U.S. patent application number 12/078860 was filed with the patent office on 2009-09-10 for high-strength hot rolled steel plate and manufacturing method thereof.
This patent application is currently assigned to NAKAYAMA STEEL WORKS, LTD.. Invention is credited to Ichiro Chikushi, Kazuaki Hakomori, Ryurou Kurahashi, Yuuji Kusumoto, Masahiko Oda, Takashi Ohtani, Fuyuki Yoshida.
Application Number | 20090223609 12/078860 |
Document ID | / |
Family ID | 39766955 |
Filed Date | 2009-09-10 |
United States Patent
Application |
20090223609 |
Kind Code |
A1 |
Hakomori; Kazuaki ; et
al. |
September 10, 2009 |
High-strength hot rolled steel plate and manufacturing method
thereof
Abstract
The present invention provides a new high-strength and Si--Cr
containing hot rolled steel plate provided with higher strength as
well as excellent workability and a method for manufacturing the
steel plate. The high-strength steel plate can be obtained by
controlling the particle size of prior austenite to be 10 .mu.m or
less, and properly selecting the coiling temperature. The steel
plate obtained includes a retained austenite phase in a volume
fraction of 5% to 20%; a martensite phase in a volume fraction
equal to or less than 10%; and a bainite phase in the remaining
volume fraction. The particle size of the retained austenite
particle is 1 .mu.m or less and the retained austenite particles
are dispersed uniformly.
Inventors: |
Hakomori; Kazuaki;
(Kobe-shi, JP) ; Kusumoto; Yuuji; (Osaka-shi,
JP) ; Yoshida; Fuyuki; (Kawanishi-shi, JP) ;
Chikushi; Ichiro; (Kawanishi-shi, JP) ; Ohtani;
Takashi; (Osaka-shi, JP) ; Kurahashi; Ryurou;
(Amagasaki-shi, JP) ; Oda; Masahiko; (Himeji-shi,
JP) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. BOX 320850
ALEXANDRIA
VA
22320-4850
US
|
Assignee: |
NAKAYAMA STEEL WORKS, LTD.
Osaki-shi
JP
|
Family ID: |
39766955 |
Appl. No.: |
12/078860 |
Filed: |
April 7, 2008 |
Current U.S.
Class: |
148/602 ;
148/320; 148/335 |
Current CPC
Class: |
C22C 38/44 20130101;
C21D 6/002 20130101; C21D 8/0263 20130101; C21D 6/008 20130101;
C21D 2211/001 20130101; C21D 9/48 20130101; C22C 38/001 20130101;
C22C 38/02 20130101; C22C 38/04 20130101; C21D 8/0426 20130101;
C21D 8/0226 20130101; C21D 2211/008 20130101; C21D 2211/002
20130101 |
Class at
Publication: |
148/602 ;
148/320; 148/335 |
International
Class: |
C21D 8/02 20060101
C21D008/02; C22C 38/00 20060101 C22C038/00; C22C 38/44 20060101
C22C038/44 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 17, 2007 |
JP |
2007-108759 |
Claims
1. A high-strength hot rolled steel plate comprising: a retained
austenite phase in a volume fraction of 5% to 20%; a martensite
phase in a volume fraction of 0% to 10%; and a bainite phase in a
remaining volume fraction, wherein a particle size of retained
austenite particles is 1 .mu.m or less, and the retained austenite
particles are dispersed in a density of seven or more particles per
10 .mu.m.sup.2.
2. The high-strength hot rolled steel plate according to claim 1,
wherein a particle size of prior austenite particles is 10 .mu.m or
less, and an average aspect ratio of the prior austenite particles
is 2.0 or less.
3. The high-strength hot rolled steel plate according to claim 1,
wherein the steel plate has a composition comprising: C (0.13 to
0.21 (% by weight)), Si (0.5 to 2.0), Mn (0.2 to 1.0), Cr (1.0 to
4.0), Ni (0.02 to 1.0), Mo (0.05 to 0.4), P (0 to 0.010), S (0 to
0.003), N (0.005 to 0.015), and remaining components including Fe
and other inevitable impurities.
4. The high-strength hot rolled steel plate according to claim 1,
the steel plate has a plate thickness of 1.0 to 3.0 mm and a
tensile strength of 1200 MPa or greater.
5. A method for manufacturing a high-strength hot rolled steel
plate, comprising the steps of: preparing a steel material having a
composition containing: C (0.13 to 0.21 (% by weight)), Si (0.5 to
2.0), Mn (0.2 to 1.0), Cr (1.0 to 4.0), Ni (0.02 to 1.0), Mo (0.05
to 0.4), P (0 to 0.010), S (0 to 0.003), N (0.005 to 0.015), and
remaining components including Fe and other inevitable impurities;
roughly rolling the steel material under conditions of:
1250.degree. C. or higher of an extraction temperature of a
reheating furnace; 1030.degree. C. or higher of a discharging-side
temperature of roughly rolling mills; and 30% or higher of a
reduction ratio for each of roughly rolling final three passes;
finish rolling of the steel material under conditions of:
950.degree. C. or higher of a discharging-side temperature of
finish rolling mills; 40% or higher of a reduction ratio for each
mill on a finish front-stage, and 0.5 or greater of accumulated
strain due to a reduction by three rolling mills on a finish
rear-stage; and cooling the steel material by air-cooling for 2 to
6 seconds, followed by water-cooling, and coiling the steel
material at a coiling temperature of 550 to 650.degree. C.
6. The method for manufacturing a high-strength hot rolled steel
plate according to claim 5, wherein upon the finish rolling, a
reduction amount of a topmost portion of the steel material is set
greater than an expected reduction amount, in one or more rolling
mills other than a mill on a final stage, wherein the reduction
amount of the top portion is set at a value increased by less than
10% of the expected reduction amount of the rolling mill, and
wherein a length to be rolled in an increased reduction amount is
within 5 m as measured from a biting site of the topmost portion of
the steel material, and thereafter the reduction amount is retuned
to the expected reduction amount.
