The present invention relates to a steel sheet and the method for manufacturing a
steel sheet. This steel sheet is, in particular, suitably used for hot stamping.
10 Priority is claimed on Japanese Patent Application No. 20 10-237249, filed
October 22,2010, the content of which is incorporated herein by reference.
I Background Art
[0002]
In order to manufacture high-strength components of a grade of 11 80 MPa or
15 higher used for automobile components or the like with excellent dimensional precision,
in recent years, a technology (hereinafter, referred to as "hot stamping") 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.
20 [0003]
In general, a steel sheet used for hot stamping contains a lot of C component for
securing formed-product strength after hot stamping and contains Mn and B for securing
hardenability when cooling a die. That is, high hardenability is a property necessary for
a hot stamped product. However, when manufacturing a steel sheet which is a material
I 25 thereof, these properties are disadvantageous, in many cases. For example, in the steel
L sheet having high hardenability, when the hot-rolled steel sheet is cooled on a Run Out
Table (Hereinafter, referred to as "ROT), the transformation from austenite to a low
temperature transformation phase such as ferrite or bainite does not complete, but the
transformation completes in a coil after coiling. In the coil, the outermost and
5 innermost peripheries and edge portions are exposed to the external air, the cooling rate
is relatively higher than that of the center portion. As a result, the microstructure
thereof becomes uneven, and the variation is generated in strength of the hot-rolled steel
sheet. Further, this unevenness of the microstructure of the hot-rolled steel sheet makes
I the microstructure after cold-rolling and continuous annealing uneven, whereby the
I
I
I
10 variation is generated in strength of the steel sheet material before hot stamping. As
1 means for solving unevenness of the microstructure generated in a hot-rolling step,
I
I
I
performing tempering by a batch annealing step after a hot-rolling step or a cold-rolling
I
step may be considered, however, the batch annealing step usually takes 3 or 4 days and
I
I
I I thus, is not preferable from a viewpoint of productivity. In recent years, in normal steel
I
I 15 other than a material for quenching used for special purposes, from a viewpoint of
I productivity, it has become general to perform a thermal treatment by a continuous
annealing step, other than the batch annealing step. However, in a case of the
continuous annealing step, since the annealing time is short, it is difficult to perform
spheroidizing of carbide by long-time thermal treatment such as a batch treatment. The
20 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
25 upper limit as the time for holding at a temperature in the vicinity of the Acl, due to a
a restriction of a length of installation. In such a short time, the carbide is cooled before
being subjected to the spheroidizing, and further, the recrystallization of the ferrite
partially delays. Accordingly, the steel sheet after annealing has an uneven
microstructure in a hardened state. As a result, as shown in FIG. 1, variation is
5 generated in strength of the material before heating in a hot stamping step, in many cases.
[0004]
Currently, in a widely-used hot stamping formation, it is general to perform
1 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
I ~ I 10 single phase temperature, it is possible to solve the variation in strength of the material
described above. Meanwhile, as disclosed in the Patent Document 1, there is a method
for manufacturing a component which employs a local heating so as to give different
strength in the component. In this method, hot stamping is performed after heating a
predetermined portion of the component. For example, if this method is employed, it is
15 possible to remain a portion which is not heated to an austenite range and has a
microstructure of the base material. In such a method, rapid heating is locally
performed, thus, the dissolving speed of the carbides when the temperature reaches the
austenite range significantly affects on the hardenability in the hot stamping and the
strength after the hardening.
20 [OOOS]
If the temperature variation exists in the sheet material for hot stamping, the
microstructure of the steel sheet does not significantly change from the microstructure of
the base material at a low temperature heated portion where the temperature reaches only
AclOC or less or non-heated portion which is not heated intentionally (hereinafter, both
25 portions are referred to as "non-heated portion"). Accordingly, the strength of the base
I 1 material before heating becomes directly the strength of the formed product. 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 is hard and has a large variation in the strength. Accordingly, there
5 is a problem in that it is difficult to manage the precision of the quality of the formed
product and press form the non-heated portion.
[0006]
In addition, in order to solve the variation in the strength of a material, when
heating at a temperature equal to or higher than A c S~O as to be an austenite single phase
10 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 strength of a material significantly increases. As the hot
stamping material, this not only becomes a reason for die abrasion in a blank before
stamping, but also significantly decreases formability or shape fixability of a non-heated
15 portion. Accordingly, if considering not only a desired strength after hot stamping
quenching, formability or shape fixability of a non-heated portion, a preferable material
before hot stamping is a material which is soft and has small variation, and a material
having an amount of C and hardenability to obtain desired strength after hot stamping
quenching. However, if considering manufacturing cost as a priority and assuming the
20 manufacture of the steel sheet in a continuous annealing installation, there is a problem in
that it is difficult to perform the control described above by an annealing technology of
the related art.
Further, there is another problem in that if the heating temperature is low and the
heating time is short in the hot stamping, carbides tend not to be dissolved in austenite
25 and a predetermined strength after quenching cannot be obtained in the hot stamped
L product.
I Citation List
~ Patent Document
[0007]
I
I 5 [Patent Document 11 Japanese Unexamined Patent Application, First
Publication No. 201 1-1 52589
I
Non-Patent Documents
[OOOS]
won-Patent Document I] "Iron and Steel Materials", The Japan Institute of
10 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. 1169
won-Patent Document 31 "Yakiiresei (Hardening of steels)--Motomekata to
15 katsuyou (How to obtain and its use)--," (author: OWAKU Shigeo, publisher: Nikkan
Kogyo Shimbun
Summary of Invention
Technical Problem
[0009]
20 An object of the present invention is to solve the aforementioned problems and
to provide a steel sheet for hot stamping in which the strength property before heating for
hot stamping is soft and even, and the hardenability is high even if the heating
temperature is low and the heating time is short, and a method for manufacturing the
same.
25 Solution to Problem
[OO lo]
The present invention employs following configurations and methods for
solving the aforementioned problems.
(I) A first aspect of the present invention is a steel sheet with chemical components
5 which include, by mass%, 0.18% to 0.35% of C, 1.0% 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 .O%
of Al, 0.005% to 0.2% of Ti, 0.0002% to 0.005% of B, and 0.002% to 2.0% of Cr, and
the balance of Fe and inevitable impurities, wherein: by volume%, a fraction of a ferrite
is equal to or more than 50%, and a fraction of a non-recrystallized ferrite is equal to or
10 less than 30%; and a value of a ratio CreICrM is equal to or less than 2, where Cre is a
concentration of Cr subjected to solid solution in an iron carbide and CrM is a
concentration of Cr subjected to solid solution in a base material, or a value of a ratio
Mne/MnM is equal to or less than 10, where Mne is a concentration of Mn subjected to
solid solution in an iron carbide, and MnM is a concentration of Mn subjected to solid
15 solution in a base material.
