[Technical Field]
[0001] The present disclosure relates to an austenitic stainless steel and a method for
manufacturing same, and more particularly, to a low-cost austenitic stainless steel
having high strength and high formability and a method for manufacturing same.
[Background Art]
[0002] Vehicle market trends are changing from conventional internal combustion engine-based
automotive industry toward battery-based eco-friendly vehicle markets. That is, conventional
internal combustion engine vehicle markets which are of high interest in middle-sized
or large-sized vehicles are changing toward battery-based vehicle markets which prefer
small-sized or lightweight vehicles.
[0003] Structural materials protecting batteries are required to have high strength in order
to protect the batteries from the risk of safety accidents such as explosions or from
external impact and for the safety of passengers, and the structural materials are
also required to be lightweight to prevent weight of small-sized or lightweight vehicles
from increasing. As well as structural materials for protecting batteries, general
structural materials have become smaller in size and higher in strength to comply
with environmental regulations. Accordingly, there is a need to develop materials
with high productivity, excellent stability, high strength, and excellent formability
applicable throughout the industry.
[0004] Stainless steels are materials applicable throughout the industry due to excellent
corrosion resistance. Particularly, austenitic stainless steels with excellent elongation
have no problem in forming complex shapes to meet various needs of customers and are
advantageous in terms of aesthetic appearance.
[0005] However, austenitic stainless steels have lower yield strength compared to common
carbon steels and are economically disadvantageous because expensive alloying elements
are used therein. Therefore, there is a need to develop stainless steels for structural
materials having high levels of yield strength and proper tensile strength with excellent
formability maintained.
[0006] In addition, there is a problem in that alloying elements constituting austenitic
stainless steels are expensive compared to elements constituting most carbon steels.
Particularly, Ni included in austenitic stainless steels may cause problems in terms
of price competitiveness because it is expensive and difficult to stably supply Ni
due to unstable supply and demand thereof due to a wide fluctuation in prices. Therefore,
there is a need to develop low-cost austenitic stainless steels in which the contents
of expensive elements such as Ni are reduced.
[Disclosure]
[Technical Problem]
[0007] To solve the above-described problems, provided is a low-cost austenitic stainless
steel having high strength and high formability.
[Technical Solution]
[0008] In accordance with an aspect of the present disclosure to achieve the above-described
objects, a low-cost austenitic stainless steel having high strength and high formability
includes, in percent by weight (wt%), greater than 0% and at most 0.08% of C, 0.2
to 0.25% of N, 0.8 to 1.5% of Si, 8.0 to 9.5% of Mn, 15.0 to 16.5% of Cr, greater
than 0% and at most 1.0% of Ni, 0.8 to 1.8% of Cu, and the remainder of Fe and other
unavoidable impurities and satisfies Expressions (1) to (4) below:
(1) Ni+0.47Mn+15N ≥ 7.5
(2) 23(C+N)+1.3Si+0.24(Cr+Ni+Cu)+0.1Mn ≥ 12
(3) 551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu) ≤ 70
(4) 11 ≤ 1+45C-5Si+0.09Mn+2.2Ni-0.28Cr-0.67Cu+88.6N ≤ 17
wherein C, N, Si, Mn, Cr, Ni, and Cu represent contents (wt%) of the elements, respectively.
[0009] In each low-cost austenitic stainless steel having high strength and high formability
of the present disclosure, a yield strength of a cold-rolled, annealed steel sheet
may be 400 MPa or more.
[0010] In each low-cost austenitic stainless steel having high strength and high formability
of the present disclosure, an elongation of a cold-rolled, annealed steel sheet may
be 55% or more.
[0011] In each low-cost austenitic stainless steel having high strength and high formability
of the present disclosure, a yield strength of a skin pass-rolled steel sheet may
be 800 MPa or more..
[0012] In each low-cost austenitic stainless steel having high strength and high formability
of the present disclosure, an elongation of the skin pass-rolled steel sheet may be
25% or more.
[0013] Also, in accordance with an aspect of the present disclosure to achieve the above-described
objects, a method for manufacturing a low-cost austenitic stainless steel having high
strength and high formability includes: preparing a slab including, in percent by
weight (wt%), greater than 0% and at most 0.08% of C, 0.2 to 0.25% of N, 0.8 to 1.5%
of Si, 8.0 to 9.5% of Mn, 15.0 to 16.5% of Cr, greater than 0% and at most 1.0% of
Ni, 0.8 to 1.8% of Cu, and the remainder of Fe and other unavoidable impurities and
satisfying Expressions (1) to (4) below; hot rolling the slab to prepare a hot-rolled
steel sheet and hot annealing the hot-rolled steel sheet to prepare a hot-rolled,
annealed steel sheet; cold rolling the hot-rolled, annealed steel sheet to prepare
a cold-rolled steel sheet and cold annealing the cold-rolled steel sheet at a temperature
of 1050°C or higher to prepare a cold-rolled, annealed steel sheet; and skin pass
rolling the cold-rolled, annealed steel sheet to prepare a skin pass-rolled steel
sheet:
(1) Ni+0.47Mn+15N ≥ 7.5
(2) 23(C+N)+1.3Si+0.24(Cr+Ni+Cu)+0.1Mn ≥ 12
(3) 551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu) ≤ 70
(4) 11 ≤ 1+45C-5Si+0.09Mn+2.2Ni-0.28Cr-0.67Cu+88.6N ≤ 17
wherein C, N, Si, Mn, Cr, Ni, and Cu represent contents (wt%) of the elements, respectively.
