The present invention relates to a hot stamped body having a non-heated portion
with small variation in hardness, and a method for manufacturing the hot stamped body.
Priority is claimed on Japanese Patent Application No. 2010-237249, filed
October 22,20 10, and Japanese Patent Application No. 20 10-289527, filed December 27,
201 0, the contents of which are 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-roliing step tends to have an
uneven microstructure in locations in hot-rolled coil. Accordingly, as means for solving
unevenness of the microstructure generated in a hot-rolling step, performing tempering by
a batch annealing step aftefa hot-rolling 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 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 difficult to perform spheroidizing of carbide to realize softness and evenness 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 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 v ~ a t i o nof the microstructure becomes a reason for variation in hardness of a hot
stamping material.
[OOOS]
Currently, in a widely-used hot stamping formation, it is general to perform
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 materihl by the &ace heating has
poor productivity since the heating takes a long time. Accordingly, a 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 &erial in a conductive state, by
modifying current density flowing to the same sheet material (for example, Patent
Document 1).
[0006]
If the temperature variation exists in the steel sheet for hot stamping by partially
heating the steel sheet, the microstructure of the steel sheet does not significantly change
from the microstructure of the base material at a non-heated portion. Accordingly, the
hardness of the base material before heating becomes directly the hardness of the
component. However, as mentioned above, the material which is subject to the
cold-rolling after hot-rolling and the continuous annealing has a variation in the strength
as shown in FIG. 1, and thus, the non-heated portion has a large variation in the hardness.
Accordingly, there is a problem in that a formed component has a variation in the collision
performance and the like and thus it is difficult to manage the precision of the quality of
the component.
[0007]
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 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 the non-heated portion.
Accordingly, if considering not only a desired hardness after hot stamping quenching,
formability or shape fixability of the non-heated portion, a preferable material before hot
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
4
I
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.
[OOOS]
Accordingly, if a formed body is obtained by hot stamping a steel sheet which is
heated so as to make a heated portion and a non-heated portion exist in the steel sheet,
I there is a problem in that the formed body one-by-one includes a variation in hardness at ~ the non-heated portion.
Citation List
Patent Document
[0009]
[Patent Document I] Japanese Unexamined Patent Application, First
Publication No. 2009-274 122
Non-Patent Documents
[OO lo]
won-Patent Document 11 "Iron and Steel Materials", The Japan Institute of
Metals, Maruzen Publishing Co., Ltd. p. 21
[Non-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. 1 169
[Non-Patent Document 31 "Yakiiresei (Hardening of steels)--Motomekata to katsuyou
(How to obtain and its use)--," (author: OWAKU Shigeo, publisher: Nikkan Kogyo
S himbun
Summary of Invention
Technical Problem
[OOll]
An object of the present invention is to solve the aforementioned problems and to
provide a method for manufacturing a hot stamped body which can suppress a variation in
a
hardness at a non-hardened portion even if a steel sheet, which is heated so as to make a
heated portion and a non-heated portion exist therein, is hot stamped, and a hot stamped
body which has a small variation in hardness at the non-hardened portion.
Solution to Problem
[OO 1 21
An outline of the present invention made for solving the aforementioned
problems is as follows.
(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 .O% to
3.0% of Mn, 0.01% to 1 .O% 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 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 heated portion at which a
highest heating temperature is equal to or higher than Ac3"C, and a non-heated portion at
which a highest heating temperature is equal to or lower than AclOC are exist, wherein the
continuous annealing includes: heating the cold-rolled steel sheet to a temperature range
of equal to or higher than Acl°C 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 "Cls; 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) In the method for manufacturing a hot stamped body according to (I), 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.
(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.
(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
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 .O% to
3.0% of Mn, 0.01% to 1 .O% 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 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 heated portion at which a
highest heating temperature is equal to or higher than Ac3"C, and a non-heated portion at
which a highest heating temperature is equal to or lower than Acl "C are exist, 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 temperature FiT in a final
rolling mill Fi in a temperature range of (Ac3 - 80)OC to (Ac3 + 40)OC, by setting time
from start of rolling in a rolling mill Fi-3 which is a previous machine to the final rolling
mill Fi to end of rolling in the final rolling mill Fi to be equal to or longer than:2.5 seconds,
and by setting a hot-rolling temperature Fim3Tin the rolling mill Fi-3 to be equal to or lower
than FiT + 100°C, and after holding in a temperature 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 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) 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 (8), wherein, when the amount of C in the steel sheet is
equal to or more than 0.18% and less than 0.25%, AHv is equal to or less than 25 and
Hv-Ave is equal to or less than 200; when the amount of C in the steel sheet is equal to or
more than 0.25% and less than 0.30%, AHv is equal to or less than 32 and Hv-Ave is
equal to or less than 220; and when the amount of C in the steel sheet is equal to or more
than 0.30% and less than 0.35%, AHv is equal to or less than 38 and Hv-Ave is equal to or
less than 240, where AHv represents a variation in Vickers hardness of the non-heated
portion, and Hv-Ave represents an average Vickers hardness of the non-heated portion.
