U.S. patent application number 12/064145 was filed with the patent office on 2008-10-02 for high strength hot rolled steel sheet containing high mn content with excellent workability and method for manufacturing the same.
This patent application is currently assigned to POSCO. Invention is credited to Kwang-Geun Chin, Seong-Ju Kim, Sung-Kyu Kim.
Application Number | 20080240969 12/064145 |
Document ID | / |
Family ID | 37771800 |
Filed Date | 2008-10-02 |
United States Patent
Application |
20080240969 |
Kind Code |
A1 |
Kim; Sung-Kyu ; et
al. |
October 2, 2008 |
High Strength Hot Rolled Steel Sheet Containing High Mn Content
with Excellent Workability and Method for Manufacturing the
Same
Abstract
A hot rolled steel sheet used for a bumper reinforceing material
or for an impact absorption material in a door of automobiles, and
a method for manufacturing the same are disclosed. The steel sheet
comprises, by weight%, C: 0.2%.about.1%, Mn: 8.about.15%, S: 0.05%
or less, P: 0.03% or less, and the balance of Fe and other
unavoidable impurities. A product of tensile strength and total
elongation (TS.times.ToLEl) of the steel sheet is 24,000 MPa % or
more. The method provides a high strength hot rolled steel sheet,
which has a high strength-elongation balance value, ensuring
excellent workability.
Inventors: |
Kim; Sung-Kyu;
(Chunlanam-do, KR) ; Chin; Kwang-Geun;
(Chunlanam-do, KR) ; Kim; Seong-Ju; (Chunlanam-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: |
37771800 |
Appl. No.: |
12/064145 |
Filed: |
August 23, 2006 |
PCT Filed: |
August 23, 2006 |
PCT NO: |
PCT/KR2006/003303 |
371 Date: |
May 27, 2008 |
Current U.S.
Class: |
420/73 ;
148/602 |
Current CPC
Class: |
C21D 8/04 20130101; C21D
8/0205 20130101; C21D 8/0405 20130101; C21D 2211/001 20130101; C22C
38/58 20130101; C21D 8/02 20130101; C22C 38/06 20130101; C21D 6/005
20130101; C22C 38/42 20130101 |
Class at
Publication: |
420/73 ;
148/602 |
International
Class: |
C22C 38/04 20060101
C22C038/04; C21D 8/02 20060101 C21D008/02 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 23, 2005 |
KR |
10-2005-0077371 |
Claims
1. A high strength hot rolled steel sheet containing a high Mn
content with excellent workability, comprising, by weight %: C:
0.2%.about.1%, Mn: 8.about.15%, S: 0.05% or less, P: 0.03% or less,
and the balance of Fe and other unavoidable impurities, wherein a
product of tensile strength and total elongation (TS.times.Tot.El)
of the steel sheet is 24,000 MPa % or more.
2. The steel sheet according to claim 1, further comprising: at
least one selected from the group consisting of Al: 0.3.about.3%,
Ni: 2.about.7% and Cu: 2.about.5%.
3. The steel sheet according to claim 1, wherein the hot rolled
steel sheet has a single phase austenite structure.
4. A method for manufacturing a high strength hot rolled steel
sheet containing a high Mn content with excellent workability,
comprising: reheating a steel slab at a temperature of
1,180.about.1,220.degree. C., the steel slab comprising, by weight
%: C: 0.2%.about.1%, MN: 8.about.15%, S: 0.05% or less, P: 0.03% or
less, and the balance of Fe and other unavoidable impurities;
finish hot rolling the reheated steel slab at a temperature of
800.degree. C. or more; and cooling the hot rolled steel sheet at a
temperature of 600.degree. C. or more, followed by coiling the
cooled steel sheet, wherein a product of tensile strength and total
elongation (TS.times.Tot.El) of the steel sheet is 24,000 MPa % or
more.
5. The method according to claim 4, wherein the steel slab
comprises at least one selected from the group consisting of Al:
0.3.about.3%, Ni: 2.about.7% and Cu: 2.about.5%.
6. The method according to claim 4, wherein the hot rolled steel
sheet has a single phase austenite structure.
7. The steel sheet according to claim 2, wherein the hot rolled
steel sheet has a single phase austenite structure.
8. The method according to claim 5, wherein the hot rolled steel
sheet has a single phase austenite structure.
Description
TECHNICAL FIELD
[0001] The present invention relates to a hot rolled steel sheet
used for a bumper reinforcing material or for an impact absorption
material in a door of automobiles. More particularly, the present
invention relates to a high strength hot rolled steel sheet
containing a high Mn content with high elongation which ensures
excellent formability, and to a method for manufacturing the
same.
