􀫺Technical Field􀫻
The present disclosure relates to an austenitic stainless steel with improved
deep drawability, and more particularly, to an austenitic stainless steel in which cracks
do not occur during a deep drawing process applied for transforming a plate into three-
5 dimensional parts.
􀫺Background Art􀫻
With the recent increase in price competitiveness, cost reduction in raw
materials applied to parts have been required. Deep drawing is an efficient method for
reducing manufacturing costs by omitting additional processes such as welding and
10 stress-removing heat treatment. Meanwhile, in the case of involving formation of
cylindrical shapes such as a cup or a battery, materials having excellent deep
drawability are required.
Austenitic stainless steel materials may be used to form in complex shapes
without causing any problem due to high elongation and have excellent work15
hardening ability, as a steel type applied to various fields involving deep drawing.
In general, austenitic stainless steels are deformed by work-hardening
occurring during cold working. In this case, it has been known that austenitic stainless
steels having excellent work-hardening ability are easily formed.
However, when austenitic stainless steels are applied to deep drawing,
20 strength is continuously increased by work-hardening, and stress is locally
concentrated, resulting in fracture.
Meanwhile, although application of intermediate heat treatment may be
considered to solve the problems of the increase in strength caused by work hardening,
there are limitations in terms of processing time/processing costs.
25 Therefore, there is a need to develop austenitic stainless steels applicable as a
deep drawing material because the intermediate heat treatment may be omitted and an
increase in strength caused by work hardening may be minimized during the deep
drawing.
30
􀋩DESCRIPTION􀋪
􀫺Disclosure􀫻
􀫺Technical Problem􀫻
Provided is an austenitic stainless steel capable of obtaining forming
processibility when applied to deep drawing by minimizing an increase in strength
5 caused by work hardening.
􀫺Technical Solution􀫻
In accordance with an aspect of the present disclosure, an austenitic stainless
steel with improved deep drawability includes, in percent by weight (wt%), 0.01 to
0.05% of C, 0.01 to 0.25% of N, 1.5% or less of Si (excluding 0), 0.3 to 3.5% of Mn,
10 17.0 to 22.0% of Cr, 9.0 to 14.0% of Ni, 2.0% or less of Mo (excluding 0), 0.2 to 2.5%
of Cu, and the balance of Fe and inevitable impurities, and satisfying Expression (1)
below:
Expression (1): Cr+Si+2*Mo+3*(Ni+Cu)+50*(C+N) 􀋻 63
wherein Cr, Si, Mo, Ni, Cu, C, and N represent the content (wt%) of each
15 element.
In addition, according to an embodiment of the present disclosure, the
austenitic stainless steel may satisfy Expression (2) below:
Expression (2): 0 < 2.4*Cr+1.7*Mo+3.9*Si-2.1*Ni-Mn-0.4*Cu-58*C-64*N-
13 < 5.5
20 wherein Cr, Mo, Si, Ni, Mn, Cu, C, and N represent the content (wt%) of each
element.
In addition, according to an embodiment of the present disclosure, the
austenitic stainless steel may further include at least one of 0.04% or less of Al
(excluding 0), 0.003% or less of Ti (excluding 0), 0.0025% or less of B (excluding 0),
25 0.035% or less of P, and 0.0035% or less of S.
In addition, according to an embodiment of the present disclosure, a true strain
value may be 0.2 or less at a maximum work-hardening exponent in Expression (3)
below:
􀀨􀁛􀁓􀁕􀁈􀁖􀁖􀁌􀁒􀁑􀀃􀀋􀀖􀀌􀀝􀀃􀄱􀀃􀀠􀀃􀀮􀄰n
30 􀁚􀁋􀁈􀁕􀁈􀁌􀁑􀀃􀄱􀀃􀁌􀁖􀀃􀁄􀀃􀁖􀁗􀁕􀁈􀁖􀁖􀀏􀀃􀀮􀀃􀁌􀁖􀀃􀁄􀀃􀁖􀁗􀁕􀁈􀁑􀁊􀁗􀁋􀀃􀁆􀁒􀁈􀁉􀁉􀁌􀁆􀁌􀁈􀁑􀁗􀀏􀀃􀄰􀀃􀁌􀁖􀀃􀁄􀀃strain, and n is a workhardening
exponent.
3
In addition, according to an embodiment of the present disclosure, a difference
between a true strain value at the maximum work-hardening exponent and a true strain
value at a work-hardening exponent of 0 may be 0.11 or more.
In addition, according to an embodiment of the present disclosure, an
5 elongation may be 35% or more.
In addition, according to an embodiment of the present disclosure, a tensile
strength may be 360 MPa or more.