(2) In the steel sheet according to the above (I), the chemical components may hrther
include one or more from 0.002% to 2.0% of Mo, 0.002% to 2.0% of Nb, 0.002% to
2.0% of V, 0.002% to 2.0% of Ni, 0.002% to 2.0% of Cu, 0.002% to 2.0% of Sn,
0.0005% to 0.0050% of Ca, 0.0005% to 0.0050% of Mg, and 0.0005% to 0.0050% of
20 REM.
(3) In the steel sheet according to the above (I) or (2), a DIinCh value which is an index
of a hardenability may be equal to or more than 3.
(4) In the steel sheet according to any one of the above (1) to (3), a fraction of a
non-segmentalized pearlite may be equal to or more than 10%.
25 (5) A second aspect of the present invention is a method for manufacturing a steel sheet
* for hot stamping, the method including: hot-rolling a slab containing chemical
components according to (1) or (2), 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
5 sheet which is subjected to cold-rolling, wherein the continuous annealing includes:
I
I heating the cold-rolled steel sheet to a temperature range of equal to or higher than AclOC
I
I 1 and lower than Ac3"C; cooling the heated cold-rolled steel sheet from the highest heating
I
I temperature to 660°C at a cooling rate of equal to or less than 10 "CIS; and holding the
I
cooled cold-rolled steel sheet in a temperature range of 550°C to 660°C for 1 second to
I
10 10 minutes.
(6) The method for manufacturing a steel sheet according to the above (5) may further
include performing any one of a hot-dip galvanizing process, a galvannealing process, a
molten aluminum plating process, an alloyed molten aluminum plating process, and an
electroplating process, after the continuous annealing.
15 (7) A third aspect of the present invention is a method for manufacturing a steel sheet
for hot stamping, the method including: hot-rolling a slab containing chemical
components according to (1) or (2), 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; and continuously annealing the cold-rolled
20 steel sheet which is subjected to cold-rolling to obtain a steel sheet for hot stamping,
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 F,T in a final rolling mill F, in a temperature range of (Ac3 - 80)"C to (Ac3 +
40)"C, by setting time from start of rolling in a rolling mill F1-3 which is a previous
25 machine to the final rolling mill F, to end of rolling in the final rolling mill F, to be equal
mill F,-3 to be equal to or lower than F,T + 100°C, and after holding in a temperature
~ I range of 600°C to Ar3"C for 3 seconds to 40 seconds, coiling is performed, and the
I continuous annealing includes: heating the cold-rolled steel sheet to a temperature range
5 of equal to or higher than (Acl - 40)"C 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 "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.
(8) The method for manufacturing a steel sheet according to the above (7) may hrther
10 include 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.
Advantageous Effects of Invention
15 [Ooll]
According to the configurations and methods according to (1) to (8) described
above, by employing the heating condition in the continuous annealing as described
above, it is possible to make the property of the steel sheet after continuous annealing
even and soft. Using the steel sheet having even property, even when the steel sheet has
20 a non-heated portion in the hot stamping process, the strength of the hot stamped product
at non-heated portion can be stabilized, and even in a case where the cooling rate after
forming is slow, sufficient hardening strength can be obtained by heating in low
temperature for short time.
In addition, by performing a hot-dip galvanizing process, a galvannealing
25 process, a molten aluminum plating process, an alloyed molten aluminum plating process,
3 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,
5 and also, rust prevention of the hot stamped product is exhibited.
Brief Description of Drawings
[OO 121
FIG. 1 is a view showing variation in hardness of a steel sheet for hot stamping
10 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.
15 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
20 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 when hot stamping by values
of Cre/CrM and Mne/MnM in the present invention.
FIG. 7A is a result of segmentalized pearlite observed by a 2000x SEM.
25 FIG. 7B is a result of segmentalized pearlite observed by a 5000x SEM.
FIG. 8B is a result of non-segmentalized pearlite observed by a 5000x SEM.
Description of Embodiments ~ [00 131
I
I
5 Hereinafter, preferred embodiments of the present invention will be described. ~
[00 141
I First, a method for calculating A c w~h ich is important in the present invention
will be described. In the present invention, since it is important to obtain an accurate
value of Ac3, it is desired to experimentally measure the value, other than calculating
10 from a calculation equation. In addition, it is also possible to measure Acl from the
same test. As an example of a measurement method, as disclosed in Non-Patent
Documents 1 and 2, a method of acquiring from length change of a steel sheet when
heating and cooling is general. At the time of heating, a temperature at which austenite
starts to appear is Acl, and a temperature at which austenite single phase appears is Ac3,
15 and it is possible to read 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
20 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
25 Ar3, however, regarding transformation in a hot-rolling step, Ar3 changes according to
hot-rolling conditions or a cooling rate after rolling. Accordingly, Ar3 was calculated
with a calculation model disclosed in ISIJ International, Vol. 32 (1992), No. 3, and a
holding time fiom AT3 to 600°C was determined by correlation with an actual
temperature.
5 [OO 1 51
(First Embodiment)
Hereinafter, a steel sheet for hot stamping according to a first embodiment of the
present invention will be described.
[00 161
10 (Quenching Index of Steel Sheet for Hot Stamping)
Since it is aimed for a hot stamping material to obtain high strength after
quenching, the hot stamping material is generally designed to have a high carbon
component and a component having high hardenability. In the present invention, the
"high hardenability" means that a DIinch value which is a quenching index is equal to or
15 more than 3. It is possible to calculate the DI,nCh value based on ASTM A255-67. A
detailed calculation method is shown in Non-Patent Document 3. Though several
calculation methods of the DI,,,h 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, in this embodiment, an equation of fB = 1 + 2.7 (0.85 - wt% C) disclosed in
20 Non-Patent Document 3. In addition, it is necessary to designate austenite grain size No.
according to an added amount of C, however, in practice, since the austenite grain size
No. changes depending on hot-rolling conditions, the calculation is performed by
standardizing as a grain size of No. 6 in this embodiment.
