U.S. patent application number 16/480785 was filed with the patent office on 2020-01-02 for hot-rolled ferritic stainless steel sheet and method for manufacturing 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, Masataka YOSHINO.
Application Number | 20200002779 16/480785 |
Document ID | / |
Family ID | 62979289 |
Filed Date | 2020-01-02 |
United States Patent
Application |
20200002779 |
Kind Code |
A1 |
YOSHINO; Masataka ; et
al. |
January 2, 2020 |
HOT-ROLLED FERRITIC STAINLESS STEEL SHEET AND METHOD FOR
MANUFACTURING SAME
Abstract
Provided is a hot-rolled ferritic stainless steel sheet which
has sufficient corrosion resistance and which can prevent the
occurrence of cracking when subjected to blanking to be formed into
a thick flange, and also provided is a method for manufacturing the
hot-rolled ferritic stainless steel sheet. A hot-rolled ferritic
stainless steel sheet having a chemical composition containing, in
mass %, C: 0.001 to 0.020%, Si: 0.05 to 1.00%, Mn: 0.05 to 1.00%,
P: 0.04% or less, S: 0.01% or less, Al: 0.001 to 0.50%, N: 0.001 to
0.020%, Cr: 11.0 to 24.0%, Ni: 0.01 to 2.00%, and Nb: 0.12 to
0.80%, with the balance being Fe and incidental impurities, wherein
the hot-rolled ferritic stainless steel sheet has a threshold
stress intensity factor K.sub.IC of 25 MPam.sup.1/2 or more.
Inventors: |
YOSHINO; Masataka; (Tokyo,
JP) ; FUJISAWA; Mitsuyuki; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE STEEL CORPORATION |
Tokyo |
|
JP |
|
|
Assignee: |
JFE Steel Corporation
Tokyo
JP
|
Family ID: |
62979289 |
Appl. No.: |
16/480785 |
Filed: |
January 12, 2018 |
PCT Filed: |
January 12, 2018 |
PCT NO: |
PCT/JP2018/000559 |
371 Date: |
July 25, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 8/0263 20130101;
C21D 8/02 20130101; C22C 38/002 20130101; C22C 38/44 20130101; C22C
38/001 20130101; C22C 38/02 20130101; C22C 38/06 20130101; C22C
38/00 20130101; C22C 38/04 20130101; C21D 2211/005 20130101; C21D
9/46 20130101; C22C 38/42 20130101; C21D 8/0205 20130101; C21D
6/004 20130101; C22C 38/52 20130101; C21D 8/0226 20130101; C22C
38/48 20130101; C22C 38/50 20130101; C21D 6/02 20130101; C22C
38/004 20130101; C22C 38/54 20130101; C22C 38/46 20130101; C22C
38/005 20130101 |
International
Class: |
C21D 8/02 20060101
C21D008/02; C22C 38/44 20060101 C22C038/44; C22C 38/48 20060101
C22C038/48; C22C 38/46 20060101 C22C038/46; C22C 38/42 20060101
C22C038/42; C22C 38/50 20060101 C22C038/50; C22C 38/52 20060101
C22C038/52; C22C 38/54 20060101 C22C038/54; C22C 38/00 20060101
C22C038/00; C22C 38/02 20060101 C22C038/02; C22C 38/04 20060101
C22C038/04; C22C 38/06 20060101 C22C038/06; C21D 9/46 20060101
C21D009/46 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 26, 2017 |
JP |
2017-012545 |
Claims
1. A hot-rolled ferritic stainless steel sheet having a chemical
composition comprising, in mass %, C: 0.001 to 0.020%, Si: 0.05 to
1.00%, Mn: 0.05 to 1.00%, P: 0.04% or less, S: 0.01% or less, Al:
0.001 to 0.50%, N: 0.001 to 0.020%, Cr: 11.0 to 24.0%, Ni: 0.01 to
2.00%, and Nb: 0.12 to 0.80%, with a balance of Fe and incidental
impurities, wherein the hot-rolled ferritic stainless steel sheet
has a threshold stress intensity factor K.sub.IC of 25 MPam.sup.1/2
or more.
2. A hot-rolled ferritic stainless steel sheet having a chemical
composition comprising, in mass %, C: 0.001 to 0.020%, Si: 0.05 to
1.00%, Mn: 0.05 to 1.00%, P: 0.04% or less, S: 0.01% or less, Al:
0.001 to 0.50%, N: 0.001 to 0.020%, Cr: 13.0 to 24.0%, Ni: 0.01 to
0.60%, and Nb: 0.12 to 0.80%, with a balance of Fe and incidental
impurities, wherein the hot-rolled ferritic stainless steel sheet
has a threshold stress intensity factor K.sub.IC of 25 MPam.sup.1/2
or more.
3. The hot-rolled ferritic stainless steel sheet according to claim
1, wherein the chemical composition further comprises, in mass %,
one or more selected from Cu: 0.01 to 1.50%, Mo: 0.01 to 2.00%, W:
0.01 to 0.20%, and Co: 0.01 to 0.20%.
4. The hot-rolled ferritic stainless steel sheet according to claim
2, wherein the chemical composition further comprises, in mass %,
one or more selected from Cu: 0.01 to 1.50%, Mo: 0.01 to 2.00%, W:
0.01 to 0.20%, and Co: 0.01 to 0.20%.
5. The hot-rolled ferritic stainless steel sheet according to claim
1, wherein the chemical composition further comprises, in mass %,
one or more selected from Ti: 0.01 to 0.30%, V: 0.01 to 0.20%, Zr:
0.01 to 0.20%, REM: 0.001 to 0.100%, B: 0.0002 to 0.0025%, Mg:
0.0005 to 0.0030%, and Ca: 0.0005 to 0.0030%.
6. The hot-rolled ferritic stainless steel sheet according to claim
2, wherein the chemical composition further comprises, in mass %,
one or more selected from Ti: 0.01 to 0.30%, V: 0.01 to 0.20%, Zr:
0.01 to 0.20%, REM: 0.001 to 0.100%, B: 0.0002 to 0.0025%, Mg:
0.0005 to 0.0030%, and Ca: 0.0005 to 0.0030%.
7. The hot-rolled ferritic stainless steel sheet according to claim
3, wherein the chemical composition further comprises, in mass %,
one or more selected from Ti: 0.01 to 0.30%, V: 0.01 to 0.20%, Zr:
0.01 to 0.20%, REM: 0.001 to 0.100%, B: 0.0002 to 0.0025%, Mg:
0.0005 to 0.0030%, and Ca: 0.0005 to 0.0030%.
8. The hot-rolled ferritic stainless steel sheet according to claim
4, wherein the chemical composition further comprises, in mass %,
one or more selected from Ti: 0.01 to 0.30%, V: 0.01 to 0.20%, Zr:
0.01 to 0.20%, REM: 0.001 to 0.100%, B: 0.0002 to 0.0025%, Mg:
0.0005 to 0.0030%, and Ca: 0.0005 to 0.0030%.
9. A method for manufacturing the hot-rolled ferritic stainless
steel sheet according to claim 1, the method comprising hot
rolling, the hot rolling including performing finish rolling
including three or more passes, wherein a temperature range for
final three passes of the finish rolling is 800 to 1100.degree. C.,
and an accumulated rolling reduction ratio for the final three
passes is 25% or more.
10. A method for manufacturing the hot-rolled ferritic stainless
steel sheet according to claim 2, the method comprising hot
rolling, the hot rolling including performing finish rolling
including three or more passes, wherein a temperature range for
final three passes of the finish rolling is 800 to 1100.degree. C.,
and an accumulated rolling reduction ratio for the final three
passes is 25% or more.
11. A method for manufacturing the hot-rolled ferritic stainless
steel sheet according to claim 3, the method comprising hot
rolling, the hot rolling including performing finish rolling
including three or more passes, wherein a temperature range for
final three passes of the finish rolling is 800 to 1100.degree. C.,
and an accumulated rolling reduction ratio for the final three
passes is 25% or more.
12. A method for manufacturing the hot-rolled ferritic stainless
steel sheet according to claim 4, the method comprising hot
rolling, the hot rolling including performing finish rolling
including three or more passes, wherein a temperature range for
final three passes of the finish rolling is 800 to 1100.degree. C.,
and an accumulated rolling reduction ratio for the final three
passes is 25% or more.
