Title: High tensile strength cold rolled steel sheet having excellent strain age hardening characteristics and the production thereof
Abstract: The present invention presents a high tensile strength cold rolled steel sheet having excellent formability, impact resistance and strain age hardening characteristics, and the production thereof. As a specific means, a slab having a composition which contains, by mass %, 0.15% or less of C, 0.02% or less of Al, and 0.0050 to 0.0250% of N at N/Al of 0.3 or higher, and has N in a solid solution state at 0.0010% or more, is first hot rolled at the finish rolling delivery-side temperature of 800° C. or above, and is subsequently coiled at the coiling temperature of 750° C. or below to prepare a hot rolled plate. Then, after cold rolling, the hot rolled plate is continuously cooled at a temperature from the recrystallization temperature to 900° C. at a holding time of 10 to 120 seconds, and is cooled by primary cooling in which the hot rolled plate is cooled to 500° C. or below at a cooling rate of 10 to 300° C./s, and furthermore if necessary, by secondary cooling in which a residence time is 300 seconds or less in a temperature range of the primary cooling stopping temperature or below and 350° C. or higher. Provided is a steel sheet containing a ferritic phase having an average crystal grain size of 10 μm or less at an area ratio of 50% or more, and if necessary, a martensitic phase at an area ratio of 3% or more as a second phase.
Patent Number: 6,899,771 Issued on 05/31/2005 to Kami, et al.
| Inventors:
|
Kami; Chikara (Chiba, JP);
Tosaka; Akio (Chiba, JP);
Osawa; Kazunori (Kurashiki, JP);
Kaneko; Shinjiro (Chiba, JP);
Yamazaki; Takuya (Chiba, JP);
Okuda; Kaneharu (Chiba, JP);
Ishikawa; Takashi (Chiba, JP)
|
| Assignee:
|
JFE Steel Corporation (JP)
|
| Appl. No.:
|
341165 |
| Filed:
|
January 13, 2003 |
Foreign Application Priority Data
| Feb 29, 2000[JP] | 2000-053923 |
| May 31, 2000[JP] | 2000-162497 |
| May 23, 2000[JP] | 2000-151170 |
| Current U.S. Class: |
148/320; 148/603; 148/652 |
| Intern'l Class: |
C22C 038/06; C22C038/12; C21D008/02 |
| Field of Search: |
148/320,330,603,652,602
|
References Cited [Referenced By]
U.S. Patent Documents
| 3673009 | Jun., 1972 | Levy.
| |
| 5123969 | Jun., 1992 | Tung-Sheng.
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| 6695932 | Feb., 2004 | Kami et al.
| |
| Foreign Patent Documents |
| 0 429 094 | May., 1991 | EP.
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| 0 608 430 | Aug., 1994 | EP.
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| 0943696 | Sep., 1999 | EP.
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| 0 999 288 | May., 2000 | EP.
| |
| 58 003922 | Mar., 1983 | JP.
| |
| 60 052528 | Jul., 1985 | JP.
| |
| 60 145355 | Dec., 1985 | JP.
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| 61272323 | Dec., 1986 | JP.
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| 04074824 | Mar., 1992 | JP.
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| 6-116682 | Apr., 1994 | JP.
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| 0 612 857 | Aug., 1994 | JP.
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| 07 090482 | Aug., 1995 | JP.
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| 8-35039 | Feb., 1996 | JP.
| |
| 08 035039 | Jun., 1996 | JP.
| |
| 08 325670 | Apr., 1997 | JP.
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| 9-296252 | Nov., 1997 | JP.
| |
| 11-80919 | Mar., 1999 | JP.
| |
| 55 141526 | Jan., 2001 | JP.
| |
Primary Examiner: Yee; Deborah
Attorney, Agent or Firm: DLA Piper Rudnick Gray Cary US LLP
Claims
1. A high yield ratio high tensile strength cold rolled steel sheet having excellent
strain age hardening characteristics with tensile strength of 440 MPa or higher
and a yield ratio of 0.7 or above, characterized in that the sheet has a composition
containing, by mass %:
0.15% or less of C;
2.0% or less of Si;
3.0% or less of Mn;
0.08% or less of P;
0.02% or less of S;
0.02% or less of Al;
0.0050 to 0.0250% of N; and
0.007 to 0.04% of Nb;
having 0.3 or more of N/Al and 0.0010% or more of N in a solid solution state,
and
furthermore containing deposited Nb at 0.005% or more, and
having the balance of Fe and inevitable impurities; and that
the steel sheet has a structure containing a ferritic phase having an average
crystal grain size of 10 μm or less at an area ratio of 50% or more, and
mainly pearlite as a residual portion.
2. A high tensile strength cold rolled steel sheet, characterized in that the
sheet further contains, in addition to the composition according to claim 1, one
group, or two or more groups of the following a to d by mass %:
Group a: one, or two or more elements of Cu, Ni, Cr, and Mo at a total of 1.0%
or less;
Group b: one or two elements of Ti and V at a total of 0.1% or less;
Group c: B at 0.0030% or less; and
Group d: one or two elements of Ca and REM at a total of 0.0010 to 0.010%.
3. A production of a high yield ratio high tensile strength cold rolled steel
sheet having excellent strain age hardening characteristics with tensile strength
of 440 MPa or more and a yield ratio of 0.7 or above, characterized in that sequentially
carried out are:
a hot rolling step wherein a steel slab that has a composition containing, by
mass %:
0.15% or less of C;
2.0% or less of Si;
3.0% or less of Mn;
0.08% or less of P;
0.02% or less of S;
0.02% or less of Al;
0.0050 to 0.0250% of N; and
0.007 to 0.04% of Nb;
and having N/Al of 0.3 or more
is heated at a slab heating temperature of 1,100° C. or higher, and
is roughly rolled to form a sheet bar, and
the sheet bar is finish rolled at a final pass draft of 25% or more at a finish
rolling delivery-side temperature of 800° C. or higher, and
is coiled at a coiling temperature of 650° C. or below to form a hot rolled
sheet;
a cold rolling step in which the hot rolled sheet is pickled and cold rolled
to form a cold rolled sheet; and
a cold rolled sheet annealing step in which the cold rolled sheet is annealed
at a temperature between the recrystallization temperature and 900° C. for
a holding time of 10 to 90 seconds, and the cold rolled sheet is cooled at a cooling
rate of 70° C./s or below to a temperature of 600° C. and below.