In addition, according to an embodiment of the present disclosure, cracks do
not occur until a fifth stage in the case of multi-stage formation at a drawing ratio of
10 1.7 to 4.3.
􀫺Advantageous Effects􀫻
According to an embodiment of the present disclosure, an austenitic stainless
steel applicable as a deep drawing material may be provided because intermediate heat
treatment may be omitted and an increase in strength caused by work hardening may
15 be minimized during deep drawing.
􀫺Description of Drawings􀫻
FIG. 1 is a graph for describing the relationship between stress and strain in a
tensile test of a material.
FIG. 2 is a graph illustrating the relationship between stress and strain together
20 with work-hardening exponent in a tensile test of an austenitic stainless steel according
to a disclosed embodiment.
􀫺Best Mode􀫻
An austenitic stainless steel with improved deep drawability according to an
embodiment of the present disclosure includes, in percent by weight (wt%), 0.01 to
25 0.05% of C, 0.01 to 0.25% of N, 1.5% or less of Si (excluding 0), 0.3 to 3.5% of Mn,
17.0 to 22.0% of Cr, 9.0 to 14.0% of Ni, 2.0% or less of Mo (excluding 0), 0.2 to 2.5%
of Cu, and the balance of Fe and inevitable impurities and satisfies Expression (1)
below.
Expression (1􀀌􀀝􀀃􀀦􀁕􀀎􀀶􀁌􀀎􀀕􀀍􀀰􀁒􀀎􀀖􀀍􀀋􀀱􀁌􀀎􀀦􀁘􀀌􀀎􀀘􀀓􀀍􀀋􀀦􀀎􀀱􀀌􀀃􀂕􀀃􀀙􀀖
30 Here, Cr, Si, Mo, Ni, Cu, C, and N represent the content (wt%) of each element.
􀫺Modes of the Invention􀫻
4
Hereinafter, embodiments of the present disclosure will be described in detail
with reference to the accompanying drawings. These embodiments are provided to
fully convey the concept of the present disclosure to those of ordinary skill in the art.
The present disclosure may, however, be embodied in many different forms and should
not be construed as limited to the 5 exemplary embodiments set forth herein. In the
drawings, parts unrelated to the descriptions are omitted for clear description of the
disclosure and sizes of elements may be exaggerated for clarity.
Throughout the specification, the term "include" an element does not preclude
other elements but may further include another element, unless otherwise stated.
10 As used herein, the singular forms are intended to include the plural forms as
well, unless the context clearly indicates otherwise.
Hereinafter, embodiments of the present disclosure will be described in detail
with reference to the accompanying drawings.
An austenitic stainless steel is a steel type used in products having various
15 shapes due to high elongation and excellent formability. Under a stress, the austenitic
stainless steel is deformed by transformation, i.e., transformation induced plasticity,
from an unstable austenite phase to a martensite phase, at room temperature.
In this regard, since the generated martensite phase has a high strength,
strength of the material also increases. In other words, both deformation and an
20 increase in strength simultaneously occur in an austenitic stainless steel by workhardening.
The work-hardening ability is represented using a work-hardening
exponent, and the work-hardening exponent varies according to strain.
Austenitic stainless steels having excellent work-hardening ability have been
known to easy formed.
25 However, when a deep drawing process, which is performed while reducing a
blank diameter, is applied to an austenitic stainless steel, strength continuously
increases in accordance with work-hardening to cause local concentration of stress in
the material resulting in fracture. Also, cracks may suddenly occur due to aging cracks.
Therefore, in the deep drawing that causes a large amount of deformation, it
30 is important to uniformly induce deformation over the entire material and minimize
variation in strength while deformation occurs. That is, in order to improve deep
drawability of the austenitic stainless steel, work-hardening should be inhibited.
5
Meanwhile, work-hardening of the austenitic stainless steel is related to the
degree of stability of an austenite phase. The work-hardening of the austenitic stainless
steel may be inhibited by increasing the degree of stability by controlling elements.
However, workability of an austenitic stainless steel represented by elongation
is derived from work-hardening 5 due to transformation induced plasticity, and thus a
decrease in work-hardening causes a problem of deteriorating workability of the
austenitic stainless steel.
The present inventors have made various studies to enhance elongation of an
austenitic stainless steel and inhibit an increase in strength caused by work-hardening
10 during a deep drawing process and have found those described below.
In the present disclosure, as a result of examining factors for preventing
fracture in an austenitic stainless steel in the case of applying deep drawing, it was
found that deep drawability of the austenitic stainless steel may be improved by
obtaining a certain amount of deformation without excessively increasing strength
15 while suppressing over work hardening by inhibiting martensite phase transformation
induced by stress. To this end, a composition of alloying elements capable of obtaining
continuous deformation without excessively increasing strength has been derived.