13. A method for manufacturing the hot-rolled ferritic stainless
steel sheet according to claim 5, the method comprising hot
rolling, the hot rolling including performing finish rolling
including three or more passes, wherein a temperature range for
final three passes of the finish rolling is 800 to 1100.degree. C.,
and an accumulated rolling reduction ratio for the final three
passes is 25% or more.
14. A method for manufacturing the hot-rolled ferritic stainless
steel sheet according to claim 6, the method comprising hot
rolling, the hot rolling including performing finish rolling
including three or more passes, wherein a temperature range for
final three passes of the finish rolling is 800 to 1100.degree. C.,
and an accumulated rolling reduction ratio for the final three
passes is 25% or more.
15. A method for manufacturing the hot-rolled ferritic stainless
steel sheet according to claim 7, the method comprising hot
rolling, the hot rolling including performing finish rolling
including three or more passes, wherein a temperature range for
final three passes of the finish rolling is 800 to 1100.degree. C.,
and an accumulated rolling reduction ratio for the final three
passes is 25% or more.
16. A method for manufacturing the hot-rolled ferritic stainless
steel sheet according to claim 8, the method comprising hot
rolling, the hot rolling including performing finish rolling
including three or more passes, wherein a temperature range for
final three passes of the finish rolling is 800 to 1100.degree. C.,
and an accumulated rolling reduction ratio for the final three
passes is 25% or more.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is the U.S. National Phase application of
PCT/JP2018/000559, filed Jan. 12, 2018, which claims priority to
Japanese Patent Application No. 2017-012545, filed Jan. 26, 2017,
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 hot-rolled ferritic
stainless steel sheet having excellent blanking workability and
being suitable for use for a flange and the like and to a method
for manufacturing the hot-rolled ferritic stainless steel
sheet.
BACKGROUND OF THE INVENTION
[0003] In recent years, laws and regulations regarding exhaust
gases from automobiles have been increasingly tightened, and
improving fuel economy is an urgent need. Correspondingly, exhaust
gas recirculation (EGR) systems, which are systems for reusing
exhaust gases generated by an automobile engine as intake gases for
the engine, are being increasingly employed. Exhaust gases that are
generated by an engine pass through an EGR cooler, which is used to
reduce the gas temperature, and are thereafter resupplied to the
engine. For circulation of exhaust gases, individual exhaust system
parts are joined together via flanges to prevent gas leakage.
Flanges used for such exhaust system parts need to have sufficient
rigidity. Accordingly, thick (e.g., 5 mm or more in sheet
thickness) flanges are used for such exhaust system parts.
[0004] In the related art, ordinary steels have been used for thick
flanges. However, flanges that are used for parts, for example,
parts of an EGR system, through which high-temperature exhaust
gases pass, are required to have sufficient corrosion resistance.
Accordingly, studies are being conducted regarding using a
stainless steel, which has higher corrosion resistance than
ordinary steels, and particularly, using a ferritic stainless
steel, which has a relatively low coefficient of thermal expansion
and is less likely to generate thermal stress, and there is a
strong need for a ferritic stainless steel sheet that has a large
sheet thickness (e.g., 5 mm or more in sheet thickness) and can be
used for thick flanges.
[0005] To meet such a market demand, Patent Literature 1, for
example, discloses a hot-rolled Nb-containing ferritic stainless
steel coil having a sheet thickness of 5.0 to 10.0 mm, the steel
having a composition containing, in mass %, C: 0.030% or less, Si:
2.00% or less, Mn: 2.00% or less, P: 0.050% or less, S: 0.040% or
less, Cr: 10.00 to 25.00%, N: 0.030% or less, and Nb: 0.01 to
0.80%, with the balance being Fe and incidental impurities. The
coil has an adjusted hardness of 190 HV or less and an adjusted
Charpy impact value at 25.degree. C. of 20 J/cm.sup.2 or more.
PATENT LITERATURE
[0006] PTL 1: Japanese Unexamined Patent Application Publication
No. 2012-140688
SUMMARY OF THE INVENTION
[0007] However, when the present inventors performed blanking using
a crank press on a hot-rolled ferritic stainless steel coil such as
disclosed in Patent Literature 1 to form a thick flange shape,
noticeable cracking occurred in a central part in the sheet
thickness of the blanked part and therefore the predetermined
flange shape was not obtained in some cases although the coil had a
sufficient Charpy impact value, and thus the coil was found to be
insufficient to be used for thick flanges. Furthermore, when a
hot-rolled coil such as disclosed in Patent Literature 1 is to be
obtained, it is necessary to immerse the coil in water, after
completion of hot rolling coiling, and keep the coil immersed for
15 minutes or more, which poses problems of manufacturability and
productivity.
[0008] Objects according to aspects of the present invention are to
provide, in order to solve such problems, a hot-rolled ferritic
stainless steel sheet which has sufficient corrosion resistance and
which can prevented the occurrence of cracking when subjected to
blanking in a crank press to be formed into a thick flange and to
provide a method for manufacturing the hot-rolled ferritic
stainless steel sheet.
[0009] The present inventors performed detailed studies to solve
the problems and consequently found that blanking for forming a
thick flange by using a processing method with a relatively high
processing speed, such as a method using a crank press, can be
accomplished without causing cracking when the threshold stress
intensity factor, K.sub.IC, of the steel sheet is increased.
Specifically, it was found that, by ensuring that the threshold
stress intensity factor K.sub.IC is 25 MPam.sup.1/2 or more, the
occurrence of cracking in the blanked edge surface when blanking
for forming a thick flange is performed can be effectively
prevented even if a processing method with a high processing speed,
such as a method using a crank press, is used, and therefore
practical use of the steel sheet for thick flanges can be
sufficiently realized.
[0010] The present inventors conducted detailed studies to solve
the problems. As a result, it was discovered that, in the case that
a thick steel sheet of more than 5.0 mm sheet thickness is to be
blanked to form a thick flange by using a processing method with a
high processing speed, such as a method using a crank press,
without causing cracking, the workability cannot be accurately
evaluated by using the Charpy impact value, which has been used in
the conventional technique, but can be accurately evaluated by
using the threshold stress intensity factor K.sub.IC, which is a
toughness evaluation index in the field of thick steel plates. The
reason for this is thought to be as follows. In the case of a thin
steel sheet of less than 5.0 mm sheet thickness, the plastic
deformation region in and near the blanked edge surface resulting
from the processing is large with respect to the sheet thickness,
and therefore the fracture phenomenon involved in the forming
cannot be uniquely defined with an approach associated with the
fracture mechanics, whereas, in the case of a thick steel sheet of
5.0 mm or more sheet thickness, the plastic deformation region in
and near the blanked edge surface resulting from the processing is
sufficiently small with respect to the sheet thickness and thus the
small scale yielding state is sufficiently satisfied, and therefore
the fracture phenomenon involved in a specific processing process
can be handled with the stress intensity factor, which is a
quantitative index associated with the fracture mechanics, and
particularly can be accurately evaluated by using the threshold
value, that is, the threshold stress intensity factor K.sub.IC.
[0011] In view of the above, the present inventors closely
investigated the relationship between the threshold stress
intensity factor K.sub.IC and the presence or absence of cracking
that may occur when blanking, using a crank press, for forming a
flange having a predetermined shape is performed. As a result, it
was found that, by ensuring that the threshold stress intensity
factor K.sub.IC is 25 MPam.sup.1/2 or more, the occurrence of
cracking in the blanked edge surface when blanking for forming a
thick flange is performed by using a crank press can be effectively
prevented, and therefore practical use for thick flanges can be
sufficiently realized.
[0012] Further, it was found that, when, for a ferritic stainless
steel containing appropriate composition, the accumulated rolling
reduction ratio (=100-(final sheet thickness/sheet thickness before
start of rolling in final three passes).times.100[%]) particularly
in the final three passes of a finish hot rolling process including
multiple (three or more) passes is appropriately controlled, the
threshold stress intensity factor K.sub.IC of the hot-rolled steel
sheet is improved.