7. The method for manufacturing a high-strength hot rolled steel
plate according to claim 5, wherein a high-grip roll having
micro-carbide particles dispersed on a surface of the high-grip
roll is used as a working roll for each finish rear-stage rolling
mill including a final rolling mill.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon the prior Japanese Patent
Application No. 2007-108759 filed on Apr. 17, 2007, the entire
contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a high-strength hot rolled
steel plate having high tension strength and superior workability,
and also relates to a manufacturing method thereof.
[0004] 2. Description of the Related Art
[0005] Recent demands for a high-strength steel plate which can
exhibit superior workability will be described, with respect to
cars, by way of example. In view of environmental protection for
the earth, it should be required to reduce the amount of exhaust
gas, such as CO.sub.2 or the like, in the field of automobiles. To
this end, it is quite essential to further reduce the weight of
each car body. In order to achieve the weight reduction of the car
body, it is necessary to enhance the strength of steel plate used
for the car body so as to reduce the thickness of the steel plate.
In addition, safety for the users should be secured in the car.
Also for this purpose, the strength of the steel plate must be
further improved.
[0006] However, increase of the strength of the steel plate may
tend to degrade its workability, and it will be difficult to apply
a higher strength steel plate to cold working, such as general
press molding or the like.
[0007] Hot pressing is a hot press working process and usually
generates a quite small amount of spring back, thus exhibiting
preferable shape freezing properties. In addition, due to a
hardening effect upon the hot pressing, this method can present
parts having significantly higher strength, with high accuracy.
However, this process requires heating the steel plate prior to
subjecting it to the hot press working, and also requires reduction
of the manufacturing scale after the hot press working. Thus, this
process has possibility to significantly deteriorate the working
efficiency. Furthermore, shorter life of the mold, which should
contact with a heated steel plate, inevitably increase the
manufacturing cost.
[0008] The elongation of the steel plate after the hot press
working will be decreased, as such a hot pressed member may tend to
be broken only due to slight deformation caused by being subjected
to some external force, impact or the like. Therefore, the steel
plate of this type is generally assessed to be of poor impact
absorbing ability. Accordingly, it is quite difficult to use such
hot pressed parts as key components for securing the safety for
cars or the like.
[0009] As a method for enhancing the strength, reinforcement by a
solid solution treatment, reinforcement utilizing precipitation,
reinforcement by grain refinement, and reinforcement utilizing a
low-temperature transforming phase can be mentioned. It is not
possible to manufacture the steel plate, for which significantly
enhanced strength is required, only by employing a reinforcing
mechanism, including the solution treatment or precipitation
requiring addition of a greater amount of alloys. Even in the case
of utilizing the reinforcement by grain refinement, the improvement
of the strength is limited although achieved to some extent. While
the reinforcement by utilizing a low-temperature transforming phase
is highly effective for manufacturing the steel plate exceeding
1200 MPa, this method can not be expected, in order to enhance
ductility which can be balanced with such improvement of the
strength.
[0010] Generally, higher strength of the steel plate may tend to
lower ductility, as such degrading the workability.
[0011] As known materials having enhanced ductility among
high-strength steel plates, there are a dual phase steel plate
consisting of ferrite and martensite phases, and a transformation
induced plasticity (TRIP) steel plate consisting of ferrite,
bainite and retained austenite phases.
[0012] The dual phase steel plate is formed by finely dispersing
the hard martensite phase in the ferrite phase. Due to the highly
hard martensite phase, significant work hardening is caused upon
transformation, thus providing higher ductility to the steel
plate.
[0013] Examples of the TRIP steel plate are described in Patent
Documents 1 and 2. The steel plate of this type containing the
retained austenite phase exhibits highly excellent ductility and
moldability both attributed to working induced transformation,
depending on the amount the retained austenite phase and the
stability to the deformation.
[0014] However, if attempting to obtain the steel plate having
strength greater than 1200 MPa, delayed fracture may be caused. The
term "delayed fracture" means a phenomenon wherein while cracking
and/or fracture is not generated upon working and assembly for
respective members, it appears suddenly during use of them. A
high-strength steel plate disclosed in Patent Document 3 is
intended to provide more preferable anti-delayed fracture
properties, by reducing a soft phase, such as the ferrite phase, as
much as possible, and by controlling the volume fraction of the
retained austenite phase to be less than 4%, relative to the
low-temperature transforming phase, such as the bainite phase
and/or the tempered martensite phase.
[0015] Patent Document 1: JP-A-No. 60-43425
[0016] Patent Document 2: JP-A-No. 9-104947
[0017] Patent Document 3: JP-B-No. 3247908
[0018] As the steel plate whose elongation properties in the cold
working are enhanced while keeping higher strength, the dual phase
steel plate and TRIP steel plate as described above can be
mentioned.
[0019] In the dual phase steel plate, a higher strength can be
achieved even in the case of a smaller additional amount of alloys,
as well as more uniform elongation properties can be obtained due
to the work hardening.
[0020] The TRIP steel plate exhibits higher ductility and has more
excellent deep drawing properties. Therefore, this material is
suitable for providing a part or member for which a complicated
shape, higher workability and more enhanced strength are
required.
[0021] The TRIP steel plate described in the Patent Document 1 is
manufactured by a method comprising: creating the ferrite phase in
the austenite phase by holding a raw material at 450 to 650.degree.
C. for 4 to 20 seconds in a cooling step after rolling, cooling it
to a temperature lower than 350.degree. C., and coiling it around a
rod material.
[0022] In the Patent Document 2, in order to promote formation of
the ferrite phase in the austenite phase in a cooling process after
the rolling, a raw material is gently cooled at Ar3 to Ar1 or
subjected to a rolling completion temperature of approximately Ar3
then cooled to a temperature within a range of 350 to 500.degree.
C., and is wound around a rod material.
[0023] Such a TRIP steel plate has a structure in which the
martensite phase, retained austenite phase and/or bainite phase is
dispersed in the ferrite parent phase, and exhibits excellent
strength and elongation properties.
[0024] However, under the condition of C.ltoreq.0.20% which can
ensure the spot welding properties, only the tensile strength as
high as 800 MPa can be obtained, as such more enhanced workability
should be desired. Accordingly, it is difficult to manufacture the
steel plate having significantly higher strength, under such
conditions.
