U.S. patent application number 17/265374 was filed with the patent office on 2021-10-07 for ti- and nb-added ferritic stainless steel excellent in low-temperature toughness of welds.
This patent application is currently assigned to POSCO. The applicant listed for this patent is POSCO. Invention is credited to Deok Chan Ahn, Il Chan Jung, Jin-Suk Kim, Jong Chul Kim, Wan-Yi Kim.
Application Number | 20210310105 17/265374 |
Document ID | / |
Family ID | 1000005680467 |
Filed Date | 2021-10-07 |
United States Patent
Application |
20210310105 |
Kind Code |
A1 |
Kim; Jong Chul ; et
al. |
October 7, 2021 |
TI- AND NB-ADDED FERRITIC STAINLESS STEEL EXCELLENT IN
LOW-TEMPERATURE TOUGHNESS OF WELDS
Abstract
A Ti, Nb-added ferritic stainless steel with excellent
low-temperature toughness of weld zone according to an embodiment
of present disclosure includes, in percent (%) by weight of the
entire composition, C: 0.004 to 0.015%, N: 0.004 to 0.015%, Si:
0.01 to 0.7%, Mn: 0.01 to 0.7%, P: 0.0001 to 0.04%, S: 0.0001 to
0.005%, Cr: 10 to 30%, Al: 0.005 to 0.04%, Ti: 0.1 to 0.5%, Nb: 0.1
to 0.6%, Ca: 0.0001 to 0.003%, the remainder of iron (Fe) and other
inevitable impurities, satisfies the following equation (1), and
Al--Ca--Ti--Mg--O-based oxide and Ti--Nb--C--N carbonitride
containing the oxide have an average diameter of 3 to 10 .mu.m and
a distribution density of 4/mm.sup.2 or more.
{(Ti+0.5*Nb)*(C+N)}/Al>0.25 (1)
Inventors: |
Kim; Jong Chul; (Daegu,
KR) ; Kim; Wan-Yi; (Pohang-si, Gyeongsangbuk-do,
KR) ; Jung; Il Chan; (Pohang-si, Gyeongsangbuk-do,
KR) ; Kim; Jin-Suk; (Pohang-si, Gyeongsangbuk-do,
KR) ; Ahn; Deok Chan; (Seoul, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
POSCO |
Pohang-si, Gyeongsangbuk-do |
|
KR |
|
|
Assignee: |
POSCO
Pohang-si, Gyeongsangbuk-do
KR
|
Family ID: |
1000005680467 |
Appl. No.: |
17/265374 |
Filed: |
November 2, 2018 |
PCT Filed: |
November 2, 2018 |
PCT NO: |
PCT/KR2018/013229 |
371 Date: |
February 2, 2021 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/001 20130101;
C22C 38/28 20130101; C22C 38/26 20130101; C21D 2211/005 20130101;
C21C 5/005 20130101; C22C 38/04 20130101; C22C 38/002 20130101;
C21D 8/105 20130101; C22C 38/02 20130101; C22C 38/06 20130101 |
International
Class: |
C22C 38/28 20060101
C22C038/28; C22C 38/26 20060101 C22C038/26; 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 8/10 20060101
C21D008/10; C21C 5/00 20060101 C21C005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 3, 2018 |
KR |
10-2018-0090874 |
Claims
1. A Ti, Nb-added ferritic stainless steel with excellent
low-temperature toughness of weld zone comprising, in percent (%)
by weight of the entire composition, C: 0.004 to 0.015%, N: 0.004
to 0.015%, Si: 0.01 to 0.7%, Mn: 0.01 to 0.7%, P: 0.0001 to 0.04%,
S: 0.0001 to 0.005%, Cr: 10 to 30%, Al: 0.005 to 0.04%, Ti: 0.1 to
0.5%, Nb: 0.1 to 0.6%, Ca: 0.0001 to 0.003%, the remainder of iron
(Fe) and other inevitable impurities, and satisfying the following
equation (1), Al--Ca--Ti--Mg--O-based oxide and Ti--Nb--C--N
carbonitride containing the oxide have an average diameter of 3 to
10 .mu.m and a distribution density of 4/mm.sup.2 or more:
{(Ti+0.5*Nb)*(C+N)}/Al>0.25 (1) (Here, Ti, Nb, C, N, and Al mean
the content (% by weight) of each element).
2. The ferritic stainless steel of claim 1, wherein the
distribution density is 4/mm.sup.2 or more and 15/mm.sup.2 or
less.
3. The ferritic stainless steel of claim 1, further comprising: any
one or more selected from the group consisting of Mo: 0.1 to 2.0%,
Ni: 0.1 to 2.0% and Cu: 0.1 to 2.0%.
4. The ferritic stainless steel of claim 1, wherein the
Al--Ca--Ti--Mg--O-based oxide satisfies the following equations (2)
to (4): %(TiO.sub.2)+%(CaO)+%(Al.sub.2O.sub.3).gtoreq.80 (2)
{%(TiO.sub.2)+%(CaO)}/{%(TiO.sub.2)+%(CaO)+%(Al.sub.2O.sub.3)}.gtoreq.0.3
(3) 0.3.ltoreq.%(CaO)/%(TiO.sub.2).ltoreq.0.8. (4)
5. The ferritic stainless steel of claim 1, wherein the
Ti--Nb--C--N carbonitride has the Al--Ca--Ti--Mg--O-based oxide as
a nucleus and is formed to surround the Al--Ca--Ti--Mg--O-based
oxide.
