U.S. patent application number 12/298959 was filed with the patent office on 2009-03-19 for high manganese high strength steel sheets with excellent crashworthiness and method for manufacturing of it.
This patent application is currently assigned to POSCO. Invention is credited to Kwang Geun Chin, Sung Kyu Kim, Il-Ryoung Sohn.
Application Number | 20090074605 12/298959 |
Document ID | / |
Family ID | 39562686 |
Filed Date | 2009-03-19 |
United States Patent
Application |
20090074605 |
Kind Code |
A1 |
Kim; Sung Kyu ; et
al. |
March 19, 2009 |
HIGH MANGANESE HIGH STRENGTH STEEL SHEETS WITH EXCELLENT
CRASHWORTHINESS AND METHOD FOR MANUFACTURING OF IT
Abstract
There are provided a high-workability high strength steel sheet
with excellent workability due to the high elongation and excellent
crashworthiness due to the high yield strength, and a method for
manufacturing of it. The high manganese steel sheet includes, by
weight: carbon (C): 0.2 to 1.5%, manganese (Mn): 10 to 25%,
aluminum (Al): 0.01 to 3.0%, phosphorus (P) 0.03% or less, sulfur
(S): 0.03% or less, nitrogen (N): 0.040% or less, at least one
selected from the group consisting of silicon (Si): 0.02 to 2.5%,
titanium (Ti): 0.01 to 0.10% and niobium (Nb): 0.01 to 0.10%, and
the balance of Fe and other inevitable impurities. The high
manganese steel sheet may be a hot-rolled steel sheet, a
cold-rolled steel sheet, or a plated steel sheet, and is suitable
for elaborate internal sheets as well as structural members of a
car body since it has press workability due to the high elongation
and high strain hardening index. Also, the high manganese steel
sheet may be used for parts such as a front side member of an
automobile since, among its characteristics, the steel sheet has an
excellent impact absorbing ability.
Inventors: |
Kim; Sung Kyu;
(Jeollanam-do, KR) ; Chin; Kwang Geun;
(Jeollanam-do, KR) ; Sohn; Il-Ryoung;
(Jeollanam-do, KR) |
Correspondence
Address: |
THE WEBB LAW FIRM, P.C.
700 KOPPERS BUILDING, 436 SEVENTH AVENUE
PITTSBURGH
PA
15219
US
|
Assignee: |
POSCO
Pohang
KR
|
Family ID: |
39562686 |
Appl. No.: |
12/298959 |
Filed: |
December 24, 2007 |
PCT Filed: |
December 24, 2007 |
PCT NO: |
PCT/KR2007/006780 |
371 Date: |
October 29, 2008 |
Current U.S.
Class: |
420/75 ; 148/620;
72/365.2 |
Current CPC
Class: |
C22C 38/06 20130101;
C22C 38/04 20130101 |
Class at
Publication: |
420/75 ; 148/620;
72/365.2 |
International
Class: |
C22C 38/04 20060101
C22C038/04; C21D 8/02 20060101 C21D008/02; B21B 1/00 20060101
B21B001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 27, 2006 |
KR |
10-2006-0135658 |
Claims
1. A high manganese high strength steel sheet with excellent
crashworthiness, comprising, by weight: carbon (C): 0.2 to 1.5%,
manganese (Mn): 10 to 25%, aluminum (Al): 0.01 to 3.0%, phosphorus
(P) 0.03% or less, sulfur (S): 0.03% or less, nitrogen (N): 0.040%
or less, at least one selected from the group consisting of silicon
(Si): 0.02 to 2.5%, titanium (Ti): 0.01 to 0.10% and niobium (Nb):
0.01 to 0.10%, and the balance of Fe and other inevitable
impurities.
2. The high manganese high strength steel sheet of claim 1, having
an austenite single-phase structure.
3. The high manganese high strength steel sheet of claim 1, having
a yield strength of 600 MPa or more and a tensile strength of 900
MPa or more.
