U.S. patent application number 16/760979 was filed with the patent office on 2020-08-20 for steel used for hot stamping, hot stamping process and formed component.
The applicant listed for this patent is EASYFORMING STEEL TECHNOLOGY CO., LTD. VOLKSWAGEN AKTIENGESELLSCHAFT. Invention is credited to Tobias OPITZ, Xiaochuan XIONG, Hongliang YI.
Application Number | 20200263271 16/760979 |
Document ID | 20200263271 / US20200263271 |
Family ID | 1000004826461 |
Filed Date | 2020-08-20 |
Patent Application | download [pdf] |
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United States Patent
Application |
20200263271 |
Kind Code |
A1 |
YI; Hongliang ; et
al. |
August 20, 2020 |
STEEL USED FOR HOT STAMPING, HOT STAMPING PROCESS AND FORMED
COMPONENT
Abstract
A kind of steel is able to achieve a high elongation with the
steel used for hot stamping by means of simple hot stamping
process. The formed component has excellent yield strength, tensile
strength and elongation. The steel used for hot stamping comprises
by weight percent 0.1-0.19% of C, 5.09-9.5% of Mn, 0.11-0.4% of V,
and 0-2% Si+Al, wherein the combination of C and V meets one of the
following two requirements: 1) 0.1-0.17% of C and 0.11-0.4% of V;
and 2) 0.171-0.19% of C and 0.209-0.4% of V.
Inventors: |
YI; Hongliang; (Suzhou,
Jiangsu, CN) ; XIONG; Xiaochuan; (Suzhou, Jiangsu,
CN) ; OPITZ; Tobias; (Braunschweig, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
EASYFORMING STEEL TECHNOLOGY CO., LTD.
VOLKSWAGEN AKTIENGESELLSCHAFT |
Chongqing
Wolfsburg |
|
CN
DE |
|
|
Family ID: |
1000004826461 |
Appl. No.: |
16/760979 |
Filed: |
October 29, 2018 |
PCT Filed: |
October 29, 2018 |
PCT NO: |
PCT/CN2018/112367 |
371 Date: |
May 1, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 9/48 20130101; C21D
6/005 20130101; C21D 8/0447 20130101; C21D 8/0405 20130101; C22C
38/12 20130101; C22C 38/04 20130101; C21D 1/673 20130101; C22C
38/02 20130101; B21D 22/022 20130101; C21D 6/008 20130101 |
International
Class: |
C21D 9/48 20060101
C21D009/48; B21D 22/02 20060101 B21D022/02; C21D 8/04 20060101
C21D008/04; C21D 6/00 20060101 C21D006/00; C22C 38/12 20060101
C22C038/12; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C21D 1/673 20060101 C21D001/673 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 2, 2017 |
CN |
201711063360.6 |
Claims
1. A steel used for hot stamping, characterized in that the steel
used for hot stamping comprises, by weight percent, the following
components: 0.1.about.0.19% of C, 5.09.about.9.5% of Mn,
0.11.about.0.4% of V, and 0.about.2% Si+Al, wherein the combination
of C and V also meets one of the following two requirements: 1)
0.1.about.0.17% of C and 0.11.about.0.4% of V; and 2)
0.171.about.0.19% of C and 0.209.about.0.4% of V.
2. The steel used for hot stamping according to claim 1,
characterized in that the steel used for hot stamping also
comprises at least one of the following components: 0.about.5% of
Cr, 0.about.0.2% of Ti, 0.about.0.2% of Nb, 0.about.0.2% of Zr,
0.about.0.005% of B, 0.about.4% of Ni, 0.about.2% of Cu, 0.about.2%
of Mo and 0.about.2% of W.
3. The steel used for hot stamping of claim 1, wherein the C
content ranges from 0.12 to 0.17%, and the Mn content ranges from
5.09 to 8%.
4. The steel used for hot stamping of claim 1, wherein the steel
used for hot stamping is provided on its surface with a coating
selecting from the group comprising an Al--Si coating, a galvanized
coating and a high-temperature oxidization coating.
5. The steel used for hot stamping of claim 1, wherein the
component ratio of the steel used for hot stamping meets the
following requirement: the actual measured value of the martensitic
transformation start temperature of the steel used for hot stamping
after hot stamping is from 150 to 280.degree. C.
6. A hot stamping process, wherein the hot stamping process
comprises: Step A: heating the steel used for hot stamping of claim
1, or a preformed component obtained by preforming the steel used
for hot stamping to a temperature ranging from 700 to 890.degree.