6. The ferritic stainless steel of claim 1, wherein the average
grain size of the solidified structure of the weld zone is less
than 150 .mu.m.
7. The ferritic stainless steel of claim 1, wherein an impact
energy of the weld zone is 90 J/cm.sup.2 or more at -30.degree.
C.
8. The ferritic stainless steel of claim 1, wherein a DBTT of the
weld zone is -25.degree. C. or less:
{(Ti+0.5*Nb)*(C+N)}/Al.gtoreq.0.25. (1)
Description
TECHNICAL FIELD
[0001] The present disclosure relates to a ferritic stainless
steel, and more specifically, Ti, Nb-added ferritic stainless steel
with excellent low-temperature toughness of weld zone.
BACKGROUND ART
[0002] The main use of ferritic stainless steel is parts for
automobile exhaust system. Mainly, the final product is made by
forming through press processing and welding these processed
products, or by expanding and forming the welded pipe. Therefore,
as an important requirement of ferritic stainless steel for
automobile exhaust system, the processing characteristics of the
weld zone are mentioned.
[0003] The welding process of ferritic stainless steel generally
melts the base metal using arc heat, and the molten metal is
rapidly cooled to form a solidified structure, and the grain size
and shape of the solidified structure have a great influence on the
workability of the weld zone.
[0004] In particular, the welding method for automobile exhaust
system has a large heat input and a wide range, which increases the
probability of cracking during subsequent processing due to
coarsening of the grains in the weld zone. In addition, coarsening
of weld zone grains has a characteristic of impairing
low-temperature toughness characteristics, and particularly, there
is a problem in that the rate of occurrence of weld zone cracks
increases rapidly during product processing in winter.
[0005] Therefore, it was found that it was necessary to refine the
solidified structure of the molten part to satisfy the weld zone
characteristics of automobile exhaust system parts.
[0006] As a solidified structure refinement technique, a
low-temperature casting method and an electromagnetic stirring are
used, but these techniques can refine the solidified structure of
the base material, but have no effect on the refinement of the
solidified structure of the molten part during welding.
[0007] In particular, the solidification condition of the weld zone
has a characteristic that the solidified structure becomes coarse
because the cooling rate is faster than that of the normal
solidification condition, so it is advantageous to grow into a
columnar crystal. Therefore, in order to refine the solidified
structure of the weld zone, it is possible by promoting non-uniform
nucleation. When the re-dissolved molten part re-solidifies during
welding, non-uniform nucleation occurs due to the remaining oxide,
which promotes nucleation and growth of the equiaxed crystal, and
the solidified structure is expected to be refined.
[0008] As an example of non-uniform using oxides of ferritic
stainless steel, Prior Document 1 discloses a technology for
refining the base material structure using Al--Mg-based inclusions.
The Prior Document 2 discloses a technique of manufacturing
stainless steel mainly using a composite oxide containing Ti and
Ca. In addition, Prior Document 3 discloses that MgO and
MgO--Al.sub.2O.sub.3 can be produced to secure a base material
structure.
[0009] However, Prior Documents 1 to 3 above focus on refinement of
the solidified structure of the base material, and do not consider
the composition of oxides or the number of sizes of oxides for the
solidified structure of the weld zone. In particular, in the case
of the weld zone, unlike the usual casting structure, the melting
temperature is high, so that the effect may be lost due to
re-dissolution of the oxide, and the cooling rate is fast, so that
the size control of the oxide for refinement is required.
Therefore, in the case of the prior documents, it cannot be said to
be a preferred method for refinement the solidified structure of
the weld zone.
[0010] (Prior Document 1) Korean Patent Application Publication No.
10-2011-0074217 (published on Jun. 30, 2011)
[0011] (Prior Document 2) Japanese Patent Application Publication
No. 2000-001715 (published on Jan. 7, 2000)
[0012] (Prior Document 3) Japanese Patent Application Publication
No. 2001-254153 (published on Sep. 18, 2001)
DISCLOSURE
Technical Problem
[0013] Embodiments of the present disclosure are intended to
provide ferritic stainless steel capable of improving
low-temperature toughness of weld zone through refinement of the
base material structure of stainless steel and the solidified
structure of the weld zone.
Technical Solution
[0014] In accordance with an aspect of the present disclosure, a
Ti, Nb-added ferritic stainless steel with excellent
low-temperature toughness of weld zone includes, in percent (%) by
weight of the entire composition, C: 0.004 to 0.015%, N: 0.004 to
0.015%, Si: 0.01 to 0.7%, Mn: 0.01 to 0.7%, P: 0.0001 to 0.04%, S:
0.0001 to 0.005%, Cr: 10 to 30%, Al: 0.005 to 0.04%, Ti: 0.1 to
0.5%, Nb: 0.1 to 0.6%, Ca: 0.0001 to 0.003%, the remainder of iron
(Fe) and other inevitable impurities, satisfies the following
equation (1), and Al--Ca--Ti--Mg--O-based oxide and Ti--Nb--C--N
carbonitride containing the oxide have an average diameter of 3 to
10 .mu.m and a distribution density of 4/mm.sup.2 or more.
{(Ti+0.5*Nb)*(C+N)}/Al>0.25 (1)
[0015] Here, Ti, Nb, C, N, and Al mean the content (% by weight) of
each element.
[0016] The distribution density may be 4/mm.sup.2 or more and
15/mm.sup.2 or less.
