TECHNICAL FIELD
[0001] The present invention relates to a hotstamped
part used for an automobile body or others,
and a method of manufacturing the hot stamped part.
BACKGROUND ART
[0002] In recent years, weight reduction of an
automotive body has been a crucial issue in the
viewpoint of protecting global environments, and
studies on the application of a high-strength steel
sheet to a vehicle body part have been actively
conducted. As the strength of a steel sheet used has
been increasing still more, consideration on
workability and shape fixability thereof have become
important. Further, since the forming load in press
forming increases as the strength of steel sheet
increases, raising the pressing capability has also
become a major issue.
[0003] Hot stamp forming (hereafter, also referred
to simply as "hot stamping") is a technique in which
a steel sheet is heated to a high temperature in an
austenite range and subjected to press forming while
it is at the high temperature. Since a softened
steel sheet is formed in the hot stamp forming, it is
possible to perform more complicated working.
Moreover, in the hot stamp forming, since rapid
cooling (quenching) is performed at the same timing
- 1 -
as the press forming to cause the structure of the
steel sheet to undergo martensite transformation, it
is possible to achieve strength and shape fixability
according to the carbon content of the steel sheet at
the same time, Further, since a softened steel sheet
is subj ected to forming in the hot stamp forming, it
is possible to significantly reduce the forming load
compared with ordinary press forming which is
performed at room temperature.
[0004] A hot-stamped part, which is manufactured
through hot stamp forming, especially a hot-stamped
part used for an automotive body requires excellent
low-temperature toughness - A hot-stamped part is
sometimes called a steel sheet member- Techniques
relating to enhancements of toughness and ductility
are described in Patent References 1 to 5. However,
the techniques described in Patent Reference 1 to 5
cannot provide sufficient low-temperature toughness.
Although Patent References 6 to 10 also disclose
techniques relating to hot press forming or the like,
they cannot provide sufficient low-temperature
toughness as well.
CITATION LIST
PATENT REFERENCE
[00053 Patent Reference 1: Japanese Laid-Open Patent
Publication No. 2006-152427
Patent Reference 2: Japanese Laid-open Patent
Publication No. 2012-180594
Patent Reference 3: Japanese Laid-Open Patent
- 2 -
Publication No. 2010-275612
Patent Reference 4: Japanese Laid-open Patent
Publication No. 2011-184758
Patent Reference 5: Japanese La id-Open Patent
Publication No. 2008-264836
Patent Reference 6: Japanese Laid-Open Patent
Publication No. 2011-161481
Patent Reference 7: Japanese Laid-Open Patent
Publication No. 07-18322
Patent Reference 8: International Publication
Pamphlet No. WO 2012/169640
Patent Reference 9: Japanese Laid-Open Patent
Publication No. 2013-14842
Patent Reference 10: Japanese Laid-Open Patent
Publication No. 2005-205477
SUMMARY OF THE INVENTION
TECHNICAL PROBLEM
[0006] It is an objective of the present invention
to provide a hot-stamped part which can achieve
excellent tensile strength and low-temperature
toughness, and a method of manufacturing the same.
SOLUTION TO PROBLEM
[0007] The prevent inventors have conducted
intensive studies on the cause of difficulty in
achieving sufficxent lov/-temperature toughness for a
conventional hot-stamped part. As a result, it has
been found that iron-based carbides precipitate
nearly all over the prior austenite grain boundary
and thereby intergranular fracture is more likely to
- 3 -
occur. The present inventors have also found that
the cooling rate during hot stamp forming is an
important factor to inhibit the precipitation of
iron-based carbides at prior austenite grain
boundary.
[0008] Accordingly, based on these findings, the
present inventors have come to conceive various
aspects of the invention described below.
[0009] (1) A hot-stamped part including:
a chemical composition represented by, in mass%:
C: 0.120% to 0.400%;
Si: 0.005% to 2.000%;
Mn or Cr, or both thereof: 1.00% to 3.00% in
total;
Al: 0:0*05% to 0. 100%;
B: 0.0003% to 0.0020%;
P: not more than 0.030%;
S: not more than 0.0100%;
0: not more than 0.0070%;
N: not more than 0.0070%;
Ti: 0% to 0.100%;
Nb: 0% to 0.100%;
V: 0% to 0. 100%;
Ni: 0% to 2.00%;
Cu: 0% to 2.00%;
Mo: 0% to 0.50%;
Ca or REM, or both thereof: 0% to 0.0300% in
total; and
the balance: Fe and impurities; and
_ 4 -
a structure represented by:
an area fraction of martensite or bainite,
or both thereof: not less than 95% in total;
a coverage factor of prior austenite grain
boundary by iron-based carbides: not more than 80%;
and
a number density of iron-based carbides in
prior austenite grains: not less than 45/^im2.
[00103 (2) The hot-stamped part according to (1),
wherein the chemical composition satisfies:
Ti: 0.005% to 0.100%;
Nb: 0.005% to 0.100%; or
V: 0.005% to 0.100%; or
any combination thereof.
[00113 (3) The' 'hot-stamped part according to (1) or
(2), wherein the chemical composition satisfies:
Ni: 0.05% to 2.00%;
Cu: 0.05% to 2.00%; or
Mo: 0.05% to 0.50%; or
any combination thereof.
[00123 (4) The hot-stamped part according to any one
of (1) to (3), wherein the chemical composition
satisfies
Ca or REM, or both thereof: 0.0005% to
0.0300% in total.
[00133 (5) A method of manufacturing a hot-stamped
part, including the steps of:
heating a steel sheet to a temperature of not
less than Ac3 point and not more than 950°C at an
- 5 -
average heating rate of not less than 2°C/sec;
then, cooling the steel sheet through a
temperature range from a Ar3 point to (Ms point -
50)°C at an average cooling rate of not less than
100°C/sec while performing hot pressing; and
then, cooling the steel sheet through a
temperature range from (Ms point - 50)°C to 100°C at
an average cooling rate of not more than 50°C/sec,
wherein
the steel sheet includes a chemical composition
represented by, in mass%:
C: 0.120% to 0.400%;
Si: 0.005% to 2.000%;
Mn or Cr, or both thereof: 1.00% to 3.00% in
total;
Al: 0.005% to 0.100%;
B: 0.0003% to 0.0020%;
P: not more than 0.030%;
S: not more than 0.0100%;
O: not more than 0.0070%;
N: not more than 0.0070%;
Ti: 0% to 0.100%;
Nb: 0% to 0.100%;
V: 0% to 0.100%;
Ni: 0% to 2.00%;
Cu: 0% to 2.00%;
Mo: 0% to 0.50%;
Ca or REM, or both thereof: 0%-0.0300% in
total; and
- 6 -
the balance: Fe and impurities, and
a maximum cooling rate is not more than 7 0°C/sec
and a minimum cooling rate is not less than 5°C/sec in
a temperature range from (Ms point - 120)°C to 100°C.
[0014] (6) The method of manufacturing the hotstamped
part according to (5), wherein the chemical
composition satisfies :
Ti: 0.005%-0.100%;
Nb: 0.005%-0.100%; or
V: 0.005%-0.100%; or
any combination thereof.
[0015] (7) The method of manufacturing the hotstamped
part according to (5) or (6), wherein the
chemical composition satisfies:
Nr: -0.05%-2.00%;
Cu: 0.05%-2.00%; or
Mo: 0.05%-0.50%; or
any combination thereof.
