is
exhibited in the case where the amount of Mg is in a rage of
0.0002% or greater, the lower limit is set to 0.0002%. However,
since an excessive addition of Mg leads to a deterioration in
oxidation resistance of the steel material, a reduction in
weldability, and the like, the upper limit is set to 0.0100%. The
content of Mg is desirably in a range of 0.0003% to 0.0020% in
consideration of refining costs.
[0082] Next, the texture of a cold rolled ferritic stainless steel
sheet of this embodiment will be described.
[0083] Regarding the texture of a cold rolled ferritic stainless
steel sheet of this embodiment, it is important that when a sheet
thickness is represented by t, an area ratio of crystal grains
having a {111} orientation (hereinafter, simply referred to as
{111}-oriented grains) in a region from the surface layer to t/4 (a
region ranging from the surface to a depth of t/4) is in a range of
20% or greater, and an area ratio of {111}-oriented grains in a
region from t/4 to t/2 (a region ranging from a depth of t/4 to the
center in the sheet thickness direction) is in a range of 40% or
greater. Furthermore, it is important that an area ratio of crystal
grains having a {011} orientation (hereinafter, simply referred to
as {011}-oriented grains) in the entire region in the sheet
thickness direction is in a range of 15% or less.
[0084] The crystal grains having a {111} orientation are crystal
grains in which the crystal orientation is indicated by plane index
{111}, that is, crystal grains in which the sheet surface (the
surface of the steel sheet) and the {111} plane are in parallel.
The crystal grains having a {011} orientation are crystal grains in
which the crystal orientation is indicated by plane index {011},
that is, crystal grains in which the sheet surface and the {011}
plane are in parallel.
[0085] The area ratios of the {111}-oriented grains and the
{011}-oriented grains can be obtained as an area ratio of the
crystal grains having the respective orientations in a plane which
is perpendicular to the surface of the steel sheet and in parallel
to the rolling direction.
[0086] Hereinafter, the reasons for restricting the texture of this
embodiment will be described.
[0087] It is a well-known fact that the Lankford value (r-value)
which is an index of workability improvement is related to the
recrystallization texture. In general, it is known that the r-value
is improved by increasing the ratio of crystal grains having a
{111} orientation. However, the crystal orientation distribution
was not uniform in the sheet thickness direction, and a
sufficiently high r-value was not necessarily secured only with the
control of the crystal orientation of a specific portion.
[0088] Accordingly, in the invention, the relationship between the
crystal orientation distribution in the sheet thickness direction
of the cold rolled steel sheet (product sheet) and the r-value was
examined in consideration of non-uniformity in the sheet thickness
direction. As a result, it was proved that area ratios of
{111}-oriented grains present in a region from the surface layer to
t/4 (t is a sheet thickness) and in a region from t/4 to t/2 are
required to be in a range of 20% or greater and in a range of 40%
or greater, respectively. In addition, it was proved that an area
ratio of {011}-oriented grains present in the entire region in the
thickness direction is required to be in a range of 15% or less. In
order to more stably secure the r-value, the area ratio of the
{111}-oriented grains is preferably in a range of 25% or greater in
the region from the surface layer to t/4 and the area ratio of the
{111}-oriented grains is preferably in a range of 45% or greater in
the region from t/4 to t/2, and the area ratio of the
{011}-oriented grains are preferably in a range of 10% or less.
[0089] FIGS. 1 to 3 show the relationship between the area ratio
(ratio) in the respective crystal orientations and an average
r-value of a product sheet.
[0090] Here, regarding the r-value, a JIS13-B tensile test piece is
collected from a cold rolled and annealed sheet and a strain of
14.4% is applied in a rolling direction, in a direction at an angle
of 45.degree. with respect to the rolling direction, and in a
direction at an angle of 90.degree. with respect to the rolling
direction. Then, an average r-value is calculated using the
following Expressions (1) and (2).
r=ln(W.sub.0/W)/ln(t.sub.0/t) (1)
[0091] Here, W.sub.0 represents a sheet width before pulling, W
represents a sheet width after pulling, t.sub.0 represents a sheet
thickness before pulling, and t represents a sheet thickness after
pulling.
Average r-value=(r.sub.0+2r.sub.45+r.sub.9)/4 (2)
[0092] Here, r.sub.0 represents an r-value in the rolling
direction, r.sub.45 represents an r-value in a direction at an
angle of 45.degree. with respect to the rolling direction, and
r.sub.90 represents an r-value in a direction at a right angle with
respect to the rolling direction.
[0093] An exhaust component required to have a complicated shape
can be subjected to sufficient working in the case where the
average r-value is in a range of 1.2 or greater. Therefore, in this
embodiment, a component having an average r-value of 1.2 or greater
is judged to have excellent workability.