[0017] The ferritic stainless steel may further include: any one or
more selected from the group consisting of Mo: 0.1 to 2.0%, Ni: 0.1
to 2.0% and Cu: 0.1 to 2.0%.
[0018] The Al--Ca--Ti--Mg--O-based oxide may satisfy the following
equations (2) to (4).
%(TiO.sub.2)+%(CaO)+%(Al.sub.2O.sub.3).gtoreq.80 (2)
{%(TiO.sub.2)+%(CaO)}/{%(TiO.sub.2)+%(CaO)+%(Al.sub.2O.sub.3)}.gtoreq.0.-
3 (3)
0.3.ltoreq.%(CaO)/%(TiO.sub.2).ltoreq.0.8 (4)
[0019] The Ti--Nb--C--N carbonitride may have the
Al--Ca--Ti--Mg--O-based oxide as a nucleus and may be formed to
surround the Al--Ca--Ti--Mg--O-based oxide.
[0020] The average grain size of the solidified structure of the
weld zone may be less than 150 .mu.m.
[0021] The impact energy of the weld zone may be 90 J/cm.sup.2 or
more at -30.degree. C.
[0022] The DBTT of the weld zone may be -25.degree. C. or less.
Advantageous Effects
[0023] Examples of the present disclosure can control the size and
distribution density of effective nucleation products in the base
metal of stainless steel by controlling the composition of Ti,
Nb-added ferritic stainless steel, and accordingly, the solidified
structure of the weld zone can be refined, and the low-temperature
toughness of weld zone can be improved.
DESCRIPTION OF DRAWINGS
[0024] FIG. 1 is a photograph showing a solidified structure of a
Ti, Nb-added ferritic stainless steel weld zone according to an
embodiment of the present disclosure.
[0025] FIG. 2 is a photograph showing the solidified structure of a
Ti, Nb-added ferritic stainless steel weld zone according to a
comparative example.
[0026] FIG. 3 is a graph showing a result of analysis of nucleation
inclusions in the center of a grain of a weld zone solidified
structure of Ti, Nb-added ferritic stainless steel according to an
embodiment of the present disclosure.
[0027] FIG. 4 is a graph showing the distribution of the number of
effective nucleation products by size of the inventive example and
comparative example according to the present disclosure.
[0028] FIG. 5 is a graph showing the number of effective nucleation
products having a size of 3 to 10 .mu.m per unit area in an
inventive example and a comparative example according to the
present disclosure.
[0029] FIG. 6 is a graph showing the results of measuring the
average grain size of the weld zone solidified structure of the
inventive example and comparative example according to the present
disclosure.
[0030] FIG. 7 is a graph showing the result of measuring the impact
energy of the weld zone of the inventive example and comparative
example according to the present disclosure.
[0031] FIG. 8 is a graph showing a result of measuring a weld zone
DBTT of an inventive example and a comparative example according to
the present disclosure.
[0032] FIG. 9 is a graph showing the correlation between the value
of Equation (1) and the average grain size of the weld zone of the
inventive example and comparative example according to the present
disclosure.
BEST MODE
[0033] A Ti, Nb-added ferritic stainless steel with excellent
low-temperature toughness of weld zone according to an embodiment
of present disclosure includes, in percent (%) by weight of the
entire composition, C: 0.004 to 0.015%, N: 0.004 to 0.015%, Si:
0.01 to 0.7%, Mn: 0.01 to 0.7%, P: 0.0001 to 0.04%, S: 0.0001 to
0.005%, Cr: 10 to 30%, Al: 0.005 to 0.04%, Ti: 0.1 to 0.5%, Nb: 0.1
to 0.6%, Ca: 0.0001 to 0.003%, the remainder of iron (Fe) and other
inevitable impurities, satisfies the following equation (1), and
Al--Ca--Ti--Mg--O-based oxide and Ti--Nb--C--N carbonitride
containing the oxide have an average diameter of 3 to 10 .mu.m and
a distribution density of 4/mm.sup.2 or more.
{(Ti+0.5*Nb)*(C+N)}/Al>0.25 (1)
[0034] Here, Ti, Nb, C, N, and Al mean the content (% by weight) of
each element.
[0035] [Modes of the Invention]
[0036] Hereinafter, the embodiments of the present disclosure will
be described in detail with reference to the accompanying drawings.
The following embodiments are provided to transfer the technical
concepts of the present disclosure to one of ordinary skill in the
art. However, the present disclosure is not limited to these
embodiments, and may be embodied in another form. In the drawings,
parts that are irrelevant to the descriptions may be not shown in
order to clarify the present disclosure, and also, for easy
understanding, the sizes of components are more or less
exaggeratedly shown.
[0037] The inventers of present disclosure have to control the size
and number of Ti--Nb--CN carbonitride including oxides that promote
nucleation of delta ferrite to act as an effective nucleation
product. As a result, the effective nucleation product formation
conditions could be derived.
[0038] The inventors of the present disclosure understand that the
size and number of Ti--Nb--CN carbonitride including oxides that
promote nucleation of delta ferrite must be controlled in order to
act as an effective nucleation product, and for this purpose,
control of components in molten steel is the key. As a result of
the experiment, the conditions for forming an effective nucleation
product could be derived.
[0039] In the present disclosure, an effective nucleation product
means an Al--Ca--Ti--Mg--O-based oxide and Ti--Nb--C--N
carbonitride including the same.