BACKGROUND ART
[0002] In automobiles, bumper reinforcing materials or impact
absorption material inside doors are directly related to safety of
passengers upon collision of the automobiles, and generally formed
of super strength hot rolled steel sheets having a tensile strength
of 780 MPa or more. In recent years, corresponding to gradually
increasing regulations for environmental pollution, high strength
steel has been increasingly used in the automobiles to increase
fuel efficiency, and investigations have progressed more and more
for commercial application of the high strength steel having the
tensile strength of 780 MPa or more.
[0003] As for the high strength steel for the automobiles, there
are dual phase (DP) steel, transformation induced plasticity (TRIP)
steel, and multi-phase steel.
[0004] Typically, a method for manufacturing a hot rolled steel
sheet for the automobiles comprises reheating a steel slab for
allowing steel components to form a solid solution again, hot
rolling the steel slab to a steel sheet having a predetermined
thickness, cooling the hot rolled steel sheet at room temperature,
and coiling the cooled steel sheet. Here, the dual phase steel is
produced in such a way of hot rolling the reheated slab at a
temperature in an austenite region, and cooling the hot rolled
steel sheet to a cooling finish temperature below Ms-temperature
for transformation from austenite to martensite. In the dual phase
steel, an increase in ratio of the martensite structure causes an
increase in strength of the steel, and an increase in ratio of the
ferrite structure causes an increase in elongation of the
steel.
[0005] However, the dual phase steel has a disadvantage in that a
high cooling rate is required to form the martensite at low
temperatures.
[0006] The TRIP steel refers to steel which has further enhanced
workability by forming retained austenite in a portion of the steel
structure. Since the TRIP steel has excellent uniform elongation by
transformation through work hardening, and typically has an
elongation of 30% for 800 MPa, which is excellent compared with
that of other super strength steel, it has a high tensile
strength-elongation balance value. However, high strength steel
having a higher level of strength has been required to satisfy
higher requirement for a light automotive body. Furthermore,
machining of more complicated components with the steel is required
for integration of the components, thereby requiring the steel to
have an elongation 30% higher than those for the same strength
level.
[0007] The multi-phase steel refers to steel which is further
increased in strength and elongation at the same time than the TRIP
steel by forming the austenite during hot rolling, and controlling
the cooling rate and the cooling finish temperature during cooling
the coiled steel sheet to form ferrite, martensite, a small
fraction of bainite, and a mixed phase of martensite and austenite
at room temperature. The multi-phase steel has superior weldability
and high yield strength due to its smaller added amount of alloy
element. However, the multi-phase steel has a disadvantage in view
of formability due to a low elongation.
DISCLOSURE OF INVENTION
Technical Problem
[0008] Therefore, the present invention has been made to solve the
above problems, and it is an object of the present invention to
provide a high strength hot rolled steel sheet containing a high Mn
content with excellent workability, which is controlled in Mn
content to 8.about.15 wt % to form a single phase austenite
structure, ensuring an excellent elongation, and to form twins upon
deformation, preventing necking which causes cracks.
Technical Solution
[0009] In accordance with one aspect of the invention, the above
and other objects can be accomplished by the provision of a high
strength hot rolled steel sheet containing a high Mn content with
excellent workability, comprising, by weight%: C: 0.2%.about.1%,
Mn: 8.about.15%, S: 0.05% or less, P: 0.03% or less, and the
balance of Fe and other unavoidable impurities, wherein a product
of tensile strength and total elongation (TS.times.Tot.El) of the
steel sheet is 24,000 MPa % or more.
[0010] In accordance with another aspect of the invention, a method
for manufacturing a high strength hot rolled steel sheet containing
a high Mn content with excellent workability comprises: reheating a
steel slab at a temperature of 1,180.about.1,220.degree. C., the
steel slab comprising by weight%: by weight%: C: 0.2%.about.1%, Mn:
8.about.15%, S: 0.05% or less, P: 0.03% or less, and the balance of
Fe and other unavoidable impurities finish hot rolling the reheated
steel slab at a temperature of 800.degree. C. or more and cooling
the hot rolled steel sheet at a temperature of 600.degree. C. or
more, followed by coiling the cooled steel sheet, wherein a product
of tensile strength and total elongation (TS.times.Tot.El) of the
steel sheet is 24,000 MPa % or more.
Advantageous Effects
[0011] As apparent from the above description, the present
invention provides a high strength hot rolled steel sheet
containing a high Mn content with excellent workability, which has
a higher strength-elongation balance value than that of TRIP steel
known as having excellent elongation in the art.
BEST MODE FOR CARRYING OUT THE INVENTION
[0012] Preferred embodiments will be described in detail
hereinafter.
[0013] For general high strength steel having ferrite structure,
deformation is caused by slip resulting from movement of
dislocations, and an increase in deformation amount results in a
higher degree of work hardening, which in turn results in
concentration of the deformation on grain boundaries, causing
necking. When the necking occurs, an increasing rate of work
hardening fails to compensate a reduction rate of a cross section
caused by the necking, resulting in failure of the steel.