Advantageous Effects of Invention
[00 131
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
hot stamping a steel sheet which is heated so that a heated portion and non-heated portion
are exist in the steel sheet, it is possible to stabilize the hardness of the non-heated portion
of the hot stamped product.
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 product is exhibited.
In addition, by employing such methods, it is possible to obtain a hot stamped
body in which, when the amount of C in the steel sheet is equal to or more than 0.18% and
less than 0.25%, AHv is equal to or less than 25 and Hv-Ave is equal to or less than 200,
when the amount of C in the steel sheet is equal to or more than 0.25% and less than
0.30%, AHv is equal to or less than 32 and Hv-Ave is equal to or less than 220, and when
the amount of C in the steel sheet is equal to or more than 0.30% and less than 0.35%,
AHv is equal to or less than 38 and Hv-Ave is equal to or less than 240, where AHv
represents a variation in Vickers hardness of the non-heated portion, and Hv-Ave
represents an average Vickers hardness of the non-heated portion.
Brief Description of Drawings
[00 141
FIG. 1 is a view showing variation in hardness of a steel sheet for hot stamping
after continuous annealing of the related art.
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 hot stamping steps of example of the present invention.
FIG. 6 is a view showing variation in hardenability wheri hot stamping by values
of Cre/CrMa nd bh)/Mn~in the present invention.
FIG. 7A is a result of segmentalized pearlite observed by a 2000x SEM.
FIG. 7B is a result of segmentalized pearlite observed by a 5000x SEM.
FIG. 8A is a result of non-segmentalized pearlite observed by a 2000x SEM.
FIG. 8B is a result of non-segmentalized pearlite observed by a 5000x SEM.
Description of Embodiments
[00 1 51
Hereinafter, preferred embodiments of the present invention will be described.
[00 161
First, a method for calculating Ac3 which 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 fiom the same test.
As an example of a measurement method, as disclosed in Non-Patent Documents 1 and 2,
a method of acquiring fiom 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,
each temperature from change in expansion. In a case of 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.
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, AQ changes according to
hot-rolling conditions or a cooling rate after rolling. Accordingly, AT3 was calculated
with a calculation model disclosed in ISIJ International, Vol. 32 (1992), No. 3, and a
holding time from Ar3 to 600°C was determined by correlation with an actual temperature.
[00 171
Hereinafter, a steel sheet for hot stamping according to the present inve$ion used
in a method for manufacturing a hot stamped body will be described.
I [OO 1 81
(Quenching Index of Steel Sheet for Hot Stamping)
Since it is aimed for a hot stamping material to obtain high hardness after
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 DIinchv alue which is a quenching index is equal to or more than 3. It is
possible to calculate the DIinchv alue based on ASTM A255-67. A detailed calculation
method is shown in Non-Patent Document 3. Several calculation methods of the DIinch
value have been proposed, regarding an equation of fB for calculating using an additive
method and calculating an effect of B, 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
I 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.
[00 191
The Dlinchv 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 DIinchv 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 DIinchv alue is equal to or
more than 3. Meanwhile, when the DLch value is extremely high, since the ferrite
transformation in the continuous annealing does not proceed, a value of about 10 is
preferable as an upper limit of the Dllnchv alue.
[0020]
(Chemical Components of Steel Sheet For Hot Stamping)
In the method for manufacturing a hot stamped body according to the present
invention, a steel sheet for hot stamping manufactured fiom a steel piece including
chemical components which include C, Mn, Si, P, S, N, Al, Ti, B, and Cr and the balance
of Fe and inevitable impurities is used. In addition, as optional elements, one or more
elements fiom 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.
[0021]
(C: 0.18% to 0.35%)
When content of C is less than 0.18%, hardened strength 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 non-heated portion which is heated to Acl
point or lower 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%.
[0022]
(Mn: 1 .O% to 3.0%)
When content of Mn is less than 1.0%, it is difficult to secure hardenability at 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
preferably 2.5%.
[0023]
(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 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
1.0%, 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%.
[09241
(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
same manner as in a case of Si. In addition, a lower limit thereof is not particularly
I
provided, however, it is difficult to have the content of less than 0.001% since the cost
significantly rises.
[0025]
(S: 0.0005% to 0.01%)
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.
[0026]
(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
provided, however, it is dificult to have the content of less than 0.001 % since the cost
significantly rises.