[0025] Even in a method of gradually cooling the raw material to a
temperature lower than 500.degree. C. without providing the gentle
cooling process en route after the rolling, the promotion of
creating a fine ferrite phase can be achieved, if setting the
rolling completion temperature at a point of approximately Ar3.
With respect to the material quality of a hot rolled steel plate
which has been subjected to rolling at a temperature of
approximately A3, however, anisotropy of the material may tend to
be undesirably greater.
[0026] Moreover, the hot rolled steel plate described in the Patent
Document 1 exhibits lower rolling workability and has a metallic
structure in which coarse ferrite particles and retained austenite
particles are present contiguously because of the temporary
stopping for the cooling at a point of approximately A1.
[0027] Hydrogen dissolved in the steel plate, which is likely to be
a cause of the delayed fracture, is a factor of determining the
crystal phase, and is trapped preferentially in the retained
austenite phase. Especially, the interface between the martensite
phase and ferrite phase having been subjected to the impact or
working, i.e., the working induced transformation site, is
considered to be a highly possible trapping site for hydrogen.
[0028] Coarser retained austenite particles will provide a more
reduced ratio of the area of the interface between the martensite
phase and ferrite phase having been experienced the working induced
transformation, as compared with the volume of the retained
austenite particles. Consequently, the concentration of hydrogen to
be trapped is increased, as such presenting a greater risk of the
delayed fracture. If the martensite phase and the retained
austenite phase coexist contiguously (in an M-A state), propagation
of the fracture is likely to be promoted, thus providing a further
increased risk of the fracture.
[0029] The high-strength steel plate described in the Patent
Document 3 is intended to enhance the anti-delayed fracture
properties by limiting the amount of the retained austenite.
However, in order to obtain excellent workability while keeping
higher strength, utilization of the retained austenite is
substantially effective. Accordingly, it is desirable if the
presence of the retained austenite will not detrimentally affect
the anti-delayed fracture properties without providing any
limitation as described above.
SUMMARY OF THE INVENTION
[0030] To address this challenge, the present inventors have
developed a new low-alloy and higher-strength steel plate and a
method of manufacturing thereof, the steel plate having a bainite
phase in which seven or more of the retained austenite particles
having a particle size of 1 .mu.m or less are finely dispersed per
10 .mu.m.sup.2 (the volume fraction is within the range of from 5%
to 20%), thereby exhibiting higher strength as well as more
preferred workability and secure anti-delayed fracture
properties.
[0031] Through many experiments, we have found that a preferably
high-strength steel plate can be obtained by employing appropriate
rolling conditions and selecting a proper composition of components
for the steel plate. Namely, higher strength and excellent
ductility as well as secure anti-delayed fracture properties can be
provided to a low-alloy steel plate, by subjecting a slab having a
proper composition of components to rough hot rolling under high
pressure conditions, completing rear-stage higher strain rolling in
a finish rolling process under high temperature conditions,
starting a cooling process after air-cooling for several seconds,
and coiling the processed material at an appropriate
temperature.
[0032] A high-strength hot rolled steel plate of the present
invention comprises: a retained austenite phase in a volume
fraction of 5% to 20%; a martensite phase in a volume fraction of
0% to 10%; and a bainite phase in the remaining volume fraction,
wherein particles constituting the retained austenite phase have a
particle size of 1 .mu.m or less. More preferably, the particle
size of prior austenite is 10 .mu.m or less, and the average aspect
ratio of the particles is 2.0 or less.
[0033] With the control of the particle size of the austenite
crystal after hot rolling to be 10 .mu.m or less (FIG. 3), the lath
structure of the bainite phase can be made fine. In addition, with
completion of uniform bainite transformation, the retained
austenite particles having a particle size of 1 .mu.m or less can
be finely and effectively dispersed in the phase with the density
of seven or more particles per 10 .mu.m.sup.2 (FIG. 8).
[0034] In this manner, superior anti-delayed fracture properties
can be obtained, even in the case of steel provided with a higher
ductility by utilizing working induced plasticity to be caused by a
relatively great amount of retained austenite.
[0035] With the control of the aspect ratio of the prior austenite
particles to be 2.0 or less (FIG. 3), anisotropy of the material,
which is drawn in both of the rolling direction and the direction
vertical to the rolling direction, can be reduced, as such
enhancing the workability (FIG. 4).
[0036] Preferably, the high-strength hot rolled steel plate of the
present invention has a composition comprising: C (0.13 to 0.21 (%
by weight)), Si (0.5 to 2.0), Mn (0.2 to 1.0), Cr (1.0 to 4.0), Ni
(0.02 to 1.0), Mo (0.05 to 0.4), P (0 to 0.010), S (0 to 0.003), N
(0.005 to 0.015), and the remaining components including Fe and
other inevitable impurities.
[0037] Such a chemical composition comprising proper types and
amounts of the selected components can facilitate formation of the
high-strength steel plate which can include the phases described
above and exhibit desired mechanical properties.
[0038] Since the alloy elements described above can constitute the
desired steel plate structure of the present invention in the steps
of cooling after hot rolling, and coiling the cooled material, Cr
and Si having greater influence on the bainite transformation are
included as major elements. With controlling of amounts of these
elements, the bainite transformation can be promoted, and formation
of the martensite phase can be suppressed, thereby to control the
entire phase to have an aimed strength.
[0039] The effect of each component will be described below.
[0040] It is preferred that the high-strength hot rolled steel
plate has the structure as described above, and also a plate
thickness of 1.0 to 3.0 mm, tensile strength (TS) of 1200 MPa or
greater, and elongation of 13% or greater (JIS No. 5 test
piece).
[0041] Namely, this steel plate can possess the structure described
above, and hence exhibit greater strength and more excellent
elongation properties.