[0016] (8) The method of manufacturing the hotstamped
part according to any one of (5) to (7) ,
wherein the chemical composition satisfies
Ca or REM or both thereof: 0 . 0005%-0.0300%
in total.
ADVANTAGEOUS EFFECTS OF INVENTION
[0017] According to the present invention, it is
possible to achieve excellent tensile strength and
low-temperature toughness.
BRIEF DESCRIPTION OF DRAWINGS
[0018] [Figure 1] Figure 1 is a schematic diagram
- 7 -
illustrating a prior austenite grain/ and iron-based
carbides that have precipitated at the grain
boundary.
DESCRIPTION OF EMBODIMENTS
[0019] Hereafter, embodiments of the present
invention will be described. A hot-stamped part
according to an embodiment of the present invention
is manufactured, as described below in more detail,
through hot stamp forming including quenching of a
steel sheet for hot stamping. Thus, the
hardenability and quenching conditions of the steel
sheet for hot stamping affect the hot-stamped part.
£0020] In the beginning, a structure of a hotstamped
part according to the present embodiment will
be described. The hot-stamped part according to the
present embodiment includes a structure represented
by: an area fraction of martensite or bainite, or
both thereof: not less than 95% in total; a coverage
factor of prior austenite grain boundary by ironbased
carbides: not more than 80%; and a number
density of iron-based carbides in prior austenite
grains: not less than 45/(im2.
[0021] (An area fraction of martensite or bainite, or
both thereof: not less than 95% in total)
Martensite and bainite, particularly martensite,
are important for achieving strength of a hot-stamped
part- If the total of the area fraction of
martensite and the area fraction of bainite is less
than 95%, it is not possible to achieve sufficient
- 8 -
strength, for example, a tensile strength of not less
than 1180 MPa. Therefore, the area fraction of
martensite and the area fraction of bainite are not
less than 95% in total. Martensite may be, for
example, either fresh martensite or tempered
martensite. The tempered martensite obtained in the
present embodiment is, for example, auto-tempered
martensite. Fresh martensite is as-quenched
martensite. Tempered martensite includes iron-based
carbides which have precipitated after or during the
cooling of tempering. Auto-tempered martensite is
tempered martensite v/hich is generated during cooling
in quenching without being subjected to heat
treatment for tempering. To achieve desired strength
more surely; i;he area fraction of martensite is
preferably more than the area fraction of bainite,
and the area fraction of martensite is preferably not
less than 70%.
[0022] The balance other than martensite and bainite
is one or more of ferrite, pearlite, or retained
austenite, for example. The amounts thereof are
preferably as low as possible.
[0023] Identification of martensite, bainite,
ferrite, pearlite, and retained austenite,
confirmation of positions thereof, and measurement of
area fractions thereof may be performed by observing
a cross-section in parallel with the rolling
direction and the thickness direction, or a crosssection
orthogonal to the rolling direction of a hot-
- 9 -
stamped part. Observation of a cross section may be
performed by, for example, etching the cross-section
with a Nital reagent, and observing it at a
magnification of 1000 times to 100000 times with a
scanning electron microscope (SEM) or a transmission
electron microscope (TEM). Other etching solutions
may be used in place of the Nital reagent. An
example of usable etching solution is described in
Japanese Laid-open Patent Publication No. 59-219473.
The etching solution described in Japanese Laid-open
Patent Publication No. 59-219473 is "a color etching
solution characterized by consisting of a
pretreatment solution and a post-treatment solution,
in which the pretreatment solution is prepared by
mixing a solution A in which 1 to 5 g of picric acid
is dissolved into 100 mL of ethanol, with a solution
B in which 1 to 25 g of sodium thiosulfate and 1 to 5
g of citric acid are dissolved into 100 mL of water,
in a proportion of 1 : 1, and thereafter adding 1.5
to 4% of nitric acid to the solution, and the posttreatment
solution is prepared by mixing 10% of the
pretreating solution with a 2% Nital solution, or
mixing 2 to 5% of nitric acid with 10 0ml of ethanol."
Crystal orientation analysis using a field emission
scanning electron microscope (FE-SEM) may also be
performed to identify structures, confirm positions
thereof, and measure area fractions thereof.
Structures may also be determined from hardness
measurement of a minute region, such as measurement
- 10 -
of micro Vic leers hardness.
[0024] The area fractions of bainite and martensite
may also be measured in the following way. For
example, a sample is obtained which has a crosssection
in parallel with the rolling direction and
the thickness direction of a steel sheet as an
observation surface, the observation surface is
electropolished, and a portion of the steel sheet at
a depth of 1/8 to 3/8 thickness thereof from the
surface is observed with an FE-SEM. In such an
occasion, each measurement is performed at a
magnification of 50 0 0 times in 10 visual fields, the
area fraction is assumed to be an average value
thereof. Observed martensite may include tempered
martensite as well. Since martensite may not be
sufficiently etched by Nital etching, the area
fractions of ferrite and bainite may be measured by
the above described method using an FE-SEM, and the
area fraction of martensite may be assumed to be the
area fraction of the un-etched portion which is
observed by the FE-SEM. The area fraction of
retained austenite may also be determined from
intensity measurement by X-ray diffraction. For
example, it may be determined from an X-ray
diffraction intensity ratio between ferrite and
austenite. Ferrite, which is made up of lump-like
grains, means a structure which does not include any
sub-structure such as a lath thereinside.
[0025] (Coverage factor of prior austenite grain
- 11 -
boundary by iron-based carbides: not more than 80%)
The coverage factor of prior austenite grain
boundary by iron-based carbides means a ratio of
portions at which iron-based carbides have
precipitated within the prior austenite grain
boundary. The portions of the prior austenite grain
boundary where iron-based carbides have precipitated
look like being covered with the iron-based carbides
when observed with microscope. If the ratio of
portions at which iron-based carbides have
precipitated within the prior austenite grain
boundary is more than 80%, intergranular fracture is
more likely to occur, and therefore sufficient lowtemperature
toughness cannot be achieved- Therefore,
the coverage "factor is not more than 80%. To achieve
further excellent low-temperature toughness, the
coverage factor is preferably not more than 70%, and
more preferably not more than 60%.
[0026] {Number density of iron-based carbides in
prior austenite grains: not less than 4 5/fim2)
Iron-based carbides in prior austenite grains
contribute to enhancement of low-temperature
toughness. If the number density of iron-based
carbides in prior austenite grains is less than
45/urn2, it is " not possible to achieve sufficient lowtemperature
toughness. Therefore, the number density
is not less than 4 5/um2, In order to achieve more
excellent low-temperature toughness, the number
density is preferably not less than 50/(xm2. If the
- 12 -
number density is more than 2 0 0/(xm2, the effect of
enhancing low-temperature toughness is saturated.
Therefore, the number density is preferably not more
than 200/|xm2.
[0027] An Iron-based carbide is a compound
consisting of iron and carbon, examples of which
include cementite (6 phase), 8 phase, and % phase. As
describe later. Si or the like may be dissolved into
and contained in iron carbide. Carbides containing
no iron, such as Ti carbides and Nb carbides, do not
correspond to the iron-based carbide.
[0028] Here, a method of determining a coverage
factor of prior austenite grain boundary by ironbased
carbides will be described with reference to
Figure 1. Figure 1 is a schematic diagram
illustrating a prior austenite grain, and iron-based
carbides that have precipitated at the grain
boundary.