Description
TECHNICAL FIELD
The present invention relates to a high tensile strength cold rolled steel sheet
which is mainly useful for vehicle bodies, and particularly, relates to a high
tensile strength cold rolled steel sheet having tensile strength (TS) of 440 MPa
or higher and excellent strain age hardening characteristics, and the production
thereof. The high tensile strength cold rolled steel sheet of the present invention
is widely applicable, ranging from relatively light working, such as forming into
a pipe by light bending and roll forming, to relatively heavy drawing. Moreover,
the steel sheet of the present invention includes a steel strip.
"Having excellent strain age hardening characteristics" in the present invention
indicates that an increase in deformation stress before and after an aging treatment
(referred to as BH amount; BH amount=yield stress after the aging treatment-predeformation
stress before the aging treatment) is 80 MPa or higher under the aging condition
of holding the temperature at 170° C. for 20 minutes after the predeformation
at the tensile strain of 5%, and that an increase in tensile strength (mentioned
as ΔTS; ΔTS=tensile strength after the aging treatment-tensile strength
before the predeformation) before and after a strain aging treatment (the predeformation+the
aging treatment) is 40 MPa or higher.
BACKGROUND ART
The reduction of vehicle body weights has been a critical issue, which relates
to the regulation of emission gas and recent global environmental problems. In
order to lighten the body of a vehicle, it is effective to reduce the thickness
of steel sheets by increasing the strength of steel sheets that are used in quantity,
in other words, by using high tensile strength steel sheets.
However, even vehicle parts of thin high tensile strength steel sheets have
to perform sufficiently well based on their purposes. The performance includes,
for instance, static strength against bending and torsional deformation, fatigue
resistance impact resistance, and the like. Therefore, high tensile strength steel
sheets for use in vehicle parts also have to have such excellent characteristics
after being formed.
Moreover, press forming is carried out on steel sheets to form vehicle
parts. However, when the steel sheets are too strong, the following problems are found:
(1) shape freezability declines; and
(2) problems such as cracking and necking are found during forming due to a decrease
in ductility. The application of high tensile strength steel sheets to vehicle
bodies has been limited.
In order to overcome this problem, steel sheets that use an extra-low carbon
steel
as a material and in which the amount of C finally remaining in a solid solution
state is controlled in an appropriate range are known as, for instance, cold rolled
steel sheets for an outer sheet panel. This type of steel sheet is kept soft during
press forming, and maintains shape freezability and ductility and maintains dent
resistance due to an increase in yield stress which utilized strain age hardening
phenomenon during the coating and baking process of 170° C.×about 20
minutes after press forming. In this type of steel sheet, C is dissolved in steel
in a solid solution state during press forming, and the steel is soft. On the other
hand, after press forming, solid solution C is fixed to a dislocation that is introduced
during the press forming, in the coating and baking process, thus increasing yield stress.
However, an increase in yield stress due to strain age hardening is kept
low in this type of steel sheet in order to prevent stretcher strains that will
later become surface defects. Thus, there is little contribution to the actual
weight reduction of parts.
Specifically, not only does yield stress have to be -increased by strain
aging but strength characteristics also have to increase so as to reduce the weight
of parts. In other words, it is desirable to make parts stronger by increasing
tensile strength after strain aging.
For applications in which appearance is not so much of a concern, proposed are
steel sheets in which a baking hardening quantity is further increased by using
solid solution N, and steel sheets which have a composite structure consisting
of ferrite and martensite and thus have improved baking hardenability.
For instance, Japanese Unexamined Patent Application Publication No. 60-52528
discloses a production of high-strength thin steel having good ductility and spot
weldability in which steel containing 0.02 to 0.15% of C, 0.8 to 3.5% of Mn, 0.02
to 0.15% of P, 0.10% or less of Al, and 0.005 to 0.025% of N is coiled at 550°
C. or below for hot-rolling, and annealing after cool-rolling is a controlled cooling
heat treatment. The steel sheet produced in the art of Japanese Unexamined Patent
Application Publication No. 60-52528 has a mixed structure consisting of a low-temperature
transformation product phase mainly having ferrite and martensite, and has excellent
ductility. At the same time, high strength is obtained by utilizing strain aging
during a coating and baking process due to N, which is actively added.
However, in the art of Japanese Unexamined Patent Application Publication
No. 60-52528, an increase in yield stress YS due to strain age hardening is large,
but an increase in tensile strength TS is small. Moreover, the fluctuation of mechanical
properties is large, so that an increase in yield stress YS is large and uneven.
Thus, it is not currently possible to expect a steel sheet that is thin enough
to contribute the weight reduction of vehicle parts.
Moreover, Japanese Examined Patent Application Publication No. 5-24979
discloses a cold rolled high tensile steel sheet having baking hardenability. The
steel sheet contains 0.08 to 0.20% of C and 1.5 to 3.5% of Mn, and the balance
Fe and inevitable, impurities as components. The steel structure is composed of
uniform bainite containing 5% or less of ferrite, or bainite partly containing
martensite. In the cold rolled steel sheet described in Japanese Examined Patent
Application Publication No. 5-24979, a baking hardening quantity, as a structure
mainly having bainite, is greater than conventionally used due to quenching in
the temperature range of 400 to 200° C. and the following slow cooling in
a cooling process after continuous annealing.
However, although a baking hardening quantity is greater than conventionally
used due to an increase in yield strength after coating and baking in the cold
rolled steel sheet described in Japanese Examined Patent Application Publication
No. 5-24979, tensile strength cannot be increased. When the steel sheet is used
for strong members, the improvement of fatigue resistance and impact resistance
cannot be expected. Thus, there still is a problem in that the steel sheet cannot
be used for applications that strongly require fatigue resistance, impact resistance,
and the like.