[0042] A method for manufacturing a high-strength hot rolled steel
plate according to the present invention comprises the steps of:
[0043] (1) preparing a slab (rolling material) having a composition
containing: C (0.13 to 0.21 (% by weight)), Si (0.5 to 2.0), Mn
(0.2 to 1.0), Cr (1.0 to 4.0), Ni (0.02 to 1.0), Mo (0.05 to 0.4),
P (0 to 0.010), S (0 to 0.003), N (0.005 to 0.015), and the
remaining components including Fe and other inevitable impurities;
[0044] (2) roughly rolling a steel material, under the conditions
of: 1250.degree. C. or higher of an extraction temperature of a
reheating furnace; 1030.degree. C. or higher of a discharging-side
temperature of roughly rolling mills; and 30% or higher of a
reduction ratio for each of roughly rolling final three passes;
[0045] (3) finish rolling of the steel material under the
conditions of: 950.degree. C. or higher of a discharging-side
temperature of finish rolling mills; 40% or higher of a reduction
ratio for each of finish front-stage first to third rolling mills
(this is the case of using six rolling mills, but first to fourth
rolling mills are used in the case of using seven rolling mills)
and 0.5 or greater of accumulated strain in the pressed state due
to three rolling mills on a finish rear-stage; and [0046] (4)
cooling the steel material by air-cooling for 2 to 6 seconds,
followed by water-cooling, and coiling the steel material at a
coiling temperature of 550 to 650.degree. C.
[0047] With the purpose of enhancing strength by obtaining a
low-alloy baitnite phase due to employment of a temperature history
(FIG. 1) for maintaining the temperature, in the steps of rapid
cooling after hot rolling by utilizing hot strip milling and
coiling the material at predetermined temperature conditions, a
uniform phase of baitnite, in which martensite and retained
austenite are finely dispersed, can be obtained, by adding Cr and
Si as major alloy elements and selecting a composition containing
lower Mn and Ni (FIG. 7(b)).
[0048] With control of precipitation of carbides due to addition of
Si and with formation of a more uniform baitnite phase, the
austenite having carbon density of 0.8% or higher can be retained
in a greater amount. In this way, a steel plate having enhanced
strength and more excellent workability can be obtained (FIG.
11).
[0049] By controlling the hot rolling finish temperature to be
950.degree. C. or higher, the aspect ratio of the prior austenite
particles can be controlled at 2.0 or less (FIG. 3).
[0050] In order to prevent biting failure of a topmost portion of
the rolling material into a roll, it is preferred that, upon the
finish rolling, a reduction amount of a topmost portion of the
steel material is reduced, as needed, as compared with an expected
reduction amount (or reduction amount originally set for a
predetermined rolling), in first to fifth rolling mills (in the
case of using six stages of finish rolling mills, while first to
sixth rolling mills are used in the case of using seven stages of
finish rolling mills), wherein the reduction amount is increased by
10% or less, as compared with the expected amount of each rolling
mill. It is also preferred that a length to be rolled in the
increased reduction amount is within 5 m as measured from a biting
position of the topmost portion of the rolling material.
[0051] In order to prevent slip occurrence between the rolling
material and the roll during the rolling process, it is also
preferred that a special high-grip roll is used as a working roll
for each of finish first to third rolling mills including the final
rolling mill.
[0052] Our test on the manufacture, which will be described below,
demonstrates that the aforementioned high-strength steel plate can
be obtained readily by employing the conditioned as provided
above.
[0053] In the high-strength steel plate of the present invention,
the retained austenite is incorporated in the baitnite phase in a
volume fraction of 5% to 20% such that it is finely dispersed with
the density of seven or more particles per 10 .mu.m.sup.2.
Therefore, both strength and workability, which are contrary to
each other, can be provided to the steel plate, and excellent
anti-delayed fracture properties can also be provided thereto.
[0054] According to a method of the present invention for
manufacturing a high-strength steel plate, the high-strength steel
plate described above can be readily and securely manufactured.
BRIEF DESCRIPTION OF THE DRAWINGS
[0055] The above and other objects, features and advantages of the
present invention will become more apparent from the following
description taken in connection with the accompanying drawings, in
which:
[0056] FIG. 1 is a graph for schematically showing a temperature
history of hot rolling in a manufacturing process of one embodiment
of the present invention;
[0057] FIG. 2 is a photograph of prior austenite particles of a
crop at a finish entrance;
[0058] FIG. 3 is a photograph of prior austenite particles;
[0059] FIG. 4 is a graph showing a relationship of finish rolling
temperatures and anisotropy of elongation;
[0060] FIG. 5 is a graph showing a relationship of a rolling
schedule and a rolling temperature;
[0061] FIG. 6 is a graph showing a relationship of a dislocation
density and a particle size of prior austenite particles;
[0062] FIG. 7 is a photograph of typical phases of sections;
[0063] FIG. 8 is a photograph of phases of sections each obtained
by an EBSP method for a steel plate manufactured under
compositional and rolling conditions according to the present
invention, and bright or light color portions designate retained
austenite;
[0064] FIG. 9 is a graph showing a deviation of a plate thickness
at a distal end of a rolling material;
[0065] FIG. 10 is a graph showing a relationship of a coefficient
of friction and a reduction ratio, depending on types of rolls;
and
[0066] FIG. 11 is a graph showing a balance between the strength
and the ductility, and a relationship of the ductility and the
amount of retained austenite.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0067] Hereinafter, one embodiment of a sheet steel used to produce
parts by working the same, for which excellent workability and
anti-delayed fracture properties are required while keeping tensile
strength of 1200 MPa or higher, and a manufacturing method of the
sheet steel will be described.
[0068] The steel plate has a composition containing the following
components: C (0.13 to 0.21 (% by weight)), Si (0.5 to 2.0), Mn
(0.2 to 1.0), Cr (1.0 to 4.0), Ni (0.02 to 1.0), Mo (0.05 to 0.4),
P (0 to 0.010), S (0 to 0.003), N (0.005 to 0.015), and the
remaining components include Fe and other inevitable
impurities.
[0069] As used herein, the term "sheet plate" means a steel plate
having a thickness of from 1.0 mm to 3.0 mm. The steel plate to be
manufactured under the above compositional conditions can be mainly
used as parts for cars, consumer electrical appliances, electronic
equipment and the like, which require higher workability and
strength. In addition, the steel plate can also be applied to
materials for steel pipes.
[0070] First, components of the steel plate will be described.
[0071] The amount of carbon (C) should be within the range of 0.13
to 0.21%.
