U.S. patent application number 10/321665 was filed with the patent office on 2004-06-24 for dual phase hot rolled steel sheet having excellent formability and stretch flangeability.
Invention is credited to Funakawa, Yoshimasa, Inazumi, Toru, Matsuki, Yasuhiro, Sun, Weiping.
Application Number | 20040118489 10/321665 |
Document ID | / |
Family ID | 32592949 |
Filed Date | 2004-06-24 |
United States Patent
Application |
20040118489 |
Kind Code |
A1 |
Sun, Weiping ; et
al. |
June 24, 2004 |
Dual phase hot rolled steel sheet having excellent formability and
stretch flangeability
Abstract
The present invention provides a process of producing a family
of hot rolled dual phase steel sheets having excellent formability
and stretch flangeability, with yield strengths of from about 500
MPa to about 900 MPa from a single steel chemistry consisting of,
by weight percent, 0.02-0.15% of C, 0.3-2.5% of Mn, 0.1-2.0% of Cr,
0.01-0.2% Al, 0.001-0.01% Ca, not more than 0.1% P, not more than
0.03% S, not more than 0.2% Ti, not more than 0.2% V, not more than
0.2% Nb, not more than 0.5% Mo, not more than 0.5% Cu, not more
than 0.5% Ni, the balance being Fe and unavoidable impurities. A
slab or ingot of this composition is reheated to a temperature of
between 1050.degree. C. and 1350.degree. C. and held at this
temperature for at least 10 minutes, then hot rolled, completing
the hot rolling at between 800.degree. C. and 1000.degree. C. The
sheet is cooled, immediately after completion of hot rolling, at a
rate of not lower than 10 C/sec., without requiring specific
cooling patterns, and coiled at a temperature of not less than 450
C. The cooling temperature is controlled to produce the desired
yield strength within the range of from about 500 MPa to about 900
MPa.
Inventors: |
Sun, Weiping; (Canton,
MI) ; Inazumi, Toru; (Ann Arbor, MI) ;
Matsuki, Yasuhiro; (Kawasaki, JP) ; Funakawa,
Yoshimasa; (Kawasaki, JP) |
Correspondence
Address: |
James L. Bean
Suite One
1423 Powhatan Street
Alexandria
VA
22314
US
|
Family ID: |
32592949 |
Appl. No.: |
10/321665 |
Filed: |
December 18, 2002 |
Current U.S.
Class: |
148/602 |
Current CPC
Class: |
C21D 8/021 20130101;
C21D 8/0263 20130101; C21D 2211/005 20130101; C21D 2211/008
20130101; C21D 8/0226 20130101 |
Class at
Publication: |
148/602 |
International
Class: |
C21D 008/02 |
Claims
We claim:
1. A method of producing hot rolled dual phase steel sheet having
excellent formability and stretch flangeability comprising the
steps of (a) providing as a starting material a steel slab or ingot
containing, by weight percent, 0.02-0.15% of C, 0.3-2.5% of Mn,
0.1-2.0% of Cr, 0.01-0.2% Al, 0.001-0.01% Ca, not more than 0.1% P,
not more than 0.03% S, not more than 0.2% Ti, not more than 0.2% V,
not more than 0.2% Nb, not more than 0.5% Mo, not more than 0.5%
Cu, not more than 0.5% Ni, the balance being Fe and unavoidable
impurities; (b) reheating the steel slab or ingot to a temperature
in the range between 1050.degree. C. and 1350.degree. C. and
holding at this temperature for a time of not less than 10 minutes;
(c) hot rolling the reheated steel slab into a steel sheet,
completing the hot rolling process at a temperature in the range
between 800.degree. C. and 1000.degree. C.; (d) cooling the hot
rolled steel sheet, immediately after completion of hot rolling, at
a mean rate not lower than 10.degree. C./sec. without requiring
specific cooling patterns, and (e) coiling the hot rolled steel
sheet at a temperature not lower than 450.degree. C. to produce a
dual phase steel sheet having a yield strength of between about 500
MPa and about 900 MPa, depending on the coiling temperature.
2. The method as defined in claim 1, wherein the coiling
temperature is between 450.degree. C. and 500.degree. C. to produce
a steel having a yield strength of at least about 800 MPa.