[0029] In the example illustrated in Figure 1, a
prior austenite grain 21 which has a hexagonal shape
in an observation surface is included in a hotstamped
part. Iron-based carbides 1 and 2
precipitate at a first side 31, iron-based carbides 3
and 4 precipitate at a second side 32, iron-based
carbides 5, 6 and 7 precipitate at a third side 33,
an iron-based carbide 8 precipitates at a fourth side
34, iron-based carbides 9 and 10 precipitate at a
fifth side 35, and iron-based carbides 11 and 12
precipitate at a sixth side 36. The length of the
- 13 -
side 31 is Li, the length of the side 32 is L2, the
length of the side 33 is L3, the length of the side 34
is L4, the length of the side 35 is L5, and the length
of the side 36 is L6. The lengths of the iron-based
carbides 1 and 2 on the grain boundary are Xx and X2,
respectively; the lengths of the iron-based carbides
3 and 4 on the grain boundary are X3 and X4/
respectively; the lengths of the iron-based carbides
5, 6 and 7 on the grain boundary are X5/ Xe and X7,
respectively; the length of the iron-based carbide 8
on the grain boundary is X8; the lengths of the ironbased
carbides 9 and 10 on the grain boundary are X9
and X10/ respectively; the lengths of the iron-based
carbides 11 and 12 on the grain boundary are Xn and
X12/ respectively. Note that "the length of an ironbased
carbide on a grain boundary" means a distance
between two points of intersection between an ironbased
carbide and a grain boundary in an observation
surface.
[0030] Then, the sum L {\xm) of the lengths of the
six sides 31 to 36 is found, and the sum X (|im) of
the lengths of the iron-based carbides 1 to 12 on the
grain boundary is found to determine a value
represented by W(X/L) x 100" {%) as a coverage
factor. Note that when determining a coverage factor
in one hot-stamped part, coverage factors are
determined for each of 10 or more prior austenite
grains included in the hot-stamped part, and an
average value thereof is assumed to be the coverage
- 14 -
factor in the hot-stamped part. A prior austenite
grain boundary is assumed to be a part which is
caused to appear by an etching solution containing
sodium dodecylbenzenesulfonate, and a prior austenite
grain and iron-based carbides have precipitated at
the grain boundary thereof are observed with an FESEM.
[0031] Although the prior austenite grain 21 which
has a hexagonal shape in an observation surface is
illustrated as an example in Figure 1, in general,
actual prior austenite grains have more complex
shapes. Therefore, in practice, sides of a prior
austenite grain are identified according to the shape
of the observed prior austenite grain, and the sum of
the lengths of each side is determined. When a
curved portion is present in a grain boundary, the
portion may be approximated to a plurality of sides.
[0032] Subsequently, the chemical composition of a
hot-stamped part according to an embodiment of the
present invention and a steel sheet used for
manufacturing the hot-stamped part will be described.
In the following description, the symbol "%", which
is the unit of each element contained in a hotstamped
part and a steel sheet used for manufacturing
the hot-stamped part, means, unless otherwise stated,
"mass%". A hot-stamped part and a steel sheet used
for manufacturing the hot-stamped part have a
chemical composition represented by: C: 0.120% to
0.400%; Si: 0.005% to 2.000%; Mn or Cr, or both
- 15 -
'"f
thereof: 1.00% to 3.00% in total; Al: 0.005% to
0.100%; B: 0.0003% to 0.0020%; P: not more than
0.030%; S: not more than 0.0100%; O: not more than
0.0070%; N: not more than 0.0070%; Ti: 0% to 0.100%;
Nb: 0% to 0.100%; V: 0% to 0.100%; Ni: 0% to 2.00%;
Cu: 0% to 2.00%; Mo: 0% to 0.50%; Ca or REM (rare
earth metal), or both thereof: 0% to 0.0300% in
total; and the balance: Fe and impurities. As the
impurities, those contained in raw materials such as
ores and scraps, and those introduced in the
production process are exemplified.
[0033] (C: 0.120% to 0.400%)
C (Carbon) is an element to enhance the strength
of a hot-stamped part. When the C content is less
than 0.120%, the effect by the above described
function cannot be achieved sufficiently. For
example, it is not possible to obtain a tensile
strength of not less than 1180 MPa. Therefore, the C
content is not less than 0.120%. To obtain more
excellent strength, the C content is preferably not
less than 0,140%, and more preferably not less than
0.150%. When the C content is more than 0.400%, the
strength is excessive, and sufficient low-temperature
toughness cannot be achieved. Further, it is also
difficult to achieve sufficient weldability and
workability. Therefore, the C content is not more
than 0.400%. To obtain more excellent lowtemperature
toughness, the C content is preferably
not more than 0.370%, and more preferably not more
- 16 -
than 0.350%.
[0034] (Si: 0.005% to 2.000%)
Si (Silicon) is an element which dissolves into
an iron-based oxide thereby enhancing hydrogen
embrittlement resistance. Although detailed
correlation between Si and the hydrogen embrittlement
resistance is not clear, it is inferred that elastic
strain at the interface between the iron-based
carbide and the matrix phase increases as a result of
Si dissolving into an iron-based carbide, and thereby
hydrogen trapping capability of the iron-based
carbide is enhanced. When the Si content is less
than 0.005%, the effect by the above described
function cannot be achieved sufficiently. Therefore,
the Si content is not less than 0.005%. To obtain
more excellent hydrogen embrittlement resistance, the
Si content is preferably not less than 0.01%, and
more preferably not less than 0.15%. When the Si
content is more than 2.000%, the effect of enhancing
the hydrogen embrittlement resistance is saturated,
and Ac3 point is excessively high, thus unreasonably
increasing heating temperature in hot stamp forming.
Therefore, the Si content is not more than 2.000%.
Considering the balance between the hydrogen
embrittlement resistance and the Ac3 point, the Si
content is preferably not more than 1.600%.
[0035] Si also affects platability and delayed
fracture characteristic. For example, when the Si
content is more than 0.005%, the platability
- 17 -
deteriorates, thus resulting sometimes in unplating.
For this reason, when a plated steel sheet is used as
a steel sheet for hot stamping, the Si content is
preferably not more than 0.500%. On the other hand,
Si increases delayed fracture characteristic.
Therefore, when a plated steel sheet is used as a
steel sheet for hot stamping, the Si content is
preferably not less than 0.500% to achieve excellent
delayed fracture resistance.
[00363 (Mn or Cr, or both thereof: 1.00% to 3.00% in
total)
Mn (Manganese) and Cr (Chromium) are important
elements for delaying ferrite transformation during
cooling in hot stamp forming, and thereby obtaining a
desired structure of a hot-stamped part to be
described below. When the total of the Mn content
and the Cr content is less than 1.00%, it is likely
that ferrite and pearlite are formed during cooling
in hot stamp forming, and a desired structure cannot
be obtained. Thus, since the desired structure has
not been obtained, it is not possible to achieve
sufficient strength, for example, a tensile strength
of not less than 1180 MPa. Therefore, the total of
the Mn content and the Cr content is not less than
1.00%. To achieve more excellent strength, the total
of the Mn content and the Cr content is preferably
not less than 1.30%, and more preferably not less
than 1.40%. When the total of the Mn content and the
Cr content is more than 3.00%, the effect of delaying
- 18 -
ferrite transformation and thereby increasing
strength is saturated. Moreover, the strength of
hot-rolled steel sheet excessively increases, and
thereby, rupture sometimes occurs during cold
rolling, and/or wear and failure of the blade to be
used for cutting is sometimes pronounced. Therefore,
the total of the Mn content and the Cr content is not
more than 3.00%. Considering an appropriate range of
strength, the total of the Mn a,nd Cr contents is
preferably not more than 2.9%, and more preferably
not more than 2.8%. When Mn is excessively
contained, embrittlement occurs caused by segregation
of Mn, and thereby, a problem such as breakage of
cast slab is more likely to occur, and also
weldability is likely to deteriorate. Although the
content of each of Mn and Cr is not particularly
limited, the Mn content is not less than 0.8%, and
the Cr content is not less than 0.2%, for example.
