SPECIFICATION
Title of Invention
METHOD FOR MANUFACTURING HOT STAMPED BODY HAVING VERTICAL
WALL AND HOT STAMPED BODY HAVING VERTICAL WALL
Technical Field
[OOO 1)
The present invention relates to a method for manufacturing a hot stamped body
having a vertical wall and a hot stamped body having a vertical wall.
Priority is claimed on Japanese Patent Application No. 20 10-237249, filed
October 22,2010, the content of which is incorporated herein by reference.
Background Art
[0002]
In order to obtain high-strength components of a grade of 11 80 MPa or higher
used for automobile components or the like with excellent dimensional precision, in
recent years, a technology (hereinafter, referred to as hot stamping forming) for realizing
high strength of a formed product by heating a steel sheet to an austenite range,
performing pressing in a softened and high-ductile state, and then rapidly cooling
(quenching) in a press die to perform martensitic transformation has been developed.
[0003]
In general, a steel sheet used for hot stamping contains a lot of C component for
securing product strength after hot stamping and contains austenite stabilization elements
such as Mn and B for securing hardenability when cooling a die. However, although
the strength and the hardenability are properties necessary for a hot stamped product,
when manufacturing a steel sheet which is a material thereof, these properties are
disadvantageous, in many cases. As a representative disadvantage, with a material
having such a high hardenability, a hot-rolled sheet after a hot-rolling step tends to have
an uneven microstructure in locations in hot-rolled coil. Accordingly, as means for
5 solving unevenness of the microstructure generated in a hot-rolling step, performing
tempering by a batch annealing step after a hot-mlling step or a cold-rolling step may be
considered, however, the batch annealing step usually takes 3 or 4 days and thus, is not
preferable from a viewpoint of productivity. In recent years, in normal steel other than a
material for quenching used for special purposes, from a viewpoint of productivity, it has
10 become general to perform a thermal treatment by a continuous annealing step, other
than the batch annealing step.
[0004]
However, in a case of the continuous annealing step, since the annealing time is
short, it is dificult to perform spheroidizing of carbide to realize softness and evenness
15 of a steel sheet by long-time thermal treatment such as a batch treatment. The
spheroidizing of the carbide is a treatment for realizing softness and evenness of the steel
sheet by holding in the vicinity of an Acl transformation point for about several tens of
hours. On the other hand, in a case of a short-time thermal treatment such as the
continuous annealing step, it is difficult to secure the annealing time necessary for the
20 spheroidizing. That is, in a continuous annealing installation, about 10 minutes is the
upper limit as the time for holding at a temperature in the vicinity of the Acl, due to a
restriction of a length of installation. In such a short time, since the carbide is cooled
before being subjected to the spheroidizing, the steel sheet has an uneven microstructure
in a hardened state. Such partial variation of the microstructure becomes a reason for
25 variation in hardness of a hot stamping material, and as a result, as shown in FIG. 1,
variation is generated in strength of the material before heating in a hot stamping step, in
many cases.
[OOOS]
Currently, in a widely-used hot stamping formation, it is general to perform
5 quenching at the same time as press working after heating a steel sheet which is a
material by furnace heating, and by heating in a heating furnace evenly to an austenitic
single phase temperature, it is possible to solve the variation in strength of the material
described above. However, a heating method of a hot stamping material by the furnace
heating has poor productivity since the heating takes a long time. Accordingly, a
10 technology of improving productivity of the hot stamping material by a short-time
heating method by an electrical heating method is disclosed. By using the electrical
heating method, it is possible to control temperature distribution of a sheet material in a
conductive state, by modifying current density flowing to the same sheet material (for
example, Patent Document 1).
15 [0006]
In addition, in order to solve the variation in the hardness, when heating at a
temperature equal to or higher than Ac3 SO as to be an austenite single phase in an
annealing step, a hardened phase such as martensite or bainite is generated in an end
stage of the annealing step due to high hardenability by the effect of Mn or B described
20 above, and the hardness of a material significantly increases. As the hot stamping
material, this not only becomes a reason for die abrasion in a blank before stamping, but
also significantly decreases formability or shape fixability of a formed body.
Accordingly, if considering not only a desired hardness after hot stamping quenching,
formability or shape fixability of a formed body, a preferable material before hot
25 stamping is a material which is soft and has small variation in hardness, and a material
having an amount of C and hardenability to obtain desired hardness after hot stamping
quenching. However, if considering manufacturing cost as a priority and assuming the
manufacture of the steel sheet in a continuous annealing installation, it is difficult to
perform the control described above by an annealing technology of the related art.
[0007]
Further, in a case of manufacturing a formed body having a vertical wall by hot
stamping, when cooling in a die, a cooling rate in a vertical wall where clearance with
respect to the die is easily generated becomes lower than in a part adhered to the die.
Accordingly, since variation in hardness generated when quenching is added with respect
to the variation in hardness in the steel sheet before heating in a hot stamping step, there
is a problem in that significant variation in hardness is generated in the formed body
having the vertical wall.
Citation List
Patent Document
[OOOS]
[Patent Document 11 Japanese Unexamined Patent Application, First
Publication No. 2009-274122
Non-Patent Documents
[0009]
won-Patent Document 11 "Iron and Steel Materials", The Japan Institute of
Metals, Maruzen Publishing Co., Ltd. p. 2 1
won-Patent Document 21 Steel Standardization Group, "A Review of the Steel
Standardization Group's Method for the Determination of Critical Points of Steel," Metal
Progress, Vol. 49, 1946, p. 11 69
[Non-Patent Document 31 "Yakiiresei (Hardening of steels)--Motomekata to katsuyou
I (How $0 obtain and its use)--," (author: OWAKU Shigeo, publisher: Nikkan Kogyo
Shimbun
Summary of Invention
Technical Problem
5 [OO 1 01
An object of the present invention is to solve the aforementioned problems and
to provide a method for manufacturing a hot stamped body having a vertical wall and a
hot stamped body having a vertical wall which can suppress variation in hardness of a
formed body even in a case of manufacturing a formed body having a vertical wall from
1;0 a steel sheet for hot stamping.
Solution to Problem
[OO 111
An outline of the present invention made for solving the aforementioned
problems is as follows.
15 (1) According to a first aspect of the present invention, there is provided a
method for manufacturing a hot stamped body including the steps of
hot-rolling a slab containing chemical components which include, by mass%,
0.18% to 0.35% of C, 1.0% to 3.0% of Mn, 0.01% to 1.0% of Si, 0.001% to 0.02% of P,
0.0005% to 0.01% of S, 0.001% to 0.01% of N, 0.01% to 1.0% ofAl, 0.005% to 0.2% of
20 Ti, 0.0002% to 0.005% of By and 0.002% to 2.0% of Cr, and the balance of Fe and
inevitable impurities, to obtain a hot-rolled steel sheet;
coiling the hot-rolled steel sheet which is subjected to hot-rolling;
cold-rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet;
continuously annealing the cold-rolled steel sheet which is subjected to
25 cold-rolling to obtain a steel sheet for hot stamping; and
6
performing hot stamping by heating the steel sheet for hot stamping which is
continuously annealed so that a highest heating temperature is equal to or higher than
Ac3"C, and forming a vertical wall,
wherein the continuous annealing includes the steps of:
5 heating the cold-rolled steel sheet to a temperature range of equal to or higher
than AclOC and lower than Ac3"C;
cooling the heated cold-rolled steel sheet from the highest heating temperature to
660°C at a cooling rate of equal to or less than 10 "CIS; and
holding the cooled cold-rolled steel sheet in a temperature range of 550°C to
10 660°C for one minute to 10 minutes.
(2) In the method for manufacturing a hot stamped body according to (I), the
chemical components may hrther include one or more from 0.002% to 2.0% of Mo,
0.002% to 2.0% of Nb, 0.002% to 2.0% of V, 0.002% to 2.0% of Ni, 0.002% to 2.0% of
Cu, 0.002% to 2.0% of Sn, 0.0005% to 0.0050% of Ca, 0.0005% to 0.0050% of Mg, and
15 0.0005% to 0.0050% of REM.
(3) In the method for manufacturing a hot stamped body according to (I), any
one of a hot-dip galvanizing process, a galvannealing process, a molten aluminum plating
process, an alloyed molten aluminum plating process, and an electroplating process, may
be performed after the continuous annealing step.
20 (4) In the method for manufacturing a ,hot stamped body according to (2), any
one of a hot-dip galvanizing process, a galvannealing process, a molten aluminum plating
process, an alloyed molten aluminum plating process, and an electroplating process, may
be performed after the continuous annealing step.
(5) According to a second aspect of the present invention, there is provided a
25 method for manufacturing a hot stamped body including the steps of:
hot-rolling a slab containing chemical components which include, by mass%,
0.18% to 0.35% of C, 1.0% to 3.0% of Mn, 0.005% to 1.0% of Si, 0.001% to 0.02% of P,
Ti, 0.0002% to 0.005% of B, and 0.002% to 2.0% of Cr, and the balance of Fe and
5 inevitable impurities, to obtain a hot-rolled steel sheet;
~ coiling the hot-rolled steel sheet which is subjected to hot-rolling;
cold-rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet;
continuously annealing the cold-rolled steel sheet which is subjected to
cold-rolling to obtain a steel sheet for hot stamping; and
10 performing hot stamping by heating the steel sheet for hot stamping which is
continuously annealed so that a highest heating temperature is equal to or higher than
Ac3"C, and forming a vertical wall,
wherein, in the hot-rolling, in finish-hot-rolling configured with a machine with
5 or more consecutive rolling stands, rolling is performed by setting a finish-hot-rolling
15 temperature FIT in a final rolling mill F, in a temperature range of (Ac3 - 60)"C to (Ac~+
80)"C, by setting a time from start of rolling in a rolling mill F1-3 which is a previous
machine to the final rolling mill FI to end of rolling in the final rolling mill Fl to be equal
to or longer than 2.5 seconds, and by setting a hot-rolling temperature F1.3T in the rolling
mill F1-3 to be equal to or lower than FIT + 100°C, and after holding in a temperature
20 range of 600°C to Ar3"C for 3 seconds to 40 seconds, coiling is performed,
the continuous annealing includes the steps of
heating the cold-rolled steel sheet to a temperature range of equal to or higher
than (Acl - 40)"C and lower than Ac3"C;
cooling the heated cold-rolled steel sheet from the highest heating temperature to
25 660°C at a cooling rate of equal to or less than 10 "CIS; and
8
holding the cooled cold-rolled steel sheet in a temperature range of 450°C to
660°C for 20 seconds to 10 minutes.
