Note: Descriptions are shown in the official language in which they were submitted.
HOT FORMABLE, AIR HARDENABLE, WELDABLE, STEEL SHEET
CROSS-REFERENCE TO RELATED APPLICATIONS
This Application claims thc benefit under 35 U.S.C. 119(e) of U.S. Provisional
Application
No. 61/935,948 filed February 5, 2014.
FIELD OF THE INVENTION
The present invention relates to steel sheet. In particular, the present
invention 10 relates to
steel sheet that can be hot formed into parts having uniform, very high
tensile strength and
high weldability.
BACKGROUND OF THE INVENTION
Modern vehicles contain an increasing portion of high-strength and ultra-high-
strength steels
in order to improve passenger safety and reduce vehicle weight. The
configuration of many
formed vehicle body parts prevents the use of cold formed advanced high-
strength steels. As a
result, hot forming followed by quenching to a martensite condition has become
a popular
means for producing ultra-high-strength steel parts.
Special steels are used for hot stamping to ensure necessary hardenability to
fit
operational parameters. Many of these special steels are designed for
quenching in water
cooled dies.
An example of such a hot stamping steel is USIBORTM, which contains (in % by
weight or wt%) 0.15-0.25%C, 0.8-1.5%Mn, 0.1-0.35%Si, 0.01-0.2%Cr, less than
0.1%Ti, less
than 0.1`)/0A1, less than 0.05%P, less than 0.03%S, and 0.0005-0.01%B. This
chemistry is
encompassed by the steel disclosed in U.S. Patent No. 6,296,805. In this
chemistry Ti and B
are necessary to achieve high mechanical properties after hot pressing in a
water cooled die.
The manufacture of high-strength parts from USIBORTM. is described in U.S.
Patent
No. 6,564,604. The process includes heating hot rolled or cold rolled blanks
above 700 C in a
furnace, transferring heated blanks to dies, press forming the blanks in the
die and keeping the
water cooled die, with the formed blank in it, closed until the part reaches
room temperature.
Rapid cooling in the water cooled die, i.e. quenching, is necessary to obtain
the martensite
structure and hence high strength. The quenched steel might have been coated
with Zn or Al-
Si prior to heat treating for hot stamping via a continuous hot dip coating
process to protect
the steel substrate from oxidation during hot stamping and from subsequent
corrosion attack.
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Although USIBORTM is widely used for hot stamping and can achieve a tensile
strength of 1500 MPa after quenching in a water cooled die, USIBORim has a
number of
disadvantages. One disadvantage is that USIBORTM containing 0.25 wt% C has
poor
weldability. In addition, the microstructure of USIBORTM is highly sensitive
to cooling rate
and displays ferrite or bainite formation if cooling rates in the water cooled
die are slow,
hence uniform distribution of strength across hot stamped parts may not be
guaranteed.
Furthermore, the hot stamping process using USIBORTm is generally long and the
productivity of the expensive equipment used for hot stamping is relatively
low. Moreover,
the ductility (e.g, elongation) of USIBORTM having a tensile strength greater
than 1500 MPa
is relatively low.
Air hardening steels are also well known. For example, W020061048009 discloses
air-
hardenable steel containing, in mass%, 0.07-0.15% C, 0.15-0.30% Si, 1.60-2.10%
Mn, 0.5-
1.0% Cr, 0.30-0.60% Mo, 0.12-0.20% V, 0.010-0.050% Ti and 0.0015-0.0040% B.
The steel
can be readily welded and galvanized. It exhibits high strength, e.g., a yield
strength of 750-
850 MPa and a tensile strength of 850-1000 MPa. However, the steel has the
disadvantage of
using large amounts of expensive elements such as Mo and V.
Patent application publication DE 102 61 210 Al describes another air-
hardenable steel
alloy for the production of automobile parts in a hot pressing process. The
alloy contains, in
mass%, 0.09-0.13% C, 0.15-0.3% Si, 1.1-1.6% Mn, max 0.015% P, max 0.011% S,
1.0-1.6%
Cr, 0.3-0.6% Mo, 0.02-0.05% Al and 0.12-0.25% V. When the steel is quenched in
a die an
upper bainite structure can be obtained without additional quenching. The
steel exhibits a
yield strength of 750-1100 MPa, a tensile strength of 950-1300 MPa, and an
elongation of 7-
16%. One disadvantage of this steel is the necessity of using a large amount
of expensive Mo
and V.