[0014] In the method for manufacturing each low-cost austenitic stainless steel having high
strength and high formability, the skin pass rolling may be performed at a reduction
ratio of 20% or more.
[0015] In the method for manufacturing each low-cost austenitic stainless steel having high
strength and high formability, the slab may have a reduction of area of 50% or more
at a high temperature of 800°C or higher.
[Advantageous Effects]
[0016] According to an embodiment of the present disclosure, provided is an austenitic stainless
steels having excellent yield strength, in which a cold-rolled, annealed steel sheet
prepared by cold annealing at a temperature of 1050°C or higher after cold rolling
has excellent yield strength and excellent elongation sufficient for forming may be
obtained after skin pass rolling performed to further increase strength. Also, a low-cost
austenitic stainless steel having high strength and high formability with high productivity
even using reduced amounts of expensive alloying elements may be provided.
[Best Mode]
[0017] A low-cost austenitic stainless steel having high strength and high formability according
to an embodiment of the present disclosure includes, in percent by weight (wt%), greater
than 0% and at most 0.08% of C, 0.2 to 0.25% of N, 0.8 to 1.5% of Si, 8.0 to 9.5%
of Mn, 15.0 to 16.5% of Cr, greater than 0% and at most 1.0% of Ni, 0.8 to 1.8% of
Cu, and the remainder of Fe and other unavoidable impurities and satisfies Expressions
(1) to (4) below.
(1) Ni+0.47Mn+15N ≥ 7.5
(2) 23(C+N)+1.3Si+0.24(Cr+Ni+Cu)+0.1Mn ≥ 12
(3) 551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu) ≤ 70
(4) 11 ≤ 1+45C-5Si+0.09Mn+2.2Ni-0.28Cr-0.67Cu+88.6N ≤ 17
wherein C, N, Si, Mn, Cr, Ni, and Cu represent contents (wt%) of the elements, respectively.
[Modes of the Invention]
[0018] Hereinafter, embodiments of the present disclosure will be described in detail with
reference to the accompanying drawings. The embodiments of the present disclosure
may, however, be embodied in many different forms and should not be construed as being
limited to the embodiments set forth herein. Rather, these embodiments are provided
so that this disclosure will be thorough and complete, and will fully convey the concept
of the invention to those skilled in the art.
[0019] Also, the terms used herein are merely used to describe particular embodiments. An
expression used in the singular encompasses the expression of the plural, unless otherwise
indicated. Throughout the specification, the terms such as "including" or "having"
are intended to indicate the existence of features, operations, functions, components,
or combinations thereof disclosed in the specification, and are not intended to preclude
the possibility that one or more other features, operations, functions, components,
or combinations thereof may exist or may be added.
[0020] Meanwhile, unless otherwise defined, all terms used herein have the same meaning
as commonly understood by one of ordinary skill in the art to which this disclosure
belongs. Thus, these terms should not be interpreted in an idealized or overly formal
sense unless expressly so defined herein. As used herein, the singular forms are intended
to include the plural forms as well, unless the context clearly indicates otherwise.
[0021] The terms "about", "substantially", etc. used throughout the specification means
that when a natural manufacturing and a substance allowable error are suggested, such
an allowable error corresponds the value or is similar to the value, and such values
are intended for the sake of clear understanding of the present invention or to prevent
an unconscious infringer from illegally using the disclosure of the present invention.
[0022] A low-cost austenitic stainless steel having high strength and high formability according
to an embodiment of the present disclosure includes, in percent by weight (wt%), greater
than 0% and at most 0.08% of C, 0.2 to 0.25% of N, 0.8 to 1.5% of Si, 8.0 to 9.5%
of Mn, 15.0 to 16.5% of Cr, greater than 0% and at most 1.0% of Ni, 0.8 to 1.8% of
Cu, and the remainder of Fe and other unavoidable impurities.
[0023] Hereinafter, reasons for numerical limitations on the contents of alloying elements
in the embodiment of the present disclosure will be described.
Carbon (C): greater than 0 wt% and at most 0.08 wt%
[0024] Carbon (C), as an element effective on stabilizing an austenite phase, is added to
obtain a yield strength of an austenitic stainless steel. However, an excess of C
may not only deteriorate cold workability due to solid strengthening effect but also
may induce grain boundary precipitation of a Cr carbide thereby adversely affecting
ductility, toughness, corrosion resistance, and the like and deteriorating welding
properties among the elements. Therefore, an upper limit thereof may be set to 0.08
wt%.
Nitrogen (N): 0.2 to 0.25 wt%
[0025] Nitrogen (N) is the most important element in the present disclosure. Nitrogen is
a strong austenite-stabilizing element effective on enhancing corrosion resistance
and yield strength of an austenitic stainless steel. However, an excess of N may cause
occurrence of defects such as nitrogen pores while a slab is made and deteriorate
cold workability due to solid solution strengthening effect. Therefore, an upper limit
thereof may be set to 0.25 wt%.