[0072] C is the most important component for stabilizing the
retained austenite. If the amount of C is less than 0.13%,
sufficient stability can not be obtained, thus an amount of C of
0.13% or greater should be required. However, if it exceeds 0.21%,
a welded portion becomes too hard and is likely to be broken. Such
a situation provides some limitation of use to the sheet steel to
be formed. Therefore, the upper limit described above is provided
to the amount of C. Namely, by setting the amount of C within the
range of 0.13 to 0.21%, it has been found that a composite
structure which accords with an intention of the present invention
can be obtained.
[0073] The amount of silicon (Si) should be within the range of 0.5
to 2.0%. Si also serves to stabilize the retained austenite. In
addition, Si enhances the strength to be obtained by reinforcement
due to solid solution. If the amount of Si is 0.5% or greater, a
preferred composite structure and material quality can be obtained.
A greater amount of Si can increase more retained austenite as well
as enhance the stability. However, if the amount of Si exceeds
2.0%, properties for balancing the strength and the ductility will
be saturated, thus the upper limit of the Si amount should be set
at 2.0% in view of reduction of the cost.
[0074] The amount of chromium (Cr) should be within the range of
1.0 to 4.0%. Cr can create the bainite phase, and enhance the
strength of the steel plate to be formed therewith.
[0075] If the amount of Cr is less than 1.0%, the amount of ferrite
is unduely increased, as such making it difficult to obtain a steel
plate with a desirably higher strength. Therefore, the Cr amount
should be 1.0% or greater. However, if it exceeds 4.0%, the
martensite phase is likely to be produced, thus making the steel
plate strength too high and hence rendering the anti-delayed
fracture properties insufficient. Therefore, 4.0% is set as the
upper limit.
[0076] The amount of manganese (Mn) should be within the range of
0.2 to 1.0%. If the Mn amount is less than 0.2%, the manufacture of
the steel plate will be difficult. Therefore, it should be 0.2% or
greater.
[0077] In order to attain higher strength, it is preferred to add
Mn as much as possible. However, if it is excessively added, the
martensite phase may tend to be produced, thus making it impossible
to obtain the intended structure according to this invention.
Therefore, the upper limit of the Mn amount should be set at
1.0%.
[0078] The amount of nickel (Ni) should be within the range of 0.02
to 1.0%. Ni can enhance the strength of the steel plate by
reinforcement due to solid solution. However, if the amount of Ni
is too increased, the martensite phase is likely to be produced.
Moreover, inadvertent addition would lead to increase of the
production cost. Thus, the upper limit should be set at 1.0%.
[0079] Molybdenum (Mo) can create the bainaite phase as is similar
to Cr, and enhance the strength of the steel plate to be formed
therewith. In addition, a hydrogen trapping effect due to Mo
carbides is useful for providing anti-delayed fracture properties
to the steel plate. However, inadvertent addition would cause
unduely restrained recrystallization as well as lead to increase of
the cost. Therefore, the Mo amount should be set within the range
of from 0.05 to 0.40%.
[0080] In order to enhance the weldability, it is necessary to
possibly reduce the amount of phosphorus (P). Thus, the upper limit
of this element should be set at 0.010%.
[0081] Also, in order to enhance the weldability, it is necessary
to possibly reduce the amount of sulfur (S). Therefore, the upper
limit of this element should be set at 0.003%.
[0082] The amount of nitrogen (N) should be within the range of
0.005 to 0.015%. As is similar to carbon, nitrogen is useful to
stabilize the austenite phase. However, its excessive existence
will cause degradation of the weldability. Thus, the range of this
amount should be set at a value of from 0.005 to 0.015%.
[0083] A slab produced to have the composition as described above
is then subjected to hot rolling after heated again or subjected to
hot rolling immediately after casting.
[0084] FIG. 1 is a graph for schematically showing a temperature
history of hot rolling in a manufacturing process of one embodiment
of the present invention, in which particles sizes of prior
austenite are also designated. The horizontal axis denotes the
elapsed time and the vertical axis denotes the temperature.
[0085] Upon providing the hot rolling, the extraction temperature
of the reheating furnace was set at 1250.degree. C. This
temperature was selected to preferentially secure the surface
temperature of 950.degree. C. after the finish, even though some
inevitable growth of austenite particles would be caused in the
reheating furnace due to such a high temperature condition.
However, the size or diameter of the austenite particles will be
lessened in the following rolling process. Therefore, it is
necessary to reduce the particle size of the prior austenite as
finely as possible before subjecting it to a finishing rolling
mill. Accordingly, in a roughly rolling process, the crystal
particle size is reduced in advance to 35 .mu.m or less, by setting
the reduction ratio of each of final three passes for roughly
rolling at 30% or greater, at a discharging-side temperature of
1030.degree. C. or higher on the discharging side of the roughly
rolling mills. FIG. 2 shows a particle size of prior austenite
after subjected to the roughly rolling, wherein the processed
material was cut by a pre-finish crop shearing machine.
[0086] For first to third rolling mills on a finish front-stage (in
the case of using six finish rolling mills, but first to fourth
rolling mills are used in the case of using seven finish rolling
mills), the reduction ratio per mill is set at 40% or higher.
Accumulated strain in the pressed state for three rolling mills of
a finish rear-stage is set at 0.5 or greater, and the finishing
rolling mill discharging-side temperature is securely set at
950.degree. C. or higher, so as to render the austenite particle
size equal to or less than 10 .mu.m. In addition, air-cooling is
provided for 2 to 6 seconds after the finish rolling, followed by
water-cooling. A coiling temperature is set at 550.degree. C. to
650.degree. C. In the above air-cooling step, the size of the
austenite particles is also controlled. Namely, during the hot
rolling step, the particle size of the prior austenite is
controlled to be 10 .mu.m or less before post-hot-rolling hot run
cooling is started, so as to control the size of the prior
austenite particles to eliminate working strain.
[0087] FIG. 3 shows a result of observation for the prior austenite
particles of the steel plate according to the present invention by
using an SEM phase observation. An average particle size of the
prior austenite particles is 9.3 .mu.m, presenting a uniformly
granulated structure. An average aspect ratio of the major axis/the
short axis is 1.7.
