Technical Field
[0001] The present invention relates to dual-phase type high-strength steel sheets, for
automobiles use, which have excellent dynamic deformation properties and exhibit excellent
impact absorption properties, and are intended to be used as structural members and
reinforcing materials primarily for automobiles, as well as to a method of producing
them.
Background Art
[0002] The applications of high-strength steels have been increasing for the purpose of
achieving lighter weight vehicle bodies in consideration of fuel consumption restrictions
on automobiles and even more applications for high-strength steel are expected as
domestic and foreign restrictions, relating to estimated impact absorption properties
in automobile accidents, become rapidly more broad and strict. For example, for frontal
collisions of passenger cars, the use of materials with high impact absorption properties
for members known as "front side members" can allow impact energy to be absorbed through
collapse of the member, thus lessening the impact experienced by passengers.
[0003] However, conventional high-strength steels have been developed with a main view toward
improving press formability, and doubts exist as to their application in terms of
impact absorption properties. Prior art techniques relating to automobile steel with
excellent impact absorption properties and methods of producing it have been developed
which result in increased yield strength of steel sheets under high deformation speeds
as an indicator of impact absorption properties, as disclosed in Japanese Unexamined
Patent Publication No. 7-18372, but because the members undergo deformation during
the shaping process or during collision deformation, it is necessary to include a
work-hardening aspect to the yield strength as an indicator of impact resistance,
and this is inadequate in tents of anti-collision safety in the prior art described
above.
[0004] In addition, since the strain rate undergone by each location upon automobile collision
reaches about 10
3 (s
-1), consideration of the impact absorption properties of the materials requires an
understanding of the dynamic deformation properties in such a high strain rate range.
Also, high-strength steel sheets with excellent dynamic deformation properties are
understood to be important for achieving both lighter weight and improved impact absorption
properties for automobiles, and recent reports have highlighted this fact. For example,
the present inventors have reported on the high strain rate properties and impact
energy absorption properties of high-strength thin steel sheets in CAMP-ISIJ Vol.9
(1966), pp.1112-1115, wherein they explain that the dynamic strength at a high strain
rate of 10
3 (s
-1) increases dramatically compared to the static strength at a low strain rate speed
of 10
-3 (s
-1), that absorption energy during crashes is increased by greater steel material strengths,
that the strain rate dependency of materials depends on the structure of the steel,
and that TRIP type steel (Transformation induced plasticity type steel) and dual-phase
(hereunder, "DP") type steel exhibit both excellent press formability and high impact
absorption properties. Also, the present inventors have already filed Japanese Patent
Applications No.8-98000 and No.8-109224 relating to such a DP-type steel, among which
there are proposed high-strength steel sheets with higher dynamic strength than static
strength, which are suitable for achieving both lighter weights and improved impact
absorption properties for automobiles, and a process for their production.
[0005] As mentioned above, although the dynamic deformation properties of high-strength
steel sheets are understood at the high strain rates of automobile collisions, it
is still unclear what properties should be maximized for automobile members with impact
energy absorption properties, and on what criteria the selection of materials should
be based. In addition, the automobile members are produced by press forming of steel
sheets, and collision impacts are applied to these press formed members. However,
high-strength steel sheets with excellent dynamic deformation properties as actual
members, based on an understanding of the impact energy absorption properties after
such press forming, are still unknown.
[0006] For press forming of members for collision safety, a combination of excellent shape
fixability, excellent stretchability (

) and excellent flangeability (hole expansion ratio ≤ 1.2) is desirable, but at the
current time no material has provided both excellent impact absorption properties
and excellent press formability.
Disclosure of the Invention
[0007] The present invention has been proposed as a means of overcoming the problems described
above, and provides dual-phase type high-strength steel sheets for automobiles use,
which have excellent impact absorption properties and excellent dynamic deformation
properties, as well as a method of producing them.
[0008] The invention further provides dual-phase type high-strength steel sheets, for automobiles,
with excellent dynamic deformation properties, which are high-strength steel sheets
used for automotive parts, such as front side members, and which are selected based
on exact properties and standards for impact energy absorption during collisions and
can reliably provide guaranteed safety, as well as a method of producing them.
[0009] The invention still further provides dual-phase type high-strength steel sheets for
automobiles with excellent dynamic deformation properties, which exhibit all the properties
suitable for press forming of members, including excellent shape fixability, excellent
stretchability and excellent flangeability, as well as a method of producing them.
[0010] The invention was devised to achieve the objects stated above by the following concrete
means.
(1) A dual-phase type high-strength steel sheets having high impact energy absorption
properties, characterized in that the final microstructure of the steel sheet is a
composite microstructure wherein the dominating phase is ferrite, and the second phase
is another low temperature product phase containing martensite at a volume fraction
between 3% and 50% after deformation at 5% equivalent strain of the steel sheet, wherein
the difference between the quasi-static deformation strength as when deformed in a
strain rate range of 5 x 10-4 - 5 x 10-3 (s-1) after pre-deformation of more than 0% and less than or equal to 10% of equivalent
strain, and the dynamic deformation strength σd when deformed in a strain rate range
of 5 x 102 - 5 x 103 (s-1) after the aforementioned pre-deformation, i.e. (σd - σs), is at least 60 MPa, and
the work hardening coefficient at 5∼10% strain is at least 0.13.
(2) A dual-phase type high-strength steel sheet having high impact energy absorption
properties, characterized in that the final microstructure of the steel sheet is a
composite microstructure wherein the dominating phase is ferrite, and the second phase
is another low temperature product phase containing martensite at a volume fraction
between 3% and 50% after deformation at 5% equivalent strain of the steel sheet, wherein
the average value σdyn (MPa) of the deformation stress in the range of 3∼10% of equivalent
strain when deformed in a strain rate range of 5 x 102 - 5 x 103 (s-1), after pre-deformation of more than 0% and less than or equal to 10% of equivalent
strain, satisfies the inequality:

as expressed in terms of the tensile strength TS (MPa) in the quasi-static tensile
test as measured in a strain rate range of 5 x 10-4 - 5 x 10-3 (s-1) prior to pre-deformation, and the work hardening coefficient at 5∼10% strain is
at least 0.13.
(3) A dual-phase type high-strength steel sheet having high impact energy absorption
properties according to (1) or (2) above, characterized in that the ratio between
the yield strength YS(0) and the tensile strength TS'(5) in the tensile test after
pre-deformation at 5% of equivalent strain or after further bake hardening treatment
(BH treatment) satisfies the inequality

, and also satisfies the inequality:

.
(4) A dual-phase type high-strength steel sheet having high impact energy absorption
properties according to any of (1), (2) or (3) above, characterized in that the average
grain size of the martensite is 5 µm or less, and the average grain size of the ferrite
is 10 µm or less.
(5) A dual-phase type high-strength steel sheet having high impact energy absorption
properties according to any of (1), (2), (3) or (4) above, characterized by satisfying
the inequality:

, and by satisfying the inequality: hole expansion ratio (d/d0) ≥ 1.2.
(6) A dual-phase type high-strength steel sheet having high impact energy absorption
properties according to any of (1), (2), (3), (4) or (5) above, characterized in that
the plastic deformation (T) by either or both a tempering rolling and a tension leveller
satisfies the following inequality.