4. A method for manufacturing a high manganese high strength steel
sheet with excellent crashworthiness, comprising: cold-rolling the
steel sheet of claim 1 at a reduction ratio of 10 to 80%.
5. The method of claim 4, wherein the rolling is selected from the
group consisting of temper rolling, dual rolling and hot final
rolling.
6. The method of claim 4, wherein the steel sheet is selected from
the group consisting of a hot-rolled steel sheet, a cold-rolled
steel sheet and a plated steel sheet.
7. The method of claim 4, wherein the steel sheet is selected from
the group consisting of a hot-rolled steel sheet, a cold-rolled
steel sheet and a plated steel sheet prepared according to at least
one operation selected from the group consisting of the following
operations (a), (b) and (c); (a) homogenizing a steel slab at 1050
to 1300.degree. C., followed by hot-rolling the steel slab at a
finish rolling temperature of 850 to 1000.degree. C. and winding
the hot-rolled steel slab at a temperature range of 700.degree. C.
or below, (b) cold-rolling the hot-rolled steel sheet at a
reduction ratio of 30 to 80% and annealing the resultant steel
sheet at 600.degree. C. or above to prepare a cold-rolled steel
sheet, and (c) plating the hot-rolled steel sheet or the
cold-rolled steel sheet using a process selected from the group
consisting of hot plating, electroplating and deposition to prepare
a plated steel sheet.
Description
TECHNICAL FIELD
[0001] The present invention relates to a high manganese steel
sheet used for a automobile steel sheet and a method for
manufacturing of it, and more particularly, to a high-workability
high strength steel sheet whose workability is excellent due to the
high elongation and crashworthiness is excellent due to the high
yield strength, and a method for manufacturing of it.
BACKGROUND ART
[0002] Motor companies have increasingly used lightweight and
high-strength materials for the purpose of preventing environmental
pollutions and improving fuel efficiency and stability, which is
one of important characteristics that materials used for structural
members should have in addition to the automobile parts. However,
the increased strength of the materials results in deterioration in
elongation of steel, and therefore a dual-phase steel, a
transformation induced plasticity steel and the like, all of which
have excellent formability, have been increasingly used to solve
the above problems.
[0003] However, high strength steel for processing, used as
structural parts and internal sheets for automobiles developed thus
far, does not satisfy workability that automobile parts require,
and therefore it is difficult to manufacture parts with complicated
structure. To solve the above problems, motor companies have used
methods of simplifying shapes of automobile parts, or molding
several parts separately and welding the parts. The welding process
is hampered by a variety of restrictions, for example, difficulty
in designing a car body since strength of a welded portion is
different from that of the parent metal, and parts characteristics
are deteriorated due to the increased strength in the welded
portion, and the manufacturing cost is extremely high since the
parts are molded separately. Accordingly, motor companies have had
continuous demands for materials having a high strength and
excellent workability so as to apply to parts with complicated
structure and improve design flexibility.
[0004] In recent years, there has been required a high strength
steel sheet having excellent formability that is able to decrease
the weight of automobiles for the purpose of the improvement of
fuel efficiency and reduce air pollution in the field of automotive
steel sheet. High strength steel, such as low carbon steel where a
matrix structure is ferrite, has been used as the automotive steel
sheet. However, when the high strength steel such as low carbon
steel is used as the automotive steel sheet, it is impossible to
ensure up to 30% elongation in high strength steels whose tensile
strength is greater than 800 MPa. Accordingly, it is difficult to
design parts easily, for example, make simple structures of parts,
since it is difficult to apply the high strength steel of greater
than 800 MPa to complicated and elaborate parts. In order to solve
the above problems, there has been proposed an austenite-based high
manganese steel sheet with excellent ductility and strength
(Japanese Patent Laid-open Publication No. 1992-259325,
International Publication No. WO02/101109). In the case of the
Japanese patent, the high manganese steel sheet has a high
ductility by the addition of high manganese, but may be easily
broken after its processing since strain hardening is serious in
its transformed region. Also, in the case of the international
patent, the high manganese steel sheet has high ductility, but its
electroplating and hot plating properties are low due to the
addition of a large amount of silicon. Also, the steel sheets have
excellent workability but deteriorated crashworthiness due to the
low yield strength.
