U.S. patent application number 14/169184 was filed with the patent office on 2014-09-18 for method of producing hot rolled high strength dual phase steels using room temperature water quenching.
This patent application is currently assigned to THYSSENKRUPP STEEL USA, LLC. The applicant listed for this patent is THYSSENKRUPP STEEL USA, LLC. Invention is credited to Stanley Wayne Bevans, Bertram Wilhelm Ehrhardt, Ranbir Singh Jamwal, Bernd Trilling, Harald Van Bracht.
Application Number | 20140261914 14/169184 |
Document ID | / |
Family ID | 51522092 |
Filed Date | 2014-09-18 |
United States Patent
Application |
20140261914 |
Kind Code |
A1 |
Jamwal; Ranbir Singh ; et
al. |
September 18, 2014 |
METHOD OF PRODUCING HOT ROLLED HIGH STRENGTH DUAL PHASE STEELS
USING ROOM TEMPERATURE WATER QUENCHING
Abstract
A process for manufacturing hot rolled high strength dual phase
steels. In some instances, hot rolled strip is continuously cooled
to less than 100.degree. C. prior to coiling using a cooling rate
between 40-70.degree. C./second. In other instances, hot rolled
strip is cooled to less than 100.degree. C. prior to coiling using
a step cooling treatment. In yet other instances, hot rolled strip
is continuously cooled to less than 100.degree. C. prior to coiling
using a cooling rate between 70-100.degree. C./second. For hot
rolled strip subjected to a cooling rate between 40-70.degree.
C./second, or subjected to the step cooling treatment, hot rolled
steel sheet with a tensile strength greater than 590 MPa is
produced. For hot rolled strip subjected to a cooling rate between
70-100.degree. C./second, hot rolled steel sheet with a tensile
strength greater than 690 MPa is produced. Also, the hot rolled
steel sheet has a ferrite plus martensite microstructure free of
pearlite and bainite.
Inventors: |
Jamwal; Ranbir Singh;
(Mobile, AL) ; Ehrhardt; Bertram Wilhelm; (Mobile,
AL) ; Bevans; Stanley Wayne; (S.W. Decatur, AL)
; Trilling; Bernd; (Calvert, AL) ; Van Bracht;
Harald; (Theodore, AL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
THYSSENKRUPP STEEL USA, LLC |
Calvert |
AL |
US |
|
|
Assignee: |
THYSSENKRUPP STEEL USA, LLC
Calvert
AL
|
Family ID: |
51522092 |
Appl. No.: |
14/169184 |
Filed: |
January 31, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61791608 |
Mar 15, 2013 |
|
|
|
Current U.S.
Class: |
148/602 |
Current CPC
Class: |
C21D 2211/008 20130101;
C21D 2211/005 20130101; C21D 8/0452 20130101; C21D 8/0463 20130101;
C21D 8/0426 20130101; C21D 8/005 20130101; C21D 8/0226
20130101 |
Class at
Publication: |
148/602 |
International
Class: |
C21D 8/00 20060101
C21D008/00 |
Claims
1. A process for manufacturing a hot rolled high strength dual
phase steel comprising: providing a steel slab having a chemical
composition in weight percent within a range of 0.06-0.09 C,
0.8-2.0 Mn, 0.4 min Cr, 0.08 max Si, 0.05 max P, 0.005-0.010 N,
balance Fe and melting impurities, the steel slab chemical
composition also having an Al/N ratio of 10.0 max and a Nb+Ti+Mo
content of 0.015 max; soaking the steel slab within a temperature
range of 1150-1300.degree. C.; hot rolling the soaked steel slab in
a roughing treatment and producing a transfer bar; hot rolling the
transfer bar in a finishing treatment and producing hot rolled
strip, the finishing treatment having an entry temperature between
950-1120.degree. C. and an exit temperature between 800-850.degree.
C.; and cooling the hot rolled strip to less than 100.degree. C.
before coiling; coiling the cooled hot rolled strip, the coiled hot
rolled steel strip having a microstructure of ferrite plus
martensite that is free of pearlite and bainite.