[0094] Regarding the measurement of the crystal orientation, a
plane in a direction parallel to the rolling direction is cut out
of the product sheet at a right angle to the sheet surface, and
orientations of crystal grains are identified over the entire
region in the sheet thickness direction using a crystal orientation
analysis apparatus EBSP (Electron Back Scatter diffraction Pattern)
to determine area ratios of {111}-oriented grains and
{011}-oriented grains. From these results, in the invention, it is
obvious that for increasing the r-value by crystal orientation
control, it is necessary to consider a fluctuation in the
{111}-oriented grain frequency in the sheet thickness direction and
also to consider the {011}-oriented grains.
[0095] FIG. 1 is a graph showing a relationship between an area
ratio of {111}-oriented grains in a region from the surface layer
to t/4 and an average r-value in a cold rolled ferritic stainless
steel sheet of this embodiment, and FIG. 2 is a graph showing a
relationship between an area ratio of {111}-oriented grains in a
region from t/4 to t/2 and an average r-value.
[0096] As can be seen from FIGS. 1 and 2, the higher the ratio of
the {111}-oriented grains is, the higher the average r-value is,
and thus it is found that the workability is improved. Furthermore,
it is found that in order to secure the average r-value of 1.2 or
greater, it is important to secure 20% or greater of {111}-oriented
grains in the region from the surface layer of the steel sheet to a
depth of t/4 and also to secure 40% of {111}-oriented grains in the
region from t/4 to t/2.
[0097] The steel component of the cold rolled ferritic stainless
steel sheet used to examine the relationships shown in FIGS. 1 and
2 includes 0.007% of C, 0.27% of Si, 0.94% of Mn, 0.03% of P,
0.0006% of S, 17.3% of Cr, 1.8% of Mo, 0.08% of Ti, 0.47% of Nb,
0.01% of N, 0.001% of B, and 0.03% of Al (the balance being Fe and
inevitable impurities).
[0098] FIG. 3 is a graph showing a relationship between an area
ratio of {011}-oriented grains in the entire region in the sheet
thickness direction and an average r-value in the cold rolled
ferritic stainless steel sheet of this embodiment.
[0099] As can be seen from FIG. 3, the higher the ratio of the
{011}-oriented grains in the entire region in the sheet thickness
direction is, the lower the average r-value is, and it is found
that the workability deteriorates. Furthermore, it is found that in
order to secure the average r-value of 1.2 or greater, it is
important to secure 15% or less of {011}-oriented grains in the
entire region in the thickness direction.
[0100] The steel component of the cold rolled ferritic stainless
steel sheet used to examine the relationship shown in FIG. 3
includes 0.007% of C, 0.27% of Si, 0.94% of Mn, 0.03% of P, 0.0006%
of S, 17.3% of Cr, 1.8% of Mo, 0.08% of Ti, 0.47% of Nb, 0.01% of
N, 0.001% of B, and 0.03% of Al (the balance being Fe and
inevitable impurities).
[0101] Next, a hot rolled ferritic stainless steel sheet for a
material for cold rolling, which is a raw material of the cold
rolled ferritic stainless steel sheet as described above, will be
described.
[0102] In the invention, the manufacturing method was examined as
well as the texture and the component composition of the cold
rolled steel sheet (cold rolled sheet); and as a result, it was
found that the texture of the cold rolled sheet is influenced by
the microstructure of the hot rolled steel sheet (hot rolled sheet
for a material for cold rolling) which is a raw material of the
cold rolled steel sheet, and thus the r-value of the cold rolled
sheet is influenced.
[0103] That is, it was found that when the microstructure in a
region from t'/4 to t'/2 (t' is a sheet thickness of the hot rolled
sheet for a material for cold rolling) in the hot rolled sheet for
a material for cold rolling is a non-recrystallized microstructure,
the cold rolled steel sheet manufactured from such a hot rolled
sheet for a material for cold rolling has a high r-value. The
region from t'/4 to t'/2 is a region ranging from a depth of t/4
from the surface of the steel sheet to the center in the sheet
thickness direction.
[0104] Specifically, as described above, for an improvement in the
r-value in the cold rolled sheet, it is effective to secure crystal
grains having a {111} orientation. Therefore, it is very important
to develop a {111} texture in the hot rolled steel sheet which is a
raw material of the cold rolled sheet and also very important for
the texture to be a non-recrystallized microstructure without being
recrystallized. That is, in the non-recrystallized microstructure,
in a cross-section which is in parallel to the rolling direction of
the hot rolled steel sheet and is perpendicular to the sheet
surface, crystal grains exhibits an orientation indicated by plane
index {111} (an orientation in which the sheet surface and the
{111} plane are in parallel).
[0105] Hereinafter, a method of manufacturing the hot rolled
ferritic stainless steel sheet for a material for cold rolling will
be described.
