U.S. patent application number 14/355117 was filed with the patent office on 2014-10-02 for hot-rolled ferritic stainless steel sheet with excellent cold cracking resistance and manufacturing process therefor.
The applicant listed for this patent is Nippon Steel & Sumikin Stainless Steel Corporation. Invention is credited to Shigeyuki Gotoh, Junichi Hamada, Ken Kimura, Yuuji Koyama, Jun Takahashi.
Application Number | 20140294660 14/355117 |
Document ID | / |
Family ID | 48574360 |
Filed Date | 2014-10-02 |
United States Patent
Application |
20140294660 |
Kind Code |
A1 |
Kimura; Ken ; et
al. |
October 2, 2014 |
HOT-ROLLED FERRITIC STAINLESS STEEL SHEET WITH EXCELLENT COLD
CRACKING RESISTANCE AND MANUFACTURING PROCESS THEREFOR
Abstract
This hot-rolled ferritic stainless steel sheet contains, in
terms of % by mass: 0.0150% or less of C, 0.01% to 2.00% of Si,
0.01% to 2.00% of Mn, less than 0.040% of P, 0.010% or less of S,
10.0% to 30.0% of Cr, 0.001% to 0.100% of Al, and 0.0200% or less
of N, with a balance being Fe and unavoidable impurities, wherein
in a cross section in a range of 1/4 to 3/4 of a sheet thickness, a
length L of all crystal grain boundaries having orientation
differences of 1.degree. or more to less than 180.degree. and a
length La of subgrain boundaries having orientation differences of
1.degree. or more to less than 15.degree. satisfy a relation of
La/L.gtoreq.0.20.
Inventors: |
Kimura; Ken; (Kimitsu-shi,
JP) ; Hamada; Junichi; (Hikari-shi, JP) ;
Takahashi; Jun; (Kisarazu-shi, JP) ; Koyama;
Yuuji; (Kitakyushu-shi, JP) ; Gotoh; Shigeyuki;
(Kitakyushu-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Nippon Steel & Sumikin Stainless Steel Corporation |
Tokyo |
|
JP |
|
|
Family ID: |
48574360 |
Appl. No.: |
14/355117 |
Filed: |
December 6, 2012 |
PCT Filed: |
December 6, 2012 |
PCT NO: |
PCT/JP2012/081693 |
371 Date: |
April 29, 2014 |
Current U.S.
Class: |
420/60 ;
164/76.1; 420/34; 420/62; 420/64; 420/70 |
Current CPC
Class: |
C21D 8/0426 20130101;
C22C 38/26 20130101; C22C 38/42 20130101; C21D 9/46 20130101; C22C
38/22 20130101; C22C 38/001 20130101; C22C 38/02 20130101; C22C
38/32 20130101; C21D 6/002 20130101; C21D 8/0463 20130101; C22C
38/60 20130101; C22C 38/44 20130101; C22C 38/06 20130101; C22C
38/008 20130101; C21D 2211/005 20130101; C22C 38/40 20130101; C22C
38/004 20130101; C22C 38/54 20130101; C22C 38/50 20130101; B21B
3/02 20130101; C22C 38/28 20130101; C22C 38/48 20130101; C22C 38/20
20130101; C22C 38/04 20130101 |
Class at
Publication: |
420/60 ;
164/76.1; 420/62; 420/34; 420/64; 420/70 |
International
Class: |
C22C 38/54 20060101
C22C038/54; 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; B21B 3/02 20060101
B21B003/02 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 9, 2011 |
JP |
2011-270092 |
Claims
1. A hot-rolled ferritic stainless steel sheet with excellent cold
cracking resistance, comprising, in terms of % by mass: 0.0150% or
less of C; 0.01% to 2.00% of Si; 0.01% to 2.00% of Mn; less than
0.040% of P; 0.010% or less of S; 10.0% to 30.0% of Cr; 0.001% to
3.00% ofAl; and 0.0200% or less of N, with a balance being Fe and
unavoidable impurities, wherein in a cross section in a range of
1/4 to 3/4 of a sheet thickness, a length L of all crystal grain
boundaries having orientation differences of 1.degree. or more to
less than 180.degree. and a length La of subgrain boundaries having
orientation differences of 1.degree. or more to less than
15.degree. satisfy a relation of La/L.gtoreq.0.20.
2. The hot-rolled ferritic stainless steel sheet with excellent
cold cracking resistance according to claim 1, which further
comprises one or more selected from, in terms of % by mass: 0.05%
to 0.70% of Nb; 0.05% to 0.30% of Ti; 0.1% to 2.5% of Mo; 0.1% to
1.5% of Ni; 0.0001% to 0.0025% of B; 0.1% to 2.0% of Cu; and 0.03%
to 0.35% of Sn, wherein in a case where either one or both of Nb
and Ti are included, the following formula (1) is satisfied,
Nb/93+Ti/48.gtoreq.C/12+N/14 (1) element symbols in the formula (1)
indicate contents of the respective elements in terms of % by
mass.
3. The hot-rolled ferritic stainless steel sheet with excellent
cold cracking resistance according to claim 1 or 2, wherein the
content of Al is in a range of more than 0.10% to 3.00%.
4. A method for manufacturing the hot-rolled ferritic stainless
steel sheet with excellent cold cracking resistance according to
any one of claims 1 to 3, the method comprising: casting ferritic
stainless steel having the steel composition according to any one
of claims 1 to 3 to generate a semi-finished product and subjecting
the semi-finished product to hot rolling under a condition where a
finishing temperature is in a range of 800.degree. C. to
1000.degree. C. to generate a hot-rolled steel sheet; subsequently,
coiling the hot-rolled steel sheet in a coil shape at a temperature
of more than 650.degree. C. to 800.degree. C.; and immersing the
hot-rolled sheet coiled in a coil shape in a water tank within 1
hour after the coiling, maintaining the hot-rolled sheet in the
water tank for 1 hour or more, and taking out the hot-rolled steel
sheet.