C. and maintaining the temperature for 0.1 to 10000 seconds; Step
B: transferring the steel used for hot stamping or the preformed
component processed in the Step A into a die for stamping so as to
obtain a formed component; and Step C: cooling the formed component
at an average cooling speed of 0.1 to 1000.degree. C./s.
7. The hot stamping process according to claim 6, characterized in
that in the Step A, the heating temperature ranges from 740 to
850.degree. C.
8. The hot stamping process according to claim 7, characterized in
that in the Step A, the heating temperature ranges from 740 to
780.degree. C.
9. The hot stamping process according to claim 7, characterized in
that in Step C, the average cooling speed is between 1 and
100.degree. C./s.
10. A formed component, wherein the formed component is obtained by
hot stamping the steel used for hot stamping of claim 1 or a
preformed component made by preforming the steel used for hot
stamping.
11. The formed component according to claim 10, characterized in
that the formed component comprises, by volume, the following
structures: 0.1 to 5% of vanadium carbide or composite
carbonitride, 2 to 15% of retained austenite, 0 to 10% of ferrite,
with the balance being martensite.
12. The formed component according to claim 10, characterized in
that the formed component has an elongation of .gtoreq.6%.
13. The formed component of claim 10, wherein the formed component
is heated and maintains the temperature within the temperature
range from 140 to 220.degree. C., and the time for the temperature
maintaining lasts for 1 to 100000 seconds.
14. The formed component according to claim 13, characterized in
that the formed component is used as a vehicle component, and the
temperature maintaining is conducted for 5 to 30 minutes during the
paint baking of the vehicle production procedure.
15. The formed component according to claim 13, characterized in
that the formed component comprises, by volume, the following
structures: 0.1 to 2% of vanadium carbide or composite
carbonitride, 5 to 25% of retained austenite, 0 to 10% of ferrite,
with the balance being martensite.
16. The formed component according to claim 13, characterized in
that the formed component has a yield strength of .gtoreq.1100 MPa,
a tensile strength of .gtoreq.1400 MPa and an elongation of
.gtoreq.10%.
Description
TECHNICAL FIELD
[0001] The present invention relates to a steel for hot stamping, a
hot stamping process and a formed component.
BACKGROUND ART
[0002] Rapid development of the automobile industry results in
safety and environmental pollution issues. On the premise of
ensuring safety, lightweighting of vehicles can effectively reduce
energy consumption and emission, and enhance vehicle performance.
Utilization of high-strength steel may decrease the thickness of
parts and satisfy the requirements for safety performance, and is
therefore a key route for lightweighting and better safety of
vehicles.
[0003] Generally speaking, the forming properties of steel decrease
as the strength thereof increases. Hot stamping is a process for
producing an ultrahigh strength vehicle part by means of forming
the vehicle part before strengthening, wherein the strengthening
mechanism is based on interstitial solid solution strengthening of
martensite. A hot stamped part has the advantages of ultrahigh
strength and shape precision, and can effectively avoid the
springback of high-strength steel during cold formation. Among the
current high-strength steels for automobiles, only the hot-stamped
steel or press hardening steel (PHS) can have a strength equal to
or greater than 1500 MPa.
[0004] In order to achieve further weight reduction, vehicle safety
structural components require that the material used shall have a
higher strength and a better ductility compared with the current
PHS, 22MnB5. In particular, the current hot stamped components can
be improved in terms of elongation.
[0005] Moreover, current coated PHS are all Al--Si coated sheets,
which are less competent than galvanized sheets in terms of
anti-corrosion performance and are difficult to be welded.
Galvanized sheets when heated to 900.degree. C. in the hot stamping
process may be severely liquefied, gasified and oxidized, which
imposes a limitation to the application of galvanized sheets to hot
stamping.
[0006] The Chinese Patent No. CN102127675A provides a steel sheet,
a warm formed part and a manufacturing method thereof. With the
components of steel disclosed, in order to obtain desired
mechanical properties, the method comprises, under the condition of
decreased hot stamping temperature, heating a material to a
temperature ranging from 730.degree. C. to 780.degree. C., and
stamping and cooling the material to a temperature that is
30.degree. C. to 150.degree. C. below Ms point (namely, normally
cooled to 150.degree. C. to 280.degree. C.), then further heating
the material to a temperature ranging from 150.degree. C. to
450.degree. C. and maintaining the temperature for 1 to 5 minutes
to stabilize it to a final state by partitioning carbon from
martensite to retained austenite. The elongation of the material
can be increased on the basis of the Transformation Induced
Plasticity (TRIP) effect of retained austenite.
[0007] However, in this method, the component must be cooled to a
particular temperature ranging from 150.degree. C. to 280.degree.
