U.S. patent application number 15/529263 was filed with the patent office on 2017-10-05 for ferritic stainless steel having excellent ductility and method for manufacturing same.
The applicant listed for this patent is POSCO. Invention is credited to Soo-Ho PARK, Jae-Hong SHIM.
Application Number | 20170283894 15/529263 |
Document ID | / |
Family ID | 54248455 |
Filed Date | 2017-10-05 |
United States Patent
Application |
20170283894 |
Kind Code |
A1 |
PARK; Soo-Ho ; et
al. |
October 5, 2017 |
FERRITIC STAINLESS STEEL HAVING EXCELLENT DUCTILITY AND METHOD FOR
MANUFACTURING SAME
Abstract
Ferritic stainless steel having a high degree of ductility and a
method for manufacturing the ferritic stainless steel are provided.
The stainless steel includes, by wt %, C: 0.005% to 0.1%, Si: 0.01%
to 2.0%, Mn: 0.01% to 1.5%, P: 0.05% or less, S: 0.005% or less,
Cr: 10% to 30%, Ti: 0.005% to 0.5%, Al: 0.01% to 0.15%, N: 0.005%
to 0.03%, and the balance of Fe and inevitable impurities, wherein
the ferritic stainless steel includes 3.5.times.10.sup.6 or fewer
particles of an independent Ti(CN) precipitate per square
millimeter (mm.sup.2) of ferrite matrix.
Inventors: |
PARK; Soo-Ho; (Pohang-si,
KR) ; SHIM; Jae-Hong; (Seoul, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
POSCO |
Pohang-si, Gyeongsangbuk-do |
|
KR |
|
|
Family ID: |
54248455 |
Appl. No.: |
15/529263 |
Filed: |
April 30, 2015 |
PCT Filed: |
April 30, 2015 |
PCT NO: |
PCT/KR2015/004410 |
371 Date: |
May 24, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/06 20130101;
C22C 38/28 20130101; C22C 38/00 20130101; B22D 11/002 20130101;
C21D 8/0205 20130101; C21D 8/0263 20130101; C21D 6/008 20130101;
C21D 2211/004 20130101; C22C 38/002 20130101; C22C 38/02 20130101;
C21D 6/005 20130101; C21D 9/0081 20130101; B22D 27/04 20130101;
C21D 8/021 20130101; C21D 2211/005 20130101; C22C 38/04 20130101;
C21D 6/002 20130101; C21D 8/0226 20130101; C22C 38/001
20130101 |
International
Class: |
C21D 9/00 20060101
C21D009/00; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C21D 6/00 20060101 C21D006/00; C22C 38/00 20060101
C22C038/00; B22D 27/04 20060101 B22D027/04; C21D 8/02 20060101
C21D008/02; C22C 38/28 20060101 C22C038/28; C22C 38/02 20060101
C22C038/02 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 26, 2014 |
KR |
10-2014-0190545 |
Apr 30, 2015 |
KR |
10-2015-0061378 |
Claims
1. Ferritic stainless steel comprising, by wt %, C: 0.005% to 0.1%,
Si: 0.01% to 2.0%, Mn: 0.01% to 1.5%, P: 0.05% or less, S: 0.005%
or less, Cr: 10% to 30%, Ti: 0.005% to 0.5%, Al: 0.01% to 0.15%, N:
0.005% to 0.03%, and the balance of Fe and inevitable impurities,
wherein the ferritic stainless steel comprises 3.5.times.10.sup.6
or fewer particles of an independent Ti(CN) precipitate per square
millimeter (mm.sup.2) of ferrite matrix.
2. Ferritic stainless steel comprising, by wt %, C: 0.005% to 0.1%,
Si: 0.01% to 2.0%, Mn: 0.01% to 1.5%, P: 0.05% or less, S: 0.005%
or less, Cr: 10% to 30%, Ti: 0.005% to 0.5%, Al: 0.01% to 0.15%, N:
0.005% to 0.03%, and the balance of Fe and inevitable impurities,
wherein the ferritic stainless steel comprises an independent
Ti(CN) precipitate and a dependent Ti(CN) precipitate formed using
a TiN inclusion as precipitation nuclei, and the ferritic stainless
steel has a P within a range of 60% or less, the P being defined by
Formula 1 below: P(%)={N.sub.S/(N.sub.S+N.sub.C)}.times.100
[Formula 1] where N.sub.S refers to the number of independent
Ti(CN) precipitate particles per unit area (mm.sup.2), and N.sub.C
refers to the number of dependent Ti(CN) precipitate particles per
unit area (mm.sup.2).