[0005] Materials that absorb impact energy in collision and has
high yield strength to prevent transformation of automobile parts
may be desirably used for the automobile parts. However, the high
manganese steel sheet has low yield strength due to its austenite
structure, and therefore the problems regarding the high manganese
steel sheet remains to be solved.
DISCLOSURE OF INVENTION
Technical Problem
[0006] An aspect of the present invention provides a
high-workability high strength steel sheet whose workability is
high due to the excellent elongation and crashworthiness is
excellent due to the high yield strength, and a method for
manufacturing of it.
Technical Solution
[0007] According to an aspect of the present invention, there is
provided a steel sheet including, by weight: carbon (C): 0.2 to
1.5%, manganese (Mn): 10 to 25%, aluminum (Al): 0.01 to 3.0%,
phosphorus (P) 0.03% or less, sulfur (S): 0.03% or less, nitrogen
(N): 0.040% or less, at least one selected from the group
consisting of silicon (Si): 0.02 to 2.5%, titanium (Ti): 0.01 to
0.10% and niobium (Nb): 0.01 to 0.10%, and the balance of Fe and
other inevitable impurities. The steel sheet according to the
present invention has an austenite single-phase structure.
[0008] The steel sheet according to the present invention has a
yield strength of 400-600 MPa or more and a tensile strength of
900-1000 MPa or more, and the steel sheet is subject to strain
hardening through the cold rolling process to have a yield strength
of 750 MPa or more and a tensile strength of 1000 MPa through the
cold rolling process.
[0009] The steel sheet including the components according to the
present invention may be cold-rolled at a reduction ratio of 10 to
80%. Here, the rolling process may be one selected from the group
consisting of temper rolling, dual rolling and hot final rolling.
Also, the steel sheet may be selected from the group consisting of
a hot-rolled steel sheet, a cold-rolled steel sheet and a plated
steel sheet.
[0010] In the method of manufacturing a cold-rolled steel sheet
according to the present invention, the cold-rolled steel sheet may
be prepared by homogenizing a steel including the above-mentioned
components at 1050 to 1300.degree. C., hot-rolling the steel at a
finish rolling temperature of 850 to 1000.degree. C. and winding
the hot-rolled steel at a temperature range of 700.degree. C. or
below, followed by cold-rolling the hot-rolled steel sheet at a
reduction ratio of 30 to 80% and annealing the cold-rolled steel
sheet at 600.degree. C. or above.
ADVANTAGEOUS EFFECTS
[0011] According to the present invention, an aspect of the present
invention is to provide a steel sheet that is suitable for
elaborate internal sheets as well as structural members of a car
body since it has high elongation and high strength. The steel
sheet can be useful to be used for parts such as a front side
member of an automobile since, among its characteristics, the steel
sheet has an excellent impact absorbing ability.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a photographic diagram illustrating a
microstructure.
[0013] FIG. 2 is a graph illustrating changes in tensile curve and
strength vs. elongation, depending on the increasing amount of a
cold-rolled steel sheet.
BEST MODE FOR CARRYING OUT THE INVENTION
[0014] Hereinafter, exemplary embodiments of the present invention
will be described in detail with reference to the accompanying
drawings.
[0015] According to the present invention, a suitable amount of at
least one selected from the group consisting of silicon, titanium
and niobium is added to enhance yield strength of a steel sheet.
Steels whose crashworthiness is excellent due to the high yield
strength may be manufactured by cold-rolling the prepared
hot-rolled, cold-rolled and plated steel sheets.
