U.S. patent application number 14/692334 was filed with the patent office on 2016-04-21 for method for warm forming of super high tensile strength steel sheet.
The applicant listed for this patent is Hyundai Motor Company. Invention is credited to Moon Ki Bae, Young Tae Kim.
Application Number | 20160107222 14/692334 |
Document ID | / |
Family ID | 55638182 |
Filed Date | 2016-04-21 |
United States Patent
Application |
20160107222 |
Kind Code |
A1 |
Kim; Young Tae ; et
al. |
April 21, 2016 |
Method for Warm Forming of Super High Tensile Strength Steel
Sheet
Abstract
Disclosed herein is a method for warm forming of a super high
tensile strength steel sheet. The method may improve productivity
and simultaneously maintain accurate strength by securing thermal
structure stability and dimensional stability during heating and
forming the super high tensile strength steel sheet. According to
an exemplary embodiment of the present invention, the method for
warm forming of a super high tensile strength steel sheet includes:
heating a steel sheet having a single-phase structure at Al
transformation temperature or less; pressing, in a mold, the heated
steel sheet to be formed in a predetermined shape; and extracting
and cooling the pressed steel sheet.
Inventors: |
Kim; Young Tae; (Gunpo,
KR) ; Bae; Moon Ki; (Hwaseong, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Hyundai Motor Company |
Seoul |
|
KR |
|
|
Family ID: |
55638182 |
Appl. No.: |
14/692334 |
Filed: |
April 21, 2015 |
Current U.S.
Class: |
420/8 ;
72/364 |
Current CPC
Class: |
C22C 38/00 20130101;
B21C 37/02 20130101 |
International
Class: |
B21J 1/06 20060101
B21J001/06; C22C 38/00 20060101 C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 16, 2014 |
KR |
10-2014-0139781 |
Claims
1. A method for warm forming of a super high tensile strength steel
sheet, comprising: heating a steel sheet having a single-phase
structure at A.sub.1 transformation temperature or less; pressing,
in a mold, the heated steel sheet to be formed in a predetermined
shape; and extracting and cooling the pressed steel sheet.
2. The method of claim 1, wherein the steel sheet has a
single-phase martensite structure or a single-phase ferrite
structure.
3. The method of claim 2, wherein the steel sheet having the
single-phase ferrite structure has improved strength by
precipitation hardening.
4. The method of claim 1, wherein a temperature of the mold used in
the pressing is controlled by heat transferred from the heated
steel sheet without additional heating and cooling.
5. A super high tensile strength steel sheet manufactured by a
method of claim 1.
6. A vehicle part comprising a super high tensile strength steel
sheet of claim 5.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] The present application claims priority to Korean Patent
Application No. 10-2014-0139781, filed Oct. 16, 2014, the entire
contents of which is incorporated herein for all purposes by this
reference.
TECHNICAL FIELD
[0002] The present invention relates to a method for warm forming
of a super high tensile strength steel sheet by simultaneously
improving strength and formability.
BACKGROUND
[0003] Recently, use of a super high tensile strength steel sheet
has been increased with the development of vehicle industries. The
super high tensile strength steel sheet refers to a steel material
that may have greater tensile strength than a high tensile strength
steel sheet having a tensile strength of about 50 kg/mm.sup.2 and
may have further improved strain hardening, quenching, annealing,
tempering, normalizing, precipitation hardening, grain refinement,
solid solution strengthening, and the like using various
strengthening mechanisms. However, the super high tensile strength
steel sheet may have reduced formability that is in inverse
proportion to the improvement of the strength.
[0004] Further, in the related art, the super high tensile strength
steel sheet may not be applied to parts having a complicated shape,
parts having a deep forming depth, or the like due to reduced
formability of the super high tensile strength steel sheet, and
thus, many attempts to improve the formability of the super high
tensile strength steel sheet by heating have been conducted.
[0005] However, because vehicle part generally require accurate
strength and precise dimensional stability, the super high tensile
strength steel sheet may not be used as the vehicle parts due to
the reduction in strength , a twist of a member, and a dimensional
change thereof.
[0006] Recently, in various efforts to solve the above problems,
the super high tensile strength steel sheet for vehicle parts has
been manufactured by a hot stamping method which may improve
formability by heating a steel sheet at a temperature of about
900.degree. C. or greater. Accordingly, the steel sheet may be
formed and obtain high strength by rapidly cooling a steel sheet
simultaneously with performing press forming using a special mold
in which cooling water is circulated.