[0088] In the case of using low-temperature rolling, such that a
rolling completion temperature is 950.degree. C. or lower, and
employing the accumulated strain, set at 0.5 or less, of three
rolling mills on a finish rear-stage, the austenite particles
become larger (10 .mu.m or larger), and the shape of each austenite
particle may tend to be flat due to rolling, thus causing increased
anisotropy. FIG. 4 shows a relationship between the finish rolling
mill discharging-side temperature (FDT) and anisotropy of
elongation. As is seen from FIG. 4, the anisotropy of elongation
appears when the FDT is 950.degree. C. or lower. This anisotropy is
defined by an equation of IC-LI/(C+L)/2 (L is an elongation in the
rolling direction, and C is an elongation in the direction vertical
to the rolling direction). A smaller value to be obtained from this
equation shows less anisotropy.
[0089] As used herein, the "strain" means a value designated by
.epsilon. in the following equation:
.epsilon.=(h.sub.0-h.sub.1)/{(h.sub.0+h.sub.1)/2}
wherein a difference, between a thickness h.sub.0 of the steel
plate on the inlet side and its thickness h.sub.1 on the
discharging side, for each stand (each stage, or each pass upon
rough rolling), is divided by an average thickness of the both
thicknesses.
[0090] As used herein, the "accumulated strain" means a value
expressed by .epsilon..sub.c in the following equation:
.epsilon..sub.c=.epsilon..sub.n+.epsilon..sub.n-1/2+.epsilon..sub.n-2/4
wherein strain of each stage (each pass) of the rear-finish three
stands is calculated by using a weighted estimation, considering
the strength of each effect to be imposed on the metal phase, and
wherein the strain to be generated on a final stage (final pass),
front-stage (pre-pass), and pre-front-stage (pre-(front-pass)) is
each expressed by .epsilon..sub.n, .epsilon..sub.n-1, and
.epsilon..sub.n-2.
[0091] In order to carry out high-temperature finish rolling, a
temperature rising process for the steel plate by utilizing heat
generated by working due to the rolling is employed. To this end,
it is important to provide a schedule of high-strain rolling for
each rear-stage rolling mill as well as to set the reduction ratio
of the front-stage stand at 40% or higher. As shown in FIG. 5, it
can be seen that the surface temperature after finishing will vary,
by 80.degree. C., depending on the type of steel, in the case of
using the same rolling size with respect to the reduction ratio
while there is a difference in exit thicknesses at roughing
mill.
[0092] Hot rolling is completed at a temperature of 950.degree. C.
or higher, and the material is then subjected to air-cooling for 2
to 6 seconds without undergoing the post-hot-rolling hot run
cooling, so as to reduce the dislocation density in the crystal
particles. In FIG. 6, changes in the austenite particle size and
changes in the dislocation density are illustrated, wherein these
data are obtained by calculation over a period from a finishing F1
rolling mill to starting the hot run cooling, in the case of
changing the rolling temperatures for the same type of steel. From
the drawing, it can be seen that the dislocation density is
significantly influenced by the rolling temperature. It can also be
seen that under high-pressure rolling conditions, the austenite
particle size will be smaller under lower temperature conditions,
provided that the processing temperature is equal to or higher than
that required for Ar3 transformation. However, under such lower
temperature conditions, the dislocation density will be higher,
thus providing a material with further increased anisotropy.
Additionally, it can be seen that while the dislocation density is
significantly reduced due to the hot run air-cooling after rolling,
effective results can be obtained by employing the cooling time
within six seconds. In this simulation model, setting the aspect
ratio at 2.0 or less can be translated into controlling the
dislocation density to be at least 2.50E +10 (.rho./cm.sup.2) or
less (this was confirmed from the results of comparison between
actual data and the simulation model). However, the reduction of
the dislocation density leads to increase of the size of the prior
austenite particle. In order to further reduce the above-described
numerical value of the dislocation density while controlling the
prior austenite particle size at 10 .mu.m or less, the
aforementioned rolling conditions (rolling temperature: 950.degree.
C. or higher, and cooling time: 2 to 6 seconds) are required.
[0093] The simulation model described above is based on Yanagimoto,
Morimoto, et al., "Iron and Steel", vol. 88 (2002), No. 11, and
each coefficient of the numerical formulae was reviewed for this
application.
[0094] While the coiling temperature was set at a value within the
range of from 550.degree. C. to 650.degree. C., the temperature
range lower than 550.degree. C. may tend to increase the martensite
phase, thus increasing the possibility of delayed fracture. On the
other hand, the temperature rang higher than 650.degree. C. will
generate more ferrite and pearlite, as such making it difficult to
obtain higher strength. In FIG. 7, sectional phases of three-types
of high-strength steel plates are shown.
[0095] Either phase shown in FIG. 7 is bainite based, in which a
photograph corresponding to FIG. 7(a) shows a martensite-rich
structure, FIG. 7(b) shows a lower-martensite and relatively fine
structure, and FIG. 7(c) shows a ferrite-containing structure. FIG.
7(b) shows a structure obtained according to the present
invention.
[0096] In the bainite-based structure, austenite is retained in
each interface between the prior austenite particles as well as in
each packet boundary and each block boundary, i.e., in the prior
austenite particles themselves. The retained austenite can be
closely and uniformly dispersed into a parent phase such that seven
or more of the retained austenite particles having a very fine
particle size, such as 1 .mu.m or less, are dispersed per 10
.mu.m.sup.2, by employing the bainite phase as the parent phase and
setting the size of prior austenite particles before transformation
at 10 .mu.m or less. FIG. 8 is a photograph of structures of
sections each obtained by an EBSP method, for the steel plate
according to the present invention, in which the bainite phase
having a body-centered cubic phase and the austenite phase having a
face-centered cubic phase are discriminated by colors. The retained
austenite phase shown by a bright color constitutes a structure in
which seven or more of the retained austenite particles having a
particle size of 1 .mu.m or less are finely and uniformly dispersed
per 10 .mu.m.sup.2.
[0097] Due to such control for the hot rolling, the bainite phase
can be obtained, in which the retained austenite particles are
finely and uniformly dispersed.