[00 1 71
2 5 The DI,,,h value is an index showing hardenability, and is not always connected
-' to strength of a steel sheet. That is, strength 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 amount of C. Even in a case where a large
amount of C is included, phase transformation of a steel sheet proceeds relatively fastly
5 as long as the DIInchv alue is a low value, and thus, phase transformation is almost
1
I
completed before coiling in ROT cooling. Further, also in an annealing step, since
I 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 DIlnch value and a large
10 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 DIInch
value is equal to or more than 3. Meanwhile, when the DIlnChv alue is extremely high,
chemical components do not fall within the range of the present invention, and the ferrite
transformation in the continuous annealing does not proceed, thus, it is not appropriate
15 for the present invention. Accordingly, the value of about 10 is preferable as an upper
limit of the DI,nch value.
[00 1 81
(Chemical Components of Steel Sheet For Hot Stamping)
The steel sheet for hot stamping according to this embodiment includes C, Mn,
20 Si, P, S, N, Al, Ti, B, and Cr and the balance of Fe and inevitable impurities. In addition,
as optional elements, one or more elements from Mo, Nb, V, Ni, Cu, Sn, Ca, Mg, and
REM may be contained. Hereinafter, a preferred range of content of each element will
be described. % which indicates content means mass%. In the steel sheet for hot
stamping according to this embodiment, inevitable impurities other than the elements
25 described above may be contained as long as the content thereof is a degree not
9 significantly disturbing the effects of the present invention, however, as small an amount
as possible thereof is preferable.
[00 1 91
(C: 0.18% to 0.35%)
5 When content of C is less than 0.18%, hardenability after hot stamping becomes
low, and the difference in strength 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 less is significantly decreased.
Accordingly, a lower limit value of C is 0.18, preferably 0.20% and more
10 preferably 0.22%. An upper limit value of C is 0.35%, preferably 0.33%, and more
preferably 0.30%.
[0020]
(Mn: 1 .O% to 3.0%)
When content of Mn is less than 1.0%, it is difficult to secure hardenability at
15 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%.
20 [002 11
(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 added for obtaining
25 desired strength after quenching. Accordingly, it is possible to contribute to
improvement of weldability which is a disadvantage of steel having a large amount of C.
Accordingly, the effect thereof is large when the added amount is large, however, when
the added amount thereof exceeds 0.1%, due to generation of oxides on the surface of the
steel sheet, chemical conversion coating for imparting corrosion resistance is
5 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%.
10 [0022]
(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
15 provided, however, it is difficult to have the content of less than 0.001% since the cost
significantly rises.
[0023]
(S: 0.0005% to 0.01%)
Since S generates inclusions such as MnS which degrades toughness or
20 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.
[0024]
2 5 (N: 0.001% to 0.01%)
9 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 difficult to have the content of less than 0.001% since the cost
5 significantly rises.
[0025]
(Al: 0.01% to 1.0%)
Since A1 has the solid-solution hardening property in the same manner as Si, it
may be added to reduce the added amount of C. Since A1 degrades the chemical
10 conversion coating or the wettability of galvanization in the same manner as Si, the upper
limit thereof is 1.0%, and the lower limit is not particularly provided, however, 0.01%
which is the amount ofAl mixed in at the deoxidation level is a practical lower limit.
[0026]
(Ti: 0.005% to 0.2%)
15 Ti is advantageous for detoxicating of N which degrades the effect of B addition.
That is, when the content of N is large, B is bound with N, and BN is formed. Since the
effect of improving hardenability of B is exhibited at the time of a solid-solution state of
By although B is added in a state of large amount of N, the effect of improving the
hardenability is not obtained. Accordingly, by adding Ti, it is possible to fix N as TiN
20 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
25 effect of improving a delayed fracture property after hot stamping can be obtained, when
1 actively improving the delayed fracture property, it is preferable to add equal to or more
I 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.
5 [0027]
(B: 0.0002% to 0.005%)
B is one of most efficient elements as an element for improving hardenability
with low cost. As described above, when adding By since it is necessary to be in a
I
solid-solution state, it is necessary to add Ti, if necessary. In addition, since the effect
10 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.
[0028]
(Cr: 0.002% to 2.0%)
15 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.
20 [0029]
(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
25 or more than 0.002%, respectively. The effect of improving toughness can be obtained
I ' 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.
5 [003 01
(Ni: 0.002% to 2.0%)
I (Cu: 0.002% to 2.0%)
I
I 1 (Sn: 0.002% to 2.0%)
I
I In addition, Ni, Cu, and Sn improve toughness with a content of equal to or
I
10 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 2.096, respectively.
[003 11
(Ca: 0.0005% to 0.0050%)
15 (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.
20 Accordingly, the contained amounts of Ca, Mg, and REM may be in a range of 0.0005%
to 0.0050%, respectively.
[0032]
(Microstructure of Steel Sheet for Hot Stamping)
Next, a microstructure of the steel sheet for hot stamping according to this
25 embodiment 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
5 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
10 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 according to this embodiment, 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
15 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
20 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)OC to (Ac3 - 10)OC.
By heating to this temperature range, in addition to that the hardened non-recrystallized
ferrite is softened by recovery and recrystallization due to dislocation movement in
annealing, it is possible to austenitize the remaining hardened non-recrystallized ferrite.
Y 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
5 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
10 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
15 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).
[0034]
20 In more detail, the steel sheet for hot stamping according to this embodiment
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 SO%,
and a volume fraction of the non-recrystallized ferrite fraction is equal to or less than
30%. When the ferrite fraction is less than SO%, the hardness of the steel sheet after the
3C; continuous annealing step becomes high. In addition, when the fraction of the
non-recrystallized ferrite exceeds 30%, the hardness of the steel sheet after the
continuous annealing step becomes high.
[0035]
5 The ratio of the non-recrystallized ferrite can be measured by analyzing an
Electron Back Scattering diffraction Pattern (EBSP). The discrimination of the
non-recrystallized ferrite and other ferrite, that is, the recrystallized ferrite and the
transformed ferrite can be performed by analyzing crystal orientation measurement data
of the EBSP by Kernel Average Misorientation method (KAM method). The
10 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
15 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
20 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.
[0036]
2 5 In addition, in the steel sheet for hot stamping according to this embodiment,
Y
(A) a value of a ratio Cre/Cr~o f 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 Mne/MnM of concentration Mne of Mn
subjected to solid solution in iron carbide and concentration M ~oMf M n subjected to
5 solid solution in a base material is equal to or less than 10.