[0014] The present invention is provided in order to solve such a
typical problem of the high strength steel, and characterized by
controlling an Mn content to 8.about.15 wt % to form a single phase
austenite structure in order to ensure excellent elongation, and by
create twins to prevent generation of necking which forms cracks
upon deformation. In other words, the steel of the present
invention has a different deformation characteristic from that of
the conventional high strength steel. Specifically, the steel
according to the present invention is twin-induced plasticity
steel, which has a high content of Mn so as to have the austenite
structure and is formed with the twins upon application of stress
to the steel. Here, unlike the dislocations causing the deformation
concentration, the twins cause little deformation concentration
upon work hardening, providing the excellent elongation. Meanwhile,
TRIP steel is subjected to transformation upon deformation, the
deformation is concentrated on martensite. However, the TWIP steel
of the present invention is not subjected to the transformation
upon the deformation, and maintains the austenite structure,
thereby providing excellent elongation.
[0015] The present invention will be described in terms of
composition and manufacturing process hereinafter.
[0016] Composition of Steel
[0017] C: 0.2.about.1 wt % (hereinafter, "%")
[0018] Carbon (C) is the most important component in steel. C has a
close relationship with all physical and chemical properties of the
steel, such as strength, toughness, corrosion resistance, and the
like, and provides the most influential effect on the properties of
the steel. If C content is less than 0.2%, not only stability of
austenite but also a fraction of a secondary phase are reduced,
thereby providing a problem of lowered strength. If the C content
exceeds 1%, not only weldability of the steel is deteriorated, but
also the fraction of the secondary phase is rapidly increased,
thereby rapidly deteriorating workability. Thus, the C content is
preferably in the range of 0.2.about.1%.
[0019] Mn: 8.about.15%
[0020] Manganese (Mn) is an austenite stabilization element, which
increases the strength of steel by improving hardenability.
According to the present invention, it is necessary to contain 8%
or more of Mn in order to obtain a stable austenite structure. If
Mn is excessively added above 15%, there are problems, such as
excessive load during steel manufacturing, deterioration in
weldability, formation of inclusions, and increase in manufacturing
costs. Thus, the Mn content is preferably in the range of
8.about.15%.
[0021] S: 0.05% or Less
[0022] Sulfur (S) is an impurity element in the steel. If S content
exceeds 0.05%, coarse MnS precipitates are generated on a hot
rolled steel sheet, deteriorating the workability and toughness of
steel. Thus, the S content is preferably 0.05% or less.
[0023] P: 0.03% or Less
[0024] Phosphorus (P) is an impurity element in the steel. If P
content exceeds 0.03%, the toughness of steel becomes deteriorated.
Thus, the P content is preferably 0.03% or less.
[0025] Meanwhile, the steel of the present invention may further
comprise at least one of Al, Ni and Cu.
[0026] Al: 0.3.about.3%
[0027] Aluminum (Al) is a ferrite stabilization element, which
resides in a solid solution state in ferrite. Al serves to enhance
the strength of steel, and is generally added to the steel as a
deoxidation agent. Meanwhile, in the present invention, Al is
effective to continuously generate twins during deformation by
increasing stacking fault energy. If Al content is less than 0.3%,
the effect of increasing the stacking fault energy is small. If the
Al content exceeds 3%, there can be problems of increasing nozzle
clogging and inclusions during the steel manufacturing or
continuous casting process. Thus, the Al content is preferably in
the range of 0.3.about.3%.
[0028] Ni: 2.about.7%
[0029] Nickel (Ni) is the austenite stabilization element, and
advantageous in terms of properties of the steel when it is added
to the steel as much as possible. If Ni content is less than 2%,
the effect by addition of Ni cannot be obtained. If the Al content
exceeds 7%, there is a problem of significantly increasing the
manufacturing costs. Thus, the Ni content is preferably in the
range of 2.about.7%.
[0030] Cu: 2.about.5%
[0031] Copper (Cu) is an element which is solid-soluted in the
austenite or forms a precipitation phase, thereby decreasing an
amount of crystal grains of the austenite and refining crystal
grains of the ferrite. In order to obtain these solid solution and
precipitation effects, it is necessary to have Cu content of 2% or
more. If the Cu content exceeds 5%, there are problems of
significantly increasing a reheating temperature while increasing
the manufacturing costs. Thus, the Cu content is preferably in the
range of 2.about.5%.
[0032] With the composition described as above, the hot rolled
steel sheet of the present invention has a single phase austenite
structure, and exhibits an elongation of 30% or more. For the steel
sheet of the present invention, a tensile strength-total elongation
balance value (TS.times.Tot.El) is of 24,000 MPa % or more. Such a
balance value is very high when considering that the TRIP steel
known as having excellent elongation has a tensile strength-total
elongation balance value less than 24,000 MPa %.