[0016] The present invention is characterized in that an austenite
single phase is prepared, and amounts of added manganese, carbon
and aluminum are suitably adjusted to improve workability due to
the presence of twins, and amounts of added silicon, titanium and
niobium are also optimized to control a microstructure, thereby
enhancing yield strength. Also, yield strength of the steel sheet
may be enhanced through the cold-rolling process, on the basis of
the fact that elongation of the steel sheet is very excellent when
the steel sheet is subject to the strain hardening process using
the twins, and therefore it is possible to ensure formability
required for automobile parts although the elongation of the steel
sheet is rather decreased in the cold-rolling process.
[0017] According to the present invention, amounts of austenite
stabilizers such as manganese and carbon are optimized to ensure a
complete austenite phase at room temperature, and the complete
austenite phase is transformed by these components to form a twin.
Also, an amount of aluminum is adjusted to control a twin-forming
rate, thereby improving tensile properties. It is important to
minimize an amount of manganese (Mn) added to lower the
manufacturing cost, and also to add a portion of carbon to reduce
the amount of the added manganese. Amounts of added carbon and
aluminum are also adjusted suitably to facilitate twin
transformation during the processing of steel. Meanwhile, it is
preferred to reduce grain sizes of the components so as to increase
yield strength of the steel sheet. For this purpose, it is possible
to further add at least one selected from the group consisting of
silicon, titanium and niobium.
[0018] According to the present invention, the steel sheet includes
a hot-rolled steel sheet, a cold-rolled steel sheet and a plated
steel sheet.
[0019] Hereinafter, the grounds for selection of the
above-mentioned components of the steel and limitation of content
ranges of the components will be described in detail, as
follows.
[0020] A content of carbon (C) is preferably in a range from 0.2 to
1.5%.
[0021] It is desirable to increase an amount of added carbon since
the carbon contributes to stabilizing an austenite phase. Because
an .alpha.'-martensite phase is formed during the phase
transformation when the amount of the added carbon is less than
0.2%, cracks may occur during processing and ductility may be
deteriorated. And, when the amount of the added carbon exceeds
1.5%, stability of the austenite phase is highly enhanced, and,
thus, workability is deteriorated due to the transition of
deformation behaviors by slip deformation.
[0022] A content of manganese (Mn) is preferably in a range from 10
to 30%, and more preferably from 10 to 25%.
[0023] Manganese (Mn) is also an essential element that stabilizes
an austenite phase. However, an .alpha.'-martensite phase that
adversely affects formability is formed when the content of the
manganese (Mn) is less than 10%, and therefore strength is
increased but ductility is seriously decreased. And, twin formation
is suppressed when the content of the added manganese exceeds 30%,
which leads to the increased strength but the decreased ductility.
And, as the content of the added manganese increases, cracks easily
occurs during the hot rolling process, and the manufacturing cost
of the steel sheet is increased due to the use of a large amount of
the expensive manganese. Therefore, the manganese is preferably
added at a content of 25% or less.
[0024] A content of aluminum (Al) is preferably in a range from
0.01 to 3.0%.
[0025] Aluminum (Al) is usually added to deoxidize steel, but added
to improve ductility in the present invention. That is, the
aluminum (Al) is an element that stabilizes a ferrite phase, but
increases stacking fault energy in a slip surface of steel to
prevent transformation into an .epsilon.-martensite phase, which
leads to the improved ductility. In addition, the aluminum
contributes significantly to minimizing an amount of the added
manganese and improving workability since it suppresses the
transformation into the .epsilon.-martensite phase even when the
manganese is present at a low content. Accordingly, because
.epsilon.-martensite is formed when the amount of the added
aluminum (Al) is less 0.01%, strength is increased but ductility is
seriously decreased. And, because twin formation is suppressed when
the amount of the added aluminum (Al) exceeds 3.0%, ductility is
deteriorated, castability is poor in a continuous casting process,
and a steel surface is seriously oxidized during a hot rolling
process, which lead to the deteriorated quality in a surface to the
product.
[0026] Contents of phosphorus (P) and sulfur(S) are preferably in a
range of 0.03% or less.