[0098] If rolling a high-strength thin plate material (having a
thickness of 2 mm or less) under high-strain and
high-reduction-ratio conditions, a biting failure at a plate top
portion and/or a slip between a roll and a rolled material during
the rolling operation is likely to occur. It has been found that
from the rolling results, binding properties at a topmost portion
of the rolling material is not problematic in the case of using a
material of TS less than 1000 MPa at the reduction ratio of 40 to
50% per each rolling mill. On the other hand, the binding failure
at the topmost portion of the rolling material will be likely to
frequently occur (rate of occurrence: 50%), at the final rolling
mill and the first to second rolling mills of the front-stage
rolling mills, if using a material of TS greater than 1000 MPa. As
a measure for addressing this problem, we have attempted to elevate
the roll grinding finish roughness Ra up to 1 .mu.m (ordinarily 0.5
.mu.m) in order to raise the coefficient of roll friction, so as to
obtain the coefficient of friction (.mu.) during rolling of 0.4
(ordinarily 0.3). In addition, we have reduced the flow amount of
the roll cooling water in order not to unduely decrease the
temperature at the topmost portion of the rolling material.
However, securely effective results could not be obtained.
Accordingly, as shown in FIG. 9, we have attempted to render the
topmost portion of the rolling material thinner, over a place
within the range of 5 m from the discharging side of the rolling
mill, so as to make a thinner plate thickness (by 10% of a
thickness finally expected). Thereafter, an inclination up to the
expected plate thickness was provided to the plate material.
[0099] As a result, the binding failure was drastically decreased
(rate of occurrence: 0%). In addition, the setting reduction amount
employed in a range from the finish front-stage rolling mills to
the rolling mills located before the finish final rolling mill was
set at a value to be obtained by adding 10% or less of an expected
set value thereto. The reduction setting time is set within two
seconds from a biting site of the topmost portion of the plate
material into the rolling mill.
[0100] With respect to the slip between the rolling mill and the
rolled material during a rolling process, if rolling a material of
TS greater than 1000 MPa, under high temperature and high pressure
conditions, with the final plate thickness being set less than 2
mm, slip is likely to occur at the final rolling mill and the
rolling mill located just before the final rolling mill. As a
phenomenon of this situation, a metallic sound is generated during
the rolling process, the rolling load of the rolling mill upon
occurrence of the slip is drastically decreased to 50% or so. Then,
the rolls become idling, and the rolled plate can not be advanced.
At this time, when pulling out a roll from each rolling mill and
measuring the rolling roughness Ra of the roll, it is measured to
be less than 0.1 .mu.m, showing a state wherein the rolled material
and the roll are likely to slip on each other. To address such a
situation, we have employed special high-grip rolls. As a result,
the occurrence of the slip could be avoided completely. The rolls
are each formed by uniformly dispersing micro-carbide particles
(particle size: less than 1 .mu.m) over the whole surface of the
roll. These carbide particles can be utilized as spikes and
supported by a hard base material. In addition, even through the
micro-carbide particles will be worn away from the surface,
micro-oxide particles will successively appear from below, thus
maintaining a stable coefficient of friction, thereby to prevent
the occurrence of slip. As shown in FIG. 10, changes of the
coefficient of friction due to the rolling process is maintained in
a more suitable range (approximately 0.3) as compared with commonly
known rolls.
[0101] FIG. 11 is a graph showing a relationship between the volume
fraction (V.gamma.) of the retained austenite in the heat rolled
steel plate produced by the manufacturing process shown in FIG. 1
and data obtained by the tensile test. FIG. 11(a) shows a
relationship between the volume fraction V.gamma. and (the tensile
strength.times.elongation). FIG. 11(b) shows a relationship between
the volume fraction V.gamma. and the elongation. As is seen from
the drawing, in the range of from 5 to 20% of the volume fraction
of the retained austenite, as the volume fraction V.gamma. is
increased, the data of the tensile strength.times.elongation and
the elongation alone are improved. The metallic phase corresponding
to the data can be considered as the lower-martensite fine bainite
phase as shown in FIG. 7(b).
[0102] The present invention was made on the above empirical
basis.
EXAMPLES
[0103] Hereinafter, examples of the present invention will be
described.
[0104] Slab materials (rolling materials) were prepared from melted
steel having each chemical composition shown in Table 1 by using a
forging method or continuous casting method. Subsequently, these
slab materials were heated again, and subjected to hot rolling, so
as to obtain hot rolled steel plates, respectively. Table 2 shows
respective conditions of the hot rolling and properties of the
materials.
TABLE-US-00001 TABLE 1 Chemical composition (%) N Classes Types C
Si Mn P S Cu Ni Cr Mo (ppm) Developed A 0.180 0.51 0.72 0.008 0.002
0.09 0.51 2.63 0.35 104 Steel B 0.180 1.00 0.40 0.009 0.001 0.07
0.16 2.97 0.33 137 C 0.182 1.44 0.33 0.008 0.001 0.08 0.11 2.99
0.31 101 Comparative D 0.188 0.26 0.42 0.008 0.002 0.11 3.87 1.96
0.65 123 Steel E 0.180 0.24 0.76 0.005 0.011 0.30 1.03 2.22 0.46 67
F 0.097 1.45 0.41 0.002 0.005 0.11 0.16 2.95 0.29 79 G 0.120 2.00
0.36 0.002 0.005 0.11 0.17 3.02 0.31 69 H 0.199 1.97 0.41 0.010
0.005 0.11 0.17 4.42 0.31 166 I 0.241 1.53 0.40 0.010 0.005 0.11
0.16 2.97 0.31 82
[0105] With respect to steel types shown in Table 1, A, B, C
designate steel plates prepared in accordance with the present
invention, while D, E, F, G, H are provided as comparative
examples.
[0106] The steel type D as one comparative example contains
significantly lower Si and is excessively rich in Ni, thus
departing from the preferred range of the present invention.
[0107] The steel type E contains significantly lower Si, thus also
departing from the range defined according to the present
invention.
[0108] The steel types F and G contains lower C, as such departing
from the preferred range of the present invention, and the steel
type I exhibits an unduely high content of C, thus also departing
from the desired range of the present invention. The steel type H
is excessively rich in Cr, as such departing from the preferred
range of the present invention.