3. The ferritic stainless steel of claim 2, wherein the P is 58% or
less.
4. The ferritic stainless steel of claim 1, wherein the independent
Ti(CN) precipitate has a particle diameter of 0.01 .mu.m or
greater.
5. The ferritic stainless steel of claim 1, wherein the independent
Ti(CN) precipitate has an average particle diameter of 0.15 .mu.m
or less.
6. The ferritic stainless steel of claim 2, wherein the TiN
inclusion has an average particle diameter of 2 .mu.m or
greater.
7. The ferritic stainless steel of claim 1, wherein the ferritic
stainless steel has an elongation of 34% or greater.
8. A method for manufacturing ferritic stainless steel, the method
comprising casting molten steel as a slab, the molten steel
comprising, by wt %, C: 0.005% to 0.1%, Si: 0.01% to 2.0%, Mn:
0.01% to 1.5%, P: 0.05% or less, S: 0.005% or less, Cr: 10% to 30%,
Ti: 0.005% to 0.5%, Al: 0.01% to 0.15%, N: 0.005% to 0.03%, and the
balance of Fe and inevitable impurities, wherein in the casting of
the molten steel, the slab is cooled at an average cooling rate of
5.degree. C./sec or less (excluding 0.degree. C./sec) within a
temperature range of 1100.degree. C. to 1200.degree. C. based on a
surface temperature of the slab.
9. The method of claim 8, wherein in the casting of the molten
steel, the slab is cooled at an average cooling rate of 5.degree.
C./sec or less (excluding 0.degree. C./sec) within a temperature
range of 1000.degree. C. to 1250.degree. C. based on the surface
temperature of the slab.
10. The method of claim 8, wherein after the casting of the molten
steel, the method further comprises: reheating the slab; obtaining
hot-rolled steel by performing a hot rolling process on the
reheated slab; and performing a hot band annealing process on the
hot-rolled steel within a temperature range of 450.degree. C. to
1080.degree. C. for 60 minutes or less.
11. The ferritic stainless steel of claim 2, wherein the
independent Ti(CN) precipitate has a particle diameter of 0.01
.mu.m or greater.
12. The ferritic stainless steel of claim 2, wherein the
independent Ti(CN) precipitate has an average particle diameter of
0.15 .mu.m or less.
13. The ferritic stainless steel of claim 2, wherein the ferritic
stainless steel has an elongation of 34% or greater.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to ferritic stainless steel
having a high degree of ductility and a method for manufacturing
the ferritic stainless steel, and more particularly, to a new kind
of ferritic stainless steel provided by improving ferritic
stainless steel having poor ductility compared to austenitic
stainless steel for use in applications requiring high ductility,
and a method for manufacturing the ferritic stainless steel.
BACKGROUND ART
[0002] Ferritic stainless steels have a high degree of corrosion
resistance even though the contents of expensive alloying elements
in the ferritic stainless steels are low. That is, ferritic
stainless steels are more competitive in price than austenitic
stainless steels. Ferritic stainless steels are used in
applications such as construction materials, transportation
vehicles, or kitchen utensils. However, ferrite stainless steels
have poor ductility and thus it is difficult to use ferritic
stainless steels instead of austenitic stainless steels in many
applications. Therefore, many efforts have been made to improve the
ductility of ferritic stainless steels and thus to increase the
applications of ferritic stainless steels.
[0003] To this end, attempts to improve the ductility of ferritic
stainless steels by limiting the total amount or number of
precipitates in ferritic stainless steels have been made. However,
meaningful results have not yet been reported.
DISCLOSURE
Technical Problem
[0004] An aspect of the present disclosure may provide ferritic
stainless steel having a high degree of ductility and a method of
manufacturing the ferritic stainless steel.
[0005] The present disclosure is not limited to the above-mentioned
aspect. Other aspects of the present disclosure are stated in the
following description, and the aspects of the present disclosure
will be clearly understood by those of ordinary skill in the art
through the following description.