Although it is a hot rolled steel sheet, proposed is a steel sheet having
higher yield stress as well as yield strength due to a heat treatment after press forming.
For instance, Japanese Examined Patent Application Publication No. 8-23048 proposes
a production of hot rolled steel plate having a composite structure mainly of ferrite
and martensite in which steel containing 0.02 to 0.13% of C, 2.0% or less of Si,
0.6 to 2.5% of Mn, 0.10% or less of sol. Al, and 0.0080 to 0.0250% of N is reheated
at 1,100° C. or higher and finish-rolling is finished at 850 to 900°
C. for hot-rolling. Then, the steel is cooled to less than 150° C. at the
cooling rate of 15° C./s or higher, and is coiled. However, although yield
stress as well as tensile strength increase due to strain age hardening in the
steel sheet produced in the art described in Japanese Examined Patent Application
Publication No. 8-23048, steel is coiled at an extremely low coiling temperature
of less than 150° C. Thus, the inconsistency of mechanical characteristics
is large and troublesome. There also have been problems in that increases in yield
stress after a press forming-coating and baking treatment are uneven, and furthermore,
a hole expanding ratio (λ) is low, so that stretch-flanging workability declines
and press forming becomes insufficient.
High tensile strength steel sheets having relatively high yield stress include
so-called precipitation strengthened steel to which carbonitride-forming elements,
such as Ti, Nb and V, are added and which is strengthened by the fine deposits
thereof. However, unlike hot rolled steel sheets that go through a sufficient thermal
insulation process after hot rolling, it is difficult for cold rolled steel sheets
to obtain enough precipitation in a short period of continuous annealing. It has
been difficult to produce a steel sheet having high yield ratios (ratios of yield
stress relative to tensile strength:, YS/TS). Particularly, when C is reduced for
weldability, it becomes more difficult to have high yield ratios, probably because
the amount of deposit itself decreases in a region where the amount of C is low,
and this is troublesome.
Furthermore, although the above-mentioned steel sheets show excellent
strength after a coating and baking treatment in a simple tensile test, strengths
are largely uneven when plastic deformation is carried out under actual press conditions.
The steel sheets are not sufficiently applicable for parts that need to be reliable.
It is an object of the present invention to break through the limitations of
the
conventional arts mentioned above, and to provide a high tensile strength cold
rolled steel sheet having excellent strain age hardening characteristics, high
formability and stable quality and thus can obtain sufficient strength after being
formed into vehicle parts, fully contributing to the reduction of vehicle body
weights, and the production thereof that can economically produce the steel sheets
without distorting the shapes thereof The strain age hardening characteristics
in the present invention target 80 MPa or more of BH amounts and 40 MPa or more
of ΔTS under the aging condition of holding the temperature at 170°
C. for 20 minutes after predeformation at 5% of tensile strain.
Furthermore, the steel sheet is also advantageously applicable to, particularly,
parts to which relatively small strain is added. Thus, it is also an object of
the present invention to provide a high tensile strength cold rolled steel sheet
having high yield ratios of 0.7 or higher so as to raise sheet yield stress and
stabilize the strength of parts.
DISCLOSURE OF INVENTION
The present inventors, in order to achieve the objects mentioned above, produced
steel sheets by changing compositions and conditions, and carried out many material
evaluations. Accordingly, it was found that both the improvement of formability
and an increase in strength after forming can be easily achieved by effectively
utilizing a large strain age hardening phenomenon due to a strengthening element
N, which has never much been conventionally actively used.
Furthermore, the present inventors realized that it is necessary to
advantageously combine strain age hardening phenomenon due to N and coating and
baking conditions of vehicles, or furthermore, heat treatment conditions after
forming actively, and that it is effective to control the microstructure of steel
sheets and solid solution N in certain ranges under appropriate hot rolling conditions
and cold rolling, cold rolling annealing conditions therefor. They also found that
it is important, with respect to composition, to control particularly an Al content
in response to a N content in order to provide stable strain age hardening phenomenon
due to N. Moreover, the present inventors realized that N can be sufficiently used
without causing a conventional problem such as room temperature aging deterioration
when the microstructure of steel sheets is composed of ferrite as a main phase
and has an average grain size of 10 μm or less.
Furthermore, the present inventors found that low yield ratios are obtained
and ductility and formability improve when the microstructure of steel sheets is
composed of ferrite as a main phase and contains a martensite as a second phase
at the area ratio of 3% or higher. At the same time, strain age hardening phenomenon
due to N can be effectively utilized, increasing strength after forming and improving
impact resistance as parts.
In other words, the present inventors found that a steel sheet having far superior
formability than conventional solid solution strengthen type C Mn steel sheets
and precipitation strengthening type steel sheets, and strain age hardening characteristics
that are not found in the conventional steel sheets mentioned above, is provided
when N is used as a strengthening element and an Al content is controlled in an
appropriate range in response to a N content; at the same time, an appropriate
microstructure and solid solution N are provided under the optimum hot rolling
conditions and cold rolling, cold rolling annealing conditions.
Furthermore, the present inventors found that a steel sheet having far
superior formability than conventional solid solution strengthening type C—Mn
steel sheets and precipitation strengthening type steel sheets, high yield ratios
of 0.7 or higher, and strain age hardening characteristics that are not found in
the conventional steel sheets mentioned above, is provided when N is used as a
strengthening element and an Al content is controlled at an appropriate range in
response to a N content; at the same time, an appropriate microstructure, solid
solution N (N in a solid solution state), and a Nb deposit (deposited Nb) are provided
under the optimum hot rolling conditions and cold rolling, cold rolling annealing conditions.
The main phase is ferrite, and the residual portion is mainly pearlite. However,
bainite or martensite at the area ratio of 2% or less is accentahle Morever, in
order to increase the precipitation of the ferritic phase, it is preferable that
the Nb deposit analyzed by a method mentioned later is 0.005% or more.
Moreover, the steel sheet of the present invention has higher strength
after a coating and baking treatment in a simple tensile test than conventional
steel sheets. Furthermore, the fluctuation of strengths is small when plastic deformation
is carried out under actual pressing conditions, and the strength of parts is stable.