Description
TECHNICAL FIELD
[0001] The present invention relates to a hot-rolled ferritic
stainless steel sheet with excellent cold cracking resistance and a
manufacturing method therefor.
[0002] The present application claims priority on Japanese Patent
Application No.
[0003] 2011-270092 filed on Dec. 9, 2011, the content of which is
incorporated herein by reference.
BACKGROUND ART
[0004] Ferritic stainless steel is used for a variety of
applications such as household electric appliances, building
materials, and automobile components. An appropriate amount of
various elements are added to this steel in the related art
according to required characteristics such as corrosion resistance,
high temperature characteristics, and the like.
[0005] It is known that the addition of Cr, Mo, or Ni is effective
for the purpose of improving corrosion resistance. Further, the
addition of Nb, Al, or Si is effective for improving high
temperature characteristics (strength and oxidation
resistance).
[0006] In general, as the addition amount of these elements
increase, the characteristics thereof are further improved;
however, manufacturability, particularly, cold cracking resistance
is degraded. For this reason, the upper limit of the addition
amount is determined.
[0007] The term "cold cracking resistance" means cracks generated
when the coil of a hot-rolled sheet (the hot-rolled sheet coiled in
a coil shape) is uncoiled and then the hot-rolled sheet is passed
through a continuous pickling line, a continuous annealing and
pickling line, a cold rolling line, and the like, and it is
considered that cracks are generated because of insufficient
toughness of the hot-rolled sheet.
[0008] With regard to steels of the ferritic stainless steel
containing many additive elements, cracks are easily generated in
the winter season where the temperature is low.
[0009] Patent Document 1 and Patent Document 2 are well-known as
solutions for improving toughness of hot-rolled sheets formed of
ferritic stainless steel with a large amount of Cr and stainless
steel to which Al is added.
[0010] Patent Document 1 discloses a technology of coiling a steel
sheet at a temperature of 400.degree. C. to 600.degree. C. after
completing finish hot rolling and then immediately rapid cooling at
a cooling rate of equal to or more than that of water cooling, as a
technology of improving the toughness of a hot-rolled sheet formed
of steel to which Cr is added at a content in a range of 25% by
weight to 35% by weight.
[0011] Patent Document 2 discloses a method of coiling a steel
sheet at a coiling temperature of 550.degree. C. to 650.degree. C.
to obtain a coiled steel strip and then immersing the coiled steel
strip in a water tank after 3 hours or less is passed from the
coiling of the steel sheet.
[0012] As described above, Patent Documents 1 and 2 disclose the
technologies as technologies of improving the toughness of the
hot-rolled steel. However, when the knowledge in the related art
was applied to various ferritic stainless steels by the present
inventors, there were cases that cold cracking was generated; and
therefore, it was understood that the knowledge was not necessarily
effective for the improvement of the toughness thereof. In other
words, the technologies in the related art are not sufficiently
effective and further improvement is required.
PRIOR ART DOCUMENTS
Patent Documents
[0013] Patent Document 1: Japanese Unexamined Patent Application,
First Publication No. H5-320764
[0014] Patent Document 2: Japanese Unexamined Patent Application,
First Publication No. 2001-26826
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0015] In light of the above problem, the present invention aims to
provide a hot-rolled ferritic stainless steel sheet with excellent
cold cracking resistance and a manufacturing method therefor.
Means for Solving the Problems
[0016] In order to solve the above-described problem, the present
inventors inspected the relationship between the conditions of
coiling a hot-rolled ferritic stainless steel sheet and the
toughness of the hot-rolled steel.
[0017] First, ferritic stainless steels whose components were
changed were hot-rolled down to a thickness of 5 mm in a laboratory
to obtain hot-rolled steel sheets. Subsequently, each of the
hot-rolled steel sheets was inserted into a furnace in which the
temperature therein was controlled to be a coiling temperature; and
thereby, a coiling treatment was simulated. The coiling temperature
(temperature inside the furnace) was changed in a range of
550.degree. C. to 950.degree. C. and the time of the coiling
treatment (heating in the furnace) was changed in a range of 0.1 h
to 100 h. Next, the steel sheets were cooled down to room
temperature by water cooling; and thereby, hot-rolled steel sheets
were produced.
[0018] Each of the obtained hot-rolled steel sheets was subjected
to a Charpy test and an impact value (toughness) at room
temperature (25.degree. C.) was evaluated.
[0019] In addition, the microstructure of each of the hot-rolled
steel sheets manufactured under the above-described various
conditions was inspected with an optical microscope and EBSP
(Electronic Back Scattering Pattern analyzing method). The
recrystallization state of the steel sheet was inspected with the
optical microscope. Additionally, presence of a subgrain boundary
in a crystal grain was inspected with the EBSP.
[0020] The measurement by the EBSP was performed by the method
described in the following embodiment. Specifically, samples for
measurement which had cross sections (L cross section) parallel to
a rolling direction and perpendicular to a sheet surface direction
were collected. The L cross section of the sample for measurement
was subjected to grinding using electrolytic grinding or colloidal
silica. In the L cross section, the range of 1/4 t to 3/4 t of the
sheet thickness t (1/4 to 3/4 of the sheet thickness) was used as a
measuring range. Among the measuring range, a crystal orientation
was measured in a measuring step (pitch) of 0.2 .mu.m in a range of
100 .mu.m.times.100 .mu.m. Determination of a crystal grain
boundary and a subgrain boundary was performed as follows. An
interface having an orientation difference of 1.degree. or more to
less than 180.degree. between adjacent measurement points was
regarded as a grain boundary. Among these, a grain boundary having
an orientation difference of 1.degree. or more to less than
15.degree. was regarded as a subgrain boundary. The obtained
knowledge is exemplified as follows.
[0021] (1) The Charpy impact value of the obtained hot-rolled steel
sheet was largely changed in a range of 5 J/cm.sup.2 to
approximately 100 J/cm.sup.2 depending on manufacturing
conditions.