[0037] (Al: 0.005% to 0.100%)
Al (Aluminum) is an effective element for
deoxidation. When the Al content is less than
0.005%, deoxidation is insufficient, and a large
amount of oxides may remain in a hot-stamped part,
particularly deteriorating local deformability.
Moreover, the variations of features increase.
Therefore, the Al content is not less than 0.005%.
For sufficient deoxidation, the Al content is
preferably not less than 0.006%, and more preferably
not less than 0.007%. When the Al content is more
- 19 -
than 0.100%, a large amount of oxides primarily
consisting of alumina remains in a hot-stamped part,
thereby deteriorating local deformability.
Therefore, the Al content is not more than 0.100%.
To suppress the remaining of alumina, the Al content
is preferably not more than 0.08%, and more
preferably not more than 0.075%.
[0038] (B: 0.0003% to 0.0020%)
B (Boron) is an element to increase hardenability
of a steel sheet for hot stamping. As a result of
increase of hardenability, it is easier to obtain
martensite in the structure of a hot-stamped part.
When the B content is less than 0.0003%, the effect
by the above described function is not achieved
sufficiently. To achieve more excellent
hardenability, the B content is preferably not less
than 0.0004%, and more preferably not less than
0.0005%. When the B content is more than 0.0020%,
the effect of enhancing hardenability is saturated,
and iron-based borides excessively precipitate,
deteriorating hardenability. Therefore, the B
content is not more than 0.0020%. To suppress the
precipitation of iron-based borides, the B content is
preferably not more than 0.0018%, and more preferably
not more than 0.0017%.
[0039] (P: not more than 0.030%)
P {Phosphorus) is not an essential element, and
contained in steel as an impurity, for example. Pis
an element that segregates in a middle portion in the
- 20 -
thickness direction of the steel sheet, thereby
embrittling a welded zone. For this reason, the P
content is preferably as low as possible.
Particularly, when the P content is more than 0.030%,
embrittlement of welded zone is pronounced.
Therefore, the P content is not more than 0.030%.
The P content is preferably not more than 0.020%, and
more preferably not more than 0.015%. Reducing the P
content is costly, and reducing it to less than
0.001% raises the cost remarkably. For this reason,
the P content may be not less than 0.001%.
[0040] (S: not more than 0.0100%)
S (Sulfur) is not an essential element and
contained in steel as an impurity, for example. S is
an element that hinders casting and hot rolling in
manufacturing a steel sheet, thereby deteriorating
weldability of a hot-stamped part. For this reason,
the S content is preferably as low as possible.
Particularly when the S content is more than 0.0100%,
the adverse effects are pronounced. Therefore, the S
content is not more than 0.0100%. The S content is
preferably not more than 0.008%, and more preferably
not more than 0.005%. Reducing the S content is
costly, and reducing it to less than 0.0001% raises
the cost remarkably. For this reason, the S content
may be not less than 0.0001%.
[0041] (O: not more than 0.0070%)
O (Oxygen) is not an essential element and
contained in steel as an impurity, for example. O is
- 21 -
an element that forms oxides, and thereby causes
deterioration of properties of a steel sheet for hot
stamping. For example, oxides that are in the
vicinity of the surface of the steel sheet may cause
a surface flaw, thereby deteriorating the appearance
quality. If an oxide is in a cut surface, it forms a
notch-shaped flaw on the cut surface, causing
deterioration of properties of a hot-stamped part.
For this reason, the 0 content is preferably as low
as possible. Particularly, when the 0 content is
more than 0.0070%, deterioration of properties is
pronounced. Therefore, the 0 content is not more
than 0.0070%. The O content is preferably not more
than 0.0050%, and more preferably not more than
0.0040%. Reducing the O content is costly, and
reducing it to less than 0.0001% raises the cost
remarkably. For this reason, the O content may be
not less than 0.0001%.
[0042] (N: not more than 0.0070%)
N (Nitrogen) is not an essential element, and
contained in steel as an impurity, for example. N is
an element that forms coarse nitrides, thereby
deteriorating bendability and hole expandability. N
also causes occurrence of blow holes during welding.
For this reason, the N content is preferably as lov;
as possible. Particularly, when the N content is
more than 0.0070%, deterioration of bendability and
hole expandability is pronounced. Therefore, the N
content is not more than 0.0070%. Reducing the N
- 22 -
content is costly, and reducing it to less than
0.0005% raises the cost remarkably. For this reason,
the N content may be not less than 0.0005%.
Moreover, from the viewpoint of manufacturing cost,
the N content may be not less than 0.0010%.
[0043] Ti, Nb, V, Ni, Cu, Mo, Ca, and REM are not
essential elements, and optional elements that may be
appropriately contained with a predetermined amount
as a limit in a steel sheet for hot stamping, and in
a hot-stamped part.
[0044] (Ti: 0% to 0.100%, Nb: 0% to 0,100%, V: 0% to
0.100%)
Ti, Nb, and V are elements that inhibit the
crystal grain growth of the austenite phase during
hot stamp forming and thus contribute to enhancements
of strength and toughness through grain refinement
strengthening of the transformed structure. Ti also
has a function of combining with N to form TiN,
thereby inhibiting B from forming a nitride.
Therefore, one or any combination selected from the
group consisting of these elements may be contained.
However, when any of the Ti content, the Nb content,
and the V content is more than 0.100%, Ti carbides,
Nb carbides, or V carbides are excessively formed,
resulting in deficiency in the amount of C, which
contributes to strengthening of martensite, so that
sufficient strength cannot be achieved. Therefore,
all of the Ti content, the Nb content, and the V
content are not more than 0.100%. Any of the Ti
- 23 -
content, the Nb content, and the V content is
preferably not more than 0.080%, and more preferably
not more than 0.050%. To surely achieve the effect
by the above described function, all of the Ti
content, the Nb content, and the V content are
preferably not less than 0.005%. That is, it is
preferable that "Ti: 0.005% to 0.100%w, "Nb: 0.005%
to 0.100%", or "V: 0.005% to 0.100%", or any
combination thereof be satisfied.
[0045] (Ni: 0% to 2.00%, Cu: 0% to 2.00%, Mo: 0% to
0.50%)
Ni, Cu, and Mo are elements that increase the
hardenability of a steel sheet for hot stamping. As
a result of increase in hardenability, it is more
likely that martensite is formed in the structure of
a hot-stamped part. Therefore, one or any
combination selected from the group consisting of
these elements may be contained. However, when
either of the Ni content or the Cu content is more
than 2.00%, or the Mo content is more than 0.50%,
weldability and hot workability deteriorates.
Therefore, both of the Ni content and the Cu content
are not more than 2.00%, and the Mo content is not
more than 0.50%. To surely achieve the effect of the
above described function, any of the Ni content, the
Cu content, and the Mo content is preferably not less
than 0.01%. That is, it is preferable that "Ni:
0.05% to 2.00%", "Cu: 0.05% to 2.00%", or "Mo: 0.05%
to 0.50%", or any combination thereof be satisfied.