2. The process of claim 1, wherein the hot rolled strip is
continuously cooled from the finishing treatment exit temperature
to RT.
3. The process of claim 2, wherein the hot rolled strip is
subjected to a cooling rate between 40-70.degree. C./sec.
4. The process of claim 1, wherein the RT hot rolled strip has a
percent volume fraction of martensite between 10-25%.
5. The process of claim 4, wherein the RT hot rolled strip has a
tensile strength of at least 590 MPa and a percent elongation to
failure of at least 17.5%.
6. The process of claim 5, further including tension leveling the
RT hot rolled strip, the tension leveled hot rolled strip giving an
increase in the as rolled hot band yield strength of at least 40
MPa.
7. The process of claim 6, further including bake hardening the
tension leveled hot rolled strip, the bake hardened hot rolled
strip having an additional increase in the yield strength of at
least 30 MPa.
8. The process of claim 1, wherein cooling the hot rolled strip to
RT includes a step cooling treatment.
9. The process of claim 8, wherein the step cooling treatment
includes cooling the hot rolled strip from the exit temperature of
the finishing treatment to a ferrite formation zone, holding the
cooled hot rolled strip within the ferrite formation zone for at
least 3 seconds and cooling the hot rolled strip to RT.
10. The process of claim 9, wherein the ferrite formation zone is
between 640-720.degree. C.
11. The process of claim 10, wherein the RT hot rolled strip has a
percent volume fraction of martensite between 10-25%.
12. The process of claim 11, wherein the RT hot rolled strip has a
tensile strength of at least 590 MPa and a percent elongation to
failure of at least 17.5%.
13. The process of claim 12, further including tension leveling the
RT hot rolled strip, the tension leveled hot rolled strip giving an
increase in the as rolled hot band yield strength of at least 40
MPa.
14. The process of claim 13, further including bake hardening the
tension leveled hot rolled strip, the bake hardened hot rolled
strip having an additional increase in the yield strength of at
least 30 MPa.
15. The process of claim 1, wherein the hot rolled strip is
subjected to a cooling rate between 70-100.degree. C./sec.
16. The process of claim 15, wherein the RT hot rolled strip has a
percent volume fraction of martensite between 20-35%.
17. The process of claim 16, wherein the RT hot rolled strip has a
tensile strength of at least 690 MPa and a percent elongation to
failure of at least 17.5%.
18. The process of claim 17, further including tension leveling the
RT hot rolled strip, the tension leveled hot rolled strip giving an
increase in the as rolled hot band yield strength of at least 40
MPa.
19. The process of claim 18, further including bake hardening the
tension leveled hot rolled strip, the bake hardened hot rolled
strip having an additional increase in the yield strength of at
least 30 MPa.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority of U.S. Provisional
Application Ser. No. 61/791,608 filed on Mar. 15, 2013, which is
incorporated in its entirety herein by reference.
FIELD OF THE INVENTION
[0002] The present invention is related to a process for making hot
rolled high strength dual phase steels, and in particular to a
process that quenches hot rolled strip to less than 100.degree. C.
before coiling.
BACKGROUND OF THE INVENTION
[0003] Methods for producing dual phase steels are known. Such
methods typically include heating a slab to a rolling temperature,
hot rolling the slab at a temperature above the Ar3 temperature for
the steel to produce hot rolled strip, and rapidly cooling the hot
rolled strip within specific cooling rates down to coiling
temperatures. Prior art such as U.S. Pat. Nos. 4,790,889;
4,561,910; and 4,421,573 teach current practices in which
continuous cooling of a hot rolled strip is performed and high
temperature coiling is conducted at temperatures between
200-500.degree. C. In addition, U.S. Patent Application Publication
No. 2004/0118489 teaches continuous cooling practices with a range
of high coiling temperatures (CTs) greater than 450.degree. C. in
order to obtain different dual phase steels having tensile
strengths between 550-800 megapascals (MPa).
[0004] The relatively high CTs are utilized due to limitations of
cooling capability, e.g. limited quantities of quenching medium
available along a laminar length of a cooling table that is part of
a hot rolling facility. In order to compensate for the limitation
of cooling capacities, alloying additions of hardening imparting
elements such as carbon, manganese, chromium, niobium and
molybdenum are added to the steels and thereby also add to the cost
of the steel.