TABLE-US-00002 TABLE 2 Examples Hot rolling conditions Tensile test
Steel Nos. Steel Type .epsilon.c FDT CT TS EI TS * EI S/W
properties Delayed fracture Note 1 A 0.85 975 655 778 24.4 18,983
.largecircle. .largecircle. Comparative example 2 A 0.80 1,006 630
1,239 13.6 16,850 .largecircle. .largecircle. Developed steel 3 B
0.99 1,024 595 1,274 14.8 18,855 .largecircle. .largecircle.
Developed steel 4 B 0.45 925 610 1,360 13.1 17,816 .largecircle. X
Comparative example 5 B 0.45 960 600 1,335 13.0 17,355
.largecircle. X Comparative example 6 C 0.84 1,000 610 1,319 14.9
19,653 .largecircle. .largecircle. Developed steel 7 D 0.92 900 410
1,440 12.0 17,280 X X Comparative example 8 E 0.67 907 636 1,143
13.0 14,859 .largecircle. .largecircle. Comparative example 9 F
0.67 931 637 1,134 13.7 15,536 .largecircle. .largecircle.
Comparative example 10 G 0.67 935 638 1,162 13.8 16,036
.largecircle. .largecircle. Comparative example 11 H 0.67 942 635
1,559 14.6 22,761 X X Comparative example 12 I 0.67 943 635 1,357
17.8 24,155 X X Comparative example
[0109] Nos. 1 to 6 in Table 2 are examples in which the steel types
A, B, C in Table 1, respectively satisfying the preferred range of
the present invention, are subjected to rolling under various
conditions.
[0110] No. 1 was prepared by using the steel type A containing
0.51% Si and by employing the hot rolling coiling temperature of
655.degree. C. In this case, the data of TS (tensile
strength).times.EL (elongation) is quite preferable, but the
tensile strength is 778 MPa, which is undesirably low.
[0111] No. 2 was prepared by using the steel type A and employing
the hot rolling coiling temperature of 630.degree. C. This example
shows the tensile strength of 1200 MPa and the elongation of 13%,
thus exhibiting excellent properties.
[0112] No. 3 was prepared by using the steel type B containing
1.00% Si and by employing the hot rolling coiling temperature of
595.degree. C. As shown in Table 2, this example is excellent in
both of the strength and the elongation. Additionally, this example
shows more enhanced properties in both of the strength and the
elongation, as compared with the No. 2 example.
[0113] No. 4 and No. 5 were prepared under unsatisfied
reduction-ratio conditions during the hot rolling, as such these
examples exhibit negative delayed fracture while satisfying the
strength and the elongation.
[0114] No. 6 was prepared by using the steel type C containing
1.44% Si and by employing the hot rolling coiling temperature of
610.degree. C. This example exhibits excellent properties in both
of the strength and the elongation, and is superior to the No. 3
example in both of the strength and the elongation.
[0115] Nos. 7 to 12 were respectively prepared by carrying out hot
rolling, using steel types of comparative examples departing from
the desired range of the composition used in the present
invention.
[0116] No. 7 was prepared by rolling, using the steel type D
containing lower Si and higher Ni. This comparative example is
insufficient in the spot welding properties (S/W properties) as
well as in the delayed fracture properties.
[0117] No. 8 was prepared by using the steel type E containing
lower Si, thus exhibiting insufficient strength and poor balance of
strength/ductility.
[0118] No. 9 and No. 10 were prepared by using the steel types F
and G both containing lower C, respectively, as such exhibiting
unduely lower strength and poor balance of strength/ductility.
[0119] No. 11 and No. 12 were prepared by using the steel types H
and I both containing excessively high C, thus exhibiting properly
higher strength and good balance of strength/ductility. However,
these comparative examples are insufficient in the spot welding
properties as well as in delayed fracture properties.
[0120] The volume fraction of the ferrite particles was measured by
observation using an optical microscope, after polishing a section
cut along the rolling direction of each steel plate and then
subjecting the polished surface to nital corrosion. The measurement
also used a commercially available image analyzer.
[0121] The volume fraction of the martensite was obtained by
measuring the martensite phase expressed by a white color in an
image analysis process during observation using an optical
microscope for a position directed to 1/4 of the plate thickness
direction, after polishing a section cut along the rolling
direction of each steel plate and then etching the polished surface
by using a liquid formed by mixing 1:1 of 4% picric acid-alcohol
and 2% sodium pyrophosphate.
[0122] The measurement of the retained austenite was carried out by
employing the X-ray diffraction by using K.alpha. ray of Cu. The
volume fraction was determined as an average of the volume fraction
of the retained austenite to be calculated from a combination of
data obtained by respectively measuring integrated intensities of
(200), (220) and (311) faces of the austenite phase and those of
(200), (211) faces of the ferrite phase, after electrolytic
polishing for a position directed to 1/2t of the plate thickness
direction.
[0123] The tensile properties (tensile strength (TS) and elongation
(EL)) were measured by subjecting each sample to a tensile test,
the sample being formed into the shape in accordance with the JIS
No. 5 test piece.
[0124] The delayed fracture properties was assessed by observation
of each sample dipped in a 1N hydrochloric acid solution for a
predetermined period of time, after forming .phi.10 mm punch holes
with a 12.2% clearance in a central portion subjected to the
tensile test, onto which 8% or more of strain had been loaded.
[0125] As described above, the high-strength steel plates obtained
by the examples, which exhibit high strength and high ductility
properties in a lower alloy composition are suitable for use as
components for manufacturing car structures.
[0126] For example, the high-strength steel plates according to the
present invention can be used as quite preferred materials, such as
center pillars for cars, which require highly excellent properties,
including sufficient tensile strength for supporting doors and
preventing deformation upon collision or the like, bendability for
press molding, deep drawability, hole extending workability for
forming an attachment hole to be used for associated equipment, and
weldability for welding the material to another car component.
[0127] Although the invention has been described in its preferred
embodiments with a certain degree of particularity, obviously many
changes and variations are possible therein. It is therefore to be
understood that the present invention may be practiced otherwise
than as specifically described herein without departing from the
scope and spirit thereof.
* * * * *