[0040] A Ti, Nb-added ferritic stainless steel with excellent
low-temperature toughness of weld zone according to an embodiment
of present disclosure includes, in percent (%) by weight of the
entire composition, C: 0.004 to 0.015%, N: 0.004 to 0.015%, Si:
0.01 to 0.7%, Mn: 0.01 to 0.7%, P: 0.0001 to 0.04%, S: 0.0001 to
0.005%, Cr: 10 to 30%, Al: 0.005 to 0.04%, Ti: 0.1 to 0.5%, Nb: 0.1
to 0.6%, Ca: 0.0001 to 0.003%, the remainder of iron (Fe) and other
inevitable impurities.
[0041] Hereinafter, the reason for the numerical limitation of the
content of the alloy component element in the embodiment of the
present disclosure will be described. Hereinafter, unless otherwise
specified, the unit is % by weight.
[0042] The content of C and N is 0.004 to 0.015%, respectively.
[0043] It was confirmed that the roles of C and N in Ti--Nb
composite added steel have a great influence on the refinement of
the weld zone structure. That is, after the oxide is formed, a
Ti--Nb composite carbonitride is formed, and delta ferrite
nucleation are generated based on the carbonitride. Here, the
content of C and N is related to the crystallization temperature of
Ti--Nb--C--N carbonitride, and must have a minimum value in order
to have an effective effect on the nucleation of delta ferrite.
Therefore, each content is limited to at least 0.004% or more, and
the influence of the content value will be described in detail
later in setting the Ti and Nb content. Also, in the case of C and
N, both elements are interstitial elements and when the amount of
addition increases, workability decreases during molding due to
lower elongation, and each maximum value is limited to 0.015% due
to lower corrosion resistance due to the formation of grain
boundary Cr carbonitride.
[0044] The content of Si and Mn is 0.01 to 0.7%, respectively.
[0045] For Si and Mn, corrosion resistance and formability must be
considered at the same time, and it is limited to 0.01% or more in
terms of corrosion resistance and 0.7% or less in terms of
workability.
[0046] Si is an element added in terms of corrosion resistance, and
if it is less than 0.01%, it is difficult to obtain sufficient
corrosion resistance. When Si exceeds 0.7%, the impurities of the
material increase, the elongation and work hardening index (n
value) decrease, and the Si-based inclusions increase, resulting in
poor workability. Therefore, the content range thereof is
preferably 0.01 to 0.7%.
[0047] Mn is an element added in terms of corrosion resistance, and
if it is less than 0.01%, it is difficult to obtain sufficient
corrosion resistance, but if it exceeds 0.7%, there is a problem
that elongation and corrosion resistance decrease due to increased
impurities in the material. Therefore, the content range thereof is
preferably 0.01 to 0.7%.
[0048] It is preferable that both P and S are low, but in
consideration of manufacturing cost, P is limited to 0.0001 to
0.04% and S is limited to 0.0001 to 0.005%.
[0049] The content of P is preferably low in terms of corrosion
resistance. Preferably, the lower limit of the content is 0.0001%
in consideration of the cost in the steelmaking process. Therefore,
it is preferable that its content range is 0.0001 to 0.04%.
[0050] The content of S is preferably low in terms of corrosion
resistance. Preferably, the lower limit of the content is 0.0001%
in consideration of the cost in the steelmaking process. Therefore,
it is preferable that the content range is 0.0001 to 0.005%.
[0051] The content of Cr is 10 to 30%.
[0052] When the content of Cr is less than 10%, corrosion
resistance as stainless steel is insufficient, and when the content
of Cr is more than 30%, formability decreases, and the content
range thereof is preferably 10 to 30%.
[0053] The content of Al is 0.005 to 0.04%.
[0054] In the case of Al, it is absolutely necessary as a
deoxidation element, but when a large amount is added, it is
difficult to improve the low-temperature toughness because it
cannot suppress the coarsening of the weld zone grains due to the
formation of an invalid oxide. Therefore, the maximum value is
limited to 0.04% for grain refinement of the weld zone while
including at least 0.005% in consideration of the deoxidization
effect.
[0055] The content of Ti is 0.1 to 0.5%.
[0056] Ti is the most important element that determines the
effective nucleation product of the present disclosure, and the
lower limit of Ti is limited to 0.1% to satisfy the composition,
size, and distribution of the effective nucleation product
suggested in the present disclosure through a series of
experiments. In addition, when a large amount is added in excess of
0.5%, linear defects due to inclusions in the final product occur
frequently due to a high melting point nitride such as TiN, so the
upper limit is limited to 0.5%.
[0057] The content of Nb is 0.1 to 0.6%.
[0058] Nb is an essential element for securing high temperature
strength of high-temperature exhaust system components, and at the
same time has an influence on the formation of effective nucleation
products. In particular, in order to secure the characteristics as
a high-temperature exhaust system component of 660.degree. C. or
higher, it must contain at least 0.1%, and if excessively added
exceeding 0.6%, the cost of raw materials is higher than the
increase in high-temperature strength, so the upper limit is
limited to 0.6%.
[0059] The content of Ca is 0.0001 to 0.003%.
[0060] In the case of Ca, as a deoxidation element, it is an
important element in the formation of effective oxides in the
present disclosure. However, when a large amount is contained, the
formation of effective oxides is suppressed and the corrosion
resistance is also adversely affected, so the maximum value is
limited to 0.003%, and the minimum value is 0.0001%, which is the
minimum value for effective oxide formation.