3. The method as defined in claim 1, wherein the coiling
temperature is between 500.degree. C. and 550.degree. C. to produce
a steel having a yield strength of at least about 600 MPa.
4. The method of claim 1, wherein the coiling temperature is within
the range of about 550.degree. C. to about 650.degree. C. to
produce a steel sheet having a yield strength of at least about 550
MPa.
5. The method of claim 1, wherein the steel sheet contain at least
about 0.03% C.
6. The method of claim 1, wherein the steel sheet contain at least
about 0.5% Mn.
7. The method of claim 1, wherein the steel sheet contain at least
about 0.3% Si.
8. The method of claim 1, wherein the steel sheet contain at least
about 0.3% Cr.
9. The method of claim 1, wherein the dual phase hot rolled steel
sheet has a micro-structure consisting of about 3-30%, by volume,
martensite islands as a hard secondary phase embedded in a
fine-grained ferrite matrix.
10. The method of claim 1, wherein step (d) comprises cooling the
steel sheet at a rate of at least about 30.degree. C./sec.
11. A hot rolled dual phase steel sheet produced by the method of
claim 1.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to hot-rolled dual
phase-structure (ferrite/martensite) steel sheet products a method
for their production. In particular, the hot rolled steel sheet has
excellent formability and stretch flangeability, as well as
improved surface quality, weldability and fatigue property.
[0003] 2. Brief Description of the Prior Art
[0004] Currently, applications of high strength steel sheets to
automotive parts are increasing in order to reduce vehicle weight
and thus improve the fuel efficiency. Among these high strength
steels, dual phase steel, which possesses a microstructure
consisting of martensite islands embedded in a ferrite matrix, is
attracting more and more attention due to its superior combination
of high strength, excellent ductility, continuous yielding and low
yield ratio. In particular, dual phase steels help to improve
vehicle crashworthiness and durability and therefore improve
passenger safety and vehicle life. Although the existing dual phase
steel products exhibit good formability, their stretch
flangeability is relatively inferior to bainitic type steels.
Therefore, there exists a limitation of applying dual phase steel
sheets to the manufacture of the parts which must undergo both
forming (or otherwise press forming) and stretch flanging process.
For this reason, dual phase steel sheets having both excellent
formability and stretch flangeability are desired.
[0005] Several method for producing hot-rolled dual phase steel
sheets are known. U.S. Pat. No. 4,790,889 discloses a method of
producing hot-rolled steel sheet having a dual-phase structure from
a slab previously produced by ingot or continuous casting. The slab
is heated up to the rolling temperature, hot rolled at a
temperature above Ar3, rapidly cooled immediately after
finish-rolling from the final rolling temperature down to the
coiling temperature at a mean rate in the range from 30.degree. to
70.degree. C./sec and without interruption, and then coiled at a
temperature in the range from 350.degree. to 190.degree. C.
[0006] U.S. Pat. No. 4,561,910 discloses a method of producing dual
phase hot rolled steel sheet having a composition consisting of
0.03-0.15% by weight of C, 0.6-1.8% by weight of Mn, 0.04-0.2% by
weight of P, not more than 0.10% by weight of Al, not more than
0.008% by weight of S, and the remainder being substantially Fe.
The heating temperature is kept to 1,1000.degree.-1,250.degree. C.,
the finishing hot rolling temperature is kept to
800.degree.-900.degree. C., the coiling temperature is kept not
higher than 450.degree. C., preferably 400.degree.-100.degree. C.,
and the cooling rate from beginning of cooling following to hot
rolling to coiling is kept to 10.degree. to 200.degree. C./sec,
according to an ordinary cooling pattern by air-cooling or
water-cooling.
[0007] U.S. Pat. No. 4,421,573 discloses a dual phase high-tensile
steel sheet having a martensite and ferrite composite structure and
a tensile strength of the order of 50-80 kg/mm.sup.2 in an
as-hot-rolled state. The steel sheet is produced by a method which
comprises preparing as a starting material a slab comprising
0.03-0.15% C, 0.5-1.0% Mn, 0.8-2.0% Cr, 0.01-0.1% Al, the balance
being essentially Fe and accompanying impurities, heating said slab
at a temperature of 1,050.degree.-1.220.deg- ree. C., hot rolling
the heated slab, completing the hot rolling at a temperature of
800.degree.-900.degree. C., thereafter cooling the hot rolled sheet
to a temperature of 350.degree.-500.degree. C., and winding the
sheet into a coil at the latter temperature.