Silicon (Si): 0.8 to 1.5 wt%
[0026] Silicon (Si), acting as a deoxidizer during a steelmaking process, is an element
effective for improving corrosion resistance. Also, Si is an effective element for
increasing yield strength of steel materials among substitutional elements. In consideration
of these effects, Si may be added in an amount of 0.8 wt% or more in the present disclosure.
However, an excess of Si, as a ferrite phase-stabilizing element, may promote formation
of delta (δ) ferrite in a cast slab, thereby not only deteriorating hot workability
but also deteriorating ductility and impact characteristics of steel materials. Therefore,
an upper limit of the Si content may be set to 1.5 wt%.
Manganese (Mn): 8.0 to 9.5 wt%
[0027] Manganese (Mn), as an austenite phase-stabilizing element added as a Ni substitute,
may be added in an amount of 8.0 wt% or more to enhance cold workability by inhibiting
formation of strain-induced martensite. However, an excess of Mn causes an increase
in formation of S-based inclusions (MnS) leading to deterioration of ductility and
toughness austenitic stainless steels and may cause formation of Mn fumes during a
steelmaking process resulting in increased manufacturing risks. Also, an excess of
Mn rapidly deteriorates corrosion resistance of products. Therefore, an upper limit
of the Mn content may be set to 9.5 wt%.
Chromium (Cr): 15.0 to 16.5 wt%
[0028] Chromium (Cr) is a ferrite-stabilizing element but effective on suppressing formation
of a martensite phase. As a basic element for obtaining corrosion resistance required
in stainless steels, Cr may be added in an amount of 15% or more. However, an excess
of Cr, as a ferrite-stabilizing element, may promote formation of delta (δ) ferrite
in a slab in large quantity resulting in deterioration of hot workability and adverse
effects on material characteristics. Therefore, an upper limit thereof may be set
to 16.5 wt%.
Nickel (Ni): greater than 0 wt% and at most 1.0 wt%
[0029] Nickel (Ni), as a strong austenite phase-stabilizing element, is added to improve
hot workability and cold workability. However, because Ni is an expensive element,
costs of raw materials may increase in the case of adding a large amount of Ni. Therefore,
an upper limit of the Ni content may be set to 1.0% in consideration of both costs
and efficiency of steel materials.
Copper (Cu): 0.8 to 1.8 wt%
[0030] Copper (Cu), as an austenite phase-stabilizing element added instead of nickel (Ni)
in the present disclosure. Also, Cu, as an element improving corrosion resistance
of steel materials under a reducing environment, may be added in an amount of 0.8
wt% or more. However, an excess of Cu not only increases costs of steel materials
but also causes liquefaction and embrittlement at a low temperature. Also, an excess
of Cu may be segregated in edges of a slab, thereby deteriorating hot workability
of steel materials. Thus, an upper limit of the Cu content may be set to 1.8 wt% in
consideration of costs, efficiency, and properties of steel materials.
[0031] The remaining component of the composition of the present disclosure is iron (Fe).
However, the composition may include unintended impurities inevitably incorporated
from raw materials or surrounding environments. In the present disclosure, addition
of other unintended alloying elements in addition to the above-described alloying
elements is not excluded. The impurities are not specifically mentioned in the present
disclosure, as they are known to any person skilled in the art.
[0032] Examples of the unavoidable impurities include phosphorus (P) and sulfur (S), and
at least one of P (at most 0.035 wt%) and S (at most 0.01 wt%) may be contained according
to an embodiment of the present disclosure.
Phosphorus (P): at most 0.035 wt%
[0033] Phosphorus (P), as an impurity that is inevitably contained in steels, is a major
causative element of grain boundary corrosion of steel materials or deterioration
of hot workability, and therefore, it is preferable to control the P content as low
as possible. In the present disclosure, an upper limit of the P content may be set
to 0.035 wt%.
Sulfur (S): at most 0.01 wt%
[0034] Sulfur (S), as an impurity that is inevitably contained in steels, is a major causative
element of deterioration of hot workability as being segregated in grain boundaries,
and therefore, it is preferable to control the S content as low as possible. In the
present disclosure, an upper limit of S may be set to 0.01 wt%.
[0035] It is important to improve yield strength of steel materials to decrease weight of
the steel materials and enhance stability. In addition, sufficient elongation should
be obtained to manufacture structural materials having various shape including battery
module cases. In addition, in order to obtain price competitiveness of austenitic
stainless steels, the amounts of expensive austenite-stabilizing elements such as
Ni need to be reduced and the amounts of elements replacing the expensive elements
such as Mn, N, and Cu should be appropriately adjusted.
[0036] However, in the case where the Ni content is reduced and Mn, N, and Cu are added,
work hardening is rapidly increased to deteriorate elongation of a steel material
and induce reduction in resistance to hot deformation, thereby deteriorating productivity,
and thus harmony of the respective alloying elements should be considered. In consideration
of the yield strength, elongation, and price competitiveness of steel materials as
described above, the composition of alloying elements may further be limited to satisfy
Expressions (1) to (4) in addition to the above-described composition.
[0037] In the present disclosure, in order to obtain excellent elongation of a cold-rolled,
annealed steel sheet prepared by cold rolling and annealing the steel material, Expression
(1) regarding a fraction of an austenite phase has been derived.