[0027]
(Al: 0.01% to 1.0%)
Since A1 has the solid-solution hardening property in the same manner as Si, it
14
t
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 of A1 mixed in at the deoxidation level is a practical lower limit.
[0028]
(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 B,
although B is added in a state of large amount of N, the effect of improving the
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
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
0.2% is set as the upper limit.
[0029]
(B: 0.0002% to 0.005%)
B is one of most efficient elements as an element for improving hardenability
3
with low cost. As described above, when adding B, since it is necessary to be in a
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.
rc'
[0030]
(Cr: 0.002% to 2.0%)
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
thereof become saturated.
[003 11
(Mo: 0.002% to 2.0%)
(Nb: 0.002% to 2.0%)
(V: 0.002% to 2.0%)
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. Accordingly, the contained amounts
of Mo, Nb, and V may be in a range of 0.002% to 2.0%, respectively.
[0032]
(Ni: 0.002% to 2.0%)
(Cu: 0.002% to 2.0%)
(Sn: 0.002% to 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.
[0033]
(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 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.
[0034]
(Microstructure of Steel Sheet for Hot Stamping)
Next, a microstructure of the steel sheet for hot stamping will be described.
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
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
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
addition, by setting the highest heating temperature to be less than Ac3"C in the heating
step and by cooling from 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 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
6
that the hardened non-recrystallized ferrite is softened by recovery and recrystallization
due to dislocation movement in annealing, it is possible 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 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 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).
[003 61
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
becomes hard. In addition, when the fraction of the non-recrystallized ferrite exceeds
30%, the hardness of the steel sheet after the continuous annealing step becomes hard.
[0037]
The ratio of the non-recrystallized ferrite can be measured by analyzing an
P Electron Back Scattering diffraction Pattern (EBSP). The discrimination of the
~ non-recrystallized ferrite and other ferrite, that is, the recrystallized ferrite and the
I I
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 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 KAM
method, since it is possible to quantitatively show the crystal 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 lo (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 pm is defined as
the ferrite other than the non-recrystallized ferrite, that is, the recrystallized ferrite and the
transformed ferrite.
[0038]
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
of Cr subjected to solid solution in a base material is equal to or less than 2, or (B) a value
of a ratio bim/MIIM of concentration MIQ 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.
[0039]
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 stampixig step, when heating at a low
19 *
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
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
CrelCr~ex ceeds 2 and the value of h / M ne~xc eeds 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 CrelCr~b e equal to or less than 1.5 and the value of h / M nto~ b e equal to
or less than 7.
The Cre/CrMa nd the h'fne/Mn~c an 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
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. 'ko 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
hot-rolling to Acl to Ac3 in the continuous annealing and performing slow cooling from
the highest heating temperature at a temperature rate 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.
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 to set the
values of Cre/CrMa nd MQJIV~IIMa s 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 m/MnM to be low.
As shown in FIG. 6, the threshold values were determined from an expansion
curve when holding C-1 in which the values of Cre/CrM and Mh&ln~ are low and C-4 in
which the values of Cre/CrM and Mm/MnM are high, for 10 seconds after heating to 850°C
at 150 "Cis, and then cooling at 5 "CIS. That is, while the transformation starts from the
vicinity of 650°C in the cooling, in a material in which the values of Cre/CrM and
m/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 C r e / Ca~n d &/MnM are high.
That is, by setting the values of Cre/CrM and Mn&hM to be low, it is possible to improve
hardenability after the rapid heating.
[0040]
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 ahd observing using the
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.
[004 11
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/CrM and m/MnM are lower.
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
2 1
r)
and the pearlite, if the ferrite is recrystallized after cold-rolling the hot-rolled steel sheet 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. 7A and 7B. 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 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. 8A and 8B.
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
point counting method.
LO0421
(First Embodiment)
Hereinafter, a method for manufacturing a hot stamped steel sheet according to a
first embodiment of the present invention will be described.
[0043]
The method for manufacturing a hot stamped steel sheet 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.
[0044]
(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 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
11 OO°C, carbide-forming elements and carbon can be subjected to
22
rf
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 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.
[0045]
When a finishing temperature of the hot-rolling is lower than Ar3"C, the ferrite
transformation occurs in rolling by contact of the surface layer of the steel sheet and a 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.
COO461
(Coiling Step)
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
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
thereof compared to the related art by control of the microstructure in the continuous
annealing.
[0047]
(Cold-Rolling Step)
In the cold-rolling step, the coiled hot-rolled steel sheet is cold-rolled after
pickling, and a cold-rolled steel sheet is manufactured.