Unexamined Japanese Patent Application No. 2006-213959 provides a method for
manufacturing hot press, high-strength, steel members with excellent
productivity. The
method uses steel sheet that contains, in mass %, 0.05 to 0.35% C. 0.005 to
1.0% Si, 0 to
4.0%Mn, 0 to 3.0% Cr, 0 to 4.0% Cu, 0 to 3.0% Ni, 0.0002 to 0.1% B, 0.001 to
3.0% Ti, <
0.1% P, < 0.05% S, 0.005 to 0.1% Al and <0.01% N, with the balance Fe and
inevitable
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impurities, where Mn+Cr/3.1+(Cu+Ni)/1.4 > 2.5%. The steel sheet is heated at
750-1300cC
for 10-6000 seconds, and then is press-formed at 300 C or
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more. After pressing, the molded product is removed from the mold and is
cooled
from 1200-1100C down to 5-40 C at a cooling speed of 0.1 C/second or more to
yield members having a martensite structure of 60% or more in area ratio. By
this
method, the step of quenching in the press mold can be eliminated. The members
obtained have little material quality variation internally, and the shape of
the members
is good, with excellent uniformity.
Unexamined Japanese Patent Application No. 2006-212663 provides a method
of manufacturing hot press high-strength steel members of excellent
formability. The
method uses steel sheet that contains, in mass%, 0.05 to 0.35% C, 0.005 to
1.0% Si, 0
to 4.0% Mn, 0 to 3.0% Cr, 0 to 4.0% Cu, 0 to 3.0% Ni, 0.0002 to 0.1% 13, 0.001
to
3.0% Ti, < 0.1% P, < 0.05% S, 0.005 to 0.1% Al and < 0.01% N, with a balance
of
Fe and inevitable impurities, where Mn+Cr/3.1 (Cu+Ni)11.4 > 2.5. The steel
sheet is
heated to 750-1300 C, is kept there for 10-6000 seconds, and then is press-
formed
two or more times at 300 C to yield members having a martensite structure of
60% or
more in area ratio. The resulting members exhibit high-strength and little
variability
in internal material quality.
The tensile strength of steel is known to increase with C content. However, an
increase in C content decreases weldability.
A need exists for a hot-formable, air-hardenable, high-strength, steel sheet
that
does not include large amounts of expensive elements, such as Mo, and, in
addition to
having little internal variability in tensile strength, exhibits excellent
weldability.
SUMMARY OF THE INVENTION
The present invention provides a high tensile strength (800-1400 MPa) steel
sheet containing (in wt%) 0.04< C< 0.30, 0.5< Mn < 4, 0< Cr < 4, 2.7< Mn+Cr<
5,
0.003< Nb< 0.1, 0.015< Al < 0.1 and 0.05< Si < 1Ø Optionally, the steel
sheet can
contain one or more of Ti < 0.2, V< 0.2, Mo<0.3 and B< 0.015. Following
austenization at or above Ac3 +20 C, the steel sheet can be hot formed in a
die and
can be cooled in the die, or in a cooling medium such as air, nitrogen, oil or
water.
The chemistry of the steel, particularly the content of Mn+Cr of from 2.7 and
5 wt%,
makes the formed sheet insensitive to cooling rate and ensures a uniform
distribution
of strength across parts independent of the time delay between operations and
final
cooling/quenching. A Nb content from 0.003 to 0.1 wt% makes the tensile
strength
less sensitive to the amount of C and reduces the amount of C needed for same
tensile
3
strength. Furthermore, since a reduction in C improves weldability, the
addition of Nb
achieves the same high tensile strength as C alone but with improved
weldability. Coating the
steel sheet with a coating of Zn, Al or Al alloy can improve the corrosion
resistance of the
steel sheet.
The present invention also provides a steel sheet consisting of, in weight%,
0.04< C<
0.30, 0.5< Mn<4, 0< Cr < 4, 2.7< Mn+Cr< 5, 0.003< Nb < 0.1, 0.015< Al < 0.1,
and 0.05< Si<
1.0, the balance being Fe and unavoidable impurities; wherein the steel sheet
has a
microstructure consisting of up to 10 area% bainite, with the remainder being
martensite; and
wherein the steel sheet has a tensile strength in the range of 800-1400 MPa.