(6) In the method for manufacturing a hot stamped body according to (5), the
chemical components may further include one or more from 0.002% to 2.0% of Mo,
0.002% to 2.0% of Nb, 0.002% to 2.0% of V, 0.002% to 2.0% of Ni, 0.002% to 2.0% of
Cu, 0.002% to 2.0% of Sn, 0.0005% to 0.0050% of Ca, 0.0005% to 0.0050% of Mg, and
0.0005% to 0.0050% of REM.
(7) In the method for manufacturing a hot stamped body according to (5), any
one of a hot-dip galvanizing process, a galvannealing process, a molten aluminum plating
process, an alloyed molten aluminum plating process, and an electroplating process, may
be performed after the continuous annealing step.
(8) In the method for manufacturing a hot stamped body according to (6), any
one of a hot-dip galvanizing process, a galvannealing process, a molten aluminum plating
process, an alloyed molten aluminum plating process, and an electroplating process, may
be performed after the continuous annealing step.
(9) According to a third aspect of the present invention, there is provided a hot
stamped body which is formed using the method for manufacturing a hot stamped body
according to any one of (1) to (S),
wherein, when a quenching start temperature is equal to or lower than 650°C,
variation of Vickers hardness AHv of the hot stamped body is equal to or less than 100,
when the quenching start temperature is 650°C to 750°C, variation of Vickers hardness
AHv of the hot stamped body is equal to or less than 60, and when the quenching start
temperature is equal to or higher than 750°C, variation of Vickers hardness AHv of the
hot stamped body is equal to or less than 40.
Advantageous Effects of Invention
[OO 121
According to the methods according to (1) to (8) described above, since the steel
sheet in which physical properties after the annealing are even and soft is used, even
when manufacturing a formed body having a vertical wall from such a steel sheet by hot
stamping, it is possible to stabilize hardness of the hot stamped body.
In addition, by performing a hot-dip galvanizing process, a galvannealing
process, a molten aluminum plating process, an alloyed molten aluminum plating process,
or an electroplating process, after the continuous annealing step, it is advantageous since
it is possible to prevent scale generation on a surface, raising a temperature in a
non-oxidation atmosphere for avoiding scale generation when raising a temperature of
hot stamping is unnecessary, or a descaling process after the hot stamping is unnecessary,
and also, rust prevention of the hot stamped body is exhibited.
In addition, by employing such methods, it is possible to obtain a hot stamped
body having a vertical wall in which, when a quenching start temperature is equal to or
lower than 650°C, variation of Vickers hardness AHv of the hot stamped body is equal to
or less than 100, when the quenching start temperature is 650°C to 750°C, variation of
Vickers hardness AHv of the hot stamped body is equal to or less than 60, and when the
quenching start temperature is equal to or higher than 750°C, variation of Vickers
hardness AHv of the hot stamped body is equal to or less than 40.
Brief Description of Drawings
[00 131
FIG. 1 is a view showing variation in hardness of a steel sheet for hot stamping
after continuous annealing of the related art.
10
FIG. 2 is a view showing a temperature history model in a continuous annealing
step of the present invention.
FIG. 3A is a view showing variation in hardness of a steel sheet for hot stamping
after continuous annealing in which a coiling temperature is set to 680°C.
FIG. 3B is a view showing variation in hardness of a steel sheet for hot stamping
after continuous annealing in which a coiling temperature is set to 750°C.
FIG. 3C is a view showing variation in hardness of a steel sheet for hot stamping
after continuous annealing in which a coiling temperature is set to 500°C.
FIG. 4 is a view showing a shape of a hot stamped product of example of the
present invention.
FIG. 5 is a view showing variation in hardenability when hot stamping by values
of Cre/CrM and Mne/MnM in the present invention.
FIG. 6A is a result of segmentalized pearlite observed by a 2000x SEM.
FIG. 6B is a result of segmentalized pearlite observed by a 5000x SEM.
FIG. 7A is a result of non-segmentalized pearlite observed by a 2000x SEM.
FIG. 7B is a result of non-segrnentalized pearlite observed by a 5000x SEM.
Description of Embodiments
[OO 141
Hereinafter, preferred embodiments of the present invention will be described.
[00 151
First, a method for calculating A cw~h ich is important in the present invention
will be described. In the present invention, since it is important to obtain an accurate
value of Ac3, it is desired to experimentally measure the value, other than calculating
from a calculation equation. In addition, it is also possible to measure Acl from the
same test. As an example of a measurement method, as disclosed in Non-Patent
11
Documents 1 and 2, a method of acquiring from length change of a steel sheet when
heating and cooling is general. At the time of heating, a temperature at which austenite
starts to appear is Acl, and a temperature at which austenite single phase appears is Ac3, ,
and it is possible to read each temperature from change in expansion. In a case of
5 experimentally measuring, it is general to use a method of heating a steel sheet after
cold-rolling at a heating rate when actually heating in a continuous annealing step, and
measuring Ac3 from an expansion curve. The heating rate herein is an average heating
rate in a temperature range of "500°C to 650°C" which is a temperature equal to or lower
than Acl, and heating is performed at a constant rate using the heating rate.
10 In the present invention, a measured result when setting a rising temperature rate
as 5 "CIS is used.
Meanwhile, a temperature at which transformation from an austenite single
phase to a low temperature transformation phase such as ferrite or bainite starts, is called
Ar3, however, regarding transformation in a hot-rolling step, AT3 changes according to
15 hot-rolling conditions or a cooling rate after rolling. Accordingly, Ar3 was calculated
with a calculation model disclosed in ISIJ International, Vol. 32 (1992), No. 3, and a
holding time from AT3 to 600°C was determined by correlation with an actual
temperature.
[00 161
20 Hereinafter, a steel sheet for hot stamping according to the present invention
used in a method for manufacturing a hot stamped body having a vertical wall will be
described.
[00 171
(Quenching Index of Steel Sheet for Hot Stamping)
2 5 Since it is aimed for a hot stamping material to obtain high hardness after
12
quenching, the hot stamping material is generally designed to have a high carbon
component and a component having high hardenability. Herein, the "high
hardenability" means that a DIlnchv alue which is a quenching index is equal to or more
than 3. It is possible to calculate the DIinchv alue based on ASTMA255-67. A detailed
calculation method is shown in Non-Patent Document 3. Several calculation methods
of the DI,,,h value have been propoged, regarding an equation of fB for calculating using
an additive method and calculating an effect of By it is possible to use an equation of fB =
1 + 2.7 (0.85 - wt% C) disclosed in Non-Patent Document 3. In addition, it is
necessary to designate austenite grain size No. according to an added amount of C,
however, in practice, since the austenite grain size No. changes depending on hot-rolling
conditions, the calculation may be performed by standardizing as a grain size of No. 6.
[0018]
The DImchv alue is an index showing hardenability, and is not always connected
to hardness of a steel sheet. That is, hardness of martensite is determined by amounts of
C and other solid-solution elements. Accordingly, the problems of this specification do
not occur in all steel materials having a large added amount of C. Even in a case where
a large amount of C is included, phase transformation of a steel sheet proceeds relatively
fastly as long as the DImchv alue is a low value, and thus, phase transformation is almost
completed before coiling in ROT cooling. Further, also in an annealing step, since
ferrite transformation easily proceeds in cooling from a highest heating temperature, it is
easy to manufacture a soft hot stamping material. Meanwhile, the problems of this
specification are clearly shown in a steel material having a high DIlnchv alue and a large
added amount of C. Accordingly, significant effects of the present invention are
obtained in a case where a steel material contains 0.18% to 0.35% of C and the DIln,h
value is equal to or more than 3. Meanwhile, when the DIlnch value is extremely high,
13
since the ferrite transformation in the continuous annealing does not proceed, a value of
about 10 is preferable as an upper limit of the DImCvha lue.
[00 1 91
(Chemical Components of Steel Sheet For Hot Stamping)
In the method for manufacturing a hot stamped body having a vertical wall
according to the present invention, a steel sheet for hot stamping manufactured from a
steel piece including chemical components which include C, Mn, Si, P, S, N, Al, Ti, By
and Cr and the balance of Fe and inevitable impurities is used. In addition, as optional
elements, one or more elements from Mo, Nb, V, Ni, Cu, Sn, Ca, Mg, and REM may be
contained. Hereinafter, a preferred range of content of each element will be
described. % which indicates content means mass%. In the steel sheet for hot
stamping, inevitable impurities other than the elements described above may be
contained as long as the content thereof is a degree not significantly disturbing the effects
of the present invention, however, as small an amount as possible thereof is preferable.
[0020]
(C: 0.18% to 0.35%)
When content of C is less than 0.18%, hardenability after hot stamping becomes
low, and rise of hardness in a component becomes small. Meanwhile, when the content
of C exceeds 0.35%, formability of the formed body is significantly decreased.
Accordingly, a lower limit value of C is 0.18, preferably 0.20% and more
preferably 0.22%. An upper limit value of C is 0.35%, preferably 0.33%, and more
preferably 0.30%.
[002 11
(Mn: 1.0% to 3.0%)
When content of Mn is less than 1.0%, it is difficult to secure hardenability at
& 14
the time of hot stamping. Meanwhile, when the content of Mn exceeds 3.0%,
segregation of Mn easily occurs and cracking easily occurs at the time of hot-rolling.
Accordingly, a lower limit value of Mn is 1.0%, preferably 1.2%, and more
preferably 1.5%. An upper limit value of Mn is 3.0%, preferably 2.8%, and more
5 preferably 2.5%.
[0022]
(Si: 0.01% to 1.0%)
Si has an effect of slightly improve the hardenability, however, the effect is
slight. By Si having a large solid-solution hardening amount compared to other
10 elements being contained, it is possible to reduce the amount of C for obtaining desired
hardness after quenching. Accordingly, it is possible to contribute to improvement of
weldability which is a disadvantage of steel having a large amount of C. Accordingly,
the effect thereof is large when the added amount is large, however, when the added
amount thereof exceeds 0. I%, due to generation of oxides on the surface of the steel
sheet, chemical conversion coating for imparting corrosion resistance is significantly
degraded, or wettability of galvanization is disturbed. In addition, a lower limit thereof
is not particularly provided, however, about 0.01% which is an amount of Si used in a
level of normal deoxidation is a practical lower limit.
Accordingly, the lower limit value of Si is 0.01%. The upper limit value of Si
is 1.0%, and preferably 0.8%.