[0061] In addition, Ti, Nb-added ferritic stainless steel with
excellent low-temperature toughness of weld zone according to an
embodiment of the present disclosure may further include, in
percent (%) by weight, any one or more selected from the group
consisting of Mo: 0.1 to 2.0%, Ni: 0.1 to 2.0%, and Cu: 0.1 to
2.0%.
[0062] The amount of Mo is 0.1 to 2.0%.
[0063] Mo may be additionally added as a composition to increase
the corrosion resistance of stainless steel, and if it is added in
an excessive amount, the impact characteristics are deteriorated,
thereby increasing the risk of breakage during processing and
increasing the cost of the material. Therefore, it is preferable to
limit the content of Mo to 0.1 to 2.0% in consideration of this in
the present disclosure.
[0064] The amount of Ni is 0.1 to 2.0%. Ni is an element that
improves corrosion resistance, and if it is added in a large
amount, it is not only hardened, but also stress corrosion cracking
may occur, so it is preferable to be 2.0% or less.
[0065] The amount of Cu is 0.1 to 2.0%. It is preferable that Cu
contains 0.1 to 1.0% to improve corrosion resistance. However, when
it exceeds 1.0%, there is a problem that workability is
deteriorated.
[0066] The remainder of the ferritic stainless steel except for the
aforementioned alloying elements is made of Fe and other inevitable
impurities.
[0067] In the case of high Cr ferritic stainless steel with Ti--Nb
composite addition, the impact energy change according to
temperature does not change rapidly. Therefore, a temperature
having an impact energy value (90 J/cm.sup.2) that decreases by 50%
compared to the impact energy at room temperature was defined as
the ductile brittle transition temperature (DBTT), and is shown in
FIG. 8. Based on the DBTT temperature, the fracture behavior
changes from ductile fracture to brittle fracture, which is the
main cause of cracking during welding zone processing under low
temperature conditions. Therefore, it is desirable that the DBTT is
low.
[0068] Based on the apparent difference in the weld zone solidified
structure even under the same steel grade and similar component
conditions and the DBTT value also changing, the solidified
structure refinement mechanism was identified, and based on this, a
present disclosure that could improve the low-temperature toughness
of weld zone was proposed.
[0069] As a result of investigating the interaction of the molten
steel composition with the refinement of the weld zone solidified
structure in addition to limiting the range of the above-described
molten steel components, the following equation (1) could be
derived.
{(Ti+0.5*Nb)*(C+N)}/Al>0.25 (1)
[0070] When the calculated value of {(Ti+0.5*Nb)*(C+N)}/Al exceeds
0.25 with the content range of Ti, Nb, C, N, and Al of the above
composition, Al--Ca--Ti--Mg--O-based oxide and Ti--Nb--CN
carbonitride including the same are easily formed. By forming such
an effective nucleation product, the weld zone solidified structure
is refined and excellent DBTT characteristics can be obtained. On
the other hand, if Equation (1) is not satisfied, the composition
of the Al--Ca--Ti--Mg--O-based oxide does not satisfy Equations (2)
to (4) to be described later, and as a result, the number of
Ti--Nb--CN carbonitrides based on the oxide is extremely low.
Accordingly, the grains of the weld zone solidified structure are
coarse with an average of 150 .mu.m or more, and the DBTT value is
also increased to -20.degree. C. or more, which lowers the
low-temperature processing characteristics.
[0071] FIG. 1 is a photograph showing a solidified structure of a
Ti, Nb-added ferritic stainless steel weld zone according to an
embodiment of the present disclosure. FIG. 2 is a photograph
showing the solidified structure of a Ti, Nb-added ferritic
stainless steel weld zone according to a comparative example.
[0072] When comparing the weld zone solidified structure of the
inventive example of FIG. 1 and the comparative example of FIG. 2,
in the case of a comparative example, it can be seen that most are
columnar crystals, and equiaxed crystals are formed in some
centers. However, in the case of the inventive example, columnar
crystals exist, but it can be seen that a fraction of fine equiaxed
crystals is widely formed.
[0073] FIG. 3 is a graph showing a result of analysis of nucleation
inclusions in the center of a grain of a weld zone solidified
structure of Ti, Nb-added ferritic stainless steel according to an
embodiment of the present disclosure.
[0074] In order to confirm the cause of the difference in the
solidified structure in the weld zone region of the inventive
examples and comparative examples of FIGS. 1 and 2, FIG. 3 shows
the results of closely observing nucleation inclusions in the
center of the equiaxed crystal with an electron microscope. In the
case of inventive example, a spherical oxide and carbonitride of
Ti--Nb--C--N surrounding it were observed, and most of the
Ti--Nb--C--N carbonitride of 3 .mu.m or more contained a spherical
oxide therein. When the spherical oxide is closely observed through
an electron transmission microscope, it can be seen that a
crystalline CaO--TiO.sub.2 phase and an Al.sub.2O.sub.3--MgO phase
exist together. On the other hand, in the case of a comparative
example, the size of Ti--Nb--C--N carbonitride was small and the
number was small, it was confirmed that the oxide composition in
Ti--Nb--C--N carbonitride was a single Al.sub.2O.sub.3--MgO phase,
an Al.sub.2O.sub.3--MgO and MgO composite phase, or an
Al.sub.2O.sub.3--MgO and Al.sub.2O.sub.3 composite phase.