[0106] (Method of Manufacturing Hot Rolled Ferritic Stainless Steel
Sheet for Material for Cold Rolling)
[0107] Next, a method of manufacturing a hot rolled ferritic
stainless steel sheet for a material for cold rolling of this
embodiment will be described.
[0108] The method of manufacturing a hot rolled ferritic stainless
steel sheet for a material for cold rolling of this embodiment
includes: manufacturing ferritic stainless steel having the
above-described steel composition; after the manufacturing of the
steel, subjecting a semi-finished product (slab) which is cast to
hot rolling under conditions where a heating temperature of a slab
is set to be in a range of 1200.degree. C. to 1300.degree. C. and a
finishing temperature is set to be in a range of 800.degree. C. to
950.degree. C. so as to form a hot rolled sheet; coiling the hot
rolled sheet at a coiling temperature of 500.degree. C. or lower;
and then, annealing the hot rolled sheet at a temperature of
925.degree. C. to 1000.degree. C.
[0109] In the hot rolling, a hot rolling strain caused by rolling
is excessively introduced in the case where a heating temperature
of a slab is in a range of lower than 1200.degree. C., and thus it
becomes difficult to perform the subsequent control of
microstructure and surface scratches become a problem. Accordingly,
the lower limit is set to 1200.degree. C. In the case where the
heating temperature is in a range of higher than 1300.degree. C.,
the microstructure after the hot rolling is grain-coarsened. Thus,
the development of the {111} texture is suppressed and the
structure may be a recrystallized microstructure. Accordingly, the
upper limit is set to 1300.degree. C. The heating temperature is
desirably in a range of 1230.degree. C. to 1280.degree. C. in
consideration of productivity.
[0110] In the hot rolling, after the slab heating, a plurality of
passes of rough rolling are performed and then, a plurality of
passes of finish rolling are performed, and thereafter, the steel
sheet is coiled in a coil shape. In this case, in the case where
the finishing temperature is in a range of lower than 800.degree.
C., surface scratches become a problem, and thus the lower limit of
the finishing temperature is set to 800.degree. C. In the case
where the finishing temperature is in a range of higher than
950.degree. C., the microstructure after the hot rolling is
grain-coarsened. Thus, the development of the {111} texture is
suppressed and the microstructure may be a recrystallized
microstructure. Accordingly, the upper limit is set to 950.degree.
C. The finishing temperature is desirably in a range of 850.degree.
C. to 930.degree. C. in consideration of productivity.
[0111] The coiling temperature is set to be in a range of
500.degree. C. or lower from the viewpoint of the suppression of
recovery of the hot rolled microstructure and the toughness of the
hot rolled sheet. That is, in the invention, in the case where the
coiling temperature is set to be a low temperature of 500.degree.
C. or lower, the {111} texture obtained through the hot rolling
process is not recovered, and while the texture is maintained, the
subsequent processes can be proceeded. The coiling temperature is
desirably in a range of 400.degree. C. to 480.degree. C. in
consideration of productivity, toughness, and coil shape. In the
case where the coiling temperature is in a range of higher than
500.degree. C., even when the annealing temperature is appropriate
in the subsequent annealing process of a hot rolled sheet,
{110}-oriented grains caused by a hot rolling shear strain
generated in the vicinity of the surface layer portion of the sheet
thickness are grown during the course of cooling to the room
temperature after coiling of a hot rolled sheet, {110}-oriented
grains encroach another orientation in the subsequent annealing
process; and thereby, {110}-oriented grains remain in the product
sheet. Since the {110}-oriented grains lead to a reduction in the
r-value, the coiling temperature is set to be in a range of
500.degree. C. or lower. In order to suppress the growth of the
{110}-oriented grains during the processes until the coiling after
the hot finish rolling, cooling is desirably performed at a cooling
rate of 50.degree. C./sec or greater.
[0112] In general, in the annealing of a hot rolled sheet after the
hot rolling, a heat treatment is performed at a temperature at
which a recrystallized microstructure is obtained. However,
non-uniformity occurs in the microstructure in the sheet thickness
direction.
[0113] In the invention, it was found that the non-uniformity in
the microstructure in the sheet thickness direction has a large
influence on the r-value of the product sheet, and also found that
as described above, in the case where the microstructure in a
region from t'/4 to t'/2 (t' is a sheet thickness) is a
non-recrystallized microstructure, the cold rolled steel sheet,
that is, the product sheet obtains a high r-value.
[0114] FIG. 4 shows a relationship between an annealing temperature
of a hot rolled sheet and an average r-value of a product sheet.