(1) Ni+0.47Mn+15N ≥ 7.5
[0038] Here, Mn, Ni, and N denote contents (wt%) of the elements, respectively.
[0039] As the value of Expression (1) decreases, the fraction of the austenite phase decreases.
When the value of Expression (1) is less than 7.5, the austenitic stainless steel
may include delta ferrite in an amount of 5% or more or phase transformation into
martensite phase occurs during cold rolling. As a result, elongation of the austenitic
stainless steel may deteriorate, and thus a lower limit of the value of Expression
(1) may be set to 7.5 in the present disclosure to obtain a sufficient elongation.
[0040] In addition, in order to obtain a high yield strength of the austenitic stainless
steel, Expression (2) has been derived in the present disclosure in consideration
that the yield strength is improved by a stress field of a steel material.
(2) 23(C+N)+1.3Si+0.24(Cr+Ni+Cu)+0.1Mn ≥ 12
[0041] Here, C, N, Si, Mn, Cr, Ni, and Cu represent contents (wt%) of the elements, respectively.
[0042] As the value of Expression (2) increases, a stress field between lattices increases
due to size difference among the alloying elements so that a limit to withstand plastic
deformation against external stress increases. When the value of Expression (2) is
less than 12, it is difficult to obtain a yield strength required in the present disclosure.
Therefore, a lower limit of the value of Expression (2) may be set to 12 in the present
disclosure to obtain high strength characteristics.
[0043] In addition, in consideration of phase transformation caused by deformation of the
austenitic stainless steel, Expression (3) has been derived in the present disclosure.
(3) 551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu) ≤ 70
[0044] Here, C, N, Si, Mn, Cr, Ni, and Cu represent contents (wt%) of the elements, respectively.
[0045] As the value of Expression (3) increases, the austenite phase is easily transformed
by an external stress. Specifically, when the value of Expression (3) exceeds 70,
the austenitic stainless steel exhibits a rapid strain-induced martensite transformation
behavior, causing non-uniform plastic processing. As a result, a problem of deteriorating
elongation of the austenitic stainless steel may occur, and thus a lower limit of
the value of Expression (3) may be set to 70.
[0046] In addition, in consideration of dislocation slip behavior of steel materials due
to deformation of the austenitic stainless steel, Expression (4) has been derived.
(4) 11 ≤ 1+45C-5Si+0.09Mn+2.2Ni-0.28Cr-0.67Cu+88.6N ≤ 17
[0047] Here, C, N, Si, Mn, Cr, Ni, and Cu represent contents (wt%) of the elements, respectively.
[0048] As the value of Expression (4) decreases, expression of cross slip of an austenite
phase by an external stress becomes difficult. When the value of Expression (4) is
less than 11, the austenitic stainless steel exhibits only a planar slip behavior
with respect to deformation and dislocation is rapidly piled up by an external stress.
As a result, problems of non-uniform plastic processing and high work hardening may
occur. Accordingly, the elongation of the austenitic stainless steel may deteriorate,
it may be difficult to perform the skin pass rolling, and hot rolling defects such
as edge cracks may occur during deformation at a high temperature, thereby causing
a problem of decreasing productivity. In consideration thereof, a lower limit of Expression
(4) may be set to 11.
[0049] On the contrary, when the value of Expression (4) is too high, cross slip frequently
occurs causing a problem of increasing plastic non-uniformity in which a stress is
concentrated in a weak part of a steel material. As strength of a steel material increases,
such embrittlement and plastic non-uniformity tend to increase, and thus elongation
of steel materials having a high strength as in the present disclosure likely deteriorates.
In consideration thereof, an upper limit of the value of Expression (4) may be set
to 17.
[0050] Since Cr-Mn steels, in which the Ni content is reduced compared to commercially available
300 series austenitic stainless steels, have inferior hot workability, an actual yield
may decrease due to occurrence of edge cracks during a hot processing and correcting
costs may increase or there may be a need to invest additional equipment to reduce
edge cracks. According to the present disclosure, excellent hot workability may be
obtained by satisfying the above-described composition of alloying elements and appropriately
designing the composition of alloying elements using Expressions (1) to (4) without
adding a separate process and equipment. According to an embodiment of the present
disclosure, the slab having the above-described composition of alloying elements may
have a reduction of area of 50% or more at a high temperature of 800°C or higher.
[0051] In the low-cost austenitic stainless steel having high strength and high formability
according to an embodiment of the present disclosure, a yield strength of a cold-rolled,
annealed steel sheet may be 400 MPa. In addition, in the low-cost austenitic stainless
steel having high strength and high formability, an elongation of the cold-rolled,
annealed steel sheet may be 55% or more. In this regard, the "cold-rolled, annealed
steel sheet" refers to a steel material prepared by treating a slab by hot rolling,
annealing, cold rolling, and annealing.
[0052] In the low-cost austenitic stainless steel having high strength and high formability
according to an embodiment of the present disclosure, a yield strength of a skin pass-rolled
steel sheet may be 800 MPa or more. In addition, according to an embodiment, particularly,
a yield strength may be 800 MPa or more and an elongation may be 25% or more. In this
regard, the "skin pass-rolled steel sheet" refers to a steel material prepared by
skin pass rolling the above-described cold-rolled, annealed steel sheet.