[0005] Prior art references (e.g., U.S. Pat. Nos. 4,502,897 and
4,325,751) teach a "step cooling" practice where cooling of the hot
rolled strip is interrupted after finishing rolling. However, the
step cooling is also coupled with relatively high coiling
temperatures such as 300-500.degree. C.
[0006] Given the above, a process for making a hot rolled high
strength dual phase steel without the deficiencies of heretofore
known systems and methods would be desirable.
SUMMARY OF THE INVENTION
[0007] A process for manufacturing hot rolled high strength dual
phase steels is provided. The process includes providing a steel
slab with a chemical composition in weight percent within a range
of 0.06-0.09 carbon (C), 0.8-2.0 manganese (Mn), 0.4 minimum (min)
chromium (Cr), 0.08 maximum (max) silicon (Si), 0.05 max phosphorus
(P), 0.005-0.010 nitrogen (N), with the balance of iron (Fe) and
incidental melting impurities known to those skilled in the art.
The steel slab chemical composition also has an aluminum to
nitrogen (Al/N) ratio of 10.0 max and a niobium plus titanium plus
molybdenum (Nb+Ti+Mo) content of 0.015 max.
[0008] The steel slab is soaked, e.g. using a soaking furnace as
known to those skilled in the art, within a temperature range of
1150-1300.degree. C. and then hot rolled using a roughing treatment
in order to produce a transfer bar. The transfer bar is hot rolled
using a finishing treatment in order to produce hot rolled strip.
The finishing treatment has an entry temperature between
950-1120.degree. C. and an exit temperature between 800-850.degree.
C. The hot rolled strip is cooled from the temperature at which it
exits the finishing treatment down close to room temperature, e.g.
<100.degree. C., after which it is coiled. Also, the coiled hot
rolled steel strip has a microstructure of ferrite plus martensite
and is free of pearlite and bainite.
[0009] It is appreciated that the cooling and coiling operations
are executed as part of a continuous process. Stated differently,
the hot rolled strip exits the finishing treatment and travels to a
coiling station and is coiled without delay or intermittent
stopping of the hot rolled sheet on the cooling table.
[0010] In some instances, the hot rolled strip is continuously
cooled from the finishing treatment exit temperature to less than
100.degree. C. (hereafter also referred to as "close to room
temperature"). In addition, the hot rolled strip can be subjected
to a cooling rate between 40-70.degree. C./second (sec). Using such
a cooling rate affords for the steel to have a percent volume
fraction of martensite between 10-25%. In addition, cooled hot
rolled strip has a tensile strength of at least 590 MPa and a
percent elongation to failure of at least 17.5%.
[0011] The room temperature hot rolled strip can also be subjected
to tension leveling, for example within a continuous pickling line,
the tension leveling resulting in an increase in yield strength of
the material of at least 40 MPa. Also, the material can be
subjected to a bake hardening treatment after it has been tension
leveled which affords an additional increase in yield strength of
at least 30 MPa.
[0012] In other instances, the cooling of the hot rolled strip to
less than 100.degree. C. includes a "step cooling" treatment. The
step cooling treatment can include cooling the hot rolled strip
from the finishing treatment exit temperature to a ferrite
formation zone for the steel composition being hot rolled. Also,
the hot rolled strip is held within the ferrite formation zone for
at least 3 seconds before being subjected to additional cooling to
less than 100.degree. C. The ferrite formation zone can be within
the temperature range of 640-720.degree. C. Also, steel strip
subjected to the step cooling treatment has a percent volume
fraction of martensite between 10-25% and strength values similar
or equivalent to the steel subjected to the continuous cooling
process described above.