[0023]
(P: 0.001% to 0.02%)
P is an element having a high sold-solution hardening property, however, when
the content thereof exceeds 0.02%, the chemical conversion coating is degraded in the
25 same manner as in a case of Si. In addition, a lower limit thereof is not particularly
* 15
provided, however, it is difficult to have the content of less than 0.001% since the cost
significantly rises.
[0024]
(S: 0.0005% to 0.01%)
5 Since S generates inclusions such as MnS which degrades toughness or
workability, the added amount thereof is desired to be small. Accordingly, the amount
thereof is preferably equal to or less than 0.01%. In addition, a lower limit thereof is
not particularly provided, however, it is difficult to have the content of less than 0.0005%
since the cost significantly rises.
10 [0025]
(N: 0.001% to 0.01%)
Since N degrades the effect of improving hardenability when performing B
addition, it is preferable to have an extremely small added amount. From this viewpoint,
the upper limit thereof is set as 0.01%. In addition, the lower limit is not particularly
15 provided, however, it is difficult to have the content of less than 0.001% since the cost
significantly rises.
[0026]
(Al: 0.01% to 1.0%)
Since A1 has the solid-solution hardening property in the same manner as Si, it
20 may be added to reduce the added amount of C. Since A1 degrades the chemical
conversion coating or the wettability of galvanization in the same manner as Si, the upper
limit thereof is 1.0%, and the lower limit is not particularly provided, however, 0.01%
which is the amount ofAl mixed in at the deoxidation level is a practical lower limit.
[0027]
2 5 (Ti: 0.005% to 0.2%)
Ti is advantageous for detoxicating of N which degrades the effect of B addition.
That is, when the content of N is large, B is bound with N, and BN is formed. Since the
effect of improving hardenability of B is exhibited at the time of a solid-solution state of
By although B is added in a state of large amount of N, the effect of improving the
5 hardenability is not obtained. Accordingly, by adding Ti, it is possible to fix N as TiN
and for B to remain in a solid-solution state. In general, the amount of Ti necessary for
obtaining this effect can be obtained by adding the amount which is approximately four
times the amount of N from a ratio of atomic weights. Accordingly, when considering
the content of N inevitably mixed in, a content equal to or more than 0.005% which is the
10 lower limit is necessary. In addition, Ti is bound with C, and Tic is formed. Since an
effect of improving a delayed fracture property after hot stamping can be obtained, when
actively improving the delayed fracture property, it is preferable to add equal to or more
than 0.05% of Ti. However, if an added amount exceeds 0.2%, coarse Tic is formed in
an austenite grain boundary or the like, and cracks are generated in hot-rolling, such that
15 0.2% is set as the upper limit.
[0028]
(B: 0.0002% to 0.005%)
B is one of most eficient elements as an element for improving hardenability
with low cost. As described above, when adding B, since it is necessary to be in a
20 solid-solution state, it is necessary to add Ti, if necessary. In addition, since the effect
thereof is not obtained when the amount thereof is less than 0.0002%, 0.0002% is set as
the lower limit. Meanwhile, since the effect thereof becomes saturated when the
amount thereof exceeds 0.005%, it is preferable to set 0.005% as the upper limit.
[0029]
(Cr: 0.002% to 2.0%)
17
Cr improves hardenability and toughness with a content of equal to or more than
0.002%. The improvement of toughness is obtained by an effect of improving the
delayed fracture property by forming alloy carbide or an effect of grain refining of the
austenite grain size. Meanwhile, when the content of Cr exceeds 2.0%, the effects
5 thereof become saturated.
[0030]
(Mo: 0.002% to 2.0%)
(Nb: 0.002% to 2.0%)
(V: 0.002% to 2.0%)
10 Mo, Nb, and V improve hardenability and toughness with a content of equal to
or more than 0.002%, respectively. The effect of improving toughness can be obtained
by the improvement of the delayed fracture property by formation of alloy carbide, or by
grain refining of the austenite grain size. Meanwhile, when the content of each element
exceeds 2.0%, the effects thereof become saturated. Accordihgly, the contained
15 amounts of Mo, Nb, and V may be in a range of 0.002% to 2.0%, respectively.
[003 11
(Ni: 0.002% to 2.0%)
(Cu: 0.002% to 2.0%)
(Sn: 0.002% to 2.0%)
2 0 In addition, Ni, Cu, and Sn improve toughness with a content of equal to or
more than 0.002%, respectively. Meanwhile, when the content of each element exceeds
2.0%, the effects thereof become saturated. Accordingly, the contained amounts of Ni,
Cu, and Sn may be in a range of 0.002% to 2.0%, respectively.
[0032]
(Ca: 0.0005% to 0.0050%)
(Mg: 0.0005% to 0.0050%)
(REM: 0.0005% to 0.0050%)
Ca, Mg, and REM have effects of grain refining of inclusions with each content
of equal to or more than 0.0005% and suppressing thereof. Meanwhile, when the
5 amount of each element exceeds 0.0050%, the effects thereof become saturated.
Accordingly, the contained amounts of Ca, Mg, and REM may be in a range of 0.0005%
to 0.0050%, respectively.
COO331
(Microstructure of Steel Sheet for Hot Stamping)
10 Next, a microstructure of the steel sheet for hot stamping will be described.
[0034]
FIG. 2 shows a temperature history model in the continuous annealing step. In
FIG. 2, Acl means a temperature at which reverse transformation to austenite starts to
occur at the time of temperature rising, and Ac3 means a temperature at which a metal
15 composition of the steel sheet completely becomes austenite at the time of temperature
rising. The steel sheet subjected to the cold-rolling step is in a state where the
microstructure of the hot-rolled sheet is crushed by cold-rolling, and in this state, the
steel sheet is in a hardened state with extremely high dislocation density. In general, the
microstructure of the hot-rolled steel sheet of the quenching material is a mixed structure
20 of ferrite and pearlite. However, the microstructure can be controlled to a structure
mainly formed of bainite or mainly formed of martensite, by a coiling temperature of the
hot-rolled sheet. As will be described later, when manufacturing the steel sheet for hot
stamping, by heating the steel sheet to be equal to or higher than AclOC in a heating step,
a volume fraction of non-recrystallized ferrite is set to be equal to or less than 30%. In
19
addition, by setting the highest heating temperature to be less than Ac3"C in the heating
step and by cooling fiom the highest heating temperature to 660°C at a cooling rate of
equal to or less than 10 "CIS in the cooling step, ferrite transformation proceeds in
cooling, and the steel sheet is softened. When, in the cooling step, the ferrite
5 transformation is promoted and the steel sheet is softened, it is preferable for the ferrite to
remain slightly in the heating step, and accordingly, it is preferable to set the highest
heating temperature to be "(Acl + 20)"C to (Ac3 - 10)OC. By heating to this
temperature range, in addition to that the hardened non-recrystallized ferrite is softened
by recovery and recrystallization due to dislocation movement in annealing, it is possible
10 to austenitize the remaining hardened non-recrystallized ferrite. In the heating step,
non-recrystallized ferrite remains slightly, in a subsequent cooling step at a cooling rate
of equal to or less than 10 "CIS and a holding step of holding in a temperature range of
"550°C to 660°C" for 1 minute to 10 minutes, the ferrite grows by nucleating the
non-recrystallized ferrite, and cementite precipitation is promoted by concentration of C
15 in the non-transformed austenite. Accordingly, the main microstructure after the
annealing step of the steel sheet for hot stamping according to the embodiment is
configured of ferrite, cementite, and pearlite, and contains a part of remaining austenite,
martensite, and bainite. The range of the highest heating temperature in the heating step
can be expanded by adjusting rolling conditions in the hot-rolling step and cooling
20 conditions in ROT. That is, the factor of the problems originate in variation of the
microstructure of the hot-rolled sheet, and if the microstructure of the hot-rolled sheet is
adjusted so that the hot-rolled sheet is homogenized and recrystallization of the ferrite
after the cold-rolling proceeds evenly and rapidly, although the lower limit of the highest
heating temperature in the heating step is expanded to (Acl - 40)"C, it is possible to
suppress remaining of the non-recrystallized ferrite and to expand the conditions in the
holding step (as will be described later, in a temperature range of "450°C to 660°C" for
20 seconds to 10 minutes).
5 In more detail, the steel sheet for hot stamping includes a metal structure in
which a volume fraction of the ferrite obtained by combining the recrystallized ferrite
and transformed ferrite is equal to or more than 50%, and a volume fraction of the
non-recrystallized ferrite fraction is equal to or less than 30%. When the ferrite fraction
is less than 50%, the strength of the steel sheet after the continuous annealing step
10 becomes hard. In addition, when the fiaction of the non-recrystallized ferrite exceeds
30%, the hardness of the steel sheet after the continudus annealing step becomes hard.
The ratio of the non-recrystallized ferrite can be measured by analyzing an
Electron Back Scattering diffraction Pattern (EBSP). The discrimination of the
15 non-recrystallized ferrite and other ferrite, that is, the recrystallized ferrite and the
transformed ferrite can be performed by analyzing crystal orientation measurement data
of the EBSP by Kernel Average Misorientation method (KAM method). The
dislocation is recovered in the grains of the non-recrystallized ferrite, however,
continuous change of the crystal orientation generated due to plastic deformation at the
20 time of cold-rolling exists. Meanwhile, the change of the crystal orientation in the
ferrite grains except for the non-recrystallized ferrite is extremely small. This is
because, while the crystal orientation of adjacent crystal grains is largely different due to
the recrystallization and the transformation, the crystal orientation in one crystal grain is
not changed. In the ICAM method, since it is possible to quantitatively show the crystal
25 orientation difference of adjacent pixels (measurement points), in the present invention,
when defining the grain boundary between a pixel in which an average crystal orientation
difference with the adjacent measurement point is within 1" (degree) and a pixel in which
the average crystal orientation difference with the adjacent measurement point is equal to
or more than 2" (degrees), the grain having a crystal grain size of equal to or more than 3
5 pm is defined as the ferrite other than the non-recrystallized ferrite, that is, the
recrystallized ferrite and the transformed ferrite.
[0037]
In addition, in the steel sheet for hot stamping, (A) a value of a ratio Cre/CrM of
concentration Cre of Cr subjected to solid solution in iron carbide and concentration CrM
10 of Cr subjected to solid solution in a base material is equal to or less than 2, or (B) a
value of a ratio Mne/MnM of concentration Mne of Mn subjected to solid solution in iron
carbide and concentration MnM of Mn subjected to solid solution in a base material is
equal to or less than 10.