[0033] Manufacturing Method
[0034] A method of manufacturing a steel sheet according to the
present invention comprises reheating a steel slab having the
composition as described above for allowing components segregated
during casting to form a solid solution thereof again, hot rolling
the reheated steel slab to a steel sheet having a desired
thickness, cooling and coiling the hot rolled steel sheet to ensure
desired properties of the steel. These steps will be described in
detail hereinafter.
[0035] First, in the method of manufacturing the steel sheet, the
steel slab having the composition described as above is reheated.
Reheating of the steel slab is performed for the purpose of solid
solutioning the components segregated during the casting, and
preferably at a temperature of 1,180.about.1,220.degree. C. If a
reheating temperature is less than 1,180.degree. C., the components
segregated during the casting are not solid soluted again. If the
reheating temperature exceeds 1,220.degree. C., the grain size of
austenite increases, and grains of ferrite become coarsened,
thereby decreasing the strength of steel.
[0036] Then, the reheated steel slab is subjected to hot rolling,
preferably, at a finish hot rolling temperature of 800.degree. C.
or more. If the finish hot rolling temperature is less than
800.degree. C., a great amount of dislocations are induced in the
ferrite formed during the hot rolling, and the ferrite is grown
during cooling or coiling, forming coarse surface grains.
[0037] Next, the hot rolled steel sheet is cooled, and coiled.
Since a final structure of the hot rolled steel sheet according to
the present invention is a single phase austenite, the hot rolled
steel sheet does not suffer from phase transformation during the
cooling. Thus, the present invention is not limited to a specific
cooling rate, and may have a typical cooling pattern including air
cooling and water cooling. In this regard, if a cooling finish
temperature is less than 600.degree. C., there is a problem of
generating excessive retained stress. Thus, the cooling finish
temperature is preferably 600.degree. C. or more.
MODE FOR THE INVENTION
[0038] The present invention will be described in detail by
reference to examples.
EXAMPLE
[0039] After reheating a steel slab having the composition as shown
in Table 1 at 1,200.degree. C. for one hour, the steel slab was
subjected to hot rolling at a finish hot rolling temperature of
860.degree. C. Then, after cooling the hot rolled steel sheet to a
temperature of 680.degree. C., samples were manufactured using the
steel sheet, and measured for strength and elongation, results of
which are shown in Table 2.
TABLE-US-00001 TABLE 1 Sample Composition (wt %) No. C Mn S P Al Ni
Cu IS1 0.45 14.9 0.003 0.02 -- -- -- IS2 0.51 9.98 0.003 0.02 1.00
2.00 2.00 IS3 0.51 9.98 0.003 0.02 1.00 2.00 3.00 IS4 0.51 9.98
0.003 0.02 1.00 5.00 2.00 IS5 0.51 9.98 0.003 0.02 2.00 2.05 2.00
IS6 0.51 9.98 0.003 0.02 3.00 2.05 2.00 IS7 0.79 9.98 0.003 0.02
1.02 2.05 2.00 CS1 0.70 8.02 0.003 0.02 1.05 2.05 2.00 CS2 0.2 4.0
0.003 0.02 0.11 -- -- IS: Inventive steel CS: Comparative steel
TABLE-US-00002 TABLE 2 Sample YS TS Uni. El Tot. El TS .times. Tot.
El No. (MPa) (MPa) (%) (%) (MPa %) IS1 395.0 933.0 38.2 37.5 34987
IS2 401.7 752.3 31.7 32.1 24149 IS3 497.0 793.6 33.7 34.8 27617 IS4
412.0 760.1 48.9 57.4 43630 IS5 464.9 763.5 41.8 46.2 35274 IS6
519.7 768.9 31.2 37.2 28603 IS7 384.4 829.3 30.6 30.6 25377 CS1
362.6 637.3 13.8 15.9 10133 CS2 497.8 628.9 14.3 27.8 17455 IS:
Inventive steel CS: Comparative steel
[0040] As can be appreciated from Table 2, Inventive Samples
1.about.7 satisfying the conditions of the present invention have
excellent strength, elongation, and strength-elongation balance
value. In particular, the Inventive Samples 1.about.7 have
strength-elongation balance values of 24,000 MPa % or more, thereby
securing excellent workability along with high strength.
[0041] On the contrary, Comparative Samples 1 and 2 do not satisfy
the condition of the present invention in terms of Mn content, and
have low strength-elongation balance values.
[0042] It should be understood that the embodiments and the
accompanying drawings have been described for illustrative
purposes, and the present invention is limited only by the
following claims. Further, those skilled in the art will appreciate
that various modifications, additions and substitutions are allowed
without departing from the scope and spirit of the invention
according to the accompanying claims.
* * * * *