Here, steel A (represented by the reference symbols and
.smallcircle. in FIG. 4) has a composition including 0.007% of C,
0.25% of Si, 0.95% of Mn, 0.03% of P, 0.0006% of 5, 17.3% of Cr,
1.8% of Mo, 0.08% of Ti, 0.47% of Nb, 0.01% of N, 0.0010% of B, and
0.03% of Al (the balance being Fe and inevitable impurities). Steel
B (represented by the reference symbols .tangle-solidup. and
.DELTA. in FIG. 4) has a composition including 0.003% of C, 0.89%
of Si, 0.65% of Mn, 0.02% of P, 0.0010% of S, 13.5% of Cr, 0.1% of
Mo, 0.008% of Ti, 0.40% of Nb, 0.01% of N, 0.0005% of B, and 0.07%
of Al (the balance being Fe and inevitable impurities). FIG. 4 also
shows a microstructure state of the region from t'/4 to t'/2 after
annealing of a hot rolled sheet. The reference symbols and
.tangle-solidup. represent a non-recrystallized microstructure, and
the reference symbols .largecircle. and .DELTA. represent a
recrystallized microstructure.
[0115] The recrystallization temperature varies with the steel
component. However, in the composition of the invention, an
appropriate annealing temperature of a hot rolled sheet can be
found in the range of 925.degree. C. to 1000.degree. C. That is, a
temperature can be found at which a non-recrystallized
microstructure (which does not change into a full-recrystallized
microstructure), that is an appropriate microstructure for the hot
rolled sheet for a material for cold rolling, is obtained at a
depth of t'/4 to t'/2 (t' is a sheet thickness of the hot rolled
sheet for a material for cold rolling). When the hot rolled sheet
for a material for cold rolling is used as a raw material of a cold
rolled steel sheet, a material with excellent formability having an
average r-value of 1.2 or greater can be obtained.
[0116] Here, in a normal manufacturing method, in the case where a
region from t'/4 to t'/2 in a hot rolled sheet for a material for
cold rolling has a recrystallized microstructure, a random crystal
orientation distribution is obtained, and thus the texture
development in the subsequent cold rolling is not sufficient and
{111}-oriented grains are not sufficiently generated after
annealing of a cold rolled sheet. On the other hand, in the case
where the region from t'/4 to t'/2 in the hot rolled sheet for a
material for cold rolling has a non-recrystallized microstructure
as in the invention, cold rolling is performed while the {111}
texture developed in the hot rolled sheet remain. Therefore, many
{111}-oriented grains are generated in the subsequent annealing of
a cold rolled sheet, and {111}-oriented grains contribute to a high
r-value.
[0117] However, in the case where the annealing temperature of a
hot rolled sheet is too low or the annealing of a hot rolled sheet
is omitted, many {110}-oriented grains, which are caused by the hot
rolling shear strain generated in the vicinity of the surface layer
portion of the sheet thickness, remain in the product sheet after
annealing of a cold rolled sheet. Since these oriented grains lead
to a reduction in the r-value, the annealing of a hot rolled sheet
is required to be performed at a temperature of 800.degree. C. or
higher. In the invention, in order to further suppress the growth
of the {110}-oriented grains having an adverse influence on the
improvement in the r-value and to adjust the average r-value to be
in a range of 1.2 or greater, the lower limit of the annealing
temperature of a hot rolled sheet is set to 925.degree. C.
[0118] In the case where the annealing of a hot rolled sheet is
performed at a temperature of higher than 1000.degree. C., the
microstructure of the region from t'/4 to t'/2 is recrystallized,
and thus the recrystallized grains in the surface layer are
coarsened and a compound of Fe and Nb (Fe.sub.2Nb), which is called
as Laves phase, is completely dissolved after the annealing of a
hot rolled sheet. Thus, the r-value is reduced. Since the Laves
phase coarsely generated through the annealing of a hot rolled
sheet becomes a nucleus generation site of the recrystallization
texture during the annealing of a cold rolled sheet, it is
desirably that Laves phase be precipitated in the material for cold
rolling.
[0119] The upper limit of the annealing temperature of a hot rolled
sheet is set to 1000.degree. C. in consideration of these points.
Furthermore, since coarsening of crystal grains and promotion of
scale generation due to high temperature annealing lead to a
reduction in surface quality such as fracture of the sheet and
scale residues, the annealing temperature of a hot rolled sheet is
desirably in a range of 925.degree. C. to 980.degree. C. in
consideration of toughness of the hot rolled sheet and a pickling
property.
[0120] (Method of Manufacturing Cold Rolled Ferritic Stainless
Steel Sheet)
[0121] Next, the hot rolled sheet for a material for cold rolling
is subjected to cold rolling to have a thickness of 2 mm, and is
subjected to a heat treatment at a temperature of 1000.degree. C.
to 1100.degree. C. according to the steel component so that the
grain size number becomes in a range of 5 to 7. Thereby, a product
sheet is formed.
[0122] Specifically, first, the cold rolling reduction ratio is set
to be in a range of 60% or greater in order to obtain
recrystallization nuclei which grow into the {111}-oriented
crystals in the cold rolled sheet. That is, in the case where the
cold rolling reduction ratio is too low, recrystallization nuclei
for recrystallization into {111}-oriented grains through the
following annealing process cannot be sufficiently generated, and
thus the r-value of the product sheet is not sufficiently improved.