- 24 -
[0046] (Ca or REM, or both thereof: 0% to 0.0300% in
total)
Ca and REM are elements that contribute to
enhancement of strength, and improvement in toughness
through structure. Therefore, Ca or REM or both
thereof may be contained. However, when the total of
the Ca content and the REM content are more than
0.0300%, castability and hot workability deteriorate.
Therefore, the total of the Ca content and the REM
content are not more than 0.0300%. To surely achieve
the effect of the above described function, the total
of the Ca content and the REM content are preferably
not less than 0.0005%. That is, it is preferable
that "Ca or REM, or both thereof: 0.0005% to 0.0300%
in total" is satisfied. REM refers to elements that
belong to Sc, Y, and elements belonged in lanthanoide
series, and the "REM content" means the total content
of these elements. Industrially, REM is often added
as misch metal, and it contains multiple kinds of
elements such as La and Ce. A metal element
belonging to REM, such as metal La and metal Ce, may
be added alone.
[0047] According to a hot-stamped part according to
the present embodiment, it is possible to achieve
excellent tensile strength and low-temperature
toughness since it has an appropriate chemical
composition and structure.
[0048] Subsequently, a method of manufacturing the
hot-stamped part according to the embodiment of the
- 25 -
present invention will be described. According to
the method described herein, it is possible to
manufacture the hot-stamped part according to the
embodiment of the present invention.
[0049] In the manufacturing method, a steel sheet
for hot stamping, which has the above described
chemical composition, is heated to a temperature of
not less than Ac3 point and not more than 950°C at an
average heating rate of not less than 2°C/sec; is then
cooled through a temperature range from a Ar3 point
to (Ms point - 50)°C at an average cooling rate of not
less than 100°C/sec while performing hot pressing; and
is further cooled through a temperature range from
(Ms point - 50) °C to 100°C at an average cooling rate
of not more than 50°C/sec. The maximum cooling rate
is not more than 70°C/sec and the minimum cooling rate
is not less than 5°C/sec in the temperature range from
(Ms point - 120)°C to 100°C.
[0050] (Heating temperature: not less than Ac 3 and
not more than 950°C)
The temperature to which the steel sheet for hot
stamping is heated is not less than Ac3 and not more
than 950°C. The steel sheet is caused to have a
structure of an austenite single phase by heating the
steel sheet to a temperature of not less than Ac3
point. It is possible to obtain a structure in which
the area fraction of martensite and the area fraction
of bainite are not less than 95%, thus obtaining a
high strength, for example, a tensile strength of not
- 26 -
less than 1180 MPa by subjecting the steel sheet
having an austenite single phase structure to
quenching. Since the structure of the steel sheet
includes ferrite when the heating temperature is less
than Ac3 point, even if such quenching of the steel
sheet is performed, ferrite grows and it is not
possible to obtain a tensile strength of not less
than 1180 MPa. Therefore, the heating temperature is
not less than Ac3 point. When the heating
temperature is more than 950°C, austenite grains
become coarse, and low-temperature toughness after
quenching deteriorate. Therefore, the heating
temperature is not more than 950°C.
[0051] The Ac3 point may be determined from the
following formula.
Ac3 point (°C) = 910 - 203>/c -. 30Mn - H C r +
44.7S1 + 400A1 + 700P - 15.2Ni - 20Cu + 400Ti + 104V
+ 31.5MO
(C, Mn, Cr, Si, Al, P, Ni, Cu, Ti, V, and Mo each
represent a content (mass%) of each component in
steel sheet.)
If Ni, Cu, Ti, V and/or Mo, which are optional
elements, is not contained in the steel sheet, the
content of any element which is not contained is
supposed to be 0 (mass%).
[0052] (Average heating rate: not less than 2°C/sec)
When the heating rate is less than 2°C/sec,
austenite grains become coarse during heating, and
sufficient low-temperature toughness and delayed
- 27 -
fracture resistance cannot be achieved. Therefore,
the average heating rate during heating to a
temperature of not less than Ac3 point and not more
than 950°C is not less than 2°C/sec. To further
inhibiting the coarsening of austenite grains, the
average heating rate is preferably not less than
3°C/sec, and more preferably not less than 4°C/sec.
Moreover, increasing the heating rate is also
effective for increasing the productivity. The
effects of the embodiment of the present invention
can be achieved even without particularly setting an
upper limit of the average heating rate. Therefore,
the average heating rate may be appropriately set
considering the capacity of the manufacturing
facility such as heating apparatuses, without
particularly setting an upper limit of the average
heating rate. Here, an average heating rate is a
value obtained by dividing a difference between a
temperature at v/hich heating is started and a heating
temperature by a time period taken for the heating.
[0053] After being heated to a temperature of not
less than Ac3 point and not more than 950°C at an
average heating rate of not less than 2°C/sec, the
steel sheet is cooled while being subjected to hot
pressing. That is, hot stamp forming is performed.
Transformation and precipitation of iron-based
carbides occur according to temperature during the
cooling. Here, the relationship between temperature,
and transformation and precipitation of iron-based
- 28 -
carbides will be described.
[0054] In the beginning, in the temperature range
from the heating temperature to the Ar3 point,
transformation such as ferrite transformation, and
precipitation of iron-based carbides do not occur.
Therefore, the cooling rate in this temperature range
does not affect the structure of a hot-stamped part.
Once the temperature of the steel sheet reaches the
Ar3 point, ferrite transformation and/or pearlite
transformation may start depending on the cooling
rate, and further once the temperature enters a
temperature range lower than the Al point, iron-based
carbides start precipitating. Therefore, the cooling
rate in the temperature range of not more than the
Ar3 point significantly affects the structure of a
hot-stamped part. Iron-based carbides precipitate
both at the grain boundary and in the prior austenite
grain, and they are more likely to precipitate at
grain boundary at a temperature of not less than (Ms
point - 50)°C, and in grain at a temperature of not
more than (Ms point - 50)°C. Therefore, it is
important to change the average cooling rate with
reference to a temperature of (Ms point - 50)°C. The
precipitation of iron-based oxides is very unlikely
to occur at a temperature of less than 100°C, and the
transformation does not occur at less than 100°C.
Therefore, the cooling rate in this temperature range
as well does not affect the structure of a hotstamped
part. Then, in the present embodiment, the
- 29 -
cooling rate in a temperature range from the Ar3
point to (Ms point - 50)°C, and the cooling rate in a
temperature range from {Ms point - 50)°C to 10 0°C are
specified.
[0055] The Ar3 point (Ar3 transformation point) and
Ms point may be found from the following formulas.
Ar3 point (°C) = 901 - 325C + 33Si - 92(Mn + Ni/2
+ Cr/2 + Cu/2 + Mo/2)
Ms point (°C) = 561 - 474C - 33Mn - 17Ni - 17Cr -
2lMo
(C, Si, Mn, Ni, Cr, Cu, and Mo each represent the
content (mass%) of each component in steel sheet.)
If Ni, Cu, Ti, V and/or Mo, which are optional
elements, is not contained in the steel sheet, the
content of any element which is not contained is
supposed to be 0 (mass%).
[0056] Since there is a correlation as described
above between temperature, and transformation and
precipitation of iron-based carbides, it is conceived
that the cooling rate is controlled for each of the
following four temperature ranges. The four
temperature ranges include a first temperature range
from the heating temperature to the Ar3 point, a
second temperature range from the Ar3 point to (Ms
point - 50)°C, a third temperature range from (Ms
point - 50)°C to 100°C, and a fourth temperature range
of less than 100°C.