[0048]
(Continuous Annealing Step)
23 * 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
the cold-rolled steel sheet in a temperature range of equal to or higher than "Acl°C and
lower than Ac3"Cm, 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 temperature
range of "550°C to 660°C" for 1 minute to 10 minutes.
[0049]
(Hot Stamping Step)
In the hot stamping step, hot stamping is performed for the steel sheet which is
heated so as to have a heated portion and a non-heated portion. The heated portion
(hardening portion) is heated to the temperature of Ac3 or higher. General conditions
may be employed for the heating rate thereof or the subsequent cooling rate. However,
si~cteh e 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 heated
portion may not be sufficiently quenched or the heat may transfer to the non-heated
portion, 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 to make the steel sheet have the heated portion and the
non-heated portion is not particularly regulated, and for example, a method of performing
electrical-heating, a method of providing a heat-insulating member on the portion that
should not be heated, a method of heating a particular portion of the steel sheet by infrared
ray radiation, or the like may be employed.
The upper limit of the highest heating temperature may be set to 1 000°C so as to
avoid the non-heated portion from being heated due to heat transfer. 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.
The heated portion means a portion at which the highest heating temperature at
I
the time of heating the steel sheet in the hot stamping process reaches Ac3 or higher, The
non-heated portion means a portion where the highest heating temperature at the time of
heating the steel sheet in the hot stamping process is within the temperature range of equal
to or less than Ac~. The non-heated portion includes a portion that is not heated, and a
portion that is heated to Acl or lower.
[0050]
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 the steel sheet in a state of including a non-heated portion, it is
possible to reduce variation of the hardness of the non-heated portion of the hot stamped
body. In detail, it is possible to realize the following AHv which represents a variation in
Vickers hardness of the non-heated portion, and Hv-Ave which represents an average
Vickers hardness of the non-heated portion.
If the amount of C in the steel sheet is equal to or more than 0.18% and less than
0.25%, AHv is equal to or less than 25 and Hv-Ave is equal to or less than 200.
If the amount of C in the steel sheet is equal to or more than 0.25% and less than
0.30%, AHv is equal to or less than 32 and Hv-Ave is equal to or less than 220.
If the amount of C in the steel sheet is equal to or more than 0.30% and less than
0.35%, AHv is equal to or less than 38 and Hv-Ave is equal to or less than 240.
[005 11
The steel sheet for hot stamping contains a lot of C component for securing
quenching strength after the hot stamping and contains Mn and B, and in such a steel
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 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
9
temperature range of "550°C to 660°C" for 1 minute to 10 minutes, and thus the
I microstructure can be obtained to be even.
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.
[0053]
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 steel sheet according to the embodiment, in the continuous annealing
step, by heating to a heating range of "equal to or higher than Acl°C and lower than
Ac3"C" which is a higher temperature range than the Acl point, heating is performed until
having a double phase coexisterice 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 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.
[0054]
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 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"CV which is
26
L1
immediately below Ac3, the ferrite slightly remains in a state where almost hardened
non-recrystallized ferrite is reverse-transformed to the austenite, and in the subsequent
cooling step at a cooling rate of equal to or less than 10 "C/s 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 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.
[0055]
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 the method for
manufacturing a steel sheet 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 af 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 550°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
transformation product occurs. In addition, when the holding time exceeds 10 minutes,
the continuous annealing installation subsequently becomes longer and high cost is
2 7 * 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 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.
[0056]
According to the manufacturing method described above, by coiling the
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 range
in which the ferrite transformation and the pearlite transformation occur, however, 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 cqlled Run-Out-Table
i
(hereinafter, ROT) from the finish rolling of the hot-rolling step to the coiling, the phase
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 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 strength 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 or lower than
Acl, significant variation in the strength is generated as shown in FIG. 1 by the variation in
28
li
the ferrite recrystallization rate caused by the variation of the microstructure 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. When heating to a temperature of equal to or higher than Ac3 to completely
remove the non-recrystallized ferrite, significant hardening is performed after the cosling
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 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.
[0057]
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 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 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 conditions, it is possible to obtain
even strength and soft hardness of the steel sheet after the annealing.
[0058]
29
9
By using the steel having the even strength, in the hot stamping step, even in a
case of employing an electrical-heating method which inevitably generates an irregularity
in the steel sheet temperature after heating, it is possible to stabilize the strength of a
component of the formed product after the hot stamping. For example, for an electrode
holding portion or the like in which thc temperature does not rise by the electrical-heating
and in which the strength of the materjal of the steel sheet itself affects the product
I
strength, by evenly managing the strength of the material of the steel sheet itself, it is
possible to improve management of precision of the product quality of the formed product
after the hot stamping.