(7) The dual-phase type high-strength steel sheet having high impact energy absorption
properties according to the invention is also a dual-phase type high-strength steel
sheet with excellent dynamic deformation properties according to (1) to (6) above,
characterized in that the chemical compositions, in terms of weight percentage, C
at 0.02∼0.25%, either or both Mn and Cr at a total of 0.15∼3.5%, one or more from
among Si, Al and P at a total of 0.02∼4.0%, if necessary one or more from among Ni,
Cu and Mo at a total of no more than 3.5%, one or more from among Nb, Ti and V at
no more than 0.30%, and either or both Ca and REM at 0.0005∼0.01% for Ca and 0.005∼0.05%
for REM, with the remainder Fe as the primary component.
(8) The dual-phase type high-strength steel sheet having high impact energy absorption
properties according to the invention is also a dual-phase type high-strength steel
sheet with excellent dynamic deformation properties according to (1) to (7) above,
characterized in that one or more from among B (≤0.01), S (≤0.01%) and N (≤0.02%)
are further added if necessary to the steel.
(9) The method of producing a dual-phase type high-strength hot-rolled steel sheet
having high impact energy absorption properties according to the invention is a method
of producing a dual-phase type high strength hot-rolled steel sheet with excellent
dynamic deformation properties according to (1) to (8) above, characterized in that
after a continuous casting slab is fed directly from casting to a hot rolling step,
or is hot rolled upon reheating after momentary cooling, it is subjected to hot rolling
at a finishing temperature of Ar3 - 50°C to Ar3 + 120°C, cooled at an average cooling rate of more than 5°C/sec in a run-out table,
and then coiled at a temperature of no greater than 350°C; and
(10) a method of producing a dual-phase high-strength hot-rolled steel sheet having
high impact energy absorption properties according to (9) above, characterized in
that at the finishing temperature for hot rolling in a range of Ar3 - 50°C to Ar3 + 120°C, the hot rolling is carried out so that the metallurgy parameter A satisfies
inequalities (1) and (2) below, the subsequent average cooling rate in the run-out
table is at least 5°C/sec, and the coiling is accomplished so that the relationship
between the above-mentioned metallurgy parameter A and the coiling temperature (CT)
satisfies inequality (3) below.