[0027] Phosphorus (P) and sulfur(S) are inevitably present in the
manufacture of a steel sheet, and therefore their contents are
adjusted to a content range of 0.03% or less. In particular, the
phosphorus (P) causes slabs to arise, which deteriorates
workability of steel. And, the sulfur (S) reacts to form coarse
manganese sulfide (MnS) which cause defects such as flange cracks,
and deteriorates hole expansibility of a steel sheet. Therefore, it
is preferred to use the minimum content of the components.
[0028] A content of nitrogen (N) is preferably in a range of 0.04%
or less.
[0029] Nitrogen reacts with aluminum during a coagulation process
to extract fine nitrides from austenite grains, which facilitate
twin formation, and the nitrogen (N) improves strength and
ductility of steel in molding a steel sheet. However, hot
workability and elongation are deteriorated since the excessive
nitrides are extracted when the content of the added nitrogen
exceeds 0.04%.
[0030] The steel with the above composition includes at least one
selected from the group consisting of silicon, titanium and
niobium.
[0031] A content of silicon (Si) is preferably in a range of 0.02
to 2.5%.
[0032] Silicon is a solid-solution strengthening element that
increases yield strength by reducing grain sizes through a
solid-solution strengthening effect. It has been known that, when
the excessive silicon (Si) is present, a silicon oxide layer is
formed on a surface of a steel sheet to deteriorate a hot plating
property. However, when a suitable amount of the silicon is added
to the steel including a large amount of the added manganese, a
thin silicon oxide layer is formed on a surface of the steel to
prevent oxidation of manganese. Therefore, the formation of a thick
manganese oxide layer on a cold-rolled steel sheet is prevented
after the rolling of the steel sheet, the corrosion in the
cold-rolled steel sheet may be prevented after an annealing process
to improve surface quality of the cold-rolled steel sheet, and it
is possible to maintain an excellent surface quality as a substrate
steel sheet of electroplating materials.
[0033] However, the increased content of the added silicon makes it
possible to form silicon oxides on a surface of a steel sheet when
the steel sheet is hot-rolled, which leads to the deteriorated
pickling property and the poor surface quality of the hot-rolled
steel sheet. And, the silicon is condensed on a surface of the
steel sheet when the steel sheet is annealed at high temperature in
the continuous annealing process and the continuous hot plating
process, and wetability of molten zinc on the surface of the steel
sheet is low when the steel sheet is hot-plated, which leads to the
poor plating property. Furthermore, the addition of a large amount
of the silicon results in the deteriorated weldability of the
steel. Accordingly, the maximum content of the silicon is
preferably 2.5%, based on the total content of the steel sheet.
Crashworthiness is associated with mechanical properties of an
inner metal seed layer but not associated with corrosiveness of the
plating layer, and a heat treatment conditions for plating a steel
sheet does not affect the mechanical properties of the high
manganese steel sheet with an austenite single phase structure.
Therefore, the preventive product has crashworthiness of a plated
product.
[0034] A content of titanium (Ti) is preferably in a range from
0.01 to 0.1%.
[0035] Titanium is a strong carbide-forming element that reacts
with carbon to form a carbide. In this case, the resultant carbide
has an effect on miniaturization of grain size since it functions
to suppress grain growth. However, the effect on miniaturization of
grain size does not appear when the content of the titanium (Ti) is
less than 0.005%, whereas the excessive titanium (Ti) is slabbed in
grain boundaries to cause grain boundary embrittlement, or a coarse
precipitate phase is excessively formed when the content of the
titanium (Ti) exceeds 0.10%, which leads to the poor effect on the
grain growth.
[0036] A content of niobium (Nb) is preferably in a range from
0.005 to 0.1%, and more preferably from 0.01 to 0.1%.