C. before being heated to a temperature ranging from 150.degree. C.
to 450.degree. C. and maintaining the temperature, in such a way
that the temperature accuracy and uniformity of the component can
be hardly controlled, and a complicated production process is
required to control the quenching temperature thereof, which is
disadvantageous to the actual production of the hot stamped
component.
SUMMARY OF THE INVENTION
[0008] An object of the present invention is to provide steel used
for hot stamping, a hot stamping process and a formed component
made therefrom. It is able to achieve a high elongation with the
steel used for hot stamping by means of simple hot stamping
process. The formed component has excellent yield strength, tensile
strength and elongation.
[0009] Technical solution 1 of the present invention relates to
steel used for hot stamping, which comprises by weight percent
0.1.about.0.19% of C, 5.09.about.9.5% of Mn, 0.11.about.0.4% of V,
and 0.about.2% of Si+Al, wherein the combination of C and V meets
one of the following two requirements: 1) 0.1.about.0.17% of C and
0.11.about.0.4% of V; and 2) 0.171.about.0.19% of C and
0.209.about.0.4% of V.
[0010] According to the technical solution 1, the steel used for
hot stamping of the present invention reduces the martensitic
transformation start temperature (Ms) and the martensitic
transformation finish temperature (Mf) of the material by addition
of austenite stabilizing elements, such as C and Mn, so as to
ensure that the quenching temperature could be set to a lower
temperature (e.g., below 100.degree. C.) to remain a reasonable
amount of retained austenite in a quenching state. Thus, the
quenching temperature may be set to room temperature, and the
temperature accuracy and uniformity are easy to control, and the
process is quite simple.
[0011] To be specific, in steel utilizing a
quenching-and-partitioning (Q&P) mechanism, the initial
quenched structure needs to comprise a substantial amount of
retained austenite as "seeds" so that carbon can be diffused from
martensite to retained austenite during the carbon-partitioning
process, thereby enhancing the stability of retained austenite to
improve the material properties. In order to enable the initial
structure to comprise a substantial amount of retained austenite,
the quenching temperature (QT) must be set between the martensitic
transformation start temperature (Ms) and the martensitic
transformation finish temperature (Mf). In the conventional Q&P
steel, for example, Ms is set to be equal to 500.degree. C., and Mf
is set to be equal to 150.degree. C. Under such circumstances, the
QT needs to be set to a temperature ranging from 200 to 300.degree.
C., which requires a particular quenching medium, like salt, oil or
a special quenching gas, for the sake of quenching. In contrast, in
the present invention, Mf is surely lower than room temperature.
Even if the QT is set to room temperature or a temperature ranging
from 0 to 100.degree. C. (with water as a medium), it is also
possible to readily obtain a structure containing a large amount of
retained austenite to guarantee the material properties.
[0012] Moreover, the steel used for hot stamping of the present
invention is added with Vanadium (V), and the precipitation of
vanadium carbide (VC) or a composite carbonitride formed of V, Ti
and Nb etc. from austenite can be controlled by means of processes.
On the one hand, crystalline grains are refined; on the other hand,
the precipitation of vanadium carbide (VC) or the composite
carbonitride consumes the C content in a matrix, thereby reducing
the C content in martensite in a hot stamping state. Reduction of
the C content in the matrix by the two mechanisms, i.e., grain
refinement and precipitation of vanadium carbide (VC) or the
composite carbonitride, guarantees the toughness of the material
after hot stamping, the elongation being .gtoreq.6% thereby
avoiding delayed cracking and meeting the requirements for welding
and assembling. When there is 0.1 to 0.17% of C, more than 0.11% of
V can ensure that enough vanadium carbide precipitates to meet the
above requirement; when there is 0.171 to 0.19% of C, more V needs
to be added for formation of vanadium carbide, and V needs to be
higher than 0.209% to meet the object of reducing the C content in
the matrix.
[0013] The steel used for hot stamping of the present invention may
also comprise at least one of the following components by weight
percent: 0.about.5% of Cr, 0.about.0.2% of Ti, 0.about.0.2% of Nb,
0.about.0.2% of Zr, 0.about.0.005% of B, 0.about.4% of Ni,
0.about.2% of Cu, 0.about.2% of Mo and 0.about.2% of W.