[003 71
The cementite which is a representative of the iron carbide is dissolved in the
austenite at the time of hot stamping heating, and the concentration of C in the austenite
is increased. At the time of heating in a hot stamping step, when heating at a low
10 temperature for a short time by rapid heating or the like, dissolution of cementite is not
sufficient and hardenability or strength 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
Cre/CrM exceeds 2 and the value of Mne/MnM exceeds 10, the dissolution of the
15 cementite in the austenite at the time of heating for short time is insufficient. It is
preferable that the value of Cre/CrM be equal to or less than 1.5 or the value of Mne/Mn~
to be equal to or less than 7.
The Cre/CrM and the Mne/MnM can be reduced by the method for manufacturing
a steel sheet. As will be described in detail in the second embodiment and the third
20 embodiment, 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
Y time. To avoid this, there are two major methods. One of them is, as described in the
second embodiment, a method of dissolving all austenite by heating the iron carbide
generated in the hot-rolling to Acl to A c ~in the continuous annealing and performing
slow cooling from the highest heating temperature to a temperature equal to or lower
5 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, as described in the third embodiment, in the cooling
step after the hot-rolling step, by completing ferrite and pearlite transformation, it is
10 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 third aspect of the
present invention, in the state of the hot-rolled sheet after the hot-rolling, it is possible to
set the values of Cre/Cr~a nd Mnelkln~a s low values. Thus, in the continuous
15 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 CrelCr~an d the Mne/Mn~to be low.
As shown in FIG. 6, the threshold values were determined from an expansion
20 curve when holding C-1 in which the values of Cre/CrMa nd Mne/Mn~ar e low, which is
within the scope of the present invention, and C-4 in which the values of Cre/CrM and
Mne/Mn~ar e high, which is not within the scope of the present invention, 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
9
values of Cro/CrM and Mne/MnM are high, clear phase transformation is not observed at a
1 temperature equal to or lower than 400°C, in the material in which the values of Cro/CrM
and Mno/MnM are high. That is, by setting the values of Cro/CrM and Mne/MnM to be
I
I
I low, it is possible to improve hardenability after the rapid heating.
I ~
5 [003 81
I A measurement method of component analysis of Cr and Mn in the iron carbide I
is not particularly limited, however, for example, analysis can be performed ~ with an
I 1 energy diffusion spectrometer (EDS) attached to a TEM, by manufacturing replica
1
I materials extracted from arbitrary locations of the steel sheet and observing using the
I
1~ 10 transmission electron microscope (TEM) with a magnification of 1000 or more. Further,
I I 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.
1003 91
15 In addition, in the steel sheet for hot stamping according to this embodiment, 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 Cro/Cr~a nd Mne/MnMa re lower.
20 If the fraction of the non-segmentalized pearlite is equal to or more than lo%, the
hardenability of the steel sheet is improved.
When the microstructure of the hot-rolled steel sheet is formed from the ferrite
and the pearlite, if the ferrite is recrystallized after cold-rolling the hot-rolled steel sheet
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
5 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
10 point counting method.
[0040]
(Second Embodiment)
Hereinafter, a method for manufacturing a steel sheet for hot stamping according
to a second embodiment of the present invention will be described.
15 [004 11
The method for manufacturing a steel sheet for hot stamping according to this
embodiment includes at least a hot-rolling step, a coiling step, a cold-rolling step, and a
continuous annealing step. Hereinafter, each step will be described in detail.
[0042]
(Hot-Rolling Step)
In the hot-rolling step, a steel piece having the chemical components described
in the above first embodiment is heated (re-heated) to a temperature of equal to or higher
than 1 1 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
25 manufactured using an electric furnace. By heating the steel piece to a temperature of
9
equal to or higher than 1 100°C, carbide-forming elements and carbon can be subjected to
decomposition-dissolving sufficiently in the steel material. In addition, by heating the
steel piece to a temperature of equal to or higher than 1200°C, precipitated carbonitrides
in the steel piece can be sufficiently dissolved. However, it is not preferable to heat the
5 steel piece to a temperature higher than 1280°C, from a view point of production cost.
[0043]
When a finishing temperature of the hot-rolling is lower than k 0 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
10 limit of the finishing temperature is not particularly provided, however, the upper limit
may be set to about 1050°C.
[0044]
(Coiling Step)
It is preferable that a coiling temperature in the coiling step after the hot-rolling
15 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
20 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.
(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.
[0046]
(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 "AclOC and
10 lower than Ac3"CV, 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.
[0047]
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
20 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"Cn, then cooled from the highest
temperature to 660°C at a cool rate of equal to or less than 10 "CIS, and then held in a
temperature range of "550°C to 660°C" for 1 minute to 10 minutes, and thus the
b microstructure can be obtained to be even.
[0048]
In the continuous annealing line, a hot-dip galvanizing process, a galvannealing
process, a molten aluminum plating process, an alloyed molten aluminum plating process,
5 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.
[0049]
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
10 manufacturing a steel sheet for hot stamping 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 coexistence with the austenite phase in
which the non-recrystallized ferrite slightly remains. After that, in the cooling step at a
15 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
20 cementite precipitation or pearlite transformation is promoted by holding in the same
temperature range.
[0050]
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
25 suppressing generation of the ferrite nucleation at the time of cooling from the austenite
b
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
5 Ac3"C" which is immediately below Ac3, the ferrite slightly remains in a state where
almost hardened non-recrystallized ferrite is reverse-transformed to the austenite, and in
the subsequent cooling step at a cooling rate of equal to or less than 10 "CIS and the
holding step of holding at a temperature range of "550°C to 660°C'' for 1 minute to 10
minutes, softening is realized by the growth of the ferrite by nucleating the remaining
10 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
15 and the hardening is realized, the temperature described above is set as the lower limit.
[005 11
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
20 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 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
5 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
10 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.
[0052]
15 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
20 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 called Run-Out-Table
Y
(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
5 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
10 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
15 variation in 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
20 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 cooling
with an effect of elements for improving hardenability such as Mn or B. Accordingly, it
is advantageous to perform coiling at the temperature range described above for evenness
of the microstructure of the hot-rolled sheet. That is, by performing coiling in the
25 temperature range of "700°C to 900°C", since cooling is sufficiently performed from the
9
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.
5 [0053]
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
10 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 in tensile strength of
15 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 and soft strength of the steel sheet
after the annealing.
[0054]
20 By using the steel sheet having even strength, even in a case where the hot
stamping step includes a local heating manner which inevitably generates the temperature
irregularity in the steel sheet after heating, it is possible to stabilize the strength of a
component after hot stamping. For example, for the portion in which a temperature
does not rise by the local heating and in which the strength of the material of the steel
8
sheet itself affects on 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.