[0037] Niobium is a strong carbide-forming element that binds to
carbon in the same manner as the titanium to form a carbide. Also,
the resultant carbide is an element that has an effect on
miniaturization of grain size since it functions to suppress grain
growth, and has a high precipitation strengthening effect by the
miniaturization of grain size and the formation of the precipitate
phase since a precipitate phase is formed at a lower temperature
than the conventional titanium. However, the precipitation
strengthening effect does not appear when the content of the
niobium (Nb) is less than 0.005%, whereas the excessive niobium
(Nb) is slabbed in grain boundaries to cause grain boundary
embrittlement, or a coarse precipitate phase is excessively formed
when the content of the niobium (Nb) exceeds 0.10%, which leads to
the poor effect on the grain growth. Therefore, a preferable
content of the added niobium is in a range from 0.01 to 0.1%.
[0038] Hereinafter, a method for manufacturing a high manganese
steel sheet will described in detail.
[0039] In general, the manufacture of a high manganese hot-rolled
steel sheet may be carried out using the continuous casting method
as in the manufacture of conventional steels. The above-mentioned
composition is homogenized in the similar manner to the general
conditions of steel, finish-rolled and wound to prepare a
hot-rolled steel sheet.
[0040] According to the present invention, a heating temperature of
a casting slab is preferably in a range from 1050 to 1300.degree.
C. when the high manganese steel sheet is hot-rolled. The maximum
heating temperature is limited to a temperature range of
1300.degree. C. This is why a grain size increases with the
increasing temperature, surface oxidation results in the decrease
in strength of steel, or a surface of a steel sheet has poor
physical properties. Also, a liquid-phase layer is formed in
columnar crystal grain boundaries of the casting slab when the high
manganese steel sheet is heated to greater than 1300.degree. C.,
which leads to the cracks during the hot rolling process.
Meanwhile, the minimum heating temperature is limited to a
temperature range of 1050.degree. C. This is why it is difficult to
ensure a temperature required for finish-rolling a steel sheet when
the heating temperature is low, and the increase in rolling load
due to the decreased temperature makes it possible to sufficiently
roll a steel sheet to a predetermined thickness. That is, since the
conventional finish rolling temperature is at least 850.degree. C.
or above, and preferably about 900.degree. C. during the hot
rolling process, the rolling load increases with the decreasing
finish rolling temperature, and therefore the unreasonable load is
inflicted on a rolling machine, and also has a bad effect on the
quality of an internal steel sheet. And, the excessively high
finish rolling temperature facilitates oxidation in a surface of
the steel sheet during the hot rolling process, and therefore the
finish rolling temperature is limited to a temperature range of
1000.degree. C.
[0041] The hot rolling process is carried out at a coiling
temperature of 700.degree. C. or below. An oxide layer is not
easily removed off during the pickling process since a thick oxide
layer is formed on a surface of the hot-rolled steel sheet and the
inner part of the hot-rolled steel sheet is oxidized when the
coiling temperature exceeds 700.degree. C. Accordingly, the
hot-rolled steel sheet is preferably hot-rolled at a low coiling
temperature.
[0042] The resultant hot-rolled steel sheet may be manufactured
into a cold-rolled steel sheet, when necessary.
[0043] The cold-rolled steel sheet is prepared by cold-rolling a
steel sheet so as to meet the shape and thickness of the steel
sheet, and a preferable cold rolling is carried out at a reduction
ratio of 30 to 80%.
[0044] The cold-rolled steel sheet is continuously annealed at a
temperature of 600.degree. C. or above. At this time, when the
annealing temperature is too low, it is difficult to ensure
sufficient workability and a sufficient level of austenite is not
formed during the phase transformation to maintain an austenite
phase at a low temperature. Accordingly, it is preferred to perform
the annealing process at an annealing temperature of 600.degree. C.
or above. Because an austenite steel whose phase transformation
does not occur easily is used in the present invention, it is
possible to ensure sufficient workability when the steel is heated
to a temperature greater than its recrystallization temperature.
Therefore, the cold-rolled steel sheet may be annealed under
conventional annealing conditions.