[0008] U.S. Pat. No. 4,502,897 discloses a method for producing hot
rolled steel sheets having a low yield ratio and a high tensile
strength due to dual phase structure by finishing the final rolling
at a temperature of 780.degree. C., rapidly cooling the steel sheet
at a cooling rate of more than 40.degree. C./sec to the temperature
range wherein the transformation of .gamma. to .alpha. is
efficiently caused, holding the steel sheet at this temperature
range for more than 5 seconds and rapidly cooling the steel sheet
at a cooling rate of more than 50.degree. C./sec from the held
temperature to a coiling temperature of 550-200.degree. C. Although
this patent allows a slight increase of coiling temperature to as
high as 550.degree. C., a step cooling pattern is involved, the
latter of which could results in a lower productivity and also is
practically difficult to be conducted during hot rolling where the
steel sheet continuously travels at a relatively high speed. With
respect to mill facility, this method entails expensive cooling
sections.
[0009] U.S. Pat. No. 4,407,680 discloses a method for producing a
dual phase steel sheet in which the steel sheet is hot rolled and
cooled to exhibit a substantially bainite structure throughout its
cross-section, and in which the steel sheet is subsequently
continuously annealed in the two phase ferrite/austenite field and
cooled to transform the austenite to martensite. This method
entails an extra continuous annealing process, with a corresponding
production cost increase.
[0010] U.S. Pat. No. 4,325,751 discloses a steel sheet displaying
high strength and formability properties. This product is
fabricated by coiling a steel sheet which has been previously
processed through a hot strip mill from an initial steel having a
very low amount of alloying compounds and having a temperature of
between 750.degree. and 900.degree. C., the coiled steel sheet
being maintained at a temperature of between 800.degree. and
650.degree. C. for a period of at least one minute, and thereafter
cooled to a temperature of below 450.degree. C., the cooling being
accomplished at a rate exceeding 10.degree. C./sec. The method
includes the additional process of step cooling after coiling,
which not only increases the production cost but also requires
adding extra facility to most existing hot strip mills.
[0011] The previously known methods or low coiling temperature
method could only produce a hot rolled dual phase steel sheet with
a limited range of mechanical properties from a single chemistry
matrix. In other words, these methods could only provide a single
grade of hot rolled dual phase steel sheet based on one chemistry
design. A variety of chemistry designs are thus required to produce
different grades of hot rolled dual phase steel sheet with
different levels of mechanical properties. This would prolong the
production time cycle and increase production cost should different
grades of hot rolled dual phase steel sheet products be requested
for various applications by different customers.
OBJECTS OF THE INVENTION
[0012] Despite the concerted activities in obtaining dual phase hot
rolled steel sheet, as evidenced in part by the patents noted
above, a need still exists to develop new manufacturing methods to
produce this type of steel sheet under conventional hot rolling
operation conditions.
[0013] As a principal object, the present invention has thereof
been made in order to advantageously avoid the above described
problems of the prior methods and has the provision of an alloy
design and manufacturing method, which has less demanding or
restrictive facility and processing requirements, for producing hot
rolled dual phase steel sheet at higher coiling temperatures
without any specific annealing and/or cooling processes following
the conventional hot rolling and coiling steps.
[0014] Although the hot rolled dual phase steel sheets obtained by
the previous patents or methods exhibit low yield ratio and good
ducility, no improved stretch flangeability has been demonstrated.
In order to meet the current property requirements of the
automotive industry, a dual phase steel sheet having the combined
properties of excellent formability and stretch flangeability as
well as improved surface quality, weldability and fatigue property
is desired.
[0015] A further object of the present invention is this to provide
a manufacturing method to produce a hot rolled dual phase steel
sheet having excellent formability and stretch flangeability as
well as improved surface quality, weldabilty, and fatigue
properties.
[0016] Another object of the present invention is to provide a
practical manufacturing method, including properly adjusting
coiling temperature, to produce a family of dual phase hot rolled
steel sheets, including hot rolled dual phase steel 550, 600, and
800, using a single chemistry matrix, the details of which will be
further demonstrated below by examples.