BRIEF DESCRIPTION OF THE DRAWINGS
The preferred embodiments of the invention will be described in detail, with
reference
to the following figures, where:
FIG. 1 shows the change in tensile strength (MPa) with C for various steel
sheet
compositions when the amount of C ranges from 0.06 to 0.12 wt%, with and
without Nb
addition;
FIG. 2 shows the change in tensile strength (MPa) with C for various steel
sheet
compositions when the amount of C ranges from 0.06 to 0.18 wt%, with and
without Nb;
FIG. 3 depicts a Continuous Cooling Transformation (CCT) diagram for a steel
according to the present invention, plotting cooling curves as temperature in
degrees C vs log
of time in seconds;
FIGS. 4a-4d are photomicrographs, taken at varying magnifications, of a steel
of the
present invention cooled at different cooling rates;
FIG. 5 is a plot of weld current vs sample number for steels of the present
invention,
the plot specifically shows the non-scatter of expulsion of the steel in spot
welding.
FIG. 6 is a collection of four (4) photomicrographs showing, from top to
bottom and
left to right, a complete spot weld of a steel of the present invention, a
higher magnification of
the base metal, heat affected zone, and the welded zone of the spot weld.
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DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention provides a steel sheet that can be hot formed into a
part having a
uniform distribution of strength and improved weldability. The steel sheet is
a low alloy steel
composition and contains, in wt%, 0.04<C< 0.30, 0.5< Mn<4, O<Cr< 4, 2.7<
Mn+Cr< 5,
0.003< Nb< 0.10, 0.015< Al< 0.1 and 0.05< Si < 1Ø Optionally, the steel
sheet can contain
one or more of Ti < 0.2, V<0.5, Mo<0.6 and B< 0.015. This chemistry makes a
sheet that after
hot forming is insensitive to
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cooling rate and ensures a uniform distribution of strength across parts
independent of
the time delay between operations and final cooling/quenching. The guaranteed
uniformity of tensile properties regardless of cooling rate in specific
locations of a
formed part can substantially increase the productivity of hot forming.
Although
tensile strength increases with increasing C. the increase in C decreases
weldability.
However, by substituting a portion of C with Nb the tensile strength increase
can be
maintained and weldability improved.
The concentrations of the various component elements of the steel sheets of
the present invention are limited for the followings reasons. The
concentrations are
given in weight % (i.e., wt%).
Carbon is essential for increasing the strength of the steel. However, if too
much C is added, welding becomes difficult. Thus, the amount of C is limited
to the
range of from 0.04 to 0.30 wt%. Preferably, the lower limit for the amount of
C is
0.06 wt%, more preferably 0.08 wt%. Preferably, the upper limit for the amount
of C
is 0.18 wt%, more preferably 0.16 wt%.
Manganese, besides being a solid solution strengthening elements, also
inhibits ferrite transformation, so it is an important chemical element for
ensuring
quenchability. However, adding too much Mn will not only encourage co-
segregation
with P and S, but also adversely affect manufacturability during steel making,
casting,
and hot rolling. Thus, the amount of Mn is limited to the range of from 0.5 to
4 wt%.
Preferably, the lower limit for the amount of Mn is 1 wt%, more preferably,
1.5 wt%.
Preferably, the upper limit for the amount of Mn is 3.5 wt%, more preferably
3.0 wt%.
Chromium is important for improving quenchability. However, too much Cr
will adversely affect manufacturability during manufacturing. Thus, the amount
of Cr
is limited to the range of from 0 to 4 wt%. Preferably, the lower limit for
the amount
of Cr is 0.2, more preferably, 0.5 wt%. Preferably, the upper limit for the
amount of
Cr is 3.5 wt%, more preferably 3.0 wt%.
The combined amount of Mn and Cr is limited to the range of from 2.7 to 5
wt% in order to make the steel insensitive to cooling rate after forming and
to ensure a
uniform distribution of strength across parts independent of the time delay
between
operations and final cooling/quenching. Preferably, the lower limit for Mn+Cr
is 3.0,
more preferably, 3.3 wt%. Preferably, the upper limit for Mn+Cr is 4.7 wt%,
more
preferably 4.4 wt%.
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Previously, small additions of Nb to FISLA steels has been known for its
significant effect on preventing austenite recrystallization and hence fine
ferrite grain
size, as well as precipitation hardening of ferrite by fine carbo-nitrides.
Also, larger
quantities of Nb have been added to high C creep resistant alloyed steels.