[0057] (First temperature range)
In the first temperature range (from the heating
- 30 -
temperature to the Ar3 point), since neither
transformation such as ferrite transformation, as
described above, nor precipitation of iron-based
carbides occur, there is no need of particularly
controlling the cooling rate. However, considering
that the average cooling rate in the second
temperature range is not less than 100°C/sec as
described later, it is preferable that the average
cooling rate in the first temperature range is not
less than 100°C/sec as well.
[0058] (Second temperature range)
In the second temperature range (from the Ar3
point to (Ms point - 50)°C), ferrite transformation
and pearlite transformation occur depending on the
cooling rate, and further iron-based carbides
precipitate in the temperature range lower than the
Al point, as described above. If the average cooling
rate in the second temperature range is not less than
100°C/sec, it is possible to avoid ferrite
transformation and pearlite transformation, thereby
making the total of the martensite area fraction and
the bainite area fraction be not less than 95%. On
the other hand, if the average cooling rate in the
second temperature range is less than 100°C/sec,
ferrite transformation and/or pearlite transformation
occurs so that it is not possible to make the total
of the martensite area fraction and the bainite area
fraction be not less than 95%. Therefore, the
average cooling rate in the second temperature range
- 31 -
-i
"1
is not less than 100°C/sec. Moreover, in the second
temperature range, iron-based carbides are likely to
precipitate at a grain boundary and the coverage
factor of grain boundary by the iron-based carbides
increases as the cooling time period in the second
temperature range increases. For this reason, to
make the coverage factor be not more than 80%, the
cooling time period in the second temperature range
is preferably shorter. From this viewpoint as well,
it is very effective to make the average cooling rate
in the second temperature range be not less than
100°C/sec. To surely obtain a desired structure, the
average cooling rate in the second temperature range
is preferably not less than 150°C/sec, and more
preferably not less than 200°C/sec. An upper limit of
the average cooling rate in the second temperature
range is not particularly specified, and in an
industrial sense, a range of not more than 500°C/sec
is practical. Here, the average cooling rate in the
second temperature range is a value obtained by
dividing the difference between the Ar3 point and (Ms
point - 50) by the time period taken for the cooling.
[0059] (Third temperature range)
In the third temperature range (from (Ms point -
50)°C to 100°C), iron-based oxides are likely to
precipitate in grains of prior austenite, as
described above. Making iron-based carbides
precipitate in grains allows to obtain excellent lowtemperature
toughness. When the average cooling rate
- 32 -
in the third temperature range is more than 50°C/sec,
precipitation in grains is deficient resulting in
that a large amount of dissolved C remains in steel
sheet, thereby deteriorating low-temperature
toughness. Therefore, the average cooling rate in
the third temperature range is not more than 50°C/sec.
To surely obtain a desired structure, the average
cooling rate in the third temperature range is
preferably not more than 30°C/sec, and more preferably
not more than 20°C/sec.
[0060] Even if the average cooling rate is not more
than 50 °C/sec/ when the cooling rate in a temperature
range from (Ms point ~ 120) °C to 100°C in the third
temperature range is more than 70°C/sec, precipitation
in prior austeri'ite grains is deficient, making it
impossible to achieve sufficient low-temperature
toughness. Therefore, the maximum cooling rate in
the temperature range from (Ms point - 120)°C to 100°C
is not more than 70°C/sec. Moreover, even if the
average cooling rate is not more than 50°C/sec, when
the cooling rate in a temperature range from (Ms
point - 120)°C to 100°C in the third temperature range
is less than 5°C/sec, ferrite excessively precipitates
during cooling, and it is not possible to make the
total of the martensite area fraction and the bainite
area fraction be not less than 95%. Moreover, the
iron-based carbides that precipitate at a grain
boundary increase so that the coverage factor of
grain boundary by iron-based oxides is more than 8 0%.
- 33 -
Therefore, the minimum cooling rate in the
temperature range from (Ms point - 120}°C to 100°C is
not less than 5°C/sec.
[0061] (Fourth temperature range)
In the fourth temperature range (less than
10 0°C), since precipitation of iron-based carbides is
very unlikely to occur, and also transformation does
not occur, as described above, there is no need of
particularly controlling the cooling rate.
[0062] Thus, it is possible to manufacture a hotstamped
part according to the present embodiment,
which has excellent strength and low-temperature
toughness.
[0063] According to the method of manufacturing the
hot-stamped part according to the present embodiment,
since appropriate temperature control is performed,
it is possible to obtain a hot-stamped part having an
appropriate structure, thereby achieving excellent
tensile strength and low-temperature toughness.
£0064] Other conditions of hot stamp forming, such
as a type of forming and a kind of die, may be
appropriately selected within a range not impairing
the effects of the present embodiment. For example,
the type of forming may include bending, drawing,
bulging, hole expanding, and flange forming. The
kind of die may be appropriately selected depending
on the type of forming.
[0065] The steel sheet for hot stamping may be a
hot-rolled steel sheet or a cold-rolled steel sheet.
- 34 -
An annealed hot-rolled steel sheet or annealed coldrolled
steel sheet, which is obtained by subjecting a
hot-rolled steel sheet or cold-rolled steel sheet to
annealing, may also be used as the steel sheet for
hot stamping.
[0066] The steel sheet for hot stamping may be a
surface treated steel sheet such as a plated steel
sheet. That is, a steel sheet for hot stamping may
be provided with a plating layer. The plating layer
contributes to enhancement of corrosion resistance,
for example. The plating layer may be an
electroplating layer or a hot-dip plating layer. The
electroplating layer is exemplified by an
electrogalvanizing layer, and a Zn-Ni alloy
electroplating layer. The hot-dip plating layer is
exemplified by a hot-dip galvanizing layer, an
alloyed hot-dip galvanizing layer, a hot-dip aluminum
plating layer, a hot-dip Zn-Al alloy plating layer, a
hot-dip Zn-Al-Mg alloy plating layer, and a hot-dip
Zn-Al-Mg-Si alloy plating layer. The coating weight
of the plating layer is not particularly limited, and
may be, for example, a coating weight within a common
range. A plating layer is provided on a heat
treated steel material in the same way as a steel
sheet for heat treatment.
[0067] Subsequently, an example of a method of
manufacturing a steel sheet for hot stamping will be
described. In the manufacturing method, casting, hot
rolling, pickling, cold rolling, annealing, and
- 35 -
plating treatment are performed to manufacture a
plated steel sheet, for example.
{0068] In casting, a slab is cast from a molten
steel having the above described chemical
composition. As the slab, a continuous casting slab
and a slab made by a thin slab caster may be used - A
process such as a continuous casting-direct rolling
(CC-DR) process, in which hot rolling is performed
immediately after a slab is cast, may be applied.
[0069] The temperature of the slab before hot
rolling (slab heating temperature) is preferably not
more than 1300°C. If the slab heating temperature is
excessively high, not only the productivity
deteriorates, but also the manufacturing cost
increases. Therefore, the slab heating temperature
is preferably not more than 1250°C. When the slab
heating temperature is less than 1050°C, the
temperature is lowered in finish rolling, thereby
causing the rolling load to increase. As a result,
not only the rollability may deteriorate, but also
shape defects may occur in the steel sheet.
Therefore, the slab heating temperature is preferably
not less than 1050°C.