[0059]
(Second Embodiment)
Hereinafter, a method for manufacturing a hot stamped steel sheet according to a
second embodiment of the present in1 ention will be described.
[0060]
The method for manufacturing a hot stamped steel sheet 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.
[006 11
(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 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 1 200°C, precipitated carbonitrides in
the steel piece can be sufficiently dibsolved. However, it is not preferable to heat the
30
9
steel piece to a temperature higher than 1280°C, from a view point of production cost.
LO0621
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
finish-hot-rolling temperature FiT in a final rolling mill Fi in a temperature range of (Ac3 -
80)OC to (Ac3 + 40)OC, by (B) setting a time from start of rolling in a rolling mill Fi-3
which is a previous machine to the final rolling mill Fi to end of rolling in the final rolling
mill Fi to be equal to or longer than 2.5 seconds, and by (C) setting a hot-rolling
temperature Fim3Tin the rolling mill Fi-3 to be equal to or lower than (FiT + 100)OC, and
then holding is performed in a temperature range of "600°C to Ar3OCV for 3 seconds to 40
seconds, and coiling is performed in the coiling step.
[0063]
By performing such hot-rolling, it is possible to perform stabilization and
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 and
holding at a temperature of equal to or lower than Ar30C in the ROT for a long time are .
important conditions.
[0064]
When the FiT is less than (Ac3 - 80)OC, a possibility of the ferrite transformation
in the hot-rolling becomes high and hot-rolling deformation resistance is not stabilized.
On the other hand, when the FiT 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
"(Ac3 - 70)OC to (Ac3 + 20)OC". By setting the heating conditions as described above, it
is possible to refine the austenite grain size after the finish rolling, and it is possible to
promote the ferrite transformation in the ROT cooling. Accordingly, since the
3 1
9
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.
For example, in a case of a hot-rolling line including seven final rolling mills,
transit time fiom a F4 rolling mill which corresponds to a third mill fiom 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.
[0066]
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 AT3, 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 Fi-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.
[0067]
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 Ar3 is the ferrite transformation
st& temperature, this is set as the upper limit, and 600°C at which the softened ferrite is
generated is set as the lower limit. A preferable temperature range thereof is 600°C to
700°C in which generally the ferrite transformation proceeds most rapidly.
9
[0068]
(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
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.
[0069]
(Cold-Rolling Step)
In the cold-rolling step, the coiled hot-rolled steel sheet is cold-rolled after
pickling, and a cold-rolled steel sheet is manufactured.
[0070]
(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
the cold-rolled steel sheet in a temperature range of equal to or higher than "(Ac, - 40)"C
and lower than Ac3"CWa, nd 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 "450°C to 660°C" for 20 seconds to 10 minutes.
[007 11
(Hot Stamping Step)
In the hot stamping step, hot stamping is performed for the steel sheet which is
heated so as to have a heated portion and a non-heated portion. The heated portion
(hardening portion) is heated to the temperature of Ac3 or more. General conditions may
be employed for the heating rate thereof or the subsequent cooling rate. However, since
33
$
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 heated portion
may not be sufficiently quenched or the heat may transfer to the non-heated portion, 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 to make the steel sheet have the heated portion and the
non-heated portion is not particularly regulated, and for example, a method of performing
electrical-heating, a method of providing a heat-insulating member on the portion that
should not be heated, a method of heating a particular portion of the steel sheet by infrared .
ray radiation, or the like may be employed.
The upper limit of the highest heating temperature may be set to 1 OOO°C so as to
avoid the non-heated portion from being heated due to heat transfer. 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.
The heated portion means a portion at which the highest heating temperature at
the time of heating the steel sheet in the hot stampiqg process reaches Ac3 or higher. The
non-heated portion means a portion where the highest heating temperature at the time of
heating the steel sheet in the hot stamping process is within the temperature range of equal
to or less than Acl. The non-heated portion includes a portion that is not heated, and a
portion that is heated to Ac1 or lower.
[0072]
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 the steel sheet in a state of including a non-heated portion, it is
possible to reduce variation of the hardness of the non-heated portion of the hot stamped
body. In detail, it is possible to realize the following AHv which represents a variation in
Vickers hardness of the non-heated portion, and Hv-Ave which represents an average
Vickers hardness of the non-heated portion.
If the amount of C in the steet sheet is equal to or more than 0.18% and less than
0.25%, AHv is equal to or less than 25 and Hv Ave is equal to or less - than 200.
If the amount of C in the steel sheet is equal to or more than 0.25% and less than
0.30%, AHv is equal to or less than 32 and Hv- A ve is equal to or less than 220.
If the amount of C in the steel sheet is equal to or more than 0.30% and less than
0.35%, AHv is equal to or less than 38 and Hv - Ave is equal to or less than 240.