Therefore, it is important that the rolling reduction ratio is set
to be in a range of 60% or greater. The rolling reduction ratio is
desirably in a range of 60% to 80% in consideration of productivity
and anisotropy.
[0123] Next, with regard to the cold rolled sheet in which the
recrystallization nuclei that grow into the {111}-oriented crystals
are generated, annealing of the cold rolled sheet is performed at a
temperature of 1000.degree. C. to 1100.degree. C. In general, in
the annealing of the cold rolled sheet, the heat treatment
temperature is determined according to the steel component in order
to obtain a recrystallized microstructure. However, in the case
where the temperature is in a range of lower than 1000.degree. C.,
a non-recrystallized microstructure is obtained in the steel
component of the invention, and thus the lower limit is set to
1000.degree. C. In the case where the temperature is in a range of
higher than 1100.degree. C., the crystal grains are coarsened, and
surface deterioration occurs during working and causes cracking.
Therefore, the upper limit is set to 1100.degree. C. The heat
treatment temperature is desirably in a range of 1010.degree. C. to
1070.degree. C. in consideration of elongation and a pickling
property.
[0124] As described above, it is possible to obtain a cold rolled
ferritic stainless steel sheet having excellent workability in
which the area ratio of the {111}-oriented grains is increased and
the {011}-oriented grains are suppressed.
[0125] The thickness of the slab, the thickness of the hot rolled
sheet, and the like may be appropriately designed. In addition, in
the cold rolling, the roll roughness and the roll diameter of a
used work roll, a rolling oil, the number of passes of rolling, the
rolling rate, the rolling temperature, and the like may be
appropriately selected. In addition, if necessary, for the
annealing of a cold rolled sheet, bright annealing may be performed
so that the annealing is performed under a non-oxidation atmosphere
such as hydrogen gas or nitrogen gas, or the annealing may be
performed in the air.
EXAMPLES
[0126] Hereinafter, effects of the invention will be described
using examples, but the invention is not limited to the conditions
used in the following examples.
Example 1
[0127] In this example, first, steel having a component composition
shown in Table 1 was melted to cast a slab, and the slab was
subjected to hot rolling to form a hot rolled sheet having a
thickness of 5.0 mm. Thereafter, the hot rolled sheet was subjected
to continuous annealing, and then subjected to pickling. The
resulting material was subjected to cold rolling to have a
thickness of 2.0 mm and was subjected to continuous annealing and
pickling to form a product sheet. Among the component compositions
shown in Table 1, steels Nos. 1 to 13 are out of the scope of the
invention, and steels Nos. 14 to 32 are out of the scope of the
invention. The component compositions out of the scope of the
invention are indicated by an underline.
[0128] In all of the cases, the hot rolling conditions were within
the scope of the invention. The heating temperature of a slab was
set to be in a range of 1200.degree. C. to 1300.degree. C., the
finishing temperature was set to be in a range of 800.degree. C. to
950.degree. C., and the coiling temperature was set to be in a
range of 500.degree. C. or lower. In addition, regarding the
annealing conditions of a hot rolled sheet, the annealing
temperature was set to be in a range of 800.degree. C. to
1000.degree. C. and to be a temperature at which a
non-recrystallized microstructure is obtained in a region of a
depth of t'/2 to t'/4 (t': a sheet thickness of a hot rolled
sheet). Thereafter, cold rolling was performed at a rolling
reduction ratio of 60%. The annealing of a cold rolled sheet was
performed at a temperature of 1000.degree. C. to 1100.degree. C.
according to the steel component so that a recrystallized
microstructure is obtained.