[0022] (2) When the microstructure of the obtained hot-rolled steel
sheet was observed with an optical microscope, three structures
were recognized which were an unrecrystallized structure, a
complete recrystallized structure, and a mixed structure of
unrecrystallized structure and recrystallized structure are
recognized In the case of the complete recrystallized structure,
the Charpy impact value of the hot-rolled steel sheet was in a
range of less than 20 J/cm.sup.2. In the case of the
unrecrystallized grain and in the case of the mixed structure of
unrecrystallized structure and recrystallized structure, it was
recognized that the Charpy impact value was in a range of 20
J/cm.sup.2 or more in some cases.
[0023] (3) The total (length L of all crystal grain boundaries) of
the lengths of the crystal grain boundaries having orientation
differences of 1.degree. or more to less than 180.degree. and the
total (length La of subgrain boundaries) of the lengths of subgrain
boundaries having orientation differences of 1.degree. or more to
less than 15.degree. were obtained through the inspection of
crystal grain boundaries using EBSP. Then, a relationship between
the ratio of La/L and the Charpy impact value was obtained.
[0024] FIG. 1 illustrates the relationship of a toughness value
(Charpy impact value) and the ratio of La/L when the coiling
conditions (temperature and time) in various ferritic stainless
steels were changed. As shown in FIG. 1, the Charpy impact value is
in a range of 20 J/cm.sup.2 or more which is high when the ratio of
La/L is in a range of 0.20 or more, and the Charpy impact value is
in a range of less than 20 J/cm.sup.2 when the ratio of La/L is in
a range of less than 0.20.
[0025] In general, the crystal grain boundary indicates an
orientation difference between adjacent crystal grains. In the case
of the complete recrystallized structure, substantially all the
crystal grains at both sides interposing the crystal grain boundary
have orientation differences of 15.degree. or more. That is,
crystal grain boundaries having orientation differences of
1.degree. or more to less than 15.degree. are not present in the
complete recrystallized structure; and therefore, the ratio of La/L
becomes closer to 0.
[0026] In the present test, in the case where the coiling
temperature was 900.degree. C., the complete recrystallized
structure was obtained in any type of steels, and the Charpy impact
value was in a range of less than 20 J/cm.sup.2 in any cases. On
the other hand, in the case where the coiling temperature was in a
range of 800.degree. C. or less and the Charpy impact value was in
a range of 20 J/cm.sup.2 or more, unrecrystallized crystal grains
appeared to be largely present in the microstructure (optical
microscope structure), and subgrain boundaries were largely present
observed by the analysis of EBSP.
[0027] The present invention can be obtained based on this
knowledge and features of one aspect of the present invention are
as follows.
[0028] (1) A hot-rolled ferritic stainless steel sheet with
excellent cold cracking resistance contains, in terms of % by mass:
0.0150% or less of C; 0.01% to 2.00% of Si; 0.01% to 2.00% of Mn;
less than 0.040% of P; 0.010% or less of S; 10.0% to 30.0% of Cr;
0.001% to 3.00% of Al; and 0.0200% or less of N, with a balance
being Fe and unavoidable impurities, wherein in a cross section in
a range of 1/4 to 3/4 of a sheet thickness, a length L of all
crystal grain boundaries having orientation differences of
1.degree. or more to less than 180.degree. and a length La of
subgrain boundaries having orientation differences of 1.degree. or
more to less than 15.degree. satisfy a relation of
La/L.gtoreq.0.20.
[0029] (2) The hot-rolled ferritic stainless steel sheet with
excellent cold cracking resistance according to (1), further
contains one or more selected from, in terms of % by mass: 0.05% to
0.70% of Nb; 0.05% to 0.30% of Ti; 0.1% to 2.5% of Mo; 0.1% to 1.5%
of Ni; 0.0001% to 0.0025% of B; 0.1% to 2.0% of Cu; and 0.03% to
0.35% of Sn, wherein in a case where either one or both of Nb and
Ti are included, the following formula (1) is satisfied.
Nb/93+Ti/48.gtoreq.C/12+N/14 (1)
[0030] Element symbols in the formula (1) indicate contents of the
respective elements in terms of % by mass.
[0031] (3) The hot-rolled ferritic stainless steel sheet with
excellent cold cracking resistance according to (1) or (2), wherein
the content of Al is in a range of more than 0.10% to 3.00%.
[0032] (4) A method for manufacturing the hot-rolled ferritic
stainless steel sheet with excellent cold cracking resistance
according to any one of (1) to (3), includes: casting ferritic
stainless steel having the steel composition according to any one
of (1) to (3) to generate a semi-finished product and subjecting
the semi-finished product to hot rolling under a condition where a
finishing temperature is in a range of 800.degree. C. to
1000.degree. C. to generate a hot-rolled steel sheet; subsequently,
coiling the hot-rolled steel sheet in a coil shape at a temperature
of more than 650.degree. C. to 800.degree. C.; and immersing the
hot-rolled sheet coiled in a coil shape in a water tank within 1
hour after the coiling, maintaining the hot-rolled sheet in the
water tank for 1 hour or more, and taking out the hot-rolled steel
sheet.
Effects of the Invention
[0033] As described above, according to one aspect of the present
invention, cold cracking resistance of a hot-rolled steel sheet can
be prevented by increasing the ratio of subgrain boundaries
influencing the toughness of a hot-rolled ferritic stainless steel
sheet which contains various elements.
[0034] Further, according to the hot-rolled ferritic stainless
steel sheet of one aspect of the present invention, cold cracking
is not generated even when continuous annealing or a pickling step
is performed after hot rolling.
[0035] Furthermore, according to one aspect of the present
invention, manufacturing yield can be increased or production
efficiency can be improved by suppressing the cold cracking of
various hot-rolled ferritic stainless steel sheets. As a result,
effects extremely useful for industries in terms of reduction in
manufacturing costs can be exhibited. In addition, energy usage can
be suppressed by the improvement of production efficiency, and this
contributes to global environmental conservation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] FIG. 1 is a graph showing a relationship between a ratio
(La/L) of a length La of subgrain boundaries having orientation
differences of 1.degree. or more to less than 15.degree. to a
length L of all crystal grain boundaries of a hot-rolled ferritic
stainless steel sheet in the present embodiment and a Charpy impact
value.