[0073]
Since the steel sheet is coiled into a coil after transformation from the austenite 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 (Acl -
40)"C to lower than Ac3"Cm, subsequently cooling fiom 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.
[0074]
In the continuous annealing I ine, 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.
[0075]
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 steel sheet for hot stamping according to the second 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 (Ac~- 40)"C and lower than Ac3"C", heating is
'I 3 5
I
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 of the ferrite in the coil, even
with the heating temperature of AclOC 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 "Cls, 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 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.
[0076]
From these viewpoints, when the temperature is less than (Acl - 40)"C, since 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 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.
[0077]
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
36
@
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 steel
sheet according to the embodiment, even in a case of heating a material having high
hardenability to a temperature right below the A c ~po int 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 dificult for the cementite precipitation or the pearlite
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 20 seconds, the
ferrite transformation, the cementite precipitation, or the pearlite transformation is
insufficient, 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.
[0078]
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 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 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,
3 7
B
by performing the continuous annealing with suitable conditions, it is possible to obtain
even strength and soft hardness of the steel sheet after the annealing.
COO791
By using the steel having the even strength, in the hot stamping step, even in a
case of employing an electrical-heating method which inevitably generates an irregularity
in the steel sheet temperature after heating, it is possible to stabilize the strength of a
component of the formed product after the hot stamping. For example, for an electrode
holding portion or the like in which the temperature does not rise by the electrical-heating
and in which the strength of the material of the steel sheet itself affects the product
strength, by evenly managing the strength of the material of the steel sheet itself, it is
possible to improve management of precision of the product quality of the formed product
after the hot stamping.
[OOSO]
Hereinabove, the present invention has been described based on the fitst
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 embodimenl, it is possible to employ the conditions of the
second embodiment.
Examples
[OOS 11
Next, Examples of the present invention will be described.

[Table 81
Material Microstructure
Steel Condition ATS TS-Ave Non-Ferrite fi-action crystallized Non-segmentalized Cr$CrM M ~ M
type No. ferrite hction pearlite hction
[ma1 [ma1 [vol.%] [vol.%] [vol.%]
1 2 1 aluminum 1 9 1 181 1 468 1
[0090]
Good
[Table 91
B
rY
C
Steel
type
A
3
4
5
6
N09.i j
1
2
3
4
5
6
7
8
9
10
condition
No.
1
2
3
4
5
6
7
8
9
10
1
plating
hot-dip
galvanizing
. .. hot-dip
galvanizing
hot-dip
galvanizing
galvannealing
hot-dip
galvanizing
-
Plating type
hot-dip
galvanizing
galvannealing
hot-dip
galvanizing
hot-dip
galvanizing
molten
26
29
21
23
18
20
32
46
12
15
53
42
43
48
214
211
180
195
Variation of hardness of
non-hardened portion ,
187
185
216
210
197
187
224
223
250
220
Hardness of
non-hardened
portion
462
468
465
462
456
459
47 1
468
465 -- --
458
468
AHv
18
12
11
46
17
18
28
17
~2 1- --
19
18
47 1
468
478
475
Hv-Ave
190
181
178
230
233
220
217
220
179 ---- - -
196
184
474
478
48 1
474
466
468
46 1
475
485
495
Chemical conversion
coating
Hv
Good
Good
Good
Good
Good
Good
Good
Good
Good -
Good
Good
Good
Good
Good
Good
Note
Non-recrystallized ferrite remaining
Insufficient ferrite transformation and cementite precipitation
Insufficient ferrite transformation
Insufficient ferrite transformation and cementite precipitation
Insufficient ferrite transformation and cementite precipitation
p-.---pp---p
Insufficient ferrite transformation and cementite precipitation
Insufficient ferrite transformation and cementite precipitation
Good
Good
Good
Good
Good
Good
Good
Good
Good
Good
Insufficient ferrite transformation and cementite precipitation
Non-recrystallized ferrite remaining
Insufficient ferrite transformation and cementite precipitation
Insufficient ferrite transformation and cementite precipitation
Insufficient ferrite recrystallization
Insufficient cementite precipitation
Steel
type
-
D
Variation of hardness of
Conditio Plating type non-hardened ortion
3
4
5
alvanizin
hot-dip
24 248
24 233
7
8
9
I alloyed I I
hot-dip
galvanizing
I molten
aluminum I 21 1 196
electroplating
plating
2 1
22