[0013] Aspects of the present invention were made based on the
above findings, and a summary of aspects of the present invention
is as follows.
[1] A hot-rolled ferritic stainless steel sheet having a chemical
composition containing, in mass %, C: 0.001 to 0.020%, Si: 0.05 to
1.00%, Mn: 0.05 to 1.00%, P: 0.04% or less, S: 0.01% or less, Al:
0.001 to 0.50%, N: 0.001 to 0.020%, Cr: 11.0 to 24.0%, Ni: 0.01 to
2.00%, and Nb: 0.12 to 0.80%, with the balance being Fe and
incidental impurities, wherein the hot-rolled ferritic stainless
steel sheet has a threshold stress intensity factor K.sub.IC of 25
MPam.sup.1/2 or more. [2] A hot-rolled ferritic stainless steel
sheet having a chemical composition containing, in mass %, C: 0.001
to 0.020%, Si: 0.05 to 1.00%, Mn: 0.05 to 1.00%, P: 0.04% or less,
S: 0.01% or less, Al: 0.001 to 0.50%, N: 0.001 to 0.020%, Cr: 13.0
to 24.0%, Ni: 0.01 to 0.60%, and Nb: 0.12 to 0.80%, with the
balance being Fe and incidental impurities, wherein the hot-rolled
ferritic stainless steel sheet has a threshold stress intensity
factor K.sub.IC of 25 MPam.sup.1/2 or more. [3] The hot-rolled
ferritic stainless steel sheet according to [1] or [2], wherein the
chemical composition further contains, in mass %, one or more
selected from Cu: 0.01 to 1.50%, Mo: 0.01 to 2.00%, W: 0.01 to
0.20%, and Co: 0.01 to 0.20%. [4] The hot-rolled ferritic stainless
steel sheet according to any one of [1] to [3], wherein the
chemical composition further contains, in mass %, one or more
selected from Ti: 0.01 to 0.30%, V: 0.01 to 0.20%, Zr: 0.01 to
0.20%, REM: 0.001 to 0.100%, B: 0.0002 to 0.0025%, Mg: 0.0005 to
0.0030%, and Ca: 0.0005 to 0.0030%. [5] A method for manufacturing
the hot-rolled ferritic stainless steel sheet according to any one
of [1] to [4], the method including hot rolling, the hot rolling
including performing finish rolling including three or more passes,
wherein a temperature range for final three passes of the finish
rolling is 800 to 1100.degree. C., and an accumulated rolling
reduction ratio for the final three passes is 25% or more.
[0014] Here, the threshold stress intensity factor K.sub.IC is a
stress intensity factor determined by cutting a CT test piece in
accordance with ASTM E399 from a sheet width central portion in
such a manner that a fatigue pre-crack is introduced in a direction
perpendicular to the rolling direction and the stress axis is in a
direction parallel to the rolling direction and testing the test
piece in accordance with ASTM E399.
[0015] Aspects of the present invention make it possible to obtain
a hot-rolled ferritic stainless steel sheet which has sufficient
corrosion resistance and which has excellent toughness to prevent
the occurrence of cracking when subjected to blanking in a crank
press to be formed into a thick flange.
[0016] In accordance with aspects of the present invention, the
phrase "sufficient corrosion resistance" means that the rust area
ratio (=rust area/total area of steel sheet.times.100 [%]) of the
evaluation surface of a steel sheet is not more than 25% in the
case where, after the surface to be evaluated is polished with #600
emery paper and the edge surface portions are sealed, the steel
sheet is subjected to 5 cycles of a cyclic salt spray test as
specified in JIS H 8502 (a test in which one cycle is as follows:
salt spraying (5-mass % NaCl, 35.degree. C., 2-hour
spraying).fwdarw.drying (60.degree. C., 4 hours, 40% relative
humidity).fwdarw.exposure to humidity (50.degree. C., 2 hours,
relative humidity.gtoreq.95%).
[0017] Furthermore, the phrase "has excellent toughness to
prevented the occurrence of cracking when subjected to blanking in
a crank press to be formed into a thick flange" means that the
threshold stress intensity factor K.sub.IC is 25 MPam.sup.1/2 or
more, as determined by cutting a CT test piece in accordance with
ASTM E399 from a sheet width central portion in such a manner that
a fatigue pre-crack is introduced in a direction perpendicular to
the rolling direction and the stress axis is in a direction
parallel to the rolling direction and testing the test piece in
accordance with ASTM E399.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0018] According to aspects of the present invention, a hot-rolled
ferritic stainless steel sheet has a chemical composition
containing, in mass %, C: 0.001 to 0.020%, Si: 0.05 to 1.00%, Mn:
0.05 to 1.00%, P: 0.04% or less, S: 0.01% or less, Al: 0.001 to
0.50%, N: 0.001 to 0.020%, Cr: 11.0 to 24.0%, Ni: 0.01 to 2.00%,
and Nb: 0.12 to 0.80%, with the balance being Fe and incidental
impurities, and the hot-rolled ferritic stainless steel sheet has a
threshold stress intensity factor K.sub.IC of 25 MPam.sup.1/2 or
more.
[0019] In a preferable embodiment, a hot-rolled ferritic stainless
steel sheet according to aspects of the present invention has a
chemical composition containing, in mass %, C: 0.001 to 0.020%, Si:
0.05 to 1.00%, Mn: 0.05 to 1.00%, P: 0.04% or less, S: 0.01% or
less, Al: 0.001 to 0.50%, N: 0.001 to 0.020%, Cr: 13.0 to 24.0%,
Ni: 0.01 to 0.60%, and Nb: 0.12 to 0.80%, with the balance being Fe
and incidental impurities, and the hot-rolled ferritic stainless
steel sheet has a threshold stress intensity factor K.sub.IC of 25
MPam.sup.1/2 or more.
[0020] The threshold stress intensity factor K.sub.IC is a stress
intensity factor determined by cutting a CT test piece in
accordance with ASTM E399 from a sheet width central portion in
such a manner that a fatigue pre-crack is introduced in a direction
perpendicular to the rolling direction and the stress axis is in a
direction parallel to the rolling direction and testing the test
piece in accordance with ASTM E399.
[0021] Embodiments of the present invention will now be described
in detail.
[0022] The present inventors performed detailed studies on factors
responsible for cracking that occurred when blanking was performed
on various types of hot-rolled ferritic stainless steel sheets
having a sheet thickness of 5.0 mm in a crank press to form a
flange shaped with a hole having a diameter of 30 mm. As a result,
it was found that, in the steel sheets in which cracking occurred,
the cracking occurred in such a manner that microcracking generated
in or near a central part in the sheet thickness of a blanked edge
surface in a direction perpendicular to the blanking direction, and
that the microcracks propagated.
[0023] The present inventors performed detailed studies on the
relationship between the generation and propagation of microcracks
and the material properties. As a result, it was found that there
is a tendency for propagation of microcracks to occur more easily
as the threshold stress intensity factor of a steel sheet
decreases. Accordingly, blanking for forming a flange such as that
described above was attempted by using various hot-rolled ferritic
stainless steel sheets (5.0 mm in sheet thickness), and as a
result, it was found that no cracking occurred in steel sheets
having a threshold stress intensity factor of 25 MPam.sup.1/2 or
more and that cracking was more likely to occur in steel sheets in
which the threshold stress intensity factor was less than 25
MPam.sup.1/2, the threshold stress intensity factors being measured
using a specified measurement method.
[0024] Accordingly, the present inventors conducted detailed
research on composition of steel and conditions for hot rolling to
perform studies on techniques for improving the threshold stress
intensity factor of hot-rolled ferritic stainless steel sheets. As
a result, the following was found. When, for a ferritic stainless
steel containing appropriate composition, a hot rolling process, in
which finish rolling including multiple passes is performed, is
appropriately controlled particularly such that the temperature
range for the final three passes is 800 to 1100.degree. C. and the
accumulated rolling reduction ratio (=100-(final sheet
thickness/sheet thickness before start of rolling in final three
passes).times.100[%]) for the final three passes is 25% or more,
rolling strains are effectively introduced not only to the surface
layer part but also to the central part in the sheet thickness.