[0013] In still yet other instances, the hot rolled strip is
continuously cooled from the exit temperature of the finishing
treatment to less than 100.degree. C. using an increased cooling
rate between 70-100.degree. C./sec. Steel strip subjected to the
enhanced cooling rate has a percent volume fraction of martensite
between 20-35%, a tensile strength of at least 690 MPa, and a
percent elongation to failure of at least 17.5%. The cooled hot
rolled strip can also be subjected to tension leveling, for example
within a continuous pickling line, which increases the yield
strength of the material by at least 50 MPa. Also, the material can
be bake hardened to further increase its yield strength by at least
30 MPa.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a graphical plot/illustration of temperature
versus time for production of high strength dual phase steels
according to an embodiment of the present invention;
[0015] FIG. 2 is a graphical plot/illustration of temperature
versus time for production of high strength dual phase steels
according to another embodiment of the present invention;
[0016] FIG. 3 is a graphical plot/illustration of temperature
versus time for production of hot rolled high strength dual phase
steels according to another embodiment of the present
invention;
[0017] FIG. 4 is an optical micrograph of a characteristic
microstructure for a hot rolled dual phase steel produced according
to an embodiment of the present invention and etched using nital
plus sodium metabisulphite;
[0018] FIG. 5 is an optical micrograph showing a characteristic
microstructure for a hot rolled dual phase steel produced according
to an embodiment of the present invention and etched with nital
plus sodium metabisulphite;
[0019] FIG. 6 is a graphical plot of percent elongation versus
tensile strength for hot rolled dual phase steel alloys having a
tensile strength greater than 690 MPa and produced according to an
embodiment of the present invention;
[0020] FIG. 7 is a graphical plot illustrating tensile strength
versus yield strength for hot rolled dual phase steel alloys having
a tensile strength greater than 690 MPa and produced according to
an embodiment of the present invention;
[0021] FIG. 8 is a graphical plot of increase in yield strength
(.DELTA.YS) versus percentage of tension leveling for hot rolled
high strength dual phase steels produced according to an embodiment
of the present invention and tension leveled on a continuous
pickling line; and
[0022] FIG. 9 is a schematic plot of strength versus percent
elongation via tension leveling, the plot illustrating the effect
of tension leveling on final yield strength for hot rolled dual
phase steels produced according to an embodiment of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0023] A process for manufacturing hot rolled high strength dual
phase steels is provided. As such, the present invention has use
for producing hot rolled steel strip.
[0024] The process affords for the manufacture of hot bands and/or
pickled and oiled products with ferrite-martensite microstructures.
The ferrite-martensite microstructures can be obtained via close to
room temperature coiling practices in combination with high cooling
rates. In some instances, hot rolled strip is continuously cooled
to less than 100.degree. C. before coiling or, in the alternative,
the hot rolled strip is subjected to a "step cooling" pattern. In a
preferred embodiment, the continuous cooling and step cooling of
the hot rolled strip is performed using a superior laminar cooling
system having reinforced cooling zones at the beginning and end of
a relatively long laminar cooling length/table available after the
last finishing stand of a hot rolling finishing treatment.
[0025] The step cooling pattern interrupts the continuous cooling
and holds the material in a ferrite phase formation region for a
given steel composition. It is appreciated that a particular
ferrite phase formation region for a given steel can be obtained
from a continuous cooling transformation (CCT) curve for the
particular grade of steel being hot rolled. The step cooling
pattern affords for ferrite percent volume fractions greater than
80% and thus provides greater flexibility for a desired and
intended strength-ductility balance of the material.
[0026] The close to room temperature coiling practice provides
material properties analogous to cold rolled and annealed strip
that is cooled to room temperature. However, the close to room
temperature cooling of the hot rolled strip as disclosed herein
avoids additional downstream cold rolling and annealing
processes.
[0027] The combination of steel alloy compositions and processes
disclosed herein provides hot rolled dual phase steels with
exceptional mechanical properties not currently available with
similar low alloyed steel compositions. The improvement is obtained
by the formation of higher volume fractions of martensite while
bypassing the formation of other low transformation products such
as pearlite and upper and/or lower bainite.
[0028] The hot rolled dual phase steels also exhibit high
sensitivity to tension elongation/work hardening (e.g., via tension
leveling), for example during pickling. As such, the inventive hot
rolled dual phase steels can exhibit an additional increase,
sometimes referred to as "kick", in yield strength when compared to
as-rolled hot band yield strengths. This kick in the yield strength
can be in the range of 50 MPa.