TABLE-US-00001 TABLE 1 Steel Component Composition (mass %) No. C
Si Mn P S Cr Mo Ti Nb N Invention 1 0.007 0.27 0.94 0.03 0.0006
17.3 1.8 0.08 0.47 0.010 Examples 2 0.005 0.10 0.71 0.01 0.0002
16.2 1.7 0.11 0.55 0.005 3 0.010 0.88 0.62 0.02 0.0012 14.2 0.5
0.09 0.32 0.013 4 0.006 0.35 0.85 0.04 0.0025 18.9 1.5 0.15 0.45
0.005 5 0.002 0.15 0.77 0.01 0.0005 17.2 1.6 0.006 0.45 0.011 6
0.003 0.21 0.89 0.02 0.0015 17.5 0.2 0.09 0.53 0.015 7 0.013 0.91
0.61 0.03 0.0011 13.5 0.1 0.007 0.35 0.013 8 0.015 0.55 0.65 0.04
0.0018 14.3 0.5 0.07 0.41 0.011 9 0.009 0.25 0.85 0.03 0.0007 17.5
1.6 0.05 0.45 0.013 10 0.008 0.23 0.91 0.04 0.0007 17.9 2.6 0.006
0.55 0.016 11 0.012 0.19 0.62 0.03 0.0008 19.1 2.8 0.07 0.53 0.006
12 0.018 0.11 0.75 0.01 0.0031 13.2 1.6 0.12 0.44 0.016 13 0.003
0.63 0.85 0.04 0.0003 15.2 1.1 0.07 0.65 0.006 Steel Component
Composition (mass %) No. B Al Cu Ni W Zr Sn Co Mg Invention 1
0.0010 0.03 -- -- -- -- -- -- -- Examples 2 0.0005 0.07 -- -- -- --
-- -- -- 3 0.0021 0.11 -- -- -- -- -- -- -- 4 0.0009 0.008 -- -- --
-- -- -- -- 5 0.0013 0.04 -- -- -- -- -- -- -- 6 0.0008 0.02 1.2 --
-- -- -- -- -- 7 0.0008 0.06 -- 1.1 -- -- -- -- -- 8 0.0008 0.03 --
-- -- -- 0.2 -- -- 9 0.0011 0.04 0.4 -- -- -- -- -- 0.0005 10
0.0009 0.05 1.5 -- 0.13 -- -- -- -- 11 0.0019 0.13 1.4 0.2 1.5 --
-- -- -- 12 0.0013 0.33 -- -- -- 0.09 -- -- -- 13 0.0030 0.45 0.4
0.3 -- -- 0.1 0.08 -- Steel Component Composition (mass %) No. C Si
Mn P S Cr Mo Ti Nb N Comparative 14 0.035 0.34 0.76 0.02 0.0009
16.8 1.5 0.09 0.41 0.011 Examples 15 0.013 0.03 0.35 0.02 0.0009
14.3 1.3 0.13 0.45 0.007 16 0.008 0.91 0.45 0.02 0.0012 14.5 0.5
0.11 0.40 0.005 17 0.011 0.75 0.66 0.08 0.0001 14.5 1.5 0.12 0.38
0.005 18 0.004 0.95 1.30 0.01 0.0164 18.8 1.1 0.16 0.42 0.013 19
0.012 0.22 0.98 0.04 0.0001 11.5 1.5 0.11 0.31 0.005 20 0.004 0.36
1.2 0.02 0.0015 14.0 3.5 0.08 0.55 0.013 21 0.005 0.54 1.15 0.03
0.0053 14.1 1.5 0.003 0.65 0.015 22 0.008 0.91 1.38 0.01 0.0015
19.5 1.5 0.18 0.12 0.005 23 0.013 0.13 0.64 0.04 0.0033 14.1 2.3
0.01 0.47 0.032 24 0.018 0.15 1.35 0.02 0.0023 16.8 0.6 0.03 0.33
0.006 25 0.002 0.11 1.05 0.03 0.0013 16.5 1.1 0.07 0.39 0.017 26
0.006 0.35 0.85 0.02 0.0023 16.8 0.6 0.11 0.31 0.006 27 0.009 0.85
0.86 0.01 0.0016 14.3 2.2 0.11 0.66 0.010 28 0.014 0.88 0.96 0.04
0.0022 14.5 1.6 0.13 0.53 0.013 29 0.012 0.26 0.95 0.03 0.0007 17.3
0.6 0.08 0.33 0.016 30 0.011 0.25 1.03 0.05 0.0011 14.1 0.7 0.07
0.41 0.013 31 0.006 0.36 1.05 0.01 0.0025 16.3 1.2 0.15 0.51 0.009
32 0.005 0.49 1.35 0.02 0.0021 18.6 1.9 0.009 0.62 0.005 Steel
Component Composition (mass %) No. B Al Cu Ni W Zr Sn Co Mg
Comparative 14 0.0008 0.03 -- -- -- -- -- -- -- Examples 15 0.0011
0.04 -- -- -- -- -- -- -- 16 0.0016 0.11 -- -- -- -- -- -- -- 17
0.0035 0.01 -- -- -- -- -- -- -- 18 0.0006 0.35 -- -- -- -- -- --
-- 19 0.0009 0.16 -- -- -- -- -- -- -- 20 0.0015 0.05 -- -- -- --
-- -- -- 21 0.0033 0.04 -- -- -- -- -- -- -- 22 0.0006 0.03 -- --
-- -- -- -- -- 23 0.0008 0.12 -- -- -- -- -- -- -- 24 0.0064 0.15
-- -- -- -- -- -- -- 25 0.0019 0.64 -- -- -- -- -- -- -- 26 0.0045
0.03 2.6 -- -- -- -- -- -- 27 0.0043 0.02 -- 3.4 -- -- -- -- -- 28
0.0013 0.05 -- -- 3.6 -- -- -- -- 29 0.0011 0.13 -- -- -- 0.45 --
-- -- 30 0.0019 0.009 -- -- -- -- 0.63 -- -- 31 0.0028 0.01 -- --
-- -- -- 0.77 -- 32 0.0009 0.01 -- -- -- -- -- -- 0.0265
[0129] Next, from the product sheet obtained in this manner, a test
piece was collected to measure ratios (area ratios) of
{111}-oriented grains and {011}-oriented grains and to evaluate an
average r-value, a high temperature strength, and oxidation
characteristics. The measurement and evaluation methods will be
described in detail.