(11) The method of producing a dual-phase type high-strength cold rolled steel sheet
having high impact energy abosorption properties according to the invention is a method
of producing a dual-phase type high-strength cold rolled steel sheet with excellent
dynamic deformation properties according to (1) to (8) above, characterized in that
after a continuous cast slab is fed directly from casting to a hot rolling step, or
is hot rolled upon reheating after momentary cooling, it is hot rolled, the hot-rolled
and subsequently coiled steel sheet is cold-rolled after acid pickling, and during
annealing in a continuous annealing step for preparation of the final product, it
is heated to a temperature between Ac1 and Ac3 and subjected to the annealing while held in this temperature range for at least
10 seconds, and then cooled at a cooling rate of more than 5°C/sec; and
(12) a method according to (11) above for producing a dual-phase type high-strength
cold rolled steel sheet having high impact energy absorption properties according
to (1) to (8) above, characterized in that in the continuous annealing step, the cold
rolled steel sheet is heated to a temperature (To) between Ac1 and Ac3 and subjected to the annealing while held in this temperature range for at least
10 seconds, and for subsequent cooling, it is cooled to a secondary cooling start
temperature (Tq) in the range of 550°C-To at a primary cooling rate of 1∼10°C/sec
and then cooled to a secondary cooling end temperature (Te) which is no higher than
Tem determined by the chemical compositions and annealing temperature (To), at a secondary
cooling rate of 10∼200°C/sec.
Brief Description of the Drawings
[0011]
Fig. 1 is a graph showing the relationship between the absorption energy (Eab) of
a shaped member during collision and the material strength (S), according to the invention.
Fig. 2 is a perspective view of a shaped member for measurement of impact absorption
energy for Fig. 1.
Fig. 3 is a graph showing the relationship between the work hardening coefficient
and dynamic energy absorption for a steel sheet.
Fig. 4 is a graph showing the relationship between the yield strength × work hardening
coefficient and the dynamic energy absorption for a steel sheet.
Fig. 5 is a general view of a "hat model" used in the impact crush test method relating
to Figs. 3 and 4.
Fig. 6 is a cross-sectional view of the shape of the test piece of Fig. 5.
Fig. 7 is a schematic view of the impact crush test method relating to Figs. 3-6.
Fig. 8 is a graph showing the relationship between TS and the difference between the
average value σdyn of the deformation stress in the range of 3∼10% of equivalent strain
when deformed in a strain rate range of 5 x 102 - 5 x 103 (1/S) and TS, as an index of the impact energy absorption property upon collision,
according to the invention.
Fig. 9 is a graph showing the change in the static/dynamic ratio with tempered rolling
for an example of the invention and a comparative example.
Fig. 10 is a graph showing the relationship between ΔT and the metallurgy parameter
A for a hot-rolling step according to the invention.
Fig. 11 is a graph showing the relationship between the coiling temperature and the
metallurgy parameter A for a hot-rolling step according to the invention.
Fig. 12 is a graph showing the annealing cycle for continuous annealing according
to the invention.
Best Mode for Carrying Out the Invention
[0012] Impact absorbing members such as front side members of automobiles are produced by
bending and press forming of steel sheets. Because impacts during automobile collisions
are absorbed by such members which have undergone press forming, they must have high
impact absorption properties even after having undergone the pre-deformation corresponding
to the press forming. At the current time, however, no attempt has been made to obtain
high-strength steel sheets with excellent impact absorption properties as actual members,
with consideration of both the increase in the deformation stress by press forming
and the increase in deformation stress due to a higher strain rate, as was mentioned
above.
[0013] As a result of much experimentation and research with the aim of achieving this purpose,
the present inventors have found that steel sheets with a dual-phase (DP) structure
are ideal as high-strength steel sheets with excellent impact absorption properties
for actual members which are press formed as described above. It was demonstrated
that such steel sheets with a dual-phase microstructure, which is a composite microstructure
wherein the dominating phase is a ferrite phase responsible for the increase in deformation
resistance by an increased strain rate, and the second phase includes a hard martensite
phase, have excellent dynamic deformation properties. That is, it was found that high
dynamic deformation properties are exhibited when the microstructure of the final
steel sheets is a composite structure wherein the dominating phase is ferrite and
another low temperature product phase includes a hard martensite phase at a volume
fraction of 3∼50% after deformation at 5% equivalent strain of the steel sheet.
[0014] Concerning the volume fraction of 3∼50% for the hard martensite phase, since high-strength
steel sheets and even steel sheets with high dynamic deformation properties cannot
be obtained if the martensite phase is less than 3%, the volume fraction of the martensite
phase must be at least 3%. Also, if the martensite phase exceeds 50%, this results
in a smaller volume fraction of the ferrite phase responsible for greater deformation
resistance due to increased deformation speed, making it impossible to obtain steel
sheets with excellent dynamic deformation properties compared to static deformation
strength while also hindering press formability, and therefore it was found that the
volume fraction of the martensite phase must be 3∼50%.
[0015] The present inventors then pursued experimentation and research based on these findings
and, as a result, found that although the degree of pre-deformation corresponding
to press forming of impact absorbing members such as front side members sometimes
reaches a maximum of over 20%, depending on the location, the majority are locations
with 0%∼10% of equivalent strain, and that by understanding the effect of pre-deformation
in this range, it is possible to estimate the behavior of the member as a whole after
pre-deformation. Consequently, according to the invention, a deformation of from 0%
to 10% of equivalent strain was selected as the amount of pre-deformation applied
to members during press forming.
[0016] Fig. 1 is a graph showing the relationship between the absorption energy (Eab) of
a press formed member during collision and the material strength (S), for the different
steel types shown in Table 5, according to an example to be described later. The material
strength S is the tensile strength (TS) according to the common tensile test. The
member absorption energy (Eab) is the absorption energy in the lengthwise direction
(direction of the arrow) along a press formed member such as shown in Fig. 2, upon
collision with a 400 kg mass weight at a speed of 15 m/sec, to a crushing degree of
100 mm. The shaped member in Fig. 2 consists of a 2.0 mm-thick steel sheet formed
into a hat-shaped section 1 with a steel sheet 2 of the same thickness and the same
type of steel, joined together by spot welding, the hat-shaped section 1 having a
corner radius of 2 mm, and with spot welding points indicated by 3.
[0017] From Fig. 1 it is seen that the member absorption energy (Eab) tends to increase
with the strength of materials under normal tensile testing, though with considerable
variation. Here, the materials in Fig. 1 were subjected to pre-deformation of more
than 0% and less than or equal to 10% of equivalent strain, and then the static deformation
strength σs when deformed in a strain rate range of 5 x 10
-4 - 5 x 10
-3 (s
-1) and the dynamic deformation strength σd when deformed in a strain rate range of
5 x 10
2 - 5 x 10
-3 (s
-1) after the pre-deformation, were measured. As a result, a classification was possible
based on (σd - σs). The symbols plotted in Fig. 1 were as follows:
- ○:
- (σd - σs) < 60 MPa with any pre-deformation of more than 0% and less than or equal
to 10%;
- •:
- 60 MPa ≤ (σd - σs) with any pre-deformation in the above range, and 60 MPa ≤ (σd -
σs) < 80 MPa with pre-deformation of 5%;
- ■:
- 60 MPa ≤ (σd - σs) with any pre-deformation in the above range, and 80 MPa ≤ (σd -
σs) < 100 MPa with pre-deformation of 5%;
- ▲:
- 60 MPa ≤ (σd - σs) with any pre-deformation in the above range, and 100 MPa ≤ (σd
- σs) with pre-deformation of 5%.
[0018] Also, when 60 MPa ≤ (σd - σs) with any pre-deformation in the range of more than
0% and less than or equal to 10% of equivalent strain, the values for member absorption
energy (Eab) during collision was equal to or greater than the values predicted from
the material strength S, thus indicating steel sheets with excellent dynamic deformation
properties as impact absorbing members for collision. These predicted values are those
shown in the curve in Fig. 1, represented by Eab = 0.062S
0.8. Consequently, (σd - σs) must be at least 60 MPa.
[0019] For improved impact absorption properties, it is basically important to increase
the work hardening coefficient, specifically to at least 0.13, and preferably at least
0.16; by controlling the yield strength and the work hardening coefficient to specified
ranges it is possible to achieve excellent impact absorption properties, and for improved
press formability it is effective to design the volume percentage and particle size
of the martensite to within a specified range.
[0020] Fig. 3 shows the relationship between the work hardening coefficient of a steel sheet
and the dynamic energy absorption which indicates the member impact absorption properties,
for a class of materials with the same yield strength. Here it is shown that increased
work hardening coefficients of the steel sheets result in improved member impact absorption
properties (dynamic energy absorption), and that the work hardening coefficient of
a steel sheet can properly indicate the member impact absorption properties so long
as the yield strength class is the same. Also, when the yield strengths differ, as
shown in Fig. 4, the yield strength × work hardening coefficient can be an indicator
of the member impact absorption properties. While the work hardening coefficient was
expressed in terms of an n value of 5%∼10% strain in consideration of the strain undergone
by members during press forming, from the viewpoint of improving the dynamic energy
absorption, work hardening coefficients of under 5% strain or work hardening coefficients
of even more than 10% strain may be preferred.
[0021] The dynamic energy absorptions for members shown in Fig. 3 and Fig. 4 were determined
in the following manner. Specifically, the steel sheet was shaped into the member
shape shown in Fig. 6 (corner R = 5 mm) and spot welded at 35 mm pitch using an electrode
with a tip radius of 5.5 mm at a current of 0.9 times the expulsion current, and then
after baking and painting treatment at 170°C x 20 minutes, an approximately 150 Kg
falling weight was dropped from a height of about 10 m to crush the member in its
lengthwise direction, and the displacement work where displacement = 0-150 mm is calculated
from the area of the corresponding load displacement diagram to determine the dynamic
energy absorption. A schematic illustration of this test method is shown in Fig. 7.
In Fig. 5, 4 is a worktop, 5 is a test piece and 6 is a spot welding section.
[0022] In Fig. 6, 7 is a hat-shaped test piece and 8 is a spot welding section. In Fig.
7, 9 is a worktop, 10 is a test piece, 11 is a falling weight (150 kg), 12 is a frame,
and 13 is a shock absorber. The work hardening coefficient and yield strength of each
steel sheet was determined in the following manner. The steel sheet was shaped into
a JIS-#5 test piece (gauge length: 50 mm, parallel width: 25 mm), subjected to tensile
test at a strain rate of 0.001 (s
-1) to determine the yield strength and work hardening coefficient (n value at 5%∼10%
strain). The steel sheet used had a sheet thickness of 1.2 mm and the steel sheet
composition contained C at 0.02∼0.25 wt%, either or both Mn and Cr at a total of 0.15∼3.5
wt% and one or more of Si, Al and P at a total of 0.02∼4.0 wt%, with the remainder
Fe as the main component.
[0023] Fig. 8 is a graph showing the relationship between the average value σdyn of the
deformation stress in the range of 3∼10% of equivalent strain when deformed in a strain
rate range of 5 x 10
2 - 5 x 10
3 (s
-1) and the static material strength (TS), as an index of the impact energy absorption
property upon collision according to the invention, where the static material strength
(TS) is the tensile strength (TS: MPa) in the static tensile test as measured in a
strain rate range of 5 x 10
-4 - 5 x 10
-3 (s
-1).
[0024] As mentioned above, impact absorbing members such as front side members have a hat-shaped
cross-sectional shape, and as a result of analysis of deformation of such members
upon crushing by high-speed collision, the present inventors have found that despite
deformation proceeding up to a high maximum strain of over 40%, at least 70% of the
total absorption energy is absorbed in a strain range of 10% or lower in a high-speed
stress-strain diagram. Therefore, the dynamic deformation resistance with high-speed
deformation at 10% or lower was used as the index of the high-speed collision energy
absorption property. In particular, since the amount of strain in the range of 3∼10%
is most important, the index used for the impact energy absorption property was the
average stress: σdyn in the range of 3∼10% of equivalent strain when deformed in a
strain rate range of 5 x 10
2 -5 x 10
3 (s
-1) high-speed tensile deformation.
[0025] The average stress: σdyn of 3∼10% upon high-speed deformation generally increases
with increasing static tensile strength {maximum stress (TS: MPa) in a static tensile
test measured in a stress rate range of 5 x 10
-4 - 5 x 10
-3 (s
-1)} of the steel material prior to pre-deformation or baking treatment. Consequently,
increasing the static tensile strength (which is synonymous with the static material
strength) of the steel material directly contributes to an improved impact energy
absorption property of the member. However, increased strength of the steel results
in poorer press formability into members, making it difficult to obtain members with
the necessary shapes. Consequently, steels having a high σdyn with the same tensile
strength TS are preferred. It was found that, based on this relationship, steel sheets
wherein the average value σdyn (MPa) of the deformation stress in the range of 3∼10%
of equivalent strain when deformed in a strain rate range of 5 x 10
2 - 5 x 10
3 (s
-1), after pre-deformation of more than 0% and less than or equal to 10% of equivalent
strain satisfies the inequality:

as expressed in terms of the tensile strength (TS: MPa) in the static tensile test
as measured in a strain rate range of 5 x 10
-4 - 5 x 10
-3 (s
-1) prior to pre-deformation, have higher impact energy absorption properties as actual
members compared to other steels, and that the impact energy absorption property is
improved without increasing the overall weight of the member, making it possible to
provide high-strength steel sheets with high dynamic deformation resistance.
[0026] Also, although the details are still unclear, it has been discovered that steel sheets
with excellent dynamic deformation properties can be obtained when, as shown in Fig.
9, YS(0)/TS'(5) is no greater than 0.7, which amount is dependent on the initial microstructure,
the amount of solid solution elements in the low temperature product phase other than
the martensite phase and the main ferrite phase, and the deposited state of carbides,
nitrides and carbonitrides. Here, YS(0) is the yield strength, and TS'(5) is the tensile
strength (TS') in the static tensile test with pre-deformation at 5% of equivalent
strain or after further bake hardening treatment (BH treatment). It was also demonstrated
that steel sheets with even more excellent dynamic deformation properties can be obtained
when the yield strength: YS(0) × work hardening coefficient is at least 70.
[0027] Furthermore, it is known that dynamic deformation strength is usually expressed in
the form of the power of the static tensile strength, and as the static tensile strength
increases, the difference, between the dynamic deformation strength and the static
deformation strength decreases. However, a small difference between the dynamic deformation
strength and the static deformation strength will mean that no greater improvement
in the impact absorption properties can be expected. From this standpoint, it is preferred
for the value of (σd - σs) to be in a range which satisfies the following inequality,
(σd - σs) ≥ 4.1 x σs
0.8 - σs.
[0028] The microstructure of a steel sheet according to the invention will now be described
in detail. As already mentioned, the martensite is at a volume fraction of 3∼50%,
and preferably 3∼30%. The average grain size of the martensite is preferably no greater
than 5 µm, and the average grain size of the ferrite is preferably no greater than
10 µm. That is, the martensite is hard, and contributes to a decrease in the yield
ratio and an improvement in the work hardening coefficient, by producing a mobile
dislocations primarily in adjacent ferrite grains; however, by satisfying the restrictions
mentioned above it is possible to disperse fine martensite in the steel, so that the
improvement in the properties spreads throughout the entire steel sheet. In addition,
this dispersion of fine martensite in the steel can help to avoid deterioration in
the hole expansion ratio and tensile strength × total elongation, which is an adverse
effect of the hard martensite. Also, because it is possible to reliably achieve work
hardening coefficient ≥ 0.130, tensile strength × total elongation ≥ 18,000 and hole
expansion ratio ≥ 1.2, it is thereby possible to improve the impact absorption properties
and press formability.
[0029] With a martensite volume fraction of less than 3%, the yield ratio becomes larger
while the press formed member cannot exhibit an excellent work hardening property
(work hardening coefficient ≥ 0.130) after it has undergone collision deformation,
and since the deformation resistance (load) stays at a low level, and the dynamic
energy absorption is low preventing improvement in the impact absorption properties.
On the other hand, with a martensite volume fraction of greater than 50%, the yield
ratio becomes larger while work hardening coefficient is reduced, and deterioration
also occurs in the tensile strength × total elongation and the hole expansion ratio.
From the standpoint of press formability, the volume fraction of the martensite is
preferred to be no greater than 30%.
[0030] Also, the ferrite is present at a volume fraction of preferably at least 50%, and
more preferably at least 70%, and its average grain size (mean circle equivalent diameter)
is preferably no greater than 10 µm, and more preferably no greater than 5 µm, with
the martensite preferably adjacent to the ferrite. This aids the fine dispersion of
the martensite in the ferrite matrix, while effectively extending the property-improving
effect, beyond simply a local effect, to the entire steel sheet, favorably acting
to prevent the adverse effects of the martensite. The structure of the remainder present
with the martensite and ferrite may be a mixed structure comprising a combination
of one or more from among pearlite, bainite, retained γ, etc., and although primarily
bainite is preferred in cases which require hole expansion properties, since retained
γ undergoes work-induced transformation into martensite by press forming, experimental
results have shown that including retained austenite prior to press forming has an
effect even in preferred small amounts (5% or less).
[0031] Also, from the standpoint of impact absorption properties and press formability it
is preferred for the ratio of the martensite and ferrite particle sizes to be no greater
than 0.6, and the ratio of the hardnesses to be at least 1.5.
[0032] The restrictions on the values for the chemical components of dual-phase type high-strength
steel sheets with excellent dynamic detonation properties according to the invention,
and the reasons for those restrictions, will now be explained.
[0033] Dual-phase type high-strength steel sheets with excellent dynamic detonation properties
which are used according to the invention are steel sheets containing the following
chemical compositions, in terms of weight percentage: C at 0.02∼0.25%, either or both
Mn and Cr at a total of 0.15∼3.5%, one or more from among Si, Al and P at a total
of 0.02∼4.0%, if necessary also one or more from among Ni, Cu and Mo at a total of
no more than 3.5%, one or more from among Nb, Ti and V at no more than 0.30%, and
either or both Ca and REM at 0.0005∼0.01% for Ca and 0.005∼0.05% for REM, with the
remainder Fe as the primary component. They are also dual-phase type high strength
steel sheets with excellent dynamic deformation properties which contain, if necessary,
one or more from among B (≤0.01), S (≤0.01%) and N (≤0.02%). These chemical components
and their contents (percent by weight) will now be discussed.
C: C is the element which most strongly affects the microstructure of the steel sheet,
and if its content is too low it will become difficult to obtain martensite with the
desired amount and strength. Addition in too great an amount leads to unwanted carbide
precipitation, inhibited increase in deformation resistance at higher strain rates
and overly high strength, as well as poor press formability and weldability; the content
is therefore 0.02∼0.25 wt%.
Mn, Cr: Mn and Cr have an effect of stabilizing austenite and guaranteeing sufficient
martensite, and are also solid solution hardening elements; they must therefore be
added in a minimum amount of 0.15 wt%, but if added in too much the aforementioned
effect becomes saturated thus producing adverse effects such as preventing ferrite
transformation, and thus they are added in the maximum amount of 3.5 wt%.
Si, Al, P: Si and Al are useful elements for producing martensite, and they promote
production of ferrite and suppress precipitation of carbides, thus having the effect
of guaranteeing sufficient martensite, as well as a solid solution hardening effect
and a deoxidization effect. P can also promote martensite formation and solid solution
hardening, similar to Al and Si. From this standpoint, the minimum amount of Si +
Al + P added must be at least 0.02 wt%. On the other hand, excessive addition will
saturate this effect and result instead in brittleness, and therefore the maximum
amount of addition is no more than 4.0 wt%. In particular, when an excellent surface
condition is required, Si scales can be avoided by adding Si at no greater than 0.1
wt%, and conversely by adding it at 1.0 wt% or greater Si scales can be produced over
the entire surface so that they are not conspicuous. Also, when excellent secondary
workability, toughness, spot weldability and recycling properties are required, the
P content may be kept at no greater than 0.05%, and preferably no greater than 0.02%.
Ni, Cu, Mo: These elements are added when necessary, and are austenite-stabilizing
elements similar to Mn, which increase the hardenability of the steel, and are effective
for adjustment of the strength. From the standpoint of weldability and chemical treatment,
they can be used when the amounts of C, Si, Al and Mn are restricted, but if the total
amount of these elements added exceeds 3.5 wt% the dominant ferrite phase will tend
to be hardened, thus inhibiting the increase in deformation resistance by a greater
strain rate, as well as raising the cost of the steel sheet; the amount of these elements
added is therefore 3.50 wt% or lower.
Nb, Ti, V: These elements are added when necessary, and are effective for strengthening
the steel sheet through formation of carbides, nitrides and carbonitrides. However,
when added at greater than 0.3 wt% they are deposited in large amounts in the dominant
ferrite phase or at the grain boundaries as carbides, nitrides and carbonitrides,
becoming a source of the mobile dislocation during high speed deformation, and inhibiting
the increase in deformation resistance by greater strain rates. In addition, the deformation
resistance of the dominant phase becomes higher than necessary, thus wasting the C
and leading to higher costs; the maximum amount to be added is therefore 0.3 wt%.
B: B is an element which is effective for strengthening since it improves the hardenability
of the steel by suppressing production of ferrite, but if it is added at greater than
0.01 wt% its effect will be saturated, and therefore B is added at a maximum of 0.01
wt%.
Ca, REM: Ca is added to at least 0.0005 wt% for improved press formability (especially
hole expansion ratio) by shape control (spheroidization) of sulfide-based inclusions,
and the maximum amount thereof to be added is 0.01 wt% in consideration of effect
saturation and the adverse effect due to increase in the aforementioned inclusions
(reduced hole expansion ratio). For the same reasons, REM is added in an amount of
from 0.005% to 0.05 wt%.
S: The amount of S is no greater than 0.01 wt%, and preferably no greater than 0.003
wt%, from the standpoint of press formability (especially hole expansion ratio) by
sulfide-based inclusions, and reduced spot weldability.
[0034] The method of applying the pre-deformation according to the invention will now be
explained. The pre-deformation may be press forming for member shaping, or it may
be working with a tempering rolling or tension leveler which applied to the steel
sheet material prior to its press forming. In this case, either or both a tempering
roller and tension leveler may be used. That is, the means used may include a tempering
rolling, a tension leveler, or a tempering roller and tension leveler. The steel sheet
material may also be subjected to press forming after being worked with a tempering
rolling or tension leveler. The amount of pre-deformation applied with the tempering
rolling and/or tension leveler, i.e. the degree of plastic deformation (T), will differ
depending on the initial dislocation density, and T should be small if the initial
density is large. Also, with few solid solution elements the introduced dislocations
cannot be fixed, and high dynamic deformation properties cannot be guaranteed. Consequently,
it was found that the plastic deformation (T) is determined based on the ratio between
the yield strength YS(0) and the tensile strength TS'(5) in the static tensile test
with pre-deformation at 5% of equivalent strain or after further bake hardening treatment
(BH treatment), or YS(0)/TS'(5). That is, YS(0)/TS'(5) is an indicator of the sum
of the initial dislocation density and the dislocation density introduced by 5% deformation,
and the amount of the solid solution elements; it may be concluded that a smaller
YS(0)/TS'(5) means a higher initial dislocation density and more of the solid solution
elements. YS(0)/TS'(5) is therefore no greater than 0.7, and is preferably provided
according to the following equation:

wherein the upper limit for T is determined from the standpoint of press formability
including impact absorption property and flexibility.
[0035] A method of producing a dual-phase type high strength hot rolled steel sheet and
a cold rolled steel sheet with excellent dynamic deformation properties according
to the invention will now be explained. In this production method, a continuous cast
slab is fed directly from casting to a hot rolling step, or is hot rolled upon reheating
after momentary cooling. Thin gauge continuous casting and continuous hot rolling
techniques (endless hot rolling) may be applied for the hot rolling in addition to
normal continuous casting, but in order to avoid a lower ferrite volume fraction and
a coarser average grain size of the thin steel sheet microstructure, the bar (cast
strip) thickness at the hot rolling approach side (the initial steel bar thickness)
is preferred to be at least 25 mm. At less than 25 mm, the mean circle equivalent
size of ferrite of the steel sheet is made coarser, while it is also a disadvantage
against obtaining the desired martensite. The final pass rolling speed for the hot
rolling is preferred to be at least 500 mpm and more preferably at least 600 mpm,
in light of the problems described above. At less than 500 mpm, the mean circle equivalent
diameter of ferrite of the steel sheet is made coarser, while it is also a disadvantage
against obtaining the desired martensite.
[0036] The finishing temperature for the hot rolling is from Ar
3 - 50°C to Ar
3 + 120°C. At lower than Ar
3 - 50°C, deformed ferrite is produced, with inferior work hardening property and press
formability. At higher than Ar
3 + 120°C, and the mean circle equivalent size of ferrite of the steel sheet is made
coarser, while it is also becomes difficult to obtain the desired martensite.
[0037] The average cooling rate for cooling in the run-out table is at least 5°C/sec. At
less than 5°C/sec it becomes difficult to obtain the desired martensite.
[0038] The coiling temperature is no higher than 350°C. At higher than 350°C it becomes
difficult to obtain the desired martensite.
[0039] According to the invention, it was found particularly that a correlation exists between
the finishing temperature in the hot rolling step, the finishing approach temperature
and the coiling temperature. That is, as shown in Fig. 10 and Fig. 11, specific conditions
exist which are determined primarily between the finishing temperature, finishing
approach temperature and the coiling temperature. Specifically, the hot rolling is
carried out so that when the finishing temperature for hot rolling is in the range
of Ar
3 - 50°C to Ar
3 + 120°C, the metallurgy parameter A satisfies inequalities (1) and (2). The above-mentioned
metallurgy parameter A may be expressed by the following equation.