[0029] In the alternative, when such an increase in yield strength
is not desired, the hot rolled dual phase steels are subjected to
less than 0.5% tension elongation, e.g. in the pickling line, and
thereby give a low yield ratio pickled product. It should be
appreciated that the sensitivity to work hardening by the inventive
steels disclosed herein can be utilized to give higher strength
after forming operations in a final formed part that is different
than the increase in strength of approximately 30 MPa imparted by
traditional prestraining and paint baking.
[0030] Regarding the chemical composition of steels disclosed
herein, no titanium is intentionally added since it is known that
titanium results in the formation of titanium carbonitrides which
deteriorates fatigue properties. As such, the hot rolled dual phase
steels of the present invention can be used for automotive
applications requiring high fatigue resistance, e.g. wheels and
rims, along with intended usage for other components such as frames
and body parts.
[0031] The process includes providing a steel slab having a
chemical composition in weight percent within a range of 0.06-0.09
C, 0.8-2.0 Mn, 0.4 min Cr, 0.08 max Si, 0.05 max P, 0.005-0.010 N,
with the balance of Fe and incidental melting impurities known to
those skilled in the art. The steel slab chemical composition also
has an Al to N (Al/N) ratio of 10.0 max and a Nb+Ti+Mo content of
0.015 max.
[0032] Thereafter, the steel slab is soaked within a temperature
range of 1150-1300.degree. C. followed by hot rolling the slab
through a roughing mill and then through a finishing mill in order
to produce hot rolled strip. The finishing mill may or may not be a
seven-stand finishing mill. After exiting the finishing mill, the
hot rolled strip is subjected to a continuous cooling treatment or
a step cooling treatment until the strip is cooled to less than
100.degree. C. prior to coiling.
[0033] The continuous cooling treatment employs average cooling
rates in the range of 40-70.degree. C./sec after the finishing
treatment. In some instances, the continuous cooling treatment is
executed within a laminar cooling section of a hot strip mill
cooling table. In the alternative, the step cooling treatment
employs a first phase of fast or rapid cooling after the final
stage of finishing rolling followed by a hold of the hot rolled
strip within a ferrite formation zone for the particular steel
chemistry being processed. The hot rolled strip is held within the
ferrite formation zone for at least 3 seconds which aids in
efficient austenite to ferrite transformation.
[0034] The hot rolled strip, whether continuously cooled or
specifically when step cooled, is subjected to a final water quench
and cooling to a target coiling temperature. The final or third
stage is a fast rapid quenching step that utilizes high cooling
rates that ensure pearlite and upper and/or lower bainite are
avoided in the final room temperature microstructure. It is
appreciated that such lower temperature products hamper the
strength and ductility of the material and thus their absence is an
improvement provided by the inventive process disclosed herein that
is not present in the prior art.
[0035] The microstructure of the hot rolled dual phase steels
processed according to embodiments of the present invention has a
volume fraction percentage of ferrite greater than 80%, which in
turn provides for an ideal strength-ductility balance for hot
rolled dual phase steels with tensile strengths greater than 590
MPa. Again, the microstructures so obtained for the hot rolled dual
phase steels are completely ferrite plus martensite with no
pearlite, bainite or any other lower transformation products
present.
[0036] In a preferred embodiment, cooling of the hot rolled dual
phase steels is achieved by utilizing six reinforced cooling zones
during a first step or first phase of cooling, followed by eight
micro cooling zones, which is then further followed by another six
reinforced cooling zones towards the end of a cooling table laminar
length. In addition, an available water capacity of greater than
15,000 cm.sup.3/hr is used to achieve desired high cooling
rates.
[0037] The term "reinforced cooling zones" refers to cooling zones
having two additional valves at both top and bottom locations. The
two additional valves provide a total of six top and six bottom
water cooling valves at each of the six cooling zones at the start
and towards the end of the laminar length of the cooling table.
Also, the term "micro cooling zones" refers to traditional laminar
cooling arrangements with four headers per cooling zone
location.