[0045] The annealed steel sheet, as prepared thus, is plated when
necessary. Here, the plating may be selected from hot plating,
electroplating and deposition processes, and the hot plating
process is preferred. The method for manufacturing a plated steel
sheet includes: continuously annealing a cold-rolled steel sheet at
600.degree. C. or above and manufacturing a hot-plated,
electroplated or deposited steel sheet. The conventional heat
treatment affect a transformation induced plasticity steel sheet
during the electroplating or hot plating process, but it is
possible to plate the inventive steels under the conventional
conditions since the inventive steels have an austenite single
phase and a low difference in mechanical properties due to the lack
of the phase transformation.
[0046] According to the present invention, one of the
above-mentioned high manganese steel sheets satisfying the
components according to the present invention, for example a
hot-rolled steel sheet, a cold-rolled steel sheet and a plated
steel sheet may be cold-rolled again at a reduction ratio of
10.about.80% to enhance yield strength. Here, the rolling process
may be carried out using one of a temper rolling process, a dual
rolling process and a hot coil process used in steel mills.
MODE FOR THE INVENTION
[0047] Hereinafter, exemplary embodiments of the present invention
will be described in detail.
Example
[0048] The inventive steels and the comparative steels are listed
in the following Table 1, and molten steel ingots were kept in a
1200.degree. C. heating furnace for one hour, and then hot-rolled.
At this time, a hot rolling finish temperature was 900.degree. C.,
and a coiling temperature was 650.degree. C. Some of the hot-rolled
steel sheets were processed into tensile test samples according to
the JIS5 standard, and the tensile test samples were tested for
tensile strength using a conventional universal testing machine.
And, the hot-rolled steel sheets were pickled and cold-rolled at a
cold reduction ratio of 50%. The cold-rolled test samples were
continuously annealed, simulated and heat-treated at an annealing
temperature of 800.degree. C. and an over-aging temperature of
400.degree. C. After the continuous annealing, simulation and
heat-treatment processes, the cold-rolled test samples were tested
for tensile strength using a conventional universal testing
machine. Meanwhile, the cold-rolled test samples were continuously
annealed, simulated and heat-treated at an annealing temperature of
800.degree. C. in a 460.degree. C. hot galvanizing bath.
TABLE-US-00001 TABLE 1 C Mn P S Al Si Nb Ti N Note 1 0.15 2.5 0.010
0.006 0.05 0.50 0.026 0.006 Comparative Steel 2 0.10 6.0 0.010
0.010 0.04 0.50 0.006 Comparative Steel 3 0.45 12.0 0.010 0.011
1.48 0.01 0.006 Comparative Steel 4 0.44 14.8 0.012 0.009 1.40 0.01
0.006 Comparative Steel 5 0.43 15.0 0.009 0.005 0.05 0.01 0.006
Comparative Steel 6 0.15 15.0 0.010 0.005 1.50 0.01 0.006
Comparative Steel 7 0.60 15.1 0.009 0.008 1.36 2.50 0.006 Inventive
Steel 8 0.60 15.2 0.008 0.005 1.50 0.01 0.006 Comparative Steel 9
0.60 24.0 0.005 0.006 0.05 0.006 Comparative Steel 10 0.59 18.8
0.010 0.004 1.64 0.49 0.006 Inventive Steel 11 0.62 17.9 0.010
0.009 1.60 0.046 0.006 Inventive Steel 12 0.62 18.2 0.010 0.008
1.60 0.076 0.006 Inventive Steel 13 0.62 18.2 0.010 0.009 1.57
0.038 0.006 Inventive Steel 14 0.62 18.6 0.010 0.009 1.61 0.006
Comparative Steel 15 0.63 18.2 0.012 0.010 0.006 Comparative Steel
16 0.66 19.2 0.010 0.008 1.66 0.026 0.006 Inventive Steel 17 0.61
18.1 0.010 0.009 1.51 0.5 0.03 18 0.63 18.3 0.010 0.009 1.52 0.5
0.05 19 0.60 17.5 0.021 0.002 1.42 0.005 0.006 Comparative
Steel