[0007] The hot stamping method may provide the steel sheet with an
improved strength of about 150 K or greater by transforming an
austenite phase of a steel sheet into a martensite phase. The hot
stamping method may not cause a spring back even after extracting
the steel sheet and may further prevent a dimension of parts from
changing, since the parts within the mold are completely cooled in
a fixed state.
[0008] However, the hot stamping method may have low productivity
to a level of 2 SPM, because a specific mold in which the cooling
water is circulated should be used, laser trimming of a draw panel
should be performed, and the steel sheet is completely cooled
within the mold.
[0009] Further, since a heating temperature of the steel sheet may
increase to an A.sub.3 transformation temperature or greater of
austenite, when the super high tensile strength steel sheet is
processed by the precipitation hardening, strengthening effect of
the steel sheet due to precipitates may be reduced.
[0010] In other words, re-crystallization may occur by
high-temperature heat applied to the steel sheet, and thus most of
the strengthening effects may be reduced and a lattice structure
and a lattice constant may be deformed depending on a change in a
crystalline phase, and as a result, a dimension of the steel sheet
may also be changed.
[0011] As such, the hot stamping method which has been currently
rapidly applied may not be considered as an optimal method for
manufacturing a super high tensile strength steel sheet for vehicle
parts. Therefore, the present invention may provide a method to
solve the above-mentioned problems.
[0012] In the related arts, a certain method for preventing
formability of a steel sheet from reducing has been reported in
"Steel Strip For The Automotive Reinforcement Parts And Method Of
Manufacturing Thereof (KR 10-0530068)" and "Quenched Steel Sheet
Having Ultra High Strength, Parts Made Of It And The Method For
Manufacturing Thereof(KR 10-0878614)".
[0013] Further, a CERTAIN method of manufacturing a steel strip for
vehicle reinforcement parts has been reported in "Steel Strip For
The Automotive Reinforcement Parts And Method Of Manufacturing
Thereof(KR 10-0530068)". The vehicle reinforcement parts
manufactured by the method including performing homogenization
treatment on aluminum killed steel at 1050 to 1300.degree. C., hot
rolling thereon at a finishing rolling temperature condition of 850
to 950 .degree. C. which is just above an Ar3 transformation point,
winding it at a temperature range of 650 to 800 .degree. C., and
performing cold rolling thereon at a cold reduction ratio of 30 to
80% may have improved heat treatment properties.
[0014] Further, in the related art, a quenched steel sheet, parts
from the steel sheet and the method of manufacturing thereof have
been developed as reported in "Quenched Steel Sheet Having Ultra
High Strength, Parts Made Of It And The Method For Manufacturing
Thereof(KR 10-0878614)". The quenched steel sheet may have improved
tensile strength by heat treatment hardening and then improved
yield strength after suffering from painting heat treatment.
[0015] However, the above mentioned technical problems such as
reduced productivity due to the use of the specific mold and the
rapid cooling in the mold, reduced strengthening effect due to the
re-crystallization, change in the lattice constant and the lattice
structure, and the like still remain in the related art.
[0016] The above information disclosed in this background section
is merely for enhancement of understanding of the background of the
invention and therefore it may contain information that does not
form the prior art that is already known in this country to a
person of ordinary skill in the art.
SUMMARY
[0017] Thus, in a preferred aspect, the present invention provides
a method for warm forming of a super high tensile strength steel
sheet. The method may increase productivity and accurate strength,
by securing thermal structure stability and dimensional stability
when the super high tensile strength steel sheet is heated and
formed.
[0018] According to an exemplary embodiment of the present
invention, a method for warm forming of a super high tensile
strength steel sheet may include: heating a steel sheet having a
single-phase structure at A.sub.l transformation temperature or
less; pressing, in a mold, the heated steel sheet to be formed in a
predetermined shape; and extracting and cooling the pressed steel
sheet.
[0019] The steel sheet may be a steel sheet having a single-phase
martensite structure or a steel sheet having a single-phase ferrite
structure.
[0020] The steel sheet having the single-phase ferrite structure
may have improved strength by precipitation hardening.