SUMMARY OF THE INVENTION
[0017] The above and other objects of the present invention are
achieved by a method for producing a dual phase hot rolled steel
sheet having excellent formability and stretch flangeability as
follows:
[0018] (a) providing as a starting material a steel slab or ingot
of a composition comprising (in weight percentages) about
0.02-0.15% carbon (C), about 0.3-2.5% manganese (Mn), about
0.1-2.0% silicon (Si), about 0.1-2.0% chromium (Cr), not more than
about 0.1% phosphorous (P), not more than about 0.2% titanium (Ti),
not more than about 0.2% vanadium (V), not more than about 0.2%
niobium (Nb), not more than about 0.5% molybdenum (Mo), not more
than about 0.5% copper (Cu), not more than about 0.5% nickel (Ni),
and 0.001-0.010 calcium (Ca), the remainder essentially being iron
(Fe) and unavoidable impurities;
[0019] (b) reheating the steel slab obtained in the above step to a
temperature in the range between 1050.degree. C. (1922.degree. F.)
and 1350.degree. C. (2462.degree. F.) and then holding at this
temperature for a time period of not less than 10 minutes;
[0020] (c) hot rolling the reheated steel slab into hot rolled
steel sheet and completing the hot rolling process at a temperature
in the range between 800.degree. C. (1472.degree. F.) and
1000.degree. C. (1832.degree. F.);
[0021] (d) cooling the hot rolled steel sheet, immediately after
completing hot rolling, at a mean rate not slower than 10.degree.
C./sec (18.degree. F./sec) without requiring specific cooling
patterns;
[0022] (e) coiling the cooled hot rolled steel sheet at a
temperature not lower than 450.degree. C. (842.degree. F.);
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] The foregoing and other features and advantages will be more
apparent from the detailed description contained herein below,
taken in conjunction with the drawings, in which:
[0024] FIG. 1 is a graph showing the relationship of tensile
strength of hot rolled dual phase steel sheets to the coiling
temperature;
[0025] FIG. 2 is a graph showing the relationship between total
elongation and tensile strength obtained on different grades of hot
rolled dual phase steel sheets manufactured according to the
present invention;
[0026] FIG. 3 is a graph showing the relationship between hole
expansion ratio and tensile strength obtained on hot rolled dual
phase steel sheets manufactured according to the present invention
as well as the comparison of this relationship with that measured
on the conventional hot rolled dual phase steel sheets produced
using the prior methods; and
[0027] FIG. 4 is a micrograph obtained using a LaPera etching
technique, which illustrates the typical dual phase structure (fine
martensite islands uniformly distributed in the fine-grained
ferrite matrix) presented in the hot rolled steel sheet of the
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] The present invention is directed to dual phase hot rolled
steel sheet and methods of making such a steel sheet. In a
preferred embodiment, the dual phase hot rolled steel sheet
manufactured according to this invention possesses a microstructure
consisting about 3-30% (in volume percentages) martensite islands
as a hard second phase embedded in a fine-grained ferrite matrix,
and exhibits excellent formability, stretch flangeability, surface
quality, weldability and fatigue property. With respect to the
preferred applications, the steel sheet can be used after being
formed and/or stretch flanged in an "as-hot-rolled" state or
optionally painted state for products such as automobile,
electrical appliances and building components.
[0029] As will be shown in more detail below, the ranges of the
chemical elements desirably contained in the dual phase hot rolled
steel sheet produced according to the present invention can be
readily obtained in the conventional manufactured process. The
preferred limitations on the composition and the reasons for these
desired limitations will now be discussed in more detail below.
[0030] Carbon is an important element affecting the hardenability
and strength of the steel sheet. It is necessary in an amount of at
least 0.02% in order to provide necessary strength for the steel
sheet. Thus, the lower limit of carbon content is 0.02% by weight
in the preferred embodiment of the present invention. In order to
secure the formation of martensite even at high coiling
temperatures, however, a more preferable lower limit of carbon is
given as 0.03% by weight in the present invention. Since carbon
present in the steel sheet in amounts above about 0.15% could
deteriorate the formability and weldability of the steel sheet, the
maximum carbon content is limited to 0.15%.