EMBODIMENTS OF THE INVENTION
[0037] Hereinafter, a hot-rolled ferritic stainless steel sheet of
the present embodiment will be described in detail.
[0038] The hot-rolled ferritic stainless steel sheet of the present
embodiment has a steel composition which contains, in terms of % by
mass: 0.0150% or less of C; 0.01% to 2.00% of Si; 0.01% to 2.00% of
Mn; less than 0.040% of P; 0.010% or less of S; 10.0% to 30.0% of
Cr; 0.001% to 3.00% of Al; and 0.0200% or less of N with a balance
being Fe and unavoidable impurities, wherein in a cross section in
a range of 1/4 to 3/4 of a sheet thickness, a length L of all
crystal grain boundaries having orientation differences of
1.degree. or more to less than 180.degree. and a length La of
subgrain boundaries having orientation differences of 1.degree. or
more to less than 15.degree. satisfy a relation of
La/L.gtoreq.0.20.
[0039] Hereinafter, the reason for restricting the steel
composition of the hot-rolled steel sheet of the present embodiment
will be described. In addition, the term "%" in regard to
compositions means "% by mass" unless otherwise noted.
[0040] C: 0.0150% or less
[0041] When C is present in a solid solution state, grain-boundary
corrosion resistance of a welded portion is degraded; and
therefore, a large amount of C is not preferable. The upper limit
of the amount of C is set to 0.0150%. In addition, the refining
time and the manufacturing costs become increased when the amount
of C is reduced not to influence the grain-boundary corrosion
resistance. Therefore, the lower limit of the amount of C is
preferably set to 0.0010%. Further, when considered from viewpoints
of the grain-boundary corrosion resistance of a welded portion and
the manufacturing costs, it is preferable that the amount of C be
set to be in a range of 0.0020% to 0.0070%.
[0042] Si: 0.01% to 2.00%
[0043] Si is an element which improves oxidation resistance.
However, workability of a product is degraded when a large amount
of Si is added; and therefore, the upper limit of the amount of Si
is set to 2.00%. On the other hand, since Si is inevitably mixed as
a deoxidizing agent, the lower limit of the amount of Si is set to
0.01%. In addition, the amount of Si is preferably in a range of
0.02% to 0.97%.
[0044] Mn: 0.01% to 2.00%
[0045] Mn is an element that improves high temperature strength and
oxidation resistance, however, the addition of a large amount of Mn
results in degradation of workability of a product as is the case
with Si. Accordingly, the upper limit of the amount of Mn is set to
2.00%. Further, since there are cases in which Mn is inevitably
mixed, the lower limit of the amount of Mn is set to 0.01%.
Furthermore, the amount of Mn is preferably in a range of 0.02% to
1.95%.
[0046] P: less than 0.040%
[0047] Since P is inevitably mixed from raw materials of Cr and the
like, there are many cases in which 0.005% or more of P is mixed,
however, P degrades ductility and manufacturability. Accordingly,
it is preferable that the amount of P be as small as possible.
However, since it is extremely difficult to perform excessive
dephosphorization and manufacturing costs increase, the amount of P
is set to be in a range of less than 0.04%.
[0048] S: 0.010% or less
[0049] Since there are cases in which S generates an easily soluble
compound and degrades corrosion resistance, it is preferable that
the amount of S be small; and therefore, the amount of S is set to
be in a range of 0.010% or less. Further, since it is preferable
that the amount of S be small from a viewpoint of corrosion
resistance, the amount of S is preferably in a range of less than
0.0050%. It is more preferable that the lower limit of the amount
of S be set to 0.0001% because a desulfurization technology is
improved in recent years. It is still more preferable that the
lower limit of the amount of S be set to 0.0005% when stable
manufacturability is considered.
[0050] Cr: 10.0% to 30.0%
[0051] Cr is a basic element necessary for securing corrosion
resistance, high temperature strength, and oxidation resistance,
and 10.0% or more of Cr is necessarily added for exhibiting the
effects thereof On the other hand, since the toughness is degraded
when a large amount of Cr is added, the upper limit of the amount
of Cr is set to 30.0%. In addition, when the amount of Cr is
larger, the strength of the structure becomes higher, and
embrittlement, which is referred to as "475.degree. C.
embrittlement", specific to a steel containing a large amount of Cr
tends to occur. Therefore, the amount of Cr is preferably in a
range of 20.0% or less.
[0052] Al: 0.001% to 3.00%
[0053] Since Al is applied as a deoxidizing element, an appropriate
amount of Al is added. When less than 0.001% of Al is added,
deoxidizing ability becomes insufficient; and therefore, the lower
limit thereof is set to 0.001%. On the other hand, the oxygen
amount can be sufficiently reduced with 0.100% of Al, and the
deoxidizing ability is substantially saturated with an addition
amount of Al exceeding 0.100%. Therefore, the upper limit of the
amount of Al is preferably 0.100% in a case of adding Al only for
the purpose of deoxidization. In this case, the amount of Al is
preferably in a range of 0.002% to 0.095%.
[0054] Further, Al has an effect of improving high temperature
strength and corrosion resistance. In a case of adding Al for the
purpose of improving high temperature strength and corrosion
resistance, the amount of Al is preferably in a range of more than
0.10% to 3.00% and more preferably in a range of 0.50% to 2.00%.
Furthermore, since the workability of a product is degraded when a
large amount of Al is added, the upper limit of the amount of Al is
set to 3.00%. The upper limit of the amount of Al is preferably
2.00% or less.
[0055] N: 0.0200% or less
[0056] As is the case with C, grain-boundary corrosion resistance
of a welded portion is degraded when N is present in a solid
solution state; and therefore, a large amount of N is not
preferable. Accordingly, the upper limit of the amount of N is set
to 0.0200%. In addition, the refining time and the manufacturing
costs become increased so as to reduce the amount of N. Therefore,
the lower limit of the amount of N is preferably set to 0.0030%.