[OOSS]
(Third Embodiment)
Hereinafter, a method for manufacturing a steel sheet for hot stamping according
to a third embodiment of the present invention will be described.
[0056]
The method for manufacturing a steel sheet for hot stamping according to the
10 embodiment includes at least a hot-rolling step, a coiling step, a cold-rolling step, and a
continuous annealing step. Hereinafter, each step will be described in detail.
[0057]
(Hot-Rolling Step)
In the hot-rolling step, a steel piece having the chemical components described
15 in the above first embodiment 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
20 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.
[0058]
Y 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)"C to (Ac3 + 40)"C, by (B) setting a time from start of rolling in a rolling mill Fi-3
5 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 Fi-3T in the rolling mill Fi-3 to be equal to or lower than (FiT + 100)"C7 and
then holding is performed in a temperature range of "600°C to Ar3"Cm for 3 seconds to 40
seconds, and coiling is performed in the coiling step.
10 [0059]
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 strength of the steel
15 sheet accompanied with a cooling temperature deviation generated after 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 Ar3"C in the ROT for a long time are
important conditions.
[0060]
20 When the F,T is less than (Ac3 - 80)"C, 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
25 of "(Ac3 - 70)"C to (Ac3 + 20)"CY'. 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 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
5 of coil cooling after the coiling.
[006 11
For example, in a case of a hot-rolling line including seven final rolling mills,
transit time from a F4 rolling mill which corresponds to a third mill from an F7 rolling
mill which is a final stand, to the F7 rolling mill is set as 2.5 seconds or longer. When
10 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
15 temperature of the steel sheet between the stands largely decreases and it is impossible to
perform hot-rolling.
[0062]
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
20 temperature of equal to or higher than Ar3, and to recrystallize the austenite at the same
temperature range. Accordingly, a temperature on the rolling exit side of the F4 rolling
mill is set to be equal to or lower than (FiT + 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
25 lower limit of F,-3T is not particularly provided, however, since the temperature on the
*, exit side of the final F7 rolling mill is FiT, this is set as the lower limit thereof.
[0063]
By setting the holding time in the temperature range of 600°C to k 0 C to be a
long time, the ferrite transformation occurs. Since the AT3 is the ferrite transformation
5 start 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.
[0064]
(Coiling Step)
10 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
15 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.
[0065]
(Cold-Rolling Step)
20 In the cold-rolling step, the coiled hot-rolled steel sheet is cold-rolled after
pickling, and a cold-rolled steel sheet is manufactured.
[0066]
(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 "(Acl - 40)"C
and lower than Ac3"CV, 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
5 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.
[0067]
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 third embodiment
10 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"C", subsequently cooling from the highest temperature to
15 660°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 second embodiment.
[0068]
20 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.
[0069]
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 third embodiment, in
addition to the second embodiment in which, in the continuous annealing step, by heating
5 to a heating range of "equal to or higher than (Acl - 40)"C and lower than Ac3"Cn,
heating is performed until having a double phase coexistence with the austenite phase in
which the non-recrystallized ferrite slightly remains, it is possible to lower the heating
temperature for even proceeding of the recovery and recrystallization of the ferrite in the
coil, even with the heating temperature of AclOC to (Acl - 40) OC at which the reverse
10 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 "CIS, compared to
the second embodiment. This shows that the ferrite transformation proceeds faster in
15 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
20 same time as ferrite transformation, and cementite precipitation or pearlite transformation
rapidly occurs by holding in the same temperature range.
i I From these viewpoints, when the temperature is less than (Acl - 40)OC, 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
5 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.
[007 11
Herein, in the holding step of holding the steel sheet in a temperature range of
10 "450°C to 660°C" for 20 seconds to 10 minutes, the cementite precipitation or the
pearlite transformation can be promoted in the non-transformed austenite in which C is
incrassated after the ferrite transformation. Thus, according to the method for
manufacturing a 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
15 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
20 of the ferrite transformation is delayed and the annealing takes long time. On the other
hand, when the temperature is lower than 450°C, the ferrite itself which is generated by
the transformation is hardened, it is difficult for the cementite precipitation or the pearlite
transformation to proceed, or the bainite or the martensite which is the lower temperature
transformation product occurs. In addition, when the holding time exceeds 10 minutes,
e 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
5 parts of the microstructure after the cooling are hardened phase, and the steel sheet is
hardened.
[0072]
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.
10 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
15 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 and soft strength of the steel sheet after the
20 annealing.
[0073]
By using the steel sheet having the even strength, even in a case where the hot
stamping step includes a local heating manner which inevitably generates the temperature
irregularity in the steel sheet affer heating, it is possible to stabilize the strength of a
@ component after the hot stamping. For example, for the portion in which a temperature
does not rise by the local heating (such as an electrode holding portion) 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
5 management of precision of the product quality of the formed product after the hot
stamping.
[0074]
Hereinabove, the present invention has been described based on the first
embodiment, the second embodiment, and the third embodiment, however, the present
10 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 second embodiment, it is
possible to employ the conditions of the third embodiment.
Examples
15 [007 51
Next, Examples of the present invention will be described.
Steel type
A
B
C
D
E
F
G
H
I
J
K
L
M
N
0
P
[Table 11
C Mn Si
(mass%)
0.22
0.22
0.22
0.23
0.28
0.24
0.2 1
0.16
0.40
0.2 1
0.28
0.26
0.29
0.24
0.22
0.23
("C)
735
725
725
720
725
740
725
735
730
735
710
755
735
730
725
725
P
1.35
1.65
1.95
2.13
1.85
1.63
2.62
1.54
1.64
0.82
3.82
1.85
1.50
1.30
1 .80
1.60
("C)
850
840
830
825
825
860
820
850
810
865
770
880
1055
850
830
840
S
-
4.8
3.5
4.2
5.2
3.8
5.4
8.0
3.4
4.1
1.8
7.1
9.2
4.6
4.1
2.2
1.3
0.15
0.03
0.03
0.05
0.10
0.85
0.12
0.30
0.20
0.13
0.13
1.32
0.30
0.03
0.04
0.03
N
0.009
0.009
0.008
0.010
0.008
0.009
0.008
0.008
0.009
0.007
0.008
0.008
0.008
0.008
0.009
0.009
0.004
0.004
0.003
0.005
0.004
0.004
0.003
0.003
0.004
0.003
0.003
0.004
0.003
0.004
0.005
0.005
Al
0.003
0.004
0.003
0.004
0.003
0.003
0.003
0.003
0.004
0.003
0.004
0.003
0.004
0.003
0.003
0.003
Ti
0.010
0.010
0.010
0.020
0.015
0.032
0.022
0.020
0.010
0.021
0.020
0.020
1.300
0.020
0.010
0.012
B
0.020
0.010
0.012
0.0 15
0.080
0.020
0.0 15
0.012
0.020
0.020
0.010
0.012
0.020
0.310
0.020
0.003
Cr
0.0012
0.0013
0.0013
0.0015
0.0013
0.0014
0.0012
0.0010
0.00 12
0.001 1
0.0012
0.0015
0.0018
0.0012
0.0001
0.0010
0.22
0.02
0.15
0.10
0.0 1
0.01
0.10
0.03
0.01
0.01
0.13
0.0 1
0.01
0.20
0.10
0.01
ACI Ac3 DI,,,h

[0078] a
[Table 31
Condition
No.