[00703 The temperature of finish rolling (finish
rolling temperature) in hot rolling is preferably not
less than 850 °C. When the finish rolling temperature
is less than 850°C, the rolling load may increase,
leading to that not only the rolling may be
difficult, but also shape defects may occur in the
- 36 -
steel sheet. An upper limit of the finish rolling
temperature is not particularly specified, and the
finish rolling is preferably performed at not more
than 1000°C. This is because, when the finish rolling
temperature is more than 100 0°C, the slab heating
temperature is excessively increased to obtain a
temperature of more than 1000°C.
[0071] The temperature in coiling the hot-rolled
steel sheet (coiling temperature) after the end of
hot rolling is preferably not more than 700°C. When
the coiling temperature is more than 7 00°C/ a thick
oxide may be formed on the surface of the hot-rolled
steel sheet, deteriorating a pickling property
thereof. When cold rolling is performed after the
coiling, the coiling temperature is preferably not
less than 600°C. This is because v/hen the coiling
temperature is less than 600°C, the strength of the
hot-rolled steel sheet may excessively increase,
thereby causing sheet rupture and shape defects
during cold rolling. Rough-rolled sheets after rough
rolling may be joined together during hot rolling to
perform finish rolling in a continuous manner.
Further, finish rolling may be performed after once
coiling the rough-rolled sheet.
[0072] Oxides on the surface of the hot-rolled steel
sheet are removed by pickling. Pickling is
particularly important to improve the hot-dip
platability on the occasion of manufacturing a hotdip
plated steel sheet, such as a hot-dip aluminum
- 37 -
plated steel sheet, a hot-dip galvanized steel sheet,
an alloyed hot-dip galvanized steel sheet, and the
like. The number of times pickling is performed may
be one or more times.
[0073] In the cold rolling, for example, a rolling
reduction ratio is 30% to 90%. When the rolling
reduction ratio is less than 30%, it may be difficult
to keep the shape of the cold-rolled steel sheet
flat. Moreover, it is sometimes difficult to achieve
sufficient ductility after cold rolling. When the
rolling reduction ratio is more than 90%, the rolling
load excessively increases, making the cold rolling
difficult. To achieve more excellent ductility, the
rolling reduction ratio is preferably not less than
4 0%, and to achieve more excellent rollability, the
rolling reduction ratio is preferably not more than
70%. The number of rolling passes in the cold
rolling, and the rolling reduction ratio for each
pass are not particularly limited.
[007 4] Annealing is performed in, for example, a
continuous annealing line or a box-type furnace. The
condition of annealing is not particularly limited,
and it is preferably of a level that allows the steel
sheet strengthened by cold rolling to be
appropriately softened. For example, the annealing
temperature is preferably within a range of 550°C to
8 50°C. By performing annealing within this
temperature range, dislocations introduced during
cold rolling are relieved by recovery,
- 38 -
recrystallization, and/or phase transformation.
[0075] As the plating treatment, for example, a hotdip
plating treatment or an electroplating treatment
is performed, The hot-dip plating treatment includes
a hot-dip aluminum plating treatment, a hot-dip
galvanizing treatment, an alloyed hot-dip aluminum
plating treatment, and an alloyed hot-dip galvanizing
treatment. According to the hot-dip plating
treatment, it is possible to achieve such effects as
inhibiting the formation of scale and enhancing
corrosion resistance. To inhibit the formation of
scale in a hot-stamped part, a thicker plating layer
is more preferable. To form a thicker plating layer,
a hot-dip galvanizing treatment is more preferable
than an electroplating treatment. Ni, Cu, Cr, Co,
Al, Si or Zn, or any combination thereof may be
included in a plating layer formed by the plating
treatment. Moreover, to improve plating
adhesiveness, a plating layer of Ni, Cu, Co or Fe, or
any combination thereof may be formed on the coldrolled
steel sheet before annealing.
[0076] Note that all of the above described
embodiments merely show examples for practicing the
present invention, and those should not be
interpreted as liming the technical scope of the
present invention. That is, the present invention
can be practiced in various forms without departing
from its technical concept or its principal features.
Examples
- 39 -
[0077] Subsequently, an example of the present
invention v/ill be described. The condition shown in
the example indicates merely one condition which is
adopted to confirm the feasibility and effect of the
present invention, and the present invention will not
be limited to the example of this one condition. The
present invention can adopt various conditions as
long as its objective is achieved without departing
from the gist of the present invention.
[0078] In this experiment, slabs v/ere cast using
steels {steel types a to r and A to H) having
chemical compositions listed in Table 1, and hot
rolling was performed under the conditions listed in
Tables 2 and 3. For some of the hot-rolled steel
sheets, cold roiling was performed after hot rolling.
For some of the cold-rolled steel sheets, plating
treatment was performed by a continuous annealing
facility or a continuous hot-dip plating facility
after cold rolling. in this way, various steel
sheets for hot stamping (a hot-rolled steel sheet, a
cold-rolled steel sheet, a hot-dip galvanized steel
sheet, an alloyed hot-dip galvanized steel sheet, or
a hot-dip aluminum plated steel sheet) were prepared.
Under a condition in which a hot-rolled steel sheet
was used as the"steel sheet for hot stamping, the
thickness of the hot-rolled steel sheet was 1.6 mm.
Under a condition in which a steel sheet other than
the hot-rolled steel sheet was used as the steel
sheet for hot stamping, the thickness of the hot-
- 40 -
rolled steel sheet was 3.2 mm, the rolling reduction
ratio of cold rolling was 50%, and the thickness of
the cold-rolled steel sheet was 1.6 mm. Blanks in
Table 1 indicate that the content of the
corresponding element was less than a detection
limit. An underline in Table 1, 2, or 3 indicates
that the numerical value thereof was out of the scope
of the present invention.
[007 9] After a steel sheet for hot stamping was
prepared, hot stamp forming was performed under the
conditions listed in Tables 4 and 5 to obtain hotstamped
part. In Tables 4 and 5, the minimum cooling
rate indicates a minimum value of the cooling rate in
a temperature range from (Ms point - 120) °C to 100°C,
and the maximum cooling rate indicates a maximum
value of the cooling rate in the temperature range
from (Ms point - 120)°C to 100°C. An underline in
Tables 4 or 5 indicates that the numerical value
thereof was out of the scope of the present
invention.
[0080] Then, measurement of tensile property,
observation of structure, and evaluation of lowtemperature
toughness for each hot-stamped part were
performed.
[0081] In the measurement of tensile property, a
tensile test specimen conforming to JIS Z 2201 was
taken, and a tension test was performed in conformity
to JIS Z 2241 to measure tensile strength. These
results are listed in Tables 6 and 7. An underline
- 41 -
in Table 6 or 7 indicates that the numerical value is
out of a desired range in the present invention.
[0082] In the observation of structure, an area
fraction of martensite, an area fraction of bainite,
an area fraction of ferrite, and an area fraction of
retained austenite, a coverage factor of prior
austenite grain boundary by iron-based carbides and a
number density of iron-based carbides in prior
austenite grains were measured.
[0083] The area fraction of martensite, the area
fraction of bainite, and the area fraction of ferrite
were determined by taking a sample which had a crosssection
in parallel with the rolling direction and
the thickness direction of the hot-stamped part as an
observation surface, polishing the observation
surface, performing Nital etching, and observing a
portion of the steel sheet at a depth of 1/8 to 3/8
thickness thereof with an FE-SEM. In the
observation, area fractions of each structure were
measured in 10 visual fields at a magnification of
5000 times for one hot-stamped part, and an average
value thereof was adopted as the area fraction of
each structure in the hot-stamped part. The area
fraction of retained austenite was determined from an
X-ray diffraction intensity ratio between ferrite and
austenite. Pearlite was not observed.