[0038]
15 The cementite which is a representative of the iron carbide is dissolved in the
austenite at the time of hot stamping heating, and the concentration of C in the austenite
is increased. At the time of heating in a hot stamping step, when heating at a low
temperature for a short time by rapid heating or the like, dissolution of cementite is not
sufficient and hardenability or hardness after quenching is not sufficient. A dissolution
20 rate of the cementite can be improved by reducing a distribution amount of Cr or Mn
which is an element easily distributed in cementite, in the cementite. When the value of
Cre/Cr~e xceeds 2 and the value of Mne/MnMe xceeds 10, the dissolution of the
cementite in the austenite at the time of heating for short time is insufficient. It is
preferable that the value of Cre/CrM be equal to or less than 1.5 and the value of
MIle/MnM to be equal to or less than 7.
The Cre/CrM and the MIle/MnM can be reduced by the method for manufacturing
a steel sheet. As will be described in detail, it is necessary to suppress diffusion of
substitutional elements into the iron carbide, and it is necessary to control the diffusion in
5 the hot-rolling step, and the continuous annealing step after the cold-rolling. The
substitutional elements such as Cr or Mn are different from interstitial elements such as C
or N, and diffuse into the iron carbide by being held at a high temperature of equal to or
higher than 600°C for long time. To avoid this, there are two major methods. One of
them is a method of dissolving all austenite by heating the iron carbide generated in the
10 hot-rolling to Acl to Ac3 in the continuous annealing and performing slow cooling from
the highest heating temperature to a temperature equal to or lower than 10 "CIS and
holding at 550°C to 660°C to generate the ferrite transformation and the iron carbide.
Since the iron carbide generated in the continuous annealing is generated in a short time,
it is difficult for the substitutional elements to diffuse.
15 In the other one of them, in the cooling step after the hot-rolling step, by
completing ferrite and pearlite transformation, it is possible to realize a soft and even
state in which a diffusion amount of the substitutional elements in the iron carbide in the
pearlite is small. The reason for limiting the hot-rolling conditions will be described
later. Accordingly, in the state of the hot-rolled sheet after the hot-rolling, it is possible
20 to set the values of Cre/CrM and MAe/MnM as low values. Thus, in the continuous
annealing step after the cold-rolling, even with the annealing in a temperature range of
(Acl - 40)"C at which only recrystallization of the ferrite occurs, if it is possible to
complete the transformation in the ROT cooling after the hot-rolling, it is possible to set
the Cre/CrM and the Mne/MnM to be low.
23
As shown in FIG. 5, the threshold values were determined from an expansion
curve when holding C-1 in which the values of Cre/CrM and hhe/MnM are low and C-4 in
which the values of Cre/CrM and Mne/MnM are high, for 10 seconds after heating to
850°C at 150 "C/s, and then cooling at 5 "CIS. That is, while the transformation starts
5 from the vicinity of 650°C in the cooling, in a material in which the values of Cre/Cr~
and Mne/MnM are high, clear phase transformation is not observed at a temperature equal
to or lower than 400°C, in the material in which the values of Cre/CrM and khe/MnM are
high. That is, by setting the values of Cre/CrM and M.ne/MnM to be low, it is possible to
improve hardenability after the rapid heating.
10 [0039]
A measurement method of component analysis of Cr and Mn in the iron carbide
is not particularly limited, however, for example, analysis can be performed with an
energy diffusion spectrometer (EDS) attached to a TEM, by manufacturing replica
materials extracted from arbitrary locations of the steel sheet and observing using the
15 transmission electron microscope (TEM) with a magnification of 1000 or more. Further,
for component analysis of Cr and Mn in a parent phase, the EDS analysis can be
performed in ferrite grains sufficiently separated from the iron carbide, by manufacturing
a thin film generally used.
[0040]
20 In addition, in the steel sheet for hot stamping, a fraction of the
non-segmentalized pearlite may be equal to or more than 10%. The non-segmentalized
pearlite shows that the pearlite which is austenitized once in the annealing step is
transformed to the pearlite again in the cooling step, the non-segmentalized pearlite
shows that the values of Cre/CrMa nd Mne/~nMare lower.
24
If the fraction of the non-segmentalized pearlite is equal to or more than lo%,
the hardenability of the steel sheet is improved.
When the microstructure of the hot-rolled steel sheet is formed from the ferrite
and the pearlite, if the ferrite is recrystallized after cold-rolling the hot-rolled steel sheet
5 to about 50%, generally, the location indicating the non-segmentalized pearlite is in a
state where the pearlite is finely segmentalized, as shown in the result observed by the
SEM of FIGS. 6A and 6B. On the other hand, when heating in the continuous
annealing to be equal to or higher than Acl, after the pearlite is austenitized once, by the
subsequent cooling step and holding, the ferrite transformation and the pearlite
10 transformation occur. Since the pearlite is formed by transformation for a short time,
the pearlite is in a state not containing the substitutional elements in the iron carbide and
has a shape not segmentalized as shown in FIGS. 7A and 7B.
An area ratio of the non-segmentalized pearlite can be obtained by observing a
cut and polished test piece with an optical microscope, and measuring the ratio using a
15 point counting method.
[004 11
(First Embodiment)
Hereinafter, a method for manufacturing a hot stamped body having a vertical
wall according to a first embodiment of the present invention will be described.
2 0 [0042]
The method for manufacturing a hot stamped body having a vertical wall
according to the embodiment includes at least a hot-rolling step, a coiling step, a
cold-rolling step, a continuous annealing step, and a hot stamping step. Hereinafter,
each step will be described in detail.
2 5 [0043]
(Hot-Rolling Step)
In the hot-rolling step, a steel piece having the chemical components described
above is heated (re-heated) to a temperature of equal to or higher than 1 100°C, and the
hot-rolling is performed. The steel piece may be a slab obtained immediately after
5 being manufactured by a continuous casting installation, or may be manufactured using
an electric furnace. By heating the steel piece to a temperature of equal to or higher
than 1 100°C, carbide-forming elements and carbon can be subjected to
decomposition-dissolving sufficiently in the steel material. In addition, by heating the
steel piece to a temperature of equal to or higher than 1200°C, precipitated carbonitrides
10 in the steel piece can be sufficiently dissolved. However, it is not preferable to heat the
steel piece to a temperature higher than 1280°C, from a view point of production cost.
[0044]
When a finishing temperature of the hot-rolling is lower than An°C, the ferrite
transformation occurs in rolling by contact of the surface layer of the steel sheet and a
15 mill roll, and deformation resistance of the rolling may be significantly high. The upper
limit of the finishing temperature is not particularly provided, however, the upper limit
may be set to about 1050°C.
[0045]
(Coiling Step)
20 It is preferable that a coiling temperature in the coiling step after the hot-rolling
step be in a temperature range of "700°C to 900°C" (ferrite transformation and pearlite
transformation range) or in a temperature range of "25°C to 500°C'' (martensite
transformation or bainite transformation range). In general, since the coil after the
coiling is cooled from the edge portion, the cooling history becomes uneven, and as a
2 6
result, unevenness of the microstructure easily occurs, however, by coiling the hot-rolled
coil in the temperature range described above, it is possible to suppress the unevenness of
the microstructure from occurring in the hot-rolling step. However, even with a coiling
temperature beyond the preferred range, it is possible to reduce significant variation
5 thereof compared to the related art by control of the microstructure in the continuous
annealing.
[0046]
(Cold-Rolling Step)
In the cold-rolling step, the coiled hot-rolled steel sheet is cold-rolled after
10 pickling, and a cold-rolled steel sheet is manufactured.
100471
(Continuous Annealing Step)
In the continuous annealing step, the cold-rolled steel sheet is subjected to
continuous annealing. The continuous annealing step includes a heating step of heating
15 the cold-rolled steel sheet in a temperature range of equal to or higher than "AclOC and
lower than Ac3"CyYa,n d a cooling step of subsequently cooling the cold-rolled steel sheet
to 660°C from the highest heating temperature by setting a cooling rate to 10 "CIS or less,
and a holding step of subsequently holding the cold-rolled steel sheet in a temperature
range of "550°C to 660°C" for 1 minute to 10 minutes.
[0048]
(Hot Stamping Step)
In the hot stamping step, hot stamping is performed for the steel sheet which is
subjected to the continuous annealing as described above after heating to a temperature
of equal to or higher,than Ac3, and a vertical wall is formed. In addition, the vertical
2 7
wall means a portion which is parallel to a press direction, or a portion which intersects
with a press direction at an angle within 20 degrees. General conditions may be
employed for the heating rate thereof or the subsequent cooling rate. However, since
the production efficiency is extremely low at a heating rate of less than 3 "CIS, the
5 heating rate may be set to be equal to or more than 3 "CIS. In addition, since the vertical
wall may not be sufficiently quenched in particular, at a cooling rate of less than 3 "CIS,
the cooling rate may be set to be equal to or more than 3 "CIS.
The heating method is not particularly regulated, and for example, a method of
performing electrical heating or a method of using a heating furnace can be employed.
10 The upper limit of the highest heating temperature may be set to 1000°C. In
addition, the holding at the highest heating temperature may not be performed since it is
not necessary to provide a particular holding time as long as reverse transformation to the
austenite single phase is obtained.
[0049]
15 According to the method for manufacturing a hot stamped body described above,
since a steel sheet for hot press in which hardness is even and which is soft is used, even
in a case of hot-stamping forming of the formed body having a vertical wall in which
clearance with the die is easily generated, it is possible to reduce variation of the
hardness of the hot stamped body. In detail, it is possible to obtain a formed body
20 having a vertical wall in which, when a quenching start temperature is equal to or lower
than 650°C, variation of Vickers hardness AHv of the hot stamped body is equal to or less
than 100, when the quenching start temperature is 650°C to 750°C, variation of Vickers
hardness AHv of the hot stamped body is equal to or less than 60, and when the
quenching start temperature is equal to or higher than 750°C, variation of Vickers
28
hardness AHv of the hot stamped body is equal to or less than 40.
[0050]
The steel sheet for hot stamping contains a lot of C component for securing
quenching hardness after the hot stamping and contains h)ln and B, and in such a steel
5 component having high hardenability and high concentration of C, the microstructure of
the hot-rolled sheet after the hot-rolling step tends to easily become uneven. However,
according to the method for manufacturing the cold-rolled steel sheet for hot stamping
according to the embodiment, in the continuous annealing step subsequent to the latter
stage of the cold-rolling step, the cold-rolled steel sheet is heated in a temperature range
10 of "equal to or higher than Acl°C and less than Ac3"CV, then cooled from the highest
temperature to 660°C at a cool rate of equal to or less than 10 "CIS, and then held in a
temperature range of "550°C to 660°C" for 1 minute to 10 minutes, and thus the
microstructure can be obtained to be even.
coo5 11
15 In the continuous annealing line, a hot-dip galvanizing process, a galvannealing
process, a molten aluminum plating process, an alloyed molten aluminum plating process,
and an electroplating process can also be performed. The effects of the present
invention are not lost even when the plating process is performed after the annealing step.