18
alvanizin
hot-dip
galvanizin
190
211
208
3 1
9
3 5
214
193
214
468 Good I Non-recrystallized femte remaining
474 Good
Hardness of
non-hardened
478 1 Good
Chemical conversion
coating Note
474
478
48 1
479
474
48 1
Good
Good
Good
Good
478
Insufficient femte transfomation and cementite precipitation
Insufficient ferrite transformation and cementite precipitation
Insufficient femte transformation and cementite ~reci~itation
Good
Good
Good
Good
Insufficient ferrite transformation
Insufficient ferrite transformation and cementite ~reci~itation
539
545
539 .-___L- Good
- --
539
Good
--
Good
Good
Good I Insufficient ferrite transformation
- - -
539
52 1
533
Insufficient ferrite transformation and cementite precipitation
536
Good
Good
Good
48 1
48 1
484
484
Good I Insufficient ferrite transformation and cementite ~reci~itation
Insufficient ferrite transformation and cementite precipitation
Good
Good
Good
Good Non-recrystallized ferrite remaining
I Variation o f , h a r y s of I Hardness of I Chemical c o y i o n
Condition Plating type non-I hardened ortion non-hardened h-0 I XTn I coatin
I LJyU I '.". I I AH" I Hv Ave I ort ti on I Hv
1
2
1 . 3 - I
1 r i1 15 178 383 I - - Good
electroplating
4
5
H
1
I " ! 2 18 179 386 Good
194
185
190
-
2 1
17
12
1
2 I
I K I 1 I I 1 - 1 I Good
hot-dip
galvanizing
2 1 Good
1 40
465
468
465
2
Good
Good
GOO^
12
47
190
32
254
349
Good
L I 1
M I 1
Good
187
208
208
46 Good
484
0
260
2 1 Poor
43
456
456
346
199
N I 1
1
2
Q
R
Note
Good
Good
Good
233
Good
Strength after hot stamping is less than 1 180 MPa
9
17
1
1
S 1 1
Cracks on end portion are generated at the time of hot
low hardenabili
545
T I 1
12 186
Poor chemical conversion coating
Poor
187
184
hot-dip
galvanizing
Poor chemical conversion coating
Hot-rolling. is difficult
466
38 3 AHv is in the range even with the method of the related art for
3 80
9
19
Good
Hot-rolling is difficult
Good
Good
182
216
468
513
Good
Good
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.
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
performed from an expansion and contraction curve by formaster, and values measured at
a heating rate of 5 "CIS 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 "microstructure of a steel strip", "Cre/Cr~", and "h4neIMn~" acquired based
on tensile strength measured from 10 portions of the steel strip after the continuous
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, as shown in FIG. 5, an
electrical-heating was performed using an electrode 2 with respect to the steel sheet 1 for
hot press, thereby heating the steel sheet for hot press so that a heated portion 1-a and a
non-heated portion 1-b are exist in thc steel sheet. Then, hot stamping was performed.
The heated portion 1 -a is heated at the heating rate of 30 "CIS until the temperature reaches
Ac3+500C, and then, without performing temperature holding after the heating, the die was
cooled at the cooling rate of not less than 20 "CIS. The hardness of the non-heated
portion 1 -b as shown in FIG. 5 was measured by obtaining average value of five points
using Vickers hardness tester with 5 kgf load, at the cross sectiorl in the 0.4 mni depth
from the surface. With respect to the hot-rolled coil, 30 parts are selected at random and
the difference between the maximum hardness and the minimum hardness was obtained as
ABv, and the average thereof was obtained as Hv - Ave. The threshold value of the AHv
is significantly affected by the amount of C of the steel material, thus, the present
invention employs the following criteria for the threshold value.
If the amount of C in the steel sheet is equal to or more than 0.18% and less than
0.25%, AHv 5 25 and Hv-Ave 5 200.
If the amount of C in the steel sheet is equal to or more than 0.25% and less than
0.3%, AHv 5 32 and Hv-Ave 5 220.
If the amount of C in the steel sheet is equal to or more than 0.3% and less than
0.35%, AHv F 38 and Hv - Ave F 240.
100941
In the tensile test, steel sheet samples were extracted from portions within 20 m
from the initial location and final location of the steel strip, and the tensile strength was
acquired by performing tensile tests in the rolling direction to obtain values of the tensile
strength at respective 5 portions in the width direction as measurement portions.
[0095]
As to the hardenability, if the chemical components are out of the range of the
present invention, the hardenability is low and thus, the variation of the hardness or the
rising of the hardness in the steel sheet manufacturing as described in the opening of this
specification does not occur. Accordingly, when the hardness of the non-heated portion
of the component is measured after hot stamping, low hardness and low variation of the
hardness can be stably obtained even if the present invention is not employed. Therefore,
this is regarded as out of the invention. More specifically, a product manufactured by
employing a condition which is out of the range of the present invention but satisfies the
above-mentioned threshold value of AHv is regarded as out of the present invention.