[0053] Hereinafter, a method for manufacturing the low-cost austenitic stainless steel having
high strength and high formability according to the present disclosure will be described.
[0054] The method for manufacturing the low-cost austenitic stainless steel having high
strength and high formability according to an embodiment of the present disclosure
includes: preparing a slab including, in percent by weight (wt%), greater than 0%
and at most 0.08% of C, 0.2 to 0.25% of N, 0.8 to 1.5% of Si, 8.0 to 9.5% of Mn, 15.0
to 16.5% of Cr, greater than 0% and at most 1.0% of Ni, 0.8 to 1.8% of Cu, and the
remainder of Fe and other unavoidable impurities and satisfying Expressions (1) to
(4); hot rolling the slab to prepare a hot-rolled steel sheet and hot annealing the
hot-rolled steel sheet to prepare a hot-rolled, annealed steel sheet; cold rolling
the hot-rolled, annealed steel sheet to prepare a cold-rolled steel sheet and cold
annealing the cold-rolled steel sheet at a temperature of 1050°C or higher to prepare
a cold-rolled, annealed steel sheet, and skin pass rolling the cold-rolled, annealed
steel sheet to prepare a skin pass-rolled steel sheet.
[0055] Reasons for numerical limitations on the contents of the alloying elements and Expressions
(1) to (4) are as described above. Hereinafter, each of the manufacturing steps will
be described in detail.
[0056] The slab having the above-described composition of alloying elements may be hot-rolled
at a temperature of 1000 to 1300°C to prepare a hot-rolled steel sheet, and then annealed
at a temperature of 1000 to 1100°C to prepare a hot-rolled, annealed steel sheet.
In this regard, annealing heat treatment may be performed for 10 seconds to 10 minutes.
[0057] Subsequently, the hot-rolled, annealed steel sheet is cold-rolled to prepare a cold-rolled
steel sheet and then annealed to prepare a cold-rolled, annealed steel sheet. Conventionally,
as a method for improving a yield strength of an austenitic stainless steel, low-temperature
annealing heat treatment was performed at a low temperature of 1000°C or below after
cold rolling as described above. The low-temperature annealing heat treatment is a
method for increasing strength using energy accumulated in the steel material during
cold rolling without completing recrystallization. However, in such an austenitic
stainless steel that has undergone low-temperature annealing heat treatment, under
pickling may occur during a subsequent picking process or aesthetic appearance may
not be obtained as well as the possibility of non-uniform quality.
[0058] According to an embodiment of the present disclosure, the hot-rolled, annealed steel
sheet is cold-rolled to prepare a cold-rolled steel sheet, and then annealed at a
temperature of 1050°C or higher to prepare a cold-rolled, annealed steel sheet. In
this case, the annealing heat treatment may be performed for 10 seconds to 10 minutes.
[0059] According to the present disclosure, excellent elongation may be obtained because
low-temperature annealing is not performed after cold rolling, and an appropriate
level of yield strength may be obtained by designing the composition of alloying elements.
[0060] The cold-rolled, annealed steel sheet according to the present disclosure may have
a yield strength of 400 MPa or more.
[0061] The cold-rolled, annealed steel sheet according to the present disclosure may have
an elongation of 55% or more.
[0062] By designing the composition of alloying elements as described above, a cold-rolled,
annealed steel sheet may have an appropriate yield strength without performing low-temperature
annealing treatment via a process which does not cause loads on production.
[0063] In addition, according to the present disclosure, high yield strength may be obtained
via adjustment of the composition of alloying elements and subsequent skin pass rolling
without performing low-temperature annealing treatment after cold rolling. According
to an embodiment of the present disclosure, the yield strength of the skin pass-rolled
steel sheet may be 800 MPa or more. The skin pass rolling may be performed at a reduction
ratio 20% or more according to the present disclosure.
[0064] Skin pass rolling may increase strength by using a high work hardening phenomenon
while the austenite phase is transformed into strain-induced martensite during cold
deformation or using dislocation pile-up of a steel material. However, elongation
of the steel material may rapidly deteriorate by skin pass rolling.
[0065] According to the present disclosure, a rapid decrease in elongation of a steel material,
which is caused by skin pass rolling, may be prevented by appropriately controlling
phase transformation and dislocation behavior by designing the composition of alloying
elements as described above. As a result, a low-cost austenitic stainless steel having
high strength and high formability, in which a skin pass-rolled steel sheet has a
yield strength of 800 MPa or more and an elongation of 25% or more, may be provided
according to an embodiment of the present disclosure.
[0066] Hereinafter, the present disclosure will be described in more detail through examples.
However, it is necessary to note that the following examples are only intended to
illustrate the present disclosure in more detail and are not intended to limit the
scope of the present disclosure. This is because the scope of the present disclosure
is determined by matters described in the claims and able to be reasonably inferred
therefrom.
{Examples}
[0067] Slabs having compositions of allying elements shown in Table 1 below were prepared
by ingot melting, heated at 1250°C for 2 hours, and hot-rolled to prepare hot-rolled
steel sheets. Then, the hot-rolled steel sheets were subjected to annealing heat treatment
at 1100°C for 90 seconds to prepare hot-rolled, annealed steel sheets. Subsequently,
the steel materials were cold-rolled at a reduction ratio of 70% to prepare cold-rolled
steel sheets and subjected to annealing heat treatment at 1100°C for 10 seconds to
prepare cold-rolled, annealed steel sheets.