[0021] A temperature of the mold used in the pressing may be
controlled by heat transferred from the heated steel sheet without
additional heating and cooling.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The above and other objects, features and advantages of the
present invention will be more clearly understood from the
following detailed description taken in conjunction with the
accompanying drawings, in which:
[0023] FIG. 1 is an exemplary graph illustrating elongation and
strength of a steel sheet having a multi-phase structure
strengthened by a strengthening mechanism for each temperature in
the related art;
[0024] FIG. 2 is an exemplary graph illustrating that a strength of
the steel sheet is changed when the steel sheet having the
multi-phase structure and an exemplary steel sheet having a
single-phase structure according to an exemplary embodiment of the
present invention are heated at about 400.degree. C. to 700.degree.
C. and then cooled; and
[0025] FIG. 3 is an exemplary graph illustrating a change in
temperature, a change in strength, and a change in elongation of an
exemplary super high tensile strength steel sheet at the time of
warm forming according to the exemplary embodiment of the present
invention.
DETAILED DESCRIPTION
[0026] It is understood that the term "vehicle" or "vehicular" or
other similar term as used herein is inclusive of motor vehicles in
general such as passenger automobiles including sports utility
vehicles (SUV), buses, trucks, various commercial vehicles,
watercraft including a variety of boats and ships, aircraft, and
the like, and includes hybrid vehicles, electric vehicles, plug-in
hybrid electric vehicles, hydrogen-powered vehicles and other
alternative fuel vehicles (e.g. fuels derived from resources other
than petroleum). As referred to herein, a hybrid vehicle is a
vehicle that has two or more sources of power, for example both
gasoline-powered and electric-powered vehicles. The terminology
used herein is for the purpose of describing particular exemplary
embodiments only and is not intended to be limiting of the
invention. As used herein, the singular forms "a", "an" and "the"
are intended to include the plural forms as well, unless the
context clearly indicates otherwise. It will be further understood
that the terms "comprises" and/or "comprising," when used in this
specification, specify the presence of stated features, integers,
steps, operations, elements, and/or components, but do not preclude
the presence or addition of one or more other features, integers,
steps, operations, elements, components, and/or groups thereof As
used herein, the term "and/or" includes any and all combinations of
one or more of the associated listed items.
[0027] Unless specifically stated or obvious from context, as used
herein, the term "about" is understood as within a range of normal
tolerance in the art, for example within 2 standard deviations of
the mean. "About" can be understood as within 10%, 9%, 8%, 7%, 6%,
5%, 4%, 3%, 2%, 1%, 0.5%, 0.1%, 0.05%, or 0.01% of the stated
value. Unless otherwise clear from the context, all numerical
values provided herein are modified by the term "about."
[0028] Hereinafter, an exemplary embodiment of the present
invention will be described with reference to the accompanying
drawings and background art and reference numerals of the foregoing
components are similarly applied unless specifically mentioned.
[0029] A method for warm forming of a super high tensile strength
steel sheet described below is an exemplary embodiment of the
present invention, and is not limited to the exemplary embodiment
but may be implemented in various forms.
[0030] As generally known in the art, for improving strength of
metal which is elastically and plastically deformed, atoms may be
prevented from moving. As such, various strengthening mechanisms of
strain hardening, quenching, annealing, tempering, normalizing,
precipitation hardening, grain refinement, solid solution
strengthening, and the like which restrict the movement of atoms
within a lattice have been used.
[0031] When the strengthening mechanism is applied to a steel
sheet, a phase change and a structure change of the steel sheet may
internally occur and thus the strength of the steel sheet may be
sharply increased, but the formability may be reduced in inverse
proportion to the strength and have inappropriate characteristics
to manufacture parts having a complicated shape. Accordingly, to
apply the steel sheet manufactured by the strengthening mechanism
to the parts having a complicated shape, the formability may be
improved by a separate heating process. That is, parts having a
predetermined shape may be manufactured by heating the steel sheet
at high temperature to increase the elongation of the steel sheet
and then performing the press forming thereon.
[0032] However, a phase change, a structure change, a change in a
lattice structure, and the like may occur in internal structure of
the steel sheet due to high heat applied to the steel sheet and
thus the strengthening mechanism may be reduced and thus, the
quenching may be performed simultaneously with the press forming
using a mold in which cooling water flows to secure the strength of
the steel sheet. Accordingly, the present invention provides a
method for warm forming of a super high tensile strength steel
sheet. Particularly, in the method of the present invention, the
strengthening mechanism and the lattice structure may not react
sensitively to temperature and additional cooling process such as
quenching or water cooling may not be used.