[0130] The methods of measuring the ratio of the crystal-oriented
grains and the average r-value are the same as the above-described
method. A plane in a direction parallel to the rolling direction
was cut out of the obtained product sheet at a right angle to the
sheet surface, and orientations of crystal grains were identified
over the entire region in the sheet thickness direction using a
crystal orientation analysis apparatus EBSP to determine area
ratios of {111}-oriented grains and {011}-oriented grains.
[0131] In addition, regarding the average r-value, a JIS13-B
tensile test piece was collected from the obtained product sheet
and a strain of 14.4% was applied in the rolling direction, in a
direction at an angle of 45.degree. with respect to the rolling
direction, and in a direction at an angle of 90.degree. with
respect to the rolling direction based on JIS Z 2254. Then, the
average r-value was calculated using the above-described
Expressions (1) and (2). In the evaluation of workability, the
workability was evaluated to be favorable in the case where the
average r-value was in a range of 1.2 or greater.
[0132] Next, regarding the high temperature strength, a high
temperature tensile test piece was collected in the rolling
direction from the obtained product sheet, and a high temperature
tensile test was performed at 900.degree. C. based on JIS G 0567 to
measure a 0.2% proof stress.
[0133] In the oxidation resistance test, a continuous oxidation
test was performed at 900.degree. C. for 200 hours in the air based
on JIS Z 2281 to evaluate breakaway and occurrence of scale
peeling.
[0134] In the case where a 0.2% proof stress which is the high
temperature strength at 900.degree. C. is in a range of 20 MPa or
greater and in the case where breakaway does not occur in
continuous oxidation in the air, a performance as an exhaust
component for a vehicle is satisfied. Therefore, a case in which
the 0.2% proof stress was less than 20 MPa was evaluated as
failing. A case in which breakaway and scale peeling did not occur
was evaluated as A (good), and a case in which breakaway and scale
peeling occurred was evaluated as B (bad).
[0135] The evaluation results are shown in Table 2.
TABLE-US-00002 TABLE 2 Crystal Orientation Ratio of Product Sheet
Ratio of Ratio of {111}-Oriented {111}-Oriented Ratio of High-
Continuous Grains Grains {011}- Average Temperature Oxidation at in
Region from in Region Oriented r-Value of Strength at 900.degree.
C. Surface Layer from t/4 Grains Product 900.degree. C. Oxidation
Test No. Steel No. to t/4 (%) to t/2 (%) (%) Sheet (MPa)
Characteristics P1 1 22 48 8 1.3 23 A Invention P2 2 36 41 9 1.2 25
A Examples P3 3 41 49 2 1.6 21 A P4 4 29 52 4 1.5 24 A P5 5 31 35
11 1.4 24 A P6 6 26 56 7 1.6 21 A P7 7 27 51 3 1.5 20 A P8 8 25 42
9 1.3 21 A P9 9 31 44 5 1.5 24 A P10 10 27 48 4 1.6 27 A P11 11 39
47 13 1.7 28 A P12 12 33 43 2 1.8 24 A P13 13 40 61 6 1.9 26 A P14
14 18 35 26 0.7 17 B Comparative P15 15 19 35 13 1.0 19 B Examples
P16 16 21 42 9 1.2 20 B P17 17 18 36 17 1.1 22 B P18 18 19 33 19
1.1 20 B P19 19 28 51 10 1.2 19 B P20 20 21 35 23 1.0 24 B P21 21
11 29 11 0.9 19 A P22 22 10 25 11 0.9 17 A P23 23 17 34 23 0.7 17 B
P24 24 22 45 19 1.0 21 B P25 25 31 41 20 1.1 21 A P26 26 25 45 26
0.8 25 B P27 27 19 35 14 1.0 22 A P28 28 21 33 23 0.8 24 A P29 29
29 41 20 0.9 22 A P30 30 31 49 25 0.8 21 B P31 31 15 25 21 0.6 21 A
P32 32 23 41 10 1.2 20 B
[0136] As is obvious from Tables 1 and 2, it is found that the
steel having a component composition specified in the invention has
a higher average r-value and more excellent workability than those
in the comparative examples. In addition, the high temperature
strength is also high and the oxidation resistance is also
excellent. Since the comparative steels Nos. 14, 15, 17, 18, and 20
to 31 have a steel component which is out of the scope of the
invention, the crystal orientation ratio of the product sheet is
out of the scope of the invention, and thus the average r-value of
the product sheet is in a range of less than 1.2. When these
materials are subjected to working into a component having a
complicated shape, there is a concern that cracking may occur. In
addition, the comparative steels Nos. 16, 19, and 32 have a
satisfactory r-value, but are insufficient in oxidation resistance
and in high temperature strength. When these are applied as an
exhaust component, there is a concern that breakage may occur
during use.