Consequently, the threshold stress intensity factor K.sub.IC of 25
MPam.sup.1/2 or more can be achieved.
[0025] The sheet thickness of hot-rolled ferritic stainless steel
sheets according to aspects of the present invention is not
particularly limited but preferably not less than 5.0 mm because a
sheet thickness suitable for a thick flange is desirable.
Furthermore, the sheet thickness is not particularly limited but
preferably not more than 15.0 mm and more preferably not more than
10.0 mm.
[0026] The following description describes reasons that, with the
above-described technique, a steel sheet after hot rolling has
rolling strains effectively introduced also to the central part in
the sheet thickness and therefore has an increased threshold stress
intensity factor K.sub.IC over the entire thickness of the steel
sheet.
[0027] When a steel sheet is subjected to rolling, the steel sheet
is elongated with the surface layer being deformed first.
Accordingly, if the rolling reduction ratio is low, the amount of
deformation in the central part in the sheet thickness is small,
and therefore almost no rolling strain is introduced to the central
part in the sheet thickness. In addition to this, in ferritic
stainless steels, recovery of work strain tends to occur during hot
rolling. Consequently, hot rolling of the related art cannot
effectively introduced work strains to the central part in the
sheet thickness because the rolling reduction ratio is
insufficient. In addition, introduced rolling strains are
eliminated and reduced by excessive recovery during hot rolling. As
a result, with hot rolling of the related art, the predetermined
threshold stress intensity factor K.sub.IC cannot be achieved.
[0028] In view of the above, the present inventors diligently
performed studies on techniques for effectively and sufficiently
introducing rolling strains to the central part in the sheet
thickness in a hot rolling process, from the standpoints of both
steel components and the method of hot rolling.
[0029] As a result, from the perspective of the method of hot
rolling, it was found that rolling strains are sufficiently and
effectively introduced to the central part in the sheet thickness
when the temperature range for the final three passes of finish hot
rolling is controlled to be appropriate and, in such a state,
rolling is performed at a high accumulated rolling reduction
ratio.
[0030] However, from the perspective of steel components, it was
found that, with a ferritic stainless steel containing
substantially no Nb, recovery during hot rolling tends to occur,
and therefore, even if the method of hot rolling proposed by the
present inventors is used, the rolling strain density that can be
obtained is insufficient and the predetermined threshold stress
intensity factor cannot be achieved.
[0031] On the other hand, it was found that, with a ferritic
stainless steel containing an appropriate amount of Nb, fine Nb
carbonitrides precipitate during hot rolling, and such fine Nb
carbonitrides suppress the movement of dislocations, and therefore,
using the method of hot rolling proposed by the present inventors
makes it possible to obtain a high rolling strain density and
ensures that the hot-rolled steel sheet has the predetermined
threshold stress intensity factor.
[0032] That is, the following was found. In accordance with aspects
of the present invention, with a ferritic stainless steel
containing an appropriate amount of Nb, the temperature range for
the final three passes of finish hot rolling is controlled to be
appropriate and, in such a state, rolling is performed at a high
accumulated rolling reduction ratio, and as a result, rolling
strains are sufficiently and effectively introduced to the central
part in the sheet thickness while recovery of rolling strains is
suppressed, and therefore the predetermined threshold stress
intensity factor K.sub.IC can be achieved.
[0033] Specifically, performing hot rolling in the following manner
was devised. For a ferritic stainless steel containing Nb in an
amount of 0.12% or more, a finish hot rolling process including
three or more passes is to be appropriately controlled such that
the temperature range for the final three passes is 800 to
1100.degree. C. and the accumulated rolling reduction ratio
(=100-(final sheet thickness/sheet thickness before start of
rolling in final three passes).times.100[%]) for the final three
passes is 25% or more.
[0034] The chemical composition of the hot-rolled ferritic
stainless steel sheet according to aspects of the present invention
will now be described. In the following description, "%" used to
indicate the chemical composition means "mass %" unless otherwise
specified.
[0035] C: 0.001 to 0.020%
[0036] If C is contained in an amount more than 0.020%, workability
and corrosion resistance in the weld zone noticeably deteriorate. A
C content as low as possible is preferable from the perspectives of
corrosion resistance and workability but is not preferable from the
standpoint of manufacturing because obtaining a C content of less
than 0.001% requires that refining be performed for a long period
of time. Accordingly, the C content is within the range of 0.001 to
0.020%. The C content is preferably not less than 0.003% and more
preferably not less than 0.004%. Furthermore, the C content is
preferably not more than 0.015% and more preferably not more than
0.012%.
[0037] Si: 0.05 to 1.00%
[0038] Si has an effect of improving corrosion resistance in the
weld zone by being concentrated in the oxide layer formed during
welding. Si is an element also useful as a deoxidizing element in
the steelmaking process. These effects are obtained when Si is
contained in an amount of 0.05% or more. The effects increase as
the content increases. However, the presence of Si in an amount
more than 1.00% is not preferable because, in the hot rolling
process, the rolling load increases and the formation of scale is
noticeable, and in the annealing process, pickling properties are
deteriorated because of the formation of a Si-rich layer in the
surface layer of the steel sheet, and consequently, an increase in
surface defects and an increase in manufacturing cost are induced.
Accordingly, the Si content is 0.05 to 1.00%. It is preferable that
the Si content not be less than 0.10%. Furthermore, the Si content
is preferably not more than 0.60% and more preferably not more than
0.40%.
[0039] Mn: 0.05 to 1.00%
[0040] Mn has an effect of increasing the strength of steel and
also acts as a deoxidizer. To obtain the effects, Mn need to be
contained in an amount of 0.05% or more. If the Mn content is more
than 1.00%, however, precipitation of MnS, which acts as a
corrosion initiation site, is promoted, which deteriorates
corrosion resistance. Accordingly, the Mn content is 0.05 to 1.00%.
It is preferable that the Mn content not be less than 0.10%.
Furthermore, the Mn content is preferably not more than 0.50% and
more preferably not more than 0.30%.
[0041] P: 0.04% or Less
[0042] P is an element inevitably present in steel, but, since P is
an element harmful to corrosion resistance and workability, it is
preferable that the P content be as low as possible. In particular,
if the P content is more than 0.04%, workability noticeably
deteriorates because of solid solution strengthening. Accordingly,
the P content is not more than 0.04%. It is preferable that the P
content not be more than 0.03%.
[0043] S: 0.01% or Less
[0044] Similarly to P, S is an element inevitably present in steel,
but, since S is an element harmful to corrosion resistance and
workability, it is preferable that the P content be as low as
possible. In particular, if the S content is more than 0.01%,
corrosion resistance noticeably deteriorates. Accordingly, the S
content is not more than 0.01%. It is preferable that the S content
not be more than 0.008%. It is more preferable that the S content
not be more than 0.003%.
[0045] Al: 0.001 to 0.50%
[0046] Al is an effective deoxidizer. In addition, Al has a higher
affinity for N than does Cr and therefore has an effect of, if N
penetrates into a weld, causing N to precipitate as an Al nitride
rather than as a Cr nitride, thereby suppressing sensitization.
These effects are obtained when Al is contained in an amount of
0.001% or more. However, the presence of Al in an amount more than
0.50% deteriorates the penetration characteristics for welding,
which deteriorates welding workability, and is therefore not
preferable. Accordingly, the Al content is within the range of
0.001 to 0.50%. The Al content is preferably not more than 0.20%
and more preferably not more than 0.10%.
[0047] N: 0.001 to 0.020%
[0048] If the N content is more than 0.020%, workability and
corrosion resistance in the weld zone noticeably deteriorate. A N
content as low as possible is preferable from the perspective of
corrosion resistance, but reducing the N content to less than
0.001% requires long-time refining, which results in an increase in
the manufacturing cost and a deterioration in productivity, and is
therefore not preferable. Accordingly, the N content is within the
range of 0.001 to 0.020%. The N content is preferably not less than
0.003% and more preferably not less than 0.005%. Furthermore, the N
content is preferably not more than 0.015% and more preferably not
more than 0.012%.