[0068] Compositions of alloying elements of each of inventive examples and comparative examples
and values obtained by substituting the contents of the alloying elements into Expressions
(1) and (4) are shown in Table 1 below.
(1) Ni+0.47Mn+15N ≥ 7.5
(2) 23(C+N)+1.3Si+0.24(Cr+Ni+Cu)+0.1Mn ≥ 12
(3) 551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu) ≤ 70
(4) 11 ≤ 1+45C-5Si+0.09Mn+2.2Ni-0.28Cr-0.67Cu+88.6N ≤ 17
Table 1
|
Elements (wt%) |
Expressi on (1) |
Expressi on (2) |
Expressi on (3) |
Expressi on (4) |
C |
Si |
Mn |
Ni |
Cr |
Cu |
N |
Comparative Example 1 |
0.06 |
0.4 |
1.1 |
8.1 |
18.2 |
0.1 |
0.04 |
9.22 |
9.27 |
5.07 |
18.00 |
Comparative Example 2 |
0.05 |
2.0 |
9.5 |
0.1 |
16.0 |
2.0 |
0.13 |
6.52 |
12.03 |
92.39 |
0.02 |
Comparative Example 3 |
0.08 |
1.0 |
7.0 |
0.5 |
15.0 |
1.0 |
0.16 |
6.19 |
11.48 |
125.22 |
10.64 |
Comparative Example 4 |
0.08 |
0.5 |
6.0 |
0.5 |
15.0 |
1.0 |
0.16 |
5.72 |
10.73 |
137.92 |
13.05 |
Comparative Example 5 |
0.06 |
1.0 |
7.0 |
0.2 |
15.0 |
2.0 |
0.17 |
6.04 |
11.42 |
109.54 |
9.29 |
Comparative Example 6 |
0.06 |
2.0 |
8.0 |
0.2 |
15.0 |
2.0 |
0.19 |
6.81 |
13.28 |
83.00 |
6.15 |
Comparative Example 7 |
0.06 |
1.0 |
8.0 |
0.2 |
15.0 |
1.5 |
0.20 |
6.96 |
12.09 |
102.08 |
12.38 |
Comparative Example 8 |
0.06 |
1.2 |
8.1 |
0.6 |
15.6 |
2.1 |
0.21 |
7.56 |
12.97 |
57.59 |
12.58 |
Comparative Example 9 |
0.06 |
1.7 |
8.9 |
0.2 |
16.0 |
2.0 |
0.21 |
7.53 |
13.68 |
55.53 |
9.23 |
Comparative Example 10 |
0.06 |
2.2 |
8.2 |
0.7 |
15.3 |
2.2 |
0.19 |
7.40 |
13.80 |
55.13 |
6.05 |
Comparative Example 11 |
0.30 |
1.0 |
6.0 |
0.2 |
16.0 |
0.0 |
0.18 |
5.72 |
16.83 |
46.44 |
21.95 |
Comparative Example 12 |
0.20 |
1.0 |
7.0 |
0.2 |
16.0 |
0.0 |
0.18 |
6.19 |
14.63 |
84.54 |
17.54 |
Inventive Example 1 |
0.08 |
1.0 |
8.9 |
1.0 |
16.0 |
1.0 |
0.20 |
8.18 |
12.95 |
63.15 |
15.17 |
Inventive Example 2 |
0.06 |
1.0 |
8.6 |
0.8 |
15.7 |
1.7 |
0.21 |
7.99 |
12.74 |
59.81 |
14.31 |
Inventive Example 3 |
0.06 |
1.0 |
9.0 |
1.0 |
15.8 |
1.3 |
0.23 |
8.68 |
13.21 |
51.76 |
16.79 |
Inventive Example 4 |
0.07 |
1.05 |
8.7 |
0.9 |
15.6 |
1.3 |
0.21 |
8.14 |
12.95 |
63.99 |
15.03 |
[0069] Yield strength, tensile strength, and elongation of the each of the cold-rolled,
annealed steel sheets of the inventive examples and comparative examples were measured.
Also, yield strength, tensile strength, and elongation of skin pass-rolled steel sheets
respectively prepared by skin pass rolling the cold-rolled, annealed steel sheets
according to the inventive examples and comparative examples by 20% were measured.
[0070] The measurement of the yield strength, tensile strength, and elongation was carried
out according to the ASTM standards, and the measured yield strength (YS, MPa), tensile
strength (TS, MPa) and elongation (EL, %) are shown in Table 2 below. Also, occurrence
of cracks in annealed materials was measured after a 180° adhesion bending test and
results are shown in Table 2 below.