[0038] A preferred embodiment also employs a long laminar cooling
section with a length greater than 150 meters (m) with
approximately 130 m of water cooling length available when using a
step of "air cooling" during a step cooling practice.
[0039] It is appreciated that the long laminar cooling section with
reinforced cooling zones affords for desired temperatures and
cooling rates for continuous cooling and/or step cooling to obtain
desired ferrite-martensite dual phase structure. For example,
austenite to ferrite transformation aided during a hold zone of
cooling in air, in combination with reinforced cooling zones during
the third or final phase of cooling, allows for the avoidance of
pearlite and other undesired transformation products after the hold
zone. In addition, cooling practices during the first and third
phases of the step cooling aid in obtaining hard martensite islands
in a ferritic matrix, which gives an ideal dual phase
strength-ductility balance of properties.
[0040] The inventive process affords for hot rolled strip to be
cooled to temperatures of less than 70.degree. C., which is
analogous to a cold rolled and annealed product cooled to room
temperature. However, it is appreciated that the inventive process
disclosed herein allows for the additional production steps of cold
rolling and intercritical annealing to be eliminated. As such, the
novel hot rolling practice provides a product that is immediately
available to be pickled and/or shipped as hot band. Stated
differently, the inventive process avoids the traditional cooling
period of 48-72 hours for hot rolled coiled steel strip and thus
reduces inventory, logistic complications, and the like.
[0041] In another embodiment of the present invention, steels
falling within the chemistry range described above and having
tensile strengths greater than 690 MPa are provided. Such steels
are subjected to close to room temperature cooling; however,
enhanced cooling rates in the range of 70-100.degree. C./sec are
used. The higher cooling rates afford for higher martensite volume
fractions to be present in the room temperature microstructure
which affords for the increase in tensile strength. Therefore,
heretofore hot rolled dual phase steels that use higher alloying
contents to achieve such tensile strengths are greatly improved
upon.
[0042] Regarding 590 MPa grades of hot rolled steel sheet, a slab
of steel having a chemical composition within the range disclosed
above can be soaked at elevated temperatures within
1150-1300.degree. C. in order to ensure that most if not all
alloying elements are in solid solution. The slab is then subjected
to a roughing treatment and/or a finishing treatment to produce hot
rolled strip having a thickness between 2.0-5.5 millimeters (mm).
The finishing treatment can have an entry temperature between
950-1120.degree. C. and an exit temperature between 800-850.degree.
C. In some instances, the lower range of temperatures for the
finishing treatment marks the temperature at which possible
two-phase rolling can take place.
[0043] After exiting the last finishing stand of the finishing
treatment, the strip can be continuously cooled to a temperature
less than 100.degree. C. before coiling. The strip is cooled using
an average cooling rate of between 40-70.degree. C./sec with the
lower cooling rate range marking the limit to prevent pearlite
formation, while the higher cooling rate range marking the limit to
prevent a higher than desired volume fraction of martensite in the
steel microstructure.
[0044] In the alternative, hot rolled strip exiting the finishing
treatment can be cooled using a step cooling practice that holds
the hot strip in a ferrite formation zone between 640-720.degree.
C. for a minimum of 3 seconds. The hold step is followed by a fast
or rapid water quench. Sufficiently high cooling rates are obtained
or employed in the first cooling phase in order for effective
targeted cooling of the material into the ferrite formation zone,
and in the third cooling phase for preventing the transformation of
low transformation products, apart from martensite.
[0045] Hot rolled strip, whether cooled via continuous cooling or
step cooling, is coiled at temperatures less than 70.degree. C. As
such, coils manufactured according to an inventive process
disclosed herein are immediately available for shipping as hot band
and/or proceeding downstream for pickling. In addition, and as
stated above, the microstructure of such steel is dual phase with
martensitic islands in a ferritic matrix.