[0049] Cr: 11.0 to 24.0%
[0050] Cr is the most important element for ensuring the corrosion
resistance of stainless steel. If the Cr content is less than
11.0%, sufficient corrosion resistance is not exhibited in an
automobile exhaust gas atmosphere. On the other hand, if Cr is
present in an amount more than 24.0%, toughness significantly
deteriorates because of formation of the .sigma. (sigma) phase,
and, in accordance with aspects of the present invention, the
predetermined threshold stress intensity factor cannot be achieved.
Accordingly, the Cr content is within the range of 11.0 to 24.0%.
The Cr content is preferably not less than 13.0%, more preferably
not less than 14.0%, even more preferably not less than 16.0%, and
still more preferably not less than 17.0%. Furthermore, the Cr
content is preferably not more than 21.5%, more preferably not more
than 20.0%, and even more preferably not more than 18.5%.
[0051] Ni: 0.01 to 2.00%
[0052] Ni is an element that improves the corrosion resistance of
stainless steel and an element that suppresses the progression of
corrosion in a corrosive environment in which the passivation film
is not formed and active dissolution occurs. Furthermore, Ni is a
strong austenite-forming element and has an effect of reducing the
formation of ferrite in a weld, thereby suppressing sensitization
due to precipitation of Cr carbonitrides. This effect is obtained
when Ni is contained in an amount of 0.01% or more. The effect
increases as the Ni content increases. If the Ni content is more
than 2.00%, however, workability deteriorates, and in addition,
stress corrosion cracking tends to occur. In addition, since Ni is
an expensive element, increasing the Ni content increases the
manufacturing cost and is therefore not preferable. Accordingly,
the Ni content is 0.01 to 2.00%. The Ni content is preferably not
less than 0.05% and more preferably not less than 0.10%.
Furthermore, the Ni content is preferably not more than 1.00%, more
preferably not more than 0.60%, even more preferably not more than
0.50%, and still more preferably not more than 0.45%.
[0053] Nb: 0.12 to 0.80%
[0054] In a hot rolling process, Nb combines with C and N and
precipitates as Nb carbonitrides. Precipitated Nb carbonitrides
have an effect of pinning the movement of dislocations and thus
preventing rolling strains introduced by hot rolling from being
eliminated through recovery. Consequently, recovery during hot
rolling is retarded, and a decrease in the rolling strain density
due to the occurrence of excessive recovery can be prevented. The
effect described above is produced when Nb is contained in an
amount of 0.12% or more. However, if the Nb content is more than
0.80%, toughness, on the contrary, may deteriorate because of the
formation of a Laves phase, and a significant increase in the
rolling load in hot rolling makes it difficult to employ the method
of hot rolling provided according to aspects of the present
invention. Accordingly, the Nb content is within the range of 0.12
to 0.80%. The Nb content is preferably not less than 0.15% and more
preferably not less than 0.20%. Furthermore, the Nb content is
preferably not more than 0.75% and more preferably not more than
0.60%.
[0055] One aspect of the present invention is a ferritic stainless
steel containing the above-described essential elements, with the
balance being Fe and incidental impurities. In addition,
optionally, one or more selected from Cu, Mo, W, and Co may be
contained, or alternatively or additionally, one or more selected
from Ti, V, Zr, REM, B, Mg, and Ca may be contained. The ranges of
the contents are as follows.
[0056] Cu: 0.01 to 1.50%
[0057] Cu is an element particularly effective for improving the
corrosion resistance of the base metal and a weld when it is in an
aqueous solution or when drops of weakly acidic water adhere
thereto. This effect is obtained when Cu is contained in an amount
of 0.01% or more. The effect increases as the Cu content increases.
If Cu is contained in an amount more than 1.50%, however, a
deterioration in hot workability may induce surface defects. In
addition, in some cases, descaling after annealing is difficult.
Accordingly, when Cu is to be contained, it is preferable that the
Cu content be within the range of 0.01 to 1.50%. The Cu content is
more preferably not less than 0.10% and even more preferably not
less than 0.30%. Furthermore, the Cu content is more preferably not
more than 0.60% and even more preferably not more than 0.45%.
[0058] Mo: 0.01 to 2.00%
[0059] Mo is an element that noticeably improves the corrosion
resistance of stainless steel. This effect is obtained when Mo is
contained in an amount of 0.01% or more. The effect improves as the
content increases. If the Mo content is more than 2.00%, however,
an increase in the rolling load in hot rolling may deteriorate
manufacturability, and the strength of the steel sheet may increase
excessively. Furthermore, since Mo is an expensive element, the
presence of Mo in large amounts increases the manufacturing cost.
Accordingly, when Mo is to be contained, it is preferable that the
Mo content be 0.01 to 2.00%. It is more preferable that the Mo
content not be less than 0.10%. Furthermore, it is more preferable
that the Mo content not be more than 1.40%. However, in a
Ti-containing steel, Mo also has an effect of deteriorating
toughness, and it is therefore preferable, when Ti is contained in
an amount of 0.15% or more, that the Mo content be 0.30 to not more
than 1.40%. When Ti is contained in an amount of 0.15% or more, it
is more preferable that the Mo content not be less than 0.40%.
Furthermore, when Ti is contained in an amount of 0.15% or more, it
is more preferable that the Mo content not be more than 0.90%.
[0060] W: 0.01 to 0.20%
[0061] Similarly to Mo, W has an effect of improving corrosion
resistance. This effect is obtained when W is contained in an
amount of 0.01% or more. If W is contained in an amount more than
0.20%, however, strength increases, and an increase in rolling
load, for example, may deteriorate manufacturability. Accordingly,
when W is to be contained, it is preferable that the W content be
within the range of 0.01 to 0.20%. It is more preferable that the W
content not be less than 0.05%. Furthermore, it is more preferable
that the W content not be more than 0.15%.
[0062] Co: 0.01 to 0.20%
[0063] Co is an element that improves toughness. This effect is
obtained when Co is contained in an amount of 0.01% or more. On the
other hand, if the Co content is more than 0.20%, workability may
deteriorate. Accordingly, when Co is to be contained, it is
preferable that the Co content be within the range of 0.01 to
0.20%.
[0064] Ti: 0.01 to 0.30%
[0065] Ti is an element having a higher affinity for C and N than
does Cr and has an effect of, by precipitating as carbides or
nitrides, suppressing sensitization caused by precipitation of Cr
carbonitrides. To obtain this effect, Ti needs to be contained in
an amount of 0.01% or more. If the Ti content is more than 0.30%,
however, TiN may precipitate excessively, and consequently, good
surface properties may not be obtained. Accordingly, when Ti is to
be contained, it is preferable that the Ti content be within the
range of 0.01 to 0.30%. The Ti content is more preferably not less
than 0.03% and even more preferably not less than 0.10%.
Furthermore, the Ti content is more preferably not more than 0.20%
and even more preferably not more than 0.15%.
[0066] V: 0.01 to 0.20%
[0067] V forms carbonitrides with C and N and suppresses
sensitization during welding and thus improves corrosion resistance
in the weld zone. This effect is obtained when the V content is
0.01% or more. On the other hand, if the V content is more than
0.20%, workability and toughness may noticeably deteriorate.
Accordingly, it is preferable that the V content be 0.01 to 0.20%.
It is more preferable that the V content not be less than 0.05%.
Furthermore, it is more preferable that the V content not be more
than 0.15%.
[0068] Zr: 0.01 to 0.20%
[0069] Zr has an effect of, by combining with C and N, suppressing
sensitization. This effect is obtained when Zr is contained in an
amount of 0.01% or more. On the other hand, if Zr is contained in
an amount more than 0.20%, workability may noticeably deteriorate.
Accordingly, when Zr is to be contained, it is preferable that the
Zr content be within the range of 0.01 to 0.20%. It is more
preferable that the Zr content not be more than 0.10%.
[0070] REM: 0.001 to 0.100%
[0071] REM (rare earth metal) has an effect of improving oxidation
resistance and inhibits the formation of a weld oxide layer
(welding temper color) and thus suppresses the formation of a Cr
depletion zone immediately below the oxide layer. This effect is
obtained when one or more REM is contained in an amount of 0.001%
or more. On the other hand, if one or more REM is contained in an
amount more than 0.100%, manufacturability, such as pickling
properties for cold rolling-annealing, may deteriorate.