For example, a part where thickness is reduced due to heavy strain is harder than
other parts and tends to be even in the weighting load capacity of (sheet thickness)×(strength),
and strength as parts become stable.
The present invention has been completed with further examinations based on the
above-mentioned knowledge.
Specifically, a first invention is a high tensile strength cold rolled
steel sheet having excellent strain age hardening characteristics with the tensile
strength of 440 MPa or higher, and preferably, a sheet thickness of 3.2 mm or less.
The steel sheet is characterized in that the sheet has a composition containing,
by mass %, 0.15% or less of C, 2.0% or less of Si, 3.0% or less of Mn, 0.08% or
less of P, 0.02% or less of S, 0.02% or less of Al, and 0.0050 to 0.0250% of N,
having 0.3 or higher of N/Al and 0.0010% or more of N in a solid solution state,
and having the balance of Fe and inevitable impurities. The steel sheet has a structure
that contains a ferritic phase having an average crystal grain size of 10 μm
or less at the area ratio of 50% or more. Moreover, it is preferable that the first
invention further contains, in addition to the composition mentioned above, one
group, or two or more groups of the following a to d by mass %:
Group a: one, or two or more elements of Cu, Ni, Cr, and Mo at the total of
1.0% or less;
Group b: one or two elements of Nb, Ti, and V at the total of 0.1% or less;
Group c: B at 0.0030% or less; and
Group d: one or two elements of Ca and REM at the total of 0.0010 to 0.010%.
Moreover, electroplating or melt plating may be carried out on the above-mentioned
high tensile strength cold rolled steel sheet in the first invention.
A second invention is a production of a high tensile strength cold rolled steel
sheet having excellent strain age hardening characteristics with the tensile strength
of 440 MPa or more. The production is characterized in that sequentially carried
out are: a hot rolling step in which a steel slab having a composition containing,
by mass %, of 0.15% or less of C, 2.0% or less of Si, 3.0% or less of Mn, 0.08%
or less of P, 0.02% or less of S. 0.02% or less of Al, and 0.0050 to 0.0250% of
N, and having N/Al of 0.3 or higher is heated at the slab heating temperature of
1,000° C. or higher and is roughly rolled to form a sheet bar, and the sheet
bar is finish rolled at the finish rolling deliver-side temperature of 800°
C. or higher and is quenched at the cooling rate of 40° C./s or above, preferably,
within 0.5 seconds after finish rolling and is coiled at the coiling temperature
of 650° C. or below to form a hot rolled sheet; a cold rolling step in which
the hot rolled sheet is pickled and cold rolled to form a cold rolled sheet; and
a cold rolled sheet annealing step of primary cooling in which the cold rolled
sheet is annealed at a temperature between the recrystallization temperature and
900° C. for the holding time of 10 to 60 seconds, and is cooled at the cooling
rate of 10 to 300° C./s to the temperature of 500° C. or below, and a
secondary cooling at the residence time of 300 seconds or less in a temperature
range between the stopping temperature of the primary cooling and 400° C.
It is preferable in the second invention that temper rolling or leveling at the
elongation percentage of 1.0 to 15% is further carried out after the cold rolled
sheet annealing step.
It is preferable in the second invention that adjacent sheet bars are joined
between
the rough rolling and the finish rolling. It is also preferable in the second invention
that one or both of a sheet bar edge heater that heats a width edge section of
the sheet bar, and a sheet bar heater that heats a length edge section of the sheet
bar, are used between the rough rolling and the finish rolling.
A third invention is a high yield ratio type high tensile strength cold rolled
steel sheet having excellent strain age hardening characteristics with the tensile
strength of 440 MPa or higher and the yield ratio of 0.7 or above, and preferably,
a sheet thickness of 3.2 mm or less. The steel sheet is characterized in that the
sheet has a composition containing, by mass %, 0.15% or less of C, 2.0% or less
of Si, 3.0% or less of Mn, 0.08% or less of P, 0.02% or less of S, 0.02% or less
of Al, 0.0050 to 0.0250% of N, and 0.007 to 0.04% of Nb, having 0.3 or higher of
N/Al and 0.0010% or more of N in a solid solution state, and having the balance
of Fe and inevitable impurities. The steel sheet has a structure that contains
a ferritic phase having an average crystal grain size of 10 μm or less at
the area ratio of 50% or more, with mainly pearlite as a residual portion. Moreover,
it is preferable that the third invention further contains, in addition to the
composition mentioned above, one group, or two or more groups of the following
a to d by mass %:
Group a: one, or two or more elements of Cu, Ni, Cr, and Mo at the total of
1.0% or less;
Group b: one or two elements of Ti and V at the total of 0.1% or less;
Group c: B at 0.0030% or less; and
Group d: one or two elements of Ca and REM at the total of 0.0010 to 0.010%.
A fourth invention is a production of a high tensile strength cold rolled steel
sheet having excellent strain age hardening characteristics with the tensile strength
of 440 MPa or more and the yield ratio of 0.7 or above. The production is characterized
in that sequentially carried out are: a hot rolling step in which a steel slab
having a composition containing, by mass %, 0.15% or less of C, 2.0% or less of
Si, 3.0% or less of Mn, 0.08% or less of P, 0.02% or less of S, 0.02% or less of
Al, 0.0050 to 0.0250% of N, and 0.007 to 0.04% of Nb, and having N/Al of 0.3 or
higher is heated at the slab heating temperature of 1,100° C. or higher and
is roughly rolled to form a sheet bar, and the sheet bar is finish rolled at the
final pass draft of 25% or more at the finish rolling delivery-side temperature
of 800° C. or higher and is quenched at the cooling rate of 40° C./s
or above, preferably, within 0.5 seconds after finish rolling and is coiled at
the coiling temperature of 650° C. or below to form a hot rolled sheet; a
cold rolling step in which the hot rolled sheet is pickled and cold rolled to form
a cold rolled sheet; and a cold rolled sheet annealing step in which the cold rolled
sheet is annealed at a temperature between the recrystallization temperature and
900° C. for the holding time of 10 to 60 seconds and is cooled at the cooling
rate of 70° C./s or below to the temperature range of 600° C. and below.