[0014] The C content preferably ranges from 0.12 to 0.17%, and the
Mn content preferably ranges from 5.09 to 8%. The inventors find
that although the yield strength of 1100 MPa can be substantially
achieved when the C content is 0.11%, the C content of more than
0.12% will further ensure that the yield strength is greater than
1100 MPa. On the other hand, although a risk of brittle cracking
during the hot stamping can be substantially avoided when the C
content is 0.19%, the C content of below 0.17% will further ensure
that the material has a good toughness in the hot stamping. In
addition, if the C content is set to be from 0.12 to 0.17%,
5.09.about.8% of Mn can just obtain a suitable martensitic
transformation start temperature so as to set the quenching
temperature to room temperature to facilitate manufacturing of
parts to the maximum extent.
[0015] The steel used for hot stamping of the present invention may
also be provided on its surface with a coating selecting from the
group comprising an Al--Si coating, a galvanized coating and a
high-temperature oxidization coating. The galvanized coating and
iron are alloyed to have a highest melting point of about
780.degree. C. Conventional steel used for hot stamping usually has
an austenitic heating temperature of over 900.degree. C. Zinc may
evaporate and the zinc-iron coating may melt during the hot
stamping, which may result in liquid zinc induced embrittlement and
reduce the strength and toughness of the steel used for hot
stamping. In addition, liquid zinc is severely oxidized at a high
temperature, and the hot stamped component must undergo a high-cost
dry ice treatment or shot-blasting treatment to remove the zinc
oxides on the surface, thereby guaranteeing subsequent painting
process. Preferably, the complete austenitizing temperature of the
steel used for hot stamping of the present invention may be lower
than 780.degree. C., and hot stamping may be conducted at a
temperature below 650.degree. C., thus meeting the requirements of
hot stamping formation of galvanized sheets.
[0016] Preferably, the component ratio of the steel used for hot
stamping meets the following requirement: the actual measured value
of the martensitic transformation start temperature (Ms) of the
steel used for hot stamping after hot stamping is from 150 to
280.degree. C.
[0017] Whereby, it can further ensure that the quenching
temperature can be set to room temperature to facilitate the
manufacturing of the parts.
[0018] The technical solution 2 of the present invention relates to
a hot stamping process, characterized in that the hot stamping
process comprises: Step A: heating the steel used for hot stamping
of technical solution 1 or its preformed component to a temperature
ranging from 700 to 890.degree. C. and maintaining the temperature
for 0.1 to 10000 seconds; Step B: transferring the steel used for
hot stamping or its preformed component processed in the Step A
into a die for stamping so as to obtain a formed component; and
Step C: cooling the formed component at an average cooling speed of
0.1 to 1000.degree. C./s.
[0019] In Step A, if the temperature is lower than 700.degree. C.,
there may occur insufficient austenization, which fails to meet the
requirement of a ferrite being 0-10%; on the other hand, if the
temperature is greater than 890.degree. C., it will lead to grain
growth and vanadium carbide dissolution and growth, thereby
resulting in poor performance. In addition, the average cooling
speed in Step C is set to 0.1 to 1000.degree. C./s, which can avoid
non-martensite structures like ferrite, pearlite, bainite to
provide the material with good hardenability.
[0020] Preferably, in Step A, the steel used for hot stamping of
technical solution 1 or its preformed component is heated to a
temperature ranging from 740 to 850.degree. C. and maintaining the
temperature. If the heating temperature is greater than 740.degree.
C., it takes shorter time for heating and may increase the
production efficiency; and if the temperature is lower than
850.degree. C., it may be helpful for better grain control and
precipitation of vanadium carbide; and preferably the temperature
maintaining time lasts for 10 to 800 seconds, a shorter heating
time may lead to non-uniform and unstable heating, and a longer
heating time may result in poor production efficiency. Further
preferably, in Step A, the steel used for hot stamping of technical
solution 1 or its preformed component is heated to a temperature
ranging from 740 to 780.degree. C. and maintaining the temperature.
If the heating temperature is lower than 780.degree. C.,
liquefaction and oxidization of galvanized sheets during hot
stamping can be better inhibited.
[0021] More preferably, in Step C, the average cooling speed is
between 1 and 100.degree. C./s. A slower cooling speed will result
in prolonged cooling time and poor production efficiency, but it is
very different to carry out the hot stamping process at a higher
cooling speed.
[0022] The technical solution 3 of the present invention relates to
a formed component, which is obtained by hot stamping the steel
used for hot stamping of the technical solution 1 or the preformed
component made by preforming the steel used for hot stamping.
[0023] Preferably, the formed component comprises, by volume, 0.1
to 5% of vanadium carbide or composite carbonitride, 2 to 15% of
retained austenite, 0 to 10% of ferrite, with the balance being
martensite.
[0024] The formed component obtained according to the technical
solution 3 of the present invention has an elongation of
.gtoreq.6%, which can meet the requirements for prevention of
delayed cracking and weld cracking.