An austenitic stainless steel with improved deep drawability according to an
embodiment of the present disclosure include, in percent by weight (wt%), 0.01 to
20 0.05% of C, 0.01 to 0.25% of N, 1.5% or less of Si (excluding 0), 0.3 to 3.5% of Mn,
17.0 to 22.0% of Cr, 9.0 to 14.0% of Ni, 2.0% or less of Mo (excluding 0), 0.2 to 2.5%
of Cu, and the balance of Fe and inevitable impurities.
Hereinafter, reasons for numerical limitations on the contents of alloying
elements in the embodiment of the present disclosure will be described. Hereinafter,
25 the unit is wt% unless otherwise stated.
The content of C is from 0.01 to 0.05%.
Carbon (C) is an element effective on stabilization of an austenite phase and
may be added in an amount of 0.01% or more to inhibit formation of martensite and
obtain strength during deformation. However, an excess of C may bind to Cr to induce
30 grain boundary precipitation of a Cr carbide, thereby deteriorating corrosion resistance.
Therefore, an upper limit the C content may be controlled to 0.05%.
The content of N is from 0.01 to 0.25%.
6
7
Nitrogen (N), like carbon, is an element effective on stabilization of an
austenite phase and may be added in an amount of 0.01% or more to obtain deep
drawability. However, an excess of Ni may form a nitride, thereby deteriorating the
surface quality, and thus an upper limit of the N content may be controlled to 0.25%.
5 The content of Si is 1.5% or less (excluding 0).
Silicon (Si) is an element serving as a deoxidizer during a steelmaking process
and used to obtain strength and corrosion resistance of an austenitic stainless steel.
However, an excess of Si, as a ferrite phase-stabilizing element, may promote
martensite transformation and precipitate intermetallic compounds such as a 􀄱 phase
10 to deteriorate mechanical properties and corrosion resistance. Thus, an upper limit of
the Si content may be controlled to 1.5%.
The content of Mn is from 0.3 to 3.5%.
Manganese (Mn), like carbon (C) and nitrogen (N), is an element stabilizing
austenite and has an effect on inhibiting an increase in strength during a forming
15 process, and thus Mn may be added in an amount of 0.3% or more. However, an excess
of Mn may form a large amount of S-based inclusions (MnS), thereby deteriorating
corrosion resistance and surface gloss of an austenitic stainless steel. Thus, an upper
limit of the Mn content may be controlled to 3.5%.
The content of Cr is from 17.0 to 22.0%.
20 Chromium (Cr) stabilizes ferrite as a basic element contained in stainless
steels in the largest amount among the elements used to improve corrosion resistance.
In the present disclosure, Cr may be added in an amount of 17.0% or more to obtain
corrosion resistance by forming a passivated layer that inhibits oxidation.
However, as excess of Cr, as a ferrite phase-stabilizing element, decreases
25 stability of an austenite phase to promote martensite transformation. Accordingly, an
increase in the Ni content increases manufacturing costs, and intermetallic compounds
􀁖􀁘􀁆􀁋􀀃􀁄􀁖􀀃􀁄􀀃􀄱􀀃􀁓􀁋􀁄􀁖􀁈􀀃are precipitated to deteriorate mechanical properties and corrosion
resistance. Therefore, an upper limit of the Cr content may be controlled to 22.0%.
The content of Ni is from 9.0 to 14.0%.
30 Nickel (Ni) is the strongest austenite phase-stabilizing element. As the Ni
content increases, an austenite phase is stabilized to soften a material, and it is essential
to include 9% or more of Ni to inhibit work-hardening caused by deformation-induced
martensite. However, use of a large amount of Ni, which is a high-priced element,
8
cases an increase in costs of raw materials. Therefore, an upper limit of the Ni content
may be controlled to 14.0% in consideration of costs and efficiency of steel materials.
The content of Mo is 2.0% or less (excluding 0).
Molybdenum (Mo) is an element effective on obtaining corrosion resistance.
However, an excess of molybdenum, 5 as a ferrite phase-stabilizing element, may
decrease stability of an austenite phase making it difficult to obtain deep drawability,
and precipitate intermetallic compounds such as a 􀄱 phase to deteriorate mechanical
properties and corrosion resistance. Therefore, an upper limit of the Mo content may
be controlled to 2.0%.