1
2
3
4
5
6
7
8
9
10
1
2
3
4
5
6
1
2
3
4
5
6
Hot-rolling
F4T
["C]
955
945
945
940
945
955
950
945
880
875
960
950
945
945
890
900
970
960
965
960
880
895
Continuous
Highest
heating
temperature
["C]
830
825
830
700
870
820
825
810
810
710
820
815
860
810
805
705
820
815
810
700
695
790
to
F7T
["C]
905
900
900
900
905
910
905
905
820
810
890
895
895
900
830
845
905
910
915
910
800
820
annealing conditions
Cooling
rate
["CIS]
3.5
4.2
4.1
4.3
4.5
13.5
5.2
4.6
4.2
4.3
3.5
5
4.5
5
3.9
4.5
5.6
5.5
5.2
4.3
4.5
3.1
coiling conditions
(Acs-80)
["C]
770
770
770
770
770
770
770
770
770
770
760
760
760
760
760
760
750
750
750
750
750
750
Holding
temperature
["C]
585
580
585
570
580
560
530
575
560
470
580
560
560
500
570
460
570
570
510
560
475
560
(Ac3+40)
["C]
890
890
890
890
890
890
890
890
890
890
880
880
880
880
880
880
870
870
870
870
870
870
Time fiom 4
stage to 7
stage
[s]
2.7
2.9
2.2
2.8
2.9
2.5
2.6
2.2
4.6
4.5
2.2
2.8
2.6
2.9
4.8
5 rl
2.2
2.8
2.3
3.0
5.2
4.5
Holding
time
[s]
320
330
320
330
300
290
300
45
310
3 5
290
300
320
310
50
pp
45
300
290
280
300
28
32
HolQng time from
600°C to
[s]
2.1
1.3
0.3
2.5
3.1
3.2
2.9
4.6
8.2
7.9
4.0
1 .O
3.0
3.0
7.2
7.6
4.0
4.0
4.0
3.0
7.5
6.5
CT
["C]
680
500
800
680
675
685
680
685
580
610
650
500
670
670
600
590
650
680
680
680
610
590

[0079]
gl
Steel
type
D
E
F
[Table 41
Condition
No
1
2
3
4
5
6
7
8
9
10
1
2
3
4
5
6
7
8
9
1
2
3
Hot-rolling
F4T
["C]
950
960
965
960
965
975
960
950
950
955
950
960
965
955
955
945
950
900
910
960
950
950
Continuous
Highest heating
temperature
["C]
700
810
775
775
800
800
700
775
700
775
800
805
795
840
800
800
795
785
700
840
830
835
to
F7T
["C]
910
910
920
915
910
930
910
920
910
915
900
890
895
890
890
895
895
830
810
910
900
920
annealing conditions
Cooling
rate
["C/s]
2.1
4.3
1.6
2.9
2.2
4.3
2.1
1.6
2.1
1.6
2.3
2.5
2.8
2.5
13.5
4.2
3.5
4.2
3.9
4.6
4.3
3.5
(Ac3+40)
["C]
865
865
865
865
865
865
865
865
865
865
865
865
865
865
865
865
865
865
865
900
900
900
coiling conditions
(Ac3-80)
["C]
745
745
745
745
745
745
745
745
745
745
745
745
745
745
745
745
745
745
745
780
780
780
Holding
temperature
["C]
500
580
580
540
540
500
680
580
550
580
575
580
580
580
580
520
575
610
460
560
585
580
Time fi-om 4
stage to 7
stage
[s]
3.2
2.1
2.0
3.3
2.3
2.9
2.1
2.1
2.2
2.3
2.5
2.5
2.9
3.1
2.2
2.2
2.3
5.3
6.4
2.2
2.1
2.1
Holding
time
[s]
324
320
405
270
405
270
324
405
324
405
325
320
328
315
300
350
45
5 5
22
325
520
320
Holding time fiom
600°C to AT3
[s]
4.0
4.0
4.0
3.0
4.0
4.0
1 .O
2.0
0.0
0.0
3.0
1 .O
1 .O
3.0
3.0
1 .O
1 .O
7.2
8.1
2.2
2.3
3.0
CT
["C]
680
680
680
680
680
680
500
500
750
750
680
500
750
680
680
680
680
595
600
675
675
450

Steel
[0080]
jl,
[Table 51
Condition
No.