It is preferable in the fourth invention that temper rolling or leveling at the
elongation percentage of 1.5 to 15% is further carried out after the cold rolled
sheet annealing step.
It is preferable in the fourth invention that adjacent sheet bars are joined
between
the rough rolling and finish rolling. It is also preferable in the fourth invention
that one or both of a sheet bar edge heater that heats a width edge section of
the sheet bar, and a sheet bar heater that heats a length edge section of the sheet
bar, are used between the rough rolling and the finish rolling.
A fifth invention is a high tensile strength cold rolled steel sheet having excellent
strain age hardening characteristics, formability and impact resistance, tensile
strength of 440 MPa or higher and, preferably, a sheet thickness of 3.2 mm or less.
The steel sheet is characterized in that the sheet has a composition containing,
by mass %, 0.15% or less of C, 3.0% or less of Mn, 0.02% or less of S, 0.02% or
less of Al, and 0.0050 to 0.0250% of N, and furthermore, one or two elements of
Mo at 0.05 tb 1.0% and Cr at 0.05 to 1.0%, having 0.3 or higher of N/Al and 0.0010%
or more of N in a solid solution state, and having the balance of Fe and inevitable
impurities. The steel sheet has a structure that contains a ferritic phase having
an average crystal grain size of 10 μm or less at the area ratio of 50% or
more, and furthermore, a martensitic phase at the area ratio of 3% or more. Moreover,
it is preferable that the fifth invention further contains, in addition to the
composition mentioned above, one group, or two or more groups of the following
e to h by mass %:
Group e: one, or two or more elements of Si at 0.05 to 1.5%, P at 0.03 to 0.15%,
and B at 0.0003 to 0.01%;
Group f: one, or two or more elements of Nb at 0.01 to 0.1%, Ti at 0.01 to
0.2%, and V at 0.01 to 0.2%;
Group g: one or two elements of Cu at 0.05 to 1.5% and Ni at 0.05 to 1.5%; and
Group h: one or two elements of Ca and REM at the total of 0.0010 to 0.010%.
Moreover, a sixth invention is a production of a high tensile strength
cold rolled steel sheet having excellent strain age hardening characteristics,
formability and impact resistance and tensile strength of 440 MPa or more. The
production is characterized in that sequentially carried out are: a hot rolling
step in which a steel slab having a composition containing, by mass %, 0.15% or
less of C, 3.0% or less of Mn, 0.02% or less of S, 0.02% or less of Al, and 0.0050
to 0.0250% of N, and furthermore, one or two elements of Mo at 0.05 to 1.0% and
Cr at 0.05 to 1.0%, having N/Al of 0.3 or higher, or furthermore, containing one
group, or two or more groups of the following e to h:
Group e: one, or two or more elements of Si at 0.05 to 1.5%, P at 0.03 to 0.15%,
and B at 0.0003 to 0.01%;
Group f: one, or two or more elements of Nb at 0.01 to 0.1%, Ti at 0.01 to
0.2%, and V at 0.01 to 0.2%;
Group g: one or two elements of Cu at 0.05 to 1.5% and Ni at 0.05 to 1.5%; and
Group h: one or two elements of Ca and REM at the total of 0.0010 to 0.010%
is heated at the slab heating temperature of 1,000° C. or above and is roughly
rolled to form a sheet bar, and the sheet bar is finish rolled at the finish rolling
delivery-side temperature of 800° C. or above and is coiled at the coiling
temperature of 750° C. or below to form a hot rolled sheet; a cold rolling
step in which the hot rolled sheet is pickled and cold rolled to form a cold rolled
sheet, and a cold rolled sheet annealing step in which the cold rolled sheet is
annealed at the temperature between (Ac, transformation point) and (AC
3
transformation point) for the holding time of 10 to 120 seconds and is cooled at
the average cooling rate of a critical cooling rate CR or higher from 600 to 300°
C. The critical cooling rate CR is defined by the following formula (1) or (2):
wherein CR is a cooling rate (° C./s); and Mn, Mo, Cr, Si, P, Cu and
Ni are contents of each element (mass %). It is preferable in the sixth invention
that the cooling is started within 0.5 seconds after the finish rolling, and quenching
is performed at the cooling rate of 40° C./s or above before the coiling.
It is also preferable in the sixth invention that temper rolling or leveling at
the elongation percentage of 1.0 to 15% is further carried out after the cold rolled
sheet annealing step.
BEST MODE FOR CARRYING OUT THE INVENTION
First, the reasons for limiting the composition of the steel sheet of the
present invention will be explained. Mass % is simply noted as % hereinafter.
C: 0.15% or below
C is an element that increases the strength of a steel sheet. Moreover, in order
to achieve important features of the present invention such as the average grain
size of ferrite at 10 μm or less, and furthermore, to maintain desirable
strength, it is preferable to contain C at 0.005% or more. However, beyond 0.15%,
a fractional ratio of carbide becomes excessive in a steel sheet, thus clearly
lowering ductility and deteriorating formability. Furthermore, spot weldability,
arc weldability, and the like clearly decline. In consideration of formability
and weldability, the content of C is limited to 0.15% or less, or preferably, 0.10%
or less. For applications requiring more preferable ductility, C is contained preferably
at 0.08% or less. For applications requiring the most preferable ductility, C is
contained preferably at 0.05% or less.
Si: 2.0% or less
Si is a useful element for strengthening a steel sheet without clearly reducing
the ductility of steel, and is preferably contained at 0.1% or more. On the other
hand, Si sharply increases a transformation point during hot rolling, deteriorating
quality and shape or providing negative effects on the appearance of a steel sheet
surface, such as surface properties and chemical convertibility. In the present
invention, the content of Si is limited to 2.0% or less. When Si is contained at
2.0% or less, the sharp increase of a transformation point can be prevented by
adjusting the amount of Mn added along with Si, and good surface properties can
be kept. Moreover, it is preferable to contain Si at 0.3% or more in a high tensile
strength steel sheet having the tensile strength TS of more than 500 MPa for a
balance between strength and ductility.