[0031] In general, manganese acts as a basic alloying element
enhancing the strength and hardenability of steel sheets and is
relatively inexpensive. An amount of at least 0.3% by weight of
manganese is necessary in order to ensure the strength and
hardenability of the steel sheet. The lower limit of manganese
content is thus 0.3% by weight in the preferred embodiment of the
present invention. Furthermore, in order to enhance the stability
of austenite and to form at least 3% by volume of martensite in the
final steel sheet, the amount of manganese needs to be more than
0.5% by weight. Therefore, it is a more preferable to contain at
least 0.5% by weight of manganese in the present invention.
However, when the amount exceeds 2.5% by weight, the weldability is
adversely affected. It is thus of importance to limit the amount of
manganese to no more than 2.5% by weight.
[0032] Silicon is an element useful for increasing the strength but
not significantly impairing the ductility or formability of the
steel sheet. An amount of at least 0.1% by weight of silicon is
required in order to improve the balance between the strength and
formability of the steel sheet. The lower limit of silicon content
is thus 0.1% by weight in the preferred embodiment of the present
invention. Moreover, silicon promotes the ferrite transformation
and needs at least 0.3% by weight to form at least 70% of ferrite
in the final steel sheet. In view of this point, a more preferable
lower limit of silicon is 0.3% by weight in the present invention.
When the content of silicon exceeds 2.0%, its beneficial effect is
saturated and the economical disadvantage is then brought out.
Accordingly, the upper limit of silicon content is preferably
defined to be 2.0% be weight.
[0033] Chromium is an element for improving the hardenability and
strength. It is also useful for stabilizing the remaining austenite
and promoting the formation of martensite while having no adverse
effects on austenite to ferrite transformation. In order to assure
these effects, the lower limit of chromium content is 0.1% by
weight in the preferred embodiment of the present invention. It is
of note that this element is particularly important in the present
invention for modifying the microstructure of the hot rolled dual
phase steel sheet to achieve excellent combinations of formability,
stretch flangeability, surface quality and weldability. In view of
the above benefits, a more preferable lower limit of chromium is
determined as 0.3% by weight in the present invention. The upper
limit of this element is preferably defined to be 2.0% by weight in
this invention for maintaining a reasonable manufacturing cost.
[0034] In principle, phosphorus exerts a similar effect to
manganese and silicon in view of solid solution hardening. When
large amount of phosphorus is added to the steel, however, the
rollability of the steel sheet is deteriorated. Besides, the
segregation of phosphorus at grain boundaries at high coiling
temperatures results in brittleness of the steel sheet, which in
turn impairs its formability, stretch flangeability, and
weldability. For these reasons, the preferred upper limit of
phosphorus content is defined to be 0.1% by weight.
[0035] Sulfur is not normally added to the steel because lower
sulfur content is preferable. However, it is present as a residua
element, the amount of which depends on the employed steelmaking
techniques. Since the present steel contains manganese, sulfur is
precipitated in the form of manganese sulfides. A large amount of
manganese sulfide precipitates greatly deteriorates the
formability, stretch flangeability and fatigue property of the
steel sheet. The preferred upper limit of sulfur content is
accordingly defined to be 0.03% by weight.
[0036] Aluminum is employed for deoxidation of the steel and fixing
nitrogen to form aluminum nitrides. Theoretically, the acid-soluble
amount of (27/14)N, i.e., 1.9 times the amount of nitrogen, is
required to fix all nitrogen as aluminum nitrides. Practically,
however, the use of at least 0.01% of aluminum by weight is
effective as a deoxidation element. Therefore, the lower limit of
aluminum content is preferably defined to be 0.01% by weight. When
the content of aluminum exceeds 0.2%, on the other hand, the
formability of the steel sheet is significantly decreased. The
preferred amount of aluminum is thus at most about 0.2% by
weight.
[0037] The alloying elements, titanium, vanadium, and niobium, have
strong effect for retarding austenite recrystallization and
refining grains. When a moderate amount of these elements is added,
the strength of the final steel sheet is properly increased. These
elements are also useful to accelerate the transformation of
austenite to ferrite after the final hot rolling. However, when the
content of each of these element exceeds 0.2% by weight, large
amounts of the respective precipitates are formed during hot
rolling, cooling and/or coiling the steel sheet. The corresponding
precipitation hardening becomes very high and the formability of
the steel sheet is markedly deteriorated. It is therefore
preferable to contain each of these elements not more than 0.2% by
weight.