However,
up to now, the effect of small additions of Nb on low to mid carbon steels
with
martensitic microstructure has not been reported in open literature. The
inventors
have discovered that a small addition of Nb to air hardenable the steels of
the present
invention reduces the sensitivity of tensile strength on the C content, and
significantly
increases strength of steel, thus reducing the quantity of C needed to achieve
a
specific tensile strength. Since carbon reduction improves weldability, the
addition of
Nb helps to achieve the desired high tensile strength with improved
weldability. To
achieve these effects, the amount of Nb is limited to the range of from 0.003
to 0.1
wt%. Preferably, the lower limit for the amount of .Nb is 0.005, more
preferably,
0.010 wt%. Preferably, the upper limit for the amount of Nb is 0.09 wt%, more
preferably 0.085 wt%.
Al in small amount is added to steel as deoxidizer. However, too much Al
results in many nonmetal inclusions and surface blemishes. AI is also a strong
ferrite
forming element and significantly increases full austenitization temperature.
These
are undesirable effects for air hardenable steels. Thus, the amount of Al is
limited to
.. the range of from 0.015 to 0.1 wt%. Preferably, the lower limit for the
amount of Al
is 0.02, more preferably, 0.03 vvt%. Preferably, the upper limit for the
amount of Al is
0.09 wt%, more preferably 0.08 wt%.
Si is effective for increasing the strength of steel sheet. However, too much
Si
creates a problem of surface scale. Thus, the amount of Si is limited to the
range of
from 0.05 to 0.35 wt%. Preferably, the lower limit for the amount of Si is
0.07, more
preferably, 0.1 wt%. Preferably, the upper limit for the amount of Si is 0.3
wt%, more
preferably 0.25 wt%.
Ti can be optionally added to the steel with B in an amount of 5 0.1 wt% to
improve quenchability. Ti combines with N at very high temperature, hence
preventing BN formation. B in solution improves quenchability. Ti beyond the
stoichiometric ratio to nitrogen is a carbide forming element. It strengthen
steel by
forming very fine carbides. It's effect is similar to Nb.
V can be optionally added to the steel in an amount of < 0.2 wt% to increase
the strength of the steel via fine precipitation. It also adds to
hardenability of steel.
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Mo can be optionally added to the steel in an amount of < 0.3 wt% to increase
strength and improve quenchability.
B can be optionally added to the steel in an amount of 5 0.005 wt% to increase
hardenability and hence strength of the steel.
The steel also contains Fe and can contain unavoidable impurities.
The steel sheet of the present invention has a martensitie microstructure that
can include up to 10% lower bainite phase. The microstructure is predominantly
martesnite. The amount of bainite can be up to 10%, preferably less than 5%
and
more preferably less than 1%.
The steel sheet of the present invention has a tensile strength in the ranee
of
800-1400 MPa. The lower limit of the tensile strength is preferably 900 MPa,
more
preferably 1000 MPa. The final strength depends mostly on carbon content in
martensite.
The steel sheet of the present invention can exhibit an eloneation in the
range
of from 4 to 9%, preferably 5 to 9%, more preferably 6 to 9%.
The steel sheet of the present invention can be made by processes that begin
with conventional steelmaking and casting processes and then follow with hot
rolling.
The cast slabs may be charged directly to a reheating furnace before hot
rolling or
cooled before doing so. There is no restriction on the finishing temperature
in the hot
rolling process other than that it should be above Ar3.
The coiling temperature after hot rolling depends on the processing after hot
rolling. If cold rolling is required to obtain the final thickness, then a
coiling
temperature between 700 C and 600 C is preferred. If the final required
thickness
can be obtained directly by hot rolling, then a coiling temperature between
600 C and
500 C is recommended.
The hot rolled sheet can be pickled. For cold-rolled products, the hot rolled
sheet can be pickled before cold rolling to the required thickness.
The hot rolled or cold rolled steel sheet can be protected from. oxidation
and/or
corrosion by coating one or both sides of the steel sheet with Zn, Al or an Al
alloy,
such as Al-Si. The coating can be performed by continuously hot dip coating
the steel
sheet.
Steel sheets with or without coatings are heated to the temperature of full
austenitization, i.e., to at least Ac3 + 5 C, before being formedõ e.g., by
stamping, in
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one or several dies to the shape desired. The hot formed part is then cooled
in a die or
in a cooling medium such as air, nitrogen, oil or water. Different cooling
media
provide different cooling rates. The formed parts exhibit uniform martensite
structure
across the parts regardless of cooling rate.