[0025] Preferably, the formed component is heated and maintained
within the temperature range from 140 to 220.degree. C., the time
for the heating and temperature maintaining is 1 to 100000
seconds.
[0026] Preferably, the formed component is used as a vehicle
component, and the heating and temperature maintaining is conducted
for 5 to 30 minutes during the paint baking of the vehicle
production procedure.
[0027] Therefore, carbon partitioning can be realized in the baking
and coating steps of the vehicle assembling procedure without
additional heat treatment process, and the coated and baked
material will be improved in terms of elongation and toughness, so
as to meet the collision performance requirement.
[0028] Preferably, the formed component after the heating and
temperature maintaining treatment comprises, by volume, the
following structure: 0.1 to 2% of vanadium carbide or composite
carbonitride, 5 to 25% of retained austenite, 0 to 10% of ferrite,
with the balance being martensite.
[0029] The formed component after the heating and temperature
maintaining treatment has a yield strength of .gtoreq.1100 MPa, a
tensile strength of .gtoreq.1400 MPa and an elongation of
.gtoreq.10%, which can meet the collision performance
requirement.
[0030] The present invention reduces the C content in initial
martensite and reduces or avoids the brittleness of quenched
martensite by setting the components of steel, thereby ensuring a
stable performance of the component under a hot stamping state and
an elongation of .gtoreq.6%, and preventing delayed cracking and
meeting the requirement for welded assembling; in addition, the
material under a hot stamping state, after baking and painting
process, can be carbon-partitioned from martensite to retained
austenite, and transformed reversely from a portion of martensite
to austenite, finally resulting in a formed component with more
than 5% of retained austenite, stable performance, a yield strength
of .gtoreq.1100 MPa, a tensile strength of .gtoreq.1400 MPa and an
elongation of .gtoreq.10%.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 illustrates an example of a heat treatment process of
the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0032] The technical solutions of the present invention will be
explained with reference to the embodiments.
[0033] Steel used for hot stamping of the present invention
comprises the following components by weight percent:
0.1.about.0.19% of C, 5.09.about.9.5% of Mn, 0.11.about.0.4% of V,
and 0.about.2% Si+Al. The steel used for hot stamping may also
comprise at least one of the following components: 0.about.5% of
Cr, 0.about.0.2% of Ti, 0.about.0.2% of Nb, 0.about.0.2% of Zr,
0.about.0.005% of B, 0.about.4% of Ni, 0.about.2% of Cu, 0.about.2%
of Mo and 0.about.2% of W, the content of which is also calculated
by weight percent. The component ratio of the steel used for hot
stamping is made in such a way that the actual measured value of
the martensitic transformation start temperature (Ms) of the steel
used for hot stamping after hot stamping is from 150 to 280.degree.
C.
[0034] The chemical components of the steel used for hot stamping
of the present invention are listed as follows for the above
reasons:
[0035] C: 0.1% to 0.19%
[0036] Carbon is the cheapest strengthening element that can
greatly increase the strength of steel by interstitial solid
solution. And the increase in the carbon content will greatly
reduce the complete austenitizing temperature (Ac3), thereby
reducing the heating temperature and saving energy. Although carbon
can greatly reduce the martensitic transformation start
temperature, the requirements of the alloy design for the
martensitic transformation start temperature being
.ltoreq.280.degree. C. and the requirements for the microstructure
of the steel must be met, and carbon is the most important
interstitial solid solution strengthening element, therefore the
lower limit of the carbon content is 0.1%. However, an excessively
high carbon content may greatly affect the mechanical performance
of steel and cause a great increase in strength and decrease in
toughness of the steel, therefore the upper limit of carbon is
0.19%, and the carbon content that is higher than the value may
cause brittle cracking of steels under the hot stamping state. More
preferably, the C content ranges from 0.12% to 0.17%.
[0037] Mn: 5.09% to 9.5%
[0038] Mn is an important element in the present invention. Mn is a
good deoxidizer and desulfurizer. Mn is an austenite stabilizing
element that can expand the austenite region and reduce the Ac3
temperature. Mn has a good effect on inhibiting the transformation
of austenite to ferrite and improving hardenability of steel. In
order to decrease the heating temperature during the heat
treatment, the lower limit of Mn is set to 5.09% so as to ensure
that the martensitic transformation start temperature is
.ltoreq.280.degree. C., and meanwhile the complete austenitizing
temperature (Ac3) of the material is guaranteed to be
.ltoreq.780.degree. C. so as to facilitate the formation of the
galvanized sheet by hot stamping. Addition of too much Mn may
result in that the material after quenching forms a brittle
martensite, therefore the upper limit of Mn is set to 9.5%. More
preferably, the Mn content ranges from 5.09 to 8%.