Technical Solution
[0006] According to an aspect of the present disclosure, ferritic
stainless steel may include, by wt %, C: 0.005% to 0.1%, Si: 0.01%
to 2.0%, Mn: 0.01% to 1.5%, P: 0.05% or less, S: 0.005% or less,
Cr: 10% to 30%, Ti: 0.005% to 0.5%, Al: 0.01% to 0.15%, N: 0.005%
to 0.03%, and the balance of Fe and inevitable impurities, wherein
the ferritic stainless steel may include 3.5.times.10.sup.6 or
fewer particles of an independent Ti(CN) precipitate per square
millimeter (mm.sup.2) of ferrite matrix.
[0007] According to another aspect of the present disclosure,
ferritic stainless steel may include, by wt %, C: 0.005% to 0.1%,
Si: 0.01% to 2.0%, Mn: 0.01% to 1.5%, P: 0.05% or less, S: 0.005%
or less, Cr: 10% to 30%, Ti: 0.005% to 0.5%, Al: 0.01% to 0.15%, N:
0.005% to 0.03%, and the balance of Fe and inevitable impurities,
wherein the ferritic stainless steel may include an independent
Ti(CN) precipitate and a dependent Ti(CN) precipitate formed using
a TiN inclusion as precipitation nuclei, and the ferritic stainless
steel may have a P within a range of 60% or less, the P being
defined by Formula 1 below:
P(%)={N.sub.S/(N.sub.S+N.sub.C)}.times.100 [Formula 1]
[0008] where N.sub.S refers to the number of independent Ti(CN)
precipitate particles per unit area (mm.sup.2), and N.sub.C refers
to the number of dependent Ti(CN) precipitate particles per unit
area (mm.sup.2).
[0009] The independent Ti(CN) precipitate may have a particle
diameter of 0.01 .mu.m or greater.
[0010] The independent Ti(CN) precipitate may have an average
particle diameter of 0.15 .mu.m or less.
[0011] The TiN inclusion may have an average particle diameter of 2
.mu.m or greater.
[0012] The ferritic stainless steel may have an elongation of 34%
or greater.
[0013] According to another aspect of the present disclosure, a
method for manufacturing ferritic stainless steel may include
casting molten steel as a slab, the molten steel including, by wt
%, C: 0.005% to 0.1%, Si: 0.01% to 2.0%, Mn: 0.01% to 1.5%, P:
0.05% or less, S: 0.005% or less, Cr: 10% to 30%, Ti: 0.005% to
0.5%, Al: 0.01% to 0.15%, N: 0.005% to 0.03%, and the balance of Fe
and inevitable impurities, wherein in the casting of the molten
steel, the slab may be cooled at an average cooling rate of
5.degree. C./sec or less (excluding 0.degree. C./sec) within a
temperature range of 1100.degree. C. to 1200.degree. C. based on a
surface temperature of the slab.
[0014] In the casting of the molten steel, the slab may be cooled
at an average cooling rate of 5.degree. C./sec or less (excluding
0.degree. C./sec) within a temperature range of 1000.degree. C. to
1250.degree. C. based on the surface temperature of the slab.
[0015] After the casting of the molten steel, the method may
further include: obtaining a hot-rolled sheet by performing a hot
rolling process on the slab; and performing a hot band annealing
process on the hot-rolled sheet within a temperature range of
450.degree. C. to 1080.degree. C. for 1 minute to 60 minutes.
Advantageous Effects
[0016] The ferritic stainless steel of the present disclosure has a
high degree of ductility.
DESCRIPTION OF DRAWINGS
[0017] FIG. 1 is a scanning electron microscope (SEM) image
illustrating the microstructure of a hot-rolled sheet of Inventive
Example 1.
[0018] FIG. 2 is a high magnification SEM image illustrating region
A in FIG. 1.
BEST MODE
[0019] The inventors have reviewed various factors to improve the
ductility of ferritic stainless steel and have acquired the
following knowledge.
[0020] (1) In general, a small amount of titanium (Ti) is added to
ferritic stainless steel to improve the corrosion resistance of the
ferritic stainless steel. In this case, however, a large amount of
Ti(CN) inevitably precipitates in the ferrite matrix of
Ti-containing ferritic stainless steel, and the Ti(CN) precipitate
becomes the main cause of ductility deterioration.