Mn: 3.0% or less
Mn is a useful element, preventing S from causing thermal cracking, and is preferably
added in response to S content. Moreover, Mn is effective in the refinement of
crystal grains as an important feature of the present invention. It is preferable
to actively add Mn to improve the quality of a material. Moreover, Mn is an element,
improving hardenability. It is preferable to actively add Mn to form a martensitic
phase as a second phase with stability. Mn is preferably contained at 0.2% or more
for fixing S with stability and forming a martensitic phase.
Moreover, Mn is an element increasing steel sheet strength, and is preferably
contained at 1.2% or more for providing strength of more than TS 500 MPa. It is
more preferable to contain Mn at 1.5% or more to maintain strength with stability.
When a Mn content is increased to this level, fluctuations of mechanical properties
and strain age hardening characteristics of a steel sheet in relation to the change
in production conditions, including hot rolling conditions, become small, thus
effectively stabilizing quality.
Mn also lowers a transformation point during a hot rolling process. As Mn is
added
with Si, it can prevent Si from increasing a transformation point. Particularly,
in products having thin sheet thickness, since quality and shape sensitively change
due to the fluctuation of transformation points, it is important to strictly balance
the contents of Mn and Si. Accordingly, it is more preferable that Mn/Si is 3.0
or higher.
On the other hand, when Mn is contained in a large amount of more than 3.0%,
the
thermal deformation resistance of a steel sheet tends to increase and spot weldability
and the formability of a weld zone tend to deteriorate. Furthermore, as the generation
of ferrite is restricted, ductility tends to clearly decline. Thus, the content
of Mn is limited to 3.0% or less. Additionally, for applications requiring good
corrosion resistance and formability, the content of Mn is preferably 2.5% or less.
For applications requiring better corrosion resistance and formability, the content
of Mn is 1.5% or less.
P: 0.08% or less
P is a useful element as a solid solution strengthening element for steel. However,
when P is added excessively, steel becomes brittle, and furthermore, the stretch-flanging
workability of a steel sheet declines. Moreover, P is likely to be segregated in
steel, which makes a weld zone brittle thereby. Therefore, the content of P is
limited to 0.08% or less. When stretch-flanging workability and weld zone toughness
are particularly emphasized, it is preferable that P is contained at 0.04% or less,
and more preferably, 0.02% or less for weld zone toughness.
S: 0.02% or less
S is an inclusion in a steel sheet, and is an element that deteriorates the ductility
of a steel sheet and also corrosion resistance. In the present invention, the content
of S is limited to 0.02% or less. For applications requiring particularly good
formability, the content is preferably 0.015% or less. Furthermore, when stretch-flanging
workability is highly required, the content of S is preferably 0.008% or less.
Moreover, in order to maintain high strain age hardening characteristics with stability,
the content of S is preferably reduced to 0.008% or less although the detailed
mechanism thereof is unclear.
Al: 0.02% or less
Al is a useful element that functions as a deoxidizer and improves the purity
of steel. Furthermore, Al is an element refining the structure of a steel sheet.
In the present invention, Al is preferably contained at 0.001% or more. On the
other hand, excessive Al deteriorates surface properties of a steel sheet, and
furthermore, solid solution N as an important feature of the present invention
is reduced. Thus, solid solution N contributing to strain age hardening phenomenon
becomes insufficient, and strain age hardening characteristics are likely to be
inconsistent when production conditions are changed. Accordingly, in the present
invention, Al content is limited to a low 0.02% or less. In consideration of material
stability, the content of Al is preferably 0.015% or less.
N: 0.0050 to 0.0250%
N is an element increasing the strength of a steel sheet due to solid solution
strengthening and strain age hardening, and is the most important element in the
present invention. N also lowers the transformation point of steel, and is also
useful for stable operation under a situation of rolling thin sheets while heavily
interrupting transformation points. By adding an appropriate amount of N and controlling
production conditions, the present invention obtains solid solution N in a necessary
and sufficient amount for cold rolled products and plated products. Accordingly,
strength (YS, TS) in solid solution strengthening and strain age hardening sufficiently
increases. The mechanical properties of the steel sheet of the present invention
are satisfied with stability, including 440 MPa or above of TS, 80 MPa or above
of a baking hardening amount (BH amount) and an increase in tensile strength before
and after a strain aging process ΔTS of 40 MPa or above.
When the content of N is less than 0.0050%, an increase in strength is unlikely
to be stable. On the other hand, when the content of N exceeds 0.0250%, a steel
sheet tends to have more internal defects, and slab cracking and the like are likely
to occur more frequently during continuous casting. Thus, the content of N is in
the range of 0.0050 to 0.0250%. For the stability of quality and the improvement
of yields in entire production processes, it is more preferable that the content
of N is 0.0070 to 0.0170%. If the N content is within the range of the present
invention, there are no negative effects on weldability of spot welding, arc welding,
and the like.
N in a solid solution state: 0.0010% or more.
In order to obtain sufficient strength and furthermore provide enough strain
age
hardening due to N in cold rolled products, steel should have N in a solid solution
state (also mentioned as solid state N) at an amount (in concentration) of 0.0010%
or more.
The amount of solid solution N is calculated by subtracting a deposited N amount
from a total N amount in steel. Based on the comparison of various analyses by
the present inventors, it is effective to analyze a deposited N amount in accordance
with an electrolytic extraction analysis applying a constant potential electrolysis.
Methods of dissolving ferrite for extraction and analysis include acid decomposition,
halogenation, and electrolysis. Among them, electrolysis can dissolve only ferrite
with stability without decomposing unstable deposits such as carbide and nitride.
Acetyl-acetone based electrolyte is used for electrolysis at a constant potential.
In the present invention, a deposited N amount by the measurement of a constant
potential electrolysis showed the best result in relation to the actual strength
of parts.
Thus, after a residue is extracted by the constant potential electrolysis,
a N content is found in the residue by chemical decomposition as a deposited N
amount in the present invention.