Further, when considered from viewpoints of the grain-boundary
corrosion resistance of a welded portion and the manufacturing
costs, it is preferable that the amount of N be set to be in a
range of 0.0050% to 0.0120%.
[0057] Moreover, in the present embodiment, in addition to the
above-described elements, it is preferable that either one or both
of 0.05% to 0.70% of Nb and 0.05% to 0.30% of Ti be included to
satisfy the following formula (1).
Nb/93+Ti/48.gtoreq.C/12+N/14 (1)
[0058] Element symbols in the formula (1) indicate contents of the
respective elements in terms of % by mass.
[0059] Nb and Ti generate precipitates together with C and N; and
thereby, Nb and Ti reduce amounts of solid-solubilized C and N. In
addition, when Nb and Ti are present in a solid solution state, the
high temperature strength and thermal fatigue characteristics of a
member are improved due to solid solution strengthening at a high
temperature. In the case where Nb is contained, 0.05% or more of Nb
is needed to be contained for fixing C and N, and it is preferable
that 0.10% or more of Nb be contained. Further, in the case where
Ti is contained, 0.05% or more of Ti is needed to be contained for
fixing C and N.
[0060] In addition, the formula (1) described above is needed to be
stoichiometrically satisfied for make all of C and N present in the
steel be in a precipitation state.
[0061] On the other hand, when a large amount of Ti is exceedingly
added, the toughness in the middle of manufacturing processes is
degraded, and the generation of surface defects becomes remarkable
in some cases. Therefore, the upper limit of Ti is set to
0.30%.
[0062] Further, addition of a large amount of Nb leads to the
degradation of workability of a product. Accordingly, the upper
limit of Nb is set to 0.70%, and it is more preferable to set the
upper limit thereof to 0.55% or less.
[0063] Moreover, in the present embodiment, it is preferable to
include one or more selected from 0.1% to 2.5% of Mo, 0.1% to 1.5%
of Ni, 0.0001% to 0.0025% of B, 0.1% to 2.0% of Cu, and 0.03% to
0.35% of Sn in addition to the above-described elements. Mo, Ni,
Cu, and Sn are elements that improve the high temperature strength
or corrosion resistance and may be added as needed. Further, Ni has
an effect of improving the toughness.
[0064] Since the improvement of the high temperature strength
becomes remarkable when any one of Mo: 0.1% or more, Ni: 0.1% or
more, Cu: 0.1% or more, and Sn: 0.03% or more is included, those
amounts are set as lower limits thereof. It is more preferable that
any one of Mo: 0.3% or more, Ni: 0.25% or more, Cu: 0.4% or more,
and Sn: 0.10% or more be included for further improving the high
temperature strength and the corrosion resistance.
[0065] Addition of large amounts of Mo, Ni, and Cu leads to
degradation of picking properties; and thereby, productivity is
degraded. Therefore, the upper limits of Mo, Ni, and Cu are
respectively set to Mo: 2.5%, Ni: 1.5%, Cu: 2.0%, and more
preferably set to Mo: 2.2% or less, Ni: 1.2% or less, and Cu: 1.4%
or less. Since addition of a large amount of Sn leads to
degradation in toughness and generation of surface defects, the
upper limit of Sn is set to 0.35% and it is more preferable to set
the upper limit thereof to be in a range of 0.20% or less.
[0066] B is an element which improves secondary workability. When a
steel is used for the purpose in which the secondary workability is
needed, B may be added when necessary. Since an effect of improving
the secondary workability is exhibited when the addition amount of
B is in a range of 0.0001% or more, this value is set to the lower
limit and it is more preferable to set the lower limit thereof to
be in a range of 0.0003% or more. Further, since there are cases in
which the addition of a large amount of B leads to the degradation
of toughness in the hot-rolled sheet and workability, the upper
limit of B is set to 0.0025%, and it is more preferable to set the
upper limit thereof to be in a range of 0.0015% or less.
[0067] In addition, as an important characteristic of the present
embodiment, in a cross section in a range of 1/4 to 3/4 of the
sheet thickness, the ratio between a length L of all crystal grain
boundaries having orientation differences of 1.degree. or more to
less than 180.degree. and a length La of subgrain boundaries having
orientation differences of 1.degree. or more to less than
15.degree. satisfies a relation of La/L.gtoreq.0.20.
[0068] The ratio between the length L of all crystal grain
boundaries and the length La of subgrain boundaries is measured by
the following method. First, samples for measurement are collected
from 10 arbitrary portions of a hot-rolled steel sheet. The
positions of the collected portions are not particularly limited.
However, in practice, there are cases where a difference in coiling
temperature is caused between a coiling-started part (top portion)
and a coiling-finished part (bottom portion) when the hot-rolled
steel sheet is actually coiled in a coil shape. Accordingly, in
such a case, it is desired to collect samples for measurement from
the top portion, the middle portion, and the bottom portion of the
hot-rolled steel sheet in order to obtain an average value of the
entire steel sheet. In regard to the width direction of the
hot-rolled steel sheet, it is desired to collect samples for
measurement from the approximate middle portion. Further, the
samples for measurement are collected such that the samples have
cross sections (L cross sections) parallel to the rolling direction
and perpendicular to the sheet surface direction.
[0069] The L cross sections of the samples for measurement are
subjected to grinding using electrolytic grinding or colloidal
silica.
[0070] At or in the vicinity of the surface layer, relatively fine
crystal grains tend to be easily generated and the toughness
thereof is excellent in some cases. Accordingly, the vicinity of
the center of the sheet thickness t, that is, the range of 1/4 t to
3/4 t of the sheet thickness t in the L cross section is used as a
measuring range.
[0071] Next, the length of the crystal grain boundaries is measured
using the EBSP by the following method. In the above-described
measuring range, a crystal orientation is measured at a measuring
step (pitch) of 0.2 .mu.m in a range of 100 .mu.m.times.100 .mu.m.