[0021] (2) The Ti(CN) precipitate includes a Ti(CN) precipitate
independently formed in the ferrite matrix (hereinafter referred to
as an "independent Ti(CN) precipitate") and a Ti(CN) precipitate
formed with the help of particles of a TiN inclusion that are
crystallized during a steel making process and function as
precipitation nuclei (hereinafter referred to as a "dependent
Ti(CN) precipitate"). The dependent Ti(CN) precipitate does not
have a significant effect on ductility deterioration when compared
to the independent Ti(CN) precipitate.
[0022] (3) Therefore, if a large amount of Ti(CN) precipitates in
the form of a dependent Ti(CN) precipitate with the help of TiN
inclusion particles functioning as precipitation nuclei, the amount
of independent Ti(CN) precipitate particles may decrease. In this
manner, the ductility of Ti-containing ferritic stainless steel may
be improved.
[0023] Hereinafter, ferritic stainless steel having a high degree
of ductility will be described in detail according to an aspect of
the present disclosure.
[0024] First, the composition of the ferritic stainless steel of
the present disclosure will be described in detail. In the
following description, the contents of elements are given in wt %
unless otherwise mentioned.
[0025] Carbon (C): 0.005% to 0.1%
[0026] Since carbon (C) markedly affects the strength of steel, if
the content of carbon (C) in steel is excessively high, the
strength of the steel may increase to an excessive degree, and the
ductility of the steel may decrease. Therefore, the content of
carbon (C) is limited to 0.1% or less. However, if the content of
carbon (C) is excessively low, the strength of steel decreases too
much. Therefore, the lower limit of the content of carbon (C) may
be limited to 0.005%.
[0027] Silicon (Si): 0.01% to 2.0%
[0028] Silicon (Si) is an element added to molten steel during a
steel making process to remove oxygen and stabilize ferrite. In the
present disclosure, silicon (Si) is added in an amount of 0.01% or
greater. However, if the content of silicon (Si) in steel is
excessively high, the ductility of the steel may decrease due to
hardening. Therefore, the content of silicon (Si) is limited to
2.0% or less.
[0029] Mn (Manganese): 0.01% to 1.5%
[0030] Manganese (Mn) is an element effective in improving the
corrosion resistance of steel. In the present disclosure, manganese
(Mn) is added in an amount of 0.01% or greater, more preferably,
0.5% or greater. However, if the content of manganese (Mn) in steel
is excessively high, the generation of Mn-containing fumes markedly
increases during a welding process, and thus the weldability of the
steel decreases. In addition, an MnS precipitate may be excessively
formed to result in a decrease in the ductility of the steel.
Therefore, the content of manganese (Mn) is limited to 1.5% or
less, more preferably 1.0% or less.
[0031] Phosphorus (P): 0.05% or less
[0032] Phosphorus (P) is an impurity inevitably included in steel,
causing grain boundary corrosion during a pickling process and
deteriorating the hot formability of the steel. Therefore, the
content of phosphorus (P) is adjusted as low as possible. In the
present disclosure, the upper limit of the content of phosphorus
(P) is set to 0.05%.
[0033] Sulfur (S): 0.005% or less
[0034] Sulfur (S), an impurity inevitably included in steel,
segregates along grain boundaries of the steel and deteriorates the
hot formability of the steel. Therefore, the content of sulfur (S)
is adjusted as low as possible. In the present disclosure, the
upper limit of the content of sulfur (S) is set to be 0.005%.
[0035] Chromium (Cr): 10% to 30%
[0036] Chromium (Cr) is effective in increasing the corrosion
resistance of steel. In the present disclosure, chromium (Cr) is
added in an amount of 10% or greater. However, if the content of
chromium (Cr) is excessively high, manufacturing costs increase
markedly, and grain boundary corrosion occurs. Therefore, the
content of chromium (Cr) is limited to 30% or less.
[0037] Titanium (Ti): 0.05% to 0.50%
[0038] Titanium (Ti) fixes carbon (C) and nitrogen (N), thereby
decreasing the amounts of carbon (C) and nitrogen (N) dissolved in
steel. In addition, titanium (Ti) is effective in improving the
corrosion resistance of steel. In the present disclosure, titanium
(Ti) is added in an amount of 0.05% or greater, more preferably
0.1% or greater. However, if the content of titanium (Ti) is
excessively high, manufacturing costs increase markedly, and
Ti-containing inclusions are formed causing surface defects.