Therefore, from the above results, the refinement of the weld zone
solidified structure could be confirmed by the oxide composed of
multiple oxide crystal phases including CaO--TiO.sub.2 phase, and
the Ti--Nb--C--N carbonitride formed by using these oxides as
nuclei.
[0075] In particular, it was confirmed that Ti--Nb--C--N
carbonitride has a high crystallization temperature compared to TiN
nitride found in a conventional Ti alone-added steel. That is, in
the case of Ti--Nb composite steel, it was found through
experiments and thermodynamic analysis that Ti--Nb--C--N
carbonitride was crystallized at a higher temperature under the
same Ti component condition than that of the Ti alone-added steel.
Therefore, Ti--Nb--C--N carbonitride is easily formed around the
effective oxide formed in the present disclosure, and as a result,
delta-ferrite nucleation easily occurs below the liquidus
temperature, thereby improving the equiaxed crystal rate of the
weld zone.
[0076] As described above, in order to confirm that Ti--Nb--CN
carbonitride causes a difference in the solidified structure of the
weld zone region, the size and number distributions of Ti--Nb--C--N
carbonitrides present in the base material of the inventive example
and the comparative example were compared and analyzed and shown in
FIG. 4.
[0077] FIG. 4 is a graph showing the distribution of the number of
effective nucleation products by size of the inventive example and
comparative example according to the present disclosure. FIG. 5 is
a graph showing the number of effective nucleation products having
a size of 3 to 10 .mu.m per unit area in an inventive example and a
comparative example according to the present disclosure.
[0078] Referring to FIG. 4, in the case of a comparative example, a
large amount of Ti--Nb--C--N carbonitride of less than 3 .mu.m is
distributed, while the number of Ti--Nb--C--N carbonitrides of 3
.mu.m or more is rapidly decreased. In the case of the inventive
example, it can be seen that a number of Ti--Nb--C--N carbonitrides
having a size of 3 .mu.m or more are distributed. Based on these
results, it can be confirmed that the refinement of the solidified
structure of the weld zone is a Ti--Nb--C--N carbonitride of 3
.mu.m or more. On the other hand, when the size of Ti--Nb--C--N
carbonitride exceeds 10 .mu.m, float separation is easily performed
on the surface of the molten part, so it cannot play the role of
delta-ferrite nucleation.
[0079] In addition, as shown in FIG. 5, as a result of comparing
the number per unit area, it was confirmed that the distribution
density should be 4/mm.sup.2 or more. However, when the number of
Ti--Nb--C--N carbonitrides exceeds 15/mm.sup.2, they form an
aggregate and this becomes a major factor of surface defects, so it
is desirable to have a distribution density of 15/mm.sup.2 or
less.
[0080] According to the embodiment of the present disclosure, as a
method for refinement of the weld zone solidified structure, it
should contain Al--Ca--Ti--Mg--O-based oxide that does not
re-dissolve in molten steel even at high welding heat and remains
in a solid state. This provides a nucleation site of Ti--Nb--C--N
carbonitride when the molten metal in the weld zone is solidified,
and as a result, the amount of equiaxed crystal formation
increases.
[0081] Usually, the oxide observed under Al deoxidation conditions
is Al--Ca--Ti--Mg--O. Al--Ca--Ti--Mg--O-based oxides include
TiO.sub.2, CaO, Al.sub.2O.sub.3, MgO, etc., and the conditions for
simultaneously forming the CaO--TiO.sub.2 phase and the
Al.sub.2O.sub.3--MgO phase, which are advantageous oxides for
nucleation of ferrite, can be predicted from the
Al.sub.2O.sub.3--TiO.sub.2--CaO ternary phase diagram. As a result
of precise analysis and statistics of oxides present in the base
material of the inventive examples and comparative examples, the
average oxide composition of the base metal with improved
low-temperature toughness of weld zone should satisfy the following
equations (2) to (4).
[0082] According to an embodiment of the present disclosure, the
Al--Ca--Ti--Mg--O-based oxide may satisfy the equations (2) to (4)
below.
%(TiO.sub.2)+%(CaO)+%(Al.sub.2O.sub.3).gtoreq.80 (2)
{%(TiO.sub.2)+%(CaO)}/{%(TiO.sub.2)+%(CaO)+%(Al.sub.2O.sub.3)}.gtoreq.0.-
3 (3)
0.3.ltoreq.%(CaO)/%(TiO.sub.2).ltoreq.0.8 (4)
[0083] According to the equation (2), the inclusions during Al
deoxidation are Al--Ca--Ti--Mg--O, and the total ratio of
%(TiO.sub.2), %(CaO) and %(Al.sub.2O.sub.3) should be 80% or more.
When the total ratio of %(TiO.sub.2), %(CaO) and %(Al.sub.2O.sub.3)
is less than 80%, it is difficult to form a CaO--TiO.sub.2 phase
effective for nucleation as it is stabilized with MgO rich oxide or
Al.sub.2O.sub.3--MgO oxide. Due to the high crystallization
temperature, it is difficult to remain in the liquid phase because
they are easily coarsened during the cooling process.
[0084] According to the equation (3), the total ratio of
%(TiO.sub.2), %(CaO) and %(Al.sub.2O.sub.3) to the total ratio of
%(CaO) and %(TiO.sub.2) which is the source of CaO--TiO.sub.2 is
set, and this is to secure a large amount of CaO--TiO.sub.2 phase,
which is advantageous as an equiaxed crystal nucleation site of the
solidified structure of the weld zone. If the ratio is less than
0.3, sufficient refinement of the average grain diameter of the
weld zone solidified structure becomes difficult.