where
FT: finishing temperature (°C)
Ceq: carbon equivalents = C + Mneq/6 (%)
Mneq: manganese equivalents =

ε*: final pass strain rate (s-1)

h1: final pass approach sheet thickness
h2: final pass exit sheet thickness
r : (h1 - h2)/h1
R : roll radius
v : final pass exit speed
ΔT: finishing temperature (finishing final pass exit temperature) - finishing approach
temperature (finishing first pass approach temperature)
Ar3: 901 - 325 C% + 33 Si% - 92 Mneq
[0040] Thereafter, it is preferred for the average cooling rate on the run-out table to
be at least 5°C/sec, and the coiling to be carried out under conditions such that
the relationship between the metallurgy parameter A and the coiling temperature (CT)
satisfies inequality (3).

[0041] In inequality (1) above, a log A of less than 9 is unacceptable from the viewpoint
of production of retained martensite and refinement of the microstructure, while it
will also result in an inferior dynamic deformation resistance σdyn and 5∼10% work
hardening property. Also, if log A is to be greater than 18, massive equipment will
be required to achieve it. With inequality (2), if the condition of inequality (2)
is not satisfied it will be impossible to obtain the desired martensite, and the dynamic
deformation resistance σdyn and 5∼10% work hardening property, etc. will be inferior.
The lower limit for ΔT is more flexible with a lower log A as indicated by inequality
(2). Furthermore, if the relationship with the coiling temperature in inequality (3)
is not satisfied, there will be an adverse effect on ensuring the amount of martensite,
while the retained γ will be excessively stable even if retained γ can be obtained,
it will be impossible to obtain the desired martensite during deformation, and the
dynamic deformation resistance σdyn and 5∼10% work hardening property, etc. will be
inferior. The limit for the coiling temperature is more flexible with a higher log
A.
[0042] The cold rolled sheet according to the invention is then subjected to the different
steps following hot-rolling and coiling and is cold rolled and subjected to annealing.
The annealing is ideally continuous annealing through an annealing cycle such as shown
in Fig. 12, and during the annealing of the continuous annealing step, it must be
kept for at least 10 seconds in the temperature range of Ac
1 - Ac
3. At less than Ac
1 austenite will not be produced and it will therefore be impossible to obtain martensite
thereafter, while at greater than Ac
3 the austenite monophase structure will be coarse, and it will therefore be impossible
to obtain the desired average grain size for the martensite. Also, at less than 10
seconds the austenite production will be insufficient, making it impossible to obtain
the desired martensite thereafter. The maximum residence time is preferably no greater
than 200 seconds, from the standpoint of avoiding addition to the equipment and coarsening
of the microstructure. The cooling after this annealing must be at an average cooling
rate of at least 5°C/sec. At less than 5°C/sec the desired space factor for the martenseite
cannot be achieved. Although there is no particular upper limit here, it is preferably
300°C/sec when considering temperature control during the cooling.
[0043] According to the invention, the cooled steel sheet is heated to a temperature To
from Ac
1 - Ac
3 in the continuous annealing cycle shown in Fig. 12, and cooled under cooling conditions
provided by a method wherein cooling to a secondary cooling start temperature Tq in
the range of 550°C-To at the primary cooling rate of 1∼10°C/sec is followed by cooling
to a secondary cooling end temperature Te which is no higher than a temperature Tem
which is determined by the chemical compositions of the steel and annealing temperature
To, at a secondary cooling rate of 10∼200°C/sec. This is a method whereby the cooling
end temperature Te in the continuous annealing cycle shown in Fig. 12 is represented
as a function of the chemical compositions and annealing temperature, and is kept
under a given critical value. After cooling to Te, the temperature is preferably held
in a range of Te - 50°C to 400°C for up to 20 minutes prior to cooling to room temperature.
[0044] Here, Tem is the martensite transformation start temperature for the retained austenite
at the quenching start point Tq. That is, Tem is defined by Tem = T1 - T2, or the
difference between the value excluding the effect of the C concentration in the austenite
(T1) and the value indicating the effect of the C concentration (T2). Here, T1 is
the temperature calculated from the solid solution element concentration excluding
C, and T2 is the temperature calculated from the C concentration in the retained austenite
at Ac
1 and Ac
3 determined by the chemical compositions of the steel and Tq determined by the annealing
temperature To. Ceq* represents the carbon equivalents iii the retained austenite
at the annealing temperature To. Thus, T1 is expressed as:

and T2 is expressed in terms of:

and the annealing temperature To, and when

is greater than 0.6,

,
and when it is 0.6 or less,

.
[0045] In other words, when Te is equal to or greater than Tem, the desired martensite cannot
be obtained. Also, if Toa is 400°C or higher, the martensite obtained by cooling is
tempered, making it impossible to achieve satisfactory dynamic properties and press
formability. On the other hand, if Toa is less than Te - 50°C, additional cooling
equipment is necessary, and greater variation will result in the material due to the
difference between the temperature of the continuous annealing furnace and the temperature
of the steel sheet; this temperature was therefore determined as the lower limit.
Also, the upper limit for the holding time was determined to be 20 minutes, because
when it is longer than 20 minutes it becomes necessary to expand the equipment.
[0046] By employing the chemical composition and production method described above, it is
possible to produce a dual-phase type high-strength steel sheet with excellent dynamic
deformation properties, wherein the microstructure of the steel sheet is a composite
microstructure wherein the dominating phase is ferrite, and the second phase is another
low temperature product phase containing martensite at a volume fraction from 3%∼50%
after shaping and working at 5% equivalent strain, and wherein the difference between
the quasi-static deformation strength σs when deformed in a strain rate range of 5
x 10
-4 - 5 x 10
-3 (1/s) after pre-deformation of more than 0% and less than or equal to 10% of equivalent
strain, and the dynamic deformation strength σd measured in a strain rate range of
5 x 10
2 - 5 x 10
3 (1/s) after the aforementioned pre-deformation, i.e. (σd - σs), is at least 60 MPa,
and the work hardening coefficient at 5∼10% strain is at least 0.13. The steel sheets
according to the invention may be made into any desired product by annealing, tempering
rolling, electronic coating or hot-dip coating.
Examples
[0047] The present invention will now be explained by way of examples.
(Example 1)
(Example 2)
[0049] The 22 steel materials listed in Table 5 (steel nos. 27∼48) were heated to 1050∼1250°C
and subjected to hot rolling, cooling and coiling, followed by acid pickling and then
cold rolling under the conditions listed in Table 6 to produce cold rolled steel sheets.
Temperatures Ac
1 and Ac
3 were then calculated from the chemical compositions for each steel, and the sheets
were subjected to heating, cooling and holding under the annealing conditions listed
in Table 6, prior to cooling to room temperature. As shown in Table 7, the steel sheets
satisfying the chemical composition conditions and production conditions according
to the invention have a dual-phase structure with a martensite volume fraction of
at least 3% and no greater than 50% and, as shown in Fig. 8, the mechanical properties
of the hot-rolled steel sheets indicated excellent impact absorption properties as
represented by a work hardening coefficient of at least 0.13 at 5∼10% strain,

, and

, while also having suitable press formability and weldability.
Table 7
Microstructure of steels |
Steel No. |
Dominant phase |
Ferrite |
Martensite |
|
Phase |
Circle equivalent diameter (µm) |
Volume fraction (%) |
Circle equivalent diameter (µm) |
Volume fraction after 5% working (%) |
27 |
ferrite |
9.8 |
100 |
-- |
0 |
28 |
ferrite |
6.4 |
86 |
3.2 |
12 |
29 |
ferrite |
6.4 |
95 |
-- |
1 |
30 |
ferrite |
6.4 |
94 |
-- |
0 |
31 |
ferrite |
5.3 |
89 |
3.1 |
11 |
32 |
ferrite |
4.8 |
82 |
2.8 |
15 |
33 |
ferrite |
5.1 |
84 |
2.9 |
12 |
34 |
ferrite |
4.8 |
75 |
2.2 |
18 |
35 |
ferrite |
5.1 |
90 |
2.3 |
10 |
36 |
ferrite |
5.5 |
90 |
2.8 |
8 |
37 |
ferrite |
6.2 |
89 |
3.1 |
11 |
38 |
ferrite |
5.8 |
81 |
3.0 |
16 |
39 |
ferrite |
5.6 |
78 |
3.2 |
18 |
40 |
ferrite |
5.6 |
87 |
3.2 |
13 |
41 |
ferrite |
4.2 |
80 |
1.7 |
16 |
42 |
ferrite |
4.5 |
78 |
2.1 |
18 |
43 |
ferrite |
4.3 |
79 |
2.2 |
19 |
44 |
ferrite |
5.0 |
79 |
2.3 |
13 |
45 |
ferrite |
4.9 |
81 |
2.1 |
1 |
46 |
ferrite |
4.1 |
42 |
2.4 |
35 |
47 |
ferrite |
4.6 |
51 |
2.6 |
25 |
48 |
ferrite |
5.6 |
88 |
2.6 |
12 |
Underlined data indicate values outside of the range of the invention. |