Therefore, the content of titanium (Ti) is limited to 0.50% or
less, more preferably 0.30% or less.
[0039] Aluminum (Al): 0.01% to 0.15%
[0040] Aluminum (Al) is a powerful deoxidizer used to decrease the
oxygen content of molten steel. In the present disclosure, aluminum
(Al) is added in an amount of 0.01% or greater. However, if the
content of aluminum (Al) is excessively high, nonmetallic
inclusions increase, causing defects in sleeves of cold-rolled
strips and deteriorating the weldability of steel. Therefore, the
content of aluminum (Al) is limited to 0.15% or less, more
preferably 0.1% or less.
[0041] Nitrogen (N): 0.005% to 0.03%
[0042] Nitrogen (N) is an element facilitating recrystallization by
precipitating austenite during a hot rolling process. In the
present disclosure, nitrogen (N) is added in an amount of 0.005% or
greater. However, if the content of nitrogen (N) in steel is
excessively high, the ductility of the steel decreases. Therefore,
the content of nitrogen (N) is limited to 0.03% or less.
[0043] The ferritic stainless steel of the present disclosure may
include 3.5.times.10.sup.6 or fewer (excluding zero) independent
Ti(CN) precipitate particles per square millimeter (mm.sup.2) of
ferrite matrix. As described above, the Ti(CN) precipitate includes
an independent Ti(CN) precipitate and a dependent Ti(CN)
precipitate formed using TiN inclusion particles as precipitation
nuclei. The dependent Ti(CN) precipitate does not have a
significant effect on ductility deterioration when compared to the
independent Ti(CN) precipitate. Therefore, only the number of
independent Ti(CN) precipitate particles is controlled in the
present disclosure. If the number of independent Ti(CN) precipitate
particles is outside the above-mentioned range, it may be difficult
to obtain a desired degree of ductility.
[0044] As described above, a method of reducing the number of
independent Ti(CN) precipitate particles is to increase the amount
of Ti(CN) precipitating using TiN inclusion particles as
precipitation nuclei. According to an exemplary embodiment of the
present disclosure, a desired degree of ductility may be obtained
by adjusting P defined by Formula 1 below within the range of 60%
or less.
P(%)={N.sub.S/(N.sub.S+N.sub.C)}.times.100 [Formula 1]
[0045] where N.sub.S refers to the number of independent Ti(CN)
precipitate particles per unit area (mm.sup.2), and N.sub.C refers
to the number of dependent Ti(CN) precipitate particles per unit
area (mm.sup.2).
[0046] In the present disclosure, the independent Ti(CN)
precipitate being the subject of control may be limited to having a
particle diameter of 0.01 .mu.m or greater. Since there is a limit
to analyzing and quantifying independent Ti(CN) precipitate having
a particle diameter of less than 0.01 .mu.m, special consideration
may not be given thereto. The upper limit of the particle diameter
of the independent Ti(CN) precipitate may not be specifically set.
However, since it is difficult to form an independent Ti(CN)
precipitate having a particle diameter of 2 .mu.m or greater, the
upper limit of the particle diameter of the independent Ti(CN)
precipitate may be set to be 2 .mu.m.
[0047] It may be preferable that the independent Ti(CN) precipitate
have an average particle diameter of 0.15 .mu.m or less. If the
average particle diameter of the independent Ti(CN) precipitate is
greater than 0.15 .mu.m, surface defects may be formed even though
the number of independent Ti(CN) precipitate particles is small.
The term "average particle diameter" refers to the average of
equivalent circular diameters of particles measured by observing a
cross-section of steel.
[0048] In addition, it may be preferable that the average particle
diameter of a TiN inclusion be within the range of 2 .mu.m or
greater. The reason for this is that a relatively coarse TiN
inclusion having an average particle diameter of 2 .mu.m or greater
forms nucleus forming sites more efficiently, and thus facilitates
the precipitation of Ti(CN). The upper limit of the average
particle diameter of the TiN inclusion is not limited. However, if
the TiN inclusion is excessively coarse, the total surface area of
the TiN inclusion may be excessively small, and thus it may be
difficult to increase the number of dependent Ti(CN) precipitate
particles. Therefore, the upper limit of the average particle
diameter of the TiN inclusion may be set to be 20 .mu.m.