[0046] The process and chemical compositions disclosed herein
provide hot rolled dual phase steel that is sensitive to tension
leveling/work hardening based on the continuous yielding phenomena
for steels. As such, hot band or pickled product provided to
customers can receive additional work hardening to provide an
increase in yield strength of around 50 MPa in a final
formed/stamped part compared to heretofore known and similar steel
grades. The formed/stamped high strength product can subsequently
be paint baked to get an additional bake hardening increase in
strength of at least 30 MPa, i.e. BH2.gtoreq.30 MPa. As such,
additional high strength can be obtained or incorporated in
as-formed product with the aid of work hardening and bake
hardening.
[0047] Turning now to FIG. 1, a temperature versus time graphical
illustration for an embodiment of the present invention is shown.
As illustrated in the figure, a slab of steel having a chemistry
within the compositional range disclosed above is subjected to a
soaking treatment between 1150-1300.degree. C. Thereafter, the
steel slab is subjected to a hot rolling roughing treatment
followed by a hot rolling finishing treatment and finally
continuous cooling to less than 100.degree. C. prior to
coiling.
[0048] The hot rolling finishing treatment has an entry temperature
between 950-1120.degree. C. and an exit temperature between
800-850.degree. C. Also, the hot rolled strip is subjected to a
continuous cooling treatment with a cooling rate between
40-70.degree. C./sec until reaching close to room temperature, and
then coiled.
[0049] With reference to FIG. 2, a steel slab is soaked and then
subjected to a hot rolling finishing treatment followed by a step
cooling procedure. The step cooling procedure includes cooling of
the hot rolled strip upon exiting the finishing treatment down to a
ferrite formation zone where the strip is held for at least 3
seconds. The ferrite formation zone is naturally a function of the
chemical composition of the strip that is being processed and is
typically between 640-720.degree. C. After the hold step, the
material is further cooled to less than 100.degree. C. before
coiling.
[0050] FIG. 3 provides a temperature versus time illustration of
another embodiment of the present invention in which a steel slab
that has been soaked and subjected to a roughing treatment and a
finishing treatment, is further subjected to rapid or enhanced
continuous cooling down to less than 100.degree. C. before coiling.
The rapid cooling is afforded with cooling rates between
70-100.degree. C./sec.
[0051] In order to provide a specific teaching of the invention and
yet not limit the scope thereof in any way, examples of the process
according to one or more embodiments of the invention are provided
below.
Example 1
[0052] Steel slabs from heat chemistries labeled 1, 2, 3, and 4 as
defined in Table 1 below and having chemical compositions (wt %)
within the range disclosed above and a thickness of approximately
255 mm were soaked at approximately 1240-1280.degree. C. and then
subjected to a roughing treatment to produce a transfer bar. The
transfer bar was subjected to a finishing treatment with an entry
temperature of 1050.degree. C. and an exit temperature of
820.degree. C. in order to produce hot strip with a thickness
between 2-5.3 mm.
TABLE-US-00001 TABLE 1 C Mn Si P S Al N Cr Ti Mo Nb Ni Heat 1 0.077
0.96 0.05 0.033 0.0008 0.035 0.007 0.42 0.004 0.01 0.001 0.03 Heat
2 0.07 0.993 0.063 0.0343 0.0031 0.0365 0.007 0.417 0.0015 0.0001
0.0021 0.01 Heat 3 0.067 0.956 0.057 0.0312 0.0024 0.029 0.007
0.411 0.0005 0.0001 0.0003 0.012 Heat 4 0.068 0.953 0.063 0.0356
0.0032 0.0306 0.008 0.423 0.0002 0.0001 0.0012 0.012 Heat 5 0.084
0.961 0.062 0.0317 0.0018 0.0368 0.008 0.433 0.0021 0.001 0.0015
0.013 Heat 6 0.081 0.966 0.063 0.0324 0.0017 0.0269 0.007 0.425
0.0023 0.0011 0.002 0.013
[0053] The strip was then continuously cooled at an average cooling
rate of between 40-70.degree. C./sec as illustratively shown in
FIG. 1 or, in the alternative, cooled using the "step cooling"
practice with an intermediate hold at 660-680.degree. C. for 3-5
seconds, followed by a fast or rapid water quench, as illustrated
in FIG. 2. The close to room temperature cooled hot bands were
coiled and available immediately after the hot rolling for
pickling. The coiled hot bands were in fact pickled in a continuous
pickling line with an applied tension leveling of 0.3% applied
thereto. Test samples taken from the pickled and oiled sheet were
subjected to standard mechanical testing with results shown in
Table 2 below.