[0052]
20 As shown in the schematic view of FIG. 2, the microstructure of the steel sheet
subjected to the cold-rolling step is a non-recrystallized ferrite. In the method for
manufacturing of a hot stamped body having a vertical wall according to the embodiment,
in the continuous annealing step, by heating to a heating range of "equal to or higher than
AclOC and lower than Ac3"CV which is a higher temperature range than the Acl point,
2 9
heating is performed until having a double phase coexistence with the austenite phase in
which the non-recrystallized ferrite slightly remains. After that, in the cooling step at a
cooling rate of equal to or less than 10 "CIS, growth of the transformed ferrite which is
nucleated from the non-recrystallized ferrite slightly remaining at the highest heating
5 temperature occurs. Then, in the holding step of holding the steel sheet at a temperature
range of "550°C to 660°C" for 1 minute to 10 minutes, incrassating of C into the
non-transformed austenite occurs at the same time as ferrite transformation, and
cementite precipitation or pearlite transformation is promoted by holding in the same
temperature range.
10 [0053]
The steel sheet for hot stamping contains a lot of C component for securing
quenching hardness after the hot stamping and contains Mn and B, and B has an effect of
suppressing generation of the ferrite nucleation at the time of cooling from the austenite
single phase, generally, and when cooling is performed after heating to the austenite
15 single phase range of equal to or higher than Ac3, it is difficult for the ferrite
transformation to occur. However, by holding the heating temperature in the continuous
annealing step in a temperature range of "equal to or higher than AclOC and less than
Ac3"Cn which is immediately below Ac3, the ferrite slightly remains in a state where
almost hardened non-recrystallized ferrite is reverse-transformed to the austenite, and in
20 the subsequent cooling step at a cooling rate of equal to or less than 10 "CIS and the
holding step of holding at a temperature range of "550°C to 660°C" for 1 minute to 10
minutes, softening is realized by the growth of the ferrite by nucleating the remaining
ferrite. In addition, if the heating temperature in the continuous annealing step is higher
than Ac3"C, since the austenite single phase mainly occurs, and then the ferrite
30
transformation in the cooling is insufficient, and the hardening is realized, the
temperature described above is set as the upper limit, and if the heating temperature is
lower than Acl, since the volume fraction of the non-recrystallized ferrite becomes high
and the hardening is realized, the temperature described above is set as the lower limit.
5 [0054]
Further, in the holding step of holding the cold-rolled steel sheet in a
temperature range of "550°C to 660°C" for 1 minute to 10 minutes, the cementite
precipitation or the pearlite transformation can be promoted in the non-transformed
austenite in which C is incrassated after the ferrite transformation. Thus, according to
10 the method for manufacturing a formed body having a vertical wall according to the
embodiment, even in a case of heating a material having high hardenability to a
temperature right below the Ac3 point by the continuous annealing, most parts of the
microstructure of the steel sheet can be set as ferrite and cementite. According to the
proceeding state of the transformation, the bainite, the martensite, and the remaining
15 austenite slightly exist after the cooling, in some cases.
In addition, if the temperature in the holding step exceeds 660°C, the proceeding
of the ferrite transformation is delayed and the annealing takes long time. On the other
hand, when the temperature is lower than 550°C, the ferrite itself which is generated by
the transformation is hardened, it is difficult for the cementite precipitation or the pearlite
20 transformation to proceed, or the bainite or the martensite which is the lower temperature
transformation product occurs. In addition, when the holding time exceeds 10 minutes,
the continuous annealing installation subsequently becomes longer and high cost is
necessary, and on the other hand, when the holding time is lower than 1 minute, the
ferrite transformation, the cementite precipitation, or the pearlite transformation is
3 1
insufficient, the structure is mainly formed of bainite or martensite in which most parts of
the microstructure after the cooling are hardened phase, and the steel sheet is hardened.
[0055]
According to the manufacturing method described above, by coiling the
5 hot-rolled coil subjected to the hot-rolling step in a temperature range of "700°C to
900°C" (range of ferrite or pearlite), or by coiling in a temperature range of "25°C to
550°C" which is a low temperature transformation temperature range, it is possible to
suppress the unevenness of the microstructure of the hot-rolled coil after coiling. That
is, the vicinity of 600°C at which the normal steel is generally coiled is a temperature
10 range in which the ferrite transformation and the pearlite transformation occur, however,
b
when coiling the steel type having high hardenability in the same temperature range after
setting the conditions of the hot-rolling finishing normally performed, since almost no
transformation occurs in a cooling device section which is called Run-Out-Table
(hereinafter, ROT) from the finish rolling of the hot-rolling step to the coiling, the phase
15 transformation from the austenite occurs after the coiling. Accordingly, when
considering a width direction of the coil, the cooling rates in the edge portion exposed to
the external air and the center portion shielded from the external air are different from
each other. Further, also in the case of considering a longitudinal direction of the coil,
in the same manner as described above, cooling histories in a tip end or a posterior end of
20 the coil which can be in contact with the external air and in an intermediate portion
shielded from the external air are different from each other. Accordingly, in the
component having high hardenability, when coiling in a temperature range in the same
manner as in a case of normal steel, the microstructure or the hardness of the hot-rolled
sheet significantly varies in one coil due to the difference of the cooling history. When
performing annealing by the continuous annealing installation after the cold-rolling using
the hot-rolled sheet, in the ferrite recrystallization temperature range of equal to br lower
than Acl, significant variation in the hardness is generated as shown in FIG. 1 by the
variation in the ferrite recrystallization rate caused by the variation of the microstructure
5 of the hot-rolled sheet. Meanwhile, when heating to the temperature range of equal to
or higher than Acl and' cooling as it is, not only a lot of non-recrystallized ferrite remains,
but the austenite which is partially reverse-transformed is transformed to the bainite or
the martensite which is a hardened phase, and becomes a hard material having significant
variation in hardness. When heating to a temperature of equal to or higher than Ac3 to
10 completely remove the non-recrystallized ferrite, significant hardening is performed after
the cooling with an effect of elements for improving hardenability such as Mn or B.
Accordingly, it is advantageous to perform coiling at the temperature range described
above for evenness of the microstructure of the hot-rolled sheet. That is, by performing
coiling in the temperature range of "700°C to 900°C", since cooling is sufficiently
I 15 performed from the high temperature state after the coiling, it is possible to form the
entire coil with the ferritelpearlite structure. Meanwhile, by coiling in the temperature
range of "25°C to 550°C", it is possible to form the entire coil into the bainite or the
martensite which is hard.
[0056]
20 FIGS. 3A to 3C show variation in strength of the steel sheet for hot stamping
after the continuous annealing with different coiling temperatures for the hot-rolled coil.
FIG. 3A shows a case of performing continuous annealing by setting a coiling
temperature as 680°C, FIG. 3B shows a case of performing the continuous annealing by
setting a coiling temperature at as 750°C, that is, in the temperature range of "700°C to
33
900°C" (ferrite transformation and pearlite transformation range), and FIG. 3C shows a
case of performing continuous annealing by setting a coiling temperature as 500°C, that
is, in the temperature range of "25°C to 500°C" (bainite transformation and martensite
transformation range). In FIGS. 3A to 3C, ATS indicates variation in strength of the
5 steel sheet (maximum value of tensile strength of steel sheet -minimum value thereof).
As clearly shown in FIGS. 3A to 3C, by performing the continuous annealing with
suitable conditions, it is possible to obtain even and soft hardness of the steel sheet after
the annealing, and accordingly, it is possible to reduce variation in hardness of the hot
stamped body having a vertical wall.
[0057]
By using the steel having the even hardness, in the hot stamping step, even in a
case of manufacturing the formed body having the vertical wall in which the cooling rate
easily becomes slower than in the other parts, it is possible to stabilize the hardness of a
component of the formed body after the hot stamping. Further, for the portion which is
15 an electrode holding portion in which a temperature does not rise by the electrical heating
and in which the hardness of the material of the steel sheet itself affects the product
hardness, by evenly managing the hardness of the material of the steel sheet itself, it is
possible to improve management of precision of the product quality of the formed body
after the hot stamping.
[005S]
(Second Embodiment)
Hereinafter, a method for manufacturing the hot stamped body having a vertical
wall according to a second embodiment of the present invention will be described.
100591
3 4
The method for manufacturing a hot stamped body according to the embodiment
includes at least a hot-rolling step, a coiling step, a cold-rolling step, a continuous
annealing step, and a hot stamping step. Hereinafter, each step will be described in
detail.
5 [0060]
(Hot-Rolling Step)
In the hot-rolling step, a steel piece having the chemical components described
above is heated (re-heated) to a temperature of equal to or higher than 11 OO°C, and the
hot-rolling is performed. The steel piece may be a slab obtained immediately after
10 being manufactured by a continuous casting installation, or may be manufactured using
an electric furnace. By heating the steel piece to a temperature of equal to or higher
than 1 100°C, carbide-forming elements and carbon can be subjected to
decomposition-dissolving sufficiently in the steel material. In addition, by heating the
steel piece to a temperature of equal to or higher than 1200°C, precipitated carbonitrides
15 in the steel piece can be sufficiently dissolved. However, it is not preferable to heat the
steel piece to a temperature higher than 1280°C, from a view point of production cost.
[0061]
In the hot-rolling step of the embodiment, in finish-hot-rolling configured with a
machine with 5 or more consecutive rolling stands, rolling is performed by (A) setting a
20 finish-hot-rolling temperature FIT in a final rolling mill F, in a temperature range of (Ac3
- 80)"C to (Ac3 + 40)OC, by (B) setting a time fiom start of rolling in a rolling mill F,-3
which is a previous machine to the final rolling mill F, to end of rolling in the final
rolling mill F, to be equal to or longer than 2.5 seconds, and by (C) setting a hot-rolling
temperature F,-3T in the rolling mill F1-3 to be equal to or lower than (F,T + lOO)"C, and
35
then holding is performed in a temperature range of "600°C to k 0 C " for 3 seconds to 40
seconds, and coiling is performed in the coiling step.
[0062]
By performing such hot-rolling, it is possible to perform stabilization and
5 transformation from the austenite to the ferrite, the pearlite, or the bainite which is the
low temperature transformation phase in the ROT (Run Out Table) which is a cooling
bed in the hot-rolling, and it is possible to reduce the variation in the hardness of the steel
sheet accompanied with a cooling temperature deviation generated after the coil coiling.