1
2
1
2
1
2
1
1
1
1
1
2
1
1
1
1
1
Hot-rolling
F4T
["C]
960
955
950
955
945
950
960
960
960
965
970
960
940
945
Continuous
Highest heating
temperature
["C]
830
760
800
790
840
750
850
860
810
750
820
810
785
795
to
F7T
["C]
915
920
905
900
905
910
920
910
910
905
930
910
905
910
annealing conditions
Cooling
rate
["CIS]
4.2
4.1
3.2
2.8
3.5
3.8
5.2
4.5
3.5
4.2
4.5
5
4.2
3.2
coiling conditions
(Ac3-80)
["C]
770
770
730
730
785
785
690
800
975
770
750
750
760
755
745
735
730
Holding
temperature
["C]
580
550
580
540
5 80
530
560
580
580
520
580
575
575
585
(Ac3+40)
["C]
890
890
850
850
905
905
810
920
1095
890
870
870
880
875
865
855
850
Holding
time
[s]
305
310
290
285
300
310
300
305
305
310
300
310
305
295
Time fiom 4
stage to 7
stage
[s]
2.4
2.5
2.6
2.7
2.8
2.6
2.9
2.3
2.5
2.9
2.5
2.9
2.1
2.2
2.4
Holding time from
600°C to Ar,
[s]
2.1
2.5
2.1
2.5
2.1
2.1
2.5
4.0
2.1
2.1
2.3
2.5
2.1
2.2
CT
["C]
685
680
675
670
680
685
680
680
670
680
680
680
610
605
6'11
1.8
P'6
8'21
8'8
2'8
8'L
S'8
2'8
2'8
8'8
P'8
6'8
S'L
6'L
8 '1
E.L
L'8
S 'L
£'PI
S'L
1'8
2'8
wwVWI
[I 8001
C
S'E
2' 1
2'2
E'E
V. I
P. I
2.1
9' 1
S'1
E' I
S'I
2. I
P' 1
S.1
E' I
S'1
£'I
2' 1
S.1
2.E
P' 1
S'I
E. I
YVJ3PJ3
0
SZ
01
0
S
S 1
S I
01
S I
S
0
S I
S I
01
02
S
5
0
0
0
OE
02
S 2
[%'10~1
uop3eg
a~greadp azge~uauBas-uo~
[9 a19e.LI
09
5
01
09
0 1
S I
0 I
S
01
01
0
0 I
S
S
S
01
S
S
0
SS
S
S
0 I
[%'10~1
uorpeg alyaj pazqelsrG3-uo~
OP
01
08
OP
SP
0 L
S9
08
OL
SP
S E
S9
0 L
58
SL
OP
SP
SE
02
S P
S9
SL
S9
[%'10~1
uoy3e.g a p a ~
a.m1~nqsoi3!~
OEL
029
SP9
589
SOL
SO9
019
SE9
585
069
OOL
06s
009
OP9
085
OZL
OIL
OZL
09L
OSL
085
06s
029
[w41
~AV-SL
56
Sf
OP
OS I
SO I
59
09
SP
S E
56
58
OE
09
S S
OE
S S
06
09
SS
0s I
S E
OP
09
['&Wl
SLV
pualen
L
9
5
P
E
2
I
9
S
P
E
Z
1
0 I
6
8
L
9
S
P
E
Z
I
'ON
uoypuo3
3 q* 3
P
m
3
w
m o m m m m w
- m m
00
0,s
P
6
[Table 71
Steel
type
D
Condition
No.
1
2
Material
ATS 1 TS-Ave
[ma1
166
62
Microstructure
Ferrite fiaction I Non-crystallized ferrite fraction I Non-segmentalized pearlite
[MPal
690
610
Cr$CrM
[vol.O/o]
40
70
M@~M
[vol.%]
55
10
fiaction
[vol.%]
5
20
3.5
1.2
13.2
7.6

E'6
2'6
6'8
8'9 1 S'I I 01 / 02
S'I
8.1
£'I
5'8
2'6
0 L
2'8
S 'L
E'EI
S 1
OE
S1
L' I
9'1
Y v W W
019
s.1
Z' 1
S'Z
OZ
0 I
[8 a19e.LI
YvJY13PJ3
0 1
01
02
OE
SI
oz
0
SO9
SOL
S6S
SL
09
S9
uog3e.g
a~!lnzad paz!~e~uadas-uo~
I
0 I
SZ
01
S I
02
S E
S9
OE
0
09L
OS9
0 L
S9
uorpeg apaj pazy~~a~slCI~-uo~
SP
S9
08
arn~3nqso~x~
I
1
1
OPI
0 L
uop3e.g atwad
S
2
b
058
OE8
089
~euaww
I
I
I
~AV-SL
MI
7
x
0s 1
OEI
SO1
uoprpuo~13
SLV
pals
Z
I
Z
I
'ON ad4
uofle~d~aa.lp~uda wa3 pw uo!lauuojsm~a lyaj luaprgnsu1 PC“=f)
uoyeuuojsmq alyaj lua~~gnsu~
uogeqdpald alpuawa3 pue uo!~euuojsues alyaj qua!DgnsuI
8up.qewal alyaj pazt~~elsL3a~-uo~
P O 0 9
P O 0 9
Po09
P O 0 9
P O 0 9
P O 0 9
Insufficient ferrite transformation and 8 Good cementite precipitation 9
Insufficient ferrite recrystallization
Insufficient cementite precipitation
9
10
Good
Good
Steel Condition Plating type I type I No.
Chemical conversion coating I
I 1 I - I Good I Non-recrystallized ferrite remaining I
I Good
13 1 hot-dip galvanizing I Good I 1
14 1 - 1 Good 1 Insufficient ferrite transformation and cementite precipitation 1
15 1 - I Good ( Insufficient ferrite transformation and cementite precipitation I
I 9 1 - 1 Good I Insufficient ferrite transformation and cementite precipitation I
6
7
8
1 10 I - I Good I
E 11 I - I Good I I
electroplating
12 I hot-dip galvanizing I Good I I
Good
Good
Good
-
Insufficient ferrite transformation and cementite precipitation
Insufficient ferrite transformation
3
4
5
I I I
18 I - I Good I I
Good
Good
Good
--
hot-dip galvanizing
6
I
I Good
- -
Insufficient ferrite transformation and cementite precipitation
Insufficient ferrite transformation
I Good I Insufficient ferrite transformation and cementite precipitation
7 I Good I Insufficient ferrite transformation and cementite precipitation
F
3
4
1
2
hot-dip galvanizing
hot-dip galvanizing
alloyed molten
aluminum plating
Good
Good
Good
Good

Steel
type
H
I
J
K
L
M
N
0
P
Q
R
S
T
[Table 11:
Condition
No.
1
2
1
2
1
2
1
1
1
1
1
2
I
1
Plating type
hot-dip galvanizing
1
1
1
Chemical conversion coating
Good
Good
Good
Good
Good
Good
Poor
Poor
Good
Good
Good
Good
Note
Strength after hot stamping is less than 1 180 MPa
Cracks on end portion are generated at the time of hot stamping forming
AHv is in the range even with the method of the related art for low hardenability.
Hot-rolling is difficult
Poor chemical conversion coating
Poor chemical conversion coating
Hot-rolling is difficult
AHv is in the range even with the method of the related art for low hardenability.
AHv is in the range even with the method of the related art for low hardenability.
- - -
Good
Good
Hot-rolling is difficult
[0087]
A steel having steel material components shown in Table 1 and Table 2 was
prepared, and 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 "steel type", "condition No.", "hot-rolling to coiling conditions", and
"continuous annealing condition". Acl and A c w~e re 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, and then, as shown in
Tables 6 to 8, "strength variation (ATS)" and "strength average value (TS-Ave)" are
acquired based on tensile strength measured from 10 portions of the continuous annealed
steel strip. 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.