Table 2
|
Cold-rolled steel sheet |
Skin pass-rolled steel sheet |
Cracks by bending test (○/×) |
YS (MPa) |
TS (MPa) |
EL (%) |
YS (MPa) |
TS (MPa) |
EL (%) |
Comparative Example 1 |
300.0 |
697.0 |
52.2 |
624.4 |
876.3 |
32.1 |
× |
Comparative Example 2 |
501.6 |
862.1 |
55 |
940.8 |
1205.3 |
20.2 |
○ |
Comparative Example 3 |
379.7 |
1133.7 |
38.1 |
850.4 |
1517.5 |
14.9 |
○ |
Comparative Example 4 |
311.4 |
1324.0 |
28.1 |
893.7 |
1652.2 |
11.9 |
○ |
Comparative Example 5 |
362.9 |
958.3 |
39.8 |
920.3 |
1430.5 |
20.3 |
○ |
Comparative Example 6 |
435.9 |
1050.8 |
58.7 |
918.1 |
1397.8 |
26.1 |
○ |
Comparative Example 7 |
461.9 |
1028.2 |
50.3 |
953 |
1413.9 |
22.8 |
○ |
Comparative Example 8 |
427.7 |
878.8 |
59.7 |
802.8 |
1210.7 |
26.7 |
× |
Comparative Example 9 |
457.6 |
888 |
58.7 |
848 |
1220.2 |
27.7 |
× |
Comparative Example 10 |
462.6 |
930.6 |
57.3 |
933.6 |
1332.6 |
23.7 |
○ |
Comparative Example 11 |
508.7 |
948.2 |
32.2 |
881.1 |
1416.8 |
15.9 |
○ |
Comparative Example 12 |
464.3 |
914.4 |
25.8 |
840.5 |
1313.5 |
12.3 |
○ |
Inventive Example 1 |
435.2 |
938.1 |
57.6 |
801.7 |
1288 |
25.5 |
× |
Inventive Example 2 |
432.5 |
878.7 |
59.5 |
841.8 |
1233.6 |
25.5 |
× |
Inventive Example 3 |
453.1 |
876.6 |
60.1 |
894.6 |
1261 |
25.3 |
× |
Inventive Example 4 |
442.7 |
860.5 |
60.4 |
851.8 |
1233.1 |
26.3 |
× |
[0071] Referring to Table 2, in the case of Inventive Examples 1 to 4 satisfying the composition
of alloying elements suggested by the present disclosure and satisfying Expressions
(1) to (4), it was confirmed that the cold-rolled, annealed steel sheets had yield
strengths of 400 MPa or more and elongations of 55% or more. In addition, referring
to Table 2, the skin pass-rolled steel sheets of Inventive Examples 1 to 4 had yield
strengths of 800 MPa or more and sufficient elongations of 25% or more even after
skin pass rolling. In addition, it was confirmed that the steel materials according
to Inventive Examples 1 to 4 had price competitiveness due to relatively low Ni contents
of 1.0 wt% or less.
[0072] Referring to Tables 1 and 2, the steel materials according to comparative examples
will be evaluated.
[0073] The steel material according to Comparative Example 1, as a commercially available
standard austenitic stainless steel, had a low yield strength because the steel material
did not satisfy the composition of alloying elements of the present disclosure and
Expressions (2), (3), and (4). Also, the commercial austenitic stainless steel of
Comparative Example 1 had inferior price competitiveness due to the high Ni content
of 8.1 wt% which is far higher than that of the Ni content according to the present
disclosure.
[0074] Because Comparative Example 2 does not satisfy Expression (1), a considerable amount
of initial delta ferrite remains in the steel material after cold rolling and annealing.
Cracks easily occur at an interface between delta ferrite phase and austenite phase
during a forming process such as bending a steel material due to a phase difference,
and thus a low value of Expression (1) involves cracks when bent. As a result, although
Comparative Example 2 exhibited a high yield strength due to the high Si content and
a high elongation, cracks occurred by the bending test due to the remaining delta
ferrite indicating inferior formability including bending characteristics.
[0075] All of the steel materials according to Comparative Examples 3 to 5 are steel types
not satisfying Expressions (1) to (4). Because Expression (1) was not satisfied, considerable
amounts of initial delta ferrite remained in the steel materials after cold rolling
and annealing, and thus formability including bending characteristics was inferior.
In addition, because Expression (2) was not satisfied, low yield strengths were obtained.
In addition, because the value of Expression (3) exceeds 100, plastic non-uniformity
easily occurs due to phase transformation into strain-induced martensite. In addition,
due to the too low value of Expression (4), serious dislocation pile-up occurred by
planar slip. As a result, elongation deteriorated. Particularly, elongations of Comparative
Examples 3 to 5, which deteriorate because Expressions (3) and (4) were not satisfied,
further deteriorated after skin pass rolling, so that physical properties of the steel
materials were not suitable as skin pass-rolled steel sheets.
[0076] In Comparative Example 6, inferior formability including bending characteristics
was obtained because Expression (1) was not satisfied and thus a considerable amount
of initial delta ferrite remained in the steel material after cold rolling and annealing.
In addition, although the steel material of Comparative Example 6 had the high yield
strength due to the high Si content and Expression (2), the elongation was not sufficient
due to effects of Expressions (3) and (4).
[0077] The steel material of Comparative Example 7 had inferior formability including bending
characteristics because Expression (1) was not satisfied and thus a considerable amount
of initial delta ferrite remained in the steel material after cold rolling and annealing.
Also, plastic non-uniformity easily occurs during deformation due to phase transformation
into strain-induced martensite because the value of Expression (3) was over 100, which
did not satisfy Expression (3). Therefore, the cold-rolled, annealed steel sheet and
the skin pass-rolled steel sheet had inferior elongation.