TABLE-US-00002 TABLE 2 Cold-Rolled/ Hot-rolled Plated Steel Steel
Sheet Sheet YS TS T-El YS TS T-El Note 1 545 646 23.7 520 800 23
Comparative Steel 2 818 1248 8 -- -- -- Comparative Steel 3 403 837
40.5 339 678 40.3 Comparative Steel 4 435 875 66.7 341 862 63.2
Comparative Steel 5 374 922 32.8 373 978 37 Comparative Steel 6 374
991 49 377 1019 52.5 Comparative Steel 7 567 979 54 514 994 66.9
Inventive Steel 8 391 893 68.7 399 894 62.9 Comparative Steel 9 353
772 25.8 -- -- -- Comparative Steel 10 609 899 45.3 448 873 63
Inventive Steel 11 587 947 55.5 521 974 66.9 Inventive Steel 12 557
943 59.4 531 969 55.3 Inventive Steel 13 452 902 70.1 482 952 63.6
Inventive Steel 14 418 887 68.3 445 932 66.5 Comparative Steel 15
349.4 963.5 41.8 -- -- -- Comparative Steel 16 560 947 54.7 492 967
63.4 Inventive Steel 17 502 924.1 61.9 Inventive Steel 18 534 965.7
54.8 Inventive Steel 19 471.3 939.9 60.4 -- -- -- Comparative
Steel
[0049] Changes in the mechanical properties of the inventive steels
and the comparative steels according to the manufacturing
conditions of the steel sheets are listed Table 2. In the Note, the
steel sheets according to the present invention represented by the
inventive steels represents hot-rolled steel sheets, and, when the
tensile tests were carried out after the continuous annealing,
simulation and heat-treatment processes, the hot-rolled steel
sheets had a tensile strength of 700 MPa or more, an elongation of
40% or more and a yield strength of 500 MPa or more. Therefore,
materials that are suitable for structural members such as
automobile members and fillers were found.
[0050] In the case of the steel sheets of test sample Nos. 1 and 2,
it is impossible to secure sufficient strength and ductility due to
the low content of the added manganese.
[0051] The steel sheets of test sample Nos. 3 to 6, 8 to 9, 14 to
15 and 19 were not suitable as the structural members since the
elongation was poor, and the yield strength was low at 500 MPa or
less due to the insufficient contents of the added carbon,
manganese, silicon and aluminum.
[0052] The steel sheets of test sample Nos. 7, 10 to 13 and 16 to
18 were suitable as the structural members since the contents of
the added carbon, manganese and aluminum were appropriate, and the
yield strength is desirable due to the addition of the silicon,
titanium and niobium.
Example 2
[0053] The high strength steel sheets with high workability and
high manganese having an austenite single phase, prepared in
Example 1, were cold-rolled again, and measured for mechanical
properties. The results are listed in the following Table 3.
TABLE-US-00003 TABLE 3 Rolling YS TS T-El amount Note 19 471.3
939.9 60.4 0 Comparative Steel 19-1 750.5 1047.2 44.6 10 Inventive
Steel 19-2 930.5 1209.7 22.2 20 Inventive Steel 19-3 1088.3 1371.3
12.5 30 Inventive Steel 19-4 1247.7 1554.2 8.6 40 Inventive Steel
19-5 1388.2 1704.1 6.8 50 Inventive Steel 19-6 1503.6 1808.6 4.6 60
Inventive Steel 19-7 1612.8 1924.5 2.8 70 Inventive Steel
[0054] As shown in Table 3, it was revealed that the yield
strengths of the high strength steel sheets are enhanced. It was
usually seen that the yield strengths are increased to 750 MPa or
more in the 10% phase transformation, and the elongation is
excellent at 44%, indicating that the high strength steel sheets
have excellent formability and crash-worthiness as the structural
members.
[0055] While the present invention has been shown and described in
connection with the exemplary embodiments, it will be apparent to
those skilled in the art that modifications and variations can be
made without departing from the spirit and scope of the invention
as defined by the appended claims.
* * * * *