Tables 9 to 11 show types of plating performed after continuous annealing. The
threshold values of "ATS" and "TS-Ave" are significantly affected by the amount of C of
the steel material, the present invention employs the following criteria for the threshold
values.
If the amount of C is 0.18% to 0.25%, ATS i 80 MPa, and TS - Ave 5 650 MPa.
If the amount of C is 0.25% to 0.3%, ATS 5 100 MPa, and TS-Ave 5 720 MPa.
If the amount of C is 0.3% to 0.35%, ATS 5 120 MPa, and TS - Ave I780 MPa.
[0088]
In the tensile test, steel sheet samples are extracted from portions within 20 m
from the initial location and final location of the steel strip, and the tensile strength is
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.
[0089]
As to the hardenability, if the chemical components are out of the range of the
present invention, the hardenability is low. Therefore, the variation of the strength or the
rising of the strength in the steel sheet manufacturing does not occur as described above,
and thus, are regarded as out of the invention since the low strength and the low variation
can be stably obtained even if the present invention is not employed. More specifically, a
steel sheet manufactured by employing a condition which is out of the range of the present
invention but satisfies the above-mentioned threshold values of ATS and TS-Ave is
regarded as out of the present invention.
Then, the manufactured steel sheet was cut, and the cut steel sheet and a die were
arranged as illustrated in FIG. 5 such that an end portion is not heated, and after locally
heating the center portion of the steel sheet, the hot stamping was performed so as to have
a shape as illustrated in FIG. 4. In the hot stamping, the rising temperature ratio of the
center portion was set to be 50 "CIS and the steel sheet was heated to the maximum heating
temperature of 870°C. The end portion was non-heated portion. 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.
[0090]
Further, since it is a precedent condition to use a material for hot stamping in the
1)
present invention, a case where the maximum strength becomes less than 11 80 MPa when
the hot stamping is performed from the temperature at which a single phase of austenite
appears, is regarded as out of the invention.
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).
[009 11
Test Examples A-1, A-2, A-3, A-9, A-10, B-1, B-2, B-5, B-6, C-1, C-2, (2-5, 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 ATs became high, and also, TS-Ave 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 precipitation in the subsequent cooling and the holding
did not proceed, the hard phase fraction after the annealing became high, and TS-Ave
became high.
[0092]
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 TS-Ave became 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 TS-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 TS-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 TS-Ave became high.
[0093]
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 DIinchv alues of 3.5,4.2 and
5.2, it was found that, when the Dlinchv alue was large, improvement of ATs and TS-Ave
was significant.
Since a steel type H had a small amount of C of 0.1696, the hardened strength
after hot stamping became 1160 MPa and is not suitable for a material for hot stamping.
Since a steel type I had a large amount of C of 0.40%, the strength after annealing
is high, and thus the formability of the non-heated portion at the time of hot stamping was
insufficient.
A steel type J had a small amount of Mn of 0.82%, and the hardenability was low.
[0094]
Since steel types K, N, and T respectively had a large amount of Mn of 3.82%, an
amount of Ti of 0.3 1 %, and an amount of Cr of 2.35%, it was difficult to perform the
Ir
hot-rolling.
Since steel types L and M respectively had a large amount of Si of 1.32% and an
amount of A1 of 1.300%, the chemical conversion coating after hot stamping 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.
[0095]
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
[0096]
According to the present invention, it is possible to provide a steel sheet for hot
stamping which has a soft and even strength property before heating in a hot stapmping
process and a method for manufacturing the same.

* CLAIMS
1. A steel sheet with chemical components which include, by mass%, 0.18% to
0.35% of C, 1.0% to 3.0% of Mn, 0.01% to 1.0% of Si, 0.001% to 0.02% of P, 0.0005% to
0.01% of S, 0.001% to 0.01% of N, 0.01% to 1.0% ofAl, 0.005% to 0.2% of Ti, 0.0002%
to 0.005% of By and 0.002% to 2.0% of Cr, and a balance of Fe and inevitable impurities,
wherein:
by volume%, a fraction of a ferrite is equal to or more than 50%, and a fraction of
a non-recrystallized ferrite is equal to or less than 30%; and
a value of a ratio CrelCr~is equal to or less than 2, where Cre is a concentration
of Cr subjected to solid solution in an iron carbide and CrM is a concentration of Cr
subjected to solid solution in a base material, or
a value of a ratio Mne/Mn~ is equal to or less than 10, where Mne is a
concentration of Mn subjected to solid solution in an iron carbide, and MnM is a
concentration of Mn subjected to solid solution in a base material.
2. The steel sheet according to Claim 1, wherein the chemical components
further include one or more from 0.002% to 2.0% of Mo, 0.002% to 2.0% of Nb, 0.002%
to 2.0% of V, 0.002% to 2.0% of Ni, 0.002% to 2.0% of Cu, 0.002% to 2.0% of Sn,
0.0005% to 0.0050% of Ca, 0.0005% to 0.0050% of Mg, and 0.0005% to 0.0050% of
REM.
3. The steel sheet according to Claim 1, wherein a DI,,,h value which is an
index of a hardenability is equal to or more than 3.
4. The steel sheet according to Claim 1, wherein a fraction of a
non-segmentalized pearlite is equal to or more than 10%.
5. A method for manufacturing a steel sheet for hot stamping, the method
comprising:
hot-rolling a slab containing chemical components according to Claim 1 or 2, 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;
and
continuously annealing the cold-rolled steel sheet which is subjected to
cold-rolling,
wherein the continuous annealing includes:
heating the cold-rolled steel sheet to a temperature range of equal to or higher
than AclOC and lower than Ac3"C;
cooling the heated cold-rolled steel sheet from a highest heating temperature to
660°C at a cooling rate of equal to or less than 10 "CIS; and
holding the cooled cold-rolled steel sheet in a temperature range of 550°C to
660°C for 1 second to 10 minutes.
6. 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.
7. A method for manufacturing a steel sheet for hot stamping, the method
comprising:
hot-rolling a slab containing chemical components according to Claim 1 or 2, 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;
and
continuously annealing the cold-rolled steel sheet which is subjected to
cold-rolling to obtain a steel sheet for hot stamping,
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 F; in a temperature range of (Ac3 - 80)"C to (Ac3 +
40)"C, by setting time from start of rolling in a rolling mill Fi-3 which is a previous
machine to the final rolling mill 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 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, 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 from a 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.
8. The method for manufacturing a hot stamped body according to Claim 7, 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.
Dated this 12.04.2013
ATTORNEY