[0049] The ferritic stainless steel of the present disclosure has a
high degree of ductility. According to an exemplary embodiment of
the present disclosure, the ferritic stainless steel may have an
elongation of 34% or greater.
[0050] The ferritic stainless steel of the present disclosure may
be manufactured by various methods without limit. For example,
according to an exemplary embodiment, the ferritic stainless steel
may be manufactured as follows.
[0051] Hereinafter, a method for manufacturing ferritic stainless
steel having a high degree of ductility will be described in detail
according to an aspect of the present disclosure.
[0052] According to the aspect of the present disclosure, the
method for manufacturing ferritic stainless steel includes casting
molten steel having the above-described composition as a slab. One
of the technical features of the method is to maximally restrict
the formation of an independent Ti(CN) precipitate by facilitating
the diffusion of titanium (Ti), carbon (C), and nitrogen (N), and
thus inducing the formation of a dependent Ti(CN) precipitate with
the help of TiN inclusion particles functioning as precipitation
nuclei.
[0053] In general, a slab produced by casting molten steel is
subjected to a cooling process to improve productivity. However,
according to the research conducted by the inventors, if a slab is
cooled at a normal cooling rate, relatively fine TiN inclusion
particles are formed in the slab, and Ti(CN) precipitates randomly
in the slab, thereby markedly increasing the number of independent
Ti(CN) precipitate particles. The reason for this is speculated as
follows: relatively rapid cooling of the slab limits the diffusion
of alloying elements in the slab, and sufficient nucleus forming
energy facilitates the formation of nuclei of a TiN inclusion and a
Ti(CN) precipitate simultaneously across the slab.
[0054] However, according to the present disclosure, after the
molten steel is cast as a slab, the slab is cooled within the
temperature range of 1100.degree. C. to 1200.degree. C. based on
the surface temperature of the slab at an average cooling rate of
5.degree. C./sec or less (excluding 0.degree. C./sec), preferably
3.degree. C./sec or less (excluding 0.degree. C./sec), more
preferably 2.degree. C./sec (excluding 0.degree. C./sec). That is,
the inventors have tried to precipitate as much Ti(CN) as possible
using TiN inclusion particles as precipitation nuclei by properly
controlling the average cooling rate of a slab within the
temperature range of 1100.degree. C. to 1200.degree. C., and thus
to decrease the number of independent Ti(CN) precipitate particles.
The inventors have found that if a slab is cooled under the
conditions described above, the number of independent Ti(CN)
precipitate particles is reduced to a target value or less. The
reason for this may be that since slow cooling guarantees a
sufficient time period for alloying elements to move, large amounts
of Ti, C, and N diffuse toward TiN inclusion particles and
precipitate in the form of Ti(CN) using the TiN inclusion particles
as precipitation nuclei. In the present disclosure, the average
cooling rate of the slab may be controlled using any method or
apparatus. For example, a heat insulating material may be disposed
around a cast strand.
[0055] As described above, the method of controlling the average
cooling rate of the slab is not limited. For example, the slab may
be cooled slowly at a constant cooling rate within the
above-mentioned temperature range, or the slab may be cooled at a
relatively high cooling rate after the slab is constantly
maintained at a particular temperature within the temperature
range.
[0056] According to an exemplary embodiment of the present
disclosure, the temperature range within which the slab is slowly
cooled may be widened to a range of 1000.degree. C. to 1250.degree.
C. to induce the formation of a coarse TiN inclusion and enable the
coarse TiN inclusion to function as nucleus forming sites more
effectively for the precipitation of Ti(CN).
[0057] According to an exemplary embodiment of the present
disclosure, the method may further include: forming a hot-rolled
sheet by performing a finish hot rolling process on the slab; and
performing a hot band annealing process on the hot-rolled sheet.
These processes will now be described in detail.
[0058] Hot band annealing process: perform within the range of
450.degree. C. to 1080.degree. C. for 60 minutes or less.
[0059] The hot band annealing process is performed to improve the
ductility of the hot-rolled sheet. Owing to the hot band annealing
process, the independent Ti(CN) precipitate may be dissolved again,
and dissolved alloying elements may be diffused, thereby further
decreasing the number of independent Ti(CN) precipitate particles.