[0038] The alloying elements, molybdenum, copper, and nickel, are
useful for improving hardenability and strength of the steel sheet.
However, all of these elements are expensive and thus the preferred
upper limit for each of these elements is defined to be 0.5% by
weight for economic reasons.
[0039] Calcium is another important element in this invention
because it helps to modify the shape of sulfides. Thus, it reduces
the harmful effect of sulfur and eventually improves the stretch
flangeability and fatigue property. Since an amount of at least
0.001% by weight is needed to secure this beneficial effect, the
lower limit of calcium content is established at 0.001% by weight
in the preferred embodiment of the present invention. It is also of
note that this beneficial effect is saturated when the amount of
calcium exceeds 0.01% by weight, so that the preferred upper limit
of this element is defined as 0.01% by weight.
[0040] Other impurities should be kept to as small a concentration
as is practicable.
[0041] By employing a steel falling within the above compositional
or chemistry constraints, the process will have less demanding or
restrictive facility and processing requirements. In fact, the
process can be carried out at most existing hot strip mills without
requiring any additional equipment or capital cost. A more specific
recitation of a preferred process includes the following steps:
[0042] (1) Prepare a steel melt having a composition falling within
the ranges discussed above.
[0043] (2) Use a conventional continuous slab caster or a
conventional ingot caster to produce a slab (or ingot) having a
thickness suitable for hot rolling into a hot rolled band,
alternatively referred to as a hot rolled steel sheet.
[0044] (3) Heat the steel slab obtained in th above step to a
temperature in the range between the 1050.degree. C. (1922.degree.
F.) and 1350.degree. C. (2462.degree. F.).
[0045] (4) Hold the steel slab at the above temperature for a time
period of not less than 10 minutes, and preferably not less than 30
minutes to assure the uniformity of the initial microstructure of
the slab before hot rolling.
[0046] (5) Hot roll the reheated steel slab into a steel sheet and
complete the hot rolling process at a temperature in the range
between 800.degree. C. (1472.degree. F.) and 1000.degree. C.
(1832.degree. F.), and preferably in the range between 800.degree.
C. (1472.degree. F.) and 950.degree. C. (1742.degree. F.) in order
to obtain a fine-grained ferrite matrix and avoid the formation of
pearlite phase.
[0047] (6) Cool the hot rolled steel sheet, immediately after
completing hot rolling, at a mean rate not slower than 10.degree.
C./sec (18.degree. F./sec). Since the steel composition and the
final product properties in accordance with the present invention
are not dependent on control of specific cooling patterns, and the
conventional runout table cooling conditions at most existing hot
strip mill are suitable for the process.
[0048] (7) Coil the cooled hot rolled steel sheet at a temperature
not lower than 450.degree. C. (842.degree. F.). The employed
coiling temperature determines the mechanical properties (yield
strength, tensile strength, and total elongation) of the final
steel sheet. Through properly adjusting the coiling temperature
within the range of 460.degree. C. to 650.degree. C., a family of
dual phase hot rolled steel sheets, say hot rolled dual phase
steels having yield strengths of 550, 600, and 800 MPa, can be
produced using a single chemistry design, the details of which will
be further demonstrated below by example.
[0049] Hot rolled steels produced by the above process can be
formed and/or stretch flanged into a desired shape for a final
application. If desired, the final component can be painted.
EXAMPLE
[0050] In the course of developing the present invention, steel
slabs having the following compositions were prepared:
1 Component Weight % C .061 Mn 1.330 Si 1.101 Cr 0.817 P 0.015 S
0.004 Al 0.045 Ti 0.012 V 0.005 Mo 0.011 Cu 0.010 Ca 0.001 Fe
balance
[0051] Each of the steel slabs was reheated to about 1232.degree.
C. (2250F) and then held at this temperature for about 2 hours.
Subsequently, these slabs were hot rolled using hot rolling
termination temperatures (or finishing exiting temperatures) ranged
from 850.degree. C. (1562.degree. F.) to 950.degree. C.
(1742.degree. F.). Immediately after hot rolling, the hot rolled
sheets were water cooled at a runout table using cooling rates
ranging from 20.degree. C./sec (36.degree. F./sec) to 70.degree.