In order to provide a high BH amount and ΔTS, the amount of solid solution
N is 0.0020% or more. For a higher BH amount and ΔTS, it is preferable that
the amount is 0.0030% or more. For a much higher BH amount and ΔTS, the amount
of solid solution N is preferably 0.0050% or more.
N/Al (ratio between N content and Al content): 0.3 or higher.
In order to have residual solid solution N with stability at 0.0010% or more
in
a product, it is necessary to control the amount of Al as an element to firmly
fix N. After examining steel sheets of various combination of N and Al contents
within the composition range of the present invention, it was found that N/Al has
to be 0.3 or higher to provide 0.0010% or more of solid solution N in a cold rolled
product and a plated product when the amount of Al is limited low at 0.02% or below.
In other words, the Al content is limited to (N content)/0.3 or less.
In the present invention, it is preferable to contain one group, or two or more
groups of the following a to d in addition to the above-noted composition:
Group a: one, or two or more elements of Cu, Ni, Cr, and Mo at the total of
1.0% or less;
Group b: one or two elements of Nb, Ti and V at the total of 0.1% or less;
Group c: B at 0.0030% or less; and
Group d: one or two elements of Ca and REM at the total of 0.00010 to 0.010%.
The Group a elements of Cu, Ni, Cr and Mo contribute to an increase in strength
of a steel sheet depending on needs, and they may be contained alone or in combination.
However, when the content is too high, thermal deformation resistance increases
or chemical convertibility and broad surface treatment characteristics deteriorate.
Thus, a weld zone hardens, and weld zone formability deteriorates. Accordingly,
it is preferable that the total content of the Group a is 1.0% or less.
The reason for containing one or both of Mo at 0.05 to 1.0% and Cr at 0.05 to
1.0%, in particular:
Both Mo and Cr contribute to an increase in strength of a steel sheet. Furthermore,
the elements improve the hardenability of steel, and are likely to generate a martensitic
phase as a second phase. In order to actively obtain a martensitic phase, the elements
are contained alone or in combination. Particularly, Mo and Cr have a function
to finely disperse a martensitic phase, and have effects to lower yield strength
and easily achieve low yield ratios. Such effects are found when each amount of
Mo and Cr is 0.05% or more. On the other hand, when Mo is contained at more than
1.0%, formability and surface treatment properties deteriorate. Thus, production
costs increase, which is economically disadvantageous. Moreover, when the content
of Cr is more than 1.0%, plating wettability deteriorates. Thus, the content of
Mo is limited to 0.05 to 1.0%, and that of Cr is limited to 0.05 to 1.0%.
The Group b elements of Nb, Ti and V contribute to provide fine and uniform crystal
grains. Depending on needs, the elements may be selected and contained alone or
in combination. However, when the content is too large, thermal deformation resistance
increases, and chemical convertibility and broad surface treatment characteristics
deteriorate. Accordingly, it is preferable that the total content of the Group
b is 0.1% or less. The reason for containing Nb at 0.007 to 0.04%, in particular:
In the present invention, Nb is an important element for visibly refining crystal
grains, increasing YS and improving yield ratios (YR=YS/TS) at 0.7 or higher, and
at the same time, achieving high strain age hardening due to N. In order to obtain
these effects, the content of Nb is preferably 0.007% or more. On the other hand,
in consideration of other nitride forming elements, Nb content is preferably limited
to 0.04% or less to maintain a required amount of solid solution N.
Deposited Nb: 0.005% or more.
For the addition of Nb in the present invention, the existing state of Nb in
steel is also important. In other words, it is preferable that Nb in a deposited
state (also mentioned as deposited Nb) exists in a constant amount so as to obtain
stable strain age hardening characteristics and 0.7 or above of yield ratios. Within
the range of a Nb content of the present invention, deposited Nb content should
be at least 0.005%. For the determination of Nb, Nb is dissolved by electrolytic
extraction with the use of acetyl-acetone based solvent and is extracted. The value
obtained by this method showed the best correlation with strain age hardening characteristics
although there are various types of dissolution methods. It is assumed that Nb
is more correlated to C than N within the range of the present invention, but the
details thereof are unknown.
The Group c element of B is effective in improving the hardenability of steel.
The element can be contained based on needs so as to increase a fractional ratio
of a low temperature transformation phase, except for a ferritic phase, and to
increase the strength of steel. However, when the content is too high, thermal
deformation declines, and solid solution N decreases as BN is generated. Therefore,
it is preferable that the content of B is 0.0030% or less.
The Group d elements of Ca and REM are useful for controlling the form of an
inclusion. Particularly, when stretch-flanging formability is required, it is preferable
to add the elements alone or in combination. In this case, when the total content
of the Group d elements is less than 0.0010%, the effect of controlling a form
is insufficient. On the other hand, when the content exceeds 0.010%, surface defects
become apparent. Accordingly, it is preferable to limit the total content of the
Group d to the range of 0.0010 to 0.010%.
Instead of the above-mentioned Group a to Group d, one, or two or more Groups
of the following Group e to Group h may be added to the composition mentioned above
in the present invention.
Group e: one, or two or more elements of Cu, Ni, Cr and Mo at the total of
1.0% or less;
Group f: one or two elements of Ti and V at the total of 0.1% or less;
Group g: B at 0.0030% or less; and
Group h: one or two elements of Ca and REM at the total of 0.0010 to 0.010%
The Group e elements of Cu, Ni, Cr and Mo contribute to an increase in strength
without reducing high ductility of a steel sheet. This effect is found at 0.01%
or above of Cu, 0.01% or above of Ni, 0.01% or above of Cr, and 0.01% or above
of Mo. Based on needs, the elements may be selected and contained alone or in combination.
However, when the content is too high, thermal deformation resistance increases,
or chemical convertibility and broad surface treatment characteristics deteriorate.
Thus, a weld zone hardens, and weld zone formability deteriorates. Accordingly,
it is preferable that the total content of the Group e is 1.0% or less.
The Group f elements of Ti and V contribute to provide fine and uniform crystal
grains. This effect is found at 0.002% or above for Ti and at 0.002% or above for
V. Depending on needs, the elements may be selected and contained alone or in combination.
However, when the content is too high, thermal deformation resistance increases,
and chemical convertibility and broad surface treatment characteristics deteriorate.