[0039] V: 0.11% to 0.4%
[0040] Vanadium is precipitated as strong carbide. Precipitation of
vanadium carbide can achieve the effect of grain refinement and
strength improvement. Vanadium carbide is precipitated from
Vanadium during the austenitizing phase and the hot stamping phase,
which, on the one hand, refines the original austenite grains, and,
on the other hand, reduces the carbon content in the matrix,
thereby keeping the carbon content in martensite at a low level
after hot stamping. The present invention controls the carbon
content in martensite after hot stamping by adding vanadium element
and precipitating vanadium carbide, in order to guarantee the
elongation and the elongation stability of the hot stamped
material. Less than 0.11% of V cannot achieve an obvious effect and
fails to meet the material design requirement of the present
invention. However, addition of a large amount of vanadium element
will lead to an increase in size of VC, and in steel cost. In order
to keep stable elongation of initial steel after hot stamping, the
V content shall be not more than 0.4%.
[0041] Si+Al: 0% to 2%
[0042] Si and Al can both inhibit the formation of carbides. When
the steel is maintained at a temperature range below the Ac1
temperature after being quenched to room temperature, Si and Al can
both inhibit precipitation of carbides in martensite and partition
carbon to retained austenite to improve the stability of austenite
and improve the product of strength and elongation of steel. In the
industrial production, too much Al may block the nozzle in the
continuous casting, increasing the difficulty in continuous
casting, and Al may increase the martensitic transformation start
temperature and the complete austenitizing temperature of the
material, which does not meet the requirement of structure
temperature control of the steel of the present invention. A high
Si content will lead to more impurities in steel. The present
invention adopts carbon-partitioning at a low temperature ranging
from 140 to 220.degree. C. During the low-temperature range, the
formation of cementite will be inhibited, and only a portion of
transitional carbides may be formed, but the portion of carbides
will not significantly affect the toughness of the material.
Addition of a large amount of Si and Al cannot inhibit the
production of transitional carbides, so the present invention does
not depend on the addition of Si+Al. The content of Si+Al in the
present invention is not more than 2%.
[0043] Cr: 0% to 5%
[0044] Cr is also an element that can improve hardenability of a
material and reduce the martensitic transformation start
temperature. Thus, the percentage of Mn and Cr in steel is
determined according to the requirements of the alloy design for
the martensitic transformation start temperature and the carbon
content in steel. Mn and Cr are added either alone or both.
Preferably, Cr is not added due to high cost.
[0045] Ti, Nb, Zr: 0% to 0.2%
[0046] Ti, Nb and Zr refine the crystalline grains of steel,
increase the strength of steel and render the steel a good heat
treatment property. The excessive low concentration of Ti, Nb and
Zr does not work, but more than 0.2% thereof will increase
unnecessary costs. The steel of the present invention can obtain a
strength of more than 1600 MPa and good elongation because of a
reasonable design of C and Mn, so preferably it does not need to
add extra Ti, Nb and Zr for the sake of cost reduction.
[0047] B: 0% to 0.005%
[0048] The segregation of B at austenite grain boundaries prevents
the nucleation of ferrite, which may greatly improve the
hardenability of steel, and significantly improve the strength of
steel after the heat treatment. The B content of more than 0.005%
cannot obviously make improvement. Since the design of high Mn
content in steel of the present invention has a high hardenability,
preferably it does not need to add extra B for the sake of cost
reduction.
[0049] Ni: 0% to 4%; Cu:0% to 2%
[0050] Ni can increase the strength of steel and maintain the good
plasticity and toughness of steel. If the concentration of Ni is
more than 4.0%, the costs will be increased. Cu can increase the
strength and toughness, especially atmospheric corrosion
resistance. When the Cu content is greater than 2%, the
processability may be deteriorated, and a liquid phase may be
formed during hot rolling, which results in cracking. The high Cu
content may also cause an increase in unnecessary costs. The steel
of the present invention can obtain a strength of more than 1600
MPa and good elongation because of a reasonable design of C and Mn,
so preferably it does not need to add extra Ni and Cu for the sake
of cost reduction.
[0051] Mo and W: 0% to 2%
[0052] Mo and W can improve the hardenability of steel, and
effectively increase the strength of steel. In addition, even if
the steel is not sufficiently cooled due to its unstable contact
with the die during the high-temperature forming process, the steel
may still have a suitable strength due to the increased
hardenability resulting from Mo and W. In the case of Mo and W
being greater than 2%, no additional effects can be achieved, and
costs will rise instead. Since the design of high Mn content in
steel of the present invention has high hardenability, preferably
it does not need to add extra Mo and W for the sake of cost
reduction.