Accordingly, when one or more REM is to be contained, it is
preferable that the REM content be within the range of 0.001 to
0.100%. It is more preferable that the REM content not be more than
0.050%.
[0072] B: 0.0002 to 0.0025%
[0073] B is an element effective for improving secondary working
embrittlement resistance after deep drawing. This effect is
obtained when the B content is 0.0002% or more. On the other hand,
if B is contained in an amount more than 0.0025%, workability and
toughness may deteriorate. Accordingly, when B is to be contained,
it is preferable that the B content be within the range of 0.0002
to 0.0025%. It is more preferable that the B content not be less
than 0.0003%. Furthermore, it is more preferable that the B content
not be more than 0.0006%.
[0074] Mg: 0.0005 to 0.0030%
[0075] Mg is an element that improves the equiaxed crystal ratio of
a slab and is effective for improving workability and toughness.
This effect is obtained when Mg is contained in an amount of
0.0005% or more. On the other hand, if the Mg content is more than
0.0030%, surface properties of the steel may deteriorate.
Accordingly, when Mg is to be contained, it is preferable that the
Mg content be within the range of 0.0005 to 0.0030%. It is more
preferable that the Mg content not be less than 0.0010%.
Furthermore, it is more preferable that the Mg content not be more
than 0.0020%.
[0076] Ca: 0.0005 to 0.0030%
[0077] Ca has an effect of refining inclusions formed during
steelmaking and continuous casting and is a component particularly
effective for preventing nozzle blockage in continuous casting. The
effect is obtained when Ca is contained in an amount of 0.0005% or
more. If Ca is contained in an amount more than 0.0030%, however,
formation of CaS may decrease corrosion resistance. Accordingly,
when Ca is to be contained, it is preferable that the Ca content be
within the range of 0.0005 to 0.0030%. The Ca content is more
preferably not more than 0.0015% and even more preferably not more
than 0.0010%.
[0078] Threshold stress intensity factor K.sub.IC: 25 MPam.sup.1/2
or more Hot-rolled ferritic stainless steel sheets according to
aspects of the present invention have a threshold stress intensity
factor K.sub.IC of 25 MPam.sup.1/2 or more, and as a result, when
subjected to blanking in a crank press to be formed into a thick
flange, the occurrence of cracking can be prevented. The threshold
stress intensity factor K.sub.IC is preferably not less than 30
MPam.sup.1/2, more preferably not less than 35 MPam.sup.1/2, and
even more preferably not less than 40 MPam.sup.1/2. The "thick
flange" is not particularly limited but may be, for example, a
flange of 5.0 mm or more sheet thickness. For example, the flange
is preferably a flange of 5.0 to 15.0 mm sheet thickness and more
preferably a flange of 5.0 to 10.0 mm sheet thickness.
[0079] A method for manufacturing the hot-rolled ferritic stainless
steel sheet according to aspects of the present invention will now
be described. In the following description, the temperature is the
surface temperature of a steel slab, a hot-rolled steel sheet, or
the like measured with a surface thermometer or the like unless
otherwise specified.
[0080] The hot-rolled ferritic stainless steel sheet according to
aspects of the present invention can be obtained by using a steel
slab having a chemical composition as described above and ensuring
that, in hot rolling including rough rolling and finish rolling
that includes three or more passes, the temperature range for
rolling in the final three passes of the finish rolling is 800 to
1100.degree. C. and the accumulated rolling reduction ratio for the
final three passes is 25% or more.
[0081] First, a molten steel with a chemical composition as
described above is prepared using a steelmaking process known in
the art, using, for example, a converter, an electric furnace, or a
vacuum melting furnace. The steel is subjected to a continuous
casting process or an ingot casting-slabbing process to form a
steel starting material (slab).
[0082] The slab is subjected to hot rolling after being heated at
1100 to 1250.degree. C. for 1 to 24 hours, or the as-cast slab is
directly subjected to hot rolling without being heated. In
accordance with aspects of the present invention, although the
rough rolling is not particularly limited, effectively destroying
the casting structure prior to the finish hot rolling provides an
advantage for grain refining in the subsequent finish hot rolling,
and therefore, further improvement in toughness due to refinement
of the metallurgical structure after hot rolling can be expected,
and accordingly, it is preferable that the accumulated rolling
reduction ratio for the rough rolling not be less than 65%.
Subsequently, rolling is performed in the finish hot rolling to
obtain a predetermined sheet thickness. In the final three passes
of the finish rolling, rolling is performed within the temperature
range of 800 to 1100.degree. C. and at the accumulated rolling
reduction ratio of 25% or more.
[0083] Rolling temperature range for final three passes of finish
hot rolling: 800 to 1100.degree. C.
[0084] Accumulated rolling reduction ratio for final three passes
of finish hot rolling: 25% or more
[0085] To ensure that the threshold stress intensity factor after
hot rolling is the predetermined threshold stress intensity factor,
it is necessary to appropriately control the temperature and the
accumulated rolling reduction ratio for rolling in the final three
passes of the finish hot rolling, thereby effectively introducing
rolling strains also to the central part in the sheet thickness
while suppressing excessive recovery during rolling.
[0086] To introduce sufficient rolling strains also to the central
part in the sheet thickness, it is necessary that, in the finish
hot rolling, the rolling temperatures for the final three passes be
within the range of 800 to 1100.degree. C. and the accumulated
rolling reduction ratio (=100-(final sheet thickness/sheet
thickness before start of rolling in final three
passes).times.100[%]) for the final three passes be 25% or more, so
that rolling strains can be effectively introduced also to the
central part in the sheet thickness while preventing the rolling
strains introduced in the final three passes from being eliminated
through recovery.
[0087] If the accumulated rolling reduction ratio for the final
three passes of the finish hot rolling is less than 25%, rolling
strains are not effectively introduced to the central part in the
sheet thickness, and as a result, the predetermined threshold
stress intensity factor cannot be achieved. Accordingly, the
accumulated rolling reduction ratio for the final three passes is
25% or more. It is preferable that the accumulated rolling
reduction ratio not be less than 30%. It is more preferable that
the accumulated rolling reduction ratio not be less than 35%. The
upper limit of the accumulated rolling reduction ratio is not
particularly limited but is preferably not more than 60% because,
if the accumulated rolling reduction ratio is excessively high, an
increase in the rolling load may deteriorate manufacturability.
[0088] If the rolling temperatures for the final three passes of
the finish hot rolling are lower than 800.degree. C., the rolling
load significantly increases with a decrease in the temperature of
the steel sheet, and therefore such temperatures are not preferable
from the standpoint of manufacturing. On the other hand, if the
rolling temperatures for the final three passes are higher than
1100.degree. C., rolling strains introduced by the rolling are
eliminated through excessive recovery, and the predetermined
threshold stress intensity factor cannot be achieved. Accordingly,
the rolling temperatures for the final three passes are within the
range of 800 to 1100.degree. C. It is preferable that the rolling
temperatures for the final three passes be within a range of 800 to
1050.degree. C. It is more preferable that the rolling temperatures
for the final three passes be within a range of 850 to 1000.degree.
C.
[0089] To prevent an excessive rolling load from being applied in a
specific pass of the final three passes of the finish hot rolling,
it is preferable that, in the final three passes, the rolling
temperature range for the first pass be 950 to 1100.degree. C., the
rolling temperature range for the second pass, which follows the
first pass, be 925 to 1075.degree. C., and the rolling temperature
range for the third pass, which follows the second pass, be 875 to
1050.degree. C.
[0090] In the method for manufacturing the hot-rolled ferritic
stainless steel sheet according to aspects of the present
invention, in the final three passes of finish hot rolling
including three or more passes, high-reduction rolling is applied
with the temperature range being controlled. If rolling in which
high-reduction rolling is applied is performed over the final four
or more passes, strains cannot be sufficiently introduced to the
central part in the sheet thickness even when the accumulated
rolling reduction ratio is unchanged because the reduction ratio is
distributed among the passes, and in addition, since the
accumulated interval time between passes increases, recovery during
transfer between passes is promoted, which reduces the strain
introducing effect, and as a result, achieving the predetermined
threshold stress intensity factor is difficult. On the other hand,
it is not preferable that the control of the rolling temperature
and the accumulated rolling reduction ratio of the finish rolling
be performed in the final two or less passes because, in such a
case, since the high-reduction rolling at the accumulated rolling
reduction ratio of 25% or more is performed in two passes, the
rolling load significantly increases, which may decrease
manufacturability. Accordingly, in the method for manufacturing the
hot-rolled ferritic stainless steel sheet according to aspects of
the present invention, the rolling temperature and the accumulated
rolling reduction ratio of the final three passes of the finish
rolling are controlled.