An interface having an orientation difference of 1.degree. or more
to less than 180.degree. between adjacent measurement points is
regarded as a grain boundary. Among these, a grain boundary having
an orientation difference of 1.degree. or more to less than
15.degree. is regarded as a subgrain boundary.
[0072] The total of the lengths of all the crystal grain boundaries
is calculated as "the length L of all crystal grain boundaries,"
and the total of the lengths of subgrain boundaries is calculated
as "the length La of subgrain boundaries." Further, the ratio of
La/L is obtained.
[0073] With regard to each of 10 samples for measurement, the ratio
of La/L is obtained in the same way, and the average value of 10
values of La/L is calculated.
[0074] In the case where the ratio of La to L is less than 0.20,
the toughness of the hot-rolled steel sheet becomes degraded to
less than 20 J/cm.sup.2; and therefore, it is necessary to set the
ratio of La/L to be in a range of 0.20 or more. As shown in FIG. 1,
when the ratio of the subgrain boundaries is higher, the toughness
of the hot-rolled steel sheet tends to be higher. Accordingly, it
is preferable that the ratio of La/L be in a range of 0.35 or more.
The upper limit of the ratio of La/L is not particularly limited,
but the ratio of La/L becomes 1 when the orientation differences of
all the grain boundaries are in a range of 1.degree. or more to
less than 15.degree.. In the test performed by the present
inventors, the ratio of La/L of 0.80 or more is not obtained.
[0075] Next, the method for manufacturing a hot-rolled ferritic
stainless steel sheet in the present embodiment will be
described.
[0076] The method for manufacturing a hot-rolled ferritic stainless
steel sheet in the present embodiment has the following steps.
[0077] (1) A step of casting ferritic stainless steel having the
above-described steel composition to generate a semi-finished
product and then, subjecting the semi-finished product to hot
rolling under a condition where a finishing temperature is in a
range of 800.degree. C. to 1000.degree. C. to generate a hot-rolled
steel sheet.
[0078] (2) Subsequent to the hot rolling, a step of coiling the
hot-rolled steel sheet in a coil shape at a temperature of more
than 650.degree. C. to 800.degree. C.
[0079] (3) A step of immersing the hot-rolled steel sheet coiled in
a coil shape in a water tank within 1 hour after the coiling,
maintaining the hot-rolled sheet in the water tank for 1 hour or
more, and taking out the hot-rolled steel sheet.
[0080] Hereinafter, the method for manufacturing a hot-rolled
ferritic stainless steel sheet in the present embodiment will be
described in detail.
[0081] First, a ferritic stainless steel having the above-described
steel composition is cast to generate a semi-finished product, and
the semi-finished product is subjected to hot rolling to generate a
hot-rolled steel sheet. Subsequently, the hot-rolled steel sheet to
which the hot rolling (finish rolling) is applied is cooled down to
the coiling temperature by water cooling, and the hot-rolled steel
sheet is coiled in a coil shape at a coiling temperature. In the
present embodiment, the finishing temperature of the hot rolling is
set to be in a range of 800.degree. C. to 1000.degree. C., and the
coiling temperature thereof is set to be in a range of more than
650.degree. C. to 800.degree. C.
[0082] In the case where the finishing temperature is less than
800.degree. C. or more than 1000.degree. C., it becomes extremely
difficult to generate crystal grain boundaries having orientation
differences of 1.degree. or more to less than 15.degree. after the
coiling. Therefore, 800.degree. C. and 1000.degree. C. are set to
the lower limit and the upper limit, respectively
[0083] Further, in the present embodiment, it is preferable not to
generate an austenite phase at the time of the hot rolling. Whether
the austenite phase is generated at the time of the hot rolling or
not is determined depending on the amount of austenite generating
elements in the steel, particularly, the amounts of C and N having
a high austenite generating ability. In the hot-rolled steel sheet
of the present embodiment, the amounts of both of C and N are
small, and the generation of the austenite phase at the time of hot
rolling is not recognized.
[0084] It is also difficult to generate crystal grain boundaries
having orientation differences of 1.degree. or more to less than
15.degree. even in the case where the coiling temperature is in a
range of 650.degree. C. or less. In the case where the coiling
temperature is in a range of more than 800.degree. C., the
recrystallization at the time of coiling progresses, and the ratio
of the crystal grain boundaries having orientation differences of
15.degree. or more to less than 180.degree. increases; and
therefore, the toughness is degraded.
[0085] Next, the hot-rolled steel sheet coiled in a coil shape is
immersed in a water tank. This is because it is necessary to
suppress the generation of precipitates that degrade the toughness
in a slow cooling step after the coiling. Here, after the
temperature of the hot-rolled steel sheet reaches the coiling
temperature by water cooling subsequent to the finish rolling, a
process in which the precipitates are generated and becomes
coarsened strongly depends on the temperature and the elapsed time
of the steel sheet after the coiling. In addition, in the case
where the hot rolling is performed under general conditions and the
coiling is performed at a coiling temperature of more than
650.degree. C. to 800.degree. C., the time from the end of the hot
rolling to the moment where the temperature of the hot-rolled steel
sheet reaches the coiling temperature is within 1 minute, and the
cooling rate during that time is in a range of 3.degree. C./sec or
more. Under such a condition of the cooling rate, precipitates
influencing the toughness are not generated during the time period
from the completion of the finish rolling to the start of the
coiling.
[0086] In regard to the generation of the precipitates that degrade
the toughness, the time during the steel is maintained at the
above-described coiling temperature becomes an important factor. In
the present embodiment, it is necessary to immerse the hot-rolled
steel sheet in a water tank within 1 hour after the coiling. In the
case where the time taken from the completion of the coiling to the
immersion in a water tank exceeds 1 hour, precipitates are
generated during that time, and the toughness is degraded by the
generated precipitates in some cases.
[0087] Further, the time for maintaining the hot-rolled steel sheet
in a water tank after immersing the hot-rolled steel sheet in a
water tank is also an important factor. It is preferable that the
immersing time for maintaining the hot-rolled steel sheet in a
water tank be in a range of 1 hour or more according to the present
embodiment.