[0053] Unavoidable Impurities Such as P, S and N
[0054] In general, P is a harmful element in steel, which may
increase the cold brittleness of steel, worsen the weldability,
reduce the plasticity and deteriorate the cold bending property.
Generally speaking, S is also a harmful element, which may cause
hot brittleness of steel, and reduce the elongation and weldability
of steel. N is an unavoidable element in steel. N is similar to
carbon in terms of function and is helpful in bake hardening.
[0055] The steel used for hot stamping or its performed component
of the present invention is hot stamped.
[0056] In one embodiment, the steel used for hot stamping or its
preformed component is heated to a temperature ranging from 700 to
890.degree. C. and maintains the temperature for 0.1 to 10000
seconds (Step A). In a process used in the experiment, the heating
temperature ranges from 750 to 840.degree. C. and the temperature
is maintained for 5 minutes. As shown in FIG. 1, the heating
temperature may be 780.degree. C. and maintaining the temperature
for 5 minutes. Then, the steel used for hot stamping or its
preformed component is transferred into a die for hot stamping
(Step B), and the formed component is cooled by air or other means
to a temperature below 100.degree. C. at an average cooling speed
of 0.1 to 1000.degree. C./s (Step C). After a period of time, the
processed component is heated and maintains the temperature within
a temperature range from 140 to 220.degree. C. for 1 to 100000
seconds for a carbon-partitioning treatment and then cooled to room
temperature. The cooling media may include, but are not limited to,
air, water, oil and die surface. In a process used in the
experiment, the heating and temperature maintaining is conducted
within a temperature from 150 to 210.degree. C. for 5 to 30
minutes. As shown in FIG. 1, the heating and temperature
maintaining can be conducted during the paint baking of the vehicle
production procedure.
[0057] Table 1 shows the components of steel used in an embodiment.
The steel can be made into a sheet by the following processes,
namely, a cast blank is maintained at the temperature of
1200.degree. C. for 3 hours and then forged into a sheet blank, the
sheet blank is maintained at the temperature of 1200.degree. C. for
10 hours before undergoing a homogenization treatment and ground to
make its superficial decarburized layer off, and then is heated to
1200.degree. C. and maintains the temperature for 1 hour before
being hot-rolled at a temperature ranging from 800.degree. C. to
1200.degree. C. to form a hot-rolled sheet. The hot-rolled pickled
sheet is maintained at the temperature of 600.degree. C. for 10
hours to simulate hooded annealing so as to reduce the strength of
the hot-rolled sheet for the sake of cold rolling, the hot-rolled,
pickled and annealed sheet is cold-rolled to be, e.g., 1.5 mm
thick, and the cold-rolled sheet is annealed to simulate industrial
cold-rolled sheet continuous annealing or coated sheet production
process to obtain a steel sheet used for hot stamping.
[0058] In the tables, BT series is the steel of the present
invention, and CT series is the compared steel, and the components
of the CT series steel extend beyond the scope of the present
invention.
[0059] Table 2 shows the processes adopted, and Table 3 shows the
properties of the formed component obtained by treating the steel
of Table 1 by means of the process shown in Table 2.
TABLE-US-00001 TABLE 1 Main Chemical Components of Steel C Mn Si V
BT1 0.15 7.5 0.2 0.15 BT2 0.15 7.5 0.2 0.25 BT3 0.17 6.4 0.19 0.34
BT4 0.12 7.43 0.21 0.18 CT1 0.22 8 1.26 -- CT2 0.24 7.3 1.21 0.25
CT3 0.17 7.2 0.2 --
TABLE-US-00002 TABLE 2 Heat Treatment Processes of Steel
austentizing temperature (.degree. C.) and time for temperature
baking No. maintaining quenching tem- baking Steel Process (min) in
Step temperature perature Time Type No. A (.degree. C.) (.degree.