In order to complete the transformation in the ROT, refining of the austenite grain size
10 and holding at a temperature of equal to or lower than Ar3"C in the ROT for a long time
are important conditions.
[0063]
When the FIT is less than (Ac3 - 8O)OC, a possibility of the ferrite transformation
in the hot-rolling becomes high and hot-rolling deformation resistance is not stabilized.
15 On the other hand, when the F,T is higher than (Ac3 + 40)OC, the austenite grain size
immediately before the cooling after the finishing hot-rolling becomes coarse, and the
ferrite transformation is delayed. It is preferable that FIT be set as a temperature range
of ''(Ac~- 70)"C to (Ac3 + 20)"CYy. By setting the heating conditions as described
above, it is possible to refine the austenite grain size after the finish rolling, and it is
20 possible to promote the ferrite transformation in the ROT cooling. Accordingly, since
the transformation proceeds in the ROT, it is possible to largely reduce the variation of
the microstructure in longitudinal and width directions of the coil caused by the variation
of coil cooling after the coiling.
[0064]
3 6
For example, in a case of a hot-rolling line including seven final rolling mills,
transit time from a F4 rolling mill which corresponds to a third mill from an F7 rolling
mill which is a final stand, to the F7 rolling mill is set as 2.5 seconds or longer. When
the transit time is less than 2.5 seconds, since the austenite is not recrystallized between
stands, B segregated to the austenite grain boundary significantly delays the ferrite
transformation and it is difficult for the phase transformation in the ROT to proceed.
The transit time is preferably equal to or longer than 4 seconds. It is not particularly
limited, however, when the transition time is equal to or longer than 20 seconds, the
temperature of the steel sheet between the stands largely decreases and it is impossible to
perform hot-rolling.
[0065]
For recrystallizing so that the austenite is refined and B does not exist in the
austenite grain boundary, it is necessary to complete the rolling at an extremely low
temperature of equal to or higher than Ar3, and to recrystallize the austenite at the same
temperature range. Accordingly, a temperature on the rolling exit side of the F4 rolling
mill is set to be equal to or lower than (F,T + 100)"C. This is because it is necessary to
lower the temperature of the rolling temperature of the F4 rolling mill for obtaining an
effect of refining the austenite grain size in the latter stage of the finish rolling. The
lower limit of F,-3T is not particularly provided, however, since the temperature on the
exit side of the final F7 rolling mill is FIT, this is set as the lower limit thereof.
[0066]
By setting the holding time in the temperature range of 600°C to Ar3"C to be a
long time, the ferrite transformation occurs. Since the AT3 is the ferrite transformation
start temperature, this is set as the upper limit, and 600°C at which the softened ferrite is
700°C in which generally the ferrite transformation proceeds most rapidly.
[0067]
(Coiling Step)
By holding the coiling temperature in the coiling step after the hot-rolling step at
600°C to Ar3"C for 3 seconds or longer in the cooling step, the hot-rolled steel sheet in
which the ferrite transformation proceeded, is coiled as it is. Substantially, although it is
changed by the installation length of the ROT, the steel sheet is coiled in the temperature
range of 500°C to 650°C. By performing the hot-rolling described above, the
10 microstructure of the hot-rolled sheet after the coil cooling has a structure mainly
including the ferrite and the pearlite, and it is possible to suppress the unevenness of the
microstructure generated in the hot-rolling step.
[0068]
(Cold-Rolling Step)
15 In the cold-rolling step, the coiled hot-rolled steel sheet is cold-rolled after
pickling, and a cold-rolled steel sheet is manufactured.
[0069]
(Continuous Annealing Step)
In the continuous annealing step, the cold-rolled steel sheet is subjected to
20 continuous annealing. The continuous annealing step includes a heating step of heating
the cold-rolled steel sheet in a temperature range of equal to or higher than ''(Ac, - 40)"C
and lower than Ac3"C", and a cooling step of subsequently cooling the cold-rolled steel
sheet to 660°C from the highest heating temperature by setting a cooling rate to 10 "CIS
or less, and a holding step of subsequently holding the cold-rolled steel sheet in a
38
temperature range of "450°C to 660°C" for 20 seconds to 10 minutes.
[0070]
(Hot Stamping Step)
4 In the hot stamping step, hot stamping is performed for the steel sheet which is
5 subjected to the continuous annealing as described above after heating to a temperature
of equal to or higher than Ac~a,n d a vertical wall is formed. In addition, the vertical
wall means a portion which is parallel to a press direction, or a portion which intersects
with a press direction at an angle within 20 degrees. General conditions may be
employed for the heating rate thereof or the subsequent cooling rate. However, since
10 the production efficiency is extremely low at a heating rate of less than 3 "CIS, the
heating rate may be set to be equal to or more than 3 "CIS. In addition, since the vertical
wall may not be sufficiently quenched in particular, at a cooling rate of less than 3 "CIS,
the cooling rate may be set to be equal to or more than 3 "CIS.
The heating method is not particularly regulated, and for example, a method of
15 performing electrical heating or a method of using a heating furnace can be employed.
The upper limit of the highest heating temperature may be set to 1000°C. In
addition, the holding at the highest heating temperature may not be performed since it is
not necessary to provide a particular holding time as long as reverse transformation to the
austenite single phase is obtained.
2 0 [0071]
According to the manufacturing method described above, since a steel sheet for
hot press in which hardness is even and which is soft is used, even in a case of
hot-stamping forming of the formed body having a vertical wall in which clearance with
the die is easily generated, it is possible to reduce variation of the hardness of the hot
39
stamped body. In detail, it is possible to obtain a formed body having a vertical wall in
which, when a quenching start temperature is equal to or lower than 650°C, variation of
Vickers hardness AHv of the hot stamped body is equal to or less than 100, when the
quenching start temperature is 650°C to 750°C, variation of Vickers hardness AHv of the
5 hot stamped body is equal to or less than 60, and when the quenching start temperature is
equal to or higher than 750°C, variation of Vickers hardness AHv of the hot stamped
body is equal to or less than 40.
[0072]
Since the steel sheet is coiled into a coil after transformation from the austenite
10 to the ferrite or the pearlite in the ROT by the hot-rolling step of the second embodiment
described above, the variation in the strength of the steel sheet accompanied with the
cooling temperature deviation generated after the coiling is reduced. Accordingly, in
the continuous annealing step subsequent to the latter stage of the cold-rolling step, by
heating the cold-rolled steel sheet in the temperature range of "equal to or higher than
15 (Acl - 40)"C to lower than Ac3"Cn, subsequently cooling from the highest temperature to
660°C at a cooling rate of equal to or less than 10 "CIS, and subsequently holding in the
temperature range of "450°C to 660°C" for 20 seconds to 10 minutes, it is possible to
realize the evenness of the microstructure in the same manner as or an improved manner :
to the method for manufacturing a steel sheet described in the first embodiment.
[0073]
In the continuous annealing line, a hot-dip galvanizing process, a galvannealing
process, a molten aluminum plating process, an alloyed molten aluminum plating process,
and an electroplating process can also be performed. The effects of the present
invention are not lost even when the plating process is performed after the annealing step.
[0074]
As shown in the schematic view of FIG. 2, the microstructure of the steel sheet
subjected to the cold-rolling step is a non-recrystallized ferrite. In the method for
manufacturing of a hot stamped body having a vertical wall according to the second
5 embodiment, in addition to the first embodiment in which, in the continuous annealing
step, by heating to a heating range of "equal to or higher than (Acl - 40)OC and lower
than Ac3"CV, heating is performed until having a double phase coexistence with the
austenite phase in which the non-recrystallized ferrite slightly remains, it is possible to
lower the heating temperature for even proceeding of the recovery and recrystallization
10 of the ferrite in the coil, even with the heating temperature ofAclOC to (Acl - 40) "C at
which the reverse transformation of the austenite does not occur. In addition, by using
the hot-rolled sheet showing the even structure, after heating to a temperature of equal to
or higher than AclOC and lower than Ac3"C, it is possible to lower the temperature and
shorten the time of holding after the cooling at a cooling rate of equal to or less than 10
15 "CIS, compared to the first embodiment. This shows that the ferrite transformation
proceeds faster in the cooling step from the austenite by obtaining the even
microstructure, and it is possible to sufficiently achieve evenness and softening of the
structure, even with the holding conditions of the lower temperature and the short time.
That is, in the holding step of holding the steel sheet in the temperature range of "450°C
20 to 660°C" for 20 seconds to 10 minutes, incrassating of C into the non-transformed
austenite occurs at the same time as ferrite transformation, and cementite precipitation or
pearlite transformation rapidly occurs by holding in the same temperature range.
[0075]
From these viewpoints, when the temperature is less than (Acl - 40)"C, since
4 1
the recovery and the recrystallization of the ferrite is insufficient, it is set as the lower
limit, and meanwhile, when the temperature is equal to or higher than Ac3"C, since the
ferrite transformation does not sufficiently occur and the strength after the annealing
significantly increases by the delay of generation of ferrite nucleation by the B addition
5 effect, it is set as the upper limit. In addition, in the subsequent cooling step at a cooling
rate of equal to or less than 10 "CIS and the holding step of holding at a temperature
range of "450°C to 660°C" for 20 seconds to 10 minutes, softening is realized by the
growth of the ferrite by nucleating the remaining ferrite.
[0076]
Herein, in the holding step of holding the steel sheet in a temperature range of
"450°C to 660°C" for 20 seconds to 10 minutes, the cementite precipitation or the
pearlite transformation can be promoted in the non-transformed austenite in which C is
incrassated after the ferrite transformation. Thus, according to the method for
manufacturing a formed body having a vertical wall according to the embodiment, even
15 in a case of heating a material having high hardenability to a temperature right below the
Ac3 point by the continuous annealing, most parts of the microstructure of the steel sheet
can be set as ferrite and cementite. According to the proceeding state of the
transformation, the bainite, the martensite, and the remaining austenite slightly exist after
the cooling, in some cases.
In addition, if the temperature in the holding step exceeds 660°C, the proceeding
of the ferrite transformation is delayed and the annealing takes long time. On the other
hand, when the temperature is lower than 450°C, the ferrite itself which is generated by
the transformation is hardened, it is difficult for the cementite precipitation or the pearlite
transformation to proceed, or the bainite or the martensite which is the lower temperature
42
transformation product occurs. In addition, when the holding time exceeds 10 minutes,
the continuous annealing installation subsequently becomes longer and high cost is
necessary, and on the other hand, when the holding time is lower than 20 seconds, the
ferrite transformation, the cementite precipitation, or the pearlite transformation is
5 insuficient, the structure is mainly formed of bainite or martensite in which the most
parts of the microstructure after the cooling are hardened phase, and the steel sheet is
hardened.