10 The content of Cu is from 0.2 to 2.5%%.
Copper (Cu), as an austenite phase-stabilizing element added instead of the
high-priced nickel (Ni), may be added in an amount of 0.2% or more to enhance price
competitiveness and deep drawability. However, 􀁚􀁋􀁈􀁑􀀃􀁗􀁋􀁈􀀃􀀦􀁘􀀃􀁆􀁒􀁑􀁗􀁈􀁑􀁗􀀃􀁌􀁖􀀃􀁈􀁛􀁆􀁈􀁖􀁖􀁌􀁙􀁈􀀏􀀃􀄰-
Cu precipitates with a low-melting point are formed to deteriorate the surface quality.
15 Thus, an upper limit of the Cu content may be controlled to 2.5%.
In addition, according to an embodiment of the present disclosure, the
austenitic stainless steel may further include at least one of 0.04% or less of Al
(excluding 0), 0.003% or less of Ti (excluding 0), 0.0025% or less of B (excluding 0),
0.035% or less of P, and 0.0035% or less of S.
20 The content of Al is 0.04% or less (excluding 0).
Aluminum (Al), as a strong deoxidizer, reduces a content of oxygen in molten
steels. However, an excess of Al may cause sleeve defects of a cold-rolled strip due to
an increase in nonmetallic inclusions, and therefore an upper limit of the Al content
may be controlled to 0.04%.
25 The content of Ti is 0.003% or less (excluding 0).
Titanium (Ti) is an element effective on corrosion resistance of a steel because
Ti preferentially binds to interstitial elements such as carbon (C) or nitrogen (N) to
form precipitates (carbonitrides), thereby reducing amounts of solute C and solute N
in the steel and inhibits formation of a Cr depletion region. However, an excess of Ti
30 may form Ti-based inclusions causing a problem in a manufacturing process and a
surface defect such as scabs, and therefore an upper limit of the Ti content may be
controlled to 0.003%.

We claim:
􀫺Claim 1􀫻
An austenitic stainless steel with improved deep drawability comprising, in
percent by weight (wt%), 0.01 to 0.05% of C, 0.01 to 0.25% of N, 1.5% or less of Si
(excluding 0), 0.3 to 3.5% of Mn, 5 17.0 to 22.0% of Cr, 9.0 to 14.0% of Ni, 2.0% or
less of Mo (excluding 0), 0.2 to 2.5% of Cu, and the balance of Fe and inevitable
impurities, and
satisfying Expression (1) below:
Expression (1􀀌􀀝􀀃􀀦􀁕􀀎􀀶􀁌􀀎􀀕􀀍􀀰􀁒􀀎􀀖􀀍􀀋􀀱􀁌􀀎􀀦􀁘􀀌􀀎􀀘􀀓􀀍􀀋􀀦􀀎􀀱􀀌􀀃􀂕􀀃􀀙􀀖
10 wherein Cr, Si, Mo, Ni, Cu, C, and N represent the content (wt%) of each
element.
􀫺Claim 2􀫻
The austenitic stainless steel according to claim 1, wherein the austenitic
15 stainless steel satisfies Expression (2) below:
Expression (2): 0 < 2.4*Cr+1.7*Mo+3.9*Si-2.1*Ni-Mn-0.4*Cu-58*C-64*N-
13 < 5.5
wherein Cr, Mo, Si, Ni, Mn, Cu, C, and N represent the content (wt%) of each
element.
20
􀫺Claim 3􀫻
The austenitic stainless steel according to claim 1, further comprising at least
one of 0.04% or less of Al (excluding 0), 0.003% or less of Ti (excluding 0), 0.0025%
or less of B (excluding 0), 0.035% or less of P, and 0.0035% or less of S.
25
􀫺Claim 4􀫻
The austenitic stainless steel according to claim 1, wherein a true strain value
is 0.2 or less at a maximum work-hardening exponent in Expression (3) below:
Expression (3􀀌􀀝􀀃􀄱􀀃􀀠􀀃􀀮􀄰n
30 wherein 􀄱 is a stress, K is a strength coefficient, 􀄰􀀃is a strain, and n is a workhardening
exponent.
21
􀫺Claim 5􀫻
The austenitic stainless steel according to claim 4, wherein a difference
between a true strain value at the maximum work-hardening exponent and a true strain
value at a work-hardening exponent of 0 is 0.11 or more.
5
􀫺Claim 6􀫻
The austenitic stainless steel according to claim 1, wherein an elongation is 35%
or more.
10 􀫺Claim 7􀫻
The austenitic stainless steel according to claim 1, wherein a tensile strength is
360 MPa or more.
􀫺Claim 8􀫻
15 The austenitic stainless steel according to claim 1, wherein cracks do not occur
until a fifth stage in the case of multi-stage formation at a drawing ratio of 1.7 to 4.3.