[0091] In the method for manufacturing the hot-rolled ferritic
stainless steel sheet according to aspects of the present
invention, it is important to control the rolling temperature and
the accumulated rolling reduction ratio of the final three passes
of the finish hot rolling, and any number of passes may be
performed in the finish hot rolling provided that the finish hot
rolling includes three or more passes; however, it is preferable
that the maximum number of passes not be more than 15 passes
because, if the maximum number of passes is more than 15 passes, a
decrease in the temperature of the steel sheet due to increased
number of contact with the rolling rolls is more likely to occur,
and consequently, for example, heating of the steel sheet from the
outside becomes necessary to maintain the temperature to be within
the predetermined temperature range, and as a result,
manufacturability may deteriorate or the manufacturing cost may
increase. It is more preferable that the maximum number of passes
not be more than 10 passes.
[0092] After the finish hot rolling, the steel sheet is cooled, and
subsequently the steel sheet is coiled to form a hot-rolled steel
strip. In accordance with aspects of the present invention, the
coiling temperature is not particularly limited, but, if the
coiling temperature is between higher than 450.degree. C. and lower
than 500.degree. C., embrittlement due to 475.degree. C.
embrittlement may occur. Accordingly, it is preferable that the
coiling temperature not be higher than 450.degree. C. or not be
lower than 500.degree. C. Coiling may be performed at a temperature
not higher than 450.degree. C. after accelerated cooling using
water mist cooling or the like is performed following the final
rolling. This is more preferable because elimination of rolling
strains due to recovery after coiling can be further
suppressed.
[0093] The hot-rolled steel sheet obtained in accordance with
aspects of the present invention may be subjected to
hot-rolled-sheet annealing to form a hot-rolled and annealed steel
sheet. The hot-rolled steel sheet provided in accordance with
aspects of the present invention has excellent toughness and
therefore can be subjected to hot-rolled-sheet annealing in a
continuous annealing line, which is avoided in the related art
because of concern over failure due to low toughness. Furthermore,
the resulting hot-rolled and annealed steel sheet may be
subsequently subjected to cold rolling and cold-rolled-sheet
annealing.
EXAMPLE
[0094] Aspects of the present invention will now be described in
more detail with reference to examples.
[0095] Molten stainless steels each having the chemical composition
shown in Table 1 were prepared by steelmaking using a 150-ton
capacity converter and involving refining using a strong-stirring
vacuum oxygen decarburization process (SS-VOD). The molten
stainless steel was continuously cast to form a steel slab of 1000
mm width and 200 mm thickness. The slab was heated at 1200.degree.
C. for 1 hour and thereafter subjected to hot rolling. In the hot
rolling, the slab was subjected to reverse rough rolling using
three stands to form a steel sheet of approximately 40 mm in
thickness, which was subsequently subjected to finish rolling
including seven passes to form a hot-rolled steel sheet. The final
three passes (fifth pass, sixth pass, and seventh pass) of the
finish rolling were performed under the conditions listed in Table
2.
[0096] The resulting hot-rolled steel sheet was evaluated as
follows.
[0097] (1) Evaluation of Threshold Stress Intensity Factor
K.sub.IC
[0098] A CT test piece in accordance with ASTM E399 was cut from a
sheet width central portion in such a manner that a fatigue
pre-crack was introduced in a direction perpendicular to the
rolling direction and the stress axis was in a direction parallel
to the rolling direction. The threshold stress intensity factor
K.sub.IC of the test piece was determined in accordance with ASTM
E399. Threshold stress intensity factors equal to or more than 25
MPam.sup.1/2 were rated as "pass", and threshold stress intensity
factors less than 25 MPam.sup.1/2 were rated as "fail".
[0099] (2) Evaluation of Corrosion Resistance
[0100] A test piece of 60.times.100 mm was cut from the resulting
hot-rolled steel sheet. The surface to be evaluated was polished
with #600 emery paper, and thereafter the edge surface were sealed
to prepare such a test piece, which was subjected to a cyclic salt
spray test as specified in JIS H 8502. In the cyclic salt spray
test, five cycles were conducted, with one cycle being as follows:
salt spraying (5-mass % NaCl, 35.degree. C., 2-hour
spraying).fwdarw.drying (60.degree. C., 4 hours, relative humidity
of 40%).fwdarw.exposure to humidity (50.degree. C., 2 hours,
relative humidity.gtoreq.95%). The evaluation surface of the test
piece after conducting five cycles of the cyclic salt spray test
was photographed, the rust area of the evaluation surface of the
test piece was measured by image analysis, and, from the ratio of
the rust area to the total area of the test piece, the rust ratio
(rust area of test piece/total area of test piece).times.100 [%])
was calculated. Rust ratios equal to or less than 10% were rated as
"pass" (.circle-w/dot.), as being particularly good in corrosion
resistance, rust ratios more than 10% and not more than 25% were
rated as "pass" (.largecircle.), and rust ratios more than 25% were
rated as "fail" (x).
[0101] The test results together with the hot rolling conditions
are shown in Table 2.
TABLE-US-00001 TABLE 1 Chemical composition (mass %) No. C Si Mn P
S Al N Cr Ni Nb Others Notes A 0.005 0.11 0.29 0.03 0.005 0.004
0.007 16.4 0.12 0.23 -- Invention example B 0.013 0.24 0.46 0.02
0.006 0.003 0.015 17.3 0.14 0.35 Mo: 0.46 Invention example C 0.012
0.30 0.44 0.02 0.003 0.002 0.013 17.6 0.12 0.41 -- Invention
example D 0.005 0.27 0.17 0.03 0.002 0.016 0.008 18.9 0.20 0.34 Mo:
1.91 Invention example E 0.017 0.58 0.14 0.03 0.004 0.004 0.018
19.5 0.44 0.46 Cu: 0.54, V: 0.12 Invention example F 0.005 0.39
0.18 0.02 0.001 0.009 0.006 22.6 0.16 0.29 Mo: 1.14, B: 0.0009
Invention example G 0.009 0.90 0.36 0.03 0.002 0.014 0.012 14.5
0.23 0.50 Mo: 0.05, Co: 0.04 Invention example H 0.007 0.81 0.23
0.03 0.001 0.377 0.007 16.9 0.18 0.47 Cu: 1.22 Invention example I
0.004 0.33 0.21 0.02 0.003 0.026 0.006 14.7 0.36 0.52 Mo: 1.53, Co:
0.04 Invention example J 0.012 0.19 0.20 0.02 0.004 0.039 0.015
15.2 0.11 0.36 Zr: 0.17 Invention example K 0.018 0.13 0.20 0.01
0.006 0.023 0.006 15.2 0.26 0.31 W: 0.15 Invention example L 0.006
0.11 0.25 0.02 0.007 0.016 0.012 16.3 0.12 0.35 Ti: 0.14 Invention
example M 0.018 0.16 0.21 0.02 0.005 0.046 0.014 23.8 0.13 0.39 --
Invention example N 0.016 0.19 0.27 0.01 0.003 0.023 0.009 21.4
0.16 0.37 REM: 0.07 Invention example O 0.007 0.19 0.18 0.03 0.002
0.033 0.017 22.0 0.14 0.38 Ca: 0.0009, Mg: 0.0028 Invention example
P 0.014 0.20 0.26 0.03 0.003 0.024 0.007 13.3 0.19 0.76 --
Invention example Q 0.011 0.17 0.21 0.04 0.004 0.020 0.013 15.2
0.20 0.12 -- Invention example R 0.019 0.14 0.27 0.03 0.004 0.023
0.006 18.3 0.11 0.83 -- Comparative example S 0.013 0.15 0.31 0.04
0.005 0.032 0.014 18.4 0.17 0.08 -- Comparative example T 0.010
0.22 0.36 0.03 0.003 0.037 0.012 17.9 0.13 0.26 Cu: 0.33 Invention
example U 0.008 0.19 0.33 0.03 0.004 0.008 0.009 11.2 0.54 0.24 --
Invention example V 0.009 0.17 0.32 0.04 0.005 0.007 0.010 16.3
0.98 0.25 -- Invention example W 0.007 0.21 0.30 0.02 0.002 0.005
0.008 16.5 1.83 0.25 -- Invention example X 0.011 0.18 0.31 0.04
0.004 0.006 0.012 11.4 1.98 0.23 -- Invention example Y 0.009 0.20
0.31 0.02 0.003 0.008 0.009 9.8 0.15 0.23 -- Comparative example Z
0.012 0.19 0.32 0.03 0.004 0.009 0.013 24.6 0.23 0.26 --
Comparative example Balance, other than above components, is Fe and
incidental impurities. Underline indicates the value is outside
range of present invention.