[0077]
FIGS. 3A to 3C show variation in strength of the steel sheet for hot stamping
10 after the continuous annealing with different coiling temperatures for the hot-rolled coil.
FIG. 3A shows a case of performing continuous annealing by setting a coiling
temperature as 680°C, FIG. 3B shows a case of performing the continuous annealing by
setting a coiling temperature as 750°C, that is, in the temperature range of "700°C to
900°C'' (ferrite transformation and pearlite transformation range), and FIG. 3C shows a
15 case of performing continuous annealing by setting a coiling temperature as 500°C, that
is, in the temperature range of "25°C to 500°C" (bainite transformation and martensite
transformation range). In FIGS. 3A to 3C, ATS indicates variation of the steel sheet
(maximum value of tensile strength of steel sheet - minimum value thereof). As clearly
shown in FIGS. 3A to 3C, by performing the continuous annealing with suitable
20 conditions, it is possible to obtain even and soft hardness of the steel sheet after the
annealing.
[0078]
By using the steel having the even hardness, in the hot stamping step, even in a
case of manufacturing the formed body having the vertical wall in which the cooling rate
4 3
easily becomes slower than in the other parts, it is possible to stabilize the hardness of a
component of the formed body after the hot stamping. Further, for the portion which is
an electrode holding portion in which a temperature does not rise by the electrical heating
and in which the hardness of the material of the steel sheet itself affects the product
hardness, by evenly managing the hardness of the material of the steel sheet itself, it is
possible to improve management of precision of the product quality of the formed body
after the hot stamping.
[0079]
Hereinabove, the present invention has been described based on the first
embodiment and the second embodiment, however, the present invention is not limited
only to the embodiments described above, and various modifications within the scope of
the claims can be performed. For example, even in the hot-rolling step or the
continuous annealing step of the first embodiment, it is possible to employ the conditions
of the second embodiment.
Examples
[0080]
Next, Examples of the present invention will be described.

[0083]
[Table 31

[0084]
[Table 41
Steel
type
D
Continuous Conditio annealing conditions
nNo
1
2
Highest heating
temperature
["CI
700
810
Hot-rolling to coiling conditions
Cooling
rate
["C/s]
2.1
4.3
Holding
temperature
["CI
5 00
580
Time from 4
stage to 7 stage
[sl
3.2
2.1
Holding
time
[sl
324
320
(Ac3+40)
["CI
865
865
F4T
["CI
950
960
Holding time from
600°C to Ar,
[sl
4.0
4.0
F7T
["CI
910
910
CT
["CI
680
680
(Ac,-80)
["CI
745
745

[0085]
[Table 51

[0087]
[Table 71

[0088]
[Table 81
Steel
type
G
Condition
No.
1
N
0
P
Material
1
1
2
1
ATS
[MPal
70
TS-Ave
W a l
635
Microstructure
30
55
30
CrdCr,
1.3
Ferrite fraction
[ v o I . ~
60
Mne/Mn~
9.2
610
600
600
Non-crystallized
ferrite fraction
[vol.%]
30
Non-segrnentalized
pearlite fraction
[vol.O/o]
10
70
75
75
20
10
15
10
15
10
1.5
1.6
1.3
6.8
7.5
8.5

[0089]
[Table 91
Steel
type
A
condition
No.
1
2
3
4
5
6
7
8
9
10
Plating type
hot-dip
galvanizing
galvannealing
hot-dip
galvanizing
Variation of Vickers hardness
AHv of the hot stamped body
when a quenching start
temperature is 600°C
55
65
67
123
132
144
135
125
65
66
Variation of Vickers hardness
AHv of the hot stamped body
when a quenching start
temperature is 700°C
44
35
3 8
78
69
85
86
72
35
48
Variation of Vickers hardness
AHv of the hot stamped body
when a quenching start
temperature is 800°C
28
25
24
48
5 5
63
65
68
22
21
Chemical conversion
coating
Good
Good
Good
Good
Good
Good
Good
Good
Good
Good
Note
Non-recrystallized
femte remaining
Insufficient ferrite
transformation and
cementite
precipitation
Insufficient fenite
transformation
Insufficient fenite
transformation and
cementite
precipitation
Insufficient femte
transformation and
cementite
precipitation
B
C
1
2
3
4
5
6
1
2
3
4
5
6
7
8
9
35
39
74
76
44
49
46
48
72
75
54
55
75
78
71
hot-dip
galvanizing
molten
aluminum
plating
hot-dip
galvanizing
hot-dip
galvanizing
hot-dip
galvanizing
galvamealing
hot-dip
galvanizing
59
62
115
119
57
59
65
67
121
126
67
72
113
114
135
27
22
66
5 1
21
25
21
25
46
48
19
22
54
5 1
55
Good
Good
Good
Good
Good
Good
Good
Good
Good
Good
Good
Good
Good
Good
Good
Insufficient femte
transformation and
cementite
precipitation
Insufficient ferrite
transformation and
cementite
precipitation
Insufficient femte
transformation and
cementite
precipitation
Non-recrystallized
femte remaining
Insufficient femte
transformation and
cementite
precipitation
Insufficient femte
transformation and
cementite
precipitation
Insufficient ferrite
10 132 69 69 Good
recrystallization
Insufficient
cementite
precipitation
[Table 101
Variation of Vickers hardness AHv
of the hot stamped body when a
quenching start temperature is
700°C
75
5 1
52
78
74
8 1
64
55
8 1
Variation of Vickers hardness AHv
of the hot stamped body when a
quenching start temperature is
600°C
121
78
82
132
115
141
121
84
128
Plating type
hot-dip
galvanizing
electroplatin
i?
Steel
type
D
Variation of Vickers hardness AHv
of the hot stamped body when a
quenching start temperature is
800°C
5 1
22
23
45
52
5 5
5 3
19
49
condition
No.
1
2
3
4
5
6
7
8
9
Chemical
conversion
coating
Good
Good
Good
Good
Good
Good
Good
Good
Good
Note
-
Non-recrystallize
d fenite
remaining
Insufficient femte
transformation
and cementite
precipitation
Insufficient femte
transformation
and cementite
precipitation
Insufficient ferrite
transformation
and cementite
precipitation
Insufficient femte
transformation
hsufficient femte
transformation
F
4
5
6
7
8
9
1
2
3
4
5
alloyed
molten
aluminum
plating
hot-dip
galvanizing
hot-dip
galvanizing
--
135
111
119
108
77
76
79
91
89
82
d femte
remaining ,
75
79
78
82
45
48
54
49
46
48
72
52
56
54
62
32
3 1
3 1
29
28
3 3
55
Good
Good
Good
Good
Good
Good
Good
Good
Good
Good
Good
Insufficient f z
transformation
and cementite
precipitation
Insufficient femte
transformation
Insufficient fenite
transformation
and cementite
precipitation
Insufficient ferrite
transformation
and cementite
precipitation
Non-recrystallize
[0091]
[Table 111
Steel
type
G
H
I
J
K
Condition
No.
1
2
3
4
5
1
2
1
2
1
2
1
Plating type
electroplating
hot-dip
galvanizing
Variation of Vickers hardness AHv
of the hot stamped body when a
quenching start temperature is
600°C
76
75
81
69
109
72
75
76
77
Variation of Vickers hardness AHv
of the hot stamped body when a
quenching start temperature is
700°C
51
52
49
44
7 1
45
55
45
44
Variation of Vickers hardness AHv
of the hot stamped body when a
quenching start temperature is
800°C
29
28
22
26
61
2 1
19
35
34
Chemical
conversion
coating
Good
-
Good
Good
Good
Good
Good
-
Good
Good
Good
Good
Good -
Good
Note
Non-recrystallize
d femte
remaining
Strength after hot
stamping is less
than 1180 MPa
Cracks on end
portion are
generated at the
time of hot
stamping forming
AHv is in the
range even with
the method of the
related art for low
hardenability.
Hot-rolling is
difficult
L
M
N
0
P
Q
R
S
T
54
59
54
55
51
43
49
39
hot-dip
galvanizing
1
1
1
1
2
1
1
1
1
1
Poor
Poor
Good
Good
Good
Good
Good
Good
Good
32
35
32
34
34
25
31
22
91
87
87
88
83
71
77
84
Poor chemical
conversion
coating
Poor chemical
conversion
coating
Hot-rolling is
difficult
AHv is in the
range even with
the method of the
related art for low
hardenability.
AHv is in the
range even with
the method of the
related art for low
hardenability.
Hot-rolling is
difficult
[0092]
A steel having ,steel material components shown in Table 1 and Table 2 was
smelted and prepared, heated to 1200°C, rolled, and coiled at a coiling temperature CT
shown in Tables 3 to 5, a steel strip having a thickness of 3.2 mm being manufactured.
5 The rolling was performed using a hot-rolling line including seven finishing rolling mills.
Tables 3 to 5 show a "steel type", a "condition No.", "hot-rolling to coiling conditions",
and a "continuous annealing condition". Acl and Ac3 were experimentally measured
using a steel sheet having a thickness of 1.6 mm which was obtained by rolling with a
cold-rolling rate of 50%. For the measurement of Acl and Ac3, measurement was
10 performed from an expansion and contraction curve by formaster, and values measured at
a heating rate of 5 "C/s are disclosed in Table 1. The continuous annealing was
performed for the steel strip at a heating rate of 5 "CIS with conditions shown in Tables 3
to 5. In addition, in Tables 6 to 8, "strength variation (ATS)", a "strength average value
(TS-Ave)", a ccmicrostructure of a steel strip", "Cre/CrM", and "Mne/MnM7' acquired
based on tensile strength measured from 10 portions of the steel strip after the continuous
i
annealing are shown. The fraction of the microstructure shown in Tables 6 to 8 was
obtained by observing the cut and polished test piece with the optical microscope and
measuring the ratio using a point counting method. After that, the electrical heating
with an electrode with respect to the steel sheet for hot stamping was performed, and the
steel sheet for hot stamping was heated at a heating rate of 30 "CIS so that the highest
heating temperature was Ac3"C + 50°C. Then, without performing temperature holding
after the heating, the heated steel sheet was hot stamped and a formed body having a
vertical wall shown in FIG. 4 was manufactured. A cooling rate of the die cooling was
set as 20 "CIS. The die used in pressing was a hat-shaped die, and R with a type of
punch and die was set as 5R. In addition, a height of the vertical wall of the hat was 50
mm and blank hold pressure was set as 10 tons.