Then, using a press die and a piece of steel sheet which was cut from the
manufactured steel sheet and electrically-heated with electrodes schematically shown in
FIG. 5, hot stamping was performed, thereby manufacturing a hot-stamped component
with a shape as illustrated in FIG. 4. In the hot stamping, the heating rate of the center
portion was set to be 50 "CIS and the steel sheet was heated to the highest heating
temperature of 870°C. The end portion of the steel sheet was a non-heated portion since
5 1
J!
the temperature of the electrode was about a room temperature. In order to easily
generate a temperature variation in the steel sheet depending on the areas of the steel sheet,
as shown in-FIG. 4, a steel sheet electrically-heated with an electrical-heating electrode
unit through which a cooling medium passes was pressed. 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.
[0096]
Further, since it is a precedent condition to use a material for hot stamping in the
present invention, a case where the maximum hardness at the hardened portion after hot
stamping becomes less than Hv 400 is regarded as out of the invention. The maximum
hardness of the hardened portion was measured at "HARDNESS-MEASUREMENT
AREA FOR HARDENED PORTION" as shown in FIG. 5 where the steel sheet is heated
to Ac3 or more and is in'close contact with the die. The hardness measurement was
conducted for 30 components to obtain the average value as similar to the hardness
measurement of the non-heated portion as mentioned above.
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).
[0097]
Test Examples A-1, A-2, A-3. 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. 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
!P precipitation in the subsequent cooling and the holding did not proceed, the hard phase
fraction after the annealing became high, and Hv - Ave 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-Ave bccame high. In Test Examples A-7, D-4, D-5, D-6,
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 Hv-Ave 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 Hv - Ave became high. 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 - Ave 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
steel material and having different DI,,,h values of 3.5, 4.2 and 5.2, it was found that, when
the DI,,,h value was large, improvement of AHv and Hv - Ave was significant. Since a
steel type H had a small amount of C of 0.16%, the hardness after quenching in the hot
stamping became lower, and it was not suitable as a hot stamped comp~nent. Since a
steel type I had a large amount of C of 0.40%, the formability of the non-heated portion
was generated 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 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 L and M
respectively had a large amount of Si of 1.32% and A1 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.
[0098]
!lr
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
[0099]
According to the present invention, it is possible to provide a method for
manufacturing a hot stamped body which can suppress a variation in hardness at a
non-hardened portion even if a steel sheet which is heated so as to have a heated portion
and a non-heated portion is hot stampcd, and a hot stamped body which has a small
variation in hardness at the non-hardened portion.

54
CLAIMS
1. 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, 1.0% 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% ofN, 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
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 heated portion at which a highest heating temperature is ,
equal to or higher than Ac3"C, and a non-heated portion at which a highest heating
temperature is equal to or lower than Acl "C are exist,
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 Ac3OC;
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 "Cls; 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 fiom 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 REh4.
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.
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, 1.0% to 3.0% oEMn, 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% ofN, 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
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 heated portion at which a highest heating temperature is
equal to or higher than Ac3"C7 and a non-heated portion at which a highest heating
temperature is equal to or lower than Ac1 OC are exist, 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 temperature
FiT in a final rolling mill Fi in a temperature range of (Ac3 - 80)OC to (Ac3 + 40)OC, by
setting time from start of rolling in a rolling mill Fi-3 which is a previous machine to the
56
(r
final rolling mill Fi to end of rolling in the final rolling mill Fi to be equal to or longer than
2.5 seconds, and by setting a hot-rolling temperature Fi-3T in the rolling mill Fi-3 to be
equal to or lower than F,T + 100°C, and after holding in a temperature range of 600°C to
Ar3"C for 3 seconds to 40 seconds, coiling is performed, and
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 fiom 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 450°C to
660°C for 20 seconds to 10 minutes.
6. The method for manufacluring a hot stamped body according to Claim 5,
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, 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.
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.
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 the amount of C in the steel sheet is equal to or more than 0.18% and less
than 0.25%, AHv is equal to or less than 25 and Hv - Ave is equal to or less than 200;
when the amount of C in the steel sheet is equal to or more than 0.25% and less
than 0.30%, AHv is equal to or less than 32 and Hv-Ave is equal to or less than 220; and
when the amount of C in the steel sheet is equal to or more than 0.30% and less
than 0.35%, AHv is equal to or less than 38 and Hv-Ave is equal to or less than 240,
where AHv represents a variation in Vickers hardness of the non-heated portion,
and Hv-Ave represents an average~vickersh ardness of the non-heated portion.
Dated this 12.04.2013
SHRIMANT/SI+QH
OF REMFRY & SAGAR
ATTORNEY FOR THE APPLICANT[S]