TABLE-US-00002 TABLE 2 Rough 5th pass 5th 6th 7th 7th Accumulated
rolling start pass pass pass pass rolling end sheet start start end
end reduction sheet thick- temper- temper- temper- sheet ratio for
K.sup.IC Steel thickness ness ature ature ature thickness final 3
[MPa Corrosion No. symbol [mm] [mm] [.degree. C.] [.degree. C.]
[.degree. C.] [mm] passes [%] m.sup.1/2] resistance Notes 1 A 40.6
14.2 969 932 895 9.8 31 34 .largecircle. Invention example 2 B 40.1
14.8 985 944 902 10.4 30 34 .circleincircle. Invention example 3 C
40.9 14.3 966 927 890 10.3 28 31 .largecircle. Invention example 4
D 40.6 14.2 989 941 908 9.8 31 29 .circleincircle. Invention
example 5 E 40.2 15.4 977 941 903 10.4 32 30 .circleincircle.
Invention example 6 F 40.5 15.9 969 930 890 10.2 36 29
.circleincircle. Invention example 7 G 39.8 14.5 985 941 903 10.3
29 32 .circleincircle. Invention example 8 H 40.3 14.5 998 948 907
10.3 29 32 .circleincircle. Invention example 9 I 40.8 15.1 979 939
904 10.2 32 36 .circleincircle. Invention example 10 J 39.9 14.7
983 937 899 10.5 29 34 .largecircle. Invention example 11 K 40.4
15.8 980 941 901 10.5 34 33 .largecircle. Invention example 12 L
40.7 14.5 983 936 897 9.8 32 28 .largecircle. Invention example 13
M 40.8 15.4 984 947 911 10.1 34 29 .circleincircle. Invention
example 14 N 40.7 15.0 972 934 900 9.9 34 28 .circleincircle.
Invention example 15 O 40.2 15.2 956 920 889 10.2 33 36
.circleincircle. Invention example 16 P 39.9 14.7 972 924 891 10.1
31 35 .largecircle. Invention example 17 Q 39.9 14.8 971 923 888
10.3 30 32 .largecircle. Invention example 18 O 40.2 9.4 964 921
888 5.3 44 34 .largecircle. Invention example 19 A 40.6 11.4 975
935 899 8.4 26 28 .largecircle. Invention example 20 C 39.8 19.3
990 947 909 13.0 33 34 .largecircle. Invention example 21 C 40.5
16.1 1097 1072 1048 10.2 37 33 .largecircle. Invention example 22 C
39.8 15.8 953 925 877 10.3 35 36 .largecircle. Invention example 23
C 40.2 14.0 1113 1072 1043 10.4 26 18 .largecircle. Comparative
example 24 F 40.4 12.7 986 948 907 10.4 18 17 .circleincircle.
Comparative example 25 F 40.4 17.7 1134 1101 1079 12.6 29 14
.circleincircle. Comparative example 26 F 39.9 17.9 794 765 Not
evaluated because excessive load resulted in incomplete rolling
Comparative example 27 R 40.7 17.5 983 941 908 12.6 28 15
.largecircle. Comparative example 28 S 40.4 17.2 982 944 906 12.3
28 18 .largecircle. Comparative example 29 T 40.7 14.7 959 933 894
10.3 30 32 .largecircle. Invention example 30 U 40.5 14.6 971 939
903 10.2 30 38 .largecircle. Invention example 31 V 40.6 14.4 979
946 905 9.9 31 31 .largecircle. Invention example 32 W 40.5 14.5
983 937 899 10.4 28 35 .largecircle. Invention example 33 X 40.8
14.7 976 943 902 10.0 32 43 .largecircle. Invention example 34 Y
40.7 14.6 972 929 888 10.2 30 31 X Comparative example 35 Z 40.7
14.6 984 948 895 10.1 31 16 .circleincircle. Comparative example
Underline indicates the value is outside range of present
invention.
[0102] In Nos. 1 to 22 and 29 to 33, in each of which the steel
composition and the hot rolling conditions satisfied the ranges of
the present invention, rolling strains were sufficiently introduced
into the steel sheet by the predetermined hot rolling, and as a
result, the predetermined threshold stress intensity factor was
achieved. In addition, the corrosion resistance of each of the
resulting hot-rolled steel sheets was evaluated, and as a result,
it was found that all of the hot-rolled steel sheets had a rust
ratio not more than 25% and therefore had sufficient corrosion
resistance.
[0103] In particular, in Nos. 2, 4, 6, 7, and 9, in which
Mo-containing steels B, D, F, G, and I were used, in Nos. 5 and 8,
in which Cu-containing steels E and H were used, and in Nos. 13 to
15, in which high-Cr-content steels M, N, and O were used, higher
corrosion resistance was achieved, each with a rust ratio not more
than 10%.
[0104] In No. 23, in which the rolling temperature of the fifth
pass (third pass from the final pass) was above the range of the
present invention, and in No. 25, in which the rolling temperatures
of the fifth pass and the sixth pass (second pass form the final
pass) were above the range of the present invention, rolling was
performed at a predetermined accumulated rolling reduction ratio,
but, because of the excessively high rolling temperature, excessive
recovery of work strains introduced by rolling occurred, and as a
result, the threshold stress intensity factor after hot rolling was
not the predetermined threshold stress intensity factor. In No. 24,
in which the accumulated rolling reduction ratio for the final
three passes was below the range of the present invention, rolling
strains were not sufficiently introduced, and as a result, the
threshold stress intensity factor after hot rolling was not the
predetermined threshold stress intensity factor.
[0105] In No. 26, in which the rolling temperatures of the fifth
pass and the sixth pass were below the range of the present
invention, because of the excessively low rolling temperatures, the
rolling load significantly increased, and, while rolling was
performed in the final seventh pass, the load exceeded the
acceptable range of the machine, and as a result, rolling could not
be completed, and the predetermined evaluation could not be
conducted.
[0106] In No. 27, in which steel R, which had a Nb content above
the range of the present invention, was used, toughness
significantly deteriorated because of precipitation of Laves phase
during hot rolling, and therefore the predetermined threshold
stress intensity factor was not achieved.
[0107] In No. 28, in which steel S, which had a Nb content below
the range of the present invention, was used, excessive recovery
occurred during hot rolling because the amount of Nb carbonitrides
precipitated was insufficient, and therefore the predetermined
threshold stress intensity factor was not achieved.
[0108] In No. 34, in which steel Y, which had a Cr content below
the range of the present invention, was used, desired corrosion
resistance was not achieved because of the insufficient Cr
content.
[0109] In No. 35, in which steel Z, which had a Cr content above
the range of the present invention, was used, .sigma. phase
precipitated because of the excessive Cr content, and as a result,
toughness significantly deteriorated, and the predetermined
threshold stress intensity factor was not achieved.
INDUSTRIAL APPLICABILITY
[0110] Hot-rolled ferritic stainless steel sheets that can be
obtained in accordance with aspects of the present invention have,
in particular, excellent blanking workability for crank press and
are, in particular, suitable for use in, for example, forming a
thick flange that is manufactured by, for example, blanking using a
crank press or another technique and which needs to have high
workability and corrosion resistance.
* * * * *