[0050] The microstructure was evaluated by the following method.
[0051] Identification of the ferrite, bainite, martensite and residual structure, observation
of the location and measurement of the average grain size (mean circle equivalent
diameter) was accomplished using a 1000 magnification optical micrograph with the
thin steel sheet rolling direction cross-section etched with a nital and the reagent
disclosed in Japanese Unexamined Patent Publication No. 59-219473.
[0052] The properties were evaluated by the following methods.
A tensile test was conducted according to JIS5 (gauge mark distance: 50 mm, parallel
part width: 25 mm) with a strain rate of 0.001/s and, upon determining the tensile
strength (TS), yield strength (YS), total elongation (T. El) and work hardening coefficient
(n value for 1%∼5% strain), the YS x work hardening coefficient and TS × T. El. were
calculated.
The stretch flanging property was measured by expanding a 20 mm punched hole from
the burrless side with a 30° cone punch, and determining the hole expansion ratio
(d/d0) between the hole diameter (d) at the moment at which the crack penetrated the plate
thickness and the original hollow diameter (d0, 20 mm).
The spot weldability was judged to be unsuitable if a spot welding test piece bonded
at a current of 0.9 times the expulsion current using an electrode with a tip radius
of 5 times the square root of the steel sheet thickness underwent peel fracture when
ruptured with a chisel.
Industrial Applicability
[0053] As explained above, the present invention makes it possible to provide, in an economical
and stable manner, high-strength hot rolled steel sheets and cold rolled steel sheets
for automobiles which provide previously unobtainable excellent impact absorption
properties and press formability and thus offers a markedly wider range of objects
and conditions for uses of high-strength steel sheets.
1. A dual-phase type high-strength steel sheets having high impact energy absorption
properties, characterized in that the final microstructure of the steel sheet is a
composite microstructure wherein the dominating phase is ferrite, and the second phase
is another low temperature product phase containing martensite at a volume fraction
between 3% and 50% after deformation at 5% equivalent strain of the steel sheet, wherein
the difference between the quasi-static deformation strength σs when deformed in a
strain rate range of 5 x 10-4 - 5 x 10-3 (s-1) after pre-deformation of more than 0% and less than or equal to 10% of equivalent
strain, and the dynamic deformation strength σd when deformed in a strain rate range
of 5 x 102 - 5 x 103 (s-1) after said pre-deformation, i.e. (σd - σs), is at least 60 MPa, and the work hardening
coefficient at 5∼10% strain is at least 0.13.
2. A dual-phase type high-strength steel sheet having high impact energy absorption properties,
characterized in that the final microstructure of the steel sheet is a composite microstructure
wherein the dominating phase is ferrite, and the second phase is another low temperature
product phase containing martensite at a volume fraction between 3% and 50% after
deformation at 5% equivalent strain of the steel sheet, wherein the average value
σdyn (MPa) of the deformation stress in the range of 3∼10% of equivalent strain when
deformed in a strain rate range of 5 x 10
2 - 5 x 10
3 (s
-1), after pre-deformation of more than 0% and less than or equal to 10% of equivalent
strain, satisfies the inequality:

as expressed in terms of the tensile strength TS (MPa) in the quasi-static tensile
test as measured in a strain rate range of 5 x 10
-4 - 5 x 10
-3 (s
-1) prior to pre-deformation, and the work hardening coefficient at 5∼10% strain is
at least 0.13.
3. A dual-phase type high-strength steel sheet having high impact energy absorption properties
according to claim 1 or 2, characterized in that the ratio between the yield strength
YS(0) and the tensile strength TS'(5) in the static tensile test after pre-deformation
at 5% of equivalent strain or after further bake hardening treatment (BH treatment)
satisfies the inequality

, and also satisfies the inequality:

.
4. A dual-phase type high-strength steel sheet having high impact energy absorption properties
according to any of claims 1, 2 or 3, characterized in that the average grain size
of martensite is 5 µm or less, and the average grain size of ferrite is 10 µm or less.
5. A dual-phase type high-strength steel sheet with excellent dynamic deformation properties
according to any of claims 1 to 4, characterized by satisfying the inequality:

, and by satisfying the inequality:

.
6. A dual-phase type high-strength steel sheet having high impact energy absorption properties
according to any of claims 1 to 5, characterized in that the plastic deformation (T)
by either or both a tempering rolling and a tension leveller satisfies the following
inequality:
7. A dual-phase type high-strength steel sheet having high impact energy absorption properties
according to any of claims 1 to 6, characterized in that the chemical compositions
of the dual-phase type high-strength steel sheet with excellent dynamic deformation
properties contains, in terms of weight percentage, C at 0.02∼0.25%, either or both
Mn and Cr at a total of 0.15∼3.5%, one or more from among Si, Al and P at a total
of 0.02∼4.0%, if necessary one or more from among Ni, Cu and Mo at a total of no more
than 3.5%, one or more from among Nb, Ti and V at no more than 0.30%, and either or
both Ca and REM at 0.0005∼0.01% for Ca and 0.005∼0.05% for REM, with the remainder
Fe as the primary component.
8. A dual-phase type high-strength steel sheet having high impact energy absorption properties
according to any of claims 1 to 7, characterized in that one or more from among B
(≤0.01), S (≤0.01%) and N (≤0.02%) are further added if necessary to the chemical
compositions of the steel of the dual-phase type high-strength steel sheet with excellent
dynamic deformation properties.
9. A method of producing a dual-phase type high strength hot rolled steel sheet having
high impact energy absorption properties according to any of claims 1 to 8, characterized
in that after a continuous cast slab is fed directly from casting to a hot rolling
step, or is hot rolled upon reheating after momentary cooling, it is subjected to
hot rolling at a finishing temperature of Ar3 - 50°C to Ar3 + 120°C, cooled at an average cooling rate of more than 5°C/sec in a run-out table,
and then coiled at a temperature of no greater than 350°C.
10. A method of producing a dual-phase high-strength hot rolled steel sheet having high
impact energy absorption properties according to claim 9, characterized in that at
finishing temperature for hot rolling in a range of Ar
3 - 50°C to Ar
3 + 120°C, the hot rolling is carried out so that the metallurgy parameter A satisfies
inequalities (1) and (2) below, the subsequent average cooling rate in the run-out
table is at least 5°C/sec, and the coiling is accomplished so that the relationship
between said metallurgy parameter A and the coiling temperature (CT) satisfies inequality
(3) below.
11. A method of producing a dual-phase type high-strength cold rolled steel sheet having
high impact energy absorption properties according to any of claims 1 to 8, characterized
in that after a continuous cast slab is fed directly from casting to a hot rolling
step, or is hot rolled upon reheating after momentary cooling, it is hot rolled, the
hot rolled and subsequently coiled steel sheet is cold rolled after acid pickling,
and during annealing in a continuous annealing step for preparation of the final product,
it is heated to a temperature between Ac1 and Ac3 and subjected to the annealing while held in this temperature range for at least
10 seconds, and then cooled at a cooling rate of more than 5°C/sec.
12. A method for producing a dual-phase type high-strength cold rolled steel sheet having
high impact energy absorption properties according to any of claims 1 to 8, characterized
in that in said continuous annealing step, the cold rolled steel sheet is heated to
a temperature between Ac1 and Ac3 and subjected to the annealing while held in this temperature range for at least
10 seconds, and for subsequent cooling, it is cooled to a secondary cooling start
temperature (Tq) in the range of 550°C-To at a primary cooling rate of 1∼10°C/sec
and then cooled to a secondary cooling end temperature (Te) which is no higher than
Tem determined by the chemical compositions and annealing temperature (To), at a secondary
cooling rate of 10∼200°C/sec.