Example 2
[0137] Next, with regard to the invention steels Nos. 1 and 6 shown
in Table 1, characteristics of cases in which the manufacturing
conditions were variously changed are shown in Table 3. The
recrystallization state is a microstructure state in the region
from t'/2 to t'/4.
TABLE-US-00003 TABLE 3 Annealing Cold of Cold Rolling Rolled Hot
Rolling Annealing of Hot Rolled Sheet Rolling Sheet Heating
Finishing Coiling Heating Recrystallization Reduction Heating Test
Steel Temperature Temperature Temperature Temperature State Ratio
Temperature No. No. (.degree. C.) (.degree. C.) (.degree. C.)
(.degree. C.) After Annealing (%) (.degree. C.) P33 1 1250 900 450
950 Non-Recrystallized 60 1050 Invention Microstructure Example P34
6 1260 910 460 970 Non-Recrystallized 60 1070 Invention
Microstructure Example P35 1 1320 900 450 950 Recrystallized 60
1050 Comparative Microstructure Example P36 1 1240 1000 460 950
Recrystallized 60 1070 Comparative Microstructure Example P37 1
1230 900 600 940 Non-Recrystallized 60 1020 Comparative
Microstructure Example P38 1 1250 920 450 1050 Recrystallized 60
1020 Comparative Microstructure Example P39 1 1250 880 470 950
Non-Recrystallized 50 1050 Comparative Microstructure Example P40 1
1240 880 430 950 Non-Recrystallized 60 950 Comparative
Microstructure Example P41 6 1100 900 440 970 Non-Recrystallized 60
1040 Comparative Microstructure Example (surface scratches was
generated) P42 6 1260 750 450 930 Non-Recrystallized 60 1040
Comparative Microstructure Example (surface scratches was
generated) P43 6 1250 850 450 750 Non-Recrystallized 60 1050
Comparative Microstructure Example P44 6 1250 830 430 950
Non-Recrystallized 60 1150 Comparative Microstructure Example
Crystal Orientation Ratio of Product Sheet Ratio of {111}-Oriented
Grains Surface Ratio of Product Layer t/4 to {011}-Oriented Sheet
Test Steel to t/4 t/2 Grains Average No. No. (%) (%) (%) r-Value
P33 1 22 48 8 1.3 Invention Example P34 6 26 56 7 1.6 Invention
Example P35 1 22 35 17 1.1 Comparative Example P36 1 18 37 16 1.0
Comparative Example P37 1 21 39 16 1.1 Comparative Example P38 1 15
29 20 0.9 Comparative Example P39 1 8 15 14 0.7 Comparative Example
P40 1 20 26 15 0.8 Comparative Example P41 6 22 45 15 1.2
Comparative Example (surface scratches was generated) P42 6 25 41
13 1.2 Comparative Example (surface scratches was generated) P43 6
15 42 22 0.9 Comparative Example P44 6 21 42 25 0.9 Comparative
Example
[0138] It is found that test Nos. P33 and P34 in which all of the
manufacturing conditions specified in the invention are satisfied
have a higher average r-value and more excellent workability than
those of the comparative examples.
[0139] It is found that in the comparative examples (test Nos. P35
to P44) which are out of the scope of the manufacturing conditions
specified in the invention, the crystal orientation ratio of the
product sheet is out of the scope of the invention, a satisfactory
average r-value of 1.2 or greater is not obtained, and thus
workability deteriorates. Therefore, when the product sheet is
subjected to working into a component having a complicated shape,
there is a concern that cracking may occur. In addition, when the
heating temperature or the finishing temperature in the hot rolling
is out of the lower limit, a satisfactory r-value of 1.2 or greater
is obtained, but surface scratches are generated.
[0140] From these results, it was possible to confirm the
above-described knowledge and also to support the evidence limiting
the above-described steel compositions and configurations.
INDUSTRIAL APPLICABILITY
[0141] As is obvious from the above description, according to the
invention, it is possible to efficiently provide a heat-resistant
ferritic stainless steel sheet having excellent workability without
requiring special new facilities. Therefore, when a cold rolled
steel sheet to which the invention is applied is applied to,
especially, an exhaust member, a social contribution ratio such as
a reduction in manufacturing costs can be increased. That is, the
invention has sufficient industrial applicability.
* * * * *