TABLE-US-00002 TABLE 2 Thickness YS_0.2% TS % Elongation (mm) (MPa)
avg. (MPa) avg. (A.sub.50) avg. BH.sub.2 (MPa) 5.3 433 622 26
>30 4.1 435 613 24 3.5 432 614 24 2.8 430 615 23 <2.3 450 631
21
[0054] FIG. 4 provides a representative microstructure for a
particular sample. As shown in the optical micrograph, a percent
volume fraction of martensite between 15-22% is present. In
addition, the microstructure is free of pearlite and bainite.
Example 2
[0055] Steel slabs from heat chemistries identified as 5 and 6 in
Table 1 above and having a thickness of approximately 255 mm were
soaked at approximately 1240-1280.degree. C. The soaked steel slabs
were subjected to roughing and finishing treatments as described
above, the finishing treatments having an exit temperature of
820.degree. C. After exiting the finish treatment, the hot rolled
strip was subjected to enhanced cooling rates between
70-100.degree. C./sec and continuously cooled to less than
100.degree. C. before coiling. FIG. 5 provides an optical
micrograph of the ferrite-martensite microstructure obtained from
the rapidly cooled material. As shown in the figure, a higher
percentage volume fraction of martensite between 25-32% was
present.
[0056] The higher cooling rates coupled with the close to room
temperature quenching practices resulted in the hot rolled strip
having a minimum tensile strength of 690 MPa and a yield ratio in
the range of 0.55-0.7. It is appreciated that the higher volume
fraction of martensite as compared to the volume fraction shown in
FIG. 4 shifts the dual phase product to the dual phase steel grade
family having a tensile strength of at least 690 MPa.
[0057] In addition to the above, FIG. 6 provides a graphical plot
of percent elongation versus tensile strength for a plurality of
steel alloys having chemical compositions within the chemistry
range disclosed herein and subjected to the high cooling rate
procedure discussed above. As shown in the figure, all
alloys/samples tested had a tensile strength greater than 690 MPa
and a percent elongation greater than 19%. In addition, FIG. 7
provides a graphical plot of tensile strength (TS) versus 0.2%
yield strength (YS) for the samples used to plot FIG. 6. As shown
by the plot in FIG. 7, the yield ratio (0.2% YS/TS) for the samples
is in the range of 0.55-0.7.
[0058] The effect of tension leveling the hot rolled strip after
being subjected to the close to room temperature coiling practice
disclosed herein is shown in FIG. 8. As shown in this figure,
tension leveling of 0.4% provided an average increase in yield
strength of approximately 47 MPa. In addition, tension leveling of
0.8% provided an average increase in yield strength of
approximately 90 MPa.
[0059] Such an increase in yield strength is schematically shown on
a plot of yield strength versus percent elongation in FIG. 9. Yield
strength for hot rolled strip without tension leveling is shown by
the curve labeled as "Hot Band" and yield strength for hot rolled
strip as a function of percent tensioning level elongation is
labeled "Hot Rolled Pickled". Also, the additional strength
provided by a bake hardening treatment is illustratively shown by
the line/curve labeled "BH". It is appreciated that FIG. 9
corresponds to material processed according to the embodiments
illustratively shown in FIGS. 1 and 2; however, the increase in
yield strength is also provided for steels processed according to
the embodiment illustratively shown in FIG. 3. As such, the
inventive steels and the process used herein can provide a kick in
yield strength via tension leveling and also by bake hardening.
[0060] It should be appreciated that the data disclosed herein is
unknown in the prior art and thus demonstrates unexpected results.
In addition, the above examples and embodiments are not meant to
limit the scope of the invention but are simply examples that
provide teaching to one skilled in the art. As such, changes,
alterations, modifications, and the like will be apparent to one
skilled in the art and yet fall within the scope of the present
invention. Therefore, it is the claims and all equivalents thereof
that define the scope of the invention.
* * * * *