[0078] The steel material of Comparative Example 8 satisfied the contents of the alloying
elements except for Cu and satisfied Expressions (1) to (4). Thus, the cold-rolled,
annealed steel sheet had excellent yield strength and elongation. However, Comparative
Example 8 had inferior hot workability due to an excessive Cu content. Evaluation
thereof will be described below in more detail with reference to Table 3.
[0079] The steel materials according to Comparative Examples 9 and 10 had inferior hot workability
due to excessive amounts of Si and Cu. Evaluation thereof will be described below
in more detail with reference to Table 3.
[0080] The steel materials according to Comparative Examples 11 and 12 had inferior formability
including bending characteristics due to a considerable amount of initial delta ferrite
remaining in the steel material after cold rolling and annealing because Expression
(1) was not satisfied. Also, plastic non-uniformity, in which stress is concentrated
on weak parts of the steel materials, increased due to frequent cross slip in Comparative
Examples 11 and 12 because the values of Expression (4) were too high. As a result,
the cold-rolled, annealed steel sheet and the skin pass-rolled steel sheet had inferior
elongation. Although effects of the stress concentrated by cross slip on elongation
are negligible in commercial steel materials, elongation significantly deteriorate
in high-strength steel materials having too high values of Expression (2) as in Comparative
Examples 11 and 12.
[0081] The austenitic stainless steel according to the present disclosure has excellent
price competitiveness due to high productivity and high actual yield due to excellent
hot workability. For comparative evaluation of hot workability, reduction of area
was measured in slabs of several comparative examples with high elongation and the
inventive examples at different temperatures. Measurement of the reduction of area
was performed according to the ASTM standards by a high-temperature tensile test,
and results are shown in Table 3.
Table 3
|
Reduction of area at different temperatures (%) |
800°C |
900°C |
1000°C |
1100°C |
1200°C |
Comparative Example 1 |
81.4 |
78.7 |
76.3 |
84.7 |
96.3 |
Comparative Example 2 |
40.3 |
43.6 |
53.4 |
66.6 |
88.2 |
Comparative Example 6 |
42.5 |
50.5 |
69.5 |
88.1 |
95.2 |
Comparative Example 7 |
52.8 |
57.2 |
69.3 |
82.4 |
95.2 |
Comparative Example 8 |
43.2 |
48.7 |
64.7 |
85.3 |
93.4 |
Comparative Example 9 |
41.2 |
48.5 |
55.2 |
68.2 |
91.0 |
Comparative Example 10 |
40.9 |
45.6 |
55.4 |
66.6 |
90.2 |
Inventive Example 1 |
53.3 |
64 |
75.1 |
88.7 |
96.6 |
Inventive Example 2 |
50.1 |
54.7 |
71.1 |
87.1 |
97.0 |
Inventive Example 3 |
60.0 |
56.9 |
66.2 |
84.5 |
95.8 |
Inventive Example 4 |
55.8 |
52.8 |
58.2 |
85.2 |
96.9 |
[0082] Referring to Table 3, it was confirmed that reductions of area of 50% or more were
obtained at a high temperature of 800°C or higher in the case of Inventive Examples
1 to 4 satisfying the composition of alloying elements suggested by the present disclosure
and satisfying Expressions (1) to (4).
[0083] As a commercial standard austenitic stainless steel, the steel material according
to Comparative Example 1 had excellent hot workability due to low amounts of Cu and
N added to reduce the amounts of Si and Ni, which are required to increase strength.
However, a large amount of Ni, which is an expensive element, is contained in the
commercial 300 series austenitic stainless steels, the 300 series austenitic stainless
steels have considerably low price competitiveness. In addition, as evaluated in Table
2, the steel material had inferior yield strength because the composition of alloying
elements and Expressions (2), (3), and (4) were not satisfied.
[0084] In Comparative Examples 2, 6, 9, and 10, excessive amounts of Si were added to improve
yield strength of the cold-rolled, annealed steel sheets and excessive amounts of
Cu replacing Ni were added for price competitiveness. The steel materials according
to Comparative Examples 2, 6, 9, and 10 had low hot workability due to excessive amounts
of Si and Cu.
[0085] Because Si and Cu, which deteriorate hot workability, were added within the ranges
suggested in the present disclosure, the steel material according to Comparative Example
7 had excellent hot workability. However, as evaluated in Table 2, the steel material
had inferior formability because Expression (1) was not satisfied and had inferior
elongation of the cold-rolled, annealed steel sheet and the skin pass-rolled steel
sheet because Expression (3) was not satisfied.
[0086] The Cu content of Comparative Example 8 exceeded the range suggested by the present
disclosure. Excessive Cu was segregated on edges or surface of slabs causing liquid
metal embrittlement, thereby deteriorating hot workability of Comparative Example
8. In Comparative Example 8, due to inferior hot workability, actual yield may decrease
due to edge cracks occurring after hot rolling, correcting costs therefor may increase,
or investment for additional equipment to reduce edge cracks may be required.
[0087] While the present disclosure has been particularly described with reference to exemplary
embodiments, it should be understood by those of skilled in the art that various changes
in form and details may be made without departing from the spirit and scope of the
present disclosure.
[Industrial Applicability]
[0088] According to the present disclosure, a low-cost austenitic stainless steel having
high strength and high formability applicable throughout various industrial fields
may be provided.