C.) (min) BT1 1-1 780; 5 room -- -- temperature 20 1-2 850; 15 room
-- -- temperature 20 1-1-200 780; 5 room 200 30 temperature 20
1-2-200 800; 5 room 200 30 temperature 20 1-3 800; 5 room -- --
temperature 20 1-4 760; 5 room -- -- temperature 20 1-5 780; 5 room
-- -- temperature 20 1-5-170 780; 5 room 170 25 temperature 20 BT2
2-1 800; 5 room -- -- temperature 20 2-2 850; 15 room -- --
temperature 20 2-3 800; 5 room -- -- temperature 20 2-4 780; 5 room
-- -- temperature 20 2-4-180 780; 5 room 180 30 temperature 20
2-4-200 780; 5 room 200 20 temperature 20 BT3 3-1 780; 5 room -- --
temperature 20 3-1-200 780; 5 room 200 20 temperature 20 BT4 4-1
780; 5 room -- -- temperature 20 4-1-200 780; 5 room 200 20
temperature 20 CT1 CT1-1 800; 6 room -- -- temperature 20 CT1-2
780; 5 room -- -- temperature 20 CT1-1-200 800; 6 room 200 30
temperature 20 CT2 CT2-1 780; 5 room -- -- temperature 20 CT2-2
780; 5 room -- -- temperature 20 CT2-2-170 780; 5 room 170 30
temperature 20 CT3 CT3-1 780; 5 room -- -- temperature 20 CT3-2
780; 5 room -- -- temperature 20 CT3-2-170 780; 5 room 170 30
temperature 20
TABLE-US-00003 TABLE 3 Mechanical Properties of Formed Component
yield strength tensile strength elongation No. (MPa) (MPa) (%) BT1
1-1 970 1889 6.7 1-2 985 1826 7.1 1-1-200 1269 1630 12.2 1-2-200
1182 1611 10.2 1-3 989 1860 8.4 1-4 944 1810 9.6 1-5 930 1827 9.5
1-5-170 1148 1722 12.4 BT2 2-1 1001 1804 10.1 2-2 1024 1722 6.8 2-3
1016 1753 8.9 2-4 1020 1776 9.9 2-4-180 1210 1625 11.2 2-4-200 1234
1594 11.8 BT3 3-1 1083 1566 7.8 3-1-200 1220 1505 10.5 BT4 4-1 984
1545 9.1 4-1-200 1158 1458 10.8 CT1 CT1-1 -- 1145 1.7(brittle
failure) CT1-2 -- 1211 1.9(brittle failure) CT1-1-200 1020 1844 8.8
CT2 CT2-1 -- 1355 2.2(brittle failure) CT2-2 -- 1327 1.9(brittle
failure) CT2-2-170 1095 1941 10.2 CT3 CT3-1 -- 1418 3.5(brittle
failure) CT3-2 -- 1390 2.8(brittle failure) CT3-2-170 1180 1729
10.6
[0060] The formed component which has not undergone the heating and
temperature maintaining treatment (baking treatment) comprises, by
volume, the following structures: 0.1 to 5% of vanadium carbide or
a composite carbonitride, 2 to 15% of retained austenite, 0 to 10%
of ferrite, with the balance being martensite. As known from 1-1,
1-2, 1-3, 1-4, 1-5, 2-1, 2-2, 2-3, 2-4, 3-1 and 4-1 of Table 3, all
of these formed components have an elongation of more than 6%.
[0061] The formed component which has undergone the heating and
temperature maintaining treatment comprises, by volume, the
following structures: 0.1 to 2% of vanadium carbide or a composite
carbonitride, 5 to 25% of retained austenite, 0 to 10% of ferrite,
with the balance being martensite. As known from 1-1-200, 1-2-200,
1-5-170, 2-4-180, 3-1-200 and 4-1-200 of Table 3, all of these
formed components have a yield strength of more than 1100 MPa, a
tensile strength of more than 1400 MPa, and an elongation of more
than 10%.
[0062] In contrast, irrespective of the heat treatment processes,
the steels CT1, CT2, CT3 in the compared examples all fail to meet
the four properties of the steel of the present invention: an
elongation of .gtoreq.6% under hot stamping state (before
carbon-partitioning); a yield strength of .gtoreq.1100 MPa, a
tensile strength of .gtoreq.1400 MPa, and an elongation of
.gtoreq.10% after carbon-partitioning (such as paint baking). In
particular, as known from CT1-1, CT1-2, CT2-1, CT2-2, CT3-1, CT3-2,
the steels CT1, CT2 and CT3 in the compared examples are very
likely to suffer from brittle cracking before carbon-partitioning,
whereas the steel of the present invention has an elongation of
.gtoreq.6% before carbon-partitioning, which helps to avoid brittle
cracking and can meet the requirement for weld assembling.
[0063] The formed component of the present invention can be used as
a high-strength component for land vehicles, including, but not
limited to, B-post reinforcers, bumpers, vehicle door
anti-collision beams and wheel spokes.
[0064] The above embodiments and experimental data are intended to
exemplarily explain the present invention. Those skilled in the art
shall understand that the present invention is not limited to these
embodiments, and can be changed without departing from the
protection scope of the present invention.
* * * * *