[0033] The method for warm forming of a super high tensile strength
steel according to the exemplary embodiment of the present
invention may include a heating process, a press process, and a
cooling process. The heating process may be a process of heating
the steel sheet to improve the formability of the steel sheet and
setting the heating temperature of the steel sheet to be A.sub.1
transformation temperature or less. Mechanical physical properties
of the steel sheet may be sharply changed depending on the heating
temperature. For example, when the temperature of the steel sheet
increases, vibration energy of the lattice may be increased and
thus an elasto-plastic behavior may be changed to a viscoelastic
behavior and thus the elongation may be increased as illustrated in
FIG. 1.
[0034] FIG. 1 is an exemplary graph illustrating the elongation and
the strength of the steel sheet which has a multi-phase structure
strengthened by the strengthening mechanism at each temperature of
room temperature, about 300.degree. C., about 400.degree. C., about
500.degree. C., and about 600.degree. C., respectively. As shown in
FIG. 1, when the temperature of the steel sheet is room
temperature, the steel sheet may have a strength of about 1200 MPa.
As the temperature increases, the strength may be reduced and the
elongation may be increased in inverse proportion to the
strength.
[0035] The increase in elongation of the steel sheet may mean
improvement of formability, and thus, when the formality is
improved, the parts having a complicated shape may be easily
manufactured by the press forming. However, the steel sheet having
the multi-phase structure in which phases such as ferrite,
austenite, and martensite are mixed may deteriorate from a phase
change, a structure change, and a change in a lattice structure,
and the like as temperature increases. As the temperature of the
steel sheet increases, the strength may be reduced, thereby failing
to solve technical problems in the related art. Therefore, in the
heating process according to the exemplary embodiment of the
present invention, the steel sheet having a single-phase structure,
not the multi-phase structure, may be applied. For example, the
steel sheet having the single-phase structure may be heated to a
A.sub.1 transformation temperature or less.
[0036] FIG. 2 is an exemplary graph illustrating that the strength
of the steel sheet is changed when the steel sheet having the
multi-phase structure and the steel sheet having the single-phase
structure are heated at about 400.degree. C. to 700.degree. C. and
then cooled. As shown in FIG. 2, when the steel sheet having the
single-phase structure is cooled, the final strength of the cooled
steel sheet may not be changed substantially compared to initial
strength before the steel sheet is heated. In contrast, as the
heating temperature increases, a final strength of the steel sheet
having the multi-phase structure may be sharply reduced after being
cooled.
[0037] In other words, when the steel sheet having the single-phase
structure, not the steel sheet having the multi-phase structure, is
strengthened by the strengthening mechanism and then is heated at
A.sub.1 transformation temperature or less, both the strength and
the formability may be improved. In particular, heat may be applied
until the A.sub.1 transformation temperature or less of the steel
sheet to prevent the dimensional stability from reducing due to the
phenomenon of the phase change, the structure change, the change in
the lattice structure, the change in the lattice constant, and the
like which may be caused when the high heat of the A.sub.1
transformation temperature or greater applied to the steel sheet
having the single-phase structure. Further, reduction of the
strength due to reduced strengthening mechanism may be
prevented.
[0038] In particular, the steel sheet having the single-phase
structure may have a single-phase structure of martensite or
ferrite to provide improved strength required for the super high
tensile strength steel sheet. The martensite may have a
needle-shaped structure and substantially improved strength and
thus may be suitably used for the super high tensile strength steel
sheet. The ferrite may have relatively less strength than the
martensite but may have improved strength by various strengthening
mechanisms, and thus, may be applied to the method for warm forming
of the super high tensile strength steel sheet according to the
exemplary embodiment of the present invention.
[0039] Further, a steel sheet of which the strength is improved by
the precipitation hardening may be applied among the steel sheets
having the ferrite structure, and thus, the improved strength
required for the super high tensile strength steel sheet may be
obtained. Since the heating temperature is also equal to or less
than the A.sub.l transformation temperature, the precipitation
hardening effect may remain without deterioration. The press
process may form the steel sheet heated by the heating process in a
predetermined shape. In particular, when the method for warm
forming according to the exemplary embodiment of the present
invention is applied, since the strength, the phase, the structure,
the lattice structure, the lattice constant, and the like of the
steel sheet are not changed, the quenching in the related art may
not be performed.