[0088] In the case where the immersing time of the hot-rolled steel
sheet in a water tank is in a range of less than 1 hour which is
short, the cooling becomes insufficient, and precipitates that
degrade the toughness of the hot-rolled steel sheet are generated
due to recuperation thereafter in some cases.
[0089] According to the hot-rolled ferritic stainless steel sheet
of the present embodiment described above, a microstructure
influencing the toughness of the hot-rolled steel sheet can be
controlled by the requirements according to the components and the
crystal grain boundaries; and as a result, cold cracking of the
hot-rolled steel sheet can be prevented.
[0090] Further, according to the hot-rolled ferritic stainless
steel sheet of the present embodiment, the cold cracking is not
generated even when continuous annealing or a pickling step is
carried out after hot rolling.
[0091] Furthermore, according to the hot-rolled ferritic stainless
steel sheet of the present embodiment, since the cold cracking can
be suppressed, the manufacturing yield can be increased and the
production efficiency can be improved. As a result, effects
extremely useful for industries in terms of reduction in
manufacturing costs can be exhibited. In addition, energy usage in
a manufacturing step can be suppressed due to improvement of
production efficiency, and this contributes to global environmental
conservation.
EXAMPLES
[0092] Hereinafter, effects of the present embodiment will be
described with reference to the examples, but the present
embodiment is not limited to the conditions used for the examples
described below.
[0093] In the present example, at first, steels having the
respective compositions listed in Table 1 were melted and cast to
obtain steel ingots (semi-finished products).
[0094] Each of the steel ingots was grinded to a thickness of 90
mm. Each of the steel ingots was subjected to hot rolling at a
finish temperature (FT) listed in Tables 2 and 3, and the steel
ingot was rolled to a thickness of 5 mm to generate a hot-rolled
steel sheet. Subsequently, each of the hot-rolled steel sheets was
cooled down to the coiling temperature (CT) listed in Tables 2 and
3 by water cooling while the steel sheet temperature after rolling
was monitored using a radiation thermometer. Here, the cooling rate
at this time was 20.degree. C./sec.
[0095] Next, each of the hot-rolled steel sheets was inserted into
a furnace in which the temperature therein was controlled by the
coiling temperature (CT) listed in Tables 2 and 3; and thereby, a
coiling treatment was simulated. Subsequently, each of the
hot-rolled steel sheets was immersed in a water tank after the
elapsed time (t) listed in Tables 2 and 3 passed. Next, each of the
hot-rolled steel sheet was maintained for the immersion time (tx)
listed in Tables 2 and 3, and then the hot-rolled steel sheet was
taken out.
[0096] Each of the obtained hot-rolled steel sheets had a complete
ferrite single-phase structure.
[0097] In addition, characteristics (ratio La/L of the length La of
the subgrain boundaries to the length L of all crystal grain
boundaries) of the crystal grain boundaries were calculated using
the EBSP in the same manner as that of the measurement method
described in the embodiment.
[0098] Subsize Charpy impact test pieces were collected from each
of the hot-rolled steel sheets in conformity to JIS Z 2202, and
metal materials were subjected to the impact test in conformity
with JIS Z 2242 by setting an impact direction to a direction
perpendicular to the rolling direction. Impact absorbing energy was
inspected under a condition where the test temperature was set to
25.degree. C.
[0099] In addition, according to the obtained results, the cold
cracking resistance (toughness) of the hot-rolled steel sheet was
evaluated by the following method.
[0100] According to the present example, in a hot-rolled steel
sheet having a Charpy impact value of less than 20 J/cm.sup.2, the
cold cracking was generated in a continuous annealing or a pickling
step which was a step thereafter, and the yield was degraded. On
the contrary, in a hot-rolled steel sheet having a Charpy impact
value of 20 J/cm.sup.2 or more, such cold cracking was not
generated. Accordingly, the cold cracking resistance of the
hot-rolled steel sheet having a Charpy impact value of less than 20
J/cm.sup.2 was evaluated as "poor" and the cold cracking resistance
of the hot-rolled steel sheet having a Charpy impact value of 20
J/cm.sup.2 or more was evaluated as "excellent". The Charpy impact
values of less than 20 J/cm.sup.2 were underlined in Tables 2 and
3.
[0101] The above-described manufacturing conditions and the
evaluation results are listed in Tables 2 and 3.
[0102] Further, in Tables 2 and 3, FT represents the finishing
temperature (.degree. C.) of hot rolling and CT represents the
coiling temperature (.degree. C.) of the hot-rolled steel sheet. t
represents the time (h) that was taken from the completion of the
coiling to the start of water cooling (start of immersion), and tx
represents the time (h) that was taken from the start of the water
cooling to the completion thereof (from the start of immersion to
the moment where the hot-rolled steel sheet was taken out).
[0103] Further, in Tables 1 to 3, the numerical values outside the
ranges designated by the present embodiment were underlined.