[0085] According to the equation (4), even if the equations (2) and
(3) are satisfied, when the %(CaO)/%(TiO.sub.2) ratio is less than
0.3, the oxide composition cannot sufficiently secure a
CaO--TiO.sub.2 phase which is advantageous for nucleation. When the
ratio of %(CaO)/%(TiO.sub.2) exceeds 0.8, the oxide composition
transitions to a coarse low melting point oxide of
CaO--Al.sub.2O.sub.3 and transitions an oxide ineffective for
nucleation.
[0086] FIG. 6 is a graph showing the results of measuring the
average grain size of the weld zone solidified structure of the
inventive example and comparative example according to the present
disclosure. As a result of comparing the size of the equiaxed
crystal of the weld zone of the inventive examples and comparative
examples, in the case of the inventive example, it can be seen that
the size of the equiaxed crystal is finer by about 40% compared to
the comparative example. Specifically, the average grain diameter
of the weld zone solidified structure of ferritic stainless steel
according to the inventive examples is 97.5 .mu.m, which is 110
.mu.m or less, but the average grain diameter of the weld zone
solidified structure of ferritic stainless steel according to
comparative examples is 167.1 .mu.m, which is 150 .mu.m or
more.
[0087] That is, the average grain size of the weld zone solidified
structure of Ti, Nb-added ferritic stainless steel with excellent
low-temperature toughness of weld zone according to an embodiment
of the present disclosure may be less than 150 .mu.m.
[0088] FIG. 7 is a graph showing the result of measuring the impact
energy of the weld zone of the inventive example and comparative
example according to the present disclosure. FIG. 8 is a graph
showing a result of measuring a weld zone DBTT of an inventive
example and a comparative example according to the present
disclosure.
[0089] Referring to FIGS. 7 and 8, the ductile brittle transition
temperature (DBTT) can be obtained from the weld zone impact energy
graph of FIG. 6, and it was evaluated as -35.8.degree. C. in the
inventive example and -17.8.degree. C. in the comparative example.
It can be seen that the inventive example has a DBTT of about
20.degree. C. lower than that of the comparative example.
[0090] That is, the weld zone impact energy at -30.degree. C. of
Ti, Nb-added ferritic stainless steel with excellent
low-temperature toughness of weld zone according to an embodiment
of the present disclosure may be 90 J/cm.sup.2 or more, and the
weld zone DBTT may be -25.degree. C. or less.
[0091] The evaluation results of the above-described
low-temperature toughness of weld zone and weld zone microstructure
results are summarized as follows. When the weld zone has a fine
solidified structure, it has low DBTT characteristics. The
refinement of the weld zone solidified structure for this was
confirmed that Al--Ca--Ti--Mg--O-based oxides that satisfies all
equations (2) to (4) along with the satisfaction of equation (1)
and Ti--Nb--CN carbonitrides with an average diameter of 3 to 10
.mu.m including the same should have a distribution density of
4/mm.sup.2 or more.
[0092] On the other hand, the oxides present in the test specimens
may appear mixed with various types, among these, in the case of a
specimen whose distribution of Ti--Nb--C--N carbonitride containing
oxides satisfying equations (2) to (4) does not satisfy the above
conditions, the weld zone solidified structure was also coarse and
the DBTT value was also high.
[0093] Therefore, in the case of Ti, Nb-added ferritic stainless
steel according to the present disclosure, the oxide whose
composition satisfies equation (1) and satisfies all equations (2)
to (4), and Ti--Nb--CN carbonitride having an average diameter of 3
to 10 .mu.m including the same, should have a distribution density
of 4/mm.sup.2 or more.
[0094] Hereinafter, the present disclosure will be described in
more detail through inventive examples.
Inventive Example 1 to 8
[0095] After manufacturing a cast steel through the process of
electric furnace (EAF)-refining furnace (AOD)-component adjustment
(LT)-tundish-continuous casting process for stainless steel
containing the composition of the base material according to
inventive examples 1 to 8 of Table 1 below, cold-rolled coils with
a final thickness of 2.0 mm were manufactured through hot rolling
and annealing, and cold rolling and annealing.
Comparative Example 1 to 7
[0096] After manufacturing a cast steel through the process of
electric furnace (EAF)-refining furnace (AOD)-component adjustment
(LT)-tundish-continuous casting process for stainless steel
containing the composition of the base material according to
comparative examples 1 to 7 of Table 1 below, cold-rolled coils
with a final thickness of 2.0 mm were manufactured through hot
rolling and annealing, and cold rolling and annealing.