To this end, the hot band annealing process may be performed at a
temperature of 450.degree. C. or higher. However, if the
temperature of the hot band annealing process is higher than
1080.degree. C., or the duration of the band annealing process is
longer than minutes, the dependent Ti(CN) precipitate may be
dissolved again, and thus the above-mentioned effects may be
decreased. The lower limit of the duration of the band annealing
process is not limited. For example, it may be preferable that the
band annealing process be performed for 1 minute or longer to
obtain sufficient effects.
[0060] As long as the above-mentioned manufacturing conditions for
the ferritic stainless steel are controlled as described above,
other conditions may be controlled according to manufacturing
conditions for normal ferritic stainless steel. In addition, the
annealed hot-rolled sheet may be subjected to a cold rolling
process and a cold rolled sheet annealing process to produce a
cold-rolled steel sheet.
[0061] Hereinafter, aspects of the present disclosure will be
described more specifically according to examples. However, the
following examples should be considered in a descriptive sense only
and not for purpose of limitation. The scope of the present
invention is defined by the appended claims, and modifications and
variations reasonably made therefrom.
Mode for Invention
[0062] Molten steels having the compositions shown in Table 1 were
prepared and were cast at a constant speed under the conditions
shown in Table 2 in order to produce slabs. The slabs were
subjected to a hot rolling process and a hot band annealing process
to obtain hot-rolled sheets. In Table 1, the contents of elements
are given in wt %, and in Table 2, the slab cooling rate is an
average cooling rate measured based on the surface temperature of a
slab within the temperature range of 1100.degree. C. to
1200.degree. C.
TABLE-US-00001 TABLE 1 Steel C Si Mn P S Cr Ti Al N A 0.012 0.25
0.16 0.031 0.003 11.0 0.15 0.040 0.012 B 0.015 0.35 0.8 0.025 0.002
12.0 0.21 0.032 0.015
TABLE-US-00002 TABLE 2 Slab Cooling Rate Hot Band Hot Band
(.degree. C./sec) within the Annealing Annealing Temperature Range
of Temperature Time Steel 1100.degree. C. to 1200.degree. C.
(.degree. C.) (min) Notes A 2 600 30 Inventive Example 1 A 2 800 15
Inventive Example 2 A 6 800 15 Comparative Example 1 B 1 900 15
Inventive Example 3 B 6 900 15 Comparative Example 2
[0063] Thereafter, the hot-rolled sheets were photographed using a
transmission electron microscope (TEM), and the number and ratio
(P) of independent Ti(CN) precipitate particles having a particle
diameter of 0.01 .mu.m or greater were measured using an image
analyzer. In addition, samples were taken from the hot-rolled
sheets based on a direction making an angle of 90.degree. with the
rolling direction of the hot-rolled sheets according to JIS 13B,
and the elongation of the samples was measured. Results of the
measurements are shown in Table 3.
TABLE-US-00003 TABLE 3 Number of Independent Ti (CN) Precipitate
Particles per P Elongation Steel Millimeters (mm.sup.2) (%) (%)
Notes A 3.1 .times. 10.sup.6 56 37 Inventive Example 1 A 2.9
.times. 10.sup.6 42 37 Inventive Example 2 A 8.9 .times. 10.sup.6
88 30 Comparative Example 1 B 2.2 .times. 10.sup.6 58 39 Inventive
Example 3 B 6.5 .times. 10.sup.6 79 32 Comparative Example 2
[0064] Referring to Table 3, Samples of Inventive Examples 1 to 3
satisfying the conditions proposed in the present disclosure had
3.5.times.10.sup.6 or fewer independent Ti(CN) precipitate
particles per square millimeter (mm.sup.2) and thus had an
elongation of 34% or greater. However, each sample of Comparative
Examples 1 and 2 had an excessive number of independent Ti(CN)
precipitate particles because the slab cooling rate was relatively
high, and thus the ductility of the samples of Comparative Examples
1 and 2 were poor.
[0065] FIG. 1 is a scanning electron microscope (SEM) image
illustrating the microstructure of a hot-rolled sheet of Inventive
Example 1, and FIG. 2 is a higher magnification SEM image
illustrating region A in FIG. 1. A particle shown in the center of
region A in FIG. 1 corresponds to a TiN inclusion particle
crystallized during a steel making process. Referring to FIG. 2
illustrating region A on an enlarged scale, a large amount of
Ti(CN) has precipitated on the TiN inclusion particle functioning
as a precipitation nucleus.
* * * * *