TABLE-US-00001 TABLE 1 Steel C Si Mn P S Cr Al N Nb Ti Mo Ni Cu Sn
B A 0.0011 0.15 0.12 0.023 0.001 22.1 0.010 0.0016 -- -- -- -- --
-- -- B 0.0029 0.43 0.31 0.033 0.003 10.9 0.004 0.0060 -- 0.15 --
-- -- -- -- C 0.0035 0.10 1.50 0.014 0.005 19.0 0.003 0.0160 0.11
0.10 0.8 1.11 -- -- 0.0006 D 0.0110 1.22 0.75 0.029 0.006 24.3
0.077 0.0095 0.55 -- -- -- -- -- 0.0024 E 0.0045 0.40 0.35 0.031
0.002 19.2 0.031 0.0119 0.21 0.15 2.10 -- -- -- -- F 0.0080 0.21
1.02 0.025 0.001 17.0 0.030 0.0130 0.53 0.12 0.31 0.10 1.25 --
0.0008 G 0.0032 0.15 0.12 0.023 0.001 22.1 0.010 0.0016 0.22 0.06
0.81 -- -- -- 0.0009 H 0.0124 0.51 0.13 0.028 0.002 19.2 0.025
0.0180 0.38 -- -- 0.29 0.44 -- -- I 0.0034 0.12 0.24 0.017 0.007
14.5 0.074 0.0061 0.11 0.10 -- -- -- 0.10 -- J 0.0058 0.52 0.46
0.025 0.001 13.9 0.031 0.0080 -- 0.14 -- -- 1.17 -- -- K 0.0081
0.95 0.22 0.022 0.002 27.1 0.006 0.0099 -- 0.30 0.48 -- -- 0.34
0.0005 L 0.0115 0.25 0.33 0.028 0.001 32.5 0.007 0.0144 -- 0.33 --
-- -- -- 0.0042 M 0.0045 0.45 0.22 0.033 0.003 21.4 1.85 0.0151
0.11 0.25 -- -- -- -- -- N 0.0028 0.08 0.11 0.020 0.001 14.3 0.033
0.0089 0.13 0.08 -- -- -- 0.03 0.0005 O 0.0033 0.25 0.09 0.028
0.001 13.5 0.025 0.0091 -- 0.07 -- -- -- 0.03 --
TABLE-US-00002 TABLE 2 Charpy impact FT CT t tx values Examples
Steel (.degree. C.) (.degree. C.) (h) (h) La/L (J/cm.sup.2) 1 A 920
675 0.4 1.5 0.51 72 Example of the present invention 2 A 780 660
0.4 3.5 0.16 15 Comparative Example 3 A 880 500 0.7 2.4 0.02 9
Comparative Example 4 B 820 710 0.3 2.2 0.46 82 Example of the
present invention 5 B 840 660 3.5 1.5 0.06 8 Comparative Example 6
B 1050 750 0.3 0.1 0.08 11 Comparative Example 7 C 850 760 0.5 11.0
0.34 65 Example of the present invention 8 C 910 780 1.2 0.4 0.04
12 Comparative Example 9 C 890 620 0.4 1.5 0.05 12 Comparative
Example 10 C 840 500 2.0 2.6 0.11 5 Comparative Example 11 D 890
670 0.3 4.2 0.45 64 Example of the present invention 12 D 925 600
0.5 0.3 0.02 14 Comparative Example 13 E 900 730 0.4 1.4 0.39 43
Example of the present invention 14 E 890 600 0.7 3.2 0.02 5
Comparative Example 15 E 910 690 2.5 0.2 0.07 3 Comparative Example
16 F 970 750 0.4 3.0 0.47 65 Example of the present invention 17 F
960 840 0.9 5.0 0.04 10 Comparative Example 18 F 1050 790 0.4 4.5
0.03 9 Comparative Example 19 G 920 780 0.3 4.0 0.37 65 Example of
the present invention 20 G 890 670 0.8 5.0 0.52 72 Example of the
present invention 21 G 910 450 0.1 0.5 0.07 10 Comparative Example
22 H 870 660 0.1 3.5 0.29 80 Example of the present invention 23 H
1040 814 0.4 2.5 0.08 10 Comparative Example 24 H 830 550 0.3 1.4
0.07 9 Comparative Example 25 H 865 680 3.5 0.3 0.05 14 Comparative
Example
TABLE-US-00003 TABLE 3 Charpy impact FT CT t tx values Example
Steel (.degree. C.) (.degree. C.) (h) (h) La/L (J/cm.sup.2) 26 I
900 355 0.5 3.0 0.07 11 Comparative Example 27 I 910 685 0.3 5.0
0.45 72 Example of the present invention 28 I 770 660 0.2 3.0 0.18
18 Comparative Example 29 J 911 670 0.2 5.0 0.65 55 Example of the
present invention 30 J 890 740 0.3 3.0 0.44 67 Example of the
present invention 31 J 980 842 0.5 1.1 0.02 15 Comparative Example
32 K 925 721 0.4 2.4 0.34 56 Example of the present invention 33 K
830 560 3.5 7.5 0.04 17 Comparative Example 34 K 924 678 0.6 0.2
0.14 6 Comparative Example 35 L 820 731 0.4 4.5 0.11 5 Comparative
Example 36 L 900 445 0.3 10.5 0.05 4 Comparative Example 37 L 860
695 0.4 4.3 0.09 3 Comparative Example 38 M 895 660 0.3 2.5 0.49 71
Example of the present invention 39 N 940 740 0.5 3.0 0.47 71
Example of the present invention 40 N 900 500 0.5 3.0 0.04 7
Comparative Example 41 N 1030 850 0.9 3.0 0.03 9 Comparative
Example 42 O 830 780 0.2 2.6 0.42 66 Example of the present
invention 43 O 950 700 2.0 2.2 0.06 8 Comparative Example 44 O 950
480 0.1 1.5 0.03 8 Comparative Example
[0104] As is clear from Tables 2 and 3, a hot-rolled ferritic
stainless steel sheet with a Charpy impact value of 20 J/cm.sup.2
or more and excellent cold cracking resistance, that is, a
hot-rolled steel sheet with excellent toughness can be obtained in
the examples of the present invention according to the present
embodiment.
[0105] On the other hand, in the comparative examples outside the
ranges designated by the present embodiment, the Charpy impact
values were low in any cases. From this result, it is understood
that the cold cracking resistance (toughness) of the hot-rolled
steel sheets in the comparative examples was degraded.
[0106] From this result, the above-described knowledge can be
verified and the grounds for limiting the above-described
respective steel compositions and the constitutions can be backed
up.
INDUSTRIAL APPLICABILITY
[0107] The hot-rolled ferritic stainless steel sheet of the present
embodiment has a Charpy impact value of 20 J/cm.sup.2 or more and
excellent cold cracking resistance. Therefore, the cold cracking
does not occur even when continuous annealing or a pickling step is
carried out after hot rolling. Accordingly, the hot-rolled ferritic
stainless steel sheet of the present embodiment can be
appropriately applied to a step of manufacturing members such as
household electric appliances, building materials, and automobile
components for which the ferritic stainless steel is used.
[0108] ti
* * * * *