The final strength can be controlled by the chemistry (in particular, the
amounts of C and Nb) and/or by heating below or above the temperature of full
austenitizati on.
EXAMPLES
50mm slabs of the chemistries shown in. Table lwere made in laboratory. The
slabs were hot rolled to 3.5mm sheets. The reheating temperature was 1220 C,
finishing temperature of 850 C and coiling temperature of 700 C. The hot
rolled
sheets were surface ground on both sides to 2.5mm thickness to remove a
decarburized surface layer that would have been caused during the laboratory
reheating process. The 2.5mm sheets were cold rolled to 1 mm (60% cold
reduction)
in a reversing laboratory cold mill. Specimens from the cold rolled sheets
were
austenitized at 900 C for 300see in a salt bath and then oil quenched. Some
samples
were instrumented with a thermocouple to measure the cooling rate during oil
quenching. Average cooling rate from 800 C' to 300 C was 150 C/s. Mechanical
properties of quenched samples were measured in transverse to rolling
direction.
Summary of the mechanical properties are given in Table 2
Tensile strength data in Table 2 plotted against carbon in the chemistry,
Figure
1. Tensile strength strongly depends on carbon, as noted by many previous
publications (for example see "Mattensite transformation, structure and
properties in
hardenable steels, G. Krauss, Hardenability concepts with applications to
steel, D.V.
Doane & J.S. Kirkaldy ed., October 24-26, .1977, page 235). However, Figure 1
also
shows that the steels with Nb have higher strength than the steel with similar
carbon
without Nb. In addition, strength of Nb added steel is less dependent on
carbon since
slope of the line fitted to the tensile strength of steels with Nb is much
less than the
one for steels without Nb. The difference in strength of steels with and
without Nb
becomes less as C is increased and both group of steels have similar strength
at 0.17%
C and higher, Figure 2.
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To determine effect of cooling rate on final strength of quenched material,
"critical
cooling rate" i.e., "the minimum cooling rate from austenitization temperature
to avoid
ferrite" was evaluated. In these experiments, Continuous Cooling
Transformation (CCT)
diagram of the steel was produced using MMC dilatometer. In these test a small
sample was
heated to 900 C and then cooled at pre-determined cooling rates while the
sample dilatation
(change in length) was measured. Different phase transformations during
cooling were
identified from the dilatation data as well as by evaluating the
microstructure and final
hardness of the cooled sample. Several cooling rates are required to construct
the CCT
diagram.
An example of such diagram is shown in Figure 3. As it seen from this figure,
ferrite
transformation does not occur at cooling rates higher than 1 C/sec.
Microstructures at 3 C/sec
and higher cooling rates shown in Figure 4-A & C show a martensitic
microstructure.
However, there is high degree of tempering at the lower cooling rates, Figure
4-B & D.
Despite tempering martensite, high hardness of 350HV was obtained at 3 C/sec
cooling rate
and it increases as the cooling rate increase. Cooling a steel of the present
invention in any
medium (air, oil, die, nitrogen) which results in cooling rates higher than 1
C/sec or
preferably higher than 3 C/sec will produce a fully martensitic ¨ high
strength steel.
Spot weldability of steels 55, 63, 81 and 141 were evaluated according to
IS018278-2
specification in homogeneous joint configuration. These tests showed non-
scattered results
under expulsion, Figure 5, with uniform microstructure of the weld nugget,
Figure 6A-D.
Table 1 and Table 2, Figure 1 and Figure 2 show that the same high tensile
strength
can be obtained when, for a C content ranging from 0.04 to 0.20 wt%, some C is
replaced
with Nb in amounts ranging from 0.003 to 0.055 wt%.
The disclosure herein of a numerical range is intended to be the disclosure of
the
endpoints of that numerical range and of every rational number within that
numerical range.
While the present invention has been described with respect to specific
embodiments,
it is not confined to the specific details set forth, but includes various
changes and
modifications that may suggest themselves to those skilled in the art, all
falling within the
scope of the invention as defined by the following claims.
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"Fable 1
. , .. = = .
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3.01
7 0.076 1.98 1.02 0.035
,
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- - - .,
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,
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3.94
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4.00
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,
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63 . 0.0769 4.08 , , 0.03 0.019 , 0.046 4.08
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2.57
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142 0.154 2.09 1.02 0.057 0.03 3.10
,
2.58
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