TABLE-US-00001 TABLE 1 equation Cr C Mn Si P S Al Ti Nb N Mo (1)
comparative 17.7 0.0068 0.219 0.362 0.0206 0.0009 0.030 0.189 0.373
0.0076 0.002 0.180 example 1 comparative 17.8 0.0062 0.244 0.405
0.0289 0.0007 0.026 0.173 0.362 0.0084 0.001 0.199 example 2
comparative 18.0 0.0063 0.266 0.376 0.0246 0.0006 0.054 0.241 0.491
0.0074 0.004 0.123 example 3 comparative 17.7 0.0091 0.223 0.436
0.0246 0.0005 0.046 0.224 0.505 0.0086 0.001 0.183 example 4
comparative 17.9 0.0070 0.236 0.358 0.0232 0.0004 0.024 0.148 0.361
0.0074 0.003 0.197 example 5 comparative 18.4 0.0065 0.237 0.201
0.0218 0.0008 0.027 0.232 0.381 0.0062 0.127 0.199 example 6
comparative 18.6 0.0070 0.225 0.184 0.0234 0.0005 0.026 0.195 0.442
0.0066 0.135 0.218 example 7 inventive 17.8 0.0103 0.216 0.395
0.0226 0.0005 0.028 0.219 0.501 0.0075 0.001 0.298 example 1
inventive 17.8 0.0120 0.237 0.357 0.0271 0.0007 0.022 0.219 0.331
0.0089 0.003 0.365 example 2 inventive 18.0 0.0093 0.208 0.384
0.0235 0.0004 0.027 0.58 0.444 0.0067 0.002 0.284 example 3
inventive 18.0 0.0093 0.208 0.384 0.0235 0.0004 0.027 0.258 0.337
0.0067 0.001 0.253 example 4 inventive 17.8 0.0097 0.233 0.405
0.0273 0.0006 0.019 0.194 0.354 0.0093 0.004 0.371 example 5
inventive 17.8 0.0075 0.222 0.382 0.0220 0.0003 0.023 0.191 0.492
0.0098 0.003 0.329 example 6 inventive 18.5 0.0059 0.228 0.204
0.0259 0.0005 0.018 0.180 0.466 0.0108 0.133 0.383 example 7
inventive 18.4 0.0060 0.219 0.173 0.0229 0.0007 0.015 0.201 0.462
0.0077 0.129 0.395 example 8
[0097] Thereafter, after welding by the GTA process in order to
evaluate the welding characteristics of the steel sheet
manufactured according to the inventive examples and comparative
examples, the grain size of the weld zone, weld zone cross-section
and surface analysis, hardness analysis, Ericsson test, and weld
zone impact energy were investigated. The molten steel components
as the main influencing factors and the types and size
distributions of internal oxides according thereto were
investigated and shown in Table 2 below.
TABLE-US-00002 TABLE 2 *effective weld zone nucleation solidified
product structure distribution average equation equation equation
density grain size DBTT (2) (3) (4) (pieces/mm.sup.2) (.mu.m)
(.degree. C.) comparative 52 0.25 0.25 2.7 190 -14 example 1
comparative 54 0.27 0.41 3.2 170 -18 example 2 comparative 58 0.18
0.33 3.7 169 -15 example 3 comparative 55 0.19 0.31 3.5 160 -18
example 4 comparative 75 0.25 0.25 3.4 169 -19 example 5
comparative 81 0.30 0.23 3.8 150 -21 example 6 comparative 83 0.28
0.27 4.0 162 -20 example 7 inventive 85 0.33 0.37 5.0 109 -31
example 1 inventive 88 0.40 0.41 5.1 110 -37 example 2 inventive 82
0.38 0.37 5.2 107 -35 example 3 inventive 84 0.38 0.42 5.5 102 -37
example 4 inventive 86 0.41 0.36 6.1 88 -40 example 5 inventive 88
0.39 0.45 6.7 102 -38 example 6 inventive 86 0.40 0.41 6.7 80 -33
example 7 inventive 87 0.54 0.44 6.7 80 -35 example 8 The
*effective nucleation product means an Al--Ca--Ti--Mg--O-based
oxide having an average diameter of 3 to 10 .mu.m and Ti--Nb--C--N
carbonitride containing the oxide.
[0098] Referring to Tables 1 and 2, inventive examples 1 to 6
satisfy the Al--Ca--Ti--Mg--O oxide composition of equations (2) to
(4) by satisfying the conditions of the equation (1). At the same
time, the distribution density of Ti--Nb--C--N carbonitride
(effective nucleation product) including this was also 4/mm.sup.2
or more. Specifically, the average grain size of the weld zone
solidified structure was smaller as 30.about.60 .mu.m than the
comparative examples 1 to 5, and the DBTT temperature was also
reduced by about 15.degree. C. compared to the comparative
example.
[0099] In addition, in the case of inventive examples 7 and 8, if
the condition of the equation (1) is satisfied in the same way for
high Cr ferritic stainless steel with Mo added, it was confirmed
that the average grain size was small and the DBTT temperature was
also low compared to comparative examples 6 and 7 of the same steel
type.
[0100] FIG. 9 is a graph showing the correlation between the value
of Equation (1) and the average grain size of the weld zone of the
inventive example and comparative example according to the present
disclosure.
[0101] In conclusion, as can be seen from the inventive examples of
Table 2 and FIG. 9, in order to secure a low-temperature toughness
of weld zone, even if the composition is included in the scope of
the present disclosure, if the equation (1) is not satisfied, it
can be seen that the average grain size of the weld zone solidified
structure is coarse because the distribution density of effective
nucleation product of 3 to 10 .mu.m cannot be secured.
[0102] In the foregoing, exemplary inventive examples of the
present disclosure have been described, but the present disclosure
is not limited thereto, and a person with ordinary knowledge in the
relevant technical field does not depart from the concept and scope
of the following claims. It will be appreciated that various
changes and modifications are possible in.
INDUSTRIAL APPLICABILITY
[0103] The ferritic stainless steel according to the present
disclosure can refine the grain size of the weld zone solidified
structure, thereby securing excellent low-temperature toughness of
weld zone.
* * * * *