Thus, it is preferable that the Group b is contained at the total of 0.1% or less.
The Group g element of B is effective in improving the hardenability of steel.
The element can be added based on needs so as to increase a fractional ratio of
a low temperature transformation phase, except for a ferritic phase, and to increase
the strength of steel. This effect is found when B is added at 0.0002% or more.
However, when the amount is too large, thermal deformation deteriorates, and solid
solution N decreases because of the generation of BN. Thus, it is preferable that
B is 0.0030% or less.
The Group h elements of Ca and REM are useful for controlling the form of an
inclusion. Particularly, when stretch-flanging formability is required, it is preferable
to add the elements alone or in combination. In this case, when the total content
of the Group h elements is less than 0.0010%, the effect of controlling a form
is insufficient. On the other hand, when the content exceeds 0.010%, surface defects
become apparent. Accordingly, it is preferable to limit the total content of the
Group d to the range of 0.0010 to 0.010%.
Subsequently, the structure of a steel sheet of the present invention
will be explained.
Area ratio of a ferritic phase: 50% or above.
The purpose of a cold rolled steel sheet of the present invention is an application
for steel sheets for vehicles and the like that is preferably highly workable.
In order to maintain ductility, the steel sheet has a structure containing a ferritic
phase at an area ratio of 50% or above. When the area ratio of the ferritic phase
is less than 50%, it is difficult to obtain required ductility as a steel sheet
for vehicles that has to be highly workable. For greater ductility, the area ratio
of the ferritic phase is preferably 75% or above. The ferrite of the present invention
includes not only normal ferrite (polygonal ferrite) but also bainitic ferrite
and acicular ferrite that contain no carbide.
Moreover, other phases, besides a ferritic phase, are not particularly
limited. However, in order to increase strength, a single phase or a mixed phase
of bainite and martensite is preferable. Additionally, in the component ranges
and production method of the present invention, retained austenite is often formed
at less than 3%.
In order to increase YS so as to improve yield ratios (YR=YS/TS) at 0.7 or higher
and to have high strain age hardening due to N, it is desirable in the present
invention that a phase (second phase), other than a ferritic phase, is a structure
composed mainly of pearlite, in other words, a structure composed of a pearlistic
single phase, or a structure that contains bainite or martensite at an area ratio
of 2% or less with the balance pearlite.
On the other hand, the composition of the steel sheet of the present invention
in which a martensitic phase is finely dispersed and yield strength is reduced
to achieve low yield ratios, is a microstructure containing a ferritic phase as
a main phase and a martesitic phase as a second phase. Additionally, when the area
ratio of a ferritic phase exceeds 97%, effects as a composite structure cannot
be expected.
Area ratio of a martensitic phase: 3% or above.
The martensitic phase as a second phase is dispersed mainly at the grain boundary
of the ferritic phase as a main phase. Martensite is a hard phase, and increases
the strength of a steel sheet by strengthening a structure. Furthermore, as moving
dislocations are generated during transformation, martensite improves ductility
and lowers yield ratios of a steel sheet. These effects become clear when martensite
exists at 3% or more. When martensite exceeds 30%, a problem such as a decrease
in ductility is found. Thus, the area ratio of martensite as a second phase is
between 3% and 30%, preferably, 20% or less. Moreover, no problems are caused when
10% or less of bainite, as a second phase, is contained in addition to martensite
in those amounts.
Average crystal grain size: 10 μm or less.
The present invention adopts a larger crystal grain size, calculated from a grain
size based on a picture of a cross-sectional structure by a quadrature in accordance
with ASTM, and a nominal grain size based on a picture of a cross-sectional structure
by a cutting method in accordance with ASTM (for instance, see Umemoto et al.:
Heat Treatment, 24 (1984), 334).
Although the cold rolled steel sheet of the present invention has a predetermined
amount of solid solution N as a product, the present inventors' test results showed
that strain age hardening characteristics fluctuate greatly even at a constant
amount of solid solution N when the average crystal grain size of a ferritic phase
exceeds 10 μm. The deterioration of mechanical characteristics also becomes
obvious when the steel sheet is kept at room temperature. The detailed mechanism
is currently unknown. However, it is assumed that one cause of inconsistent strain
age hardening characteristics is crystal grain size, and that crystal grain size
is related to the segregation and precipitation of alloy elements to a grain boundary,
and furthermore, the effect of work and heat treatments thereon. Thus, in order
to stabilize strain age hardening characteristics, a ferritic phase should have
an average crystal grain size of 10 μm or less. It is also preferable that
ferrite has an average crystal grain size of 8 μm or less in order to further
increase a BH amount and ΔTS with stability.
The cold rolled steel sheet of the present invention having the above-mentioned
composition and structure has a tensile strength TS of 440 MPa or higher and excellent
strain age hardening characteristics. The cold rolled steel sheet has excellent
workability and impact resistance.
When TS is below 440 MPa, the steel sheet cannot be applied for structural members.
Additionally, in order to broaden the applications, it is desirable that TS is
500 MPa or above.
"Having excellent strain age hardening characteristics" in the present invention
indicates, as described above, that an increase in deformation stress before and
after an aging treatment (referred to as BH amount; BH amount=yield stress after
the aging treatment-predeformation stress before the aging treatment) is 80 MPa
or higher under the aging condition of holding the temperature at 170° C.
for 20 minutes after the predeformation at the tensile strain of 5%, and that an
increase in tensile strength (referred to as ΔTS; ΔTS=tensile strength
after the aging treatment-tensile strength before the predeformation) before and
after a strain aging treatment (the predeformation+the aging treatment) is 40 MPa
or higher.
A prestrain (predeformation) amount is an important factor regulating strain
age
hardening characteristics. The present inventors assumed deformation styles that
are applicable to steel sheets for vehicles, and examined the effect of a prestrain
amount on strain age hardening characteristics. As a result, they found that (1)
deformation stress in the deformation styles can be regulated by a uniaxial equivalent
strain (tensile strain) amount, except for the case of extremely deep drawing;
(2) a uniaxial equivalent s