[0040] The press process may use various methods for forming a
steel sheet. However, according to an exemplary embodiment of the
present invention, using a generally used mold, cooling may not be
performed at the time of the press forming, and thus cooling time
may not be required. Further, the temperature of the mold may be
controlled by heat transferred from the steel sheet heated without
additional heating and cooling, and thus the generally used mold
may be used, unlike a specific mold including cooling water as used
in the related art.
[0041] The air cooling process may extract and then cool the steel
sheet from the press process. In the related art, the steel sheet
may be extracted from inside of the mold after the steel sheet is
cooled to prevent the shape of the pressed steel sheet from
changing and the steel sheet from reducing. However, according to
an exemplary embodiment of the present invention, extracting and
cooling the steel sheet may be completed without performing
additional cooling after the press process.
[0042] Further, as described above, since the strength, the phase,
the structure, the lattice structure, the lattice constant, and the
like of the steel sheet may not be changed, there is no need of
using the cooling method such as cooling water and quenching that
requires substantial period of time . For example, the air cooling
may be applied effectively. For example, when the heated and
pressed steel sheet is cooled, the steel sheet may be cooled by air
as being immediately extracted without waiting, and as a result, a
production efficiency may be substantially improved.
[0043] FIG. 3 is an exemplary graph illustrating the change in
temperature, the change in strength, and the change in elongation
of an exemplary super high tensile strength steel sheet according
to the exemplary embodiment of the present invention during an
exemplary warm forming process at a temperature range of
200.degree. C. to 600.degree. C. according to the exemplary
embodiment of the present invention. Particularly, the temperature
may be maintained less than the A.sub.1 transformation temperature
of the applied steel sheet. As shown in FIG. 3, when the
temperature of the steel sheet increases, the strength may be
gradually reduced to a predetermined level and the elongation may
be improved in inverse proportion thereto.
[0044] However, when the heated steel sheet is conveyed and mounted
in the mold, the temperature of the steel sheet may be
substantially reduced. Since the additional heating apparatus is
not mounted in the mold and thus the temperature of the mold is in
a less state than that of the heated mold, when the heated steel
sheet contacts the mold, the temperature may be sharply reduced.
Further, to the contrary, the reduced strength may be increased
again and thus the predetermined level may be maintained, and the
elongation may be substantially reduced and then the predetermined
level may be maintained.
[0045] The elongation may be substantially reduced and then
maintained at the predetermined level at the time of the press
process due to the temperature of the steel sheet that may be
sharply reduced and then maintained at the predetermined level. As
such, the elongation may be reduced and thus the pressed steel
sheet may not be easily deformed and the shape of the steel sheet
may be maintained.
[0046] Moreover, when the pressed steel sheet is immediately
extracted without additional process and then cooled by air, the
temperature of the steel sheet may be reduced to the room
temperature over time, and to the contrary, the strength may be
further increased and the elongation may be reduced, and thus the
further forming may not be performed.
[0047] As described above, according to various exemplary methods
for warm forming of a super high tensile strength steel sheet,
improved strength of the steel sheet may be obtained simultaneously
with improved formability when forming the parts having a
complicated shape thereby improving the productivity.
[0048] The present invention may provide various advantages as
follows.
[0049] First, the present invention may improve the productivity
since phase change may not occur and additional cooling process may
not be required.
[0050] Second, the present invention may perform the heating at the
A.sub.1 transformation temperature or less to minimize the waste of
heating time and energy.
[0051] Third, the present invention may improve strength while
manufacturing the parts having a complicated shape.
[0052] Fourth, the present invention may manufacture the super high
tensile strength steel sheet having substantially improved
dimensional stability and thermal structural stability.
[0053] Fifth, the present invention may maintain the processing
hardening effect applied to the steel sheet.
[0054] The present invention is described in detail with reference
to the exemplary embodiments but the scope of the present invention
is not limited to the specific exemplary embodiment but needs to be
construed by the accompanying claims. Further, it will be to be
understood that the present invention may be variously modified and
changed by those skilled in the art without departing from the
scope of the present invention.
* * * * *