C./sec (126.degree. F./sec) down to various coiling temperature
ranging from 460.degree. C. (860.degree. F.) to 650.degree. C.
(1202.degree. F.). The final thickness of the hot rolled steel
sheets processed in this way ranged from 2.5 mm to 6.0 mm in order
to meet the specific requirements for different applications.
[0052] Test pieces were cut from the resulting hot rolled steel
sheets in a direction along the hot rolling direction, and then
machined into specimens for standard ASTM tensile testing to
measure the corresponding mechanical properties. The obtained
testing data have demonstrated that the mechanical properties,
especially tensile strength, depend largely on the employed coiling
temperature, as shown in FIG. 1. Therefore, a family of dual phase
hot rolled steel sheets (hot rolled DP 550, 600, and 800) were
produced from these slabs which have the same chemical compositions
by properly adjusting coiling temperature. Some typical mechanical
property results obtained according to standard ASTM tensile
testing on these hot roll steel sheets are presented in TABLE 1 in
the first order of product grade and second order of product
thickness in view of their final applications. As clearly
demonstrated in TABLE 1, several grades of hot rolled dual phase
(HR DP) steel sheets with excellent ductility or formability can be
readily manufactured using a single chemistry design through
properly adjusting the coiling temperature, which is based on the
present invention. This is also demonstrated by FIG. 1.
2TABLE 1 Coiling Coiling Yield Tensile Total Product Thickness
Temperature Temperature Strength Strength Elongation Grade (mm)
(.degree. C.) (.degree. F.) (MPa) (MPa) (%) HR DP 550 2.5 610 1130
397 577 30.2 3.7 607 1125 418 584 31.3 3.9 621 1150 389 564 31.1
5.5 629 1165 391 556 33.3 HR DP 600 3.7 532 990 418 625 28.9 5.5
502 935 397 662 32.5 HR DP 800 3.9 488 910 614 838 19.6
[0053] The total elongation values obtained during the above
standard ASTM tensile testing on the hot rolled dual phase steel
sheets with the final thickness of 2.5 mm, 3.7 mm, 3.9 mm, and 5.5
mm are also presented in FIG. 2 as a function of tensile strength.
The excellent total elongation-tensile strength relationship is
clearly revealed in this figure for different grades of these steel
sheet products, which further demonstrates the excellent
formability associated with these hot rolled steel sheets.
[0054] In view of application or customer requirements, more test
pieces were cut from some of the hot rolled steel sheets in a
direction perpendicular to the hot rolling direction, and then
machined into specimens for standard JIS No. 5 tensile testing to
measure the corresponding mechanical properties. Some typical
results obtained during these tests are given in TABLE 2, which
demonstrate again the excellent ductility or formability exhibited
by the dual phase hot rolled steel sheets produced based on the
present invention.
3TABLE 2 Coiling Coiling Yield Tensile Total Product Thickness
Temperature Temperature Strength Strength Elongation Grade (mm)
(.degree. C.) (.degree. F.) (MPa) (MPa) (%) HR DP 550 2.5 610 1130
414 578 36.0 5.5 629 1165 348 551 35.0 6.0 649 1200 351 577 32.5 HR
DP 600 3.5 538 1000 395 640 29.5 5.5 502 935 386 662 33.8
[0055] Square test specimens of about 80 mm by 80 mm were also cut
from some of these hot rolled dual phase steel sheets, and then
prepared for standard JSFT hole expansion testing to determine the
corresponding stretch flangeability. The obtained results on the
hot rolled dual phase steel sheets with the final thickness of 3.9
mm, 3.7 mm and 2.5 mm are given in FIG. 3 as a function of tensile
strength. As also compared in this figure, the obtained values of
hole expansion ration are much higher than those measured on the
conventional hot rolled dual phase steel sheets produced using the
prior methods, which demonstrates the excellent stretch
flangeability pertinent to the hot rolled dual phase steel
manufactured according to the present invention.
[0056] Finally, the microstructure of the presently invented hot
rolled steel sheets was examined. One of the typical micrographs
obtained using a "LePera" etching technique is given in FIG. 4. As
is illustrated by and can be readily observed in this micrograph,
fine martensite islands (white) are uniformly distributed in the
fine-grained ferrite matrix. It is such a dual phase structure that
provides not only the excellent formability but also the excellent
stretch flangeability.
* * * * *