[0093]
The quenching was performed by setting the quenching start temperature to
5 600°C, 700°C, to 800°C, variation of Vickers hardness AHv of the vertical wall of the hot
stamped body of being evaluated for each. For the hardness of the vertical wall, the
hardness of the cross section in a position of 0.4 rnrn from the surface was acquired from
the average of 5 values with a load of 5 kgf using a Vickers hardness tester. Evaluation
results of the "variation of Vickers hardness AHv of the hot stamped body when a
10 quenching start temperature is 600°C", the "variation of Vickers hardness AHv of the hot
stamped body when a quenching start temperature is 700°C", and the "Variation of
Vickers hardness AHv of the hot stamped body when a quenching start temperature is
800°C" are shown in Tables 9 to 11.
[0094]
15 For the chemical conversion coating, a phosphate crystal state was observed
with five visual fields using a scanning electron microscope with 10000 magnification by
using dip-type bonderised liquid which is normally used, and was determined as a pass if
there was no clearance in a crystal state (Pass: Good, Failure: Poor).
100951
2 0 Test Examples A-1, A-2, A-3, A-9, A-10, B-1, B-2, B-5, B-6, C-1, C-2, C-5, C-6,
D-2, D-3, D-8, D-10, E-1, E-2, E-3, E-8, E-9, F-1, F-2, F-3, F-4, G-1, G-2, G-3, G-4, Q-1,
R-1, and S-1 were determined to be good since they were in the range of the conditions.
.s
In Test Examples A-4, C-4, D-1, D-9, F-5, and G-5, since the highest heating temperature
in the continuous annealing was lower than the range of the present invention, the
non-recrystallized ferrite remained and AHv became high. In Test Examples A-5, B-3,
and E-4, since the highest heating temperature in the continuous annealing was higher
than the range of the present invention, the austenite single phase structure was obtained
at the highest heating temperature, and the ferrite transformation and the cementite
5 precipitation in the subsequent cooling and the holding did not proceed, the hard phase
fraction after the annealing became high, and AHv became high. In Test Examples A-6
and E-5, since the cooling rate from the highest heating temperature in the continuous
annealing was higher than the range of the present invention, the ferrite transformation
did not sufficiently occur and AHv became high. In Test Examples A-7, D-4, D-5, D-6,
10 and E-6, since the holding temperature in the continuous annealing was lower than the
range of the present invention, the ferrite transformation and the cementite precipitation
were insufficient, and AHv became high. In Test Example D-7, since the holding
temperature in the continuous annealing was higher than the range of the present
invention, the ferrite transformation did not sufficiently proceed, and AHv became high.
15 In Test Examples A-8 and E-7, since the holding time in the continuous annealing was
shorter than the range of the present invention, the ferrite transformation and the
cementite precipitation were insufficient, and AHv became high. When comparing Test
Examples B-1, C-2, and D-2 and Test Examples B-4, C-3, and D-6 which have similar
manufacturing conditions in the steel type having almost same concentration of C of the
20 steel material and having different DIInchv alues of 3.5, 4.2 and 5.2, it was found that,
when the DI,,,h value was large, improvement of AHv was significant. Since a steel
type H had a small amount of C of 0.16%, a quenching temperature after the hot
stamping became lower, and it was not suitable as a hot stamped component. Since a
steel type I had a large amount of C of 0.40%, cracks on the end portion were generated
p m r
at the time of hot stamping. A steel type J had a small amount of Mn of 0.82%, and the
hardenability was low. Since steel types K and N respectively had a large amount of
Mn of 3.82% and an amount of Ti of 0.3 lo%, it was difficult to perform the hot-rolling
which is a part of a manufacturing step of a hot stamped component. Since steel types I,
and M respectively had a large amount of Si of 1.32% and an amount of Al of 1.300%,
the chemical conversion coating of the hot stamped component was degraded. Since a
steel type 0 had a small added amount of B and a steel type P had insufficient
detoxicating of N due to Ti addition, the hardenability was low.
[0096]
In addition, as found from Tables 3 to 11, although the surface treatment due to
plating or the like was performed, the effects of the present invention were not disturbed.
Industrial Applicability
[0097]
According to the present invention, even with a case of manufacturing a formed
body having a vertical wall from the steel sheet for hot stamping, it is possible to provide
a hot stamped body having a vertical wall which can suppress the variation in hardness of
the formed body.

CLAIMS
1. A method for manufacturing a hot stamped body, the method comprising:
hot-rolling a slab containing chemical components which include, by mass%,
5 0.18% to 0.35% of C, 1.0% to 3.0% of Mn, 0.01% to 1.0% of Si, 0.001% to 0.02% of P,
0.0005% to 0.01% of S, 0.001% to 0.01% of N, 0.01% to 1 .O% of Al, 0.005% to 0.2% of
Ti, 0.0002% to 0.005% of B, and 0.002% to 2.0% of Cr, and the balance of Fe and
inevitable impurities, to obtain a hot-rolled steel sheet;
coiling the hot-rolled steel sheet which is subjected to hot-rolling;
cold-rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet;
continuously annealing the cold-rolled steel sheet which is subjected to
cold-rolling to obtain a steel sheet for hot stamping; and
performing hot stamping by heating the steel sheet for hot stamping which is
continuously annealed so that a highest heating temperature is equal to or higher than
15 Ac3"C, and forming a vertical wall,
wherein the continuous annealing includes:
heating the cold-rolled steel sheet to a temperature range of equal to or higher
than AclOC and lower than Ac3"C;
cooling the heated cold-rolled steel sheet from the highest heating temperature to
20 660°C at a cooling rate of equal to or less than 10 "CIS; and
holding the cooled cold-rolled steel sheet in a temperature range of 550°C to
660°C for one minute to 10 minutes.
2. The method for manufacturing a hot stamped body according to Claim 1,
wherein the chemical components further include one or more from 0.002% to
2.0% of Mo, 0.002% to 2.0% of Nb, 0.002% to 2.0% of V, 0.002% to 2.0% of Ni,
0.002% to 2.0% of Cu, 0.002% to 2.0% of Sn, 0.0005% to 0.0050% of Ca, 0.0005% to
0.0050% of Mg, and 0.0005% to 0.0050% of REM.
5 3. The method for manufacturing a hot stamped body according to Claim 1,
the method further comprising performing any one of a hot-dip galvanizing process, a
galvannealing process, a molten aluminum plating process, an alloyed molten aluminum
plating process, and an electroplating process, after the continuous annealing.
4. The method for manufacturing a hot stamped body according to Claim 2,
the method further comprising performing any one of a hot-dip galvanizing process, a
galvannealing process, a molten aluminum plating process, an alloyed molten aluminum
plating process, and an electroplating process, after the continuous annealing.
15 5. A method for manufacturing a hot stamped body, the method comprising:
hot-rolling a slab containing chemical components which include, by mass%,
0.18% to 0.35% of C, 1 .O% to 3.0% of Mn, 0.01% to 1.0% of Si, 0.001% to 0.02% of P,
0.0005% to 0.01% of S, 0.001% to 0.01% of N, 0.01% to 1.0% ofAl, 0.005% to 0.2% of
Ti, 0.0002% to 0.005% of B, and 0.002% to 2.0% of Cr, and the balance of Fe and
20 inevitable impurities, to obtain a hot-rolled steel sheet;
coiling the hot-rolled steel sheet which is subjected to hot-rolling;
cold-rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet;
continuously annealing the cold-rolled steel sheet which is subjected to
cold-rolling to obtain a steel sheet for hot stamping; and
performing hot stamping by heating the steel sheet for hot stamping which is
74 ~3
continuously annealed so that a highest heating temperature is equal to or higher than
Ac3"C, and forming a vertical wall,
wherein, in the hot-rolling, in finish-hot-rolling configured with a machine with
5 or more consecutive rolling stands, rolling is performed by setting a finish-hot-rolling
5 temperature FIT in a final rolling mill F, in a temperature range of (Ac3 - 80)"C to (Ac~+
40)"C, by setting time from start of rolling in a rolling mill FI-3 which is a previous
machine to the final rolling mill Fl to end of rolling in the final rolling mill F, to be equal
to or longer than 2.5 seconds, and by setting a hot-rolling temperature FP3T in the rolling
mill FI-3 to be equal to or lower than FIT + 100°C, and after holding in a temperature
10 range of 600°C to Ar3"C for 3 seconds to 40 seconds, coiling is performed,
the continuous annealing includes:
heating the cold-rolled steel sheet to a temperature range of equal to or higher
than (Acl - 40)"C and lower than Ac3"C;
cooling the heated cold-rolled steel sheet from the highest heating temperature to
15 660°C at a cooling rate of equal to or less than 10 "CIS; and
holding the cooled cold-rolled steel sheet in a temperature range of 450°C to
660°C for 20 seconds to 10 minutes.
6. The method for manufacturing a hot stamped body according to Claim 5,
20 wherein the chemical components further include one or more from 0.002% to
2.0% of Mo, 0.002% to 2.0% of Nb, 0.002% to 2.0% of V, 0.002% to 2.0% of Ni,
0.002% to 2.0% of Cu, 0.002% to 2.0% of Sn, 0.0005% to 0.0050% of Ca, 0.0005% to
0.0050% of Mg, and 0.0005% to 0.0050% of REM.
7. The method for manufacturing a hot stamped body according to claim 5,
I
the method hrther comprising performing any one of a hot-dip galvahizing process, a
galvamlealing process, a molten aluminum plating process, an alloyed molten aluminum
plating process, and an electroplating process, after the continuous annealing.
5 8. The method for manufacturing a hot stamped body according to Claim 6,
the method further comprising performing any one of a hot-dip galvanizing process, a
galvannealing process, a molten aluminum plating process, an alloyed molten aluminum
plating process, and an electroplating process, after the continuous annealing.
10 9. A hot stamped body which is formed using the method for manufacturing a
hot stamped body according to any one of Claims 1 to 8,
wherein, when a quenching start temperature is equal to or lower than 650°C,
variation of Viclters hardness AHv of the hot stainped body is equal to or less than 100,
when the quenching start temperature is 650°C to 750°C, variation of Vickers hardness
15 AHv of the hot stamped body is equal to or less than 60, and when the quenching start
temperature is equal to or higher than 750°C, variation of Vickers hardness AHv of the
hot stamped body is equal to or less than 40.
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Dated this 12/04/20 13
ATTORNEY FOR THE APPLICANT