[0084] The coverage factor of prior austenite grain
boundary by iron-based carbides was obtained by the
method described with reference to Figure 1. That
- 42 -
is, for each hot-stamped part, a value represented by
W(X/L) x 100" (%) was determined.
[0085] In the evaluation of low-temperature
toughness, a Charpy impact test was performed at -
120°C. Then, evaluation was made such that a result
was graded as a pass (O) when it exhibited an
absorption energy, which was obtained by converting a
measured absorption energy to that of a specimen
having a thickness of 10 mm, of not less than 50 J/cm2
and a percent ductile fracture of not less than 50%,
and was graded as a fail (X) when it did not satisfy
either one or both of them.
[0086] As listed in Tables 6 and 7, in inventive
examples, in which all the conditions were within the
scope of the""present invention, it was possible to
achieve a tensile strength of not less than 1180 MPa
and excellent low-temperature toughness. On the
other hand, in comparative examples, in which any one
or more kinds of conditions were out of the scope of
the present invention, it was not possible to achieve
a tensile strength of not less than 1180 MPa and/or
excellent low-temperature toughness.
[0087] In conditions a-7, b-7, c-7, n-7, and q~7,
since the heating temperature of hot stamping was too
low, the area fractions of martensite and bainite
were deficient so that the desired tensile strength
was not achieved.
[0088] In conditions a-8, b-8, c-8, n-8, and q-8,
since the average cooling rate in the second
- 43 -
temperature range was too low, the area fractions of
martensite and bainite were deficient so that the
desired tensile strength was not achieved. Moreover,
the coverage factor by iron-based carbides increased
so that excellent low-temperature toughness was not
achieved.
[0089] In conditions a-9, b-9, c-9, n-9, and q-9,
since the minimum cooling rate in the temperature
range from (Ms point - 120)°C was low, the area
fractions of martensite and bainite were deficient in
the hot-stamped part so that the desired tensile
strength was not achieved- Moreover, the coverage
factor by iron-based carbides increased so that
excellent low-temperature toughness was not achieved.
[0090] In conditions a-10, b-10, c-10, n-10, and q-
10, since the maximum cooling rate in a temperature
range from (Ms point - 12 0)°C to 10 0°C in hot stamping
was too high, precipitation of iron-based carbides in
grains of prior austenite was deficient so that
excellent low-temperature toughness was not achieved.
[0091] In conditions a-11, b-11, c-11, n-11, and q-
11, since the average cooling rate in a third
temperature range in hot stamping was too high,
precipitation of iron-based carbides in grains of
prior austenite was deficient so that excellent lowtemperature
toughness was not achieved.
[0092] In conditions A-l, B~l, C-l, D-l, E-l, F-l,
G~l, and H-l, since the chemical compositions were
out of the scope of the present invention, a tensile
- 44 -
strength of not less than 1180 MPa and/or excellent
lov-?-temperature toughness were/was not achieved. For
example, in condition B-1, the C content was too high
so that the strength was excessively high and
excellent lov/-temperature toughness was not achieved.
In condition F-l, since the total of the Mn content
and the Cr content were too high, excellent lowtemperature
toughness was not achieved.
[0093] [Table 1]
- 45 -
j
INDUSTRIAL APPLICABILITY
[0100] The present invention may be utilized for
industries for manufacturing and utilizing, for
example, a hot-stamp part used for automobiles, and
others. The present invention may also be used for
industries for manufacturing and utilizing another
machine structural part.

CLAIMS
[1] A hot-stamped part comprising:
a chemical composition represented by, in mass%:
C: 0.120% to 0.400%;
Si: 0.005% to 2.000%;
Mn or Cr, or both thereof: 1.00% to 3.00% in
total;
Al: 0.005% to 0.100%;
B: 0.0003% to 0.0020%;
P: not more than 0.030%;
S: not more than 0.0100%;
O: not more than 0.0070%;
N: not more than 0.0070%;
Ti: 0% to 0.100%;
Nb:* 0% to 0.100%;
V: 0% to 0.100%;
Ni: 0% to 2.00%;
Cu: 0% to 2.00%;
Mo: 0% to 0.50%;
Ca or REM, or both thereof: 0% to 0.0300% in
total; and
the balance: Fe and impurities; and
a structure represented by:
an area fraction of martensite or bainite,
or both thereof: not less than 95% in total;
a coverage factor of prior austenite grain
boundary by iron-based carbides: not more than 8 0%;
and
a number density of iron-based carbides in
- 53 -
prior austenite grains: not less than 4 5/u.m2.
[2] The hot-stamped part according to claim 1,
v/herein the chemical composition satisfies:
Ti: 0.005% to 0.100%;
Nb: 0.005% to 0.100%; or
V: 0.005% to 0.100%; or
any combination thereof.
[3] The hot-stamped part according to claim 1 or 2,
wherein the chemical composition satisfies:
Ni: 0.05% to 2.00%;
Cu: 0.05% to 2.00%; or
Mo: 0.05% to 0.50%; or
any combination thereof.
[4] The hot-stamped part according to any one of
claims 1 to 3, wherein the chemical composition
satisfies
Ca or REM, or both thereof: 0.0005% to
0.0300% in total.
[5] A method of manufacturing a hot-stamped part,
comprising the steps of;
heating a steel sheet to a temperature of not
less than Ac3 point and not more than 950°C at an
average heating rate of not less than 2°C/sec;
then, cooling the steel sheet through a
temperature range from a Ar3 point to (Ms point -
50)°C at an average cooling rate of not less than
100°C/sec while performing hot pressing; and
then, cooling the steel sheet through a
temperature range from (Ms point - 50)°C to 100°C at
- 54 -
an average cooling rate of not more than 50°C/sec,
wherein
the steel sheet comprises a chemical composition
represented by, in mass%:
C: 0.120% to 0.400%;
Si: 0.005% to 2.000%;
Mn or Cr, or both thereof: 1.00% to 3.00% in
total;
Al: 0.005% to 0.100%;
B: 0.0003% to 0.0020%;
P: not more than 0.030%;
S: not more than 0.0100%;
O: not more than 0.0070%;
N: not more than 0.0070%;
Tir0% to 0.10 0%;
Nb: 0% to 0.100%;
V: 0% to 0.100%;
Ni: 0% to 2.00%;
Cu: 0% to 2.00%;
Mo: 0% to 0.50%;
Ca or REM, or both thereof: 0%-0.0300% in
total; and
the balance: Fe and impurities, and
a maximum cooling rate is not more than 70°C/sec
and a minimum cooling rate is not less than 5°C/sec in
a temperature range from (Ms point - 120)°C to 100°C.
[6] The method of manufacturing the hot-stamped part
according to claim 5, wherein the chemical
composition satisfies:
- - 55 -
Ti: 0. 005%-0.100%;
Nb: 0.005%-0.100%; or
V: 0.005%-0.100%; or
any combination thereof.
[7] The method of manufacturing the hot-stamped part
according to claim 5 or 6, wherein the chemical
composition satisfies:
Ni:0.05%-2.00%;
Cu: 0 . 05%-2.00%; or
/{St
Mo: 0.05%~0.50%; or
any combination thereof.
[8] The method of manufacturing the hot-stamped part
according to any one of claims 5 to 1, wherein the
chemical composition satisfies
Ca or tt"EM or both thereof: 0.0005%-0.0300%
in total.